Mamoru Kaiho

The effect of acid treatment of coal on H2S evolution during pyrolysis in hydrogen

Abstract The amount of H2S evolved during pyrolysis was analyzed by g.c.-m.s. for four types of raw coal samples, Taiheiyo, Datong, Wandoan and Mequinenza. These coals were treated with HNO3 or HF HCl solution, in which inorganic sulfur was eliminated by the acid treatment. In the case of HNO3-treated samples, the evolution of H2S in the low temperature region, below 500°C, was reduced by the chemical oxidation. At high temperatures, the amount of H2S evolution from acid-treated coal was markedly increased compared to the raw coals, especially Taiheiyo and Wandoan, through the elimination of minerals. From X-ray fluorescence analysis and X-ray powder diffraction of coal and char, it was confirmed that calcium in the raw coal has an important role for the fixation of sulfur during pyrolysis.

Stoichiometric Approach to the Analysis of Coal Gasification Process

Coal is a solid fuel and less convenient for storage and transportation than petroleum and natural gas. In addition, it usually holds undesirable compounds containing S, N, and so on. Gasification converts coal into H2, CO, and CH4 by the reaction with gasifying agents such as O2 and H2O. There are three types of commercialized processes, 1) fixed bed gasifier: lump coal is gasified in a shaft reactor at 900~1000°C, 2) fluidized bed gasifier: crashed coal is gasified in a fluidized reactor at around 900°C, and 3) entrained bed gasifier: pulverized coal is gasified by burner system at 1350~1600°C. In order to improve the performance of 500~2000t/d plants, operating conditions should be appropriately determined and controlled based on the understanding of chemical reaction process that occurs in gasifier. It has been believed that coal supplied to gasifier is decomposed thermally to produce gases such as H2, CO, CO2, H2O, and CH4, tar, and char. Tar and char react with O2 and H2O supplied to form H2, CO, CO2 and CH4. However, it is quite difficult to understand gasification mechanism only based on the kinetics of above reactions.

Study on reaction characteristics of coal hydrogasification.

In order to estimate a suitable condition for the production of high calorie gas by the hydrogasification of coal, Taiheiyo coal was gasified continuously with hydrogen, and the yield of each product or the composition of gas produced was measured. The experiments were carried out under conditions of 714-775°C and 0-50 kg/cm2G. The feed rate of the coal ranged from 0.256 to 1.47 kg/hr and of hydrogen ranged from 0.113 to 1.059 m3/hr.The volume of methane produced was in the range of 0.2 to 0.3 m3/kg-coal, and it depended strongly on the feeding ratio of coal to hydrogen. The other factors, such as the hydrogen pressure and the coal feed rate, did not affect obviously on the evolution of methan. The yield and the viscosity of tar decreased with the residence time of gas in a reactor. The coal conversion was ranged from 0.38-0.55 kg/kg-coal and it depended on the feeding ratio of coal to hydrogen. The total heat value of the residual char was in good linear relation to the coal conversion.

Study on Gasification of Coal under Pressure Hydrogasification at Lower Temperature Region

It has been known that the evolution of methane shows the maxima at about 550°Cand 750°C when a coal was heated in the atmosphere of pressurized hydrogen under non-isothermal condition.In order to make clear the hydrogasification phenomena in the visinity of the maximum at lower temperatare, 550°C, Taiheiyo coal and its carbonized products were gasified under a hydrogen pressure of 40 atm at constant temperatures, 400-600°C, for 5 hours to which the small scale batch-type gasifier was heated from outside electrically. Discussions were made taking both the changes with time in evolution rate of gaseous hydrocarbons and the conver-sions of carbon and hydrogen in the starting material to gaseous, liquid and residual products into account.It is suggested that at lower temperature below about 600°C the gaseous hydrocarbon is evolved not by the reaction between the hydrogen and the carbon itself in the coal but that between the hydrogen and the limitted active sites in the coal which are excited by the in-crease of temperature but depressed remarkably by the stop of the increase of temperature. The active sites in the carbonized coal heat-treated below reaction temperature were as effective as those in the untreated raw coal, while those in the carbonized coal above reaction tem-perature disappeard almost completely resulting in no evolution of gaseous hydrocarbons.

Studies on Gasification of Coal under Pressure Hydrogasification of Taiheiyo Coal under Non-isothermal Conditions

In order to clarify the hydrogasification reaction of coal, the change in weight loss and the corresponding gaseous products were measured at various hydrogen pres-sures under non-isothermal conditions.The evolutions of gaseous hydrocarbons by hydrogasification began from about 420°C, reached their respective maximam rate at the same temperature of about 550°C and then decreased irrespective of the hydrogen pressure. After showing the minima at about 600°C, the formations of methane and ethane again increased to the clear second maxima. It was found that below 600°C the ratio of the formation rate of ethane to that of methane remained constant regardless of the reaction temperature, but increased with increase in hydrogen pres-sure. This suggests that some active portion of carbon in coal structure will simultaneously produce gaseous hydrocarbons including ethylene and propane.Above 600°C, methane was predominatingly produced from fixed carbons at a rate proportional both to the hydrogen pressure and to the weight of residual coal.