Björn Kjellström

Alkali retention/separation during bagasse gasification: a comparison between a fluidised bed and a cyclone gasifier

Abstract Biomass fuelled integrated gasification/gas turbines (BIG/GTs) have been found to be one of the most promising technologies to maximise electricity output in the sugar industry. However, biomass fuels contain alkali metals (Na and K) which may be released during the gasification processes and cause deleterious effects on the downstream hardware (e.g. the blades of gas turbines). Much research has therefore been focused on different kinds of gas cleaning. Most of these projects are using a fluidised bed gasifier and includes extensive gas cleaning which leads to a high capital investment. Increasing alkali retention/separation during the gasification may lead to improved producer gas quality and reduced costs for gas cleaning. However, very little quantitative information is available about the actual potential of this effect. In the present work, comparative bench-scale tests of bagasse gasification were therefore run in an isothermal fluidised bed gasifier and in a cyclone gasifier to evaluate which gasification process is most attractive as regards alkali retention/separation, and to try to elucidate the mechanisms responsible for the retention. The alkali retention in the fluidised bed gasifier was found to be in the range of 12–4% whereas in the cyclone gasifier the alkali separation was found to be about 70%. No significant coating of the fluidised beds bed material particles could be observed. The SEM/EDS and the elemental maps of the bed material show that a non-sticky ash matrix consisting of mainly Si, Al and K were distributed in a solid form separated from the particles of bed material. This indicates the formation of a high temperature melting potassium containing silicate phase, which is continuously scavenged and lost from the bed through elutriation.

Development of a sugar cane residue feeding system for a cyclone gasifier.

Abstract For the smooth and continuous operation of a cyclone gasifier, the fuel must be fed continuously and without interruption or large fluctuations. A feeding system for bagasse/cane trash powder was therefore designed, built and tested. It consisted of a feeding bin with four feeder screws in the bottom which deliver the fuel to two downcomers from which the fuel is injected by steam into the gasifier. During the first tests, the low bulk density and cohesive characteristics of a crushed bagasse/cane trash powder were found to cause an accumulation of the fuel in the feeding system, creating difficulties for the flow into the gasifier. In addition, once the flow of the crushed bagasse/cane trash powder is interrupted by a build-up in the downcomer channels, the crushed bagasse/cane trash powder becomes progressively compacted into a dense structure, resulting in blockage of the discharge. It was found possible to eliminate this problem by changing the shape of the slivers of the crushed bagasse/cane trash powder to render them more homogeneous. This was achieved by pelletizing the crushed bagasse or cane trash before grinding it to powder.

Effects of Steam Injection and Stochiometry on Cyclone Gasification of Wood Powder

Atmospheric gasification of wood powder in a cyclone has been studied. A cyclone gasifier has been designed where the wood powder is injected into the cyclone with air or air/steam as transport media. The effects of stochiometry and steam injection on the gasification temperature and gas composition are investigated. The experimental results are compared with theoretical equilibrium calculations.The fuel flow has been 26 kg/h corresponding to a thermal input of 140 kW. Equivalence ratio was varied between 0.15 and 0.4. Wood powder has been injected with steam mass flows of 50–80% of the fuel flow. The gas from the cyclone gasifier was analysed regarding CO, H2, CH4 and CO2. The lower heating value of the gas varies between 4 and 6 MJ/Nm3 with lower values for steam injected gasification. Compared to theoretical calculations the air/steam injected measurements showed better agreement with equilibrium predictions than only air injection.Copyright

Alkali Separation in Steam Injected Cyclone Wood Powder Gasifier for Gas Turbine Application

Cyclone gasification of wood powder at atmospheric pressure has been studied. The cyclone gasifier works as a particle separator as well and the wood powder is injected into the cyclone with air or air/steam as transport medium. The effects of stochiometry and steam injection on the gasification temperature and separation of char particles are investigated. The experimental results are compared with theoretical predictions. The amount of Potassium (K) and Sodium (Na) that can be separated together with the char is also studied since this is of special interest if the gas is used to operate a gas turbine. The fuel flow has been 26 kg/h corresponding to a thermal input of 140 kW. The equivalence ratio was varied between 0.15 and 0.4. Wood powder has been injected with air only or air/steam with steam mass flows of 50–80% of the fuel flow. It has been possible to separate 20–40% of the potassium and 10–20% of the sodium supplied with the wood. The resulting alkali content in the gas entering a turbine at a temperature of 850°C is between 4 and 8 mg/kg gas.