Influence of the mass ratio of pulverized-coal in fuel-rich flow to that in fuel-lean flow on the gas/particle flow and particle distribution characteristics in a 600 MWe down-fired boiler

Abstract

Fuel rich/lean combustion is an important means of reducing NOx in coal-fired boilers. The mass ratio of pulverized-coal in fuel-rich flow to that in fuel-lean flow (MRL) is a key parameter in fuel rich/lean combustion that affects the gas/particle flow characteristics and in turn the coal ignition and NOx formation. In this work, using a 1:40 scale model furnace and a particle dynamics anemometer measurement system, the effects of various MRLs (6:4, 7:3, 8:2 and 9:1) on the gas/particle flow characteristics in a 600 MWe down-fired boiler incorporating multiple-injection and multiple-staging were studied. The results demonstrate that airflows injected from the front-arch burners flow downwards along the front wall and then turn upward in the furnace center zone, forming a U-shaped flow pattern. Increasing the MRL significantly raises the maximum particle volume flux under the fuel-rich flow nozzle, which favors advanced pulverized-coal ignition and improves the fuel rich/lean combustion in the boiler. The velocity decay of the downward gas/particle flow is also accelerated. The particle volume flux in the hopper zone is gradually reduced while the location of the maximum particle volume flux gradually moves away from the hopper declined wall. This scenario avoids the downward flame flushing the furnace hopper and prevents both hopper wall overheating and tube bursts in the actual down-fired boiler. Based on these results, an MRL of 9:1 is recommended as optimal.