Ờ Å ỊÙ× Ư Ờ Biomass Gasification with CO2 in a Fluidized Bed Yongpan Cheng, Zhihao Thow, Chi-Hwa Wang PII: DOI: Reference: S0032-5910(14)01023-7 doi: 10.1016/j.powtec.2014.12.041 PTEC 10702 To appear in: Powder Technology Received date: Revised date: Accepted date: November 2014 18 December 2014 22 December 2014 Please cite this article as: Yongpan Cheng, Zhihao Thow, Chi-Hwa Wang, Biomass Gasification with CO2 in a Fluidized Bed, Powder Technology (2014), doi: 10.1016/j.powtec.2014.12.041 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain ACCEPTED MANUSCRIPT RI PT Biomass Gasification with CO2 in a Fluidized Bed Department of Chemical and Biomolecular Engineering, National University of NU a SC Yongpan Chenga, b, Zhihao Thowa, Chi-Hwa Wanga,b* Singapore, Engineering Drive 4, 117585, Singapore NUS Environmental Research Institute, National University of Singapore, 5A MA b AC CE P TE D Engineering Drive 1, #02-01, 117411, Singapore Submitted to Powder Technology December 2014 *Corresponding author Tel.: +65-65165079; Fax: +65-67791936; E-mail address: chewch@nus.edu.sg (C H Wang) ACCEPTED MANUSCRIPT Abstract: Biomass gasification with CO2 as the gasifying agent is a promising way to relieve the energy PT shortage and minimize CO2 emission In this study, Eulerian method was used to study the CO2 RI gasification of biomass in a fluidized bed gasifier It was found that the CO2 percentage in the gasifying mixture with air, CO2-to-biomass ratio, the moisture content of wood chips, the SC woodchip size had great influence on the gasification performance With the increasing CO2-to- NU biomass ratio, the mole fraction of CO in the producer gases would be increased while those of H2 and CO2 vary in the opposite trend When CO2 mass percentage in the gasifying agent was MA 60%, the fractions of CO and CH4 in the producer gas reached the maximum, as well as the lower heating value and cold gas efficiency, so this was the optimal condition when the gasifier had the D best performance The moisture content and particle size of the woodchips had negative effects TE on the gasification performance, because of the lower heating value of producer gases, cold gas efficiency and CO2 conversion ratio were both reduced with increasing moisture content and AC CE P particle size This study offers a promising way to integrate the gasification of renewable biomass with CO2 capture, and may be helpful in the design and operation of biomass gasifier Keywords: CO2 capture; Biomass gasification; Renewable energy; Numerical simulation ACCEPTED MANUSCRIPT Introduction The explosive increasing energy consumption is one of the critical challenges throughout the PT world, and currently significant percentage of the consumed energy comes from fossil fuels, such RI as petroleum, coal and natural gases According to Song [1], the total energy consumption in the 20th century was about 10,048 million tons of oil equivalent, with 24% from coal, 39% from SC petroleum, 23% from natural gas, 6% from nuclear power, and only 8% from renewable energy, NU including hydroelectric power, biomass, geothermal, solar and wind energy On one hand, the limited and non-renewable fossil fuels have been consumed rapidly and will be depleted in the CO2 emission (about 41%) MA near future; on the other hand, energy generation from fossil fuels is also the major source for [2], which is the major greenhouse gas contributing to global D warming Furthermore, the emission of NOx and SOx can result in acid rain, which is also a great TE threat to the environment In order to resolve the energy shortage and relieve global warming, biomass is considered a potential new and clean energy source Biomass is a CO2 neutral and AC CE P environmentally friendly energy source, as it is formed by the plant photosynthesis process, which absorbs CO2 from the atmosphere Furthermore, biomass can be converted into gaseous, liquid and solid fuels, so it is convenient for storage and transportation In the literature, there are many studies on the topic of biomass utilization, covering various subjects such as fermentation, combustion and gasification Fermentation does not