By Nirmalya Deb
The coronavirus pandemic has hit the manufacturing sector hard, not just because of the scarcity of manpower during the prevailing lockdown period but also because demand has hit rock bottom with the construction, automobiles and consumer goods sectors being thrown o? gear.
Blast furnace (BF) operators, as a result, have been compelled to slash production. Manufacturers have also been compelled to idle assets or opt for plant shutdowns. SteelMint focuses on the technical aspects related to shutting down or restarting BFs –and what it means for Indian steel makers – at a time when consumption has hit a nadir and the lockdown-imposed stagnation continues to throttle the market.
To shut down or not…
“First of all it is important to understand the implications of stopping a blast furnace,” says a senior engineer at an integrated steel plant. “BFs produce gas batteries and the gas is very important for the entire plant. They consume about 35% and the rest 65% goes to the coke ovens, rolling mills and boilers. Of course, there are replacements, but they are costly and qualitatively inferior to BF gas. BF gas is clean and ideal for hot strip mills and coke ovens. So the many impacts of stopping a BF are felt across the plant whether it’s sintering, coke making or the steel melt shop as well as the di?erent mills and total despatches.”
However, market conditions often weigh in on the uninterrupted functioning of BFs. “Recently US Steel has announced that it is not restarting operations at two of its big BFs. One of them was due to come back after repair but they are not restarting it. This is an example of idling, ie, not using functional assets with an eye on weak market conditions. But shutting down has its own technologies, with a revival plan and the costs involved as well as the total time taken,” says an industry veteran and erstwhile head of operations of an integrated steel plant.
“Shutting down a furnace means either blowing it out in a hot condition or banking,” he continues. “We know that air and oxygen are constantly pushed in through the tuyeres.
Blowing out involves stoppage of charging in a controlled manner. So what happens is that within the furnace the extremely hot gases coming from the bottom impart heat to the material charged from the top in an ambient temperature. In case of blowing out, we stop charging iron ore, ?uxes, etc from the top but the material at the bottom of the furnace keeps getting charged. But the outgoing gases, in that case, will undergo a drastic change in calori?c value as well as temperature. In fact, the gases are so hot that they could damage charging equipment at the top which are costly. Nowadays all BFs have a Paul Wurth arrangement which is costly. So we cool the gas through either inert gas injection, i.e. through nitrogen, or water spraying or, in rare cases, steam. This doesn’t take very long. In a BF working full capacity it takes about eight hours for whatever is charged from the top to come out of the bottom. Even when it is running at high speed with an eye on e?ective control, it takes 15-16 hours. So, in case of blowing out or blowing down, operators ?rst stop charging, thereby allowing the charge to descend to the bottom of the furnace, all the while tapping normally. When it reaches the tuyere level the process is stopped.”
When operators are blowing down a furnace for major repairs, for example in the hearth bottom area, they might opt for salamander tapping. Nowadays, with advanced state-of-the-art equipment and e?cient blasting methods salamander tapping has become very rare unlike in the past. But idling, banking or blowing down have got nothing to do with salamander tapping. This is only done when a furnace needs to be shut down for a long period for hearth repairs or relinements, he explains.
“Unavailability of raw materials or lack of manpower at steel melting shops, as for instance during strikes, may necessitate the idling or blowing down of BFs. When the market is weak and manpower is not available, steel producers opt for production curtailments, especially the integrated producers with signi?cant capacities. They usually opt for blowing down one or more BFs at their plants while the other BFs continue to operate in a throttled manner. Blowing out some BFs this way naturally brings down the total production. It is important to note that the time taken for blowing out and again restarting the furnace, roughly 3-9 days on average, depends on the technical competence of the team of operators,” he adds.
Banking is another method of stopping a furnace which involves charging extra coke into the furnace with an eye on the total time period for which the furnace would remain idle.
“Now, with time, the cooling system starts accepting heat, the whole atmosphere starts accepting heat. So one can’t stop cooling altogether although it can be reduced. But whatever heat is lost must be kept in the furnace because once it is restarted the coke and other materials will be charged from the top and they will take time to descend.
Given the current market condition both methods can be adopted by BF steel makers. However, supply of raw materials is more or less steady and, given the fact that the government is supporting continued production, the integrated producers would be loath to stop operations altogether. Of course, some BFs would be blown down or idled while others would be banked and production would come down,” avers the expert on BF operations.
“There is another way of cutting down production which is called throttling. As part of this method, blowing in of air into the furnace is reduced. Suppose at a given plant 3,000 metre/cube of air is blown in during an entire hour. This could be suddenly reduced to 190 metre/cube per hour. So operations remain all the same while production is scaled down. So blowing down or throttling or even banking are decisions taken with regard to consumption patterns, availability of manpower and the need for capital repairs. Restarting a furnace aGer blowing it down takes about three to nine days. If you don’t take into account the cost involved in asset idling, the start- up cost, considering the average size of furnaces operated by the primary sector in India, is close to INR 1.5 crore approximately,” he explains.
Lack of demand prevails
“Bfs are running although production has inevitably been scaled down. Manufacturers have to decide whether to scale down production by 80%, 60% or 40% depending on the cost e?ciency. Naturally, if production goes below 40%, e?ciency is reduced. As far as raw materials supply is concerned there are few bottlenecks even aGer the imposition of the lockdown but lack of demand prevails with the infrastructure and construction as well as the auto sectors coming to a standstill. Throttling is the preferred choice for Indian BF steel makers today. However, other processes such as blowing down or banking are also adopted depending on the requirements of speci?c plants and the volume of production to be reduced or the time period for which assets are to remain idle. Although, if the lockdown persists beyond 21 days or more, piling inventories at stockyards could overwhelm manufacturers and more drastic production cuts could follow,” says a senior executive of M.N. Dastur.