Fuel nitrogen conversion combustors

A Study ot Fuel Nitrogen Conversion in Jet-Stirred Combustors... 

Table II. Combustor < > Results from fuel nitrogen conversion experiments - toluene fuel. t TFN NO hc soar (ms) / % of v, % ofv, vol. %v, % of v fuel N TFN" las CH fuel C ...

Data from the present set of experiments suggest that the conversion of fuel nitrogen to TFN in jet-stirred combustors depends upon the equivalence ratio and average residence time of gases within the combustor, the fuel type and certain physical characteristics of the combustors. However, the effects of these primary variables on fuel nitrogen conversion appear to be related to their effects on the concentrations of unburned hydrocarbons and soot in the exhaust gases. These effects and their relationships to unburned hydrocarbon and soot concentrations are discussed below. 

The observed differences among the data obtained at similar JSC and LFJSC operating conditions suggest that comparisons of data from the various combustors must allow for possible effects of combustor type in the results. Extensions of the results to larger, commercial size gas turbine combustors or to similar laboratory combustors should also be made with extreme caution. Qualitative trends such as the dependence of fuel nitrogen conversion on HC concentrations should be valid within a given combustor however, quantitative numerical results may depend strongly on specific combustor characteristics. For example,... 

A further point is that the equilibrium levels of TFN under fuel-rich combustion conditions are very low. The stable form of nitrogen is N2. In practical combustors, however, equilibrium is not attained because of the slow rates of both chemical and physical (mixing) processes. The chemical processes consist of reactions convering fuel-nitrogen species to N2, the reaction of NO with hydrocarbon species to form HCN, and the subsequent slow conversion of HCN to N2. 

In this paper we report on factors which affect the conversion of fuel nitrogen to TFN in laboratory jet-stirred combustors which serve to simulate the primary zone in a gas turbine. The independent variables in the experiments were fuel type (aliphatic isooctane vs. aromatic toluene), equivalence ratio (fuel-to-oxygen ratio of combustor feed divided by stoichiometric fuel-to-oxygen ratio), average gas residence time in the combustor, and method of fuel injection into the combustor (prevaporized and premixed with air vs. direct liquid spray). Combustion temperature was kept constant at about 1900K in all experiments. Pyridine, C5,H5N, was added to the fuels to provide a fuel-nitrogen concentration of one percent by weight. 

Jonke (33) studied the NO emissions from a coal-flred, fluid-bed combustor operating at a bed temperature below 1300°K and observed 580 ppm NO emissions with both N2-O2 and argon-02. This corresponds to about 25% conversion of the fuel nitrogen. 

The first of these approaches uses controlled air addition to provide a fuel rich primary zone followed by fuel lean intermediate and dilution zones within the gas turbine combustor. The fuel rich primary zone, with an equivalence ratio of about 1,4, promotes the conversion of the ammonia contained in the fuel to nitrogen rather than to NO, The intermediate zone conqiletes the combustion of the fuel to achieve low CO emissions at an equivalence ratio near unity, whilst the dilution zone provides the correct temperature profile to suit the inlet to the gas turbine. The primary zone of the combustor is fully iti ingement cooled. This ensures that the local equivalence ratio... 

Due to the relatively low combustion temperatures in the gas turbine combustors when burning product gas thermal NO, is very low. Total NO emissions can however be higher compared to operation on liquid tijel with steam injection due to the conversion of fuel bound nitrogen into NO. From Figure 7 below the influence of the amount of fuel bound nitrogen is evident. The recorded levels of alkalines have been below 0.1 ppm wt. 

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