| 摘要: | 本研究目的在於探討稻稈氣化與結合熱氣體淨化(Hot gas cleaning)過程，相關反應動力特性、提昇產氫效率、微量污染物去除、微量金屬分佈特性，以及預測形成之物種。本研究採用氣泡式流體化床氣化爐，控制條件包括氣化反應溫度800 °C及當量比0.2~0.4。熱氣體淨化系統主要由沸石、?燒白雲石及活性碳所組成，操作溫度控制在250 °C。為進一步探討熱氣體淨化系統之性能，本研究亦嘗試使用自製鎳基催化劑取代沸石，同時評估試驗吸附劑之吸附容量及其再生性能。
根據動力學分析結果顯示，在稻稈裂解及氣化條件下，活化能分別為75.40 kJ/mol及89.56 kJ/mol。在稻稈裂解/氣化過程中，主要氣相產物為一氧化碳、二氧化碳、甲烷、氯化氫、二氧化硫、脂肪烴、乙酸及一些微量氣體化合物，如酚及甲醇等。利用熱氣體淨化系統提昇產氣之氫氣濃度，結果顯示可從6.82%增加至9.83%。在控制當量比0.2之條件下，合成氣最大之低位發熱量，約為6.09 MJ/Nm3。同時研究結果指出，熱氣體淨化系統可成功應用在微量污染物去除。其中利用熱氣體淨化系統對整體焦油去除效率，趨近於70%，氯化氫及硫化氫去除效率，則分別約95%及80%。此外，為探討熱氣體淨化系統對提昇產氫及焦油產量最小化之影響，研究以自製鎳基催化劑取代沸石進行評估。研究結果顯示，當沸石催化劑及熱氣體溫度從250 劑調整至400 ℃時，氫氣及一氧化碳分別由7.31%及14.57%增加至8.03%及17.34%，而焦油去除效率則在70%至90%之間變動。當使用沸石取代自製鎳基催化劑及熱氣體淨化系統溫度控制在250 ℃時，氫氣含量顯著地從6.63%增加至12.24%，此係由於碳氫化合物(焦油)及甲烷含量減少。
在氣相中微量金屬鋅、鉻、鎘及鉛之分佈比例，隨著當量比增加而增加。根據XRD物種鑑定分析結果顯示，粒狀物之物種晶相分別為SiO2、KClO3、K2SO4、K2Si4O9、CdSiO3、PbCl2及Pb8Zn(SiO7)3。另熱動力平衡模擬結果顯示，稻稈氣化主要之氣相物種包括KCl(g)、NaCl(g)、KO(g)、K2O(g)、ZnCl2(g)、CrO2Cl2(g)、CuCl2(g)、PbCl2(g)、PbO(g)及Cd(g)。此外，熱氣體淨化系統對於金屬去除之效率比較，依序為K>Cr>Ca>Pb>Mg>Cd>Na>Zn>Cu，另活性碳對於Pb、Cd、Cr、Ca、K及Mg，具有較佳的吸附性能。
熱氣體淨化系統中各吸附劑對於硫化氫吸附容量之結果顯示，其吸附容量依序為活性碳>?燒白雲石>沸石，其中沸石、?燒白雲石及活性碳之吸附容量，分別為每公克吸附劑可吸附2.22, 3.16以及 5.88 mg 硫含量。此外，經過四次吸附再生循環試驗，沸石及活性碳在350 °C操作條件下，可達完全再生之效果，且持續具有穩定的硫吸附容量。本研究使用之三種吸附劑，其硫化氫及氯化氫之貫穿及飽合曲線極為迥異，其中氯化氫之最佳吸附性能，主要由白雲石及沸石所提供，至於硫化氫之最佳吸附性能，則以活性碳為主。整體而言，本研究稻稈氣化處理過程結合熱氣體淨化系統，可有效改善稻稈氣化產氣之氣體品質，及減少微量污染物排放，研究成果同時可提供研究者獲得更多合成氣污染控制之重要資訊。
;This study investigates the kinetic behavior, enhanced hydrogen yield efficiency, trace pollutants removal, trace metals partitioning characteristics, and speciation formation prediction in rice straw gasification combined with hot gas cleaning (HGC). A bubbling fluidized bed gasifier was used by controlling the temperature at 800 °C and equivalence ratio (ER) ranging from 0.2 to 0.4. The hot gas cleaning system consists of zeolite, calcined dolomite, activated carbon, and operated at 250 °C. To further determine the HGC system performances, the prepared Ni-based catalyst was replaced for zeolite as an adsorbent in the HGC system. The tested adsorbents capacity and regeneration performances were also studied in this research.
