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The influence of tar composition and concentration on fouling, emission and efficiency of micro and small scale gas turbines by combustion of biomass derived low calorific valued gas (TARGET)


It is a declared goal of the European Union, to increase the share of biomass for power production. Gasification of biomass and combustion in an gas engine or turbine is the most efficient way of power production. However, for gas turbines tars in the lcv gas are a big problem and can result in fouling, increased emissions and failures during operation. In order to avoid this problems, intensive gas cleaning is up to now recommended because limits and effects in gas turbine combustion are not known. This makes biomass gasification less cost competitive then combustion systems. The focus of this project is on the effect of tars on fouling and emissions of turbines and micro-turbines and especially on interactions between gasifier, gas cleaning and (micro) turbine. Only by an integrated approach it will be possible to provide customised and more cost competitive biomass based IGCC systems.
Reliable tar analysis is necessary to study the influence of tars on fouling/emissions and interactions between gasifier, gas cleaning and (micro) turbine. This work was performed by IVD and KTH. They improved their methods, being the enhanced on-line overall tar analyser and the solid phase adsorption method for easy tar sampling and species analysis. Both methods were tested in different gasification rigs, including pressurised fluidised bed and fixed bed downdraft reactor. KTH furthermore investigated catalytic cracking of tars on cheap iron catalyst; the results were promising as compared to the use of dolomite. Compression of (tarry) biomass derived LCV gas was studied and a selection of a compressor was made: a screw compressor. The compressor was installed and tested in an integrated setup of atmospheric fixed bed, cyclones, scrubber and sawdust filter. Problems arose with increased tar amounts in the oil of the compressor. This led to an improved comparatively cheap gas cleaning component. Two new design LCV gas combustors for application in small and microturbines were developed, delivered and tested with LCV gas, also including model tar species (naphthalene). Somewhat increased CO levels were measured. CFD modelling (both 2D and 3D) was performed for both combustors.

The results of modelling and experiments were compared and qualitatively good agreement regarding main species and CO emissions were obtained, although validation on velocity and temperature profiles should be performed. Also further model development for soot is to be done. It was intended to apply a microturbine for testing on (tarry)LCV gas. Due to the unavailability of this equipment and budget limititations, this testing could not be accomplished, though task parts were shifted and taken over (combustor testing with tar injection). Also, the influence of tars was studied during operation of an advanced small-scale power cycle consisting of a pressurised fluidised bed gasifier, high temperature gas cleaning and gas combustion. The influence of process conditions like pressure, temperature and air stoichiometry on tar formation and gas-cleaning performance was studied. Regarding high temperature gas cleaning, high filtration efficiencies were attained of >99.67%, resulting in solids loads of the gas of the order of 1-10 mg/mn3. It was found that steam addition to the gasifier improved the filtration behaviour, probably due to avoidance of blinding (cracked tar products/soot). Regarding the combustor emissions, low CO values were attained (below EU emission standards). NOx emissions depended strongly on the applied fuel: the N content of these fuels. Furthermore, tests with an integrated installation of a 1MW fixed bed gasifier, wet gas cleaning were performed with the following main activities, lead by HoSt: - Optimisation of the existing low temperature gas cleaning intended for operation of a downstream advanced micro turbine. - Experiments based on the new configuration with the gas turbine.

The focus of the research program was directed towards the first subject because 1) the gas turbine did not become available in time and 2) because of the modifications necessary regarding the gas cleaning as a result of the first gas compressor tests. The budget available for the workpackage was therefore mainly spent on optimisation of the gas cleaning. Before the project started, the following configuration was available for compressor tests: - Downdraft atmospheric gasifier - Cyclones (2) - Water scrubber - Saw dust filter - Booster In the cyclones dust is removed from hot fuel gas which enters the cyclones with a temperature of 500 C. Behind the cyclones the gas flows through a water scrubber. In the scrubber the gas is cooled down to a temperature of +/- 30 C and saturated. This unit removes a large part of the tars as well as contaminants like HCl and NH3. After scrubbing the saturated gas is led through a saw dust filter. In the filter remaining tars -in the form of aerosols- are removed. During compressor tests it was shown that the tar content behind the gas cleaning was too high for the gas compressor to operate stably over a long time frame without frequent, costly oil refreshment. Also a very positive conclusion could be drawn: the screw compressor oil succeeded in removal of almost 95% of the tars. Calculations with the tar condensation model showed that removal of this tar amount would avoid condensation during gas compression. Based on these promising results HoSt decided to develop the oil scrubber so as to be able to compress the gas and fire it in an advanced micro gas turbine. Device testing proved the capability to remove 95% of tars from the gas. Finally, the results of the project were summarized in an applicable form for industry, with the composition of the consortium ensuring future implementation of the results.

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