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Cloud chamber technology uses electrically charged water droplets to attract fine particles and remove them from exhaust streams. At the Saint-Gobain Advanced Ceramics Boron Nitride (formerly Carborundum) plant in Amherst, N.Y., the technology has exceeded required performance criteria and helped a plant with an air pollution control problem remain a part of the local industrial community.
Boron nitride (BN) is produced by heating a mixture of boric acid and tricalcium phosphate in an electric furnace with an atmosphere of ammonia. The compound is a fine white powder with a particle size of about 1 micron. Saint-Gobain is a leading supplier of BN products in various forms to the marketplace. The company hot-presses BN into machineable solid shapes, which may be used as fixtures in hot environments or as crucibles to contain molten metal. The semiconductor industry, one of Saint-Gobain's most important markets, uses BN solid diffusion planar sources.
Any plant that handles and processes chemicals to produce their product is by law-if not for other reasons-concerned about environmental compliance. As production at its Amherst plant grew, Saint-Gobain piloted a five-stage wet electrostatic precipitator (ESP) to control its particulate emissions. A primary performance requirement was that the effluent's opacity rating, after particle removal, had to be less than 20%. The wet ESP system failed in this application, and Saint-Gobain sought an alternative method of particulate control.
In addition to the issue of particulate removal, the area in which the plant was located had evolved over the years into a high-tech industrial park, which created pressure to eliminate the plume at the stack. In response, Saint-Gobain chose to try the Cloud Chamber System (CCS) offered by Tri-Mer Corp. of Owosso, Mich., to control its particulate emissions.
The Problem of ParticulateThe U.S. Environmental Protection Agency (EPA) has announced new emission restrictions (PM2.5) to regulate particulate matter that is 2.5 microns or smaller in size. The EPA defines particles in this range as "fine," whereas particles larger than 2.5 microns are "coarse." Fine particles remain airborne long after being released, and they can spread over large areas, posing a substantial health threat if they remain lodged in the human respiratory tract after inhalation. This is especially true of sub-micron particles, which are often a major component of industrial process exhausts. These particles have a strong tendency to scatter light and produce highly visible emission plumes or cause visible haze.
Both state and federal authorities regulate hazardous air pollutants that often have a large sub-micron fraction. Moreover, regulating agencies have given notice that industry should expect even tighter fine particle requirements in the future. With ever-decreasing mass emission limits, industry is looking for new methods to control fine particulate pollution, especially in the sub-micron range.
PM2.5 particles are difficult to capture, and the challenges increase dramatically at sub-micron sizes. To complicate matters, many PM2.5 exhaust streams also contain gases that are regulated. Standard treatment methods are not designed for fine particle-gas mixtures, especially when they contain condensable constituents that form particles when entering the atmosphere.
Conventional scrubbers only capture coarse particles, although they are effective for soluble gases that contribute to acid rain. Conversely, specially constructed fabric filters (baghouses) can capture fine particles but are ineffective on gases. Dry electrostatic precipitators are effective only on coarse particles. Both wet and dry electrostatic precipitators are ineffective for sub-micron particles, and they are not gas removal devices. In contrast, CCS technology simultaneously treats sub-micron, fine, coarse and condensable particulates-plus all soluble gases-with a high degree of effectiveness.
How the Technology WorksBased on advances in electrofluidics, the CCS treats sub-micron particulate pollution in the same way that a good lightning storm clears haze from the atmosphere: It uses charged water droplets to remove fine particles from process exhaust streams (see Figure 1).
CCS technology works by passing a dirty gas stream through a chamber that contains a carefully generated "scrubbing cloud" of high-density, charged water droplets. The gas passes first through a preconditioning chamber (PCC), which uses a water spray to cool the incoming gas stream, provide an initial stage of gas scrubbing to remove coarse particles (larger than 10 microns) and, through super-saturation, cause ultra-fine particles (less than 0.1 micron) to "grow" into larger sizes for easier removal.
The gas stream then passes into the cloud generation vessel (CGV), where the scrubbing cloud is formed. Inside this vessel, billions of charged droplets rapidly interact with the particle-bearing process stream. When a particle and a droplet pass within 20 microns of each other, electromagnetic forces cause mutual attraction and the particle, being less massive by orders of magnitude, is pulled into the droplet. Each individual water droplet becomes a particle collector. The amount of contact between the exhaust stream and the CGV cloud determines the efficiency of particle removal. Depending on the application, there can be a single CGV or a second CGV (usually with oppositely charged scrubbing droplets) for higher performance levels.
After the droplets collect particles from their interaction with the exhaust stream, they coagulate together and "rain" into a sump at the bottom of the system. Captured particles agglomerate within the sump, settle out and are removed as low-volume slurry from the bottom. Since the charged droplets act as particle collectors, there is no need for fibrous filters, collector plates, venturi throats, layered pads, bags or cartridges. If there is also gas removal, then the pH in the sump is controlled.
Relatively clean collected water from the top of the sump is re-circulated by pump to the charging head, which generates billions of charged water droplets that form a cloud to complete the cycle and begin it again. The charging head is located in the front section of the CGV and draws a maximum of 10 Watts per 1000 cfm.
Finally, the mist eliminator removes excess moisture from the gas stream before it exits the system. A simple chevron mist eliminator, similar to those found on conventional wet scrubbers, provides 99.5% droplet removal efficiency in droplets 15 microns in size.
Several factors are involved with optimizing the effectiveness of a particular CCS application, including droplet size, droplet charge, particle size, particle charge, particle retention time and electric field effect. For each application, a computer simulation can be run to analyze these factors, along with expected inlet loading, gas type and concentration. Gases to be treated, if any, are taken into consideration. The simulation results are used to determine the ideal system configuration in terms of re-circulation flow, gas-to-cloud contact time and vessel size. Any required fine-tuning of parameters occurs during operational startup.
Effective ControlThe function of the CSS at the Saint-Gobain BN facility in Amherst is to collect and process boric acid emissions from the plant's induction furnaces, operating at 2000°C, which produce the boron nitride. Before installation, Tri-Mer guaranteed that the CCS would achieve less than 5% opacity (which it actually exceeds in performance) and would beat output regulations of 0.05 grains per dry standard cubic foot. In fact, the CCS comes in at 0.0055 grains per dry standard cubic foot.
The technology also helped to environmentally transform the plant into a clean member of the local industrial community. According to Saint-Gobain, after six years of operation, the CSS performs reliably day in and day out. The system has never experienced unscheduled downtime, and its normal maintenance costs have been minimal.
For more information regarding the control of particulate emissions, contact Tri-Mer Corp. at 1400 Monroe St., Owosso, MI 48867; (989) 723-7838; fax (989) 723-7844; e-mail firstname.lastname@example.org; or visit www.tri-mer.com.