Reactivity of heat-treated coals in air at 500 °C

Twenty one US coals, of widely ranging rank, have been carbonized under controlled conditions to 1000 °C, and the reactivity in air at 500 °C of the resulting chars or cokes has been measured by a gravimetric method. The reactivities lie within a well-defined band when plotted against rank of the parent coal. The lower-rank coal chars are more reactive than those prepared from high-rank coals. In extreme cases, the reactivity found for a Montana lignite char is some 100 times as great as that obtained for a char produced from a Pennsylvania low-volatile coal. Variation of reactivity with heat-treatment temperature (600 to 1000 °C) has been studied for three coals. As heat-treatment temperature increases, there is a decrease in reactivity. Some results are reported on the effects which mineral matter and pore structure have on the reactivity parameter. Chars containing high concentrations of magnesium and calcium impurities are most reactive. The amount of macro and transitional porosity in a char has a marked influence on reactivity.     

Importance of carbon active sites in the gasification of coal chars

A demineralized lignite has been used in a fundamental study of the role of carbon active sites in coal char gasification. The chars were prepared in N₂ under a wide variety of conditions of heating rate (10 K min^?1 to 10⁴ K s^?1), temperature (975–1475 K) and residence time (0.3 s–1 h). Both pyrolysis residence time and temperature have a significant effect on the reactivity of chars in 0.1 MPa air, determined by isothermal thermogravimetric analysis. The chars were characterized in terms of their elemental composition, micropore volume, total and active surface area, and carbon crystallite size. Total surface area, calculated from C0₂ adsorption isotherms at 298 K, was found not to be a relevant reactivity normalization parameter. Oxygen chemisorption capacity at 375 K and 0.1 MPa air was found to be a valid index of char reactivity and, therefore, gives an indication, at least from a relative standpoint, of the concentration of carbon active sites in a char. The commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with their active surface areas. The importance of the concept of active sites in gasification reactions is illustrated for carbons of increasing purity and crystallinity including a Saran char, a graphitized carbon black and a spectroscopically pure natural graphite.     

Calcium catalysis in air gasification of cellulosic chars

Cellulosic chars containing calcium have been prepared from carboxymethyl cellulose in the calcium form and from pure cellulose containing sorbed calcium acetate, at several heat treatment temperatures (HTTs) in the range 400–900°C. The chars have been gasified in air at 300°C. The results indicated a general decrease in reactivity with increasing HTT. However, instead of a monotonic decrease in reactivity reported previously for coal chars containing indigenous or added calcium, the reactivity versus HTT curve consisted of three distinct regions which probably reflect the transformations being undergone by the catalyst species on increasing the HTT. Crystalline CaO was detected by XRD only in chars heated to 1000°C, at which temperature the catalyst was no longer effective. The relationships between gasification rate and catalyst concentration and also mode of addition have been determined using chars of HTT 600°C. As found by earlier workers with some coal chars, the rate reached a maximum with increasing calcium content and then declined. Chars containing sorbed calcium showed a relatively early decline of rate. They also showed less reactivity than the chars containing ion-exchanged calcium at all calcium concentrations.     

The estimation of char reactivity from coal reflectogram

Measurements of the intrinsic reactivity of chars to oxygen are increasingly being sought as an indicator of the combustion potential of fuels. The coal reflectogram has been used to characterize the chemical properties of coal and its resultant char structure. In this study, six Australian coals varying in rank were separated using density separation technique to obtain vitrinite and inertinite rich fractions. Chars were obtained from these density fraction samples in a Drop Tube Furnace (DTF) at 1673 K. The reactivity of the chars was measured non-isothermally in a Thermal Gravimetric Analysis (TGA) in the temperature range of 573–1073 K. The results suggested that with the increase in the coal rank, the maximum reactivity of chars derived from vitrinite rich fractions decreases, while the reactivity of chars derived from inertinite rich fractions decreases with the increase in the inertinite content in samples and has no obvious relationship with rank. The kinetic parameters were derived using data from non-isothermal TGA after accounting for changing in surface area with conversion. The frequency factor is found to decrease with increasing coal FMR, defined as the summation of each reflectance value multiplied by its frequency, for a constant activation energy (E=146 kJ/mol). This suggests that the behavior of a maceral is characterized primarily by its reflectance distribution instead of the type of its parent coal.     

