Bituminous Coal Char Research Articles

Structural evolution of a bituminous coal char related to its synchronized gasification behavior with H2O and/or CO2

This work aims to investigate the structural evolution of the char during gasification under a single or mixed atmosphere of H2O and/or CO2 with the synchronized investigation of the effect of the varying char structure on the gasification reactivity. The experimental char was prepared from a bituminous coal at 1000°C. The changes of the char structure along with the progress of the carbon conversion during gasification were characterized using N2 adsorption, SEM, and Raman spectroscopy. The results revealed that H2O showed a more dramatically change on the char structure than CO2 and two reactants had different reaction pathways. The different pathways of reactants affected the evolution manners of the char structure and different gasification reactivity of char was related to structural evolution. The specific reaction rate between the char and CO2 decreased monotonously with increasing carbon conversion. However, the opposite trends are observed when H2O exist, either H2O alone or the mixtures of H2O and CO2. The char gasification reactivity was under the common effect of physical and chemical structure. In terms of the mixture of H2O and CO2, the significant specific surface area caused by H2O provided more active sites for CO2. The interactions between H2O and CO2 promoted reaction between C and CO2 (C + CO2 → CO) in mixtures of H2O and CO2, leading to higher amount of CO and higher specific reaction rate than calculated.

Gasification Kinetics of Bituminous Coal Char in the Mixture of CO2, H2O, CO, and H2

The gasification kinetics of bituminous coal char was investigated in a mixture of CO2, H2O, CO, H2, and N2 under isothermal conditions. In addition, the impacts of gasification temperature, gasification time, and gas composition on the gasification process were analyzed. As the experimental results suggest, there is a significant increase of the carbon conversion degree of bituminous coal char not just when gasification temperature and time increase, but also when H2 and CO concentration decreases. The kinetics of bituminous coal char in the gasification process was successfully modeled as a shrinking unreacted core. It is concluded that the gasification of bituminous coal char is controlled by an internal chemical reaction in the early stage and diffusion in the later stage. The activation energies of bituminous coal char gasification for different stages were studied. Moreover, it is proposed for the first time, to our knowledge, that the diffusion-control step is significantly shortened with the decrease of the CO2/H2O ratio. As scanning-electron-microscopy results suggest, bituminous coal char gasified in CO2/H2O = 1/3 atmosphere has numerous inner pores (0–5 m). Therefore, in the process of gasification, the inner pores provide a gas channel that reduces the gas diffusion resistance and thus shortens the diffusion-control step. These results can serve as a reference for industrialized application of the technology of coal gasification direct reduced iron.

Release of Ca during coal pyrolysis and char gasification in H2O, CO2 and their mixtures

The release characteristics of Ca during a Chinese bituminous coal pyrolysis and char gasification in different gasifying agents was studied. Residual chars with different carbon conversion levels were obtained by gasifying in a quartz fixed-bed reactor under H2O, CO2, and H2O/CO2 atmospheres at 800–1000 °C. Sequential chemical extraction and microwave digestion were used to determine the modes of occurrence of Ca in chars. X-ray diffraction and scanning electron microscopy were used to analyze the ash composition and the morphology of minerals. The contents of Ca in chars were measured by atomic absorption spectroscopy. The results indicate that water-soluble form and HCl-soluble form Ca transforms mainly into gas phase during pyrolysis. The release ratio of Ca at 800 °C under different atmospheres is H2O/CO2 >H2O > CO2. The main reason is that minerals are more prone to sintering in CO2 atmosphere. The interaction of H2O and CO2 can expand pores, which precipitate the minerals to the char surface and release into the gas phase. Increasing temperature induces the collapse of the pore structure which changed the release rule of Ca. The composition of the ash obtained from gasification at 1000 °C in CO2, H2O and H2O/CO2 are nearly same.

Characteristics of high temperature co-gasification and ash slagging for Victorian brown coal char and bituminous coal blends

