Research On Gas-particle Flow Characteristics For A660MW Down-fired Boiler With Over-fire Air

Down-fired combustion technology is one of the techniques applied for burninghard-to-burn coal as for its special boiler structure, it has become the main boiler forfiring low volatile coal fuels with dozens years developing. However, practicaldown-fired boiler operations still suffer from the problem of unstable combustion, highcarbon content in the fly ash, serious slagging and a high level of NO_xemissions.This paper focus on high NO_xemissions from a660MWeFW down-fired boilerunit, experiments were carried out using particle dynamics anemometer (PDA) on atwo-phase test facility of a small scale-modeling and its ratio is1:15. This studyinvestigate into Gas/Particle flows characteristics in the boiler with the over-fire air onthe arches.This paper includes two parts:The gas/particle flows characteristics were studied with OFA ratio change whilethe total volume unchanged. The result show that with the OFA ratio decrease, thepenetration depth of the fuel rich air-flow increase, As the OFA ratio increases from0%to20%the fuel rich air-flow penetrate under the center of F-tier secondary air.andthe pulverized coal particles trajectory become longer, which is better to burn outWhile OFA ratio is25%it only penetrate above the center of F-tier secondary air.Which is disadvantageous to the burnout of coal particle? With the OFA ratio increase,the rigid of OFA become strength. When the OFA ratio increase from0to15%, OFAcannot penetrate to the center of the furnace. And it cannot mix with the up-flowinggas which increases the carbon content in fly ash. When OFA ratios are20%and25%,OFA can mix with the up-flowing gas and penetrate into the center of the furnace.Comprehensive consider the penetration depth of pulverized coal in the verticaldirection and OFA flow in the horizontal, an optima OFA ratio is20%isrecommended.Under the condition of the OFA ratio is20%determined previous, by reducing theF layer secondary air nozzle area, the velocity is same without OFA equipment. Thegas/particle flow characteristics in the furnace were investigated and compared withthe OFA ratio is20%and without OFA equipment. The result indicates that the increasing F-tier velocity has little influence on the zone under the arch compared tothe zone near the furnace hopper. The higher F-tier secondary air can help the fuel-richpenetrating a deeper depth than the OFA ratio is20%and accordance without OFAequipment. So the OFA ratio is20%and reducing the F layer secondary air nozzle areacan prolong the pulverized coal trajectory Increase the time of pulverized coal in thefurnace and the degree of burnout of pulverized Therefore, F-tier secondary velocityshould increase in practication with the OFA ratio is20%in the future design.