Assessment of potassium concentration in biochar before and after the after-burner of a biomass gasifier

Abstract

Small scale biomass gasifiers are gaining continuously attention due to the renewability of the fuel, the high feed-in tariffs and the novel systems with high efficiency that are becoming available in the market. Gasification is a thermochemical conversion process which mainly produces a gaseous fuel, i.e. syngas, but also other byproducts. The most important ones are the solid phase residues, i.e. char and ash which represent 2-5 % of the initial input by mass. The small scale of operation does not allow the operators to develop centralized efficient strategies for managing char and ash. Thus, onsite strategies have been developed like the after-burner which converts part of the char and the alkali and heavy metals from solid to gas in order to reduce the concentrations of specific substances within the environmental limit. Commercial software are not able to properly model the operation of an after-burner and, therefore, not all the product streams of interest can be calculated. This work introduces a three phase thermodynamic model that aims to assist the operation of commercial software in modelling an after-burner. The key aspect of this three-phase model is that it uses the yield of char as a control variable for modelling the concentration potassium and how it fluctuates among the different phases. Combustion of potassium has the characteristic of creating several intermediate oxides that distribute among the solid and the gaseous phase for a wide range of operating conditions contrary to the majority of heavy and alkali metals which concentrate completely in the solid phase until a tipping point when they completely convert to gaseous products. The model returns results consistent with the onsite measurements and also is able to provide all the additional data, i.e. the molar fractions of gases and the amount of reactive oxygen, which can assist the optimization of the after-burner.