“What do you mean mill-scale formation would reduce if I start
using Emulsified Fuels, I sell the
mill-scale!!”
But at what rates? I
asked.
“I always get at least 800 Rupees a ton,” boasted the Executive
Director . “Well considering that steel ingots cost about Rs.25,000
per ton, wouldn’t it make more sense to cut down on waste generation,
rather than selling it at 3% of its value?”
Silence…
Sometimes, the logic and fallacies governing Indian business
practices defy all convention, so visiting a new client I am always
mentally prepared to hear the most outlandish of claims or experience
the most bizarre of procedures. The above conversation was held at the
office of a mid-sized SME Steel rebar manufacturer and my pitch was how
converting to Emulsified HFO would save them far more money by reducing
metal oxide losses in their process than via fuel economy. After all,
metal loss through burning and scaling comprises a major loss in any
steel and non-ferrous re-heating furnace.
People outside the metal industry are mostly unaware that heating and
reheating of most metal creates a significant amount of waste
comprising of different metal oxides. In ferrous industries the term
commonly used for such oxides is Mill-Scale, in non-ferrous plants like
Zinc and Aluminum it is called Dross.
Here by their own admission, the company was generating over 5 metric
tons of mill-scale per day amounting to a loss of Rs.1,25,000/-(almost
2000 USD) in raw material cost. That’s $ 60,000 per month, of which
they proudly recovered back a little over 3%.
Mill-scale consists of iron oxides like FeO and Fe2O3 which first
form when ingots or billets are re-heated to red-hot temperatures inside
the furnace and later again when they are rolled into different
sections and profiles. The scales comprise of flakes of metal that fall
off in slivers ranging from a few millimeters to several inches in
length and thickness. In an average steel re-heating and rolling
operation the quantum of mill-scale generated is close to 3% of its
daily process capacity though it can go up as high as 7%.
The metal scaling that takes place during the rolling process due to
the exposure of the red-hot steel to air is what actually should be
termed as mill-scale. And unfortunately not much can be done to reduce
its formation as it would involve purging the entire area with a fluxing
gas; a very costly and technically impractical process.
The scale that form inside the furnaces due to the prolonged exposure
of the steel to extremely high temperatures and atmospheric oxygen is
correctly known as Furnace-Scale. Reducing the peak temperature and
stringent control of combustion and excess air inside the furnace can
reduce this type of scale formation, through it is an absolute nightmare
to implement real life.
A similar process takes place in an Aluminum and Lead furnaces where
oil fired burners are used for re-melting the metal or for holding the
molten metal till the time it is poured or cast. Here, instead of
forming flakes, the oxide layer forms as a thick semi-solid layer on top
of the molten metal. Various chemical fluxes are used to reduce the
dross formation, nonetheless a considerable amount of metal is lost to
forming oxides. However, a positive differentiation between the two
types of metallic oxides is that unlike steel scale, dross fetches a
descent price in the market, though still a fraction of that of finished
metal.
It is also an easily provable fact, that combustion of Emulsified
Fuel in the metal reheating process drastically reduces mills-scale and
dross formation. In fact, following the instantly measurable reduction
of NOx levels and spike in O2 levels in the flue gases, the next easiest
quantifiable aspect directly attributable to usage of emulsified fuels
is the reduction of metal scaling and dross.
This phenomenon is accomplished by a simple factor: emulsified fuel needs extremely low levels of
Excess Air because of the complete combustion of fuel due to
Secondary Atomization. So combustion reduces most of the oxygen leaving very little for the purpose of oxidization.
Secondly, the super heated steam released by the emulsified fuel
combustion ensures that the flame’s peak temperatures doesn’t exceed
1300 degrees and the temperature spread is uniformly distributed with no
hot-spots. This limitation of temperature is also what inhibits oxide
formation both on the metal as well as in gas. So the reduction of
metallic oxides losses always goes in tandem with decline in NOx levels
in the flue gas.
Lastly, the traces of steam quickly radiates out the latent heat due
to having a higher thermal transfer co-efficient than air and so it
cools down slightly, increasing in density and settles down acting like a
flux insulating the heated metal from air of more specifically oxygen.
After converting a 100 Ton per day rebar mill to emulsified HFO, the
amount of burning loss and mill-scale fell down from 3.5 % to 1.6 %.
Moreover, instead of the 2-3 mm thick and 30-40 cm long slivers
constantly peeling off steel ingots earlier, we found only tiny
mica-like flakes not exceeding .06 mm in thickness.
In terms of cost calculations, considering today’s average price for
steel ingots at Rs 25,000, the reduction of mill-scale by almost 2% in a
100TPD rolling mill, amounts to a savings of Rs. 47,500 per day or
22,619 USD per month.
And now if we calculate only in terms of fuel saving, considering the
industry average usage of 36 litres of HFO per metric ton of steel, at
Rs.25,000 per KL, the daily consumption of fuel would be 3600 litres
costing Rs. 90,000. Now considering the( highly unlikely) best case
scenario of gaining a maximum of 10% savings in fuel consumption by
switching over to Emulsified HFO, would only save Rs.9000 per day or
4286 USD per month.
Switching over to Emulsified fuels in metallic furnaces saves way
more money when considering the reduction in waste oxide formation than
in fuel saving. Besides it’s way easier to measure and quantify.
Its a pity the Executive Director from the beginning of the article,
failed to see the logic till the very end or may be he just didn’t want
to loose of his Rs 800 per ton of mill-scale monthly revenue.
Ravi Deka is an energy management adviser and one of the pioneers in introducing Fuel Emulsification concepts in the country.
www.rade.co.in
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