CHAPTER 1
Introduction: the Sulfur Problem
1 Sources of Sulfur and Major Uses
Sulfur has been known since the beginning of history. It occurs uncombined in nature, and it is a major global pollutant when oxidised to sulfur dioxide. Sulfur compounds are used extensively in the modern industrialised world. Their main use is for the synthesis of sulfuric acid by the contact process. In this, sulfur or sulfide minerals are converted to sulfur dioxide by heating them in air. The sulfur dioxide is then oxidised to sulfur trioxide in air over a supported potassium sulfate promoted vanadia catalyst at ca. 500 °C. The sulfur trioxide is absorbed in 98% sulfuric acid in ceramic packed towers and diluted to the desired concentration with water. Sulfur trioxide cannot be allowed to react directly with water, since this would result in the formation of a mist of sulfuric acid droplets that would pass right through the absorber and into the atmosphere. The contact process is very efficient, accounting for less than 2% of emissions into the atmosphere as sulfur dioxide. Sulfuric acid is mainly used in fertilisers. Other sulfur compounds are also used in a range of other industries including rubber vulcanising, leather processing and in the production of paper, cellulose, rayon, and many pharmaceuticals, fungicides and insecticides.
Hydrogen sulfide and organic sulfides are found in a variety of feedstocks. One of the major sources is the solid fossil fuel feedstock coal, which is derived from the partial degradation of plants. Coal can contain 0.1 to 6 wt% sulfur depending on its source. The chemical composition of coal is complex and nonstoichiometric but is typically comprised predominantly of carbon, hydrogen and oxygen with smaller amounts of nitrogen and sulfur. Bituminous coal, which contains 80% carbon, has a high calorific value but also a high sulfur content. Low sulfur coals such as anthracite are more desirable but supplies of these are diminishing as they have already been heavily mined. Approximately half of the sulfur in a typical coal will be found as pyrites (FeS2). The remainder is found as organically bound sulfur, sulfate and H2S.
Another source of sulfur is crude oil. Crude is comprised predominantly of hydrocarbons with smaller amounts of sulfur, nitrogen and oxygen, and traces of heavy metals such as vanadium and nickel. The sulfur content of crude oil depends on its origin. North African crudes contain ca. 0.2 wt% sulfur, midcontinental US crudes contain ca. 0.2–2.5 wt% sulfur and Venezuelan crudes 2–4 wt% sulfur. The sulfur in crude oil is present as organic sulfur compounds, H2S and small amounts of elemental sulfur. When processing crude oil it is separated into fractions by distillation. The fractions are defined by either their boiling point range or carbon chain length as appropriate; the fractions and their typical uses are detailed in Table 1.1. The atmospheric residual fraction is the residue left after distillation at 350 °C at 1 atmosphere. At higher temperatures, the crude oil is distilled under vacuum to form heavy gas oils, and the residue left from this is known as the vacuum residual. It has an equivalent boiling point of ~560 °C at 1 atmosphere pressure.
The sulfur content increases with increase in the overall molecular weight of the fraction. The main sulfur compounds are organic sulfides or disulfides, mercaptans and thiophenes in the low boiling fractions. The sulfur is found mainly as thiophene derivatives such as benzo- and dibenzothiophenes in the higher boiling fractions.
Other anthropogenic sources of sulfur include: (i) industrial gas streams which contain sulfur as carbonyl sulfide, carbon disulfide, low molecular weight mercaptans and thiophene; (ii) natural and refinery gases which contain sulfur as mercaptans, COS and thiophene; (iii) synthesis gas (CO + H2) containing sulfur as COS and CS2; and (iv) emissions from vehicle exhausts.
