Pressure drop in fixed beds of non-spherical biomass char particle mixtures

Abstract

Biomass gasification is an important engineering process for the production of fuel gases. The pressure drop of gas flowing through fixed beds is an important parameter for the design and operation of gasifiers. Pressure drop over gasifiers with biomass feedstocks is not as widely researched as fossil fuel feedstocks. Biomass char particles were used as the packing material to make fixed bed particle mixtures. Five batches of particles ranging from +2.36mm and -13.2mm were used to make binary and ternary mixtures for bed voidage and pressure drop experiments. Experiments were done to gain information on the particles’ shape characteristics. These shape characteristics were used in modelling the bed voidage and pressure drop. Pressure drop and bed voidage determination experiments were conducted with mono-sized, binary and ternary particle mixtures. The voidage models produced satisfactory results with percentage errors of less than 10%. The two linear bed voidage models used, namely the Aim-Golf model and the linear packing density model. The Aim-Golf model modelled bed voidage for binary mixtures with a percentage error of 4.99%. The linear packing density model modelled bed voidage for binary mixtures with a percentage error of 3.82%. The non-linear models used for binary mixtures were the Toufar model and the modified Kwan model. The Toufar model modelled bed voidage for binary mixtures with a percentage error of 3.94%. The modified Kwan model modelled bed voidage for binary mixtures with a percentage error of 4.25%. The three non-linear models for ternary mixtures were the Wong, Chang and De Larrard models. The Wong model had a percentage error of 8.22%. The Chang model had a percentage error of 9.95%. The De Larrard model had a percentage error of 12.0%. Five fundamental pressure drop models were used for modelling. The Ergun equation and four of its modified versions were tested. The modified versions included the Montillet, Tallmadge, KTA and Foscolo equations. These equations did not yield satisfactory results. The Ergun equation and its modifications could not model pressure drop for mixtures of char particles with percentage errors of less than 17%. Three statistical models were used to model pressure drop through mixtures of char particles with percentage errors of less than 10%. The statistical models include quadratic, Padé and 3rd-order polynomials. Their modelling percentage errors were 9.0%, 8.4% and 8.1% respectively. These models only considered interactions between the particles of which the mixtures were made and the fluid gas flow velocity. This study found that more research is required to study the effects of particle interactions when modelling pressure drop through packed beds of char particles. These interactions may in future be included in the fundamental pressure drop models.