Fine Char Research Articles

Studies of Biochars Generated from Pilot-Scale Downdraft Gasification

<abstract><title><italic>Abstract.</italic></title> Biochars were generated with a pilot-scale downdraft gasifier using wood chips as the raw material. Biochars were collected from the bottom of the gasifier (coarse char) and from the filter (fine char). They were characterized by scanning electron microscopy (SEM), elemental analysis, thermogravimetric analysis (TGA), and higher heating value (HHV) and compared with the raw material and commercial activated carbon. Coarse char showed a larger average particle size than fine char. Coarse char also had higher carbon content (47.2%) than fine char (32.4%). Both biochars had much lower carbon content but higher mineral content than commercial activated carbon (Filtersorb 200; 88.1%). Both coarse char and fine char retained considerable HHV (14.5 and 10.0 MJ kg^-1 respectively), but their HHVs were lower than that of wood chips (17.7 MJ kg^-1) and much lower than that of Filtersorb 200 (30.7 MJ kg^-1). Fine char showed higher thermal stability than coarse char. Both biochars were more thermally stable than wood chips but less stable than activated carbon.

Filtration of fast pyrolysis char fines with a cross-flow moving-bed granular filter

A cross-flow moving-bed granular filter was investigated for its performance to filter the fast pyrolysis char-laden gas with an aim to elucidate the influence of the number of filtration stages, the filter media size and mass flow rate on the collection efficiency of the filter and the pressure drop across the bed. The results showed that the two-stage filtration gave higher collection efficiency than the one stage, especially for small filter media size. The decrease of the filter media size significantly increased the collection efficiency, while slightly increased the pressure drop. In addition, the increase of the mass flow rate led to the reduction in the collection efficiency. Within the scope of the parameters investigated, the maximum collection efficiency of the granular filter was 99.79% which occurred when applying single stage filtration with the filter media size and the mass flow rate of 0.425–0.600 mm and 8 g/min, respectively.

Experimental and Kinetic Investigations of CO2 Gasification of Fine Chars Separated from a Pilot-Scale Fluidized-Bed Gasifier

The CO2 gasification behaviors of two fine chars separated from a pilot-scale fluidized-bed gasifier were studied in a thermogravimetric analyzer (TGA) within the temperature range of 1000–1300 °C. The physical properties of fine chars were examined by scanning electron microscopy (SEM), N2 adsorption, and X-ray diffraction (XRD). The differences in gasification reactivity and related properties between the fine chars and the corresponding experimental-produced coal chars were also compared. The results show that the fine chars have higher ash content, larger Brunauer–Emmett–Teller (BET) surface area, and better gasification reactivity than the corresponding coal chars. The gasification reactivity of fine chars was promoted by the catalytic alkali and alkaline earth metals (AAEMs) but inhibited by the enrichment of the ash layer in a higher carbon conversion range or the ash melting at a higher temperature. In addition to the AAEMs, the reactivity of different fine chars is mainly influenced by their pore and carbon crystalline structures. The kinetic investigation reveals that the modified random pore model (MRPM) and shifted-modified random pore model (S-MRPM) perform more reasonably than the random pore model (RPM) in some special conditions. Moreover, the reactivity of fine chars increases, and the reaction shifts from chemical reaction control to gas diffusion control as the gasification temperature increases.

Entrained-flow gasification of coal under slagging conditions: Relevance of fuel–wall interaction and char segregation to the properties of solid wastes

Entrained-flow slagging gasifiers are characterized by operating conditions that promote ash migration/deposition onto the reactor walls, whence ash is drained as a molten phase. Experimental investigation on ashes generated by full-scale plants suggested that both char entrapment inside the melt and carbon-coverage of the slag can occur. Because of the wide range of spatial and temporal scales involved in these phenomena, numerical simulation of the fate of the flying fine char particles is a very difficult task. This work illustrates how different numerical modeling approaches can be jointly used to understand segregation patterns of char particles in full-scale entrained-flow coal gasifiers operated in the slagging regime. A multilevel approach has been developed for this purpose. RANS-based simulations of the full-scale geometry with coal particle injection and tracking aimed to obtain the general behavior of the flow field and particle trajectories. Simulations enabled to estimate the effect of swirl and tangential flow on the bulk-to-wall char particle deposition rate. Then, RANS results were adopted in a more detailed numerical model based on the solution of the filtered Navier–Stokes equations. In this last model, a turbulence LES approach for the Eulerian gas phase was applied. The equations of particles motion were solved via a Lagrangian particle tracking algorithm with the TrackToFace method. Simulations were performed involving a level of detail that allowed to obtain a clear picture of the multiphase flow behavior responsible for char deposition phenomena. This multilevel approach enabled the assessment of the char particle deposition rates and the nature of char–slag interaction (segregation/entrapment) that are likely to occur in full-scale slagging gasifiers. Results of numerical simulations have been critically discussed in the light of experimental findings. They represent a useful source of information for the implementation of constitutive equations and parameters in design-oriented reduced compartmental models.