Emissions from car exhausts have been much reduced in recent years by fitting three-way catalytic converters. The catalysts are comprised of platinum and rhodium dispersed on ceria-alumina mixed oxides coated on a monolith. The monolith is a magnesia-alumina silicate that has been extruded into a series of parallel channels. The role of the catalyst is to effect the three-way conversion of small hydrocarbons, carbon monoxide and nitric oxide formed in the exhaust of the petrol engine into water, carbon dioxide and nitrogen. This will lower emissions of volatile organic carbon and nitrogen oxides which contribute to acid rain formation. However, sulfur may be emitted as H2S and COS rather than SO2 under these conditions, and H2S is a known neurotoxin. COS is very unreactive and is retained in the troposphere for a long time. The troposphere is the lower half of the earth's atmosphere. The upper layer of the atmosphere is known as the stratosphere and begins at around 15 km above the surface of the earth. Hydrocarbons and CO are converted to CO2 and thus contribute to greenhouse gas emissions and thus global warming. It is clear that there are no easy solutions to control pollution levels in the environment.
There are also substantial natural reserves of sulfur, the most important of which are biogenic sources, sea spray and volcanoes. The biogenic sources originate from bacterial reduction of sediments to H2S in the sea and release of dimethyl sulfide from sea organisms. Most of the H2S is redeployed in bacterial oxidation, so that dimethyl sulfide is the major biogenically produced sulfur compound in the oceans, giving rise to a concentration of 0.01 ppb sulfur in sea spray from this source.
Volcanoes are the main natural source of sulfur dioxide. There are over 550 volcanoes in the world that are classed as active since they have erupted during historic time. When a volcanic eruption takes place, molten rock rises to the surface and it either flows in streams of glowing lava, or the molten material is violently ejected into the atmosphere together with large amounts of volcanic ash. Gases are also emitted during the eruption. The gases are mainly comprised of water vapour but are also accompanied by variable amounts of nitrogen, carbon dioxide and sulfur gases (mostly SO2 and H2S). Small quantities of CO, H2 and chlorine are also sometimes formed. Figure 1.1 shows a photograph of the Puu Oo vent from the Kilauea volcano in Hawaii and was taken in September 1983. The Kilauea volcano is the most active in the world and the photograph shows a stream of lava erupting from the vent of the volcano.
The molten rock, which is also called the magma, only contains a few percent of these gases, but they can have a catastrophic effect on the environment. The eruption of Laki fissure in Iceland in 1783 is one of the earliest recorded volcanic eruptions. Toxic gases, presumably SO2, were emitted when the volcano erupted and they formed a haze which reduced the sunlight intensity right across Europe. Three quarters of the livestock in Iceland died, and the haze caused the air temperature to drop so that the crops failed. This was then followed by the coldest winter in 225 years. There was insufficient food to support the population and ca. 24 % of the Icelanders died of starvation. A more recent example is the eruption of Mount Pinatubo in the Philippines in June 1991. Approximately 20 million tonnes of SO2 along with 3 to 5 km3 of particulate matter (mostly volcanic ash) were ejected into the atmosphere. 9 One meteorological report showed that 1992 temperatures were at a ten year low in the northern hemisphere and a fifteen year low in the southern hemi- sphere following the eruption. Volcanic eruptions can also create considerable long term social disruption to nearby communities, as demonstrated by the volcanic eruption of the Soufriere hills volcano on Montserrat which lasted from July 1995 to March 1998. The entire population on the southern half of the island had to be relocated.
Although the effects of volcanic activity are devastating, natural sources, of which volcanoes are the biggest component, account for less than 30% of sulfur emissions, so that man is by far the biggest contributor to atmospheric sulfur levels. The major anthropogenic source of SO2 is from coal fired power stations. This could be avoided by completely removing SO2 from effluent gases, but this is not economically or technologically achievable. However, many techniques exist for substantially reducing S02 emissions, and they will be discussed in Chapter 4 of this monograph. Other sources of SO2 emissions include oil-refinery operations, oil-fired energy generation, copper smelting and sulfuric acid manufacture.
