Prediction and Validation of Major Gas and Tar Species from a Reactor Network Model of Air-Blown Fluidized Bed Biomass Gasification

Abstract

The thermochemical conversion of biomass via gasification offers a promising approach to producing fungible substitutes for petroleum-derived fuels and chemicals. The kinetic study of the gas-phase reactions of biomass gasification is key to understanding fluidized bed biomass gasification (FBBG). Under typical operating conditions for air-blown FBBG (700–1000 °C), tars exist in the product gas in significant quantities (2–50 g/Nm3). Predicting the formation and evolution of tars in a FBBG reactor model is particularly important as they introduce several operational and cleanup challenges in practice. However, such predictions require implementation of detailed chemical kinetic mechanisms due to the large number of species and competing conversion pathways involved. A detailed gas-phase mechanism has been proposed by the CRECK modeling group at Politecnico di Milano encompassing the secondary pyrolysis, cracking, and oxidation reactions of the devolatilization species of biomass, as well as the oxidation and combustion reactions of the resultant gas-phase hydrocarbon species. In this work, a one-dimensional reactor network model (RNM) of an air-blown fluidized bed gasifier utilizing this detailed chemistry model is developed and validated for the prediction of major gas-phase species and tar compounds. It is found that this RNM is able to accurately predict the syn-gas production and total tar concentration given a modification of water gas shift and/or CO oxidation kinetics to account for catalytic effects of the biomass ash and char. Additionally, validation of the predicted tar composition is attempted against available experimental measurements. Good agreement is achieved for single-ring aromatic and oxygenated tar compounds, while it is found that polycyclic aromatic hydrocarbons are underpredicted by more than an order of magnitude. Finally, the conversion pathways of representative devolatilization products of biomass—levoglucosan, xylofuranose, and p-coumaryl—are analyzed in the context of syn-gas and tar formation routes.