Efficiency of introducing a steam screw-rotor machine to the heating power plant circuit

The aim of the work is to increase the efficiency of the Nizhnekamsk CHPP-1 (combined heat and power plant) by introducing a steam screw-rotor machine (SSRM) into the thermal circuit. It is proposed to exclude the passage of steam from the exit of the turbine through the pressure reduction and desuperheating station (PRDS) for own needs. Superheated steam is diverted to be sent to a steam screwrotor machine installed parallel to the PRDS. This technical solution will allow to obtain steam used in low pressure deaerators, as well as electricity for own needs of the CHPP. The article presents the operating parameters, as well as the calculation results of the backpressure turbine. A feasibility study was carried out for the introduction of SSRM into the plant’s thermal circuit: the equivalent fuel and electricity savings for own needs were calculated, as well as the payback period of the project for introducing a steam screwrotor machine. In the course of the calculations, the following results were obtained: a decrease in the specific consumption of equivalent fuel for the production of 1 kWh of electricity by 1.9 g; saving of equivalent fuel during the implementation of the SSRM will be 13 tons per year, which also entails a reduction in emissions into the environment; Electricity production for own needs is 8100 kWh, the payback period for the project to introduce a steam screw machine in the thermal circuit of a thermal power plant is 5 years.


Introduction
The aim of the work to find out the possibility of parallel production of 0.12 MPa auxiliary steam at the Nizhnekamsk CHPP-1 and 1250 kW of electric energy with a frequency of 50 Hz supplied to the CHPP auxiliary electric bus. To achieve this goal, it is proposed to introduce a steam screw-rotor machine into the thermal circuit of the heating power station, as well as evaluate the economic effect of this technical solution [1][2].
Steam is used from the extraction of cogeneration turbines is used at the Nizhnekamsk CHPP-1 to obtain 0.12 MPa steam for heating demineralized water in low pressure deaerators (LPD). In winter, with a lack of extraction from these turbines, steam is used from regulated industrial steam extraction of the counterpressure turbine. Superheated steam passes after the turbine through a pressure reduction and desuperheating station PRDS 14/1.2 (the parameters are shown in Table  1) with its pressure and temperature reducing there. To exclude the steam reduction and to use the existed differential pressure, it is proposed to introduce a steam screw-rotor machine (SSRM) in parallel with PRDS 14/1.2. This technical solution will allow simultaneously receiving 0.12 MPa steam and 1,250 kW of electrical energy [3][4][5][6][7].
The SSRM is developed in Russia. It is a unique equipment without any foreign analogues. The design of the SSRM, its assemblies and systems is covered with about 25 patents in Russia and abroad [8]. The steam screw-rotor machine (SSRM) is a volumetric rotary machine. According to its operation principle, it is an inverted dry screw compressor.
Structurally, the SSRM consists of a housing where two working bodies -rotors are located with sliding bearings, seals and other components and parts. The power unit consists of a steam screw-rotor machine (SSRM) and a generator connected through a compensating sleeve and installed on a concrete foundation with supports, the generator frame and base plates [9][10][11][12].

Methods
The used methods include computational mathematics, heat and mass transfer theory, methods of technical and economic calculations in the power energy industry, and thermodynamic analysis of power plants. Application software packages were used for calculations and plotting graphical dependencies.
A thermal calculation of the backpressure turbine was performed to determine the parameters of the steam at the inlet to the steam screw-rotor machine. The calculation followed the method presented in [13][14][15]. The calculation results are shown in Table 2.
Dpr is steam flow rate for the production, kg/s; hse is steam parameters in regulated industrial steam extraction, kJ/kg; hd is the feedwater temperature after the deaerator, kJ/kg; DPRDS is the steam flow rate at the PRDS, kg/s; Heat consumption of the power generating turbine using PRDS or SSRM: The efficiency factor of power generation when using SSRM: The difference in the specific consumption of equivalent fuel for power generation with using PRDS and SSRM: .  (15) where CER-costs of the equipment repairs, 2629 US dollar.

Results and Discussion
An estimated calculation of the economic efficiency of the SSRM implementation is given in tables 3 and 4. The above calculations show that the payback period of a project to upgrade the thermal circuit of Nizhnekamsk CHPP -1 based on the SSRM will be 5 years. The economic effect in 2020 will be 166,152 USD and will grow from year to year, given the change in the indexation factor.

Conclusions
In this paper, we consider the option of upgrading the thermal circuit of the Nizhnekamsk CHPP -1 with the installation of a 1300kW steam screw-rotor machine. The proposed solution will allow getting 0.12 MPa auxiliary steam (including for the LPD) and at the same time additionally generating 1.25 MW of electricity by eliminating the need for steam throttling through a 14/1.2 atm PRDS. A feasibility study was also carried out for the SSRM into the thermal circuit of the CHPP. Savings of equivalent fuel in case the SSRM implementation were defined along with the plant auxiliary power saving, as well as the payback period of the proposed solution.