We've noted in a couple of reports that residual, un-combusted Carbon in Coal Fly Ash impairs it's utility as a fine, reactive, property-enhancing aggregate or admixture in Portland Cement Concrete.
Such residual Carbon interferes with air entrainment additives, which additives enable the formation of very tiny air bubbles in Concrete that improve it's resistance to freeze-thaw cycles and subsequent and related chemical corrosion. As explained by the Portland Cement Association via:
Cement & Concrete Basics: Air-Entrained Concrete | Portland Cement Association (PCA); "One of the greatest advances in concrete technology was the development of air-entrained concrete in the late 1930s. Today, air entrainment is recommended for nearly all concretes, principally to improve resistance to freezing when exposed to water and deicing chemicals. However, there are other important benefits of entrained air in both freshly mixed and hardened concrete. Air-entrained concrete contains billions of microscopic air cells. These relieve internal pressure on the concrete by providing tiny chambers for the expansion of water when it freezes".
And, as we've also noted in a few previous reports, with details accessible via:
there exists a fairly well-known beneficiation treatment for Coal Ash called "Carbon Burn Out", which we haven't really done a very good job of documenting or explaining for you, although it is described in general terms in our above-cited report, "Virginia Converts Coal Ash to Cash".
And, as seen in our report of:
West Virginia Coal Association | Wisconsin Cleans Ammonia from Coal Ash | Research & Development; concerning, in part:"United States Patent 6,755,901 - Ammonia Removal from Fly Ash; 2004; Assignee: Wisconsin Electric Power Company; Abstract: A method and apparatus for the application of beat to remove ammonia compounds from fly ash, thereby making the fly ash a marketable product is disclosed. The method includes the steps of providing an amount of fly ash wherein at least a portion of the amount of fly ash comprises particulates having ammonia compounds affixed to the particulates, and exposing the fly ash to flowing air having a temperature of at least 1,500F... such that the fly ash is maintained in the flowing air until the fly ash reaches a temperature of at last 900F... . The method ... further comprising: recovering heat from the particulate material after the particulate material has been removed from the flowing air (and) using the heat recovered from the particulate material to preheat the second amount of fly ash. This Invention relates to the treatment of coal ash to remove ammonia compounds that contaminate the ash as part of post-combustion treatments of exhaust gases to remove nitrogen compounds. In 1990, the US EPA put into place the Clean Air Act Amendments which were designed to reduce the emissions of 'greenhouse gases'. Among the emissions covered are the nitrogen compounds NO and NO2 referred to generically as NOx. NOx is generated through the combustion of coal and its generation is directly affected by combustion temperature, residency time and available oxygen. Several technologies have been developed to meet the mandated NOx reduction limits. The NOx reduction technologies fall into two major categories. One category includes technologies that modify or control the combustion or firing characteristics. The effect of these approaches has been an increase in residual unburned carbon in the coal ash";
an increased amount of unburned Carbon in Coal Ash is associated with some types of pollution controls which also add ammonia, another objectionable contaminant, to Coal Ash, but which can also be removed through a Carbon Burn Out process.
We note, too, that, as seen in:
West Virginia Coal Association | University of Kentucky Prepares Coal Ash for Market | Research & Development; concerning: "United States Patent 6,533,848 - High Quality Polymer Filler and Super-Pozzolan from Fly Ash; 2003; Assignee: The University of Kentucky; Abstract: A novel method for producing fly ash material with a range of particle sizes (as specified) is provided utilizing superplasticizers. The method produces fly ash material suitable for use as filler material in the plastics industry and super pozzolan for the concrete industry. A method of providing fly ash with a mean particle size in the range (specified) comprising: slurrying fly ash with water in the presence of a superplasticizer; bringing said slurry to a pH of 7.5-10.5; and elutriating the resulting slurry in a column/hydraulic classifier";
there are other ways of preparing, cleaning and classifying Coal Ash, as well.
But, one variation or another on methods to combust the residual Carbon in Coal Ash seems to be the preferred route to Ash beneficiation, especially since additional heat energy can be recovered from the process and utilized in constructive ways.
We'll be documenting more of that as we go along, and we think some will find it surprising how widely the technology is being employed, and, be surprised as well that some specialty contractors are in the business of engineering and installing such facilities.
