Experimental Studies And Numerical Simulations On HAP Low-NOx Combustion Technology In A Down Fired Boiler

Down-fired boilers are widely applied for burning anthracite and lean coal owing to the long distance of flame penetration and large amounts of recirculation flue gas. However, there are many problems such as high NOx emissions, hardness to burnout and serious slagging in daily operation of down-fired boilers. Aiming at these problems, a new combustion technology based on deep-staging combustion concept is proposed in this paper. It is hot air packing low-NOx technology (HAP) which necessitates the hot-air ports adding in the furnace ash hopper (SA-H) and furnace bottom (SA-B) as well as a secondary-air declination. These retrofitting methods are supposed to decrease NOx emission as well as unburnt carbon content in fly ash and avoid serious slagging through applying the deep-air-staging, packing pulverized coal flow, increasing the pulverized-coal residence time.This dissertation investigates the aerodynamic flow fields, combustion characteristics and NOx emissions in a pilot3.5MW facility with HAP technology in a step-by-step route of cold small-scale single-phase modeling experiments, cold state single-phase numerical simulations, hot tests, and hot state numerical simulations. The results can provide a theoretical foundation and references for industrial applications of HAP low-NOx technology.Cold small-scale single-phase modeling experiments evaluate the characteristics of aerodynamic flow fields and the effect of various factors after applying HAP technology. By comparing with the prior art without HAP technology, HAP technology can reach a deeper penetration depth of primary air and a higher filling ratio of the lower furnace, and it also can disappear air flow washing the furnace wall and slight slagging in furnace. Moreover, HAP technology also has a much better airflow distribution and larger recirculation flue gas without dead air recirculation in the ash hopper zone. The measurements were carried out for different injection angles of the wall secondary air and hopper secondary air as well as the location of hopper secondary air. The results indicate that the case with45°wall secondary air and horizontal fed hopper secondary air equipped on upper location perforns the best. It is found that the simulation results are consistent well with measured values, which verified HAP’advantages of reaching a deeper penetration depth of primary air and larger recirculation zone without a dead recirculation in the ash hopper zone. Simulation experiments also show that the wall secondary air with45°and up-located horizontal-fed hopper secondary air is the best choice.Guizhou anthracite combustion tests were carried out to evaluate the boiler performance applying HAP technology on a3.52MW down-fired boiler. The results indicate that HAP technology can avoid generating large amount of NOx in the high-temperature lower furnace. Bottom secondary air and OFA have great impact on unburnt carbon content of pulverized coal particles but little impact on temperature distribution; hopper secondary air influences NOx emission a lot. The tests also indicate that NOx emission can increase as the excess air coefficient goes up while temperatures increase firstly and then become stable. It is better for the excess air coefficient to be1.1, which can have a lower NOx emission and unburnt carbon content of pulverized coal particles. The unburnt carbon content of fly ash can control in the range of5%±3%and NOx emission700±200mg/m3by HAP technology. Compared with the normal down-fired technology, HAP technology has relative reductions of27.23%-61.68%of NOx emissions without dropping down boiler efficiency.Finally we did hot state numerical simulations of HAP technology. The results show that a deeper penetration depth of primary air and longer residence time of pulverized-coal particles appeare in the furnace with HAP technology and a large amount of recirculation flue gas backflow can ensure sufficient heat for the pulverized-coal ignition. The contrast case generates NOx emissions a lot in the middle part of the lower furnace and ash hopper zone while HAP technologies do not. The dead recirculation zone can be eliminated and a reductive atmosphere is formed, which goes against to NOx generation. Overall, simulated and experimental results consistent well and each case with HAP technology have NOx, reductions of about50%compared with the contrast case. NOx, emission increases with the excess air coefficient increase, and grows faster if the excess air coefficient is greater than1.3.