Air Pollution Control Innovations

Wet Scrubbers for Gasifier Gas Cleaning

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I gave recent presentations at the International Biomass Conference in Minneapolis, MN and the International Thermal Treatment (IT3) Conference in San Francisco, CA on wet scrubbers for gasification.  Below is the paper abstract. A free download of the paper and presentation is available by clicking the links below. The paper discusses two common tar management approaches regarding syngas cleaning:

1. Thermal Tar Destruction Systems 
2. Tar Removal Systems

ABSTRACT

Concern for global climate change coupled with high oil prices has generated new interest in renewable energy sources.  Many innovative companies are working to commercialize these sources using gasification to convert waste to energy and fuels.  Gasification is a thermal conversion process which produces synthetic gas (syngas).  With proper cleaning, syngas can be used to fuel an internal combustion engine (ICE) to drive a generator, and produce electricity.  Waste heat is recovered from the system to improve the overall plant efficiency. 

During gasification, various pollutants may be produced depending on the type of gasification process and the make-up of the waste feedstock.  The feedstock can vary from biomass, municipal solid waste (MSW), to even medical or hazardous waste.  The pollutants involved can include large to sub-micron particulate matter, tars, and acid gases.  A key challenge to commercializing gasification is designing a syngas cleaning system that removes pollutants to a level that is tolerated by the ICE (or fuels and chemical production system) and also meets emission standards. This paper will discuss different approaches to tar removal and control strategies for the various pollutants. 

Please click on the below icons to download the IT3 conference white paper and the International Biomass Conference presentation. 

Medical Waste to Energy Conversion Using Plasma Gasification Melting

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I gave recent presentations at the International Thermal Treatment (IT3) Conference in San Francisco, CA and the AWMA Conference in Calgary, Canada.  The paper is co-authored with Liran Dor, CTO of EER - Environmental Energy Resources Ltd.  The paper discusses an environmentally friendly way of converting medical waste to energy using EER’s Plasma Gasification Melting (PGM) and Envitech’s wet scrubbing technology. 

ABSTRACT 

A plasma gasification melting (PGM) technology has been developed to transform waste into synthesis gas and products suitable for construction materials.   The core of the technology was developed at the Kurchatov Institute in Russia and has been used for more than a decade for the treatment of low- and intermediate-level radioactive waste in Russia. It is applicable to municipal solid waste (MSW), municipal effluent sludge, industrial waste and medical waste.

Plans are currently underway to build a plant in the US to recycle medical waste using the PGM technology into a high calorific Syngas and a benign residue.  Both output materials may be considered secondary materials since they have commercial use in other processes.  Current plans include the production of steam which will be sold as a commodity to nearby industrial users. 

The Syngas is fed into a Heat Recovery Steam Generator (HRSG) to produce superheated steam for use as heat or electricity generation using a steam generator. The Syngas leaving the HRSG will enter an Air Pollution control (APC) system for post process gas cleaning.  The APC system will use a wet scrubber system that has successfully achieved low emission standards on other typical combustion processes.  This paper will discuss how these technologies are combined to create an economically viable and environmentally friendly solution for converting medical waste into energy.

Please click on the below icon to download the AWMA and IT3 conference white paper. 

Wet Scrubbers for meeting the new HMIWI MACT Standard

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I gave recent presentations at the International Thermal Treatment (IT3) Conference in San Francisco and the Annual AWMA conference in Calgary, Canada that discusses the new hospital/medical/infectious waste incineration (HMIWI) MACT standard and implications for existing systems.  Below is the abstract.  A free download is available by clicking on the link below.  The paper presents emissions data on several scrubber systems and discusses how these relate to the new rules.  I also discuss cost effective strategies to comply with the new rules using add-on controls. 

ABSTRACT

On October 6th, 2009, proposed revisions to the New Source Performance Standards (NSPS) and Emission guidelines (EGs) for the Hospital/Medical/Infectious Waste Incinerators (HMIWI) Standards became final. These regulations, originally promulgated in 1997, were established under Section 129 of the Clean Air Act (CAA) and serve as the maximum achievable control technology (MACT) standards for hospital, medical, and infectious waste incinerators.

