The disposal of municipal, industrial, agricultural and other types of wastes is a problem that continues to grow with the increase of population and with growth of production, especially in the industrially developed countries.
The USA generates 11 billion tons (10 billion metric tons) of solid wastes each year. Solid wastes are waste materials that contain less than 70% water. This class includes such materials as household garbage, some industrial wastes, some mining wastes, and oilfield wastes such as drill cuttings. Liquid wastes are usually wastewaters that contain less than 1% solids. Such wastes may contain high concentrations of dissolved salts and metals. Sludge is a class of wastes between liquid and solid. It usually contains between 3% and 25% solids, while the rest of the material is water-dissolved materials.
Federal regulations classify wastes into three different categories, such as non-hazardous wastes (e.g., a household garbage), common hazardous wastes (such as those having ignitability or reactivity) and special hazardous wastes (e.g., radioactive and medical).
There are many different methods of disposing of wastes of which most common is disposing to landfills. In a modern landfill, refuse is spread thin, and the compacted layers are covered by a layer of clean earth.
Another method of wastes disposal is incineration. The myth that burning makes wastes disappear has lead to incineration emerging as a widely used method for disposing many kinds of wastes, including hazardous wastes. However, along with elimination of the wastes, incinerators generate very toxic gases. Moreover, incinerator ashes are contaminated with heavy metals, unburned chemicals, and new chemicals formed during the burning process. These ashes are then buried in landfills or dumped in the environment. In other words, incineration is a method where industry can break down its bulk wastes and disperse it into the environment through air, water, and ash emissions. Thus, the industry masks today's waste problems and passes them onto future generations.
Existing data shows that burning hazardous wastes, even in “state-of-the-art” incinerators, produces heavy metals, unburned toxic chemicals, and generates new pollutants—entirely new chemicals formed during the incineration process.
An alternative method of waste disposal is pyrolysis of the wastes. Pyrolysis is the chemical decomposition of materials by heating in the absence of oxygen. Many different methods and apparatus are known in the art for pyrolytic processing of wastes.
For example, U.S. Pat. No. 4,308,807 issued in 1982 to S. Stokes discloses an apparatus for pyrolytically treating municipal and other wastes composed of solid heat-decomposable materials by radioactive heating from an open flame and convective heating. The invention is aimed at a recovery of a part of an initial energy input. The source of initial heat energy input can vary from gasification of renewable fuel such as wood chips and the like, to burning of a primary fuel but preferably augmented by burning of at least a portion of the gaseous pyrolytic decomposition gases produced as a result of treatment of the wastes. The apparatus is capable of handling the wastes both prior to and after the pyrolysis process. The pyrolysis temperature is relatively low and ranges from 250° C. to 600° C. The untreated wastes are dried directly in the pyrolysis reactor. By-products contain liquid fuel, e.g., oil. Although the heat generated by the apparatus is reused in the waste-processing system, the apparatus does not generate thermal energy as its final product. Another disadvantage is that gas produced in the process is discharged into the atmosphere without preliminary cleaning.
U.S. Pat. No. 5,678,496 issued in 1997 to D. Buizza, et al. describes a method and a plant for pyrolytic treatment of wastes that contain organic materials, particularly municipal solid wastes. The method comprises loading the wastes onto transport trolleys, inserting the trolleys with the waste into a treatment tunnel that has a pyrolysis chamber, carrying out pyrolysis of the wastes by indirectly heating the wastes to a temperature of 500° C. to 600° C., discharging the gaseous-phase substances generated by the pyrolysis from the chamber, and removing the trolleys from the tunnel for unloading the solid residues remained in the trolleys. The gas obtained as a result of pyrolysis is subjected to pre-cleaning by bicarbonate treatment that involves the use of nucleating and/or adsorbent agents, such as sodium bicarbonate or activated charcoal fines. The pre-cleaned gas is dissipated into the atmosphere, while the carbon residue of pyrolysis is utilized.
U.S. Pat. No. 5,868,085 issued in 1999 to A. Hansen, et al. discloses a system for pyrolysis of hydrocarbon constituents of waste material. The system includes a treatment unit featuring a retort with an ellipsoidal cross-section forming a first retort half and a second retort half. The material to be treated is selectively deposited in only one half of the retort at a time during any given period of system operation This is done to avoid abrasion and wear of the half not in use, thus prolonging the life of the retort component. For improving efficiency of the system, gases that formed in the pyrolysis are retuned directly into the interior of the retort. Prior to discharge into the atmosphere, the gaseous products of pyrolysis are cleaned from pollutants by using a plurality of selectively detachable gas injection tubes, which provide fuel for thermal oxidation of the gases. Each injection tube can be removed for cleaning independently of any other injection tube, and removal can be accomplished without disrupting operation of the system. The system is provided with a dryer that uses solar energy, which significantly limits the areas where the system can be used.