The large scale industrial use of sulfur-containing gas, oil and solid fuel feedstocks makes it essential to clean up both feedstocks and refinery effluent in order to minimise production costs and prevent extensive pollution of the environment. Thus, sulfur contaminants emitted into the atmosphere ultimately form acid rain (Section 3), and sulfur contaminants found in industrial feedstocks cause plant pipeline corrosion at concentrations in excess of 3 ppm. Sulfur contaminants can also poison catalysts used in the processing of feedstocks. A nickel on alumina catalyst, for example, is used in steam reforming in which steam reacts with methane at 800 °C at 35 bar to form synthesis gas (carbon monoxide and hydrogen). The catalyst is severely poisoned by sulfur compounds which behave as Lewis-type bases donating electrons into the unfilled d orbitals of the metal.
2 Environmental Sulfur Levels
Sulfur dioxide and sulfate concentrations have been determined across the UK in recent years. They show that S02 emissions have decreased from 240000 metric tonnes (measured as S) in 1987 to 140000 metric tonnes in 1992–1994. Sulfur as sulfate has decreased from 230 000 metric tonnes to 200 000 metric tonnes in the same period. The average concentration of SO2 in the atmosphere in the UK today is ca. 33 µg m-3 most of which comes from anthropogenic sources; SO2 emissions in urban areas are considerably higher than those in rural areas. This figure is considerably smaller than it was in the past; the average urban SO2 concentrations were 188 µg m-3 in 1958, 144 µg m-3 in 1970 and 73 µg m-3 in 1977 in the UK. This reduction in emissions is partly accounted for by the enforcement of the Clean Air Acts of 1956 and 1968 and conversion of housing to smokeless fuels, natural gas and electricity. Although emissions from low household chimneys have decreased, emissions from the taller chimneys associated with power stations have considerably increased. Taller chimneys will cause the sulfur to be carried away from the immediate vicinity so that localised sulfur levels will be reduced, but it will then be deposited elsewhere. In the absence of a source of moisture such as clouds, sulfur dioxide can travel hundreds of kilometres to eventually be precipitated as acid rain when it comes into contact with moist air. In Sweden, for example, where the effects of acid rain on lakes and streams has been quite severe, only one tenth of the pollution originates from atmospheric SO2 emissions in Sweden; a further tenth can be attributed to UK sulfur emissions and the remainder is from industrial regions in northern Europe. It has to be said, though, that now we are aware of this problem, legislation has been introduced to reduce/eliminate acid rain substantially. A series of environmental action programmes has been developed, which included remedial measures to reduce SO2 emissions. New measures such as encouraging the use of 'cleaner' energy resources, recycling of waste and conservation of natural resources have also been introduced. One interesting effect of the reduction in sulfur emissions has been that soils in some areas are now so sulfur deficient that sulfur is having to be added to the fertilisers used on this ground!
Species other than SO found in the atmosphere include HS, dimethyl sulfide [(CH3)2S], dimethyl disulfide [(CH3)2S2], carbonyl sulfide (COS) and carbon disulfide (CS2). H2S, (CH3)2s and (CH3)2S2 are rapidly oxidised to SO and thus have lifetimes of only a few days in the atmosphere. Carbonyl sulfide and carbon disulfide are much longer lived species and are found in the troposphere. Carbonyl sulfide in particular is a problem since it is present in the troposphere at concentrations of ca. 500[micros]g m-3. Concentrations of carbon disulfide are less than one tenth those of the carbonyl sulfide. H2S, COS and CS2 react with hydroxide radicals to form SH radicals in the atmosphere.
OH• + CS2 [right arrow] COS + SH• (1.1)
OH• + COS [right arrow] COS + SH• (1.2)
OH• + HS2 [right arrow] H2O + SH• (1.3)
The SH radicals are then further oxidised to SO2.
The hydroxide free radicals are formed in the atmosphere by photolysis of ozone. This involves the splitting of an ozone molecule by the absorption of solar radiation, followed by reaction with water to give the hydroxide free radicals.
