Application of Energy-saving Technology of Air Compressor System in Thermal Power Plant

Air compressors are very commonly used in industrial and mining enterprises Thermal Power Plant. Most of the industrial enterprises use screw air compressors, because of its high reliability, it has a series of advantages such as simple structure, less wearing parts, low maintenance cost, low exhaust temperature, and it can compress gases with relatively high moisture content. As a result, screw compressors have a high rate of utilization, rapid development and large market share in the market. But it is because it is mostly used in industrial and mining enterprises, and mostly in the form of air compressor group, and the motor capacity is generally large, most of them are in continuous operation all year round, so there is a great potential for energy saving.

1. The Development History of Air Compressor

In the 1930s, Prof. Alf Lysholm of the Royal Institute of Technology in Sweden was the first to design a screw compressor to meet the need for research on gas turbines. The increasing demand for air production has led to the continuous development and application of screw compressors.

Following the development of the screw compressor, the UK was the first country to obtain a license from SRM to manufacture screw compressors. Subsequently, Europe, the United States, Japan and others successively obtained the right to manufacture licenses. They began the production and sale of screw air compressors. But at the beginning, although the screw air compressor has some advantages, there are still many shortcomings.

Until 1957, the oil-injected screw compressor was put into the market, which further improved the shortcomings of the previous screw compressor, the screw compressor has been widely used.

In 1961, screw refrigeration air compressors and screw process air compressors were successfully developed. These two types of screw air compressors largely expanded the diversity of screw air compressors. Since then, a wide range of screw air compressors have been introduced.

With the continuous progress of science and technology and the further development of products, such as oil-free screw air compressor, completely oil-free screw air compressor, micro-oil screw air compressor  appeared one after another. The superiority of screw air compressor has been continuously played and widely used.

2. Air Compressor Working Principle and Structure Composition: Thermal Power Plant

Air compressors can be categorized into two main types according to their working principle: volumetric type and power type. Screw air compressors belong to the volumetric type, here is an example of how a twin-screw air compressor works: The yin and yang rotors and their supporting bodies constitute a volume change that produces a continuous working process of suction and compression and exhaust. If the twin-screw compressor is used in the field of thermal power generation, it will be equipped with a gas after-treatment system and a corresponding dryer.

• Electrical parts: motor, control system, operation panel, etc.
• Compressor part: main head, pressure adjustment valve, check valve, oil cut-off valve, minimum pressure valve, solenoid valve, etc.
• Cooler section (air-cooled): cooling fan, oil-air cooler, etc.

3. Problems of the Air Compressor System in Thermal Power Plants

Air supply unit for the compressed air system of two 600MW units of a thermal power plant is equipped with a total of 15 screw compressors (single compressor rated at 250KW, 375KW, 355kW). 4 air compressors are required for the operation of the instrument air supply system of 2 units, and 11 air compressors are required for the operation of the ash removal system. The outlet pressure of each instrument air compressor is around 0.72MPa, and the outlet pressure of the ash removal air compressor is around 0.5MPa. More problems affecting the energy consumption of the compressor have arisen since its long-term operation. There are several main points:

The air compressors in the power plants are all in the air compressor cluster mode, with multiple units supplying air, he air compressor unit is in manual control mode, with the operator turning on the appropriate number of air compressors according to the supply pressure. Because the air compressor group has no automatic chain start-stop function, resulting in low performance of air compressor group operation control and low efficiency; due to the number of days of operation, it is mostly in the 6kV, 10kV bus section power supply, so its power consumption is larger.

In the process of compressed air system operation, the screw air compressor is frequently loaded and unloaded, resulting in high temperature of the exhaust gas, which is often between the alarm value and the value of the jumping machine, causing hidden danger to the safe operation of the unit.

In order to ensure the system pressure, the number of screw air compressor operating units is on the high side, and the pressure reaches the high-pressure unloading value, which makes the air compressor unloading operation for a long time. Because the system air consumption may change frequently with high and low peaks depending on the time of regular ash blowing in the ash conveyor system, it leads to large fluctuations in the exhaust pressure of the screw air compressor. The ash conveying unit has low conveying efficiency (low ratio of conveyed ash to gas, low back pressure of conveyed ash) If there is a lack of coordinated control of multiple ash transfer units, problems such as large fluctuations in air consumption in the ash transfer system can occur, leading to poor operating efficiency of the compressor fleet.

