MODELLING OF FLUIDIZED BED BIOMASS GASIFICATION

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1 hemcal and Process Engneerng 0, 3 (), DOI: 0.478/v MODELLING OF FLUIDIZED BED BIOMASS GASIFIATION IN THE QUASI-EQUILIBRIUM REGIME FOR PRELIMINARY PERFORMANE STUDIES OF ENERGY ONVERSION PLANTS Jacek Kalna Slesan Unversty of Technology, Insttute of Thermal Technology ul. Konarskego, Glwce, Poland Thermodynamc equlbrum-based models of gasfcaton process are relatvely smple and wdely used to predct producer gas characterstcs n performance studes of energy converson plants. However, f an unconstraned calculaton of equlbrum s performed, the estmatons of product gas yeld and heatng value are too optmstc. Therefore, reasonable assumptons have to be made n order to correct the results. Ths paper proposes a model of the process that can be used n case of defcency of nformaton and unavalablty of expermental data. The model s based on free energy mnmzaton, materal and energy balances of a sngle zone reactor. The constrant quasequlbrum calculatons are made usng approxmated amounts of non-equlbrum products,.e. sold char, tar, H 4 and H 4. The yelds of these products are attrbuted to fuel characterstcs and estmated usng expermental results publshed n the lterature. A genetc algorthm optmzaton technque s appled to fnd unknown parameters of the model that lead to the best match between modelled and expermental characterstcs of the product gas. Fnally, generc correlatons are proposed and qualty of modellng results s assessed n the aspect of ts usefulness for performance studes of power generaton plants. Keywords: bomass gasfcaton, fludzed beds, thermodynamc equlbrum. INTRODUTION Thermal converson of bomass nto a gaseous fuel by means of gasfcaton s consdered nowadays as one of the most attractve technologes for O emsson reducton and fossl fuel savngs. Ths s manly due to a hgh level of power generaton effcency of potental power plants. Nevertheless, a conceptual desgn, optmzaton and feasblty studes are requred to demonstrate savng potental and economc proftablty of bomass to energy converson projects. Nowadays the most documented, matured and commercalzed bomass gasfcaton technology s the fludzed bed process. In practce ths technology domnates the market wthn the power range above 0 MW (chemcal fuel energy nput). For research and development purposes fludzed bed reactors are bult startng from the power of about 00 kw (Bolhàr-Nordenkampf et al., 00b). It has already been demonstrated n projects ARBRE (UK), Värnamo (Sweden) and Güssng (Austra) that a varety of technologcal schemes of power plants can be desgned and successfully operated wth the medum scale reactors. Due to a relatvely hgh power output fludzed bed gasfers are more sutable for plants wth gas turbnes. Nevertheless, examples of plants wth gas engnes also exst (Bolhàr-Nordenkampf et al., 00b; Wu et al., 008). *orrespondng author, e-mal: jacek.kalna@polsl.pl Download Date //6 0:6 PM 73

2 J. Kalna, hem. Process Eng., 0, 3 (), There s a number of studes of ntegrated bomass gasfcaton energy converson systems avalable nowadays n the lterature. Most of them are based on the thermodynamc equlbrum approach towards modellng gasfcaton. Some examples are gven by Brown et al. (009), Buragohan et al. (00), Kalna (00), Klmantos et al. (009) and Schuster et al. (00). The equlbrum modellng of gasfcaton reactors s also utlsed by software avalable for computatonal analyss and smulaton of energy plants, that s e.g. the ycle Tempo (Delft Unversty of Technology, ). In general, the equlbrum approach s regarded as satsfactory for ths problem, assumng that one s aware of ts lmtatons (Gòmez-Barea et al., 00). Smple mathematcs and ndependence (to a certan extend) from the reactor desgn parameters are the advantages. However, f the equlbrum approach s appled for an analyss of a fludzed bed reactor, the results typcally show an overestmated mass converson effcency, heatng value of the producer gas and generaton of hydrogen and carbon oxde. The yelds of methane and hgher hydrocarbons are usually notably underestmated. Mevssen et al. (009) presented that the dfferences between predcted and expermental data can be sgnfcant. Prns et al. (007) clam that the equlbrum model gves the hghest gasfcaton effcency that can be possbly attaned for a gven fuel. In practce, t s dffcult to reach equlbrum condtons for sold carbon converson at gasfcaton temperatures below 000º, whch s the case wth fludzed bed reactors. A composton of a producer gas that s closer to equlbrum one can be obtaned usng ether n-bed catalyst or a secondary catalytc bed reactor (Asadullah et al., 003; orella et al., 998; Kurkela et al., 009). Gl et al. (999) examned the results of dfferent expermental studes of pne chps gasfcaton n bubblng fludzed slca sand bed usng ar, pure steam and steam-o mxtures as gasfcaton agents. Smlar expermental condtons were taken nto account. The reported H content n the dry producer gas was wthn the range of %, % and % for the three agents respectvely. The concentraton of O was %, % and % and the concentraton of H 4 was. - 6.%, % and %. It can be easly concluded that these concentratons are far from equlbrum ones calculated for the same values of excess ar rato λ. Gòmez-Barea et al. (00) conducted an extensve revew of modellng of bomass gasfcaton n bubblng and crculatng fludzed bed reactors. Accordng to the authors fludzed bed gasfers have to be modelled usng revsed pseudo-equlbrum models, or, n some extreme cases, by detaled flow models. Good assumptons for calculatons are requred to obtan an agreement between modellng and expermental results. Typcally, some emprcal parameters, such as concentraton of methane n the product gas and/or carbon converson effcency, are ncluded n a model. The constraned equlbrum models that nowadays can be found n the lterature are usually tuned usng a reactor specfc expermental data. onsequently, the models have lmted predctve capabltes. Another modellng approach s based on the so called quas-equlbrum temperatures. An example has been gven by Brown et al. (009). They estmated yeld and composton of a producer gas usng a parametrc stochometrc model where the equlbrum of gasfcaton reactons was calculated at a temperature lower than the real process temperature. The model was calbrated usng expermental data from a plot crculatng fludzed bed reactor. Accordng to Prns et al. (007) such an approach s mpractcal as the temperature used for calculaton appears to be ndependent from the real process temperature and hardly predctable wthout expermental data. In ths paper an attempt s made to develop a general constraned quas-equlbrum model of the fludzed bed gasfcaton process. It s expected that the model would be able to predct a producer gas yeld, composton and heatng value wth an acceptable accuracy. These parameters have sgnfcant nfluence on mass and energy balance as well as on the economc evaluaton of a project. They also determne performance of machnery such as gas engnes and gas turbnes nstalled downstream of a gasfer. An extensve study of the lterature has been made to make reasonable assumptons. Then, the model has been formulated and verfed aganst the publshed expermental results. 74 Download Date //6 0:6 PM

