Combustion and co-combustion characteristics of Searsia lancea and Tamarix usneoides with coal

Abstract

The co-combustion of coal with biomass is one of the promising technologies that can offer cleaner and sustainable electricity. This option is attractive as it holds the potential to reduce CO2, NOx and SOx emissions from coal combustion, which are associated with climate change. As emission standards and regulations become stricter, existing coal power stations will struggle to comply as various technologies like Flue Gas Desulfurization (FGD) are too expensive to be integrated with an existing power plant. The co-firing of coal with biomass would be a suitable approach for South Africa which is largely dependent on coal, as the country continue using its available coal resources to meet its energy demand, while complying with the stipulated environmental regulations. However, with the poor physicochemical properties of many biomass sources, it is important to investigate the suitability of each biomass species in order to determine their co-firing potential before being used. Two indigenous South African trees, Searsia lancea and Tamarix usneoides were planted to control Acid Mine Drainage (AMD) in mining areas. The trees were planted as part of the AngloGold Ashanti (AGA) mine rehabilitation program in collaboration with the University of the Witwatersrand. The tree species were chosen because of their ability to absorb toxic minerals in their different parts, particularly in the leaves. Different parts of the trees were evaluated for their combustion and co-combustion potential with two South African coals, a discard and a run of mine coal. The physicochemical characteristics and thermal properties of the coals, raw biomass and their blends were investigated using a thermogravimetric analyzer (TGA), a bomb calorimeter, and an elemental analyzer (Elementar vario EL cube analyser). The oxides of major and minor elements present in the biomass, coal and their blends were obtained from X-ray Fluorescence (XRF) techniques. The emissions of CO2, NOx and SOx during biomass, coal and coal-biomass co-combustion were monitored using Multi Gas Analyzer (MGA 11). Significant differences in fuel characteristics were found between both tree species and their divided plant COMPONENTS and the coals used in the co-firing tests. Searsia lancea’s leaves possessed a calorific value of 19.21 MJ/kg compared to 16.73 MJ/kg from the discard coal utilized. All the Searsia lancea blends, irrespective of trial site location, possessed lower ash contents (3.77 ii to 6.60%) relative to Tamarix usneoides plant blends which were found to have ash contents of 7.48 to 8.98%. The leaves of both tree species were found with the highest ash content and concentrations of alkali elements leading to higher slagging and fouling ratios. The coals utilized were seen to ignite and burnout completely at higher temperatures than biomass. Searsia lancea and Tamarix usneoides displayed higher combustion reactivity than discard coal and run of mine coal. Searsia lancea harvested from the Mispah site and Tamarix usneoides harvested from the Madala site are better fuels, in terms of reactivity compared to all other samples. A decrease in ignition temperature was noted for the discard coal when co-fired with both Searsia lancea and Tamarix usneoides. There was also an increase in the coal reactivity as the percentage of biomass in the blend increased. The activation energies for the 100% biomass were found to be lower than the activation energies for the coals utilized in this study, indicating that less energy is required for biomass combustion than for coal combustion. The activation energy for the first stage combustion of Searsia lancea was 32.98 kJ/mol, and 24.94 kJ/mol for its second stage combustion reaction. Tamarix usneoides had an activation energy of 37.39 and 26.23 kJ/mol in its first and second stage combustion, respectively. The biomass used were found with higher amounts of Na2O and K2O than discard coal, indicating that the use of biomass alone for power generation might be detrimental to boilers. The addition of coal to the biomass was found to reduce the concentrations of Na2O and K2O, thereby lowering the slagging and fouling potential of the biomass. The emitted gases, that is, CO2, SOx and NOx from the discard coal were reduced by the blending and co-firing of the biomass with coal. This study has demonstrated that both indigenous tree species used for the phytoremediation of contaminated mine sites, could serve as a suitable and compatible valuable source of biomass for combustion and co-combustion with South African high ash coal.