Incineration or thermal decomposition?

Although some part of the waste we produce, we can safely store at landfills, turn into compost or alternative fuel, there are kinds of dangerous waste, which are quite a problem. The only way to get rid of them properly is high-temperature processing – incineration or thermal decomposition. The latter method is much better, because it allows us to retrieve most of the substances the waste is composed of, whereas Nonrenewable part of the waste is turn to pure energy. That is why the right solution is not combustion of toxic and expensive fossil fuels nor building new harmful mining plants.

Hence, there exists an efficient method of waste recycling which does not entail production of another waste material, but allows to process it into useful product – this method is plasma technology, and it has been consequently implemented by many companies worldwide for more than 40 years. The great majority of devices of the processing line are solutions commonly known, proven and widely used in the energetic, chemical and petrochemical industries for a long time. The most innovative devices are plasma torches with is power system. The invention originated in connection with the competition for outlining the method of recycling rich materials used in the production of shuttles and spacecrafts, which was organised in 1977 by NASA. The winning company Westinghouse Plasma Corporation (WPC) proposed recycling of these materials with use of plasma. Since then, the process has constantly been improved. Five years ago on the market appeared its fourth generation – most current and electrically very economical, allowing to sell as much as 80% of energy produced by a single plant.

Plasma torch

Last decade has brought solutions, which allow exploitation of the plant in efficiently economical, emission-free and environment-friendly way. Thanks to it, plants of conventional thermal recycling started to go out of date.

It is estimated that nowadays there are ca. 20 plants exploiting the gasification technology of Westinghouse for waste recycling. Most of them are plants dedicated to recycling of dangerous waste (ammunition, asbestos, biomedical waste), and part of them serve to vitrification in high-temperature the secondary waste from conventional grate-incineration plants. These are not sole plants of thermal waste processing, the work of which relies on plasma torches.

Companies providing the technology of plasma waste recycling are:

  • Alter NRG/Westinghouse;
  • Integrated Environmental Technologies;
  • InEnTec;
  • Plasco Energy Group;
  • ScanArc Plasma Technologies (PyroArc);
  • Advanced Plasma Power (APP);
  • Solena;
  • GeoplasmaInenTec;
  • ZeGen.

In the world, there are several hundred plants of various size that, in the process of waste recycling, use electricity and high temperature generated via ARC plasma torches.

In Poland, there are several ongoing development projects concerning creation of plants for thermal waste recycling, the majority of them, however, are dedicated to utilisation of communal waste, and the implementation of them has begun some years ago, and that is why this particular technology has been selected, since it required lesser investment input.

How does it work?

Let us begin with the sole plasma. Plasma is a heavily ionised matter resembling a gas state (e.g. oxygen). To obtain it industrially, a considerate amount of energy is to be provided to the gas via electric arc. The energy forces the atoms of the gas to “lose” their electrons, and this is what eventually makes plasma a perfect conductor of electricity and heat. It can also easily transfer this energy onto surrounding matter. Plasma heats up every kind of matter to a very high temperature, and that is why it is used in e.g. cutting metals, ceramics and even rocks. On cooling down it becomes an ordinary gas again. Complex substances heated up by plasma decompose into particles of simple compounds such as carbon monoxide, carbon dioxide or hydrogen. This process is called gasification, that is decomposition without combustion. It takes place in temperatures near 1000ºC with access of oxidant factor such as air or steam.

The idea of plasma waste recycling

After cooling down the particles does not form the previous compounds, but remain as three types of primary matter:

Synthesis gas (syngas)

It is a product of decomposition of organic matter contained in waste. In its composition and properties is similar to a raw natural gas. after being purified from useless ingredients (soots, excess of steam, chlorine, nitrogen, sulphur and excess of carbon dioxide) – is composed almost only of carbohydrates, hydrogen, carbon monoxide and carbon dioxide. This enriching treatment enhances its calorific value which is on a par with this of a natural gas. The gas is burnt in gas turbines, and only products of this stage are: energy, steam and carbon dioxide which are released to the atmosphere. Carbon dioxide is, however, not coming directly from the fossil fuels, thus it does not contribute to the greenhouse effect. In the planned plant, syngas will be used mainly in generating electric and thermal energies, yet it is not its only application. Syngas can be used as a material for the production of synthetic fuel oil. It can also be condensed and sent in cisterns to chemical factories. There, it is used in production of plastics, oils, lubricants, etc.

Vitrite

Vitrite, one of the by-products of the process, is a mixture of chemical compounds – mainly oxides of silica, calcium and halides of various metals. In the liquid state is has properties of volcanic lava (molten rock). Cooled, it resembles pieces of firestone or basalt, and has similar properties. It is used in production of building materials, e.g. mineral wool, warm bricks, gravel or pad stones. Mineral wool, also known as stone wool, is an isolating material of mineral origin used in building industry. It is made of several materials including basalt stone, dolomite or calcium aggregate, and also more and more often – the recycled materials (mineral briquet). The idea to make mineral wool has been born during the research on the Kilauea Volcano in the Hawaii – in time of its eruption molten pieces of rocks were thrown out in form of lava – it has been observed that cooling down in the strong wind they turned into stone fibres.

Mixture of various metals that has got molten out of various kinds of waste material

The process of recycling metals from waste is analogical to the one which takes places in huge blast furnaces, where smelting of the pig iron consists in separation, in high temperature, compounds of metals from the ore. In our case the process is simpler. The fractioning in temperatures concerns those metallic products, the earlier separation of which was not economical. A classical example of this may be a car tire – in this case, a part of flange, wiring, and frame are sunk in the rubber monolith. The mixture of metals as the heaviest ingredient of the gasified materials precipitate to the bottom, from where it is taken onto further processing – with use of e.g. centrifuge. Smelted metals are sent to the factories.

The heat-producing factor used in the process is plasma, which is counted among the type of cold plasma (4000-30000 K), and it is produced in plasma torches, whereas we can speak of hot plasma when it reaches temperatures over 30000 K – such plasma can be found inside stars.

Invention of a safe way to reach high temperatures of the stream of cold plasma gave an opportunity of a very efficient – in comparison to a standard combustion – waste recycling. Thanks to the impact of high temperature and high-density energy considerably quickened the process and thus its efficiency.

You can find more information about plasma gasification in the FAQ section.

Scheme of municipal waste recycling process in a reactor of Westinghouse Plasma Corporation

The produced syngas can be used directly in:

  • production of electric energy;
  • production of fuels (synthesis of liquid hydrocarbons);
  • synthesis of methanol.

Waste without waste

In an energy plant employing the method of plasma waste recycling no single trace is left of the recycled waste! What is more, the products obtained in the process can be entirely re-used in various branches of industry. What is more, the materials made of them can be entirely used in other branches of industry. It should be emphasized that this technology is the first method of generating energy that does not pollute the environment (quite the contrary – it cleanse the environment).

There is no doubt that the method employing plasma arcs as the main source of energy is a natural step in the development of the technology of thermal recycling. Hence, it seems that as a safer and more efficient method it will, continue to supplant currently used technologies.

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