The present work introduces an innovative concept for converting biomass, in this case wood, to electricity in a process that is decentralized and has a very high electrical efficiency. The concept is based on an absorption enhanced reforming (AER) biomass gasification process, the product gas of which is then converted to electricity at 5bar absolute pressure in a hybrid system consisting of a solid oxide fuel cell (SOFC) and a micro gas turbine.
Prior to conversion in the SOFC, the AER gasification product gas must be cleaned of unwanted tars. This is achieved by reforming the tars with the steam in the gas at a nickel catalyst. Typical tar reformer operating temperatures of 900°C require a large external heat supply, due mainly to the parasitic steam reforming of methane, which is endothermic in nature. Is it possible to operate the tar reformer at a temperature between 600°C and 700°C, the amount of external heat required can be reduced, since the chemical equilibrium of the methane reforming reaction is thereby moved to the reactant side of the equation, resulting in a lower rate of methane conversion.
A one-dimensional, isothermal tar reformer model was created in Microsoft Excel in order to test the viability of tar reforming in the 600°C to 700°C temperature range and to calculate the expected methane concentrations at the reformer outlet. The model inputs are the reaction kinetics of both the tar reforming and methane reforming reactions, and were produced by the laboratory-scale test rig developed specifically for the purposes of this work. Steam reforming experiments using naphthalene as a model tar showed that stationary, reproducible conditions are achieved over time, despite the fact that the nickel catalyst undergoes deactivation due to coking. A hyperbolic approach that takes deactivation into account is derived for the speed of naphthalene reforming under stationary conditions. The relevant parameters for 650°C and 700°C could be determined experimentally.
The result of this work is that tar reforming in the 650°C to 700°C temperature range is possible and that the concentration of individual components at the tar reformer outlet can be calculated according to Gibbs equilibrium. Because of the reduced rate of methane conversion for a reforming temperature of 650°C, the required fuel gas for external heat input in the tar reformer can be reduced from 1.78MW (for a reference temperature of 900°C) to 0.86MW. This increases the electrical efficiency of the overall process from 58.6% to 61.5% (relative to the gasification raw gas).