The low ash and high iron content of Victorian brown coal (VBC) makes it difficult to achieve desirable slagging for entrained-flow gasification. This study investigates the blending of VBC char with a high-ash bituminous coal for entrained-flow gasification. An industrially pyrolyised VBC char, a silica-rich bituminous coal char (BCC), and VBC char with BCC blends in three different blending ratios (20%BCC, 40%BCC and 80%BCC) are selected for high temperature gasification investigation in a lab-scale drop tube furnace (DTF) for temperatures ranging from 1000 to 1300 °C with different CO2 volume fractions. Scanning electron microscopy (SEM) coupled with elemental mapping, advanced synchrotron XANES and Mössbauer spectroscopy (MS) are employed to clarify the slagging propensity and iron transition behaviours during the blend co-gasification. It is found that VBC char, even in large sized particles, achieves a high char conversion and more than 50% CO fraction in syngas at 1300 °C, while BCC shows a lower char conversion and only 15% CO fraction in the same condition, due to only small particles being consumed. The blending of BCC into VBC lowers the char conversion compared to that of VBC alone due to the ash interactions. However, the element (Fe/Ca/Mg) loss during gasification reduces remarkably upon the addition of BCC, showing that the silica from BCC captures these elements. In addition, VBC generates more than 20% metallic iron out of total Fe-species during gasification, and the amount increases when temperature increases, which is undesirable for slag discharge. In contrast, blending 20% BCC with VBC char reduces α-Fe from 15% to about 7%, and reduces Fe3O4 from 12% to less than 6%. This is because the silica in BCC can preferentially capture the iron-bearing compounds before they are reduced into metallic iron. Additionally, the unreacted carbon is covered by the slag, stopping it from reducing the Fe2+ to metallic iron Fe0. The reduction of metallic iron is closely linked to the increase on the ash slagging extent, which is beneficial to slag discharge in entrained-flow gasifiers.

The carbon dioxide gasification characteristics of biomass char samples and their effect on coal gasification reactivity during co-gasification

The carbon dioxide gasification characteristics of three biomass char samples and bituminous coal char were investigated in a thermogravimetric analyser in the temperature range of 850–950 °C. Char SB exhibited higher reactivities (Ri, Rs, Rf) than chars SW and HW. Coal char gasification reactivities were observed to be lower than those of the three biomass chars. Correlations between the char reactivities and char characteristics were highlighted. The addition of 10% biomass had no significant impact on the coal char gasification reactivity. However, 20 and 30% biomass additions resulted in increased coal char gasification rate. During co-gasification, chars HW and SW caused increased coal char gasification reactivity at lower conversions, while char SB resulted in increased gasification rates throughout the entire conversion range. Experimental data from biomass char gasification and biomass-coal char co-gasification were well described by the MRPM, while coal char gasification was better described by the RPM.

Study on CO2 Gasification Reaction during the Combustion of Pulverized Coal Char Particle

Although much research on the effect of CO2 gasification on char combustion has been reported, few studies give clear information on the proportion of carbon consumed by the gasification and oxidation respectively. In this work, a drop-tube furnace is used to study the combustion of Huangling bituminous coal char in different oxygen and carbon dioxide concentrations at different residence times. It is found that with increasing CO2 content the carbon conversion ratio decreases first and then goes up in both oxygen content of 21 and 36 vol.%. There always exists a minimal point around CO2 content of 8-15 vol.% along with increased residence time during pulverized coal char combustion. Meanwhile, a preliminary formula is proposed to calculate the overall carbon conversion ratio of char and the proportion of carbon consumed by the gasification and oxidation respectively.

O2/CO2 and O2/N2 combustion of bituminous char particles in a bubbling fluidized bed under simulated combustor conditions

Most of the time char particles in commercial FB units encounter considerably lower oxygen concentrations than that in the entrance, say between 2% and 10%, and the effects of the oxy-fuel atmosphere on the conversion history of char particles need to be clearly treated under these conditions. In the present work, an experimental study of combustion of bituminous coal char was carried out in a laboratory FB in O2/CO2 and O2/N2 atmospheres under simulated FB combustor conditions at O2 concentrations of 4–10% v/v, bed temperatures of 800–900 °C and char particle sizes of 2–8 mm by continuously measuring the concentrations of O2 and CO in the flue gas. The results indicate that the conversion of char is controlled by diffusion of O2 in the boundary layer of the particle in O2/CO2 and O2/N2 environments and that the gasification of char in O2/CO2 is limited by chemical kinetics. A char conversion model, taking into account the mass transfer from the bed to the particle and the gasification kinetics of char, was built, based on the experimental results and the intrinsic reactivity of char obtained in TGA tests. Simulations, carried out under the same conditions as in the FB experiments, give good agreement in terms of burnout time and instantaneous reactivity during the conversion of single particles. Simulated results prove that the low diffusivity of O2 in CO2 is the main reason for the decreased reaction rate of the char particle in O2/CO2 compared with O2/N2. The contribution of gasification to the consumption of char is more notable at high bed temperature (900 °C) and coarse particles (8 mm), particularly at lower oxygen concentration (4–6% v/v).

