Modelling of solid waste gasification process for synthesis gas production

Transcription

1 Jurnal f BAVANAM Scientific & Industrial & SASTRY: Research MODELLING SOLID WASTE GASIFICATION PROCESS FOR SYNTESIS GAS Vl. 7, Sept - Oct 13, pp Mdelling f slid waste gasificatin prcess fr synthesis gas prductin Anjireddy Bhavanam* and R C Sastry Department f Chemical Engineering, Natinal Institute f Technlgy, Warangal 56, India Received 19 March 13; revised May 13; accepted 8 May 13 This study presents a mathematical mdel t predict synthesis gas ( ) cmpsitin frm selected slid wastes (municipal waste, animal waste and agricultural waste) in a dwndraft fixed bed reactr. This mdel assumes chemical and thermdynamic equilibrium with prducts inside the reactr. Effect f temperature in gasificatin zne, equivalence rati and misture cntent f bimass n gas cmpsitin was investigated. Mdel helps t knw the behaviur f different bimass types and is a useful tl fr predicting perating parameters f dwndraft gasifiers with this bimass. Keywrds: Equilibrium, Equivalence rati, Municipal slid waste, Synthesis gas Intrductin Gasificatin is ne f the efficient ways t cnvert energy embedded in bimass. Cnversin f municipal slid wastes (MSW) t electrical energy can cnserve mre valuable fuels and lessen any harmful impact n envirnment. Prductin f transprtatin fuels r hydrgen frm MSW thrugh gasificatin is an attractive ptin bth ecnmically and envirnmentally. Cuntries like India, where there is n natinal pwer grid, distributed generatin (DG) f pwer has a great ptential fr supplying pwer t remte areas 1, where energy resurces are available lcally. In a dwndraft fixed bed, bimass underges cmbustin, pyrlysis and gasificatin. Babu & Chaurasia 3, reprted mdelling and simulatin f varius znes in gasificatin prcess. Altafini et al 5 simulated an equilibrium mdel based n minimizatin f Gibbs free energy fr wd waste. Melgar et al 6 cmbined chemical equilibrium and thermdynamic equilibrium f glbal reactin t predict final cmpsitin f synthesis gas. Zainal et al 7 studied an equilibrium gasificatin mdel based n equilibrium cnstants, t simulate gasificatin prcess in a dwndraft gasifier. This study presents a mathematical mdel t predict synthesis gas ( ) cmpsitin frm selected slid wastes (MSW, animal waste and agricultural waste) in a dwndraft fixed bed reactr. *Authr fr crrespndence anjireddy.bhavanam@gmail.cm Experimental Sectin Bimass Characteristics Varius bimasses [MSW, animal waste (AW) and agricultural waste {grundnut shell (GS)}] that differ in cmpsitin were selected as feedstck fr this mdel. MSW is hetergeneus in nature cntaining cmbustibles (paper, yard clippings, wd waste, fd waste, plastics and textiles) and nn-cmbustibles (glass, metals and ther inrganic wastes). It has less carbn cmpared t AW and GS. Als, ash cntent is mre in MSW than AW and GS. Increase f ash cntent in bimass decreases ptential value fr synthesis gas prductin. These bimass fuels were tested in rder t investigate synthesis gas cmpsitin at different gasificatin temperatures. Methdlgy in Mdel Frmulatin Frmulatin f mathematical mdel was based n fllwing assumptins: i) All carbn cntent in bimass is cnverted int gaseus frm and residence time is high enugh t achieve thermdynamic equilibrium; ii) Ash in feedstck was assumed inert in all gasificatin reactins; iii) All gaseus prducts are assumed t behave as ideal gases; iv) Reactin was aut-thermal and n external surce f heat was applied, prcess is cmpletely adiabatic s that n heat lsses ccur frm gasifier and amunt f air was varied t achieve desired reactin temperature in gasifier; and v) Sulphur and chlrine cntents in bimass were neglected. Chemical cmpsitin f bimass was taken t be in the frm C x O y N z and gasificatin reactin can be written as

