Energy Conversion and Management 88 (2014) 693–699
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Energy Conversion and Management j o u r n a l h o m e p a g e : : w w w . e l s e v i e r . c o m / l o c a t e / e n c o n m a n
Date palm waste gasification in downdraft gasifier and simulation using ASPEN HYSYS M. Bassyouni Bassyouni a,b, Syed Waheed ul Hasan a, M.H. Abdel-Aziz a,c, , S.M.-S. S.M.-S. Abdel-hamid Abdel-hamid b, Shahid Naveed d, Ahmed Hussain e, Farid Nasir Ani f ⇑
a
Department of Chemical and Materials Engineering, King Abdulaziz University, Rabigh 21911, Saudi Arabia Department of Chemical Engineering, Higher Technological Institute, Tenth of Ramdan City, Egypt c Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt d Punjab Institute of Contemporary Sciences, 5.5 KM Raiwind Road, Lahore, Pakistan e Department of Nuclear Engineering, King Abdulaziz University, Jeddah 21589, Saudi Arabia f Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, UTM 81310 Johor Bahru, Malaysia
b
a r t i c l e
i n f o
Article history: Received 23 April 2014 Accepted 27 August 2014
Keywords: Biomass gasification Downdraft gasifier ASPEN HYSYS Simulation Steam to biomass ratio
a b s t r a c t
The present research research aims to study the simulation of date palm waste gasification gasification using ASPEN HYSYS. A steady state simulation of downdraft gasifier firing date palm leaves has been developed. The model is able to predict syngas composition with sound accuracy and can be used to find optimal operating conditions ditions of the gasifier. gasifier. Biomass Biomass is defined defined as an unconv unconventi entiona onall hypothe hypothetica ticall solid compon component ent in HYSYS. HYSYS. A set of six reacto reactorr mode models ls simula simulates tes vario various us react reaction ion zone zoness of the down downdra draft ft gasifi gasifier er in accord accordan ance ce with with its hydrody hydrodynam namics. ics. Biomass Biomass decompo decompositio sition n into constitue constituents nts in the pyrolys pyrolysis is zone zone is modeled modeled with with a converconversion reactor. The combustion of char and volatiles in the combustion zone are modeled with equilibrium and Gibbs reactor models respectively. The gasification zone is modeled with a Gibbs and equilibrium reactor. The results of simulation are validated against experimental results of a parametric variability study on a lab scale gasifier. The proportion of synthesis gas increase as temperature increases (concentration, tration, molar fraction fraction,, and partial pressure pressure). ). CO2 and CH4 in the product product gases were also found found to decreas decrease e with with increasi increasing ng temper temperatur ature. e. At 800 C, the exit gas reaches reaches a stable stable molar molar composition composition (H2 = 56.27% 56.27%,, CO = 21.71% 21.71%,, CO2 = 18.24% 18.24%,, CH4 = 3.78%). 3.78%). Increasin Increasing g steam steam to biomass biomass ratio increase increasess CO2 and and H2 at the expen expense se of CO, CO, govern governed ed by shift shift react reaction ion.. Steam Steam indu inducti ction on incre increase asess the the metha methane ne conten contents, ts, thereby improves the heating value of the product gas. 2014 Elsevier Ltd. All rights reserved.
1. Introduction The world is shifting to renewable sources of energy owing to depleting oil reserves Aleklett et al. [1] [1],, unpredictable supply and price of petroleum [19] petroleum [19],, and high CO2 concentrations [14] concentrations [14] causing causing greenhouse effect. Biomass; the fourth largest fuel source on earth [29] is seen seen to have have a major ajor shar share e in futu future re ener energy gy supp supply ly due due to its its abundance abundance and renewable renewable nature [4] [4].. Carbon Carbon dioxide dioxide produc produced ed during energy generation is soon consumed by the growing biomass during photosynthesis, photosynthesis, making the fuel carbon neutral. neutral. As biomass is diverse in nature, it is converted into different biofuels depend depending ing upon upon its chemic chemical al compos compositio ition. n. Biomas Biomasss contain containing ing sugar sugar and starch starch are usually usually saccharifi saccharified ed into bioetha bioethanol nol [35], [35], ⇑ Corresponding author at: Department of Chemical and Materials Engineering, King Abdulaziz University, Rabigh 21911, Saudi Arabia. E-mail address:
[email protected] (M.H.
