Traditional Fuel Firing systems

CFBC Boiler and Types

Advantages of CFBC boiler

Environmental friendliness of CFBC

NOx Control

SOx Control

Potential Failure problems and Prevention.

The Traditional modes of burning solid fuels like coal or lignite are

Static Mode

Suspension Mode

Mass burning (MB) and traveling grate (TG) stokers burn solid fuel in static mode with the fuel resting on a grate.

Most Traditional power plants use pulverized fuel (PF) firing which burns the fuel in suspension mode in transport condition.

Between these two extremes of burning, as static and suspension modes, the intermittent one is the fluidized mode.

PF firing Technology has a long history, the roots way back to 200 years, when it is started with Cement industry and next by steel industry. Then Finds its way to power generation.

PF firing technology shifted the Power generation from 15 MW to as high as 1300 MW in early 1970.

When a solid fuel such as coal is reduced to the consistency of talcum powder and fired in an open furnace, the resulting combustion is almost equal to oil or gas firing—in

Speed

controllability, and

Heat release.

A coal particle burns out between 1 and 2 s, depending on its volatile content, similar to oil, and the combustion is most complete at >99% carbon burn up.

For high-volatile matter (VM) coals and lignites, the combustion efficiency can be as high as 99.7%.

The limitations relate to the inability of PF to

Deal with variation in fuel without risking fouling and slagging of boilers

Provide multi-fuel firing

Fire fuels with >40% total moisture unless there is enough VM such as in brown coals

Burn very low-volatile fuels such as petcoke

Combustion is very poor in fuels with

Gross calorific value (GCV) <2000 kcal/kg

A burden >65% (ash and H2O)

Very high S

Have step less load turndown of more than —70 to 100%

PF firing has been the preferred method for solid fuel firing in large quantities for utilities since last 8 decades.

However, in the last few years, circulating fluidized bed combustion (CFBC) boilers have begun to disturb this equilibrium by offering reliable solutions in the areas not served well by PF firing.

The limitations of PF boilers were recognized decades ago. In the absence of a better alternative, they have been accepted.

Principles of Operation

Types of FBC

Advantages and

Limitations

With the advent of the CFBC technology, remedial measures

have since been found to some extent for the limitations of

PF boilers. Circulating fluidized bed combustion boilers

Can address all the above issues with PF firing

Are nearly as efficient for conventional fuels

Are more operator friendly, with very few moving parts and

controls

Have lower O&M costs if erosion issues are not encountered

Offer better environmental friendliness and ensure against

emerging requirements

Present little danger of explosion

Traditional methods of firing failed to address the emerging emission norms early and late 70’s in a cheaper way and addressed these with a secondary gas cleaning system

The more and more stringent pollution limits, pushed the cost of secondary gas cleaning system to a new height. FGR – Flue Gas recirculation or

SCR – Selective Catalytic reduction for NOx

FGD – Flue Gas De-Sulphurisation for SOx

Pollution limits of SOx and NOx Played a vital role in a new way of combustion.

A cleaner and cooler combustion was the only way to meet strict levels of SOx and NOx.

Fluidized bed combustion (FBC) is

burning of various solid fuels in the

fluidized state—a condition where a gas-

solid mixture behaves like a free-flowing

fluid.

With the right proportioning of air

pressure and the proper sizing of fuel,

the air-solid mixture behaves like a fluid.

The fluid bed experiences progressively

more turbulence, as air velocity is

increased.

FBC is the combustion in this state—

bubbling FBC (BFBC) at the lower end

and circulating FBC (CFBC) at the higher

end.

Based on the fluidized region of operation, the FBC boilers emerged in following types

BFBC or AFBC – Bubbling/Atmospheric Fluidized bed combustion boilers

CFBC – Circulating fluidized bed combustion boilers

PFBC – Pressurized Fluidized bed combustion boilers

In the 1980s and early 1990s, both technologies were developed simultaneously, BFBC boilers in the United States and CFBC boilers in Europe. Both shared the fluidization principle. As the contours and the limits of the technologies grew sharper, it became clearer that they were more complementary than competing.

The classical CFBC Boiler operates at the higher end of the fluidized bed regime, just lower than the transport phase. It includes a fluid bed expanded all the way to the roof of the combustor instead of restricting the bed to the lower part as in a BFBC boiler.

In the Classic 3 Ts of perfect combustion

Time

Turbulence

Temperature

PF fired boilers makes the combustion more efficient by keeping all the 3 Ts in optimized condition.

