MX2011000275A - Targeted reagent injection for slag control from combustion of coals high in iron and/or calcium. - Google Patents

Abstract

Disclosed is a process that increases the output of a combustor fired with coal having high iron and/or calcium content, by reducing the tendency of slag to form on heat exchange surfaces and changing the nature of the slag to make it easier to remove. The process includes combusting a slag-forming coal, having high iron and/or calcium content, with an overall excess of oxygen; moving the resulting combustion gases though heat exchange equipment under conditions which cause cooling of slag formed by burning the fuel; and prior to contact with said heat exchange equipment, introducing aqueous aluminum trihydroxide in amounts and with droplet sizes and concentrations effective to decrease the rate of fouling, and preferably, increase the friability of the resulting slag. Desirably, the aluminum trihydroxide reagent is introduced in the form of an aqueous liquid and computational fluid dynamics is employed to determine flow rates and select reagent introduction rates, reagent introduction location(s), reagent concentration, reagent droplet size and/or reagent momentum. In a preferred aspect, the feed rate will up to about 6 pounds ATH per ton and preferably with up to about 2 pounds Mg(OH)2 per ton of coal. A process is also provided for cleaning and maintaining cleanliness of a combustor.

Description

REAGENT INJECTION ADDRESSED FOR SCREEN CONTROL OF THE COMBUSTION OF HIGH CARBON IRON AND / OR CALCIUM FIELD OF THE INVENTION The invention is related to a process that increases the output of a combustion system with charcoal burning of high content of iron and / or calcium, reducing the tendency of slag formation on the surface of the heat exchanger equipment by changing the nature of the scum to make its removal easier.

BACKGROUND OF THE INVENTION The combustion of coal, like that of other fossil fuels, is invariably less efficient than desired and can be a source of pollution. The maintenance of the operation of the combustion system with high efficiency controlling the quality of the emissions, is. essential to obtain the necessary energy to boost our economy while preserving the quality of the air we require for survival. Because efficiency and emissions are interrelated and some technological solutions have proven to be competitive with one another, it has been difficult to achieve both. The economic operation of power plants and incinerators is in the public interest and new technologies are essential for this effort.

Fuel selection plays an important role in mitigating some pollution problems but can not eliminate them. Some bituminous coals such as those in the Appalachian and Illinois basins are important in many plants designed for coal where the economy limits other options.

The slag formation trend and the properties of the same produced by coals with high content of iron have been a main concern for the engineers and operators of combustion plants for decades. There are a number of factors that impact the physical and chemical properties of the slag. See, for example; Fossil Combustion Energy, 1991, Joseph G. Singer, P. E., editor, Chapter 3, Combustion Engineering.

However, as established by today's industry, there is a compromise between the selection of low-cost coal and the current economy in energy production where slag becomes a problem. Slag accumulation is a problem that causes a decrease in heat transfer and sometimes leads to long periods of time used for cleaning. A problem related to coal, are the large amounts of ash and fine particles formed that must be captured and eliminated. Currently they have been used additives to control the formation of slag and its properties but the additives can cause stress in the recovery systems of solids used, in terms of volume.

Consequently, optimum control of the slag has sometimes been compromised because the solid recovery systems can not effectively remove the necessary totality of solids. This is a problem especially for old plants where increasing the collection capacity of solids is not an option.

Making the problem more complex, there is the fact that the carbons react in different ways with additives depending on their composition. As a general rule, there is no known formula that makes it possible to direct all the different coal compositions with the suitable additives to effective levels that can be handled adequately by the solid recovery equipment. The discovery of a particular charcoal composition and additive regime is highly sought after to ensure that economic energy can then be supplied by generating sufficient income for effective control of pollution.

There is a need for an improved process that controls slag formation more effectively, especially with problematic fuels such as carbons containing sulfur that play an increasing role in the formation of slag as well as those with high iron and / or calcium content to improve the efficiency of the boiler and the economy.

