Chemical Engineering January 2018 - 50

FIGURE 2. Thermal oxidizers
are differentiated from
conventional combustors
because they use excess
oxygen to complete oxidation
reactions at a suitably
high temperature
exchanger to pre-heat the process
gas. This type of system is limited
by the combustible concentration
of the process gas and is limited to
components that will not poison the
catalyst. For the correct applications,
a catalytic thermal oxidizer can offer
high destruction efficiency and low
NOx and CO emissions.
Ultra-low-emissions models
A new class of advanced combustor
called the certified ultra-low emissions
burner can achieve high destruction
efficiencies and extremely
low NOx and CO emissions. This
type of combustor has been used
in environmentally sensitive areas
to achieve best-available control
technology (BACT) emissions levels.
These devices use surface pre-mix
combustion to create short-lived,
low-temperature flames that are extremely
efficient. This reduces flame
temperature, resulting in low NOx,
but also can provide destruction efficiency
of up to 99.99%.
Selection guidance
When selecting a combustion system,
emissions and destruction efficiency
have become the primary
criteria. Across the industry, there
is pressure, and sometimes economic
incentives as well, to reduce
NOx and CO emissions, along with
emissions of volatile organic compounds
(VOCs). With this in mind,
the first step should be to determine
what local emissions guidelines
apply to the specific device, and
48
to evaluate the benefits in reducing
emissions. There are several cutoff
points to be aware of. A simple enclosed
combustor will achieve about
98% destruction efficiency. This can
be achieved by almost any diffuse
flame burner without any additional
effort. Up to 99.5% destruction efficiency
can be achieved with a
temperature-controlled combustor,
a regenerative thermal oxidizer or
a catalytic thermal oxidizer. Above
that, a direct-fired thermal oxidizer
or an ultra-low-emissions combustor
is required. Although the difference
between 99.5% and 99.9% destruction
efficiency may seem
small, half a percent improvement
on destruction
efficiency represents five
times greater emissions
for the former compared
to the latter.
NOx emissions. NOx
emissions have traditionally
been especially hard
to control. NOx is formed
through various mechanisms
during combustion,
which, if not addressed,
can create large amounts
of this pollutant. A diffuse
flame burner will generate
the largest amount of NOx,
so typically any enclosed
combustor, even temperature-controlled
models,
will
produce relatively large
amounts of NOx. This includes
direct-fired thermal
oxidizers, which, even
FIGURE 3. Regenerative thermal oxidizers capture the heat from
oxidation, which is retained by a ceramic media and is then used
to heat and oxidize the process gas
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though they offer great destruction
efficiency, do not greatly improve over
simpler combustors in NOx generation.
There are several low-NOx burners
and designs that can improve NOx
emissions for combustors and directfired
thermal oxidizers. Other solutions
are also available, such as ammonia
injection, but that can prove to
be quite expensive. Regenerative and
catalytic thermal oxidizers can offer
low-NOx emissions.
Fuel efficiency. Fuel efficiency is another
important, yet frequently overlooked,
consideration for selecting
an enclosed combustion system.
Where a fuel source, such as natural
gas, is inexpensive, it is not always
feasible to invest in a higher-cost
system to increase fuel efficiency,
but many products are still worth
considering. Regenerative thermal
oxidizers offer the greatest fuel efficiencies,
recovering up to 98%
of thermal energy. These systems
are ideal for low-concentration and
high-flowrate applications. Catalytic
thermal oxidizers can also operate
at high thermal efficiencies by
incorporating heat exchangers to
pre-heat the process gas before it
passes through the catalyst. Finally,
a recuperative thermal oxidizer can
be used to pre-heat process gas
to increase fuel efficiency, or it can
be used to recover heat to use in
another process in the plant. This
can put the thermal energy gener
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Chemical Engineering January 2018

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