Air Pollution Control Innovations

Andy Bartocci

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Refinery Thermal Oxidizer SO2 scrubber and Sulfuric Acid Mist Candle Filter

Posted by Andy Bartocci on Wed, Apr 08, 2020 @ 10:00 AM

Sulfuric acid mist arrangementWaste 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.

candle filter


Design Plant 1 Plant 2
Flow rate, acfm 10,000 4,500
Inlet temp, oF 1,600 376
SO2 removal > 90% > 95%
H2SO4 removal > 99% > 99%







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

Download Literature

Topics: Scrubbers, SO2 Scrubber, Acid Gas, SO3 Aerosol, Packed Bed Absorbers, Packed Bed Scrubbers, Wet Scrubbers

BP Amoco Purified Terephthalic Acid (PTA) Plant Quencher

Posted by Andy Bartocci on Wed, Jan 29, 2020 @ 09:00 AM

Venturi_Quencher_BP_AmocoBP Amoco was engaged in expanding production capacity for purified terephthalic acid (PTA) at the CAPCO 6 and Zhuhai 1 petrochemical plants in China. PTA is used in the manufacture of polyester resin and polyethylene terephthalate (PET) plastic bottles. It is also widely used in the pharmaceutical, food, textile and packaging industries. The expansion projects were part of efforts to achieve process simplifications and lower capital costs. Project goals were to reduce plot area and the number of equipment pieces by 40%

The process includes up to 530,000 acfm of corrosive exhaust gases that are cooled to the adiabatic saturation temperature. Cooling large flow rate gases is challenging due to potential void spaces in the water spray system. Splitting flows into smaller exhaust trains is a common method to overcome this challenge. However, this adds ductwork and installation cost. For capital cost reductions, the customer wanted to saturate the gas in a single train in a horizontal arrangement to conserve space and to minimize ductwork.

BP Amoco PTA Venturi QuencherThe customer selected a custom engineered Envitech horizonal quencher. The material of construction is Hastelloy C276 to provide corrosion resistance at high inlet temperatures. A proprietary internal water injection and throat design splits the stream inside the vessel to ensure turbulence for mixing gases and water. Gases accelerate through the throat areas in combination with a recirculated water spray system. A small pressure drop cools gases over a wide flow rate operating range and minimizes the number of nozzles. The water spray header is removable to facilitate nozzle replacement and maintenance. The vessel is 13 feet in diameter in a horizontal arrangement. This allows the quencher to be shop fabricated so that inspection and quality assurance are done in a controlled environment. The unit is shipped as one piece.

The Envitech quenchers were installed in 2001 and operate to the design parameters summarized below. The proprietary design contributed to BP Amoco’s goals to reduce footprint area, process equipment pieces, and capital cost.


  • Max flow rate: 530,000 acfm
  • Inlet temperature: 560oF
  • Inlet pressure: 518 In. W.C. 
  • Guarantee: Cooling to +/- 40oF of the saturation temperature

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

Download Literature

Topics: Venturi scrubbers, Scrubbers, quenchers, Wet Scrubbers

Pharmaceutical Fermentation H2S Scrubber

Posted by Andy Bartocci on Tue, Jan 21, 2020 @ 06:00 AM

A major pharmaceutical company is implementing an expansion project in their Midwest facility. It includes two production Fermentors,Fermentation H2S Scrubber a secondary feed tank and secondary make-up tank. Combined vent gases contain up to 1,500 ppmv (24 lb/hr) of Hydrogen Sulfide (H2S) in air.

H2S is a highly toxic gas that can kill or seriously harm human health when exposed to it. For general industry the OSHA permissible exposure limit (PEL) is 20 ppm for an 8 hour period. The specification requires 99.9% removal which achieves less than 1.5 ppmv, well below the PEL.

Installation is on the first floor with a limited footprint area of 18 ft L x 8 ft W. Redundancy is required for pumps and ID fans. The customer desires maximum integration and pre-assembly to limit installation work on site.

The customer selected an Envitech H2S packed bed scrubber. The scope includes a fiber reinforced plastic (FRP) packed bed absorber, instruments, recirculation pumps (1 operating, 1 spare), pre-assembled CPVC piping, valves, and fittings with pre-mounted instruments, metering pumps, ID fans (1 operating, 1 spare), interconnect duct, and stack.
The stack passes through a second floor before breeching the roof.  Fans are located near the outlet on a mezzanine. A custom arrangement accounts for obstacles and interferences in the installation space.

