As the United States seeks greener sources of energy, the demand for waste-to-energy (WTE) facilities that produce refuse-derived fuel (RDF) could grow. Thirteen of the 89 WTE facilities currently operating in the United States are RDF projects in which municipal solid waste (MSW) is processed to prepare a fuel for existing or new solid fuel boilers. Unlike WTE facilities that produce energy through mass burn technology, facilities that produce RDF perform fairly extensive processing of MSW to increase the quality of the fuel. Processing has the benefit of extracting non-combustible recyclables, such as metals and glass, from the waste stream.
The increased demand for RDF production is likely to come from two sources. The first is the requirement of many conversion technologies — such as gasification or pyrolysis — for a fuel source of a specific particle size and other specific characteristics. This requirement leads the companies that are developing and marketing these technologies to mandate front-end MSW preparation prior to their conversion process.
The second, and perhaps more significant, source is the movement toward using renewable fuels to provide for our nation's energy needs. Last summer, the U.S. House of Representatives passed the American Clean Energy and Security Act of 2009 (H.R. 2454). The Senate has yet to vote on the legislation. The bill would require retail electricity suppliers to meet 20 percent of demand through renewable electricity and efficiency measures by 2020. In addition, the U.S. Environmental Protection Agency (EPA) recently issued a final rule for its renewable fuel standard (RFS) that is more inclusive of MSW than the U.S. Department of Energy's definition, which designated only the biogenic portion of MSW (estimated to be about 60 percent of the waste stream) as renewable. EPA's broadened definition may prompt those states that do not include MSW or MSW-derived fuels in their definition of "renewable" to adopt the EPA standard. Also, a presidential executive order issued in October 2009 includes MSW as a renewable energy fuel source and encourages increased federal use of renewable energy.
This movement to use renewable fuels could have a significant impact on solid waste disposal. Replacing just 10 percent of the coal used to generate electricity in the United States in 2007 with RDF would create a potential demand for 225 million tons of RDF — significantly more than might be available if all the waste currently destined for landfills were diverted for RDF processing!
Refuse-Derived Fuel in the United States
The oil shortage of the 1970s created interest in processes that could use the energy content in MSW. EPA demonstration grants helped fund a number of innovative waste processing projects that included refuse-derived fuel. In one early RDF project that took place from 1972 to 1976, the city of St. Louis and the Union Electric Company produced RDF from city waste and co-fired it with coal in existing coal-suspension-fired utility boilers.
RDF production facilities in Chicago; Duluth, Minn.; Monroe County, N.Y.; Milwaukee; and Robbins, Ill., also began sending their RDF to existing utility boilers. Ranging in size from 150 tons per day (TPD) to 2,000 TPD, most of these facilities used dry shredding to achieve the size reduction needed for the combustion process. An EPA demonstration project in Franklin, Ohio, introduced the use of hydropulpers to reduce the size of the waste. Two commercial projects were constructed using hydropulpers: a 3,000 TPD facility in Hempstead, N.Y., in 1976, and a 3,000 TPD plant in Miami-Dade, Fla., in 1979.
While a substantial number of facilities were built in the 1970s and 1980s, no new RDF production facilities at WTE plants have been built in the United States since 1996 when the Robbins, Ill., facility was completed. Thirteen U.S. WTE facilities that produce RDF continue to operate. Many of these facilities have been modified since their original design and construction to correct issues that developed during operations. Today, several dirty MRFs and MSW composting facilities also produce and sell RDF, including facilities in Medina, Ohio; Truman, Minn.; and the Isle of Wight, United Kingdom.
Refuse-derived fuel is produced from MSW though a number of processes to meet requirements for particle size, moisture content and non-combustible content dictated by the thermal unit that will receive the RDF. At its simplest, MSW is shredded to a maximum particle size to produce RDF. More often, additional steps are taken to remove non-combustible materials and control the particle size. Removing non-combustible metals and glass from the waste stream improves the RDF by increasing heating value and reducing the amount of ash produced.
