Co-operation in anti-pollution

STM Technologies' clients are very concerned about environmental issues, wanting the ability to meet the most stringent anti-pollution laws anywhere in the world. In order to satisfy these requests, STM Technologies (STM) made contact with many wet electrofilter producers. After thorough analysis, STM expressed its full confidence in TurboSonic's proposal and together developed what the companies believe to be the most efficient and reliable wet electrofilter to be used in glass-wool manufacturing.

The first application was in a Chilean glass-wool plant, completely designed and supplied by STM Technologies. After the upgrade to increase the capacity of this plant, the air pollution control equipment was also improved by installing a specially designed TurboSonic SonicKleen™ Wet Electrostatic Precipitator (WESP). The goal was to decrease atmospheric emissions as much as possible, while recycling all of the process water used to flush electrodes so that it could be used for binder preparation. These goals were realised and exceeded. The emissions were lower than expected, as presented in this article.

Emissions control for mineral wool production

Air emissions from mineral wool production at the forming and curing areas consist mostly of particulate matter (PM) and volatile organic compounds (VOCs). The PM is typically made up of solid particulates swept up in the ventilation hoods and condensed VOCs are released from the binders used during the curing process.

Particulate matter (PM)

The US Environmental Protection Agency (EPA) defines PM as any material that condenses or sublimates at or above the temperature of the sampling filter. This temperature is usually 120°C but can vary depending on measurement method used. The PM from this process typically consists of two components as follows:

1. Mechanically generated fibre or dust, either generated from the material handling process prior to fibre entering the former, or generated within the former itself. This PM is considered to be relatively coarse, typically over 2µm in diameter, but may also contain a fraction of particulate smaller than 2µm.

2. Condensed VOCs. These can include the condensed VOCs mentioned above. They are typically sub-µm liquid droplets that are formed as a fine aerosol as the gas cools. These liquid droplets are generally between 0.2-0.7µm in diameter and are responsible for creating a blue-haze effect due to light refraction from stack gases.

PM emissions control

The wet electrostatic precipitator (WESP) (also known as an electrofilter) is an ideal device for collection of fine filterable particulate matter (FPM) and condensable particulate matter (CPM). The WESP is typically operated in conjunction with a wet scrubber. The two devices perform different and complementary functions, as follows:

Wet scrubbers

Wet scrubbers cool the gas stream to adiabatic saturation temperature and condense VOCs with boiling points above the saturation temperature. They remove large PM (2-5µm and larger) and can take emissions down to 50mg/Nm³. They also absorb acidic gases and water-soluable VOCs.

The wet scrubber removal mechanism for PM is mainly particle impaction with water droplets that are generated in the scrubber. Simply put, in order to maximise particle impaction, the wet scrubber is required to generate droplets sized relative to the particles they are impacting with, in a turbulent gas flow. Therefore, to collect finer PM, the scrubber needs to generate finer water droplets, which require more energy to produce. In general, removal of PM less than 5µm in size in a wet scrubber requires an inordinate amount of energy and is typically not used for collection of submicron PM.

WESP (Electrofilter)

WESPs provide additional removal of PM after the wet scrubber and are sometimes referred to as 'polishing devices,' taking emissions below 10mg/Nm³. They remove all sizes of PM including sub-µm solid PM and remove condensable organic particulates at sub-µm levels. They also eliminate mist.

WESPs utilise electrostatic attraction as the main particulate removal mechanism. This method uses a high voltage discharge electrode to generate corona in the gas passages, which then imparts a charge in the passing PM. The charged PM is then attracted electrostatically to grounded collection electrodes. The collected particulate is then removed from collection plates.

The way the particulate is charged is in actuality dependent on the particle size, but in general, the WESP is able to collect all sizes of particulate including sub-micron particulate at relatively low energy usage.

