Idea Transcript
Proceedings of the 18th Annual North American Waste-to-Energy Conference NAWTEC18 May 11-13, 2010, Orlando, Florida, USA
NAWTEC18-3567 HIGH EFFICIENCY WASTE TO ENERGY POWER PLANTS COMBINING MUNICIPAL SOLID WASTE AND NATURAL GAS OR ETHANOL Sergio Guerreiro Ribeiro University of Brasil – COPPE-UFRJ Rio de Janeiro, RJ 21945-970, Brazil
Tyler Kimberlin Omega Energy Consulting Fort Collins, CO 80525, USA
ABSTRACT 1. INTRODUCTION A new WTE (Waste-to-Energy) power plant configuration combining municipal solid waste and gas turbines or landfill gas engines is proposed. The system has two objectives: increase the thermodynamic efficiency of the plant and avoid the corrosion in the MSW (Municipal Solid Waste) boiler caused by high tube metal temperatures. The difference between this concept and other existing configurations, such as the Zabalgarbi plant in Bilbao, Spain, is lower natural gas consumption, allowing an 80% waste contribution to the net energy exported or more. This high efficiency is achieved through four main steps: 1. introducing condensing heat exchangers to capture low temperature heat from the boiler flue gases; the stack temperature can drop to 70°C; 2. high steam temperatures in external superheaters using hot clean gases heated with duct burners; 3. mixing the exhaust gases of a small gas turbine with hot air preheated in a specially designed heat exchangers. The resulting temperature of this gas mixture is almost the same as a standard gas turbine but with the flow similar to that of a large machine with a higher O2 content; 4. After the duct burner and heat exchangers, the oxygen content of the clean gas mixture is still high, nearly 18%, and the temperature is approximately 200°C. The gas is then used as combustion air to the MSW boiler such that all the energy stays in the system. The efficiency can be as high as 33% for the MSW part of the plant and 49% for the natural gas system. Since the natural gas consumption is almost ten times less than the existing designs, it can be replaced by landfill gas or gasified ethanol or biodiesel. Currently an 850 ton/day plant is being designed in Brazil in partnership with a large power company. Other advantages include, self generation of internal power and lower steam superheating temperatures in the MSW boiler. This concept can be used with any grate design.
Conventional WTE plants burn waste on specially designed grates and the hot flue gases generate steam in a boiler. Due to the very corrosive nature of these flue gases, [1], the steam temperature and pressure are limited to 400°C / 40 bar resulting in low thermodynamic efficiencies, around 22%, for power generation. One way to overcome this difficulty is to combine a natural gas turbine with a waste incinerator in such a way that the superheated steam produced in the MSW boiler is further heated using the “clean” exhaust from a gas turbine in an external superheater. Many WTE plants have been built using this concept, the most important one being the Zabalgarbi plant, Figure 1. This power plant generates 100 MWe gross and the thermodynamic efficiency for the MSW portion of the fuel is approximately 30%. For natural gas the efficiency is around 50%. The disadvantage of this scheme is that 75% or more of the electric energy produced comes from natural gas. Although in some cases, this can be a good solution from an energy point of view, it is not as environmentally desirable since natural gas is a fossil fuel and contributes to global warming, cancelling the benefits of landfill diversion. Also natural gas prices can vary unpredictably and it may not be economical to dispatch such plants. However, WTE plants have to run with a high availability which poses additional problems to the grid operator. 2. OPTIMIZED COMBINED CYCLE – OCC The proposed concept, named Optimized Combined Cycle - OCC, greatly reduces the amount of natural gas needed to increase the efficiency of MSW combustion. With OCC, 80% or more of the net energy comes from MSW allowing the natural gas to be replaced by fuels not commonly available in large amounts, including landfill gas or biogas from anaerobic digestion. Another possibility is to replace natural gas with gasified bio-fuels such as ethanol or biodiesel using the LPP Combustion,
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LLC C, a Maryla and-based ccompany, process p [2]. The efficciency of the e MSW can reach values of more than 33% % and the na atural gas e efficiencies are a higher th han a gas turbine if it was w used in a standalone e combined cycle he natural ga as efficiency approachess 50% without MSW. Th n for small gas turbiness around 5 MWe. The OCC even (Opttimized Co ombined Cycle) conccept has other adva antages succh as being specially suited for high moissture MSW as well as for small in ncinerators using refra actory walls. Neverthelesss large wate erwall boilers s can emp ploy the sche eme with ma any advantag ges as discu ussed here ein.
between 600 b 0°C and 700 0°C, with du uct burners (11) ( and (12) to adjusst the steam m superheatting tempera ature. To in ncrease the overall efficiency of the plant, the am mount of n natural gas used u in the d duct burners s must be op ptimized, the steam cyycle efficiencyy increased (higher presssure and emperature and reheating), the stack losses minimized m te b lowering the waste b by boiler flue ga as temperature, and lo owering the combustion excess air .