3 Modellng of fludzed bed bomass gasfcaton n the quas-equlbrum regme Due to complexty and knetc lmtatons of the process t s almost mpossble to develop a precse general model of a fludzed bed gasfer. Accordng to Gl et al. (999) the fnal composton of a producer gas s nfluenced by at least 0 operatonal parameters concernng the reactor and feedstock. In ths work the parameters that are assumed to have a key mpact on modellng results are: carbon converson effcency, yelds of methane and hgher hydrocarbons (ncludng tar), char composton, yeld of NH 3 and heat losses. These are the parameters that reduce the amount of carbon and hydrogen avalable to equlbrum calculaton and nfluence the process temperature. L et al. (004) proposed to correct devatons between model-predcted and expermentally measured product gas yeld and composton by usng certan amounts of sold carbon and methane that do not take part n calculatng the equlbrum. They defned carbon and hydrogen avalablty functons as smple correlatons of the ar rato. The functons were establshed usng a reactor specfc expermental data. Accordng to Kurkela et al. (009) carbon converson effcency of modern ar/o fludzed bed gasfcaton reactors s between 96-99%. In allothermal gasfers, where pure steam s used as the gasfcaton agent and heat s suppled from an external source, the converson effcency s between 85-9 %. Van der Mejden et al. (00) modelled an autothermal fludzed bed gasfcaton of bomass usng the approxmated sngle value of carbon converson effcency of 90%. In the case of allothermal steam gasfcaton they calculated converson effcency usng a lnear functon of temperature that gave the values from 7% to 86% wthn the range from 800 to 900º. Van der Drft et al. (00) presented expermental values of carbon converson effcency of gasfcaton of dfferent woody bomass fuels usng ar as the gasfcaton agent n the range of 85-97%, and the average value was 9%. De Souza-Santos (004) clams that due to fludzaton process requrements t s very dffcult to acheve the effcency n excess of 95%. A hgh carbon converson requres a long partcle resdence tme and a specal desgn of the reactor. The problem s partly solved n pressurzed systems and n oxygen blown reactors. onverson effcency s also nfluenced by the type of n-bed catalyst (Asadullah et al., 003). A comprehensve study of gasfcaton n pressurzed fludzed bed reactors was presented by De Jong (005). It can be concluded from the presented results that the hgher s the value of the excess ar rato λ the hgher s the carbon converson effcency. Gasfcaton of mscanthus wth λ n the range of resulted n carbon converson between 8 and 9%. The carbon converson observed durng gasfcaton of wood wth λ n the range of vared very lttle from 96.6 to 98.7%. Hgher carbon converson effcency of wood was attrbuted manly to smaller partcle sze and hgher reactvty. It can be observed that the contents of methane and hgher hydrocarbons n a producer gas (excludng tar) depend lnearly on the values of λ. The total content of hydrocarbons n the product gas vared from.48 to 5.03 for both analysed fuels. Measurements of the composton of the product gas at dfferent heghts of the reactor column show that varatons of the gas composton are relatvely small along the freeboard. An analyss of the expermental results also shows that 50 70% of the fuel ntrogen forms NH3 and 4.5 4% forms HN. Mcco et al. (999) presented that carbon converson effcency ncreases both wth ar to fuel rato and process temperature. The effcency asymptotcally approaches ts maxmum value wth ncreasng reactor heght. At the bottom of the freeboard carbon converson sharply approached the value measured at the ext of the freeboard. At the most favourable condtons (.e. T = 900º and λ = 0.35), carbon converson at the reactor ext reached a maxmum value of 97%. The mnmum carbon converson effcency was 70% (at λ = 0.5 and T = 700 º). The presented results showed concentraton of methane n the producer gas at the level above 5.7%. A general estmaton of tar content n the dry gas obtaned from a fludzed bed reactor s 0 g/nm 3 Mlne et al. (998). Most of the reported results from gasfcaton tests show tar content below ths fgure. However, n some unfavorable condtons as much as 00 g/nm 3 can be generated. Van der Download Date //6 0:6 PM 75

