Technology Options for Municipal Solid Waste to Enrgy Project- MEDA- 2000

8/8/2019 Technology Options for Municipal Solid Waste to Enrgy Project- MEDA- 2000

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Sudhir Kumar

Technology options for municipal solid

waste-to-energy project

TERI Information Monitor on Environmental Science 55555(1): 111

Maharashtra Energy DevelopmentAgency, 191 -A, MHADA Commerci al

Complex, Yerawada, Pune 41 1 006E-mai l: [email protected]

AAAAAbsbsbsbsbstrtrtrtrtractactactactact T here are various options available to convert solidwaste to energy. Mainly, the following types of

technologies are available: (1) sanitary landfill,

(2) incineration, (3) gasification, (4) anaerobicdigestion, and (5) other types. Sanitary landfill is the

scientific dumping of mun icipal solid waste due to which

the maturity of the waste material is achieved faster and

hence gas collection starts even during the landfill

procedure. Incineration technology is the controlled

combustion of waste with the recovery of heat, to

produce steam that in turn produces power through

steam turb ines. About 75% of weight reduction and

90% of volume reduction is achieved through burning.

A gasification technology involves pyrolysis underlimited air in the first stage, followed by higher

temperature reactions of the pyrolysis products to

generate low molecular weight gases with calorific value

of 10001200 kcal/nm3. T hese gases could be u sed in

internal combustion engines for direct power generation

or in boilers for steam generation to produce power. In

biomethanat ion, the putrescible fraction of waste is

digested anaerobically (in absence of air), in specially

designed digesters. U nder this active bacterial activity,

the d igested pulp produces the combustible gas methane

and inert gas carbon dioxide. The remaining digestate isa good quality soil conditioner. O ther t echnologies

available are pelletization, pyro-plasma, and flash

pyrolysis. All these technologies have merits and

demerits. The choice of technology has to be made

based on the waste, quality, and local conditions. The

best compromise would be to choose the technology,

which (1) has lowest life cycle cost, (2) needs least land

area, (3) causes practically no air and land pollution,

(4) produces more power with less waste, and (5) causes

maximum volume reduction.


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TIMES (TERI Information Monitor on Environmental Science) Volume 5, Number 1 (June 2000)

IntrIntrIntrIntrIntroductionoductionoductionoductionoductionCities in M aharashtra typically produce M SW(municipal solid waste) of 0.40.5 kg/person/day. T he rapid u rbanization an d ind ustrializationhas increased the pollution load on t he urb an

environment to unmanageable and alarmingpro por tion s. T he existing landfill sites are full be-yond capacity. It is difficult to get new dum pingyards and if at all available, they are far from thecity and this adds to the exorbitant cost of trans-portation. It is high time the municipal corpora-tions, state governm ents, and policy makers takeup the matter seriously. The best option is to re-duce the volume by effective treatm ent of thewaste. In recent years, the waste-to-energy projecthas gained attention due to its double benefit ofresource generation and pollution abatement.

Man y mun icipal corporations have shown

keen interest though an d t he high initial cost ofsuch projects were the main inhibiting factors.Rising to the occasion, some private investors /promot ers have offered to take up such projectson BOO (bu ild own operate) basis. T he maintask is to choose a promoter and specifically, tochoose a particular technology that is beneficialand environm ent- friendly. T he objective of thisarticle is to review the available technology optionsin the context of their merits/demerits, based onthe waste quality and quan tity.

TTTTTececececechnology optionshnology optionshnology optionshnology optionshnology optionsInitially, th ere was a tend ency to u se well-proventechnology such as steam tu rbines, using con-ventional boilers with MSW as feed. Subse-quently, many other technologies weredeveloped an d field-tested. M any other tech-nologies are read y for field trial following suc-cessful laboratory tests. It is worth noting herethat all demonstration an d full-scale plants areavailable in the West (P arker and R oberts 1985)and t hey are yet to b e launched com merciallyund er Ind ian conditions. Although m any differ-ent t ypes of R&D pr ojects have been taken u p inIndia and abroad, only commercially successfulprojects have been described here since descrip-

tion of R&D projects is beyond the scope of thispaper . T here are ma inly the following types oftechn ologies available on comm ercial scale.P Sanitar y landfillP IncinerationP GasificationP Anaerob ic digestionP Other types

As described in F igure 1, M SW without segre-gation could be used either in sanitary landfill or

mass burning to p roduce power. H owever, aftermechanical segregation, an energy-rich fuelcalled RD F (refuse derived fuel) is obtained,which can be u sed to produ ce power eitherthrou gh biochemical or thermal rout. In bio-

chemical rout, only anaerobic digestion has beenused commercially while in the case of thermalrout, b oth pyrolysis and RD F b urning have beenused successfully for commercial purposes. Eachof the categories is described below.

SanitSanitSanitSanitSanitararararary landfy landfy landfy landfy landfil lil lil lil lil lWhat is sanitary landfil l?