Wet scrubbers are currently used on many hospital, medical, and infectious waste incinerators in the United States.  The new emission limits exceed the design capability of most of these systems, primarily with respect to particulate matter (PM), lead (Pd), cadmium (Cd), and mercury (Hg). As a consequence, new control strategies are needed to meet the more stringent standards. 

This paper presents a cost effective control strategy for meeting the new limits and discusses how the strategy has been implemented on similar hazardous waste incinerator scrubbers.

Please click on the below icon to download a white paper on this topic: "Wet Scrubber Control Strategy to Meet the New Hospital/Medical/Infectious Waste Incinerator Standard"  

Photo Credit: bravosixninerdelta

Coal Dryer Venturi Scrubber Systems

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The US Department of Energy (DOE) has funded R&D in coal gasification in recent years as part of a strategy to reduce green house gases.  One aspect of this technology is the use of coal dryers to dry the coal before feeding it into the gasifier.  This requires a coal dryer scrubber which can be comprised of a Venturi scrubber followed by a condenser tower shown in the sketch.

The exhaust gas from the dryer passes through a Venturi scrubber for particulate removal then through a condenser tower to condense water vapor in the gas stream.  The gas passes through a mist eliminator at the top of the condenser tower to remove water droplets in the gas stream.  Re-circulated water in the Venturi throat is collected in the sump of the condensertower.  Gas flow rates for these processes are relatively large and can exceed 300,000 acfm.  Because of the large gas flows, the condenser tower can be as large as 20 feet in diameter or larger.  The Venturi scrubber (shown in the image on the left) must have a special throat design to account for the large gas flow rate. The Venturi throat design is discussed in  in the previous blog post for Venturi Scrubber Throat Design for Large Gas Flow Processes.

Venturi Scrubber Throat Design for Large Gas Flow Processes

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In previous blog posts, I have discussed how a key to particle collection in a Venturi scrubber is maintaining uniform water distribution across the Venturi throat to collide with particles.  This presents a challenge for large volumetric gas flow rate processes.  The difficulty becomes getting water across a large cross sectional area  without any void spaces for particles to escape through.  Often times, the solution may be to simply split the gas flow into multiple trains.  However, this increases capital costs for additional ductwork and piping and takes up more real estate. It is always desirable to minimize the equipment footprint and maintain the gas flow in one train.

To achieve this objective, Envitech uses a proprietary Venturi throat design that has been used on large gas flow rates processes, including foundries and purified teraphthalic acid (PTA) plants. The proprietary design has an internal construction that ensures uniform water distribution throughout the Venturi throat cross sectional.

The adjacent image shows a picture of an Envitech Venturi/Quencher constructed from Hastelloy C276 used for a PTA plant with a gas flow rate of 530,000 acfm.  This type of Venturi design may be used on other large gas flow rate processes like a coal dryer system for a coal gasification plant which can have a gas flow rate as large as 300,000 to 400,000 acfm. 

Marine Scrubber System to Remove SO2 from Diesel Engine Exhaust Gas Using Seawater (SO2 Seawater Scrubber)

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On March 18^th, 2010 I participated on a panel discussion for the Cruise Lines International Association's Inc. (CLIA) Exhaust Gas Scrubber (EGS) Workshop in Miami, Fl. The workshop was professionally managed by BMT Designers & Planners, a navy architecture and marine engineering firm.   The panel was comprised of potential marine exhaust gas scrubber vendors.  The intent of the workshop was to provide information to cruise line participants to assess the maturity of the industry and the likelihood that exhaust gas cleaning systems will be a feasible response to the challenges of changes in regulations.

