What Is The Difference Between a Chain Grate Boilers And a Reciprocating Grate Boilers?

In industrial production, biomass boilers, as core equipment for energy conversion, play a key role in providing steam or heat energy for industries such as textiles, chemicals, and electricity. Their performance directly affects production efficiency, energy consumption, and environmental compliance. As the core component of the boiler combustion system, the type of grate is the key factor in determining the applicability of the boiler.

Chain grate boilers and reciprocating grate boilers are the two most widely used grate types in the industrial field. There are significant differences between the two in terms of combustion principle, fuel adaptability, and operating costs. This article will systematically analyze the structure, working principle, and core differences of the two boilers to provide a reference for industrial users to select equipment.

1. What Is a Chain Grate Boiler?

A chain grate boiler is a layer-fired boiler that uses a continuously moving ring chain as a fuel delivery and combustion carrier. Its core design concept is to achieve full automation of fuel processing from feeding to burning through the uniform movement of the grate, thereby ensuring the stability of steam output.

Structurally, the chain grate boiler consists of five core systems:

• Fuel supply system:including hopper, coal feeder and distribution device, responsible for spreading the fuel evenly on the grate surface;
• Chain grate assembly:composed of driving sprocket, driven sprocket, chain and grate plate, the grate plate is mostly made of cast iron or heat-resistant steel, and can withstand high temperature of 800-1000℃;
• Combustion chamber:a closed space around the grate, with water-cooled wall tubes arranged on both sides and superheater or convection tube bundle on the top;
• Flue gas treatment system: including economizer, air preheater, dust collector (bag or electrostatic type) and induced draft fan;
• Control system: monitors the grate speed, furnace temperature and steam pressure through sensors to achieve automatic adjustment.

Limited by mechanical strength and operational safety, chain grates have clear size restrictions: the width is usually controlled at 1.5-4.5 meters and the length is 5-8 meters, which makes the evaporation capacity of a single boiler mostly 10-120 tons/hour, and the rated working pressure is generally 1.25-3.82 MPa.

In industrial scenarios, chain grate boilers are widely used in textile printing and dyeing (requiring 1.0-1.6 MPa saturated steam), papermaking (requiring continuous steam for drying), food processing and other industries, especially suitable for burning bituminous coal, lignite, biomass molded fuel (such as wood chip pellets, straw briquettes) and other fuels with stable physical properties.

2. Working Principle of Chain Grate Boiler

The operation of chain grate boiler is a continuous process of multi-link coordination, and its core mechanism can be divided into the following steps:

Fuel transportation and pretreatment: After the fuel enters the hopper through the coal feeder, it is evenly distributed on the grate by the coal spreader, and the thickness is controlled at 100-150mm. As the chain grate moves at a speed of 0.5-2 m/min, the fuel first enters the preheating and drying zone (temperature is about 300-500℃), and uses the radiant heat from the furnace and the primary air (sent in from the wind chamber below the grate) to remove moisture (water content drops to below 15%).

Staged combustion process:

• Ignition zone:When the fuel moves to the middle of the grate, it is ignited by the high-temperature flue gas and the heat radiation from the grate, and the volatile matter (such as methane and hydrogen) burns first, and the temperature rises to 800-1000℃;
• Combustion zone:Fixed carbon burns violently, requiring a large amount of secondary air (sprayed from both sides of the furnace) to supplement oxygen, and the temperature can reach 1200-1400℃. The heat released in this area accounts for more than 70% of the total heat;
• Burnout zone:The remaining coke is burned out at the end of the grate, and the ash is discharged into the ash hopper through the slag remover. The unburned carbon content is usually controlled below 5%.

Heat transfer and steam generation: The high-temperature flue gas generated by combustion transfers heat in three ways:

• Radiative heat transfer: The high-temperature flame in the furnace directly heats the water-cooled wall tubes, and the working medium (water) in the tubes absorbs heat and partially evaporates;
• Convection heat transfer:When the flue gas flows through the superheater and the convection tube bundle, the heat is transferred to the working medium by flushing the tube wall;
• Heat conduction: The metal parts of the grate conduct heat to the bottom of the fuel layer to assist combustion.

