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CONSTRUCTION AND OPERATION PRINCIPLES OF CHAIN GRATE BOILER

Chain grate boilers are widely used in various industries such as food, textile dyeing, plastic, rubber, animal feed, sugar, paper, and packaging.

1. What is a Chain Grate Boiler?

- A chain grate boiler is a device that uses heat from combustion fuel to heat water into steam, serving production or power generation needs. The generated steam has a suitable temperature and pressure to meet the use requirements.

- The chain grate boiler has a combustion chamber with the highest mechanical automation level among fuel-grate/dumping chamber types. Its distinctive feature is the chain grate's endless movement and the fuel layer's simultaneous movement. However, the chain grate cannot be designed wider than 4.5m and longer than 8m to ensure operational conditions. This limits the maximum combustion chamber capacity to about 40MW, producing steam at 120 tons per hour.

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2. Structure of Chain Grate Boiler

- Fuel Feed System: Designed to be automatic or semi-automatic, including equipment such as loaders, fuel storage hoppers, bucket conveyors, conveyor belts, screw conveyors, etc. It is flexibly controlled. The fuel feed system may also be equipped with a mass flow meter to determine the fuel consumption for the boiler.

- Combustion Chamber - Boiler Body: Designed for thorough fuel combustion and efficient heat absorption for steam generation. Includes components such as the chain grate, combustion chamber, primary air fan, secondary air fan, radiant heat absorption tube bundle, and steam generation tube bundle.

- Water Heat Recovery System, Air Heat Recovery System: Utilized to harness heat from flue gas, increasing the boiler's efficiency.

- Dust Filtration System: Can use baghouse dust filtration, electrostatic precipitators, or Venturi wet scrubbers to treat boiler flue gas dust. These dust filtration devices ensure the boiler's flue gas meets the strictest environmental standards.

- Exhaust Fan and Smokestack: The exhaust fan draws flue gas from the boiler combustion chamber and pushes it into the smokestack, expelling it into the environment. The exhaust fan also maintains negative pressure in the combustion chamber to prevent combustion incidents, ensuring safety for operators and the boiler system.

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3. Operating Principles

- The combustion fuel is automatically metered and introduced into the boiler through the fuel feed system. The fuel hopper is located outside the combustion chamber area. Gravity ensures an even distribution of fuel on the grate, and then the fuel moves with the chain grate into the boiler's combustion chamber. During the chain's movement, different stages of the fuel combustion process occur along the length of the grate. The ash produced at the end of the combustion process is pushed into the ash hopper by an ash-pushing device.

- The combustion process (oxidation process) is a chemical reaction between fuel elements and oxygen, producing light and heat. The main oxidizing agent is O2 (taken from the air) supplied into the chain grate combustion chamber through airboxes. Fuel elements (C, H, O, N, S, etc.) undergo oxidation during combustion, producing smoke.

- As the combustion process proceeds, the released heat is transferred to the combustion chamber's steam tubes. Water in the steam tubes is heated to boiling point, producing steam. The steam-water mixture rises and concentrates in the steam drum. The steam drum is used to separate steam from the steam-water mixture.

- Unvaporized water in the steam drum is returned to the steam tubes through a downcomer system placed outside the wet wall (to avoid heat absorption). The water in the downcomer tubes is not heated, having a higher density than the steam-water mixture in the steam tubes, creating a closed-loop natural circulation.

- When the boiler's operating pressure approaches the limit or exceeds it, a circulation pump is used to assist water movement through the steam tubes. The steam exiting the steam drum is saturated, and if passed through a superheater to increase its temperature, it becomes superheated steam.

- The flue gas produced during combustion when passing through the boiler's flue gas flow channels, retains significant thermal energy. Therefore, the boiler system uses a feedwater heater and an air preheater to utilize this heat, avoiding energy wastage and ensuring the highest boiler efficiency. After passing through the dust filtration system, the flue gas is pushed out through the exhaust fan and expelled into the environment.

- Water supplied after degassing (removing O2 and CO2 dissolved in water) is pumped into the feedwater heater. Here, water is heated to the appropriate temperature and then moves into the boiler's steam drum. Note that the feedwater heater should not be designed too large to avoid steam generation in the feedwater heater, creating significant resistance in the water supply to the boiler.

- Outside air is drawn into the air preheater by a fan from the external environment to maximize the heat from the flue gas, at around 110-140°C. The flue gas temperature should not be too low to prevent corrosion of the smoke tubes due to condensation (oxidation of SO2 or NOx in the gas to form acid).

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4. Pros and Cons of Chain Grate Boiler

- Pros:

  • Chain grate boilers use a variety of combustion fuels, suitable for burning large-sized particles, up to 50mm.
  • Simple construction, easy to operate.
  • Efficient cooling of the chain grate, as only half of its length is in direct contact with the burning fuel during operation.
  • Easy to adjust the load as needed.
  • The boiler system operates in automatic mode, optimizing labor costs.

- Cons:

  • Chain grates are mechanical moving parts, making them prone to damage and requiring high maintenance costs.
  • Boiler efficiency is not high due to heat loss through ash.
  • Large-capacity boilers cannot be manufactured because the efficiency and combustion chamber are not guaranteed.

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