O3 + hb [right arrow] O2 + O* (1.4)
O*+ H2O [right arrow] 2HO• (1.5)
It is important not to consider sulfur em1ss1ons in isolation, as they are accompanied by other pollutant gases resulting from domestic and industrial stack emissions and emissions from motor fuel combustion. Other main pollutants include smoke and particulates, nitrogen oxides, CO, CO2, HCl, hydrocarbons and heavy metals. Hydrocarbons, heavy metals, smoke and particulates are a health hazard, particularly to asthmatics, nitrogen oxides contribute (along with SO2) to acid rain formation (see Section 3), and CO2 contributes to global warming. HCl and chlorine compounds, used in aerosol spray cans, refrigerants and solvents, have increased the amount of chlorine in the atmosphere, and this breaks down to chlorine radicals which damage the ozone layer. Ozone breakdown occurs in the stratosphere. Chlorofluorocarbons (CFCs) were used for many years as inert, non-toxic, non-flammable compounds in refrigeration, for example. Unfortunately, they decompose photochemically in the stratosphere to chlorine which again catalyses the decomposition of ozone. CFCs were phased out completely from 1st January 1996. Since then, more 'environmentally friendly' non-flammable, non-toxic replacements have been sought for use as refrigerants. These include hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). These materials break down in the troposphere where there is very little ozone. However, HCFCs still contain chlorine which is released into the troposphere when the HCFCs break down and some of this chlorine may find its way into the stratosphere and therefore still contribute to ozone breakdown. The best alternative CFC replacements are therefore the HFCs and indeed, the HCFCs have now been largely phased out. 15 The main effect of the depletion of the ozone layer is to increase the amount of ultraviolet radiation reaching the earth's surface leading to an increased risk of eye cataracts and skin cancer in the human population.
Sulfur dioxide is a respiratory irritant at concentrations > 1 ppm, especially when it is combined with soot. It can cause pleurisy, bronchitis and emphysema in susceptible individuals. 14 Its effects were particularly marked before legislation was introduced to stop coal being used as a fuel in cities. Most British coal is bituminous and has a high tar and hydrocarbon content. This causes it to smoke considerably when it is burned. Smoke in a humid environment, such as that commonly found in British winters, acts as nucleation centres for fog formation, and this combination of smoke and fog is known as smog. One of the worst incidents from smog pollution occurred in London in 1952 when four thousand people died. A cold black sulfurous smog was trapped over the city by a blanket of warm air for almost a week. This irritated the bronchial tubes of individuals inhaling the smog so that they flooded with mucus and the people choked to death. It was thought that the smoke and sulfur dioxide caused the deaths, but it is now thought that the effects of the sulfurous smog were accentuated by the formation of highly acidic particles.
3 Acid Rain
The sulfur dioxide that is released into the atmosphere generates acid rain. Acid rain is extremely detrimental to the environment, causing lakes and soils to become acidified and resulting in the death of fish and the poisoning of trees. Acid rain also damages buildings, particularly limestone buildings, by accelerating the rate of decay of stone and increases the rate of corrosion of metal structures such as bridges. Acid rain was first identified by Robert Angus Smith in England in 1872, but the detrimental effects of acid rain were not identified until 1961. Since then steps have been taken to limit acid rain formation by controlling gas emissions into the atmosphere. Although sulfur dioxide is the main precursor to acid rain formation, nitrogen oxides are also involved in acid rain formation. Traces of HCl are also found in acid rain, and they are thought to originate from the reaction of sulfuric acid with atmospheric sodium chloride originating from evaporation of seawater.
Sulfur dioxide in the atmosphere can be oxidised by a variety of routes, e.g. gas-gas reactions such as the interaction of hydroxide free radicals with S02 to generate HSO•3:
HO• + SO2 [right arrow] HOSO•2 (1.6)
HOSO•2 is very reactive and is eventually further oxidised to sulfate either in water droplets or on nitrogen-containing molecules such as NO. Reactions also take place in solution and may involve a catalyst such as manganese, e.g.:
SO2 + H2O2(aq) [right arrow] SO3(aq) + H2O [right arrow] H2SO4 (1.7)
