CA2888538A1 - A non-selective and non-catalytic flue gas treatment system and method - Google Patents

A non-selective and non-catalytic flue gas treatment system and method Download PDF

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CA2888538A1
CA2888538A1 CA2888538A CA2888538A CA2888538A1 CA 2888538 A1 CA2888538 A1 CA 2888538A1 CA 2888538 A CA2888538 A CA 2888538A CA 2888538 A CA2888538 A CA 2888538A CA 2888538 A1 CA2888538 A1 CA 2888538A1
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flue gas
water
quenching
pollutants
scrubber
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Hisham Younis
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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/46Removing components of defined structure
    • B01D53/48Sulfur compounds
    • B01D53/50Sulfur oxides
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/46Removing components of defined structure
    • B01D53/54Nitrogen compounds
    • B01D53/56Nitrogen oxides
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/46Removing components of defined structure
    • B01D53/62Carbon oxides
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/46Removing components of defined structure
    • B01D53/64Heavy metals or compounds thereof, e.g. mercury
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/46Removing components of defined structure
    • B01D53/68Halogens or halogen compounds
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/46Removing components of defined structure
    • B01D53/72Organic compounds not provided for in groups B01D53/48 - B01D53/70, e.g. hydrocarbons
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/73After-treatment of removed components
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/74General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/75Multi-step processes
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/74General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/77Liquid phase processes
    • B01D53/78Liquid phase processes with gas-liquid contact
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23JREMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
    • F23J15/00Arrangements of devices for treating smoke or fumes
    • F23J15/006Layout of treatment plant
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23JREMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
    • F23J15/00Arrangements of devices for treating smoke or fumes
    • F23J15/02Arrangements of devices for treating smoke or fumes of purifiers, e.g. for removing noxious material
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D2251/00Reactants
    • B01D2251/10Oxidants
    • B01D2251/104Ozone
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D2251/00Reactants
    • B01D2251/30Alkali metal compounds
    • B01D2251/304Alkali metal compounds of sodium
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D2251/00Reactants
    • B01D2251/60Inorganic bases or salts
    • B01D2251/604Hydroxides
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D2257/00Components to be removed
    • B01D2257/20Halogens or halogen compounds
    • B01D2257/204Inorganic halogen compounds
    • B01D2257/2045Hydrochloric acid
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D2259/00Type of treatment
    • B01D2259/12Methods and means for introducing reactants
    • B01D2259/124Liquid reactants
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34Chemical or biological purification of waste gases
    • B01D53/46Removing components of defined structure
    • B01D53/48Sulfur compounds
    • B01D53/50Sulfur oxides
    • B01D53/501Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound
    • B01D53/504Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound characterised by a specific device
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23JREMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
    • F23J2215/00Preventing emissions
    • F23J2215/10Nitrogen; Compounds thereof
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23JREMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
    • F23J2215/00Preventing emissions
    • F23J2215/20Sulfur; Compounds thereof
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23JREMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
    • F23J2215/00Preventing emissions
    • F23J2215/40Carbon monoxide
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23JREMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
    • F23J2215/00Preventing emissions
    • F23J2215/60Heavy metals; Compounds thereof
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23JREMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
    • F23J2219/00Treatment devices
    • F23J2219/20Non-catalytic reduction devices
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23JREMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
    • F23J2219/00Treatment devices
    • F23J2219/40Sorption with wet devices, e.g. scrubbers
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23JREMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
    • F23J2219/00Treatment devices
    • F23J2219/80Quenching
    • Y02A50/2341

Abstract

A non-selective and non-catalytic flue gas treatment system and method for the substantial removal of nitrogen oxides, sulfur oxides, hydrocarbons, carbon monoxide, heavy metals and particulate matter from flue gas by subjecting the gas to quenching with a spent alkaline solution to cool down the flue gas and wash out pollutants from the flue gas, condensation to further cool down the flue gas and wash out additional pollutants, the reaction with least one strong oxidant to make residual pollutants more water-soluble and wet scrubbing with an alkaline solution to remove residual pollutants. The system and method generates wastewater that is treated to produce water for recirculation through the treatment system and method, sludge for disposal and a brine solution for safe discharge to the sewers. The system and method minimizes chemical consumption, minimizes sludge production, conserves water and recovers otherwise wasted thermal energy.

