Air Pollution Control Innovations

Thermal oxidation of fossil fuels or other sulfur containing material generates sulfur dioxide, SO₂. Petroleum refineries, secondary lead smelters, paper and pulp manufacturers, geothermal power generators, waste incinerators, and mineral processors are the primary emitters of SO₂.

SO₂ contributes to respiratory illness and aggravates existing heart and lung conditions.   It contributes to acid rain, damaging vegetation, sensitive ecosystems, and waterways. It is one of the six common criteria pollutants. Criteria pollutants are subject to primary and secondary National Ambient Air Quality Standards (NAAQS) under the federal Clean Air Act. Primary standards prevent adverse effects on human health.

Packed bed absorbers are a common wet scrubber technology for removing SO₂. Absorbers use sodium hydroxide (NaOH), often referred to as caustic, or soda ash (Na₂CO₃) to neutralize SO₂. Relative to other air pollution control technologies, packed bed absorbers achieve high removal efficiency, possess a low capital cost, are highly automated, and require minimal maintenance with high reliability.

When absorbed into water, SO₂ solubilizes to sulfite (SO₃). SO₃ requires further oxidation to stabilize in water. If left unoxidized, SO₃ increases the chemical oxygen demand of the wastewater and can convert back to SO₂ resulting in toxic offgas.

One way to oxidize the wastewater is through forced oxidation. This oxidizes sulfite (SO₃) reaction products to sulfate (SO₄). Forced oxidation can increase the size, complexity, and operating cost of the system. The PFD image below shows a block diagram of a thermal oxidizer SO₂ scrubber with forced oxidation. Waste, fuel, and air combust in a thermal oxidizer. Sulfur compounds in the waste oxidize to SO₂. After the thermal oxidizer an evaporative quencher cools the gas to its saturation temperature, typically 180^°F or lower. The quencher sprays water into the gas, cooling it. Some of the water evaporates, increasing the gas water content. The gas then enters a packed bed absorber. Packing provides mass transfer to facilitate absorption of SO₂ in the gas into the recirculated water. Caustic in the recirculated water reacts with dissolved SO₂ by the reaction shown below:

SO₂ + 2NaOH -> Na₂SO₃ + H₂O

The reaction occurs at a pH near neutral. Excess water from the quencher and packed bed collects in the scrubber sump.

When the process requires SO₃ oxidation or stabilization, aeration is integrated into the system. Caustic and air inject into the sump. Oxygen in the air oxidizes SO₃. An aeration diffuser assembly promotes the transfer of oxygen into the water to facilitate the oxidation reaction.

2SO₃^2- (aq) + O₂ (g) -> 2SO₄^2- (aq)

The oxidation reaction is very fast. The limiting step is dissolving oxygen into the water to allow SO₃ oxidation to occur. In the case of a low sulfur load, aeration can occur in the sump with little impact on the scrubber size. In the case of a high sulfur load, the scrubber sump requires substantial modification to sufficiently oxidize the SO₃. For excessive sulfur loads, oxidation may need to take place in separate oxidation tanks.

It should be noted that forced oxidation is uncommon. Most industrial SO₂ packed bed absorbers don’t require forced oxidation. High sulfur load packed bed absorbers are also uncommon. For high sulfur loads, the operating cost of sodium-based reagents make higher capital cost alternatives more palatable. Other options include a limestone spray tower, a dual alkali scrubber, or a lime injection bag house.  Each alternative is substantially more capital cost, more complex to operate, and require higher maintenance. When pursuing a forced oxidation SO₂ absorber, it’s important to select a vendor capable of properly sizing and designing equipment for both SO₂ absorption and SO₂ oxidation.

Click on the link below to download SO₂ scrubber literature.

For economic reasons, a refinery desires to maintain refining capacity during periodic maintenance shutdown of their sulfur recovery unit (SRU).  A temporary caustic scrubber had been used to remove sulfur from sour fuel gas  during shutdown but was quite expensive and logistically difficult.  Several technologies were evaluated to replace the temporary scrubber for a permanent, lower cost solution. Considered technologies included a reverse jet scrubber, fiber film contactor, and an Envitech packed bed caustic scrubber.

The fuel gas enters the absorber horizontally at the bottom of the vessel and travels vertically upward, counter-current to downward flowing water.  Scrubbing water is collected in the sump and is re-circulated to the top of the packed bed.  A dilute solution of plant-supplied sodium hydroxide is metered into the scrubber recirculation line to neutralize H₂S and is controlled by the recirculation liquid pH.  A blowdown stream from the recirculation line purges the system of reaction products. A mist eliminator at the top of the vessel removes droplets before exiting the scrubber.

