Co-combustion of Searsia lancea and Tamarix usneoides with high ash coal

Abstract

This study utilized two indigenous South African trees, Searsia lancea and Tamarix usneoides, which had been planted for phytostabilisation of heavy metal and Acid Mine Drainage (AMD) contaminated in soils on AngloGold Ashanti mining properties. After twelve years of growth, the two species were harvested from four different trial sites, and divided into different plant components including root, wood, twigs and leaves. These components were analysed for their physicochemical properties, both individually and in combination, with specific emphasis on their properties for direct combustion and co-combustion with coal. Significant differences in fuel characteristic were found between both tree species and their divided plant COMPONENTS and the coal used in the cofiring tests. Searsia lancea leaves possessed a calorific value of 19.21 MJ/kg compared to 16.73 MJ/kg from the coal utilized. All the Searsia lancea blends, irrespective of trial site location, possessed lower ash contents (3.77–6.60%) relative to Tamarix usneoides plant blends which were found to have ash contents of 7.48–8.98%.The Derivative curve (DTG) obtained from the combustion tests conducted on the two tree species indicated that both exhibited relatively low ignition temperatures and high reactivities. Raw 100% Tamarix provided the highest combustion efficiency of all the samples tested. The emission test conducted on the combustion of the Searsia lancea samples, from the Madala site, along with its coal blends, shows a clear trend in the reduction of the SOX and NOX emission between the raw 100% Tamarix, its blends and the coal utilized. This preliminary test shows that both indigenous tree species are capable of being used as a suitable and compatible valuable source of biomass for combustion and co-combustion with South African high ash coal although minor qualitative and performance characteristics occur between them.