Waste floor covering is an economical alternative fuel
By Norman W. Martin, wTe Corp.
Carpet manufacturers call it carpet-derived-fuel, but power plants that can use it call waste carpeting an economical alternative fuel
According to recent Environmental Protection Agency (EPA) data, 1.7 millions tons per year (tpy) of post-consumer carpet (PCC) waste are generated by residential and commercial sources. Monsanto and other major nylon producers responded to the magnitude of this figure and their increased environmental sensitivity by sponsoring a project to assess alternatives to landfill disposal of PCC materials.
Many alternatives to landfill disposal exist for waste carpet. These include demonstrated recycling options, such as the reuse of thermoplastic resins for polymer applications and carpet reuse in landfill capping systems. Not all materials can be recycled economically, hence, thermal recycling alternatives to landfills are used, including combustion as a substitute fuel in power plant boilers.
Carpet waste combustibility
One of the most obvious attributes of PCC is its combustibility, potentially allowing it to serve as a fuel or fuel supplement for solid-fuel combustion systems. Because PCC in its as-generated form usually consists of large rolls of whole carpet, a certain degree of processing is necessary before it can serve as a viable fuel stock. Thus, we define Ocarpet-derived-fuelO (CDF) as post-consumer carpet that has been processed for fuel use.
Potential markets for CDF primarily involve applications with the least processing expense and where effects on combustion equipment performance will be minimal. Extremely large combustion devices meet both objectives. Their geometry doesn?t require small particle size fuel and their firing rate using conventional fuel is so large that supplementary CDF injection causes minimal effect on boiler operating characteristics.
Power plant boilers, cement kilns and some industrial boilers, such as those used in the pulp and paper industry, fill this need. Many of these are fired with coal, a virtual prerequisite for a CDF application. They commonly have one billion Btu per hour firing rates, which is one order of magnitude greater than the CDF co-firing rate equivalent to a regionally effective reclaim program.
Estimates show that generation of PCC is approximately proportional to the general population. Thus, almost half of this waste is generated within an approximate 100-mile radius of major metropolitan centers. Fortunately, the availability of coal-fired power plants follows the population centers and corresponding PCC generation reasonably well.
Fuel characteristics and emissions
CDF fuel qualities seem amenable with large coal-fired boilers. One indicator of fuel compatibility is its analysis. This includes moisture and ash, as well as the elemental ingredients of carbon, hydrogen, oxygen, nitrogen and sulfur. Chlorine and fluorine also are elements of interest. Table 1 lists the ranges of these elements, moisture and ash in CDF, and compares the same characteristics in pulverized coal and No. 6 fuel oil. Note that the CDF analyses were performed recently under the auspices of wTe Corp. on several varieties of Nylon carpet. Although the sample was small, the results are felt to be representative.
Compared with coal, CDF ash content is much higher, generally a concern with solid fuels. However, the ash composition is not only extremely benign, but benefits emissions control by decreasing sulfur and other acid gases due to its high lime content (state-of-the-art acid gas scrubbers often use similar calcium compounds as sorbent materials). The ash should also act as a flux for slag, a performance benefit for wet-bottom boilers.
CDF has a slightly lower carbon content and heating value than coal, but its higher hydrogen content offsets this and the higher oxygen content plays a minor role in increasing CDF?s combustibility.
Nitrogen is the most important, though troubling, component in CDF. So-called Ofuel-boundO nitrogen is a major contributor to NOx emissions, a problem that is exacerbated by high combustion temperatures found in power plant boilers. Large industrial solid-fuel boilers typically use stoker grates with high excess air, resulting in lower temperatures, so that the potential NOx emissions are not a major concern.
Nitrogen in CDF is five times higher than coal. However, the right system for feeding CDF into the furnace may ameliorate the situation. Staged combustion, for example, reduces NOx formation and normally is accomplished by using a substoichiometric primary combustion zone followed by a superstoichiometric zone.
If the CDF is fed to the combustion zone in small bales or guillotined carpet roll sections, combustion may occur in stages, which will minimize NOx formation. Nevertheless, the nitrogen content of the CDF is a legitimate concern that can only be addressed and quantified during a test program using the actual combustion device.
