POWER July 2016 - 63

FUELS
Understanding and Mitigating
Metallurgical Effects of Coal
Blending and Switching
Fuel blending and switching has become the norm in response to regulatory
and market forces, but many older boilers were designed for a particular
fuel. Understanding the effects of different fuels on combustion system
metals is crucial for ensuring smooth operations.
Rama S. Koripelli, PhD
I
s there a clear distinction between good
and bad fuel to burn in a particular boiler?
Apparently not! Some fuels may contain
higher heating values, but they may also
possess characteristics that cause emissions
problems, as well as operational and reliability
challenges. Other fuels may be more environmentally
friendly, but may contain higher
moisture content and lower heating values.
Ideally, the " right fuel " is that for which a
boiler is designed or retrofitted to specifically
burn.
Congress extensively amended the Clean
Air Act in 1990. Following those changes, the
Environmental Protection Agency began imposing
more stringent emissions regulations
in the mid-1990s, which led to fuel blending
and switching becoming the norm. Of
course, there are other factors-such as cost,
availability of mines, and meeting full-load
requirements in a dynamic environment-
that have also influenced fuel blending or
switching decisions.
Pros and Cons of PRB Coal
The use of Powder River Basin (PRB) coal,
whether as a blend or a switch, has tremendously
increased in response to economic
and regulatory changes. PRB coal gained
a tremendous amount of interest for its
abundant availability, as well as its lower
sulfur content, which results in lower SOx
emissions.
PRB coal has higher moisture content
than other coals due to its porous structure.
For that reason, PRB coal offers a lower heating
value (around 8,000 Btu/lb) compared to
high-sulfur bituminous coals (about 13,000
Btu/lb). Even so, PRB coal often has economic
advantages over the latter.
There can also be significant fuel flexibility
advantages when blending coal. The
low-sulfur characteristics of PRB coal and its
63
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POWER | July 2016
relatively low cost make it attractive for the
power industry.
Some other major differences between
PRB and bituminous coals are found in concentrations
of ash, calcium oxide, and magnesium
oxide. The concentrations of these
oxides in PRB coal are much higher than in
bituminous coals, which affects the ash melting
temperature and radiant heat absorption
capacity in the furnace area. Reduced radiant
heat absorption capacity in the furnace area
is a product of higher reflectivity or lower
emissivity of the deposit. This reduced heat
transfer capacity in the furnace impairs the
thermal efficiency by raising the furnace exit
gas temperature (FEGT).
Slagging and Fouling Problems
Slagging and fouling may be elevated in
high-temperature superheater or reheater circuits
due to a combination of staged combustion
and higher FEGT (Figure 1). Low-NOx
burners and overfire air can further exacerbate
this situation.
The reducing conditions formed during
staged combustion promote the formation
of hydrogen sulfides and porous metallic
sulfides on the pressure parts of the boiler
or furnace. These sulfide scales are more
porous and less protective than oxides. Reducing
conditions also promote carburization
of T91 (Figure 2) and stainless steel,
resulting in a loss of corrosion and oxidation
resistance.
Although PRB coal contains lower ash
content, it requires higher throughput to
meet full-load conditions. Therefore, erosion
problems may be exacerbated because
1. Bridging the gap. Secondary combustion contributes to high furnace exit gas temperatures,
resulting in slagging and fouling in high-temperature circuits. Courtesy: David N. French
Metallurgists
http://www.powermag.com

POWER July 2016

Table of Contents for the Digital Edition of POWER July 2016

Contents
POWER July 2016 - Cover1
POWER July 2016 - Cover2
POWER July 2016 - Contents
POWER July 2016 - 2
POWER July 2016 - 3
POWER July 2016 - 4
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