How to improve combustion efficiency and reduce consumption in thermal power plants

　　According to the survey, thermal power generation is currently the main force in the development of electric power. The power generation of thermal power plants mainly depends on boiler combustion, that is, the chemical reaction between fuel and oxygen. The combustion efficiency control in the combustion process of a thermal power plant boiler is extremely important. Increasing the combustion efficiency can ensure the economical and safety of the boiler operation, maximize the utilization of thermal power plant resources, and increase the electricity production rate. At the same time, with the gradual advancement of the national green environmental protection policy, thermal power plants also need to focus on the utilization of fuel in the power generation process and the impact on non-renewable energy in the direction of improving power generation technology. In short, it is to reduce consumption and reduce emissions. . Therefore, while improving the efficiency of fuel conversion, thermal power plants must also reduce their own energy consumption, reduce pollution emissions, improve the comprehensive competitiveness of thermal power plants, and adapt to the needs of harmonious society development.

　　First,Problems faced by thermal power plants

　　The efficiency of large-capacity boilers in modern thermal power plants is generally 90-94%. However, due to the inaccurate control of combustion efficiency in most thermal power plants, the combustion of carbon is incomplete, and the carbon content of fly ash is 10%-13%. The boiler efficiency is only 85.3%-88.66%, greatly affecting the safety and economy of boiler operation. In addition, with the increasingly serious problem of environmental pollution and the implementation of ultra-low emission reform of thermal power plants, the environmental protection department has put forward more stringent emission standards for pollutant gases, and even shut down some production enterprises that do not meet the pollution discharge standards, 2018 The introduction of environmental protection tax has accelerated the pace of enterprises seeking technological innovation and reducing pollution emissions. Therefore, how to improve combustion efficiency and minimize energy loss and pollution emissions is the primary consideration for the high-quality development of modern thermal power plants.

　　Energy consumption analysis of thermal power plants

　　Second, controlling the excess air coefficient is the key to improving boiler efficiency

　　The purpose of improving boiler efficiency is to ensure safe, economical and environmentally friendly operation of the boiler on the basis of the amount of steam required for external electrical loads and the quality of the qualified steam. Specifically summarized as:

　　1. Improve the economics of boiler operation

　　1) Minimize various losses by running optimization adjustments to improve boiler efficiency;

　　2) Optimize the coal blending mode to provide boiler operation economy;

　　3) Ensure the normal and stable steam pressure, steam temperature and evaporation of the boiler, reduce the flow rate of the reheater desuperheating water, etc., in order to improve the thermal efficiency of the whole unit;

　　4) For every 10 °C decrease in main steam temperature, the coal consumption for power generation is about 0.93g/kWh; for every 10°C decrease in reheat steam temperature, the coal consumption for power generation is about 0.75g/kWh;

　　5) For every 10t/h increase of the desuperheater water flow, the coal consumption of power generation is about 0.08~0.12g/kWh; for every 10t/h of reheater desuperheating water flow, the coal consumption of power generation is about 0.52~0.63g/kWh.

　　2. Improve the safety of boiler operation

　　1) Improve the stability of boiler operation;

　　2) Ensure that the boiler burns completely flame and evenly fills the furnace to prevent the boiler from running flame deflection;

　　3) Reduce the temperature deviation of the horizontal flue outlet;

　　4) reduce the generation of a reducing atmosphere around the water cooling;

　　5) Reduce boiler slagging and prevent burning of the burner;

　　6) Ensure that the water wall, superheater, and reheater are not overheated.

　　In order to improve the thermal efficiency of the boiler, the heat loss of the boiler should be appropriately reduced. The heat loss of the boiler mainly includes the heat loss of the exhaust gas, the heat loss of the incomplete combustion of the gas, the heat loss of the incomplete combustion, the heat loss of the boiler and the physical heat loss of the ash. Five kinds. The excess air coefficient is one of the main factors affecting the heat loss of the boiler, and it is also an important economic indicator of combustion technology for boiler operation.

　　The so-called excess air ratio, that is, the ratio of the actual air supply to the theoretical air demand when the fuel is burned. The excess air coefficient specified in the boiler discharge standard is related to the boiler type and power. The specific provisions are: for coal-fired boilers, the power is less than or equal to 45.5 MW, the excess air coefficient is 1.8, the power is greater than 45.5 MW, and the excess air coefficient is 1.4. For gas or oil-fired boilers, the excess air ratio is 1.2.

