Parametric Investigation of a Downdraft Gasifier Using Numerical Modelling for Different Biomass Materials in Myanmar

Transcription

1 S. Han, M. M. Oo, and T. M. Htike / GMSAN Intenational Jounal 1 (018) Paametic Investigation of a Downdaft Gasifie Using Numeical Modelling fo Diffeent Biomass Mateials in Myanma Sithu Han *, Mya Mya Oo, and Thein Min Htike Abstact Myanma is an agicultual county and biomass mateials such as wood, bamboo, ice husk and ice staw ae the most abundant biomass esouces. Theefoe, thee is a huge potential of biomass gasification, which can povide the powe to the ual population which ae still unelectified. It has also been epoted that thee have been some applications of downdaft gasifie fo this pupose in some pats of the county. Howeve, the widespead application is still limited because of the need to futhe investigate its pefomance and envionmental concen. In this study, the pefomance of diffeent biomass species in Myanma is investigated using the numeical model of downdaft gasifie unde diffeent opeating conditions. A one-dimensional model of a downdaft gasifie is developed by consideing both pyo-oidation zone and eduction zone. Fistly, species compositions (Cha, CO, CO, H O, H, CH and N ) in pyo-oidation zone have been solved consideing the chemical equilibium of the species at a given tempeatue of gas. Net, compositions obtained fom the chemical equilibium model wee fed to the kinetic model as initial inputs. Then, the eduction zone has been solved based on the chemical kinetic of eduction eactions by the cha consideing along the length of the zone to pedict the final gas compositions. The esults ae veified with the values fom the liteatue. Finally, the model was applied to investigate the potential of fou diffeent biomass mateials (wood, bamboo, ice husk and ice staw) available in Myanma unde diffeent opeating conditions such as equivalence atios and biomass moistue contents. It was found that all the fou biomass fuels in Myanma can be used in downdaft gasifie to poduce the combustible gas which has a potential application in intenal combustion engines. Keywods Biomass, downdaft gasifie, poduce gas, eduction zone, numeical modelling. 1. INTODUCTION As Myanma is planning pope enegy mi to fulfil gowing demand, moe investigation on the potential of diffeent enegy esouces should be done inclusively to find possible altenatives. Among available enewable esouces [1], biomass is paticulaly abundant waste mateials ecept its application as animal feed in the ual aeas. Most of biomass mateials ae found as bypoduct in Myanma since main souces of evenue in Myanma is agicultue, timbe logging and animal eaing. Theefoe, biomass would be the most viable fom of enegy in Myanma and thee have been some applications of biomass gasifies fo ual electification and SME sectos []. Although othe enewable enegy esouces such as sola and wind enegy ae being planned in Myanma and studied [3], investigation of potential of biomass gasification fo electification is still needed especially focusing on bette undestanding of biomass gasification inside the eacto. Biomass gasification is a pocess of convesion of solid cabonaceous fuel into combustible gases (known as poduce gas with caloific value) by patial Sithu Han is with the Chemical Engineeing Depatment, Yangon Technological Univesity, P.O. Bo 11011, Gyogone, Insein Tsp, Myanma. Mya Mya Oo is with Yangon Technological Univesity, Myanma. She is now with the Chemical Engineeing Depatment. Thein Min Htike is with Yangon Technological Univesity, Myanma. He is now with the Depatment of Mechanical Engineeing. * Coesponding autho: Sithu Han; Phone: ; E- mail: sithuhan76@gmail.com. combustion. This poduce gas can be used to un small ecipocating engines, boile, pocesses heates. Inside a gasification unit, fou pocesses can be identified: dying, devolatilization (pyolysis), combustion and eduction. In the dying pocess