Reaction: A chemical reaction is a dissociation, recombination, or rearrangement of atoms.

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

Reburning : Reburning entails the injection of natural gas, biomass fuels, or other fuels into a coal-fired boiler above the primary combustion zone—representing 15 to 20 percent of the total fuel mix—can produce NOx reductions in the 50 to 70 percent range and SO2 reductions in the 20 to 25 percent range. Reburning is an effective and economic means of reducing NOx emissions from all types of industrial and electric utility boilers. Reburning may be used in coal or oil boilers, and it is even effective in cyclone and wet-bottom boilers, for which other forms of NOx control are either not available or very expensive.

Recombinant DNA: DNA that has been artificially introduced into a cell, resulting in alteration of the genotype and phenotype of the cell, and is replicated along with natural DNA. Used in industrial micro-organisms to produce more productive strains.

Recovery boiler: A pulp mill boiler in which lignin and spent cooking liquor (black liquor) is burned to generate steam.

Reforestation:  Reestablishing a forest by planting or seeding an area from which forest vegetation has been removed.

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.

Refractory lining: A lining, usually of ceramic, capable of resisting and maintaining high temperatures.

Refuse-derived fuel (RDF): Fuel prepared from municipal solid waste; noncombustible materials such as rocks, glass, and metals are removed, and the remaining combustible portion of the solid waste is chopped or shredded.

Regeneration Cut: A cutting strategy in which old trees are removed while favorable environmental conditions are created for the establishment of a new stand of seedlings.

Reproduction: (a) The process by which young trees grow to become the older trees of the future forest. (b) The process of forest replacement or renewal through natural sprouting or seeding or by the planting of seedlings or direct seeding.

Residual Stand: Trees left in a stand to grow until the next harvest. This term can refer to crop trees or cull trees.

Residues: Bark and woody materials that are generate in primary wood-using mills when round wood products are converted to other products.

Residuum (plural residua, also known as resid or resids): The nonvolatile portion of petroleum that remains as residue after refinery distillation; hence, atmospheric residuum, vacuum residuum.

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.

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.

Renewable diesel : Defined in the Internal Revenue Code (IRC) as fuel produced from biological material using a process called "thermal depolymerization" that meets the fuel specification requirements of ASTM D975 (petroleum diesel fuel) or ASTM D396 (home heating oil). Produced in free-standing facilities.

Renewable energy resource: An energy resource that can be replaced as it is used. Renewable energy resources include solar, wind, geothermal, hydro, and biomass. Municipal solid waste (MSW) is also considered to be a renewable energy resource.

Renewable Fuel Standards: Under the Energy Policy Act of 2005, EPA is responsible for promulgating regulations to ensure that gasoline sold in the United States contains a minimum volume of renewable fuel. A national Renewable Fuel Program (also known as the Renewable Fuel Standard Program, or RFS Program) will increase the volume of renewable fuel required to be blended into gasoline, starting with 4.0 billion gallons in calendar year 2006 and nearly doubling to 7.5 billion gallons by 2012. The RFS program was developed in collaboration with refiners, renewable fuel producers, and many other stakeholders.

Renewables Portfolio Standards/Set Asides : Renewables Portfolio Standards (RPS) require that a certain percentage of a utility's overall or new generating capacity or energy sales must be derived from renewable resources, i.e., 1% of electric sales must be from renewable energy in the year 200x. Portfolio Standards most commonly refer to electric sales measured in megawatt-hours (MWh), as opposed to electric capacity measured in megawatts (MW). The term "set asides" is frequently used to refer to programs where a utility is required to include a certain amount of renewables capacity in new installations.

Reserve margin: The amount by which the utility's total electric power capacity exceeds maximum electric demand.

Return on investment: (ROI) The interest rate at which the net present value of a project is zero. Multiple values are possible.

Rotation: Period of years between establishment of a stand of timber and the time when it is considered ready for final harvest and regeneration.

Rotten tree: A live tree of commercial species that does not contain a saw log now or prospectively primarily because of rot (that is, when rot accounts for more than 50 percent of the total cull volume).

ROI: Return on investment.

Rough tree : (a) A live tree of commercial species that does not contain a saw log now or prospectively primarily because of roughness (that is, when sound cull, due to such factors as poor form, splits, or cracks, accounts for more than 50 percent of the total cull volume) or (b) a live tree of noncommercial species.

Round wood products: Logs and other round timber generated from harvesting trees for industrial or consumer use.

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