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Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 1: Experimental investigation
Chemical Engineering Science 66 (2011) 2141–2148
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Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 1: Experimental investigation Qixiang Xu a, Shusheng Pang a,n, Tana Levi b a b
Department of Chemical and Process Engineering, University of Canterbury, New Zealand CRL Energy Ltd., Wellington, New Zealand
a r t i c l e i n f o
abstract
Article history: Received 5 October 2010 Received in revised form 27 January 2011 Accepted 10 February 2011 Available online 20 February 2011
Biomass and coal are important solid fuels for generation of hydrogen-rich syngas from steam gasification. In this work, experiments were performed in a bench-scale gasifier to investigate the effect of coal-to-biomass ratio and the reaction kinetics for gasification of chars of biomass, coal and coal–biomass blends. In the gasification of these chars, steam was used as the gasification agent, while nitrogen was used as a gas carrier. The gasification temperature was controlled at 850, 900 and 950 1C. Gas produced was analysed using a micro-GC from which carbon conversion rate was also determined. From the experiments, it is found that the coal and biomass chars have different gasification characteristics and the overall reaction rate decreases with an increase in the ratio of coal–to-biomass. The microstructure of the coal char and biomass char was examined using scanning electronic microscopy (SEM), and it was found that the biomass char is more amorphous, whereas the coal char has larger pore size. The former enhances the intrinsic reaction rate and the latter influences the intra particle mass transportation. The difference in mass transfer of the gasification agent into the char particles between the two fuels is dominant in the char gasification. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Steam gasification Producer gas Coal and biomass blend Solid char Kinetics Char conversion rate
1. Introduction Heavy use of fossil fuels by human beings has caused serious concerns relating to the shortage of future energy and the negative impacts on environment, due to the green house gas (CO2) emissions. Extensive studies have been conducted recently in order to find alternative and sustainable sources for future energy and fuels. Biomass, originating from crops and trees, has attracted increasing interest as the full cycle of biomass growing and energy utilisation is considered to be largely carbon neutral. However, due to low energy density and the scattered distribution of the biomass, handling and transportation costs are high and this has hindered the potential for biomass utilisation for energy and fuels (Collot et al., 1999; McKendry, 2002). In order to reduce the production costs, the biomass can be blended with a proportion of high density coal for energy production through gasification. Gasification is an effective and efficient method for converting solid carbonaceous matters to hydrogen-rich syngas, called producer gas. This gas can either be directly used for generation of power and heat or be further synthesized to produce liquid fuel and chemicals. Of the various types of gasification technologies, fluidized bed gasification has advantages of high heat and mass transfer rates,
n
Corresponding author. E-mail address: [email protected] (S. Pang).
0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.02.026
and uniform temperature distribution in the gasifier, therefore, it is widely used in large scale operations. The overall process of solid fuel gasification within the fluidized bed reactor can be divided into two main steps after the initial short drying: (1) fast pyrolysis of the raw materials and (2) subsequent gasification of resultant chars. The former is a short process generating solid char and volatile gases. The latter consists of a series of heterogeneous reactions of the chars with gasification agent (air, oxygen or steam), and reactions among reactant and resultant gases. The char gasification process is a much slower conversion process compared to the initial pyrolysis, thus it is dominant in the whole gasification process (Everson et al., 2006). Extensive studies on gasification of either pure coal or pure biomass have been found in the literature. However, limited studies are reported on co-gasification of blended biomass and coal, and there is an apparent lack of fundamental understanding of the interactive effect of coal and biomass during the co-gasification. Due to the differences in blending ratio and diversity in fuel (biomass and coal) properties, the mechanism of co-gasification of the blend is complicated and the effects of the blending ratio on the gasification process is still unclear (Franco et al., 2003; Pan et al., 2000; Pinto et al., 2003, 2007). Coal, particularly the low quality coal such as lignite, contains significant amount of metal elements, which have catalytic effect on the gasification process (Clemens et al., 1998). Co-gasification of the blended biomass and coal has great potential for an effective use of existing and renewable sources in the future (Kumabe et al., 2007).
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In current practice of co-gasification, the coal and the biomass are fed into the gasifier after mixing, however, due to density difference between biomass and coal, the two fuels are segregated in feeding and fluidisation, and thus the coal and the biomass are gasified separately. This is undesirable since the difference in residence time of the two fuels will limit the robustness of cogasification operation. In order to ensure the biomass and the coal to bind together through the whole gasification process, one solution is to pre-mix and press the mixture into pellets before being fed to the gasifier. In this case, the co-gasification of the blend is uniform although the gasification characteristics may differ from the intrinsic reactions of each fuel, because the interaction between these two fuels and the effective internal mass transfer resistance can be significant in the large sized pellets (Paviet et al., 2008). Gasification kinetics of coal and biomass chars have been experimentally studied in recent years. Fermoso and his colleagues (Fermoso et al., 2010) investigated the reactivity of chars derived from a bituminous coal, residues of chestnut and olive stones, and the blends of these three fuels. From this work, no significant interactions were detected for the blend chars of bituminous and chestnut, but significant interactions were observed for the blend chars of bituminous and olive stones. Cousins and colleagues (Cousins et al., 2006) studied the char reactivity of Daw Mill coal with carbon dioxide. They found that the char activity is negatively affected by temperature, pressure and char diameter. Contrarily, the investigation by Asadullah et al. (2009) showed that the char reactivity of Australian mallee wood has positive relationship with particle size, due to the increase in the retention time of the intrinsic metal elements. Czechowskia and Kidawaa (1991) studied the reactivity of Janina bituminous coal char generated at 900 1C and found that the morphological changes of chars during the steam gasification promotes the reactivity at an internal surface of the char pore structure. In the study on radiata pine char by Cetin et al. (2005), apparent char reactivity was found to be increased with total surface area. From the literature review, intrinsic char reactivity as an important fuel property is generally higher for the biomass than for the coal (Kastanaki and Vamvuka, 2005; Mastsumoto et al., 2009), and the blended char reactivity can be increased by adding biomass to the coal (Zhu et al., 2008). The difference in the char reactivity between the two fuels is attributed to the differences in their structures and properties as the biomass char generally has lower density and is structurally more amorphous (Klose and Wolki, 2004). By the addition of biomass to coal, the effective exposed area is increased, hence the intrinsic reactivity of the coal is enhanced (Lu et al., 2002; Sadhukhan et al., 2009). The objectives of this study are to both experimentally and theoretically investigate the gasification reactivity of chars of biomass, coal and their blends, and to quantify the effects of the biomass–coal ratio on the gasification process. This paper will present the experimental work in which the gasification characteristics (kinetics and gas composition) are investigated and the microstructural differences between the coal char and the biomass char are examined. The results from this study will be used in development and validation of a mathematical model of the solid char gasification, which will be presented in the subsequent paper (Xu et al., 2011). The information generated from this work will be used in the subsequent work on development of a full scale gasifier model.
