(327b) New Strategies for Polygeneration: Hybrid Natural Gas Reforming and Coal Gasification Techniques for Production of Methanol, Electricity, and Fischer-Tropsch Fuels

Polygeneration is the conversion of fuels such as coal, natural gas, and biomass into multiple products such as synthetic natural gas, hydrogen, methanol, diesel, gasoline, dimethyl ether, and electricity. Polygeneration systems can be constructed with some flexibility, such that the product mix can be altered in response to changing market conditions over the lifetime of the plant. This provides financial security and reduced risk in the face of rapid price fluctuations and an uncertain future. Additionally, the incorporation of multiple inputs or outputs provides many opportunities for energy integration which can lead to overall efficiency improvements.

In this work, novel polygeneration processes which convert coal and natural gas to methanol, Fischer-Tropsch products (i.e., diesel, naphtha) and electricity with CO₂ capture capability are presented. To create the liquid products, syngas must be generated with a H₂/CO ratio of about 2.0. The key characteristic of each process lies in how this ratio is achieved. For example, coal gasification is commonly considered, which can be used to generate syngas with a H₂/CO ratio of about 0.7-1.3 and then upgraded through the addition of H2 gas from an external source (such as solar-powered electrolysis of water) or through the water-gas-shift reaction. However, the exothermic, high-temperature gasification step (~1300°C) is inherently inefficient since the high-temperature waste heat is underutilized for steam generation at best and discarded at worst.

Instead, we explore the combination of natural gas reforming with coal gasification. Natural gas can be reformed by adding steam at high temperatures to create syngas with a H₂/CO ratio of greater than 3.0. This is then mixed with coal-derived syngas to obtain the desired ratio, avoiding the need for H₂ generation or the water-gas-shift reaction. However, because the reforming step is both high-temperature and highly endothermic, the reformer can be heat integrated with the gasifier, such that the high-temperature generated by the gasification reaction can be used to provide for the heat needs of the reformer directly. This synergistic configuration creates efficiency improvements throughout the plant and has interesting consequences when considering environmental issues such as CO₂ capture and water consumption. Using Aspen Plus simulations and nonlinear optimization techniques, a techno-economic analysis is presented which compares several design options to highlight these improvements. The effects on the optimal design and profitability caused by changes in the prices of the fuels and products are discussed, as well as the impact of a potential carbon tax.