污水污泥热解制取富氢燃气实验及机理研究.pdf.doc.doc
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1、 华中科技大学博士学位论文污水污泥热解制取富氢燃气实验及机理研究姓名:熊思江申请学位级别:博士专业:市政工程指导教师:章北平2010-11-07 华 中 科 技 大 学 博 士 学 位 论 文根据 Coats-Redfern法,采用 11种常见机理函数对不同升温速率下干、湿污泥的热解主体阶段进行线性模拟,并结合 Malek 法筛选出最为合理的机理方程,求解其表观动力学参数,并建立动力学方程。在保证热解充分进行的条件下,升温速率对于干燥污泥热解主体阶段的活化能影响不大,但湿污泥在该阶段的活化能则随着升温速率的提高而增加;同一升温速率下,湿污泥热解主体阶段的活化能大于干燥污泥。利用气相色谱-质谱联
2、用仪和傅里叶红外光谱仪等检测设备,研究了不同含水率污泥在不同温度下热解所得主要产物的分布和组成,探讨了温度和含水率对热解产物特性的影响规律,进而对产物的形成途径及热解机理进行了系统分析。结果表明,水蒸气在大分子有机化合物的裂解过程中起着至关重要的作用,水分的参与促进了有机质蒸汽重整反应的发生,有利于CO和H2等气体的生成,在减少有毒物质和结焦现象产生的同时,提高了富氢燃气的产量;另外,水蒸气还促进了挥发份的析出,使残炭中的有机质含量大幅减少,热解更加完全。当湿污泥被送入热解炉时,水分迅速脱除形成高温水蒸气,同时挥发份析出,一部分热解初期产物和焦炭会与高温水蒸气发生蒸汽重整反应和气化反应,生成
3、H2、CH4、CO2和CO等气体;随着温度继续升高,大分子有机物发生二次裂解,生成大量H2、CO和C2、C3等小分子气体,还有一部分有机质通过缩合、环化、芳香化反应,形成大分子PAHs,同时产生大量H2,因此H2的大幅增长标志着焦油二次反应的发生。此外, PAHs的形成过程可用Diels-Alder反应机理解释。对 1000下干燥污泥和含水率为 84%的湿污泥的热解过程进行了质量和能量衡算分析,通过干、湿污泥热解系统建立的质量平衡误差分别在 1.0%和 6.4%左右;湿污泥的热解反应热 Qp远高于干燥污泥,但其中有超过 70%的能量被用于水分蒸发和水蒸气的升温,实际热裂解工艺热 Qpyroly
4、sis则明显低于干燥污泥;热解过程可回收大量高品质的富氢燃气、焦油及固体残炭,其中热解气不但可直接用于热解系统的供热,还可作为管道煤气或化工原料,焦油也可在回收加工后当做石油的替代燃料,固体残炭则可作为建筑材料;烟气余热的回收利用对湿污泥热解制气技术的推广具有十分重要的作用。从拓展湿污泥热解形式,提高产品附加值,降低热解成本的角度出发,研究了V 华 中 科 技 大 学 博 士 学 位 论 文干、湿污泥和秸秆的掺混比例对共热解产物分布、热解气组成、热值及气体能量转化率的影响,结果表明随着污泥掺混比例的提高,共热解产生的气体和焦油的产率减少,固体残炭产率显著增加,相同的掺混比例下,湿污泥/秸秆共热
5、解的气体产率均大于干燥污泥/秸秆共热解的气体产率,这是由于在污泥/秸秆共热解过程中,秸秆的添加可补充碳源的不足,并提高原料的挥发分含量,水分可作为气化剂和氢源,有利于富氢燃气的生成,但随着湿污泥掺混比例的进一步升高,原料挥发份含量下降,碳氢化合物的析出减少,气体产率显著降低,且 H2、CO和 CnHm的产量也明显下降,气体整体产量的减少和高热值 CnHm气体比率的降低也造成了气体热值和相应气体能量转化率的下降。关键词:湿污泥 热解 资源回收 机理 能量平衡 混合热解VI 华 中 科 技 大 学 博 士 学 位 论 文AbstractDue to the ever increasing amou
6、nt of sewage sludge and increasingly stringentenvironmental standards, the problem of sewage sludge disposal is becoming more seriousthan before. Meanwhile, as a type of biomass, sewage sludge has the potential forenergy-oriented use because of its high organic content and heating value. As an econo
7、micand environmental acceptable route for waste disposal, pyrolysis is one of the mostpromising methods of sludge resource utilization in the future and has attractedincreasingly worldwide attention. The moisture content of sewage sludge has an importanteffect on the pyrolysis process. On the one ha
8、nd, the increasing moisture content improvesthe energy consumption of pre-drying process, leading to a cost increase in the pyrolysis;on the other hand, the presence of water in the raw sewage sludge favors the production ofpyrolysis gas and the generation of hydrogen-rich fuel gas. This thesis stud
9、ied thepyrolysis process of wet sewage sludge at high temperatures which completing the drying,pyrolysis and gasification of wet sewage sludge in a single thermal process. The mainfindings of this study are summarized as follows:The effects of heating mode, temperature, moisture content and heating
10、rate on thecharacteristics of products yield and gas composition were analyzed in a tubular furnace.The results indicate that the main components of pyrolysis gas include H2、CO、CO2、CH4 、C2H2、C2H4、C2H6、C3H6 and C3H8. High quality hydrogen-rich fuel gas can beobtained when wet sewage sludge is sent in
11、to the reactor quickly at the setting temperature.High temperature is an advantageous circumstance for reducing the yields of solid andliquid and increasing the production of hydrogen-rich gases. The increase of moisturecontent favors the steam reforming of the volatile compounds and a partial gasif
