Reactivities of 34 coals under steam gasification

The reactivities of 34 coal chars of varying rank with H₂O have been determined to examine the effect of coal rank on the gasification rate of coal char. The reactivities of chars derived from caking coals and anthracites (carbon content > 78 wt%, daf) were very small compared with those from non-caking (lower-rank) coals. The reactivities of low-rank chars do not correlate with the carbon content of the parent coals. To clarify which factor is more important in determining the reactivity, the evolution of CO and CO₂ from char, the moisture content of char and the amount of exchangeable cations were determined for these low-rank coals or their chars. These values were considered to represent the amount of active carbon sties, the porosity and the catalysis by inherent mineral matters, respectively. It was concluded that the amount of surface active sites and/or the amount of exchangeable Ca and Na control the reactivity of low-rank chars in H₂O.     

Effect of pressure on gasification reactivity of three Chinese coals with different ranks

The gasification reactivities of three kinds of different coal ranks (Huolinhe lignite, Shenmu bituminous coal, and Jincheng anthracite) with CO₂ and H₂O was carried out on a self-made pressurized fixed-bed reactor at increased pressures (up to 1.0 MPa). The physicochemical characteristics of the chars at various levels of carbon conversion were studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), and BET surface area. Results show that the char gasification reactivity increases with increasing partial pressure. The gasification reaction is controlled by pore diffusion, the rate decreases with increasing total system pressure, and under chemical kinetic control there is no pressure dependence. In general, gasification rates decrease for coals of progressively higher rank. The experimental results could be well described by the shrinking core model for three chars during steam and CO₂ gasification. The values of reaction order n with steam were 0.49, 0.46, 0.43, respectively. Meanwhile, the values of reaction order n with CO₂ were 0.31, 0.28, 0.26, respectively. With the coal rank increasing, the pressure order m is higher, the activation energies increase slightly with steam, and the activation energy with CO₂ increases noticeably. As the carbon conversion increases, the degree of graphitization is enhanced. The surface area of the gasified char increases rapidly with the progress of gasification and peaks at about 40% of char gasification.     

Kinetics of coal char gasification at elevated pressures

The effects of temperature, pressure, steam flow rate and CO₂/H₂O ratio of gasifying agent on the pressurized gasification of Linnancang coal char were investigated. A correlation of kinetic data was developed for coal chars from coals of different ranks at 30 kg cm ⁻² and 950 °C. The catalytic effects of Ca, Na and Fe catalysts on the gasification activity, activation energy and methane recovery were studied.     

Gasification studies of onakawana lignite

Onakawana lignite was gasified in air, steam and an air + steam mixture in a fixed bed reactor. The extent of devolatilization was determined by pyrolysis in nitrogen. The composition of products, expressed in terms of H₂/CO ratio, was temperature dependent. The ratio decreased with increasing temperature. During steam gasification the ratio decreased from 4.6 to 2.6 when temperature increased from 700° to 990°C. The addition of air to steam resulted in a marked decrease of this ratio. Steam gasification reactivity of chars prepared from Onakawana lignite at 500°C and 800°C were studied in the temperature range of 650°C to 1000°C. The carbon conversion results were fitted into equations describing the continuous and shrinking core models. The char prepared at 500°C was much more reactive than the one prepared at 800°C. Product distribution expressed as the H₂/CO ratio, was favourable in the temperature range. For comparison, the Kentucky #9 coal and chars derived from this coal were used as referee materials. The reactivity of these chars was markedly lower than that of chars derived from Onakawana lignite.     

Reactivity of Australian coal-derived chars to carbon dioxide

The reactivities to CO₂ of four chars derived from Australian coals at 610 °C, were measured thermogravimetrically. Reaction rates in 100% CO₂ (total pressure, 101 kPa) varied from 0.026 mg h^?1 mg^?1 at 803 °C for char derived from a Lithgow coal to 6.3 mg h^?1 mg^?1 at 968 °C for a Millmerran coal char. Activation energies for the four chars were in the range 219–233 kJ mol^?1. The results show that for Lithgow (Hartley Vale) coal char, reactivity increases with CO₂ concentration and decreasing particle size. The apparent reaction order for this char with respect to CO₂ concentration was found to be 0.7. For different chars, reactivity is inversely proportional to the rank of the parent coal. No general correlation has been established between total mineral content (ash) and char reactivity.     

