Furnace eng SP

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INDEX

Sl.

No.

Contents Page

No.

1. Furnaces for SRRM 1

2. Design Aspects 7

3. Fuel & Fuel Properties 11

4. Design of Burners 38

5. Insulation Aspects 42

6. Heat Recovery 45

7. Statutory and Safety requirements 52



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CHAPTER-1

FURNACES FOR SRRM

Rerollers use reheating furnaces for rolling:

REHEATING FURANCE

Furnace Types-Reheating furnaces are divided into two general classes:

1. Batch type.2. Continuous type, including pusher-type, rotary hearth-type, walking-beam-type,

walking-hearth type and roller-hearth-type furnaces.

Batch furnaces

Batch furnaces are those in which the charged material remains in a fixed position on

the hearth until heated to rolling temperature. Continuous furnaces are those in whichthe charged material moves through the furnace and is heated to rolling temperatureas it progresses through the furnace. Batch furnaces are the older type and are used forheating all grades and sizes of steel. However, small billets seldom have been heatedin this type since the introduction of continuous furnaces. Batch furnaces are firedwith either gaseous or liquid fuel, with preheated or cold air for combustion. The airmay be preheated by regenerators and the furnace firing reversed from one end to theother, as in open-hearth furnaces. Batch furnaces, in which the air is preheated byrecuperators, are not reversed and firing is continuous from one or both ends,depending upon the location of the gas outlet port. The steel to be heated in a batchfurnace commonly is charged and drawn through front doors by a charging machine.Batch furnaces vary in size from those with hearths of less than a square metre (only afew square feet), with a single access door, to those about 6 meter (20 feet) in depthby 15 metres (50 feet) long, with five or six doors.

Pusher- Type Furnaces

Continuous pusher-type furnaces were designed initially for heating billets and smallbloom sections. The hearths were relatively short in length and were slopeddownward longitudinally towards the discharge end to permit an easy movement of billets through the furnace. In early designs the furnaces were fired by burners locatedat the discharge end and the billets were heated by the hot gases flowing through thefurnace above the top surface the steel toward the charging end. Pushers were used to

push forward the charge of cold billets. The flow of gas and steel in the furnace werecounter-current. The, modern continuous pusher-type furnace has been altered inmany respects from those of early design, although a large number of the older ones,particularly billet-heating furnaces, are still used. Longer furnaces generally areconstructed now. Some have hearths about 24.5 to 32 metres (80 to 105 feet) long,with top and bottom firing, and contain preheating, heating and soaking zones. Thehearths usually are constructed level. Recuperators are utilized to provide waste-heatrecovery.



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The steel to be heated in a continuous furnace can be charged either from the end orthrough a side door. In either case, the steel is moved through the furnace by pushingthe last piece charged with a pusher at the charging end. As each cold piece is pushedinto the furnace against the continuous line of material, a heated piece is removed.The heated piece is discharged by several methods, such as through an end door bygravity upon a roller table which feeds the mill, or pushed through a side door to the

mill table by suitable manual or mechanical means or withdrawn through the end doorby a mechanical extractor.

A distinctly different type of continuous reheating furnace is the rotary-hearth type. Itis used frequently for heating rounds in tube mills and for heating short lengths of blooms or billets for forging. The rotary-hearth type permits the external walls androof to remain stationary while the hearth section of the furnace revolves.

Walking-Beam-Type Furnaces

The walking-beam concept for conveying materials is not new. However, in the caseof furnaces, the idea at first was applied successfully only to furnaces that operated at

maximum temperatures of about 1065°C (1950°F), compared with reheating furnacesthat must heat steel to temperatures up to 1315°C (2400°F). The early furnacesemployed alloy steel walking beams that were exposed directly to the heat of thefurnace and were subject to heat corrosion and warping and did not retain enoughstrength at the higher temperatures. Difficulties were met in trying to seal the slotsbetween the walking beams and the fixed portion of the hearth, so that control of furnace pressure and atmosphere presented problems.

Walking-beam furnaces that operated successfully in the higher ranges of temperaturebegan to be used in Europe in the early 1950's, being first applied to the reheating of billets and blooms. They are now used here and abroad to reheat slabs as well asbillets and blooms. The walking beam may consist of water-cooled steel memberstopped with refractories in such a manner that only the refractories are exposeddirectly to the heat of the furnace. Alternatively, the beams and supports may be con-structed of water-cooled tubular sections (with "buttons" on the top surfaces to keepthe hot steel from direct contact with the water-cooled tubes). Walking-beam furnacescan be designed for side or end charging and discharging. Either hydraulic ormechanical methods can be used to actuate the beams. Cross firing with side-wallburners above and below the stock being heated and heating with radiant-type burnersin the furnace roof or in both the roof and below the stock have been employed.

Walking-Hearth-Type Furnaces

In a walking hearth furnace, travel of the work through the heating chamber followsthe same general path as in the walking-beam furnace. The main difference in methodof conveyance in these two furnace types is that, in the walking-hearth furnace, thework rests on fixed refractory piers. These piers extend through openings in the hearthand their tops are above the hearth surface during the time when the work is stationaryin the furnace. The furnace gases can thus circulate between most of the bottomsurface of the work and the hearth.

To advance the work toward the discharge end of the furnace, the hearth is raised



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vertically to first contact the work and then raise it a short distance above the piers.The hearth then moves forward a preset distance, stops, lowers the work onto its newposition on the piers, continues to descend to its lowest position and then movesbackward to its starting position toward the charging end of the furnace to await thenext stroke.

GENERAL CONSIDERATIONS IN FURNACE-TYPE SELECTION

Batch-Type Furnaces

Some of the particular advantages of batch-type furnaces are:

1. They provide means for heating steels of various types and sizes, which can beheated more properly in separate batches than when mixed with other types in thesame furnace, especially when specific heating practices are required.

2. They are suitable as a reservoir for holding hot steel directly from the primary millfor later rolling in the /finishing mills.

3. They can be operated to heat steel to temperatures above 1315oC (2400°F) moresatisfactorily than a continuous furnace. Steel can be given a "wash heat," whendesirable, without trouble from the pieces sticking together. (Steel is sometimes"washed" or heated until an earlier oxide scale is melted and a new scale formed,to reduce surface defects.)

Some of the important disadvantages of batch furnaces are:

1. High capital investment per unit of production.

2. Low hearth area efficiency. That is, the hearth in the conventional type is notutilized fully because of interference of door jambs and clearance required insidethe furnace for handling the stock with charging machines.

3. High man-hours per ton of heated steel.

4. Lack of flexibility for heating steel slowly in the lower temperature range.Furnaces must be cooled to charge high-carbon steel and some alloy steels.

5. Length of pieces to be heated is limited by tendency to bend when they areremoved.

Pusher-Type Furnaces

The advantages of the pusher-type continuous furnaces are:

1. High production per dol1ar investment.

2. Low man-hours per ton of steel heated.

3. Greater ease in charging and drawing steel.



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4. High hearth area efficiency.

5. Better means for controlling the rate of heating at all temperature levels. Gradualrise in temperature permits charging al1 grades of cold steel without coolingfurnace.

6. Less trouble from temperature inequalities between each succeeding piece drawn.

7. High production per square foot of ground space occupied.

8. Can be built for any reasonable length of piece to be heated, resulting in highermill yield, because of fewer crop ends from lengths beyond batch-furnacepossibilities.

Some of the important disadvantages of pusher-type continuous furnaces are:

1. Lack of flexibility for heating efficiently smal1 orders of different lots of steel or

thicknesses. Heating time must be increased to suit the heating cycle of the piece,requiring lower heat transfer rate which may be detrimental to the heating of theother grades and sizes in the furnace.

2. Trouble from water-cooled skid maintenance, and from building up of scale onhearth, which results in piling of steel in furnace, due to climbing of pieces overeach other, particularly if square contact is lost between adjacent pieces, whenpressure is applied by the pusher.

3. Face of contacting surface of stock must be square to prevent piling.

4. Expensive to empty furnace at end of schedule.

5. Difficulty in pushing mixed sizes through furnace.

6. Where water-cooled skids are used, thickness of product is limited to 300 to 350mm (12 to 14 inches).

7. Presence of visibly colder "stripes" on the hot steel caused by contact with water-cooled skids, unless adequate soaking time is provided.

Walking-Beam Furnaces

Some of the important advantages of walking-beam furnaces are:

1. Pieces can be separated from one another on the hearth, eliminating stickers.

2. Pile-ups are reduced.

3. Furnace retention time is reduced.

4. Furnace can be emptied easily from either end by activating the beam



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mechanisms.

5. No line contact with water-cooled skids occurs, thus eliminating troublesome skidmarks (also called "stripes" or "black marks") on the heated stock.

6. There is no rubbing or friction between the stock and the hearth, minimizing

hearth wear and stock damage.

7. By selecting the proper number of walking beams, better hearth utilization can beobtained when charging mixed sizes.

8. The potential for the extension of overall furnace length to improve the utilizationof furnace waste gases and reduce fuel consumption. A similar advantage is notavailable with other furnace types because of limitations on overall furnace length.

Some of the disadvantages of walking-beam furnaces are:

1. Complexity of the walking-beam system.

2. Inherently higher cost per ton of capacity.

3. Precautions must be taken to avoid problems from scale that drops off the stock being heated.

4. Maintenance of hearth seals and hearth refractory.

In general, the same advantages and disadvantages of the walking-beam furnace applyto walking-hearth furnaces.

Some features in both batch and continuous furnaces are worthy of note:

1. Regenerators or recuperators act as a reservoir of heat supply, which is especiallyvaluable for efficient soaking of steel.

2. Continuous furnaces provided only with top firing require longer hearths for equalproduction than those with top and bottom firing, but, in the case of pusher typefurnaces, do not require a special soaking zone to eliminate cold spots on the work caused by contact with water-cooled skids.

3. Continuous furnaces with single-zone firing have higher scale losses and greatertendency to cause decarburization of high-carbon steel than the top-and bottomfired furnaces, since the steel is in the furnace longer. Decarburization is caused

primarily by hydrogen and water vapor combinations in furnace gases, andincreases almost directly with the time the steel is at elevated temperatures. Freeair and carbon dioxide' in the furnace atmosphere cause decarburization to a lesserdegree. The scaling of steel is practiced sometimes deliberately to remove thedecarburized surface layer.

