Thermal Processes , ‘-.. \
Municipal solid w~ste’is high in organic contentand may also ny II ill II and hazardous materials, Thus~ the comb’;~on of MSW at hi II \11111 tures, sufficient to kill infectious aqents and produce energy ne! hllll’
\
ash, has been attractive to communities that either have no landfill’. lit ‘”,
landfilling is too costly. The burning of dumps is, of course, ancient history, as thi I’. III I
ger permitted in most countries due to air pollution and wat I I” 1111
problems. Engineered MSW combustion facilities, called incineratol ,
first used in Europe and later adopted by many larger municip li\1I III
United States. For the most part, these early incinerators were poorly desiqmu l
operated and produced not only horrendous air pollution but also WI II ”
ficient in volume reduction and disinfection.’ The technology of incineration advanced with time, es (‘11.111
Europe, and with strict air pollution control and better managem nt (” I I waste input stream, incineration remained an option for many munic ‘I 10IIt II
There was always the notion among combustion engineers that in illt I III
was an unfortunate operation. It was, and is, a method of wastinq 1111 I
High-energy material is introduced as the feed and low-energy rn.ill” I
produced. Why can’t this energy be recovered?
‘The incinerator constructed in the 1940 in Durham, NC, was derisively called the
“Durham toaster,”
II Iii 1’1 I) I I I (J II 11 III willi II IHII ill Y I’m wi I
y bulklln J I hut th s Y t m 11 Hlel . h next designs w r
o I-fired power plant wh r w ter was converted to steam
us d for generating electricity. Again, it took some time to
propriate technology since MSW is quite different from coal
ves in a combustor. It is more corrosive, it has higher water
often it is unpredictable.
II X step in the development of MSW combustion was a better
m processing of the feed. This feed material can also be I with coal or other fossil fuels and burned in so-called co-fired
III II n units. the two oil embargos in the 1970s, many communities embraced
n rgy as a solution to managing their solid waste and also for pro-
f III I”l wable energy. In 1980, the United States combusted 2.7 million
“I M W. By 1990, 29.7 million tons of MSW were combusted. During Ir I ,there were other factors that contributed to the rapid growth
nts, such as favorable tax treatment, tax exempt financing, and
ower-purchase agreements from utilities.
lit w ver, after this decade of expansion, very few new plants have 11I11t. In fact, 22 years after 1990, the amount of MSW combusted
leased to 29.3 million tons. In the 1980s, California had planned
III I rous waste-to-energy plants from San Francisco to San Diego. In
VI nly three were built in the late 1980s with all of the other planned
, I ing abandoned. The last planned California waste-to-energy plant
I I ated by the San Diego County Board of Supervisors in 1991. There
III ny reasons why new plants have not been built in the United States, I the inability to site new plants and the high capital and operating
hi build a plant compared to the cost of landfilling.
N vertheless, in the year 2012, 11.7% of the MSW produced in the
d States was combusted in facilities that included energy recovery.” he United States, in Europe, modern plants have continued to be
Municipal solid w~ste’is high in organic contentand may also ny II ill II and hazardous materials, Thus~ the comb’;~on of MSW at hi II \11111 tures, sufficient to kill infectious aqents and produce energy ne! hllll’
\
ash, has been attractive to communities that either have no landfill’. lit ‘”,
landfilling is too costly. The burning of dumps is, of course, ancient history, as thi I’. III I
ger permitted in most countries due to air pollution and wat I I” 1111
problems. Engineered MSW combustion facilities, called incineratol ,
first used in Europe and later adopted by many larger municip li\1I III
United States. For the most part, these early incinerators were poorly desiqmu l
operated and produced not only horrendous air pollution but also WI II ”
ficient in volume reduction and disinfection.’ The technology of incineration advanced with time, es (‘11.111
Europe, and with strict air pollution control and better managem nt (” I I waste input stream, incineration remained an option for many munic ‘I 10IIt II
There was always the notion among combustion engineers that in illt I III
was an unfortunate operation. It was, and is, a method of wastinq 1111 I
High-energy material is introduced as the feed and low-energy rn.ill” I
produced. Why can’t this energy be recovered?
‘The incinerator constructed in the 1940 in Durham, NC, was derisively called the
“Durham toaster,”
II Iii 1’1 I) I I I (J II 11 III willi II IHII ill Y I’m wi I
y bulklln J I hut th s Y t m 11 Hlel . h next designs w r
o I-fired power plant wh r w ter was converted to steam
us d for generating electricity. Again, it took some time to
propriate technology since MSW is quite different from coal
ves in a combustor. It is more corrosive, it has higher water
often it is unpredictable.
II X step in the development of MSW combustion was a better
m processing of the feed. This feed material can also be I with coal or other fossil fuels and burned in so-called co-fired
III II n units. the two oil embargos in the 1970s, many communities embraced
n rgy as a solution to managing their solid waste and also for pro-
f III I”l wable energy. In 1980, the United States combusted 2.7 million
“I M W. By 1990, 29.7 million tons of MSW were combusted. During Ir I ,there were other factors that contributed to the rapid growth
nts, such as favorable tax treatment, tax exempt financing, and
ower-purchase agreements from utilities.
lit w ver, after this decade of expansion, very few new plants have 11I11t. In fact, 22 years after 1990, the amount of MSW combusted
leased to 29.3 million tons. In the 1980s, California had planned
III I rous waste-to-energy plants from San Francisco to San Diego. In
VI nly three were built in the late 1980s with all of the other planned
, I ing abandoned. The last planned California waste-to-energy plant
I I ated by the San Diego County Board of Supervisors in 1991. There
III ny reasons why new plants have not been built in the United States, I the inability to site new plants and the high capital and operating
hi build a plant compared to the cost of landfilling.
N vertheless, in the year 2012, 11.7% of the MSW produced in the
d States was combusted in facilities that included energy recovery.” he United States, in Europe, modern plants have continued to be
built. h IfF r n uro I~ Il1crl h wn n II lill Chapter 2. Theoretically, the combustion of refuse pro uc dye 111111111 is sufficient to provide about 5 to 15% of the electrical pow r n h III’ II residents. Refuse therefore is not an insignificant source of power.
While some would argue that waste-to-energy technology’ tilll II
passed, the renewed focus on domestically produced renewabl< 1111
may cause communities to once again consider waste-to-en i<IY I’ nuclear catastrophe in Fukushima has lighted the discussions IIII1 II
options for alternative energy generation. Principally, just burying th WI
in landfills is a loss of materials and energy, and therefore, countri ” wit, mainly landfill their wastes have a high potential to increase their ‘,11111
renewable energy from waste. Internationally, waste-to-energy (W II )
energy-from-waste (EtWl, as it is called in Europe, continues to b “wl,1 used alternative to landfilling.
While data about global installations are scarce, a recent report ‘.1 11
Today, almost 2,200 WTE plants are active worldwide. They heVI
a disposal capacity of around 270 million tons of waste per y ,II
More than 200 thermal treatment plants with a capacity of OWl 60 million annual tons were constructed between 2009 and 2013
We estimate almost 500 new plants with a capacity of aboul
160 million annual tons to be constructed by 2023.42
Many of the international plants incorporate advanced features that It I
been developed in the last decades.
7-1 HEAT VALUE OF REFUSE
One of the earliest measures of heat energy still widely used by American 1’1111 neers is the British thermal unit (Btu), which is defined as that amount of’ 1’111’1 I necessary to heat one pound of water one degree Fahrenheit. The internatiou.rll accepted unit of energy is the Joule. Other common units for energy ,III’ lit calorie and kilowatt-hour (kWh)-the former used in natural sciences, L1w I.IIill in engineering.
Another expression of heat value, used by the oil industry’s Intern ,1 I 11111 I Energy Agency (lEA) and Organization for Economic Co-operation ,III I
I>a • I )II III(‘
Ultimate Analysis , ,,’,H,I uualvsis uses the chemical makeup of the fuel to approximate its h al III III’ III st popular method using ultimate analysis is the DuLong equation, I 11111 gillally was developed for estimating the heat value of coal:
IItll/lb = 145 C + 620( H – ~O) + 40 S
,. :, II, r and S are the weight percentages (dry basis) of carbon, hydrogen, oxy- I “Id ulfur, respectively. The Dulong formula is cumbersome to use in pra Lie’,
I It ill)I’ n t give acceptable estimates of heat value for materials other than oa1.1
1 Energy Conversions
To Multiply By
Calories Joules kWh toe Btu Joules kWh toe Btu Calories kWh toe Btu Calories Joules toe Btu Calories Joules kWh
252 1054 2.93 X 10-4 2.52 X 10-8 3.97 X 10-3 4.18 1.16 X 10-6 1.00 X10-1O 9.49 X 10-4 0.239 2.78 X 10-7 2.39 X 10-11 3413 8.62 X 105 3.60 X 106 8.60 X 10-5 3.97 X 107 1.00 X 1010 4.18 X 10-10 1.16 X 104
Table 7-2 Typical Ultimate Analyses of MSW
An th ‘I’ • 11I.1I1~II fOI (Mllll1,1t11l1\ Ill’ 11’11 vIIIII’ 01 , ’11’ I’ III II, analysis is2
It plastics, percent by weight of total MSW,on dry basis food waste, percent by weight of total MSW,on dry basis
‘) paper, percent by weight oftotal MSW,on dry basis W water, percent by weight, on dry basis
Illig ” gression analysis and comparing the results to actual measurern nts II V 11I , an improved form of a compositional model is suggested:”
Illu/ib = 1238 + lS.6R + 4.4P + 2.7G – 20.7W
Btu/lb = 144 C + 672 II -1-.2 41.11 S 10,8 N where C, H, 0, S, and N are again the weight p r ntc S ( lry ill’ hydrogen, oxygen, sulfur, and nitrogen, respectiv ly, in th mbusnhk-I: II I the fuel. That is, the sum of all of these percentages must add I I O()I~o
Other approaches are used to estimate the heat value of W(\$II’H 11111 Iii its elemental composition and a correction term for the mOiSIUf’\’101111 III empirical formula according to Boie” as given in source [44] giv s ll1\’ ‘II I I II value (NCV), which is another term used for the Low HeatValu (LlIV) 01 I III!
NCV [MJ/kg] = 34.8 C + 93.9 H + 10.5 S + 6.3 N – 10. () ” where: C = carbon content [kg/kg], H = hydrogen content Ikg/kt41, ~l content [kg/kg], N = nitrogen content [kg/kg], 0 = oxygen cant III 1I1f,./1 W = water content [kg/kg].
As mainly organics contribute to the energy content, only the nJ:ljOl I II HI typically found in organic and biochemical compounds are consi 1\’11 Ii III I formula.
The elemental analysis can be conducted using standard mctlu«] III those published by the American Society for Testing and Materials (t\~ II’- I) elements C, H, N, and S are measured in one procedure by thermally ti1’11III11 ing the sample at high temperatures (typically> 1000°C) in a catalyt I I The introduction of oxygen produces the gases N02, CO2, H20, ami, I’ II are separated in a gas-chromatograph (GC) and analyzed by thermo-cmulu ” Moisture is usually determined by thermo-gravimetric (TG) measun’uu-ut oxygen can be measured by pyrolysis of the sample in helium gas wi \I’ll II is tied up as carbon monoxide. The CO can then be measured with ill Table 7-2 lists some typical values found in MSW.
V lu
H V I Ii I 11111/11 I IY W I h’
70 0 14,000
7500 10,000
7500 2800 8000
60 300 300
3000
7-1-2 Compositional Analysis Ultimate analysis provides a theoretical maximum of the heat value but till take into account any inefficiencies or chemical interactions that Illip.111lit during combustion. An alternative to ultimate analysis is compositionai 11111’/, which are based on the actual content of the MSW sample. One such fOll1lll1.1 I
Btu/lb = 49R + 22.S(G + P} – 3.3W
/~= plastics, percent by weight, on dry basis t’ = paper, percent by weight, on dry basis , = food wastes, percent by weight, on dry basis
W = water, percent by weight, on dry basis
II ‘v n more exact compositional analysis is possible if the composition or t W r processed fuel is known. The heat value of the complex fuel can b
I 111.11 \ I by using typical heat values of its components, as listed in Table 7-3.The 111.11 n is shown in Example 7-l.
Constituent Average of Three Different Samples of MSW
Carbon Hydrogen Oxygen Sulfur Nitrogen
51.9 7.0
39.6 0.37 1.1
rocessed refuse-derived fuel has the following composition:
mponent Fraction by Weight, Dry Basis
0.50 0.10 0.30 0.10Source: [4J
Note: The numbers in this table have been adjusted for zero inerts and zero moisture.
./,——
typic I v lu s in 7,
0.50 (7200) + 0.10 (2000) + 0.30 (14,000) + 0.10 (60) = 8 611111/11,
Some of the components of refuse may not always be tuned finely 1’11111111 such a compositional analysis. Paper, for example, comes in many C(lH’l\llIit I each of these categories has its own heat value, as shown in Tabl 7-tl.
If the waste composition is known in terms of the wet weight. lill’ II II II of moisture has to be subtracted from the composition fractions. Typical nlill III concentrations of MSW components are shown in Table 7-5. The allul,lIllll heat value for a wet sample is shown in Example 7-2.
