Impact of ammonium sulfate and kaolin on ash deposition during co-firing of straw pellets and pulverized coal

This study investigated the effects of additives on reduction in ash deposit formation while co-firing straw pellets with pulverized coal. A pulverized-fuel-fired combustion system was used to co-combust sub-bituminous coal and herbaceous biomass. Kaolin (Al₂Si₂O₅(OH)₄) and ammonium sulfate ((NH₄)₂SO₄) were used to determine the influence of additives on ash deposit reduction. Experiments were performed in an 80 kW_th scale combustion system for various proportions of additives (proportions of 1 wt% and 4 wt% of fuel). Weight of deposited ash was measured concurrently. In the experimental results, different trends from several previous researches were observed regarding effectiveness of the additives: Addition of kaolin increased deposition by up to 19%, while ammonium sulfate increased deposition by up to 36%. Thermodynamic equilibrium of all experimental cases was calculated based on ash composition, and we reveal that in our experimental cases kaolin promotes formation of K-silicates of low melting temperature, while ammonium sulfate reacts with KCl to increase melting temperatures for K-Al-silicates. This demonstrates that the fuel-specific choice of additives is important to effectively reduce ash deposition.