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Influence of Coal Quality on Boiler Combustion

September.22 Coal

For the current coal supply tensions and the boiler combustion problems caused by coal quality changes, we must face the reality. In the running process, we need to positively test, explore, and make corresponding practical operational solutions. And we should also work hard to adjust the combustion running of boiler to ensure that the combustion outlet temperature can reach the standard There is supposed to be less running fault on the boiler and its auxiliary equipment to keep the whole process safe, stable and economic.

1. Combustion Process of Coal Sample: 
Once the coal sample gets into the furnace, it usually has four stages until it is combusted completely.

a. Moisture Evaporated Stage: 
when the temperature reaches around 105℃, the moisture shall be evaporated totally;

b. Volatile Combustion Stage:
In the combustion process of volatile matter, the coal sample will constantly absorb heat, the temperature will keep on increasing, then subsequently volatile matter will be separated out. When the temperature reaches the ignition point, the volatile matter will start its combustion. The combustion speed is quite fast. It usually only takes 1/10 of the whole combustion time. 

c. Coke Combustion Stage:
After the volatile matter combustion, the remaining solid composed of carbon and ash is coke. The coke temperature increases quickly. The fixed carbon is combusted violently to release a large amount of heat. The combustion speed and burnout degree mainly depend on this stage.

d. Burnout Stage:
In order to improve the efficiency, the coke in the ash is required to be burnt out as much as possible to reduce the heat loss.

2. Good combustion must have three conditions as below
a. Temperature      
The higher the temperature is, the faster the combustion is. The furnace chamber temperature is usually around 900~950℃ (Circulating Fluidized Bed Boiler). 

b. Air 
The faster the air flow is, the faster the combustion is.

c. Time
The coal is required to have enough combustion time in the furnace chamber.  When the coal burns, there is a layer of ash shell around the coal. The CO, CO2 generated by the combustion will move towards all round, among which, the CO will become CO2 after combustion when it have enough oxygen. That is to say, when the carbon particle burns, the ash shell is surrounded by one layer CO and one layer CO2. If the oxygen in air needs to fully contact the carbon, the oxygen is required to penetrate two gas layers. Therefore, it is very easy to remove the outer gas layers by speeding up the air flow to scour the carbon particle;

At the same time, strengthen the disturbance to make the oxygen directly contact with carbon particle, which can speed up the combustion. If the oxygen is not enough for the disturbance, the coal cannot be burnt out. In this case, there will left some carbon nucleus without combustion in ash residual. What is more, some of the CO without combustion may be released together with smoke. As for coal particle with large size, it takes longer combustion time, because if the stop time is too short, the combustion cannot be complete. In the daily running, user is required to supply sufficient oxygen, adopt secondary air to strengthen the disturbance and increase the combustion temperature. 

3. The Requirements for Coal Type
Coal has a certain selectivity. Bituminous coal with volatile matter over 15%, weak caking of ash melting point higher than 1250 ℃ , moderate size, heat value over 18800~21000kJ/kg is the most appropriate.

The ash content in coal is required to be 10%~30%. Pulverized coal (0~15mm) should not exceed 50%~55%; Pulverized coal (0~10mm) should not exceed 30%. And the coal particle size cannot exceed 20mm.
The moisture content in coal is recommended as below:

When the Pulverized coal (< 3mm) is 20~40%, the moisture in coal is recommended to control within 5%. When the Pulverized coal (< 3mm) is 80%, the moisture in coal is recommended to control within 8%. When the Pulverized coal (< 3mm) is ~100%, the moisture in coal is recommended to control within 10%.

4. Problem Existed in Coal Quality
a. There are more ash content in coal quality;

b. Coal particles have uneven size;

c. There are a lot of impurities in coal;

d. Coal has lower heat value;

e. It is not easy to ignite;

f. Moisture content is uncertain;

g. The slag has high concentration of carbon content;  

In conclusion, to ensure the combustion stability, it is better to use the designed coal type or the similar coal type for the boiler. In recent years, the coal supply becomes more tense and diversified. The coal quality is quite different than that of before. These factors cause great influences on the combustion stability and normal heating supply.

5. Influence of Coal Quality on Boiler Combustion
Heat value generated by coal is the key factor to prove the coal quality. When the heat value generated is lower than a certain value, it will not only lead to unstable and incomplete combustion, but also cause the boiler flameout, which may make the outlet temperature difficult to reach the standard. This will affect its normal running.

Volatile matter can precipitate and combust at low temperature. With the combustion, the temperature of coke particle increases quickly, which makes it convenient for ignition and combustion. What is more, volatile matter precipitation has increased the internal space and external response area for coking, which shall speed up the combustion.

Therefore, if there is more volatile matter, the nonflammable solid carbon in coal will be less, the pulverized coal will be easier to be burnt out, response areas will increase a lot, which will speed up the combustion.

If there is less volatile matter, the ignition temperature of pulverized coal flow increases significantly, which makes the ignition difficult. In this case, it takes longer time to reach the ignition point. The combustion stability reduces. The flame center shifts up. Heat absorbed by furnace chamber reduces, while the heat absorbed by convective heating surface increases. The temperature of exhaust in the rear part rises up. Besides, the exhaust loss will increase a lot.

In the combustion process, the ash content of coal will not generate heat. And furthermore, it needs to absorb heat.

When there is more ash content, the heat generated is less. It is not easy to ignite under this circumstance, which may lead to ignition delay. At the same time, the furnace chamber temperature reduces. The burnout degree of coal decreases. The combustibles in fly ash increases lot.

