FUELS AND HEAT POWERFUELS, FURNACE, AND FIRINGINTRODUCTIONFuel is a substance which, when burnt, i.e. on coming in contactand reacting with oxygen or air, produces heat. Thus, the substancesclassifed as fuel must necessarily contain one or several of thecombustible elements: carbon, hydrogen, sulphur, etc. In the processof combustion, thechemical energyof fuel is convertedintoheatenergy.To utilize the energy of fuel in most usable form, it is reuired totransform the fuel from its one state to another, i.e. from solid to liuidor gaseous state, liuid to gaseous state, or from its chemical energyto some other form of energy via single or many stages. In this way,the energy of fuels can be utilized more e!ectively and e"ciently forvarious purposes.Fuels are also any materials that store potential energy in formsthat can be practicably released and used for wor# or as heat energy.The concept originally applied solely to those materials storing energyin the formof chemical energy that could be released throughcombustion,$%& but the concept has since been also applied to othersources of heat energy such as nuclear energy 'via nuclear fssion ornuclear fusion(.Fuels arecontrastedwithother methods of storingpotentialenergy, such as those that directly release electrical energy 'such asbatteriesandcapacitors( or mechanical energy'suchas)ywheels,springs, compressed air, or water in a reservoir(.History%The frst #nown use of fuel was the combustion of wood or stic#sby *omo erectus near +,,,,,,,, 'two million( years ago. Throughoutmost of human history fuels derived from plants or animal fat were theonly ones humans used. -harcoal, a wood derivative, has been usedsince at least .,,,, /-0 for melting metals. It was only supplanted byco#e, derived fromcoal, as 0uropean forests started to becomedepleted around the %1th century. -harcoal briuettes are nowcommonly used as a fuel for barbecue coo#ing. -oal was frst used as a fuel around %,,, /-0 in -hina. 2ith thedevelopment of the steam engine in the 3nited 4ingdom in %5.6, coalcame into more common use as a power source. -oal was later used todrive ships and locomotives. /y the %6th century, gas extracted fromcoal was being used for street lighting in 7ondon. In the +,th and +%stcenturies, the primary use of coal is to generate electricity, providing8,9 of the world:s electrical power supply in +,,;. Fossil fuels were rapidly adopted during the industrial revolution,becausetheyweremoreconcentratedand)exiblethantraditionalenergy sources, such as water power. They have become a pivotal partof our contemporary society, with most countries in the world burningfossil fuels in order to produce power.-urrently the trend has been towards renewable fuels, such asbiofuels li#e alcohols.Problems with Felsect and shouldpropagateclearof anysolidob>ect. Dnyobstructionwill de@atomize the fuel particles thus a!ecting combustion and createblac#smo#e. If )ameimpingesonthestoc#, therewouldbeincrease in scale losses 'Mefer Figures 8.%5 and 8.%1(.Fi,re 1#$@ He!t Distribtio& i& Fr&!*e !&' Fi,re 1#$AAli,&me&t o) 9r&ers i& Fr&!*eb.( If the )ames impinge on refractories, the incomplete combustionproducts can settle and react with the refractory constituents athigh )ame temperatures. 1,c.( The )ames of di!erent burners in the furnace should stay clear ofeach other.If they intersect, ine"cient combustion would occur.It is desirable to stagger the burners on the opposite sides.d.( The burner )ame has a tendency to travel freely in thecombustion space >ust above the material. In small furnaces, theaxisof theburner isnever placedparallel tothehearthbutalways at an upward angle. Flame should not hit the roof.e.( The larger burners produce a long )ame, which may be di"cultto contain within the furnace walls. ?ore burners of less capacitygivebetter heat distributioninthefurnaceandalsoincreasefurnace life.f.( For small furnaces, it isdesirabletohavealong)amewithgolden yellow colour while fring furnace oil for uniform heating.The )ame should not be too long that it enters the chimney orcomesout throughthefurnacetopor throughdoors. Insuchcases, ma>or portion of additional fuel is carried away from thefurnace."