Improving By-product Gas Utilisation in Steel Milling Operations

The South African steel-making industry remains globally uncompetitive due to strenuous financial conditions and increasing energy costs. Declining production figures, decreasing exports and increasing imports have adversely affected the market profitability. Energy contributes 20% towards the total operational costs of an integrated steel enterprise. Maximum usage of alternative energy sources is required to alleviate costs. By-product gases are combustible energy carriers, containing almost 18% of the energy input of production processes. Coke oven gas (COG) was found to be competitive with natural gas and applicable for high-temperature requirements, such as in steel reheating furnaces. However, fluctuations in the calorific value (CV) and composition of COG cause combustion and temperature instabilities. As such, quick reactions from human operators are required to adjust the air-fuel ratio (AFR) to maintain furnace stability using a trial-and-error method. This causes energy inefficiencies and production losses. A methodology was developed to predict and adjust the AFR according to the CV of COG using adaptive control strategies. The control strategy uses historic behavioural patterns of the furnace to determine the required AFR. Pilot studies on a billet mill reheating furnace have shown a 95 – 98% improvement in AFR adjustment accuracy. Potential cost savings of R7.5 million/annum in natural gas purchases were identified for this case study. Investigations into applying the developed solution to similar challenges is encouraged.