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 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.
[4] Bennett P. Conroy A., 1996, “Coal Quality Impact Model”, ACARP Final Report C3091, October 1996.