U.S. Pat. No. 6,178,899 issued in 2001 to M. Kaneko, et al. describes an installation for pyrolytic treatment of industrial and household wastes for carbonizing waste-containing organic substances in a condition sealed from an air so as to separate the wastes into a pyrolysis gas and a pyrolysis residue. The pyrolysis process is carried out at a high temperature in the rage of 1000° C. to 1200° C. In a gas cracking step of the process, pyrolysis gas is reacted with an oxide component for thermally decomposing high molecular hydrocarbon in the pyrolysis gas. The heat generated by the oxidization reaction produces a cracked gas that contains low molecular hydrocarbon. The product then passes through a residue cooling step for cooling and solidification of the pyrolysis residue. The obtained solid residue is mechanically crushed and sorted for obtaining a pyrolysis char consisting essentially of a pyrolyzed organic substance and inorganic components. The obtained pyrolysis char is burned at a high temperature by being mixed with fuel and oxygen or air for melting and gasification of the carbon component to obtain a gasified gas that contains low molecular hydrocarbon. Thus, the pyrolysis gas and the pyrolysis residue obtained from the pyrolysis furnace are treated separately. The cleaned gas obtained in the process is stored in a gas holder and can be supplied to a gas engine, a boiler, the pyrolysis chamber, or any other unit as required. Although the gas is cleaned in several steps, the system does not provide cleaning from CO2, and this does not allow increase of the calorie-content in the produced gaseous fuel.
U.S. Patent Application Publication No. 20059939650 published in 2005 (inventors C. Cole, et al.) describes a pyrolytic waste-treatment reactor supported in a manner that causes minimal movement or flexing of the chamber under effect of temperature changes. Wastes are mixed in the reactor by means of a single bladed shaft.
U.S. Pat. No. 6,619,214 issued in 2003 to W. Walker discloses a method and apparatus for treatment of wastes. The apparatus comprises four major cooperating subsystems, namely a pyrolytic converter, a two-stage thermal oxidizer, a steam generator, and a steam turbine driven by steam generated by the steam generator. In operation, the pyrolytic converter is heated without any flame impinging on the reactor component, and the waste material to be pyrolyzed is transported through the reaction chamber of the pyrolytic converter by a pair of longitudinally extending, side-by-side material transporting mechanisms. However, the system is designed only for afterburning of the gas and does not teach the subsequent use of the gaseous product as a fuel for a power generator. Although the waste feeder is made in the form of two parallel augers, the latter transport the wastes without effective mixing.
U.S. Patent Application Publication No. 20070113761 published in 2007 (inventors C. Cole, et al.) discloses a pyrolytic waste-treatment reactor with dual knife gate valves. The apparatus has a thermal chamber, a feed-stock inlet coupled to the thermal chamber for feeding the waste material into the thermal chamber, a heater that heats the thermal chamber, and at least one dual knife gate valve positioned in the apparatus for restricting the passage of the waste material through an interior space of the apparatus and for limiting introduction of gas into the thermal chamber. The dual knife gate valve has at least one movable blade that moves toward another blade. The wastes are moved without effective mixing.
U.S. Patent Application Publication No. 20070186829 published in 2007 (inventors C. Cole, et al.) discloses a variable speed pyrolytic waste treatment system comprising a pyrolysis chamber and a movement mechanism adapted to move waste through the pyrolysis chamber at different speeds along the length of the pyrolysis chamber. The main conception of this system is to vary the rate of movement of material through a pyrolysis chamber. In particular, material might move at a slower rate when it first enters the chamber and move at a faster rate after it has been heated and as is moved toward the chamber exit. The movement mechanism contains two shafts that are provided with screwed which covert into blades. The shafts rotate in opposite directions and therefore take the material from the central area and transfer it to the periphery of the chamber.
U.S. Patent Application Publication No. 20070289507 published in 2007 (inventors W. Parrott, et al.) discloses a system, method and apparatus for pyrolyzing wastes to produce a gaseous fuel and any remaining material in a plurality of chambers The gaseous fuel may be sent to an afterburner where other elements may be added to generate heated gas having a predetermined temperature. The heated gas may be used in a variety of devices, such as a heat exchanger, or the like. However, the system is not provided with a material crusher and dryer.
