Available online at ScienceDirect. Energy Procedia 63 (2014 ) GHGT-12

Transcription

1 Avalable onlne at ScenceDrect Energy Proceda 63 (2014 ) GHGT-12 Energy for CO 2 capture: use of an auxlary bomass combned heat and power unt Zakeh Khorshd a,b, Mnh T. Ho a,b, Danne E. Wley a,b, * a The Australan Cooperatve Research Centre for Greenhouse Gas Technologes (CO2CRC), Australa b School of Chemcal Engneerng, UNSW Australa, 2052 UNSW Sydney, Australa Abstract One of the challenges for applyng capture to exstng fossl power plants s the sgnfcant amount of energy requred for CO 2 capture and compresson leadng to plant output loss. Ths study nvestgates the use of an auxlary bomass combned heat and power unt to provde the energy requred for MEA post-combuston capture n a coal-fred power plant. By capturng CO 2 from the bomass unt, negatve emssons can be acheved. Low bomass prces or hgh bomass and coal prces, or/and an ntermedate ncentve schemes make auxlary unts proftable. However, for negatve emssons, low bomass prces and hgh ncentve schemes are requred The The Authors. Publshed Publshed by Elsever by Elsever Ltd. Ltd. Ths s an open access artcle under the CC BY-NC-ND lcense Selecton ( and peer-revew under responsblty of GHGT. Peer-revew under responsblty of the Organzng Commttee of GHGT-12 Keywords: Bomass; Auxlary unt; Coal-frel plant; CO 2 capture; Carbon prce; Renewable energy certfcate; Negatve emssons 1. Introducton Post-combuston capture s the most lkely capture technology that can be appled to fossl fuel power plants n the near future. However, one of the man barrers for mplementng ths capture method n exstng power plants s the sgnfcant amount of energy requred for CO 2 capture and compresson usng currently avalable technology. Usng a conventonal MEA solvent process for post-combuston capture, the energy requred can be as much as 30% * Correspondng author. Tel.: ; fax: E-mal address: d.wley@unsw.edu.au The Authors. Publshed by Elsever Ltd. Ths s an open access artcle under the CC BY-NC-ND lcense ( Peer-revew under responsblty of the Organzng Commttee of GHGT-12 do: /j.egypro

2 Zakeh Khorshd et al. / Energy Proceda 63 ( 2014 ) of the net electrcty output. If CO 2 capture s appled at large scale, the output loss assocated wth the mplementaton of capture would pose a serous challenge to meetng current and future electrcty demand. To avod output loss from the power plant, an auxlary energy source can be used to supply the energy requred for CO 2 capture and compresson. Usng renewable energy n the auxlary unt, grd loss can be avoded wthout ncreasng fossl fuel consumpton. Among possble renewable energy sources, bomass has some advantages. It can provde a constant electrcty output, whle wnd or solar energy mght be temporarly unavalable. In addton, to acheve hgher emsson cuts, the CO 2 emtted from carbon neutral bomass can also be captured to obtan negatve emssons. Negatve emssons play a crtcal role n meetng strngent emsson targets. From an economc pont of vew, electrcty generaton from renewable bomass can create renewable energy certfcates (RECs), and CO 2 capture from the auxlary bomass unt can result n negatve emssons that may be rewarded n emsson tradng schemes. These ncentves may make the combnaton of an auxlary bomass unt wth a coal plant and capture unt economcally attractve n comparson wth other avalable optons for CO 2 mtgaton from exstng coal-fred plants. In ths paper, usng auxlary bomass unts to supply the energy requred for capture n coal-fred power plants s evaluated. We nvestgate usng bomass auxlary unts n two dfferent cases. In the frst case, the CO 2 s only captured from the coal power plant, and the CO 2 emtted from the auxlary bomass unt s released to the atmosphere. For the second case, CO 2 s captured from both the power plant and the auxlary unt. The objectve of ths paper s to examne the economc feasblty of usng auxlary bomass unts under dfferent fuel prces and ncentve schemes. 2. Methodology The performance and economcs of the coal-fred power plant, CO 2 capture unt and auxlary bomass CHP unt are evaluated usng the Integrated Carbon Capture and Storage Economcs Model (ICCSEM) developed by UNSW Australa (The Unversty of New South Wales) for the Cooperatve Research Centre for Greenhouse Gas Technologes (CO2CRC). ICCSEM covers the entre carbon capture and storage (CCS) chan, however CO 2 transport and storage costs and emssons are not ncluded n the analyss n ths paper. ICCSEM has a process module for estmatng process flow and szng equpment as well as an economc module for estmatng costs. In ths paper, the coal plant s a 500 MW sub-crtcal, pulverzed fuel plant wth 35 % net effcency (HHV), 0.88 t CO 2 /MWh emsson ntensty, and 85 % capacty factor. The capture technology mplemented s CO 2 absorpton usng a solvent of 30 wt% MEA n water, and the capture rate s assumed to be 90 %. The major energy requrement for the capture unt s the heat for solvent regeneraton [1, 2]. Besdes the heat, electrcty s also requred for pumps and fans n the solvent absorpton/regeneraton system as well as for compressng the CO 2 product to a hgh pressure (e.g. 100 bar) to make t ready for transport [3]. Table 1 shows propertes of the capture unt such as specfc heat and electrcty requrements. For CO 2 capture usng MEA, the coal plant should be equpped wth flue gas desulfurzaton (FGD) and selectve catalytc reducton (SCR) to decrease the content of SO x and NO x n the flue gas, because the presence of these mpurtes results n hgh solvent loss [4]. The auxlary unt s a combned heat and power (CHP) unt that s szed based on the amount of heat requred for CO 2 capture. The electrcty generated by the CHP unt s a by-product of steam generaton, and s estmated usng the power to heat rato of the CHP unt. The maxmum value for power to heat rato s assumed to generate the hghest possble amount of electrcty from bomass. Table 1 shows other detals of the CHP unt. Three dfferent cases are consdered: Coal plant wth capture (Coal-Cap): In ths case, the coal plant tself supples the energy requred for CO 2 capture and compresson. Coal plant wth capture and auxlary bomass unt (postve emssons) (Aux-Pos): In ths case, the energy requred for CO 2 capture and compresson s suppled by the auxlary bomass CHP unt. CO 2 s only captured from the coal plant and the CO 2 produced by the auxlary bomass CHP unt s vented to the atmosphere. Coal plant and auxlary bomass unt wth capture (negatve emssons) (Aux-Neg): In ths case, the energy requred for CO 2 capture and compresson s suppled by the auxlary bomass CHP unt, but unlke the prevous case, CO 2 s captured from both the coal plant and the auxlary bomass CHP unt.

