Gasification Reaction Pathways of Condensable Hydrocarbons

Abstract

Dual fluidized bed (DFB) gasification of biomass generates numerous species with large differences in size and boiling point. At the heavier (tar) end, the produced species range from benzene to coronene. In this work, a method for elucidating the pathways of tar evolution is applied to previously presented measurements that satisfy the carbon balance of the Chalmers 2–4-MW DFB gasifier. In addition to quantifying the cold gas and tar, the measurements yield information regarding the amount and C, O, H composition of unknown condensable species (UCS). The reaction pathways were identified by means of fitting a model to the performed measurements. The employed solver varies freely the reaction rate coefficients of three global reactions (mimicking dissociation and reactions with hydrogen and steam) per modeled group, as well as the carbon distribution coefficients within a predefined reaction scheme. The mature tar (excluding primary tar) spectrum is divided into phenols, furans, benzene, naphthalene, pyrene, and one-, two-, and three-ring aromatic components. In addition, UCS that are considered to contain primary tar are divided into four subgroups, to encompass two levels of reactivity with varying composition. Ultimately, the solver converges, yielding a reaction scheme that is based on the findings of earlier works and that describes the creation of mature tar from UCS. Furthermore, the importance of individual reaction routes is discerned for the pertinent measurements. Thus, it is demonstrated that the maturation of secondary tar species (e.g., toluene and phenol) is not in itself sufficient to describe the formation of the tar spectrum