Gasification reactivity of Chilean coals

Gasification reactivities of raw and acid-washed coal chars obtained from the three most important coal-bearing regions in Chile have been determined in 0.1 MPa of oxygen using a thermobalance. Oxygen chemisorption capacities of the demineralized chars were also measured gravimetrically at 373 K in 0.1 MPa of oxygen. The subbituminous coals of Catamutun and Peckett are more reactive than the bituminous coals of Lota and Trongol due to the catalytic effect of their inorganic constituents. However, in the absence of catalytically significant mineral matter, coal rank is not an important parameter of char reactivity. The reactivity of chars based on carbon active surface areas, estimated from gravimetric chemisorption measurements, agreed very well with the previously reported value based on active surface areas obtained in a volumetric system. These results supply additional evidence that active surface area is the fundamental parameter that can explain most of the observed differences in the kinetic behaviour of coal chars and carbons in general.     

Gasification performance of torrefied Timothy hay and spruce wood chars in a CO2 environment

Timothy hay abundantly available in New Brunswick, Canada, is mostly used for animal feed and bedding. Upgrading biomass using Torrefaction method can offer benefits in its waste management, energy density and energy conversion efficiency. Temperature and residence time play an important role in the torrefaction process. Meanwhile, CO₂ gasification is also a promising thermochemical conversion process due to its potential to reduce net GHG emissions and tune syngas composition. This study investigates the impact of torrefaction parameters on isothermal and non-isothermal CO₂ gasification of Timothy hay and spruce chars. Timothy hay chars exhibited higher CO₂ gasification reactivity than chars from spruce. The physicochemical properties analysis indicated that higher reactivity of Timothy hay char was mainly attributed to the high amount of alkali and alkaline earth metal (AAEM) content, relatively large BET surface area, a high number ofactive sites, and a low crystalline index. Moreover, in both experimental cases, char derived through a high heating rate and high residence time conditions exhibited improved gasification performance, which was attributed to the generation of large amounts of AAEM (Ca and K) and high specific surface area. Co-gasification results during non-isothermal processes under CO₂ showed the presence of larger interactions in coal char/Timothy hay char blends than that of coal char/spruce char blends. For both experimental conditions, interactions were enhanced once the char prepared from high heating rate and high residence time was gasified with coal char. Thus, the proposed approach is a sustainable way of conversion of Timothy hay under CO₂ environment.     

Effect of oxygen chemisorption on char gasification reactivity profiles obtained by thermogravimetric analysis

The gasification reactivity of an Illinois No. 6 bituminous coal char was determined in oxygen and carbon dioxide using thermogravimetric analysis (TGA). Extensive tests were carried out to ensure the absence of diffusional limitations. Measurements of chemically controlled rates were verified by analysing the activation energies for reactions of the char at various conversion levels. The effect of stable carbon-oxygen complex formation on TGA reactivity profiles was investigated. For disordered carbons (e.g. coal chars) gasified in oxygen, the results showed that the observed differences between reactivity profiles obtained by TGA and those obtained by product gas analysis (e.g. non-dispersive infrared spectroscopy, i.r.) can be attributed to significant amounts of stable complex being formed during the initial stages of reaction. The fact that TGA reactivity profiles become equivalent to i.r. reactivity profiles, when corrected to account for stable complex formation, suggests that the former may not be accurate representations of the variations in intrinsic reaction rates and should be used with caution when attempting to validate proposed models of char gasification kinetics. The extent to which stable complex forms during char gasification was used to explain the observed differences in the reactivity profiles obtained for reactions of char in oxygen and carbon dioxide.     