2. Experimental: materials and procedure In the experiments, lignite and wood (Eucalyptus nitens) were selected as the test materials. Lignite is a typical low quality coal
that has vast reserves in New Zealand and E. nitens is a fast growing plantation hardwood species. Before the experiments, the coal and the biomass were dried to about 10% moisture content, and then milled separately to a particle size under 450 mm. After this, the two feeds were thoroughly mixed in batches with blending ratios of coal-to-biomass of 20:80, 50:50 and 80:20. Each of the mixtures, as well as pure lignite and pure biomass, were compressed into pellets of 6 mm diameter and 10 mm length. The choice of the pellet size was to match the size of pellets that are used in practical domestic burners. The cylindrical pellets were formed by placing the feed into a mould and applying a pressure of 1.6 MPa. As the objective is to investigate the char gasification process, the pellets of all batches were firstly placed in an oven at 900 1C for 7 min to generate solid chars. This char generation process completely removed the volatile components. The properties of lignite, biomass and their chars are listed in Table 1. For the gasification experiments, the char particles were tested in a bench-scale gasifier as shown in Fig. 1, which consists of a nitrogen and water feeding system, a steam generator, a tube reactor, a gas cooler and an on-line gas analyser (Micro-GC). During the experiment, the solid chars were held by a porous quartz frit near the bottom of the vertical tube reactor, which was placed into an electrically heated oven. The temperature in the reaction zone during the experiment was held at either 850, 900 or 950 1C. In each run, once the set temperature was reached, the water pump was started and the steam generator was turned on. The gas mixture of nitrogen and generated steam was then fed to the top of the tube reactor and the steam reacted with the solid char particles. The producer gas generated from the gasification process flowed out with the carrier gas (nitrogen) via the outlet at the reactor bottom, and then passed through a water bath to remove water vapour in the gas. Finally the vapour-free producer gas was continuously analysed using the Micro-GC from which the producer gas releasing rate and its composition were obtained, and the actual steam gasification reactivity was determined. In each run, 1 g samples of the chars were loaded in the tube reactor. The nitrogen flow rate was controlled at 600 mL/min. By testing the five blended coal-to-biomass ratios (0:100; 20:80, 50:50, 80:20 and 100:0) at three operation temperatures (850, 900 and 950 1C), a total number of 15 experimental runs were carried out. For each condition, three tests, including two replicates, were performed. In order to understand the difference in gasification performance between the coal and the biomass, and to investigate the effects of two fuel blending, the microstructure and morphology of chars of coal, biomass and a 50:50 blend were examined using an SEM. Porosities of the biomass char and the coal char were determined in terms of the mean pore diameters based on the SEM images. In order to investigate the effect of the vapour diffusion resistance within the char particle, a separate set of experiments Table 1 Relevant properties of lignite and Eucalyptus nitens wood and derived chars.
Moisture (wt%) Ash (wt%) Volatile (wt%) Fixed carbon (wt%) Total carbon (wt%) Total hydrogen (wt%) Total nitrogen (wt%)
Raw lignite
Raw wood
Lignite char
Wood char
19.1 4.9 41.9 34.1 50.6 3.67 0.55
5.4 0.38 81.5 12.7 47.5 5.57 0.03
0.8 12.8 5 81.4 82.9 – –
0.7 3.2 4 92.1 91.9 – –
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T Nitrogen
Steam generator & Preheater
Water
Exhaust GC
Water bottle & dehumidizer
Gas cooler
Char
Fig. 1. Experimental setup of the bench-scale gasifier system.
Fig. 2. Profile of producer gas generation rate from gasification of biomass chars at 900 1C.
Fig. 3. Profile of producer gas generation rate from gasification of chars with coal-to-biomass ratio of 20:80 at 900 1C.
was conducted using milled chars of coal and biomass, which had much smaller diameters than the original char particles.
3. Results and discussion 3.1. Char gasification characteristics For each experimental run, the composition and generation rate of the producer gas as a function of elapsed time have been determined from the continuous Micro-GC analysis. The results for an operation at 900 1C are shown in Figs. 2–6 for coal-tobiomass ratios of 0:100 (pure biomass); 20:80; 50:50; 80:20 and 100:0 (pure coal), respectively, as average values for the three replicate runs. In the figures, the nitrogen content was not shown as it only acted as a carrier and was not involved in the gasification reactions. The gas components of the char gasification producer gas were hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2), which will be used for further analysis. In Figs. 2–6, the gas composition of a given gas species can also represent the gas production rate of the species as the carrier gas (N2) had a constant volumetric flow rate (600 mL/min). Therefore, high concentration of a gas component means high production rate of this gas component. From the results presented in Figs. 2–6, it is seen that the char gasification can be divided into three stages: initial heat-up and slow gas production stage, a fast gas production stage and a final falling-rate gas production stage.
Fig. 4. Profile of producer gas generation rate from gasification of chars with coal-to-biomass ratio of 50:50 at 900 1C.
The first stage took about 3 min and the gas production reached the maxima by the end of this stage. During this stage, the maximum values of gas production rate for different fuels were different. The maximum H2 compositions (mol/mol) at the end of the first stage of gasification for the five different coal-to-biomass
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Fig. 5. Profile of producer gas generation rate from gasification of chars with coal-to-biomass ratio of 80:20 at 900 1C.
Fig. 7. Carbon consumption rate for steam gasification of chars of pure biomass, pure coal and blended biomass and coal with coal-to-biomass ratio of 50:50.
