PFB Air Gasification of Biomass, Investigation of Product Formation and Problematic Issues Related to Ammonia, Tar and Alkali

Fluidised bed thermal gasification of biomass is an effective route that results in 100 % conversion of the fuel. In contrast to chemical, enzymatic or anaerobic methods of biomass treatment, the thermal conversion leaves no contaminated residue after the process. The product gas evolved within thermal conversion can be used in several applications such as: fuel for gas turbines, combustion engines and fuel cells, and raw material for production of chemicals and synthetic liquid fuels.

This thesis treats a part of the experimental data from two different gasifiers: a 90 kWth pressurised fluidised bubbling bed gasifier at Lund University and a 18 MWth circulating fluidised bed gasifier integrated with gas turbine (IGCC) in Värnamo.

A series of parallel and consecutive chemical reactions is involved in thermal gasification, giving origin to formation of a variety of products. These products can be classified within three major groups: gases, tars and oils, and char. The proportion of these categories of species in the final product is a matter of the gasifier design and the process parameters. The thesis addresses the technical and theoretical aspects of the biomass thermochemical conversion and presents a new approach in describing the gasification reactions.

There is an evidence of fuel effect on the characteristics of the final products: a mixture of plastic waste (polyethylene) and biomass results in higher concentration of linear hydrocarbons in the gas than gasification of pure biomass. Mixing the biomass with textile waste (containing aromatic structure) results in a high degree of formation of aromatic compounds and light tars.

Three topic questions within biomass gasification, namely: tars, NOx and alkali are discussed in the thesis. The experimental results show that gasification at high ER or high temperature decreases the total amount of the tars and simultaneously reduces the contents of the oxygenated and alkyl-substituted poly-aromatics in the product gas. There is an indication that the tars are the products of the stepwise destruction of the primary structure of the biomass. Increased temperature favours dissociation of the heavy tar compounds to lighter structures.

During gasification a part of the fuel-bound nitrogen (fb-N) converts to ammonia which forms NOx in the following combustion steps of the product gas. The degree of conversion to ammonia is dependent on the process parameters and generally increases with increasing ER and temperature until a total carbon conversion is achieved. The mechanisms of the release of the fb-N and also the routes to minimise the ammonia in the product gas are discussed.

In a gasification plant alkali metals can be the reason beyond problems such as agglomeration of the bed material, deposit formation on cold surfaces and erosion and corrosion of the ceramic and metallic parts. The experimental results show that the type of alkali from the fuel has a crucial importance in causing the alkali-related problems.