EPA Guidelines for Residential Fireplaces
These days, fireplaces are used primarily for aesthetic effects and secondarily as a supplemental heating source in houses and other dwellings. Wood is the most common fuel for fireplaces, but coal and densified wood "logs" can also be burned. The user also intermittently manually adds fuel to the fire. Fireplaces can be divided into 2 broad categories: 1: Masonry (generally made of brick and/or stone, assembled on site, and integral to a structure) and 2: Prefabricated (usually made from metal, and installed on site as a package with appropriate duct work).
Masonry fireplaces typically have large fixed openings to the fire bed as well as dampers above the combustion area in the chimney which help limit losses of heat when the fireplace is not being used. Some masonry fireplaces are designed or retrofitted with doors and louvers to reduce the intake of combustion air while they are being used.
As mentioned, prefabricated fireplaces are usually equipped with glass doors and louvers to reduce the intake of combustion air, and some are surrounded by ducts through which floor level air is drawn by natural convection, heated, and then returned to the room. Many varieties of prefabricated fireplaces are now available on the residential market. One general class is the freestanding fireplace, the most common variety of which consists of an inverted sheet metal funnel and stovepipe directly above the fire bed. Another class is the "zero clearance" fireplace, an iron or heavy-gauge steel firebox lined on the inside with firebrick and surrounded by multiple steel walls with spaces to allow for air circulation. Some zero clearance fireplaces can be inserted into existing masonry fireplace openings, and for this are sometimes referred to as "inserts".
Some of these units are equipped with close-fitting doors and have operating and combustion characteristics that are similar to wood stoves. Masonry fireplaces usually heat a room through radiation, and a significant fraction of the combustion heat is lost in the exhaust gases and through the fireplace walls. Moreover, some of the radiant heat entering the room goes toward warming the air that is pulled into the residence to make up for that which is drawn up the chimney. The net effect is that masonry fireplaces are generally inefficient heating devices. And in cases where combustion is poor, where the outside air is cold, or where the fire is allowed to smolder (thus drawing air into a residence without producing significant radiant heat energy), a net heat loss may occur in a residence that is using a fireplace. Fireplace heating efficiency may be improved using several methods that either reduce the excess air rate or transfer back into the residence some of the heat that would normally be lost in the exhaust gases or through the walls of a fireplace.
As noted above, such measures are commonly integral design features in prefabricated units. As a result, the energy efficiencies of prefabricated fireplaces are slightly higher than those of masonry fireplace models.
Emissions And Controls
Fireplace emissions, caused mainly by incomplete combustion, include particulate matter (PM)(mainly PM less than 10 micrometers in diameter [PM-10]), carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx) and volatile organic compounds (VOC). Significant quantities of combustibles that are not burnt are produced because fireplaces are inefficient combustion devices, having high uncontrolled excess air rates and without any sort of secondary means of combustion. The latter is especially important when wood burning because it contains highly volatile content matter, typically 80 percent by dry weight.
External Combustion Sources
Hazardous air pollutants (HAPs) are a minor, but potentially important, component of wood smoke. A group of these HAPs known as polycyclic organic matter (POM) includes potential carcinogens such as benzo(a)pyrene (BaP). POM results from the combination of free radical species formed in the flame zone, primarily as a consequence of combustion that is incomplete. Under reducing conditions, radical chain propagation is enhanced, allowing the buildup of complex organic material such as POM. The POM is generally found on or in smoke particles, although it is probable for there to be some sublimation into the vapor phase.
Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions are all produced during wood combustion in residential fireplaces. Most of the fuel carbon in wood is converted to CO2 during the combustion process, but because of the inefficient combustion, low combustion temperatures, and large amounts of excess air, a much higher ratio of carbon monoxide to CO2 is produced in an open fireplace burning wood than in airtight wood stoves or wood-fired boilers. This formation of carbon monoxide combined with incomplete combustion acts to slightly reduce CO2 emissions when compared to other types of wood combustion. However, CO2 emitted from this type of source may not increase the total atmospheric CO2 because emissions may be offset by the uptake of CO2 by re-growing biomass.
During the combustion process, nitrous oxide is formed through a complex series of reactions and its formation is dependent upon many factors. Although no test data is available, it is assumed that N2O emissions from residential fireplaces would be significantly higher than either or commercial wood-fired boilers or wood stoves because of the combination of low combustion temperatures and high amounts of excess air.
Methane emissions are at their highest during periods of low-temperature combustion or incomplete combustion, both of which occur frequently in residential fireplaces. VOC emissions for residential fireplaces are high compared to other wood combustion sources. Typically, conditions that favor the formation of N2O also favor emissions of CH.
Another important constituent of wood smoke is creosote. This tar-like substance will burn if the fire is hot enough, but at insufficient temperatures, it may leave deposits on surfaces in the exhaust system. Creosote deposits are a fire hazard in the flue, but they can be reduced if the chimney is properly insulated to prevent creosote condensation or if the chimney is cleaned regularly to remove any buildup.
To decrease PM and CO emissions from fireplaces, combustion needs to be improved. Combustion efficiency improves as the burn rate and flame intensity increase. Non-catalytic fireplace inserts reduce emissions by directing the unburned hydrocarbons and CO into an insulated secondary chamber, where mixing with fresh, preheated makeup air occurs and combustion is then enhanced.
Emissions from fireplaces are highly variable and are a function of operating practices and many wood characteristics. In general, conditions which promote a fast burn rate and a higher flame intensity enhance the secondary combustion, thereby lowering emissions. Conversely, higher emissions will result from a slow burn rate and a lower flame intensity. Such generalizations apply particularly to the earlier stages of the burning cycle, when significant quantities of combustible volatile matter are being driven out of the wood. Later in the burning cycle, the charcoal that remains burns with relatively few emissions.