November/December 2019 issue of Glass Worldwide
SMART FURNACE DESIGN TO MEET THE GLOBAL REQUIREMENTS OF PERFORMANCES (EMISSIONS, ENERGY CONSUMPTION)
The glass melting process is by nature an energy-intensive process. The specific energy consumption for melting of a typical container glass could be approximately estimated in the range of 3.8 – 5.8 MJ/kg molten glass, depending on melting technology and process/product requirements.
As melting energy accounts for about 60-70% of the total energy consumption in a container glass plant, it is obvious the necessity to develop and use energy-efficient technologies. Mainly natural gas or fuel oil are used for combustion in the glass melting process; sometimes, especially in furnaces producing colored glass, electric boosting is used in support of the main fuel.
The use of electricity as main energy source for melting process is used only in small furnaces and often in countries where it is generated from nuclear power or hydro-power plants. The energy from electricity applied via electrodes is more efficiently transferred to the glass compared to the energy released to the glass from the combustion; however, based on average energy efficiency of a power plant plus the transport losses, only about 40% of the fossil fuel energy contents will be converted effectively into electrical energy (the primary energy equivalent of 1 kWh is about 9 MJ). In terms of emissions, natural gas contains hydrocarbons, nitrogen and a small amount of carbon dioxide and no other impurities (sometimes could contain very small additions of mercaptans applied to give the natural gas the typical smell for safety reason).
Fuel oil instead contains several impurities as Vanadium, Nickel, Sodium, and Sulphur. The quantity of emissions, such as CO2and NOx, is related to the fuel consumption and resulting flue gas volume flows. Energy efficiency improvement will not only lead to lower energy costs but will also goals to lower release of pollutants to the atmosphere with a particular attention to the emissions of greenhouse gases such as CO2.
The energy demand of a glass melting furnace depends on various parameters such as furnace design, type of furnace, furnace insulation and also on the operating conditions (air excess in the combustion process, the composition of the glass, recycled cullet level in the batch, etc..).
The flue gas heat recovery system is an important matter for the energy consumption of a glass furnace; almost all glass plant using air-fired glass furnaces are equipped with a combustion air preheater using the heat contents of the exiting combustion flue gases. Obviously, the energy consumption of glass furnaces increases with the age of the furnace due to increased cooling of the sidewalls, increasing open joints and leakage, worsening of the efficiency to the heat recovery system (i.e. fouling of the regenerator checkers), deterioration of the insulation and also the application of repairs during the furnace campaign have an impact.
End-port fired regenerative furnaces equipped with well-insulated regenerator chambers can preheat the combustion air up to more than 1250°C and are actually the most energy-efficient furnaces types. The flue gases at regenerators’ exit have a temperature of 450 – 550°C, with residual heat which can be used to preheat the cullet or batch (up to 250 to 375°C with energy-saving up to 15%) or for other secondary uses (i.e. HVAC).
Cross-fired regenerative furnaces appeared, on average, less energy efficient than end-port fired regenerative glass furnaces; it could be due to the structural heat losses of the burner ports and relatively large outside regenerator surface area. Glass furnaces equipped with recuperative system reach a much lower air preheat temperature, about 400 – 750 °C, so generally, demand a much higher specific energy supply.
Oxygen-fuel fired furnaces results much more efficient compared to recuperative furnaces but, considering the energy consumption for oxygen production (cryogenic distillation or VPSA system), appears less efficiency (and more expensive) compared to end port air-fuel fired (5-10% less efficient in average). However, new oxygen-fired furnace design, for instance, equipped with a batch pre-heater system, present energy efficiencies similar to end-port fired regenerative furnaces, while on environmental point of view present lower impact on NOx emissions.
Melting glass furnaces that use an high level of waste glass cullet in the batch show much lower specific energy consumption levels compared to furnaces with a low waste glass cullet ratio; on average, the specific energy consumption decreases about 0.28 – 0.3% of the specific energy consumption of a normal batch (not using waste glass cullet) for a 1% increase of waste glass cullet in the batch. In the case of flint glass production the use of recycled post-consumer glass is limited due the impurity that may contains which may lead to glass color deviations, but for the production of green or amber glass cullet levels in the batch may exceed the 80% of the entire mass.
Author: Gabriele Campani, Project Engineer Falorni Tech