Nitric acid plants, emissions

Although often it is considered that a single reaction mechanism occurs in the selective reduction of NO by ammonia, data show that instead different mechanisms are possible and that too depending on the type of catalyst and reaction conditions (feed composition, reaction temperature) - one mechanism may prevail over the others [31b], However, not considering this aspect and making extrapolation regarding the reaction mechanism from one catalyst to another or to different reaction conditions may lead to erroneous conclusions. In addition, it is important to consider all possible opportunities to develop new kinds of catalysts, for example, for the combined removal of NO and N20 from nitric acid plant emissions [25],... 

A different case is the SCR of N2O to eliminate this greenhouse chemical from nitric acid plant emissions or waste fluid bed combustors [19]. In this... 

The presence of NO2 in the feed (in nitric acid plant emissions, or when the feed is pretreated by NTP) induces a significant change in the reactivity and in the reaction mechanism. Koebel et al. [36] studied the low temperature behavior of the SCR process with feed gases containing both NO and NO2 and observed the presence of two main reactions ... 

Flue gas treatment (FGT) is more effective in reducing NO, emissions than are combustion controls, although at higher cost. FGT is also useful where combustion controls are not applicable. Pollution prevention measures, such as using a high-pressure process in nitric acid plants, is more cost-effective in controlling NO, emissions. FGT technologies have been primarily developed and are most widely used in Japan. The techniques can be classified as selective catalytic reduction, selective noncatalytic reduction, and adsorption. 

Ammonium Nitrate Plants - In ammonium nitrate plants, wet scrubbers can be considered for prill towers and the granulation plant. Particulate emissions of 0.5 kg/t of product for the prill tower and 0.25 kg/t of product for granulation should be the target. Similar loads for ammonia are appropriate. Other effluents that originate in a nitrogenous fertilizer complex include boiler blowdown, water treatment plant backwash, and cooling tower blowdown from the ammonia and nitric acid plants. 

The plant disposes of two waste streams gaseous and aqueous. The gaseous emission results from the ammonia and the artunonium nitrate plants. It is fed to an incinerator prior to atmospheric disposal. In the incinerator, ammonia is converted into NOj,. Ehie to more stringent NO regulations, the conqmsition of ammonia in the feed to the incinerator has to be reduced from 0.57 wt% to 0.07 wt%. The lean streams presented in Table 9.5 may be employed to remove ammonia. The main aqueous waste of the process results from the nitric acid plant. Due to its acidic content of nitric acid, it is neutralized with an aqueous ammonia solution before biotreatment. 

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

There are four main applications of the SCR-NH3 process for the reduction of NO in the emissions (1) power plants, (2) gas turbines, (3) waste incineration and (4) nitric acid plants. Although, often, specific distinction is not made between these cases and the same catalysts are assumed to be applicable in all cases, there are significant differences in terms of composition of the emissions and space velocities. A specific difference between the first three cases (combustion) and the latter (nitric acid plants) regards NO/N02 ratio which is typically close to 20 for combustion processes and close to 1 for the nitric acid plants. Furthermore, no S02 is present in the latter case. 

While vanadia- on titania-based catalysts can be used for both the classes of applications, there are other types of catalysts such as those based on copper [31b], which show good performances in case of mixtures of N0/N02 (nitric acid plants), while performances are worse when applied to emissions from catalytic processes. 

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

Low-temperature activity promotion is an issue in mobile (diesel) applications, but may not be a critical issue in several stationary applications, apart from those where the temperature of the emissions to be treated is below 200°C (for example, when a retrofitting SCR process must be located downstream from secondary exchangers, or in the tail gas of expanders in a nitric acid plant). In the latter cases, a plasmacatalytic process [91] could be interesting. In the other cases, the use of NTP together with the SCR catalyst is not economically viable. However, the synergetic combination of plasma and catalysts has been shown to significantly promote the conversion of hazardous chemicals such as dioxins [92], Although this field has not yet been explored, it may be considered as a new plasmacatalytic SCR process for the combined elimination of NO, CO and dioxins in the emissions from incinerators. 

