Rich-lean combustion systems

In the following, a review of the traditional and novel concepts of catalytic combustion for GTs is addressed, with emphasis on the requirements and challenges that the different applications open to catalysis. The most relevant characteristics of PdO-supported catalysts and of transition metal-substituted hexaaluminates (which have been most extensively considered for lean combustion applications) are described, along with those of noble metal catalysts adopted in rich combustion systems. 

Although gas turbine combustion systems operate with overall air/fuel ratios which are quite fuel-lean, perhaps three times stoichiometric, stabilization of the combustion process requires that a portion of the combustor, the primary zone, operate stoichiometric or fuel-rich. Under fuel-lean conditions, fuel-bound nitrogen can be converted directly to N0X. Under fuel-rich conditions, fuel-bound nitrogen can be converted to HCN and NH3 in addition to N0X. Of course, in either case, the most desirable product of converted fuel nitrogen would be molecular nitrogen,... 

Figure 2.2 shows NO and CO as functions of the 02/CH4 stoichiometry for CH4 combusted with an oxidizer consisting of 95% 02 and 5% N2. This represents one type of oxygen-enhanced system. The graph shows the strong dependence of both pollutants on the stoichiometry. At fuel-rich conditions (stoichiometry < 2), NO decreases, while CO increases. Also, the fuel efficiency is reduced since the fuel is not fully combusted. At fuel-lean conditions (stoichiometry > 2), NO increases, while CO decreases. Again, the fuel efficiency is reduced because the excess oxidizer carries heat out of the process. In order to minimize both CO and NOx, the combustion system should be operated close to stoichiometrically, which also maximizes the fuel efficiency. 

The effects of probe materials, such as metal and quartz, as well as the probecooling requirements, have been investigated for sampling gases in combustion systems.12 Several studies have found that both metal and quartz probe materials can significantly affect NO measurements in air/fuel combustion systems, especially under fuel-rich conditions with high CO concentrations.13 14 However, the NO readings were not affected under fuel-lean conditions. 

These systems were tested in several catalytic reactions, giving promising results for methane rich combustion and ethanol steam reforming (BaRhxZr(i. x)03), and for methane lean combustion (BaPdxZr(i x)03). 

As decreases below 1.0, the amount of fuel relative to the amount of oxidant decreases and the mixture becomes lean. In chemical terms, the system is overoxidized, and, if the fuel concentration drops too low relative to the oxidant, combustion cannot take place. When is greater than 1.0, there is more fuel relative to the oxidant, the mixture is rich, and the system is imderoxidized, which can also be taken to an extreme at which combustion is impossible. An imderoxidized system favors production of the less oxidized product and releases less heat. 

Tests are needed to determine the effects of multicomponent lean and rich mixtures on the performance of deflagration and detonation flame arresters. Combustion of lean mixtures can result in spin and galloping detonations which have more focused and higher pressures, and thus are of greater concern with respect to the structural integrity of flame arresters and other pipeline devices (e.g., fast-closing valves). Lean mixtures are more prevalent than stoichiometric mixtures in most manifolded vent systems. 

From other more recent studies of NO formation in the combustion of lean and slightly rich methane-oxygen-nitrogen mixtures as well as lean and very rich hydrocarbon-oxygen-nitrogen mixtures, it must be concluded that some of the prompt NO is due to the overshoot of O and OH radicals above their equilibrium values, as the Bowman and Seery results suggested. But even though O radical overshoot is found on the fuel-rich side of stoichiometric, this overshoot cannot explain the prompt NO formation in fuel-rich systems. It would appear that both the Zeldovich and Fenimore mechanisms are feasible. 

In fuel-rich systems, there is evidence [8, 22, 23] that the fuel-nitrogen intermediate reacts not only with oxidizing species in the manner represented, but also competitively with NO (or another nitrogen intermediate) to form N2. This second step, of course, is the reason that NO yields are lower in fuel-rich systems. The fraction of fuel nitrogen converted to NO in fuel-rich systems can be as much as an order of magnitude less than that of lean or near-stoichiometric systems. One should realize, however, that even in fuel-rich systems, the exhaust NO concentration is substantially greater than its equilibrium value at the combustion temperature. 

The NSR technology has been also applied to diesel engines, and is most reliable and attractive method for lean-burn combustion vehicles. Diesel particulate-NOx reduction system (DPNR) method is used to realize the simultaneous and continuous reduction of particulate and NOx is also recommended. This catalyst system is DPF combined with NSR catalyst. Soot on catalyst is removed during NOx reduction by occasional rich engine modification. Many other catalyst systems with NSR catalyst have been also developed. With decreasing S content in fuel and successive development of... 

A lean NOx trap (LNT) (or NOx adsorber) is similar to a three-way catalyst. However, part of the catalyst contains some sorbent components which can store NOx. Unlike catalysts, which involve continuous conversion, a trap stores NO and (primarily) N02 under lean exhaust conditions and releases and catalytically reduces them to nitrogen under rich conditions. The shift from lean to rich combustion, and vice versa, is achieved by a dedicated fuel control strategy. Typical sorbents include barium and rare earth metals (e.g. yttrium). An LNT does not require a separate reagent (urea) for NOx reduction and hence has an advantage over SCR. However, the urea infrastructure has now developed in Europe and USA, and SCR has become the system of choice for diesel vehicles because of its easier control and better long-term performance compared with LNT. NOx adsorbers have, however, found application in GDI engines where lower NOx-reduction efficiencies are required, and the switch between the lean and rich modes for regeneration is easier to achieve. 

Recent concepts to achieve NOx purification from lean exhaust gas are emerging. In the NSR system, the material favors the reactive adsorption of NO to N02 which is stored as a nitrate complex in 02 rich atmosphere. The exhaust gas is then switched to a stoichiometric or HC rich environment in which the nitrate is thermodynamically unstable. The stored NOx is then released and catalytically reacts with excess HC species in the exhaust gas to form N2. In the SNR system, the NOx are temporarily stored on an adsorbent and periodically recirculated to the combustion chamber to be decomposed in the combustion process. In these processes the key point is the adsorption/desorption of N02 which has been... 

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