Integrated Pressurized Gasification of Biomass for Small Gas Turbines

Nowadays small and micro gas turbines are increasingly used for on-site cogeneration. Although this means a dramatic increase in energy efficiency, liquid or gaseous fossil fuels are used. At the same time, there is interest in using solid biomass for electricity production in a distributed generation scheme. Unfortunately, conventional gas turbines can not burn solid biomass. This paper presents a thermodynamic simulation of a pressurized gasification system integrated into a gas turbine Brayton cycle, in order to study the feasibility of using biomass. The critical process of burning the resulting low-calorific-value gas is experimentally scrutinized for feasibility using a LOWNOX-LPP combustor and a synthetic gas. The results are promising. The thermochemical processing of biomass gasification using air produces a fuel barely suitable for gas turbines. Biomass gasification using air with no cooling provides outlet temperatures in the range from 700 K to 900 K and heat values of around 6 MJ/kg, being the tar content low and in a gaseous state, especially with down-draft moving bed gasifiers. Fuel gas cleaning is possible in a cyclone particle separator without previous cooling. This system requires a special design to bear the high temperature and to separate the ash particles down to turbine tolerance, without causing too much pressure loss and clogging. Additional hot filtering using porous media is also feasible. Two benefits arise due to high temperature cleaning. Firstly, tars do not condense but burn in the combustor increasing the biogas calorific value. Secondly, the thermal enthalpy from gasification is recovered. Therefore, the whole biomass primary energy is injected in the working gases, except heat losses. On the other hand, the gasification reactor needs to work pressurized (at a slightly higher pressure than the combustor pressure) thus requiring an additional power to overcome the head loss caused by the biomass bed and the cyclone particle separator. Thermodynamic analysis shows that under favourable conditions, a fall of around 1% of the overall efficiency appears. The heat value of gas and fuel-to-air ratio of the mixture affect flame stability inside the combustor. Nevertheless, with a swirl stabilized flame it has been demonstrated that it is possible to obtain stable flames down to 6 MJ/kg heating value with low CO and NO_x emissions. For these preliminary experiments synthetic gases representing gasifiers output have been used. Using the integrated scheme here studied for feasibility, off-the-shelf gas turbines could be transformed into biomass burning micro-cogenerators, thus contributing to greenhouse gases emission reduction.