Optimization of combustion characteristics of palm kernel-based biofuel for grate furnace

Abstract

Grate firing is one of the main competing technologies in biomass combustion for steam and electricity generation. Ash generated in the furnace during combustion process would greatly reduce the boiler thermal performance and may lead to unscheduled shutdown. The focus of this study is to optimize the combustion characteristics of the mixture of palm kernel shell (PKS) and selected additives (Al2O3, MgO and CaO) to develop a fuel mixture of low ash yield and higher heating value (HHV). D-Optimal Design under Cross Methodology of Design Expert (6.08) was employed to mix the components alongside various particle sizes. The mixed samples were ashed in a muffle furnace (848 K) to a constant weight and their HHV were determined using Ballistic Bomb Calorimeter. Combustion test based on optimized PKS additive mixture was conducted with a 5 kW grate furnace from which the effects of varying the ratio of primary to secondary air flow rate on temperatures and flue gas compositions from the furnace were measured. The ash obtained after combustion process was characterized using X-ray diffraction (XRD) for the purpose of identifying the mineral phase compounds that are present in PKS and PKS-additive ash. The optimum composition obtained for the process was 2.5, 0.0, 5.0, 92.5% and 5.50 mm for additives (Al2O3, MgO, CaO), PKS and particle size, respectively. The composition resulted in lowest ash yield (0.56%) and HHV (20.64 kJ/g). The coefficient of determination (R2) (0.7951 and 0.7344) and least-square errors (0.19 and 0.024) of the prediction model indicated a close fitness to the experiment results obtained for ash yield and HHV. Primary to secondary air ratio of (40:60) recorded maximum temperature (1058 K), minimum level of CO (285 ppm) and 6% oxygen. XRD results showed excellent interaction between PKS and additives. The appearance of potassium-alumino silicate (KAlSiO4) in the PKS-additive ash prevented the release of potassium chloride which has the ability to increase ash deposition and corrosion.