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ISSN Print: 1064-2285
ISSN Online: 2162-6561
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NOx FORMATION AND EMISSION CHARACTERISTICS OF A 600 MW UNIT UNDER OFF-DESIGN CONDITIONS
ABSTRACT
With the rapid spread of renewable energy, pulverized-fired (PF) power plants are forced to participate in load cycling to ensure peak modulation and balance electricity demand and supply. The load fluctuation for PF power plants has led to a new focus problem in the study of NOx formation and emission characteristics under off-design conditions. The difficulty in this study is due to the complex physical processes and large-scale research objects. An integrated simulation method is proposed to investigate the NOx formation and removal processes of a 600 MW unit under off-design conditions systematically. The method combines the computational fluid dynamics (CFD) method, the boiler thermodynamic calculation method, and the NOxremoval model. The CFD method is adopted to analyze the combustion and NOx formation processes and obtain the NOx formation value. The NOx removal model is proposed to study the selective catalytic reduction (SCR) denitration process and calculate the NOx removal efficiency accurately. At the operating condition of the SCR device, the boiler thermo-dynamic calculation method is used to calculate the flue gas temperature and velocity in each heat exchanger, including the economizer outlet (i.e., SCR inlet). The formation value of NOx and its removal efficiency can facilitate the acquisition of the NOx emission value. The temperature in the furnace decreased with the load, while the oxidizing atmosphere remained unchanged from the boiler maximum continue rate (BMCR) to the turbine heat acceptance (THA) condition and then increased from the THA to the 50% BMCR condition. The NOx formation value decreased from 211 mg·m-3 of BMCR to the lowest value (204 mg·m-3) of THA and then increased to 245 mg-m-3 of 50% BMCR. The SCR inlet temperature and the flue gas flow rate decreased with the PF plant load reduction. The aforementioned factors coupled with the NOx formation value initially decreased the NOx removal efficiency from the BMCR to the THA condition but rapidly increased until the 50% BMCR condition. The NOx emission value increased from 31.15 mg·m-3 to 37.79 mg·m-3 , 39.68 mg·m-3, and 41.04 mg·m-3 during the load reduction from BMCR to THA, 75% BMCR, and 50% BMCR conditions, respectively.
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