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  • Product name: Membrane Wall Hearth Thermal Center: Energy-Saving, Environmental Protection, No Clogging
  • Product ID: N001
  • Shelf time: 2015-06-15
  • Views : 73

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Savings in electricity of running for 2 to 3 years can recover the cost of the Thermal Center (compared with traditional ones).

1. Composition of membrane wall hearth thermal center

1. Travelling grate combustion apparatus
2. Membrane wall main hearth
3. Drum
4. The second membrane wall hearth
5. Convection area for waste heat and heat conduction oil
6. Air regulating door

7. Flue regulating door
8. Steam convection area
9. The first-level dust removal
10. Flue-gas mixing chamber
11. The second-level dust removal
12. Steam gas heat exchanger
13. Thermal deoxidizer
14. Soft water tank
15. Water treatment equipment
16. Draft fan

As for membrane wall hearth thermal center, a membrane wall generating steam is designed on the top of travelling grate. It applies M-shaped fume flow commonly used by biomass boiler. In the convection area, it is divided into three paths: heat conduction oil convection pipe area, water steam convection pipe area and hot fume. Such three paths of fume are regulated mainly by controlling the heat production quantity of heat conduction oil. On the basis of guaranteeing that thermal power of heat conduction oil satisfies production demand, steam consumption volume is appropriately distributed

2. Heat consumption characteristics in the wood-based panel industry

Taking heat consumption of medium-density fiber board with annual productivity of 200 to 300 thousand m3for example, thermal medium parameter characteristics of thermal center in the density board industry is introduced.

Table 1 (take heat consumption of medium-density fiber board with annual productivity of 200 to 300 thousand m3for example)


Heat consumption equipment

Heat consumption medium

Heat consumption parameter

Heat consumption percentage %




Q= 18t/h  P=1.2Pa  (12.6MW)



Wax melting


Q=0.7t/h  P=0.3Pa  (0.3MW)



Glue making


Q=3t/h   P=0.4Pa  (2.09MW)



Fiber drying

Hot fume



Hot fume 52


Hot press

Heat conduction oil



Heat conduction oil 9.5


Chip preheating


Q= 2.5t/h  P=0.3Pa 1.74MW



Glue spray system







(1.0MW, max 1.3MW)



Total thermal power


Steam 38.5

 Heat consumption percentage of three media: steam 38.5% (including steam consumption of thermofiner 26%), heat conduction oil 9.5%, and hot fume 52%.
Design parameters of steam: P = 1.6Mpa, Q = 28t/h;
Design parameters of heat conduction oil: P = 1.0MPa, Q = 4.65MW (oil output temperature 280
Design parameters of hot fume: Q=25.5MW (350
smoke temperature drying fan)

3. Membrane wall hearth thermal center: power parameters of three media

Table 2 (take heat consumption of medium-density fiber board with annual productivity of 200 to 300 thousand m3for example)



Thermal medium


Production equipment


Auxiliary machine


Auxiliary machine parameter


Auxiliary machine power/KW





Membrane wall and convection pipe


Draft fan



Used jointly with hot fume wind



Water feeding pump

Q=30t/h H=220m




Thermal oil


Waste heat conduction oil furnace


Draft fan of heat conduction oil furnace




Thermal oil circulation pump





Hot fume wind


Membrane wall second hearth exhaust and convection pipe exhaust


Total draft fan





If the drying fan is near to thermal center (<25m), 450KW draft fan is optional.



Temperature regulating fan




Total power (power of other auxiliary machines is not listed)

User can compare this value with the total power of draft fan and circulation oil pump of three thermal media in other thermal centers.


760/310 (near distance)


4. Characteristics of membrane wall hearth thermal center

1) Membrane wall hearth can directly generate steam. There is no need to generate steam by using thermal oil secondary transfer and no need to equip it with steam generator, so that the total covering area of equipment is smaller without secondary circulation pump of steam generator.

2) Thermal oil is designed to supply heat for necessary equipment. The thermal power is small; oil pipe diameter of thermal oil system is small; oil consumption quantity is small; circulation oil pump power is small; both oil and electricity is saved.

3) Membrane wall hearth applies traditional M-shaped structure of biomass boiler which can control combustion time and hearth output temperature and maximum control generation of harmful gas. The second vertical shaft burnout chamber cannot only reduce fume temperature and solidify dust particles in fume, but also sedimentates 70% of large grained dust in the lower 180° turn-round chamber, to avoid clogging in convection pipe from back area.

4) The front and back arches of the hearth apply water pipe water cooling bracket structure. After the arches are molded, they are firm, stable and durable.

5) The covering area of the overall equipment is small and it can be arranged outdoor.

6) Installation power of the overall equipment is low.

7) Heat absorption quantity of membrane wall increases as thermal load increases. Therefore, the total steam generation quantity will be more than equipment steam consumption quantity. If the steam is emptied, thermal energy will be wasted. Therefore, a steam-gas heat exchanger must be equipped at the front of back dryer, so as to transfer waste steam to warm air and let it enter drying pipe. Condensed water returns to soft water tank or thermal deoxidizer. Such process is completed by steam pressure inside drum and no additional power consumption is needed.

8) Electric control valve used for controlling pressure must be equipped in the steam pipe. The system will transmit waste steam to steam-gas heat exchanger to ensure the drum pressure will be within the set scope.

