HK1074416B - Hazardous waste treatment method and apparatus - Google Patents

Description

Method and equipment for treating harmful waste

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent application serial No. 60/378,357, filed at 5/8/2002, the disclosure of which is incorporated herein by reference.

Technical Field

The invention relates to a system for treating industrial waste gas or harmful waste.

Background

In addition to being a serious source of pollution, hazardous waste affects human health through its toxicity, deflagration, corrosivity, reactivity, and infectivity. Hazardous waste is typically disposed of by land treatment, incineration and recycling. However, as some inappropriate waste treatment events, such as the emission of toxic substances from waste incineration and landfills (e.g. dioxin from incineration and toxic leachate from landfills), begin to create serious health and ecological problems, public concern has led to increased legislation and stricter environmental protection policies. These policies have led to the search for other efficient, reliable and cost-effective alternative processing approaches.

Many methods based on plasma arc are proposed for destroying various forms of organic and inorganic hazardous waste, converting the hazardous waste into a flammable synthesis gas for power generation, and converting all non-flammable materials into a stable glassy state that can be safely disposed of. However, these methods are considered inefficient and require significant capital and operating costs.

In general, two basic plasma arc techniques, namely plasma torches (transferred and non-transferred modes) and graphite electrode plasma arc (a.c. or d.c.) systems, have been proposed to generate plasma arcs for hazardous waste destruction or conversion processes.

Systems using plasma torches are generally less energy efficient than systems using graphite electrodes because more energy is lost to the cooling water of the plasma torch. Especially when a metal plasma torch is placed inside a hot reactor/vessel and operated therein, the efficiency of the plasma torch is typically less than 70%. Thus, the plasma torch is effective for gas heating and special material processing or manufacturing processes, and has no practical and economic value for material dissolution. Further, when air is used as the plasma working gas, Nitrogen Oxides (NO) are generated due to the reaction of nitrogen in the air plasma working gas with oxygen and hydrocarbons in the container/reactor at high temperature_x) And Hydrogen Cyanide (HCN). Furthermore, the vapour generated in the vessel may condense to the surface of the metal cover of the plasma torch. Thus, carbon black/soot, along with non-free toxic materials, will deposit and accumulate on the cold wet metal cover without causing complete destruction of the hazardous waste. When the plasma torch is to be removed from the vessel for maintenance, workers are therefore subjected to toxic materials.

The lifetime of the electrode and the stability (performance) of the plasma arc generated by the plasma torch also depend on the atmosphere inside the vessel/reactor. Therefore, the operation of the plasma torch system is more complicated than that of the graphite electrode plasma arc system. Metal plasma torches require high pressure cold water for cooling the internal components. The chemical and electrical conductivity of the cooling water must be monitored and adjusted to prevent chemical corrosion and ore deposition inside the torch. These require the use of expensive auxiliary equipment, thus increasing both production capital and operating costs.

Other systems use graphite electrode plasma arc technology. These systems can lead to severe oxidation of the graphite electrode or excessive formation of fine carbon black/soot in the byproduct gas stream. A combined system of a.c. and d.c. graphite electrodes was developed to provide both arc generation and joule resistance heating in the cell. Other techniques use a concentric electrode system and a single top d.c. graphite electrode with a conductive bottom for melting and gasification. However, the single top d.c. graphite electrode system must maintain the electrical conductivity of the bottom electrode at all times, especially when the bottom electrode of the cold vessel/reactor is covered by a layer of slag that is not electrically conductive at low temperatures.

It has been found that the kinetics of soot formation during pyrolysis of hydrocarbons under slightly reducing conditions are very high. Thus, soot is always produced during the gasification process of the reducing plasma arc and must be removed prior to the downstream air pollution control system. Increasing the residence time of the by-products inside the vessel/reactor or increasing the operating temperature helps to remove the carbon black. However, increasing the residence time requires the use of larger equipment or a reduction in the throughput of added waste. Therefore, some systems have been proposed that include an afterburner or thermal oxidizer to enhance the reaction kinetics by the turbulent environment as a secondary gas treatment process to ensure complete combustion. However, these methods of generating high heat for the oxidation process use air and fuel. Thus, a secondary waste stream such as nitrogen oxides may be generated in these systems under such an oxidizing atmosphere.

