The off-gas of the oxygen converter contains considerable chemical energy which is being increasingly utilized with off-gas collection. From the economic point of view, it would be advantageous in the converter process to return at least a part of this energy to the process. With full combustion of CO to CO2 and the utilization of this heat in the converter, sufficient energy would be available to increase the scrap rate to about 55 % of the total charge. But it is known that complete combustion of CO to CO2 cannot be reached in contact with liquid iron because of the required high oxygen potential, which then would significanty oxidize iron.
The energy obtainable by post combustion can thus only be utilized if the post combustion takes place in the gas space and the enormous amount of heat released thereby will be transferred from the gas phase back into the bath. If a larger portion of the post combustion energy remained in the off-gas, the off gas temperature would increase so drastically that the refractory lining of the converter would be destroyed.
The post combustion ratio in the converter is dependent upon the balance between the
oxidation of CO produced to CO2 and the reduction of CO2 to CO. It is considered that the post combustion ratio depends on the results of complicated heterogeneous reactions in a nonsteady state. Under the assumption that the main reactions of post combustion are the formation of CO2 by the reaction between the oxygen jet and CO in the atmosphere and the formation of CO by the reaction between CO2 produced as described above and carbon contained in the steel, the post combustion ratio was studied by Hirai. M, Tsujino.
Post Combustion in LD process
In the top blowing converter we have two problems, if the height of the lance is high the stirring is low and if the height of the lance is low the stirring is high but the control of post combustion is very difficult (the amount of post combustion is low).
The waste gases in a conventional BOF contain 80-90 % CO with the balance CO2 as a result of the primary decarburization reaction which is the basis of the BOF process. This large volume of CO is a substantial potential source of energy if a suitable method can be developed to combust this CO to CO% within the BOF vessel and transfer the heat liberated in the combustion reaction back into the metal bath.
Effect of Lance Height on Post Combustion
The lance height is the distance between the lance tip and the liquid surface before blowing.An increase of lance
height leads to a higher post combustion ratio.
It was observed by Gou H., Irons G.A. et al. that the oxygen lance height had a very significant influence on PCR and HTE. When the lance was raised from 4 to 5 m, the PCR increased from 16 to 23 %. In their trials, bath temperatures were not measured; the bath temperatures quoted in Table were measured in earlier trials in the same vessel. The blowing time in each heat lasted approximately 19 minutes; stable operation was achieved from the 5 to 15 minute marks. Three operational conditions were examined:
(i) a 4 m lance height early in the vessel campaign;
(ii) a 4 m lance height late in the campaign when the inner dimensions of the vessel were increased due to refractory erosion; and
(iii) a 5 m lance height, also late in the campaign.
In most studies, similar increases in PCR and decreases in HTE with increasing lance height have been noted. The conventional interpretation of this phenomenon is that more CO is entrained into the oxygen jet to increase PCR. For a given blowing rate, if the DCR (Decarburization Rate) increases, more primary oxygen is used to generate carbon monoxide; consequently, less primary oxygen is available to post combust the carbon monoxide to carbon dioxide.
Raising the lance height softens the blowing, so that less oxygen is transferred to the bath. This fundamental relationship has not been previously appreciated 131. The post combustion ratio is decreased by lowering a position of blowing of post combustion oxygen by elevating a position of the lance. Conversely, when the position of the lance is lowered, the post combustion ratio is decreased.