Fig. 6. Profile of producer gas generation rate from gasification of coal chars at 900 1C. Fig. 8. Effect of blending ratio on char yield in the pyrolysis of char forming.
ratio chars were, respectively, 25% (Fig. 2), 36% (Fig. 3), 35% (Fig. 4), 27% (Fig. 5) and 29% (Fig. 6). In the second and third stages of the gasification, both the gas production rate and the trends were significantly different for chars with different coal-to-biomass ratios, and thus the total gasification time was also different. Therefore, the coal-to-biomass ratio has significant effects on the gas production rate, and hence the gasification characteristics. In Fig. 2, for the gasification of pure biomass char, the producer gas composition curves maintained a relatively constant rate for about 10 min in the second stage after reaching the maximum value, and then decreased towards zero in the third stage when the char conversion was completed. In contrast, for the gasification of pure coal (Fig. 6), the second and third stages are not distinguishable, and the gasification rate underwent an exponential decay approaching zero towards the end of the gasification process. It is interesting to note that once coal was added into the biomass to form a blend, curves of gas production rate and composition for blended char are more identical to the pure coal gasification as shown in Figs. 3–5. In Fig. 3, the gas composition profile was significantly changed from the pure biomass (Fig. 2) although only 20% coal was added. The maximum gas compositions for H2 and CO2 at the end of the first stage were much higher than those in the gasification of pure coal chars. Based on the gas production rate and gas composition from the char gasification, the solid carbon conversion rate can be determined as the solid chars are only converted to the three gases (H2, CO and CO2) from the gas analysis. The results for steam gasification at 900 1C are shown in Fig. 7 for chars of pure biomass, pure coals and blended coal and biomass at the blending
ratio of 50:50. The carbon conversion rate can be used for further investigation of the gasification performance of various blending ratios of coal and biomass. It can be seen that the pure biomass char conversion rate maintained at a relative constant rate, after the initial heating up period, followed by a falling rate, while for the pure coal char, the carbon conversion rate continued decreasing after reaching the maximum value in the initial heating up period. The carbon conversion rate profile of blended char was similar to that of the pure coal char. The identical gasification characteristics between the blended char and the coal char can be attributed to a number of factors. In the char generation process before gasification, the biomass lost more volatile components than coal, thus the actual char mass formed from the biomass was less than that from the coal. Fig. 8 shows experimental results on the percentage char yield compared to the fresh fuel mass before the char generation. The char yield was only 15% for the pure biomass and this was increased linearly to 40% for the pure coal. In other words, the 50:50 coalto-biomass blend would be changed to the ratio of coal char-tobiomass char of 72.7:27.3 in the blend char. It can then be speculated that in the char generation of blended coal and biomass pellets, a higher proportion of the biomass was lost compared to the coal, thus leaving more space for the coal char to cover. In addition, the microstructures between the pure biomass and coal are different, and the coal becomes more influential than the biomass when being blended together. This will be discussed later in this paper.
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3.2. Effect of blending on apparent char reactivity The apparent gasification reactivity of the char quantifies char conversion per unit time relative to the remaining carbon mass (m) at the given time (t), which is defined as r¼
1 dm 1 dX ¼ m dt 1X dt
ð1Þ
where X is the conversion ratio of converted carbon to the initial total carbon mass (m0) during the gasification, which is determined by X ¼ 1
m m0
ð2Þ
The initial total carbon mass in the char (m0) was obtained as the mass difference between the char being loaded into the reactor and the ash remaining after the gasification test. The remaining carbon mass (m) at the given time can be determined as the difference between the initial total carbon mass and the accumulated carbon in the gases generated from the char gasification Z t dC m ¼ m0 MWc dt ð3Þ dt t¼0
Fig. 10. Apparent gasification reactivity as a function of char conversion rate at 900 1C.
In which MWC is the molar mass of the carbon in the gases generated from the char gasification (12 g/mol). dC=dt is the solid carbon consumption molar rate, which can be determined from the GC gas analysis results. Since the flow rate of an N2 feed was maintained constant through the experiment, the molar flow rate of each gaseous species can be obtained from the instantaneous GC data, therefore, the molar consumption rate of solid carbon (dC=dt) at any given time can be calculated by summating the molar flow rate of all gaseous species, which contain carbon (CO and CO2) in this study vCO þ vCO2 PVN2 dC ¼ ð4Þ dt vN2 RT where vCO, vCO2 and vN2 are the instantaneous volumetric percentage of CO, CO2 and N2 in the producer gas mixture; VN2 is the volumetric flow rate of nitrogen feed, 600 mL/min in this case. P and T are the pressure and temperature of nitrogen at the feed point. The results of the gasification reactivity for chars with various coal-to-biomass ratios are shown in Figs. 9–11 for gasification tests at 850, 900 and 950 1C, respectively. From these figures, it
Fig. 9. Apparent gasification reactivity as a function of char conversion rate at 850 1C.
Fig. 11. Apparent gasification reactivity as a function of char conversion rate at 950 1C.
can be seen that all of the chars, except for the pure biomass chars, had similar reactivity from the start until the char conversion reached 0.4. The biomass char reactivity tended to be lower initially with a clear difference for gasification test at 950 1C. However, after this point (0.4 char conversion), the reactivity of pure biomass char increased rapidly and became the highest towards the end of the gasification. In the meantime, the gasification reactivity for the coal char tended to be the lowest, or near the lowest, whereas the reactivity of the blended chars fell between these two extreme cases. Based on these findings, it is concluded that adding coal-to-biomass has a negative effect on the gasification reactivity, although the effect is reduced with increasing the gasification temperature. The influence of gasification temperature has been examined by comparing the elapsed time, when the char conversion was completed and the results are shown in Fig. 12. The results clearly demonstrate that with higher temperatures, the elapsed time for complete conversion of the chars was reduced, thus the gasification reactivity was increased. The influence of gasification temperature can be due to the reduced difference in apparent activation energy between biomass and coal chars at higher temperatures. Fig. 12 also shows that the complete carbon conversion time increased with the coal proportion in the blended chars, which is consistent with the reactivity analysis.
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3.3. Morphology of the chars
3.4. Analysis of char gasification mechanism
The SEM images are illustrated in Fig. 13 from which it can be seen that in micrometre scale, the biomass char matrix (Fig. 13a) is significantly more amorphous than the coal char (Fig. 13c). For the biomass char, there are a large amount of finer voids between the carbonaceous matters forming the structure of thinner clusters. For the coal char, the carbonaceous materials are more likely to be segregated in the form of more compacted clusters and large cracks. These differences can be used to explain the different gasification characteristics between the biomass char and the coal char. For the biomass char, the fine voids can allow more uniform gas transfer through the gasification process, thus high gasification reactivity can be maintained and increased with the elapsed time. For the coal char, the large cracks make it possible for fast gas transfer in the early stages of the gasification. However, the compacted clusters mean high resistance for the gas transfer from the compacted clusters to the large cracks. The microstructure of the blended char shows the major characteristics of the coal char, where large cracks and compacts clusters are observed. The observed differences in the char microstructures are consistent with previous findings of gas production and carbon conversion rates during the gasification process of these chars.
