HK1206299B - Methods for removing contaminants from exhaust gases - Google Patents

Description

Method for removing pollutants from exhaust gas

Cross Reference to Related Applications

This application claims priority from U.S. provisional patent application No. 61/640,128 filed on day 4, 20, 2012.

Background

The present invention provides a method for removing a contaminant selected from the group consisting of nitrogen oxides, sulfur oxides, particulates, heavy metals, and acid gases from a gas stream. More specifically, the present invention provides a method for removing contaminants from gas streams produced in marine and land-based engines and other combustion devices, wherein seawater is used to scrub the combustion gas stream.

While co-scrubbing nitrogen oxides produces nitrate as an objectionable byproduct, the present invention can also treat exhaust streams from chemical, metallurgical, partial and complete combustion processes by removing pollutants such as nitrogen oxides, sulfur oxides, hydrochloric acid and particulate matter.

The captured nitrogen oxides are separated from other pollutants in the gas stream. This results in a smaller volume of nitrate-containing liquid stream that needs to be treated before it can be discharged or recycled.

Combustion and chemical processes typically produce gas streams containing pollutants that need to be removed before the gas stream is discharged into the atmosphere.

Large ocean going cargo vessels, ferries, ocean liners and naval warships use low cost hydrocarbon fuels containing sulfur, chlorine, nitrogen and metal compounds in hydrocarbons, which produce exhaust gases containing pollutants such as acid gases, particulate matter and heavy metals. According to new legislation, these large emissions require purging of the fuel stream before being vented to atmosphere. Water scrubbing using seawater is one of the more widely used techniques in many technologies and plants for the removal of acid gases such as sulfur oxides, chlorine, hydrochloric acid, etc., particulates and other contaminants. Seawater is weakly alkaline and in a once-through mode neutralizes the acidic components present in the seawater to form salts, allowing these salts to be discharged back into the ocean in an environmentally safe manner.

Many industrial plants using wet scrubbers, such as fluid catalytic cracking regenerators and generators along the coast, use seawater for scrubbing in a once-through or cyclic mode.

During combustion, in addition to sulfur oxides, hydrochloric acid, chlorine, and other acid gases, nitrogen oxides are also formed for several reasons, such as high flame temperature (thermal NO)_X) Nitrogen-containing compound present in fuel (fuel NO)_X) Or nitrogen-containing components in materials that are subjected to combustion temperatures (such as waste incineration).

Nitrogen oxides formed at temperatures above 1300F (about 704 c) take predominantly the form of nitric oxide, NO. Nitric oxide is the major component of nitrogen oxides produced during combustion. Nitric oxide is almost insoluble in water and therefore water scrubbing removes only negligible amounts of nitric oxide from the nitrogen oxide stream. Coal, solid fuels, heavy oils, and other carbon feedstock oils, when combusted, form exhaust gas streams containing particulate matter and other objectionable contaminants such as heavy metals, e.g., mercury, which may or may not be effectively scrubbed by the water scrubbing operation.

Among all absorption processes, ozone-based processes, as described in U.S. Pat. nos. 6,162,409, 5,206,002 and 7,303,735, provide a variety of pollutant removal methods and have been applied to flue gases generated from gas and coal fired boilers to remove a variety of pollutants, including nitrogen oxides, sulfur oxides, particulates, and the like. Industrially, ozone-based processes are also practiced in reducing emissions in metal pickling processes, fluid catalytic cracking units (FCC), and metal recovery furnaces.

The processes as described in U.S. patent nos. 6,162,409, 5,206,002, 6,649,132, and 7,303,735 take advantage of the chemical nature of nitrogen oxides to react with ozone to form higher oxides of nitrogen, particularly the pentavalent or higher forms. These oxides are readily soluble in water and are easily removed by wet scrubbing. The stoichiometric amount of ozone required to convert 1 mole of nitrogen oxides in the NO form to the pentavalent form is about 1.5 moles of ozone. If the nitrogen oxides are NO₂In the form of (a), this amount is reduced to 0.5 mole of ozone.

