INCINERATION SYSTEM FOR THE DESTRUCTION OF HAZARDOUS WASTES

Abstract: A process and apparatus for the controlled incineration of hazardous wastes comprising a two stage combustion of the waste into gases. The gases are thereafter cooled and cleaned by passing through primary (17) and secondary (20) scrubbers for cleaning. The invention also concerns an apparatus for the incineration of hazardous waste. The system cleans the gases produced by the incineration process so that hazardous materials are completely incinerated and convert­ ed into environmentally safe products at the site of their production.

Description: The present invention relates generally to the treatment of potentially hazardous waste materials, and more particularly to an improved process and apparatus for incinerating such materials. Prior to the present awareness of the hazardous nature of numerous waste materials, these materials were typically either dumped or were allowed to enter the ecosystem in other ways. Examples of such practices were the widespread dumping of wastes containing chlorinated hydrocarbons onto land or the release of toxic heavy metals, such as those produced in metal plating or metal finishing industries, into sewerage systems and hence to the sludges produced from sewage treatment or land farming. Such materials can and have entered the human food chain either through leaching by rain of the hazardous material into the underground water table or from food grown on land that has been contaminated by these substances. Numerous attempts have been made to address such environmental problems. In the past, heavily contaminated soil or sludge was often transported to a site remote from population centers or from sources of drinking water. The contaminated soil or sludge was then deposited into "safe" landfills after fixation. An attractive technical approach to the problem of disposing of hazardous waste material is offered by incineration which facilitates the conversion of chlorinated hydrocarbons into less hazardous by-products such as carbon dioxide and hydrogen chloride, or heavy metals into environmentally safe oxides. For this method to be successful, however, two requirements must be met. First, the incineration must be complete since partial oxidation of chlorinated hydrocarbons may result in the production of compounds that are more harmful than the original material. These harmful compounds may be either in the form of exhaust gases or residual ashes, or both. Second, all harmful combustion products must be removed completely from the gas stream produced by the incineration and thereafter rendered harmless. Thus far, incineration processes have not met these two essential requirements. One problem is presented by the complexity and sensitivity of the control system required for effecting a complete combustion. Another obstacle has been the size and cost of incinerators capable of eliminating industrial quantities of waste. The size and cost of adequate scrubbing systems for cleaning the resulting gases are similarly daunting. Moreover, the cost efficiency of existing incineration systems is significantly reduced by the need for mechanical stirring or vibration within the scrubbing systems. Another formidable limitation is the opposition by the public toward both the location of such an incinerator in their vicinity and the transportation of hazardous wastes thereto. As a result, incineration plants have generally been very large, costly, and, by necessity, stationary. Consequently, large quantities of hazardous waste materials have had to be carted from the waste-producing location over long distances on public roads with an obvious concomitant risk to the environment. In addition, the resistance by neighborhoods to the siting of such incinerating plants in their localities has grown to such a level as to inhibit the construction and use of this possibly beneficial technology. It is accordingly an object of this invention to provide a process for the complete incineration of hazardous waste into environmentally safe exhaust gases and residual ash. It is another object of this invention to provide such a process wherein all harmful combustion by-products are removed from the gas stream produced by the incineration process and thereafter rendered harmless. It is another object of this invention to provide such a process wherein harmful particulate matter is removed from the gas stream by passing the gas stream through a scrubber chamber without the need for mechanical vibration or stirring. The International Journal of Air Pollution Control and Hazardous Waste Management 37 (1987) May, No. 5 at pages 558-586 in a review of hazardous waste incineration describes apparatus generally in accordance with the precharacterising clause above, but in which the final separation of combustion solids is achieved in a Venturi scrubber involving injection of a scrubbing liquid into the exhaust gas stream as it passes through a high velocity constriction. Also, US-A-36 73 769 describes a gas scrubber in which entering gas is quenched, driven through a Venturi into a turbulent bed of water to impinge upon a submerged baffle and then aspirated upwardly through slots imparting a cyclonic flow to the gas to spin out solids. In accordance with a first aspect of the invention, there is now provided a process for treating hazardous waste by incineration, comprising the sequential steps of: (a) incinerating hazardous waste material in a first combustion chamber to produce a heavy solid residue, fine solids and vaporous by-products of combustion, and in a second combustion chamber to produce off-gas; (b) cooling the off-gas; (c) cleaning the cooled off-gas by passage through an atomized water spray and a water bath and into a secondary scrubber chamber; and (d) releasing the cleaned off-gas into the atmosphere, characterised in that the off-gas is first passed through the atomised water spray, is then deflected to assume a tortuous path through a Venturi tube to repeatedly contact the scrubbing surfaces of the Venturi tube, and is finally passed into the water bath in a brisk stirring or turbulent action. In accordance with a second aspect of the invention, there is provided also apparatus comprising a scrubber chamber adapted to contain a water bath at its lower end, an