Description

Description FIELD OF INVENTION
The present invention relates to a non-selective, non-catalytic flue gas treatment system and method comprising quenching, condensation, oxidation and wet scrubbing to substantially remove nitrogen oxides, sulfur oxides, heavy metals, hydrocarbon, carbon monoxide and particulate matter from flue gas.
PRIOR ART
The majority of power generation plants, a number of manufacturing operations and industrial operations that use industrial boilers burn fossil fuel to generate heat and electricity. Waste to energy plants that utilize biomass or municipal solid waste as fuel, combust organic waste or convert it to gas then combust the gas to generate heat and electricity. Following energy utilization the combusted fuels end up as flue gases that are emitted to the atmosphere. These flue gases contain air pollutants that come from chemicals present in the fuel and compounds generated during the combustion of such fuels.
The main air pollutants in flue gas emissions are Sulfur Oxides (SO2 and S03, referred to as SOx), Nitrogen Oxides (NO and NO2, referred to as NOx), Hydrochloric Acids (HCI), Hydrocarbons (CxHy) , Carbon Monoxide (CO), Heavy Metals and particulate matter. These air pollutants have come under increasingly more stringent air emission regulations over the past few years. Permitted discharge concentrations of these pollutants are only expected to be lowered in the future.
SOx compounds dissolve in rain and condensing flue gas humidity to form a sulfuric acid solution that come back to the ground as acid rain damaging vegetation and aquatic life.
Acid rain also damages buildings and infrastructure. The concentration of SOx in the flue gas generated by the combustion of certain types of coal and heavy oil can be as high as 5,000 mg/Nm3. Flue gas generated by the combustion of natural gas or municipal solid waste contains lower amounts of SOx. Most of the SOx in flue gas are the form of SO2 with 1-2% in the form of S03. Both gases are highly soluble in water. SO2 dissolves in water forming H2S03 which quickly oxidizes to form sulfuric acid (H2SO4), while S03 reacts directly with water to form H2SO4. The boiling point for a concentrated 64%
H2504 solution is 149 oC
(300 oF). Therefore SO2 and S03 exist only in their gaseous state in an unsaturated flue gas at 149-204 oC
(300-400 oF).
NOx compounds are toxic gases that can cause long term adverse effects to the lungs. They also react with volatile organic compounds in the flue gas in the presence of heat and sunlight to form ground level ozone which causes respiratory problems in children and the elderly. NOx compounds under atmospheric conditions react with other compounds in polluted air to form nitric acid which contributes to the damaging effects of acid rain. The concentration of NOx in flue gases can be in the range of 60 mg/Nm3 in flue gas from the combustion of natural gas to 1000 mg/Nm3 in flue gas from the combustion of coal or heavy oil. Most of of the NOx gases exists as NO with less than 5% as NO2.
While the solubility of the more dominant NO in water is only 1.5 g/L at 15 oC, NO2 hydrolyses in an aqueous solution to form nitric acid (HNO3) and NO. Nitric acid has a boiling temperature of 83 C (181 F).
Therefore both NO and NO2 remain in their gaseous state in hot flue gases.
HCI is a strong acid gas that contribute to the damaging effects of acid rain.
HCI concentrations in flue gases generated by the combustion of fossil fuels are much lower than their concentrations in flue gases generated by municipal solid waste. HCI is highly soluble in water.
CxHy are a group of hundreds of chemicals that end up in the flue gas due to the incomplete combustion of hydrocarbon based fuels. Some of these chemicals like methane (CH4) contribute to the greenhouse effect while many others are carcinogenic like benzene, polycyclic aromatic hydrocarbons (PAH) and dioxins. Hydrocarbon concentrations in the flue gas can reach a few hundreds of milligrams per Nm3 of flue gas.
Fossil fuels and municipal solid waste contain varying concentrations of heavy metals such as Mercury, Cadmium, Tallium, antimony, arsenic, lead, chromium, cobalt, copper, manganese, nickel, vanadium, tin and their compounds. Heavy metals cause a range of chronic and acute carcinogenic and neurological health effects.
Particulate matter in flue gas are a mixture of tar, metal oxides, silica oxides, elemental calcium and salts of different compounds that become entrained in the flue gas during combustion. Depending on the geological source of the fossil fuel or the composition of combusted municipal solid waste, these particulate matter particulate matter can contain uncombusted hydrocarbons and traces to large amounts of arsenic, beryllium, cadmium, chromium, hexavalent chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium. Particulate matter also contain dioxins and Poly aromatic hydrocarbons (PAH) compounds that are attached to the structure of the particulate matter. While current flue gas filtration technology captures more than 99% of particulate matter. The remaining less than 1% of fly ash that escape filtration are fine particulates that have come under tighter regulation over the past few year. While particulate matter cause respiratory problems, toxic or carcinogenic chemicals that are attached to the particulate matter cause cumulative human health issues and ecological damage when the particulate matter eventually fall back to the ground.
Like Nitrogen Oxides, CO in ambient air can enter reactions that form ground level ozone which causes respiratory problems in children and the elderly. Cumulative CO emissions can eventually alter the low CO concentration in ambient air.
Traditional flue gas treatment systems incorporate pollutant-specific processes and reagents that typically target the removal of particulate matter, SOx or NOx. The removal of other pollutants comes as a side benefit due to the ability of a process or reagent to partially absorb but not substantially remove one or more of the other pollutants. When required, other processes to remove mercury are used. Cyclones, electrostatic precipitators and baghouse filter are the conventional methods of particulate matter removal.
2 The first SOx removal system by a utility was installed in 1931 at the coal-fired Battersea Power Station in London England. Most installed SOx removal systems ¨commonly known as flue gas desulfurization (FGD) systems- are scrubbers using a dry, semi dry or a wet slurry of lime or limestone as the desulfurization reagent. FGD systems come with expensive solid reagent crushing, grinding and pulverization equipment.
Wet slurry scrubbers are the preferred FGD system at large fuel burning power generation plants and often come with high maintenance slurry regeneration equipment. SOx removal efficiencies for traditional FGD systems exceed 90%. Acid gases like HCI and HF will also be absorbed by a FGD system.
The majority of the fairly water-insoluble NOx gases are not absorbed by the FGD system except for a small percentage that dissolves in the slurry as HNO3 acid.
Selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) technologies for NOx removal were first commercialized in the 1970s. SNCR involves injecting urea or ammonia in the combustion chamber of the boiler where the temperature must be maintained within the high and narrow range of 870 -1,200 C (1,600 -2,200 F). At these temperatures urea or ammonia react with NOx to form the harmless nitrogen gas. Due to reaction time and temperature control difficulties, removal efficiencies for SNCR systems range from 40%-75%. SCR involves injecting urea, anhydrous ammonia or aqueous ammonia into the flue gas then passing the gas through a catalyst chamber. A catalyst made of a precious or a base metal inside the catalyst chamber converts NOx to nitrogen gas. SCR systems are prone to catalyst plugging and performance reduction by different contaminants in the flue gas. Urea is safer to store than ammonia. However, in SCR system urea must first be thermally decomposed to ammonia in a separate reactor before it is injected in the catalyst chamber.
Anhydrous ammonia needs no further processing but is classified as a hazardous material that is subject to strict transportation, handling and storage regulations. Aqueous ammonia at 19% concentration is considered safe to transport, handle and store but contains 19% of the active ingredient therefore comes with a high transport and storage cost. SCR systems operate at a low temperature range of 165 -600 C (325 -1,100 F) and have a NOx reduction efficiency of 60-90%
The SNOXTM process, developed by Ha!dor Topsoe in the 1980s has been used by several flue gas treatment installations across the world. The process involves a catalytic NOx reduction to nitrogen using ammonia and a separate catalytic reduction of SO2 to S03 followed by passing the hot flue gas through a condenser where the gas is cooled below a temperature of 100 C for S03 gas to hydrate to water vapour which is then condensed and recovered as sulfuric acid solution. Like all catalytic processes the process is highly dependent on keeping the catalyst beds clean.
Trona or sodium bicarbonate sorbent injection with or without another alkaline compound has been used as FGD for small, low-sulfur flue gas treatment systems. The alkaline compound can be magnesium carbonate, calcium carbonate, magnesium hydroxide or calcium hydroxide. The dry sorbent is injected in and mixed with the flue gas. The sorbent absorbs most of the SOx, HCI, HF and knockout particulate matter. Activated carbon is often injected with the sorbent to help capture fine particles which include fines of elemental heavy metals or their compounds. The heavy metals are adsorbed and absorbed by the powdered activated carbon then a particulate matter removal technology captures the powder as part of the captured fly ash. A sorbent and activated carbon injection followed by filtration can help remove close to 90% of flue gas heavy metals.
3 U.S. patent no. US5674459 A claims a system for removing sulfur dioxide, nitrogen dioxide and other contaminants from a flue gas using hydrogen peroxide as the reagent to convert the oxides of sulfur and some of the oxides of nitrogen (NO, NO2, etc.) to more water soluble compounds. Acids formed as a result of this conversion, namely sulfuric acid (H2 SO4) and nitric acid (HNO3) can then be neutralized with an aqueous solution or a slurry of limestone.
U.S. patent application no. US20130089482 Al claims methods for removing acid gases from flue gas by cooling the flue gas with a sprays of aqueous solutions of alkaline earth hydroxides, sodium hydroxide, potassium hydroxide, ammonia and ammonium hydroxide to condense a portion of the water vapor on the gas and for the reagent used to neutralize the dissolved acid gases to their respective dissolved salts in water. Hydrogen peroxide is also added to oxidize SO2 to 503 and NO to NO2 gases which react with water forming sulfuric and nitric acids which are neutralized by the alkaline compound in the water to form water soluble salts.
U.S. patent no. US7455820 B2 claims a process for removing sulfur dioxide and nitrogen oxides from flue gas by reacting the strong oxidant chlorine dioxide with the flue gas in an alkaline reaction solution so as to simultaneously remove sulfur dioxide and nitrogen oxides from the flue gas.
Chlorine dioxide is a dangerous gas. The patent describes preparing chlorine dioxide on site Chlorine dioxide from an acidic solution of NaC102 or NaCI03.
US. Patent application no. US20140241971 Al claims a process for removing sulfur dioxide from flue gas by threating the gas through a semi-dry lime treatment scrubber, flowed flue gas quenching with a water spray at low pH to remove mercury and ammonia, followed by a packed bed wet scrubber with an alkaline solution of sodium hydroxide (described in the patent application as a condenser) to capture residual sulfur dioxide. The process does not claim the removal of other pollutants. The process uses fresh water both for quenching and wet scrubbing the flue gas. If commercialized the process is expected to consume a large amount of sodium hydroxide because the acidic moisture formed during quenching would still react with sodium hydroxide inside the wet scrubber.
U.S. patent no. US5206002 A claims a process to treat flue gas by cooling the gas through a series of heat exchangers, then removing the formed acidic condensate from the flue gas. The condensate washes a good portion of pollutants out of the flue gas. The gas is then ozonated to oxidize remaining pollutants before the gas is quick-washed inside the flue gas ducts with an alkaline solution to capture fine particulate matter, dissolve oxidized pollutants and react with dissolved salts to form water soluble salts.
The success of the process relies heavily on the ability of condensation in an acidic flue gas environment and a single pass of oxidation with one gaseous oxidant followed by a quick wash with an alkaline solution to effectively clean the flue gas. The process does not claim the need to pre-remove large particulate matter which will foul the exchange surfaces of the heat exchanger and increase the consumption of both ozone and alkaline solution. The acidic moisture of the flue gas retains lots of the acid gases, heavy metals and particulate matter. While ozone alone is one of the strongest oxidants known to man, ozone generators come with a high capital and operating cost. Therefore, ozone should be introduced in more optimal conditions where it only reacts with residual stubborn pollutants.
This is more important in flue gas treatment applications because reaction times are limited to a few seconds due to the high flow rates of flue gases. Proper scrubbing is required to ensure the removal of residual pollutants after oxidation.
4 Commercialized flue gas treatment systems continue to apply pollutant-specific reagents and processes to remove the larger concentration pollutants of S0x, NOx and particulate matter. The use of powdered activated carbon is known to reduce heavy metals levels. The reduction of certain other pollutants comes as a side benefit to an applied pollutant-specific process.
Prior art includes attempted to introduce the use of non-selective strong oxidants to oxidize several pollutants to more water soluble compounds making them easier to remove through a wet scrubber.
However, strong oxidants are more expensive than lime, limestone, urea or ammonia. The use of strong oxidants in flue gas treatment has not beep successfully commercialized yet, primarily due to the high chemical consumption of the currently proposed methods. These methods tend to rely on applying strong oxidants as early as possible in the treatment sequence where the strong oxidant and supporting alkaline solution end up reacting with most pollutants in the flue gas. Due to the often high flue gas flow rates, the reaction time inside wet scrubbers is often limited to a few seconds. This makes it even more difficult for the strong oxidant and alkaline solution to achieve near complete removal of all pollutants. Prior art does not address the simultaneous and effective removal of the host of hydrocarbons and heavy metals that are present in the flue gas along with the other main pollutants.
SUMMARY OF THE INVENTION
The system and method described in this invention provide a flue gas treatment process that achieves near complete and non-selective removal of regulated air pollutants in the flue gas by subjecting the gas to sequential or combined treatment steps where each step conditions the flue gas for more efficient treatment through the remaining steps.
The treatment starts with quenching the hot flue gas with a spent alkaline solution collected from the last main step of the treatment system and method. A portion of the water in the alkaline solution evaporates therefore cooling down the flue gas. The rest of the water in the alkaline solution washes out particulate matter and dissolves water-soluble acid gases and hydrocarbons while the high pH of the spent alkaline solution neutralizes water soluble acids to water soluble salts. Residual oxidants in the spent solution also oxidize some pollutants in the flue gas. The partially treated flue gas then enters a condenser with an external cooling medium to further cool down the gas and condense excess moisture out of the gas as wastewater condensate that washes out and captured particulate matter, soluble acid gases and hydrocarbons. The reactive higher pH of the condensing vapor promotes the capture of more particulate matter and acid gases that would not have been captured had condensation been carried out alone or before quenching with the alkaline solution. Quenching before condensation also reduces particulate matter fouling of the heat exchange surfaces and corrosion problems caused by acidic condensate.
The flue gas exiting the condenser contains far less pollutants making it possible for the remaining chemical separation steps to effectively achieve near complete removal of all residual pollutants while maintaining a low chemical consumption. The chemical separation steps first comprise oxidizing residual pollutants in the flue gas with at least one non-selective strong oxidant.
Oxidation converts pollutants that have no or low solubility in water to water-soluble compounds.
Oxidization of certain elemental metals and entrained metal ions forms metal compounds that dissolve or precipitate in water. Since a strong oxidant needs only to oxidize a small amount of air pollutants, some of the most powerful but normally expensive strong oxidants to use like ozone or hydrogen peroxide can now be used while keeping chemical consumptions costs low. When possible, it's best to use both ozone and hydrogen peroxide because the two strong oxidants react with each other, forming the more powerful and non-selective hydroxyl radicals. The reaction rates for hydroxyl radicals with most pollutants in the flue gas are 106 to 109 times the reaction rates for ozone alone with the same compounds.
This is a major benefit for ozone/ hydrogen peroxide treatment considering that residence times for flue gases inside oxidation or = wet scrubbing vessels are only a few seconds due to the often high floe rates of flue gases.
Following oxidation, the flue gas is thoroughly washed with an alkaline solution inside a wet scrubber. The water in the solution dissolves residual preexisting and newly formed water soluble compounds while capturing residual pre-existing and newly-formed fine particulate matter. The alkaline compound in the alkaline solution reacts with dissolved pollutants to form more stable water-soluble salts and particulate matter. Depending on the type of the selected wet scrubber, the scrubber may have an alkaline solution recirculation loop to increase the utilization of the alkaline solution.
Wastewater treatment equipment treat the spent alkaline solution to generate sludge for disposal and treated wastewater. The condensate from the condensation step and the treated wastewater are processed through water treatment equipment that recover most of the water as demineralized or partially demineralized makeup water for the system's wet scrubber or the plant's boiler. The water treatment equipment generate a small stream of brine that can be safely discharged to the sewers.
The captured by flue gas condensation can be used to pre-heat boiler makeup water and boiler condensate return. Heat can also be extracted from the flue gas using raw combustion air. This otherwise lost heat will raise the efficiency of the plant's boiler, therefore reducing fuel consumption which reduces greenhouse emissions. The clean flue gas can be used to capture high purity carbon dioxide using carbon dioxide capture technologies. The clean carbon dioxide can be sold to different industries. To prevent the formation of a water vapor plume, the flue gas is re-heated through a steam heat exchanger to a temperature above local ambient temperatures before the flue gas is discharged to the atmosphere.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows the preferred embodiment of this invention DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
SOx and NOx concentrations in raw flue gas generated by the combustion of coal or heavy oil are much higher than their concentrations in raw flue gas from the combustion of natural gas or municipal solid waste. However flue gas generated by the combustion of municipal solid waste contains higher concentrations of HCI, particulate matter, heavy metals and uncombusted hydrocarbons than normally found in flue gas generated by the combustion of fossil fuels.
The second column in Table 1 shows a typical raw flue gas composition generated by a municipal solid waste to energy plant. The third column in Table 1 shows an assumed flue gas composition using higher concentrations for all pollutants including values for SOx and NOx that would emitted by heavy-oil or high-sulfur-coal fired power plants. The assumed worse composition will be used to demonstrate the pollutant removal efficiency of the system and method disclosed in this invention.
Table 1-Typical and Assumed composition of raw flue gas for the purpose of evaluating the pollutant removal efficiency of the system and method disclosed in this invention =
Air Pollutant Typical Composition of Raw Assumed Concentration Flue Gas generated by the in Raw Flue Gas combustion of municipal (mg/ Nm3) solid waste (ring/Nm3) SOx 200-1,000 5,000 NOx 350-500 1,000 HC1 500-2,000 2,000 PM 1000-5,000 5,000 Mercury 0.05-0.5 1 Cd, TI <3 4 Total Heavy Metals <50 100 Organic matter as 1-10 100 Methane Dioxin/ Furans 1-10 20 (ng-I-TECt/Nm3) In the preferred embodiment of this invention ozone and hydrogen peroxide are the preferred strong oxidants because they generate the more powerful hydroxyl radicals that ensure near complete oxidation of pollutants. Sodium Hydroxide is the preferred alkaline scrubbing compound because it reacts with all acid gases in the flue gas. Most of the chemical reactions with the strong oxidants, water and the alkaline compound take place throughout the entire treatment sequence. These reactions, including sample representative reactions with heavy metals and organics, are:

