草浆黑液流化床燃烧直接碱回收的试验研究及数值模拟

发布时间：2018-09-04 19:21

【摘要】：黑液是造纸碱法煮浆段产生的废液,其COD(化学需氧量)排放量占整个造纸过程污染物排放量的90%。因此,黑液的治理就是解决造纸废水污染问题的关键。黑液的主要成分为木质素、半纤维素、碳水化合物的降解物和煮浆段残余的烧碱,因此它也是一种重要的能量和碱的来源。目前,流化床碱回收技术已被证明是一种经济有效的黑液处理方法,该技术可以在利用高温无害化处理黑液的同时回收利用黑液燃烧所产生的热量和碱。但是,当使用流化床燃烧氯、钾和硅元素含量更高的草浆黑液时会出现严重的床料烧结问题,这成为使用流化床碱回收法处理草浆黑液的最大障碍。此外,传统的碱回收工艺流程较为复杂且需要消耗额外能源,因此其经济性被大幅降低。针对以上问题,本文提出草浆黑液流化床燃烧直接碱回收技术,在解决床料烧结问题的同时简化传统碱回收工艺流程。本文首先进行了不同温度和床料条件下的草浆黑液流化床燃烧试验研究,并根据试验得出的最佳运行条件进行了流化床直接碱回收试验。随后,通过试验研究了不同浸泡条件下湿法预处理对于草类制浆原料中氯和钾元素浸出效果的影响。最后,创建与本文所用流化床试验台尺寸相同的三维模型,并根据试验过程中得出的具体操作参数对草浆黑液的流化床燃烧过程进行了数值模拟研究。首先,通过钙基沸石为床料的草浆黑液流化床燃烧试验研究,得出了不同温度下床料的烧结特性、钙基沸石抑制烧结的机理、氯和钾元素气相析出特性以及NO_x和SO_2的排放特性。发现钙基沸石具有良好的抑制烧结效果,它可以保证草浆黑液在700-850°C下稳定燃烧4小时且不出现流化停滞现象。HCl是草浆黑液燃烧时氯元素气相析出的主要形式,且钙基沸石会与钾元素反应从而抑制其析出。其次,通过以碳酸钙为床料的草浆黑液流化床燃烧试验,得出了碳酸钙抑制烧结的机理、碳酸钙对氯和钾元素气相析出特性以及NO_x和SO_2排放特性的影响。随后根据试验结果挑选最佳燃烧条件进行了流化床直接碱回收试验,验证了直接碱回收工艺可行性并计算了碱回收率。发现碳酸钙具有强于钙基沸石的抑制烧结效果,且以碳酸钙为床料的流化床燃烧直接碱回收工艺是可行的,其碱回收率超过90.1%。第三,通过草类制浆原料的湿法预处理试验,研究了浸泡温度、浸泡时间以及浸泡液种类对氯和钾元素浸出效果的影响。发现稻秆和麦秆中分别有大约95.18%的氯元素和85.12%的钾元素可通过湿法预处理去除。浸泡温度的提升能够明显增加稻秆和麦秆中氯和钾元素的浸出速度以及浸出率。此外,0.25mol/L的氢氧化钠溶液不会影响氯和钾元素的浸出,但0.25mol/L的柠檬酸溶液能提高钾元素的浸出率。最后,基于欧拉-欧拉双流体模型,认为颗粒相与气相均为连续相,考虑黑液固形物在炉内的热解、气体反应和异相反应过程,同时依据流化床燃烧试验得到的最优操作参数对草浆黑液的流化床燃烧特性进行了数值模拟研究,得到了气固两相的速度分布、浓度分布、压力分布、湍动特性、各气相产物的浓度分布以及温度场分布。对比燃烧气体产物组分的模拟值和试验值,结果表明本文建立的草浆黑液流化床燃烧模型可靠。
[Abstract]:Black liquor is the effluent from alkaline pulping section of papermaking. Its COD (Chemical Oxygen Demand) accounts for 90% of the total pollutant discharged from papermaking process. Therefore, the treatment of black liquor is the key to solve the pollution problem of papermaking wastewater. It is also an important source of energy and alkali. At present, the fluidized bed alkali recovery technology has been proved to be an economical and effective black liquor treatment method, which can recover heat and alkali from black liquor combustion while using high temperature harmless treatment of black liquor. In addition, the traditional alkali recovery process is more complex and needs to consume additional energy, so its economy is greatly reduced. In view of the above problems, this paper proposes a fluidized bed reactor for straw pulp black liquor. Combustion direct alkali recovery technology can solve the problem of bed material sintering and simplify the traditional alkali recovery process. Firstly, the experiment of straw pulp black liquor fluidized bed combustion under different temperature and bed material conditions was carried out, and the experiment of direct alkali recovery in fluidized bed was carried out according to the best operating conditions. The effects of wet pretreatment on leaching efficiency of chlorine and potassium in straw pulping raw materials under different soaking conditions were studied. Finally, a three-dimensional model with the same size as the fluidized bed test bed used in this paper was established, and the fluidized bed combustion process of straw pulping black liquor was numerically simulated according to the specific operating parameters. Firstly, the sintering characteristics of straw pulp black liquor at different temperatures, the mechanism of calcium-based zeolite suppressing sintering, the gas phase precipitation characteristics of chlorine and potassium, and the emission characteristics of NO_x and SO_2 were obtained by the experimental study on the combustion of straw pulp black liquor with calcium-based zeolite as bed material. HCl is the main form of chlorine vapor phase precipitation in straw pulp black liquor combustion, and calcium-based zeolite reacts with potassium to inhibit its precipitation. Secondly, the mechanism of calcium carbonate inhibiting the sintering of straw pulp black liquor in fluidized bed combustion with calcium carbonate as bed material is obtained. The effects of calcium carbonate on the gas phase precipitation characteristics of chlorine and potassium and the emission characteristics of NO_x and SO_2 were studied. Then the best combustion conditions were selected to carry out the direct alkali recovery test in fluidized bed. The feasibility of the direct alkali recovery process was verified and the alkali recovery rate was calculated. Moreover, the direct alkali recovery process with calcium carbonate as bed material is feasible. The recovery rate of alkali is more than 90.1%. Thirdly, the effects of soaking temperature, soaking time and soaking liquor types on the leaching efficiency of chlorine and potassium were studied by wet pretreatment of straw pulping materials. Chlorine and potassium in rice straw and wheat straw can be removed by wet pretreatment. The leaching rate and leaching rate of chlorine and potassium in rice straw and wheat straw can be significantly increased by increasing soaking temperature. In addition, 0.25 mol/L sodium hydroxide solution does not affect the leaching of chlorine and potassium, but 0.25 mol/L citric acid solution can improve the leaching rate of potassium. Finally, based on the Euler-Euler two-fluid model, it is considered that the particle phase and the gas phase are continuous phase, and the pyrolysis, gas reaction and heterogeneous reaction process of the black liquor solid in the furnace are considered. Velocity distribution, concentration distribution, pressure distribution, turbulence characteristics, concentration distribution and temperature field distribution of the gas products in the two phases of straw pulp were studied.
【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：X793

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