The focus of this work was to further develop steam biomass gasification by means of the absorption enhanced reforming process (AER) and to design future industrial plants for generation of renewable substitute natural gas. For this purpose, laboratory scale experimental investigations with a bubbling fluidised bed reactor and a dual fluidised bed reactor were performed and a model for the absorption enhanced reforming process for generation of renewable substitute natural gas was developed.
The key aspect of the experimental work on the bubbling fluidised bed reactor was the measurement of pyrolysis and steam gasification behaviour in the presence of limestone based bed material. Spheres of beech wood were employed as fuel. The resulting amount of biomass char, which significantly influences the internal energy balance of a dual fluidised bed reactor, and product gas yield were determined as a function of sphere size, temperature and residence time.
A unique feature of the absorption enhanced biomass gasification is the possibility to adapt the product gas stoichiometry for downstream methane synthesis. Therefore, the CO2 sorption behaviour and the homogeneous water gas shift reaction were characterised by experimental investigations on the dual fluidised bed reactor as a function of the driving partial pressure difference from fluidized bed inlet to the thermodynamic equilibrium and the specific CO2 content of the limestone based bed material. Both key reactions of the absorption enhanced reforming process were determined in detail in order to describe the interrelation of bed material behaviour (e.g. attrition stability) and process conditions (e.g. bed material circulation).
A process chain for the generation of renewable substitute natural gas was simulated, taking into account the experimental results. It was investigated how the biomass particle size, the temperature, the steam-to-biomass ratio and the deviation of both the CO2 sorption reactions and the homogeneous water gas shift reaction from the thermodynamic equilibrium influence the efficiency and the quality of product gas and substitute natural gas. In addition, the process chain was modelled with integration of hydrogen generated by electrolysis from renewable electricity in order to show the possibility of upgrading the limited biomass resources.
The experimental and simulated results of steam biomass gasification by means of the absorption enhanced reforming process can be used for the engineering of an optimised dual fluidised bed reactor and for the design of a future plant for the generation of renewable substitute natural gas from biomass residues.