We regret that our excerpts won't do justice to the full technical discussion of the recent United States Patent we bring to your attention herein; a great deal of insight could be gained from it. And, we will forbear offering additional observations on the seemingly wide but almost secretive employment of Coal Ash beneficiation technologies, as mentioned above, since the length of our excerpts, as it is, will likely tax the capacity of email; and we want to make certain we include enough information concerning this disclosure of a very recent improvement in Carbon Burn Out technology, intended to further improve the product quality of Coal Fly Ash:
"United States Patent 8,234,986 - Method and Apparatus for Turbulent Combustion of Fly Ash
Date: August, 2012
Inventors: Jimmy Knowles and Richard Storm, SC and NC
Assignee: The Sefa Group, Inc., Lexington, SC
(Interestingly, "Sefa" is a formalized acronym for "Southeastern Fly Ash"; and, it is a company that has been in the business of profitably processing and marketing Fly Ash for various commercial applications for a number of years. More can be learned via:
The SEFA Group, Inc.: Private Company Information - Businessweek; "The SEFA Group, Inc. produces fly ash for the construction industry in Southeast and Mid-Atlantic regions. It offers pozzolan-grade, concrete, flowable fill, and asphalt fly ash, as well as fly ash for utilities. The SEFA Group, Inc. was formerly known as Southeastern Fly Ash Company, Inc. and changed its name to The SEFA Group, Inc. in December 2001. The company was founded in 1976 and is based in Lexington, South Carolina. It has manufacturing locations/operations in South Carolina, North Carolina, Virginia, Alabama, Maryland, and Tennessee"; and:
The SEFA Group; "Today's construction and tomorrow's environment can benefit greatly by using concrete designed with fly ash. Fly ash can transform ordinary concrete into "high performance concrete". Fly ash has greater long-term durability and , therefore, concrete structures built with fly ash concrete will last longer. Using fly ash as a resource, rather than disposing of it in landfills like a waste material, reduces the amount of other natural resources used in construction today. And since structures built with fly ash concrete will last longer, fewer resources will be depleted in the future. Using fly ash in this way exemplifies sustainable development. Using fly ash in concrete is good for the environment in some obvious - and some not so obvious ways. Obviously, fly ash is an industrial by-product. If fly ash were not recycled, it would have to be disposed of in landfills. Also, using fly ash in concrete means less cement is needed. The production of cement is energy intensive and releases large quantities of carbon dioxide (CO2) into the atmosphere. For every ton of cement produced, approximately one ton of CO2 is released into the atmosphere. Fortunately, when fly ash is used in concrete, less cement is used. The energy that would have been required to produce that cement is saved. And for every ton of fly ash used in concrete, there is approximately one less ton of CO2 released into the atmosphere.When concrete hardens, about 25% of the cement forms a by-product called calcium hydroxide. This compound contributes nothing to the strength of the concrete, it dissolves in water and may leach out of the concrete. Or worse, it could form combinations with other materials that could seep into the concrete and cause premature deterioration. Fortunately, fly ash reacts with this calcium hydroxide, and chemically combines with it to form stable cemetious bonds. This pozzolanic reaction increases the durability of the concrete in two ways. First, since the fly ash chemically combines with the calcium hydroxide, it is tied-up and cannot react with other materials that may seep into the concrete. Also, as these extra bonds develop between the fly ash and the calcium hydroxide in the cementious paste matrix, the permeability of the concrete is reduced. Less water or other impurities can penetrate into the concrete to corrode reinforcing steel or otherwise damage the concrete. Fly ash closely resembles the volcanic ashes used in the production of the earliest known hydraulic cements some 20,000 years ago, near the small Italian town of Pozzuoli (which later gave its name to our modern day pozzolans). The volcanic ashes were used in a number of well known Roman structures including the Coliseum and aqueducts which survive to date. In addition to using in modern concrete, some other examples of how fly ash can be beneficially used in sustainable ways are in asphalt and in flowable fill. Architects and engineers recognize fly ash concrete as a more durable and a more sustainable building material".
The SEFA Group's web site is well-worth examination by anyone genuinely interested in starting to think of Coal Ash in terms of being a resource, rather than as a waste; and, in truth we could have, perhaps should have, devoted our presentation herein to their company and their commercial accomplishments.
However, of importance, please note, in the above, SEFA's confirmation of one fact we have earlier reported and documented from other sources, i.e.: The manufacture of Portland-type Cement generates a lot of CO2, in large part from the calcination of limestone, via the reaction CaCO3 + Heat = CaO + CO2, to form the basic component of Portland-type Cement, Calcium Oxide. And, that is in addition to any CO2 generated by the combustion of fuel to generate the heat for the calcination. Any Cement which can be replaced by Fly Ash will result in the prevention, very roughly, of an equivalent weight of CO2 from being emitted by the Cement manufacturing process.)
Abstract: An apparatus for processing fly ash comprising a heated refractory-lined vessel having a series of spaced angled rows of swirl-inducing nozzles which cause cyclonic and/or turbulent air flow of the fly ash when introduced in the vessel, thus increasing the residence time of airborne particles. Also disclosed is a method of fly ash beneficiation using the apparatus.
Claims: A method for reducing the carbon content of small particulate combustion products said small particulate combustion products consisting essentially of fly ash or fly ash with chemical residue and/or contaminants, said small particulate combustion products being a product of a previous combustion and containing unburned carbon and incombustible matter, the method comprising:
a) introducing a feed of said small particulate combustion products into a swirling and generally upward helical flow within a pneumatic transport solid gas reaction vessel and thereby suspend said small particulate products in a finely-divided, separated state within said flow, said vessel having a top portion, a bottom portion, a side wall between the top portion and the bottom portion, an interior, and an exit, the exit being at the top portion of the vessel, the interior being defined by the top portion, the bottom portion, and the side wall, the interior having an upper section and a lower section and having a substantially uniform cylindrical cross-sectional area between the top portion and the bottom portion, said feed being introduced into an open area in said lower section;
b) at least initially heating the interior of said vessel so as to heat said small particulate combustion products;
c) introducing at least one of air or another gas into said Vessel through said side wall to create said swirling and generally upward helical flow within said vessel, to prevent said small particulate combustion products from forming a particulate bed within said vessel, and to combust at least some of said unburned carbon in said small particulate combustion products to reduce the carbon content and the particle size of the feed of said small particulate combustion products, the introducing of at least one of air or another gas occurring below said introducing of said feed, substantially all of said reduced carbon content and reduced particle size small particulate combustion products exiting said vessel in a flow through the exit at the top portion of the vessel;
d) separating the reduced carbon content and reduced particle size small particulate combustion products from the flow exiting the vessel via the exit at the top of the vessel in a gas-solids separator to provide said reduced carbon content and reduced particle size small particulate combustion products as an output product, said gas-solids separator including a first outlet leading to a processed material collector and a second outlet capable of returning processed material to said vessel;
(e) collecting the output product in a product silo for storage or transport, the collected output product having a reduced carbon content and reduced particle size relative to said feed,
(f) the suspended flow of small particulate combustion products being heated to a temperature within said vessel which is above (i) the fusion temperature of the fly ash mineral matter, (ii) the fusion temperature of chemical and mineral residues present in the raw feed fly ash, or (iii) the ignition temperature of any residual unburned carbon, and,
(g) the suspended flow of small particulate combustion products being maintained in the finely-divided, separated state by cooling the suspended flow of small particulate combustion products to a temperature that is less than the ash fusion temperature after step (c) and before step (d).