While the processes described in these patents are effective at achieving low nitrogen oxide emission levels in the treated gas stream, they produce nitrate/nitric acid in scrubber cleaning. The nitrate/nitric acid needs to be disposed of and discharged in an environmentally safe manner or must be used to produce useful byproducts. These all add to the expense of nitrogen oxide treatment.

When seawater is used as the scrubbing medium, it is used in a once-through mode due to its limited alkalinity. This produces a large amount of liquid discharge from the wet scrubber. When ozone is added to remove nitrogen oxides, the purge stream will contain nitrates that need to be treated before being discharged back into the ocean.

The present invention overcomes the problems that have existed in previous approaches. The contamination of scrubber clean-up with nitrate is mitigated so that large volumes of seawater can be used in a single pass through the scrubbing medium, which can be safely discharged without additional treatment. A separate scrubber for removing nitrogen oxides is also not required, thus minimizing capital investment in retrofitting acid gas (sulfur oxides, hydrochloric acid, etc.) or particulate scrubbing plants with nitrogen oxide control.

Disclosure of Invention

In one embodiment of the present invention, a method for removing contaminants from a gas stream is disclosed, comprising the steps of:

a) flowing a gas stream containing contaminants into a scrubber;

b) contacting the gas stream containing contaminants with a scrubbing medium comprising seawater;

c) contacting the gas stream containing contaminants with ozone; and

d) recovering the gas stream free of contaminants.

In another embodiment of the present invention, a method for removing contaminants from a gas stream is disclosed, the method comprising the steps of:

a) flowing a gas stream containing contaminants into a scrubber;

b) contacting the gas stream containing contaminants with a scrubbing medium comprising seawater;

c) flowing a gas stream containing contaminants into a droplet separator in fluid communication with a scrubber;

d) contacting the gas stream containing contaminants with ozone; and

e) recovering the gas stream free of contaminants.

The gas stream being treated is typically a flue gas stream from a combustion or chemical process. Typically these flue gas streams are from ships or operating units near the ocean where the seawater is abundant. These flue gas streams typically contain contaminants selected from the group consisting of: particulates, sulfur oxides, nitrogen oxides, acid gases, and heavy metals (e.g., mercury).

These pollutants, especially sulfur oxides and nitrogen oxides, will react when in contact with ozone. These reactions also produce by-products such as nitrates and nitric acid, which can be recovered for other operational uses or which can be disposed of and disposed of in an environmentally responsible manner.

The scrubber is typically selected from the group consisting of: spray type, venturi type, rod, packed bed and tray column scrubbers. The scrubber employed in the process of the present invention should be of sufficient size to allow the ozone to mix with the gas stream and remain in contact with the contaminants for a sufficient time to oxidize the contaminants.

Ozone will be added in a stoichiometric amount greater than the amount of nitrogen oxides present in the gas stream.

The gas stream containing oxidized nitrogen oxides will come into contact with a droplet separator where the nitric acid present in the treated gas stream will condense and may be captured in the liquid state.

The scrubber may further comprise a device selected from the group consisting of: cooling coils, demisters and electrostatic precipitators (ESP). These devices can be used to aid in the condensation of certain reaction products of the reaction between the contaminants present in the gas stream and ozone as the gas stream passes through the scrubber. These condensed reaction products may be recovered and recycled or treated for disposal.

It is preferred to have seawater flow into the scrubber through a system of one or more distributors so that the seawater is more freely contacted with the contaminant-containing gas stream to be treated. The seawater may be continuously fed into the scrubber or used in a cyclic manner, as desired by the operator.

Drawings

FIG. 1 is a schematic diagram of a scrubber that includes adding ozone to a flue gas stream.

Figure 2 is a schematic diagram showing another scrubber fed with seawater and ozone and nitrate/nitric acid for clean recovery.

Detailed Description

The flue gas is quenched and scrubbed with seawater in a wet scrubber, which may be of the standard spray type, venturi type, rod, packed bed, tray tower or other scrubber commonly used in industrial operations for acid gas and particulate removal. EDV scrubbers such as those from Belke Technologies, Inc. or Dynawave from MECS, Inc. may be used in the present invention. These scrubbers remove particulates, sulfur oxides, hydrochloric acid, and other contaminants removed by the wet scrubber.