have high requirement for the feedstock, such as limitation on the moisture content, but the process is quite slow and usually need large reactors to ensure significant output of producer gas Comparatively, combustions are fast processes, so the reactors can be quite compact However, the exhaust gases from biomass combustion can include dust, NOx, SOx or heavy metals etc, which are costly to be removed Therefore, compared with fermentation and combustion, gasification is an effective and clean way to convert biomass into useful fuels and chemical feedstocks [3, 4] With proper cleaning of the fuels produced, they can be directly used in electricity and heat production devices, ACCEPTED MANUSCRIPT such as internal combustion engines, gas turbines and fuel cells [5, 6] So far there have been a great amount of reviews on the study of biomass [7-14], however, biomass gasification with CO2 PT as gasifying agent is seldom addressed RI Gasification with CO2 has several advantages [15], for example, no energy is required for vaporization; the H2/CO ratio in producer gases can be easily adjusted to meet the specific SC requirement; CO2 can produce more volatiles in a reactive char, so the gasification efficiency can NU be improved Finally the gasification of CO2 instead of nitrogen can lead to flue gas with high percentage of CO2, which is suitable for direct recovery and recycle of CO2 CO2 has been MA successfully used to gasify coal to produce syngas and relieve CO2 pollution [16-18] The thermochemical processes involved in biomass gasification with CO2 are similar to those in coal D gasification with CO2 As the reaction of CO2 with carbon is highly endothermic, and highly TE energy-intensive, usually the mixture of CO2 and O2 or CO2 and steam is used as the gasifying agents Renganathan et al [19] carried out a thermodynamic analysis on CO2 utilization for AC CE P gasification of carbonaceous feedstocks using Gibbs minimization approach, and found that when CO2 was combined with steam or oxygen as a gasifying agent, the requirement for carbon dioxide and energy could be reduced, as well as the carbon dioxide conversion Furthermore, the ratio of hydrogen/carbon in syngas could be varied in a wide range Irfan et al [16] reviewed the effect of different parameters on coal-char gasification in CO2 stream, such as the effect of coal rank, pressure, temperature, gas composition, catalyst and the minerals inside the coal, heating rate, particle size and reactor types As the extension of the study, they also reviewed the kinetics and reaction rate equations for coal-char gasification in low temperature and high temperature regions under low and high pressures Mani et al [20] studied the reaction kinetics and mass transfer of wheat straw char gasification with CO2 through thermogravimetric apparatus (TGA); the effects of temperature and particle size on the diffusion and surface reactions were identified Also by virtue of TGA, Butterman and Castaldi [15] examined the gas evolution, mass decay behavior, ACCEPTED MANUSCRIPT energy contents of several woods, grasses and agricultural residues using a mixture of CO2 and steam with different proportions Garcia et al [21] carried out the catalytic CO2 gasification of PT pine sawdust at a relatively low temperature and atmospheric pressure; the influence of the RI catalyst weight/biomass flow rate ratio was analyzed on product distribution and gas composition As fluidized bed gasifier has high reaction rates and effective mixing inside, it is widely used in SC medium and large scale biomass gasification Oevermann et al [22] simulated wood gasification NU in a two-dimensional bubbling fluidized bed reactor with an Eulerian-Lagrangian approach The influence of wood feeding rate on the gasification was studied With a similar method, Xie et al MA [23] simulated the gasification of forestry residues in a three-dimensional fluidized bed Their numerical model could predict the product gas composition and carbon conversion efficiency, in D good agreement with experimental data In addition, the flow regimes, profiles of particle species, TE and distribution of gas compositions