4. A level hearth in a continuous furnace eliminates the stack effect of hearthssloping upwards towards the charging end. This stack effect draws cold air intothe furnaces at the hot end causing higher fuel consumption and scale losses.



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5. Batch-type furnaces used for heating in a production line require supplementaryfurnaces for preheating certain grades of alloy and high-carbon steels for transferinto the hotter furnaces. The preheating zone of a continuous furnace makes thisunnecessary.

6. Side-discharge continuous furnaces have less air infiltration at the hot end thanend-door discharge furnaces. End-door discharge of the usual gravity type inducescold air into the furnace by the stack effect at the discharge section of the furnace.End-door discharge, however, is mechanically simpler for removing the heatedstock; particularly slabs and heavy blooms.



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CHAPTER-2

DESIGN ASPECTS

Structural Features

The methods and materials used in furnace construction have an important bearing onfuel efficiency since breakdown and delays may be a source of heat waste.

The design of structural frame work of the furnace is based on the well knownprinciples mechanical construction. A special point on its bearing on fuel efficiency isthat robustness is essential, since, repeated heating and cooling of structural memberstogether with concomitment expansion and contraction of refractory materials usedcan, in course of time induce a remarkable degree of distortion of the frame work.Large furnaces of insufficient robustness have been known to be several feet out of alignment after long use where sufficient strength and allowance for expansion havenot been applied. Then cracks in brickwork develop more readily causing air leakageor flame emission and lowering of furnace efficiency. Cast iron plates and structural

of parts exposed to high temperatures for which special materials such as heatresisting alloys must be used. The development of these materials during recent yearshas resulted in considerable progress in mechanization of furnace practice. Suchmaterials are used extensively up to about 1000oC and special work even at highertemperature.

The must be resistant to sealing and have sufficient hot strength. The last knownproperty is known creep strength. Other important property of material is resistance tocarburizing to the effect of repeated heating and cooling, heat diffusivity, malleabilityand machinability.

Furnace Chambers & Roofs

Provided the abutment of the arch, or skew-back is adequately supported by a strongframe work with tie rods the arch must rise on heating. In selecting brick work, theexpansion characteristics of the refractory to be used must be known; also whetherthere is any after contraction on firing to the temperature at which the roof is to beoperated. Roofs are constructed of specially shaped end bricks abutting on theskewback to allow for this variation to be corrected by natural expansion duringheating.

Furnace roofs fail either by yielding of skew backs or by spalling and wear. spallingcan weaken an arch and finally reaches the condition of collapse. It may be urged with

good reason that planned maintenance is preferable to the policy of striving for theabsolute limit of the durability.

Side Walls and Hearths

If the side wall forms the support of the roof its correct construction is of primeimportant. The major fault to be avoided in the construction of the lower part of thefurnace lining is lack of adequate strength of supporting girders of the hearth which



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should be securely anchored to the buck stays. In high temperature furnaces side wallsupport should be provided by the use of strong cast iron plates.

Adequate in hearth in modern furnace practice is imperative and it is practicable to beobtaining material capable of with standing heaviest hearth load.

Foundation

Discussing design of foundation as a factor is entering into problem of correctpractice in normal operation since it features of initial construction. Thin, nonventilated brick work hearths which are, however capable of being readily altered cancause overheating of concrete; dehydration of which beings at 260oC and is completedat 480

oC. Limiting thickness are D/6 for furnace working at below 1205

oC and D/8

for 870oC, D being the shortest dimension of hearth for 30 cm.

Hearth support, skids and hangers

Wide range of mechanical device and hangers are used to enable stock to be moved

readily through continuous furnaces. Apart from waste of fuel and output excessiveheat losses may result from water cooling but its use is inevitable. Skid pipes areusually of hydraulic section and must be securely anchored in the furnace hearts. Theymay be fastened into the anchorage in such a manner as to admit of being turned toexposed new and unknown surfaces for further service or alternatively welded strips,may be used to take the wear. Safe guarding the water supply and preventingblockages in pipes are obvious precautions.

The main objection to the use of water cooling in skids arises from cold patches in thestock; finishing periods of heating must then be made by transfer to solid refractoryhearth. The use of dry skids, hearth plats and mechanical conveyors introduces a fieldof furnace practice of a specialized type.

Jambs, Doors and Opening in side walls

Opening in side walls can be a cause of in efficiency, in furnaces operating at atemperature above 1000oC. These openings loose heat at very high rate from directradiation to atmosphere. Distorted buck stays, piers and jamb reinforcement cause airin leakage and flame loss. For proper fitting of doors is no longer possible. Modernrefractory practice in the provision of high temperature insulating materials makespracticable more robust and better doors. A most important feature of furnacespractice is attention to the sealing doors to prevent such leakages. Water cooled doorsmay be essential but the heat losses due to the cooling action should always be

examined in relation to the general economy.

A badly fitting discharge door on a reheating furnace having a gap 25 mm wide 600mm long along its top age will give a heat loss of approximately 50,000 KCal an hourdue to escaping gases if the furnace is operating at 1100oC with an exhausttemperature of 700oC and with a normal furnace pressure. This clearly shows the losswhich can be obtained from a comparatively small opening.



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AUXILIARY PLANT

In addition to the furnace proper there is often much ancillary equipment that requiresattention if the best results are to be obtained.

Dampers

The function of a damper is to regulate the draught; accordingly it becomes a primeinstrument of the furnace economy. The control should always be situated in such aposition as to be readily accessible and preferably also permit of sighting the flamewhen adjustments are made. The choice of the material from which the damper ismade depends upon condition of operation. It must not cracks or warp nor must brickswork become dislodged. It should more easily in its seating and be readily capable of fine adjustment, since, quite a small movement may mean a waste of many tons of fuel. Allowance should be made for its expansion and suitable covers or leakage sealsprovided to prevent the inflow of cold air to the flue. If the furnace is temporarilyshutdown the damper should always be closed. Closing the damper when shuttingdown the furnace is equivalent to turning of the fuel valves, since, it prevents cold air

in leakage.

Water cooled dampers of special design or damper constructed of heat resistant alloysare necessary for highest temperature.

Fans

Fan design and selection are matters for specialists. These fans should be maintainedproperly by regular lubrication and cleaning. Particular care should be paid to inletside of the fan to prevent foreign matter from being drawn into working parts. A fangives a ready means of registering the air flow to the furnace either by pipe,attachment with orifice gauge placed of the air inlet or by the use of orifice gauge inthe supply main. Straightening grids may be used in difficult condition to preventswirl effects.

Control Valves

Control valves regulate the amount of fuel entering the furnace. For every furnacethere is an ideal rate of consumption of fuel for the particular heating operation. Allvalves should be examined, cleaned and adjusted regularly to ensure that they are ingood working order.

Burners

Accurate distribution of temperature is matter of importance, frequently calling for theuse of gaseous and liquid fuels for all types.

Refractory aspect

Refractory material plays very important role in designing a furnace. Differentrefractory material like refractory bricks, castables, refractory blocks, and shapedrefractories are used in different parts of the furnaces. The refractoriness, high



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temperature resistance, cold and hot strength etc of a refractory material are of muchconsideration while designing the furnaces.

For example

In simple continuous reheating furnace roof is made of 60% Al2O3 or 42% Al2O3

bricks. Soaking zone bottom is cast with 70% Al2O3 low cement castables. Burners

are case with 60% Al2O3 low cement castable. Chimney is relined with fireclaybricks.



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CHAPTER-3

FUELS & FUEL PROPERTIES

Fuels are either solids, liquids or gases. They may be obtained naturally ormanufactured from other, naturally occurring fuels. Table-1 list some of the common fuels.

Table-1 : Some common fuels

Type Natural or primary fuels Manufactured or secondaryfuels

solid Anthracite coalBituminous coalLignitePeatWood

CokeCharcoalBriquettes

Liquid Petroleum Tar, Petroleum distillates(Gasoline, Diesel etc.)

Gaseous Natural gas Coke-Oven gas, Producer gas,Water gas, Blast furnace gas,Acetylene

Primary Solid Fuels

The primary solid fuels are the peats, lignites, and various grades of coals which haveoriginated from the vegetal matters. This has been confirmed by the fossil remains of fragments of plants, such as leaves, cellular matters, trunks etc. In the distant pastcarboniferous period, about 300 m years ago, large quantities of plant materialsunderwent decay under water, and then metamorphosed into coals by the action of

heat and pressure. Such processes continue in all ages. About 15-20 m thick vegetalmatter yields a meter thickness of bituminous coal seam. Actual coal seams can bemany meters thickness which means that enormous quantities of plant matterunderwent decay.

On dry ground, fallen vegetable matter is attacked by the oxygen of the air and isgradually converted into water and carbon dioxide so that eventually no trace remains.Under water, however, the course of decomposition is different and at first isinfluenced by the action of bacteria. The bacterial action continues until the productsof decay become so concentrated that the bacteria are destroyed. Then heat andpressure of overlying rocks, clays and other materials became the main operativefactors. These lead to the metamorphosis into coal over periods.

Stages of Coalification

The coalification process has been divided into several stages. Table-2 lists themalong with some of their typical characteristics. Peat is the stage next to wood andshows very high percentage of oxygen and hydrogen. This is due to entrappedmoisture. After that the major chemical change is gradual decrease of hydrogen andoxygen and increase in the percentage of carbon.



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Table-2 : Various stages of coalification and their approximate characteristics

Stage Proximate Anal. % Ultimate Analysis, %

VolatileMatter

Fixedcarbon

Moisture H O C

Gross Cal.Value,cal/g

Wood 6.3 43.1 49.5 3,200

Peat 26 11 56 8.3 63.0 21.0 2,000Lignite 35 23 35 6.6 42.1 42.4 4,000

Sub-BituminousCoal

28 45 24 6.1 33.8 55.3 5,200

Bituminous Coal 27 63 3 5.2 7.5 78.0 7,700

Semi-bituminouscoal

15 75 2 4.1 4.2 80.0 7,800

Semi-Anthracitecoal

9 77 3 3.6 4.9 78.4 7,300

Anthracite coal 1 88 3 1.9 4.4 84.4 7,400

The deposits of all these modifications of wood are found throughout the world. Now-a-days it is generally acknowledged that more than one factor led to the difference inthe stage of coalification of different deposits. These are listed:

(a) Not all the plant constituents decay to the same extent.(b) If the acid products of bacterial action accumulates then the decay does

not proceed fast and may hinder coalification. A more alkaline clayremoves the acid products more and thereby helps coalifications.