The steam-water mixture enters the steam drum along the riser. After steam-water separation, the saturated steam enters the superheater and is heated to 300-450℃ (depending on the process requirements), and the water flows back to the water-cooled wall through the downcomer to form a natural circulation (a forced circulation pump is required when the pressure is ≥16 MPa).

Waste heat recovery and flue gas purification:

• After leaving the convection tube bundle, the high-temperature flue gas (about 600-800℃) first heats the boiler feed water (raised to 150-200℃) through the economizer, and then heats the combustion air to 200-300℃ through the air preheater, and finally the flue gas temperature drops to 120-150℃ (to avoid acid dew point corrosion);
• The purification stage adopts the "bag filter + desulfurization tower" combined process, and the particle emission concentration can be controlled below 30 mg/m³, and the SO₂ concentration meets the national standard requirement of 50 mg/m³.

3. What Is a Reciprocating Grate Boiler?

A reciprocating grate boiler is a layer-fired boiler that realizes fuel transportation, turning and combustion through the reciprocating motion of the grate. Its core feature is the use of alternating fixed grate plates and movable grate plates. The movable grate plates are driven by an eccentric shaft to make reciprocating motions (stroke 50-100mm, frequency 3-10 times/minute), pushing the fuel forward and enhancing the combustion reaction.

Structurally, the core components of a reciprocating grate boiler include:

• Reciprocating grate assembly:divided into several sections (usually 3-5 sections), each section is staggered with fixed grate pieces and movable grate pieces, the grate pieces are cast with high chromium cast iron or heat-resistant steel, and can withstand high temperatures of 1000-1200℃;
• Drive system:composed of a motor, a reducer, an eccentric wheel and a connecting rod, which controls the movement rhythm of the movable grate;
• Segmented air chamber:The air chamber is set under the grate according to the combustion stage, and the air volume of each area can be adjusted independently (such as less air in the drying area and more air in the combustion area);
• Combustion chamber and heat exchange system: The furnace volume is large (20%-30% higher than that of the chain grate boiler), which is convenient for high-humidity fuel combustion. The heat exchange surface layout is similar to that of the chain grate boiler, but the superheater area is usually larger.

The evaporation capacity of a single reciprocating grate boiler is mostly 6-60 tons/hour, with a rated pressure of 1.25-2.5 MPa. Due to its strong adaptability to fuel, it is widely used in biomass power generation (straw and rice husk combustion), waste incineration power generation, small-scale regional heating and other fields.

4. Working Principle of Reciprocating Grate Boiler

The combustion process of reciprocating grate boiler is characterized by stage and controllability, and its operating mechanism is as follows:

Fuel Layered Combustion Process:

• Feeding and drying: The fuel is fed into the front end of the grate through a screw feeder, and the reciprocating motion of the movable grate pushes the fuel into the furnace, and the water evaporates in the front section (temperature 300-400℃) (the moisture content drops from 40% to below 20%);
• Ignition and volatile combustion:After the fuel enters the middle section, it is ignited by high-temperature radiation, and the volatiles burn in an oxygen-rich environment. The turning of the movable grate loosens the fuel layer to avoid local hypoxia;
• Coke burnout: When the fuel moves to the rear section, the fixed carbon is fully burned under sufficient oxygen, and the thrust of the movable grate pushes the ash to the slag discharge port, and the unburned carbon content can be controlled below 3%.

Mechanical agitation and air mixing mechanism: When the moving grate moves forward, the fuel is pushed up and slides forward, and then the moving grate moves backward, and the fuel falls due to gravity and flips. This process can break up the fuel agglomerates, expose the fresh surface, and enhance the contact with oxygen. At the same time, the segmented air chamber adjusts the air volume according to the combustion stage (such as the excess air coefficient of 1.1-1.2 in the drying zone and 1.3-1.5 in the combustion zone) to further improve the combustion efficiency.

