Modeling and Experimental Studies on CO2 Gasification of Coarse

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Modelling and Experimental Studies on CO2 Gasification of Coarse Coal Char Particle Ashok Prabhakar, Anup Kumar Sadhukhan, Biswajit Kamila, and Parthapratim Gupta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03241 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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Modelling and Experimental Studies on CO2 Gasification of Coarse Coal Char Particle Ashok Prabhakar, Anup Kumar Sadhukhan*, Biswajit Kamila, Parthapratim Gupta

Department of Chemical Engineering, National Institute of Technology, Durgapur-713209, West Bengal, India

ABSTRACT: Gasification study of a single Indian sub-bituminous coal char particle, is carried out in the temperature range of 880-910 oC in pure CO2 atmosphere. Two temperatures 900 oC and 800 oC are used for char preparation and the char obtained at higher temperature is found to be more reactive. A fully transient non-isothermal model is developed incorporating the reaction kinetics along with transport limitations. Spatial and temporal variation of thermo-physical properties like thermal conductivity, diffusivity and density of the gas mixture and variable specific pore surface area and accessible porosity are included in the model. The model computation shows that the combined kinetics and heat transfer model is more effective to predict the experimental findings of the present authors and that reported in literature. The simulation study is carried out to assess the effect of reaction temperature, particle size and char reactivity on the particle temperature, conversion, gasification rate and CO and CO2 mass fractions within the porous volume of the particle.

Keywords: coal char, gasification, fully transient, partial differential equation, pore surface area, simulation.

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1. INTRODUCTION Coal is the leading source of energy in the 21st century as it fulfilled nearly half of the global energy demand over the past few decades, especially in the power sector.1 The role of coal is expected to continue to remain dominant as resources like oil and natural gas are depleting fast. The energy production from coal by combustion process emits a lot of greenhouse gases like CO2 which causes global warming. The use of alternative technologies like integrated gasification and combustion cycle (IGCC) and carbon capture and storage (CCS) may reduce the CO2 emission in environment and the resultant global warming significantly. The present research thrust for controlling the global warming is focussed towards the sequestration of CO2 from the flue gases of power plants and oil refineries. However the technique of CO2 sequestration followed by underground storage is not yet attractive due to inefficient capture rates, requirement of large storage sites and high cost. The other alternatives for CO2 sequestration are deep ocean sequestration and conversion of CO2 into mineral carbonates. Apart from various technical issues, both these processes need intensive capital investment. The researchers including Woycenko et al.2 recommended that the CO2 concentration in the gaseous or liquid stream should be at least 90% for its injection into the deep sea. As the flue gas from power plants and oil refineries generally contains only 12-15% CO2, it is difficult to capture the CO2 from it. Oxy-fuel combustion is an attractive alternative, though it suffers from a high adiabatic flame temperature. Recycling of flue gas with enriched oxygen may avoid high flame temperature as well as provide higher CO2 content (90% and above) in the flue gas.3 Gasification using CO2 is found to be one of the more cost effective routes for utilization of CO2 than conventional carbon capture technologies.4 The Boudouard reaction between coal char and CO2 may play a major role for reduction of greenhouse gas emission from industrial combustors and burners. Investigations on the gasification of coal using CO2 is crucial for


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better understanding of gasification characteristics, kinetics and design of industrial gasifiers. The gasification process of coal consists of two major steps; pyrolysis of raw coal and subsequent gasification of the residual char. Generally the char gasification rate being much slower than that of coal pyrolysis rate, the gasification of char is the rate determining step.5 Therefore the knowledge on reaction kinetics along with the mass and heat transport limitations during char gasification may provide the vital insight in the design of various types of industrial gasifier.6 A large number of studies on gasification kinetics for coal char-CO2 has been reported in literature.3 Most of the studies are limited to char fines in Thermogravimetric Analyser. Varying physicochemical properties of different carbonaceous materials affect the gasification reactivity in the presence of CO2. Sawettaporn et al.7 carried out the experimental investigation on the gasification of two different type of coal chars (Ban Pu lignite A and Lampang sub-bituminous B ) in the size range of 75-250 µm in pure CO2 atmosphere at the temperature range of 900-1100 oC. The char was prepared in the drop tube fixed bed reactor under nitrogen flow in the temperature range of 500-900 oC. The reactivity of char produced at the lowest pyrolysis temperature was found to be highest and the chars produced at the same pyrolysis temperature showed higher conversion at a given time with increasing gasification temperature. However, Wu et al.8 contradicted the effect of pyrolysis temperature on the gasification reactivity of coal char in CO2 atmosphere as observed by Sawettaporn et al.7. Li et al.9 investigated experimentally Chinese bituminous coal char of particle size of less than 150 µm at a temperature range of 970-1150 oC in CO2. They reported that increase in gasification temperature significantly increases carbon conversion and reduces the complete conversion time. Kim et al.10 used twelve different type of coal chars obtained from various coals having different volatile matters and ash content in the experimental gasification study with char


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particle size less than 250 µm at the temperature range of 1050-1400 oC. They found that the gasification temperature, coal type and particle size greatly affect the gasification reactivity. The gasification reactivity at 1350 oC was 6.9 times higher than that of 1050 oC for all twelve different type coals. They found that the gasification reaction shifted from chemical control regime to pore diffusion control with increase in the reaction temperature. They observed that the catalytic effect of alkali compounds in minerals decreased with increase in temperature. Experimental investigations were carried out by Mandapati et al.11 using four different type of coal char fines (less than 150 µm) at the temperature range of 800-1050 oC to study the inhibition effect of CO at different partial pressure during gasification reaction. Onedimensional kinetic-diffusion model was used to determine the contribution of diffusion resistance. Guangwei et al.12 carried out experimental studies in TGA apparatus in isothermal and non-isothermal condition on anthracite coal char fines (74 µm) at 1200 oC. While both fractional conversion profile and the DTG curve shifted towards the high temperature zone with the increase in heating rate under non-isothermal condition, the rise in final gasification temperature reduced the gasification time. Under isothermal condition, rise in gasification temperature increased the maximum gasification rate and reduced the time required to reach the peak rate. The experimental study by Lahijani et al.13 revealed that there is significant influence by alkali and alkaline earth metal on the CO2 gasification reactivity for both low rank coal char and biomass char. Kajitani et al.14 investigated the kinetics of coal char gasification in the temperature range 1100-1500 oC in a pressurized drop tube furnace, and observed the peak reaction rate at a carbon conversion of 40%. The reaction orders of gasification reaction with CO2 and steam were reported to be 0.73 and 0.86 respectively. Experimental data on pore surface area of partially gassified char was fitted with the random pore model and the structural parameter, ψ, was found to be 3 in case of CO2 and 14 in case of mixture of CO2 and oxygen as the