3 6794 Zakeh Khorshd et al. / Energy Proceda 63 ( 2014 ) Table 1- Key techncal assumptons Temperature of steam used for solvent recovery Pressure of steam used for solvent recovery Specfc heat requrement of the capture unt Specfc electrcty requrement of the capture unt Heat-to-electrcty equvalence factor Power to heat rato of the CHP unt Effcency of the bomassfred CFB boler Value Reference 134 C [5] 3 bar [5] 4,370 kj th/kg CO 2 captured [6] 500 kj e/kg CO 2 captured [6] 0.17 [7] 0.2 [8] 0.8 [9] The fuel consumed n the coal-fred power plant s Australan black coal, and the bomass feed stock used n the auxlary CHP unt s wood chps. Wood chps are assumed to be receved clean, and wth a mosture content of 20 wt% due to open storage [10]. Table 2 shows the ultmate analyss and emsson factors of the black coal and wood chps. Table 2- Ultmate analyss (mass fracton on a dry bass) and emsson factors for coal and wood chps Wood chps [10] Black coal [11] C (% wt) H (% wt) O (% wt) N (% wt) S (% wt) Emsson Factor (t CO 2 / GJ) The plant economcs s estmated based on a dscountng perod of 35 years begnnng from It s assumed that the coal plant captal cost s completely wrtten off at the start of the dscountng perod. CO 2 capture and auxlary CHP unts are added to the exstng coal plant and become operatonal n the year Implementaton of capture n 2020 s assumed, as t s a lkely tmeframe at whch CO 2 capture wll be deployed at a large scale. When the capture unt s nstalled at the power plant, t s assumed that some modfcatons to the plant wll also be undertaken to extend the plant lfe for an addtonal 30 years. Thus, the operatng lfe span of the plant s extended to The captal and operatng costs of the capture unt nclude the costs for the FGD, the SCR, and CO 2 separaton and compresson unts. The COE s calculated usng a present value calculaton (equaton (1)): COE n 1 [( Capex TotOpex ) /(1 d) ] n 1 [( NetPower ) /(1 d) ] (1)

