Assessment of highly turbocharged oxygen production cycles coupled with power generation systems working under oxycombustion

Resumen

This thesis assesses oxygen production cycles based on membranes in three industrial situations, emphasizing power production operating under oxycombustion. The primary motivation is the reduction of pollutant emissions while not affecting the system's thermal efficiency.Thus, a thermoeconomic analysis of a membrane-based oxygen production cycle is performed to assess the viability of these facilities in the context of a ceramic plant. The cycle is driven by recycling gases within the plant and uses turbochargers and heat exchangers to compress and heat the air for oxygen obtention. Two configurations were studied, finding an optimum oxygen production cost of 31e/t was found, being competitive when compared with an average wholesale market price of 50e/t. Compared with other oxygen production methods, this cycle exhibits a competitive behavior regarding oxygen purity, production, and energy consumption. The promising results of this analysis motivate the study of similar configurations working in two oxycombustion contexts: a power plant and a spark-ignition engine. Two oxygen production methods operating with a power production plant (Graz cycle) are compared in the first context. The power plant uses cryogenic air separation as its oxygen source, the baseline in this analysis. Therefore, two membrane configurations are considered: three-end and four-end membranes. A medium-temperature stream within the power production cycle is the energy source to drive the membrane cycles. Both cases are compared with the baseline Graz cycle operation. The three-end membrane-based cycle improves the baseline efficiency by 0.61% and the four-end by 2.30 %. The oxygen production requires less power consumption in the membrane cases than in the baseline, increasing the net power output. Thus, membrane-based cases display a promising performance, with possible integration within an oxycombustion power plant. In the second context, the membrane-based cycle is coupled within an oxycombustion spark-ignition engine, where different operation conditions are evaluated regarding fuel consumption and energy availability for oxygen production. The energy source to drive the membrane-based cycle is the exhaust gases stream. As a first step, different oxygen concentrations and engine compression ratios are studied at medium speed, comparing the performance with the engine's conventional operation. Medium oxygen concentration (30 %) was found to be optimum. This concentration allows the operation at a high engine compression ratio. Secondly, a full-load study in a wide range of engine speeds is made. The oxycombustion engine achieves a sustainable operation at the studied speeds, reaching the reference full-load power values. Similar fuel consumptions regarding the most efficient conventional case are achieved when the engine compression ratio is elevated under oxycombustion. Thirdly, operative limits regarding part-load and altitude operation are found. The fuel consumption behavior of the oxycombustion case is similar to a conventional turbocharged engine at part-load while improving a naturally aspirated engine operation. The minimum achievable load is between 40 to 50% of the maximum load, depending on the engine compression ratio. The membrane cycle operation is affected at lower loads. On the other hand, the system shows a suitable performance up to 4000 m. Thus, it can be concluded that the membrane-based oxygen production cycle exhibits flexibility to work in a wide range of available energy, displaying a suitable performance according to the requirements. Additionally, possible advantages in energy consumption and operative costs could be found when a careful design is performed.