Investigation of Char–Slag Interaction Regimes in Entrained-Flow Gasifiers: Linking Experiments with Numerical Simulations

Entrained-flow gasifiers are characterized by operating conditions that promote ash migration/deposition onto the reactor walls, whence the ash is drained as a molten phase. Experimental investigation on ashes generated by full-scale plants suggested that both char entrapment inside the melt and carbon-coverage of the slag can occur. Because of the wide range of spatial and temporal scales involved in these phenomena, numerical simulation of the fate of the flying fine char particles is a very difficult task. Simulations based on a Eulerian large eddy simulation (LES) approach for the turbulent gas phase and a Lagrangian particle tracking (LPT) approach for the solid phase have been performed to explore, in a simplified flow configuration, near-wall particle segregation and the role of relevant parameters (in particular, particles’ inertia and slag layer restitution coefficient). Results of numerical simulations have been critically discussed with reference to the experimental observations. Numerical results confirm that near-wall accumulation of particles may be extensive, and promoted by large Stokes numbers and by small restitution coefficients.

Application of char products improves urban soil quality

AbstractUrban soils are a key component of the urban ecosystem but little research has considered their quality and management. The use of char or partially combusted char products as a soil amendment is becoming popular worldwide because of perceived benefits to fertility and the potential for increasing carbon sequestration. In this study, we assessed the effect of applying coarse and fine char material on the quality of four different types of soil‐based root‐zone mixes typically used for turfgrass and general landscaping in Singapore: clay loam soil, approved soil mix (ASM, 3 soil:2 compost:1 sand), 50:50 (sand/soil) and 75:25 (sand/soil). Char briquettes made from sawdust were mixed thoroughly at rates of 25, 50 and 75% by volume with the soil mixes. Results showed that addition of char (both coarse and fine) significantly enhanced the carbon content of the mixes, with the largest increase being associated with the 50% and 75% additions. Soil nutrients (total N, extractable P, K, Ca and Mg) and mean weight diameter of aggregates were also significantly increased following the application of char. The clay loam and the 50:50 and 75:25 soil mixes were more responsive to the addition of char than was ASM.

A new application of bagasse char as a solar energy absorption and accumulation material

ABSTRACTThis study is concerned with the relationship between carbonisation conditions and properties of the resultant bagasse char, and the challenging proposal for the use of fine bagasse char particles as high-performance solar light collectors. Bagasse char was obtained by carbonising raw bagasse at temperatures of 200–900°C for three hours in nitrogen (N2) gas. Characterisation of the resultant bagasse char was performed by elemental analysis (EA), scanning electric microscope (SEM) observation, estimation of colour, nominal bulk density and evaluation of specific surface area and pore structure by N2 gas absorption/desorption. The most typical property of the resultant bagasse char is its unique pore structure, which is clear in SEM observation. The largest specific surface area of bagasse char in this study was about 600 m2/g, which is as large as commercial activated carbon materials. Macro-, meso- and micro-porous structures in the bagasse char induce many important characteristics, such as increased hydrophilicity, very low bulk density and excellent light absorption and accumulation. We also suggest the use of bagasse char as an excellent heat insulation material. The energy used for air conditioning in a private house or office building can be decreased by more than 50–60% by use of this insulator on the roof or walls.