It would be beneficial to have a system and method for treating exhaust from a waste treatment system that at least partially suffers from these disadvantages.

Disclosure of Invention

The present invention provides a system for treating exhaust gas from a waste treatment system, such as a graphite electrode arc gasification system, that reduces carbon black in the exhaust gas while avoiding the production of nitrogen oxides and other pollutants. The system includes an afterburner using a plasma torch having a nitrogen-free working gas, which in one embodiment is a mixture of carbon dioxide and oxygen. The plasma arc ionizes the working gas, thus forming atomic oxygen, which helps to remove carbon black from the exhaust.

In one aspect, the present invention provides an apparatus for treating an exhaust gas from a waste treatment system. The apparatus includes a refractory-lined cylindrical chamber having an input port for receiving the off-gas and an output port, and a DC powered plasma torch positioned within the chamber proximate the input port, the torch receiving a working gas, the working gas comprising a mixture of carbon dioxide and oxygen. The plasma torch heats the chamber and the exhaust gas is converted in the chamber to an output gas which is ejected through an output port.

In another aspect, the present invention provides a method for treating an exhaust gas from a waste treatment system. The method comprises the following steps: the method includes receiving the exhaust gas at an output port of the refractory-lined cylindrical chamber, heating the refractory-lined cylindrical chamber by ionizing a working gas comprising a mixture of carbon dioxide and oxygen using a DC-powered plasma torch located within the chamber proximate the input port, thereby converting the exhaust gas to an output gas, and outputting the output gas from the cylindrical chamber.

In yet another aspect, the present invention provides a waste treatment system for treating hazardous waste. The waste treatment system includes a primary waste treatment stage that receives hazardous waste and produces a by-product waste gas and a secondary waste treatment stage that couples the primary waste treatment stage and receives the waste gas. The secondary waste treatment stage comprises a refractory-lined cylindrical chamber having an input port for receiving the off-gas and an output port, and a DC powered plasma torch located within the chamber proximate the input port, the torch receiving a working gas comprising a mixture of carbon dioxide and oxygen. The plasma torch heats a refractory-lined cylindrical chamber and the exhaust gas is converted to an output gas which is ejected from an output port.

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

Brief Description of Drawings

In the following, by way of example, reference is made to the accompanying drawings which show a specific embodiment of the invention, and in which:

FIG. 1 shows a waste treatment system diagram of the present invention;

FIG. 2 shows a top block diagram of the waste treatment system; and

figure 3 shows a cross-sectional view of the cyclonic oxidation chamber of the present invention.

Like numbers in different figures represent similar parts.

DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION

Referring first to figures 1 and 2, figure 1 shows a diagram of a waste treatment system 100 of the present invention, and figure 2 shows a top block diagram of the waste treatment system 100. The system 100 includes a graphite electrode d.c. plasma arc gasifier/melter 4 and a plasma torch cyclone oxidizer 3. The waste material is introduced into a gasifier/melter 4 which melts the non-combustible material and decomposes the organic material. The off-gas produced by the gasifier/melter 4 is introduced into the cyclonic oxidizer 3. The cyclonic oxidizer 3 then treats the exhaust gas according to the present invention. The gasifier/melter 4 may also be considered a gasification/vitrification chamber.

The gasifier/melter 4 is preheated to a temperature above 1500 c by melting scrap steel in the gasifier/melter 4 before charging the hazardous waste into the gasifier/melter 4 of the graphite electrode d.c. arc for destruction. The gasifier/melter 4 is refractory lined and the sidewalls and roof of the d.c. arc gasifier/melter 4 are water cooled to minimize mechanical and chemical corrosion due to melting to extend the life of the refractory. The refractory system serves to contain the melt and minimize heat loss from the gasifier/melter 4. The refractory material is also chemically compatible with the slag and by-product gases produced.

