14-01-2011, 11:37 PM
ABSTRACT
This paper brings out the significance of fluidized combustion technology. The atmospheric as well as pressurized fluidized bed
combustion is discussed. Development of fluidized bed boilers and
their present status in the fluidized combustion technology is also
presented. Fluidized combustion technology is the superior one when
compared with other combustion technologies present in India today.
With out making any more developments in the advanced gas turbine
technology, it is possible to generate electricity in an economically cheap way with the help of a PFBC. This aspect of combustion technology is also discussed in the paper
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CONTENTS
Introduction
An Outlook to Fluidized Bed Combustion
General Considerations of Fluidized Bed Combustion
Fluidized Bed Combustion of Solid Fuels
Development of Fluidized Combustion Boilers
Prototype Boiler
Salient Features of the Boiler
Model Boiler
Pressurized Fluidized Bed Combustion
Pressurized Fluidized Bed Combustor
Performance Characteristics of PFBC
Areas of Problems in PFBCs
Conclusion
INTRODUCTION
Whenever air is passed through a fixed or packed bed of particles, air passes through the interstitial gaps between the particles. As the air flow rate is steadily increased, a point reaches where the pressure drop across the bed balances with the weight of the particles per unit cross sectional area of bed. At this point the bed is said to be fluidized and the velocity of air flowing through the bed at this point is said to be the minimum fluidizational velocity, Umf.
As the air flow rate increases further, the particles are buoyed up and the bed behaves like a fluid having motion, with drag forces exerted by the fluid on the particles exceeding the weight of the particles. At this point/stage, the particle mixing takes place at a high degree and an equilibrium between the gas and the particles is rapidly established.
So fluidization is an operation by which fine solids are transformed into a fluid like state through contact with gas(or liquid) and the fluidized bed behaves similar to a liquid.
Here the velocity with which the air flows into the bed is the superficial velocity, U. As we further increase the superficial velocity, it reaches a state where the particles starts getting entrained in the air stream. The superficial velocity of air at which the particle entrainment starts is called the terminal velocity, Ut .
AN OUTLOOK TO FLUIDIZED BED COMBUSTION
Coal is the main fossil fuel available for use in India. Out of the total coking coal available, only 40% is available as primary coking coal. For the purpose of steam generation, this coal is utilized in fluidized bed. It is evident that only low grade coals with high ash content will be generally available for steam generation. Combustion of coal particles can be done when they are dropped into a fluidized bed of hot (say 8500c) inert particles fluidized with air. However in the case of coal particles burning in a hot free air stream, the coal surface temperature can greatly exceed the fusion temperature of the coal ash so that the ash melts and can stick to surfaces down stream. In fluidized bed, however the burning carbon surface can be controlled at a temperature below the ash fusion temperature. So in common with other types of coal burning, when coal is burned in fluidized beds the volatile matter is emitted rapidly while the char which is left after emission of volatiles burn slowly.
GENERAL CONSIDERATIONS OF FLUIDIZED BED
COMBUSTION
There are wide ranges of design for fluidized bed combustors. The factors which influence combustion in a FBC or the important characteristics defining a FBC are described below.
a) 1:- The depth of bed can be set at many considerations such as containment of tubing or adequate depth for fines which burn out in the bed. So a wide range of reaction environments may occur. Refer fig.1. Here the reaction environments ranges from gasification to oxygen rich combustion.
Fig.1. Typical Zones of Fluidized Bed Reactors
b) Fluidizational Velocity:- A bed operating near the incipent fluidizing velocity is characterised by little mixing and high temperature gradients. Prediction of peak particle temperature is essential in this region. At very high fluidizing velocities above the terminal velocity of particles, a lean phase region exists above the bed and the bed can become circulating or fast fluidized bed.
c) Particle Size:- The particle size dominates the fluidized bed characteristics since many of the gas transfer properties are predetermined by this characteristic. Size distribution can influence segregation and particle packing characteristics and thus they influence combustion.