2 61 J SCI IND RES VOL 7 SEPT - OCT 13 C x O y N z m w Ox g (O 3.76N ) x 1 x x 3 x Ox 5 C (z/x g 3.76)N (1) Where, mles f water vapur m w can be calculated as m w = Mlecular weight f bimass (Mb) * [ Misture cntent f bimass (m) / [18 (1 Misture cntent f bimass (m) () Majr reactins that ccur inside gasifier are as C (3) f (98) = kj/ml (Buduard Reactin) C O () f (98) = 13.1 kj/ml (Water Gas Reactin) Eqs (3) and () can be cmbined as single reactin knwn as water-gas shift reactin as O (5) f (98) = -.kj/ml Other prminent reactins in gasificatin prcess are as C C...(6) f (98) = -7.9 kj/ml (Methanatin Reactin) C O 3... (6a); f (98) = 5.31 kj/ml (Steam refrming f methane) Where C is assumed as pure slid and activity is taken as unity. Eqs (5) and (6) are tw majr reactins that ccur in gasificatin prcess. Equilibrium cnstant fr these equatins as functin f their mlar cmpsitin can be written as P P n n X3X K 1 = = = P P n n X1X O O P C n C n tt X K 5 = = = n tt n X P (7) (8) Gibbs free energy [Eq. (9)] is used in determining value f K 1 and K. Fr a given ideal gas, Gibbs free energy is a strng functin f reactin temperature and a weak functin f pressure 8. ln K ( T ) = GT (9) RT GT = X i G f, T, i (1) i Where, G f, T, i is empirically calculated as c d e g f T i = at T bt T 3 T,, ln f gt f, i 3 T Values f a-g and enthalpy f frmatin at standard reference state f 98 K and 1 atm pressure are taken frm Prbstein & icks 9. Eqs (11-13) can be written by balancing C, and O mles, respectively as x 1 x 3 x 5= =1 (11) x x x 5 = xm w (1) x 1 x x 3 = ym w x g (13) Ttal enthalpy cntent in any chemical species is sum f its chemical enthalpy and sensible enthalpy and can be written at reference temperature and pressure f 98 K and 1 atm as f X 3 Bimass f z 3.76 X g m 98 w ( ) P 98 P f O ( l ) dt X dt N ( vap ) = X 1 f O 98 f P O 98 P dt X 5 dt X f C 98 P 98 PC dt. dt...(1) Eq. (1) acts as cnstraint fr gasificatin prcess and frms basis fr adjusting amunt f air t be supplied. Amunt f air is adjusted in such a way that ttal enthalpy f reactants is equal t that f prducts in gaseus frm 1. In this study, LV is calculated in dry basis f bimass and was calculated using Eq. (9) as LV =.187(81C3-6(O-S)-6(9m)) (kj/kg), where C,, O, S are carbn, hydrgen, xygen and sulphur fractin in bimass respectively and m is misture cntent in bimass n dry basis. C p can be determined using an empirical relatin as C p (T) = c 1 c Tc 3 T c T 3 (kj/kg) (15) Sensible heat f each gas species was fund by integrating Eq. (15) frm ambient temperature t

3 BAVANAM & SASTRY: MODELLING SOLID WASTE GASIFICATION PROCESS FOR SYNTESIS GAS 613 Table 1 Ultimate analysis fr selected bimass feedstcks (dry basis, wt%) Sl N. Bimass C % % O % N % Ash % 1 Municipal slid waste (MSW) Animal waste (AW) Grund nut shell (GS) Table Synthesis gas cmpsitin at 15% misture cntent and at 8 C fr varius bimass feedstcks Sl N. Bimass C N 1 Municipal slid waste Animal waste Grund nut shell Synthesis gas gas cmpsitin Cmpsitin, (vl. %) %) C N Synthesis gas cmpsitin (vl.%) (vl. %) C N Misture Cntent (% wet basis) Misture Cntent (% wet basis) a) b) Synthesis Synthesis gas gas cmpsitin Cmpsitin (vl. ( vl.%) %) C N Misture Cntent (% wet basis) c) Fig. 1 Effect n synthesis gas cmpsitin at 8 C f misture cntent in: a) typical municipal slid waste; b) animal waste; and c) grund nut shell gasificatin temperature. Values f c 1 -c are taken as reprted by Reid et al 11. Mdel was run with elemental cmpsitin btained frm ultimate analysis f bimass, gasificatin temperature, misture cntent and equivalence rati as inputs fr MSW, AW and GS feedstcks. Synthesis gas cmpsitin was determined by slving 6 equatins (Eqs 7-8, 11-1) using MATLAB 1 prgramming by Newtn-Jacbi iteratin methd. Results and Discussin Table.1 presents ultimate analysis fr selected bimass feedstcks (dry basis, wt%) 13,1. Table. gives synthesis gas cmpsitin at 15% misture cntent and

