MXPA06010738A - Bromine addition for the improved removal of mercury from flue gas - Google Patents

Abstract

Bromine-containing compounds (10), added to the coal (16), or to the boiler combustion furnace (14), are used to enhance the oxidation of mercury, thereby enhancing the overall removal of mercury in downstream pollution control devices. The method is applicable to utility power plants equipped with wet FGD systems, as well as those plants equipped with spray dryer absorber FGD systems.

Description

ADDITION OF BROMINE FOR IMPROVED REMOVAL OF MERCURY FROM CHIMNEY GAS FIELD AND BACKGROUND OF THE INVENTION The Emission Standards, as set forth in articles in Amendments to the Clean Air Act of 1990 as established by the Environmental Protection Agency of the United States (EPA), required the estimation of hazardous air pollutants from power plants. In December 2000, EPA announced its intention to regulate mercury emissions from service kettles heated with coal. Service kettles heated with coal are a major known source of anthropogenic mercury emissions in the United States. Elemental mercury and many of its compounds are volatile and will therefore leave the kettle as constituents in very small quantities in the flue gases of the kettle. Some of these mercury constituents are insoluble in water, which makes them difficult to capture in conventional wet and dry scrubbers. So new methods and processes are needed to calculate these constituents in very small amounts of the flue gases. kettle. Mercury appears in flue gases from the combustion of mineral coal in both solid and gas phases (mercury confined to particles and mercury vapor phase, respectively). The so-called mercury confined to particles is actually mercury vapor phase adsorbed on the surface of ash or carbon particles. Due to the high volatility of mercury and - many of its compounds, the majority of mercury found in flue gases is mercury in the vapor phase. Mercury in the vapor phase can manifest as elemental mercury (metallic mercury vapor, elemental) or as oxidized mercury (vapor phase species of various mercury compounds). Speciation, which refers to the form of mercury present, is a key parameter in the development and design of mercury control strategies. All efforts to devise new control strategies for mercury emissions from power plants should focus on this characteristic of mercury. Particle collectors in use in electric service plants, most commonly electrostatic precipitators (ESP) or cloth filters (FF), sometimes called household bags, provide high-efficiency removal of mercury confined to particles. Fabric filters tend to exhibit better particulate-confined mercury removal than ESPs by providing a filter cake on which particulate mercury is trapped as the flue gas passes through the filter cake. If the filter cake also contains constituents that will react with the mercury such as unreacted carbon or even activated carbon, then the filter cake can act as a site to facilitate the gas-solid reactions between the gaseous mercury and the carbon particles. solid. If a power plant is equipped with a Chimney Gas Desulfurization (FGD) system then either wet scrubbers or spray dry absorbers (SDA) can remove significant amounts of oxidized mercury. Oxidized mercury, which typically manifests in the form of mercury chloride, is soluble in water, making it treatable to removal in sulfur dioxide scrubbers. Elemental mercury, insoluble in water, is less likely to be purified in conventional scrubbers. The removal of elemental mercury, therefore, remains a major problem in the search for cost-effective mercury control techniques. Numerous studies have been, and continue to be, conducted to develop cost-effective procedures for the control of elemental mercury. Many of the studies have focused on the injection of a carbonic sorbent (for example, pulverized activated carbon)., or PAC) in the upstream of the stack gas from the particle collector to adsorb the mercury in the vapor phase. The sorbent, and its charge of adsorbed mercury, are subsequently removed from the flue gases in a downstream particulate trap. Adsorption is a technique that has often been successfully applied for the separation and removal of tiny amounts of undesirable components. The PAC injection is used, commercially, to remove mercury from the exhaust gases of the municipal waste combustion chamber. The PAC injection removes both oxidized and elemental mercury species, although the efficiencies of the removal are higher for the oxidized form. Although this procedure appears attractive at early work, the economy of high injection rates can be prohibitive when applied to service plants heated with coal. More refined studies are now in progress to define more precisely what can and can not be achieved with the PAC. Still other studies seek to increase PAC technology. One technique subjects