The industry is evaluating alternatives for meeting upcoming SOx emission limits under Annex VI of Marpol 73/78.  The SOx emission limits will require ships to achieve at least a SOx reduction equivalent to 0.1% sulfur fuel by 2015.  This requirement can be met by using more expensive, low sulfur fuel, or by scrubbing the exhaust gas stream.  The rules essentially require > 97% SOx removal assuming 3.5% sulfur fuel.   The International Maritime Organization (IMO) has issued Guidelines for Exhaust Gas Cleaning Systems, Annex 4, Resolution MEPC.170(57), adopted April 4^th, 2008 to specify the requirements for testing, survey certification, and verification of exhaust gas cleaning (EGS) systems to ensure compliance with Annex VI.   

Envitech first started evaluating the marine scrubber application in early 2008 at the request of one of the major cruise lines.  The cruise line was interested in working with a company that could apply industrial air pollution control equipment experience to marine diesel exhaust streams on board a ship.   Envitech has deployed many particulate and acid gas scrubbers on a wide range of combustion processes including a seawater scrubber for an industrial waste incinerator at a pharmaceutical plant.  Many of these systems are similar process requirements for a diesel engine exhaust.   As a result of our evaluation Envitech developed, and recently filed a patent application for, the Hysea Marine Scrubber which is a hybrid seawater scrubber system.  We introduced this technology to the industry during the CLIA EGS workshop.

The Hysea Marine Scrubber uses available seawater alkalinity to scrub SOx.  The system is chemically assisted with caustic solution (NaOH) to achieve high SOx removal and reduced water flow rates.  The chemical consumption is minimal and estimated to be less than 7% of the usage of a closed loop, recirculation system.  The system is designed to provide flexibility to operate in two modes:

• Open Loop/Caustic Reduced Mode - Continuous, once-though liquid discharge.
• Closed loop/bunkering Mode - Re-circulated seawater with a small discharge stream that can be temporarily bunkered on board the ship.

The discharge liquid in both operating modes is treated to meet regulatory requirements.  Because chemical assistance with caustic substantially reduces the water flow rate, the water treatment system becomes more manageable on board a ship.  The water treatment system also re-oxygenates the water to meet chemical oxygen demand (COD) standards.

The table below shows a comparison of three different marine scrubber configurations, including:

• Open Loop - Using once through seawater
• Closed Loop - Using re-circulated water
• HySea Marine Scrubber - Using chemically assisted Seawater

A comparison of the operating parameters highlights the reduced water and power consumption of the hybrid system compared to an open loop system.  It also shows the substantial caustic reduction compared to a closed loop system.  The main advantages of the Hysea Marine scrubber include:

• Reduced seawater flow rates - 75% - 80% Reduction
• Reduced power consumption - 70% - 75% Reduction
• Smaller piping - Simplified installation
• Smaller water treatment system - Simplified installation
• High removal efficiency -  0.1% sulfur fuel equivalent
• Including low alkalinity seawater conditions
• Operating flexibility to bunker a low flow discharge stream
• Reliance on reliable and proven process technology
• Water discharge that exceeds  discharge requirements
• Water treated for chemical oxygen demand (COD)

Although the Hysea scrubber was designed for ship board use for a diesel engine exhaust, the same design principals also apply to acid gas scrubbing for land based industrial processes that have access to seawater.

A lot of interest in Marine exhaust gas cleaning systems was expressed during the EGS workshop. However, the cruise line industry is still evaluating the full range of options for complying with Annex VI of Marpol 73/78.  The general consensus of the panel participants is that exhaust gas cleaning is not only technically feasible, but provides a compelling financial case as a means for meeting the new regulations.

Photo Credit: Saint Seminole

Wet Electrostatic Precipitator (WESP) Control of Heavy Metals

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Last November I made a blog post describing the use of a wet electrostatic precipitator (WESP) for meeting metals emisssions.  This topic will be discussed in greater detail in an Envitech paper being presented at the 2010 A&WMA 103rd annual conference in Calgary, Canada.  

The metals of concern can include mercury, arsenic, lead, cadmium, nickel and others, depending on the process.  A control strategy using a wet electrostatic precipitator (WESP) in conjunction with sub-cooling was used on a secondary lead smelter to meet more stringent emission standards.  This approach achieved > 98% removal of arsenic and > 92% removal of lead and other condensed metals downstream of a bag-house.  This substantially reduced the plant's cancer risk index and helped to meet reduced fence line lead emission limits.   

