Hydrogen sulfide atmospheric oxidation

Solutions of iron chelates can be used to remove hydrogen sulfide and oxides of sulfur and nitrogen in industrial gas scmbbing processes (41,50,51) before flue gases are released to the atmosphere. 

Sulfur forms several oxides that in atmospheric chemistry are referred to collectively as SOx (read sox ). The most important oxides and oxoacids of sulfur are the dioxide and trioxide and the corresponding sulfurous and sulfuric acids. Sulfur burns in air to form sulfur dioxide, S02 (11), a colorless, choking, poisonous gas (recall Fig. C.1). About 7 X 1010 kg of sulfur dioxide is produced annually from the decomposition of vegetation and from volcanic emissions. In addition, approximately 1 X 1011 kg of naturally occurring hydrogen sulfide is oxidized each year to the dioxide by atmospheric oxygen ... 

Atmospheric and vacuum distillation units (Figures 4.3 and 4.4) are closed processes, and exposures are expected to be minimal. Both atmospheric distillation units and vacuum distillation units produce refinery fuel gas streams containing a mixture of light hydrocarbons, hydrogen sulfide, and ammonia. These streams are processed through gas treatment and sulfur recovery units to recover fuel gas and sulfur. Sulfur recovery creates emissions of ammonia, hydrogen sulfide, sulfur oxides, and nitrogen oxides. 

A substantial portion of fhe gas and vapors emitted to the atmosphere in appreciable quantity from anthropogenic sources tends to be relatively simple in chemical structure carbon dioxide, carbon monoxide, sulfur dioxide, and nitric oxide from combustion processes hydrogen sulfide, ammonia, hydrogen chloride, and hydrogen fluoride from industrial processes. The solvents and gasoline fractions that evaporate are alkanes, alkenes, and aromatics with relatively simple structures. In addition, more complex... 

Emissions to the atmosphere from ammonia plants include sulfur dioxide (SOj), nitrogen oxides (NOJ, carbon monoxide (CO), carbon dioxide (COj), hydrogen sulfide (HjS), volatile organic compounds (VOCs), particulate matter, methane, hydrogen cyanide, and ammonia. The two primary sources of pollutants, with typical reported values, in kilograms per ton (kg/t) for the important pollutants, are as follows ... 

The gaseous component typically contains hydrocarbons, hydrogen sulfide, ammonia, mercaptans, solvents, and other constituents, and is either discharged directly to the atmosphere or is combusted in a flare. The major air emissions from blowdown systems are hydrocarbons in the case of direct discharge to the atmosphere and sulfur oxides when flared. 

Hydrogen sulfide in the air is oxidized at a relatively slow rate by molecular oxygen (02) but at a much faster rate by hydroxide (OH) radicals, forming the sulfhydryl radical and ultimately sulfur dioxide or sulfate compounds (Hill 1973 NSF 1976). Sulfur dioxide and sulfates are eventually removed from the atmosphere through absorption by plants and soils or through precipitation (Hill 1973). 

Although hydrogen sulfide does not react photochemically, it may be transformed to sulfur dioxide and sulfate by nonphotochemical oxidation reactions in the atmosphere. Its atmospheric residence time is typically less than 1 day (Hill 1973), but may be as high as 42 days in winter (Bottenheim and Strausz 1980). 

However, hydrogen sulfide in industrial atmospheres is rapidly oxidized to sulfur dioxide and sulfuric acid. 

The second term in Equation (6.8) corresponds to the sinks for sulfide in the water phase that, according to Figure 4.4, are primarily caused by oxidation in the water phase and emission into the sewer atmosphere. Pomeroy and Parkhurst (1977) propose values for Nat two levels,/V=0.96 and A=0.64. The first value corresponds to a median buildup of sulfide, whereas the last value is a conservative estimate for prediction of sulfide buildup in a sewer. The second term of Equation (6.8) shows that the removal of sulfide from the water phase is considered a 1-order reaction in the sulfide concentration. The term also includes elements related to the reaeration and, thereby, the emission of hydrogen sulfide [cf. Equations (3) and (6) in Table 4.7 and Section 4.3.2],... 

Ventilation of sewers may not only reduce the hydrogen sulfide concentration in the sewer atmosphere but also the moisture that is a fundamental requirement for establishment of microbial activity on the sewer walls. It is important that the ventilation be well controlled otherwise, odorous problems in the vicinity of the sewer network may occur. In some cases, operational procedures like treatment of the vented air, e.g., by wet oxidation, by chemical scrubbing or by passing a biofilter, may need to be considered. 

The model is, in several aspects, a simplification of the processes that occur. As an example, it is important—and also possible when further data are available from experiments — to expand the model by including the oxidation of sulfide in a gravity sewer at low DO concentrations and the emission of hydrogen sulfide into the sewer atmosphere. From a general point of view, however, it is considered important only to deal with the most important aspects to keep the number of processes low and not include too many process parameters that are site specific. 

Extension of the WATS model to integrate further dry-weather processes is considered important. Examples of such extensions are the description of the wastewater quality and nitrite/nitrate transformations under anoxic conditions and the emission of hydrogen sulfide into the sewer atmosphere followed by its transformation (oxidation) at the sewer walls. 

The integrated aerobic-anaerobic WATS model has changed this situation. As an example, it is possible to use the model in a gravity sewer with changing aerobic and anaerobic conditions. As previously stressed, a number of in-sewer processes still need to be dealt with. Examples are the anoxic transformations and the processes related to the extended sulfur cycle, particularly, the oxidation of sulfide and the emission of hydrogen sulfide into the sewer atmosphere, including its further oxidation at the sewer walls. Combined use of empirical and conceptual models is still needed. 

The simulations depicted in Figure 8.8 also show that arather low hydrogen sulfide concentration is predicted in the gravity sewer. Only minor problems related to hydrogen sulfide production may therefore arise. Until now, the WATS model did not include sulfide release to the sewer atmosphere, sulfide oxidation or sulfide precipitation that may further reduce the concentrations shown. The predicted sulfide concentrations are, therefore, maximum levels. In case a natural capacity of iron salts in the wastewater to precipitate sulfide is inadequate, the sulfide concentrations are considered at a level that can be relatively easily controlled. 

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