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Comprehensive effects of different inorganic elements on initial biomass char-CO2 gasification reactivity in micro fluidised bed reactor: Theoretical modeling and experiment analysis

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Abstract

In this study, a theoretical model of biomass char gasification reactivity was developed, focusing on the catalytic effect of inorganic elements on the char gasification process. A comparison with previous research results shows that the catalytic ability of K in a fixed bed reactor is stronger than that of Ca, while the catalytic ability of Ca in a fluidised bed reactor is stronger than that of K. The migration and transformation of K and Ca in a fixed bed reactor and fluidised bed reactor are compared. In the fluidised bed reactor, a larger proportion of Ca is transformed into an ion-exchanged state than K, which is contrary to the experimental results in the fixed bed reactor. Then, according to the equivalent-volumetric impregnation method, the saturated loading ratio of K was determined to be 35%, and the full catalytic ratio of K was determined experimentally. Finally, eight typical biomass char samples were selected to perform experiments at different temperatures in the micro fluidised bed reactor to determine the char gasification reactivity, and the results were compared with the calculated values of the model. Results show that the model can effectively predict the char gasification reactivity both in trend and accuracy.

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The intrinsic kinetics of char combustion were commonly investigated using Thermogravimetric analysis (TGA) in previous works at low temperatures to prevent oxygen limitations and temperature deviation. However, the low temperatures caused excess test time because the reaction rate was too slow. In this study, the micro-fluidized bed (MFB), which has effective heat and mass transfer, was used to investigate the intrinsic kinetics of char combustion at higher temperature within less time. TGA was used to check the reliability of MFB. Results suggested that for fine particles (74-100 μm), particle temperature deviation and gas distortion could be disregarded in MFB. For four coal samples, the activation energy measured by MFB was similar to the result measured by TGA, but the test time using MFB was greatly shortened due to the higher temperatures. The kinetic parameters measured by MFB were used to predict char oxidation rate. The predicted rate fit the TGA experiment well at lower temperatures. These results demonstrated the reliability of MFB and TGA measurement. However, at higher temperatures, the combustion rate in TGA was limited by oxygen diffusion, suggesting that TGA measurement should be carried out at relatively low temperatures to prevent oxygen diffusion limitation. Instead, MFB measurement was still valid at higher temperatures due to the effective mass and heat transfer.
Article
The catalytic effect of sodium on the demineralized Shengli (SL⁺) lignite char microstructure and the performance of steam gasification were studied. Various sodium compounds including NaNO3, CH3COONa, Na2CO3 and NaOH were loaded on the demineralized coal samples, respectively, and the steam gasification was tested on the fix-bed reactor. The char samples were characterized by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS) and FT-IR spectroscopy. Experimental results showed that sodium hydroxide loaded samples exhibited the highest gasification reactivity among the coal samples prepared. With the increase of the alkalinity of sodium compounds, the carbon crystallite structure tended to be disordered. In the process of pyrolysis, the introduction of sodium species promoted the ring-opening and polycondensation process of the chemicals in the coal samples. The possible reaction mechanism might be inferred that the sodium ions may replace the hydrogen ions in the oxygen-containing functional groups to form sodium phenolate intermediate, which may be critical for the catalytic effect of sodium species during gasification. It was speculated that the ring-opening of the condensation aromatic nucleus was the rate-limiting step in the whole process of gasification.
Article
A study was carried out in both inert and CO2 atmosphere to investigate the interaction mechanism of K2CO3 with graphite and coal char during the catalytic gasification of coal char. The formation of K2CO3 catalytic precursors was identified by using series of different techniques. The results obtained in an inert atmosphere show that the interaction between K2CO3 and carbon readily took place at low temperature. For the K2CO3 loaded graphite, the intermediate of CnK clusters formed by the interactions of K2CO3 with carbon prevented the evaporation of K at low temperature. At temperatures above 800 °C, however, the CnK clusters were unstable, and decomposed to release K through vaporization. On the contrary, for the K2CO3 loaded demineralized coal char, due to the presence of oxygen groups, K was tightly bonded to carbon during heating, forming a complex precursor containing O and K with high thermal stability. The reaction pathway of carbon-K2CO3 interactions in an inert atmosphere are proposed for both char and graphite. The results of DRIFTs experiment suggest that CO2 is chemisorbed by the K- catalytic precursors during CO2 gasification to form potassium phenolate species and a covalently bound carbonate group (dimethyl carbonate). It is believed that these species act as active intermediates in the gasification process.
