Steam Gasification of Low-Rank Coals with Ion-Exchanged Sodium Catalysts Prepared Using Natural Soda Ash

Ion-exchange reactions of brown and sub-bituminous coals with natural soda ash, composed of >99% Na2CO3, have been studied at 20–40 °C without any pH-adjusting reagents, and the pyrolysis and subsequent steam gasification of the resulting Na+-exchanged coals have been conducted using a fixed-bed quartz reactor at 700 °C. When the Na+ concentration and pH of an aqueous mixture of coal and soda ash are monitored during the ion-exchange process, both values decrease at a greater rate with brown coal with a higher content of COOH groups, indicating that ion exchange of Na+ with H+ of the COOH group is the predominant process. About 65% of COOH can be exchanged with Na+ ions under optimal conditions, irrespective of the coal type. The reactivity of these raw coals in steam at 700 °C is similar, with char conversions of less than 20 mass %, even after 2 h of reaction. Exchanged Na promoted the gasification of both coals at this temperature, but the rate profiles were different: conversion of brown coal increased linearly with time and reached nearly 100% at 1 h, whereas sub-bituminous coal needed approximately 2 h to be gasified completely. The temperature dependence of the conversion with this coal revealed that the use of a Na catalyst can lower the reaction temperature by about 120 °C, and the apparent activation energies were estimated to be 190 and 120 kJ/mol without and with the catalyst, respectively, from Arrhenius plots of the initial specific rate. The scanning electron microscopy–electron probe microanalysis and X-ray diffraction analysis of Na-containing chars recovered after pyrolysis and gasification suggested that the Na catalysts were finely dispersed at the initial stage of the reaction but that they may be deactivated by the formation of sodium silicates at high char conversions at temperatures higher than 90%, even at the low temperature of 700 °C.