F 2 – Optimized Combined Cycle Fig. e Scheme
arbi Plant Concept in Bilbao, Spain n [4] Fig. 1 – Zabalga D ON OF THE PROCESS 3. DESCRIPTIO Consider Figure F 2. The e power is generated g byy one sma all gas turbine (10), a HP (High Pressure) steam s turbine (17) and a LP (Low Pressure) stteam turbine (18). The HP steam iss superheate ed in the MSW W boiler (6) up to a co orrosion safe e temperaturre and, optio onally, to a higher h temp perature in the t external superheaterr (3). Similarlly the LP steam s is reh heated in (5) below 400 0°C first by MSW M flue gas then furrther reheate ed in the exte ernal reheate er (2). After the extern nal superheater, (3) the e flue gas is at perature T2, above 400°C, and can be b used in (1 13) to temp preh heat the air, from (9), beffore being mixed m with the e gas turbine exhaust (Y). This ha as two effects: it reduce es the ount of naturral gas in the e duct burne ers and incre eases amo the O2 content of the gas turbine t exha aust. After th he air heater (13), the flue gasses from the gas turbine e may preh preh heat the boiler feedw water in the e optional heat exch hanger (25)) and then be used as part off the com mbustion air in n the MSW boiler. b Corrosion iss avoided byy using one or more extternal erheaters (2)) and (3) hea ated by the clean c gas exh haust supe com ming from the gas turbine (10) mixed with w preheate ed air at (9 9) and (13). This mixturre is heated d to tempera atures
Since the flue gas te emperature leaving l the APC A (Air Pollution Con P ntrol) with drry scrubbers is 140 to 17 70°C, we c can recover this ene ergy using condensin ng heat e exchangers ( (CHX) made e of glass tu ubes, teflon tubes or te eflon coated d steel tube es, built by Swiss comp pany Air F Fröhlich [3].. Combustio on air can n be prehe eated to te emperature Tair2 usin ng a CHX (Condensin ng Heat E Exchangers) air heater (9 9) and CHX economizer (8) used o preheat the to t feedwate er close to the deaera ator (23) te emperature. In a good de esign, the sta ack (16) tem mperature c be as low can w as 70°C allowing not only o the senssible heat r recovery but also the lattent heat from water con ndensing in ncreasing the heat transferred from the e waste c combustion in the boiler ((14) to the stteam. The flu ue gas at T which is cooler and has a lowerr O2 contentt can be T9 p partially recirculated as combustion n air to con ntrol the w waste comb bustion temperature an nd to reducce NOx fo formation in the t MSW furrnace. We ca an also run the t plant w without the gas g turbine (10) by incrreasing the pure air f flow Y, and natural gas duct firing (11) ( and (12 2) during m maintenance e periods. In this case, th he amount of o energy p produced by the natural gas approx ximately matcches the p plant parasitiic load and most of the energy exp ported by the plant will come from the wastte. The natural gas e efficiency willl be lower since it is liimited by the steam c cycle efficien ncy, howeve er, such a plant without the gas turbine would d be a good d solution iff landfill gass can be u utilized, particularly beca ause it is ge enerally ava ailable in liimited quanttities. This iss also a go ood solution from an
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environmental point p of vie ew, because e all the power p prod duced will come c from waste, inc cluding the plant para asitic load. In some ca ases it is be etter to use a gas engin ne as osed to a gas g turbine. Gas engines differ from m gas oppo turbines with re espect to th heir use in combined cycle t ways: allmost all the e heat rejectted in appllications in two gas turbines goe es to the exh haust flue gas s. In gas eng gines, ubstantial pa art of the he eat loss occ curs in the water w a su cooling the cylinders. Thus w we can introd duce an addittional dwater prehe eater before e or after th he optional heat feed exch hanger (25),, to capture e the heat from the en ngine cooling system to increase e the efficiency of the plant whic ch at the sam me time redu uces the nee ed for a heatt sink to co ool the engin ne. Gas engine exhaust has h an O2 co ontent of 7-11%, 7 comp pared to a g gas turbines 13-16%. Mixing M hot ambient air from