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Process Heaters MACT Standards Development
e-mail: eddinger.jim@epa.gov
Background
Process heaters are being considered for regulation of hazardous air pollutant (HAP) emissions under section 112 of the Clean Air Act, as amended (CAA). The definition of "process heater" currently in use is as follows:
"Process heater" means an enclosed device using controlled flame where the device's primary purpose is to transfer heat:
- To a process fluid, or
- To a process material that is not a fluid, or
- To a heat transfer material for use in a process unit (not including generation of steam).
The universe of process heaters has been divided into two categories, indirect-fired and direct-fired, with the respective definitions being as follows:
"Indirect-fired process heater" means any process heater in which the combustion gases do not mix with, or exhaust to the atmosphere from the same stack(s), vent(s), etc. with any gases emanating from the process or material being processed.
"Direct-fired process heater" means any process heater in which the combustion gases mix with and exhaust to the atmosphere from the same stack(s), vent(s), etc. with gases originating with the process or material being processed.
Thus, indirect-fired units are used in situations where direct flame contact with the process material is not wanted because, among other reasons, of the problems of contamination and ignition of the material. Direct-fired units are used where such problems are not a factor. Emissions from indirect-fired units consist entirely of products of combustion (including those of incomplete combustion). Emissions from direct-fired units, on the other hand, consist both of products of combustion along with emissions of the process material. Thus, emissions to the atmosphere from indirect-fired process heaters are generic to the fuel in use and are common across a wide range of industrial sources while those from direct-fired units are unique to the given process and may vary widely both within a given industrial process (if the process material is changed) and between industrial sources (where widely varying process materials may be handled).
Because of this difference in emission characteristics, work-to-date has focused on the indirect-fired unit population, of which there may be tens of thousands in the United States. Information currently available indicates that the majority of such units are contained in the following industrial sources:
Petrochemical industry (petroleum refineries, chemical manufacturing plants, etc.). Process heaters are used to effect a chemical reaction in the feedstock through the application of heat. The majority of units are fired with a gaseous fuel (e.g., natural gas, refinery process gas) with oil also being used. The units may range widely in size but no units are known to be controlled for anything other than nitrogen oxides (NOx). Emissions of HAPs are believed to be low.
Asphalt concrete industry. Process heaters are used to heat the asphalt storage tanks to maintain viscosity. These units are believed to be rather small and to burn natural gas or oil. (Note that the drum dryer, the major part of the asphalt concrete plant, is not considered an indirect-fired process heater.)
Gypsum industry. Certain types of gypsum kettles may be considered indirect-fired process heaters. Most, if not all, are believed to be fired with natural gas. Little else is known at this time.
Iron and steel industry. Certain types of heat treaters (annealing ovens, etc.) are believed to be indirect fired. Most are believed to be fired with gaseous fuels. Little else is known at this time.
Wood and forest products industry. Oil heaters (commonly called "Konus heaters") are indirectly fired to maintain oil viscosity. Wood may be used as the fuel. Some units are known to have particulate matter controls but the impact of these controls on HAPs is not known at this time.
As noted above, direct-fired process heaters are not being considered at this time under this project. Many such units are being considered under separate MACT-development projects. Examples of such projects include those for portland cement plants, lime kilns, brick and clay manufacturing plants, fertilizer plants, and asphalt concrete plants.
Types of fuel used
The majority of process heaters burn gaseous fuels (including natural gas, refinery process gas, coke oven gas, blast furnace gas, etc.) or fuel oils. Wood is also used. It is not believed that any indirect-fired process heater uses coal but that is still being explored.
Pollutants emitted
Information is still being developed on emissions of HAPs from process heaters but concentrations are generally believed to be low, often near the emission test method detection limit. Aldehydes and naphthalene are two of the compounds believed to be emitted by gaseous fuel-fired units. Oil-fired units would also be candidates for trace metal emissions.
Controls
There are no specific controls for HAPs on process heaters. Controls for NOx are frequently used but available information indicates that these controls have no impact on HAP emissions. The impact on HAP emissions of particulate control devices used on wood-fired process heaters is not known.