Indirect gasification production of biomethane for use in heavy-duty state-of-the-art gas engines

Licentiate thesis, 2015

The climate targets set for the European transport sector have stimulated intensive research by groups in academia, the energy industry, and vehicle manufacturing in the Gothenburg region into biomethane production via indirect gasification of lignocellulose biomass and the development of advanced gas engine technologies. This work presents the results of a comprehensive study of biomethane production and utilization in heavy duty engines. The different steps in the biomethane chain (biomass drying, gasification process, and combustion) are assessed, and opportunities for improving the efficiency of utilization of biomass resources are evaluated. The biomethane chain is investigated through a well-to-wheel (WtW) analysis of the newly built GoBiGas plant (Gothenburg, Sweden), in combination with three state-of-the-art gas engines technologies: spark-ignited (SI); dual fuel (DF); and high-pressure direct injection (HPDI). Opportunities for improving the biomethane process are focused on the drying system and on the dual fluidized bed gasifier. An advanced drying system for the dual fluidized bed gasifier, which uses low-temperature steam as the drying medium and recovers the evaporated moisture as a gasification agent, is evaluated. A method for simulating the process that occurs in the dual fluidized bed gasifier using experimental data is introduced, with the aim of exploiting the extensive body of information derived from pilot and demonstration gasifiers in relation to process optimization and techno-economic analyses. The uncertainty that arises from the measurements is assessed stochastically and transferred to process parameters. The WtW analysis shows that emissions from biomethane are reduced by 73%, 46%, and 68% when used in the SI, DF, and HPDI engines, respectively, as compared to using NG and LNG. The evaluation of the drying process reveals a theoretical energy efficiency of 95% when combined with a DFB gasifier and an exergy efficiency of 53%, values that are considerably higher than those obtained with other drying systems. Through interpolation ii and extrapolation of the experimental data, the proposed modeling method is demonstrated to be a flexible tool for simulating the gasifier under several operational conditions Comparisons of the data from different measurement set-ups demonstrate that a detection rate of ≥95% for the carbon in the produced gas is necessary to keep the uncertainty at <3% and to estimate the char conversion and oxygen transport rates in the gasifier. Overall, the results of this study indicate that the current biomethane chain achieves considerable reductions in emissions compared to the use of fossil fuels, and that there is significant potential for further improvements.