|J.V. Vankov||Kazan State Power Engineering University, Kazan 420066, Russia|
|R.R. Rotach||LLC “KER-Avtomatika”, Kazan 420006, Russia|
|S.V. Laptev||1. Plekhanov Russian University of Economics, Moscow 117997, Russia 2. Financial University under the Government of the Russian Federation, Moscow 125993, Russia|
|S.G. Ziganshin||Kazan State Power Engineering University, Kazan 420066, Russia|
|O.V. Afanaseva||Peter the Great St. Petersburg Polytechnic University, St. Petersburg, 194021, Russia|
article discusses how a steam screw-rotor machine (SSRM) can be introduced as
an energy-saving measure at chemical plants. The difference between a SSRM and
traditional blade steam turbine installations is also shown. It is proposed
that SSRMs be introduced into the thermal circuits of the chemical plants in
parallel with the existing pressure reduction and desuperheating station (PRDS)
30/16 and PRDS 15/8 to make it possible to use the potential energy of steam.
Passing through the SSRM, the steam decreases its pressure to the required
parameters and is then directed to the technological needs of the enterprise.
This additionally generates electrical energy that can be used for the plant's
own needs. Based on the calculation results, it is proposed that two SSRMs with
a capacity of 1.4 and 1.6 MW be installed. This technical solution will make it
possible to generate about 20 GW of electricity per year for the plant's own
needs. The payback period of the project will be 5.5 years.
Cogeneration; Energy saving; Expander; Pressure Reduction and Desuperheating Station (PRDS); Steam Screw-Rotor Machine (SSRM)
The aim of this work is to increase the efficiency of chemical enterprises by introducing a screw-rotor machine (SSRM) into their heat-technological scheme; this will also generate additional electrical energy for their own needs. This topic is relevant since, in Russia, chemical enterprises use about 12% of the country’s total energy consumption.
The energy intensity of the chemical industry is estimated at 15–17% on average. For a number of industries, such as the production of synthetic rubbers, the share of energy resources reaches 20–22% in the cost of production.
Chemical companies try to reduce the cost of heat and electricity by purchasing energy resources from external suppliers, buying energy sources from generating companies, or introducing their own generating capacity.
At the chemical plants under consideration, steam for their technological needs is generated and purchased at nearby thermal power plants. However, the parameters of this steam (temperature and pressure) are too large for the conditions required in the enterprises. To reduce the temperature and pressure of the initial steam, chemical enterprises are equipped with a number of pressure reduction and desuperheating stations (PRDSs).
However, when using a PRDS, steam loses its efficiency when throttled. The unused potential of the throttled steam can be realized in turbines (as a rule, these are bladed steam turbine units) when they are installed instead of a PRDS. At the same time, additional electrical energy will be generated for the company’s own needs.
This article discusses the use of a steam screw-rotor machine (SSRM) instead of steam-turbine installations. The SSRM is essentially a new type of steam engine. More than 20 patents have been obtained in Russia and abroad for the design of SSRMs, their components and systems.
According to the principle of operation, a SSRM (Figure 1) is a volumetric machine; that is, the expansion of steam occurs in a closed, changing space of the working cavity. This cavity is formed by two helical cavities of the master and slave rotors, as well as the body, and is called a paired cavity (Berezin, 2010a; Ziviani et al., 2016; Rotach et al., 2019b).
High-pressure steam enters the SSRM through the intake window in the housing from one end of the rotors. After filling the groove with steam between the teeth, the steam is cut off and, with further rotation of the rotors in the groove (steam cavity), the volume expansion of the steam portion occurs. At the end of the expansion, the groove communicates with the exhaust windows in the housing at the other end of the rotors. The spent steam is supplied to the network for technological needs (Berezin, 2010b; Smith et al., 2011).
Figure 1 The principle of operation of the SSRM: (1) initial filling of the paired cavity; (2) the expansion of the steam; and (3) exhaust steam output
The main advantage of a SSRM compared with the steam turbine power plants available on the market is that steam turbine installations are designed for almost a single combination of flow and steam pressure at the entrance to the machine and at the exit from it (this combination of steam conditions determines the power of the machine). That is, there are only a certain number of capacities available for steam turbine installations. At the same time, the parameters of steam used by enterprises vary greatly, with the most common power range being 200–2000 kW. A SSRM does not have a certain number of capacities. With a single basic machine design, it is possible to produce a machine of the required power in the abovementioned range. Such variability significantly expands the range of SSRM applications (Berezin et al., 2009a; Guzairov and Akhmetshin, 2009; Rotach et al., 2019a).
