Oxy-gas combustion is the perfect and most efficient combustion, compared to all other combustion applications. It is true that all other applications use oxygen from the air as a comburent and today, air is still free, but oxygen, being a by-product of air, requires processing and distribution, which you must be paid for.
Oxy-gas burners have an optimal efficiency, as they use only oxygen as comburent. The oxygen/natural gas ratio is about 2:1, compared to the air/natural gas ratio, which is 10:1, i.e. 5 times less fume volume, as we do not have to heat 79% of the nitrogen in the air, and therefore NOx emissions are practically negligible.
Figure 1 shows the percentage of thermal power available as a function of flue gas temperature at different oxygen levels. For example, at a temperature of 1095°C (2000°F) with standard air at 21% O2, the available heat is 46%. This means that, if the power supplied to the furnace is 1000 kW, only 460 kW are available; on the other hand, with 100% O2, we increase to 75%, 750 kW.
Also, the addition of nitrogen from the air to the process not only create environmental problems, increases fuel costs.
Figure 2 shows a furnace that, at 1095°C (2000°F), with 100% oxygen, instead of air, will consume approximately 38% less fuel. Less fuel burned is not only a reduction in the fuel bill; it also means less m3 of flue gas emitted. Reducing the volume of fumes reduces CO2 emissions into the atmosphere.
Reduced fuel consumption also brings other, more subtle benefits.
– As less combustion products enter the furnace, the residence time increases, increasing energy efficiency.
– Typically, furnaces are adjusted so that there is 1-2% excess oxygen in the flue gas. This is done to compensate for the variable calorific value of natural gas. If the excess O2 is not consumed in the combustion process, it remains available in the furnace for slag formation (AL203). If less fuel is needed, less oxygen is needed. This means that 1-2% of excess O2 is in fact less oxygen, which means less available oxygen passes through the furnace for slag formation.
In fact, Oxy-gas combustion appears to be an optimal situation; however, there are other considerations to consider:
– The flame temperature of a standard ambient air/gas burner is 1760°C, in contrast, an oxy-gas flame can reach temperatures higher than 2800°C.
– This high temperature achieves increased power and heating ratios, which often leads to increased production, for the same heating chamber.
– However, in the initial part of the melting cycle with a direct load process, convective heat transfer to the load is essential. The transfer rate is based on the volume, temperature and velocity of the gases. Oxy-gas combustion reduces the volume of gases considerably.
– A 38% reduction in gas consumption would normally be very attractive; however, this must be offset by the cost of oxygen. What is the cost per molten ton? Will the melting rate be affected?
– 0 ppm NOx is no practical, due to air intakes around doors, etc. This air contains nitrogen, which will produce some NOx.
Advantages of Oxy-Gas Combustion:
– Increased energy efficiency and fuel savings
– NOx reduction, particulates, CO, CO2 emissions
– CAPEX Cost saving equipment, no fans, smaller burner size,…
– Combustion gases reduction, with less fumes flow and filtration requiremenst.
– Regenerators and heat exchanger elimination
