POWER October 2013 - 44

COAL-TO-GAS CONVERSIONS
Practical Considerations for
Converting Boilers to Burn Gas
The enormous reserves of natural gas at comparatively low prices and increasing
regulatory pressure to reduce emissions from coal-fired generation
combine to make natural gas today's fuel of choice. What happens to existing
coal-fired plants? The preferred solution is to " refuel " with natural gas.
Nathan Schindler and Lawrence Berry
M
any utilities have taken advantage
of relatively low-cost natural gas
to address environmental pressures
and upgrade their power generation portfolio
in one of three distinct ways-replace,
repower, or refuel.
A common approach has been to shutter
an existing coal-fired plant and " replace " it
with a new high-efficiency combined cycle
plant. Another approach is to " repower " an
existing coal-fired plant. A typical repowering
project retires the coal-fired steam generator
but retains the steam turbine portion
of the plant. Combustion turbines and heat
recovery steam generators provide the replacement
steam (see " Repowering South
Mississippi Electric Power Association's J.T.
Dudley, Sr. Generation Complex " in the August
2013 issue). Repowering should cost less
than building a new combined cycle plant but
the thermal efficiency will also be less. The
third option is to " refuel " the existing plant
with natural gas. The advantages of this approach
are very low capital cost compared to
the other two options, minimal impact on the
existing firing system, fast implementation,
and the fuel flexibility offered because the
coal-firing option can often be retained.
Refueling a boiler from burning coal or
oil to natural gas impacts the entire boiler
gas path, from the forced draft (FD) and induced
draft (ID) fans to combustion stoichiometry
and combustion air requirements. On
the steam side, the changes in combustion
product temperature, quantity, and composition
affect the velocities and heat absorption
within the furnace, superheater, and
economizer sections and thus steam flow and
temperature. If a boiler fuel switch is in your
future, the first step must be a comprehensive
feasibility study that evaluates gas- and
steam-side boiler modifications and the impact
of those changes on boiler emissions,
heat transfer, boiler efficiency, steam production
and superheat temperature.
Modern natural gas burners are capable of
44
burning natural gas cleanly and completely.
Due to its absence of sulfur, atomic nitrogen
and ash, natural gas combustion produces significantly
fewer emissions of NOx
compounds, and particulates than coal or oil.
Power Boiler Modifications
Power boilers, typically from 10 MW to 800
MW in capacity, usually have multiple burners
arranged one of two ways: wall-fired or
tangent-fired (also called corner-fired). The
wall-fired approach features multiple burners
on several levels, usually on the front and
back boiler walls. The tangent-fired arrangement
has burners located in each of the four
corners of the boiler, often in multiple levels.
The four burners on each level are designed
to fire in unison, creating a swirling combustion
zone (commonly referred to as the " fireball " )
in the center of the furnace. In order to
control superheat temperatures, the tangentfired
configuration can tilt burners up and
down in unison, thus moving the flame up or
down within the furnace.
Wall-fired burners are typically round.
They can be fired from a single wall or can
be placed on opposite walls and can be arranged
on a single level or multiple levels.
Wall-fired burners have integrated registers
or dampers to balance air to each burner and
to isolate burners that are not firing. These
mechanisms help ensure that each burner has
the design fuel-to-air ratio under a range of
operating conditions. Existing burner assemblies
can often be modified to add natural gas
without necessitating a full burner replacement,
thus allowing co-firing capability with
the original fuel (coal and/or oil).
In many cases existing burners can be
modified to accept a low-NOx
natural gas
burner manifold. This approach permits reuse
of existing burner dampers and actuators,
windbox dimensions, mounting plates, and
throat configuration, which significantly reduces
installation cost and time. Even when
the register must be replaced, front wall preswww.powermag.com
POWER
| October 2013
sure parts are usually not required to convert
a coal or oil burner to natural gas (Figure 1).
Proper air distribution and flow to and around
, CO, sulfur
the burner entry is also a key factor in any burner
system capable of achieving its expected performance.
In a typical fuel conversion project
in order to ensure the airflow to each burner is
optimized, an airflow model of the combustion
air system is usually performed.
Low-NOx
natural gas burners rely on fuel
and air staging to reduce peak flame temperature.
Increasing pressure drop through the
burner can reduce burner flame length. The
FD fans on most coal boilers have sufficient
capacity for a natural gas burner, even with
a greater pressure loss, although the fan capacity
must be evaluated during the feasibility
investigation.
Low-NOx
natural gas burners offer significantly
higher turndown than coal or oil burners.
Natural gas burners are capable of 10:1 turndown,
while oil burners are limited to approximately
6:1 and coal burners are limited to 3:1.
This adds to the boiler's firing flexibility and
consumes less fuel in standby modes compared
1. Switch hitter. Low-NOx gas burner
retrofits capable of burning coal are available if
the ability to co-fire coal is to be retained. The
burner shown in the photo is capable of burning
gas or pulverized coal. Typical natural gas
pressures at the burner are 12 to 15 psig. Courtesy:
Combustion Components Associates
http://www.powermag.com
POWER October 2013

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