Juneau Commission on Sustainability
Biomass energy is defined as energy derived from renewable organic materials such as wood products, agriculture residue, fish waste, waste water plant sludge, etc. In Southeast Alaska we most commonly think of biomass as wood products such as cordwood, wood pellets, and wood chips.
From an energy perspective, biomass is simply solar energy, stored as a solid chemical fuel. Through the process of photosynthesis, plants absorb CO2 out of the atmosphere and store that carbon in the structure of the plant. Harvesting trees and burning them releases the energy stored in those carbon structures.
Heating with biomass can be very cost effective compared to heating with oil or electric resistance. For example, a wood pellet stove can provide heat for approximately two-thirds the cost of providing that heat with oil or electric resistance at current Juneau prices. Modern biomass heating systems are efficient, clean, economical, and convenient.
In terms of CO2 emissions, biomass energy is ‘carbon neutral.’ Whenever any carbon-based fuel is combusted, CO2 is a resulting byproduct. It doesn’t matter if it’s wood or natural gas, oil, or coal. The difference between wood and fossil fuels is that the carbon contained in trees was extracted in the atmosphere, and will return to the atmosphere when the tree dies and decays, or is burned. The forest and the atmosphere are in equilibrium – burning biomass energy takes advantage of that natural cycle, and as long as new trees are allowed to grow up to replace the trees that are harvested, the system is carbon neutral. Burning fossil fuels such as oil introduce ‘new’ carbon to the atmosphere, thus increasing the overall concentration of CO2. Harvesting and transporting wood requires the use of some fossil fuels, so it is probably more accurate to state that biomass is ‘carbon advantaged’ compared to any of the fossil fuel alternatives.
Biomass Fuel Sources
This is the simplest and often the cheapest form of residential heating energy in Juneau. Residents can harvest their own fuel in the Juneau area, or purchase firewood from local suppliers. Firewood can be burned in fireplaces, fireplace inserts, free standing wood stoves, or wood boilers.
While energy from cordwood can be relatively inexpensive, there are several other considerations to keep in mind:
- If a building doesn’t have an existing chimney suitable for a cordwood system, installing a new chimney can be fairly expensive and involved. It will require a building permit and should only be installed by a knowledgeable party familiar with applicable fire and building codes.
- Firewood must be ‘seasoned’ (air dried) in order to achieve the the cleanest burn and best efficiency. In our cool, wet climate, that can take a year or more to achieve.
- Firewood must be stored in a dry, covered location. That can consume a considerable space footprint and add to the cost of the system.
- Firewood can be heavy and awkward to handle.
- Because wood-stoves cannot self-ignite or self-feed, cordwood systems generally require regular attention for lighting, stoking, and adjusting.
- Though modern wood stoves have improved significantly over the older stoves, wood stoves still have much higher emissions of particulate matter than other sources of heating energy. While this typically does not represent a significant problem in our area, during certain weather conditions the Mendenhall Valley experiences an ‘Air Emergency’ where cordwood burning is prohibited (https://juneau.org/lands/air-quality). This situation varies from year to year, but typically burning is restricted less than a week cumulative during a winter.
Many of the disadvantages identified above are eliminated with advanced cordwood boilers, such as the Garn which burns wood in a large batch, storing the heat in an insulated water storage tank for controlled delivery to a building’s heat distribution system (www.garn.com).
Wood pellets are manufactured in a pellet mill. The feedstock is typically waste sawdust and shavings from some value-added milling process, but it can also be sourced from chipping up whole trees that are not suited for sawmill utilization. Pellets are manufactured according to the standards established by the Pellet Fuels Institute (http://pelletheat.org/pfi-standards/pfi-standards-program/). The pellet mill compresses the sawdust under heat and pressure and extrudes the pellets through a die – the process resembles a pasta-spaghetti maker. The heat and pressure cause the sawdust to ‘melt’ and bind together to create firm, uniform-sized cylindrical pellets. No additive glues or binders are used to manufacture pellets. Pellets are manufactured to standard quality grades, with the principal factor being ash (mineral) content – less ash means a cleaner burn and less frequent cleaning of pellet burning equipment. Most pellets available in Juneau meet the ‘Premium’ pellet standard.
Pellets have some very significant advantages over other forms of biomass fuel:
- They are a uniform in size and shape. This is critical for the fuel handling components such as feed augers, and provides a standard combustion profile which is helpful for precise control settings.
