Combustion Process Improvements for Power Plants

The primary contributors to combustion process conditions and their effects include:

Coal

Flue gas flow rate
Flue gas moisture content
Fly ash resistivity
Fly ash inlet loading
Fly ash particle size

Coal Mills

Fly ash particle size
Unburned carbon (LOI)

Furnace

Base load/swing load operation
Flue gas flow rate

Burners

Flue gas temperature
Fly ash resistivity
Unburned carbon (LOI)

Air Pre-Heaters

Rotation
Gas flow pattern
Gas temperature pattern
SO₃distribution pattern

Coal

Bituminous coals from Eastern mines, sub-bituminous and lignite coals from Western mines, and lignites from Texas mines are substantially different from each other in the combustion process. Coal blending is now used for operational and financial benefits. This results in a wide range of boiler and precipitator operating conditions.

Precipitating fly ash from difficult coals can be improved with conditioning systems. However, the furnace and its associated equipment can still cause problems in the precipitator, particularly coal mills, burners, and air pre-heaters.

Coal Mills

The setting of the coal mills and classifiers defines the coal particle size which in turn impacts the fly ash particle size. Larger coal particles are more difficult to combust, but larger fly ash particles are easier to collect in the precipitator.

Furnace

Base-load operation of the boiler is usually better for precipitator operation than swing-load operation due to more stable operating conditions. Boiler operation at low loads may be as problematic for the precipitator as operating the boiler at its maximum load level, due to fallout of fly ash in the ductwork, low gas temperatures, and deterioration of the quality of the gas velocity distribution.

If low load operation cannot be avoided, the installation of additional gas flow control devices in the inlet and outlet of the precipitator may prove beneficial.

Coal Burner

The operation of coal burners, together with the setting of the coal mills and their classifiers, affects the percentage of unburned carbon (LOI or UBC) in the fly ash. The use of Lo-NOx burners increases this percentage, and causes re-entrainment and increased sparking in the precipitator. Further, the UBC tends to absorb SO₃, which in turn increases the fly ash resistivity. Over-fire air optimization or coal-reburn systems may reduce UBC in the fly ash.

Air Pre-heater

Regenerative air pre-heaters cause temperature and SO₃ stratification in the downstream gas flow. This problem is more severe in closely coupled systems, where the precipitator is located close to the air pre-heater. Depending upon site-specific conditions, flow mixing devices may be installed in the ductwork to the precipitator, or flue gas conditioning systems may be used to equalize the gas flow characteristics.

Fly Ash & Flue Gas Conditioning

Flue gas and fly ash characteristics at the inlet define precipitator operation. The combination of flue gas analysis, flue gas temperature and fly ash chemistry provides the base for fly ash resistivity. Typically, fly ash resistivity involves both surface and volume resistivity. As gas temperature increases, surface conductivity decreases and volume resistivity increases.

In lower gas temperature ranges, surface conductivity predominates. The current passing through the precipitated fly ash layer is conducted in a film of weak sulfuric acid on the surface of the particles. Formation of the acid film (from SO₃ and H₂O) is influenced by the surface chemistry of the fly ash particles.

In higher gas temperature ranges, volume conductivity predominates. Current conduction through the bodies (volume) of the precipitated fly ash particles is governed by the total chemistry of the particles.

Fly ash resistivity can be modified (generally with the intent to reduce it) by injecting one or more of the following upstream of the precipitator:

Sulfur trioxide (SO₃)
Ammonia (NH₃)
Water

Sulfur Trioxide & Ammonia Conditioning Systems

In most cases, a sulfur trioxide conditioning system is sufficient to reduce fly ash resistivity to an acceptable level. The source of sulfur trioxide can be liquid sulfur dioxide, molten elemental sulfur, or granulated sulfur. It is also possible to convert native flue gas SO₂ to SO₃.

In some instances, ammonia alone has been proven a suitable conditioning agent. It forms an ammonia-based particulate to increase the space charge. The source of ammonia may be liquid anhydrous or aqueous ammonia, or solid urea.

Finally, sulfur trioxide and ammonia may be used in combination. This solution has been successful because it can lower fly ash resistivity and also form ammonia bisulfate. The latter increases the adhesion of particles, and thus reduces re-entrainment losses.

Water Injection

The injection of water upstream of the precipitator lowers the gas temperature and adds moisture to the flue gas. Both are beneficial in cold-side precipitator applications. However, care must be taken that all of the water is evaporated and that the walls in the ductwork or gas distribution devices do not get wet.