(Some additional extended Claims describe variations on the above; and, it gets a little complicated. We really don't want to give short shrift to their work, but suffice it to say that it is a process for the pretty thorough oxidative, and mechanical, removal of contaminants from the Coal Ash, with some efficiencies gained by the internal recycling of heated product.)
(Some additional extended Claims describe variations on the above; and, it gets a little complicated. We really don't want to give short shrift to their work, but suffice it to say that it is a process for the pretty thorough oxidative, and mechanical, removal of contaminants from the Coal Ash, with some efficiencies gained by the internal recycling of heated product.)
Background and Field: The present disclosure relates to methods for processing fine particulate matter, such as coal fly ash, to improve its characteristics, such as by reducing the residual carbon, and removing contaminants, such as mercury, ammonia, and the like. The present disclosure also relates to apparatus for processing fly ash as well as a fly ash product.
(SEFA's full exposition of background is reading we recommend. We can't do it real justice in our excerpts, as long as they might prove to be; but, if anyone does have an interest in the fullest employment of our Coal Ash resources, the full Disclosure, like SEFA's web site, is well worth the study. We are including, though, much of their discussion on residual Carbon's negative effect on air entraining efforts, so that you might get some better understanding of why all of this is important.)
Coal fly ash is produced by coal-fired electric and steam generating plants and other industrial facilities. Typically, coal is pulverized and blown with air into the combustion chamber of a boiler where the coal immediately ignites, generating heat and producing a molten mineral residue. Boiler tubes extract heat from the boiler, cooling the flue gas and causing the molten mineral residue to harden and form ash. Coarse ash particles, referred to as bottom ash or slag, fall to the bottom of the combustion chamber, while the lighter fine ash particles, termed fly ash, remain suspended in the flue gas. Prior to exhausting the flue gas, fly ash is removed by particulate emission control devices, such as electrostatic precipitators or filter fabric baghouses.
The American Coal Ash Association reports that 70,150,000 tons of fly ash was produced in 2003 and that 27,136,524 were beneficially utilized, while the remainder was disposed in lagoons and landfills. The most prevalent utilization application for fly ash (12,265,169 tons) was in concrete as pozzolan. Pozzolans are siliceous or siliceous and aluminous materials, which in a finely divided form and in the presence of water, react with calcium hydroxide at ordinary temperatures to produce cementitious compounds.
A substantial portion of fly ash particles are reactive glass, which will combine with alkali hydrates, especially calcium hydroxide, that are formed as cement hydrates in plastic concrete. This chemical reaction is referred to as a pozzolanic reaction and the result of this reaction is a stable cementious bond, similar to the bond that results through the hydration products of cement ... .
Coal fly ash is produced by coal-fired electric and steam generating plants and other industrial facilities. Typically, coal is pulverized and blown with air into the combustion chamber of a boiler where the coal immediately ignites, generating heat and producing a molten mineral residue. Boiler tubes extract heat from the boiler, cooling the flue gas and causing the molten mineral residue to harden and form ash. Coarse ash particles, referred to as bottom ash or slag, fall to the bottom of the combustion chamber, while the lighter fine ash particles, termed fly ash, remain suspended in the flue gas. Prior to exhausting the flue gas, fly ash is removed by particulate emission control devices, such as electrostatic precipitators or filter fabric baghouses.
The American Coal Ash Association reports that 70,150,000 tons of fly ash was produced in 2003 and that 27,136,524 were beneficially utilized, while the remainder was disposed in lagoons and landfills. The most prevalent utilization application for fly ash (12,265,169 tons) was in concrete as pozzolan. Pozzolans are siliceous or siliceous and aluminous materials, which in a finely divided form and in the presence of water, react with calcium hydroxide at ordinary temperatures to produce cementitious compounds.
A substantial portion of fly ash particles are reactive glass, which will combine with alkali hydrates, especially calcium hydroxide, that are formed as cement hydrates in plastic concrete. This chemical reaction is referred to as a pozzolanic reaction and the result of this reaction is a stable cementious bond, similar to the bond that results through the hydration products of cement ... .
The strength-producing characteristics of fly ash allow for a lower amount of cement than would otherwise be needed. The value of fly ash as pozzolan is generally related to the cost of the portion of cement that is replaced by the fly ash.