Table 7-4 Heat Value of Various Types of Paper
Type of Paper Heat Value, Btu/lb Dry Weight
Newspaper Cardboard Kraft Beverage and milk boxes Boxboard TIssue Colored office paper White office paper Envelopes Treated paper Glossy paper
7520 6900 6900 6800 6700 6500 6360 6230 6150 6000 5380
Source: [8]
Table 7-5 Typical Moisture Contents of MSW
Component Typical Moisture, Percent
Food waste Paper Cardboard Plastics Textiles Rubber Leather Garden trimmings Wood Glass Nonferrous metals Ferrous metals Dirt, ashes, other fines
70 6 5 2
10 2
10 60 60 2 2 3 8
Source: Adapted from [9]
11’1)( lilt II 1)( II Vill\(
hewn in Table 7-5, 6% ofthe w t weight of paper is water that does III I tribute to the heat value. Hence, the first term from the calcula- III 1\ In xample 7-1 is multiplied by 1.0 – 0.06 = 0.94, and so on.
,50 (7200) 0.94 + 0.10 (2000) 0.30 + 0.30 (14,000) 0.98 + 0.10 (60) 0.98 = 7570 Btu/lb
Proximate Analysis I (1111III te analysis, it is assumed that the fuel is composed of two typ S or IIII ,iI : v latiles and fixed carbon. The amount of volatiles can be estimat d I y
II W .ight when the fuel sample is burned at some elevated temperature, su h 1,110 ir oooe, and the fixed carbon is estimated by the weight loss when the
111\111’1/1 ombusted at 950°C. A commonly used proximate analysis equation for 1IIIliltingthe heat value of refuse is
I3tujlb = 8000A + 14,500B
A = volatiles, fraction of all dry matter lost at 6000e B = fixed carbon, fraction of all dry matter lost between 6000e and 950°
1111111’I’ form of a proximate analysis equation is Btu/lb = 2500D – 330W
D = fraction volatile material, dry basis, defined as weight loss at 80aoe W = fraction water, dry basis
III nding on whether the moisture is included or not, the results are presented 1I111/lbas received or as Btujlb on dry basis. Table 7-6 lists some representativ
III 1111′.volatiles, and fixed carbon fractions for several refuse components.’?
4 Calorimetry ,1,.,Imetry is the referee method of determining the heat value of mixed fuels.
I III(‘ 7-1 shows a schematic sketch of a bomb calorimeter. The bomb is a stainless- , I I h 11 that screws apart. The ball has an empty space inside into which th ‘ 11111to be combusted is placed. A sample of known weight (such as a small II t I of coal) is placed into the bomb, and the two halves are screwed shut. Oxygen 11th I high pressure is then injected into the bomb, and the bomb is placed in
J
Table 76 Typic W mp n nt
r tlon by Wight
Component Moisture Volatile Fix d
Mixed paper 0.102 0.759 0.084 Yard waste 0.752 0.186 0.045 Food waste 0.783 0.170 0.036 Polyethylene 0.002 0.985 0.001 Wood 0.200 0.697 0.113
Source: [10] Note: If the values in Table 7-6 are to be used in the proximate analysis equation, th 1,,11 111111′ have to be recalculated on the basis of dry matter.
an adiabatic water bath with wires leading from the bomb to a sourc 01 1’1i’! IIII I current. By means of a spark from the wires, the material in the ste I h.1I1111111 busts and heats the bomb, which in turn heats the water. The temper, (1111′ I I’ll I the water is measured with a thermometer and recorded as a function 1)1 I III Figure 7-2 shows the trace of a typical calorirn eter curve.
To electrical contact
Thermometer
Stirrer
Figure 7-1 Bomb calorimeter used to measure heat value of a fuel.
A” (III (I III (J III ()III ()!III
Ii • •
I I I I I I R I I I I I I
It Ti I______——‘1,,…,. ..'” rrz •.•• ;:-.:7…1′. :- Preperiod – •.•.!.•.·—Rise period –_ •…..•·I-postp riod
I 9 10 11 12 13 14 15 16 17 IH I
·········l.. I I I J J J
I I
0.63R
o 1 2 3 4 5 6 7 8 Time,min
– — – —— , 7·2 Temperature/time trace from a bomb calorimeter.
r accurately, Figure 7-2 is a plot of temperature (T) versus time (t) an I is I11III .1 thermogram. Because the initial (preperiod) and final (postperiod) slo] ·s I !II th rrnogram may be different and because the temperature rise due t a IIIIIIl al reaction is usually nonlinear, it is necessary to decide on a procedur LO
tililott) the true net temperature rise (~TJ in the experiment. A common meth d II Ixtrapolate the postperiod slope backward in time and the preperiod slope
” 011′ I in time (the dotted lines in Figure 7-2). The net temperature rise (~’/:J III II’ measured from the difference of these two extrapolated lines at some inter-
III tllll ‘ time in the reaction period. The exact location of this intermediate time I I” 11 I on the calorimeter and on the reaction being studied. A frequently used 1111lion for estimating this time is to measure ~T at the point where the shaded I I II the figure are equal. It can be shown that these shaded areas are equal Iii II the temperature increase is approximately O.63t..T. From the thermogram,
III rI r~rence between the initial temperature (T) and the final temperature (Tf) is ” II In perature rise (~T) of the reaction.
The water container of a bomb calorimeter is well insulated, so no heat is 11111 d to escape from the system. All of the energy liberated during the combus-
ill used to heat the water and the stainless steel bomb. The heat energy is calcu- li II as the temperature increase of the water times the mass of the water plus th ‘
./
b rnb. Sin ‘ II’ alorlc III 1’/1n d all Ill’ unu 11111 oj’ ‘1\ ‘I~ II’ ‘ • .11 101,1 temperature of on gram r wat r 11 U W’ ‘l’ C ‘I. iUH, nud I flowing th ‘ HI 1111 water in the calorimeter, it is possible l (I ult t ‘ lit’ .n r y in nlorlcs. I 1111 the weight of the sample, its heat value can b c I ul l d.
Each calorimeter is different and must be standardized using a 111.1111111 which the heat of combustion is known precisely. Plus, the heal gcn ‘1’:11″” ” combustion of the ignition wire must be taken into account if a urate 011,,11 are required. Typically, benzoic acid is used as the standard. A benz ic ,I( hi I” specially manufactured for this purpose is combusted in a bomb caloriuu-u I, , the temperature rise (LlT) is determined from the thermogram and used to I ,ii, III the heat capacity of the calorimeter C . That is,
u
UMb LlTCu
where Cv = heat capacity of the calorimeter, caIre U = heat of combustion of benzoic acid, cal/g, “C
Mb = mass of benzoic acid pellet, g LlT = rise in temperature from thermogram, “C
For very accurate determinations, the heat generated by the combustion 01 1111 III wire must be induded in the calculations.
[6318M” + 1643MJ C = —“———”’–
v LlT
where Cu = heat capacity of the calorimeter, caWe
Mb = mass of benzoic acid tablet used, g Mill = mass of ignition wire used, g LlT = temperature rise, “C
Note that 6318 is the heat of combustion of benzoic acid expressed as cal/I-\,1111 1643 is the heat of combustion of nickel chromium wire, cal/g.
A benzoic acid pellet weighing 5.00 g is placed in a bomb calorimeter along with 0.20 9 fuse wire. The benzoic acid is ignited, and the tem perature rise is 3.56°C. What is the heat capacity of this calorimeter?
c = [6318(5.00) + 1643(0.2)] _ 0 v 3.56 – 8966cal/ C
1/ (; ‘f’
I’
M
h ” t valu of unknown nl:11 .rial, Ijg rise in t mperatu r rrOI11UI zrrnogram, mass f the unknown material, g h at apacity of the calorimeter, caWe
u evLl T = 8966 X 4.72 = 4231 cal/g M 10.00
1111 onversion from cal/g to Btu/lb is 1.78 X cal/g = Btu/lb, so that Ih fu I has a heat value of 7531 Btu/lb.
II lmportant aspect of calorimetric heat values is the distinction betw .cn ,,, II, (1/ Illg value and lower heating value. The higher heating value (l-IIIV) is
I IliI 1 the gross calorific energy, while the lower heating value (LHV) is also II II the net calorific energy. The distinction is important in the design of
I I I Oil units. III \ lorimeter, as organic matter combusts, the products of combustion arc II ) arbon dioxide and water. The water produced is in vapor form. As th ‘ I II )1, however, this water condenses, yielding heat that is measured as part
” 1IIIHp rature rise. The HHV is calculated by including the contribution due II te« n: heat of vaporization (the heat required to produce steam from wat 1’). II It (‘ ndensation does not occur in a large furnace. The hot flue gases carry
III v por outside the furnace, and condensation cannot take place. The LlIV II Ill’ HHV minus the latent heat of vaporization that has occurred in the
II 1,t10rLmeter.For design purposes, the LHV is a much more realistic numb ‘I’, IIH’1i n engineers usually express heat values in terms of HHV. When h ‘nl III I’ fuse or other fuels are specified, therefore, it must be dear whether Ihe
I I I are expressed as HHV or LHV. 1\ ,1′ ny engineering practices, heat value refers only to HHV. In this I ‘XI, nth rwise specified, heat value refers to HHV. Although calorimetry is th ‘
I I 1 method of measuring heat value of a fuel, it does not actually sirnulat ‘
th }h<vi r or-II al III ‘1111II lull·s I I’ (0 IIII I!’ III, ‘l’II’I”,lil .11 I .1111111’1II I I whyth lIIIV numb r v r slim, tes the f1 11I,II11’ill value In )l1llll. 111111’11\1I j ence of metals, the incornpl L cornbusti n r rg, nl ,( 11I th Wtll’l 11’11\III a gaseous state.
Some metals, most notably aluminum, will oxidize at Slimell’llliI III temperatures to yield heat. The oxidation of aluminum can b d s rilwd ,I’
4Al + 302 –‘> 2(AlP3) + heat This reaction is highly exothermic, yielding 13,359 Btu/Ib (31,070 1<//1(11) MSW sample contains a significant amount of aluminum, the m asur -d 111’,11III in the calorimeter will reflectthis exothermic reaction because th 1’111111’1,111111I combustion in the calorimeter is so high. In a typical full-scale cornbusnu, III ever, the temperature is not sufficiently high to force the oxidation r “It 111111111111
The second problem with the use of calorimetric heat values is 11l.11I Itll all organic material will oxidize in a calorimeter, this will not occur in ,I lull I II combustor. The amount of unburned organics can vary from 2 to 5%, d ‘Ill 1111111 on the effectiveness of the operation. In one laboratory study, samples or HI )1 II combusted for different times in various furnace temperatures, and lh(‘ 11″11111111 ash was analyzed for calorific value. The results, pictured in Figure 7- r shuw lit
19
Absolute calorific value, 19,410 kJlkg
13
10 15 20 25 Time in furnace, min
o 5 30
Figure 7-3 MSW combustion requires time to achieve full energy recovery. Sour: I’ I I
II 1\’1\ 01Ii11I ‘ I .• I 11’11i’ tlill’, uul 1’1/1\111111111′, tI III11)\)I’ll ‘ I. III I I I’ ‘,I, 011 why III IV norm.tll II 1’1• I 111111.• the ’11’rgy which (;111h’
111’11,1 (10111rh ‘ wastes ln m WI’I\ pltlill 111’1.1 I Iht I , I r . Ira lion of’ tl'” 1111[I’,IV” Ill, wm I lant is gast’llll, w,III’I, 1\ I’ 11 t mrn n t Install ‘(ill
I II I 10 I’C v r this n rgy, whi ‘Ii I, ,II, () known a “latent h at.” This dirrc.’I’ 1111III,illlly I’ ‘n ts th liff ren c bctwc -n 1.1IV n I HI-IV. Theref re. if II slich II tI’lI,t’l’ units ar irnplemerue I, il is m re appropriate to use LlIV inst ‘ad or
IIII V,ilII’S t al uJate the pOL nu Ily re overable energy from wast s. Such I II 11m” h assumes that the water vapor produced when the fuel is burn xl is I I I’d via th xhaust. In this context it is importantto verify which assurn ptions
II \ I I “11 LIS d to calculate the overall energy efficiency of a process. Th result III It II ~ r nt using LHV or HHV to calculate the process’s efficien y. In most I Ill, v, lues are given for HHV; however, in the literature this is not lwayx
I 111111 a nsistent manner. II<‘ j LIS MSW is such a heterogeneous and unpredictable fuel, engi ncci S
II1ItI II ‘d to have “rules of thumb” for estimating the heat values. For MSW, one III III thumb is that one ton of MSW produces 5000 lb of steam, and this st .run Ilhlll ‘6600 kW of electricity. This value, however, depends on the combustlon
1111111)gy and the heating value of the MSW.
