What are the main factors that affect the boiler efficiency and how to adjust them in normal operation?

Analysis of factors affecting 1 boiler efficiency

In order to improve the efficiency of the unit, as far as the boiler is concerned, on the one hand, it is necessary to reduce all kinds of losses as much as possible by adjusting the operation mode; On the other hand, steam parameters should be improved, and the amount of desuperheating water and sewage discharge should be reduced. Among all the losses, exhaust heat loss and mechanical incomplete combustion are the main losses, so effectively reducing these losses can improve boiler efficiency.

1. 1 Main factors affecting heat loss of exhaust gas.

The main factors affecting the heat loss of exhaust gas are exhaust gas temperature and exhaust gas quantity. Generally speaking, the exhaust heat loss will increase by 0.6% ~ 1% for every increase of exhaust temperature. The smoke exhaust quantity is mainly determined by the excess air coefficient and the moisture in the fuel, and the moisture in the fuel is determined by the composition of the coal entering the furnace. The main factors affecting exhaust temperature and exhaust volume are analyzed as follows.

1. 1. 1 air leakage

Air leakage refers to furnace air leakage, pulverizing system air leakage and flue air leakage. Air leakage directly leads to the increase of exhaust heat loss. Practice has proved that every time the air leakage coefficient of the furnace increases by 0. 1, the exhaust temperature will increase by 3 ~ 8℃ and the exhaust heat loss will increase by 0.2% ~ 0.4%.

(a) air leakage in the furnace. Among all the air leaks, the air leak at the bottom of the boiler has the greatest influence. When the water seal at the bottom of the boiler is lost or the furnace is crushed by large coke, a large amount of cold air will leak from the bottom of the boiler, which will seriously affect the economy and safe operation of the boiler. Another common place where the furnace leaks air is the fire hole and manhole door, especially the fire hole. When it is not closed or fastened after closing, it is easy to blow the hole cover open during soot blowing, resulting in cold air leakage.

(b) air leakage in the pulverizing system. The air leakage of pulverizing system mainly comes from pulverized coal pipeline. During operation, the 1 ~ 2 pulverizing system is usually shut down, and the air intake shows zero, but actually there is still cold air leaking in. This is mainly because the adjusting door is not completely closed.

(c) air leakage in the flue. Under the condition of constant oxygen content, the flue air leakage will also squeeze out the primary and secondary air volume, which will increase the exhaust temperature. Another hazard of flue air leakage is that the cold air leaked into the flue does not participate in combustion. Because the oxygen meter is installed at the flue gas inlet of the air preheater, the air leakage in the rear flue will make the oxygen display value greater than the actual value, which may make the combustion air volume insufficient in actual operation and cause oxygen-deficient combustion in the furnace.

1. 1.2 Ash slagging in the heated area

Ash slagging in heating area mainly includes air preheater plugging, furnace dust accumulation, flue dust accumulation and so on. The ash blocking of air preheater seriously affects the heat transfer effect and makes the exhaust temperature rise greatly. Dust accumulation in furnace and flue will reduce the heat absorbed by steam from high-temperature flue gas, thus increasing the inlet flue gas temperature of air preheater, the heat transfer temperature difference of air preheater and the exhaust temperature.

1. 1.3 external factors

The external factors affecting the exhaust gas temperature are mainly the ambient temperature (that is, the inlet temperature of the air preheater) and the composition of the coal entering the furnace. The change of ambient temperature will make the heat transfer temperature difference of air preheater change accordingly, so the exhaust temperature will also change with the season.

1.2 Main factors affecting heat loss of incomplete combustion

1.2. 1 coal quality

When the volatile content in fuel is high, pulverized coal is easy to catch fire, the combustion process is stable, and the heat loss of incomplete combustion is small. If the ash content in the fuel is high and the combustion stability is poor, the burnout performance of coal is poor due to the isolation effect of ash. The influence of moisture on combustion is mainly to make ignition difficult, reduce the temperature in the combustion zone, and make it difficult for pulverized coal to burn out.

1.2.2 fineness of pulverized coal

The finer the pulverized coal, the larger the surface area, the easier it is to catch fire, and the shorter the combustion time, the more complete the combustion. But if the pulverized coal is too fine, it will increase power consumption and reduce boiler efficiency.

1.2.3 air volume

Excessive air coefficient in the furnace is too small, fuel combustion is incomplete, and the existence of incomplete combustion products in flue gas will bring the threat of secondary combustion to boiler operation. The excessive air coefficient in the furnace is too large, and the heat loss of flue gas is large, which can not achieve the effect of economic operation.

1.2.4 oxygen

The operating oxygen content of the boiler directly affects the economy of the boiler. It can be seen from the table 1 that under different operating loads, the oxygen content is too high, which leads to the heat loss of flue gas and increases the consumption of wind power; On the contrary, although the power consumption of the wind turbine is reduced, the combustible materials of the fly ash are increased, and the heat loss of incomplete combustion is increased.

