COMBUSTION the process industry, the fuel burnt i In Is an enclosed space in whi burnt in furnaces may be a solid, liquid or gas. The ich heat is : eet the more ‘comm ont aalateel eat is produced by the chemical oxidation of a fuel. Solid fuels : Prine . i p: Soll ee incipally coal which is a mixture of carbon, hydrogen, non sett ath water and sulphur coke and to some extent wood, began b in fuels : Principally hydrocarbons obtained by distillation of a crude oil igetroleum ie: Gaseous foe + Principally natural gas which usually contains 80 to 95 methane, the pace being ethane, propane and small quantities of other gases, light hydrocarbons, peatylene ct. ‘The rapid reaction of a fuel with oxygen is known as combustion. In combustion operation, tbe union of carbon, hydrogen and sulphur of fuel takes place with oxygen. When a fuel is feened, the carbon in the fuel reacts to form either CO, or CO, hydrogen reacts to form HO ant Auphur eocts to form SOz, The combustion is termed as complete combustion ifthe products of i tion are COz, HO and SQ; A combustion reaction in which CO is formed is referred to {s partial or incomplete combustion. Examples : (+0, + COz complete combustion ofcarbon. =. Gilly +50 +3CO, + 4H,0 complete combustion of propane. Gilly + i Oy > 3CO +4H,0 partial combustion of propane. The combustion products of a fuel (COz, HO and frequently CO and SO,) are relatively vorhless as compared to the fuels burned to obtain them The significance of combustion reactions lies in the tremendous quantities of heat teased during the course of combustion Process: ‘The heat released is used to produce steam, piste during te cour ye turiues tlodac eleczicity. T= the process 1nusir7» Ep heat leased by burning a fuel is used to supply tee eee stent reresiae Ts Pacing aeons, air is the source of oxygen in the most Ami 07 Me’ ea oduct gases that leaves a combustion chamber [containing CO2, CO, B40 Q2, 802 (S02) and Ni/is referred to as the stack gases or flue Bases In the analysis of flue gases, the term akin a to denote the component mole fractions of the flue gases that on a wet basis ‘a dry basis ‘used to denote the component mole fractions F ; nisins water and composition of 8 OY One scring water present in the flue gases). ‘the flue gases without water (.e-» Calorific Van jues of Fuels : ing value of the fuel is the negative of Th also known as the heating value Gs ola va fay al ee Pe ing vl pe Nandard heat of combustion is always D6 o ee niuted when a unit mass of fuel Hs The calorific value of a fuel ia defined °° ™pletely burnt with pure oxygenStoichiometry : uel rea When a fuel is burnt, the hydrogen ae iets water is present in the flue gases a8 VaPOls To associated with water vapour) and hence this q) purpose. ting value) of a fuel The net calorific value (net heating value or a eat in the vapoul value of the fuel when water in the combustion pro i i roduc ENGI em EPROPS ta. meee The latent heat of vaporisation of water can De mee CUA alue or gro vapours are condensed. The gross-calorific value (higher the combustion gaia value) ofa fuel isthe calorific value of the fuel when water in the omit produ liquid state. ie. itis equal tothe net calorific value ofa fuel to which the latent heat of vapours is added. The gross calorific value is abbreviated as GCV, higher he HEY, lower heating value as LHY, net calorific value as NCV and net heating val The NCV and GCV of the fuels are usually reported at 298 K (25 °C). These expressed in ki/kg of fuel, kd/mol of fuel or kfm? of gaseous fuel, To caleulate the net calorific value of a fuel from the gross calorific value of the | vice-versa, we must calculate the moles of water produced when a unit mass of burned. Let n be the mol of water produced, Then GCV =NCV +n AHy [H,0, 298 K (25 °C) kJ/mol of fuel where AHy [H,0, 298 K (25 °C)] is the heat of vaporisation