4 61 J SCI IND RES VOL 7 SEPT - OCT 13 Fig. Effect f temperature n dry synthesis gas cmpsitin in selected range f temperatures (5-1 C) at 8 C fr varius bimass feedstcks btained frm Mdel. Effect f Misture Cntent n Synthesis gas Cmpsitin Bimass misture is ften expressed n a dry basis. If W wet kg f wet bimass becmes W dry after drying, its dry basis (M dry ) is expressed as M dry = W wet W dry / W dry. This can give a misture percentage greater than 1% fr very wet bimass, which might be cnfusing. Therefre, basis f misture shuld always be specified. Wet-basis misture is M wet = W wet W dry / W wet. Increasing misture cntent (-8 wt%), cncentratin f hydrgen ( ) increased fr MSW ( vl%; Fig. 1a), AW ( vl%; Fig. 1b) and GS ( vl%; Fig. 1c), and started decreasing thereafter with further increase in misture cntent. Whereas carbn mn xide () decreased fr MSW frm 7.19 t 1.39 vl%, fr AW frm 1.99 t.33 vl% and fr GS frm t 3.6 vl%, with an increase in misture (-5 wt%). With increase in misture cntent (-5 vl%), carbn di xide ( ) cncentratin increases fr MSW ( vl%), fr AW ( vl%) and fr GS ( vl%). ighest percentage f methane (C ) is fund assciated with GS fllwed by AW and MSW. Variatin f methane% in all feedstcks is almst cnstant. As assumed that the prcess is cmpletely adiabatic, additinal air flw is required with an increase in misture cntent t generate heat required t sustain desired temperature. This can be seen in increase in cncentratin f N with increase in misture cntent. In an actual gasificatin prcess, if this air flw is nt supplemented, a decrease in gasifier temperature is bserved. Small increase in cncentratin is vershadwed by rapid decrease f with an increase in misture cntent. Effect f Temperature n Synthesis gas Cmpsitin and n Gasificatin Prcess with Municipal Slid Waste (MSW) Gasifier temperature was varied frm 5 C t 1 C. At very lw temperature (5 C), carbn present in MSW (Fig. ) is nt utilized cmpletely s synthesis gas prductin is nt gd but with increasing temperature mre carbn is xidized and rate f cnversin increases. At lw temperatures bth unburnt carbn and C are present in synthesis gas but as temperature increases, carbn is cnverted int in accrdance with Buduard reactin. C is cnverted int by reverse methanatin reactin, resulting in increasing perating temperature f gasifier that favurs prductin f and, cnsequently heating value f gas imprves. Accrding t Buduard reactin, as gasifier temperature increases, vlume fractin f increases and that f decreases. Water gas reactin suggests that high temperature increases prductin f bth and. Accrding t methanatin reactin, vlume fractin f C in synthesis gas decreases and that f increases with increase in temperature. At higher temperatures, yield f and starts reducing, als attributed t water gas reactin. C prductin decreases sharply at temperature abve 5 C. Effect f Equivalence Rati (ER) n Synthesis gas Cmpsitin ER is defined as the rati f the amunt f air actually supplied t gasifier and stichimetric amunt f air. ER (<1.) gasificatin = (air/bimass) / (air/bimass)

