Wood Combustion and Outdoor Wood Boilers - Background

Public Information Sheet - a summary of highlights of the regulation.

Good Operating Practices for Outdoor Wood Boilers - a summary of operating practices.

Complaints and Problems from Wood Burning Boilers

Th outdoor wood boiler e Department is aware some of the outdoor boilers are causing air quality problems in their neighborhood. Some existing boilers do not have the benefit of recent cleaner design changes that better reduce the particulate matter and hydrocarbons in their exhaust; some have been sited too close to residential buildings and may also not have adequate stack height to disperse their exhaust.

Persons that have complaints about an outdoor wood boiler can report them here or they can call the DEP toll free in Augusta at 1-800 452-1942 or the nearest DEP Regional Office -- Presque Isle at 1-800 769-1053, Bangor at 1-888-769-1137, or Portland at 1-888-769-1036.

Introduction

Maine residents have a long tradition of meeting their home heating needs by burning fire wood in residential wood stoves and furnaces. This is particularly true of those living in rural communities. Even after other home heating fuel options became available, many people have continued to rely on wood fuel, in whole or in part, to heat their homes.

As these communities grew, both in terms of population and geographic area, so did the numbers of operating wood-fired stoves and furnaces, until the volume of wood smoke emissions approached or exceeded the capacity of a given locale to disperse or otherwise dilute these emissions. In those communities where emissions from wood combustion are not adequately dispersed (due to local conditions of topography, meteorology, and/or wood stove density) measurable impacts on the environment and human health are documented. Residential wood combustion has thus become recognized as a significant source of air pollution.

The Combustion Process

Under ideal conditions, the complete combustion of wood yields carbon dioxide, water vapor, and a small quantity of mineral residue or ash. More typically, wood combustion is not complete and its emissions contain particulate matter, carbon monoxide, nitrogen oxides, volatile organic compounds, ash, and trace amounts of sulfur oxides.

The quantity and character of the emissions from wood combustion varies with the wood species combusted as well as with the nature and completeness of the combustion process itself. In order for the combustion process to occur, four requirements have to be met: there must be a sufficiently high temperature to initiate the pyrolysis, or thermal decomposition, of the wood fuel; there must be sufficient time at combustion temperatures to initiate the combustion of the pyrolitic gasses, and sufficient turbulence to ensure the thorough mixing of oxygen and the combustible gasses; lastly, there must be sufficient oxygen present to sustain the combustion process. Thus, ideally, at the beginning of the combustion process, the moisture contained within the wood is boiled off. As temperatures increase the pyrolysis of the wood begins as gaseous vapors and tar droplets. At sufficiently high temperature, if oxygen is present, these gases will burn with a visible flame. The remaining material is essentially carbon, with some quantity of mineral residue; the carbon is consumed as the coals of the fire, and the mineral constituents remain as the ash, following the completion of the combustion process.

Under normal conditions of operation, residential wood stoves and furnaces do not lend themselves to the complete combustion of the wood fuel. As batch-fed appliances, deficiencies in one or more of the requirements for combustion (i.e. temperature, time, oxygen, and turbulence) will occur at some point during the burn cycle (even under the best of circumstances), and inevitably result in the loss of pyrolitic gasses to the atmosphere. In fact, from the start of a fire in the woodstove until the last bit of fuel is consumed, the combustion chemistry in the firebox is in a state of flux and may never achieve (or achieve only for a portion of the burn) optimal conditions for efficient combustion. Operational controls such as damping down the woodstove to reduce the rate of fuel consumption and/or heat output creates an oxygen deficient condition in the firebox which further inhibits the combustion of the pyrolitic gasses, increasing emissions vented to the atmosphere.

Newer wood-fired home heating appliances have improved residential wood combustion processes significantly in terms of both heating efficiency and emissions. These improvements have been realized through the use of combustion air distribution systems, firebox heat management designs, and/or the use of catalysts to reduce combustion temperature thresholds. Notwithstanding the improved design characteristics, it is simply not possible to avoid intermittent or short term deficient combustion conditions without the use of auxiliary combustion management systems. In addition, the air quality benefits accruing from the turnover of older to newer wood stove technologies have not kept pace with the density of operating wood-fired residential heating units in many communities. Thus, the air quality issues associated with residential wood combustion persist.

Wood Smoke Constituents and Public Health

By design, residential woodstoves are naturally drafted and combustion typically occurs in an air-rich environment. Because of poor air/fuel mixing, however, within the same firebox a portion of the combustion air may not be sufficiently mixed with the combustion gasses to burn them completely; while elsewhere in the firebox, an abundance of combustion air may cool the gasses enough to prevent the combustion reactions from being completed. Both conditions result in reduced combustion efficiency, generating carbon monoxide and promoting the formation of fine particulates. The fine particulates are predominantly in the form of solid and condensed organic products of incomplete combustion which are ultimately vented to the atmosphere, visible as wood smoke.

