Design & Simulation of a CO2 purification Unit for Inherent Carbon Capture in IGCC power plants

Abstract

Integrated Gasification Combined Cycles (IGCC) offer the potential of utilizing the heating value of a high-carbon containing fossil fuel to transform it into electrical power with a higher efficiency than pulverized coal boilers, reducing the specific emissions of the plant. Emissions can be driven to substantially lower levels when CO2 capture technology is implemented in the plant. Research and technology development focus on different pathways to capture carbon related emissions in IGCC. A growingly interesting option consists of inherent carbon capture where a metallic oxygen carrier withdraws O2 from the compressed air stream releasing it in a N2 free environment where combustion of syngas takes place, also known as chemical looping combustion (CLC). This can be achieved by a cluster of gas switching combustion (GSC) reactors alternatively in reduction and oxidation mode achieving a steady state operation.Although the GSC provides an inherent separation of combustion gases and air, a certain mixing with depleted air (consisting mainly of N2) is unavoidable as a consequence of the switching valve mechanism. When fluidized beds are employed in the reactor cluster, this mixing effect is even more pronounced because of the better mixing properties of this reactor type. After heat is retrieved from the combustion gases stream and condensed water is knocked out, a purification unit is necessary to increase the CO2 concentration to at least 96% mol, which is the typical specification for cost effective CO2 transport. In this work the design of this unit and an assessment of its performance with varying inlet compositions and operating conditions is carried out in the process flowsheet simulator Unisim Design R451 from Honeywell, using Peng-Robinson as the thermodynamic package to calculate stream properties. The unit consists of a two temperature flash vessels with cold boxes. The refrigeration required to cool down the inlet stream to the unit is accomplished by Joule Thompson expansion valves and gas (re-heated)expansion. Because of vapour liquid equilibrium constraints at temperatures and pressures specified in the flash vessels, a certain fraction of this CO2 will be present in the gas stream exiting the unit and will be vented to atmosphere as a purge, resulting in an increase of plant emissions. The remaining captured CO2 will be routed to the CO2 compressors to reach the transport pressure specification (150 bar). Thus, the CO2 purification efficiency (expressed as % of captured CO2 relative to that without venting) and the energy consumption per kg of CO2 captured are used as key performance indicators of this unit. This comprehensive study effectively assesses the problem of purifying a CO2 rich stream delivered by an IGCC power plant configuration that can potentially result in a drastic reduction of GHG emissions, with a significantly smaller energy penalty than conventional CO2 capture technologies.