EMISSION CONTROL SYSTEM UTILIZING CHLORINE DIOXIDE AND HYDROGEN PEROXIDE

CROSS-REFERENCE TO RELATED APPLICATIONS

5 [0001] This application claims the benefit of the earlier filed US Provisional

Application Serial No. 60/737,015 filed November 14, 2005, and US Patent- Application Serial No. 11/466,624 filed August 23, 2006, the contents of which are incorporated herein by reference.

10 ORIGIN OF THE INVENTION

[0002] The invention described herein was made in part by an employee of the United States Government and may be manufactured and used by and for the Government of the United Stales for governmental purposes without the payment of any royalties thereon or therefor.

15

FIELD OF THE INVENTION

[0003] The present invention relates generally to emission control and in particular to the control of emissions from combustion sources.

20 BACKGROUND OF THE INVENTION

[0004] Control of emissions from fossil fuel combustions sources addresses a major environmental problem. The Environmental Protection Agency (EPA) through the Clean Air Act regulates the emissions from fossil-fuel-fired power plants. Initial regulations were focused on oxides-of-nitrogen (NOx) and oxides-of-sulfur (SOx) 25 emissions, but newer regulations will include provisions to control heavy metals (Hg, etc.) and carbon dioxide.

[0005] Gas streams from combustion processes are often scrubbed, i.e., contacted with water or water solutions, to remove many of their contaminants. However, these scrubbing processes often produce hazardous waste streams that must 30 be dealt with. Furthermore, scrubbing of nitric oxide (NO) is generally ineffective. To remove this contaminant, the nitric oxide is typically converted to nitrogen dioxide

-?-

(NO ₂ ) prior to scrubbing. Various oxidizing agents have been utilized for this conversion. However, more effective oxidizing agents are also more unstable, may not be cost effective, and may be difficult or dangerous to store.

[0006] For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative methods and apparatus for treating combustion gas streams.

SUMMARY OF THE INVENTION [0007] Methods and apparatus utilizing chlorine dioxide (ClO ₂ ) to reduce NOx emissions arc described herein. Such methods and apparatus may be stand-alone systems or may further be incorporated into more encompassing systems, such as systems utilizing hydrogen peroxide to reduce SOx, NOx, and mercury or other oxides- of-metal emissions. The apparatus is modular and components can be added or removed depending upon the specific requirements for a given removal operation.

[0008] For one embodiment, the invention provides a method of treating a gas stream for removal of at least oxides of nitrogen. The method includes generating chlorine dioxide in a continuous reaction process, mixing the chlorine dioxide with the gas stream upon generation, maintaining a desired level of nitric oxide in the gas stream by regulating a generation rate of the chlorine dioxide, and scrubbing the gas stream with a hydrogen peroxide solution, thereby producing a first scrubber liquor. The method further includes maintaining a desired hydrogen peroxide concentration in the first scrubber liquor by regulating a flow of the first hydrogen peroxide solution.

[0009] For another embodiment, the invention provides a method of treating a gas stream for removal of at least oxides of sulfur and oxides of nitrogen and separating Uic resulting sulfuric and nitric acids. The method includes scrubbing the gas stream with a first hydrogen peroxide solution, thereby producing a first scrubber liquor that contains primarily only oxides of sulfur, and maintaining a desired hydrogen peroxide concentration in the first scrubber liquor by regulating a ⁽ low of the first hydrogen peroxide solution. The method further includes removing a by-product stream containing sulfuric acid after attaining a desired sulfuric acid concentration in the first scrubber liquor. The method further includes a demister to reduce the transport of sulfuric acid mist. The method further includes oxidizing the gas stream using a

chlorine dioxide mixture adapted to convert nitric oxide to nitrogen dioxide after scrubbing the gas stream with the first hydrogen peroxide solution and, after oxidizing the gas stream, scrubbing the gas stream with a second hydrogen peroxide solution. The method still further oxidizes elemental mercury to oxides-of-mercury with chloride dioxide and, after scrubbing the gas stream in a second hydrogen peroxide solution that contains nitric acid, dissolves the oxidcs-of-mercury.

