Emission Assessment of Rice Husk Combustion for Power Production

Transcription

1 Emsson Assessment of Rce Husk Combuston for Power Producton Thpwmon Chungsangunst, Shabbr H. Gheewala, Suthum Patumsawad Open Scence Index, Energy and Envronmental Engneerng waset.org/publcaton/514 Abstract Rce husk s one of the alternatve fuels for Thaland because of ts hgh potental and envronmental benefts. Nonetheless, the envronmental profle of the electrcty producton from rce husk must be assessed to ensure reduced envronmental damage. A 10 MW plot plant usng rce husk as feedstock s the study ste. The envronmental mpacts from rce husk power plant are evaluated by usng the Lfe Cycle Assessment (LCA) methodology. Energy, materal and carbon balances have been determned for tracng the system flow. Carbon closure has been used for descrbng of the net amount of CO 2 released from the system n relaton to the amount beng recycled between the power plant and the CO 2 adsorbed by rce husk. The transportaton of rce husk to the power plant has sgnfcant on global warmng, but not on acdfcaton and photo-oxdant formaton. The results showed that the mpact potentals from rce husk power plant are lesser than the conventonal plants for most of the categores consdered; except the photo-oxdant formaton potental from CO. The hgh CO from rce husk power plant may be due to low boler effcency and hgh mosture content n rce husk. The performance of the study ste can be enhanced by mprovng the combuston effcency. Keywords Envronmental mpact, Fossl fuels, Lfe Cycle Assessment (LCA), Renewable energy, Rce husk T I. INTRODUCTION HAILAND s energy supply comes mostly from fossl fuels, a large percentage of whch are mported from other countres causng concern for energy securty. Moreover, fossl fuels combuston s assocated wth emssons of CO 2, SO 2 and NO X leadng to envronmental mpacts. The proposed soluton for these problems s usng renewable energy sources nstead of conventonal (fossl) energy sources [1, 2]. The Energy Conservaton Promoton Fund commttee (ENCON Fund) authorzed the Energy Polcy and Plannng Offce (EPPO) to subsdze the Small Power Producers (SPP) producng electrcty from bomass for sellng to the Electrcty Generatng Authorty of Thaland (EGAT) [3]. Ths paper s focused on rce husk power plant because t s one major energy sources from the agro-ndustral sector. Rce husk s consdered to be an envronmentally frendly fuel Thpwmon Chungsangunst and Shabbr H. Gheewala are wth the The Jont Graduate School of Energy and Envronment, Kng Mongkut s Unversty of Technology Thonbur, 91 Pracha Utht Rd, Bangmod, Tungkru, Bangkok, Thaland. Suthum Patumsawad s wth Department of Mechancal Engneerng, Kng Mongkut s Unversty of Technology North Bangkok, 1518 Pbulsongkram Rd, Bangsue, Bangkok 10800, Thaland (correspondng author to; e-mal: stt@kmutnb.ac.th).. because t can mtgate CO 2, SO X and NO X emssons when compared wth conventonal fuel [4]. However, the emssons from rce husk energy producton have not been assessed n quanttatve way n Thaland. To answer that, Lfe Cycle Assessment (LCA) has been used to evaluate the envronmental profle of rce husk power plant. All the data; resource and materal usage, emssons and wastes as well as energy usage n the energy producton wll be based on 1 MWh of electrcty producton for comparng the results wth conventonal power producton II.RICE HUSK ENERGY OVERVIEW Rce s cultvated n every regon of Thaland. Rce husk, whch accounts for 20% by weght of rce, comes from rce mllng process as by-product. Generally, a large amount of rce husk s dumped as waste whch results n waste dsposal problem and methane emssons. Moreover, the low densty of rce husk can cause t to be ar-borne easly resultng n breathng problems, f nhaled. Rce husk can be converted to a useful form of energy to meet the thermal and mechancal energy