One Size Doesn’t Fit All
When We Energies completed its control technology review, or Best Available Control Technology analysis, for its 50-MW biomass power plant now operating in Rothschild, Wis., it evaluated numerous combustion technologies, ultimately selecting a circulating fluidized bed (CFB). That was for several reasons, not the least of which was low generation of nitrogen oxide (NOx), says Terry Carrol, We Energies asset manager. “It allows a much more controlled temperature profile throughout the combustion zone, so the generation of uncontrolled NOx is lower than many other technologies. It also promotes complete combustion of fuel, has excellent fuel mixing and low residence time—the three keys of combustion: time, temperature and turbulence.”
CFB technology is considered state-of-the-art for biomass combustion because of its ability to accommodate biomass’s heterogeneous nature, as it often varies in moisture and ash content, Carroll adds. “Some technologies don’t lend themselves to that sort of fuel diversity over time.”
Other BACT mechanisms We Energies chose include selective catalytic reduction and a fabric filter baghouse. “The SRCR (selective regenerative catalytic reduction) system is a widely understood and available control for NOx,” Carroll says. An SRCR doesn’t have a catalyst bed, such as a larger installation like a coal power plant does, so ammonia is injected directly into the boiler at just the right temperature—1,400 to 1,500 degrees Fahrenheit—a range in which the ammonia reacts with NOx and dissolves it back into water and nitrogen, reducing emissions in the flue gas stream.
A fabric filter baghouse was chosen due to its extremely high particulate collection efficiency, a decision that We Energies made to ensure it would comply with pending Boiler MACT rules, which were undergoing the final rulemaking process at the time.
While We Energies’ aforementioned control technology decisions may be categorized as devices, contrary to what its title suggests, Best Available Control Technology does not simply refer to equipment selections. Rather, as defined by the U.S. EPA under the Clean Air Act, it is an emission limitation based on the maximum degree of reduction of emitted air pollutants achievable through currently available methods, systems, and techniques while taking economic, energy, environmental and other costs into consideration.
While equipment choices are factors in BACT determinations, other project aspects are also considered. In the case of We Energies, that includes good combustion practices, storage and handling systems, and even paved roads for dust mitigation. “All of our biomass handling systems are in enclosures,” Carroll explains. “Our conveyor galleries are completely enclosed, as is the fuel storage building, and we have a very extensive set of dust collection points. All of the conveyor galleries, fuel receiving hoods, and conveyor transfer points are under negative pressure and pulled into a dust collector—a fabric filter baghouse of its own, a smaller version for which bags collect the cake, they pulse, drop it into a cone and it’s conveyed out.”
Yet another BACT mechanism put into place at We Energies is a lime injection system. “Upstream of the baghouse we inject hydrated lime, which floats around in the exit off-duct and clings to the baghouse, where it is able to capture any sulfur dioxides or chlorides that come in with the biomass,” Carroll says. “That was also put into place in anticipation of IB MACT rules. I’m not sure we’d need it today, but we like having it, it’s inexpensive to operate and it does a good job of allowing us to get an extra dose of air cleanup.
While some biomass power plants may end up utilizing many of the same technology choices We Energies made, state BACT requirements vary, and one particular emission control device is not an across-the-board solution for any specific emission.
“You’ll hear people say, regenerative selective catalytic reduction is BACT for NOx, and that’s not correct at all,” explains Douglas Morrison of Environmental Law Northwest. “BACT for NOx is the emission limit that could be achieved over the life of the unit if RSCR or some other technical and economically feasible control device is installed. It is an emission limitation—not a control—that represents the lowest achievable emissions considering energy, environmental and technological issues, that the unit can meet over its lifetime. That’s a critical point.”
While there is no definite BACT solution for any given pollutant, some states do have presumptive BACT, says Brandon Mogan, project engineer at Geosyntec Consultants. “That’s simply because there’s been enough BACT analyses done for those types of emission sources and pollutants that there is a fairly good understanding of what will be economically feasible. In states that have presumptive BACT, you can say “We’re going to install an SCR” and that’s considered BACT, and you don’t have to go through the whole BACT analysis process. In other states, you can’t do that, you must go through the process.”
