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Can Oxygen Enrichm ent Boost CupolaFurnace Production?
Modern Casting, August, 2001 by Seymour Katz
Oxygen enrichment has the potential to enhance cupola output and
lower melting costs, butother developments in cupolas through the
years also can stake a claim to increasedproductivity.
Oxygen (O) enrichment is a versatile tool that provides
important benefits for cupolaoperation. The benefits, reported for
a wide range of foundries, are listed in Table 1 and havebeen
recognized for more than 70 years. Not until the mid-1960s,
however, was the cost of Olow enough to make its use cost
effective. For the most part, foundries have pursued Obenefits
because of economic factors. The two major factors for O use are
the replacement ofcostly charge materials with less expensive ones
and the ability of low capital means toincrease cupola output. Thus
far, environmental factors, such as reduced greenhouse gasemissions
via reduced coke consumption, have not figured as strong incentives
for foundriesto further increase the use of O. This situation might
change in the future.
How Oxygen Enriches
All the benefits from O enrichment derive from the fact that
adding O to the blast air is thesame as increasing the blast air
rate, without adding extra nitrogen. Because less nitrogen is
in the gas phase, the heat produced from combustion of coke
increases the temperature of thegas. In turn, that increase
transfers heat to the iron drops as they pass through the melt
zone.Higher iron temperatures create the conditions that permit
improvements in a cupolaoperation.
The methods used to introduce additional O into the cupola are
classified into threecategories: diffuser enrichment, tuyere
injection and well injection. In the diffuser method, Ois
introduced upstream of the tuyeres through a diffuser installed in
the windbox. Withtuyere injection, injector tubes direct pure O
through each tuyere (or every other tuyere).The third and least
accepted method is well injection in which O is injected into the
coke bedbeneath the tuyeres using water-cooled injectors.
Modes of Enrichment
All the observed benefits of O enrichment cannot be realized
simultaneously. Four basicmodes of O enrichment define the nature
of the benefits that are obtainable. These are:
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* add O;
* add O and reduce coke;
* add O and reduce blast air;
* add O and reduce blast air and coke.
Table 2 relates the benefits found in Table 1 with each of the
modes listed above. The mostimportant benefits are highlighted in
red. Salient comments concerning the benefits andneeded precautions
are given below.
1 Add O--T his mode produces the highest iron temperatures and
largest gains in carbon (C)and silicon (Si) content. It also is
useful for melting at rates that exceed the capacity of the
airblast system.
2 Add O and reduce coke--This mode produces maximum melt rates
as it provides thehighest O-to-coke ratio. The iron temperature and
alloy recovery are lower than in theprevious case. The amount of
coke that can be removed from the charge in this mode is
limited. Although O raises the temperature in the melt zone, the
combustion of C supplies theenergy for melting. In a perfect
system, with complete combustion to carbon dioxide and noenergy
losses, about 5% coke is needed. Realistic values are much higher.
As the amount ofcharged coke is reduced, the cupola atmosphere
becomes more oxidizing and increasinglylarger amounts of the alloy
are oxidized. Eventually, the costs of the C and Si losses
weighheavily on the cost of increasing the melt rate.
Add O and reduce blast air--This mode of operation is useful
when holding a constant meltrate is necessary. A benefit of reduced
air volumes is a reduction on the load placed on the hotblast and
emission systems. Through its studies, the British Cast Iron
Research Assn.
(BCIRA) established that, at a given coke level, the total O
mainly determines the melt rate.Thus, replacing free air with
costly O must be accompanied by an application that
providessufficient savings to make the replacement attractive. If
considerable air is removed,attention needs to be given to the
consequence of poorer blast penetration and reducedenergy
production due to the generation of greater amounts of CO.
Add O and reduce blast air and coke-Coke rates can be reduced as
O is added to maintain aconstant iron temperature, which is
possible because of the higher combustion zone gastemperatures when
the blast is enriched with O. Depending on the relative amount of
blastair and coke removed, the melt rate can either increase or
decrease.
Early Oxygen Delivery
Early in the application of continuous O enrichment, gas
generally was introduced to the blastmain. The BCIRA conducted
extensive studies in the 1960s and 1970s that indicated thebenefits
were small compared to the introduction of the same amount of O
through eachtuyere (tuyere injection). This manner of introduction
had the benefit of eliminating O lossesdue to leaks in the blast
main and tuyere plumbing. More importantly, a
high-velocity,injected-O stream was able to penetrate deeper into
the cupola than the blast air, keepingcombustion away from the
cupola wall and reducing energy losses. In keeping with
thisphilosophy, O producers developed supersonic O injectors in the
1980s as a way to further
accomplish blast penetration.
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