Article
Different concentrations (0.1 and 1 M K⁺/Na⁺) of salt solutions (K2CO3, Na2CO3, NaOH and NaCl) were used to impregnate alkali in sawdust. After devolatilization, char samples were gasified at different temperatures (750–900 °C) under CO2 in a macro-thermogravimetric analyzer for gasification kinetics. Morphologically, three classes of chars could be identified. Chars experiencing the highest catalytic influence were in Class-2 (0.5 M K2CO3 and 1 M NaOH) with a swollen and molten surface. In contrast, Class-1 (wood char like) and Class-3 (with salt deposits) chars showed moderate and low catalytic effect on gasification reactivity respectively. It is believed to be related to char surface swelling and alkali salt used. At 850 °C or below, the reactivity increased linearly (Class-1 and Class-3 Char) with initial alkali content up to 2200 mmol alkali/kg of char (except for NaCl). The same reaction rate was maintained until 3600 mmol/kg of char of alkali loading (Class-2) and then decreased. However, no trend was observed at 900 °C due to drastic change in reactivity of the samples, probably due to alkali transformation. Among the salts, K2CO3 (0.5 M) was found to be the most suitable for catalytic gasification due to its high catalytic activity in combination with relatively low carbon leaching.
Article
The ignition and combustion characteristics of Zhundong lignite (ZDL), with and without washing to remove different forms of inherent sodium, were investigated. Water washed (ZDL-WW) and acid washed (ZDL-AW) samples were prepared by soaking the raw lignite (ZDL-Raw) in ultrapure water and 0.5 M hydrochloric acid (HCl), respectively, at 60 °C for 24 h. A single particle of a ZDL sample, ca 2.5 mm in diameter, was suspended on a silicon carbide fibre (142 µm) tip and burned in air at 1123 K in a furnace. The time-resolved ignition and combustion behaviours of the single particles were observed with the aid of combined use of a shortwave infrared camera, a CCD camera, which enabled the determination of the ignition mechanism, ignition time, burnout time and burning rate. A flame emission spectrometer was used to identify the presence of sodium in the flame. The ignition of all ZDL samples followed the joint hetero-homogeneous mechanism in the present work. Upon the homogeneous ignition, ZDL-Raw exhibited a soot free yellowish translucent flame while ZDL-WW and ZDL-AW showed sooty flames. The ignition time followed the order of ZDL-Raw>ZDL-WW>ZDL-AW while the burning rate followed the opposite order. These observations were attributed to the catalytic effect of sodium in the lignite whose amount was varied due to the water and acid washing. Sodium ions were detected in the flame of ZDL with and without washing and the intensity of sodium signal also followed the order of ZDL-Raw>ZDL-WW>ZDL-AW. It is believed that sodium ions released in the flame promoted catalytic cracking of large tar fragments and oxidation of soot precursors.
Article
Cooling char, which is prepared in an inert atmosphere and then reacted with oxygen after cooling, has been widely used in the kinetic study of char combustion. However, there is no cooling process in commercial combustion systems and thus the impacts of the cooling process on char reactivity remain unclear. To illustrate this question, a two-step reaction analyzer was developed to study the combustion kinetics of in-situ char, which was produced at a constant high temperature in an inert atmosphere and then directly reacted with oxygen at different temperatures. Two types of Chinese coal, Shenhua sub-bituminous coal and Yangquan anthracite, were selected as samples. The results showed that the reaction rate of in-situ char was about 1.1–1.25 times of that of rapid cooling char, suggesting that the cooling process has a significant effect on char reactivity. Then the combustion rate and structure of chars cooled at different cooling rates were investigated to determine the reason for char deactivation during the cooling process. The cooling char reactivity decreased with the decreasing cooling rate, but the specific surface area did not decreased considerably. This result revealed that char deactivation during the cooling process was independent of specific surface area. As the cooling rate decreased, the oxygen chemisorption capacity (active surface area) of cooling char decreased for Shenhua coal, indicating that the observed char deactivation during the cooling process might be caused by the decrease of active surface area. In addition, Yangquan anthracite, as a high rank coal, was less sensitive to the cooling process due to its higher carbon crystallites and provision of fewer active sites for oxidation.