Effectiveness of K2CO3 and Ni as catalysts in steam gasification

The catalytic steam gasification of 34 coals ranging from anthracite to peat was conducted in a thermobalance at 1023 K. When the difference in the reactivities of coals with and without catalyst is taken as a measure of the catalyst efficacy, that of K₂CO₃ was found to be extremely large and almost independent of coal rank. The effectiveness of Ni depends on the coal type to a great extent and it was very large for several low rank coals. SEM observation and EDAX analysis showed that the K₂CO₃ catalyst was finely dispersed over the char and such a good dispersion was common for all the coals examined. The dispersion state of Ni catalysts depends on the coal type, Ni catalysts were well dispersed over low rank coal chars which showed high reactivity in the Ni-catalysed gasification.     

Low-temperature air oxidation of caking coals. 1. Effect on subsequent reactivity of chars produced

The effect of preoxidation of two highly caking coals in the temperature range 120–250 °C on weight loss during pyrolysis in a N₂ atmosphere up to 1000 °C and reactivity of the resultant chars in 0.1 MPa air at 470 °C has been investigated. Preoxidation markedly enhances char reactivity (by a factor of up to 40); the effect on char reactivity is more pronounced for lower levels of preoxidation. For a given level of preoxidation, the oxidation temperature and the presence of water vapour in the air used during preoxidation have essentially no effect on weight loss during pyrolysis and char reactivity. An increase in particle size of the caking coals reduces the rate of preoxidation as well as subsequent char reactivity. Preoxidation of caking coals sharply increases the surface area of the chars produced. Compared to heat treatment in a N₂ atmosphere, pyrolysis in H₂ of either the as-received or preoxidized coal results in a further increase in weight loss and a decrease in subsequent char reactivity.     

Enhancement of lignite char reactivity to steam by cation addition

Chars produced from lignites typically have much higher reactivities to gasification than those produced from bituminous coals. This has been attributed previously to the presence of carboxylate salts of inorganic constituents on the lignites. Upon charring of the lignites, the carboxylate salts decompose leaving behind well dispersed inorganic constituents which act as catalysts for gasification. In this study, a raw lignite has been treated with HCl and HF to demineralize it and to increase its carboxyl content prior to exchanging selected cations with the hydrogen on the carboxyl groups. Up to 2.14 mmol of calcium per g of coal could be added using this procedure. Addition of varying amounts of calcium to the lignite resulted in the production of chars containing calcium contents ranging from 1.1 to 12.9 wt %. Such addition resulted in a rectilinear increase in reactivity of the char to steam with increasing amount of calcium added. Maximum reactivity attained was over ten times the reactivity found for the char produced from the raw lignite. At comparable molar loadings of metal cations onto the acid-treated lignite, the chars subsequently produced had reactivities in steam in the order: K >Na ≈ Ca >Fe >Mg. Char reactivity could also be enhanced by the addition of cations to nitric acid-treated char which had been produced, in turn, from demineralized lignite.     

CO2 and steam gasification of a grapefruit skin char

A kinetic study on the gasification of carbonised grapefruit (Citrus Aurantium) skin with CO₂ and with steam is presented. The chars from this agricultural waste show a comparatively high reactivity, which can be mostly attributed to the catalytic effect of the inorganic matter. The ash content of the carbonised substrate used in this work falls around 15% (db) potassium being the main metallic constituent. The reactivity for both, CO₂ and steam gasification, increases at increasing conversion and also does the reactivity per unit surface area, consistently with the aforementioned catalytic effect. Lowering the ash content of the char by acid washing leads to a decrease of reactivity thus confirming the catalytic activity of the inorganic matter present in the starting material. Saturation of this catalytic effect was not detected within the conversion range investigated covering in most cases up to 0.85-0.9. Apparent activation energy values within the range of 200-250 kJ/mol have been obtained for CO₂ gasification whereas the values obtained for steam gasification fall mostly between 130 and 170 kJ/mol. These values become comparable with the reported in the literature for other carbonaceous raw materials including chars from biomass residues and coals under chemical control conditions.     