[0010] For a further embodiment, the invention provides a system for the removal of at least oxides of nitrogen from a gas stream. The system includes an oxidation tower coupled to receive the gas stream and a chlorine dioxide mixture, a scrubber coupled to receive an exit gas stream from the oxidation tower and coupled to receive a hydrogen peroxide solution and a recycled liquor from the scrubber, a demister to reduce nitric acid mist, a neutral izer coupled to receive an exit gas stream from the scrubber that adjusts the pH to 7.0 ± 2.0, and a chlorine dioxide generation and injection system for continuous generation of the chlorine dioxide mixture and injection into the oxidation tower.

[0011] For a still further embodiment, the invention provides a system for the removal of at least oxides of sulfur and oxides of nitrogen from a combustion flue gas. The system includes a heat exchanger to transfer heat from a raw Hue gas stream to the processed flue gas stream before it enters the flue exhaust stack, a water or a cooler/heat exchange coupled to receive a raw flue gas stream as it exits from the heat exchanger, a first scrubber coupled to receive an exit gas stream from the water or a cooler and coupled to receive a first hydrogen peroxide solution, a solids removal system coupled to receive a first scrubber liquor from the first scrubber and to provide a recycled scrubber liquor to the first scrubber after removal of solids from the first scrubber liquor, and a control system to maintain a desired hydrogen peroxide concentration in the recycled scrubber liquor and to maintain a desired sulfuric acid concentration in the recycled scrubber liquor. The system further includes a first demister coupled to receive an exit gas stream from the first scrubber, an oxidation tower coupled to receive an exit gas stream from the demister and a chlorine dioxide mixture, and a second scrubber coupled to receive an exit gas stream from the oxidation lower and coupled to receive a second hydrogen peroxide solution and a recycled liquor from the second scrubber. The system further includes a second demister coupled to

receive an exit gas from the second scrubber. The system still further includes a neutralizer coupled to receive an exit gas stream from the second demister coupled to the second scrubber and a chlorine dioxide generation and injection system for continuous generation of the chlorine dioxide mixture and injection into the oxidation tower.

[0012] The invention further includes methods and apparatus of varying scope.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a block schematic of an emission control system in accordance with an embodiment of the invention.

[0014] FIG. 2 is a schematic of a hydrogen peroxide concentration control system for use in accordance with one embodiment of the invention.

[0015] FIG. 3 is a hydrogen peroxide reaction vessel subsystem for use in accordance with one embodiment of the invention. [0016] FIG. 4 is a functional schematic of a chlorine dioxide generation and injection system in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical, and chemical changes may be made without departing from the spirit and scope of the present invention. It is noted that the drawings arc not to scale unless a scale is provided thereon. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.

[0018] Emission control systems in accordance with the invention address environmental pollutants SOx, NOx, and heavy metals. Such systems are designed to reduce emissions of these environmental pollutants to the levels established, for example, by the EPA (US Environmental Protection Agency). This emission control system provides a method based on hydrogen peroxide to reduce the SOx, NOx, and metal and metal oxide emissions from combustion sources to acceptable levels as established by the EPA. In addition, useful by-product streams of sulfuric acid, nitric acid, salts of these acids, and feedstock for oxides-of-metal production may be isolated.

[0019] FIG. 1 is a block schematic of an emission control system in accordance with an embodiment of the invention. The process starts with a gas stream, such as raw flue gas 150 after the particulates have been removed. There are several options for this design and steps can be omitted or alternate unit operations may be substituted for the general processes depending on the requirements of the installation. These alternate steps are noted during the description of the process. [0020] The first step of the process is to use a heat exchanger 102 to transfer the heat form the incoming flue gas to the exiting flue gas. A further embodiment ofthe invention is to use the heat transferred to the exiting flue gas to raise the flue gas temperature to increase the stack gas velocity. A further embodiment of the inventions is to improve the scrubbing efficiency by lowering the flue gas temperature. It is desirable to reduce the SOx emissions first before removing the NOx emissions because it permits the separation of sulfuric and nitric acids and it makes removal ofthe latter more efficient, so in the preferred embodiment of the order of scrubbing improves the overall efficiency and lowers overall cost as well. Keeping in mind that most of the NOx pollutants are NO and not NOx, and that NO is not soluble in water and SOx is water soluble in the presence of hydrogen peroxide, this allows the separation of sulfuric and nitric acids. NO must first be oxidized to NO ₂ before it can be scrubbed in the hydrogen peroxide solution.