Article
The existence form of sodium in Xinjiang coals was studied by extraction with distilled water, ammonium acetate and hydrochloric acid step by step. The extraction liquid and the residual coal samples were analyzed using an ion chromatography and an inductively coupled plasma atomic emission spectroscopy. The influences of different forms of sodium on the combustion characteristics of the high sodium coal were evaluated by the ignition temperature, the burnout temperature and the combustion characteristic index. The results indicate that the sodium existing in the coals is mainly in the form of water-soluble sodium, while the proportions of acid-soluble sodium and insoluble sodium are smaller. The particle size and inner pore structure of coal may have great influences on the water-soluble sodium content in coal samples, while the organic sodium contained in different particle size ranges of coal remains relatively constant. The water-soluble sodium and chlorine in different coals differ from each other. The water-soluble sodium would increase the ignition temperature and burnout temperature and decrease the combustion characteristic index, while the organic sodium would have catalysis on the combustion of coal.
Article
The Grain-Thermo-Balance (GTB) rig was built for generation of the char, at high heating rates, and for experimental determination of its oxidation rates. A zero-dimensional mathematical model for calculating oxidation rates of millimeter-size char particles was developed to serve as a char particle sub-model in a CFD-based software for predicting performance of fluidized bed boilers. When morphology of the char was determined using mercury porosimetry and the char oxidation rates in the kinetic regime were measured using TGA, the zero-dimensional Shrinking Particle Model was able to reproduce the GTB measurements well - with exception of the last 2% burnout where due to a model singularity the calculated temperatures exceeded the measured values. When the char was generated in TGA, at low heating rates, its intrinsic reactivity was four times lower and the reactivity decrease was attributed to alterations to the char morphology (40% slow-down) and annealing (60%).
Article
The existence form of minerals in Wucaiwan (WCW) coal were studied by extraction, XRD, SEM-EDX and ICP-OES. The influences of different forms of mineral on the combustion characteristics and the fusibility of ash were evaluated. The results indicate that the original minerals in WCW coal mainly include calcite, anhydrite, kaolinite, quartz and pyrite. The modes of occurrence of sodium in WCW coal are mainly in the form of water-soluble sodium. The organic bound sodium, magnesium and calcium can promote the combustion of coal to a certain extent. The fusibility of WCW coal ash is mainly influenced by the mol ratio of iron to calcium. The ash melting points decreases with increasing the Fe2O3/CaO mol ratio.
Article
In order to research the effects of ash removal on the structure and combustion characteristics, the ash of Shengli lignite were removed by water-washing and acid-washing, respectively. The oxygen-functional group, pore size distribution and combustion performance of coal samples were characterized by Fourier infrared spectrum (FT-IR), Nitrogen adsorption method and TG-DTG analysis technique. The results show that the effect of water-washing on the lignite structure is not remarkable. The acid-washing increases the amount of oxygen-functional group, significantly decreases cumulative pore volume from 1.5 nm and reduces specific surface area from 5.13 m2/g of raw coal to 2.41 m2/g. The kinetic analysis of lignite pyrolysis reveals that the combustion reaction orders of 2, 2 and 1 are found to be an effective mechanism for SLR, SLD and SLA, respectively. During the volatile matter releasing, the activation energy of lignite SLR, SLD and SLA is 5001.30, 3698.95, 17110.08 J/mol, respectively. While during the combustion of coal coke, the activation energy of lignite SLR, SLD and SLA is 40991.58, 40065.07, 44712.98 J/mol, respectively. ©, 2015, China University of Mining and Technology. All right reserved.
Article
In combustion and gasification reactors, solid fuels such as coal or biomass undergo a pyrolysis step to form volatiles and a char. Pyrolysis is combined with an oxidation by O2, H2O and/or CO2 of the resultant char. Here we propose an original approach to model the oxidation of a single char particle. The intrinsic chemical reactivity of the char surface is assessed by the reactive surface area (RSA). The evolution of the RSA upon the conversion of the particle (burn-off) is modelled by a specific kinetic law similar to the law used to model the evolution of active sites in catalysis. This approach is applied on experimental results taken from the literature for coal and wood chars for O2, H2O or CO2 gasification. The evolution of the total surface area (TSA) is modelled by the random pore model. Diffusional limitations are also accounted for.