Chemical Reactions with Ozone:
R1: SO2 + 03 4 3S03 R2: NO + 03 4 NO2 + 02 R3: NO2 + 03 4 NO3 + 02 R4: NO3 + NO2 4 N205 R5: 2HCI + 03 4 02 + H20 + Cl2 R6: 2C0 + 203 4 2CO2 + 202 R7: An example of oxidation of simple organics oxidation:
15CH4 (Methane) + 1003 4 15CO2 + H20, R8: An example of oxidation of Polycyclic aromatic hydrocarbons (PAHs):
3C20H12 (Benzo(a)pyrene) + 46 03 -> 60 CO2 + 18 H20 R9: Hg + 03 4 Hg0 + 02 R10: Elemental Metal or Ions of Metals+ 03 4 Metal Oxide + 02 Generation and Chemical Reactions of Hydroxyl Radicals:
R11: 2 03 + H202 4 2 *HO+ 3 02 R12: Acid Gas + =HO 4 Acid Gas in highest oxidation state making the acid gas more soluble in water (Different Mechanisms) R13: Organics + .HO --> CO2 + H20 + Inorganic Salts (Different Mechanisms) R14: Elemental Metal or Ions of Metals + =HO 4 Metal Oxide + H20 (Different Mechanisms) Chemical Reactions with Hydrogen Peroxide:
R15: H2S03(aq) + H202(aq) 4 H2SO4(aq) + H20(1) R16: 2NO + 3H202 2HNO3 + 2 H2O
R17: 2NO2 + H202 -> 2HNO3 R18: Soluble Organics +H202 -> CO2 + H2O + Inorganic Salts R19: Elemental Metal +H202 4 Metal Oxide + H2O
Chemical Reactions with Water:
R20: SO2 (g) + H2O (I) -> H2S03 (aq) R21: S03 (g) + H20 (I) --> H2SO4 (aq) R22: 3 NO2 (g) + H2O (I) -> 2HNO3 (aq) + NO (g) R23: N205 (g) [ From R4] + H2O (I) 2HNO3 (aq) R24: HCI (g) + H20(1) HC1(aq) + H20(1) Chemical Reactions with Sodium hydroxide in Solution:
R25: H2503 + 2NaOH -> Na2S03 + 2H20 R26: H2SO4 + 2NaOH 4 Na2SO4 + 2H20 R27: NO+ NO2 + 2NaOH --> 2NaNO2 + H20 R28: 2NO2 + 2NaOH 4 NaNO2 + NaNO3 + H2O
R29: HNO3 + NaOH --> NaNO3 + H20 R30: NCI+ NaOH --> NaC1+ H20 R31: RCOOH (Organic Acid) + NaOH 4 RCOONa 4-H20 R32: C6H5OH (Phenol) + NaOH C6H5ONa + H20 R33: CdC12 (aq) + 2NaOH (aq) Cd(OH)2 (s) + 2NaC1 (aq) The temperature of flue gas exhaust from a steam boiler after energy utilization is typically around 149-204 oC (300-400 oF). Depending on the type of fossil fuel used, the water vapor content of the flue gas can range from 5-20%. This water vapor comes from the humidity in the combustion air and from the oxidation of hydrogen that is part of the molecular structure of fuel to water vapor. Additional water vapor in the flue gas can come from the moisture content of the fuel itself such as moisture trapped in coal and municipal solid waste. Flue gas from the thermal utilization of biogas can contain as high as 30%
water vapor.

