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XI. SOLID FUEL BURNING SYSTEMS
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Contents
1. INTRODUCTION
2. WHAT ARE THE SUB-SYSTEMS
3. AIR HANDLING
4. FUEL HANDLING SYSTEM
5. IGNITION ENERGY SYSTEM
6. PRODUCTS OF COMBUSTION SYSTEM
7. MAIN BURNERS AND BOILER FURNACE
8. MAINTENANCE
9. CONCLUSIONS
FIG.XI-I COLD PRIMARY AIR SYSTEM
FIG.XI-2 AIR FLOW CONTROL CHARACTERISTICS
FIG.XI-3 DRIFTING AS FRICTION FACTOR IN 300mmCOAL PIPE
FIG.XI-4 WINDBOX ASSEMBLY-TYPICAL CUT-AWAY VIEW
FIG.XI-5 AUXILIARY AIR COMPARTMENT IN WINDBOXASSEMBLY - TYPICAL
FIG.XI-6 AUTOMATIC WINDBOX DAMPER CONTROL SETTINGFUEL AIR DAMPERS
FIG.XI-7 WIND BOX TO FURNACE DIFFERENTIAL Vs LOAD
FIG.XI- 8 IGNITION ENERGY REQUIREMENT
FIG. XI-9 VIEWS OF TANGENTIALLY FIRED FURNACE SHOWINGNOZZZLE TILTS
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XI. SOLID FUEL BURNING SYSTEMS
1. INTRODUCTION
The primary function of fuel burning system in the process of steam generation is to providecontrolled, efficient conversion of the chemical energy of the fuel into heat energy. Satisfactoryboiler operation requires that the four ingredients viz., air, fuel, ignition energy and productsof combustion be properly ratioed, directed and sequenced so that the furnace never can containan explosive mixture. Basically the fire side safeguard system supervises the flow andprocessing of fuel, air, ignition energy and the products of combustion. In the followingparagraphs we shall see the system requirements for air, fuel (solid fuel), ignition energy andproducts of combustion for control purposes. The discussions are mainly confined to thetangentially fired furnaces which are being supplied by us for most of the utility and industrialboilers.
2. WHAT ARE THE SUB-SYSTEMS
The fuel burning system should function so that the fuel and air input is ignited,continuously and immediately upon its entry into the furnace.
The total fuel burning system required to do this consists of sub-systems for
(a) Air handling
(b) Fuel handling
(c) Ignition
(d) Combustion product removal
(e) Main burners and boiler furnace.
3. AIR HANDLING
This sub-system should be capable of supplying properly air to the main burners on a continuousand uninterrupted basis. It should be capable of providing the required air fuel ratio over theentire range of the burning.
The total air required for combustion is divided into primary air and secondary air. The primaryair is that portion of the total air which is sent to the mill. The air dries the coal in the mill asthe coal is getting pulverised, transports the acceptable coal particles to the furnace, and suppliesoxygen for the combustion of volatiles. The secondary air otherwise also known as auxiliaryair helps complete combustion.
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3.1 Primary Air Handing System
Though various types of air systems are adopted in boiler designs, the cold primary air systemshown in Figure XI-1 is the one which is offered for our present utility boilers. As the nameimplies, the primary air fan handles clean cold air. The fan is located upstream of the airpreheater and a separate primary air system is maintained through the airheater. An approximatethumb rule is that the percentage of volatiles is equal to the percentage of primary air in thetotal air.
3.2 Controls
Factors governing the control of the primary air are :
a) Coal drying requirement
b) Fuel pipe velocities to ensure that particles remain air borne
c) Aerodynamic flow pattern within the mill
d) Proper air fuel ratio.
The optimum air flow characteristics as shown in Figure X1-2 are chosen for controlpurposes so that the correct air fuel ratio along with non-drifting flow (Refer Figure X1-3) ismaintained through the pulverlsed fuel pipes.