Ozone is then added to the scrubbed gas stream. As described in U.S. patent nos. 7,303,735 and 7,766,995, oxidation of nitrogen oxides is achieved by mixing ozone with the scrubbed gas stream and providing sufficient reaction time. Moderate oxidation of the nitrogen oxides in the presence of moisture will form water-miscible nitric acid.

The flue gas containing oxidized nitrogen oxides is then passed through a droplet separator to reduce residual mist or through controlled cooling on condensation surfaces. The droplets in the separator and/or on the condensing surface provide sufficient surface area for dissolving, condensing, absorbing and removing nitrogen oxides.

The droplets are collected to form a small stream of water in a droplet separator/condenser and removed, if necessary neutralized for safe discharge or for by-product use.

The flue gas stream to be treated contains nitrogen oxides. When ozone is mixed with the gas stream, the nitrogen oxides are oxidized. If all nitrogen oxides are in the form of nitric oxide, the amount of ozone required to convert the nitric oxide to dinitrogen pentoxide is 1.5 moles of ozone per mole of nitrogen oxides. For each mole of NO₂Only 0.5 mole of ozone is required. Thus, ozone in the range of about 0.5 to 1.5 moles per mole of nitrogen oxides may be added to the flue gas stream to be treated. The oxidation of nitrogen oxides to dinitrogen pentoxide involves many reactions, but for the sake of brevity these reactions can be simplified as follows:

NO+O₃→NO₂+O₂(1)

NO₂+O₃→NO₃+O₂(2)

NO₂+NO₃→N₂O₅(3)

the magnitude of reaction (1) is faster than reaction (2). To some extent, reactions (1), (2) and (3) are continuous reactions. NO₂Has limited solubility and therefore unless the reaction proceeds to form specific NO₂Higher oxides have limited removal of nitrogen oxides in wet scrubbers. To form higher oxides without the need to add excess ozone, it is necessary to mix the ozone thoroughly and provide the necessary reaction time while minimizing back mixing. To achieve good nitrogen oxide removal, some theory may be applied. For example, ozone can be introduced into the gas phase using a distributor that distributes ozone uniformly throughout the cross-section of the flue gas stream. A gas flow path for mixing oxygen may be selected in which the gas flow is turbulent. The rate of injection of the ozone-containing gas stream into the flue gas stream can be maintained at least 2 times the flow rate of the flue gas stream, preferably3 times or more.

Modern tools such as Computer Fluid Dynamics (CFD) models can be applied to ensure adequate mixing of ozone in the flue gas stream in a minimum of time.

A conical or diverging nozzle in the distributor can rapidly distribute ozone into the cross section of the flowing flue gas stream. Ozone can be mixed with a quantity of dilution gas and directed into the distributor for mixing with the gas stream containing nitrogen oxides. Ozone can also be introduced in a co-current or counter-current direction.

When oxidized, nitrogen oxides are converted to their pentavalent form. The gas stream leaving the nitrogen oxide treatment zone is saturated with steam. The dinitrogen pentoxide will react with the moisture in the gas phase to form nitric acid in the gas phase:

N₂O₅+H₂O→2HNO₃(g) (4)

completely water soluble HNO₃(g) Will immediately dissolve in the condensed or condensed water droplets.

HNO₃(g)→HNO₃(l) (5)

Some N due to extremely soluble₂O₅Will dissolve directly in the condensed or condensed water droplets.

N₂O₅+H₂O(l)→2HNO₃(l) (6)

If alkali or alkaline earth metal hydroxides, carbonates or bicarbonates are present in the coagulated droplets, they neutralize the nitric acid to form nitrates. If it is condensed water vapour, it will still be nitric acid. In the present invention, the removal of nitrogen oxides by ozone in a gas stream can be achieved without mixing nitrate/nitric acid with the remaining contaminants without using a double scrubber.