Sanitary landfill is the scientific dumping of MSWusing an engineering facility that requires detailedplanning and specifications, careful construction,and efficient operation (O Leary and Walsh 1991a).T here are m ainly three types of sanitary landfillsnamely (1) area method, (2) ramp method, and ( 3)

trench m ethod. In all the method s the site is firstselected considering the following factors.1 It should be at least 10 000 ft (3048 metres)

away from th e airport.2 It should not be located in wetlands.3 It should not be in flood- or earthquake-prone

areas.4 It should have a stable soil structure.

T he proper land preparation is carried outby having (1) 60-cm compacted solid liner,

Municipalsolid waste

Sanitary land fill

Mechanicalsegregation

Biochemicalcovertion

Anaerobicdigestion

Methane EthanolOil, gas

and CHARSteam

Hydrolysis/fermentation

Pyrolysis RDFburning

Refuse derivedfuel

Thermalconversion

Ferrous

Aluminum

Glass

Mass burning

Figure 1Figure 1Figure 1Figure 1Figure 1 Options for energy production from MSW (munici-pal solid waste)


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(2) flexible HDPE (high density poly ethylene)geo-membrane liner, (3) geo-textile liner,(4) 30-cm drainage material layer, and (5) 60-cmprot ective layer. T he pr ovisions for gas collection(throu gh 1.25 cm d iameter perforated poly vinyl

chloride pipe) and leachate collection (15 cmdiameter slotted H DP E pipe) are made. T hesolid wastes are land filled b y spreading th in lay-ers, compacted to t he smallest p ractical volum eand covering it each day or p eriodically withsome suitable substitute m aterial in a way thatminimizes environmental problems. Successivelayers are built up until a depth of 1012 feet(304365 cm) is achieved. Finally, it has to becovered with 60 cm o f soil layer for final closure(OLeary and W alsh 1991 b , c, d).

D uring the landfill procedure, at least 40%moisture mu st be maintained to achieve maxi-mu m microbial degradation. Periodically theleachate collection in the bot tom needs to b epum ped out to d rying beds specially preparedfor this purpose. Due to scientific landfilling, thematurity is achieved faster and hence gas collec-tion starts even du ring the landfill procedure.T he gas generation and com plete extraction areachieved even after closure (say up to 10 years).This is faster than the ordinary landfill where gasextraction con tinues even u p to 50 years. Figure2 shows clearly how, after final soil cover andmatu ring, the comb ustible gas could be used forgenerating power while leachate removal andtreatment are carried out simultaneou sly. Com-

post retrieval is an optional choice depending onsite condition a nd comm ercial feasibility.

Why sanitary landfil l?Unscientific and ordinary landfilling is the com-mon practice for M SW disposal in In dia andmany oth er Asian cou ntries. Even in westerncountries it was a common practice till the lateseventies after which it was legally banned. Theordin ary landfills are supp osed t o adversely af-fect the environm ent. T he major concern is thegroundwater pollution. T he chemical reactions,during degradation in p resence of moisture and

rainwater, allow the pollutant chemicals to dis-solve and flow into the groundwater reserve,which is later used for drinking in the nearbyareas. Besides this, CH

4(methane) and other

toxic gases evolve which are d angerou s for thevegetation an d p opulation nearby. U ncheckedmethane is an explosive gas and is hazardous asit may cause fires. CH

4released to th e atmos-

phere is responsible for global warming 10 timesmore deadly than that of CO

2(carbon dioxide)

Alarmed by these factors, the ordinary landfills

were legally banned vide the regulation FR-5/91

in the U S and similar regulations in E uropeancoun tries. O nly Sanitar y landfill was allowed.

UtilizationConcentration of C H

4over 25% only is worth

extracting for energy production. A typical LFG(land fill gas) has C H

4concentration of 50%.

T he LF G could be ut ilized directly for heating,as medium-heating value gas (raw gas), high-heatin g value gas (filtered gas) an d a lso forpower generation in IC ( internal combustion)engines, and in gas and steam t urbines. H ow-ever, the m ost economic option is the d irect u se

for process heating and boiler fuel. In the powergenerat ion projects, the cost of engines or turb inesis more than 60% of the total plant cost. T he onlyway to make the power generation viable is to forcethe utilities to purchase power from LFG at highercost (which is the case in the U K and the U S). Asurvey carried out in the US in 1992 showed thefollowing status (Thorneloe 1992).P T otal LFG plants (120 M W): 144P LF G p lants with IC engines (Caterpillar,

Coop er Superior and Wun kesha make): 61

Site selection

Scientific land

preparation

Layerwise sanitaryland filling

Final soil coverand maturing

Combustible gascollection

Gas engine

Electricgenerator

Compostsegregation

Final compost

Leachate pumpingout

Water treatment

Treated water

Figure 2Figure 2Figure 2Figure 2Figure 2 Flow diagram of an MSW (municipal solid waste)power plant based on sanitary landfil l technology


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P LFG plants with gas turbines (Super Centaurmakes): 21

P LF G p lants with steam turbines (Fu ji Electri-cal make): 3

P D irect use of LF G as pipeline gas: 6

P Con version of LF G to d iesel: 1

A typical LFG power generation plant in the UScosts roughly 15 million dollars per megawatt(1992 price) of which the generators cost 970thousand dollars. This shows that direct burning ismost cost-effective (35004000 kcal/m3).

