Gas separation: processes – Electric or electrostatic field – With addition of solid – gas – or vapor
Reexamination Certificate
2002-09-16
2004-09-28
Chiesa, Richard L. (Department: 1724)
Gas separation: processes
Electric or electrostatic field
With addition of solid, gas, or vapor
C095S065000, C095S071000, C095S216000, C095S230000, C096S052000, C096S074000, C096S356000, C096S360000, C423S395000, C423S397000
Reexamination Certificate
active
06797035
ABSTRACT:
FIELD OF THE INVENTION
The present invention is a method and apparatus for removing undesired particles, such as fly ash, from gas streams. More particularly, the present invention embodies an improved approach for removing such undesired particles by selectively introducing oxidants into the gas stream.
BACKGROUND OF THE INVENTION
Environmental standards for particulate emissions from coal-fired electrical power plants, petroleum refineries, chemical plants, pulp and paper plants, cement plants, and other particulate-emitting facilities are becoming increasingly more demanding. For example, air quality standards in the United States now require power plants to remove more than 99% of the particulates produced by coal combustion before the flue gas can be discharged into the atmosphere. The term “particulate” within the meaning of these restrictions generally refers to fly ash and other fine particles found in flue gas streams and can include a host of hazardous substances such as those listed in 40 CFR §302.4 (e.g., arsenic, ammonia, ammonium sulfite, heavy metals and the like. As environmental standards tighten, there is a corresponding need for a more efficient means of particulate removal.
An electrostatic precipitator is a commonly used device for removing electrically particulates from the gas streams produced by plants and refineries. In a typical electrostatic precipitator, undesired particle-laden gases pass negatively charged corona electrodes which impart a negative charge to the particulates. The charged particulates then migrate towards and collect on positively charged collection plates and are intermittently removed by various techniques, including sonic horn blasts or rapping of the collection plates. Electrostatic precipitators may employ a common stage or separate stages for both the charging and collection of particulates.
In utility applications, there are two types of electrostatic precipitators. Cold-side electrostatic precipitators are located on the downstream side of the air preheater or heat exchanger (which transfers heat from the flue gas to the air to be fed into the furnace) and therefore operate at relatively low temperatures (i.e., temperatures of less than about 200° C.). Hot-side electrostatic precipitators are located on the upstream side of the air preheater and therefore operate at relatively high temperatures (i.e., at least about 250° C.).
Many hot-side electrostatic precipitators suffer from problems related to the resistivity of collected particulates. Such problems can cause a deterioration of the particulate collection efficiency of the electrostatic precipitator (and higher particulate emissions) and excessive ESP power consumption. These problems can be caused by sodium depletion of collected particulates on the collection plates. It has long been known that ionic charge transfer through the collected particulate layer at high temperatures can be degraded as available charge carriers, namely sodium cations, are depleted from the collected particulate layer. This phenomenon is commonly referred as sodium depletion. The problems can also be caused by the inherently high resistivity of particulates and/or resistivity problems during low load or at colder temperatures.
Additives, such as sulfur trioxide, ammonia, and various surface conditioning additives (such as sulfuric acid) that are effective under cold-side conditions are generally ineffective under hot-side conditions because of different charge conduction mechanisms. Referring to
FIG. 1
, under cold-side conditions (which exist at operating temperatures less than the critical temperature) surface conduction of charge is believed to be the predominant charge conduction mechanism while under hot-side conditions (which exist typically at operating temperatures more than the critical temperature) volume conduction of charge through the particulates is believed to be the predominant charge conduction mechanism. As used herein, the “critical temperature” is the temperature corresponding to the highest attainable resistivity of a particulate (which is commonly located at the top of a bell-shaped curve as shown in FIG.
1
). As can be seen from
FIG. 1
, particulate electrical resistivity varies with temperature over as much as two orders of magnitude at normal process temperatures for hot-side ESPs.
For the reasons set forth above, an effective flue gas conditioning treatment under hot-side conditions therefore should prevent or substantially delay long-term ion (e.g., sodium) depletion and moderate resistivity for highly variable particulate compositions and process temperatures.
One conditioning method for controlling particulate resistivity that has had some success under hot-side conditions has been bulk addition of sodium into the coal feed to the boiler. Typically, from about 0.5 to about 4% by weight sodium (relative to the weight of the ash in the coal) is added to the coal feed as a sodium sulfate or soda ash. The sodium is co-fired with the coal in the boiler resulting in the sodium being incorporated into the particulates as sodium oxides.
The bulk addition of sodium to the coal feed can, however, cause problems. For example, bulk sodium addition can cause boiler slagging and boiler and economizer fouling due to the high sodium content of the particulates (substantially negating any gains by reduced ESP cleaning). Bulk sodium addition can lead to the consumption of excessive amounts of alkali material (and a commensurate increase in operating costs) and to higher gas temperatures downstream of the boiler (that can lead to duct and electrostatic precipitator structural problems). Bulk sodium addition may not effectively control sodium depletion, because the added sodium charge carriers are contained as sodium oxides in the bulk particles. Therefore, a thin layer of collected particulates next to the plate that is never rapped clear (or removed from the plate) will still be subject to sodium depletion and higher electrical resistivity as the sodium ions in the collected thin layer migrate to the plate. Once sodium depletion is established in the collected thin particulate layer, bulk sodium addition becomes less and less effective over time. Compared to the absence of bulk sodium addition, it is common that the prolonged bulk addition of sodium may delay or extend time between ESP cleaning by only a few months. Bulk sodium addition often cannot be performed on an intermittent or as-needed basis and thereby fails to provide control over short-term particulate resistivity. The sodium content of the coal supply is a major contributor to the electrical resistivity of the resulting fly ash. The sodium content can range from less than 0.5% to more than 2% depending on the coal supply. Coal sodium content is variable over a period of days to weeks with a lag time of several hours from when new coal is loaded into the feed bunkers to its full effect on ESP performance. There is no real-time feedback to determine optimum sodium content in the as-fired coal. Therefore, the bulk sodium addition rate is adjusted based on observed changes in stack opacity and ESP power. Bulk sodium rate adjustments are made several hours after as-fired coal changes and the effects of a rate change do not take effect for several additional hours.
Another hot-side conditioning method is to inject sodium-precursor chemicals, notably carboxylic acid salts, into the flue gas stream as a finely atomized liquid spray. This conditioning method is discussed in detail in U.S. Pat. No. 6,267,802. The conditioning mechanism is enrichment of sodium ion charge carriers on the collected particulates. Advantages compared to bulk sodium addition to the coal include the co-precipitation of chemical and sodium ion charge-carriers with the particulates, the use of only a small fraction of the material required for bulk sodium addition, the avoidance of detrimental boiler slagging and fouling, and the rapid and precise adjustment of additive application rate.
Sodium precursor chemicals, however, may be unable to address shor
Baldrey Kenneth E.
Durham Michael D.
ADA Environmental Solutions LLC
Chiesa Richard L.
Sheridan & Ross P.C.
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