Developing a by-product gas controller to improve on-site electricity generation for iron and steel manufacturing
High production costs and low demands have placed the South African iron and steel manufacturing industry under severe pressure. The steel manufacturing industry is therefore forced to streamline internal structures and reduce production costs as a means to ensure sustainable operations. Electricity is the second largest energy source used in iron and steel manufacturing. A typical South African steel producing plant consumes in the vicinity of 960 MWh over 24 hours. South African industrial electricity tariffs increased by 250% additional to inflation from 2003 to 2016. Historical electricity tariff trends and Eskom's allegations to future tariff increases projects that electricity rates have not yet stabilised. The iron and steel making process produces combustible gases. These gases are recovered as a by-product and used for fuel at different stages in iron and steel manufacturing. By-product gases are also used to generate electricity through the on-site power generation plant. The gas distribution network is kept safe by flaring excess by-product gas. Day-to-day imbalances in the production and consumption of by-product gases are responsible for a significant portion of flaring. Addressing control of these imbalances will reduce flaring of by-product gases. Up to 550 TJ of energy is lost annually due to by-product gas flaring on a South African iron and steel manufacturing facility. The combination of iron and steel manufacturing being electricity intensive and South African electricity tariffs ever increasing led to the objective of this study. The objective of the study is to decrease electricity expenditure by increasing on-site power generation. Improving by-product gas utilisation increases on-site power generation. Increasing on-site power generation reduces electricity expenditure. A human operator, additional to several other responsibilities, reactively controls the by-product gas networks. Manual control is complicated, labour intense and requires constant concentration. The demanding complexities of the system accompanying many parameters lead to the operators frequently missing electricity generation opportunities. In this study, a methodology is developed to improve by-product gas utilisation. A controller is the mechanism responsible for the improved by-product gas utilisation. The controller continuously determines the electricity generation rate according to the by-product gas availability. The proposed controller uses instantaneous gas holder levels to determine the available by-product gas. The methodology and control proved valid after a case study was implemented. The control was incorporated into the Supervisory Control and Data Acquisition (SCADA) system. Inputs from plant personnel refined the control. Complexities led to a comprehensive Measurement and Verification (M&V) study. The results from the case study proved that the controller could reduce flaring by 20%. A control benefit of more than R4.8 million per annum was determined. A project sustainability strategy concludes the study. The strategy revolves around continual awareness of the control performance. An automated reporting system was used to generate and distribute reports among relevant parties.
- Engineering