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gasifying agent. In a similar study by Ochoa et al.15 reported the value of structural parameter, ψ, as 4.7 for sub-bituminous coal char and 7 for high volatile bituminous coal char, where gasification reaction was carried out in the temperature range of 900-1260 oC and particle size less than 250 µm. Kajitani et al.16 further performed the gasification experiment with bituminous coal char in the size range of 30-40 µm and observed that the pore diffusion controls the gasification rate in the temperature range of 1000-1400 oC and the presence of CO with CO2 inhibits the reaction rate. Liu et al.17 showed that the gasification reaction of coal char is kinetically controlled in the temperature range of 700-1000 oC while it is pore diffusion-controlled in the temperature range of 1000-1400 oC. Hence it is apparent that the pore diffusion plays an important role during the gasification of millimetre-size char particle at higher temperature. Veca and Adrover18 studied the effect of gasification temperature for millimetre-size char particle (0.125-1.4 mm). They incorporated shrinking core model along with intrinsic reaction kinetics with the effect of pore diffusion through ash layer incorporated through Thiele modulus. The model suffers from the limitation that it does not give the information of concentration profile as a function of space co-ordinate and time. Jayaramam and Gokalp19 studied experimentally the effect of structural characteristics of partially converted char on gasification rate in presence of steam and CO2. They reported that the heating rate during devolatilization has significant effect on the reactivity of the resulting char during gasification. At higher temperature the devolatilization is rapid causing more pore openings than that at lower temperature which may provide the higher accessible porosity responsible for pore diffusion of CO2 within the interior of the char particle and reaction occurs throughout the entire volume of the char particle. Nearly all the investigation in the field of gasification of coal char in CO2 atmosphere is primarily limited to fine char particles incorporating isothermal kinetic model. Veca and Adrover18 assessed the effect of pore diffusional resistance in 1.4 mm particle size without


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incorporating the details of pore surface area, diffusion of reactant and product gases though pores, temperature gradient within the particles, endothermic heat of reaction of the gasification reaction, and temperature and composition dependent diffusivities of the gaseous components. In the present work the single porous coarse char particle obtained from sub-bituminous coal of Indian origin (Jharia) is used for the gasification study in pure CO2 and CO2–N2 atmosphere in the temperature range of 880-910 oC. The temporal profiles of the particle temperature and mass loss due to the chemical reactions are determined experimentally. A fully transient, non-isothermal Volume Reaction Model (VRM), incorporating heat transfer and reaction kinetics coupled with pore diffusion, is developed to validate the experimental observation of the present authors as well as that reported in literature. The non-equimolar multicomponent diffusion described by Stefan Maxwell equation along with variable thermophysical properties is also incorporated in the model. The model is expected to provide the vital inputs for integration of reactor modelling through CFD analysis.

2. MODEL 2.1. Gasification Chemistry. A single spherical porous char particle is assumed to react with the surrounding gas mixture containing CO2 and N2. The convective heat and mass transfer co-efficient depends on the particle size and operating conditions within the reactor. During the heterogeneous gasification reaction the CO2 gas diffuses through the pores from bulk gas phase and reacts within the internal pore surface area of the char particle and, consequently, the reaction rate depends on the pore surface area. Initially the gasification reaction is limited within the pores around the particle exterior surface and gradually more pores are accessible to the gaseous reactant due to internal reactions, causing the reaction to proceed throughout the entire volume of the particle according to Volume Reaction Model


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(VRM). The char particle is assumed to consist of reactive carbon and non-reactive ash. During gasification only reactive component of the char particle takes part in the heterogeneous surface reaction inside the pore of the particle to produce CO which diffuses out towards the bulk phase. The chemical reaction between solid carbon and gaseous CO2 may be represented by Boudouard reaction. k1 C ( s )  CO 2 ( g )  2CO ( g )

E 2 Rs  k s0 exp  pCO2 T moles of CO 2 consumed s m of pore surface  RT 





As one mole of CO2 reacts with solid carbon producing two moles of CO, the inward diffusion flux of CO2 is hindered by the outward diffusion flux of CO, causing non-equimolar counter diffusion. 2.2. Pore Structure and Surface Area. Higher char surface area leads to higher

gasification reactivity.20 During the char preparation stage by pyrolysis, pore enlargement and pore blocking phenomena greatly influence the development of porous structure of the resulting char.21 The pore structure of the char particle also undergoes a lot of changes during gasification reaction with CO2. As explained by Mahamud22 the opening of the pores of the reacting char during the course of gas-solid heterogeneous reaction results from the transformation of the micropores into the macropores through the intermediate formation of mesopores. As a result, the mass transfer resistance decrease due to the increase in accessible porosity, ɛ and the effective diffusivity, Dke. ε0 and εf, the initial and final accessible porosity, were measured experimentally using a mercury porosimeter and porosity at any conversion was estimated by linear interpolation using the following equation.