U.S. Patent Application Publication No. 20080236042 published in 2008 (inventor J. Summerlin) discloses a rural municipal waste-to-energy system and method converting waste to fuel gas through anaerobic digestion (AD) and pyrolysis/gasification. Fuel gases derived during the processes are combined and used to generate electricity. The amount of municipal solid wastes normally collected by a municipality may be inadequate in volume to produce enough fuel gas with which to generate electricity sufficient to serve the entire community. It may be necessary, therefore, for additional wastes from the surrounding area to be included. The municipality must have its own public electric utility to be able to charge rates necessary for profitability. A waste-drying step is not included, and the syngas is mixed with a biogas without any preliminary cleaning.
U.S. Patent Application Publication No. 20090020052 published in 2008 (inventors F. Becchetti, et al.) describes a process for solid waste treatment, and particularly municipal solid waste, with recovery of the thermal energy, which is based on the general pyrolysis process modified in order to improve, on the one hand, the energy yield and, on the other, to reduce the quantity of unusable solid residues to be sent to the waste disposal, the unusable solid waste being limited to 10-15% of the total weight of the initial residue. The process and relative plant include a boosted treatment of the incoming waste, with a preliminary separation into three solid fractions, the first one of which is separately subjected to a preliminary drying step and the third one undergoes further shredding. The process and relative plant also include a section for recovering energy from the pyrolysis coke, wherein the latter is subjected to a thermochemical treatment with the production of a further quantity of synthesis gas. The system offers drying only of those wastes the fraction of which does not exceed 80 mm, while waste fractions ranging from 80 mm to 300 mm are not subject to drying. The syngas is subjected to afterburning without pre-cleaning, although the flue gas obtained after afterburning allows generation of steam for use in a boiler and is cleaned prior to exhaust into the atmosphere. Electricity is generated by a steam turbine. The patent does not teach utilization of the coke residue that remains after the process.
Known in the art also are plasma waste processing systems (see, e.g., U.S. Pat. No. 7,394,041 issued in 2008 to W. Choi). However, the plasma process is applicable only to treatment of gaseous products. Therefore, prior to treating the wastes by plasma, it is necessary to covert the solid wastes into a gaseous state. For example, U.S. Pat. No. 5,544,597 issued in 1996 to S. Camacho discloses plasma pyrolysis and vitrification of municipal wastes. Municipal mixed solid wastes are delivered to a processing facility where it is compacted before being placed into a reactor. The compaction apparatus serves to remove most of the air and some of the water from the waste as well as to seal the reactor against air infiltration. A transfer apparatus, in response to a signal relating to the height of waste in the reactor, sequentially deposits blocks of compacted waste in the top of the reactor when the height is low. The reactor has a pivotally and extensively mounted plasma arc torch as a heat source which is effective to pyrolyze organic waste components to generate desired by-product gases. Air and steam are added in controlled quantities to improve the operational efficiency and the by-product gas composition. The residual materials which do not pyrolyze are melted and cooled into a substantially inert vitrified mass.
A disadvantage of plasma pyrolysis systems is that such systems require gasification of solid wastes which is associated with additional expenses. Another problem is associated with the generation of hazardous gases such as dioxins and furanes which are formed after cooling of the plasma products to a temperature below 500° C., if the wastes were treated at temperature above 1200° C. for less than 2 sec.
Another disadvantage of plasma systems is that such systems work at temperature of 2500 C to 5000 C. Such temperatures deteriorate structural materials of the system and lead to frequent and expensive maintenance and repairs.
Analysis of the known waste-processing systems based on pyrolytic treatment of wastes shows that a majority of these systems involves afterburning of the syngas either in a separate burning chamber or in a separate section of the pyrolysis reactor. The flue gases obtained as a result of afterburning are used for generation of steam or electric energy in steam turbines that have energy-generation efficiency lower than gas-turbine co-generators. Some systems offer to use the syngas for generation of energy in gas-turbine generators. However, in some cases the syngas is used as a component for a mixture with biogas without pre-cleaning, and in other cases the syngas is not separated from CO2 and therefore has low energy-generation efficiency. Another disadvantage of the known waste-processing systems is a lack of an efficient mechanism for feeding and mixing materials in pyrolysis chambers, a relatively high metal-to-power ratio in the structure of the pyrolysis chamber and, hence, high manufacturing cost, and incomplete cleaning of the gases, which are exhausted through the upper channel of the chamber together with the syngas, from fine waste particles.