4 J. Kalna, hem. Process Eng., 0, 3 (), Mejden et al. (00) assumed generc tar concentraton n a dry raw producer gas from fludzed reactors of 30 g/nm 3. Gl et al. (999) establshed the so-called "representatve" values for tar contents n a dry producer gas. These values are - 0 g/nm 3 for gasfcaton wth ar, 4-30 g/nm 3 for gasfcaton wth steam-o mxtures and g/nm 3 for gasfcaton wth steam. The yeld of tar from FIFB gasfer s between.5 and 4.5 g/nm 3 (Bolhàr-Nordenkampf et al. 00a; Hofbauer et al., 997). orella et al. (006) presented that the amount of tar from ar gasfcaton of bomass decreases wth process temperature and ar to fuel rato. On the other hand t ncreases wth heat loss and bomass mosture content. The paper presents that only a relatvely dry bomass (% wt.) gasfed wth the ar equvalence rato ER = 0.35 results n tar content below g/nm 3. The authors concluded that the two most crtcal operatonal condtons are relatvely hgh equvalence rato and good n-bed materal whch determne the knetc constants of the process.. MODEL FORMULATION Wthn ths work a sngle compartment model of a gasfcaton process s beng formulated. It s assumed that a wet sold bomass undergoes thermal converson, that can be descrbed usng the followng global stochometrc reacton: n H O N S + n H O( l) + n H O( g) + λn H 4 b H H b c 4 O c b3 N b4 H 4 c3 H O H 4 tar t H O t O t3 O H O NH 3 H O NH 3 O mn H O( g) N N + n N N SO = n SO H H The process also nvolves ash and argon from the atmospherc ar whch are regarded as nert substances. Bomass fuel s defned by a proxmate and ultmate analyss. The total mass composton from the ultmate analyss of bomass s: c + h + o + s + n + w + ash = () Usng ths data the number of moles of each substrate enterng the system wth the fuel can be determned: c n = ; M h n H = ; M H o n O = ; M O s n S = ; M S n n N = ; M The stochometrc formula of bomass n Equaton () can be calculated: b n H = ; n b n O = ; n b n N 3 = ; n b n = N O H O O () w n H O = (3) M S 4 (4) n A bottom feedng of bomass nto the reactor s taken nto account. The operatng temperature s consdered to be wthn the range of The reactor conssts of two zones, the so-called bed zone and freeboard. Wthn the bed zone flash pyrolyss oxdaton and char gasfcaton take place. In the freeboard zone there are only homogenous reactons n the gas phase. Any sold partcle of char present n the freeboard s consdered to be chemcally nactve. Gaseous products passng from the bed to freeboard are O, O, H, H 4, H 4, H O, N, tar and SO (f sulphur s present n the fuel). Sold products leavng the reactor are ash and unconverted char. The bed materal s assumed to have some catalytc propertes and the resdence tme of char partcle n the bubble bed s hgh. Therefore, t can be assumed that carbon converson effcency s hgh and gas composton s close to the equlbrum. 76 Download Date //6 0:6 PM

5 Modellng of fludzed bed bomass gasfcaton n the quas-equlbrum regme The composton of the product gas s determned assumng the state of thermodynamc equlbrum n gaseous phase. Usng the approach based on Gbbs free energy mnmzaton, the objectve functon can be formulated: ls G = n μ mn (5) = Gven that the chemcal potental s equvalent to the partal free enthalpy of components n the mxture and the mxture s a perfect soluton of deal gases at a specfed pressure, p, and temperature, T, t s possble to wrte the objectve functon n the followng form: ls = 0 [ g ( T, p) + RT ln z ] mn Where the partal Gbbs free energy of a pure component s: g n (6) 0 0 ( T, p) h ( T ) Ts ( T p) 0, = (7) H, O, H 4, O and H O are consdered to be the only products of the gas phase reactons at the equlbrum. Tar, char, NH 3, H 4 and a porton of H 4 yeld are consdered to be non-equlbrum products of the process. These components are just wthdrawn from the substrates and bypass the calculaton of equlbrum. The constrants for the mnmzaton of the objectve functon (6) are substance balances of carbon, hydrogen and oxygen: b n n n n n n t n 0 (8) = O O H 4 H tar = 4 ( c n + t n + 3n ) 0 b = n H + n H O + nh O nh n H O nh n 4 H 4 tar NH = (9) 3 ( n + n ) n n ( n + c n + t n ) 0 b3 = n O + λ n O mn + H O H O O SO O H O 3 tar = (0) The total number of moles of the raw producer gas at the output of the gasfer s: n = n () pg H O H 4 H 4 O H O N SO Ar tar NH 3 It s assumed that remanng solds consst of ash and unconverted char H c O c N c3. The mass of carbon n the char s determned usng the assumed value of carbon converson effcency, whch can be gven by: n n ε = () n To calculate the amounts of hydrogen and oxygen n the char the emprcal correlatons developed by Rchard et al. (00) are used: = 0.55exp T c [ ( 73) ] =.7exp T c [ ( 73) ] (3) (4) The amount of ntrogen n the char s assumed to be equal to 40% of fuel ntrogen. The remanng 60% of ntrogen present n bomass forms ammona NH 3 (Van der Drft, 00). Download Date //6 0:6 PM 77

