Patent Application
20160151760
This disclosure is related to agents for and improvements in the capture of fly ash (e.g., produced from the combustion of coal) with an electrostatic precipitator.
Electrostatic precipitators (ESPs) have been used in many industries; for example cement, refinery and petrochemical, pulp and paper and power generation. Although the physical operation of a precipitator is simple and essentially the same for each industry, involving particle charging, collection, dislodging and disposal, the sizing of a precipitator is more complex.
The typical equation used in precipitator sizing is the modified Deutsch equation: Efficiency=1−e−(A/V·w)
Particulate resistivity is used to describe the resistance of a medium to the flow of an electrical current. By definition, resistivity, which has units of ohm-cm, is the electrical resistance of a dust sample having a volume of 1 cm3. Resistivity levels are generally broken down into three categories: low; under 1×108 ohm-cm, medium; 1×108 to 2×1011 ohm-cm, and high; above 2×1011 ohm-cm.
Particles in the medium resistivity range are the most acceptable for electrostatic precipitators. Particles in the low range are easily charged; however upon contact with the collecting electrodes, they rapidly lose their negative charge and are re-entrained into the gas stream to either escape or to be recharged by the corona field. Particles in the high resistivity category may cause back corona which is a localized discharge at the collecting electrode due to the surface being coated by a layer of non-conductive material.
Resistivity is influenced by flue gas temperature and conditioning agents, such as flue gas moisture and ash chemistry. Conductive chemical species will tend to reduce resistivity levels while insulating species, such as SiO2, Al2O3 and Ca will tend to increase resistivity. In those cases where high resistivity is encountered, such as the utility industry when low sulfur coal is being fired, flue gas conditioning with SO3 can reduce resistivity to a more optimum value thus reducing the size of the precipitator that is needed.
Electrostatic precipitators are also grouped according to the temperature of the flue gas that enters the ESP: cold-side ESPs are used for flue gas having temperatures of approximately 204° C. (400° F.) or less; hot-side ESPs are used for flue gas having temperatures greater than 300° C. (572° F.).
In describing ESPs installed on industrial and utility boilers, or municipal waste combustors using heat recovery equipment, cold side and hot side also refer to the placement of the ESP in relation to the combustion air preheater. A cold-side ESP is located behind the air preheater, whereas a hot-side ESP is located in front of the air preheater. The air preheater is a tube section that preheats the combustion air used for burning fuel in a boiler. When hot flue gas from an industrial process passes through an air preheater, a heat exchange process occurs whereby heat from the flue gas is transferred to the combustion air stream. The flue gas is therefore “cooled” as it passes through the combustion air preheater. The warmed combustion air is sent to burners, where it is used to burn gas, oil, coal, or other fuel including garbage.
A first embodiment is a composition effective for reducing the particulate resistivity of hydrated lime and capturing acidic gases from flue gas, the composition includes a supported carbonate that comprises 5 wt. % to 50 wt. % of a carbonate and 50 wt. % to 95 wt. % of a support; wherein the carbonate is an alkali metal carbonate selected from the group consisting of a carbonate, a bicarbonate, and a mixture thereof.
Another embodiment is a process of manufacturing a supported carbonate that includes admixing a support and an alkali metal carbonate selected from the group consisting of a carbonate, a bicarbonate, and a mixture thereof; providing sufficient water to the admixture to dissolve at most 50 wt. % of the carbonate; and then removing sufficient water from the admixture to provide a dry, flowable particulate.
Still another embodiment is a process wherein acidic gases are removed from a flue gas, the process including injecting, into the flue gas at a location upstream of an electrostatic precipitator (ESP), a composition that includes a supported carbonate which comprises an alkali metal carbonate selected from the group consisting of a carbonate, a bicarbonate, and a mixture thereof; and which comprises 5 wt. % to 50 wt. % of the alkali metal carbonate and 50 wt. % to 95 wt. % of the support; and then collecting fly ash from the flue gas in the ESP.
