The objective of any Air Pollution Control (APC) system is to limit the quantity of pollutants entering the environment with the lowest possible capital expenditure and operating costs — while maintaining maximum reliability and uptime.
APC systems address a number of issues, including:
- Total Particulate Matter (TPM)
- Acid Gases
- Greenhouse Gases
- Volatile Organic Compounds (VOC) and other Organic Vapors
- Hazardous Air Pollutants (HAP)
- Non-Condensed Gases (NCG)
- Aerosols, Dioxins, and Oxides of Nitrogen (NOx)
Frequently, a number of these pollutants can be found in a total plant exhaust stream. That’s why NESTEC offers a total system evaluation, with both dry and wet options, to find the best and most economical solution.
NESTEC offers comprehensive Smart Solutions for clean air and energy conservation. Some of the details are summarized on our website in the NEWS section in the November 2020 issue, and available through the following link:
Other considerations should also be included, such as choosing a wet or dry system and the tradeoffs between cost of energy versus capital outlays to find the best and most economical solution. Process changes should also be investigated, such as switching to the production of low-VOC coatings, feedstock selection, plant-siting to avoid non-attainment areas or those with complex terrain with other environmental issues, and better capture systems to reduce the exhaust air to be treated with the APC equipment.
Particle loading in the flue gas from any industrial process is measured in grains/dry std. ft3 (gr/dSCF) or mg/dry std. m3 (mg/dSCM) or mg/normal m3. The particle loading will vary widely depending upon the gas velocity, particle composition, particle size, particle density, and the nature of the upstream process and feedstock, for example wood combustors with white or bark fuel. The smaller the particle, the more easily it is carried by the flue gas, even at relatively low gas velocity.
For example, wood industry actual wood combustor test date:
- white wood ash = 0.45%
- Bark wood ash = 1.8 %
The wet-versus-dry decision is far from clear cut. Some industries tend to use dry systems (for example, the ESPs used in coal-fired power plants), while the wood industry has utilized a combined wet TPM and VOC/HAP control with wet electrostatic precipitator (WESP) on the dryer exhaust prior to the RTOs for wet particulate control.
When unwanted wet particulates must be removed from the flue gas stream, options typically include cyclones, venturis, wet electrostatic precipitators (WESP), and wet scrubbers. Each has its own characteristic operating curve. Figure 1 (below) provides a comparison for several. Both a specific Wet Electrostatic Precipitator (WESP) and a dry multi-complex cyclone (MCCyclone) that is utilized in the wood industry are also included. Note that the collection efficiency for nearly all devices begins to fall off for smaller particles.
Preferences change over time as new and more stringent regulations are enacted. Disposal options for the removed particulate matter, wet sludge, stabilized mixture that is formed during treatment is another major factor in the decision making process. For example, cement kilns, where baghouses are used to capture Cement Kiln Dust (CKD), produces a salable product. Whereas a wet capture system would form unsaleable material and system-fouling cement solids.
In the wood industry, fly ash from the wood combustor was the primary factor in adding a WESP prior to the RTO to minimize heat exchange media degradation due to the wood combustor TPM. However, the addition of a wet particulate control device has added additional considerations in the overall air emission control system selection.
- Wet systems add a second emission control requirement for the water/particulate discharge.
- The wood industry has a zero-water discharge requirement
- Added costs for chemical treatment of the water
- Added costs in disposal
- Added potential freezing issues and associated capital costs and operating for heat tracing
- The Added water to the process during the quench saturation of the exhaust stream for a typical wood dryer adds another 5 to 6% additional energy load or fuel operating cost for the RTO. It also adds additional capital cost due to the increased size of the RTO.
- Discharging recirculation water to the wood fuel can lead to increased sodium carry over to the RTO media and increased degradation as well as corrosion issues.
- Wet systems increase NOx emissions.
NESTEC offers a total air emission control evaluation for both wet and dry systems in the wood industry with their associated benefits and advantages.
NESTEC Inc. has helped successful companies increase their air emission control operating efficiencies and reduce both capital and/or operating costs. We are confident that we can help your facility with any present or future air emission control issues that threaten your bottom line.
For assistance from our extensive experience, call or e-mail NESTEC Inc. (Office: 610-323-7670) for a free evaluation of your VOC/HAP and/or particulate control equipment and service needs.
wholden@NESTECinc.com 610-323-7670 ext. 103 (service)