In the preferred embodiment of this invention, the hot flue gas is subjected to four separate steps of quenching, condensation, oxidation and wet scrubbing that are integrated with associated wastewater/
water treatment and recirculation equipment. As per Fig.1and with reference to the above mentioned chemical reactions R1-R33 and the physical and chemical properties of air pollutants in flue gas and water as described at the beginning of the description of prior art, the flue gas after energy utilization (1) is subjected to:
Quenching:
In the preferred embodiment on this invention, the flue gas enters a venturi quencher/ scrubber (2) and is immediately sprayed with a spent alkaline solution (64) of sodium hydroxide (NaOH) through small openings (3) at the mouth of the venturi scrubber. The mixture of flue gas and alkaline solution accelerates through the narrow throat of the scrubber which increases turbulence in the gas causing the atomization of the sprayed alkaline solution in the gas. A portion of the atomized liquid evaporates in the hot gas therefore quenching the gas to below or just above 100 oC (212 oF).
The colder gas becomes saturated or partially saturated with the alkaline water vapor. The rest of the scrubbing liquid droplets coat the particulate matter with moisture. As the gas starts to expand through the conical shape under the throat, the excess liquid droplets wash'the particulate matter out of the flue gas to the bottom of the scrubber's liquid collection basin (4). Solid metal oxides, hydroxides or other metal compounds are also washed out of the flue gas as part of the washed particulate matter.
A significant portion of the 502 and 503 gases dissolve in the alkaline water and react with it to form water soluble salts according to reactions R20, R21, R25 and R26. HCI also dissolves in the alkaline as salt according to reactions R24 and R30. The solubility of NO, which is the dominant NOx gas, is very low at 100 oC (212 oF). However, NO2 reacts with the alkaline water as per reactions R22 and R29 forming a water soluble salt. The high pH of the scrubbing liquid promotes the precipitation of metal compounds and particulate matter that would otherwise remain volatile in the flue gas.
As per the example in reaction R33, the alkaline solution will also form metal hydroxides (solid or soluble) out of ionic metals in the water vapor content of the flue gas.
The alkaline solution collection vessel (4) below the venturi scrubber (2) is configured to force the flue gas to hit the far wall away from the vessel's exit so that excess alkaline solution and/or partially condensed flue gas moisture that are laden with particulate matter collect at the bottom of the vessel.
A pump (66) recirculates a portion of the spent alkaline solution (65) through the scrubber's alkaline solution feed line (64). Recirculation maintains a liquid-to-gas ratio of 2.7 to 5.3 liters per actual rn3 (20 to 40 gallons per 1,000 actual fe) inside the venturi scrubber to achieve high particulate matter and acid gas scrubbing efficiencies. A combination of control valve (67), a variable speed drive controlled recirculation pump (66) and/ or flow meters control the recirculation rate based on the flue gas flow rate.
The mixture of excess spent alkaline solution and/or partially condensed flue gas moisture that collects at the bottom of the venturi scrubber's basin (32) is sent to a dissolved air floatation unit (34) for further treatment. The control valve (33) at the bottom of the venturi scrubber controls the flow to the dissolved air floatation unit bases on a liquid level signal for the liquid level inside the venturi scrubber's basin.

