On the power plants, the main equipment consuming over 80 % of energy is energy boiler and turbine, so the most of attention with respect to resources is given to them. At present, the fuel preparation systems with mills fans (MF) are widely used on coal-fired power plants that perform the functions of fuel drying of flue gases selected from the top part of the combustion chamber of the boiler, grinding and transporting of pulverized coal to the burners of the combustion chamber (Fig. 1). To a large extent the efficiency of the boiler is determined by optimal work of pulverization systems. Various combinations of coal-pulverization systems, loading them with fuel and structural features, are greatly affected by heat loss with mechanically incomplete combustion and exhaust gases, as well as to maintain the set parameters of fresh steam, steam reheat, etc. In addition, the significant costs of expensive oil in the modes of the boiler firing and lighting the torch affect economic viability of coal-fired power plants operating with MF.

Fig. 1
figure 1

Scheme of the mills fans

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Improving energy efficiency of the system of production, transportation, and consumption of energy is an impotent problem, covering a variety of tasks. On the one hand, analytical and experimental dependences of the technical and economic performance of main power equipment have long been known [2, 3]. However, for production it would be nice to have a tool to quickly determine the technical and economic performance of the power unit. These tasks during the operation of equipment appear a lot and many can be solved by conducting time-consuming tests, mostly by contractors for the very substantial funds, either on their own, but without change in the energy characteristics of the equipment, which is often meaningless. One way to solve these time-consuming tasks is the creation of complex software, which will improve the efficiency of decision making to improve the efficiency of production.

The influence of structural features of MF on the efficiency of the high-power boilers is clearly seen in the example of the reconstruction of the mills, installed at power unit with the capacity of 215 MW, of the branch of JSC “INTER RAO—Elektrogeneratsiya” Kharanor HPS [4]. Three boilers TPE-216 are installed on Kharanorskaya SDPT, the third power unit with upgraded boiler TPE-216m. In the energy balance of power system installed capacity of the Trans-Baikal Territory, Kharanorskaya SDPT is 41 % and UFMC (utilization factor of maximum capacity) at the 68 %. In this situation, improvement of the efficiency of the power plant is a very urgent task. Note that among coal plants Kharanorskaya SDPT is currently one of the best in Russia in such indicators as SFCE (specific fuel consumption for electricity), which is 349.5 g/kWh. With the commissioning of the third unit at bringing it to the design performance, it will be more economical. The project fuel of KHPS is low-grade brown coal of Kharanorsky incision \( (Q_{i}^{W} = 11983.4\, {\text{kJ}}/{\text{kg)}} \) and mill equipment has been designed by taking into account its characteristics. The brown coal of Urtuyskiy incision is burned at the station since 1999 \( (Q_{i}^{W} = 15,503\, {\text{kJ}}/{\text{kg}}). \) Burning of non-project fuel led to unjustified reduction of technical and economic parameters (TEP), by increasing the flue gas temperature from 170 to 179 °C, excessive consumption of electricity for own needs (due to underloading of MF and increase of \( q_{4} \) by 4.4 %). Decrease of TEP has necessitated the modernization of pulverization, because the reliability and efficiency of steam boilers with high-power direct injection system of pulverization (MF) largely depends on quality of fuel, the loading level of mills by it, opportunities for temperature regulation of the drying agent, structural features of MF. Managing these factors, it is possible to find the optimal variant of coal-pulverization systems operation, achieving the most efficient operation of boiler [5].

There are a number of urgent tasks, which will greatly improve the techno-economic indices of Kharanorskaya SDPT. They are optimization of heat supply system, development of a reliable model of the boiler which allows to estimate changes in the regime; investigation of the efficiency of steam distribution system turbines at partial loads, etc. To achieve the objectives, we need a valid mathematical equipment model. The model can be used traditionally: heat balance method [2] or exergy method [6–8]. Consider the use of exergy method for determining the efficiency of boiler–turbine system. Exergy method is based on the fundamental laws of thermodynamics [9–11].

In boiler–turbine system, the greatest interest is the boiler, as exactly it happens in fuel heat utilization. Boiler TPE-216 has a U-shaped layout and 6 mill fans. For many years, staff of the station, together with the research staff from Trans-Baikal State University, analyzed, adjusted, and brought to optimal level mills, that is to the minimal losses in the system. Figure 1 shows a schematic work dust system with mills fans (1—boiler, 2—raw coal bunker, 3—mill fan, and 4—dust discriminator).

Hot gases for drying are taken from an upper portion of the combustion chamber of the boiler 1. To ensure that the temperature of the drying agent was permissible for conditions of safe operation of CF, additive exhaust gas is carried by exhauster gas recirculation. Fuel comes from the raw coal hopper 2 and dehumidified by the exhaust gases downstream exhaust. Coal is grinding in the mill fan 3 to the required size and sent to the dust discriminator 4, which is distributed in three tiers.

To reduce ventilation (air-powder flow), fan mills proposed a technical solution presented in Fig. 2. Its essence lies in the return of the dust-gas mixture back into the mill. On the positive side, we put the decrease ventilation and exhaust gas temperature due to the less use of gas recirculation. The negative effects include reduction of the turnaround time of the mill, due to the higher abrasion of impeller blades.

Fig. 2
figure 2

Windmill fan with installed internal recycling

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A further step to increase fuel efficiency is to reduce the number of employees up to three mills on the loads less than 185 MW. It may be noted that the power unit operates stably at three mills and at a high load in the case of high calorific value of the fuel.

