What are the hazards of long term low load operation of circulating vulcanized bed boilers?

The hazards of low-load operation are:

1.Affect the normal production: the bed temperature is easy to fluctuate, it is not easy to adjust, the bed temperature will be low, and the return section is not easy to control, which may cause flameout;

2.Affect the life of the internal heat exchange pipeline of the boiler: when running at low load, there will be an imbalance between the amount of coal and the amount of ash returned, resulting in uneven heating of the heat exchange pipeline, which will easily lead to local overheating of the pipeline, causing overheating of the pipe wall, and increasing the probability of pipe bursting during long-term operation;

3.Increase the corrosion of the tail heat exchange pipeline: Long-term low-load operation, the exhaust gas temperature will decrease, and the probability of low-temperature corrosion of the heat exchange pipeline arranged at the tail will increase;

4.Affect the thermal efficiency of the boiler: low furnace temperature will cause incomplete combustion, and the boiler efficiency will definitely decrease.

5. Affect economic benefits.

Personally, I think there are the following harms:

1. If you want to run at low load, the thickness of the material layer will inevitably be low, and the material layer will penetrate, which will make the bed temperature deviation larger;

2. At this time, the primary air volume should be kept at the lowest fluidized air volume, there is a risk of poor fluidization, once the poor fluidization occurs, then coking may occur, and it is not far from stopping the furnace, that is, it will take time to clean up the coking after the furnace is stopped;

3. The ash separated by the cyclone separator will be less, and the return cycle may be destroyed to a certain extent; Some 75-ton boilers are set up with high-temperature superheaters between the return leg and the furnace, and the general cyclone separator is 2, if one of them is not returned, it will bring great harm to the superheater; In addition, the abnormal return will also bring violent fluctuations to the bed temperature;

4. In the long run, it will affect the maximum output of the boiler, and it will be difficult to reach the rated load at that time;

5. If the follow-up equipment has a steam turbine, the low-load operation of the steam turbine is also harmful;

The minimum load of 75t h circulating fluidized bed boiler should not be less than 38t h (50% of the rated load).

Hazards of long-term low loads:

1.Affect the normal production: the combustion is unstable, the bed temperature is easy to fluctuate, it is not easy to adjust, the bed temperature will be low, and the return section is not easy to control, which may cause flameout;

2.Affect the life of the internal heat exchange pipe of the boiler: low-load combustion will cause an imbalance in the amount of coal and ash return, resulting in uneven heating of the heat exchange pipeline, which will easily lead to local overheating of the pipeline, cause overheating of the pipe wall, and increase the probability of pipe bursting during long-term operation;

3.Increase the corrosion of the tail heat exchange pipeline: Long-term low-load operation, the exhaust gas temperature will decrease, and the probability of low-temperature corrosion of the heat exchange pipeline arranged at the tail will increase;

4.Affect the thermal efficiency of the boiler: low furnace temperature will cause incomplete combustion, and the boiler efficiency will definitely decrease.

5.It is easy to cause ash in the tail heat exchange area, and the heat exchange efficiency is greatly reduced.

The influencing factors of the desulfurization efficiency of circulating fluidized bed boiler are (dry desulfurization in the furnace).

1. The purity of limestone, the activity of limestone, and the granularity of limestone;

2. The temperature of the bed. The reaction with the s in the coal is actually the calcium oxide decomposed by the limestone at 800-850, if the bed temperature is not enough, the limestone does not decompose, and the desulfurization efficiency is of course low.

3. The level of coal sulfur. Coal has a high sulfur content and low relative efficiency.

In the operation of circulating vulcanized bed boiler, the change of primary air volume has the following effects on the lower bed temperature:

1. Combustion efficiency.

Changes in primary air volume can affect the rate and extent of fuel combustion in the boiler. When the primary air volume increases, the oxygen** in the fuel combustion area increases, and the combustion efficiency also increases, resulting in an increase in the lower bed temperature. On the contrary, when the primary air volume decreases, the oxygen ** is insufficient, the combustion efficiency of the celery is reduced, and the bed temperature may slow down.

2. The temperature difference in the whole area.

Changes in primary air volume can affect the flow rate and residence time of the bed in the circulating cooler. When the primary air volume increases, the bed material flow rate increases and the bed stock residence time in the circulating cooler decreases, which may lead to an increase in the temperature difference in the distribution area and an increase in the bed temperature. On the contrary, when the primary air volume decreases, the flow speed of the bed material slows down, the residence time is prolonged, the temperature difference in the whole area may be reduced, and the bed temperature may decrease.

