Ammonia formation

Acceptable levels of ammonia in the gas stream are dictated by local regulations. Gasification systems must meet emissions regulations however, the concentrations of ammonia are relatively low with common feedstocks in most gasifiers. 

Ammonia in the product stream is undesirable because it can result in the formation of NOx emissions when the product gas is burned. Cleanup of the ammonia is therefore required for systems in locations with strict NOx regulations. 

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Clearly, strong 0-H interaction occurs leading to dissociation of ammonia, formation of OH groups, dehydroxylation, and surface imide formation. The NH(a) species has a characteristic N(ls) value of 398 eV, i.e., 1 eV greater than N(a) and 1 eV less than NH2(a). 

The in-series two-step pathway described above is able to account for the temporal evolution of the product which is observed during the reduction of stored NO, with nearly complete N2 selectivity at the beginning of the reduction process and significant ammonia formation near the end of the regeneration step (see Figure 13.17). 

FIGURE 4.22 The plug on the left is soaked in hydrochloric acid and that on the right in aqueous ammonia. Formation of ammonium chloride occurs where gaseous hydrogen chloride and ammonia meet. The reaction occurs closer to the HC1 plug because HC1 has the greater molar mass and thus its molecules diffuse more slowly. 

The increase in amide-like ammonia formation can be explained by the mechanisms given in Reactions 20-27. From these it would be expected that the yield of amide-like nitrogen would be doubled by the presence of oxygen since in the absence of oxygen (Reactions 20-23) two a-carbon free radicals are needed to form one amide group. In the presence of O2, every a-carbon free radical which combines with O2 is transformed into an amide group (Reactions 20 and 24-27). It is also possible that in the presence of O2, amide-like ammonia arises from reactions in the side chains. One such possibility for PGA is discussed in the next section. 

In the reduction of nitrite, Horold found that supports with lower surface areas showed improved selectivity for nitrogen over ammonia. The same study found that silica supports had much higher selectivity (i.e. much less ammonia formation) than alumina supports. (Horold et al. 1993)... 

If a reacting solution is acid quenched then a small amount of hydrazine, derived probably from an intermediate reduction product, is formed. The rate of ammonia formation is linear in PN2 and depends on the square of vanadium concentration, but the actual rate is a function of dioxygen pressure, metal contaminants, etc. The mechanism is believed to follow the pattern discussed above, with dinitrogen bound between two dinuclear pairs of vanadium(II) ions (232, 233), but the precise identity of the fixing species remains a mystery. 

Indirect methods of lowering blood ammonia are, in the main, more effective than the chemical techniques. These include withdrawal of protein from the diet (M3, P7) and antibiotic therapy (F3, S6). A completely protein-free diet leaves little nitrogen in the gut for bacterial action. Antibiotic therapy, particularly by die oral route (S10, W5), so alters the gut flora as to diminish the number of ammonia-forming organisms. It must be kept in mind that urea is always present in the gut as a substrate for ammonia formation even on a nitrogen-free diet. 

Genzel Y, Ritter JB, Konig S, Alt R, Reichl U (2005) Substitution of glutamine by pyruvate to reduce ammonia formation and growth inhibition of mammalian cells. Biotechnol. Prog. 21 58-69. 

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