Logs, wood chips, brush clippings, and grasses all serve as woody biomass—renewable fuels suitable for producing heat and power. Burning woody materials is the original home heating system since there is evidence that prehistoric peoples burned woody materials in caves to provide heating—millenia before anyone recognized that coal or other materials could be burned to produce heat. And even in the not-so-distant past, such as in Colonial America, burning wood logs in a fireplace or wood stove was the principal means for heating a home. Today, wood-burning fireplaces and stoves remain a viable means of providing heat, although the use of traditional designs is now usually more a matter of nostalgia and atmosphere than practical function.
From 19th Century Fireplaces to Institutional Boilers
Where they still exist in today's homes, these old-fashioned open fireplaces are more for ambiance than for heating, because the design is inherently inefficient in terms of energy use. While an open fireplace feels cozy and warm in the immediate area around the fireplace, the natural convection currents actually cause an open fireplace to suck indoor air up and out of the house through the chimney. More than one homeowner has noticed that while a crackling fireplace blazes in the den or family room, it is also sucking in cold air around windows and doors in other parts of the house. In addition, the traditional wood fireplace or wood stove only makes use of roughly 15 percent of the potential energy stored in wood, so these older designs are increasingly giving way to more energy-efficient methods.
Wood fires also have the very real drawback of creating air pollution. In some parts of the country, there are legal restrictions on the use of wood-burning fireplaces and stoves. In Colorado, for example—a setting where you might think that a wood-burning fireplace is almost mandatory—wood smoke is such a pollution hazard that uncertified wood-burning fireplaces and stoves in new construction are not allowed at elevations below 7,000 feet, and where they are used, they must be specially engineered units designed as principle heat sources, not for ornamental use. At certain times of the year, deemed "high-pollution days," wood-burning in uncertified fireplaces may be forbidden entirely.
For this reason, where wood-burning is desired as an actual home-heating strategy, the systems now usually involve sealed furnaces or pellet stoves that burn compressed biomass pellets. There are also fireplace and stove designs that use sealed burning chambers and mechanical air circulation features. A vibrant industry exists for converting older fireplace units to more energy-efficient "certified" designs. The design is aimed at burning the wood at higher temperatures that minimize the particulate matter that escapes as smoke and wrings as much energy as possible from the wood burned as fuel. The EPA has a certification program for wood stoves and fireplace inserts. Of the approved styles, catalytic wood stoves burn more cleanly (emitting no more than 4.1 grams of particulate per hour).
On a larger scale, woody biomass can operate boilers that heat schools, offices, institutions and manufacturing facilities. The largest wood-powered facilities typically produce both heat and electricity at the same time. Such thermal and electric power “cogeneration” systems are actually the most energy efficient.
Like any power source, woody biomass has its benefits and challenges as a fuel source.
Pros of Wood-Burning Systems
- The net fuel cost is cheaper than heating oil, natural gas, or coal.
- Woody biomass can be grown and purchased locally, thereby benefitting local economies.
- Fuel (in the form of logs, wood chips, brush clippings, grasses, and lumber yard waste) is widely available, renewable, and sustainable.
- Fuel prices are relatively stable.
- Pellet stoves, which burn pellets made of compressed woody biomass, are relatively non-polluting and are approved by the EPA.
- With proper engineering, wood-burning systems emit fewer pollutants into the air than coal and oil.
- Over its lifecycle, biomass is a carbon-neutral fuel source.
- Using forest wastes improves forest health. By removing deadfalls from the forest, the practice can reduce forest-fire hazards.
Cons of Wood Burning Systems
- When wood burns, it releases hazardous gases (e.g., nitrogen oxide and carbon monoxide) and soot (particulate matter). To avoid pollution issues, fireplaces, wood stoves, and wood energy appliances and facilities must be properly designed and receive permits that meet air quality regulations and standards.
- Constant use requires a continuous supply of logs, wood chips or other biomass.
- Wood systems require more space to store bulky fuel.
- Waste ash that remains after burning needs proper, safe disposal.
- Most pellet stoves and sealed fireplaces require some electricity to operate circulating fans.
- The upfront, capital cost of building a sizeable wood energy facility can be high; it can take years to realize any savings.
- Compared with conventional gas or oil boiler systems, wood systems require a larger boiler to handle the fuel.
- Automated wood chip conveyor systems and fuel-handling equipment must be monitored closely to prevent jams and system shutdowns.
- Wood chip fuel varies by size, moisture content, and energy content. Standard un-dried, or “green,” fuel contains 30 to 55% water, which slows combustion.
- Equipment to dry wood chips and improve efficiency is very expensive. (Note: dry wood is highly flammable and requires a sophisticated boiler system.)