When the ash content increases, the carbon particles may be surrounded by ash shell. The combustion speed on the surface of carbon particles reduces lot, leading to incomplete combustion. Furthermore, the fly ash concentration becomes higher, resulting in serious wear of economizers and air heaters, etc. With the increase of dust removal, the physical heat loss of fly ash and slag increases, and the thermal efficiency of the boiler is reduced.

The particle size of coal has great influence on the combustion of the boiler.

When the particles are excessive large, in the combustion process, the coal particles have too short time to stay in the boiler. Coke in coal cannot be burnt out. The carbon content in slag rises, and the physical heat loss of boiler slag increases.

When the particles are excessive small, in the combustion process, the fine pulverized coal will be easily brought out by wind. The carbon cannot have a good contact with oxygen. Meanwhile, the pulverized coal is also easily to be blew up by air, and then brought out with smoke, which increases the heat loss of fly ash. That is why we need to properly adjust the air flow according to coal particles.

As for the effect on characteristics, the moisture content to certain limits is consistent with the volatile matter. Small amount of moisture content is good for the fire.

From the perspective of the combustion kinetics, moisture vapor in high temperature has a catalytic effect on combustion. It can accelerate the coke combustion of pulverized coal, improve the flame emissivity. In addition, when the moisture vapor decomposes, it will generate H and OH to improve the thermal conductivity of the flame. However, if the moisture content is too much, the ignition heat will also increase. At the same time, some of the combustion heat shall be used to make the moisture become vapor to reduce the smoke temperature in furnace, which is not good for the ignition.

Coal impurities will not only absorb heat produced by combustion, but also prevent the coal from contacting with oxygen. The influences on combustion will reduce the thermal efficiency of the boiler, increase the amount of boiler slag ash running, increase the slag and ash removal quantity in the operation of the boiler, and bring great harm to the safe operation of the boiler.

6. Influence of Coal Quality on Operation of Boiler and Auxiliary Equipment:
When coal quality entering the furnace is quite different with the boiler design and coal quality requirements, it will bring the following adverse effects to the boiler combustion and auxiliary equipment:   

a. If the coal quality is very poor, the boiler ignition and operation will be quite difficult. It will be difficult to burn, easy to extinguish, and seriously affect the boiler outlet temperature to reach the standards. 

b. The furnace chamber is easily coking, leading to serious wear to the heating surfaces of convection bank, economizer, air-preheater. Furthermore, it also very easy to accumulate ash. The resistance of the boiler increases. These factors will affect boiler thermal efficiency violently.

c. After combustion, coal with high concentration of ash content will not only affect the dust removal effect, but also increase the operation load of ash removal and ash removal system. In this case, the operation failure may occur, which could lead to adverse effects on the working environment and the external environment.

d. When the coal has high concentration of sulfur, it may lead to high temperature corrosion of water wall, low temperature corrosion of   boiler tail flue, economizer, air preheater, etc. These factors may cause boiler tube explosion, and affect the running state of boiler.

7. Following response are recommended
a. Improve the operator’s operating skills. Make sure all the operators master the coal quality analysis skills, especially the calorific, volatile matter, ash content and particle size etc., so that they will be able to adjust the combustion according to different coal quality.

b. Strengthen all types of technical skills for co-firing and blending combustion. Explore different coal with air flow and other operating parameters by continuous combustion adjustment experiments. On the basis of the experiments, we can keep exploring the coal blending techniques to improve the utilization ratio of poor quality coal, reducing the heating operation cost.

c. Strengthen the combustion adjustment of the boiler. Ensure that the coal and air have proper quantities and enough mixed contact. The furnace chamber is required to keep in high temperature to help combustion. Adjust boiler loading according to the specified operation. To make the boiler run perfectly with the best parameters, operator needs to monitor the furnace pressure, exhaust temperature, oxygen, carbon dioxide, etc.  And then check and adjust the boiler for the incomplete combustion caused by uneven carbon particle size and so on.

d. Strengthen the management of coal transportation. Different types of coal is supposed to be stacked separately. If necessary, use a different burning coals at different times, or use the coal in different proportions. In the process of coal transportation, the coal worker must communicate with the stoker on duty in time. For the coal with lower moisture content or more pulverized coal, operator can take a proper stirring to sort out the impurities in, crush the coal with larger size, and adjust the crusher.

e.Enhance the inspection and maintenance of boiler combustion equipment and auxiliary equipment. Eliminate the faults of boiler and its auxiliary equipment in time (Especially the boil sealing, sample feeder, high temperature separator, superheater, economizer, air pre-heater, and dust removal equipment, etc. ).

f. Strengthen the control of boiler air flow and furnace temperature. To keep the furnace chamber in high temperature is beneficial for the ignition and combustion.

g. Enhance the coal storage. Take effective measures to prevent coal from weathering and spontaneous combustion. Reduce the coal quality to increase the difficulty of spontaneous combustion. 

h. Enhance the control and management of coal quality. Explore a better and more stable coal market. Timely and accurately grasp the industrial analysis data of coal, and give the data to each workshop. This data can help the operation personnel to select the most proper coal type that is suitable for the boiler.

i. Adopt more mature advanced technology and equipment to change the combustion status.

8. Conclusion
With the increasingly tight supply of coal, the coal quality may have great changes at any time. To explore and research different coal types that is suitable for the current available boilers, maximally reduce the negative influences of the poorer coal quality on combustion, achieve the optimal operation of the boiler, all of these measures, will not only improve the overall economic efficiency, but also ensure the normal operation of the entire system.