Influence of carbonization conditions on the gasification of acacia and eucalyptus wood chars by carbon dioxide

Gasification rates of cubic shaped acacia and eucalyptus wood chars were measured thermogravimetrically in a carbon dioxide atmosphere at temperatures in the range 810–960 °C. The effects of wood species and carbonization conditions, such as temperature, heating rate and soaking time, were determined. Both reactivity and the activation energy for the gasification of wood chars were found to be strongly influenced by the carbonization conditions employed during their preparation and wood type. The reactivities of both the acacia and eucalyptus wood chars decreased with increasing preparation temperature; while the activation energy for their gasification increased. Slow carbonization (heating rate: 4 °C min⁻¹) led to the production of wood chars having lower reactivities and higher activation energies than those of the wood chars prepared under rapid carbonization (heating rate: 30 °C min⁻¹) at the same temperature. With increasing soaking time, at carbonization temperatures of 800 and 1000 °C, the reactivity of resulting wood chars was reduced. The results also show that the reactivities of acacia wood chars are higher than those of similarly prepared eucalyptus wood chars.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na in coal on char reactivity

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.     

Catalytic gasification of char from co-pyrolysis of coal and biomass

The catalytic gasification of char from co-pyrolysis of coal and wheat straw was studied. Alkali metal salts, especially potassium salts, are considered as effective catalysts for carbon gasification by steam and CO₂, while too expensive for industry application. The herbaceous type of biomass, which has a high content of potassium, may be used as an inexpensive source of catalyst by co-processing with coal. The reactivity of chars from co-pyrolysis of coal and straw was experimentally examined. The chars were prepared in a spout-entrained reactor with different ratios of coal to straw. The gasification characteristics of chars were measured by thermogravimetric analysis (TGA). The co-pyrolysis chars revealed higher gasification reactivity than that of char from coal, especially at high level of carbon conversion. The influence of the alkali in the char and the pyrolysis temperature on the reactivity of co-pyrolysis char was investigated. The experimental results show that the co-pyrolysis char prepared at 750 °C have the highest alkali concentration and reactivity.     

Reactivity of heat-treated coals in carbon dioxide at 900 °C

Reactivities of sixteen 40 × 100 (U.S.) mesh U.S. coals charred to 1000 °C were measured in carbon dioxide at 900 °C. Chars derived from coals with less than 80% carbon, on a dry-ash-free basis, were the most reactive. These chars also gave the widest spread in reactivity. Plots of inorganic element content in the chars versus reactivity showed that magnesium and calcium are important to char reactivity. Six coals were acid-washed with hydrochloric acid and four coals were further demineralized with hydrofluoric acid. Most acid-treated coals showed a decrease in reactivity; but two coals of high rank increased in reactivity. This increase in reactivity is attributed to the creation of additional porosity as a result of mineral matter removal and thus a reduction in resistance to carbon dioxide diffusion to reactive sites. Two demineralized and two original coals were divided into four size ranges and chars were produced from each size of each coal. Gasification rates increased monotonically with decreasing particle size reacted.     

Changes in char reactivity and structure during the gasification of a Victorian brown coal: Comparison between gasification in O2 and CO2

Char reactivity is an important factor influencing the efficiency of a gasification process. As a low-rank fuel, Victorian brown coal with high gasification reactivity is especially suitable for use with gasification-based technologies. In this study, a Victorian brown coal was gasified at 800 °C in a fluidised-bed/fixed-bed reactor. Two different gasifying agents were used, which were 4000 ppm O₂ balanced with argon and pure CO₂. The chars produced at different gasification conversion levels were further analysed with a thermogravimetric analyser (TGA) at 400 °C in air for their reactivities. The structural features of these chars were also characterised with FT-Raman/IR spectroscopy. The contents of alkali and alkaline earth metallic species in these chars were quantified. The reactivities of the chars prepared from the gasification in pure CO₂ at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O₂ also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O₂ and pure CO₂) are oxidising conditions, the results indicate that the reaction mechanisms for the gasification of brown coal char at 800 °C in these two gasifying atmospheres are different. FT-Raman/IR results showed that the char structure has been changed drastically during the gasification process.     

INFLUENCE OF CHAR PROPERTIES ON REACTION RATES

The reaction of coal char is affected by the char morphology, which is dependent on the temperature and pressure at which the char is prepared. Char properties, such as surface area (by CO₂ adsorption) and diffusion coefficients of CO₂ in char have been measured for chars prepared at 900-1200°C and at pressure to 16 atm at 900°C. The diffusion coefficient results strongly indicate Knudsen or activated surface diffusion. The surface areas and diffusion coefficients decrease in general at higher preparation temperatures, but have a maximum at 1000°C. There is an apparent relationship between these observations and the reactivity results which demonstrate unusual behavior above 1000°C.     