The influence of granular flow rate on the performance of a moving bed granular filter

The goal of this study is to evaluate the effect of granular flow rate on the performance of a moving bed granular filter designed for hot gas filtration of fine char particles (dust) produced during fast pyrolysis of biomass. The filter employs a counter-current configuration, in which down-flowing granular material spreads out at the bottom of the filtration vessel to form an interfacial area where the dusty gas enters the granular bed and much of the gas cleaning is hypothesized to occur. This study uses a real-time particle counter to measure the instantaneous filtration efficiency during cold flow tests of the filter. Differential pressure measurements at various locations within the granular bed are used to assess the level of char dust hold-up over time. These experiments reveal a critical granular residence time below which the filter must be operated to achieve filtration efficiencies exceeding 99%. Operating above the critical value causes the filter to “clog” and decrease in efficiency. The clogging is characterized by a critical dust volume fraction as determined through a fixed bed filtration test. The filter is found to accumulate most of the dust at the interfacial region. Also the interfacial region is more efficient than the downcomer section of the granular bed in removing dust. Decreasing residence time of granular material in the filter reduces the hold-up of char dust in the filter, which is expected to mitigate coking reactions of organic vapors when the filter is used to remove char from fast pyrolysis gas streams.

Aerosol-based method for investigating biomass char reactivity at high temperatures

An aerosol-based method was proposed and developed to characterize particles fragmented from biomass chars during oxidation. The chars were prepared from both wood and miscanthus pellets under various pyrolysis conditions. Char fragments with aerodynamic diameters in the range of 0.5–10 μm were suspended and transported in a reactive gas through an aerosol reactor, which was heated by an electric oven. The oxidation of char particles in the reactor was investigated by determining on-line the particle size distributions before and after passage through the reactor using an aerodynamic particle sizer (APS) spectrometer. The interpretation of APS data was evaluated by both experiments and models in which the fine char particles were assumed to keep either constant density or constant diameter during the oxidation process. The results indicate that the aerosol-based method can be used to determine reaction kinetics of char particles in the high-temperature range, where oxidation is normally controlled by diffusion limitation if measuring with the conventional techniques. The application of the aerosol method indicated that high pyrolysis temperature and prolonged retention time will reduce the char reactivity.

Experiments on chemical looping combustion of coal with a NiO based oxygen carrier

A chemical looping combustion process for coal using interconnected fluidized beds with inherent separation of CO 2 is proposed in this paper. The configuration comprises a high velocity fluidized bed as an air reactor, a cyclone, and a spout-fluid bed as a fuel reactor. The high velocity fluidized bed is directly connected to the spout-fluid bed through the cyclone. Gas composition of both fuel reactor and air reactor, carbon content of fly ash in the fuel reactor, carbon conversion efficiency and CO 2 capture efficiency were investigated experimentally. The results showed that coal gasification was the main factor which controlled the contents of CO and CH 4 concentrations in the flue gas of the fuel reactor, carbon conversion efficiency in the process of chemical looping combustion of coal with NiO-based oxygen carrier in the interconnected fluidized beds. Carbon conversion efficiency reached only 92.8% even when the fuel reactor temperature was high up to 970 °C. There was an inherent carbon loss in the process of chemical looping combustion of coal in the interconnected fluidized beds. The inherent carbon loss was due to an easy elutriation of fine char particles from the freeboard of the spout-fluid bed, which was inevitable in this kind of fluidized bed reactor. Further improvement of carbon conversion efficiency could be achieved by means of a circulation of fine particles elutriation into the spout-fluid bed or the high velocity fluidized bed. CO 2 capture efficiency reached to its equilibrium of 80% at the fuel reactor temperature of 960 °C. The inherent loss of CO 2 capture efficiency was due to bypassing of gases from the fuel reactor to the air reactor, and the product of residual char burnt with air in the air reactor. Further experiments should be performed for a relatively long-time period to investigate the effects of ash and sulfur in coal on the reactivity of nickel-based oxygen carrier in the continuous CLC reactor.