2 optimization and adjustment of boiler operation mode

2. 1 water level adjustment

The purpose of water level regulation is to ensure the balance of steam and water, that is, the main steam quantity is equal to the water supply quantity, so as to prevent the water level from fluctuating too much. If the water level is too low, there may be steam in the downcomer, which will reduce the flow head of circulating water and reduce the safety of natural water circulation. If the water supply is interrupted and the boiler runs continuously, a "dry pot" may appear in a few tens of seconds. If the water level is too high, the height of the steam-water separation space will be reduced, and the steam will carry water, and the steam quality will become worse (the mechanical carrying of steam will increase the salt content). In severe cases, the steam bubble will be filled with water, resulting in a large amount of water carried in the steam. The steam with high salt content will seriously scale the superheater, which will lead to overheating and explosion of the pipe wall, and will also cause water impact on the main steam pipeline and steam turbine, which will affect the safe and economic operation of the equipment. In addition, it is necessary to keep the balance between water quantity and evaporation to prevent water shortage (or full water) accidents.

During operation, the feed water flow is adjusted in time according to the load change, and the water level in the pot is continuously monitored by the water level gauge. The boiler water level should always be kept at the normal water level line, and it is allowed to fluctuate within 50 ~ below the normal water level line. In order to keep the water level normal, the water level should be adjusted according to the load of the boiler. When the boiler is running at low load, the water level should be slightly higher than the normal water level to prevent the water level from falling too low when the load increases; When the boiler is running at high load, the water level should be slightly lower than the normal water level to avoid the water level rising too high when the load is reduced. In boiler operation, the water supply should be stable and uniform, because the change of water level will make the steam pressure and temperature fluctuate. The time and mode of water supply should be appropriate. The water supply time interval is too large, and it is difficult to stabilize the steam pressure if there is too much water supply at one time; Don't feed water when the combustion is weakened, which will cause the steam pressure to drop.

2.2 steam pressure adjustment

Steam pressure is one of the main parameters of boiler operation monitoring, which refers to the outlet pressure of superheater. Its level directly affects the safety and economy of steam turbine equipment. Generally, the deviation between the numerical value and the rated value does not exceed 0.05-0. 1 MPa.

The change of steam pressure is influenced by external and internal factors. External factors mainly refer to the normal increase or decrease of external load and the large-scale load rejection in accident situations. The change of steam pressure is actually the result of the destruction of boiler evaporation and external load balance. The load change is objective for the boiler, so the steam pressure is the adjustment of boiler evaporation. Because evaporation mainly depends on combustion conditions, the adjustment of steam pressure is actually the adjustment of fuel quantity and air quantity. No matter what kind of disturbance changes the steam pressure, it is necessary to change the coal supply and air volume, and consider adjusting the bubble water level and steam temperature.

When the load decreases, the steam pressure increases. If the water level is high at this time, the fuel quantity and air volume should be reduced first to weaken the combustion, and then the water supply should be reduced or suspended appropriately to stabilize the steam pressure and water level within the rated range, and then the combustion and water supply should be adjusted according to the normal situation; If the water level is low, the water supply should be increased first. After the water level is normal, the combustion and water supply should be properly adjusted according to the steam pressure and load.

When the load increases, the steam pressure decreases. If the water level is high at this time, the water supply should be reduced or suspended, and the fuel quantity and air supply quantity should be increased to strengthen combustion and increase evaporation to meet the load demand, so that the steam pressure and water level can be stabilized within the rated range, and then the combustion and water supply quantity should be adjusted according to the normal situation; If the water level is low, the fuel and air supply should be increased first, and the water supply should be gradually increased while the combustion is strengthened to keep the steam pressure and water level normal.

2.3 Adjustment of combustion

Combustion regulation is mainly to ensure proper fuel quantity to meet the needs of external load, and at the same time to ensure stable and economical combustion.

Combustion adjustment mainly includes the adjustment of fuel quantity and air volume, that is, the adjustment of air-coal ratio. The adjustment of primary air is carried out by professionals, and generally does not move after adjustment, mainly according to the volatile content in coal. For high volatile coal, higher primary air volume and primary wind speed can be used, which is beneficial to economy; For low volatile coal, low primary air volume and low primary wind speed should be adopted to ensure the ignition time first. The fuel quantity and air quantity should be kept in a reasonable proportion and fully mixed; The furnace should be kept as high as possible to prevent cold air from entering the furnace and reduce heat loss. Monitor the exhaust temperature, CO2 and O2 content, and adjust the combustion state in time; Keep the grate running smoothly to prevent uneven combustion. It is necessary to maintain the negative pressure operation of the furnace to prevent the leakage of combustion gases and reduce the thermal efficiency.

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In the process of boiler operation, it is necessary to adjust the operating water level, steam pressure, steam temperature and combustion of the boiler, and also to discharge pollutants and soot blowing to ensure the normal operation of the boiler and provide guarantee for normal power generation.