1.1 Grate firing refers to combustion of stationary fuel on the boiler grate, also known as fixed-bed combustion.

Boilers adopting grate firing include chain grate boilers, fixed grate boilers, vibrating grate boilers, reciprocating grate boilers, etc.

Currently, chain grate boilers are widely applied among industrial boilers.

1.2 Grate-fired boilers burn slowly. The combustion process shows obvious staged and layered features, generally divided into four phases: preheating and drying of fresh coal, volatile release and ignition, main combustion of fixed carbon, and burnout & slag formation.

Little or no oxygen is required for coal preheating and the final burnout stage, while abundant oxygen is needed for the central main combustion zone. Therefore, zoned air supply is adopted.

The front and rear sections feature low combustion temperature with low air supply and low wind pressure, prone to excess air.

The middle and rear sections have intense fuel combustion and high furnace temperature, requiring large air volume and high wind pressure, which easily leads to oxygen deficiency.

1.3 Fuel remains static on the grate; there is no relative motion between air and fuel, or between fuel and grate. The mixing of fuel and air and fuel heating conditions are poor.

Combustion progresses gradually from the upper layer to lower layer of the fuel bed, and from the fuel surface to coke core. Combustion is slow and requires long residence time.

1.4 When the feed coal has high moisture, poor coal quality and low furnace temperature, fresh coal is hard to ignite.

Large fuel lumps cannot be fully burned, so coke is discharged together with slag.

For over-dry pulverized coal, narrow gaps in the fuel bed result in poor air permeability. Coupled with low primary air pressure, air hardly penetrates the fuel bed and combustion becomes abnormal.

2.1 Poor coal adaptability

Only suitable for bituminous coal with high volatile content and calorific value above 5000 kcal/kg.

2.2 Low combustion efficiency

High heat loss from mechanical incomplete combustion. Especially when burning low-grade coal, slag has high carbon content and causes energy waste. For example, the low heating value of slag from some chain grate boilers can exceed 2500 kcal/kg.

2.3 Weak heat transfer inside furnace

Only simple radiative heat transfer is available. Furnace temperature is greatly limited by the grate; excessive temperature will burn out the grate. Boiler thermal efficiency is low, around 65% in general.

2.4 Severe coal leakage and air infiltration

Gaps inevitably exist on the grate, and coal feeding openings are oversized, leading to serious coal leakage and air ingress.

2.5 Bulky structure

Chain grate consumes a large amount of steel. The grate is prone to jamming, damaging the reducer and even burning out the motor.

2.6 High cost for desulfurization and denitrification

In-furnace desulfurization and denitrification as in fluidized bed boilers are unavailable. Flue gas emissions may fail national environmental standards, risking production shutdown amid tightening environmental regulations.

If external desulfurization is adopted, equipment investment rises sharply and increases the risk of unrecovered investment.

2.7 High carbon content in slag and fly ash

Due to low combustion efficiency, boilers often run under overload conditions. Carbon content ≥14% in slag and ≥30% in fly ash.

3.1 During combustion, the mixture of solid particles and gas flow in fluidized bed boilers exhibits fluid properties:

3.1.1 Fluidity

Bed materials inside the fluidized bed keep flowing continuously; stationary dead material is not allowed.

3.1.2 Fluid shapelessness

The fluid shape changes with air ducts and vessel geometry.

3.1.3 Uniform density

Material density is roughly consistent across the bed cross-section.

3.1.4 Uniform temperature

Temperature is basically uniform everywhere inside the fluidized bed.

3.1.5 Consistent liquid level height

The boiling height of bed materials, i.e., fluid liquid level, remains roughly the same.

3.1.6 Uniform pressure on the same cross-section

Pressure at each point on the same cross-section height inside the bed is basically consistent.

3.2 Intense fluidization motion of fuel particles in the fluidized bed brings the following combustion benefits:

3.2.1 Good mixing condition

Fuel and air mix well, and the whole outer surface of particles is wrapped by air.

3.2.2 Shorter combustion time

Mutual collision between particles easily peels off ash formed on particle surfaces during combustion, accelerating combustion and shortening burnout time.

3.2.3 Good coal ignition condition

Fresh coal mixes rapidly and moves with high-temperature bed materials upon entering the furnace, achieving favorable heating and ignition conditions.

3.2.4 Uniform combustion temperature

Temperature distribution inside the fluidized bed is nearly uniform. Combustion reaction is intense and consistent, unlike grate-fired boilers where intense reaction only occurs in the main combustion zone while heating and burnout zones have low temperature and slow reaction.

3.2.5 Low carbon content in slag

Slag generated from fluidized bed combustion is discharged periodically by operators based on burnout status.

Sufficient fuel residence time delivers high combustion efficiency. Carbon content in cooled slag of fluidized bed boilers is far lower than that of grate-fired boilers.

