Information model of the structure of the heat and power system of pulp and paper production and a systematic approach to its improvement

The stages of the system analysis of an industrial energy system with a complex structure are considered by the example of pulp and paper production. The software of the structural and thermodynamic stages of the analysis is presented. The results of thermodynamic calculations of the elements of the energy system by the optimal sequence are given. A variant of the system modernization using a heat pump is presented.


Introduction
Pulp production is one of the largest industrial consumers of thermal energy. Pulp and paper production is a complex structure with many elements that interact with each other and with the environment. The interaction of elements is often of a reverse nature, which leads to recycling and numerous iterations in the calculations of the system and in the selection of measures for the modernization of production [1 -3].
To reduce the number of iterations in the calculation of the scheme, it is proposed to use the method of structural modeling and the development of an information model of production based on it. This approach simplifies the structure of multi-element production schemes and can be the basis for conducting thermodynamic calculations of the system. The structural modeling method formed the basis of the developed software [4 -7], which can be used when conducting a system analysis of production and making decisions on the return of waste energy flows to the technological circuit.

Stage of structural analysis
The object is the heat technology scheme of pulp and paper production at JSC "Polygraphboard", Balakhna (Russia), including 110 elements, 193 streams [7]. All elements and threads are represented as an oriented graph. The scheme is decomposed by weak links (flows that are not included in the closed contours of the scheme) into separate blocks. Let us dwell on one of the blocks selected as a result of decomposition (fragment - Figure 1). This unit is also the most energy-intensive sector of pulp and paper production. The stages of the program are considered (Figure 2, Figure 3). The scheme is represented as a matrix (Figure 2 (a)); the number in the matrix is the stream number. The matrix is multiplied according to the rules of Boolean algebra (Figure 2 (a)). At certain stages of the matrix multiplication, there are closed sequences -cycles (Figure 2 (b)). Next is the minimum number of streams, the conditional discontinuity of which will allow to carry out a sequential thermodynamic calculation of the circuit with a minimum of iterations ( Figure 2

Stage thermodynamic analysis
The software provides for the possibility of carrying out thermodynamic calculations based on the found sequence and determining the efficiency of the apparatus ( Figure 4). Let us enter the flow parameters when entering the scheme matrix at the structural analysis stage and entering formulas for the scheme elements ( Figure 4 (a)). Also provides for the calculation of performance indicators of the elements of the system (Figure 4 (b)); explanation of symbols in the Figure 3 (b).
The obtained parameters of the flows of the block of the cardboard machine in the form of the initial data and the results of the thermodynamic calculation are presented in table 1. The values of thermal and exergic power are determined by temperature, flow rate, pressure for each flow. The greatest exergetic power among the outlets is at the condensate stream 113 and at the air stream 117.
Further, the possibility of returning the energy of these flows to this scheme is considered. For this purpose, a thermal and exergy calculation of the elements included in the block is carried out [8 -11]. The results of the exergic calculation are shown in table 2.
From table 2 it is seen that for those devices in which there is no heat and mass transfer, the efficiency is high, therefore, there is no need to make their modernization. And at devices 33 -37, the efficiency is much lower. Therefore, it is necessary to carry out the modernization of these devices, namely, to realize the return of the energy of the streams of the greatest exergy potential to these devices [12,13].

Conclusions
Condensate flow 13 was previously used in the scheme under consideration in the system for collecting and returning condensate, therefore it is not considered in the . 2. Stages of the program for the structural analysis of complex schemes (a, b, c, d).
proposed return system. The circuit for returning the airflow 117 [14,15] is shown in Figure 5. It has been proposed to use a heat pump in the drying part of a cardboard machine for utilizing the heat flow of spent moist air 17 in order to bring the parameters of the spent moist air to the required parameters of heating dry air. Cold air enters the condenser of the heat pump 2, where it is heated to the required technology temperature of 110 °C and sent to the drying chamber 5. The air temperature in the drying chamber decreases and the a) b)  Moisture is removed from the product and this moisture is transferred to the air stream. Next, moist air with a temperature of 74 °C is sent to the evaporator of the heat pump 4, where it is cooled. Simultaneously with the cooling process, the precipitated moisture is removed. In the evaporator 4, the exhaust moist air acts as a source of thermal energy, the potential of which rises in the heat pump.
The results of the calculations showed that the heat of the outgoing air is sufficient to heat the air entering the drying unit.  Using the scheme with a heat pump, 6245.89 kW of additional thermal energy was obtained through the use of 2942.44 kW of wet flue gas energy. In the case of using a drying installation with a conventional air heater, an additional 3789.24 kW of energy would be consumed in the electric heater.
As a result of the use of the heat pump during the drying process, the exergic coefficient of system utilization [12] of the block under consideration increased from 49.5 % to 65.6 %.