With resident time of 1-2 seconds, more turbulence and very high temperature burning causes the combustion more effective.

Temperature

Turbulence Time

As the FBC operates lower temperature range, from 800°C to

950°C., where as the PF fired boilers operates more than

1200 to 1500° c.

The Negative effect of the lower temperature of FBC range of

boilers in Combustion (3 Ts) has been over come by

increasing the other 2 Ts, Turbulence and resident time.

The Combustion efficiency in fact increased drastically

because of high turbulence in the bed and longer residence

time, despite being operated in lower combustion

temperature.

High fan power for fluidization reduces net output per unit

fuel by 1% compared to PF, if deNOx and deSOx units are not

large enough or are absent.

Tube and refractory erosion issues are not fully resolved in

CFBC Boilers.

Single unit sizes of 1000 MWe and above are proven in PF,

whereas in CFBC, units >300 MWe are still under initial

operation.

Developed in 1970s and commercialized in 1980s in Europe

Pulverized fuel boiler supremacy has been challenged

seriously after its reign of more than half a century by the

CFBC boiler.

CFBC boilers replaced PF fired boilers literally under 100

MW category.

Besides the expensive hot cyclone, the second-generation designs such as cold cyclone, compact, and U-beam are available in industrial range.

For large utility boilers, full — circulation types give a more compact arrangement. The expanded bed designs can also meet the utility boiler requirements.

Present-day designs can be broadly categorized as follows:

1. Full circulation

A. Hot cyclone design

B. Compact design

2. Expanded bed

A. Cold cyclone design

B. U-beam/no-cyclone design

Classic CFBC boiler PF fired Boiler

CFBC boilers are gaining widespread use at least in the sub-100 MW sizes, due to

Fuel flexibility-Capability to burn almost any fuel

Excellent multi-fuel flexibility

High combustion efficiency

Environmental friendliness

In situ and very convenient desulfurization

Very low NOx generation

Low O&M costs

No slagging and fouling of tubes

Good to excellent load response

Simpler ash handling

One of the major advantages of a CFBC boiler is its fuel flexibility, ability to operate in vast range of fuels. The Fig. shows the fuel range in which the CFBC can operate.

Heat Release (MW/H)

Evaporation (TPH)

100 200 300 400 500 600 1800

DeNOx and DeSOx

Or

De-nitrification and De-Sulfurization

In FBC boilers operating at ~850°C, there is an inherent lower production of NOx auto­matically, as the combustion temperature is low.

Even In PF boilers, Flame temperature reduction from 1480-1500 to 1250° C will impact NOx generation by 10 fold.

These boilers also offer a very convenient way of reducing the SOx emission within the furnace enclosure by the reaction with lime stone

This desulfurization is adopted only for coals and other solid fuels with medium to high sulphur.

CFBC Boilers keeps an Upper hand on environmental friendliness with cheaper way of DeNOx and DeSOx capabilities.

Nox Formation

NOx Types

De-Nitrification - DeNOx

During Combustion in Furnace, At elevated temperatures, oxygen combines with nitrogen to form nitrogen oxides and other complex compounds collectively called NOx.

Nitrogen oxides are of environmental concern because they initiate reactions that result in the formation of ozone and acid rain, which can cause health problems, damage buildings, and reduce visibility.

NO reacts to form NO2, which reacts with other pollutants to form ozone (O3).

Three Kinds of NOx formed during Fuel Combustion

Fuel NOx

Prompt NOx

Thermal NOx

Nitrogen is present in fuel and combustion air.

Fuel NOx forms when Nitrogen in fuel reacts with oxygen in combustion air.

These fuel bound Nitrogen accounts for 50% of total NOx emission from coal and oil

combustion.

Prompt NOx results when fuel hydrocarbons break down and recombine with

nitrogen in air (this reaction generally takes place before the flame tip).

Accounts for 15-20% of total NOx Emission.

Thermal NOx forms when Nitrogen in air reacts with Oxygen along with intense

heat. These kind of Nox Rate of formation increases,

Exponentially with Temperature

And Directly Proportional to Oxygen (O2) concentration.

Much higher temperatures of >1200°C (2200°F) are needed to

Form Thermal NOx, which are fortunately not feasible for FBC.

As all the FBC boilers generally operates at 800° C to 950°C.

In PF fired boilers which operates in this temperature Range the

NOx emission will be generally higher than CFBC due to higher

combustion Temperature.

