The Technical And Economic Analysis Of The NOx Control Methods For Large Capacity Boilers

The pollution from coal combustion has been the biggest pollution source in China, which impose a significant threat to the environment. Efficient and clean utilization of coal and reducing the pollutant emission are important issues for China—a country with great coal resource and consumption. NO* is one of the main pollutant emissions from coal combustion and should be strictly controlled especially for coal-fired utility boilers, the main consumer of coal. In order to meet the increasingly NO_x emission limit, effective NO_x control technology should be applied to the coal-fired boilers.NO_x emissions can be controlled either by using the low NO_x combustion technology or post-combustion removal process. Both the NO_x reduction efficiency and technology cost should be considered for NO_x control technology selection. This work was involved in the technical and economic analysis of the NO_x control methods for large capacity boilers, the performance of several NO_x reduction methods were investigated combined with engineering cases and comprehensive economic analysis was performed for various NO_x control techniques.In this work, an air staging modification scheme for a 410 t/h tangential fired boiler was introduced. This boiler met serious furnace slag and superheater overheating problem during the past. To avid the serious furnace slag and high temperature corrosion when implement air staging, a novel low NO_x combustion configure named as "the Compound multi-function combustion system" was applied for this modification. The performance of the multi-function combustion system was evaluated using the numerical simulation technology, which shows that this system has clear effect on the prevention of furnace slag and high temperature corrosion. During the practical operation, nearly 40% NO_x reduction efficiency was achieved, the slag and corrosion problems were solved well also.An application example of advanced reburning, combination of coal reburning and SNCR, was investigated by means of numerical simulation and field tests. From this work, the following conclusions were obtained. To avoid furnace slag and high temperature corrosion which may occur during coal reburning, the fuel rich-lean pulverized coal burner could be applied and the size of tangential combustion circle should be moderate. The reasonable arrangement for each elevation of burners, especially for the reburning and OFA burners, are required to improve the performance of reburning, both on the NO_x reduction efficiency and carbon burnout. Also, the air distribution among each elevation burners should be reasonable.The OFA and reburning fuel jet penetration, mixing and dispersion in the furnace influence the performance of reburning system significantly. In this work, Large Eddy Simulation (LES) and standard k-εturbulence model were employed separately to simulate a round and a elliptic turbulent jet in cross flow with the Reynold number of 16600. The numerical results of the trajectory show a good agreement with the previous experimental data. Simulation also captured the dominating vortex systems: shear layer vortical roller, Counter-rotating Vortex Pair (CVP) which initially begins in the near-field and dominates the far-field, and upright vortex in the wake. The differences of the vortex systems determine that the elliptic turbulent jet has stronger penetrability than the round turbulent jet.In this work, the technical features of a SCR system introduced from abroad was studied. The performance of this SCR system, both on NO_x reduction and ammonia slip, was evaluated during the commissioning tests.This work introduced a economic analysis model for the NO_x control methods for large capacity boilers. The capital and annual cost of various NO_x control methods were assessed for several domestic coal-fired boilers using this model, and the suggestion for NO_x control technology selection was put forward.