the PAC to an impregnation process in which elements such as iodine or sulfur are incorporated into the carbonic solvent. Such processes can produce sorbents that bind more strongly with adsorbed mercury species, but also result in the cost of the solvent significantly higher. The speciation of. Mercury in vapor phase depends on the type of mineral coal. Bituminous mineral carbons in the eastern United States tend to produce a higher percentage of oxidized mercury than sub-bituminous and lignite coal from the west. Mineral coals from the west have low chloride content compared to typical eastern bituminous mineral coals. It has been recognized for several years that a loose empirical relationship is maintained between the content of mineral carbon chloride and the degree by which mercury manifests itself in the oxidized form. Fig. 1 (Source: Senior, C.L. Behavior of Mercury in Air Pollution Control Devices on Coal-Fired Utility Boilers, 2001) illustrates the relationship between the chlorine content in mineral coal and the speciation of mercury in the vapor phase. An important reason for the significant dispersion in the data of Fig. 1 is that the oxidation of the mercury depends in part on the specific characteristics of the kettle as well as the fuel. The oxidation reactions of mercury proceed through the reaction mechanisms, both homogeneous and heterogeneous. Factors such as the conversion step of the boiler and the temperature profiles of the combustion air preheater, composition of the flue gas, characteristics and composition of the fly ash, and the presence of unburned coal have all been shown to affect the conversion of elemental mercury to oxidized mercury species. Felsvang et al. (U.S. Patent No. 5,435,980) teaches that the removal of mercury from a system heated with coal using an SDA system can be improved by increasing the chlorine-containing species (e.g., hydrogen chloride) in the exhaust gases. chimney. Felsvang and colleagues further teach that this can be achieved through the addition of a chlorine-containing agent to the combustion zone of the kettle, or through the injection of hydrochloric acid (HCl) vapor into the upstream of the gases of the SDA. These techniques are claimed to improve the performance of mercury removal from PAC when used in conjunction with an SDA system. BRIEF DESCRIPTION OF THE INVENTION It is an object of this invention to produce significant technical and commercial advantages over the prior art. The present inventors have determined through the experimental test that the use of bromine containing compounds, added to the mineral coal, or to the combustion furnace of the boiler, are significantly more effective than the compounds containing chlorine in improving the oxidation of mercury, thus allowing the complete removal of mercury in the downstream contamination control devices . Second, the technique is applicable to power plants equipped with wet FGD systems as well as those plants equipped with SDA systems. The wet FGD is the sulfur dioxide removal system of choice for most coal-fired facilities around the world. Approximately 25% of coal-fired power plants in the United States are equipped with wet FGD systems. The various characteristics of the novelty characterizing the invention are indicated with particularity in the claims appended to and forming a part of this description. For a better understanding of the present invention, its operating advantages and the specific benefits attached to its uses, reference is made to the accompanying drawings and the descriptive matter in which the preferred embodiments of the invention are illustrated. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph illustrating the relationship between the mercury content of mineral coal and the mercury speciation for mineral carbons in the United States.; Fig. 2 is a schematic illustration of a first embodiment of the present invention involving the addition of bromine for the removal of mercury from flue gases; Fig. 3 is a graph of the test data illustrating the effect of the addition of a particular halogen, calcium bromide, CaBr2, on the total vapor phase mercury produced during the combustion of the mineral coal, according to the present invention; Fig. 4 is a schematic illustration of a coal-fired electric service plant configuration comprising a kettle equipped with an SDA and a downstream particle collection means such as a cloth filter (FF) or a precipitator electrostatic (ESP); Fig. 5 is a schematic illustration of a coal-fired electric service plant configuration comprising a kettle equipped with a downstream particle collection means such as a cloth filter (FF) or an electrostatic precipitator (ESP); and Fig. 6 is a schematic illustration of a coal-fired electric service plant configuration comprising a kettle equipped with a particle collecting means such as a cloth filter.

