METHOD AND COMPOSITION FOR UTILIZING LIME-UREA HYDRATES TO SIMULTANEOUSLY REDUCE NOX AND SOX IN COMBUSTION EFFLUENTS

Abstract: Methods and compositions are provided for simultaneous reduction of NO x and SO x in combustion effluents and process gas streams, employing a sorbent material of calcium or dolomitic lime hydrated with aqueous carboxamides, optionally with carbinol reductants, in amounts sufficient to reduce the NO x and SO x concentrations from the effluents. The sorbent material may be introduced as dry powders or wet hydrates in effluent streams at temperatures ranging from 400° to 2400°F.

Description: The burning of fossil fuels, for example in a utility power plant boiler, produces combustion effluents that include undesirable sulfur and nitrogen compounds, primarily nitric oxide (NO) and sulfur dioxide (SO₂). Combustion processes are known to produce other environmental contaminants such as unburned hydrocarbons and carbon monoxide which are also released in the flue gas in various concentrations. The nitric oxide is oxidized in the atmosphere to nitrogen dioxide, which subsequently reacts with hydrocarbons in the presence of sunlight to form photochemical "smog". Prior research has shown that emissions of sulfur dioxide into the atmosphere can combine with rainwater to produce acidic deposition or "acid rain" which damages lakes, streams, fish populations and forest preserves. Recent environmental research, particularly in West Germany, concerning atmospheric transport, chemistry and deposition associated with "acid rain" suggests that substantial forest damage is also caused by nitrogen oxides. Prior human health effects research in Tennessee has been cited by the U.S. Environmental Protection Agency (EPA) as justification for emission standards to control nitrogen oxides produced by combustion processes. Combustion products from automotive internal combustion engines, fossil fuel power plants, process furnaces, incinerators and the like all contribute to the production of nitric oxide. Control of nitric oxide production has been directed toward modifications in the combustion process or removal of the nitric oxide from the combustion products prior to discharge into the atmosphere. Examples of these control measures were discussed at the EPA/Electric Power Research Institute (EPRI) Joint Symposia on stationary combustion NO x control held in Dallas, Texas in 1983 and Boston, Massachusetts in May, 1985. There have been numerous efforts to effectively remove nitric oxide from combustion effluents. One of the major difficulties has been that many of these devices only operate well with relatively large oxygen excess. This is true of power plant boilers, process furnaces, gas turbines, diesels, stratified charge engines, and spark ignition engines with thermal reactors. These devices generally contain flue gases with oxygen concentrations from 0.1 to 10 volume percent and nitric oxide concentrations from 100 to 4000ppm. Thus, oxygen is present in large excess with respect to nitric oxide. While it is well known how to reduce both nitric oxide and oxygen with large quantities of a reducing agent, desirably, a process should reduce nitric oxide selectively. Effluents from the combustion processes named previously may contain substantial concentrations of sulfur dioxide in proportion to the sulfur in the fuel. Since coal is an economically attractive fuel in the U.S. for large fuel consumers including electrical utility boilers, there have been numerous processes developed to control sulfur emissions including wet scrubbers, spray dryers, fluidized bed combustors, and in-furnace sorbent injection processes (also known as LIMB). One of the most prevalent methods of removing SO₂ from combustion effluents has been the wet scrubber. However, utilities have expressed disappointment with wet scrubbers in terms of power consumption, capital cost, frequent failures, corrosion, etc. A dry SO₂ removal process is frequently preferred where the spent sulfur capture material can be collected and removed along with the fly ash. Fluidized bed combustion systems show promise as an alternative for new power plants, but retrofit applications are very costly. Current research and development efforts for retrofit applications have been directed at in-furnace sorbent injection processes leading to dry SO₂ removal as discussed at the recent EPRI/EPA First Joint Symposium on Dry SO₂ Control Technologies in San Diego, CA in November, 1984. One of the primary limitations of most current NO x and SO x control technologies is that they require separate costly equipment and process control measures that occasionally degrade plant performance and reliability. Therefore, there is an important need for a simultaneous NO x and SO x control technology that utilizes