inside the reactor were also discussed Gerber et al [24] modeled wood gasification in a bubbling fluidized bed reactor using char as bed material with an AC CE P Eulerian method The product gas concentration and temperature were investigated under different operating conditions and model parameters Blasi [12] reviewed modeling in chemical and physical processes of wood and biomass pyrolysis, especially the chemical kinetics in primary reactions described by one- and multi-component mechanisms, and secondary reactions of tar cracking and polymerization Gomez-Barea and Leckner [11] reviewed the modeling of biomass gasification in fluidized bed The mathematical reactor models for biomass and waste gasification in fluidized bed were presented As biomass gasification experiments are usually quite expensive and time-consuming, numerical simulation provides an efficient alternative way to carry out such studies for providing guidance to design the gasifier and optimize the experiments In this study, the Eulerian method will be used to study CO2 gasification of biomass in a fluidized bed The effects of compositions of CO2 and air in gasifying mixture, moisture content of biomass, particle size on the biomass ACCEPTED MANUSCRIPT gasification will be studied in details The gasification performance will be evaluated in terms of gas composition and temperature, axial profiles of gas species, lower heating value of the PT producer gases, cold-gas efficiency and fractional CO2 conversion This study integrates biomass gasification with CO2 captures, which cannot only increase the power supply of renewable energy, SC RI but also reduce CO2 emissions effectively NU Mathematical formulation MA 2.1 Physical model The numerical simulation of biomass gasification in this study was based on the lab-scale D bubbling fluidized bed gasifier, which was operated at the Institute of Energy Engineering at TE Berlin Institute of Technology [24], as shown in Fig.1 The freeboard zone and the bubbling AC CE P fluidized bed zone have inner diameters of 0.135 m and 0.095 m respectively The gasifying agent (mixture of air and CO2) was introduced at the bottom of gasifier, while the woods were fed into the gasifier through a fuel inlet with diameter of 0.05m at 0.08 m above the bottom The producer gases escaped from the outlet with diameter 0.03m at 0.05m below the top Instead of inert bed materials, char was selected as bed materials, as done by [24] The use of char as bed materials had some potential advantages, for example, the char had the capability to decompose tar [25, 26], and it did not need to be regenerated in that char was a byproduct of biomass gasification, the pressure loss in the reactor was also lower due to its low density compared with traditional bed materials The wood and char had the constant diameter of 2mm and 4mm, and had the density 605 kg/m3 and 450 kg/m3, respectively The gasifier was initially filled to a char bed height of 350 mm with volume fraction of 0.63 In order to quickly activate the biomass gasification, the initial temperature of the char bed and gas was at 1000 K The preheated gasifying agent with different compositions of air and CO2 were continuously introduced at a ACCEPTED MANUSCRIPT 970K from the bottom of the gasifier during the entire operation time, the wall of the gasifier was set as 970K as well 10 kg/hour of wood at 623 K was fed into the gasifier through fuel inlet PT under atmospheric pressure RI 2.2 Governing equations SC In this study, due to the high solid fraction in the bubbling fluidized bed, the Eulerian method was used to simulate the biomass gasification The solid char was considered as the continuum, and NU the solid fluctuating energy was described with granular temperature from the kinetic theory [27] The phases were able to interpenetrate into each other, and the sum of all the volume mass MA fraction was unity The accumulation of mass in each phase was balanced by the convective mass fluxes The biomass gasification included the fluid flow, heat and mass transfer, as well as the AC CE P 2.2.1 Gas phase TE D chemical reactions; they were governed by the following