Glass and Ceramics, Vol. 51, Nos. 11-12, 1994

CONSERVING RESOURCES

A R C P L A S M A T R O N S F O R B U R N I N G F U E L IN I N D U S T R I A L INSTALLATIONS UDC 662.951.1:662.959.2/3:662.4:537.5

N. A. Gatsenko and S. I. Serbin

The construction materials industry in CIS countries is a complicated set of specialized production branches which prepare the vast list of construction materials, articles, and structures. Conserving energy has now become one of the key problems in different branches of the national economy, including in such power-consuming plants as in fabrication of different types of glass and ceramic articles. The problem of conserving energy in using high-temperature thermotechnical installations (HTI), including rotating furnaces, spray dryers, glass- and fritmelting furnaces, and boiler installations, is especially pressing. This is because of: the significant consumption of fuel and energy resources; the characteristic high level of the thermodynamic potential of saving energy, i.e., the important gap between the theoretically possible and actually realized levels of the efficiency of utilization of the energy supplied; the limitation of reserves of tile fuels used; replacement of primary fuels by more expensive energy carriers (artificial fuel, electric power), partial or total substitution of atmospheric air by oxygen in heating; the special importance of the ecological factor (HTI pollute the environment with products from both heating and industrial processes). Incomplete combustion increases significantly even in nominal conditions in conversion of thermotechnical installations to heavier fractions of mazout and gas from fractions of a percent to 2-3 % and more in conditions of partial loads. Moreover, sooting, corrosion, and erosion of the eIements of the circulating part of industrial installations increase, causing additional losses and decreasing the efficiency and resources of the installations. When heavy liquid, gaseous, and coal-dust fuels are used, their ignition is complicated, and more powerful energy sources and new ignition systems which raise the speed and reliability of starting up the installations and provide for automated processes for burning the fuel are required. However, until recently, the specific power consumption for industrial processes in the construction materials industry was many times higher than the theoretical minimum. This result is the consequence of using predominantly traditional methods of calculations and existing installations and equipment in improving energy-saving installations. While not belittling the practical importance of this direction, it is impossible not to note that power consumption is not radically reduced: new technical solutions are required to do this. One of the promising directions in the development of new energy-saving technologies is acting more effectively on ignition and combustion of fuel mixtures in the combustion chambers of devices for plasma initiation of burning of fuel [1]. The state of a substance in which it contains positively and negatively charged particles capable of conducting an electric current and obeying the laws of magnetic gas dynamics is considered to be plasma. Plasma is also characterized by a high particle temperature. Hot (high-temperature) and cold (low-temperature) plasma is distinguished. Hot plasma has very high conductivity, degree of particle ionization close to unity, and the particle temperature attains tens of millions of degrees. Such plasma is found in the interior of the Sun and other stars. In contrast to hot plasma, cold plasma is obtained in terrestrial conditions, the degree of ionization is of the order of 1-10%, and the temperature varies within the limits of 3000-50,000 K. This type of plasma is used for industrial purposes. We will subsequently discuss low-temperature plasma and will use the term "plasma" to designate it. Akrnolinsk Institute of Construction Engineering; Nikolaevsk Ship Building Institute. Keramika, Nos. 11-12, pp. 34-36, November-December, 1994. 0361-7610/94/1112-0383512.50

9

Plenum Publishing Corporation

Translated from Steklo i

383

Plasma-forming

6, T2, v2. Plasma

!! medium

~ ~/ ]/

Fig. 1. Very simple diagram of a plasmatron.

1

Air

Fuel

2 3

4

5

6

Fig. 2. Diagram of a plasma chemical reactor: 1) cathode holder; 2) body; 3) anode; 4) swirler; 5, 10, 13) openings; 6) exhaust nozzle; 7) sleeve; 8) channel; 9) body of ignition unit; 11) screw; 12) cathode; 14) sleeve. Different gases, so-called plasma-forming media, are used to obtain plasma in plasma-forming devices. It can be single- and multicomponent. Such gases as argon, helium, nitrogen, and hydrogen are used as a single-component plasmaforming medium. Low-temperature plasma has been successfully used in production of acetylene from natural gas, disilene, hydroxylirnine, and nitrogen oxide from air. Ion-plasma spraying, carbothermal reduction of metal oxides, and reduction of metal chlorides with hydrogen have also been conducted. For industrial purposes, arc discharge, which has a number of advantages compared to other methods, is the best method of obtaining low-temperature plasma. The advantages are: possibility of obtaining plasma for a long time and with relatively high efficiency; possibility of obtaining plasma from solid, liquid, and gaseous media of almost any chemical composition; convenience of operation with respect to regulation of parameters during the industrial process; possibility of obtaining plasma in a vacuum and at high pressures; possibility of using standard power sources. An electrotechnical apparatus in which the plasma-forming medium is heated due to heat exchange with the arc discharge should be considered a low-temperature plasma generator or arc plasmatron. Different methods are used to intensify the heat exchange processes. The simplest scheme of a plasmatron should be considered the one in which the arc burns between electrodes and the plasma-forming medium passes through it (Fig. 1). The plasma-forming medium at the inlet to the plasmatron has flow rate G, temperature T 1, and velocity V1, the flow rate remains as previously at the outlet, but due to heating and expansion of the gas, its temperature T2 and velocity V2 increase [2]. The low-temperature plasma torch has a thermal and chemical effect on the fuel--air mixture. The significant increase in the number of active sites (atoms, radicals, ions, and electron gas) ensures energetically more efficient heating than traditional flame methods and accelerates combustion reactions. The completeness of combustion of the fuel is sufficiently high regardless of the operating mode of the installation. The system of plasma chemical intensification of combustion consisting of a plasma chemical burner and power source is designed for initiation of ignition and stabilization of fuel combustion in the combustion chambers of power and industrial installations. 384