heat exchanger e (1 13) increase es the c of th he gas engine exhaust to that of a gas O2 content turbines exhaus st, usually hig gher. In con ntrast with na atural c plants, where only gas turbines are gas combined cycle used d, we can employ either gas engines or turb bines, choo osing the be est solution ffor each parrticular case. This has special adva antages for small machines, say below 2 MWe, where ga as engines are more efficient e than n gas turbines. In the propo osed scheme, the comb bustion air fo or the MSW W boiler is preheated p be etween 200°C C and 230°C C and the O2 content is s close to 18 8%. This help ps to reduce e NOx form mation and to o vaporize th he water in the t MSW ea arly in the combustion grate. This is particularrly advantag geous ure waste th hat otherwis se would re equire for high moistu o promote continuous com mbustion. addiitional fuel to 4. NUMERICAL RESULTS The actual design of a WTE plant using OCC requ uires extensiive calculatio ons in order to optimize e the para ameters gov verning the project. p Des sign requirem ments vary y for diffe erent locations as well as MSW M charracteristics. For F example, in Brazil, th here is a tax cut in elec ctricity sales for power pla ants up to 30 MWe. Alth hough tipping fees are very low, un nder US$ 20 0/ton, power costs as is almost twice are higher than in the USA. Natural ga m be redu uced to a po oint in the international price and must ch the MSW W efficiency is optimized d with respe ect to whic econ nomic feasib bility. The ssmall amoun nt of natural gas need ded opens the door ffor a WTE//gas plant 100% 1 rene ewable, exce ept for plastic cs, allowing the t use of eth hanol [2] or o landfill gas s. To reach the optimum design point, we have eloped speciific OCC plan nt software making m it pos ssible deve to quickly q run hundreds o of cases, varying v not only thermodynamic quantities but b also plan nt configurations, MSW W properties s, as well ass economic parameters. This softw ware was validated v (An nnexes A and a B) using g the Gate eCycle comp puter program showing an a almost pe erfect
agreement between a b the two calculattions for a particular p c case of OCC configuratio on. To emphasize the ad dvantages off the concep pt, Figure 3 represents s a case where the Zab balgarbi MSW W boiler, 7 792 TPD (m metric tons p per day) of 1,850 Kcal//Kg LHV (Low Heating g Value) was ste correspon nding to 71 MWth, M is c combined wiith a 5.5 MW We General Electric (G GE) GE5 g turbine, instead gas i of a GE LM6000 (46 MWe) as a shown in n Figure 1. The T LM6000 0 has an ope en cycle effic ciency of 4 41% while th he GE5 value is 30.7%. In a pure combined c cycle, Generral Electric liists the effic ciencies as 51% 5 and 4 43%, respec ctively. Con nsidering tha at the natu ural gas e efficiencies in n the MSW/g gas plant are the same as if the s same amoun nt of gas wa as used in a standard combined c cycle plant, as a described by Korobitsy yn [1], we ha ave: Z Zabalgarbi MSW appa arent efficienc cy = 31.66% % Total Nat G Gas consump ption = 152 MWth M O OCC
MSW appa arent efficienc cy = 34.51% % Total Nat G Gas consump ption = 21.84 4 MWth
It can be e seen that th he natural ga as needed de ecreases by a factor of b o seven and the MSW efficiency is s almost 1 10% higher. The actual e efficiencies fo or the OCC case c can b calculated be d and are sho own below: OCC MSW actual O a efficien ncy = 32.65% % O OCC Nat Gas efficiency = 49 9.06% A naturall gas efficien ncy of 49% on this scalle, 21.84 MWth, is no M ot achievab ble in any internal com mbustion m machine ava ailable. Also, a pure com mbined cycle e system fo this amo for ount is not e economical and in prac ctice, the m maximum effficiency tha at can be obtained us sing gas e engines this size is under 40%. Of co ourse the nattural gas c consumption ecreased with a corres sponding can be de lo ower value for f the MSW W efficiency as a functio on of the d design requirrements. Forr an apparen nt MSW effic ciency of 3 30%, approximately 80% % of the net power p will co ome from w waste. These e results are summarized d in Table 1.
T Table 1 – Orriginal Bilbao Plant x Sa ame MSW boiler witth OCC.