   0   f   0 

(1)

The global carbon conversion was experimentally measured by estimating ash content of the partially gasified char samples at various time instances. The global carbon conversion for experimental studies was estimated using the following equation


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

W0  W 

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(2a)

W0  Wash 

The constancy of the mass of ash throughout the progress of reaction is assumed. Possible reduction of silica in ash at very high temperatures may question the applicability of this assumption. However, as the particle temperature in the present study is below 1000 oC, the chance of reduction of silica is quite remote thermodynamically and the assumption of constancy of the mass of ash is reasonably valid. A number of authors have used this assumption like Jayaramam and Gokalp19, Malekshahian and Hill20, Kim et al.10 in investigations on gasification and combustion of coal. The local carbon conversion for simulation studies was calculated using the local carbon content as per the following equation.

 1 

Wc Wc 0

(2b)

S is the instantaneous specific pore surface area per unit volume of solid char which changes with the extent of local carbon conversion (ξ) following the random pore model.23-25 S0 is the specific pore surface area initially available at zero carbon burn off. The parameter ψ may be obtained by fitting the experimental values of S (ξ) at different carbon conversion to eq. (3).

S  ( 1  ξ). 1  ψ ln ( 1  ξ) S0

(3)

2.3. Governing Equations for Conservation. Governing equations for the conservation of mass and energy within the porous solid particle phase along with the initial and boundary conditions are presented below. The component mass balance in terms of the mass fraction of individual gaseous component may be presented as:

Yk ,s   1  1    Rv  k M k ( s  g ,s Yk ,s )  [ 2 (r 2 N tg ,s M av,s Yk ,s )]  Dke,s  g ,s 2  r 2 t  r  r  r r r  

(4)

Rv  R s S

(5)


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In the above equation of solid particle phase the convective term contains the molar flux N tg ,s , arising due to non-equimolar chemical reaction and the diffusive term contains the effective diffusivity Dke, s where the effect of porosity has been included. Total mole balance of the gas mixture: 3 ( .ct , s ) 1  2  2 (r N tg ,s )  k Rv t r r k 1

(6)

The mass balance equation of solid carbon in char particle may be expressed as: dWc   Rv M c dt

(7)

Rv varies spatially due to the spatial variation of temperature and composition of the gaseous components. Hence Wc is dependent both on time and space co-ordinate. Energy balance:  1  2 1   2 Ts (c ps  sTs )  2 (r N tg ,s M av ,s c psTs )  s 2  r t r r r r  r

   Rv (H ) 

(8)

The gas mixture present in the pore volume is considered to be in thermal equilibrium with the solid particle. The heat transfer within the porous char proceeds through conduction, while at the external particle surface (boundary) it follows both convective and radiative modes of heat transfer.

Initial conditions: At t = 0 Yk , s (r ,0)  Yk0,s (r )

(9)

Ts (r ,0)  Ts0 (r )

(10)

N tg,s (r ,0)  0

(11)

Wc  Wc0

(12)


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Boundary Conditions: Solid Particle Phase, at r = 0 (Particle centre) N tg , s  0  Dke , s

Yk , s

At r = rs

 e

r 0

r

T  e s r

 Dk e,s

(13)

r 0

N tg , s c g ,s

Yk , s

r 0

0

(Due to spherical symmetry)

 N tg , s M av , s c pg , s Ts

r 0

(14)

0

(Particle surface)

Yk ,s r

Ts r



r  rs

2.4.

r  rs



N tg ,s c g ,s

Yk ,s

r  rs

 N tg , s M av, s c pg, s Ts

 k m (Yis  Yib )

r  rs

(15)

 hc (Ts  Tb ) r  r   r (Ts4  Tb4 ) s

r  rs

(16)

Estimation of Thermo-physical Properties. The thermo-physical properties

involved in the conservation equations are usually treated as constant in most of the works reported in literature. However, in the present work the properties like specific heat, thermal conductivity, heat of reaction, etc. are assumed to vary with temperature and composition. The Chapman–Enskog equation was used to calculate the viscosity of the gaseous components while the Wilke method was used in order to determine the viscosity of the gas mixtures. The effective thermal conductivity of the porous char is assumed to depend both on the thermal conductivity of gas mixture in the pores and the thermal conductivity of the solid char. The more details on various equations and expressions for thermo-physical properties are available in the previous work by the present authors.30 The Chapman–Enskog equation is employed to determine the binary diffusion coefficients Dkj for various gaseous components. Considering the presence of a multicomponent gas mixture, the effect of other gaseous components on Dkj is incorporated through Stefan–


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Maxwell equation. Finally, the effective diffusivity is calculated using the following equation incorporating the influence of porous texture of the particle. Dk e,s 

 Dk e 

(17)

The tortuosity factor, τ, is assumed to be 2.2.30 The convective heat transfer co-efficient at the particle surface during gasification under the flow of CO2 gas is estimated using the Ranz and Marshall correlation.26 The mass transfer co-efficient for gaseous species is estimated using the Baxter and Robinson correlation.27

3. EXPERIMENTAL 3.1. Char Preparation from Coal. The Indian sub-bituminous Jharia coal particles in the size range of 10-12 mm were heated in the nitrogen atmosphere for one hour at temperatures of 900 and 800 oC to form char # 1 and char # 2 respectively. Each particle was filed to be shaped into sphere. The particle dimension in three mutually perpendicular directions were measured using a vernier scale. The equivalent diameter of the particle was reported as the cubic root of the three measured dimensions, and the sphericity of the particle was taken as the ratio of the equivalent diameter to the longest dimensions. Char particles with equivalent diameter of 8 ± 0.06 mm and sphericity of 0.88 ± 0.02 were chosen for experiments. The proximate and ultimate analysis of resulting char are furnished in Table 1. As the coal is heated for a long period of time, most of the volatiles are released leaving behind the solid char as residue and both the resulting chars show almost the same proximate and ultimate analysis. The higher temperature causes faster removal rate of the volatiles from the inner depth of the coal particles creating more pore openings (Sadhukhan et al.30) and greater pore surface area during the heterogeneous surface reaction of carbon with CO2.


