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Characteristics of exhaust smoke from biomass-fired boilers and treatment methods

boiler exhaust smoke
Image of exhaust smoke from a biomass boiler

1. Characteristics of exhaust smoke of biomass-fired boiler

The process of biomass burning in the boiler is the process of burning the solid fuel layer in the manual combustion chamber. The air needed for combustion is blown by the fan under the recording surface and goes up to participate in the combustion process. High-temperature combustion gases pass through rows of heat exchange tubes of the furnace. After transferring heat to the furnace water which causes the water to generate steam, the furnace smoke is blown out out by the exhaust fan through the chimney.

The exhaust gas flow from the chimney has a still high temperature of about 120 ~ 150 oC, depending largely on the furnace structure. The composition of exhaust smoke includes fuel combustion products, mainly CO2, CO, N2 gases, accompanied by a small amount of volatile substances. The fuel doesn't have time to burn completely, excess oxygen and ash fly away in the smoke stream. The composition of exhaust smoke can vary greatly depending on the burning mode on the burn surface.

  • In good burning mode, exhaust smoke carries very little volatiles in the firewood, so it can be seen that You can see that the gas is clear or light gray in color.
  • When the combustion mode is bad, lacks oxygen and the fire chamber temperature is low, the exhaust smoke is dark gray to black due to soot particles condensing from the particles. Heavy carbide-hydrogen particles are abundant in exhaust smoke. 

When burning wood, the composition of substances in exhaust smoke varies depending on the type of fuel, but the amount of emissions birth is relatively stable. To calculate, we can use the value VT2 0 = 4.23 m3 /kg, which means that when burning 1kg of firewood, produces 4.23 m3 of exhaust gas at a temperature of 20oC.

The amount of ash in the exhaust smoke is a part of the unburned amount and the amount of impurities combustible fuel, the amount of impurities usually accounts for 1% of dry fuel weight. Ash is carried along by the burning smoke into the furnace smoke stream, creating a certain amount of dust in the exhaust smoke. The concentration of this dust fluctuates greatly because it depends on the actions of the furnace worker. The largest amount of dust is generated when workers add fuel to the furnace or "turn the furnace". It also depends on the speed of combustion gas flow in the furnace and the furnace structure. Dust in wood-burning boiler exhaust has particle size from 500µm to 0.1µm, concentration ranges from  200-500 mg/m3. When firewood is wet or rotten, more smoke with small particles comes out.

2. Methods of treating biomass boiler exhaust smoke

Factors affecting pollution load

  • Humidity of fuel : Limit putting wet fuel into the furnace because during the burning process, the amount of water in the fuel evaporation will cool the burning surface, hindering the evaporation of volatiles in the fuel, causing The combustion chamber generates a lot of smoke. 
  • Provide a sufficient amount of air blowing: Most combustion chambers use fans to blow air into the furnace to help the fuel burn easily. Evenly distributing this amount of air into the bottom of the grate is very necessary to make the fuel burn evenly across the grate surface. On the other hand, only blow in enough air to burn the fuel avoid cooling the furnace and flying ash into the exhaust. 

Principle diagram of dust filter for biomass fired boiler

Principle diagram of dust filter for Biomass boiler

Measures to control air pollution

Basic mitigation measures that will be applied are:

a. Cluster cyclone dust suppressor

Special attention to environmental issues, the dust collector used for biomass-fired boilers is the cluster Cyclone type, researched and developed by Naan. Its careful design and efficiency have been proven on all Naan boilers installed. The Cyclone beam dust suppressor is a beam of single cyclones that uses the effect of centrifugal force to separate dust particles, ensuring the capture of over 90% of ash and dust flying along the smoke path.

  • The smoke flow enters the Cyclone chum thanks to the suction force of the exhaust fan placed behind it. Then the smoke stream is evenly distributed to the Cyclone elements. At the beginning of the sections from the Cyclone, the smoke stream begins to spiral in a circular motion thanks to the spiral blades. This is called “outer spiral flow”.
  • Due to the increasing smoke flow velocity, caused by centrifugal force, large dust particles in the smoke are pushed radially towards the wall. Cyclone element. This happens to all large sized particles because the centrifugal force is exceeded by the spiral motion.
  • When the particle hits the wall it will be slowed down by friction. These particles will be separated from the main air stream and pushed down the cone by the downward component of the velocity. Here, theSolid particles are separated from the Cyclone element and go down to the ash collection hopper of the Cyclone unit.
  • The smoke flow, when reaching the conical bottom part of the Cyclone magnetic part, will change direction and move back up. above and form an “inner helix”. The gas flow rotates around the central cylinder of the Cyclone element and exits at the outlet.
  • Dust settles into the ash collection funnel and is discharged through the rotating valve and screw periodically. Dust discharged through the rotary valve and screw will be bagged for convenient transportation and cleaning.

