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).
Venturi scrubbers are used to remove particulate from the exhaust gas of industrial sources. They are highly efficient at removing particulate 1-micron in size and larger. Venturi scrubbers are used in solid waste incineration, waste-to-energy production, mining, biosolids sludge processing, plastics production and coal gasification. In many of these applications, the Venturi is used on the back end of a dryer or thermal destruction device.
In accordance with Bernoulli's equation, inlet gas accelerates at the converging section, increasing gas-liquid contact. As water is injected perpendicular to the gas flow, the accelerated gas particles are captured by water droplets upon collision. The resulting droplets aggregate through the diverging section and are separated from the process gas by the mist eliminator (ME) in the entrainment separator (ES).
The ability of the mist eliminator to remove water droplets from the gas stream can have a significant impact on the scrubber performance. Any water droplets that "escape" the ME will carry entrained particulate which can foul a stack test and increase the measured outlet emissions. The amount of pressure drop consumed by the ME can impact scrubber performance. Minimizing ME pressure drop allows a higher pressure drop across the Venturi which increases the particulate capture efficiency in the Venturi throat.
The mist eliminator efficiency is heavily impacted by velocity. Therefore, it is critical to achieve even flow distribution before reaching the ME. The ability to distribute the flow uniformly will allow the system to operate more effectively under a wider range of inlet gas flow rates.
Flow studies were performed to evaluate the capability of three different designs:
- Envitech design
- Design from an ES supplier (referred to as Config. 1)
- Design from a customer based on past experience (referred to as Config. 2).
All three systems have their respective "plate" design to help improve the flow distribution.
Differential pressure (∆P) across the mist eliminator is a direct indicator of how well distributed the flow is; the lower the ∆P, the more evenly the flow is distributed. The graph below compares the ∆P across the mist eliminator and the plate respectively for the three designs. The ideal ∆P across ME was obtained at uniform inlet gas flow through the ES. Envitech's design has the lowest ∆Ps, reflecting that the plate distributes the flow most effectively and as a result the ∆P across ME is the closest to ideal.
To provide a visual illustration, the cut plots below were obtained from flow studies showing the velocity(y) distribution prior to the plate, before and after mist eliminator. A zone with high velocity (red) was observed in all designs before entering the plate. The plate breaks up the hot spot and re-distributes the flow. The flow pattern before and after the ME for the Envitech design is the most homogenous among the three which is consistent with the pressure drop results.
The flow studies enabled the Envitech Venturi Scrubber to be optimized three ways as follows:
- Reduced the material cost wiwthout compromising scrubber ability to agglomerate and remove particles.
- Refined the flow distribution to expand the process window in the entrainment separator.
- Reduced the pressure drop across the ES which allows a higher pressure drop across the Venturi throat.
The purpose of a wet electrostatic precipitator (WESP) is to remove particulates that are smaller than one micron from a gas stream. One of the common industrial applications is downstream of standard pollution control equipment on power plants or pulp mill facilities. Power plants typically use wet electrostatic precipitators after all other pollution control equipment for polishing. Pellet mill facilities use wet electrostatic precipitators upstream of RTOs to prevent buildup on the RTO media. Particulate removal is achieved by sending process gas through an array of high voltage electrodes and grounded collectors. Particles in the gas are charged and then attracted to the collector surface; clean gas continues up the WESP.
The outlet section of a WESP houses the insulator compartments. Insulator compartments support a high voltage grid which holds the discharge electrodes in place. This arrangement separates the high voltage grid from the grounded section. The introduction of purge air through the insulator compartment reduces dust and moisture buildup on the walls of the compartments. This not only keeps the insulator bushing clean but also maintains a positive air pressure to prevent process gases from entering the compartment.
The diagram above shows how the WESP is connected to the insultator compartment with a shroud. Operating the system at minimum purge air flow helps reduce operating cost as less energy will be required to heat up the purge air. The caveat here is that inadequate flow will result in wet process gases swirling into the insulator compartment through the shroud from the WESP. In an attempt to optimize our operating conditions, flow simulation was performed to analyze the flow pattern of purge air through the shroud at varying inlet flow rates.
