Behavior of Slagging Deposits during Coal and Biomass Co-combustion in a 300 kW Down-Fired Furnace

Coal and biomass co-combustion in existing utility boilers is a promising option of mitigating the fossil energy crisis and reducing the gaseous emissions of NOx, SOx, and CO2. However, ash-related problems, including fouling, slagging, and corrosion cause damage to the heat exchange tube and reduce boiler efficiency. In an attempt to give better insights into the slagging behavior during coal/biomass combustion, an experimental investigation was conducted to study the growth of slag when coal was co-fired with wood and corn stalk in a 300 kW pilot-scale furnace. For comparison, combustion of pure coal was also conducted. During the experiments, biomass proportions of 5 and 10% by weight were examined. Slags formed on an oil-cooled deposition probe were collected, sampled, and analyzed using scanning electron microscopy and X-ray diffraction (XRD). The change in slag thickness with time was obtained by a charge-coupled device monitoring system. With two thermocouples in the probe, the heat flux through the slag could be measured. The slag from pure coal combustion showed a layered structure with different levels of compactness and hardness. The heat flux decreased by 31.7% as the slag grew to 5.19 mm. The results showed that co-firing wood significantly inhibited the slagging behavior. Especially in the 10% wood case, hardly any slag was collected from the probe. Nevertheless, co-firing corn stalk resulted in severe slagging, with a slag thickness of 5.5 and 6.1 mm for two blend ratios. The formation of bubbles in the deposits together with greater deposit thickness caused heat transfer deterioration. XRD results revealed that the influence of co-firing biomass and corn stalk caused quite different changes to mineral species from wood. It was observed that fly ash under different biomass co-firing conditions differed little on mineral compositions.