Making the most of South Africa’s low-quality coal: Converting high-ash coal to fuel gas using bubbling fluidised bed gasifiers

Abstract

South Africa has abundant resources of high-ash and other low-quality coals. The aim of this work is to investigate the possibility of using fluidised bed gasification technology to convert these coals into clean fuel gas. The fuel gas can be used for process heating or for power generation using the IGCC (Integrated Gasification Combined Cycle) process. A high-ash coal from the Waterberg coalfield was tested in a bubbling fluidised bed gasifier using various gasification agents and operating conditions. The results of the tests show that when air and steam are used as the gasification agents, the calorific value of the gas is too low (2.9 MJ/Nm3) for efficient power generation using the IGCC process. The calorific value of the gas can be increased to 4.85 MJ/Nm3 if oxygen-enriched air (34% oxygen) and steam are used as the gasification agents. Calculations show that combustion of this gas using air can produce flue gas temperatures of up to 1 500 °C, which would result in high IGCC plant efficiencies. The effect of temperature and residence time on the conversion efficiency of coal in the gasifier using oxygen-enriched air was also investigated. The results show that the coal conversion increases by increasing the temperature and residence time of coal char in the gasifier. The results of the pilot-plant tests are being used to calibrate a simulation model that can be used to determine the optimum temperature, residence time and oxygen-enrichment levels for large-scale fluidised bed gasifiers using Grootegeluk coal from the Waterberg coalfield.