重型工業爐數值模擬分析

Abstract

本研究係以商業套裝模擬軟體ANSYS FLUENT模擬重型工業爐的燃燒行為，分別為熱風爐與切線式燃煤鍋爐。熱風爐的研究目的為找出高爐氣(BFG)及空氣的最低預熱溫度並保持爐內燃燒的穩定性。數值模擬分析主要使用一氧化碳(CO)及氫氣(H2)的四步化學反應機制，並且考慮熱傳導現象的影響。在參考案例中，廢氣出口與爐頂溫度隨時間變化之趨勢在模擬與實驗中皆相同，並且得到最佳的模擬格點為168萬。根據各種不同預熱溫度等參數分析，當預熱溫度高於473 K時，火焰結構達到穩定及對稱性。由結果顯示，建議將高爐燃氣及空氣的溫度預熱至520 K以上，此時火焰長度為燃燒室半徑的4.24倍。 切線式鍋爐的主要研究目的為找出阿達羅粉煤與焙燒木屑顆粒的最佳混合比例，本研究針對六種不同的混燒比例進行數值分析(0%(純煤)、20%、40%、60%、80%、100%(純焙燒木屑)，並採用兩種不同熱裂解機制(pryo1與pryo2)進行參數研究。pryo1參數條件參考文獻[39]及[40]，pryo2則使用中鋼公司所提供之參數。由模擬結果可之，爐內最高溫度通常發生於13 m的位置，該位置也為主要的燃燒區域，其pryo1與pryo2模擬預測的最高溫度分別為1,493 K 與 1,658 K。使用pryo2之熱裂解反應式模擬純煤(0%)及純木頭(100%)的燃燒，可發現該位置(13 m)之平均溫度相差了159 K，由結果顯示可知，燃燒效率並不會因為焙燒木屑的取代而降低太多(pryo1則相差79 K)。最後，添加焙燒木屑能使NOx濃度降低，燒比例在40%以上時，NOx的排放量有大幅降低的趨勢，當比例達到80％，NOx排放的下降速率才逐漸減緩。由結果得知，而純焙燒木屑(100%)比純煤(0%)的燃燒能降低87.1 ppm的NOx濃度。

This research applied the commercial code, FLUENT, adopt four-step chemical mechanism to simulate the burning behaviours of blast furnace gas (BFG) in a hot-blast stove. The main purpose of FLUENT is to find the lowest preheated BFG and air temperatures while maintaining a stable burning operation to save energy. The emphases were on utilizing the four-step chemical mechanism for both CO and H2 combustion in BFG and considering the influence of heat transfer through the stove wall. In the reference case, the trends of the experimental and simulated temperatures of the dome and hot blast with time were the same, and the grid number used was 1,680,000. In the present parametric study of the preheated air temperatures, the results indicated that the preheated temperature must be above 473 K, and the flame structures, stable and symmetrical. According to the results, the suggested preheated BFG and air temperatures are 520 K and above, and the flame length, times larger than the chamber diameter. The research of tangential coal fired boiler is to use Adaro coal powders mixed with Torrefied wood pellets in a boiler. The proportions of Torrefied wood in the mixed fuel are 0% (pure coal), 20%, 40%, 60%, 80% and 100% (pure Torrefied wood), respectively. Two types of devolatilization mechanisms termed as pyro1 and pyro2 are used in the parametric studies. Pyro1 is adopted from references [39] and [40], whereas pyro2 is derived from the measurements by CSC. The simulation results show that the highest temperature usually occurs at 13m-height position, corresponding to the main combustion area in the boiler. The highest temperatures are 1,493K by using pyro1 and 1,658 K by pyro2, respectively. The temperatures and NOx emissions at 13m-height as a function of Torrefied-wood proportion. By using the devolatilization process of pyro2, it shows that the temperature difference in such location is only about 159K between 0 % and 100 % wood ratios, indicating that the combustion efficacy is reduced not too much by using the wood completely to replace the coal. (The corresponding value is 79K for pyro1) It also shows that the decrease of NOx emission is apparent as the proportion of Torrefied-wood is more than 40 %. As the proportion reaches 80 %, the declining rate of NOx emission is being slowed. The NOx emission by burning the case 40% of Torrefied wood in the solid fuel is only about 87.1 ppm less than that by burning pure Adaro coal (0% wood).