　　In the actual description, the power of some boilers is t/h, and its conversion relationship with MW is: 0.7MW=1t/h. For example, a 45.5MW boiler is equivalent to a 65t/h boiler. The higher the excess air coefficient of the boiler, the lower the combustion efficiency of the boiler, so the coefficient of the coal-fired boiler is higher than that of the fuel-fired boiler, and the coefficient of the small coal-fired boiler is higher than that of the large coal-fired boiler. The higher the excess air ratio, the higher the oxygen can be. For boilers above 65t/h, the theoretical oxygen content of the flue gas is 6%, while the boiler below 65t/h has a theoretical oxygen content of 9.3%.

　　Excessive or excessively low air ratio is not conducive to combustion: the excess air coefficient is large, indicating that there is too much air supply in the furnace, which not only reduces the furnace temperature, deteriorates the combustion, but also increases the amount of flue gas, thereby causing an increase in the heat loss of the boiler exhaust gas. The boiler efficiency is reduced, and the power consumption of the fan and the power consumption rate of the thermal power plant and the coal consumption of the power supply are increased. If the excess air coefficient is too small, the incomplete combustion heat loss of the gas and the incomplete combustion heat loss of the solid may be increased, and the energy utilization cannot be maximized. . The best way to do this is to ensure that the fuel is more O2 and burned as much as possible, the lower the excess air coefficient, the more economical the combustion.

　　Table 1. Excess air coefficient versus efficiency for a typical power plant boiler

　　To ensure that the fuel burns properly, the excess air ratio must be controlled within the appropriate range. The amount of CO, O2 and CO2 in the flue gas is usually analyzed to determine the excess air coefficient. Generally, the combustion gas analyzer is used to extract the flue gas to analyze the gas composition in real time, and the excess air coefficient is calculated, or the online flue gas analyzer is used to continuously monitor the content of CO, O2 and CO2 in the flue gas, and the excess air coefficient is converted. Gas sensors in combustion efficiency analyzers or online flue gas analyzers can be classified into electrochemical and infrared according to different measurement principles.

　　Due to the simple structure and small size of the electrochemical sensor, most of the combustion efficiency analyzers use the sensor of this principle to measure CO and O2 respectively, and calculate other thermal parameters such as CO2 and air excess coefficient. However, in actual use, due to different flue pressures of different furnace bodies, the sampling flow rate of the analyzer often causes different or fluctuating, and the electrochemical sensor is susceptible to the sampling flow, thereby reducing the test accuracy. Secondly, due to the presence of other gases such as SO2 and NOx in the flue gas, cross-interference will be exerted on the CO measurement. In addition, electrochemical sensors are prone to "poisoning" when the concentration of exhaust gas is high, resulting in complete failure of the sensor.

　　Compared with electrochemical sensors, infrared sensors have the advantages of good anti-interference, no influence on sampling flow, long life, etc. The infrared sensor can simultaneously measure CO and CO2, and can also expand the measurement of SO2, NOx and other gases when needed. Provide more reference for fuel combustion control. At the same time, the infrared sensor also has the advantages of high sensitivity, high precision, wide measuring range, etc. These features are beneficial to achieve accurate calculation of the air excess coefficient. For example, the combustion efficiency analyzer Gasboard-3400P and the online flue gas analyzer Gasboard-3000 use infrared gas analysis technology combined with long-life electrochemical sensor technology to simultaneously measure CO, CO2, O2 content in flue gas and expand The gas content of SO2, NOx, etc. is measured.

　　In addition to measuring multiple gas contents simultaneously, the Gasboard-3400P can automatically calculate the excess air ratio. However, due to its portable design, the Gasboard-3400P is not suitable for continuous online monitoring of the boiler combustion process, and the Gasboard-3000 can be satisfied. The combustion efficiency analyzer and the online flue gas analyzer have become an indispensable important equipment for improving the combustion efficiency of thermal power plants. Correct use and reasonable control of the thermal operation parameters of the device will reduce the amount of combustion air and exhaust air, and save the fan. Power cost; reduce the heat loss caused by excess air in the flue gas, and excessive combustion supply, to achieve energy saving; reduce the emission of pollutants such as NOX, SO2, and extend the service life of the combustion device.

　　Third. Ultra-low emissions have become an important issue in the quality development of thermal power plants.

　　Ultra-low emission refers to the high-efficiency synergistic removal of integrated system technology in the process of power generation operation and end treatment of coal-fired boilers in thermal power plants, so that the concentration of atmospheric pollutants reaches the emission limit of natural gas turbines. The dust emission does not exceed 5mg/m, the sulfur dioxide does not exceed 35mg/m, the nitrogen oxides do not exceed 50mg/m, and the special emission limit of the key areas of coal-fired boilers specified in the “Air Pollutant Emission Standards for Thermal Power Plants” (GB13223-2011) The decline was 75%, 30% and 50% respectively. It was first proposed by Zhejiang Energy Group in 2011 and is a new benchmark for the clean production level of coal-fired generating units.