Fluidized bed combustion of pelletized biomass and waste-derived fuels

The fluidized bed combustion of three pelletized biogenic fuels (sewage sludge, wood, and straw) has been investigated with a combination of experimental techniques. The fuels have been characterized from the standpoints of patterns and rates of fuel devolatilization and char burnout, extent of attrition and fragmentation, and their relevance to the fuel particle size distribution and the amount and size distribution of primary ash particles. Results highlight differences and similarities among the three fuels tested. The fuels were all characterized by limited primary fragmentation and relatively long devolatilization times, as compared with the time scale of particle dispersion away from the fuel feeding ports in practical FBC. Both features are favorable to effective lateral distribution of volatile matter across the combustor cross section. The three fuels exhibited distinctively different char conversion patterns. The high-ash pelletized sludge burned according to the shrinking core conversion pattern with negligible occurrence of secondary fragmentation. The low-ash pelletized wood burned according to the shrinking particle conversion pattern with extensive occurrence of secondary fragmentation. The medium-ash pelletized straw yielded char particles with a hollow structure, resembling big cenospheres, characterized by a coherent inorganic outer layer strong enough to prevent particle fragmentation. Inert bed particles were permanently attached to the hollow pellets as they were incorporated into ash melts. Carbon elutriation rates were very small for all the fuels tested. For pelletized sludge and straw, this was mostly due to the shielding effect of the coherent ash skeleton. For the wood pellet, carbon attrition was extensive, but was largely counterbalanced by effective afterburning due to the large intrinsic reactivity of attrited char fines. The impact of carbon attrition on combustion efficiency was negligible for all the fuels tested. The size distribution of primary ash particles liberated upon complete carbon burnoff largely reflected the combustion pattern of each fuel. Primary ash particles of size nearly equal to that of the parent fuel were generated upon complete burnoff of the pelletized sludge. Nonetheless, secondary attrition of primary ash from pelletized sludge is large, to the point where generation of fine ash would be extensive over the typical residence time of bed ash in fluidized bed combustors. Very few and relatively fine primary ash particles were released after complete burnoff of wood pellets. Primary ash particles remaining after complete burnoff of pelletized straw had sizes and shapes that were largely controlled by the occurrence of ash agglomeration phenomena.

A Fluidized Bed Biomass Combustion Model with Discretized Population Balance. 1. Sensitivity Analysis

A model of a 1 MWth atmospheric bubbling fluidized combustor burning waste wood fuel (feeding) is presented. The model incorporates both the solid and gas phases. The bed is assumed to consist of two phases, of which the emulsion phase takes both gas and solids into account, while the bubble phase consists only of gas. A wide size distribution of biomass feed, representative of the actual boiler feed, has been assumed. The model calculates the gas composition, velocities, and other important hydrodynamic parameters in both the emulsion and bubble phase. A particle size distribution model is included to calculate elutriation losses of fine char particles. This approach is novel in the sense that a population balance for fine particle class is derived, using the well-known mass balance principles, and a coupled discretized population balance equation, valid for the whole particle size range, is presented. The model takes into account devolatilization, fragmentation, and attrition of the solid phase along with gaseous profiles. It includes nine components for which differential equations have been derived and solved to calculate species concentration at any point along the bed height. In total, 20 particle size classes have been considered, 10 of which are considered as feed and the rest as fine classes. The model aims to assess the effect of different parameters on boiler performance and gaseous emissions. A sensitivity analysis of the gaseous emission profiles with respect to different variables and parameters defining different submodels has been carried out. A homogeneous NOx model has been included with individual kinetic parameters for relevant species. It has been found that gas hydrodynamics play a significant role, and the system can be optimized using these parameters.

Formcoke production from char fines of hard brown coals by air curing

The purpose of the present study was to explore the technical feasibility of formcoke production from non-coking bright hard brown coals and pitch as a binder by air curing. The briquettes obtained from the chars of Soma coals carbonised at 500, 600, 700, 800 and 900°C with 12% pitch were air cured in a streaming gas containing 16%O2(air + N2) for 120 min at 270°C and the formcoke strength was found to be 10–40% higher than that of raw briquettes. Under the prevailing conditions, the formcoke strength and the apparent specific gravity of the green and formcoke briquettes increased as the carbonisation temperature increased, reaching a maximum at the optimum carbonisation temperature of 800°C. M40 and M10 values and the compressive strength of SK800 formcokes were determined to be 92·3, 7·7% and 23·5 MPa respectively being similar to those of conventional cokes produced at coke ovens. The optimum curing conditions (pitch content, curing time and temperature) were determined using TK800 char. The optimum curing time for TK800 briquettes at 220°C was determined to be 90 min. The formcoke strength of TK800 briquettes subjected to a thermal treatment at 220°C was lower than that of the briquettes air cured at the same temperature. The optimum curing temperature for TK800 briquettes with 10% pitch and cured in a gas stream containing 16%O2 in terms of formcoke strength was determined to be 285°C. M40 and M10 values and the compressive strength of TK800 formcokes were determined to be 91, 8·1% and 19·6 MPa respectively. The experimental results reveal the advantages of the method because the formcokes produced possess suitable properties for various metallurgical processes and smokeless fuel consumption. The gas and tar obtained during the carbonisation of raw coal are the added benefits.

