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

Furnace emissions are reduced and thermal efficiency is very high in electric furnaces, but wider use has been
limited by operating costs and technical considerations. Electric melting can only be installed in a furnace at
rebuild. Higher alkali insulating wool fiberglass can be produced in cold-top all-electric furnaces up to 200 tpd,
but has been determined to be uneconomical and not technically viable. One float glass plant in the United
Kingdom experimented with an electric furnace to demonstrate the principle of cold-top electric melting. On a
pilot scale, the plant was successful for producing a range of exotic glasses for which the emissions would have
been difficult to control in a conventional furnace. The experiment also showed that it is not economically
viable to operate a full-scale float glass line (>500 tpd) with electricity.
Based on current practice, electric furnaces may be viable for continuous operations according to the following
guidelines:
• furnaces below 75 tpd are generally viable;
• furnaces in the range of 75 to 100 tpd may be viable in some circumstances;
• furnaces greater than 150 tpd are generally unlikely to be viable.
The viability of choosing electricity as a fuel source depends mainly on the price differential between electricity
and fossil fuels. Average electricity costs per unit of energy are two to three times the cost of fuel oil and can
vary up to 100 percent from region to region. Electric furnaces are thermally efficient, two to four times better
than air-fuel furnaces, and can be more competitive than air-fueled furnaces in the 15 to 100 tpd range of
throughputs because of their lower specific heat losses. Site specific factors can also influence a decision to use
an electric melter, such as prevailing energy costs; product quality requirements; available space; costs of
alternative abatement measures; prevailing legislation; ease of operation; and the anticipated operating life of
alternative furnaces. To compare costs, 292 Kw is one million Btu. At 5 cents per Kw a million Btu of electric
costs $14.60, and this is higher than most natural gas priced from $6 to 7 per million Btu’s. Looking at the
multiple of efficiency, with electric being near 85 percent, makes it affordable with increasing gas prices.
However, $.05 per Kw is becoming a low number as well.
Completely replacing fossil fuels as a heat source for a glass-melting furnace eliminates such combustion
products as oxides of sulfur, thermal NOx and carbon dioxide. Other emissions arise from particulate carryover
and decomposition of batch materials, particularly CO2 from carbonates, NOx from nitrates, and SOx from
sulfates.
The emission of volatile batch components is lower than in conventional furnaces due to the reduced gas flow
and the absorption, condensation, and reaction of gaseous emissions in the batch blanket, which usually covers
the whole surface of the melt. Furnaces are usually open on one side and gaseous emissions and heat from the
melt cause air currents. Some form of ventilation is needed whether by natural draft or extraction to allow dust,
gases, and heat to escape without entering the workplace.
The waste gas emitted by natural draft will be very low volume but may have high dust and chemical
concentrations (chlorides, sulfates, NOx and other toxic vapors) and poor dispersion characteristics. Dust
emissions can be controlled by extraction to a dust abatement system, which is usually a bag filter due to the
low volume involved, allowing for low dust emissions and treatment of halide emissions by dry scrubbing if
necessary.
Although electric furnaces have lower capital costs than conventional furnaces, which when annualized
compensate partially for the higher operating costs, the furnaces have shorter campaign lives. They require
rebuild or repair every two to six years, compared to five to 14 years for conventional furnaces.
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