12、ication ofthe solid char residue, which leads to the generation of H2 and other fuel gases and theheating value of generated gas could reach over 12 MJm-3. Additionally, high heating rateprolongs the residence time of the volatiles in the hot zone, resulting in an increase innon-condensable gases an
13、d a decrease in the yields of tar and char.The kinetic experiments of dried sewage sludge and wet sewage sludge with amoisture content of 84 wt% were carried out by using thermogravimetry - differentialthermal analysis (TG-DTA) at different heating rates. The TG curves of dried sludge haveVII 华 中 科
14、技 大 学 博 士 学 位 论 文an obvious weight loss peak, indicating that the pyrolysis process of dried sewage sludgecan be divided into 3 stages: dehydration, main stage of pyrolysis and carbonization stage;while the TG curves of wet sewage sludge have two significant weight loss peaks,indicating that its pyr
15、olysis process can be divided into 4 stages: dehydration, early stageof pyrolysis, main stage of pyrolysis and carbonization stage. The pyrolysis process shouldbe promoted by high heating rate, although the effect of high heating rate on the pyrolysisof wet sewage sludge is much more significant tha
16、n that on the pyrolysis of dried sewagesludge. The Coats-Redfern method was employed to seek the most probable kineticmechanism function of the main stage of pyrolysis from 11 kinetic functions. The Malekmethod was used to screen the best mechanism equation, and the kinetic parameters werecalculated
17、 according to the mechanism function determined. With a sufficient pyrolysis,the effect of heating rate on the activation energy of the main stage of pyrolyzing driedsewage sludge is insignificant; however, the activation energy of the main pyrolysis stageof wet sewage sludge increases with heating
18、rate increasing. Moreover, the activationenergy of the main pyrolysis stage of wet sewage sludge is proved to be much higher thanthat of dried sewage sludge at the same heating rate.Sewage sludge with different moisture content was pyrolyzed at differenttemperatures in a tubular furnace. The distrib
19、ution and composition of pyrolysis productswere detected by GC-MS and FTIR, respectively, in order to investigate the influence oftemperature and moisture content on the characteristic of pyrolysis products. Further, apreliminary mechanistic understanding of wet sewage sludge decomposition from star
20、tingconstituents to final products was developed. The results show that the high moisturecontent of wet sewage sludge favors the steam reforming reaction of volatile compounds,which promotes the production of hydrogen-rich fuel gas and reduces the generation oftoxic substances and coking phenomena.
21、Meanwhile, the presence of moisture favors theejection of the volatile matter of sewage sludge, and reduces the organic content of char.Normally, a steam-rich atmosphere would be generated as soon as the wet sewage sludgeis sent into the furnace at high temperature, which gave rise to in situ steam
22、reformingreaction with volatile compounds and a partial gasification of the solid char reidue,generating H2、CH4、CO2 and CO etc. With temperature increasing, a secondary crackingof large organic compounds would occur, generating a large amount of H2, CO, C2 and C3hydrocarbons; simultaneously, some PA
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