The close-up view of the shroud shown on the bottom was taken facing the insulator compartment with WESP at the back. The green/blue color indicates air flow towards the WESP (into the page), while the red/yellow represents flow towards the insulator (out of the page). This illustrates a positive air flow into the WESP achieved at minimal flow rate. Under this operating condition, energy savings will be maximized while ensuring no swirling of wet process gas into the insulator compartment.
Envitech will be attending the International Biomass Conference & Expo in Minneapolis, MN April 8-10, 2013. We will exhibit at Booth 807. During the conference I will be presenting in the Pellets & Densified Biomass track between 8:30 am and 10:00 on Wednesday, April 10th. The topic will be "High Performance Wet Electrostatic Precipitator (WESP) for Pellet Mills".
A large portion of wood pellets produced in North America are exported to Europe to support aggressive low-carbon fuel incentives. Growth of the export market has lead to
large scale pellet mills with production capacities ranging from 500,000 to 750,000 tons of pellets per year.
These mills use wood dryers to remove moisture from incoming feed material. A wet electrostatic precipitator (WESP) collects particulate (PM) and condensed volatile organic compounds (VOC) from the dryer exhaust. This is required to protect a downstream regenerative thermal oxidizer (RTO) from fouling. The RTO destroys volatile organic compounds (VOCs) before exhausting to the atmosphere. The process flow conditions are large and typically several hundred thousand cubic feet per minute.
This presentation will discuss a high performance WESP design to treat the pellet mill dryer exhaust. The design leverages experience from several large flow rate applications including a coal dryer scrubber, secondary lead smelter WESP, and sulfite pulp mill recovery boiler WESP. The presentation will focus on how the design achieves high performance.
Authors: Zach Schulz and Andy Olds
Recently, Envitech was tasked by a leading waste oil refiner in Southern California to supply and install a wet scrubber on a process gas afterburner. The goal of the project was to reduce the sulfur emissions from the facility to avoid the SOx (sulfur oxides) reclaim program in the Southern California Air Quality Management District. The SOx reclaim program is a cap and trade program that requires emitters to secure or purchase the right to emit sulfur into the atmosphere. Details of the project can be found in the article "High Efficiency SO2 Scrubber Case Study for a Waste Oil Re-Refiner" which outlines the incredibly stringent SO2 emissions standards that Envitech's wet scrubber successfully met.
Envitech; holding a general contractor's license in California; has the capability to not only supply the wet scrubber but to also supply and install the support equipment for the project. As the project developed, the waste oil supplier expanded Envitech's scope to include nearly all of the work required for the wet scrubber, including the supply, installation, and warranty for the ductwork from the existing afterburner to the wet scrubber system.
Envitech faced some unusual challenges with the ductwork for the existing afterburner. The afterburner itself is composed of refractory lined carbon steel, with an average exit temperature of 1600°F and with excursions to 2000°F. Exhaust gas from the afterburner can also contain 1000ppm sulfur dioxide (SO2) with the potential of some sulfur trioxides (SO3). Further, 90% of the time the gas passes through a heat exchanger that reduces the temperature to as low as 600F, with outages on the heat exchanger once a week. Thus, Envitech had to provide an exhaust duct that could handle temperatures from 600F to 2000F with an elevated concentration of sulfur compounds, cope with thermal cycling, and operate near the acid dew point.
Envitech consulted with Rolled Alloys to determine the best alloy for the design conditions. Separately, Envitech evaluated the cost and expected lifetime of refractory lined duct.
RA 253 MA was chosen for this application for its great resistance to high temperatures up to 2000°F. It also has a very lean nickel content (11%) which is beneficial for sulfur bearing environments at high temperatures. After working with Rolled Alloys and its subcontractors, Envitech found that the cost of a refractory lined duct, including installation, was slightly higher than that of the RA 253 MA material suggested by Rolled Alloys. Further, Envitech found that insulated RA 253 MA material would have a longer expected lifetime than the refractory lining especially due to the thermal stress created by the temperature cycling and the aggressive nature of the sulfur in the gas.
