US coal fired stations are the source of one per cent of all global mercury emissions. A new rule should reduce US emissions by three quarters.

Janet Wood, Acting Editor, PEi

On 15 March, the US Environmental Protection Agency (EPA) issued the Clean Air Mercury Rule, regulating mercury emissions from coal fired power plants. The Rule makes the US the first country in the world to regulate mercury emissions from coal fired plants, but it is likely to face legal challenges from environmental groups because the regulation is based on a ‘cap and trade’ regime.

Opponents argue that the new rule is too closely tied to the Clean Air Interstate Rule (CAIR) and that the cap and trade approach will mean that in some states, emissions could increase. But the EPA said its modelling showed that the Rule would “significantly reduce the majority of the coal fired power plant mercury emissions that deposit in the USA“ and the biggest reductions would be in areas where mercury deposition is currently the highest. It said that it makes sense to address mercury, SO2 and NOx emissions simultaneously and that CAMR together with CAIR, would reduce utility emissions of mercury by 70 per cent, from 48 tons a year to 15 tons.

The Clean Air Mercury Rule establishes standards of performance limiting mercury emissions from new and existing coal fired power plants and creates a market-based cap-and-trade programme that will reduce nationwide utility emissions of mercury in two phases. The first phase caps emissions at 38 tons from 2010, down from the current level of 48 tons, and emissions will be reduced by taking advantage of “co-benefit” reductions – that is, mercury reductions achieved by reducing sulphur dioxide (SO2) and nitrogen oxides (NOx) emissions under CAIR. In the second phase, due in 2018, coal fired power plants will be subject to a new cap, which will reduce emissions to 15 tons upon full implementation. New coal stations (with construction starting on or after 30 January 2004) will have to meet new source performance standards and fit within the caps.

Figure 1: The US contribution to global mercury emissions
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The new cap and trade system will be based on EPA’s Acid Rain Programme, which EPA says reduced SO2 emissions faster and at far lower costs than anticipated. EPA has assigned each state and two tribes that have coal fired power stations an emissions “budget” for mercury (see Table). Each must submit a State Plan revision detailing how it will meet its mercury budget. The rule includes a model cap and trade programme and states may either include the rule in state regulations, or they may adopt regulations that mirror the necessary components of the model trading rule. The states are not obliged to join the trading scheme but the Agency believes most will do so. State implementation plans will be required some time within the next two years and operators believe mercury monitors will be required by January 2008.

The state and tribal emission budgets are permanent, regardless of growth in the electric sector. The mandatory emissions caps will decline over time.

As well as being used for SO2 reductions in the US, the cap and trade approach has found favour elsewhere to achieve swift reductions in pollutants: the EU’s Emissions Trading Scheme for CO2 reductions, now entering operation, for example. EPA now says the flexibility of allowance trading creates financial incentives for coal fired power plants to look for new and low-cost ways to reduce emissions and improve the effectiveness of pollution control equipment. This marks a change from the maximum achievable control technology (MACT) approach favoured by many lobby groups and initially explored by EPA – it was reported that in a presentation to an industry trade group in 2001, it was said that MACT could reduce mercury emissions by 90 per cent within four years. Industry groups, who have been saying that the technologies were too expensive and not commercially viable, appear to have won the battle over using MACT or trades.

Using the cap and trade approach meant EPA had to revise its December 2000 finding that it is “appropriate and necessary” to regulate mercury as a hazardous air pollutant. That finding would have required the agency to order the industry to use the MACT approach and force individual plants to cut emissions.

In its final ruling, EPA said that both systems would establish emissions limits. However, under the trading system reductions are capped permanently and nationwide emissions can only go down. In comparison, EPA said that in the MACT approach, which sets standards based on technology performance, each plant must meet a specific emissions limit. But the benefits are not always permanent because the MACT limit is set as an emission rate per billion Btu of coal burnt. With shifts in coal use and with economic growth, more coal burning would mean the amount of mercury emitted would start to rise again.

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Cap and trade also allows unused allowances to be ‘banked’ for future use and EPA said this was an incentive for technology innovation. Southern Company said it had argued strongly in favour of the cap and trade approach. Southern’s Larry Monroe argued that the industry “needs very robust equipment so it is very risk-averse. If we have a fixed MACT approach it is unlikely that the industry would take chances on new equipment. But with cap and trade you get rewarded for taking risk because you may have more allowances to sell, and if the risk fails you can buy allowances. So it is a technology-enabling regulatory approach.”

