Release to the Gas Phase of Potassium, Chlorine, and Sulfur from Biomass Fuels under Dual Fluidized Bed Gasification Conditions
The transformation and release of ash-forming elements during the thermal conversion of biomass are conducive to ash-related problems, for example, agglomeration of the bed material in fluidized beds. The occurrence and severity of ash-related problems largely depend on the extents to which potassium (K), chlorine (Cl), and sulfur (S) are released from biomass fuels to the gas phase during thermal conversion processes. To predict and better mitigate these problems, knowledge of the extent, and the mechanism by which of each of these elements is released from the fuel to the gas phase under relevant operational conditions is required. As K is also a catalyst for char and tar conversion, understanding its transformation and release is also important, as it could be deactivated by other ash-forming elements, such as silicon and phosphorus.
In the present work, the transformation and release of K, Cl, and S from biomass under conditions relevant to dual fluidized bed gasification were investigated. The main aim was to assess the influences of operational conditions and fuel ash characteristics on the extents to which these elements are released to the gas phase during the main stages of fuel conversion, i.e., devolatilization, char gasification, and char combustion. The obtained information is relevant for: 1) the optimization of operational conditions towards the minimization of ash-related problems; and 2) the maximization of the beneficial effects of ash transformation during thermal conversion.
The study was carried out using a laboratory-scale bubbling fluidized bed reactor operated in the temperature range of 550°–900°C. The release of K, Cl, and S from biomass fuels during devolatilization, char gasification, and char combustion were quantified using a mass balance that linked the mass of each element in the virgin fuel to that in the solid residue obtained at the end of each experiment. To gain further insights into the release mechanisms, chemical fractionation, Brunauer-Emmett-Teller (BET) surface area measurements, and chemical thermodynamic equilibrium modeling were carried out.
For a typical temperature range within which the gasification chamber of dual fluidized bed gasifiers is operated (800°–900°C), most of the Cl and virtually all of the S in the virgin biomass follow the raw gas out of the gasification chamber, while most of the K follows the char and bed material to the combustion chamber. During char combustion, further release of K occurs; this K would follow the flue gas out of the combustion chamber unless retained on the bed materials. Furthermore, the complete conversion of the organic matrix of the char increases the risk of K reacting with other ash-forming elements, such as Si in the fuel or in the bed material, which may render the K inactive for char and tar conversion. Therefore, unless measures are taken to capture and maintain the K in a catalytically active form in the combustion chamber with subsequent recirculation to the gasification chamber, catalytic char and tar conversion may be hindered. The transformation and release of K, Cl, and S as functions of temperature, retention time, atmosphere, and fuel ash characteristics are also discussed.