of water at 298 K (25 °C The GCV calculated will be ki/mol, If 'm’ is the quantity of water produced in kg mass of fuel is burned and ‘a’ is the latent heat of water vapours at 298 K (25 °C) in k. GCV and NCV ofa fuel in kJ/kg of the fuel are related by a GCV = NCV+mi Latent heat of water vapours, 4, at 298 K (25 °C) is 2442.5 kJ/kg. GOV = Nov + {Wt% hydrogen) (9) (i) cts with oxygen to produce w, tent heat of vaporisation js f heat is not available for 100 kJ/kg tes the fuel under consideration is a mixture of combustible substances, its is — HV = 3x (HV) - 4 where (HV) is the heating value ofthe ith combu, expressed in units of energy per unit mass me components, and when they are expressatny a° fractions of the fuel components, | °2 1" Units of energy per mole then the x's ar@ The proximate analysis of coal involy ae 7 and fixed earbon (obtained by deducting % monger VM a mosture, vola analysis of coal involves determination of cartes hyd Mand % ash from 100) coal. The oxygen content of coal Togen, nitrogen, sulphu ee een eee al is obtained by dedu ig iphur, Air Requirement For any combustion process, jc which will combine with carbon, hyd burning a fuel as it being an inex : ian tPensive source of Yaloable do to nerenae ee acl Drastie ta rang eA rable ¢ e onversion of the y rete P imelve react 7 all i sro invariably ron with more ar than none” supply ggattant: Hence, combus . oxygen in the stoichi stible substance. When the heatin n the x;'s are the weight fracti eting sum of % of elementsoretical oxygen : It i to the combustion Sones amount of oxygen needed for complete combustion of all the el oS with oxygen to woretical nix : thts it form H,0 and all sulphur reacts to form SO2- ee air required to bare of oer contains the theoretical oxygen, i., it is the fuel co He peogen into HO and all sulphur es egos converted into COz, Theoretical air requirement of any fuel is calculated as _ Theoretical oxygen requirement in moles (9.5) 0.21 Hee ‘qheoretical air requirement in moles yretical air. actual air supply — ical air req uy) theoretical air re wizement) ua (8.6) Percent excess air = ae : theoretical air requirement % excess oxygen = % excess air ‘The quantities of actual air supplied and theoretical air requires { units or in molar units in equation (8.6). The actual air supply is obtained knowing the % excess and thi d can be used either in eoretical air requirement by relation : . é FY Fo ‘Actual air supply = Theoretical air requirement [: + oxess| (8.7) .ed rate of a fuel and the stoichiometric equation for complete combustion ‘alculate the theoretical oxygen demand/requirement and air feed rate. the fuel, you ean ited above is applicable for the calculation of equation for the calculation of % excess air ci Nacoss oxygen. The % excess air and % excess Oxygen ore one and the same terms «a oan then treat it as % excess O, => % excess O» = % excess air). The ‘antity of fuel does not depend on how much fuel is af the percent excess air depends only on the theoretical air and ‘0, actually consumed or whether the combustion is ‘as actual air requirements are expressed in eg., kg per kg fuel or m? per kg fuel. For jn kmol into the amount of air in kg, the average For converting the amount of air from kmol/mol ideal gas law. “depends upon the type of fuel burnt. Gaseous fuels require very more excess air than for gaseous fuel and solid fuels uire higher amounts of excess ® than that ‘required for liquid fuels. Gaseous fuels are 1 with’s co 15% excess air while quid and solid fuels are burnt with 10 to 50% excess air. Ex. 8.1: Crude oil is analysed to contain 879% carbon, 12.5% hydrogen and 0.5% sulphur weight). Calculate the net calorifie value of crude oil at 298 K (25 °C). Pata . Gross colerific wala pious ALOE? °C) is 45071 kJ/kg oil. Latent heat of water vapour ‘at 298 K (26°C) = 2442.5 