Бурение грунтовых зондов, установка энергетических колодцев

Nitrogen enters the gasifier in two forms, either as molecular nitrogen, generally in the gasification agent (but also as a component in gaseous feeds), or as organic nitrogen in the fuel. Although the bulk of the nitrogen in the synthesis gas is present as molecular nitrogen, most gasifiers produce small amounts of HCN and NH3. There is little literature on the formation of nitrogen compounds in gasifiers. It is, however, possible to draw inferences from the well-researched mechanisms of NOx formation in combustion flames.

Fuel-derived formation of HCN and NH3 is far greater than that formed from molecular nitrogen, so that in most cases the latter can be neglected. Fuel nitrogen is often contained in structures with N-H and N-C bonds, which are much weaker than the triple bond in molecular nitrogen. The typical mechanism for NO formation during complete combustion can be depicted as follows:

Initially, fuel nitrogen is converted to HCN, which rapidly decays to NH{ (i = l,2,3), which under combustion conditions, where sufficient oxygen is present, reacts to form NO and N2 (Smoot 1993). Under gasification conditions, the oxidation of NHj radicals does not take place, and in the presence of a large hydrogen surplus, the nitrogen remains as HCN and NH3. Research on NOx formation indicates that HCN is the principle product when the nitrogen in the fuel is bound in aromatic rings, whereas NH3 appears to be the principle product when the nitrogen is bound in amines. The proportions of HCN and NH3 formed, therefore, vary in accordance with the fuel characteristics.

Only in the partial oxidation of natural gas, where no chemically bound fuel nitrogen is present, is it necessary to recognize that at least some thermal HCN and NH3 formation does take place. Since thermal HCN and NH3 formation is a function of the actual temperatures in the flame zone and thus of individual burner perform­ance, one can only rely on the experience of licensors with their own burner designs to provide data on the expected HCN and NH3 formation.

Typical Concentrations

Typical concentrations of nitrogen compounds in various syngases are shown in Table 6-3. It is unclear whether the figure of 0.05 ppmv NOx given by Rowles for oil gasifica­tion (Slack and James 1974) was really measured or just represents the limit of detect­ability. For raw gas from a Koppers-Totzek gasifier Partridge (1978) provides a figure of 70ppm NO. In the same source he gives a figure of 150ppm for the oxygen content. These are both much higher than figures quoted for other entrained-flow processes and may well be due to the ingress of air and/or poor mixing of reactants in the gasifier.

Effects of Nitrogen Compound Impurities

Ammonia has a very high solubility in water (two orders of magnitude higher than C02). One effect of this is that ammonia is seldom removed from the wash or quench water of carbon removal systems. Sufficient ammonia is then recycled in the scrubber wash water and partially stripped out by the syngas in the scrubber such that the potential for full ammonia removal in the syngas water wash is seldom realized.

Where chlorine is present, typically when gasifying coal, ammonia will combine with the chlorides to form ammonium chloride (see Section 6.9.3).

In methanol plants, ammonia (and also nitrogen oxides) can contribute to the formation of amines on the methanol synthesis catalyst. The presence of amines is not permitted in internationally accepted methanol specifications (e. g., U. S. Federal Specification, Grade AA) and can only be removed from the raw methanol with an ion exchanger (Supp 1990). It is, therefore, preferable to ensure the absence of nitrogen compounds in the synthesis gas upstream of the synthesis itself.

Hydrogen cyanide also has a high solubility in water and other physical wash solutions. If the main acid gas removal (AGR) is based on a physical solvent, then an HCN pre-wash can be integrated with the main system. It can also be removed by a water wash, although it should be noted that the high solubility also has its downside, namely the cost of regeneration.

Care should be exercised when using an amine AGR on a gas with a high HCN content, since although amines will remove it satisfactorily the acidic cyanide will react with the amine and degrade it. This problem should be examined as part of the AGR selection process.

Any HCN or NO entering a raw gas shift will be hydrogenated to ammonia (BASF). For some catalytic processes, such as Fischer-Tropsch, HCN acts as a poison (Boerrigter, den Uil, and Calis 2002).

Nitrogen oxides require particular attention in ammonia plants. In the liquid nitrogen wash (LNW) of an ammonia plant they will form a resin with any unsaturated hydro­carbons in the gas, and this resin is “extremely susceptible to spontaneous detonation” (Slack and James 1974). In most plants the molecular sieve immediately upstream of the liquid nitrogen wash (LNW) represents the last line of defense against ingress of both NOx and unsaturated hydrocarbons into the cold box. If Rectisol is used as the acid-gas removal system, for instance, the unsaturated hydrocarbons would already be removed at this stage. Where a raw gas shift is installed, both nitrogen oxides and unsaturated hydrocarbons would be hydrogenated on the catalyst.

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