The system will be installed and utilized during a  planned maintenance shutdown in 2023.  The design parameters are summarized below.

• Inlet flow rate – 1,000 acfm, 7,300 scfm
• Inlet temperature – 110^oF
• Inlet pressure - 117 psia
• Design pressure - 200 psig, full vacuum
• Vessel - T316SS per ASME VIII division 1 or 2 with code stamp
• Inlet H₂S - 8,200 ppmv
• Removal efficiency > 99.3%

Click on the link below to download literature on this application.

A catalyst production facility operates a calciner that generates a small stream of hot dirty gas. The exhaust is cleaned using a particulate/SO₂ scrubber followed by a vertical entrainment separator.  The configuration is problematic for operation.  The scrubber requires a long duct run from near grade to the scrubber inlet flange.  There is a U-shaped bend to the inlet flange.   Particulate condenses in the duct and plugs over time.  The process is routinely shut down to clean out accumulated material. This limits production capacity.  The customer sought to redesign the scrubber to increase production while maintaining as much of the original equipment as possible.

The customer selected Envitech to redesign and supply a scrubber to be retrofit into the existing system.  The scrubber inlet was replaced with a high efficiency Hastelloy Venturi scrubber for particulate removal.  The Venturi length was significantly decreased and oriented at an angle to the entrainment separator inlet.  The shorter distance and angled orientation significantly reduced the duct run from the calciner outlet to the scrubber inlet, minimizing fouling potential.

The top of the entrainment separator was replaced with a packed bed absorber to neutralize and remove SO₂.  Structural modifications to the existing vessel ensures the  new packed bed is well supported.

After the Venturi, the gas enters the bottom of the packed bed and travels vertically upward, counter current to downward flowing water.  Excess water from the Venturi and the packed bed collects in a common sump.  Caustic solution injected into the recirculation line neutralizes acid gases.  Liquid recirculates back to the Venturi throat and top of the packed bed.  The gas passes through a vertical entrainment separator above the packing to remove water droplets before exiting the system.

The scrubber was put into service in early 2021.  A continuous emissions monitoring system (CEMS)  confirms emission limits are met. The duct clean out time has been reduced from 24 to 36 hours to < 1 hour.  System uptime has improved from < 60% to > 80%, enabling higher production capacity. The Venturi scrubber is significantly smaller in size, making replacement cost less expensive.   The scrubber is designed to meet the process conditions below:

• Flow rate: < 1,000 acfm
• Inlet Temp, 400 ^oF
• Particulate removal: > 99.8%
• SO₂ removal: > 99.9%

Click on the link below to download literature about the catalyst Calciner scrubber.

A dual purpose, low flow scrubber is needed for a facility that extracts lithium from geothermal brine. The scrubber treats a chlorine (Cl₂) rich stream from a chlor-alkali process and HCl from a caustic storage tank vent line.  The scrubber is installed outdoors in a desert environment with significant seismic and wind requirements.  Temperatures reach 113^oF in the summer.  The furthest emission source is 70 ft away. The scrubber will use and induced draft (ID) fan to pull exhaust gas from the upstream sources. The fan must be corrosion resistant and capable of pulling a low flow rate draft  across the pressure drop of the inlet ductwork and scrubber.

The customer selected an Envitech packed bed scrubber.  The scope of supply includes a fiber reinforced plastic (FRP) packed bed absorber, instruments, control system with HMI display, pre-assembled recirculation pump, piping, valves, and fittings, interconnect duct, and an FRP ID fan.  Instruments are pre-mounted in the piping and pre-wired to the control system.

The Cl₂ scrubbing reaction is exothermic.  A dilution air damper on the HCl tank vent line dilutes pollutant concentrations to a range that keeps the scrubber from getting too hot.  It also provides enough flow for the minimum cross-sectional area needed to house the packing.  The packed bed vessel and ductwork are self-supporting for wind and seismic.  A high-performance mesh pad at the top of the scrubber removes acid aerosol droplets formed in the scrubber.  The recirculation liquid operates at a high pH. The scrubber needs to achieve 99.98% removal for the worst-case loading conditions.  The scrubber height and mesh pad  work together to achieve performance guarantees.

The scrubber will be delivered in the first quarter of 2022 and is designed to meet the following design conditions:

• Inlet flow rate – 400 acfm
• Inlet temperature – Ambient
• Max HCl and Cl2 discharge limit < 5 ppmv
• Removal efficiency > 99.98%

The California Air Resource Board (CARB) is considering new rules in California affecting chrome plating processes for metal finishers. An estimated 100,000 people participate in the chrome plating supply chain. A hundred and fifty chrome plating operations are in California. Eighty to ninety of those are for defense and aerospace applications.

