With its landfills rapidly filling to capacity, one community in Chicago has devised an important alternative to landfilling that will not only process waste, but also recycle and convert it to energy.
A combination recycling and waste-to-energy (WTE) facility in the Village of Robbins, Ill., a suburb of Chicago, can accept, separate and process the 1,600 tons of residential and commercial waste it receives daily.
Of that solid waste, 25 percent by weight is recycled, while the other 75 percent is converted to refuse-derived-fuel (RDF). At an 85 percent capacity rate, the facility can handle about 1.5 million tons of solid waste annually.
The RDF fuels twin circulating fluid bed (CFB) boilers that provide steam for a turbine generator system producing more than 50,000 kilowatts, leaving boiler ash residue as the only by-product to be landfilled.
Upon delivery, unacceptable and unprocessable wastes are removed from the solid waste. Two processing lines then separate the recyclable and combustible material. On each, a primary trommel opens trash bags, breaks glass and separates any material less than six inches in size.
Using a horizontal hammermill, the material not removed by the primary trommel is shredded to pieces no larger 3.5 inches.
That material is sent to a two-stage trommel screen, where it's separated into three waste streams. The first handles glass and organic materials, including yard and food wastes that are then conveyed to the glass recovery system, yielding organic-free glass that can be used as building material and road aggregate. Organic materials are sent off-site to be processed into compost, which in turn can be used as soil conditioner.
Several overhead belt magnets located in the processing system recover 92 percent of the ferrous metals (about 5 percent of total solid waste). Eddy current separators remove about 65 percent of aluminum in the solid waste (mainly cans).
Shredders provide uniform-sized fuel with an estimated value of 6,170 British thermal units per pound. The RDF is placed in storage, where an estimated three-day fuel supply is stored before being sent to the CFB boiler feed system.
Two high-tech CFB boilers, supplied by Foster Wheeler Power Systems, Clinton, N.J., a partner in developing the facility, burn up to 600 tons of RDF to produce 229,000 pounds of superheated steam per hour. Currently, this is the world's largest circulating, fluidized bed WTE unit in operation.
Unlike conventional combustors, CFB combustors burn waste in a hot, fluid suspension of material that is trained in an upward flow of air rather than on a grate or hearth. This fluid bed consists of RDF mixed with intensely hot particles of a screen bed ash and sand mixture.
CFB technology yields a combustion efficiency of 99 percent, high boiler efficiency (81 percent versus 70 to 75 percent for other types of boilers), low combustion temperatures, reduced nitrogen oxide (NOx) emissions, more stable combustion and cleaner stack emissions than other technologies.
In addition, there are twin state-of-the-art air pollution control systems: selective, non-catalytic reduction and dry blue-gas scrubber/baghouse. The result: low carbon monoxide and NOx emissions.
The system is environmental friendly, having eliminated the need of addition MSW transportation to landfills, and the plant produces cleaner electricity than coal-fired power plants typically used in this region.