As shown in fig. 2, two graphite electrodes pass through the top of the gasifier/melter 4. The electrode clamps 16 and 17 clamp the graphite electrode and are connected to the d.c. power supply 2. The electrode clamp 16 is connected to the negative pole of the power source 2, while the electrode 17 is connected to the positive pole. Electrode clamps 16 and 17 are attached to electrode arm 15 as part of an auto-responsive electrode guidance system that moves electrode arm 15. The automated responsive electrode guide system moves the electrode arm 15 to adjust the mutual position of the two graphite electrodes and the position relative to the molten material in the hearth of the gasifier/melter 4. Adjusting the relative position of the graphite electrodes affects the length of the arc. An electrode sealed at the top serves to condition the electrode while preventing ingress of ambient gases and escape of by-product gases out of the d.c. arc gasifier/melter 4.

The viewport 25 of the gasifier/melter 4 allows for uniform entry of scrap steel. The positive electrode clamped by the electrode clamp 17 is buried in the scrap steel, while the negative electrode clamped by the electrode clamp 16 is located above the solid scrap steel. The cathode electrode is then slowly lowered until an arc is created between the anode electrode and the scrap steel. Scrap steel begins to melt at temperatures in excess of 1500 c to form a molten bath. When the scrap steel is completely melted in the hearth, the cathode electrode is then raised to obtain a long arc length while the anode electrode remains submerged in the molten bath.

The system 100 includes a feed mechanism for introducing hazardous solid waste to the gasifier/melter 4. In other embodiments, rather than receiving the solid hazardous waste directly, the gasifier/melter 4 receives toxic byproducts from a primary chemical or incineration process. The main chemical or incineration processes produce toxic by-products that are reduced to non-toxic stable slag in the gasifier/melter 4.

In this embodiment, the feed mechanism includes a conveyor 20 and a gas-tight chamber 22 connected to the gasifier/melter 4. A gas-tight door 21 separates the conveyor 20 from the gas-tight chamber 22, and a water-cooled gas-tight door 24 separates the gas-tight chamber 22 from the gasifier/melter 4.

Waste can be distributed by the conveyor 20 through the gas-tight door 21 to the gas-tight chamber 22. After a batch of waste has been distributed to the gas-tight chamber 22, the gas-tight door 21 is closed. Next, the gas-tight chamber 22 is evacuated by opening the vacuum control valve 14 to remove air in the gas-tight chamber 22. Then, the vacuum control valve 14 is closed and the carbon dioxide control valve 13 is opened to refill the gas-tight chamber 22 with carbon dioxide to prevent by-products from escaping the d.c. arc gasifier/melter 4 when the water-cooled gas-tight door 24 is opened. The gas-tight chamber 22 comprises a high temperature resistant hydraulic cylinder 23 for advancing the waste material within the gas-tight chamber 22. When the gas-tight door 24 is fully open, the hydraulic ram 23 pushes the waste along a side or top chute into the d.c. arc gasifier/melter 4. Once the waste is pushed into the gasifier/melter 4, the hydraulic ram 23 is retracted to its original position in the gas-tight chamber 22. The water cooled gas-tight door 24 is then closed, the valve 13 is closed, and the vacuum valve 14 is opened to remove the carbon dioxide in the gas-tight chamber 22 until the gas-tight door 21 begins to open to receive the waste from the conveyor 20 again, thereby completing the feeding cycle of the solid waste.

For liquid or gaseous hazardous waste, the waste is metered and pumped into the molten bath in the d.c. arc gasifier/melter 4 via retractable high temperature resistant atomizing nozzles located in the side walls. Steam was used as the carrier gas and to clean the liquid/gas feed line.

Inside the gasifier/melter 4, the wastes are placed in an extremely high temperature atmosphere, and an electric arc is generated between the cathode electrode 16 and the molten iron. The organic matter in the waste is split into its respective atomic forms. Due to the extremely high temperature conditions, the formation of dioxin/furan can be completely avoided. The incombustible matter containing the metal and the glass is melted and mixed with the molten iron to produce liquid slag and metal in the hearth. Slag and metal are sometimes removed from the d.c. arc gasifier/melter 4 by opening a tap hole 19 with a drill bit. Thermocouples are installed in the side walls, top and bottom to control the rim height (freeboard) and the temperature of the refractory material. If the refractory material and the high edge temperature start to drop, the electrode power is increased by increasing the current or cathode voltage. The internal pressure of the arc gasifier/melter 4 is kept at a negative value at d.c. to avoid the release of by-product gases into the ambient air by the exhaust fan of the air pollution control system 8.