4 Zakeh Khorshd et al. / Energy Proceda 63 ( 2014 ) where Capex and TotOpex are the captal and total operatng costs n the th year, d s the dscount rate, n s the dscountng perod and NetPower s the total sent out electrcty n the th year. The total operatng cost (TotOpex ) n equaton (1) ncludes plant fxed and varable non-fuel operatng costs, fuel cost (for coal and bomass), CO 2 penaltes/credts and REC revenues. Although there s no CO 2 credt for negatve emssons n emsson tradng schemes, we assumed that negatve emssons would be rewarded by the same value as carbon prce n the near future. Based on the regulatons, REC revenues ncluded n the total operatng cost (equaton (1)) are only allocated to electrcty produced from bomass not thermal energy [12]. CO 2 credts and REC revenues are treated as negatve operatng costs for the COE calculaton. The key economc parameters are presented n Table 3. All costs are n constant 2011 US dollars. An exchange rate of 1 Australan dollar to 1 US dollar s used. The ranges shown for fuel prce, carbon prce and REC n Table 3 are used for performng senstvty analyss n secton 3. Table 3- Key economc assumptons Varable Baselne value Range Reference Dscount rate (real) 7 % 5 15 % [13] Dscountng perod Project lfe 30 y y [13] Captal cost for coal plant lfe extenson 35 y 1,619 $/kw e a Captal cost for steam turbne CHP unt 5,150 $/kw e b 4,100 6,200 $/kw e Fxed operatng cost for steam turbne CHP unt 54 $/kw e [14] Varable operatng cost for steam turbne CHP unt [14] 35 $/MWh e [14] Captal cost for CFB boler 138 $/kw th [9] Fxed operatng cost for CFB boler 4 % of the captal cost Varable operatng cost for CFB boler 5 $/MWh th [9] Wood chps prce 4.5 $/GJ 3 9 $/GJ [15] Coal prce 1.5 $/GJ 1 7 $/GJ [16] Carbon prce 23 $/t CO $/t CO 2 [13, 17] REC prce 40 $/MWh e $/MWh e [18, 19] a The captal cost requred for plant lfe extenson s estmated as 50 % of a new plant [20]. The captal cost for new coal-fred power plants s obtaned from the EPRI report [21]. b The captal cost data are the average cost for a 5 MW e CHP unt. Captal costs for larger unts have been scaled usng a power law (Capex sze 2 = Capex sze 1 (sze 2/sze 1) m-1 ) where the scalng factor m = 0.7 [14]. 3. Results Table 4 summarzes energy requrements for the CO 2 capture unt, the energy generaton from the auxlary bomass CHP unt and the total sent-out electrcty for the dfferent cases. Because the auxlary unts are szed based on the heat requrement of the capture unt, the electrcty produced n the auxlary unt s hgher than the electrcty requred by the CO 2 capture and compresson unts by 41 MW for Aux-Pos and 105 MW for Aux-Neg. In ths paper, we assume that ths excess electrcty can be sold n the market. The overall electrcty output n Table 4 refers to the total of the sent-out electrcty from the coal plant and the excess electrcty from the auxlary CHP unt. For Aux-Neg, the energy requred for the capture unt and also the energy produced by the auxlary unt are hgher than the other cases because a hgher amount of CO 2 s captured (from both the coal plant and the auxlary bomass CHP unt). Thus, a larger capture unt and consequently a larger auxlary unt are requred.

5 6796 Zakeh Khorshd et al. / Energy Proceda 63 ( 2014 ) Table 4- Energy requred for the capture unt, the energy produced from auxlary unt and the total sent-out electrcty Coal-Cap Aux-Pos Aux-Neg Capture unt Total heat requred (MWth) Total electrcty requred (MWe) Auxlary bomass unt Total heat produced (MWth) NA Total electrcty produced (MWe) NA Electrcty output Total sent-out electrcty (MWe) The fuel flow rate and CO 2 emssons are summarzed n Table 5. Two dfferent emsson ntenstes are presented n Table 5; actual emsson ntensty and net emsson ntensty. Actual emsson ntensty takes nto account the CO 2 emtted by the combuston of both the coal and the bomass, whereas net emsson ntensty only accounts for the CO 2 emtted from the coal. Accordng to Table 5, the actual emsson ntensty for Aux-Pos and Aux-Neg s hgher than that for Coal-Cap because the amount of CO 2 emtted per unt of energy from bomass s hgher than coal (see bomass and coal emsson factors n Table 2). In addton, the effcency of the coal plant s hgher than that of the auxlary bomass CHP unt. However, neglectng the CO 2 emtted from the carbon neutral bomass, the net emsson ntensty for Aux-Pos and Aux-Neg s lower than that of Coal-Cap by 0.04 and 1.07 t CO 2 /MWh respectvely. Table 5- Performance comparson for dfferent cases Coal-Cap Aux-Pos Aux-Neg Coal used (Mt/y) Bomass used (Mt/y) NA CO2 emsson - Coal (Mt CO2/y) CO2 emsson - Bomass (Mt CO2/y) NA Actual emsson ntensty (t CO2/MWh) Net emsson ntensty (t CO2/MWh) Fgure 1 shows the breakdown of the cost of electrcty (COE) for the dfferent cases. The term CO 2 penaltes/credts refers to the combnaton of emsson penaltes due to the carbon prce and all the revenues from CO 2 credts ganed through negatve emssons. If the energy requrement of the capture unt s provded by the auxlary bomass CHP unt, and the CO 2 emtted from that unt s released to the atmosphere (as n Aux-Pos), the COE decreases slghtly (by 2%) compared wth the coal plant wth capture (Coal-Cap). The lower COE s due to a hgher electrcty output, a lower CO 2 emsson penalty and the producton of REC revenues for the electrcty generated from bomass. If CO 2 s captured both from the coal plant and the auxlary bomass CHP unt (Aux-Neg), the COE s 20% hgher than for the coal plant wth capture (Coal-Cap). Ths s because the cost of the larger capture unt wth larger energy requrements cannot be compensated by the revenues obtaned from the combnaton of the addtonal electrcty produced, CO 2 emsson credts and RECs.

