Process Integration Opportunities for Synthetic Natural Gas (SNG) Production by Thermal Gasification of Biomass

Licentiate thesis, 2010

Synthetic natural gas (SNG) from gasified biomass is one promising option to produce renewable transport fuels. This thesis presents a process integration study investigating thermal gasification of biomass for the production of SNG and identifies critical conversion steps for the overall process performance. A base case process consisting of an indirect gasification unit followed by tar reforming, aminebased CO2 separation, isothermal methanation and, finally, compression, H2-purification by membrane separation and gas drying is presented. Based on the lower heating value (LHV) of the wet fuel feedstock, the estimated conversion efficiency from biomass to SNG is 69.4 %. The process mass and energy balances are obtained by using flow-sheeting software and are analysed by using pinch methodology. The integration studies performed highlight the significant potential for improvement of the overall process performance offered by integrated feedstock drying. In particular, steam drying and low-temperature air drying – using available process excess heat – are shown to influence the process performance favourably. The integration of SNG production with existing combined heat and power (CHP) steam power plants is proven to be a promising option to efficiently convert excess heat of the SNG process to electricity. The process integration study performed shows that an increased level of thermal integration leads to an increase in electricity production attributed to the SNG process (100 MWLHV dry fuel input) from 2 to 4.9 MW when using steam drying for feedstock drying, and from 0.5 to 5.6 MW for air drying, without any negative effects on SNG yield. Alternative integration opportunities for biomass gasification not aiming at SNG production specifically, but at replacing fossil fuels for power production, are also highlighted. Biomass gasification integrated to a fossil natural gas combined cycle plant results in high biomass-specific electrical efficiencies of up to 49.6 %.