Turning to FIG. 1, a scrubber assembly is shown. Flue gas containing contaminants selected from particulates, sulfur oxides, nitrogen oxides, and acid gases flows through line 1 into scrubber assembly 10. The flue gas containing the pollutants will rise through the scrubber assembly 10 and first come into contact with seawater, which first flows through line 3 into the cooling coil or demister a. The seawater will exit the cooling coil or demister a through line 5 and flow downwardly to combine with the sweep gas as it enters the scrubber assembly 10 through line 1. Line 6 will redirect the seawater through some distributors C so that the flue gas will be in contact with the seawater and utilize this contact for quenching.

Flue gas that has been wetted by seawater will continue to rise through scrubber assembly 10 and will come into contact with ozone entering scrubber assembly 10 through line 2. Based on the size of the scrubber assembly 10, there will be sufficient room for the ozone to react with the nitrogen oxides and sulfur oxides in the flue gas stream. This will allow the necessary contact time during which ozone will react with nitrogen oxides and sulfur oxides. Part of the reaction product will be nitric acid which will condense or condense on tray means D and will be discharged from scrubber assembly 10 via line 9 for neutralization and may be allowed to be reused or disposed of in an environmentally friendly manner.

The treated gas stream will continue to rise through the scrubber assembly 10 and will exit as exhaust through line 4. The seawater used as the scrubbing medium in the scrubber assembly 10 may be once through or recycled based on the operator's preference. Used detergent is captured at the bottom of the scrubber assembly 10 and can be pumped through line 7 by means of pump B and removed from the scrubber assembly 10 through line 8.

In another embodiment of the invention, the ozone adding means is a separate device from the washing assembly. In fig. 2, the wash assembly 20 is attached to a droplet separator F. At the top of the droplet separator F, ozone is injected via line 13. Flue gas flows in through line 11 and will rise through scrubber assembly 20 where it will contact seawater flowing through line 12 into distributor E. The mist scrubbed flue gas will leave the scrubber assembly 20 and enter the droplet separator F.

At the top of the droplet separator F the flue gas will be contacted with ozone where the nitrogen oxides and sulphur oxides present in the flue gas stream will be oxidized and their reaction products nitrate and nitric acid will be collected and removed via line 15. This stream can be processed and neutralized and either or disposed of in an environmentally friendly manner. The treated flue gas stream will leave the droplet separator F via line 14 and be discharged to the atmosphere. The seawater used as scrubbing medium can be used in a once-through manner or recycled and used for several cycles of treating flue gas. The used seawater will exit the bottom of the scrubber assembly 20 through line 16 and be assisted in discharge through line 17 by pump G.

Alternatively, a wet electrostatic precipitator (ESP) may be used in place of the cooling coil or demister device at the top of the scrubbing assembly shown in fig. 1 and 2. Wet ESPs condense nitric acid/nitrate on the trays. The scrubbing of sulfur oxides and acid gases is achieved by quench or wet ESP wet zone, the small downstream section between the wet zone and the charged tray being the oxidizer for nitrogen oxides. Seawater is used in the wet zone to remove sulfur oxides, hydrochloric acid, and other contaminants, while the charged trays primarily trap particles, droplets, and nitrogen oxides.

In other embodiments, the present invention may employ the use of an aqueous medium in a wet scrubber instead of seawater in a single pass or circulation through spray nozzles or through packed bubbles or a tray tower. The flow of liquid and gas may be counter current or co-current. The spray may be projected onto the walls of the scrubber, as is the case in EDV scrubbers.

When present in the flue gas, mercury or other heavy metals are oxidized along with nitrogen oxides and may also be removed along with nitric acid/nitrates on the condensing/condensing surfaces. Downstream of the wet scrubber a dry adsorbent in fluidized form or in a fixed bed can be used to adsorb moisture and oxidized nitrogen oxides/nitric acid.

The present invention oxidizes nitrogen oxides by the addition of ozone downstream of the wet scrubbing stage, thereby separating nitrogen oxide removal products such as nitric acid and/or nitrates by scrubbing other contaminants present in the treated gas stream. The scrubbing is preferably carried out in a once-through manner with seawater in a scrubber, such as along shore and on-board industrial facilities, and in which the discharge of nitrates is avoided.

