Lecture 4 - Introduction to Combustion PDF

Hurricane Ida’s Fallout Before storm: 1.8 million barrels of oil and 2.2 Bcf of gas After storm: 300,000 barrels of oil Total delay of 20 million barrels of oil Meanwhile, underwater pipeline is leaking https://www.wsj.com/articles/hurr icane-idas-fallout-continues-to- cripple-u-s-oil-produc...

Hurricane Ida’s Fallout Before storm: 1.8 million barrels of oil and 2.2 Bcf of gas After storm: 300,000 barrels of oil Total delay of 20 million barrels of oil Meanwhile, underwater pipeline is leaking https://www.wsj.com/articles/hurr icane-idas-fallout-continues-to- cripple-u-s-oil-production- 11631104257?mod=saved_cont ent Climate change opinion map https://climatecommunication.yale.edu/visualizations- data/ycom-us/ Question 2018 2021 Global warming is happening? 70% 72% Global warming will harm me personally? 41% 47% Discuss global warming at least 36% 35% occasionally? Corporations should do more to address 68% 70% global warming? Introduction to Combustion ME 4180 – Energy Systems and Sustainability Prof. Mario Medina Department of Mechanical Engineering Agenda and Outline Objective Ability to balance chemical reactions Familiarize ourselves with combustion terminology Evaluate emissions criteria Agenda Global chemical reactions Stoichiometric combustion Working with mixtures Equivalence ratio Emission Index Enthalpy of combustion Adiabatic flame temperature NOx formation mechanisms Introduction to combustion Combustion is primary source of power in the world (mobile and stationary) >80% and includes coal, NG, and oil State of fuel Liquid Solid Gaseous Mixedness of fuel and oxidizer Premixed Non-premixed or unmixed or diffusion flames Ratio of fuel to oxidizer Fuel-to-air ratio (mass or mole basis) % excess air Stoichiometry Equivalance ratio % theoretical air Introduction to combustion Combustion is conversion of chemical energy to thermal energy From chemical bonds (hydrocarbon) to heat (high temperature) Need chemistry models to represent process Combustion chemistry is modeled using either global reaction or a series of detailed/elementary reactions Chemistry models of combustion 1) Elementary reactions: hundreds to thousands of steps CH4 + O2 → CH3 + HO2 CH3 + O2 → CH3O + O Too complicated for our needs, requires non-linear pde solver to determine products, heat release etc. 2) Global reactions: simplified* one step (reactants  products) CH4 + 2(O2 +3.76 N2) → CO2 + 2H2O + 7.52 N2 Good basic understanding of the products of combustion under a range of air levels No information on intermediate species Global reactions Characteristics of global or overall reactions 1) Reactants  products (“converted to”) 2) Unidirectional reactions; process cannot be reversed 3) Idealization of real process; don’t really happen this way 4) Created by assuming products then apply conservation of mass (or elements H, C, N, O) 5) Written on a mole basis CH4 + 2(O2 +3.76 N2) → CO2 + 2H2O + 7.52 N2 Complete combustion Global reactions that assume complete combustion assume no excess O2 All H  H2O All C  CO2 All N2 remains N2 Complete combustion equates to: 0% excess air = 100% theoretical air = stoichiometric = φ = 1.0 Example – iso-octane and air combustion 1a) What is the global reaction for complete combustion of iso-octane in air? Working with mixtures Defining mole and mass fractions: Let’s determine the mole fraction of oxygen in air: Working with mixtures We can convert between mass and mole fraction using molecular weights/molar mass: 𝑦𝑦𝑖𝑖 𝑀𝑀𝑀𝑀𝑖𝑖 𝑛𝑛𝑖𝑖 𝑀𝑀𝑀𝑀 𝑚𝑚𝑖𝑖 𝑛𝑛𝑖𝑖 𝑛𝑛𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝑥𝑥𝑖𝑖 𝑀𝑀𝑀𝑀𝑖𝑖 𝑥𝑥𝑖𝑖 = = 𝑀𝑀𝑀𝑀𝑗𝑗 𝑖𝑖 𝑦𝑦𝑖𝑖 = = 𝑀𝑀𝑀𝑀𝑗𝑗 =∑ 𝑛𝑛𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 ∑𝑗𝑗 𝑛𝑛𝑗𝑗 𝑛𝑛 𝑚𝑚𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 ∑𝑗𝑗 𝑛𝑛𝑗𝑗 𝑛𝑛 𝑗𝑗 𝑥𝑥𝑗𝑗 𝑀𝑀𝑀𝑀𝑗𝑗 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 Let’s determine mass fraction of oxygen in air: Example – iso-octane and air combustion 1b) What is the air to fuel ratio for this reaction on a mole basis? 