PLASMA HEATING IN CLOSED CIRCULATION SYSTEM

Abstract: This invention relates to the use of a plasma arch furnace for the heating of gases, liquids and solid materials. There is provided a closed circulation loop where the gas or the gas mixture in the loop is controlled with regard to composition, pressure and temperature to achieve optimal chemical processing of materials which are fed into the loop. The reacting gas is fed into the loop in stochiometric quantities to avoid simultanous presence of nitrogen and oxygen in the plasma arch furnace. The basic carrier ga may be any one optimal for the process.

Description: Plasma heating in closed circulation system This invention relates to the use of a plasma- arch for the heating of gases, liquids and solid materials. More specifically the invention relates to such heating in a closed circulation system. As well known from the state of the art, the plasma arch was originally used commercially to produce nitrous gases so called "noxes" N0 1 N0 2 N 0 3 etc. Gases, i.e. air, were sent through a plasma arch reactor chamber at a reaction temperature of 4- 5000°C. In connection with fertilizer production, there were developed very reliable and robust reactor constructions. The socalled Birkeland-Eide ovens or heaters - (BEH) were in commercial use for nearly 35 years (1905-1940) . In the later years there have been proposed new industrial appli¬ cations for the use of the plasma arch, for example for the heating of industrial gases used in chemical processing and for the destruction of waste. The advantages in using a plasma arch reactor are the ability to generate extremely high temperature up to 4-5000°C and with the possibility of reclaiming most of the energy as high grade heat with no pollution from fossile fuels. Thus, there are known several patented plasma arch reactors and processes intended for such use, f.example as known form the recent Norwegian patents Nos. 161759 and 162260. The Plasma can be described as the state to which a gaseous phase of any chemical structure arrives when its molecules dissociate themselves and their atoms are polarized. The plasma is formed by a mixture of electrical, neutral gas molecules and of molecules which are dissociated into positive ions and negative electrons. Thus, with a high polarity differential there is a visquous state at a temperature of 4000°C or higher. As mentioned above plasma torches are currently known for the heating of industrial gases used in some steel processing and for industrial chemical processes. A recent application which seems to be even more interesting is for the destruction of problem waste at high temperatures. There are, however, some inherent, basic difficulties in utilizing the plasma arch system for the above application, when operating it as a straight-through flow system. Firstly, when oxygen and nitrogen si ultanously are present in the plasma zone noxes will inevitably occur. Secondly, when the material to be destructed is fed upstream of or directly into the plasma zone, the stability of the reactor may be affected, and it may shut down or stop at random. According to one known method the formation of noxes may be reduced or prevented when the gas stream which passes through the reactor consists of steam. Such a method may be applicable for special processes, but may otherwise be unsuitable. The use of appreciable amounts of steam will be uneconomical and not optimal from an energy consumption point of view. Furthermore the presence of appreciable amounts of moisture may intefere with the reactions kinetics in the reaction chamber. According to the present invention there is provided an improved- utilization of plasma heating thorugh the establishment of a closed circulation system in the form of a gas loop with the plasma arch reactor integrated with the loop and utilized only for the purpose of heating. A special chemical reactor will take care of the reactive part of the system, and a cooler in introduced downstream of the reactor, whereby the loop system will consist of high temperature and low temperature section or circuit. Essential for the loop system is that it is operated with a controlled gas atmosphere as well as with regard to gas composi¬ tion as with regard to pressure and temperature and typically with a carrier gas as well as a reaction gas. The carrier gas may be provided from an external source and continuously be fed into the loop, or being gases produced in the loop reaction zone. Other essential and characteristic features of the loop system are defined in the patent claims 1-5. The loop system will be described as utilized in a technical scale process for the combustion or destruction of waste and with reference to the accompanying drawings, where Fig. 1 shows a flow diagram of a Birkeland- Eide (BE) waste disposal loop, while Fig. 2 illustrates a BE waste disposal loop for the combustion of dioxin waste. The main components of the loop system are a plasma oven 1, a gas blower or compressor 4 and heat exchangers/coolers 3 for the production of steam or hot water and a separate reactor 2. The gas loop consists of two main temperature zones. A high temperature zone with a temperature suitable to generate the combustion reaction or the chemcial reaction required f.example 1000-2000°C. A low temperature zone, where the temperature is lowered and limited to the highest temperature suitable for a gas blower, f.example 300-500°C. In the gas loop or circuit a suitable carrier or circulation gas B is applied for the relevant process. If a special reaction gas is used this is introduced as A. Formation of nitrous and nitric gases in the plasma oven 1 can be eliminated and avoided by not permitting the presence of 0 2 or N 2 simultanously in the oven. If air is to be employed as reaction gas, 0 2 -free circulation gas in the plasma oven 1 is obtained with the introduction of air A between the plasma oven and the reaction chamber and securing that added air is providing stochiometric amounts of oxygen. Alternatively, a catalyst 5 of conventional type (palladium) may be incorporated in the low temperature zone. Hereby excessive 0 2 may be removed through the catalytic combustion with f.ex methanol. Finally the BE loop system according to Fig. 1 has an outlet for ash E in the reaction chamber and an outlet D for combustion gases which passes through a cooler/condenser 6 with an extra outlet for condensate. Because the combustion and other forms of chemical reaction are controlled with stochiometric amounts of reaction gases, such as 0 2 , there is made provisions for; minimum expenditure or comsumption of electrical energy in the plasma oven and minium losses through release of gas (bleed) or of energy to the surroundings. This again will give practically 100% recovery as thermal heat, of the total electrical energy consumption. The energy balance is maintained because the spent or used electric energy pluss/minus reaction energy are balanced by the continous removal of the same amount of energy from the circuit as thermal energy, i.e. in the form of steam and hot water. The mass balance is maintained through the system because the added mass, in the form of gases, liquids or solid materials are led out from the circuit through equivalent amounts of reaction products and lost carrier gases. Contrary to corresponding open systems, the closed loop system allows the use of expensive carrier gases, as f.ex. H 2 , A and C0 2 . The release or losses of such gases may be kept low and sometimes at nearly zero level. The intensity of the reaction can easily be controlled through the percentage of reactant gas in the circulating gas. The reaction chamber may be constructed according to the type of materials to be processed. For solid waste material f.ex., it will be convenient to employ a rotating oven or a fluidized bed reactor. For gases and liquids the chemical reaction may take place in C0- current flow in a reaction tube, formed as serpentine with a cyclon at the outlet. Example - Destruction of dioxin containing waste A liquid mass from a purification plant containing 75% water and 25% solid sludge was presented for destruction. The sludge consists of inorganic salts and carbon, initially in the form of active carbon. Dioxin is also absorbed in the carbon and the whole mass has a ph of 4 or lower. The total volume of liquid from the purification plant is appr. 360 m 3 /year. The combustion was carried out in a BE waste disposal loop system according to Fig. 2. The liquid sludge from the tank 9, segrega¬ tion avoided by a stirrer was forwarded by a pump 8 and preheated by passing through the heat exchanger 3 into the reaction chamber 2 in the form of a serpentine tube. The reaction products were led into the cyclone separator 7, with ash outlet at the bottom, the reaction gases were led through heat exchanger 3 to preheat the liquid sludge, and thereafter through a coiler 10 incorporat¬ ing a waste heat boiler system. The bleed gas was passed through the cooler/condensator 6 where condensed H 0 and HC1 were drained out and the remaining CO vented to atmosphere. A blower or com¬ pressor 4 provided in the low temperature part of the loop provided the necessary gas circulation through the system. The destruction process was carried out in an atmosphere of 0 2 or air at a temperature of 1500°C or higher. The resulting reaction products were C0 , H 2 0, HCL and solid ash (approx. 5% of total weight of waste) and consequently these gases were also utilized as circulation gas in the loop system. (The reaction chamber was formed as a serpentine tube, ending up in a cyclon for ash particles.) At the inlet of serpentine tube preheated waste is fed into the hot circulation gas. At the same point 0 is introduced. The ash collected in the cyclon is batchwise removed. The gas is recirculated through heat exchanger/cooler and cooled down to 400°C before entering the gas blower 4. The energy demand of the BE-heater is approximately 50 kW, which may be split on as 26 kW for the heat of evaporation and 15 kW for superheating to >2000°C. This is total energy requirement without heat recovery. If heat is recovered the energy consump¬ tion will be reduced to approx. 25 kW.