The gas produced by the gasifier/melter 4 is treated in a cyclonic oxidizer 3. The cyclonic oxidizer 3 is connected to the gasifier/melter 4 to receive the by-product gas produced by the gasifier/melter 4. In a particular embodiment, the by-product gas produced at the d.c. arc gasifier/melter 4 may include carbon monoxide, hydrogen, light hydrocarbons, carbon black, and small amounts of carbon dioxide. Carbon black/soot due to its fine particle size often presents serious operational problems in downstream energy recovery and air pollution control systems. Furthermore, the carbon black/soot can act as nucleation sites for modifying toxic organic compounds. The exhaust gas enters the cyclonic oxidizer 3 tangentially at a high velocity, thereby creating a cyclonic condition within the cyclonic oxidizer 3. In one embodiment, the cyclonic oxidizer 3 is positioned substantially horizontally, sloping slightly downwardly from the upstream end to the downstream end.

Reference is now made to fig. 3 of the present invention, which shows a cross-sectional view of the cyclonic oxidizer 3. A vertical refractory offgas duct 26 is used to connect the d.c. arc gasifier/melter 4 and the cyclonic oxidizer 3. The off-gas pipe 26 injects the by-product gas tangentially into the bottom side of the cyclonic oxidizer 3 adjacent its upstream end. The vertical offgas duct 26 minimizes the pressure drop between the d.c. arc gasifier/melter 4 and the cyclonic oxidizer 3 to improve the flow of offgas to the cyclonic oxidizer 3. The efficiency of the oxidation reaction is increased by the strong internal mixing between the by-product gas and the injected atomized oxygen and steam caused by the intensity of the cyclonic action within the cyclonic oxidizer 3.

In another embodiment, the cyclonic oxidizer 3 treats off-gas resulting from a primary chemical reaction or incineration process, in which case the off-gas is routed directly to the cyclonic oxidizer 3. In this case, the gasifier/melter 4 may be unnecessary.

The cyclonic oxidizer 3 includes a d.c. plasma torch 18 at its upstream end. The plasma torch 18 preheats the cyclonic oxidizer above 1300 c. The plasma torch 18 is powered by the d.c. power supply 1. In a particular embodiment, the d.c. power supply 1 for the plasma torch 18 is independent of the d.c. power supply 2 for the gasifier/melter 4 so that if the power supply 2 for the gasifier/melter 4 fails, it can be ensured that the cyclonic oxidizer 3 can continue to operate. The cyclone oxidizer 3 is lined with refractory 32 and thermocouples 27, 28 and 29 are installed along the side of the inner refractory 32 for controlling the hot face temperature. If the temperature is below 1350 c during the heat treatment, the power of the plasma torch 18 or oxygen injection is increased. The operation of the plasma torch 18 may be controlled by the process controller 6 (see fig. 2) through a feedback loop. The process controller 6 of the present invention may include a microcontroller suitably programmed to execute a set of instructions or functions for performing control steps and providing control signals.

The plasma torch 18 uses a mixture of carbon dioxide and oxygen as the plasma working gas. The gases are initially mixed in a dynamic mixer 5. The dynamic mixer 5 responds by adjusting the composition of the gas mixture and controlling the flow rate of the gas mixture according to the desired operating conditions and the requirements of the plasma gas. In a particular embodiment, the volume content of oxygen in the gas mixture is between 15% and 25%, preferably 21%. An oxygen sensor is included in the dynamic mixer 5 to monitor the oxygen content in the gas mixture. The use of carbon dioxide and oxygen as the plasma working gas avoids the formation of nitrogen oxides and hydrogen cyanide. The dynamic mixer 5 may receive control signals from a process controller 6.