6 J. Kalna, hem. Process Eng., 0, 3 (), The tar present n the products s modelled as a mxture of benzene 6 H 6, naphthalene 0 H 8 and phenol 6 H 5 OH. The molar composton of the tar s assumed as follows: 6 H 6-30%; 0 H 8-50%; 6 H 5 OH - 0%. The yeld of the tar s calculated n grams per kg of dry bomass fuel (g/kg db ) usng the followng formula: ( f + 30)( w ) m tar = δ 90 H O + db (5) To estmate the yeld of methane t s assumed that a porton of methane from pyrolyss remans unconverted durng the process and goes nto the product gas. Ths amount of H 4 s assocated wth fuel hydrogen content. Therefore, t can be wrtten as: ( n H ) eq n H = n 4 H + 4 δ (6) The yelds of hydrocarbons H, H 4 and H 6 are represented by equvalent yeld of H 4 that s assumed to be equal to: n = δ n (7) H 4 3 H The varables ε, δ, δ and δ 3 are four undetermned parameters of the model. In the process of model verfcaton these parameters are vared to obtan the best agreement of modelled results wth expermental data. Fnally, there are 5 unknowns to be determned by the mnmzaton of the objectve functon (6). These are n" H, n" O, n" O, n" HO and (n" H4 ) eq. The task can be solved by the method of undetermned Lagrange multplers. In ths case the modfed objectve functon to be mnmzed takes the form: ( T, p) ls 0 g 3 F = n + ln n ln n pg + λ k bk mn (8) RT = k= Mnmzaton procedure now requres a soluton of eght equatons wth eght unknowns (ncludng λ, λ, λ 3 ). Fve equatons result from the condton of zerong the partal dervatves of functon F wth respect to the unknowns: F = 0 n Equatons (8) for gas components take the fnal form: 0 3 ls g ( ) T, p n = n pg exp λ kakj (0) RT k= j= The remanng three equatons are substance balances (8), (9) and (0). The temperature T s determned from the energy balance of the process. For kg of a wet bomass at the nput temperature 98 K the balance of the process takes the form: (9) LHV db ( w) + n MLHV wr + c + MLHV n Δh H T 98K + + Q n = j n j MLHV j + Δh T j 98K ( mcharcchar + mashcash )( T 98) + Qout () The heat capacty of the char s calculated usng the followng formula (Thunman et al., 00) c char 6 = T T T 9 0 T, J/kgK () Download Date //6 0:6 PM

7 Modellng of fludzed bed bomass gasfcaton n the quas-equlbrum regme The heat capacty of the ash s modelled usng a correlaton developed by Krov (965), that for ash takes the followng form: 4 c ash = T, kj/kgk (3) A code has been wrtten usng Engneerng Equaton Solver (EES) to solve the reactor model. Propertes of substances are calculated usng JANAF and NASA tables attached to the software package as the external routnes. 3. RESULTS AND DISUSSION In order to verfy the model the results of calculatons were compared wth expermental data publshed n the lterature. The frst set of expermental data was taken from Mcco et al. (999). The gasfed feedstock was beech wood of the followng characterstcs (dry bass): volatles 84.90%, fxed carbon 4.07%, ash.03%; mass composton: 4.8%, H 6.03%, N 0.%, O 50.54%; LHV 8380 kj/kg. Detaled results of gasfcaton are reported for the temperature T = 800º and equvalence rato ER = 0.5. The composton of the dry product gas was: H - 5.0%, O - 9.0%, H 4-6.3%, H x -.30%, O - 8.0%, N %. The carbon converson effcency of the experment was 78% (read from the chart). In the smulaton the assumed values of ε, T and ER were kept the same as the reported expermental ones. To fnd values of the undetermned parameters δ, δ and δ 3 the EES embedded genetc algorthm optmzaton procedure was appled. The mnmzaton of the sum of squares of relatve devatons between the measured and calculated concentratons of the product gas components the objectve was: ( z ) e ( z ) ( z ) m Γ = mn (4) e The number of ndvduals was set to 64 and the number of generatons was set to 56. The optmzed undetermned parameters were: δ = 0.004, δ = 0.3 and δ 3 = The calculated composton of the dry product gas (tar and argon free) was: H %, O - 5.7%, H %, H x -.37%, O -.74%, N %. The calculated lower heatng value of the gas was kj/kmol versus 7054 kj/kmol calculated for the expermental composton. The nfluence of undetermned parameters on the objectve functon Γ (4) s shown n Fg.. The relatve devatons of gas heatng value are shown n Fg.. Fg.. Objectve functon Γ (4) as a functon of undetermned parameters δ, δ and δ 3 Download Date //6 0:6 PM 79

