Published on March 23, 2014
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
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
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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