MATERIALS AND THERMAL BALANCES
1 Combustion Air 1111111’rgy from the sun is stored using the process of photosynthesis in orga 11ic IIItll’ III ,and this energy is slowly released as the organic materials decem POSl’, 111111lis, of course, receive their energy from plants by eating them and extra ti I1g III II Y G r their maintenance needs as well as cell growth. In simplest of terms, thr
I III lit> ynthesis process is xC02 + sunlight + nutrients + xH20 –‘> nutrients· (H2CO), + x02
1111(Ill O).represents an infinite variety of carbohydrates. The degradation or the It II ‘n rgy organics is then
nutrients· (H CO) + ° –‘> xCO + xH ° + nutrients + heat energy2 x 2 2 2 I Itll1bustion of the organic fraction of refuse is simply a very rapid decomposition 11111 s, which is strongly exothermic. The end products from the combustion or 1111hydrocarbons are essentially the same as in slower aerobic biochemical decem- 1″1’ II 11that reaches the final low-energy products.
f interest to engineers is the amount of heat produced in the combustion 1lllllion. The heat value of pure materials can be estimated from thermodynarn
II r example, the combustion of pure carbon is a two-stage reaction:
C + 0 –‘> CO + 10,100 Jig III
CO + ° –‘> CO2 + 22,700 Jig
./
although it is usually xprcss ‘<J ,II, singlt’ I’ ‘II( ( 011: C + 02 —,”> CO2 + 32,800 Jig
The amount of oxygen necessary to oxidize some hydrocarbon is kl10W11 chiometric oxygen. Consider the simple combustion of carbon:
C + 02 —,”> CO2 + heat That is, one mole of carbon combines with one mole of mole ular OXVI\I II molecular weight of carbon is 12 and of oxygen 16, so it takes two 0 \”1\1II 32 grams of oxygen to react with 12 grams of carbon. The stoirhiomruf is then 2.67 g 02/g c.
Calculate the stoichiometric oxygen required for the combu tion I” methane gas.
CH4 + 202 ~ CO2 + 2H20 That is, it takes 16 grams of methane (12 + 4) to react with 2 X ;.> 16 = 64 grams of oxygen. (The molecular weight is 16, there ar IWI I oxygens, and there are two moles.) Thus, the stoichiometric oxyqltll required for the combustion of methane is 64/16 = 4 g O/g CH~.
Normally, refuse is not burned using pure oxygen, however, and aii’ \I as the source of oxygen. A correction factor therefore is needed to recognm lit the air is 23.15% oxygen by weight. Dividing the stoichiometric oxygen by (I I yields stoichiometric air.
Calculate the stoichiometric air required for the combustion III methane gas.
Since the stoichiometric oxygen from Example 7-5 is 4 g O/g ClI,,, the stoichiometric air requirement is 4/0.2315 = 17.3 9 air/q methane.
I lilt!. (1·i\I(\ how II I (W ‘I’ pl.llIl 111111111′ , W.II I’ is h ‘aiel! 10 • (‘1111 III n It I, uul IIH’, team if! us’ I to turn ,\ uul lit’, wh (II drlv ‘S, g n ‘ral )1′, This dla- III I \II h’ In I lin t l (sill pi’ ‘ll’IH b,II.II\(‘ wh ‘I’ en >rgy I” has I .qual I /11I11 (’11 ‘I’gy w, st I in the OI1Vl’l’ 1)11 I use ul n rgy) plus II n rgy ( lJ- II II I 11 (\1 . b x. lxpr ss c\ (IS an cqu: (i )11:
rat of energy
CONSUMED
rate of energy
PRODUCED
rat r energy + OUT
rate of energy
IN till I l’, ‘n rgy is never produced or consumed in the strict sense; it is simj Iy
“I I Ii 11 f I’m. iii I ,111pr C sses involving materials can be studied in their steady-state c n-
1111, If 111 d as no change occurring over time, energy systems also can be in ” 1.11’, If there is no change over time, there cannot be a continuous accurn 1I- .Ii I I ‘f) rgy, and the equation must read
11′. I f energy IN] = [rate of energy OUT] I II e f the energy out is useful and the rest is wasted,
I’, I of rate of rate of energy + energy USED WASTED
nput and useful output from a black box are known, the efficiency of the I an be calculated as
energy USED Ii = X 100
energy IN Ii II: efficiency (%).
Cold water.–
Fuel– Combustion –Steam
I 7-4 Operation of a fossil-fuelpower plant.
A coal-fired power plant uses 1000 Mg (m 9 r ms, 0 100 I’!I, commonly called a metric tonne) of coal per day. Th en r y v dill of the coal is 28,000 kJ/kg (kilojoules/kilogram). The pi nt ro hilI 2.8 x 106 kWh of electricity each day. What is the electric I ffi ‘I(]I IIY of the power plant?
Energy In = (28,000 kJ/kg x 1000 Mg/day) X (1 X 103 kg/M ) = 28 X 109 kJ/day
Useful Energy Out = 2.8 X 106 kWh/day X (3.6 X 106) J/kWh X 10 I I I = 10.1 X 109 kJ/day
Efficiency (%) = (10.1 X 109)/(28 X 109) X 100 = 36%
One of the most distressing problems in the production of ele tri( itv 11111 MSW-or from any fossil fuel for that matter-is that the power plants ,III II
. than 40% efficient. The reason for such low efficiency is that the waste SI(‘,IIII11111 be condensed to water before it again can be converted to high-pressuu ‘.11,1111 Using the previous energy balance:
where Qo = energy flow in Qu = useful energy out Qw = wasted energy out
Note that energy flow can be in any number of terms such as kl/sec or Btu/Iu III efficiency of this system, as previously defined, is
E(%) = (Qu)/(Qo) X 100 From thermodynamics, it is possible to prove that the greatest efficient y (II wasted energy) can be achieved with the Carnot heat engine, and this effi(il III\ I determined by the absolute temperature of the surroundings. The efficiency I” III Carnot heat engine is defined as
T – T E (%) =’ 0 X 100
C T[
where Ee = Carnot efficiency, % T[ = absolute temperature of the boiler, OK = °C + 273 To = absolute temperature of the condenser (cooling water), OK
III’ I I’. t po, II I’ 111″11111, III I II wui hl ,~I(‘1ll 11111 I IH’ i!’
Ii Ii I
II ‘1’, – ‘1’(1
( II ‘1’, Idl III IO,tI b il I’ generate SL ‘( III with I rnp ratures as high as GO 0 r whil ‘
I III IIIlI-ntal r stri lions limit nd ns r water temperature to about 20° .Thus, I I’ I ‘ t d fficiency is
) (GOO + 273) – (20 + 273)
Ii (% = ) X 100 = 66% I (600 + 273
1111\ I’, I i( nt also experiences energy losses due to hot stack gases, evaporation, to I 1111 I )SS ‘S, ete.; these losses typically reduce the efficiency from the the I’ ‘Ii ;11 11111 III 1 % to a value in the range of 47-38% for plants of the size in th rallg(‘ I II 0.1 MW, respectively. Typically larger plants of the same technology I(‘IHI II 11101’ fficient. As a comparison, the efficiency of nuclear power stati ns ill \I I rlnnd ranges from 30 to 35%. In contrast, a gas-fired combined-cycle lower
II 1111, whi 11 uses methane as fuel, can reach efficiencies as high as 58%.’1(. Mo I I’ll WTE plants can recover heat and/or power. Heat, in the forn or
1111, an be used for district heating or process heating. District heating is lypi Ii I I Illy needed during the winter months while process heating is needed y ‘;1r 1111111. II wever, if the process heat is for a specific industrial operation, th .rc is I I th risk that the industry could reduce operating hours or close duri ng Ilw 1’1 I 11111g life of the WTE plant. Power, in the form of electricity, can usually he Id 1110 an electrical grid continuously. However, the price paid for the electrici ty Illd flu tuate depending on the demand.
II using an extraction turbine which has openings in its casing for extra IIIor c portion of the steam at some intermediate pressure before condensing
It lI’tn, ining steam, it is possible to generate both steam and electricity. Thus lhe 1111 HUH of steam produced can be adjusted based on the demand for steam.
‘Inee the production of electric power from the steam represents an energy I 111 efficiency of producing electricity will be lower than the efficiency for
,1111111 ing steam. Figure 7-5 shows the heat and power efficiencies for 29 WI’I’: 11111 in Switzerland in 2006. To determine overall efficiency, the power and heat II I rncy numbers are added together. Thus, those plants that only produce power III tl’i ity) have an efficiency of 13 to 23% (heat efficiency = 0). On the other
IIIII I, those plants that produce heat (steam) and power (electricity) have a much III II ‘I’ fficiency.
II summary, the efficiencies for power generation are different from plan I plant. Whereas an older WTE plant would only convert about 20% of the
IIIIHYcontained in MSW to power, the currently most efficient WTE plant in III I( rdam converts more than 30% of the net energy content to power; that is, 111111 850 kWh perton of waste. It is therefore known as Waste Fired Power Plant
II P). The concept and principles for this high-efficiency plant are described II I)UrCe[45].
Energy recoveryefficiency of Swiss in ‘in ‘m(IlI’S In 00(> to.7o.————–11—————————
0.65,…_….._..:c——————————~ __…….. Bern. • Zuchwil0.60r—“‘–<__……..;;:—-=.=:..><–=——————~
__…….. • ZurichHagenholz0.55f—-~o;:–~—-=-=======————– ~0.50r———–~_….._..~——————————————— ~ <, La Chaux-de-Fonds-0 0.45 _….._…
~ _….._…•Weinfelden~ 0.40r——————-~__……..~~H}7=~==~————————–= c-, -……:::.orgen ~ 0.35r———————~~_….._..~—-~———————— 5 “<, • Winterthur~ 0.30I—————————-~__……..:—————————~ •.. Tridel• “‘”DurcbschnittSchweiz~ 0.25r———————_.———-‘ •.•~;::—-_——————–~
::r: St.Gallen • ~hs (AG) 0.20r—————~B~a-z-en’h-e~id’·–.-Jo-s-ef-st-r-as-se—.T~r~i~;::—·~——.-T-h-u-n—–~ 0.15r—————————B-ie~1-·~L~;·z-er1n-.·•.C~o~)lo~)mQ~ii~eerr:—T~-.–.–B–I–(S{\.,.., _….._..• urgl UC1S , ‘I 0.10 ~~”. _….._..• Dietikon
Hinwil “<,0.05r————-~———–S-I·o-n——~~.~~·~G~,a~m~s~e=n~~”-. —••7Po-s~ic-IIX Nlederurnen.J .Oftrin1l:eJiO.OOL———L———~—-+—-L–~~~~~~~~~~—
0.00 0.05 0.10 0.15 0.20 Monthey 0.25 Power recoveryefficiency
arre 7-5 Heat and power recovery efficiencies of 29 WTE plant plants in Switzerland. Till’ ,III ” indicates the minimal efficiency to obtain subsidies in order to cover the true costs fOI 1)( ‘WI I
aeration: that is, KEV subsidies. Adapted from source [51J.
7-2-3 Benefit/Cost Fossil fuel power plants buy fuel and produce power. The hope is that till the fuel (plus cost of operation) is less than the income from the sale or till I'” This definition of efficiency does not apply to solid waste heat recovery I.” II I however, since the fuel has a negative cost. People actually pay the plant III fuel (solid waste). The effectiveness of a MSW energy recovery facility is 11t1’1 I calculated as a benefit/cost ratio where the cost (capital cost, operating (II~I, cost of residue disposal) is compared to the benefit (sale of energy and lipplill I charged to the users).
If such a calculation produces a number less than l.0, meaning 11111 II operation loses money, the tipping fees have to be increased. Increased lippllill I could cause users to go to other facilities, which would result in even lawn II Ii III from tipping fees. As an alternative, there could be subsidies provided by 1\4 I ment programs. In some cases this subsidy takes the form of increased I\’V4 I II from the sale of the energy. For example, in Switzerland 50% of the energy 411111 in MSW is accounted as “renewable,” which allows the operators ofWTE pl,IIII sell this energy at a higher rate.
II
I”.l of I at +
11 JTl ulated rate of
heat out inthe
tack gases steam rate of heat out rate of rate of
as latent heat out heat loss due h at of vaporization in the ash to radiation s is in a steady state, the first term (accu~~lati.on) is zero, and n be balanced. The best way to illustrate this 1S with an exampl
th’
To steam
To vaporization
radiation
From •water To
From •fuel
To ash
Black box showing energy flow in a combustor.
A combustion unit is burning RDF consisting of 85% org nl ,1 )” water, and 5% inorganics (inerts) at a rate of 1000 kg/h. A Wilt the heat value of the fuel as 19,000 kJ/kg on a moisture-fr I 1’1 Assume the combustion unit is refractory lined with no water w II II111 no heat recovery. Assume that the air flow is 10,000 kg/h and tl til” under- and overfire air contributes negligible heat. Assume furth I 111111 5% of the heat input is lost due to radiation and that 10% of th 1\11 I remains uncombusted in the ash, which exits the combustion ch 1111)1 , at 800°e. The specific heat of ash is 0.S37 kJ/kgre, and the sp 1111 heat of air is 1.0 kJ/kgre. What is the temperature of the stack 9 “,’
(a) Heat from the combustion is
1000 kg/h X 0.S5(organics) X 0.9 X 19,000 kJ/kg = 14,535,000 I Jill (b) The heat due to water vaporization is
1000 kg/h X 0.10 X 2575 kJ/kg = 257,500 kJ/h where 0.10 is the 10% water in the RDF and 2575 kJ/kg is tlu- latent heat of vaporization for water.