requrement for the mlls themselves. Ths helps mnmze the waste problem n addton to convertng rce husk to a renewable energy resource. Smaller mlls can sell the husk to power plants set up under the SPP scheme mentoned earler. However, the cost of rce husk s ncreasng now due to ts usage for other applcatons such as; cement addtve [5], fertlzaton n felds [6] and chcken ncubaton [7], etc., whch makes the nvestment n energy producton from rce husk hgher than before. III. POWER PLANT BACKGROUND The Northeastern regon s partcularly mportant as one of the major rce growng belts of the country [2]. Due to the envronmental and economc scenaros, the Tha government has supported renewable energy producton from ndgenous sources. One project that has been conducted under ths support s a 10 MW plot plant n Ro-Et provnce, usng rce husk as feedstock for water tube boler type. Approxmately 255 tons of rce husk s suppled daly from the nearby rce mll (located 2 km from the power plant) as the man source and others from 7 to 125 km. The water supply (138 tons) s from the Sh rver water and 24 MWh of electrcty are requred for power producton n one day. The maxmum power producton capacty s 10.2 MW and the mnmum s 6.5 MW, dependng on raw materals avalablty. The surplus 625

2 Open Scence Index, Energy and Envronmental Engneerng waset.org/publcaton/514 electrcty s sold to EGAT for the commtment under the SPP scheme whch has been desgned for 21 years. Raw water s pretreated by demneralzaton before feedng to boler and coolng tower. Water from boler blow down (flash tank) and coolng tower blow down s sent for treatment, whch ncludes ph balance, coagulaton and demneralzaton. Sold wastes are sludge from waste water treatment, bottom ash and fly ash. IV. METHODOLOGY LCA s an envronmental assessment methodology developed to evaluate the nputs and outputs of systems and to convert them nto envronmental burdens assocated wth a product, process or actvty over the entre perod of ts lfe from the extracton and processng of raw materal from whch t s made, through the manufacturng, packagng and marketng process, and the use, reuse and mantenance of the product, and on to ts eventual recyclng or dsposal as waste at the end of ts useful lfe. The ISO/EN14040 defnes LCA framework n four phases: Goal defnton and scopng, Inventory analyss, Impact assessment and Interpretaton [8]. A. Goal Defnton and Scopng Goal Defnton and Scopng s the frst phase of LCA, whch s used for defnng the objectves of the study and the system boundares for provdng envronmental nformaton [8]. The goal for ths study s to conduct a comparatve LCA between electrcty producton at a rce husk power plant and conventonal power generaton n Thaland. Energy and materal balances are conducted to evaluate the effcency and check the data consstency. As rce husk s consdered as a waste of rce producton, the system boundary ncludes only energy generaton and transportaton of rce husk to the plant (Fg. 1). Rce cultvaton and producton are not ncluded n the system boundary. Power plant constructon s also excluded from the system boundary as ts lfe s qute large and hence ts contrbuton to power producton wll be relatvely small. Functonal unt, whch s the bass of comparson, s 1 MWh of electrcty. releases n power producton. Ths step ncludes materal and energy balances, carbon closure and data analyss. Accordng to the conservaton law, matter and energy cannot be created or destroyed [9]. Hence, materals/ energy nputs must be equal to materal/ energy outputs. The calculatons n ths phase have been done for checkng the consstency of the data. Materal balance deals wth materal flow, calculaton of the balance, C balance and C closure. Energy calculatons nclude energy flow, energy balance, Sankey dagram, overall effcency, combuston (boler) effcency and turbne effcency. Materal flow and Materal