6 Zakeh Khorshd et al. / Energy Proceda 63 ( 2014 ) Fgure 1- Breakdown of baselne cost and COE for dfferent cases Fgure 2 and Fgure 3 show the effect of the fuel prces and ncentve schemes on the COE. Note that n these fgures other economc parameters are constant at the baselne values shown n Table 3. Fgure 2 shows that the slope of lnes decreases as the total sent-out electrcty ncreases. That s because the coal flow rate s the same for all cases (see Table 5), but the sent-out electrcty s hgher for cases wth an auxlary bomass CHP unt (see Table 4). As the sent-out electrcty ncreases, the coal cost component (expressed n $/MWh) decreases (as shown n Fgure 1) even though the absolute flow rate of the coal remans the same. Ths also means that the COE (expressed n $/MWh) depends on the coal prce. As shown n Fgure 2, at a low enough bomass prces, usng an auxlary bomass CHP unt results n a lower COE than f the auxlary unt s not nstalled. However, the advantage of the use of the auxlary bomass CHP unt decreases as the coal prce decreases. For the same bomass prce, the COE of Aux-Pos s always lower than the COE of Aux-Neg. At a hgh bomass prce, usng an auxlary bomass CHP unt to provde the energy for capture only from the coal plant can result n a COE that s lower than that for not usng the auxlary bomass CHP unt, provdng the coal prce s hgh enough. However, a hgh bomass prce makes CO 2 capture both from the coal plant and the auxlary bomass CHP unt economcally unattractve. Fgure 2- Effect of fuel prces on the COE for a fxed carbon prce of 23 $/t CO 2 and REC of 40 $/MWh at a bomass prce of 3 $/GJ and 9 $/GJ

7 6798 Zakeh Khorshd et al. / Energy Proceda 63 ( 2014 ) From Fgure 3, at fxed fuel prces and a selected REC, a lower carbon prce s requred to ncentvze the use of an auxlary bomass CHP unt wthout capture than wth capture. However, at a hgh enough carbon prce, capturng CO 2 both from the coal plant and the auxlary bomass CHP unt wll always be the cheapest opton, provded that carbon credts are produced by achevng negatve emssons. Fgure 3-Effect of ncentve schemes on COE for a fxed coal prce of 1.5 $/GJ and bomass prce of 4.5 $/GJ (for dark blue and dark red lnes) and 3 $/GJ (for lght blue and lght red lnes) at a REC of 0 $/MWh and 100 $/MWh 4. Concluson Ths paper evaluates the performance and economcs of applyng an auxlary bomass CHP unt to a 500 MW sub-crtcal black coal-fred power plant wth CO 2 capture n order to supply the heat and electrcty requred for capture. The results show that when CO 2 s only captured from the coal plant, 41 MW of addtonal electrcty s produced by the CHP unt whle the emsson ntensty drops from 0.88 t CO 2 /MWh to 0.08 t CO 2 /MWh. When CO 2 s captured both from the coal plant and the auxlary bomass CHP unt, 105 MW of addtonal electrcty s produced by the CHP unt and negatve emssons are acheved (-0.95 t CO 2 /MWh). In terms of plant economcs, low bomass prce and the rght combnaton of ncentve schemes would favor the use of auxlary bomass CHP unts. To make the achevement of negatve emssons (.e. capture CO 2 from both the coal plant and the auxlary bomass CHP unt) economcally attractve, hgher ncentve schemes are requred than for capture from the coal plant alone. Acknowledgements The authors acknowledge fundng provded by the Australan Government through ts CRC program to support ths CO2CRC research project. One of us (ZK) acknowledges scholarshp support provded by UNSW Australa. References [1] Wang M, Lawal A, Stephenson P, Sdders J, Ramshaw C. Post-combuston CO 2 capture wth chemcal absorpton: A state-of-the-art revew. Chemcal Engneerng Research and Desgn. 2011;89(9): [2] Peeters ANM, Faaj APC, Turkenburg WC. Techno-economc analyss of natural gas combned cycles wth post-combuston CO 2 absorpton, ncludng a detaled evaluaton of the development potental. Internatonal Journal of Greenhouse Gas Control. 2007;1(4):

