Causes and solutions for coal feeder failure in boilers

Why Coal Feeder Coal Break Is a Fatal Hidden Danger To Boiler Operation

Coal feeder coal break, in common parlance, is that the raw coal can't be normally delivered to the coal mill or furnace, it seems to be a small fault, but it can directly lead to the interruption of the boiler's fuel supply. For coal-fired power stations, the boiler is the core equipment, once the fuel supply is out of gear, the stable operation of the entire unit will be threatened.

This kind of failure will not only cause the unit load to plummet and shut down, but also may cause more serious accidents such as furnace fire, main fuel trip, i.e. MFT, which will directly increase the downtime and operation and maintenance costs. Whether it is power plant operators, maintenance engineers, or boiler equipment owners, understand the root cause of coal feeder breakage in order to prevent in advance, quickly dispose of, and minimize losses.

Today, in conjunction with my years of front-line operational experience, the coal feeder coal break those things to make it clear to everyone: what signal after the coal break, why will the coal break, how to quickly investigate, how to prevent the usual, the novice can also be a look at the will.

What Abnormalities Will Occur In The Boiler After The Coal Feeder Breaks Coal?

Instant Operating Symptoms, a Glance Can Be Found

After the coal feeder breaks, the equipment parameters will immediately appear abnormal, experienced operators through the monitoring screen can quickly determine. The most intuitive is the coal feeder motor current plummet - because there is no raw coal load, the motor running resistance is reduced, the current will instantly fall to a lower level.

If the coal feeder is driven by frequency conversion, there will also be a large deviation between the speed command and the actual speed, and there will even be a strange noise of idling. At the same time, the parameters of the coal mill will also change synchronously, the differential pressure of the grinding bowl will drop significantly, and the outlet temperature will increase rapidly.

Changes in the furnace chamber are also obvious, the flame will become darker and fluttering, the flame monitor may issue an early warning of flame instability, and in severe cases, it will also trigger the fire extinguishing alarm, all of which are direct manifestations of insufficient fuel supply.

Secondary Effects On Boiler Performance That Cannot Be Ignored

As the coal cutoff time is prolonged, the abnormality will spread to the entire boiler system. The main steam pressure and temperature will decrease rapidly, and the superheater and reheater outlet pressures will also decrease simultaneously, which will directly lead to a reduction in the turbine load.

Negative pressure in the furnace chamber will appear first positive and then negative fluctuations: the beginning of fuel reduction, combustion is weakened, the amount of flue gas is reduced, the furnace chamber will appear positive; if the air volume is not adjusted in a timely manner, too much wind will make the furnace chamber into a negative pressure, the boiler operation to bring greater risk.

More importantly, if the problem of coal breakage continues to be unresolved, some units will trigger chain alarms such as coal feeder coal breakage and coal mill coal loss, which will seriously lead to coal mill tripping, and even trigger MFT action, resulting in unit shutdown.

The Core Reason For Coal Breakage Of Coal Feeder, Mainly Divided Into Five Categories

Many people encounter coal breakage, the first reaction is that there is not enough coal, but in fact, the essence of coal breakage is that the original coal conveying path is blocked, the common reasons are mainly concentrated in the original coal warehouse, the coal feeder itself, the drive control system, the quality of the coal, and the environment of operation of these five aspects, of which the original coal warehouse problem is the most common.

Raw Coal Bunker Problems

1. Raw coal bunker arching and bridging: this is the most likely problem on the side of raw coal bunker, mostly related to raw coal moisture and particles. If the moisture of raw coal is more than 12%, it is easy to become sticky and stick to the silo wall; coupled with uneven particles of raw coal and more fine powder, it will form an arch in the silo, which will lead to unsmooth coal transportation and ultimately lead to coal breakage. If the raw coal contains more clay components, the viscosity will be stronger, easier to form adhesion and compaction in the silo wall, and after a long time, it will block the mouth of the coal, interrupting the raw coal transportation.
2. Raw coal bin material level is too low or empty: this situation is mostly caused by human management or equipment failure. For example, the coal conveying system is not timely added to the warehouse, resulting in depletion of raw coal in the warehouse; or the material level meter malfunctions, misreporting the high material level, the operator mistakenly believes that there is coal in the warehouse, not timely added to the warehouse, which ultimately leads to the empty warehouse and cut off the coal.
3. Under the coal pipe blockage: under the coal pipe is the raw coal from the warehouse to the coal feeder must pass through, once blocked by foreign matter, it will be directly cut off coal. Common foreign objects are stones, wood, iron wire, etc. in the raw coal, which may also be tools left behind during maintenance. In addition, the gate valve of the lower coal pipe is mistakenly closed or jammed, which will also lead to the raw coal not being able to pass through, especially if the gate valve is not reset in time after maintenance, this problem is very likely to occur. There are also times when the residual raw coal hardens and accumulates in the pipe, which can also slowly block the pipe.

