Habitat: The area where a plant or animal lives and grows under natural conditions. Habitat includes living and nonliving attributes and provides all requirements for food and shelter.

Hammermill : A device consisting of a rotating head with free-swinging hammers which reduce chips or wood fuel to a predetermined particle size through a perforated screen.

Hardwood: One of the botanical groups of dicotyledonous trees that have broad leaves in contrast to the conifers or softwoods. The term has no reference to the actual hardness of the wood. The botanical name for hardwoods is angiosperms. Short-rotation, fast growing hardwood trees are being developed as future energy crops. Examples include: Hybrid poplars (Populus sp.), Hybrid willows (Salix sp.), Silver maple (Acer saccharinum), and Black locust (Robinia pseudoacacia).

Hazardous fuels reduction (Forest fuels treatments/thinning): Forest management treatments, often within or near wildland-urban interface zones, designed to reduce wildfire threat to people, property, or structures.

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 rate: The amount of fuel energy required by a power plant to produce one kilowatt-hour of electrical output. A measure of generating station thermal efficiency, generally expressed in Btu per net kWh. It is computed by dividing the total Btu content of fuel burned for electric generation by the resulting net kWh generation.

Heat transfer efficiency: useful heat output released / actual heat produced in the firebox.

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

Heating value: Higher heating value (HHV) is the potential combustion energy when water vapor from combustion is condensed to recover the latent heat of vaporization. Lower heating value (LHV) is the potential combustion energy when water vapor from combustion is not condensed.

Heavy (crude) oil: Oil that is more viscous that conventional crude oil, has a lower mobility in the reservoir but can be recovered through a well from the reservoir by the application of secondary or enhanced recovery methods.

Hectare: Common metric unit of area, equal to 2.47 acres. 100 hectares = 1 km.

Heteroatom compounds: Chemical compouds that contain nitrogen and/or oxygen and/or sulfur and/or metals bound within their molecular structure(s).

Hemicellulose: Hemicellulose consists of short, highly branched chains of sugars. In contrast to cellulose, which is a polymer of only glucose, a hemicellulose is a polymer of five different sugars. It contains five-carbon sugars (usually D-xylose and L-arabinose) and six-carbon sugars (D-galactose, D-glucose, and D-mannose), and uronic acid. The sugars are highly substituted with acetic acid. The branched nature of hemicellulose renders it amorphous and relatively easy to hydrolyze to its constituent sugars compared to cellulose. When hydrolyzed, the hemicellulose from hardwoods releases products high in xylose (a five-carbon sugar). The hemicellulose contained in softwoods, by contrast, yields more six-carbon sugars.

Herbaceous plants: Non-woody species of vegetation, usually of low lignin content such as grasses.

Herbaceous energy crops: Perennial non-woody crops that are harvested annually, though they may take two to three years to reach full productivity. Examples include: Switchgrass (Panicum virgatum), Reed canarygrass (Phalaris arundinacea), Miscanthus (Miscanthus x giganteus), and Giant reed (Arundo donax).

Hexose: Any of various simple sugars that have six carbon atoms per molecule (e.g. glucose, mannose, and galactose.)

HFCS: High fructose corn syrup.

Higher heating value (HHV) (or gross calorific value or gross energy or gross heat) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor such as steam produced to recover the latent heat of vaporization. More specifically, it is the heat produced by combustion of one unit of substance at constant volume in an oxygen bomb calorimeter under specified conditions. The conditions are: initial oxygen pressure of 2.0-4.0 MPa (20-40 atm), final temperature of 20°-35°C, products in the form of ash, liquid water, gaseous CO2 and N2, and dilute aqueous HCl and H2SO4. It is assumed that if significant quantities of metallic elements are combusted, they are converted to their oxides. In the case of materials such as coal, wood, or refuse, if small or trace amounts of metallic elements are present, they are unchanged during combustion and are part of the ash. see Lower heating value.

High Grading : A harvesting technique that removes only the biggest and most valuable trees from a stand and provides high returns at the expense of future growth potential. Poor quality, shade-loving trees tend to regenerate and dominate high- graded sites.

Hog: A chipper or mill which grinds wood into an acceptable form to be used for boiler fuel.

Holocellulose: The total carbohydrate fraction of wood; cellulose plus hemicellulose.

Horsepower (electrical horsepower; hp): A unit for measuring the rate of mechanical energy output usually used to describe the maximum output of engines or electric motors. 1 hp = 550 foot-pounds per second = 2,545 Btu per hour = 745.7 watts = 0.746 kW.

Hybrid: The offspring of genetically different parents The term is applied as well to the progeny from matings within species and to those between species. Hybrids combine the characteristics of the parents or exhibit new ones.

Hydrocarbon: An organic compound that contains only hydrogen and carbon.

Hydrocarbonaceous materials: A material such as bitumen that is composed of carbon and hydrogen with other elements (hetero elements) such as nitrogen, oxygen, sulfur, and metals chemically combined with the structures of the constituents; even though carbon and hydrogen may be the predominant elements, there may be very few true hydrocarbons (qv).

Hydrocracking: A process in which hydrogen is added to organic molecules at high pressures and moderate temperatures; usually used as an adjunct to catalytic cracking.

Hydrodesulfurization: The removal of sulfur by hydrotreating (qv).

Hydrogenation: Treatment of substances with hydrogen and suitable catalysts at high temperature and pressure to saturate double bonds.

Hydroprocesses: Refinery processes designed to add hydrogen to various products of refining.

Hydrolysis: The conversion, by reaction with water, of a complex substance into two or more smaller units, such as the conversion of cellulose into glucose sugar units.

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

Hydrotreating: The removal of heteroatomic (nitrogen, oxygen, and sulfur) species by treatment of a feedback or product at relatively low temperatures in the presence of hydrogen.

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