On a Numerical Model for Gasification of Biomass Materials

Transcription

1 1st WSEAS Int. Conf. on COPUAIONAL CHEISRY, Caro, Egypt, December 9-31, On a Numercal odel for Gasfcaton of Bomass aterals ahd Vaez 1, ohammad Passanddeh-Fard, and ohammad oghman 3 Department of echancal Engneerng Ferdows Unversty of ashhad, ashhad, IRAN Abstract: In ths paper, a thermochemcal equlbrum model s used to predct the performance of a downdraft gasfer. Numercal results are shown to be n good agreement wth those of the experments. Dfferent bomass materals are tested usng the model and forest resdual s shown to be the most energetc one. For ths materal, the gasfcaton temperature, syngas composton and calorfc value are calculated. he effects of mosture content, ar/fuel rato, ar nlet temperature and steam/fuel rato are also nvestgated. he ar nlet temperature s found to be the only way to ncrease the gas calorfc value and cold gas effcency. he steam/fuel rato, on the other hand, plays a key rule n controllng the gasfcaton temperature and H /CO rato. Keywords: Renewable Energy, Bomass, Gasfcaton, hermochemcal Equlbrum, athematcal odelng 1 Introducton Bomass as a new source of energy has drawn world wde attenton durng the last decade. Postve rate of consumpton of fossl fuels, negatve rate of ther natural reservors and restrctng envronmental rules have created awareness for the need to dentfy alternatve sources of fuel such as bomass. Byproducts of actvtes lke agrculture or food processng are categorzed as bomass materals. hese materals were used to dspose n open lands creatng serous envronmental problems. Compostng, recyclng and ncneraton have been used as alternatve methods for waste handlng [1]. ore recently, gasfcaton of bomass materals has been ntroduced as another knd of waste to energy (WE converson; a process to convert carbonaceous materals to synthetc gases such as CH 4, CO, CO and H. One of attractve features of ths technology nclude the ablty to produce a clean syngas product that can be used ether for generatng electrcty or producng chemcals. It must be understood that gasfcaton s not combuston. A combuston process needs stochometrc feed of ar/oxygen, whle gasfcaton process s performed at sub-stochometrc condtons (30% to 70% of stochometrc ar/oxygen. In some cases, ntrogen and/or steam are also njected n order to control the gasfcaton condton and volume of products. he many advantages of gasfcaton over combuston make t feasble to revew the possbltes of syngas producton as a snk for bomass materals whle observng the envronmental regulatons. In ths technology, sold feed materals are gasfed n a reactor such that vrtually all of ther contents are converted nto fuel gas wth calorfc values typcally 3±6 J/m 3 (natural gas s 34 J/m 3 wth most of the energy beng avalable from H and CO. After cleanng, ths gas can be used to run small recprocatng engnes, bolers, process heaters etc. Bomass gasfcaton s a complex process wth many mportant controllng parameters such as ar/fuel rato and mosture content. As a result, mathematcal models have been ntroduced for predctng the performance of gasfers and as tools for ther desgn optmzaton [ 4]. One of these models, the equlbrum modelng, was used by many researchers. Although, thermodynamc equlbrum n realty never takes place n a gasfcaton process [5], several works were performed to demonstrate the applcablty of the equlbrum model for ths process. hese models especally work well n hgh temperatures that occur n entraned flow gasfers (above 1500K [6]. Some equlbrum models were based on the mnmzaton of Gbbs free energy [7-8] whle others were based on equlbrum constant. Altafn and randola [4] smulated a bomass gasfer, based on mnmzng the Gbbs free energy. he bomass gasfcaton process was also modeled by Zanal et al. [9] based on equlbrum constants. Lapuerta et al. [10] predcted the producer gas composton as a functon of the fuel/ ar rato by means of an equlbrum model. A descrpton of a 1 Graduate student and correspondng author,3 Unversty professors