Reactivity of bituminous coal chars during the initial stage of pulverized-coal combustion

The reactivities of pyrolysed and partially burned char particles prepared in an entrained-flow reactor have been investigated. The results indicate that chars collected at the end of the active devolatilization stage are more reactive than those collected before or after this stage. Deactivation of the pyrolysed chars was accompanied by the development of micropores. The chars produced from a liptinite-rich fraction of an HVA bituminous coal showed higher reactivities than those generated from an inertinite-rich fraction. It is suggested that residual volatiles play a more important role in determining char reactivity than the microporosity and the optical anisotropy of the chars. A new expression for TGA reactivity is suggested for use in deriving char combustion kinetics. Relatively constant activation energies of ≈ 125 kJ mol⁻¹ were obtained for chars prepared from a wide range of coal precursors. Calculated char combustion rates at high temperatures extrapolated from such reactivity parameters were in agreement with experimentally determined rates.     

The SMSI between supported platinum and CeO2–ZrO2–La2O3 mixed oxides in oxidative atmosphere

CeO₂–ZrO₂–La₂O₃ (CZL) mixed oxides were prepared by citric acid sol–gel method. The as-received gel was calcined at 500, 700, 900 and 1050 °C to obtain the so-called C5, C7, C9 and CK, respectively. The C5, C7 and C9 powders were impregnated with H₂PtCl₆ and then calcined at 500 °C to prepare P5C5, P5C7 and P5C9, respectively. The impregnated CK powders were calcined at 500, 700 and 900 °C to prepare P5CK, P7CK and P9CK, respectively. The XRD and XPS analyses show that the surface distribution of Pt is evidently influenced by the structural and textural properties of the support. The CO adsorption followed by FTIR reveals that the dispersion and the chemisorption sites of Pt are reduced as the calcination temperature of CZL support increases. The chemisorption ability of the CK samples is even completely deactivated. The encapsulation mechanism, which has been applied to explain the so-called strong metal–support interaction (SMSI) after reductive treatment, is introduced here to demonstrate the abnormal observations though the samples were prepared in oxidative atmosphere. The HRTEM results also confirm this explanation. The effects of oxygen vacancies, the chemisorption sites on the Pt surface and Pt/Ce interfacial sites on the three-way catalytic activities are discussed.     

Influence of inorganic additives on oxygen chemisorption on cellulosic chars

Chemisorption of oxygen on cellulosic chars is the initial step leading to gasification and is a significant factor in controlling chemical reactivity and heat release in smoldering and glowing combustion of cellulose. Oxygen chemisorption kinetics have been determined for chars (HTT 550°C) prepared from cellulose and cellulose treated with inorganic additives. Elovich kinetic analysis indicates that combustion behavior can be correlated with chemisorption kinetics. Addition of the same inorganic additives by grinding with pure cellulose chars had little or no effect on chemisorption kinetics. These data indicate that the mode of action on inorganic additives in enhancing or inhibiting the solid phase combustion of cellulose chars involves their influence on char functionality developed during pyrolysis. Chemisorption of oxygen on chars results in a decrease in free radical concentration, and heat treatment at 400°C in flowing nitrogen restores the original concentration. However, free radical concentrations do not differ significantly between additive treatments over most of the temperature range studied. Therefore, combustion behavior cannot be explained strictly in terms of changes in free radical concentration and other functional groups must also play a significant role.     

Thermogravimetric assessment of combustion characteristics of blends of a coal with different biomass chars

In connection to future energy demand and fossil fuel crisis particularly in India, biomass is gaining its importance for possible use as co-fuel. In India varieties of biomass products are available which do have tremendous potentiality for co-combustion with pulverized coal. Based on the emerging need, detailed investigations are felt necessary to examine the compatibility of different kind of biomass with coal and to select suitable blend composition(s) before utilizing those biomass products in utility operation as co-fuels. This study elaborates the lab scale findings of combustion experiments in DSC-TGA apparatus with a typical Indian coal, two biomass samples and low temperature biomass chars (300 and 450 °C) as well as with ‘blends of low temperature chars and coal’. Conventional TGA parameters, activation energy and ignition index of different blends were estimated which provided elaborate information on their basic combustion features. Results of non-isothermal combustion studies in general depict that blends containing less than 50% biomass char are better performing as compared those with higher biomass char content. Lowering of activation energy and improvement of reactivity in major combustion zone were also observed in the coal/biomass-char blends. Improvement of ignition index of the blends of coal with 300 °C chars over expected weighted mean values was noticed. Such attempts may help to identify appropriate biomass-type, blend proportion for a given coal and to derive some specific advantages with respect to particular combustion practice.     