5. Advantages of membrane wall thermal center: energy saving, environmental protection, no clogging

5.1 Energy saving

Energy saving includes two aspects: firstly, energy saving in fuel combustion and thermal utilization efficiency; secondly, energy saving in equipment installation total power.

5.1.1 High efficiency of fuel combustion and thermal utilization

 In the combustion process of biomass fuel, if air distribution is not suitable or combustion time is insufficient, it will result in loss due to incomplete combustion. Because the composition of biomass solid fuel is different from that of coal, so the combustion process is different. 

Composition of biomass solid fuel and coal

Fuel type















Biomass solid fuel

















From the above table it can be seen that the biomass fuel has the following characteristics:

(1) Low carbon content and low fuel thermal value.

(2) Relatively large hydrogen content and volatile component. In biomass fuel, most carbon joins with hydrogen to constitute low molecular hydrocarbon which will break down into volatile substance. Therefore, it is easily ignited. In the early stage of combustion, the precipitation quantity is large, so the air distribution must be suitable and it must be ensured that the combustion environment has enough temperature for complete combustion. If air distribution is not suitable, the furnace temperature will not be enough and flame with black edge will be easily generated to result in incomplete combustion and black smoke.

(3) Large oxygen content in biomass fuel, flammability, low air consumption during combustion.

(4) Low sulfur content and low sulfur in fume.

(5) Low ash content, small density, easiness to be blown to heating surface in back area by wind.

(6) Small density, loose texture, flammability, easiness for burnout, low carbon content in ash, low loss due to incomplete solid combustion.

As for the above characteristics, the following two points must be ensured in combustion apparatus design and air distribution:

a) Select suitable travelling grate and design enough combustion volume.

When fuel is burning on grate, a large grate segment can push and roll a thick (about 300mm) fuel to reach the effect of intensifying ventilation and combustion.

Thermal load of grate area is 600750KW/m2 (510,000~650,000Kcal/h m3);

       Design value of hearth volume thermal load is 100~140KW/m3 (86,000120,000Kcal/m3).

      b) Suitable air distribution, fractional combustion

Air distribution is divided into primary air and secondary air. Primary air is supplied under grate and secondary air is supplied above grate.

Theoretical air supply quantity of biomass fuel is 4~5m3/kg and excess air coefficient is about 1.4. Secondary air accounts for 30%~40%. Some straw fuel is 50%. The position for secondary air to enter hearth is at high temperature throat of front and back arches. Such position has short front and back distance and secondary air can easily go through it so as to supply air evenly to volatile gas and satisfy high-efficiency combustion.

The speed for secondary air to enter hearth is set about 60m/s, so as to ensure that air and combustible gas can be fully and evenly mixed to reach burn-out effect.

       Steam, thermal oil and hot fume are the three thermal media that are necessary for the wood-based panel manufacturers. However, the percentage of heat consumption of each thermal medium is different. As for heat consumption percentage of density fiber boardTable 1, steam is 38.5%, head conduction oil is 9.5% and hot fume is 52%. From above Table 2, it can be seen that installation power of membrane wall thermal center is the lowest. Users can compare the data of the 2 tables by themselves.

5.2 Environmental protection

Harmful gases emitted from combustion in boiler mainly include CO, SO2, NOx, dioxin, etc. By means of secondary air distribution, fractional combustion, enlarged hearth sizes and heating surface, prolonger combustion time of solid particles and volatile component in hearth, the effect of maximum burnout and harmful gas control is reached.

5.3 No clogging

5.3.1 Characteristics of biomass fuel ash

Ash of biomass fuel contains relatively large amount of calcium ion, sodium ion and potassium ion. In common conditions, it shows fusion status under 850~1000. If it is processed into particle fuel, it will not show fusion status until under 1198~1213. However, thermal center of wood-based board enterprise basically doesn’t combust processed particle biomass fuel, therefore biomass fuel ash shows fusion status and is easily adhesive to pipe wall under 850~1000.

5.3.2 The reason for no clogging in membrane wall hearth thermal center

 Since the hearth has water cooling membrane wall and mature “M”-shaped fume flow is applied, the designed hearth outlet temperature is 850; after entering the second vertical shaft hearth (burnout chamber), fume goes downward vertically to fume turn-round chamber at lower part, now the fume temperature is reduced to 750, and then it rolls and goes to thermal oil convection area, water-steam convection area and hot fume area. Now its temperature to enter convection area is about 750and ash is solidified and won’t produce adhesive “cement ash”.

6. Suggestion

     From the aspect of energy saving and running cost, the location of the thermal center to be designed closer to the drying fan. The best efficiency of power-saving can be achieved with the distance between the startup chimney and the drying fan ≤25m.
           Savings in electricity of draft fan and circulation pump running for 2 to 3 years can recover the cost of the thermal center!

7. Typical users of membrane wall hearth thermal center

7.1 Guilin Longteng Wood Industry Co., Ltd.: 200~250 thousand m3/year, 49MW (4200×104Kcal/h)
7.2 Shandong Tangkou Density Board Factory: 120 thousand m3/year, 23.5MW
7.3 Hubei Dongrun Wood Industry Co., Ltd.: 150 thousand m3/year, 29MW (2500×104Kcal/h)
7.4 Jingdezhen Dingcheng Wood and Plastic New Material Co., Ltd.: 120 thousand m3/year, 23.5MW (2000×104Kcal/h)

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