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For this case, it is feasib ble to replace e natural gass with dfill gas, gasified ethanol,, or biodiese el [2]. We can n use land a sm maller gas tu urbine, gas engine e or jusst the duct burner depe ending on th he economicss and the avvailability of these t fuelss. The T Optimiz zed Combine ed Cycle co oncept has been exam mined by the e Austrian P Patent Office e in Vienna under u the Patent Co ooperation Treaty (PC CT) Internattional 08/000347. It has been given appllication No. PCT/BR 200 the statements of Novelty (N), Inventiv ve step (IS)) and ustrial applica ability (IA). Indu
F 3 - OCC Applied to Zabalgarbi Boiler. Fig. 4 CO2 EMISSIONS 4. The nexxt item to co onsider is ho ow the use of fossil natural gas will affectt the globa n al warming related e emissions. F Italian wa For aste, Consonni et al [6] showed that the total carbon conttent is 27.6% % and the re enewable f fraction is 16.0%, 1 i.e., 58% of the t total ca arbon is r renewable. T This seems to o be true for several locations. Considerr a conventional WTE plant, p without natural g gas, burning 792 TPD of a 1.850 Kc cal/Kg LHV, with the s same carbon n characterisstics as the Italian waste e, and a c capacity facttor of 90%. This corres sponds to a thermal in nput of 71 MWth. M With 22% efficien ncy, the total electric p power produ uced will be 15.62 MWe e. The annu ual fossil C 2 emission CO n from MSW W burning wou uld be: CO2 from MSW W (fossil) = 792 7 x 365 x 0.9 0 x 0.116 x (44/12) = 110,660 TP PY (Tons perr year) onsider the same MSW W boiler with OCC Now co cconsuming 21.84 MWtth of naturral gas (N NG) and p producing 33 3.89 MWe. Considering, for f simplicity,, that NG iss equal to pure p methan ne (CH4), burning b one MWh of n natural gas will w produce 0.2 ton of CO C 2. The ann nual CO2 e emission from m NG would be: C 2 from NG CO G in OCC=21.84 x 0.2 x 7884 7 = 34,43 37 TPY Since the 21.84 MW Wth of NG spent s would result in 33.89 - 15.62 3 2 = 18.27 MWe additional power, ge enerating the same po ower using NG alone with 40% efficiency e or this amount), one wou uld need (maximum achievable fo o burn 18.27 7 / 0.4 =45.68 MWth of NG. N This wou uld result to
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in 72,028 TPY of CO2. The difference of 37,591 TPY of avoided CO2 is due to the efficiency improvement of OCC. This corresponds to 34% of the CO2 emissions of the fossil fraction of the MSW. If we replace natural gas with landfill gas or ethanol, this will increase to 65%. Additionally, the avoided methane from landfill diversion will correspond to approximately 338,000 TPY of CO2 meaning that, even for the NG case, the annual net CO2 sequestration would be 265,000 TPY of CO2 for a 792 TPD WTE plant. 5. CONCLUSIONS This process allows WTE to be feasible at very modest tipping fees. Developing Countries that could not afford the costs of landfill diversion will be able to stop burying their organic wastes. Also Europe, North America and Japan could benefit from this concept and apply the surplus of resources from lower tipping fees in other ways to mitigate global warming. The OCC concept can be generalized to other types of thermal electric power plants such as sugarcane bagasse fuel for which the efficiency improvement can surpass 50% with very modest increase in the investment. ACKNOWLEDGMENTS I would like to express my gratitude and deepest admiration for Professor Nickolas J. Themelis, Chair of WTERT, who probably does not realize that the seeds he planted when he visited Rio de Janeiro in 2006 are about to germinate into large trees.
REFERENCES [1] Korobitsyn, M.A., “New and Advanced Energy Conversion Technologies. Analysis of Cogeneration, Combined and Integrated Cycles” – Laboratory of Thermal Engineering of the University of Twente – 1998. [2] LPP Combustion, “Dispatchable Renewable Energy: Gas Turbines Can Burn Liquid Biofuels as Cleanly as Natural Gas”- Renewable Energy World March 10 12, 2009. [3] Air Fröhlich - Flue Gas Heat Exchangers Catalog (http://www.airfrohlich.com/). [4] Martin, J., “Global Use and Future Prospects of Waste-to-Energy Technologies” - Fall Meeting Columbia University, Oct.7-8, 2004. [5] Alison Smith, Keith Brown, Steve Ogilvie, Kathryn Rushton, Judith Bates, “Waste Management Options and Climate Change” - Final report to the European Commission - DG Environment - July 2001 [6] S. Consonni, M. Giugliano, M. Grosso, “Alternative strategies for energy recovery from municipal solid waste Part A: Mass and energy balances” - Waste Management 25 (2005) 123 135. [7] Reference Document on the Best Available Techniques for Waste Incineration, Integrated Pollution Prevention and Control - EUROPEAN COMMISSION – August 2006.
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ANNEX A A PLANT SPECIFIC C OCC MODE EL
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ANNEX B GAT TECYCLE O OCC MODEL L
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