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• Conducted a numerical study on co-firing biomass with coal in a pressurized combustor. • Quantititively assessed the reaction pathways of coal and biomass inside the furnace. • Analized NOx evolution in pressurized oxy-fuel combustion. As the next evolution in oxy-fuel combustion technology, pressurized oxy-combustion (POC) can potentially improve the process efficiency and economics. Biomass combustion is considered to be nearly CO 2 neutral, and thus, a negative CO 2 balance can be achieved by co-firing biomass in an oxy-fuel combustion process. However, this concept has not yet been demonstrated for the POC system. To address this need, and validate the feasibility of co-firing under these conditions, simulation investigations are conducted based on a pressurized pilot-scale facility. The influences of biomass co-firing ratio on the combustion characteristics and pollutant emissions are estimated. It is evident from the results that the diffusion flame formed by the fuel-oxidant reaction has a good fullness within the furnace, with the maximum flame temperature under each condition not exceeding 1950 K. Biomass typically contains higher moisture and volatile mass fractions compared to coal, leading to a lower temperature around the fuel inlet. In the POC scenario, char gasification plays a prominent role in the process of char consumption. In 100 % coal and 100 % biomass cases, the char consumption caused by the gasification reaches 80 % and 69 % respectively. The predicted NOx emissions in 0 %Bio, 50 %Bio, and 100 %Bio cases are no more than 10 ppm. Furthermore, NOx reduction by volatile and char is quantitatively estimated. The employed low equivalence ratio, coupled with the enhanced NOx reduction by volatile and char can be applied to account for the low NOx emissions in the POC system. This investigation is to provide a comprehensive assessment regarding the combustion and pollutant emissions within the pilot-scale pressurized oxy-combustor during co-firing biomass with coal. [ABSTRACT FROM AUTHOR]

In this work, the influence of KCl concentration on the gasification characteristics of lignite coal char between 800–1100 °C is studied through the experiments of temperature programmed gasification and isothermal gasification using a thermogravimetric analyzer. The gasification kinetics, characteristic parameters, and the reaction mechanism of catalytic gasification is explored using the random pore model (RPM). In view of temperature programmed gasification, the gasification rate of coal char is relatively slow when the temperature is below 700 °C, and only when the temperature is higher than 700 °C, the gasification starts to accelerate. Results show that with the existence of a catalyst, the temperature required for gasification reaction is reduced and the gasification reaction rate is increased. Furthermore, the higher concentration of KCl leads to the shorter half reaction time, the higher gasification rate, and the stronger catalysis. In addition, the activation energy of AW-char (the char from acid-washed coal) is the highest, while the activation energy and the energy level required for the gasification reaction are reduced by adding KCl. Based on the analysis of the catalytic mechanism, it is found that the unified mechanism of catalytic gasification of alkali and alkaline earth metals is applicable for the KCl catalysis on coal char gasification. [ABSTRACT FROM AUTHOR]

The high concentration of CO 2 and low concentration of O 2 at relatively low temperatures of 1123-1223 K are typical reactive atmospheres in the fuel reactor of a chemical looping with oxygen uncoupling (CLOU) unit. Char within this environment can be converted via lean-O 2 oxidation and/or rich-CO 2 gasification. Conventional models, which either considered the oxidation only or considered the oxidation and gasification contributions independently by simply adding the two individual conversion rates together, seem arbitrary. Therefore, it is necessary to quantitatively explore the contributions of the oxidation and the gasification reactions as well as their interactions in CLOU conditions through more detailed particle-resolved simulations. In this work, a single particle model which described a spherical reacting porous particle with its reacting boundary layer was developed to investigate the conversion characteristics of pulverized coal char in lean-O 2 and/or rich-CO 2 conditions (analogous CLOU conditions). The heterogeneous reaction kinetics developed by Tilghman and Mitchell [35] and the simplified GRI-Mech 3.0 gas-phase reaction kinetics were adopted. It was found that the char conversion rate in CLOU conditions could be basically modeled as the sum of the full oxidation consumption rate and the partial gasification consumption rate, i.e. r mix = φ 1 r gasi + r oxid. The interactions between the oxidation and gasification reactions were elaborated in two aspects: on the one side, the oxidation of coal char in CLOU conditions is within the weak zone II burning region, where the oxidation reaction only happens peripherally and is nearly unaffected by the high concentration of CO 2 ; on the other side, the gasification reaction within the external layer of the coal char particle is inhibited by the oxidation reaction, consequently the gasification reaction only happens inside the coal char particle where O 2 cannot penetrate into. Based on these understandings, a correlation between the coefficient, φ 1 , and the effectiveness factor of the oxidation reaction, η , was proposed to describe the overall char conversion rate in CLOU conditions. Through fitting the simulation results under typical CLOU conditions, the quantitative relationship between φ 1 and η was finally attained. [ABSTRACT FROM AUTHOR]

