3.2.6 Fuel and limestone handling system

Fuel is usually fed into the front wall and loop seal. Feeding into the loop seal has an

advantage of covering more area than the feeding into the front wall due to better

distribution. The number of fuel feed points per unit area is determined from the fuel

characteristics and the degree of lateral mixing in the specific design of combustor.

Typically, one feed point per 7 to 38 square meters can be used. The rate of fuel feed is

automatically controlled in response to the main steam header pressure.

Fuel is fed into the combustion chamber through four fuel feed chutes on the front wall

and four loop seal chutes that feed into the rear wall. Fuel feed air bustles help admit fuel

to the boiler. The flow of fuel is controlled according to the steam demand signal. The

combustor front wall feed arrangement is shown in Figure 3.6. Coal travels from a drag-

chain feeder to a drag-chain conveyor through a rotary valve, feed chute, expansion joint

and into the chamber.

Limestone is usually employed as sorbent for SO2 removal. The limestone from the silo

drops down a standpipe onto a rotary feeder. The volumetric feeder discharges the

limestone into a rotary valve that seals the feeder from the transport air. The limestone is

then pneumatically conveyed to the feed points on the furnace. It is injected into the

boiler at eight locations. Each feed point location at the boiler has an expansion joint and

boiler-isolating valve. The feed point connections at the boiler are made through

secondary air nozzles. The rate of limestone feed is automatically controlled in response

to SO2 emission and the required level. A sketch of the limestone system arrangement is

shown in Figure 3.7.

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Figure 3.6 Solid fuel feed system

Figure 3.7 Limestone feed system

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3.2.7 Air and flue gas system

Figure 3.8 shows a typical CFB boiler air and flue gas schematic. Air supply to the CFBC

boiler is divided into primary and secondary air. Primary air is supplied by a fan through

the grid to fluidize the bed material and for combustion in the lower part of the

combustor. The grid is composed of nozzles that have to be properly designed to avoid

problems of bed material leakage into the windbox, pluggage or erosion of nozzles.

Common nozzle types include cap nozzles, pi-tail nozzles and directional nozzles. For

some designers, in order to reduce the impact of change in boiler load on the pressure

drop across the grid, a portion of the primary air can be injected above the grid from the

combustor walls. Secondary air is supplied another fan and is injected into the combustor

through the side walls in order to create staged combustion leading to lower Nox

emission. The number of secondary air injection nozzles and the injection levels vary

among boiler designers. Nonetheless, the primary air to total air split varies from 50-75%

depending on fuel type and boiler design. The removal of flue gas from the combustor is

assisted by an induction draft fan system. In addition, high pressure air is used to fluidize

the loop seal area and for classifying of particles in the bottom ash drain classifier or ash

cooler system.

Gases from the combustion chamber flow to the top of the chamber and are drawn into

the hot cyclones by action of the two induced draft fans (IDF). Both fans must be running

to attain maximum continuous rating (MCR) of the boiler. Normally, temperature and

pressure in the cyclone area are expected to be between 815-927o C and a furnace

pressure of negative 172 Pa. Hot dust-laden gases flow over the reheater, superheater,

econimizer sections and the tubular air heater located in the back pass. After transferring

heat to these surfaces, the temperature of flue gases is gradually reduced to about 149 o C

leaving the final air heater. At this point, flue gas enters the baghouse/ESP and then IDF.

The ID fans discharge the cleaned combustion gases to ducts. The duct leads to the

common stack for discharge to the atmosphere. During normal operation both ID fans are

running. If one of the ID fans stops the boiler is limited to 50% MCR steam flow.

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Figure 3.8 Air and flue gas schematic

3.2.8 Ash handling system

Final particulate clean-up is achieved by using a baghouse or electrostatic precipitator.

Fly ash particles leaving a cyclone are typically less than 100 m in diameter and have a

mean size of approximately 30 m. It has been found that the majority of carbon loss is

in fly ash particles smaller than 15 m. Captured fly ash particles are pneumatically

carried to the ash silos for storage. In some cases, the fly ash stream may be diverted to

an ash reinjection system. Ash to be reinjected passes through air locks where it is

conveyed to the combustor through a pneumatic transport line. Fly ash reinjection is used

in some cases for improved combustion efficiency and lime stone utilization.

Bed ash is removed from the bottom ash drain system. In order to maintain a constant bed

pressure and good bed quality, large ash particles are removed from the bottom ash drain

system. The bottom ash drain system can be in the form a fluidized bed cooler or ash

drain pipe with water cooled screw conveyer. For fluidized bed cooler, the superficial

velocity is used to control the classification of particles. High pressure air is used to

classify the material flowing out of the combustor ash drain pipe systems. The hot bed

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material is fed into a variable-speed water-cooled screw conveyor before discharging to

the ash handling system.

For disposal or utilization, fly ash and bottom ash can be removed from the storage silo

by either a dry unloading spout or through a twin-paddle mixer ash conditioner. In

general fly ash has a higher unburned carbon and free lime content than bottom ash. For

example, the unburned carbon content for coal fired CFB fly ash is approximately 4-9%

where as the bottom ash is about 0.2 –4%. The free lime content as Ca(OH)2 is about 20-

40% for fly ash and 10-30% for bottom ash. Landfill is the main ash disposal method. In

order to reduce the volume of waste, ash utilization methods for agriculture, sludge

stabilization, and cement and concrete production have been developed.