The table below provides a summary of other processes facing similar challenges including the additional removal efficiencies that can be required downstream of existing air pollution controls.  Most of the processes are from combustion sources and use a range of air pollution controls including bag-houses, packed bed absorbers, and Venturi scrubbers.  In some cases a combination of controls are used. Despite existing controls, very low concentrations of heavy metals can be emitted.  In the case of bag-houses for instance, the operating temperature may be in a range that some of the metals are in a gas phase.  In such case they will not be collected by particulate control devices. In other cases, the concentrations of submicron, condensed phase heavy metals may exceed the removal capability of controls like packed bed absorbers or Venturi scrubbers.

*Refers to additional removal efficiency after the upstream controls. 

The performances achieved on a secondary lead smelter using a WESP, suggest the approach can be used on these other processes to remove residual concentrations of condensed metals.  In the case of mercury, the ability to remove it with a WESP depends on whether it is in a condensed form. This requires reliable speciation data to make that determination.  More specifically, the mercury must be in a particulate or oxidized form for it to be removed by a WESP.

Please click on the below icon to download a white paper on this topic from the 2010 A&WMA's 103 Annual Converence in Calgary, Canada: "Wet Electrostatic Precipitator (WESP) Control for Meeting Metals Emission Standards".

NOx Scrubbers Using Packed Bed Absorbers

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NOx refers to a class of pollutants that is any binary compound of nitrogen and oxygen and is a main contributor to what is commonly referred to as smog.  It is most commonly produced in combustion processes, but can also be generated in non-combustion processes like metal refining, picking baths, or nitric acid manufacturing to name a few.  The following link: EPA NOX technical bulletin, provides a good overview of NOx as a pollutant, but focuses mostly on NOx formed by combustion processes.

This blog post focuses on non-combustion NOx abatement using packed bed scrubbers.  In general a NOx Scrubber System can be comprised of a single packed bed absorber to several other system components, including:

• Quencher
• Stage 1 Packed Bed: Conversion of NO to NO2
• Stage 2 Packed Bed: Absorption and Reduction of NO2
• Stage 3 Packed Bed: H2S Odor Control.

The actual configuration will depend on a combination of factors including:

The temperature of the inlet gas

• The ratio of NO/NO2
• The Outlet NO limit
• Preferences of the plant regarding the risk of H2S odors.

Each component is further described below.

Quencher - The quench stage is used to cool the gas to the saturation in the case the inlet gas is hot. This is done with an evaporative quencher constructed of metal.

Stage 1 Packed Bed: Conversion of NO to NO₂ - This stage is used in the case there is a sufficiently high concentration of NO in the inlet gas stream or sufficiently low outlet concentration limit for NO.  NO is essentially insoluble in water, but it can be quickly oxidized to NO₂ by chlorine dioxide (ClO₂) or ozone (O₃).  This can be done using a packed bed, but the reaction of NO actually occurs in the gas phase. The packed bed functions as a ClO₂ generator and static mixer, or as a static mixer for ozone injected in the form of an aqueous solution. ClO₂ can be generated by the reaction of sodium chlorite (NaClO₂) with a strong acid. Sulfuric acid (H₂SO₄) is commonly used, but if the air being scrubbed contains enough nitric acid fumes, it may not be necessary to add much H₂SO₄.  Most NO_x scrubbers don't have an oxidation stage. When NO_x is generated by the reaction of nitric acid with metals, it usually consists mainly of NO₂. NO is invisible, and it is much less toxic than NO₂. (The Threshold Limit Value for 8-hour workplace exposure to NO is typically 25 ppm_v, vs. 3 ppm_v for NO₂.)

