SMS group offers the tube and wire industry a broad range of reheating and heat treatment furnaces. The portfolio includes pusher furnaces for small billets as well as large walking beam furnaces for reheating of big size blooms or slab. Furthermore, SMS group also delivers a broad variety of rotary hearth furnaces for seamless pipe mills or forge shops, as well as provides a strong know-how in combustion technology. Most of these reheating furnaces make use of direct flame burners having typically a rating from 500 to 3,000 kW. With ever stricter environmental regulation and increasing need for environmental protection SMS group is always seeking to contribute by researching innovative technologies for containing the pollution generated by its furnaces. One of its efforts in this direction is the designing, prototyping, testing and patenting of a remarkable new type of low emission burner, now commercialized under the product name of SMS-ZeroFlame. This burner follows the steps of a long line of flameless burners present in the market since the last 20 years, but boldly surpasses the performances of any akin by offering unprecedented features. Flameless combustion occurs when fuel and comburent jets mix in the following conditions (referred to as natural gas combustion):
- Oxygen concentration in comburent jet is less than 13 volume percent.
- Temperature of the mixed jet is above 670 degrees celsius.The dilution of oxygen in the air jet is obtained by “entrainment”, that is the phenomenon that occurs when a free jet is developing in a gas volume: the free jet sucks particles of the external gas because of a momentum exchange with the still gas. The overall entrainment is called recirculation. Theory and experiments both show that an effective recirculation depends on the diameter of the jet nozzle, once that the temperatures of the two gases (the jet and the flue gas inside the furnace chamber) are set.
However, the flameless burners have some limitations, i.e.:
- There is an upper limit to the power that can be supplied by the burner.
- The flames produced by the burner are very long.
- The turn down of the burner is limited
SMS-ZeroFlame Burner
The SMS-ZeroFlame burner was developed specially to overcome the limitations of the flameless burners available on the market, providing the following performances:
- Stable low NOx flame at low furnace temperature.
- Flameless condition with ultra-low NOx emissions for furnace temperature above 750 degrees celsius.
- Booster function: increasing of the rating without increasing the flame length keeping the NOx emissions low.
- Flame length modulation: limitation of the flame length at any rating keeping the NOx emissions low.
- Increasing of turn down range.
The result is obtained by the integration of two different flames (see illustration below) having the following characteristics:
- An inner flame, swirled, visible and stable, produced in under- stoichiometric conditions.
- An external flame, over-stoichiometric, not visible and diluted.
Due to the swirl, the inner flame is shorter than the external one so, also if it supplies the same rating, the total length of the flames is defined only by the flameless condition. The SMS-ZeroFlame flameless design allows integrating these two flames having different characteristics and at the same time leaving the overall NOx production unchanged. This feature allows the burner to supply an overboost without increasing the flame length, by adding the inner flame to the flameless working mode. The flexibility of the burner allows a finer control with respect to a usual flameless burner that has a limited turndown.
Another innovation the SMS-ZeroFlame burner brings to the users is the possibility to control the flame length, making possible the installation of large-size burners even in small furnace chambers, and to supply more power without increasing the flame length.
By a precise modulation of the combustion air and fuel distribution between the two sub-burners it is possible to modify and control the flame length at the same rating (figure on the right). In the past, this feature was obtained by double air burners, but in that case the short flame, produced by a strong swirled external air, produced very high NOx emissions because, in any configuration, a stable flame was produced. The turndown can reach up to 25 percent of the total available rating by acting on the distribution of the flames, always favouring the flameless condition to minimize NOx emissions.
The new ZeroFlame burner of SMS group provides furnace operators with an effective tool for managing the combustion process with tremendous flexibility and outstanding performance.