[0021] The second step in the process is to use the cooling chamber 103 to provide additional cooling ofthe flue gas that exits the heat exchanger. The cool water is returned to the cooling pond 104.

[0022] An alternate first step of the process can be to use a cooling and wash chamber 102 to cool and remove some of the particulates in the entering flue gas 150. Process water from chamber 103 is provided to wash chamber 102. Particulates, wash water and heat are sent from chamber wash 102 to the sedimentation and cooling pond 104. Water is then re-circulated from pond 104 back to water chamber 103.

[0023] The cooled or washed flue gas is fed from cooling or wash chamber 103 to a scrubber tower 105 for the removal of SOx and/or heavy metals. Scrubber lower 105 uses hydrogen peroxide from hydrogen peroxide storage 112 to oxidize sulfurous acid (H ₂ SO ₃ ) to sulfuric acid (H ₂ SO ₄ ) to prevent reemission of SO ₂ . Hydrogen peroxide storage 112 preferably provides aqueous hydrogen peroxide of approximately 35 percent to 70 percent by volume, and more preferably at approximately 70 percent by volume. λs the scrubber liquor pH decreases due to the formation of sulfuric acid, most of the heavy metal oxides, including Hg, etc., are dispersed as metal oxides and/or dissolved hydroxides are converted to sulfates. The remaining un-dissolved particulates and insoluble sulfates arc removed with a solids removal system 106, e.g., a solid-bowl centrifuge, a continuous belt filter, or other system for the continuous removal of solids from a liquid stream. FIG. 1 depicts the use of a centrifuge 106 as the solids removal system as but one example.

[0024] Centrifuge 106 continuously removes the solids and circulates the scrubber liquor through valve 109 back to scrubber tower 105 for continuous scrubbing and cleaning the flue gas. When the scrubber liquor (sulfuric acid) reaches the desired concentration, the cleansed scrubber liquor is discharged from the centrifuge 106 through valve 1 10 then drained to sulfuric acid storage 1 11, which can then be utilized for fertilizer or industrial uses. The solids from centrifuge 106 are discharged to the recycle solids container 107. Soluble metals and metal oxides present in the cleansed scrubber liquor may further be removed as insoluble solids upon neutralization of the sulfuric acid as may be performed, for example, during the production of fertilizer.

[0025] The concentration of hydrogen peroxide in the first scrubbing mixture is maintained at a predetermined level, e.g., 0.1 to 5 percent by volume. Concentration of the first scrubbing mixture may be maintained using a hydrogen peroxide controller of the type described below. Additional detail of such a controller may be found in U.S. Patent No. 6,039,783 issued March 21, 2000, to Lucck, ct al. and U.S. Patent No. 6,641 ,638 issued November 4, 2003, to Lucck, et al.

[0026] When the flue gas exits the scrubber tower 105 it contacts the dcmister 108, where the mist that contains sulfuric acid coalesces. The coalesced mist is returned to the scrubber tower 105 and the desulfurized flue gas flows to the neutralizer 118, if only the SOx system is used. IfNOx is present, the neutralization is bypassed and the flue gas flows directly to the oxidation tower 1 13. Residual acid gases are neutralized with a base in neutralizer 118, the cleansed and neutralized flue gas passes through the heat exchanger, and then exits through the exhaust slack. Alternately, an exhaust fan could be used in place of the heat exchanger at the outlet of the neutralizer 118, if desired, to boost the cleansed and neutralized flue gas out of the exhaust stack as processed flue gas 152.