Supercritical water gasification of coal is a clean and efficient method for coal utilization which can convert coal into H 2 and CO 2 . In order to further reduce costs, a novel two-step cascade utilization method was proposed in this study: conducting traditional pyrolysis first and then gasifying the pyrolysis char in supercritical water. The influences of different pyrolysis operating parameters on gaseous products and char gasification in supercritical water were investigated. Quartz tube reactors were used to ensure the complete collection of gaseous products in pyrolysis process. The experimental results showed that both carbon and hydrogen conversion efficiency increased with temperature, and the increasing trend became not obvious after reaction for 5 min. The thermo-gravimetric curves showed that volatilization removal process was completed at the pyrolysis time of 5 min and higher pyrolysis temperatures were beneficial to the subsequent gasification process. The result also showed that residual weight was 15%–20% of the initial weight. Hydroxyl radicals kept stable during pyrolysis process with the absorption peak intensity increasing first and then decreasing, and mineral substance disintegrated gradually as time increased. As pyrolysis temperature increased, the peak of C C double bonds decreased, turning into stable functional groups and carbonyl group increased. Dispersive pores occurred at the surface of coal as residence time increased with particle size decreasing, specific surface area and reactivity increasing. The results might be used for the design of a cascade utilization system based on coal gasification in supercritical water. [ABSTRACT FROM AUTHOR]

The measurement of char gasification reactivity and kinetic parameters by TGA is generally complicated with mass transfer limitations. There is a reactant gas concentration gradient in the stagnant gas region between the mouth of the TGA crucible and the surface of the char bed because of gas consumption by the sample. The counter-diffusions between the reactant and product gases occur in this stagnant region. The Maxwell-Stefan equation has been used to estimate the distribution of gas concentration in the stagnant gas region. However, there is some uncertainty associated with semi-empirical gas diffusion coefficients used in the equation. In this study, we propose a new method to estimate the distribution of CO 2 concentration in the stagnant gas region during CO 2 gasification of coal char in TGA. The external effectiveness factors obtained in this way were in a reasonably good agreement with the value obtained by the traditional method widely used in literature. [ABSTRACT FROM AUTHOR]

Reaction mechanism takes place in char gasification was tested by kinetic studies. A modified kinetic model (thin reaction zone model) was developed to evaluate the mechanisms controlling the process of gasification. The model was compared to the experimental data of char gasification using steam as gasification agent. The gasification was carried out at 600-800∘C for 60 min. The model assuming the mass transfer of steam through the ash layer controls the reaction rate work well. The average error at operating temperatures of 600, 700, and 800∘C was 2.89, 1.75, and 3.26% respectively. [ABSTRACT FROM AUTHOR]

A multi-stage fluidized bed (MSFB) by configuring the distributor with an overflow standpipe between its neighboring stages was developed to couple the powder coal pyrolysis with its resultant char gasification for co-production of tar and syngas. This work succeeded in the smooth operation of MSFB for coal staged conversion. The three modes of coupling pyrolysis and gasification in terms of the one-stage, two-stage and three-stage bed characterized by temperature drop from the bottom up were investigated to evaluate the quality of the liquid and gas products. Coupling low- and mid-temperature tandem coal pyrolysis with high-temperature char gasification in the MSFB improved the quality of tar and syngas. The obtained tar yield was over 80% of the Gray-King assay tar yield and its light tar fraction (boiling point <360 °C) was as high as 70–80% in the MSFB. Syngas with CH 4 content of 5.2 vol.% was produced that was suitable for SNG production. Inside the reactor, the flow direction of pyrolysis volatiles toward the temperature drop avoided the deep secondary reaction of tar. Syngas and steam from the bottom gasification section could contribute to the formation of light tar and CH 4 by affecting the top coal pyrolysis. A comparison with the typical pyrolysis processes suggested that the MSFB process had its own advantages in treating powder coal to produce the high-quality tar and syngas. [ABSTRACT FROM AUTHOR]

Biochar, as an important renewable and carbon-natural energy, shows plentiful prospects to be used in the ironmaking industry. In this study, the effect of iron ore content and temperature on the reduction of iron ore-biochar pellets in nitrogen atmosphere were investigated. Results indicated that with the decrease of carbon to iron ratio, the iron ore indirect reduction was promoted more than char conversion which resulted in an decrease value of CO:CO 2 . Pellets reduction at 900 °C found that 60% iron ore in pellet exhibited considerable carbon conversion and the highest iron ore reduction amount. With the increase of reaction temperature, the char gasification reaction was promoted prior to the iron ore reduction, resulting in an increase of CO:CO 2 ratio. The wustite and metallic Fe gradually increased, while hematite and magnetite gradually decreased, with the increase of temperature. Kinetics study found that carbon conversion was controlled by one-dimensional diffusion, while the iron phase reduction reaction was controlled by three-dimensional diffusion, and the increasing activation energy as the reaction proceeded restricted the iron reduction process. The intrinsic reaction behavior, thermal kinetics, and technical analysis may guide the application of bioenergy in the ironmaking industry. [ABSTRACT FROM AUTHOR]

