Experimental Study on Combustion Characteristics in a CFB during Co-firing of Coal with Biomass Pellets in Thailand

Abstract

This paper studies the effect of operating conditions, including excess air ratio and biomass shares in fuel mixtures on emissions (CO, NO_x, SO₂) and ash characteristics from coal combustion during co-firing of coal with biomass pellets. The experiments were performed in a circulating fluidized bed (CFB) reactor of 80 kW_th capacity. The domestic biomass, including rice straw, eucalyptus and empty fruit bunch were used in pellet form as the supplementary fuels for co-firing with coal. In the experiments, the excess air ratios were varied over a range of 1.28–2.10, and the fuel feed rate was kept at 14 kg/h. The mass fractions of biomass pellets were at 25 wt% and 50 wt%. The combustion time was around eight hours after reaching a stable temperature. Flue gas emissions (CO, NO_x, SO₂) were measured with a TESTO 350 XL flue gas analyzer. In order to investigate the effect of blending biomass pellets with ash characteristics, fly ash and bottom ash were collected and analyzed by X-Ray Fluorescence (XRF) and a Scanning Electron Microscope with Energy Dispersive X-ray Spectrometer (SEM-EDS), respectively. The results showed that the excess air could promote complete combustion with decreasing CO emission. With the further increase in excess air, CO emission increased because of the insufficient residence time for complete combustion and the cooling effect. During co-firing experiments, the bed temperature in riser decreased by about 150–200°C as compared to coal combustion. NO_x emissions for co-firing with rice straw pellets and empty fruit bunch pellets were lower than the emissions from coal. At high excess air, NO_x concentration for co-firing of coal with 50 wt% eucalyptus pellets was higher than the concentration for coal alone. SO₂ emissions for co-firing with biomass pellets in all cases were lower than those from coal. The bed agglomeration was not observed in the spent bed during co-firing of coal with biomass pellets up to 50 wt%. However, the bed particles were coated with layers of elemental compositions corresponding with fuel ashes. FactSage software in the “Phase Diagram” module was used to predict the melting temperature of ash when coating the bed particles. The highest and the lowest melting temperatures of ash were 1734°C and 1410°C for coal combustion and co-firing of coal with empty fruit bunch at 50 wt%, respectively.