Thermal mode of the condenser of a pyrolysis bioenergy plant with recuperation of secondary thermal energy

. The article describes the technological scheme and the principle of operation of a pyrolysis bioenergy plant (PBEP) with a recuperative heat exchanger, which simultaneously provides hot water, gaseous and liquid biofuels to the consumer. The results of mathematical modeling of heat exchange processes in the condenser of a pyrolysis plant are presented in order to identify optimal thermal parameters for cooling the biomass pyrolysis steam. A mathematical model of the heat balance of a “mixing-displacement” type condenser (along the length of the coil) of a pyrolysis plant has been developed for calculating, selecting and thermotechnical analysis of the working cycle of the condenser, which allows to obtain the values of optimal parameters of the temperature regime. The simulation results show that the considered variants of pipes with diameters of 15 mm and 20 mm fully meet the technological requirements of condenser cooling.


Introduction
Under the current conditions, pyrolysis is considered worldwide as a technology for solving a complex environmental and economic problem of rational waste disposal [1].
One of the priority directions for improving the energy saving efficiency of existing pyrolysis plants are the following technological operations [2][3]:  Increasing the use of secondary fuel and energy resources.  Maximum use of heat recuperation.  Solving the problem of increasing the energy efficiency of installations by optimizing the operating modes of technological installations. To date, a number of studies and review papers on biomass (BM) pyrolysis have been published, which focus on improving the energy efficiency of pyrolysis plants using a twostage circuit at low-and high-temperature pyrolysis sites, installations with external combustion of fuel and the use of solar energy [4][5][6][7][8][9][10].
A comparative analysis of the technical and energy characteristics of bioenergy plants shows that the energy intensity of BM processing in existing plants ranges from 0.3 to 1.5 MJ/kg [11]. In this regard, the development of energy-efficient bioenergy plants with the recuperation of secondary thermal energy in order to reduce the energy intensity of BM preprocessing is relevant.

Material and metods
Based on the conducted scientific research on the pyrolysis of plant BM and hydrocarboncontaining waste in order to reduce the energy intensity of the pyrolysis plant, we have proposed an experimental pyrolysis plant with a recuperative heat exchanger for pyrolysis of the processing of hydrocarbon waste to obtain alternative fuels ( Figure 1).
The experimental PBEP works as follows. BM for preliminary preparation is loaded into the BM preparation tank 1. The prepared BM passes into the pyrolysis reactor 3. A water jacket 2 is located outside the pyrolysis reactor. At the beginning of the process, under the influence of reserve fuel 15, using a burner device 4, the pyrolysis reactor 3 is heated to a temperature sufficient to start the pyrolysis process. In the future, the biogas of the gas tank 11 is partially used to heat the pyrolysis reactor using a control valve 12 and heat recuperation of the recuperative heat exchangers 8 and 14. The temperature inside the reactor 3 rises, and pyrolysis of the loaded BM occurs, and the water jacket 2 removes heat from the heating zone of the burner device into the upper layers of BM. As a result of pyrolysis and subsequent destruction in the reactor, the vapor-gas component leaves through the pipeline of the vapor-gas mixture 5 and passes through the condenser 6. To intensify the pyrolysis process, cold water is supplied to the condenser using a circulation pump 17. During heat recuperation between the walls of cold water pipelines and the combined-cycle gas mixture, cold water is heated and supplied to the consumer. The condensed vapor-gas mixture decomposes into pyrolysis liquid and pyrogas. The pyrolysis liquid enters the pyrolysis liquid container 7, part of the pyrolysis liquid is supplied to the consumer. A recuperative heat exchanger 8 is located in the pyrolysis liquid container, in which there is water. With the help of a circulation pump 9, heat recuperation occurs between the pyrolysis liquid and the water jacket 2. The decomposed pyrogas passes through the pyrogas purification unit 10 and accumulates in the gas tank 11. Biogas is supplied to the consumer from the gas tank. A recuperative heat exchanger 14 is located in the water jacket, which makes it possible, with the help of a circulation pump 16, to utilize the heat of the outgoing flue gases into the water jacket.
The analysis of the conducted studies shows that the condenser is the main heat exchange device, which provides the energy cycle of the pyrolysis plant. To select the rational parameters of the PP condenser, in this paper, heat exchange processes are simulated in non-stationary temperature conditions.
To develop a mathematical model based on the schematic diagram of the PBEP, we present the design scheme of the "mixing-displacement" type condenser in Figure 2. Taking into account the stationarity of the process (the time derivative is zero), the differential equation of the thermal balance along the length of the coil [12,13]: Where, the total length of the displacement zone (coil). Starting conditions: l=0; Heat balance of the condenser [14][15]: (2) Or: Integrate equation (1): (4) Or ,then we get the following final equation: The main thermal engineering parameters of the PBEP condenser are given in Table 1.

Discussions
The results of theoretical studies based on the method of mathematical modeling of heat exchange processes in a condenser in the cooling mode with different diameters of cooling pipes (Ø10, Ø15 and Ø20 mm) show that the cooling pipes of a condenser with a diameter d = 15 mm more efficiently provides the thermal mode of the installation.
The water temperature at the outlet of the condenser with a pipe diameter d = 15 mm ranges from 25 to 82 0 S and depends on the cooling water flow rate (Figure 4).

Conclusions
Based on the performed computational studies and simulation results, the following conclusions can be drawn:  The water consumption in the condenser 0.01 kg/s and 0.02 kg / s provides an outlet temperature of 40 = 82 0 C.  The cooling water consumption of 0.01-0.02 kg/s with a pipe diameter of d = 20 mm is more acceptable for performing a cooling cycle of a vapor-gas mixture of pyrolysis.