Fluidised bed gasification of South African coals – experimental results and process integration

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 at a high efficiency into clean fuel gas and/or electricity. The fuel gas can be used for process heating or for power generation using the Integrated Gasification Combined Cycle (IGCC) process. This paper presents research on two areas – experimental work and process integration using pinch analysis. A high-ash coal from the Waterberg coalfield was tested in a bubbling fluidised bed gasifier at the CSIR 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. Parallel to the above experimental work, the University of Pretoria has undertaken pinch analysis to optimize the use of energy generated by IGCC power plants. The focus was on the steam path (subsystem) of IGCC power plants and no alterations were done on the syngas path of the plant. A case study on the world’s largest capacity IGCC plant, the Elcogas plant, revealed that an increase in steam turbine power output from 135 MW to 145.6 MW can be achieved by applying pinch analysis. This increase in steam turbine power output results in a gross increase in efficiency from 47% to 51.6%. The new design however requires a significantly larger heat exchange area to exchange the extra energy that was not exchanged in the preliminary design. It is therefore recommended that a cost analysis should be done to determine whether the new design would be cost effective when compared to the preliminary design.