(c) Differences in heat and pressure at various deposits also contributetowards differences. At a given point in a coal field, the carbon contentincreases as we go down vertically. Some lignite deposits are located

just at the surface.

(d) The time is also a factor. Some of the lignite deposits are of morerecent origin.

Coals are extremely complex substances, and are mixtures of various complexchemical compounds produced from the constituents of initial vegetal matter. Someextraneous mineral matters also get entrapped mechanically. A considerable volumeof research has been done in order to determine the physical and chemical nature of coals.

Assessment of Coals

The assessment of the quality and grade of coals is commonly done by three tests.

Proximate Analysis

This is a very widely employed test because it is easy to do. The results are obtainedfast and are meaningful from the utilization point of view. The method is however,arbitrary and does not have a theoretical basis. Therefore it is carried out by strictlyobserving some standard procedures, such as the ASTM procedure given below:

(a) Moisture (M) is obtained by drying 1 gm. of sample for 1 hr at 105oC.The weight loss represents moisture.



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(b) Volatile Matter (VM) is obtained by heating 1 g. of coal of a certainparticle size range for 7 minutes at 9500C in the .absence of air. Theloss of weight represents VM + M.

(c) Ash (A) is the residue after complete combustion in a muffle at 700 -750oC.

(d) Fixed Carbon (FC) = 100 - %M - % VM - %A

The results of proximate analysis can be recalculated on Moisture-Free as well as Ashand Moisture Free basis. In the former case

%VM + %A + %FC = 100whereas in the latter case,%VM + %FC = 100

Classification of coals

The properties of the coals vary enormously. Coal seams always differ from eachother. Again, in the same seam compositions and properties may vary to some extent.

The purpose of coal classification is to simply divide coals into several classes whichmeet one or two special requirements.

A number of classifications have been proposed to date. The most successful systemshave been based on the evaluation of the quality of the coal substance, leaving theinfluence of size and content of ash or other impurity to be assessed as a secondarymatter by the experts within the industry. For thorough classification, one has to takeinto account details of the physical and chemical characteristics.

Pulverized Coal

Pulverized coal is coal in a fine state of division. It is widely used now-a-days forheating furnaces, as well as in heating the boiler of the large power stations. As amatter of fact, its consumption has increased significantly over the years since theFirst World War. Pulverized coal is fed into the combustion chamber like a thick suspension in air. It is burnt like a gaseous fuel or a fuel oil. As a result, these threecan be easily interchanged. It has all the advantages of gaseous fuels, such as highefficiency, more compact design of burner and combustion chamber, adaptability to awide variety of fuels, easy and flexible control etc. as well because mechanizedmining automatically produces them.

LIQUID FUELS

Liquid fuels are those combustible or energy-generating molecules that can beharnessed to create mechanical energy, usually producing kinetic energy; they alsomust take the shape of their container. Most liquid fuels, in widespread use, are orderived from fossil fuels; however, there are several types, such as hydrogen fuel (forautomotive uses), which are also categorized as a liquid fuel.

Fossil fuels which are also liquid fuels come from dead animals and plants which diedmany millions of years ago. The most notable of these is gasoline.



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Gasoline

Gasoline is the most widely used liquid fuel. Gasoline is made of hydrocarbonmolecules forming aliphatic compounds, or chains of carbons with hydrogen atomsattached. However, many aromatic compounds (carbon chains forming rings) such asbenzene are found naturally in gasoline and cause the health risks associated with

prolonged exposure to the fuel.

Production of gasoline is achieved by distillation of crude oil. The desirable liquid isseparated from the crude oil in refineries. Crude oil is extracted from the ground inseveral processes, the most commonly seen may be beam pumps. To create gasoline,petroleum must first be removed from crude oil.

Gasoline itself is actually not burned, but the fumes it creates ignite, causing theremaining liquid to evaporate. Gasoline is extremely volatile and easily combusts,making any leakage extremely dangerous. Gasoline for sale in most countries carriesan octane rating. Octane is a measure of the resistance of gasoline to combustingprematurely, known as knocking. The higher the octane rating, the harder it is to burn

the fuel, which allows for a higher compression ratio. Engines with a highercompression ratio produce more power (such as in race car engines). However, suchengines actually require a higher octane fuel.

Diesel

Conventional diesel is similar to gasoline in that it is a mixture of aliphatichydrocarbons extracted from petroleum. Diesel may cost more or less than gasoline,but generally costs less to produce because the extraction processes used are simpler.

After distillation, the diesel fraction is normally processed to reduce the amount of sulfur in the fuel. Sulphur causes corrosion in vehicles, acid rain and higher emissionsof soot from the tail pipe (exhaust pipe). In Europe, lower sulfur levels than in theUnited States are legally required. However, recent US legislation will reduce themaximum sulphur content of diesel from 3,000 ppm to 500 ppm by 2007, and 15 ppmby 2010. Similar changes are also underway in Canada, Australia, New Zealand andseveral Asian countries.

A diesel engine is a type of internal combustion engine which ignites fuel bycompressing it (which in turn raises the temperature) as opposed to using an outsidesource, such as a spark plug.

Biodiesel

Biodiesel is similar to diesel, but has differences akin to those between petrol andethanol. For instance, biodiesel has a higher cetane rating (45-60 compared to 45-50for crude-oil-derived diesel) and it acts as a cleaning agent to get rid of dirt anddeposits. It has been argued that it only becomes economically-feasible above oilprices of $80 (£40 or €60 as of late February, 2007) per barrel. This does howeverdepend on locality, economic situation, government stance on biodiesel and a host of other factors- and it has been proven to be viable at much lower costs in somecountries. Also, it gives about 10% less energy than ordinary diesel. NOTE: As with



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14% for ethanol and yeast. NOTE: Making butanol from oil produces no such odour,but the limited supply and environmental impact of oil usage defeats the purpose of alternative fuels. The cost of butanol is about $0.57-$0.58 per pound ($1250-$1320per ton or $8 approx. per gallon) - so another drawback is its high cost in proportionto ethanol (approx. $1.50 per gallon) and methanol.

On June 20 2006, DuPont and BP announced that they were converting an existingethanol plant to produce 9 million gallons of butanol per year from sugar beets.DuPont stated a goal of being competitive with oil at $30-$40 per barrel withoutsubsidies, so the price gap with ethanol is narrowing.

Hydrogen

Hydrogen as a fuel is a feasible option for future use as a fuel. Liquid hydrogen is animportant consideration because it has a higher density than its gaseous counterpart.Liquid hydrogen would be stored in cryogenic tanks. Its application would be mostuseful in fuel cells where hydrogen would react with oxygen (obviously this is readilyavailable in the air) to create electricity which would power the vehicle.

Unfortunately, widespread use of liquid hydrogen is several decades away. Theirapplication is plagued with several serious problems including production, which maystill involve fossil fuels, durability of the fuel cells to common roadway conditionssuch as bumps, the impracticability of conversion of older cars and difficulties withstorage and handling.

Converting energy from another form of fuel to hydrogen would lead to unavoidablelosses, so the final user would get, from burning hydrogen, far less energy than hecould get directly using the fuel.

Coal Tar Fuels

A number of liquid fuels and one solid fuel, pitch, are produced by the distillation of coal tar. These fuels are named as CTF-50, CTF-100, CTF-200, CTF-250, CTF-300and CTF-400 respectively. The numbers indicate temperatures in degree fahreignheight (oF). These fuels have a low sulphur content rarely above 1.0% by weight.They contain only small quantities of inorganic matter and amount of ash neverexceed 0.75% by weight. In the creosote and the less viscous tarry fuels the ashcontent is almost negligible. The general properties have been indicated in table givenbelow.



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Table-I



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Typical ultimate analysis of coal tar fuels are given in table-II

Table-II

It has been demonstrated by Flame Radiation Research Association that for equal heatinput the emmissivity of flame of CTF 200 is about 18% greater than that of similargrades of petroleum oil. This fact emphasizes the value of CTF for metallurgicalpurpose.

Gaseous fuels

Gaseous fuels offer a number of advantage over other fuels : cleanliness, absence of ash, ease of handling end control flexibility and good combustion characteristics.Moreover the cal. value/vol can be controlled and adjusted by mixing various gases.That is why gas heating is preferred wherever it does not become uneconomic.

Gaseous fuels are a mixture of several compounds in varied proportions. The commoncompounds can be classified into two groups: combustibles and diluents, the formerproviding the heat by combustion. The common combustibles and diluents are listed.

Combustibles Diluents

Hydrogen Nitrogen

Carbon monoxide Carbon dioxide

Methane Water

Ethane (C2H6)

Ethylene (C2H6)

and other hydrocarbons.



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Stochiometric Combustion

Combustion characteristics of gaseous compoundsCompound Vol. of O2 per vol.

of compoundVol. of flue gas per vol. of

compound using stoichiometric airNet. Cal. value,

in Kg.cal/m3

H2 0.5 2.88 2332

CO 0.5 2.88 3034

CH4 2.0 10.52 8560

C2H6 3.5 18.16 15110

C2H4 3.0 15.28 14480

C2H2 2.5 12.40 13440

Commercial gaseous fuels

Table presents the analyses and calorific values of some gaseous fuels. For aparticular gas, there is a range. Therefore the figures should be taken as some typical

values only.

Typical analyses and calorific values of dry gaseous fuels

Analysis in vol. %Gaseous fuel

H2 Co CH4 C2H

6

Otherhydro-carbon

N2 CO2 O2

Net. Cal.value, inKg.cal/m

3

Blast furnace gas 3.7 26.3 0.0 0.0 0.0 57.1 12.9 0.0 926

Procedure gas 15.0 24.7 2.3 0.0 0.8 52.2 4.8 0.2 1390

Coke-oven gas 57.0 5.9 29.7 1.1 3.4 0.7 1.5 0.0 5120

Water gas 49.7 39.8 1.3 0.1 0.0 5.5 3.4 0.2 2720Oil gas 50.8 10.2 27.6 - 3.5 5.1 2.6 0.2 4700

Natural gas 0.0 0.0 94.5 0.5 0.5 4.0 0.2 0.2 8500

The source of these gases will be briefly described now.

Blast furnaces gas is a by-product of the iron blast furnace. Coke is charged in theblast furnace and air is blower from the bottom. The oxygen of the air reacts withcoke producing CO, and CO2. The exhaust gas therefore contains primarily CO, CO2

and N2 and is known as the blast furnace gas. Although its calorific value is low,enormous quantities are produced justifying its utilization. Therefore it is cleaned free

of dust particles and is utilized in the steel plant for heating.

