In this chapter, the following terms have the meanings indicated. Terms Defined. “Boiler” means an enclosed fossil or other fuel-fired combustion device used to produce heat and to transfer heat to recirculating water, steam, or other medium. “Combined cycle system” means a system comprised of one or more combustion turbines, heat recovery steam generators, and steam turbines configured to improve overall efficiency of electricity generation or steam production. “Combustion turbine” means an enclosed fossil or other fuel-fired device: That is comprised of a compressor, a combustor, and a turbine; and In which the flue gas resulting from the combustion of fuel in the combustor passes through the turbine, rotating the turbine. “Fossil fuel” means natural gas, petroleum, coal, or any form of solid, liquid, or gaseous fuel derived from such material. “Fossil fuel-fired” means: The combustion of fossil fuel, alone or in combination with any other fuel, where fossil fuel actually combusted comprises more than 50 percent of the annual heat input on a Btu basis during any year; or For a new unit, the combustion of fossil fuel, alone or in combination with any other fuel, where fossil fuel is projected to comprise more than 50 percent of the annual heat input on a Btu basis during the year that the new unit begins combusting fossil fuel. “New unit” means: A non-trading large NOx unit that is installed after May 1, 2018; or An existing unit that is modified to meet the definition of §B(8) of this regulation. “New unit set aside” means remaining NOx ozone season emission tons available for new units. “Non-trading large NOx unit” means a fossil-fuel fired stationary boiler, combustion turbine, or combined cycle system unit with; A maximum design heat input greater than 250 mmBtu per hour; or A name plate capacity greater than 25.0 MW. “Ozone season” means May 1 through September 30 of any calendar year.