Study On Pulverized Coal Combustion And NO_x Emission Characteristics In High-Temperature Air And Enriched-oxygen Environment

The oxygen-enriched and high temperature air combustion (HTOEC) is apromising pulverized coal combustion technology. In the paper, the combustion andNO_xformation characteristic of the pulverized coal under the high temperature andoxygen-enriched conditions was fully investigated through literature study, theoreticalanalyses and experimental study, model burner assessment and numerical simulation.First, in the laboratory, TGA, drop tube furnace and entrained flow reactor (Henckenburner) were used to study the combustion kinetic and ignition behaviors of the coalparticle in oxygen-enriched and preheating conditions. Then, a model HTOECcoal-fired burner was developed. A test rig was established and a set of experiments hasbeen conducted to validate the combustion and NO_xemission performance of the modelHTOEC burner. At the same time, corresponding studies were conducted numericallyby using the Eddy Dissipation Concept (EDC) model with several improvements, andthe predictions were consistent with the experimental results. And the improvements,which could be also applied to the general simulation of the turbulent combustion ofpulverized coal include (1) A new method to estimate the volatile content based on theproperties of the coalï¼›(2) The consideration of the intermediate reactions of the volatilematter in the homogeneous reaction and NO_xformation;(3) Adoption of the extinctiontheory proposed by Magnussen to cease the homogeneous chemical reaction when itscharacteristic time scale is greater than that of the turbulence;(4) The noveltwo-mechanism-zone method.The main conclusions are as follows:1) Based on the laboratory experiments, the increase of oxygen concentration andheating rate could cause the reduction of the ignition time and ignition temperature ofthe pulverized coal particles, enhancing their combustion intensity and burnt-out.2) The HTOEC model burner experiments showed that with the special burnerstructure and proper flow organization, the HTOEC method can be achieved for thepulverized coal with a small flow rate of oxygen-enriched flow. The HTOEC modelburner has excellent performance in flame stabilization, fuel flexibility, assisted-oilsaving and low NO_xemission, and thus has a great potential for industrial application. 3) The numerical simulation results showed that under the high temperature andoxygen-enriched conditions, the flame temperature and combustion intensity of thepulverized coal jet increases, and the release rate of the volatile matter rises. With asmall flow rate of oxygen-enriched flow, consistent with the experimental results, NO_xemission could be remarkably reduced. NO_xemission first decreases and then increaseswith the oxygen concentration in the oxygen-enriched air flow, and the turning pointhappens at about50%.4) With the consideration of the intermediate reactions of the volatile matter andthe adoption of the extinction theory developed by Magnussen, the perdition accuracyof the EDC model increases in simulating the turbulent pulverized coal combustion.Combining the EDC model with the skeletal mechanizing and the traditional postprocessing method, and correction of the experimental data, NO_xemission of theturbulent pulverized coal combustion can be predicted with an acceptable accuracy.Dividing the computational domain into a core reaction zone and a non-core zone andlimiting the complex chemistry in the core reaction zone can effectively save the CPUcost and thus allow the industrial application for the modified EDC model with higheraccuracy.