This work, performed in the context of Gaya project, focuses on the characterization and modelling of the effect of the pyrolysis heating rate on the steam gasification of char from woodchips. An experimental work was performed based on thermogravimetry and solid characterization of char samples prepared under different conditions. The results showed that gasification apparent reactivity increased with the increase of heating rate. The faster chemical kinetics of char prepared under high heating rate could be related to its higher catalytic inorganic content together with a more reactive carbonaceous structure. The increase of heating rate favours porosity development and the enlargement of pores, which would have a positive effect on gasification reaction rate by enhancing gas diffusion within the solid. The effect of heating rate on char gasification was included in a model previously developed, by correlating the chemical kinetics, particle density and diffusion rate to the char yield, which was selected as a suitable indicator of the heating rate. The new version of the model, considering the variability of heating rate during char formation, was validated through accurate predictions of the experimental results. [ABSTRACT FROM AUTHOR]

A high temperature stage microscope to investigate the temperature effect caused by particle size on char gasification is applied in this study. Experiments were carried out with different particle sizes for raw chars and chars on molten slag surface, respectively. Heat transfer models were built for the raw char of two temperature distributions and char particle on molten slag, respectively. Results showed that reaction layer temperature of raw char decreased in the reaction dominant while char on molten slag had higher temperature. Temperature difference between two distributions increased with the initial particle size, indicating the temperature effect on large particles was obvious. Shrinking core model was applied and modified herein coupled with the modification of reaction layer temperature and reaction area. Model prediction and experimental data showed good agreements of carbon conversion and reactivity index for raw char and char on molten slag, respectively. © 2016 American Institute of Chemical Engineers AIChE J, 63: 716-724, 2017 [ABSTRACT FROM AUTHOR]

Based on percolation theory, the discrete model of char particle gasification with structure evolution under various distinct regimes was developed. Initial structure of individual char particle was modeled as a simple cubic lattice consisting of a large number of small sites. Sites arranged in the char lattice were randomly classified as either carbon, ash, or macropore, depending upon the proximate analysis data and the porosity of coal char. Suggested model was used to study the effects of structure parameter and reaction condition on the gasification reactivity and the evolution of char structure. Simulation results showed that under external diffusion controlled regime, the gasification rate decreased as reaction advanced for char particles with lower porosity (porosity < 0.15), and the gasification rate vs. conversion curves exhibited maxima for large char particles with higher porosity (porosity > 0.15). Besides, the particle sizes of coal char decreased slowly in the early stage of gasification, and then they decreased slightly fast. The gasification reaction of fine coal chars under the external diffusion controlled region could inhibit the particle fragmentation and reduce the formation of fine ash particles to a certain degree. [ABSTRACT FROM AUTHOR]

A pseudo-steady state model of reaction and diffusion has been constructed to model the non-isothermal gasification of single char particles by CO 2 . The model uses a Cylindrical Pore Interpolation Model for intraparticle mass transfer, Ergun’s Langmuir–Hinshelwood equation for the intrinsic rate of gasification, an experimentally determined function f ( X ) for the pore evolution as a function of the conversion X , the Stefan–Maxwell equations to describe external mass transfer and the equation of energy for energy balance. Experiments were conducted to measure rates and extent of conversion, at various temperature and for various particle sizes, of lignite char and the less reactive activated carbon. The simulation results have been compared with experimental measurements, with excellent agreement being obtained. The results show that for the gasification of chars, the experimentally determined function f ( X ) predicts the rate of reaction of particles of various sizes well, all being at the same temperature. However, for the CO 2 gasification of char particles, at least, an f ( X ) evaluated at one temperature does not predict experimental measurements well at other temperatures, possibly because there are contributions from multiple types of active sites within the particles. However, multiple sites are also not reflected in most published pore models. [ABSTRACT FROM AUTHOR]

The effect of inherent minerals in coal on gasification in H 2 O and CO 2 mixtures was investigated, and the synergistic effect was explained. First, low-temperature ash that has the same state as the inherent minerals in raw coal was prepared by low-temperature ashing with an oxygen plasma at 150 °C. The minerals were identified by X-ray diffraction. Then, the coal was successively demineralized with deionized water, HCl, HF, and HCl/HF. Char gasification reactivities of the successively demineralized coals with different mixtures of H 2 O and CO 2 were investigated using a thermogravimetric analyzer at 800 °C. The gasification reactivity closely correlated with the Ca content in successively demineralized coals. The calcite present in coal catalyzes the gasification reaction, causing the synergistic effect. [ABSTRACT FROM AUTHOR]