Unfortunately, some of the unintended consequences of (described) pollution control methods have negatively impacted the utilization of the coal fly ash, especially as pozzolan in concrete. According to the ACAA less than 17.5% of the fly ash produced in 2003 was used as pozzolan. However, in many parts of the U.S., the demand for fly ash as pozzolan is significantly higher than the local supply of pozzolan-grade fly ash. A major reason for this shortage of "quality" fly ash is caused by changes in the characteristics of fly ash produced in the U.S. due to changes made to coal-burning operations and to the increased use of post-combustion pollution control techniques implemented by electric utilities to meet lower emission limits.
According to the U.S. Department of Transportation's Federal Highway Administration, changes in boiler operations and/or air emissions control systems at power plants will continue to alter the quality of fly ash produced. Factors that impact ash quality in this way include: a reduction in the pozzolanic reactivity; the presence in the fly ash of excessive unburned carbon; and, chemical residuals from post-combustion emission control.
It would be desirable to beneficially alter the characteristics of coal fly ash, especially the fly ashes that have been negatively affected by the aforementioned pollution control methods. Generically, such processes are called beneficiation processes; specifically, it would be desirable to have a thermal beneficiation process that is especially designed to alter these particular characteristics.
There are three major issues which affect the value and utility of fly ash used as pozzolan in concrete: the pozzolanic, strength-producing characteristics; the air-entraining characteristics; and, the presence of foreign residual chemicals.
Due to the lower combustion temperatures necessary to reduce the formation of thermal NOx, commercial operation of low-NOx burners produce coal ash that has not been exposed to the high-temperature operating conditions employed before low-NOx combustion techniques were implemented. The operating temperatures employed for low-NOx combustion may be below the melting point of individual constituents of the mineral matter contained in the coal being burned, especially for the higher ash fusion bituminous coals. Consequently, the amount of mineral mater that has become molten and then air-cooled, thereby forming reactive glass, may be reduced significantly due to low-NOx combustion of coal.
The pozzolanic reaction of fly ash ... is ... known to increase the long-term durability of concrete. When fly ash is used as pozzolan in concrete, the density of the concrete paste matrix may increase significantly and, therefore, the durability of the concrete may be significantly greater than with ordinary portland cement alone. Increasing the glass to crystalline ratio of fly ash will increase the pozzolanic reactivity, making the concrete more impervious and, therefore, more durable.
A second major issue affecting the utilization and value of fly ash - the air-entraining characteristics of the fly ash - is also related to the impact of fly ash on concrete durability. Particularly, one aspect of concrete durability, namely, freeze-thaw durability, may be negatively impacted by the presence of fly ash, especially by the presence of unburned carbon that remains in fly ash ... .
There are many factors that can affect the durability of concrete to cycles of freezing and thawing; however, the single greatest impact on freeze thaw durability derives from the presence and uniform distribution of air voids in the hardened cement paste matrix with optimum spacing and size.
When hardened concrete begins to freeze, residual water inside concrete will also freeze; when water freezes its volume increases 9%. The expanding ice forces water into the unfrozen regions of the cement binder. This movement of water creates large hydraulic pressures and generates tensile stress. Although concrete has excellent strength in compression, its tensile strength is less than 10% of the compressive strength. When the tensile stress exceeds the tensile strength of the concrete, cracking and deterioration occurs. A network of air voids with the proper spacing and size distribution in the hardened cement paste matrix allows the water to expand and migrate deeper into the concrete, reducing the hydraulic pressure and tensile stress in the concrete.
Air is naturally entrapped in the cement paste of plastic concrete through the folding and shearing action of the mixing process. However, the entrapped air voids are large and not stable in concrete without the use of surface active agents, commonly called surfactants. Surfactants can be used in the production of concrete to reduce the surface tension of water. Consequently, large air voids will divide into smaller, more stable air voids. Air entraining agents (AEAs) are commonly used as surfactants in the production of concrete designed to increase freeze-thaw durability.
Residual unburned carbon in fly ash can have a high adsorptive capacity for AEAs. More specifically, there are certain active sites on the carbon surface, which are typically nonpolar, that preferentially adsorb AEAs from the aqueous phase. The rate of AEA adsorption will vary according to the type, amount, and/or level of activated carbon surface area, requiring a varying increase in AEA dosage to maintain the desired entrained air void system or else resulting in an inconsistent level of entrained air in the hardened concrete, which will ultimately affect the strength and/or durability of the concrete by degrading the air void system. There is also an increased risk of over-dosing the AEA and creating an elevated entrained air content, which would negatively impact the strength of the hardened concrete.
The use of fly ash as pozzolan is typically controlled by specifications that effectually limit the amount of unburned carbon that can remain in fly ash used as pozzolan. Most specifications prescribe a maximum limit for the Loss On Ignition (LOI) of fly ash used as pozzolan in concrete. LOI is a percent-by-weight measure of the residual combustible material, primarily carbon, in the fly ash. The strength-producing characteristics of a fly ash are relatively unaffected by LOI levels up to and above 12%; therefore, the low maximum limits prescribed by most of the controlling specifications for fly ash as pozzolan in the U.S. are not necessary to assure the strength-producing characteristics. Instead, the intent of these low maximum LOI limits is to assure adequate air-entraining characteristics for pozzolan-grade fly ash used to produce air-entrained concrete. The concrete industry also references specific LOI values for fly ash to predict and/or monitor the air-entraining characteristics of the various fly ashes available in the marketplace and there is a general perception that lower LOI levels equate to higher quality.
