AU2009238232A1 - Methods and apparatus for waste treatment by melt decomposition assisted with plasma arc heating - Google Patents

Description

P/00/011 Regulation 3.2 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "METHODS AND APPARATUS FOR WASi E TREATMENT BY MELT DECOMPOSITION ASSISTED WITH PLASMA ARC HEATING" The following statement is a full description of this invention, including the best method of performing it known to me/us: 1 METHODS AND APPARATUS FOR WASTE TREATMENT BY MELT DECOMPOSITION ASSISTED WITH PLASMA ARC HEATING FIELD OF THE INVENTION 5 The present invention relates to a system for treating wastes including hazardous wastes by melt decomposition assisted with plasma arc, More specifically, it relates to a system that utilizes an ultra-high temperature field created by plasma arc heat and/or Joule heat to make gasifying agent react with the organic substances of the wastes to 10 produce hydrogen-rich gas with high heating value, which can be purified and then used for power generation or recovering thermal energy. The inorganic substances of the wastes can be melted to produce vitreous slag, which can be treated by water quenching and then directly used as the building materials; and the metals of the wastes can be directly cast 15 into metal ingots and recycled. BACKGROUND OF THE INVENTION Plasma arc can produce a ultra-high temperature field of 3000-10000*C, and therefore is widely applied in the field of waste 20 treatment. Under the ultra-high temperature and heat capacity of plasma arc, organic substances can be decomposed into combustible micro-molecule gases and compounds with stable chemical properties, such as C, CnHm, CO, and H 2 . Inorganic compounds can be melted and decomposed into extra fine substances, which either are vitrified and into 25 the molten pool, or float in the exhaust gas. The molten metals stay at the bottom of the furnace. The four systems respectively disclosed in US Patent No. 4644877 by Barton et al, US Patent No. 4431612 by Bell et al, US Patent No.