Thermal NOx <2000 mg/NM3with Normal burners

NOx 600 mg/Nm3 with low NOx burners

Low NOx is achieved in PF boilers with Low NOx burners with

lower flame temperature and better air staging

Fuel NOx is formed in FBC boilers also as equal to PF, the analysis results data says that, almost all the nitrogen in fuel may be converted to NOx.

For 1% of Nitrogen in the fuel, the possible potential NOx emission will be approximately 3800 Mg/Nm3.

This Fuel NOx then, Largely reduced to Elemental Nitrogen again by the Presence of the strong reducing agents in the form of

Char (C – Carbon) and

CO (Carbon Monoxide) in the bed

In CFBC boilers the combustion happens in two stages,

The primary Combustion in Furnace bed with primary air and secondary

combustion in free board area with Secondary and Tertiary air.

55 to 60 % in Furnace Bed – Primary Combustion

40 to 45 % in Free board area - Secondary Combustion

Due to this staged combustion, the furnace bed will be in Sub-

Stoichiometric conditions, this will increase the active Carbon (char) and

Carbon Monoxide (CO) in furnace bed.

The Final resulting NOx emission in CFBC will be almost 50% that formed

in BFBC boilers.

In CFBC boilers, PA forms only 50 to 60% of total air required for combustion. Remaining part of air is taken care by SA

Figure, captures the effect of Secondary Air on the NOX emissions.

Higher the SA air %, lower the Nox emission.

NOX emissions effort­lessly stay at <200 mg/N m3 on 6% O2 when 30% or more SA is given.

Secondary air %

De-Sulphurisation

Limestone Requirement

(Dis)Advantages

In-bed desulfurization is a breakthrough in CFBC boilers.

Lime stone-sulphur as SO2 reaction within the furnace bed,

along with the combustion reaction and the resultant gypsum

exit with ash is simplification personified.

A good understanding of the mechanics of this reaction and

the limitations is very necessary for ensuring realistic emission

of Sox.

It also helps to correctly set up the limestone and ash handling

systems that meet the present and future requirements.

5 to 6 % Sulphur in fuel (typically pet coke) can lead to 10000

mg/NM3 of SOx in exit flue gas.

Sulphur Capturing in CFBC has been done by adding

Limestone (CaCO3) or dolomite (MgCO3) along with

bed material as sulphur absorbent.

The absorbent dosed along with fuel.

The Capturing process is done in two different

reactions, the reactions are

Calcination

Sulphation

The Limestone (Calcium Carbonate – CaCO3) added into the furnace along with fuel decomposes as Calcium oxide (CaO) is called Calcination. The Reaction is,

During this Process of Calcination Limestone generates 44% of Carbon dioxide (CO2). 100 Kg of Pure Limestone will decomposes and gives

56 Kg calcium oxide and 44 kg Carbon dioxide.

CaCO3 CaO + CO2

In case of Dolomite addition, it decomposes as Magnesium Oxide and emits Carbon dioxide.

During this process 84 kg of pure dolomite decomposes as,

40 kg of Magnesium oxide and

Emits 44 kg of Carbon Di Oxide

MgCO3 MgO + CO2

During Combustion, the Sulphur in fuel reacts with Oxygen in combustion air and gives SO2 (sulphur dioxide).

32 Kg of Sulphur Reacts with 32 kg of Oxygen and gives 64kg of SO2.

Each Kg of sulphur in fuel will give 2 kg of Sulphur dioxide.

The sulfation reaction is the reaction of solid CaO with gaseous SO2 in the presence of oxygen (O2). The sulfation reaction is normally referred to as the reaction which yields solid CaSO4 as the final product

56 Kg of Calcium oxide Reacts with

64 kg of SO2 along with 16 Kg of Oxygen and

gives 136 Kg of Calcium Sulphate – CaSO4.

This By product of CaSO4 is also called as dry Gypsum.

The Theoretical requirement of Lime vs sulphur is 1 mol of Ca per 1 mol of Sulphur (Ca/S ratio is 1) in molar units.

The trend shows the DeSox efficiency vs the molar ratio. At 2% molar ratio the SOx capturing efficiency is >95%.

In Mass fractions, 1 kg of suphur requires 1.25 kg (40/32 kg) of Calcium (Ca). Ca/S Molar Ratio

In terms of CaO or calcium oxide the mass requirement is 1.75 (56/32) kg CaO/kg of Sulphur.

As, the availability is in Limestone as CaCO3, the mass requirement of Limestone is equal to 3.125 Kg / kg of Sulphur (100/32).