Producer gas is generated by passing a mixture of air and steam through a red-hot bedof coke or coal. The reactions are primarily the following:

2C + O2 = 2CO (exothermic)H2 O + C = H2 + CO (endothermic)



- 20 -

The water gas is generated by allowing steam to react with red-hot coke or coal. It ispreferred to producer gas when a high calorific value is desired. Oil gas is obtained bythermal cracking of oil and is popular in localities where oil is cheap.

Natural gas is found in the earth, either in a separate source or along with petroleum.It has the same marine origin as petroleum.

Gasification of coal

Earlier we have given est imates of coking and non-coking coal reserve of the country.It has been shown that the non-coking varieties of coal are far more abundant. Thesevarieties, with high contents of volatile matter and moisture, are essentially used forsteam and gas generation in railways, power houses, cement factories etc. Cokingcokes yield a dense product on carbonization and are vitally needed for metallurgicalapplications. Some less strong cokes, known as soft cokes find domestic applications.

Wastage of coal in SRRM sector

Major usages of coal, namely railway traction through steam locomotives, powergeneration through steam boilers and turbines etc, cause a colossal waste of energy.The efficiency of steam locomotive is 7-12% as compared to 28-30% for dieselengines and 35% in electric engines. The efficiency of thermal power generation is, atthe most, 30-35 percent. The remaining energy is lost mainly because steam used todrive the turbine cannot reach a temperature greater than what the machinery canstand.

A far greater proportion of coal is wasted during railway transportation which isresponsible for about 90% of coal despatches. The steam locomotive efficiency beingabout 7-12 percent it has been estimated that the locomotive is consumes a great equalamount to transport 20-30 mt. of coal throughout the country. Moreover, dependenceon railway transportation has often given rise to irregular supply.

In view of the high cost of coal transport it becomes necessary to locate large thermalpower stations close to collieries and washeries and transit the power to load centresby transmission lines. However, there is excessive wastage during power transmission- some 20% compared to about 5.7 and 7.7 in West Germany and France,respectively. There is also power pilferage. The break even point between railtransport and electric energy for distances greater than 700 Km. for loads greaterthan 200 MW at 200 KV or 375 MW at 440 KV. However, it should be noted thatsuch transmission necessarily implies power losses.

Gasification of coal

A fuel bed may be shallow or deep. If the bed is very shallow, the carbon burnswholly to carbon dioxide. With a deeper bed the carbon dioxide first formed, andwater vapour present in the incoming air are brought into contact with red-hot carbonand certain other reactions occur, to promote which is the function of gasification. Adeep fuel bed naturally offers the best conditions for these further reactions, thoughthey may occur to a lesser extent in the shallow fuel bed of the boiler furnace. A fuel



- 21 -

Chemical reactions in a gas producer

bed of the order of 3 feet deep or more is used for gasification for the manufacture of producer gas.

It is convenient to consider these reactions as confined to sever zones though theyoverlap into the zones above and below. The gases passing into the base of the fuelbed may comprise air and steam and when reacting in the fuel bed give rise to

chemical and thermal effects of great importance.

The ash zone at the base of the fuel bed serves to protect the grate from the intense

heat and helps to distribute the air and steam over the bed.

Above the ash zone is the oxidation zone, at which the reaction C+O2 = CO2

proceeds, free oxygen disappearing about 4 to 5 inches above the top of the layer of ash. This reaction generates heat and provides practically the whole of the heatrequired for the subsequent gasification reactions.



- 22 -

The CO2, accompanied by nitrogen and steam, travels upwards into the reducing zone.Here a reaction between CO and carbon takes place resulting in the production of thecombustible gas, carbon monoxide, thus:

C+CO2 = 2CO

This reaction (unlike most other combustion reactions) results in the absorption of heat. Heat must be supplied to the substances taking part in it; heat is absorbed fromthe red hot fuel bed and its surroundings, the temperature falls, and the reaction“slows down”. Consequently, the extent to which this reaction continues dependsupon the temperature, and upon the time available. Equilibrium values, which areattained only by sustained contact of dry air with carbon in the absence of otherdisturbing factors, are given in Table 7.

It will be seen that the percentage of CO produced increases with rise in temperature.In practice, although a temperature of 800°-900°C. would appear to give asatisfactorily high percentage of CO, higher temperatures are generally required toenable the reaction to proceed at a useful rate.

Equilibrium Figures for Reaction between Carbon Dioxide and Carbon

Temperature Composition of gasoC CO2 (%) CO (%) N2 (%)

600 13.76 11.74 74.50

700 5.85 24.95 69.20800 1.35 32.30 66.35

900 0.30 34.10 65.601000 0.08 34.42 65.50

1100 0.03 34.52 65.451200 0.01 34.56 65.43

The velocity of chemical reactions increases very rapidly with rise in temperature andis correspondingly low at lower temperatures.

Owing to the absorption of heat by the reactions which take place, as the gases passupwards through the reduction zones of the producer, the temperature of the fuel bedfalls; hence the velocity of the reactions taking place is lower than expected. Theresult is that true equilibrium conditions are not usually attained with times of contactand temperatures common in the reduction zone of industrial producers.

Since the reactions in the combustion zone give predominantly CO2, the gas formed attemperatures of 600o - 1000oC, contain in practice more CO2 than is indicated bytable. At higher temperatures of the order of 1,200oC, the reaction might pro esubstantially to completion, but owing to the influence of other factors this rarelyoccurs.

In a producer operated or “blown” with dry air, the presence of CO2 in the producergas represents a waste of fuel, because some carbon has been burnt completely to CO2

and sensible heat has been liberated within the producer instead of subsequently in the



- 23 -

furnace where the producer gas is burnt. When steam is added to the blast, however,not all the CO2 represents waste, as will be shown below. In any case, it is clearlydesirable that the temperature in the gasification zone be as high as possible. On theother hand, the gas outlet temperature should be as low as possible in order to avoidexcessive loss of sensible heat if the gas is cooled before being used.

At temperatures above about 800°C steam reacts with carbon according to theequation: C+H2O=CO+H2 with absorption of heat. Steam is, therefore, added to theblast partly to moderate the temperature in the combustion zone in order to reduceclinker formation and partly to utilize the gasification zone more effectively bygenerating additional CO and H2. The higher temperature at which the reaction takesplace the greater is the extent to which the steam is decomposed by the carbon and,therefore, the greater is the amount of CO and H2 formed, until at temperatures of theorder of 1,100° C. the steam is almost completely decomposed. The effect of temperature on the reaction when steam only is passed over or through red-hot carbonis illustrated by the experimental figures given in Table.

TABLE: THE ACTION OF STEAM ON CARBON: ANALYSES OF GASES

Temperature Steamdecomposed

CO2 CO N2

oC (%) (%) (%) (%)

674 8.8 29.8 4.9 65.2

838 41.0 22.9 15.1 61.9954 70.2 6.8 39.3 53.5

1010 94.0 1.5 49.7 48.81125 99.4 0.6 48.5 50.9

These figures do not correspond to the attainment of chemical equilibrium, but give

some indication of the extent to which the reaction may proceed under practicalconditions. In the case of a producer where the steam is mixed with a large volume of air, the relative proportions of CO2, CO, and H2 in the products would differconsiderably from the above, but the general trend with changing temperatures wouldbe the same.

In the temperature range 600—1,000° C. another reaction, which takes place betweenCO arid undecomposed steam and is known as the water-gas “shift” reaction,becomes important. This is represented by the equation:

CO + H2O = CO2 + H2

It will be noticed when steam is present the CO 2 formed according to the equationdoes not represent a waste of carbon, because the CO which reacts with the steam isreplaced by its own volume of hydrogen, which has approximately the same calorificvalue and the final gas will have the same heat content on a volume basis. It should benoted, however, that the dilution of the final dry gas by the CO2 formed in thisreaction has the undesirable effect of lowering its calorific value. As in the case of dryblast operation, a high yield of combustible gases requires a high temperature in thereduction zone, where CO and H2 are formed with absorption of heat. The necessaryheat is provided wholly by the combustion reaction in the oxidation zone, and this



- 24 -

heat must (a) heat up the fuel in the reduction zone to the necessary temperature, and(b) compensate for the absorption of heat in the reduction of CO and steam.

All the reactions are equilibrium reactions, and under suitable conditions may proceedin the reverse direction to those here indicated. At any temperature there is anequilibrium composition of the substances taking part in the reaction which is more

closely approached as time and temperature are adequate; an example was given whendealing with the reaction

C + CO2 = 2CO

At the top of the flue bed, the fresh fuel (if coal is used) is distilled, permanent gases,water and tar being added to the gases leaving the fuel bed. Any water contained inthe incoming fuel is evaporated at this stage.

The quantity of steam used has an important influence on the reactions. This isdiscussed in detail in the chapter on gas producers. It is obviously important that asmuch of the steam as possible shall be converted into gas, and in its conversion into

gas shall produce as much CO and H2 as possible.

The net results of the various reactions here described can be followedexperimentally. The course of the changes that take place has been determined byexamination of gas samples withdrawn from closely adjacent sampling points in thefuel bed of a coke-fired producer.

Composition of Gases in the Fuel Bed of a Gas Producer

Composition of gases – Percent by Vol.Zone Height of sampling pointabove grate in.

O2 CO2 CO H2 H2O

Ash 5 18.4 - - - 13.4

Oxidation 7.5 - 17.6 2.8 - 13.110 - 11.5 12.2 - 12.2

15 - 7.2 20.4 4.0 8.1

Primary reduction

20 - 5.9 23.3 7.0 4.7

Secondaryreduction

30 - 4.6 25.6 8.2 3.5

Top of fuel bed 40 - 4.7 27.0 8.5 2.7

If, as in some older producers air only is blown through the fuel bed, the resulting gasis similar to blast furnace as, thou containing less CO2 and proportionately more CO.

The common type of gas producers are vertical cylindrical chambers and coal is fedfrom top as shown in figure.

Various zone of gas producers are shown in the figure. It is probable that carbonmonoxide is, strictly speaking, the primary product of the combustion of carbon withoxygen; but while free oxygen is still present in the gas surrounding the particles of fuel this carbon monoxide burns to carbon dioxide immediately it is formed.



