陽極氣體回收機制對乙醇固態氧化物燃料電池之影響

Abstract

本篇論文檢視關於陽極廢氣回收對乙醇蒸氣重組固態氧化物燃料電池所產生之影響，依照回收率不同觀察系統整體表現。為達此目的，本研究使用Matlab/Simulink架構一完整的固態氧化物燃料電池之穩態模型，並可觀察系統中各子單元於不同回收率下其溫度、熱量、氣體流量等的改變。 本研究所模擬之系統為5kW等級之固態氧化物燃料電池，其內電池堆之燃料利用率為70%，並設定進料之水醇比為Rae = 3。燃料前處理系統中，使用乙醇蒸氣重組進行主要產氫反應。文獻上模擬乙醇轉氫的化學式中甚少模擬Methanation的反應，但是由於陽極廢氣回收機制可能導致二氧化碳濃度過高，進而影響一氧化碳的濃度與Methanation的反應量，因此本研究將Methanatoin與Water-gas-shift的反應加入乙醇重組器與SOFC電池堆的反應中，並利用Van’t Hoff’s equation，以期能夠更精確地模擬不同溫度下反應物的濃度。 由模擬結果得知，系統整體之熱效率及電效率皆會隨陽極廢氣回收率增加而被提升，然而系統內之乙醇重組器和燃料電池堆之反應效率卻會隨回收率增加而降低。此現象乃因為回收氣體中包含高濃度的二氧化碳，使得重組反應及陽極電化學反應氣體皆被大幅稀釋。在無陽極氣體回收機制下，系統的熱效率（定義：(後燃氣排放氣體含熱-所有需熱單元所需熱能總和)/(輸入乙醇流量*乙醇低熱值)）為29.83%，系統的發電效率（定義：電池堆輸出電能/(輸入乙醇流量*乙醇低熱值)）為40.75%；在無二氧化碳移除機制下，陽極氣體回收率於0.2~0.8時可提升系統的熱效率由33.44％至61.83％，系統的發電效率由43.07％至50.01％；在加入二氧化碳移除機制下，陽極氣體回收率於0.2~0.8時可壓制系統的熱效率由29.27％至26.09％，系統的發電效率由43.26％至53.65％。

In this research, the property of anode exhaust gas recirculation of a solid oxide fuel cell (SOFC) system equipped with an ethanol steam reforming fuel pre-processing unit is investigated. An integrated steady state system with recycling unit has been built in Matlab/Simulink to examine the effect of recirculation ratio change. In the previous research, the water gas shift reaction and methanation reaction is rarely applied in reforming reaction and electricity generating unit. However, in the system with recycling mechanism, the CO2 density is high. The CO density and methanation reaction configuration inside the whole system are greatly affected by the rising CO2 density. Thus, these two equations are added in both the reformer and full cell stack of this study to construct a more precise simulation. A 5kW system with fuel utilization 70% and water-to-ethanol ratio (Rae) 3 is presented. The chemical equilibrium configuration of reformer and solid oxide fuel cell are calculated by adopting the theory of Van’t Hoff’s equation. The thermal and electricity efficiency are enhanced with the increasing recycling rate. However, the performance of reformer and fuel cell stack is lowered with the increasing Rrec. The gas component has been great diluted with the rising amount of carbon dioxide in recycled gas. Without the anode gas recirculation design, the heat efficiency is 29.83%, the electricity efficiency is 40.75%. With the anode gas recirculation from Rrec 0.2~0.8, the electricity efficiency increases monotonically from 43.07% to 50.01%, the heat efficiency increases from 31.44% to 33.83%. After removing CO2, the efficiencies from Rrec 0.2~0.8 are 26.28% to 29.06% for heat efficiency, and the electricity efficiency from 29.27% to 26.05%, from 46.26% to 53.65%, respectively.