H2SFermentation scrubber vesselVent gases enter the bottom of the absorber and travels vertically upward. A removable spray nozzle in the quench zone pre-scrubs and cools the process vent stream. Recirculated water sprayed over the packed bed mixes with counter current upward flowing gas. A dilute solution of sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) metered into the recirculation line neutralizes and oxidizes acid gases. Chemical addition is controlled by pH and ORP.

An entrainment separator at the top of the absorber removes water droplets before exiting the scrubber.

The equipment has been delivered and will be installed in early 2020.  Design and performance parameters are summarized below.


  • Inlet flow rate: 3,000 acfm
  • Inlet temperature: 102oF to 212oF
  • H2S inlet concentration: 1,500 ppmv 
  • H2S Removal: > 99.9%

Click on the link below to download a case study and other packed bed scrubbing literature.

Download Literature

Topics: Scrubbers, Packed Bed Absorbers, Packed Bed Scrubbers, Wet Scrubbers

Renewable Energy Food Digester Ammonia Scrubber

Posted by Andy Bartocci on Tue, Jan 14, 2020 @ 07:00 AM

The University of California, Davis (UCD) operates an anaerobic biodigester that converts 50 metric tons of Env Lab Scrub Sketchfood waste per day into 100 MMBtu/d of renewable energy biogas. The digester is sited near campus at a decommissioned landfill.

The biodigester produces a digestate liquid waste stream. It’s odor and high ammonia concentration render it unsuitable for composting or direct land-applied fertilizer. To avoid expensive hauling costs, a distillation column strips ammonia out of the digestate into the vapor phase so that the digestate can be treated in an existing Wastewater Treatment Plant (WWTP). An Ammonia Scrubber Package is needed to meet air emission limits and to recover a saleable liquid fertilizer product from the vapor phase ammonia.

Ammonia is an odorless gas that is harmful to human health with a permissible exposure limit (PEL) of 50 ppm. The scrubber must have high removal efficiency and be capable of treating a low flow rate exhaust and be robust and reliable.

The customer selected an Envitech ammonia lab scrubber. The scope of supply includes a fiber reinforced plastic (FRP) packed bed absorber, instruments pre-wired to a control system wLab Scrub Mktg 1ith HMI, pre-assembled pump, piping, valves and fittings, ID fan with VFD, and free-standing interconnect duct and stack.

Ammonia laden stripped gas enters the scrubber and travels vertically upward through the packed bed, counter-current to downward flowing recirculation liquid. Scrubbing liquid is collected in the sump and is re-circulated to the top of the packed bed. Sulfuric acid (H2SO4) is metered into the recirculation line to neutralize ammonia and is controlled by pH. A blowdown stream purges ammonia sulfate (NH4)2SO4 reaction products which can be sold as fertilizer. A mist eliminator at the top of the scrubber removes entrained water droplets before exiting through an induced draft fan and stack.

The ammonia scrubber will be commissioned in the 1st half of 2020 and will meet the design parameters and performances below.



  • Inlet flow rate: 1,000 acfm (capable of up to 2,000 acfm)
  • Inlet temperature: 160oF
  • Gas Composition:  Air with 1.4% mole ammonia
  • Ammonia Removal: > 97.5% permitted (capable of > 99% removal).

Click on the link below to download a case study and other packed bed scrubbing literature.

Download Literature

Topics: Scrubbers, Packed Bed Absorbers, Packed Bed Scrubbers, Wet Scrubbers

Peat Dryer Horizontal Venturi Scrubber

Posted by Andy Bartocci on Thu, Jan 09, 2020 @ 05:30 AM

An agricultural solutions company, American Peat Technology, transforms locally harvested unprocessed peatPeat Dryer Horizontal Venturi into a consistent granular media. The media is used as a natural microbial carrier to inoculate millions of acres of food crops each year. The transformation process relies on drying harvested material in industrial rotary dryers. A fraction of the material becomes entrained in the flue gas as particulate emissions. The facility is expanding operations to double the capacity. A scrubber solution is needed to reduce total plant particulate emissions.

The customer selected two identical Envitech horizontal Venturi scrubbPeat Dryer Horizontal Venturi_long viewers. The scrubbers are comprised of a variable throat Venturi scrubber and a chevron style entrainment separator mounted on an equipment skid. The scope of supply for each scrubber includes a control system, re-circulation pump with pre-assembled piping, valves and fittings. Instruments are pre-mounted in the piping where possible and pre-wired to a junction box on the skid. The horizontal configuration provides a compact design to fit inside the building with easy maintenance access to valves and instruments.

