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%

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.

Secondary lead smelters recycle lead bearing scrap metal, primarily lead acid car batteries, into elemental lead or lead alloys.  Metal from the batteries are remelted in blast or reverb furnaces and then refined in secondary smelters. The batteries contain high amounts of sulfur which oxidizes to SO2 in the furnaces.

An unutilized secondary lead smelting facility was retrofitted with new process equipment to restart operations. Air pollution control equipment was needed to achieve greater than 96% removal of peak loads of up to 4,500 lb/hr of SO2 from the furnace exhaust. 

The customer selected an Envitech packed bed scrubber to meet emission requirements.  Three combustion sources are combined in a duct header into a forced draft fan. The fan provides motive force through the scrubber.

The first scrubber step  is an evaporative quencher to cool the gas to saturation. The quencher is constructed from T316SS and is a low pressure drop Venturi to provide turbulence for rapid quenching with a wide turn-down ratio. A fiber reinforced plastic (FRP) elbow connects the quencher to a 10 foot diameter FRP absorber vessel.

Gas from the quencher passes vertically upward through a packed bed, counter-current to downward flowing recirculated water.  Scrubbing water and excess quench water are collected in a common sump and is recirculated to the top of the packed bed and quencher. 

A pre-assembled recirculation pump skid with redundant pumps was supplied with the scrubber.  Instruments were pre-mounted and pre-wired to a control box on the skid.

A dilute solution of plant-supplied sodium hydroxide is metered into the scrubber recirculation line to neutralize acid gases and is controlled by the recirculation liquid pH.  A blowdown stream purges the system of reaction products and is controlled by conductivity.  Blowdown liquid is treated by separate oxidation tanks to convert sulfite reaction products to sulfates.

After the packed bed, the gas passes through a chevron style mist eliminator above the packing material to remove water droplets.  A wash header below the mist eliminator provides a periodic wash to keep the chevrons clean.  Finally, the gas exits the system and is exhausted through a stack.

The scrubber has been operational since 2010 with good result.  Below is a summary of design and performance results.

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

Waste oil is recycled and refined into low sulfur marine diesel and other industrial fuels at West Coast refineries. Waste gas is sent to thermal oxidizers for volatile organic compound (VOC) destruction. Sulfur compounds in the waste gas are oxidized to SO2 and removed by a packed bed scrubber. A fraction of SO2 converts to sulfur trioxide (SO3) before entering the scrubber. SO3 further converts to sulfuric acid (H2SO4) and generates a submicron liquid mist upon quenching the gas. New ground level pollutant regulations require removal of sulfuric acid mist before exhausting the flue gas to atmosphere. A multi-pollutant solution is needed to remove both SO2 and sulfuric acid mist. 

The customer selected an Envitech SO2 scrubber, candle filter system. The arrangement includes a quencher to cool the gas to saturation, a caustic packed bed absorber to remove SO2, and a candle filter to capture sulfuric acid mist. Internal ducts with outlets near grade simplifies ductwork between the caustic scrubber and candle filter and between the candle filter and ID fan. The scrubber comes with pre-assembled pump skids. Instruments are pre-mounted in pre-assembled piping and pre-wired to a junction box to reduce installation time and cost.

Both scrubbers have been shipped to the sites. The smaller system was put into service in early 2020. The scrubbers meet the design conditions summarized below.

DESIGN PARAMETERS

Click on the link below to download a case study and related wet scrubber literature.

As an equipment supplier of custom engineered wet scrubber equipment, Envitech frequently provides lunch and learns (L&L’s) to engineering companies to help educate engineers about the basics of available technology.

Figure 1 below is a summary chart of predominant wet scrubber technology options. The main product categories include packed bed absorbers, Venturi scrubbers, and wet electrostatic precipitators (WESPs). Each wet scrubber type serves a different purpose and is used in different circumstances. For instance, packed bed absorbers are primarily used to remove gaseous emissions like SO2, HCl, or HF. We often receive packed bed absorber inquiries for particulate removal, however, this would be a misuse of technology. Packed bed absorbers remove some particulate but they are not nearly as efficient as other options.

Figure 1: Wet scrubber technology summary

Venturi scrubbers are used for particulate removal. Just like we sometimes receive packed bed absorber inquiries for particulate removal, we’ll occasionally get Venturi scrubber inquiries for acid gas removal. This would also be a misuse of technology. Venturi scrubbers achieve some acid gas removal, but they have poor mass transfer compared to a packed bed absorbers.  

Venturi scrubbers use mechanical forces to remove particulate. Particles are captured through a process of impaction between particles in the gas and water droplets in the Venturi throat. A high differential velocity is created between particles and droplets by accelerating the gas in the throat. A pressure drop in the throat provides energy to capture the particles. Smaller particles less than 1 micron in size avoid capture by behaving like gas molecules and finding slip streams around the water droplets. Venturi scrubber performance drops offs exponentially for submicron particulate. Overall removal efficiency may be limited for a gas stream with a high concentration of submicron particulate. Venturi scrubbers are a good choice for industrial dryers or other applications with large size particulate.

