In PFBC, the combustor and hot gas cyclones are all enclosed in a pressure vessel. Both coal and sorbent have to be fed across the pressure boundary, and similar provision for ash removal is necessary. For hard coal applications, the coal and limestone can be crushed together, and then fed as a paste, with 25% water. As with atmospheric FBC (CFBC or BFBC), the combustion temperature between 800-900°C has the advantage that NOx formation is less. SO2 emissions can be reduced by the injection of a sorbent, and its subsequent removal with the ash.

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GUJARAT TECHNOLOGICAL UNIVERSITY
Chandkheda, Ahmadabad
Affiliated
Government Engineering College,
Bharuch
A project report on
PRESSURIZED FLUIDIZED BED COMBUSTION BOILER
Under subject of
POWER PLANT ENGINEERING
B.E. SEMESTER-VII
Branch- Mechanical
Presented by:
Sr. No Name of the student Enrolment number
1 RATHOD PRASHANAT 160140119094
2 RATHWA RAHUL 160140119095
3 RAVAL KRUNAL 160140119096
4 SAKHIWALA MAHIR 160140119098
Faculty guide:-
PROF. U.V. JOSHI

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INTRODUCTION
The major portion of the coal available in India is of low quality, high ash
content and low calorific value. The traditional grate fuel firing systems have
got limitations and are techno-economically unviable to meet the challenges of
future. Fluidised bed combustion has emerged as a viable alternative and has
significant advantages over conventional firing system and offers multiple
benefits – compact boiler design, fuel flexibility, higher combustion efficiency
and reduced emission of noxious pollutants such as SOx and NOx. The fuels
burnt in these boilers include coal, washery rejects, rice husk, bagasse and other
agricultural wastes. The fluidized bed boilers have a wide capacity range- 0.5
T/hr to over 100 T/hr.
In PFBC, the combustor and hot gas cyclones are all enclosed in a pressure
vessel. Both coal and sorbent have to be fed across the pressure boundary, and
similar provision for ash removal is necessary. For hard coal applications, the
coal and limestone can be crushed together, and then fed as a paste, with 25%
water. As with atmospheric FBC (CFBC or BFBC), the combustion temperature
between 800-900°C has the advantage that NOx formation is less. SO2
emissions can be reduced by the injection of a sorbent, and its subsequent
removal with the ash.

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Mechanism of Fluidised Bed Combustion
When an evenly distributed air or gas is passed upward through a finely divided
bed of solid particles such as sand supported on a fine mesh, the particles are
undisturbed at low velocity. As air velocity is gradually increased, a stage is
reached when the individual particles are suspended in the air stream – the bed
is called “fluidised”. With further increase in air velocity, there is bubble
formation, vigorous turbulence, rapid mixing and formation of dense defined
bed surface. The bed of solid particles exhibits the properties of a boiling liquid
and assumes the appearance of a fluid – “bubbling fluidized bed”. At higher
velocities, bubbles disappear, and particles are blown out of the bed. Therefore,
some amounts of particles have to be recirculated to maintain a stable system -
"circulating fluidised bed". This principle of fluidisation is illustrated in Figure .
Fluidization depends largely on the particle size and the air velocity. The mean
solids velocity increases at a slower rate than does the gas velocity, as illustrated
in Figure. The difference between the mean solid velocity and mean gas
velocity is called as slip velocity. Maximum slip velocity between the solids and
the gas is desirable for good heat transfer and intimate contact.
If sand particles in a fluidised state is heated to the ignition temperatures of
coal, and coal is injected continuously into the bed, the coal will burn rapidly
and bed attains a uniform temperature. The fluidised bed combustion (FBC)
takes place at about 840°C to 950°C. Since this temperature is much below the
ash fusion temperature, melting of ash and associated problems are avoided.
“T”s that is Time, Temperature and
Turbulence. In FBC, turbulence is
promoted by fluidisation. Improved
mixing generates evenly distributed heat at
lower temperature. Residence time is
many times greater than conventional grate
firing. Thus an FBC system releases heat
more efficiently at lower temperatures.
Since limestone is used as particle bed,
control of sulfur dioxide and nitrogen
oxide emissions in the combustion
chamber is achieved without any
additional control equipment.This is one of
the major advantages over conventional
boilers.Combustion process requires the
three

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PRESSURISED FLUID BED COMBUSTION:
Pressurised Fluidised Bed Combustion (PFBC) is a variation of fluid bed
technology that is meant for large-scale coal burning applications. In PFBC, the
bed vessel is operated at pressure upto 16 ata ( 16 kg/cm2).
The off-gas from the fluidised bed combustor drives the gas turbine. The steam
turbine is driven by steam raised in tubes immersed in the fluidised bed. The
condensate from the steam turbine is pre-heated using waste heat from gas
turbine exhaust and is then taken as feed water for steam generation.
The PFBC system can be used for cogeneration or combined cycle power
generation. By combining the gas and steam turbines in this way, electricity is
generated more efficiently than in conventional system. The overall conversion
efficiency is higher by 5% to 8%.
At elevated pressure, the potential reduction in boiler size is considerable due to
increased amount of combustion in pressurized mode and high heat flux through
in-bed tubes. A comparison of size of a typical 250 MW PFBC boiler versus
conventional pulverized fuel-fired boiler is shown in the Figure.