Since the moisture content of the coal received in the power plant varies, controls are to beprovided for adjusting the air temperature at the inlet of the mill so as to get an optimum airfuel mixture temperature at the outlet of the mill. The mill outlet temperature varies between60 and 800 C depending on the rank of the coal. Dampers are provided in the air ducting so asto achieve the above control. Should the mill outlet temperature reach a value of 900C orabove, provision is also made to suddenly shut off the hot air supply. Thus the variousrequirements enumerated above for the primary air are met with.
3.3. Secondary Air System
The secondary air which is handled by the FD fan passes through the regenerative airheaterand to the windbox connecting duct which supplies the secondary air to a pair of windboxes.The secondary air is divided into two parts, namely, fuel air and auxiliary air. Fuel air is thatair which immediately surrounds the fuel nozzles. Since this air provides a covering for thefuel nozzles it is also called mantle air. Auxiliary air is admitted through compartments aboveand below the fuel nozzles (Refer Figures X1-4 and XI-5). Dampers are provided in thewindbox compartments so that the correct quantities of air to the individual compartmentscan be modulated to achieve better combustion in the furnace.
3.4 General
In order to ensure safe light off condition the preoperational purge air flow (at least 30% ofthe full load air flow) is maintained during the entire warm up period until the unit load hasreached a point where the air flow must be increased to accommodate further load increase.
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The 30% air flow is maintained in order to ensure an air rich furnace atmosphere with enoughexcess air for good combustion. After the unit is online the total amount of air flow is afunction of the unit load. Proper air flow at a given load depends upon the characteristics ofthe fuel fired and the amount of excess air required.
The function of the windbox compartment dampers is to proportion the amount of secondaryair admitted to an elevation of fuel compartments in relationship to that admitted to adjacentelevation of auxiliary air compartments.
Windbox compartment damper positioning affects the air distribution as follows
Opening up the fuel air dampers or closing down the auxiliary dampers increases the air flowaround the fuel nozzle. Closing down the fuel air dampers or opening the auxiliary dampersdecreases the air flow around the fuel stream,
The correct proportioning of secondary air between the fuel compartments and auxiliarycompartments depends primarily on the burning characteristics of the fuel. It influences thedegree of mixing, the rapidity of combustion and the flame pattern within the furnace.
3.5 Fuel Air Damper Controls
All fuel air dampers are normally closed. They open at a definite time interval after theassociated feeders are started. These dampers are to be modulated with reference to the amountof fuel fed to that elevation of fuel nozzles. The fuel air damper opening can be effectivelyused for the control of flame front position. They are fully opened when both FD fans are off.
On coal fired jobs the characteristics of the fuel dictates the fuel air damper openingcharacteristics.
3.5.1 Fuel Air Damper Setting
More fuel air shifts the flame front further away and makes the flame unstable and consequentlyhazardous situations develop. Low to medium volatile coals require least fuel air to be suppliedwhile high volatile coals require larger quantity of fuel air to keep the coal flame away fromthe nozzle tip at the desired distance, (vide Figure XI-6). Low fuel air results in burningwithin the nozzles and hence overheating and deformation of the nozzle and the consequentdeteriorating of the combustion process and even impingement of flame, leading to failure ofthe boiler tubing; the life of the burner nozzle is also considerably reduced.
3.6 Auxiliary Air Damper Controls
During the furnace purge period and initial operation of the unit (upto 30% loading) allelevations of auxiliary dampers modulate to maintain a predetermined windbox to furnacedifferential- All these dampers are modulatetd on elevation basis. When the unit load exceeds30% loading, the windbox to furnace differential is changed to a higher value. At this pointthe auxiliary air dampers which are associated with non-operating compartments are closedin time sequence, starting with upper elevations and progressing to the lowest elevation.These damper controls are done to achieve the windbox to furnace differential as shown inFigure XI-7. The optimum windbox damper settings for a particular unit depends on theconditions that are present on the particular unit. In general the factors which determine thesetting are:
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a) Ignition stability
b) Ignition point relative to fuel nozzles
c) Overall combustion condition in furnace.