[email protected] (M.H. Abdel-Aziz).
http://dx.doi.org/10.1016/j.enconman.2014.08.061 0196-8904/ 2014 Elsevier Ltd. All rights reserved.
whereas having oils and fatty acids are converted into biodiesel [3].. Lignocellulosic biomass is usually combusted or gasified into [3] synthesis gas (CO and H 2). Lignocellulosic biomass can be hydrolyzed to bioethanol as well but this process is not cost effective yet and is maturing for industrialization [7,21,22] [7,21,22].. Gasification is a thermochemical process to convert biomass into synthesis gas, which can be used directly to run engines engines or can be converted converted into liquid fuels via Fisher–Tropsch Fisher–Tropsch process [6] [6].. Hydrogen Hydrogen separated from from synth synthesi esiss gas gas can be used used to powe powerr fuel fuel cells, cells, which which are highly highly efficien efficientt and environ environmen mental tally ly friendl friendly y compar compared ed to gasoline gasoline engines. Saudi Arabia is among the largest date producing countries of the world having more than 22 million date palm (Phoenix dactylifera) trees [15]. [15]. A date palm tree produces produces around around 20 kg of dry leave leavess a year year [2] [2],, contr contribu ibutin ting g to an annu annual al prod produc uctio tion n of 440 440 thou thou-sand tons of date palm leaves (DPL) waste. This waste is burnt in farmlands farmlands which causes environmen environmental tal problems problems [16]. [16]. DPL DPL can can
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be gasified to synthesis gas which can further be processed into liquid fuels, adding to oil exports of the country while managing the waste dumping and burning issues. During During gasification, biomass is reacted with a limited amount of oxygen in the presence of a gasifyin gasifying g agent agent (steam, (steam, air or pure pure oxygen oxygen). ). The oxygen oxygen in air oxidizes a portion of biomass, generating heat which helps to mainta maintain in the gasifier gasifier temper temperatu ature re and drives drives endoth endotherm ermic ic gasifica gasifica-tion tion reacti reaction ons. s. The The heat heatin ing g valu value e of syng syngas as depe depend ndss on the the gasifi gasificacation medium medium used; used; steam steam gasifica gasification tion results results in syngas syngas with with a heating heating value value of 10–18 MJ/N MJ/N m3 [5] [5].. In Europe, Canada and U.S., around 75 percent of the gasification plants are using downdraft gasifiers [18] [18].. The motivation lies in its compatibility compatibility with internal combustion engines engines (ICE), low tar contents contents (0.015–3.0 g/m3), and short startup time. The objective of this research is to develop a steady state simulatio lation n of a down downdr draft aft bioma biomass ss gasifi gasifier er to pred predict ict the the comp compos ositi ition on of the resulting syngas using a commercial process simulator and to perform a sensitivity analysis of the simulation. ASPEN HYSYS is used used to deve develop lop the the simul simulati ation on.. A few few studie studiess are are avai availab lable le on simsimulation ulation of biomass biomass gasificat gasification ion using using ASPEN ASPEN HYSYS HYSYS [23,25,11]. [23,25,11]. ASPEN PLUS is the usual choice for simulation of biomass gasification [24,26,8,9,36 [24,26,8,9,36]]. The reason is that ASPEN PLUS can better handle solid components compared to ASPEN HYSYS as it has inbuilt library models for solid properties calculations. calculations. Moreover, Moreover, the customization and user defined operations are easier to develop in ASPEN PLUS as it uses the FORTRAN code; a customary language for numerical calculations. Whereas, ASPEN HYSYS is powered by Visual Basic which is mostly used for software development.