Despite the huge literature on biomass char gasification with CO 2 or H 2 O, ambiguity still hovers over the issue of char gasification in complex atmospheres. Gas alternation gasification experiments, in which the reacting gas is changed during the reaction, were performed with CO 2 /H 2 O at 900 °C for small (200 μm) and large (13 mm) Low Heating Rate (LHR) beech wood char particles to assess the potential influences that CO 2 and H 2 O can have on each other during the char gasification reaction. The results showed no influence of a first gasification atmosphere on the char reactivity under the second one. The char reactivity to a specific gas at a certain conversion level was the same as if the gasification reaction was operated from the beginning with the same atmosphere composition. The purpose of this paper is to bring understanding keys to this lack of influence of previous gasification conditions on the char reactivity. Characterization of the chars throughout the conversion by measuring the total surface area and the active surface area was first performed. Then a transport limitation analysis based on the Thiele modulus was considered. It was concluded that the two gasses develop different porosities in the char, however, the Thiele modeling results and active surface area analysis indicate respectively that gasses diffuse preferentially in large macro-pores and that the concentration of active sites evolves similarly during both gasification reactions. This similarity in the diffusion mechanism as well as in the evolution of the concentration of active sites could be a plausible explanation for the only-dependent conversion reactivity observed in the gas alternation gasification experiments. [ABSTRACT FROM AUTHOR]

Char is the solid product of coal pyrolysis and its utilization is closely related to the scale of coal pyrolysis and tar processing. Char gasification is a promising choice for char utilization as it can provide hydrogen, which is needed for tar hydrogenation and chemical synthesis. However, reliable and mature coal char gasification technology has not yet been developed. The fluidized bed has been adopted as a gasifier in many studies, and its gasification temperature is relatively low. In this work, the feasibility of char gasification using a fluidized bed has been studied. A pyrolyzed bituminous coal char was gasified in a mixture of O2 and steam using a circulating fluidized bed. The height of the fluidized bed is 2360 mm and the inner diameter is 66 mm. The reactor is made of Ni-Cd alloy steel, and a loopseal was used for solid circulation. In the experiments, the gasification temperature was varied between 870 and 900 °C, and the effects of O2/char and steam/char mass ratios on the gasification results were analyzed. The volume fraction of combustible gas, dry gas yield, carbon conversion, and lower heating value of the best case were 43.92 %, 1.64 Nm3 kg−1, 0.85, and 5.17 MJ N m−3, respectively. In addition, a simple model and a complex model were established by using Aspen Plus software. The two models were used to simulate char gasification, and the simulation results were validated with the experimental results. The results indicated that the complex model is more accurate than the simple model and is more suitable to simulate char gasification in a circulating fluidized bed. [ABSTRACT FROM AUTHOR]

Char particles of high ash Indian coal with different sizes are produced using thermogravimetric method at different heating rates. Particle diameter and porosity changes during devolatilization significantly affect char gasification rates. The effect of coal particle size on char production under various heating rates is reviewed. Char gasification is affected by operating conditions such as reaction temperature, char production method, and particle size in addition to chemical composition and physical structure of char. The measurements of the steam and CO 2 gasification rate of the produced chars are performed by an isothermal thermo-gravimetric analysis at the temperatures of 900, 950, and 1000 °C under ambient pressure conditions. It is found that the reactivity of char gasification is increased along with heating rate of char. Three kinetic models are applied to describe the varying conversion rate: volumetric model, grain model, and random pore model. The activation energy of the gasification is varied based on the char generation method and particle sizes. The activation energy is varying from 122 to 177 kJ mol −1 for steam gasification and 130 to 214 kJ mol −1 for CO 2 gasification to different sized char particles. The effect of char heating rate on gasification is more pronounced in the activation energies of smaller sized particles. [ABSTRACT FROM AUTHOR]

Char particles of high ash Turkish coal of two sizes were produced by pyrolysis in an atmospheric pressure thermo-gravimetric apparatus using 3 heating rates (100 K/min, 500 K/min and 800 K/min). It is observed that the particle size (for particles between 0.8 and 3 mm) did not change the pyrolysis process results. Char gasification rates in CO 2 , steam and mixture of CO 2 + steam are investigated in the same TGA system over the temperature range of 850–950 °C. The pyrolysis heating rate for char formation is observed to have a marked influence on the subsequent gasification reactivity of the char. The results indicate that the char produced from high heating rates show improved gasification rates. Smaller particles exhibit higher char–CO 2 and char–steam gasification rates. Increasing the temperature from 850 to 950 °C, leads to a significant reduction of the time required for 50% char conversion both for small and large particles. The maximum reaction rate is shifted to higher conversion degrees when the chars are produced from high heating rates. The gasification rate of char–H 2 O strongly depends on the H 2 O partial pressure. Gasification rates in CO 2 ambiance are much lower than those in steam. For CO 2 –steam blended ambiance, no obvious CO 2 inhibition effect is observed. But this topic requires more investigation. Kinetic parameters of the char particles are estimated using three different kinetic models. The obtained activation energies for char gasification essentially depend on the particle size and the three kinetic models give very close activation energy values for all the tested conditions. [ABSTRACT FROM AUTHOR]