2 P 1st WSEAS Int. Conf. on COPUAIONAL CHEISRY, Caro, Egypt, December 9-31, two-zone model n a downdraft gasfer was adapted by Jayah et al. [7] and Schuster et al. [11]. An expermental study was also descrbed by Zanal et al. [1]. A zero-dmensonal model presented n ths paper s based on the thermochemcal equlbrum of the process usng equlbrum constants. Numercal model s frst valdated by a comparson between calculated results and two separate experments avalable n the lterature [-4]. he effect of ultmate analyss on the calorfc value s studed for 0 dfferent common bomass materals and the most energetc one s selected as a sample for rest of the paper. he nfluence of mportant parameters of a gasfcaton process such as: the ar/fuel rato, bomass mosture content, ar preheatng, and steam njecton on the gasfcaton characterstcs are also nvestgated. hese characterstcs are the syngas composton, gasfcaton temperature, calorfc value of the producer gas and the cold gas effcency. ethodology he man assumptons of the developed model are as follows. he gasfer reactor s assumed to be adabatc and the resdence tme of the reactants s supposed to be hgh enough to reach chemcal equlbrum. In addton, all carbon n the bomass s assumed to be gasfed and, therefore, the formaton of char s neglected. o develop the model, the chemcal formula of feedstock s defned as CHxOyNz. he global gasfcaton reacton can be wrtten as follows: CHxOyNz + wh O n H + n CO + n H CO ( z / m N ( lq CO + sh O CO + n ( vap HO + m( O N H O + n CH + CH4 4 (1 where x, y, and z are numbers of atoms of hydrogen, oxygen, and ntrogen per one atom of carbon n the feedstock; respectvely, and w, m, and s are the amounts of mosture, ar, and steam per one kmol of feedstock, respectvely. All nputs on the left-hand sde of Eq.(1 are defned at 5 C.On the rght-hand sde, n s the number of moles of the speces whch s also unknown. olar quantty of water per one kmol of bomass can be wrtten as [9]: bm C w ( H (1 C O where and are the masses of the bomass bm H O and water; respectvely, and C s the mosture content. Ar/fuel rato can be calculated as ( α + 0.5β 0.5γ for a fuel wth a chemcal formula of C [13]. Durng the gasfcaton α H βoγ Nξ process, only between 30% and 70% of stochometrc ar s used; therefore, we assume m to be a fracton of the calculated stochometrc ar. In addton, the enthalpy of formaton for bomass can be obtaned as follows [14]: 1 h f, bm LHVdb + vh f, (3 where h f bm prod s the enthalpy of formaton, LHV s the lower heatng value and s the stochometrc coeffcent of the speces. o obtan the fve unknown speces of the producer gas, fve equatons are requred whch are generated usng mass balance and equlbrum constant relatonshps. Consderng the global gasfcaton reacton n Eq.(1, the frst three equatons are formulated by balancng each chemcal element consstng of carbon, hydrogen and oxygen. he remanng two equatons are obtaned from the equlbrum constant of the reactons that occur n the gasfcaton zone whch are gven as: Boudouard reacton: C + CO CO ( 4 Water-gas reacton: C + HO CO+ H ( 5 ethane reacton: C + H CH 4 (6 Hgman and van der Burgt [15] showed that Eqs.(4 and (5 can be combned to gve the water gas shft reacton as: Water-gas shft reacton: CO + H O CO + H (7 For the model n ths study, n addton to an assumpton of thermodynamc equlbrum, all gases are assumed to be deal and all reactons to occur at the atmospherc pressure. herefore, the equlbrum constants, whch are functons of temperature, for the methane and the water-gas shft reactons can be wrtten based on the followng: K ( x x v P ( n total v v (8 where s mole fracton of the speces n the o deal gas mxture, v s the stochometrc number, P s the standard pressure (1atm and n total s the total number of moles of product gas. As a result, the equlbrum constants for the water-gas shft and the methane reactons wll be as follows: ( nco ( n H k 1 (9 ( n ( n CO H O

