The conversion of biomass to synthetic natural gas (SNG) draws great interest in the world because it is a sustainable energy resource, where it can replace the fossil natural gas and reduce environmental problems. Common technologies for CH4 production are based on the gasification of biomass at high temperature followed by CO and CO2 methanation, but it is energetically costly and complex, requiring separate reforming stages due to the heavy tar production from the gasification process, and multiple cooling stages of the methanation due to the large exothermicity of this equilibrium driven reaction.
Therefore, the main focus of this research was to attempt to address these issues by introducing the low temperature steam reforming (LTSR) process of bio-oil for CH4 production via fast pyrolysis of biomass using palm empty fruit bunch (PEFB) as the biomass feedstock, a significant renewable waste of the palm oil industry, currently underexploited. One advantage of proposing the pyrolysis route vs. gasification, was the conversion of PEFB into bio-oil without generation of heavy tars, and at lower temperature than gasification due to the lower endothermicity of the chemical process favouring oil product rather than gas. Another advantage was the lower exothermicity of the subsequent methanation step by using bio-oil as feed rather than CO and CO2. It was intended that bringing closer the enthalpy changes of the gasification by pyrolysis and of the methanation by feedstock substitution, would improve the efficiencies of heat transfers between the two.
Chemical Equilibrium and Applications (CEA) program was used to analyse thermodynamic equilibrium for conversion PEFB bio-oil to CH4 using LTSR process. It was found that CH4 production was favoured in the 130–330 ºC range and at around molar steam to carbon ratio of 3 at atmospheric pressure. Using the optimum conditions observed from the thermodynamic equilibrium calculations, the experimental feasibility of CH4 production from acetic acid as single compound bio-oil surrogate via LTSR was performed at bench scale by using nickel-calcium aluminate (Ni/Ca-Al2O3) catalyst in a packed bed reactor. The optimum conditions for CH4 production were obtained at 400 ºC and S/C of 2 with 15.7 wt.% at atmospheric pressure. As undesirable carbon formation on the catalyst was observed during the experiments, it is suggested to operate at higher pressure (20–30 bar), which is commonly used in the CO and CO2 methanation industrial processes. Based on the Aspen Plus simulation results for the full flow biorefinery of CH4 production from PEFB via fast pyrolysis followed by LTSR of the bio-oil, the estimated thermal efficiencies were 74.3% (net power and heat demand not included in the process) and 81.1% (net power and heat demand included in the process) were comparable to the current biomass gasification technology to CH4 production via syngas followed by CO and CO2 methanation.
