SPECIAL REPORT/GLASS MANUFACTURING: Capturing Particulate
Inside a CCS, billions of charged droplets rapidly interact with the particle- bearing process stream. When a particle and droplet pass within 20 microns of each other, electrical forces cause mutual attraction and the particle is pulled into the droplet. Each individual water droplet becomes a particle collector (see Figure 1).
Principle of OperationThe CCS process is generally accomplished in three stages. In a typical system, a particulate-laden gas stream (up to 2500°F) enters a pre-conditioning chamber (PCC) where it is cooled, and coarse particles are removed by spray impaction. Fine particles (under 2.5 microns, as defined by the U.S. Environmental Protection Agency) pass through the chamber, causing ultra-fine particles (under 0.1 microns) to "grow" to a few tenths of a micron through agglomeration and coagulation under special conditions created in the PCC.
If commingled pollutant gases are present, the primary stage of gas phase scrubbing takes place in the PCC as well. The CCS simultaneously handles acid gases, including HCl, HF, H2SO4, HNO3, SO2, Cl2 and H2S, as well as alkaline vapors such as ammonia and amine compounds.
The pre-conditioned flow next enters the first of two cloud generation vessels (CGVs). The first generation vessel contains positively charged droplets. Any residual coarse particles are easily eliminated here. Fine particles are captured by the charged droplets with very high effectiveness on submicron, condensable and conditioned ultra-fine particles. Secondary gas-phase scrubbing occurs simultaneously.
The partially treated flow then continues into the second CGV, which contains negatively charged droplets. As with the oppositive polarity, water droplets act as the collectors, and only the droplets-not the pollutant particles-are charged. Each droplet is capable of collecting thousands of particles as it passes through the vessel. This stage substantially removes the remaining submicron, ultra-fine and condensable particulate. Cleaned air passes through a mist eliminator and goes to the stack.
Captured particles are removed from the bottom of the system as a very thin, low-volume slurry. Relatively clean water from the top of the sump is recirculated to the charging grid, where it is recharged, minimizing water use and completing the cycle. The amount of water required for the blowdown discharge of captured pollutants is also minimal. If pollutant gas is removed, the pH in the sump is controlled and the wastewater is treated in a separate sump.
In typical glass applications, CCS can handle high concentrations of particulate loadings up to 1.3 grains per dry standard cubic foot at efficiencies of 99%. Gas temperature, particle solubility, resistivity and reactivity do not affect collection performance, nor do load flux or loading constituents, thanks to the physics of the charged clouds.
CCS smoothly accommodates changes in flow volume and can be turned down over a wide range, typically 10 to 1 or better. CCS systems do not have fabric filter bags to maintain and change, and they offer a very fire-safe approach.
Other TechnologiesCCS technology is sometimes confused with electrostatic precipitation (ESP), a common method of particulate collection, because both use an electric charge. Aside from this superficial similarity, the technologies are strikingly different; only CCS uses charged droplets. Rather than attempting to charge the particles and move them to a distant collector, the CCS charges only the collectors (that is, the water droplets) and moves the collectors to the particles.
ESP devices charge particles in the gas stream, causing them to migrate through an electrostatic gradient to metal plates for collection. In general, dry ESPs are unsuitable for submicron particle applications because of particle size, resistivity, back corona, re-entrainment and other issues. However, wet ESPs that use water to condition the particles and rinse them from the collectors can be effective on some kinds of fine particulate in the submicron range. Acceptable results have not been attainable with ESPs on many pollutants common to glass manufacturing, however.
Another sharp contrast between ESP and CCS technology is power consumption. Electrostatic precipitation creates a power draw that increases proportionally with the number of particles. CCS requires just 6-10 watts per 1000 cfm, regardless of particle loading. According to vendor literature, wet electrostatic precipitators use 1000-3000 watts per 1000 cfm.
The main power draw for CCS operation comes from pumps that recirculate water to the charging heads. CCS chambers are open, with no packing or baffles. Pressure drop for the entire system is low (between 1.0 and 1.5 in. w.g.), which translates to low fan energy requirements. While CCS recirculates (but does not consume) more water than a wet electrostatic precipitator, CCS uses substantially less total power to operate.