8 J. Kalna, hem. Process Eng., 0, 3 (), The second set of expermental data was taken from the work presented by Narváez et al. (996). The data s presented n Table. The gasfed feedstock was pne sawdust of the followng characterstcs (dry bass): volatles 8 to 83%, fxed carbon 6 to 7%, ash 0.5 to.%; mass composton: %, H 5.7%, N 0. to 0.3%, O 44.%, S 0.03%; LHV 8000 to 8400 kj/kg. Agan the optmzaton procedure was appled to fnd the values of undetermned parameters. In ths case, however, the value of ε was not known so t was added to the set of decson varables. Fg.. Relatve devaton of heatng value of the gas as a functon of undetermned parameters δ, δ and δ 3 Table. Expermental data presented by Narváez et al. (996) Bomass mosture, %wt ER Freeboard temperature, º Bed temperature, º Dry tar free gas composton, %vol H O O H H N oncentraton of tar, mg/nm Gas yeld, Nm 3 /kg daf VLHV, MJ/Nm Stochometrc oxygen requrement, kmol/kmol of gas Download Date //6 0:6 PM

9 Modellng of fludzed bed bomass gasfcaton n the quas-equlbrum regme It was found that the mnmzaton of the objectve functon Γ (4) resulted n relatvely hgh devatons of the heatng value of the gas and stochometrc oxygen requrement for ts combuston. In most cases also carbon converson effcency was too optmstc. Therefore, the objectve functon was modfed to the followng form: Ve Vm VLHVe VLHVm Γ = mn V + e VLHV, (5) e The dea behnd ths modfed approach was to mnmze errors of the estmaton of the product gas chemcal energy flow. In ths way the energy balance of a plant s close to the realstc one. The results of the calculaton for a fxed value of freeboard temperature are gven n Table. Table. Predcted product gas characterstcs. Dry tar free gas composton, %vol H O O H H N oncentraton of tar, mg/nm Gas yeld, Nm 3 /kg daf VLHV, MJ/Nm Heat losses, % of fuel energy nput Stochometrc oxygen requrement Relatve devatons from data presented by Narváez et al. (996) H O O H H N oncentraton of tar, mg/nm Gas yeld, Nm 3 /kg daf VLHV, MJ/Nm Stochometrc oxygen requrement alculated values (EES genetc algorthm) δ δ δ ε Download Date //6 0:6 PM 8

10 J. Kalna, hem. Process Eng., 0, 3 (), In another test also the temperature of the process was added to the set of decson varables for optmzaton procedure. It was found that n both cases the relatve devatons from the expermental values of VLHV, gas yeld and stochometrc oxygen requrement were reduced. It was also concluded from the results that a correlaton between the calculated carbon converson effcency ε and equvalence rato ER can be establshed. Ths observaton s n agreement wth some publshed data L et al. (004). Therefore, the generated results were put together wth other values of carbon converson effcency that were collected from the lterature for wood gasfcaton (De Jong, 005; Mcco, 999; Van der Drft, 00). A sngle varable functon has been establshed and t s shown n Fg. 3. The lack of expermental data does not allow for better estmaton of nfluence of process parameters such as pressure, temperature, presence of n-bed catalyst and steam delvered on converson effcency. Fg. 3. arbon converson effcency of authothermal ar gasfcaton of wood n fludzed bed gasfer as a functon of equvalence rato A smlar correlaton of ε as a functon of ER was presented by L et al. (004) for an expermental FB gasfer. However, the correlaton developed n ths work results n hgher values of carbon converson effcency. The expermental data presented by De Jong (005) were used n another analyss. The gasfed feedstock conssted of wood pellets of the followng characterstcs (as receved bass): volatles 74.9%, fxed carbon 6.5%, ash 0.3%, mosture 8.4%; mass composton: 47.0%, H 6.5%, N 0.5%, O 46.%, S 0.%; HHV 8600 kj/kg. After the optmzaton usng the objectve functon (5) t was found that the varatons of the optmal values of undetermned parameters were: δ = 0. to 0.96, δ = 0.05 to 0.0 and δ 3 = 0.06 to It was not possble to establsh any reasonable correlaton between the observed values and excess ar rato λ. Therefore, t was fnally decded to use the average values from all the test runs of the model. These values are δ = 0.40, δ = 0. and δ 3 = A comparson between the expermental and modellng results, that were obtaned usng carbon converson effcency gven n Fg. 3 and the average values of δ, δ and δ 3, s presented n Table 3. The calculatons were made usng a gven expermental freeboard temperature. 8 Download Date //6 0:6 PM

11 Modellng of fludzed bed bomass gasfcaton n the quas-equlbrum regme Table 3. omparson of model results wth expermental data presented by De Jong (005) run ER Freeboard temperature, K Bed temperature, K Gasfcaton pressure, kpa Steam to ar mass rato Dolomte to fuel rato Expermental characterstcs of the product gas O, %vol H, %vol H 4, %vol H 4, %vol H 6, %vol O, %vol H O, %vol N, %vol Ar, %vol HHV, MJ/Nm ε Stoch. oxygen requrement alculated characterstcs of the product gas O, %vol H, %vol H 4, %vol H 4, %vol O, %vol H O, %vol N, %vol Ar, %vol HHV, MJ/Nm 3 (tar free wet gas) ε Stoch. oxygen requrement Relatve devatons between calculated and expermental values HHV, MJ/Nm ε Stoch. oxygen requrement Download Date //6 0:6 PM 83