When analyzing the operation of the power unit in a 3-mill mode, implementation of technical solutions in the range of 160–190 MW load produces an average reduction of exhaust gas temperature on (Fig. 3):

  • 6 °C—the burning of coal from Kharanor;

  • 13 °C—the burning of coal from Urtuyskiy.

Fig. 3
figure 3

Dependence of the exhaust gas temperature at the 3- and 4-mill mode when burning: a Kharanor lignite, b Urtuyskiy lignite [15]

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Each of the above technical solutions can be accurately evaluated by the assessing changes in exergy of the exhaust gases. In general, the specific exergy is called a specific efficiency of the heat flow or the flue of the working body in a reversible thermodynamic process of the system state changes from the initial settings until an equilibrium state with the ambient medium [10–13].

e = (i1 − i0) − T0 (S1 − S0) kJ/kg, where with the index 1—parameters of the working body, with the index 0 are environmental parameters; i—enthalpy kJ/kg; S—entropy kJ/kg K; T—temperature K.

Exhaust gas parameters with the parameters of the environment, temperature 20 °C:

  • i0 = 175 kJ/kg

  • s 0 = 6.89 kJ/kg K.

Parameters of exhaust gases at a temperature of 153 °C:

  • i 153 = 1490 kJ/kg

  • s 153 = 6.95 kJ/kg.

Parameters of exhaust gases at a temperature of 168 °C:

  • i 168 = 1638 kJ/kg

  • s 168 = 6.99 kJ/kg K.

Flow rate of exhaust gases is 670,000 m3/h.

Fuel consumption is 135 t/h with calorie 15840 kJ/kg.

Let us find exergy flows in absolute terms, knowing that E = e * G, where G—coolant flow.

  • E 153 = 241.46 MW;

  • E 168 = 266.82 MW;

  • E fuel = 594.0 MW.

The percentage of saved fuel energy can be determined from the following relationship:

$$ \begin{aligned} \Delta & = \frac{{E_{168} - E_{153} }}{{E_{\text{fuel}} }} \cdot 100, \\ \Delta & = 4.27\,\% . \\ \end{aligned} $$

Determined from the following expression with the loss leaving gases:

$$ q_{2} = \frac{{\left\lfloor {I_{yx} - (a_{yx} - \beta^{\prime } \cdot I_{{ 0 {\text{prs}}}} - \beta^{\prime } \cdot I_{0xv} )} \right\rfloor }}{{Q_{p}^{p} }} $$

  • \( q_{2}^{153} \,\% , \)

  • \( q_{2}^{168} \,\% . \)

Changing the efficiency can be estimated as follows:

Knowing that the efficiency of the boiler unit, excluding losses from exhaust gases 98 % find that the amount of fuel will decrease by 1.01 times. Fuel consumption at rated 135 t/h and the number of hours of use of installed capacity 5957 h, with the SFCE of 68 %, the cost of conventional fuel 1500 rub/Tce. Economic effect will be 6450 rubles/year. Reducing such as a key indicator of SFCE is approximately 1.5 g/kWh, which is very important [14]. At a cost of less than 1000 thousand. Rub. the payback period for the event is less than 1 year.

With the large share of the load of Kharanorskaya SDPT in balance grid and freeze rates for 2014, such events play an important role in maintaining profits of energy companies and improve the competitiveness of the electricity market in Russia.

In addition to the quality of the fuel and the level of loading of mills, the reduced efficiency of boilers with MF determines the need for expensive fuels combustion in modes of kindling boilers and lighting the torch. Currently by Kharanor HPS is used fuel oil as a starting fuel, the value of which in terms of conventional fuel considerably exceeds the cost of coal. Existing technologies of substitution of fuel oil in the modes of kindling on coal-fired thermal power plants (TPP) are adapted for boilers with direct injection. In Gusinoozerskaya, HPS plasma-fuel boilers for kindling and lighting the torch were introduced with the use of plasma-fuel systems (PFS) directly from the mills [16]. Implementation of PFS on boilers with direct injection is only possible when using a hammer mill (HM). Using the MF in standard modes of plasma-fuel kindling [17] is not possible, due to their inability to work at temperatures of drying agent (flue gas) lower than the calculated values. Using PFS modes of kindling in most types of boilers and lighting of the torch is a proven technology and defines a significant economic effect, due to displacement of fuel oil from the fuel balance of thermal power plants (TPP) [16–18]. To use the efficiency of PFS on the boilers with MF, expected to use the scheme (Fig. 4).

Fig. 4
figure 4

Scheme of plasma-fuel boilers kindling with MF

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Figure 4 shows a diagram of plasma-fuel boilers kindling with MF (1—boiler, 2—regular system of pulverization with MF, 3—ignition bunker of dust, 4—plasma-cyclone system (PCS) [19], and 5—plasma torches of kindling and lighting the torch). In the kindling mode with PFS-5, to replace the fuel oil used PCS-4, as a generator of high-temperature gases, which are used as drying agent in MF. To carry out the ignition operation of the PCS provides the hopper dust.-3. The issue of filling of the hopper kindling should be consistent with the requirements of the explosion protection of transportation and storage of dust.

MF is promising device of dust preparation boilers since they have low energy production and supply of dust as compared to other methods. Also it has simple construction and compactness. Their reliability and cost efficiency largely depends on the quality of fuel, the loading level, and the possibility of controlling the temperature of the drying agent. Managing these factors, the best operating coal-pulverization systems can be found, achieving the most efficient operation of the boiler unit. Using of PFS with it will allow them to exclude from the balance of TPP expensive fuel, allowing to increase the efficiency of the entire plant.