The present invention effectively utilizes seawater to scrub sulfur oxides, hydrochloric acid and other gases. U.S. Pat. No. 5,206,002 describes a process for oxidizing nitrogen oxides with ozone in flue gas. Condensation surfaces, droplet/mist separators or wet electrostatic precipitators (ESP) for capturing oxidized nitrogen oxides are described in us patent No. 6,162,409.

The process of the present invention eliminates contamination of nitrate/nitric acid in a particulate scrubber or a wet scrubber for acid gases, but also eliminates the need for a separate scrubbing device for removing nitrogen oxides and minimizes the nitrate/nitric acid containing purge stream that can be neutralized for disposal in an environmentally responsible party.

The small stream of nitric acid-containing liquid collected in the droplet separation device or cooler and/or condenser can be neutralized, treated and disposed of in an environmentally safe manner or sold/used as a by-product.

The separation of other contaminants in the wet scrubber leaves the nitrate/nitric acid in a less contaminated form, which limits the biological processes that can be used to decompose the nitrate/nitric acid into nitrogen.

The invention is not limited to seawater scrubbing only, but may be used in conjunction with any industrial wet scrubber intended for particulate and/or acid gas scrubbing.

The use of a single wet scrubber for removing nitrogen oxides without mixing nitrate/nitric acid with other contaminants is less expensive, both in terms of capital and operating costs, than prior art processes that require multiple scrubbers.

The volume of nitrate/nitric acid formed using the process of the present invention is also minimized and the absence of other contaminants in the aqueous effluent stream makes the stream a useful byproduct such as fertilizer. Although the present invention has been described in terms of specific embodiments thereof, it is evident that many other forms and modifications of the invention will be apparent to those skilled in the art. It is intended that the appended claims be interpreted as including all such obvious forms and modifications as fall within the true spirit and scope of the present invention.

Claims (15)

1. A method for removing contaminants from a gas stream, comprising the steps of:

a) flowing the contaminant-containing gas stream into a scrubber;

b) contacting the contaminant-containing gas stream with a scrubbing medium comprising seawater;

c) flowing the contaminant-containing gas stream into a droplet separator in fluid communication with the scrubber;

d) contacting the contaminant-containing gas stream with ozone, wherein the liquid droplets in the droplet separator provide sufficient surface area to dissolve, condense, absorb and remove nitrogen oxides, and the droplet separator collects the liquid droplets to form a small water stream and is removed; and

e) recovering the contaminant-free gas stream.

2. The method of claim 1, wherein the gas stream is flue gas from combustion and chemical processes.

3. The method of claim 1, wherein the contaminant is selected from the group consisting of: particulates, acid gases and heavy metals.

4. The method of claim 3, wherein the acid gas is selected from the group consisting of sulfur oxides and nitrogen oxides.

5. The method of claim 1, wherein the contaminant reacts with ozone.

6. The method of claim 1, wherein the scrubber is selected from the group consisting of: jet, venturi, rod, packed bed and tray columns.

7. The method of claim 1, wherein ozone is mixed with the contaminant-containing gas stream for a sufficient time to oxidize the contaminant.

8. The method of claim 1, wherein the ozone is flowed in an amount that is greater than a stoichiometric amount of an amount of nitrogen oxides present in the gas stream.

9. The method of claim 1, wherein the oxidized nitrogen oxide-containing gas stream is contacted with a droplet separator.

10. The method of claim 1, further comprising a device present in the scrubber selected from the group consisting of: cooling coil, defroster and electrostatic precipitator.

11. The method of claim 1, wherein the seawater is flowed into the scrubber through one or more distributors.

12. The method of claim 1, wherein there is sufficient time for contact between ozone and the gas stream.

13. The method of claim 1, wherein the seawater is used in a once-through manner or recycled.

14. The method of claim 1, wherein ozone is injected into the droplet separator.

15. The method of claim 1, wherein nitrate and nitric acid are produced by contacting the gas stream containing contaminants with ozone.