Cause analysis and measure of coking in CFB boiler

Cause analysis and measure of coking in CFB boiler

Circulating fluidized bed (CFB) boiler technology is an efficient and clean combustion technology developed rapidly.Coking is a common fault in the operation of CFB boiler, which directly affects the safe and economical operation of boiler.Based on the running characteristics of CFB boiler, this paper analyzes the main reasons for coking of CFB boiler and discusses how to prevent coking of CFB boiler.

Key words: CFB boiler coking reasons and measures

An overview of the

Large CFB boiler is a power plant boiler developed in recent years, its design and operation are to be improved by accumulating experience, and some problems inevitably appear in the operation.Through the investigation of large CFB boiler of 440t/h class which has been put into production in China, it is found that the coking of CFB boiler is the most common and serious problem compared with conventional pulverized coal boiler.Bad treatment will seriously affect the safety and economic operation of CFB boiler, and further development and application of CFB boiler.Therefore, the analysis of coking reason of CFB boiler and the solution will improve the stable operation level of large CFB boiler.

1. Coking phenomenon and reasons

The coking phenomenon of CFB boiler in operation mainly include: (1) CRT display bed temperature, bed pressure is very uneven, combustion is very unstable, the relevant parameters fluctuations, large deviation.The coking early (local) material layer differential pressure drop, coking serious, material layer differential pressure increased sharply.We see a rapid decline in the oxygen content, almost to zero.In addition, furnace negative pressure increases, a volume of air, wind chamber wind pressure fluctuations.5. Both load, pressure and steam temperature are down.6 dregs are not smooth, bed slag tube blockage, single or multiple slag mouth put slag or slag in the loose porous sintering coke (local coking);While observing the flames, partial or large areas will appear white.

2. Cause analysis of coking:

Low-temperature coking occurs when the overall temperature of the bed is lower than the deformation temperature of ash slag and the coking is caused by local overtemperature or low-temperature sintering. Low-temperature coke blocks are loose with many embedded unsintered particles.The overall temperature level of the bed is higher and the coking phenomenon formed when fluidization is normal is called high temperature coking, the high temperature coke block is basically molten on the surface, after cooling is dark brown and mixed with a few pores.The slowly growing coke blocks in the operating bed temperature, bed pressure and fluidization are called progressive coking, which is difficult to detect.Coking in furnace is due to the high temperature, low temperature of anti-coking coking, progressive coking coal and oil burning for a long time and fluidized coking caused by abnormal, whether it is what causes of coking, once the clinker in the bed material and with the passage of time, the focal piece will be more and more big, cause the fluidization difficulties, blocked discharge pipe, finally forced to stop the furnace.

Analysis of possible causes of coking in production operation: 1, coal burning, bed material melting point is too low, in bed temperature at a low level can lead to coking.The flow rate is low, the flow is bad in normal time.Primary air volume is too small, below the critical fluidized air volume, material fluidization is not good.The air pressure at the bottom of the furnace is too low, and the resistance of the air distribution board is relatively low (generally the resistance of the air distribution board should be 25 ~ 30% of the resistance of the whole material layer). The uneven distribution wind leads to poor fluidization in the furnace, local blowing through occurs in the bed, while the other parts are not enough to supply wind, the temperature of the bed is high, and the material is bonded, thus forming coke block.The damage of the hood, resulting in uneven cloth wind board cloth, part of the material layer does not flow.Effects of returned shipments.The return material wind is too small to cause the return material of the return material abnormal or return material suddenly due to the collapse of refractories and blocked or due to the material poor high release circulation ash leakage out of control and other reasons, return material can not return to the furnace, resulting in high bed temperature and coking.If the pressure and steam temperature are maintained by adding coal, the bed temperature will be coking on the bed due to the double effect of returning material not returning to the furnace and adding coal.5) the bed temperature measuring device fails, the bed temperature gauge is not accurate, causing the operation personnel to misjudge or do nothing about the high temperature of a single point of bed.6 operation of the bed temperature monitoring lax caused by excessive temperature.Improper operation during holding fire, cold air enters the furnace.Being long-term overload operation or boiler load increased too fast, improper operation.'levies and furnace material layer too thin or too thick.⑽ furnace castable cave in large area, causing local fluidization, overheating and coking.⑾ during operation due to coal feeder operation is not normal, is not allowed to coal measure and coal is overmuch, cause local overtemperature bed.⑿ in the operation of the boiler, the wind for a long time, ratio of coal, excessive to coal.