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In order to measure the centre temperature of the char particle, a pin hole was made with the help of 1 mm drill bit up to the centre of the particle. The centre point temperature was measured during the course of gasification using a fine sheathed Chromel-Alumel thermocouple. The thermocouple wires were flexible enough to provide negligible impact for mass and heat transfer operation.28 The thermocouple was fixed into the hole using high temperature resistant iron-cement sealant (Figure 1 b). 3.2. Gasification of Char Sample. Gasification two char samples was carried out in the CO2 environment in the isothermal mass loss apparatus (Figure 1 a). A tubular reactor, called mass loss apparatus, having 60 mm inside diameter and 1 m of reactor height, was made of Inconel alloy. The insulation of mica sheet was enclosed around the super canthal heating wire wrapped around the reactor. One more layer of asbestos rope was coiled on the mica sheet followed by a thick layer of plaster of Paris casting. The inside temperature of the reactor was controlled by a PI controller. An electronic micro balance, fitted right above the mass loss apparatus, monitored the instantaneous mass of the char sample. A basket containing the char particle was hung from the weighing scale into the furnace, kept at a desired constant temperature. Two char particles were used in each run in CO2 environment (99.9 %) for gasification study. 3.3. Estimation of Pore Surface Area and Porosity of Partially Reacted Char. Partially gasified char samples at different time instants were withdrawn from the reactor and quickly quenched by the liquid nitrogen to prevent possible oxidation by air. A part of the char sample was crushed and burned in a muffle furnace to estimate the carbon conversion. The remaining part was used for measurement of specific surface area at various conversion levels using Nitrogen adsorption technique at −196 °C with automatic sorption analyzer (Autosorb-1, Model No. ASIC-9 by Quantachrome instruments. Brunauer–Eemmett–Teller (BET) analysis software available with the instrument was used to determine the specific


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surface area. The initial and final porosity of the char particle was determined by using Mercury porosimeter (Pore-Master-GT Model No. PM-33-6). The more details on the determination of specific pore surface area and the pore parameters are available elsewhere.30

4. RESULTS AND DISCUSSION The present model incorporates heat and mass transfer resistances within the particle to predict the spatial profiles of temperature and conversion which affect the gasification behaviour significantly. Spatial and temporal variation of thermo-physical properties like thermal conductivity, diffusivity and density of the gas mixture and variable specific pore surface area and accessible porosity are included in the model. Such generalised nonisothermal, non-equimolar, fully transient volume reaction model with variable thermosphysical and structural properties has not much been reported in literature so far. A finite element solver (COMSOL Multiphysics) is employed to solve the non-linear coupled mass and energy balance equations.

4.1. Internal Surface Area and Porosity Measurement. The internal surface area of both the partially gasified chars are determined at different conversion levels. The initial surface areas are estimated to be 23.8 cm2/g for char # 1 and 17 cm2/g for char # 2 and the char # 1 is found to be more reactive than char #2. The internal surface area of partially gasified char # 1 of 8 mm diameter at a reactor temperature of 910 oC for four different conversion level of 0.15, 0.45, and 0.75 and 0.85 are found to be 23.7, 24.8, 16.7 and 5.1 m2/g respectively. Similarly for char # 2 the internal surface area of partially gasified char during gasification for different conversion level of 0.15, 0.45, 0.75 and 0.95 is found to be 20.0, 19.0, 12.14 and 3.5 m2/g respectively. The porosity at the zero carbon conversion and at complete burn-off is found to be 0.15 and 0.65 respectively. The experimentally observed


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surface area at various conversion is fitted with random pore model and the pore parameter ψ found to be 6 and 2 for char 1 and char 2 respectively.

4.2. Validation of the Model with Experimental Results. The conversion profile for char gasification at various time instants are obtained from experimental runs carried out in CO2 environment at 910 oC for 8 mm char particle (Figure 2). Most of the works on gasification reported in literature18,19 considered only isothermal kinetic model. Such kinetic model predicts the experimental findings quite well for fine particles up to 0.5 mm where hardly any temperature gradient exists within the particle. However the char gasification in presence of CO2 proceeds through endothermic Boudouard reaction, which is prominent at temperatures above 700 oC

29

, when the Gibbs free energy becomes negative (ΔG =

ΔH−TΔS). The endothermic gasification reaction generates a temperature gradient within the particle. The model developed in this work is validated extensively with the experimental results carried out by the authors for the purpose. It is observed that the conversion profile predicted from the isothermal kinetic model shows a mean deviation of about 8% from the experimental results, while the deviation is only 2.5% for fully transient coupled kinetic-heat transfer model (Figure 2). The time required for 94 % conversion for 8 mm char particle predicted by isothermal kinetic model only is 142 min, whereas the combined model predicts 165 min, which is very near to the experimental value of 173 min. However, for very small particles the heat transfer resistance is not significant. For 0.5 mm char particle (char #1) the experimentally observed conversion times for 25%, 48%, 82 % and 95% conversion during gasification with CO2 are found to be 6.6, 11, 20 and 29 min respectively. The combined kinetic-heat transfer model predicts these to be 6.50, 11.10, 20.20 and 28.80 min respectively with the maximum deviation of only 1.5%. Similar computation


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using the kinetic model, neglecting the heat transfer resistance, exhibits these values as 6.48, 11.07, 20.14 and 28.65 min respectively. This shows that the kinetic model is adequate for the particles having size less than 0.5 mm. Figure 3 indicates that the experimental conversion profile matches well with the model prediction for two different char particles at reactor temperatures 880 oC and 910 oC. In both cases the maximum relative main error are observed to be about 3%. It is observed experimentally that the burn-out times (time required to achieve the 99% of conversion) for char # 1 at 910oC and at 880oC are 117 min and 188 min respectively, while for char # 2 these are 212 min and 350 min respectively. The higher the reactor temperature lower is the burn-out time and vice versa and the same trend is followed by both the chars. Due to higher reactivity, the char #1 it takes less time to react than char #2 under identical gasification conditions. The combined kinetic-heat transfer model predicts these experimental findings well. Hence, it is concluded that the initial surface area and pore parameter play a significant role in gasification process. The model also predicts the experimentally observed particle temperature profiles very well (Figure 4 a & b). It is observed that particle is first heated up to 896 oC by convective and radiative heat transfer in about 60 s after which the particle centre temperature starts falling due increase in the rate of endothermic gasification reaction up to 882 oC at 3480 s, where the conversion is 0.70. Subsequently, due to ash layer build up near the particle surface, the reaction becomes ash layer diffusion controlled. The reaction rate decreases, resulting in less heat absorption due to gasification reaction, leading to the rise of the centre temperature gradually to the reactor temperature (Figure 4b). Figure 5 exhibits a close match between the model-predicted and experimentally observed gasification reaction rate with time for chars # 1 and # 2 at two different reactor temperatures. At the outset the reaction rate is insignificant due to low initial particle temperature. The


