Thermodynamic analysis of the biomass-to-synthetic natural gas using chemical looping technology with CaO sorbent

This study evaluates the biomass-to-synthetic natural gas (SNG) using calcium looping gasification (CLG) with CaO sorbent (CLG-SNG) via thermochemical methods. The CLG-SNG process consists of three steps in sequence: steam gasification in situ CO₂ capture using CaO sorbents, gas cleaning, and methanation. The concept of interconnected fluidized beds was adopted for repeated carbonation/calcination cycles of CaO sorbents in the gasification unit. A process simulation was conducted based on the chemical equilibrium method using Aspen Plus. Then, the effects of some key variables on the thermodynamic performances, such as the gas composition, yield of SNG (Y _SNG), cold gas efficiency (η_cold), the overall energy efficiency (η), exergy efficiency (ψ) of the process, and the unit power consumption (W_SNG) were investigated. The variables include CaO-to-biomass ratio (Ca/B) in the range of 0.7-1, steam-to-biomass ratio (S/B) in the range of 0.1-1.5, and gasification temperature (t_G) in the range of 600-700 C. At Ca/B = 0.83, i.e., a stoichiometric number of SN = 1, the CH₄ content in SNG and W_SNG each reach the maximum while the Y_SNG reaches the minimum. With S/B increasing from 0.1 to 1.5, CH₄ content in SNG gradually decreases while W_SNG shows an increasing tendency. Y_SNG, η_cold, η, and ψ reach the maximum at S/B = 0.6 (i.e., when the gasifier reach the heat equilibrium). Generally, lower t_G values are favorable for the thermodynamic performances (mainly Y_SNG and η_cold) of the CLG-SNG process. The optimal performances demonstrate that the CLG-SNG process has a strong competitiveness, compared to the traditional SNG production process.