Welcome to the Glass Age

114 For larger furnaces with higher pull rates, the higher volumes and lower wall losses make recuperators or regenerators sensible. Gas-fired furnaces can be cheaper than the efficient electric melter. This was historically so in most countries because electricity was generated from fossil fuels, and typically 2.5 to 3x more costly per kWh than the fuel alone. Even small electric furnaces are 70-85% thermally efficient. While a fuel fired furnace without a recuperator at a low pull is only 10% efficient, adding a regenerator improves efficiency to 45% and an oxy-gas fired furnace, can achieve 50% efficiency. Most common all-electric melters produced 10-30 TPD, sometimes up to 80 TPD. They were round or hexagonal to avoid heat losses via the furnace walls and to allow more easily distributed batch charging and electric connections. Figure 7.4 shows a larger rectangular melter at 80 TPD. These cold top electric melters used the batch cover as a heat insulating blanket, conserving heat inside the melt. They were called vertical melters, as the glass melts on the surface near the batch, refines at lower levels and flows out via a bottom throat into a working end/distributor. To maintain batch coverage and hence an insulating blanket, the cullet content was usually below 50%. Electric melters were mostly used for high quality clear glasses and crystal (lead) glasses, as the redox (color) control is best managed with this process. During the 1970 global oil crisis, some glass producers, especially in the United States converted their regenerative furnaces to all electric melters. They retained the infrastructure and horizontal configuration because other shapes were difficult to incorporate into their existing space; sidewall losses are less important at higher pull rates. The future of carbon free melting —electric, hydrogen or hybrid? Currently, 95% of all glass melting uses fossil fuels, mostly natural gas or heavy oil; but industries are now strongly encouraged to follow the Paris Climate Agreement guidelines and are seeking to minimize CO 2 emissions. Many but not all countries are enforcing rules, with penalties for carbon emissions and benefits for reductions. Either way, the glass industry knows its consumers expect low-carbon or carbon-free production, so are working to achieve this while remaining competitive amongst themselves and with other packaging materials. Four key technologies for carbon reduction exist, in addition to those already discussed. They are: • Cold top all electric vertical melting (AEM). • Hydrogen combustion (replacing natural gas in regenerative or oxy-gas furnaces). • Horizontal hot top electric melting (H 2 EM) also referred to as hybrid melting. • Horizontal hot top hydrogen electric melting (H 3 EM). The question is: What is the best solution —not just now— but for 2030? 2050? After 2050? Hydrogen Currently, truly green hydrogen produced by electrolysis using renewable electric energy is the first choice, but there is simply insufficient available. Even with low electric pricing, hydrogen at 6 € /kg is three times too costly to compete with natural gas. So, in most regions it would be uneconomic, without state subsidy. More research on hydrogen combustion is needed, specifically the effect on the molten glass and refractories of water concentrations approaching 100% in the combustion atmosphere. Certainly concentrations near 50% in the combustion atmosphere of oxy-gas furnaces created problems. Using electricity to break water into H 2 and O 2 by electrolysis is expensive and is only now reaching 70% efficiency levels. However, expectations are that investment costs should decline while efficiency continues to increase so that,