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Biomass Combustion Energy Self Assessment
The Natural Resource Conservation Service (NRCS) biomass heating self assessment tool has been developed to increase renewable energy utilization. Before considering biomass for space or water heating, it is recommended that you maximize energy efficiency measures for your current system first as energy efficiency measures will almost always have a faster payback and return on investment than energy production. Suggestions for energy efficiency improvements are offered through our Energy Self Assessment Tools. The Biomass Energy tool considers the cost effectiveness of using biomass fuels such as cord wood, corn, rye, wood pellets or prairie grass pellets to fuel a furnace or boiler versus using fossil fuels. The tool evaluates the annual cost of a variety of biomass fuels based on the amount of heat you would like displaced by the chosen biomass fuel compared to a fossil fuel.
This tool does not provide a site-specific renewable energy assessment. It evaluates thermal energy use based on user input.
Frequently Asked Questions:
Conventional wood burning fireplaces are generally not efficient since they burn wood with little control. It is estimated that 90% of the heat generated in a fireplace leaves through the flue. Because fireplaces typically burn uncontrollably, and at lower temperatures relative to commercial biomass burning systems, there is a higher risk of creosote build up, and chimney fires. Fireplaces with sealed doors are highly recommended for new installations because they draw combustion air from outside, have a heat exchanger to capture heat and the fire burn rate can be controlled by the air inlet damper.
Fireplace inserts/stoves are available that can be installed into an existing masonry fireplace. These units have higher heat recovery and higher efficiency than a standard fireplace. They can burn wood or other biomass feedstocks (usually pellets) in a controlled environment. These systems should have sealed doors and may have a viewing window on the front door. Wood is typically loaded into the front of the fireplace insert as it would be with a conventional fireplace. If it is a pellet stove the pellets will be loaded into a hopper, usually above the combustion chamber.
Wood Stoves:
Wood stoves are also a popular way to utilize biomass heat and are available to burn cord wood or pelleted fuels. Pellet stoves will generally be more efficient than cord wood stoves and can operate for long hours without manual loading of fuel. Stoves may draw in air from the surrounding room, which may create a cold draft of incoming outside air or they often can be piped so they draw combustion air from outside. The higher efficiency pellet stoves can be direct vented, eliminating the need for a chimney which reduces installation costs. Wood stoves can take up more living space than a fire place and may be difficult to retrofit into an existing building.
Furnaces and Boilers:
Biomass fuel (wood, pellets, or corn) can also be used in both furnace and boiler applications. These systems can be located both indoors or outdoors. Stand-alone outdoor combustion systems have become very popular in recent years. Most outdoor wood boilers are very inefficient ranging from 20 to 50% efficiency typically. In 2008 many outdoor boiler manufacturers introduced new boilers to meet the EPA’s new emission standard for outdoor boilers. These units generally have higher efficiency and emit 70 to 90% less smoke emissions. Pellet fuel boilers and furnaces generally do not have issues with emissions because the unit is always burning and the fueling rate is decreased as the heating requirement is reduced. Biomass-fueled furnace systems are for forced-air heating applications, and biomass boilers are used for supplying domestic hot water or space heating.
Outdoor wood-fire boiler | Pellet/biomass boiler |
Biomass boiler systems use water or steam as the heat transfer medium; a furnace heats the air directly. In a boiler, water can be heated in tubes surrounding the firebox chamber, or heat can be transferred to a water jacket that surrounds most of the firebox. When heat is required, a circulator pump circulates the heated water to a heat exchanger where the heat is transfered to the medium to be heated (air or liquid). The boiler control system regulates the damper control to increase or decrease the heating rate to maintain the boiler water temperature. If the boiler is fueled by pelleted fuels, the pellet feed rate and the air flow can be regulated to control the water temperature. If the boiler is located outdoors, insulated supply and return lines are run to the building(s) where heating is needed. A furnace works the same way as a boiler except air is blown around the fire box to transfer heat from the fuel to the air. Furnaces are typically located inside of the building to be heated because they need to be connected to the building’s air distribution system. A boiler is often easier to connect to an existing force-air furnace system because it provides more flexibility in the placement of the boiler. Piping is installed between the boiler and the air distribution system and a heat exchanger to transfer heat from water to air is installed in the air duct. When heat is required the furnace fan and a circulating pump is operated to circulate heated water through the heat exchanger from the boiler. |
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Pellet furnace |
Wood Stoves: Obtaining wood may require labor for timber collection, wood splitting, or the purchase of wood. Wood is usually sold by the “cord”, which is 128 cubic feet, or a volume 4 feet high, 4 feet wide, and 8 feet long. A cord may have an adjective in front of it such as face or fireplace. A fireplace cord is two, 4 feet high by 8 feet long stacks of fireplace length wood, usually about 16-18” long while a face cord is one stack of fireplace length wood. Wood should be stored in a dry area, with easy access for transport to the wood-burning boiler or furnace. It is recommended to avoid using freshly cut green lumber as the moisture content will hinder complete combustion and produce excessive smoke and creosote. The frequency of loading the firebox of the wood burner will depend on the size of the firebox chamber, type of wood, moisture content of wood and the heat demand on the system. Some systems require filling once a day, while others can require filling every couple of hours, which could result in waking up in the middle of the night to load the firebox. Wood will typically be handled four to five times before it is burned. If there is an electrical power interruption, most wood stoves or fireplaces will work but their ability to recover heat is reduced if fans are required. Outdoor wood boilers require electricity to operate circulation pumps, dampers, and draft controls.
