POWER January 2013 - 14

Safety a Main Theme at Asian Coal
Users' Meeting
Power plant operators, managers, and other professionals from
across Southeast Asia met in Hong Kong in early November for
the second annual Asian Sub-Bituminous Coal Users' Group meeting,
created to share information and best practices related to
safety, handling, combustion, characteristics, and risk management
of the fuel. This year's co-hosts were CLP Power and HK
Electric. The event was organized by the Powder River Basin Coal
Users' Group and TradeFair Group, which publishes POWER.
Presentations during the two-and-a-half-day event focused on
boiler management and coal-handling best practices.
Danny S. Lau, engineer I (materials handling), with Hong Kong
Electric Co., said a number of benefits come with the use of low-
calorific value (LCV) coal, which includes subbituminous varieties.
He struck a conference theme by saying subbituminous coal
also presents a number of problems to users such as increased fire
risk, coal spillage and fugitive dust, and generation unit derates.
" These problems must be mitigated to prevent any catastrophic
failure of coal-handling and combustion equipment, " he said. At
the Lamma Power Station near Hong Kong, where he works, coal
yard operations were reengineered and coal-burning equipment
was modified to accommodate increased use of LCV coal.
Lau said the high moisture content of LCV coal adversely affects
both pulverizer performance and the combustion process. As a result,
mill inlet temperatures at the power plant were restricted to below
200C to minimize the risk of a mill fire when handling LCV coals. The
mill outlet temperature also had to be lowered from 75C to around
60C to 65C. At the Lamma station, in-mill drying is the accepted
method of preparing coal for pulverized fuel burning, and Lau reiterated
industry standards of achieving a proper dryness in the coal by
manipulating primary airflow and temperature.
These standards were achieved at the Lamma plant in part
by modifying the mills. These modifications included installing
a dynamic classifier, which helped improve the fineness of the
pulverized fuel; installing a dynamic vane wheel to improve mill
airflow; changing the separators to deflectors to minimize the
accumulation of residual coal; and installing a mill inerting system
to admit steam into the system in case of a fire.
1. Asian coal users' confab. Delegates to the second annual
Asian Sub-Bituminous Coal Users' Group meeting in Hong Kong mingle
prior to the start of a conference session. The meeting drew power
generators from across Asia and North America to discuss the safe,
efficient, and economic use of sub-bituminous coal by generating companies.
Source: POWER, David Wagman
In addition, the mill and the boiler were retuned to handle
LCV coals in an effort to obtain optimal operation and system
performance. The tuning involved adjusting the mill outlet temperature
in accordance with a coal fuel ranking system. Under the
system, bituminous coal with a calorific value between 7,800 and
6,380 kcal/kg was classified as " A, " high-caloric value subbituminous
coal of between 6,380 and 5,800 kcal/kg was classified
as " B, " low-caloric value subbituminous coal of between 5,800
and 4,600 kcal/kg was classified as " C, " and lower-caloric value
subbituminous coal of less than 4,600 kcal/kg was classified as
" D. " (Note that 1 kcal = 3.97 Btu and 1 kg = 2.2 pounds.)
Boiler control parameters were adjusted, depending on the
classification of the coal being burned. For example, existing
boiler control function curves had been set for high-caloric value
coal (6,300 kcal/kg), but that practice resulted in an oversupply
of combustion air when LCV coal was used. This had the dual effects
of reducing boiler efficiency and increasing the coal flow.
Lau said that Lamma station operators learned that, based on the
tuning results, excess O2
could be trimmed 1% at full load and
0.5% at half load. This adjustment enhanced boiler efficiency
and alleviated unit derating when LCV coal was burned.
Lau reported several improvements to plant operations as a result
of the modifications. First, plant output increased when two
types of LCV coal labeled " A " and " B " were burned. Following new
settings that placed the mill outlet temperature at 70C, excess O2
at 3.2%, and the induced draft fan blade opening at 83%/77%,
electrical output using coal A rose some 28.3 MW from a base of
322.4 MW to 350.7 MW. Auxiliary power consumption dropped by
0.31%, and boiler efficiency rose 0.79%. New settings applied to
coal B combustion resulted in an increase of 17.3 MW from a base
of 348.6 MW, to 365.9 MW. Auxiliary power consumption dropped
by 0.74% and boiler efficiency rose 0.53%.
" The improvement in boiler efficiency was attributed to reduc2.
Korean coal connection. Sung-Won Ha (right), senior manager
with Korea South-East Power Co., answered questions following
his presentation at the second annual Asian Sub-Bituminous Coal Users'
Group conference, which was held in Hong Kong in early November.
Source: POWER, David Wagman
14
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POWER January 2013

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