As legislation grows in volume and stringency, monitoring and quantifying emission pollutants in an accurate and transparent manner are becoming priorities, write Mike Hayes and Roberto Parola

Coal-fired power plants are the largest man-made source of mercury emissions

Credit: The Linde Group

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.

An indestructible chemical element, mercury is found both naturally and as an introduced contaminant in the environment, mainly from high-temperature industrial processes such as alkali and metal processing; incineration of coal and oil in electric power stations; foundries; waste combustion and oil and gas processing.

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 tonnes 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.

Current anthropogenic sources are responsible for about 30 per cent of annual mercury emissions into the air. A further 10 per cent comes from natural geological sources and the remaining 60 per cent is from re-emission of previously released mercury that has built up in surface soils and oceans.

Natural sources of atmospheric mercury include volcanoes, geologic deposits of mercury and volatilization 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. Because 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 kg of mercury is enough to render almost two million kg of fish unsafe to eat.

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 European Union 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 industrialized nations, emissions are growing in other regions, especially in East and Southeast Asia, South America and sub-Saharan Africa where the use of mercury in artisanal and small-scale gold mining operations in remote locations still has a big impact. All 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, gastrointestinal complaints and lungs.

UNEP and Minamata Convention

The United Nations Environment Programme (UNEP) has been working to address mercury issues since 2003 and its current mercury programme has two main facets. An intergovernmental negotiating committee is working to develop a global legally binding instrument to strengthen global action on mercury. In tandem, a UNEP Global Mercury Partnership aims to protect human health and the global environment from the release of mercury and its compounds by minimizing and, wherever feasible, ultimately eliminating global, anthropogenic mercury releases to air, water and land. Mercury rapidly moved up the pollution control agenda in the EU, the US and Asia prior to the legally binding UNEP global treaty on mercury implemented in 2013.

Reducing mercury is an ongoing process

Credit: The Linde Group

A UNEP report stated that “mercury is a substance that can be transported in the atmosphere and in the oceans around the globe travelling hundreds and thousands of kilometres from where it is emitted. The global environmental threat to humans and wildlife has not receded despite reductions in mercury discharges, particularly in developed countries. Indeed, the problems remain and appear, in some situations, to be worsening as demand for energy, the largest source of human made mercury emissions, climbs.”

In January 2013, 140 countries signed the Minamata Convention, the objective of which was to protect human health and the environment from anthropogenic emissions and releases of mercury and mercury compounds.

The Convention is named after the Japanese city of Minamata, which experienced a severe incidence of mercury poisoning after industrial wastewater was discharged into Minamata Bay. The Convention sets out controls and reductions across a range of products, processes and industries where mercury is used, released or emitted. These range from medical equipment such as thermometers, batteries and energy-saving light bulbs to the mining, cement and coal-fired power industry sectors.

The treaty also addresses the direct mining of mercury, export and import of the metal and safe storage of waste mercury.

Europe and the US

The United States Environmental Protection Agency (EPA) has long supported the efforts of UNEP and other partners to address mercury, securing an agreement for UNEP to conduct a Global Mercury Assessment that has provided clear scientific evidence of mercury’s global reach. In late 2011 the, EPA finalized the Mercury and Air Toxics Standards (MATS), the first national Clean Air standards to reduce emissions of mercury and other toxic air pollutants from new and existing coal- and oil-fired power plants.

Coal generates a third of global electricity

Credit: The Linde Group

It is estimated that the top three sectors responsible for anthropogenic emissions are coal burning (24 per cent), primary production of non-ferrous metals (10 per cent) and cement production (9 per cent), while oil refining counts for approximately 1 per cent.

The US emits in the region 50 tonnes of mercury per annum into the atmosphere, with around 70 per cent being released from coal combustion plants; the similar estimation for the EU is 87.5 tonnes, with around 50 per cent being released from coal combustion plants, followed by another 25 per cent represented by cement and non-ferrous metal production.

To further address mercury use and pollution in the EU, the Community Strategy concerning mercury was adopted in 2005 and reviewed in 2010. It focuses on mercury emissions to air, the banning of mercury exportation (including certain mercury compounds) and enforcing restrictions on products containing mercury and industrial processes using mercury.

In regard to industrial emissions of mercury, the Industrial Emissions Directive (IED) – applicable for combustion plants with a size greater than 50 MW – addresses the issue via the Reference documents on the Best Available Techniques (BREF).

The behaviour of mercury throughout a processing plant is quite complex and can vary substantially at the duct level. Monitoring mercury continuously or periodically several times a year typically permits better knowledge of the real performance of a plant – the more data available, the more accurate is the description of the plant’s behaviour in term of mercury emissions. Furthermore, between the two emissions monitoring systems the continuous one currently offers a lower mercury detection limit.

The best available technique associated with the reduction of mercury emission levels to the air from coal and lignite combustion vary in the range of 1-10 μg/m3 depending on the thermal input (lower or higher than 300 MW) and whether it is an existing or new plant. Typically, a new plant of equal size has a lower emission level than an old one.

