Pyrolysis is widely used as an industrial process to produce charcoal from wood, coke and coke gas from wood, coke and coke gas from coal, and fuel gas and pitch from heavy petroleum fractions. In spite of these industrial uses, the pyrolysis of MSW has not been as successful. No large-scale pyrolysis units are used for MSW operation in the United States as of April 30, 1995.
Only one full-scale MSW pyrolysis system was built in the United States. A simplified flowsheet of the Occidental Flash Pyrolysis System is shown in Figure 10.12.2. The front-end system consists of two stages of shredding, air classification, trommeling, and drying to produce finely divided RDF. Because of the short residence time of RDF in the reactor, this process is described as flash pyrolysis. The heat required for the pyrolysis reaction in the reactor is supplied from recirculation of the hot char. The hot char is removed from the reactor, passed through an external fluidized bed in which some air is added to partially oxidize the char, and recirculated to furnish energy for the endothermic pyrolysis reaction which yields the liquid byproducts.
The end products were gases, pyrolytic oil, char, and residues. The liquid product had several noxious qualities making it a poor substitute for Bunker C fuel oil. It was corrosive, requiring special storage and fuel nozzles, and was more difficult to pump and smelled poorly. These qualities resulted largely from highly oxygenated organics (including acids). Furthermore, the oil produced had a moisture content of 52%, not the 14% predicted from the pilot plant results. The increase in moisture in the oil decreased the energy content to 3600 Btu/lb, compared to the 9100 Btu/lb predicted by the pilot plant tests. The 100 tpd plant was built in El Cajun, California but never ran successfully and was shut down after only two years of operation.
The principal causes for the failure of pyrolysis technology appear to be the inherent complexity of the system and a lack of appreciation by system designers of the difficulties of producing a consistent feedstock from MSW (Tchobanoglous, Theisen, and Vigil 1993).
Although systems such as the Occidental Flash Pyrolysis System were not commercial successes, they produced valuable design and operational data that can be used by future designers. If the economics associated with the production of synthetic fuels change, pyrolysis may again be an economical, viable process for the thermal processing of solid waste. However, if gaseous fuels are required, gasification is a simpler, more cost-effective technology.
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