Article
This research investigates the catalytic properties of char which was recovered directly from a biomass gasifier. Poplar wood was gasified in steam and CO2 environments in a fluidized bed reactor at temperatures ranging from 550 to 920 °C. Char was composed of 85% carbon with concentrations of N, H, and S between 0.3% and 3%, depending on gasification conditions. The inorganics (Ca, K, Na, P, Si, Mg) were quantified, revealing that Ca was present in the highest concentration (0.5-1%), followed by K, ranging from 0.1% to 0.8%. The char had catalytic activity for decomposition of methane, which was used as a model molecule. The quantity of inorganics in the char was modified by acid washing in 16% aqueous HCl, which removed >95% of Ca, K, P, and Mg from the char. This resulted in an 18% decrease in the quantity of methane reacted compared to the original char sample, demonstrating that inorganics, which only make up approximately 2% of the char, play a significant role in its catalytic activity for methane cracking reactions. In addition, carbon was found to play an important role in the catalytic activity of the char, both as a catalyst and a support on which the inorganics were dispersed. The activity of carbon free ash was approximately 90% lower than that of char, and deactivated to have no measurable activity after 45 min on stream, demonstrating the importance of carbon and dispersed inorganics for catalytic activity. When char was heated to 1000 °C in N2, inorganics and oxygen migrated to the surface of the char, covering the carbon surface in a metal oxide layer. This decreased the catalytic activity by approximately 40%. Acidic (e.g. carboxylic, lactones) and basic (e.g. carbonyl, pyrone) oxygen functional groups were identified on the char surface. However, acidic oxygen groups desorbed at reaction temperatures, so these groups likely do not participate in cracking reactions.
Article
Studies have shown that both char particle diameter and apparent density vary during char conversion at high temperatures. To account for such variations, power-law expressions have been used to correlate rp/rp,0 and ρp/ρp,0 with mp/mp,0. The parameters in these relations are constants, thus this approach fails to account for variations in the functional relationship between mass, size, and apparent density as mass conversion proceeds. To overcome this limitation, a model for the mode of particle conversion has been developed that permits the variation in size and apparent density with mass loss to depend upon the Thiele modulus, which varies during char conversion. The rate with which the particle radius decreases is shown to be given by the ratio of the time derivative and the spatial derivative of the particle density at the surface of the particle. The model presented can be used to describe the mode of conversion of reactive porous particles in a range of different applications such as entrained flow gasifiers, pulverized coal burners and circulating fluidized bed combustors. There are no free tunable parameters in the model.
Article
A comprehensive description of catalytic effects during char gasification under various conditions relevant to biomass gasification was made. A three-parallel reaction model was proposed to describe the dynamic change in catalytic activity of ash during gasification of biomass char particles. Three different regimes of conversion were identified by analyzing char reactivity experiments conducted in a vertical TGA with nine biomasses under a wide range of operating conditions (temperature: 1023–1123 K, pressure: 0.1–3.0 MPa and gasification mixtures of CO2–CO–H2O–H2): (1) catalytic char gasification with deactivation of catalyst, (2) non-catalytic char gasification, and (3) catalytic char gasification with small amount of stable ash, without suffering deactivation. A model including the three regimes was developed and the measurements were used to fit the kinetic coefficients. It is shown that the model accurately predicts the reactivity of biomass char in CO2–CO mixtures during the whole range of conversion. It was detected that char gasification maintains the catalytic activity during the entire conversion process when: (i) biomasses having small amount of silicon was used, and (ii) steam is used as part of the gasification agent. The model is still useful as predicting tool for these two conditions but its physical significance is contestable on the light of the model developed. For the conditions where the model is valid, it is shown that the model is a useful tool as sub-model in reactor simulations, predicting the conversion rate of single particles fast and accurately at different stages of conversion. The aspects that need to be further investigated for expanding the applicability of the model were identified.