Coal combustion efficiency in CFB boiler

The carbon content in the fly ash from most Chinese circulating fluidized bed (CFB) boilers is much higher than expected, thus directly influencing the combustion efficiency. In the present paper, carbon burnout was investigated both in field tests and laboratory experiments. The effect of coal property, operation condition, gas-solid mixing, char deactivation, residence time and cyclone performance are analyzed seriatim based on large amount of experimental results. A coal index is proposed to describe the coal rank, defined by the ratio of the volatile content to the coal heat value, is a useful parameter to analyze the char burnout. The carbon content in the fly ash depends on the coal rank strongly. CFB boilers burning anthracite, which has low coal index, usually have high carbon content in the fly ash. On the contrary, the CFB boilers burning brown coal, which has high coal index, normally have low carbon content. Poor gas-solid mixing in the furnace is another important reason of the higher carbon content in the fly ash. Increasing the velocity and rigidity of the secondary air could extend the penetration depth and induce more oxygen into the furnace center. Better gas solid mixing will decrease the lean oxygen core area and increase char combustion efficiency. The fine char particles could be divided into two groups according to their reactivity. One group is “fresh” char particles with high reactivity and certain amount of volatile content. The other group of char particles has experienced sufficient combustion time both in the furnace and in the cyclone, with nearly no volatile. These “old” chars in the fly ash will be deactivated during combustion of large coal particles and have very low carbon reactivity. The generated fine inert char particles by attrition of large coal particles could not easily burn out even with the fly ash recirculation. The fraction of large coal particles in coal feed should be reduced during fuel preparation process. The cyclone efficiency controls the particle residence time in CFB loop, especially for that of the fine particles. So the cyclone efficiency, especially the cut size, will greatly influence the carbon content in the fly ash.

04/00401 The influence of fine char particles burnout on bed agglomeration during the fluidized bed combustion of a biomass fuel: Scala. F. et al. Fuel Processinn Technology. 2003. 84. (1–3). 229–241

Batch fluidized combustion of 5 mm particles of a petroleum coke has been carried out in 40 mm I.D. combustors to investigate the generation of fines due to the abrasion of bed solids on the surface of burning particles. The experimental technique used allowed the time resolution of attrited fines generation, providing detailed curves of attrition rates as a function of time for oxygen concentration in the fluidizing gas ranging from 21 to 0.75%. The influence on combustion has been shown, by comparison with results of purely mechanical attrition of particles in nitrogen-fluidized beds. The change of particle size as a function of burn-off and fractional time for complete conversion, the number of particles present in the bed throughout each run, and sizes of the attrited fines have been determined. The modifications produced in the texture of the cortical region of the coke particles by combustion have been examined under an optical microscope and related to the “activation” of the surface with respect to attrition.A descriptive model of combustion assisted attrition of particles of a petroleum coke burning in a fluidized bed has been proposed. It is based on the assumption that the coke particle is divided into elements burning at different rates, because of their different accessibility to oxygen. Elements reacting more slowly emerge as asperities on the external surface of the particle. They eventually burn or are abraded by bed solids forming, in this latter case, the attrited fines. The model provides a coherent framework for all the experimental data. In particular, it gives an explanation of the typical shape of attrition curves with a maximum somewhere between the beginning of the burning and burn-out, in terms of the competing effects of the “activation” of the particle surface to form detachable asperities and its reduction with burn-off.

The influence of fine char particles burnout on bed agglomeration during the fluidized bed combustion of a biomass fuel

