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

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.

Thermo-environomic evaluation of the ammonia production

François Maréchal, Matthieu Perrenoud, Laurence Tock

Within the global challenge of sustainable energy supply and greenhouse gas emissions mitigation, carbon capture and storage and the deployment of renewable resources are considered as promising solutions. In this study the production of ammonia mainly used in the fertilizer industry and responsible for around 2-3% of the world greenhouse gas emissions is analyzed. Considering natural gas and biomass as a resource and the option of CO2 capture and storage, different process configurations are systematically compared with regard to energy, economic and environmental considerations. A consistent thermo-environonomic optimisation approach combining flowsheeting, process integration techniques, economic performance evaluation, life cycle assessment and multi-objective optimisation is applied for the conceptual process design and competitiveness evaluation. It is highlighted that the quality of the process integration is a key factor for improving the performance by valorizing the heat excess through electricity cogeneration. Including CO2 mitigation in the ammonia production allows to reduce the emissions, but leads to a slight efficiency decrease due to the energy consumption for the CO2 compression. For the natural gas fed process yielding an energy efficiency around 65%, the overall life cycle emissions can be reduced to 0.79kgCO2/kgNH3 with CO2 capture compared to 1.6kgCO2/kgNH3 without capture. Considering the biogenic nature of the carbon in the biomass, the emissions drop to -1.79kgCO2/kgNH3 for the biomass process having an energy efficiency of 50%. The economic competitiveness highly depends on the resource price and the introduction of a carbon tax. This study reveals the potential of the decarbonisation of the fertilizer industry. This article is protected by copyright. All rights reserved

Wiley-Blackwell2015

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

The role of process integration techniques in the thermo-economic optimization of energy conversion systems

François Maréchal, Laurence Tock

Within the goal of reducing greenhouse gas emissions and supplying sustainable energy, complex integrated energy conversion systems have to be designed by taking into account thermodynamic, economic and environmental considerations simultaneously. The process performance highly depends on the quality of the process integration and on the energy conversion, in particular, on how the heat requirements are balanced and how the combined heat and power generation is integrated. In chemical processes, the heat requirements after heat recovery are typically satisfied by the energy conversion system, while for fuel and electricity producing processes, the requirements are balanced by waste and process streams available on-site. The optimal internal heat recovery and utility integration can be computed by applying heat cascade models that are solved by using mathematical programming techniques. The mass and energy balances of the process unit operations are therefore satisfied by the optimal energy conversion system integration without having to explicitly model the heat exchanger network. The presented thermo-economic optimization approach combines flowsheeting and process integration techniques with economic evaluation and life-cycle assessment in a multi-objective optimization framework. The advantage of including the process integration model in the design process is that the influence of the process design and operation is reflected on the thermo-environomic performance of an energy balanced system in which the heat supplied from energy resources is integrated. The methodology has been successfully applied to study CO2 capture concepts in power plants using natural gas, coal or biomass resources, to analyse thermo-chemical H2 production processes and to evaluate different biofuel processes producing Fischer-Tropsch fuel, methanol, dimethylether and H2 by biomass gasification. The potential performance improvement through process integration is revealed for each case. In particular, it is highlighted how the optimization of the combined cycle valorizing the excess heat and of heat pumping options improve the process efficiency by simultaneously maximizing the combined production of fuel, captured CO2, heat and power. This shows that this computer-aided process design strategy combining flowsheeting, energy integration, cost assessment and multi-objective optimization is a powerful tool to systematically generate different process options, assess trade-offs and reveal process improvements through process integration.

2013

The role of process integration techniques in the thermo-economic optimization of energy conversion systems

François Maréchal, Laurence Tock

2013