Figure 3 illustrates the axial temperature distributions along the FBC height for fuel studied at 50% excess air. As can be seen from the figure, coal combustion gives higher bed temperature (y = 0-40cm) but lower freeboard temperature (y = 450-120cm) in comparison to biomass. Then, all the temperatures shows start to fall from 120 cm above distributor plate indicating that most of the combustion was completed. This significant combustion behaviour can be explained by the devolatilization process of the fuel . With high volatility (more than 50%) and low ignition temperature (250-350°C), biomass (rice husk and palm kernel shell) will start to devolatilize upon feeding at 45 cm of the FBC height (freeboard region) and was mostly burned before it reached the bed region. While coal with low volatility (30%) and higher ignition temperature (400-600°C) will travel down to the bed and completed combustion in the bed region. This was also greatly influenced by settling velocity of the fuel particles which correspond to the fuel particle size and fluidizing velocity . Those explain why palm kernel shell has higher bed temperature than rice even though the volatility is almost similar (see Table l).This was due to the fact that greater particle size contributed to a greater devolatilization time and settling time. The greater settling time, the higher the freeboard temperature due to greater volatile combustion contributing to higher combustion efficiency providing the bed temperature is maintained within the range of 800-900oC.
Figure 3. Axial temperature profile for co-combustion of coal with rice husk and palm kernel shell combustion in the case of excess air = 50%
Significant increment of carbon combustion efficiencies was noted with coal addition to biomass fraction (see Figure 2).The improvement can be attributed to an increase in bed temperature, Figure 3, which is caused by the addition of fixed carbon content in the mixture.
This fixed carbon, from coal, burns in the bed while the volatile gas burns in the freeboard region. Thus, there is more chance for fuel conversion carbon to carbon dioxide as the coal fraction increases and less volatile and tend to escape combustion, because of the reduced biomass concentration. Furthermore, this can be explain by the fact that biomass fuels with lower density (about half) compared to coal tend to burn in freeboard and coal tends to burn in the bed region. Therefore, the addition of coal in biomass increases the amount of fixed carbon reaching the bed resulting in higher bed temperatures. This observation agrees with the results of Abelha et al. (2003) and Suksankraisorn et al. (2003) who investigated the cofiring of coal and chicken litter and co-firing of lignite with municipal solid waste in a fluidised bed combustor, respectively.