Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
1999-01-26
2001-04-24
Lillis, Eileen D. (Department: 3673)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C106S705000, C106S708000, C406S045000
Reexamination Certificate
active
06221001
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed generally to a method for wet handling of fly ash particles removed from flue gas and specifically to a method for wet handling of fly ash that uses a retardant to inhibit solidification of the fly ash.
BACKGROUND
In coal burning power plants, fly ash particles are entrained in the waste gas and must be removed prior to discharge of the waste gas into the environment. Common methods for removal of the fly ash include electrostatic precipitators, baghouses, and scrubbers.
One method for disposal of the collected fly ash particles is to slurry the collected particles and transport the slurry by pipeline to a suitable disposal site. This disposal method requires little handling of the fly ash and, therefore, can be relatively inexpensive to perform.
The disposal method, however, can have drawbacks. The slurry can solidify in the pipeline during transportation of the slurry to the waste disposal site, particularly when the slurry pump malfunctions or other flow stoppages occur. The solidified deposits resemble concrete and can partially or totally block the pipeline, thereby interrupting operation of the plant. Such deposits can be very difficult and expensive to remove. Because solidification is more likely to occur in higher calcium fly ashes, power plants using this waste disposal method have had to use more expensive, lower calcium coal to generate power.
SUMMARY OF THE INVENTION
Objectives of the present invention include providing a pipeline transportation system in which slurried fly ash particles, particularly high calcium content fly ash particles, are unlikely to solidify in the pipeline during transportation.
The method of the present invention removes undesired particles from a gas stream by the following steps:
(a) removing the undesired particles from the gas stream to form collected particles;
(b) forming the collected particles into a slurry; and
(c) transporting the slurry in a conduit to a disposal site, wherein the slurry includes a retardant to inhibit substantially the solidification of the slurry in the conduit.
The undesired particles are typically fly ash particles that are formed by the combustion of coal. The fly ash particles commonly have a calcium content ranging from about 5 to about 50 and more commonly from about 15 to about 30 wt %.
The retardant can be any compound(s) that is able to inhibit the solidification of the slurry in the conduit.
The most preferred scale inhibitors include molecularly dehydrated phosphate and more particularly hexametaphosphates, pyrophosphates, tripolyphosphates, phosphonates, and mixture thereof. As used herein, “molecularly dehydrated phosphates” refers to the class of phosphate compounds, the members of which are formed by molecular dehydration; that is, the members are formed by removal of the water of constitution, which is water that forms an integral part of the molecule in contrast to attached water or water of crystallization. Molecularly dehydrated phosphates not only preferentially react with dissolved metals such as calcium, iron, magnesium and manganese but also have dispersing properties that aid in the removal of existing water-soluble metal salts.
In a preferred retarded mixture, at least most of the retardant is phosphates. More preferably, the retardant mixture contains at least about 25% by weight and more preferably from about 5 to 40% by weight phosphates.
The particularly preferred retardant mixture includes a hexametaphosphate and tripolyphosphate. More preferably, the retardant mixture includes from about 4 to about 38% and even more preferably from about 20 to about 35 wt % hexametaphosphate and more preferably from about 1 to about 5% and even more preferably from about 2 to about 4 wt % tripolyphosphate.
While not wishing to be bound by any theory, it is believed that the preferred scale inhibitors preferentially react with calcium before hydration can occur and thereby inhibit the reaction of the calcium ions with silica or alumina and the subsequent precipitation of calcium silica and calcium alumina deposits in the conduit. Such deposits can cause partial or total blockages of slurry flow through the conduit. Accordingly, the removal of calcium is believed to inhibit solidification of the slurry in the conduit.
The amount of the retardant in the slurry can be important to the performance of the additive. Preferably, the concentration of the retardant in the slurry ranges from about 0.01 to about 0.20 g/l and more preferably from about 0.035 to about 0.14 g/l. The ratio of the retardant to the undesired particles ranges from about 0.5 to about 15 g/kg and more preferably from about 1.0 to about 10 g/kg.
The retardant is preferably introduced as an aqueous liquid composition into the slurry or into the liquid used to form the slurry. The concentration of the retardant in the aqueous composition preferably ranges from about 0.01 to about 0.20 g/l and more preferably from about 0.035 to about 0.14 g/l. The rate of introduction of the aqueous composition into the slurry or the liquid used to form the slurry ranges from about 10 mls/min to about 2 l/min . The volumetric ratio of the aqueous composition to the slurry preferably ranges from about 0.1:10,000 to about 1.5:10,000 and more preferably, from about 0.2:10,000 to about 1.0:10,000.
The above-described method can offer many benefits, especially to utilities. For example, the equipment to implement the process (e.g., a storage tank and metering pump) is relatively inexpensive to purchase and install, is highly reliable, and is easy to use. The retardant can be environmentally safe, non-hazardous, non-toxic, and relatively inexpensive. The retardant can inhibit not only cementation but also build-up of deposits caused by the precipitation of water-soluble calcium and magnesium salts and prevent the precipitation of dissolved calcium, magnesium, iron, and manganese. The rate of injection of the retardant to the water or slurry is adjustable to match undesired particle cementing characteristics. The retardant is preferably in liquid form, which is easy to feed to the slurry. Finally, the retardant permits utilities, who have a wet flyash handling system, to switch to coals producing high calcium flyash without having to perform expensive retrofits of the ash handling system.
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Calgon Coproration; “Relative Efficiency of Phosphates Used in Boiler Water Conditioning”; 4 pages. (Undated).
Baldrey Kenneth Eugene
Comer John Philip
Sterner Robert P.
Wurster John
ADA Environmental Solutions LLC
Kreck John
Lillis Eileen D.
Ross P.C. Sheridan
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