Thermodynamic analyses of synthetic natural gas production via municipal solid waste gasification, high-temperature water electrolysis and methanation

The production of synthetic natural gas (SNG) from syngas derived through waste gasification is an attractive option for converting waste into useful fuel. However, the carbon conversion ratio of traditional SNG production is usually below 50% due to the limited amount of H₂, while the excess CO₂ needs to be removed. To realize a higher utilization of carbon, a novel SNG production system combining a municipal solid waste (MSW) gasifier, a solid oxide electrolyzer cell (SOEC) and a methanation unit is proposed and analyzed by thermodynamic modeling using ASPEN Plus®. Comparison is made with a similar system without the SOEC unit. The SOEC unit is used to electrolyze water into H₂ required for higher CH₄ production, thus eliminating the need of CO₂ removal. Theoretical calculations show that the integration between SOEC with the gasifier produces H₂-rich (greater than18 vol% H₂) SNG without CO or CO₂, allowing direct use of existing town-gas pipelines for easy transportation. With an equivalence air ratio of 0.3 for the gasification process, the carbon conversion ratio can achieve 98% with an efficiency of 67% when the ratio of H₂O supplied to the SOEC unit to the carbon in the MSW reaches 2.75. In comparison, the system without SOEC unit can only achieve a carbon conversion ratio of 44% with an efficiency of 78%. Parameter selection of the proposed system in practical implementation is discussed considering the trade-off between the carbon conversion ratio and the efficiency. At the end, preliminary economic analysis also shows that the proposed system is financially attractive.