With this information, Envitech presented the RA 253 MA option to the waste oil refiner and jointly agreed that the RA 253 MA material was the best option for the ductwork. Envitech's subcontractors built the ductwork out of Rolled Alloys RA 253 MA material on-time and on-budget, with startup occurring in May, 2012. The ductwork is still in service with no reported issues.
With the help of Rolled Alloys, Envitech was able to provide ductwork that was easy to install, cost-effective, and within budget, contributing to success on the project that has enabled the waste oil refiner to avoid the SOx reclaim program by lowering its emissions.
The project was considered a success by all parties and Envitech is currently working with the waste oil refiner on a second wet scrubber project with similar design conditions and intends to use the RA 253 MA material.
The above article was jointly written by Zach Schultz of Rolled Alloys and Andy Olds of Envitech. The article was co-published on the Rolled Alloys Technology blog and Envitech's Air Pollution Control Innovations blog.
Envitech is an air pollution control equipment supplier serving industrial, medical, refinery and utility customers since 1993. Their website is www.envitechinc.com. You can contact Andy Olds directly at email@example.com.
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 TRIGTM. 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.
Emissions from marine vessels contribute to global smog forming pollutants. In response, the International Maritime Organization (IMO) has adopted new regulations in MARPOL Annex VI for progressive reduction of NOx, SOx, and Particulate. The new regulations require ships to achieve 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 using a marine diesel scrubber. The scrubbers must achieve greater than 97% SOx removal with 3.5% sulfur fuel. A cost effective scrubber is needed to help ship operators comply with these rules and avoid high cost ultra-low sulfur fuel.
The Envitech HYSEA DeSOx scrubber combines 30 years of experience achieving low emissions on stationary sources with patent pending technology. Scrubber features include:
- Innovative inlet design allowing a vertical bottom direct connect to minimize space.
- Proprietary quencher to cool the gas to saturation and remove particulate.
- Flow works modeling to ensure uniform gas distribution for maximum performance.
- Flexibility for open loop operation to minimize operating cost and closed loop operation for navigation in inland water ways and SECA zones.
- High performance, foul-resistant mass transfer media to minimize pressure drop and parasitic load.
- Skid mounted pump skid with integrated control panel.
Please click on the icon below to download a case study on the Envitech HYSEA DeSOx scrubber.
A secondary lead smelter in Southern California recovers lead from used automobile batteries. The process uses a combination of combustion processes including kilns and furnaces. Despite extensive control equipment including bag-houses, HEPA filters, and wet scrubbers, low concentrations of heavy metals were being emitted from various stack sources. Cancer risk index standards were being exceeded due to the close proximity of residents to the plant. To be compliant with state regulations a solution was needed to reduce these emissions. The special circumstances of nearby residents and a stringent cancer risk index required the facility to meet emission limits well below any other secondary lead smelter in the country.
A case study is now available to describe how an Envitech high performance Wet Electrostatic Precipitator was used to solve this problem.
Please click on the icon below to download the case study.
A waste oil re-refiner in the Western United States will increase production to nearly 20 MM gpy. The state requires a direct fired heater to reduce SO2 emissions by 95%. Although packed bed scrubbers are common for this type of process, a vertical scrubber design can be expensive to retrofit into an existing facility. The facility was looking for a solution to meet or exceed emission standards and be relatively simple to install and maintain.
A case study is now available to describe how an Envitech high efficiency SO2 scrubber was used to solve this problem.
Please click on the icon below to download the case study.
A 582-megawatt (MW) electric generating plant in Kemper County Mississippi is being built. The process uses locally mined lignite coal for fuel which contains 40% moisture. The coal must be dried before it is converted to syngas in the gasifier.
Six (6) fluidized bed dryers dry the coal with a combined exhaust gas flow rate of 2.1 MM cfm. A cost effective means was needed to remove particulate and moisture from the exhaust gas before it is recycled back to the dryers.
A case study is now available to describe how an Envitech Venturi scrubber-condenser solves this problem.
Please click on the icon below to download the case study.
Follow the link to a previous blog post to download a presentation on this project from the 2012 Coal-Gen Conference.