balance The characterstcs of rce husk n Table 1 have been determned by CHONS analyss method, mosture content analyss and bomb calormeter analyss. The emsson data n Table 2 was determned based on JIS code method [10]. Materal balance can be calculated as presented n Fg. 2 (Eq. 1), carbon balance s Fg. 3 and carbon closure s Fg. 4 (Eq. 2). M RH + M A + M RW = M FA + M BA + M FG + M WE + M WD, (1) Where; M RH s mass flow rate of rce husk, M A s mass flow rate of ar nlet, M FA s mass flow rate of fly ash, M BA s mass flow rate of bottom ash, M RW s mass flow rate of raw water, M WE s mass flow rate of water evaporaton and M WD s mass flow rate of water dscharge. The dfference n materal nput and output works out to 3.79 t/h, whch s about 4.34 % of the total materal nput. Fg. 2 Materal balance Fg. 1 Scope and system boundary of the study B. Inventory Analyss Inventory Analyss s the dentfcaton and quantfcaton of materals, resources and energy usages, and envronmental Fg. 3 Carbon balance The data n Table 1 show that the man elemental components of rce husk are carbon and oxygen, and there s lttle ntrogen and sulphur content. The man emsson s CO 2, whle CO, SO 2 and NO 2 are very low (Table 2). Thus only 626

3 carbon balance has been done for ths study. Carbon nput to the power plant comes manly from rce husk where the carbon content s 42.2 %. Total carbon output durng the power producton conssts of 10 % CO 2 and 0.08 % CO n 81,523.6 m 3 /h of flue gas, 1.3 % carbon content n 1,560 t/h of fly ash and 2.05 % carbon content n 323 t/h of bottom ash. There s also a small nput from the CO 2 n the ar whch has been adjusted for. TABLE 1 ELEMENT COMPOSITION OF RICE HUSK SAMPLE Parameter Result Std. Dev. Bass C 42.2 % 0.99 Dry H 5 % 0.06 Dry O 36 % 2.16 Dry N 0.7 % 0.15 Dry dstance s 4 km/round trp whch consumes 0.50 L of desel ol. Carbon emsson from transportaton s 0.20 kg-c/h (base dstance) n the form of CO 2, CH 4 and CO. Carbon emssons from the rce husk power plant are 4, kg-c/h as CO 2, kg-c/h as CO, and 26.9 kg-c/h as unburned carbon. Carbon closure = {( C nput C output )/ C nput } 100 % (2) When; C nput = RH + T (3) C output = T + UA + FG CO (4) Where; C nput s total carbon content n (kg-c/h), C output s total carbon content n (kg-c/h), RH s carbon content n rce husk (kg-c/h), T s carbon released from the transportaton (kg-c/h), UA s carbon released as unburned carbon n total ash (kg-c/h), FG CO2 s carbon released wth flue gas as CO 2 (kg-c/h) and FG CO s carbon released wth flue gas as CO (kg- C/h). Open Scence Index, Energy and Envronmental Engneerng waset.org/publcaton/514 S * - Dry Total mosture 11 % 1.11 As receved Heatng value MJ/kg 0.07 Dry *: Below detecton lmt of nstrument whch s 1 % TABLE II STACK EMISSIONS DATA [10] Parameter Value, v./ v**. kg/h kg/mwh CO 2 10 % 16, , CO 0.08 % NO ppm SO 2 16 ppm TSP 12.7 ppm Fly ash 1,560 kg/h 1, Bottom ash 323 kg/h Volume of flue gas; 81,523.6 m 3 /h, 160 C Carbon closure of the system s llustrated as Fg. 4. The flows that cross the system boundares ndcate carbon nflows and outflows from the system. Overall, the outflows of carbon equal to the nflows of carbon n rce husk and carbon n desel ol. Carbon content n rce husk s 4, kg-c/h and n desel ol s 0.20 kg-c/h. Rce husk s transported from rce mll to the power plant n trucks. The base case transportaton TABLE III STACK DATA OF CONVENTIONAL [15] Fg. 4 Carbon Closure The amount of carbon recycled by rce husk energy producton s % of total carbon nput. About 1.38 % of total carbon nput, whch are carbon emssons from transportaton of rce husk to power plant and CO from ncomplete combuston of rce husk, are released to the atmosphere and contrbute to global warmng. Energy flow and Energy balance A useful way of chartng energy flow s to use a Sankey dagram. Sankey dagram of rce husk energy producton can be used for descrbng the losses at every sub process of rce husk energy producton. The equaton of Sankey dagram calculaton s Eq.5. Energy nput = Electrcty + Waste heat + Total loss (5) * Weghted average of coal, ol and gas-fred power plant 627