The European Parliament has also recently issued the Medium Combustion Plants Directive (MCPD) applicable for combustion plants with size between 1 MW and 50 MW, from 2018 for new plants and from 2025 for existing installations.

Although the MCPD doesn’t currently include mercury within the controlled pollutants, measures to control other pollutants such as sulphur dioxide, nitrogen oxides and dust offer some co-benefits in terms of mercury removal.

For example, Electrostatic Precipitators (ESP) and Bag Filters (BF) offer a co-benefit of dust emissions reduction from flue gases generated by combustion plants burning solid or liquid fuels by capture of particle-bound mercury. Both systems are generally applicable to both new and existing plants.

Also the Selective Catalytic Reduction (SCR) process, used to reduce the emissions to air of the nitrogen oxides (NOx), offers the co-benefit of reducing mercury emission thanks to greater oxidation of elemental mercury. A study related to mercury emissions in coal-fired power plants concluded that a conventional SCR system oxidized 13 per cent of the elemental mercury, and that ammonia injection before SCR might have a positive effect on the adsorption of mercury onto particulate matter.

Moreover, SCR systems can be enhanced by the addition of catalysts to specifically convert elemental mercury to oxidized mercury, which can be subsequently captured by a wet Flue Gas Desulphurization (FGD) scrubber. In fact, the FGD scrubber offers the co-benefit of sulphur oxides (SOx) emissions reduction by solubilizing and capturing the oxidized mercury.

However, SCR systems are generally not applicable to new plants of <100 MW or operated less than 500 hours per year. Similarly, FGD scrubbers are not applicable to new plants operated less than 500 hours per year and there may be technical and economical restrictions for applying to new plants of <300 MW. In both cases the technical and economical restrictions become more relevant for retrofitting existing plants.

Bearing in mind the recent ratification of the Minamata Convention (February 2016) and calls from some non-governmental organizations – for example the European Environmental Bureau – to include mercury in the revised National Emissions Ceiling (NEC) directive, the expectation is that mercury may indeed be included in the scope of MCPD in a future update.

It may also result in state-of-the-art abatement techniques becoming mandatory on all existing and new large coal-fired power plants by including the relevant requirements in the sector-specific BREF document under the IED regime.

Advanced systems and methods are now required to measure ever lower concentrations of pollutants as emission limits tighten. Increased measurement accuracy will become paramount as pollutants such as nitrous oxide, methane and possibly mercury are introduced to trading markets in the EU and US.

The change will mean that once a monetary value comes into play, measurement accuracy becomes an economic target as well as an environmental one.

As legislation and action plans grow in number and stringency, the importance of monitoring and quantifying emission pollutants in an accurate and transparent manner are becoming priorities.

NEGOTIATING POTENTIAL ROADBLOCKS

In US, The Linde Group was the first company to offer the market gaseous mercury calibration standards for the monitoring and detection of emissions from power generation plants.

Linde has worked alongside the United Nations Environment Programme for nearly a decade to identify technology to reduce the overall amount of mercury in the atmosphere, resulting in a 1-60 μg/m3 range of globally high precision gaseous mercury standards to calibrate analytical instruments operating at industrial processing plants.

Typical analytical instruments in this application include Atomic Absorption Spectrometers (AAS) and Inductively Coupled Plasma (ICP) mass spectrometers. The AAS is generally used to determine the mercury content of raw natural gas coming into a facility from the oil and gas fields. Ensuring speed of response and a direct online read, the instrument also plays a key role in the process control instrumentation loop in natural gas processing, to ensure that mercury has been successfully removed before the gas enters the compression train.

In developing its gaseous mercury standards, the Linde research programme negotiated several potential roadblocks to success.

These included the different forms of mercury that could be utilized, the ability to insert a known quantity of mercury into a cylinder, ensuring the stability of the calibration gas mixture over an extended period of time and determining the precise concentration of the calibration gas mixture.

The final calibration gas mixture standard attracted a lot of attention because of its ability to deliver accurate, real-time results on-site, compared with conventional sorbent sampling tubes, which take time to render a result.

The gaseous mercury standards are sold in 4.0 m3 aluminium cylinders with a concentration range from 1-60 μg/m3, approximately 700 parts per trillion up to 12-13 parts per billion.

Through the efforts of the Research and Development Programme at Linde, and our propriety cylinder passivation procedures, gaseous mercury calibration standards are supplied with a guaranteed stability period of six months.

Linde also supplies special regulators to accompany these cylinders, which ensure that no amalgam attaches to the regulator. In terms of traceability, the standards have been certified by the National Institute of Standards and Technology (NIST) in the US and these values are used to calibrate Linde’s own instruments and to name the mixtures it produces.

Mike Hayes and Roberto Parola

Mike Hayes is Global Product Manager, Specialty Gases & Specialty Equipment at Linde USA. Roberto Parola is Global Product Manager, Specialty Gases & Specialty Equipment at Linde Germany.