Elements in CDF
The sulfur content of CDF is low compared to coal, which offsets the potential environmental negatives of the nitrogen. Sulfur and other acid-gas emissions will also be improved because of the sorbent nature of CDF combustion ash.
Chlorine content of coal and CDF is similar, causing no immediate concerns regarding HCl, dioxins and other chlorine-associated problems.
Trace amounts of fluorine have been found in CDF. Literature on fluorine in coals indicates a range of 10 ppm to 295 ppm. In Table 1 the average of 10 ppm and 295 ppm is used before normalization for coal fluorine. Comparing CDF?s fluorine content, approximately 25 percent of the average coal figure, one would not anticipate a significant problem caused by the fluorine in CDF. CDF?s natural lime content also would tend to capture HF, as would the tremendous lime loading present in a cement kiln.
Table 2 lists key components of the proximate analysis for CDF and coal. The relatively high volatile matter and low fixed carbon of the CDF indicate quick combustion and temperatures much lower than coal. Whereas much time and temperature (and perhaps a very small particle size) is required to burn out the carbon in coals, CDF?s volatility is anticipated to be much more forgiving with respect to combustion conditions.
Bomb calorimetry indicates that CDF combustion liberates approximately
30 percent less heat energy than coal on a gravimetric basis. However, CDF contains 20 percent ash and that ash
Besides the relative compositions of CDF and coal, it also is necessary to compare the characteristics of ash that is generated by each. Table 2 lists the range of CDF ash composition (determined during recent laboratory analyses) and the ranges of ash composition for typical bituminous coals.
The most notable features of CDF ash are its lower silicon dioxide content, higher calcium oxide and higher magnesium oxide compared to coal. It is anticipated that the calcium oxide (lime) will have a beneficial effect on SO2 and acid-gas emissions by absorbing sulfur and chlorine, and in wet-bottom boilers it will have a beneficial effect on slag fluxing.
The lower silicon dioxide (sand) in CDF may be beneficial from a heat transfer surface erosion standpoint, but no significant effect is expected. Similarly
to calcium oxide, the magnesium oxide will act as a sorbent material for acid gases such as SO2 and HCl generated by both CDF and coal combustion. There also is a potential for insignificant fouling due to MgO.
Table 3 lists ash softening temperatures for CDF ash. Temperatures, shown for both oxidizing and reducing conditions, are not remarkable. They indicate a probable compatibility with wet and dry-bottom combustion systems.
Tests also were performed on samples of CDF ash to determine its leachate characteristics by using the TCLP test advocated by EPA for use in evaluating the toxicity due to heavy metals of municipal solid waste (MSW) ash in landfills (Table 4). Note that the heavy metals in the CDF results from pigments used during manufacture to color the carpet. All metals extraction results are less than one-tenth of regulatory limits, except for selenium which is half to two-thirds of the limit.
Other properties of CDF are shown in Table 5. The basic, uncompressed bulk density of CDF is approximately 5 pounds per cubic foot. The bulk density test was performed on small squares cut from larger carpet pieces. For whole carpet rolls, the bulk density is probably approximately 50 percent higher. It has also been determined that 2-foot by 3-foot by 4-foot bales of shredded carpet weigh approximately 500 pounds, or approximately 20 pounds per cubic foot.
Table 5 also shows that CDF can contain up to 78 percent moisture as-received. With this much water, CDF would not liberate enough heat during combustion to sustain a flame. After complete removal of surface moisture by mechanical means, the remaining bound moisture can be as much as 56 percent. Moisture levels this high won?t eliminate self-sustaining combustion, but thermal efficiency is poor and overall flame temperature is adversely affected. Thus, any CDF combustion concept must include weather protection at every step from initial collection to the boiler front.
The last property of CDF investigated was its tendency to become sticky or melt at elevated temperatures. Study of such a property is useful when designing conveying and feeding systems that are near combustion devices that operate at temperatures between 370 F and 490 F.
Fuel substitution rates
CDF volatility is different enough from coal to cause concern that heat may be released differently in a boiler causing wall slagging and other problems in industrial boilers. Hence, it is prudent to limit CDF fuel substitution to 10 percent. In a 100-MW boiler, a 10 percent fuel substitution rate corresponds to a CDF feedrate of approximately 4 tph.