During the char gasification using steam as the gasification agent, there are three distinct steps which influence the overall gas production and carbon conversion rates: (1) diffusion of the steam molecules into the char particles, (2) heterogeneous reaction on the surface of the char micro-pores and (3) gaseous products move out from the particles by diffusion under gas concentration gradient within the particle and by bulk gas flow, due to gradient of partial pressure. In the non-catalytic gas solid reactions between the steam and the solid char, if both the vapour diffusion resistance and intrinsic reaction rate are high enough, the vapour molecules will initially react near the particle surface before they further diffuse into the centre of the solid particle. On the other hand, if the vapour diffusion rate is significantly faster comparing with intrinsic reaction rate, the vapour concentration within the particle is uniform and the whole solid volume would have similar conversion rate, thus the diffusion of resultant gases is dominant. In this case, the overall reaction rate will be similar to the intrinsic reaction (Gupta and Saha, 2003). The resistance of inward diffusion of water vapour molecules into the char particles can be significant for the apparent char size of greater than millimetre level (Paviet et al., 2008). In this study, the char particle size was large enough to generate significant diffusion resistance that has competing effect with heterogeneous reactions. From Fig. 2, it has been noticed that the overall gasification rate of biomass char maintained relatively constant after the initial heat-up period, which indicates an inhomogeneous reaction rate during the gasification, and the effective reaction volume of the biomass char can be limited by the vapour diffusion resistance. Therefore reactions occurred at and near the char particle surface, while the surface continuously moved inward with the char conversion. On the contrary as shown in Fig. 6, the coal char gasification exhibited typical first order reaction kinetics from which exponential decay in the reaction rate was observed. This confirms that vapour diffusion resistance within the coal char can be negligible in the gasification compared to the intrinsic reaction rate. Therefore, the gasification reactions between the water vapour and the solid char occur within the whole volume of the char particle, because the vapour can penetrate into the char particle through the large cracks. In this case, the resistance for the gas diffusion through the compact clusters plays a dominant role. In order to verify the above hypothesis for the char gasification mechanism, experiments were conducted using finer char particles of biomass and coal. The results of comparison between pellet char and pulverised chars are shown in Fig. 14 for carbon consumption rate and in Fig. 15 for char conversion rate (X).
Fig. 12. Effect of gasification temperature and blending ratio on complete conversion time of the solid chars.
Fig. 13. The electronic microscopic scanning images of surface of (a) pure biomass char, (b) 50:50 blended char and (c) pure coal char. All images have a magnification of 350.
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Fig. 15. Effect of particle size on the char conversion rate: (a) biomass char and (b) coal char.
Fig. 14. Effect of particle size on the carbon consumption rate: (a) biomass char and (b) coal char.
From these figures, it is observed that for the coal char, the carbon consumption rate and the overall char conversion rate were only increased slightly after being milled to finer particles with smaller diameters. This confirms that the vapour can easily diffuse into the coal char particles, thus the particle diameter is not a critical factor. However, for the biomass chars, the carbon conversion rate and overall char conversion rate of the milled chars were increased significantly compared to the original biomass pellet chars. In the milled biomass char particles, the particle diameter was significantly reduced, thus the surface area to the particle mass was much higher, enhancing the carbon conversion rate significantly in the first and second stages of the gasification. The difference in the vapour diffusion resistance between the biomass char and the coal char can be attributed to the distinct structural properties of these two solids. From the SEM analysis, the biomass char is much more amorphous than the coal char, hence the specific effective surface area is larger and the intrinsic reactivity is enhanced. On the other hand, the diffusion rate is relatively slower due to the smaller pore size, which limits the inward diffusion of water vapour. However, the coal char shows denser cluster and large cracks, which result in a larger effective pore diameter. Based on the above mechanisms, a mathematical model has been developed to simulate the gasification process of biomass char, coal char and blended biomass and coal char. The model is validated using the experimental results. This will be presented in the subsequent paper (Xu et al., 2011).
4. Conclusions Gas production rate, carbon conversion rate and reactivity for gasification of biomass char, coal char and blended coal and biomass chars have been experimentally investigated in this work. The sample char particles were produced from the pellets of coal (lignite), E. nitens and their blends at three different blending ratios. A series of gasification tests were carried out in a benchscale gasifier at three different temperatures. The composition of the producer gas and gas production rate was continuously analysed by using a Micro-GC. Based on the gas analysis results, carbon conversion rate and gasification reactivity were determined. From the study, the following conclusions are drawn: (1) The gasification of biomass chars, coal chars and the chars of blended biomass and coal can be represented by three stages: initial heat-up and slow gasification stage, fast gasification rate stage and falling rate stage. In the initial heat-up stage, the gasification rate increases to the maximum values, which vary with the coal-to-biomass ratio. The overall conversion rate of biomass char maintains at a relatively constant rate after the initial heat-up period, and this is followed by a falling rate in the last period of the process. For gasification of the coal char, the reaction rate decreases continuously after the maximum value is reached. The reaction rate of the blended coal and biomass chars has intermediate characteristics with the trend similar to the coal char gasification. This is because the biomass loses more mass during the char generation. (2) The overall reactivity of the blended coal–biomass char decreases with the incremental fraction of the coal in the
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blend, and the effect of the coal tends to be reduced at higher gasification temperatures. (3) The SEM image analysis indicates that the biomass char has a more amorphous structure; however, the effective pore size is smaller than coal char. The coal char has a more packed cluster structure and large cracks. The chars of blended biomass and coal are more similar to the coal char. (4) The distinct gasification characteristics of the coal char and the biomass char are due to the differences in gas diffusion within the char particles. The internal resistance to the gas diffusion in the biomass char is significant and an intrinsic reaction is fast due to its amorphous structure, hence, the significant gradient in the gas concentration along the char radius results in inhomogeneous overall reaction rate through the char particle. However, in the gasification of the coal char, the denser structure and larger effective pore size make the overall gasification more homogeneous, and the diffusion of vapour and resultant gases through the dense clusters plays an important role.
