| Nowadays, it is generally accepted that the increasing emission of CO2 from fossil fuel combustion is the main contributor to greenhouse effect and global warming. Chemical-looping combustion (CLC) is regarded as a promising novel combustion technology, which provides an inherent feature of isolating CO2 during the combustion process with a low energy penalty. Over the past years, studies on CLC of gaseous fuels have been extensively conducted. At present, attention has been paid to the CLC technology with solid fuels (represented by coal). The relevant studies mainly involve the following aspects:design and operation of CLC reactors, performance investigation of oxygen carriers (OCs), and numerical simulation of flow patterns and reaction performance of CLC reactors. The present work is devoted to these aspects by both experimental and simulation methods.A novel coal-fired CLC method based on the high-flux circulating fluidized bed (HFCFB) technology was developed in this study. The HFCFB riser was selected as the fuel reactor (FR), which could provide sufficient solids holdups and favorable gas-solid contact over the whole reactor height to promote the efficiencies of reaction and heat transfer. The cross-flow moving bed was employed as the air reactor (AR) because of its advantages in terms of low pressure drop, steady solids flow, and compact structure, which could be simply placed in the middle of the CFB downcomer to enhance the simplicity and stabilization of the whole system. In addition, a specific two-stage separation system was designed to achieve the selective of large oxygen carrier particles and fine combustible particles from the fuel reactor outlet, which should be beneficial for the carbon capture efficiency and fuel conversion. In detail, the first-stage separator was a low efficiency inertial separator where most of the OC particles could be separated to the AR for regeneration, but the fine unreacted char particles would continue on to the second-stage separator. The second-stage separator was a high-efficiency cyclone where most of the unreacted char particles could be separated and recirculated to the fuel reactor for further reactions.A lean iron ore with low iron content was innovatively proposed as the OC. Performance tests of the OC with coal gases were performed in a laboratory thermogravimetric analyzer (TGA) reactor. Consecutive reduction-oxidation cycles were firstly carried out to evaluate the cyclic stability and the agglomeration tendency of the OC. The effects of temperature, fuel gas concentration, and fuel gas composition on the reduction reactions were further investigated. On the basis of the TGA results, the integrated rate of reduction (IRoR) method and the shrinking-core model (SCM) were adopted to determine the kinetics of the global reduction reaction of the OC and acquire relevant kinetics parameters necessary for the later simulation work.The cold experimental apparatus for the novel coal-fired CLC method was established. The steady operation of the whole system and the flexible adjustments of operation parameters were realized, primarily verifying the feasibility of this novel system. The achievement of high solids fluxes (max up 500 kg/m2s) in the CFB riser greatly enhanced the solids holdups and hence the gas-solid contacts in the FR. The cross-flow moving bed as the AR showed the advantages of low pressure drop, continuous solids flow, and large gas-solids contact area. The specific two-stage separation system showed high selective separation efficiency and global separation efficiency, which could ensure the efficient separation among the OC particles, unreacted char particles and the flue gases at the outlet of FR with the next hot system. The pressure adjustment between the two reactors could control the gas flow directions in the two reactors and restrain the gas bypassing, thereby ensure high concentration and capture efficiency of CO2 in the hot system.Built upon the operational experience achieved with the cold experimental apparatus, a pilot-scale hot unit of the novel coal-fired CLC method was further established. Experiments were conducted using Shenhua bituminous coal as the fuel and the lean iron ore as the OC. After commissioning, the hot unit has been successfully running for a total duration of over 50 h with both stability in operation and good reaction performance, verifying the feasibility of the novel coal-fired CLC method. The effects of fuel reactor temperature on the carbon capture efficiency and the fuel conversion were also studied. During the long-time operation under the high temperature and high-flux conditions, the lean iron ore OC showed adequate reactivity and oxygen transport capacity, good cyclic stability, high resistance to attrition, and low tendency for agglomeration, indicating this natural lean iron ore used in this study should have great potential for applications with future commercial coal-fired CLC power plants.On the basis of the kinetics parameters acquired in the TGA tests and the experimental results from the pilot-scale hot unit, a comprehensive three-dimensional numerical model was developed to simulate the coal-fired CLC process with the novel method. Both gas-solid flow and chemical reactions were considered. The simulation object was a HFCFB fuel reactor by reference to our hot experimental system. The gas-solid flow patterns, species distributions, fuel conversion, reaction rates and other important characteristics were predicted. The simulation results showed a good agreement with the experimental data, verifying the rationality of the model. On this basis, the model was further used to predict the effects of some important operating conditions on the gas-solid flow behaviors and reaction performance of the fuel reactor, achieving the supplement and extension of the experimental studies. |
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