Yet another embodiment is a composition for reducing the particulate resistivity of hydrated lime consisting essentially of about 50 wt. % to about 95 wt. % of a phyllosilicate and about 5 wt. % to about 50 wt. % of a supported sodium salt; the sodium salt having an atomic mass percentage that is about 20% to about 60% sodium.
Herein, compositions are disclosed as including about an amount of a first agent and about an amount of a second agent. These amounts, often expressed as ranges, are weight percentages of the listed agents and optionally additional agents. Notably, the compositions herein often include water, for example, incorporated into the composition, as waters of hydration of a specific or multiple agents, or intercalated into porous or layered structured, and, importantly, water is not included in the determination of weight percentages of agents in a composition. That is, the provided weight percentages are the values for the dry (or theoretically dry) composition.
Described herein is a process of enhancing the collection of fly ash without the addition of SO3 to the flue gas. Preferably, the process is essentially free of or completely free of the addition of SO3 to the flue gas; less preferably, the process includes a reduction but not the elimination of the addition of SO3 to the flue gas. The described process includes the reduction of the resistivity of the fly ash and thereby the enhanced collection of the fly ash in an electrostatic precipitator (ESP). Importantly, the process includes the collection of the agent (i.e., the particulate resistivity aid) that affects the resistivity of the fly ash.
As used herein, fly ash has its commonly understood meaning; that is, fly ash is the (silicate, aluminate, and other) non-combustible solid particulates that result from the combustion of fossil fuels, including coal, petroleum, and lignites. The fly ash produced from the combustion process has a resistivity measured in ohm-cm. Herein, the “native fly ash resistivity” is the resistivity of the fly ash after exiting a boiler and before the resistivity is augmented by adding chemicals to the fly ash. That is, the native fly ash resistivity is the resistivity of the produced fly ash as it reaches an ESP taking into account, for example, inline processing units (e.g. selective catalytic reduction (SCR) units) which might affect the resistivity of the fly ash between the boiler and the ESP. As used herein, the “admixture resistivity” is the resistivity of an admixture of the fly ash and the herein described particulate resistivity aid. Notably, native fly ash resistivity and admixture resistivity change as a function of temperature, any comparison between resistivities, be it fly ash resistivities and/or admixture resistivities, are at the same temperature or within a sufficiently small temperature range to negate the effect of temperature on the resistivity.
In a first embodiment, the process of enhancing fly ash collection includes providing a flue gas that includes fly ash and combustion gases from a coal fired boiler; injecting or adding into the flue gas a particulate resistivity aid (e.g., forming an admixture that includes the fly ash and the particulate resistivity aid); and then collecting the fly ash and particulate resistivity aid (the admixture) with a cold side ESP. Preferably, the process enhances the collection of fly ash from the flue gas without adding SO3 to a flue gas.
In another embodiment, the process of enhancing fly ash collection includes providing a flue gas at a temperature of about 120° C. or about 150° C. to about 250° C. or about 300° C., the flue gas including fly ash with a resistivity (native fly ash resistivity) in a range of about 1011 to about 1014 ohm-cm, preferably a resistivity above 2×1011, and combustion gases from a coal fired boiler; injecting into the flue gas a particulate resistivity aid; and then collecting the fly ash and particulate resistivity aid with a cold side ESP. Preferably, the fly ash resistivity is reduced to about 108 to about 1011 ohm-cm or about 2×1011 ohm-cm (admixture resistivity), more preferably the admixture resistivity is below 2×1011 ohm-cm.
In still another embodiment, the process of enhancing fly ash collection can include providing a flue gas that includes fly ash and combustion gases from a coal fired boiler that is burning Powder River Basin coal; injecting into the flue gas a particulate resistivity aid thereby reducing a resistivity of the fly ash by at least about one order of magnitude (ohm-cm); and then collecting the fly ash and particulate resistivity aid with a cold side ESP.