equations The conservation equation for the mass fraction of the species i in the gas phase could be written as [24]:  g  gYgi t     g  g vgYgi   mi ,g   s s mi ,s (1) n With additional momentum terms between gas phase and solid phases, the gas phase momentum equation could be written as:  g  g vg t    g  g vg vg    g p   g   g   s s I sg   g  g g n The momentum exchange term between solids phases and gas phases I sg  K sg  vs  vg   msg  vs  vg  (2) could be expressed as: (3) ACCEPTED MANUSCRIPT The dominant force in the gas and solid phase momentum balances was the inter-phase momentum transfer, which was represented as drag force; herethe Syamlal-O’Brien drag model PT was used for calculating the fluid-solid drag force [28] n The total enthalpy of the gas phase, SC t     g  g vg H g     qg   s s hgs Ts  Tg   msg hsg   H g (4) was defined as follows: NU  g  g H g RI The energy equation in gas phase could be expressed as: MA Tg H g  i Ygi H i  i Ygi   C p ,i dT H i0 Tref ,i  (5)  Tref  D 2.2.2 Solid phase TE The conservation equations for the mass of the solids phase s could be written as: AC CE P n n  s  s     s  s vs   l s msl   s s mi ,s (6) t The solid phase s momentum equation could be written as: n   s  svs      s  svsvs   p   s  mss I sl  I sg   s  s g (7) t The energy equation in solid phase s could be expressed as: n  s  s H s     s  s vs H s   qs   s s hgs Ts  Tg   msg hsg   H s (8) t Since the optical thickness of the gasifier was large, P-1 model worked reasonably well to compute the radiative heat transfer [29] The P-1 model assumed the propagation of radiation energy into an orthogonal series of spherical harmonics [30] ACCEPTED MANUSCRIPT The transport equation for incident radiation, G, could be written as [30]:   1 PT       s   C s  G  G  4 T  SG (9) RI 2.3 Reaction model SC 2.3.1 Pyrolysis In this model, three kinds of chemical processes were considered: (1) pyrolysis of wood, (2) NU homogenous gas phase reactions and (3) heterogeneous char reactions The reaction model MA included a system of three phases, which were one gas phase and two solid phases (wood solid phase and char solid phase) The gas phase was assumed to include eight components only: CO, D CO2, H2, CH4, O2, H2O, tar and N2 Since the temperature of introduced wood was higher than the TE wood drying temperature, the drying process was not modeled in this project For simplification, the dried woods were assumed to enter the gasifier with 10% wet basis of moisture in the form of AC CE P water vapor at the fuel inlet Modeling of pyrolysis of wood was the most challenging step in the model Since pyrolysis was a very complicated thermochemical process, two one-step global reactions were applied to model both primary and secondary pyrolysis For simplification, the chemical formulae of char and all non-condensable hydrocarbon gases were considered as pure carbon and methane respectively The composition of tar was usually relevant to condensed aromatics, so it was quite reasonable to model tar as phenol [31] Since the temperature was just high enough, it was assumed that all tars that produce from the primary pyrolysis were cracked in the secondary pyrolysis [22] Due to the complex composition of the products of both primary and secondary pyrolysis, the composition of the product from pyrolysis of wood was developed from the experimental data available in the literature Table showed the product’s mass fraction for both primary and secondary pyrolysis from experimental data ACCEPTED MANUSCRIPT 0.20 PT RI 0.10 20% CO2 SC CO mole fraction 0.15 40% CO2 0.05 2.1 NU 2.0 2.2 2.3 MA 0.00 1.9 60% CO2 80% CO2 2.4 2.5 2.6 AC CE P 0.50 (a) CO TE D CO2/biomass mass ratio 20% CO2 40% CO2 CO2 mole fraction 0.45 60% CO2 80% CO2 0.40 0.35 0.30 0.25 0.20 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 CO2/biomass mass ratio (b) CO2 40 ACCEPTED MANUSCRIPT 0.07 20% CO2 40% CO2 60% CO2 PT RI 80% CO2 0.05 SC H2 mole fraction 0.06 0.03 1.9 2.0 2.1 NU 0.04 2.2 2.3 2.4 2.5 2.6 AC CE P 0.10 0.08 CH4 mole fraction (c) H2 TE D MA CO2/biomass mass ratio 20% CO2 40% CO2 60% CO2 80% CO2 0.06 0.04 0.02 0.00 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 