Conceptual Thermodynamic Cycle and Aerodynamic Gas Turbine Design - on an Oxy-fuel Combined Cycle
Abstract: The world is today facing a serious problem with global warming, which is heading towards an appallingly high temperature level. The greater part of the overall climate science community agree that global warming is caused by the greenhouse effect, which depends largely on emitted CO2 emissions from the combustion of fossil fuel. The agreement at the COP 21 climate meeting in Paris (December 2015) was that global warming must be limited to no more than + 2.0 C, with the aim of keeping it below + 1.5 C. Accomplishing this requires
a concerted effort in several different areas, for example through increased energy efficiency, more renewable energy sources and the utilization of carbon capture and sequestration (CCS) technology. The oxy-fuel combined cycle (OCC), which is the topic of this thesis, is a subcategory of oxy-fuel combustion which, in turn is one of the three main technologies for CCS today. The key idea with oxy-fuel combustion is to avoid mixing the CO2 formed in the combustion with
the non-condensable nitrogen, as occurs at the combustion in a conventional combined cycle power plant (CCPP). This is achieved by combusting the gas fuel with pure oxygen (O2) and thereby forming a combustion product consisting of only steam (H2O) and carbon dioxide (CO2). The CO2 can then be separated downstream of the HRSG by condensing out the H2O and thereby leaving a pure stream of CO2 for sequestration. The OCC consists of a topping Brayton cycle and a bottoming Rankine cycle and has many similarities with a conventional CCPP. In the OCC, however, the flue gas leaving the HRSG is recirculated back to the gas turbine units compressor inlet, instead of being emitted to the atmosphere as in a CCPP. Thereby, the combustion products also act as the working medium in the topping (gas turbine) cycle. The working medium has a composition of about 85 wt.-% CO2, 10 wt.-% H2O and a few percentage points of enriched nitrogen and argon, which follows with the oxygen stream as impurities. The CO2-rich working medium has significantly different gas properties, compared to air and conventional flue gas. This affect the design of the topping cycle, the exhaust heat utilization in the HRSG, the design requirements for the gas turbine unit and the aerodynamic design of its compressor and turbines. One of the major effects on the design, is the requirement for a higher gas turbine pressure ratio than for a conventional CCPP, as a result of the lower isentropic exponent for the CO2-rich working medium. This thesis takes the OCC concept to the next technical readiness level not just by identifying, optimizing and proposing a cycle design for a 115 MWel OCC. It also addresses the conceptual design of a gas turbine unit suitable for an OCC and a quite detailed conceptual aerodynamic design for the gas turbines unit’s turbomachineries, i.e. one compressor and two turbines. The work investigated the performance levels to be expected from both the entire OCC, the embedded gas turbine unit and its turbomachineries. The proposed gas turbine unit was a single-shaft gas generator with a free direct-driven power turbine. The conceptual turbomachinery design of the compressor, the compressor turbine and the power turbine covered the conceptual design loop of the 1D mid-span, the 2D through-flow, and the 3D steady-state calculations. The compressor design was a 16-stage design, with a mass flow of 220 kg/s and a pressure ratio of 31.0. The turbine design was a two-stage compressor turbine and a four-stage power turbine. The oxy-fuel combined cycle was calculated to have an overall net efficiency of 48.2%, which includes the energy cost for the CO2 compression to 140 bar and the external O2 production in an air separation unit (ASU).
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