FGC Theory
Flue Gas Conditioning | Sulfur Trioxide | Ammonia
Theory of SO3 & NH3 Flue Gas Conditioning
The concept of artificially modifying flyash resistivity is not new. For almost eighty years it has been recognized that by varying the quantity of SO3 in the flue gas, the performance of an electrostatic precipitator can, in many instances be improved.
Precipitator performance depends upon the physical and chemical properties of the flue gas and particulate treated. In a power plant, the type of coal burned, the furnace design, and the overall operation of the boiler govern these properties. The composition, temperature and pressure of the gas govern the basic particle charging capability of the precipitator while particle size, particle concentration, and electrical resistivity of the ash affect both the charging and collecting capability of the precipitator.
The chemical composition of the flyash varies widely. Major constituents of most flyashes are silica, alumina and iron oxides, and to a lesser extent, sodium and calcium. Silica and alumina are present in the ash primarily in the form of silicates, which contribute to the typical glassy appearance of the particles. The specific quantities of these constituents are also major contributors to flyash resistivity.
Flyash resistivity depends upon a number of factors, including not only the chemical composition, but the flue gas temperature, the moisture content, and the SO3 content in the flue gas. At typical air heater gas outlet temperatures, (250° - 350°F), surface conduction over the flyash particles predominates and is heavily dependent on the moisture and SO3 levels. At higher temperatures, volume conduction through the particles predominates.
Sulfur occurs in coal as organic and inorganic compounds. When coal is burned, more than 95% of the sulfur becomes SO2. A small fraction is converted to gaseous SO3. When the flue gas temperature drops below approximately 600°F, SO3 begins to react with water vapor to produce sulfuric acid vapor. The reaction is essentially complete when the temperature drops to about 300° - 350°F, where precipitators normally operate. Thus, in a strict sense conditioning results from sulfuric acid vapor, rather than SO3, being absorbed onto the surface of the flyash particles.
Some flyashes do not readily absorb the sulfuric acid vapor, generated naturally from sulfur in coal or from SO3 Flue Gas Conditioning, which would be expected to be of sufficient quantity for flyash resistivity modification. The primary reason for this occurrence is the silica, alumina and iron. When the sum of these three constituents is high, the surface characteristics of the ash become more glass-like and less absorbent. This is analogous to trying to moisten glass or Teflon - it does not occur to an appreciable extent. In these instances, the addition of ammonia (NH3) has proven to be beneficial.
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Sulfur Trioxide as a Conditioning Agent
When coals with high sulfur contents are burned, there is generally enough SO3 formed to bring the flyash resistivity into a range which results in good precipitator operation. However, when switching to a coal with low sulfur content an insufficient amount of naturally occurring SO3 is present for resistivity modification, and precipitator performance deteriorates. Thus, the purpose of the SO3 injection is to simply supplement the SO3 which is formed naturally to modify the resistivity to that which produces optimum precipitator performance.
Over the years, many SO3 containing chemicals and processes-including sulfuric acid, oleum, liquid SO3 and catalytic conversion from SO2 have been tried. However, the sulfur-based, catalytic conversion process, due to safety, simplicity and cost considerations, is the predominant system in use today.
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Ammonia as a Conditioning Agent
Ammonia conditioning in a cold side precipitator involves interaction with sulfuric acid to form a fine fume of ammonium sulfate and ammonium bisulfate. These particles alter the electrical characteristics of the flue gas between the discharge and collecting electrodes producing a space charge enhancement of the electric field. This effect is well documented and arises when fine fume is charged in the precipitator and the electric field is thereby increased, with an accompanying increase in the charge level of the flyash particles and the collecting field near the plates.
Ammonia is used as a coagulating agent to create larger flyash particles and provide a flyash that is receptive to the available SO3. When the flue gas temperature is above about 300°F, ammonium bisulfate melts and become a semi liquid and acts as a "glue" when mixed with flyash. This produces highly cohesive, relatively large particles, resulting in high efficiency due to reduced rapping losses and re-entrainment. Opacity readings are greatly affected by fine flyash particles, creating larger particles results in less fines and thus less fines are expelled through the stack. This physical effect is also called agglomeration.
A secondary use for ammonia injection is to combine with excess sulfur trioxide in the gas stream, thus raising the acid dew point. This has the effect of making control of the system less critical of the system with respect to avoidance of possible corrosive conditions when the flue gas temperature is reduced downstream of the precipitator. Over conditioning of flue gas with SO3 is more forgiving as NH3 may neutralize excess SO3.
Currently, many coal fired utility boilers employ simultaneous injection of SO3 and NH3 on a continuous basis. In some cases only NH3 injection is required due to the fact that the naturally occurring SO3 content in the flue gas is sufficient for resistivity modification.
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