2 5280757 by Carter et al, and US Patent No. 5284503 by Bitler et al, have many drawbacks. These drawbacks include: not enough time for heating, mixing and retention for a wide range of wastes is provided to produce high-quality and non-infiltration vitreous substances. Moreover, the hearth 5 dimensions and the feeder design have very strict limit, because the furnace walls must be relatively close to the electric arc plasma since the electric arc plasma is the only heat source. Due to the limited hearth dimensions, very high thermal stress is often generated on the furnace walls. The plasma arc generator employs water-cooled metal electrodes, 10 which have short service life and low thermal efficiency, and therefore their application is limited. In the two waste treatment systems assisted by plasma arc as respectively disclosed in Chinese Patent No. ZL96192788.7 by Charles (USA) et al and Chinese Patent No. ZL200310121342.0 by Hongzhi 15 SHENG et al of Institute of Mechanics, Chinese Academy of Sciences, fixed graphite electrodes are used as plasma arc generator, and inductive heating and resistance heating are used in combination in the furnace, bringing light to practical application. However, the apparatus is complex and the electric power consumption is huge, and also it is difficult to 20 initiate and strike arc, and a great deal of fly ash is exhausted from the furnace. Though such systems have been applied in some projects, there are still restrictions in their development. In the Chinese Patent Application No. 200480004183.9 by Tsangaris (Canada) et al, a plurality of plasma arc torches are designed, wherein, a 25 plasma arc torch with three degrees of freedom is arranged. In the system disclosed in the patent application, all input waste can fall into the plasma arc area, and therefore can be melted and decomposed quickly and completely. The resulting fly ash together with the exhaust gas pass 3 through a downward inclined structure of furnace roof and the spraying of additive and water vapor is helpful for a portion of the fly ash to fall into the molten pool. However, the system has a drawback that it totally employs water-cooled metal plasma arc torches, as results in high energy 5 consumption, low efficiency, and short service life, and inadequate quantity of fly ash dropping into the molten pool. In the Chinese Patent Application No. 200480042882.2 by HWANG, Soon Mo (Korea), an inclined water-cooled metal plasma arc torch is disclosed, wherein, the ionized carrier fluid that is blown out at a high 10 speed is utilized to form swirling gas flow in the reactor, so that the fly ash carried by the gas flow is absorbed under the centrifugal force onto the inside walls or into the molten pool in the reactor, and then melted. Due to the limitation of structure, only a non-transfer-arc water-cooled metal plasma arc torch can be used, and the wastes can not be heated directly; 15 instead, the wastes can only be melted and decomposed by the relatively far radiation heat; therefore, the formal industrial application of the system is limited. In the Chinese Patent Application No. 200710023338.9 by Jialiang DING et al, the difficulty in plasma arc striking on graphite electrode is 20 settled by utilizing an assistant electrode. However, the problem of fly ash reduction is not solved successfully yet. CONTENTS OF THE INVENTION The systems for waste treatment by melt decomposition with 25 plasma arc described above have many drawbacks. The first drawback is: all of the systems totally employ electric power, and therefore have high energy consumption and are adverse to power generation of the generated hydrogen-rich gas; because the generated power fails to meet 4 the power demand of the systems itself in some cases, the economic issue is often in doubt. The second drawback is: the wastes usually can not be introduced into the ultra-high temperature area created by plasma arc heat since such introduction may cause contamination of the electric 5 arc electrodes and subsequent instability of electric arc, and because the wastes are introduced to the peripheral downstream area away from the electric arc, and is indirectly heated by the torch gas, the retention time of the wastes in ultra-high temperature area is shortened relatively, resulting in incomplete decomposition of the wastes; though the above problem 10 has been solved in some systems, other problems such as overheating of furnace wall are not solved. The third drawback is: the discharge of fly ash severely affects subsequent exhaust gas purification and increases abrasion of the power generation system; even if the fly ash is melted for the second time, it will consume energy again. 15 In conclusion, no prior art can provide an effective method for melting and decomposing all types and all forms of wastes (including hazardous wastes) into compounds that are suitable for treatment without polluting the environment, with low energy consumption. Therefore, the object of the present invention is to overcome such 20 drawbacks, and provides a system and a method for waste treatment, which ensures all components of wastes can be decomposed completely by plasma arc while maintaining low electric power input, and ensures long service life of the refractory lining material, and keeps a great deal of fly ash produced by melt decomposition be melted in the furnace, wherein 25 the plasma arc can be initiated from the electrodes easily, the electrodes are easy to control, the vitreous substances can be discharged easily, the equipment can be operated flexibly, and a variety of heating forms coexist and complement to each other.