2. Experimental setup The schematic of downdraft gasifier used in this study representing reactions reactions and temperatures temperatures of different different zones is illustrated illustrated in Fig. in Fig. 1. 1. In the drying drying zone, zone, biomass biomass is heated heated around around 150 C which which removes moisture contents. Passing down to pyrolysis zone, DPL start startss to brea break k downat downat 180 180 C into into charcoa charcoal, l, non-co non-conde ndensa nsable ble gases gases (H2, CH4, CO, CO2, H2O), and tars (condensable higher hydrocarbons). Pyrolysis zone has a limited oxygen supply from the lower bed so pyrol pyrolysi ysiss takes takes place place in a fuel fuel rich flame, flame, also called called flamin flaming g pyrolysis. Tars are burned in the combustion zone producing heat and the remaining amount cracks into lower hydrocarbon while
Table 1
Gasification reactions. Name of reaction
Reaction
Incom Incomple plete te oxidat oxidatio ion n Oxidation Water gas Boudouard Shift Hydro drogasi gasifi ficatio ation n Ammo Ammoni nia a form format atio ion n Hydrogen sulfide formation
C + 0.5O 0.5O 2 CO C + O2 CO2 C + H2O C O + H2 C + CO CO2 2CO CO + H2O CO2 + H2 C + 2H2 CH4 N2 + 3H2 2NH3 H2 + S H2S ?
?
?
?
?
?
?
?
Heat of reaction (25 C) (kJ/mol)
Number
283 394 +131 +172 41.2 74.8 46.1 21
G-1 G-2 G-3 G-4 G-5 G-6 G-7 G-8
passing over hot ash and unconverted charcoal at the bottom of the gasifiers gasifiers,, resultin resulting g small small tars in synthe synthesis sis gas compare compared d to other other gasifier designs. Combustion zone (800–1500 C) lies sandwiched between pyrolysis and gasification zones and maintains the temperature profile of the gasifier at steady state. It supplies heat to endothermic endothermic reactions in gasification gasification zone (800–1000 C) where CO2 and H2O coming from the combustion zone reacts with char to form form synth synthesi esiss gas, gas, whichis whichis colle collecte cted d from from the the botto bottom m of the the gasgasifier. Major reactions taking place in various zones of gasifier are shown in Table in Table 1 [20] 1 [20].. DPL was analyze analyzed d using using thermo thermogr gravim avimetr etric ic analysis analysis (TGA), (TGA), shown in Fig. in Fig. 2. 2. The The deta details ils of the the pyro pyrolys lysis is kineti kinetics cs of DPL DPL have have been been studie studied d by the sixth author, published elsewhere [28] elsewhere [28].. The downdraft gasifier at Gasifi Gasificat catio ion n Resear Research ch Labo Laborat rator ory y of UET UET Laho Lahore re has has been been used used in this study, shown in in Fig. Fig. 3(a). 3(a). Biomass is fed from the top of the gasifier with the help of a scre screw w feed feeder er at a rate rate of 8 kg/h kg/h.. The The syng syngas as prod produc uced ed in the the gasi gasifie fierr was was passe passed d thro throug ugh h a cyclo cyclone ne to remo remove ve tars tars and and ash prese present nt in the the gas. gas. The The syng syngas as prod produc uced ed was was flared flared and and show shown n again against st the the pictu picture re plate in Fig. in Fig. 3(b). 3(b). The gasifier was operated in two different servo contro controll mode modess to find find out out the the chan change ge in the the compo composit sitio ion n of synth syntheesis gas with gasifier temperature and S/B ratio. The experimental results obtained were used to validate the simulation results of ASPEN HYSYS.
3. ASPEN HYSYS model A steady steady state state equilib equilibriu rium m model model for has been develope developed d for biomass gasification gasification using ASPEN HYSYS. The unit operations have been arranged in confluence confluence with hydrodynam hydrodynamics ics of downdraft downdraft gasifier.
3.1. Assumptions The following assumptions were made to model the downdraft gasifier.
Fig. 1. Schematic of downdraft gasifier showing reaction and temperature zone.