d) Pressure:- The operating pressure in the combustor influences the fluidization properties and thus effect the reaction mechanism.
e) Fuel and Ash Properties:- These properties and the addition of the standard additives such as limestones can influence the way a fluidized bed combustor is operated, thus the ash should have a low softening temperature and the bed temperature should be low enough to avoid sintering. But coal of high sulphur may have lime added to the coal feed so as to reduce the emission of sulphur-di-oxide.
f) 1:- This is controlled to avoid sintering and sometimes to optimize additive performance. At low temperatures, burning rates can be substantially reduced since reactions become much slower. In addition to the above mentioned factors, the geometry of the combustor, the fuel feeding method and the air feeding method also affects the fluidized bed combustion process.
The process of FBC done with the help of fluidized bed combustors is thus applied for the generation of steam.
FLUIDIZED BED COMBUSTION OF SOLID FUELS
The fluidized bed consists of inert particles such as sand through which fluidizing air can be passed. Usually the air is passed through a porous plate to the bed of particles. This porous plate is called the Distributor. Fig.2 shows different types of distributors commonly used.
Fig.2. Types of Distributors commonly used
In the process of fluidized bed combustion also, the fluidizing air is passed to the bed by means of a distributor plate at the bottom. Fig.3.
Fig.3. Fluidized Bed Combustor
The bed normally contains only a small percentage of burning fuel - about 1 or 2 % - so that the inert particles carry the heat away from the burning particles, thus keeping their temperature below that at which ash melts. The temperature of bed when burning solid fuels is commonly controlled to about 8500c. So heat is required to be removed from the bed by using cooling tubes. The bed temperature during start up should be raised to the solid fuel ignition temperature, which may vary from 4500c to 5000c depending on the type of fuel by burning a fuel gas or oil in the bed. The initiated input to the bed by burning gas or oil is gradually reduced till the main fuel starts to deliver heat and thus self sustaining combustion is established. Fig.4.
Fig.4. Startup transient of a FBC
Flue gas desulphurization is acheived by adding a sorbent such as limestone to the fluidized bed, where sulphur is absobed in a solid form. This sorbent is used as a bed material into which fuel is fed. A combustion temperature of 8500c is about the optimum for the absorption of sulphur oxides by lime. Fluidized beds can burn a wide variety of fuels quite effeciently. If we consider the case of low grade fuels, they can be burned satisfactorily only by means of fluidized bed combustion.
DEVELOPMENT OF FLUIDIZED COMBUSTION BOILERS
The fluidized bed combustion has formed very useful inputs for the design of commercial-sized units. But several areas like coal feeding, lighting up, part load operation and corrosion and erosion which are of vital importance to large-sized units, are still to be investigated for finding commercially acceptable solutions.
An important fact to be noted is that fluidized combustion offers a susperior combustion technology and has special relevance to our conditions. The objective of atmospheric pressure fluidized combustion boiler is to design, construct and operate atmospheric pressure FCB systems of sufficiently large scale so as to establish the economic viability and commercial acceptability of technology in industrial as well as utility areas.
PROTO TYPE BOILER
The fig.5 shows the prototype Fluidized Combustion Boiler Flow Diagram.
Fig.5. Prototype Fluidized Combustion Boiler
The boiler is a controlled circulation, balanced draft unit having an occassional water cooled and compartmentalized furnace chamber with spiral bed tubes and a disengaging chamber with a closely pitched convection bank meant for serving the dual purpose of cooling the product gas as well as of screening off the elutriated bed particles. A finned tube economizer follows through this tube bank. Flue gas flows through a twin cyclone seperator to the stack. The boiler could be operated as a forced draft system by leading the flue gas directly to chimney. Otherwise ID fan with inlet guide vane control system would help to run the boiler as a balanced draft system.
Seperate coal feed nozzles are provided in each compartmental bed and are located above the distributor plate. A common ash removal port is kept concentrically in the bed discharging over flow ash from the bed down to ash pit below the air box.
