Abstract:

This article investigated the effects of coal bed thickness, which could change heating rate and product distribution, on pyrolysis of coal in a rectangular radial flow fixed-bed reactor. Increasing coal bed thickness prolonged the time required to heat up coal at bed center to the preset temperature, increased the yield of pyrolysis products water and gas, decreased the yield of char and tar oil, but the content of light component in tar oil tended to increase, and the calorific values of char and gas slight decreased. For example, at furnace wall temperature of 900℃, when coal bed thickness increased from 45 mm to 105 mm, the tar oil yield lowered by 12.7% from 7.17%(mass, the same below) to 6.26% (dry coal basis), char decreased from 80.0% to 77.0% and heat value from 24348.5 kJ·m^-3 to 20649.2 kJ·m^-3, and the light fraction in tar oil in tar raised by 8.5% from 67.0% to 72.7%, and water and gas yields increased from 6.96% and 5.91% to 8.85% and 7.90%, respectively. It was found that the heat value of char was varied with the bed position and more coal bed thickness, more difference: on the radial direction the heat value was higher for the char at region with higher temperature than for that with lower temperature, while on the axial direction the heat value for all char at different region was almost the same. The calorific value of the char in the bed increased gradually from the high-temperature zone to low-temperature zone, indicating the fact that the deeper pyrolysis of coal occurred in the high-temperature zone. It is well known that there is the flow of gas and liquid products by pyrolysis from high temperature zone to low temperature zone and heavier fraction in tar oil can be trapped by low-temperature coal layer if coal bed was thicker. The trapped heavy fraction can then be cracked to form light fraction once the temperature of this coal layer raised enough high as the pyrolysis process proceeded. This indicated that the total tar oil yield decreased due to formation of much light fraction for the case of thicker coal bed. As a consequence, controlling the flow of pyrolysis product from high-temperature zone to low-temperature zone is critical for manipulation of the secondary reactions of primary pyropysis products and thereby for increase of the final yield and quality of the expected pyrolysis products. The study clarified again that in the pyrolysis reactor the optimal matching between the flow of gaseous pyrolsyis product and the gradients of temperature and tar composition is critical for realization of the selective cracking of heavy species and for improvement of the yield and quality of pyrolysis tar product.

Key words: coal pyrolysis, radial flow reactor, coal bed thickness, secondary reaction, reaction control, moving bed with internals

摘要：

考察了方形径向流固定床煤热解反应器中变化煤层厚度对料层升温速度及煤热解产物分布特性的影响。随着料层厚度增加,导致煤热解反应要求的时间增长,热解水和气的产率相应增加,焦油和半焦收率逐渐降低,但焦油中轻质组分(沸点低于360℃组分)含量呈升高趋势,半焦和煤气热值稍许降低。如,加热壁温度900℃、从45 mm至105 mm增加煤料层厚度时,焦油产率从7.17%(质量,下同)下降到6.26% (相对干基煤),但焦油中的轻焦油组分含量则从67%升至72.7%,半焦产率由80.0%降至77.0%,热解水和煤气产率分别由6.96%和5.91%增至8.85%和7.90%,煤气热值则由24348.5 kJ·m^-3下降至20649.2 kJ·m^-3。所得半焦的热值径向上由高温侧向低温侧逐渐降低,煤料层越厚、热值降幅越大,而相同煤料层厚度处与加热壁平行的同一轴向平面上的半焦热值基本相同。针对研究的反应器,气相热解产物在反应器内沿径向(横向)由高温料层区向低温料层区流动。在该过程中伴随着热解产物对远离加热壁的低温煤料的传热、热解生成重质组分的冷凝和在煤/半焦颗粒表面的吸附截留,进而在低温料层进一步升高温度时发生二次裂解等物理化学过程。反应器内煤层厚度越大,上述各种伴随的物化作用越显著,从而明显影响煤料层的升温及热解特性。

关键词: 煤热解, 径向流反应器, 料层厚度, 二次反应, 反应调控, 内构件移动床