(1) Steady state isothermal isothermal process. (2) Instantaneou Instantaneouss devolatilization devolatilization after introducing palm leaves into the gasifier [27] [27].. (3) Uniformly Uniformly sized particles with sphericity equal to one. (4) The diameter of the particles stays constant during during gasification in accordance with the shrinking core model. (5) Biomass Biomass is modeled modeled on dry ash free (DAF) basis for simplicity, therefore biomass char contains carbon only. (6) The entire amount of sulfur in biomass reacts to form H 2S only [30] only [30].. (7) Only ammoni ammonia a (NH3) is formed formed during during gasifica gasification tion,, No oxides oxides of nitrogen are produced [30] [30].. (8) Tars are considered considered to be non-equilibrium non-equilibrium products products to simplify hydrodynam hydrodynamics ics [10] [10]..
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Fig. 2. Pyrolysis Pyrolysis of date palm leaves in thermobal thermobalance ance (sample (sample weight weight = 11.45 11.45 mg, heating heating rate = 25 C/min).
Table 2
Characteristics of date palm leaves. Proximate analysis Moisture Volatile matter Fixed carbon Ash
5.0 78.1 5.2 11.7
Ultimate analysis C H N S O DPL density (kg/m3) Aver Averag age e part partic icle le size size (mm (mm) DPL flow rate
49.4 5.8 1.2 1.3 42.3 298 5–10 5–10 8 kg/h
and defining pure components, assigning a property package for carrying out flash and physical properties calculations, and defining reactions which can be embedded into any unit operation during the simulation process. As HYSYS does not have biomass as a library componen component, t, date palm leaves was modeled modeled as a solid hypothetical component component within Hysys, using ultimate analysis, given in Table Ta ble 2. Peng–R Peng–Robin obinson son equatio equation n of state state (EOS) (EOS) is selected selected as propproperty package to calculate the physical properties of components, and has been reported to well estimates the physical properties in an IGCC power plant simulation [25] [25].. The gasification reactions (see Table (see Table 1) 1) are defined as equilibrium reactions in SBM, specifying equilibrium constants as a function of temperature.
3.3. Simulation description The gasificatio gasification n of DPL in downdr downdraft aft gasifier gasifier is simula simulated ted in three main stages of biomass decomposition, decomposition, volatiles combustion and and char char gasifi gasificat cation ion.. The The proc process ess flow flow diagr diagram am of the the simul simulati ation on is shown in Fig. in Fig. 4, 4, with description of main unit operation models in Table 3. 3.
Fig. 3. (a) Experim Experimenta entall downdra downdraft ft gasifier gasifier assembly, assembly, (b) flare of synthesis synthesis gas against the picture plate.
3.2. Simulation basis manager Simulation Basis manager (SBM) is the welcome interface for a simulation project in ASPEN HYSYS and helps mainly in selecting
3.3.1. Decomposition of biomass A conver conversion sion reactor reactor model model in HYSYS; HYSYS; BM Breakd Breakdown own simulat simulates es the decomposition decomposition of biomass biomass which closely represents a pyrolysis pyrolysis process in downdraft gasifier in terms of its functionality. Biomass defined as a hypothetical component in HYSYS is split into its constituting conventional components of carbon, hydrogen, nitrogen, oxygen and sulfur, using ultimate analysis. As biomass is fed on a dry ash free (DAF) basis, char from BM Breakdown consist of pure carbo carbon. n. The The stream streamss Comb Comb Feed Feed and and Char Char in the the simul simulati ation on represent represent volatile matter and fixed carbon respectively, defined
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696
Fig. 4. ASPEN HYSYS steady state simulation of downdraft gasifier firing date palm leaves.
