Air Pollution Control Innovations

Envitech recently got noticed in a local news story by Michael Chen of KGTV Channel 10 News, “San Diego Companies Could Help Clean China’s Air”.  The story is about how California’s Governor Jerry Brown’s diplomatic trip to China could lead to opportunities for local San Diego companies like Envitech.  During his visit, Gov. Brown signed a pact that will pave the way for California companies to help China measure and improve its air quality.   As a leader in industrial air pollution control equipment, Envitech has process technology that can be used in China for reducing hazardous air pollutants (HAPs) and pollutants that contribute to regional haze like sulfur dioxide (SO₂). These technologies have been applied to many processes in North America including a coal gasification plant, hazardous waste incinerators, lead smelters, sulfite pulping mills, waste oil re-refiners, geothermal plants, and mining and mineral processing to name a few.  Envitech has pursued several opportunities in China through 3^rd party customers and will have one installation starting up later this year.

For more information on Envitech's capabilities, please download our product brochure.

In 2009 I wrote a blog piece about the new EPA rules promulgated for the hospital, medical, and infectious waste incinerator (HMIWI) maximum achievable control technology (MACT) standard.  The compliance dates for these rules are fast approaching.  Facilities with existing equipment must demonstrate compliance to the new standards by October 2014.  Envitech is already under contract with several facilities to retro-fit existing medical waste incinerator scrubbers with add-on control equipment to meet the new standards. 

The emissions reduction challenge with the new rules can be seen in the adjacent graph which compares the difference between the 2007 MACT standard to the new MACT standard.   Stack emissions must meet substantially lower limits for Cd, Pb, and Hg. In many cases, this requires add-on controls capable of greater than 90% removal of sub-micron condensed metals.  Most facilities are putting on a re-heat and filter package to remove the condensed metals.  A few will use wet electrostatic precipitators (WESP) which are more expensive.   The ability to meet the new rules using a re-heat and filter package has been demonstrated for lead and cadmium on a commercial and industrial waste incinerator (CISWI).  The WESP capability has been demonstrated for reduction of lead emission achieved at a secondary lead smelter in California.

Another emissions reductions challenge is dioxins/furans (D/F).  Emission limits for D/F have been reduced from 125 ng/dscm Total and 2.3 ng/dscm TEQ (corrected to 7% O2) to 25 and 0.6 ng/dscm, respectively.  These emission limits are too low to be met with carbon injection.  An add-on control package of re-heat and carbon bed absorber is required to meet the new limits.

Solutions to these challenges exist and facilities are taking steps to meet them.  Click on the link below to download the HMIWI MACT Rule paper from the 2010 International Conference of Thermal Treatment Technologies and Hazardous Waste Combustors (IT3/HWC).

Mississippi Power Company recently released a Youtube video providing an update on the Kemper County Coal Gasificaton Integrated Combined Cycle (IGCC) Project.  The project is a 582-megawatt power plant currently under construction.  The facility will convert locally mined lignite coal into energy using a state of the art coal gasification process call Transport Integrated Gasification, or TRIG^TM.  The process enables a 65% CO₂ reduction making green house gas emissions equivalent to similar size natural gas combined cycle power plant

The lignite coal is very wet and needs to be dried before it is gasified.  An Envitech wet scrubber-condenser system is used in the material handling/drying train.  The system is comprised of a Venturi scrubber and packed bed condenser.  The wet scrubber equipment treats 2.1 MM cfm of dryer exhaust and can be seen in the lower left corner of the screen 28 seconds into the video.

Part of the CO₂ reduction comes from CO₂ capture using 200 ft solvent absorbers.  The CO₂ will be piped to another location in MS and used for enhanced oil recovery. This will allow an increase in oil production of approximately 2M barrels per year. Some milestones/features of the plant include:

• Installation is 70% complete
• Start-up planned during the summer of 2013
• The plant will be a zero liquid discharge facility
• Approximately 2,500 workers are currently on site
• Over 12,000 construction jobs will be created during the course of onstruction
• About 1,000 permanent positions will be created once the facility is open.

Click on the icon below to download a free presentation from the 2012 Coal-Gen conference on the coal dryer wet scrubber system.

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.

Please read our case study on reducing SO2 emissions for ships docked at ports by clicking the link below.

Photo Credit: Saint Seminole

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.