There are several processes in commercial use that aim to significantly reduce the LOI of moderate to high LOI fly ashes--to a level below 3% by weight, specifically triboelectric separation and carbon combustion. It should be noted that carbon makes up most of the measured LOI (to within about 10%); however, as previously discussed, it is the adsorptive capacity of the fly ash, especially the active carbon sites, for air entraining agents and not the LOI per se, that impacts the marketability of the fly ash. At this time, there is a growing realization that lower LOI fly ashes do not assure superior, or even adequate, air-entraining characteristics for many fly ashes.
Therefore, regardless of the specific reduction of LOI through combustion, it would be desirable to beneficially alter the air-entraining characteristics of the processed fly ash by reducing the overall adsorptive capacity of the fly ash.
A third major issue affecting the utilization and marketability of fly ash also derives from operational changes in the coal-burning process, specifically the presence of residual chemicals and/or particulate matter deposited in or adsorbed on the coal fly ash during the coal-burning operation and/or subsequent flue gas treatment processes, especially those processes intended to reduce air pollution. These changes in coal-burning operations are intended to reduce the emissions of particulate matter; polluting gases, such as sulfur and nitrogen oxides; and heavy metals, such as mercury, or other toxic emissions, especially those considered to be persistent bioaccumulative toxins.
There are several different techniques for the reduction of each of the above pollutants and coal-burning operations often utilize a combination of some or all of these pollution control techniques in order to meet the targeted emission levels. One example of these pollution control techniques, namely, flue gas conditioning, is used to enhance precipitator performance. This technique deliberately deposits foreign chemicals, particularly ammonia, sulfur, and other proprietary chemicals, on the coal fly ash. This technique actually "conditions" the fly ash by coating the particles with these chemicals, changing the surface conductivity and, therefore, the resistivity of the fly ash. These chemicals may also create a space-charge effect and improve the cohesiveness of the fly ash particles.
Injecting these chemicals in the hot flue gases will improve the efficiency of electrostatic precipitators and, therefore, the collection rates for the coal fly ash. However, the collected fly ash will have increased levels of ammonia, sulfur oxides, and/or other residual chemicals which are known to negatively impact the marketability of fly ash as pozzolan at high concentration levels.
Additional pollution control techniques include, but are not limited to, fuel switching and/or blending, the use of low-NOx burners, flue gas treatment to enhance the performance of NOx scrubbers, e.g., selective catalytic reduction (SCR), non-selective catalytic reduction, selective auto-catalytic reduction, etc., as well as the use of flue gas desulfurization (FGD) scrubbers, etc. All these techniques have specific effects on the fly ash which negatively impacts the marketability of fly ash as pozzolan.
For example, there are several dry FGD scrubbing techniques that are used in coal burning operations to decrease the emissions of sulfur oxides, such as lime spray drying, duct sorbent injection, furnace sorbent injection, and circulating fluidized bed combustion. The use of any of these techniques can result in a single, comingled by-product stream consisting of coal fly ash and spent lime sorbent. The general make-up of the residual particulate matter collected following these coal burning and dry FGD scrubbing operations, often generically referred to as "spray dryer material," are a heterogeneous combination of coal fly ash and a blend of calcium sulfate and calcium sulfite compounds.
The chemical composition of spray dryer material residues depends on the sorbent used for desulfurization and the proportion of fly ash collected with the FGD residues. The fly ash in dry FGD materials has similar particle size, particle density, and morphology to those of conventional fly ashes, but FGD materials have lower bulk densities. The difference in bulk density is due to variations in the chemical and mineralogical characteristics of the reacted and unreacted sorbent. Dry FGD materials contain higher concentrations of calcium and sulfur and lower concentrations of silicon, aluminum, and iron than fly ash.
Typically, dry FGD materials usually will not conform to the controlling specifications for pozzolans (e.g., ASTM C-618), due to the varying chemistry and glass content, the presence of high levels of calcium sulfate, and the generally heterogeneous nature of dry FGD materials. Therefore, they cannot be reliably used as pozzolan, especially for pozzolan in concrete structures.
Unfortunately, some of the unintended consequences of (described) pollution control methods have negatively impacted the utilization of the coal fly ash, especially as pozzolan in concrete. According to the ACAA less than 17.5% of the fly ash produced in 2003 was used as pozzolan. However, in many parts of the U.S., the demand for fly ash as pozzolan is significantly higher than the local supply of pozzolan-grade fly ash. A major reason for this shortage of "quality" fly ash is caused by changes in the characteristics of fly ash produced in the U.S. due to changes made to coal-burning operations and to the increased use of post-combustion pollution control techniques implemented by electric utilities to meet lower emission limits.
According to the U.S. Department of Transportation's Federal Highway Administration, changes in boiler operations and/or air emissions control systems at power plants will continue to alter the quality of fly ash produced. Factors that impact ash quality in this way include: a reduction in the pozzolanic reactivity; the presence in the fly ash of excessive unburned carbon; and, chemical residuals from post-combustion emission control.
It would be desirable to beneficially alter the characteristics of coal fly ash, especially the fly ashes that have been negatively affected by the aforementioned pollution control methods. Generically, such processes are called beneficiation processes; specifically, it would be desirable to have a thermal beneficiation process that is especially designed to alter these particular characteristics.
There are three major issues which affect the value and utility of fly ash used as pozzolan in concrete: the pozzolanic, strength-producing characteristics; the air-entraining characteristics; and, the presence of foreign residual chemicals.