In these embodiments, the process, preferably, reduces particulate emissions (e.g., fly ash emissions) from the ESP by at least about 10%, about 20%, about 30%, about 40%, or about 50%. In multi-field ESPs, the reduction in particulate emissions can be measured after each field. In one preferable example, a first-field ESP collected mass fraction is increased by at least 5%. That is, the percentage of particulates collected by the first-field in the ESP is increased by at least 5% (e.g., from about 90% to about 95%).
In these embodiments, the particulate resistivity aid, preferably, includes a particulate support and a resistivity aid. Preferably, the particulate support carries the resistivity agent, where carrying includes any physio-chemical relationship between the particulate support and the resistivity agent. That is, carrying can include the adhesion of the resistivity agent to a surface of the particulate support, the ionic or electrostatic bonding of the resistivity agent to a surface of the particulate support, the intercalation of the resistivity agent into the particulate support, or into or between layers of the particulate support. Preferably, carrying excludes mixtures of the particulate support and resistivity agent that completely dissociate upon mixing with a gas or dispersion into a gas. Even more preferably, the particulate resistivity aid consists essentially of the particulate support carrying the resistivity agent.
The particulate support can be selected from silicates, aluminates, metal oxides (e.g., transition metal oxides such as titanates, vanadates, tungstates, molybdates, and ferrates; and alkali and/or alkali earth oxides such as calcium oxides), polymeric supports, and mixtures thereof. Examples of particulate supports include but are not limited to phyllosilicates (e.g., vermiculite, montmorillonite, bentonite, and kaolinite) allophane, graphite, quartz, and mixtures thereof.
Preferably, the particulate support does not affect the resistivity of the fly ash, that is, does not affect the native fly ash resistivity. More preferably, the particulate support does not reduce the native fly ash resistivity. Even more preferably, the particulate support does not reduce the native fly ash resistivity by a factor greater than about five when added to the fly ash in an amount less than about 50 wt. %, 25 wt. %, 10 wt. %, 5 wt. %, or 2.5 wt. %. Still more preferably, the particulate support, when free of the resistivity agent, has a particulate support resistivity that is equal to or greater than the native fly ash resistivity.
The particulate resistivity aid includes a resistivity agent carried by the particulate support. The resistivity agent, preferably, affects the resistivity of the fly ash. In an example, an unsupported resistivity agent may be capable of affecting the resistivity of the fly ash but the supported resistivity agent has been found to have an enhanced effect on the resistivity of the fly ash. That is, the activity (as measured in the reduction of the native fly ash resistivity) of the supported resistivity agent is greater than the unsupported resistivity agent on a gram/gram basis of resistivity agent. For example, one kilogram of supported resistivity agent (carried by sufficient quantity of the particulate support) has a greater activity than one kilogram of unsupported resistivity agent. The supported resistivity agent activity is enhanced (when compared to the unsupported resistivity agent activity) despite the resistivity of the particulate support (when free of the resistivity agent). Compositionally, the resistivity agent can include iron, copper, tin, titanium, calcium, sodium, and mixtures thereof. In one preferable example, the resistivity agent includes the sulfide of iron, copper, tin, titanium, calcium, sodium, or mixtures thereof. The sulfide can be a terminal sulfide, a polysulfide, or a thiolate. One particularly preferable combination for the resistivity agent includes copper and sulfur (e.g., a copper sulfide). Another particularly preferable combination for the resistivity agent includes sodium and sulfur (e.g., a sodium sulfide).
One particularly preferable particulate resistivity aid consists of the particulate support carrying a resistivity agent. Here, the particulate support is a phyllosilicate, preferable a bentonite. The resistivity agent can be one or more compounds carried by the phyllosilicate but includes a water-soluble, alkali metal salt. The water-soluble, alkali metal salt can be selected from a sodium salt, a potassium salt, and a mixture thereof; preferably, the water-soluble, alkali metal salt is a sodium salt (e.g., sodium chloride, trona, sodium carbonate, sodium bicarbonate, sodium hydroxide, or mixtures thereof). Notably, the resistivity agent can include, in addition to the water-soluble, alkali metal salt, a transition metal (e.g., a first row transition metal) or a transition metal compound.
