These were one of the easiest pollutants to control; all that is required for good control is sufficient excess air and adequate mixing between the burning coal, its decomposition products, and the air. To get complete combustion with imperfect m nixing, one must supply excess air in addition to that ded for stoichiometric combustion. The amount of excess air to be used is determined by economics. At zero excess air, some valuable fuel escapes unburned to pollute the atmosphere Large amounts of excess air lower the combustion temperature by diluting the combustion (fracticts, and carry away more heat in the exhaust gas. This lowers the furnace's efficiency furnaces operate with 5 to 30 percent excess air. Autos have variable excess air naing on load. The optimum amount of excess air for voc destruction is generally optimum for fuel efficiency; air pollution control officers try to induce furnace operators to use the timum amount for voc destruction mixing problem is especially difficult in flares. These are safety devices used in oil refineries and many other processing plants. All vessels containing fluids under pressure have high-pressure elief valves that open if the internal pressure of the vessel exceeds its safe operating value. Al ousehold water heaters have such a valve to prevent tank rapture in some unlikely but not impossible circumstances. With a hot water heater, if the valve opens, hot water drops onto the floor. In the case of a large petroleum-processing vessel(distillation column, cracker, isomerize tc. )the material released is an inflammable VOC, which cannot safely be dropped on the floo The outlets of a refinery's relief valves are piped to a flare(or"flare stack"), which is an elevated mix air into the gas being released. These flares handle significant amounts of vocs only during process upsets and emergencies at the facilities they serve. When there is a small release, the steam jets can often mix the gas and air well enough that there is practically complete combustion For a large release the mixing is inadequate, and the large, bright orange, smoky flame from the flare indicates a significant release of unburned or partly burned voc In the coal combustion process one difficulty, even in well-designed modem furnaces, is that some articles of coal and some hydrocarbons pass out of the flame zone before they can be combusted These are called soot. In modem steam boilers this soot will collect in parts of the furnace where it is too cold for soot to burn, typically on the tubes in which the water is boiled or the steam rheated. If soot is allowed to collect there it will impede heat transfer and make the boiler less efficient. The cure for this problem is a soot blower, which is typically a fixed or moving steam jet that blows high-pressure steam onto the surface of the tubes to remove this soot normally, soot blowing is required only a few minutes per day. Soot dislodged in this way exits the furnace as short-period emissions of black smoke. Most public relations officers ask plant engineers to do all soot blowing at nigl Biological Oxidation(Biofiltration) as discussed above. the ultimate fate of vocs is to be oxidized to co, and H,o. either in our engines or furnaces, or incinerators, or in the environment. Many microorganisms will carry out these reactions fairly quickly at room temperature. They form the basis of most sewage treatment plants(oxidizing more complex organic materials than the simple voCs of air pollution interest Microorganisms can also oxidize the vOCs contained in gas or air streams. The typical biofilter (not truly a filter but commonly called one; better called a highly porous biochemical reactor) consists of the equivalent of a swimming pool, with a set of gas distributor pipes at the bottom covered with several feet of soil or compost or loam in which the microorganisms live. The tonta mn aed vas nters through the watrer ton aipes an he ssi sland y hep to eg b oizeal bw ithe microorganisms that live there these devices have soil depths of 3 to 4 ft void volumes of 50%, upward gas velocities of0.005 to 0.5 ft/s, and gas residence times of 15 to 60 s. They work much better with polar VOCs, which are fairly soluble in water than with HCs whose solubility is much less. The microorganisms must be kept moist, protected from conditions that could injure them, and in some cases given nutrients. Because of the long time the gases must spend in them, these devices are much larger and take up much more ground surface than any of the other devices discussed in this chapter. In spite of these drawbacks, there are some applications for which they are economica 10.5 Summary 1. VOCs are emitted from e variety of sources and have a wide variety of individual components, each with its own properties. We use VOCs mostly as petroleum-based fuels and solvents. The emissions come from fuel and solvent usage, transportation, and storage 2. The control alternatives are prevention, concentration and recovery, or oxidation 3. Some of these control options can also be used for non-VOC emissions, e.g., incineration for odor control of HS, adsorption for SOz or mercury vapor and leakage control for any process source 10-610－6 These were one of the easiest pollutants to control; all that is required for good control is sufficient excess air and adequate mixing between the burning coal, its decomposition products, and the air. To get