Reactivity of heat-treated coals in hydrogen

Reactivities of eighteen 40 × 100 mesh U.S. coals charred to 1000 °C have been measured in H₂ at 2.7 MPa and 980 °C. The char-hydrogen reaction usually occurs in two stages: a slow induction period followed by a constant-rate region. Reactivities of various chars in the initial stage (R_i) decrease, in general, with increasing carbon content of the parent coals, whereas reactivities in the constant-rate region (R_c) are essentially independent of the rank of the parent coals. Reactivities of chars in H₂ differ markedly from those in air and CO₂. Results of surface-area measurements of chars and activation energies for the hydrogasification reaction suggest that during the induction period the reaction is diffusion-controlled whereas in the constant-rate region it is chemically controlled. Upon removal of mineral matter, R_i values generally decrease but R_c values show a random variation. Removal of mineral matter from coals prior to their carbonization brings about profound changes in surface area and porosity of chars. The effect of char particle size on reactivity is considered.     

The characterization of surface properties and steam reactivities of two Spanish coals of high ash content

Studies were made on two Spanish coals of high as content (a semi-anthracite and a high volatile bituminous coal) and on the coals after heat treatment and on acid demineralisation in HCl and HF. X-ray diffraction revealed that the mineral matter content of the coals included quartz, siderite and aluminosilicate minerals; siderite and the aluminosilicates were decomposed by heat treatment. Mineral matter content was substantially reduced by acid treatment, but the metallic element content, as revealed by EDAX, remained similar to that of the raw coals. N₂ and CO₂ adsorption isotherms and mercury porosimetry show that the coals contain mainly micropores and macropores, the semi-anthracite having the greater microporosity. For bituminous coal, macro- and micro-porosity increase substantially upon heat treatment; for anthracite there is a smaller increase in macroporosity and a decrease in microporosity. Microporosity in both coals is unaffected by acid demineralisation, but macroporosity is increased. The steam reactivity of the bituminous coal char is greater than that of the anthracite coal char, and demineralisation of the coals increases steam reactivity. N₂ surface areas and steam reactivities for both coal chars follow trends with parent coal rank previously established for US coals.     

Catalysis of gasification of coal-derived cokes and chars

Calcium is the most important in-situ catalyst for gasification of US coal chars in O₂, CO₂ and H₂O. It is a poor catalyst for gasification of chars by H₂. Potassium and sodium added to low-rank coals by ion exchange and high-rank coals by impregnation are excellent catalysts for char gasification in O₂, CO₂ and H₂O. Carbon monoxide inhibits catalysis of the CH₂O reaction by calcium, potassium and sodium; H₂ inhibits catalysis by calcium. Thus injection of synthesis gas into the gasifier will inhibit the CH₂O reaction. Iron is not an important catalyst for the gasification of chars in O₂, CO₂ and H₂O, because it is invariably in the oxidized state. Carbon monoxide disproportionates to deposit carbon from a dry synthesis gas mixture (3 vol H₂ + 1 vol CO) over potassium-, sodium- and iron-loaded lignite char and a raw bituminous coal char, high in pyrite, at 1123 K and 0.1 MPa pressure. The carbon is highly reactive, with the injection of 2.7 kPa H₂O to the synthesis gas resulting in net carbon gasification. The effect of traces of sulphur in the gas stream on catalysis of gasification or carbon-forming reactions by calcium, potassium, or sodium is not well understood at present. Traces of sulphur do, however, inhibit catalysis by iron.     

