Thermal DeNOx process

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... 

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... 

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. 

NOx-removal catalysts are offered by Haldor-Topsoe, KTI, and others. The Thermal DeNOx process offered by ExxonMobil is non-catalytic. 

Hgure 10-5. Temperature dependency of the THERMAL DeNOx " process. (Exxon, 19821... 

Figure 10 8. THERMAL DeNOx " process flow diagram. Haas, 1992A Hurst, 1983t...

The efficiency of the Thermal DeNOx process (Lyon, 1975) for conversion of NO to N2 in combustion systems has stimulated considerable interest in the reaction between NH2 and NO. It is now generally accepted that this reaction proceeds directly, but controversy continues regarding the probable elementary reaction channels, their individual rate constants, and the overall rate constant at high temperatures. Miller et al (1981) have recently reviewed this reaction from a theoretical and modeling viewpoint here we emphasize recently reported experimental data. 

The Possibility of Modifying Thermal DeNOx Chemistry to suit Diesel Engines The Use of Other Radicals NH2 is not the only kind of radical known to be capable of reducing NO. Hydrogen abstraction from HNCO can form the free radical NCO which is known to react rapidly with NO, reducing it to N20. The initial report of the RAPRENOx process8 claimed that HNCO was capable of reducing NO at temperatures as low as 592°C,but subsequent work showed that reduction of NO by HNCO produces much more N20 than does the reduction with Nt and that the two processes have essentially identical temperature windows. Since the reaction sequence... 

Denox A generic name for processes for removing nitrogen oxides from flue gases by catalyzed reaction with ammonia. See also Thermal DeNOx. 

The temperature of DeNOx reaction (function 3) comparing Figure 5.5a (NO TPD, Cat nr) to Figure 5.5b - TPSR in the presence of a three-function catalyst (CoPd/HMordenite, Cat I ), in a complete flowing feed N0/HC(CH4)/02 (excess) -the temperature of DcNOx is that of the NO thermal desorption. According to the model, the catalyst will have to produce C H O . (CH3OH, HCHO) (function 2) to proceed to the DcNOx process, as discussed in Section 4.2. 

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