It's also worth considering how CCS contrasts with wet scrubbers, another particulate collection technology. Wet scrubbers treat water-soluble gases and achieve removal efficiencies of 90-99%. The scrubbers expose the pollutant gas to water and, through chemistry and Brownian motion, cause the gas to be absorbed. Generally speaking, packed bed-wet scrubbers incorporate packing small spherical plastic forms inside a scrubbing column. The packing increases the surface area of the water to facilitate absorption of the pollutant gas.
In most cases, water recirculating through the wet scrubber is mixed with pH-adjusting chemicals. These bind to the absorbed gas and prevent saturation of the gas into the water, thus preventing the gas from stripping back out into the air.
Wet scrubbers remove coarse particles by simple impaction with the water. This is ineffective for fine particles. Because of their lightweight, fine particles are pushed out of the path of the water droplets and are forced to follow the streamlines around the immobile packing material.
In addition to the charged droplets, which are the heart of its technology, CCS makes use of basic wet scrubbing. In the preconditioning chamber, coarse particles are captured by impaction, and much of the soluble gas is removed from the airstream by standard wet scrubbing absorption. Small particles lack both the momentum and Brownian motion, however. In contrast, these same small particles are captured easily by the charged droplets of a CCS.
The high-temperature applications that are commonplace within glass manufacturing are well-suited to the CCS. The gas saturation temperature is achieved using a metal cooling sleeve. Air-to-air heat exchangers that provide venting to atmosphere are also an option, as is heat recovery for use in the production of steam or electricity.
Some high-temperature, high-volume exhaust streams can even be turned into cash-positive opportunities. By lowering exhaust flow temperatures through the extraction of usable heat, the CCS and associated equipment can be efficiently downsized.
Pilot StudyGlass furnace exhaust from container/flat glass manufacturing has proven to be an ideal application for CCS technology. Following other successful applications in the glass industry, a pilot study evaluating the performance of a CCS was completed in the summer of 2005.
The study's purpose was to evaluate the ability of the CCS to treat particulate and sulfur dioxide (SO2) emissions from glass furnace exhaust. Data from the pilot study was the basis on which Tri-Mer designed commercial systems for installation on existing and future furnaces as required by state and federal regulations. In addition, the pilot study was used to optimize the hardware configuration, demonstrate reliability and validate the process control design of the CCS.
Third-party testing was contracted by a glass company to source-test the inlet and outlet of the CCS using EPA Method 5/202 for particulates. Three one-hour tests were conducted for each of four differently configured glass melting furnaces. The results of these tests showed that the CCS could remove filterable particulates in excess of 95%, and total particulates (including condensables) in excess of 90%.
Other tests conducted independently by Tri-Mer, and accepted by experts with the hosting glass company, showed that the CCS could remove total particulates in excess of 99%. A summary of the tests conducted with more advanced methods and equipment is shown in Table 1. Performance results on container glass indicate over 99% capture of total particulate, both filterable and condensable. These tests were conducted by Tri-Mer using a modified Method 5 to account for "drift water," an artifact of the early generation pilot equipment used on this test.
These levels of performance on both particulate matter and SO2 comfortably exceed those required to comply with container glass emission regulations for the installation location. They also exceeded any expected requirements for locations worldwide. The evaluations showed that the CCS system could operate continuously with no reductions in performance or impacts to the system that might cause concerns relative to long-term commercial operations. This conclusion was strongly supported by long-term 24/7 operations of CCS systems in sectors of the industry other than glass melting furnaces.
While the pilot and first systems are for oxy-fired furnaces, the CCS can also be used on air-fuel furnaces. When combined at the Tri-Mer factory with selective catalytic reduction (SCR), the integrated system offers several benefits, including NOx removal. In addition, particulate and sulfur compounds that can poison the SCR catalyst are cleaned from the air stream before reaching the SCR, optimizing removal efficiencies and catalyst life. Analysis by two major glass companies has shown that the integrated CCS-SCR system may be preferable over a conversion to oxy-fired furnaces.
The CCS is also a low-maintenance device, with well-documented high uptime. Annual or semi-annual preventative maintenance is all that the open vessel design requires, since standard high-quality pumps are the only moving parts. When the cost of ownership is calculated, performance and ease of operation stand out as qualities that make CCS deserving of serious consideration.
For more information regarding particulate control, contact Tri-Mer Corp., 1400 Monroe St., Owosso, MI 48867; (989) 723-7838; e-mail Kevin.firstname.lastname@example.org; or visit www.tri-mer.com.