Bio-SNG from Thermal Gasification - Process Synthesis, Integration and Performance

Doctoral thesis, 2013

Biomethane or synthetic natural gas (Bio-SNG) produced from gasified renewable woody biomass is a promising option for replacing fossil natural gas. The complete interchangeability with natural gas in all its conventional applications such as in the power generation, transportation and chemical industry sector is of particular interest. This work presents results from a comprehensive process integration study of different process alternatives for Bio-SNG production from gasified biomass. The influence of the main conversion steps in the process chain – drying, gasification, gas cleaning, methanation, and gas upgrade – on the overall process performance is investigated. Process bottlenecks and both heat and material integration opportunities are highlighted. Using future energy market scenarios the energetic, economic, and carbon footprint performance of the investigated processes are evaluated from a system perspective clearly showing the sensitivity of the obtained results to underlying assumptions. It is shown that drying of the biomass feedstock prior to gasification using excess process heat – using steam drying or low-temperature air drying technology – is an important aspect for improving the process energy efficiency. The results also indicate that indirect and direct gasification technologies perform equally well within the overall Bio-SNG production process. Existing infrastructure in the form of biomass-fired combined heat and power plants based on fluidised bed combustion technology presents interesting opportunities for integrating indirect gasification for Bio-SNG production, with beneficial effects on the cogeneration of electricity from the Bio-SNG process excess heat. The choice of methanation technology between fixed and fluidised bed is not a critical one with respect to process integration, since both technologies allow for efficient heat recovery and consequent cogeneration. For gas upgrade, in particular removal of CO2 from the product gas, amine based separation is shown to achieve better energy efficiency and economic performance than membrane based or pressure swing adsorption processes. Preliminary estimations of Bio-SNG costs are significantly higher than current natural gas prices, thus dedicated and long term policy measures are necessary in order to stimulate Bio-SNG production. The process integration aspects presented in this thesis can contribute to reducing production costs by increasing energy efficiency and in consequence increasing economic robustness of Bio-SNG process concepts.