3. Bed pressure distribution.

Changes in primary air volume also affect the gas flow regime and pressure distribution in the bed. When the primary air volume increases, the gas flow rate increases, the bed pressure may increase, and the bed temperature will increase accordingly. Conversely, when the primary air volume decreases, the gas velocity decreases, the bed pressure may decrease, and the bed temperature may decrease.

It is worth noting that the bed temperature is affected by a combination of factors, and the change in the primary air volume is only one of them. In addition to the primary air volume, other factors such as fuel type and quality, secondary air volume, bed temperature control strategy, etc., will have an impact on the bed temperature. Therefore, in actual operation, it is necessary to comprehensively consider multiple factors, and carry out reasonable control and adjustment to achieve the ideal bed temperature control effect.

(Circulating fluidized bed) is developed on the basis of bubbling bed boiler (boiling furnace), so some theories and concepts of bubbling bed can be applied to circulating fluidized bed boilers. But there is a big difference. The early circulating fluidized bed boiler had a relatively high fluidization velocity, so it was called a fast-circulating circulating bed boiler.

The basic theory of fast bed can also be applied to circulating fluidized bed boilers. The basic theories of bubbling beds and fast beds have been studied for a long time and have formed certain theories. To understand the principle of circulating fluidized bed, it is necessary to understand the theory of bubbling bed and fast bed, as well as the dynamic, combustion and heat transfer characteristics of materials from bubbling bed to turbulent bed and fast bed in various states.

A fluidization:

When there is fluid in the solid particles, as the fluid velocity gradually increases, the solid particles begin to move, and the friction between the solid particles also increases, when the flow rate reaches a certain value, the friction between the solid particles is equal to their gravity, each particle can move freely, and all solid particles exhibit a phenomenon similar to the fluid state, which is called fluidization. For liquid-solid fluidized solid particles, the particles are evenly distributed in the bed, which is called "scatter" fluidization. For the solid particles of gas-solid fluidization, the gas does not flow uniformly through the bed, the solid particles are divided into groups for turbulent movement, and the porosity in the bed changes with the difference of position and time, this fluidization is called "poly" fluidization.

Circulating fluidized bed boilers belong to the "poly" fluidization. Solid particles (bed), fluid (fluidized wind), and the equipment that completes the fluidization process are called fluidized beds.

2. Critical fluidization velocity.

1.For a bed composed of particles of uniform particle size, when the gas flow rate through the fixed bed is very low, with the increase of wind speed, the bed pressure drop increases proportionally, and when the wind speed reaches a certain value, the bed pressure drop reaches the maximum, which is slightly greater than the bed static pressure, if the wind speed continues to increase, the fixed bed will suddenly unlock, and the bed pressure drop will drop to the bed static pressure. If the bed is composed of wide sieved particles, its characteristics are:

Before the large particles move, the small particles in the bed have been partially fluidized, and the unlocking phenomenon of the bed from fixed bed to fluidized bed is not obvious, but the phenomenon of stratified fluidization often occurs. The minimum velocity required for a particle bed to transition from a stationary state to fluidization is called the critical fluidization velocity. As the wind speed increases further, the bed pressure drop is almost constant.

The general fluidized wind speed of circulating fluidized bed boiler is 2 3 times the critical fluidization velocity. 2.Factors influencing the critical fluidization velocity:

1) The thickness of the material layer has little effect on the critical flow velocity. (2) The critical flow velocity increases when the equivalent average diameter of the material layer increases. (3) The critical flow velocity increases when the density of solid particles increases.

It should be a circulating fluidized bed boiler, which generally refers to a boiler in which coal-fired particles are circulated in the furnace under the action of the boiler blower.

Bed temperature drops are generally caused by:

a. The coal quality is poor, the calorific value is reduced, and the oxygen content of the flue gas increases, so the bed temperature should be increased by increasing the coal feed.

b. The fuel particle size is small. A part of the smaller coal in the coal bunker is concentrated into the furnace, and the fine coal particles stay in the dense phase area for a short time, resulting in a decrease in the combustion share of the dense phase area, and the bed temperature is reduced, the correct adjustment should reduce the primary air volume and increase the secondary air volume, and the coal volume should not be increased, so as not to cause the increase in the combustion share of the upper space of the furnace and cause the overtemperature coking of the returner.

c. The oxygen index remains unchanged, the bed temperature decreases slowly, and the entire combustion system is decreasing, and the boiler load remains unchanged. This is due to the increase of circulating materials, which increases the heat exchange coefficient of the heating area, resulting in a decrease in furnace temperature, and some circulating ash should be released to make the furnace temperature rise.

