1. Technical Field
The invention relates to a method and apparatus for separating the lower density fly ash particles from the overall mixture of various density particles contained in raw coal ash. More particularly, the particles of fly ash separated and collected by this method include generally solid particles with pervasive internal and external porosity that are frothy in appearance, thick walled hollow particles with a specific gravity in the range of from 1.0 to 2.0 and a minor amount of thin walled hollow particles with a specific gravity of less than 1.0 known as cenospheres. Specifically, the method involves changing the momentum of the exhaust gas; either by drastically changing the diameter of the feed stream to drastically change the velocity of the gas, or by de-energizing at least the first static electric charging field of an electrostatic precipitator (ESP) through which the exhaust gases pass on their way to the stack of the power plant whereby these hollow, porous and other low density particles fall out and are collected in a collection hopper.
2. Background Information
Various types of filler materials have been used for decades to improve the properties and lower costs of various industrial and consumer products including concrete blocks; plastic composites such as shower stalls, automobile body panels, sinks and countertops; roofing materials; tires and other rubber products; caulking compounds; paper; and a multitude of other applications. For example, in plastic composites these fillers are commonly utilized to enhance their structural and mechanical properties, to improve the composite""s fire resistance, to thicken or stiffen the pre-formed mix prior to molding and to reduce costs.
These fillers include natural or mineral fillers such as clay, talc and calcium carbonate, and synthetic fillers, such as glass beads, ground polymers and ceramics. Both mineral and synthetic fillers have proven to be useful fillers in materials ranging from ceramics to a variety of plastics including thermosetting plastics such as polyesters, epoxies and phenolics and thermoplastics such as polyethylene, polypropylene, acrylic lattices, as well as many other resin systems.
Generally, the selection of a filler for a specific application is based upon its physical characteristics (e.g. color, density, shape, thixotropic effect, reactivity, particle size distribution and handling features) and the mechanical properties of the filler (e.g. hardness and strength) and the resulting properties of the filled system. One of the important properties of a filler is its specific gravity.
When specific gravity is examined, two classes of fillers become apparent, namely high density mineral fillers, and low density materials, typically processed minerals or synthetic materials.
Most of the commonly used and economically priced fillers are high density minerals, usually having a specific gravity of 2.6 and higher. They are generally mined, are plentiful and include calcium carbonate (limestone), clay, silica and talc.
However, when the overall weight of the filled system is a concern, lower density fillers are much more desirable. As a result, the demand for economically priced low density fillers is increasing. Low density fillers are generally not plentiful in nature and therefore are man-made or derived by processing natural minerals. Low density fillers are generally expensive and include glass or plastic microspheres, hollow glass beads, expanded ceramic spheres and cenospheres.
Raw fly ash, a product created when coal is combusted in a generation facility is plentiful, and is being generated from coal fired power plants. More particularly, there are currently hundreds of coal fired power plants in the U.S. alone. These plants burn well in excess of one billion tons of coal per year, and as such, coal combustion by-products including fly ash have become one of the nation""s most abundant resources. Growth is further expected as nuclear power loses preference to more standard power sources such as coal. As a result, the need to safely dispose of fly ash, and the need to develop an economical use for fly ash is ever increasing. Fly ash is currently used as a filler material in many applications but has never achieved the status of a major filler. Some of the reasons why fly ash have been unable to capture a large portion of the filler market are its color, its difficulty to handle due to dustiness, its wide particle size distribution, its hardness, and the inconsistency of its composition and properties. Most sources of fly ash have a specific gravity in the range of 2.1-2.3, which is less that than the common mineral filler, but this apparent advantage is not enough to offset the disadvantages.
Fly ash with a low specific gravity, whether hollow or solid has been usually separated from raw ash using water as the separation medium. Those particles having a specific gravity less than 1.0 will float and the remaining portion of the ash sinks to the bottom of the separation pond. The cenospheres must be collected from the ponds, usually by a skimming process, cleaned of other floating materials, dried, and often further processed. Cenospheres, when they are present in raw fly ash, typically represent one percent or less of the weight of the ash. However, cenospheres have a specific gravity of less that 1.0, and are very valuable as a low density filler, selling at a price significantly higher than other types of ash. Similarly, all low density ash, when cleaned and processed, is extremely valuable as a filler material.