- 25 -

GAS PRODUCERS

PRODUCER GAS is made by blowing air and steam through a bed of incandescentsolid fuel under such conditions that the combustible matter of the fuel is convertedinto combustible gases. Gas obtained in this way finds application in many industrialprocesses.

The account of the principles of gasification indicates that in the simplest case, inwhich dry air is blown through a bed of coke of sufficient depth and at a sufficientlythigh temperature, the gas obtained consists almost exclusively of carbon monoxideand nitrogen in the ratio of about 1:2 by volume, with only traces of carbon dioxide

and hydrogen. If coal is used under similar conditions, the gas contains additionalhydrogen and methane derived from the volatile matter of the coal. In the earliestproducers, of about a century ago, such conditions were used and gave gases of thefollowing compositions and calorific values:



- 26 -

Composition of gas made (%)Fuel used

CO2 O2 CO H2 CH4 N2

Coke 1.5 - 32.5 1.0 - 65.0

Coal 1.0 - 31.0 5.0 2.0 61.0

15oC & 760 mm Hg

Such conditions, however, do not provide the maximum thermal efficiency or themost satisfactory operating conditions, because the heat liberated in the combustion of carbon-to-carbon monoxide is much greater than would be sufficient to maintain asuitable temperature in the bed of fuel. Consequently, the fuel bed temperaturebecomes excessively high and much of the potential heat of the fuel leaves theproducer as sensible heat in the gas. Another undesirable consequence of the hightemperature is the fusion of the ash, with the formation of clinker.

The addition of steam to the blast was introduced later to increase the thermalefficiency and moderate the temperature of the fuel bed by genera ting additionalcombustible gas by the reaction of steam with carbon,

C + H2O = CO + H2

which takes place with absorption of heat. Typical gas compositions obtained in thisway are:

Composition of gas made (%)Fuel used

CO2 O2 CO H2 CH4 N2

Coke 6.0 - 27.0 12.5 0.6 53.9

Coal 6.0 - 26.0 15.0 2.5 50.5

15o

C & 760 mm Hg

It will be observed that these gases contain appreciable proportions of carbon dioxide.This does not necessarily imply that any carbon has been burnt with out contributingto the thermal value of the gas made: it is due to the “water gas shift” reaction

C + H2O = CO2 + H2

Composition of Producer Gas

The major constituents of producer gas made from either coal or coke, however,under normal operating conditions, has a composition within the range

CO2 CO H2 CH4 N2

3-9 20-30 11-20 0-3 46-55 percent



- 27 -

FUEL FOR GAS PRODUCER

Description of coal

Volatile matter per cent(dry, m.m.-free)

Caking properties

3.5-6.0Anthracite

6.1-9.0Non-caking

9.6-11.5Dry steam11.6-13.5

Non-caking

13.6-15.0 Weakly caking

15.1-17.0 Medium cakingCoking Steam

17.1-19.5 Strongly caking33.1-37 Medium caking

>37 Medium caking33.1-37 Weakly caking

>37 Weakly caking33.1-37 Very weakly caking

>37 Very weakly caking

33.1-37 Non-caking

High volatile

>37 Non-caking

The characteristics of the ash of the fuel are i A low ash content is desirable but notessential, and fuels containing more than 15 per cent, of ash can be gasified quitesatisfactorily in producers equipped with mechanical grates. The ash fusiontemperature, however, is of great importance. If it is low, and particularly if the ash isvery heterogeneous, a high proportion of steam in the blast is required to avoid clinkertroubles, as will be appreciated from the general description of the fuel bed givenabove. A high fusion temperature of the order of 1,400° C., therefore, allows gas of ahigher calorific value to be made. It is important that attention should be paid tocontrol of clinker formation by control of the blast saturation temperature when the

fuel supply is variable or when a change is made in the source of supply. Foranthracite and coke the lowest permissible ash-fusion temperature is about 1150° C.,measured under reducing conditions.

Practical operation of a producer

Following are the operational practice for good operation of a producer:1) Fuel charging.2) Leveling and Pocking.3) Care of condition of fuel bed.4) Removal of Ash "Ashing".5) Blast.

6) Observation of gas temperature.



- 28 -

Conversion of gases to rich gas

It is possible to convert most of this gas into methane and thereby almost double itscalorific value and thus make it comparable to natural gas as an energy source. A flowsheet of process is given below:

PROCESS FLOW CHART

STEPS IN COAL GASIFICATION FOR HIGH CV GAS

High calorific value gas

C+O2 = CO2C+H2O = CO + H2

C+2H2 = CH4

Reactions

Steam, Air/Oxygen

Coal

Coal preparation

Devolatilization

Char + Gas

Gasification

Removal of Tar &Dust

Raw Gas

Shift conversion

Purification ex-sulphur removal

MethanationNi, catalyst

Removal of CO2

2CO+2H2 = CH4+ CO2

Steam

Tar Dust

CO2 +H2 = CO+ H2O



- 29 -

Combustion of fuel

All the fuels contain basic elements such as carbon, hydrogen, sulphur or itscompounds. The combustion of fuel is described with the help of few simple chemicalequations as given below:

Pulverized fuel and its burner design aspect

PULVERIZED fuel firing is a method whereby the finely crushed fuel, generallyreduced to a fineness such that 75-85 per cent, passes a 200 B.S. sieve, is carriedforward by air through pipes directly to burners or to storage bins from where it ispassed to burners. When discharged into the combustion chamber the mixture of airand fuel burns in a manner very similar to that of the combustion of a gas. The use of pulverized fuel has increased rapidly constitutes the principal form in which coal isused.

The increasing proportion of fines in the coal raised since machine-mining becamegeneral has rendered pulverized fuel the most satisfactory way of using fine, low-grade coal.

There is less likelihood of bonded deposits accumulating on the heating surfaces thanwith stoker firing. In addition, while there is, with stoker thing, a limit to thetemperature to which the combustion air can be pre-heated, with P.F. firing the limitis much higher. This has an important effect on the overall cycle efficiency whereregenerative heating leads to a high feed temperature.

The economic motives for the introduction and development of pulverized-fuel firingin power stations and large industrial furnaces can be listed as these:



- 30 -

1) Efficient utilization of the cheaper low-grade coal.2) Flexibility in firing with ability to meet fluctuating loads.3) Better reaction to automatic control.4) In furnaces, the ability to produce a high-temperature flame at the correct

position.

5) The ability to use high combustion air temperatures.

For installation of a P.F. plant is scaling for higher or lower capacity.

Preparation of Pulverized Fuel

Selection of fuels

Bituminous coal was the first fuel to be fired in pulverized form but subsequently themethod has been applied to other coals. The fuels now successfully burnt in powderedform include bituminous coals, the semi-bituminous and anthracitic coals, hard pitch,

and brown coal which may contain up to 65 per cent of inherent moisture. Theanthracites must, for reasons explained below, be more finely ground than the normalrun of bituminous coals but unfortunately are harder and more difficult to pulverize.All fuels that are to be pulverized must be nearly of quite dry, and this applies moreparticularly to the low-volatile fuels. If the coal is initially too wet the material mayclog in the hoppers and feeders or even in the mill, which will suffer a reduction inoutput. The moisture that gives rise to these deleterious effects is the surface moisture,i.e. that which can be removed from the coal by air-drying, and not the inherentmoisture which is removed by drying at over 100°C. One of the first matters to beconsidered, therefore, is how the coal is to be brought into the proper condition forpulverizing. If the moisture content of the coal is not unduly high, it will be dried outsufficiently by the passage of pre-heated air (at 500°—700° F.) through the grindingmill. If it contains too much moisture to be dried in this manner an auxiliary drier isnecessary.

Drying the coal

It is always objectionable, but often unavoidable, for the coal entering the mills to bewet. The moisture is carried forward in the air-stream into the furnace where it lowersthe flame temperature, increases the volume of waste gases, and in general has adeleterious effect on combustion; clearly it is desirable that the moisture content of the incoming pulverized fuel be as low as possible.

In order to enable the coal to be ground fine enough, its excess superficial moisturemust be completely, or almost completely, removed. In the system most generallyused this is effected by passing through the grinding mills the air, preheated as justexplained, to 500—700° F., that is to be used in the combustion chamber as primaryair. After pulverization, the dried-coal/air suspension leaves the drier at a temperatureof about 150° F. This temperature is selected as being low enough to avoid anypossibility of coking at the burner or pre-ignition of the coal in the pipeline betweenthe mill and the burner, but high enough to avoid condensation of the moisture drivenoff in drying.



- 31 -

The coal supply gravitates from the storage bunker, through an automatic weigher andfeeder mechanism, to the pulverizing mill in the basement. The coal is ground on ahorizontal rotating table by three spring loaded rollers running in contact with thelayer of coal, while a continuous blast of hot air from the duct dries the moisture outof it and transports the resulting powder, through the medium of the exhauster fan,

each of the four corner burners.

Choice of system

The bin-and-feeder is not favoured in modern power stations, on account of dangersfrom explosion of coal/air mixtures and the tendency of the stored pulverized fuel tocake. In addition, maintenance costs, losses through vents, capital costs and operatingcosts are all against the system. Consequently the unit system is used. The bin-and-feeder system still finds application in special circumstances, for example when firingseveral metallurgical furnaces on one works.

Mechanism of combustion of pulverized fuel

Factors influencing the time required for the combustion of a particle of coal includecoal properties such as rank and size, temperature, and aerodynamic conditions in thefurnace into which the particle is injected.

Studies of the ignition and combustion of particles of coal injected into a hot furnacein a stream of air indicate that combustion takes place in three stages, (a) pre-ignitionduring which changes in shape and size (depending on the rank of the coal) and someevolution of volatile matter occurs, (b) ignition and combustion of the volatile matter,(c) combustion of the residual carbon.

During combustion a particle is surrounded by an “atmosphere” of combustionproducts through which the oxygen has to diffuse to react with the carbon.Experiments have shown that the time for the combustion of a 200-mesh (76µ)particle of coal containing 30-40 per cent. volatile matter is approximately 0.3 second,and that the time required for combustion varies approximately as d1.5, being thediameter of the particle. Higher burning rates can be attained with relative motionbetween the air and the burning particle to increase the rate of diffusion to the carbonsurface. With small particles however, as is mentioned later, this is difficult to achieveas the particle rapidly achieves the velocity of the air stream into which it isintroduced.

