Modelling of Biomass Syngas Combustion with CFD

Abstract: Gas turbines integrated with biomass gasification in a combined cycle power plant (Bio-IGCC) provides a path to power production with very high efficiency. Over 60% fuel-to-power efficiency has been demonstrated with natural gas. The fast ramp and relatively low cost make Bio-IGCC via gas turbines the ideal complement to intermittent power from wind turbines and PV cells. With stricter pollutant emissions and in order to enable the use of renewable fuels there is a great interest in improving fuel flexibility. The drawback of syngas from biomass gasification is that its properties vary depending on the feedstock and gasification principle and they are significantly different from conventional fuels. The TECFLAM swirl burner geometry, which is designed to be representative of common gas turbine burners, was selected for an initial comparison between a typical hydrocarbon fuel and syngas. The advantage with this geometry is that detailed experimental measurements with methane are publicly available. A two-stage approach was employed with development and validation of an advanced CFD model. The validated model was used to compare the flame shape and other characteristics of the flow between methane, 40% hydrogen enriched methane and four typical syngas compositions. It was found that the syngas cases experience lower swirl intensity due to high axial velocities that weakens the inner recirculation zone. The syngas compositions used are representative of practical gasification processes and biomass feedstocks. The demonstrated strong correlation between laminar flame speed and flame shape using the CFM combustion model could be used as a rule of thumb to quickly judge whether the flame would be detrimental to the function of the combustion system. To speed up the parametric analysis needed and to test more configurations a Two-Step One Way coupled method was assessed. This is a common approximation in CFD that is used to solve complex problems with limited computational resources. The test case used for the assessment was the CeCOST burner that uses strong swirl for flame stabilization. Only isothermal flow was investigated to eliminate the influence from flow – chemistry interactions. This method effectively divides the domain in two parts, one downstream and one upstream. The assumption behind this method is the downstream part should not have a big influence on the upstream part and hence it could be solved separately. These parts are then only coupled in one way, with the downstream part being dependent on the upstream solution, where its inlet boundary conditions are extracted, but having no influence on it. From the comparison it was found that the full solution and the approximations were in good qualitative agreement. However, there were some minor quantitative discrepancies, and it was proposed that the explanation for the differences could be the slightly different solution approaches that were used for the full simulation (URANS) and the two approximate solutions (RANS). The speed-up from using the approximate method was close to one order of magnitude. Nonetheless, an artificial steady inlet could not reproduce all the dynamic phenomena created by the geometry of a swirler, hence, for the continuation a full CeCOST burner domain was used. LES modelling was also employed to be able to identify smaller structures that would affect flame stability. Using LES and the Artificially Thickened Flame model fine flame differences were identified between methane and a syngas composition that relates to Black Liquor gasification.