Glossary of Biomass Heating Terms

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Appliance efficiency: The ratio of output energy to input energy when the combustion system is running under design conditions. Also called steady state efficiency.

Ashing auger: An auger, operated manually or by a motor, used to remove ash from the base of a furnace or boiler setting. Also called ashing screw.

Backpressure turbine: A type of steam turbine that produces low-pressure steam exhaust, which can be used as the source of heat for space heating or other uses.

Backup system: An alternate fuel combustion system used to provide heat when the primary system is out of service or unable to meet the full heat load.

Bag house: A type of particulate removal device used with very large biomass heating plants.

Bio-gas: A gas produced from biomass. Can be used as a combustion fuel.

Biomass: Any organic matter that can be burned for energy. Here used as synonymous with wood in its various forms.

Blast tube: A short connecting passage between a combustor and a boiler or other heat exchanger. Hot combustion gases from the primary chamber pass through the tube, sometimes with the addition of secondary or tertiary combustion air.

Boiler: A heat exchanger used to extract heat from hot combustion gases and transfer the heat to water. The boiler output can be either hot water or, if the water is allowed to boil, steam.

Bole chips: Woodchips produced from the main stems or trunks of trees, excluding branches and tops.

Bottom ash: Ash that collects under the grates of a combustion furnace.

Btu: British thermal unit, a standard unit of energy equal to the heat required to raise the temperature of one pound of water one degree Fahrenheit.

Btu meter: A device for measuring energy flow over time. Used to measure boiler heat output or energy consumption. For a hot water boiler, a Btu meter includes a water flow meter and temperature sensors that give the increase in temperature between the return water and output water.

Bucket elevator: A solid fuel handling device that lifts the fuel vertically.

Burnback: Movement of flame from the combustion chamber back along the incoming fuel stream.

Calorific value: The energy content of a fuel, expressed in units such as Btu per pound.

Carbon burn-out: The end of the combustion process in which all uncombined gaseous and solid carbon is oxidized to carbon dioxide.

Char: Carbon-rich combustible solids that result from pyrolysis of wood in the early stages of combustion. Char can be converted to combustible gases under certain conditions, or burned directly on the grates.

Char reinjector: A device that collects unburned char at certain locations in large boilers and injects it back into the primary combustion zone, both to keep it from going up the stack and to capture its energy through recombustion.

Chipper: A large device that reduces logs, whole trees, slab wood, or lumber to chips of more or less uniform-size. Stationary chippers are used in sawmills, while trailer-mounted whole-tree chippers are used in the woods.

CHP: The acronym for 'combined heat and power.' CHP is the simultaneous production of heat and electrical power from a single fuel.

Close-coupled gasifier: A biomass combustion burner that produces combustible gases under controlled conditions in the primary combustion chamber or combustor, and burns the gases to produce heat in an adjacent chamber.

Cogeneration: Combined heat and power (CHP). A term used in industrial settings, now being displaced by the more descriptive term CHP.

Combined-cycle gas turbine: A type of high-efficiency turbine for burning gas to produce electricity. Can be used to burn the output bio-gas produced by a biomass gasifier.

Combined heat and power (CHP): The simultaneous production of heat and electrical power from a single fuel.

Combustion efficiency: The efficiency of converting available chemical energy in the fuel to heat, typically in excess of 99% in biomass burners. Efficiencies of conversion to usable heat are much lower.

Combustor: A freestanding primary combustion furnace, usually located adjacent to the boiler or heat exchanger. Exhaust gases from the combustor pass into and through the boiler before exiting to the stack.

Commissioning: The process of verifying that a new heating plant meets the performance specifications called for in the installation contract.

Complete combustion: Combustion in which all carbon and hydrogen in the fuel have been thoroughly reacted with oxygen, producing carbon dioxide and water vapor.

Cyclone separator: A flue gas particulate removal device, which creates a vortex that separates solid particles from the hot gas stream.

Day bin: An intermediary solid fuel storage bin that holds enough fuel to last approximately one day. Could be designed with the capacity to feed the combustion system for a weekend.

Demand charges: A class of charges typically found in commercial and industrial electric rates. Demand charges reflect the cost placed on the utility of the maximum number and size of all the electricity-consuming devices in use at any one time during a billing period.

Design/build: A design and contracting process under which the contractor bears ultimate responsibility for the design and function of the equipment or system installed.

Design specifications: For mechanical systems, specifications (and drawings) produced by the owner’s mechanical or design engineer. Design specifications become part of the contract for the installation. The designer bears ultimate responsibility for the design and function of the system.

DHW: Domestic hot water.

Direct-burn system: A biomass combustion system in which the primary combustion chamber is located under and directly connected to the combustion chamber of the boiler itself.

Discount rate: In economic analysis, the interest rate that reflects the rate of return the owners could get if their money was invested elsewhere.

District heating: The use of a single boiler plant to provide hot water or steam for heating a number of buildings in a locality.

