Sulfur compounds, reduced emissions

In the manufacture of varnish, heat is necessary for formulation and purificahon. The same may be true of operations preparing paints, shellac, inks, and other protective or decorative coahngs. The compounds emitted to the atmosphere are gases, some with extremely low odor thresholds. Acrolein, with an odor threshold of about 4000 /xg/m, and reduced sulfur compounds, with odor thresholds of 2 are bofh possible emissions... 

The most widely used pulping process is the kraft process, as shown in Fig. 6-11, which results in recovery and regeneration of the chemicals. This occurs in the recovery furnace, which operates with both oxidizing and reducing zones. Emissions from such recovery furnaces include particulate matter, very odorous reduced sulfur compounds, and oxides of sulfur. If extensive and expensive control is not exercised over the kraft pulp process, the odors and aerosol emissions will affect a wide area. Odor complaints have been reported over 100 km away from these plants. A properly controlled and operated kraft plant will handle huge amounts of material and produce millions of kilograms of finished products per day, with little or no complaint regarding odor or particulate emissions. 

Power boilers at pulp and paper mills are sources of particulate emissions, S02, and NOx. Pollutants emitted from chemical recovery boilers include S02 and total reduced sulfur compounds (TRS). 

Sulfur Dioxide. Both flame photometric and pulsed fluorescence methods have been applied to the continuous measurement of S02 from aircraft. In the flame photometric detector (FPD), sulfur compounds are reduced in a hydrogen-rich flame to the S2 dimer. The emission resulting from the transition of the thermally excited dimer to its ground state at 394 nm is measured by using a narrow band-pass filter and a photomultiplier tube. 

The relative contribution that each reduced sulfur compound makes to the total sulfur flux is often of interest because the various compounds behave differently once they enter the atmosphere. Terrestrial biogenic sulfur emissions are dominated by COS (38%), DMS (35%) and H2S (21%). Emissions of CS2 and DMDS together represent about 6% of the total. DMS emissions dominate during the summer season with 41% of the total. 

Deposition of reduced sulfur compounds to enclosed surfaces could also be measured by adding low loss (5 ng/min, GC Industries) permeation tubes to the inlet of the chambers. This permeation rate adds an additional sulfur load which varied typically between 2 and S times that already within the enclosure. These studies were limited to surfaces where large temporal changes in natural emissions were not occurring, and ultimately became limited by the reliability of the permeation devices. 

Biogenic Sulfur Emissions from the Ocean. The ocean is a source of many reduced sulfur compounds to the atmosphere. These include dimethylsulfide (DMS) (2.4.51. carbon disulfide (CS2) (28). hydrogen sulfide (H2S) (291. carbonyl sulfide (OCS) (30.311. and methyl mercaptan (CH3SH) ( ). The oxidation of DMS leads to sulfate formation. CS2 and OCS are relatively unreactive in the troposphere and are transported to the stratosphere where they undergo photochemical oxidation (22). Marine H2S and CH3SH probably contribute to sulfate formation over the remote oceans, yet the sea-air transfer of these compounds is only a few percent that of DMS (2). 

The acid gas from the Sul find regenerator must be disposed of in an environmentally acceptable manner. The Claus process offers an effective means for converting nearly all of the sulfur in the acid gas to saleable elemental sulfur. The tail gas from the Claus plant still contains some sulfur compounds. To minimize sulfur emissions from the plant, the Claus tail gas can be fed to a Shell Claus ff-gas Jreating (SCOT) unit where most of this sulfur is recovered and recycled to the Claus plant. With use of the SCOT Process, additional marketable sulfur is recovered within the Claus plant while tail gas sulfur emissions are substantially reduced, to typically less than 250 ppmv. 

On a global scale, natural emissions of reduced sulfur compounds account for about 50% of the total sulfur flux into the atmosphere (1-3). Hence, it is important to understand the natural sulfur cycle in order to establish a "base line" for assessing the significance of anthropogenic perturbations (primarily SO2 emissions). Dimethylsul-fide (DMS) is the predominant reduced sulfur compound entering the atmosphere from the oceans (4-9), and DMS oxidation represents a major global source of S(VI). The atmospheric oxidation of DMS can be initiated by reaction with either OH or NO3. In marine environments, however, NO3 levels are typically very low and DMS is destroyed primarily by OH ... 

Two objectionable air emissions have characterized pulp mills for years the sulfur dioxide of the sulfite pulping mill and the malodorous reduced sulfur compounds (TRS) (mercaptans and hydrogen sulfide) of the kraft mill. Still another less noxious air emission is the particulate matter from steam boilers. Coal-burning boilers also emit sulfur dioxide, as is well known. The treatments for the particulate emissions are shown in Table 28.7. 

Gasification of coal containing organic nitrogen and sulfur compounds generates trace amounts of ammonia and other toxic and corrosive impurities. The ammonia thus formed must be removed prior to combustion of the synthetic gas to reduce the emission of NOx in the gas turbine. The decomposition of ammonia... 

Typical examples of the first approach, upgrading or improving the fiiel, are the catalytic removal of sulfur and aromatic compounds in automotive fuels [8,9]. A shift from the use of coal to the use of natural gas as a fuel in many industrial applications has led to reduced emissions, due to the favorable composition of natural gas as compared to coal. However, future developments of combustion processes will most likely include the use of more low-grade fuels, such as heavy fuel oils [10]. The use of coal will increase again, which can be related to its relative abundance. Finally, low-Btu fuels, such as gasified biomass or gasified coal will play an important role [11]. 

The principal challenge of the sulfur removal processes is to reduce the sulfur compounds in the syngas to a low enough level to meet the requirements of downstream processes and prevailing emission regulations, and to do so as economically as possible. This would not be a problem if only H2S were present, since H2S is easily removed by most processes down to very low levels. However, the solubility of COS in both types of solvents is much lower than that of H2S, making it more difficult to remove. 

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