High efficiency power production from biomass and waste

Two-stage gasification allows power production from biomass and waste with high efficiency. The process involves pyrolysis at about 550°C followed by heating of the pyrolysis gas to about 1300°C in order to crack hydrocarbons and obtain syngas, a mixture of H2, CO, H2O and CO2. The second stage produces soot as unwanted by-product. Experimental results are reported on the suppression of soot formation in the second stage for two different fuels: beech wood pellets and Rofire pellets, made from rejects of paper recycling. Syngas obtained from these two fuels and from an industrial waste fuel has been cleaned and fed to a commercial SOFC stack for 250 hours in total. The SOFC stack showed comparable performance on real and synthetic syngas and no signs of accelerated degradation in performance over these tests. The experimental results have been used for the design and analysis of a future 25 MWth demonstration plant. As an alternative, a 2.6 MWth system was considered which uses the Green MoDem approach to convert waste fuel into bio-oil and syngas. The 25 MWth system can reach high efficiency only if char produced in the pyrolysis step is converted into additional syngas by steam gasification, and if SOFC off-gas and system waste heat are used in a steam bottoming cycle for additional power production. A net electrical efficiency of 38% is predicted. In addition, heat can be delivered with 37% efficiency. The 2.6 MWth system with only a dual fuel engine to burn bio-oil and syngas promises nearly 40% electrical efficiency plus 41% efficiency for heat production. If syngas is fed to an SOFC system and off-gas and bio-oil to a dual fuel engine, the electrical efficiency can rise to 45%. However, the efficiency for heat production drops to 15%, as waste heat from the SOFC system cannot be used effectively. The economic analysis makes clear that at -20 €/tonne fuel, 70 €/MWh for electricity and 7 €/GJ for heat the 25 MWth system is not economically viable at the price level of 1600 €/kWe expected for SOFC systems in the near future and 200 €/kWth for two stage gasification and gas cleaning. In order to reach break-even, the electricity price would have to rise to 141 €/MWh. For break-even at 70 €/MWh, the price of both main parts would have to drop to e.g. 500 €/kWe for the SOFC and to 80 €/kWth for the gasification and gas cleaning. At the higher equipment price level, break-even may already be reached at an electricity price of 87 €/MWh if more favourable conditions apply, i.e. -100 €/tonne fuel, CO2 credits of 10 €/tonne, cheap labour and full use of the available heat. In that case, the simple pay-back time would become 9.3 years. In those favourable conditions, the simple pay-back time of the base case Green MoDem system with dual-fuel engine would be 4.9 years. Addition of an SOFC system would increase the simple pay-back time to 10.5 years.