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Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 1: Experimental investigation Qixiang Xu a, Shusheng Pang a,n, Tana Levi b a b
Department of Chemical and Process Engineering, University of Canterbury, New Zealand CRL Energy Ltd., Wellington, New Zealand
a r t i c l e i n f o
abstract
Article history: Received 5 October 2010 Received in revised form 27 January 2011 Accepted 10 February 2011 Available online 20 February 2011
Biomass and coal are important solid fuels for generation of hydrogen-rich syngas from steam gasification. In this work, experiments were performed in a bench-scale gasifier to investigate the effect of coal-to-biomass ratio and the reaction kinetics for gasification of chars of biomass, coal and coal–biomass blends. In the gasification of these chars, steam was used as the gasification agent, while nitrogen was used as a gas carrier. The gasification temperature was controlled at 850, 900 and 950 1C. Gas produced was analysed using a micro-GC from which carbon conversion rate was also determined. From the experiments, it is found that the coal and biomass chars have different gasification characteristics and the overall reaction rate decreases with an increase in the ratio of coal–to-biomass. The microstructure of the coal char and biomass char was examined using scanning electronic microscopy (SEM), and it was found that the biomass char is more amorphous, whereas the coal char has larger pore size. The former enhances the intrinsic reaction rate and the latter influences the intra particle mass transportation. The difference in mass transfer of the gasification agent into the char particles between the two fuels is dominant in the char gasification. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Steam gasification Producer gas Coal and biomass blend Solid char Kinetics Char conversion rate
1. Introduction Heavy use of fossil fuels by human beings has caused serious concerns relating to the shortage of future energy and the negative impacts on environment, due to the green house gas (CO2) emissions. Extensive studies have been conducted recently in order to find alternative and sustainable sources for future energy and fuels. Biomass, originating from crops and trees, has attracted increasing interest as the full cycle of biomass growing and energy utilisation is considered to be largely carbon neutral. However, due to low energy density and the scattered distribution of the biomass, handling and transportation costs are high and this has hindered the potential for biomass utilisation for energy and fuels (Collot et al., 1999; McKendry, 2002). In order to reduce the production costs, the biomass can be blended with a proportion of high density coal for energy production through gasification. Gasification is an effective and efficient method for converting solid carbonaceous matters to hydrogen-rich syngas, called producer gas. This gas can either be directly used for generation of power and heat or be further synthesized to produce liquid fuel and chemicals. Of the various types of gasification technologies, fluidized bed gasification has advantages of high heat and mass transfer rates,
n
Corresponding author. E-mail address: [email protected] (S. Pang).
0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.02.026
and uniform temperature distribution in the gasifier, therefore, it is widely used in large scale operations. The overall process of solid fuel gasification within the fluidized bed reactor can be divided into two main steps after the initial short drying: (1) fast pyrolysis of the raw materials and (2) subsequent gasification of resultant chars. The former is a short process generating solid char and volatile gases. The latter consists of a series of heterogeneous reactions of the chars with gasification agent (air, oxygen or steam), and reactions among reactant and resultant gases. The char gasification process is a much slower conversion process compared to the initial pyrolysis, thus it is dominant in the whole gasification process (Everson et al., 2006). Extensive studies on gasification of either pure coal or pure biomass have been found in the literature. However, limited studies are reported on co-gasification of blended biomass and coal, and there is an apparent lack of fundamental understanding of the interactive effect of coal and biomass during the co-gasification. Due to the differences in blending ratio and diversity in fuel (biomass and coal) properties, the mechanism of co-gasification of the blend is complicated and the effects of the blending ratio on the gasification process is still unclear (Franco et al., 2003; Pan et al., 2000; Pinto et al., 2003, 2007). Coal, particularly the low quality coal such as lignite, contains significant amount of metal elements, which have catalytic effect on the gasification process (Clemens et al., 1998). Co-gasification of the blended biomass and coal has great potential for an effective use of existing and renewable sources in the future (Kumabe et al., 2007).
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In current practice of co-gasification, the coal and the biomass are fed into the gasifier after mixing, however, due to density difference between biomass and coal, the two fuels are segregated in feeding and fluidisation, and thus the coal and the biomass are gasified separately. This is undesirable since the difference in residence time of the two fuels will limit the robustness of cogasification operation. In order to ensure the biomass and the coal to bind together through the whole gasification process, one solution is to pre-mix and press the mixture into pellets before being fed to the gasifier. In this case, the co-gasification of the blend is uniform although the gasification characteristics may differ from the intrinsic reactions of each fuel, because the interaction between these two fuels and the effective internal mass transfer resistance can be significant in the large sized pellets (Paviet et al., 2008). Gasification kinetics of coal and biomass chars have been experimentally studied in recent years. Fermoso and his colleagues (Fermoso et al., 2010) investigated the reactivity of chars derived from a bituminous coal, residues of chestnut and olive stones, and the blends of these three fuels. From this work, no significant interactions were detected for the blend chars of bituminous and chestnut, but significant interactions were observed for the blend chars of bituminous and olive stones. Cousins and colleagues (Cousins et al., 2006) studied the char reactivity of Daw Mill coal with carbon dioxide. They found that the char activity is negatively affected by temperature, pressure and char diameter. Contrarily, the investigation by Asadullah et al. (2009) showed that the char reactivity of Australian mallee wood has positive relationship with particle size, due to the increase in the retention time of the intrinsic metal elements. Czechowskia and Kidawaa (1991) studied the reactivity of Janina bituminous coal char generated at 900 1C and found that the morphological changes of chars during the steam gasification promotes the reactivity at an internal surface of the char pore structure. In the study on radiata pine char by Cetin et al. (2005), apparent char reactivity was found to be increased with total surface area. From the literature review, intrinsic char reactivity as an important fuel property is generally higher for the biomass than for the coal (Kastanaki and Vamvuka, 2005; Mastsumoto et al., 2009), and the blended char reactivity can be increased by adding biomass to the coal (Zhu et al., 2008). The difference in the char reactivity between the two fuels is attributed to the differences in their structures and properties as the biomass char generally has lower density and is structurally more amorphous (Klose and Wolki, 2004). By the addition of biomass to coal, the effective exposed area is increased, hence the intrinsic reactivity of the coal is enhanced (Lu et al., 2002; Sadhukhan et al., 2009). The objectives of this study are to both experimentally and theoretically investigate the gasification reactivity of chars of biomass, coal and their blends, and to quantify the effects of the biomass–coal ratio on the gasification process. This paper will present the experimental work in which the gasification characteristics (kinetics and gas composition) are investigated and the microstructural differences between the coal char and the biomass char are examined. The results from this study will be used in development and validation of a mathematical model of the solid char gasification, which will be presented in the subsequent paper (Xu et al., 2011). The information generated from this work will be used in the subsequent work on development of a full scale gasifier model.
