Paper Mill Gains Operational Flexibility While Reducing Emissions
A paper company in Wisconsin was contemplating expensive precipitator modifications to reduce emission levels. Through modeling we discovered the same result could be achieved for much, much less by making relatively simple gas flow distribution changes.
"With Neundorfer's help, we were able to increase fuel ash content and boost precipitator ash carbon content without raising stack emissions."
Midwest Paper Mill
When a good solution isn’t good enough, and cost is a barrier to the most obvious solution, what do you do?
You look at the problem differently.
As part of its process to optimize boiler performance, a Wisconsin paper mill consulted with Neundorfer to affordably reduce emission levels. The situation was the result of a fuel change, which had led to more ash load and higher carbon content in the ash. With Neundorfer’s help, the company found a way to effectively handle the extra ash, keep stack emissions low, and avoid spending an arm and a leg on upgrades.
Before bringing in Neundorfer, the mill was contemplating making expensive ESP modifications, since coal boiler operations and fuel mix were already optimized. As it turned out, however, there was a more affordable way to reach the same end result.
Finding a Solution
Neundorfer started the process by creating a 3-D model of the unit, and then used Computational Fluid Dynamics (CFD) analysis and a quantitative performance prediction model to identify areas where gas-flow patterns needed to be improved.
Results of the CFD/performance prediction model suggested three problems. First, the inlet ductwork’s shape seemed to be creating uneven velocity and air distribution. Second, velocities across the outlet face suffered from significant variations. Third, during rapping there was an unfavorably high level of dust and ash re-entrainment.
All three problems pointed to gas-flow as the culprit for why the mill was experiencing undesirable opacity levels and an inefficient unit. By optimizing gas-flow distribution, Neundorfer predicted the mill could reduce its emissions, drop its system pressure, improve precipitator efficiency and eliminate ash re-entrainment.
To determine how best to resolve the problem from a gas-flow distribution standpoint, Neundorfer analyzed CFD results using a computer performance model used to quantitatively predict the effectiveness of different solutions. After running 29 tests, Neundorfer recommended six improvements:
- Add inlet nozzle egg-crate vanes in order to distribute the air evenly across the face of the precipitator and perforated plates, helping to reduce re-entrainment from rapping.
- Add inlet perforated plates and change the porosity to direct and distribute the air across the inlet face more uniformly.
- Lengthen the hopper baffles from 3′-6″ to 14′-1.5″ to help prevent ash re-entrainment and clogging in the hoppers.
- Install a perforated plateon the outlet face to create a higher average velocity toward the bottom of the duct and eliminate the variation in velocities across the plane.
- Add turning vanes in the outlet ductwork to disperse the air more evenly and reduce pressure drop as it exits the precipitator.
- Upgrade the transformer/rectifier control system to further reduce emissions.
Neundorfer helped the paper company decide which of the recommended modifications would best fit into its planned outage schedule and budget. The unit’s outlet was modified with new turning vanes and 27% perforated plate. Also, four expanded metal screens were removed from the inlet nozzle and replaced with egg-crate vanes.
Neundorfer predicted that, just by making these three changes, the mill would reduce stack emissions by 22%. Initial results were very positive: fuel ash content increased, as did carbon levels in the precipitator ash, but stack emissions didn’t rise.
Even after the modifications were installed, Neundorfer continued working with the paper company to make sure unit issues were resolved. Neundorfer’s support continues as planning continues for future improvements.