(375c) Optimal Energy Polygeneration System Design Under Different Economic Scenarios

Polygeneration is a concept where multiple energy products are generated in a single plant by tightly integrating multiple processes into one system. Compared to conventional single-product systems, polygeneration systems have many economic advantages, such as potentially high profitability and high viability when exposed to market fluctuations.

The optimal design of an energy polygeneration system that converts coal and biomass to electricity, liquid fuels (naphtha and diesel) and chemical products (methanol) with CO2 capture under different economic scenarios is investigated. In this system, the Integrated Gasification Combined Cycle (IGCC), the Fischer-Tropsch (FT) process and the gas-phase methanol synthesis process are integrated in one plant. Syngas is produced by gasification of coal and/or biomass; purified by a cleaning process to remove particles, mercury, sulfur and carbon dioxide; and then split to different downstream sections such as the gas turbine, FT process and the methanol process. Production rates of electricity, liquid fuels and methanol can be easily adjusted by varying the syngas split fraction to each downstream process. In this work, the optimal design with the highest net present value under the same dry feedstock input rate is determined by optimizing equipment capacities, stream flow rates and stream split fractions. A global optimization solver, BARON, is used to guarantee the global optimality of the solutions.

The results reveal that the optimal design of polygeneration systems is strongly influenced by economic conditions such as feedstock prices, product prices, and potential emissions penalties for CO2. Over the range of economic scenarios considered, it can be optimal to produce a mixture of electricity, liquid fuels, and methanol; only one each; or mixtures in-between. Moreover, the optimal design of the polygeneration system is quite sensitive to the carbon emissions tax policy. For example, as the carbon tax increases, the optimal production portfolio changes from electricity and liquid fuels to electricity and methanol because of the difference in how liquid fuels and chemicals are treated under likely carbon tax policies. The optimal biomass/coal feed ratio significantly increases when the carbon tax increases or the biomass price decreases. At carbon taxes above $30/tonne, CO2 emissions are significantly reduced due to the addition of carbon sequestration and storage capability to the plant. An economic analysis of the optimal polygeneration designs yielded a higher net present value than comparable single-product plants.