Since the solid residue from low volatile coals and anthracites is relatively uncreative,these fuels are more difficult to burn than coals of lower rank. Consequently theyshould be more finely ground and used with less excess air, whilst heat losses shouldbe avoided by providing refractory walls backing the ignition zone. These precautionsare essential to secure satisfactory operation.



- 32 -

The combustion of pulverized coal in practice

Burners

The main requirements of a burner are that it shall ensure rapid and intimate mixingof fuel and air, and maintain an adequate supply of oxygen surrounding each fuelparticle until its combustion is completed. The need for intimate and rapid mixing of

fuel and air within the furnace dominates the design of the burner. According towhether a short or a long flame is required the burner is designed for turbulent orparallel flow.

Fig: P.F.burner.

In designing burners it is assumed that the coal/air ratio can be correlated with the rateof flame propagation. Any appreciable lack of balance between these two will causeeither loss of ignition or back-firing. This is illustrated in Fig. which indicates thespeed of flame propagation with various fuel/air mixtures and with different types of coal. It will be observed that the maximum rate of flame propagation, for low volatilecoals is at a point where the air/coal ratio is low, and this ratio must be still furtherreduced for anthracite.



- 33 -

To a major degree the volatile content of the fuel controls the stage of ignition, which-is the point where combustion becomes self-supporting. The temperature of ignitionof bituminous coal is significantly lower than that of anthracite and this is anotherfactor of which account must be taken in burner design. To overcome this difficulty,arrangements have been made in certain installations to extract part of the air from thefuel/air mixture at the burner by the use of an “air separation” burner, and to control

the ignition by regulating the quantity of air extracted, the vented air beingsubsequently introduced into the combustion chamber at a suitable point as secondaryair.

In direct-fired systems the air/coal ratio is important since the amount of air requiredfor sweeping the mill and conveying the fuel may affect the composition of theair/fuel mixture at the burner. Some systems incorporate an additional air controlwhich enables the composition of the fuel/air mixture to be adjusted and reduces theneed to relate the burner design closely to the air-flow characteristics of thepulverizer.



- 34 -

Fuel distribution

It is important to secure uniform distribution of fuel to the several burners feeding afurnace and to avoid local differences in concentration Attempts have been made toachieve this by situating the exhaust fans and fuel piping symmetrically in relation tothe burners. These attempts have not been completely successful and mechanical

distributors have been found to be necessary.

Air pre-heating

The importance of high furnace temperature to secure a high rate of radiant heattransmission from burning fuel to the receiving surfaces is clear. Air preheat will helpto heat the fuel more quickly and will thus enable the processes of distillation andcombustion previously described to be completed in a shorter time. The mainpractical advantage of preheating air, however, is to reduce the flue gas loss.

It is impossible to prescribe exact temperatures for either the primary air or thesecondary air. If conveying air is used for drying coal and also, as secondary air the

ultimate temperature will depend on the extent of drying upper limit of hot drying air.Limit of the temperature of that air when used as primary air being about 150oF. Theuse of a much higher temperature for secondary air is, of course, desirable, but herealso there are limits according to the nature of the combustion cycle. An unduly hightemperature cannot be used in the case of high volatile bituminous coals owing to therisk of their incipient distillation: with lower volatile coals and anthracites, of course,much higher temperatures are possible, i.e. up to 600

oto 700

oF

3.

One of the main problems in pulverized-fuel firing is to shorten the time required forcombustion. The development of combustion chamber design has been markedthroughout its history by the endeavour to burn the maximum amount of coal dust inthe minimum space and in minimum time. To this end, means were sought forgrinding the coal to a very fine sub division at an economic cost. But for a givenfineness, shortening of the combustion time is mainly the aerodynamic problem of bringing air and fuel together within the furnace as quickly as possible. This meansthat the air, waste gas and fuel particles must be in a state of violent motion, orturbulence, relative to one another. Early designers sought to achieve this through theuse of burners causing a turbulent or whirling motion, skillfully introducing primary,secondary and sometimes tertiary air. Such burners promote quick ignition but it is acharacteristic of pulverized-fuel firing, in distinction to stoker-firing, that it isextremely difficult to influence the tail of combustion. This cannot be done by burneragitation or by introducing secondary air at points farther along in the path of theburning dust.



- 35 -

Characteristics of Naptha

Naptha is a class-A product as per Petroleum Act, 1934.

Density : 0.70 to 0.75 (0.7165gm/ml at 15oC)

Calorific value (HHV), Kcal/Kg : 11,200Flash point : <23 oCResidue on evaporation : 5 mg/ 100 ml max.IBP ( Initial boiling point) : 45 oCFBP (Final boiling point) : 156

oC

Total Sulphur : 100 ppmAromatic, % volume : 11.5Olefines, % volume : NilLead Content : 3 ppmReid vapour pressure RVP at 38 oC : 0.42 kg/cm2

Carbon/Hydrogen ratio : 5.5Viscosity at 37.8 oC : < 0.5 CST

Characteristics of High Speed Diesel Oil (HSDO)

Specification of Diesel Fuel Corresponds to IS:1460-1974

Acidity, inorganic : NILAcidity, total, mg KOH/g, max : 0.50Ash, % Wt. max. : 0.01Carbon residue (Ramsbottom) % wt. max. : 0.20Cetane number, min. : 42Pour point, oC max. : 6Copper Strip corrosion for 3 hrs. at 100 oC : Not worse than No.1Distillation, % recovery at366 oC, min.

: 90

Flash point, Abel oC, mm. : 32Kinematic viscosity, CST. at38oC

: 2 to 7.5

Sediment, % wt. max. : 0.05Total sulphur, % wt. max. : 1.0Water content, % V max. : 0.05Total sediments, mg/100 ml max. : 1.0The composition is as givenbelow

: H2 = 13.6%C = 85.4%

S = 1%

In accordance with the definition of classification, HSDO is Class - B petroleum.



- 36 -

Characteristics of Light Diesel Oil (LDO)The specification of light diesel fuel conforms to IS:1460-1974Acidity, inorganic : NIL

Ash, % Wt. max. : 0.02

Carbon residue (Ramsbottom) % wt. max. : 1.5

Pour point,o

C max. : 12 - 18Copper Strip corrosion for 3 hrs. at 100 oC : Not worse than No.2

Flash point, Abel oC, mm. : 66

Kinematic viscosity, CST. at38oC

: 2.5 to 15.7

Sediment, % wt. max. : 0.10

Total sulphur, % wt. max. : 1.8

Water content, % V max. : 0.25

Total sediments, mg/100 m

max.

: 1.0

Since the flash point of LDO is just above 65 oC, this is a class-C petroleum as per definition.

Characteristics of Low sulphur heavy stock (LSHS) Oil

The specification of LSHS (including other grades of furnace oils) shall conform to IS-1593-1982

Acidity, inorganic : NIL

Ash, % by mass, max. : 0.10

Gross CV, Kcal/kg : 10,500

Flash po int (Pensky Morten closed), °C : 72

Kinematic viscosity, in centistokes at 50°C, max. : 500

Sediment, % by mass, max. : 0.25

Sulphur, total, % by mass, max. : 1.75

Water content, % by vol. ,max : 1.00

Note:1. Although the above characteristics specify total sulphur as 1.75% by mass max., M/s IOC

in a letter have categorically stated that the sulphur content of LSHS will vary from batchto batch and refinery to refinery and the maximum sulphur may go upto 4% by mass.

2. In accordance with the definition of classes of petroleum, LSHS is a class-C petroleum.



- 37 -

Low Viscosity (LV) Grade Furnace OilThe specification of LV Grade furnace oil shall also Correspond to IS-1593-1982

Acidity, inorganic : NIL

Ash, % by mass, max. : 0.1

Gross CV, Kcal/kg : ~10,000Flash point (Pensky Morten closed), °C : 66

Kinematic viscosity, in CST at 50°C, max. : 80

Sediment, % by mass, max. : 0.25

Sulphur, total, % by mass, max. : 3.5

Water content, % by vol. ,max : 1.0



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CHAPTER-4

DESIGN OF BURNER

Designing of a burner is essential for proper fuel utilization and to meet the requiredflame length and also proper utilization of furnace space.

Liquid fuel burners should meet the following requirements:

i) they should atomize liquid fuel and mix it thoroughly with air.ii) ensure stable combustion and form an uninterrupted flame of a desired

length.iii) should be reliable in operation, simple in design and convenient for

cleaning.

In industrial furnace, liquid fuels are mostly atomized before burning. The mainstages of burning of liquid fuel are (a) atomization (b) ignition which is preceded andfavoured by inter-mixing, preheating and evapouration; and (c) combustion of liquid

fuel droplets.

a) The fuel is disintegrated into droplets by mean of an atomizer. The process isonly possible when pressure of moving atomizer exceeds the force of surfacetension. Equating the pressure of atomiser and the surface tension of liquidfuel we can obtain the following formula for size of droplet.

1r 300

lW2

Where W = relative velocity of atomizer m/sec.

l = atomizer density Kg/m3.

b) Ignition occures as atomized liquid fuel begins to evaporate upon entering amedium at high temperature, the vapour air mixture that forms at the surfaceof droplets ignites first, the temperature at which this vapour-air mixturesignites is called ignition temperature of fuel.

As has been found evaporation increases with smaller size of droplets. Theboiling point of liquid fuel is lower than the ignition point. It has been foundexperimentally that the pattern of ignition of liquid fuel at a temperature upto500oC is the same as that of homogeneous mixture in which O2 initially

penetrates fuel molecules and then the chain process of gradual oxidationtakes place. At the temperature above 500oC fuel molecules under gopreliminary thermal cracking. The process of ignition is accelerated withincreasing temperature.

c) Combustion of droplet: All the stages of fuel combustion occur either at thesurface of fuel droplet or near it. Burning droplet together form a flame.



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As has been shown experimentally the similarity between combustion of single droplet and atomized fuel exist. It relates both completeness of combustion and the flame temperature. The combustion of liquid fuel mainlyoccurs in vapour phase.

Of great practical importance is the rate of burning of liquid fuel droplets.

Combustion of large droplets are more stable and such droplets exist longerthan smaller ones.

The full time of burning of droplets, is expressed as follows:

lQv

t = r2 sec.2(Tm - Td)

Where t = full time of burning of a droplet

l = density of liquid fuelQv = latent heat of vapourization of liquid fuelTm = temperature of gaseous mediumTd = temperature of droplet surface

= thermal conductivity of liquid fuel

The total time of burning is-tt = t1 + t2t1 = time of burning of gaseous volatilest2 = time of burning of coke residue

Usually t1 = 0.5 - 0.6 sec.t2 = 0.3 - 0.35 sec.