SUMMARY OF THE INVENTION An object of the invention is to provide an improved technology for the control of slag in combustion systems using fuels with a tendency to slag.

Another objective is to provide a process for the control of slag coming from the combustion of coal with a high content of iron and / or calcium, reducing the use of chemical products.

Another objective is to provide a process for the removal of slag from the surface of the heat exchangers, produced by the combustion of coal with a high content of iron and / or calcium, reducing the use of chemicals.

An additional but more specific goal is to provide a process to more effectively control the slag, decreasing the amount of downtime associated with slag removal.

A more specific objective of some aspects of the invention is to achieve the aforementioned objectives improving, at the same time, the efficiency of the combustion system.

These and other objectives will be. achieved by the present invention in, at least, its main aspects which provide an improved process for slag control in combustion systems fed with slag-forming coal, with high content of iron and / or calcium.

In one aspect, the invention provides a process for reducing the cohesiveness and / or adhesiveness of the slag in a combustion system, thereby decreasing the level of fouling, comprising: the burning of slag-forming coal with a high content of iron and / or calcium with a total excess of oxygen; the elimination of. the resulting combustion gases by means of heat exchange equipment under conditions which cause the slag formed by the combustion of the coal to cool and, prior to contact with said heat exchanger equipment, the introduction of aqueous aluminum trihydroxide in amounts, sizes of drop and effective concentrations to decrease the level of soiling and, preferably, increasing the friability of the resulting slag.

In an important aspect, the aluminum trihydroxide reagent is introduced in liquid form using the Computerized Fluid Dynamics for determine the yield and select the speed and force of introduction of the reagent, the sites of introduction, the concentration and the size of drop.

In another important aspect, the magnesium hydroxide is introduced as an aqueous mixture together with the mixture of aluminum trihydroxide.

In another aspect, the invention provides a process for cleaning hot surfaces containing the slag formed by introducing aqueous aluminum trihydroxide in amounts, droplet sizes and effective concentrations to make contact with the fine particles resulting from the drying of the mixture and with the existing slag deposits.

In another aspect, the invention provides a process for cleaning and maintaining a combustion system comprising an initial dosage rate of approximately 1,362 to 2,724 kg (3 to 6 pounds) of aluminum trihydroxide per ton of coal and 0.454 to 0.908 kg. (1 to 2 pounds) approximately magnesium hydroxide per ton of coal for a sufficient time to reduce the slag followed by a second dosage of about 10 to 50% of the initial values to keep the combustion system clean and operating efficiently. Other important aspects are described in the following description.

BRIEF DESCRIPTION OF THE FIGURES The invention will be better understood and its advantages will become more apparent when the following detailed description is read together with the accompanying figures in which; Fig. 1 is a schematic view of the physical representation of the invention.

Fig. 2 is the photograph of a sample of slag obtained after 24 hours of action of the aluminum trihydroxide in a combustion system operated with high iron content coal as set forth below.

DETAILED DESCRIPTION OF THE INVENTION First reference will be made to Fig. 1 which is a schematic view of the physical representation of the invention. Fig. 1 shows a large combustion system, of the type used to: produce steam for electric power generation, steam process and heating or incineration processes. The coal is fed by the burners 20 and 20a and is burned with air in the combustion zone 21.

It is favorable for the invention that coal with a high content of iron and / or calcium have, for example, a iron content above 15%, (from 20 to 35% based on the weight of the ash and expressed as Fe2Ü3) and / or a calcium content greater than 5% (from 20 to 25% based on the weight of the ash and expressed as CaO). It is also an advantage of the invention that the slag can be efficiently controlled even for coal with a significant sulfur content, for example, about 1% with an approximate range of 3 to 5%. Here and everywhere in this description, the percentages are by weight.

