POWER October 2013 - 50

COAL-TO-GAS CONVERSIONS
ate their gas igniters or warm-up guns when recovering
from coal-related equipment derating.
This typically requires only a modest capital
project to increase the size and duty cycle capability
of the unit. If new waterwall penetrations
are required, the capital cost and complexity of
the project can increase significantly. Often, gas
igniters and warm-up guns can be upsized inplace
to yield as much as 20% to 25% total heat
input to the steam generator.
The greatest benefit of employing natural
gas cofiring with coal is increasing the fuel
flexibility of the unit. There are many ways
in which increased fuel flexibility can benefit
a coal-fired unit, such as these:
■ Lower-cost coals can be purchased with
less concern for lost capacity. For example,
a unit that operates most efficiently
using only coals with moisture content
of 27% or less (a typical Southern Powder
River Basin coal) can likely push this
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specification to include the lower-cost,
higher-moisture coals from the Northern
Powder River Basin (PRB).
■ More aggressive spot market purchasing
can be employed. A " finicky " unit that suffers
from a variety of exclusively coal-related
derates can sometimes find great values
on the used equipment market, knowing
that if the coal-related equipment isn't up
to the task, natural gas can supplement the
heat input and maintain the desired load.
■ Power stations that are unsure about the
quality of their coal stockpiles or that suffer
from poor or erratic blending practices
can put those fears to rest. If for some
reason too much poorer-quality coal is
stocked into the bunkers or silos, then
operators can modulate natural gas use to
make up the difference.
■ Unexpected outages of parallel equipment
(such as coal mills) can be hedged by having
natural gas available to make up lost load.
■ Coal inventory levels can be reduced. In
the event that significant unexpected coal
supply restrictions occur, natural gas use
can be maximized to reduce the coal pile
draw-down. Conversely, should natural
gas supply restrictions occur unexpectedly,
coal use could be increased to maintain
the unit generation plan.
■ Some emissions control retrofits may be
reduced in scope, delayed, or avoided, depending
upon the coal quality, level of gas
cofiring that is intended, and the plant's
regulatory environment.
Provided that the gas pipeline is of sufficient
size, few fuel-handling system modifications
are required for natural gas cofiring. Using
recent Black & Veatch studies as a guideline,
capital costs for implementing natural gas cofiring
(outside of new pipeline costs) can range
from $10,000 to $100,000 per MW.
Coal to Natural Gas Conversion. There
For more information, contact NAES at 425.961.4700
or email sales@ naes.com
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POWER | October 2013
are advantages to converting an existing coalfired
power station to one fueled entirely by
natural gas, although it is a higher-risk option
than operating as a dual-fuel unit. Achieving
the unit's maximum continuous rating (MCR)
when burning coal or natural gas typically
requires significant physical modifications
to the boiler. If the goal is to decommission
the unit for coal use frequently, the existing
burner penetrations can be used for the installation
of 100% capacity natural gas burners.
Heat transfer surface area modifications
are typically required and can be completed
during the same outage when the burners are
replaced. (See " Natural Gas Conversions of
Existing Coal-Fired Boilers " in the August
2011 issue at powermag.com.) Using recent
Black & Veatch feasibility studies as a
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POWER October 2013

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