Influence of coal preoxidation and the relation between char structure and gasification potential

A study was carried out to ascertain the effects of coal preoxidation and carbonization conditions on the structure and relative gasification potential of a series of bituminous coal chars. Chars were prepared from two freshly mined bituminous coals and preoxidized samples derived from them. Carbonization conditions included a wide range of heating rate (0.2–10000K s^?1), temperature (1073–1273 K) and time (0.25–3600 s). Char properties were characterized in terms of analysis of char morphology, surface area, elemental composition, and gasification reactivity in air. Over the range of conditions used, preoxidation substantially reduced coal fluid behaviour and influenced macroscopic char properties (char morphology). Following slow heating (0.2 K s^?1), preoxidized coals yielded chars having higher total surface areas and higher reactivities toward gasification in air than did similar chars prepared from fresh coal. Following rapid heating (10000 K s^?1) and short residence times (0.25 s), chars prepared from preoxidized and fresh coals exhibited similar microstructural and chemical properties (surface area, $\text{C}\text{H}$ ratios, gasification rates). Carbonization time and temperature were found to be the critical parameters influencing char structure and gasification potential.     

Gasification kinetics of five coal chars with CO<Subscript>2</Subscript> at elevated pressure

For five coals, the reactivity of char-CO₂ gasification was investigated with a pressurized thermogravimetric analyzer (PTGA) in the temperature range 850-1,000 C and the total pressure range 0.5-2.0 MPa. The effect of coal rank, initial char characteristics and pressure on the reaction rate were evaluated for five coal chars. The reactivity of low lank coal char was better than that of high rank coal char. It was found that Meso/macro-pores of char markedly affect char reactivity by way of providing channels for diffusion of reactant gas into the reactive surface area. Over the range of tested pressure, the reaction rate is proportional to CO₂ partial pressure and the reaction order ranges from about 0.4 to 0.7 for five chars. Kinetic parameters, based on the shrinking particle model, were obtained for five chars.     

Kinetics of gasification of Douglas Fir and Cottonwood chars by carbon dioxide

Arrhenius kinetic parameters have been determined for the CO₂ gasification of chars (heat treatment at 1000 °C) prepared from well-characterized samples of a hardwood, a softwood and a Montana lignite. The effects of pre-pyrolysis addition of inorganic salts of the alkali, alkaline earth and transition metal groups to the wood samples have also been determined. The reactivities of the chars of the cottonwood and lignite samples exceeded that of Douglas fir char by a factor of four to seven between 700 and 900 °C. The reactivity of the wood char was related to the inorganic content of the sample. There was very little difference in the reactivity of chars prepared from the hardwood and the softwood after treatment with similar quantities of inorganic salts. The inorganic content of the lignite char was more than five times greater than that of cottonwood char, but its reactivity was similar. The carbonates of sodium and potassium were equally effective gasification catalysts. The transition metal salts were the most effective catalysts initially, but they lost their activity well before the gasification was complete. The data indicate that treatment of wood with aqueous salts results in replacement of some of the natural minerals by ion exchange, and that these exchangeable ions play a major role in controlling reactivity of the chars.     

The catalytic steam gasification of chars from various sources by K2CO3

Gasification of a char prepared from hydrocracked residuum was compared with the gasification of chars prepared from bituminous and sub-bituminous Canadian coals, wood and graphite. Each material was mixed with 10 mass per cent K₂CO₃ and pyrolyzed up to 900°C. The yield of char was inversely proportional to the amount of volatile matter in the original material. The char prepared from hydrocracked residuum was different from the others. The other chars all followed zero-order gasification kinetics. Gasification of char prepared from the residuum was first-order in the solid. The development of a liquid phase during the pyrolysis of the residuum to char may explain this difference. The gasification rate of the char. from residuum was slower than the rates with the two coal chars and the wood char, but faster than the gasification rate of graphite. A combination of transient experiments and X-ray photoelectron spectroscopic (XPS) measurements indicated that hydrogen was formed almost instantaneously when steam reacted with the char. XPS spectra at liquid nitrogen temperature indicated that during gasification the formation of carbon oxygen bonds proceeded in the following sequence: COH, CO and CO.     

