Large-Scale Production and Use of Biomethane

Doktorsavhandling, 2016

Societal ambitions to create an economy based on renewable resources, require the development of technologies transforming these resources into energy-carrying products and biomaterials. Dual fluidized bed (DFB) gasification represents a key technology for achieving sustainability targets, as it is a scalable and highly efficient route for the conversion of biomass. The development of DFB technology has led to the construction of the GoBiGas (Gothenburg-Biomass-Gasification) demonstration plant, in 2014. The GoBiGas plant is a world-first advancement for large-scale production of biofuels as it represents a substantial scaling up of the gasification technology combined with downstream biomethane synthesis. However, to ensure the desired breakthrough of biomass-based products, it is necessary to improve the profitability of gasification plants, through increasing their size, efficiency and identifying opportunities with high economic feasibility for the transport, energy, and chemical sectors. This thesis presents an exploration of potential improvements for the up-scaling of the biomethane process to a commercial scale. The work summarises and places in context the experience acquired in the research groups at Chalmers and Göteborg Energi AB, including the experience gained from the dedicated experiments in the Chalmers Gasifier and during the commissioning phase of the GoBiGas plant. A method for analysis of the experimental data is introduced, with the goal of improving the quality of the simulations of large-scale gasification processes. The method is applied to the evaluation of the DFB gasifier at the GoBiGas plant, which is presented in the thesis and used as references for further investigations. Some of the measures investigated to increase the profitability of a large-scale plant were proposed in this work, including: an advanced biomass steam dryer integrated with the gasifier, power-to-gas conversion via direct heating of the DFB gasifier and co-production of biomethane with intermediate products for other chemical industries. Furthermore, the utilization of biomethane as fuel for heavy duty vehicles was evaluated within a project in collaboration with Volvo AB. The well-to-wheel approach was applied to calculate the emissions related to three state-of-the-technologies: spark-ignited, dual fuel and high-pressure direct injection. The evaluation of the DFB gasifier at GoBiGas has shown high fuel conversion, with char gasification of ~54%, and the fraction of the volatiles converted to methane of ~34%mass. The cold gas efficiency for GoBiGas was calculate in 71.7%LHVdaf using dried biomass (8% moist). The simulation of the DFB gasifier in a large-scale optimised process showed a cold gas efficiency up to ~85%LHVdaf using fresh biomass (40% moist) and an advanced drying systems. The chemical efficiency of such a plant was calculated in ~72% LHVdaf, which is more than 20pp higher than the current GoBiGas design. Owning to the efficient conversion of the biomass in the gasifier, the co-production of biomethane and other intermediate chemicals represents a feasible opportunity to increase the profitability of the plant. The chemical efficiency of such processes was estimated between 72% and 85% therefore, there is no substantial advantage to produce biomethane, unless biomethane is the desired end-product.  As fuel for heavy-duty vehicles, biomethane reduces the emissions compared to diesel by 30 - 41 gCO2e per MJbiomass, with the biomethane produced at the GoBiGas plant. The emission saving can be increased to 43 - 54 gCO2esaved/MJbiomass if biomethane is produced at large scale.  Following the demonstration at a commercial scale, biomethane is established as a biofuel with a high environmental impact, although the gap between the current status and its potential application is highlighted.