3.2.9 Fuel Characteristics:

One of the key advantages of CFB boilers is fuel flexibility. In order to design properly

the boiler and auxiliary equipments for burning various types of fuels, fuel characteristics

must first be determined. Proximate, ultimate and heating value analysis are all

important. Since the ash and sorbent forms the bed material, the ash composition of the

fuel has an impact on maintaining the proper bed inventory for optimum operation. The

friability of fuel ash and sorbent affect the split of fly ash and bottom ash, there by

influencing the design of the ash handling equipment. In some cases, where the fuel has a

very low ash content or very friable ash, make-up bed material, such as sand, may be

needed. Fuel moisture content affects the flue gas volume and leads to an increase in the

volume of the boiler for a given operating velocity. Fuel moisture content also affects the

handling of the fuels. The fuel nitrogen and sulfur content together with the local

emission regulations dictate the emission control strategy. The alkali content (sodium,

potassium) and chlorine content of the fuel also need to be determined since they

profoundly influence the potential for agglomeration or fouling. In addition, the form of

the alkali (organically or inorganically bound) is important. The fuel particle size varies

with fuel reactivity and for coal, the top size varies from 6 to 25 mm. Typically, the fuel

particle size for more reactive fuels such as sub-bituminous coals and lignite can be

bigger than for less reactive fuels such as bituminous coal and anthracite.

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3.3 IR-CFB Boiler

The Internal Recirculation (IR-CFB) boiler is distinctive in design due to the use of U-

beam impact-type particle separators as opposed to cyclone separators.

In the IR-CFB boiler, most of the entrained solids recirculate within the furnace, being

captured and returned directly to the furnace by the U-beam impact-type particle

separator (Figure 3.9). Most of the fines passing the U-beams are collected by the

secondary multi cyclone dust collector/separator(MDC) or by the first field(s) of the

electrostatic precipitator (ESP) and are also recirculated to the furnace.

Figure 3.9 U-beam separators – plan view

The two-stage solids recirculation provides increased residence time to maximize fuel

burnout and sorbent utilization. This provides a high rate of gas solids reaction for

combustion, good sulphur capture if required at relatively low calcium-to-sulphur molar

ratios (Ca/S). low Nox emissions, a high rate of heat transfer to the furnace walls and

predictable temperature profile for the entire furnace height.

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U-Beam Solids Separators

The solids separation system is a key element to any IR-CFB boiler design, influencing

both capital and operating costs of the unit. The boiler has two stages of primary solids

separators: in-furnace U-beam separators and external U-beam separators. The two rows

of in-furnace U-beams are able to collect more than 75% of the solids entering the

primary separators. A particle storage hopper is located at the bottom of the external U-

beams. The separated solids are recycled internally into the furnace via discharge ports

from the transfer hopper (Figure 3.10).

Figure 3.10 IR-CFB Boiler Scheme

Water-cooled air plenum and bubble cap nozzles

The wind box or air plenum is completely made of water-cooled panels except at the rear

wall. The bubble caps are fitted on the distributor water-cooled floor panel. The bubble

caps are designed to distribute the air uniformly, preventing the back sifting of solids

even at low load operation, create good turbulence and promote fuel, limestone and bed

material mixing in the primary zone.

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Design conditions and fuel data for the KCIL( Renukoot, Uttar Pradesh) IR-CFB boiler

are listed in Table 3.1.

Table 3.1 KCIL (Kanoria Chemicals & Industries Ltd, Renukoot) IR-CFB performance data @100 percent MCR (18 MW Thermal Output for Captive Power requirements).

Design & Predicted

Performance test data

STEAM CONDITIONSSteam flow (kg/hr.) 105 000 103 000Steam pressure (MPa) 6.4 6.2Steam temperature (°C) 485 483FW temperature (°C) 180 180Steam temperatureControl range (% MCR) 60-100 60-100Turndown 3.5:1 4:1Flue Gas temperature leaving air heater (°C) 140 130-140Coal flow rate (kg/hr) 25 760 21 760Furnace bed temperature (°C) 860 865-880Upper furnace temperature (°C) 878 865-880Furnace bottom ΔP (mm wc) 610 600-680Furnace upper ΔP (mm wc) 340 300-380Boiler efficiency (%) 87.9 88.8Excess air (%) 20 16-20PERFORMANCE COAL ANALYSISPROXIMATE ANALYSIS (% BY WT)Ash 45.0 37.40Moisture 10.0 9.40Sulphur 0.4 0.22Volatile matter 18.0 25.70Fixed Carbon 24.0 27.28ULTIMATE ANALYSIS (% BY WT) Carbon 32.00 40.00Hydrogen 2.10 3.20Oxygen 9.82 8.83Sulphur 0.40 0.22Nitrogen 0.68 0.91Moisture 10.00 9.40Ash 45.00 37.40Higher Heating Value(kCal/kg) 3500 3910Coal size (mm) 6.4 x 0 6.4 x 0Mid size (d50), micron (in.) 750 (0.03) 1200 (0.05)

EMISSIONSNox (ppm) 100 70Particulate before ESP (mg/Nm3) 300 <300SO2 w/o limestone (mg/Nm³) <1200 <650CO - -

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