提高豬場沼氣發電效益之研究

Abstract

從豬場廢水處理的厭氧發酵過程中所產生的沼氣是一種天然的再生能源，主要由甲烷(60~70%)及二氧化碳(20~30%)所組成，而這兩種氣體均為溫室效應氣體，其中甲烷所造成的影響還遠高於二氧化碳的二十倍。甲烷是一種可燃氣體，可透過引擎或者發電機進行燃燒產生動能或電能，燃燒後的二氧化碳則可以利用微藻進行光合作用達到二氧化碳移除和轉化產生質柴油而形成一個碳循環。在台糖豬場不但建立了廢水再利用、二氧化碳減量、轉酯化技術還進行沼氣發電。於厭氧處理所產生的沼氣中含有高濃度的二氧化硫，可透過生物除硫反應器進行移除，除硫後的沼氣才能提供給30kW發電機進行發電。 本論文首先應用富養燃燒繼續於本研究中，添加氧氣混合於燃料中並進到活塞式引擎內燃燒。本研究添加3%氧氣後，在260L/min沼氣流率的條件下，發電功率可以提高到28.2kW、熱效率可增加到30.2%，而甲烷使用率幾乎達到100%，並且在較低甲烷流率的條件下依然可以正常運轉(220L/min)。再來本論文將探討利用廢熱回收系統預熱不同進氣溫度之影響，當過剩空氣比>9.5時，熱效率會隨著甲烷濃度增加而提高，而當過剩空氣比>1.3時，預熱進氣溫度所提高沼氣發電機的效益會比較明顯。本研究進行沼氣除濕後，200、220、及240L/min沼氣流量最大的發電功率，在過剩空氣比=1時，分別可以達到21.55kW、24.78kW以及26.35kW，在比較沼氣沒有除濕的的情形下，發電功率分別提高了4.7、5.9和2.7%。接著本研究安裝了點火系統，其中包含火星塞壓力感測器及旋轉編碼器，以記錄汽缸內壓力和活塞引擎區柄軸角度。結果發現在上死點前13度(BTDC13)點火可以提供最高的發電功率，減少或增加點火時間皆會導致較低的發電功率輸出，相較於其他提早或延遲的條件下，BTDC13條件下點火會得到比較低的有效平均壓力變異係數(CoVIMEP)，這代表有效平均壓力(IMEP)較為穩定，除此之外可以發現CoVIMEP越低甲烷使用率越高。最後進行相同裝置容量活塞式引擎以及渦輪引擎在額定功率15-30kW的比較，從結果可以雖然發現渦輪引擎比起活塞式引擎運轉較為穩定，但是渦輪引擎在低負載界線為15kW而活塞式引擎低負載的界線為8kW。在使用沼氣為燃料所帶來的經濟效益如下，在台灣3,000頭豬場規模，每年可以產145,000度電，以及減少3,000公噸的CO2，而如果豬場規模達到10,000頭豬隻，每年可以產495,000度電，以及減少10,000公噸的CO2，而預計回收成本年限分別為13.6年以及5.6年。

Biogas generated from the anaerobic treatment of wastewater in a swine farm is a kind of natural renewable energy source, mainly consisting of CH4 (60~70%) and CO2 (20~30%). In this thesis, the enhancements of 30kW piston engine performance (i.e. power generation) are discussed intensively. The first, an oxyfuel combustion technology was applied. The extra oxygen mixed with the fuel, and the mixture flew into the piston engine. With 3% oxygen-enriched air, the maximum power generation, thermal efficiency, and CH4 consumption percentage increased up to 28.2 kW, 30.2%, and approximately 100%, respectively, for a biogas supply rate of 260 L/min, and the engine can operate normally at a lower limited biogas supply rate of 220 L/min. The second, the effect of preheating the inlet gas to different temperatures was investigated by applying a waste-heat recovery system. The thermal efficiency increases with increasing methane concentration only when λ (excess air ratio) > 0.95, although on the relatively rich side (λ < 0.95), there is no benefit. The improved generator performance obtained by preheating the inlet gas is apparent when the excess air ratio is relatively high, such as when λ > 1.3. The third, the maximum power outputs of biogas supply at 200, 220 and 240L/min after dehumidification withλ=1.0 are 21.55kW, 24.78kW and 26.35kW. In comparison with the corresponding ones without dehumidifying, the increases in power generation are 4.7, 5.9 and 2.7%, respectively. The fourth, a complete ignition system, consisting of a spark-plug pressure sensor and a rotary encoder, was installed to record the in-cylinder pressure and crank angle of piston cylinder. The optimum spark timing is located at 13 degrees before top-dead center (BTDC13), which supplies the highest power generation. Delaying or advancing the optimum spark timing leads to poorer power outputs. The spark timing of BTDC13 possess a lower coefficient of variation in indicated mean effective pressure (CoVIMEP) than the delayed and advanced timings, where the engine performs at more stable indicated mean effective pressures (IMEP) during combustion. In addition, it was found that the lower CoVIMEP results in a higher CH4 consumption ratio. Finally, this work conducted a series of comparison tests by using piston and turbine engines, respectively, under the loads varying from 15 to 30kW. The results showed that the operation of turbine engine is more stable than that of piston one in such range. However, the lower load limit is 15kW for turbine engine, whereas piston engine still can be operated at as low as 8kW. As to the economic benefits of using biogas, it estimated that a swine farm with a scale of 3,000 heads in Taiwan can generate 145,000 kWh of electricity and decrease 3,000 tons of CO2 per year. If the scale rises to 10,000 heads, then, the power generated increases to 495,000 kWh and CO2 reduction is 10,000 tons. The durations of cost recovery are 13.6 and 5.6 years, respectively.