Thermal NOX, formation

NOx. When 02 enrichment is used in an existing air/fuel combustion system, there may be an increase in NOx emissions,41 which is due to the increased flame temperature that increases thermal NOx formation. This is discussed in more detail in Chapter 2. 

NOx formation can occur by three mechanisms a) the Zeldovich mechanism for thermal NOx formation, b) the prompt mechanism and c) the fuel nitrogen mechanism. For clean nitrogen-free fuels, only the first two mechanisms occur. 

NOx formation occurs by a complex reaction network of over 100 free-radical reactions, and is highly dependent on the form of nitrogen in the waste. Nitro-compounds form N02 first, and then NO, approaching equilibrium from the oxidized side. Amines form cyano intermediates on their way to NO, approaching equilibrium from the reduced side. Using air as the oxidant, NO also forms from N2 and 02. This last is known as thermal NO. ... 

A partial combustion is achieved in a first zone with excess fuel, after which air is added under intense turbulence. The final combustion occurs under lean conditions in the second zone. The rich conditions in the first zone ensure low NOx levels and combustion stability, whereas the large air excess in the second zone, and hence the low temperature, avoids the formation of thermal NOx. This approach yields 50% reduction in NOx emissions today [27] as compared to conventional diffusion flame combustors. The problem with... 

Thermal NOx The formation of thermal NO, is described by the Zeldovich mechanism ... 

Catalytic incineration (complete air oxidation) for the purification of gas streams is now quite commonly used in many applications (1-7), being preferred in these over thermal (non-catalytic) incineration and adsorption methods. It can offer advantages over thermal incineration in terms of costs, size, efficiency of destruction, and minimization of thermal NOx by-product formation. The catalytic incineration systems are now commonly employed in such applications as exhaust emission purification from a variety of industrial processes (including manufacture of organic chemicals and polymers) and air-stripping catalytic processes used to clean contaminated water or soil. 

The use of noble metals in catalytic combustion applications allows for fuel lean feeds and lower gas temperatures, thus avoiding the formation of thermal NOx... 

The chemistry of the conversion of coke-bound nitrogen on the FCC catalyst into NOx, though not yet very clear, is analogous to the three modes of NO formation in ordinary combustion,namely thermal-NOx, prompt NOx and fuel-NOx- The conversion of fuel/coke nitrogen first to HCN and NH3 and then to NOx is schematically shown in Figure 8. 

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). 

Most models assume that the fuel-bound nitrogen that is released by the devolatilization of coal is in the form of HCN, or some instantaneous transforms of HCN, which in turn form the base species of NO formation. It is believed that the HCN not only contributes to fuel NOx formation but also to some destruction of NOx and that the net formation might depend on the chemical as well as the thermal state of the mixture. The global chemical reactions involved for coal flames might therefore be expressed as... 

Conventionally NO formation is divided into thermal NOx and fuel NOx, with the former coming from either the Zeldovich mechanism... 

Catalytic combustion can produce a stable surface flame at low fuel/air ratio and temperatures as low as 1300°C, and this avoids the formation of thermal NOX. Operating costs are also significantly lower.Compression np to an oper-... 

The prompt mechanism predominates at low temperatures under fuel-rich conditions, whereas the thermal mechanism becomes important at temperatures above 2732 °F (1500 °C). Due to the onset of the thermal mechanism the formation of NOx in the combustion of fuel/air mixtures increases... 

PAN is known to play an important role in tropospheric chemistry. As discussed in this section, its thermal decomposition releases both N02 and an organic free radical, so that it can act as an NOx reservoir and ultimately as a source of OH in the dark. In addition, PAN is a strong lachrymator (eye irritant), is mutagenic in certain bacterial assays, and is phytotoxic to plants. Because of these broad effects on a variety of systems, its formation and reactions have been studied in some detail. 

As discussed in other chapters of this book and summarized in Chapter 16, the formation of tropospheric ozone from photochemical reactions of volatile organic compounds (VOC) and oxides of nitrogen (NC/) involves many reactions. Concentrations are therefore quite variable geographically, temporally, and altitudinally. Additional complications come from the fact that there are episodic injections of stratospheric 03 into the troposphere as well as a number of sinks for its removal. Because 03 decomposes thermally, particularly on surfaces, it is not preserved in ice cores. All of these factors make the development of a global climatology for 03 in a manner similar to that for N20 and CH4, for example, much more difficult. In addition, the complexity of the chemistry leading to O, formation from VOC and NOx is such that model-predicted ozone concentrations can vary from model to model (e.g., see Olson et al., 1997). 

The coupling of heterogeneous reactions on the catalyst surface and homogeneous gas-phase reactions, as discussed in the previous section, is important for the design and operation of a catalytic combustor with maximum temperatures over 900 C, which is the case for gas turbine combustors. It is worth pointing out that the ignition of the fuel-air mixture over the catalyst at much lower temperatures than possible for homogeneous gas-phase combustion is the reason why catalytic combustors can operate at flame temperatures as low as 1100 C [53]. Hence, the formation of thermal NO, which is the most important type of NOx for gas turbine combustors, is practically avoided. 

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