27 January 2023

What Is CFB Technology | CFBC Boiler | Topic Covered -Topic 2. What Is CFB Technology | # 2

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What Is CFB Technology

Contents:-

1. History
2. General Advantages
3. CFB System
– Furnace
– Air Distributor
– Fuel/limestone feed
– Solids recycle device
– External heat exchanger (EHE)
– Air/gas/particle flow-path

 

1. History

  1. less than 20 years old
  2. Modern and mature technology to burn coal and other solid fuels
  3. More than 400 CFB boilers in operation worldwide
  4. Unit capacity: from 5MW to 250MW (electricity)
  5. General description:
    • Use fluidized bed principle;
    • Crushed (6~12 mm) fuel and limestone are injected into the lower furnace. The particles are suspended in a stream of upwardly flowing air that enters the bottom of the furnace through air distribution nozzles;
    • Balance of combustion air is admitted above the bottom of the furnace as secondary air;
    • Combustion takes place at 815~925°C, with uniform combustion condition in chamber;
    • Fine particle (< 450 microns) are elutriated upward in furnace with flue gas of 4~6 m/s.
    Particles are then collected by the solids separators and circulated back into the furnace.
    Individual particles may recycle anything from 10 to 50 times
    • Particles’ circulation provides efficient heat transfer to furnace walls and longer residence
    time for carbon and limestone utilization.

2. General Advantages

􀂾 Fuel Flexibility

􀂎 The relatively low furnace temperatures (815~925°C) are below ash softening temperature of nearly all fuels;
􀂎 Therefore, furnace design is independent of ash characteristics

􀂾 Low SO2 Emissions

􀂎 Limestone is an effective sulfur sorbent in temperature range of 815~925 °C
􀂎 SO2 removal efficiency: >95%

􀂾 Low NOx Emissions

􀂎 Very low NOx emission, thanks to
– Low furnace temperature
– Air staging to the furnace

􀂾 High combustion efficiency (even for difficult-to-burn fuels)

􀂎 Very long solids residence time in furnace due to re-circulation
􀂎 Vigorous solids/gas mixing in furnace
􀂎 These two compensate negative effect of low furnace temperature

3. CFB System

3.1 Furnace

􀂎 Operating at velocities corresponding to pneumatic transportation regime
• CFB furnace is high, in order to allow for a long residence time of the gas;
• Furnace cross-section is selected based on flue gas superficial velocity.
􀂎 Furnace enclosure is made of gas-tight membrane water-cooled walls
􀂎 Refractory to protect against corrosion & erosion
• Thin layer of refractory on all lower furnace walls;
• In dense bed, an ultra high strength abrasion-resistant low cement alumina refractory of 16~25 mm thick is applied over a dense pin studded pattern.
􀂎 Furnace temperature is precisely controlled by maintaining proper inventory
􀂎 Combustion Air
• Primary air: through distributor into furnace bottom, typically 60~70% of total air
• Secondary air: introduced through over fire nozzles & material injection points into the top of lower dense bed, 30~40% of total air. Several levels.
• Primary zone: the region below the lower secondary air level.
– The primary zone has reduced cross section to provide good mixing and promote solids entrainment at low load;
– Startup burners, fuel fed points & secondary ash recycle points located.

􀂎 Dense bed vs. Dilute bed (freeboard region)
• Dense bed: In the primary zone [~4.25 m/s];
– Design for uniform distribution & intensive mixing of PA & bed solids
– High mass-transfer rate provides intense combustion & sulfation;
– High heat capacity of the bed allows burning any kind of fuels;
– Sub-stoichiometric conditions there limit NOx formation;
– Special erosion-protection ways needed.
• Dilute bed: In the middle and upper furnace [~6 m/s], design for
– Used as a disengagement zone: most of material carried-over from bed can settle and return to the bed inside furnace
– Served as a post-combustion chamber: sufficient residence time for fuel burnout and sulfur capture
– Height required to burnout volatiles and to settle entrained particles
The former is often larger than the latter; so most of the entrained solids in freeboard region re-circulate within furnace
– High solid inventory for better heat transfer rates & sorbent reaction
Solids densities are relatively high (level of 10 kg/m3 gas): very good
for gas-solid reactions and for heat transfer
– Heat transfer surface of the enclosure walls
– Good mixing of secondary air and combustion products

• Transition between them is gradual.

3.2 Air Distributor

3.3 Fuel/limestone feed

3.4 Air/gas/particle flow-path

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