12 J. Kalna, hem. Process Eng., 0, 3 (), Fg. 4. Dstrbuton of enthalpes of substances leavng the reactor calculated usng varous approaches towards equlbrum modellng Table 4. omparson of results from dfferent thermodynamc equlbrum based models Approach Full equlbrum Equlbrum wth ε Quas-equlbrum δ δ δ ER Product gas composton (tar free), %vol O H H H H O H O N Ar Tar free gas LHV, kj/nm ε Dry gas yeld, Nm 3 /kg db Process temperature, K Tar yeld g/nm 3 dry gas V VLHV V VLHV dg dg F dg dg Q V VLHV dg dg Q Download Date //6 0:6 PM

13 Modellng of fludzed bed bomass gasfcaton n the quas-equlbrum regme Eventually a comparson was made between dfferent equlbrum approaches towards modellng of a gasfcaton process. The reactor s authothermal and atmospherc ar at 98 K s used as the gasfcaton agent. The calculatons were performed for wood of the heatng value LHV db = 7680 kj/kg and mosture content of 5%. Accordng to Van der Drft et al. (00) and orella et al. (006) heat losses from the gasfer were assumed to be 3% of the total heat released. The results are presented n Table 4. The share of enthalpy of partcular outlet product n the total reactor outlet energy s presented n Fg. 4. It can be observed that dfferences between the results are qute consderable. It s mportant that the devaton n dry gas total enthalpy s at the level of 0-30 %, dependng on the modellng approach. It may have a sgnfcant nfluence on the results of an the examned energy system study. Table 5. omparson of modellng results wth expermental data presented by Hofbauer et al. (997) and Pfefer et al. (004). Expermental results Gasfcaton temperature, º In-bed catalyst none olvne N-olvne H, %vol N, %vol.79 n.a. n.a. O, %vol H 4, %vol O, %vol H 4, %vol H 6, %vol 0.55 n.a. n.a. hgher x H y, %vol 0.3 n.a. n.a. LHV, kj/nm V g, Nm 3 dry gas/kg of dry bomass n.a Results of calculaton δ δ δ H, %vol N, %vol O, %vol H 4, %vol O, %vol H 4, %vol H 6, %vol hgher x H y, %vol LHV, kj/nm V g, Nm 3 dry gas/kg of dry bomass V g estmated usng corelaton gven by Fercher et al. (998) The model was also used to smulate an FIFB process. The results presented by Hofbauer et al. (997) and Pfefer et al. (004) were used for tunng and valdaton of the model. In the work presented by Hofbauer et al. (997) the gasfed feedstock were wood chps of the followng characterstcs (wet Download Date //6 0:6 PM 85

14 J. Kalna, hem. Process Eng., 0, 3 (), bass): volatles 73.3%, fxed carbon 4.0%, mosture.%, ash 0.6%, LHV 5600 kj/kg; mass composton (dry bass): 5.5%, H 6.3%, N 0.%, O 44.3%, S 0.05%. Pfefer et al. (004) used wood chps of the characterstcs as follows (dry bass): 49.0%, H 6.5%, N 0.%, O 44.3%, S 0.05%; LHV 70 kj/kg. Durng the calculatons the temperature of the process was assumed the same as gven n the cted publcatons. Three values of carbon effcency were tested. The frst one was ε = 0.85 as suggested by Schuster et al. (00), the second one was ε = 0.89 as suggested by Fercher et al. (998) and the thrd one was calculated from the correlaton presented by Van der Mejden et al. (00) that resulted n sgnfcantly lower values. It was found that at the hghest value of ε the hghest devaton was observed between the reported and calculated dry tar free gas yeld values. At the lowest value of ε calculated usng the correlaton of Van der Mejden et al. (00) the gas yeld was n a relatvely good agreement wth the experment but the generaton of tar and the composton of the gas were radcally dfferent. At the value of ε = 0.85 the results of the calculaton were n a relatvely good agreement wth the measurements (for a system usng a nckel-olvne catalyst). The only relatvely hgh devaton was observed n the case of gas yeld. However, the calculated values were closer to the values of gas yeld estmated from the correlaton gven by Fercher et al. (998). The results are presented n Table 3. Fnally, for allothermal gasfcaton n a catalytc bed usng steam as the gasfcaton agent a sngle value of carbon converson effcency of 0.85 s suggested. There were not enough data to decde the values of δ, δ, δ 3 for ths process. For ntal studes the values gven n Table 5 can be used. 5. ONLUSIONS Thermodynamc equlbrum based models of fludzed bed bomass gasfcaton f not constraned result n too optmstc results. An overestmated product gas yeld and heatng value can lead to a sgnfcant error of fnancal proftablty analyss of a bomass-to-energy plant project. To correct results and forecast economc ndces wth a better accuracy t s recommended that gasfcaton reactors are modelled usng the constrant thermodynamc equlbrum approach. Four parameters can be specfed to perform quas-equlbrum calculatons. These are the amounts of the non-equlbrum products,.e. sold char, tar, H 4 and H 4. After tunng and valdaton of the model the average values of the constrants were proposed. It was found that predctons of the producer gas composton usng the quas-equlbrum approach very relatvely close to the values reported for wood by several references. It has to be, however, stressed that despte a consderable number of expermental and analytcal studes of fludzed bed bomass gasfcaton process only few publcatons contan data sutable for valdaton of models. Ths concluson s n lne wth observatons made by Gòmez-Barea et al. (00). Therefore, there s a need for future work amed at updatng the elaborated values of model parameters. It was observed n dfferent runs of the model that f the calculated content of H n the product gas ncreases, the content of O decreases as compared to the expermental values. As these gases have smlar heatng values and the same stochometrc oxygen requrement t can be concluded that equlbrum models are good for systems wth gas engnes and turbnes. In a possble analyss of a fuel cell system or gas combuston knetcs the results of equlbrum based calculatons can lead to more sgnfcant errors. Ths work was carred out wthn the frame of research project no. N N , ttled: Analyss and optmzaton of dstrbuted energy converson plants ntegrated wth gasfcaton of bomass. The project s fnanced by the Polsh Mnstry of Scence. 86 Download Date //6 0:6 PM

