PROCESS FOR OPERATING A COAL-FIRED UTILITY BOILER

Abstract

A process for operating a coal-fired utility boiler. The process has the steps of (a) providing coal and one or more additives in proximity to one or more burners in the boiler; (b) providing air to the boiler; and (c) burning the coal and the one or more additives in the boiler to generate heat and an exhaust gas. Also disclosed are a burner and a coal-fired boiler.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority based on U.S. Provisional Application No. 61/425,894, filed Dec. 22, 2010, and U.S. Provisional Application No. 61/349,452, filed May 28, 2010, both of which are incorporated herein by reference in their entireties.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates to a process for operating a coal-fired utility boiler. The disclosure further relates to a burner useful in a coal-fired utility boiler. The disclosure further relates to a coal-fired utility boiler having a burner therein.

2. Description of the Related Art

Utility boilers or furnaces are employed in industry for generation of heat, production of steam, and generation of electricity utilizing steam. Utility boilers typically have a furnace therein where a fuel is oxidized or burned to generate heat. Along with generating heat, utility boilers will generate or evolve an exhaust gas and/or flue gas that will contain carbon dioxide, residual oxygen (unreacted), inert air components, i.e., nitrogen and argon, and emissions, such as sulfur-based and nitrogen-based compounds. Exhaust gas and/or flue gas is typically treated and then vented to the atmosphere.

A variety of problems are encountered when operating utility boilers. Such problems generally relate to slag deposition, fouling, cleanliness, efficiency, and emissions.

Emissions problems relate to sulfur-based emissions, such as sulfur dioxide (SO₂), sulfur trioxide (SO₃), and sulfuric acid (H₂SO₄); nitrogen-based emissions (NO_x), such as nitrous oxide (NO) and nitrogen dioxide (NO₂); mercury-based emissions (Hg) and particulates. Free sulfur trioxide in an exhaust gas and/or a flue gas imparts an undesirable opaque appearance (a blue haze or trailing plume) to the gas when vented to the atmosphere. Free sulfuric acid can cause corrosion of process surfaces in utility boilers as well as acid rain in the atmosphere. Particulate emissions are made up of unburned carbon and ash. Unburned carbon is formed when burning of oil in a boiler is incomplete. Particulates have to be captured and/or vented the atmosphere. Particulates also absorb and transport free acid. Ash is naturally present in coal. Particulates also present cleanliness and industrial hygiene problems.

Slag deposition can take the form of one or more layers caked/baked onto process surfaces. The one or more layers typically contain silica, aluminum, calcium, and metal complexes of vanadium with sodium, nickel, iron, or magnesium. Slag can deposit on the surfaces of tube bundles or other heat transfer devices within the utility boiler denuding a boiler's heat transfer efficiency.

Chemical additives have been employed in the art in utility boilers to reduce or minimize slag deposition, fouling, cleanliness, efficiency, and emissions in utility boilers and exhaust gas or flue gas. Such additives have been added directly to utility boilers and/or to fuel intended for such boilers.

Addition of chemical additives to coal-fired utility additives has proven problematic. Additives are frequently used in a liquid form for ease of use and precise metering of amounts added. When liquid-based additives are sprayed onto feed coal, the additives are usually absorbed or adsorbed into the feed coal. When the treated feed coal reaches the boiler, the additive may volatilize prior to the combustion of the char or matrix of the coal. Thus, the additive is substantially unavailable to assist in or to otherwise be present during oxidation or burning of the coal. Direct introduction of additives into coal-fired boilers has likewise heretofore been ineffective due to non-homogeneous or ineffectual distribution of additives within the boiler. The potential benefits of the additives have been compromised or denuded by such non-homogeneous or ineffectual distribution and/or have required excess make-up amounts of additives.

It would be desirable to have a process for operating a coal-fired utility boiler in which additives can be effectively introduced such that slag deposition, fouling, cleanliness, and undesirable emissions can be reduced or minimized. It would also be desirable to have a process for operating a coal-fired utility boiler that affords enhanced operational efficiency.

