Carbon dioxide (CO2) capture and final mitigation technologies need to be considered as part of an overall strategy to reduce greenhouse gas emissions, which contribute to global warming. Systems, which integrate coal gasification with combined-cycle energy production, and CO2 capture technologies, are an important option for future coal-based power generation. Literature studies show that integrated gasification combined cycle (IGCC) systems have the highest energy efficiency and the lowest environmental impact among available technologies for coal-based electric power generation. Membranes also offer low operating cost compared to other competitive CO2 separation processes, such as absorption, cryogenic, PSA, etc. In this study the use of a hybrid coal gasification system, consisting of a gasifier, a shift reactor and a membrane separator, has been examined. With this scheme the gasifier stream is enriched in CO2 and thus, its final recovery and disposal becomes more efficient. Two alternative separation options were studied:
(a) low temperature, polymer membrane separation, and
(b) high temperature, ceramic membrane separation.
The mass and energy balance analyses of these hybrid IGCC schemes were performed by combining a developed membrane simulation software and an Aspen Plus simulator. The capital cost in terms of energy production was estimated for each case by using the material and energy balance results in the design and cost equations for the process equipment. The composition of the effluent stream, the energy impact and the capital cost, were examined for various gasifier operating conditions (temperature, pressure, feed composition). The energy analysis shows that the introduction of a Shift reactor and of polymer or ceramic membranes results in a appreciabe energy penalty (almost 10%), which is identical for both cases. However, CO2 removal can be over 50%, compared to conventional IGCC CO2 emissions, in the case of polymer membranes. The impact of gasifier operating conditions (pressure, temperature and feed composition) on total energy output and on CO2 purity is significant. The estimated capital cost of the IGCC base case scheme (without shift rector and membranes) is in good agreement with the respective literature data. The use of polymer and especially of ceramic membranes results in an increase of the capital cost, mainly due to the high cost of the membrane equipment. The energy and cost analysis of the alternative cases show that CO2 removal in this hybrid IGCC scheme is technologically feasible. The future development of low cost ceramic and/or polymer materials for industrial membrane separation use, is expected to offer high potential for significant reduction in capital cost.