5 Generally, the present invention provides a method and an apparatus for treatment of wastes by melt decomposition assisted with plasma arc heating; said apparatus comprises: a reactor that is cooled by water and/or air externally and lined with refractory material, and an ultra high 5 temperature field provided by means of plasma arc heat or Joule heat to assist melt decomposition of the wastes; in the ultra-high temperature field, under the action of some gasifying agent such as oxygen or air or water vapor, the organic substances are decomposed into micro-molecule gases such as carbon monoxide and hydrogen, while the 10 inorganic substances are melted into vitreous slag and molten metals, which are discharged from the reactor through corresponding channels. The first characteristic of the apparatus according to the present invention is: the apparatus has one or more air-tight waste feeders to feed the wastes into the reactor continuously or periodically; wherein, the 15 air-tight waste feeders can be rotary blade wheel feeders, screw feeders, or double-gate feeders, and the feeders can be arranged on the top of the reactor or on the opposite side of exhaust port of the reactor. The second characteristic of the apparatus according to the present invention is: the apparatus has one or more feeding ports for additive or 20 gasifying agent; addition of these gasifying agents can not only ensure smooth process of chemical reaction but also facilitate the quick rotation of the exhaust gas carrying fly ash in the reaction chamber, so that the fly ash can be absorbed under the centrifugal force of quick circulation of waste gas into the molten vitreous substances on the inside walls or at 25 the bottom of the reactor and then melted and decomposed again. If the wastes only contain inorganic substances, the gasifying agent feeding valve can be simply closed. The third characteristic of the apparatus according to the present 6 invention is: the apparatus utilizes at least a fixed plasma arc cathodic electrode to assist heating and at least a movable anodic electrode to provide an ultra-high temperature field to assist melt decomposition at a quick or flashing speed; the anodic electrode can change position to 5 perform arc striking, and plasma arc heating or Joule heating to assist secondary treatment or final treatment of slag, so as to facilitate the vitrified slag and molten metals to discharge from the reactor. All cathodes are distributed at both sides of the reactor and arranged opposite to each other at an appropriate angle, so that the wastes 10 surrounded by the plasma arc can fall into the correct position. After the arc is started, the anode can be moved to an appropriate position, so that the anode can be used for plasma arc heating, or the electrode can be inserted into the slag layer of the molten pool and used for Joule heating. The fourth characteristic of the apparatus according to the present 15 invention is: a voltage signal obtaining source is arranged at the bottom of the molten pool and through the voltage signal obtaining source, the voltage differences between a plurality of electrodes (including the cathodic and anodic electrode) and the spot of the voltage signal obtaining source can be obtained, and thereby the in/out, 20 ascending/descending and movement of the electrodes can be adjusted to adapt to variations of the working condition in the reactor. The fifth characteristic of the apparatus according to the present invention is: the electrodes are inclined at an angle of 20~70* relative to the bottom surface of the reactor, so that not only the heat loss can be 25 reduced to reduce energy consumption of the electrodes, but also the hearth capacity can be increased to facilitate the wastes to fall into the hearth and thus react with the gasifying agent. The sixth characteristic of the apparatus according to the present 7 invention is: the reactor can be tilted integrally, and the exhaust gas can flow out through the hollow shaft, which is dynamically butt jointed to the exhaust gas purification system in a sealed manner, and the hollow shaft can be cooled by water to increase its ablity to bear the furnace weight; 5 when the reactor is tilted integrally, the vitreous slag and molten metals can be discharged more easily, and therefore the operation flexibility and practicability of the apparatus is improved. Generally, the method according to the present invention comprises: in a reactor that is cooled by water and/or air externally and lined with 10 refractory materials, and in an uniform ultra-high temperature area where plasma arc heat or Joule heat exists, feeding solid and/or liquid and/or hazardous wastes into and passing through the ultra-high temperature area, choosing additive and/or gasifying agent in appropriate quantity to react with the organic wastes to decompose and recombine the organic 15 wastes into micro-molecule exhaust gases such as carbon monoxide and hydrogen, and melting the inorganic substances and metals; discharging the molten substances from the reactor as vitreous slag and metals, and respectively recycling them as resources; after purification, the exhaust gas can be used for energy recovery, such as power generation. 20 The first characteristic of the method according to the present invention is: the exhaust gas that carries fly ash rotates in the reactor quickly, so that the fly ash is absorbed under the centrifugal force of quick circulation of waste gas into the molten pool on the inside walls or at the bottom of the reactor and then melted and decomposed again. 25 The second characteristic of the method according to the present invention is: the gasifying agent is selected from oxygen, air, water vapor and/or the mixture thereof, and it can be heated by residual heat to 100*~1100OC in a variety of forms.

