DeNOx processes

Exxon Thermal DeNOx Similar to SCR, the Exxon Thermal DeNOx process utilizes the NO /ammonia reaction. However, this process does not use a catalyst to aid the reaction. Rather, tightly controlled temperatures are used to steer the reactions. Optimum reaction temperatures are found between IbOOT (871°C) and... 

The backbone of the DeNOx process over mononuclear TMI encaged in zeolites can be epitomized in the form of three interconnected cycles associated with the formation of the N2 and 02 reaction products (Figure 2.6), inferred from the steady state and transient rate data combined with spectroscopic evidence for surface species and... 

Their decomposition into NO and 02 is apparently the most difficult step of the whole DeNOx process and requires elevated temperatures (Figure 2.24). Most likely it takes place in two following steps ... 

Pietrzyk, P., Gil, B. and Sojka, Z. (2007) Combining computational and in situ spectroscopies joint with molecular modeling for determination of reaction intermediates of deNOx process -CuZSM-5 catalyst case study, Catal. Today. doi 10.1016/j.cattod.2006.09.033. 

This question has to be answered to completely understand the DeNOx process, and design the final efficient catalyst, according to the nature of reductant and the experimental conditions (more particularly, temperature window). 

Finally, Iwamoto and Takeda [9] assumed that the decomposition of RNOx species leads to the formation of oxygenated compounds CxHyOz which could be the intermediates of the global DeNOx process. This last assumption corresponds to a fundamental step of the model described in this chapter. 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

If the preceding requirements are fulfilled, then the DeNOx process (function 3) does not need a large amount of reductant, as it is very often claimed the stoichiometry of 2NO -t-C H CX, = N2 + xCO/CO, + v/2 H20 should be considered. Clearly, it is generally impossible to avoid the competition between the Oads left by NO and the Oads due to 02 dissociation, for the total Q II/l. oxidation on function 3 (this competition corresponds to a kinetic coupling of at least two catalytic cycles, through Oads [13]). Both of them contribute to the total oxidation of reductants. 

From 320°C on, methane starts being consumed leading directly to C02. The beginning of C02 formation corresponds to the maximum of N02 desorption. Steady-state results (figure not shown) confirm that simultaneously with C02 formation, N2 formation also occurs (deNOx process effectively starts at 320°C). Methane reacts with N02, probably leading to the formation of oxygenated species and NO [18] according to reaction ... 

NO is then reduced to N2 and mild oxygenated species are completely oxidised to C02, by reacting with the adsorbed oxygen species left during the NO reducing process, so regenerating the active sites responsible for the deNOx process ... 

DeNOx (1) A Denox process for removing nitrogen oxides from the gaseous effluents from nitric acid plants. The oxides are reduced with ammonia, over a catalyst containing potassium chromate and ferric oxide. Developed by Didier Werke in the 1980s. 

A further interaction comes into play when the thermal DeNOx process is used to reduce NO,.. When stack gases cool and initial sulfur is present in the fuel, the S03 that forms reacts with water to form a mist of sulfuric acid, which is detrimental to the physical plant. Furthermore, the ammonia from the thermal DeNO, process reacts with water to form NH4HS02—a glue-like, highly corrosive compound. These S03 conditions can be avoided by reducing the S03 back to S02. Under stack (post-combustion) temperatures, the principal elementary reactions for S03 to S02 conversion are... 

P. Glarborg, K. Dam-Johansen, J.A. Miller, R.J. Kee, and M.E. Coltrin. Modeling the Thermal DeNOx Process in Flow Reactors. Surface Effects and Nitrous Oxide Formation. Int. J. Chem. Kinetics, 26 421-436,1994. 

Thermal DeNOx Process Costs and Requirements in Application to Boilers and Furnaces With a modest 1.5 to 1 excess of NH3 over NO t he Thermal DeNOx reaction is capable of reducing NOx to levels which in an ideal case would be very low. For NFf at 290/ ton this corresponds to a cost of 161/ton of NOx removed. In some applications hydrogen is not needed but in others it is. For some of the latter applications it is readily available and inexpensive. In others hydrogen must be generated by NHg decomposition. For a situation in which H2 at 2/1 ratio to NF is needed, the NFf thus used would cost 214/ton of NOx removed. 

Subsequent work by other research groups (see Schmidt and Bowman and references cited therein3) resulted in the development of improved computer models. The number of elementary reactions used was increased to 127. These additional reactions while they are unimportant under the range of conditions used in the Thermal DeNOx process, improve the model because they allow it to make predictions beyond this range. The adjustable parameters used in our model were replaced with experimentally determined rate constants. 

The sequence of reactions 2,3, and 4 has a chain branching factor of 2, but, the overall chain branching factor for 1, 2, 3, and 4 is 2(1- a) where a is the ratio of the rate constant for reaction 2 to the sum of the rate constants of reactions 1 and 2. Since reaction 1 is faster than 2, a is less than one. This means that for a self sustaining chain reaction to occur another reaction which provides additional chain branching is necessary. It is generally agreed that in the Thermal DeNOx process reaction 5 has that role... 

DENOX A Denox process engineered by Kinetics Technology International for retrofitting on gas turbine engines for generating electrical power. First operated in 1995. 

The ratio of NO/NO2 in exhaust gases of power plants is usually about 95/5. The nitrogen oxides, during the DENOX process, are reduced by NH3 at the catalyst surface while the following reactions occur... 

All these remarks recall the work on several system where an oxygenated HC is involved as a reaction intermediate in the DeNOx process. Zn has a promoting effect in an oxidising atmosphere contrary to the reactivity under hydrogen where inhibiting effects are usually observed ]. The role of Zn (or ZnO) could be to favour the adsorption and formation of the reaction intermediate. The nature of the interaction between Pt and Zn still need to be elucidated. 

Figure 5.9 A schematic of the Thermal DeNOx process. Source [76].

There are a number of processes developed for noncatalytic NOx removal. The thermal DeNOx process (Figure 5.9) and the NOx out process (Figure 5.10) are typical examples. In the first, the gas phase reaction between NOx and ammonia occurs at 870—1200°C using air or steam as a carrier gas ... 

Fellows, W. D., Application of the thermal deNOx process to glass melting furnaces, 1989 Fall International Symposium, American Flame Research Committee, International Flame Research Foundation, 1989. 

Shimizu. K. and Oda, T. DeNOx process in Hue gas combined with nonthcrmal plasma and catalyst IEEE Trans. Ind. AppUcat.. 1999, 35, 1311-I3I7... 

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