Utilization of the city garbage in Donetsk is one of the sharpest ecological problems. The city is growing, the amount of wastes is increasing, every year the ecological situation is becoming worse. The attitude towards this problem practically does not change.

Today in Donetsk region about 4 billions of tons of industrial and 400 millions of m3 of domestic wastes are accumulated. Annually these volumes increase almost 50 million and 6 million tons correspondingly. On average per capita there is 800 tons of the accumulated industrial wastes, that is almost 2 times more than the index within Ukraine. The area which is taken by wastes in dumps and stores makes 6.2 000 hectares or is more than 2% of area’s general territory. One Donetsk citizen produces about 0.9 kg of solid domestic wastes (SDW) daily, which makes 1.4 million m3 on the city scale annually.

Normative accumulation of solid domestic wastes in Donetsk makes 360 000 t/year (1000 t/day).

The wastes are complex multicomponent mixtures of organic and inorganic compounds in most cases.

As the composition and aggregate state of industrial wastes to a large extend depend on the character of production. That is why it is necessary to carry out special analyses of physical and chemical properties and wastes composition, to develop technology of thermal treatment (neutralization) that ensures minimal harmful emissions to the environment in every particular case. The most promising are the technological schemes that presuppose complex thermal processing of different in terms of aggregate state wastes from each district of the city.

Three basic methods of the thermal processing and utilization of SDW are distinguished: layered incineration of the initial (unprepared) wastes in waste-burning cauldrons, layered incineration of specially prepared wastes (without ballast fractions) in power cauldrons with natural fuel or in the cement furnaces, pyrolysis of wastes, which were preliminary prepared or not.

The first two methods are mostly widespread and studied. However while using them a lot of contaminating substances are emitted: gaseous (carbon oxide and dioxide, sulphur dioxide, nitric oxides, anhydrous hydrogen fluoride and bromine hydrogen, compounds of heavy metals, incomplete combustion products, among which we should note polyaromatic hydrocarbons, and the so-called dioxins), flue ash, carcinogenic component, possessing properties of a catalyst (facilitates the synthesis of dioxins) and being a wonderful sorbent of dioxins transports them in the atmosphere, slag (the third class of danger), sewage waters (2.5 m3/t of processed SDW).

Among these the most dangerous and harmful (beginning with 10-12 gr/ m3) and indisputable destroyers of living organisms are artificial toxic dioxin-type substances and also polyaromatic compounds of benzpyrene type and oxides of heavy metals. Figuratively speaking, the incinerating facilities of open-flame type have become the “refuse of the sky”.

For solid and gaseous substances recovery the incinerating factories are equipped with devices which make almost 30% of the capital expenditures of building such a factory.

However, as the experience of the use of this high-performance equipment has shown, it does not eliminate the ecological danger of the open-flame incineration, foremost the problems of dioxins. On this account the UNO imposed a moratorium on mass incineration of SDW by the described above method.

Except the ecological disadvantages of direct incineration, this method has a lot of other drawbacks, namely: properties of flue ash and acid smoke gases, which do not allow to make the steam temperature higher than 400 ?C, and pressure higher than 40 bar, as thus the term of service of a superheater, even made of stainless steel, does not exceed five years, impossibility to change SDW to a state when it is possible to burn it not in the stratified cauldrons an a furnace-bar grate, but in cauldrons with the chamber furnace, when the fuel burns in the suspension state.

All the enumerated disadvantages worsen the economic indices of work of power-generating facilities, using SDW as fuel, as: owing to low steam indices the efficiency of transformation of thermal energy into electric decreases, which influences commercial expedience of theSDW use as fuel, because of the necessity to conform to strict European ecological standards considerable expenses on acquisition and installation of a complete set of multi-stage gas- and dust-trapping units take place, while at TEPS smokes gas treating is carried out by relatively cheap electric air filters, expenses take place on sterilization and utilization of ash-and-slag wastes.

As an alternative to direct incineration in many countries the technologies of SDW pyrolysis are being developed – thermal decomposition of their organic part without oxygen and dissolution of inorganic components.

The use of the pyrolysis technology has some advantages compared to incineration:

1. Considerable decrease of air and water contamination;

2. Almost complete utilization of potential energetic and material resources of SDW;

3. Produced pyrolized gas, except for heating potential, has high chemical value.

Today in the world there is a great variety of technologic execution of the pyrolysis: low-, medium- and high-temperature. However all of them have ecological drawbacks, among which the most important is the unsolved problem of dioxins, toxic hydrocarbon and heavy metals oxides. The problem solving has been transferred (as in the case of open incineration) to external (beyond the reactor) facilities of gas treatment, which only partially decrease the content of dioxins (e.g. with the help of filters).

Thus, it is possible to establish, that the economic decision of the problem of solid waste treatment within the operating normative ecologic requirements to thermal treatment SDW has not been found yet. However, at the same time, despite the enumerated economic peculiarities of SDW thermal treatment, it is possible to say that the attention and interest to this technique of SDW disposal has not disappeared. It is supported by the energetic value of these wastes, which is testified by the Euro Parliament decision to regard them as a renewable source of energy.

The analysis of the world experience in incineration, generalized for almost 40 years, allows to draw a conclusion that elimination of ecologic danger of SDW thermal treatment with the help of dust- and gas-cleaning systems as well as with the SDW preparation to incineration is unpromising.