8 Zakeh Khorshd et al. / Energy Proceda 63 ( 2014 ) [3] Rao AB, Rubn ES, Berkenpas MB. An ntegrated modelng framework for carbon management technologes. IECM Techncal Documentaton: Amne-based CO 2 Capture and Storage Systems for Fossl Fuel Power Plant. Carnege Mellon Unversty, Pttsburg, [4] Rubn ES, Chen C, Rao AB. Cost and performance of fossl fuel power plants wth CO 2 capture and storage. Energy Polcy. 2007;35(9): [5] Romeo LM, Bolea I, Escosa JM. Integraton of power plant and amne scrubbng to reduce CO 2 capture costs. Appled Thermal Engneerng. 2008;28(8 9): [6] Raksajat A, Ho MT, Wley DE. Reducng the Cost of CO 2 Capture from Flue Gases Usng Aqueous Chemcal Absorpton. Industral & Engneerng Chemstry Research. 2013;52(47): [7] Oyenekan BA, Rochelle GT. Strpper models for CO 2 capture by aqueous solvents. In Proceedngs of the Seventh Internatonal Conference on Greenhouse Gas Technologes (GHGT-7); 2004; Vancouver, Canada. [8] EPA. Catalog of CHP technologes. U.S. Envronmental Protecton Agency (EPA), Combned Heat and Power Partnershp, [9] Basu P, Butler J, Leon MA. Bomass co-frng optons on the emsson reducton and electrcty generaton costs n coal-fred power plants. Renewable Energy. 2011;36(1): [10] CCPC. Bomass Co-frng, A fnal phase 3 report, Prepared by CCPC Techncal Comttee. Canadan Clean Power Coalton (CCPC), [11] Australan Government. About the Renewable Energy Target. Clean Energy Regulator: [12] Spero C. Utlsaton of Queensland's low and hgh volatle coals n power generaton EU-Australa Coal Conference; September 2001; Germany. [13] Allnson GW, Fmbres Wehs G, Ho MT, Neal P, McKee G, Rchards M, Wley DE. CO2CRC CCS Economc Methodology and Assumptons [14] Bruckner T, Chum H, Jäger-Waldau A, Kllngtvet A, Gutérrez-Negrín L, Nyboer J, Musal W, Verbruggen A, Wser R. Annex III: Cost Table. In IPCC Specal Report on Renewable Energy Sources and Clmate Change Mtgaton. Cambrdge Unversty Press, Cambrdge, Unted Kngdom and New York, NY, USA.: [15] Parratt & Assocates. Scopng Borefneres: Temperate Bomass Value Chans. Prepared for Botechnology Innovaton Polcy Secton, Pharmaceutcals, Health Industres & Enablng Technologes Branch, Innovaton Dvson, Department of Innovaton, Industry, Scence and Research, Canberra Australa, [16] ACIL Tasman. Calculaton of energy costs for the BRCI. Prepared for the Queensland Competton Authorty: [17] Australan Government. Gude to Carbon Prce Lablty under the Clean Energy Act Clean Energy Regulator: [18] AFMA Australan Fnancal Markets Report. Australan Fnancal Market Assocaton In partnershp wth Australan Trade Comsson, [19] ACIL Tasman. Achevng a 20 % RET. ACIL Tasman. Prepared for TRUenergy, [20] Wembrdge N, Polon L, McGmpsey N, Hall N. Thermal Power staton Advce: Report for the Electrcty Commsson. PARSONS BRINCKERHOFF, [21] EPRI. Australan Electrcty Generaton Technology Costs Reference Case EPRI, 2010.