In a gas-fired furnace heating application, the first step is to analyse the different technologies available. Two of the most interesting technologies are regenerative burners and oxy-fuel burners.
Combustion fundamentals
The most common fuel used in furnaces is natural gas, which is more than 90% methane (CH4). Air, consisting of approximately 20% oxygen (O2) and the rest nitrogen (N2), is usually used as a combustion agent.
The reaction of natural gas with air requires approximately 10 volumes of air for every 1 volume of natural gas, reacting with the free O2 and transforming it into carbon dioxide (CO2) and water vapour (H2O), generating heat, leaving the N2 in the air free and hot.
Comparison of oxy-fuel combustion with regenerative fuels
From a thermodynamic point of view, oxy-fuel combustion is more efficient, as we do not have to heat the inert N2. However, regenerative combustion considers heat recovery, taking advantage of the high exhaust gas temperature and depends on the heat recovery efficiency of the regenerative system.
Oxy-gas burners have an optimal efficiency, as they use only oxygen as comburent. The oxygen/natural gas ratio is approx. 2:1, compared to the air/natural gas ratio, which is 10:1, i.e. 5 times less flue gas volume, as we do not have to heat the nitrogen in the air, and therefore NOx emissions are practically negligible.
In addition, the addition of nitrogen from the air to the process not only poses environmental problems, but also increases fuel costs.
Control with oxy-fuel burners is simpler, with smaller pipes and valves, without requiring supply or exhaust fans; whereas, with regenerative burners it is more complex with hot gas regeneration cycles and requires more space and periodic maintenance service.
It is true that the availability of oxygen suppliers nearby and reliable, with prices that we must analyse and put in the balance of the project.
Application in aluminium recycling
In the case of aluminium recycling, considering that the material load may contain a percentage of volatile organic elements (paints, lacquers, oils, …), the stoichiometric combustion is not sufficient for the incineration of additional combustible elements, generating flames and uncontrolled emissions, for which an addition of extra oxygen is necessary.
With regenerative burners, it is normal to add an oxygen lance that is active during the process of incineration of volatiles and, with Oxy-gas burners, the combustion ratio is regulated, controlling with an O2 probe at the flue gas outlet.