What are sulfur oxides?
Sulfur oxides (SOx) are compounds of sulfur and oxygen molecules. Sulfur dioxide (SO2) is the predominant form found in the lower atmosphere. Sulfur oxide is the common term for many types of sulfur and oxygen containing compounds such as:
- Lower sulfur oxides(SnO, S7O2 and S6O2)
- Sulfur monoxide(SO)
- Sulfur dioxide(SO2)
- Sulfur trioxide(SO3)
- Higher sulfur oxides(SO3 and SO4 and polymeric condensates of them)
- Disulfur monoxide(S2O)
- Disulfur dioxide(S2O2)
These molecules are colorless, but have a very distinct and strong scent and taste that can be detected when the gas is of high concentration.
Sulfur monoxide, SO, is a common sulfur oxide compound that is rarely found outside of space. When concentrated or condensed it becomes S2O2, or disulfur dioxide (see below). Under extreme laboratory conditions sulfur monoxide can be produced by treating sulfur dioxide with sulfur vapors in a glow discharge. In spaced sulfur monoxide is detected around one of Jupiter’s moons, Lo, in the atmosphere and in the plasma torus. It has also been found in the atmospheres of Venus and the Hale-Bopp comet as well as the interstellar medium. In biological chemistry, sulfur monoxide may have some biological activity, the formation of transient SO in porcine coronary artery has been inferred from the reaction products. Because it is rare in occurrence it is difficult to fully understand the hazardous of sulfur monoxide but as sulfur dioxide it is both toxic and corrosive.
Sulfur dioxide, one of the more popular sulfur oxides, is a poisonous gas with a smell that is often described as a just struck match. Sulfur dioxide can be can be liquefied at room temperatures under moderate pressures. As a liquid it freezes at -37° C (-99.4° F) and boils at -10° C (+14° F) under atmospheric pressure. Sulfur dioxide, like sulfur monoxide is often found in space. In the atmosphere of Venus it is the third-most significant gas at 140 ppm. Interestingly, on Venus it is a contributor to global warming and is a key component of the chemical reactions in the planets atmosphere.
Sulfur trioxide, SO3, is a significant pollutant in its gaseous form, and is the primary component of acid rain. When perfectly dry, sulfur trioxide vapor is completely invisible; when in liquid form, its transparent. Sulfur trioxides fumes profusely, even being used as a smoke agent. The odorless vapor is extremely corrosive, due to its formation of sulfuric acid mist. Sulfur trioxide is an essential reagent in sulfonation reactions, in the creation of dyes, pharmaceuticals, and detergents. Unaltered sulfur trioxides will cause serious burns when ingested, or inhaled, as its highly corrosive. It is important to know that sulfur trioxide should be handled very carefully, because it reacts very violently with water. The combination of sulfur trioxides with water creates sulfuric acid, which is highly dangerous, and corrosive.
Where are sulfur oxides found?
Sulfur oxides are mainly produced when gases containing sulfur oxides undergo combustion, through the roasting of metal sulfide ores, high-sulfur coal burning powerplants. Vehicles can also produce sulfur oxides, especially heavy gas burners. The most common source for sulfur dioxide is nature, accounting for between 35-65% of emissions, from things like volcanoes. Data on the composition of SOx from combustion and other man-made sources indicate that about 98 percent of emitted SOx is sulfur dioxide; the remaining fraction of typical SOx emissions is sulfur trioxide and its derivatives.
What are Sulfur Oxides used for?
Sulfur oxides have a great many different number of applications. Though primarily used in the production of sulfuric acid, SOx’s it is also used as a disinfectant, or detergents and bleaches. They are also used as a preservative in foods, most commonly dried fruits and meats. Another common use for sulfur oxides is as a refrigerant, as it absorbs heat; though it is extremely reactive, and rarely used in this way anymore. Have you ever heard of sulfite-free shampoo? That means it is free of sulfur oxides, since they are commonly used in shampoos. Sulfur dioxides are considered one of the most important chemical compounds in the chemical industry. The negative side effects are detrimental to the environment when they are the result of the combustion of sulfurous fossil fuels, like coal and oil.
Why are Sulfur Oxides a concern?
Air pollution is caused by a cocktail of complex chemical substance, of which sulfur oxide is a main component. The extent of their negative effects on the environment, agriculture, and human health, though not totally known, seem to be incredibly serious. Sulfur oxide emissions from man-made sources often consist primarily of sulfur dioxides. Although natural sources of sulfur oxides are much more common than industrial sources, the concentrated emissions from technological processes, in particular fossil fuel combustion, are much greater than natural contributions in more heavily industrialized regions. These regions experience greater, and more obvious adverse reactions from abundant sulfur oxide emissions.
