Thermodynamic and Economic Evaluation of an IGCC Plant Based on the Graz Cycle for CO2 Capture

The IEA World Energy Outlook 2009 predicts a considerable growth of the world’s primary energy demand and states that fossil fuels will remain the dominant source of primary energy. Among them coal will increase its share because of its vast reserves, its relatively even global distribution and its low prices compared to oil and gas. On the other hand the burning of coal emits larger quantities of CO₂ than oil and gas. As CO₂ is the leading cause for global warming, the use of coal for power generation demands a clean coal technology with carbon capture and storage (CCS). Therefore in this work it is suggested to combine a coal gasification unit with a Graz Cycle power plant, an oxy-fuel technology of highest efficiency. The firing of the syngas from coal gasification with pure oxygen avoids the expensive pre-combustion CO₂ sequestration and leads to a working fluid of CO₂ and steam, where CO₂ is captured by simple steam condensation. In contrast to this, the more conventional technology is to send the syngas to a water-shift reactor and a CO₂ scrubber so that a fuel containing mainly hydrogen is obtained which can be fired in a conventional combined cycle plant. In order to evaluate these two competing technologies a thermodynamic simulation as well as an economic cost analysis of both power cycles is performed. It turns out that the achievable efficiency of the Graz Cycle plant is — despite of the increased oxygen demand — far higher than that of a plant of conventional capture technology due to the avoidance of shift reaction and scrubbing. The following economic analysis shows mitigation costs of 22.5 €/ton CO₂ avoided for the Graz Cycle plant compared to 33 €/ton for an IGCC plant with CO₂ capture.