Liquid burner meets the requirement of atomisation, mixing, stable-combustion anduninterrupted flame of desired length. It should be simple in design and convenientfor cleaning. It is divided into two large groups as low pressure and high pressureburners.

Comparative characteristic of Low & High pressure liquid burner

# Characteristic Low Pressure

burner

High Pressure burner

a) Atomizer blow, air Compressed air, streamb) Atomizing pressure

KNm

2

2.94 - 8.82 Compressed air, 588-784Steam 588-1774

% of atomizing air 100 7-12c)% total air for combustion 0 88-93

d) Ultimate temperature of airpreheat oC

300 Heating of secondary air notlimited



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e) Consumption of atomizerper kg of fuel oil

-- 0.6-0.8

f) Speed of exit of atomizerfrom burner mouth m/sec

50-80 330, in some cases higherspeeds are possible

g) Degree of atomization(diameter of droplets) mm

upto 0.5 0.05

Low pressure burners are calculated to determine the outlet Xnal area for liquid fueland air

f f = Ab = mm2

f f Pf

Where,A = Co-eff equal to 195.625 if P is measured in N/m2.b = fuel oil flow rate Kg/hr.

f = 0.2-0.3.Pf = Fuel oil pressure N/m2.

f = Fuel oil density Kg/m3.

KVaf a =

a Pa a

fuel oil densities 950 - 960 Kg/m3.f a = Xnal area mm2.Va = air flow rate m

3 /hr.

a = air density Kg/m3.

a = coefficient 0.7 - 0.8.

Pa = total head of air N/m

2

.

High pressure of special design are employed in cases where a sharp bright flame isessential. A example is shown in the figure below:



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Channel 1) for fuel oil has a constant cross-sectional area. The size of steam/air slit isvaried by moving a fuel tube. 2. This can be done on loosening nut 3. This method of adjustment is quite complicated and in practice the fuel oil supply pipeline andpipeline for air are provided with valves which are easy to control the flows. Thisresults in proper atomizing effect and increased consumption of speed.

The velocity of atomizer efflux does not exceed that of sound 330 m/sec.The burner produces a long flame 2.5 - 4m.

The amount of steam to atomize 1 kg of preheated fuel oil is 0.4 to 0.6 Kg and that of compressed air from 0.6 to 0.8 m

3.

Basic dimensions of this type of burner for different capacities can be found in below.

BASIC DIMENSIONS OF OIL BURNER

Outlet diameter (mm) Productivity (Kg/hr)BurnerNo. For fuel

oil

For steam

Diameterof fuel oil

pipe(inches)

I II III

1 2 4.5 3/8 3 7 10

2 3 5.5 3/8 6 20 303 4 7.0 1/2 12 40 60

4 5 8.0 1/2 19 60 905 6 9.0 1/2 27 80 120

6 7 10 1/2 38 100 1507 8 11 1/2 50 130 180

8 10 13 3/4 70 180 240

9 13 16 3/4 125 250 320

10 16 20 3/4 200 350 400

Different pressure ranges required for I, II & III have been indicated below:

I - For fuel oil pressures 4.9 KN/m2

II - For fuel oil pressure 58.8 KN/m2

Steam/air/pressure before burner 294.2-494.3 KN/m2

III - Fuel pressure 196-245 KN/m2

Steam/air pressure above 490 KN/m2

Using Laval nozzle and a diffuser another class of burner has been designed whichgives efflux velocity of 750 m/sec or more with higher degree of oil atomization.

Oil burners particularly for coal tar fuels have been also in use and workingsatisfactorily using two stage emulsification of fuel by steam. Advantage of this classof burners is in its working at lower pressure.



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CHAPTER-5

INSULATION ASPECTS

Introduction

The need for efficient thermal insulation has become more important with higheroperating temperatures and increasing energy costs. Prevention of heat leakage by

judicious application of thermal insulation is the simplest method of achievingsubstantial economy in energy consumption. With the advent of technology ininsulating materials, many upgraded alternatives are available to engineers forapplication in industry. Though thermal insulation is equally relevant for cryogenicapplications, this booklet is limited to medium temperature ranges (50 to 600

oC).

Insulation for high temperatures of 600oC, which is normally encountered inindustrial furnaces, is covered in the booklet on 'Refractories' in this series. Theillustrative examples given in the booklet are meant for helping the reader understandand appreciate the importance of insulation. However they should not construed to beideal or normative. It is always desirable to make individual calculations for thermal

insulation on account of diversity of the factors which are subject to change withtime, type of industry, fuel and a host of other parameters. The approaches / methodsgiven in this booklet may have to be supplemented by appropriate references of thebooks / handbooks and the data to be obtained from the insulation Manufacturers / Suppliers to tackle the practical problems.

Functions of Insulation

Thermal insulation serves several functions such as:i) Saving of energyii) Fire protectioniii) Conservation of products

iv) Control of temperaturev) Increased productionvi) Better working conditions

Refractory lining

A typical refractory lining is indicated below for effective heat saving and to ensuremaximum life. Different types of refractories and other insulating materials arerequired for furnace proper, flue offtake and flue duct from furnace to chimney,chimney, doors, burner blocks etc.

Design of the wall thickness may be worked out based on data for these tables.

Selection of working layer for different zones is done on the basis of furnace insidetemperature at these zones. Typical material selection is indicated below.

Indicative lining pattern of Furnace Roof

Soaking Zone: 250 mm thick special shaped bricks made of 60% AluminaRefractories backed by 50 mm thick Insulating castable.



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Heating Zone: 250 mm thick special shaped Refractories 60% Alumina qualitybacked by 50 mm thick Insulating castable.

Pre-Heating Zone: 250 mm thick special shaped Refractories 40% Alumina qualitybacked by 50 mm thick Insulating castable.

Indicative lining pattern of Furnace side walls and end walls

Discharge end wall: 230 mm thick 60% Alumina quality Ref ractories backed by 115mm hot face insulation bricks, 115 mm thick Mica Insulation bricks (lS-2042) and 75mm thick Calcium silicate blocks insulation and 5 mm thick asbestos sheet.

Side walls (Heating & Soaking zones): 230 mm thick 60% Alumina qualityrefractory bricks backed by 115 mm thick hot face insulation bricks, 115 mm thick Mica Insulation bricks / Cold face insulation bricks, 75 mm thick Calcium silicateblock insulation and 5 mm thick asbestos sheet.

Side walls (Preheating Zone): 230 mm thick 40% Alumina quality (IS-8) refractory

bricks backed by 115 mm thick Hot face insulation bricks, 115 mm thick MicaInsulation bricks, 75 mm thick calcium silicate block insulation and 5 mm thick asbestos sheet.

End wall (Charging side): 230 mm thick 40% Alumina quality( IS-8) refractorybricks backed by 115 mm thick Hot face insulation bricks, 115 mm thick MicaInsulation bricks, 75 mm thick calcium silicate block insulation and 5 mm thick asbestos sheet..

Flue Off take/down corners: 150 mm thick ceramic fibre blanket (RT128) to be heldin position by heat resisting (Stainless steel) studs and washers in case of flue port atthe roof. If the flue line is below the hearth, lining pattern will be same as that of charging side end wall.

Soaking Zone of the Hearth: 150mm thick High Alumina fire bricks (60% Alumina)backed by 75 mm thick 40% Alumina quality firebricks, 230 mm thick cold faceinsulation bricks and 5 mm thick asbestos sheet.

Heating Zone of the Hearth: 150 mm thick High Alumina quality fire bricks (60%Alumina) backed by 75 mm thick 40% Alumina (IS-8) Firebrick, 115 mm thick Hotface insulation bricks, 115 mm thick cold face insulation bricks and 5 mm thick asbestos sheet.

Pre-Heating Zone of the Hearth: 150 mm thick High Alumina fire bricks (45%Alumina) backed by 75 mm thick 40% Alumina quality firebricks, 115 mm thick Hotface insulation bricks, 115 mm thick cold face insulation bricks and 5 mm thick asbestos sheet.

Charging end Doors: 175 mm thick RT128 grade ceramic fibre module backed by25 mm thick mineral wool insulation blanket.



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Discharge end Doors: 225 mm thick RT128 grade ceramic fibre module backed by25 mm thick mineral wool insulation blanket.

Recuperator area: 115 mm thick lS-8 quality firebricks backed by 115 mm thick Mica insulating bricks and 50 mm thick calcium silicate block insulation.

Flue duct from Recuperator up to chimney: 115 mm thick IS-6 quality firebricksbacked by 115 mm thick Mica insulating bricks.

Structural Design

Main structures of the furnace consist of bottom and top ties beams, wall plates androof structures. There are different arrangements for these structures. Some of thearrangements are described below.

Bottom and top tie beams

Bottom and top tie beams are made of ISMB 150 to ISMB 250. The bottom tie beams

are placed on RCC foundation at span of 500 to 1000 mm. Adequate space betweenthe bottom tie beams is required for cooling the hearth of the furnace. On the bottomtie beams 10 to 12 mm thick M.S plate is provided and on this plate hearth isconstructed.

Top tie beams are tied to the side walls. Top tie beams carry the weight of the roof.Normally beams of size ISMB-250 to 350 at a spacing of 500-750 mm are used fortypical furnace inside widths of 3000 to 4500 mm.

Walls

There are two types of wall construction, viz., (i) Panel construction and (ii) In situwelding. In the panel construction, wall plates of length 4000 to 5000 mm and of required width are prefabricated and are fixed to the bottom the tie beams with bolts.Similarly the top tie beams are fixed with bolts. This construction has advantage of dismantling provision, when required. In the in situ welding walls all the constructionis done at the site and all the assembly of tie beams and side walls is done by sitewelding.

Roof

In case of arch roof, function of structures is to hold the side walls. Where as in caseof flat roof, function of roof structures, which include roof tie beams, is to carry the

weight of refractory of the roof as well as hold the walls. For suspending therefractory of the roof there are different metallic hanger arrangements.



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CHAPTER-6

HEAT RECOVERY

In any industrial furnace the products of combustion leave the furnace at atemperature higher than the stock temperature. Sensible heat losses in the flue gases,

while leaving the chimney carry 35 to 55 percent of the heat input to the furnace.Typical quantities of waste heat available in different operations are listed in Table-1below:

Furnace Flue Gas Temperature (oC)

Small billet heating furnace 500-800

The higher the quantum of excess air and flue gas temperature, the higher would bethe heat availability. The sensible heat in flue gases can be generally salvaged by thefollowing methods.