8 The third characteristic of the method according to the present invention is: water vapor is injected as an additive at the exhaust outlet of the reactor, and the speed at the exhaust outlet can be adjusted to facilitate the fly ash with a small quantity of solid to return to the molten 5 pool formed by the movable anode. The fourth characteristic of the method according to the present invention is, the carrier gas for plasma arc can be selected from oxygen, air, hydrogen, argon gas, nitrogen, synthetic gas produced in the reaction chamber, and/or the mixture thereof. 10 The fifth characteristic of the method according to the present invention is: the reaction chamber is under slightly negative pressure, so that the gas produced by decomposition will not leak out, and therefore the operating environment and safety are ensured. The sixth characteristic of the method according to the present 15 invention is: the reaction chamber can be tilted integrally, the vitreous slag and molten metals can be discharged from the same channel or different channels, and the vitreous slag can be treated by water quenching. The seventh characteristic of the method according to the present invention is: the cathodic and anodic electrodes can be made of graphite 20 material, so that the energy consumption and operating cost are reduced, and the graphite electrodes can be replaced and connected as convenient as steel making in electric arc furnaces, without stopping the furnace. 25 DESCRIPTION OF THE DRAWINGS The technical scheme of the present invention will be described in details with reference to the accompanying drawings. Fig.1 is a 3D schematic diagram of the back and sides of the main 9 structure of the reactor according to one aspect of the present invention; Fig.2 is a 3D schematic diagram of the front and sides of the main structure shown in Fig.1; Fig.3 is a top view of the hearth shown in Fig.1; 5 Fig.4 is a vertical sectional view of the center of furnace body shown in Fig.1; Fig.5 is a transversal sectional view of the center of furnace body shown in Fig.1. As shown in Fig.1, 2, 3, 4, and 5, according to one aspect of the 10 present invention, the apparatus comprises a water and/or air-cooled reactor mounted on a base, and the reactor has a cubodi tiltable body, a cambered and inclined furnace roof, three graphite plasma arc electrodes being arranged on it. It can been seen from the drawings that the reactor comprises cambered and inclined furnace roof 1, cubodi furnace body 2, 15 chamfered cubodi furnace bottom 3 and furnace base 4. On the shell of furnace roof 1, a base 5 that can be connected to air-tight feeders, a vapor inlet 10, a joint flange 11 for measuring negative pressure, a thermocouple insertion hole 16 for measuring furnace temperature, a base 6 connected to a water/air cooled camera for 20 monitoring working condition in the furnace, and a flange hole 22 that is suitable for mounting an infrared temperature sensor or camera, are arranged. The entire furnace roof is in a double-layer water or air cooled structure, and lined with cast refractory materials. On the shell of furnace body 2, three brackets 8 for supporting, lifting, 25 and feeding graphite electrode,which hold cathodic electrode 7(-) and anode 7(+), an anode carriage 9 and associated drive mechanism 18, a left rotary hollow trunnion 19, a right rotary trunnion 19A, primary gasifying agent feeding inlet 13 and secondary gasifying agent feeding 10 inlet 21, and a water inlet 20 for quenching vitreous slag and molten metals, are arranged. The furnace body with cooling water inlet/outlet or air inlet/outlet 15 comprises double-layer water or air cooled steel enclosure, refractory liners 26, 28, and refractory walls 25. 5 On the shell of furnace bottom 3, a lead-out hole for neutral point voltage signal source 7(E) , which is conductive and earthed on the furnace bottom, a preheated or air-cooled gasifying agent inlet/outlet 14, and a cooling water inlet/outlet 17 for forced cooling of molten pool on the furnace bottom and prolonging service life of the liner, are arranged. The 10 shell of furnace bottom 3 comprises double-layer water/air cooled steel enclosure and refractory material 26. As shown in Fig. 4 and FIg. 5, a water vapor injector 27 is arranged at the center of exhaust outlet 31. It can be seen from Fig.3, 4, and 5: after the arc is started and the 15 molten pool is created by moving the anode 7 (+) to get close to the cathode 7(-), the wastes are fed; the positions of cathodic and anodic electrodes are adjusted depending on the size of the molten pool; increase the distance between cathode and anode as far as possible on the premise of not interrupting the plasma arc, so as to expand the 