[0027] The analytical system used for the control system, as initially described in U.S. Patent No. 6,039,783, contains a unique process that controls the concentration of hydrogen peroxide. The process is controlled by a programmable logic controller (PLC) designed to measure the concentration of hydrogen peroxide and to add additional hydrogen peroxide as required to maintain the preset concentration. In addition to the hydrogen peroxide controller, the control system contains two commercial controllers, one for pH and the other to measure the concentration of sulfuric acid, nitric acid, or salts of these acids. These commercial controllers are used to maintain a target pH or concentration and to add reagents as required. Details of the design and operation of thc control system arc given in the following section.

[0028] The block flow diagram for a hydrogen peroxide controller suited for use with embodiments of the invention is shown in FIG. 2. The hydrogen peroxide PLC that controls the operations starts the first sequence by pulling a sample into the system from sample point 231 with pump 232 and pumping it through valve 233. The pH probe 247 and conductivity probe 248 arc exposed to the sample before passing the sample into metallic filter 234 where a portion of thc sample passes through the filler 234 and the remainder bypasses the filler 234 and washes the residues back to sample return 245. The Filtrate that passed through filter 234 continues through valve 235, multiport valve 236, and sample loop 237. From sample loop 237 the sample returns to multiport valve 236 and then exits and returns back to sample return 245. The sequence of events described above serves to collect a sample from the sample point 231 , filter the sample, fill the sample loop that has a known volume, and return the unused sample to sample return 245. While the sample loop 237 is being filled, the metering pump 238 pulls sodium hypochlorite from container 239 and injects a known

volume into reaction cell 240 through mulliport valve 236. The second sequence is triggered by the PLC, which sends a signal to rotate multiport valve 236 and pump a second quantity of sodium hypochlorite, but this time it is used to displace the sample from the sample loop 237, which is pumped into the reaction cell 240. The reaction of sodium hypochlorite with hydrogen peroxide produces water, sodium chloride, and oxygen, which causes an increase in pressure in the reaction cell that is sensed by the pressure transducer 241 . Calibration data programmed into the PLC for pressure change as a function of hydrogen peroxide concentration is used to control the concentration of hydrogen peroxide in the system. If the measured concentration is below the set concentration, a pump is activated to transfer hydrogen peroxide from storage. The hydrogen peroxide pump stops when the measured concentration is greater than the set concentration. This cycle is repeated continuously to maintain the set concentration of hydrogen peroxide. While the concentration is being measured, the filter 234 is back-flushed with water 243 through valve 244 to remove particulates captured by the filter 234. When the back-flush starts, valve 233 closes and pump 232 stops and the water 243 pushes through the filter 234 and back to the sample return 245. In addition, the contents of the reaction cell may be expelled through valve 242 to sample waste 246 at this time. Once the back-flush is complete, the PLC returns the process to the first sequence and sampling process start over again. [0029] FIG. 3 shows the details of the reaction cell 240. The reaction cell body

351 has an internal chamber 352 that is attached to a gauge guard 353 that protects the pressure transducer 241. Sodium hypochlorite and the sample are pulled through sodium hypochlorite inlet 355 by metering pump 238. Once the reaction in the pressure cell is complete, it is discharged through valve 242 to sample waste 246. The reaction cell is fabricated preferably from a machinable corrosion-resistant polymer.

While the foregoing brief description of the control of hydrogen peroxide concentration is included to aid the reader, a more detailed description is provided in U.S. Patent No. 6,039,783 and U.S. Patent No. 6,641,638 as noted previously.

[0030] The p H may be controlled with commercially-recognized devices, such as Rosemont's model 0054pH/ORP-08 pH controller and a model 306P-02010055 pH probe. The pH probe is item 247 in FIG. 2 and it is used to continuously measure the pH when the acid is neutralized to produce a fertilizer. The pH controller system has proportional algorithms that adjust the pump feed rate as the pH set-point is approached. For one embodiment, the pH is controlled to a level of between 7.0 and ±

2.0 by adding a base, e.g., potassium hydroxide. The concentration of sulfuric acid and/or nitric acid, and salts of these acids, is controlled with the Rosemont model 1054B%l -99 controller. The conductivity probe model 228-02-21-54-61 is item 248 in FIG. 2. Once calibrated for the specific ion used in the system, the proportional control algorithms adjust pumping rate for the base used to form the salts.