SUMMARY OF THE DISCLOSURE

According to the present disclosure, there is a process for operating a coal-fired utility boiler. The process has the steps of (a) providing coal and one or more additives in proximity to one or more burners in the boiler; (b) providing air to the boiler; and (c) burning the coal and the one or more additives in the boiler to generate heat and an exhaust gas. Useful functionalities for the one or more additives include (i) reduction in slag formation, (ii) generation of oxygen, (iii) reduction in acid formation, (iv) reduced fouling, (v) reduction in sulfur compound formation, (vi) reduction in nitrogen compound formation, (vii) reduction in particulate formation, and (viii) capture of heavy metals.

Further according to the present disclosure, there is a burner. The burner has a first conduit for transporting a fuel; a second conduit for transporting air; a third conduit for transporting an additive; and an igniter adapted to initiate combustion of the fuel and the air. The first conduit, the second conduit, and the third conduit are longitudinally coextensive with each other. The third conduit is positioned within the first conduit and the second conduit. Either the first conduit or the second conduit is positioned within the other. The burner is adapted to maintain a continuous flame after initiation of combustion.

Further according to the present disclosure, there is a coal-fired utility boiler. The boiler has a furnace chamber adapted to combust coal and air and one or more burners positioned within the chamber. The one or more burners each include a first conduit for transporting the coal, a second conduit for transporting air, a third conduit for transporting an additive, and an igniter adapted to initiate combustion of the fuel and the air. The first conduit, the second conduit, and the third conduit direct the coal, the air, and the additive proximal to the igniter. The one or more burners each are adapted to maintain a continuous flame after initiation of combustion.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of a boiler system useful in carrying out the process of the present disclosure.

FIG. 2 is a cross-sectional view of an embodiment of a burner of the present disclosure.

FIG. 3 is a cross-sectional view of the burner of FIG. 2 showing the burner in operation with a flame envelope.

FIG. 4 is a perspective view of another embodiment of a burner of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

In the present disclosure, additives are introduced into the boiler in proximity to the burner(s), i.e., in proximity to the flame(s) (jet(s)) thereof to reduce slag deposition, fouling, amount of undesirable emissions, and enhance cleanliness. Introduction of additives in proximity to the burner substantially prevents premature volatilization of the additives (compared to pretreatment of coal) and allows the additives to be present and/or burned substantially concurrently with the char or matrix of the coal. Further, introduction of additives in proximity to the burner substantially mitigates the effects of non-homogeneous or ineffectual distribution of additives within the boiler as the additives are readily available for burning and/or chemical treatment or modification of the coal or its combustion products and byproducts as the coal is burned. The mitigation of the effects of the non-homogeneous or ineffectual distribution of additives also reduces or eliminates the amount of make-up additives required.

Preferably, the additives are introduced directly to the flame(s) produced by the burner(s). More preferably, additives are injected directly into or on the flame(s) by means of lances. In a preferred embodiment, additives are injected in liquid form through a hollow lance or probe within a lance. The lance or probe preferably has an atomizer at its distal end to effect spraying into or on the flame(s).

The additives are most preferably injected into the internal recirculation zone (IRZ) of the flame. The IRZ typically extends about 0.5 diameters to about 4 diameters downstream and preferably about 0.5 diameters to about 2 diameters downstream of the burner. The term “diameters” correlates to the diameter of the burner throat. Injection of additives into the IRZ functions to stabilize the flame and promote complete combustion of the coal. An IRZ can be formed by burner aerodynamic hardware, e.g., a swirler, and impeller, and a flame stabilizer. The location of an IRZ relative to an embodiment of a burner is depicted by way of illustration in FIG. 3.

An embodiment of the process of the present disclosure is set forth in FIG. 1 in the form of a boiler system 10. System 10 has a boiler 12. Feed stream 14 provides a conduit for feeding a fuel, a first additive, a second additive to boiler 12 through burner 17. Feed streams 20 and 22 provide conduits for feeding water and air, respectively, into boiler 12. Burner 17 has an igniter (not shown) for the purpose of initiating a flame. Exit stream 24 delivers steam produced in boiler 12. Exit stream 26 delivers exhaust gas.

