Unit A-5 Introduction to Plant Operations and the Environment PDF

Unit A-5 • Introduction to Plant Operations and the Environment Objective 2 Describe the theory of operation of separators/collectors and monitoring of flue gas particulates. As was mentioned earlier, fly ash varies greatly in size. There are several methods of removing it from flue gas, including...

Unit A-5 • Introduction to Plant Operations and the Environment Objective 2 Describe the theory of operation of separators/collectors and monitoring of flue gas particulates. As was mentioned earlier, fly ash varies greatly in size. There are several methods of removing it from flue gas, including: a) Mechanical separation using inertia and centrifugal force to cause the fly ash to drop out of suspension into a hopper or collector, b) The use of fabric filters to trap the particles, and c) The use of electrostatic precipitators that use electrical charges to pull particles out of a gas stream. These methods are briefly discussed here. The Furnace/Boiler Draft section covers the full details of their construction and operation. Mechanical Collectors Mechanical collectors use different methods to remove particulates from the flue gas stream. a) Collectors may be designed to cause abrupt changes in direction. Since particulate particles are more massive than flue gas, the particles cannot change direction easily, so they fall from the gas path. b) Collectors may be designed to slow down the ash by putting barriers in the flue gas path. This causes the ash to impinge upon the barriers and drop out by gravity. c) Collectors may be designed to cause flue gas to take a circular path so that centrifugal force flings the small particles out of the flue gas. The collection efficiency is best with the larger, heavier particles. Particles removed from the flue gas stream fall by gravity into hoppers for disposal. Figure 2 shows the first two methods. Figure 2 – Particle Removal by Change in Direction (a) and Impingement (b) B Gas entry C Gas exit A C A B Dust Hopper Hopper Particles impinge against small baffles in the gas path and fall out. (a) 3-6 (b) 4th Class Edition 3 • Part A Liquid and Solid Emissions • Chapter 3 Collectors that use centrifugal force are often called “cyclone” separators (Fig. 3). This collector uses centrifugal force as well as an abrupt change in direction to cause the particles to fall out. Figure 3 – Cyclone Separator 5. Clean gas is discharged from outlet 1. Dirty gas enters and is whirled by vanes 2. Centrifugal action separates dust 4. Clean gas rises through centre to outlet tube 3. Dust drops into hopper (Courtesy of Western Precipitation Corp.) The efficiency of these types of collectors is dependent on the size of the particles in the flue gas path. Mechanical collectors are unable to remove particles smaller than about 10 microns. They cannot, on their own, meet the stringent regulations and emission limits of modern plants. They can however be used to “pre-clean” the flue gas of the larger particles before passing the flue gas through more efficient types of collectors. Fabric Filters Fabric filters trap dust on fine cloth filters or bags, usually tubular in shape. Figure 4 provides an overview of their typical operation. Several bags are usually enclosed in a large chamber, which is called a baghouse. As the flue gas flows through the bags, fine fly ash particles adhere to the fabric surface. The fabric filter obtains its maximum dust-removal efficiency during this period of ash buildup. After a fixed operating period, the bags are cleaned by one of the following methods. 1. Mechanical shaker 2. Rapper system 3. Using reverse pulses of compressed air. This common method shakes the bags and breaks off the dust cake that adhere to the bags. In all three methods, gravity causes the fly ash to fall into hoppers. The ash is then collected for disposal. Immediately after cleaning, the filtering efficiency is reduced until the buildup of collected ash takes place again. The fabric filter can be applied in any process area where dry collection is desired and where the temperature and humidity of the gas will not damage the cloth. For particulate matter, efficiencies above 99% can be achieved with fabric filters. 4th Class Edition 3 • Part A 3-7 Unit A-5 • Introduction to Plant Operations and the Environment Figure 4 – Baghouse Operation Clean Flue Gas Out Tubesheet Fabric Filter Bags Flue Gas and Particulate In Ash Out Electrostatic Precipitators Electrostatic precipitators create an electric charge on the particles to be collected and then gather the charged particles by electrostatic forces onto collecting electrodes. This supplied voltage is between 10 000 and 100 000 volts DC. Periodically, the collecting electrodes are cleaned by rapping or vibrating. The dust falls into hoppers. An overview of this process is shown in Figure 5. 3-8 4th Class Edition 3 • Part A Liquid and Solid Emissions • Chapter 3 Figure 5 – Electrostatic Precipitator Operation Electrical Field Charged Particles Collector Electrode at Positive Polarity Discharge Electrode at Negative Polarity Inlet Dirty Gas Flow Clean Gas Exit High Tension Supply from Rectifier Uncharged Particles Particles Attracted to Collector Electrode and Forming Dust Layer 10 000 to 100 000 volts D.C. Rapper Collecting Electrode (positive) Discharging Electrode (negative) Hopper Electrostatic collector puts charge on small particles; draws them from gas. Of all the processes in place, the electrostatic precipitator is the most efficient at removing particulate from flue gases. Depending on the flow velocity and the particulate size, it can remove 99.9% of the particulate. 4th Class Edition 3 • Part A 3-9