| This 30,000 SCFM regenerative thermal oxidizer has an HCL scrubber. |
By Rich Grzanka Is your plant in compliance with the new national standard for hazardous air pollutants? With a November deadline, it’s changing how plants view their process streams. Chemical plants may be required to invest substantial capital in air pollution control equipment thanks to a new EPA regulation. Batch and continuous chemical manufacturers are required to be compliant with the National Emission Standard for Hazardous Air Pollutants (NESHAP) for Miscellaneous Organic Chemical Manufacturing by November or apply for an extension. Extensions are not automatic. Section 112 of the Clean Air Act authorizes the EPA to create national standards for the control of hazardous air pollutant (HAP) emissions. These standards are directed toward source categories where national emission standards for hazardous air pollutants have been developed. The chemical industry is well represented on the list of source categories, and production processes for paints, coatings, adhesives and resins are included. This regulation is applied to all major source emitters of one or more of the HAPs listed in section 112a of the Clean Air Act. A major source is defined as a stationary source located within a contiguous area under common control that emits, or has the potential to emit, in aggregate 10 tons per year or more of one HAP or 25 tons per year or more of any combination of HAPs. By definition, the standard is designed to catch miscellaneous processes that have the potential to emit HAPs. The source categories are therefore broad and include manufacturers of organic chemicals in batch or continuous processes and manufacturers of quaternary ammonium compounds, ammonium sulfate produced with caprolactam or hydrazine as well as major sources of toluene, methanol, xylene, hydrogen chloride, methylene chloride, hydrochloric acid or creesols. Because emission limits are based on a rolling 365-day average, facilities that reach their yearly limit have to discontinue their process until the initial 365 days have passed. To avoid a shutdown, many plants will be installing air pollution control systems if there is even a chance of exceeding the limit. Historically, EPA regulations have concentrated on the actual production emissions from equipment. This rule is process-based, not equipment based, focusing on the miscellaneous steps that are necessary to production in a chemical manufacturing plant.Once it has been determined that a process stream requires control, characterizing the emission stream becomes a primary concern. By nature, many streams will have variable gas flow conditions and various HAP loadings. Classification of the emission stream as a high-volume-low-concentration (HVLC) stream or a low-volume-high-concentration (LVHC) stream will effect technology selection and operational design. To determine the emission stream, you need to calculate the lower explosive limit (LEL) of the process stream. If it is above the LEL, consider one of three approaches. • Check oxygen levels. If less than 3-5 percent oxygen, the stream is suitable for transport to the thermal oxidizer burner system as an inert stream. Once injected into the thermal oxidizer, additional air can be introduced into the system to allow safe oxidation. • Dilute the process stream with air to 50 percent of the LEL at the process collection point for transport to the thermal oxidizer with LEL monitors per National Fire Protection Association (NFPA) requirements. • Dilute with air to 25 percent of the LEL for transport without LEL monitors per NFPA. When selecting oxidation technology, consider that thermal oxidation is an effective technology for the destruction of organic HAPs and volatile organic compounds (VOCs). The three Ts of oxidation — time, turbulence, temperature — and the presence of oxygen in sufficient quantities will ensure proper combustion. These principals are applied to the molecules of regulated chemicals; the reaction destroys polluted air streams by converting them to carbon dioxide, water vapor and heat. HAP and VOC destruction efficiencies of 95-99 percent are typical but 99 percent and above are possible. The most straightforward method of oxidation is a direct-fired thermal oxidizer. It is typically used in LVHC streams. The process stream is directly injected into the combustor and destroyed. The heating value of the compounds usually makes up a significant portion of the heat required for combustion, therefore keeping the addition of supplemental fuel (natural gas or fuel oil) to a minimum. Another method is a recuperative thermal oxidizer. If the LEL of the process stream is somewhat lower, the need for supplemental fuel goes up. The addition of a metallic heat exchanger allows for heat recovery of 70 percent, which helps to preheat the process stream and keep fuel usage to a minimum. Catalytic oxidizers take an extra step toward reducing fuel consumption and therefore operating costs. A precious metal catalyst in the air stream reacts with the compounds and reduces the temperature required to drive the oxidation process. Catalytic oxidizers can be somewhat susceptible to deactivation or poisoning by particulates, metals and other compounds.Regenerative thermal oxidizers allow the highest thermal energy recovery (95 percent) and ensure that fuel consumption is reduced, even for air streams with contaminant loads below 5 percent. Large beds of ceramic heat exchange media allow for this high thermal energy efficiency. For the most dilute streams, a zeolite concentrator can be used to collect compounds on a rotary wheel, which can then be desorbed into a smaller air stream and oxidized more efficiently in an appropriate thermal oxidizer. Many of the process streams requiring oxidation will have halogenated compounds. Through oxidation, acid gas forms. In some cases, a secondary acid gas scrubber will be required to completely clean the air before emitting it to the atmosphere. In conclusion, the very nature of the new EPA regulation provides for a wide range of process streams that will require some type of process emission control. The flow and concentration variability of many different types of process emissions further complicates the selection of abatement technology. A full evaluation of all possible controls is required.
More on MON from EPA Details about the new National Emission Standard for Hazardous Air Pollutants for Miscellaneous Organic Chemical Manufacturing, also known as the Miscellaneous Organic NESHAP or MON regulation, are available at www.epa.gov/ttn/atw/mon/monpg.html. About the Author: Rich Grzanka is a regional vice president with Anguil Environmental Systems Inc., 8855 N. 55th St., Milwaukee, WI 53223, which specializes in air pollution control systems. He has a bachelor’s degree in mechanical engineering and a master’s degree in business administration and has served as a member of numerous industry associations. Questions about this article can be addressed to him at 973-543-8923. Additional information is available at www.anguil.com.