The char-CO gasification reactions in isothermal and pressurized conditions were studied kinetically by high-pressure thermo-gravimetric analyzer, and the primary objective of this paper was to study the effects of pressure, temperature and char rank on the intrinsic reaction kinetics of the char-CO gasification by the Langmuir-Hinshelwood model. The results indicated that the order of reactivity sequence at the same temperature and pressure was Char A > Char B > Char C (Char A from subbituminous coal, and Char B and C from bituminous coals). With the increase in the pressure and temperature, both the intrinsic reaction rate at the same carbon conversion and the carbon conversion efficiency at the same reaction time increased. The results also showed that the gasification rate experienced an initially slow increase, followed by a rapid increase, and finally a decrease corresponding to the increase in the carbon conversion efficiency. It implied that the Langmuir-Hinshelwood model was incompatible to thorough presentation of the complete conversion of the char-CO gasification, but was well agreeable to the intrinsic reaction kinetics of char-CO gasification when the carbon conversion efficiency was low at the initial stage of the carbon conversion (0-0.6). Based on this, the intrinsic reaction kinetic models of the selected char samples were built. [ABSTRACT FROM AUTHOR]

The gasification of carbonaceous char into synthesis gas species presents a viable thermochemical conversion pathway of both fossil fuel-based and biomass-based solid fuels. Observations of these synthesis gas species—particularly hydrogen—inhibiting gasification rates have been documented for decades. Finite-rate surface kinetic reaction mechanisms that can be applied to a variety of reactor models and conditions are needed, particularly those which describe gasification inhibition. To quantify the effects of hydrogen inhibition reactions in gasification conditions, existing surface kinetic mechanisms were employed in simulations and compared with experimental data available in the literature. A computationally inexpensive method to model the surface chemistry in simple gasification reactors is utilized in this study—where appropriate, these experiments in the literature were simulated as perfectly stirred reactors. When used in comparison with kinetically controlled experiments, this method can readily facilitate the identification of missing reaction steps in surface mechanisms. Results reveal the inability of current surface mechanisms in predicting experimentally observed inhibition trends. Modeling the inhibition with an adsorption–desorption pair of reactions based on a combination of theoretical and empirical considerations was found to improve the overall model when these reactions were appended to a current surface mechanism. Reasonable agreement with experimental data is found with regard to the kinetics of carbon gasification reactions in the inhibiting presence of hydrogen. [ABSTRACT FROM AUTHOR]

Carbon dioxide was considered as a co-gasifying agent in a coal gasification reactor. The work presented herein describes the simulation results for the process and the experimental data on coal char gasification with CO addition as the rate-controlling step for the entire process. To study the potentially beneficial effect of the introduction of CO into the gasification system, several simulations were conducted using the commercial process simulation software ChemCAD 6.3. The results of a Gibbs equilibrium reactor were evaluated. The Boudouard reaction is a critical path for the development of this process, and the kinetics were studied experimentally. Four chars derived from the pyrolysis of Polish coals of different origins were selected for the experiments. The kinetic characteristics of this system were examined using a custom-designed pressurized fixed-bed reactor. To determine the effect of pressure on the gasification rate, several preliminary studies on the gasification of coal chars were performed isothermally at the temperature of 950 °C and pressures of 1, 10, and 20 bars. In contrast to the thermodynamic calculations, the experimental data revealed that increasing the CO pressure leads to a higher reaction rate for medium-rank coal chars and low-rank lignite coal char, resulting in higher efficiency for carbon monoxide production. The pressure influences the reactivity more strongly when varied from 1 to 10 bars; a further increase in pressure affects the rate almost insignificantly. The observed behavior representing the changes in carbon conversion degree during gasification is satisfactorily described by the grain model. [ABSTRACT FROM AUTHOR]

Highlights: [•] Intrinsic gasification rate formulas for coal and coke char from 850°C to 1000°C. [•] The effectiveness factor with an nth-order model was developed to study the diffusion. [•] The experimental effectiveness factor agrees with the calculated result. [•] The char with higher reactivity possesses stronger pore diffusion resistance. [ABSTRACT FROM AUTHOR]

Highlights: [•] Calcium additive synergized with K2CO3 towards gasification of ash-free chars. [•] A main mechanism for the synergistic effect (SE) was the formation of a eutectic. [•] Another mechanism for SE was due to the organic binding of calcium on char. [Copyright &y& Elsevier]

Abstract: Investigating the release characteristics of alkali and alkaline earth metallic species (AAEMs) is of potential interest because of AAEM’s possible useful service as catalysts in biomass thermal conversion. In this study, three kinds of typical Chinese biomass were selected to pyrolyse and their chars were subsequently steam gasified in a designed quartz fixed-bed reactor to investigate the release characteristics of alkali and alkaline earth metallic species (AAEMs). The results indicate that 53–76% of alkali metal and 27–40% of alkaline earth metal release in pyrolysis process, as well as 12–34% of alkali metal and 12–16% of alkaline earth metal evaporate in char gasification process, and temperature is not the only factor to impact AAEMs emission. The releasing characteristics of AAEMs during pyrolysis and char gasification process of three kinds of biomass were discussed in this paper. [Copyright &y& Elsevier]