(c) The heat loss due to radiation is
0.05 X 16.15 X 106 = S07,500 kJ/h (d) The heat lost in the ashes is both the sensible heat in the ash “,
as well as the unburned fuel. The total amount of ash is the in II’, plus the unburned organics or
[1000 kg/h X 0.05 (inorganics)] + [1000 kg/h X 0.S5 (organics) X 0.1] = 135 kg/h
If the temperature of the ash is soooe, the heat loss due to the’ hot ash is
soooe X 135 kg/h X 0.837 kJ/kg/Oe = 90,396 kJ/h
where 0.S37 kJ/kgre is the specific heat of ash. (e) The heat lost in the stack gases is then calculated by subtraction, 01
0= 14,535,000 – 257,500 – 807,000 – 90,396 – X or X = 13,400,000 kJ/h. If the total air is 10,000 kg/h, and if the’ specific heat of air is 1.0 kJ/kg/oe, the temperature of the stack gases is
13,400,000 kJ/hr ° 10,000kg/hr1.0kJ/kgre = 1340 e
which is about 2,500°F-quite hiqh.” These hot gases could b run through a heat exchanger to produce steam as a valuable product of the combustion process.
11111 ‘ 1111111 ( p,lI 1It’Il hnv II Ii In 111111 \II I i’l toll O/’wiI,li’, III lI(h
r liS thcr I’m u I
I A h mu t not exceed a percent combustible level. , I~xh ust gas from the boiler must be within a predetermined temp raLurc I’:lng.
II Iw( rit ria ensure complete combustion of the solid waste and recov ry of’ II 111111″ nd both criteria can be easily monitored.
COMBUSTION HARDWARE USED FOR MSW
.11 I a t when refuse was burned without recovering energy, the units w ‘I’ • 1111 II ,IS incinerators, a name no longer used by the industry because of the sorry .1″ I of these facilities. Poor design, inadequate engineering, and inept operati 11 ‘11111111 ‘d Laproduce an ash still high in organics and smoke that (even in the days
1111111, Industrial air-pollution control) caused many communities to shut d wn II’ 1111 In rators.
Without energy recovery, the exhaust gas from these units was very h l. I I’ 1.11 ir pollution control for particulates consisted of a dry cyclone. As the
’11I1i1’m nt for particulate control increased, electrostatic precipitators (ESPs) I II uired for control of particulates, but the hot exhaust gas exceeded th ‘
I ••pl.lble inlet temperature for ESP. If a wet cyclone were used prior to the ESP to .,Ii III gas, any moisture carryover would cause severe corrosion problems in the
I I’ II became very difficult to upgrade old incinerators, and most of them were llill I wn.
1 Waste-to-Energy Combustors It Idl’I’.I1 combustors combine solid waste combustion with energy recovery
II III’ 7-7). Such combustors have a storage pit for storing and sorting the inca m- III I ·fuse (Figure 7-8), a crane for charging the combustion box Figure 7-9, ;t 1IIIIIIIIstion chamber consisting of bottom grates on which the combustion OCCLIrs I’ 1″1′ 7-10), the furnace or combustion chamber, the heat recovery system of pip s III which water is turned to steam (Figure 7-11), the ash-handling system, and the III ,wllution control system. The units take their name from the fact that the com III II n chamber is lined with refractory bricks that are heat resistant, very much 1111 the firebricks in the home fireplace. These units produce steam in a boiler l’II,\l d at the top of the combustion chamber.
Overhead Feed t am crane hopper generator Sind
Scrubber
Solid waste
storagel~~~=t~~~~Jb~~~~~~~~”L-~~~ pit
grate conveyor
I ‘·8 Storage pit with water misters to control dust. (Courtesy William Wilif II)
Figure 7-7 A typical municipal solid waste combustor. (Courtesy William A. WOrl(·II)
I’ll heart of the combustion process is the chamber in which the combusti n 1111 , In most units, the refuse is moved through the combustion chamber on tI
I II IIg rate, and the design and operation of these grates often determines the III I S r failure of the entire process. The functions of the grates are to provi I 1I111111n so that the MSW can be thoroughly burned, to move the refuse down and
1111111111 the combustion chamber, and finally, to provide undetfire airto the refuse 11111111 h penings in the grates. The underfire air both assists in the combustion and II rl t h grates. Some newer grates are also water cooled. The control of underf re II ,II an important variable in maintaining a desired operating temperature in III t I bustion chamber. Most refuse combustors operate in the range of 1800 to IHltl”P (980 to 1090°C), which ensures good combustion and elimination of odors.
The temperature within the combustion chamber is critical for successful I” Illli n. If it is too low, say below 1400°F (770°C), then many of the plastics will III h rrn, resulting in poor combustion. Above 20000P (1090°C) slagging of met- I II the ash could become a problem. Thus, the window for effective operation
1IIIt large, and close control needs to be kept on the charge to the combustion II 11111 r and the amount of overfire air and underfire air.
the amount of excess air is increased, the temperature drops. The rela- 1111 hip between the availability of air and the temperature in the combustion II uuber is shown in Figure 7-12. Note that at stoichiometric air quantities the IIII irature increases to intolerable levels. As shown in Figure 7-12, maintaining umperature of 2000°F (1090°C) in the combustion chamber requires about
tlllllViI excess air. Also note that the temperature drops if the unit is operated in the
-,
-9 Crane for moving M W, ( urtesy William A. Worrell)
(a)
Fixed grate
)
II ydraulically operated moving grates
Hydraulic or mechanical arm pushes grate up
(c) (b)
(d)
7-10 Grates in an MSW combustor, The underfire air is blown through the hol II \ es. (Photo courtesy Hitachi, Zosen Inova) The drawings show three types of gr ,Iprocating, (c) rocking, (d) traveling. (Courtesy P. Aarne Vesilind)
” I,
‘hlll’KII1I1 dllll(‘
Flue gases–..
To expansion ch.unhcr and gas scrubber-Rotary kiln
Figure 7-11 Boiler tubes for steam generation. (Courtesy William A. Worr II) Residue conveyors
starved-air mode, which provides less than stoichiometric air to the COilIIII I III unit. Such combustors are discussed next.
The air blown into the combustion chamber above the refuse is 10.’11 III overfire air, and its purpose is to provide the oxygen necessary for comhll~II’t II well as to enhance the turbulence in the combustion chamber.
7·13 Rotary kiln.
I odification of the combustion chamber is the rotary kiln, shown in 11I1 7-13. In this unit, the refuse travels down an ignition grate by gravity and
Itt ,I I taring kiln where the combustion occurs. Rotary kilns provide the most tlltld~’1 e of any grate system and thereby enhance the rate and completion ( r 11111111. lion.
1’1 furnace walls of modern combustors are lined with metal tubing through I I II w ter is circulated. This water wall is then part of the boiler or heat recov ry II Ill.
‘I’ll water tubes protect the combustion chamber housing by transferring II III nl into the water. Figure 7-14 shows how the water wall is placed in the 11110\ ‘J, Figure 7-14 shows a close-up photograph of a water wall in an MSW 11111 ustor.
‘l’he steam-generating system has been a source of problems with solid II’ fuels-in particular the generation of superheated steam, which is needed
111’11 nerating electricity. The superheated steam, between 600 and 950°F (315 III ‘i l O’C). allows the use of multistage high-efficiency turbine generators. The 11,1\1, L gas from the combustion unit is still very hot when it encounters the
1111 I hater tubes, causing rapid corrosion of the tubes. These corrosion problems I “I I’ quired the retrofitting of refractory materials on the tubes, which decreas ‘s III rfficiency of heat transfer. One solution has been to move the superheater
1111’ La the back of the boiler.
4000
J.L.
°Qr3000…. ;j•.. t1l
[2000 / S / Q)
E-=< 1000
OL—~–~–~—L __-L_ -50 0 +50 +100 +150
Excess air, % above stoichiometric
Figure 7-12 Excess underfire air and temperature relationship in MSW combusuun
./ . m r du I n 1M W , tu/l
6′ 0 4000 0
Air-cooled team condenser
1 1 25 32 39 14 16 20 24 28 71 66 55 44 33 4.3 3.9 3.2 2.3 1.
Charging -t”j–..\.I—1 chute
ncy of steam production also depends on the quality of the I’ll I. As 7-7, the production of steam drops dramatically with increas sin fuel as in well as the fraction of noncombustibles.
Modular Starved-Air Combustors.ncinerator stoker unit
Ii1I II IYI of combustion unit is the modular starved-air combustor, shown ill 1111’I 1 r,:. These units are characterized by a two-stage combustion system, wi th I I I lc e being operated in a starved-air mode, producing a large quantity f
1″111_I arbon that is then burned using a fossil fuel in the second stage. Th sc II 1111b, tch fed using a double-ram system, as shown in Figure 7-16, and rang I I II In 15 to 100 tons per day. Typically, such units do not incorporate h at
” I I _Y terns. Modular combustors are useful in cases where the waste has to be combusL I
I 1111 [uantity is insufficient to warrant the construction of a large refractory- oil I w ter-wall combustor. Modular units are also flexible in that more units
I II purchased as the need arises. For example, if the average production of II I W.I te is 100 tons per day, three 50-ton/day units can be purchased with
I. Illf ining idle as a standby and for scheduled maintenance. One of the most I. I I d applications of these units is in the destruction of some hazardous I Illd , such as biohazards waste from hospitals.
Pyrolysis and Gasification IIII),\/s is destructive distillation or combustion in the absence of oxygen. The 11111I’ of pyrolysis include a solid, a liquid, and a gas. In true pyrolysis, heat .Id \ I to the complex organic feed. For example, if the feed material is pure
Ihillls , gaseous, liquid, and solid products are formed”:
I , H 4 (g), H
2 (g), CO(g), HP(g), C2H4(g) (ethylene), ete.
I” rls: Levoglucosan (C 6 H
10 0
S ‘ b.p. 384°C), Formic acid (CHP2’ b.p.lOO.8°C),
I 11l1~ldehyde(C Z H
4 0
Z ‘ b.p. 131.3°C), Naphtalene (ClOHS’b.p. 218°C), tars, ete.
,101: har modification of pyrolysis isgasification, in which a limited quantity of oxygen
Itlll duced as pure oxygen, air, or as water, and the resulting oxidation producesiJre 7-14 Water-wall tubes lining the furnace of an MSW combustor. (Courtesy W,·II,·”\farrell) .1111
Aurouuuic llilid ‘Ill londcr (1’I.)111()1’ ‘Iy controllcd-I’ .cip« ’01’ b tween right and left incinerators)
. /~~~~~~~~Ul~ rung end dome Hopper ry lined) door Ram feeder
(hydraulic)
Door open
Secondary chamber Lining: castable refractory
Stack __
Lining: castable refractory
~as burner JO Btu/h) Primary chamber (760 ft3)
Lining: fire brick lower section castable refractory upper section,
Fire door
am feed system for a modular incinerator.
II r.u IIg problems. The pyrolysis of predictable and homogeneous fuels (such I 111.11) bagasse) would seem reasonable and logical, and these units ar
IIrl, Th heterogeneous and unpredictable nature of MSW has resulted in 1111111 allures. III II c lit decades, Japan has started to use gasification technology to combust III N goya, a full-size gasification plant opened in 2009. This plant was built
111j1l11’l the existing three conventional waste-to-energy plants and processes It I ‘Wand ash from these existing waste-to-energy plants. By melting the ash ,,1111 nventional waste-to-energy plants, the gasification plant can convert I “111l a useful product that does not have to be landfilled. Unfortunately, , I If tion technology that significantly reduces ash generation and has very 11’1 missions also costs significantly more than a conventional waste-to-energy
II .tnd produces less energy. This challenge (see Figure 7-18) is of general
-n door
Natural gas burners (1,000,000 Btu/h] Hydraulic unit
.5 Modular combustor.
enough heat to make the system self-sustaining. Thus, the gasification rr.u tlllll be exo- or endothermic, depending on the amount of heat and oxygen addl II
The process of pyrolysis or gasification can be manipulated III II to achieve a desired end product. Four general modes of operation (I’,) I II identified: See Table 7-8.
The choices of these variables determine the products obtained 1111111 pyrolysis system, as shown in Figure 7-17. At very high temperatures, till’ 1’111 I is mostly gas, while at low temperatures, mostly solid product results. ‘)
Pyrolysis (and gasification) has a lot to recommend it theoretically.” ‘III cess is environmentally excellent, producing little pollution, and it results III production of various useful fuels. It would seem therefore that pyrolyzuu; would be an ideal application. Unfortunately, pyrolysis has had a SOil V iii I with MSW. Large facilities were constructed in the 1970s to produce h()111 11’1 fuel (oil from garbagel}” and solid fuels, and all of these facilities failed I” I
18 Four General Modes of Operation for Manipulation of Pyrolysis
proceeds at a very slow rate of temperature increase, generally less than 1°C/sec, and the final temperature range is between 500 and 750°e. takes place at a more rapid temperature rise, of 5 to 100°C/ sec, and reaches temperatures of between 750 and 1000°e. occurs when the temperature rise is fast, between 500 and 106°C/sec, The temperatures reached with this process are over 1000°e. takes place when the temperature rise is essentially instantaneous, of over 106°C/sec. The temperatures attained in this process exceed 1200°e.
