High energy consumption and low coal-fired efficiency are common problems in coal-fired boilers. There are many causes. The most direct reason is that the coal quality and the design of the boiler are not well matched, and the fuel combustion is not complete.

Improve the combustion quality of coal in coal-fired boilers and minimize the loss caused by incomplete combustion.

There are three main types of loss of incomplete combustion of coal in coal-fired boilers:

1. Coal ash

2. Boiler leakage

3. Fly Ash

The methods adopted are:

1. Increase the fuel surface area to increase the coal burning speed;

2. Choose a reasonable type of coal according to the boiler type, combustion method and other parameters;

3. Strictly control excess air entering the furnace, and provide enough burning time to ensure that the fuel and air are fully mixed;

Chain Boiler Transformation Plan

The boiler of a certain unit burns Type II bituminous coal with a low calorific value of 2093KJ/kg, and the boiler operating efficiency is less than 70%.

Modification Plan

1) Two sets of medium temperature cyclone separators are arranged after the superheater and before the economizer.

2) The overall frame of the boiler remains unchanged, and the position of the steam drum remains unchanged.

3) Replace all induced draft fans, blowers and secondary fans.

4) Separation equipment, coal feeder and fluidized bed adopt mature shaping equipment.

5) Add a set of crushing and screening device to the coal part.

6) All radiant heating surfaces are changed to membrane water walls.

7) The thermal instrument is partially retained, and some control points are appropriately added.

8) A set of high-temperature inertial separators are arranged after the furnace outlet and before the superheater, and the structure after modification

Retrofit design highlights

1) Induced draft fan and blower. Calculate the smoke and wind resistance, and use high-pressure blower and induced draft fan accordingly.

2) Burning part. Remove the coal feed hopper, coal gate, grate, ash hopper and slag outlet of the original chain furnace, add fluidization and air distribution system, the air distribution plate is fixed on the membrane wall, and the equal pressure air is arranged under the orifice plate In the chamber, a coal feeding system and a limestone adding system are added, and secondary air is arranged around the furnace.

3) Monitoring instrument. Monitoring the furnace temperature is very important for operation, and there are at least two furnace temperature gauges.

And the distribution is reasonable. The return device should also be equipped with a thermometer, so that the furnace can judge whether the return is normal. In addition, the air pressure gauges in each air chamber, separator, and return device are equally important. As many basic air pressure gauges as possible are arranged to facilitate the operation of the furnace.

4) Cyclone separation system. The cyclone separation system is a medium temperature separation. It is installed in the gap between the furnace and the tail shaft. The boiler column is used to increase the support. The flue gas is drawn from the back wall of the low temperature section of the superheater and enters the left and right cyclone separators in two ways. After passing through the separator, it is led out from the upper outlet header and directly enters the shaft economizer inlet flue. The lower part of the cyclone separator adopts a U-shaped material return device, and is equipped with a material return system and an ash discharge system.

5) The heating surface of the water wall. The position of the boiler and steam drum remain unchanged, the header downcomer is completely removed, the water wall is also completely removed, the radiant heating surface is changed to the membrane wall heating surface, and the downcomer is re-arranged.

6) Feeding part. The fuel feed uses two spiral positive pressure coal feeders, and a limestone feed device for desulfurization can be added at the front wall feed hopper as needed. A set of crushing and screening devices should also be added to control the particle size of coal in the range of 0-8mm, so that the boiler can achieve higher combustion efficiency.

7) Superheater and the heating surface of the tail. The heating surface of the original boiler economizer is adjusted, the high temperature section of the original economizer is removed, the air preheater remains unchanged, the position of the superheater does not move, the low temperature section superheater needs to reduce part of the heating surface, and the heating area of the high temperature section remains unchanged.

8) Inertial separation system. The inertial separation system is suspended on the top of the furnace with stainless steel round steel, and is arranged in two staggered rows, arranged behind the furnace outlet and before the high-temperature superheater. The separation efficiency can reach 60%, and the resistance is 200~300Pa. It is characterized by low thermal inertia, low resistance, simple structure and convenient layout, which minimizes the adverse effects of particle rebound and airflow backmixing. The material is heat-resistant Cast steel, the separation element does not have a relatively large wear problem.

After Transformation Effect 

After the boiler is modified by the above method, the design parameters are reached. The superheated steam temperature is 450℃, the feed water temperature is 150℃. The boiler evaporation capacity is between 33-40t/h, the superheated steam pressure is 3.5-3.8MPa, and the boiler thermal efficiency is ≥85%. Efficiency ≥98%, it can burn coal anhydride stone with low calorific value less than 10886kJ/kg, particle size 0~8mm

The overall design success is mainly reflected in:

1) No design defects were found during the trial operation, and no furnace shutdown due to design defects occurred.

2) The boiler output is between 33-40t/h, which can reach full load and operate stably.

3) No accident occurred during the operation of the furnace wall.

4) After the boiler is in operation, the blackness of the flue gas is lower than Ringelmann level 1, and the emission of sulfur dioxide also meets the requirements.

5) The combustion efficiency is relatively high.

6) The separation and return system operates normally and reliably.