2. Experimental: materials and procedure In the experiments, lignite and wood (Eucalyptus nitens) were selected as the test materials. Lignite is a typical low quality coal
that has vast reserves in New Zealand and E. nitens is a fast growing plantation hardwood species. Before the experiments, the coal and the biomass were dried to about 10% moisture content, and then milled separately to a particle size under 450 mm. After this, the two feeds were thoroughly mixed in batches with blending ratios of coal-to-biomass of 20:80, 50:50 and 80:20. Each of the mixtures, as well as pure lignite and pure biomass, were compressed into pellets of 6 mm diameter and 10 mm length. The choice of the pellet size was to match the size of pellets that are used in practical domestic burners. The cylindrical pellets were formed by placing the feed into a mould and applying a pressure of 1.6 MPa. As the objective is to investigate the char gasification process, the pellets of all batches were firstly placed in an oven at 900 1C for 7 min to generate solid chars. This char generation process completely removed the volatile components. The properties of lignite, biomass and their chars are listed in Table 1. For the gasification experiments, the char particles were tested in a bench-scale gasifier as shown in Fig. 1, which consists of a nitrogen and water feeding system, a steam generator, a tube reactor, a gas cooler and an on-line gas analyser (Micro-GC). During the experiment, the solid chars were held by a porous quartz frit near the bottom of the vertical tube reactor, which was placed into an electrically heated oven. The temperature in the reaction zone during the experiment was held at either 850, 900 or 950 1C. In each run, once the set temperature was reached, the water pump was started and the steam generator was turned on. The gas mixture of nitrogen and generated steam was then fed to the top of the tube reactor and the steam reacted with the solid char particles. The producer gas generated from the gasification process flowed out with the carrier gas (nitrogen) via the outlet at the reactor bottom, and then passed through a water bath to remove water vapour in the gas. Finally the vapour-free producer gas was continuously analysed using the Micro-GC from which the producer gas releasing rate and its composition were obtained, and the actual steam gasification reactivity was determined. In each run, 1 g samples of the chars were loaded in the tube reactor. The nitrogen flow rate was controlled at 600 mL/min. By testing the five blended coal-to-biomass ratios (0:100; 20:80, 50:50, 80:20 and 100:0) at three operation temperatures (850, 900 and 950 1C), a total number of 15 experimental runs were carried out. For each condition, three tests, including two replicates, were performed. In order to understand the difference in gasification performance between the coal and the biomass, and to investigate the effects of two fuel blending, the microstructure and morphology of chars of coal, biomass and a 50:50 blend were examined using an SEM. Porosities of the biomass char and the coal char were determined in terms of the mean pore diameters based on the SEM images. In order to investigate the effect of the vapour diffusion resistance within the char particle, a separate set of experiments Table 1 Relevant properties of lignite and Eucalyptus nitens wood and derived chars.
Moisture (wt%) Ash (wt%) Volatile (wt%) Fixed carbon (wt%) Total carbon (wt%) Total hydrogen (wt%) Total nitrogen (wt%)
Raw lignite
Raw wood
Lignite char
Wood char
19.1 4.9 41.9 34.1 50.6 3.67 0.55
5.4 0.38 81.5 12.7 47.5 5.57 0.03
0.8 12.8 5 81.4 82.9 – –
0.7 3.2 4 92.1 91.9 – –
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T Nitrogen
Steam generator & Preheater
Water
Exhaust GC
Water bottle & dehumidizer
Gas cooler
Char
Fig. 1. Experimental setup of the bench-scale gasifier system.
Fig. 2. Profile of producer gas generation rate from gasification of biomass chars at 900 1C.
Fig. 3. Profile of producer gas generation rate from gasification of chars with coal-to-biomass ratio of 20:80 at 900 1C.
was conducted using milled chars of coal and biomass, which had much smaller diameters than the original char particles.
3. Results and discussion 3.1. Char gasification characteristics For each experimental run, the composition and generation rate of the producer gas as a function of elapsed time have been determined from the continuous Micro-GC analysis. The results for an operation at 900 1C are shown in Figs. 2–6 for coal-tobiomass ratios of 0:100 (pure biomass); 20:80; 50:50; 80:20 and 100:0 (pure coal), respectively, as average values for the three replicate runs. In the figures, the nitrogen content was not shown as it only acted as a carrier and was not involved in the gasification reactions. The gas components of the char gasification producer gas were hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2), which will be used for further analysis. In Figs. 2–6, the gas composition of a given gas species can also represent the gas production rate of the species as the carrier gas (N2) had a constant volumetric flow rate (600 mL/min). Therefore, high concentration of a gas component means high production rate of this gas component. From the results presented in Figs. 2–6, it is seen that the char gasification can be divided into three stages: initial heat-up and slow gas production stage, a fast gas production stage and a final falling-rate gas production stage.
Fig. 4. Profile of producer gas generation rate from gasification of chars with coal-to-biomass ratio of 50:50 at 900 1C.
The first stage took about 3 min and the gas production reached the maxima by the end of this stage. During this stage, the maximum values of gas production rate for different fuels were different. The maximum H2 compositions (mol/mol) at the end of the first stage of gasification for the five different coal-to-biomass
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Fig. 5. Profile of producer gas generation rate from gasification of chars with coal-to-biomass ratio of 80:20 at 900 1C.
Fig. 7. Carbon consumption rate for steam gasification of chars of pure biomass, pure coal and blended biomass and coal with coal-to-biomass ratio of 50:50.