Merits and demeritsThe merits of sanitary landfill are many as com-pared th ose of the conventional ones. It redu cesemission of CH

4, non-CH

4organic compounds,

and toxicants (e.g. vinyl chloride, toluene, ben-

zene, etc.) into the atm osphere. Land and watercontamination d ue to leachate migration is mini-mized. T he main advantage is the protection ofpublic health and environment. The menace ofbirds and rod ents is reduced. F ire hazard isminimized d ue to r egular extraction of gas. T heproblem of smelly odour wafting to th e nearbylocality is compar atively reduced .

The major objections to sanitary landfills arehigh initial costs for design and construction,public opposition when selecting the site, andincreasingly, the co ncern for recovery of materi-als instead of disposal. T he general tend ency isdum p yard n ot in m y backyard. Even after pre-

cautions, the obn oxious gas emission t o the at-mosphere can never be checked completely andhence some odour is bound to b e there.The efficiency of leachate and gas extraction isalso dou btful. T he problems of pests androdents are m inimized bu t never completelyeliminated. Even after keeping the sanitarylandfill area aesthet ically presentab le, the realestate value of the nearby area is reduced(T horneloe 1992).

IncinerIncinerIncinerIncinerIncinerationationationationationT he scientific sanitary landfills also have ma ny

problems. T he m ain problem is the availabilityof land located where transportation is economi-cally viable, and with minimum public objection.Accumulation of such a large volume of wastefor long time is dangerous for the en vironm ent.H ence t he b est way to solve the problem is toreduce the volume by burning. Even 90% volumereduction can be achieved by burning. But uncon-trolled burning causes air pollution and the heatthus generated is wasted and incineration is a

practical solution. Incineration technology is thecontrolled combustion of waste with the recoveryof heat to produce steam that in turn producespower through steam tu rbines (Bhide andSunderesan 1983). Figure 3 shows that MSW

after pretreatment is fed to the boiler of suitablechoice wherein high pressure steam is used toproduce power through a steam turbine. Properair pollution con trol measures are taken and ashfrom the boiler is dum ped in the n earby landfill.T he choice and t ypes of boilers suitab le aredescribed below.


Refuse derivedfuel

RDFburningboiler

Flue gas

Air pollutioncontrol

Stack

Steam

Steam turbine

Ash

Landfill

Electricgenerator


Ferrous

Aluminum

Glass

Figure 3Figure 3Figure 3Figure 3Figure 3 General flow diagram of an MSW (municipal soli dwaste) power plant based on incineration t echnology

TechnologyDepending upon the pretreatment methodology,there are m ainly two types of MSW com bustiontechn ologies available.1 Unp rocessed solid waste combustion technol-

ogy (also known as mass burning)2 Processed solid waste combustion technology

(also known as RD F b urning)


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Mass burningT his is the most com mon technology whereinMSW is burned withou t significant fuel prepara-tion as discarded form. The MSW undergoesonly limited processing to remove non-com busti-

ble and oversized items. Mass-burn technologiesinclude water wall furnace, refractory furnace,rotary kiln furnace, water-cooled rotary combus-tion furnace, and controlled air furnace. E xceptsome design changes, in all types of furnaces, themass burning of M SW is primarily performed ona grate system that enables combu stion air to beprovided throu gh the fuel bed with a variety ofalternative methods of feeding fuel to the grate.MSW fuel feeder typically includes a feed hop-per and hydraulic ram th at pushes fuel from thebottom of the grate. Furn ace wall can either bewater- or air-cooled (Bjeldanes and Beard 1996) .

RDF burningT his techn ology involves various processes toimprove physical and chemical properties ofsolid waste. Basically, RD F systems are u sed toseparate MSW into combustible and non-com-bustible fractions. T he com bustible material iscalled RDF and can be used in boilers. T heM SW r eceiving facility includ es an en closedtipping floor called m un icipal waste receivingarea, with a storage capacity equal to about twodays of typical waste deliveries. T he sorted M SWis then fed to either of the two equal capacityprocessing lines. E ach pr ocessing line includesprimary and secondary trom mel screens, threestages of magnetic separation, eddy current sepa-ration, a glass recovery system, and a shredder.D ue to red uction in fuel particle size and reduc-tion in non-com bustible material, RD F fuels aremore homogeneous and easier to burn than theMSW feedstock. RDF has been successfullyburned in a variety of stroker boilers and in sus-pension as a stand-alone fuel in bub bling andcirculating fluidized bed combustion technologyboilers. It needs lower excess air and henceworks at b etter efficiency. Also, han dling iseasier since non-combustibles have been already

removed. The RDF burning technology includesspreader stroker fired boiler, suspension firedboilers, fluidized bed units, and cyclone furnaceunits (Bjeldanes and Beard 19 96).

