Volatile organic compounds (VOCs) are unburned hydrocarbons of higher molecular weight than methane. Sources of VOCs include combustion products, automobile exhaust solvents, and paints, to mention a few. When released into the atmosphere, VOCs contribute to the formation of harmful ozone and are health hazards, particularly because of their high molecular weights. [Unburned hydro­carbons (UHCs) are similar to VOCs but are of lower molecular weight and characterized as methane.]

Good combustion techniques and the maintenance of high combustion temperatures minimize VOC formation; however, they also increase NOx. In chemical plants, incineration is generally adopted to minimize the emission of VOCs. There are two types of oxidizers. Thermal oxidizers combust the VOCs along with natural gas and maintain a gas temperature of 1500-1800°F with a few seconds of residence time, which destroys the VOCs. Catalytic oxidation requires a lower temperature, 500-700°F, and therefore consumes less natural gas. Heat recovery boilers may be used behind incinerators for recovering energy from the flue gases, as discussed in Chapter 2. VOCs in packaged boilers are reduced by using good combustion techniques. Oxidation catalysts also reduce VOCs but are expensive.