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Page 16 of 33

particle then gets heated up resulting in an increase in the reaction rate which reaches a maxima and then drops due to the consumption of the solid reactant. The gasification rate of the more reactive char # 1 at 910 oC shows the maximum gasification rate of 4.62x10-8 kg/s after 1300 s at the conversion of 0.24, while at 880 oC the maximum gasification rate of 3.12x10-8 kg/s is achieved after 2900 s at the conversion of 0.38. Similar trend is observed for the less reactive char # 2 with maximum reaction rates of 1.65x10-8 and 2.82x10-8 kg/s achieved after the conversions of 0.27 and 0.33 at 880 oC and at 910 oC respectively. The char #1 being more reactive due to its higher initial pore surface area and higher value of pore parameter (ψ), its gasification rate is higher than that of char #2 at the same reactor temperature. Consequently, the gasification is completed within 117 min for char # 1, while the char #2 takes longer duration of 212 min at 910oC. However, the total area under the gasification rate vs time curves for both char #1 and char #2 are the same at two temperatures, indicating the same total mass of solid reactant in the reacting char getting consumed at complete conversion at two different temperatures. In the present study the variation of surface area with carbon conversion is modelled using random pore model25. According to this model, the surface area starts increasing from the initial surface areas of 23.8 cm2/g for char # 1 and 17 cm2/g for char # 2 with carbon conversion, reaches a maxima and then reduces to a very small value for a fully converted char. The reaction rates, being dependent on the specific pore surface area (Eq. 5), also show the similar trend (Fig. 5). At the reactor temperature of 880 oC too, reaction rate profiles exhibit similar but less steep trends as the rate of reaction is slower. Fig. 5 shows that the rate profile for char #1 at 880 oC and that of char #2 at 910 oC are comparable. This is due to the fact that the higher kinetic resistance at 880oC for char #1 is compensated by higher initial pore surface area and the value of the pore parameter in this case.


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Figure 6 validates experimentally the model prediction of the reaction time to achieve 50% (Figure 6a) and 99% (Figure 6b) conversions at different reactor temperatures. Both t0.5 and t0.99 decrease with the rise in reactor temperature which was also reported by Jayaraman and Gokalp.19 Two different chars reveal similar trend though with different slopes. Char # 1, being more reactive, takes lesser time than char # 2 to reach a given conversion at all reactor temperatures. The experimentally determined values of t0.5 and t0.99 at different reactor temperatures are in good agreement with the model prediction. t0.5 increases more than three times and four times respectively for char # 1 and char # 2 when the reactor temperature decrease from 910 to 840 oC respectively. t0.99 increases by about four times for both chars on decreasing the reactor temperature from 910 to 840 oC. It is observed that though there is a wide difference between t0.5 and t0.99 at lower reactor temperature for both chars due to their difference in reactivity, the difference gradually diminishes as the reactor temperature increases. 4.3. Validation of the Model with Published Experimental Results. The model is further validated against the experimental results reported by Jayaraman and Gokalp19 for gasification of 3 mm char particle in CO2 and N2 atmosphere. The kinetic-heat transfer model shows a better fit than the kinetic model at both the reactor temperatures (Figure 7) which reinforces the merit of the model developed. The experimentally observed reaction rate19 is also found to match the present model prediction well (Figure 8). The maximum relative mean error is within 3% in both cases. The model also shows good validation with the experimental results of Veca and Adrover18 who gasified sub-bituminous coal char of diameter 1.0-1.4 mm in pure CO2 environment under various temperatures in Thermogravimetric Analyser (Figure 9). 4.4. Simulation Studies


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4.4.1. Effect of Particle Size. The average particle temperature decreases with the particle size due to additional heat transfer resistance (Figure 10). The small reacting char particle of size up to 0.5 mm remains nearly at the reactor temperature of 950 oC except during the initial heating period of about 10 s. This is due to the fact that for small particles the heat absorbed by the endothermic gasification reaction is balanced by the inward heat transfer by convection and radiation through the external surface of the particle. There exists hardly any temperature gradient within the particle as the heat transfer resistance is almost negligible. But as the particle size increases, the heat absorbed by endothermic gasification reaction is higher than that supplied by the heat transfer through the external surface, causing the particle temperature drop lower than the reactor temperature. It is observed that the 4 mm particle is first heated to 939 oC, lower than the reactor temperature of 950 oC, in 20 s and then the particle temperature starts falling upto the minima of 933 oC at 1200 s. Then the endothermic reaction rate starts falling due to depletion of active reacting carbon mass and the particle temperature stars rising, eventually reaching the reactor temperature. The 8 mm particle is heated up to 930 oC in 40 s and its temperature then starts falling upto the minima of 920 oC at 1290 s beyond which it increases gradually to reach the reactor temperature. The particle temperature is one of the important parameters which dictates the conversion profile of char particle during gasification as higher temperature enhances the reaction rate. The conversion at a given time instant decreases with the increase in particle size (Figure 11). The inaccuracy resulting from the use of isothermal kinetic model is demonstrated by comparing the conversion profiles predicted by the isothermal kinetic model and by combined kineticheat transfer model for different particle sizes at reactor temperature of 950 oC. The conversion with kinetic-heat transfer model is lower due to additional heat transfer resistance, particularly for larger char particles. Consequently, t0.99 predicted by isothermal kinetic model is much lower than that predicted by combined kinetic-heat transfer model.