The energy content of RDF depends on its initial properties and the pre-processing performed on the incoming MSW. The more screening and air classification that takes place, the higher the energy content and the lower the ash content. However, with more screening and air classification, the quantity of RDF produced will be lower.
Facility designers have taken different approaches to producing RDF. More processing and refinement is prudent when using RDF as a supplemental fuel than when using it in a dedicated boiler, which can be designed to withstand a lower quality fuel. Another benefit of a higher level of RDF preparation is that more non-combustible materials can be separated from the MSW. Visual inspection at the time of tipping or prior to taking any size reduction steps is prudent to help remove items — such as bulky items or propane tanks — that may cause machine stoppages or explosions. Taking additional steps to make the RDF have properties closer to coal — such as further size reduction drying, and densifying the RDF — will add significant costs to fuel preparation, but may be necessary depending upon how the RDF is to be used.
The Ames, Iowa, facility which started operating in 1975, is a good example of how MSW can be processed into an engineered fuel to be used in an existing coal-fired boiler. MSW is processed there through shredding, magnetic separation, air classification, and screening and eddy-current separation for both fuel preparation and materials recovery. The Robbins, Ill., facility was designed to recover recyclable materials and produce RDF that had a particle size of less than 3.5 inches. The plant, built and operated by Foster Wheeler, consisted of two 85 tons per hour (TPH) lines designed to produce RDF for circulating fluidized bed boilers. Although the Robbins plant operated successfully, it was closed as a MSW facility due to a combination of political/permit/economic problems. The facility currently operates by processing biomass (wood chips).
Great Britain and Europe, while more widely recognized for having mass burn facilities, are also home to many refuse-derived fuel projects. In Slough, England, a coal-fired power station was converted in 2001 to co-fire coal with biomass and non-recyclable waste. Some 260 TPD of renewable fuel is prepared from non-recyclable waste at a separate but nearby facility. The power station co-fires these waste fuel cubes along with coal and other prepared biomass fuels from wood wastes, paper mill sludge, and even hay bales.
Another project in Ravenna, Italy, uses fluid bed boiler technology. The POLYNT SpA Waste Gas Incinerator and Waste Heat Boiler processes approximately 50,000 tons per year of MSW and delivers delivering approximately 7 megawatts of electricity. The system uses both RDF and prepared fuels from selected paper, wood and plastic waste streams. Shredding, size classification and magnetic separation steps are included in preparing the fuel.
In Canada, Dongara Pellet Plant LP has a 200,000 tons-per-year MSW facility in Vaugh, Ontario, which began operating in the summer of 2008. The facility produces an engineered fuel with a heat content of 10,000 to 12,000 BTU per pound.
It is important for companies to gain an understanding of the capital and operating costs for refuse-derived fuel production as well as the operating regime required to properly and safely operate and maintain this conversion technology process. Special attention should be paid to materials recovery opportunities so that revenue can be generated and higher levels of recycling can be achieved. Prospective markets should be contacted and their specification requirements well understood so that process design can incorporate equipment and operations that will produce saleable products.
RDF has clearly moved beyond the 1970s as an experimental technology and is now well-positioned to become a more important element in the waste industry. RDF allows us to reduce our dependence on oil and coal, recover energy and certain recyclables from MSW, and divert waste from landfills. In the years ahead, we can expect conversion companies to continue to refine RDF technologies for their facility operations. RDF can help us reach the goal of having MSW no longer be a wasted resource but a source from which to extract raw materials and energy on an industrial scale. This is the promise of RDF.
Want to Know More?
Harvey Gershman will speak in further detail on this issue at "The Resurgence of Waste-to-Energy and Conversion Technologies" session at WasteExpo on Tuesday, May 4. The session, which is part of the Technology track, will run from 10:15 a.m. to 11:45 a.m.