Combustion Engines: Major Air Pollutants And Their Sources

The 19th-century invention of the internal combustion engine has had a significant environmental impact. These engines, which operate through the ignition and combustion of a fuel-air mixture, produce a variety of air pollutants. The type and amount of pollutants depend on the appliance, its installation, maintenance, venting, and the type of fuel used. Common combustion pollutants include carbon monoxide, nitrogen dioxide, particulate matter, and sulfur dioxide. Other pollutants include unburned hydrocarbons, aldehydes, and polycyclic aromatic hydrocarbons (PAHs). These pollutants have adverse effects on human health, the climate, and the ecosystem.

Nitrogen dioxide, nitrogen monoxide, and nitrogen oxides

Nitrogen oxides are highly reactive chemical compounds formed by various combinations of oxygen and nitrogen at high temperatures during the combustion process. Nitrogen dioxide (NO2) and nitric oxide (NO) are the two principal nitrogen oxides associated with combustion sources. They are formed during the combustion of fossil fuels such as coal, oil, methane gas (natural gas), diesel, and waste plants. The higher the combustion temperature, the more nitric oxide is generated. Diesel engines, in particular, are a significant source of nitrogen oxides.

Nitrogen oxides are a major air pollutant, contributing to particle pollution and the formation of ground-level ozone. Ground-level ozone is a harmful pollutant that affects the lungs, causing coughing and shortness of breath, damaging and inflaming airways, and aggravating asthma. It is formed through the photochemical reactions of nitrogen oxides, carbon monoxide, and volatile organic compounds in the presence of strong sunlight and UV radiation.

Nitrogen dioxide is a harmful pollutant that has adverse effects on the respiratory systems of both humans and animals, increasing the risk of stroke and asthma in children. It also contributes to the formation of ground-level ozone and secondary particulate matter when combined with other atmospheric compounds such as ammonia. High levels of nitrogen dioxide are associated with indoor combustion sources, including tobacco smoke, gas, wood, oil, kerosene, and coal-burning appliances, such as stoves, ovens, and heaters.

Nitrogen monoxide, also known as nitric oxide, is another nitrogen oxide associated with combustion sources. While it is rapidly oxidized in ambient conditions to form nitrogen dioxide, it still has adverse health effects. It dissolves in water vapour to create acid rain and contributes to the formation of ground-level ozone. Nitrogen monoxide is emitted in smaller amounts compared to nitrogen dioxide, with 90-95% of nitrogen oxides emitted as nitric oxide and only 5-10% as nitrogen dioxide.

The impact of nitrogen oxides and other air pollutants from combustion engines on human health and the environment is significant. Efforts to reduce emissions and improve air quality are crucial to mitigate their harmful effects.

Carbon monoxide

The toxicity of carbon monoxide lies in its ability to bind to hemoglobin in the blood, inhibiting oxygen transport and making it difficult for the body's cells to bind to oxygen. This lack of oxygen damages tissues and cells, causing health issues such as breathing difficulties, exhaustion, dizziness, and other flu-like symptoms. Exposure to high levels of carbon monoxide can even lead to death.

Indoor exposure to carbon monoxide is a significant concern. In homes, particularly in developing countries, the burning of biomass fuels and tobacco smoke are the primary sources of carbon monoxide. Clogged chimneys, wood-burning fireplaces, decorative fireplaces, gas burners, and supplementary heaters without proper safety features can vent carbon monoxide into indoor spaces. Tobacco smoke, including from motor vehicles in attached garages, is another major source of indoor exposure.

Additionally, combustion of low-grade solid fuel and biofuels in small stoves or fireplaces can generate lethal levels of carbon monoxide emissions if the flue gases are not adequately vented outdoors. At the beginning of combustion, the pollutants released are mainly particulate matter, but as combustion progresses, carbon monoxide becomes the dominant pollutant.

Identifying carbon monoxide poisoning is crucial, and it is essential to consider the victim's environment and immediate past living or work situation. Moving the victim to fresh air is the first step, and while carbon monoxide will eventually be eliminated from the blood through normal ventilation, emergency measures should be initiated immediately as serious health damage can occur before this happens.