4 Fg. 5 descrbes the Sankey dagram. Energy nput s only from rce husk whch s defned as 100%. Electrcty produced accounts for 23.14% of the total energy nput. Waste heat s the heat rejected at condenser (61.07%). Total loss conssts of energy losses at boler, losses at steam turbne and losses from dstrbuton whch are accounted as %. The recycled energy s the energy content n ar nlet and feed water, whch are heated by the flue gas before releasng to the stack. Loss at boler ncludes heat loss n flue gas, fly ash, bottom ash, radaton loss, water blow down dscharge loss. At the steam turbne, there s loss durng converson of thermal energy to mechancal energy and mechancal energy to electrcty along wth the waste heat durng condensaton. Where; CD s combuston effcency-drect method (%), E Out s energy output from boler (GJ/h), E RH s energy n rce husk (GJ/h), E FW s energy n feed water to boler (GJ/h) and E A s energy n ar nlet to boler (GJ/h). Open Scence Index, Energy and Envronmental Engneerng waset.org/publcaton/514 RICE HUSK ENERGY 100% WASTE HEAT 61.07% BOILER ENERGY USED CONDENSER 13.15% ELECTRICITY TOTAL LOSS TURBINE 23.14% Fg. 5 Sankey dagram Energy balance n Fg. 6 s also a part for checkng the consstency of process flows. The calculaton of overall energy balance s based on the law of conservaton of energy. The calculaton of energy balance s Eq.6. E RH = E FA + E BA + E FG + E WE + E WD + E R&Ub + E ST + E Elec (6) Where; E RH s energy n rce husk (GJ/h), E FA s energy n fly ash (GJ/h), E BA s energy n bottom ash (GJ/h), E FG s energy n flue gas (GJ/h), E WE s energy n water evaporaton (GJ/h), E WD s energy n water dscharge (GJ/h), E R&Ub s radaton and unaccounted loss at boler (GJ/h), E ST s heat rejecton (GJ/h) and E Ele s electrcty (GJ/h). The dfference n nput and output energy from energy balance calculatons, n terms of percentage s 2.64 %. The calculatons of overall effcency [11], combuston effcency [12, 13] and turbne effcency [14] can be used for calculatng the producton effcency. % O = (E g E u ) 100/ (LHV M RH ) (7) Where; O s overall effcency (%), E g s energy generated (MJ/h), E u s energy utlzed n power plant (MJ/h), LHV s Low Heatng Value of rce husk (MJ/kg) and M RH s mass flow rate of rce husk (kg/h). % CD = E Out 100/(E RH + E FW + E A ) (8) Fg. 6 Energy balance. ( v + v + v + v) % CI = 100 [ 100 %] (9) Total energy nput Where; CI s combuston effcency-indrect method (%), '' s heat loss due to flue gas (%), '' s heat loss due to mosture n fuel (%), '' s heat loss due to hydrogen n fuel (%), 'v' s Heat loss due to mosture n ar (%), 'v' s heat loss due to ncomplete combuston (%), 'v' s heat loss due to unburned C n ash (%) and 'v' s radaton loss (%). WT % ST = (10) W s Where; ST s steam turbne/sentropc effcency (%), W T s work turbne (kj/h), W S s work sentropc (kj/h). The rce husk power plant produces 9.5 MWh electrc power from 10,625.5 kg of rce husk. The Low Heatng Value of rce husk s 12, kj/kg. Thus, the overall converson system effcency can be defned as %. The combuston effcency or boler effcency by nput-output method s % (Eq. 8) and heat loss method s % (Eq. 9). The dfference between the two methods may be due to unaccounted losses. The range of boler effcency generally obtaned s 85 to 90 % [14]. Thus, the combuston effcency s n the range of reference. The range of steam turbne effcency s generally between 35 to 55 % [14]. The result of turbne effcency calculaton s % (Eq. 10), whch s n the upper part of the range. C.Impact Assessment The total emssons to ar are from two sources vz., electrcty producton and transportaton. These can be calculated as; E P,T = E P + E T (11) Where; E T = D FE EF (12) E g 628