1c) What is the air to fuel ratio for this reaction on a mass basis? Example – iso-octane and air combustion 1d) What is the mole fraction of the fuel in the reactants and what is the significance of the value? Equivalence ratio More convenient to compare fuel-to-air ratio of any combustion process to the stoichiometric combustion process Composition changes lead to different calculations for lean and rich conditions Emissions index Emissions index is a measure of emissions intensity relative to the fuel Can be used to determine amount of CO2 from carbon-based fuels using theory Example – Impact of vehicle CO2 What were the annual CO2 emissions from gasoline burning cars in the United States? 46% of a barrel of oil is converted to gasoline The U.S used 32.54 exajoules in oil consumption BP, “Statistical Review of World Energy 2021” https://www.bp.com/content/dam/bp/business- sites/en/global/corporate/pdfs/energy-economics/statistical-review/bp-stats-review-2021-full-report.pdf Impact of vehicle CO2 We estimated ___ billion metric tonnes of CO2 per year Consumed 112 billion gallons of gasoline Emitted 984 teragram (Tg) of CO2 EPA, “Fast Facts: U.S. transportation sector GHG”, https://www.epa.gov/greenvehicles/fast-facts-transportation-greenhouse-gas-emissions Enthalpy of combustion Energy released in a combustion process by breaking chemical bonds and forming stable products Enthalpy of combustion = heat of combustion = heating value = ∆hc Calculated theoretically or measured directly using calorimeter Two different definitions, depends on water phase in products If liquid, ∆hc is the higher heating value (HHV) If gas, ∆hc is the lower heating value (LHV) Values of enthalpy of combustion HHV LHV of fuel Exothermic ∆hfg,water [/kg fuel] Alkanes (C-C) ∆hfg,fuel About the same From Fundamentals of Thermodynamics © Wiley & Alkenes Sons, 7th Ed., Borgnakke and (C=C) Sonntag, Wiley, 2009. Values of enthalpy of combustion Aromatics (Rings) Iso-octane ≈ n-heptane ≈ kerosene ≈ lower From Fundamentals of large per kg, small per vol Thermodynamics © Wiley & Sons, 7th Ed., Borgnakke and Sonntag, Wiley, 2009. Adiabatic flame temperature Following a similar process, we can calculate the highest temperature using enthalpies of formation Adiabatic flame temperature (Tad) for given mixture under equilibrium As φ decreases, dilution with N2 decreases Tad As φ increases, incomplete combustion decreases Tad Effects of fuel on Tad Tad,max about the same for most HC fuels; dominated by hf,CO2 and hf,H2O Key exceptions are H2 and C2H2, where Tad,max,H2 = 2400 K Effects of pressure on Tad Tad increases with pressure due to radical recombination: CO+O  CO2 OH + H  H2O These are exothermic reactions There are diminishing returns after 10 atm Effects of equivalence ratio on major products Our previous assumption of major products is valid Mostly O2, N2, H2O, and CO2 in exhaust for fuel lean Mostly N2, H2O, CO2, CO and H2 in exhaust for fuel rich At φ = 1.0, some CO, O2 and H2 are still produced due to a balance of Tad, cp (the specific heat of the mixture), and