The combustion of biomass char in a bubbling fluidized bed is hereby addressed, with specific reference to the influence that the combustion of fine char particles may exert on ash deposition and bed agglomeration phenomena. Experiments of steady fluidized bed combustion (FBC) of powdered biomass were carried out with the aim of mimicking the postcombustion of attrited char fines generated in the fluidized bed combustion of coarse char. Experimental results showed that the char elutriation rate is much smaller than expected on the basis of the average size of the biomass powder and of the carbon loading in the combustor. Samples of bed material collected after prolonged operation of the combustor were characterized by scanning electron microscopy (SEM)–EDX analysis and revealed the formation of relatively coarse sand–ash–carbon aggregates. The phenomenology is consistent with the establishment of a char phase attached to the bed material as a consequence of adhesion of char fines onto the sand particles. Combustion under sound-assisted fluidization conditions was also tested. As expected, enhancement of fines adhesion on bed material and further reduction of the elutriation rate were observed. Experimental results are interpreted in the light of a simple model which accounts for elutriation of free fines, adhesion of free fines onto bed material and detachment of attached fines by attrition of char–sand aggregates. Combustion of both free and attached char fines is considered. The parameters of the model are assessed on the basis of the measured carbon loadings and elutriation rates. Model computations are directed to estimate the effective size and the peak temperature of char–sand aggregates. The theoretical estimates of the effective aggregate size match fairly well those observed in the experiments.

Char Attrition from Australian Black Coals in Pressurized Fluidized Bed Combustion

Char attrition is the main mechanism for the fine char generation in PFBC. The char attrition rates were significantly different for each of the two coal chars tested. Specific char attrition rate increased with the surface porosity of burning char particles. A coal with higher ratio of telocollinite/inertinite contents formed chars in PFBC with larger pores or higher porosity during devolatilization, which resulted in a higher specific attrition rate.

Fluidized bed combustion of tyre derived fuel

Mechanistic aspects of the fluidized bed combustion of tyre derived fuel (TDF) have been analyzed both experimentally and theoretically. The time-resolved rates of carbon fines elutriation during the fluidized bed combustion of TDF have been measured during batch experiments in a bench scale reactor at 850 °C, under different operating conditions. Experimental results indicated that both gas superficial velocity and oxygen partial pressure exert influence upon the overall fixed carbon combustion efficiency. The efficiency increases slightly with the oxygen concentration and significantly if the gas superficial velocity decreases. Experimental data are further analyzed in the framework of a fluidized bed combustor model especially suited for high-volatile solid fuels feedings. The model takes into account phenomena that assume particular importance with this kind of fuels, namely fuel particle fragmentation in the bed and combustibles segregation and postcombustion above the bed. Experimental and model results indicate that the efficiency of the fluidized bed combustion of TDF is controlled by the competition between combustion and entrainment of char fines and volatile matter released in the early stage of fuel conversion.

Coal topping in a fluidized bed system

A batch-fed bench-scale pressurised fluidised bed combustion (PFBC) facility was built to research the in-bed processes generating fine char particles during PFBC of Australian black coals. Elutriation of such particles is responsible for the combustion inefficiency and their combustion in the filter cake on high-temperature ceramic filters may contribute to the formation of ‘sticky ash’ and consequent cleaning difficulties. Measured carbon elutriation varied from coal to coal and the results agreed qualitatively with large-scale PFBC performance, allowing definition of satisfactory and unsatisfactory performance criteria. Carbon elutriation in PFBC was correlated tentatively with crucible swelling number although the range of values of the crucible swelling number needs to be extended. Carbon elutriation in PFBC did not correlate with volatile matter for the five Australian coals investigated in contrast with previous PFBC pilot plant studies on four Northern Hemisphere coals. The higher swelling coal produced a significantly larger average macro-pore diameter after devolatilisation in PFBC, which caused greater attrition of fine char from the particle surface during the char burnout stage.

Ignition temperature of coal carbonizates

A batch-fed bench-scale pressurised fluidised bed combustion (PFBC) facility was built to research the in-bed processes generating fine char particles during PFBC of Australian black coals. Elutriation of such particles is responsible for the combustion inefficiency and their combustion in the filter cake on high-temperature ceramic filters may contribute to the formation of ‘sticky ash’ and consequent cleaning difficulties. Measured carbon elutriation varied from coal to coal and the results agreed qualitatively with large-scale PFBC performance, allowing definition of satisfactory and unsatisfactory performance criteria. Carbon elutriation in PFBC was correlated tentatively with crucible swelling number although the range of values of the crucible swelling number needs to be extended. Carbon elutriation in PFBC did not correlate with volatile matter for the five Australian coals investigated in contrast with previous PFBC pilot plant studies on four Northern Hemisphere coals. The higher swelling coal produced a significantly larger average macro-pore diameter after devolatilisation in PFBC, which caused greater attrition of fine char from the particle surface during the char burnout stage.