15 Modellng of fludzed bed bomass gasfcaton n the quas-equlbrum regme SYMBOLS ash mass fracton of ash n wet bomass, kg/kg a kj the number of partcles of element k (, O, H ) n the gas component j b ndcator of constrant equaton c mass fracton of carbon n wet bomass, kg/kg c heat capacty, kj/kg/k H bobnb3sb4 stochometrc formula of bomass H c O c N c3 stochometrc formula of char t H t O t3 stochometrc formula of tar molar fracton of steam n gasfcaton agent f g G g h h H O 0 0 T h 98 K gas Gbbs free energy, kj standard partal free enthalpy of pure component, kj/kmol mass fracton of hydrogen n wet bomass, kg/kg standard enthalpy of pure component, kj/kmol Δ physcal enthalpy of component at ts temperature T, kj/kmol l lqud LHV dry bass lower heatng value of bomass, kj/kg ls number of gaseous components m mass, kg M molecular mass, kg/kmol MLHV molar lower heatng value of gaseous product, kj/kmol n mass fracton of ntrogen n wet bomass, kg/kg wb n number of moles of component, kmol n prmary number of moles of component n bomass, kmol n number of moles of product, kmol o mass fracton of oxygen n wet bomass, kg/kg wb p pressure, kpa r heat of evaporaton of water, kj/kg H O R unversal gas constant 8.34 kj/kmolk s mass fracton of sulfur n wet bomass, kg/kg wb 0 s standard entropy of pure component, kj/kmolk T temperature, K Q n heat delvered to the process, kj Q out heat heat lost from the process, kj V producer gas yeld, Nm 3 VLHV volumetrc lower heatng value of the gas, kj/nm 3 w mass fracton of lqud water n wet bomass, kg/kg molar fracton of component n the mxture z Greek symbols δ model undetermned parameters ε converson effcency Γ, Γ objectve functons for optmzaton λ excess oxygen factor; Lagrange multplers chemcal potental of component μ Download Date //6 0:6 PM 87

16 J. Kalna, hem. Process Eng., 0, 3 (), subscrpts ash related to ash b, b, b 3,b 4 stochometrc numbers of mols of components H, O, N n bomass, kmol c, c, c 3 stochometrc numbers of mols of components H, O, N n char, kmol carbon char related to unconverted char db dry bass dg dry gas e related to expermental value eq resulted from equlbrum calculatons H hydrogen H O water element ndcator j phase or product ndcator k constrant ndcator m related to modeled value N ntrogen O oxygen O mn mnmum number of moles of oxygen requred for total combuston pg product gas S sulfur t, t, t 3 stochometrc numbers of mols of components H, O, N n tar, kmol wb wet bass REFERENES Asadullah M., Myazawa T., Ito S., Kunmor K., Tomshge K., 003. Demonstraton of real bomass gasfcaton drastcally promoted by effectve catalyst. Appl. atal. A: Gen., 46, DOI:0.06/S X(03) Bolhàr-Nordenkampf M., Rauch R., Bosch K., Acherng., Hofbauer H., 00a. Bomass HP Plant Güssng usng gasfcaton for power generaton. Proc. Internatonal onference on Bomass Utlsaton, Thaland. Bolhàr-Nordenkampf M., Bosch K., Rauch R., Kaser S., Tremmel H., Acherng., Hofbauer H., 00b. Scaleup of a 00 kwth plot FIFB-gasfer to a 8 MWth FIFB-gasfer demonstraton plant n Güssng (Austra). Proc. st Internatonal Ukranan onference on Bomass For Energy, Kyv, Ukrane. Brown D., Gassner M., Fuchno T., Maréchal F., 009. Thermo-economc analyss for the optmal conceptual desgn of bomass gasfcaton energy converson systems. Appl. Therm. Eng., 9, DOI:0.06/j.applthermaleng Buragohan B., Mahanta P., Moholkar V.S., 00. Thermodynamc optmzaton of bomass gasfcaton for decentralzed power generaton and Fscher-Tropsch synthess. Energy, 35, DOI:0.06/j.energy orella J., Orío, Aznar P., 998. Bomass gasfcaton wth ar n fludzed bed: Reformng of the gas composton wth commercal steam reformng catalysts. Ind. Eng. hem. Res. 37, DOI:0.0/e98054h. 3 orella J., Toledo J. M., Molna G., 006. alculaton of the condtons to get less than g tar/m n n a fludzed bed bomass gasfer. Fuel Process. Technol., 87, DOI:0.06/j.fuproc Delft Unversty of Technology, ycle-tempo 5.0. A program for thermodynamc modelng and optmsaton of energy converson systems. De Jong W., 005. Ntrogen compounds n pressursed fludsed bed gasfcaton of bomass and fossl fuels. PhD thess, TU Delft, Optma Grafsche ommuncate, Rotterdam (avalable at: De Souza-Santos M. L., 004. Sold fuels combuston and gasfcaton. Modelng, smulaton, and equpment operaton. Marcel Dekker Inc., New York, Basel. Fercher E., Hofbauer H., Fleck T., Rauch R., Veronk G., 998. Two years experence wth the FIFBgasfcaton process. Proc. 0th European onference and Technology Exhbton, Würzburg. Germany. 88 Download Date //6 0:6 PM