the feedstock is heated and its tempeatue inceases, thus wate undegoes vapoization. Devolatilization occus as the tempeatue of feedstock ises, and pyolysis take places conveting feedstock into cha and hydocabon gases. eduction o gasification is the esults of seveal chemical eactions involving cabon, steam, hydogen and cabon dioide among othes. The combustion pocess povides the themal enegy equied fo the gasification pocess, by consuming some of the cha o dy feedstock and in some cases the volatiles within the gasifie []. Modelling of gasification system is of geat impotance in ode to pedict optimum opeating conditions and pocess paametes to obtain acceptable poduce gas quality. Diffeent simulation models used fo this pupose may be classified into themodynamic equilibium model, kinetic model, computational fluid dynamic model and atificial neual netwok. Giltap et al. have investigated kinetic-based eaction model fo the eduction zone of a downdaft gasifie to pedict the composition of the poduce gas unde steady-state condition [5]. A constant cha eactivity facto was used in his study. Assuming cacking of pyolysis poducts into equivalent amount of CO, CH and H O limited the accuacy of this model and esulted in an ove pediction of methane faction at the gasifie outlet. Babu and Sheth modified Giltap s model by incopoating a vaiation of the cha eactivity facto [6]. Finite diffeence method was applied to pedict the 17

2 S. Han, M. M. Oo, and T. M. Htike / GMSAN Intenational Jounal 1 (018) tempeatue and composition pofiles along the eduction zone of the downdaft gasifie. It was obseved that an eponential vaying of the cha eactivity facto gave the bette esult fo both tempeatue and gas composition when compaed to the epeimental data of Jayah et al. [7]. A. K. Shama has developed the themodynamic equilibium and kinetic modelling of cha eduction eactions in a downdaft biomass gasifie in ode to pedict the status of un-conveted cha in addition to gas composition, caloific values, convesion efficiency, eit gas tempeatue by compaing these two models [8]. In anothe study, oy et al. have built a numeical model of downdaft gasifie based on chemical equilibium in the pyo-oidation zone and finite ate kinetic-contolled chemical eactions in the eduction zone [9]. It was epoted that the model pediction ageed well with the tempeatue distibution and gas composition obtained fom the epeiments at cha eactivity facto fo 100. It was found that the heating value of the poduce gas inceased with the incement in equivalence atio and deceased in the biomass moistue content. In the pesent wok, the main focus is set on the investigation of potential of fou available biomass esouces in Myanma whose compositions have been studied by Min Lwin Swe [1]. Fo the analysis, two sepaate sub-models in the downdaft gasifie - one fo pyo-oidation zone and anothe fo eduction zone following the concept by oy et al. [9]. The sub-model fo pyo-oidation zone is based on poposal by Zianal et al. [10] and the sub-model fo eduction zone is studied based on poposals by Giltap et al. [5] and Babu and Sheth [6]. The models used in this wok ae discussed in the second pat, methodology. A paametic investigation is also pefomed to study the effects of equivalence atio and biomass moistue content on the gas composition and lowe heating value. esults ae epoted and discussed on the thid pat of the pape and concluded in the last pat.. METHODOLOGY.1 Model Fomulation The pesent model consists of two distinct zones: pyooidation zone whee the dying, pyolysis and oidation eactions occu, and eduction zone whee the gasification o eduction eactions take place in the downdaft gasifie. The sub-model fo the pyo-oidation of the downdaft gasifie has been solved based on the chemical equilibium appoach consideing wate gas eaction and methanation eaction, as the eactions occu at a easonably high tempeatue thee. The composition of diffeent species (C, CO, CO, H O, H, CH and N ) and the tempeatue of the gas which flows fom the pyo-oidation zone to the eduction zone ae obtained. The sub-model fo the eduction zone is established based on chemical kinetics of eduction eactions by cha consideing along the length of zone to pedict final gas composition.. Sub-model fo pyo-oidation zone In the pesent study, this pyo-oidation step is modelled using a single step global eaction as follows; CH O who mo 3. 