5 Open Scence Index, Energy and Envronmental Engneerng waset.org/publcaton/514 The envronmental mpact potentals are calculated by Eq. (13) [8]: EP ( j) Q EP( j) (13) Where EP(j) s the emsson s potental contrbuton to the envronmental mpact category (j), Q s the magntude of emsson of substance () and EF(j) s the substance s equvalency factor for the envronmental mpact category (j). Dstance, km/round Dstance, km/round TABLE IV IMPACT POTENTIAL OF TOTAL AIR EMISSIONS GLOBAL WARMING POTENTIAL Total amount (kg CO 2-eq/ MWh) From transportaton (kg CO 2-eq/ MWh) (50.12%*) (50.29%*) (51.41%*) (53.39%*) (56.35%*) ACIDIFICATION POTENTIAL Total amount (kg SO 2-eq/ MWh) From transportaton (kg SO 2-eq/ MWh) (0.03%*) (0.07%*) (0.35%*) (0.86%*) (1.71%*) Dstance, km/round PHOTO-OXIDANT POTENTIAL Total amount (kg C 2H 2-eq/ MWh) From transportaton (kg C 2H 2-eq/ MWh) E-05 (0.01%*) E-05 (0.01%*) E-04 (0.06%*) E-04 (0.15%*) E-03 (0.31%*) * percent of total mpact Table 4 presents the selected mpact potentals of total ar emssons from both sources. Transportaton has sgnfcant nvestment cost snce rce husk has very low densty. The rce husk requrement of the factory s met by 13 trucks/day. Usually the rce husk s transported to the power plant from a rce mll 2 km away. However, at tmes when the rce husk has also to be purchased from sources about 125 km from the power plant. The long dstance of rce husk transportaton to the plant ste can contrbute global warmng potental up to 56 % of the lfe cycle amount for a round trp dstance of 250 km, but ts contrbuton to acdfcaton and photo-oxdant formaton s not sgnfcant (1.7% and 0.3% respectvely at 250 km/round trp). It s therefore nterestng to calculate the transportaton dstance at whch the global warmng potental from rce husk power plant wll equal that of conventonal power plant. Ths dstance works out to about 35,925 km/round. Obvously, ths s an unreasonably hgh dstance whch wll never occur n practce. Hence, we can safely conclude that the performance of rce husk power plant s better than conventonal power plant for global warmng. Table 5 shows the results of envronmental mpact potentals comparson between rce husk power plant and the average value of conventonal power plants n Thaland. The data requred for calculaton for rce husk power plant s presented n Table 2 [10] and Table 3 [15] s for average value of conventonal power plants. The envronmental mpact potentals have been calculated by the characterzaton equaton, Eq.(13). TABLE V ENVIRONMENTAL IMPACT POTENTIALS COMPARISON [10, 15] Impact category Global warmng (kg CO 2- eq/mwh) Acdfcaton (kg SO 2- eq/mwh) Photo-oxdant formaton (kg C 2H 4- eq/mwh) Nutrent enrchment (kg N-eq/MWh) Sold waste (kg ash/mwh) Rce husk Coal 1, Ol Natural gas Comb ned* n.a. n.a. n.a. n.a. * Combned value of % natural gas, % coal and 0.02% ol. n.a. Not avalable. D.Interpretaton The results from nventory analyss ndcate that materal and energy balances are consstent. The small dfference of materal balance may be due to loss n ppes and tubes, and unsteady state of the producton. The dfference of carbon balance s 1.42 % whch s very small and may be due to mnor varatons n rce husk characterstcs and averagng of measurements. Carbon closure s % of total carbon nput. Thus, about 1.38 % of total carbon nputs contrbute to global warmng. Sankey dagram and energy balance ndcate that the overall effcency of ths plant s about 23 %. Ths nformaton can be used for mprovement of the power plant effcency by dentfyng the source and amount of losses. The maxmum heat loss occurs durng water condensaton. To mprove the power plant effcency one way suggested s to nstall heat exchanger for trappng waste heat from coolng tower for usng n other processes, such as for dryng rce husk before 629

