CZ349992A3 - Process and agent for reducing concentration of contaminants in combustion products - Google Patents

Abstract

The invention presented relates to a process and composition for the reduction of nitrogen oxides and sulfur oxides in the effluent from the combustion of a carbonaceous fuel. The composition is a slurry which includes calcium hydroxide effective for the reduction of sulfur oxides in the effluent; a nitrogenous treatment agent effective for the reduction of nitrogen oxides in the effluent; and water. The process permits reductions of sulfur dioxide beyond those found when calcium hydroxide is used alone, without sacrificing nitrogen oxides reductions.

Description

The present invention relates to a novel process and a means for achieving a substantial reduction in the concentration of nitrogen oxides. where x is an integer, usually · 1 or 2. and sulfur oxides SOx, where x is an integer, usually 2. or 3, in the flue gas of the carbonaceous fuel.

At. the use of fossil fuels in suspension-heated boilers, such as multi-purpose boilers, produces temperatures above about 1094 ° C, most commonly, above 1205 ° C. At these temperatures, nitrogen and sulfur in the fuel can be oxidized to form acidic gases. At such high temperatures, NOx can also be formed from nitrogen in the atmosphere, since such high temperatures cause the formation of free oxygen and nitrogen radicals and? their chemical reaction to form nitrogen oxides. These impurities can furthermore be formed in flue gases. other incineration plants, such as municipal waste incineration plants or industrial boilers or heaters. Generally speaking, sulfur and nitrogen oxides can be formed in the flue gas of all plants where the fuel contains sulfur.

Both nitrogen oxides and sulfur oxides are difficult impurities found in the flue gases of the boilers heated as described above and constitute the main components of acid rain. In addition, it is believed that nitrogen oxides may undergo a process known as photochemical smog formation by a series of reactions in the presence of some hydrocarbons. They can also contribute to the warming of the atmosphere, commonly referred to as the greenhouse effect.

Thus, there is a need to find a means for effectively reducing the concentration of nitrogen oxides and sulfur oxides in the flue gas by a simple operation. '

BACKGROUND OF THE INVENTION j

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Means and procedures for reducing the concentration of NOx and / or SOx are the subject of intensive research. Due to greater importance. currently attached to environmental issues can be expected. That these studies will continue intensively. >

To date, several effective processes for reducing the concentration of nitrogen oxides by selective gas phase reactions have been described. Na-C, an example of Lyon in U.S. Pat. No. 3,900,554 discloses reducing NO in the flue gas with aqueous ammonia solutions, optionally together with another reducing agent, such as hydrogen, at a temperature of up to about 1150 ° C.

In U.S. Pat. No. 4,208,386 to Fun, Men, and Sotter. _r which is generally considered to improve Lyon process is described is caused reindeer. by introducing aqueous urea solutions to reduce || Nitrogen oxide treatment by a free-radical-mediated selective gas phase process. Furthermore, Hofmann, Sprague and -¾ Sun described the use of carbamate. ammonium to reduce the concentration of ¾ nitrogen oxides in U.S. Patent 4,997,631. The use of other nitrogenous agents to reduce the NOx concentration in flue gas is described, for example, in U.S. Patent No. 5,017,347 to Epperly, Sullivan and Spragua, and U.S. Patent No. 5,057,293 to Epperly, Peter-Hoblyn. Shulofa Jr. . Sullivan, Spragua and O'Leary. j

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Said agents are nitrogenous Although uniquely effective in reducing the concentration {N0 _x in exhaust gas, but the patent Li- j.

There is no indication in the literature that, without further processing or addition of other reagents, they would facilitate the reduction of the sulfur oxide concentration.

The literature on reducing SOx concentration contains numerous examples of the use of limestone-containing compositions;

CCaCO3), quicklime (CaaOH) or slaked lime (CaCOHIA) and other similar compounds as sulfur scrubbing agents

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for flue gas. Such procedures are described, for example, by Kurod et al. In US Patent 4,687,649 and by Frank et al. In US 4,540,555. In addition, Torbov in US 4,555,996 showed that it is preferable to use calcium compounds in suspension rather than dry.

While these processes are effective in reducing the SO 2 concentration in the flue gas, there is no indication that the efficacy of these compositions and processes can be increased and effectively used to reduce both the SOx and NOx concentrations simultaneously. The latest literature even demonstrates the opposite.