Effects of alkaline metal on coal gasification at pyrolysis and gasification phases

A Chinese high-rank coal was acid-washed and ion-exchanged with Na and K to prepare the H-form, Na-form and K-form coals. After pyrolysis, H-form, Na-form and K-form chars and two additional H-form chars (acid washed Na-form and K-form chars) were prepared to investigate the effects of alkaline metal (AM) on coal gasification at the pyrolysis and gasification phases. The H-form char had the highest pryolysis rate; the H-form char had a relative low gasification rate. The AM loaded coals exhibited relative low pyrolysis rate, while the corresponding chars had high gasification reactivity. Acid-washing reduced the reactivities of Na-form and K-form chars. AM inhibited the progress of graphitization of the base carbon resulting in a more reactive char of less ordered crystalline carbon structure. A kinetic model incorporating AM-catalyzed gasification and non-catalytic gasification was developed to describe the gasification rate changes in the char conversion for AM-catalyzed gasification of chars.     

Effects of coal carbonization conditions on rate of steam gasification of char

Effects of carbonization conditions on char reactivity in steam gasification were evaluated by a gravimetric method, using 12 coals varying widely in rank, type and source. The carbonization variables examined were

• 1.(1) heating rate (5–420K min⁻¹) in steam atmosphere;
• 2.(2) gaseous atmosphere (N₂,H₂,H₂O andCO₂);
• 3.(3) incomplete devolatilization in N₂ (final temperature 200–800 °C);
• 4.(4) quenching of incompletely devolatilized char; and
• 5.(5) complete carbonization (900–1400 °C).

The char reactivity to steam depended on the kind of coal but was almost independent of the carbonization conditions of heating rate, gaseous atmosphere and quenching at temperatures below ≈ 1000 °C. Carbonization above 1100 °C reduced the char reactivity, for example by a factor of 7 to 10 at 1300 °C compared with 900–1000 °C, depending on the parent coal. The char deactivation brought about by increasing carbonization temperature could be correlated with a decrease in the micropore volume of the char, unless graphitization was significant.     

Effect of pre-oxidation on reactivity of chars in steam

The effects of pre-oxidation of char from Taiheiyo coal, a non-caking bituminous coal, in the 400–550 °C temperature range on its gasification reactivity with N₂-H₂O at 0.1 MPa (steam partial pressure of 13.2 kPa) have been investigated. The pre-oxidation of char markedly enhances gasification rates at temperatures between 800 and 900 °C. Reactivity is found to parallel the burn-off level during preoxidation at low temperatures (400–430 °C), whereas at relatively high temperatures (480–550 °C), the burn-off level only affects the reactivity slightly. The amount of CO and CO₂ evolved from the preoxidized char by heat treatment is proportional to the burn-off level at low temperatures (400–430 °C), being closely related to the enhancement of the gasification reactivity in steam.     

Gasification reactivity of char from dried sewage sludge in a fluidized bed

The gasification reactivity of char from dried sewage sludge (DSS) applicable to fluidized bed gasification (FBG) was determined. The char was generated by devolatilizing the DSS with nitrogen at the selected bed temperature and was subsequently gasified by switching the fluidization agent to mixtures of CO₂ and N₂ (CO₂ reactivity tests) and steam and N₂ (H₂O reactivity tests)._. The tests were conducted in the temperature range of 800–900 °C at atmospheric pressure, using partial pressure of the main reactant in the mixture (CO₂ or H₂O) in the range of 0.10–0.30 bar. Expressions for the intrinsic reactivity (free of diffusion effects) as a function of temperature, partial pressure of gas reactant (CO₂ or H₂O) and degree of conversion were obtained for each reaction. For the whole range of conversion it was found that the char reactivity in an H₂O–N₂ mixture was roughly three times higher than that in a mixture with the corresponding partial pressure of CO₂. The reactivity was only influenced by particle size greater than 1.2 mm in the tests with steam at 900 °C. It was demonstrated that the method of char preparation greatly influences the reactivity, highlighting the importance of generating the char in conditions similar to that in FBG.