# (!i&t!i&i&, O8timm O8er!ti&, Tem8er!tre o) Fr&!*e%It is important to operate the furnace at optimum temperature.The operating temperatures of various furnaces are given in Table 8.8.or advantage of this type of system is that it can be retro@ftted to an existing furnace structure to increase production capabilitywithout having to alter the existing exhaust gas ducting arrangements.15ustable valves, the unit beinglocated in the front or rear wall.*orizontal fringwiththeturbulent typeof burner is set upusually in the front 'front wall fred( or rear walls of the furnace.Thisburnerconsistsof aninnerconeforprimaryairandfuelwhich is given a rotarymotion as it passes throughthe burner.Thismixes with a stream of rotating secondary air before burning.Im8!*t Firi&,Dform of verticalfring,consists of burners located inan archlow in the furnace or in the side walls and directed toward the furnacedoor, with high velocities of both primary and secondary air. This typeof fring is used exclusively in wet@bottom or slagging type furnaces.This is the arrangement with the type of burner used with slagtap furnaces where the ash is #ept in a molten state on the furnace)oor and tapped o! as and when necessary.6.Cor&er or T!&,e&ti!l Firi&,Is characterized by burners located in each corner of the furnaceand directed tangent to a horizontal, imaginary circle in the middle ofthefurnace, therebyma#ingthefurnacetheburnerine!ect, sinceturbulence and intensive mixing occur where the streams meet.Tangential fring is a method of fring a fuel to heat air, which isthe most common one in thermal power stations.The )ame envelope rotates ensuring thorough mixing within thefurnace, providing complete combustion and uniform heat distribution.The most e!ective method for producing intense turbulence is bytheimpingement of one)ameonanother. This actionis securedthrough the use of burners located in each of the four corners of thefurnace, close to the )oor or the water@screen. The burner nozzles areso directed that the streams of coal and air are pro>ected along a linetangent to a small circle, lying in a horizontal plane, at the center ofthefurnace. Intensivemixingoccurswherethesestreamsmeet. Dscrubbing action is present which assures contact between thecombustible and oxygen, thus promoting rapid combustion andreducing carbon loss. D relative motion, similar to that of a cyclone, isimpartedtothe)amebody, whichspreadsoutandfllsthefurnacearea. Theignitionat eachburner is aidedbythe)amefromthepreceding one.2ith tangential fring the furnace is essentially the burner,conseuently air and coal uantities need not be accuratelyproportional to the individual fuel nozzle assemblies. Turbulenceproduced in the furnace cavity is su"cient to combine all the fuel andair. This continuously insures uniform and complete combustion so thattest performance can be maintained throughout daily operation. 2ith65other types of fring the fuel and air must be accurately proportioned toindividual burners ma#ing it di"cult to always eual test results.2ith this type of fring, combustion is extremely rapid and short)ame length results. The mixing is so intense that combustion ratesexceeding E;,,,, /tuG 'ftE`h( or E., #2GmE are practical under certainconditions. *owever, since there is considerable impingement of )ameover thefurnacewallsit isabsolutelynecessarythat theybefullywater cooled.Thissweepingofthewater@cooledsurfaces, inthefurnace, bythe gas increases the evaporation rate. Thus, in addition to absorptionby radiation from the )ame envelope, there is transfer by convection,and the resulting furnace temperatures are lower than with other typesofburners,eventhoughthe heat liberationrates maybe somewhathigher. Tangentially@fredfurnacesareusuallycleanintheupperzoneand, as a result, both the furnace and the boiler are comparatively freefrom ob>ectionable slag deposits.9rie) EC8l!&!tio& o) T!&,e&ti!l Fel Firi&, SystemIn this system, burners are set at each corner of the furnace andare directed to stri#e the outside of an imaginary circle in the centre ofthe furnace./ecause the streams of fuel so stri#e each other, extremely goodmixing is obtained.