Biomass Gasification with CO2 in a Fluidized Bed

The moisture content and particle size of the woodchips had negative effects on the gasification performance, because of the lower heating value of producer gases, cold gas efficiency an

Yongpan Cheng, Zhihao Thow, Chi-Hwa Wang

PII: S0032-5910(14)01023-7

DOI: doi: 10.1016/j.powtec.2014.12.041

Reference: PTEC 10702

To appear in: Powder Technology

Received date: 3 November 2014

Revised date: 18 December 2014

Accepted date: 22 December 2014

Biomass Gasification with CO2 in a Fluidized Bed, Powder Technology (2014), doi:

10.1016/j.powtec.2014.12.041

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Biomass Gasification with CO2 in a Fluidized Bed

Yongpan Chenga, b, Zhihao Thowa, Chi-Hwa Wanga,b*

H2 and CO2 vary in the opposite trend When CO2 mass percentage in the gasifying agent was 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 best performance The moisture content and particle size of the woodchips had negative effects

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 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

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1 Introduction

The explosive increasing energy consumption is one of the critical challenges throughout the world, and currently significant percentage of the consumed energy comes from fossil fuels, such

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 petroleum, 23% from natural gas, 6% from nuclear power, and only 8% from renewable energy, 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 near future; on the other hand, energy generation from fossil fuels is also the major source for

CO2 emission (about 41%) [2], which is the major greenhouse gas contributing to global warming Furthermore, the emission of NOx and SOx can result in acid rain, which is also a great 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 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,

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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

as gasifying agent is seldom addressed

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 requirement; CO2 can produce more volatiles in a reactive char, so the gasification efficiency can

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 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 gasification with CO2 As the reaction of CO2 with carbon is highly endothermic, and highly 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 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,

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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 pine sawdust at a relatively low temperature and atmospheric pressure; the influence of the 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 medium and large scale biomass gasification Oevermann et al [22] simulated wood gasification

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 [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 good agreement with experimental data In addition, the flow regimes, profiles of particle species, 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 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

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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 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,

but also reduce CO2 emissions effectively

2 Mathematical formulation

2.1 Physical model

The numerical simulation of biomass gasification in this study was based on the lab-scale bubbling fluidized bed gasifier, which was operated at the Institute of Energy Engineering at Berlin Institute of Technology [24], as shown in Fig.1 The freeboard zone and the bubbling 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

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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 under atmospheric pressure

2.2 Governing equations

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 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 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 chemical reactions; they were governed by the following equations

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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 was used for calculating the fluid-solid drag force [28]

The energy equation in gas phase could be expressed as:

CO2, H2, CH4, O2, H2O, tar and N2 Since the temperature of introduced wood was higher than the 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 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 1 showed the product’s mass fraction for both primary and secondary pyrolysis from experimental data

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After the mass balance of the chemical components, the stoichiometric coefficients of each chemical component in the two one-step pyrolysis reactions were determined The chemical equations of both primary and secondary pyrolysis were shown as follows:

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The water-gas shift reaction was modeled as a chemical equilibrium reaction that favored forward reaction at relatively low temperature to produce more H2 and CO2 and reversed reaction at high temperature to produce more CO and H2O [23] The Arrhenius rate expression for homogeneous gas phase reactions was shown in Table 2

2.3.3 Heterogeneous Reaction

The heterogeneous reactions between char and gases (O2, H2O and CO2) were modeled Char particle was assumed as a spherical particle that was surrounded by a stagnant boundary layer, so the gas species must diffuse through the layer before the char surface reactions occur [33].The overall rate of partial oxidation of char with O2 (Reaction 7) was expressed as the mixture of kinetic and mass transfer diffusion controlled [24] A User Defined Function written in C programming language was used to incorporate the sub-model On the other hand, the kinetic rates of char heterogeneous reactions with CO2 and H2O (Reactions 5 and 6) were much slower than the partial oxidation of char, so these two reactions were assumed to be kinetically controlled reactions only The rate expression for char heterogeneous reactions was shown in Table 3

Furniture wood waste Dalbergia Sisoo was used as the biomass in the gasification simulations

The proximate analysis and ultimate analysis of the Dalbergia Sisoo were reported in Table 4 Based on the ultimate analysis of the wood, the wood was modeled as the hydrocarbon

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2.5 Solution methodology

The commercially available software FLUENT 14.0 was used to simulate the biomass gasification To avoid instability and poor convergence, an unsteady model was used with a small time step size of 1.0-6 s It took around 2.5-3 seconds for this gasification to reach steady state, so for each case the physical time around 7 s was simulated The phase-coupled SIMPLE algorithm was used for the pressure-velocity coupling, and the second-order upwind scheme was adopted to discretize the governing equations, and a residual of less than 10-4 for all the variables was imposed as the stopping criterion For simplicity, the bubbling fluidized bed was considered as a two-dimensional geometry, which was discretized with 4390 cells with maximum mesh interval