Table 3
Description of reactors in the simulation. Reactor
HYSYS reactors family
Reaction zone
Description
BM Brea Breakd kdo own
Conv Conver ersi sion on
Pyro Pyroly lysi siss
Volatile Combus to tor
Gibbs
Gasifi asifier er-A -A Gasifier-B CO shif shiftt reac reacto torr
Equilib ilibri riu um Gibbs Equi Equili libr briu ium m reac reacto torr
HS reactor
Conversion
Mode Models ls the the deco decom mposi positi tion on of biom biomas asss (a hypo hypoth thet etic ical al com compone ponent nt)) into into con convent ventio iona nall constituent component component This reactor models the combus ti tion of volatiles and tars gener at ated dur in ing the pyrolys is is of DPL using Gibbs free energy minimization approach These ese three ree reac reacto torsco rscoll llec ecttivel ively y model del the the DPL char char gasi gasifi ficat cation ion proce rocess ss basedoneq asedonequ uilib ilibri riu um data Gasi Gasifie fierr-A, A, and and COShift COShift Reac Reacto torr mode modell the the comb combus usti tionand onand wate waterr gas gas reac reacti tion onss resp respec ecti tive vely ly in gasification process using equilibrium data Gasifier-B models the main gasification reactions by multiphase chemical and phase equilibrium calculations using Gibbs free energy minimization approach Models the complete convers io ion of sulf ur ur into hydr og ogen sulfide in a conversion reactor
Combustio stion n Gasi Gasific ficat atio ion n
in accordance with the proximate analysis of the parent fuel (see Table 2). 2).
3.3.2. Volatiles combustion Assuming Assuming combustion of volatile matter (VM) follows Gibbs equi equili libr briu ium m, it is model odeled ed with with a Gibb Gibbss reac reacto torr in HYSY HYSYS, S, named Volatile Combustor. Combustor. VM feed to the Volatile Combustor, Combustor, called called H. Comb Comb Feed Feed contain containss a small small amoun amountt of carbon, carbon, representin senting g gaseo gaseous us carbo carbon n in the the volat volatile ile matt matter er.. Carb Carbon on in H. Comb Comb Feed Feed can be calcula calculated ted by the difference difference method method using using prox proxim imat ate e analy analysis sis data data.. The The mode modelin ling g of VM comb combus ustio tion n is carried out in accordance with the hydrodynamics of downdraft gasifier gasifier.. The small diffusio diffusional nal effects of synthesi synthesiss gas in upper upper zones zones have have been accommod accommodated ated in the simulatio simulation n by the Oxygen stream stream leaving leaving X-101. X-101. The combus combustio tion n produc products ts (CO and H2O) of volatile matter have their share in the gasification reactions; therefore Flue Gas stream from Volatile Combustor in the simulation is recycled to the gasification reactor Gasifier-B using recycle operation RCY-1. 3.3.3. Char gasification The gasificatio gasification n process process is modeled modeled as a set of equilib equilibriu rium m and Gibbs Gibbs reactor reactorss in ASPEN ASPEN HYSYS, HYSYS, modeli modeling ng variou variouss zones zones of
downdraft downdraft gasifier. The gasification reactions (see Table 1) 1) were defined defined as six equilib equilibrium rium reactions reactions in HYSYS HYSYS simulat simulation ion Basis Basis Manager, Manager, specifying the variation variation of the equilibrium constant constant of each reaction with temperature temperature [20]. [20]. Gasifier-A, Gasifier-A, an equilibrium equilibrium reactor reactor models models the char combus combustion tion reaction reactionss in the air so that that the exiting streams; streams; Gasif-1 and Gasif-1 Gasif-1 Solids are in chemical chemical and physical equilibrium. Gasifier-A closely models the combustion tion zone zone of down downdr draf aftt gasi gasifie fier. r. The The exit exitin ing g stre stream amss from from Gasifier Gasifier-A -A along along with with Steam Steam enter enter Gasifie Gasifier-B r-B;; a Gibbs Gibbs reactor reactor modmodeling gasification zone of downdraft gasifier. It models Water gas, Bourda Bourdard, rd, and Mathen Mathenatio ation n reaction reactionss using using Gibbs Gibbs free energy energy minminimization method at equilibrium. Gasifier-B and CO Shift Reactor collectively simulates the gasification zone of the gasifier. CO Shift Reactor Reactor is an equilib equilibriu rium m reactor reactor which models models water water gas shift shift reaction, completing the gasification process. The entire synthesis gas gas stream stream is passe passed d thro throug ugh h HS React Reactor or which which mode models ls the the conv converersion of solid sulfur in hydrogen sulfide with a conversion reactor assuming complete conversion. The exit streams from HS Reactor and Volatile Combustor merge at MIX-101, resulting in synthesis gas naming Gas Mix. The entire moisture in the synthesis gas is separated separated in Dewatering unit, which simulates simulates the knockout drum downstream the gasifier. Syn-Gas represents the dry synthesis gas obtained from the gasification of date palm leaves.
