For energy polygeneration processes (the simultaneous generation of multiple energy products such as liquid fuels, chemical products, and electricity in a tightly integrated system), the key defining characteristic is the manner in which syngas is created with an appropriate H2:CO ratio. Many options are possible, primarily using some combination of coal and/or biomass gasification, natural gas reforming, the water gas shift reaction, hydrogen blending, and the use of absorption or adsorption separation processes. However, the ability to design energy efficient and cost-effective systems depends strongly on the synergies that can be harnessed between differing product mixes, the cleaning needs of each fuel, the market prices of the feedstock and products, and the type of carbon emission regulations.
In this work, we present several promising new process designs to convert coal, biomass, and/or natural gas into a mixture of methanol, naphtha, diesel, and electricity. For example, we consider the combination of natural gas reforming with coal gasification to produce syngas with an H2:CO ratio of 2:1, ideal for the Fischer-Tropsch and methanol processes. In addition to the traditional approaches, novel heat-integrated systems are proposed in which both steps are performed within the same unit, such that the high-temperature waste heat produced by coal gasification can be use to satisfy the natural gas reforming heat needs directly. This synergistic configuration provides efficiency improvements throughout the plant, as well as interesting environmental benefits. In addition, the use of solid oxide fuel cells (SOFC) as the primary electricity generation source enables significant advantages for cost effective CO2 capture. With this approach, synergistic efficiency improvements can be made throughout the process, particularly in the areas of sulfur and CO2 removal. Using detailed flowsheet simulations, the efficiencies, costs, and environmental benefits of these proposed designs strategies are compared with several traditional concepts.