So, every 1% of sulphur in Fuel requires 3.125% of pure Limestone.

Both sulfation and calcination reactions start at ~700°C (1300°F) and are optimum at 840-850°C

Bed temperature vs DeSOx efficiency trend shows that, the Limestone Consumption is lowest at this temperature range.

At the Ca/S molar ratio of 4 or 3 the DeSOx efficiency is >95%.

Bed Temperature ° C

The sulfation takes place on the surface of the lime particle in the bed and so the core of the particle fails to participate.

Some sulfur in fuel, which is inorganically bound, does not oxidize to SO2.

Some SO2 escapes when sorbent is less or accompanies the volatile matter (VM) of fuel.

Available Limestone purity is lower than optimum—typically —92%. Maintaining the bed temperature around 800-900 ° C is important for de-

sulphurisation because

Calcination is not complete at Temperature <800° C

Sulfation reaction falls off rapidly beyond 850°C because CaSO4 formed on the sur­face of CaO melts due to high temperature and forms a coating on CaO, and isolates that for further reaction.

Within the residence time SO2 gas molecules do not encounter reactive solid CaO particles despite high bed turbulence.

Bed Temperature

Particle Resident Time

Bed Quality

Gaseous Environment (O2%)

Furnace Pressure

Chemical Composition (purity of limestone)

Porosity of Limestone

Surface area

Particle Size

CaO

• Calcium Oxide

SO2

• Sulphur Dioxide

O2

• Oxygen

• -8974.8 KJ/kg

CaSO4

• Calcium Sulphat

CaCO3

• Calcium Carbonate

• +1783 KJ/kg

CaO

• Calcium Oxide

CO2

• Carbon DiOxide

The Calcination process of Limestone is an Endothermic reaction. As shown in the figure below, each kg of CaCO3 will consume 1783 kJ/kg heat from furnace.

The Sulphation process of CaSO4 formation is an Exothermic reaction in which during the reaction like combustion this process will yield 8974.8 kJ/kg of heat per kg of Calcium Oxide.

So Each Kg of CaCO3 gives 3243 kJ/kg of Heat energy as final result.

This is an UnTapped advantage of In Bed Sulphur Capturing

The calcination and sulfation reactions in the bed, which are endo — thermic and exothermic, respectively, alter the heat marginally.

With a Ca/S ratio >2, there is a net loss,

whereas at <2, there is a net gain in heat.

Sensible heat loss. With coals having high ash and employing desulfurization, the bed ash discharge at —850°C can represent loss as high as 5%. As the bed ash formed will be 20-25% of total ash

Fan credits. Forced draft (FD)/primary air (PA) fans consume a lot of power in fluidizing the combustion air.

The churning of air in the fan casing to produce such a high pressure heats the air, the power for which is provided by the fan.

Cyclone radiation loss. Radiation loss taken from the standard American Boiler Manufacturers Association (ABMA) chart does not account for the losses of cyclones.

The CaO formed during De-sulphurisation process is equal of cement and excess limestone dosing can solidify in bed during shutdown and leakage conditions.

It also can block bed ash drain pipes after getting cooled.

Limestone Dosing in bed increases the Nox emission.

If the fuel has higher moisture which can condense in the APH and allow the CaO to settle and solidify. Which can reduce the heat transfer performance of APH.

Tube Leakage in first pass can make the bed as cemented concrete due to the CaO content.

Failure Potentials in CFBC boilers

Furnace Explosions

PA duct explosions

HGG explosions

A few CFBC Boilers have suffered furnace explosion in the past.

As regards CFBC boilers, these are comparatively newer generation of technology and explosion avoidance measures are not clearly understood by the operating engineers.

There are three necessary elements which must occur simultaneously to cause a fire:

Fuel

Heat and

Oxygen.

These elements form the three legs of the fire triangle.

By removing any one of these elements, a fire becomes impossible.

For example,

if there were very little or no oxygen present, a fire could not occur regardless of the quantities of fuel and heat that were present.

Likewise, if insufficient heat were available, no concentrations of fuel and oxygen could result in a fire.

Fuel

Oxygen

Ignition source

Fire

For an explosion to occur, In any case, there are five necessary elements which must occur

simultaneously:

Fuel

Heat

Oxygen

Suspension

Confinement

Fuel Confinement

Suspension

Heat

Oxygen

Explosion

These form the five sides of the explosion pentagon. Like the fire triangle, removing any one of these requirements would prevent an explosion from Confinement propagating.