The air for combustion is supplied by a fan 22 and a duct 24 and is preferably heated by gas to gas heat exchangers (not shown) which transfer heat from the duct (not shown) to the outlet end of the combustion system. The hot combustion gases rise and flow through the heat exchangers which transfer this heat to the water for steam generation. Other heat exchangers, including an economizer (low flow, not shown) may also be provided according to the particular design of the system. The slag left untreated would tend to form on the surface of these heat exchangers which are positioned within specific combustion systems based on important design considerations. An advantage of the present invention is that the techniques for the elaboration of models such as the Computerized Fluid Dynamics is initially used for direct treatment with chemicals (especially those identified as effective for some particular types of coal according to the invention) in the optimal sites for the reduction and / or control of the slag formation maintaining an efficient operation of the combustion system.

A series of suitable injectors, preferably assisted by atomization of air in each bank of injectors 30 and 30a are provided for the introduction of the aluminum trihydroxide alone or mixed with magnesium hydroxide from the containers 40 and 40a respectively. Both aluminum trihydroxide and magnesium hydroxide are preferably aqueous, as is suitable for mixtures and / or solutions. The supply lines 41 are shown as double lines in the scheme.

The valves 42 are represented by the common symbol ¾¾ and the temperature sensors 44 are represented by the common symbol U. Both the valves and the temperature sensors are connected to the controller 46 by electrical conductors 48 shown with dotted lines.

These valves, temperature sensors and electrical conductors are only illustrative and the skilled worker, using the principles described herein, will strategically place them to provide appropriate control signals and responses. The controller 46 may be a general purpose digital computer programmed according to a predetermined rate with both power and feedback applications.

The aluminum trihydroxide which has been identified as effective according to the invention to greatly reduce slag deposition or to clean previously deposited slag from problematic carbon types is also known under other names such as aluminum hydroxide and alumina. hydrated Without considering the form that aluminum trihydroxide has as a raw material, it is preferable that it be mixed with water for its transfer to tank 40 through associated lines 41 with or without chemical stabilizers at suitable concentrations for storage and handling, for example , at least about 25% and preferably at least 65% solids by weight.

As will be described, the concentration and flow velocity will be determined initially by models to ensure that the quantity is adequately supplied. appropriate chemicals in the correct place of the combustion system in the correct physical form to obtain the desired results of slag reduction and ease of cleaning. For use in the process it is diluted as determined by Computerized Fluid Dynamics within the range of approximately 0.1 to 10% and narrower from approximately 1 to 5%. When the aqueous aluminum trihydroxide comes into contact with the hot gases in the combustion system, it is believed to be reduced to very small particles (nanoparticles) for example, less than 200 nanometers, and preferably below 100 nanometers. The average particle size of 50 to 150 nanometers is a useful range for the process of the invention. To achieve this size, it is important that the aluminum trihydroxide is introduced in aqueous form. It is believed that small particles interrupt the normal crystallization of the crystal that forms the slag.

Regardless of the mechanism involved, it is another advantage of the invention that the slag thus formed be highly friable and easily broken with brushing and can be removed manually.

A significant advantage of the invention is that the friability of the slag formed is increased making it easier to remove. The invention also retards or eliminates slag formation. In a favorable way, at high doses, can now remove the already formed slag. It should be understood by "increasing the friability of the slag" that the slag after treatment requires less strength per unit area to be removed than that formed under the same conditions but without the treatment. The term "remove slag" means that the weight of the slag adhering to the heater, particularly the surface of the heat exchangers, is reduced from its initial values by the treatment of the invention.

There are several additional and useful advantages of the invention such as the reduction of S03 from carbons high in sulfur, the reduction of drip pressure in heat exchangers, the possibility of using lower cost coal, lower CO generation, lower generation of CO2 due to the increase in fuel consumption, better heat transfer, less downtime, higher performance, clean in line, cleaner heat exchanger surfaces, possibility to clean the combustion system in its entirety and the possibility to run all the loads with greater efficiency.