complete combustion with imperfect mixing, one must supply excess air in addition to that needed for stoichiometric combustion. The amount of excess air to be used is determined by economics. At zero excess air, some valuable fuel escapes unburned to pollute the atmosphere. Large amounts of excess air lower the combustion temperature by diluting the combustion products, and carry away more heat in the exhaust gas. This lowers the furnace's efficiency (fraction of heating value of the fuel transferred to whatever is being heated). Large industrial furnaces operate with 5 to 30 percent excess air. Autos have variable excess air, depending on engine load. The optimum amount of excess air for VOC destruction is generally higher than the optimum for fuel efficiency; air pollution control officers try to induce furnace operators to use the optimum amount for VOC destruction. The mixing problem is especially difficult in flares. These are safety devices used in oil refineries and many other processing plants. All vessels containing fluids under pressure have high-pressure relief valves that open if the internal pressure of the vessel exceeds its safe operating value. All household water heaters have such a valve to prevent tank rapture in some unlikely but not impossible circumstances. With a hot water heater, if the valve opens, hot water drops onto the floor. In the case of a large petroleum-processing vessel (distillation column, cracker, isomerizer, etc.) the material released is an inflammable VOC, which cannot safely be dropped on the floor. The outlets of a refinery's relief valves are piped to a flare (or "flare stack"), which is an elevated pipe with pilot lights to ignite any released VOCs. Many have steam jets running constantly to mix air into the gas being released. These flares handle significant amounts of VOCs only during process upsets and emergencies at the facilities they serve. When there is a small release, the steam jets can often mix the gas and air well enough that there is practically complete combustion. For a large release the mixing is inadequate, and the large, bright orange, smoky flame from the flare indicates a significant release of unburned or partly burned VOC. In the coal combustion process one difficulty, even in well-designed modem furnaces, is that some particles of coal and some hydrocarbons pass out of the flame zone before they can be combusted. These are called soot. In modem steam boilers this soot will collect in parts of the furnace where it is too cold for soot to burn, typically on the tubes in which the water is boiled or the steam superheated. If soot is allowed to collect there, it will impede heat transfer and make the boiler less efficient. The cure for this problem is a soot blower, which is typically a fixed or moving steam jet that blows high-pressure steam onto the surface of the tubes to remove this soot. Normally, soot blowing is required only a few minutes per day. Soot dislodged in this way exits the furnace as short-period emissions of black smoke. Most public relations officers ask plant engineers to do all soot blowing at night. Biological Oxidation (Biofiltration) As discussed above, the ultimate fate of VOCs is to be oxidized to CO2 and H20, either in our engines or furnaces, or incinerators, or in the environment. Many microorganisms will carry out these reactions fairly quickly at room temperature. They form the basis of most sewage treatment plants (oxidizing more complex organic materials than the simple VOCs of air pollution interest). Microorganisms can also oxidize the VOCs contained in gas or air streams. The typical biofilter (not truly a filter but commonly called one; better called a highly porous biochemical reactor) consists of the equivalent of a swimming pool, with a set of gas distributor pipes at the bottom, covered with several feet of soil or compost or loam in which the microorganisms live. The contaminated gas enters through the distributor pipes and flows slowly up through soil, allowing time for the VOC to dissolve in the water contained in the soil, and then to be oxidized by the microorganisms that live there. Typically these devices have soil depths of 3 to 4 ft, void volumes of 50%, upward gas velocities of 0.005 to 0.5 ft/s, and gas residence times of 15 to 60 s. They work much better with polar VOCs, which are fairly soluble in water than with HCs whose solubility is much less. The microorganisms must be kept moist, protected from conditions that could injure them, and in some cases given nutrients. Because of the long time the gases must spend in them, these devices are much larger and take up much more ground surface than any of the other devices discussed in this chapter. In spite of these drawbacks, there are some applications for which they are economical, and for which they are used industrially . 10.5 Summary 1. VOCs are emitted from a wide variety of sources and have a wide variety of individual components, each with its own properties. We use VOCs mostly as petroleum-based fuels and solvents. The majority of our VOC emissions come from fuel and solvent usage, transportation, and storage. 2. The control alternatives are prevention, concentration and recovery, or oxidation. 3. Some of these control options can also be used for non-VOC emissions, e.g., incineration for odor control of H2S, adsorption for SO2 or mercury vapor, and leakage control for any process source