Thompson 'and Mužio disclose in U.S. Pat. Nos. 4,731,233 and 4,795-5586 a method and composition that can simultaneously reduce the NOx and SOx concentration according to the patent owners. in the flue gas. The disclosed composition comprises calcium oxide and urea hydrate, used as a sorbent to remove NOx and SOx from the flue gas. There is no indication of synergies from these data. In addition, DeMichele and Quattroni. EP 0,373,351 discloses the use of a mixture of urea, calcium hydroxide and water in conjunction with electrostatic purification to remove nitric oxide and sulfur oxides. It is reported that when ¹ · calcium oxide in the dispersion is present in an amount of at least 35% is necessary to use a dispersant Na- basis. polysaccharide or lignin sulfonate. DeMichele and Quattroni directly claim that their process is better than with dry calcium oxide, but they do not make any comparison with the independent use of the calcium oxide slurry, respectively. urea.

In U.S. Patent 5,058,514. disclose Mozes, Manga 1 and Thampt suspension injection processes for SOa and NOx removal using a calcium carbonate and nitrogen precursor such as urea. No interaction found. According to the inventors, the advantage of using this method lies in the independent effect of both components, i.e. without loss of potency compared to the independent use of the individual components.

Mozes and coworkers found the absence of inter- ference significant and surprising. No information or mention of achieving synergy using urea and calcium carbonate is given.

SUMMARY OF THE INVENTION

SUMMARY OF THE INVENTION The present invention provides a composition and method for substantially reducing the concentration of SOx in flue gas while reducing the concentration of NOx. The composition is a slurry comprising calcium hydroxide, a calcium sorbent effective to remove sulfur oxides from flue gas, and a nitrogenous agent effective to remove nitrogen oxides from flue gas.

The composition may be injected into the flue gas in the form of a suspension. It has been found that the use of this composition can achieve a greater reduction in the concentration of sulfur oxides than with the components alone, without limiting the reduction in the NOx concentration.

Thus, the composition according to the invention makes it possible to simultaneously reduce the concentration of nitrogen oxides and sulfur oxides in the flue gas. The composition comprises calcium hydroxide, which is selected for its effectiveness in removing sulfur oxides from the flue gas. Further, the composition comprises a nitrogenous agent selected for its NOx removal efficiency in the flue gas. The composition according to the invention is introduced into the flue gas in the form of a suspension. The reduction in the concentration of sulfur oxides achieved is higher than expected from the use of calcium hydroxide alone.

Although many calcium-based sorbents for SO 2 removal are known, including limestone, calcium carbonate, calcium hydroxide, and mixtures thereof, it has been found that only calcium hydroxide (which is meant to include calcined CaO) forms a synergistic mixture with a nitrogenous agent. The efficacy of other sorbents, such as calcium carbonate, has been found to be consistent with Mozes, Mangal, and Thampi data, ie there is no decrease in efficacy when used with urea, but there is no sys-

nergie.

Generally, the sorbent must contain at least about 10% calcium hydroxide; The amount of calcium sorbent in the composition of the invention may vary depending on the amount of SO2 in the flue gas and the amount of nitrogenous agent in the composition. Preferably, the calcium hydroxide is present in the composition in a molar ratio of calcium to base concentration (i.e., after pretreatment) of SO 2 of about 1: 2 to about. 4-1, preferably about 1 = 1 to about 3.-1. Generally, calcium hydroxide constitutes at least about 10, preferably between about 15 and about 50% of the suspension of the invention. When considering the dry weight of the components of the suspension, i.e. without diluent, calcium hydroxide is up to about 97% and preferably between about 65 and about 97%.

Nitrogenous reagents suitable for the preparation of the composition of the invention. of the disclosure include compounds disclosed, for example, in US Patents 4,997-631, 5,017,347 and 5,057,293. These include urea, amo7, niac, hexamethylenetetramine (HMTA), ammonium carbamate and various ammonium salts. A preferred component of the NOx removal composition of the invention is urea.

The nitrogenous reagents are preferably present in a ratio of nitrogen in the reagent to the base concentration (i.e., after pretreatment) of nitrogen oxides in the flue gas in the range of about 0.5 to about 3.5.

