Improving by-product gas utilisation in steel milling operations
Iron and steel manufacturing are energy-intensive processes. Rising energy and operational costs have placed South African steel manufacturers under financial strain, while they struggle to remain afloat within an extremely competitive global market. Declining production and export figures, as well as increasing imports, have adversely affected the sustainability and profitability of the local market. It is, therefore, imperative for local manufacturers to implement operational change strategies without additional capital investment requirements in order to save energy and reduce costs. Hot-rolling processes of steel milling operations use reheating furnaces to elevate the temperature of the steel to a state of plastic deformation prior to rolling. The reheating furnaces combust fuel gases such as by-product gases or natural gas for thermal energy. They consume 70 % of the energy of the hot-rolling processes. By-product gases are combustible carriers of energy. Coke oven gas (COG) carries 18 % of the energy input of the coke production process. The energy content of COG was found to be competitive with natural gas and compatible for high-energy requirement applications, such as in reheating furnaces. The challenge arises in that the calorific value (CV) of COG fluctuates over time. These fluctuations prove problematic during the control of COG combustion processes as the air-fuel ratio needs to be adjusted accordingly. Furnace operators typically rely on a trial-and-error method for air-fuel ratio adjustments in response to the fluctuating calorific value of coke oven gas (CV of COG). This causes furnace temperature instabilities, energy inefficiencies and production losses. Additionally, COG has a high impurity content inherent from the coal. These impurities clog CV of COG analysing equipment. Several alternative tools are available to estimate the unavailable CV of COG; however, these tools do not account for the fluctuating nature of COG and only provide static estimated values. Based on these challenges, a need was identified to facilitate more stable operations of the furnace. The aim of this study entails the development of a new methodology to adjust the air-fuel ratio according to the incoming CV of COG and the current furnace operating conditions. The historic behaviour of the furnace is used in the control strategy. To address the recurring events where CV of COG data is unavailable, a novel methodology is developed for a time-continuous estimation of CV of COG for stable process applications. A seven-step methodology was developed whereby an appropriate method can be selected for the determination of CV of COG, based on the available data and measuring equipment. Three methods are presented, with varying accuracy and consistency. Pilot studies were undertaken on a billet mill reheating furnace in order to validate the air-fuel ratio adjustment methodology. The results verified that the air-fuel adjustments were within 94 % - 98 % accurate, resulting in a significant improvement in temperature stability. A reduction of 0,9 GJ/t in the energy intensity was determined. Based on the average natural gas maximum price of R 141/GJ, an annual potential cost saving of approximately R 7,5 million on natural gas purchases can be realised.