The solution of the problem has to be made at a higher level, namely at the stage of compounds of dioxin type. Due to this not only the ecologic cleanliness of SDW neutralization is defined, but also its competitiveness at the world market. The technology of thermal treatment has to be measured so that it excludes the synthesis of extremely toxic substances of dioxin type, and less harmful pollutants are caught with dust- and gas-cleaning systems. This is possible only in case of correction of order and conditions of the process of SDW thermal treatment having found the reasons of dioxin formation.

In our opinion the desired solution can be found by correction of the order of the high-temperature pyrolysis process.

The reason for dioxin formation in modern pyrolysis installations is that the currently used pyrolysis reactors work on the principle of blast-furnace process (ideal from the heat-exchange point of view). The process is accompanied by oxidation, as well as reduction of elements.

In such reactors heat-generation zone (tuyere area) is at the bottom and is formed by burning of remainders of undecomposed organics (carbon residue), which go from the bottom to the top. At the same time combustion is maintained by the air (oxygen) injection, hot substances of two types are formed (their temperature is 1500 – 1600 ?C): gaseous (volatile), and liquid (slag). Within the given temperatures dioxins decompose, but in the outlet area in the cold zone (temperature 250-700?C) the conditions for dioxin formation are created.

In addition, under the influence of carbon monoxide (CO) some heavy metals are reduced from oxides, for example some fusible metals (mercury, cadmium). Instead of getting to slag they “fly”, becoming volatile, getting into pyrolized gas. The formed pyrolized gas is the mixture of gaseous phase of all the products, obtained gradually at different heights of the reactor. It comprises heavy hydrocarbons, halogen-organic compounds (dioxins), steams of reduced heavy fusible metals. Thus, the problem of dioxin and other polluting substances formation is not solved.

It remains unsolved even if the dioxins are emitted within the standard limits after gas-cleaning systems, as for dioxins (because of their special properties: permanence and ability to accumulate like radioactive substances) the criterion of maximum permissible concentration (MPC) is inadmissible, i.e. there is no safe quantity of dioxins.

On the basis of the carried out research the main principles of ecologically safe process of high-temperature pyrolysis have been defined. Their keeping ensures maximum possible prevention of formation of harmful (toxic) substances in the pyrolized gas under the conditions of obligatory treatment of these gases from less harmful (nontoxic) and dust components with the help of common cheap systems. Here belong:

1. The use of high temperatures at intervals, grounded ecologically and technologically. Thus, under temperatures higher than 1427?C the formation of benzpyrene is eliminated. However, the indicated temperature does not provide the necessary conditions for furnacing of refractory mineral components of SDW. For stable furnacing of slag-components of the wastes it is necessary to maintain the temperature of about 1600-1700?C. this level of temperatures ensures not only complete furnacing of SDW mineral components, but also guaranteed dissolution of all organic toxins, including dioxin framework.

2. The creation in the reactor of deoxidizing atmosphere (under the enumerated high temperatures) with carbon surplus, which is present in many components of undissolved SDW. Deoxidizing atmosphere of the reactor excludes the new formation of dioxins, as freed oxygen under the mentioned temperatures will combine rather with carbon (according to thermodynamic rules), creating CO, than with chlorine, which, in its turn, will combine with hydrogen (HCl). Moreover, oxygen in the conditions of high temperatures (more than 1200?C) in the presence of carbon is combined with sulphur, forming sulphur dioxide (SO2). As CO, HCl, SO2 and other substances are stable compounds, the material base for second forming of dioxins while gas stream cooling is liquidated. It is also has to be mentioned that the stable combination of chlorine by hydrogen in the defined temperatures interval is ensured under 10%-surplus of water vapor over stoichiometric consumption. This guarantees the complete withdrawal of chlorine out of reactors in the form of hydrogen chloride and excludes its use in other combinations. At the same time the deoxidizing atmosphere with carbon surplus under 1600-1700?C are ideal conditions for reduction of all dangerous heavy metals from complex toxic compounds to pure (elemental metals), which are ecologically friendly. The reduced metals are disseminated in the slag and taken out of the reactor. After the vitrificated slag is cooled they appear to be safe and improve constructional qualities of the slag.

3. The filtration of the pyrolized gas before it is exhausted by the reactor through the layer of hot carbon. It is necessary to prevent dioxin compounds and heavy metals slippage out of the reactor. This is more effective than filtration of gas through the layer of activated carbon in sorption machines.

4. “Tempering” of the pyrolized gas at the reactor outlet (quenching to temperatures lower than 200?C, under which the combination of halogens with oxygen – dioxins – is impossible). Thermodynamic equilibrium, which was under high temperatures, when dioxins were absent, is “frozen”. Now the technical realization of this theoretical principle is not known yet. We suggest using for this purpose the vortical effect, which appears in the circulating flow of viscous compressed fluid. It is realized in a simple system which is called a vortex tube. During the process of SDW thermal neutralization the fine flue ash is bound to appear. It is active dioxin and heavy metals compounds sorbent. It is trapped from the gas flow by internal gas purifying systems (electric air filters). During the operation the filters get hot, which can cause desorbtion of harmful substances from the ash surface. The same takes place while using traditional units of pyrolysis. However, in this case there is no danger of harmful substances emission, as theoretically we prognosticate the absence of the mentioned substances in the pyrolized gas, i.e. material sorbed substances.