Pellet and Corn Stoves: Pellet and grain stoves have hoppers and automatic fuel feeding systems. The hopper can be an integral part of the stove, a separate unit that sets next to the stove or a large bulk bin that is typically located outdoors but within close proximity of the boiler or furnace. The hoppers can be loaded with bags of fuel or augered from a bulk bin. Fuel can be purchased in bags, one ton totes or bulk delivered by a truck with an auger to transfer the pellet fuel into the bin. Bags of pellet fuel or grains are typically available in 40- or 50-pound bags and are available in many retail outlets and feed stores. The frequency a hopper needs to be filled will depend on the hopper or bin size, stove capacity and the heat demand. An auger moves pelleted fuel or grain from the hopper into the fire chamber under thermostat control. It is important to maintain the hopper and auguring mechanisms according to manufacturer specifications to assure proper fuel loading. The auger mechanisms require electricity to operate, and therefore will not run during a power outage unless backup power is available.
Biomass combustion system maintenance is important to maintain safe and efficient operation. Creosote and ash may build up on the holding grate, walls and floor of the fire box. Corn and pellet combustion may create “clinker” build up, a solid lump of minerals on the grate of the firebox. Some systems have built-in scraping systems which allow for regular, easy removal of the clinker build-up. Creosote and/or clinker build-up needs to be removed as it may contain combustible materials which could ignite.
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Wood Pellets
Wood pellets are typically available in 40 to 50 pound bags from a variety of retail outlets or bulk from distributers. The pellets are 1/4” in diameter and 1” to 1-1/2” long. There are three grades of pellets: Premium, Standard and Utility. The grade is based on the maximum ash and moisture content.
Fuel Property Premium Standard Utility Ash % ≤ 1.0 ≤ 2.0 ≤ 6.0 Moisture % ≤ 8.0 ≤ 10.0 ≤ 10.0 Bulk Density
lb./cubic foot40 – 48 38 - 48 38 - 48 Fines % ≤ 0.5 ≤ 1.0 ≤ 1.0 -
Cord Wood
Cord wood represents wood which is cut to length and split, and has a heat value of 8600 Btu per pound. All types of wood will burn, however hardwoods such as oak, walnut, hard maple and locust will have the highest energy value per cord of wood. The moisture content of wood has a direct effect on how well it will burn and the net energy available. Green wood is about 50% moisture while air dried wood will generally be about 20%. Un-split wood with bark attached will take longer to dry than wood that has been split or the bark removed. The type of wood will affect how long it takes to dry. Generally, allowing it to air dry for two summers will result in the best heating value. Wood requires many years to mature for harvest, whereas corn and grass pellets can be made from an annual crop.
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Corn and small grains
Corn used in corn burners is typically the same type of corn used for livestock feeding. Corn harvested for feed is dried to 14 or 15% moisture content but at this moisture content it can be difficult to keep the fire burning at low feed rates. Many pellet stove manufacturers recommend using corn that is dried to 11-12% moisture content for fuel. It is important for corn fuel to be free of dust and foreign matter larger than 3/8” diameter. Typically, there may be a few pieces of cob and stalk in the corn which shouldn’t cause problems. Some feed mills will clean the corn to remove the fine materials and apply a light coating of oil to reduce dust. The net energy content of corn at 15% moisture is 6800 to 7000 Btu per pound. A bushel of corn weighs 56 pounds, so a bushel of corn will contain about 390,000 Btu of energy or about the same energy content as 4-1/4 gallons of LP gas, 2.8 gallons of heating oil, or 3.9 therms of natural gas. Other grain crops can also be used in a multi-fuel stove such as rye or other small grains. Corn can often be mixed 50% with wood pellets in a wood pellet stove and perform well. Corn, rye and other small grains are planted, grown, and harvested annually.
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Mixed grass / biomass pellets
Pellets can be made of grasses such as switchgrass, big blue stem, or a variety of other prairie-type grasses, or from crop residues such as straw or corn fodder or a mixture. Pellets are sold in 40 to 50 pound bags or may be available in bulk, depending on your distance from a processor. Btu values per grass pellet type will vary, and the grass pellets used in the Biomass Energy Calculator are assumed to have a Btu value of 7200 Btu per pound. Grasses are perennial crops that are planted once and re-grow yearly for many years before needing to be replanted. Grasses are typically cut and harvested for pelleting one or two times a year. Sulfur and chloride contents in grasses and crop residues can cause corrosion to a boiler or stove over time unless the boiler firebox and flue are made of corrosion resistant materials like stainless steel. Ash content of grass/biomass pellets will be higher than wood pellets and typically range from 5 to 8%.