The kinetic behaviors result shows the activation energy of rice straw is 75.40 kJ/mol and 89.56 kJ/mol under pyrolysis and gasification conditions, respectively. The main gas phases that occur during rice straw pyrolysis/gasification include CO, CO2, CH4, HCl, SO2, aliphatic hydrocarbons, CH3COOH, and some trace gas compounds containing phenol, methanol, etc. The hydrogen concentration of produced gas was also increased from 6.82% to 9.83% with the HGC system used. The maximum syngas LHV was approximately 6.09 MJ/Nm3 as ER controlled at 0.2. Meanwhile, the experimental results indicated that the HGC system for removing trace pollutants has successfully developed. The overall tar removal efficiency was nearly 70% by the HGC system. The HCl and H2S removal efficiencies were approximately 95% and 80%, respectively. Moreover, the enhancing H2 generation and minimizing tar yield using the HGC system combined with prepared Ni-based catalyst was investigated. When zeolite catalyst and hot gas temperature were adjusted from 250 ℃ to 400 ℃, H2 and CO increased slightly from 7.31% and 14.57% to 8.03% and 17.34%, respectively. The tar removal efficiency varies in the 70% to 90% range. When the zeolite was replaced with prepared Ni-based catalysts and HGC operated at 250 ℃, H2 contents were significantly increased from 6.63% to 12.24% resulting in decreasing the hydrocarbon (tar), and methane content.
The trace metals Zn, Cr, Cd, and Pb partitioning in the gas phase were increased with equivalence ratio increased. The crystalline phases of some elements in the particulates were found as SiO2, KClO3, K2SO4, K2Si4O9, CdSiO3, PbCl2, and Pb8Zn(SiO7)3 by XRD identification. The thermodynamic equilibrium simulation results confirmed the dominant gaseous species produced from rice straw gasification, such as KCl(g), NaCl(g), KO(g), K2O(g), ZnCl2(g), CrO2Cl2(g), CuCl2(g), PbCl2(g), PbO(g), and Cd(g). Besides, the metals removal by the hot gas cleaning system was found in decreasing order as: K > Cr > Ca > Pb > Mg > Cd > Na > Zn > Cu. Activated carbon showed a good performance for adsorbing Pb, Cd, Cr, Ca, K, and Mg.
The H2S adsorption capacity order decreased as the activated carbon > calcined dolomite > zeolite. The adsorption capacity of zeolite, calcined dolomite, and activated carbon are 2.22, 3.16, 5.88 mg S/g adsorbents, respectively. In addition, the tested zeolite and activated carbon could be fully regenerated at 350 °C with a stable sulfur adsorption capacity during four adsorption regeneration cycles. In this study, breakthrough curves obtained with several adsorbents for H2S and HCl were shown. Very different breakthrough and saturation times were observed. About HCl, the best adsorption performance was obtained with calcined dolomite and zeolite, in the case of H2S, the best adsorption performance was obtained with activated carbon. In conclusion, the HGC system is proposed as an effective way for improving the syngas quality and reducing trace contaminants emission in rice straw gasification. The results gained from this study could significantly support researchers obtaining more information about the control of syngas contaminants. |