The skid arrangement allows the scrubbers to be shop assembled and tested prior to shipment.  The combination of a variable throat Venturi with a chevron style mist eliminator allows a wide turn-down ratio while maintaining high performance. The use of a wet scrubber instead of a bag-house eliminates the potential for bag fires that could be caused by occasional hot embers entrained in the gas.  The facility is also able to make use of the latent heat absorbed by the scrubber, making use of hot recirculated scrubber water to thaw out frozen peat in the winter before it is fed to the dryers. 

The scrubbers were commissioned in December 2019 and are now operational. Both scrubbers are used for two dryers providing a wide operating range. The scrubbers meet the design parameters listed below:


  • Inlet flow rate: 25,000 acfm
  • Inlet temperature: 185oF
  • Inlet Particulate: 15 lbs/hr 
  • Particulate Removal: > 99%

Click on the link below to download a case study and other Venturi scrubbing literature.

Download Literature

Topics: particulate control, Venturi scrubbers, Scrubbers, Wet Scrubbers

Pigment Manufacturing Ammonia Recovery System

Posted by Andy Bartocci on Tue, Jan 07, 2020 @ 07:00 AM

A pigment manufacturer operates batch reactors that emit very low volumes of highly concentrated ammoniaAmmonia Recovery Scrubber. Batches occur over a 20-hour period and ammonia concentrations varies from <1 to 96%.

Ammonia is an odorless gas with a permissible exposure limit (PEL) of 50 ppm. Ammonia can also form explosive mixtures in air with a lower explosive limit (LEL) of 15% and upper explosive limit (UEL) of 28%.

The customer needs to recover a 25-28% (w/w) aqueous solution of ammonia for re-use as a raw material while meeting air emission limits. The recovered ammonia can have no potential silicone due to sensitivity of the end-product. The system must be robust and safe to operate.

The customer selected a custom designed Envitech ammonia recovery system based on Envitech’s proprietary design for low flow rate applications. The scope of supply includes a compressor, a packed bed concentrator, a packed bed absorber, instruments, PLC and HMI control system, and an equipment skid containing pre-assembled vessels, instruments, control system, pumps, piping, valves, fitting, and instruments with pre-wiring. Panel and components are explosion proof. Vessels are 316SS fabricated to pressure vessel ASME code for 90 PSI. All components are specially selected to ensure no possible silicone contamination.

The gas is drawn out of the reactor using a specially selected compressor ideally suited to handle corrosive, low flow rate gases and to generate high pressures while generating an upstream vacuum.

The concentrator uses recirculated water to absorb and concentrate ammonia. A 2nd stage absorber recovers residual ammonia to meet air emission limits.
Blowdown from both stages purge concentrated ammonia for re-use as raw material.

The recovery system will be commissioned in early 2020 and will meet the design parameters and performances below.


  • Inlet flow rate: 50 acfm
  • Inlet temperature: 68oF
  • Carrier gas composition:  N2
  • Peak ammonia concentration: 96%
  • Concentration of recovered ammonia: > 25%
  • Ammonia removal: > 98% 

Click on the link below to download an ammonia recovery system case study and other scrubbing literature.

Download Literature

Topics: Scrubbers, Packed Bed Absorbers, Packed Bed Scrubbers, Wet Scrubbers

Refinery Catalyst Regeneration SO2 Scrubber

Posted by Andy Bartocci on Mon, Dec 16, 2019 @ 06:59 AM

A Saudi petrochemical plant operates a hydrogenation reactor to Refinery Catalyst Regen SO2 Scrubber renderingreduce organic compounds using a catalyst. In-situ regeneration is desired to restore the catalyst for additional processing. This improves operational efficiency and safety. The facility is evaluating technology options to achieve this goal. One approach is to
contact the catalyst with a carrier gas at elevated temperature to promote oxidation, resulting in SO2 emissions.

A wet scrubber is needed with this approach to remove SO2 before exhausting to atmosphere. The scrubber will be installed outdoors in a harsh coastal environment with summer temperatures exceeding 114oF. It will operate 3 to 5 days every 18 months and needs to be stored properly to maintain operational condition between uses.

The scrubber is to be built in conformance with over 12,000 pages of detailed refinery specifications with extensive quality assurance requirements.  Because of a tight schedule, a scrubber engineering order is initiated in parallel to evaluating other technology options. This reduces scrubber delivery time if the carrier gas oxidation approach is selected.

