Chemical Engineering January 2018 - 48

Environmental Manager
Enclosed Combustion Equipment
and Technology
A thorough understanding of enclosed combustion technologies will help engineers
to select the proper equipment to balance emissions-control performance with
efficiency, costs and complexity
Anu D. Vij
Ship & Shore Environmental
S
election of an enclosed
combustion solution is a
complex process that involves
various environmental
and operational requirements
that are unique for each application.
There is great benefit in selecting
the correct equipment, since it
can translate into decreased capital
and operational costs by preventing
over-designing or under-designing
the equipment. Different technologies
offer tradeoffs between initial
cost, operating expense and complexity,
emissions, fuel efficiency
and destruction efficiency. This article
describes the main categories
of enclosed combustion devices
and presents a few of the advantages
and tradeoffs to aid in selecting
the proper equipment for a
particular case.
Enclosed flares
The simplest enclosed combustion
device is an enclosed flare. An enclosed
flare is simply meant to hide
the flame and does not make a particular
effort to increase combustion
efficiency or reduce emissions.
Enclosed flares provide cooling and
combustion air through natural draft.
The enclosed flare burner is simple
and can be an anti-flashback type, a
high-pressure type or a forced-draft
type. A forced-draft type of device
is used when the process gas has a
tendency to produce smoke. It utilizes
a blower to provide 20-40% of
the stoichiometric air to the fuel gas
near the burner tip. Enclosed flares
typically operate at around 98%
destruction efficiency.
Destruction efficiency is defined
as
46
the
difference between
the
FIGURE 1. These steam-generating thermal oxidizers with waste-heat boilers represent an example of an
enclosed combustion technology being used for emissions control in a degassing system that ultimately
generates inert gases
amount of pollutants entering the
system and the amount of pollutants
exiting the system divided by
the mass of pollutants entering the
system, expressed as a percentage.
Depending on the pollutants to be
destroyed, a minimum destruction
efficiency is needed to meet regulations.
Thus, the selection of the
combustion equipment depends on
the destruction efficiency needed.
In order to reach higher levels of
destruction efficiency, we start off
with an enclosed flare design and
add temperature control and assist
gas. This can be called a vapor combustor,
or in some cases, a thermal
oxidizer. The vapor combustor can
maintain higher temperatures in the
chamber, which allows it to maintain
a destruction efficiency of up to
99.9%. Residence time is typically
around 0.7 seconds for these types
of combustors.
A thermocouple is used to monitor
system temperature and control
the opening of the louvers or
the flow of assist gas to maintain
a desired chamber temperature of
1,400 to 1,800°F. Different burners
can be used, including forced-draft
burners for smokeless combustion
of heavy hydrocarbons, antiflashback
burners and low-NOx
(oxides of nitrogen) burners.
Thermal oxidizers
To reach higher levels of destruction
efficiency and lower emissions, we
can turn to thermal oxidizers (Figure
1). Thermal oxidizers employ a
wide variety of designs, but can be
generally split into three main categories:
direct-fired thermal oxidizers;
regenerative thermal oxidizers;
and catalytic thermal oxidizers. The
main difference between a thermal
oxidizer system and a combustor is
that a thermal oxidizer does not rely
on directly igniting the process gas
for oxidation. A thermal oxidizer sustains
the proper conditions for oxidization
of the combustible materials
present in the process gas by maintaining
an operating temperature
sufficiently above the auto-ignition
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