Fossil fuel sulfur content

The environmental, health, and cultural impacts of acid rain were well documented in the late 20th century. Acid rain results from combustion of the sulfur and nitrogen contents of fossil fuels. Sulfur and nitrogen oxides combine with atmospheric water to form acids. The prevalence of mobile (e.g., automotive and aircraft) sources globally hints at the requirement for control of Title IV compounds and precursors. 

Acid Dew Point For fossil fuels, the acid dew point temperature is that temperature at which the actual mixed acid vapor pressure equals the mixed acid vapor saturation pressure. The mixed acid dew point can be approximated by the sulfuric acid dew point (Fig. 24-56). It can be described as a function of the SO3 and water content of the flue gas (Huijbregts). These concentrations result from the sulfur, hydrogen, and free water content of the fuel the relative humidity of the air and the amount of excess air used. Using the equation of Ver-hoff, where T is degrees K and P is mm Hg (see OUces, A.G.) ... 

Acid rain. Lakes in some areas of the world are now registering very low pH s because of excess acidity in rain. This was first noticed in Scandinavia and is now prevalent in eastern Canada and the northeastern U.S. Normal rainfall is 5.6 (because of CO2 in the air forming H2CO3). However, excessive use of fossil fuels (especially coal) with high sulfur and nitrogen content cause sulfuric and nitric acids in the atmosphere from the sulfur dioxide and nitrogen oxide products of combustion. Some rain in the Adirondack Mountains of upper New York State has been measured with a pH of 3.0. This problem is not specific to the chemical industry but should be of concern to all of us. 

Whitehurst, Isoda, and Mochida write about catalytic hydrodesulfurization of fossil fuels, one of the important applications of catalysis for environmental protection. They focus on the relatively unreactive substituted di-benzothiophenes, the most difficult to convert organosulfur compounds, which now must be removed if fuels are to meet the stringent emerging standards for sulfur content. On the basis of an in-depth examination of the reaction networks, kinetics, and mechanisms of hydrodesulfurization of these compounds, the authors draw conclusions that are important for catalyst and process design. 

Sulfur chemistry is important both in combustion and in the petrochemical industry. Most fossil fuels contain sulfur, and also biofuels and household waste have a sulfur content. As a consequence sulfur species are often present in combustion processes. Knowledge of gas-phase sulfur chemistry occurring in combustion has bearing on pollutant emissions and on system corrosion. Air pollution by SO2 still constitutes a major environmental concern and search for control techniques has motivated research also on high-temperature homogeneous sulfur chemistry. However, more recent work on sulfur chemistry has been concerned mainly with the effect of sulfur on other pollutant emissions, such as NO and CO, and with the SO3/SO2 ratio, which is important for the corrosive potential of the flue gas and for formation of sulfur containing aerosols. 

Biomass differs from conventional fossil fuels in the variability of fuel characteristics, higher moisture contents, and low nitrogen and sulfur contents of biomass fuels. The moisture content of biomass has a large influence on the combustion process and on the resulting efficiencies due to the lower combustion temperatures. It has been estimated that the adiabatic flame temperature of green wood is approximately 1000°C, while it is 1350°C for dry wood [41]. The chemical exergies for wood depend heavily on the type of wood used, but certain estimates can be obtained in the literature [42]. The thermodynamic efficiency of wood combustors can then be computed using the methods described in Chapter 9. 

Sulfur dioxide is generated in flue gas as a result of combustion of fossil fuel in, for example, thermal plants. Although the sulfur dioxide content in the flue gas is usually small, namely below about 0.1-0.4 vol.% [86], the volume of the produced gas is so large that considerable amounts of sulfur dioxide contaminate the atmosphere, and therefore proper desulfurization becomes important. Here, absorption of SO2 by the sodium method is considered as part of the combined chemical/biological process shown in Fig. 9.21. The process includes SO2 removal, NaHCCh recovery, and elemental sulfur production [70]. 

One of the properties of fossil fuels of interest to refiners is metal content. Vanadium in particular is troublesome in refinery operations (2). In some crude oils it occurs in concentrations up to 1000 ppm (3). Of the other metals in petroleum, only nickel is known to be present in concentrations approaching that of vanadium. These two metals are com-plexed with porphyrins and other ligands, the exact nature of which is unknown (4), Crude oils that are high in sulfur are usually high in vanadium (5), and it is interesting to determine whether or not this criterion also applies to tar sands. 

In recent years the sulfur dioxide content of the air has risen, and so also have the nitrogen dioxide levels. This is due to increased use of fossil fuels and the fact that oxides of nitrogen are produced when some fuels and other materials burn in petrol engines. Catalytic converters on car exhausts help to reduce the amount of the oxides of nitrogen and carbon monoxide in the air, and so help to lower car emission pollution . 

Sulfates are discharged into water from mines and smelters, and from kraft pulp and paper mills, textile mills, and tanneries. Atmospheric sulfur dioxide, formed by the combustion of fossil fuels and by metallurgical roasting processes, may contribute to the sulfate content of surface waters. Sulfur trioxide, produced by the photolytic or catalytic oxidation of sulfur dioxide, combines with water vapor to form dilute sulfuric acid, which falls as acid rain . The environmental fate and transport of sulfate are inextricably linked to the physical and chemical processes active in the earth s sulfur cycle. 

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