Rules regulating emissions of mercury from power plants have been finalized in the USA. No mercury control technologies have yet reached commercialization, but utilities such as Southern Company are playing a key role in helping the industry find a solution.

Siân Green

In June 2005, the Electric Power Research Institute (EPRI) of the USA awarded a Technology Transfer Achievement Award to Southern Company. The award recognises the contribution that the company is making to help the electric power industry develop and evaluate technologies for controlling mercury emissions from coal fired power plants.

In making the award, EPRI noted that in collaboration with the US Department of Energy (DOE), EPRI and other power utilities, Southern is conducting full-scale tests on four different mercury control technologies at the Yates power plant, operated by Southern’s Georgia Power subsidiary. “The project team and employees of Plant Yates are to be commended for offering their plant, their time and their skills to evaluate and improve not just one mercury control technology, but four,” said Hank Courtright, vice president of generation at EPRI.

The timing of Southern Company’s efforts is no coincidence: in March 2005 the US Environmental Protection Agency (EPA) issued final rules for regulating mercury emissions from coal fired power plants [see sidebar]. The new regulations require a 70 per cent reduction in mercury emissions, phased in over the next 12 years. The rules presume the commercial availability of mercury controls in the near future, and the pressure is on the industry to develop the technologies that will help it achieve compliance.

Figure 1. The pilot scrubber unit at Plant Yates. It is a 1 MW JBR (Jet Bubbling Reactor), as compared to the 100 MW full-size JBR, which can be seen behind the pilot.
Click here to enlarge image

Southern Company has therefore decided to take a leadership role in the development and demonstration of mercury control technologies. “Southern wants to be proactive in developing new technologies,” says Mark Berry, principal research engineer at Southern. “The [power] industry is very conservative and any new technology must be proven to be robust and reliable before widespread use. Vendors therefore need to prove the reliability of new technology and they cannot do this alone, they need partners. Although there are risks, we see benefits to Southern as well as to the industry as a whole.”

Full scale tests

With legislation pushing the industry hard, Southern’s work has helped to make real in-roads into the commercialization of mercury control technologies, and initially focussed on testing one of the most promising mercury control technologies: activated carbon injection (ACI).

In 2000, Southern’s Gaston plant – operated by its Alabama Power subsidiary – was chosen by the DOE to carry out the country’s first full-scale mercury control testing. The utility deployed and tested an EPRI-patented ACI technology known as Toxecon, which works by injecting activated carbon sorbents into the flue gas upstream of the baghouse system designed to collect fly ash. The sorbents adsorb mercury and are then collected in the baghouse.

The ACI technology tested at the Gaston plant was developed by ADA-Environmental Solutions and won an R&D 100 Award in 2003. Initial tests carried out in 2001 revealed an average mercury reduction of 80 per cent, and in 2002 the DOE selected the Gaston plant to carry out further testing. The $2.1 million project finished in 2004.

Testing at Yates

As work at the Gaston plant finished in mid-2004, so testing of ACI technology began at Georgia Power’s Yates plant. The objective of this project was to further advance research on a different plant configuration and coal type. An important part of this project was investigating the effectiveness of ACI with small ESPs, says Berry.

“The majority of ACI testing has been carried out on plants with large ESPs but many plants in the US have relatively small ESPs,” notes Berry. Tests at Yates have revealed that 70 per cent removal of mercury is possible, but that ACI may not be as effective on plants with small ESPs as it is on plants with large ESPs.

Southern has tested a number of activated carbon sorbents at Yates. It has found that the carbon can cause erratic behaviour in the ESP, a factor that could give rise to maintenance issues. Testing of the technology is on-going.

Figure 2. The storage silo at Plant Gaston where the powdered activated carbon is stored. Underneath this storage bin is a feeder which measures out the correct amount of activated carbon.
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Yates is also carrying out pilot-scale mercury control tests using another EPRI patented technology known as Mercury Capture by Adsorption Process (MerCAP). This technology involves the installation of gold plates at the outlet of the flue gas desulphurization (FGD) unit. This is a novel technology in the early stages of development, but offers the promise of lower-cost mercury removal.

MerCAP tests will be carried out on a slipstream FGD at Yates in early 2006, according to Berry. In the course of the tests, Southern will also examine the regenerative capacity of the gold plates by removing them from the plant, removing the mercury and reinstalling them for further tests.

Other technologies to be tested at Yates include low temperature oxidation catalysts, and the use of additives. Low temperature oxidation catalysts would fit in the back end of an ESP to oxidize a high percentage of the mercury in the flue gas for capture by a downstream SO2 control device (e.g., FGD system). This technology is intended for use at plants that burn coals producing large proportions of elemental mercury and that want to maximize the multi-pollutant capture capabilities of their SO2 control. Additives could be used to sequester the mercury captured by the SO2 scrubber so it does not attach to the gypsum. Gypsum, a byproduct of the scrubber, is used for wallboard and to improve the growth of peanuts and other agricultural products.

Research centre

But Southern’s contribution to mercury control research does not end at the Yates project. The company announced in January 2005 that it is to launch the USA’s first mercury research centre. The purpose of the centre – located at the Christ plant, which is owned and operated by Southern’s Gulf Power subsidiary – is to study different methods of reducing mercury emissions from power plants.

The research centre will consist of a 5 MW slipstream facility at the Christ plant, near Pensacola, FL. It will install and test various state-of-the-art technologies to assess long-term performance and reliability in reducing mercury emissions. It will also be open to other companies or researchers suggesting or conducting new or different treatment technologies.

The first phase of testing at the centre will evaluate five different advanced control technologies using a portion of the plant’s emissions for the research. Technologies scheduled for initial testing are estimated to cost $5 million and will include a selective catalytic reduction unit, a rotary air pre-heater, a baghouse, an ESP, and a wet limestone scrubber. As the research continues other methods may be discovered and added to further research.

Mercury falling: EPA rules put industry under pressure

Mercury was recognised as a toxin in the 1960s, but emissions to the air were the last to be regulated in the USA. The country’s 600 or so large coal fired power plants, which account for around 50 tonnes of mercury air pollution per year, are now the focus of regulation.

The US Environmental Protection Agency (EPA) formally issued its Clean Air Mercury Rule (CAMR) in March 2005. The two-phase rule creates performance standards and established permanent, declining caps on mercury emissions for coal fired power plants. This makes the USA the first country in the world to regulate these emissions.

The CAMR will work in tandem with EPA’s new Clean Air Interstate Rule (CAIR), which tightens existing regulations on SO2, NOx and particulates. By 2018, CAIR and CAMR will reduce mercury emissions from coal fired power plants by 70 per cent. Reductions will be achieved through a market-based cap and trade system.

Under the cap and trade system, the EPA will assign individual states an emissions budget for mercury. During the initial phase of the scheme, plants that meet or exceed emissions limits ahead of time can bank credits for use later on or can sell them to plants that will not be able to meet the limits.

Reductions in mercury emissions can be achieved by modifying existing SO2 or NOx control devices, but new approaches will be required to meet the final 2018 emission limits. Given the scale and importance of the electric utility industry, it is essential that any control technology be thoroughly tested before being commercially deployed. Long-term testing on different plants burning different coals is also essential.