Notably, the particulate resistivity aid has a ratio of the particulate support to the resistivity agent. The ratio is, preferably, in a range of about 1:1 (about 50 wt % resistivity agent) to about 99:1 (about 1 wt % resistivity agent) by weight, or in a range of about 4:1 (about 20 wt % resistivity agent) to about 19:1 (about 5 wt % resistivity agent) by weight. For example the particulate resistivity aid can include about 0.5 wt. %, about 1 wt. %, about 2 wt. %, about 3 wt. %, about 4 wt. %, about 5 wt. %, about 10 wt. %, about 15 wt. %, about 20 wt. %, about 25 wt. %, about 30 wt. %, about 35 wt. %, about 40 wt. %, about 45 wt. %, or about 50 wt. % or the resistivity agent.
The manufacture of the particulate resistivity aid can be by any method that provides the resistivity agent carried by the particulate support. One example is an incipient wetness process wherein the resistivity agent and particulate support are sheared with sufficient liquid (preferably water) to facilitate an interaction or reaction between the resistivity agent and particulate support, and then the removal of all or most of the liquid. The particulate resistivity aid is, preferably, not manufactured by the dry blending of the particulate support and the resistivity agent as dry blending procedures typically produce a mixture of the materials not the herein disclosed particulate resistivity aid. In limited circumstances, dry blending is possible when the blended materials are sufficiently solvated (e.g., hydrated) to generate free solvent (water) during the blending process.
The process of enhancing fly ash collection further includes the injection of the particulate resistivity aid into the flue gas. The location for the injection of the particulate resistivity aid can be between an air preheater and the ESP or upstream/before the air preheater. When the particulate resistivity aid is injected before the air heater, the particulate resistivity aid flows through the air preheater before being collected by the ESP.
In one preferable example, the particulate resistivity aid is injected into the fly ash to produce produces an admixture of the fly ash and particulate resistivity aid that includes about 0.1 wt. % to about 5 wt. % or about 0.1 wt % to about 1 wt % of the particulate resistivity aid; for example, an admixture that includes about 0.1 wt. %, about 0.2 wt. %, about 0.3 wt. %, about 0.4 wt. %, about 0.5 wt. %, about 0.6 wt. %, about 0.7 wt. %, about 0.8 wt. %, about 0.9 wt. %, about 1 wt. %, about 1.25 wt. %, about 1.5 wt. %, about 1.75 wt. %, about 2 wt. %, about 2.5 wt. %, about 3 wt. %, about 3.5 wt. %, about 4 wt. %, about 4.5 wt. %, or about 5 wt. % of the particulate resistivity aid. As the fly ash travels through the flue ducts at an average weight per hour, the particulate resistivity aid can be injected into the flue duct and thereby the flue gas and mixed with the fly ash at an average weight per hour to yield the fly ash-particulate resistivity aid mixture that includes about 1 wt. % to about 5 wt. % of the particulate resistivity aid. For example, if 80 kg of fly ash is produce per hour by a coal fired boiler, the particulate resistivity aid can be injected into the flue duct carrying the fly ash at a rate of about 0.8 kg (about 1 wt. %) to about 4 kg (about 5 wt. %) per hour.
Yet another embodiment includes a composition effective for reducing the particulate resistivity of hydrated lime and capturing acidic gases from flue gas. This composition can include, consist essentially of, or consist of a supported carbonate. The supported carbonate is, preferably, a composition of about 5 wt. % to about 50 wt. % of a carbonate and about 50 wt. % to about 95 wt. % of a support, where the carbonate is carried by (e.g., chemabsorbed to) the support. The supported carbonate can be a composition that includes 10 wt. % to 50 wt. % of the alkali metal carbonate and 50 wt. % to 90 wt. % of the support; 20 wt. % to 40 wt. % of the alkali metal carbonate and 60 wt. % to 80 wt. % of the support; or 25 wt. % to 35 wt. % of the alkali metal carbonate and 65 wt. % to 75 wt. % of the support.
