Gaseous pollutants conversion

As these conversion equations show, in addition to the molecular weight of a measured gaseous pollutant, the temperature and pressure at the time of the measurement must be known. Because this information is often not given in the literature, no attempt has been made in this report to convert to a common unit. 

The heterogeneous batch conversion system illustrated in Figure 31G is phenomenologically even more complex than the homogeneous batch conversion system. Heterogeneous batch conversion is found in small-scale applications, for example wood stoves. Research activity is also very high in this area [43,44]. The big problems are gaseous pollutants in the emissions and the low efficiency of the stoves. 

Transport from the atmosphere to land and water Dry deposition of particulate and gaseous pollutants Precipitation scavenging of particulate and gaseous pollutants Adsorption of gases onto particles and subsequent diy and wet deposition Transport within the atmosphere Turbulent dispersion and convection Atmospheric transformation Diffusion to the stratosphere Photochemical degradation Oxidation by free radicals and ozone Gas-to-particle conversion... 

Typically, the reaction temperature is in the range of 300° to 900°F, and is determined on the basis of the material being oxidized, the catalyst used, and the conversion desired. The temperatures required for 99% conversion of a large number of gaseous pollutants over Hal-dor Tops0e metal oxide catalysts are given in Table 13-7. 

In these discussions we will thus use the following explicit definition of a chemical measurement in the atmosphere the collection of a definable atmospheric phase as well as the determination of a specific chemical moiety with definable precision and accuracy. This definition is required since most atmospheric pollutants are not inert gaseous and aerosol species with atmospheric concentrations determined by source strength and physical dispersion processes alone. Instead they may undergo gas-phase, liquid-phase, or surface-mediated conversions (some reversible) and, in certain cases, mass transfer between phases may be kinetically limited. Analytical methods for chemical species in the atmosphere must transcend these complications from chemical transformations and microphysical processes in order to be useful adjuncts to atmospheric chemistry studies. 

When hydrocarbons such as octane are burned, the release of energy is accompanied by the conversion of highly ordered reactant molecules into relatively disorganized gaseous products such as CO, and H20. Unfortunately, gasoline combustion is inefficient that is, other substances such as the poisonous molecule carbon monoxide (CO) that pollute the environment are also released. 

Carbon monoxide is quite Stable in the atomosphere and is probably converted to CO2, but the rate of this conversion (not known exactly) is low. Its a poisonous inhalent and no other toxic gaseous air pollutant is found at such relatively high concentrations in the urban atmosphere. 

Coal-conversion processes under development are directed towards producing either gaseous or liquid feedstocks which approximate in composition to petroleum-derived feedstocks. They can then be utilized directly in existing petrochemical plant and processes. To achieve this, however, two problems must be overcome, which are a consequence of the differing natures of coal and oil. Firstly, the H C ratios are different for coal and for petroleum-derived liquid feedstocks. Secondly, significant amounts of heteroatoms are present in coal, particularly sulphur which may reach levels as high as 3%. The sulphur has to be removed for two reasons (i) on combustion it will form the atmospheric pollutant SO2, and (ii) it is a potent catalyst poison, and most of the downstream petrochemical processes are catalytic. However, its removal from coal is difficult and it is therefore removed from the conversion products instead. 

Particulate pollutants are emitted from many sources. Additionally, particles are formed in the atmosphere by both chemical and physical conversions from natural and anthropogenic gaseous substances. Particulate pollutants cover a size range from <10 nm to >100 pm. The major proportion of the aerosol below 1 pm is generally man-made, including sulfates from SO2 oxidation and carbon from vehicle exhausts, for example. Particles of a greater size are frequently natural (e.g., soil-derived, marine aerosol) but this division cannot be regarded as absolute. 

Both gaseous and aqueous NO remediation via catalytic means involve similar species as intermediates and products but the processes exhibit key differences. Even the catalysts most active for gas-phase reduction require high temperatures (upward of 475 K) for appreciable conversion [41] while the aqueous processes occur at room temperature. Pt, Pd, and Rh are the key precious metals used in gas-phase treattnent [41], while only Pd-based catalysts seem to be appropriate for aqueous-phase application [42, 43]. These similarities in pollutants, intermediates and products, as well as the differences in the choice of precious metal catalysts and operating conditions offer an opportunity to explore a rich chemistry across all three phases of matter. Therefore, aqueous-phase NO remediation, like gas-phase NO remediation, remains an active area of research. 

Particulate carbon as soot, carbon black, coke, and graphite originates from auto and truck exhausts, heating furnaces, incinerators, power plants, and steel and foimdry operations, and composes one of the more visible and troublesome particulate air pollutants. Because of its good adsorbent properties, carbon can be a carrier of gaseous and other particulate pollutants. Particulate carbon surfaces may catalyze some heterogeneous atmospheric reactions, including the important conversion of SO2 to sulfate. 

Querol et al. [25] have presented an overview on the methodologies for zeolite synthesis from the fly ash. The authors have detailed the conventional alkaline conversion processes, with special emphasis on the experimental conditions to obtain high cation exchange capacity (CEC) zeolites. They have reported that zeolitic products having CEC up to 300 meq./lOO g, can be obtained from high-glass fly ash by direct conversion and the main application of this material is the uptake of heavy metals and ammonium ion from polluted water. It has been clarified that some of the zeolites synthesized, are useful as molecular sieves to absorb water molecules from gas streams or to trap SO2 and NH3 from low water gaseous emissions based on their pores and molecular sizes as depicted in Fig. 7.4. 

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