The spent alkaline solution entering the dissolved air floatation unit can be passed through an air cooled heat exchanger using raw combustion air as the cooling medium. Cooling this spent alkaline solution improves the pollutant removal efficiency of the dissolved air floatation unit.
The particulate matter removal efficiency for venturi scrubbers using water alone is typically 90-99%. The soluble acid gas removal efficiency for the venturi scrubbers is typically 30-60%. The use of the spent alkaline solution as the scrubbing medium will increase both scrubbing efficiencies. For the purpose of illustrating the expected removal efficiencies for the entire system and method disclosed in this invention, a minimum of 30% S0x/HCI removal efficiency and 90% particulate matter removal efficiency will be considered for this step of quenching with the spent alkaline solution.
In other embodiments of this invention, the quenching apparatus can have a different configuration as long as it serves the same purpose of using the spent alkaline solution from the rest of the process to cool down the flue gas by evaporating a portion of the spent alkaline solution while the rest of the solution washes out particulate matter and dissolves acid gases out of the flue gas.
Condensation:
In the preferred embodiment of this invention, the quenched and therefore saturated or partially saturated flue gas (5) enters a condensation vessel (6) that is fitted with a shell-and-tube or plate-and-frame heat exchange (7).
Cold water enters the heat exchanger at the top side (8) and exists at the bottom side (9). The cold water absorbs latent and sensible heat from the flue gas therefore reducing the temperature of the flue gas to a desired dew point temperature that is well below the boiling temperature of water. The flue gas at this new temperature exits the heat exchanger at a new saturation equilibrium with water vapor. The amount of water vapor at this new equilibrium is less than the amount of water vapor in the gas after quenching. The excess moisture falls to the bottom of the condensation vessel as condensate that dissolves more acid gases out of the flue gas as per chemical reactions R20-R24. The condensate also washes out more particulate matter, leaving only residual acid gases, fine particulate matter, volatile organics and fine elemental metals and metal ions in the flue gas.
Using cold water at 10-15 OC (50-59 OF), a flue gas exit temperature of 30-40 OC (86-104 OF) can be achieved. The flue gas water vapor content at this exit temperature range is 4-14%. While the water vapor content of the flue gas after condensation may not be too different from the original 5-20% water vapor content of raw flue gas, the quenching with the high pH spent alkaline solution then the condensation of the new mixture of vaporized alkaline solution and original water vapor in the flue gas is what leads to the substantial removal of pollutants.
The flue gas leaving the heat exchanger passes through a mist removal pad (10) that coalesces small condensed moisture droplets into larger heavier droplets that fall back to the bottom of the condensation vessel (38). The mist removal pad (10) is fitted with water spray nozzles (11) that intermittently clean the mist removal pad with a small spray of hot clean water.

The condensate that collects at the bottom of the condensation vessel (38) is sent to a baffled clarifier (40) to for the removal of settled solids before further treatment as will be described later in the disclosure of this invention. A control valve (39) controls the flow of the condensate to the baffled clarifier (40).
The condensate entering the baffled clarifier (40) can be passed through an air cooled heat exchanger using raw combustion air as the cooling medium. Cooling the condensate improves the pollutant removal efficiency of the dissolved air floatation unit.
Condensation of flue gas that has been pre-quenched with an alkaline solution is better than direct condensation of raw flue acidic gas. Quenching removes large particulate matter that may foul the heat exchange surfaces of the heat exchanger. Condensation of the pre-quenched flue gas produces a less corrosive condensate with a pH closer to 7 and promotes the reaction of pollutants and precipitation of particulate matter in the more alkaline condensate droplets as they separate from the flue gas. In contrast, direct condensation of raw flue gas generates a more corrosive condensate with a pH less than 4 and leaves more pollutants dissolved in the acidic flue gas.
The solubilities of SO2 in water at 40 OC is about four times its solubility at 100 OC. Therefore, with proper contact time through the heat exchanger and the presence of trace alkaline compound in the flue gas, condensation is expected to remove more than 80% of the SO2 gas out of the flue gas. The physical surfaces of the heat exchanger help the falling condensate wash fine particulate matter. Packed beds are known to remove 90-99% of particulate matter. Conservatively, and for the purpose of demonstrating the expected removal efficiencies for the entire system and method disclosed in this invention, condensation of the pre-quenched flue gas is considered to remove minimum of 80% of SOx and 80% of particulate matter which includes particulates of heavy metal compounds.
In another embodiment of this invention, and depending on the flue gas flow rate and the amount of spent alkaline solution available for quenching, direct condensation of the flue gas to the desired flue gas exit temperature of 30-40 OC (86-104 OF) can be achieved with just direct injection or spray of the spent alkaline solution in the flue gas without the need for the condensing heat exchanger. In this case, the condensation vessel (6) will serve as a condensate separation vessel without the heat exchanger (7). For better condensate separation, the empty condensation vessel will be configured as a round cyclone with a bottom tangential flue gas inlet (5). The tangential entrance forces the flow gas to spiral upwards against the wall of the cyclone. The generated centrifugal force pushes the water droplets that carry the pollutants against the internal wall of the cyclone causing the droplets to lose their inertia, coalesce and fall to the bottom of the cyclone (38).
In yet another embodiment of this invention, the flue gas quenching apparatus along with an adequate amount of spent alkaline solution are enough to condense and collect wastewater moisture out of the flue gas therefore eliminating altogether the need for the condensation vessel (6).

Oxidation with a First Strong Oxidant:
In the preferred embodiment of this invention, the flue gas after condensation enters a venturi injector (12) connected to an ozone suction port (13) that is connected to an ozone generator. The ozone generator is not shown on Fig 1. to keep the drawing simple. The flue gas accelerates through the narrow structure of the venturi injector therefore drawing and mixing the ozone gas with the flue gas. Table 2 show the oxidation potential for the top strong oxidants known to man. Ozone is the third strongest oxidant known to man after fluorine the hydroxyl radical (.HO):
Table 2- Oxidation Potentials for Strong Oxidants Oxidant Oxidation Potential, V
Fluorine 3.0 Hydroxyl radical 2.8 Ozone 2.1 Hydrogen peroxide 1.8 Potassium permanganate 1.7 Chlorine dioxide 1.5 Chlorine 1.4 With reference to chemicals reactions R1-R10:
Ozone oxidizes residual SO2 gas into 503 according to chemical reaction Rl.
S03 is highly reactive in water forming H2S03(aq) acid according to R21. Ozone also reacts with residual NO and NO2 gases according to reactions R2-R4. While NO2 is soluble in water, NO which is the dominant NOx gas in the flue gas has a very low solubility in water even at low temperatures. The end product from the reaction of ozone with NO and NO2 is N205 which reacts readily with water forming nitric acid (HNO3(aq)).
Since HCI is highly soluble in water, most of the original HCI content of the flue gas would have been removed during quenching and condensation. Therefore only residual HCI would be present in the flue gas at the ozone venturi injector (12). Ozone reacts with the residual HCI
according to chemical reaction R5 forming 02, H20 and chlorine gas (Cl2). The chlorine gas formed from the reaction of HCI with 03 reacts with the second strong oxidant hydrogen peroxide (H202), forming HOCI
weak acid which then reacts with the alkaline scrubbing solution of NaOH forming sodium hypchlorite (Na0C1) or bleach. Na0C1 itself is a strong oxidant. Due to the low concentrations of the formed Na0C1, its reactions with pollutants are secondary to Ozone and Hydrogen peroxide reactions. Any residual NaOCI
remain in the spent alkaline solution until consumed by pollutants in the quenching step.
As per chemical reaction R6, ozone oxidizes CO to CO2.
Chemical reactions R7 and R8 show reactions between ozone and a simple hydrocarbon like methane and a complex (PAH) like Benzo(a)pyrene wherein the organic compound is completely oxidized to CO2 and H20. Although fast, ozone reactions with organics are selective and limited to reactions with unsaturated aromatic and aliphatic compounds and well as reactions with certain functional groups in complex compounds converting these compounds to different more simple compounds.

