Syngas Production with Thermo-Chemically Recuperated Gas Expansion Systems: An Exergy Analysis and Energy Integration Study

In spite of the large degree of energy integration in the modern syngas production units, the highly endothermic reactions of steam methane reforming and the combined steam and power generation still require a huge amount of energy that is typically supplied by an expensive natural gas-fired furnace at very high temperatures. Since normally only half of the energy supplied by the furnace is used to carry out the reforming reactions, the remaining heat recovery must be performed in a separate convection train (HRCT). Additionally, the high temperature effluent of the secondary reformer is generally cooled down by producing low temperature steam, increasing the process irreversibility and the losses associated to the excessive amount of condensate. Thus, in this paper, the advantages of introducing a chemically recuperated gas turbine (CRGT) concept to simultaneously carry out the endothermic chemical reactions and recover the exergy available from the autothermal reformer effluent are discussed. In this way, higher temperatures and higher conversions can be attained with lower driving forces. The power required by the air compression as well as other ancillary systems (e.g. air separation unit, carbon capture system, boiler feedwater pumps and recompressor) can also be supplied, while the exergy of the exhaust gases from the turbine is used more efficiently than in typical steam generation systems. Thus, more compact and integrated syngas production plants can be envisaged. Moreover, by introducing more advanced cogeneration features such as air enrichment, process gas reheating and incremental levels of pressures in the reaction-driven components, energy savings together with drastically reduced atmospheric CO2 emissions and irreversibility can be achieved in the frontend syngas production section. As a result, the average specific exergy destruction and exergy fuel consumption of the proposed CRGT-based configurations are, respectively, 12.0% and 2.7% lower, whereas the specific atmospheric CO2 emissions are cut down up to 25%, when compared to the conventional syngas production process.