Status and costIn 1996, t here were 140 operating MSW com-bustion un its in th e US p rodu cing an aggregateof 2400 MW of electricity (Bjeldanes and Beard1996). D ifferent t ypes of combu stion technolo-

gies have been adop ted and run comm ercially. Inthe case of mass burn types the unit size variesfrom 503200 T PD (tonnes per day). T he oper-ating efficiency of 450580 kWh/tonne has beenachieved except in refracto ry furn aces where it is

comparatively less, i.e. arou nd 100 kWh/tonne.T he capital cost varies from 80 000 to 200 000dollars depend ing upon th e unit size and t ype oftechnology. All the mass-burn systems generatepoor quality of ash, and the flue gas thus formedhas higher pollution con tent.

T he RD F-t ype plants also have different com-mer cially-available techn ologies working at dif-ferent sites. T he unit sizes are 300500 T PD .T hese plan ts work at slightly higher efficiency,i.e. 550600 kWh/tonne. T he project cost variesfrom 85 000 to 160 000 dollars per T PD. Aspredicted, th ey produce better q uality of ash andash d isposal is less problemat ic. Specially de-signed fluidized-bed combu stors produ ce lesspollutants throu gh the flue gas. T he fluidized-bed technology, being new, represents only 23%of the total power plants on M SW.

Merits and demeritsT he most important merit of incineration technol-ogy is the size reduction that eases the p roblem ofwaste dumping. About 75% of weight reductionand 90% of volume reduction is achieved throughburning. All the organic matt ers, whetherputrescible or not, are redu ced to ash. M SW canbe used even without much fuel preparation. A

complete sterilization is achieved d ue to burn ing ofeven biologically hazardous waste, thus avoidingepidemics. MSW can also be co-fired as an ad di-tional source in coal-based power plants.

T he dem erit of such p lants is the air pollutionthat can never be avoided even in highly sophis-ticated power plants. Th e additional cost of thecomplete pollution control systems is about 30%of the power plant cost, which makes it finan-cially unattractive to the already high investmentsystem. T he pollution is caused mainly due toparticulate matter, CO

2, SO

2(sulphur dioxide),

,N O

x(oxides of nitrogen), d ioxin, and furan. T he

last two are supposed t o be hazardous to health.D ioxin and furan are th e generic terms referringto a special family of chlorinated benzene r ingcompou nds (1) polychlorinated d ibenzo-p-dioxins and (2) polychlorinated dibenzo-p-furans. Totally 210 different types of toxins andfurans exist bu t t he tetra series (containing fourchlorine atom s) are believed t o be t he m osttoxic. Also the ash contains toxic elements suchas arsenic, cadmium, lead, and mercury and


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treating the ash for the pollutants beyond limit isanother costly affair. Another operational prob-lem found in such plants is the add itional main-tenance cost to solve the problems of fouling andslagging in the fur nace.

GasifGasifGasifGasifGasificationicationicationicationicationT he extraction of maximum heat from a givenfuel depends upon the efficiency of mixing thefuel with oxygen or air. T his is perfectly achievedin th e case of gaseous fuels. T hat is why conver-sion of solid waste int o gaseous fuel is consid-ered one of the best options. As described inFigure 4 , M SW after pre-treatment is fed intothe main gasification chamber wherein biomassis converted into gas, which, in tu rn, p rodu cespower after cooling and cleaning through gas

engine connected to electric generator. A gasifieressentially carry out pyrolysis und er limited air inthe first stage followed by higher temperaturereactions of the pyrolysis products to generatelow molecular weight gases such as CO (carbon

monoxide), C H 4, hydrogen, nitrogen, etc. Thegas known as pro du cer gas has the calorific valueof 10001200 kcal/nm 3, which could be used inIC engines for direct power generation or in boil-ers for steam generation to p roduce power.

ProcessT he M SW that is received is sorted to removethe non -combustible materials. T he remainingorganic fraction (RD F) is shredd ed to achievethe feedstock size of less than 5 cm. T he mo is-ture content of feed is maintained at less than20% to maximize the heat recovery.

