In the overall operation of a thermal power plant, heat use is a very important factor that greatly affects its energy efficiency. The basic operating principles and main measures to improve thermal efficiency for thermal power plants have been outlined in this article, and some practical recommendations for thermal power plants operating in Vietnam's power system has been proposed to consider and apply optimally.

1. Overview of the thermal system in a thermal power plant

In a steam turbine power plant, fuel is burned in a boiler to convert it to heat. Water takes the heat of the flame due to the burning of the fuel and of the smoke stream to boil and evaporate. The generated saturated steam is fed to the superheater and continues to receive heat and increase the temperature to become superheated steam. The superheated steam coming out of the superheater is routed to the turbine by a pipeline. In a turbine, the steam with high temperature and pressure expands to produce work and then exits into a condenser. The steam will then be cooled by river water, sea water or by air to condense back into condensate. The condensate is then pumped into the boiler to repeat the thermal cycle. The above working principle follows a cycle called Rankine.

The principle heat diagram of a 300MW steam turbine thermal-power unit is shown in the figure below.

In coal-fired power plants in Vietnam, most of them are equipped with units with a capacity of 300MW, using high-pressure and subcritical steam. In recent years, new thermal power plants are almost equipped with large capacity units (500MW, 600MW) with subcritical or supercritical steam parameters. The thermal power plants of Thai Binh 2, Vung Ang 1, Vinh Tan 2 etc. use a unit with a capacity of 600MW, with subcritical steam. The Vinh Tan 4 thermal power plant uses a 600MW unit with supercritical steam parameters.

Figure 1. Thermal diagram of the 300 MW unit

Thermal efficiency of the unit

Thermal efficiency of a thermal power unit is defined as a percentage determined by the ratio of amount of produced electricity divided by the energy of the input fuel. Fuel energy can be determined through high calorific value (HHV) or low calorific value (LHV). HHV will be higher than LHV about 5 to 10%.

The thermal efficiency of the entire power generation process in a thermal power unit using a steam-extractor heat-recovery turbine according to the Rankine cycle is calculated as follows:

hunit  = hLH.htt.hoi(TB).ht,R(TB).hm.hg = hLH.htt.hTB

In which:

- hLH is the thermal efficiency of the boiler, if determined by the inverse balance method: hLH = 100% - q2 - q3 - q4 - q5 - q6 where qi is the heat loss components in the furnace a little bit.

- htt is the refrigerant/medium transfer efficiency on the entire plant's pipeline diagram which is referred to as the heat loss on the new steam path from the boiler to the turbine. This efficiency depends on the amount of refrigerant leaking from the scheme, the temperature of the leaking medium and the temperature of the pipeline insulation.

- hTB = hoi(TB).ht,R(TB) is the efficiency of the turbine device, including: the turbine's own internal efficiency is ht,R(TB) and the thermodynamic efficiency of the cycle thermal process ht,R(TB).

- hm is the mechanical efficiency of the turbine-generator unit.

- hg is the efficiency of the generator.

The economic efficiency of a thermal power unit depends mainly on the above efficiency components. In which the value of Rankine cycle efficiency ht,R(TB) has the strongest influence on the overall efficiency of the whole unit.

Today, large capacity units with new steam overcritical or supercritical parameters with a reasonable number of heat recovery heaters and suitable intermediate superheat parameters can achieve the thermodynamic efficiency of Rankine cycle up to ht,R(TB) = 0.50 ÷ 0.55 and thus achieve the thermal efficiency of turbine equipment  ht,R(TB) = 0.426 ÷ 0.478, for the whole unit will achieve hunit = 0.39 ÷ 0.44.

2. Improve thermal efficiency of thermal power plants

            For thermal power plants in operation, the following solutions can be applied entirely or partly to optimize heat use, in other words to improve thermal efficiency or reduce heat dissipation of the plant. Most of these measures have been proven in practice at thermal power plants in Vietnam as well as in the world.

- Boiler and auxiliary equipment: maintaining optimal excess-air volume, optimally adjusting the ratio of wind levels; using additives to enhance fuel combustion efficiency; strengthening monitoring, assessment and minimizing water, vapor leakage and air, smoke leakage; equipping intelligent dust-blowing system; applying low-temperature heat source to preheat coal (if possible) etc.

- Upgrading and improving old turbines, strengthening maintenance and repair work such as for adhesion/stick on turbine blades, cross section of transmission lines, wear of inserts, wing wear, steam leaks, etc. Those problems need to be overcame in time to improve the internal efficiency of the turbine itself. Up to 50% of the losses in the turbine itself are due to steam leaks in the shaft seals and blade tip inserts. Improving (for example, replacing turbine blades with new design technology to achieve higher efficiency) or upgrading old turbines is also a measure that can achieve more (1 ¸ 3)% of its efficiency , depending on the status.

