Pulverized fuel combustion systems

Nitric oxide (NOx) is one of the main pollutants in combustion systems and rotary kilns are no exception particularly for pulverized fuel combustion. NOx formation depends on three factors, namely (i) the amount of nitrogen present in the fuel, (ii) the combustion temperature, and (iii) the stoichiometric conditions for the combustion reaction. Hence NOx production is classified into fuel NOx, thermal NOx, and prompt NOx. Some of the mechanisms for the formation of these species during pulverized coal combustion in rotary cement kilns have been described in commercial CFD packages (e.g., FLUENT, CINAR). 

Combustion Systems. Combustion systems vary in nature depending on the nature of the feedstock and the air needed for the combustion process (54). However, the two principal types of coal-buming systems are usually referred to as layer and chambered. The former refers to fixed beds the latter is more specifically for pulverized fuel. 

Initial preheating of the combustion chamber by gas or oil is normally required in order to provide the necessary temperature environment to release the volatiles that provide the stabilization in the base of the flame. Some small PF systems have used another fuel for flame support, but this compromises the economics. A typical pulverized fuel burner is shown in Figure 24.17. 

In the entrained-bed combustor, the feed coal must be introduced to the combustor as small. The particles (generally <200 mesh) are carried by the gas into the furnace and travel in a suspended state through a hot zone where they are consumed. After combustion, approximately 20% of the ash falls to the bottom of the furnace and is removed there. Combustion gases, which contain about 80% of the ash, pass out of the furnace and are treated to remove the remaining particulates (usually in an electrostatic precipitator) and sulfur compounds (usually in a stack gas scrubber). This type of combustion system (PCC) is in common use for large-scale utility boilers. Higher combustion rates can be attained in pulverized fuel combustors than in fixed-bed combustors. 

The thermodynamic analysis of NO formation during fuel combustion is additionally improved by inclusion of trajectory construction problem in it, which is illustrated in Fig. 2,a and b. The Fig. 2 presents the calculation results for the Kansk-Achinsky coal pulverized combustion. Residence time for the reacting agent in combustion zone is taken equal to a second. Fig. 2,a shows solutions to the problem of determining attainable states the state with maximum possible concentration of nitrogen oxide and the state of final equilibrium. The latter was calculated not at the complete system of conditions (29), (30) that determine the region Dj (y), but with an accoimt of the condition for monotony (29) only. Application of the notion of equilibrium that is not related to kinetics in this case... 

Pulverized coal (PC) boilers are the most common large combustion systems for the generation of electricity in the US and the industrialized economies of the world. PC boilers include both wall-fired boilers and tangentially-fired (T-fired) boilers wall-fired boilers include both front wall and opposed wall configurations. Like cyclone boilers, they have the potential to use petroleum coke as a fuel provided that a sulfur dioxide scrubber has been installed on the unit. The vast majority of the petroleum coke fired in the USA is burned in PC boilers due to their dominance of the industry. Again the low volatility in the petroleum coke limits its use in PC boilers the typical cofiring percentage is on the order of 20 - 30 percent (calorific value basis). 

For central station power generation the open cycle system using electrically conducting coal combustion products as the working fluid is employed. The fuel typically is pulverized coal burned directly in the MHD combustor, although in some plant designs cleaner fuels made from coal by gasification or by beneficiation have been considered (8—10) (see Fuels, synthetic). 

Sohd fuels are burned in a variety of systems, some of which are similar to those fired by Hquid fuels. In this article the most commonly burned soHd fuel, coal, is discussed. The main coal combustion technologies are fixed-bed, eg, stokers, for the largest particles pulverized-coal for the smallest particles and fluidized-bed for medium size particles (99,100) (see Coal). 

Ptilverizers The pulverizer is the heart of any solid-fuel suspension-firing system. Air is used to dry the coal, transport it through the pulverizer, classify it, and transport it to the burner, where the transport air provides part of the air for combustion. The pulverizers themselves... 

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