D ensification of RD F is not required, whichsaves significant capital expenses and operat ingcost. T he RD F is fed into the feed hopper withan agitator and hydraulically driven feed auger.The feedstock then passes to the main thermalreactor where at high temperature (9001200 C)the RD F is converted into gas. Ash is removedfrom the base of reactor in a closed system.T here are no fugitive airborne em issions fromthe ash systems. T he gas flows from th e top ofthe r eactor vessel through pipes equipped withinternal cleaners to mechanical gas cleaner ves-sel. Th en th e gas is passed thr ough a series ofmechanical cleaners to remove char, particulate

matt er, and any un-reacted solids carried by thegas stream. T he gas is then cooled throu gh heatexchangers to the tem perature as required byeither IC engine, turbine, or boiler. A high volt-age and low amperage electrostatic precipitatorcompletes the gas cleaning and cooling process.A self-contained oil/tar/water separ ation systemreceives condensate from the electrostaticprecipitator. O ils and tars are separated and re-injected into th e reactor. T he waste heat recov-ered from the reactor is used to preheat thefeed-stock and red uce moisture to an acceptablelimit ( Solar Energy Research Institute 1 979).

The quality of gas is regularly monitoredthrou gh a gas meter. As per the requirement , thegas is fed to IC engines to prod uce power. T heengine emissions are usually quite low. Anotheropt ion is to use th e gas in gas-fired b oilers forsteam generation, which, in turn, p rodu cespower through steam turb ines.

StatusM any such plants have been reported to beworking in the U S and Can ada successfully.


Refusederivedfuel

Biomass

gassifictionunit

Flue gas Gas engine

Airpollutioncontrol

Electricgenerator

Stack

Combustibleproducer gas

Producergascooling/ cleaning

Ash

Landfill


Ferrous

Aluminum

Glass

Figure 4Figure 4Figure 4Figure 4Figure 4 Flow diagram of an MSW (municipal solid waste)power plant based on gasifi cation t echnology


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They range in capacity from 100 to 400 TPD,installed between 1980 and 1992. After gainingthe experience of a decade, setting up of man ycomm ercial plants are un der way. It is expectedthat some plants of MSW gasification will be

installed on BOO basis (Parker and Roberts1985).

Merits and demeritsOne of the merits of gasification is that handlinggas is easier th an solid fuel. T he overall gasifica-tion to thermal efficiency is higher than directburn ing. Weight reduction of 75% and volumereduction of 90% are achieved. T his reduces thedaily amount to be dum ped at the dum pingyard. It produ ces only 8%12% ash as comparedto 15%20% during incineration. The most at-tractive aspect is the pollution control. Practi-

cally no hazard ous gas is expelled into t heenvironm ent. M oreover, limited plant area isrequired as compared to other contemporarytechnologies.

T he demerits are the regular maintenancerequired for the cleaning system. In case of inef-fectiveness or failure of the cleaning system , th etar and volatile gases cause damage to IC en-gines. T herefore, the b etter option is to use thegas for th erm al applications. T he design is fuel-and size-specific and looses versatility. T he so-phisticated cooling, handling of cleaning system,and regular operation and maintenan ce make it acostly affair. I n som e cases the tar s and volatile

gases are no t recirculat ed as feed for cracking.The release of such chemicals is poisonous to thevegetation an d soil environment (Reed an dDas 1988).

BiomeBiomeBiomeBiomeBiomettttthanationhanationhanationhanationhanationMunicipal solid waste is a heterogeneous wasteand contains the following fractionsP Pu trescible fraction . T his is also called digest-

ible fraction and contains biodegradable or-ganic matter such as kitchen waste, vegetablemarket waste, paper, grass cutting, and yardtrimmings. P utrescible fraction represents

40% of MSW in Ind ia.P Combustible fraction. Also known as refrac-

tory organ ics, these are eith er slowly digestibleor indigestible organic matter such as wood,plastics, rub ber, and other synthetics. T heyrepresent around 20% of MSW in India.

P Inert fraction . T hey are typically non -digest-ible and non-com bustibles such as stones,sand, glass, and m etals. T hey represent 15%of the MSW in India.

P Remaining 25% is the moisture conten t.

T he pu trescible fraction is ideally suited to pro-duce biogas and the rem aining slurry is a goodfertilizer. T here are different types of biogas tech-nologies discussed in t he next section. H owever,Figure 5 shows that the MSW is subjected to me-

chanical segregation to obtain p utrescible fraction,i.e. RDF. The digestible organic fraction thus ob-tained is kept as pulp in hydrolysis tanks for break-ing them into smaller molecules. T he hydrolysedpulp is then fed into anaerobic digestion tanks.H ere it is digested anaerobically (in absence of air),in th e specially designed digesters. Un der t his ac-tive bacterial activity, the digested pulp producesthe combustible gas CH

4, and inert gas, CO

2. The

CH4

gas is then used to produce power through abiogas engine connected to electric generator. Theremaining d igestate (slurry) is a soil conditioner ofgood quality and free from pathogens. With thehelp of a solid/liquid separator, organic fertilizer isobtained and the treated water can be safely usedfor irrigation.