· <Λ This ratio can be referred to as the normalized stoichiometric ratio or NSR '. The normalized stoichiometric ratio is the ratio of the concentration of reducing radicals, such as NHx radicals (NHx radicals, where x is an integer, are considered to be a reagent-derived group and facilitate a series of reactions by which NO _{x is} reduced to N 2). to the concentration of nitrogen oxides in the flue gas and can be expressed as [/χ] / [Ν0χ1. If the reduction chemistry is not sufficiently defined, a molar ratio of reagent to NOx concentration may be used instead of NSR; Thus, the term NSR as used herein also includes, as appropriate, the molar ratio.

The nitrogenous reagent is preferably present in an amount of about 3 to about 35% by weight of the total composition without diluent (i.e., water) and about 1 to about 15% of the total suspension. If urea is added in an aqueous solution, water is not taken into account when calculating the weight of the urea added.

Generally, the weight ratio of sorbent to nitrogen reagent in the suspension of the invention is about 1.5-1 to about 33 = 1, preferably about δ

2.7 = 1 to about 22 = 1. In addition to the nitrogenous reagent and sorbent, the remainder of the composition contains water or other diluent sufficient to form a suspension. Generally, the suspension contains about 20 to about 70, preferably about 26 to about 50% by weight solids.

By way of example, assuming a waste gas velocity of 28,000 m ³ / h (1,000,000 SCF / h) and a SO2 concentration of 1,000 ppm, and a NOx concentration of 300 ppm (which are commonly found), a suspension of Ca (OH) a and urea in water It can be found to have a sorbent to SO2 ratio of 2 (or 174.4 kg / h) and a NS dj of 2 (21.20 kg / h). The result is a sorbent to urea ratio

8.22.

The suspension thus obtained is then introduced into the off-gas under conditions favorable to the reduction of the concentration of the impurities present. Generally, the slurry is introduced into the gas at a temperature of about 816 to about 1205 ° C.

The suspension according to the invention is preferably obtained by mixing the calcium hydroxide suspension with a concentrated nitrate solution in the desired ratio. If the slurry is not to be introduced into the flue gas immediately after its preparation, it may be desirable to add suitable stabilizers to the slurry over time. for which it should be stored before use.

The suspension thus prepared is then introduced, for example by injection, into the treated flue gas. This ensures uniform mixing and sufficient penetration of the suspension. Injection takes place from nozzles located in the pre-spaced openings and allowing the suspension to be distributed evenly across the cross-sections, passing through my planes at different flue gas flow levels.

Because of the need to distribute the suspension uniformly throughout the desired flue gas cross-sections to effectively remove impurities, the nozzles must be selected in an efficient manner. Conventional nozzles are known to the skilled person, some of which are suitable for this purpose. Suitable individual nozzles for injecting an aqueous suspension include Turbotak ^a . Other suitable nozzles are described in patent application PCT / EP91 / 00952 entitled Process and Apparatus far Minimizing Pollutant Concentrates in Combustion Gases, filed on. names Chavla, von Bergmann and Pachaly 21.5. 1991, and in DE-26 27 830 C2, published 11-11.1982, it is likely that under certain conditions the amount of water or other diluent in the suspension according to the invention may extinguish the sorbent and the nitrogenous agent. In fact, the time required to evaporate the diluent may delay the effect of the reagents and processing at a lower temperature than initially expected. This may occur (but not necessarily) when the solids content of the suspension is below about 38%, preferably below about 28% by weight.

If this occurs, the suspension may further comprise an accelerator such as oxidized hydrocarbons such as ethylene glycol, ammonium salts of organic acids such as ammonium acetate and benzoate, heterocyclic hydrocarbons with at least one ring oxygen, such as furfural. molasses, sugar, 5- or 6-membered heterocyclic hydrocarbons with at least one ring nitrogen, for example pyridine and pyrrolidine, hydroxyamino hydrocarbons such as milk or skimmed milk, amino acids, proteins and monoethanolamine, and various other compounds described as effective removal agents nitrogen oxides in flue gas. These accelerators which are ^'T ient present in an amount of about 0.5 to about, 25 wt%, causing a reduction in flue gas temperature at which the suspension · · efficient.

