International EPC (Engineering, Procurement and Construction) company specialized in Energy, Defense, Steel and Industry
Optimal Sequencing of burners ignition in a slabs reheating furnace for a hot rolling mill to reduce fuel consumption and emissions.
The reheating furnace is an important step in the steel-making process and represents a significant energy (gas) cost factor in the whole chain.
It is used at a hot-rolling mill to reheat steel slabs emerging from continuous casting, from room temperature to 1250°C, in order to roll them into thin steel strips (thickness reduction from 200mm to 5 – 20mm).
Slabs are reheated using gas or oil burners installed along the lateral walls of the furnace. Every minute a control system computes the heat to be delivered to each burner for the next minute, depending on the slab temperature increase profile and online measurements. For diverse reasons, this set point can vary a lot, between 0 and 100% from burner to burner and from minute to minute.
Burners are usually driven in a proportional mode to deliver constant power throughout each minute. The valve is opened according to a set point expressed as a percentage of nominal power.
Based on the observation that a burner operates at its optimal yield (in terms of gas consumption and emissions) when opened at nominal power (100%), and that this yield can drop significantly if the burner is operated far from its nominal
power, our client has developed a new “On-Off” ignition technology. The principle of this technology is to operate the burners in 0-1 mode and to control the set point via the duration of the 0 and 1 sequences.
Using this technology the burners consistently operate at optimal yield.
However a new difficulty has to be tackled. Due to the rapid and frequent changes of “on and off” states, the overall heat demand of the furnace, and hence the fuel and air flows, can present huge instantaneous variations which are not acceptable given the slower dynamics of the fuel-feed and air-vent systems.
The challenge is then to build smart 0-1 sequences for all the burners, to deliver the appropriate heat at each one while minimizing overall heat demand variations.
This is a complex combinatorial problem that calls for state-of-the-art optimizing techniques. N-SIDE was delighted to take up this challenge and is proud to have enabled this environmentally friendly technology.
To address this problem, N-SIDE has developed a state-of-the-art, tailor-made “Tabu search” algorithm that computes an optimal ignition sequence for all burners and achieves a reduction of heat demand variations by one order of magnitude.
The main challenge was to compute an on-line solution for any furnace setup (from 20 to more than 200 burners) in less than 10 seconds in order to embed the algorithm within the furnace control system with no perturbation of the feedback loop dynamics (no delay).
The optimal burners’ ignition sequencing was the required component to make the On-Off technology possible and hence enable reduction in gas consumption and emissions at the reheating furnace of 10%.
This translates for a 3Mtpy hot strip mill into 6M$/y of energy savings.
Due to simpler and more robust 0-1 valves, lower maintenance is an additional benefit.
Today this sequencing program is embedded in the reheating furnace automation systems of several hot rolling mills all over the globe and is the core component in one of our client’s flagship technologies for steel applications.