The SonicKleen™ WESP

The SonicKleen WESP uses a hexagonal downflow tubular style arrangement. While there are many different configurations of WESPs, (such as tubular versus plate style, round tube versus hexagonal tube and upflow versus downflow), TurboSonic prefers to use a downflow tubular style WESP.

A tubular style WESP ensures that all gas is treated and there is no slippage of untreated gas. This is important for applications where low outlet emissions are required. A downflow configuration is used because condensable organic material is sticky by nature and tends to stick to dry surfaces. The falling film of liquid is formed by collection of water droplets to keep the tube wetted, preventing build-up of dust and organic material.

Hexagonal tubes were chosen because they provide the highest collection area per kilogram of material. This configuration allows the smallest footprint.

Sizing factors

The required WESP size is calculated using the traditional Deutsch-Anderson equation. The variable that determines WESP size is migration velocity (ωm), which is determined using the following parameters:

• Gas composition,
• Particle type,
• Particle-size distribution.

Design factors that affect performance include voltage spacing, electrode design, field intensity, residence time, flow distribution, electrode alignment system, insulator compartment design and mist elimination during flush periods.

The SonicKleen WESP is focused on optimising all of the above factors to ensure that maximum

performance is delivered, while at the same time attention is given to minimising maintenance. While cost considerations are important, they take a secondary role so that the best components can be used to minimise failure modes common to precipitators. Some examples of this design philosophy are:

• Rugged ceramic insulators rated for >100kV,
• Insulator compartments designed so that insulators are out of the gas stream,
• Conservative gas expansion transitions, with double gas flow distribution screens to ensure equal treatment of all of the gas flow,
• 'Lifetime' rigid mast electrodes with three-point alignment hardware,
• All stainless steel construction for long life,
• Patented hood mist eliminator for elimination of water droplet discharge during flushing,
• Well-designed flushing system to ensure all surfaces are flushed and kept clean.

Basic Deutsch-Anderson equation

The Deutsch-Anderson equation is a dimensional equation describing the performance of a WESP:

η = 100*(1-e(-EMV*SCA/508))

where...

η = efficiency (%)

SCA = Specific Collecting Area (ft2/1000ACFM)

EMV = Migration velocity (cm/s)

EMV is a performance parameter of a WESP with collecting area A operating with gas flow Q and efficiency η.

Case study - El Volcan, Chile

Up until 2009 the exhaust from the El Volcan glass wool insulation production line, which is partly owned by insulation giant Saint-Gobain, vented to the atmosphere via a stack. Following treatment using an existing wet scrubber, emissions were in the region of 67-70mg/Nm³, with emissions from the stack visible to the naked eye. Chilean law demands maximum emissions of 56mg/Nm³ (interruptible) and 28mg/Nm³ (non-interruptible). If emissions could be reduced to under 10mg/Nm³ the plant would be able to make savings on permitting costs.

In addition to this, the original scrubber required eight or more hours of cleaning every month, causing a large amount of downtime for the plant.

A SonicKleen WESP was quoted to two specifications, first to meet the 28mg/Nm³ requirement and an option to increase the collection area and residence time to meet the 10mg/Nm³ lower permit limit. The plant selected the larger unit for purchase based on cost benefit analysis, demonstrating that there is only a small incremental cost for significantly higher performance.

The WESP was installed and started up in June 2009. Most of the equipment was fabricated in Chile and a local contractor carried out the installation. Actual performance at startup was measured at <2mg/Nm³, with typical emissions for ongoing operation at 7-8mg/Nm³. There were no visible emissions. The estimated power usage is 85kW. These and other summary operating parameters are provided in the table below.

The SonicKleen™ WESP has operated successfully for almost two years with little or no attention or maintenance required other than annual inspection and cleaning.

Table 1: Expected and actual operating parameters.

Conclusion

A properly designed and installed WESP can meet the most stringent PM emissions regulations and be a trouble free, low maintenance device for glass-wool manufacturing plants. The SonicKleen WESP from TurboSonic meets these requirements.