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1. What is a wood fired boiler?

Wood boilers (or wood-fired boilers) are industrial boilers that use biomass fuels such as firewood, sawdust, or wood chips, etc. as the main fuel to burn to generate heat, creating steam for industrial production processes.

On the market, wood-fired boilers are designed and classified in many ways:

- Classification by structure: Tube-type wood-fired boilers, round-type wood-fired boilers

- Classification by operating method: Static grate wood-fired boilers, chain grate wood-fired boilers and fluidized bed wood-fired boilers.

Unlike traditional boilers that use fuel from oil or coal, wood-fired boilers utilize renewable biomass sources that are readily available in nature or from by-products, helping businesses save fuel costs, reduce CO₂ emissions, and be environmentally friendly. Thanks to these advantages, wood-fired boilers are gradually becoming indispensable equipment in many manufacturing fields today. Here are some common applications:

• Agricultural processing industry: Wood-fired boilers are used to dry agricultural products such as coffee, pepper, cashews, cereals, fruits, etc., helping to maintain product quality and prolong shelf life.
• Food and beverage industry: Providing steam to pasteurize milk, soft drinks, steam food, and support the production process of confectionery and fast food.
• Textile industry: Generates heat for washing, ironing, dyeing, steaming fabrics, etc.
• Construction materials manufacturing industry: Provides heat for the drying process of bricks, ceramics, porcelain, mortar, shortening waiting time and improving the quality of finished products.
• Rubber and plastic industry: Used for drying, heating molds, and stabilizing temperature in the production process.

2. Structure and Operating Principle of Wood-Fired Boiler

2.1 Structure of Wood-Fired Boiler

The basic structure of a wood-fired boiler includes:

• Combustion chamber: This is where the process of burning wood takes place to generate high heat (1300 - 1900°C). The combustion chamber is designed with high-temperature resistant materials such as bricks or cast iron, ensuring the combustion process is continuous and effective.
• Steam pipe system: Its task is to transfer heat from hot smoke to water, help the water in the furnace quickly reach boiling temperature and form steam.
• Dust filter and chimney system: It treats exhaust gas, removes dust, ash, and toxic gases before releasing them into the environment to minimize negative impacts on human health and the environment.
• Steam drum: It is the part that stores the generated steam, helping to coordinate and provide stable steam to the equipment used in the factory.

2.2 Operating principle:

The operation of a wood-fired boiler begins when fuel (dry wood) is fed into the combustion chamber and burned to generate a large amount of heat. This heat is transferred through the steam generating tube system, causing the water inside to reach boiling temperature and convert into high-pressure steam. The steam is then stored in the steam drum and led through the pipe system to the equipment that needs to be used.

The exhaust smoke generated from the combustion process will be led through three levels of heat exchange chambers (3 passes) to maximize the remaining heat, while cooling the smoke before going through the treatment system and discharging through the chimney.

Note: To ensure the efficiency and longevity of the wood-fired boiler, during operation, it is necessary to:

• Check and ensure the quality of the water supply to meet technical standards.
• Maintain the combustion chamber temperature in the optimal range of 800 - 850°C to limit dust accumulation, create ash flow and ensure stable combustion.

3. Biomass Boiler Technology

Biomass boiler is a cost-effective and environmentally-friendly solution, using biomass fuels such as firewood, sawdust, rice husk, bagasse, etc. to generate steam for industrial production. Common technologies include: chain grate/push grate technology and fluidized bed technology.

Let's compare the characteristics of each technology through the table below:

	Chain grate/push grate technology	Fluidized bed technology
Combustion efficiency	About 70 – 75%	High 85 – 87%
Suitable fuel type	Large and uneven firewood, bark, sawdust	Diverse biomass: rice husk, sawdust, bagasse, coffee husk...
Operation characteristics	Simple operation	High technology, requires professional staff
Emission level	Generate a lot of dust and exhaust gas, requiring additional treatment system	Emission level meets environmental standards (QCVN 19:2009/BTNMT)
Maintenance	Regularly because the system is prone to dust and ash accumulation.	Less due to even and clean burning
Investment costs	Low	Higher initially but long term savings

Based on the comparison table above, Fluidized Bed Technology is considered an ideal choice for large-scale enterprises that require a stable and continuous steam source, while aiming to improve production efficiency, reduce long-term operating costs and minimize CO₂ emissions.

4. Advantages and limitations of wood-fired boilers

4.1 Advantages:

• Cost savings: The cost of wood fuel is 40 - 60% lower than coal or oil, helping factories significantly reduce fuel costs in the long term.
• Environmentally friendly: Firewood is a renewable fuel, generating less CO₂ than fossil fuels, contributing to reducing pollution and protecting the environment.
• Abundant fuel sources: Especially in areas with wood processing industries, waste firewood and agricultural by-products are always available, ensuring stability in fuel supply.
• High flexibility: Can be combined with other boiler systems or upgraded technology to increase efficiency and operation, suitable for many types of production.

4.2 Limitations:

• Pollution emissions: Emissions contain CO, CO₂, SO₂, NO₂, and ash dust, requiring an effective treatment system.
• Impact of firewood moisture: Wet firewood reduces combustion efficiency, making it difficult to control temperature.
• High maintenance requirements: The furnace is prone to dust, requiring periodic cleaning to maintain efficiency.