An embodiment of a burner is generally referenced by the numeral 40 and is shown in cross-section in FIG. 2 and in operation in FIG. 3. Burner 40 has an inlet tube 42, which has a lance 44 therein and extending longitudinally therealong. Burner 40 has a coal head 46 and a throat 48 through which coal (in pulverized or dust form) and air enter burner 40, respectively. Throat 48 extends radially and circumferentially and has a plurality of blades 60 that are adjustable and function to spin air to assist in the formation of an internal recirculation zone (IRZ). Throat 48 has a shroud 58 that can be actuated as desired to partially or completely cover throat 48. FIGS. 2 and 3 show shroud 58 in closed position. Coal dust and air are directed toward the distal end or flame area of burner 40 by fuel conduit 49. Inlet tube 42 and lance 44 extend longitudinally within the fuel conduit 49. FIG. 2 depicts the direction and movement of pulverized coal and air through burner 40. One or more additives 50 are sprayed into an IRZ at the distal end of burner 40 via lance 44. An igniter (not shown) ignites the pulverized coal and the one or more additives 50 to form a flame, which in shown in FIG. 3 as flame envelope 55. Alternately, the one or more additives 50 can be sprayed into the flame through an igniter instead of or in addition to lance 44. A plurality of nozzle blades 64 is adjustable, extend radially and circumferentially, and function to spin the coal dust so as to shape the flame. A plurality of stabilizer blades 66 is adjustable, extend radially and circumferentially, and function to spin the air so as to shape the flame and enhance formation of an IRZ. Stabilizer blades 66 are depicted in FIGS. 2 and 3. Burner throat 51 extends circumferentially and defines a burner throat diameter (BTD). The formation of IRZ 54 with a corresponding low pressure zone 52 (shown in sketch in FIG. 2 and shown in full in FIG. 3) at the distal end of lance 44 allows for high levels of combustion and efficient operation of the boiler.

Another embodiment of a burner is generally referenced by the numeral 140 and is shown in perspective in FIG. 4. Burner 140 is similar to burner 40 of FIGS. 2 and 3. Burner 140 has an inlet tube (not shown), which has a lance therein (not shown) and extending longitudinally therealong. Burner 140 has a coal head 146 and a throat 148 through which coal (in pulverized or dust form) and air enter burner 140, respectively. Throat 148 extends radially and circumferentially and has a plurality of blades 160 that are adjustable and function to spin air to assist in the formation of an IRZ. Throat 148 has a shroud 158 that can be actuated as desired to partially or completely cover throat 148. FIG. 3 shows shroud 158 in open position. Pulverized coal and air are directed toward the distal end or flame area of burner 140 by fuel conduit 149. The inlet tube and the lance extend longitudinally within fuel conduit 149. One or more additives are sprayed into a flame (not shown) at the distal end of burner 140 via the lance. Igniter 162 ignites the pulverized coal and the one or more additives to form the flame. Alternately, the one or more additives can be sprayed into the flame through igniter 162 instead of or in addition to lance 44. A plurality of nozzle blades 164 is adjustable, extend radially and circumferentially, and function to spin the pulverized coal so as to shape the flame. A plurality of stabilizer blades (not shown) can be situated radially outside of blades 164. The stabilizer blades can be adjustable, extend radially and circumferentially, and function to spin the air so as to shape the flame and enhance formation of an IRZ. Burner throat 151 extends circumferentially and defines a burner throat diameter (BTD).

Various additives are employed in the present disclosure for controlling slag formation, fouling, emissions levels, and the like. Additives useful in controlling such process variables include metal compounds of lithium, magnesium, calcium, selenium, manganese, iron, cerium, copper, platinum, aluminum, and zirconium. Useful compounds include the following: oxides, carbonates, carboxylates, salicylates, naphthenates, and sulfonates, hydroxides, salicylates, nitrates, borates, and bromides.

