Steam Gasification of Low-Rank Coal with a Nanoscale Ca/Na Composite Catalyst Prepared by Ion Exchange

Ca²⁺ and Na⁺ cations were separately or consecutively ion-exchanged with an Indonesian low-rank coal using saturated aqueous solutions of Ca(OH)₂ and/or soda ash at 20-40 °C without pH adjustment. This was done by adding aqueous solutions of the soda ash and/or calcium hydroxide to a dispersion of coal particles in deionized water, while monitoring the Na⁺ and Ca²⁺ concentrations and pH of the resulting mixture. A 3.5 mass % Ca²⁺-exchanged coal, a 1.6 mass % Na⁺-exchanged coal and a 3.8 mass % Ca²⁺- and 1.0 mass % Na⁺-exchanged coal were obtained. The catalytic pyrolysis and subsequent gasification of these ion-exchanged specimens were performed in a fixed bed quartz reactor at 650, 700, or 750 °C. X-ray diffraction analyses of the samples pyrolyzed at 700 °C found no evidence of Na or Ca species, indicating that these were present as nanoscale particles. The Ca²⁺/Na⁺-exchanged coal underwent steam gasification more readily than the other samples, with complete char conversion of this material following 1 h of processing at 700 °C. The use of a binary metal catalyst was also found to lower the required reaction temperature by more than 100 °C.