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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 NO x , SO x , 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.

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This paper presents an experimental study on bubble formation in the process of sintering of Zhundong coal and corn stalk (CS) ash blends in a horizontal chamber furnace. The effect of the blend ratio of biomass ash was investigated. Bubble parameters, such as number, area, and porosity, were measured on the basis of a metalloscope equipped with a charge-coupled device camera and digital image processing technique. After sintering experiments, ash and condensed matter in the bubbles were sampled and analyzed by X-ray diffraction. In addition, a chemical equilibrium calculation was conducted to reveal the influence of biomass ash on the formation of bubbles. The experimental results show that a 50% CS blend has the greatest melting degree and the formation time of bubbles is earlier than other cases, while a low ratio of CS ash has limited influence on bubble formation and mineral composition. The formation mechanism of bubbles is proposed on the basis of the results. The condensed matter in the bubbles mainly contains NaCl, CaSO4, and KCl. The results indicate that chlorine promotes the transformation of alkali metals to gaseous phase. The chemical equilibrium calculation verified the experimental results.