SALIENT FEATURES OF THE BOILER
A circular cross section is the ideal choice for fluidization. In the present design, octagonal shape has been adopted, which is a compromise between practicable shapes that can be easily fabricated and the ideal one. The octagonal section changes to a square shape in the convection zone with transaction taking place immediately below the convection bank. The square section is necessary to facilitate ease in positioning and maintaining the convection tubes. Increased cross section above the bed is anticipated to aid seperation of some fraction of dust particles from the gas stream.
The octagonal bed chamber is divided into a number of compartments. Each compartment has independent coal and air feeding arrangement with associated controls. It is possible to operate the boiler with some of the compartments completely shut off and this arrangement is specially intended to facilitate effecient low load operation. The compartmental system can also facilitate quick start up and fast raising of load because of the high heat storage capacity of the beds.
A number of circularly coiled tubes - Pancake - with changing radius in the horizontal planes having a common inlet header is adopted in the prototype. The inlet header is placed coaxially with the overflow ash disharging pipe there by cooling the later. The concept of pancake coil will enable the bed to be fluidized with minimum disturbance. Moreover the staggered pitching arrangement adopted in these coils is expected to eliminate the problems of gas channeling.
Effecient low load operation and the process of lighting up (bringing up the bed material to a temperature of 823K) is also made possible through such a prototype boiler.
MODEL BOILER
The main objective of making the model boiler is to prove some of the system concepts and to predict performance of the prototype boiler for different operating variables and with different coals.Through a model boiler the following facts have been established :
a) Fluidization in a compartmentalized bed with heat transfer tubes immersed in it has not created many problems.
b) Light up procedures with gas burners and paraffin have been established.
c) Particle settling on the immersed bed tubes has not been experienced.
d) Over flow discharge of solid material from a common central port is possible.
e) It is possible to burn coal in a single compartment while other compartments are not activated with out causing any operational difficulties.
PRESSURIZED FLUIDIZED BED COMBUSTION
If a coal fired fluidized bed combustor is operated at elevated pressure, the products of combustion can be expanded through a gas turbine to produce electricity. The low temperature (750 to 9500c) at which coal can be burned in fluidized beds minimizes the volatilization of alkali metals in the fuel and avoids any sintering of the coal ash.
Pressurized fluidized bed combustion has 3 potential advantages compared with atmospheric operation :-
a) There is an increase in specific power output and hence a potential reduction in capital cost. Fig.6 shows the effect of operating pressure and fluidizing velocity on bed area.
Fig.6. Effect of Operating Pressure and Fluidizing Velocity on Bed area
b) By combining a gas turbine and a steam cycle, the effeciency of power generation can be increased.
c) Emission of oxides of Nitrogen are substantially reduced.
By conducting researches on PFBC, it is found that by combining a steam turbine plant and a gas turbine, a significantly higher effeciency of electricity generation is possible than that from either a gas turbine or a steam turbine plant alone. Fig.7 shows a combined cycle plant with a pressurized fluidized bed combustor.
Fig.7. A Combined cycle plant with a PFBC
PRESSURIZED FLUIDIZED BED COMBUSTOR
The size of a pressurized fluidized bed combustor (PFBC) for a given rate of heat release is smaller than the one operated at atmospheric pressure. The power output of a PFBC is high as compared to an atmospheric pressure FBC. So PFBC has significantly higher effeciency. More over, the emission of pollutants can also be controlled at low levels.
There are two types of PFBC plants differing in the inlet temperature of the gas to gas turbine :-
a) PFBC plants with a low temperature gas turbine (450-5000c) which is just enough for a gas turbine to drive the compressor. Fig.8 shows a pressurized fluidized bed plant with a low temperature gas turbine.
Fig.8. Pressurized Fluidized Bed Plant with
a Low Temperature Gas Turbine
b) PFBC plants with a high temperature gas turbine where gas turbine is located directly after the fluidized bed and is driven by exhaust gas at 800-8500c. Fig.9 shows a PFBC with a high temperature gas turbine. Such a gas turbine produces more power, in excess of that needed to drive the compressor. It is of compact design and its potential effeciency is 5 to 10 % higher than conventional plant.
