以微波誘發農林廢棄物焙燒技術之研究

Abstract

生質物之焙燒技術又可稱為輕微裂解技術，而生質物透過此焙燒反應可改善其原本不利於後續能源轉換程序之特性。本研究係利用單模聚焦式的微波照射方式，誘發生質廢棄物進行熱焙燒反應。與傳統加熱方法相比，微波誘發焙燒技術為一快速、高效能且更為經濟之技術。在本研究中主要是以稻殼和甘蔗渣作為研究對象，以工業用微波爐提供能量來源，使其在無氧狀況下進行焙燒反應。藉由改變不同之操作參數，包括：微波功率、反應時間、原物料之水分含量及原物料之粒徑大小等，以得到反應之最適合之操作條件。 由實驗結果發現，微波功率的增加可提升熱焙燒反應的升溫速率及最高反應溫度，且原物料之減量比亦會增加。然而過高之微波功率由於其升溫速度太快、反應溫度較高，使焙燒反應轉變成劇烈裂解反應；太低之微波功率由於升溫速率、及反應溫度過低，使焙燒反應效果不完全，故本研究建議以微波功率250 W-350 W做為微波誘發焙燒技術之最適操作條件。 焙燒固相產物之熱值分析結果顯示，在微波功率250 W、反應時間20 min及350 W、10 min，稻殼之熱值提升率最高可達26%；在微波功率250 W、反應時間15 min，甘蔗渣之熱值提升率最高可達57%。且本研究與傳統加熱焙燒技術所需之反應時間相比，則大幅縮短許多，僅需不到20 min。 此外，稻殼之水分含量在微波功率較低（200 W）之條件下，其對焙燒反應效果影響較大，其熱值可提升21%（5%水分含量）及25%（10%水分含量）；而於微波功率較高（300 W）之條件下，對焙燒反應效果並無太大之影響。而稻殼之粒徑大小亦會影響微波誘發焙燒反應之效果。粒徑較小之原物料在進行焙燒反應時，其反應效果會超乎預期，而使得反應由熱焙燒反應轉變成劇烈的熱裂解反應使得固相產物之熱值下降，如：粒徑＜200 mesh之稻殼，其熱值不但沒有獲得提升，反而降低3%。元素分析結果顯示，其C元素含量變化大致隨著微波功率及反應時間增加而增加；O元素含量則相反；H元素含量則無太大變化。微波誘發焙燒反應可降低生質物的O/C ratio和H/C ratio，與傳統加熱焙燒反應相比，其下降幅度更大且反應所需時間更短。如：以微波功率300 W，10 min內，C元素含量提高48%、O元素含量減少85%、O/C ratio減少89%，H/C ratio減少近99%。 上述結果顯示，與傳統加熱焙燒相比，微波誘發稻殼及甘蔗渣之焙燒技術不僅能大幅的縮短反應時間，且能夠達到比傳統加熱焙燒更好的焙燒效率，故微波誘發焙燒技術為一具有開發潛力之技術。

Torrefaction is a mild pyrolysis process which improves properties of fuel. This study utilized single-mode microwave irradiation to induce torrefaction of biomass wastes. Comparing to the conventional method, microwave-induced torrefaction is more rapid, efficient and economical. In this study, two types of agricultural residues, rice husk and sugarcane residue were torrefied under an inert atmosphere in an industrial microwave device. . Different parameters, including microwave power level, processing time, water content of biomass and particle size of biomass, were varied in the experiments to optimize the process. Based on the experimental results, it could be concluded that increasing microwave power enhanced the heating rate and the maximal temperature of torrefaction. However, it is worthwhile noting that torrefaction might go beyond the original expectation when the microwave power was relatively too high. On the other hand, when the microwave power fell below a level, the reaction would not reach completion. This study suggests that the microwave power should be set between 250 W - 350 W for torrefaction of these two agricultural residues. Furthermore, the caloric value enhancement of processed rice husk could reach 26% under a microwave power of 250 W. With regards to processed sugarcane residue, the caloric value enhancement could achieve 57% under the same operating condition. This study also investigated the effects of water content and particle sizes of biomass on the torrefaction process. Under lower microwave power settings, water content of biomass, ranging 5 to 10%, helped to increase mass reduction and caloric value. Moreover, particle size of biomass was also an important process parameter. When the particle size was too small, the torrefaction process would adversely go beyond expectation and ended up with a reduction in the caloric value. Microwave-induced torrefaction also changed properties of the solid char. Elemental analysis showed that carbon content of the solid char also increased with the microwave power applied. However, oxygen content of the solid char decreased during the process. It implied that microwave-induced torrefaction is more efficient than the conventional method for O/C ratio reduction. In conclusion, microwave-induced torrefaction can shorten the processing time when compared to the conventional method. It can achieve more efficient torrefaction. The results of this study suggest that microwave-induced torrefaction be a promising technology with a great potential.