Circulating Fluidized Bed Combustion

Chungen Yin CHY@iet.auc.dk

CHY

CIRCULATINGFLUIDIZED BEDCOMBUSTION(CFB)BOILERS

Agenda

Topic 1. What Is Fluidization?Topic 2. What Is CFB Technology?Topic 3. Solids Separator of CFB.Topic 4. Why Is CFB Clean?Topic 5. Why Select CFB?Topic 6. How Build CFB?Appendix: Limitations Of CFB.

CHY

Topic 1: What Is Fluidization?

ISSUES:

1. Principles of Fluidization- Incipient fluidization- Bubbling bed- Slugging flow- Turbulent regime- Fast fluidization

2. Classification of Particles

CHY

CHY

1. Principles of Fluidization

Figure 1. Different stages of fluidization

Incipient FluidizationFluidization is a process in which a bed of particles is converted to afluid state by means of an upward flow of gas (or liquid);When a fixed bed of particles is exposed to upward gas-flow, individualparticles gradually tend to move apart and bed expansion becomesnoticeable (expanded bed). Pressure drop increases with rising gas flow;At high airflow rates, a point is reached at which pressure drop becomesequal to bed weight, which enters into a state of incipient fluidization

CHY

With air flow increasing, a bed of solids material will experience following stags:

Fixed Bed Expanded Bed Incipient Fluidization Bubbling Bed

Slug FlowTurbulent RegimeFast Fluidization(Pneumatic Transportation)

Bubbling BedAny additional airflow causes (inherently unstable) rising bubbles in bed• The bubbles originate at distributor, detach from it, rise and inflate, merge or

split, and eventually reach the surface of the bed.• Distributor is a device that supports the bed & distributes airflow into the bedRising bubbles brings about a steady circulation of bed material• push aside particles and aspire them in their wake;• result in a thorough and steady mixing of bed materials;• At sufficiently high bubbling rates, floating light or settling dense material are

spread homogeneously throughout the bed.A high rate of heat transfer in the bed is achieved• The bed temperature tends to be strictly homogeneous in any case.A high mass exchange between gas and fuel particles is also achievedProbably, gas short-circuiting sometimes occurs to a certain extentA bubbling bed resembles a boiling liquid, such as:• The horizontal boundary between the fluidized bed and air phase above it;• Fluid-like features of the bed, which can flow out of a hole or over a weir;• The principle of communicating fluidized vessels;• Light materials to float and dense materials to sink to achieve separation;• The hydrostatic pressure, which rises with the depth in the bed;• The steady but erratic movement of individual particles

CHY

Slugging FlowFB is operated using a gas velocity well above incipient fluidization• In fluidized state, pressure drop no longer increases significantly with gas flow.

When gas flow further increases, slugging may occur.• slugging is a situation, occurring frequently in narrow slender beds;• bubbles grow in size and coalesce so that they cover entire bed cross section;• the slugs push bed material upwards until it rains through them,

temporarily breaking up the slug;• slugging regime is undesirable, because

it is accompanied by erratic pressure shocks and a rather poor gas/particle contact

Turbulent RegimeWhen gas flow increases further, the bed is termed turbulent in a riser

Fast FluidizationWhen gas flow increases even further, the bed is termed fastCFB operates at velocities, corresponding to pneumatic transportation

CHY

2. Classification of Particles

Geldart's classification

- Homogeneously fluidizable powders;- Powders fluidizable with a bubbling regime, e.g. sand; - Cohesive powders, difficult to fluidize, e.g. cement and fly-ash;- Pellets, fluids;- Others able with a bubbling regime, e.g. plastic pellets, corn.

CHY

Topic 2: What Is CFB Technology?