Abstract: The effect of particle size, porosity and reactor temperature/reaction rate constant on the progress of a char particle conversion has been investigated numerically by solving the transport equation inside a reacting char particle. Numerical simulations have been conducted for three cases that include two extreme cases and one general case. The two extreme cases correspond to a very large Damkohler number (3.2607×103) and a very small Damkohler number (0.0042). The third case corresponds to an intermediate value of Damkohler number. For the very large Damkohler number case, concentration profiles of the gasifying agent showed a steep gradient across the particle and the reaction occurred mostly in outer layer of the particle. This behavior corresponds to a diffusion controlled process. For the very small Damkohler number case, gasifying agent concentration was a straight line parallel to the x-axis, with a y-axis value of the surrounding concentration. The reaction occurred homogeneously across the particle and the degree of conversion was only a function in time. This behavior corresponds to a chemically controlled process. The total conversion of the char particle as a function of time has also been calculated for different particle sizes, initial porosity and reaction rate constant. Variation in conversion profiles as a function of time due to variation in initial porosity and reaction rate constant were limited to a certain extent. Very high initial porosity values tend to shift the process towards a chemically controlled one; any further increase in porosity does not have a positive effect on the conversion–time relationship. Very high reaction rate constants tend to shift the process towards diffusion controlled process. Kinetic parameters have been determined experimentally using a chemically controlled process. The obtained parameters have been used in the model to determine the progress of char particle conversion at an increased reactor temperature of 1000°C. The model has been compared to experimental results at the same temperature (1000°C). The results showed very good agreement. [Copyright &y& Elsevier]

Abstract: A mathematical model for the group combustion of pulverized coal particles was developed in a previous work. It includes the Lagrangian description of the dehumidification, devolatilization and char gasification reactions of the coal particles in the homogenized gaseous environment resulting from the three fuels, CO, H2 and volatiles, supplied by the gasification of the particles and their simultaneous group combustion by the gas phase oxidation reactions, which are considered to be very fast. This model is complemented here with an analysis of the particle dynamics, determined principally by the effects of aerodynamic drag and gravity, and its dispersion based on a stochastic model. It is also extended to include two other simpler models for the gasification of the particles: the first one for particles small enough to extinguish the surrounding diffusion flames, and a second one for particles with small ash content when the porous shell of ashes remaining after gasification of the char, non structurally stable, is disrupted. As an example of the applicability of the models, they are used in the numerical simulation of an experiment of a non-swirling pulverized coal jet with a nearly stagnant air at ambient temperature, with an initial region of interaction with a small annular methane flame. Computational algorithms for solving the different stages undergone by a coal particle during its combustion are proposed. For the partial differential equations modeling the gas phase, a second order finite element method combined with a semi-Lagrangian characteristics method are used. The results obtained with the three versions of the model are compared among them and show how the first of the simpler models fits better the experimental results. [Copyright &y& Elsevier]

Abstract: Experimental investigation of the combustion of an air-dried Victorian brown coal in O2/N2 and O2/CO2 mixtures was conducted in a lab-scale drop-tube furnace (DTF). In situ diagnostics of coal burning transient phenomena were carried out with the use of high-speed camera and two-colour pyrometer for photographic observation and particle temperature measurement, respectively. The results indicate that the use of CO2 in place of N2 affected brown coal combustion behaviour through both its physical influence and chemical interaction with char. Distinct changes in coal pyrolysis behaviour, ignition extent, and the temperatures of volatile flame and burning char particles were observed. The large specific heat capacity of CO2 relative to N2 is the principal factor affecting brown coal combustion, which greatly quenched the ignition of individual coal particles. As a result, a high O2 fraction of at least 30% in CO2 is required to match air. Moreover, due to the accumulation of unburnt volatiles in the coal particle vicinity, coal ignition in O2/CO2 occurred as a form of volatile cloud rather than individual particles that occurred in air. The temperatures of volatile flame and char particles were reduced by CO2 quenching throughout coal oxidation. Nevertheless, this negative factor was greatly offset by char-CO2 gasification reaction which even occurred rapidly during coal pyrolysis. Up to 25% of the nascent char may undergo gasification to yield extra CO to improve the reactivity of local fuel/O2 mixture. The subsequent homogeneous oxidation of CO released extra heat for the oxidation of both volatiles and char. As a result, the optical intensity of volatile flame in ∼27% O2 in CO2 was raised to a level twice that in air at the furnace temperature of 1273K. Similar temperatures were achieved for burning char particles in 27% O2/73% CO2 and air. As this O2/CO2 ratio is lower than that for bituminous coal, 30–35%, a low consumption of O2 is desirable for the oxy-firing of Victorian brown coal. Nevertheless, the distinct emission of volatile cloud and formation of strong reducing gas environment on char surface may affect radiative heat transfer and ash formation, which should be cautioned during the oxy-fuel combustion of Victorian brown coal. [Copyright &y& Elsevier]

