Power-to-Gas concepts integrated with syngas production through gasification of forest residues - Process modelling

Climate change and global warming caused by the emission of CO2 from fossil fuels utilization are one of the most challenging environmental threat mankind is facing nowadays. As motives to reduce CO2 emissions, the attentions to replace the fossil fuels with renewable energy sources such as biomass, sun and wind is high on agenda in the recent decades. However, these renewable resource suffer from some drawbacks: the intermittent nature of the electricity from the sun and the wind destabilize the electric grid and energy from biomass is not easily accessible to be used in the transport sector for example, which is the major CO2 emitter. By a thermo-chemical process called gasification, biomass can be reacted to produce syngas (mixture of CO2 CO, H2, CH4) which can be used to synthesize secondary bio-fuels such as biogas, methanol and Fischer-Tropsch. Through power-to-gas technology the intermittent electricity can be used in the gasification system. Gasification and Power-to-gas technology together can work in synergy enhancing the production of secondary biofuels, while increasing the integration of intermittent energy sources in the energy system and even stabilizing the electric grid. This project is concerned with the modeling of biomass gasification in a pressurized, oxygen-blown, fluidized bed gasifier and integration of power-to-gas technology in the gasification system. The model is based on experimental data available in a literature and developed by flowsheeting in ASPEN PLUS. The model includes processes such as biomass drying, biomass gasification, methanation of the syngas and the Sabatier process. Four different layouts of the Sabatier process are developed to investigate and compare the thermodynamic performance (energy and exergy efficiency), economic performance (operational profits) and operational flexibility of layouts when integrating power-to-gas concept in the gasification system. The layout upstream of the Sabatier reactor is identical for every scenario and it has a CH4 yield of 0, 24kgCH4/kgdrybiomass. An important aspects of the different layouts is in the CO2 removal unit position and utilization: the CO2 can be fed to the Sabatier reactor either mixed in the gas coming from the methanation unit, or pure. In the former case all the CO2 is injected but only the unreacted has to be separated at the end of the process, whereas in the latter almost all the CO2 is removed, but then only the desired amount is injected into the reactor. Energy and exergy efficiencyof the system is in the range of 0,55 - 0,8 and 0,35 - 0,4 respectively, while the operational revenues can peak 0, 22USD/kWhdrybiomass. Concerning the operational performances (economic, thermodynamic and flexibility) it was noticed that feeding the Sabatier reactor with the entire mix of gases coming from the methanation unit, and separating the unreacted CO2 afterwards was the most advantageous scenario, but it may lead to higher investment cost