Diesel emissions are the organic, inorganic, and volatile particles released from the complete or incomplete combustion of diesel in engines and generators. The distribution of emitted diesel particles consists of particles with diameters less than 50 nm and the rest with diameters in the range 50 – 500 nm. The former is known as the nucleation mode whereas the latter is known as the accumulation mode. What is diesel in the first place? Very good question which we will answer shortly. Diesel fuel is the common term for the petroleum distillate fuel oil sold for use in vehicles. Diesel is named after the inventor of the compression ignition engine, Rudolf Diesel. He patented his original design in 1892. Diesel fuel is refined from the crude oil at petroleum refineries. The parent crude oil determines the quality of diesel which also affects the types of diesel emissions in the cycle from source to application. Diesel is a complex mixture of hydrocarbons with boiling points in the range 150 to 380 degrees celcius. Small amounts of sulfur, nitrogen, and oxygen are present in diesel. The emitted volatile mass fraction consists mainly of an organic fraction of unburned hydrocarbons, polyaromatic hydrocarbons, nitrates, and sulfates. Most of these are released as nanoscale liquid droplets, which form the nucleation mode and/or as a condensed film onto the surface of accumulation mode soot particles. Nucleation mode particles are composed mostly of species that have solid materials in trace concentrations. The non-volatile core from both accumulation and nucleation modes contains mainly carbon and possibly metal oxides from the engine lubricant. The article details the different chemistries of components emitted from diesel and conclude with answering the question – Can diesel emissions cause cancer?
Chemistry of diesel emissions – carbon monoxide, oxides of nitrogen and sulphur.
Diesel engines always run on an overall lean mixture. This means that at an air to fuel ratio of 20:1 and higher up to 100:1. At 20:1 air/fuel ratio, there is 30% more air than chemically necessary for complete combustion and therefore diesel exhaust gases contain significant amount of oxygen. Concentration of oxygen in diesel exhaust gases varies from 5 to 17%, depending on the engine load. The ideal exhaust gas should contain nitrogen, carbon dioxide and water as products from the reaction of air and diesel fuel. Deviation from ideal combustion behavior leads to generation of several by-products. These by-products are what we are calling emissions. Although they are generated in small quantities, they are either toxic or have negative environmental impacts. The major emissions generated in diesel combustion are what we will discuss moving forward. Carbon monoxide (CO) is a colorless, odorless, and highly toxic gas. It is produced under excessively rich combustion conditions. The major factors influencing CO formation are lack of oxidants, low gas temperature and short residence times. Oxides of nitrogen (NOx) including nitric oxide (NO) and nitrogen dioxide (NO2). Both oxides are formed at the high combustion temperatures and pressure encountered in diesel engine fuel combustion. NO is a colorless, odorless gas which is formed from the reaction of nitrogen and oxygen at high temperatures. NO2 is formed from the oxidation of NO and it is a toxic red brown gas with an unpleasant odor. It is very reactive and has strong oxidation properties. Sulfur in diesel is found in exhaust gas as sulfur dioxide (SO2). It is a colorless gas with a characteristic-irritating odor. Oxidation of SO2 produces sulfur trioxide (SO3). SO3 condenses with water to form sulfuric acid.
Chemistry of diesel emissions – hydrocarbons, PAHs, and diesel particulate matter.
Diesel engines comprise of many hydrocarbon species. Hydrocarbons have an irritating odor and some of the hydrocarbons are toxic and carcinogenic. Two major sources of hydrocarbons in diesel exhaust are unburned fuel and lubricating oil. Formation of hydrocarbons emissions occurs in the lean out flame region which is too lean to burn and in the spray core, which is too rich to burn. Hydrocarbons are oxidized to form carbon dioxide and water. Other sources of hydrocarbon are quenching of flame on cold cylinder walls, spray tail which is the large droplets in the last part of injected fuel and misfire. Once released into the atmosphere, hydrocarbons undergo photochemical reactions leading to the formation of ground level ozone and photochemical smog. Polynuclear aromatic hydrocarbons (PAHs) are hydrocarbons with two or more benzene rings. They are present in diesel and their concentrations vary between 1.5 to 2.5%. A small fraction of between 0.2 to 1% of total diesel PAHs survives combustion and is found in exhaust gas. They attract special attention because majority of these compounds are known to be human carcinogens. Diesel particulate matter (DPM) is the most unique and difficult to control emission from diesel engines. It is a complex mixture of elemental carbon, heavy hydrocarbons, and sulphate. The major source of particulate emission includes loss of oil control, over fueling, poor combustion, misfire, and intermittent malfunctions of injection system such as nozzle dribble, secondary injections and after injections. DPM are responsible for diesel engines characteristic black smoke. Carbonaceous particles in DPM agglomerate and absorb other species to form structures of complex physical and chemical properties.
Can diesel emissions cause cancer?
The discussion has extensively highlighted the origins and chemistry aspect of diesel emissions. We now seek to answer the question – Can diesel emissions cause cancer? It is important to mention that concerns for health effects from exposure to diesel exhaust arose in the mid-1900s and stimulated development of emissions regulations and research to improve the technology and characterize potential health hazards. Fast forward, in 1989 the International Agency for Research on Cancer classified whole diesel exhaust as “probably carcinogenic to humans”. The non-volatile carbon and metal oxides are slightly soluble in lung fluid. They generate reactive oxygen species such as superoxide, hydrogen peroxide, and hydroxyl radicals, that cause oxidative stress on epithelial cells. Combustion of non-homogenous mixtures in diesel engines emit soot and particulate matter along with relatively high NOx concentrations. These chemical components cause various lung related diseases and high concentration exposure continuously results in cancer. Diesel exhaust particles in the 2000s were classified as “reasonably anticipated to be a human carcinogen” based on evidence of elevated lung cancer in occupational groups exposed to diesel exhaust. The turn of the millennium has also seen more supportive evidence supporting that diesel emissions cause cancer. Research data shows that there are mutagenic and/or chromosomal effects of diesel and its organic constituents. Having mentioned all the facts, the article concludes by the definition of cancer. It is a family of diseases characterized by new and uncontrolled growth of tissues. Be sure to look out for the next article which focuses on ways of reducing diesel emissions in mining.
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REFERENCES
Maximilien et al. 2017. Diesel engine exhaust exposures in two underground mines. International Journal of Mining Science and Technology.Saarikoski et al. 2017. Particulate matter characteristics, dynamics, and sources in an underground mine. Aerosol Science and Technology.
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