only one set of equipment and preferably is a dry process that requires little or no equipment in direct contact with the combustion effluent stream. Efforts have been made to avoid the problems of wet scrubbing SO₂ control systems wherein the gas contacts a finely divided SO₂ sorbent material that has been dried to a powder by the heat of the flue gas stream. These systems have posed some problems, but importantly, the commonly used sorbent materials have no ability to also reduce NO x emissions. Therefore, a separate NO x control system is required. Fluidized bed systems have been developed for SO₂ control, but desulfurization efficiency is low if the bed is operated at temperatures to minimize NO x emissions and their NO x control efficiency is poor when optimized for cost effective sulfur capture. In addition, development problems exist with heat removal from the bed without coolant tube burnout. Also, most systems require the spent reactants to be regenerated or subject to conversion treatment to obtain useable by-products. For these reasons, utility and environmental research organizations have expressed a need for simultaneously removing NO x and SO x from a combustion effluent stream with a dry process that can be retrofit to an existing boiler without requiring major lower furnace modifications with a fluid bed or large space consuming wet scrubbers at the stack. It is also preferable to have a dry spent reactant material that can be collected by a conventional existing precipitator or baghouse. In US-A-4,208,386 there is disclosed a method for reducing NO x in combustion effluents by contacting urea with combustion effluents containing oxygen and oxides of nitrogen an elevated temperature. The urea is claimed to selectively reduce the oxides of nitrogen. In US-A-3,900,554 there is disclosed a method for the use of ammonia for selectively reducing nitric oxide. This method employs a thermal homogeneous gas phase method of selectively reducing NO in the presence of O₂ and does not require the use of an associated solid catalyst. In US-A-3,565,575 there is disclosed a process for scrubbing nitrogen oxides from a nitric acid plant using an aqueous urea solution. In this process the nitrogen oxides are first dissolved in the solution to form nitrous acid which reacts with the urea to form nitrogen, carbon dioxide and water. In US-A-4,388,877 there is disclosed a method and composition for carrying out the combustion of fossil fuels in a fluidized bed with reduced emissions of sulfur, carbon and nitrogen oxides, and unburned hydrocarbons. The capture of sulfur and nitrogen oxides occurs within the bed material where combustion simultaneously occurs. Calcium carbonate, dolomites and calcium oxides are used as sorbents, and the sorbent is combined with catalytic metal substances that are impregnated onto the matrix. In US-A-4,424,197 a process is disclosed for adsorbing sulfur dioxide from flue gas by using stoichiometric amounts of a specially prepared calcium oxide material. High lime utilization is achieved by flash calcining sand-size grains of calcium carbonate which results in calcium oxide particles comprising a highly voided skeletal structure of very large surface area and large pore volume. In DE-A-2752918, NO x and SO x are scrubbed from an effluent gas stream by use of an aqueous scrubbing medium containing calcium hydroxide and urea. In the invention, sulfur oxide- and nitrogen oxide-containing gas streams are treated with a sorbent material from lime (high calcium or dolomitic) previously hydrated with aqueous amides, particularly carbonic acid amides. The sorbent materials are prepared by hydrating high calcium lime or dolomitic lime with aqueous carboxamide solutions, particularly urea solutions, in appropriate ratios based on the relative concentrations of nitrogen and sulfur oxides in the effluent stream. Other additives may be included in the hydrating solution. Uncomplexed water is then removed to yield dry particles. The particles are introduced into the effluent stream, either dry or in fine water droplets, at temperatures in the range of 200°C to 1315°C (400° to 2400°F). The presence of ancillary reductants or catalytic substances in the hydration process or added in combination with the sorbent material will be usual for operations in the lower portion of the temperature range. Dry spent reactant may be collected along with substantially inert CaSO x products, while NO x results in nitrogen formation. This includes effluents from utility power plant boilers, industrial and commercial boilers, process furnaces, MHD combustors, stationary and mobile gas turbine engines, stationary and mobile spark ignited and diesel engines, incinerators, and any combination thereof, such as supplementary-fired combined cycle power plants. The