The material balance of process of plasma-chemical conversion of polymer wastes into synthesis gas

The process of conversion of polymer wastes in the flow of water-steam plasma which are created by the liquid electrodes plasma generators was experimentally studied. The material balance was calculated. The regularities of the participating of hydrogen and oxygen which contained in the water-steam plasma, in formation of chemical compounds in the final products were revealed.


Introduction
It is known that the water-steam plasma is the most attractive option of energy carrier of plasmachemical processes, including plasma waste treatment processes [1,2]. It provides a number of advantages. Water-steam plasma does not contain the ballast components (such as nitrogen in the composition of the air plasma). Therefore, the thermal efficiency of energy carrier becomes substantially higher. Formation mechanisms of harmful oxides such as oxides of nitrogen and sulfur are suppressed in the water-steam plasma. This ensures the most favourable environmental conditions. The water-steam plasma is enriching synthesis gas by hydrogen due the carbon oxidation of raw materials by water-steam: С + Н 2 О → Н 2 + СО. This increases the amount of the final product. There are other positive effects that accompany the practical application of water-steam plasma which are may be noted [3,4].
Currently arc plasma is used mainly in the processes of recycling [5]. Energy carrier plasma is generated by blowing out the arc discharge using various gases, including water-steam. However, in practice, there are additional technical difficulties at using water-steam in arc plasma torches. In real industrial settings in the first place required an efficient steam generator for obtaining superheated steam. The protection of the refractory electrodes from direct exposure to water-steam needed. And also to take steps to prevent moisture condensation on the surfaces of the conductive elements.
Applying of liquid electrodes plasma generators based on gas discharge with liquid-electrolyte cathode eliminates the above-mentioned negative factors. The plasma stream is forming from electrolytic fumes. Evaporation of electrolyte takes place under the influence of heat coming from the gas discharge.

Experiment
In this work, the liquid-cathode plasma generator was used, allowing to create a plasma flow of electrolyte vapor at temperature up to 1800°С and the mass flow rate of up to 3,0 g/s [6,7]. Previously used pieces of plastic film (PE material) and bottles made of polyethyleneterephthalate (PET material) were as raw material. Processing of raw materials was in a sequence: 1) thermal decomposition in the environment of water-steam at a temperature of 770-800 K; 2) conversion to synthesis gas in plasmachemical reactor in the temperature range 1670-1720 K. Some results of experimental studies published in [8][9][10][11][12][13]. The adopted simplifications and source data. In the calculations the balance of the main chemical elements that make up the reagents was considered. These are: carbon C, hydrogen H and oxygen O. Material balance implies equality of the masses of individual chemical elements in the primary components (polymer, raw materials and water) and in the end product in all physical phases: solid, liquid and gaseous.
The limited options which available direct measurement, do not allow the calculation material balance in full, taking into account changes of the chemical composition of a reacting mixture during the whole duration of the process. In this regard, for the calculations selected the steadiest regime, which in time is located at a sufficient distance from the initial and final stages of the process. However, even in this case there is an uncertainty in the numerical values of several process parameters. In particular, remain unknown the exact values of the mass arrival rate of raw material components (chemical elements C, H and O) in the plasma chemical reactor, where are formed the final products of plasma chemical reactions. Therefore, the calculation of the material balance is only possible under certain simplifying assumptions. As such, in this work, we adopted the following simplifications.
1. Thermal decomposition of the raw material occurs uniformly. The source reagents are received in the plasma-chemical reactor with mass velocity ṁ = k•m/t. Here kis the coefficient taking into account the formation of solid residue; mis the mass of raw material; tis the period of time during which there is an intensive thermal decomposition of raw materials of volatile components.
2. The mass rate of the chemical elements C, H and O in a plasma-chemical reactor is constant and their ratio to each other is equal to the atomic mass ratio in a conditional formula of raw materials. For PE-raw materials (conditional formula С 2 Н 4 ): ṁ C : ṁ H = 24 : 4.
It is necessary to consider when compiling a conditional formula of the raw material polyethyleneterephthalate about forming of a solid residue of carbon. In the plasma-chemical reactor is receives only the volatile components of the thermal decomposition of polyethyleneterephthalate. Therefore, in the composition of the reagents that acts in plasma-chemical reactor, C atoms less than the original feedstock. This leads to changes in the stoichiometric coefficients in the chemical formula.
In the experiments, the mass of the solid residue amounted to an average of 10% from the initial weight of raw materials. With this in mind, conditional formula is written as С 8  From table 1 follows that the end products in plasma-chemical process are mainly generated in the gas phase. Significant amounts hydrogen, carbon oxides and methane are formed (tables 3 and 4). In small amounts are present acetylene, ethylene and benzene. Thus in the total mass of gaseous end products the content of heavy chemical elements O and C more, than lightweight H.  For comparative analysis more informative is the number of atoms, i.e. the molar content, rather than mass. These data are obtained by the recalculation are shown in figures 1 and 2 in the form of diagrams.  As seen from the presented diagrams that PE-raw does not contain atoms O, and in the final products they appear. At the same time there is a significant addition of atoms H. Similar patterns are present in the case of PET-raw. In both cases, the atoms H and O are transferred from the steam.