            - Efficiently operate the cooling tower and cooling water system for condenser :

The efficiency of heat transfer at the condenser is directly and greatly dependent on adhesion/stick and non-condensable air leakage. Thereby it will have a very strong and sensitive effect on the cycle efficiency. The operation needs to well control the rate of adhesion/stick and build-up in the condenser tube as well as the air leakaging entering the vacuum system at the end of the low-pressure turbine and at the condenser. The pressure difference between 2 water boxes in / out of the condenser is most important information to identify the degree of adhesion/stick and blockage or perforation of the condenser pipe. Constant control of air inlet to the condenser vacuum system through control of condensate supercooling and air-vapour mixture flow after the intake manifold (Ejectors).

The adjustment of condenser vacuum maintenance, causes of condenser vacuum reduction, methods of maintaining vacuum, the number of circulating pumps put into operation, the control of air leaks and the formation of adhesion/stick growth in the condenser tube, etc. are considerations for the operation of the condenser in particular and the circulation system in general.

            - Minimize self-consumption: increasing the use of power-saving devices for self-consumption motors, such as installating inverter, paying special attention to equipment that is often under-loaded.

            For units that have to operate with frequent load changes, the installation of a speed-changing flow control device (ie. inverter or hydraulic coupling) for the supply pump will have great power-saving benefits. For units of 300MW size it should still give preference to the electric supply pumps with inverters over the pumps powered by auxiliary turbines. Today, with the manufacture of large-sized inverters and certain improvements made to low-voltage turbines, units up to 600MW are even acceptable for economically operating when using powered supply  pumps (with inverter) compared to using powered supply pumps (by auxiliary turbine).

            - Operate the unit at low pressure when running under load (ie. operation according to slip parameters):

            For units that often have to change the load and run at low load, adjust to run in the new steam pressure mode with the appropriate value, lower than the rated pressure. However, in which load mode it is necessary to maintain the pressure at what value is an issue that needs to be optimally calculated according to the efficiency of the heat diagram and according to the allowable adjustment capacity of that unit for good backup when there is a need to change the load. By reducing the new steam pressure at a specified value corresponding to the generated load at the above value, it will help to reduce energy loss due to large throttling at the steam control valve in the turbine and reduce self-used steam consumption for auxiliary turbine or self-used electricity for supply pump.

- Enhance modern measuring and control equipment with optimal support for computer-operated processes;

- Renovate, upgrade or install new measurement equipment systems for optimal controling and monitoring performance in real time.

- Increase awareness for operators and operation managers about the factors that affect the decline in unit performance.

- Increase the combined use of solar energy, for example, equip a solar battery system or a solar thermal system.

- Implement an energy management system to achieve continuous and sustainable improvement of plant-wide energy efficiency. Thermal power plants can adopt different models in the process of building energy management systems. However, with its large scale and complex equipment system, the ISO 50001:2018 energy management system standard is the optimal choice.

3. Recommendations for thermal power plants operating in the Vietnamese power system

            Operational management to maintain and improve the thermal efficiency of the plant is a requirement in itself but is also the responsibility of thermal power plants. Energy efficiency indicators (heat efficiency, fuel consumption) are also important quality indicators. Therefore, the thermal efficiency index certainly receives great attention from both senior leaders as well as employees of thermal power plants. The Law on Economical and Efficient Use of Energy also clearly stipulates the obligations and responsibilities of energy production entities, including thermal power plants, in ensuring the required energy efficiency parameters.

In order to improve heat efficiency and comply with legal requirements, thermal power plants can consider the following recommendations:

- For senior leadership: The involvement of the plant's senior leaders is a key factor to form, promote and maintain the continuous and sustainable improvement of the thermal efficiency improvement process of the plant. The commitment of senior leadership is demonstrated through the enactment of the energy policy, the formation of the energy management structure as well as the commitment to dedicate the necessary resources to the implementation of activities.

- Train: to raise awareness and understanding of officials and employees about activities and measures to improve energy efficiency. The necessary training should be implemented early and firstly.

- Monitor and evaluate: to build and apply processes, indicators, measuring equipment systems for monitoring and evaluating energy efficiency.

- Develop solutions and projects to improve the thermal efficiency of the plant. The implementation roadmap usually starts from measures to reduce energy loss, waste and from energy efficiency measures that require small investment or non-investment. Next step is implementation the solutions that require medium and large investments.

- Consider applying an energy management system according to ISO 50001:2018./.

MSc. Bui Thanh Hung, HUST

& Vietnam Energy Conservation and Energy Efficiency Association

References

[1] Nguyen Van Manh, Bui Thanh Hung, Pham Van Tan, Do Manh Hung and associates. Handbook of energy saving in thermal power plants. Report on the task designated by the Ministry of Industry and Trade, 2014.

[2] Jie Xiong, Haibo Zhao, Chao Zhang, Chuguang Zheng, Peter B. Luh. Thermoeconomic operation optimization of a coal-fired power plant. Energy 42 (2012).

[3] Mr. Santosh Mahadeo Mestry. Major energy saving potential in thermal power plant & effective implementation of EC Act 2001 in Power Sector. Reliance Energy.

[4] Operational documents of thermal power plants of Pha Lai, Uong Bi, Hai Phong, Mong Duong 2.