Au«; »untic (Ilild ‘III loud ‘,. (I’ ‘1IlOICly conn-:::roll ‘d-r “j pro ’11(‘ ‘s b .twc ‘II righ~ and left ill in raters)
. /~f!~~~~~~~~ Full-opening end dome Hopper (refractory lined) door Ram feeder
(hydrauli.> c)
Secondary chamber Lining: castable refracto .ry
Primary cha,c:nber (760 ft3) Lining: fire ~rick lower section
castes- ble refractory , uppe:: .r section,
Fire door
Stack _
Lining: castable refractory
Natural gas burner (3,500,000 Btu/h)
Inspection door
Ash removal pad ~
Natural gas burners Hydraulic uOit (1,000,000 Btulh)
Figure 7-15 Modular combustor.
enough heat to make the system self-sustaining. Thus, the gasification rC,1( II11I1 be exo- or endothermic, depending on the amount of heat and oxygen add!’d
The process of pyrolysis or gasification can be manipulated i II III to achieve a desired end product, Four general modes of operation (I I)) 1.111 identified: See Table 7-8.
The choices of these variables determine the products obtained (111111 pyrolysis system, as shown in Figure 7-17, At very high temperatures, th« 1111111 is mostly gas, while at low temperatures, mostly solid product results. 12
Pyrolysis (and gasification) haS a lot ~ore~ommend it theoretically I I ‘111f 1 cess is environmentally excellent, producmg httle pollution, and it results III production of various useful fuels. It would seem therefore that pyrolyziIII would be an ideal application. Ullfortunately, pyrolysis has had a son I’ It I I with MSW, Large facilities were collstructed in the 1970s to produce bOlir III, fuel (oil from garbagel}” and solid fuels, and all of these facilities failed II •
I yst m for mo LlI~ll illcill I 1/<11,
VJ 100.;;:; 1l 90 iJ ~ 80 ::l..;;70 Q)
.~ 60 u Q)
;;; 50 ro ~ 40
~ 30ill) ~ 20
…… … , ‘,Solid ,,
” “- “- “- … – – – Oil..•… ” – – – – – – -,- ‘– – — .•…- ——- —,- – ………… / /”
.- ——- ..- Gas — Io
200 700 800300 400 500 600 Temperature, degrees celsius
=igure 7-17 Effect of temperature in the formation of pyrolysis products. SOUl (I’ i” 1.1 ~eport. Environmental Factors of Waste Tire Pyrolysis, Gasification, and Liquerdt III1/1 =alifornia Integrated Waste Management Board, July 1995, table 4-1, p. 4-4. © I’JiIII II l1e California Department of Resources Recycling and Recovery (CaIRecycle).
nature. To obtain high quality materials which are inert for disposal or Willi II be recovered in new products, energy is required which will be lost for lhl’ I I I generation; that is, for the distribution of power to the grid. However, ill 11111I judge if the treatment is justified, an overall assessment is needed that (1111I\d the additional benefits.
>-‘-<Q) :>o u Q) ‘-<>- OIl’-<Q)
d3
oc.S•…«l.!::l•…’-< Q)
.S c~
.S•… «l U~.~ o•… Q)
Q L- ~
Materials recovery Detoxification, inertization
Figure 7-18 Detoxification and materials recovery go hand in hand (left), whel”” these measures compete with energy generation (right). To achieve sirnultan (lIlly high energy and materials recovery is therefore very challenging.
IIIII
HIIIII lion systems are characteriz d as ither mass burn units or refuse-derived II I (I I II) units. A mass burn unit has no pre-processing of the solid waste pri r ,llIlltl\ fed into the combustion unit. The solid waste is loaded into a feed hut, IlIi II I ‘,1 Is t the grates: the solid waste in the chute provides an air lock, whi h “,1 Iii’ em unt of combustion air to be controlled. From the chute, the solid I I. uiov directly onto the combustion grate.”
I” nn RDF system, the solid waste is processed prior to combustion t 1111I’ n n ombustible items and to reduce the size of the combustible fra – 1111thus producing a more uniform fuel at a higher heat value. The RDF is f d 11111If h f rotary feeder and injected into the combustion unit above the grat .
HIli rornbustion takes place above the grate, with the remaining combustion 1111tillg on the grate. This is referred to as semi-suspension firing. If no grat I In I all combustion occurs in the air, this would be called suspension [iring.
I 111\,i n firing is common when burning pulverized coal but is not generally 1’1’1 .rl I to RDF.
n initial advantage of an RDF plant is that the heat value of the fuel is uniform, and thus, the amount of excess air required for combustion is
0111 I. The amount of combustion air used is important, because if ther 1II’IIm ient oxygen in the combustion chamber, a reducing atmosphere is
II .lil’ I, which leads to corrosion problems. For RDF systems, the excess air iii v ‘ stoichiometric requirements) is about 50%, while in mass burn plants
I. I IIIIS of the large variation in fuel value between items) about 100% excess It I n ded. However, with modern furnace design and flue gas recirculation,
s air requirements in mass burn plants have been significantly reduced of RDF plants or below. When pre-processing the solid waste, some of
III II( t ntial problem items-such as batteries that contain mercury-can be III IV d.l? In addition, by removing noncombustible material prior to burning,
,\ h is produced. While there appear to be several theoretical advantages of RDF over mass
11111plants, they have had their share of operating problems. Processing solid I II i not easy, and RDF plants have encountered corrosion and erosion prob-
I III , Whether there is an advantage to incurring the additional cost related to the 1IIIItition ofRDF over mass burn is still unclear; however, many more mass burn lilli, have been built than RDF plants.
Ir the RDF is produced from mixed paper that has been sorted as part of III I cling effort, the fuel is a valuable resource. The characteristics of shredded II I d paper as a fuel are shown in Table 7-9. The data in this table are the average III” obtained from 12 different communities.”
The heat value of mixed paper is surprisingly high, considering the high ash IlIll’nt. Overall, the HHV of mixed paper is about 7200 Btu/lb (16,700 kJ/kg).IS
Table 7-9 Characteristics of Mixed P p r s R fu .[) rlv d u I
Component Average as Percent of Tot I WIlli —————-_ .._————————
7.3 9.9
82.8 7.1
41.1 6.2 0.1 0.1 0.1
Moisture Ash Volatile Fixed carbon Carbon Hydrogen Nitrogen Sulfur Chloride
Average Concentration as Parts per Million (mg/l J)
Arsenic Barium Beryllium Cadmium Cobalt Chromium Copper Manganese Mercury Molybdenum Nickel Lead Antimony Selenium Tin Zinc
0.48 46.21
0.70 0.55 6.78 6.48
18.12 27.34
0.05 7.42 6.92 7.51 3.88 0.06 7.92
149.21
Source:[18]
The amount of heat value contributed by various components of mixed Ihll’ varies, as shown previously in Table 7-4 and in Appendix D. In addition I (I 1111 1 , papers, some other secondary fuels can become valuable sources of CIH”I fiV. shown in Table 7-10.
The American Society for Testing and Materials (ASTM) has dcvvlu]« t designations of refuse-derived fuels, as shown in Table 7-1l. RDF-1 is lid I refuse without any processing, while RDF-2 is refuse that has been shredded lit homogenizes the fuel and makes it much easier to handle in combustion: HIli is shredded refuse from which most of the inorganic materials have been rClllllV I Such a fuel would be produced in a typical materials recovery facility (MRF), wll II might process mostly pre-separated solid waste. Further shredding into ,I “U, (RDF-4) (Figure 7-19) or pelletized into dog-food-sized pellets (RDF-5) ruuh improves the usefulness of the fuel (Figure 7-20).20 The last categories, RDI: () 1111 t 7, have been tried on a pilot basis but have not been found to be successful I full-scale plants.
mbu tlon Prop rtl
r nt M ur II lV lu , tu/lb P r nt A h 5 160 2,0
5 7180 3.6 1 11,600 22.1
35 5800 5.0 1 16,200 1.8
38 5230 0.2
11 ASTM Refuse-Derived Fuel Designations
11\ Description
I I Unprocessed MSW (the mass burn option). I I MSW shredded but no separation of materials. I I Organic fraction of shredded MSW. This is usually produced in a materials
recovery facility (MRF) or from source-separated organics such as newsprint.
I I Organic waste produced by a MRF that has been further shredded into a fine form (almost powder), which is also called fluff.
I , I Organic waste produced by a MRF that has been densified by a pelletiz r or a similar device. These pellets often can be fired with coal in existing furnaces.
I I Organic fraction of the waste that has been further processed into a liquid fuel, such as oil.
I , / Organic waste processed into a gaseous fuel.
I III 7-19 Fluff, or finely shredded RDF. (Courtesy P. Aarne Vesilind)
Figure 7-20 Pellets used as RDF. (Courtesy NCRR)
One advantage of RDF over mass bum is that the fuel can be stored III 11111 storage containers, as shown in Figure 7-21, and burned as needed=-unlil« I11I II MSW in mass bum facilities, where the storage time is limited.
7-4 UNDESIRABLE EFFECTS OF COMBUSTION
The combustion of fuel such as RDF can have undesirable side effects. III Ihl discussion, we cover the waste heat generated by a power plant, the ash rcqu lilt disposal, the materials that can escape in the stack and cause air pollution, ,11111 lit production of carbon dioxide.
7-4-1 Waste Heat MSW is a low-grade fuel that can be used for the production of steam. TIl(‘ 1111 I generates this steam, which is used for driving turbines, but the exhaust sf II III
I, ure 7-21 Bulk storage unit. (Courtesy P.Aarne Vesilind)
111101 the turbine has little industrial use (unless it is located sufficiently close to hulldings that it can be used for heating).
Often the exhaust steam is condensed back to water using either a water ruoler or air-cooled condenser or heat exchanger. Usually the boiler water is reused In the boiler, because it is too expensive to be used only once. Small amounts-less 111,10 IO%-are discharged, and fresh water is added to the recycled water to keep IIssolved solids low.
If a water-cooled condenser is used and the water used to 0 I the steam I discharged into a watercourse, the result can be serious dclct ‘ri LISecologi- I,ll effects in streams, rivers, and estuaries. To prevent this (rom Lining, heat III’charges are governed by environmental regulations. ‘I’yp] ally, the limit on heat discharges is that the temperature of the receivi “1\ w,11 [ cannot be raised hy more than 1°C.
Th 31 ulation of’th ‘I(‘mp .rarurc of n I “(‘ v Ill{ bo Iy o(‘wal{‘I’IN ~IIIIIIIII ward. Heat energy is easy lO analyz by n r y I alan 8, sln the qllil/II rv III ‘ energy in a material is simply its mass tim sits bs IUl l rnpcratun-, ‘I’I”~ if the heat capacity is independent of temperature. In particular, ir phu II’ I It do not OCOlr(as in the conversion of water to steam), an en rgy baluur r hll energy in terms of the quantity of heat is
[ heat ] = [mass Of] X [absolute tempe~atureJ energy energy of the material
Energy flows are analogous to mass flows. When two heat-energy flowN .\1. II bined, for example, the temperature of the resulting flow at equilibrium II I lated using the energy balance:
0= (heat energy IN) – (heat energy OUT)
or, stated in other words, assuming two input and one output stream, 0= (T1Q] + TP2) – (T3Q3)
or
where T = absolute temperature Q = flow; mass/unit time (or volume constant density)
1 and 2 = input streams 3 = output stream
The mass/volume balance is Q3 = Q] + Q2
Although strict thermodynamics requires that the temperature be expu: I Id I absolute terms, in the conversion from Celsius (C) to Kelvin (K), the 27 \ I1III cancels out, and T can be conveniently expressed in degrees Celsius. I~(‘I.il] t I O°C= 273°K.
A coal-fired power plant discharges 3 m3/sec of cooling water oil 800e into a river,which has a flow of 15 m3/sec and a temperature ~” 20°e. What will be the temperature in the river immediately below the discharge?
f th riv n b th ught ‘f
,0. 1
-I- T 2 Q
2
‘,- Q 3
[(80 -I- 273)(3)J -I- [(20 -I- 273)(15)] ,j = (3 + 15) = 303°K
I 03 – 273 = 30°e. Note that the use of absolute temperatures is II ‘t necessary since the 273 cancels out (the right-hand side of the
1 ation is really 30 + 273).