Stage 2 Packed Bed: Absorption and Reduction of NO₂ - NO₂ reacts only slowly with caustic solutions, and when it does, a competing reaction with water converts part of the NO₂ back to NO. So scrubbing NO₂ using NaOH alone is very inefficient.  For efficient removal of NO₂, a strong reducing agent that reacts faster (usually sodium hydrosulfide: NaHS) is added as required to maintain an ORP of about -400 mV in the scrubbing solution. NaOH is added as required to maintain pH ≥ 12.5, in order to minimize H₂S emissions.

Stage 3 Packed Bed: H₂S Odor Control - This stage is optional. If the pH and ORP settings in Stage 2 are adjusted properly, there will be little H₂S released from that stage. However, the strongly alkaline hydrosulfide solution in Stage 2 is a severe environment for pH and ORP probes, so the probes will tend to get out of calibration faster than they would in a scrubber operating at lower pH levels. Some customers with NOx scrubbers prefer to install a caustic scrubbing stage as a second line of defense against odor emissions.  This may be more important to plants located near other businesses or a residential community.  The wastewater from Stage 3, containing excess NaHS and excess NaOH, can be recycled to the sump of Stage 2 in order to reduce chemical usage there.

Please click the icons below to view a video of a packed bed scrubber and quencher video, respectively.

Photo Credit: monovinyl

Venturi Scrubber Operation for Variable Flow Conditions

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Venturi scrubbers are commonly used in pollution control systems as particulate control devices. Particles are collected primarily according to their aerodynamic size through inertial mechanisms.  Good particle collection is achieved by maintaining a high differential velocity between particles in the gas stream and water droplets in the Venturi throat.  A high differential velocity is created by reducing the cross sectional area in the Venturi throat and thereby creating a pressure drop.  The reduction in area accelerates the particles relative to water that is injected into the throat perpendicular to the gas flow.  As particles collide with the water droplets they become entrained. The particle laden droplets are then collected in the Venturi sump and are purged in a blowdown stream.  

A key to Venturi performance is therefore maintaining a constant pressure drop across the throat. This is relatively straightforward if you have a process with a constant flow rate. However, many processes have variable flow rates.  An incinerator or kiln comes to mind where there are changing flow rates throughout the process cycle. In many cases the variation may be as high as 4:1 or 6:1 from the maximum to minimum flow rate.  This ratio is often called the turn-down ratio.  Three methods of maintaining a constant pressure drop for variable flow conditions are discussed below:

• Reflux Damper
• Variable Throat
• Manual Inserts

Reflux Damper - A reflux damper is often used on Venturi scrubber systems for solid waste combustors.  A solid waste combustor can be an incinerator, kiln, gasifier, or plasma reactor.  The Venturi is designed for the maximum flow condition.  When the gas flow decreases, ambient air is recycled to the Venturi inlet through a pneumatically actuated damper to make up the difference.  The ambient air is recycled from the downstream side (clean side) of an induced draft fan which is used to pull the gas through the system.  The damper modulates to maintain the combustor draft pressure based on a 4-20 mA control signal from a draft sensor mounted in the combustor chamber.

The flow rate is equal to the design gas velocity times the cross sectional area.  As the flow rate decreases the cross sectional area must be reduced to maintain the design gas velocity.  For this reason a reflux damper is particularly recommended for smaller gas flows because it is easier to modulate than for a variable throat. This is because the gas velocity of a reflux damper is about 1/6^th the gas velocity of a Venturi throat.  A reflux damper is therefore less sensitive to flow rate variation.  This makes it easier to tune and maintain the control loop.  Another advantage of a reflux damper is the recycled gas is clean because it has already passed through the Venturi. Therefore there is no potential for fouling the damper blade from particulate in the gas.

The adjacent photo shows a 400 lb/hr medical waste incinerator scrubber with a Venturi inlet flow rate of 1,200 scfm.  The reflux damper can be seen as the white horizontal duct from the ID fan outlet to the Venturi inlet on the right hand side of the rectangular condenser/absorber box.