Non-isothermal study of gasification process of coal char and biomass char in CO2 condition

AbstractNon-isothermal method was used to study gasification characteristics of three coal chars and one biomass char. Four chars were made from anthracite coal (A), bituminous coal (B), lignite coal (L), and wood refuse (W), respectively. The gasification process was studied by random pore model (RPM), unreacted core model (URCM) and volumetric model (VM). With an increase in metamorphic grade, the gasification reactivity of coal char decreased, and the gasification reactivity of biomass char was close to that of low metamorphic coal char. With an increase in heating rate, the gasification of all samples moved towards high temperature zone, and the whole gasification time decreased. It was concluded from kinetics analysis that the above-mentioned three models could be used to describe the gasification process of coal char, and the RPM fitted the best among the three models. In the RPM, the activation energies of gasification were 193. 9, 225. 3 and 202. 8 kJ/mol for anthracite coal char, bituminous coal char and lignite coal char, respectively. The gasification process of biomass char could be described by the URCM and VM, while the URCM performed better. The activation energy of gasification of wood refuse char calculated by the URCM was 282. 0 kJ/mol.

Study on coal char ignition by radiant heat flux.

The study on coal char ignition by CO2-continuous laser was carried out. The coal char samples of T-grade bituminous coal and 2B-grade lignite were studied via CO2-laser ignition setup. Ignition delay times were determined at ambient condition in heat flux density range 90–200 W/cm2. The average ignition delay time value for lignite samples were 2 times lower while this difference is larger in high heat flux region and lower in low heat flux region. The kinetic constants for overall oxidation reaction were determined using analytic solution of simplified one-dimensional heat transfer equation with radiant heat transfer boundary condition. The activation energy for lignite char was found to be less than it is for bituminous coal char by approximately 20 %.

Gas emissions from blends of bituminous coal and biomass chars under different pyrolysis conditions

ABSTRACTIn this work, thermal behavior and gas emission characteristics of coal, different kinds of biomass chars, and their blends were evaluated by using the thermogravimetric analyzer coupled with a mass spectroscope. The chars were prepared from wheat straw at 300°C and 500°C under N2 and CO2 atmosphere, respectively. The results show that the pyrolysis temperature has a greater effect on the combustion characteristics of biomass char than the pyrolysis atmosphere. NOx emissions were reduced effectively with the addition of biomass chars. The study also reveals that there is an obvious improvement in the combustion behavior of coal when blended with biomass chars.

Emissivity of burning bituminous coal char particles – Burnout effects

The investigation of burnout effects on the char particle emissivity in the spectral range from 1.25 to 5.5µm is presented. Single coal particles of a bituminous coal were combusted in a flat flame burner under oxy-fuel conditions (20.1mol%). The emissivity was determined using a test-rig, which also measures particle temperature and diameter, both being necessary input parameters for single particle emissivity, in the visual spectral range. The infrared radiation was measured spectrally resolved with a fiber spectrometer (1.25–2.25µm) and integrated with an InSb detector (2.4–5.5µm). The emissivity decreases clearly with progressing burnout: E.g. at 1255nm the emissivity decreases from 0.49 to 0.38. The effect is larger in the spectral range from 1.25 to 2.25µm, but still visible in the longer wave length range.

Adsorption of Heavy Metals in the Presence of a Magnetic Field on Adsorbents with Different Magnetic Properties

The application of static magnetic fields during adsorption as an unconventional method for the removal of heavy metals such as Cd2+ and Zn2+ was studied in this work. Two different materials were employed as adsorbents: activated carbon from bituminous coal and bone char. The magnetic characterization of these materials reveals that the activated carbon from bituminous coal is a ferromagnetic material, while the bone char is a paramagnetic material. Both adsorbents were used as received; i.e., no additional modification was employed to alter or improve their magnetic properties. Adsorption studies included adsorption isotherms at 30 °C, at pH 5, and equilibrium times of 72 h. The adsorption results obtained depended on the magnetic nature of both the adsorbent and the adsorbate. It was determined that because of its ferromagnetism (hence, its chemical composition), the activated carbon performed better when an external magnetic field was present during the adsorption process. An increase of 63% and 15% i...

The influence of K2CO3 and KCl on H2 formation during heat treatment of an acid-treated inertinite-rich bituminous coal–char

Thermogravimetric-mass spectrometry (TG-MS) studies were conducted in order to investigate the effect of potassium carbonate (K2CO3) and potassium chloride (KCl) on the conversion and hydrogen evolution of chars derived from acid-leached South African inertinite-rich bituminous coal. K2CO3 and KCl (0.5, 1, 3, 5 potassium ion mass percentages) were loaded to the acid-leached (demineralized) coal samples prior to charring. The resulting ‘doped’ coal–char samples were subjected to heat treatment in a CO2 atmosphere up to 1200 °C, and the thermogravimetric curves, as well as the temperature ranges of evolution of hydrogen, were investigated. The results obtained indicate that the temperatures at maximum rate of mass loss (from the DTG curves), as well as the temperatures at maximum rate of hydrogen (H2) evolution (from the MS curves) are lowered with increasing potassium salt loadings. The relative coal gasification reaction reactivity (1/Tmax) determined from the DTG curves increases with increasing potassium salt loading and increases more for K2CO3 than for KCl. The catalytic influence of K2CO3 on the CO2 gasification of the acid-treated coal sample was found to be greater than that of KCl at a loading of 5 % potassium ion mass percentage.

