(249b) Modeling of an Indirect Coal-Biomass to Liquids (CBTL) Plant with CO2 Capture and Storage (CCS)

Coal to liquids plants produce substantial amount of CO₂ in comparison to the petroleum –based fuel production processes. However, if moderate amount of biomass is co-fed with the coal, the life cycle CO₂ emissions can be substantially reduced¹. Further reduction in CO₂ emissions is possible by optimally designing these plants while ensuring that they remain cost-competitive with conventional petroleum-based processes. With this motivation, a high-fidelity model of an indirect coal-biomass to liquids (CBTL) plant with CO₂capture and storage (CCS) has been developed. The plant is integrated with a combined cycle plant that produces steam and electric power by utilizing light hydrocarbons and excess heat that is available from the CBTL plant.

In the indirect CBTL plant, syngas is produced from a coal-biomass feed in a fluidized-bed gasifier. A water gas shift (WGS) reactor is used to obtain the desired H₂/CO ratio in the syngas for maximizing the amount of liquid produced in the slurry-phase Fischer-Tropsch (FT) reactor while satisfying the desired extent of CO₂ capture in the overall process. A significant portion of the light gases produced in the FT reactor is recycled back via an autothermal reformer (ATR) in order to increase the fuel yield with low utility penalty². The remaining portion is sent to the pressure swing adsorption (PSA) H₂ unit to satisfy the H₂ requirement in the product upgrading section. The liquid products are sent to the product upgrading section where a novel integrated hydrotreater is used to achieve higher thermodynamic efficiency and smaller footprint than the traditional configuration where multiple hydrotreaters are used. The straight-run gasoline obtained from the FT crude is processed in an isomerization unit in combination with a catalytic reforming unit (CRU) to obtain the gasoline product with desired specifications. The off-gas from the entire process is used in a combined cycle plant that uses a gas turbine integrated with a heat recovery steam generator (HRSG) coupled with a steam turbine to produce electric power. The steam required in the CO₂ capture units is extracted from the steam turbine. Substantial amount of CO₂ is captured before the FT reactor by using a dual-stage, selective physical solvent-based process. For selection of the optimal post-FT CO₂capture technology, three candidate technologies, namely Selexol, MEA and MDEA/PZ, are evaluated.

The plant-wide model of the CBTL plant including the combined cycle plant has been developed in Aspen Plus integrated with additional models developed in Excel. The steam turbine model involves stage-by-stage calculations and has been developed in Matlab. In this presentation, we will discuss details of the models for all key unit operations. In addition, we will present results from the optimization studies that show that the energy penalty due to CO₂capture can be substantially reduced while having negligible effect on the fuel yield.  

References

1. Affordable, Low Carbon Diesel Fuel from Domestic Coal and Biomass, DOE/NETL 2009/1349, January 2009

2. Steynberg, A.P.; Dry, M.E. Fischer-Tropsch Technology, Elsevier Science: San Diego, 2004.