ISSUES:

1. History2. General Advantages3. CFB System

- Furnace- Air Distributor- Fuel/limestone feed- Solids recycle device- External heat exchanger (EHE)- Air/gas/particle flow-path

CHY

1. History

less than 20 years old

Modern and mature technology to burn coal and other solid fuels

More than 400 CFB boilers in operation worldwide

Unit capacity: from 5MW to 250MW (electricity)

General description:• Use fluidized bed principle;• Crushed (6~12 mm) fuel and limestone are injected into the lower furnace. The particles aresuspended in a stream of upwardly flowing air that enters the bottom of the furnacethrough 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 residencetime for carbon and limestone utilization.

CHY

CHY

2. General AdvantagesFuel Flexibility

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

Low SO2 EmissionsLimestone is an effective sulfur sorbent in temperature range of 815~925 °CSO2 removal efficiency: >95%

Low NOx EmissionsVery 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-circulationVigorous solids/gas mixing in furnaceThese two compensate negative effect of low furnace temperature

CHY

3. CFB SystemFurnace part5 – Primary air inlet4 – Air distributor8 – Hot gas generator3 – Coal-limestone feeder6 – Secondary air nozzle14 – Bed drain pipe12 – Water wall

Solids recycle device

External heat exchanger (EHE)9, 10 – EHE (not-necessary in CFB)7 – Fluidized air13 – Circulator (moving bed)

• Air Distributor

• Fuel/limestone feed

• Air/gas/particle flow-path

FurnaceOperating 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 overfire 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 mixingand promote solids entrainment at low load;

- Startup burners, fuel fed points & secondary ash recycle points located

CHY

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 entrainedsolids in freeboard region re-circulate within furnace

- High solid inventory for better heat transfer rates & sorbent reactionSolids densities are relatively high (level of 10 kg/m3 gas): very goodfor 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.

CHY

CHY

Air DistributorLocated at the bottom of the furnace

Having dual functions• Support the bed;• Distribute the primary air

An example (right)• Distributor with bubble-caps nozzles• Designed to

- Distribute air uniformly;- Prevent back-sifting of solids at low load- Create good turbulence for fuel/sorbent mixing in primary zoneBig bed-drain pipes in distributor, designed to,

• Drain some bed-materials on regular to maintain proper inventory in the bed;• The inventory can be indicated by pressure-drop across dense bed;

it can affect the bed temperature and thus the furnace temperature.

Fuel/limestone Feed

One of major challenges• high fines & moisture

Over-bed feed option• commonly used manner

Fuel feed points• normally in front & rear walls• number of points:

achieve even fuel distribution

CHY

CHY

Air/gas/particle flow-path

PA blowers Pre-heatersFreeboard

SA blowers Pre-heaters

Distributor Dense bed

Upper dense bed & lower dilute bed

Solids separatorRear passBaghouseor ESPStack

Fuel & limestonefeeders Separator Rear pass Most of the remaining

Finer solids are collectedBy Baghouse or ESP

Stack(with few finestparticles)

Dense bed Freeboard

Part of solidsFall down alongFurnace wallUnder gravity

Most of solids are collectedAnd returned back to dense bed

A few are collected by the ash silounder the rear pass

Topic 3: Solids Separator of CFB

ISSUES:

1. Solids Separator

2. Cyclone CFB- Hot Cyclone CFB- Cold Cyclone CFB- Compact Cyclone CFB

3. IR-CFB (Internally Recycled)

4. Comparison: Cyclone-CFB vs. IR-CFB

CHY

1. Solids SeparatorOne of the most important key components in CFB• The main distinguishing feature of a CFB boiler is the separator.

Located at the furnace gas outlet

For collecting bed material entrained in flue gas and return them back to bed• Bed material contains fuel ash, unburned fuel, utilized & unutilized limestone;• Collection & re-circulation results in excellent fuel burnout & limestone utilization

Two mainstreams of separators: cyclone type vs. impact-separator• Cyclone: the most commonly used separator in industry by now;

having high separation efficiency;immediately following the furnace along gas path;separating solids from gases which have left the furnace;Cyclone CFB boilers.

• Impact-Separator: a two-stage solid separation system;1st stage being an impact-type solids separator located at furnace exit;Majority of solids collected by it are Internally Recycled within furnace;IR-CFB boilers.