Kinetic Analyses of Gasification and Combustion Reactions of Carbonaceous Feedstocks for a Hybrid Solar/Autothermal Gasification Process To Continuously Produce Synthesis Gas

A hybrid solar/autothermal steam gasification process to continuously produce synthesis gas from carbonaceous feedstocks is considered. Langmuir–Hinshelwood-type reaction mechanisms for gasification with CO2 and H2O(v) and combustion were used with non-isothermal thermogravimetry to determine kinetic parameters for three feedstocks: (1) activated charcoal, (2) bituminous coal char, and (3) miscanthus char. Temperature ranges of 550–960 and 1120–1270 K and concentrations of 10–40% O2–Ar and 20–100% H2O–CO2 were evaluated for combustion and gasification, respectively. Carbonaceous feedstock microstructures were examined using scanning electron microscopy. Surface area and porosity were characterized using Brunauer–Emmett–Teller analysis.

Numerical investigation on separate physicochemical effects of carbon dioxide on coal char combustion in O2/CO2 environments

The thermophysical and chemical effects of CO2 on the combustion characteristics of a single char particle in O2/CO2 environments remain unclear because these effects are intercoupled during the complex process of coal char combustion. A numerical method was proposed to adjust the char particle temperature in an O2/CO2 environment back to that in an O2/N2 environment by replacing an appropriate portion of CO2 with argon because of the molar heat capacity of Ar < N2 < CO2, and to quantitatively isolate the effects of CO2 on bituminous coal char combustion in specially designed combustion environments with the continuous-film model. The numerical result shows that the char particle temperature in a 21% O2/79% CO2 environment is lower by 493 K than that in a 21% O2/79% N2 environment because of the combined effects of lower local gas temperature and endothermic char gasification reaction with CO2. The char particle temperature increases significantly with the oxygen concentration, and the char temperature in a 52.5% O2/47.5% CO2 environment is equivalent to that in air environment. Most importantly, the effects of CO2 on bituminous coal char combustion in O2/CO2 environments are quantitatively isolated with specially designed combustion environments (21% O2/79% N2, 21% O2/11% CO2/68% Ar, 52.5% O2/47.5% CO2 and 52.5% O2/30% CO2/17.5% Ar). The relative contributions of the oxygen concentration, thermal, and chemical effects on the char combustion rate are 82.1%, 11.2%, and 6.7%, respectively, at an ambient gas temperature of 1200 K for bituminous coal char of 91 µm. The results show that the oxygen concentration effect is the most important factor, followed by the thermal effect and the chemical effect of char gasification reaction with CO2.

Study on the surface active reactivity of coal char conversion in O2/CO2 and O2/N2 atmospheres

There has been a controversy about the reactivity of coal/char combustion under oxyfuel and air conditions. The conversions of NCP anthracite and DT bituminous coal chars were studied under different atmospheres, using simultaneous thermogravimetry and differential scanning calorimetry (TG–DSC). A model of the intrinsic reaction rate was developed to obtain the variable activation energy at different temperatures and char conversion ratios in terms of a TG–DSC curve only. In the endothermic stage, the chemisorption obeys the same mechanism in O2/CO2, O2/N2 and O2 atmospheres, in which the reaction order is about 0.48 in the nth-order model, while in CO2 is a zeroth-order reaction; both of them are not affected by coal rank. The chemisorption activation energy of CO2 is less than that of O2. The chemisorption activation energies in O2/CO2 are more than that in either 100% CO2 or 100% O2, showing that both O2 and CO2 compete for the same active sites. The char-O2 chemisorption is inhibited by the char-CO2 chemisorption, resulting in a lower reactivity in O2/CO2. Subsequently, in the exothermic stage, the NCP and DT char combustions obey different mechanisms, in which the reaction orders are 1.65 and 1, respectively. According to the significant differences in variable activation energies obtained from the TG and the DSC data, the char reaction can be divided into a multi-step chemisorption oxidation stage and a one-step burning stage. Compared with TG method, DSC method is more suitable for describing the reactive characteristics of the multi-step stage. The results show that the char-O2/CO2 chemisorption partially shares common active sites with a complicated multi-step mechanism. The trends of variable activation energies with char conversion ratio at different O2 concentration in O2/CO2 are similar. The activation energy is not significantly affected by CO2, and is about 178±11kJ/mol. The intrinsic reaction mechanisms of DT char combustion in O2/CO2 and O2/N2 have no fundamental difference; however, the NCP char combustion is affected by both CO2 and oxygen partial pressure.