The carbonate is, preferably, an alkali metal carbonate. Herein, the term alkali metal carbonate includes carbonates, bicarbonates, and a mixture thereof; that is alkali metal carbonate is used as a generic term for alkali metal salts (e.g., lithium salt, sodium salt, potassium salt) that include a carbonate anion. In another instance, the carbonate can be an alkali earth metal carbonate; that is an alkali earth metal salt (e.g., a magnesium salt, a calcium salt) that includes a carbonate anion.
The support is, preferably, a solid phase temperature resistant material. Examples of supports include phyllosilicates, silicates, aluminates, aluminosilicates, graphite, activated carbon, fly ash, transition metal oxides, and mixtures thereof. In a preferable example, the support is selected from the group consisting of a silicate, an aluminate, an aluminosilicate, and a mixture thereof. In a more preferable example, the support is selected from the group consisting of bentonite, montmorillonite, kaolinite, and a mixture thereof.
In one instance, the alkali metal carbonate is a sodium salt. For example the alkali metal carbonate can be selected from the group consisting of sodium sesquicarbonate (e.g., trona), sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), and a mixture thereof. Preferably, the alkali metal carbonate is sodium sesquicarbonate or sodium carbonate; more preferably, the alkali metal carbonate is sodium carbonate; even more preferably the alkali metal carbonate is sodium sesquicarbonate.
In another preferable example, the supported carbonate consists essentially of a support selected from the group consisting of a silicate, an aluminate, an aluminosilicate, and a mixture thereof; and an alkali metal carbonate selected from the group consisting of sodium sesquicarbonate, sodium carbonate, sodium bicarbonate, and a mixture thereof. In one instance the support is selected from the group consisting of bentonite, montmorillonite, kaolinite, and a mixture thereof. In another instance, the support is a synthetic silicate, aluminate or aluminosilicate. In still another instance, the support is a phyllosillicate.
In still another example, the composition is an admixture of the supported carbonate and hydrated lime. In this example, the composition can be an admixture that includes 1 wt. % to 25 wt. % of the supported carbonate; 5 wt. % to 20 wt. % of the supported carbonate; or 5 wt. % to 15 wt. % of the supported carbonate. In one preferable instance, the composition is an admixture that consists essentially of, or consists of, 1 wt. % to 25 wt. % of the supported carbonate and the hydrated lime.
One particularly preferable aspect of the admixture is approximately consistent particle sizes and densities. That is, in a preferable example, the particulates that make up the admixture each, individually (e.g., the supported carbonate and the hydrated lime), have approximately equal particle sizes. Again, in a preferable example, the particulates that make up the admixture each, individually (e.g., the supported carbonate and the hydrated lime), have approximately equal densities. More preferably, the particulates that make up the admixture each, individually (e.g., the supported carbonate and the hydrated lime), have approximately equal particle sizes and densities. Herein, an admixture where the supported carbonate and the hydrated lime have approximately equal particle sizes would, preferably, yield a single Gaussian distribution of particles sizes by common techniques.
Still another embodiment is the process of manufacturing the supported carbonate. The manufacturing process can include admixing the support and an alkali metal carbonate. Preferably, the admixing comprises mechanically shearing the support and the alkali metal carbonate. Mechanical shearing methods may employ extruders, injection molding machines, Banbury® type mixers, Brabender® type mixers, pin-mixers, and the like. Shearing also can be achieved by introducing materials at one end of an extruder (single or double screw) and receiving the sheared material at the other end of the extruder. Optionally, materials can be added at intermediate locations in the extruder. The temperature of the materials entering the extruder, the temperature of the extruder, the concentration of materials added to the extruder, the amount of water added to the extruder, the length of the extruder, residence time of the materials in the extruder, and the design of the extruder (single screw, twin screw, number of flights per unit length, channel depth, flight clearance, mixing zone, etc.) are several variables which control the amount of shear applied to the materials.