[0031] Returning to FIG. 1, for removal of NO _x emissions, the neutralized flue gas from neutral izcr 1 1 8 flows to NO oxidation tower 113, where nitric oxide (NO) is oxidized to nitrogen dioxide (NO ₂ ). Alternatively, the flue gas may be passed directly from the demister 108 to the NO oxidation tower 113 for later neutralization, eliminating neutralizer 118. Chlorine dioxide (ClO ₂ ) from chlorine dioxide feed system 114 is fed into NO oxidation tower 113.

[0032] The chlorine dioxide fed to oxidation tower 113 reacts with NO to convert it to NO ₂ and HNO ₃ as follows:

2NO + ClO ₂ + H ₂ O → NO ₂ + HCl + HNO ₃ Both the HCl and HNO ₃ may be removed by subsequent scrubbing.

[0033] The oxidized NO in the flue gas flows from the oxidation tower 1 13 to the scrubber tower 115 where it is captured as nitric acid in an acidic hydrogen peroxide scrubber liquor generally as follows: 2NO ₂ -I- H ₂ O ₂ → 2HNO ₃ The concentration of hydrogen peroxide in the second scrubbing mixture ranges from 0.1 percent to 5 percent by volume and is controlled by a second hydrogen peroxide controller determining make-up hydrogen peroxide from the hydrogen peroxide storage 112 to add to the re-circulating scrubber liquor from pump 116. The cleansed flue gas that exits from scrubber tower 1 15 passes into neutralizer 119, where base is added to neutralize any residual acid gases by adjusting the pH to 7. Once neutralized, the cleansed flue gas exits through the heal exchanger, and then to the exhaust stack as processed flue gas 152. Alternately, an exhaust fan could be used in place of the heat exchanger at the outlet of the neutralizer 1 19, if desired, to boost the cleansed and neutralized flue gas out of the exhaust stack as processed flue gas 152. The mixed acid solution containing nitric acid and hydrochloric acid is then sent to mixed acid storage 1 17 for recovery or disposal.

[0034] Although chlorine dioxide is recognized as an unstable gas, the various embodiments generate the chlorine dioxide gas at the time of use and, optionally, providing for concurrent dilution with air. Chlorine dioxide may be produced continuously by mixing a solution of hydrogen peroxide (H ₂ O ₂ ) and sulfuric acid (H ₂ SO ₄ ) with the solution of sodium chlorate (NaClOs). The produced chlorine dioxide may be mixed, as it is formed, with air to keep the gas phase concentration below 10 percent.

[0035] FIG. 4 is a functional schematic of a chlorine dioxide feed system 414 in accordance with an embodiment of the invention. The chlorine dioxide is generated through a reaction of a sodium chlorate solution 460 and a hydrogen peroxide and sulfuric acid solution 462. The sodium chlorate solution 460 is metercd into a reaction vessel 468 using a metering pump 464. The hydrogen peroxide and sulfuric acid solution 462 is mctered into the reaction vessel 468 using a metering pump 466.

[0036] For one embodiment, the hydrogen peroxide and sulfuric acid solution 462 is produced using 37.6 wt% of sulfuric acid (98 wt%) and 7 wt% of hydiogen peroxide (50 wt%) in water. The solution can be prepared by slowly adding sulfuric acid (98 wt-%) to water, allowing that to cool, and then adding hydrogen peroxide (50 wt- %). For a further embodiment, the sodium chlorate solution 460 contains 50 wt% sodium chlorate (NaClO ₃ ) in water. λ solution having a specific gravity of 1.4 may be prepared by mixing equal weights OfNaClO ₃ and water. The solution can be prepared by slowing adding sodium chlorate to water and stirring until the solids arc dissolved.