Due to the lower combustion temperatures necessary to reduce the formation of thermal NOx, commercial operation of low-NOx burners produce coal ash that has not been exposed to the high-temperature operating conditions employed before low-NOx combustion techniques were implemented. The operating temperatures employed for low-NOx combustion may be below the melting point of individual constituents of the mineral matter contained in the coal being burned, especially for the higher ash fusion bituminous coals. Consequently, the amount of mineral mater that has become molten and then air-cooled, thereby forming reactive glass, may be reduced significantly due to low-NOx combustion of coal.
The pozzolanic reaction of fly ash ... is ... known to increase the long-term durability of concrete. When fly ash is used as pozzolan in concrete, the density of the concrete paste matrix may increase significantly and, therefore, the durability of the concrete may be significantly greater than with ordinary portland cement alone. Increasing the glass to crystalline ratio of fly ash will increase the pozzolanic reactivity, making the concrete more impervious and, therefore, more durable.
A second major issue affecting the utilization and value of fly ash - the air-entraining characteristics of the fly ash - is also related to the impact of fly ash on concrete durability. Particularly, one aspect of concrete durability, namely, freeze-thaw durability, may be negatively impacted by the presence of fly ash, especially by the presence of unburned carbon that remains in fly ash ... .
There are many factors that can affect the durability of concrete to cycles of freezing and thawing; however, the single greatest impact on freeze thaw durability derives from the presence and uniform distribution of air voids in the hardened cement paste matrix with optimum spacing and size.
When hardened concrete begins to freeze, residual water inside concrete will also freeze; when water freezes its volume increases 9%. The expanding ice forces water into the unfrozen regions of the cement binder. This movement of water creates large hydraulic pressures and generates tensile stress. Although concrete has excellent strength in compression, its tensile strength is less than 10% of the compressive strength. When the tensile stress exceeds the tensile strength of the concrete, cracking and deterioration occurs. A network of air voids with the proper spacing and size distribution in the hardened cement paste matrix allows the water to expand and migrate deeper into the concrete, reducing the hydraulic pressure and tensile stress in the concrete.
Air is naturally entrapped in the cement paste of plastic concrete through the folding and shearing action of the mixing process. However, the entrapped air voids are large and not stable in concrete without the use of surface active agents, commonly called surfactants. Surfactants can be used in the production of concrete to reduce the surface tension of water. Consequently, large air voids will divide into smaller, more stable air voids. Air entraining agents (AEAs) are commonly used as surfactants in the production of concrete designed to increase freeze-thaw durability.
Residual unburned carbon in fly ash can have a high adsorptive capacity for AEAs. More specifically, there are certain active sites on the carbon surface, which are typically nonpolar, that preferentially adsorb AEAs from the aqueous phase. The rate of AEA adsorption will vary according to the type, amount, and/or level of activated carbon surface area, requiring a varying increase in AEA dosage to maintain the desired entrained air void system or else resulting in an inconsistent level of entrained air in the hardened concrete, which will ultimately affect the strength and/or durability of the concrete by degrading the air void system. There is also an increased risk of over-dosing the AEA and creating an elevated entrained air content, which would negatively impact the strength of the hardened concrete.
The use of fly ash as pozzolan is typically controlled by specifications that effectually limit the amount of unburned carbon that can remain in fly ash used as pozzolan. Most specifications prescribe a maximum limit for the Loss On Ignition (LOI) of fly ash used as pozzolan in concrete. LOI is a percent-by-weight measure of the residual combustible material, primarily carbon, in the fly ash. The strength-producing characteristics of a fly ash are relatively unaffected by LOI levels up to and above 12%; therefore, the low maximum limits prescribed by most of the controlling specifications for fly ash as pozzolan in the U.S. are not necessary to assure the strength-producing characteristics. Instead, the intent of these low maximum LOI limits is to assure adequate air-entraining characteristics for pozzolan-grade fly ash used to produce air-entrained concrete. The concrete industry also references specific LOI values for fly ash to predict and/or monitor the air-entraining characteristics of the various fly ashes available in the marketplace and there is a general perception that lower LOI levels equate to higher quality.
There are several processes in commercial use that aim to significantly reduce the LOI of moderate to high LOI fly ashes--to a level below 3% by weight, specifically triboelectric separation and carbon combustion. It should be noted that carbon makes up most of the measured LOI (to within about 10%); however, as previously discussed, it is the adsorptive capacity of the fly ash, especially the active carbon sites, for air entraining agents and not the LOI per se, that impacts the marketability of the fly ash. At this time, there is a growing realization that lower LOI fly ashes do not assure superior, or even adequate, air-entraining characteristics for many fly ashes.
Therefore, regardless of the specific reduction of LOI through combustion, it would be desirable to beneficially alter the air-entraining characteristics of the processed fly ash by reducing the overall adsorptive capacity of the fly ash.
A third major issue affecting the utilization and marketability of fly ash also derives from operational changes in the coal-burning process, specifically the presence of residual chemicals and/or particulate matter deposited in or adsorbed on the coal fly ash during the coal-burning operation and/or subsequent flue gas treatment processes, especially those processes intended to reduce air pollution. These changes in coal-burning operations are intended to reduce the emissions of particulate matter; polluting gases, such as sulfur and nitrogen oxides; and heavy metals, such as mercury, or other toxic emissions, especially those considered to be persistent bioaccumulative toxins.
