水煤漿與生質煤漿在挾帶床氣化爐氣化特性的數值模擬

Numerical Simulation Study of Gasification Characteristics in Entrained Bed Gasifier with Coal Water Slurry and Bio-slurry

潘則瑋(Ze-Wei Pan)

指導教授 ： 楊授印

國立虎尾科技大學/工程學院/機械與機電工程研究所/碩士(2013年)

摘要

煤是目前所有化石能源中存量最多的種類約占72%，煤本身的高能量密度特性，因此有較高的經濟效益。煤的組成複雜，直接使用的過程中易產生大量的汙染物，為提高煤轉換的經濟效益同時降低汙染物排放對環境所造成的危害，淨煤技術就成為替代能源發展主要方向。IGCC為淨煤技術的一種，粉煤通過氣化爐的轉換產生合成氣，並搭配二氧化捕捉技術及其它防治措施，降低二氧化碳與污染物排放到大氣中，合成氣利用渦輪機進行發電或發展氫能。氣化步驟是IGCC系統中重要的一個階段，因此氣化過程會影響整體系統的能源效益，本研究主要目的為提高IGCC系統在氣化階段的轉換效率。
氣化的轉換過程包含蒸發、裂解、燃燒與還原反應，反應過程又可分為氣相與固相反應，本研究使用的燃料為水煤漿，由細微粉煤顆粒與水混合製成漿體燃料，因此利用數值模擬探討水煤漿氣化在轉換過程化學及物理特性。以SIMPLE solver計算壓力與速度的偶合，紊流的計算以standard的k-ε模型簡化，化學反應過程中物種的擴散現象以species transport模型求解，且燃料為液滴包覆顆粒的兩相流，因此利用離散相模型追蹤顆粒進入到反應器中的變化與反應過程，水煤漿液滴的輻射與吸熱行為使用DO 模型。
以碳轉換率與冷氣體效率作為指標，結果顯示在O/C ratio0.5到0.8的範圍內，O/C ratio增加因為在燃燒的程序中較好有較高的碳轉換率，以O/C ratio 0.8作為最佳化進料參數。在固定進料O/C ratio探討不同的操作壓力對於氣化過程的影響，由常壓到20個大氣壓，結果顯示壓力增加使反應較為集中而提高燃燒溫度，限縮了燃燒區域，影響質量與熱的擴散，且增加液滴停滯時間，因此當壓力增加時，降低氧氣濃度與液滴之間的影響。水煤漿液滴在常壓的環境中影響最為顯著，受到反應器距離影響，較大的液滴在蒸發過程中所需距離較長，縮減了氣化過程的區域，因此降低碳轉換率，此外在高壓時液滴穿透深度下降，增加在氣化爐內的時間，能夠改善液滴的問題。水煤漿濃度從55%到75%的範圍中，濃度越高水的含量減少，反應區有較多的熱，因此輻射吸熱的現象越明顯。根據水煤漿在操作壓力、液滴大小與濃度的分析結果，得到在常壓、液滴400μm以及濃度55%時氣化性能較好，以上述參數為條件，進一步討論5%到20%咖啡渣焦碳製成生質煤漿的氣化分析。模擬結果發現，當焦碳與煤漿的混合比例增加會讓揮發物提早產生，因為焦碳揮發物含量比煤炭高。這樣的結果造成生質煤漿添加比例增加時氧化反應較好，提高碳轉換率，但卻降低了冷氣體效率，表示氣化性能下降。

關鍵字

水煤漿 ； 生質煤漿 ； 氣化 ； 數值模擬 ； 挾帶床

並列摘要

Coal is the largest stock of the fossil fuel around the world, that was about 72%. It content high energy density to own high potential performance of economics. It emitted large amount the pollutions in the coal using process directly, that reduced the competitiveness of coal. Therefore, how to make the coal to be a clean energy for using is an important issue in the world, IGCC is an opportunity of clean coal using that couple with capture carbon dioxide technology (CCS) and other pollution controller unit to reduce pollution in the process. The products of IGCC are hydrogen and carbon monoxide (syngas) mainly. Syngas was employed to burn in gas turbine to generate the electrical power. The gasification process is an important process in IGCC, in that dominates the efficiency of clean coal technology.
, There are several sub-reactions and phase reactions in coal gasification such as evaporation, pyrolysis, combustion, reforming and homogenous and heterogeneous reaction, respectively. Coal-water slurry and bio-char slurry were employed to investigate the gasification process by numerical methods in this study. SIMPLE methods and standards k-ε model were employed to solve problems of pressure-velocity and turbulence, respectively. To track droplet via phase change used the discrete phase model and the DO model was for radiation characteristics calculation in droplet. The carbon conversion efficiency and cold gas efficiency were the performance index of gasifier in this study. These results of study showed that the efficiencies of combustion in gasification were optimized when O/C ratio were between 0.5 to 0.8. Therefore, we investigated the pressure and droplet size effect in coal water slurry in O/C ratio of 0.8. The evaluated pressure in gasifier confined the combustion regime to increase the flame temperature and reduce the transport of heat and mass of slurry and oxidizer in pressure of 1 to 20 atm. In addition, the evaluated pressure effect affected the penetration length and droplet residence time that were parameters of gasifier. operating parameters of In the pressure 1bar, droplet 400 μm and concentration 55% have better gasification. By the above conditions, 5% -20% of the coffee grounds char are mixed into the slurry, made into a bio-slurry and analyzed. There results show that char is added to improve levels of volatile matter. Carbon conversion rate is better, but cold gas efficiency decreases.

並列關鍵字

coal water slurry ； bio-char slurry ； gasification ； numerical simulation ； entrained bed

參考文獻

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