A Walk In Our Boots

A biomass cogeneration plant manager provides an overview of events that occur during a typical scheduled outage.
By Stacy Cook | December 29, 2015

Koda Energy LLC is a 24-MW, combined-heat-and-power biomass plant located in Shakopee, Minnesota, that came online in 2009. It is a baseload facility that must be online and producing energy for our customers as much as possible, but we shut down occasionally to make repairs, do maintenance, and perform upgrades and cleaning. Work performed during a typical scheduled outage might include defouling and slag removal to restore heat transfer surfaces to full capability; cleaning and inspecting areas that are inaccessible while the plant is operating; reparation of equipment that cannot be accomplished safely while online; completing improvement projects or modifications; or to perform code work—insurance company or OEM recommended—and ultrasonic tube thickness inspections in normally inaccessible areas of the plant.

Mapping Out Plans
Our outage process planning begins well in advance of our shutdowns. We begin by developing a list of projects, parts, and contractors required to complete the work. All parts, safety supplies, and expendable materials are ordered in advance so that they arrive at least a week prior to the outage start date. We also coordinate staff, contractor, inspector, fuel supplier, and customer schedules, and move parts and equipment inside the plant within close proximity to where they will be used.

The actual outage begins by reducing fuel load in the plant several hours, or even a day or two, prior to reducing output. This is done to ensure that our entire fuel system is empty and ready to inspect and repair once we shut down. We will notify our power purchaser, Xcel Energy, and Rahr Malting—our neighbor, who supplies us with fuel and we supply with process steam—of what time we anticipate ceasing electricity production. We then begin reducing the electrical output of the plant, and take our steam turbine generator offline.

At this point, the plant is no longer producing electricity, but it is still online as a thermal heating system for Rahr malting. One at a time, Rahr will switch its kilns to natural gas heat, while gradually reducing the firing rate and steam pressure in the boiler. This begins to reduce the tube temperature in the boiler—the warm-up and cool-down rate is limited to 100 degrees Fahrenheit per hour.  Once only a small amount of thermal load remains, we will continue “cooking down” the boiler temperature and pressure, until they are no longer adequate to support the thermal load. The last energy user is switched over to backup systems, and the boiler is shut down to continue cooling.

In the meantime, the operations crew is busy preparing all systems to be opened up. Steam, water, fuel, and electrical systems must be isolated, drained, and brought to zero energy potential. LOTO (lock out, tag out) sheets will be completed, and all locks hung. Ash systems are emptied out, and ash hauled off site.

LOTO is a safety procedure done to ensure machines are properly shut off and can’t be started up prior to completion of maintenance or servicing work, while it is in a hazardous state or a worker is in contact with it. Equipment must be isolated and rendered inoperative before work begins, and sources are then locked. A tag is placed on the lock identifying the worker who has placed it. Koda lists all isolations and lock outs on a board and adds or modifies as new projects are added or completed.
Once the LOTOs are complete and the temperatures are acceptable for personnel, the operations crew will begin cleaning combustibles from hot work areas and ash and slag from boiler internals. They will also create scaffold access, run power cords, and provide lighting for all the work areas. All Koda employees work 12-hour days during our outages, with two crews on nights and days. Pictured below are some projects completed during our last outage in October.

Koda Energy completed many more (unpictured) projects during this outage, including the upgrading of software for the control system, replacement of the internal housing wear parts on a hammer mill, cleaning and inspection of the electrostatic precipitator internals, replacement of all the lighting fixtures in the fuel unload building, replacement of an ash belt and drag chain flights, replacement of the seals on the turbine’s extraction nonreturn valves, calibration of instruments, replacement of steam and water valves, and repair ration of  leaks in the plant.

By the time an outage is complete, we will know the condition of nearly everything that cannot be directly observed while the plant is in operation. That enables us to begin a list of work items and improvements to include in our next outage planning cycle.

In preparation to come back online, LOTOs are cleared, all valves and breakers are placed in their startup positions, and the boiler is warmed back up at a rate of 100 degrees F per hour. The steam piping is warmed and we begin taking on thermal load, bringing the turbine back online. Output is gradually increased until we’ve reached capacity.

An outage at Koda Energy is a very busy time, and our team is in a state of constant motion to get all work completed with as little down time as possible.  Our most recent outage was our quickest turnaround to date—the boiler was shut down at 1:00 a.m. on a Friday morning, with all work completed by Monday morning. Warming of the boiler began on Tuesday morning, and the facility was back in full output by the afternoon.

Outages are performed to clean, inspect, repair, modify, and improve plant components and its operations. If done safely and efficiently with proper planning, it will more than likely ensure the facility operates problem-free until the next scheduled outage.

Author: Stacy Cook
Plant Manager, Koda Energy