Slag control agents can be employed in the process of the present disclosure to prevent buildup of slag deposits within the furnace of the utility boiler and other process surfaces during the combustion of coal. Conversion of undesirable vanadium compounds, such as vanadium pentoxide and sodium vanadium pentoxide, to more innocuous vanadium compounds helps to prevent or reduce catalysis of sulfur dioxide to sulfur trioxide, corrosion of process surfaces due to acid exposure, and deposition of vanadium compounds on process surfaces inside the utility boiler.

Useful slag control agents include, but are not limited to, the following: magnesium hydroxide; magnesium oxide; magnesium carbonate; and magnesium organometallic compounds, such as magnesium carboxylate, magnesium salicylate, magnesium naphthenate, and magnesium sulfonate. Preferred slag control agents are magnesium hydroxide, magnesium oxide, and organometallic magnesium carboxylate with magnesium carbonate overlay.

Oxygen-generating agents can be employed in the process of the present disclosure to provide additional oxygen at the situs of oxidation or burning in the furnace, which allows the percent of excess air, i.e., excess oxygen, supplied to the utility boiler to be reduced and/or minimized. Lower excess air content reduces the amount of SO₃ and NO_x formed. Use of the oxygen-generating agent also reduces the incidence of unburned carbon, i.e., reduces incidence of particulate formation, due to more efficient combustion or burning. Use of the oxygen-generating agent also reduces the amount of coal required to produce a given amount of energy hence enhancing power generation efficiency and economy and lowering per capita emission levels. Reduction of unburned carbon also reduces the incidence and retention of sulfuric acid, which is absorbed by unburned carbon.

Useful oxygen-generating agents include, but are not limited to, the following: calcium nitrate, calcium organometallic compounds, calcium salicylate, calcium sulfonate, overbased calcium carboxylate, iron oxides, iron carboxylates, iron organometallic compounds, iron sulfonates, barium oxide, barium carbonate, barium carboxylate, barium organometallic compounds, and barium sulfonate. Preferred oxygen-generating agents are the calcium compounds. Most preferred oxygen-generating agents are calcium nitrate and calcium carboxylate.

Acid mitigation agents can be employed in the process of the present disclosure to reduce or minimize the amount of acidic compounds in the boiler and the exhaust gas and/or flue gas. Particularly, the agent reacts with sulfuric acid to form innocuous, non-acidic compounds thereby reducing acid emissions in the exhaust gas and/or flue gas and corrosion of process surfaces within the boiler. Acid mitigation agents can either neutralize or absorb/adsorb acids. Examples of acid mitigation agents include magnesium oxide, magnesium hydroxide, magnesium carbonate, sodium bicarbonate carbonate, and calcium carbonate.

Fouling prevention agents can be employed in the process of the present disclosure to reduce or minimize buildup on process surfaces within the boiler and maintain operational efficiency. Examples of fouling prevention agents include magnesium oxide, magnesium hydroxide, magnesium carbonate, and sodium borate.

Oxidizer agents can be employed in the process of the present disclosure to (i) reduce or minimize excessive air addition to the furnace and (ii) help convert mercury and heavy metal constituents to an oxidized form that is easier to capture. Examples of oxidizer agents include calcium bromide, calcium chloride, and sodium bromide.

Heavy metal capture agents can be employed in the process of the present disclosure to reduce or minimize mercury emissions. A preferred heavy metal capture agent is a mercury capture agent. Examples of mercury capture agents include calcium sulfide, calcium polysulfide, and sodium sulfide. Heavy metal capture agents are preferably incorporated outside the boiler into one of its auxiliary devices. For example, the agent can be injected into the exhaust system or added to a flue gas desulfurization unit. The agent can take the form of a dry or wet system.

The additives can be used in any known product form, such as a mineral ore, a powder, or liquid. Liquids may be water-based, oil-based, or a combination thereof. Liquids may take any known liquid form, such as solutions, slurries, suspensions, dispersions, or emulsions. Liquid forms are preferred since they can be injected or sprayed with precision via conventional pumping and metering devices. A preferred means of adding additives into the burner is via injection in liquid form in proximity to a burner.

