Pressurized entrained flow gasification of pulverized biomass : Experimental characterization of process performance

Sammanfattning: The anthropogenic emissions of greenhouse gases (e.g. CO2), mostly connected to the use of fossil fuels, to the atmosphere have increased during the last century and there is significant evidence that this is the main reason for the recent global temperature rise. Urgent action is necessary to steer the energy systems on to a safer path by reducing the CO2 emissions to mitigate further climate changes. Sustainable production of CO2-neutral bio-based transportation fuels is one alternative to reduce the dependence on fossil fuels and to reduce the net CO2 emissions from road traffic.One of the goals in the national energy strategy of Sweden is that the vehicle fleet should be independent of fossil fuels by 2030. Pressurized, O2 blown, entrained flow gasification of forest residues followed by methanol production is one of the suggested routes for the production of synthetic motor fuels that could help reach this goal. One of the benefits with entrained flow gasification is that a syngas with high quality is generated. The high syngas quality is necessary for the subsequent synthesis to biofuels. However, there are still a number of hurdles to overcome before large-scale commercial application of this technology can be done. In this context, a pressurized O2 blown entrained flow biomass gasifier (PEBG) was built to demonstrate the ability to gasify pulverized biomass without extensive pretreatment other than drying and milling. This thesis focuses on the understanding of the dominating mechanisms during entrained flow gasification of solid biomass. Athorough process characterization was performed to study the effect of the most important process parameters, e.g. the O2 stoichiometric ratio (λ), the process pressure, the fuel load and the fuel particle size distribution. In addition to this, experiments were performed with different types of wood residues. The wood residues were pretreated differently in order to study the effect of the fuel pretreatment (e.g. torrefaction) and/or the fuel composition. The process performance was measured in terms of syngas yield, the syngas composition and the gasification process efficiency. In some cases, special attention was placed on the syngas particulates that were examined with respect to particle composition and morphology in order to increase the understanding of the particle formation process.The results in this work showed that the maximum cold gas efficiencies CGEpower and CGEfuel from gasification of stem wood were 76 % (at λ=0.30) and 71 % (at λ=0.35), respectively. This work also showed the importance of minimizing the heat losses from the gasifier in order to achieve as high CGE as possible. It was therefore concluded that a well-insulated refractory lined gasifier is the preferred alternative regarding reactor design to maximize the CGE. To generate a high quality syngas, the gasifier should preferably be operated close to the maximum design load, with a λ aiming for process temperatures above 1400 °C in order to keep the levels of CH4 andother unwanted syngas species low. At this temperature, the experimental results suggest that a plug-flow residence time of 3 sec was sufficient to reduce the syngas concentrations of CH4 and C6H6 below 1 mol-% and 100 ppm (on a dry and N2 free basis), respectively. Finally, the syngas particulates from gasification of stem wood were composed mainly of soot with concentrically stacked graphitic layers. It was concluded that the soot yield and the soot particle size could be reduced if the gasifier was operated at high λ. Gasification of bark-based fuels with higher amount of ash indicated lower concentration of particulates in the syngas. Especially K and Zn seemed to affect the syngas particle morphology.