Focus on Operational Flexibility Reduced Derates, Improves Capacity
As the hopper evacuation system struggled to keep up with the collection profile of the unit, nightly derates were required to maintain operations. With improve performance and ash collection profiles, the plant was able to increase capacity, run consistently at high load, and save on maintenance costs.
"Not only were we able to drastically reduce opacity and PM, we were able to increase the revenue from our fly ash sales and improve system capacity. "
A 440MW mine mouth plant had to have nightly derates due to its hopper evacuation system design and poor operation of the ESP.
The ESP, the primary particulate control equipment, had 12” plate spacing and four collecting fields in the direction of gas flow. The unit began to have issues from erosion that led to failing wires in the first field frames. In 2009 it was determined that the first field required a rebuild. The plan was to upgrade the first field with rigid discharge electrodes while retaining the 12” plate spacing, but several vendors recommended 16” plate spacing, which saved on time and material cost. In 2010 the changes were implemented and the first field was changed to 16” plate spacing with RDEs. For several years, the unit ran adequately, but slowly degraded in performance.
The unit burns lignite, which is a high ash content fuel. The inlet field hopper evacuation system is a high capacity pressure system, whereas the back fields use a lower capacity vacuum system. Considering the expected collection profile of an ESP, the design should be adequate. However, as the inlet field’s collection efficiency began to degrade, the back fields were seeing increased ash loading and were forced to collected a bulk off the ash. In 2016 it became necessary to derate the unit for 8-12 hours each night in order to allow the ash evacuation system to catch up. The lower capacity vacuum system on the back three fields were not able to handle the amount of ash they were collecting.
During high load, opacity at the stack hovered around 8%. The overwhelmed ash system was leading to lost generation. It was also impacting the ash sales from the unit due to the lower quantity and lack of size segregation which required classifiers to remove the larger particles. The OEM was approached to perform an inspection of the ESP and make recommendations to improve the collection of the inlet field. The recommendations were followed, but minimal improvements were gained.
Neundorfer was contacted to assist the plant in evaluating options to improve performance. The challenges of timing and budget were constraints that had to be factored during the evaluation process. What could be done within a short period of time and with lowest impact on budget as this project was not part of a planned budget?
The first step was to understand the limitations of the current systems and the multitude of factors that are influencing performance and field collection. Once these factors were understood, the most cost-effective methods of improving performance and increasing operational flexibility could be understood. The next step would be implementing the tools that would allow for feedback into the system’s operation, offering insights into operational adjustments and system optimization. The background of the issue, knowledge of the system limitations, and the focus on performance, operational flexibility, and system optimization allowed for the improvements and tools implemented to achieve the end result. Improving the performance of the entire unit afforded the opportunity to optimize power input, system operations, and the hopper evacuation system.
It was determined that the Transformer Rectifier (TR) sets provided combined with the wider plate spacing and low resistivity could not properly charge the ash particles so they could be properly collected. Neundorfer recommended replacing the existing 82kV TR sets with 100kV units.
In November of 2016, Neundorfer began the outage work with the support of their preferred subcontractors. Both contractors were experienced with the scope of work and with the skills required to implement a time-sensitive critical path project. This was a short outage, with 6 days available for the work. Of course, best laid plans can go awry, quickly! The outage started off with a broken crane that couldn’t be fixed. The contractors were flexible and they were able to around the crane issues, until two smaller cranes arrived to work on opposite sides of the ESP. The weather cooperated and the work was completed as scheduled. The unit was back on-line, and the original issues that initiated the project could be addressed.
With the additional power input into the precipitator, software and remote support provided insights into operational improvements. These included adjustments to field powering and the rapping scheme. It also allowed for the hopper evacuation system to be monitored in connection with field performance. Through trials along with data and performance review, operational schemes were put into place that reducing hopper cycling time.
As a result, the unit has been able to run at full load 24/7 without ESP or hopper evacuation issues. Opacity has been reduced to 3% and PM emissions by over 60%, indicating a decrease in ash loading to the scrubber. Most importantly, the inlet field is collecting 15% more ash, which reduces the ash loading on the rest of the ESP by at least 25%. The new ash collection patterns have led to segregation of particle sizes, which increases the value of the fly ash. Improvements to the ash evacuation system has resulted in drastically reduced cycle times, which increases capacity and will offer future maintenance savings. By focusing on building operational flexibility and connecting the data from various systems, ESP performance was improved, hopper evacuation system capacity was increased, and maintenance needs were reduced.