20 plasma arc area and facilitate all wastes to fall into the ultra-high temperature field along the direction indicated by the arrow 32 for melting and decomposition; increase the feeding amount and the air inlet valve at the primary gasifying agent feeding inlet 13 is started, to feed one or more of preheated gasifying agents such as oxygen, oxygen-rich air, and air in 25 appropriate quantity, and then the air inlet valve at the secondary gasifying agent feeding inlet 21 is adjusted to feed preheated gasifying agent (e.g. preheated water vapor) in appropriate quantity; the feeding amount of gasifying agents is to be controlled to convert the carbon (C) in 11 the exhaust produced by decomposition into carbon monoxide (CO) as much as possible, so as to reduce residual carbon (C) and ensure heat value of the hydrogen-rich gas as high as possible, so as to facilitate recycling the energy carried in the exhaust gas; in addition, the exhaust 5 gas rotates quickly in the direction indicated by the arrow 29 in the reactor, so that the fly ash in the exhaust gas is absorbed on the inside walls of the reactor and in molten pool in the reactor under centrifugal force; excessive fly ash accumulated on the inside walls of the reactor can be blown flow into the molten pool by the air, and remains in the reactor as 10 much as possible under quick rotation of the exhaust gas. At the swirling center of exhaust gas (i.e., at the exhaust gas outlet 31 on the refractory wall 25), an adjustable water vapor nozzle 27 is provided to intercept the residual fly ash in the exhaust gas for a second time and at the same time reduce the flow rate of exhaust gas at the outlet so as to prolong the dwell 15 time of exhaust gas in the reactor; the gasifying agent sprayed through the gasifying agent feeding port 10 on the furnace roof participates in the reaction and also assists to suppress the fly ash in the exhaust gas into the molten pool. The direction indicated by the arrow 30 is the gasifying agent feeding direction and position. This is one of the main 20 characteristics of the method according to the present invention. The inorganic substances in the wastes is melted to form the molten pool. As the electrode 7 (+) is moved away from the electrode 7(-) gradually, the molten pool is gradually increasing. The potential difference U+/U- between electrode 7(+)/7(-) and electrode 7 (E) can be obtained 25 due to existence of the neutral earthing point 7(E) arranged on the furnace bottom, so that the lifting and feeding of the cathodic and anodic electrodes can be controlled separately and conveniently by comparing U+/U- with the preset voltage values, and thereby the anode 7(+) can be 12 conveniently switched to plasma arc heating or Joule heating mode and accomplish control of working condition in the furnace as required. This is also one of the main characteristics of the method according to the present invention. 5 The vitreous slag and molten metals in the molten pool can be conveniently cast through separate channels or the combined water inlet 20 by tilting the reactor; in addition, before and after casting, the water inlet 20 can be sealed conveniently to ensure air tightness of the reactor; the casting time can be chosen appropriately to keep negative pressure 10 in the furnace and prevent leakage of exhaust gas. This approach can simplify operation and improve practicability of the system, and is also one of the major characteristics of the method according to the present invention. The exhaust gas produced in the reactor is led out from the outlet 31, 15 and butt jointed to the exhaust gas purification system through the gas chamber 24 and the air-tight hollow water-cooled shaft 19. The arrangement of hollow water-cooled shaft 19 is helpful for integral tilting of the reactor and prevents the exhaust gas from interruption and leakage. This is also one of the main characteristics of the method and apparatus 20 according to the present invention. Those skilled person in the art can easily identify the essential characteristics of the present invention from above description, and thereby make changes and modifications to the embodiments of the present invention without departing from the spirit and scope of the 25 present invention, so as to adapt to various purposes and conditions, such as the appearance and shape of the apparatus, the feeding amount and position of gasifying agent, the nature, quantity, and positions of electrodes, and the position of waste feeding port on the furnace body, 13 etc. Therefore, such changes and modifications shall be completely and reasonablely deemed as falling into the entire equivalent protection scope of the present invention as defined by the claims.

