Furnace combustion and burners family
Three different models of heavy oil high pressure burners are available : models 300L, 400L and 500L.
They are three different generations of burners. All require high pressure air to fracture the oil in very small droplets which provides a more effective combustion. The burners are also designed to obtain the desired smaller and shorter flame which is kept inside the burner port preventing radiation heat to reach the pellets.
The energy required for firing the pellets in the indurating furnace comes from two sources. The pellet composition provides a large part of the fuel either with the addition of carbon in the pellet material or from the exothermic reaction coming from the change of phase of the ore in the firing process (for instance with magnetite ore).
In the travelling straight grate process, the second source of heat comes from fossil fuel injected through burners located in burner ports on both sides of the furnace. Fuel could be in different forms but the most common ones are liquid fuel (light oil or more frequently heavy fuel oil) or gaseous fuel (natural gas or CO gas coming from other processes of the steel making facilities).
Metal 7 focused on heavy fuel oil firing, which at first made sense given all the Canadian pelletizing plants operated with this fuel.
At the end of the 1980’s, the only combustion and burner technology available for straight grate furnaces using liquid fuel was the low pressure atomizing burners. Others had tried high pressure atomizing burners but they had not been successful.
Low pressure burners are not efficient at all. They require a large volume of air for their operation. The air cools the outer tube of the burner. It atomizes the oil in the furnace and shapes the flame. It is claimed the air is also required for combustion purposes but it could be questioned since the indurating furnace operates in excess of air and oxygen.
The air fed to the burners is cold air. Consuming a large volume of this air has a negative impact on the energy consumption of the furnace has a significant volume of energy is required just to heat this cold air.
Plus the low pressure burners do not atomized satisfactorily the oil. Its stays in large droplets and it is common to see unburn oil reaching the top of the pellet bed degrading the quality of the pellets in the upper layers of the bed.
The flame shape of low pressure burners is also terrible. It is not stable. It is long and large. With this technology, it is common to see a large part of the flame protruding out of the burner port. That is not good. It is necessary to keep the flame itself far from the pellet bed. Pellets have to be fired with hot gases, by convection heat and not by the radiation heat emanating from the flame.
A large and long flame also has a detrimental effect on the refractory material in the burner ports and furnace walls/ceiling. The appearance of hot spots on the furnace created by damage to the refractory is an major cause for stoppage of the furnace and pelletizing line. Cold shutdowns are required for the repair in the refractory causing major production losses.
Metal 7 oriented its research toward the development of new oil burners that address all the flaws in the low pressure burners.
Its first generation of burners was introduced by Metal 7 in 1989, the model 300L high pressure burners. Through this combustion/burner technology and the following model 400, the company emerged has the leader of high pressure oil burners for straight grate induration process. Most of the pelletizing plants using oil as a fuel have converted from the former low pressure to Metal 7’s high pressure burners.
In 2016, Metal 7 innovated yet again in launching its model 500L burner, which is again and upgrade of the previous generations consuming even less air for its operation.
Features of Metal 7’s burner technology:
a- We use compressed air for the atomization which transforms the oil in very fine droplets. This is the state-of-the-art in maximizing efficient combustion with a lower quantity of oil consumed per ton of pellets;
b- Reduce consumption of cold air atomizing the fuel and cooling the burner. This is one of the main contributions of Metal 7 burner technology. The lower the amount of cold air fed to the furnace, the more energy-efficient the pellet firing process will be. The reduction of the consumption of air is also the main difference between the 300L and the 400L and between the 400L and the new 500L.
c- It does exactly what those burners should do. It produces a short and narrow flame heating the air coming from the downcomers to bring it to the required temperature for the firing process. Pellets are not exposed to the flame. The flame shape is also optimized to protect the refractory in the burner port and the furnace;
d- Self-cleaning nozzle design. The coking of oil on the tip of the burner is frequent with other technologies. This has been addressed in our design. Furthermore, the design of the nozzle prevent clogging or the return of oil in the air line;
e- Turn down ratio of 7:1 or more which enables us to use only one model of nozzle for every zones of the furnace. Furthermore, the ratio could be increased further by using an air/oil control system such as those provided by Metal 7.
f- The burners fit in all existing installations. Other than bringing compressed air to the burner, the conversion to Metal 7’s combustion/burner technology does not require modifications to the furnace or burner ports. Plus, the design is very flexible and would be adapted to the requirement of your furnace. It would be sized to your energy requirement and the required flame diameter and length;
g- The expertise of Metal 7 in material and surface engineering contributes to the development of the selection of most effective heat resistant materials for the fabrication of all the critical parts.
Metal 7’s engineering group supports its customer in the implementation of the new technology. They also offer control valve panels providing extra constant regulation of the air/fuel ratio to optimize combustion and fuel efficiency at all operation regime. The improved control provided by the panels gives extra benefits in increasing, for instance, the turn down ratio of the burners.
A- Oil energy savings and/or increase in induration productivity
Depending on the existing burner technology in the plant and the generation of Metal 7 burners selected by the customer, savings of more than 12 % in oil consumption are attainable. The return on investment would be very rapid, and even more so given that the burner technology of Metal 7 can be retrofitted without much modification on the burner port or furnace equipment;
The gain in combustion/burner efficiency could also be used to boost production in the furnace, assuming the other department of the pelletizing plant could produce more green pellets and the customer market could absorb the extra output. In such case, the gains would be much higher in consideration of the lower production marginal cost for the extra tons of pellets.
B- Better quality of the pellets
Better firing of the pellets with a more appropriate combustion technology means improvements in the average pellet physical and metallurgical properties. That is particularly true for the upper layer of pellets of the pellet bed, and they are not exposed to radiation heat from the flame or to unburned oil;
C- Decrease of CO2 and NOx emission
Lower oil consumption per ton of pellets and better heat distribution improve CO emission. It is even more applicable to the new model 500L burner;
D- Higher availability of the furnace
Better flame shape and combustion control prevents damage to the refractory and creation of hot spots on the burner ports or furnace walls.
The burners require low maintenance. They are not subject to clogging, coking on the tip and returning of oil in the air line is not likely to happen.
Therefore, the conversion to Metal 7 combustion/burner technology has a positive impact on the induration furnace availability. This means a higher pellet output per year.
A study conducted for the evaluation of the savings with the conversion to 500L burners for a specific pelletizing demonstrated the following gains:
- Decrease of energy consumption: 496 MJ/H ( or a saving of US $ 725 000/year for this plant);
- Energy consumption reduction: 496 MJ/H ( or a saving of US $ 725 000/year for this plant);
- Total annual cost savings, combining energy savings and increases in productivity of the furnace: Over US $ 2,9 M/year.
In this example, the return on investment would be of few months...