Claims: Claims 1. A plasma heating and reaction system, involving a plasma arch furnace for raising the temperature of a reaction gas and/or carrier gas to desired high level, a reaction chamber, and means for transporting the gas through the reaction chamber, c h a r a c t e r i z e d i n t h a t there is provided a closed circulation loop where the gas or gas mixture loop is controlled with regard to compo¬ sition, pressure and temperature to achieve . optimal chemical processing of materials which are fed into the loop. 2. System according to claim 1, c h a r a c t e r i z e d i n t h a t the reacting gas is fed into the loop between the plasma are furnace (1) and the reaction chamber (2) and in stochiometric quantities to avoid simultanous presence of nitrogen and oxygen in the plasma arch furnace and that there are copiers/heat exchangers (3, 10) to establish a high and a medium temperature zone to make the chemical reaction take place at relevant conditions and enable a blower (4) to circulate the medium hot gas or gases. 3. System according to claim 2 c h a r a c t e r i z e d i n t h a t the basic carrier gas may be any one optimal for the process. 4. System according to claim 2 , c h a r a c t e r i z e d i n t h a t the reactant gas may be air, 0 2 , H 2 or anyone optimal for the purpose. 5. System according to claim 2 c h a r a c t e r i z e d i n t h a t the closed gas loop can be used for production of chemi¬ cals, for drying of materials, for steam production by electrical power and for the destruction of waste materir als like dioxines, chlorinated hydrocarbons such as PCB etc.

Inventors: HENRIKSEN NOROLF, HENRIKSEN, NOROLF
Applicants: NORSK HYDRO AS, NORSK HYDRO A.S
Application year: 1991
Main classification: F23G5
IPC codes: F23G5