Other factors affecting substitution rate include permits for testing feed rates, carbon burnout as related to boiler design and ash handling system capacity. Emissions also may limit fuel substitution rates. For example, the sooting phenomenon experienced when using CDF under reduced load, or turndown, would cause excessive particulate emissions and thus limit fuel substitution. Availability of the carpet derived fuel at economical rates, affected mostly by local tipping fees, is another factor that may limit use.
Appropriate cofiring candidates
Wet-bottom pulverized coal and cyclone-fired boilers are good candidates for CDF cofiring where NOx is already being controlled or not a concern. In addition, fluidized bed combustors (FBC?s) with adiabatic beds would be easily adaptable to CDF firing. Because bed temperature in these units are controlled with excess air, it is possible that CDF could be fed with a simple blowpipe arrangement.
Traveling grates are rarely found in utility power plants because their excess air requirements cause unacceptable thermal efficiency. However, they are found in the pulp and paper industry. They require very little fuel preparation and are commonly used in industry for firing a variety of coals and other solid fuels such as bark (hog fuel or slash). Fuels with a very high ash content are handled easily and most boilers would require minimal modifications.
Pulverized coal (PC) fired boilers with dump grates that are used for refuse derived fuel co-firing situations would also be a good candidate for cofiring of highly prepared (small particle size) CDF. On the other hand, dry-bottom PC boilers without grates, often used in the utility industry, probably are not adaptable for co-firing, because they do not have a system in place to handle bottom ash.
CDF-fired system economics
An economic model for a PC, wet-bottom (slag-tap) boiler at an unnamed location has been developed. Potential economics of a similar full-scale CDF application could take place in many locations throughout the United States.
This model assumes that the unit is located within a 100 mile radius of a service population of 3.3 million who generate 23,000 tpy of PCC (.007 tpy per capita). If the estimated capture rate of PCC varies between 50 percent and 70 percent (according to distance), the estimated capture of PCC is 15,000 tpy, and the shipping requirement from remote collection points to the station is approximately 840,000 ton-miles.
A preliminary estimate of capital costs associated with a full-scale waste carpet venture is approximately $3.4 million. These costs include provisions for gathering and handling the waste carpet, processing it to CDF, and feeding the CDF to a boiler.
The capital cost estimate assumes that a business entity (e.g., the power plant operator) purchases and distributes one special covered trailer per 100,000 people to centralized locations (33 trailers required) where retailers can deposit rolls of carpet.
Ejector trailers were used in the pricing model because quick unloading is needed and because their cost is similar to other candidate container systems such as walking-floor trailers. Note that such costs are considerably higher than dumpsters.
The capital cost estimate assumes that over-the-road trucking cost is completely expensed (no capital required) by contracting with local tractor operators for the hauling. The capital cost does include an allowance for modifications to the power plant receiving area. It also provides for a 200-foot conveyor that has an inclined section for elevation changes to take carpet fuel to the boiler. The concept further assumes a minimum handling approach where the fuel is shipped and off-loaded in full rolls. Manually loaded on the conveyor, the rolls are guillotined into mini-rolls and fed through an airlock into the furnace.
A general estimate of operating and maintenance costs associated with the concept is approximately $850,000 per year. This includes the hauling cost for transporting the trailers to and from the power plant at $419,000 per year. A preliminary economic summary of the full-scale concept indicates a net operating income of $766,000 and a four-year simple payback on capital assuming that prevailing tipping fees average $55 per ton and the cost of coal is $1.50 per million Btu. The displacement of coal by CDF on a Btu basis would be 8 percent.
There are strong economic and technical indications that post consumer carpet waste can be effectively and economically recycled as a carpet-derived-fuel in highly populated areas where tipping fees at solid waste landfills are elevated. As opposed
to landfilling, combustion offers a perpetual recycling vehicle. It displaces fossil fuel. It simultaneously solves disposal problems for retailers and government administrators. The next step subsequent to this study should involve a full scale test at a power plant or cement kiln located within 100 miles of a major metropolitan area. Such a test will confirm the technical, financial and availability issues postulated in this study. END
Norman Martin, now chief technologist for American Ref-Fuel, was, until recently, vice president engineering at wTe Corp. He holds master?s and bachelor?s degrees in mechanical engineering from LeHigh University. He has written several publications on fluidized-bed combustion and was awarded a Patent for a timed fuel injection system.