Experimental Study And Numerical Simulation On Micronized-Coal Reburning And Deep Air Staging

Nitrogen oxides (NO_x) are one of the major gaseous pollutants emitted from the coal-fired boilers. With the increasingly strict emission standards, the NO_x reduction for utility coal-fired boilers has been an important research topic.Micronized coal reburning and air staging are two kinds of typical in-furnace NO_x reduction technologies with advantages of high NO_x reduction efficiency and cost effective feature. Micronized coal reburning is capable of providing more than 50% NO_x reduction without increase in the fly ash carbon content. And the NO_x emissions can be reduced to 350mg/Nm~3 with the use of deep air staging in the supercritical and ultra-supercritical boiler firing bituminous coal. This work was involved in the study on these two NO_x reduction technologies for the utility coal-fired boilers.Based on 863 Programme, micronized coal reburning was demonstrated at a 200MW tangentially coal-fired boiler equipped with the ball type pulverizer system. Micronized coal contained in the tertiary air was used as the reburning fuel and the oxygen level of the tertiary air was reduced with the use of flue gas recirculation. During the performance tests, the effects of the operating variables on the NO_x emissions and boiler performance were investigated, including OFA velocity, pulverizing system operating mode, FGR, overall excess air, overall coal fineness and micronized coal content in the tertiary air. The results of the long-term tests show that NO_x emissions can be reduced by approximately 40% under the optimal operating condition, with no significant impact on boiler performance. The object of 50% NO_x reduction was not achieved mainly because of the insufficient reburning coal fraction and the short reburn zone residence time.In this work, the detailed numerical simulation on tertiary air reburning was carried out. During the simulation, the gas temperature profile in the furnace, flyash carbon content and NO_x emissions were predicted in different conditions of micronized coal content in the tertiary air and OFA nozzles elevation. The results indicate that NO_x reduction rate of 56% can be obtained without adverse impact on boiler performance with satisfying the following conditions: the OFA nozzles are shifted upward in a proper range, each zone in the furnace is operated at reasonable stoichiometry, and the micronized coal content in the tertiary air is increased to 20%. The 1000MW dual circle tangential firing boiler equipped with the low NO_x concentric firing system (LNCFS) is an advanced boiler-manufactured technology. Deep air staging was carried out with the use of CCOFA, SOFA and CFS jets. In this work, a physical isothermal flow model study was conducted to investigate the fluid mechanic performance. During the tests the following phenomena was observed. When the air flow was evenly distributed among all of the nozzles, dual tangential circles were established in the single furnace without mutual disturbance. Both circles will offset if the air distribution was nonuniform in half part of the furnace. The residual rotation at upper furnace can be eliminated by using the counter-tangential SOFA air, which is possible to cause the reverse air rotation in the upper furnace. So the levels and angles of counter-tangential SOFA nozzles should be chosen appropriately as to achieve the reasonable counter-tangential moment.In order to reduce the conversion of char-N to NO_x, a new idea was put forward in which the Over Fire Air was divided into several stages and entered the furnace at different elevations. Several multistage SOFA schemes were designed for the 1000MW dual circle tangential firing boiler equipped with the LNCFS system, and the numerical simulation technology was applied to evaluate the feasibility. The simulation results show that multistage SOFA schemes can reduce the rebound extent of NO_x concentration obviously when the first-stage OFA was injected into the furnace. The NO_x emissions at the furnace exit are determined by the NO_x formation quantity in the latter stages of burnout zone. In order to achieve lower NO_x emissions at the furnace exit, char-N should be promoted to release as much as possible in the first and/or second stage of burnout zone where the oxygen content was lower relatively. The effectiveness and feasibility of multistage SOFA schemes should be demonstrated more strictly before practical application.