CHY

CHY

2. Cyclone CFBHot-Cyclone

Use plate/refractory as inner linerOperate at high temperature, ~850 °CMain problems

Mechanical erosion: at cyclone inlet• High temperature (~850) • High particle concentration (~10) • High gas velocity (~25)• Severe attack angle

Chemical wear:• result from fuel alkali & sulfur;• chemical reaction with refractory

Thermal wear:• from rapid temperature change• result in cracking or spalling

Two thick layers of refractory coverage• an insulating layer (~20cm)• an abrasion-resistant layer (~12cm)

Result in high thermal inertia of system;long start-up or cool-down period;high operation costs

CHY

Cold-Cyclone CFBFormed from steam-cooled tubing;Aimed at reducing refractory maintenance;Features:• Refractory can last years due to low surface temperature (~550°C);• Having one layer of abrasion resistant over the cyclone tubes;• Providing minimum operating costs due to less refractory maintenance & heat

loss from the shell to ambient;• Having a somewhat higher capital costs

CHY

Compact-Cyclone CFBAn improved CFB design• Developed since late 1980s• 1st unit commissioned in 1993

Design features• Flat walls instead of curved walls• Gas & entrained solids from furnace

come in through a tall, narrow openingin its center & out through two outletsin its roof

• Its inlet relative to its outlet imparts aswirl to flow, causing solids separationjust as if separator walls were curved

• Its walls formed from panels of tubingcooled by water or steam

• Inner walls covered with thin refractory• Outer walls covered with insulation &

lagging, to reduce heat loss• Low operating costs at a reduced

capital cost• Combine advantages of hot- & cold-

cyclones

CHY

Demonstration

One CFB, with the largest compact separator (deep 7.3 m), is in successful operation in Indonesia

CHY

3. IR-CFB (Internally Recycled)Developed since 1990Two-stage solid separation• 1st stage: an impact-type solids separator, i.e., an array of U-beams,

- located at furnace exit & collect most of the entrained solids in gas• 2nd stage: located in lower gas temperature region of boiler convection pass- using a mechanical dust collector (MDC) or the first field of an ESP;- collecting the finest fraction of circulating solids.

CHY

In-furnace U-beams• Collected solids returned directly to bed along furnace rear-wallExternal U-beams• Collected solids returned first to a storage hopper, then to the bed along rear-wall

Scheme of U-beams Separator

CHY

Design TemperatureContinuous Operation, °CHigh alarm, °C

~ 825~ 855

Beam SizeDepth, mmWidth, mmLength, mm

11~ 176~ 160As required

Material Stainless steel

Rows of U-beamsIn-Furnace U-beamsExternal U-beams

112 (Generally)~ 7 (Depending on fuel fired)

CHY

ESP

1st & 2nd field

Heatexchangers

In-furnaceU-beams

ExternalU-beams

118

100

18

75% 90%

97.5

2.5 0.2

0.1

2.2

Solids recycle flows in IR-CFB (Denoted with )

Two-path model in furnace• Upward in furnace-center;• Downward near-wall.

1st stage: U-beams • Collect most of solids;• Solids Internally-Recycled

into bed along rear-wall.

2st separation stage• Collect & return the finest

solids that can not becaptured by U-beams;

• Only a small amount.

CHY

Most of the entrained solids areseparated by U-beams at furnaceexit, returned internally to lowerfurnace by gravity, falling as acurtain along rear furnace wall.

In lower furnace, these solids arere-entrained by PA & SA, andcarried back to furnace exit.

This intensive solids back-mixingprovides uniform distribution &optimum residence time.

Finer solids, not collected by U-beams, are carried by the gasthrough convection pass;then collected by secondaryseparator; and re-circulated tothe lower furnace.