When the gas mixture is ionized in the plasma arc region at temperatures in excess of 5000 ℃, carbon dioxide separates into carbon monoxide and very reactive atomic oxygen species. The combination of reactive atomic oxygen and the increased turbulent environment within the cyclonic oxidizer 3 allows for the effective conversion and destruction of carbon black/soot and volatile toxic materials in the by-product gas. The particulates in the by-product gas melt and can form a molten layer remaining on the side wall by the centrifugal force generated by the cyclonic action of the cyclonic oxidizer 3. The molten material flows down to a more downstream side bottom portion fitted with a nozzle 33 connected to a container 34 to receive the molten material. The molten material is then solidified and removed within the vessel 34 and returned to the d.c. arc gasifier/melter 4 where the slag forms a glassy substance.

Oxygen and steam are metered and injected through control valves 10 and 11 into the cyclonic oxidizer 3 as oxidant. The gas is atomized through the high temperature resistant atomizing nozzles 30 and 31. The process controller 6 includes an online exhaust gas monitoring sensor for analyzing the by-product gas composition: carbon monoxide, hydrogen, hydrocarbons and carbon dioxide. From the analyzed data, the process controller 6 can quickly send process control signals to the control valves 10 and 11 to control the injection of oxygen and steam. For low heating value waste, the cyclone oxidizer 3 completely converts the by-product gas into water and carbon dioxide to produce a clean exhaust gas to the atmosphere by increasing the injection amount of oxygen and or steam until the total concentration of light hydrocarbons and carbon monoxide is less than 20 ppm. For high calorific value waste, the final by-product gas may be a high quality combustible synthesis gas for power generation. At carbon dioxide concentrations above 3%, the injection of steam and/or oxygen is reduced. And when the concentration of carbon dioxide is less than 1%, the injection of oxygen and steam is increased.

Referring again to fig. 1, enthalpy in the byproduct gas produced from the cyclonic oxidizer 3 may be recovered by a heat exchanger 7 to produce hot water or steam for improving overall process efficiency. The vapor is recycled to the liquid/gas waste feed system as a carrier gas and applied to the cyclonic oxidizer 3 as an oxidant. The cooled gas is processed by an air pollution control system 8 before the final product gas is stored as a combustible synthesis gas containing mainly hydrogen and carbon monoxide or before the final product gas is compressed in a compressor 9 to produce liquefied carbon dioxide.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. Certain further applications and modifications of the present invention will be apparent to those skilled in the art. The particular embodiments discussed above are therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (29)

1. An apparatus for treating an exhaust gas from a waste treatment system, comprising:

(a) a refractory-lined cylindrical chamber having an inlet port for receiving the exhaust gas and an outlet port; and

(b) a DC powered plasma torch located within the chamber proximate the input port, the torch receiving a working gas comprising a mixture of carbon dioxide and oxygen and no nitrogen;

wherein said plasma torch heats said chamber thereby converting the exhaust gas into an output gas which is ejected through said output port,

further comprising:

a dynamic mixer connected to the plasma torch and providing the working gas, the mixer receiving a supply of oxygen gas and a supply of carbon dioxide gas.

2. The apparatus of claim 1, wherein the mixture of carbon dioxide and oxygen comprises 15-25% oxygen by volume.

3. The apparatus of claim 1, further comprising an oxygen injector in communication with the chamber for injecting nebulized oxygen and a steam injector in communication with the chamber for injecting nebulized steam.

4. The apparatus of claim 3, wherein the oxygen injector and the steam injector comprise a heat resistant atomizing nozzle fluidly connected to the chamber.

5. The apparatus of claim 3, further comprising a sensor coupled to the output port for analyzing the composition of the output gas, and a process controller coupled to the sensor for receiving data from the sensor and to the injector for controlling the injection of oxygen and steam.

6. The apparatus of claim 5, wherein the mixer mixes the gas supply in response to a control signal from the process controller.

7. The apparatus of claim 1, wherein the plasma torch comprises a plasma region operating at a temperature greater than 5000 ℃.

8. The apparatus of claim 1, further comprising a temperature sensor within the chamber, and wherein the temperature within the chamber is maintained at greater than 1300 ℃.