The process for most coals works best with a combination of aluminum trihydroxide and magnesium hydroxide. While some coals, for example, with low composition of silicates can be burned with few problems attributed to the slag, the use of magnesium hydroxide, at least at the beginning, is preferable.

The magnesium hydroxide reagent can be prepared preferably from brines containing calcium and other salts, usually from underground pits or from seawater. The dolomitic slime is mixed with these brines to form calcium chloride and magnesium hydroxide in solution, the latter is separated from the solution by precipitation and filtration. This form of magnesium hydroxide can be mixed with water with or without stabilizers at concentrations suitable for storage and handling, for example, with 25 to 60% solids by weight. For use in the process, it is diluted as determined by Computerized Fluid Dynamics within the range of 0.1 to 10%, more narrowly from 1 to 5%. When in contact with the effluent in the combustion system, it is believed to be reduced to nanoparticles of less than 200 nanometers preferably less than 100 nanometers. Particles of average size of 50 to 150 nanometers are the useful range for the process of the invention. Other forms of MgO may also be used where necessary or desirable, for example, a "light burn" or "caustic" may be employed in the desired particle range.

For the best achievement of these effects, the invention will mainly take advantage of Computerized Fluid Dynamics to project initial performances and select the rate of introduction of the reagent, site of introduction, concentration, strength and drop size. Computerized Fluid Dynamics is a well understood science and is used with a total benefit in this case where it is desired to supply a minimum amount of the chemical for maximum effect.

It is observed in a very significant manner that the amount of reagent will be sub-stoichiometric in terms of the affectation of the melting point of the slag often considered as the controlling factor in the slag control. In accordance with the present invention, there is good evidence, in addition to the amount of reagent employed relatively small, that the results of the invention are due to the physical disruption of slag formation by possible chemical bonds and kinetic effects not explained by the literature.

Tests have shown that the initial feed rates determined by Computerized Fluid Dynamics can be used with good effects and then be adjusted based on the observed results. As a guide for feeding speeds, the initial value, for the best economy in combustion systems operating in a manner similar to that exemplified below, may be greater than 2,724 kg (6 pounds) of aluminum trihydroxide (as dry reagent) or 3,632 kg (8 pounds, as a mixture to 65 - 70%) per ton of coal.

For example, when added as a 70% blend, effective amounts of 0.454 to 2.724 kg (1 to 6 pounds) of mixture will be effective, more narrowly from 0.908 to 1.362 kg (2 to 3 pounds).

It is also preferred to also use approximately 0.908 kg (2 pounds) of magnesium hydroxide mixture (with 50 to 60% solids) per ton of carbon. For example, when added as a mixture to 60% solids, approximate amounts of 0.227 to 0.908 kg (0.5 to 2 pounds) of aqueous mixture of magnesium hydroxide are used per ton of carbon, more narrowly from 0.318 to 0.454 kg (0.7 to 1 pound) of mixture. The mixtures are diluted as necessary, typically from a solids concentration of about 5% (for smaller applications) to about 35% or more.

The weight of the slag adhered to the combustion system, particularly to the surface of the heat exchangers, is reduced from its initial values by the treatment of the invention especially when aluminum trihydroxide and magnesium hydroxide are used at high concentrations within the upper ranges, for example, from 1,362 to 2,724 kg (3 to 6 pounds) of aluminum trihydroxide and from 0.454 to 0.908 kg (1 to 2 pounds) of magnesium hydroxide per ton of carbon. This ability to remove the slag provides the possibility of establishing a cleaning and maintenance regime where the initial dose is just as mentioned to remove the slag, and with the dose reduced between 10 to 50% from its initial values for maintenance of the clean combustion system and operating efficiently.

It is essential for the optimum slag remediation, according to the invention, that the correct initial concentrations and rates of introduction be calculated and employed for the most effective physical form of the aluminum trihydroxide and, optionally, the magnesium hydroxide, to be introduced to the chamber. of hot gas combustion 20 to allow adding the reagent with the desired effect.

