Effects of the addition of coal ash on the transformation of different types of inorganic K in biomass during gasification

Potassium (K) is a major inorganic element in biomass. The transformation of K during biomass gasification can cause slagging, ash deposition and agglomeration problems. The addition of coal ash rich in Si and Al is one of the effective methods to solve these problems. In most biomass, K mainly exists in the form of water-soluble inorganic salts, such as K₂CO₃, K₂SO₄ and KCl. However, the content of these K salts varies significantly with the species of biomass, which leads to the significant differences in the agglomeration and slagging behaviors during gasification. Therefore, the research utilized pine wood doped with K₂CO₃, K₂SO₄ and KCl to simulate different types of inorganic K in biomass. The effect of coal ash on the agglomeration in the fluidized-bed reactors (silica sand as the bed material) and the transformation of different types of inorganic K in biomass was studied systematically during steam and CO₂ gasification. Although two kinds of coal ash with different mineral compositions were used, the K₂CO₃-doped and K₂SO₄-doped pine wood react with both kinds of ash to form kaliophilite (KAlSiO₄) during gasification. However, under the CO₂ atmosphere, for the KCl-doped pine wood, most of KCl is mainly retained by physical adsorption because the ability to form KAlSiO₄ is weaker than that under the steam atmosphere, which results in a lower K retention ratio under the CO₂ atmosphere. In addition, the K retention ratio of KCl-doped pine wood is significantly lower than that of pine wood loaded with K₂CO₃ and K₂SO₄, which suggests that KCl in biomass tends to release and reacts with coal ash only to a limited extent. Therefore, KCl in biomass is more likely to cause ash deposition or corrosion problems. However, the results of thermodynamic equilibrium calculations indicate that the reaction between the three types of K salts and coal ash mainly forms leucite or feldspar, which are significantly different from the experimental results. Lastly, although the addition of coal ash cannot stop the formation of agglomerates, it inhibits the formation process of low melting-point K silicates generated from the reaction between silica sand and K-salt-doped pine wood. Also, coal ash makes the agglomerates to be broken more easily, thus mitigating the agglomeration problem in fluidized-bed reactors effectively.