Abstract: Char–CO2 gasification reactions in the presence of CO and char–steam gasification reactions in the presence of H2 were studied at the atmospheric condition using a thermogravimetric apparatus (TGA) at various reactant partial pressures and within a temperature range of 1123K–1223K. The char was prepared from a lignite coal. The partial pressure of H2 and CO varied from 0.05 to 0.3atm. The experimental results showed that Langmuir–Hinshelwood (L–H) kinetic equation was applicable to describe the inhibition effects of CO and H2. The kinetic parameters in L–H equations were obtained. Interactions of char gasification by steam and CO2 in the presence of H2 and CO were discussed. It was found that the kinetic parameters determined from pure or binary gas mixtures can be used to predict multi-component gasification rates. The results confirmed that the char–steam and char–CO2 reactions proceed on separate active sites rather than common active sites. [Copyright &y& Elsevier]

Abstract: The non-isothermal thermogravimetric method (TGA) was applied to a bituminous coal (PT), two types of biomass, chestnut residues (CH) and olive stones (OS), and coal–biomass blends in order to investigate their thermal reactivity under steam. Fuel chars were obtained by pyrolysis in a fixed-bed reactor at a final temperature of 1373K for 30min. The gasification tests were carried out by thermogravimetric analysis from room temperature to 1373K at heating rates of 5, 10 and 15Kmin−1. After blending, no significant interactions were detected between PT and CH during co-gasification, whereas deviations from the additive behaviour were observed in the PT–OS blend. However, for the two coal–biomass blends, the gasification behaviour resembled that of the individual coal, as this component constituted the larger proportion of the blend. The temperature-programmed reaction (TPR) technique was employed at three different heating rates to analyze noncatalytic gas–solid reactions. Three nth-order representative gas–solid models, the volumetric model (VM), the grain model (GM) and the random pore model (RPM) were applied in order to describe the reactive behaviour of the chars during steam gasification. From these models, the kinetic parameters were determined. The best model for describing the reactivity of the PT, PT–CH and PT–OS samples was the RPM model. VM was the model that best fitted the CH sample, whereas none of the models were suitable for the OS sample. [Copyright &y& Elsevier]

Abstract: Two Chinese coals were used in this study and coal chars were prepared at different temperatures. High temperature gasification of coal chars with CO2 was investigated in a bench scale fixed-bed reactor and the transformations of minerals from these two coals were also studied from 1100 to 1500 °C. Mineral matters produced at different temperature and ash generated after gasification were collected and analyzed by XRD and FTIR. It was found that the iron oxides were only catalytic mineral matters existing at high temperature. And gasification behaviors above ash melting temperature were different for different mineral composition, especially the content and form of iron oxide, which not only accelerates the gasification reaction, but also reduces the influence caused by melting minerals. [Copyright &y& Elsevier]

Abstract: Flameless combustion is a well known measure to reduce emissions in gas combustion but has not yet been fully adapted to pulverised coal combustion. Numerical predictions can provide detailed information on the combustion process thus playing a significant role in understanding the basic mechanisms for pollutant formation. In simulations of conventional pulverised coal combustion the gasification by or is usually omitted since its overall contribution to char oxidation is negligible compared to the oxidation with . In flameless combustion, however, due to the strong recirculation of hot combustion products, primarily and , and the thereby reduced concentration of in the reaction zone the local partial pressures of and become significantly higher than that for . Therefore, the char reaction with and is being reconsidered. This paper presents a numerical study on the importance of these reactions on pollutant formation in flameless combustion. The numerical models used have been validated against experimental data. By varying the wall temperature and the burner excess air ratio, different cases have been investigated and the impact of considering gasification on the prediction of NO formation has been assessed. It was found that within the investigated ranges of these parameters the fraction of char being gasified increases up to 35%. This leads to changes in the local gas composition, primarily CO distribution, which in turn influences NO formation predictions. Considering gasification the prediction of NO emission is up to 40% lower than the predicted emissions without gasification reactions being taken into account. [Copyright &y& Elsevier]

• Various forms of metal elements in coal were determined by extraction experiment. • Gasification experiments were performed at 1073–1223 K to verify the model. • The self-developed char reactivity model was universal for coal samples. • Effect of preparation conditions on char physicochemical structure was analysed. • The model may be suitable for predicting the in situ char gasification reactivity. This study was performed to analyse the adaptability of the self-developed char reactivity model and reveal the influence of preparation conditions on the char gasification reactivity. Firstly, six coal samples were selected for the step-by-step extraction of the metal elements. The forms of K elements in the different coal samples were quite different. Na elements were mainly water-soluble, ion-exchanged and acid-soluble states, whilst Ca, Mg and Fe elements were mainly ion-exchanged, acid-soluble and insoluble states. Then, the coal char gasification rate was determined using thermogravimetric analysis (TGA) in the temperature range of 1073–1223 K to verify the self-developed char reactivity model. Results showed that the model could accurately predict the reaction rate of the char samples under different gasification temperatures. Finally, a typical coal sample was selected to prepare char under various heating rates, cooling rates and annealing times and characterised by CO 2 and N 2 adsorption–desorption, Raman, FTIR, XPS and TGA. Results showed that with prolonged annealing time and decreased heating and cooling rates, the proportion of micro-pores in char increased, the particles tended to possess a graphitisation structure. In addition, the surface reaction functional groups disappeared, reducing the char gasification reaction rate. Comparison of the model calculated values and the experimental values of the char gasification reaction rate obtained under different preparation conditions showed that the self-developed model could provide a reference for the prediction of in situ char gasification reaction rate. [ABSTRACT FROM AUTHOR]

