6971395

3,000-6,000

a Calorific value in MJ/kg (Btu/lb) as fired.

a Calorific value in MJ/kg (Btu/lb) as fired.

The shredding, which can be done wet or dry (as received), converts the refuse into a relatively homogeneous mixture. The shredding is usually done by hammermills or crushers. This shredding operation is costly both in terms of energy and maintenance. One reference gives 1977 maintenance costs of 60 cents/ton of waste.16

Problems with fire, explosions, vibrations, and noise are common in the shredding operation.

Density separation increases the fuel's heating value, minimizes wear on transporting and boiler heat-transfer surfaces, and makes the ash more usable. Resource recovery can be an important by-product, with separation of metals and glass for resale.

16.4.3 Pyrolysis and Other Processes

Pyrolysis is the thermal decomposition of material in the absence of oxygen. The product can be a liquid or a gas suitable for use as a fuel. There are many py-rolysis projects in the research and development stage. An apparently successful pyrolytic heat-recovery system is described in Ref. 17. The new plant saved $53,000 the first year while disposing of 90% of the firm's waste. It was expected that the system would pay out in approximately three years. Emissions were said to be below standards set by EPA. A second pyrolytic heat recovery is being installed to take care of expanding needs.

Anaerobic digestion processes, similar to those used in wastewater treatment facilities, can also be used to convert the shredded, separated waste into a fuel. About 3 scf of methane can be produced from about 1 lb of refuse. In this process the shredded organic material is mixed with nutrients in an aqueous slurry, heated to about 140°F, and circulated through a digester for several days. The off-gas has a heating value of about 600 Btu/scf but can be upgraded to nearly pure methane.

Solid fuel pellets can also be prepared from refuse which are low in inorganics and moisture and with heating values around 7500 to 8000 Btu/lbm (17,000 to 19,000 kJ/kg). Some pellets have been found to be too fibrous to be ground in the low- and medium-speed pulverizers that might normally be found in coal-fired plants.

16.4.4 Refuse Combustion

The major problems in firing refuse in steam generators seems to be fouling of heat-transfer surfaces and corrosion. Fouling is caused by slag and fly-ash deposition. It is reduced by proper sizing of the furnace, by proper arrangement of heat-transfer surfaces, and by proper use of boiler cleaning equipment.17

Corrosion in RDF systems is usually due to:

1. Reducing environment caused by stratification or improper distribution of fuel and air.

2. Halogen corrosion caused by presence of polyvinyl chloride (PVC) in the refuse.

3. Low-temperature corrosion caused when some surface in contact with the combustion gases is below the dew-point temperature of the gas.

It appears that many existing coal-fired boilers can be modified to use suitably prepared refuse as a fuel.

One type of refuse that is both abundant and readily usable in certain types of furnaces and steam generators is tires. Paper mills, cement plants and electric utilities are among those who have discovered this abundant and readily available fuel, with a higher heating value than coal and with less production of pollutants such as nitrous oxides and ash.18

Old tires have been a problem in landfills because their shape permits the trapping of water and methane. Tire piles which catch fire are difficult to extinguish and may burn for days, spewing black smoke into the air and oozing oil into the ground. Burning tires as fuel solves the difficult problems created by the U.S. inventory of old tires, estimated to be about 850 million, and the 250 million tires added to the piles each year. Added incentives using old tires are offered by most state governments and tire collectors typically charge a tipping fee for taking a used tire. The company or utility operating the furnace can assume responsibility for direct collection or hire that done by a vendor.

Most of the experience in using tires as a fuel in the U.S. electric utility industry has been with cyclone-fired boilers, which make up less than 10 percent of utility capacity. Little modification is required in these type boilers except for the conveyer system bringing the fuel to the combustion chambers. The metal bead wire around the rim of the tires must be removed prior to combustion and any other metal left after burning must be removed magnetically. The tires are typically burned in cyclone-fired boilers as chips about 1 inch square and as a small percentage of the total fuel. Slightly larger sizes have been successfully burned in stoker-fired units where the fuel sits on a moving grate near the bottom of the boiler.

Tires are more difficult to burn successfully in pul-verized-coal boilers except as whole tires in wet-bottom furnaces operating at high temperatures, around 3200°F. In one project whole tires were fed into the boiler at 10 second intervals by a conveyor and lock hopper system. There appears to be good promise of burning tires in fluidized-bed combustion systems, particularly if they are designed in advance to handle such fuel. In the near future it is likely that all tires being discarded will find their way directly into use in some material manufacturing, such as road surfacing, or will be used as a fuel. The split between the various uses will be determined primarily by the economics.

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