inlet for off-gas from a combustion system at its upper end, a water spray device located adjacent the inlet, deflecting means so positioned adjacent the inlet to deflect the off-gas in a tortuous path down a Venturi tube the upper portion of which is connected to the deflecting means and the lower portion of which is positioned to be immersed in the water bath, and an outlet for scrubbed off-gases located substantially above the water bath. The present invention is directed to an incineration system for the destruction of hazardous waste. In accordance with the broad aspects of the present invention, waste in the form of sold, liquid or sludge is introduced into and incinerated in a first combustion chamber to produce ash, particulate solids and vaporous materials. The ash by-products are discarded from the first combustion chamber and the remaining particulate solids and vaporous materials are introduced into and incinerated in a second combustion chamber to produce off-gases. Generally, the source of oxygen in the first and second combustion chambers is air. The off-gases are removed from the second chamber and are passed over cold water circulating coils so as to lower the temperature of the off-gases to at least 540°C (1000°F) to enable filtering of particulates of solid residue therefrom. After being subjected to an atomized water spray, the cooled gases are passed through a water bath and are then released into a primary scrubber chamber. Advantageously, the cooled gases are introduced into the primary scrubber without the need for mechanical vibration or stirring by deflecting the cooled gases down a Venturi tube into the water bath. The gases released from the primary scrubber are subsequently passed through a secondary scrubber chamber to remove any aerosol bubbles and to remove any residual contaminants. Cleaned gases are exhausted after treatment in the secondary scrubber chamber is complete. The invention is described below by way of example described in the followed specification as considered with only with reference to the accompanying drawing which is a schematic of the inventive incineration system. In practicing the present invention, as illustrated in Fig. 1, solid or liquid forms of contaminated waste material are fed into an open end (1) of a first combustion or rotary furnace chamber (3), which is held in a horizontal, slightly inclined position. On the end of rotary furnace chamber (3) opposite open end (1), a flame or burner (2) is arranged so as to give a counter-current incineration of the feed material within combustion chamber (3). The counter-current incineration permits the lighter combustion products to pass to the upper portions of combustion chamber (3) whereas the heavier combustion products remain at the bottom of chamber (3). In this way, less waste materials travel through the system. The heavier solid residue from this combustion event is removed at an opening (4) in rotary furnace chamber (3) whereas partially combusted fine solids and vaporous materials are carried in a gas stream in a tangential direction into a second combustion chamber (6). These particulate solids and vaporous materials pass from first combustion chamber (3) into second combustion chamber (6) through open end (5). Another flame or burner (7) arranged at the opposite end of second chamber (6) provides a co-current horizontal mode of combustion leading to a full incineration of the particulate and gaseous matter. The tangential approach into second combustion chamber (6) imparts a spinning motion to the gas stream carrying the combusted fine solids and vaporous materials. Thus, there is an increase in residence time within second chamber (6) without the depositing of particulate waste material in chamber (6). The pressures and temperatures utilized in first combustion chamber (3) and second combustion chamber (6) are not critical. Generally, the temperature will be within the range of about 760-870°C (1400-1600°F) and the pressure will be within the range of about 108-115 kPa (1-2 lbs). A temperature of about 815°C (1500°F) and a pressure of about 108 kPa (1 lb) are generally preferred. Generally, the oxygen source in first combustion chamber (3) and second combustion chamber (6) will be air. The off-gases emanating from second chamber (6) exit open end (8) of second combustion chamber (6) and enter a compartment (23) containing a system of cold water-circulating coils (10) through open end (9). The off-gases are first passed horizontally over cold water-circulating coils (10). The temperature of the gases is thereby lowered to about 540°C (1000°F) resulting in a reduction of the volume of the gases before introduction into a primary scrubber (17). At the entrance (12) of primary scrubber (17), the cooled gases are subjected to a spray of atomized water (13) to further cool the gases and to remove any remaining heavy particulate matter. The gases are then directed by deflectors (14) down a Venturi tube (15) into a water bath (16) located at the lower end of primary scrubber compartment (17). The deflectors (14) are positioned at the end of Venturi tube (15). Due to the initial severe angle of deflection of at least 135° from the horizontal, the gases assume a tortuous path through Venturi tube (15), and repeatedly hit the scrubbing surface. The gases then enter water bath (16) in a direction and velocity so as to produce a brisk stirring action or turbulence. This action is produced entirely by the change of direction of the gas flow and does not require any moving mechanical parts. The pH of the resulting solution can be easily controlled by introducing suitable acids or bases into water bath (16). Generally, the pH of the resulting solution is within the range 6.5 - 8.5. In the continuation of treatment, the gases or vapors released from water bath (16) pass from primary scrubber (17) to secondary scrubber (20). In one embodiment of the invention, secondary scrubber (20) may be a column packed with solids in which the solids in the column bed are coated with a film of continuously running water introduced through an aperture (21). The packing material consists of plastic irregular-shaped pieces which cannot nest or compact. This stage is to remove any aerosol bubbles produced by the turbulence in the preceding scrubbing stage. It also