A single compressor will operate with loading and unloading based on upper and lower pressure limits, which is basically, the compressor inlet valve closes when the exhaust pressure of the compressor reaches a certain pressure, and then re-opens the compressor inlet valve when the exhaust pressure decreases to a certain level. The time between intervals depends on the size of the storage tank, pipeline volume, the production site of the gas situation and the permissible pressure fluctuations in the pipeline network, usually the pressure fluctuations in the pipeline network should be controlled within the range of 0.1MPa. Only some parts of the components have the capacity to adjust the energy mode, for example, the U.S. GD or domestic Fusheng model is equipped with a rotary valve and other components, according to the size of the exhaust pressure to adjust the response to the inlet valve opening. However, this is also limited to the early products before 2010, the energy-saving effect is not very obvious after a long time of operation, because of its complex structure, high failure rate in long-term operation, and many late products, so the function has been canceled. The operation control logic of the air compressor cluster is too simple, resulting in low performance of the operation control of the air compressor cluster.

4. Selection of Energy-saving Retrofit Solutions for Air Compressors Thermal Power Plant

4.1 Waste Heat Utilization of Air Compressors

4.1.1 Overview of the Retrofit

The production of compressed air consumes a lot of energy and its exhaust temperature is around 80~110℃. Much of this heat is dissipated in the form of rear-mounted coolers.In addition to wasting the waste heat of the exhaust gas, an empty water-cooled cooler is provided to cool this heat.And no matter whether it is air-cooled or water-cooled, it is a secondary waste of energy that costs water and electricity. The solution is currently available through complete waste heat recovery solutions already on the market, which can achieve more than 70% energy recovery.

4.1.2 Scope of Application

When the air compressor is in operation, the heat generated is recovered through a heat exchanger, and then the hot water is circulated to the hot users through a circulation pump, which provides hot water to the users while satisfying the cooling of the air compressor exhaust. The principle is suitable for the following living applications:

Domestic water heating;

• Central air-conditioning system heating water and hot water for floor heating;
• Pre-heating of boiler make-up water;
• Heating of hot water cleaning process;
• Plant heating and other liquid media heating.

4.1.3 Transformation Advantages

• Compared with electric boiler, solar hot water system saves energy more significantly.
•  It does not consume electricity or oil, no energy consumption, which solves the cooling problem of air compressor.
•  It is easy and convenient to install and occupies little space.
•  Compared with other water heaters, it drastically reduces the cost and saves space.
• The heating system is low carbon and environmentally friendly, does not produce carbon monoxide, sulfur dioxide and other harmful gases, no black smoke, noise, oil pollution, and truly realize zero emissions.
•  A large amount of waste heat is effectively utilized, thus saving the electricity and fuel costs for the enterprise due to the use of electric heat, boilers and so on.
• The control part can be set to integrate the whole machine or control it separately.

4.2 Intelligent Compressed Air System Optimization Techniques

4.2.1. Retrofit Overview

Under the premise of satisfying the safety and stable production of electric power, the compressed air system is designed in accordance with the concept of energy saving of compressed air, we carry out reasonable and effective control of the air compressor air production regulating system, introduce air compressor groups, balance the operating time, control strategies, improve system efficiency, reduce the flow rate used by the system, optimize the system supply pressure and other technical means, which optimizes the operation of current compressed air systems and minimizes their air and energy consumption. At the same time, due to the optimization of energy-saving pneumatic conveying, the frequency and flow rate of pneumatic ash conveying are reduced, the wear and tear of the ash conveying pipeline is lowered, the maintenance cost of the pipeline and pneumatic valve is reduced, and the energy-saving purpose of the compressed air system is achieved.