~—————————————————–_I •’Ill most states restrict thermal discharges to a less than 1°C rise aboy
I 1IIIlhint stream temperature, the heat in the cooling water must be elis- 11’111 I into the atmosphere before the cooling water is discharged. Various
III ar used for dissipating this energy, including large shallow ponds d I ) ling towers. A cutaway drawing of a typical cooling tower is shown I I ur 7-22. Cooling towers represent a substantial additional cost to th
111’11111 11 of electricity. I\v n with this expense, watercourses immediately below cooling water dis-
h “11 t re often significantly warmer than normal. This results in the absence or I IIlIfing hard winters and the growth of immense fish. Stories about the size I II 11 aught in artificially warmed streams and lakes abound, and these places
I II III not only favorite fishing sites for people but winter roosting places for 1111. Wild animals similarly use the unfrozen water during winter when other
III hll ‘ waters are frozen. The heat clearly changes the aquatic and terrestrial eco- 11’111, and some would claim that this change is for the better. Everyone seems to
I \I Il from having the warm water. Yet changes in aquatic ecosystems are often IHIIIdictable and potentially disastrous. Heat can increase the chances of various III’ f disease in fish, and heat will certainly restrict the types of fish that can exist
III t Ii . warm water. Many cold-water fish (such as trout) cannot spawn in warmer “111”1′; they will die out, and their place will be taken by fish that can survive
( 11111s catfish and carp). It is unclear what the net environmental effect is due to ” l’l’nmental restrictions on thermal discharges-such as “no more than 1°Crise
hlll’,nperature.” iven the potential disadvantages of water-cooled condensers, the obvious
111I Lionis why not use an air-cooled condenser. In an air-cooled condenser, air is \I lid to cool the exhaust steam and condense it back to water. Air-cooled condens- I ,II’ more expensive and less efficient than water-cooled condensers.
water vapor
(steam)
.1 .;: I
”
.1 I I I
”
I I I I
” olll
[III
Cooled water 0(
Figure 7-22 Cooling towers typically used in fossil-fuel and solid waste cornbir.t« ill facilities.
7-4-2 Air Pollutants In the United States, much has been done to minimize the impact of waste-to 1’1111 gy plants on air pollution. The Clean Air Act required all large and small pl.uu II be retrofitted to meet Maximum Achievable Control Technology (MACT) by )(11111 and 2005, respectively. As shown in Table 7-12, this retrofit significantly r(‘dull I
~ -=- ~~ .–~- -~
I I n r m I IW t ombu Ion U It
P r nt 1990 ml I n (Ii Y) missions (tpy) R du tl n
Q basis* 4400 15 991 57 2.3 96
9.6 0.4 96 170 5.5 97
atter 18,600 780 96 57,400 3,200 94 38,300 4,600 88 64,900 49,500 24
emissions are in units of grams per year toxic equivalent quantity (TEQ), using 1989 factors; all other pollutant emissions are in units of tons per year. Source: [33J
s. The EPA recognized this air-emission reduction by concluding thai produce electricity with less environmental impact than almost any
e of electricity.” In Switzerland, where over 50% of waste is burned, of waste is not a significant source of air pollution (see Figure 7-23).
ir pollutants of concern in municipal waste combustion can be gases or particulates. Another classification is primary pollutants and llutants. Primary pollutants are products of the combustion process shown to be harmful in the form they are emitted. Secondary
re those that are formed in the atmosphere as a direct result of the primary pollutants.
– —
I I I I I I I I I I I I I I 1 I I I I I I I I -, I I 1 I I I I I T I I T I I I I I I I I I I I I I I I I I I I I I I I I I I l_ I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I –
% 10% 20% 30% 40% 50% 60% 70% 80% 90’1., IO(
— — – — — — Share of all emissions from combustion in Switzerland (purple bars) in compai i:,(III missions in Switzerland (100%). Source [51].
11111III/fur n I III toxicity
II I IIIis ion I H plants
,,1111’1’ sure 1IIIIlhustion
The a d,1 In d as I I II/lda ry po
IIt,11 n be 1IIIIhilants a I11I S, i n of
ioxin
Hg Cd
Zn Pb
PMI0 N20 HCI
CO NMVOC
NOx S02
o
II ure 7-23 t” the total e
It ,IIII’I LIP n by sunlight to yield ozone (03)’ which is a compound not erniue I 11111Ir m sources and hence is considered a secondary pollutant. Ozone r a Is
lill II Ir arbons to form a series of compounds that includes aldehydes, organi It!, ind ~oxy compounds. The atmosphere can be viewed as a huge rea lion
‘~I,wh rem n~w compounds are being formed while others are being destroy d. I h) formatl?n of p~otochemi~al smog is a dynamic process that b gins
Itlt (I’. productlO? of nitrogen oxides from automobiles, industrial fa ilili 8, 111\ MSW .combustlOn. As the nitrogen oxides react with sunlight, 03 and other 11\111118ale pro.duced. The hydrocarbon level similarly increases at the beginning I1111I,y and then drops off in the evening.
rh escape of heavy metals with the emission gases has been another con rn lilt ornbustion of MSW.Lead, cadmium, and mercury have been the most studi I ,III,. I r sent the ~etals of most likely health concern which are presently regulal I
11\1 I th ~lean Air Act. Mercury is especially difficult to control, because it volalil- II I’ adily and escapes with the gaseous emissions. Difficulties in the corur I of
•It ,IIyare exacerbated by the difficulties in controlling its inclusion in the wasI II ,III\’ I.Iowever, the ?e.st ~ractice for controlling mercury emissions is dry s rub- III, II l’v~ted carbon mJ~~on, and fabric filters, which result in a greater than ~% 11111n 10 mercury emissions!” The effect of efficient scrubbers is also shown ill III17-24 which is showing the emissions of different MSW incinerators.
nether concern with solid waste combustion is the formation r g/olltl/ “IIIIIIg gases. The e~rth acts as a reflector to the sun’s rays, receiving the rad i.uion “” (II sun, reflecting some of it into space (called albedo), and adsorbing tltl’ I only to reradiate this into space as heat. In effect, the earth acts as :1 W,WI’
“‘ I I’l, r, receiving the high-energy, high-frequency radiation from the SIIiI 111It! III rrung I?ost of it into low-energy, low-frequency heat to be radiat d h,1tI 11111 ” .In this manner, the earth maintains a balance oftemperatur ,so 111,11
[ energy from the sun] = [energy radiated back to spacel
IN OUT
lInfortunately,. s?me gases-such as methane (CH,J and (,111111111\ I ,)-adso.rb radiation at wavelengths approximately the S,III\I I II 11,111n trying to find its way back to space. Because the 1′:1(11.111111I th.’ atmosphere by these gases, the temperature of the ,\111111Itlil I I II the earth. The system works exactly like a greenhouse III till !
Be aus t.1t, ‘Ol-ilhu (\ )1\ 01 M,’W \ (OI’lllh’X PIOI’I’ , III III \IIH\(‘ 1,lltl reactions can occur that produc un I ‘S !’,II Il’ \11t’ ,
A particularly difficult probl m is Ih ‘ sul I’ll r xld ‘s PI’ dLI ‘diu 1’i11f.1 (‘01111III tion. In the presence of high temperature, ih sulfur in th (I I en 0111\)1111’\III oxygen to produce sulfur dioxide.
S + 02~S02 Sulfur dioxide is itself a primary pollutant and can cause respiratory lisllI'” 111,1 even property damage. But sulfur emissions also can be a secondary \1(1111111I when sulfur dioxide (in the presence of water vapor and oxygen) all II II1I further to sulfur trioxide:
02 + 2S02 ~ 2S03 Sulfur trioxide can then dissolve in water to form sulfuric acid,
S03 + H20 ~ H2S04 The sulfuric acid, in combination with the hydrochloric acid produced ill Ihl’ 11\I tion of chlorine compounds and the nitric acid produced from nitrogen 11It! produces acid rain. Normal, uncontaminated rain has a pH of about 5.6, \llli 1111 rain can be as low as pH 2 or even lower.
The effect of acid rain production from the burning of coal has h(,I’I’ rI astating. Hundreds of lakes in North America and Scandinavia have b« 111111 acidic that they no longer can support fish life. In a study of Norw gi,1I1111\ more than 70% of the lakes having a pH of less than 4.5 contained no fish, \ I II nearly all lakes with a pH of 5.5 and above contained fish. The low pI I 1111\11111 affects fish directly, but contributes to the release of potentially toxic met, I~’11111 aluminum, thus magnifying the problem. In North America, acid rain has ,1111I I wiped out all fish and many plants in 50% of the high mountain lakes 11II Adirondacks. The pH in many of these lakes has reached such levels or ,1( 11\11 to replace the trout and native plants with acid-tolerant mats of algae.
The deposition of atmospheric acid on freshwater aquatic systems P,ollll I the EPA to suggest a limit of from 10 to 20 kg SO;2 per hectare pet WIll “Newton’s law of air pollution” is used (what goes up must come down), it I I I to see that the amount of sulfuric and nitric oxides emitted from powci 1’1111, municipal combustors, cars, and many other sources is vastly greater Ih,1I1It. limit. For example, just for the state of Ohio alone, the total annual em iSSlIll1 I 2.4 X 106 metric tons of SO2 per year. If all of this were converted to S() I j 1\ deposited on the state of Ohio, the total would be 360 kg per hectare pCI yr,1I
But not all ofthis sulfur falls on the folks in Ohio, and much of it is l’XIII”t by the atmosphere to places far away. Similar calculations for the sulfur l’lli _III for the northeastern United States indicate that the rate of sulfur emission» H 5 times greater than the rate of deposition.
Another secondary pollutant of concern in municipal waste com hiI’lllilt the formation of photochemical smog. The well-known and much-discus’« II I Angeles smog is a case of secondary pollutant formation. Table 7-13 lists ( 1\ It plified form) some of the key reactions in the formation ofphotochemic,ll’ 111
The reaction sequence illustrates how nitrogen oxides formed in the llllll\l. tion of fuels (such as gasoline and municipal waste) and emitted to the auno jilt
Impllfl d II
I LI ht I 2
-I- NO -I- (HC)x -I- 02 -I- HC -I- NO -I- 02 + N02
,JllIllhlllllltllllCAI In
N
°N02 02 HCO° HC03° Aldehydes, ketones, t. HC02° + N02 03 + HC02° Peroxyacetyl nitrates
Figure 7-24 Mercury emissions during combustion of MSW. They stron Iy I” II I on the implemented air pollution control (APC) system, the operation COIi( tn II III the amount of mercury in the waste. Strong variations can occur over a I I II i’ II II I The data given in the Figure are ordered with increasing concentrations (II<i111 I show no underlaying physical-chemical relationship or trend. Source: [251
(short-wave, high-frequency radiation) passes through the greenhouse 1′,1.1 . the long-wavelength, low-frequency heat radiation is prevented from eS(‘,’I1 lip gases that adsorb the heat energy radiation are properly referred to as ,~II’f /III gases, since they in effect cause the earth to heat up just like a greenhouse
Not all gases have the same effect in global warming. Methane, [01 I ,11111 is much more potent as a greenhouse gas than carbon dioxide. The ~II 1 ill greenhouse warming potential (GWP) of methane depends on the time ‘I II common time scale considers 100 years. The Intergovernmental Panel 011( 11111 Change (IPCe) has adopted the GWP value for methane in the past sevci ,II llill from 21 in 1996 to 34 in 20l3S3 It therefore can be argued that mcrh.un 34 times more effective as greenhouse gas than CO2, and therefore the pro.hn II of CO2 in solid waste combustion is far better for the global environment ill,1I1I’ production of methane in landfills. The requirement of flaring the gases in I IIIIII landfills is directly related to the reduction in the production of global w.u utl) gases. According to the EPA, nearly one ton of Cf), equivalent emissions is .IVIIIII for every ton of MSW combusted by a waste-to-energy plant.
The combustion of renewable fuels such as paper and wood does not I (111111 ute to the increase in global carbon dioxide and thus does not increase the 1I1I of global warming. The combustion of a wood fiber results in the productlnn i carbon dioxide and water, but this is exactly what occurs when that same I ” decays naturally. Combustion simply increases the rate of decay. Thus III(‘ 11111 deleterious gases produced by a MSW combustor come from plastics and any (II” products made from fossil fuels.
Control of Particulates The Clean Air Act emission standards allow states to set strict emissions limit« II various sources. Municipal waste combustors are regulated under Subpart I~.’III
~1 OOOr——————-l U)
“‘:::;s .::: ..•… .9 0 1000 ‘” ‘”.~ §S •…. Q).::: 100 >-,0′-< .~;:j:::::i
b] 10:::;S-O .::: ;:j
8. 160oooooo————.-l
00000 00
o 00
o
Distribution of data
111,p/dl IllIlll.lIlI 1111,WIIl'( It’d 10 11111’11i1,111()!101’111’ ‘Illis. Oil. ,\II 1\ 11101’ ,I r I ) I • ‘milll’d Ihrolllo\h
1.111,1\’ nusc n I’m, i air h,IS :lholll 1)1111II 1″11, 111’1″ [ulr 111111 I rnalrualn lit \I 7
‘ Vt) (.) yg n (by v lumc) allow .11111101111stnn 1<1’1 m ng v: ri LI$I laurs.
