Glass Melting Technology: A Technical and Economic ... - OSTI

periods. Natural gas is the preferred fossil fuel for glass melting in the United States, with heat content ranging
from 900 to 1000 Btu/ft 3 . Light to heavy fuel oil has heat content between 135,000 and 155,000 Btu/US gal.
Heavy fuel oil (#5, #6) is viscous at low temperatures and must be heated before being fed to burners, where it
is atomized with compressed air for combustion or mechanically atomized to save 7 percent energy by avoiding
heating the cold atomizing air to flame temperature.
Gas-fired regenerative furnaces are about 20 to 35 percent or as high as 41 percent efficient when comparing
actual energy consumption to theoretical requirements. Container furnaces are higher efficiency than the Color
Television (CTV) furnace. When oxygen is substituted for air, oxygen reduces the fuel required to melt a unit of
glass. For a well-engineered soda-lime glass furnace, fuel reduction with conversion to oxy-fuel is typically 10
to 15 percent. Oxy-fuel firing can reduce energy consumption by eliminating the majority of the nitrogen from
the combustion atmosphere and reducing the volume of waste gas emissions by 60 to 80 percent. Energy
consumption can be reduced because the atmospheric nitrogen does not have to be heated to the temperature of
the flames, and a lower volume of hot combustion products exit the furnace.
However, the most flexible furnaces are electrically boosted, fossil fuel tank furnaces that use combined energy
sources rather than a single fuel. By directly applying electrical energy to molten glass by electrodes, glass is
melted more efficiently—2 to 3.5 times greater than by using fossil fuels. But production of electricity from
fossil fuel at the power plant is only about 30-percent efficient.
Electric furnaces lose less heat from the structure and have no costly regenerators or recuperators to repair or
replace. Electric furnaces are about 85 percent efficient due to the high thermal insulation of the batch (blanket)
on the melt surface. In addition the water-cooling jackets on molybdenum electrodes pull additional heat. The
first 3 percent of furnace energy applied nearly offsets these heat losses through electrode contacts. Less than 10
percent of melters are all electric after more than 70 years of application and 25 percent are oxy-fuel after only
14 years.
III.2. Traditional furnace designs
Heat to melt glass is provided by burning fossil fuels above a bath of continuously fed batch material and
continuously withdrawing molten, founded glass from a furnace. For melting and refining the glass, the
temperature depends on the formulation of the melt but is between 2372 and 2822˚F (1300 and 1550˚C) Heat
transfer is dominated by radiative transmission from the refractory superstructure that is heated by the flames to
up to 3002˚F (1650˚C), and from the flames themselves. In each furnace design heat input is arranged to
recirculate convective currents within the melted batch materials to ensure consistent homogeneity of the
finished glass that is fed into the forming process. The mass of molten glass in the furnace is held constant and
the mean residence time is about 24 hours of production for container furnaces or about 72 hours for some float
glass furnaces.
Traditional designs in operation in current glass manufacturing are regenerative, recuperative, oxy-fuel fired,
electric, mixed-fuel furnaces pot/day tank, unit melters. A furnace is chosen based on
the requirements of a glass manufacturer.
• Regenerative furnace
More than 50 percent of industrial glass furnaces in the US are regenerative furnaces. The maximum
theoretical efficiency of a regenerator is 80 percent because the mass of waste gases from a furnace exceeds that
of the incoming combustion air and the heat capacity of exhaust gases exceeds that of combustion air. The
efficiency of the furnace is limited by cost and structural losses are greater as the size of regenerators increases.
A regenerative furnace design with greater than 70 to 75 percent efficiency is difficult to conceive.
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