Cement Plant Gains Consistent Operations
A Midwestern cement plant needed to balance cost, outage time, and performance gains in order to make the best use of short outages to address the weak links in the air pollution control equipment. The result was a plan for incremental projects that would achieve the greatest performance and reliability gains with the lowest overall capital investment.
As stricter air pollution control standards become reality, many facility owners are faced with the challenge of improving air pollution control equipment efficiency within an existing footprint. At one Midwestern Cement plant, precipitator reliability was impacting plant production and its ability to burn alternative fuels. Gas flow velocity modifications often represents the most cost-effective method of improving performance.
Neundorfer, Inc, a company that specializes in the optimization of air pollution control equipment, was commissioned to construct and test a 1/12th‑scale model of the plant’s electrostatic precipitator (ESP). The objective of the study was to identify realistic options to improve the reliability and performance of the ESP.
The ESP operates at an input temperature between 365-410 degrees Fahrenheit, it handles between 30 -45 tons of dust per hour and air flow between 380,000 – 420,000 SCFM. Neundorfer constructed the model in the ESP’s current condition to establish the baseline and evaluate the potential for improvement. The optimal ESP operation is to achieve 85% of velocities within 115% of the average velocity and 99% within 140% of the average. The baseline condition demonstrated by the model indicated that 77% of velocities were within 115% of the average velocity with a 37% standard deviation. This information was then input into a mathematical performance model that had been calibrated to current emissions information. Both the air flow measurements and the performance model indicated that improvements to gas flow velocity distribution would yield significant performance gains.
Ideally, air flow velocity is also minimized above the hoppers, turbulence is reduced, factors that would lead to re-entrainment are reduced, and the air flow under and over the treatment zone of the ESP is minimized. For these factors, the baseline condition indicated there was significant room for improvement, with high velocities (160ft/min; 0.25m/s) measured just above the hopper areas and high velocities skewing flow around the treatment zone. The results also indicated that velocities measured above the hoppers would likely lead to dust re-entrainment.
After pertinent modifications were completed and tested, the final solution reduced velocities to 74% within 115% with a 25% standard deviation and, more importantly, hopper velocities were reduced by 40%. Although Neundorfer standards for velocity distributions were not met, the solution was one that best accounted for design complexity, outage duration, and construction feasibility. Furthermore, the performance model indicated that achieving these air flow velocities would result in a 25-30% reduction in outlet emissions, and the hopper velocity decrease would significantly lower re-emissions. The proposed solution involved lowering the porosity of the existing perforated plates, allowing the option for either replacing the perforated plate or reusing the current plates and blanking off pre-determined rows of holes. It also included installing vanes on the inlet nozzle floor, vertical baffles between fields 4 and 5, and replacing the outlet flange perforated plate with one that had an optimized porosity to achieve the desired results. This approach allowed for a combination of potential projects to be identified, modeled, and planned to achieve the highest performance and reliability gains with the lowest capital investment. Although the flow modifications would result in 25 – 30% reduction in emissions, the outage required to implement the modifications was not readily available. To initially improve performance while minimizing downtime, other performance options were selected for implementation. Short, planned outages were utilized to make improvements to the powering systems and address weak links in the internal component reliability. These resulted in a 54% increase in KVA, and 37% decrease in opacity, even during the historically difficult months. These improvements provided enough impact to allow for the flow modifications to be completed during future outages that allowed the plant to make the best use of outage time and budget resources.