Many power producers have goals to use renewable resources in their electrical energy mix—by either regulation or economic incentive. At the ARC Industry Forum this week in Orlando, Minnesota Power’s Luther Kemp presented the company’s plans to expand renewable sources of energy using wind and biomass to achieve 25% of their generating capacity by 2025 using wind, water, and wood.
He shared a project at the M.L. Hibbard Renewable Energy Center using two boilers fueled by wood, coal, and natural gas. The facility supplies 1/3 of steam to the local paper and recycle mill next to the facility. The goal of the project was to increase boiler wood burn percentage up to 100%. The mill and other sources in the area supplied the wood biomass, which was transported over to the boiler.
There were several technical challenges in this project. The first was the requirement for a design and control strategy to handle a fuel source with wide variability. Also, the boiler had been through several fuel source conversions from coal to oil to multi-fuel: wood/coal/natural gas. The paper and recycle mill placed large steam load swings back on the boiler. From a mechanical standpoint, some of the air dampers were inoperable.
The operation required two operators to handle the manual control to adjust for the load swings and fuel BTU variability. The manual operations also led to high fuel carryover instead of efficient, optimal combustion. Beyond the technical challenges, the facility experienced a 40% of expertise through retirement. Luther noted this trend will continue over the next several years given their staff demographics.
These challenges led to the need to develop a method of control to handle the technical challenges and require fewer operators to run the boiler. Luther and the plant staff partners with the Emerson industrial energy team and Jansen, which provides an overfire air system. This system provides high velocity air nozzles that are installed above the grates to help capture the unburned fuel from escaping the furnace. For improved airflow, new ducts were designed with improved measurement locations. New airflow devices were added to provide repeatable results.
New air damper actuators and locations with position feedback as well as upgraded control devices based on new technology were added to address control-related issues. For the control strategy, the plant used a DeltaV system and boiler optimization package, based on model predictive control (MPC). The optimizer within the boiler optimization package provided fuel cost optimization. The optimizer allocates boiler demand individually to all three fuels to determine the most economic method of production based upon fuel cost.
Automatic control was established for the undergrate, overfire and gas burner air, which work in cascade to maintain the desired excess air on the boiler. As required, the operators can adjust fuel limits or other constraints. With the completion of the project, the air systems operate in cascade and adjust automatically according to demand. Also, the undergrate air is now used to improve unit speed of response to load swings. Per the project goals and as they continue to work through constraints, the boiler is approaching 100% wood-based fuel operation and is able to handle the changing steam load swings from the mill—all with greater energy efficiency. The boiler now requires a single operator to manage operations.
You can read a fuller account of this story in a Control magazine article, Going Green with Fuel Use Optimization.