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Figure 12a and 12b show the model predicted temporal variation of mass fraction of CO2 and CO respectively at the centre of the char particle for different particle sizes during gasification in presence of pure CO2 environment. At the outset, mass fraction of CO2 is 1 as the pores are filled with the reacting gas CO2 only. As the reaction proceeds, CO is formed by gasification reaction causing the mass fraction of CO2 to decrease gradually. When the active carbon content of the char falls significantly due gasification reaction, the reaction rate starts decreasing along with an increase in the CO2 mass fraction with the profile showing minima. Though a similar trend is observed for all particle sizes, lower CO2 mass fraction is exhibited for larger particles due to increase in the diffusion resistance. A reverse trend is predicted by the model for the CO mass fraction which starts with zero and passes through a maxima due to its generation by gasification reaction (Figure 12b).

4.4.2. Effect of Reactor Temperature. Figure 13 shows the variation of model predicted gasification rate with conversion at different reactor temperatures. At lower reactor temperature the conversion profile is flat but with rise in temperature it becomes steeper. The reaction rate increases with time as the particle gets heated up and the reaction proceeds throughout the entire char volume due to the porous nature of the char. However as the conversion increases the active carbon content and the active reaction surface area decrease causing the reaction rate to fall making the profiles pass through a maxima which is prominent at higher reactor temperature. Both char # 1 and char # 2 show similar rate profiles at various reactor temperatures. However, as char # 1 has higher reactivity due to higher initial surface area and higher pore parameter values, its reaction rate is much greater than that of char # 2 at a given conversion and reactor temperature.


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5. CONCLUSION 1. The experimental findings show that the gasification reactivity of pyrolysed char depends on the pyrolysis temperature. The char produced at higher pyrolysis temperature is more reactive due to the increased pore surface area and accessible porosity. 2. The model developed by the authors is found to predict the experimental results of the authors and published by other researchers18,19 well. 3. The centre particle temperature first increases due to the particle heat-up; then decreases due to the endothermic gasification reaction, and finally increases gradually to the reactor temperature. The model predicts these experimental observations nicely. This effect is more dominant for larger particles. Particles up to 0.5 mm size remain almost at the reactor temperature after the initial particle heat up due to negligible heat transfer resistance. 4. The gasification rate increases with time, reaches a maxima and then decreases again, the effect being more pronounced at higher temperature. 5. The CO2 mass fraction at the centre of the particle decreases with time, reaches a minima and then increases again with the CO2 mass fraction exhibiting a mirror image. This effect is more prominent for larger particles.

AUTHOR INFORMATION Corresponding author: Telephone: +91-9434788048. E-mail: [email protected].

ACKNOWLEDGMENTS The authors acknowledge the financial support received from the Department of Science and Technology (DST), the Government of India. The Fund for Improvement of S&T Infrastructure (FIST) program was utilized to upgrade the combustion laboratory. Most of the experimental work has been carried out in the Combustion Engineering Laboratory at the Department of Chemical Engineering of the National Institute of Technology, Durgapur, India. 20 ACS Paragon Plus Environment

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Energy & Fuels

NOMENCLATURE cg = Concentration of gaseous component (mol m-3) cp = Heat capacity (J kg−1 K−1) ct = Total concentration of gaseous mixture (mol m-3) Dke = Effective diffusivity of component k (m2 s-1) Dp = Particle diameter (mm) E = Activation energy (J mol−1) ks0 = Pre-exponential factor of heterogeneous reactions (mol K m−2 atm−1 s−1) M = Molecular weight (kg mol-1) Ntg,s = Total molar flux of gas mixture inside the pore (mol m-2 s-1) pco2 = Partial pressure of CO2 (atm) r = Radial distance (m) R = Universal gas constant 8.314 (J mol−1 K−1) Rs = Surface reaction rate of CO2 gasification (mol m−2 s−1) Rv = Volume reaction rate (mol m-3 s-1) S = Specific pore surface area (m2 m−3) T = Temperature (K) t = Time (s) W=instantaneous mass of solid char (kg) Wash= mass of ash in char sample (kg) Wc = Instantaneous mass concentration of carbon in solid char (kg m-3) W0 = initial mass of solid carbon (kg) Yk = Mass fraction of component k Greek letters


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ΔH = Heat of reaction (J mol−1) ε = Porosity εf = Char porosity at full carbon conversion ε0 = Char porosity at zero carbon conversion ϵr = Char emissivity γk = Stoichiometric coefficient for component k λ = Thermal conductivity of char (J m−1 s−1 K−1) λe = Effective thermal conductivity of char (W m-1 K-1) ρ = Density (Kg m-3) σ = Stefan–Boltzmann constant, 5.67×10-8 (W m-2 K-4) ψ = Pore parameter τ = Tortuosity factor of char ξ = Local carbon conversion in solid char

Subscripts av = Average value b = Bulk value c = Carbon g = Gas phase s = Solid phase

REFERENCES (1) World Energy Outlook, IEA, 2011, pg 353. (2) Woycenko, D.M.; van de Kamp, W.L.; Roberts, P.A. Combustion of pulverized coal in