¹ Such accelerators are described in U.S. Patents

US 4,719,092; US 4,888-164. US 4,877,591, US 4,803,059, in PCT patent application entitled Process Far The Reduction of Nitrogen Oxides in an Effluence with publication number WO 89/02870, filed in the names of Epperly, Sullivan and Sprague on September 22, 1988. in PCT patent application entitled "Process for the Reduction of Nitrogen Oxides in an Effluent" with publication number WO 89/03242 filed in the name of Epperly, Sullivan and Sprague on October 14, 1988; and US Patents 4,830,839; -704.

The use of a composition of the invention to reduce the concentration of NOx and SOx by the method of the invention may be part of a multi-stage treatment process to reduce the content of impurities in the flue gas. Such processes are described, for example, in U.S. Pat. Nos. 4,777,024 to Epperly, Peter-Hoblyn, Shulof Jr., and Sullivan, and in U.S. Patent 5,057,293 to Epperly, Peter-Hoblyn, Shulof Jr., Sullivan, Spragua and O'Leary.

For example, in the first step of such a process, NOx and SOx are reduced by means of the composition of the invention. In the second step, either the composition of the invention (possibly in a different ratio) or other agents for removing either NO _x or SO _x can be used. Another possibility is to perform a first stage with a urea or ammonia solution and a second stage with a composition of the invention, or a first stage with a composition of the invention and a second stage with a urea, ammonia or hexamethylenetetramine solution, and so on. Other components, for example, a complexing agent for copper, may be used with the composition of the invention.

Surprisingly, it has now been found that the use of the composition according to the invention results in a synergistic decrease in the concentration of sulfur oxides in the flue gas as compared to the reduction that can be achieved by separate introduction of the individual components without significantly affecting the NOx9 reduction achieved.

L? BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail with reference to the accompanying drawings, in which Fig. 1 is a graphical representation of the results of Example 1 and Fig. 2 is a graphical representation of the results of Example 2.

j | The invention is illustrated by the following examples

Non-limiting examples are provided to further illustrate the invention. Unless otherwise indicated, all parts and percentages are by weight and are based on the weight of the product at the particular process stage.

Example 1

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A pilot plant furnace with refractory lining is used as the apparatus; a. Bottom heating output. 14.7 kV, with an internal diameter of 15.2 cm and a total length of about 4 m, capable of burning natural gas or coal. During natural gas combustion, the gaseous combustion environment is simulated by doping the fuel with ammonia (which is oxidized to form nitrogen oxides) and sulfur dioxide. The furnace works with tangential and axial air with a total flow rate of 0.39 m ³ / min, including a 50% excess.

gaseous emissions shall be sampled and passed through heated pipes to continuous emission monitors. Sulfur dioxide concentration analysis is performed by an ultraviolet analyzer downstream of the particle separator and heated sampling line. (about 350 PCs). The removal percentage is calculated from the removal values at the sorbent to sulfur ratio stoichiometry between 1 and 3 = 1 and from the curve taken by the regression method and interpolated to removal at the sorbent to sulfur ratio 2 ^; 1- Nitrogen oxides are analyzed by chemi-condensation. a method that records NO * concentrations not including nitrogen dioxide; however, earlier tests show that nitrogen dioxide concentration is less than 5% of the total nitrogen oxide concentration. All emission results of gases ¹ and

They are corrected for zero concentration. 02 Before the test, the basic concentrations of impurities in the flue gas are determined by injecting deionized water in an amount equivalent to the injected reagents. The test is carried out by injecting dry calcium hydroxide as a sorbent, dosed with a K-tron weight loss twin g screw calibrator, calibrated before and after each operation: dosing a 30% solids calcium hydroxide slurry into the Turbotak nozzle. a calibrated peristaltic pump and a sorbent-urea suspension, where urea in the form of an aqueous solution is dosed into the suspension by means of a calibrated double injection needle pump.

Both the dry sorbent and the suspension are injected with water-cooled probes, which are injected coaxially with the process gas. The dry sorbent probe U® injects 15.7 total XFR airflow to ensure sorbent transfer and dispersion. Turbotak suspension probe uses air <18% of total air flow through the furnace)

k. atomization of droplets.

The droplet size distribution of the suspension leaving the Turbotak nozzle. is determined using a Munhall particle size analyzer which shows that the suspension has a droplet size distribution with a D 50 of 13 µm and a D 90 of 88 µm. The injection point temperature is determined using a suction pyrometer and a R-type thermocouple.

