CFD software designs for MACT compliance

When Severstal North America, an integrated steelmaker in Dearborn, MI, rebuilt one of its blast furnaces recently, we faced a challenge: the emission control system had to be designed for a casthouse that didn’t yet exist. The project would have been difficult – maybe nearly impossible – without upfront computational fluid design (CFD) software.

Computational fluid design allowed the project developers to simulate the air flow rates for the tilting runner duct.

The redesigned casthouse included a new set of hoods, ductwork, fabric filtration equipment, and fans. Under U.S. Environmental Protection Agency (EPA) regulations, casthouse particulate emissions must be controlled with what is called Maximum Achievable Control Technology (MACT). MACT compliance is measured by stack emission concentrations and the opacity of emissions that escape from the casthouse building. The Michigan Dept. of Environmental Quality (MDEQ) also has established an opacity limit. In our case, the casthouse needed to meet U.S. and Michigan environmental regulations by capturing 98% of emissions.

MACT compliance is usually based on actual observations of casthouse operations to determine emission intensities, crosswind speeds and vertical rise rates. But, Severstal could not do that because the rebuilt casthouse was very different in configuration and operation from previous ones.

Our solution was to use CFdesign software from Blue Ridge Numerics to conduct a comprehensive design study that simulated the new equipment under every conceivable operating condition.

Setting up the model
The first step was to set up the CAD geometry in CFdesign to include the physical boundaries and internal blockages to fluid flow within the casthouse. The geometry of the model, in this case created in MicroStation software (www.bentley.com), included the physical bounds of the casthouse as well as the furnace, tuyere platform, iron and slag runners, hoods and other equipment.

Simulations generated by CFdesign software accurately predicted performance of Severstal North America’s new casthouse emissioncontrol system in early design stages. This image is one simulation of the air flow at the furnace taphole.

After setting up the geometry in MicroStation, the model was meshed in CFdesign, which provides built-in tools for simulating the conditions that the new equipment and structure would face. For the casthouse model, we needed to simulate the temperature and velocity of incoming air; temperature, velocity, fume and combustion components associated with the casting and pouring of hot metal; and air flow rates for the tilting runner duct, taphole hoods and slag pot shanty duct.

To achieve and accurate simulation of specific conditions for the new casthouse design, heat and material balance calculations and video of similar installations were analyzed to establish fume rise velocities, intensities, and horizontal movement, especially at the taphole — the most concentrated source for emissions.

Information describing these processes must be incorporated into the model as boundary conditions to accurately evaluate the operation of the fume hoods that control emissions. In this case, input information was derived from previous experience at similar casthouse operations, where heat and material balance calculations were coupled with video analyses.

Establishing the parameters
After the model geometry and simulation information were entered, the initial hood designs were incorporated into the model to evaluate their performance.

Once again, heat and material balance calculations and video of similar casthouses were analyzed to establish fume rise velocities, intensities and horizontal movement, especially at the taphole, which is the most concentrated source for emissions. From these previous studies, the following parameters were established:

• Metal temperature – 2,800°F;
• Slag temperature – 2,700°F;
• Rise rate/vertical velocity – 900 ft./ min. at the taphole;
• Cross drafts – 450 ft./min. at the taphole (approximately 5 mph) and 900 ft./ min. under the tilters.

Other values that were assigned based on previous studies included the correspondence between fume intensity and emissions, and the fume densities for the taphole, tilting runner and slag pot shanty.

Simulating real-world conditions
After establishing the fundamental geometry and base set of conditions, finding the optimal design was an iterative fourstep process:

1. Using initial hood designs and ventilation rates, determine capture efficiencies for tapholes, iron tilters, and slag pot areas.

2. If initial hood/ventilation combinations do not achieve the 98% emission control rate, revise the geometry of the hood and/or the ventilation rate.

3. Re-run the model with the revised hood/ventilation design.

4. Repeat steps 2 and 3 until acceptable emission control is achieved, then apply the acceptable model parameters to the emission control system design.

Each time a simulation was run, CFdesign performed a series of complex calculations and then compared those to determine the degree to which they agreed. Then, the software automatically adjusted the model and reran the simulation to achieve close convergence for small subsections in terms of temperature, velocities and other parameters. It required 225 to 250 iterations of each model to achieve convergence and calculate a valid capture efficiency.

Capture efficiency for the casthouse model was computed as the probability that a fume particle generated at each source (taphole, tilter or slag shanty) would be captured by each hood and subsequently drawn through the ductwork to the bag house. The next – and most important – step in the calculation was to determine fume capture percentages for each individual operation.

The entire study entailed dozens of CFD simulations to arrive at an optimal mix of hood configurations and ventilation volumes. A summary of the key results from CFdesign is shown in table. In addition to numerical results, CFdesign provided static and dynamic images that created a better understanding of what was occurring during the simulations.

Accurate and cost-effective

Upfront CFD using CFdesign software proved to be an indispensable analytical method for optimizing design of the emission control system for the Severstal casthouse. It enabled us to successfully establish ventilation volumes, hood configurations and volume distribution profiles for 50 separate operating scenarios for the new blast furnace. All this was done in three months, without costly physical testing.

At this time, the new furnace is fully operational and in compliance with the MACT regulations. Upfront CFD proved to be an accurate and cost-effective method to predict actual performance of the casthouse emission control system under realworld conditions.