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Table 4
Accuracy of the predicted composition of synthesis gas. Mode of operation of gasifier
Mean error H2
Gas composition with temperature Gas composition with S/B ratio
0.050133 0.039651
CO CO
CO2
CH4
0.270876 0.044724
0.115872 0.088646
0.362044 0.112359
4. Model validation The simulation results have been validated with experimental data of DPL gasification gasification in a lab scale downdraft gasifier. gasifier. The accuracy of the simulation results is estimated using the sum squared deviation method [12] method [12].. n
RSS
y X ¼
ke
k
¼1
MRSS
y
yke
kp
2
¼ RSS n
Mean error
¼
ð1Þ ð2Þ
p ffiffi ffi ffi ffi ffi ffi MRSS
ð3Þ
where where RSS and MRSS are ranked ranked set samplin sampling g and median median ranked set sampling and y ke and y kp are experimental and simulation tion compos compositio itions ns of syngas syngas constit constituen uentt (H2, CO, CO, CO2, or CH4) respectively. Table respectively. Table 4 shows shows the accuracy accuracy of the predict predicted ed compos composiition tion of synthesis synthesis gas for temperat temperature ure and steam/b steam/biom iomass ass ratio ratio varvariati iation on expe experi rime ment nts. s. Expe Experi rime ment ntal al dete determ rmin inat atio ion n of gas gas compos compositio ition n has been detecte detected d using using flue gas analyze analyzerr (MGA5 (MGA5 plus). All mathematical calculations were performed using Matlab 10 software. Hydrogen and methane show the lowest and highest deviation respectively in both the operation modes of the gasifier. The deviation of predicted composition of methane from experimental values has been reported in various studies of simulation of biomass gasification gasification [36,24]. [36,24]. As methane is the only hydrocarbon in the synthesis synthesis gas, a possible possible explanati explanation on is the software software replaces methane for every possible hydrocarbon reaction.
5. Results and discussion The steady state simulation developed is tested for its accuracy by simulating variation of synthesis synthesis gas composition with temperature ature and steam/b steam/biom iomass ass (S/B) (S/B) ratio ratio on ASPEN ASPEN HYSYS. HYSYS. The resultin resulting g value valuess are are comp compare ared d with with the the expe experim rimen enta tall valu values es obta obtain ined ed from from a downdraft gasifier firing DPL for the same set of experiments.
5.1. Effect of temperature Fig. Fi g. 5 shows shows a compari comparison son of experim experiment ental al and simulat simulation ion results of change in exit gas composition for a temperature range of 650–800 C in downdraft gasifier. At a constan constantt S/B ratio, the gasifier gasifier tempera temperatur ture e is varied varied by varying the air flow rate which is the same as the varying equivalence ratio (ER). Therefore, an increase in temperature or equivalence ratio has the same effect over the composition of synthesis gas. It can be seen from Fig. 5 that the composition of hydrogen and carbon carbon monoxid monoxide e increase increasess with with increasi increasing ng tempera temperatur ture, e, while methane and carbon dioxide decreases. A similar trend has been been observe observed d for downdr downdraft aft gasifier gasifier with with variou variouss kinds kinds of biomass biomass [31–34].. [31–34] In ASPE ASPEN N HYSYS HYSYS,, all gasifi gasificat catio ion n react reaction ionss have have been been mode modeled led as an equilib equilibriu rium m reaction reaction except except oxidatio oxidation n reactio reactions ns of carbon; carbon; which which have have been modele modeled d as conver conversion sion reactio reactions. ns. Water Water gas reacreaction (G-3) is the fundamental reaction giving rise to hydrogen in
Fig. 5. Variation of synthesis gas composition with change in gasifier temperature at S /B = 1.5. 1.5.