As per chemical reactions R9 and R10 Ozone reacts with elemental metals and ions of metals to form particulates of metal oxide.
It takes a few milliseconds for ozone to react with air pollutants in the flue gas. In the preferred embodiment of this invention, a length of flue gas duct (14) is provided to allow at least one second of reaction time with ozone before the addition of hydrogen peroxide. A reaction duct is better than a reaction vessel because it increases the chances for the ozone molecule to bump into the air pollutant molecules. In the preferred embodiment of this invention ozone is injected at least twice the stoichiometric amount required to react with air pollutants in the flue gas entering the ozonation venturi injector (12). This ensures that enough ozone is available to react with pollutants, then react with hydrogen peroxide as described below.
Oxidation with a Second Strong Oxidant:
In the preferred embodiment of this invention hydrogen peroxide (H202) is added to the ozonated flue gas through the H202 venturi injector (15). As the flue gas accelerates through the narrow structure of the venturi injector the flue gas draws H202 from the venturi injector's suction port (16) and mixes it with the flue gas. The venturi injector's suction port is connected to a concentrated H202 solution storage tank. The storage tank is not shown on Fig. 1 to keep the drawing simple.
Hydrogen peroxide reacts with ozone forr..ing the highly reactive and non-selective hydroxyl radicals (0H0) strong oxidant according to chemical reaction R11. The hydroxyl radicals have a very short half-life of 10 p.S at a 10-4 M concentration. The reaction rates for these radicals with oxidizable compounds in gas or water are more than a million to a billion times the reaction rates for ozone with the same compounds.
Table 3 compares the reaction rate constants for ozone and hydroxyl radicals for some groups of organic compounds.
Table 3 Reaction rate constants (k, M-1 s-1 ) of ozone vs. hydroxyl radical Compound 03 =HO
Phenols 103 109-1011 N-containing organics 10-102 108-1010 Aromatics 1-102 108-1010 Ketones 1 109-1010 Alcohols 10-2 ¨ 1 108-109 The instantaneous and indiscriminate reactions of these radicals with oxidizable compounds are complex and can take different paths depending or the presence of certain other chemicals in gas or water. Acid gases are oxidized to the higher oxidation states of these gases which are readily reactive with water forming the corresponding acids. Organics are oxidized to carbon dioxide, water and inorganic salts.
Carbon monoxide is oxidised to carbon dioxide. Elemental metals and ions of metals are oxidized to their particulate metal oxides. In the preferred embodiment of this invention, a length of flue gas duct (17) is provided to allow at least one second of reaction time with hydroxyl radicals before the flue gas enters the wet scrubber (18). A reaction duct is better than a reaction vessel because it increases the chances for the hydroxyl radicals to bump into the air pollutant molecules.
In the preferred embodiment of this invention, ozone is added through the ozone venturi injector (13) at least twice the stoichionnetric amount that is required to react with air pollutants in the flue gas. After these reactions, the amount of ozone in the flue gas entering the H202 venturi injector (15) should be more than half the amount of ozone added through the ozone venturi injector (13). In the preferred embodiment of this invention, the amount of hydrogen peroxide added through the H202 venturi injector (15) is more than the stoichiometric amount required to react with the ozone in the flue gas entering the H202 venturi injector (15). This minimum ratio ensures the decomposition of the remaining ozone gas in the flue gas and the generation of enough hydroxyl radicals to oxidize any residual oxidizable pollutants. While the dominant reactions after introducing H202 are chemical reaction R11-R14 for hydroxyl radicals, some H202 molecules may react directly with air pollutants according to reactions R15-R19.
Any H202 that remains in the flue gas becomes miscible with the alkaline scrubbing solution of NaOH
inside the wet scrubber. H202(aq) reacts with residual air pollutants inside the wet scrubber (18) or the quencher/ scrubber (2) according to reactions R15-R19.
In other embodiments of this invention other strong oxidants that are capable of converting SO2 and NO
gases to SO3 and NO2 (or N205) gases which are substantially more reactive with water are used.
Wet Scrubbing with an Alkaline Solution:
In the preferred embodiment of this invention, the flue gas containing residual pollutants that have been oxidized to particulates or compounds that are highly reactive with water enters near the bottom side of a vertical packed bed wet scrubber (18) where the flue gas rises through a countercurrent flow an aqueous solution of sodium hydroxide (24). The packing media (19) consists of small randomly packed saddles, rings or engineered shapes of inert plastic or metal that provide a high mass transfer surface area between the flue gas and the alkaline scrubbing solution. A perforated redistribution plate (21) redistributes the falling alkaline scrubbing solution to prevent channeling through the packed media.
In the preferred embodiment of this invention, the alkaline solution is prepared by adding a concentrated NaOH solution (58) to the solution in pipe (24). Depending of the flow rate of the flue gas and amount of water required to properly scrub the residual pollutants, the water used to dilute the concentrated NaOH
solution can be a fresh supply of water (58) or the recirculated treated water (53) leaving the reverse osmosis system (50) or a mixture of both. Valves (59) and (57) control the flow of the concentrated NaOH
solution and fresh makeup water respectively. Excess treated water (54) due to a high flue gas moisture content can be recovered through valve (55) for further utilization in the plant or as boiler-makeup water.
The NaOH solution is sprayed into the flue gas through an array or spray nozzles (20) at the top of the wet scrubber (18). As the alkaline solution falls through the packed bed, the water reacts with the acid gases according to chemical reactions R20-R24. Na0H(aq) reacts with the resulting acids to form soluble sodium salts according to chemical reactions R25-R30. As per the examples in chemical reaction R31 and R32, the NaOH solution also reacts with organic acids and phenols forming water soluble organic compounds. As per the example in chemical reaction R33, the NaOH solution reacts with ions of metals and some elemental metals to form soluble or particulate Metal Hydroxides.
A pump (60) recirculates a portion of the spent alkaline solution (33) through the scrubber's alkaline solution feed line (20). Recirculation maintains a liquid-to-gas ratio of 2.7 to 5.3 liters per actual rn3 (20 to 40 gallons per 1,000 actual ft3) inside the packed bed scrubber to achieve high particulate matter and acid gas scrubbing efficiencies. A combination of control valve (31), a variable speed drive controlled recirculation pump (60) and/ or flow meters control the recirculation rate based on the flue gas flow rate.
The rest of the spent alkaline solution (63) becomes the feed scrubbing solution for the venturi quencher/
scrubber (2) of the raw flue gas. A check valve (61) prevents that portion of the spent alkaline solution from draining out of pipe (64) at all times. Automatic control valve (62) controls how much spent alkaline solution is sent to the venturi quencher/ scrubber (2).
To ensure there is excess NaOH to react with pollutants inside the wet scrubber, the amount of NaOH
solution maintained at the alkaline solution inlet of the wet scrubber is at least twice the stoichiometric amount required to react with all pollutants in the flue gas (17) entering the wet scrubber (18). Since only residual pollutants are present in the flue gas (17) entering the wet scrubber (18), NaOH consumption is very low compared to direct flue gas scrubbers even at three times the stoichiometric ratio. The NaOH
that remains in the spent alkaline solution (63) reacts with air pollutants inside the venturi scrubber (2).
Before the flue gas leaves the wet scrubber (18), it passes through a mist removal pad (22) that coalesces moisture droplets into larger heavier droplets. These droplets mix back with the sprayed alkaline solution (20). The mist removal pad (22) is fitted with water spray nozzles (23) that intermittently clean the mist removal pad with a small spray of hot clean water.
Properly sized gas-liquid packed bed wet scrubbers using an NaOH solution are known to dissolve 90-99%
of water soluble compounds. Wet scrubbers also remove 99-99.9% of particulate matter down to 1 micron size. Since most pollutants are removed during the quenching and condensation steps then residual pollutants are made water-soluble or as particulate matter during the oxidation step, the wet scrubber using a strong alkaline solution of NaOH will easily remove more than 99% of the remaining low concentrations of residual soluble and particulate pollutants. For the purpose of evaluating the overall pollutant removal efficiency of the said system and method, a minimum of 99%
removal efficiency for all remaining flue gas pollutants is considered for the wet scrubber (18).
In the preferred embodiment of this invention, the flue gas (25) leaving the wet scrubber passes through a steam heated heat exchanger (26) to raises the temperature of the flue gas to an acceptable temperature above ambient temperatures to prevent the formation of a water vapor plume in the plant's surroundings. Plans steam (27) is used to heat the flue gas. The steam leaves the heat exchanger as condensate (28) that can be used as boiler make up water. The treated flue gas (29) can be discharged to the atmosphere through the plant's stack or further utilized to capture high quality carbon dioxide gas that can be sold to different industries.

