Fuel-rich stage

The NOx storage-reduction (NSR) catalyst, developed by Toyota and other companies, offers a solution based on a two step process, in which the engine switches periodically between a long lean-burn stage and a very short fuel-rich stage. The NSR catalyst combines the oxidation activity of platinum with a NOx storage compound based on barium oxide. Figure 10.10 illustrates the principle of operation. 

In the lean-burn stage all exhaust components are oxidized by the Pt catalyst and NO is oxidized to NO2. The latter reacts with the basic storage compound BaO to yield Ba(N03)2. In the fuel-rich stage which only lasts for seconds, the reducing agents CO, H2, and hydrocarbons in the exhaust stream are able to reduce the Ba(N03)2 to give N2, CO2 and H2O. Figure 10-3 illustrates the overall process. 

Burch. T. E., Tillman, F. R., Chen, W. Y., Lester, T. W., Conway, R. B., and Sterling, A. M.. 1991, Partitioning of Nitrogenous Species in the Fuel-Rich Stage of Rebuming, Energy and Fuels, American Chemical Society, March/April, pp. 231-237. 

Similar behavior was observed for LNG clouds during both continuous and instantaneous tests, but average flame speeds were lower the maximum speed observed in any of the tests was 10 m/s. Following premixed combustion, the flame burned through the fuel-rich portion of the cloud. This stage of combustion was more evident for continuous spills, where the rate of flame propagation, particularly for LNG spills, was very low. In one of the continuous LNG tests, a wind speed of only 4.5 m/s was sufficient to hold the flame stationary at a point some 65 m from the spill point for almost 1 minute the spill rate was then reduced. 

Reburning is a process involving staged addition of fuel into two combustion zones. Coal is fired under normal conditions in the primary combustion zone and additional fuel, often gas, is added in a reburn zone, resulting in a fuel rich, oxygen deficient condition that converts the NO, produced in the primai y combustion zone to molecular nitrogen and water. In a burnout zone above the reburn zone, OFA is added to complete combustion. 

FIGURE 7.7 Schematic of a low-NOx/SOx pulverized coal burner. The addition of oxygen is staged to produee an initial fuel-rich zone in the burner that results in reduced emissions of nitrogen oxides. Courtesy, M. P. Heap, Energy and Environmental Research Corporation. 

Other observations of the reaction of hydrazine and nitrogen tetroxide substantiate the production of non-equilibrium combustion products. Non-equilibrium product concentrations were found in combustion gases extracted from a small rocket combustion chamber through a molecular beam sampling device with direct mass spec-trometric analysis (31) (39). Under oxidizer rich conditions excessive amounts of nitric oxide were found under fuel rich conditions excessive amounts of ammonia were found. A correlation between the experimentally observed characteristic velocity and nitric oxide concentration exists (40). Related kinetic effects are postulated to account for the two stage flame observed in the burning of hydrazine droplets in nitrogen dioxide atmospheres (41) (42). 

Thus, the most likely survivors of fuel-rich, first-stage pyrolysis are unsubstituted and oxygen substituted aromatics and nonbasic nitrogen compounds. These are the precursors to soot and N0x formation during oxygen-rich, second-stage combustion. 

Compared to PFD and ASD, CSD SRC has the potential for producing a low carbon (<10%) fly ash under low N0X> staged combustion conditions if flame temperature can be maintained sufficiently high during both fuel rich and fuel lean stages, thereby making the CSD SRC fly ash amenable to collection in electrostatic precipitators. 

In the case of air staging, the conversion of fuel nitrogen in the fuel-rich zone follows a reaction path through the rapid initial formation of cyanides, their subsequent conversion to amines, and eventual conversion to molecular nitrogen (middle column in Fig. 2). The combustion is then completed in a fuel-lean zone by injecting additional amounts of combustion air. 

---