7) Since the operation, a number of coal types have been tested successively, all of which can be stably burned, and no boiler shutdown due to switching of coal types has occurred.

DZL Coal-Fired Boiler Changed to Gas-Fired Boiler

With the continuous improvement of global environmental protection requirements, coal-fired boilers are increasingly unable to adapt to the trend of "energy saving and environmental protection". At present, it has become a trend to transform coal-fired boilers into gas-fired boilers. Practical needs require comprehensive evaluation.

Coal-Fired Boiler Gas Transformation Advantages:

1. Gas reformation of coal-fired boilers can save space and land.

2. The gas boiler adopts artificial intelligence control to reduce labor costs.

3. The noise produced by the gas boiler is relatively low.

4. Natural gas is a clean energy source, which is extremely beneficial to energy conservation and emission reduction.

DZL coal-fired boiler Transformation Plan :

1) According to the structural characteristics of the boiler and the consideration of reducing the transformation workload, the DZL coal-fired boiler is still an external fired boiler after being changed to a gas-fired boiler.

2) Remove the coal scuttle. If the boiler furnace space is relatively large, the chain grate can not be removed, but two layers of refractory bricks are laid on the chain grate, and then the brick joints are compacted with refractory cement to form the dry bottom of the gas boiler . If the boiler furnace space is relatively small and the flame diameter is larger, the chain grate should be removed, and the air chamber should be moved down to a certain size or removed together with the air chamber, and a refractory dry furnace bottom should be established between the base and the boiler support.

3) Calculate the gas heat and smoke and wind forces to determine the output of the gas boiler and the capacity of the blower (induced). Especially when the convection smoke pipe adopts the threaded smoke pipe, the smoke and wind resistance should be carefully calculated. 

4) According to the resistance of the flue duct, the blast type automatic control burner can be selected, and the blower can be removed at this time.

After calculation, the capacity of the induced draft fan is sufficient to overcome the resistance of the flue duct, and the blower can also be removed.

When a simple multi-stage jet burner is used, the burner should be equipped with control devices including high-temperature ignition, flame monitoring, and flameout protection.

If you choose a split type (actually a combustion head without a blower) burner, the blower can still be used and moved to the front of the furnace, but the blower volume needs to be approved, because the blower has a larger margin for gas.

5) Take out the screw slag remover, build the ash chamber with refractory bricks, and seal it tightly to form a dry furnace bottom.

6) The inspection furnace door can be changed to an explosion-proof door or reserved as an inspection door when the boiler gas is used.

7) The burner port is opened on the front wall of the boiler. The front arch can be retained and extends down to the bottom of the furnace. It is helpful to fix the burner when the burner is installed.

8) Before putting oil or gas into use, the soot in the convection smoke pipe should be cleaned and the coal ash on the surface of the water pipe in the boiler furnace should be removed; at the same time, the scale on the water side should be cleaned to improve the heat transfer of the heating surface of the boiler. ability.

9) Oil and gas boilers with positive and negative pressure combustion should pay special attention to sealing with the outside world.

10) If the length of the boiler is equivalent to the flame length, a secondary reflection arch can be installed in the furnace of the boiler to increase the radiation intensity of the oil-fired gas boiler, as shown in Figure 16-15. Even dry the bottom of the furnace.

When a coal-fired boiler is changed to an oil-fired gas-fired boiler, in order to increase the volume of the boiler furnace, the front and rear arches of the original boiler should be removed as much as possible. Only when the secondary radiation area is set up on the basis of ensuring a larger boiler volume can the effect be achieved. After the transformation, the volume radiation of the flame and flue gas is increased, and the low and long back arch slope is removed, and the whole is retained. Part of the back wall is used as a smoke partition.

Of course, things have two sides, with pros and cons.

Coal-Fired Boiler Gas Reformation Shortcomings:

1. The combustion efficiency is relatively poor

Horizontal internal-combustion gas-fired boilers use micro-positive pressure chamber combustion, with high combustion efficiency. Equipped with advanced burners, the combustion efficiency can reach 99.5%; while coal-fired boilers can only be converted to gas-fired boilers with negative pressure combustion, and the combustion efficiency is about 85%. .

2. Inflexible control

Most of the burners use fully automatic burners, but the resistance of coal-fired boilers is relatively large, and it is necessary to add an induced draft fan to ensure normal operation.

3. The heat loss is relatively large

The volume of coal-fired boilers is much larger than that of horizontal internal combustion gas-fired boilers, the insulation materials are also relatively poor, the surface temperature is high, and the heat dissipation loss is large. After the coal-fired boiler gas reformation, its actual efficiency is 70%, while the thermal efficiency of the coal-fired boiler design is 78%, and the thermal efficiency of the gas-fired boiler design is 90%. By comparison, the operating cost will be about 20% higher than that of the gas-fired boiler. .

Conclusion

In the process of coal-fired boiler gas transformation, it is necessary to carefully consider the feasibility of the plan, make decisions based on the actual situation of the coal-fired boiler (use time, operating life, maintenance cost, etc.), and budget the transformation cost in advance, because there are It is more convenient and economical to purchase a new gas boiler at the time of renovation.

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