Gas separation: processes – Electric or electrostatic field – With addition of solid – gas – or vapor
Reexamination Certificate
1999-06-17
2001-07-31
Chiesa, Richard L. (Department: 1724)
Gas separation: processes
Electric or electrostatic field
With addition of solid, gas, or vapor
C095S072000, C096S052000, C096S074000, C252S192000, C423S243080
Reexamination Certificate
active
06267802
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related generally to the conditioning of gas streams for particulate removal and specifically to the conditioning of gas streams for particulate removal using an electrostatic precipitator, particularly at high temperatures.
BACKGROUND
Environmental standards for particulate emissions by 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 flyash produced by coal combustion before the flue gas can be discharged into the atmosphere. 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 undesired particles from the gas streams produced by plants and refineries. As used herein, “undesired particles” refers to any particulate matter such as flyash, that is desired to be removed from a gas stream. In a typical electrostatic precipitator, undesired particle-laden gases pass negatively charged corona electrodes which impart a negative charge to the undesired particles. The charged particles then migrate towards positively charged collection plates and are 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 undesired particles.
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 no more 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., more than about 250° C.).
Many hot-side electrostatic precipitators suffer from problems related to the resistivity of collected undesired particles. Such problems can cause a deterioration of the particulate collection efficiency of the electrostatic precipitator and excessive power consumption. These problems can be caused by sodium depletion of collected undesired particles on the collection plates, inherently high resistivity of undesired particles, 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 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 is the predominant charge conduction mechanism. As used herein, the “critical temperature” is the temperature corresponding to the highest attainable resistivity of an undesired particle (which is commonly located at the top of a bell-shaped curve as shown in FIG.
1
).
One conditioning method for controlling high temperature 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 3% by weight sodium is added to the coal feed as a sodium sulfate or soda ash. The sodium is co-fired with the coal in the boiler and is incorporated into the undesired particles as various sodium oxides. However, the bulk addition of sodium to the coal feed can cause serious problems, such as boiler slagging due to high sodium flyash, the consumption of excessive amounts of alkali material and a commensurate increase in operating costs, higher gas temperatures downstream of the boiler that can aggravate duct and electrostatic precipitator structural problems, and an inability to supply the additive on an intermittent or as-needed basis.
SUMMARY OF THE INVENTION
Objectives of the present invention include providing an electrostatic precipitator that can remove sufficient undesired particles from a gas stream to comply with pertinent environmental regulations; increasing the efficiency of electrostatic precipitators in the removal of undesired particles from a gas stream, preferably without significantly increasing capital and operating costs and without undue power consumption; increasing electrostatic precipitator efficiency without the use of toxic additives; increasing electrostatic precipitator efficiency by methods and apparatuses that are readily adaptable to existing designs; and reducing undesired particle reentrainment during removal of undesired particles from a collection surface. Related objectives include increasing electrostatic precipitator efficiency without inducing boiler slagging, without excessive consumption of alkali material, without increasing gas stream temperatures downstream of the boiler, and using an additive that can be supplied on an intermittent or as-needed basis.
In one embodiment of the present invention, a process is provided for removing undesired solid particles from a gas stream that can realize these and other objectives. The process includes the steps of:
(a) contacting with the gas stream a composition including an organometallic compound;
(b) collecting on at least one collection surface in a collection zone a solid aggregate including at least a portion of the composition or a derivative(s) thereof and at least a portion of the undesired solid particles; and
(c) removing the agglomerate from the collection surface. As used herein, “agglomerate” refers to a cluster or accumulation of undesired particles and/or particles of the organometallic compound or a derivative thereof; a “carboxylic acid” refers to any acid having both a carboxyl (hydroxyl (OH) and carbonyl (C═
0
)) group of the form R-COOH where R is a linked organic structure to the carboxylic group (COOH); a “collection surface” is any surface which collects undesired particles (e.g., an electrode or a porous filtration surface); and “contacting” refers to any technique for inputting the composition into the gas stream, such as by spray nozzles, drip emitters, venturi eductors and the like.
The organometallic compound is preferably any organic compound that decomposes at the gas stream temperature to produce an inorganic metal oxide after injection into the gas stream. “Decomposition” refers to the constituents of the organometallic forming other compounds as a result of thermal or chemical decomposition, chemical reaction, or otherwise. The inorganic metal oxide is preferably an oxalate, carbonate, hydroxide, oxide and mixtures thereof with a carbonate and oxide being more preferred. It is desired that the organometallic compound have a melting point that is less than and a boiling point that is more than (i.e., is substantially nonvaporizable or free of vaporization) at the temperature of the gas stream to produce a liquid additive of the injection and a relatively low molecular weight (i.e., preferably no more than about 180 daltons). More preferably, the organometallic compound is a monocarboxylic acid (metal) salt and even more preferably the monocarboxylic acid salt has 3 or fewer carbon atoms (the carbon of the terminal group being counted as part of the chain) and is selected from the group consisting of a metal acetate, a metal formate, a metal propionate and mixtures thereof and even more preferably a metal acetate, a metal formate and mixtures thereof. As will be appreciated, a metal salt of a carboxylic acid is formed when a metal cation substitutes fo
Baldrey Kenneth Eugene
Bisque Ramon Edward
Durham Michael Dean
Jackson Douglas W.
ADA Environmental Solutions LLC
Chiesa Richard L.
Sheridan & Ross P.C.
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