Chemical-looping combustion of solid fuels in a 100 kW dual circulating fluidized bed system using iron ore as oxygen carrier

Chemical-looping combustion (CLC) is an innovative carbon-capture technology with potential to drastically reduce the cost of capture. By using a circulating bed material to transfer oxygen from the combustion air to the fuel, air and fuel are never mixed and the CO2 can be obtained as a separate flue gas stream, undiluted by N2. In other words, carbon capture is inherent to the CLC process. Here, we present findings from a 100kW chemical-looping combustor for solid fuels. The oxygen carrier used in the present tests was an iron ore from Tierga, Spain. In total, 26h of operation using bituminous coal and wood char as fuel was achieved. The highest gas conversion with bituminous coal was 87%, and the highest gas conversion using wood char as fuel was 93%. The carbon capture efficiency with bituminous coal was 94–98%, which is lower than what has been observed with ilmenite. For the wood char, the carbon capture was even lower. The fate of nitrogen and sulfur was investigated. It was found that 84wt% of the S-containing gas was SO2, and only 16wt% was H2S. The nitrogen analysis indicates that the fuel-N converted to gas was distributed as 10wt% NO, 37wt% NH3, and 53wt% N2. It was found that the oxygen-carrier particles retained their high reactivity throughout the operational period. Furthermore, the expected lifetime of the iron ore was found to be approximately 300h.

Temperature induced development of porous structure of bituminous coal chars at high pressure

Abstract The porous structure of chars affects their reactivity in gasification, having an impact on the course and product distribution of the process. The shape, size and connections between pores determine the mechanical properties of chars, as well as heat and mass transport in thermochemical processing. In the study the combined effects of temperature in the range of 973–1273 °K and elevated pressure of 3 MPa on the development of porous structure of bituminous coal chars were investigated. Relatively low heating rate and long residence time characteristic for the in-situ coal conversion were applied. The increase in the temperature to 1173 °K under pressurized conditions resulted in the enhancement of porous structure development reflected in the values of the specific surface area, total pore volume, micropore area and volume, as well as ratio of the micropore volume to the total pore volume. These effects were attributed to the enhanced vaporization and devolatilization, as well as swelling behavior along the increase of temperature and under high pressure, followed by a collapse of pores over certain temperature value. This proves the strong dependence of the porous structure of chars not only on the pyrolysis process conditions but also on the physical and chemical properties of the parent fuel.

The role of ash layer in syngas combustion in underground coal gasification

Underground coal gasification (UCG) proceeds generally in the presence of an ash layer on the coal (or char) surface. The ash layer would not only increase the mass transfer resistance to gases as reported in the literature but also lead to reactions of O2 with syngas (CO and H2) generated from the ash–char interface. This paper studies the effects of ash layer in a specially designed one-dimension gasification assembly using a bituminous coal char by analyzing the ash and the gases components. The ash content of the char is 26.3wt.% and the deformation temperature (DT) is 1460°C. It is found that at 1300°C the ash layer does not reduce the O2 mass transfer until it is thicker than 1.3mm. The ash layer, however, allows O2 to react with CO generated from the ash–char interface to form CO2, which raises the local temperature in the ash layer leading to local ash melting and transfers the char reaction from partial oxidation by O2 in the absence of an ash layer to the Boudouard reaction, i.e. between char and CO2.

Reaction Kinetic Analysis of Bituminous Coal Char Gasified by CO2

CO2-gasification behaviors of Shenfu bituminous coal from China were investigated by thermogravimetric analyzer within the temperature range of 1173–1323 K, and the pressure from 0.1 to 3.0 MPa. The effects of the temperature and pressure on the CO2 gasification of Shenfu char and the relationship between the gasification rate and gasification time in reactor were analyzed. It is found that the reaction rate presents normal distribution with gasification time and a normal distribution model is proposed to fit the kinetics data. Compared with the random pore model, the normal distribution model can be used to describe the gasification rate varying with gasification time at different temperature and pressure. The gasification time tm for maximum gasification rate obeys the following equation as the function of reaction temperature: . rm, r0 were calculated by a normal distribution model following Arrenius law.