Thermodynamics of combustion

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Information about Thermodynamics of combustion

Published on March 23, 2014

Author: AlfaSharma



Describes thermodynamics of combustion and temperature-enthalpy diagram

Alfa Sharma M.Tech(1st yr.) Redg.No-13307002 Centre for Green Energy Technology Pondicherry University INDIA-6050143/23/2014 1

INTRODUCTION  Major thermodynamic functions of combustion processes which fundamentally influence the utilisation of fuels in diverse appliances are given as; (i)Heat of combustion (ii)Equilibrium constant of reactions during combustion (iii)Enthalpy of combustion systems (iv)Flame temperature 3/23/2014 2

HEAT OF COMBUSTION  Also known as potential heat of fuel.  Can be calculated by applying Hess’s law.  Heat of combustion(Hc) of carbon depends on its allotropic form.  In physics and thermo chemistry,β-graphite is used as a basis of heat of formation(ΔHf =0). But in technical processes amorphous carbon like coke carbon is the basis. 3/23/2014 3

Contd......  Application of Hc :- Calorific value can be determined easily from heat of combustion. For example, For carbon , CV = 97000/12 = 8083Kcal/kg For any substance, GCV = ΔHc /22.4 .........(1) for CH4 = 212798/ 22.4 = 9500 Kcal/Nm3 %η(efficiency of combustion) = (potential heat in flue gas +potential heat in refuse) potential heat in fuel ×100 3/23/2014 4

ENTHALPY OF COMBUSTION SYSTEM  Heat transfer from the combustion gases takes place at fairly constant pressure not far from the atmospheric. ...............(2) Where, Q - heat absorbed by surrounding -ΔH – enthalpy decrease of the system  Enthalpy change is the change of heat content of system at constant pressure.  Knowledge of enthalpy is in relation to a reference state(0ᴼC,760 mm) is sufficient. Q = -ΔH 3/23/2014 5

CONTD...............  Enthalpy of a gas at temperature tᴼC is, ΔHt = Cp(0-t) × t1 ..............................(3) Where, Cp(0-t) – mean specific heat between reference temperature 0ᴼC and the given temp. tᴼC expressed in Kcal/Nm3 ᴼC(volume basis) and Kcal/kg ᴼC(in weight basis) Hence the enthalpy change between two temperatures t1 &t2 can be determined by, ΔHt(1) – ΔHt(2) = ( Cp(0-t1)×t1) – ( Cp(0-t2)×t2) .............(4) 3/23/2014 6

 For mixture of gases, (Cp(0-t))mix = Xa × (Cp(0-t))a + Xb ×(Cp(o-t))b ..........(5) where, (Cp(0-t))a and (Cp(0-t))b - mean specific heat of components of a and b. Xa and Xb - volume fraction or weight fraction of components. Hence enthalpy of mixture of gases is given by, (ΔH(0-t))mix = (Cp(0-t))mix × t ...................................(6)  Specific heat and enthalpy of combustion gases are used in calculation of flame temperature , heat loss with flue gas and furnace efficiency. 3/23/2014 7

EQUILIBRIUM CONSTANTS OF COMBUSTION REACTION  Reactions like dissociation of water vapour, carbon dioxide are endothermic in which a part of heat of combustion gases into potential heat in form of Hc of combustible components formed. H2O ↔ H2 + ½ O2, ΔH = +57798Kcal H2O↔ OH + ½ H2, ΔH = +67858 Kcal CO2 ↔ CO + ½ O2, ΔH = +67636 kcal H2 ↔ 2(H) , ΔH = +104178 Kcal O2 ↔ 2(O) , ΔH = +118318 Kcal 3/23/2014 8

 Combustion of fuels is rendered incomplete at high temperature has 3 effects; (i) Combustion efficiency is lowered (ii) Temp. of system falls (iii)Dissociation is an increase in volume and no. of moles of gases. 3/23/2014 9

Enthalpy- Temp Diagram  On complete combustion, ΔH(flue gas) = Σ(ΔH(theoretical flue gas) + ΔH(excess air)) ...(7)  For a given fuel enthalpy of flue gas depends on function of two variables i.e. temp.& excess air.  At temp. above than 1600ᴼC endothermic effect of dissociation reaction takes place which forms the basis of enthalpy- temperature diagram(Ht- or It- diagram).  Ht- diagram covers the temp. range of 100- 2500ᴼC and shows relationship between enthalpy and temp. for different air content of flue gas.  It is useful in rapid workout of problems concerning heat loss from flue gas and flame temperature. 3/23/2014 10

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FLAME TEMPERATURE  It is the average temperature attained by combustion products of a mixture of fuel and oxidant.  Classified into 4 types as; (i)Theoretical flame temperature (ii)Adiabatic flame temperature (iii)Actual flame temperature (iv)Maximum adiabatic flame temperature or Maximum flame temperature  The theoretical flame temp. is not a tangible concept while others are. 3/23/2014 12

 Theoretical flame temp. is the resultant temp. obtained when combustion of fuel is complete and entire heat of combustion goes to heat the products of combustion .  But in reality combustion never completed at high temperature owing to dissociation reactions.  Adiabatic flame temp. Come into picture when endothermic effect of dissociation reaction taken into account which is lower than theoretical flame temp..  Whereas actual flame temp. is the resultant average temp. of combustion products as always some heat is losed to the surrounding of the system.  All the above types of flame temperature depends on composition of fuel- oxidant mixture. (a) Quantity of oxidant is low- incomplete combustion (b) Quantity of oxidant is large – dilute the products and heat taken away 3/23/2014 13

Theoretical flame temp. has maximum value at stoichiometric composition of fuel and oxidant. With increase of temperature the degree of dissociation markedly increases enhancing the heat loss. Hence maximum adiabatic flame temperature is realised when fuel is slightly in excess of stoichiometric composition. Again flame temperature of a fuel is much higher in oxygen than in air because of high N- content of air takes away a significant quantity of heat. 3/23/2014 14

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Flame Temp. Calculation  By balancing a heat equation between fuel and air on one hand and combustion products on the other hand as given below, CN + ΔHfuel +AΔHair = VΔHwg + Qloss + Qdiss. ........(8) Where, CN - Net calorific value of fuel (in Kcal/Nm3) ΔHfuel – Enthalpy of fuel above reference temperature (in Kcal/Nm3) ΔHair – Enthalpy of air above reference temperature ΔHwg - Enthalpy of combustion products above reference temperature A – Air supplied V – Combustion gases produced (in Nm3) Qloss – Heat loss to the surrounding Qdiss. - Heat loss by dissociation 3/23/2014 16

But, ΔHwg = tf Cp wg,(0-tf) – tCp wg,(0-t) ...........(9) tf – flame temp. t - reference temp.(25ᴼC) Cp wg,(0-tf) – Mean specific heat of combustion products between tf and 0ᴼC Cp wg,(0-t) – Mean sp. heat of combustion products between t and 0ᴼC. Hence, tf = CN+ ΔHfuel + AΔHair + Qdiss – Qloss + Vt Cp wg,(0-t) VCp wg,(0-tf) ....(10) 3/23/2014 17

 Flame temp. has significance as it governs thermal efficiency of transference of heat from flames to heating surface. η =(Tf – Ts)/Tf = 1-Ts/Tf ..................(12) • Hence higher the Tf value ,greater is the efficiency. 3/23/2014 18

REFERENCES:- Fuel &Combustion, S. Sarkar (3rd edition ,Universities Press) Thermodynamics for Chemists, S. Glasstone Basic Thermo-chemistry, I.L.Levine 3/23/2014 19

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