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Heat recovery steam generators are complex industrial systems with multiple variations depending on their application and energy source.
Two-pressure HRSGs
Two-pressure HRSGs are the most common models. They have high- and low-pressure sections. The high-pressure section increases the steam's temperature and pressure, which improves the turbine's performance. Two-pressure HRSGs maximize energy recovery in combined cycle plants.
HRSGs with supplementary firing
HRSGs with supplementary firing include combustors inside the system. They add fuel to the combustors to heat the air. This makes the temperature in the system rise. More steam will be generated, and it will increase the overall efficiency of the combined cycle and open cycle power plants.
Single-pressure HRSGs
This type of generator has evaporating, boiling, and superheating functions in one single-pressure drum, making it more convenient to manufacture. However, the single-pressure HRSGs have a disadvantage of lower efficiency compared to two- or three-drum HRSGs. Thus, this type of HRSG is typically applied in open-cycle gas turbine plants.
Multiple-pressure HRSGs
Multiple-pressure HRSGs consist of high-, medium-, and low-pressure drums. They usually have re-heat functions. The combustion gases go through the low- and medium-pressure drums first. After that, they are re-heated in the moderately high-pressure drum. The multiple pressure levels in this type of HRSG greatly improve the heat exchange efficiency. In addition, the re-heating feature promotes a further increase in the generator's overall efficiency. As a result, it becomes a great choice for use in combined-cycle power plants.
Capacity:
It refers to the maximum amount of water the feed pump can supply, generally Expressed in tons/hour or kg/h.
Pressure:
Represents the operating pressure of the HRSG, which determines the boiling point of water and the formation of steam. Usually, it is written in bar or MPa.
Temperature:
Includes the inlet temperature of the feed water and the outlet temperature of the steam, often in degrees Celsius (°C).
Structure:
The kind of HRSG structure, such as single-drum, double-drum, tower-type, etc.
Heat exchange surface area:
The total heat exchange surface area of the HRSG, which is usually represented in square meters (m²).
Efficiency:
It means the energy conversion efficiency rate of the HRSG, which represents the utilization of the waste heat. It usually may be from 70% to 90%.
Regular cleaning:
Regularly clean inner and outer surfaces to prevent excessive scale or dirt, and impact the thermal efficiency and safety of HRSG.
Inspection of the main components:
Check the main parts of the HRSG, such as the gas pipes, pressure vessels, insulating materials, etc., to guarantee there are no signs of damage, leakage, or corrosion, and ensure the equipment's integrity and reliability.
Replacement of consumable parts:
Regularly replace the HRSG of consumable parts, such as gaskets, sealing rings, or filters, etc., and ensure the sealing and filtration performance of the equipment.
Monitoring and adjustment of operating parameters:
Regularly monitor the operating parameters of the HRSG, such as temperature, pressure, flow, etc., and adjust them in accordance with the prescribed values to ensure the stable and efficient operation of the equipment.
Preventive maintenance:
Preventive maintenance for periodic inspection and repair is crucial. Follow the prescribed maintenance schedule and carry out routine inspections and repairs to ensure the good state of the HRSG.
HRSGs have diverse applications, with the majority being used in the power generation and oil and gas industries.
Power Generation
HRSGs are commonly used in combined-cycle power plants, which are regarded as some of the most efficient methods for transforming thermal energy into electrical energy. The factors leading to this high efficiency have been discussed above. The power generation efficiency of an HRSG can reach as high as 90% or more. Through the systematic and effective recovery of waste heat, the utilization of energy is maximized, and CO2 emissions are reduced, making a notable contribution to the mitigation of global warming.
Oil and Gas Industry
Heat Recovery Steam Generators play a crucial role in the oil and gas sector. They are instrumental in refineries, where they recover waste heat from oil refining processes to generate steam that fuels other industrial processes. In gas transmission and compression stations, HRSGs utilize waste heat from gas compressors to produce steam for driving gas turbines and further gas compression. Additionally, in the liquefaction of natural gas, HRSGs contribute by recovering heat to facilitate the liquefaction process.
Aside from the aforementioned, other industries making use of an HRSG are the cement manufacturing industry and the food and beverage industry, among others. This shows how the application of an HRSG is wide and diverse.
Energy requirements analysis:
Assess the energy requirement of the enterprise and determine the installation capacity according to the steam volume and operating temperature pressure.
Fuel type:
Choose an HRSG compatible with the fuel used by the enterprise. Different types of HRSGs are suitable for different fuels.
Operating efficiency:
Consider the operating efficiency of the HRSG, including energy conversion efficiency and waste heat utilization rate.
Equipment size and installation:
Consider whether the size of the HRSG meets the space available and the install requirements of the enterprise.
Automated control system:
Choose an HRSG with automated control systems for monitoring, data acquisition, and remote control functions.
Safety features:
Consider the safety features of the HRSG, such as pressure relief valves, safety interlocks, etc., to ensure safe operation.
Noise and emission control:
Consider the design of HRSGs for noise and emissions, which comply with the relevant environmental standards and regulations.
Q1: What does HRSG stand for in the power industry?
A1: In the power industry, HRSG stands for heat recovery steam generator.
Q2: Why is an HRSG crucial in a Combined Cycle Power Plant?
A2: An HRSG is instrumental in a Combined Cycle Power Plant. The machine contributes to making the plant 60% more efficient by utilizing a gas turbine's exhaust to create steam. The steam then powers a steam turbine, allowing for the recovery of waste heat that would otherwise go unused.
Q3: What types of HRSG are there?
A3: There are three main types of heat recovery steam generators used in the industry.
Wide fin HRSGs:
Wide-fin HRSGs are characterized by low gas-side pressure drops. They keep high thermal efficiency, yet their performance relies heavily on fin spacing and fin configurations.
Narrow fin HRSGs:
Narrow-fin HRSGs possess higher gas-side pressure drops. When there are high gas flow rates, narrow-fin HRSGs are more favorable.
Compact HRSGs:
Compact HRSGs have heat transfer areas integrated within the overall equipment. They have low gas pressure drops and are suitable for limited installation spaces.
Q4: What are the key components inside an HRSG?
A4: Super heater: The super heater increases the temperature of saturated steam to dry saturated steam. The gas coils are the main heat exchanging components of the super heater.
Economizer:
It raises the temperature of feed water before it enters the evaporator and is turned into steam. Normally an economizer is placed below the evaporator.
Evaporator:
The evaporator consists of water and steam coils. It turns water into steam by utilizing the heat from the flue gas. The evaporator is normally placed above the economizer and below the super heater.
Q5: What are the benefits of an HRSG?
A5: An HRSG recovers waste heat energy that would go unused, improving overall energy efficiency. The generator's compact design allows for flexible installations in different setups. It's fully automatic and has low operating costs.