Coal Feeder Mechanical Failure

1. Belt or chain jamming: the belt, scraper or chain of the coal feeder is the core component for conveying raw coal. If the raw coal is mixed with large pieces of coal, iron wire and other foreign materials, it may jam these parts, resulting in transportation interruption. Belt deflection, slippage is also a common problem, belt slippage will lead to insufficient transmission of speed, can not effectively transport raw coal; if the belt is seriously worn or broken, it is necessary to shut down for maintenance, which directly affects the operation of the unit.
2. Failure of cleaning device: The scraper cleaner at the bottom of the coal feeder is mainly used to clean up the residual raw coal in the warehouse and prevent the accumulation of coal. If the cleaner is jammed and worn out, it will lead to the accumulation of raw coal at the bottom, which will slowly hinder the transportation of raw coal and eventually lead to coal breakage. Especially after a long period of operation, the mechanical parts of the sweeper will be worn out, if they are not serviced and replaced in time, the problem of coal accumulation will become more and more serious, and the risk of coal breakage will also increase.
3. Failure of sealing air system: The coal feeder needs to rely on the sealing air to maintain pressure and prevent the hot air in the coal mill from backing up. If the sealing air pressure is insufficient and lower than the inlet air pressure of the coal mill, the hot air will pour into the coal feeder, drying and arching the raw coal, forming the phenomenon of bridging, leading to coal breakage. Failure of the fan of the sealing air system and leakage of the pipeline may lead to insufficient sealing air pressure, which can be easily overlooked during daily inspections.

Drive And Electrical System Problems

1. Motor or power supply failure: Once the motor of a coal feeder has a problem, it will simply stop running. Common motor failures include overload tripping, bearing damage, or power supply interruption, all of which will cause the motor to be unable to drive the coal feeder to run normally, triggering coal breakage. Especially, the wear of motor bearings will lead to abnormal operation and heat, which, if not detected in time, will cause damage to the motor and affect the whole conveying system.
2. Damage to reducer or coupling: reducer and coupling are the key components for transmitting rotational speed. Worn gears and insufficient lubrication of the reducer, or broken couplings and misaligned shafts will lead to interruption of rotational speed transmission, and the coal feeder will not be able to work normally. Long-term high-load operation, coupled with inadequate lubrication, is the main cause of this type of failure, daily maintenance needs to focus on lubrication.
3. Failure of control system or DCS signal: Nowadays, most of the power stations adopt DCS, i.e. decentralized control system, to control the operation of coal feeder. If the DCS sends out wrong commands, or the speed sensor or frequency conversion device, i.e. VFD, fails, it will lead to the coal feeder's command to run but not actually rotate. For example, the speed sensor failure will lead to abnormal speed feedback, DCS can not accurately control the speed of the coal feeder; VFD failure will directly affect the motor speed, and ultimately lead to coal breakage.

Coal Quality Factors

Problems with the coal quality itself will also increase the risk of coal breakage. In addition to the previously mentioned high moisture, the raw coal has too much ash and too large particles, which will also affect the conveyance. Excessive ash will lead to poor fluidity of raw coal, and large particles are easy to jam coal feeder parts. In addition, raw coal mixed with foreign materials during transportation, such as stones, metal shavings, wood, etc., is also an important cause of coal feeder jamming and coal breakage, which requires a good screening before the raw coal is put into the silo.

Environmental And Operational Factors

Extreme weather will affect the raw coal delivery, such as severe cold weather will lead to raw coal freezing, lumps can not be normal under the coal; after a long period of shutdown, the raw coal in the silo will be compacted and lumps, and it is also easy to break coal when restarting. Improper operation of the operator can also cause problems, such as load adjustment is too fast, the coal feeder can not adapt to changes in the amount of coal in a timely manner; daily preventive maintenance is not in place, the equipment is not timely investigation of hidden problems, will increase the probability of coal breakage.

Classification Of The Causes Of Coal Breakage: Mechanical, Material, And Control Of Three Types Of Core Problems

In order to facilitate rapid investigation, we can divide the causes of coal breakage into three categories, clearly distinguish the direction of the fault, and reduce the time of investigation.

Mechanical failures are mainly concentrated in the coal feeder itself and related transmission components, such as belt jamming, reducer damage, cleaning device failure, etc., this kind of failure needs to be shut down and overhaul to solve.

Material flow problems are mainly related to the raw coal bunker and coal quality, such as arching of raw coal bunker, clogging of lower coal pipe, high moisture of raw coal, and at the core of the raw coal can not be sent out, and this kind of problems can be solved by daily adjustment and dredging.