[0037] The reaction vessel 468 is preferably of a material resistant to the oxidizing power of the reagents, e.g., glass-lined, polyethylene-lined, Teflon®-lincd, etc., and equipped with a mixer 470. To initiate the reaction, it may be desirable to add crystalline sodium chlorate to some hydrogen peroxide and sulfuric acid solution 462 in the reaction vessel 468, and then meter in further hydrogen peroxide and sulfuric acid solution 462 and sodium chlorate solution 460 after the reaction has begun, e.g., after approximately a 5 minute delay. By metering in at approximately stoichiometric molar quantities of the reagents 460 and 462, a desired production rate of chlorine dioxide may be maintained.

[0038] For one embodiment, desired rale for production of chlorine dioxide is an amount sufficient to reduce NO emissions from a flue gas stream 484 to a desired level when mixed with the flue gas 484 as described below. The reaction of the reagent solutions 460 and 462 may proceed generally as follows: 2NaClO ₃ + H ₂ SO ₄ + H ₂ O ₂ → 2ClO ₂ + O ₂ + Na ₂ SO ₄ + 2H ₂ O

Due to dilution as the reaction proceeds, the reaction vessel 468 may need to be drained occasionally for the removal of excess water and sodium sulfate (Na ₂ SO ₄ ).

[0039] The reaction vessel may further include an air intake 472 to maintain the reaction vessel 468 at approximately ambient pressures and to provide an air sweep for dilution of the produced chlorine dioxide. For one embodiment the air intake 472 is passive and provides make-up air as a chlorine dioxide/air mixture 474 is pulled from head space of the reaction vessel 468 by an eductor 476. Alternatively, the air intake 472 may include regulated pressure feed to push the chlorine dioxide/air mixture 474 out of the reaction vessel 468. If air intake 474 is not passive, the eductor 476 may be eliminated. Note that while air is used in this example, an inert gas may also be substituted.

[0040] The eductor 476 is operated using a pressurized air feed 478, resulting in a further diluted chlorine dioxide/air mixture 480 for feed into flue 482. Note that the flue may represent the oxidation tower 1 13 of FIG. 1. In this example, the incoming flue gas 484 would represent the output of the neutralizcr 1 18 or the dcmislcr 108 of FIG. 1, and the outgoing flue gas 486 would represent the output of the oxidation lower 113 fed to scrubber 1 15.

[0041] The particulate material in the SOx scrubber liquor is a mixture of fly- ash and insoluble sulfates that include mercury and other heavy metals. These solid materials are removed from the scrubber sump with a solids removal system, such as a continuous filter, centrifuge, or a combination of a continuous filter and a centrifuge. To further improve the separation efficiency, the scrubber sump can be divided into two compartments. The first compartment is designed to receive the scrubber liquor that is returning from the scrubber tower and the second compartment is designed to receive the cleansed scrubber liquor and the overflow from the first compartment. The

scrubber pump draws the scrubber liquor from the middle of the second compartment and the continuous filter or centrifuge draws from the bottom of the first compartment. This two compartment configuration can facilitate maximizing the concentration of particulates going to the filter or centrifuge and minimizing the particulates going to the scrubber pump.

[0042] Methods and apparatus for controlling emissions have been described. Some methods utilize hydrogen peroxide to reduce SOx and mercury (or other oxidcs- of-metal) emissions prior to treatment to reduce NOx. For removal of NO, chlorine dioxide is generated continuously and fed into the gas stream to be treated. By utilizing methods and apparatus in accordance with the invention, combustion flue gas streams can be treated for the removal of NOx, as well as SOx and oxides-of-metal, while isolating useful by-products streams of nitric acid, sulfuric acid, salts of nitric acid, salts of sulfuric acid and solids for the recovery of the heavy metals. One of the significant advantages of the present invention is the fact that the process can be run continuously, with measuring and adjustments made in real time while the process is being performed. Computer monitoring can initiate flow changes of reagents in response to automatic measurements to maintain desired process conditions.

[0043] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.