Wet electrostatic precipitator (WESP) are the third type of wet scrubber summarized in the table. Like Venturi scrubbers, they are also particulate removal devices. They differ from Venturi scrubbers in a couple of ways, 1.) electrical, not mechanical forces are used to capture particulate, and 2,) they are efficient at capturing submicron particulate. Figure 2 shows a performance comparison between a WESP and Venturi scrubber. It can be seen that performance drops off dramatically for Venturi scrubbers for particles less than 1 micron in size. WESP’s on the other hand remove particles regardless of particle size.

Figure 2: WESP, Venturi scrubber performance comparison versus particle size.

The summary in Figure 1 also shows how each wet scrubber technology differs in regulatory control. A packed bed absorber is typically controlled for recirculation flow rate and liquid pH. A Venturi scrubber is controlled by recirculation rate and pressure drop. A WESP is controlled by voltage. Control limits are typically spelled out in the operating permit.

Finally, some examples of applications are given for each type of wet scrubber. It should be noted that there are many applications that have multiple types of pollutants. A hazardous waste incinerator, for instance, contains particulate, acid gases, and specific heavy metals like cadmium and lead. A fraction of particulate is submicron in size and difficult for a Venturi scrubber to remove. It is common for different types of wet scrubbers to be combined into a multi-pollutant device. Figure 3 shows a common arrangement for an incinerator scrubber. The gas is first cooled in a quencher. A packed bed absorber removes acid gases. A Venturi scrubber removes particulate and a WESP removes the submicron particulate and heavy metals.

Figure 3: Incinerator wet scrubber arrangement

Wet scrubbers can also be combined with dry scrubbers in certain circumstances. An upstream bag-house can remove particulate followed by a packed bed absorber for acid gas removal. A cyclone can be used to knock out large particulate before using a Venturi scrubber for the remaining particulate. A cyclone helps to minimize blowdown and water consumption. In some cases, a dry filter or carbon bed absorber can be integrated downstream of a wet scrubber for mercury and/or dioxin/furan. Click here to read a blog piece about an example of a wet scrubber combined with a carbon bed.

This about covers wet scrubber basics. If you’re with an engineering company and want to discuss scheduling a lunch an learn, please give Envitech a call. You can click on the icon below for a set of Envitech brochures.

Click on the icon below to download an Envitech brochure.

Envitech will be attending and exhibiting at the 37th International Conference on Thermal Treatment Technologies and Hazardous Waste Combustors (IT3/HWC) in League City, TX on October 2nd and 3rd, 2019. If you happen to be attending the conference, please stop by the Envitech booth or find me at the conference to say hello.

Three keynote panels will include high level experts and cover hot topics in the industry:

• PFAS Overview, Regulatory Approach, Testing and Destruction
• Emerging Air Quality Monitoring
• Lessons Learned from the United Kingdom's 2018 Novichok Nerve Agent Incident

Papers presented in technical sessions cover:

• Technologies and Trends in Incineration
• Plastics Recycle and Reuse
• Emission Monitoring
• Waste-to-energy, Emission Monitoring, Pyrolysis 

Envitech will present papers on the following two topics.

Technology Solutions for Sulfuric Acid Formation and Removal in Liquid Waste and Waste Gas Thermal Oxidizers

Petrochemical plants, refineries, and waste-oil re-refiners operate liquid waste or waste gas thermal oxidizers. The thermal oxidizers need a wet scrubber to neutralize and remove SO2. Flue gas entering the scrubber contain some sulfur trioxide (SO3) which is converted to sulfuric acid (H2SO4) in the quencher. Sulfuric acid is a submicron liquid aerosol that passes through the downstream packed bed absorber. Some facilities are now being regulated for H2SO4. This paper evaluates and compares candle filters versus wet electrostatic precipitators (WESP’s) for H2SO4 removal in these applications.

Sewage Sludge Incinerator (SSI) Mercury Control Technologies

Waste water treatment facilities operating sewage sludge incinerators (SSI) can reduce sludge volume and disposal costs by combusting dewatered sewage sludge. Emissions are regulated by the US EPA Maximum Available Control Technology (MACT) standard 40 CFR Part 60 and 62 to control particulate, lead (Pb), cadmium (Cd), SO2, HCl, dioxins/furans, and mercury (Hg). Many SSI’s need a control device specifically for mercury. This paper evaluates two mercury control technologies: sulfur‐impregnated activated carbon and Gore sorbent polymer catalyst (SPC) modules. Several facilities have used sulfur-impregnated activated carbon but safety issues have arisen due to fires which have shut down some systems. The Gore SPC modules are a relatively new technology with at least seven installations. A comparison is made of capital cost, operating cost, mercury removal efficiency, fire and performance risks based on incineration of 3,000 lbs/hr of sewage sludge. Finally, an overview is provided for an Envitech SPC mercury control scrubber operating at one facility.

Click on the icon below to download an Envitech brochure.