There are several different techniques for the reduction of each of the above pollutants and coal-burning operations often utilize a combination of some or all of these pollution control techniques in order to meet the targeted emission levels. One example of these pollution control techniques, namely, flue gas conditioning, is used to enhance precipitator performance. This technique deliberately deposits foreign chemicals, particularly ammonia, sulfur, and other proprietary chemicals, on the coal fly ash. This technique actually "conditions" the fly ash by coating the particles with these chemicals, changing the surface conductivity and, therefore, the resistivity of the fly ash. These chemicals may also create a space-charge effect and improve the cohesiveness of the fly ash particles.
Injecting these chemicals in the hot flue gases will improve the efficiency of electrostatic precipitators and, therefore, the collection rates for the coal fly ash. However, the collected fly ash will have increased levels of ammonia, sulfur oxides, and/or other residual chemicals which are known to negatively impact the marketability of fly ash as pozzolan at high concentration levels.
Additional pollution control techniques include, but are not limited to, fuel switching and/or blending, the use of low-NOx burners, flue gas treatment to enhance the performance of NOx scrubbers, e.g., selective catalytic reduction (SCR), non-selective catalytic reduction, selective auto-catalytic reduction, etc., as well as the use of flue gas desulfurization (FGD) scrubbers, etc. All these techniques have specific effects on the fly ash which negatively impacts the marketability of fly ash as pozzolan.
For example, there are several dry FGD scrubbing techniques that are used in coal burning operations to decrease the emissions of sulfur oxides, such as lime spray drying, duct sorbent injection, furnace sorbent injection, and circulating fluidized bed combustion. The use of any of these techniques can result in a single, comingled by-product stream consisting of coal fly ash and spent lime sorbent. The general make-up of the residual particulate matter collected following these coal burning and dry FGD scrubbing operations, often generically referred to as "spray dryer material," are a heterogeneous combination of coal fly ash and a blend of calcium sulfate and calcium sulfite compounds.
The chemical composition of spray dryer material residues depends on the sorbent used for desulfurization and the proportion of fly ash collected with the FGD residues. The fly ash in dry FGD materials has similar particle size, particle density, and morphology to those of conventional fly ashes, but FGD materials have lower bulk densities. The difference in bulk density is due to variations in the chemical and mineralogical characteristics of the reacted and unreacted sorbent. Dry FGD materials contain higher concentrations of calcium and sulfur and lower concentrations of silicon, aluminum, and iron than fly ash.
Typically, dry FGD materials usually will not conform to the controlling specifications for pozzolans (e.g., ASTM C-618), due to the varying chemistry and glass content, the presence of high levels of calcium sulfate, and the generally heterogeneous nature of dry FGD materials. Therefore, they cannot be reliably used as pozzolan, especially for pozzolan in concrete structures.
(Please keep in mind, though, that all of the above does not apply to the use of Coal Ash, and of "FGD materials", as raw materials in the making of the Cement itself, as described, for one example in:
West Virginia Coal Association | Pittsburgh Converts Coal Ash and Flue Gas into Cement | Research & Development; concerning: "United States Patent 5,766,339 - Producing Cement from a Flue Gas Desulfurization Waste; 1998; Assignee: Dravo Lime Company, Pittsburgh; Abstract: Cement is produced (from) mixture of a flue gas desulfurization process waste product containing 80-95 percent by weight calcium sulfite hemihydrate and 5-20 percent by weight calcium sulfate hemihydrate, aluminum, iron, silica and carbon (and) wherein said source of aluminum and iron comprises fly ash".)
In addition to the altered by-product particulate matter generated through the use of these various clean air strategies, air emissions from some of these pollution control techniques have in and of themselves resulted in other air pollutants. For example, at many power plants, when flue gas undergoes selective catalytic reduction of NOx, high levels of SO3 are emitted from the stack. The SO3 is visible as a "blue plume" and quickly condenses into a mist of sulfuric acid, damaging the health of humans, animals, and plant life and destroying real property.
Therefore, coal-burning operations employing combinations of certain air pollution control techniques are now being forced to mitigate the unintended consequences of their actions by further altering their operations with additional flue gas treatments to limit emissions of blue plume (SO3) aerosols or other condensable particulate matter yet to be determined and/or publicly reported in the literature.
In summary, coal burning operations have changed and will continue to change in order to comply with federally mandated and/or self-imposed limits on air emissions. These changes in coal-burning operations include, but are not limited to, the use of low-NOx burners; fuel blending/switching, flue gas conditioning with ammonia or sulfur to enhance precipitator performance; flue gas treatment to enhance the performance of NOx scrubbers; and/or FGD scrubbers to reduce the emissions of particulate matter or polluting gases, such as sulfur and nitrogen oxides.
Examples of residual chemicals and foreign particulate matter that may be deposited in or adsorbed on coal fly ash include, but are not limited to: (1) ammonia and/or SO3(solid) from flue gas conditioning; (2) ammonia from NOx reduction scrubbing and/or slip; (3) the chemical residuals from injecting hydrated lime, magnesium hydroxide, sodium bicarbonate carbonate, ammonia, sulfur, sodium bisulfate, magnacite, magnesium silicate, magnesium oxide, etc. for mitigating blue plume, i.e., SO.sub.3(gas); and, (4) mercury-laden sorbents such as activated carbon from mercury scrubbing.
The presence of any of these foreign residual chemicals and/or particulate matter will negatively impact the utilization of fly ash in general and will especially negatively affect the value of fly ash marketed as pozzolan in concrete.