- They have a uniform, and low, moisture content. Low moisture means that the pellets burn efficiently and cleanly.
- Because they have been ‘densified’ in the pellet making process, they contain a lot of energy per cubic foot. This makes pellets cheaper to transport and to store because their energy is more condensed.
- Due to their shape, pellets ‘flow’ like a liquid to some extent. This allows for simple bulk storage and feeding systems. Much of the equipment for the storage and handling of pellets is based on grain storage and
- handling – a technology which has been refined over centuries.
- Since pellets adhere to manufacturing standards, they are a commodity product. Pellets from multiple sources and multiple mills will be suitable for whatever pellet burning equipment you install.
Pellet stoves are essentially a ‘drop-in’ replacement to Toyo or Monitor stoves. They typically consume about the same amount of floorspace, have similar installation requirements, and produce similar amounts of heat. Self-igniting and thermostatically controlled, pellet stoves are very efficient and convenient. Their installed cost is comparable to the cost of installing an oil stove and fuel tank. One significant difference is that most pellet stoves don’t have outdoor bulk fuel storage tanks, so they must be periodically filled with pellets. Typically, a pellet stove will run for a day or two on a 40 lbs bag of pellets in the winter; although that’s highly variable due to the weather, how warm the house is kept, etc.
Pellet stoves require slightly more maintenance than a comparable oil stove, with periodic cleanings during the year and the emptying of the ash bin. These activities are easily within the capabilities of most home owners, and typically take around 30 minutes. The small amount of ash produced by a pellet stove can be disposed of in a garden or compost bin, or spread on the ground. It’s wood ash and contains no environmentally toxic compounds.
Pellet boilers are a potential replacement to an oil or electric boiler. Very similar to an oil boiler, a pellet boiler combusts pellets and heats water or steam for building heating. Pellet boilers come in many sizes from residential to commercial, and feature sophisticated combustion and emissions management systems. Pellet boilers are typically fed from an external bulk pellet silo, meaning that en entire season’s worth of fuel can be stored onsite, depending on the size of the silo.
Because pellets burn far more cleanly than cordwood systems, they produce much lower levels of air emissions, and are not subject to burn bans in the Mendenhall Valley.
Most of the pellets consumed in Southeast Alaska are shipped up from the Lower 48. Currently there are two pellet mills in Alaska. One large mill, Superior Pellets in Fairbanks. (http://www.superiorpelletfuels.com/Superior_Pellet_Fuels_Home.html).
The other is a small mill, Tongass Forest Enterprises in Ketchikan. (http://www.akforestenterprises.com/).
As more facilities convert from oil to pellet heating, these mills will grow and additional mills will be established. As more mills come online, more of Southeast Alaska’s pellets will be supplied from regional sources of material, meaning less money leaves our region for energy purchases, remaining in our local economy. And as this local energy economy develops, the cost of pellets will decrease.
The Forest Service’s stated goal is to transition 30% of oil heating over to wood pellet heating over the next decade.
Wood chips represent another form of biomass fuel. Chips, or ‘hog fuel’ (a coarser, lower grade of chip made by grinding wood) are typically used in larger commercial-scale boilers. They are often the cheapest form of wood fuel. The City of Craig operates a chip boiler to heat several buildings and a swimming pool complex. Chip boilers have been successfully installed in schools in Montana and Idaho, as well as in the Northeast, where there are nearby mills producing wood chips. If Juneau ever installs a large multi-building district energy system, chips could potentially be the fuel media of choice, but generally wood chips are not suitable for a residential or small facility installation.
The supply of wood feedstock to meet our biomass fuel needs is an often-raised question. Will we be clear-cutting the Tongass just to heat our buildings? Will the Forest Service ever permit enough wood harvest to meet our needs? These are complex questions of course, but the short answer is that the amount of wood required to annually meet our thermal energy needs is a tiny fraction of the available harvest. Supplying Juneau’s heating energy needs will not deplete the Tongass of wood – it would barely be noticeable. More challenging are the economics of harvesting wood from the Tongass – due to the remoteness, the rugged terrain, and the lack of infrastructure, it can be very expensive to harvest and transport wood fuel. As the demand for biomass fuel grows,economies of scale will kick in, making the economics work better all around.