This investigation involved the formation and release of precursors of NOx, which were HCN and NH3, during gasification of coal and its char. Gasification was carried out in a fixed bed reactor at atmospheric pressure. The reactor allowed coal particles to be heated up rapidly and held for a pre-specified period of time at peak temperature. The influence of coal rank and coal particle size on the release of N-containing compounds during coal gasification with CO2 is discussed. Gasification agents and coal gasification temperature were two key factors on the amount of nitrogenous compounds released. Results showed that with an increase of reaction temperature a great amount of NH3 was formed during gasification with steam: The yield of NH3 was highest at 800 °C during gasification with CO2. The volatiles in coal played the key role in the formation of HCN and NH3 during coal gasification under steam atmosphere. Volatiles were the main source of the formation of HCN and NH3, mainly from the nascent char thermal cracking, whose procedure could be promoted by H2O(g): The yield of HCN during coal gasification had no strong relation with gasification agents and increased with an increase in gasification temperature. A reasonable mechanism for the formation of nitrogenous compounds during coal gasification was suggested in this study, which could explain some results in the literature on pyrolysis and gasification of coal. [Copyright &y& Elsevier]

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 0.5 wt% 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. [Copyright &y& Elsevier]

• Ion exchange of Na+, K+, Ca2+, Mg2+, Ca2+, Co2+ and Ni2+ on coal is performed. • Metal ion-exchanged coal pyrolysis and its char gasification were investigated. • The char decrease and the gas increase during metal ion-exchanged coal pyrolysis. • Ion exchange increases char gasification reactivity: K+ > Na+ > Mg2+ > Co2+ > Ni2+. • Mechanisms of coal pyrolysis and its char gasification are concluded. The effects of ion-exchanged metal cations (Na+, K+, Ca2+, Mg2+, Co2+ and Ni2+) on the pyrolysis reactivity of Shendong demineralized coal (SD-coal) and the gasification performance of pyrolysis chars were investigated by Py-GC/MS, TG-FTIR, FBR (the fixed-bed reactor), FTIR, SEM, XRD and CO 2 reactivity. The results show that the order of pyrolysis weight loss rates is SD-coal-Ni > SD-coal-Co > SD-coal-Mg > SD-coal-Na > SD-coal-Ca > SD-coal-K. The char yields decrease and the pyrolysis gas yields increase during the pyrolysis of metal ion-exchanged coals compared with those of SD-coal. Ion exchange of Co2+ and Ni2+ increases the coal tar yields, especially the yield of coal tar obtained from SD-coal-Co pyrolysis increases to 9.45 %. Ion exchange of alkali and alkaline earth metal cations is generally conducive to the formation of naphthalene and its derivatives in coal tar. The catalytic effect of these metal cations on coal pyrolysis is Na+ > K+ > Ni2+ > Ca2+ > Mg2+ > Co2+. The values of L c and N ave in chars derived from metal ion-exchanged coal pyrolysis are reduced compared with SD-coal char. The effect of ion-exchanged metal cations on the gasification reactivity of chars is K+ > Na+ > Mg2+ > Co2+ > Ni2+. And on this basis, the mechanisms of metal ion-exchanged coal pyrolysis and char gasification are concluded. [ABSTRACT FROM AUTHOR]

The gasification experiments of coal chars with CO2 were carried out isothermally, with K, Ca, Ni, and Zn chloride catalysts, adopting a thermal gravimetric analyzer (TGA) from 800 to 1100 °C. The kinetic characteristic of the samples were described using the volumetric model (VM), the grain model (GM), and the random pore model (PRM). The morphology patterns of the samples were tested applying X-ray diffraction (XRD) and the catalytic mechanisms concerning the phase changes were proposed. The results confirm that the gasification rate and char reactivities are enhanced by K, Ca and Ni chlorides, while ZnCl2 inhibited the process. The catalysis ability shows the following cation order: Ca > K > Ni > Zn. Among the models described above, PRM was proven to give the best fitting value and hence adopted to kinetics parameters calculation. The activation energies in promoting conditions were lower than that of the uncatalyzed cases. In view of the catalytic mechanism, the K metals tend to form intermediate complexes and repeatedly connect with coal char, while the Ca species may follow the oxidation-reduction mechanism and the Ni metals catalyze the gasification process. [ABSTRACT FROM AUTHOR]