Refusederived

fuel

Hydrolysistank

Anaerobicdigestion tank

Sludge asfertilizer

Treatedwater

Electricgenerator

Methanegas

Biogasengine

Digestedsludge

Solid/ liquidseperator


Ferrous

Aluminum

Glass

Figure 5Figure 5Figure 5Figure 5Figure 5 Flow diagram of an MSW (municipal solid waste)power plant based on anaerobic digesti on technology


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TTTTTececececechnologyhnologyhnologyhnologyhnologyDepending upon the M SW quality and quantity,local environm ental and climatic cond ition, thereare various com mer cially viable techn ologiesavailable globally. In all the t echno logies the

following steps are involved.P Sorting. T he pu trescible fraction is separated

either manually or mechanically.P Particle size reduction. T o provide maximum

surface area to the bacteria, the particle sizereduction is carried out by using screw cut-ting, milling, drum ming, pulping, or shred-ding m achines.

P D igestion. T he mat erial is then fed intoanaerobic digesters for gas generation.

P Post treatm ent. T he slurry or digestate is ma-tured for two to four weeks to make an agri-culture or horticulture quality fertilizer or soil

conditioner.

T he core of the whole technology is theanaerobic digestion. T here are m any technolo-gies for effective digestion which differ from eachother depend ing upon their digestion param-eters. T hey are briefly explained below (IE A1997; Roos and M oses 1997).

Dry batchT he M SW is fed b atchwise in the d igester afterinoculating it with the slurry of the previousbatch. T he load is allowed to d igest for 2030days till maximum gas is recovered. Commer-cially it is available as BOC EL L pr ocess. It hasthe typical disadvantages of instability and mate-rial handling, pertaining to batch process.

Dry continuousDRANCO , VALORGA, and KOM POG AS arethe co mm ercially available technologies in t hiscategory. T hey operate at th e solid con centrationof 20%40% and they achieve high loading ratesand minimize the requirement of water. Recy-cled effluent is mixed with fresh charge of M SWduring semi-continuou s feeding. T hey are ideallysimilar for thermophilic digestions (5055 C).

Wet continuousREF CO M t echnology uses the MSW solid con-centration < 10% u sing large amoun t of water.T his is most ideal if mixed with sewage sludge,animal slurries, and industrial waste. T o avail thelarge disposal of liquid, the effluent liquids areused to dilute the feed. T his technology is usuallynot chosen if it is to be used only for M SW.

Wet multistageBT A and PAQU ES designs are commercial tech-nologies available in t his category. M SW is con-verted int o slurry with water and fermen ted inthe first stage. T he slurry with volatile acids is

then converted to biogas in th e second stageusing high rate anaerobic digesters such as theanaerobic filter or UASB (upflow anaerobic sew-age blanket) digesters. Th e major disadvantage isthe com plexity in d esign and operation. H ighlyskilled man power is also requ ired.

Sequencing batchT his is a new concept known as SEBAC but yetto be fully comm ercialized. T his is similar to th edry batch type technology except that leachatefrom the base of one digester is used to inoculateand remove the volatile acid from other digester

in th e series. T he digestion is usually allowed for2030 d ays in one digester and the d igesters arefilled every week in sequence so that continuousgas supply is available throu gh th e centr alizedgas collection system. High solid content over30% is used in it. T he advantage of this design isthat volatile acids are not allowed to accumulateand anaerobic bacteria keeps on getting foodcontinuou sly. T he on ly disadvantage is the lackof continuous feed.

Status and costAnaerobic digestion technology using mix-waste,

bio-waste, and manu re is working on a com mer-cial basis in at least 15 cou nt ries, mostly inEuro pe, with capacity ranging from 4000 to220 000 ton nes per year. Only a few of themwork purely on MSW but the number of plantsworking solely on M SW is increasing, andman y new ones are on the p ipeline. T here aresome promoters such as ENKEM , CIC ON ,GE NL , and N ST LER who have offered to takeup such p lants using MSW on BOO basis inInd ia. On an average they have estimated t hetotal cost of the plant at 80100 m illion ru peesper 100 T PD capacity of the system. It has beenobserved that the cost of anaerobic digestion

plants have reduced with time and is expected tobe most com petitive as compared to th e otheroptions. T he m ost attractive aspect is environ-ment al gain, i.e. no pollution and soil condition-ing (IEA 1997).

Merits and demeritsAn anaerobic d igestion technology has manymerits.


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P Useful products such as biogas and compostare obtained.

P D rastic redu ction of p athogens is achieved.P Needs comparatively much less land.P N o release of greenh ouse gases to environm ent .

P No problem of odour.P Aesthetically good looking and hence no prob-

lem of real estat e loss.P Comparatively more cost effective from the

point of view of life cycle cost.

Besides having so m any m erits it suffers fromsome dem erits.P It is a slow process and cannot accept shock

loading.P N on-biodegradable organic fraction (i.e. re-

fractory organics) cann ot b e digested andhence, needs to b e disposed of without treat-ment or, additional technology (such as gasifi-

cation) needs to be emp loyed for therefractory organics.

P T he compost prod uced is not directly usefulfor soil, and needs special treatment toremove undesired materials such as glass,metals, and pathogens. T his adds to the costof the plants.