76mN 1 3 (1) H O H CH N C CO CO whee, α and β ae the numbe of atoms of hydogen and oygen pe numbe of atom of cabon in the biomass obtained by ultimate analysis using empiical data studied by Min Lwin Swe fo Myanma biomass as shown in Table 1 [1]; m is the es of oygen in ai and w is the amount of moistue content in biomass; 1,, 3,, 5, 6 and 7 ae the numbe of e of poduct species: C, CO, CO, H O, H, CH and N espectively. Since the numbe of e fo N is known, thee ae si unknown species in the poduct. Moeove, numbe of e of O, m is unknown in the eactant. Hence, altogethe, thee ae seven unknown species epesenting the si unknown species of the poduce gas and one unknown fo oygen content fo the eaction. Necessay equations fo solving these unknowns wee geneated by using mass balance, equilibium constants elationships and enegy balance. Balancing each chemical species in Eq. (1) esults in; C : 1 () H : m w (3) O: w () N : m (5) In this model, the themodynamic equilibium was assumed fo all chemical eactions in the gasification eaction. All gases wee assumed to behave ideally and all eactions fom at 1 atm pessue. Nitogen is consideed to be inet, but it dilutes the final enegy of density of gas. The equilibium modelling is developed based on equilibium constants fo each eaction in equations, (6) and (8). Wate gas shift eaction CO 1 (6) g HO g CO g Hg ; H K (7) Methanation eaction C 75 (8) s Hg CHg ; H 6 5 K (9) Two equilibium equations ae equied to detemine the equilibium constants K 1 and K in equations, (7) and (9). The equilibium constant is a function of tempeatue only fo each eaction consideed and thei epessions ae deived using data epoted in Zainal et al. [10]. The enegy balance fo gasification pocess was then 18

3 S. Han, M. M. Oo, and T. M. Htike / GMSAN Intenational Jounal 1 (018) incopoated into the equilibium model. The kinetic and potential enegy changes in the steam wee neglected and the steady state enegy flow equation was applied. Theefoe, the heat balance fo this pocess, assumed to be adiabatic was in Eq. (10); H H fch H eactan ts poducts (10) H f biomass wh fh O( l) H fho( g ) H fco 3H fco H fho( g ) 6 C T 5C pco C ph 3 pco C 6 pch C pho 3.76mC pn (11) C 173 ( s) CO g CO g ; H CO 1 cfk1 CO Keq,1 : Wate gas eaction c k CO H F H O Keq, C 131 s HO g CO g Hg ; H 3: Methanation eaction. (1) (13) whee, H f is the enthapy of fomation in kj/k and its value is zeo fo all chemical elements at standad tempeatue and pessue. T epesents the tempeatue diffeence between any given state and at efeence state. C p is the specific heat at constant pessue in kj/k K and is a function of tempeatue. The system of the equations (-5), (7), (9) and (11) has been pefomed to evaluate the gas factions and the tempeatue at the end of the pyo-oidation zone. System of equations wee solved by using Newton s Jacobi iteation in MATLAB. The esults of this sub-model wee then tansfeed to the kinetic model of the eduction zone as input bounday conditions. Table 1. Ultimate Analysis fo Diffeent Biomasses Available in Myanma (Dy Basis, Weight Pecentage) [1] Biomasses C H O N S ice Husk Bamboo Wood ice Staw Sub-model fo eduction zone The model fo eduction zone is based on the kinetically contol chemical eactions within the downdaft gasifie opeating unde steady-state conditions. It was oiginally pesented by Giltap et al. []. Duing the eduction pocess, the five simultaneous eactions 1 to 5 take place. These eactions ae selected fom the