Fly ash is usually collected by electrostatic
precipitators or fabric filters, which operate at efficiencies of
greater than 99%. In electrostatic precipitators the electrical
resistivity of the fly-ash is an important factor affecting the
performance of the precipitator. Resistivities greater than
1012W.cm are considered undesirable, since high
electrical resistance can lead to back ionisation, which reduces
collection efficiency. The collection efficiency of high
resistivity fly ashes may be improved by dosing the flue gas with
additives such asSO3 or
NH3,or by using
intermittent or pulse-energising techniques to control the voltage
in the precipitator.
The precipitator performance of blends generally
is near to or slightly better than the component coal with the best
precipitator performance.
Fabric filters are made of a woven, knitted or
felted textile in the shape of cylindrical bags through which the
flue gas is passed. The flyash is collected on the bag and then
removed by shaking, reversed gas flow, sonic horns or air pulses.
The properties of the flyash that affect the performance of fabric
filters relate to how the flyash compacts on the bag. This
influences the pressure drop through the bag and the flyash
adhesive strength, which influences the force required to remove
the dust layer.
ACARP has published a summary of on the
precipitator performance of Australian bituminous
coals. Hall [1] reviewed the effects of process
conditions on electrostatic precipitator (ESP) performances, in
this review Hall listed the most important factors affecting
precipitator performance as:
- Ash
Resistivity - as controlled by ash chemistry, gas temperature
and density, gas composition - especially H20,
S03/H2S04 or the presence of other
specific conditioning agents. Resistivity basically controls the
allowable ESP operating current densities, hence operating
voltages, useful power input and size of ESP equipment
required.
-
Inlet Particle Size Distribution and Concentration - as
influenced by boiler type, fuel properties and process factors.
Senior and others (1993) showed that the particle size distribution
greatly influences precipitator performance.
-
Electrical Energization - the heart of the electrostatic
precipitation process comprising the electric field strengths for
particle charging and collection by means of electric forces
applied directly to the particles, per se. Said electric fields and
copious supply of negative gas ions for particle charging are
provided by a high voltage corona discharge maintained between
suitable electrodes, e.g. parallel plate ducts with coplanar
discharge electrodes centred therein. Ion mobilities,
voltage/current characteristics and electric fields are influenced
by gas and ash properties. The electrical energization system
and conditions most strongly influence achievable effective
particle migration velocities.
-
Specific Collecting Area (SCA) - ESP total collecting area
per unit gas flow rate. Important influences on SCA include
efficiency requirements, electrode geometry, ESP duct width and
electric energy density.
- Dust
Loss Factors - as influenced by particle adhesive and cohesive
properties, gas distribution quality, gas sneakage, local air
inleakage, gas turbulence conditions, gas velocity erosion and rap
reentrainment, excessive ESP sparking, saltation, and long term
reliability factors.
Burnout is also important as the presence of
unburnt carbon can alter the electrical properties of the fly ash,
and adversely affect dust emission levels.
Blending can have a large influence on ESP
performance as demonstrated during trial burns at a Queensland
power station using blends of Meandu coal with Jeebropilly and
Wilkie Creek coals [2]. Meandu coal, the coal normally fired, has
approximately 28% ash, which is comprised of 71% quartz. This makes
the ash quite difficult to precipitate efficiently. In these trials
it was found that the particulate emissions of the blends decreased
to less than a quarter of the usual emissions. The main reason for
this decrease was attributed to a decrease in ash resistivity due
to greater amounts of magnesium and sodium ions in the ash of the
blended coals. Spero [3] also noted those from the Walloon Coal
Measures had superior ESP performance.
ESP efficiency can be calculated using the
modified Deutsch equation which is given below. The effective
migration velocity (ωk) in this equation is the
parameter that is normally related to the ash properties of a coal.
The Specific Collection Area (SCA) is the size of the
precipitator.
The general application of correlations for the
prediction of effective migration velocity, based on ash analysis
alone, needs to be treated with the same degree of caution as the
wide application of predictive
slagging
indices. This is because it is the surface
composition of the fly ash that is important not the bulk
composition. The figure below shows the comparison between actual
pilot scale performance and that predicted by a correlation base of
ash analysis (Bennett, 1996). The superior ESP performances of the
coal from the Walloon Coal Measures (Coal 72) and blends using this
coal are shown in this figure.[1] Hall, H.J., 1990. “The effects of
fireside process conditions on electrostatic precipitator
performance in the electric utility industry”, Proceedings:
Eight Particulate Control Symposium, San Diego, California, Nov.,
1990.
[2] Whelband, B., 1999, “Influence of coal
blending on electrostatic precipitator performance at Tarong Power
Station”, Department of Chemical Engineering, The University
of Queensland, Thesis & Seminars, 1999.
[3] Spero,. C., 1997, “Walloon coals: their
properties and power station performance”, Report to:
Queensland Department of Mines and Energy QTHERM Program, Austa
Energy Report RTM 97/003. October 1997.