OtOtOtOtOther ther ther ther ther tececececechnologieshnologieshnologieshnologieshnologiesT here are other techn ologies that h ave been

reported to be tried on experiment al levels butare yet to be comm ercially exploited. T hey aredescribed below (Bhide and Sund eresan 1983) .

PelletizationPelletization is a process of prod ucing fuel pelletsfrom solid waste. The complete process involvesdrying, removal of non-combustibles, grinding,mixing, and produ ction of pellets under highpressure. Usually, the conversion time is 25 min-utes. T he calorific value of raw garbage isarou nd 1000 kcal/kg while the p ellets also knownas RDF have the calorific value around 4000Kcal/kg. About 1520 tonnes of fuel pellets canbe produ ced after treatmen t of 100 tonnes ofraw garbage. T hese pellets could be u sed for

heating in the boilers and the steam t hus gener-ated, in tur n, is used to produ ce power. A powerplant of 5 MW based on RD F will need 12 acres(4.85 hectares) of land and 600 TPD of rawgarbage. It will cost 220240 million rupees(1996 price). T wo such plants are reported to beworking in Bangalore and M um bai.

Plasma arc (pyro-plasma process)This system uses a heat source called a plasmaarc flame. Two electrodes are precisely shaped

and distanced . A highly ionized gas is passedbetween them and high voltage discharge occursbetween the electrodes causing a hot plasmazone to be created. T he plasma gas aroun d theelectrodes is of extremely high temperature rang-

ing from 560030 000 F (309316 649 C). Atsuch a h igh tem perature th e molecules withinthat zone dissociates into their individual atoms.T hereafter, quen ching allows for th e contro lledcooling of the hot plasma gas. T he reintegrationprocess produces synthesis gas. Since the processoccurs in the vacuum th e intermediary produ cts(pollution causing) N O

xand SO

x(oxides of sul-

phu r) are n ot formed. It takes care of all organicmatt ers whether biodegradable or not. T he costis expected to be Rs 4050 million rupees. Afully commercial plant is yet to be installed. Onesuch demo nstration plant is expected to be in-stalled in T aiwan.

Garret flash pyrolysis processT his low temperat ure p yrolysis process yieldsfuel oil. In this system , plan t refuse is initiallycoarse shredded to less than 50 mm size, air clas-sified to separate organics, and dried in an airdrier. T he organic portion is then screened,passed thro ugh a hamm er mill to reduce theparticle size to less than 3 m m, an d th enpyrolysed in a reactor at atmospheric pressure.T he heat exchange system allows pyrolytic con-version of th e solid waste to a viscous at 500 C.

Status of MSW power projects inIndiaT he first M SW power plant was a project withincineration techn ology using 300 T PD waste toproduce 3.5 MW power installed at Timarpur,Delhi, with financial aid from the Danish gov-ernment and the M NE S (Ministry of Non-con-ventional Energy Sources), Government ofIndia, during 1989/90. The plant has been re-ported to be ru nning without com mercial viabil-ity, probab ly due t o some design p roblem andtherefore, finally was closed. The lesson from theabove plant h ad taught all the concerned agen-

cies to take up the future projects carefully. Atthe same time, th e mun icipal corporations wereskeptical about ventur ing in this field. A ray ofhope cam e from the private investors / promoterswho offered to take up the waste-to-energyprojects with their own investment on BOO basisdur ing 1993/94. T he proposals on BOO basiswere invited from private investor by many mu-nicipal corporations. As a result the municipalcorporation signed agreements with th e privateprom oters wherein, t he mu nicipal corporation


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was expected to provide land and garbage free ofcost while the prom oter would run the plantcommercially. Only a few agreements appear tohave been converted into real projects.

T he statu s of projects in Ind ia is as follows.

P An MSW project with 500 TPD capacity hasbeen installed at Hyderabad M unicipal C or-poration by Selco International Lt d whereinthe waste is converted into energy-rich fuelpellets. T he energy plant installed with t hehelp of the MN ES and the APTD C (AndhraPradesh T echnology Development & Promo-tion Cen tre) is expected to expand further totreat 1500 TP D of waste to produce 20 MWpower.

P N agpur Mu nicipal Corporation signed anagreement with ENBEE Infrastructure Ltd,Bhopal, for installation of a 500 T PD 4 M Wwaste-to-power project using biogas technol-ogy with foreign collaboration. The construc-tion work started at the site after the projectreceived the sanction of subsidy from theMN ES and loan from IRED A (Indian Renew-able Energy Development Agency Ltd) an dHUDCO (Housing and Urban DevelopmentCorporation Ltd). The administrative ap-proval from the Governm ent of Maharashtrahas been already accorded t o the p roject.

P T he other m unicipal corporations who are inthe process of signing agreements with privatepromoters are Mumbai 1000 TPD (10 MW);Pune 450 T PD ( 4 MW); Solapur 300 TPD

(3 MW); and Kalyan 600 TP D (6 MW) allin Mah arashtra. It is also reported that theCh ennai Mu nicipal Corporation is planing totake up a 600-T PD project.