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Energy & Fuels

a mixture of oxygen and recycled flue gas. Report No. JOU2-CT92-0093, IFRF Doc F98/Y/4, Summary of the APG research program, 1995. (3) Irfan, M.F.; Usman, M.R.; Kusakabe, K. Coal gasification in CO2 atmosphere and its kinetics since 1948 : A brief review. Energy 2011, 36, 12–40. (4) Skodras, G.; Nenes, G.; Zafeiriou, N. Low rank coal-CO2 gasification : Experimental study, analysis of the kinetic parameters by Weibull distribution and compensation effect. Appl Therm Eng 2015, 74, 111–8. (5) Ahmed, I.I.; Gupta, A.K. Particle Size , Porosity and Temperature Effects on Char Conversion. AIAA 2011, 49, 1–16. (6) Feng, B.; Bhatia, S.K. On the validity of thermogravimetric determination of carbon gasification kinetics. Chemical Engineering Science 2002, 57, 2907–20. (7) Sawettaporn, S.; Bunyakiat, K.; Kitiyanan, B. CO2 gasification of Thai coal chars : Kinetics and reactivity studies. Korean J. Chem. Eng 2009, 26, 1009–15. (8) Wu, S.; Gu, J.; Zhang, X.; Wu, Y.; Gao, J. Variation of Carbon Crystalline Structures and CO2 Gasification Reactivity of Shenfu Coal Chars at Elevated Temperatures. Energy & Fuel 2008, 22, 199–206. (9) Li, P.; Yu, Q.; Xie, H.; Qin, Q. Wang K, CO2 Gasification Rate Analysis of Datong Coal Using Slag Granules as Heat Carrier for Heat Recovery from Blast Furnace Slag by Using a Chemical Reaction. Energy & Fuel 2013, 27, 4810-4817. (10) Kim, Y.T.; Seo, D.K.; Hwang, J. Study of the Effect of Coal Type and Particle Size on Char-CO2 Gasification via Gas Analysis. Energy & Fuel 2011, 25, 5044–54. (11) Mandapati, R.N.; Daggupati, S.; Mahajani, S.M.; Aghalayam, P.; Sapru, R.K.; Sharma, R.K. et al., Experiments and Kinetic Modeling for CO2 Gasi fi cation of Indian Coal Chars in the Context of Underground Coal Gasification. Ind. Eng. Chem. Res 2012, 51, 15041-15052. (12) Wang, G.; Zhang, J.; Shao, J.; Li, K.; Zuo, H. Investigation of non-isothermal and isothermal gasification process of coal char using different kinetic model. Int J Min Sci Technol 2015, 25, 15-21. (13) Lahijani, P.; Alimuddin, Z.; Rahman, A.; Mohammadi, M. Bioresource Technology CO2 gasification reactivity of biomass char : Catalytic influence of alkali , alkaline earth and transition metal salts. Bioresour Technol. 2013, 144, 88–95. (14) Kajitani, S.; Hara, S.; Matsuda, H. Gasification rate analysis of coal char with a pressurized drop tube furnace. Fuel 2002, 81, 2–6.


Energy & Fuels

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(15) Ochoa, J.; Cassanello, M.; Bonelli, P.; Cukierman, A. CO2 gasification of Argentinean coal chars: a kinetic characterization. Fuel Process Technol 2001, 74, 161-176. (16) Kajitani, S.; Suzuki, N.; Ashizawa, M.; Hara, S. CO2 gasification rate analysis of coal char in entrained flow coal gasifier. Fuel 2006, 85, 163–169. (17) Liu, G.; Tate, A.G.; Bryant, G.W.; Wall, T.F. Mathematical modeling of coal char reactivity with CO2 at high pressures and temperatures. Fuel 2000, 79, 1145–54. (18) Veca, E.; Adrover, A. Isothermal kinetics of char-coal gasification with pure CO2. Fuel 2014, 123, 151–157. (19) Jayaraman K; Gokalp I. Effect of char generation method on steam , CO2 and blended mixture gasification of high ash Turkish coals. Fuel 2015, 153, 320–327. (20) Malekshahian, M.; Hill, J.M. Effect of Pyrolysis and CO2 Gasification Pressure on the Surface Area and Pore Size Distribution of Petroleum Coke. Energy & Fuel 2011, 25, 5250–5256. (21) Gale, T.K.; Bartholomew, C.H.; Fletcher, T.H. Decreases in the Swelling and Porosity of Bituminous Coals during Devolatilization at High Heating Rates. Combustion and Flame 1995, 100, 94–100. (22) Marı, M. Textural changes during CO2 activation of chars : A fractal approach. Applied Surface Science 2007, 253, 6019–6031. (23)Kumar, A.; Gupta, P.; Kumar, R. Characterization of porous structure of coal char from a single devolatilized coal particle : Coal combustion in a fluidized bed, Fuel Process Technol 2009, 90, 692–700. (24) Krishna, R.; Wesselingh, J.A. The Maxwell-Stefan approach to mass transfer. Chemical Engineering Science 1997, 52, 861–911. (25) Bhatia, S.K.; Perlmutter, D.D. A Rondom Pore Model for Fluid-Solid Reactions. AIChE J  1980, 26, 379–386. (26) Ranz, W. E.; Marshall, W. R. Evaporation from Drops. Chem. Eng. Prog 1952, 48, 141-146. (27) Bharadwaj, A.; Baxter, L.L.; Robinson, A.L. Effects of Intraparticle Heat and Mass Transfer on Biomass Devolatilization : Experimental Results and Model Predictions. 24 ACS Paragon Plus Environment

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Energy & Fuel 2004, 18, 1021–1031. (28) Hayhurst, A.N.; Parmar, M.S. Measurement of the Mass Transfer Coefficient and Sherwood Number for Carbon Spheres Burning in a Bubbling Fluidized Bed. Combustion and flame 2002, 130, 361-375. (29) Hunt, J.; Ferrari, A.; Lita, A.; Crosswhite, M.; Ashley, B.; Stiegman, A.E. Microwave-Specific Enhancement of the Carbon-Carbon Dioxide (Boudouard) Reaction. J. Phys.Chem 2013, 117, 26871-36880. (30) Sadhukhan, A.K.; Gupta, P.; Saha, R.K. Modeling and Experimental Studies on Combustion Characteristics of Porous Coal Char : Volume Reaction Model. Int J Chem Kinet 2010, 42, 299–315.


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Table 1. Proximate and Ultimate Analysis of Coal Sample Proximate Analysis (db) (Wt. %)

Ultimate Analysis (daf)

Raw Coal

Raw Coal

(Wt. %)

Volatiles

19.0

Carbon

86.46

Ash

28.8

Nitrogen

1.82

Fixed Carbon

51.2

Hydrogen

4.51

Sulphur

0.15

Oxygen (by diff.)

