Hybrid furnaces combine the combustion of a fuel (mostly natural gas) with a highly increased proportion of electric power. For the combustion, an end-fired as well as a cross-fired setup can be used. This includes furnaces with regenerators, recuperator and oxyfuel combustion. If the overall electric share is aimed to be over 40 to 50 %, the latter is the preferred solution. Compared to their classic pendant with little to no boosting, the overall energy consumption is reduced by approximately 5 to 10 %, while CO2 emissions from combustion decrease 20 to 45 %.
HORN hybrid furnaces are specially designed to combine the classic heating method of combusting a fossil fuel with the usage of more electric power by using electrodes in the melting tank. The high electric share leads to a reduction of the fuel consumption and therefore to lower temperatures in the superstructure. In combination with a lower flue gas volume (even more for an oxyfuel furnace), this requires a modification of the superstructure design and possibly of the refractory materials to avoid cold spots and the subsequent damage.
In the melting tank, a high number of electrodes are necessary. They can be installed as side or bottom electrodes depending on steelwork, position of burners, glass composition, etc. and preferably form “convection zones” in the tank. In these areas, the typical convection is preserved, despite the influence of all the electrodes, to support the melting process.
To some degree, a hybrid setup offers flexibility regarding the energy contribution between fossil and electric and also for the pull rate.
Currently, hybrid concepts are used for furnaces with smaller pullout of 50 to 400 t/d and/or specialty glass, but in general the maximum furnace size is approximately the same as for the classic fossil furnaces (750 t/d and even larger).