Volumetric heat release rate inside the furnace is also much higher.

3.2.6 Excellent load regulation capacity

CFB boilers feature outstanding load adjustment performance. The maximum stable load ramp-up rate reaches 7% per minute, and stable load ramp-down rate up to 12% per minute. With guaranteed circulation ratio, the minimum stable operating load can reach 25%.

3.2.7 Fast banked fire and startup

CFB boilers with adiabatic dense-phase zone can achieve instant banked fire, dropping load from 100% to 0 without any damage to boiler structure.

It facilitates emergency handling and simple maintenance. After banked fire, ultra-hot, hot and warm startups can be performed to quickly resume production and reduce outage losses.

3.3 Strong heat storage capacity

The dense-phase zone in fluidized bed serves as a large heat reservoir.

Under normal operation, newly fed coal only accounts for 3%~5% of bed materials. It is quickly enveloped by high-temperature bed materials and heated to normal combustion temperature for ignition.

Heat absorbed for heating fresh coal imposes little impact on bed temperature and generally does not disrupt normal combustion.

3.4 Wide fuel adaptability

3.4.1 Capable of burning various high-grade and low-grade fuels, such as high-quality and low-grade bituminous coal, anthracite, peat, unburned slag discharged from grate-fired boilers, etc.

3.5 Two-phase low-temperature combustion

The fluidized bed uses broadly sized particles of 0~8 mm. Fine particles are pneumatically conveyed out of the fluidized bed and burn in the suspension zone. Thus, simultaneous combustion occurs: coarse particles burn in the dense-phase zone in the lower furnace, while fine particles burn in the dilute-phase zone in the upper suspension section.

3.6 Staged air supply

Oxygen-deficient combustion is adopted in the dense-phase zone at bed temperature of 850℃~950℃ to minimize NOₓ generation. The upper oxygen-rich combustion zone ensures complete burnout of carbon monoxide produced by oxygen-poor combustion in the lower zone.

3.7 High boiler thermal efficiency

Combustion efficiency reaches up to 97% for anthracite and 98~99.5% for bituminous coal; boiler thermal efficiency exceeds 85~92%.

3.8 High-temperature high-efficiency separator with high separation efficiency

The high-temperature high-efficiency separator is the core component of CFB boilers. Our high-efficiency separator achieves over 99% separation efficiency with resistance around 700 Pa and no coking during operation.

3.9 Flue gas recirculation technology

A recirculation fan extracts part of flue gas and feeds it back into the furnace for flue gas recirculation. It improves flue gas distribution inside the furnace, prevents ash deposition and enhances heat transfer. It reduces flue gas volume, lowers excess air coefficient and raises boiler thermal efficiency.

3.10 High boiler heating load technology

Our air distributor area ensures full fuel combustion and sufficient high-temperature flue gas supply downstream, securing flue gas heat load and boiler heating load. CFB boiler heating load can reach 100%~120%, while chain grate boilers only reach 60%~70%.

3.11 Superior environmental performance

3.11.1 In-furnace desulfurization technology

With CFB combustion technology, high desulfurization efficiency can be achieved by feeding a relatively small amount of limestone powder. At Ca/S molar ratio of 1.5, more than 90% of SO₂ is captured. It features low cost and high desulfurization efficiency, ensuring raw SO₂ emission ≤100 mg/Nm³.

The operating cost of in-furnace desulfurization is less than 2 RMB per ton of steam.

3.11.2 In-furnace denitrification technology

Adopting two-stage low-NOₓ combustion at high and low temperature, staged air supply and low-temperature combustion minimize harmful components SO₂ and NOₓ in flue gas. Simple in-furnace denitrification can directly meet local industrial boiler NOₓ emission standard ≤200 mg/Nm³.

3.12 Low carbon content in slag and fly ash

Cyclone separators capture fly ash and return it to the furnace for further burnout, improving combustion efficiency and lowering carbon content in fly ash. Slag carbon content ≤2%, fly ash carbon content ≤4%.

3.13 Remarkable energy saving effect

Energy saving ≥30% compared with chain grate boilers.

3.14 Low exhaust gas temperature

According to production and boiler operating conditions, additional heating surfaces can be installed at the tail flue to increase load, reduce exhaust gas temperature, improve thermal efficiency and cut exhaust heat loss.

3.15 Long service life of heat transfer oil

The heat transfer oil heating surface is not in direct contact with high-temperature flame. Heat transfer oil is less prone to overheating decomposition and carbonization, ensuring safe operation of heat transfer oil.

3.16 Good comprehensive utilization of fluidized bed ash and slag

Ash and slag can be used for brick making, cement admixture, concrete aggregate, aggregate for plastic products, fertilizer production and so on.