How to Account for Carbon Reduction in Electric Melting

Whether emissions rise or fall when the same glass furnace is converted to electric heating often cannot be answered in a single sentence. The difference usually lies in which stages are included in the boundary. Agreeing the basis at the review meeting saves more time than arguing the conclusion first.

Emissions from flame melting are concentrated in fuel combustion and are relatively direct to measure; emissions from electric melting are transferred with purchased electricity and follow the grid mix. Put the two in the same table and the emission factors for combustion and for electricity, together with the certified output of each route, must be shown at the same time; otherwise there is no common basis. Molybdenum electrodes carry and distribute current in an electric melting furnace, and their condition affects power consumption; the mechanism is described further in the process technology section.

Book Combustion and Electricity Separately

Fuel is converted using actual consumption and net calorific value, electricity using purchased quantity multiplied by the applicable emission factor. If one side uses measured values and the other estimates, the two tables are not on the same basis. The more defensible approach is to list direct emissions and purchased electricity emissions in separate columns so that reviewers can see whether a difference comes from the process route or from the power supply.

Another common problem is the time basis. Fuel and electricity are often counted over different periods, one monthly and one per batch, and mixing them makes emissions per unit of product fluctuate irregularly. Results are stable only when both data sets cover the same batch, or when both are aggregated annually and then allocated.

The Denominator Is Certified Output, Not Feed Input

The denominator for emissions per unit of product should be certified output of the same specification. How the electric melting furnace performs in heating rate, refining time and temperature uniformity changes the electricity consumption per ton of glass melt; a few percentage points of yield variation can cancel out part of the apparent gain when converted to the unit product basis. Comparing actual output over the same production period and specification is closer to reality than using design capacity. The resistive loss in the electrode body depends on size and purity; at the selection stage, current density and loss range can be checked against the product specification pages.

Power Mix Changes the Direction of the Conclusion

Where the share of low-carbon electricity is high, electric melting has a clear emissions advantage; on a grid dominated by coal, that advantage is diluted. Results can differ between plants of the same group, or between years at the same plant. It is therefore more reliable to model the electricity emission factor as high, medium and low scenarios than to fix on a single number.

Tariffs also belong in the scenario. The operating cost of electric melting is sensitive to electricity price, so a lower-emission option is not necessarily economic, and the reverse also holds. Presenting emissions and cost side by side gives a more complete basis for the decision.

Items That Are Easily Left Out

1. Heat storage and losses of the furnace body: power consumption during cold start is often underestimated and should be allocated by start-up frequency.
2. Power supply losses: transformers, busbars and power control equipment each carry a stage of efficiency loss.
3. Auxiliary systems: the power drawn by cooling water circulation, dust collection and exhaust fans must be included.
4. Raw material conditions: changes in batch moisture content and particle size alter melting energy per ton.
5. Remelt losses: the repeated energy of reworking rejects should not be booked at zero cost.

Once boundary, denominator and power-supply scenario are stated clearly, the comparison has value for discussion.