Charter Steel: Sustainable Scrap Preheat System

Background

The foundation of Charter Steel’s business is recycling millions of tons of scrap steel. In 2020 alone, its Saukville, Wisconsin, plant recycled approximately 550,000 tons of scrap steel. Recycling scrap steel is a dusty process, impacting air quality and the working environment for facility employees. The company sought to implement a new scrap preheat system to improve air quality, employee engagement, and energy performance.

In conventional electric arc furnace (EAF) scrap preheat systems, exhaust gases from the furnace are passed through a scrap preheating system where the gases supply heat to the scrap steel material to raise its temperature before charging in the EAF vessel. Scrap preheating typically raises the temperature of the scrap several hundred degrees above the ambient temperature and reduces the amount of energy required for melting the scrap. The lower cost option would have been to install an exhaust gas cooling system that would meet the primary goal of air quality improvement. Instead, Charter Steel implemented an alternate scrap preheat system that not only reduced dust inside the plant but also reduced greenhouse gas emissions and energy use at the plant.

Solutions

The design, engineering, construction, installation, testing, and commissioning of the new scrap preheat system and its supporting equipment took more than two years. Charter Steel worked with a metal manufacturing specialist to complete the installation and commissioning. The EAF scrap preheat system is a custom retrofit to the plant and one of eight such installations in the United States and 74 worldwide, most of which were installed at new construction sites.

Aside from high capital costs for the project, a major hurdle was that melt shop operations team members had to significantly modify their approach to steelmaking. All automated set-points for the steel charging system had to be reconfigured. Previously, around 100 tons of scrap steel were dropped, or “charged,” into the open electric arc furnace (EAF) from above, creating a surge of cold scrap steel, reducing furnace temperature, and requiring make-up of a large amount of lost heat. With the new scrap preheating process, the EAF roof is kept in place and a series of conveyors slowly feeds scrap metal into the EAF furnace. The 2,400-degree Fahrenheit off-gas exhaust is now drawn over the steel scrap while on the conveyor, preheating the scrap to between 600-800 degrees Fahrenheit and reducing furnace electrical and coal/gas energy demands. The hot scrap steel is then dropped into a large bath of molten steel that is heated by the electrode. This is different from the prior process, which slowly lowered the electrode onto a pile of cold steel.

Since this was a retrofit system, the operations team had many issues getting back to normal production volumes and quality. Issues identified by the team included:

About 25% of the heat was diverted to other products due to variation in specifications.
The process was not capable of consistently removing enough chromium from the steel at the EAF.
The EAF needed to operate at less than full power to preserve anode life.
Off-gas temperatures were high, resulting in excess water usage to cool the off-gas.
All crews were running the EAF differently.
Burners were still oversized.
Connecting car tips (connection of conveyor to furnace) were failing prematurely.
Scrap tracking was not accurate, resulting in inconsistent charge weights.
The draw on the EAF was not properly tuned for each step of the profile.
Top feed coal was not an efficient source of energy.
Bucket charge weights were not standardized.

To address these issues, Charter Steel formed a smaller employee team focused on understanding and optimizing the new scrap reheat process. This team worked closely with the Charter Steel’s internal energy department to use energy management tools that measure and monitor relevant variables impacting energy performance. A new EAF energy dashboard was created to provide relevant data for operators and engineers to monitor. Using this dashboard, the team working on the scrap preheat stabilization was able to shift their focus toward energy reduction. They utilized a consultant to conduct an EAF energy balance. New data sets were provided on key EAF energy metrics. The calculations revealed that adding excess carbon showed no improvement in energy being transferred to the steel. The team concluded most top-feed coal was being burned up in the top layer of slag and the heat from the reaction was exiting through the off-gas system. The consultant recommended using less coal and the next step was to figure out the best solution to reduce usage.

Armed with this data, the C-crew furnace operator and the furnace area manager focused on reducing carbon usage. They combined their years of experience and expertise to begin the process of EAF profile modifications to reduce carbon usage. They trialed dozens of new profiles and quickly identified how the EAF was performing in each step and phase of a new profile. They stayed in constant communication on the performance of each new trial and discussed where they could continue to reduce coal from the process. Once a trial proved to be successful, a new profile was published and updated for the other crews. During the project, they were aided by the new energy dashboard to be able to view the performance of a new profile within 24 hours. They also used the dashboard to see the entire energy picture and quickly understand the impact of each change on total energy consumption.

Other Benefits

The initial goal of the scrap preheat project was to improve indoor air quality for employees. The results were more impactful and widespread than Charter Steel initially anticipated. They included the following impacts:

73% reduction in melt shop particulate matter and decreased potential of associated fugitive emissions.
300,000 pounds/year reduction in 100% carbon electrode consumption.
5.6 million pounds/year reduction of coal (1,000 lb. reduction per “heat” or batch).
3.5 million pounds/year reduction of listed hazardous waste in the form of baghouse dust due to lower velocities in the exhaust duct.
16.8 Mcf/year reduction in natural gas consumption.
2.25 million kWh/year reduction in electricity consumption.
7,500-ton reduction/year in scrap usage due to improved yield.
943 fewer gallons of water used per heat or 5.9 million fewer gallons of water/year.

This all contributed to a nearly $4 million per year payback and an approximately 14,000-ton annual reduction in greenhouse gas emissions.

Additionally, there were many less quantifiable improvements related to the project, including:

Extended life of major water-cooled parts.
Better control of energy use and profiles.
Reduced deposits clogging the exhaust duct.
Off-gas temperatures significantly reduced.
Consistency between crews on the EAF profile(s) used.
Discovered the need to create different EAF profiles for each scrap mix.
Discovered the need for different profiles based on the number of heats on the anode.

Throughout the process, Charter Steel empowered and engaged employees to continuously improve how the scrap preheat system was designed and operated. Because it was a retrofit project, there were unique challenges that required input from vendors, site employees, and experts at energy conferences and forums. It was critical to apply the latest energy management tools and dashboards to track variables and identify ways to impact those variables. Providing data daily was key in the efficient evaluation of trials and process changes. Charter Steel now looks at energy in real-time instead of relying on a lagging indicator. To replicate this success, the Charter Steel Saukville energy team joined the Cleveland site to start leveraging the new knowledge. There is already an EAF energy dashboard in Cleveland and the teams are working on optimization. Charter Steel is also sharing this equipment-specific dashboard with all of its other plants to help them understand and optimize their significant energy users.

ANNUAL ENERGY USE

ANNUAL ENERGY COST

ANNUAL WATER USE

ANNUAL WATER COST

ANNUAL PLANT ELECTRICITY USE

ANNUAL PLANT ELECTRICITY COST

IN BUILDING ENERGY USE

IN BUILDING ENERGY COST

ACROSS NETWORK USE

ACROSS NETWORK COST

ANNUAL EMISSIONS

USE

COST

Charter Steel: Sustainable Scrap Preheat System

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