UNIT 1 COMBUSTION THERMODYNAMICS BY DEEPA M S

UNIT 1 COMBUSTION THERMODYNAMICS BY DEEPA M S & VARUNA TANDON

Introduction • Thermodynamics essential to designing processing systems for biorenewable resources (Net energy output must be positive!!!) • Fundamental concepts include –Mass balances –Energy balances • These lectures not a substitute for a course in engineering thermodynamics

Kinds of Systems • Isolated system –neither mass nor energy enters the system • Closed system –mass does not enter or leave the system (no restriction on energy flow) • Open system –both mass and energy can flow through the system

Describing mass flow through an open system

Mass Balances for Combustion Processes • The fuel-oxygen ratio (F/O) = mass of fuel per mass of oxygen (Sometime, F/O ratio could be written based on mole rather than mass. ) • The equivalence ratio ( ) • (F/O)stoichiometric is the Fuel-Oxygen ratio at which exactly all the available oxygen is used to burn the fuel completely • The advantage of using equivalence ratio over fuel– oxidizer ratio is that it does not depend on the units being used.

Mass Balances for Combustion Processes (Continues) Example Consider a mixture of one mole of ethane (C 2 H 6) and one mole of oxygen (O 2). F/O ratio of this mixture based on the mass of fuel and O 2 is: F/O ratio of this mixture based on the number of moles of fuel and O 2 is: 2 0. 5

• To calculate the equivalence ratio, we need to first write out the stoichiometric reaction of ethane and oxygen Based on Mass: (F/O)actual (F/O)stoichiometric Equivalence Ratio 0. 938 Based on Mole: 0. 5 30/112 = 0. 268 1/(7) = 0. 143 3. 5

Mass Balances for Combustion Processes (Continues) • The air in excess of the stoichiometric amount is called the excess air Actual Air X 100 • Theoretical Air (%) = Stoichiometric Air • Excess Air (%) = Actual Air – Stoichiometric Air X 100

Describing energy flow through an open system

Energy Balance for Open System *he or hi = specific enthalpy (energy / mass) *he or hi = specific molar enthalpy (energy / mole) *He or Hi = enthalpy (energy)

Energy Balance for Open System • Within each inlet and outlet stream, we can have a multiple species. In this case, we need to add the enthalpy contribution from each and every species in the stream: . Sm h . - Smh . S . - Snh p p - p p n ph p Hp r r r Hr (Based on Mass) (Based on Mole)

Energy Balance for Open System Example:

Let us assume that 1 kmole/hr of biogas is produced by anaerobic digestion of animal waste consists of 60% of CH 4 and 40% of CO 2 (molar basis). The biogas reacts with 1. 2 kmol/hr of O 2 to form CO 2 and H 2 O (no other products). Biogas + O 2 T = To = 298 K Q T = T 2 = 1500 K CO 2 + H 2 O 0. 6 CH 4 + 0. 4 CO 2 + 1. 2 O 2 CO 2 + 1. 2 H 2 O

We want to calculate Q under steady state condition for this example with following additional info. Specific Molar Enthalpy (k. J/kmol) T (K) CH 4 O 2 CO 2 H 2 O 298 - 8, 682 9, 364 9, 904 1500 - - 71, 078 57, 999 o The standard enthalpy of reaction ( h. R) is -890, 330 k. J/kmol of CH 4 at 298 K. Step #1: Do Energy Balance = - = H

Step #2: Calculate H Reactants H Products H HR (To) T To = 298 K T 2 = 1500 K H = HR(To) + H = (-890, 00)*(0. 6) + [1*(71, 078 -9, 364) + (1. 2)*(57, 999 -9, 904)] = -414, 770 k. J/hr

Energy Balances • For well-characterized fuels, standard enthalpies of reaction can be calculated from tabulations of specific enthalpies of formation, , of chemical compounds from their elements at a standard state: h p( r ( - npr and nrp are the stoichiometric coefficients for reactants and products of a chemical reaction Example: Calculate the standard heat of reaction for the dehydrogenation of ethane: C 2 H 6 C 2 H 4 +H 2

• Most biomass fuels are not well characterized in terms of their chemical constituents –Often simpler to perform calorimetric tests on biomass fuels to determine enthalpy of reaction

Thermodynamic efficiency • Every energy conversion process can be characterized by its thermodynamic efficiency