Effects of volatile-char interactions on the reactivity of chars from NaCl-loaded Loy Yang brown coal

In a fluidised-bed gasifier, char particles are in constant contact with the volatiles and the products from the gasification and thermal cracking of volatiles. The highly reactive nature of volatiles as well as the vulnerable structure of char from brown coal means that there are strong interactions between volatiles and char. The purpose of this study is to investigate the effects of volatile-char interactions on the reactivities of chars from a Victorian brown coal. NaCl-loaded and acid-washed Loy Yang brown coal samples were pyrolysed in a novel fluidised-bed/fixed-bed reactor at 900 °C that has been specially designed to investigate the volatile-char interactions. Char reactivity in air was measured in a thermogravimetric analyser (TGA) under conditions minimising mass transfer limitations. The oxidation of char with air in the TGA showed apparent kinetic compensation effects between the apparent activation energies and pre-exponential factors. The presence of the apparent kinetic compensation effect is a reflection of the heterogeneous structure of char having sites of a wide range of energy levels. Our results have clearly shown that volatile-char interactions can lead to drastic decreases in char reactivity due to the volatilisation of Na and the changes in char structure. The reactivities of chars from the pyrolysis of the catalyst-free H-form Loy Yang brown coal provided unequivocal evidence for the changes in char structure after volatile-char interactions. For chars from the NaCl-loaded Loy Yang brown coal, it appears that the condensation of ring structures in char as a result of volatile-char interactions could have also led to changes in the dispersion of Na catalyst.     

Pyrolysis and combustion kinetics of Fosterton oil using thermogravimetric analysis

Thermogravimetric analysis (TGA) was used to examine the thermal behavior of Fosterton oil mixed with reservoir sand. TGA experiments were performed in nitrogen and air atmospheres at the heating rate of 10 °C/min up to 800 °C. In this study, four sets of TGA runs were performed to examine the thermal behavior of Fosterton whole oil, and the coke sample derived from the whole oil. Similar to previous studies in the literature, we also observed low-temperature oxidation (LTO), fuel deposition (FD), and high-temperature oxidation (HTO) in the non-isothermal combustion experiment. Higher activation energy values were obtained in reaction regions at higher temperatures. The mean activation energy for whole oil in nitrogen and air atmospheres was 33 and 126 kJ/mol, respectively. Fresh coke samples derived from whole oil were subjected to isothermal combustion at different temperatures from 375 to 500 °C. Arrhenius model was used to obtain the kinetic parameters from the TGA data. From the model, the Arrhenius parameters such as activation energy (E = 127 kJ/mol) and the pre-exponential factor (A = 1.6 × 10⁸/min) were determined for the coke combustion. The results showed a close agreement between the kinetic model and experimental data for different combustion temperatures. It was observed that the apparent order of combustion reaction for different temperatures approach unity.     

神木煤显微组分半焦燃烧特性

引　言煤是由许多有机显微组分和少量矿物质组成的有机岩石 ,煤的岩相显微组成是确定煤类型的重要特征 ,因此在研究煤的反应性时应同时考虑到煤的岩相显微组成才能得到较为符合实际的结果 .近年来 ,对显微组分热解半焦燃烧反应性的研究已有报道[1～ 4 ] .Ｋａｎｄｉｙｏｔｉ等[5] 对南非煤的研究结果表明 ,在 70 0℃下所得热解半焦的燃烧反应性随镜质组(Ｖｉｔｒｉｎｉｔｅ)含量的增加而升高 ,随丝质组 (Ｆｕｓｉｎｉｔｅ)含量的增加而降低 .但对同一煤种 ,在 15 0 0℃所得热解半焦的燃烧反应性正好相反 .而对南非煤(87%Ｃ ,ｄａｆ) ,随丝质…