serves to remove any residual materials not cleaned by the previous treatment. Finally, the cleaned gases are exhausted through a chimney (22) into the atmosphere without danger or contamination. Control of the combustion process which is essential to "clean" burning is achieved in the present invention by a reliable process in which gas temperatures are measured and controlled automatically via a microcomputer-directed feedback system that operates the air intake control valves at the location of first combustion chamber (3). Thus both incineration chambers (3) and (6) can be run at predetermined and continually maintained temperatures. More specifically, the microcomputer feedback system is a continuous monitoring system comprising a sampling probe, a heat gas transport line, sampling pumps, an oxygen 0₂ and carbon monoxide CO analyzer, a manually calibrated analyzer system and a data recorder. Once the combustion conditions such as temperature and pressure ranges are established, the microcomputer feedback system will monitor the operation of the incineration system under the desired treatment conditions. If operation conditions deviate from the predetermined ranges, a fail safe will be initiated. A further aspect of the invention is the facility to remove particulate and gaseous incineration products by introducing chemicals to inactivate or bind the water soluble hazardous materials in primary scrubber system (17). Specifically, the inventive system permits the use of very reactive compounds in primary scrubber stage (17) followed by water or less reactive materials in secondary scrubber (10). Examples of these facilities are: sodium hydroxide solution present in both scrubbers to remove the hydrogen chloride produced by combustion of chlorinated hydrocarbons; and hydrazide or sodium borohydride in the first scrubber stage to chemically reduce to metal those metal oxides which are the incineration by-products from heavy metal contaminated sludges. The present invention provides a further advantage in that the inventive incineration system can be transported to any desired location. The incineration system of the present invention is comprised of first combustion chamber (3), second combustion (6) in communication with first combustion chamber (3), compartment (23) containing a system of cold water circulating coils (10), wherein compartment (23) is in communication with second combustion chamber (6), primary scrubber (17) in communication with compartment (23), and secondary scrubber (20) in communication with primary scrubber (17). The chambers and parts comprising the incineration system can either be transported as a unit or be assembled at the site of hazardous waste. In this way, hazardous materials can be treated and disposed of at the site of their production, thus eliminating the need for costly and dangerous transportation of such materials. It will be appreciated that the waste incinerator system of the present invention as described above meets the objects set forth in that it is mobile and allows for the treatment of hazardous waste at the site of production.

Claims: A process for treating hazardous waste by incineration, comprising the sequential steps of: (a) incinerating hazardous waste material in a first combustion chamber (3) to produce a heavy solid residue, fine solids and vaporous by-products of combustion, and in a second combustion chamber (6) to produce off-gas; (b) cooling (10) the off-gas; (c) cleaning the cooled off-gas by passage through an atomized water spray (13) and a water bath (16) and into a secondary scrubber chamber (20); and (d) releasing (22) the cleaned off-gas into the atmosphere, characterised in that the off-gas is first passed through the atomised water spray, is then deflected (14) to assume a tortuous path through a Venturi tube (15) to repeatedly contact the scrubbing surfaces of the Venturi tube, and is finally passed into the water bath (16) in a brisk stirring or turbulent action. A process according to claim 1, wherein the hazardous waste material is passed through the first combustion chamber in counter-current to the combustion products of the first combustion. A process according to claim 1 or claim 2, wherein the heavy solid residue produced in the first combustion chamber is discarded and the fine solids and vaporous by-products are introduced into the second combustion chamber. A process according to any one of claims 1 to 3, wherein the incineration in the first and second combustion chambers is controlled by a temperature probe and a computer feed-back system. A process according to any one of claims 1 to 4, wherein the off-gas is cooled by passage over a system of cold water circulating coils (10) to a temperature of at least 535°C (1000°F). A process according to any one of claims 1 to 5, wherein the pH of the water bath is controlled. A process according to any one of claims 1 to 6, wherein the off-gas is passed in the secondary scrubber chamber over a plurality of irregularly shaped plastics pieces which are coated with a film of flowing water. Apparatus comprising a scrubber chamber (17) adapted to contain a water bath (16) at its lower end, an inlet (12) for off-gas from a combustion system at its upper end, a water spray device (13) located adjacent the inlet, deflecting means (14) so positioned adjacent the inlet to deflect the off-gas in a tortuous path down a Venturi tube (15) the upper portion of which is connected to the deflecting means and the lower portion of which is positioned to be immersed in the water bath, and an outlet (18) for scrubbed off-gases located substantially above the water bath. Apparatus for use in carrying out the process of claim 1, the apparatus comprising in series communication (a) a first combustion chamber (3); (b) a second combustion chamber (6); (c) a compartment (23) containing a system of cooling coils (10); (d) a primary scrubber (17); and (e) a secondary scrubber (20), characterised in that the primary scrubber comprises apparatus according to claim 8. Apparatus according to claim 9, wherein the first combustion chamber comprises a rotary furnace.

Inventors: PHILLIPS PATRICK A, PHILLIPS, PATRICK, A.
Applicants: VESTA TECHNOLOGY INC, VESTA TECHNOLOGY, INC.
Application year: 1988
Main classification: F23D14
IPC codes: F23G5, F23J15, B01D53