dissociation. Effects of equivalence ratio on minor products OH is a good indicator for hottest region in combustion chamber; OH peaks at ~ Tad,max NO peaks at φ = 0.8 NO levels are HIGH = 1000’s of ppm NO regulations are ~10 ppm Trade-off between CO and NO; as φ increases CO increases; as it decreases NO increases Also trade-off between soot/NOx; as soot increases, NOx decreases and vice versa NOx emissions Primary mechanism for combustion generated NOx 1) Thermal NOx  created at high temperatures 2) Prompt NOx  created by fuel rich conditions 3) Fuel NOx  created when fuel contains N General observations: NOx formation is very sensitive to temperature NO is direct combustion product, which is quickly converted to NO2 (brown haze, LA smog) Premixed flames (e.g. spark-ignition engines, gas turbines) Equivalence ratio is primary parameter controlling temperature Non-premixed flames (e.g. diesel engines) Temperature is higher than premixed flames  more NOx Supplemental Material Adiabatic flame temperature Tad: the adiabatic flame temperature = highest temperature that can be achieved by a given mixture (F+O) taken to equilibrium under adiabatic (no heat loss) condition. Appendix: One-step global reaction: ΣRi → ΣPj Closed system (total mass conserved despite chemical reaction) A. Constant volume adiabatic const. vol 1st law (integrated): RQP + RWP = UP − UR ⇒ UR = UP B. Constant pressure (more common in practical devices), PP = PR = P 1st law (integrated): RQP + RWP = UP − UR where RWP = PVP − PVR adiabatic ⇒ UR + PRVR= UP + PPVP HR = HP Effects of preheat on Tad Tad linear increase with preheat temperature Some of the energy is consumed by dissociating reactants Have to be careful because density changes Thermal NOx Thermal NO (Zeldovich) Mechanism Global: N2 + O2 ↔ 2NO Actual: N2 + O → NO + N Ea = 315 kJ/mol (Rxn1) N + O2 → NO + O Ea = 26 kJ/mol (Rxn2) R1 is highly sensitive to temperature, slow (i.e. rate-limiting) O atom originally comes from chain-branching reactions (H + O2 → OH + O) during combustion Extended Zeldovich mechanism N + OH ↔ NO + H Ea = 0 kJ/mol (Rxn3) Thermal NOx Reaction Rate of Zeldovich NOx Based on equilibrium model, 1/ 2 d [NO]  K p Pref   moles  [ 2] [ 2]  1/ 2 1/ 2 = 2k N1   T O N  vol ⋅ sec  0 dt  R  where kN1 = the rate coefficient of reaction N1 = Aexp(−Ea/R0T) (large Ea: highly temperature sensitive!!) [NO] = concentration of NO [moles/vol] Kp = equilibrium constant, function of temperature Pref = reference pressure (e.g. 1 atm) at reaction R0 = universal gas constant (= 8.314 kJ/kmol-K) T = temperature [O2], [N2] are determined at equilibrium conditions. - NO production rate increases rapidly with temperature (through kN1 mostly) - The equilibrium model tends to underpredict NO production due to the neglect of super-equilibrium O in combustion gases. Thermal NOx Prompt NOx Prompt NOx has 3 sources: 1. Super-equilibrium concentrations of O and OH combined with the thermal NOx mechanism 2. Cyanogen kinetics HCN formed by CH + N2 (called the Fenimore mechanism) 3. N2O kinetics (nitrous oxide) O+N2+M  N2O + M N2O+O  NO + NO Prompt NOx is associated with fuel rich conditions Prompt NOx is not as T dependent as thermal NOx Fuel NOx Fuel NOx can be unavoidable due to N chemically bound within the fuel structure Fuel NOx is primarily a concern for coal and propellants Nitrogen is a trace impurity in some crude oils Coal = C176H144O8N3 RDX = research department explosive; hexahydro- 1,3,5-trinitro-1,3,5 triazine HMX = high melting explosive; octahydro-1,3,5,7- tetranitro-1,3,5,7 tetrazocine