\lId uou, ih ~ d ral I’ gulauons llmlt IIII’ l’lltlssi ns t I ss than 10% I a ity . III’ lmpl st d vi s f r orurollin PMII ulat s are settling chamuers, which
11111’n thing more than wid’ pln cs in ih exhaust flue where larg r parti I ‘s 11111’out, usually with a barn t slow the emission stream. Obviously, Oldy 11111′ pal’li ulates (>100 urn) can be efficiently removed in settling charnb rs,
I IIII’ ‘tit’ LISd (if ever) only as pretreatment in modern combustion units . 11111111’1.’J’S that transfer heat from the exhaust gas to combustion air are exam I s
I III 1\ hamber. (III \ lbly the most popular, economical, and effective means of controlling
till III \I’ from many industrial sources is the cyclone, similar to the cy Ins .I 1111removing shredded particles following air classification (see Chapt r ). I11I hows a simple single-stage cyclone and a bank of high-eff illy 11111I,’I’h dirty air coming to the cyclone is blasted into a conical cylind r rr II 111t’rline. This creates a violent swirl within the cone, and the heavy solids I \11 I) the wall of the cylinder, where they slow down due to friction, slid II Ill’ ne, and finally exit at the bottom. The clean air is in the middle f th
1\110111.md exits out the top. Cyclones are not sufficiently effective for removing III “Iii les and thus need to be backed up by other particulate-removal devi s .
II/ll< ( r fabric) filters used for controlling particulates (Figure 7-26) op r~t III mmon vacuum cleaner. Fabric bags are used to collect the dust, wh I h
I I I i’ P riodically shaken out of the bags. The fabric will remove nearly all par- t! III , , including submicron sizes. Bag filters are widely used in many industria I 1’111.111ns. including MSW combustion. The basic mechanism of dust removal I II I l filters is thought to be similar to the action of sand filters in water quality II’lf (111 nt. The dust particles adhere to the fabric due to entrapment and surfac
I I ‘I’b yare brought into contact by impingement and/or Brownian diffusion. \ I I,d ric filters commonly have an air-space-to-fiber ratio of 1:1, the removal
1″lIlisms cannot be simple sieving. lit; crubber (Figure 7-27) is another method for removing large particulates.
II ‘/’I ient scrubbers promote the contact between air and water by violent 1111n a narrow throat section into which the water or a chemical slurry is
1111″I d. Generally, the more violent the encounter (hence the smaller the gas 11111. r water droplets), the more effective the scrubbing. Scrubbers are used in
Iumbustion mostly for the removal of gaseous pollutants, but they also hel p III I .moval of particulates. Wet scrubbers, which use water sprays, are efficient 11\ I lit have three major drawbacks:
I. They produce a visible plume, albeit only water vapor. The lay public eldom differentiates between a water vapor plume and any other visible
plume, and hence, public relations often dictate no visible plume . . The waste is now in liquid form, and some manner of water treatment is
necessary. t The ash is wet, and recovery of metals is difficult.
(a)
-1W••••••••_! I I
~:::;
~ Solids
(h)
:Jure 7-25 Cyclones. (a) Simple cyclone, (b) simple cyclone, (c) bank of high-efficiency clones. (Courtesy P. Aarne Vesilind)
IIIh ‘sh t
I oppel vnlv ‘
uti ‘I’ I Oil ct v live (puis posirion)
!tPUIS d nirop ba ac 81 WPIP
Filtering module
Outlet manifold
Bypass valve
Filter b g
Cleanin modul
tInlet manifold
To ash removal syst ‘Ill
I lire 7-26 Bag filters. (Courtesy Bundy Environmental Technology, Inc.)
Dry scrubbers inject a chemical slurry such as lime. This type of scrubber does III! I reduce a visible plume, and the waste is a powder-not a liquid.
Electrostatic precipitators (Figure 7-28) are widely used in power plants. The 1′,11 I I ulate matter is removed by first being charged by electrons jumping from
1111 lectrode to the other. The negatively charged particles then migrate to the lit I Livelycharged collecting electrode. The type of electrostatic precipitator shown in II Ill’ 7-28 consists of a negatively charged wire hanging down the middle of posi- II ely charged plates. The particulates collect on the plates and must be removed by It,lIIgingwith a hammer. Electrostatic precipitators have no moving parts, require only “‘(Iricity to operate, and are extremely effective in removing submicron particulates.
:1.- “. ~”‘..
Inlet baffle diffuser
,’.
…—– Clean gas 0111
Water jets
,.j–tlf— Tangential inlrl for dirty gas
…- Water in ____ Flushing jets
directed downward
‘~”‘~f——~~~~~i~;d particles out
Figure 7-27 Scrubbers used for air pollution control. (Courtesy William A. We 1111 II)
-;
;\’ll1gI1S 0111
N gative Ie trod’ ‘OIIIH’ ‘lI’d -1:–__ ,——- to electric pow r sou r ‘r
-=-‘1—— Negatively char xl wire
Grounded collecting ph1I(~ with positive harp,’
t~~i’f,,”’F-;” Dirty gas in
~——– Hopper to discharge
—————————————– 7-28 Electrostatic precipitator. (Courtesy P. Aarne Vesilind)
ntrol of Gaseous Pollutants I III ontrol of gases involves the removal of the pollutant from the gaseous emis- luns. a chemical change in the pollutant, or a change in the process producing III’ pollutant. In MSWcombustion, where the fuel composition is seldom under 11I1[ro\’the gaseous removal processes must be robust and effective.
W l S rubbers, wh (11(“\11I ‘ It. t’t! 101p,lIll,1I 1,111((d,lIl’ I ‘illOV,II, \,111″ remove gaseous polluu nls I y simply uis, lvlng lil{ III in the water, AIII’IIIIII II a chemical may be inject dint th s rubb I’ w tcr, whi I 111 n ITi\(‘l.~wlrh II( pollutants. This is the basis for most So.2 removal t chniqu s, as diseUH \’t! HI I Because of the moisture carryover, wet scrubbers are usually pia d a 11’\ 1111I I or baghouse.
Dry scrubbers are very effective in controlling sulfur dioxid s. A linu 111111 is injected into this unit, but the liquid evaporates due to the high ICII’IIII.l1I1I of the exhaust gas. The lime slurry also reduces hydrogen chloricl in III(‘ I’ 11,111 gases.
In simple terms, the reaction in the dry scrubber is Ca(o.HL + heat ~ Cao. + Hp So.2 + CaO ~ CaSo.3
or if limestone is used, So.2 + CaCo.3 ~ CaSo.4 + Co.2
Waste-to-energy combustors also are a source of nitrogen oxides that 1,111II II to the formation of photochemical smog and contribute to acid rai n. N1111111I oxides, designated by the general expression No.,J are produced in Iwo \ ( Thermal NO x results from the reaction of excess oxygen (from air) wiIh IIIIIIII• , (from air) at high temperatures. Fuel NOx are produced when the nitr g(,11III Iii fuel produces the oxide during combustion. Generally, the thermal No..I’repl I ‘. III only about 25% of the total nitrogen oxide production.”
Unlike sulfur, nitrogen cannot be readily removed from fossil rIll'” III reduction of nitrogen oxide is accomplished by improved combustion conu«l .11111 using a DeNo.x system that converts nitrogen oxides to nonpolluting nitro):I’1Ipi N2, and water vapor.
Covanta, the largest waste-to-energy provider in the United Sta tvx, IIII developed a combustion control system for existing plants that call 11’11111 nitrogen oxide emissions to 50% below EPA standards and another sysuur 1111 new plants that can reduce nitrogen oxide to 70% below EPA standards IIII systems greatly reduce the amount of excess oxygen in the furnace needed III complete combustion, and for new plants, they also provide a furn.u I’ ,I I recirculation system.
In addition to combustion control, one of two types of DeNO, SYSII’11II used. The selective non-catalytic reduction (SNCR) process involves the injcc 11011ul ammonia and steam into the furnace. This is an attractive retrofit option to l’xl’lI” plants because of the low capital cost to retrofit the equipment.
In a selective catalytic reduction (SCR) process, a catalyst is used to al II I ‘ higher reductions than are possible with a SNCR process. The catalysts all’ II II sitive to particulate contamination and are typically located after the sCllIIIIII and baghouse. However, for the catalyst to function properly, the flue gas 111’1II to be 450°F (230°C) and thus must be reheated prior to the catalyst. Thci dill some processes have the SCR system before the wet scrubber and bagll()l1~ So that the catalyst can work properly, an electrostatic filter needs to be i111 pll mented before the SCR step. While many United States plants use the SNI I process, no plant uses the SCR process. However, the SCR process is being II I”
hun I w rl
Waste receiving
and storage Combustion and boiler
Flue gas treatment Residue handling and treatment
, TIpping hall :2 \Yastr pit 3 Waste pit ventilation 4 Waste crane
5 Peed hopper
6 Ram(~r
1 HitilChl Zosen lnova grate S Ram bottom ash extrectcr 9 Bottom ash handling
10 Primary air intake 11 Primary air fan 12 Primary air distribution 13 Secondary air fan 14 Flue gas learculation fan lS Peer-pass boiler 16 Boller drum
17 Electrostatic pr«ipitator 18 SCR DeNOx and catalyzer 19 EconomiseJ
20 Gaslgas heat exchanger 21 Quench 22 Wet scrubber 2) Fabricfilter 24 Induced draft (an 2S Silencer
26 Emissions measurement
27 SUd<
28 Ash conveyfng s)’Stem 29 RMldue silo
I, ur 7-29 Modern WTE Plant, Thun, Switzerland (Courtesy Hitachi, Zosen Inova)
III pit nts in both Europe and Japan. While the SCR process achieves a high ‘I’ \I II( en-oxide reduction compared to the SNCR process, it is more expensiv ‘ III 11 tall and operate.
The control of air pollutants from MSW combustion plants has continued t I)Iv ,A typical scheme used in European facilities, shown in Figure 7-29, consists or:
1. A boiler operating at high temperatures (>1200°C), destroying dioxins that may have been produced during the combustion of the bulk materi- als at lower temperatures or entered the burning chamber with the sol id waste.
2. An electrostatic precipitator for particle removal. 3. A SCR Denox system with a catalyzer in order to reduce No.x’ 4. An economizer and a gas-gas heat exchanger to increase efficiency. 5. A quench with water. G. A wet scrubber for removing acid gases. 7. A fabric filter to capture fine particles.
The cleaned gases are then emitted from a stack. Table 7-14 lists typical gas emissions values reported by a WTE plant in
wltzerland.”
Gas Emissions In 2014′ of an WTE PI nt which d It 0 r tl n In 2006, The discharge gas flow corresponds to 52,900 Nm Ih (1 Nm3 qu I 1 norm I cubic meter reported at oce and 1013 mbar)
Measured values R( ,it” Ihtll
Requirements Guaranteed August 19th, «lIdlll , Limit given by law of the plant values 2014 l low Iltlll
— flow rate Nm3/h 52,900
mq/Nrn” 10 10 5 0.6 1}1l (I mq/Nrn” 20 10 5 1.4 () I (I’ rnq/Nrn” 2 1 0.5 0.12 <J1l (I’ rnq/Nrn” 50 50 20 2.3 9″‘1 mq/Nrn” 80 200 50 39 ~) I I mg/Nm3 5 aucune 5 1.1 III (J rnq/Nrn” 0.1 0.05 0.05 0.0086 91 ~ mq/Nm” 0.1 0.05 0.05 < 0.0005 (N’, mq/Nrn” 1 0.5 0.5 0.061 <)’I “” mq/Nrn” 50 50 25 7 tll’ (I’
furans ngTE/Nm3 0.1 0.1 0.1 0.0058 911 ,…
ance on Air Pollution Control
y Equivalents
7-4-3 Dioxin Of particular concern in waste combustion is the production of dioxin, 11111 III emission concentrations have been shown and discussed in previous h,11’111 Dioxin has become a big issue since the chemical accident in Seveso (11,11 I occurred in 1976, and the following chapter is devoted to dioxin in cornbusrluu Dioxin is actually a combination of many members of a family of organic I I 1111 pounds called polychlorinated dibenzodioxins (PCDDs). Members of this fam ilv ,II characterized by a triple ring structure of two benzene rings connected by a p,dl I” oxygen atoms (Figure 7-30). A related family of organic chemicals are the po/)’1 1111′ rinated diben-zofurans (PCDFs), which have a similar structure except that thr 11’11 benzene rings are connected by only one oxygen. Since any of the carbon sin- 1111 able to attach either a hydrogen or a chlorine atom, the number of possibiliurs I great. The sites that are used for the attachment of chlorine atoms are idem Ii I II by number, and this signature identifies the specific form of PCDD or PCI)Jo’ IIII example, 2,3,7,8-tetra-chloro-dibenzo-p-dioxin (or 2,3,7,8-TCDD in shorrh.uul] has four chlorine atoms at the four outside corners, as shown in Figure 7-3 I ‘1111 form of dioxin is especially toxic to laboratory animals and is often identi 1I\’t I ,I a primary constituent of contaminated pesticides and emissions from W;)Sl!’ III energy plants.”
All of the PCDD and PCDF compounds have been found to be extrcuu II toxic to animals, with the LDso for guinea pigs being about 1 /-Lg/kgbody wl”I)\liI
u 7 30 A dioxin molecule.