industrial waste gas effluent or process gas stream may be from any process that includes both nitric oxide and sulfur dioxide in the gas stream. This includes smelters, ore roasters, waste treatment processors or incinerators; and coal, peat, shale, tar sand, coke, petroleum crude, or natural gas processing. Fuel treatment, gasification, desulfurization or processing to produce synthetic liquids or solids derived from other fuels are also included. While the method will find use primarily with effluents from fossil fuel combustion, the method may also find use in waste processing, metal processing and the process chemical industry, where gas streams result in elevated temperatures containing both nitrogen and sulfur oxides. For example, effluents streams from smelters, ore roasters, hazardous waste incinerators, coal gasification, crude processing and coal or oil desulfurization processes frequently require the control or reduction of both NO x and SO x . The sorbent material will be introduced into the gas stream, where the gas stream is at a temperature of at least 200°C (400°F) and preferably at least 700°C (1300°F), more preferably about 870°C (1600°F) and generally not exceeding about 1315°C (2400°F), usually not exceeding about 1040°C (1900°F). In the lower region of the temperature range, usually other additives will be present for enhancing removal of nitrogen and sulfur oxides. Additives which may be employed include catalysts, such as titanium dioxide and vanadium pentoxide on aluminum support, which aids in the removal of the nitrogen oxides. In the middle region of the temperature range, e.g., about about 700°C (1300°F), an ancillary organic reductant, particularly a polyol, e.g., a carbohydrate, may be present as part of the sorbent material or added concurrently. The polyol should be oxidizable at temperatures of less than about 815°C (1500°F) and will for the most part be carbohydrates of from 6 to 12 carbon atoms, particularly fructose, glucose and sucrose. The effluent should have at least 0.1 volume percent oxygen and not more than about 20 volume percent oxygen, preferably having from about 0.1-10 volume percent oxygen. The pressure in which the removal process is carried out is not critical and may be varied widely. Generally, the pressure will be from about 0.1 to 100 atm. The residence time for the reaction will generally range from about 0.001 to 10s. The material which is injected into the stream may be injected dry as a fine powder, slurried as a fine spray or molten as a fine spray. Various techniques can be employed for introducing the sorbent material. These techniques include high pressure nozzles, pneumatic gas injection, either air or combustion product gas, rotary atomizers, etc. The sorbent material may be introduced into the effluent stream in finely divided form as a dry powder or aqueous slurry droplets. The sorbent is injected into the effluent stream so as to provide a substantially uniform distribution of the material in the effluent stream. The sorbent material may be introduced at various sites along the path of the effluent stream, preferably in the boiler. Streams which are encountered will generally have from about 300 to 5000ppm of SO₂ and about 100 to 1500ppm of NO x , depending upon the source of the effluent stream, for example, the source of the fuel, and the conditions under which the effluent stream is formed. The sorbent particles will generally be of a size in the range of about 0.5µm to 20µm more usually 1µm to 10µm where the wet sorbent drop sizes of the slurry will usually be from about 10µm to 750µm while the dry sorbent particles will usually be from about 20µm to 250µm. The surface area will usually be from about 10 to 50, usually 20 to 40m²/g. The droplets will usually have less than 50 weight percent, usually less than about 20 weight percent free (uncomplexed) water. The amount of sorbent material will provide from about 5 to 95% reduction of SO x and from about 5-80 mol. percent reduction of NO x . The hydrating material is formed by dissolving the carboxamide compound in water or aqueous solution, where other materials may be present. Various carboxamide materials may be employed, such as urea, uric acid, semicarbazide, formamide, etc., particularly those compounds composed solely of nitrogen, hydrogen, carbon and oxygen, where the nitrogen is bonded to at least 1 hydrogen, usually at least 2 hydrogens and carbon is bonded solely to nitrogen and oxygen. To the aqueous medium is added the high calcium, magnesium, dolomitic or hydraulic quicklime. The reactants react to form the sorbent compound. Assuming complete hydration of 100% pure quicklime (CaO), the water of hydration is 24.3% for high calcium and 27.2% for true dolomite where the values of magnesium lime are intermediate, depending