waste-to-energy: Motors Turned On Savings At WTE Plant

Keith Voight

How did RTC use VFD at its WTE to save $37,000?

Operators at the city of Fergus Falls' (Minn.) Regional Treatment Center (RTC), looking for ways to cut costs at its waste-to-energy (WTE) facility, installed variable frequency motor drives (VFD) in the plant's burners to adjust its fuel and air mixtures, maximizing burner and fuel efficiency.

Like many recycling facilities, RTC uses natural gas to supplement non-recyclable trash as a fuel source. Because different materials burn at different heat levels, RTC's biggest challenge was adjusting the fuel-air mixture accurately to maintain proper burner temperatures without wasting gas.

With the help of the Otter Tail Power Co., the city installed the VFDs and cut its electricity use by 534,360 kilowatts per year and natural gas requirements by 63.8 percent, saving Fergus Falls more than $37,000 in the first year. Additionally, maintenance costs were reduced, and overall system efficiency and quality improved. The change also knocked 7 percent off RTC's heating costs, benefiting taxpayers.

The city's burner contains a pair of primary combustion chambers, each with an attached secondary combustion chamber. Garbage travels through the burn chambers where it is dried and burned in the primary chamber. It then passes to the secondary chamber, where higher temperatures oxidize the remaining materials and gas.

Previously, pressurized draft fans supplied air flow to the chambers. Louvered inlet dampers, responding to changes in chamber temperatures, oxidize the remaining materials and gas.

Meters to plot the fan motors' minute-by-minute energy consumption revealed that the older louver system caused dramatic swings in electrical demand.

Fans ran 24 hours a day, even when the louvers were closed and additional air wasn't needed.

Engineers call this "deadheading": Fans run while the louvers are closed, wasting energy and creating damaging friction without moving any air. In addition, the fans caused vibrations that were wearing down the circuit boards and sensors of the louver control.

Otter Tail engineers estimated that by "deadheading" the primary and secondary fans on the burner units, RTC wasted 27.5 horsepower annually - costing more than $9,000 yearly in electricity.

The engineers recommended VFDs as a cost-effective and workable alternative. VFDs take alternating current (AC) and convert it to direct current (DC). The DC then is switched into AC wave form, enabling the drive to adjust motor speeds according to heat levels, eliminating the louver system.

The VFDs were installed and the louvers permanently opened. When operations were monitored, not only did electricity consumption drop more than expected, but other advantages appeared: chamber temperatures held, refuse burned more completely and maintenance costs decreased. In addition, total energy requirements and gas consumption were reduced.

Many of these additional savings flowed from replacing the louvers' slow mechanical adjustment. They had caused temperature vacillation, air leaks into the chamber when the louvers were closed and over-aeration of the fuel mixture. The thinned fuel mixture also wasted natural gas.

In addition to the $37,300 saved annually in energy costs, the power company included a rebate. The bottom line? Fergus Falls' VFD investment paid for itself in 32 days.

For more information on VFDs, contact your electric utility company's industrial marketing department.

Agreement USA Waste Services Inc., Houston, recently announced that a subsidiary, Empire Sanitary Landfill Inc., reached an agreement with the U.S. Attorney for the Middle District of Pennsylvania in which Empire will pay an $8 million fine for campaign finance offenses. However, the U.S. Attorney has advised USA Waste that since all actions to which this fine relates occurred prior to the landfill's acquisition, the company will not be implicated in any of these proceedings. In addition, the fine will not result in a current charge to the company's earnings, but will be included as part of Empire's purchase price.

Investment Rating As part of its ongoing rating program, Standard and Poor's has confirmed an "A" Rating for the Delaware Solid Waste Authority's (DSWA) Solid Waste System Revenue Bonds. In addition, DSWA has earned a rating outlook of "stable."