Results on the cyclic calcination and recarbonation of calcined dolomite at atmospheric pressure and at 300 psig are presented. The effects of various reaction parameters (i.e., temperature, pressure, and gas composition) on the recarbonation rate and on the change in solid reactivity with cycling were explored. Addition of steam to the recarbonation atmosphere resulted in nearly a two order of magnitude increase in the recarbonation rate. Increasing the temperature and the partial pressure of carbon dioxide for calcination gave lower recarbonation rates and accelerated the loss of solid reactivity with cycling. Reactivation by subsequent calcination in nitrogen was demonstrated.

w Kinetic data are given for the first time for reaction of half-calcined dolomite with H2S (55O-80O0C, 0.005-0.2 atm of H2S in gas at 1 atm). The reaction can be used to remove H2S from a fuel gas made by gasifying coal with air and steam at high pressure. Gas at 20 atm from a dilute-phase slagging gasifier can be desulfurized in an operation at about 920°C to reduce the SO2 emission from power generation to roughly that expected from coal containing 0.15 sulfur. Gas at 20 atm from an ash-agglomerating fluidized-bed gasifier can be desulfurized to an even greater extent in an operation at about 760°C. In light of data given here for 700" and 8OO"C, the reaction can be expected to proceed to completion under these conditions at a faster rate than that atforded by the reaction of fully calcined dolomite, reported earlier. Data at lower temperatures reveal kinetic curiosities which may prove important to understanding of the reverse reaction, useful in regenerating half-calcined dolomite and releasing H,S.quires (1967, 1968) proposed half-calcined dolomite as

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It has been reported that Illinois No. 6 coal is modified by exposure to 50 atm of steam at temperatures between 320 and 360 O C in such a way as to give dramatically improved liquid yields in steam pyrolysis and mild extraction. Here we investigate the character of steam-modified coal. Steam-treated coal swells considerably more than the raw coal in water. This implies that the steam-treated coal is more hydrophilic. It exhibits a lower degree of hydrogen bonding and has an IR spectrum different from that of raw coal. Elemental analysis of treated coal shows a significant decrease in organic oxygen content. A mild 0-alkylation of treated coal with labeled methyl iodide, however, introduces twice the enrichment by 13C as does the same procedure carried out on raw coal. It may be concluded that steam-modified coal contains twice as many hydroxyl groups as raw coal. It is postulated that steam reacts with the ether linkages in coal, forming hydroxyl groups and thereby substantially reducing the presence of an important covalent cross-link in the coal structure.(1) Graff, R. A.; Brandes, S. D. Energy Fuels 1987,1, 84.

Pretreatment of coal by reaction with subcritical steam enhances its performance in direct liquefaction. Illinois No. 6 coal, first reacted with 51 atm of steam for 15 min at 340 °C, was liquefied in a coal injection autoclave to provide rapid heating. Liquefactions were carried out with raw and pretreated coal at high-severity (400 °C, 30 min) and low-severity (385 °C, 15 min) conditions under 1500 psia of hydrogen with tetralin as the donor solvent. Substantial improvement in product liquid quality is realized provided the pretreated coal is protected from oxygen and heated rapidly to liquefaction temperature. Under low-severity conditions, the oil yield is more than doubled, going from 12.5 to 29 wt %. Since previous work pointed to the destruction of ether cross-links by water as the dominant depolymerization mechanism during pretreatment, tests were conducted with several aromatic ethers as model compounds. These were exposed to steam and inert gas at pretreatment conditions and in some cases to liquid water at 315 °C. α-Benzylnaphthyl ether and α-naphthylmethyl phenyl ether show little difference in conversion and product distribution when the thermolysis atmosphere is changed from inert gas to steam. Hence, these compounds are poor models for coal in steam pretreatment. The otherwise thermally stable 9-phenoxyphenanthrene, on the other hand, is completely converted in 1 h by liquid water at 315 °C. At pretreatment conditions, however, mostly rearranged starting material is obtained. Therefore, 9-phenoxyphenanthrene, though less reactive, is a model for ether linkages in coal.