Energy services company (ESCO): A company that provides a broad range of energy services to a building owner, typically including the financing and installation of energy improvements under a contract that allows some of the dollar savings to accrue to the company.

Excess air: The amount of combustion air supplied to the fire that exceeds the theoretical air requirement to give complete combustion. Expressed as a percentage.

Fly ash: Airborne ash carried through the combustion chamber by the hot exhaust gases, and typically deposited in the passages of the boiler heat exchanger.

Flying Dutchman: A device commonly installed in round fuel silos to knock fuel down into the base of the silo, for transport by the fuel handling equipment to the combustion appliance.

Furnace: The primary combustion chamber of a biomass burner. The term also refers to warm-air heating appliances.

Gasification: The pyrolysis reaction in which heated biomass is converted to combustible gases in the primary combustion zone. Also refers to the conversion of char to combustible gases in the absence of oxygen and to the overall process of converting biomass, in an oxygen-starved environment, to combustible medium-Btu-content gases that are not immediately burned, but are cooled and cleaned to be used in a variety of ways.

Gasifier: A combustion device that produces bio-gas from solid biomass. Also shorthand for close-coupled gasifier.

Gasify: To convert solid biomass into combustible gas.

Grates (or combustion grates): Slotted or pinhole grates that support the burning fuel and allow air to pass up through the fuel bed from below.

Green biomass fuel: Biomass fuel that has not been significantly dried, with approximately the same moisture content as at harvest.

Heat exchanger: A device that transfers heat from one fluid stream to another. The most common heat exchanger in biomass combustion systems is the boiler, which transfers heat from the hot combustion gases to boiler water.

Heat load: The demand for heat of a building at any one time, typically expressed in Btus/hour or million Btus/hour. Peak heat load refers to the maximum annual demand for heat, and is used in sizing heating plants.

Heat transfer medium: A fluid (either water, steam, or air) that carries heat from the combustion system to the point of use.

Heating consumption: The annual total amount of heat a building requires. Can be expressed in energy units (million Btus) or fuel units (tons of biomass, gallons of oil, kilowatt hours of electricity).

Hog: Shorthand for hog mill, a device used to grind up various forms of biomass into chip-sized pieces.

Hogged fuel: Biomass fuel produced by grinding up various forms of wood and bark, possibly mixed with sawdust. Often refers to a variable low-quality fuel. If produced from clean, high-quality dry scrap, can be a very high-quality fuel.

Hydronic: Refers to a water-based heat distribution system that uses either hot water or steam.

Induced draft fan: A fan mounted at the discharge of the boiler, before the stack, to keep furnace pressure at the correct level and assure proper movement of flue gases up the chimney. Also called the ID fan.

Injection auger: The final fuel auger that moves the solid fuel into the combustion zone. In particular, an auger that forces fuel through an aperture onto the grates.

Life-cycle cost analysis: A method of economic analysis that includes all costs associated with a course of action for the lifetime of the equipment being installed. Includes price and cost inflation over time, and accounts for the time-value of money.

Live-bottom trailer: A self-unloading tractor trailer with a hydraulically operated moving floor, which is used to push the biomass fuel load out the back of the trailer. Typically filled directly by the chipper in the mill or in the woods.

Metering bin: A small bin in the fuel feed stream, just upstream of the combustion device. Allows a precise feed rate, or metering, of the fuel to the fire.

Mill chips: Woodchips produced in a sawmill. Typically produced from slabwood and other unmerchantable wood from debarked green saw logs.

MMBH: A unit that characterizes the size or peak output of a boiler, equal to one million Btus per hour.

MMBtu: A unit of energy equal to one million Btu (each M represents 1,000). In boiler or system sizing, also represents 1 MMBtu per hour.

Modulating fuel feed: A fuel feed system that adjusts the feed rate up or down in response to changes in the heat load.

Moving floor trailer: See live-bottom trailer.

Multi-chamber system: A variation on the two-chamber combustion system in which there is a connecting refractory-lined chamber between the combustor and boiler to give a longer flame path to enhance completeness of combustion.

Multi-clone (or multi-cyclone): A particulate removal device that includes a number of cyclone separators.

Municipal wood waste (MWW): Wood from sources like urban demolition and construction debris, urban tree waste, land and right-of-way clearing, and chipped pallets.

Nominal inflation rates: Price inflation rates including the rate of general inflation in the economy.

NOx: Oxides of nitrogen. Air pollutants that can be released from various types of combustion processes, including biomass combustion.

On/off fuel feed: A fuel feed system that delivers fuel to the grates on an intermittent basis in response to boiler water temperature and load variations. Efficient combustion is typically achieved during on cycles and during high-load conditions. In low-load conditions, and while off-cycle, combustion is less efficient.

Over-fire air: Combustion air supplied above the grates and fuel bed. Also called secondary combustion air.

