Grate Boiler Modification or Transformation

Boiler modification is the concern of our customers in Indonesia in recent years, and each user has different situations and different requirements. Some users just want to improve the efficiency of the long period operation of boiler and limited by the cost budget; while some users have to make the boiler modification due to the requirements of the policy or the change of fuel.

An Indonesian customer in Year 2020, we call it BW, used a 50 t/h coal-wood mixed combustion inverted chain grate boiler from a domestic boiler manufacture in 2007. BW wants to reduce operating costs by changing the high combustion value fuel to a mixture of low calorific value coal and waste wood. BW needs to reform the boiler for this purpose, and the load of the reformed boiler can meet the previous demand. 

Here is the previous boiler parameters:

1.    Boiler rated evaporation: 50 t/h

2.    Boiler saturated steam pressure: 3.82 MPa

3. Boiler superheated steam temperature: 450 ℃

4. Boiler feed water temperature: 150 ℃

5. Design cooling air temperature: 30 ℃

6. Boiler efficiency: 81 %

7. Boiler exhausting flue gas temperature: 180 ℃

8. Previous design fuel: coal with 5252Kcal/kg : waste wood = 8:2

The target after modification will be:

1.  Boiler rated evaporation: 40~50 t/h

2.  Boiler saturated steam pressure: 3.82 MPa

3.  Boiler superheated steam temperature: 450 ℃

4.  Boiler feed water temperature: 150 ℃

5.  Design cooling air temperature: 30 ℃

6.  Boiler efficiency: 81 %

7.  Boiler exhausting flue gas temperature: 145 ℃

8.  Previous design fuel: coal with 5252Kcal/kg : waste wood = 8:2

Coal analysis:

       Q _netar     3000 ~4000  KCAL/kg

       Waste wood combustion value：2600KCAL/kg

Waste wood crushed to less than dimension 40X40X5mm

How about the modification layout?

Each heating surface of the boiler can meet the operation requirements, so only the furnace is modified to ensure that the boiler can reach the rated evaporation and improve the thermal efficiency of the boiler when the calorific value of the fuel is low.

We change the coordination of the front and rear arches to make the flow rate of the flue gas at the roar more reasonable, increase the heating area of the furnace, add a secondary membrane water wall to the front wall, increase the heat absorption of the furnace, reduce the outlet temperature of the furnace, and protect the superheater.

In order to improve the combustion conditions, reduce coal consumption, increase the average temperature of the furnace, we increase the rear arch tube and the rear arch secondary air.

In order to improve the combustion degree of volatile matter in waste wood, we choose to change the structure of the secondary air nozzle on the front wall to increase the outlet flow rate, so that the secondary air has good rigidity and penetrating power.

Will the structure change??

Steam drum: adding 4 more holes in the drum for the new membrane water wall, the up and down tubes with specification φ89x5, others no change.

Water cooling system: The original design has no rear arch, this arrangement is not conducive to the timely combustion of low calorific value fuel, for this reason, it is necessary to transform the lower part of the rear wall to form a rear arch, and at the same time, a bend structure is adopted for the pipe part to arrange the secondary air, and the front arch is connected. It has good coordination, enhances the mixing and heat carrying capacity of high temperature flue gas, and avoids coking caused by local high temperature.

Superheater and temperature reduction system: no structure change but adding wear cover for the superheater.

Economizer: no structure change but adding wear cover.

Air preheater: no structure change, but user consider to change the enamel lined air preheater.

Combustion system: change the air nozzle to level structure;

Furnace wall: no change.

Boiler piping layout: adding drainage pipe for front water wall, others no change.

Secondary air draft: Change the position of the secondary air on the rear wall and the structure of the nozzle, to introduce the high-temperature flue gas and a large number of high-temperature hot particles into the rear of the furnace, which can ensure that the low-calorific value fuel absorbs heat and ignites in time, improves the combustion efficiency, and reduces the the incomplete combustion loss of the gas and the carbon content of the fly ash are beneficial to reduce the blackness and dust concentration of the flue gas.

Soot blower device: no change.