Experimental study of thermal conductivity of pyrolysised materials by means of a flat layer

Recycling of tires is currently a very important task. One of the areas of recycling tires is their low-temperature pyrolysis to produce marketable products – liquid fraction and a solid coke residue. For the development of the pyrolysis installation it is important to know the thermal conductivity of the coke residue at different temperatures of pyrolysis of initial material. As a property of matter, thermal conductivity depends in general on temperature and pressure. For materials with some structure, such as porous materials, the thermal conductivity depends on the characteristics of the structure. The thermal conductivity of the porous coke residue at pyrolysis temperatures of 300 0C, 400 0C, 500 0C and atmospheric pressure was studied experimentally at the laboratory unit of the department of “Theoretical basis of heat engineering” using the method of the flat layer in the temperature range 5…100 0C. Experimentally proved temperature dependencies of the coefficient of thermal conductivity of the coke residue are built to improve the accuracy of calculations of constructive and regime parameters of the pyrolysis installation.


Introduction
Around the world the question of utilization of the fulfilled automobile tires is urgent as the structure and composition of rubber do them extremely steady against biological decomposition. Annually in the world more than 285 million fulfilled automobile tires appear [1]. Growth of volumes of rubber waste and limited opportunities for their burial result in need of development of new, innovative methods of utilization and processing of the fulfilled tires.
Gasification of tires with receiving synthesis gas (CO and H 2 ) [2], and also pyrolysis with receiving liquid fuel, gaseous fraction and solid fraction -the coke residue or industrial carbon [3] are among perspective thermochemical methods of processing. The number of researches of process of pyrolysis of the crushed automobile tires, including with different additives of carbon-containing materials [4 -10] is executed.
Pyrolysis of the crushed automobile tires can be realized in the rotating drum furnaces. For modeling of thermal work and engineering development of such pyrolysis furnaces it is important to know heat conductivity of the coke residue at different temperatures of pyrolysis of initial material (figure 1). Besides, data on heat conductivity of the fine coke residue unloaded from the furnace are necessary for search of effective ways of its cooling.

Description of the method
According to Fourier's hypothesis, the vector of heat flux density is proportional to temperature gradient vector: Constant of proportionality  in (1) is called the thermal conductivity of substance and is the physical property of substance. Generally the thermal conductivity of solid bodies depends by nature substances and on temperature.
The method of the flat layer belongs to stationary methods of definition of thermal conductivity of materials. If the temperature field stationary and temperature changes only on thickness of the flat layer, then the heat flow through the layer of material is defined by the equation: where Q -the heat flow, W;  -thickness of the flat sample of material, m; 1 t -material temperature on the hot surface, o C ;

Description of experimental installation
For the research of heat conductivity of materials in Heat pumping Systems laboratory of Theoretical basis of heat engineering (TOT) department the experimental installation (figure 2) was created.
The principle of operation of the installation is put into implementations of the method of the flat layer and consecutive measurement of dynamic temperature fields of installation to rearch quasisteady conditions of work [11,12]. In steady conditions calculation of the thermal conductivity of the studied material is made.
The sample of the studied material represents disk volume with a diameter of 130 mm and 6 mm thick (thickness of samples can be increased to 15 mm). Samples are located between the heater and the refrigerator. They have to adjoin densely to hot (heater) and cold (refrigerator) surfaces of the device. Density of contact of all elements of the device is reached by purity of processing of the     The installation is equipped with system of the automated data collection and processing as a part of the ADC USB4718 module connected to the computer and the corresponding software allowing to write down and process signals of 8 thermocouple channels with the different time step. At the same time the measuring circuit provides automatic corrective action on temperature of the cold thermocouple junctions which are at the room temperature. Room temperature t room is taken by the precision mercury thermometer. For determination of the power consumed by the electric heater voltage drop (U, V) and current (I, A) by means of the voltmeter and the ampermeter is measured.

Processing of results of the experiment
The data which are taken off in the experiment are processed in specially created MS Excel TM template, the example of such table is presented in figure 7 for temperature of pyrolysis 300 0 C. Such representation well illustrates the main idea of the experiment. Set the power of the heater and control difference of temperatures in the layer. The difference of temperatures depends on the material thermal conductivity, and by means of further data processing [13] we will calculate its values.
Calculate the value of the heat flow determined by the power of the electric heater W, watt where q -heat flux through samples, W/m 2 Applying the technique (3)(4)(5)(6)(7) to data of the experiment (figure 7) at the schedule of loading submitted in figure 9a, we will receive change of the thermal conductivity in time -figure 9b.
On graphics of the figure 9b the site to quasisteady conditions is fixed and for final values of the thermal conductivity by means of the least-squares method dependence λ(tm) is obtained.

Results of numerical and experimental definition of the thermal conductivity
In the figure 10 as the example results of one of experiments on definition λ(tm) for temperature of pyrolysis 300 0 C are presented. Similar graphis are received for temperatures of pyrolysis 400 0 C and 500 0 C. As a result of carrying out not less than three series of experiments for each sample are shown that at temperatures of pyrolysis of 300 0 C, 400 0 C, 500 0 C and atmospheric pressure it is possible to describe thermal conductivity of the porous coke residue by linear relation: Coefficients of the equation (8) for temperatures of pyrolysis 300 0 C, 400 0 C, 500 0 C are presented in table 1.

Conclusion
The thermal conductivity of the porous coke residue at pyrolysis temperatures of 300 0 C, 400 0 C, 500 0 C and atmospheric pressure was studied experimentally at the laboratory unit of the department of "Theoretical basis of heat engineering" using the method of the flat layer in the temperature range 5... 100 0 C. The surface temperature of the test material was measured by 6 K-type thermocouples, two of them laid on surfaces of refrigerators, and the rest on the heated surfaces and inside the heater. By using ADC boards, thermal EMF of the thermocouple is transferred to the computer in real time, providing high precision control of reaching the stationary mode of operation. Thermal parameters of the installation were controlled also with the thermal imager, which allowed to more accurately determine the thermal balance of the experimental modes. The experimentally proved temperature dependencies of the coefficient of thermal conductivity of the coke residue are built to improve the accuracy of calculations of constructive and regime parameters of the pyrolisys installation.