Study On Characteristics Of Nitrogen Emissions Of Blast Furnace Gas And Pulverized Coal Blended Combustion In Oxygen-rich Environment

Combined combustion power generation technology has significantly improved theutilization of secondary energy blast furnace gas which produced in steel productionprocess. Currently a total capacity of combined combustion boiler in service is about8000MW. However, most of the gas in the blast furnace gas is non-combustible, theproportion of the combustible gas component is only less than25%, which causesinstability of combustion process and decreases of the boiler efficiency when blendingwith the pulverized coal. In addition, there are still diffusing oxygen excess problems inthe steel industry. Using of the diffuse oxygen is a very important work to achieve energyconservation task to the steel enterprises. Take a300MW blast furnace gas/pulverizedcoal combined combustion boiler of a steel plant owned power station as the object ofstudy, numerical simulations on NOx emission characteristics in the conditions ofmicro-oxygen-enriched (O2/N2) and oxygen-enriched (O2/CO2) are carried out by theFLUENT software. Firstly, over fire air ratio is optimized by the numerical simulation ofthe combustion process under the conditions of different over fire air ratio with the heatblending ratio20%of blast furnace gas. Then the influence of different O2concentrationson temperature field and NO concentration distribution were studied in the conditions ofmicro-oxygen-enriched and oxygen-enriched with the optimal ratio of over fire air. Theresult shows: within the over fire air ratio25%, the overall NO concentration decreaseswith growth of the over fire air ratio. While the over fire air ratio is out of the certainrange, it will cause burning flame against the wall, uneven distribution of the temperaturefield, flue gas flow field is destructed. Taking the heat transfer in the furnace and the NOproduction two aspects into account, the optimal boiler operation over fire air ratio isrecommended within25%; To the micro-oxygen-enriched conditions, the highesttemperature is in the upper region of the burner, the temperature distribution has acorresponding relationship with the concentrations of the components, and the hightemperature region is characterized by high carbon monoxide concentration, low carbondioxide and low oxygen concentration. With the increase of O2concentration, the furnacetemperature field significantly elevates, but in the corner exit off the flame, thetemperature tends to be similar. The NO concentration also increases with the oxygenconcentration elevating which is because of the increase of furnace temperature; To theoxygen-enriched conditions, in the bottom of the furnace and the burner zone, the fluegas temperature increases with the growth of O2concentration, but in the area above theburner, showing an opposite trend, because flue gas and furnace wall radiation heat transfer coefficient increases which causes the radiation heat transfer increasing. The NOconcentration increases with the growth of O2concentration which is because of theincreasing of O2concentration in furnace.Numerical simulations on combustion process in different blast furnace gas blendingratio are carried out in the condition27%O2concentration in the micro-oxygen-enrichedand35%O2concentration in the oxygen-enriched conditions, the results shows: Furnaceexit CO concentration are reduced to a very low levels in two conditions which shows agood burnout. The temperature fields in the furnace shows a decrease trend with thegrowth of the blending ratio, the trend is obviously in the upper of the boiler. The NOconcentration decreases with the blending ratio elevating, the decreasing trend slows withthe increase of blending ratio.