Decorative chrome plating is utilized in the consumer marketplace. Functional plating is utilized by the aerospace and defense industries and includes hard plating and chromic acid anodizing. Hard plating provides extremely hard, corrosion resistant coatings with exceptional wear, abrasion, and corrosion resistance. Chromic acid anodizing creates a thin aluminum oxide film used for flight-critical aluminum components subjected to high stresses, such as landing gear. Functional plating processes must meet strict OEM and MIL-SPEC defense requirements.

Many chrome plating operations started out in remote locations. Communities have built up around these facilities over time. Close proximity between communities and plating facilities increase public health risk. CARB had proposed new rules to mitigate these risks by requiring facilities to transition from the use of hexavalent chromium for decorative and functional plating to alternative technologies, largely based on trivalent chromium technologies. Processes affected by the proposed transition timeline are strictly controlled and require extensive qualification. Because industry viewed the timeline as extremely aggressive CARB has withdrawn the proposed rules and will propose new rules in the months ahead.

Existing hexavalent hard and decorative chromium electroplating and chromic acid anodizing facilities need to meet a hexavalent chromium limit of 0.0015 milligrams/ampere-hour as measured after an add-on air pollution control device. The size of the air pollution control (APC) system depends on the ventilation gas flow rate. A typical large plating operation may be comprised of several plating tanks with a ventilation flow rate of 40,000 to 50,000 acfm. The APC technology is comprised of a multi-stage mist elimination followed by HEPA filters for final polishing. The above figure provides an isometric view of a system. The footprint is 45 ft by 30 ft with a height of 31 ft to the top of the vessel. The vessel and ductwork are constructed from fiber reinforced plastic (FRP) to provide structural integrity to meet wind and seismic requirements as well as long life. FRP doubles the life of equipment compared to lower cost polyvinyl chloride (PVC) material which has also been used for material of construction.

Vent gases enter the bottom of the mist eliminator vessel and passes vertically upward through mesh pad stages in series. A recirculation pump recirculates dilute chromic effluent from the vessel sump to provide a continuous wash on some of the mesh pad stages. A periodic fresh-water wash is applied to the other stages. Wash water falls by gravity and is collected in the vessel sump. A blowdown stream from the discharge side of the pump purges the system of collected contaminants into a separate storage tank.

After the final mesh pad stage, the gas passes through an internal duct to exit the mist eliminator vessel near grade into an induced draft fan. For additional performance, the gas then splits into two separate lines into a re-heater duct to heat the gas above the dew point before entering a HEPA filter for final contaminant removal. Reheat minimizes condensation fouling in the HEPA filters. After the HEPA filters the gases enter a common stack before exhausting to atmosphere.

The challenge for meeting the emission limits is the mesh pad performance is dependent on the aerosol droplet size formed by the bath solution. Mesh pad suppliers provide performance characteristics down to a specific droplet size. Multi-stage mesh pads may be sufficient to meet the 0.0015 milligrams/ampere-hour limit. Downstream HEPA filters provide additional assurance performance guarantees are met under all conditions.

Click on the link below to download Envitech brochures.

There are several different waste incinerator source categories controlled by EPA standards under the Clean Air Act (CAA). These include hazardous waste combustors (HWC), sewage sludge incinerators (SSI), municipal solid waste (MSW) incinerators, commercial and industrial solid waste (CISWI) incinerators, and Hospital, Medical, and Infectious Waste Incinerators (HMIWI). Each incinerator type has its own Maximum Achievable Control Technology (MACT) standard which establishes technology based limits for emitted HAPs. MACT standards are part of the National Emission Standards for 

Hazardous Air Pollutants (NESHAP) and are applied to source categories that pose adverse risk to human health by the emission of hazardous air pollutants (HAPs).  The HMIWI MACT standard for medical waste incinerators is the most challenging of the incinerator source categories. This standard controls particulate (PM), hydrogen chloride (HCl), sulfur dioxide (SO2), lead (Pb), cadmium (Cd), mercury (Hg), dioxins/furans (D/F), nitrous oxide (NOx), and carbon monoxide (CO). Emission limits depend on the incinerator size and weather it is a new or existing source. Small incinerators are less than 200 lb/hr of waste throughput, medium incinerators are between 200 lb/hr and 500 lb/hr, and large incinerators are greater than 500 lb/hr.

Envitech recently completed a project for two medical waste incinerators at a Midwest research facility. These are the first new medical waste incinerators installed in the United States since Envitech installed a 525 lb/hr medical waste incinerator at a research facility in Galveston, TX in 2013.