3 1st WSEAS Int. Conf. on COPUAIONAL CHEISRY, Caro, Egypt, December 9-31, n CH 4 ( k ( n H (10 he values of the equlbrum constants, whch are mere functons of temperature, can be calculated usng the Gbbs free energy: ΔG lnk ~ R (11 where R ~ s the unversal gas constant and ΔG s the standard Gbbs functon of formaton. he dependence of ΔG on temperature can be wrtten as follows: d( ΔG / R h f ~ (1 d R where s the enthalpy of formaton wth a value h f of zero for all chemcal elements n a reference state. herefore, based on Eq.(11 we wll have: d ln k h f ~ ( 13 d R Snce the heat of formaton s a functon of, Eq.(13 can be ntegrated as follows: h f ln k ~ dt + I (14 R he amounts of ΔG and h f can be found n chemcal engneerng handbooks [17]. Jarungthammachote et al. [16] multpled k [see Eq. (10] by a factor to mprove the performance of the model. he same factor s used n ths model. he temperature of the gasfcaton zone needs to be determned n order to calculate the equlbrum constants [Eq.(11]. For ths reason, a balance of ether energy or enthalpy s performed on the gasfcaton process usually assumed to be adabatc. If the temperature n the gasfcaton zone s and that of the nlet state s assumed to be 98K, the enthalpy balance for ths process can be wrtten as: n h n h + Δh ( 15 j f, j ( f,, j react prod where Δh represents the enthalpy dfference between any gven state and the reference state. It can be approxmated by: Δh 98 C ( d p (16 where C p ( s the specfc heat at constant pressure whch s only a functon of temperature. It s defned by an emprcal relaton as [17]: C3 C5 C ( [ ] [ p C1 + C + C4 ] (17 C3 C5 snh( snh( When the equlbrum constants are defned, a system of equatons wll be obtaned that need to be solved. Because of the non-lnear nature of some of the equatons, the Newton Raphson method s used whch can be solved wth an nteratve procedure. he procedure starts wth an ntal guess for the gasfcaton temperature. he set of equatons are then solved to obtan the producer gas composton whch n turn s used to determne the new gasfcaton temperature [usng Eq. (14]. hs teratve procedure contnues untl the gasfcaton temperature does not change wthn a certan lmt (much less 1 K n successve teratons. 3 Results and dscusson In ths secton, the results of the presented model for producer gas composton are frst compared to those of the experments performed by Jayah et al. [] and Altafn et al. [4] as shown n able 1. Jayah et al. [] calbrated ther model by the methane content. In ths model, the thermal losses n the gasfer were not taken nto account; therefore, the predcted values for methane are slghtly less than those of the experments. he slght dfferences n other components (seen n able 1 may be attrbuted to the smplfyng assumptons of the model such as: consderng all gases to be deal, and assumng no char or resdue n the gasfcaton process. As seen from the table, the thermochemcal equlbrum model can predct the measured values wth a hgh accuracy (RS<. able 1 he comparson of expermental data [,4] wth the predcted results Reference [] a Reference [4] b Exp odel Exp odel H CO CO CH N CV J/m RS c error a Rubber Woo d -18.5% mosture content b Sawdust -10% mosture content

