Philip L. Walker

Importance of carbon active sites in the gasification of coal chars

Abstract 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 N 2 under a wide variety of conditions of heating rate (10 K min −1 to 10 4 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 C0 2 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.

Importance of catalyst dispersion in the gasification of lignite chars

Lignite chars were prepared in N2 under widely varying conditions of pyrolysis heating rate, temperature, and residence time. Their reactivities were measured by isothermal thermogravimetric analysis in 0.1 MPa air. A major decrease in char reactivity was observed with increasing severity of heat-treatment conditions. The relatively high gasification reactivity of lignite chars, compared to those obtained from higher rank coals, is due to the catalytic effect of the initially very highly dispersed CaO on the char surface. Char deactivation is caused primarily by CaO crystallite growth, measured by X-ray diffraction line broadening. When the reactivities of the various chars are expressed as turnover frequencies, i.e., per unit catalyst site, differences in observed rates of about 200 times are reduced to within 1 order of magnitude. Thus, it has been shown that the commonly observed and heretofore empirically treated lignite char deactivation with increasing severity of pyrolysis conditions can be correlated with a decrease in a measurable fundamental property of the chars: catalyst dispersion.

Catalysis of gasification of coal-derived cokes and chars☆

Abstract Calcium is the most important in-situ catalyst for gasification of US coal chars in O 2 , CO 2 and H 2 O. It is a poor catalyst for gasification of chars by H 2 . Potassium and sodium added to low-rank coals by ion exchange and high-rank coals by impregnation are excellent catalysts for char gasification in O 2 , CO 2 and H 2 O. Carbon monoxide inhibits catalysis of the CH 2 O reaction by calcium, potassium and sodium; H 2 inhibits catalysis by calcium. Thus injection of synthesis gas into the gasifier will inhibit the CH 2 O reaction. Iron is not an important catalyst for the gasification of chars in O 2 , CO 2 and H 2 O, because it is invariably in the oxidized state. Carbon monoxide disproportionates to deposit carbon from a dry synthesis gas mixture (3 vol H 2 + 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 H 2 O 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.

Effect of lignite pyrolysis conditions on calcium oxide dispersion and subsequent char reactivity

A demineralized North Dakota lignite was loaded with 2.9 wt% Ca by ion exchange. Chars were prepared by pyrolysis in N2 at 1275 K and residence times between 0.3 s and 1 h. Major differences were observed in their subsequent reactivities in 0.1 MPa air. X-ray diffraction analysis was carried out to obtain information on the state and dispersion of the Ca species on the various chars. The results clearly indicate that CaO is the predominant species responsible for catalysis of lignite char gasification. It is concluded that pyrolysis residence time also has a profound effect on CaO dispersion. Thus, a correlation was established between a fundamental physical property (catalyst dispersion) and the observed gasification behaviour of lignite chars prepared under different pyrolysis conditions.

Reactivity of heat-treated coals in steam

Abstract Reactivities of seventeen 40 × 100 mesh (U.S.) coals charred to 1000 °C have been measured at 910 °C in 0.1 MPa of a N2H2O mixture containing water vapour at a partial pressure of 2.27 kPa. Char reactivity decreases, in general, with increasing rank of the parent coal. The chars show a 250-fold difference in their reactivities. Results suggest that gasification of chars in air, CO2 and steam involves essentially the same mechanism and that relative gasification rates are controlled by the same intermediate oxygen-transfer step. Removal of inorganic matter from raw coals prior to their charring or from chars produced from raw coals decreases the reactivities of lower-rank chars, whereas reactivities of higher-rank chars increase. Addition of H2 to steam has a marked retarding effect on char reactivity in most cases. However, in a few cases H2 acts as an accelerator for gasification. The effect of particle size, reaction temperature and water-vapour pressure on char reactivity is considered.

Thermal behaviour of mineral fractions separated from selected American coals

Abstract The thermal behaviour of four samples of mineral matter, whose quantitative mineralogical compositions had previously been determined, was investigated using thermogravimetric and derivative thermogravimetric analysis methods. The mineral fractions were initially separated from the coal substance by a low-temperature ashing technique. The decomposition of the individual mineral species associated with coals is briefly reviewed. A synthetic standard mineral mixture was examined under various combustion atmospheres. X-ray diffraction analysis was utilized to study the high-temperature phases yielded by the mineral fractions. Ash-fusion data were also obtained, and can be understood in relation to the corresponding mineralogical analyses. No evidence was found of solid-state interactions below 1100 °C.

FTIR studies of Saran chars

FTIR spectroscopy has been applied to a study of oxidized Saran chars. A pregrind technique is applied during KBr pellet preparation which, along with inherent advantages of FTIR, produces much higher quality spectra than could previously be obtained. Bands occurring in the spectra of the oxidized carbons at 1720 and 1585 cm−1 are assigned to carbonyl vibrations in COOH groups and to aromatic ring stretching modes, enhanced in intensity by oxygen containing functional groups, respectively. The intensity of the 1720 cm−1 band is found to vary linearly with % burn-off in the carbon sample. It is suggested that the activity of the 1585 cm−1 band which is found to increase in intensity as a function of % burn-off, is caused mainly by an increase in oxygen containing functional groups at advancing levels of burn-off. A mechanism is postulated for the formation of surface anhydrides based on IR results. This involves early formation of anhydride structures followed by conversion of anhydrides to acidic groups via hydrolysis with atmospheric moisture.

Catalysis of lignite char gasification by exchangeable calcium and magnesium

A Montana lignite was pretreated in either HCI—HF or ammonium acetate. The former treatment replaced cations associated with carboxyl groups by hydrogen, as well as removing essentially all mineral matter. The latter treatment replaced cations by ammonium ions but left the mineral matter intact. The pretreated lignites were then loaded with varying amounts of Ca and Mg, separately or jointly, by ion exchange. Reactivities of chars produced from these exchanged lignites, as well as the raw and pretreated lignites, were determined in air, CO2 and steam. Gasification of exchanged lignites was strongly catalysed by Ca; its activity was not affected by the presence of Mg on the char. At a comparable Ca loading, gasification rates of the 1273 K raw lignite char in the various atmospheres was higher than that of the acid treated 1273 K char but lower than that of the ammonium acetate treated 1273 K char. The former finding is attributed to chlorine retention in the lignite and char; the latter, to enhanced sulphur release during lignite pyrolysis.

Inorganic constituents in American lignites

Abstract Both the discrete mineral phases and the ion-exchangeable inorganic constituents of lignites from Texas, Montana, and North Dakota have been studied. The ion-exchangeable cations and the carboxyl groups with which they are associated were characterized by ion exchange methods utilizing ammonium acetate and barium acetate respectively. Na, K, Mg, Ca, Sr, and Ba were found to be present in all three coals, but significant variations in the relative and absolute concentrations of all the cations were observed. It was found that Ca and Mg were the most abundant cations and that 40–60% of the carboxyl groups in the raw coals were exchanged with cations. The discrete mineral phases in these lignites were studied by semi-quantitative X-ray diffraction and infrared spectroscopy. The importance of the cations in this analysis was shown when the mineralogical analyses of the low-temperature ash (LTA) of the coals with the cations removed and the raw coals were compared. Results show that up to 50% of the LTA of these raw coals can be attributed to the existence of metal cations and that fixation of sulphur, carbon, and oxygen to form carbonates and sulphates is the major reason for this contribution.

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

Abstract 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.