(FF) or an electrostatic precipitator (ESP) and a wet stack gas desulfurization system (FGD). DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Referring to the drawings generally, where similar numbers designate the same or functionally similar elements by all the various drawings, a first embodiment of the present invention is illustrated in FIG. 2. A reagent containing bromine 10 it is added to the combustion furnace 14 of the boiler 12, either directly or by premixing it with the incoming mineral coal 16. The bromine species released during the combustion process improve the oxidation of the mercury as the combustion gases pass through the kiln. and, in particular, through the sections of the cooler of the direction of the boiler 18 and the combustion air preheater 20. The increased fraction of the mercury manifested in the oxidized form improves the removal of the mercury in the systems of combustion. downstream contamination control such as the wet FGD 22 and SDA 24 systems, and the systems and PAC injection. As described herein, the experimental results indicate that the addition of bromine also results in an increased fraction of mercury confined to particles. This improves the removal of mercury through the collectors of particles 26 such as cloth filters (FF) and electrostatic precipitators (ESP) The removal of elemental mercury from the coal combustion gases generated by the electric service plants through the application of a conventional PAC injection process is very expensive. The present invention promises to significantly reduce the cost of mercury removal in coal-fired power plants in two ways. The first, increasing the fraction of mercury that manifests itself in oxidized and particulate-confined forms, improves the removal of mercury in conventional pollution control systems such as particle collectors 26 and wet FGD systems 22 and SDA 24. This reduces, or it can completely eliminate the need for PAC injection to remove elemental mercury. Second, the increased fraction of oxidized mercury also improves the removal of mercury through a PAC injection process, due to the higher reactivity of oxidized mercury with the PAC. The present invention was tested in a Small Kettle Simulator (SBS) facility of 5 million Btu / hr. The SBS went on to approximately 4.3 million Btu / hr with a sub-bituminous mineral coal from the western United States. During these tests the flue gases that exist in the first SBS kettle are passed through a spray dryer absorber (SDA) for the removal of the sulfur dioxide, and then through a cloth filter (FF) for the removal of fly ash and spent solvent from SDA's FGD system. An aqueous solution of calcium bromide (CaBR2) was injected into the combustion chamber 14 through a heat boiler (not shown). Fig. 3 illustrates the removal of mercury through the SDA / FF system. It can be observed that in the injection of calcium bromide, the mercury in the vapor phase leaving the system decreased from its initial value of approximately 6 μg / dscm to approximately .2 μg / dscm. It can also be observed that the mercury in the vapor phase at the entrance of the system also decreases in the addition of the calcium bromide. This is due to the fact that calcium bromide also improves the formation of mercury confined to particles (the combined mercury in particles is not shown in the graph, since the online mercury analyzer used only detects the mercury species in the vapor phase). These results identify that the current invention can offer a cost effective method for the removal of elemental mercury from coal combustion flue gases. In the preferred mode, an aqueous solution of calcium bromide is sprayed onto the crushed coal 16 before the coal 16 is pulverized for combustion. The aqueous solution is easily handled and measured in the mineral coal 16, the mineral coal pulverized 28 intimately mix the reagent of bromide 10 with the mineral coal 16, and the transport system of the pulverized mineral coal 30 to the various charcoal burners mineral (not shown) ensures a uniform distribution of the reagent through the kettle oven 14. There are many alternative routes to implement the invention as would be apparent to one of skill in the art. Based on the tests carried out, it is believed that adequate mercury removal can be achieved when the mineral coal 16 is treated with up to about 1000 ppm bromine of the bromine-containing reagent 10; particularly between about 100 and about 200 ppm of bromine of the bromine-containing reagent 10. As will be appreciated by those skilled in the art, some non-zero amount of bromine is supplied in order to apply the principles of the invention; The upper limit of the interval is, as a practical matter, limited by the possible increased corrosion potential that could be created. In another embodiment the fuel of the coal-fired boiler 16 may include bituminous, sub-bituminous, and lignite carbons and mixtures thereof. In still another embodiment, the reagent containing bromine 10 could comprise, but is not limited to, alkaline earth metal bromides, hydrogen bromide (HBr) or bromine (Br2). In yet another embodiment, the reagent containing bromine 10 can be fed into the combustion zone of the kettle 14 in gaseous, liquid or solid form. In yet another embodiment, the electrical service plant configurations may include plants equipped with an SDA 24 and a particle collector 26 (FF or ESP) (Fig. 4), a particle collector 26 (FF or ESP) (Fig. 5), or wet collector FGD 22 and particle 26 (FF or ESP) (Fig. 6). In still another embodiment, the invention can be used in a coal-fired plant equipped with a selective catalytic reduction (SCR) 32 system for the control of nitrogen oxides, as the SCR catalysts have been shown to promote the oxidation of mercury elementary if the correct species (in this case bromine species) are present in the flue gases. In yet another embodiment, the removal of the mercury can be further improved by using a sorbent injection system in connection with the present invention. Such carbon sorbents include, but are not limited to, powdered activated carbon (PAC), charcoals and charcoal produced from coal and other organic materials, and unburned carbon produced by the combustion process thereof. While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, those skilled in the art will appreciate that changes can be made in the form of the invention covered by the following claims without departing from of such principles. For example, the present invention can be applied to the new construction of the fossil fuel boiler that requires the removal of mercury from the flue gases produced therein, or to the replacement, repair or modification of existing fossil fuel boiler facilities . In some embodiments of the invention, certain features of the invention can sometimes be used to favor without a corresponding use of the other features. Accordingly, there are other alternative embodiments that would be apparent to those skilled in the art and based on the teachings of the present invention, and which are proposed to be included within the scope and equivalents of the following claims of this invention.

