The Application of Waste Coal Fly Ash as a Support for a Heterogeneous Catalyst for Biodiesel Production and a Comparison with the Engine Characteristics Using a Homogeneous Catalyst

Coal fly ash is a waste product of coal-fired boiler combustion. The fly ash consists of fine glass microspheres with a relatively large surface area. This study aims to synthesize catalysts using waste coal fly ash as a support and calcine them with alkaline catalysts. Fly ash with a large surface area was synthesized using sodium methoxide to prepare a heterogeneous catalyst, which was then used to transesterify palm oil to produce biodiesel. Subsequently, engine performance and exhaust emissions from biodiesel combustion were evaluated. The experimental results indicate that biodiesel prepared with the heterogeneous catalyst exhibited lower fuel consumption and brake-specific fuel consumption (BSFC) than biodiesel prepared with the homogeneous catalyst, and it had a higher brake thermal efficiency. The emission concentrations of CO, CO2, and NOX are higher in biodiesel produced with homogeneous catalysts, whereas the O2 emission concentration from biodiesel prepared with a heterogeneous catalyst is lower than that of homogeneous biodiesel. Moreover, the biodiesel produced using a heterogeneous catalyst consisting of coal fly ash containing sodium methoxide appeared to have a lower excess air ratio and heating value, while exhibiting a higher exhaust gas temperature and fuel/air equivalence ratio. Using waste coal fly ash to synthesize a heterogeneous catalyst improves biodiesel properties, thereby advancing the circular economy.