Article
The isothermal differential characteristics of the gas–solid reaction occurring in a micro-fluidized bed reactor were studied using the indigenously developed Micro-Fluidized Bed Reaction Analyzer (MFBRA). The combustion of graphite powder in micrometers was taken as the model reaction because of its negligible internal diffusion and chemical-reaction simplicity. With minimized inhibitions from both the internal and external diffusions, the reaction in MFBRA at a preset temperature was analyzed by using the isothermal kinetic approach, resulting in an activation energy of 165 kJ/mol and a pre-exponential factor of 106 1/s. The reaction was further found to be subject to the nucleation and growth model expressed by G(α) = −ln(1 − α). Measuring this reaction in TG via the programmed heating method resulted in the similar activation energy and the same reaction function model (by extrapolating to zero conversion). Comparing with the non-isothermal approach for TG that involves complicated mathematical calculations, the isothermal differential approach for MFBRA allowed the separation of the temperature effect (i.e., the reaction rate constant) and kinetic function model, thus providing a simple and reliable determination of the gas–solid reaction kinetics.
Article
To study the catalytic role of alkali and alkaline earth metallic species and eliminate their negative impact during biomass thermal utilization, different leaching methods have been applied in numerous experiments. Thus it is necessary to investigate the potential influence on biomass physicochemical structure using different agents. Rice straw was selected to study the demineralization impact on physicochemical structure and pyrolysis characteristics. It is shown that strong acid leaching exhibited higher removal efficiency of minerals, but it introduced more notable impact on physicochemical structure of biomass comparing to water and weak acid leaching. Different leaching methods give chance to study catalysis characteristics of intrinsic metals on biomass thermal reaction. Contrast to alkaline earth metals especially Ca hindering thermal decomposition, alkali metals promoted this reaction obviously. In addition, comparing to physicochemical structure changes created by leaching process, the influence of removal of minerals played the dominant role in biomass thermal behavior.
Article
This study investigates the influence of alkali (Na, K), alkaline earth (Ca, Mg) and transition (Fe) metal nitrates on CO2 gasification reactivity of pistachio nut shell (PNS) char. The preliminary gasification experiments were performed in thermogravimetric analyzer (TGA) and the results showed considerable improvement in carbon conversion; Na-char>Ca-char>Fe-char>K-char>Mg-char>raw char. Based on TGA studies, NaNO3 (with loadings of 3-7wt%) was selected as the superior catalyst for further gasification studies in bench-scale reactor; the highest reactivity was devoted to 5wt% Na loaded char. The data acquired for gasification rate of catalyzed char were fitted with several kinetic models, among which, random pore model was adopted as the best model. Based on obtained gasification rate constant and using the Arrhenius plot, activation energy of 5wt% Na loaded char was calculated as 151.46kJ/mol which was 53kJ/mol lower than that of un-catalyzed char.
Article
Modeling of biomass gasification in bubbling and circulating fluidized bed (FB) gasifiers is reviewed. Approaches applied for reactor modeling, from black-box models to computational fluid-dynamic models, are described. Special attention is paid to comprehensive fluidization models, where semi-empirical correlations are used to simplify the fluid-dynamics. The conversion of single fuel particles, char, and gas is examined in detail. The most relevant phenomena to be considered in modeling of FB biomass gasifiers are outlined, and the need for further investigation is identified. An updated survey of published mathematical reactor models for biomass and waste gasification in FB is presented. The overall conclusion is that most of the FB biomass gasification models fit reasonably well experiments selected for validation, despite the various formulations and input data. However, there are few measurements available for comparison with detailed model results. Also, validation of models with data from full-scale FB biomass gasification units remains to be done.
Article
A new reaction model expressing the gasification behavior of char particles is presented. This model can treat the gasification behavior of char particles that have complicated structures. The model assumes that, before the reaction, a char particle is a three-dimensional cube. The cube is composed of a large number of small, randomly arranged lattices classified as either char, ash, or macropores, depending on the proximate analysis data of the char. The diffusion of an oxidizing agent and the reaction between the char and the agent in the particle are analyzed in order to evaluate the reaction according to the reaction conditions and char particle shapes. The calculated results reveal the relationship between the char structure and the char reactivity. When the porosity is high, the reaction occurs on both an internal void surface and an external surface. The wall thickness of the char particle, determined by the size, porosity, and shape of the particle, is one of the most important factors in the diffusion rate in the particle. Therefore, the wall thickness contributes remarkably to the transition temperature between the chemical-reaction rate control regime and the pore-diffusion rate control regime. The variations in particle size, specific surface area, and reaction rate show different behavior owing to the reaction rate control regime. The fragmentation behavior also depends on the reaction rate control regime. As the diffusion controls the reaction rate, the conversion point at which the fragmentation occurs shifts to the initial stage of the reaction. In the case of small char particles, temperature has only a small effect on fragmentation behavior.
