The Problem

Environmental catastrophes resulting from improper treatment/disposal of different types of wastes have caused increased public awareness of the growing problem of waste generated in all sectors of the public, industry, and government. Waste minimization and recycling can only provide a partial solution to the growing problem. The United States generates approximately 200 million tons of solid waste every year (about 4 lb/person/day) and this amount are projected to increase at a rate of 1% annually [21]. Therefore, stringent measures must be taken in order to provide a better and permanent solution to the problem. Thermal destruction of wastes can provide an increasingly important role in this area. This includes the application of three fundamental reacting processes of pyrolysis, gasification, and combustion. The existing destruction technologies that have been used include mass-burn-type incineration, fluidized bed, rotary kiln, molten salt bed, low- or high-temperature oxygen/air enriched systems, and low- or high-temperature starved air systems. More recently electric heating, microwave, and plasma-assisted systems have also appeared [22]. Thermal destruction offers distinct advantages over the other methods since it provides maximum volume reduction, permanent disposal, and energy recovery while the by-products can be used in several ways, such as material for building and roadbed construction [23]. For certain waste streams, under certain conditions, the by-product material can be very hard. As an example, the titanium and nitrates present in the waste material can form titanium nitrate at high temperatures. This is a very tough and strong material. This can be possible only with a controlled process so that the compound formed may be isolated from the other compounds in the by-products. Of all the permanent treatment technologies, thermal destruction provides the highest overall degree of destruction. In addition it provides maximum volume and mass reduction, maximum energy recovery, and the by-products can be nonleachable.

The disposal of municipal solid wastes (MSW) has traditionally been via landfills (about 83% of the waste generated) since the method is most convenient. Some of the gases released by this method [e.g., greenhouse gases and volatile organic compounds (VOCs)] are high and unacceptable. In addition the odors released into the environment are unacceptable. Other methods for waste disposal include incineration (6%) and recycling (11%) [24, 25]. The most common practice for the disposal of MSW has been landfill. The landfill disposal, therefore, creates the problem of odor, generation of toxic and other gases (e.g., methane and carbon dioxide), and intrusion of leachate generated from landfill site into soil and groundwater [26]. With the land prices continuously escalating and land becoming scarce, this method is unsatisfactory in addition to the environmental needs. As for the thermal destruction, special interest in air toxic organic pollutants and trace metals emission from incinerators came after the risk assessment findings toward human life [24]. Some of these metals (e.g., arsenic, cadmium, chromium, and beryllium) are very hazardous to humans in addition to being carcinogenic. In addition to the concern over pollutants such as NOx, SO2, HCl, CO, CO2, unburned hydrocarbons, soot and particulates, the emission of dioxins, furans, volatile organic compounds, and metals have received increased attention from many countries around the world. The concern over pollutants, produced as by-products from direct result of combustion process [27], is common to all incineration systems.

While several methods are being used to treat the wastes, incineration has been widely used to provide the highest degree of destruction [21, 22, 24] for a broad range of waste streams, even though combustion contributes to pollution. Thermochemical behavior of cellulose and surrogate solid wastes is provided here. The goal is to provide the further knowledge and tools to destroy solid wastes while simultaneously providing energy recovery and reduction of toxic byproducts.

The previous studies [21, 22, 27, 28] have shown that the formation of low-molecular-weight gases at elevated temperatures leads to a reduction in the molal mass of the product gas mixtures (e.g., by more than 100% at temperatures approaching 6000 K as compared to 1000 K). At these temperatures, destruction of the waste to molecular level occurs. Pyrolysis at elevated temperatures, using, for example, plasma gas, is most suitable to thermally destruct the solid wastes. Results also show that the gas composition from pyrolysis is significantly affected by the temperature and chemical properties of the solid waste material. In addition there is a dramatic increase in the volume of gas generated and heating value of the gas from pyrolysis at pyrolysis temperatures above 3000 K. There is a 250-300% increase in the volume of gas produced at temperatures approaching 6000 K as compared with 1000 K. The heating value of the gases generated is increased by about 225-350% over the same temperature range. The amount as well as heating value of the gases can be controlled via oxygen enrichment to air. Based on these observations, ultra-high-temperature operating systems, such as plasma arc systems, appear promising for the disposal of solid wastes, in particular when the space requirements are of concern. The volume of gas generated with combustion is much less with oxygen than normal air [29]. However, high-temperature chemistry and chemical kinetics are not fully understood, and comprehensive information on the high-temperature chemical kinetics is lacking. A thermochemical database for the solid wastes is required for the advanced high-temperature thermal destruction system. Therefore, fundamental studies must be carried out to obtain the basic information on the thermal destruction of solid wastes. Most studies in the literature have been on systems, so that any data at the fundamental level will assist in providing good understanding of the thermal destruction process. Comprehensive studies require details of chemistry and fluid dynamics for both organic and inorganic portions of the wastes. Information on cellulose and surrogate waste helps provide information on real wastes. It must be recognized that real wastes have poorly defined spatial and temporal composition so that scientific information can only be gained from surrogate wastes. Emphasis has been placed on the pyrolyzed gas characteristics as influenced by the waste composition, pyrolysis temperature, and gaseous environment surrounding the solid waste. Special interest has been on examining the effect of waste properties (as affected by presorting the waste) and operational parameters (surrounding temperature and chemical composition) on solid residue and product gas composition during pyrolysis using experimental and numerical studies.

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