Transformation and Release of Potassium, Chlorine, and Sulfur from Biomass under Conditions Relevant to Dual Fluidized Bed Gasification

Licentiate thesis, 2015

The transformation and release of ash-forming elements during thermal conversion of biomass are conducive to ash-related problems, for example, agglomeration of bed material in fluidized beds. The occurrence and severity of these problems largely depend on the extent to which K, Cl, and S are released from biomass during thermal conversion processes. Therefore, in order to predict and effectively mitigate these problems, knowledge of the mechanisms and the quantity of the primary release of each of these elements from the fuel under relevant operational conditions is required. In the present work, the release of K, Cl and S from biomass under conditions relevant to dual fluidized bed gasification was investigated. Emphasis was placed on the effect of steam as a fluidization medium and oxidant during char conversion. The study was carried out in a laboratoryscale bubbling fluidized bed reactor in the temperature range 550–900 °C. To avoid concentrations of K, Cl, and S in the fuel closer or lower than the detection limits of instruments used for chemical analysis, wheat straw which typically has a higher concentration of these elements than many other biomass fuels was selected. The release of K, Cl, and S from the wheat straw during devolatilization, char gasification, and char combustion was quantified with a mass balance linking the mass of each element in the wheat straw to that in the solid residue obtained at the end of each experiment. To gain further insight into the release mechanisms, leaching and Brunauer-Emmett-Teller (BET) surface area measurement of the wheat straw and some of the solid residues as well as chemical thermodynamic equilibrium modeling were carried out. The results from all the analyses indicate that, at temperatures > 700 °C, the evaporation of KCl and other volatile salts of K during devolatilization are significantly limited by pore diffusion owing to a compact char matrix. However during char gasification, the conversion of the char with steam renders the char less compact by widening and/or creating new pores in the char. Consequently, the resistance to the diffusion of volatile salts out of the char matrix decreases thereby facilitating their release. The conversion of the char is also conducive to the release of char bound-K and S, especially at 900 °C. Based on the release of each of the elements per char gasified, it was inferred that, at temperatures > 800 °C, the release of K can to some extent be decoupled from the release of S and Cl by maximizing the extent of char conversion in the gasification chamber. The extent to which the char is gasified was also found to affect the extent to which the elements are released during char combustion. On the basis of all the results obtained and literature on the transformation and release of ash-forming elements during thermal conversion processes, it can be asserted that, at operational temperatures > 800 °C, the release of K, Cl, and S from biomass in both chambers of a dual fluidized bed gasifier can be controlled to some extent by controlling the extent of char conversion in the gasification chamber.