CO2/biomass mass ratio (d) CH4 Figure Effect of CO2/biomass mass ratio on the producer gases under different CO2 percentages in gasifying agent 41 RI PT ACCEPTED MANUSCRIPT 40% CO2 0.60 60% CO2 0.55 MA 80% CO2 0.50 D 0.45 0.40 0.35 2.0 AC CE P 0.30 1.9 NU 20% CO2 TE Cold gas efficiency 0.65 SC 0.70 2.1 2.2 2.3 2.4 2.5 2.6 CO2/biomass mass ratio Figure Effect of CO2/biomass ratio on the cold gas efficiency under different CO2 percentages in gasifying agent 42 PT ACCEPTED MANUSCRIPT NU SC MA 1.9 2.0 D 2.1 TE Low heating value,MJ/m RI 2.2 20% CO2 40% CO2 60% CO2 80% CO2 2.3 2.4 2.5 2.6 AC CE P CO2/biomass mass ratio Figure Effect of CO2/biomass ratio on the low heating value under different CO2 percentages in gasifying agent 43 ACCEPTED MANUSCRIPT PT RI 0.5 SC 0.4 NU CO2 conversion ratio 0.6 0.3 D 2.0 2.1 2.2 60% CO2 80% CO2 2.3 2.4 2.5 2.6 TE 0.1 1.9 40% CO2 MA 0.2 20% CO2 AC CE P CO2/biomass mass ratio Figure 10 Effect of CO2/biomass ratio on the CO2 conversion ratio under different CO2 percentages in gasifying agent 44 PT ACCEPTED MANUSCRIPT SC RI 0.4 NU 0.2 H2 CH4 AC CE P TE D 0.1 0.0 CO CO2 MA Mole fraction 0.3 10 15 20 25 Moisture contents, % Figure 11 Effect of moisture contents in the woodchips on the mole fractions of producer gases 45 SC NU Low heating value MA 0.7 AC CE P 0.5 10 D 0.6 Cold gas efficiency CO2 conversion rate TE CGE/CO2 conversion ratio 0.8 15 20 Low heating value (LHV),MJ/m RI PT ACCEPTED MANUSCRIPT Moisture contents, % Figure 12 Effect of moisture contents in the woodchips on the cold gas efficiency, CO2 conversion ratio and low heating value 46 ACCEPTED MANUSCRIPT PT 0.6 0.5 RI CO CO2 SC CH4 0.4 NU 0.3 0.2 MA Mole fraction H2 TE 0.0 D 0.1 10 AC CE P Wood particle size,mm Figure 13 Effect of wood particle size on the mole fractions of producer gases 47 ACCEPTED MANUSCRIPT Cold gas efficiency CO2 conversion rate Low heating value PT RI 0.8 SC 0.7 NU 0.6 0.5 0.4 MA CGE/CO2 conversion ratio 0.9 6 8 Low heating value, MJ/m 1.0 10 AC CE P TE D Wood particle size, mm Figure 14 Effect of wood particle size on the cold gas efficiency, CO2 conversion ratio and low heating value 48 SC RI PT ACCEPTED MANUSCRIPT Table1 Mass fractions of gas species for primary and secondary pyrolysis from NU experimental data [24] CH4 CO2 Primary pyrolysis 0.270 0.056 Secondary pyrolysis 0.5633 0.0884 H2O H2 0.386 0.256 0.032 0.1110 - 0.0173 AC CE P TE D MA CO 49 SC RI PT ACCEPTED MANUSCRIPT NU Table Reaction rates and kinetic parameters for the homogeneous gas phase reactions  1.67  108  r1 =3.98  1014 exp   [CO] [O2 ]0.25[H 2O]0.5  RT   (2)  1.09  108  r2 =2.196  1013 exp   [H ] [O2 ] RT   (3)  1.26  107  r3 =4.40  1011 exp   [CH ]0.5 [O2 ]1.25  RT   [5] [40] AC CE P (4) TE D MA (1)  1.26  107  r4 =2.78  10 exp    RT    [CO2 ][H ]   [CO][H 2O] K p T    [5] [22]  4.546  107  K p T  =0.0265 exp   RT   50 SC RI PT ACCEPTED MANUSCRIPT NU Table Reaction rates and kinetic parameters for the char heterogeneous reactions  1.47  108  r5 =1.33T exp   [H 2O]0.6  RT   (6)  1.62  108  0.6 r6 =4.4T exp    [CO2 ] RT   (7) r7  [35] MA (5) TE D [35] [24] AC CE P Kr Kd [O2 ] Kr  Kd  9.23  107  K r =2.3T exp   , RT   Kd = ShDg dp Dg =3.13(Ts /1500)1.75 p0 / p , Sh   0.6 Re1/ Sc1/3 Re  d p  g vg  vs / g 51 NU Table Characteristics of Dalbergia Sisoo [36] SC RI PT ACCEPTED MANUSCRIPT MA Proximate Analysis (% by wt dry basis) Volatile Ash Carbon (FC) Matter (VM) 15.70 80.40 AC CE P TE D Fixed Calculated LHV (MJ/kg) 3.90 17.83 Ultimate Analysis (% by wt dry basis) Carbon 48.6 Hydrogen Oxygen Nitrogen 6.2 44.87 0.33 52 PT ACCEPTED MANUSCRIPT SC RI Graphical abstract NU Electrical heater 0.134 m MA Flare 1.1 m Flow controller CO2 tank Woodchips 0.65 m AC CE P TE D Gasifier Gas Chromatography Char 0.095 m Preheater Flow controller Air tank 53 ACCEPTED MANUSCRIPT Highlights Biomass co-gasification with CO2 can capture CO2 and produce renewable energy PT 60% CO2 in gasifying agent is the optimal case for biomass gasification The lower heating value of synthetic gas is at maximum in the optimal case AC CE P TE D MA NU SC RI Moisture content and particle size have negative effects on gasification performance 54