Functionally, fly ash is the finely divided ash material carried in the stack gases from the furnace of power plants which consume powdered and pulverized coal, and is collected before it leaves the stack usually in an electrostatic precipitator or other type of collector. The problem associated with disposal of fly ash is large because the tonnage produced by some utility companies is quite high. Numerous attempts have been made to utilize the material, and the suggestion that lightweight aggregate materials might be prepared from fly ash is the result of such an attempt.
The present invention provides a method for economically separating a low density fraction of ash from the entire particulate range of fly ash created as a by-product from the combustion of coal. The resultant low density ash has a specific gravity of 1.6-1.99. More particularly, the resultant ash is at least ten percent lower in density than raw fly ash and produces very little dust when handled. Although not as light as the thin walled cenospheres, the ash created from the present invention is significantly more durable, and is primarily composed of a frothy, relatively solid particles with both internal and external porosity and larger thick walled hollow particles. In addition to its increased durability over cenospheres, the material created by the present invention is plentiful, comprising more than thirty percent of the total raw fly ash produced, and is less expensive to produce and process than cenospheres. The product created by the present invention thus has advantages when compared to both raw fly ash and cenospheres, making it a valuable low density filler produced in an economical manner.
The need thus exists for an ash that which is relatively inexpensive to produce, which may be separated from the existing raw fly ash with limited cost, and which provides for a low specific gravity for use as a filler material. The need also exists for an apparatus and system for separating the above described ash for collection and use as a filler material.
Objectives of the invention include providing an improved device, system and method of separating and collecting the lower density fly ash particles including semi-solid frothy particles with internal and external porosity, thick walled hollow particles and a smaller amount of thin walled hollow particles (cenospheres) from the other higher density solid particles contained in raw fly ash.
A further objective is to provide a low density ash particle separation and collection method and device that effectively separates the particles by size and density, without the use of water and subsequent de-watering and drying processes.
A further objective is to provide such a low density ash particle separation and collection method and device that lessens the amount of ash disposed in land fills.
A further objective is to provide such a low density ash particle which has both internal and external porosity.
A still further objective is to provide a mixture of low density ash particles having a size in the range of from 30 microns to 400 microns.
A further objective is to provide such a low density ash particle separation and collection method and device that produces an inexpensive source of low density mineral filler.
A further objective is to provide such a low density ash particle separation and collection method and device that produces large quantities of low density ash particles.
A further objective is to provide such a low density ash particle separation and collection method and device that produces a low density filler that is useful in plastics, ceramics, concrete and other materials.
A further objective is to provide such a low density ash particle separation and collection method and device that is inexpensive to operate and uses existing equipment such as electrostatic precipitators.
A further objective is to provide such a low density ash particle separation and collection method and device that incorporates one or more or all of the above objectives and advantages.
A further objective is to provide such a low density ash particle separation and collection method and device which accomplishes its objective in a dry environment.
A further objective is to provide such a low density ash particle separation collection method and device which substantially decreases the momentum of the gas stream in order to remove low density fly ash from the gas stream.
These and other objectives and advantages of the invention are obtained by the invention which includes a method of separating lower density ash from the exhaust gas stream which contain the entrained fly ash, and then collecting the low density ash that drops out below the de-energized fields of the electrostatic precipitator. The invention further includes an electrostatic precipitator or similar device that is designed in such a manner to cause the gas or air stream to rapidly decelerate causing the entrained particles to slow down and fall out suspension. The low density fly ash particles having less momentum (relative to their size and therefore subject to greater deceleration due to drag forces) tend to fall out sooner (i.e. in the inlet portion, first hoppers, of the precipitator or similar device). The denser ash particles, with greater momentum and affected to a lesser extent by drag forces, are carried further into the precipitator. The precipitator or other device is, in effect, a hollow body with a larger cross sectional area than the inlet and outlet ducts. The electrostatic precipitator further includes a plurality of energizable fields associated with one or more collecting plates longitudinally positioned next to one another within the hollow body where the plurality of energizable fields includes a first energizable field nearest the inlet. The electrostatic precipitator further includes a de-energizing mechanism for de-energizing the first energizable field(s) resulting in the separation of the lower density ash from the overall ash stream. The ESP is equipped with collection hoppers below the plurality of energizable fields. The ash falls out of the precipitator body, due to gravity, and is collected in the hoppers. Typically a hopper would span one collecting plate having one or more electrostatic fields associated therewith. The ash can be removed from individual hoppers or from rows of hoppers by modifying the existing transport system or by installing a separate system to extract the collected low density ash.