POWER March 2013 - 26

FUELS
crease in plant efficiency. This is not the
case with CFB plants, which are less sensitive
to coal HGI.
Where low-grade coals with high sulfur
content (>1% sulfur by weight) are being
used, the auxiliary power requirements related
to sulfur removal in CFB and PC plants
are negatively affected, the consequence being
a reduction in plant efficiency. In CFB
plants, this increase in auxiliary power consumption
is experienced in ash handling due
to the increase in sorbent requirement for
desulfurization and a consequent increase in
bottom ash mass flow. In PC plants, this increase
in auxiliary power consumption is experienced
at the desulfurization equipment.
Desulfurization accounts for 15% and 13%
of the total auxiliary and miscellaneous loads
for the table's specific CFB and PC plants,
respectively.
The far right columns of the table illustrate
similar CFB and PC plants using low-grade
coal but with sulfur content less than 1%, and
show a decrease in sulfur removal-related
auxiliary power requirements to 13% and 6%
for the CFB and PC plant, respectively. This
reduction translates into higher efficiencies
in both plants. Note that the single largest
auxiliary power requirement in all conventional
Rankine cycle steam plants is from the
boiler feedwater pumps. Therefore, although
the auxiliary power requirements for mills/
crushers and sulfur removal are important in
the choice of boilers, they account for a small
percentage of overall plant auxiliary power
requirements and have marginal impact on
overall plant efficiency.
Emissions Control. For power plant
projects requiring World Bank (WB) financing
or financiers and for host countries
requiring adherence to WB standards,
proper emissions control equipment capable
of achieving the prescribed emissions
limits must be installed. PC boilers require
additional equipment in the form of
flue gas desulfurization (FGD) scrubbers
to achieve WB emission limits on SOx
.
In CFB plants, this can be achieved with
in-situ capture by the direct addition of
limestone into the boiler furnace without
the need for additional equipment. Sulfur
content alone does not determine the
grade of the coal, as some higher-grade
coals exhibit higher sulfur contents than
lower-grade coals. High sulfur content is
an indicator of the coal grade and affects
plant performance, as discussed previously.
Regardless of the coal grade, the SOx
capture methods for CFB and PC plants
remain as introduced above.
NOx control in PC plants can be achieved
using selective catalytic or selective noncatalytic
reduction (SCR/SNCR) equipment
and low-NOx
burners. CFB plants inher2.
Showing Improvement. The percentage change in CAPEX and efficiency with calorific
value (CV) for typical CFB and PC plants is illustrated. In general, the screening curves
assume that as the efficiency of the plant increases with increasing CV, the CAPEX decreases
(see Figure 4). For example, for a hypothetical CFB plant, a CV of 14,000 kJ/kg corresponds
to an efficiency of 31.6% and CAPEX of $2,320 for a baseline design. If the quality of the fuel
were improved to 21,210 kJ/kg, then the efficency increases 6%, to 33.5%, and the CAPEX
decreases 15%, to $2,022/kW. Naturally, the baseline design CAPEX depends on many sitespecific
design features as well as the contracting methods and equipment suppliers. Source:
Parsons Brinckerhoff Africa
CFB efficiency
16
14
12
10
8
6
4
2
-2
13,500 14,500 15,500 16,500 17,500 18,500 19,500 20,500 21,500
Calorific value (kJ/kg)
26
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CFB CAPEX
PC efficiency
PC CAPEX
ently operate below temperatures at which
NOx is typically formed (1,500C). The lower
operating temperature of CFB plants is also
ideally suited for the in situ capture of SOx
.
The low operating temperature of CFB boilers
is usually sufficient for non-degraded airshed
situations; however, in degraded airshed
situations, SCR/SNCR may be installed to
achieve the prescribed NOx limits.
Particulate matter (PM) control in both
CFB and PC plants is identical and requires
the use of electrostatic precipitators (ESPs)
or baghouse fabric filters. Some low-grade
coals exhibit high silica and alumina content
in their ash, which increases ash resistivity,
thus reducing the PM collection efficiency
of ESPs. Low-sulfur coals also exhibit high
ash resistivity and may necessitate the use of
baghouse fabric filters. For CFB plants with
in-situ SOx
removal, the use of ESPs is not
recommended. Baghouse fabric filters are
therefore the preferred PM control technology
as they are unaffected by ash resistivity.
Generally, if a coal mine can ensure a coal
specification within a suitable range for a
PC plant over the life of the plant (approximately
30 years), PC technology can be used.
Achieving this range will normally require
more downstream preparation (beneficiation)
of the coal feedstock, especially if the coal
supply consists of discards from coal mining.
Where securing this range cannot be ensured,
and where the range of coal feedstock is
likely to be inconsistent and varied, or if the
coal mine is unwilling to invest in beneficiation,
then a CFB plant is the better choice.
Most often, where discard coal is the source
of feedstock, CFB plants are preferred.
Project Economics
Because low-grade coals have low energy
content, larger quantities will be required
to achieve a certain power plant output than
would be required using higher-grade coal
with higher energy content. The farther away
the power plant is from the mine(s), the
greater the fuel OPEX, especially where lowgrade
coals are used. Up to a 40% increase
in boiler coal consumption can be required
by decreasing the utilized coal's CV from 20
MJ/kg to 14 MJ/kg. This could translate into
significant coal transportation costs and, consequently,
a higher OPEX.
Where a power plant's owners intend to
source from a single mine, that plant can
be designed for the specific low-grade coal
being supplied. When a power plant is not
at a mine mouth, it may be worthwhile to
improve the coal quality and thereby lower
the cost of transportation to minimize coal
OPEX. For a fixed power plant capacity
and distance from a coal mine, a lower
grade of coal at a lower price will incur
POWER | March 2013
Percentage change

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POWER March 2013

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