b. Cloth bag dust filter:

After the smoke and dust pass through the Cyclone beam dust collector, to achieve thorough environmental treatment, Naan has researched and included the dust filter Cloth bags operate completely automatically. Filtering efficiency reaches 99.9% and can filter even very small particles thanks to the filter aid layer.

  • Dust out each compartment, The chambers are isolated by a door system isolated by a pneumatic cylinder system. The chamber to be shaken will be isolated from the smoke path, ensuring thorough dust removal, increasing the life of the dust filter bag. The remaining bag chamber system remains intact. Ensure ventilation cross-section to filter dust when the furnace is operating. Controlled via differential pressure or controlled by dust removal timer.
  • The smoke flow after leaving the Cyclone will enter the cloth bag filter from the bottom of the device and go from the outside to the inside bag through a filter cloth thanks to the suction force of the exhaust fan. Initially, dust particles larger than the gaps between fabric fibers will be retained on the fabric surface according to the sieve principle. Smaller particles adhere to the surface of the filter fabric due to collision, gravity and electrostatic attraction. Gradually the collected dust layer thickens, forming a filter aid membrane through which dust is captured. 
  • After a period of time, the dust layer will be very thick, increasing the resistance to air flow. We must remove the layer of dust on the fabric surface to avoid overloading the exhaust fans, making it impossible for airflow mixed with dust to enter the filter bag. This operation is called filtration recovery.
  • The dust layer is cleaned out of the bag by spraying compressed air directly into the bag. When passing through the bag, the dust is separated from the surface of the fabric bag. Bags are cleaned one row at a time for which the timer is activated. Each pipe passes through each row of fabric bags carrying a stream of compressed air. The air pipes have a nozzle placed above the bag to spray air directly into the fabric bag. Reverse pulses and air flow interruptions occur very quickly. This method quickly improves dust membranes to help filter dust effectively.
  • Compressed air injected into the cloth bag dust filter is directed from a compressed air tank with a pre-set pressure. When the compressed air pressure in the tank drops to the lower rated pressure, the air compressor operates to supply compressed air into the air compressor tank until the tank reaches the upper rated pressure.
  • Cloth bag dust filter device Pneumatic cleaning can operate continuously and without interruption in the cleaning process of the air stream because the compressed air injection is very small compared to the total volume of air containing dust. 
  • Short cleaning cycle, reduces recirculation and dust does not settle.  In addition, the continuous cleaning feature allows them to operate with a higher rate of air passing through the fabric, so space requirements are lower.
  • Dust settles in the dust collection hopper and is discharged through the rotating valve periodically. Dust discharged through the rotary valve will be bagged for convenient transportation and cleaning.

c. Dust suppression tank:

To achieve thorough environmental treatment, Naan has researched and introduced a wet dust suppression tank system.
  • The dust removal tank is constructed with zigzag walls to create a flow for the smoke line to collide with the water membrane. Dust will be deposited in the tank according to the principle of inertia and impact with the water film.
  • The smoke coming out of the fabric bag filter will enter the wet dedusting tank before being sent through the chimney and discharged into the environment.

3. Conclusion

Treatment of exhaust fumes from biomass boilers is not only a technical task but also a commitment of manufacturers to the environment and human health. This process requires a deep understanding of the factors that affect the quality of exhaust smoke and effective approaches to minimize negative impacts.

With the application of measures With advanced treatment methods such as Cyclone beam dedusters, cloth bag dust filters and wet dedusting tanks, minimizing the environmental pollution load from boiler flue gas has become more feasible and effective than ever. These methods are not only technical solutions but also a continuous commitment to improving environmental quality and protecting public health.

The future of furnace production and operation Biomass steam is a constant journey to optimize performance, minimize environmental impact and ensure everyone's health. This is not only a technical task but also a social responsibility that we need to carry out persistently and persistently.

Naan thank you for reading the entire article. If you have any questions or comments, please leave a comment below.