CHY

4. Comparison: Cyclone-CFB vs. IR-CFB

Boiler Feature Hot-Cyclone Cold-Cyclone Compact-cyclone IR-CFB

SeparationSystem

Single-stage(100% efficiency for particles of d>100 micron;50% efficiency for particles of d=40~60 micron)

Two-stage(100% for d>80 micron50% for d<20 micron)

Upper furnacedensity, kg/m3 8~11 5~8 8~11 11~16

FurnaceTemperatureControl

Controlled in designusing heat exchanger;Or pre-determinedby boiler design

Controlled by coldash recycle rate. Temperature spanacross furnaceheight is up to 100°C.

Same ashot cyclone CFB

Designed value can be controlled thin +/-5°C interval by adjusting secondary solids recycle rate.

RefractoryThicknessCov. Area

~75 mm ~75 mm ~50 mm 15~50 mmLower furnace, cyclone, recycle loop

Entire furnace, cyclone

Same as Hot-Cyclone CFB

Lower furnace, U-beam zone enclosure walls

Hot-temperatureexpansion joints 3~5 / cyclone None NoneDepend on

arrangement

CHY

(Continued)

Boiler Feature Hot-cyclone Cold-cyclone Compact-cyclone IR-CFB

Furnace velocity 4.9~5.5 m/s

Furnace exitvelocity, m/s

22~26 -- 6.4~9.8

High-pressure air Required forJ-valves

1.5~2.0 1.0~1.5 ~1.5 ~1.0 (1st & 2nd stages)

Auxiliary powerconsumption Higher Moderate LowerHigher

4.0~4.5 m/s 4.9~5.5 m/s 4.9~7.3 m/s

22~26

Required forJ-valves

Required forsiphons Not Required

Total pressuredrop acrossseparator, kPa

Topic 4. Why Is CFB Clean?

ISSUES:

1. SO2 Removal

2. NOx Reduction

3. CO & Other PICs (Products of Incomplete Combustion)

4. Particulate Emissions

CHY

1. SO2 Removal

Sulfur capture in CFB process is achieved by adding limestone

Limestone: >95% CaCO3; and a few impurities such as MgCO3

Chemical reactions involvedCaCO3 ------> CaO + CO2 (endothermic reaction: -425 kcal/kg CaCO3)

CaO + ½ O2 + SO2 ------> CaSO4 (exothermic reaction: +3740 kcal/kg S)

Influenced by factors, such as• Sulfur content• Ca/S molar ratio (typically 2~2.5)• Limestone reactivity• Furnace temperature• Gas and solids residence time• Limestone particle size.

Bed temperature in CFB [815°C to 925°C] is ideal for removal of SO2/SO3 by limestone, more than 90% SO2 can be removed from CFB.

CHY

2. NOx ReductionNOx formation

N2 + O2 ------> 2 NO & 2 NO + O2 ------> 2 NO2

Main NOx sources• Fuel NOx: from N in fuel• Thermal NOx: from N2 in combustion air

Influenced by factors, such as• Furnace temperature• Excess air• Bed stoichiometry• limestone feed-rate• N & volatile matter in fuel

Low Temperature & Staging Air lead to a very low NOx emission• At T < 1500°C, thermal NO can be ignored; fuel NOx production is also limited• 60%~70% air pass distributor into dense bed: sub-stoichiometric / fuel-rich

condition limits NOx production.

Very low CFB-process NOx: typically lower than 150ppm

Further NOx reduction (~50%): by easy NH3 injection before/after furnace exit

CHY

3. CO & Other PICsPICs, normally including• CO;• PCDD - polychlorinated dibenzo-p-dioxins (often referred to as dioxins);• PCDF - polychlorinated dibenzofurans (often referred to as furans);• VOC - volatile organic compounds;• PAH - polycyclic aromatic hydrocarbons

Produced due to incomplete combustion, affected by one or all of• Overall low and/or fluctuating temperatures;• Uneven temperature distribution across the combustion chamber;• Inconsistent fuel feed;• Insufficient turbulence;• Insufficient residence time.