on the MgO content. Practically, an excess of moisture over the theoretical amount is required to achieve complete hydration since some water will be lost through evaporation brought by on the heat of hydration. The slurry preparation contains about 68-73% free water. The hydration may occur at ambient conditions or at elevated pressures up to 2.1MPa (300psia). Typically, high calcium lime will be hydrated at atmospheric pressure, while elevated pressures are preferred for the dolomitic materials. The hydration temperature will generally range from about 1°C (33°F) to 132°C (270°F), the upper limit being controlled by the stability of the amide compound. Any uncomplexed water is removed to provide "dry" particles substantially free of uncomplexed water. The amount of amide used during the hydration process can be tailored to the SO x and NO x concentrations in the combustion effluent and the desired level of SO x and NO x removal. Typically, the injection of SO x neutralizing capability will be at a rate such that the Ca/S or (Ca+Mg/S) molar ratio is in the range of 0.1-5, preferably 1-3. The amount of urea in the hydration solution may be such that the molar ratio of urea to NO x is in the range of .01-10, preferably 0.5-2. In an alternative statement, the carboxamide amino nitrogen to NO x equivalent ratio may be from 0.2 to 20. For example, a boiler firing an eastern high sulfur bituminous coal will exhibit SO₂ and NO x concentrations in the combustion products of the order of 3000ppm SO₂ and 800ppm NO x . If the SO₂/NOx sorbent is to have a Ca/S molar ratio of 2 and a urea/NOx ratio of one half, then the hydration of a high calcium CaO lime should include 0.071g of urea per gram of CaO. Other ratios can be determined using the same methodology. The weight ratio of lime to carboxamide will vary widely depending upon the type of lime, the composition of the carboxamide, the composition of the effluent stream to be treated, and the like. Generally the weight ratio will be in the range 2:1 to 25:1, preferably in the range 5:1 to 20:1. In addition, other additives may be used during the hydration process. These can be classified as additives that (1) change the physical properties of the resulting hydrated sorbent material or (2) change the physical properties and remain present in the final hydrated material. Examples of the first class include materials that have boiling temperatures less than that of water (i.e., 100°C) such as the alkanols of 1-2 carbon atoms, i.e., ethanol or methanol. Examples of the second include sugars such as sucrose and fructose, calcium acetate, NaCl, and CaCl₂. The polyols, e.g., sugars, will be present in from about 0.1 to 5 parts per part of lime. Additives containing carbon and hydrogen that remain in the hydrated materials extend the temperature range of the process below 815°C (1500°F). The following examples are offered by way of illustration and not by way of limitation. EXPERIMENTAL Example 1 A known amount of calcium oxide (CaO) was hydrated with a water solution containing known amounts of urea (NH₂-CO-NH₂). The resulting dry powdered SO₂/NOx sorbent containing both Ca(OH)₂ and NH₂-CO-NH₂ was injected into combustion products containing SO₂ and NO x . The combustion products were generated by burning known amounts of natural gas and air in a 15cm (6″) diameter combustion tunnel. The SO₂ at the point where the sorbent was injected was controlled by adding SO₂ to the natural gas. The nitric oxide at the point of sorbent injection was controlled by adding NH₃ to the natural gas, a portion of which is converted to nitric oxide during combustion of the natural gas. The temperature at the point of sorbent injection was measured by a thermocouple and controlled by (1) water cooled heat exchanger tubes upstream of the point of sorbent injection and (2) the amount of natural gas burned. The amount of oxygen in the combustion products was controlled by varying the relative amounts of natural gas and air. The sorbent was contacted with the stream of combustion products by using a screw feeder to add the sorbent to an air stream. The resulting sorbent air stream was then injected into the stream of combustion products. The change in SO₂ and nitric oxide was measured with continuous gas analysers. The following Table 1 indicates the results. The data in test Group 1 of Table 1 demonstrate that hydrating CaO with a solution of H₂O and urea results in simultaneous SO₂ and NO removal when injected into a stream of combustion products containing SO₂ and NO. Furthermore, the data in Group 1 (Table 1) demonstrate that the presence of the urea in the resulting hydrated compound does not detrimentally affect the reaction between CaO and SO₂. In the next study, the same equipment described above was used to demonstrate the