Biomass gasification for the production of methane

Biomass is very promising as a sustainable alternative to fossil resources because it is a renewable source that contains carbon, an essential building block for gaseous and liquid fuels. Methane is the main component of natural gas, which is a fuel used for heating, power generation and transportation. In The Netherlands, the contribution of natural gas to the primary energy consumption is almost 50% (Source: Energy Research Centre of The Netherlands [ECN]) and it is a fuel with a well-developed pipeline distribution network and infrastructure. There are different biomass conversion routes to methane depending on the water content of the biomass feed. The thermochemical conversion route to convert relatively dry biomass into methane is conventionally envisaged in a two-step process: In the first step, biomass is gasified (with heat demand, high T / low P) and in the second step methane is formed (with heat release, low T / high P) in a separate reactor. In this configuration there is no heat integration possible between the two process stages. In this thesis, a new gasification concept is investigated, termed self-gasification, that overcomes, inter alia, the issue of heat integration. The concept entails an intermediate temperature (700-800°C) and pressure (25-35 bar) steam gasifier, where recycled ash components -contained in the biomass itself- serve as potential “catalysts” for char (from biomass pyrolysis) gasification, methane formation, gas conditioning and tar cracking. The focus of the present research lies on process evaluation and study of the influence of biomass ashes on the aforementioned reactions. Ashes are present in different concentrations in targeted biomass feeds for gasification; ranging from ~0.5 wt.% in “clean wood” to ~15 wt.% in chicken litter. For that reason alone, results presented in this thesis are not limited to the self-gasification concept where methane is the final product, but give valuable information for other biomass gasification processes as well. Subjects such as gasification under pressure, methane formation and the effect of naturally occurring ash in biomass are dealt with in this thesis. The concept is now ready to be studied in an integrated bench-scale unit for further evaluation.