By David Wagman, Managing Editor
Environmental rules are frequently seen as all mandate with little or no upside for a power generator. Rules are written and power generators are given a deadline after which they face fines and other penalties should they end up in non-compliance.
A couple of recent items serve as reminders that the rules are not monolithic and that power generators may even have options when it comes to compliance strategies.
First, Robynn Andracsek, senior environmental engineer with Burns and McDonnell, writes in our new “Clearing the Air” column (starting on page 8) that the Clean Air Act and its amendments can be so convoluted that if a particular section of the Act doesn’t make sense then you are probably reading it correctly. She explains how the Clean Air Act (along with its various interpretations by regulators and the courts) actually may encourage power plants to pollute, especially in advance of planned major capital projects.
Second, Mike Wood, Business Manager of SOLVAir Products for Solvay Chemicals, recently described to me how he is working with power generators to try to uncover benefits that may be achieved through a proactive approach to environmental compliance. Such an approach means looking not just at a single emission issue, but at a range of pollutants. By taking a more comprehensive approach, the operator may be able to negotiate with state environmental regulators; for example, offering to overcomply in some areas while continuing to work to achieve compliance in others. The idea is to reach an agreement that moves toward reducing a broad range of emissions while acknowledging the sometimes high cost of environmental compliance.
As one example, Wood says that a 150 MW power plant in the Northeast could remove 50 to 60 percent of its SO2, some of its mercury and 10 to 20 percent of its NOX. The SO2 credits could be sold (for around $400 to $500 a ton in today’s market, according to Wood). NOX credits similarly could be sold for $1,000 to $2,000 a ton. The effect would be to build goodwill with environmental regulators and partially offset the treatment cost through sale of the credits.
“If you spend $500 a ton to treat the emissions, this strategy makes good business and good environmental sense,” Wood says. “In other words you lay the table.”
He points to the approach some operators take to reducing NOX using selective catalytic reduction systems. The focus tends to be on controlling NOX by itself and failing to control other pollutants such as SO2, SO3 and mercury. SCR systems can cause SO3 to form because SO2 reacts on the same catalyst being used to control NOX.
“You get a blue plume, acid mist or SO3,” Wood says. “It coalesces and travels for miles.” Outcry over the blue plume trailing from smokestacks leads to further regulation and control equipment. An approach that considers the cause, effect and treatment of a number of pollutants may make more sense and address issues before they can become a problem.
For example, mercury abatement becomes easier to accomplish when SO3 is removed from the flue gas. That’s because SO3 is thought to interfere with mercury’s adsorption onto carbon. Tests show in many cases that once the SO3 is removed mercury removal goes “way up,” Wood says. “You get both acid gas and heavy metal control.”
Wood says that a $3 million to $5 million capital investment may be required to install a dry sorbent injection (DSI) system to make a 150 MW power plant capable of achieving 40 to 60 percent SO2 control. New construction would include a silo and feeder package to store the dry powder and deliver it to a mill. The dry sorbent powder is blown directly into the duct and is collected in an electrostatic precipitator unit or baghouse.
In turn, the power plant would be able to control virtually all SO3, achieve a roughly 50 percent SO2 compliance and possibly record a 10 to 20 percent reduction in NOX while achieving mercury removal benefits.
As a rule of thumb, Wood says that DSI systems injecting Trona at a rate of 1 to 2 NSR (normalized stochiometric ratio) can achieve 40 to 60 percent SO2 control. An injection of 3 to 4 NSR can achieve 80 to 90 percent control. One power plant in the mid-Atlantic states has shown an 80 percent SO2 control with a 4 NSR.
To be sure, Mike Wood is an advocate for dry sorbent injection. But even then he recognizes that Trona probably does not fit every situation. If a power plant has an SO2 compliance threshold over 90 percent, then dry injection probably is not the appropriate solution, he says. A more economical option for the operator might be to pursue dry injection to control SO3 and mercury and use a “standard scrubber” for SO2.
“Some plants see dry injection as an interim measure,” Wood says, which may allow them to delay some capital expenditures.
So despite appearances, environmental issues are not always cut-and-dried nor monolithic and unyielding. New columns like “Clearing the Air” are one way we aim to offer greater insights into environmental compliance. And watch in September for an exclusive series of online articles addressing environmental topics. Power Engineering is publishing this series in our bi-weekly enewsletter. As always, I look forward to hearing what you, our readers, have to say.