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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.

BACT Basics

“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

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

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.



“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


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