17 Modellng of fludzed bed bomass gasfcaton n the quas-equlbrum regme Gl J., orella J., Aznar M.P., aballero M. A., 999. Bomass gasfcaton n atmospherc and bubblng fludzed bed: Effect of the type of gasfyng agent on the product dstrbuton. Bomass Boenergy, 7, DOI:0.06/S (99) Gòmez-Barea A., Leckner B., 00. Modelng of bomass gasfcaton n fludzed bed. Prog. Energy ombuston Sc., 36, DOI:0.06/j.pecs Hofbauer H., Veronk G., Fleck T., Rauch R., 997. The FIFB gasfcaton process, In: Brdgwater, A.V., Boocock, D. (Eds.), Developments n thermochemcal bomass converson, Vol., Blacke Academc & Professonal, Glasgow, Kalna J., 00. Retrofttng of muncpal coal fred heatng plant wth ntegrated bomass gasfcaton gas turbne based cogeneraton block. Energy onvers. Management, 5, do:0.06/j.enconman Krov N.Y., 965. Specfc heats and total heat contents of coals and related materals at elevated temperatures. BURA Monthly Bulletn, 9, Klmantos P., Koukouzas N., Katsadaks A., Kakaras E., 009. Ar-blown bomass gasfcaton combned cycles (BG): System analyss and economc assessment. Energy, 34, DOI:0.06/j.energy Kurkela E., Kurkela M., 009. Fludzed-bed gasfcaton of bomass for syngas applcatons: omparson of gasfcaton process alternatves. Proc. Internatonal onference on Thermochemcal onverson Scence "tcbomass009". hcago, USA. L X.T., Grace J.R., Lm.J., Watknson A.P., hen H.P., Km J.R., 004. Bomass gasfcaton n a crculatng fludzed bed. Bomass and Boenergy, 6, DOI:0.06/S (03) Mevssen N., Schulzke T., Unger.A., Mac an Bard S., 009. Thermodynamcs of autothermal wood gasfcaton. Envron. Prog. Sustan. Energy, 8, 3, DOI: 0.00/ep Mcco F., Moersch O., Splethoff H., Hen K.R.G., 999. Generaton and converson of carbonaceous fne partcles durng bubblng fludsed bed gasfcaton of a bomass fuel. Fuel, 78, DOI:0.06/S006-36(99) Mlne T.A., Evans R.J., Abatzoglou N Bomass gasfer Tars : Ther nature, formaton, and converson. Report No. NREL/TP Natonal Renewable Energy Laboratory, Golden, olorado, USA. (avalable at: Narváez I., Orío A., Aznar M. P., orella J., 996. Bomass gasfcaton wth ar n an atmospherc bubblng fludzed bed. Effect of sx operatonal varables on the qualty of the produced raw gas. Ind. Eng. hem. Res., 35, 0-0. DOI: 0.0/e Pfefer., Rauch R., Hofbauer H., 004. Hydrogen-rch gas producton wth a catalytc dual fludsed bed bomass gasfer. Proc. nd World onference and Technology Exhbton on Bomass for Energy, Industry and lmate Protecton, 0-4 May, Rome, Italy. Prns M. J., Ptasńsk K. J., Janssen F.J.J.G., 007. From coal to bomass gasfcaton: omparson of thermodynamc effcency. Energy, 3, DOI:0.06/j.energy Rchard N., Thunman H., 00. General equatons for bomass propertes. Project report. (avalable at: Schuster G., Löffer G., Wegl K., Hofbauer H., 00. Bomass steam gasfcaton an extensve parametrc study. Boresour. Technol., 77, DOI:0.06/S (00)005-. Thunman H., Nklasson F., Johnsson F., Leckner B., 00. omposton of volatle gases and thermochemcal propertes of wood for modelng of fxed or fludzed beds. Energy Fuels, 5, DOI: 0.0/ef00097q. Van der Drft A., Van Doorn J., Vermeulen J.W., 00. Ten resdual bomass fuels for crculatng fludzed-bed gasfcaton. Bomass Boenergy, 0, DOI:0.06/S (00) Van der Mejden.M., Vernga H.J., Rabou L.P.L.M., 00. The producton of synthetc natural gas (SNG): A comparson of three wood gasfcaton systems for energy balance and overall effcency. Bomass Boenergy, 34, DOI:0.06/j.bomboe Wu., Yn X., Ma L., Zhou Z., hen H., 008. Desgn and operaton of A 5.5 MWe bomass ntegrated gasfcaton combned cycle demonstraton plant. Energy Fuels,, DOI: 0.0/ef Download Date //6 0:6 PM 89