Croll-Reynolds Company, Inc.

Chemical Aspects of NOX Scrubbing

With the passage of the 1990 Clean Air Act Amendments, many chemical and metal industries and utility producers will now be required to further limit the amount of NOX (Nitrogen Oxides) produced. NOX is a precursor to ozone in the atmosphere, and is believed to be a major contributor to acidic deposition (acid rain).

Formation of ozone:

NO2 + Hv  NO + O (Photochemical Reaction)   

O + O2 + M O3 + M, where M - Energy accepting third body.

NOX is produced in a variety of different processes, including combustion equipment, gas turbines, incinerators, kilns and power plants. NOX also is emitted as by product from many metal treatment processes where nitric acid is used as an oxidant. Plating or catalyst recovery involves the reaction of nitric acid and transition metals also forming NOX. Substantial amounts of NOX also can be generated in the specialty chemical industry when nitric acid is used as a reagent [1].

The denitration processes for removal of NOX are classified into two groups: in one, NOX is absorbed by means of solutions, and in the other NOX is reduced to N2 by means of a reducing gas under the presence of a catalyst.

Selective catalytic reduction (SCR) is a chemical process that changes the oxides of nitrogen into N2 and H2O. The reactions take place at a temperature of 600-750°F in the presence of a catalyst. Ammonia is injected into the exhaust gases prior to their passing into the SCR. NOX removal efficiencies with SCR range from 80 to 90% [2]. An NH3/NOX mole ratio of 1.0 to 1.5 is normally used although the theoretical ratio is about 0.67. Although a portion of the excess ammonia decomposes in the reactor, a considerable amount of it would remain in the treated gas and may cause problems. For example, ammonia may combine with SO3, which is present in a small amount even after the wet scrubbing, to form ammonium bisulfate which condenses in a heat exchanger. However, catalysts are affected by dust and most are poisoned by sulfur and chlorine compounds.

For treatment of contaminated gas, a wet removal process may be carried out first to remove dust and poisonous chemicals, but in this case, the gas temperature drops to 120-150° and must then be reheated to 600-750ûF, a large heat exchanger and a considerable amount of fuel are needed. In addition, mists from the scrubber may cause corrosion of the heat exchanger and contamination of the catalyst. Over a period of time, the materials in the catalyst-ceramics and zeolites degrade and must be replaced depending upon the industrial sources of the gases. Because the catalysts are made up of heavy metals, disposal of spent catalysts can also be a problem.

In recent years, many wet processes for NOX removal have been developed with the aim of removing NOX and SOX simultaneously. Scrubbing process also used to remove NOX from NOX rich gases is produced in a relatively small amount at metal-dissolving, nitric acid and chemical plants, etc.

This article describes the NOX chemical reaction associated with wet removal processes. The aim, then, is to provide a reasonably clear and uncomplicated basis for evaluation of potential treatment methods to help determine which may be practically applicable in a given situation.

Several oxides of nitrogen are found in the atmosphere but only nitric oxide (NO) and nitrogen dioxide (NO2) are important as air pollutants. The symbol NOX is frequently used to represent the composite of the two. The other nitrogen oxides seldom occur in appreciable quantities and then only under special conditions.

Essentially, NOX contains nitric oxide and nitrogen dioxide in varying proportions. This fluctuating ratio, and the fact that these compounds exhibit quite different properties when contacted with water (as would occur in a wet scrubber) complicate the treatment of NOX.

NO2 gas has fairly high solubility and reactivity to water and in aqueous solutions or alkalis as compared with NO, and can be removed by wet scrubbing. On the other hand, gaseous NO is only slightly soluble in water and is not very reactive with typical aqueous solutions. Nitric oxide does react with oxygen as follows:

    2NO+O2 = 2NO2

This equation implies the coexistence of NO and NO2. Calculated equilibria indicates that the stability of NO2 decreases with increasing temperature. Nevertheless, from an equilibrium standpoint, the absolute concentration of NO2 increases with temperature while the ratio of its concentration to that of NO decreases with increasing temperature. The equilibrium concentration of NO varies with temperature; it is negligible below 1000°F but quite significant above 2000°F. (Table 1)

Table 1. Calculated Equilibrium concentration of Nitrogen Oxides

For reactions:

    N2 + O2 2NO
    2NO + O2 2NO2

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