4. FUEL HANDLING SYSTEM
The principal function of pulverised coal burners is to distribute fuel and air evenly in afurnace. Even distribution of fuel and air means that every square of the crosssection of thestream of burning mixture in the furnace gets its share of fuel and air, and this uniformity ofdistribution is maintained along the full length of the path of the burning mixture through thefurnace. It the distribution is not uniform certain parts of the furnace get too much fuel andtoo little air. In the parts of the furnace which get too much fuel the temperature is high andthere may be heavy deposit of slag in the furnace. Lack of air in one part of the stream causesincomplete combustion even though high overall excess air is maintained at the furnace outlet.
It is comparatively easy to get a uniform distribution of coal and air in the furnace which isequipped with corner firing. Bends in the fuel pipes may cause uneven distribution of coalacross the section of primary air stream. Replaceable kicker blocks are provided in the fuelinlet bends so that they will redistribute the air fuel mixture in a uniform pattern before entryto the furnace. The pressure loss in the fuel piping is equallsed., Optimum air coal ratiorequired for adequate transport characteristics, ignition stability and flame propagation of theair coal stream which Is delivered via the coal nozzle, are chosen during design stage Importanceis also given to the material selection, the layout of fuel handling system and to the easymaintainability of the system components.
5. IGNITION ENERGY SYSTEM
The functional requirement of a fuel burning system is to supply an uninterrupted flammablefurnace input and ignite it continuously as fast as it is introduced and immediately upon itsappearance in the furnace. Thus no explosive mixture can accumulate in the furnace since thefurnace input is effectively consumed and rendered inert. Ignition takes place when theflammable furnace input is heated above the ignition temperature. Supply of correct ignitionenergy for the furnace input is a substantial task. There are many factors which establish therange of combinations of ignition energy quality, quantity and location that can provide asatisfactory furnace input ignition rates at any instant.
5.1 Major Factors Deciding the Ignition Energy Requirements
Six major factors determine the total ignition energy required. They are :
a) Fuel qualityb) Fuel preparationc) Air preparationd) Burner product distributione) Total air fuel ratiof) Main burner flow rate
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5.2 Supply of Ignition Energy
Ignition energy is usually supplied in the form of heat. Ignition energy is basically dividedinto two parts namely inherent ignition energy and auxiliary ignition energy. Inherent ignitionenergy is heat that a fuel burning system retrieves from the products of combustion and usesto ignite the furnace input. This heat may be in the form of flame or recirculated product ofcombustion.
Without ample ignition energy the flame will not be stable and combustion will not be complete.At low loads the inherent ignition energy will be less and hence to get the optimum requiredignition energy additional ignition energy is introduced into the furnace. This additional ignitionenergy is called the auxiliary ignition energy. Figure XI-8 brings out the requirements ofignition energy.
5.3 P. F. Fired Boilers
In PF fired boilers auxiliary ignition energy is provided by oil guns and by ignitors. Since thecost of these fuels are large, the system designs are made so as to provide adequate energywith minimum use of premium fuel and for short period the operation of the system is dealtseparately.
The total ignition energy system should provide a reliable safe method for igniting andstabilising each main flame envelope in a furnace without manual manipulation of the ignitionenergy. Since various arrangements are possible in a furnace for the provision of ignitionenergy, it is necessary to provide for in the control system to check the adequacy of the ignitionenergy.
6. PRODUCTS OF COMBUSTION SYSTEM
This sub-system should be capable of removing furnace gases over the entire operating rangeof fuel burning system while maintaining the furnace pressure within design limitations. Aprimary function is to remove inert combustion products so that the furnace fuel and air inputcan be continuously and immediately ignited. Controls are provided in the fans for effectivelycarrying out this function.Also the analysis of CO
2 ,O
2 and CO in the products of combustion is very valuable in
determining the combustion efficiency and air infiltration. There is no perfectly reliable meansof measuring the air actually admitted to the furnace and the only means of determining theamount of such air is from the analysis of products of combustion called flue gas.