3. Technical measures to prevent coking

At present, the coking of CFB boiler is the most common and serious problem.Bad treatment will seriously affect the safe and economic operation of boilers, and further development and application of CFB boilers.Therefore, the analysis of coking reason of CFB boiler and the solution will improve the stable operation level of large CFB boiler.

Firstly, good and stable coal quality must be guaranteed, especially the indexes of grain size, fineness, gangue and melting point must be strictly controlled.Before ignition, the fluidization test must be done carefully, and the fluidization and thickness of the base material should be observed on the spot to ensure that it is qualified.Good aerodynamic field in furnace can effectively control secondary combustion of cyclone separator and avoid over temperature coking of combustion chamber, cyclone separator and reclaimant.

Secondly, the CFB boiler should try to shorten the start time, otherwise the oil-coal combustion time is too long, improper adjustment is easy to occur coking, especially in the initial coal combustion stage, a large number of coal into the furnace can not be completely burned, it is easy to stick together with the unburned oil to form local high temperature coking.At the initial stage of ignition, when the bed temperature reaches the coal temperature, the coal should be immediately cast, the stable consequences of combustion intermittent oil, including in the process of accident treatment, timely cut off the oil, make kerosene combustion time shorten, prevent coking.For coal from the conference, the phenomenon of the bed temperature rising, and the furnace point move to coal for a long time can cause the accumulation of combustible causing deflagration phenomenon, the volatilization of lower coal ignition and operation process should pays great attention to the accumulation of combustible components in order to avoid deflagration phenomenon, has just started for coal, shall not be too fast, too hard, follow the principle of a small amount of discontinuity, the first single coal feeder point move a little to confirm the oxygen chamber of a stove or furnace decline, such as coal bed temperature rise again and gradually extend some dynamic coal and increase the time for coal.Before 700℃, it is better to use inching to feed coal, and it is forbidden to feed coal continuously. The timing of coal feeding can be carried out according to the change of oxygen. Before 750℃, the amount of coal feeding must not exceed 15t/h.

At the same time, in order to ensure safe and stable operation, the uniform distribution of air should be ensured during the ignition process, and the slag should be discharged timely in the later stage of the ignition process.To avoid low-temperature coking, the best way is to ensure that the prone zone is well fluidized, the particles mixed quickly and evenly or in the normal fluidized state, so that the temperature is uniform, can prevent coking.

Finally, strictly implement the operation rules of each manufacturer to ensure the safe operation of the return roots blower equipment.Avoid slagging in the return valve due to local dead zone.The charging capacity of the feed valve should be strictly controlled within 1% of the total air volume of the boiler, so as to prevent unburned carbon particles from reburning in local areas and avoid slagging in the feed valve.

The coking of CFB boiler has many subjective and objective reasons, such as design, manufacture and operation.Design, manufacturing units, should also carry out quality return visit, summarize experience, strive to constantly improve the design, solve structural hazards, optimize the overall design.For operation and maintenance personnel, it is necessary to improve the theoretical level of large circulating fluidized bed boiler technology, at the same time, learn from the operating experience of similar units, analyze the causes of coking, implement various technical measures to prevent coking, and accumulate operating experience in practice.In this way, the coking of CFB boiler can be controlled and prevented.

conclusion

CFB boiler has become a hot spot in thermal power plant and industrial boiler field due to its good environmental protection performance.However, many circulating fluidized bed boilers have problems such as insufficient output, short operating cycle, high ash and slag incomplete combustion loss, and large smoke exhaust loss to varying degrees. These problems are caused by improper design and unreasonable operation mode, which can be improved through optimization and adjustment.

Longest running cycle of the circulating fluidized bed boiler in many unit has reached 3000 hours or so, wear of no.1 again after a period of time management, on the basis of the wear current governance, thought to open some, the author explores the reason and mechanism of wear and tear to some seriously, some preventive measures, the level of governance to strengthen, wear this paper must be a better way of corresponding management, will also extend its continuous operation time, reach or exceed 3500 hours no problem.Circulating fluidized bed boiler this new product will be in the national power and heating play a bigger role in the enterprise to will gradually replace the current market for its unique merits of the running of any type, for society and the enterprise develop his great economic benefit, social benefit and environmental benefit, contribute to ease tensions in the coal.

reference

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Cen ke method, et al. Principle design and operation of circulating fluidized bed boiler. Beijing: China electric power press, 1998

4. Yang jianhua, qu weidong, Yang yibo, et al. Technical characteristics of 440t/h circulating fluidized bed boiler in xinxiang thermal power plant.China electric power press, 2002

5. 440t/h ultra-high pressure reheat CFB boiler manual, dongfang boiler co., LTD., China electric power press, 2000.