effect of temperature within the preferred range of 800°C to 1300°C, Table 1, Test Group 2. These data demonstrate that nitric oxide removal is maximum at a temperature between 915°C to 1020°C and that SO₂ removal does not vary to any large degree over this temperature range. Following the test procedures already described, CaO was hydrated with a water-urea solution containing urea at a concentration twice as great as used in the previous tests. The resulting SO x /NO x sorbent was injected into the combustion product stream containing SO₂ and NO. The results of these tests are shown in Table 1 as Test Group 3. These data demonstrate that (1) the level of nitric oxide removal can be adjusted relative to the SO₂ removal by changing the concentration of urea in the hydrating solution, and (2) the SO x /NO x sorbent can be altered to accommodate varying SO₂ and NO concentrations by changing the concentration of urea in the hydrating solution. Example 2 A known amount of calcium oxide (CaO) was hydrated with a water solution containing known amounts of urea (NH₂-CO-NH₂) and sugar (sucrose, C₁₂H₂₂O₁₁). The resulting dry powder was injected into combustion products containing SO₂ and NO x using the procedure described in Example 1. The following Table 2 indicates the results. The data in Table 2 demonstrate that the optimum temperature at which NO is removed by the hydrated SO x /NO x sorbent can be reduced by the addition of a soluble organic material to the hydrating solution. The organic material being such that it will be incorporated with the sorbent during the hydration process. Example 3 A known amount of calcium (CaO) was hydrated with a water solution containing known amounts of urea and ethanol. The resulting dry powder was injected into combustion products containing SO₂ and NO x using the procedures described in Example 1. The following Table 3 indicates the results. The results in Table 3 demonstrate that urea can be added to a hydrating solution containing other solvents, such as ethanol, and that the resulting powdered sorbent will simultaneously remove SO₂ and NO when injected into a combustion product stream containing SO₂ and NO. The subject invention provides a number of advantages in employing a solid sorbent for combined SO x /NO x removal. The novel hydrated material is easily adjusted during manufacture to accommodate various ratios of SO x to NO x . Only a single sorbent needs to be stored and injected into a combustion product stream for both SO x and NO x removal. Incorporation of the NO x reductant into the calcium matrix yields a sorbent which can be safely handled compared to other NO x reactants such as ammonia.

Claims: A method for simultaneously reducing sulfur and nitrogen oxides from a gaseous oxygen-containing stream at a temperature in the range of 200°C (400°F) to 1315°C (2400°F) said method comprising: preparing a composition by hydrating lime in an aqueous medium containing a carboxamide, removing any uncomplexed water to provide dry particles of the composition, introducing the composition into the stream at a predetermined ratio as a sorbent for the sulfur and nitrogen oxides. A method according to claim 1 wherein the temperature is at least about 700°C (1300°F). A method according to claim 1 or claim 2 wherein the stream has a volume percent oxygen of at least about 0.1. A method according to any one of the preceding claims wherein the calcium to sulfur molar ratio is from 0.1 to 10. A method according to any one of the preceding claims wherein carboxamide amino nitrogen to NO x equivalent ratio is from 0.2 to 20. A method according to any one of the preceding claims wherein said aqueous medium has up to about 80% of an alkanol of from 1 to 2 carbon atoms. A method according to any one of the preceding claims wherein the composition is introduced into the stream as said particles substantially free of uncomplexed water. A method according to any one of claims 1 to 6 wherein the composition is introduced into the stream as said particles in droplets containing up to about 20% water. A method according to any one of the preceding claims wherein the particles are of a size in the range 0.5 to 20 µm. A method according to any one of the preceding claims wherein said lime is quicklime. A method according to any one of claims 1 to 9 wherein said lime is dolomitic lime. A method according to any one of the preceding claims wherein the carboxamide is urea. A composition comprising dry particles having hydrated lime and carboxamide, having a size in the range 0.5 to 20 µm, and the weight ratio of lime to carboxamide being in the range 2:1 to 25:1.

Inventors: MUZIO, LAWRENCE J., THOMPSON, RICHARD E.
Applicants: MUZIO, LAWRENCE J., THOMPSON, RICHARD E.
Application year: 1987
Main classification: B01D53
IPC codes: B01D53