- 學位論文
液態天然氣冷能應用於富氧燃燒發電與二氧化碳捕捉系統之可行性分析
Feasibility Analysis of the Oxy-fuel Power Generation System with CO2 Capture using the Liquefied Natural Gas Cold Energy
摘要
全球各地液態天然氣(LNG)每年的進口總量持續提高，為了將進口燃料之能源效率達到最大化以及降低液態天然氣氣化過程對環境的衝擊，液態天然氣氣化過程所釋放之冷能量的回收與再利用受到各界的重視。Zhang et al. (2010) 提出一個結合發電與液態天然氣氣化的雙循環系統 (COOLCEP-C) ，主要是利用液態天然氣的冷能量，使得富氧燃燒的發電系統得以同時進行高效發電和二氧化碳捕捉。 COOLCEP-C是一個類朗肯循環 (Rankine cycle) 的系統，將二氧化碳作為工作流體 (working fluid)，推動渦輪機以進行發電，同時液態天然氣作為此系統中的冷能量來源用於液化二氧化碳，使得液態二氧化碳能夠使用能耗較低的加壓裝置幫浦(Pump)進行加壓。在送入渦輪機前，首先氣化高壓的液態二氧化碳，而氣化過程會吸收大量的熱能，亦可以為其他系統提供冷能量，高壓的氣態二氧化碳與氧氣結合後送入燃燒器(Combustor)，以提高通過渦輪機前後的氣體焓的變化(Enthalpy change)。接著，將水分去除後，液化二氧化碳，再將燃燒過程中所產生的二氧化碳，進行分離捕捉(capture)。 但是，COOLCEP-C系統中存在一些缺點。首先，二氧化碳捕捉的過程中將在二氧化碳液化過程中排放的所有氣體進行加壓冷凝，而捕獲的二氧化碳濃度僅有88.90 mol%, 需要後續的處理，故被認為是一個耗能且充滿疑慮的策略。接著，Zhang et al. (2010) 中假設，除水器 (H2O Separator) 可以將混合氣體中的水蒸氣完全分離，其工程技術可行性令人質疑。因此，本研究針對系統中的二氧化碳捕捉與除水器進行改良。此外，本研究也將液態天然氣氣化循環中的操作壓力從73.5 bar 提高至150 bar, 同時新增一個天然氣渦輪機在此循環中，以提高系統的總發電量。COOLCEP-C經過改進後，命名為MCOOLCEP-C。隨後，為了提高系統的能源效率，本研究對CO2 Pump的壓力與Combustor 的溫度進行調整。同時，本研究也提出另外三個改良系統，分別是RMCOOLCEP-C, HIMCOOLCEP-S 和 HIRMCOOLCEP-S。RMCOOLCEP-C是在MCOOLCEP-C的燃燒器與其渦輪機的部分加入再加熱(reheat)的步驟的系統。將MCOOLCEP-C中的CO2 Compressor移除，再進行熱整合，則產生HIMCOOLCEP-S。而在HIMCOOLCEP-S中加入再加熱的步驟，則命名為HIRMCOOLCEP-S。接著，針對這些系統進行靈敏度分析、最適化與經濟評估，以提高系統整體的表現。 在最適化後， RMCOOLCEP-C系統在淨利與二氧化碳回收率上表現最佳，分別為52.58 MUSD 與98.60%。而其被捕獲的二氧化碳濃度也高達99.94 mol%。 值得注意的是最適化的RMCOOLCEP-C系統，操作在高溫高壓，其設備所使用的材料與技術必須謹慎選擇與改良。在相同的基準(Basis)下，最適化的HIMCOOLCEP-S 系統的能源效率高達69.01%，是本研究中最高的，這表示此系統能夠使用較少的燃料產生等量的電力。但HIMCOOLCEP-S 系統的總發電量將較低，因此將此提高系統的二氧化碳流量能夠改善此問題，使其成為一個能源效率高達且總發電量客觀的發電系統。
並列摘要
As the annual importation of liquefied natural gas (LNG) increases gradually, LNG cold energy recuperation and reutilization during regasification process is attached great importance in order to enhance energy efficiency of imported fuel and decrease the environmental impact. Zhang et al. (2010) proposed a novel CO2-capturing oxy-fuel power system with LNG coldness energy utilization (COOLCEP-C) that has integrated both power generation cycle and LNG regasification cycle to form a high energy efficiency system. Like a Rankine cycle, COOLCEP-C system uses CO2 as working fluid driving turbines to generate electricity and LNG acts as refrigerant to liquefy CO2 before CO2 is being pressurized by pump. In addition, low temperature pressurized CO2 stream (about -50oC) can acts as a cold energy source for other systems during evaporation. In order to enhance the enthalpy differences between the inlet and outlet stream of the gas turbine, the evaporated CO2 is then mixed with O2 stream and sent to the combustor before driving the turbine. The CO2 that produced from the combustion is captured after condensing. However, there are still some drawbacks existing in the COOLCEP-C system. The CO2 capture section in the system that compresses and condenses the mixed gas stream, and the concentration of the Captured CO2 stream is only 88.9 mol%, so that it is recognized as an energy-intensive and questionable strategy. Secondly, it is not realistic as what Zhang et al.(2010) assumed that the H2O in the CO2 stream can be perfectly removed before sending to the condensation unit. Thus, the intensifications for CO2 capture section and H2O separation section are applied to the system. Besides, the LNG pump pressure is increased from 73.5 bar to 150 bar and an additional natural gas (NG) turbine has also installed to the LNG regasification cycle in order to generate more electricity. The modified COOLCEP-C system is named as MCOOLCEP-C. Furthermore, the energy efficiency can be improved by increasing the CO2 Pump pressure and the Combustor temperature. Hence, the RMCOOLCEP-C, HIMCOOLCEP-S and HIRMCOOLCEP-S are proposed. The RMCOOLCEP-C is the system that applying the reheat procedure in the section of Combustor and CO2 turbine in the MCOOLCEP-C. When the CO2 Compressor in MCOOLCEP-C is removed and carried out the heat integration, the HIMCOOLCEP-S is then produced, and the HIMCOOLCEP-S with reheat procedure is name as HIRMCOOLCEP-S. After that, the sensitivity analysis, optimization and economic evaluation are carried out to enhance the performance of system. The optimized RMCOOLCEP-C system shows its superiority to annual net profit and CO2 recovery which are 52.58 MUSD and 98.60% respectively on the basis of recirculating CO2 flow rate at 100 kg/s and LNG flow rate at 100 kg/s. The concentration of captured CO2 stream is 99.94 mol%. In this case, the application criteria is the technology and the material of the instruments should be improved to deal with the operating conditions with high temperature and pressure. With the same basis, optimized HIMCOOLCEP-S system has the highest energy efficiency (69.01%), it reflects that the system requires lesser fuel to produce electricity. However, the netpower output of the HIMCOOLCEP-S system is relatively low if compared with other systems, therefore, the increment of the recirculating CO2 flowrate for the optimized HIMCOOLCEP-S system leads to achieve a high energy efficiency power system with considerable net power output.
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