POWER March 2013 - 25

FUELS
to power plant economic viability. Power
plant OPEX constitutes variable and fixed
operation and maintenance costs (such as
coal, limestone, fuel oil, water, and labor).
The OPEX discussion is limited to the cost
of coal, including transportation, as a major
contributor to power plant operating costs.
Power plant and coal mine relationships and
their interactions with CAPEX and OPEX
depend on the specific circumstances of a
project, which can be numerous. As a result,
the described relationships and interactions
have been limited and simplified somewhat.
Plant Technology
Subcritical power plants are known to achieve
efficiencies between 30% and 36% on average,
while supercritical and ultrasupercritical
power plants can achieve efficiencies up to
40% and 45%, respectively. The CV of the
coal generally affects the plants' ability to
achieve these efficiencies in that the lower the
coal CV, the less likely a plant will operate in
the higher regions of its efficiency class. This
is especially true when a plant has not been
designed to burn a specific grade of coal. To
maximize a power plant's potential to utilize
low-grade coal, plant technology should be
selected that is least likely to be affected by
coal quality.
Most of the technologies used in thermal
coal power generation are independent of
the coal being used. The main technologies
of concern are boilers, their associated fuelhandling
and -processing equipment, and
their emissions control technology.
Boiler Technology. Generally, a conventional
pulverized coal (PC) boiler will
function properly, provided there is minimal
deviation from the range of coal specification
for which it was designed. Significant variations
in coal characteristics in PC boilers such
as VM content are dangerous and could lead
to tube rupture and boiler explosions. Archfired
PC boilers by Foster Wheeler have been
developed to address low-volatile coals but
not a wider range of VM coal feed for a specific
design. Figure 1 depicts the relationship
between PC and circulating fluidized bed
(CFB) boilers and coal grade.
Varying and inconsistent coal types can
be processed into an acceptable fuel by
several available coal beneficiation processes,
in which case a PC boiler can be
used, even for low-grade coals. However,
where minimal downstream coal preparation
is available, the CFB boiler is generally
more capable of handling coal quality
inconsistencies. Although CFB boilers are
capable of handling a wider range of fuels
by virtue of their fuel firing/coal combustion
system, they are limited by their thermal
design to a specific range of fuel. For
March 2013 | POWER
this reason, multifuel combustion or cofiring
with dual fuels such as biomass and
coal in CFB boilers is not always achievable.
An
important aspect of boiler choice is the
auxiliary power consumption associated with
fuel processing and emissions control. In PC
boilers that use mills to grind coal, this is
important from a fuel processing viewpoint.
Generally, higher plant auxiliary loads result
in lower plant efficiency. The finer pulverized
coal required for a PC boiler results in
higher auxiliary load requirements and lower
plant efficiency. Therefore, for a specific
plant design, a lower HGI will result in higher
auxiliary power to achieve the required
coal fineness and, consequently, a lower
overall plant efficiency. CFB boilers, on the
other hand, do not require fine coal, and so
a PC plant's grinding mills are replaced by
crushers in CFB plants, which reduce coal
processing auxiliary power consumption requirements
(see table).
Should the coal HGI decrease for the PC
plant, there will be a consequent increase
in auxiliary power requirements and a de1.
Combustion characteristics. A conventional PC boiler will function properly, provided
there is minimal deviation from the range of coal specification for which it was designed.
Source: Adapted from S.J. Goidich, " Supercritical Boiler Options to Match Fuel Combustion
Characteristics, " Foster Wheeler North America, 2007
40
35
30
25
20
15
10
5
PC arch-fired
Anthracite,
petroleum coke
PC wall-fired
Bituminous, subbituminous, lignites
CFB
High-ash waste coals
10
20
30
Volatile content (% dry ash free)
Power distribution summary. The first two data columns illustrate typical auxiliary and
miscellaneous loads as a percentage of total auxiliary load required by a PC and CFB plant of
the same capacity burning a low-grade coal with an HGI of 50. The last two columns illustrate a
decrease in sulfur removal-related auxiliary power requirements to 13% and 6%, respectively,
for the CFB and the PC plant. Source: Chudi Egbuna
Plant system
Boiler PA & SA fans (%)
Boiler ID fan (%)
ESP (%)
Cooling tower fans (%)
BFP and booster pumps (%)
Other pumps (%)
Miscellaneous and other auxiliaries (%)
Transformer losses (%)
Ash handling (%)
Boiler fuel delivery (%)
Sulfur scrubbing equipment (%)
Total (%)
Low-grade coal (HGI = 50)
CFB plant
PC plant
17
9
2
3
32
6
12
1
15
3
100
6
10
2
2
27
8
9
1
9
13
13
100
Low-grade coal (S <1%)
CFB plant
PC plant
16
9
3
3
33
7
12
1
13
3
100
6
10
3
2
30
8
10
1
10
14
6
100
Notes: BFP = boiler feed pump, ESP = electrostatic precipitator, HGI = Hargrove Grindability Index, ID = induced draft,
PA = primary air, S = sulfur, SA = secondary air.
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25
40
50
Higher heating value (MJ/kg)

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POWER March 2013

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