Determination of Technical and Economic Indicators of Thermal Power Stations Directly From the Turbine Regime Characteristics

The distribution of costs for electricity and heat production is accomplished by a number of fairly conditional methods. Cost analysis and allocation must be carried out on the basis of objective technical and economic criteria. The application of a method is determined by the introduced regulatory frameworks, energy markets and the prices of energy products. There are two fundamental methods – "physical" and "exergy". The physical method is based on the distribution of costs, and the exergy on the second law of thermodynamics. The article reviews and analyzes the tariff policy of the Thermal power plant. Objective criteria have been identified to serve as a means of forecasting and controlling fuel consumption. The thermal efficiency of a Thermal power plant has been evaluated. A comparison was made with similar Thermal power plants. A fundamentally new approach is proposed to determine the main technical and economic criteria of the plant. The proposed method allows determining indicators that uniquely characterize the thermal and economic efficiency of the plant without the need for diversification of fuel costs


Introduction
Determination of the indicators of thermal and economic efficiency of thermal power plants and as a consequence of the formation of tariffs for electricity and heat, in combined energy production, in purely mathematical terms, it is a simple Diophantine equation [1,2], one unknown, which is on determined in advance depending on physical, technical or economic criteria, as appropriate. Already at the beginning of the development of thermal power plants, a clear method for visualization of solutions, called the Ginter's triangle, was developed - Fig. 1, [3][4][5]. If the Cartesian coordinates of the ordinate axis represent the costs of electricity production and the abscissa axis of heat, there are two extreme positions in which one component is the maximum and the other zero. The straight line between them forms in the first quadrant a right-angled triangle with the origin of the coordinate system. As the total cost does not change, all distribution options are the working points lying on the hypotenuse. The orthogonal projection of each operating point on the coordinate axes defines a single solution with two values for electricity and heat. When allocating costs to a third product, such as desalinated water, additional technical and economic criteria must be analysed as a cost allocation tool to determine the contribution of each product to the total production cost [6][7][8]. Then the Ginter's triangle transforms and becomes a pyramid. The operating points are already in the plane and the solutions are their projections on the three axes. Numerous options for solving the problem of diversification of fuel costs are essentially reduced to the application of one of the two fundamental methods known as "physical" and "exergy". The physical method is based on the distribution of costs, in proportion to the amount of fuel consumed for each type of energy on the basis of heat balance, while the exergy method of the second law of thermodynamics, characterizing the quality of different types of energy. The main difference between the two methods of analysis is that, is that the exergy takes into account, not so much the quantitative equivalents of energy flows, but the energy losses during its transformation. In their developments a number of authors [9][10][11][12][13][14][15] have made sufficiently in-depth analyses and comparisons of both approaches, as well as of the hybrid "Energy" method, also known as the "ORGRES method" [16,17]. All known methods of fuel distribution and the subsequent costs between electricity and heat are proving to be equally conditional and should be applied with great care. They give completely different results and therefore no one of them can serve as a basis for shap- Engineering ing the tariff policy of thermal power plants under market conditions. The experience of the leading countries in the European Union, the legal framework introduced [18] and the analysis carried out [19] convince us of the need to apply exclusively economic mechanisms, by abandoning attempts to bind pricing solely with the problems of distribution fuel on TPP.

Fig. 1. Ginter's triangle
The object of the study is the TPP at an oil refinery. The plant was put into operation in 1963, after which it was gradually expanded and modernized. In terms of composition and technology, he has the characteristic features of his time. Designed to operate on a combined basis, the electrical load is determined predominantly based on the thermal consumption of the plant.
The technical and economic indicators and the energy tariffs are determined by the distribution of the fuel for the respective type of production, by the imposed normative physical method. With the privatization of the refinery and the switchover and in private hands, the conditions under which the power plant operated significantly changed. Due to the changes, the energy capacities were separated from the refinery in an independent energy enterprise with the activity of extraction of electricity, heat and desalinated water. From the very beginning, the question arose of setting fair rates at which the new company would sell its products. Although the Energy and Water Regulatory Commission sets the tariffs for industrial unit plant thermal power plants and district heating companies using the same methodology [20,21] there is a significant difference between them. While National Electric Company is obliged to buy electricity from district heating companies, even though it has no economic interest, nothing can oblige factories to buy electricity from their former power plants at certain prices, provided that twice as cheap energy is available on the market. The difficulties in operating the market conditions of the plant are further compounded by the sharp decline in the oil refinery's thermal loads, due to the limitation of the volume of main production and the management's deliberate policy to reduce the heat consumption in technological installations [24]. The economic survival of the thermal power plant necessitates an unconditional restriction of electricity production to the possible technical minimum in accordance with the new heat loads [25].
This, in turn, results in the exploitation of out-of-date and spent main and auxiliary equipment in technological regimes far from the optimal ones with increased costs.
The purpose of the study is instead of the "physical" method, to determine the technical and economic characteristics of the TPP directly from the operating characteristics of the turbine. The advantages of the proposed method are outlined and the tariff policy of the plant when operating under market conditions is analysed.

