NOX contamination

Selective Catalytic Reduction (SCR) SCE is a process to reduce NO, to nitrogen and water with ammonia in the presence of a catalyst between 540-840 F (282-449 C). Ammonia is usually injected at a 1 1 molar ratio with the NOx contaminants. Ammonia is used due to its tendency to react only with the contaminants and not with the oxygen in the gas stream. Ammonia is injected by means of compressed gas or steam carriers. Efficiencies near 90% have been reported with SCR. See Exxon Thermal DeNO. ... 

The cements containing active photocatalytic titania nanoparticles have widespread applications to create environmentally clean surfaces. These applications include self-cleaning surfaces, anti-soiling, depollution of VOCs and NOx contaminants and antifungal/microbial activities [521-528]. The relevant photocatalytic processes may occur both at the air-solid interface and at the liquid-solid interface. 

Air pollution, especially nitrogen oxides (NOx) contamination from the combustion of hydrocarbons, is a particularly serious problem in urban areas. Despite serious efforts toward emission control, the concentrations of NO often exceed the air-quality standard, especially in large cities. Ti02 photocatalysis appears to be a promising technology for the removal of low concentrations of NO from ambient air [79]. Daikin Industries has demonstrated the efficient removal of NOx from indoor air using a photocatalyst coated on activated carbon [25]. According to their results, the indoor NOx concentration decreases from 0.1 ppm to 0.06 ppm (the air-pollution standard) in 25 minutes. 

Although the PEMFC contamination caused by NO was reported for a wide range of NOx concentrations (from 16 ppb to 1480 ppm) [21, 34, 35, 49, 50], it is difficult to draw a general conclusion about NOx contamination because the results in the literature were obtained under different conditions. 

Recoverability of Fuel Cell Performance after NOx Contamination... 

There are mainly two proposed mechanisms for NOx contamination recovery NO reduction and NOx oxidation. Mohtadi et al. [35] proposed a NOx reduction mechanism [35] ... 

Recoverability from NOx contamination has also been studied by several other groups [10,11,22,23]. It is generally agreed that NOx contamination is fully recoverable by replacing the NOx-containing air with neat air. However, if the PEM fuel cell is continuously exposed to NOx for too long (e.g., 500 h), the cell performance may only be partially recoverable [23]. 

Since only a few groups have studied NOx contamination mechanisms, there is as yet no consensus about these processes. Thus far, two primary mechanisms for NOx contamination have been proposed the NOx reduction mechanism and the NOx oxidation mechanism. Mohtadi et al. [11] proposed the reduction mechanism, suggesting that NO2 was electrochemically reduced and thereby competed with O2 for Pt sites. The product of NO2 reduction was NH4+ (via equation (3.8)). NH4+ has been reported to be an ionomer poisoning species that might affect the ionomer and/or catalyst-ionomer interface. 

Usually the gas stream entering the scrubber contains other oxides of nitrogen in addition to NO2. Any residual NO in the gas stream leaving the scrubber tends to oxidize on mixing with the outside air. A colorless exit stack cannot be assured by the use of simple liquid scrubbing of a gas stream containing NOx contaminants. 

The removal of a wide variety of pollutants by means of non-thermal plasma has been reported aliphatic hydrocarbons [1-3], aromatics [4-7], chlorinated hydrocarbons [4,8-10], as well as inorganic contaminants such as S02, H2S [11,12] and NOx, which will be discussed in detail in this chapter. 

Acid rain and air pollution are very important problems that must be solved in the future because such pollution has major effects on terrestrial and aquatic ecosystems. At present, one of the most significant problems is removal of NOx, which are produced during high-temperature combustion and are an important group of air contaminants. In particular the decomposition or reduction of nitrogen monoxide (NO) is a major target to be achieved. 

By far the largest component in smog chamber studies is air. It is especially imperative therefore that the air used to dilute the NOx and VOC is clean. Ambient air and commercially supplied air generally contain sufficient organic and NOx impurities that extensive purification is needed to reduce these contaminants to low-ppb levels or below. One such purification system is described by Doyle et al. (1977) others are described in the papers on chamber facilities cited earlier. 

Environmental impact. Contamination by fallout from downwind transport of emitted particulate matter from the oil-shale power stations has been demonstrated throughout northeastern Estonia, as far as c. 140 km northwards to Finland and as far as 100 km to the southwest within Estonia (Jalkanen et aL 2000). Indeed, Teinemaa et al. (2002) and Jalkanen et aL (2000) examined the morphology of particles in power station fly ash (see Fig. 9) and found similarities in structure, size and chemical composition of particles collected from moss surfaces in northeastern Estonia and southeastern Finland. Although the emissions of SO2 and NOx from the BPP and EPP are generally similar to, or lower than, those from conventional... 

Often, many simultaneously occurring pollutants or contaminants determine an environmental problem. In industry, agriculture, and households, products are often mixtures of many compounds. The process of production and consumption is accompanied by emissions and consequently by contamination. One example is the use of toxaphene in the past, a very complex mixture of polychlorinated camphenes, as a pesticide. Technical toxaphene consists of more than 175 individual compounds. A second example is industrial and domestic emissions resulting from the combustion of fossil fuels. The emissions contain both a mixture of gases (SO2, NOx, CO2, etc.) and airborne particulate matter which itself contains a broad range of heavy metals and also polycyclic aromatic hydrocarbons (PAH). 

Flue-gas from boilers fired with liquid or solid fuels contains fly-ash and gaseous contaminants such as CO, NOx, SO2, or volatile organic compounds (VOCs). Emission regulations require their removal, which is achieved by a sequence of after-treatment processes. The after-treatment usually comprises a filter to remove solid particulates operated at approximately 150 °C, a wet scrubber for the removal of SO2 with an alkaline solution operated at approximately 50 °C, and finally a selective catalytic reduction (SCR) unit, which converts NOx to N2 with the help of NH3 at approximately 370 °C (Fig. 15.1) [4]. During this process, the flue gas is cooled down and then heated up again, which requires additional heat transfer equipment, with its inherent energy losses. 

SAFETY PROFILE Deadly poison by ingestion, skin contact, subcutaneous, and possibly other routes. Human mutation data reported. Questionable carcinogen. A powerful systemic poison. In 1985 over 150 people in California exhibited toxic effects from eating watermelons contaminated with aldicarb. When heated to decomposition it emits very toxic fumes of NOx and SOx. 

SAFETY PROFILE A poison by ingestion. Moderately toxic by intraperitoneal and subcutaneous routes. Questionable carcinogen with experimental tumorigenic data. May be contaminated with the carcinogen p-naphthylamine. Mutation data reported. When heated to decomposition it emits toxic fumes of NOx. See also AROMATIC AMINES. 

The use of catalysts, furthermore, is required in the processes of wastewater purification by reducing treatments. Catalysts also find also application as complementary technologies to other wastewater treatment methods, such as in the control of odour, VOC, N2O and NOx emissions from wet oxidation treatments (for example, in the wet oxidation of industrial sludges), and of odours and VOC emitted from biological processes (aerobic and anaerobic). Although usually commercial catalysts are used in these cases, there are often unpredicted effects in treating complex mixtures and thus more specific catalysts would be preferable. The same is valid for catalysts used to convert stripped VOC from contaminated groundwater. 

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