5. Treatment of wood-fired boiler exhaust gas: Effective solutions

During operation, wood-fired boilers emit a large amount of exhaust gas containing many polluting components such as CO, CO₂, SO₂, NO₂, ash dust and volatile organic compounds (VOCs). Accordingly, the exhaust gas temperature ranges from 120°C to 350°C depending on the structure and operation method of the furnace. In particular, the ash dust concentration can reach 200 - 500 mg/m³, exceeding the allowable threshold if there is no suitable treatment system.

If not properly treated, these exhaust components not only pollute the air, seriously affecting human health (such as respiratory and cardiovascular diseases), but also corrode equipment, reduce machine life and destroy the regional ecosystem.

Here are some effective methods of treating exhaust gas from wood-fired boilers:

5.1 Absorption method using alkaline solution:

This method uses alkaline solution such as Ca(OH)₂ to enter the absorption chamber to directly contact the exhaust gas stream containing toxic gases such as SO₂, NO₂ and CO. Through neutralization reaction, toxic gases will be converted into non-toxic salts, thereby cleaning the exhaust gas before being released into the environment. The treatment efficiency can reach 80 - 90%, especially effective for boilers with large and stable exhaust gas flow.

5.2 Activated carbon adsorption method:

Activated carbon adsorption method is a highly effective solution in removing volatile organic compounds (VOCs), dioxins, furans and unpleasant odors in exhaust gas. The operating principle is based on the ability to adhere toxic gas molecules to the porous surface of activated carbon, thereby cleaning the exhaust gas stream before releasing it into the environment. With a treatment efficiency of up to 95 - 98%, the activated carbon adsorption system has low investment and operating costs, is easy to install, does not take up much space, and is very flexible in practical applications.

5.3 Dust filtration method

Cyclone: This is a mechanical dust filter, operating on the principle of centrifugal force. When the exhaust gas stream carrying dust enters the cyclone tangentially, the centrifugal force will push large and heavy dust particles towards the wall of the device. Then, these dust particles lose their kinetic energy, fall to the bottom of the funnel and then are collected. This method effectively removes coarse dust, reducing the load for subsequent processing stages.

Fabric filter bag: Fabric filter bag systems are widely used to treat exhaust gas containing fine dust with an efficiency of up to 99%, especially effective with dust less than 10 micrometers in size. The air stream carrying fine dust will be led into the filter chamber, where dust particles larger than the distance between the fabric fibers will be retained by the barrier effect. At the same time, a layer of small dust will accumulate and adhere to the surface of the bag, forming a secondary dust filter layer to increase the filtering efficiency. This system is suitable for factories with high requirements for output exhaust gas quality.

Wet filter tower: Wet filter tower operates by spraying water mist or absorbent solution into the exhaust gas stream. Small dust particles and toxic gases in the exhaust gas are wetted, separated from the exhaust gas, and swept to the bottom of the tower to be collected, deposited and processed. Suitable for systems with high temperature exhaust gas, helping to cool the gas and significantly reduce the concentration of pollutants.

Benefits of investing in an exhaust gas treatment system:

• Minimize air pollution and negative impacts on public health.
• Ensure compliance with industrial emission standards according to QCVN 19:2009/BTNMT on dust and inorganic concentration.
• Contribute to enhancing the image and reputation of the enterprise, demonstrating responsibility to society and the environment.
• Increase the life and performance of machinery and equipment by eliminating corrosive elements from exhaust gas.

In conclusion, exhaust gas treatment of wood-fired boilers is an inseparable part of the modern production process. This is not only an environmental protection solution but also the key to sustainable business development in the future.

6. Notes when using wood-fired boilers

To ensure stable, safe and environmentally friendly operation, businesses need to pay attention to some important factors when using wood-fired boilers:

- Choose suitable fuel: Prioritize the use of dry firewood with low humidity (under 20%), no impurities to ensure complete combustion, limit the generation of toxic gases and fouling of the combustion chamber.

- Proper fuel storage: Firewood should be stored in a well-ventilated area, with a roof and high floor to avoid mold, rot, affecting fuel quality.

- Proper operation: Monitor and adjust the appropriate combustion chamber temperature, control steam pressure, ensure the water supply meets quality standards, avoid scale and corrosion of equipment.

- Periodic maintenance: Periodically clean the combustion chamber, steam generator, dust filter system and chimney to maintain heat transfer efficiency and prolong the life of the equipment.

- Choose a reputable supplier: Cooperate with experienced boiler suppliers, provide in-depth technical support and full warranty to ensure long-term investment efficiency.

7. Conclusion

Wood-fired boilers are an efficient, environmentally friendly energy solution and are suitable for the current green production trend. With the ability to use biomass fuel, this device not only helps businesses save operating costs but also contributes to reducing greenhouse gas emissions, protecting public health and the ecosystem.

However, to maximize the efficiency of wood-fired boilers, it is extremely important to pay attention to the exhaust gas treatment system as well as apply Biomass combustion technology. This is considered a strategic investment for the long-term, safe and sustainable development of the business.

As a leading unit in the energy industry in Vietnam, Thuan Hai Corporation is proud to accompany more than 500 domestic and foreign enterprises, successfully implementing more than 135 boiler projects to meet diverse needs in terms of capacity, cost and environmental standards.

If your business is looking for an efficient, economical and optimal wood-fired boiler solution, do not hesitate to contact us for the earliest consultation and support!