MASS FLOW RATE ON PROCESS OF PLASMA PYROLYSIS

The paper describes the experimental examination of thermal utilization of used rubber. The research was carried out to examine the influence of rubber powder mass flow rate on the process of plasma pyrolysis of rubber. An arc plasma generator has been applied. Ar and mixture of Ar and H 2 were used as plasma gas. The analysis of composition of the gaseous products was done by the infrared absorption spectroscopy. All the rubber introduced to the plasma jet was decomposed. The outgoing gas did not contain any of toxic chemical compounds, like NO x or HCN.


Introduction
Even though, that there have been applied numerous methods to utilize rubber wastes (LIANG et al. 2020, SOVJAK et al. 2019, HUILIN et al. 2019) the problem of used rubber has not been solved satisfactorily yet ( fig. 1). Almost every landfill in Poland has a special spot covered with used tires well beyond the landfill capacity. This situation seems to be Thermal utilization of rubber waste can be not only a solution to the environmental problem but it might be a profitable activity as well. Decomposition of rubber in the plasma jet may be a source of hydrocarbons and syngas which exquisitely can reduce costs of utilization process of rubber by using those gases as fuel. Moreover, plasma pyrolysis of rubber might reduce amount of side products comparing to other thermal utilization methods of rubber waste.
Amongst the physical and thermal methods of utilization of rubber waste plasma pyrolysis brings a special attention and interest. That kind of pyrolysis is applied more seldom than thermal pyrolysis (maximum process temperature -1000 K). Plasma pyrolysis has much bigger potential though (Wielgosiński 2011, Tang andHuang 2004). Plasma pyrolysis offers very high process temperature, which can range from 6000 K to 18000 K (Majewski and Dębski 2012). So high process temperature provides plasma pyrolysis with huge advantage over thermal pyrolysis or ordinary incineration, especially considering decomposition of any substance or purity of gaseous products of rubber thermal utilization. That high temperature is able to decompose almost every substance and material.
The process of plasma pyrolysis has already been examined (Szuszkiewicz et al. 2001) considering plasmatron power influence on the course of plasma pyrolysis of rubber.
However, a dependence of rubber powder mass flow rate on plasma pyrolysis of rubber has not been investigated yet.
The aim of the research described in the paper was to determine the influence of amount of rubber delivered to the plasma jet on the composition of the gaseous products in the process during plasma pyrolysis. The secondary aim of the research was to investigate the influence of mass of rubber on concentration changes of the gaseous products as a potential source of energy.

Experimental set-up
The research on plasma pyrolysis of rubber has been carried out using a plasmatron ( fig. 4). The source of the heat is a plasma jet generated with plasma gas ionized by electric arc in the plasmatron (Majewski 2011, Mikoś 1987. The maximum electric power of the plasmatron in all the experiments equalled up to 30 kW. Ar was used as a primary plasma gas. It is an inert gas and so it does not influence composition of the gaseous products. Also, H2 was used as a secondary plasma gas. On the basis of the additional experiment (Szuszkiewicz 2007) it was found that in the mixture of plasma gas H2 should not exceed rate of 3.6 %. Higher amount of H2 in the plasma gas could be dangerous. H2 is a molecular gas and it plays the role of a gas stabilizing the plasma jet.
Additionally, it increases the power of a plasmatron.
The plasma generator is a PN-120 type plasmatron ( fig. 5). It has been produced by the Institute of Nuclear Research in Świerk, Poland. The cathode (3) of the plasmatron is made of tungsten. The anode (2) is made of pure copper. The anode also serves as a nozzle, which shapes the plasma jet. The plasma generator is electrically DC supplied. That construction is the most popular amongst all the plasmatrons. The DC electric supply (6, 7) simplifies the electric arc initiation and makes the plasmatron construction easier, less expensive and more dependable.
Since the temperature in the axis of the plasma jet is very high and equals up to 18000 K (Chamollo et al. 2018), both electrodes of the plasmatron have to be cooled intensively. This task is obtained by the water cooling system (6, 7).
To ensure that the reactions of decomposition and synthesis processes of the rubber are not influenced by the ambient air, the plasmatron was placed in the reactor ( fig. 4). The reactor is an open tank, so the outgoing gas will not cause the overpressure inside of it. The temperature of the reactor is lowered by the separate water cooling system ( fig. 4). The other task of the reactor cooling system is to provide intensive quenching inside of it.
The source of the examined rubber was used tires. The rubber was obtained from the vulcanization plant. The exact chemical composition of rubber mixtures has never been revealed by the tires company. It always is kept as a secret. Nevertheless, Chang et al. (1996) revealed the elemental composition of rubber mixture used in the USA: C -86.84 %, H2 -7.17 %, incinerated mineral compounds -3.78 %, S -1.89 %, N2 -0.3 %, O2 -0.02 %.
The rubber was delivered to the plasma jet by a fluidal feeder ( fig. 4). The size of the rubber particles have to be amounting up to hundreds micrometers to get entirely decomposed. Granulation of rubber powder was investigated in Szuszkiewicz et al. (2007).
The investigation of plasma pyrolysis of rubber was carried out in two separate series of experiments. In the first experiment plasma pyrolysis was carried out in the Ar plasma gas, at constant plasmatron power and constant plasma gas flow rate (tab. 1). The rubber powder mass flow rate was a variable and ranged from 0.05 kg/h to 8.04 kg/h. In the second experiment plasma pyrolysis was carried out in the Ar + 3.6 % H2 plasma gas (tab. 2). Also the rubber powder mass flow rate was a variable and ranged from 0.05 kg/h to 8.04 kg/h.