Particulates: Very small solid airborne particles. A source of air pollution that can result from biomass combustion.

Performance specifications: For mechanical systems, specifications used in design/build and turnkey contracting. Set forth the owner’s minimum requirements for how a system will be configured and function.

Pile burner: A type of biomass combustion burner in which a pile of fuel burns on the grates. Primary combustion air comes from above the grates, not below.

Process steam: Steam used as a high-temperature medium for a variety of industrial purposes.

Pyrolysis: The oxidation process by which solid wood is converted to intermediate combustible gases and combustible solids through a variety of thermo-chemical reactions.

Real inflation rates: Price inflation rates that do not include the general inflation rate in the economy.

Refractory: A material resistant to high temperatures that is used to line combustion chambers in order to reflect heat back to the fire and to keep furnace temperatures steady.

Retention time: The transit time of hot gases from the point in the combustion process where the last combustion air is added to the beginning of the heat exchanger. The period during which carbon burnout takes place.

Rotary airlock: A device used to pass solids such as incoming fuel or fly ash from a multi-cyclone without passing air. Can be used to prevent burnback or the introduction of boiler room air into the exhaust gases through a multi-cyclone.

Seasonal efficiency: The efficiency of a heating system averaged over an entire heating season.

Sensitivity analysis: A part of economic analysis used to determine how sensitive the results of the analysis are to changes in the input variables.

Setting: A base on which a boiler or combustor sits, used to elevate a boiler. Houses the grates and primary combustion zone in a direct-burn system. Can form the connecting chamber in a multi-chamber system.

Shared savings: A form of energy project financing in which the party supplying the financing and/or installation gets a share of the dollar savings resulting from the reduction in energy consumption.

Simple payback: A method of economic analysis in which cost effectiveness is based on installed cost and first-year savings. Also refers to the number of years it takes an improvement to pay back the investment, computed by dividing the installed cost by the first-year energy savings.

Sizing: The process of specifying the size (measured in MMBtu/hour or MMBH) of a heating plant.

SOx: Oxides of sulfur. Air pollutants implicated in acid rain caused by combustion of fossil fuels. Modern wood systems have 1/6 the sulfur dioxide emissions of fuel oil.

Stack: The chimney of a combustion system.

Stack emissions: The components of the hot combustion gases (including particulates) exiting from the stack.

Stack temperature: The temperature of the combustion exhaust gases passing into the chimney. One indicator of appliance efficiency.

Steady state efficiency: See appliance efficiency.

Stem: The main trunk of a tree, exclusive of branches and top.

Stoker: An auger or other device for feeding solid fuel into the combustion zone.

Summer boiler: A small boiler sized to meet the summer or off-season heating load.

Suspension burning: A type of combustion in which fuel is blown into the combustion chamber, with some or all of the solid fuel particles burning in the air (in suspension).

TA study: Technical assistance study under the federal Institutional Conservation Program (ICP).

Tertiary air: Combustion air in addition to under-fire and over-fire air, injected downstream in the flame path to increase turbulence and aid in carbon burnout.

Tertiary heat exchanger: A heat exchanger that removes latent heat from the exhaust gases by cooling them below the condensation point.

Tramp air: Unintentional, uncontrolled air entering the combustion chamber.

Tramp metal: Metal found in biomass fuel (nails, chainsaw chain, tools, etc.).

Turn-down ratio: An index of the range over which efficient combustion can be achieved by a biomass burner. Calculated by dividing the maximum system output by the minimum output at which efficient, smoke-free combustion can be sustained (for example, with a maximum of 2.4 MMBtu and a minimum of .4 MMBtu, the turn-down ratio is 6:1).

Turnkey: For mechanical systems, a contracting process under which the contractor has full responsibility for design and for the complete installed package of work. The owner accepts the completed system once the contractor has demonstrated that the system meets the performance specifications.

Two-chamber system: A combustion system in which the primary combustion furnace, or combustor, is separate from the boiler, with the two connected by a constricted opening or a blast tube. The boiler combustion chamber forms the secondary chamber.

Ultimate analysis: Laboratory analysis that tells the percentage components of the elemental constituents of a fuel, including water and ash.

Under-fire air: Combustion air added under the grates. Serves the function of drying the fuel, cooling the grates, and supplying oxygen to the pyrolysis reactions.

Van: A delivery trailer (the trailer of the term tractor trailer).

Volatiles: Fuel constituents capable of being converted to gases at fairly low temperatures.

Walking floor trailer: See live-bottom trailer.

Wet scrubber: A flue gas particulate removal device that uses a water spray to capture and remove small, gas-entrained solid particles. Used only in very large biomass burners.

Whole-tree chips: Woodchips produced in the woods by feeding whole trees or tree stems into a mobile chipper, with discharge directly into a delivery truck.

Woodchips: Small rectangular pieces of wood (approximately 1” x 2” x 1/2”) produced by either a mill chipper or a whole-tree chipper.