Variable Throat - A variable throat Venturi is another common method of maintaining a constant pressure drop across a Venturi scrubber system.  A valve is integrated into the Venturi throat.  At maximum flow, the valve is fully open. As the flow decreases, the valve closes to reduce the cross sectional area accordingly.  The variable throat can be a damper blade, butterfly valve, plumb bob, or pinch valve.  As discussed above, variable throats are generally more suitable for larger gas flow processes.  Consideration should be given to the potential for fouling from particulate build up on the valve.  Particulate can accumulate and get stuck behind a butterfly valve, damper blade or on the shaft of a plumb bob. This can impede the ability to adjust or modulate the throat.  The potential for this type of fouling may depend on the nature of the particulate. Envitech often uses variable throat Venturi's on industrial dryer applications.  Variable throat Venturi's were discussed in a previous blog post, Venturi Scrubber: Adjustable Throats. 

Manual Inserts - A third approach for maintaining a constant pressure drop is the use of manual inserts.  This approach might be taken for a process that has distinct flow rates for long periods of time. It might also be used in situation where the design conditions are uncertain, say for a pilot or demonstration plant.   The use of manual inserts provides a way of designing flexibility into the equipment.

Please click on the icon below to view a video of a variable throat Venturi.

Wet Electrostatic Precipitator Control for Meeting Metals Emissions

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There are several industrial processes that face the challenge of meeting increasingly aggressive metals emission standards.  In many cases these standards exceed the capability of existing air pollution control equipment which can include bag-houses and packed bed absorbers. Some of these processes include:

• Secondary lead smelters
• Lead refining
• Refinery sludge incinerators
• Geothermal energy plants

Some of the metals of concern can include mercury, arsenic, lead, cadmium, nickel and others, depending on the process.  To achieve more stringent metals emission standards, three features should be designed into the air pollution control system. 

• Removal of the bulk particulate load
• Sub-cooling the gas
• Wet Electrostatic Precipitator (WESP) for polishing

Removal of the bulk particulate load - Removal of the bulk particulate load may be required if there is a high particulate concentration from the upstream process.  This will be the case for secondary lead smelters, lead refining, and refinery sludge incinerators.   Geothermal energy plants will not have this requirement.  A bag-house will be used for secondary lead smelters and lead refining.  A wet Venturi scrubber system can be used for refinery sludge incinerators   Removal of the bulk particulate minimizes space-charge effects inside the WESP. Space-charge effects occur when particles interact and repel each other. This reduces WESP performance because it interferes with the migration of charged particle to the tube wall for collection.

Sub-cooling the gas - Some of the volatile metals may be in the vapor phase when they pass through a bag-house at higher temperature.  In this case they will pass right through the bag-house and will not be collected.   Sub-cooling the gas is done after the bag-house and uses a condenser/absorber to cool the gas below the saturation temperature.  Sub-cooling is beneficial because it condenses as much of the volatile metals as possible so they can later be removed as particulate.  In the case where a Venturi is used to remove bulk particulate, like for a refinery sludge incinerator, it has the added benefit of condensing water onto the particulate and condensed metals.  This increases their diameter making them easier to remove in a Venturi scrubber.  Sub-cooling also reduces the gas volume, which helps to reduce the size and cost of a downstream WESP.

Wet Electrostatic Precipitator (WESP) for polishing - A wet electrostatic precipitator is used as a final particulate polishing stage.  The performance is relatively independent of the particle size so it is highly effective at sub-micron particulate control.  In some cases, a condenser/absorber (C/A) for sub-cooling can be integrated into the conditioning section of an upflow WESP.  This was successfully done at a secondary lead smelter downstream of bag-house.  The C/A was also used to neutralize SO2 in the gas stream. The integrated C/A and WESP achieved > 98% removal of arsenic and > 92% removal of lead and other condensed metals after the bag-house.  This substantially reduced the plants cancer risk index and helped to meet more stringent fence line lead emission standards. 

To download a free white paper on wet electrostatic precipitator for a secondary lead smelter, click the icon below.

To view a free wet electrostatic precipitator video, click on the icon below. 

Photo Credits

Geothermal by Louis Falcon

Refinery by Szeke