The primary pollution problem in nitric acid plants is the abatement of NOx in tail gases. Three options exist to reduce these emissions to acceptable levels 1) Capture the NOx and convert it to additional nitric acid, 2) Capture the NOx and convert it to nitrate-nitrite sales, or 3) Render the NOx harmless by converting it to non-polluting compounds. The processes that have been developed to reduce emissions at existing and new plants can be classified into four general categories Absorption, Adsorption, Selective Catalytic Reduction (SCR) and Non-Selective Catalytic Reduction91. 

Schwefer, M., Maurer, R. and Groves, M., Reduction ofNitrous Oxide Emissions from Nitric Acid Plants, Prepared for Presentation at Nitrogen 2000, Vienna, Austria, March, 2000. 

The primary pollution problem in nitric acid manufacture is the abatement of nitrogen oxides (NOx) in tail gases. In the United States, gaseous emissions from newly constructed nitric acid plants must be limited to 1.5 kilograms of NOx per tonne of nitric acid (100% basis) produced, with a maximum stack opacity of 10 percent. Modem acid towers, with extended sections, can reduce NOx emissions to less than 200 parts per million. 5... 

A few classicaV studies on the reactivity of HCs to reduce NOx with catalysts indicated that the use of such reductants for controlling mobile NOx emissions was quite attractive to the automotive industry, thereby the advent of a new type of HC-SCR technology in the mid-1980s. An example may be the treatment process of the tail gas from nitric acid production plant via ammonia oxida-tion. The process includes the usual injection of excessive amounts of HCs over supported noble metals such as Pt, Pd and Rh to eliminate the yellowish stack plume due to 0.1 - 0.5% NOx, mainly NO2, from the nitric acid plant. 

Postformation nitrogen oxide emission control measures include selective catalytic and noncatalytic reduction with ammonia, which between them are used by some 900 power station installations worldwide [51]. The catalytic removal methods are 70-90% efficient at NOx removal, but are more expensive to operate than the noncatalytic methods which are 30-80% efficient. Ammonia or methane noncatalytic reduction of NOx to elemental nitrogen is also an effective method which is cost-effective for high concentration sources such as nitric acid plants (Chap. 11). NOx capture in packed beds is less expensive, but this method is not particularly effective [23]. It is also not a very practical method either for utilities or for transportation sources. Two-stage scrubbing has also been proposed as an effective end-of-pipe NOx control measure. The first stage uses water alone and the second uses aqueous urea. 

As shown, the catalyst displays an excellent performance and durability for N2O decomposition under realistic conditions for nitric acid plants. Since this reaction is a remediation technolo of gas emissions, with this catalyst a double environmental targeting is attained both the preparation and application minimize waste. 

Selective non catalytic reduction (SNCR) with NH3 is limited to industrial boilers in consequence of the relatively narrow temperature range for the reaction. Selective catalytic reduction (SCR) by ammonia has high efficiency and it can be used for many stationary sources, especially for nitric acid plants [1], and it is based on the catalytic pairing of nitrogen atoms, one fi-om nitric oxide, one fi om ammonia. This method, however, is imsuitable for small sources and vehicles. As far as automotive emission is concerned nonselective catalytic reduction (NSCR) by hydrocarbons, CO and H2 from the exhaust stream has been reported over various catalysts recently [1,3,4]. 

Environmental Impact [14] - The main impact on the environment is from NOx emissions to the atmosphere. NOx may contribute to acid rain and ground-levcl ozone. Nitrous oxide could theoretically contribute to greenhouse effects or affect the stratospheric ozone layer however, N2O emissions from nitric acid plants are small compared with other sources. 

The specific air emission limits and guidelines for existing nitric acid plants in Europe are given in Table 19.5 and for new plants in Table 19.6 [13]. 

Table 19.5. Air Emission Limits for Existing Nitric Acid Plants... 

Table 19.6. Air Emission Guidelines for New Nitric Acid Plants in Europe...

N2O has recently received much attention because it greatly contributes to the greenhouse effect and leads to a severe damage of the ozone layer in the stratosphere. Generally, N2O can be produced by both natural and anthropogenic sources. In comparison with the natural sources, N2O emissions, which should be decreased in the short period, are related to the chemical and energy industries. The major N2O emission of chemical production results from adipic acid and nitric acid plants (Ertl, Knozinger, Schuth, Weitkamp, 2008). 

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