沼氣發電對溫室氣體的減量及排放與環境衝擊之評估

Abstract

沼氣定義為再生能源中生質能源的一種氣態燃料，可用於產熱及發電。完善的利用過剩沼氣可以有效降低沼氣中CH4、CO2等溫室氣體直接排放所造成的衝擊，並能有效減少能資源的浪費。本研究以國內最大的污泥厭氧處理廠-八里汙水處理廠為基礎進行污泥及廚餘厭氧處理產生沼氣發電之生命週期評估以探討其可行性。研究內容主要分為兩部分。第一部分為利用燃氣渦輪機及往復式發電機燃燒沼氣發電，分析排放廢氣組成及發電機的性質，並建立第二部分生命週期評估所需要之參數。第二部分是利用生命週期評估分析，進行八里污水處理場之污泥厭氧消化產生沼氣至發電的探討，並設立以廚餘進行厭氧消化產生沼氣至發電的比較衝擊分析。最後根據發電機的燃燒排放，分析沼氣發電與化石能源發電之CO2的排放比較。 第一部份的結果顯示，當輸出功率超過20 kw時，傳統往復式沼氣發電機的NOx排放濃度約為燃氣渦輪機的20倍;輸出功率越高，濃度差異越大。每一度電對於氣體排放控制及燃燒效率的比較上，往復式發電機CO的排放量為4.8×10-2 kg、NOx為1.92×10-2、燃燒效率(combustion efficiency, CE)為0.97;燃氣渦輪機CO的排放量為4.1×10-3 kg、NOx為1.63×10-3 kg、CE為0.99。燃氣渦輪機在CO及NOx的排放量約為往復式發電機的十分之一。 第二部分的結果顯示，沼氣發電於整體生命週期的分析以厭氧消化的程序影響衝擊最大。例如，污泥厭氧消化及燃氣渦輪機燃燒沼氣發電兩程序之衝擊程度分別為10-3 pt及9.81×10-5 pt，廚餘作為厭氧消化的原料比起污泥產生的效益更好，因其有機質含量高，產出沼氣量大使其利用厭氧消化產沼氣發電對環境衝擊為以污泥作為原料的一半。以廚餘及污泥為原料產生沼氣發電之環境衝擊程度分別為5.6×10-4 pt及1.11×10-3 pt。不考慮厭氧消化產生沼氣之二氧化碳排放，使用沼氣發電比起傳統化石能源發電對於二氧化碳排放的比較上，以燃氣渦輪機作為沼氣發電的基準，計入前處理設備(如乾燥機及壓縮機)比之能源投入，每發一度電會排放266 g CO2 eq，比起煤平均至少可以減少734 g CO2 eq;天然氣可減少203 g CO2 eq;石油可減少574 g CO2 eq。以燃氣渦輪機作為沼氣發電替代台灣綜合電力及化石燃料電力，至少可以分別降低51.25%及67%的二氧化碳排放量。故沼氣發電的應用可有效解決目前本國過剩沼氣的處理問題，更可以作為化石燃料的替代能源，有效減少溫室氣體排放的影響。

Biogas is one of gaseous fuel biomass energies in renewable resourcse. It can be used for heating and power generation. Improvement of the use of surplus biogas can effectively decrease the impact of greenhouse gas emissions of CH4 and CO2 in biogas on global warming, and reduce wasting energy resource. In this study, the life cycle assessment was performed to investigate the feasibility of adopting Bali sewage treatment plant, which is the largest of anaerobic sludge treatment plant in Taiwan, for the anaerobic treatment of sludge and foodwaste to produce biogas and generate the electricity. The study consists of two parts. The first part employed gas turbine and reciprocal engine to generate electricity from biogas. The composition of exhausts and performance characteristics of power generators were exmined to provide information of parameters needs for the second part. The second part conducted life cycle assessment to investigate the process of anaerobic digestion of sludge to produce biogas and generate electricity at Bali sewage treatment plant. The scenario of process of anaerobic digestion of foodwaste was also assessed. Finally, according to the emissions of exhausts of the generator, comparation of CO2 emissions of power generation from biogas with fossil fuel was made. The results from part I show, as the output power exceeds 20 kW, emission concentration of NOx using conventional reciprocal engine is about 20 times than that using gas turbine. The higher the output power, the greater the difference in NOx concentration. For 1 kWh output electricity, the emitted amount from reciprocal engine were 4.8×10-2 kg CO and 1.92 ×10-2 kg NOx with combustion efficiency (CE) of 0.97. Those from gas turbine were 4.1×10-3 kg CO and 1.63×10-3 kg NOx with CE of 0.99. The amount of exhausts of CO and NOx from gas turbine are about one-tenth of those from reciprocal engine. The results from part II indicate that the anaerobic digestion unit process exhibits the highest environmental impact in the whole processes from the input of raw matterials to the output of power generated from biogas. The impact extents of anaerobic digestion of sludge and power generation via gas turbine using biogas are 10-3 pt and 9.81×10-5 pt, respectively. The benefits of anaerobic digestion of foodwaste as a raw material are better than those of sludge. Because of the high content of organic matter of raw material of foodwaste and its large biogas output, the environmental impact using foodwaste is in half comparing with sludge. The impact extents for power generation via gas turbine using biogas with raw inputs of foodwaste and sludge are 5.6×10-4 pt and 1.11×10-3 pt, respectively. Without counting the CO2 emission from the production of biogas by anaerobic digestion, for generating 1 kWh, the gas turbine using biogas emitted CO2 of 266 g with 734 g, 203 g and 574 g less than those of power generators using coal, nature gas and oil, respectively. The use of available biogas for gas turbine in replacing the composed energy sources and sole fossil fuels for electricity generation of Taiwan can reduce at least 51.25% and 67% of CO2 emission, respectively. Therefore, the application of power generation from biogas can effectively not only solve the problem of the surplus biogas, but also offer an alternative energy to fossil fuels, effectively reducing the impact of greenhouse gas emissions.