Table 4 compares the expected concentrations of air pollutants in the treated flue gas, using the assumed dirty flue gas composition stated in table1.
Table 4- Expected stack emissions assuming an unusually dirty flue gas is treated and using known percent removal efficiencies for the apparatuses described in the preferred embodiment of this invention.
Concentration Concentrations after Quenching after Concentrations after through a Condensation, Exit Oxidation then Alkaline venturi scrubber, Typical Assumed Exit Temperature Temperature Solution Scrubbing in a Connpositi- Max. around 100 C, around 40 C (104 packed bed-(3) on of Raw Conc. in (212 F)- (1) F)- (2) Air Flue Gas Raw Flue Pollutant from a Gas municipal (mg/
Exit Exit Air Min. %
WTE plant Nm3) Min. %
Conc. Conc. Min. % Discharge (mg/Nm3) Remova Rennov (mg/ (mg/ Removal Conc. (mg/
I al Nm3) Nm3) Nm3) SOx 200-1,000 5,000 30% 3,500 80% 700 99% 7.0 NOx 350-500 1,000 5% 950 5% 903 99% 9.0 HCI 500-2,000 2,000 30% 1,400 80% 280 99% 2.8 CO 2-50 100 0% 100 0% 100 99% 1.0 PM 1000-5,000 5,000 90% 500 80% 100 99% 1.0 Mercury 0.05-0.5 1 80% 0.20 80% 0.04 99% 0.0004 Cd, T1 <3 4 80% 0.80 80% 0.16 99% 0.002 Total Heavy <50 100 80% 20 80% 4.0 99% 0.04 Metals Organic matter as 1-10 100 50% 50 50% 25.0 99% 0.25 Methane Dioxin/Fu rans 1-10 20 50% 10 50% 5.0 99% 0.05 (ng- I-TEQ
/Nm3) Based on minimum known removal efficiencies for Venturi Scrubbers Based on the chemical and physical properties of pollutants in water at low temperature Oxidation with strong oxidants makes acid gases and organics readily reactive in an NaOH solution.
Oxidation also converts metals to particulates of metal oxides that are washed by the solution especially when a packed bed scrubber is used.

Table 5 compares the quality of this treated gas with the allowable air emissions limits in the European Union and the province of Ontario- Canada:
Table 5-Quality of treated flue gas compared to allowable air emissions limits in the European Union and Ontario-Canada Assumed Percent Air Ontario, Percent Maximum European of Discharge Overall Canada of Conc. in Union Current Air Pollutant Conc. Removal Limits Current Raw Flue Limits European (mg/ Efficiency (mg/ Ontario Gas (mg/ (mg/ Nm3) Union Nm3) Nm3) Limits Nm3) Limits SOx 5,000 7.0 99.9% 50 14.0% 56 12.5%
NOx 1,000 9.0 99.1% 200 4.5% 198 4.6%
HCI 2,000 2.8 99.9% 10 28.0% 27 10.4%
CO 100 1.0 99.0% 50 2.0% 40 2.5%
PM 5,000 1.0 100.0% 10 10.0% 14 7.1%
Mercury 1 0.0004 100.0% 0.1 0.4% 0.02 2.0%
Cd Cd, TI 4 0.002 100.0% 0.1 1.6% N.A.
=0.007 Total Heavy 100 0.040 . 100.0% 1 (Total) 4.0% N.A. N.A.
Metals Organic matter as 100 0.250 99.8% 10 2.5% 33 0.8%
Methane Dioxin/Furans (ng- I-TEQ 20 0.050 99.8% 0.1 50.0% 0.08 62.5%
/Nm3) Wastewater and Water Treatment:
In the preferred embodiment of this invention, the spent alkaline solution (33) leaving the venturi scrubber's separation vessel (4) in fed to a dissolved air floatation unit (34). The unit is fitted with coagulant and flocculent dosing pumps. The dosing pumps are not shown to keep the drawing Fig 1 simple and limited to the main components of the said system and method. The coagulant knocks dissolved organics out of solution and brings fine particles together as small flocs of particles. The flocculent brings the small flocs together as large flocs of sludge.
Atmospheric air is diffused as small bubbles inside the body of the dissolved air floatation unit. As the air bubbles rise inside the unit they lift the separated flocs to the surface of the unit where a skimmer skimmed the collected sludge and discharges it as wet sludge (35). The wet sludge can be discharges as is for further drying or treatment offsite. The treated water (36) is mixed with the condensate (38) that leaves the condensate separation vessel (6). A check valve (37) prevents the condensate from entering the dissolved air floatation unit. The high pH water leaving the dissolved air floatation unit reacts with the condensate knocking out more pollutants as heavy particles. The baffled clarifier (40) separates the newly formed sludge (41) which is mixed with the wet sludge (35) that leaves the dissolved air floatation unit (34). The treated water (43) becomes the inlet stream to a pump (44) that pumps the treated water through an activated carbon bed (46). The activated carbon bed captures fine particulates, metal fines and dissolved organics from the water. If any, excess treated water (47) is discharged to the sewers using control valve (48). The majority of the treated water (49) is fed to a reverse osmosis unit (50) that separates 60-80% of this water as partially demineralized water (53) and a brine solution (51). The brine solution goes to the sewers as a saline solution that is safe to discharge to the sewewers. Valve (52) controls the recovery rate of the reverse osmosis unit. The partially demineralized water (53) is recirculated back through the said treatment system and method as feed water to the packed bed wet scrubber (18).
Any excess demineralized water (54) is recovered through valve (55) for further use in the plant as cooling water or boiler makeup water. If needed fresh water (56) is added to the recirculated water through valve (57).
The amount of removed water (54) or added fresh water (57) is dependent on the moisture content of the raw flue gas, selected liquid-to-gas ratio inside the packed bed wet scrubber (18) and the recovery rate of the reverse osmosis unit.
In other embodiments of this invention, other wastewater and water treatment units can be used to achieve the same purposes of conditioning the wastewater to meet local sewer discharge regulations and further treat an adequate percentage of the treated wastewater to generate demineralized or partially demineralized water for recirculation through the system and method of this invention.
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 put together 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 described claims and the equivalents thereof.