The customer selected an Envitech scrubber for the engineering order after extensive evaluation. The equipment scope includes a quencher, packed bed absorber, instruments, recirculation pump, pre-assembled piping, valves, and fittings with pre-mounted instruments. Pre-mounted instruments are pre-wired to control boxes. Components meet Class I, Div 2 area classification.

The quencher and vessel are designed using T316SS with internal and external coating. Piping Saudi refineryis T316SS conforming to ASME B31.1. Welds are specified to be magnetically and X-ray tested using certified welders.

The design submittal includes more than 90% of final engineering deliverables. The design order was complete and meets the design conditions outlined in the table below.

Although designed as stationary equipment, this type of scrubber can also be designed for mobility for use in other applications or acid gases. Alternate materials of construction are available such as FRP or alloy steels.


  • Inlet flow rate: 3,200 acfm
  • Inlet temperature: 510oF
  • Carrier gas composition:  N2
  •  Inlet SO2: 64 lb/hr
  • Outlet SO2:  < 50 ppmv
Click on the link below to download a case study for a refinery catalyst regeneration SO2 scrubber and other scrubbing literature.

Download Literature

Topics: Scrubbers, Packed Bed Absorbers, Packed Bed Scrubbers, Wet Scrubbers

Mineral Processing Venturi Scrubber

Posted by Andy Bartocci on Wed, Dec 04, 2019 @ 08:04 AM

US Magnesium is implementing an electrolytic expansion project located in Rowley, UT. A scrubbing system isUS Mag Venturi Scrubber needed to remove particulate from a chlorine gas stream that originates in the anode compartments of electrolytic cells that produce magnesium metal. A significant portion of the particulate is less than 1 micron in size, making it difficult to remove.

Chlorine is an important by-product of the electrolytic process. Scrubbed chlorine flows to a chlorine plant for further processing and liquefaction. The scrubbing equipment must be designed and constructed in a manner to minimize air in-leakage and chlorine emissions. Materials of construction must be selected to withstand severe corrosive characteristics of > 90% chlorine (Cl2) gas.

Particulates in the gas leads to maintenance associated with downstream filters in the chlorine plant. It is therefore necessary to maximize removal efficiency to minimize filter maintenance costs.

The customer selected an Envitech high efficiency Venturi Scrubber. The equipment includes a fixed throat fiber reinforced plasUS Mag Venturi Scrubber Fanstic (FRP) Venturi scrubber and horizontal chevron style entrainment separator. Additional scope of supply includes instruments, pre-assembled pump skid with redundant titanium pumps (1 operating and 1 spare), pre-assembled CPVC piping, redundant fans with titanium wheel and housing, interconnect FRP ductwork, dampers, and equipment skid and platforms.

Because a large fraction of particulate is submicron in size and difficult to remove, the fans and vessels are designed to operate at -70” water column pressure drop.

An Envitech Venturi scrubber was installed on the original process in 1999. The new scrubber for the expansion project was installed in 2011. Both systems continue to operate with high reliability and uptime. The table below summarizes key design parameters for the new Venturi scrubber.


  • Inlet flow rate: 8,200 acfm
  • Inlet temperature: 130oF
  • Gas Composition:  > 90% Cl2 gas
  • Pressure drop: 60" W.C.
  • PM Removal: > 75%

Click on the link below to download a case study and other Venturi scrubbing literature.

Download Literature

Topics: particulate control, Venturi scrubbers, Scrubbers, Wet Scrubbers

Wet Scrubber Basics

Posted by Andy Bartocci on Tue, Sep 10, 2019 @ 09:02 AM


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 summaryEnv wet scrubber technologies

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.

Envitech WESP Venturi Performance

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

Env 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.

Download Literature



Topics: particulate control, Venturi scrubbers, Scrubbers, SO2 Scrubber, wet electrostatic precipitators, Acid Gas, Packed Bed Absorbers, Packed Bed Scrubbers, Wet Scrubbers

International Conference on Thermal Treatment Technologies and Hazardous Waste Combustors (IT3/HWC)

Posted by Andy Bartocci on Mon, Aug 26, 2019 @ 10:55 AM

IT3 Logo

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.Candle filter 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

Mercury Control ScrubberWaste 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.

Download Brochure



Topics: particulate control, Scrubbers, SO2 Scrubber, wet electrostatic precipitators, Acid Gas, Incinerator Scrubber, SO3 Aerosol