Considerable fuel saving can be affected by preheating the combustion air. The heat

saving devices used for this purpose are normally the recuperator and the regenerator.The benefit of preheated combustion air are -

1. Saving in fuel consumption.2. Increase in flame temperature.3. Improvement in combustion.4. Reduction in initial heating time.5. Reduction in scale loss.

In a recuperator, heat exchange takes place between the flue gas and the airthrough metallic or ceramic walls. On the other hand, in a regenerator, the fuel gasesand the air to be heated are passed alternatively through a heat strong medium,thereby resulting in transfer of heat.

A comparison of the advantages of recuperator over a regenerator is shown below:

Regenerator Recuperator

Requirement of size and space Higher Lower

Where used Class and Steelmelting furnaces

Smaller engineeringfurnaces

Requirement of civil work Large Small

Initial installation andmaintenance cost

High Low

Regenerator:

The regeneration which is preferable for large capacities has been very widely used inglass and steel melting furnaces. Important relations exist between size of the furnace(and regenerator), time between reversals, thickness of brick, conductivity of brick



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and heat storage ratio of the brick.

In the regenerators, the time between the reversals is an important aspect; longperiods would mean higher thermal storage and hence large regeneration and highercost. Further more, long periods of reversal result in lower average temperature of preheat and consequently reduce fuel economy.

Accumulation of dust on the bricks and slagging of the brick surfaces, cause theefficiency of heat transfer in the regenerator to decrease as the furnace becomes old.Heat losses from the walls of the regenerator and leakage of air inwards during thegas period and out during the air period also cause an apparent decrease in the heattransfer coefficient. Taking into effect all the above factors, the operating with naturaldraft can be taken as 4.0 to 6.5 Kcal/m2 / oC.

Besides, the checker brickwork heat can be salvaged from flow gases by means of ceramic balls or rotary regenerators. This device consists of multiples of slightlyseparated metal plates supported in "frame attached to a slowly moving rotor shaft,

which is arranged edge on to the gas and air flow. As these plates pass; progressivelythrough the gas stream, they give up heat to the air before re-entering the hot gasstream, thus maintaining the regenerative cycle. Seals are provided to reduce airinfiltration into the gas. Soot blowers are located periodically.

FIG ILLUSTRATION OF A REGENERATIVEFURNACE

REGENERATOR



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Recuperators

A recuperator is a heat exchange between the waste gases and the air to be pre-heated. It usually consists of a system of ducts or tubes, some of which carry the airfor combustion to be pre-heated: the others contain the sources of waste heat.

The recuperators may be of ceramic or metallic types. The ceramic recuperators arebulky and offer considerable resistance to transfer of heat because of lowconductivity and also have a greater tendency for leakages. Metallic recuperators.however, are less prone to leaks and thermal expansions and can be controlled.Metallic recuperators are easier to maintain and install and involve less initial cost.Due to the above reason, ceramic recuperators are not widely in use.

Some of the common flow arrangements encountered in recuperators are depicted.

However, in a cross flow recuperator the air arid hot gases flow at right angles to eachother. The considerations for design and selection of a metallic recuperator must takeinto account.

Waste gas temperature Desired air/fuel gas pre-heat

Initial cost and maintenance cost

Materials available for use in recuperator

Operating pressure, on the fuel gas and combustion air side. as well aspermissible pressure drops in the recuperator system

Availability of space for installation of recuperator

Campaign life of furnace and reliability desired in the system

Metallic recuperators can be of three basic types depending on the method of heattransfer, viz. radiation, convection, combined convection and radiation type.

GAS DOWNFLOW AIR

AND GAS COUNTERFLOW SINGLE PASS

GAS UPFLOW AI R

COUNTERFLOW THREEPASS

GAS UPFLOW AND DOWNFLOW

AIR COUNTERFLOW SINGLE PASS

GAS UPFLOW AND DOWNFLOWAIR COUNTERFLOW SINGLE PASS

GAS UPFLOW AND DOWNFLOWAIR COUNTERFLOW TWO PASS

GAS DOWNFLOW AI RPARALLEL FLOW THREE

PASS



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Radiation Recuperator

In a radiation recuperator, the products of combustion enter the recuperator throughan opening in the furnace roof while air flows at a higher velocity through a narrowannulus between the outer and inner walls. The outer shell is insulated. Fins areprovided on the inner shell to increase the heat transfer area. This type of a simplerecuperator is particularly suitable for small forging furnaces. This is shown ahead.



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METALLIC RADIATION RECUPERATOR

Radiation recuperators can operate with waste gas temperatures in the range of 1000

to 1500oC to pre-heat air up to 600oC. The advantages of radiation recuperator overconvection is that the heat transfer is intensive through radiation and also offerspossibility of a higher air pre-heat. Other advantages are low-pressure drop in fuel gasside because of larger cross sections; thereby enhancing the possibility of using dustladen waste gases.

Convective Recuperator

The shell and tube type recuperator are called convective recuperator. As shown inthe Figure below the cold air is carried through a number of parallel small diameterstubes, while the outgoing hot gas enters a shell surrounding the tubes and passes overthe cool tubes one or more times in a direction normal to their axis. Shell and tube

recuperator are generally more compact and have a higher effectiveness than radiantrecuperator. This is because of the larger heat transfer area made possible though theuse of multiple tubes and multiple passes of the gases through baffling.



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In order to overcome the temperature limitation of metallic recuperators, which isabout 1000oC on the gas side, ceramic tube recuperators have been developed. Thematerials of ceramic recuperators allow operation on the gas side up to 13000C andon the preheated airside up to 850°C. Ceramic recuperators have limited use becauseof difficulties in maintaining tight air/water gas seals and also due to their

susceptibility to thermal shock. Later development have led to various" kinds of shortsilicon carbide tubes joined by flexible seals located in the air headers.

It is important when installing a recuperator in the fuel system to ensure that draughtloss is minimized, otherwise furnace operation and performance may suffer. Increasein the fuel gas temperature or decrease in air preheat temperature after therecuperator, indicates the need for cleaning the recuperator tubes.

Cross Flow Recuperator

These recuperator are used in re-rolling mill furnaces by directing hot fuel gases at90° angel to the direction of flow of air through the tubes.

The heat exchange in surface required by a cross flow recuperator is greater than thatrequired by a counter flow recuperator, for equal heat transfer. For a given heatexchange areas, the heat transfer of parallel flow, cross flow and counter flow areroughly as 2 to 2.8 to 3, under average condition.

Combined radiation and convection recuperator

The advantages of tradition and convention recuperators are combined in thearrangement of heat exchanging surfaces. In this system, a tube bundle is arranged ina ring inside a double shell type recuperator.



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A case study of a galvanizing furnace in a steel tube plant showed significant fuelsavings on installation of a metallic recuperator as illustrated below:

System: heating of combustion air in a recuperator Average consumption of furnace oil in the furnace: 3000 litres/day (24 hrs) Temperature of flue gases before heat recovery: 5800C

Temperature of fuel gases after installation of recuperator at the exit: 300o

C Temperature of combustion air at recuperator outlet: 320oC Quantity of heat recycled back to the furnace by preheating of combustion air:

171,000 Kcal/hr Quantity of furnace oil saved per day: 500 litres / day Monthly saving in furnace oil: 500 x 25 = 12500 ltrs.

Payback period on investment: 1½ months Quantitatively, every 21oC rise in combustion air temperature results in one

percent fuel oil savings However, the quantum of savings is normally greater with higher flame

temperature, reduced excess air levels and higher productivity of the furnace.



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CHAPTER-7

STATUTORY AND SAFETY REQUIREMENTS

1. Every tank or receptacle for storage of petroleum in bulk (other than a well-head tank)

shall be constructed of iron or steel in accordance with the relevant Indian Standardsor specifications approved by the chief controller (Ref. Page-103 of the PetroleumRules, 1976)

However, API-R21615-1987 recommends that underground petroleum storage tanksmay also be manufactured from Fibreglass Reinforced Plastic (FRP).

2. When 2 or more tanks are installed in one enclosure, the total capacity of the tanks inthe enclosure shall not exceed 60,000 Kl in case of conventional fixed roof tanks and1,20,000 Kl in the case of floating roof type tanks.

A combination of fixed and floating roof tanks can be also located in the same

enclosure/premises but the total capacity in this case shall not exceed 60,000 Kl.

3. CI valves are not permitted in oil installations.

4. Height of storage tanks shall not exceed one and a half times their diameter or 20 inwhichever is less.

5. An air space of not less than 5% of the total storage capacity shall be considered whiledesigning oil storage tank capacity. (Ref. Pt.4 of page—104, Petroleum Act, 1934)

6. All tanks for the storage of petroleum in bulk installed above or below ground shall beprotected against corrosion by use of protective coatings and cathodic protection.

7. Tanks after erection or repair shall be tested with water pressure. The water used fortesting shall be free from petroleum and shall not be passed through any pipe or pumpused for the conveyance of petroleum.

8. Ministry of Petroleum prescribes that for large industries either a full rake of 76 tank wagons or half rake of 38 tank wagons shall be procured and unloading points shallbe accordingly provided.

9. Every storage and handling facility shall, at all times, maintain the distances from anyother facility in accordance with table-I & table—II.

10. Grouping of petroleum products for storage purposes shall be based on the productclassification. Class- A and /or Class-B petroleum can be stored in the same dykedenclosure. Class-C should preferably be stored in separate enclosure. However, whereclass-C is stored in a common dyke along with Class- A and/or Class—B, all safetystipulations applicable for class A or B should apply.

11. No fire water/foam ring main shall pass through dyked enclosure.



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12. Piping from/to any tank located in a single dyked enclosure should not pass throughany other dyked enclosure. Piping within the dyked enclosure should be minimised.

13. Tanks shall be arranged in maximum two rows so that each tank is approachable fromthe road/area surrounding the enclosure. However, tank having capacity of 50, 000 m3

and above shall be laid in a single row.

14. Any tank having a diameter more than 30 m should be separated with fire walls fromother tanks.

15. Firewalls should be constructed by limiting the aggregate capacity of each group of tanks within 20,000 m3.

16. The minimum distance between a tank shell and the inside of the dyke wall shall notbe less than half of the tank diameter.

*****

Furnace eng SP

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