　　Table 2. Table of relevant atmospheric pollutant emission concentration limits

　　For the new emission standards, the technology originally applied to high-concentration flue gas monitoring has been difficult to accurately measure the SO2 and NO concentrations under ultra-low emission conditions, which poses a great challenge to the existing flue gas concentration monitoring technology. The current ultra-low emission modification is the implementation of the above-mentioned flue gas emission standards. A large number of engineering applications and repeated tests have shown that CEMS in the current ultra-clean emission environment cannot guarantee the existing CEMS under the condition of “ultra-low emission”. Accurate measurement. The main reasons include: 1) adsorption of the sampling pipeline, loss of gas to be tested. 2) The gas to be tested is dissolved in water, causing gas loss and corrosion of the pipeline.

　　3) The analysis instrument has a large measuring range, resulting in low accuracy and large error.

　　The range and accuracy of any monitoring technology are adaptable, and the field operation requirements can be met without arbitrarily modifying the range by software. Therefore, in addition to the need for a suitable pretreatment system, the performance of the gas analyzer itself is also critical. The characteristics of low SO2, NO concentration, high flue gas humidity and low temperature in ultra-low emission flue gas are currently available on the market. An ultra-low-range flue gas analyzer based on ultraviolet differential absorption spectroscopy gas analysis technology, such as the self-controlled Gasboard-3000UV, is introduced. The unique algorithm can simultaneously measure the SO2, NO and O2 contents in the flue gas. Water has no effect on the measurement of SO2 and NO. The sampling flow in the flue gas has no effect on the measurement of SO2, NO and O2. There is no cross interference between the multi-component measurement gases, the anti-interference ability is strong, the measurement accuracy is high, and the detection limit is 0.1mg. /m, to meet the national environmental protection ultra-low emission requirements. In addition, the instrument optomechanical system has a high signal-to-noise ratio and high stability design, which improves the stability of the instrument.

　　Ultra low range ultraviolet flue gas analyzer Gasboard-3000U

　　At present, there are two ways to apply the ultraviolet flue gas analyzer in flue gas monitoring:

　　1. Direct measurement CEMS

　　The direct measurement type CEMS uses the flue as an open absorption tank to directly and continuously measure the gas in real time. It does not require a pretreatment system, is easy to install, has a small amount of maintenance, and is not subject to smoke and water mist in the flue within a certain range. Impact, but when the smoke or water mist is high, this method is ineffective, the consumables are more expensive, and maintenance requires professionals, especially when the instrument wind is disabled, the equipment is easily contaminated by the flue gas and the data is invalidated, and It is not easy to do comparison experiments on standard materials at the site.

　　2, fully removable CEMS

　　The ultraviolet flue gas analyzer used in the fully-removed CEMS is to place the optical platform in the protection box, and the gas to be tested needs to be pre-processed before the gas is measured. The installation of the pre-treatment system is cumbersome and the maintenance is large. However, the consumables are relatively cheap, the measuring optical path is not easy to be polluted, the maintenance personnel do not need special specialization, and the scope of application is wide, and it is easy to realize the standard substance comparison experiment on the spot.

　　Since the relevant national standards currently use dry-based calculations in the calculation of gaseous pollutant emissions, the direct measurement formula derives the moisture-based concentration; the direct-measured on-site CEMS standard-to-substance ratio Sexual experiment is more troublesome, which is not conducive to the acceptance of equipment; the actual situation of domestic flue working conditions is more complicated. In the case of harsh environment, direct measurement method can not be used normally, and it is more troublesome due to more interference factors. The measurement accuracy is degraded; direct measurement maintenance requires more professional staff, which will bring a lot of burden to the enterprise. Therefore, fully-extracted CEMS, such as Gasboard-9050, a continuous pollution monitoring system for stationary pollution sources, will be the application trend of the ultra-low emission monitoring market in the future.

　　Fourth, the conclusion

　　With the gradual deepening of the national green environmental protection concept, the requirements for emission reduction and emission reduction are becoming more and more stringent. Thermal power plants must rely on advanced science and technology, green management methods, and use gas analyzers to improve boiler efficiency, reduce energy consumption, and reduce pollution emissions. Reliable data support. In order to enhance their comprehensive competitiveness and adapt to the needs of the development of a harmonious society.

——Author：Gas monitor