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it was found that the variation trend was quite similar, i.e with the increasing height of gasifier the temperature decreases, with largest variations in the middle region of gasifier, this was because the endothermic gasification reaction mainly happened in this region The predicted temperatures were about 40-50 K lower than the experimental values This deviation was quite satisfactory for such gasifier with complex processes inside As the main reactions and kinetics in the biomass gasifier with air or CO2were the same, after validation with the experimental data,

our numerical model could be applied to the co-gasification of biomass with air and CO2

3.2 Effect of CO2-to-wood mass ratio

To synergistically combine the exothermic oxidation reactions of char with the endothermic reactions of CO2 with char, mixtures of air and CO2 with different fractions were used for wood gasification During the simulation, the wood feeding rate was kept constant at 10 kg/hr, the mass percentage of CO2 in gasifying gas varied from 20 to 80% It was noted that at fixed CO2

percentage, the increasing CO2/biomass mass ratio meant that more CO2was supplied to the gasifier, and the air supplied to the gasifier would also be increased, as well as the oxygen supply due to its fixed percentage in air

3.2.1 Distributions of gas temperature, fraction and velocity

Figure 3 showed the temperature contour plots of gas phase and char solid phase within the gasifier at 60 wt% CO2 in the gasifying agent and CO2-to-biomass mass ratio 2.056 It was seen that at the lower region of the fluidized bed, the temperature distribution was quite uniform due to the exothermic char oxidation reactions, while in the upper freeboard region, the gas temperature was reduced because the endothermic char gasification reactions with CO2 and H2O mainly occurred in this region From Fig 3(c) it was found that near the inlet of woodchips, the gas velocity was much higher than that in other regions, because rapid devolatilization happened, and

in this process the solid wood chips were converted to gases with large volume expansion

producer gases

3.2.2 Distributions of mole fractions of species

Figure 5 and Figure 6 showed the distributions and axial variations of gas species within the symmetric fluidized bed gasifier at 60 wt% CO2 in the gasifying agent and CO2-to-wood mass ratio 2.056 at steady state It could be found that at the height of 0.05m, O2was completely consumed due to the oxidation reactions of char and volatiles of pyrolysis to generate heat The highest mole fraction of CO2was observed at the location near the wood inlet because of the oxidation of CO Also, mole fraction of CO increased while that of CO2 dropped significantly after a height of 0.1m since the endothermic char gasification with CO2 (Boudouard reaction) and reversed water-gas shift reaction were dominated Because the mole fraction of CH4was mainly influenced by the pyrolysis of wood which mostly occurred in the lower part of the gasifier, the rise of mole fraction of CH4was negligible after the height of 0.06m On the other hand, mole fraction of H2 slightly increased after the height of 0.08m due to the endothermic char gasification with H2O (water-gas shift reaction) to produce H2 and CO However, since the rate of reversed water-gas shift reaction was larger than the rate of forward water-gas shift reaction, some of the

non-H2was consumed with CO2 to produce CO and H2O

3.2.3 Mole fractions of producer gases

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Figure 7 showed the effect of CO2/biomass mass ratio under different percentages of CO2 in the gasifying agent on the producer gas composition at the outlet As the biomass feeding rate was kept at 10 kg/hr, the increasing CO2/biomass mass ratio meant that more CO2was supplied to the gasifier, as well as more air or oxygen, therefore the reaction temperature was higher, and the endothermic Boundouard reaction (Reaction 5) was favored Furthermore, the reverse water-gas shift reaction (Reaction 4) was also enhanced due to high operating temperature Both reactions could convert CO2 into CO, so the mole fractions of CO always increased while the trend of CO2

variation was in contrast with the increasing CO2/biomass mass ratio

On the other hand, due to the reverse water-gas shift reaction, more CO2 and H2were consumed to produce CO and H2O at higher CO2-to-biomass ratio; the mole fractions of H2 showed a decreasing trend The dependence of mole fraction of CH4on CO2-to-biomass mass ratio was not significant because the mole fractions of CH4were mainly affected by the pyrolysis of wood under constant feeding rate

It was known that high concentration of CO2 shifted the water-gas shift reaction in the backward direction to produce more CO Henceforth, the increased CO2 percentage in the gasifying agent from 20% to 60% was found to increase the production of CO However, the production of CO slightly dropped from CO2 composition of 60% to 80% in the gasifying agent This might be due

to excessive CO2, the amount of air introduced into the gasifier being not enough to maintain the gasifier at a relatively high operating temperature through the oxidation of char and volatiles of pyrolysis, so the water-gas shift reaction in the backward direction was weakened, leading to low

