The upgrades have yielded a boiler process that operates more efficiently and with less required operator intervention, all while remaining in strict compliance with all regulations and process constraints. As with any process, more efficient and automatic operation will result in less required maintenance and lower overall lifecycle costs.
Because of the success of the project, a similar system is being developed to control the second boiler in Duluth.
The Duluth project began in the summer of 2010, and progress was so encouraging that Minnesota Power decided to begin a similar project in May 2010 at the Rapids Energy Center (REC) in Grand Rapids, Minn., to improve controls of the steam header (Figure 3).
REC provides steam, air and electrical power to the nearby Blandin paper mill—and the mill supplies 300 tons of wood waste daily to the power plant. At REC, steam is the main product and power to the grid is a byproduct.
Like Duluth, the Grand Rapids plant had older technology, some of which was piecemealed together, and its operation required frequent manual intervention by our operators. The mill's steam demand varied widely, making control of the steam header very difficult.
Another issue with the plant operating routine was that a sudden drop in steam demand from the paper mill was often met by diverting steam to the atmosphere. A main goal of any upgrades would be to divert this excess steam to the condensing turbines in order to generate the maximum amount of power at all times.
Reliable and efficient operation of the steam header was dependent upon the skill of the operator, and efficiency varied from operator to operator. Our goal was to automate control of the steam header, to deliver as much steam as possible to the turbines in order to generate as much power as possible, and allow for the operators to focus on the cause of the header upset versus reacting to the problem.
In the fall of 2010, the project team began testing to ensure that the steam system transmitters and valves were working properly. We worked with Novaspect, the local Emerson representative, to inspect and test the field devices and to address issues that were found. We then implemented Emerson's Predict-Pro and SmartProcess Header solution on the DeltaV process automation system.
We set up the software solution on a DeltaV system at Novaspect's offices in Grand Rapids, configured Predict-Pro and SmartProcess Header to represent our boiler and steam header situation, and ran simulations. We trained four operators on the simulator, and they accepted the new system quickly.
We installed the header optimization solution on the REC DeltaV system in April 2011, started it up, did some tuning, and it's been running near perfectly ever since.
Prior to the SmartProcess Header, two of our pressure relief valves (PRVs) were regularly open more than 50%. Now the PRVs open only during turbine outages. This increased flow through the turbines allows an increase in electrical power generation, which will contribute to an estimated one year payback. The biggest gain is an easy-to-use header control package that will help our new work force tremendously as our experienced operators retire in the next five years.