Fig. 6. Profile of producer gas generation rate from gasification of coal chars at 900 1C. Fig. 8. Effect of blending ratio on char yield in the pyrolysis of char forming.
ratio chars were, respectively, 25% (Fig. 2), 36% (Fig. 3), 35% (Fig. 4), 27% (Fig. 5) and 29% (Fig. 6). In the second and third stages of the gasification, both the gas production rate and the trends were significantly different for chars with different coal-to-biomass ratios, and thus the total gasification time was also different. Therefore, the coal-to-biomass ratio has significant effects on the gas production rate, and hence the gasification characteristics. In Fig. 2, for the gasification of pure biomass char, the producer gas composition curves maintained a relatively constant rate for about 10 min in the second stage after reaching the maximum value, and then decreased towards zero in the third stage when the char conversion was completed. In contrast, for the gasification of pure coal (Fig. 6), the second and third stages are not distinguishable, and the gasification rate underwent an exponential decay approaching zero towards the end of the gasification process. It is interesting to note that once coal was added into the biomass to form a blend, curves of gas production rate and composition for blended char are more identical to the pure coal gasification as shown in Figs. 3–5. In Fig. 3, the gas composition profile was significantly changed from the pure biomass (Fig. 2) although only 20% coal was added. The maximum gas compositions for H2 and CO2 at the end of the first stage were much higher than those in the gasification of pure coal chars. Based on the gas production rate and gas composition from the char gasification, the solid carbon conversion rate can be determined as the solid chars are only converted to the three gases (H2, CO and CO2) from the gas analysis. The results for steam gasification at 900 1C are shown in Fig. 7 for chars of pure biomass, pure coals and blended coal and biomass at the blending
ratio of 50:50. The carbon conversion rate can be used for further investigation of the gasification performance of various blending ratios of coal and biomass. It can be seen that the pure biomass char conversion rate maintained at a relative constant rate, after the initial heating up period, followed by a falling rate, while for the pure coal char, the carbon conversion rate continued decreasing after reaching the maximum value in the initial heating up period. The carbon conversion rate profile of blended char was similar to that of the pure coal char. The identical gasification characteristics between the blended char and the coal char can be attributed to a number of factors. In the char generation process before gasification, the biomass lost more volatile components than coal, thus the actual char mass formed from the biomass was less than that from the coal. Fig. 8 shows experimental results on the percentage char yield compared to the fresh fuel mass before the char generation. The char yield was only 15% for the pure biomass and this was increased linearly to 40% for the pure coal. In other words, the 50:50 coalto-biomass blend would be changed to the ratio of coal char-tobiomass char of 72.7:27.3 in the blend char. It can then be speculated that in the char generation of blended coal and biomass pellets, a higher proportion of the biomass was lost compared to the coal, thus leaving more space for the coal char to cover. In addition, the microstructures between the pure biomass and coal are different, and the coal becomes more influential than the biomass when being blended together. This will be discussed later in this paper.
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3.2. Effect of blending on apparent char reactivity The apparent gasification reactivity of the char quantifies char conversion per unit time relative to the remaining carbon mass (m) at the given time (t), which is defined as r¼
1 dm 1 dX ¼ m dt 1X dt
ð1Þ
where X is the conversion ratio of converted carbon to the initial total carbon mass (m0) during the gasification, which is determined by X ¼ 1
m m0
ð2Þ
The initial total carbon mass in the char (m0) was obtained as the mass difference between the char being loaded into the reactor and the ash remaining after the gasification test. The remaining carbon mass (m) at the given time can be determined as the difference between the initial total carbon mass and the accumulated carbon in the gases generated from the char gasification Z t dC m ¼ m0 MWc dt ð3Þ dt t¼0
Fig. 10. Apparent gasification reactivity as a function of char conversion rate at 900 1C.
In which MWC is the molar mass of the carbon in the gases generated from the char gasification (12 g/mol). dC=dt is the solid carbon consumption molar rate, which can be determined from the GC gas analysis results. Since the flow rate of an N2 feed was maintained constant through the experiment, the molar flow rate of each gaseous species can be obtained from the instantaneous GC data, therefore, the molar consumption rate of solid carbon (dC=dt) at any given time can be calculated by summating the molar flow rate of all gaseous species, which contain carbon (CO and CO2) in this study vCO þ vCO2 PVN2 dC ¼ ð4Þ dt vN2 RT where vCO, vCO2 and vN2 are the instantaneous volumetric percentage of CO, CO2 and N2 in the producer gas mixture; VN2 is the volumetric flow rate of nitrogen feed, 600 mL/min in this case. P and T are the pressure and temperature of nitrogen at the feed point. The results of the gasification reactivity for chars with various coal-to-biomass ratios are shown in Figs. 9–11 for gasification tests at 850, 900 and 950 1C, respectively. From these figures, it
Fig. 9. Apparent gasification reactivity as a function of char conversion rate at 850 1C.
Fig. 11. Apparent gasification reactivity as a function of char conversion rate at 950 1C.
can be seen that all of the chars, except for the pure biomass chars, had similar reactivity from the start until the char conversion reached 0.4. The biomass char reactivity tended to be lower initially with a clear difference for gasification test at 950 1C. However, after this point (0.4 char conversion), the reactivity of pure biomass char increased rapidly and became the highest towards the end of the gasification. In the meantime, the gasification reactivity for the coal char tended to be the lowest, or near the lowest, whereas the reactivity of the blended chars fell between these two extreme cases. Based on these findings, it is concluded that adding coal-to-biomass has a negative effect on the gasification reactivity, although the effect is reduced with increasing the gasification temperature. The influence of gasification temperature has been examined by comparing the elapsed time, when the char conversion was completed and the results are shown in Fig. 12. The results clearly demonstrate that with higher temperatures, the elapsed time for complete conversion of the chars was reduced, thus the gasification reactivity was increased. The influence of gasification temperature can be due to the reduced difference in apparent activation energy between biomass and coal chars at higher temperatures. Fig. 12 also shows that the complete carbon conversion time increased with the coal proportion in the blended chars, which is consistent with the reactivity analysis.
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3.3. Morphology of the chars
3.4. Analysis of char gasification mechanism
The SEM images are illustrated in Fig. 13 from which it can be seen that in micrometre scale, the biomass char matrix (Fig. 13a) is significantly more amorphous than the coal char (Fig. 13c). For the biomass char, there are a large amount of finer voids between the carbonaceous matters forming the structure of thinner clusters. For the coal char, the carbonaceous materials are more likely to be segregated in the form of more compacted clusters and large cracks. These differences can be used to explain the different gasification characteristics between the biomass char and the coal char. For the biomass char, the fine voids can allow more uniform gas transfer through the gasification process, thus high gasification reactivity can be maintained and increased with the elapsed time. For the coal char, the large cracks make it possible for fast gas transfer in the early stages of the gasification. However, the compacted clusters mean high resistance for the gas transfer from the compacted clusters to the large cracks. The microstructure of the blended char shows the major characteristics of the coal char, where large cracks and compacts clusters are observed. The observed differences in the char microstructures are consistent with previous findings of gas production and carbon conversion rates during the gasification process of these chars.