Claims (26)

Claims:
1- A system and method for removing nitrogen oxides, sulfur oxides, hydrocarbons, carbon monoxide, carbon heavy metals and particulate matter from flue gas by passing the flue gas through a treatment sequence comprising:
quenching with an aqueous solution to reduce flue gas temperature and capture some air pollutants condensation to remove condensate that is laden with more captured air pollutants oxidation with at least one oxidant to make residual pollutants more soluble in water or become particulate matter.
wet scrubbing with an alkaline solution to dissolve and react with residual water soluble compounds and wash residual particulate matter out of the flue gas.
wastewater treatment equipment to remove pollutants out of the collected wastewaters as sludge water treatment equipment to recover and recirculate treated water back through the flue gas treatment sequence while generating brine for safe disposal to the sewers of further drying
2- The system and method of claim 1 wherein said condensation precedes said quenching.
3- The system and method of claim 1 wherein said quenching and condensation occur inside one apparatus.
4- The system and method of claim 1 wherein said quenching, condensation and oxidation occur inside one apparatus.
5- The system and method of claims 1, 2, 3 or 4 wherein said wet scrubbing precedes said oxidation.
6- The system and method of claims 1, 2, 3 or 4 or 5 wherein said oxidation and wet scrubbing occur inside one apparatus.
7- The system and method of claims 1, 2, 3 or 4, 5 or 6 wherein more than one apparatus is used to subject the flue gas to any single one of the quenching, condensation, oxidation or wet scrubbing with an alkaline solution steps or a combination of two or more of the said four flue gas treatment steps.
8- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the said aqueous solution that is used for quenching is a mixture of 0-100% cold water to 0-100% spent alkaline solution that is collected from the said wet scrubbing step.
9- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the flue gas quenching apparatus comprises but is not limited to a venturi scrubber, a spray tower scrubber, a tray scrubber, a packed bed scrubber or a fluidized bed scrubber, with or without a quenching fluid recirculation loop.
10- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein a water or air cooled heat exchanger or series of heat exchangers are used to condense water vapor out of the flue gas as condensate.
11- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the raw flue gas temperature is low enough or the quenching fluid recirculation rate is high enough that the quenched flue gas is directly cooled below its dew temperature, therefore eliminating the need for further condensation using the heat exchanger(s) described in claim 10.
12- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein a flue gas-condensate separation vessel comprising but is not limited to a wet cyclone separator, a spray tower a packed bed tower or a vessel with no internals is used to separate flue gas condensate from the flue gas after the quenching or condensation steps.
13- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the flue gas exiting the flue gas-condensate separation vessel described in claim 12, passes through an optional mist eliminator to prevent entrained condensate moisture from leaving the vessel.
14- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the said minimum of one oxidant is selected from the group of air, ozone, hydrogen peroxide, nitric acid, sodium chlorite, sodium chlorate, sodium hypochlorite, sodium perchlorite, potassium hypochlorite, potassium perchlorite, potassium chlorate, potassium chlorite, potassium permanganate, chlorine dioxide.
15-The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein an additional oxidant is selected from the group of air, ozone, hydrogen peroxide, nitric acid, sodium chlorite, sodium chlorate, sodium hypochlorite, sodium perchlorite, potassium hypochlorite, potassium perchlorite, potassium chlorate, potassium chlorite, potassium permanganate, chlorine dioxide.
16- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein each oxidant is added to the flue gas using a venturi injector, an eductor or at least one spray nozzle.
17- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein an oxidation reaction zone comprising a length of flue gas duct or a separate vertical or horizontal vessel is used to provide enough reaction time for the strong oxidant to react with residual air pollutants.
18- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the quenching and oxidation steps further detailed in claim 9, 10, 11, 12 and 13 are sufficient enough to eliminate the need for one or both of the oxidation steps described in claims 14, 15, 16 and 17 predominantly when flue gas contaminants are highly soluble in water.
. .
19- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the said alkaline solution is formed by mixing one or more of sodium oxide, sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium oxide, potassium hydroxide, potassium carbonate, calcium oxide, calcium hydroxide or calcium carbonate with demineralized water, partially demineralized water, city water, well water or lake water that has low total dissolved solids
20- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the wet scrubbing apparatus comprises but is not limited to a venturi scrubber, a spray tower scrubber, a tray scrubber, a packed bed scrubber or a fluidized bed scrubber, with or without an alkaline solution recirculation loop.
21- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the treated flue gas leaving the wet scrubbing apparatus described in claim 20 passes through a mist eliminator to keep entrained moisture in the wet scrubbing apparatus.
22- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the spent alkaline solution leaving the quenching step and wastewater condensate leaving the condensation step are further cooled using cooling water or air for combustion.
23- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the treated flue gas passes through a steam or hot water heated heat exchanger to raise the temperature of the flue gas to an acceptable temperature above ambient temperatures to prevent the formation of a water vapor plume,
24- The system and method in claims 1, 2, 3, 4, 5, 6 or 7 wherein the spent alkaline solution is treated through a wastewater treatment system comprising but not limed to a clarifier, dissolved air floatation unit, microfiltration unit or nanofiltration unit followed by an activated carbon bed and cartridge filter, as deemed necessary, to generate sludge for disposal and treated water for discharge to the sewer or further processing.
25- The system and method of claim 1, 2, 4, 5, 6 or 7 wherein the said condensate is treated through a wastewater treatment system comprising but not limed to a clarifier, dissolved air floatation unit, microfiltration unit or nanofiltration unit followed by an activated carbon bed and cartridge filter, as deemed necessary, to generate sludge for disposal and treated water for discharge to the sewer or further processing.
26- The system and method of claim 1, 2, 3, 4, 5, 6 or 7 wherein the treated water leaving the said wastewater treatment system is further processed through a water treatment system comprising an activated carbon bed, a cartridge filter and reverse osmosis or ion exchange technologies to generate a stream of demineralized water or partially demineralized water to be circulated back as make up water for the wet scrubber or the plant's boiler, while generating a second stream of saline solution for discharge to the sewers.
CA2888538A 2015-04-21 2015-04-21 A non-selective and non-catalytic flue gas treatment system and method Abandoned CA2888538A1 (en)

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CN107983121A (en) * 2017-12-21 2018-05-04 聂玲 A kind of fume recovery handles environmental protecting device
CN109694146A (en) * 2018-12-04 2019-04-30 海南天美源环境保护设备工程有限公司 A kind of processing method of consumer waste infiltration liquid
CN109847578A (en) * 2019-03-15 2019-06-07 北京环境工程技术有限公司 A kind of various pollutants of fume cooperation-removal method

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN107983121A (en) * 2017-12-21 2018-05-04 聂玲 A kind of fume recovery handles environmental protecting device
CN107983121B (en) * 2017-12-21 2020-07-31 重庆凯杰林环保工程有限公司 Smoke and dust recovery processing environment-friendly device
CN109694146A (en) * 2018-12-04 2019-04-30 海南天美源环境保护设备工程有限公司 A kind of processing method of consumer waste infiltration liquid
CN109847578A (en) * 2019-03-15 2019-06-07 北京环境工程技术有限公司 A kind of various pollutants of fume cooperation-removal method

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