111’1′ P DD nor PCDF compounds have found any commercial us and ill” wt! manufactured. They do occur, however, as contaminants in other organic
Itl Illl e Is.28 V c ri us forms of dioxins have been found in pesticides (such as Ag’l1l
I 1,lllH’, widely used during the Vietnam War) and in various chlorinated organk ,11I’1lI! Is such as chlorophenols. Curiously, recent evidence has not born’ 0111 1111 am level of toxicity to humans, and it seems likely that dioxins are a tually II 11-rmful than they might appear from laboratory studies. The chemi al spill III “v so was expected to result in a public health disaster based on extrapo!a- lit Ill, from animal experiments, but thus far, this has not materialized. I1C I I “‘t t of the dose suffered by the children in Sevesco was 3,000,000 I g 01″ dill In/kg of body weight-compared to the EPA risk-specific level of 0.006 pg/kg lit 1\ I weight or the Canadian tolerable daily intake of 10 pg/kg body W igh l.
1111 ugh the children suffered from chloracne (a temporary skin condition), none lt.t • s emed to have the more serious cancers predicted. Nevertheless, di xins III’ able (at very low concentrations) to disrupt normal metabolic processes, and
tit h s caused the EPA to continue to place severe limitations on the emission r II II in from WTE plants.”
Dioxins emitted from waste-to-energy facilities come from two sou I’ s: d Il iI1S that are in the waste and are not combusted in the furnace and de 11011(1 d nxins that are created during combustion.” Tests on waste-to-energy plants that luvc included slug loads of chlorinated plastics (thought to be the main precursors
(I (I
o
o Cl (I
ure 7-31 A particularly toxic form of dioxin: 2,3,7,8-TCDD.
Metals Combustibles Ferrous metal Nonferrous metal Glass Ceramics Mineral, ash, other
16.1 4.0
18.3 2.7
26.2 8.3
24.1
1M Iinin 11 xin ~ rmatlen ] hnvv produrcd IW}\IIYI’ 1′,\111.,11 Nevl’llh’II’, I estimates that a signlf ant part r .uvir 11111 ‘l1(nl dioxins (01’ nil 101111 , III I difurans) comes, from combustion.
There is little doubt that waste-to-energy plants rnit tra ‘:\11\011111 I I ins, but nobody knows for sure how the dioxins original. Som . dl(l~ II I waste, and this may escape in the bottom ash, the fly ash, or 1111’ 1111 II I the other hand, it is also likely that any combustion pro ess I ha I 11.11 • I U amounts of chlorine produces dioxins, and these are sim p Iy • 11 cu Ii IIi 11111 the combustion process. Another theory is that the high temperatures 1\ III bustion unit destroy the dioxins but that new dioxins form as lll” (‘Idlllil I cooled. The presence oftrace quantities of dioxins in emissions fWIII Will “I and fireplaces seems to confirm this view. Whatever the mechanism, il 1111 III that it is not a simple chemical equation and that many pa ra I lcl 11′,\1 IIHI occurring to produce the various forms of dioxins and difurans.”
It might be well to remember here that the two sources of risk, W II pi versus fireplaces, are clearly unequal. The effect of the latter on 11 III 1\,11 I II III greater than the effect ofWTE plant emissions. But the fireplace is a /111/””’,,, whereas the WTE plant is an involuntary risk. People are willing to :I( 11’1’1 III tary risks 1000 times higher than involuntary risks, and they are lherdl”l III vehemently oppose WTE plants while enjoying a romantic fire in tlH’ 1111 II1I
AI
Material Percent by Weight
ustion comes perilously close to being classified as A h from MSW comb 11 sed is an extraction procedurb th EPA The test usua Y u .
11111..11’0 us waste y e . 1 d the amount of metal extracted IS m ~a- 1\111 th = is shaken ~ith a so ~en~;~ times the drinking water standard, the I’ d, If this concentratlOn excee; . pecial and very expensive disposal. I II lassified as hazardous ~n req~Hes s ften do not pass the test, and it II \I h is measured by itself. ~~~~~n~~~~~~~Sb~ttomash, however, the mixture ,III Hi d as hazardous. ~om for a nonhazardous waste. However, sorn “I ,Ct n meets th: requHeme~ts ntial of the waste, such as by defining
I.H1 n also ~onslders t~e tOXICpote. II hazardous compounds. Under this III I n entratlons for toxic and potdenna y t 1 even if it was combined with 11″ Oil h. the fly ash would be teste separa ~ Y , II( lL mash. ,.’ h li e or limestone usually produces an ash
‘1’1′ atment of the emlsslOns WIt . 1m. ., the metals in a less soluble. d thos helps m mamtammgtill’ (Ikalme range, an 1 . II oblematic than bottom ashes. In dll\l(lde form. ~l~ as?es ~re p~tent~d :::r:~ralkaline material such as cemen.t.
lilli’ es a stablhzatlOn ISpe orm h h g I d to good results. Ash disposal IS., fl h .th wood as es as e .I 1\, uuxmg y as es Wl .. I lid waste landfills If the ash IS corn-. I . gular mumopa so 1 . . . 11111’1 In speoa. or. m re hi has 1950 kg/m3 (3300 Ib/yd3). At this density
I \I I the density increases to as ig . . I 1 X 10-9 cm/see. , . bl ith a permeabIlIty as ow as
, I h i highly impermea e: w~ . roduced and landfill space becomes too As more and more as.h IS emg P h alternative uses are being sought.
hll\1 to use it for the disposal of s~~ i:si~cinerated and where landfill space I 1111 ipe. where a large fract~onhof .b uite successfuJ.24Some of the uses of
, IY pensive, ash processmg as een q 11111 tude 111,1 Ibase material II11tural fill 01 ,IV ‘1 drainage ditches ,Ipping strip mines \ Ing with cement to make building blocks
7-4-4 Ash Power plants produce both bottom ash and fly ash. Bottom ash is rcrovru II It the combustion chamber and consists of the inorganic material as w(’11 ,I I 1 unburned organics (>3%), while fly ash is the particulates removed frolll 1111 I gas. Considering both types of ash together, MSW combustion typi ally ,111,1 75 to 80% reduction in material by weight (and 90 to 95% reduction by Vldlll! Thus, about 25% of the original mass is ash with a high density of a\)()111 , III 1070 kg/m” (1200 to 1800 lb/yd'”. The materials in typical MSW ash an’ Nltll I Table 7-15.
The major problem with ash from MSW combustion is the presence III III metals. Table 7-16 shows a representative array of some heavy metals Inlll\ I combined fly ash and bottom ash from a MSW waste-to-energy unit.
Table 7-15 Materials Found in Typical MSW Ash
Source: [21]
111( lditl )11,,Ii’ll.,(‘10111tvtSW comhu, 11011(OIlt.1 111111H’\,II.,III(II II !I’I’I/ III less steel, copper, an I (lIUl11il1l1l11,an I th ‘S’ nn I ~.I” -\” III xl in ,\ III ‘(1\,11\I.d I ration process. This process is in r asingly .asll lc r I” both bottom ilild Ilv I 11 For example, concentrations of zinc in fly ash s I” th rd I” (‘ 1l1,IK”111.1 zinc concentrations in some ores, and copper can be recov r d fr rn I 0111111111II The recovery of the metals has the secondary benefit of redu ing the loxl, II’.Ii I ashes and thus facilitating their disposal.
7-5 FINAL THOUGHTS
Earle Phelps was the first to recognize that most environmental 1(‘1\111,1″, decisions are made using what he called the principle of expediency. II ,,11111II engineer known for his work with stream sanitation and the dev ‘IOIlIIII III “I the Streeter-Phelps oxygen sag curve equation, Phclps described xpcdl.-ru \ “the attempt to reduce the numerical measure of probable harm, or Ill(‘ 1111\11II measure of existing hazard, to the lowest level that is practicable and II/I IIII within the limitations of financial resources and engineering skill.” III’ II I ” nized that “the optimal or ideal condition is seldom obtainable in PI’, rt il I \III that it is wasteful and therefore inexpedient to require a nearer appro.u II III \I than is readily obtainable under current engineering practices and aI jLlslill ,III costs.” Most importantly for today’s standard setters, who often find it tIIII” III to defend their decisions, he advised that “the principle of expediency I II logical basis for administrative standards and should be frankly stated ill 11111 defense.”
Phelps saw nothing wrong with the use of standards as a kind of speed IIIIII! on pollution affecting human health. He also understood the laws of dirninnluu returns and a lag time for technical feasibility. Yet he always pushed toward 1111111 ing environmental hazards to the lowest expedient levels. Just as utilitarianism I II ethical model that can resolve moral dilemmas, Phelps’s expediency prin ipl, , III be used to resolve the moral dilemmas of setting environmental regulations III regulator must balance two primary moral values-do not deprive liberty, .uu] ill no harm. Setting strict regulations would result in unwarranted reduction in III11’11 while the absence of adequate regulations can damage public health. By usiuj; III principle of expediency, the regulator can establish the proper balance and n’sl Ii 1 I moral dilemma.
All ethical models, if they are to be useful, need adequate information. I IIii”I the utilitarian ethical model, for example, a just decision is possible only il II” amount of happiness and unhappiness that results from decisions can be ( ,III\I lated. Similarly, the regulator must have scientific evidence on pollutant qua 1111111 concentrations, vectors, and health effects to make a just environmental de. i~dlill Unfortunately, there always will be gaps between what we know and wh.u \ would like to know about environmental hazards, and the absence of adcqi 11\1 scientific knowledge makes the regulatory decision difficult.
A classic case is the setting of the dioxin standard for MSW combusilnn The best we can do at this time, in the absence of adequate information, i’l 111
f renees
Wilson, D. L. 1972. “Prediction of Heat f Combustion of Solid Wastes from
Ultimate Analyses.” Environmental cience and Technology 13 (June).
, Brunner, C. 1994. “Waste-to-energy.” In Handbook of Solid Waste Management, F. Keith (ed.). New York: McGraw-Hill.
I, ‘n st Method E1037-84, Standard Test Method for Measuring Particle Size Distribution of RDF-5, ASTM, Philadelphia, Pa., 1996.
I, Neissen, W. R. 1977. “Properties of Waste Materials.” In Handbook of Solid Waste Management, D. G. Wilson (ed.). New York: Van Nostrand Reinhold.
Ali Khan, M. Z. A. and Z. H. Abu-Ghararah. 1991. “New Approach for Estimating Energy Content of Municipal Solid Waste.” Journal of the Environmental Engineering Division ASCE 117, no. 3:376-380.
/I, Liu, J. and R. D. Paode. 1996. “Modeling the Energy Content of MSW Using Multiple Regression Analysis.” Journal of the Air & Waste Management Association 46, no. 7:650-656.
,1111 11111,IId ,I low 1\ Wi’ (\I W 11111111I UIIIIIIIII II III! 1If1<lOWIl1111 ,I 1(‘ 111111I willi II. 1)1’ ,II ‘1)1 ‘X))’ I ‘III III I lit. III11lnrd too III II ,\11(10\11)W ‘Ii. IIi ill’S not to wony nl ill II dl »(111I’ll Ii I till ,’1’111 h ‘l11i , I Is ‘X “”nply hi II, III I w’ sh uld bc n ‘1’11’1. /\, IlI’l( ‘I 1\1(1111111in b 1111111II (ln rr SI nd rd. Hut in IIll’ 111’1\1111I11′,W’ sct th
I I P’I\’I1l. l’lunlly, W know that mbusu II rcmnlns I r bl matic due to th r leas r
Itl I heavy rn tals such as mer ury. With impr ved air pollution control equip- lilt 111/III’/) \ m tals are removed from th mission gases and captured with th fly liltI I Oll m a h s. The volatile metal salts that are transferred to the fly ashe an III “lIy I a h d out when these ashes are exposed to the weather. Thus the ash is I” III(d b I’ landfilling. However, the long-term behavior oflandfills containing I II I I’, d fly ashes is not well known, and there is the potential that the weather- III pro ss s may lead to a release of the toxic metals to the environment OV’I”
,III long l rrn. Today’s expedient decision could have a significant environmental 11111′,1(‘1in the future.
7. Brunner, C. and S. Schwartz .. I ‘Hi \ and Resource Recovery from WII,I/I’ Ridge, N.J.: Noyes.
8. Erdincler, AU. and P. A Vesi 1111 d, “Energy Recovery from tvtixcd JI Waste Management and aesl’II/I’iI 11:507-513.
9. Wilson, D. G. 1977. Handbook 0/, Waste Engineering. New York: V,I Nostrand Reinhold.
10. Neissen, W. R. 1995. Combustion 1/ Incineration Processes: Appli(‘(l/Ili/l Environmental Engineering, nd New York: Marcel Decker.
11. Vesilind, P. A, W. P. Martello, ,’lId B. Gullett. 1981. “Calorimcuy I Derived Fuels.” Conservation and Recycling 4, no. 2:89-()7,
12. Wen, C. Y. and E. S. Stanley. 11),/1 Conversion Technology. Read inil, Addison -Wesley.
13. Levy, S. J. 1974. “Pyrolysis or Mill Solid Waste.” Waste Age (octot
14. Pyrolysis. 1973. Washington, 1),( National Center for Resource Recovery.