pevious eseach wok of Shama [8]. eaction 5 (wate gas shift) can be neglected as it does not affect the ate fo that paticula moistue to biomass atio, howeve, hee in this study the shift eaction is included as one pat of gasification eactions. Table shows the values of the fequency factos and activation enegy fo each eaction [11]. The cha eactivity facto was intoduced by Babu and Sheth [6] to the model by Giltap et al. [5]. The value of the cha eactivity facto may vay between 1 and 1000 and sometimes eponentially and linealy, fo the pesent model it is taken as : Boudouad eaction C 75 s Hg CHg ; H CH 3 cfk3 H Keq,3 : Steam efoming eaction CH 06 H g O g CO g 3H g ; H cfk CH. H O 3 CO. H Keq, 5: Wate gas shift eaction CO 1 g HO g CO g Hg ; H 5 cfk5 CO. HO CO. H Keq,5 (1) (15) (16) Table. Fequency Factos and Activation Enegy [11] eactions A i (1/s) E i (kj/) The sub-model fo the eduction zone assumes a cylindical fom of eduction zone with a unifom cosssection in the aial diection and neglects vaiations of gas popeties in the adial diection. The a (fo si gas species) and enegy balances esult in the following set of nine diffeential equations with the coesponding numbe of unknown paametes [7]. dv i dn 1 V n (17) dt V 1 n c i H i c T V dp P dv (18) 19

4 S. Han, M. M. Oo, and T. M. Htike / GMSAN Intenational Jounal 1 (018) dv dp 1 n c n 1183 n c H i i i n T V n c dp V T P V V gas ai c (19) (0) The set of nine diffeential equations was integated using MATLAB pogam based on the fouth-ode unge-kutta method to obtain concentations of si gas species, the tempeatue, the velocity and pessue along the length of the eduction zone. 3. ESULTS AND DISCUSSION 3.1 Model Validation The pesent model was validated with Jayah s epeimental data fom the liteatue [7] at the same paametes as shown in Table 3. Table shows compaison between the pesent model and epeimental esults by dy basis fo poduce gas. The compositions of the poduce gas pedicted by pesent model wee in good ageement with that of the poduce gas adopted by Jayah s epeimental data, ecept that fo hydogen content. The hydogen content pedicted is moe than epeimental value, while maimum discepancy fo the othe species content is less than 0%. Table 3. Paametes used fo Model Validation with Jayah s Epeimental Data [7] Bed length (z) Paametes Supeficial Velocity (V) Initial Tempeatue (T) Pessue (P) Moistue Content (MC) 0.75 m Values m/s 100 K 1atm Cha eactivity Facto (CF) % (Dy Basis) Table. Compaison of the Pesent Model esults with Jayah s Epeimental Data [7] Species Jayah (Epeiment) Pesent Model Pecentage discepancy CO CO H CH N Fig. 1 pesents the tempeatue vaiation along the length of the eduction zone of the gasifie, pedicted fom the pesent model. It can be seen obviously that tempeatue declines along the length of the eduction zone as the eduction eactions ae endothemic eactions in natue. In addition, tempeatue decays vey fast ove a shot distance fom the inlet of the eduction zone. Howeve, afte the tempeatue eaches 95 K, at a distance of 0. m fom the inlet, it hadly dops any futhe in the emaining length of the gasifie because of the fact that all almost all the cha enteing the eduction zone gets consumed within the initial length of 0. m, leading to almost no futhe eduction eaction in downsteam. Fig. 1. Vaiation of tempeatue along the length of the eduction zone. 3. Influence of moistue content (MC) on poduce gas Fig. shows the effects of moistue content in the biomass feedstock (ice husk) on the composition of poduce gas pedicted by the pesent model at E=0.3. The moistue content is vaied fom 0% to 0% and the gas composition is pesented on dy basis. It was obseved that the content of hydogen and cabon dioide in the poduce gas incease with the incease in moistue content, while the cabon monoide decease significantly. The tend in the vaiation of H, CO and CO concentation in the poduce gas at diffeent moistue content of the biomass