The temperature at the injection nozzle is calculated by extrapolating the temperature values from the downstream inputs. Sorbent is injected!

with a calcium to sulfur molar ratio corrected to 2 = 1. If urea is present in the suspension, it is present at a NSR of about 2.

The results are shown on & br. 1, which shows that the presence of urea increases the ability of calcium hydroxide to remove sulfur dioxide from the gas. |

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Example 2.

The procedure of Example 1 is repeated, but measured. with reducing the concentration of nitrogen oxides. These results are shown in Fig. 2 and show that there was no substantial change in the removal of nitrogen oxides ... caused by the presence of calcium hydroxide as a sorbent.

These examples are to be construed as merely illustrative and not limiting of the scope of the invention. Furthermore, it must be assumed., The practitioner is able, without ^- departing from the scope of the invention to carry out, within its scope various modifications and. Variations of the described solutions.

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Claims (17)

What is claimed is: 1. A method for reducing the concentration of impurities in a flue gas of a carbonaceous fuel, the method comprising: a) forming a suspension comprising (i) at least ae-i 10% by weight of a calcium sorbent containing calcium hydroxide and effective in removing sulfur oxides from the flue gas. C11) aedr 1 to aei. (I) water; and (b) the suspension is introduced into the flue gas under conditions effective to reduce the concentration of sulfur dioxide and nitrogen oxides in the flue gas. Method according to claim 1, characterized in that. That the flue gas temperature at the slurry introduction point is about 4816 to about 1205 ° C. 45 » The method of claim 2, wherein the calcium hydroxide is present in the slurry in an amount of about 15 to about 50% by weight. The method of claim 3, wherein the calcium hydroxide přít is present in an amount corresponding to the molar ratio of calcium to the base concentration of sulfur dioxide and? 41 ^; 2 to a fiktfá 4: 1 The method of claim 1, wherein the nitrogenous reagent comprises urea, ammonia, hexamethylenetetramine, ammonium carfoamate, ammonium salts, or mixtures thereof. jy 6. The process of claim 5 wherein the nitrogenous reagent is present in the slurry in an amount of from about 1% to about 4% by weight. The method of claim. 6 ,. characterized in that the nitrogenous reagents are present in a ratio of NSRs of 0.5 to 3.5 and 3.5. *) ·. The process according to claim 1, wherein the suspension # comprises from 20 to 70% by weight of the solids. 3. A process according to claim 1, wherein the suspension is formed by mixing a concentrated solution of the nitrogenous reagent into the calcium hydroxide suspension, whereupon the suspension thus formed is immediately injected into the flue gas stream. 10. The method according to claim 1, characterized in that it also comprises at least one stage of a multi-stage process for removing the purity of the flue gas. $ 11. The method of claim 10, wherein the next step of the multistage process comprises introducing a solution of urea, ammonia, hexamethylenetetramine, or mixtures thereof. The method of claim 1 wherein the weight ratio of calcium hydroxide to nitrogen reagent is aert · 1.5 = 1 to 33 = 1. A composition for removing impurities from a flue gas of a carbonaceous fuel, comprising a suspension comprising a) at least -4% by weight of a calcium sorbent containing calcium hydroxide effective to remove sulfur oxides in the flue gas. b) 1 to ae4 of 15% by weight of nitrogenous reagent; - 1 ^ _ ··>. * Při active at. removal of nitrogen oxides in flue gas, and \ (c) water. j »v The composition of claim 13, wherein the calcium hydroxide is present in the suspension in an amount of about 15 to about 50% by weight. { Composition according to claim 14, characterized in that the calcium hydroxide is present in an amount corresponding to a molar a calcium to basic sulfur dioxide concentration of about 1 to about 4 = 1. 16. A composition according to claim 13, characterized in that:. wherein the nitrogenous agent comprises urea, ammonia, hexamethylenetetramine. ammonium salts or mixtures thereof The composition of claim. 16, wherein the nitrogenous agent is present in the suspension in an amount of about 1 to about. 15 wt. The composition of claim 17, wherein the nitrogenous agent is present in an NSR ratio of about 0.5 to about 3.5. HER The composition of claim 13, wherein the suspension comprises about 20 to about 70% by weight of the solids part. t The composition of claim 13, wherein the weight ratio of calcium hydroxide to nitrogenous reagent is about 1.5 = 1 to about 33 = 1.