POWER March 2014 - 24

Practical Considerations
for Converting Industrial
Coal Boilers to Natural
Gas
Increasing Environmental Protection
Agency (EPA) restrictions pertaining to
emissions from coal-fired power plants,
the increasing cost of coal operations, and
the decreasing cost of natural gas provide
strong arguments for converting coal-fired
boilers to natural gas-firing ones. Coalfired
boilers have emissions that are potentially
noncompliant with the Maximum
Achievable Control Technologies (MACT)
rule and Mercury and Air Toxics Standards
(MATS), which will require modifications
to most coal-firing systems.
Where should coal boiler owners start
when evaluating the various conversion
options for their facilities? A comprehensive
understanding of various design
alternatives and their implications is crucial
to optimizing the initial capital cost,
operating cost, safety, and reliability of
the facility.
Conversion Benefits
The primary benefits of converting a coal
boiler to fire natural gas are a more costeffective,
cleaner, more efficient, and reliable
source of steam. As compared with
installing a new boiler, converting an
operational coal-fired unit to natural gas
typically requires a lower capital investment,
enables the most condensed schedule,
and makes use of the existing asset.
Although the solution is not new (see
" Natural Gas Conversions of Existing
Coal-Fired Boilers " in the August 2011 issue
of POWER, online at powermag.com)
and has been covered recently in two
Special Report articles (see " Practical
Considerations for Converting Boilers to
Burn Gas " and " Utility Options for Leveraging
Natural Gas " in the October 2013
issue of POWER), the topic continues to
be relevant due to evolving EPA regulations
and public perceptions of climate
change initiatives.
For most industrial boiler owners, reutilization
of the existing asset is the
most desirable course of action. Converting
an existing boiler is typically 15% to
30% of the cost of installing a new natural
gas boiler. Additionally, converting coal
boilers to firing natural gas provides the
following major benefits.
Cleaner Operation. Natural gas burns
cleaner than coal, because it does not contain
significant amounts of sulfur, atomic
nitrogen, particulate, or ash. Additionally,
natural gas allows for more precombustion
24
controllability for lower emissions, such as
NOX
and CO, without the need for extensive
flue gas treatment equipment.
Operating Cost Savings. For several
years the cost of natural gas has been decreasing
while the cost of coal has been
increasing. By using only natural gas instead
of coal, facilities can eliminate operations,
maintenance, and environmental
costs associated with coal/ash storage
and handling. With no ash carryover, natural
gas avoids ash buildup, which reduces
heat transfer, meaning the boiler can
maintain its efficiency.
Increased Boiler Flexibility. Converting
to natural gas improves boiler flexibility
and turndown capability. Coal-fired
boilers do not react to sudden load swings
as effectively as a converted natural gas
boiler. Further, coal-fired boilers have a
limited turndown capability, thus limiting
their effectiveness during low steam
demand. Bringing a coal-fired boiler back
online after a shutdown requires much
more time than is required by a natural
gas boiler.
Determining Emissions Limitations
Is the Key First Step
So you have a coal boiler that appears
to be a perfect candidate for conversion.
Where do you start? CO and NOX
emissions
limitations are of primary importance
when converting boilers to natural gasfiring.
These emission limitations dictate
the requirements of the burner design,
air/fuel mixing technology, and resulting
flame temperature. Emission limitations
affect combustion airflow requirements,
refractory requirements, amount of flue
gas recirculation, duct sizing, and damper
control, all of which have an effect on the
forced draft (FD) fan and induced draft
(ID) fan requirements.
Getting too far along in the design
of the project and then finding that the
emissions limitations have changed could
mean starting from scratch with a new
design. Working with a qualified environmental
consulting firm is a great way
to make sure that the project is getting
started on the right track.
Vertical or Horizontal Firing?
There are two typical arrangements for
mounting the most important part of
the conversion-the burners: horizontal,
wall-mounted burner design and vertical,
upward-mounted burner design. For both
arrangements, the following considerations
are important.
Heat Input Required. How many Btu
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of natural gas must be burned to achieve
the operational steam output at the required
pressure and temperature?
Boiler Furnace Geometry. Furnace
dimensions significantly affect the burner
design and burner placement. If the boiler
has superheater tubes in the radiant area,
the distance from the grate to the lowest
part of the superheater tubes is also critical
to the design.
Backend Modifications Needed.
Does a baghouse or other flue gas equipment
need to be bypassed or removed?
Does a cyclone separator need to be
" gutted " to reduce the flue gas pressure
drop, or is the ID fan so oversized that
additional flow restrictions are needed to
utilize the fan? A cost-benefit analysis
of replacing the ID fan versus installing
flow restrictions may be advised. Installing
a variable frequency drive (VFD) for
the ID fan may also make sense.
Impact of the Conversion on the
Boiler. Depending on burner placement,
a conversion from coal to gas changes
the energy release slightly. Natural gas
flames produce lower radiational energy,
thus radiant heat transfer in the lower
furnace is less, while the convective heat
transfer is increased through the backpass/economizer.
On
the steam side, the changes in combustion
temperature, quantity, and composition
affect the velocities and heat
absorption within the furnace, economizer,
and superheater sections and thus has
an effect on steam flow and temperature.
A boiler impact study may be needed to
evaluate the gas and steam processes
that may be affected by the conversion.
This analysis can provide an impact evaluation
of boiler emissions, heat transfer,
boiler efficiency, steam production, and
steam temperature.
Natural Gas Flow and Pressure
Availability. A new main gas line or a
pressure regulation station with a new
tap into an existing main gas line may
be needed.
Electrical Distribution System Capacity.
Does the current system have the necessary
capacity available for the project?
Existing Boiler Condition. Is the boiler
clean and in good repair? Is retubing or
similar work needed?
Code Requirements. Boiler code requirements
differ in some areas for natural
gas versus coal-fired boilers. This may
require boiler safety relief valve (SRV)
replacement or recertification, boiler feedwater
delivery pressure and flow capacity
changes in relation to boiler maximum
POWER | March 2014
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POWER March 2014

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