7. MAIN BURNERS AND BOILER FURNACE
In tangential firing, the furnace constitutes the burner. Fuel and air admitting nozzles arearranged in the four corners of the furnace so that they will direct the air fuel streams tangentto an imaginary circle. The long flame corner mounted type of firing symbolises the singleflame envelope method of firing. The advantages of this type of firing are well known. Eachpulveriser supplies one elevation or tier of burners and the p.f. is on elevation basis. Theburners are designed to have adequate maintainability.
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7.1 Burner Tilt
An additional feature in the present wind boxes is the on-load tilting facility (See FigureXI-9) of all the four corner windbox nozzles in the vertical plane. These nozzles can be tiltedup and down through 600 (i.e ± 300). The effect of tilting is a change in effective furnace size.This feature helps in controlling the steam temperature and also aids in burning fuels withwider variation in properties.
7.2 Flame Scanner
Flame sensing devices known as flame scanners are also provided in the corner windboxes.Hydrocarbon fuels emit ultraviolet radiations basically because of the chemical oxidation ofthe hydrogen atom. The flame scanners pick up these radiations and prove the presence offlame. Cooling air is required for the operation of the scanners.
7.3 Special Coal Burner
High turn down split coal nozzles and riffled coal nozzles are being developed against futurerequirements and desired improvements over existing coal burning equipment. The high turndown split coal nozzle relies for its improved performance on :
i) The hot gas recirculation zone created in between the two halves of the split coal noz-zle.
ii) the increased coal concentration on the upper half of the split coal nozzle due to thecentrifugal action of the fuel inlet elbow and
iii) the deceleration of the pulverised coal particles on impingement on the walls of thesplit coal nozzle tip.
It helps to reduce fuel oil consumption in boilers and thus reduce the operating cost. It ispossible to retrofit the split coal nozzle in place of existing standard coal nozzles’ The riffledcoal nozzle uses riffled splitter plates and nozzle tips. It relies for its improved performanceon (i) the local eddies formed on the primary air fuel stream and (ii) the increased mixing withfuel and air close to the nozzle tip.Direct Ignition of Pulverized Coal (DIPC) using Ultra High Energy Electric Arc (UHEA) andLow Calorific Value Coal Gas Warmup and Stabilization System are under different stages ofdevelopment.
8. MAINTENANCE
Unique feature in the present tilting tangential burners is the possibility of changing the coalnozzle and nozzle tips from outside the furnace. The coal nozzle tip is not hinged with theburner frame. This makes it possible to remove the coal nozzle and nozzle tips which aremore prone to maintenance from outside the furnace.
During every overhaul it is necessary to inspect the coal nozzle and tips before and after
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cleaning and the damaged parts are to e replaced. It is also advisable to check the free operationof nozzle tilt arrangement and dampers with their drive mechanism during each overhaul.In the fuel piping area in addition to replacement of bends and leaky parts, it is better to checkthe orifice dimensions over a period of time and it is necessary to change the orifice foroptimum performance. Easy changing of wearing parts is accomplished by the provision offuel pipe couplings.
9. CONCLUSIONS
Brief outlines of the requirements of the controls necessary for efficient usage of fuel aredealt with in the above paragraphs. This treatment may aid in picturing the evolution of thepresent day equipment and their requirement for better performance.
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FIG. XI-I COLD PRIMARY AIR SYSTEM
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FIG.XI-2 AIR FLOW CONTROL CHARACTERISTICS
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FIG.XI-3 DRIFTING AS FRICTION FACTOR IN 300mmCOAL PIPE
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FIG. XI-4 WINDBOX ASSEMBLY-TYPICAL CUT-AWAY VIEW
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FIG. XI-5 AUXILIARY AIR COMPARTMENT INWINDBOX ASSEMBLY-TYPICAL
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FIG. XI-6 AUTOMATIC WINDBOX DAMPER CONTROL SETTINGFUEL AIR DAMPERS
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FIG. XI-7 WIND BOX TO FURNACE DIFFERENTIAL Vs LOAD
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FIG. XI- 8 IGNITION ENERGY REQUIREMENT
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FIG. XI-9 VIEWS OF TANGENTIALLY FIRED FURNACE SHOWINGNOZZZLE TILTS
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