Typical Alkaline Minerals’ Migration And Transformation Mechanism And Its Influence To Ash Characteristics During Biomass Combustion

Biomass fuels which have renewable, low pollution, zero emissions of carbon dioxide and other advantages are the ideal alternative to fossil fuel energy. Biomass fuel prone to form fouling and slagging , fluidized bed material agglomerates and other problems during combustion process because of its higher levels of alkali metals, which hampered the large-scale application of biomass fuels seriously. This paper mainly focuses on typical alkaline minerals’migration and transformation mechanim and its influence to ash characteristics during biomass combustion,for solving alkali metal problems provide a theoretical reference.Wheat straw, rice straw, corn straw combustion process was calculated using thermodynamic equilibrium calculation software . Different temperatures and different S element content, different Cl content on the migration and transformation of alkali metal were calculated.The result of influence that temperature on the migration of alkaline minerals show that KCl （g）, K₂Cl₂ （g） is the mainly form in the gas phase of three kinds of biomass; S content on the alkali metal content change calculations show that KAlSi3O8 is the intermediate product of K₂SO₄; When the total Cl content reached over 0.00486mol, potassium elements of the phase composition does not vary with Cl elements content change .Based on the analysis of thermodynamics, different temperatures, different sulfur content, different content, different Cl and different ashing time on the migration and transformation of alkali metal is studied on small experimental stage. K element content of solid phase decreases with increasing temperature during wheat straw combustion process; K element in the gas phase first increases and then decreases with the S element increase, K element remains in solid phase is in the form of K₂SO₄, K element in the gas phase increases as the raw material Cl element increases, K elements in the gas phase mainly in the form of KCl （g）, and K₂Cl₂ （g） when the temperature is 800℃.The study of the influence that different alkali metal content of wheat samples to the ash composition shows that K₂O content in ash samples was first increased and then decreased with the increase of the sample K elements, the trend of SiO₂ and K₂O content is just the opposite trend; SEM images show that the ash samples appear significant agglomeration behaviorwhen the K element content of wheat samples reaches 4.02%; Ash melting point analysis showes that ash melting point first increases and then decreases with K elements content of rice straw increase,the lowest melting point temperature is in K content of 3.75% , XRD analysis showed that high melting point compounds KAlSiO₄ in ash samples.