The goal was to reduce natural gas usage and pollutant output while maintaining thermal and operational characteristics of the furnaces used at the United States Mint’s plants in Philadelphia and Denver.
To achieve this goal, the protective atmosphere used in the annealing process was switched from the traditional exogas generated by natural gas combustion to a mixture of high purity gas at a ratio of 97% nitrogen to 3% hydrogen. Both are created in cabinet generators near the furnaces, utilizing electricity and water provided by the local utility company.
The result has been an overall reduction of the output of carbon dioxide and other greenhouse gas emissions by 2,000 metric tonnes per year. As the Mint’s power providers continue to transition to renewable sources, the reduction of greenhouse gases is set to increase by as much as a further 900 metric tonnes per year.
It has also resulted in a reduction of energy required to ventilate the production area by eliminating the production of carbon monoxide (which is a byproduct of natural gas combustion to generate the exogas atmosphere).
In addition, the furnaces were retrofitted with higher efficiency recuperative burners to further reduce natural gas consumption, down by 50% from 1.415 million cubic meters to 708,000 cubic meters.
A further benefit has been an increase in blank quality, since the nitrogen/hydrogen atmosphere reduces the amount of oxides forming on the blanks during the annealing process. This in turn has reduced the use of chemicals during the post -processing and cleaning of the coin blanks and hence less wastewater output.
Safety at the facilities has also improved due to the elimination of flammable environment and the reduction of carbon monoxide generated.