synthe synthesis sis gas and higher higher temper temperatu ature re favors favors the format formation ion of hydrog hydrogen en owing owing to its endoth endotherm ermic ic behavio behavior. r. As hydrog hydrogen en is among among the reactan reactants ts in the hydrog hydrogasifi asificati cation on reactio reaction n (G-6), (G-6), higher higher temper temperatu ature re shifts shifts equilib equilibriu rium m backwar backwards ds for this this exothe exotherm rmic ic reaction, saving hydrogen from consumption. Shift reaction (G-5) is also exothermic in behavior and higher temperature favors carbon monoxide instead of hydrogen. Thus, the overall effect is a net increase increase in hydrog hydrogen en compos compositio ition n at higher higher temper temperatu atures. res. The effect effect of each reaction reaction on the final final gas compositi composition on for increase increase in temperature has been tabulated in Table 5. 5. The core gasification gasification reactions; Water gas (G-3) and Boudouard (G-4 (G-4)) prod produc uce e carbo carbon n mono monoxid xide e and and their their endo endoth therm ermic ic natu nature re is in confluence with higher temperature. Therefore the amount of carbon monoxide increases with increase in temperature in the gasifier. ifier. Althoug Although h shift shift reaction reaction (G-5) produc produces es hydrog hydrogen en at the expense of carbon monoxide, this reaction shifts the equilibrium backwards backwards at higher temperature, temperature, saving CO from consumption. consumption. Thus, the overall effect of G-3, G-4 and G-5 is a net increase in the concent concentrat ration ion of carbon carbon monoxid monoxide e at higher higher temper temperatu atures. res. Moreover, Moreover, it is apparent apparent from Ta Table ble 5 that the higher temperatur temperatures es do not not favor favor meth methan ane e and and carbo carbon n dioxi dioxide de prod produc uctio tion n in the the synth syntheesis gas. As a result their amount in the syngas reduces consistently at higher temperatures. At temper temperatu atures res higher higher than than 700 C, the the simul simulat ated ed resu results lts are are in better better agreem agreement ent with with experim experiment ental al results results for hydrog hydrogen en (see Fig. 5). 5). Carbon monoxide monoxide results simulate simulate the experimental experimental results well well at temper temperatu atures res higher higher than than 750 C. Carb Carbon on dioxid dioxide e and and meth meth-ane production are underestimated in at a lower temperature in simulation results but the results become well in agreement with experimental results at temperatures higher than 700 C. At lower temper temperatu atures, res, air steam steam gasificat gasification ion of biomass biomass produc produces es more more tars which reduce the amount of hydrogen in syngas. The deviation in the simulated and experimental experimental results for hydrogen, predominantly at low temperatures arises by ignoring production of tars in the simula simulation tion.. This This results results in undere underestim stimatio ation n of carbon carbon dioxide dioxide as the the equi equilib libriu rium m shift shiftss backw backwar ards ds in shift shift react reactio ion n (G-5 (G-5)) owing owing to higher concentration of hydrogen.
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Table 5
Effect of high temperature on the final gas composition through gasification reactions.
5.2. Effect of steam to biomass ratio (S/B) The effect of increase in steam to biomass ratio (S/B) has been studied in downdraft gasifier using ASPEN HYSYS and the simulation results were compared with the experimental values, shown in in Figs. 6–9. 6–9. Saturate Saturated d steam at 193 kPa was used and S /B ratio has has been been vari varied ed from from 0.5 to 2 usin using g a SET SET oper operat atio ion n in the the simulation.
Fig. Fig. 8. Effect Effect of steam steam to bioma biomass ss (S /B) on carbon dioxide compositio composition n in the product gas (T : 800 C, biomass biomass flow rate: 8 kg/h). kg/h).
