Chain grate boiler is a device that uses heat from burned fuel (coal, firewood, rice husk, biomass, …) to heat water into steam to serve production or electricity generation needs. Depending on the purpose of use, it creates a steam source with suitable temperature and pressure to meet the needs of use. Boilers can be divided into 3 types: energy boilers, industrial boilers, and civil boilers.

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Chain grate boilers are composed of the following main components:

a. Fuel supply system into the furnace:

The fuel supply system includes equipment such as excavators, fuel hoppers, bucket elevators, conveyor belts, screw conveyors, etc., which are arranged to be controlled flexibly, quantitatively and automatically to supply fuel to the boiler. In addition, the fuel supply system is also equipped with a mass scale to determine the amount of fuel consumed by the boiler.

b. Chain grate (combustion chamber) – Furnace body:

The fuel supply hopper is located outside the combustion chamber area. Thanks to the effect of gravity, the fuel is spread evenly on the grate surface, then the fuel goes along the chain grate into the boiler’s combustion chamber. During the movement of the grate, the stages of the fuel combustion process occur in turn and occupy certain areas along the length of the grate. The slag created at the end of the combustion process is pushed down into the slag hopper thanks to the slag scraper.

The combustion process of fuel is the chemical reaction between the chemical elements of the fuel and oxygen, which emits light and generates heat (combustion is an oxidation process). The main oxidizing agent is oxygen (taken from the air) supplied into the chain grate combustion chamber through the air boxes. The combustible elements in the fuel (C, H, O, N, S, …) are oxidized during the combustion process, the product after the combustion process is called smoke.

Khi quá trình cháy diễn ra, nhiệt lượng toả sẽ truyền nhiệt cho các dàn ống sinh hơi đặt xung quanh buồng đốt. Nước trong các ống của dàn ống sinh hơi được đun nóng đến sôi và sinh hơi. Hỗn hợp hơi nước sinh ra được đưa lên tập trung ở bao hơi. Bao hơi dùng để phân ly hơi ra khỏi hỗn hợp hơi nước. Phần nước chưa bốc hơi có trong bao hơi được đưa trở lại các dàn ống sinh hơi qua hệ thống ống xuống đặt ngoài vách ướt (để không hấp thụ nhiệt). Nước đi trong các ống nước xuống không được đun nóng, có trọng lượng riêng lớn hơn hỗn hợp hơi nước ở trong các dàn ống sinh hơi. Điều đó tạo nên sự chênh lệch trọng lượng cột nước làm cho môi chất chuyển động tuần hoàn tự nhiên trong vòng tuần hoàn kín. Nhưng khi áp suất làm việc của nồi hơi đạt trạng thái cận tới hạn hoặc tới hạn trở lên thì phải sử dụng bơm tuần hoàn để hỗ trợ nước chuyển động qua dàn ống sinh hơi. Hơi ra khỏi bao hơi là hơi bão hòa, nếu cần chất lượng hơi quá nhiệt thì đưa hơi này qua bộ quá nhiệt để gia nhiệt thành hơi quá nhiệt có nhiệt độ cao.

C. Chain grate boiler water and air recovery system

The exhaust fumes are generated during the combustion of fuel, after passing through the convection tubes of the boiler, the energy from this heat source is still very large. Therefore, the boiler system must use a feedwater heater and a feed air heater to take advantage of this heat source, avoid wasting energy and ensure the highest efficiency of the boiler.

Water from the feedwater tank after being deaerated (removing dissolved oxygen (O2) and carbon dioxide (CO2) in the water) is pumped into the water heater. Here, the water is heated to a suitable temperature, then continues to move into the boiler’s steam drum. However, the water heater should not be designed too large, to avoid the unwanted situation of water generating steam in the water heater, creating great resistance on the water supply line to the boiler.

Chain grate boilers are the type of combustion chamber with the highest level of mechanization among the types of combustion chambers that burn fuel in layers. The characteristic of this type of combustion chamber is that the chain grate moves endlessly and the fuel layer moves simultaneously with the grate.

Air from the outside environment is supplied by the fan into the air dryer to maximize the heat of the exhaust smoke brought out, the smoke after passing through the air dryer is about 110 -140oC. The exhaust smoke temperature should not be too low to avoid the problem of chimney corrosion due to the phenomenon of condensation in the smoke (SO2 or NOx in the smoke is oxidized into acid).

d. Dust filtration system:

The dust filtration system can use a bag filter, electrostatic dust filter or wet filter tower to treat dust for the boiler. These dust filtration devices will filter and separate dust particles in the exhaust smoke, ensuring that the exhaust smoke when exiting the chimney satisfies all parameters of environmental standards.

e. Exhaust fan and chimney:

The exhaust fan is a device that sucks smoke from the boiler combustion chamber, and at the same time pushes the smoke into the chimney and releases it into the environment. The exhaust fan is also a device that maintains negative pressure in the combustion chamber to avoid positive furnace accidents, which are dangerous for the operator and the boiler system.

Advantages of chain grate boilers:

Suitable for burning large-sized fuels, with particle sizes up to 50mm, diverse fuels;

Simple structure, easy operation;

Good heat dissipation of the chain grate, because during operation only half of the length of the chain grate is in direct contact with the burning fuel;

Easy to adjust the load according to demand (in case of low steam load);

The boiler system operates in automatic mode, requiring little operator labor;

Types of fuel that can be used for chain grate boilers:

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