For example,

if fuel, heat, oxygen, and confinement occurred together in proper quantities, an explosion would still not be possible without the suspension of the fuel.

However, in the above case, a fire could occur and the explosion will not be possible.

If the burning fuel were then placed in suspension by a sudden blast of air, water or steam, all five sides of the explosion pentagon would be satisfied and an explosion would be imminent.

This is where, the CFBC boiler is prone to failure in some critical cases

Furnace explosions in CFBC boilers are rare when both bed and free board temperatures are above 760 °C. Chances of explosions are very high when these temperatures are below 540 °C.

Though not fully established, yet chances of explosions cannot be ruled out when bed temperature remains between 540 °C and 760 °C.

All Fluidized bed boilers are exposed to this risk of explosion when they are stopped/tripped on loaded condition.

The explosions mostly occur in CFBC when

Boiler is restarted after a trip out.

Boiler is restarted after a short period of Hot stoppage

Fast cooling of boiler is resorted to following a tube leakage especially when the leaking water falls on the bed

The risk is very high when the leakage water wet the bed partially one sided and other side of the bed is very hot.

During hot stoppage/tripped condition…

It has good amount of fuel along with hot bed.

A Typical 100 TPH boiler will have 40 to 50 MT of hot bed material with ~5% of coal at 850°C and Confined in the furnace.

It meets the 3 legs of explosion pentagon,

Heat – above 850 ° C

Fuel – Sufficient enough

Confined

Such cases when ever the system gets other 2 legs of explosion pentagon Fuel Suspension (instant) and Oxygen, it explodes immediately.

There is a possibility of these to happen when the boiler is restarted after hot stoppage with any abnormality likes of tube leakage.

There are cases where, the PA duct has been exploded during a boiler start up.

When ever there is trip out on CFBC boiler,

The total fluidizing bed comes to static condition.

During this, the PA/FD air passed thru’ PA nozzle and entered into the fluidized zone will be pushed back thru’ the same nozzle to wind box due to the heavy weight of bed material.

These phenomenon causes the combustion air with high temperature rushes to wind box taking along some amount of fine fuel with CO as the burning process is not complete.

These un burnt ready to fire fuel (CO and Char) will cause more CO formation in the wind box and remain in the PA duct.

When ever the boiler is started after the trip, PA duct will get good amount of Oxygen as fresh air.

Our Analysis after every hot tripping says that, the O2% in PA duct and wind box will go down as low as to 12% during hot trip which directly indicates that, there is some amount other gases formation during tripping – expected to be Carbon monoxide.

This will close all the legs of the Explosion pentagon

Heat

Fuel

Oxygen

Confined (PA duct enclosed in all direction)

Suspended fuel (CO)

With PA/FD has been started, the Wind box is exposed to high heat of bed material will cause a explosion.

This is mainly applicable for Under bed burners or Hot gas generators

of cold cyclone boilers.

HGG explosion occurs due to consecutive start up failures of HGG in

Flame failure condition.

This start up failure on flame failure will lead to a accumulation of

sprayed Liquid fuel in HGG, when the fire catches the accumulated fuel

causes a explosion.

Fuel shut off valve passing also causes HGG explosion. The failed valves

can allow the HSD to pass thru’ and get dumped in the HGG chamber.

When the actual firing starts this accumulated fuel get exploded.

Fuel should never be fed into the furnace continuously for more than 12

seconds when there is no fire.

Furnace is completely purged of the explosive mixture and then fired.

Fuel supply is stopped immediately if fire/flame is not established and re-

purging is surely done before restart.

Correct air fuel ratio is maintained so that dust concentration within

explosive limits is never achieved.

Do not start PA/FD fan once the boiler is stopped due to tube leakage.

Continue to run ID and SA fans. Once the SA fan is tripped do not restart

during leakage condition. It can give required Oxygen to the gases formed in

bed due to water injection which leads to an Furnace explosion.

Provide a vent in PA duct for venting out the CO formed during Sudden tripping of

boilers.

Interlocking with an O2 analyser will further reduce the potential PA duct explosion.

Don’t start the Under bed burners or HGG’s without sufficient purging of furnace.

Continuous tripping of HGG on Flame failure during start up can lead to explosion.

Avoid restarting of HGG before ensuring that the HSD/Fuel sprayed inside HGG

chamber without burning is totally drained out or purged out.

Isolate fuel from HGG immediately after burner cut off.

Always take oil burner support when ever the bed temperature drops down below

540°C.