Steam generation and heat recovery: Similar to the chain grate boiler, the reciprocating grate boiler generates steam through radiation and convection heat exchange, but because the combustion is more intense (the furnace temperature is 50-100℃ higher than the chain grate), its heat transfer coefficient is higher (about 20-30 W/(m²・K)). The waste heat recovery system also includes an economizer and an air preheater, but because the fuel has a high ash content, a membrane wall structure is often used to reduce ash accumulation.

5. The Core Difference Between Chain Grate And Reciprocating Grate Boilers

5.1 Combustion Efficiency Comparison

Combustion completeness: The combustion efficiency of reciprocating grate boilers can reach 88%-92% due to sufficient fuel stirring and precise air distribution; the efficiency of chain grate boilers is mostly 82%-87% due to the relatively static fuel layer. Especially when the fuel humidity is greater than 25%, the chain grate is prone to "crusting", resulting in incomplete local combustion.

Ash characteristics: The mechanical stirring of the reciprocating grate makes the unburned carbon content in the ash less than 3%, while the carbon content of the chain grate is usually 5%-8% due to the thicker fuel layer (150-200mm).

Load adjustment capability: The reciprocating grate can quickly adjust the combustion intensity by changing the grate movement frequency (3-10 times/minute), and the load fluctuation range is ±30%; the chain grate needs to be adjusted by changing the coal feed and grate speed, and the response is slow, with a load fluctuation range of ±20%.

5.2 Differences In Fuel Adaptability

Chain grates have strict requirements on the physical properties of fuel (particle size, humidity, impurity content), while reciprocating grates can adapt to more complex fuel characteristics through mechanical turning and segmented combustion.

5.3 Environmental Impact Analysis

Pollutant emissions:

• Particulate matter (PM): Due to the full combustion of the reciprocating grate, the PM emission concentration is about 20-50 mg/m³; due to the small disturbance of the fuel layer, the PM emission of the chain grate is slightly higher (30-60 mg/m³), and more powerful dust removal equipment is required;
• Nitrogen oxides (NOx): The furnace temperature of the chain grate is relatively low (1200-1300℃), and the thermal NOx generation is less (150-250 mg/m³); due to the local high temperature, the NOx emission of the reciprocating grate is relatively high (200-350 mg/m³), and a low-nitrogen burner is required;
• Sulfur dioxide (SO₂): It mainly depends on the sulfur content of the fuel. The emission levels of the two are similar, and a desulfurization tower is required to be treated to less than 50 mg/m³.

Ash treatment: Chain grate boilers have a large amount of ash (about 15%-20% of the fuel amount), requiring a continuous slag removal system; reciprocating grates have a small amount of ash (10%-15%), and the ash is highly active (due to high combustion temperature), making them more suitable as building materials.

Environmental compliance: In areas where GB 13271-2014 "Boiler Air Pollutant Emission Standards" are implemented, reciprocating grates need to add denitrification devices to meet the standards, while chain grates can simplify the treatment process when burning low-sulfur coal.

5.4 Cost Comparison Analysis

Initial investment: Taking a 20 t/h boiler as an example, the chain grate boiler equipment investment is about 1.5-2 million yuan, and the reciprocating grate boiler has a complex mechanical structure and an investment of about 2-2.5 million yuan, which is 25%-30% higher.

Operation cost:

• Fuel cost: The reciprocating grate is highly efficient, and its annual fuel consumption is 5%-8% lower than that of the chain grate (calculated based on 8,000 hours of annual operation, it can save 100,000-200,000 yuan in fuel costs);
• Electricity cost:The motor power of the chain grate is relatively small (about 15-20 kW), and the reciprocating grate has a complex drive system, with a power of about 25-30 kW, and the annual electricity cost is 30,000-50,000 yuan higher;
• Labor cost: The chain grate is easy to operate, requiring 1-2 people/shift; the reciprocating grate requires a dedicated person to adjust the grate frequency and air volume, requiring 2-3 people/shift, and the annual labor cost is 50,000-80,000 yuan higher.

