The feasibility of waste heat recovery and energy efficiency assessment in a steel plant
Gerdau Manitoba Mill (Gerdau) at Selkirk, Manitoba is one of the biggest energy consumers in the province of Manitoba. This research analysis undertaken at Gerdau evaluated opportunities for energy efficiency, including the following six areas: 1) recovering waste heat to preheat billets, 2) upgrading the charge end in the reheat furnace, 3) recovering waste heat to preheat combustion air in the ladle preheater, 4) replacing direct-fired natural gas heaters with indirect-fired natural gas heaters, 5) Oxyfuel combustion, and 6) “tap to tap time” control in the eccentric bottom tapping (EBT) furnace in the melt shop. As part of this research, end-user distribution was analyzed and energy losses were assessed. An end-use analysis found that the melt shop that includes the EBT furnace is the biggest consumer of electricity consumption (kWh) and electric demand (kVa), which accounted for 68.7% and 73.6 % respectively. The 2010 delay time in the power-off time of EBT furnace at Gerdau was found to be 762.3 hr/yr. Further research to analyze the cause of each downtime at Gerdau is recommended to determine how these unplanned downtime can be reduced in the EBT furnace. The reheat furnace is the biggest natural gas consumer at Gerdau with 437,563 MCF in 2010. Flue gas losses from the reheat furnace are the biggest energy losses in the gross heat distribution with 26,874,657 Btu/hr. Energy losses from hearth and roof by heat transmission are the biggest energy losses in the net heat distribution during operation, which accounted for 8.9%. The average thermal efficiency in the reheat furnace at Gerdau is 58.9% ± 3.6%. Compared to peak capacity, idle and partial operations of the reheat furnace and idling were found to be less efficient. The opportunities that are considered feasible and recommended to Gerdau are: 1) recovering waste heat to preheat billets, 2) upgrading the charge end in the reheat furnace, 3) recovering waste heat to preheat combustion air in the ladle preheater, 4) replacing direct-fired natural gas heaters with indirect-fired natural gas heaters. These are both good for the environment, reducing fuel use and emissions and providing a good payback period and annual savings. Many opportunities are available for reducing energy as provided in Table A, which shows emissions reductions, costs, energy savings and payback. Oxyfuel combustion is not deemed feasible without considering productivity improvement as oxygen cost is more than natural gas saving. A number of incentive programs, including those from Manitoba Hydro, are applicable to Gerdau. However, a number of barriers to accessing these, particularly as regards tax incentive programs, should be explored to see if these barriers can be overcome.