GT, kg/h

_~.,,,.~f

5001 400

300

~v ~

,/"

200 /

0

I G,2

o,l

0,2

0,3

0,4 PT, MPa

Fig. 3. Input-output characteristics of the reactor: GTI: fuel flow rate through the first channel; GT2: fuel flow rate through the second channel; total fuel flow rate Gs = GT1 -F GT2. Ct~ ...o

45

40 35 30 0

0,05

0,1

0,15

0,2 Pi, MPa

Fig. 4. Angular characteristic of the reactor. The basic fuel (gas, liquid, or solid) is burned in the jet of products formed in the plasma chemical burner as a result of the reaction of low-temperature (1000-5000 K) air plasma and auxiliary fuel. Studies on heating and thermal conversion of gaseous and liquid fuels with generated arc devices applied to thermotechnical installations for construction industry enterprises have recently been conducted in the Department of Electrical Engineering and Automation of Production at the Akmolinsk Institute of Construction Engineering in collaboration with the Nikotaevsk Ship Building Institute. Projects were developed and plasmatrons with 5 and 1.5 kW power have been fabricated for rotating furnaces for firing of "keramzit" operating on furnace mazout at the KBI Plant and a Silikatchik GKP heating boiler for plasma chemical intensification of fuel combustion. The plasma chemical reactor whose diagram is shown in Fig. 2 can be used to initiate ignition of liquid hydrocarbon fuels and improve the characteristics of operation of different heating devices. An electrospark candle which has a copper cathode with a hafnium emission insert attached to the electrode with a screw thread is used as the cathode holder. The cathode holder is installed in the body of element 3, which is simultaneously the anode of the plasmatron and the mixing chamber of the reactor. Body 2 and sleeve 14 with openings and channels for fuel and air, respectively, are mounted concentric to the anode. Exhaust nozzle 6 contains screws which are positioned coaxially to fuel feeders 10 in the anode and are used to clean these openings. Parts 2, 3, 6, and 14 are made of stainless steel and are connected by argon-arc welding. Air swirler 4 is welded to the body of start-up unit 9 in which the plasma chemical reactor is installed with a screw thread. In starting up the plasma chemical reactor, the arc is initiated by high-voltage spark-over of the interelectrode gap between cathode 12 and anode 3. The electric arc excited in this way is carried by the plasma-forming air current, which acquires swirl in passing through tangential openings 13 in anode 3, into the arc channel of the anode, and subsequently heats up between the immobile electrode spot in the emission insert of cathode 12 and is moved along the periphery by the electrode spot on anode 3. The arc shunting site on the anode is cooled by the fuel passing into openings 10 in the anode through a ringshaped cavity formed by parts 2, 3, and 6. As a result of heat-exchange with the electric arc, the air is heated to plasma temperatures in the arc channel and mixed with the fuel ejected through openings 10 in the mixing chamber.

385

Mixture formation and the plasma chemical reaction are completed in the channel of exhaust nozzle 6. The current of plasma chemical products is subsequently mixed with air entering the start-up unit through openings 5 and is directed to the heating device for initiation of ignition and stabilization of combustion of the basic fuel. The input-output characteristics of the first and second fuel channels are shown in Fig. 3, and the angular characteristics of the reactor are shown in Fig. 4. The results of the experimental studies and experimental and experimental-industrial verification of the plasma systems for intensification of ignition and combustion of mixtures of different composition demonstrated their significant advantages over standard combustion devices: broadening of the range of start-up and stable operation of power plants, increasing the coefficient of completeness of fuel combustion, decreasing mechanical incomplete burning, the possibility of realizing the optimum technological parameters of the processes, excluding breakdown situations during operation, and decreasing the laboriousness of putting in operation. The method of plasma thermochemical preparation and combustion of fuel is thus an effective means of rationally utilizing energy resources and reducing emissions of nitrogen oxides and accompanying components in stack gases. Implementation of the method of plasma-initiated fuel combustion in high-temperature thermotechnical installations will reduce consumption of primary energy resources to 20% by ensuring complete combustion of the fuel. Use of the plasma method of thermochemical preparation of fuel will make it possible to decrease harmful emissions of aggressive components in stack gases to 30-40%.

REFERENCES .

2.

386

N. A. Gatsenko and I. M. Lamonov, "Prospects for the development of plasma technology for ignition and combustion of fuel in thermotechnical installations," Steklo Keram., No. 9, 21-22 (1992). A. V. Bolotov and G. A. Shepel', Arc Processes and Devices for Processing Materials [in Russian], Alma-Am (1979).