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Mar-2012

Combating the threat of mercury

Emerging legally binding action on mercury emissions highlights the importance of monitoring and quantifying emissions from gas operations

Michael Hayes and Katrin Åkerlindh, Linde Gases
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Article Summary
Mercury has been elevated to the status of a pollutant of global concern owing to some of its unique toxic properties, which pose environmental and health risks. Mercury is found both naturally and as an introduced contaminant in the environment, mainly from high-temperature industrial processes such as oil and gas processing, alkali and metal processing, incineration of coal and oil in electric power stations, foundries, and waste combustion.

In the past, mining was a substantial source of mercury in some areas. For example, the hydraulic placer-gold mines of the Sierra Nevadas in the US released several thousand tons of mercury to the environment from the 1860s to the early 1900s. The US Geological Survey believes that high levels of mercury in fish, amphibians and invertebrates downstream of hydraulic mines are a result of historic mercury use.

Atmospheric mercury
Natural sources of atmospheric mercury include volcanoes, geologic deposits of mercury and volatilisation from the ocean. Although all rocks, sediments, water and soils naturally contain small amounts of mercury, some local mineral occurrences and thermal springs are naturally high in mercury.

Long-range atmospheric deposition is the dominant source of mercury over aquatic and terrestrial ecosystems. Since it is an element, mercury is not biodegradable and, although its form and availability to living organisms may change over time, mercury endures in the environment. Converted by bacterial action in lakes and waterways to a more toxic form known as methylmercury (CH3Hg), it can bioaccumulate in fish and shellfish. Mercury is so toxic that just one kilogram of mercury is enough to render almost two million kilograms of fish unsafe to eat.

Global mercury cycle
Once it has entered the so-called “global mercury cycle”, methylmercury becomes concentrated as it is transferred up the food chain to birds, animals, marine mammals and humans in a process known as biomagnification. Through this cycle, mercury can contaminate entire food webs, posing a serious threat to ecosystem health and particularly to the higher order species in the food chain, ultimately impacting on human health.

Almost all the mercury in lakes in the EU has been deposited via atmospheric transport from sources abroad, while the amount being used and released in the world is still increasing. Coal-fired power-generating plants, owing to the nature of the fossil fuel employed, are the largest man-made source of mercury emissions, while mercury is also found in many everyday household goods, such as lighting and electrical appliances, batteries, medical equipment, older dental fillings, jewellery, paint, thermometers, barometers, manometers, thermostats, pharmaceuticals and pesticides. When these products are discarded, mercury can be released to the environment in a variety of ways during the transport of the waste, its incineration, the post-incineration disposal of residuals such as ash, and in landfills.

Although mercury use has gone down in industrialised nations, emissions are growing in other regions. The burning of coal in small-scale power plants and residential heaters, particularly in Asia, are major sources of current emissions. These emissions are likely to increase significantly because of the economic and population growth in these regions.

Mercury has been found to be responsible for a spectrum of adverse human health effects, including permanent damage to the nervous system, in particular 
the developing nervous system, affecting learning ability and neuro-development in young children. It can be transferred from a mother to her unborn child, making children and women of childbearing age vulnerable populations, especially those living in close proximity to industrial plants. It also affects the kidneys and lungs.

Plant safety and structural integrity

In gas production and processing plants, trace levels of mercury could range between 0 and up to 2000 µg/Nm3 in products, depending on the source. It poses a formidable threat to the safety of humans and capital equipment, because of this propensity to amalgamate with the materials of construction used for pipelines and equipment.

For instance, should mercury amalgamate to the pump system material or to a turbine blade, it could throw these systems out of kilter and cause immense structural damage. Liquid metal embrittlement (LME), which weakens the original structure of steel, aluminium and other metals in process plants, is the main threat. LME is a form of cracking that occurs when certain molten metals come into contact with structural alloys. The most commonly affected materials include carbon steel, low-alloy steels, high-strength steels, 300 Series stainless steel, nickel-based alloys and the copper alloys, aluminium alloys and titanium alloys. Not all alloys are susceptible to LME, as this manifestation only occurs where specific pairings of liquid metal and structural alloy are present.

LME introduces acute risk to affected industrial plant. Mercury-induced corrosion of heat exchangers constructed of aluminium have led to catastrophic failures in the past, and common industry practice to avoid these incidents is to reduce the mercury content below the detection limit (0.01 μg/Nm3) in an early phase of product processing. In a worst-case scenario, structural failure could cause a massive explosion in an oil refinery or LNG facility, resulting in catastrophic loss of life, excessive damage to capital equipment and long-term plant downtime.
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