Article
Six coals with different ranks and different ash contents have been used to study the effect of demineralization on N2 formation during coal pyrolysis. Chars obtained after pyrolysis have been also gasified with carbon dioxide at 1000 °C to investigate the influence of the demineralization on char gasification reactivity. The pyrolysis results show that the demineralization by acid washing drastically changes N2 formation profiles and decreases nitrogen conversion to N2 for low rank coals; on the other hand, the demineralization has little effect on N2 formation for high rank coals. Addition of 0.5wt% Fe promotes N2 formation from the demineralized coals, but the catalytic effect depends on the coal type. It is found that the Fe remarkably promotes N2 formation from the demineralized low rank coals, but the effect is much smaller for high rank demineralized coals. These observations suggest that the existing state of Fe-containing minerals and added Fe catalyst is important for catalytic N2 formation during coal pyrolysis. Gasification results show that the demineralization lowers char gasification reactivity not only for low rank coals but also for high rank coals.
Article
Several compounds of alkali and alkaline earth metals have been tested as catalysts for the gasification in carbon dioxide and in steam, under isothermic conditions, of chars from a bituminous coal with high inorganic matter content from Peñarroya (Córdoba, Spain).The results show that some catalysts, and especially the alkali metal hydroxides, produce a deactivation for char gasification in both atmospheres. Such a deactivating effect is only observed when very low catalyst concentration is used. Char reactivity varies with the catalyst concentration and depends on the catalyst type (alkali or alkaline earth metal compound). NaAc and NaOH bring about a large increase of char reactivity at higher catalyst concentrations. By contrast, CaAc2 and CaCl2 produce a more substantial increase of char reactivity at comparatively lower catalyst concentrations. At high catalyst concentration and gasification temperature, the order of catalytic activity found is CaCl2 > NaAc > CaAc2> NaOH > NaCl for gasification in steam and NaAc > NaOH > CaAc2 > CaCl2 > NaCl for gasification in carbon dioxide.
Article
Pyrolysis of pine and gasification of pine chars was studied in this work, focusing on the influence of organically bound metals. Selective leaching of the major ash-forming elements in pine wood was performed with different acids, namely, nitric, sulfuric, hydrochloric and oxalic acids. No other major changes in the chemical composition of the biomass were observed except the removal of the metals. The effect of organically bound sodium, potassium, magnesium and calcium was studied in both pyrolysis and gasification. Removal of the metals had a positive effect on the pyrolysis, resulting in higher bio-oil, lower char and gas yields.
Article
The purpose of this study is to investigate the catalytic effects of Na as NaCl or as sodium carboxylates (–COONa) in Victorian brown coal on the char reactivity. A Na-exchanged coal and a set of NaCl-loaded coal samples prepared from a Loy Yang brown coal were pyrolysed in a fluidised-bed/fixed-bed reactor and in a thermogravimetric analyser (TGA). The reactivities of the chars were measured in air at 400 °C using the TGA. The experimental data indicate that the Na in coal as NaCl and as sodium carboxylates (–COONa) had very different catalytic effects on the char reactivity. It is the chemical form and dispersion of Na in char, not in coal, that govern the catalytic effects of Na. For the Na-form (Na-exchanged) coal, the char reactivity increased with increasing pyrolysis temperature from 500 to 700 °C and then decreased with pyrolysis temperature from 700 to 900 °C. The increase in reactivity with pyrolysis temperature (500–700 °C) is mainly due to the changes in the relative distribution of Na in the char matrix and on the pore surface. For the NaCl-loaded coals, when Cl was released during pyrolysis or gasification, the Na originally present in coal as NaCl showed good catalytic effects for the char gasification. Otherwise, Cl would combine with Na in the char to form NaCl during gasification, preventing Na from becoming an active catalyst. Controlling the pyrolysis conditions to favour the release of Cl can be a promising way to transform NaCl in coal into an active catalyst for char gasification.