Claims (15)

1. An apparatus for treating wastes and/or hazardous wastes by melt decomposition assisted with plasma arc, wherein, in a reactor that is cooled by water or air externally and lined with refractory materials, an assisting melt decomposition ultra-high temperature field for waste is provided by plasma arc heating or Joule heating; in the ultra-high temperature field, under the action of gasifying agent composed of some oxygen and/or air and/or water vapor, organic substances are decomposed into micro-molecule gases, such as carbon monoxide and hydrogen, while inorganic substances is melted to form vitrified slag and molten metals, and then they are discharged from the reactor through corresponding channels.

2. The apparatus according to claim 1, wherein, said apparatus has one or more air-tight waste feeders, to ensure the wastes can be fed into the reactor continuously or periodically.

3. The apparatus according to claim 1, wherein, said apparatus has one or more additive or gasifying agent feed ports; addition of the gasifying agent can not only ensure the smooth process of chemical reaction but also facilitates the quick rotation of the exhaust gas that carries fly ash in the reaction chamber, so that the fly ash can be absorbed under the centrifugal force of quick circulation of waste gas into the molten vitreous substances on the inside walls or at the bottom of the reactor and then be melted and decomposed again.

4. The apparatus according to claim 1, wherein, said apparatus utilizes at least a fixed cathodic electrode for assisting plasma arc heating and at least a movable anodic electrode, to provide an ultra-high temperature field for melt decomposition at a quick or flashing speed; the anodic electrode can change its position to perform arc striking, and plasma arc 15 heating or Joule heating, to assist secondary treatment or final treatment of slag, so as to facilitate the vitrified slag and molten metals to discharge from the reactor.

5. The apparatus according to claim 1, wherein, a voltage signal obtaining source is arranged at the bottom of the molten pool and through the voltage signal obtaining source, the voltage differences between a plurality of electrodes (including cathodic and anodic electrode) and the spot of the voltage signal obtaining source can be measured, and thereby the in/out, ascending/descending and movement of the electrodes can be adjusted according to the voltage differences, so as to adapt to variations of the working condition in the reactor.

6. The apparatus according to claim 1, wherein, the electrodes are inclined at an angle of 20~70* relative to the bottom of the reactor.

7. The apparatus according to claim 1, wherein, the reactor can be tilted integrally, and the exhaust gas flows out through a hollow shaft, which is butt jointed to the exhaust purification system dynamically in a sealed manner.

8. A method for treating wastes and/or hazardous wastes by melt decomposition assisted with plasma arc, wherein, in a reactor that is cooled by water or air externally and lined with refractory materials, in an ultra-high temperature area where plasma arc heat or Joule heat exists, feeding solid and/or liquid hazardous wastes into and passing through the ultra-high temperature area, choosing additive and/or gasifying agent in appropriate quantity to react with the organic wastes to decompose and recombine the organic wastes into micro-molecule exhaust gas containing carbon monoxide and hydrogen, and melting the inorganic substances and metals; the molten substances discharged from the reactor become vitreous slag and metals, and are recycled as resources; 16 after purification, the exhaust gas can be used for energy recovery, such as power generation.

9. The method according to claim 8, wherein, said method comprises: the exhaust gas that carries fly ash can quickly rotate in the reactor, so that under the centrifugal force of quick circulation of waste gas, the fly ash is absorbed into the molten vitreous substances on the inside walls or at the bottom of the reactor and then melted and decomposed again.

10. The method according to claim 8, wherein, the gasifying agent refers to the mixture of oxygen and/or air and/or water vapor, and it can be heated by residual heat to 100 0 -1100*C in a variety of forms.

11. The method according to claim 8, wherein, said method comprises: injecting water vapor which is used as additive at the exhaust gas outlet of the reactor, and adjusting the speed at the exhaust gas outlet, to facilitate the solid fly ash carrying some solid to return to the molten pool formed by the movable anode.

12. The method according to claim 8, wherein, the carrier gas for plasma arc can be selected form oxygen, and/or air, and/or hydrogen, and/or argon, and/or nitrogen, and/or synthetic gas produced in the reaction chamber, and/or the mixtures thereof.

13. The method according to claim 8, wherein, the reaction chamber is under slightly negative pressure.

14. The method according to claim 8, wherein, the reaction chamber can be tilted integrally, the vitreous slag and molten metals can be discharged from the same channel or different channels, and the vitreous slag can be treated by water quenching. 17

15. The method according to claim 8, wherein, the cathodic and anodic electrodes can be made of graphite and/or water-cooled metal materials.