Selected pathways for a more efficient conversion of biomass in cogeneration or fuel production

Among renewable energy sources biomass plays a major role. Several paths for improving the conversion of biomass with minimum burden on the environment are being pursued worldwide. This paper reviews some of the most recent results from research at the Swiss Federal Institute of Technology of Lausanne. On the theoretical and experimental sides it covers the improvement of biogas combustion engines using combustion prechambers both with spark ignition or autoignition in order to reduce the emissions while keeping a high efficiency level, the integration of ORC heat recovery cycles on biogas cogeneration engines and tests with real biogas in Solid Oxide planar Fuel Cells. On the process modelling side the paper reviews the environomic (environment, economic and energetic) optimisation of the design of biofuel production by means of gasification and fuel reforming. Thermo-economic process modelling and integration techniques are coupled with a multi-objective optimisation algorithm to target the best process technology and operating conditions for the trigeneration of fuels, heat and power.

Thermo-economic optimization of a Solid Oxide Fuel Cell - Gas Turbine system fuelled with gasified lignocellulosic biomass

Priscilla Caliandro, François Maréchal, Laurence Tock, Adriano Viana Ensinas

Within the context of sustainable energy supply and CO2 emissions reduction a Solid Oxide Fuel Cell (SOFC) - Gas Turbine hybrid system, fuelled with gasified woody biomass for medium scale applications (8MWth,BM of dry biomass) is studied in detail. The system consists of an air dryer moisture removal unit, a gasifier, a hot cleaning section made of a particulate removal unit (cyclone and candle filter) and a two-stage TAR removal unit, a SOFC and a gas turbine with optional CO2 capture. This modern technology has the advantage of using a renewable and CO2-neutral source and to be economically competitive at medium scales. The competitiveness of different process options is systematically compared by applying a coherent approach combining flowsheeting, energy integration and economic evaluation in a multi-objective optimization framework. This analysis reveals the importance of process integration maximising the heat recovery and valorising the waste heat, by cogeneration for example. The studied process options include indirect circulating fluidized bed (using steam or oxygen as gasification agent) and Viking gasifier, atmospheric or pressurized systems and optional pre-reforming in the hot gas cleaning. To close the thermal energy balance, a fraction of the produced syngas can be burnt. The energy integration results reveal that the steam production for the gasification and reforming are key parameters (S/B and S/C ratio) defining the process performance. A multi-objective optimization maximizing the efficiency and minimizing the capital investment costs is performed with respect to the operating conditions and the process configuration in order to assess the trade-offs and identify optimal process designs. The analysis shows the potential of the system converting woody biomass into electricity with energy efficiency greater than 70%.

2013

Thermo-chemical H2 production: Thermo-economic modeling and process integration

François Maréchal, Laurence Tock

Within the global challenge of climate change and energy security, hydrogen is considered as a promising decarbonized energy vector to be used in electricity production and transportation. In this paper, the thermo-chemical production of hydrogen by natural gas reforming and by lignocellulosic biomass gasification are analyzed, compared and optimized by developing thermo-economic models. Combining flowsheeting with process integration techniques, thermo-economic analysis and life cycle assessment (LCA), a systematic comparison of different process options with regard to energy, economic and environmental considerations is made. The choice of the technologies is optimized together with the operating conditions using multi-objective optimization. In both natural gas and biomass based H2 pathways, a CO2 removal step is included during the H2 purification which allows for CO2 capture and further sequestration. The potential for greenhouse gas mitigation is assessed and compared with conventional plants without capture based on the CO2 avoidance cost and the overall CO2 equivalent emissions computed from the life cycle chain. The system’s performance is improved by introducing process integration valorizing the waste heat by the combined production of heat and power. The H2 application purpose and the corresponding required purity are key factors defining the process performance. The trade-offs between competing thermoenvironomic (i.e. energy, economic and environmental) objectives are finally assessed using a multi-objective optimization. For natural gas based H2 production overall energy efficiencies up to 80% and production cost of 22-110 USD/MWH2 are computed compared to around 60% efficiency and 75-263 USD/MWhH2 for biomass based processes having the advantage of using renewable resources. The CO2eq emissions are reduced by more than 6.4kgCO2eq/kgH2 for NG and 20kgCO2eq/kgH2 for BM processes compared to the cases without CO2 capture. The competitiveness on the energy market depends strongly on the resource price and on the imposed CO2 taxes. Our study shows that the thermo-chemical hydrogen production has to be analyzed as a polygeneration unit producing not only hydrogen but also captured CO2 and electricity.

2011

H2 processes with CO2 mitigation: Thermo-economic modeling and process integration

François Maréchal, Laurence Tock

Within the challenge of greenhouse gas reduction, hydrogen is regarded as a promising decarbonized energy vector. The hydrogen production by natural gas reforming and lignocellulosic biomass gasification are systematically analyzed by developing thermo-economic models. Taking into account thermodynamic, economic and environmental factors, process options with CO2 mitigation are compared and optimized by combining flowsheeting with process integration, economic analysis and life cycle assessment in a multiobjective optimization framework. The systems performance is improved by introducing process integration maximizing the heat recovery and valorizing the waste heat. Energy efficiencies up to 80% and production costs of 12.5-42USD per GJH2 are computed for natural gas H2 processes compared to 60% and 29-61 USD per GJH2 for biomass processes. Compared to processes without CO2 mitigation, the CO2 avoidance costs are in the range of 14-306USD/tCO2,avoided.The study shows that the thermo-chemical H2 production has to be analyzed as a polygeneration unit producing hydrogen, captured CO2, heat and electricity.

2012

Thermo-economic optimization of a Solid Oxide Fuel Cell - Gas Turbine system fuelled with gasified lignocellulosic biomass

Priscilla Caliandro, François Maréchal, Laurence Tock, Adriano Viana Ensinas

2013

Thermo-chemical H2 production: Thermo-economic modeling and process integration

François Maréchal, Laurence Tock

2011