The scope of supply includes two medical waste incinerator scrubbers and a water treatment system to treat the blowdown from both incinerators. The incinerators are permitted as new, medium size incinerators. Ozone injection is integrated into the system to meet a NOx limit of 67 ppmv. The systems include pre-assembled pumps, piping, valves, and fittings to minimize installation time and cost. The pre-assembly provides long term rigidity, consolidation of space, longer up-time, and improved safety for operators. A description of the process arrangement can be found in this link to an earlier blog post.

Stack testing was performed in June 2021 for both incinerators. Test results confirm the Envitech system reduced emissions well below MACT standard limits, providing a comfortable margin for compliance over the range of operating conditions and waste feed. Below is a summary of stack test performance.

Click on the link below to download literature on medical waste incinerator scrubbers.

A common wet scrubber air pollution control application is the removal of sulfur dioxide (SO2) and related compounds from combustion processes. This class of compounds is often referred to as SOx. The formation of SOx and SO2 occurs from sulfur bearing fuels or materials oxidizing to SO2 upon combustion. SO2 has negative health effects and can contribute to respiratory illness, especially in children, the elderly, and individuals with pre-existing conditions.   In addition to health effects, SO2 contributes to acid rain which can harm plants, trees, rivers, streams, and lakes. SO2 also reacts with other compounds in the atmosphere to form small particles that contribute to particulate matter (PM) pollution and regional haze otherwise known as smog. Regulatory agencies often control SO2 not only to minimize harmful health affects but to reduce regional haze.

Packed bed scrubbers are often considered the best available control technology (BACT) for SO2 removal. Below is a summary of SO2 exhaust streams Envitech has treated using packed bed scrubbers.

• Waste oil refinery waste gas thermal oxidizer and direct fired heater
• Refinery sulfur recovery unit (SRU) thermal oxidizer
• Geothermal power generation regenerative thermal oxidizer
• Secondary lead smelter furnace
• Mineral processing furnace
• Catalyst regeneration kiln
• Ceramic tile kiln
• Hazardous waste combustor
• Medical waste incinerator
• Marine diesel engine

In these applications Envitech has treated SO2 loads as high as 48 tons per day and achieved efficiencies exceeding 99.9% removal. Gas flow rates can vary from < 500 cfm to more than several hundred thousand cfm.

HOW DOES THE TECHNOLOGY WORK?

An example of a large refinery/petrochemical thermal oxidizer SO2 scrubber application illustrates the technology. Below is a summary of design conditions. Liquid discharge limits for chemical oxygen demand (COD) and biochemical oxygen demand (BOD) require oxidation of the blow

down.

Design Conditions

• Flow rate: 300,000 acfm
• Temperature: 1560^oF
• SO2: 300 lb/hr
• Chemical oxygen demand (COD): < 200 mg/l
• Biochemical oxygen demand (BOD: < 50 mg/l
• SO2 removal > 99%

The above figure shows the scrubber equipment arrangement. Waste gas, fuel, and air are fed into the thermal oxidizer. Combustion in the thermal oxidizer generates exhaust gas of 300,000 acfm @ 1,560^oF with 300 lb/hr of SO2. The first step of the scrubbing process cools the gas to saturation using adiabatic cooling through evaporation of water. Excess water flows into the packed bed absorber sump.

The next step is SO2 absorption via mass transfer promoted by high efficiency packing media. The gas flows vertically upward, counter-current to downward flowing recirculation water. Neutralization via caustic addition improves the SO2 absorption rate. An entrainment separator at the top of the packed bed absorber removes water droplets in the gas before exiting the system. Water from the absorber sump is recirculated to the quencher and to the top of the packed bed. Make-up water replaces evaporation and blowdown losses.

The SO2 load is low enough that the scrubber absorber sump can serve as an oxidation tank. Air is sparged into the sump to oxidize sulfites (SO^-2₃) to sulfates (SO^-2₄) in order to meet COD and BOD limits. Caustic addition ensures sulfate formation and minimizes sulfur dioxide off-gassing. Larger SO2 loads may require external oxidation tanks.

The adjacent figure shows a typical arrangement to handle 300,000 acfm of gas flow. The system is modularized with 12 feet diameter shop fabricated vessels. Recirculation pump skids and aeration pump skids are pre-assembled in the shop. Instruments are pre-mounted in the piping assembly where possible and pre-wired to a junction box on board the skid. Shop fabrication and assembly minimizes installation time and cost.

Advantages of a packed bed scrubber includes:

• High removal efficiency
• Proven technology
• Low capital cost
• Automatic operation
• High reliability, low maintenance

Click on the link below to download literature about SO2 scrubbing.