During the char gasification using steam as the gasification agent, there are three distinct steps which influence the overall gas production and carbon conversion rates: (1) diffusion of the steam molecules into the char particles, (2) heterogeneous reaction on the surface of the char micro-pores and (3) gaseous products move out from the particles by diffusion under gas concentration gradient within the particle and by bulk gas flow, due to gradient of partial pressure. In the non-catalytic gas solid reactions between the steam and the solid char, if both the vapour diffusion resistance and intrinsic reaction rate are high enough, the vapour molecules will initially react near the particle surface before they further diffuse into the centre of the solid particle. On the other hand, if the vapour diffusion rate is significantly faster comparing with intrinsic reaction rate, the vapour concentration within the particle is uniform and the whole solid volume would have similar conversion rate, thus the diffusion of resultant gases is dominant. In this case, the overall reaction rate will be similar to the intrinsic reaction (Gupta and Saha, 2003). The resistance of inward diffusion of water vapour molecules into the char particles can be significant for the apparent char size of greater than millimetre level (Paviet et al., 2008). In this study, the char particle size was large enough to generate significant diffusion resistance that has competing effect with heterogeneous reactions. From Fig. 2, it has been noticed that the overall gasification rate of biomass char maintained relatively constant after the initial heat-up period, which indicates an inhomogeneous reaction rate during the gasification, and the effective reaction volume of the biomass char can be limited by the vapour diffusion resistance. Therefore reactions occurred at and near the char particle surface, while the surface continuously moved inward with the char conversion. On the contrary as shown in Fig. 6, the coal char gasification exhibited typical first order reaction kinetics from which exponential decay in the reaction rate was observed. This confirms that vapour diffusion resistance within the coal char can be negligible in the gasification compared to the intrinsic reaction rate. Therefore, the gasification reactions between the water vapour and the solid char occur within the whole volume of the char particle, because the vapour can penetrate into the char particle through the large cracks. In this case, the resistance for the gas diffusion through the compact clusters plays a dominant role. In order to verify the above hypothesis for the char gasification mechanism, experiments were conducted using finer char particles of biomass and coal. The results of comparison between pellet char and pulverised chars are shown in Fig. 14 for carbon consumption rate and in Fig. 15 for char conversion rate (X).
Fig. 12. Effect of gasification temperature and blending ratio on complete conversion time of the solid chars.
Fig. 13. The electronic microscopic scanning images of surface of (a) pure biomass char, (b) 50:50 blended char and (c) pure coal char. All images have a magnification of 350.
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Fig. 15. Effect of particle size on the char conversion rate: (a) biomass char and (b) coal char.
Fig. 14. Effect of particle size on the carbon consumption rate: (a) biomass char and (b) coal char.
From these figures, it is observed that for the coal char, the carbon consumption rate and the overall char conversion rate were only increased slightly after being milled to finer particles with smaller diameters. This confirms that the vapour can easily diffuse into the coal char particles, thus the particle diameter is not a critical factor. However, for the biomass chars, the carbon conversion rate and overall char conversion rate of the milled chars were increased significantly compared to the original biomass pellet chars. In the milled biomass char particles, the particle diameter was significantly reduced, thus the surface area to the particle mass was much higher, enhancing the carbon conversion rate significantly in the first and second stages of the gasification. The difference in the vapour diffusion resistance between the biomass char and the coal char can be attributed to the distinct structural properties of these two solids. From the SEM analysis, the biomass char is much more amorphous than the coal char, hence the specific effective surface area is larger and the intrinsic reactivity is enhanced. On the other hand, the diffusion rate is relatively slower due to the smaller pore size, which limits the inward diffusion of water vapour. However, the coal char shows denser cluster and large cracks, which result in a larger effective pore diameter. Based on the above mechanisms, a mathematical model has been developed to simulate the gasification process of biomass char, coal char and blended biomass and coal char. The model is validated using the experimental results. This will be presented in the subsequent paper (Xu et al., 2011).
4. Conclusions Gas production rate, carbon conversion rate and reactivity for gasification of biomass char, coal char and blended coal and biomass chars have been experimentally investigated in this work. The sample char particles were produced from the pellets of coal (lignite), E. nitens and their blends at three different blending ratios. A series of gasification tests were carried out in a benchscale gasifier at three different temperatures. The composition of the producer gas and gas production rate was continuously analysed by using a Micro-GC. Based on the gas analysis results, carbon conversion rate and gasification reactivity were determined. From the study, the following conclusions are drawn: (1) The gasification of biomass chars, coal chars and the chars of blended biomass and coal can be represented by three stages: initial heat-up and slow gasification stage, fast gasification rate stage and falling rate stage. In the initial heat-up stage, the gasification rate increases to the maximum values, which vary with the coal-to-biomass ratio. The overall conversion rate of biomass char maintains at a relatively constant rate after the initial heat-up period, and this is followed by a falling rate in the last period of the process. For gasification of the coal char, the reaction rate decreases continuously after the maximum value is reached. The reaction rate of the blended coal and biomass chars has intermediate characteristics with the trend similar to the coal char gasification. This is because the biomass loses more mass during the char generation. (2) The overall reactivity of the blended coal–biomass char decreases with the incremental fraction of the coal in the
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blend, and the effect of the coal tends to be reduced at higher gasification temperatures. (3) The SEM image analysis indicates that the biomass char has a more amorphous structure; however, the effective pore size is smaller than coal char. The coal char has a more packed cluster structure and large cracks. The chars of blended biomass and coal are more similar to the coal char. (4) The distinct gasification characteristics of the coal char and the biomass char are due to the differences in gas diffusion within the char particles. The internal resistance to the gas diffusion in the biomass char is significant and an intrinsic reaction is fast due to its amorphous structure, hence, the significant gradient in the gas concentration along the char radius results in inhomogeneous overall reaction rate through the char particle. However, in the gasification of the coal char, the denser structure and larger effective pore size make the overall gasification more homogeneous, and the diffusion of vapour and resultant gases through the dense clusters plays an important role.
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