Exploratory experimental and theoretical studies of cyclone gasification of wood powder

Sammanfattning: This thesis describes an exploratory experimental and theoretical study of gasification of wood powder in a cyclone gasifier. The generated gas could be used to operate a gas turbine in a combined cyclone power plant. The objective has been to develop the understanding of cyclone gasification by experimental studies of the performance of a cyclone designed in principle as a separation cyclone and by comparisons between the experimental results and theoretical predictions. The experiments were carried out with commercial Swedish wood powder fuels, injected with air or steam/air mixture through two diametrically opposite tangential inlets and gasified at atmospheric pressure in cyclones of two different configurations with a volume of about 0.034 m3. The studies show that stable gasification of this fuel can be obtained for a specific fuel feeding rate of about 5 MW/m3 cyclone volume for equivalence ratios above 0.15 and that the equivalence ratio had to be kept below about 0.4 in order to avoid material temperatures above 950oC. A cyclone with a short outlet pipe, designed as a conventional separation cyclone was found to give lower char conversion than a modified cyclone with a long outlet pipe. The heating value of the gas was found to be approximately 4.5 MJ/kg. The dust load in the product gas was measured to between 1000 and 2500 mg/Nm3. It was possible to separate at least 40-60% of the potassium and 60-90% of the sodium supplied with the wood. The alkali that left the cyclone with the product gas appear to be in solid or melted phase in the unseparated char particles and consequently not vaporised during gasification. As the K and Na were assumed to remain within the particles during gasification, it was concluded that to reduce the amount of alkali metals in the product gas it would be necessary to improve the particle separation efficiency. The results of the theoretical modelling, using the existing models in the commercial software CFX 4.2, show with 3-dimensional modelling of the cyclone flow that RSM turbulence models give better agreement between predicted and observed velocity distributions than k-e models. Nevertheless, the latter give acceptable predictions as regards particle separation. Comparisons between predicted and experimental performance of cyclone gasifiers show that the heating value is over-predicted and the carbon conversion and alkali carry over are under-predicted. The results show that further development of the theoretical models is necessary.