The implementation of Fluid Dynamics Computerized in the invention can be carried out as provided in US Pat. No. 7,162,910 to Smyrniotis et al. The equipment for particle removal (not shown) can be used to eliminate particles before the passage of the effluent.

In another alternate form of the invention, the combustion catalysts and / or the chemicals for effluent treatment can be added to the fuel, to the combustion zone or otherwise as described, for example in Patent No. 7,162,960 of the United States for Smyrniotis and collaborators.

The following examples are presented to further explain and illustrate the invention and should not be taken as limitation in any consideration.

Example 1 This example illustrates the introduction of trihydroxide of baked aluminum burning 540 tons of coal per day. Coal is a mixture of Illinois Basin and bituminous Appalachian coals, obtaining the following results as combined: Sample 1 2 3 Humidity, H 11.28 10.85 10.19 Ashes,% 14.91 13.63 13.91 Volatile matter,% 36.03 35.04 Fixed coal,% 39.49 40.86 Total,% 100 100 Sulfur,% 3.95 4.44 HHV, kJ / kg 24, 762.89 24, 953.16 For the test, aluminum trihydroxide is fed at 70% by weight in aqueous mixture in a proportion of 2.27 kg (5 pounds) per ton of coal consumed from two banks of three injectors cooled with air and positioned on the opposite wall where they locate two series of pulverized coal burners, one bank at an elevation between the two series of burners and the other at a higher elevation of the highest series of coal burners. The mixture is diluted to a concentration of aluminum trihydroxide of 35% by weight. The density of the mixture before dilution is 1679 kg per liter (14 pounds per gallon) approximately which m that the feed rate is 0.73 m ~ 3 (193 gallons) per day, approximately 2.27 kg (5 pounds) per ton of coal. Based on this test it is estimated that an effective feed rate for this particular combustion system will be from .454 to 2,724 kg (1 to 6 pounds) of aluminum trihydroxide mixture per tonne of coal, more precisely from 0.908 to 1.362 kg. (2 to 3 pounds) per ton.

Example 2 This example illustrates the effect of the introduction of magnesium hydroxide to a combustion system by burning 540 tons of carbon per day in addition to the aluminum trihydroxide fed in Example 1. The coal was a mixture of Illinois Basin and Bituminous Appalachian coals as set forth in Example 1.

The magnesium hydroxide was fed as a mixture at 0.908 kg (2 pounds, 50 to 60% by weight) per ton of coal consumed. The density of the mixture was approximately 1439 kg per liter (12 pounds per gallon). Therefore, the feed rate was approximately 0.34 m ~ 3 (90 gallons) per day. As before, we fed the aluminum trihydroxide mixture to 2.27 kg (5 pounds) per ton of coal consumed. The density of aluminum trihydroxide was approximately 1,679 kg per liter (14 pounds per gallon) with the feed rate being approximately 0.73 rrT3 (193 gallons) per day.

Based on this test, we estimate that the optimum feed rate for the best economy for this particular combustion system is 0.227 to 0.908 kg (0.5 to 2 pounds) of magnesium hydroxide mixture per ton of carbon, for example 0.454 kg (1 pound) per ton, more approximately 0.454 to 2.724 kg (1 to 6 pounds) of aluminum trihydroxide mixture per tonne, for example from 0.908 to 1.362 kg (2 to 3 pounds) per ton. Fig. 2 is the photograph of a slag sample obtained after 24 hours of operation with trihydroxide feed aluminum alone, the slag was unexpectedly friable.

The above description is intended to teach the worker with ordinary skill how to practice the invention. It is not the intention to detail all those modifications and obvious variations which will become apparent to the skilled worker when reading the description. It is the intention, however, that all those obvious modifications and variations are included in the field of the invention which is defined by the following claims. The claims assume the coverage of the claimed components as well as the steps in any sequence that is effective to meet the intended purposes therein unless the context expressly indicates otherwise.