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Working of PFBC boiler:
In this type of the system, boiler exit gas contain enough energy about
temperature 850 to 900 C to drive a gas turbine. The power output of gas
turbine is utilized to run the air compressor and the electric generator. The
product of combustion have to be sufficient clean for gas turbine to accept
without excessive erosion, corrosion or fouling of the turbine Hence, the flue
gases along with unburnt carbon leaving the combustion chamber are passed
through a cyclone separator where the unburned carbon particles are
superheated from the gas due to centrifugal action from the mixture, separated
carbon particles their combustion. The flue gases from cyclone separator are
now passed through a dust collector .This combined operation of gas turbine
with the boiler steam turbine provides an efficient combined cycle operation.

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ADVATNGAES OF PFBC BOILER
 High Efficiency
 Reduction in Boiler Size
 Fuel Flexibility
 Ability to Burn Low Grade Fuel
 Ability to Burn Fines
 Pollution Control,Low Corrosion and Erosion
 Easier Ash Removal – No Clinker Formation
 Less Excess Air – Higher CO2 in Flue Gas
 Simple Operation, Quick Start-Up
 Fast Response to Load Fluctuations
 Provision of Automatic Ignition System
 High Reliability, Reduced Maintenance
 Quick Responses to Changing Demand
 High Efficiency of Power Generation
DISADVANTAGES OF PFBC BOILER
 It is difficult to control.
 Plant life is low

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STATUS OF PFBC TECHNOLOGY:
The first demonstration plant of capacity of 130 MWe (-i- 224 MW| co-
generation) has been operating in Stockholm, Sweden since 1991 meeting all
the stringent environmental conditions. Another demonstration plant of 80
MWe capacity is operating in Escatron, Spain using 36% ash black lignite. The
third demonstration plant of 70 MWe at TIDD station, OHIO, USA was shut
down in 1994 after a eight year demonstration period in which a large amount
of useful data and experience were obtained. A 70 MWe demo plant operated at
Wakamatsu from 1993 to 1996. Presently a 350 MWe PFBC power plant is
planned in Japan and another is on order in USA (to be operated at SPORN).
UK has gathered a large amount of data on a 80 MWe PFBC plant in
Grimethrope during its operation from 1980-1992 and is now offering
commercial PFBC plants and developing second generation PFBC. ABB-
Sweden is the leading international manufacturer which has supplied the first
three demonstration plants in the world and is now offering 300 MWe units
plants.
A proposal by BHEL for a 60 MWe PFBC plant is under consideration , with
the Government of India.

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OPPORTUNITIES IN INDIA
India’s actual energy requirement is being fulfilled from coal of about 63
percent. Today India has back up of coal for about coming 100 + yrs. India is
the top most coal producing country in the world. Also India is the highest
consumer of coal of about 339000000.total capacity of power generation in
2014 through coal is about 148478.39 MW. Which is about 59.51 percent but
due to increasing demand of energy India government is establishing no. Of
power plant and government is also electrifying rural area for completing their
needs. Now a day’s private sector is taking deep interest in generating through
their new power plant concept but his major problem is f environment problem
so for meeting these needs FBC is introduced. As per experts upcoming demand
of India will rise up to 218 GW.
In India the content of carbon is low and toxic elements are really low. The
value of Indian coal is really low On an average for generating power of about a
kWh India uses 0.7 kg of coal whereas USA uses about 0.45 kg of coal for same
productivity. This is because coals have different quality. As compare to other
countries India has GCV of about 4500 kcal/kg which is not as good as compare
to others.
Indian ministry of environment and forests has copulated the use of beneficial
coals where the content of ash is about 34 percent in urban Areas power plant.
From last 30 years India has installed power plant of capacity of 50Gws but due
to such a long periods the efficiency of power plant has been reduced and their
reliability and output is also affected.
So for overcoming these problems the concept of PFBC has been came into
lime light and soon it become commercially valuable alternative and become
favourable in many situations. In all the solid fuels this technology has been
implemented such as boifuels, peat, lignite, anthracite, culm array of toxic
wastes.