9. The device of claim 1, wherein the chamber is horizontally disposed and wherein the chamber comprises an upstream end, a downstream end, and a sidewall therebetween.

10. The apparatus of claim 9, wherein the plasma torch penetrates the upstream end, the input port comprising an inlet tube that passes tangentially through the sidewall and is proximate to the upstream end.

11. The apparatus of claim 1, wherein the working gas consists essentially of carbon dioxide and oxygen.

12. The apparatus of claim 1, wherein the working gas consists of carbon dioxide and oxygen.

13. A method for treating an exhaust gas from a waste treatment system, comprising the steps of:

(a) receiving the flue gas at an input port of a refractory-lined cylindrical chamber;

(b) heating the chamber by ionizing a working gas comprising a mixture of carbon dioxide and oxygen and no nitrogen using a DC powered plasma torch located proximate to an input port within the chamber, thereby converting the exhaust gas to an output gas; and

(c) the output gas is output from the chamber.

14. The method of claim 13, wherein the mixture of carbon dioxide and oxygen comprises 15-25% oxygen by volume.

15. The method of claim 13, further comprising the step of injecting atomized oxygen and atomized steam into the chamber.

16. The method of claim 15, further comprising the steps of analyzing the output gas composition and controlling oxygen and steam injection based on the analyzing step.

17. The method of claim 16, further comprising the step of mixing a supply of oxygen and a supply of carbon dioxide gas in a dynamic mixer to produce said working gas.

18. The method of claim 13, wherein the ionizing step is performed in a plasma region having an operating temperature greater than 5000 ℃.

19. The method of claim 13, further comprising the step of determining a temperature within the chamber, wherein the temperature is maintained at greater than 1300 ℃.

20. The method of claim 13, wherein the working gas consists essentially of carbon dioxide and oxygen.

21. The method of claim 13, wherein the working gas consists of carbon dioxide and oxygen.

22. A waste treatment system for treating hazardous waste, comprising:

(a) a primary waste treatment stage that receives hazardous waste and produces a by-product waste gas;

(b) a secondary waste treatment stage connected to the primary waste treatment stage and receiving the waste gas, the secondary waste treatment stage comprising:

(i) a refractory-lined cylindrical chamber having an input port and an output port for tangentially receiving the exhaust gas; and

(ii) a DC powered plasma torch located within the chamber proximate the input port, the torch receiving a working gas comprising a mixture of carbon dioxide and oxygen and no nitrogen;

wherein the plasma torch heats the chamber, thereby converting the exhaust gas into an output gas within the chamber, the output gas being ejected through the output port.

23. The waste treatment system of claim 22, wherein the mixture of carbon dioxide and oxygen comprises 15-25% oxygen by volume.

24. The waste treatment system of claim 22, wherein said primary waste treatment stage comprises a gasification/vitrification chamber and a transfer system connected to said gasification/vitrification chamber through a gas-tight door, said transfer system adding solid hazardous waste to said gasification/vitrification chamber.

25. The waste treatment system of claim 22, wherein said primary waste treatment system comprises a gasification/vitrification chamber and an inlet pipe connected to said gasification/vitrification chamber, said inlet pipe feeding liquid or gaseous hazardous waste to said gasification/vitrification chamber.

26. The waste treatment system of claim 22, wherein the primary waste treatment stage comprises a graphite electrode plasma arc gasifier/melter.

27. The waste treatment system of claim 26, wherein the graphite electrode plasma arc gasifier/melter comprises a pair of spaced apart graphite electrodes, each sandwiched by a corresponding electrode clamp attached to a movable electrode arm, wherein the electrode arms are operable to adjust the relative distance between the pair of spaced apart graphite electrodes or the relative distance between the electrodes and the molten material within the graphite electrode plasma arc gasifier/melter, thereby adjusting the length of the arc.

28. The waste treatment system of claim 22, wherein the working gas consists essentially of carbon dioxide and oxygen.

29. The waste treatment system of claim 22, wherein the working gas consists of carbon dioxide and oxygen.