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Waste treatment plasma technology

The modern level of industrial development has significantly aggravated the waste treatment problem. Most of the developed countries every year adopt stricter legislation in field of waste control. Major industrial corporations and municipal bodies have to make large investments in recycling of waste. This is steadily increasing the investment attractiveness of the development of ecologically sound waste treatment technologies.

The traditional method for waste destruction is their incineration in special plants, is not equally applicable to all types of waste. For instance, traditional incineration of hydrocarbons containing
halogens yields highly toxic dioxins. For this reason, the most toxic waste are kept in special storages because there are no adequate their treatment technologies.

DTAT plasmachemical reactor

One of the most promising technologies for the treatment of toxic waste is plasmochemical technology based on high-temperature plasmochemical treatment and complete decomposition of the treated waste by means of arc plasma yielding a valuable energy resource, i.e., synthesis gas, which is a mixture of CO and H2. The main advantages of plasmochemical technology are that it can universally be applied to various kinds of waste and can be implemented in relatively small-sized mobile technological modules. At the same time, wide practical application of plasma technologies is hindered by the absence of powerful and reliable arc plasma torch with a sufficiently long time of continuous operation. The existing arc plasma torches require costly inert gases as plasma-forming agent, whereas, from the viewpoint of plasmochemical reactions, the optimum plasma-forming gas is actually steam. An electric arc in powerful torches results in the erosion of electrodes, which are to be replaced periodically thus necessitating frequent stoppages of the plant. We have worked out a basically new plasma torch, which have no limits on the electrodes service life. Based on the new plasma torch a DTAT plasmochemical reactor for destructing most hazardous organic waste.


DTAT plasmochemical reactor: Basic idea

Inside a steel airtight case there are two reaction chambers with special baths (see picture). Before the first start the metal scrap is fed to the baths. An electric arc is generated between these bath and as it is burning the metal melts. Unlike traditional transferred arc, which is used in a number of plasmochemical reactors and plasma furnaces, both ends of an arc in our torch rest on a melt (plasma double transferred arc torch, DTAT). The waste is fed to the melt surface. Plasmochemical reaction of gasification proceeds in the reactor chambers at the temperature of 1300iN. Gasification products are removed from the chambers of the plasmochemical reactor and fed to the system of gas purification. During the operation, a layer of molten slag is formed on the metal surface. The slag is removed as it is accumulated or continuously.


The new DTAT reactor has some competitive advantages over plasmochemical reactors based on the traditional plasma torches.

Technical advantages:

Complete absence of electrodes erosion.
The surface of molten metal serving as electrodes is not subject to erosion; therefore, the service life of the new plasmochemical reactor is characterized by practically unrestricted service life of the electrodes.

Steam can be used as a plasma-forming gas.
In known analogues, argon, nitrogen or air are usually used as a plasma-forming gas. Then the corresponding air or nitrogen plasma is mixed with reactants, i.e., steam, and waste. In order to improve the efficiency of the reactor operation, steam is to be used as a plasma-forming gas. However, when steam is used it intensifies the erosion of the electrodes. In our case, liquid metal employed as electrodes makes it possible using steam as a plasma-forming gas.

The reactor capacity can be increased practically without restriction.
As is known, the plasma torches capacity can be enhanced by increasing the discharge current. The increase in the discharge current also intensifies the electrodes erosion process. Making uses of liquid electrodes, which are not subject to erosion, gives new potential of increasing plasma torches capacity.


Competitive advantages


As compared with incinerators:

1. Ability to dispose of hazardous highly toxic chlorinated or fluorinated hydrocarbons.
2. Full compliance of the technology with modern ecological requirements, absence of dioxins in exhaust gases.
3. Possibility of a one-stage treatment of different waste (including incombustible) without preliminary sorting.
4. Reduction in capital costs due to the decreased reactor volume and gas purification costs.
5. Production of commodities, i.e. synthesis gas and chemically inert granulated vitrified slag (synthesis gas can be used as a raw material for chemical production or as a fuel for the generation of heat or electric power, and the slag can be used in construction industry).
6. Removal of costs on ash dumps maintenance and ash processing.
7. Potential extraction of valuable metals contained in waste.
8. Possibility of mobile plants for waste treatment.


As compared with plasmochemical reactors based on traditional arc torch:

1. Drastic improvement in reliability and extension of uninterrupted operation.
2. Use of steam as a plasma-forming gas instead of costly inert gases.
3. More thorough waste processing.
4. Reduction in operating costs.
5. Potentially “unrestricted” increase in the reactor power.



Demonstrational version

Is intended for the destruction of highly toxic (including incombustible) waste in the steam plasma. The reactor operates at reduced pressure, which fully excludes penetration of the waste decomposition products into the atmosphere.

The experimental destruction of TCD (trichlorodiphenyl) transformer oil showed that dioxin concentration in the synthesis gas does not exceed 0.05 ng/m3 (EPA standard is 0.1 ng/m3), which implies that the plasmochemical reactor is fully applicable to the destruction of organochlorine wastes.


DTAT plasmachemical reactorDTAT plasmachemical reactor


Electric arc power

Plasma-forming gas

Consumption of plasma-forming gas

Capacity

Working temperature in reaction chambers


up to 200 kw

nitrogen, air, CO2, steam

3-5 g/s

up to 100 kg/hr

1300oC


Industrial standart

DTAT plasmachemical reactor
click image to enlarge

Power - 500 kw (0.5 Mw)

Productiveness - up to 1500 tons of toxic wastes per year


Reactor tests

The procedure of the reactor tests was as follows. Liquid chlorine-containing waste was fed to the reaction chamber to the melt surface. Gasification reaction between the waste and the steam yields synthesis gas (a mixture of hydrogen, CO and HCl). The produced synthesis gas is promptly cooled and quenched simultaneously being purified from HCl in centrifugal and bubbling chamber.

The gasification scheme of banned transformer oil is shown on the figure. The dioxin concentration in the obtained synthesis gas was measured in the course of experiments. The results are shown in the table.





Mixture (isopropyl + alcohol + benzol + CCl4)

TCB transformer oil


Chlorine concentration, % weight.

Plasma-forming gas

Temperature in the reactor, ºC

Concentration of dioxins in the obtained synthesis gas, TEQ, ng/Nm3


10%

air

1100

20


20%

steam

1300

0,02


40%

steam

1300

0,05



The experimental destruction of TCB (trichlorobiphenyl) transformer oil showed that dioxin concentration in the synthesis gas does not exceed 0.05 ng/m3 (EPA standard is 0.1 ng/m3), which implies that the plasmochemical reactor is fully applicable to the destruction of organochlorine wastes.



International Scientific Center
on Termophysics and Energetics

Russia, 630128, Novosibirsk,
Kutateladze st., 7/11
Tel/Fax: +7 (383) 3305542

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