So where does the process begin? The very first step is determining pollutants that require BACT, according to both federal and state rules. Federal rules require BACT as part of the PSD New Source Review process. “The Prevention of Significant Deterioration Program applies to new, major stationary sources or major modifications at existing major stationary sources,” Mogan says. “So if I’m building a new plant, I look at all of the emissions generating equipment I’m going to have and calculate my potential to emit. If I have potential to emit over 250 tons per year of any regulated pollutant, then I’m considered a major source under PSD. The only exception for that are greenhouse gases; that’s a different threshold.”
If deemed a major source, a developer must go through the whole the PSD process, including a BACT analysis, for every pollutant that will be emitted in what is noted as “significant quantities.” What constitutes “significant quantities” is defined in the PSD rules, for each regulated pollutant. “Some have different significant thresholds, some are much lower than 250 tons,” Mogan says. “For example, NOx is 40 tons per year.”
States can be stricter, but not less restrictive, than federal requirements. Many states have even established their own BACT requirements, some which require BACT for even minor sources. “In Wyoming, they say that basically any new source will have to apply BACT, but they do use presumptive BACT,” Mogan says. “Other states just adopt the federal rules, so BACT is not required unless you are a major source.”
After emissions subject to BACT are identified, the next step in a BACT analysis is an evaluation of all control devices that could potentially be considered BACT. Once that’s complete, ones that aren’t technically feasible for an application should be eliminated from consideration, Mogan says. “Recently I did a project for a combustion turbine, and some can use steam injection or water injection to control NOx. This particular unit couldn’t be modified because the combustion chamber was too small for water injection, so we eliminated that one; it wasn’t technically feasible.”
Once technically unfeasible options are eliminated, remaining devices should be ranked from highest efficiency to lowest efficiency. In areas of nonattainment, it is required that developers use U.S. EPA Lowest Achievable Emission Rate Standards, the most stringent air pollution standard above BACT. If LAER does not apply, a BACT analyses proceeds with an economic study that examines the annual cost of the selection and considers capital costs, maintenance and operating costs, and annual costs of operating the control device versus the amount of pollution control achieved from the scenario.
“The actual number is dollars per ton of pollutant removed,” Mogan explains. “If that number is too high, you determine that control scenario is economically unfeasible and eliminate it. You may also look at secondary environmental impacts—for example, if you use a thermal oxidizer to control carbon monoxide, you’ll make CO2, which is a secondary impact associated with using the thermal oxidizer. At the end of the day, you’ll have a control scenario that you’ll argue is BACT.”
Most states require developers to follow the top-down procedure, which is intended to derive the most stringent limit possible. “You should work through that process, although legally speaking, it’s not mandatory, just guidance,” Mogan explains. “If whatever process you go through gets you to a BACT emission limit that is defendable and supportable, it doesn’t matter, but if you don’t follow it, there will be a lot of suspicion about how you got your numbers.”
In brief, the EPA defines the top-down process as a method in which all available control technologies are ranked in descending order of effectiveness. The applicant first examines the most stringent, or top alternative, which is established as BACT unless the applicant can demonstrate, and the permitting authority in its informed judgment agrees, that technical considerations, or energy, environmental, or economic impacts justify a conclusion that the most stringent technology is not achievable in that case. If the most stringent technology is eliminated, then the next most stringent alternative is considered.
Technical and Economic Feasibility
In Port Angeles, where Morrison assisted Nippon Paper with its 20-MW biomass boiler project, environmentalists argued the project should have installed RSCR, which requires an external heat source to run the catalyst. “Port Angeles does not have natural gas, so that was a critical engineering, economic and technological issue that weighed against using RSCR for a basis for BACT emission limit,” Morrison says. “They weren’t going to ship in propane or liquid natural gas on a barge and set up additional storages to get a little extra NOx reduction; it just doesn’t make sense.” So while an RSCR might be BACT for another project, that wasn’t the case for Nippon Paper.