In this process, the alkali metal carbonate is preferably selected from the group consisting of a carbonate, a bicarbonate, and a mixture thereof.
The process further includes providing sufficient water to the admixture to dissolve at most 50 wt. % of the carbonate. More preferably, sufficient water is added to the admixture to dissolve at most 25 wt. % of the alkali metal carbonate; even more preferably, sufficient water is added to the admixture to dissolve at most 10 wt. % of the alkali metal carbonate.
Following the admixing, the process includes removing sufficient water from the admixture to provide a dry, flowable particulate. That is, sufficient water is removed from the admixture to “dry” the admixture to a moisture content where particulate portions of the admixture do not adhere to each other. Notably, a dry admixture can include an amount of water that is insufficient to cause agglomeration of the particulates.
Yet another embodiment is a process wherein acidic gases are removed from a flue gas. This process includes injecting, into the flue gas at a location upstream of an electrostatic precipitator (ESP), a composition that includes the above-described, supported carbonate. The process then includes collecting fly ash from the flue gas in the ESP.
In one instance, the alkali metal carbonate is selected from the group consisting of a carbonate, a bicarbonate, and a mixture thereof. In another instance, the supported carbonate includes 5 wt. % to 50 wt. % of the alkali metal carbonate and 50 wt. % to 95 wt. % of the support.
In one example of this process, the composition injected into the flue gas is an admixture of the supported carbonate and hydrated lime. In this example, this admixture comprises 1 wt. % to 25 wt. % of the supported carbonate, 2 wt. % to 22 wt. % of the supported carbonate, 5 wt. % to 20 wt. % of the supported carbonate, 5 wt. % to 15 wt. % of the supported carbonate, or 5 wt. % to 10 wt. % of the supported carbonate. In another example of this process, the composition injected into the flue gas consists essentially of the supported carbonate and the process further include injecting hydrated lime into the flue gas. In this example, the supported carbonate and the hydrated lime mix in the flue gas and can be collected at the ESP.
Still yet another embodiment is a composition for reducing the particulate resistivity of hydrated lime that is, is essentially, or consists essentially of a phyllosilicate and a supported sodium salt. Preferably, the composition includes about 50 wt. % to about 95 wt. % of a phyllosilicate and about 5 wt. % to about 50 wt. % of a supported sodium salt and/or lithium salt. Preferably, the sodium salt has an atomic mass percentage that is about 20% to about 60% sodium; the lithium salt has an atomic mass percentage that is about 5% to about 30% lithium. In one preferable example, the phyllosilicate is selected from the group consisting of bentonite, montmorillonite, kaolinite, and a mixture thereof. In another preferable example, the sodium salt is selected from the group consisting of sodium chloride, sodium bromide, sodium hydroxide, sodium carbonate, sodium bicarbonate, and a mixture thereof. In a more preferable example the sodium salt is selected from the group consisting of sodium hydroxide, sodium carbonate, sodium bicarbonate, and a mixture thereof. In a still more preferable example, the sodium salt is free of halides. In still another example the lithium salt is selected from the group consisting of lithium chloride, lithium bromide, lithium hydroxide, lithium carbonate, lithium bicarbonate, and a mixture thereof. In a more preferable example the lithium salt is selected from the group consisting of lithium hydroxide, lithium carbonate, lithium bicarbonate, and a mixture thereof. In a still more preferable example, the lithium salt is free of halides.
This disclosure claims the benefit of priority to U.S. Pat. No. 13/945,304, filed Jul. 18, 2013, the disclosure of which is incorporated herein in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US13/77943 | 12/27/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13945304 | Jul 2013 | US |
| Child | 14905555 | US |