The amount of slag control agent employed can vary depending upon a variety of process and composition conditions. Typically conditions include type of slag control agent selected, load or feed rate of the fuel, amount and type of oxygen-generating agent used, amount or feed rate of air, and impurity composition of the fuel. When a liquid form of the slag control agent is used, the amount employed will typically vary from about 1:2000 to about 1:6000 agent:agent, volume:volume.

The amount of slag control agent or other additives employed in coal can be expressed in terms of parts per million (ppm) and weight percent. The amount of slag control agent(s) employed can also be expressed in terms of “times stoichiometry” in reference to a certain emission, such as SO₃. Dosage typically ranges from about 100 ppm to about 5 weight percent based on the weight of the coal and may vary depending on the type of additive.

The amount of oxygen-generating agent employed can vary depending upon process and composition conditions. Conditions include type of oxygen-generating agent selected, load or feed rate of coal, amount and type of slag control agent used, amount or feed rate of air, and impurity composition of the coal. When a liquid form of the oxygen-generating agent is used, the amount employed will typically vary from about 1:1000 to about 1:10000 and preferably about 1:2500 to about 1:4000 agent:coal, volume:volume. Expressed as a function of weight, the amount of slag control agent(s) employed typically varies from about 25 ppm to about 3 weight percent.

In the present disclosure, additives are introduced to a boiler system in proximity to the burners. It is possible, however, to add other additives (of the same or different composition) to other areas of the boiler or to an auxiliary device thereof. An auxiliary device is an inlet or outlet apparatus or component of a boiler outside of the furnace or direct heating section thereof. For instance, heavy metal capture agents, such as mercury capturing agents, are sometimes added at an exhaust system and/or a flue gas desulfurization unit as an alternative to, or in addition to, introduction into a furnace or direct heating section thereof.

Coal useful in the process of the present disclosure can be of any form known in the art, such as anthracite, bituminous, sub-bituminous, lignite, pet coke, and charcoal. Coal is preferably pulverized prior to introduction into the boiler.

The burner(s) can be fueled by any known useful burner fuel, such as a flammable gas or fuel oil. Useful flammable gases include natural gas, propane, and butane.

Conventional process components and equipment may be utilized to control and vary the feed rates of the coal and/or additives. Examples of useful process components and equipment include, but are not limited to, flow limiting/controlling devices such as valves; pumps; fans; and conveyor belts.

The process of the present disclosure is preferably carried out substantially continuously.

Additional teachings regarding the use of additives in coal-fired utility boilers are found in U.S. Ser. No. 12/319,994, filed Jan. 14, 2009, which is incorporated herein by reference in their entireties.

U.S. Provisional Patent Application No. 61/349,452, filed May 28, 2010, entitled “A Process for Operating a Coal-fired Utility Boiler”, is incorporated herein by reference.

It should be understood that the foregoing description is only illustrative of the present disclosure. Various alternatives and modifications can be devised by those skilled in the art without departing from the disclosure. Accordingly, the present disclosure is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.

Claims

1. A process for operating a coal-fired utility boiler, comprising:

a) providing coal and one or more additives in proximity to one or more burners in the boiler;

b) providing air to the boiler; and

c) burning the coal and the one or more additives in the boiler to generate heat and an exhaust gas.

2. The process of claim 1, wherein the one or more additives are provided directly to one or more flames produced by the one or more burners.

3. The process of claim 2, wherein one or more burners form one or more internal recirculation zones when lit into which the one or more additives are directed.

4. The process of claim 2, wherein the one or more additives are provided by transmission through one or more hollow lances.

5. The process of claim 4, wherein the one or more additives are provided by transmission through one or more hollow lances extending longitudinally within and through the one or more lances.