Considering the above situation, it is obviousthat t here is no comm ercially running powerproject based on MSW. Probably the Nagpurproject will be first of its kind involving th e In -dian private investor with foreign collaboration.

ConclusionConclusionConclusionConclusionConclusionThe above discussions about the options of vari-

ous technologies show that the choice of tech-nology has to be done judiciously. Sanitarylandfill is a good choice if sufficient land is avail-able and proper care is taken to tr eat theleachate collector from th e bottom of thelandfill. Also th e area near the land fill shou ld bekept aesthetically presentable. As regards incin-eration, it has the advantage of 90% of volumeredu ction. T he techn ology is also available lo-cally. O f course, on e has to b e very careful tomake sure th at flue gas coming out of the stack

mu st not pollute the environment. T he gasifica-tion technology is relatively new and appears tobe suitable for th e treatm ent of solid waste. Itdoes not cause atm ospheric pollution and worksat higher efficiency. Biomethanation is the most

widely used technology all over the world, moreso in Europe. T he main advantage is that it is anenvironment-friendly and reliable technology.T he only disadvantage is that it cann ot treatnon-b iodegradable organic fractions. Other tech-nologies such as p yrolysis, p elletization, an dplasma arc are new and are yet to be com mer-cially established.

It is obvious from the above discussion that notechn ology is perfect. All of them have meritsand demerits. T he choice of technology has tobe m ade based on the waste quality and localconditions. T he best compromise would b e tochoose th e techn ology that fulfils these criteria.P Lowest life cycle costP N eeds least land areaP Cau ses practically no air an d land p ollutionP Produces more power with less wasteP Causes maximum volume reduction.

T he recent tren d is that m any private inves-tors / promot ers are now offering to take up th eprojects on BOO basis. T he whole project is tobe run on th eir own investmen t. O bviously, theytake greater risk and the investment is not possi-ble unless they are sure about the techn o-eco-nomic viability. In that case any of the above

technologies could be accepted b y the m unicipalcorporations as long as the promoter fulfils allthe technical, legal, financial, and pollution con-trol requirements. T he best and judicious choicelies in opting for a promoter who provides thetechnology with a well-balanced compromise.

AAAAAcccccknoknoknoknoknowwwwwledgledgledgledgledgementsementsementsementsementsT he author feels obliged to the Director, ME D A(M aharashtra En ergy D evelopment Agency)since presentation of above information was pos-sible only due to regular encouragement fromthe D irector, MED A to spread the programme

on a large scale in M aharashtra. T he trainingprogramm e on Waste-to En ergy in th e USsponsored by the MNES had exposed the authorto the wider range of knowledge in this field onan intern ational level. H ence, the au thor is in-debted to the MNES.

RRRRRefefefefeferererererencesencesencesencesencesBhide A D and Sun deresan B B. 1983

Processing m ethod for the future solid waste man-

agement in developing countries.pp. 124134


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New D elhi: Indian National Science D ocumentat ion

Centre

Bjeldanes M N and Beard G V Z. 1996

Resource recovery

In Power Plant Engineering, pp. 710732New York: Chapman and Hall.

IEA (International Energy Agency). 1997

System and m arket overview of anaerobic

digester, pp. 121

[Anaerob ic D igestion Activity]

Paris: IEA

OLeary P an d Walsh P. 1991a

Introduction to so lid waste landfills

W aste A ge22(1): 4250

OLeary P an d Walsh P. 1991b

Landfilling principles

W aste A ge22(1): 109114

OLeary P an d Walsh P. 1991c

Landfill gas movem ent, control and uses

W aste A ge22(6): 114124

OLeary P an d Walsh P. 1991d

Sanitary landfill design procedures

W aste A ge22(9): 97105

Parker C and Roberts T (eds.). 1985

Energy from w aste an evaluat ion of convers ion

technologies

London: Elsevier Applied Science Publishers. 217 pp.

Reed T B and Das A. 1988Handbook of biom ass downdraft gasifier engine

system

Golden Colorado, U SA: Solar Energy Research

Institute [SERI/SP]

Roos K F and Moses M A (eds). 1997

A man ual for developing biogas system at com -

mercial farms in U S

Ag ST AR Handbook

Washington, D C: U S Environmen tal Protection

Agency [EPA-430-B-97-015]

Solar Energy Research Institute. 1979

A Survey of Biom ass Gasification.Vol. I and Vol. IIGolden Colorado, U SA: Solar Energy Research

Institute [SERI / TR-SERI]

T horneloe S A. 1992

Landfill gas recovery / utilisation - o ptions and

economics

Paper presented at the 16th Ann ual Conference on

Energy From Biomass and W astes

Organized b y the Institute of G as T echnology,

Orlando, Florida, 5 March 1992

Technology Options for Municipal Solid Waste to Enrgy Project- MEDA- 2000

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