In practical combustion, PICs produced most probably due to• Poor furnace design

where combustion gases follow “cool” path avoiding the turbulent hot zones;• transient upset conditions, often caused by fuel heat-value / feed-rate fluctuations

It causes rapid devolatilization of fuel, depleting or lowering local O2 in furnace

CHY

A delicate tradeoff between complete combustion and low emissions• Too high temperature, too much O2, NOx will increase largely;• Time, Temperature & Turbulence: the 3T’s of combustion to achieve this balance.

Widely accepted that CFB operates at low levels of PICs, due to• Strong turbulence in CFB provides ultimate mixing, helpful for carbon to form CO2;• Uniform & complete combustion feature in CFB;• CO level is used often as an indicator of other PICs.

CHY

4. Particulate Emissions

Very low, believed less than the level of 20 mg/Nm3

• Most of particles are collected by high-efficiency separators & returned to the bed;• Few fine particles remaining in gas are further removed by bag-house (or ESP)

CHY

Topic 5. Why Select CFB?CHY

Fuel Size

PC plays a major role in electricity generation worldwide, issues needed to face:fuel in-flexibility, environmental aspects & higher maintenance costs, etc.

Description CFB Boiler PC Boiler Benefit of CFB

6-12 mm >70% of < 75 micron Lower crushing cost

Fuel Range(ash & moisture) Up to 75% Up to 60% Wider range of fuels

High S Fuel (1~6%) Inject limestone Need FGD plant Cheaper SO2 removal

Auxiliary Fuel(oil or gas) Up to 20~30% Up to 60% Consume less oil/gas

CombustionTemperature, °C 840~900 Lower NOx formation

& Easy SO2 removal1350~1500

Fuel Residence Time Compensate low T effectLow (~5 seconds)High (tens of s)

CHY

Combustion Efficiency

Description CFB Boiler PC Boiler Benefit of CFB

Good (may >98%) Better (normally >99%)

Heat Transfer Coeff. Higher (double PC) Lower Compacter furnace

NOx, ppm<200 <250 with FGD Lower emission, cheaper

Boiler Efficiency, % High A little higher

Auxiliary Power Use Slightly higher If FGD used, CFB lowerLower

Capital Cost8~15% higher with FGD & SCR5~10% lower without FGD&SCR5~10% higher

(Continued)

SO2, ppm<100 <100 with SCR No SCR or SNCR

Operation & Maintenance cost Due to less moving units5~10 % higher5~10% lower

8~15% lower

Topic 6. How Build CFB?

ISSUES:

1. CFB Design Considerations

2. Furnace Design Considerations

CHY

1. CFB Design Considerations

Furnace design;Separator design;Recycle system design;Fuel and limestone feed system design;Auxiliary equipment selection;

CHY

2. Furnace Design Considerations

Superficial fluidizing velocity;Bed temperature;Furnace exit gas temperature;Furnace aspect ratio;Furnace height;Furnace gas residence time;Primary air / secondary air split;Suspension density and pressure differential;Furnace heating surface to control bed temperature;Solid circulation rate (or solid suspension density);Excess air;Over-fire air nozzles;Furnace pressure parts, including:• distributor nozzles arrangements;• circulation, and so on.

CHY

Limitations of CFBCHY

Difficult for Large-Scale UsePrimarily intended for utilization of low grade, low volatiles, high ash coalsthat are difficult to pulverize in smaller capacity units.Difficult to scale up to 700~1000MWe range• mainly because of large number of feed points it requires to ensure uniform

distribution of the coal in the bed.Most CFB units in operations: 250~260MWe; Newly built: up to 300MWe

Maximum Particle Size limited to 300mm

Relatively high pressure-drop required to form fluidization

Fluidized bed regulation and control are not straightforward

Possibility of sintering of bed material limits operating temperature

Limited operating experience with fluidized bed combustors

Overall carbon conversion efficiency is a little lower than PC.Thermal efficiency is a little lower than PC, by ~3% for ~150MWe units.