It is known that sulfur oxides contribute to air pollution, and smog, reduced visibility, as well as playing a hugely negative role in the homeostasis of living things. These molecules are considered irritants, and have been linked with issues with the eyes, ears, and throat, as well as reduced lung function in humans. They have also been linked to respiratory destress; with young children, elderly, and asthmatics being the most at risk.
Emissions of SOx can negatively affect many different types of vegetation, including agriculture crops. Plants that are exposed to SOx in high amounts, or over prolonged periods of time, produced a smaller yield, had less leaves, and even died prematurely. Acid rain, which is primarily made up of sulfur oxides, can strip a tree of its leaves, and destroy soil quality. Unlike used soil, which can be rested, and reused, soil ruined by acid rain is nearly impossible to ever use for crops, again.
Sulfur Oxide molecules can affect non-living things as well. As a part of acid rain, SOx can contribute to the erosion of stone, and building materials. This can put roads, buildings, bridges at risk, endangering human lives, and businesses. Acid rain has been a serious issue at stone quarries for decades, and has been getting worse every year. SOx can also accumulate in dust particles. These particles are very erosive to metal, paint, and stone. Headstones are particularly susceptible to acid filled dust particles, and acid rain; their corrosion leaves behind a lot more than financial hardship, but emotional distress, as well.
- Removal of sulfur from coal, before combustion, or of sulfur oxides after the combustion
- Power plants can implement the use of reduced sulfur fuels
- Switch to nuclear power generation as no sulfur oxides are emitted from nuclear plants
- Improving the efficiency of fuel to electricity, reducing pollution emissions per unity of electricity
How do we treat Sulfur Oxides?
Since many industrial processes do result in the creation of sulfur oxides, there are ways to “clean” or dispose of sulfur oxide molecules.
A catalytic converter is a device that converts pollutants and toxic gases into a less toxic substance, by catalyzing a redox reaction. SOx catalytic abatement involves acerium-containing magnesium aluminate spinel catalysts, that oxidizes the SOx, chemsorbs it, then releases it as hydrogen oxide. The catalyst comes in varying arrangements depending on the application. Gulf Coast Environmental Systems offers multiple design solutions for catalytic SOx abatement, to best fit the operational parameters of its customers.
Sodium Bicarbonate Injection is often used for the removal of Sulfur Trioxide. Sulfur Trioxide (SO3) is often formed in certain combustion equipment treating sulfur based compounds. The SO3 is a very fine particle that post-combustion scrubbing cannot remove. The formation of SO3 in the Thermal or Catalytic Oxidizer combining with water molecules mainly forms Sulfuric Acid (H2SO4) which significantly contributes to a visible white vapor plume at concentrations as low as 3-4 ppmv and is often detectable on an opacity monitor. Injection of fine milled Sodium Bicarbonate or Baking Soda in a dry powder form collects the SO3. This will reduce the SO3 ability to react with water due to the newly formed larger particulate size. As a result the injection shall reduce or eliminate the contribution to the visible exhaust stack plume.
Another method for SOx abatement is “Scrubbing.” The term scrubbing is often misused, when referring to the removal of VOCs. There are many different types of scrubbing, but the most effective form for SOx removal is wet scrubbing. Wet scrubbers are used to treat or clean sulfur oxides from a process exhaust air stream. An SOx wet scrubber passes the SOx containing exhaust gas through several chambers, containing a carefully created scrubbing “cloud” of water. These chambers contain a high level of droplets, that capture the irritant molecules, while circulating. The liquid containing the SOx is collected in the form of droplets, and then treated prior to discharge, or reused elsewhere in a facilities processing system This system is optimized to the lowest energy consumption, and keeps emissions within stipulated limits, regardless of the sulfur oxide content in the exhaust. SOx scrubber systems are available in 3 different loops; open, closed, or hybrid systems. Wet scrubbers are an extremely versatile form of pollution control, which allows GCES to custom design each scrubber system to achieve optimal performance in the removal of SOx from air released into the atmosphere. Wet scrubbers are extremely effective in their design, in that they are often the only system that can be used to treat both particulate matter and gases in a single pollution control devise.
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Additional articles in the GCES series ‘Abating Hazardous Air Pollutants’ include:
Part 2: Chlorine Abatement
Part 6: SOx, the compounds of sulfur and oxygen molecules including Sulfur Monoxide, Sulfur Dioxide and Sulfur Trioxide
Part 11: Sulfuric Acid – H2SO4
Part 12: Ethylene Oxide – EtO
Part 13: PFAS as Emerging Contaminates