The principle environmental and public health impacts associated with residential wood combustion are derived from the fine particulate fraction of the wood smoke emissions. Fine particulates are solid particles and liquid droplets characterized by having an aerodynamic (equivalent) diameter of 2.5 m m or less. Particles in this size range tend to remain airborne for extended periods of time, lending themselves to long range transport and deposition; particles in this size range also possess light scattering properties, contributing to regional haze and other visibility issues.

From a public health perspective, exposure to airborne fine particulate is a concern because the small size of the particulate defeats the body's filtering mechanisms, enabling the particles to be drawn deep into the lung. Researchers have long recognized links between elevated levels of airborne particulate to serious health issues and increased mortality among susceptible populations. Emissions from residential wood combustion in particular is a concern due to the preponderance fine particulates in wood smoke, as well as for the toxicity of some of the chemical compounds in wood smoke.

Exposure to airborne fine particulate can cause eye, nose, lung, and throat irritation; it can impair lung function and aggravate existing medical conditions such as asthma and heart disease. Chronic exposure to concentrations of fine particulate has been associated with increases in respiratory and cardiovascular related hospital admissions and mortality. People with breathing problems such as bronchitis, emphysema or pneumonia, people with heart problems, babies, children, and the elderly are particularly vulnerable to exposure to fine particulate.

In addition to the size-related hazards associated with the fine particulate fraction of wood smoke, many of the chemical compounds that comprise the fine particles are in of themselves toxic. The emissions of organic compounds generated by the incomplete combustion of the wood fuel include a group of organic compounds called polycyclic organic matter (POM). These compounds include a subgroup called polycyclic aromatic hydrocarbons which are identified as hazardous air pollutants under Title III of the U. S. Clean Air Act of 1990.

Chemical compounds which may also be present in wood smoke include benzene, chlorinated dioxins, formaldehyde and other aldehydes, nitrogen oxides, carbon monoxide, and sulfur oxides.

Outdoor Wood-fired Boilers

During the latter part of the 1980's and throughout the 1990's an innovative technology began to appear in the residential heating appliance market. These units are frequently referred to as outdoor wood boilers. An outdoor wood boiler is essentially a free standing insulated structure in which a firebox, vented to a chimney, is surrounded by a water jacket. The unit operates as follows: wood fuel is combusted in the firebox, heating the water, which is then circulated via underground piping to a residence or other structure in order to provide space heating and/or domestic hot water.

In terms of air emissions, outdoor wood boilers are problematic. The firebox of the outdoor wood boiler is surrounded by a water jacket, in order to promote the effective heat transfer from the fire to the water reservoir. The net effect of the water jacket, however, is to reduce the combustion temperature of the fire, which in turn reduces the efficiency of the combustion process. Recalling the discussion above, lower combustion temperatures result in reduced combustion efficiency and an increase in emissions of the products of incomplete combustion.

Combustion efficiencies are degraded further by the demand/response nature of outdoor wood boiler operation. When the temperature of the heating water falls below a specified set point, additional combustion air is supplied to the firebox via either a fan or through the opening of a combustion air damper, increasing combustion and transferring the heat of that combustion to the water jacket; at an upper water temperature set point the fan is shut off and/or the combustion air damper is closed. During periods of low or no demand the fire smolders in the firebox; at these lower temperatures some portion of the smoke condenses on the internal surfaces of the unit, only to be reignited and/or vented as the fire is ramped up to meet a new demand. The emission 'pulses' associated with load changes exacerbate an already significant emission discharge into the ambient air.

Another operational issue related to emissions is the typical year round operation of outdoor wood boilers where they are installed for the dual purposes of space heating and hot water supply. The reduced demand on these units during the warmer months means that low oxygen, low temperature conditions in the firebox will be maintained for longer and more frequent periods than would occur during colder times of the year, and will generate significantly more emissions while operating in this manner. This condition is exacerbated when the outdoor wood boiler's design capacity is oversized relative to the structure(s) being served.

Outdoor wood boilers are typically installed some distance from the buildings they are intended to heat, that distance is often governed by the cost of installation, heat losses over distance, and convenience. Almost invariably, however, the outdoor wood boilers are located close enough to the buildings they serve and have short enough chimneys to be well within the meteorological cavity or wake of the structure. The cavity or wake region of the structure is, in effect, an eddy where emissions from the outdoor wood boiler chimney have little opportunity to disperse; consequently, the concentration of wood smoke within this region is likely to increase to significant and unhealthful levels. This effect can be especially pronounced in more densely populated neighborhoods.

General Info