CO production In addition, the lower operating temperature for the high percentage of CO2was less favorable for the endothermic Boudouard reaction, so it also led to the reduction in CO production

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When CO2 percentage in gasifying agent was lowered from 80% to 40%, mole fractions of H2

increased because the decreased percentage of CO2 relative to air reduced the reverse water-gas shift reaction, but when CO2 percentage was decreased further from 40% to 20%, the mole fractions of H2 decreased because more H2was consumed through the oxidization reaction On the other hand, mole fractions of CH4 increased fromCO2 composition of 20% to 60 % in gasifying agent due to less contribution of oxygen towards oxidation of CH4, and then decreased from CO2

composition of 60% to 80% in the gasifying agent since the rate of wood pyrolysis decreased with the decreasing operating temperature At a given CO2-to-biomass ratio, mole fractions of

CO2 increased with increasing mass percentage of CO2 in the gasifying agent due to more unreacted CO2

From the above analysis it was found that the concentrations of CO and CH4 reached the maximum when the CO2mass percentage was 60% in the gasifying agent, and the concentrations

of H2 reached the maximum when CO2 percentage was 40% As the sum of CO and CH4

concentrations was much higher than that of H2, it might be considered that 60% CO2 percentage

in the gasifying agent was the optimal operating condition for this gasifier, which could also be the proof-of-concept design from the view-point of cold gas efficiency and low heating value, as seen in Fig 8 and 9

3.2.4 Lower heating value and cold gas efficiency

Lower heating value (LHV) is defined as the amount of heat released by fully combusting a specified quantity without the latent heat of vaporization of the water in the combustion product [4]

The lower heating value of fuelLHVfuelcan be estimated by [37]:

2

LHV =33.9Y +102.9Y -11.2Y -2.5Y [MJ/kg] (21)

and CH4 in the producer gas Because higher operating temperature could be achieved with higher

= Volumetric flow rate of producer gas, m3/s

= Mass flow rate of fuel, kg/s

= Lower heating value of producer gas, MJ/m3

= Lower heating value of fuel, MJ/kg

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CO2-to-biomass ratio, more non-combustible components (CO2, H2O) could be converted into combustible components (CO, H2), the LHVgas increased with increasingCO2-to-biomass mass ratio [38]

According to Eq (26) CGE was proportional to the product of LHVgas and producer gas flow rate,

as both the producer gas flow rate and LHVgas increased with CO2-to-biomass ratio, the increase

of CGE with CO2-to-biomass ratio was more significant compared to that of LHVgas At a given

CO2-to-biomass ratio, LHVgas and CGE increased fromCO2 composition of 20% to 60% in the gasifying agent, but they would drop when CO2 percentages increased further to 80% in the gasifying agent It followed a similar trend to the variation of mole fraction of CO and CH4 with

CO2 percentage in the gasifying agent, because CO and CH4 accounted for a large fraction in the producer gases and were responsible for the higher LHVgas and CGE.As a result, 60% of CO2 in the gasifying agent could be defined as the optimal percentage, where CO2 addition positively compensated for the burnt fuel gas, due to its highest LHVgas and CGE

3.2.5 CO2 conversion ratio

CO2 conversion ratio is the ratio of the amount of CO2 converted in the gasification process to the amount of CO2 introduced into the gasifier [19]

The effect of CO2-to-biomass mass ratio on fractional CO2 conversion ratio for different mass percentages of CO2 in the gasifying agent was shown in Fig 10 Because larger CO2-to-biomass mass ratio could lead to higher operating temperature, which contributed to higher rate of the endothermic Boudouard reaction and reversed water-gas shift reaction, the predicted CO2

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conversion ratios increased with increasing CO2-to-biomass ratio for all mass percentages of CO2

in the gasifying agent At a given CO2-to-biomass ratio, CO2 conversion ratios increased with increasing mass percentages of CO2 in the gasifying agent because the higher concentration of

CO2 favored the Boudouard reaction and reverse water-gas shift reaction, so the utilization of

CO2 for char gasification and reverse water-gas shift reaction were relatively higher [19]

3.3 Effect of wood moisture contents

The moisture content of biomass was one of the main characteristics which affected gasification performance greatly [39] From the numerical results in section 3.2, it was shown that the gasifying agent with 60% of CO2 could produce optimal results for the producer gas compositions, LHVgas as well as CGE Therefore, in the study on the effect of moisture contents on the gasification process, the gasifying agent was composed of 40% of air and 60% of CO2 and CO2-to-biomass mass ratio was 2.49, and the moisture content of woodchips ranged from 5% to 20% wet basis

