Processing Rice Straw and Pennisetum by Microwave-Induced Torrefaction

生質物焙燒技術為一種輕微裂解之技術，透過此熱化學技術可改善生質物之特性，以利於生質物後序轉換程序。微波誘發焙燒技術可以克服傳統焙燒技術的缺點，譬如升溫速率低、處理時間長以及生質物表面和核心之間嚴重的溫度梯度等。本實驗以稻稈和狼尾草作為研究對象，並以工業用微波爐(單模聚焛式)提供微波來源，通以氮氣使其在無氧的環境下進行焙燒反應，並調整微波功率介於150 W~350 W之間。其他操作參數包括：微波功率、反應時間及反應溫度等，以期找出最佳之操作條件。微波焙燒反應前後，會探討其固態產物熱值變化、O/C比例、近似分析，元素含量、重量損失、及其三相產物之能量分配及Energy yield。 稻稈在300 W反應時間25分鐘及350 W反應時間10~15分鐘條件下，熱值提升最多可達30~34%；狼尾草則在300 W反應時間25分鐘及350 W反應時間10分鐘下，熱值提升最多可達55~56%。與傳統焙燒方法比較，反應時間明顯縮短許多，反應時間視微波功率而有所調整，但都在25分鐘前即可反應完畢。 稻稈和狼尾草在150 W反應時間10分鐘之條件下，可取得最佳的Energy yield分別為78.0%和79.6%。此外，在固定微波功率(350 W)將生質物加熱到不同溫度250℃和300℃時，發現稻稈和狼尾草其重量保留百分比和Energy yield在250℃和300℃條件下差異不大，250℃時稻稈和狼尾草之重量保留百分比分別為43.7%和42.9%，在300℃為42.9%與41.6%，而250℃下稻稈和狼尾草之Energy yield分別為52.9%與59.8%，300℃下為54.7%與56.9%。因此，較高功率微波焙燒時，溫度對重量保留百分比與Energy yield的相關性較低，反而是反應時間和微波功率相關性較高。 固態產物之元素分析結果顯示，C元素會隨微波功率提升而上升，而O元素會隨微波功率和反應時間增加而降低，H元素則隨微波功率升有些微下降。整體而言，O/C及H/C比值會隨著微波功率和反應時間增加而遞減。粒徑分析方面，可發現粒徑大小隨微波功率提升而越小且粒徑分布越均勻。 氣態產物以CH4、CO2、CO和H2居多，和少量C2H6及C3H8。在150 W-10 min條件下，稻稈和狼尾草之氣態產物以CO (54.91%和43.07%)最多，H2(19.59%和28.31%)次之，跟傳統加熱法比較可使溫室氣體CO2的產量降低。液相產物主要以Phenol、Benzofuran, 2,3-dihydro-、Phenol, 4-ethyl-以及Phenol, 4-methyl-為主。 由上述結果顯示，微波誘發焙燒與傳統加熱焙燒相比，其達到的加熱速率更快且可迅速達到高溫。產出的固態產物具疏水性和穩定性且易儲存之特性，能源密度也有提升的趨勢。Torrefaction is a mild pyrolysis process which used to improve the properties of the biomass. Microwave-induced torrefaction can minimize the disadvantages of conventional torrefaction methods such as low heating rate, long processing time and severe temperature gradient between central and surface of biomass. Microwave irradiation is single-mode from the industrial microwave device. Two agricultural residues, rice straw and pennisetum were torrefied under different microwave power in nitrogen ambient. In this study, microwave power level was set between 150W~350W. Different operating conditions, including microwave power, processing time and temperature, were undertaken to examine optimization of the process for the biomass. After torrefaction, solid product would be investigated its caloric value, O/C ratio, proximate analysis, elemental analysis, mass reduction, energy distribution of three phase products and energy yield. The caloric value of solid product was enhanced 30~34% in the condition that rice straw was torrefied under 300 W for 25 minutes and 350 W for 10~15 minutes. With regards to pennisetum the caloric value was enhanced 55~56% in the condition that pennisetum was torrefied under 300 W for 25 minutes and 350 W for 10 minutes. In Comparison with conventional torrefaction, microwave-induced torrefaction could shorten processing time significantly, and make reaction go completely within 25 minutes. The energy yield of rice straw and pennisetum which were torrefied under 150W for 10 minuntes could reach 84.9% and 79.6% respectively. When the biomass was heated to 250℃ and 300℃ under 350 W, the results indicated that mass yield and energy yield of rice straw and pennisetum under 250℃ were similar to those under 300℃. The mass yield and energy yield of rice straw at 250℃ was 43.7% and 52.9% separately, and at 300℃was 42.9% and 54.7% separately；pennisetum at 250℃ was 42.9% and 59.8% separately, and at 300℃ was 41.6% and 56.9% separately. Therefore, temperature had less correlation to mass yield and energy yield under high level microwave power. By contract, processing time and microwave power were the key parameters of the reaction. Elemental analysis showed that carbon content of solid product increased with the increase of microwave power. However, oxygen content decreased with the increase of microwave power and processing time, and hydrogen content had slight decrease. Overall, O/C ratio and H/C ratio decreased with the increase of microwave power and processing time. The size of torrefied char was smaller and more uniform with the increase of microwave power. The majority of gas products were CH4, CO2, CO and H2, with a few C2H6 and C3H8. Under the 150 W-10 min operating condition, The percentage of CO content of rice straw and pennisetum were 54.91% and 43.07%, and H2 19.59% and 28.31%. Comparing with conventional method, microwave -induced method could reduce greenhouse gas CO2. The major compounds in liquid products were Phenol、Benzofuran, 2,3-dihydro-、Phenol, 4-ethyl- and Phenol, 4-methyl-. In conclusion, the heating rate of microwave-induced torrefaction was faster than conventional torrefaction and could easily reach high temperature. This method could save time and energy. The solid products provided stability and hydrophobic characteristic for longer storage, and had a tendency to increase the energy density of solid products