Methodology of the study
The purpose of this work is to analyse the tariff policy of the oil refinery at market conditions and to propose a fundamentally new approach for determining the main technical and economic indicators of the plant without diversifying fuel costs. Engineering Two tasks arise from the stated goal. The first task is to determine objective technical and economic indicators to serve as a means of forecasting, normalizing and controlling fuel consumption, as well as criteria for evaluating thermal efficiency when compared to similar plants. The second task is to find market-based mechanisms for shaping the tariff policy of the plant.
The application of any variant for dividing the total cost of fuel by type of production leads to the solution of the first task, with the proviso that each method implies the expediency of using it. Considering the fact that only one turbine is required to cover the loads PT-50-130/555, a new approach is proposed that is based on the machine's operating characteristics. The method allows determining indices that uniquely characterize the thermal and economic efficiency of the plant without the need for diversification of fuel costs.
It is known that the algebraic form of the mode diagram of a turbine with two steam extractions, depending on the electrical (W), industrial (D P ) and district heating load (D T ) is of the type: where the coefficients represent respectively the specific costs of fresh steam for idling d 0 [t/h] and production of electricity d Е [t/Mwh], as y P and y Т are the coefficients for not using the enthalpy drop respectively for industrial and district heating steam extraction. By definition: The dependences (2) In their physical sense, they represent the specific consumption of fresh steam for the heat dissipated through industrial and district heating steam extraction. According to the technical data of the turbine [22] at nominal steam parameters and regeneration included equation (1) is specified: It follows from the equation of the straight balance of the steam generator: According to (6), the efficiency (η) is reduced from 0,9 to 0,85 due to losses in steam lines to turbine shut-off valve and the need to maintain both turbine bypasses in hot reserve. Thus, the conditional fuel consumption Bу, [kJ/kg] of the steam turbine installation is: In equation (7) The back condensate is received and qualifies in the TPP, but is not returned to the heat scheme, but is added to the desalinated water and used as process water in the refinery. For this reason, its share in the balance of the plant a bc =0 and dependencies (8) are modified in: In enthalpy of the additional water i aw =126[kJ/kg], is obtained: After the transformations, equation (7) takes the following final form: In fact, the plant consumes more fuel than the formula (11). The reason is that significant both steam extractions are used for internal consumption at the plant, such as keeping a hot reserve on the oil fuel, heating the air in the steam heaters before the boilers, liquid fuel dispersion in burners, for heating and more. The determination of this energy should be carried out by a balance sheet and reported by adding as absolute value to d 0 [t/h], or use "estimated disposable" instead of "lower working" combustion heat of formula (7).
The second task cannot be solved by the traditional method of diversification of fuel consumption. In determining the conditions and arrangements for converting the plant to an independent legal entity, it became apparent that if the plant sells the extracted electricity at prices close to market prices, and the heat at the cost determined by the physical method, due to the high fuel value, the plant would not be able to function as an enterprise because the operating point is obtained below the line of profitability (hypotenuse) of the Ginter's triangle, point Cw in Fig. 1 Engineering Finding a satisfactory solution is possible using the known method of "the split profitability". In the presence of a market price for one product (electricity), a contract is concluded and its sale is carried out without determining the components of its prime cost. The financial result of the conversion is deducted from the total costs (constant and variable), the remainder being related to the production of the other product (heat). In the Ginter's triangle - Fig. 1, this distribution corresponds to Cr. The significant share of desalinated water in the total receipts transforms the task into three-dimensional, with the geometric interpretation changing from a triangle to a pyramid - Fig. 3. Using this approach eliminates the need to separate fuel consumption between electricity Fig. 3. and heat. This is the practice in most countries of the European Union.
Thus, the price of thermal energy С Т [€/MWh] is determined by the following formula: where the value of variable costs (S v ), fixed costs (S p ), the value of electricity realized (S Е ) and the value of realized water desalinated (S dw ) are measured in [€], and the released heat energy Q T [MWh]. Obviously: As The value of the fuel is also the sum of the costs for the different types of fuels used in the thermal power plant: Here with S bf [€], S m [€] and S bg [€] the costs for: boiler fuel, natural gas and factory fuel gases are indicated.
The fixed costs S P [€] are determined by the formula: where sums (S s ) of materials not included in variable costs, depreciation, repair costs, interest on loans, etc. are summed up. Add and regulated profit (S win ) which should ensure some return on capital.