6 Open Scence Index, Energy and Envronmental Engneerng waset.org/publcaton/514 use as feed stock. The dfference of energy balance s 2.64 % whch may be due to loss n ppes and tubes. Transportaton of rce husk to power plant has sgnfcant effect on global warmng potental. However, t wll reach global warmng potental of conventonal power plants only at very long transportaton dstances whch wll never occur n practce because the cost of transportaton wll be too hgh. The mpact assessment results show that the mpact of global warmng potental of rce husk energy (17.16 kg CO 2 - eq/mwh) s far less than the combned value of fossl fuels plants ( kg CO 2 -eq/mwh) because CO 2 from bomass s consdered as greenhouse gas neutral. Sulphur and ntrogen contents n rce husk are low when compared wth coal and ol. In addton, the combuston temperature of rce husk s lower than 900 C, preventng the formaton of thermal NO X. Hence, acdfcaton and nutrfcaton potentals of rce husk plant (1.06 kg SO 2 -eq/mwh and 0.39 N-eq/MWh) are lesser than combned value of fossl fuels plants (2.34 kg SO 2 - eq/mwh and 0.73 N-eq/MWh) even though fossl fuels plants have NO X and SO X removal equpment nstalled. Only n case of natural gas, the acdfcaton potental s lesser than the rce husk power plant because very less SO X s produced from natural gas plant ( kg/mwh). NO X produced from natural gas plant (1.36 kg/mwh) s slghtly hgher than rce husk power plant (1.31 kg/mwh). Photochemcal oxdant formaton s from CO emssons of rce husk plant (0.34 kg C 2 H 2 -eg/mwh) and s hgher than the combned value of fossl fuels plants (0.08 kg C 2 H 2 -eg/mwh). Ths may be due to the low combuston effcency of the rce husk power plant resultng n part from the hgh mosture content n rce husk. For sold waste potental (dsposal amount) only the data from rce husk power plant s avalable as 9.81 kg ash/mwh. Hence, the comparson can not be made. Nevertheless, t must be mentoned that both bottom and fly ash are utlzed and hence, not gong to the landfll. Also, credts must be provded for the materal they dsplace. Ths has not been ncluded n ths study. The results present that most envronmental mpacts potentals from rce husk power plant are lesser than the fossl fuels plants. V.CONCLUSION Materal balance, energy balance and carbon balance help to confrm the process flow data and understandng the producton process. The consstency calculatons ndcate that the emssons data are relable. Losses from the balance calculatons may be caused from the unsteady plant operaton and averagng of data. Carbon balance and carbon closure can be used for gudance to reduce the amount of carbon emssons emtted to the atmosphere whch contrbute to global warmng. The carbon doxde emssons produced from rce husk power plant wll be re-absorbed by the bomass carbon cycle. Thus global warmng potental comes only from fossl fuel consumed for fuel transportaton and CO from ncomplete combuston of rce husk. The amount of carbon emsson contrbutng to global warmng s about 1.38 % of total carbon nput. Energy losses are ndcated by the converson effcency and the effcency of the equpment. Waste heat loss at condenser s very hgh at about 61 % of total energy nput. To ncrease the overall effcency as well as reduce wastage of energy, waste heat at condenser may be utlzed for dryng rce husk before combuston. Boler effcency and turbne effcency are n the standard range of reference. However, f the effcences can be mproved the producton effcency wll ncrease and CO emsson wll reduce also. Transportaton has no sgnfcant contrbuton to acdfcaton and photochemcal oxdant formaton. When consderng the whole system ncludng the electrcty producton and transportaton of rce husk to the power plant, transportaton has sgnfcant effect on global warmng and transportaton cost. 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