“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.
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