Results
To summarize the results and analyse the applicability of the proposed method Fig. 2 presents the prices of the main energy resources for the refinery from the energy market and the supply from TPP for a period of 18 months (June 2018 to February 2020). It is evident that the price of natural gas for the period increased by 28,77 %, while the average price of electricity purchased from the free market for the main activities of the refinery during the same period increased by only 3,3 %. The price of electricity from the thermal power plant follows this trend and increases only by 2,6 %, while the price of heat increases by 54 %, practically equalizing with the price of electricity. The price of desalinated water has not changed for the period. The values of heat and electricity costs obtained in the calculation by the proposed method are compared with the "physical" method. As the method introduces separate specific fuel consumption for the released heat energy for each steam extraction, as well as a specific idle consumption, it is not necessary to introduce summer and winter consumption norms. Thus, the specific fuel consumption for electricity generation no longer depends on the heat load and its seasonal consumption. A comparison of the obtained results with the physical method is shown in Table 1.

Discussion
The proposed method for determining thermal efficiency is logical and thermodynamically justified [23]. The results are plausible with regeneration turned on and insignificant deviations from the design fresh steam parameters. The method is more precise than the physical method because instead of a generic one, it introduces for each steam extraction separate specific fuel consumption for the released heat energy, as well as a specific idle expense. The technical and economic indicators thus determined can be easily used for operational planning purposes -it is sufficient to know the expected consumption of electricity and heat for industrial and heating purposes for the period considered. There is no need to introduce summer and winter consumption rates, because the specific fuel consumption for electricity production is no longer dependent on heat load and its seasonal consumption. The parallel operation of another turbine is not economically justified, and even if for some reason, it is short-lived and cannot cause serious inaccuracy. However, if this is a fact for a longer period it is no problem to consider the relative share for each machine in the coefficients of equation (4). The method becomes difficult to apply in parallel operation of more than two different turbines. In this case, the thermodynamic approach presented in [22] seems more acceptable. Engineering The analysis of the results shows that the determination of the cost price and, accordingly, the cost of the heat energy of formula (8) have some fundamental disadvantages: -all contingent fixed costs incl. depreciation, staff salaries, repairs and more at the Electro factory and the Chemical factory, which have nothing to do with heat production, are an integral part of the cost of this product; -all costs for reagents relate to heat, while only a small fraction of them (hydrazine, sodium phosphate) are related to its production. In the same wrong way, only the costs of auxiliary resources, such as nitrogen technical, instrumentation, air, raw water, et al are written off in the heat energy alone; -the cost of own electricity of the entire plant indirectly (through fuel) is also calculated in the cost of heat energy. Fig. 3 shows the Ginter's modified triangle of pyramid constructed with summarized data for the period.

Conclusion
The general conclusion from the study that can be made is that the equalization of the prices of heat and electricity is thermodynamically unjustified. Buying expensive fuel and keeping the sale price of electricity leads to an increase in the price of heat. The economic conditions under which the plant operates require the cost of heat to be determined solely by the costs associated with its production. In the absence of funds for manufacturing activity, other means of financing need to be sought. The implementation of the proposed method gives credible results that can be used in the operation of the plant. Because the method is based on the diagram of the PT-50-130/555 turbine, it has significant advantages over the physical method.