was attibuted to the fact that eaction contibuted moe than eaction 1 with highe moistue content in conveting the cha in the eduction zone. eaction inceases the fomation of hydogen and poduces one e of CO instead of two es of CO as in case of eaction 1. Theefoe, the CO concentation is educed as the eaction pevailed ove the eaction 1. The concentation of CO in the poduce gas inceases because of the wate-gas homogeneous eaction which completed the combustion of some cabon monoide and poduced hydogen and cabon dioide. The concentation of CH vaies with moistue content accoding to eaction 3 and eaction. Although nitogen is an inet gas, the faction of its concentation is deceasing because the concentations of the othe gases ae changing in the poduce gas. 130

5 S. Han, M. M. Oo, and T. M. Htike / GMSAN Intenational Jounal 1 (018) Fig.. Effects of moistue content on poduce gas fo ice husk at E = Influence of equivalence atio (E) on poduce gas Fig. 3 depicts the effect of equivalence atio on the gas composition, pedicted by the pesent model at MC = 0.. The equivalence atio has been consideed in the ange of 0.1 to 0.5. It was seen that the content of hydogen, cabon dioide and methane slightly decline with the incement in equivalence atio, while the cabon monoide and nitogen incease appaently. A highe value of E epesents a highe amount of ai which leads to moe amount of CO poduction in pyooidation zone and moe amount of N enty with the ai flow. With an incease in E fom 0.1 to 0.5, inceased amount of CO in pyo-oidation zone was conveted into CO accoding to eaction 1 in the eduction zone. poduce gas and ae esponsible fo lowe heating value. It was obseved that the effect of inceasing moistue content was the decease in LHV of the poduce gas. This tend can be eplained that eduction in cabon monoide content was moe than the incease in hydogen content as the incease of moistue content. Fo the moistue content highe than 0., Lowe heating values of wood, bamboo and ice staw ae highe than that of ice husk. The heating values of poduce gas fo wood, bamboo, ice husk and ice staw ae between 6.3MJ/Nm 3 and 7.MJ/Nm 3 which ae heating values fo poduct gas based on gasifying agent, ai as the incease in the ange of moistue content studied. The natue of the vaiation of LHV shows that the eduction in LHV with the incease in moistue is moe significant at the highe moistue level. Theefoe, the quality of the poduce gas is pogessively pooe in lowe heating value as the moistue content inceases. Howeve, heating values fo all fou types of biomass fuel ae in the ange of to 7MJ/Nm 3 which is typical heating value of poduce gas necessay fo using it as a fuel in intenal combustion engine. Fig.. Effects of moistue content on LHV of poduce gas fo diffeent biomasses. Fig. 3. Effects of equivalence atio on poduce gas fo ice husk at MC = Influence of moistue content (MC) on LHV of poduce gas The vaiation of lowe heating value (LHV) fo the fou diffeent biomasses against moistue content is shown in Fig.. The heating value is calculated using the composition of the poduce gas. Cabon monoide, methane and hydogen ae the main components of the 3.5 Influence of equivalence atio (E) on LHV of poduce gas Fig. 5 epesents the lowe heating values fo all biomasses ae function of the vaiation of equivalence atio with moistue content of 0%. As seen in the figue, the pedicted heating values fo all biomasses incease with the equivalence atio. The faction of CO content and N content significantly ise up with the ise in equivalence atio, while the content of H, CO and CH wee dopped. This behaviou can be claified that the incease in cabon monoide content is moe than the eduction in hydogen content as incease in equivalence atio. At any given equivalence atio, the wood, ice staw and bamboo have the highest heating value and the amount of heating values of poduce gas fo ice husk is the lowest. This is because of the fact that the content of cabon pecent in feedstock of wood, bamboo and ice staw is highe than that of ice husk. Howeve, lowe heating value of these biomass mateials ae lying within typical ange of heating value ( to 7MJ/Nm 3 ) fo poduce gas based on gasifying agent, ai. 131

