基于化学链燃烧处置污泥的新方法及磷资源回收机理

Sewage sludge is generated during wastewater treatment, and consists of organic and inorganic substances, as well as pathogens and disease-causing micro-organisms. Sludge incineration enjoys a combination of several advantages that are not found in other treatment alternatives, including a large reduction of sludge volume to a small stabilized ash, and thermal destruction of toxic organic constituents. One important issue of the conventional air-fired incineration of sludge is the emissions of pollutants gases including dioxins and furans, NOx, SO2, as well as HCl and CxHy. Phosphorus is an element vital for all living organisms, and the world's reserves of phosphate rock are becoming depleted. Phosphorus recovery from sludge ash is an essential aspect during sludge incineration treatment, and it is a relatively new sludge treatment technique. A novel technology for thermal treatment of sewage sludge based on chemical looping combustion (CLC) is proposed in the project. Due to O2 nonexistence in the fuel reactor, the potential production of polychlorinated dibenzodioxins and dibenzofurans can be eliminated, and NOx formation is restrained during the process of chemical looping combustion of sludge. Thus, the catalytic gasification reaction mechanism between oxygen carrier and alkali metals in sludge ash will be revealed in this project. The coupling reaction mechanism between the sludge gasification product and oxygen carrier, the transfer process of sludge gasification products in the subtle channels of oxygen carriers, the regular patterns involving mass transfer, heat transfer and momentum transfer in the multiphase flow and multiphase complex chemical reaction system will be obtained. Furthermore, the mechanism of collaborative control of pollutants in sludge conversion will be investigated. The migration behavior of phosphorus in sludge ash during CLC process is investigated. Phosphorus speciation in the sludge ash of fuel reactor will be identified based on the analysis of X-ray diffraction and X-ray fluorescence. During CLC process, both of the steam presence and the reduction condition in the fuel reactor result in the potential production of water-soluble phosphate. The water-soluble phosphate is favorable as agricultural fertilizers. Therefore, it can be indicated that CLC process is a favorable technology for sewage sludge treatment. The CLC sludge ash can be treated with acid to efficiently recover phosphorus and thus can be considered a potentially renewable source of phosphorus. A study of phosphorus recovery from CLC sludge ash will be investigated by adding sulfuric acid as well as hydrochloric acid to ash for the elution of phosphorus. The effects of acid concentration, temperature and time on phosphate extraction and heavy metal leaching are studied.

作为污水处理的终端产物，生活污泥的产量急剧增加。污泥含有病原体、重金属和持久性有机物等有毒有害物质，极易对环境造成二次污染。磷是人类和各种动植物生命活动必需的营养元素之一，磷资源的匮乏直接导致农作物不能正常生长，危及人类生存；构建磷素的持续循环体系，回收和利用污泥中磷，补偿全球磷矿的衰竭。基于化学链燃烧对生活污泥进行处置，不仅可以克服传统污泥焚烧方式和污染防治模式的局限，而且有望同时实现污染物协同脱除和磷资源回收利用。研究选择适合于污泥化学链燃烧的高反应性能的铁矿石载氧体，揭示污泥化学燃烧反应机理、多相复杂体系中热质传递与控制理论，污泥中磷、碱金属与载氧体协同催化污泥热解气化－载氧体还原反应动力学、气体污染物生成与竞争反应转化机理，污泥中氮、磷、硫、氯、碱金属以及矿物质对载氧体反应活性的影响，污泥灰中碱金属及矿物质对磷迁移、富集的反应机理，污泥灰烬回收磷的技术路线以及抑制重金属的溶出特性。

污泥中含有大量的有机物、无机物、有毒物质、营养物质（氮、磷、钾等）以及致病的微生物等。磷资源有限的储量以及磷单线性迁移过程造成了磷资源的匮乏。回收利用污泥中的磷，有利于补偿全球磷资源衰竭。化学链燃烧是一种新型的燃烧技术，其通过载氧体在两个反应器之间循环，完成了燃料的燃烧过程。与传统燃烧相比，化学链燃烧实现能量的梯级利用，具有CO2富集及低NOx排放等优点。本项目系统研究了污泥化学链燃烧特性及氮磷迁移转化行为。在污泥化学链燃烧过程中，污泥颗粒仅在燃料反应器内转化，没有未燃尽的焦炭颗粒随载氧体进入到空气反应器。燃料反应器温度的升高以及水蒸气浓度的增加均能显著提高污泥的燃烧特性。在长时间运行过程中赤铁矿具有良好的抗磨损性能。采用水泥-赤铁矿为载氧体时，污泥在低温下的燃烧效率显著提高；但是，载氧体中存在少部分Fe2O3还原到FeO，载氧体表面出现轻微的团聚现象。污泥化学链燃烧过程中生成的氮氧化物为NO，其排放量显著低于空气焚烧过程中氮氧化物（NO、NO2、N2O）的总排放量。采用水泥-赤铁矿时，污泥的燃烧效率和燃烧过程中NO的生成率均显著提高；采用Cu-Fe载氧体时，污泥的碳转化率、燃烧效率以及载氧体的反应活性均显著提高，NO生成率降低。在污泥化学链燃烧过程中，磷主要有三个迁移途径：CLC-挥发分磷、CLC-飞灰磷以及CLC-底灰磷。约60%左右的磷富集在污泥飞灰中。相比于污泥焚烧灰中有效磷的含量（1.80%），经化学链燃烧后，CLC-飞灰中有效磷的含量显著提高。磷与赤铁矿相互作用，导致赤铁矿反应活性显著下降。在长时间的循环反应过程中，污泥灰沉积在赤铁矿载氧体表面，与赤铁矿发生相互作用，降低了赤铁矿的反应活性。虽然污泥灰有大量铁和钙的存在，但由于污泥灰孔隙率和比表面积非常低，不利于还原性气体的反应。总之，采用化学链燃烧处置污泥不仅实现污泥的高效转化，而且实现污染物脱除以及磷资源的回收利用。