5 BAVANAM & SASTRY: MODELLING SOLID WASTE GASIFICATION PROCESS FOR SYNTESIS GAS 615 Synthesis gas cmpsitin (vl.%) T = 8 C T=8C C N T = 85 C Equivalence rati a) b) T = 9 C c) Fig. 3 Effect f equivalence rati n dry synthesis gas cmpsitin at gasificatin temperature f: 8 C; b) 85 C; and c) 9 C pltted at three different temperatures [8 C (Fig. 3a), 85 C (Fig. 3b), and 9 C (Fig. 3c) fr MSW, indicated that the maximum synthesis gas cmpsitin was bserved at ER ranging.3 t. at all temperatures. Fig. Cmparisin f synthesis gas cmpsitin btained frm mdel t experimental results stichimetric. As ER is increased, amunt f xygen supplied t gasifier increases, due t which cnversin f carbn present in the fuel increases. But excess amunt f xygen xidizes fuel cmpletely and prductin f synthesis gas declines. ER in gasifier has been varied frm. t.8. Initially, amunt f and is cmparatively high due t increased cnversin f fuel but after a certain limit (.3), prductin f synthesis gas decreases due t cmplete cmbustin f feed. Graphs Mdel Validatin Cmparing this mdel with the wrk f Senapati & Behera 15, predicted results frm the mdel were fund in gd agreement with experimental results (Fig. ). wever, relatins becme less accurate with increase in ash cntent in bimass materials because a reactin with ash and heat absrbed by ash is ignred in the mdel. Als, perfect adiabatic cnditins are difficult t achieve in practical gasifiers, resulting in sme discrepancy between predicted and experimental results. As temperature increases, predicted values frm this mdel and relatin becme mre realistic. Cnclusins Cncentratin f in synthesis gas increases with increase in misture cntent and it attains maximum at 8-3%. Cncentratin f decreases with increase in misture cntent. With increase in temperature, cncentratin f and in synthesis gas increases

6 616 J SCI IND RES VOL 7 SEPT - OCT 13 and attains maximum at 8-9 C. Cncentratin f and in synthesis gas decreases with increase in ER frm. t.8 at all temperatures. Maximum values attained at lwer ER arund.3. Mdel predicted cmpsitin f synthesis gas matches well with experimental data reprted in literature. Thugh chemical r thermdynamic equilibrium may nt be reached within gasifier, this mdel prvides designer with a reasnable predictin f maximum achievable yield f a desired prduct. wever, it cannt predict influence f hydrdynamic r gemetric parameters, like fluidizing velcity, r design variables, like gasifier height by using this mdel. References 1 Sharma D C, Transfrming rural lives thrugh decentralized green pwer, Futures, 39 (7) Ga N & Li A, Mdeling and simulatin f cmbined paralysis and reductin zne fr a dwndraft bimass gasifier, Energy Cnvers Manage, 9 (8) Babu B V & Chaurasia A S, Mdeling, simulatin, and estimatin f ptimum parameters in pyrlysis f bimas, Energy Cnvers Manage, (3) Babu B V & Chaurasia A S, Mdeling fr pyrlysis f slid particle: kinetics and heat transfer effects.energy cnversin and management, (3) AltafiniCarls R, Wander P R & Barret R M, Predictin f the wrking parameters f a wd waste gasifier thrugh an equilibrium mdel, Energy Cnvers Manage, (3) Melgar A, Juan F P, Laget & rill A, Thermchemical equilibrium mdelling f a gasifying prcess, Energy Cnvers Manage, 8 (7) Zainal Z A, Ali R, Lean C & Seetharamu K N, Predictin f perfrmance f a dwndraft gasifier using equilibrium mdelling fr different bimass materials, Energy Cnvers Manage, (1) Smith J M, VanNess C & Abbtt M M, Intrductin t Chemical Engineering Thermdynamics, 5th ed (McGraw-ill, New Yrk) 1, Prbstein R F & icks R E, Synthetic Fuels (McGraw-ill,) De Suza-S M L, Slid Fuels Cmbustin and Gasificatin Mdeling, Simulatin and Equipment Operatin (Marcel Dekker., Inc, New Yrk), Reid R C, Prausnitz J M & Pling B E, The Prperties f Gases and Liquids, th ed (McGraw-ill, New Yrk) 1987, MATLAB, Cmputer sftware, in Mathwrks, Inc. (Natick, Massachusetts) Raveendran K, Ganesh A & Khilar K C, Inûuence f mineral matter n bimass pyrlysis characteristics, Fuel, 7 (1995) Phyllis, Database fr Bimass and Waste (Energy research Centre f the Netherlands, the Netherlands). 15 Senapati P K & Behera S, Experimental investigatin n an entrained flw type bimass gasificatin system using ccnut cir dust as pwdery bimass feedstck, Bires Technl, 117 (1)