Fig.9. Preesurized Fluidized Bed Plant with a
High Temperature Gas Turbine
PERFORMANCE CHARACTERISTICS OF PFBC
a) Combustion Effeciency:- The main variables influencing combustion effeciency are combustion temperature, excess air and superficial gas residence time. However in commercial designs, superficial gas residence time is so large or the excess air is so great that combustion effeciency will always be high-99% or greater.
b) Heat Transfer:- Heat transfer rates have been measured in PFBCs at pressure upto 10bar and fluidizing velocities upto 2.5m/s. Over these ranges, no significant effect of pressure has been detected. But at very high Reynold’s number there may be a change to a new fluidization regime and that the accompanying change in heat transfer mechanism will result in heat transfer rates that are pressure dependant.
c) Sulphur Retention:- Pressurized operation brings about radical changes in Sulphur retention characteristics of sorbents. They concern the effects of temperature and the effectiveness of the two main types of sorbents - limestone and dolomite.
It can be seen that at atmospheric pressure the Sulphur retention effeciency increases with temperature upto 800 to 8500c and then falls sharply, but in high pressure the effeciency increases with temperature over the whole range.
d) Nox emissions:- Experimental data have shown that increasing pressure reduces the Nox emissions (approximately in proportion to the square root of pressure), although the reasons for this are not fully understood.
e) Alkali emissions:- The emission of alkalis is of considerable importance because of their influence on hot corrosion of gas turbine blades. This type of attack is initiated when alkali salts react with Sulphur-di-oxide and oxygen to form alkali sulphates which condense on hot blade surface. In PFBCs the emission of such alkalis is reduced.
Moreover the emitted alkali metal will be most probably in harmless form due to reaction with alumino-silicates in the coal ash.
AREAS OF PROBLEMS IN PFBCs
a) Gas Cleaning:- The crucial questing regarding the successful application of PFBC is whether cleaning of gases from the combustor sufficiently to avoid (or to reduce to an acceptable level) erosion of the gas turbine is possible or not. The actual amount of dust elutriated depends on the excess air, ash content of coal, nature of ash, Sulphur content of coal and the fluidizing conditions.
In order to avoid such complications a gas clean up system with control equipments for depressurizing the collected dust is a necessity. Such a system is expensive too.
b) Load changing and start up:- The combination of a PFBC and a gas turbine leads to complications in changing load and in start up which will be resolved only when large scale plant is in operation. The exact method of varying load will depend on the type of cycle and on the type of gas turbine. But however some generalisations or a compromise can be made. This won’t fully satisfy both the cycles.
c) Coal Feeding:- It is experimentally evident that coal feeding also brings out some problems during the development of the pressurized operations.
CONCLUSION
The atmospheric FCB units are always lower in capital cost than conventional coal fired boilers. The development problems of atmospheric FCB units are relatively simple compared with pressurized systems. Therefore earlier commercialization can be anticipated in such.
Combined cycle power generation using pressurized fluidized bed combustion offers the possibility of significantly cheaper electric power from coal, in an environmentally acceptable plant. This can be acheived without the development of more advanced gas turbine technology. Eventhough there are some problems in the development of PFBCs, a compromise can be made to solve these problems partialy satisfying the requirements.
REFERENCE
Y.P.Abbi, M.Banerji, M.K.Ghosh, K.T.U.Malliah and H.N.Sharan, “Development of Fluidized Combustion Boilers for Indian Coals”, AICHE SYMPOSIUM SERIES, 1978.
Kodali H.V.Prasad, “Reactivity of a high ash Indian coal towards CO2”, Indian institute of chemical technology, Hyderabad, FUEL, Vol.69, December 1990.
J.R.HOWARD, “Fluidized Beds Combustion and Applications”, Department of Mechanical Engineering, The University of Aston in Birmingham, UK.
P.K.NAG, “Power Plant Engineering - Steam and Nuclear”.
