Application And Therotical Study On Direct Ignition With Oxygen-enriched Air For The Pulverized-coal Fired Boiler Of Power Plant

The primary energy resource of oil and coal are valuable and limited. For the sustainable utilization of the energy resource, saving oil and improving coal utilization efficiency have got more and more attention in power plant. Against the background, a novel technique of no-oil ignition——pulverized-coal direct ignition with oxygen-enriched air, was proposed and investigated in the paper.The concept of pulverized-coal direct ignition with oxygen-enriched air was described as: the oxygen-enriched pulverized-coal flow is ignited directly in cold conditions and able to combust stably without other heat source. And the basic principle is that the combustion heat release rate and the heat capacity of flue gas per unit volume, both of which are bound to increase greatly responsible for high concentration of oxygen in pulverized coal flow, preserve a proper level of flame temperatures, backflow heat and ignition distance, and thus provoke a steady combustion.Given that the brand new concept and technique of pulverized-coal direct ignition with oxygen-enriched air, the basic theories of combustion and practical application in engineering are studied thoroughly and in detail in the paper. The related work includes:(1) The combustion characteristics of the pulverized-coal with different oxygen concentrations are studied by thermal analysis technology. Conclusions can be drawn that, ignition temperature and ignition heat of pulverized coal are reduced at high oxygen concentrations. But it is limit between the differences of coal quality. The combustion rate, theoretical combustion temperature and heat capacity of flue gas which increasing more times enabled the spontaneous and stable combustion of pulverized coal in cold conditions. Meanwhile, the combustion characteristics of coal char are studied with thermal analysis. Combustion kinetic parameters of coal char are calculated by the mathematic optimization Algorithm (Differential Evolution, DE). The DE separation method was proposed and the kinetic parameters of the coal char combustion with multi-stage at high oxygen concentration were studied. (2) Experimental study was done in a large horizontal furnace, which was involved with testing the feasibility of pulverized-coal direct ignition with oxygen-enriched air by using swirl burner and straight burner, as well as tuning the combustion parameters. In the cold furnace, the cold pulverized-coal flow from the swirl burner could be ignited by a torch when the primary air oxygen concentration is above 30.1% and the secondary air oxygen concentration is above 23.2%. In this case, the mixed air oxygen concentration was 25.1%. Furthermore that from the straight burner was ignited directly, with the minimum oxygen concentration 27.9% and the minimum pulverized-coal concentration 0.32kgC/kgA. In the oxygen-enriched air condition, the pulverized-coal flow was ignited rapidly after leaving the nozzle. The flame was singular bright and completed more quickly.(3) For cutting down the consumption of oxygen, two oxygen enrichment modes were proposed:partial oxygen enrichment and overall oxygen enrichment. By adding oxygen into the center of the flow from the edge of the blunt body, partial oxygen enrichment consumed oxygen only one third of that of overall oxygen enrichment which adding oxygen into the primary air pip. Based on the straight burner of an 420t/h boiler, an oxygen-enriched straight burner was specially designed for direct ignition with oxygen-enriched air. The combustion characteristics of the new straight burner were simulated by means of numerical computation. The result shows that the primary air velocity, pulverized-coal concentration and the flow velocity of oxygen for the partial oxygen enrichment burner have their own optimum values, while for the overall oxygen enrichment burner, the combustion of the pulverized-coal flow is strengthened significantly with the increasing of oxygen concentration.(4) The technique of pulverized-coal direct ignition with oxygen-enriched air was studied on its project development, engineering application and experiments in a storage pulverized-coal boiler of 125MW unit with straight combustion burners. It deserved great success. Either partial oxygen enrichment or overall oxygen enrichment, the oxygen-enriched burner can both be ignited directly in cold conditions; the combustion of the pulverized-coal flow is steady with bright flame and absence of black smoke. Some meaningful results can be obtained from the experiments. The combustion efficiency of oxygen-enriched ignition exceeds 90%. The requirements of raising temperature and boosting pressure of the boiler could be completed with oxygen-enriched ignition burner, which has a higher combustion efficiency of 87% (non-exact sampling) than other no-oil ignition techniques. Oxygen-enriched burner was qualified as the main burner to bear the designed load of the boiler. What's more, oxygen-enriched burner owns a good ability to adapt to the low loads to 25MW without additional oil or oxygen help. Finally, there was obscure influence on the primary air system operating.(5) Pulverized-coal direct ignition with oxygen-enriched air leads to fewer investment cost than other techniques of no-oil (or tiny-oil) ignition if it was used on more than one boiler in plant. Because of the higher coal combustion efficiency with the oxygen-enriched burner, there are also fewer operating costs, except the obscure difference of the auxiliary fuel (oxygen, oil, and electricity) costs.(6) The coal ash was produced under the condition of oxygen-enriched combustion and studied on the slag characteristics. The results show that the oxygen-enriched combustion increases somewhat the ash fusion point and that the oxygen-enriched ignition does not result in a significant deterioration of slag trends. The fusion procedure of the coal ash was investigated with the method of thermal analysis. The results indicated that the atmosphere mainly affects the fusion degree of the mineral matters rather than the characteristic temperature of mineral matters in coal ash. Further investigation on the evolution of mineral matters in the course of fusion was performed based on the characteristic temperatures of thermal analysis.