The deterioration of fly ash quality referenced above negatively impacts the value, marketability and, therefore, the utilization of fly ash in the U.S. Specifically, a reduction in the pozzolanic reactivity reduces the strength-producing characteristics; excessive unburned carbon is associated with poor air-entraining characteristics; and chemical residues in the fly ash can negatively impact the marketability of fly ash as pozzolan in these and other ways, creating additional technical and aesthetic concerns.
It would be desirable to economically increase the value and utility of fly ash in the marketplace by improving those characteristics of fly ashes that have been identified by the concrete industry as being deleterious to the production of quality concrete; specifically, it would be desirable to improve pozzolanic reactivity or strength producing characteristics, air-entraining characteristics, and contamination from chemicals used for flue gas treatment.
In addition to the altered by-product particulate matter generated through the use of these various clean air strategies, air emissions from some of these pollution control techniques have in and of themselves resulted in other air pollutants. For example, at many power plants, when flue gas undergoes selective catalytic reduction of NOx, high levels of SO3 are emitted from the stack. The SO3 is visible as a "blue plume" and quickly condenses into a mist of sulfuric acid, damaging the health of humans, animals, and plant life and destroying real property.
Therefore, coal-burning operations employing combinations of certain air pollution control techniques are now being forced to mitigate the unintended consequences of their actions by further altering their operations with additional flue gas treatments to limit emissions of blue plume (SO3) aerosols or other condensable particulate matter yet to be determined and/or publicly reported in the literature.
In summary, coal burning operations have changed and will continue to change in order to comply with federally mandated and/or self-imposed limits on air emissions. These changes in coal-burning operations include, but are not limited to, the use of low-NOx burners; fuel blending/switching, flue gas conditioning with ammonia or sulfur to enhance precipitator performance; flue gas treatment to enhance the performance of NOx scrubbers; and/or FGD scrubbers to reduce the emissions of particulate matter or polluting gases, such as sulfur and nitrogen oxides.
Examples of residual chemicals and foreign particulate matter that may be deposited in or adsorbed on coal fly ash include, but are not limited to: (1) ammonia and/or SO3(solid) from flue gas conditioning; (2) ammonia from NOx reduction scrubbing and/or slip; (3) the chemical residuals from injecting hydrated lime, magnesium hydroxide, sodium bicarbonate carbonate, ammonia, sulfur, sodium bisulfate, magnacite, magnesium silicate, magnesium oxide, etc. for mitigating blue plume, i.e., SO.sub.3(gas); and, (4) mercury-laden sorbents such as activated carbon from mercury scrubbing.
The presence of any of these foreign residual chemicals and/or particulate matter will negatively impact the utilization of fly ash in general and will especially negatively affect the value of fly ash marketed as pozzolan in concrete.
The deterioration of fly ash quality referenced above negatively impacts the value, marketability and, therefore, the utilization of fly ash in the U.S. Specifically, a reduction in the pozzolanic reactivity reduces the strength-producing characteristics; excessive unburned carbon is associated with poor air-entraining characteristics; and chemical residues in the fly ash can negatively impact the marketability of fly ash as pozzolan in these and other ways, creating additional technical and aesthetic concerns.
It would be desirable to economically increase the value and utility of fly ash in the marketplace by improving those characteristics of fly ashes that have been identified by the concrete industry as being deleterious to the production of quality concrete; specifically, it would be desirable to improve pozzolanic reactivity or strength producing characteristics, air-entraining characteristics, and contamination from chemicals used for flue gas treatment.
Summary: A feature of the present disclosure is to provide a method and apparatus to thermally treat and, thereby, beneficially alter certain characteristics of low-Btu value fine particulate matter, especially coal fly ash, increasing the value of the processed fine particulate material, especially as pozzolan, over the value of by-product fly ash which has not been processed or otherwise beneficiated. This process is designed to expose fly ash to high temperatures in order to effect certain physical and/or chemical changes which will increase the pozzolanic reactivity and/or the amount of reactive glass surface area, improve the air-entraining characteristics by decreasing the level of activated carbon, and reduce the presence of chemical residuals deposited in and/or on the fly ash during flue gas treatment."
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And, believe it or not, there is even quite a lot more to it. We included as much as we did to give you some idea of the unintended consequences of the single-minded drive to choke off our supply economical and abundant Coal-based electricity, through overzealous mandate of pollution control regulations.
Fortunately, American ingenuity is irrepressible, and practical-minded folks like the good people at SEFA are driven to find solutions to problems caused them by others.
The point is, though, that Coal Ash is a valuable raw material resource. We can, as herein, even though Ash has been contaminated with things like "unburned carbon" and "residual chemicals", profitably gather it up, clean it up, and, take it to market as a high-performance additive that is "known to increase the long-term durability of concrete"; and, thus, help to safeguard and maximize our collective, national return on investment made in our concrete infrastructure.
We will be returning again to SEFA and their productive recycling of Coal Ash in future reports; but, for now, it is once again demonstrated: Coal Ash, rather than being some sort of hazardous waste we must somehow, at great and inordinate expense, find some way to dispose of to the satisfaction of the concerned but ill-informed among our fellow citizens, is a valuable raw material resource, the better employment of which in Cement and Concrete could help us to build highways and bridges that are longer-lasting and entail, through the replacement of some Portland Cement with Coal Ash, a direct reduction in CO2 emissions related to their construction.