In the power range of 200–2000 kW, SSRMs have a number of undoubted advantages over other types of expanders. These include their maneuverability, quick starting and stopping times, large range of power control (20–100%), high dynamics and controllability, frequent stops of the unit are allowed, there is a low load on the foundation (rotation of the rotors in opposite directions ensures balanced operation of the machine and minimizes vibration and force effects on the foundation), high resource (up to 100–150 thousand hours) due to the absence of mutual contact of the rotors and, accordingly, have better mechanical wear (Smith et al., 2001; Quoilin et al., 2010; Bianchi et al., 2018).
A special feature of the SSRM design is the presence of a guaranteed gap between the master and slave rotors. The consequence of this is the absence of mutual contact of the screws and friction between them (Berezin et al., 2009b; Sang-Yoon et al., 2010; Dumont et al., 2018).
An important advantage of the SSRM, which distinguishes it from a number of other machines, is its efficient operation on two-phase media; for example, on wet steam. The liquid phase in the gas during rotation is thrown to the periphery of the screw and flows into the gap between the body and the screws which reduces overflows, thereby contributing to an increase in efficiency (Berezin et al., 2009a; Xia et al., 2015; Dumont et al., 2017). Figure 2 shows the flow of the SSRM workflow in p-V coordinates.
Figure 2 SSRM cycle (p – working fluid pressure; V – working fluid volume): (1)–(2) filling the steam cavity with steam through the inlet window; (2)–(3) volume expansion of steam; (3)–(4) outlet pipe pressure relief; (4)–(6) “squeezing” steam from the steam cavity
To date, SSRMs in the Russian Federation operate at the following facilities:
Regarding application abroad, SSRMs can be found:
In a SSRM, the main characteristics of water vapor used as a working fluid are (Berezin et al., 2009b; Giuffrida, 2017): steam pressure at the entrance to the SSRM P = 0.4...2.1 MPa; steam temperature at the entrance to the SSRM t = 145...350°C; consumption of steam passing through the SSRM 5 ... 50 t/h; and steam humidity is not regulated.
SSRMs are designed to work 100,000 hours, have a geometric degree of expansion of ? ? 5, and a rational power range of ~ 0.2...2.0 MW (Berezin et al., 2009a; Giuffrida, 2017). Installation of SSRMs is complex, consisting of a power unit and the following systems (Berezin, 2010a; Smith et al., 2011): steam inlet and outlet; a drainage system; a lubrication system; barrier water systems; a cooling system; automatic control, monitoring, and protection; and a generator, switch, control, and protection circuits.
There are three types of SSRMs for power plants: an autonomous mode, a parallel network mode, as well as one for driving actuators. When operating in parallel mode, the power plant operates on the electric network of the enterprise, covering part of its own electricity needs and thereby reducing its consumption from the external power grid (Berezin et al., 2009a; Rane et al., 2013). Thus, when using the potential energy of water vapor that would be lost when reducing steam in existing PRDSs, SSRM installation provides energy-saving measures that allow electrical energy to be generated without the cost of additional fuel (Orosz et al., 2013).
The introduction of a SSRM in the technological scheme of a chemical enterprise will allow the company to obtain steam of the necessary parameters for its technological needs, namely 1.6 and 0.8 MPa, and at the same time will allow additional electricity to be generated by eliminating steam throttling through the PRDS 15/8 and PRDS 30/16. The result of this implementation will be the annual generation of more than 20 GW of electricity for the company’s own needs.
The calculations show that, on average, the payback period of the project will be 2.5 years. And with the introduction of a SSRM in parallel with the PRDS 15/8 and PRDS 30/16, the company will be able to save 840,900 USD in 2021. Due to the annual cost indexation for equipment and materials, the savings will only grow.
Installation of a SSRM instead of the PRDS leads to a reduction of the exergy losses of the system. Joint production of electricity does not disrupt the operation of the main production of the chemical enterprise. The steam used in the SSRM has thermodynamic parameters sufficient for use for the main purpose and is sent to the plant’s technological needs and its heat exchangers. In addition, the introduction of a SSRM has a positive environmental aspect. Since no organic fuel is used to generate electricity, no pollutants are emitted.
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