3.3.1 Mole fractions of producer gases

Figure 11 showed the effect of wood moisture content on the producer gas composition It could

be observed that the mole fraction of H2 increased while that of CO decreased with moisture content varying from 5% to 20% wet basis However, with the same variation in moisture content, the increase in the mole fraction of CO was found to be more pronounced as compared to that in the mole fraction of H2 with the same variation interval in moisture content [40] As expected, mole fraction of CO2 also increased with moisture content from 5% to 20% wet basis This was because the increase of partial pressure of H2O vapor led to an increased rate of the forward water-gas shift reaction and a decreased rate of the reversed water-gas shift reaction, i.e more

CO2 and H2was produced than CO and H2O Also, since the operating temperature decreased with moisture content, higher moisture content was less favorable for the endothermic char

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gasification with CO2 and H2O to produce CO Moreover, mole fraction of CH4 decreased with moisture content from 5% to 20% wet basis because more energy was lost as sensible heat of water vapor, thus it weakened the pyrolysis of wood, leading to lower CH4 production

3.3.2 Lower heating value, cold gas efficiency and CO2 conversion ratio

Besides the compositions of producer gas, the moisture content also affected the LHVgas, CGE and CO2 conversion ratio greatly The effects of wood moisture content on LHVgas, CGE and CO2

conversion ratio were shown in Fig.12 The predicted LHVgas decreased with increasing moisture content of wood from 5% to 20% wet basis, because there was a greater reduction in mole fraction of CO than the increase in mole fraction of H2 [41] Since the producer gas flow rate increased while LHVgas decreased with moisture content, the decrease of CGE with moisture content was less significant as compared to LHVgas [42].At a constant percentage of CO2 in the gasifying agent, the numerical results showed that mole fraction of CO2 increased with increasing wood moisture content, but the CO2 conversion ratio showed a decreasing trend Therefore, in order to achieve high fractional CO2 conversion, it was still recommended to dry the high moisture wood to a certain degree before introducing the wood into the gasifier

3.4 Effect of wood chip size

The particle size of wood was also an important parameter which affected the quality of producer gas and gasification performance The preparation and reduction of biomass to a required size was associated with an energy cost [43] The effect of particle size of wood was investigated with moisture content of 10% wet basis and 0.28m/s inlet velocity of gasifying agent (40% of air and 60% of CO2) on gasification performance in the fluidized bed gasifier For convenience, it was assumed that the particle size of chars, which were produced from the pyrolysis of wood and/or used as bed materials, was half of the particle size of woods [24]

3.4.1 Mole fractions of producer gases

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The effect of wood particle size on producer gas composition was shown in Fig.13 It was found that the mole fraction of CO2 increased while those of CO, H2 and CH4 decreased with increasing wood particle size from 2 to8mm Because partial oxidation of char with O2was assumed to be controlled by both kinetics and mass transfer, an increase in char particle size inhibited the mass transfer of O2 to the char surface This led to more O2 being consumed by the oxidation reactions

of volatiles to increase the production of CO2 On the other hand, with larger wood particles, the overall heat exchange surface area was reduced, hence the effective transfer of heat was weakened from gas phase to both wood and char particle phases, then the wood particles would experience less complete endothermic pyrolysis, resulting in a decrease of CO, H2 and CH4

production [44] Also, since the temperature of char phase decreased with increasing char particle size, the rates of endothermic char gasification with CO2 and H2O decreased with increasing particle size from 2 to 8mm Thus, it led to the reduction in CO and H2 production and CO2

conversion

3.4.2 Lower heating value, cold gas efficiency and CO2 conversion ratio

The effect of wood particle size on LHVgas, CGE and CO2 conversion ratio was shown in Fig.14

It was found that LHVgas decreased with increasing wood particle size from 2 to 8 mm due to the decreased mole fraction of CO, H2 and CH4, similar to the trend observed by Yin et al [43] Furthermore, both LHVgas and CO2 conversion ratio were decreased with increasing wood particle size Hence it was shown that smaller wood and char particle sizes were preferred in order to improve the performance of the gasifier

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the particle size of wood chip and char on the gasification performance were evaluated in terms of the gas compositions and temperature, lower heating values, cold gas efficiency and CO2

conversion ratio The following observations were made throughout the study,

1 With increasing CO2-to-biomass mass ratio, more oxygen would be available for the exothermic oxidation reactions, resulting in higher producer gas temperature, and higher mole fractions of CO in the producer gas, but lower mole fractions of H2 and CO2

2 At 60 w% of CO2 in the gasifying agent, the mole fractions of CO and CH4 could reach the maximum The lower heating value of producer gas and cold gas efficiency also had the highest values, so this was the optimal operating condition for the biomass gasification with CO2

3 The moisture content and particle size of woodchips had negative effects on the gasification performance in that the lower heating value of producer gas, cold gas efficiency and CO2

conversion ratio were all reduced with increasing moisture contents and particle size

This study might offer insights on biomass co-gasification with CO2, and aid designs of biomass gasifiers with high performance

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