Particulate matter

The invention of the internal combustion engine in the 19th century revolutionized transportation. However, it has posed, and continues to pose, a significant environmental threat. The combustion of fuel in engines produces harmful pollutants, including particulate matter (PM).

PM is generated through the incomplete combustion of hydrocarbons, resulting in soot or cenospheres. Soot consists of carbonaceous particles produced through the gas-phase combustion process, while cenospheres are formed through the direct pyrolysis of liquid hydrocarbon fuels. PM emissions also contain solid particles from unburned fuel, soluble organic fractions (SOF) from unburned lubricants, and sulfides and other fuel additives. The composition of PM emissions varies, including both solid and liquid particles such as carbonaceous particles, nitrates, and sulfates.

PM emissions are closely related to the quality of fuel and lubricating oil, as well as the degree of combustion in a diesel engine. Two-stroke engines, in particular, lack catalytic converters, allowing unburnt fuel to escape from the exhaust and contributing to higher emissions of PM. Additionally, the oil mixed with the fuel to lubricate the engine's moving parts can create additional pollutants during the combustion process, including higher emissions of PM.

PM 2.5, particulate matter with a size of 2.5 μm or less, is of significant concern due to its ability to easily penetrate the lungs and cause respiratory diseases. It is formed through the combination of nitrogen oxides, created during high-temperature combustion, with ammonia and volatile organic compounds (VOCs). Studies have indicated that there is likely no safe level of exposure to PM 2.5, and exposure to particulate pollution has been linked to a variety of adverse health effects, including respiratory symptoms, decreased lung function, aggravated asthma, irregular heartbeat, and premature death in individuals with heart or lung diseases.

Furthermore, fine particulate matter generated from combustion engines has been described as toxic to human health and has been the subject of studies investigating its role in the spread of the COVID-19 pandemic.

Hydrocarbons

The combustion of hydrocarbons can also lead to the formation of other harmful pollutants, such as benzene, which has been associated with adverse health effects. In addition, the oil used to lubricate engine parts can create additional pollutants during the combustion process, including higher emissions of hydrocarbons.

To reduce hydrocarbon emissions from combustion engines, various strategies have been implemented. These include the use of emissions-control devices, such as catalytic converters, and the development of cleaner-burning engines. Regulatory actions, such as the Clean Air Act in the United States, have also played a crucial role in mandating the reduction of air pollutants from gasoline use.

Sulfur dioxide and sulfur trioxide

The combustion of fossil fuels in power plants and other industrial facilities is a major source of sulfur dioxide (SO2) and sulfur trioxide (SO3) emissions. These sulfur oxides are formed through the oxidation of sulfur compounds in the fuel, with most of the sulfur converting to SO2 and a small percentage undergoing further oxidation to become SO3.

SO2 is a reactive gas with a strong odour that can cause respiratory issues, reduced visibility, corrosion, and harm to vegetation. It is released into the atmosphere primarily through the burning of sulfur-containing fuels, such as coal, and ends up in flue gas or exhaust. The average concentration of SO2 in the air is around 10 mg/m3.

SO3, while present in smaller amounts compared to SO2, also has significant environmental and operational impacts. It can be released as an acid aerosol, creating visible plumes and contributing to acid rain. Additionally, SO3 can lead to the corrosion of surfaces in heat recovery zones, especially at temperatures below the acid dew point.

The formation of SO3 is influenced by various factors, including oxygen concentration, SO2 concentration, and the presence of catalytic materials. The conversion of SO2 to SO3 is a complex process that requires further investigation, especially in the context of oxy-fuel combustion.

Both SO2 and SO3 pose risks to human health and the environment. They react with water vapour in the atmosphere to form sulfurous acid and sulfuric acid, respectively, which are harmful for breathing and damaging to ecosystems. Efforts have been made to reduce sulfur emissions through the production of ultra-low sulfur or sulfur-free fuels, as well as implementing strict environmental regulations for sulfur content in fuels.

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