7.06

Char #1 (prepared at 900 0C)


Volatiles

2.10

Carbon

Ash

33.24

Nitrogen

1.76

Fixed carbon

64.66

Hydrogen

0.37

Sulphur

0.02

Oxygen (By difference)

8.45

89.40



Volatiles

2.70

Carbon

Ash

32.77

Nitrogen

1.76

Fixed carbon

64.53

Hydrogen

0.57

Sulphur

0.04

Oxygen (By difference)

8.73

88.90


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(a) (b)

Temperature measurement

Weighing balance

Temperature measurement

Temperature controller

Rotameter

Iron-cement

Computerized data acquisition

Drilled channel 1 mm

Isothermal mass loss apparatus

Reactor tube Heating coil and insulation

8 mm Gas Preheater

CO2

Figure 1. (a) Schematic of the isothermal mass loss apparatus, (b) Measurement of the particle centre temperature. 1.0 1

2

0.8

Char conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.6

1. Kinetic Model 2. Kinetic and Heat Tranfer Model

0.4

Model

0.2

Experimental 0.0 0

2000

4000

6000

8000

10000

12000

Time (s)

Figure 2. Comparison of experimental data of the carbon conversion with model prediction: Gasification of char # 2 (dp = 8 mm, Tb = 910 oC) in presence of CO2.


Energy & Fuels

1.0

Char conversion

0.8

0.6 Model 0.4

Experimental

0.2

0.0 0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

Time (s)

Figure 3. Comparison of experimental data of the carbon conversion with model prediction: Gasification of coal char (dp = 8 mm) in presence of CO2.

(a) (b) 1000 910 800

920 910

Temperature (oC)

Temperature (oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600 Model Experimental

400

900 890

Model Experimental

880 870 860

200

850 0

0 0

20

40

3000

6000

9000

12000

15000

60 Time (s)

Time (s)

Figure 4. Comparison of experimental data of the char centre temperature with model prediction: (a) Temperature profile at initial stage (b) Temperature profile for whole time span, Gasification of char # 1 (dp = 8 mm, Tb = 910 oC) in presence of CO2.


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Carbon gasification rate (kg/s)

5.0E-08 Char 1 at 910 oC 4.0E-08 Char 1 at 880 oC

Model

3.0E-08

Experimental Char 2 at 910

oC

2.0E-08 Char 2 at 880 oC 1.0E-08

0.0E+00 0

2000

4000

6000

8000

10000

12000

Time (s)

Figure 5. Comparison of experimental data of the gasification rate of char in presence of CO2 with model prediction: (dp = 8 mm).

(a)

(b)

300

1000 Experimental

250

Experimental

200

800

Model

Model

t0.99, min

t0.5, min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

150 100 50

600 400 200

0

0

840

880

920 Temperature

960

1000

840

(oC)

890

940

990

Temperautre (oC)

Figure 6. Comparison of experimental data of reaction times to achieve conversion of (a) 50% and (b) 99% with model prediction: gasification of char in presence of CO2 (dp = 8 mm).


Energy & Fuels

1.0 950 oC 0.8

Char conversion

900 oC 0.6

Present model Isothermal kinetic model

0.4

Expt_Jayaraman and Expt_Kandasamy et al. Gokalp19 [19]

0.2

0.0 0

500

1000

1500

2000

2500

3000

3500

Time (s)

Figure 7. Comparison of experimental data19 of the carbon conversion with model prediction: Gasification of char (dp= 3 mm) in presence of CO2.

0.14 Expt_Jayaraman and Expt_Jayaraman 19 Gokalp and Gokalp[19]

950 oC 0.12

Reaction rate (min-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Model

0.1 0.08 900 oC 0.06 0.04 0.02 0 0

0.2

0.4

0.6

0.8

1

Char conversion

Figure 8. Comparison of experimental data19 of carbon gasification rate with model prediction: Gasification of char (dp= 3 mm) in the presence of CO2 atmosphere.


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1 900 oC 850 oC

Char conversion

0.8

0.6 Expt_Elisabeta et18al. Expt._Veca andVecca Adrover [18] Model

0.4

0.2

0 0

20

40

60

80

100

120

140

160

180

200

Time (min)

Figure 9. Comparison of experimental data18 of the carbon conversion with model prediction: Gasification of char (-1.4 +1.0 mm) in presence of CO2.

(a)

(b)

950 950 0.5 mm 920

Average particle temp (oC)

Average particle temp (oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

4 mm 8 mm

890 860 830 800

0.5 mm

940 930

4 mm 920 8 mm

910 900

0

20

40

60

0

Time (s)

1000

2000

3000

4000

5000

Time (s)

Figure 10. Effect of char particle size on average particle temperature of char # 1 (Tb = 950 o

C): (a) Initial period and (b) Overall reaction regime.


6000

Energy & Fuels

1.0

Char conversion

0.8

0.6 Kinetic and heat transfer model 0.4 Kinetic model 0.2

0.0 0

500

1000

1500

2000

2500

3000

3500

4000

4500

Time (s)

Figure 11. Effect of char particle size on the conversion profile of char # 1 (Tb = 950 oC).

(b)

(a) 1

0.4

Y_CO/(Y_CO2+Y_CO)

Y_CO2/(Y_CO2+Y_CO)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.9

0.8

0.3

0.2

0.1

0.0

0.7 0

2000

4000

6000

8000

10000

0

2000

Time (s)

4000

6000

8000

10000

Time (s)

Figure 12. Effect of particle size on mass fraction of CO2 and CO at particle centre at different particle diameter for char # 1 (Tb = 900 oC) (a) CO2 (b) CO.


12000

Page 33 of 33

0.05 char 1 0.04

Reaction rate (min-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

char 2 1000 oC

0.03

0.02

950 oC

0.01

900 oC

0.00 0.0

0.2

0.4

0.6

0.8

1.0

Char conversion

Figure 13. Effect of reactor temperature on the reaction rate.


Modeling and Experimental Studies on CO2 Gasification of Coarse

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