A refinery is upgrading an SO2 quencher-scrubber treating incinerator exhaust from a thermal oxidizer of a sulfur recovery unit (SRU).   The quencher is a re-purposed eductor type Venturi that is at end of life and will be replaced.  Scrubber recirculation water passes through a heat exchanger to subcool the gas, eliminating make-up water.  Upstream heat recovery is removed which increases the gas flow rate to the scrubber.

The customer selected Envitech to provide a replacement quencher and to make scrubber modifications to accommodate higher gas flow.  The new quencher is an Envitech design sized to fit into existing footprint, platforms, and flange connections.  Water injection through open ports in the quencher throat eliminates a spray nozzle to improve reliability and maintenance.  Recirculated water to the quencher is significantly reduced. Existing pumps are oversized but reused by recirculating excess water from the discharge to the pump return.  Scrubber packing and mist eliminator are redesigned using high performance components for larger gas flow and reduced pressure drop.

Envitech performed a thermal study using Solidworks^TM modeling on the flange connection between the ductwork and mating quencher inlet flange.  Study results were used to ensure proper material selection.  Scope of supply includes two 90^o refractory lined duct elbows connecting to the quencher.

The elbows are insulated with a rain shield. The connecting 90^o elbow to the quencher is mitered with a flange and transition section using high temperature alloy.  All supplied equipment is compliant with refinery quality and design specifications.  Coordination with the end-user and 3^rd party engineering firm ensures fit-up and proper mechanical design for interconnecting ductwork and connections.

The scrubber upgrade meets the below design parameters and allows the plant to safely to operate with higher flow while re-using a substantial amount of existing equipment.

Click on the link below to download literature about this application.

Industrial facilities are increasingly called upon to consider the use of wet electrostatic precipitators (WESPs) for emissions control as regulations for PM 2.5 and specific heavy metals become more stringent. Facility and Environmental, Health, and Safety (EHS) Managers may need to become familiar with WESP technology and how they can be applied to their facilities.

WESPs date back to the 1970’s and are a tried-and-true method for removing submicron particulates, aerosols, SO₃, opacity, and condensed heavy metals. They are generally robust and suitable for 24/7 continuous operations. Their use is found in steel melting furnaces, secondary lead smelters, hazardous waste combustors (HWC), medical waste and sewage sludge incinerators, wood products and pellet manufacturing, mineral wool and glass manufacturing, silicon monomer manufacturing, semiconductor manufacturing, and even industrial fryers.

The adjacent figure illustrates WESP’s primary use. The graph shows removal efficiency on the vertical axis and particle size on the horizontal axis. The red curve shows typical WESP performance.   The dotted blue curve shows typical Venturi scrubber performance. A comparison shows that a Venturi scrubber is highly effective at removing particles greater than 1 micron in size. However, performance drops off rapidly for particles below 1 micron. WESP performance is relatively immune to particle size and maintains high performance for particles below 1 micron. This capability is derived from the use of electrical forces for particle removal compared to a Venturi scrubber which uses mechanical forces. WESP’s are generally higher capital cost than Venturi scrubbers but lower operating cost. They are used in cases where performance cannot be achieved with a Venturi scrubber or other, lower cost control device.

WESPs are often integrated with other control technologies and used as a polishing device at the end of a process. This is shown in the adjacent illustration for a typical waste incinerator. Perhaps the most important aspect to understand about WESP technology is the relationship between performance, size, and cost. Higher performance requires more collection area, larger footprint, and more cost. This is different than other control technologies like a Venturi scrubber or packed bed scrubber. For these devices, size and cost is primarily determined by the gas flow rate while operating cost is determined by performance. Size and cost for a WESP on the other hand is determined not only by gas flow rate but also removal efficiency. It is therefore important to have a good understanding of the range of inlet particulate concentrations and outlet emission limits. Specifying performance of 90%, 95%, or 99% removal will make a substantial difference on size and cost.

WESP technologies can vary greatly between vendors which can have an impact on footprint, maintenance, and long-term performance. Some of these differences include:

• Tube geometry (square, round, or hexagonal)
• Tube size
• Tube length
• Operating voltage
• Electrode construction
• Electrode alignment mechanism
• Bottom support grid requirement
• Gas distribution mechanism

It’s recommended that facilities evaluating WESP technologies dedicate time to understand differences in WESP designs and potential impacts on operations. Evaluation may also include how the WESP is integrated with other upstream equipment needed to meet multipollutant emissions criteria.

WESP technology is a substantial investment for any facility but may be the best available control technology for a submicron particulate application.

Click on the link below to download WESP scrubber literature.