Circulating fluidized bed boiler technology is a high-efficiency, low-pollution and clean combustion technology that has developed rapidly in the past ten years. Internationally, this technology has been widely used in the fields of power station boilers, industrial boilers and waste treatment and utilization, and has developed to large-scale circulating fluidized bed boilers with a scale of hundreds of thousands of kilowatts. The research, development and application in this area in China have also gradually emerged, and hundreds of circulating fluidized bed boilers have been put into operation or are being manufactured. The next few years will be an important period for the rapid development of circulating fluidized beds.

(1) The primary air volume is too large or the bed pressure is too high during operation.

2) Coal feeder failure or coal blockage, not found in time, resulting in long-term coal cut-off.

3) The return material of the return device is abnormal, and a large amount of circulating ash is returned to the furnace, so that the coal being burned cannot be fully burned, and the bed temperature drops rapidly.

4) When the boiler is running at low load, the operation is not properly adjusted.

5) The coal quality changes greatly, the volatile matter or calorific value is too low, and the operator does not adjust it in time.

Limestone or dolomite is added to the ** bed together with fuel coal particles, and the combustion and desulfurization processes are carried out at the same time.

In particular, the formation of Cas04 reduces the amount of SO1 and realizes desulfurization.

In the process of its removal, the desulfurizer can be mixed with the fuel in the bed for a long time, so the general desulfurization efficiency is high, and the use efficiency of the desulfurizer is also high.

SO2 is a pollutant that seriously harms the atmospheric environment, SO2 reacts with water vapor to form sulfuric acid, and it falls to the ground together with rainwater to form acid rain. NOx includes NO, NO2 and NO3, of which NO® is also one of the main causes of acid rain, and it also participates in photochemical processes to form photochemical smog, leading to the destruction of the ozone layer. When coal is heated to 400oC, it begins to decompose first to H2S and then gradually oxidizes to SO2.

The equation for the chemical reaction is.

fes2+2h2→2h2s+fe

h2s+o2→h2+so2

The most important factors affecting the formation of SO2 are bed temperature and excess air coefficient, and the higher the SO2 is when the bed temperature increases and the excess air coefficient decreases. The most commonly used desulfurizer in the combustion process of circulating fluidized bed is limestone, and when the bed temperature exceeds its combustion temperature, the calcination decomposition reaction equation is:

CaCO3 cao+CO2 absorbs 183kJ mol desulfurization reaction equation at this time:

cao+so2+1/2o2→caso4

In this way, SO2 becomes CaSO4 after chemical reaction, which cures SO2 and achieves the effect of desulfurization.

The inlet sulphur requirement for a 160 ton circulating fluidized bed boiler depends on the environmental standards and operating conditions of the bed boiler. In China, emission standards for industrial boilers are formulated by the environmental protection department in accordance with national and local environmental protection requirements. Here are some common industrial boiler emission standards:

GB13271-2014 "Boiler Air Pollutant Emission Standard": This standard stipulates the emission standards and sensitive sections of industrial boilers, including the emission limits of pollutants such as nitrogen oxides, sulfur dioxide, and particulate matter in flue gas.

GB5468-2017 "Industrial Boiler Water Quality": This standard specifies the standards for the water quality of industrial boilers, including the requirements for sulfate ions, total hardness, chloride ions and other indicators in water.

GB T16157-2014 "Code for Energy Audit of Industrial Boilers": This standard specifies the specifications for energy audit of industrial boilers, including the calculation and evaluation of boiler energy consumption, etc.

In practice, for the inlet sulfur requirements of 160 tons circulating fluidized bed boiler, the above standards can be referred to, combined with the specific environmental protection standards and use conditions, to determine the limit value and management requirements of sulfur emissions. If more specific information is required, it is advisable to consult an engineer or relevant technician.

Circulating fluidized bed pot round which furnace is an efficient and environmentally friendly boiler equipment, and its inlet sulfur requirements are very strict. According to national standards and industry norms, the inlet sulfur content of 160 tons circulating fluidized bed boiler should be controlled below 50mg nm. This is because sulfur is a harmful sail cavity sail substance that can cause harm to the environment and human health.

If this standard is exceeded, it will cause pollution to the atmospheric environment, affect air quality, and even cause serious consequences such as acid rain.

In order to meet this standard, circulating fluidized bed boilers need to take a series of measures, such as using low-sulfur fuels, optimizing the combustion process, and installing desulfurization equipment. Among them, desulfurization equipment is one of the most commonly used methods, which can effectively reduce the sulfur content of boiler emissions. At present, the commonly used desulfurization equipment includes wet desulfurization and dry desulfurization, and the specific choice needs to be decided according to the actual situation of the boiler and the use environment.

In conclusion, it is very important to control the sulfur content at the inlet of the circulating fluidized bed boiler, which can not only protect the environment and human health, but also improve the operating efficiency and safety of the boiler.