Electrical/control system failure is the command or power out of the problem, such as motor failure, DCS signal abnormality, frequency conversion device failure, this kind of failure needs electrical professionals to troubleshoot and deal with.

In addition, human operation errors, such as adding silo in time, not reset the equipment after maintenance, may also trigger coal breakage, which needs to strengthen the operator's training and management.

How To Quickly Diagnose The Specific Cause Of Coal Breakage?

Focus On Monitoring These 4 Key Parameters

When encountering coal breakage, you don't need to blindly investigate, focus on monitoring 4 parameters, you can quickly lock the direction of the fault. The first is the coal feeder motor current, current drop is mostly no coal or load reduction, current fluctuation or sudden rise may be jammed.

Secondly, the coal mill outlet temperature, the temperature rises rapidly, the probability is that the coal is insufficient, can be initially judged as a broken coal. Then is the original coal bin material level indication, material level is too low or empty warehouse, directly indicates that the reason for coal break in the original coal bin side.

Finally, the sealing air pressure, the sealing air pressure is insufficient, may be the sealing air system failure, resulting in hot air backflow triggered by the coal break. Through these four parameters, the scope of investigation can be quickly narrowed down.

Step By Step Investigation, Efficiently Find The Root Cause

investigation can be in accordance with the order from simple to complex, first look at the raw coal bin material level, to confirm whether it is empty or low material level, if so, timely addition of silo can be solved; if the material level is normal, and then check whether the coal feeder is running normally, there is no abnormal noise, jamming.

If the coal feeder is running normally, then check the sealing air pressure to see if there is hot air backflow; finally, check the drive and control system, check whether the motor, reducer, DCS signal is normal, and gradually find the specific failure point.

This way of checking can avoid blind operation, save fault disposal time and reduce the loss of unit downtime.

How To Prevent Coal Breakage Of Coal Feeder? It Is Enough To Do These 5 Points

Coal break seems to be sudden, but it can be prevented. Combined with the first-line operation and maintenance experience, do the following five points, you can significantly reduce the probability of coal breakage, to ensure the stable operation of the boiler.

1. Control the moisture of raw coal. Through coal blending or drying treatment, the moisture of raw coal will be controlled below 12%, to avoid excessive viscosity of raw coal, resulting in arching and clogging, and at the same time, do a good job of screening the raw coal, to reduce the mixing of foreign objects.
2. Install air cannons or vibrators in the raw coal silo. When encountering arching in the silo, air cannons or vibrators can be used to unblock the silo, avoiding the use of iron bars and other hard objects to stab the silo wall to prevent damage.
3. strengthen the daily inspection. Regularly check the belt, scraper and cleaning device of the coal feeder, adjust the belt tension in time, and replace the worn parts; at the same time, regularly calibrate the material level meter and rotational speed sensor to ensure accurate parameter feedback.
4. Monitor the sealing wind system. Regularly check the sealing air blower and pipeline to ensure stable sealing air pressure and avoid hot air backflow; when the air pressure is found to be insufficient, investigate the leakage point and deal with it in time.
5. Optimize automation and alarm logic. Add coal feeder coal break-automatic load reduction chain to avoid untimely manual intervention leading to the expansion of the accident; at the same time, improve the alarm mechanism to ensure that the operator can be reminded in time when the failure occurs.

High Quality Coal Feeding System, Why Can Reduce The Risk Of Coal Breakage?

In addition to daily maintenance, the selection and design of coal feeding system is also very important. Reliable gravity feeder can control the coal quantity more accurately and reduce the probability of jamming and skidding; advanced control system can monitor the equipment parameters in real time, find out the abnormality and warn in time.

The integrated design of the boiler and fuel system can make the operation of the coal feeder more compatible with the boiler load and reduce the coal breakage caused by too fast load adjustment. For old power stations, upgrading the core components of the coal feeder system can also effectively reduce the risk of coal breakage and improve the stability of unit operation.

In the long run, investing in a high-quality coal feeding system can reduce downtime and operation and maintenance costs, improve boiler efficiency, and bring more revenue for the power plant.

Summarize

Coal feeder coal breakage is a common fault in coal-fired boiler operation, but its nature is preventable and can be quickly disposed of. Most of the coal breakage faults originated from raw coal bunker arching, coal feeder mechanical failure or control system abnormality, as long as the coal quality control, daily inspection and system optimization, you can significantly reduce the risk.

When encountering a coal break, focus on monitoring key parameters, investigate the root cause according to the steps, and quickly restore the fuel supply, so as to avoid the expansion of the accident. For power stations, paying attention to the maintenance and upgrading of the coal feeding system is the only way to ensure the stable operation of the boiler and improve the overall operational efficiency.