6 S. Han, M. M. Oo, and T. M. Htike / GMSAN Intenational Jounal 1 (018) Fig. 5. Effects of equivalence atio on LHV of poduce gas fo diffeent biomasses.. CONCLUSION The numeical model has been developed fo the simulation of the pefomance of a downdaft gasifie consideing two sepaate zones: pyo-oidation zone and eduction zone to pedict the final gas compositions and lowe heating values. A paametic study has been pefomed with one dimensional model at diffeent equivalence atio and moistue content. The compositions and lowe heating values of the poduce gas wee detemined fo fou diffeent biomasses available in Myanma; wood, bamboo, ice staw and ice husk. Fom the analysis, it was concluded that the incease in moistue educed the CO faction in the poduce gas and incease the H and CO.The CH content emained nealy constant with the change in moistue content. The incease in moistue content fo all aw mateials deteioated the quality of the poduce gas and its lowe heating value educed. The pedicted lowe heating value fo all aw mateials was inceasing with the equivalence atio. The heating values of biomass mateials with ai as a gasifying agent in this study ae to 7MJ/Nm 3, which is in ageement with the values epoted in [1]. Moeove, the study by Kaupp and Goss [13] also epoted that poduce gas with such lowe heating value can be used as fuels in ecipocating intenal combustion engines. Theefoe, it can be implied that poduce gas that can be geneated using fou biomass fuels in Myanma poposed by this study can be used in ecipocating intenal combustion engines. Based on the esults in this study, it can be concluded that all biomasses available in Myanma has high potential to be used as fuel in biomass gasification and some othe biomass esouces should be studied in futue woks. Moe epeimental investigations should also be done to fully veify its pefomance in intenal combustion engines. EFEENCES [1] Min Lwin Swe, 009. Distibuted Powe Geneation fom ice Husk Gasification in ual Myanma. M.E Thesis, Chiang Mai Univesity, Thailand. [] amchanda Pode, Gayati Pode, and Bouca Diouf, 016. Solution to Sustainable ual Electification in Myanma. enewable and Sustainable Enegy ev. 59, [3] Thaakan P., and Achaya J. S., 01. Scoping offgid enewable enegy oppotunities in Myanma: Mandalay egion and Chin State. Asian Development Bank (TA63-EG, TA-751) [] Basu, P., 010. Biomass Gasification and Pyolysis: Pactical Design and Theoy. Elsevie, USA. [5] D. L. Giltap et al., 003. A Steady state model of gas-cha eactions in a downdaft gasifie. Sol Enegy, 7: [6] B. V. Babu and P. N. Sheth, 006. Modelling and simulation of eduction zone of downdaft biomass gasifie: Effect of Cha eactivity facto. Enegy Conves Manage; 7; [7] Jayah T.H., Aye L., Fulle. J., and Stewat D. F., 003. Compute Simulation of a Downdaft Wood Gasifie fo Tea Dying. Biomass Bioenegy; 5; [8] A. K. Shama, 008. Equilibium and kinetic modelling of cha eduction eactions in a downdaft biomass gasifie: A compaison. Sola Enegy; 8; [9] P. C. oy, A. Datta, and N. Chakaboty, 013. Kinetic model of the eduction zone of a downdaft biomass gasifie. ICETSD; ISSN 50-59; volume 3; [10] Z. A. Zainal,. Ali, C. H. Lean, K. N. Seethaamu, 001. Pediction of pefomance of a downdaft gasifie using equilibium modelling fo diffeent biomass mateials. Enegy Conves Manage; ; [11] Sameen Hameed, Naveed amzan, Zaka-u ahman, Muhammad Zafa and Sheema iaz, 01. Kinetic Modeling of eduction zone in biomass gasification. Enegy Conves Manage; 78: [1] KAUPP, A., and GOSS, J.. (198). Chapte IV, Small Scale Gas Poduce-Engine Systems. [13] KAUPP, A., and GOSS, J.. (198). Chapte VII, Small Scale Gas Poduce-Engine Systems. 13