ki/kg. Sol. : Basis : 1 kg of rude oil. 5 i Hydrogen burnt = Hydrogen in crude oil = 0.125 kg 1 ay ser H,0 to Um? (volume units), We The excess air requirement depe excess air. Liquid fuels requireeee kmol H,0 = 18 kg of HO 38 9.195 = 1-125 ke 2 1 kmol Hi 2 kg of H Water produced = 2025 oT BMS Latent heat of water vapour at 28 K (25°) = 1125 x7 ae = Gross calorific value ~ Latent a ater | = 45071-27478 = 42323.2 kJ/kg oil by wt x9%A Net calorific value % hydrogen Ney acy "1100 12.5 x9 x 2442.5 = 6801 =o = 4233.2 ki/kg of oil cose : the net calorific value (NCV) at 298 K ( ata ee cH Brass ty weight) and containing sulphur to the extent of 1.3% Data: The GCV of the fuel oil at 298 K (25 °C) = 41785 kilkg Latent heat of water vapour at 298 K (25 °C) = 2442.5 kd/kg Sol. : Basis : 1kg of sample of fuel oil. It contains 0.013 kg of sulphur and balance being C and H (as % S = 1.3). Let x and y be kg of carbon and hydrogen in fuel oil. Amount of C and Hin fuel oil = 1-0.013 = 0.987 kg x+y = 0.987 x : : p and J = 9.38... (given C/H ratio by weight x, = 9.33y ; 933y+y = 0987 1033y = 0987 ; Y = 0.095654kg and of hydrogen burnt = 0.09554 kg x = 0.89146 kg H+} 0, > 1,0 1kmol H = 1 kmol H,0 2kgofH, = 18kg of HO wi B ater produced = 7 x 0.00554 » C607 eg NOV = GCV ~ma = 41785 ~ 0.8597 x 24495 = 39686,2 7 Ex. 8.3 : The gross he; fuel ofl a sash td Cae a (ORY) of Saeous Sol, 1 Basis + 1 mol of gaseous value (} : CH 43. 4 800, 4. ie 4 vm d & aeAmount of UH burnt = Oates CH +20 + co, 8,0 2 CoH, +3.50, 200, + 38,0 mount of water produced by reaction (1) _ 96 * 3g %0.7514 = 1.6906 kg mount of water produced by reaction (2) = 54 ap % 0.2486 = 0, Total water produced = 1.6906 4 5, mee GCV of natural gas = > x; (NCV); + heat lost due to vaporisation of water 0.7514 x 50163.75 + 0.2486 x 47621.33 + 5222.31 = 547654 kJ/kg natural gas ame as previously obtained... Ans. . GCV and NCV] of the natural gas \/fx. 87 : Calculate the gross and net calorific values (i at 298 K (25 °C) having the following molar composition : CHy: 89.4%, CyHg : 5%, CsHg: 1.9%, m-CiHyo : 1%, C02: 0.7% and Ny : 2% Data: Component GCV, kJ/mol NCV, ki/mol CH, 890.65 80262 CpHs 1560.69 1428.64 CjHs 219.17 2048.11 CH aer.40 gees Ss, Specific volume at 298 K (25 °C) and 101.3 kPa = 24.465 m*/kmol. Sol. : Basis : 1 mol of natural gas. 0.007 mol Ie contains 0.894 mol CH, 0.05 mol CoH, 0.019 mol Calls, 0.01-mel C4Haw ba and 0.02 mol No. y is is sum of . 2 (calorific value) of the gas mixture In case of a gas mixture, the heating value 2 cea sent in it. beating values (calorific values) of in ened + 0.019% 221917 +001 2877.40 GOV. = 0.994% 89065+ 0.05157 a i HS lh: 1428.64 + 0.019 x 2049.11 + 0.01 x 2657.32 Nev = 0894 E = 854.37 kJ/mol natural 625 ion @O REDMINOTE 9 PRO MAX CO AI QUAD CAMERAStoichiomet . ‘Alternate calculation of NCV Combustion ae oo 20, > 002+ Biase CyHlg+ 3.502 + 2002+ SRE c Hs +50 > 3002 ee , Cal + 6.502 > 4002+ 5H 5 4 - 9+7 x 0.01 Total water formed = 7 x08D4+7 x 0.05 + 0010 Seg.y = E, 064 mol Weight of water produced = 2.064% 18 = 87.152 g = 37.152x 1031 k Heat lost due to vaporisation of water 2442.5 = 87,152 10° x 1 90.83 kJ/mol fuel 945.21 - 90.83 F y 854.38 kJ/mol natural gas “i Mayg. = Average molecular weight of natural gas 0.894 x 16 + 0.05 x 30 + 0.019 x 44 + 0.01 x 58 + 0,007» " NCV = 18.088 945.21 x 1000 Gov = "18.083 = 5256.2 kIicg _ 854.37 x 1000 CV i= Sania pag ae 47234.1 kSikg Specific volume at 298 K (25 °C) and 101. ‘SkPa = 24.465 m3/cmol cov = S821:100 oe a 24.465 = 38635.2 kJ/m3 854.37 x 1 NCV 000 i ay "24465 = 84922.13 kJ/m3 by Ex. 8.8 : The gross calorific value of | its net calorific value using | quid acet, ) is ae one at 298 K (25 0G) i Sol. : Basis: 1 mol of liquid acim, nate at 298 K (25 ogy, ee The combustion Teaction j, ig; CH; COCH, + 4 When 1 mele scetone burt 4° ohn oe of Water are Prod; "* So9am a 286 pop 54g = = 54109 eg