(440f) Simulation, Heat Integration and Rectisol-Based Decarbonisation for the Production of Synthetic Natural Gas from Biomass Gasification and Landfill Gases

The outline of this research is to develop a systematic heat-integrated gas purification process, followed by methanation for the production of synthetic natural gas or substitute natural gas (SNG). SNG production from biomass is currently considered as an alternative to reduce dependency on high-priced natural gas import and an opportunity to reduce greenhouse gases by applying CO₂ Capture and Sequestration (CCS) technologies. Biomass is thermally converted (e.g. gasification) to produce CO₂, CO, H₂O and H₂ gases that are cleaned to remove CO₂ before being converted to SNG (methane) via a methanation process.

The focus of this project is to develop a comprehensive simulation framework of heat-integrated decarbonized process suitable for the removal of contaminants from biomass-based produced gases to meet SNG production specifications.

In this study, a detailed heat-integrated steady-state simulation framework for biomass thermal conversion, water gas shift (WGS) reaction, gas purification by Rectisol process and SNG production via methanation process has been developed. The main objective was to explore heat recovery opportunities, cleaner operation by reducing contaminants and optimised selectivity with minimal cost. The simulation framework was constructed using Aspen HYSYS (V.2006) process simulator, which is capable to predict the optimised operating conditions of gasification, methanation and Rectisol process at which containments and solvent loss are minimized. Optimization software (SPRINT) is used for data extraction and analysis of heat integration opportunities across the entire process model.

In this project, a biomass (straw) was initially pyrolyzed to mainly produce a gas stream consisting of CO₂, CO, H₂O and H₂ along with tar and char. These streams were then mixed with landfill gases that comprise of methane (CH₄) (40-60%), CO₂ and H₂S as major components that also considered as major greenhouse gases (GHG). Furthermore, these gases are contaminated with sulphur, nitrogen, oxygen, siloxanes, water, halogenated compounds and many other chemicals which require complex gas clean-up process. This mixture was gasified to produce a gas stream consisting mainly of CO₂, CO, H₂O, H₂ and H₂S. The gas stream was then fed to a water gas shift reactor to increase H₂/CO ratio to 3. This gas mixture was cleaned up by a Rectisol process to remove CO₂, H₂S and other containments. The cleaned gas (CO, H₂O, H₂) is finally fed to a methanation process to convert it virtually to pure methane (CH₄), which is the desired final product (i.e. SNG).

The Rectisol process uses refrigerated methanol as a solvent, where 99% removal of CO₂ and H₂S that are particularly present in landfill gases and product gas from biomass gasification was achieved. Two-level refrigeration cycle was developed to maintain the operating temperature of the overall gas purification process. The methanation route was selected for the production of SNG. Methanation is an exothermic reaction and hence, a comprehensive sensitivity analysis was carried out to optimize the process by effective removal of heat of reaction and to achieve maximum methane (CH₄) formation. This analysis showed that four reactors were used to achieve maximum conversion CO to CH₄ which corresponded to CO conversion of 97.8%. After an in-depth heat integration study of the overall process model, 2.5 MW of steam at 320 °C was generated and 0.45 MW process heat recovery was achieved, which resulted in energy savings.

In summary, this project establishes process simulation framework and heat-integrated design of biomass gasification, gas purification and methanation technology. Extensive heat integration strategies have been established based on the following criteria; maximum heat recovery, cleaner and economical production of SNG. The research focused on determining optimal operating conditions for WGS reactor to maintain H₂/CO ratio of 3, for Rectisol Process to maintain 99% removal of CO₂ and H₂S and for methanation process to maximise the conversion rate to SNG. The following conclusions have been drawn from this research:

1. The overall efficiency of SNG production depends significantly on the energy content of the biomass feedstock.
2. Optimizing the refrigeration system for the Rectisol Process plays a vital role in minimizing the total energy requirement of the whole process.
3. Environmental and sustainability factors should always be taken into account prior to selecting the energy conversion process.
4. To meet the SNG specification regarding composition, relative density, calorific value and Wobbe Index; the methanation process must be optimised for maximum conversion of CO to CH₄.
5. Systematic heat integration of the overall process is essential to reduce the utility requirements and provides opportunity for heat and power generation.
6. The cost of electricity from steam generation and cost of carbon capture were estimated to be a function of biomass feedstock cost. It is concluded from this study that biomass feedstock cost is absolutely critical in determining the overall operating cost.