Claims (16)

1. CLAIMS 1. A method for removing a portion of the elemental mercury in a flue gas created during the combustion of a fossil fuel, characterized in that it comprises: providing a reagent containing bromine to the flue gas; promote the oxidation of elemental mercury with the bromine-containing reagent; create an oxidized form of mercury from elemental mercury; and remove the oxidized mercury from the flue gas.
2. 2. The method according to claim 1, characterized in that the fossil fuel is mineral coal.
3. The method according to claim 1, characterized in that the step of providing the bromine-containing reagent comprises the step of treating the fossil fuel with the reagent containing bromine before combustion.
4. 4. The method according to claim 1, characterized in that it comprises the step of treating the flue gas with the reagent containing bromine.
5. 5. The method according to claim 1, characterized in that the bromine-containing reagent is provided in an aqueous form.
6. 6. The method according to claim 1, characterized in that the bromine-containing reagent is provided in a solid form.
7. 7. The method of compliance with the claim 1, characterized in that the bromine-containing reagent is provided in a gaseous form.
8. The method according to claim 3, characterized in that it further comprises the step of spraying the fossil fuel.
9. The method according to claim 8, characterized in that the spraying step occurs after the treatment step.
10. The method according to claim 2, characterized in that the mineral coal is treated with up to about 1000 ppm of bromine of the bromine-containing reagent.
11. The method according to claim 10, characterized in that the mineral coal is treated with between about 100 and about 200 ppm bromine of the bromine-containing reagent.
12. The method according to claim 1, characterized in that a substantial portion of the elemental mercury in the flue gas is oxidized.
13. The method according to claim 1, characterized in that it further comprises the step of using a humid flue gas desulfurization apparatus to remove a substantial portion of the oxidized mercury from the flue gas.
14. 14. The method according to the claim 1, further characterized comprises the step of using a spray dryer chimney gas desulfurization apparatus to remove a substantial portion of the oxidized mercury from the chimney gas.
15. 15. The method of compliance with the claim 1, characterized in that it further comprises the step of using a sorbent injection system to remove a substantial portion of the oxidized mercury from the flue gas.
16. 16. The method according to claim 15, characterized in that the sorbent comprises powdered activated carbon.