Concepts for the production of biomass derived fuel gases for gas turbine applications

Amongst the available alternatives of regenerative energy sources, biomass may play an important role in the future. Biomass represents stored solar energy, accessible whenever there is demand. But so far, its large potential has only been used to a small extent. This is partly due to the fact that solid biomass is a common fuel for the production of heat, but is hardly used in combined heat and power (CHP) plants. For the conversion of solid fuels, combustion in combination with a Rankine cycle is the only well established technology available today, thus acceptable net efficiencies (> 25% LHV) are only possible in complex, rather large scale plants. However, since the occurrence of biomass differs depending on region and season, and because of its low energy density and thus reduced transportability, biomass is particularly suited for decentralized applications, i.e. for the generation of heat and power (1) in the small power range. This paper discusses strategies for the integration of biomass gasification / pyrolysis and gas cleaning processes into different gas turbine schemes. Rather than reviewing all possible aspects of and concepts for the usage of biomass in gas turbine applications, this paper evaluates some key aspects (pressurization, tar removal, gas cleaning, choice of gas turbine cycle) of directly fired gas turbine concepts with regard to small scale applications. The paper shows that tar is a key problem. Especially for small scale applications solutions based on the combustion of tars, like pressurized biomass conversion in combination with hot gas cleaning, prove to be simple and efficient. This paper also compares the characteristics of integrated gasification cycles, e.g. by demonstrating difficulties connected with the integration of steam blown allothermal gasification (reforming). Furthermore, it is shown that recuperated gas turbines are well suited for highly efficient, yet simple small scale applications. Test results indicate that a mild apyrolysis is an alternative technology for the production of comparatively clean medium heating value gases. Integrating the use of pyrolysis gas and charcoal into a simple gas turbine plant, however, can not easily be achieved.