4.2.2 Main Elements of Transformation: Thermal Power Plant

•  Install a high-precision flow demand control system between the gas supply system for instrumentation and ash, and compensate the excess flow to the pneumatic ash conveying system on the premise of ensuring constant pressure gas supply to the instrumentation system.
•  Install the energy-saving control system of the air pressure station for instrumentation and ash.
•  Add the pneumatic ash conveying energy-saving control system to the pneumatic ash conveying program control system.

4.2.3 Advantages of Retrofitting

Under the premise of meeting the safety and stable production of electric power, design the compressed air system according to the concept of compressed air energy saving, so as to optimize the current compressed air system related to the operation of the equipment, and minimize its gas and energy consumption, which plays an important role in the important indicators of the power plant and significantly reduces the plant’s electricity consumption rate.

4.3 Air Compressor Frequency Conversion Retrofit Technology

4.3.1 Overview of Retrofit

According to the actual operating conditions of the air compressor, the frequency converter is used to control the air compressor, and the frequency conversion air compressor is converted to frequency operation through electrical control, so as to realize the frequency conversion air compressor motor drive, which greatly reduces the rate of power consumption of the motor.

4.3.2 Content of the Renovation

•  The original industrial frequency system is kept unchanged when the system is transformed, and the inverter system is increased to achieve the interlocked switching of industrial frequency/inverter. When the frequency converter fails, the original industrial frequency system is utilized.
•  Setting the air pressure fixed value through the control panel of the inverter, the air pressure signal is converted into an electric signal and fed back to the PID regulator inside the inverter. The regulator compares the signal with the constant pressure value and then outputs a control signal, and the frequency converter outputs the frequency according to this signal and adjusts the speed of the motor to keep the air pressure stable, so that the air compressor is always in a power-saving operation state.

4.3.3 Transformation Advantages: Thermal Power Plant

•  The motor is started from low speed with low inrush current, and the motor and air compressor run smoothly.
•  As the frequency converter has strong protection function, it reduces the equipment failure rate.
•  The system pressure is more stable and the product quality is improved.

4.4 Permanent Magnet Synchronous Motors for Air Compressors

Similar to the frequency control, the power factor of permanent magnet synchronous motor can reach 0.98, the efficiency is more than 96%, and the average power saving rate reaches 15%~25%, which belongs to the high efficiency energy-saving motor. Configuring permanent magnet motors on air compressors can greatly reduce the power consumption rate of air compressors and lower the rate of factory electricity consumption.

Equipment should be replaced in time. For example: for the emergence of aging lines should be replaced in a timely manner, but also can be added to the discharge clamps, clamping insulators, etc.; for a single transformer, we should try to reduce the radius of the power supply, to avoid failures due to overloaded operation.

4.5 Air Compressor Cluster Piping Parallel System

4.5.1 Retrofit Overview

Most of the power plant air compressor groups are one air compressor corresponding to one dryer running in series. Multiple ash removal air compressors and dryers cannot be operated separately. In the air compressor or dryer maintenance you must stop the air compressor or dryer operation, so you need to install parallel piping, valves, so that the air compressor and dryer parallel operation. So that more than one dryer can be used as a backup for each other.

4.5.2 Content of the Renovation

•  Modify and install parallel stainless steel piping between the air compressor and the dryer.
•  Each air compressor outlet and group dryer inlet is equipped with electric stainless steel ball valves, manual stainless steel ball valves and flanges.
•  According to the system equipment, each air compressor outlet can enter the mother pipe of each group dryer inlet.
•  The air compressor and dryer can be operated separately after installation.

4.5.3 Transformation Advantages: Thermal Power Plant

Adding parallel piping and valves can make the air compressor and dryer run in parallel, and when the air compressor or dryer is overhauled, the air compressor or dryer can run separately, so that the 6 dryers can be used as standby for each other. So that the equipment can run safely and stably.

5. Conclusion

This article introduces the basic structure of the screw air compressor, the operation mode, the working characteristics and the development history of the screw air compressor, analyzes the screw air compressor in the thermal power plant in the prevailing problems. According to the characteristics of its high energy consumption, it lists the energy-saving technological transformation projects of air compressors that are more widely used today, and provides a basis for the efficient and economic operation of air compressors in power plants. Read more blogs.