Maintenance cost: The chain grate mainly replaces the chain and grate pieces, and the annual maintenance cost is about 30,000-50,000 yuan; the reciprocating grate needs to regularly replace the active grate pieces, eccentric shaft bearings, etc., and the annual maintenance cost is about 80,000-120,000 yuan, which is 1.5-2 times higher.

5.5 Differences In Operation and Maintenance

Daily operation: Chain grate boilers are easy to operate, mainly monitoring the coal feed, grate speed and steam pressure, and are suitable for novice operators; reciprocating grates need to adjust the grate frequency and segmented air volume in real time according to the fuel characteristics, and require higher skills from operators.

Maintenance focus:

• Chain grate: Regularly check the chain tension (droop ≤ 20mm) and grate gap (≤ 5mm) to prevent deviation or jamming;
• Reciprocating grate: Focus on maintaining the drive system (bearing lubrication, connecting rod gap) to avoid active grate jamming (grate gap needs to be cleaned once a week).

Equipment life: The main life of the chain grate is about 10-15 years (grate slices are replaced every 3-5 years); the main life of the reciprocating grate is about 8-12 years due to more severe mechanical wear (active grate slices are replaced every 2-3 years).

6. Industry Application And Preference

6.1 Advantages Of Chain Grate Boilers

Textile and printing and dyeing industry: 1.0-1.6 MPa stable saturated steam is required. The continuous operation characteristics of the chain grate can ensure that the steam pressure fluctuation is ≤±0.05 MPa, meeting the temperature stability requirements of the dyeing process.

Papermaking industry: Pulping and drying processes require a large amount of steam (20-100 tons/hour for a single plant), and coal is mostly used as fuel (stable supply). The low maintenance cost of the chain grate can reduce the risk of downtime.

Chemical industry: high requirements for steam quality (superheated steam temperature fluctuation ≤±5℃), stable combustion of the chain grate, precise control of steam parameters, suitable for fine chemical production.

6.2 Advantages Of Reciprocating Grate Boilers

Biomass power generation industry: burning agricultural waste such as straw and rice husk (humidity 20%-30%, irregular shape), the fuel adaptability of the reciprocating grate can reduce pretreatment costs, and the efficiency is high, and the coal consumption for power generation is as low as 300-350 g/kWh.

Garbage treatment industry: when burning municipal solid waste (MSW), the mechanical stirring of the reciprocating grate can handle impurities such as metal and glass in the garbage, avoid grate jamming, and achieve garbage reduction (reduction rate ≥ 90%).

District heating industry: need to cope with load fluctuations (high during the day and low at night), the rapid adjustment ability of the reciprocating grate can reduce the number of starts and stops, and save 5%-10% energy annually.

7. Selection Decision Guide

7.1 Core Selection Factors

Fuel characteristics: uniform fuel shape, humidity <20% → priority chain grate; diverse fuel, humidity >25% or impurities → priority reciprocating grate.

Steam demand: stable steam volume (fluctuation < ±10%) → chain grate; large steam volume fluctuation (more than ±20%) → reciprocating grate.

Economic evaluation:

• Short-term project (<5 years) → chain grate (low initial investment);
• Long-term project (>10 years) → reciprocating grate (lower life cycle cost).

Site and environmental protection: small site (such as old plant renovation) → chain grate (compact structure); strict environmental protection requirements (such as Beijing-Tianjin-Hebei region) → reciprocating grate (easy to meet standards).

7.2 Selection Flow Chart

1. Determine the fuel type and characteristics (particle size, humidity, impurity content);
2. Calculate steam demand and load fluctuation range;
3. Evaluate the initial investment budget and project cycle;
4. Combine environmental protection standards and site conditions to narrow the selection range;
5. Conduct a life cycle cost analysis (LCCA) to determine the optimal solution.

8. Conclusion

Chain grate boilers and reciprocating grate boilers are not "substitutes" but "complementary choices" for different working conditions: chain grates are stable, economical, and easy to operate, suitable for continuous production scenarios with a single fuel and low cost; reciprocating grates are efficient, flexible, and highly adaptable, suitable for diversified scenarios with complex fuels and high environmental protection requirements.