Fig. 6. Effect Effect of of steam steam to bioma biomass ss (S /B) on on hydrogen hydrogen composit composition ion in the product product gas (T : 800 C, biomass biomass flow rate: 8 kg/h). kg/h).
Fig. 9. Effect of steam to biomass (S /B) on methane composition in the product gas (T : 800 C, biomass biomass flow rate: 8 kg/h). kg/h).
Fig. 7. Effect of steam to biomass (S /B) on carbon monoxide composition in the product gas (T : 800 C, biomass biomass flow rate: 8 kg/h). kg/h).
The aim of introducing steam in the gasification is to increase the heating value of the resulting gas owing to increased methane and hydrog hydrogen en conten contents. ts. Injecti Injecting ng steam steam shifts shifts the equilib equilibriumright riumright in water gas reaction (G-3) making carbon monoxide and hydrogen. gen. Carbon Carbon monoxid monoxide e drives drives the equilib equilibrium rium forward forward in shift shift reaction (G-5), resulting in higher concentration of hydrogen and carbon carbon dioxide. dioxide. Higher Higher amoun amounts ts of Hydrog Hydrogen en result result in higher higher methane methane concentration concentration in the hydrogasificat hydrogasification ion reaction (G-6).
M. Bassyouni et al. / Energy Conversion and Management Management 88 (2014) 693–699
The overall effect of injecting steam is an increased concentration of hydr hydrog ogen en and and meth methan ane e as show shown n in Figs Figs.. 6 and 9, which which increases the heating value of syngas. The predicted values of carbon monoxide monoxide and carbon dioxide dioxide in the syngas are in good agreement ment with with the the experi experime ment ntal al resu result, lt, show shown n in Figs Figs.. 7 and 8 respectively. Methane composition shows good agreement in the beginning in Fig. 9 but deviates widely at higher S/B values. This is due due to drop drop in tempe temperat ratur ure e as a resul resultt of injec injecti ting ng high higher er amounts of low pressure saturated steam in the gasifier at higher S/B ratios, which favors the high tars formation. These results are in good good agreem agreement ent with with previo previous us related related studies studies [17,13], [17,13], the the author authorss reporte reported d that that the S/B ratio ratio has significant significant effect on the yield.
6. Conclusions ASPEN HYSYS is used to set up an equilibrium model for a lab scale scale downdr downdraft aft biomass biomass gasifier gasifier at steady steady state state to predict predict the synthesis thesis gas composi compositio tion. n. The model model simula simulates tes the variou variouss zones zones accorda accordance nce with with the hydrod hydrodyna ynamic micss of a downdr downdraft aft gasifier gasifier.. A proprocess flow diagram (PFD) with various unit operations represents the simulation, which models date palm leaves as a hypothetical component component and processes processes it through through a set of equilibrium air steam gasification reactions to get syngas composition. The model is able to predict predict the perfor performan mance ce of the gasifier and the simulatio simulation n results are in good agreement with the experimental results. For a sensitivity analysis of the simulation, gasifier temperature and S/B ratio were varied varied and the results results were were compare compared d with with the experimental results. At higher higher temper temperatu atures, res, the perfor performan mance ce of gasifier gasifier improv improves. es. It results in higher hydrogen and carbon monoxide concentration in synth synthesi esiss gas gas which which incre increase asess the the heati heating ng value value of the the gas gas and and cold cold gas efficienc efficiency. y. Carbon Carbon dioxide dioxide and methan methane e concen concentrat tration ion decreas decrease e with with increasi increasing ng temper temperatu ature. re. High High steam steam to biomass biomass ratio ratio improv improves es the heating heating value value of the gas by increasi increasing ng the concent concentrati ration on of hydrogen and methane but more carbon monoxide is produced.
Acknowledgments This work was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under Grant No. (1433829-402). 829-402). The authors, therefore, therefore, acknowledge acknowledge with thanks DSR technical and financial support.
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