And, like Seattle, Spokane has worked hard to encourage recycling as a means to reduce volumes of waste headed to these landfills. In fact, Spokane County alone boasts a 41% recycling rate for the last decade. Where the two cities differ, however, is in the manner in which the residual nonrecyclable waste is dealt with. Seattle packs much of its waste into intermodal containers for shipment by rail to an Oregon landfill, while Spokane uses its as fuel in one of the state’s largest waste-to-energy facilities.
Whether one method is better than the other is open to debate, as is the question whether burning municipal solid waste (MSW) is, in fact, even an environmentally friendly means of disposal.
What’s undeniable, however, is the fact that with each day the Spokane waste-to-energy facility (WTEF) diverts more than 800 tons of waste from area landfills, the life expectancy of those landfills has been dramatically increased, and enough electricity from the facility is generated to power more than 13,000 homes. The site’s success is the result, in large part, of the efficient waste transfer system the city has put in place to ensure a continuous MSW stream to its boilers.
Burns Round the Clock
Built in 1991 at a cost of $110 million ($135 million when costs for new transfer stations are factored in), the WTEF seemed, at the time, an expensive and somewhat risky venture. Nearly 15 years later, however, the site has more than proved its worth. Owned by the city of Spokane and operated by Wheelabrator Technologies Inc., of Houston, TX, the WTEF burns approximately 400 tons of MSW per day in each of two furnaces in a year-round, 24-hour operation. Concerns about the risk of the plant emissions have proved unfounded; the facility regularly meets strict pollution control standards set and monitored by the EPA, the Washington State Department of Energy, the Spokane Air Pollution Control Authority and the Spokane County Health Department. According to Ross Davenport, the solid waste management’s landfill/transfer station foreman, the decision to move away from landfill disposal was an easy one.
“Back in the 1980s both city and county landfills were not just filling up: They were polluting the local environment to such a degree that they were cited for Superfund cleanup,” he says. “An alternative method to dispose of the area’s waste was sorely needed, and waste-to-energy made the most sense from both and environmental and financial standpoint. Bolstered by an excellent recycling rate in the area, the WTEF has allowed us to really make a positive impact on waste disposal in this area.”
Anatomy of The WTEF
The system in place at the Spokane WTEF uses a pair of mass-burn, waterwall boilers, fed by overhead refuse cranes and ram feeders. MSW is contained in a pit measuring 142-feet by 50-feet by 40-feet, sufficient volume to hold about 5,000 tons of MSW or a week’s worth of fuel. Cranes pick the refuse, fluff it to reduce the density and load it into the feed hopper. Once in the furnace, waste is subjected to a combustion temperature in excess of 2,500°F and moved using a pair of Von Roll reciprocating grates. After incineration, waste is reduced in volume by 90%, leaving behind only ferrous metals and ash. Ferrous metals are removed for subsequent recycling, and ash is handled via a semi-dry, vibrating pan conveyor and removed for disposal, via rail, to an area monofill.
As mentioned, the WTEF has proved a clean technology, the result of a selective noncatalytic reduction process including dry scrubbers, fabric filters and carbon injection. The resultant electricity—26MW at capacity—is sold to area power companies including Avista and Puget Sound Energy Corp.
Improving the Load
Getting the massive volumes of waste needed to feed the WTEF involves routing material from throughout the area to either of Spokane’s two transfer stations. There, MSW collected from the area’s 420,000 residents is dumped onto a tipping floor and push-loaded into 48-foot trailers staged in a pit for shipment to the facility. Efforts to make that transport process as efficient as possible include removing unwanted items, tamping down material, and repositioning loads in the trailers themselves. According to Davenport, the city has recently upgraded the equipment used in that process.
“In the past we used a number of different methods for loading trailers for transfer of waste. At our North County [Recycling Transfer Station] location, we used a machine that was similar in design to what we have now but had outlived its efficiency and had gotten too expensive to repair and maintain. At our Valley Recycling [Transfer Station] location it was a different story altogether. There we were using a backhoe loader to reach into the pit to tamp the material down. Because very little downward pressure could be exerted before the loader’s wheels came off the tipping floor, this was anything but an efficient way of maximizing a load.”
Stand-Up Performance
To make matters even more interesting, new bid specifications for replacement of the existing loader included items that could not be met by the manufacturer of the previous unit.
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| A Model 2100 Builtrite Handler helps maximize output at one of Spokane's waste transfer stations. |
Davenport says that, because they had been using a Builtrite Handler (Northshore Manufacturing, Two Harbors, MN) to load out inert materials from their waste-to-energy operation, they already had a familiarity and level of satisfaction with their performance.
“Northshore was able to meet the new specs and won the bid to install a pair of Model 2100 Builtrite Handlers—one into the North County [Recycling] Transfer Station in 2004 and another into the valley location in 2005,” he says. “It was definitely a wise move on our part. They’ve proved excellent performers, maximizing volumes of the trailers as they leave the station, and doing so with almost no unplanned, maintenance-related downtime. And, while most people don’t realize it, here in Spokane we actually do see some temperature extremes on both the cold and hot side during the course of a year—and the handlers seem immune to it all.”
To the uninitiated, Davenport says, some of the requested modifications might seem a bit strange. To the operators themselves, however, they are just what the doctor ordered.
“The most obvious thing we do differently is to allow our operators to work from either a sitting or standing position,” he says. “The windshield of the unit is designed with an inward tilt from top to bottom which affords the operator a much better view down into the pit and the trailers. So we had the joysticks repositioned in such a way that, if the operator wants to stand and look directly down into the pit — and many of our workers prefer that position — it is easy and comfortable to do so. However, it is equally comfortable to work while seated, as is normally done.”
All About Efficiency
Additional efforts to maximize trailer loads include pushing waste directly into an SSI Model 4500 preload compactor.
Doing so produces a bale measuring 7 feet by 7 feet by 33 feet at rates that can peak as high as 100 tons per hour.
While trailer weights are maximized at 42,000 lbs, compacting the material allows for use of a lighter trailer, thereby reducing transportation costs even further.
“It’s really all about efficiency in this part of the operation,” says Davenport. “In order for the WTEF to function as it was designed and be profitable in doing so, it needs a steady stream of material delivered in as efficient a manner as possible.
“The equipment we have in place at our transfer stations—and at the plant itself—helps make that happen.”
Proof of the Spokane waste-to-energy facility’s success is in the numbers: The site produced 179,121 megawatt hours (MWh) of electricity in 2004.
After ensuring its own operational energy needs were met, the facility still sold over 148,000 MWh to Puget Sound Energy, ensuring a net revenue to the Regional Solid Waste System of better than $12.6 million.
Writer LARRY TROJAK is president of Trojak Communications, located in Ham Lake, MN.
DE - July/August 2006
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