Generation and reduction of nitrogen oxides in firing different kinds of fuel in a circulating fluidized bed

Abstract

791The process through which nitrogen oxides are generated in a fluidized bed in the course of burning the particles of solid kinds of fuel was studied on an experimental setup made in the form of an electric fur nace fitted with a cylindrical heating chamber with an inner diameter of 50 mm and an air distribution hood welded into the bottom. During the experiments, we checked the temperature of fluidized bed composed of 460 μm fractions of fused alumina (or ash particles 2.5 mm in size) and air flowrate. In addition, the com position of combustion products was continuously measured using different types of gas analyzers (GIAM 5 for CO 2 , Bekman 951 A for NO x , GIAM 5M for CO, and DET V for О 2 ) and a computerized data acquisition and processing system. Brown coal from the Moscow region, Berezovo coal, Donetsk anthracite, Kuznetsk lean coal, undersized coke of Kuznetsk coal, Estonian shale, and Volga shale were used as fuel [1].For carrying out the experiments, we sieved fuel fractions with particles having medium sizes of 0.8, 1.5, 2.5, 4, 6, and 8.5 mm; small weight charges (0.5-2 g) were fired in the fluidized bed at different temper atures.Experiments involving preliminary blowing of flu idized bed with air at a temperature of 900°С showed that so called thermal nitrogen oxides (i.e., those gen erated from the molecular nitrogen contained in air according to Zeldovich' mechanism [2]) are not gen erated under the conditions of the experiment. In view of this, all generated nitrogen oxides were subse quently regarded as fuel ones, originated from the nitrogen contained in fuel.The data shown in Fig. 1 illustrate how nitrogen oxides and carbon dioxide are generated in firing small weight charges (0.5 g) composed of particles of coal from the Moscow region and anthracite. For all coals with a high content of volatiles, an interconnec tion between the changes of CO 2 and NO x concentra tions is observed: during the combustion of volatiles, a synchronous and very rapid growth of С NO and concentrations takes place, and during the combus tion of coke residue these concentrations decrease gradually to zero. If fuel with an insignificant content of volatiles is fired, the maximum of CO 2 and NO x concentrations is not so prominent: nitrogen and car bon oxides are generated more uniformly. For quanti tatively estimating the generation of CO 2 and NO x , we calculated their relative masses and that have released by a certain moment of time τ using the following expressions:where τ b o is the experimentally determined full time for which the fuel charge burns out.At the combustion stage, nitrogen oxides are gener ated more intensely than СО 2 . The relative fraction of nitrogen oxides released for the time of visible combustion of volatiles is a quantitative parameter using which this intensity can be estimated. Figure 2a shows this parameter as a function of the content of volatiles in the fuel.It should be noted that the authors of the majority of proposed calculation procedures [3][4][5] Federal University, ul. ...

Modern design solutions in the design of boiler units, ovens and fireboxes burning solid fuel in a fluidified bed

“…It should also be noted that in a fluidized bed there is practically no formation of so-called "thermal" nitrogen oxides [13]. In addition, NO x levels are significantly reduced thanks to the staggered air supply.…”