Today, the Port Angeles facility is equipped with an electrostatic precipitator (ESP), heat recovery systems, NOx controls and is a low-emitting unit, Morrison says. But during the development and permitting stages, it struggled with its BACT decision, mostly on account of environmental group appeals. “They challenged all of our BACT decisions, air permits and how we calculated emissions, but we won it all.”
Mogan reiterates the significance of factoring in technical feasibility, as one of his clients working to install a 6-MW biomass boiler to supply process heat for the greenhouses at their facility, and was required to perform a BACT Analysis for PM, PM10, NOx, and CO. One of the control devices identified for the control of PM was a baghouse, which are typically installed on larger units at facilities that have full-time boiler staff to monitor potential fire or safety issues. The greenhouse, however, did not have such a staff member. “It was deemed technically unfeasible from that standpoint,” Mogan says. “In our case, the secondary controls scenario, an ESP was very close in terms of control efficiency. “
During the BACT determination process, the U.S. EPA’s BACT Clearinghouse database provides helpful guidance, as it is a publically available collection of BACT and other technology-based decisions.However, problems may arise when permit limits representing a different project’s BACT decision are posted, and a similar project chooses those limits without doing proper due diligence. “A lot of times those plants aren’t even built—largely because they can’t meet those limits,” Morrison says.“[Clearinghouse posts] don’t establish a valid basis for a BACT decision, though that’s the way it’s done in a lot of instances. For example, one might find a 0.01 NOx limit in Vermont, and try to meet that. But taking a closer look, that plant in Vermont never got built, or it just started operating, and has no data to show it can actually comply with that limit. Going to the BACT Clearinghouse and cherry-picking out the lowest limit for each pollutant isn’t a good permitting practice—it will result in plants that don’t get built.”
Furthermore, emissions from one source should not be compared to another source. “An example with biomass is—and this is evident when you look at the Boiler MACT rules—different emission limits are set for different types of boilers,” Morrison says. “A vibrating grate stoker boiler has a very different emissions profile than a fluidized bed boiler, so if your choice is to install a grate-fed stoker boiler, you have to be real careful about using emissions profiles from one to set BACT. If your design is to build a coal-fired power plant, you don’t set BACT on the level of emissions that a natural gas plant is able to meet. Environmental groups will try to do this to drive your emissions limit as low as possible—cherry-pick low emissions profiles from other types of sources and try to get them imposed on your project.”
Mogan emphasizes the importance of site-specific analyses for BACT. “It might turn out that because of the size of your emission source, or an existing source that you’re modifying, it will cost more to knock down trees to make a whole new pad for the control device, thus rendering it uneconomically feasible.”
And vendor quotes generally provide generic numbers, rather than site-specific. “That [vendor quote] should be used as a starting point, and site-specific engineering and land costs should be added in,” Mogan says. “It’s your property—you know where your control device needs to sit, and that will dictate how much duct and piping you need to install. Or maybe you already have a pollution control system for your plant and you need to figure out what type of new equipment you’ll need to integrate new control system. These may be big factors in overall economic feasibility [of BACT solutions].”
Author: Anna Simet
Managing Editor, Biomass Magazine
How a Baghouse Works
Baghouses consist of four basic components: a filter medium (fabric), filter cleaning device, collection hopper and shell. These cylindrical bags hang vertically in fabric filter shells. The number of bags in each shell varies from a few hundred to a few thousand or more. Dirty gas is pushed (positive pressure baghouse) or pulled (negative pressure baghouse) through the fabric filter by a fan. As the gas passes through the filter, dust in the gas stream collects in a dust cake on the inside or outside of the bags. When the bags are cleaned, the collected particles fall into a hopper and are removed. Baghouses come in three main classifications, based on how they are cleaned—pulse jet, mechanical shaker and reverse air.
BACT vs. LAER
“There are several other notable differences between BACT and LAER. BACT is evaluated on a case-by-case basis, where LAER is more uniform for a class or category of source. This case-by-case evaluation of BACT has a large scope of concerns, including energy, environmental and economic impacts. The LAER definition is very rigid and narrower, allowing little argument in the decision other than what is "achieved in practice" and what is the class or category of source. As a result, highly similar sources can have different BACT requirements, but should not, in theory, have different LAER requirements.” –California Air Resources Board