Evolution characteristics of alkali/chlorine in coal biomass co-combustion ash and subsequent utilization of ash resources

The use for cement production represents a significant way for the subsequent utilization of ash from co-combustion of coal and biomass. However, the introduction of biomass will affect the characteristics of alkali/chlorine in ash, directly impacting the cement production. While numerous studies have been carried out on the release characteristics of alkalis and chlorine during combustion processes, as well as the co-combustion of biomass with high content of alkali-active mineral, research on the characteristics of alkali/chlorine in ash and the co-combustion of biomass with low content of alkali-active minerals is notably lacking. To promote the utilization of ash resulting from coal and biomass co-combustion, the evolution characteristics of alkali/chlorine in ash from the co-combustion of coal with bamboo shavings (low in alkali-active minerals) and corn stalks (high in alkali-active minerals) was investigated. The investigation was carried out under varying conditions, including different temperatures, oxygen concentrations, and blending ratios, in order to study the impact of different operating conditions. The results show that the alkali content in the ash from the co-combustion of coal and corn stalks at temperatures ranging from 900 ℃ to 1 200 ℃ reaches a minimum value of 6.72% at 1 000 ℃. Meanwhile, the chloride ion content decreases as the temperature rises, ranging from 0.024% to 0.052%. The impact of O₂ concentrations on alkali/chlorine in ash is weakened after biomass introduced at 16%–24% O₂ concentrations. Specifically, corn stalk reduces the variation rates of alkalis/chlorine in coal ash from 40.4%/26.0% to 2.61%/2.08%. At blending ratios of 10%–50% bamboo shavings, the content of alkalis/chlorine in the ash gradually increases, with variation rates reaching up to 17.1%/76.9%. Besides, due to the similar content of alkali-active mineral in corn stalk ash compared to coal and the low ash yield of bamboo shavings, the variation in the content of alkali-active minerals in the ash under different combustion conditions is less than 10.1%. Further, the impact of different operating conditions on the proportion for cement production of ash is studied, which shows that compared to the combustion of bamboo shavings alone, the proportion for cement production of ash from co-combustion of bamboo shavings and coal increased by 17%–22%. Considering both the coal combustion and cement production processes, it is estimated to result in a reduction of 1.01–1.49 tCO₂ per tonne of cement. Based on the research findings on the evolution characteristics of alkalis/chlorine and alkali-active minerals in ash, the optimum co-combustion conditions for different biomass with coal are recommended, with the objectives of reducing carbon emissions and achieving high ash proportion for cement production.