The Rule is likely to undergo a legal challenge from environmental groups, who argue that the cap and trade approach is unlawful and that the limits are not strict enough.

Making the reduction

Whether or not any challenge to the rule is accepted, the industry is clear that there is still much work to be done to achieve the proposed reductions. “There is no mercury control technology that exists today that can achieve the reduction levels finalized in the Clean Air Mercury rule, let alone the 90 per cent reductions advocated by some activists,” Scott Segal of the Electric Reliability Coordinating Council told National Public Radio. However, research is under way.

Larry Monroe said he could list “a dozen or maybe two dozen” approaches to mercury reduction, and he highlighted three that were being developed. One uses activated charcoal injected into the flue gas to adsorb the mercury, which is then removed via baghouses or an electrostatic precipitator. The Electric Power Research Institute (EPRI) has enhanced this process by adding a chemical injector to increase the mercury absorption. Thirdly a ‘blank page’ approach was looking at rethinking the design of new coal plants with integrated emissions control systems, such as the Powerspan eco process, which uses a plasma process and an ammonia scrubber to remove SO2, NOx and mercury.

Test programme

One test programme was conducted at Sunflower Electric’s 360 MW Holcomb station near Garden City, Kansas, as part of an $8.8 million contract with the US Department of Energy’s National Energy Technology Laboratory. EPRI chose Holcomb because it has emissions control equipment typical of many power plants being planned that will burn PRB coal. It is equipped with a dry scrubber and a high efficiency fabric filter.

Three options were tested:

  • Injection of a new chemically enhanced sorbent. Mercury removal averaged slightly over 90 per cent at a relatively low injection rate, comoparted to near-zero with standard air pollution controls
  • Blending a modest amount of a western bituminous coal with the primary PRB coal. Mercury emissions decreased by 80 per cent without sorbent.
  • Combining activated carbon injection and a proprietary additive. This resulted in much higher mercury removal rates than when activated carbon was used alone.

EPRI is also partnering with a second utility, Entergy, and ADA-ES to test EPRI’s Toxecon IITM at Entergy’s Independence Steam Electric Station (ISES), part of a $5.5 million cooperative agreement awarded by the DOE.

One utility, Southern Company’s Gulf Power, has announced it will launch its own Mercury Research Centre to study different methods of reducing mercury emissions. “Currently, there are no stand-alone commercially available technologies to control mercury emissions,” said Charles Goodman, senior vice president of research and environmental policy. “By assessing various methods, we hope to gain a better understanding of today’s best available control technologies in playing a role in the reduction of mercury emissions.”

Figure 2: A mercury testing programme was conducted at Sunflower Electric Holcomb station, Kansas
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The research centre will install and test various technologies to assess long-term performance and reliability, and the facility will is also open to other companies or researchers suggesting or conducting new or different treatment technologies at the centre.

Five years and counting

For the US industry, the mercury rulemaking has come at an uncertain time, when it is already gearing up for huge capital investment to reduce SOx and NOx levels to meet the CAIR regulations. Southern Company’s Larry Monroe pointed out that “Capital equipment for CAIR is expected to cost the industry $50-60 billion” in the form of scrubbers and various other emissions reduction technologies. What the industry does not yet know is exactly how much mercury reduction it will get as a side effect of the CAIR investment.

With SOx and NOx the US industry can build on overseas experience for practical technology, costing and operational experience. That history does not yet exist for mercury. “We have fairly imperfect models and predictions,” says Monroe. “We will have some mercury control,” from these measures, but he says the chemistry is complex and not understood well enough to allow operators to predict benefits – and certainly not enough to jeopardise SOx and NOx reductions. For example, Monroe said bench-scale tests suggested that operating SCR at around 50ºF (25ºC) lower temperatures may improve mercury capture without altering the SOx capture, and although NOx capture would be lower that could be balanced by adding more catalyst. But that needed lots of thought, experience and bench scale testing, he said.

In practice, Monroe said he and most operators would install to meet SOx and NOx requirements and then optimise for maximum mercury gain. At this stage, with the effect of SAIR still unclear, the industry is just relieved that phase 2 of the CAMR will not apply until after SAIR is implemented. “The SOx and NOx reductions are such an overwhelming task that at the moment the industry doesn’t have time, energy or money” for mercury, said Monroe.