4 1st WSEAS Int. Conf. on COPUAIONAL CHEISRY, Caro, Egypt, December 9-31, c RS N ( Exp Num D In order to compare varous bomass sources of energy, gasfcaton of 0 dfferent samples are smulated under the same condtons (bomass wth zero mosture content and stochometrc coeffcent equal to 0.46 to evaluate ther producer gas calorfc values. hese samples were selected across fve major groups of bomass materals [18] consstng of grasses and straws; wood fuels; urban waste fuels; wood fuel blends; nuts, pts and shells. he results of the smulatons for the twenty samples are shown n Fg. 1. As seen from the fgure, forest resduals are found to produce the hghest calorfc value (4.73 J/m 3. Ultmate analyss of ths bomass (forest resdual s 50.31%C, 4.59%H, 39.99%O, 1.03%N, 0.11%S and a HHV of equal to J/kmol. An mportant parameter for a bomass materal s ts mosture content whch depends on many factors such as the producton mechansm, and storng and transportaton condtons. herefore, analyzng the effect of mosture content on producer gas composton and gasfcaton characterstcs s of great mportance. he effect of mosture content on the producer gas composton s shown n Fg.. As observed, an ncrease n mosture content from 0% to 40% leads to a 31.8% ncrease of H and 89.8% of CO, but a 4.69% decrease of CO. he effect of mosture content on calorfc value and gasfcaton temperature s shown n Fg.3. It can be seen from Fgure.1 Comparson between calorfc values obtaned va gasfcaton of 0 dfferent bomass materals the fgure that ncreasng the mosture content reduces the calorfc value of the producer gas. Although the calorfc values of H and CO (energetc gases are close to each other (41.8 J/kmol and 8.9 J/kmol, snce the decrease of CO s greater than the ncrease of H, the resultant calorfc value of the producer gas s decreased. Fgure 3 also shows that ncreasng the mosture content reduces the gasfcaton temperature. hs s because a greater mosture content means more requred energy for vaporzaton. he above results are n good agreement wth the numercal results of Zanal et al. [9] and Jarungthammachote et al. [16]. o nvestgate other parameters, the effect of ar nlet temperature s also studed. It was found that ar preheatng can ncrease the calorfc value of the producer gas. For an ncrease of ar temperature from 300K to 600K, the producer gas composton was not changed consderably; however, the calorfc value was ncreased by 0.6% and the

5 1st WSEAS Int. Conf. on COPUAIONAL CHEISRY, Caro, Egypt, December 9-31, Fgure. Effect of mosture content on gas composton In addton, t can be used to adjust H /CO rato and ncrease the volume of CH 4 n the producer gas. Fnally, we study the nfluence of four effectve parameters on cold gas effcency (also known as the frst law of thermodynamcs effcency defned as the rato of the energy leavng the gasfer to the energy enterng t (.e. bomass and mosture. Evaluatng the energy of the producer gas and that of the feedstock n the same state (reference temperature, the cold gas effcency s wrtten as: Heatng value n producer gas (J CGE ( 18 Heatng value n feed stock (J Fgure 6 shows the results of the model for the effect of mportant parameters on CGE. Whle ncreasng the ar nlet temperature causes a slght ncrease of Fgure.3 Effect of mosture content on gasfcaton temperature and calorfc value of the producer gas gasfcaton temperature by 130K. he slght ncrease of calorfc value (only 0.6% does not justfy the ncrease of the ar nlet temperature because of the consderable ncrease of the gasfcaton temperature (by 130K whch s not favored. Gas composton varatons aganst ar/fuel rato are plotted n Fg.4. As t was dscussed before, the ar/oxygen stream of a gasfcaton process should be between 30% to 70% of the stochometrc ar/oxygen. If ths porton s ncreased, the process nears the usual combuston wth a tendency to produce CO and N (domnant products of combuston process. hs wll ncrease the temperature and consderably decrease the calorfc value of the producer gas. he effect of steam/fuel rato on calorfc value of the producer gas and the gasfcaton temperature s shown n Fg. 5. Steam n gasfcaton s called "moderator" because t can be used as a means for controllng the gasfcaton temperature (see Fg. 5. Fgure.4 Effect of ar/fuel rato on gas composton cold gas effcency, other parameters have a reverse effect. 4 Concluson A thermochemcal equlbrum model was developed for a bomass gasfer n order to calculate the composton of the producer gas and nvestgate the gasfcaton characterstcs. he predcted results agreed well wth those of the experments avalable n the lterature. he model