6. The process of claim 1, wherein the one or more additives are provided by transmission through one or more lances.

7. The process of claim 1, wherein the one or more additives are provided in a liquid form.

8. The process of claim 2, wherein the one or more additives are provided in a liquid form.

9. The process of claim 1, wherein the one or more additives are provided in a solid form.

10. The process of claim 1, wherein the one or more additives provide one or more functionalities selected from the group consisting of (i) reduction in slag formation in the boiler, (ii) generation of oxygen in the boiler, (iii) reduction in acid formation in the boiler and the exhaust gas, (iv) reduced fouling in the boiler, (v) reduction in sulfur compound formation in the boiler and the exhaust gas, (vi) reduction in nitrogen compound formation in the boiler and the exhaust gas, (vii) reduction in particulate formation in the boiler and the exhaust gas, and (viii) capture of heavy metals in the boiler and the exhaust gas.

11. The process of claim 10, wherein the one of more functionalities provided is (ii) generation of oxygen and (vii) reduction in particulate formation.

12. The process of claim 11, wherein the one or more additives is one or more oxygen-generating agents selected from the group consisting of calcium hydroxide, calcium oxide, calcium carbonate, calcium carboxylate, calcium organometallic compounds, calcium sulfonate, iron hydroxides, iron oxides, iron carbonates, iron carboxylates, iron organometallic compounds, iron sulfonates, barium hydroxide, barium oxide, barium carbonate, barium carboxylate, barium organometallic compounds, and barium sulfonate.

13. The process of claim 10, wherein the one or more additives is one or more slag control agents selected from the group consisting of magnesium hydroxide, magnesium oxide, magnesium carbonate, magnesium organometallic compounds, magnesium carboxylate, magnesium salicylate, magnesium napthenate, and magnesium sulfonate.

14. The process of claim 10, wherein the one or more additives is one or more acid mitigation agents selected from the group consisting of magnesium oxide, magnesium hydroxide, magnesium carbonate, sodium bicarbonate carbonate, and calcium carbonate.

15. The process of claim 10, wherein the one or more additives is one or more fouling prevention agents selected from the group consisting of magnesium oxide, magnesium hydroxide, magnesium carbonate, and sodium borate.

16. The process of claim 10, wherein the one or more additives is one or more heavy metal capturing agents, wherein the one or more heavy metal capturing agents is one or more mercury capturing agents.

17. The process of claim 16, wherein the one or more additives is one or more mercury capturing agents is selected from the group consisting of calcium sulfide, calcium polysulfide, and sodium sulfide.

18. The process of claim 1, wherein the coal is provided via a first conduit, wherein the air is provided via a second conduit, wherein the one or more additives is provided via a third conduit, wherein the first conduit, the second conduit, and the third conduit direct the coal, the air, and the one or more additives proximal to one or more flames produced by the one or more burners.

19. The process of claim 18, wherein the first conduit, the second conduit, and the third conduit direct the coal, the air, and the one or more additives directly to one or more flames produced by the one or more burners.

20. A burner, comprising:

a first conduit for transporting a fuel;

a second conduit for transporting air;

a third conduit for transporting an additive; and

an igniter adapted to initiate combustion of the fuel and the air,

wherein the first conduit, the second conduit, and the third conduit are longitudinally coextensive with each other, wherein the third conduit is positioned within the first conduit and the second conduit, wherein either the first conduit or the second conduit are positioned within the other, wherein the burner is adapted to maintain a continuous flame after initiation of combustion.

21. The burner of claim 20, wherein the first conduit is positioned within the second conduit.

22. The burner of claim 20, wherein the fuel is coal.

23. The burner of claim 20, wherein the burner is adapted to form an internal recirculation zone after initiation of combustion.

24. A coal-fired utility boiler, comprising:

a furnace chamber adapted to combust coal and air, and

one or more burners positioned within the chamber,

wherein the one or more burners each include a first conduit for transporting the coal, a second conduit for transporting air, a third conduit for transporting an additive, and an igniter adapted to initiate combustion of the fuel and the air, wherein the first conduit, the second conduit, and the third conduit direct the coal, the air, and the additive proximal to the igniter, wherein the one or more burners each are adapted to maintain a continuous flame after initiation of combustion.

25. The boiler of claim 24, wherein the first conduit, the second conduit, and the third conduit are longitudinally coextensive with each other, wherein the third conduit is positioned within the first conduit and the second conduit, wherein either the first conduit or the second conduit are positioned within the other.