Post-consumer carpet (PCC) is expected to increase from 1.7 million tons in 1991 to 2.8 million tons by the year 2000, according to the U.S. Environmental Protection Agency. Since very little information exists on the combustion of PCC, wTe Corp., Bedford, Mass., is investigating the suitability of PCC as a fuel supplement.
Aside from combustion, alternatives to landfilling the materials range from recycling with face fiber recovery and repolymerization, to reuse as daily landfill cover or in final landfill capping systems.
PCC, which typically consists of large rolls of whole carpet, must be processed into a viable feedstock, or carpet-derived fuel (CDF). The processing method depends on the characteristics of the boiler and the nature of the CDF feed system. For example, large industrial or utility boilers can accept sheared segments of PCC rolls, or "mini rolls," whereas fluidized bed combustors and certain types of suspension-fired boilers, such as those used for refuse-derived fuel, may require more costly shredding techniques.
The investigation compared CDF to coal. CDF reportedly measured favorably as a fuel supplement. For example, although the heating value of CDF is lower than pulverized coal, the more volatile CDF can combust quickly and easily, even at temperatures lower than those of coal combustion.
The most significant differences between CDF and coal are in the carbon content and the ash. For example, whereas CDF had a lower carbon content than coal (47 percent, compared to 70 percent), the CDF produced 13 percent more ash.
Nitrogen is CDF's most important - and somewhat troublesome - component, which could contribute to NOx emissions, especially at the high combustion temperatures of utility boilers and cement kilns. However, the chlorine and sulfur contents of CDF were not significant problems.
A large amount of the ash produced from the combustion of CDF was made up of lime. This is beneficial since lime absorbs the sulfur and chlorine in sulfur dioxide and acid gas emissions. The lime also helps the combusted material stay molten. The ash softening temperatures for CDF are probably compatible with most combustion systems. In addition, heavy metals in the CDF ash are minimal, so toxicity is below regulatory limits.
It is also important to consider that carpet pieces can have a high moisture content. With this much water, CDF may not be able to sustain a flame during combustion. As a result, the PCC and the carpet-derived fuel must be protected from the elements during collection, processing and storage.
Given the characteristics of CDF, almost any large coal-fired industrial or utility boiler could cofire the material. The only potential problem is that the high furnace temperatures and the relatively high nitrogen in the CDF would add to NOx emissions. Possible firing systems include the wet-bottom pulverized coal boilers used by the utility companies; the traveling grate systems used in the pulp and paper industry; cement kilns; and fluidized bed combustors.
To evaluate if the project is economically feasible, the study considered a city of 3.3 million people within a 100-mile radius. The population would generate approximately 23,000 tons per year of PCC. The project would require an estimated capital investment of $3.5 million, which includes the costs of placing 30 to 35 trailers at strategic locations throughout the city, receiving and processing the CDF at the cofiring facility and setting up a boiler feed system.
The estimated annual operating costs are approximately $850,000. This assumes that the facility will use a large boiler with a simple guillotine shear to convert the PCC into "mini rolls." In addition, the project would provide a four-year simple payback on capital (see chart on page 12), if CDF costs $1.50 per million Btu and if the CDF combustion facility charges a $55 per ton tipping fee.
The wTe investigation was commissioned by the Monsanto Co., Allied Signal Inc. and E.I. Du Pont de Nemours and Co. Although the investigation found that PCC may be a viable supplement to coal in large industrial and utility boilers, only a full-scale demonstration will validate PCC's technical and financial promise.