6 1st WSEAS Int. Conf. on COPUAIONAL CHEISRY, Caro, Egypt, December 9-31, temperature and ncrease the H /CO rato, a fndng rarely reported n the lterature. he developed model n ths study can be used to smulate gasfcaton of other types of bomass materals and predct the effect of mportant varables n optmzaton of a bomass downdraft gasfer. Fgure.5 Effect of steam/fuel rato on calorfc value of the producer gas and gasfcaton temperature Fgure.6 Influence of four effectve parameters on cold gas effcency was then employed to evaluate the capablty of dfferent bomass materals to produce energy. Among twenty dfferent bomass materals of fve major famles, forest resduals had the hghest calorfc value. he effects of mosture content, ar nlet temperature, ar/fuel rato and steam/fuel rato on gasfcaton characterstcs were nvestgated. Although, the ncrease of ar nlet temperature was the only way to ncrease the producer gas calorfc value and cold gas effcency, t also ncreased the gasfcaton temperature whch was not favorable. Steam njecton was found to reduce the gasfcaton References [1] orrs, Waldhem L, Energy recovery from sold waste fuels usng advanced gasfcaton technology, Waste anagement, 1998, Vol 18, pp [] Jayah.H, Aye Lu, Fuller R.J, Stewart D.F, Computer smulaton of a downdraft wood gasfer for tea dryng, Bomass & boenergy, 003, Vol 5, pp [3] D Blas C, Dynamc behavor of stratfed downdraft gasfers, Chemcal engneerng scence, 000, Vol 55, pp [4] Altafn C.R, Wander P.R, Barreto R., Predcton of workng parameters of a wood waste gasfer through an equlbrum model, Energy Converson & anagement, 003, Vol 44, pp [5] Chowdhury R, Bhattacharya P, Chakravarty, odelng and smulaton of a downdraft rce husk gasfer, Energy Res, 1994, Vol 18, pp [6] Altafn C.R, A chemcal equlbrum model of the coal gasfcaton process based on the mnmzaton of the Gbbs free energy, Florence World Energy Research Symposum, Florence, Italy, 1997, p [7] Ruggero, anfrda G, An equlbrum model for bomass gasfcaton process, Renew Energy, 1999, Vol 16, pp [8] L X, Grace J.R, Watknson A.P, Lm C.J, Ergudenler A, Equlbrum modelng of gasfcaton: a free energy mnmzaton approach and ts applcaton to a crculatng fludzed bed coal gasfer, Fuel, 001, Vol 80, pp [9] Zanal Z.A, Al R, Lean C.H, Seetharamu K.N, Predcton of performance of a downdraft gasfer usng equlbrum modelng for dfferent bomass materals, Energy converson & management, Vol 4, 001, pp [10] Lapuerta, Hernandez J, naut F.V, Horllo A, hermochemcal behavour of producer gas from gasfcaton of lgnocellulosc bomass n SI engnes. SAE Paper seres, 001 [11] Schuster G, Offler G, Wegl K, Hofbauer H, Bomass steam gasfcaton an extensve parametrc modelng study, Boresour echnol, 001, Vol 77, pp

7 1st WSEAS Int. Conf. on COPUAIONAL CHEISRY, Caro, Egypt, December 9-31, [1] Zanal Z.A, Al R, Quadr G.A, Seetharamu K.N, Expermental nvestgaton of a downdraft bomass gasfer, Bomass & Boenergy 00, Vol 3, pp [13] Borman G.L, Ragland K.W, Combuston engneerng, Newyork: cgraw Hll, 1998 [14] de Souza-Santos, Sold fuels combuston and gasfcaton - odelng smulaton and equpment operaton, NewYork: arcel Dekker Inc, 004 [15] Hgman C, van der Burgt, Gasfcaton, USA: Gulf Professonal Pub, 003 [16] Jarungthammachote S, Dutta A, hermodynamc equlbrum model and second law analyss of a downdraft waste gasfer, Energy, Vol 3, 007, pp [17] Perry R.H, Green D.W, Perry's chemcal engneers' handbook, Newyork: cgraw Hll, 1999 [18] les.r, Baxter L.L, Bryers R.W, Jenkns B., Oden L.L, Alkal deposts found n bomass power plants: a prelmnary nvestgaton of ther extent and nature, Natonal Renewable Energy Laboratory, 1995