Results and discussion
The rubber powder was decomposed in two different plasma gases: in pure Ar and in mixture of Ar and 3.6 % H2. Plasma pyrolysis was carried out in the function of the rubber powder mass flow rate.
The exemplary absorption spectrum for pyrolysis of the rubber in the Ar plasma was presented in the fig. 6. There were identified infrared bands of C2H2, CH4, CO, CO2 and the common band of C2H2 and CH4. The band of H2O is also visible in the absorption spectrum but it is only a residue of water vapor present in the reactor prior to the experiment.
The presence of H2 in the outgoing gas could not be verified because of the technical specification of the absorption spectrometer. Although, according to the literature (Chang et al. 1996), H2 is present in the outgoing gas.
No bands of NOx are present in the absorption spectra. No bands characteristic for HCN were found in the spectra, neither. The analysis of the spectra ( fig. 6) confirms that there are no toxic chemical compounds. It means that pyrolysis of rubber in the Ar plasma brings about no undesirable compounds in the outgoing gas. So, there is no need to install any filtration or cleaning systems in the test-stand set-up.
The analysis of the gaseous products in the outgoing gas lets find out that the concentration of C2H2 is increasing as the rubber powder mass flow rate grows ( fig. 7). The concentration function of C2H2 is monotonically increasing.
The concentration functions of the other identified gaseous products (CH4, CO and CO2) of pyrolysis of the rubber in the Ar plasma ( fig. 8, fig. 9, fig. 10) are not monotonic.
The analysis of the absorption spectra of pyrolysis of the rubber in the Ar + 3.6 % H2 plasma shows off more numerous products of the process. Except for C2H2, CH4, CO and CO2, identified also in the spectra of pyrolysis of the rubber in the Ar plasma, also C3H8 (propane) and C4H10 (butane) were found for pyrolysis in the Ar + 3.6 % H2 plasma ( fig. 11).
Obviously, presence of H2 in the plasma gas stimulated synthesis of bigger number of hydrocarbons.
According to the analysis of the spectra ( fig. 11), neither toxic nor harmful gases have been identified in the outgoing gas for pyrolysis of the rubber in the Ar + 3.6 % H2 plasma.
During the experiment the rubber powder mass flow rate was being changed in the range from 0.05 kg/h up to 8.04 kg/h. The increase of the rubber powder mass flow rate caused the concentration of C2H2, C3H8 and C4H10 was increasing ( fig. 12, fig. 13, fig. 14).
The concentration functions of the three gases were monotonic.
While the rubber powder mass flow rate was increasing the concentration functions of CH4, CO and CO2 were changing non-monotonically ( fig. 15, fig. 16, fig. 17).
Vast majority of the products of plasma pyrolysis of rubber is gas (over 99 %) (Szuszkiewicz et al. 2001). The analysis of the solid state products has been done by the atomic absorption spectrometry, flame photometry and absorption spectroscopy FTIR. The analysis revealed that solid state products were mainly soot. It contained: chemical compounds with SO2 group and twelve elements, namely Pb, Zn, Cu, Fe, Mn, Cd, Cr, Ni, Ca, Mg, Na and K.

Conclusions
The carried out research positively verified all the assumed aims. Plasma pyrolysis seems to be a successful method for utilization of rubber waste. Generally, the increase of the rubber powder mass flow rate brings positive effect concerning the increase of production of most of the gaseous products, especially hydrocarbons.
The detailed conclusions according to the experimental research reveal all the benefits of the plasma pyrolysis of rubber. All the rubber powder introduced to the plasma jet was entirely decomposed. As the products of plasma pyrolysis mainly gas and small amount of soot were obtained.
Despite no filters had been applied no toxic compounds, like NOx, HCN or SO2, were identified in the gaseous products of plasma pyrolysis of the rubber.
Application of H2 as a secondary plasma gas had not only a chemical importance for plasma pyrolysis. H2 contained in the plasma gas influenced the increase of the electric current in the plasma jet. Also, H2 stimulated numerical amount and amount of hydrocarbons coming into being during plasma pyrolysis. Concentration of the gaseous products for the Ar + H2 plasma is bigger than for the Ar plasma.
The increase of the rubber powder mass flow rate did not increase the production of all the gaseous products. The increase of the rubber powder mass flow rate for the Ar plasma made the concentration of C2H2 monotonically increased. The concentration of all the other identified products of plasma pyrolysis changed non monotonically.
The increase of the rubber powder mass flow rate in the Ar + H2 plasma resulted in the monotonic increase of the concentration of C2H2, C3H8 and C4H10 and non monotonic concentration change of all the other identified gaseous products of plasma pyrolysis.