Claims (10)

NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following is claimed as property CLAIMS

1. A process for reducing the cohesiveness and / or adhesiveness of the slag in a combustion system thus decreasing the level of soiling, characterized in that it comprises: Burn the slag-forming coal with a high content of iron and / or calcium, with a total excess of oxygen. Mobilize the resulting combustion gases by the heat exchange equipment under conditions that cause the cooling of the slag formed by the burning of fuel; and prior to contact with said heat exchange equipment, the introduction of aqueous aluminum trihydroxide in amounts, drop sizes and effective concentrations to reduce the level of slag fouling.

2. A process, according to claim 1, characterized in that the treatment is effective to increase the friability of the resulting slag.

3. A process, according to claim 1, characterized in that the aluminum trihydroxide reagent is introduced in the form of an aqueous liquid and the Computerized Fluid Dynamics is used to determine the initial flow rate and to select the rate of introduction of the reagent, introduction site (s), introduction force , concentration and drop size.

4. A process, according to claim 1, characterized in that the subsequent treatment includes the introduction of magnesium hydroxide in amounts, drop size and effective concentrations to decrease the level of slag fouling.

5. A process, according to claim 1, characterized in that the treatment comprises the introduction of more than 2,724 kg (6 pounds) of an aqueous mixture of aluminum trihydroxide per ton of carbon and above 0.908 kg (2 pounds) of hydroxide of magnesium per ton of coal.

6. A process for the elimination of slag deposits in a combustion system by burning - charcoal characterized because it comprises: Introduce, in hot combustion gases in the combustion system, aluminum trihydroxide in carities, drop sizes and effective concentrations to eliminate slag deposits.

7. A process, according to claim 6, characterized in that the subsequent treatment includes the introduction of magnesium hydroxide in amounts, drop sizes and effective concentrations to decrease the level of slag fouling.

8. A process, according to claim 6, characterized in that the treatment comprises the introduction of more than 2,724 kg (6 pounds) of an aqueous mixture of aluminum trihydroxide per ton of carbon and above 0.908 kg (2 pounds) of hydroxide of magnesium per ton of coal.

9. A cleaning and maintenance process of a combustion system comprising an initial dosage regime of approximately 1,362 to 2,724 kg (3 to 6 pounds) of aluminum trihydroxide per tonne of coal and 0.454 to 0.908 kg (1 to 2 pounds) approximately of magnesium hydroxide per ton of coal for a sufficient time to reduce the slag followed by a second reduction by dosing approximately 10 to 50% of the initial values to keep the combustion system clean and operating efficiently.

10. A process, according to claim 9, characterized in that the subsequent treatment includes the introduction of magnesium hydroxide in amounts, sizes of drop and effective concentrations to reduce the level of slag fouling. SUMMARY OF THE INVENTION It is discovered, a process that increases the output of a combustion system with charcoal burning of high content of iron and / or calcium, reducing the tendency of slag formation on the surface of the heat exchangers and changing the nature of the slag to make its removal easier. The process includes the combustion of slag-forming coal with a high content of iron and / or calcium with a total excess of oxygen; mobilizing the gases resulting from the combustion by heat exchange equipment under conditions that cause cooling of the slag formed by the burning of the fuel and, prior to contact with the heat exchange equipment, introducing aqueous aluminum trihydroxide in amounts, size of drop and effective concentrations to reduce the level of fouling and, preferably, increasing the friability of the resulting slag. Preferably, the aluminum trihydroxide is introduced in the form. of aqueous liquid using Computerized Fluid Dynamics to determine the flow velocity and select the rate and force of introduction, site (s), concentration and drop size. Preferably, the feed rate will be greater than 2,724 kg (6 pounds) of aluminum trihydroxide per ton and 0.908 kg (2 pounds) of magnesium hydroxide per ton of coal. A cleaning and maintenance process of the combustion system is also provided.