Introduction to combustion

Transcription

1 Introduction to combustion EEN-E005 Bioenergy D.Sc (Tech) ssi Kaario

2 Motivation Why learn about combustion? Most of the energy in the world, 70-80%, is produced from different kinds of combustion processes Understanding of what is combustion àhow to make it more efficient or how to reduce emissions This lecture is about fundamental flames ~ 80% from combustion

3 Bunsen burner

4 Butane global chemistry...

5 Premixed Bunsen flame Flame speed Butane air

6 Premixed methane air mixture combustion Residence time [s] Kilpinen, 1997

7 Premixed methane air mixture combustion Residence time [s] Kilpinen, 1997

8 Fundamental gas flames Premixed flame F. Williams 1971 M. Hupa 000

9 Flame speed in a premixed H - case M. Ghaderi Masouleh 016 Flame front marked with hydroperoxyl (H ) radical

10 Laminar flame speed M. Ghaderi Masouleh et al., Fuel 016 SF=single fuel

11 Diffusion ---- premixed combustion

12 Fundamental gas flames Non-premixed flame M. Hupa 000 Diffusion flame

13 Diffusion flame chemistry C(s) à C

14 A. Wehrfritz 016 Non-premixed flame

15 Properties of premixed and diffusion combustion processes Premixed Diffusion -No soot -Poor radiator -Chemical reactions determine reaction rate -flame speed can be determined -Sooting flame -Good raditive heat transfer -Mixing determine reaction rate -Cannot define burning velocity -applications (stove, furnace..) -safety issues

16 Reaction scheme Complete combustion N y x H y xc N y x y x H C y x ø ö ç è æ ø ö ç è æ ø ö ç è æ = N v v air In

17 Reaction scheme Complete combustion with oxygen C H æ ç x è y 4 - z ö ø xc x y z y H Compound is incombustible if 1 z ³ x For example C C y

18 Lambda st i i n n, = l st N st st i N i n n n n n n,,, = = st n n, = l st i i n x n n x n st,, = = L st L = l, where is available air / fuel ratio and is the stoichiometric air / fuel ratio

19 Lambda ( ) N y x y x H y xc N y x y x H C y x ø ö ç è æ ø ö ç è æ - ø ö ç è æ ø ö ç è æ l l l l

20 Practise writing reaction scheme Write down reaction scheme for CH (acetylene) with Lambda=1 Lambda=

21 If Lambda is not known If you know your fuel and air amount, what is your Lambda? 1. Define the stoichiometric air consumption -For very many hydrocarbons, it is close to L st ~15 (-For methane it is a bit higher 17.1). What is your available air / fuel ratio L? -30g of air and 1g of fuel à L = 30/1 = Lambda is now (assuming L st = 15) l = 30 = 15

22 Combustion Kinetics Global reaction: Elementary reactions M * = has higher energy than M Rate law (Law of mass action) aabb à ddee = Elementary reaction rate a, b, are reaction orders respect to species A, B... = Can be used only with elementary reactions M = third body molecule brings (through collision) the energy needed to split the molecule, or takes away energy and stabilizes the combination of e.g. C

23 Combustion Kinetics Arrhenius relation for rate coefficient k k = A e - E a RT A = frequency factor E a = activation energy Low activation energy E à low temperature sensitivity High activation energy E à high temperature sensitivity Ea=30000 Ea= reaction rate [-] Temperature [K]

24 Energy diagram k = A e - E a RT Released energy in the reaction Reaction proceeds if enough heat from combustion is provided back in order to the cross the threshold energy Reactants à Products

25 Methane chemistry Detailed methane (CH 4 ) chemistry involves 53 species and 35 reaction (GRI 3.0 mechanism)

26 Radicals 1. A radical has an unpaired electrone H H H :.. : H ßM H :. : H H - H. Structure is incomplete. Therefore, almost always when they collide they react. 3. Consequently, they are very reactive Examples of radicals: -, H, H, H, CH 3, CH, CH, C, NH, NH, N, CN Whole combustion chemistry is due to radicals

27 Ignition and Radicals (0) chain initiation: (1) chain carrying: () chain branching: (3) Chain terminating: M H CH M CH M H H M H C C = = = 3 * 4 * * * 3 M H M H M M = = H H M C M C = = * 3 * * M H M H H M M = =

28 Ignition With CH fuels ignition takes place only after a certain ignition delay time. During the ignition delay period, radical pool population is increasing but the fuel consumption rate and temperature increase are low Finally, the radical pool becomes large enough to consume a significant fraction of fuel, and rapid ignition takes place.

29 A. Wehrfritz 016 Ignition in a spray flame

30 Ignition delay times in a spray flame As a function of concentration As a function of injection pressure A. Wehrfritz et al. 016 H. Kahila et al. 017

31 Ignition NTC behavior (Negative Temperature Coefficient) Two-stage ignition (Warnatz, Maas, Dibble, 1999)

32 Ignition Ando and Sakai, SAE LT = Low temperature oxidation Another view: Different set of species responsible for low temperature and high temperature oxidation

33 Hydrocarbon xidation Formaldehyde Decomposition into lighter components

34 When are Global mechanisms valid?... Global mechanisms are valid when we can assume complete combustion Typically this is the case when combustion takes place at high temperature (fast reactions) and there is enough time available Then we can use global mchanisms and e.g. look at how much air/ is needed for a combustion process, or how much C and H are produced per burned fuel amount etc However, for low temperature combustion processes (slow chemistry) including ignition phase, and short combustion durations (incomplete combustion), global mechanisms fail to predict the combustion event

35 Combustion II: Flame Development EEN-E005 Bioenergy D.Sc (Tech) ssi Kaario

36 Contents Turbulence and its significance Details of combustion and flame propagation mechanisms in premixed and non-premixed cases

37

38 Turbulence in flow Turbulenceà chaotically swirling flow Turbulent flow consists of vortices (eddies) of different size The biggest vortices are of the size of the flow geometry The eddies are breaking up into smaller eddies (kinetic energy is transported from the bigger eddies to smaller ones) At the smallest turbulent scale (Kolmogorov scale) kinetic energy is dissipated into heat Smallest scales dissipate quickly. Thereby, in order to have small scale turbulence and efficient mixing, first large scale turbulence must be created Fluctuations Turbulence = increased mixing!

39 Effect of turbulence Turbulence has significant effect on combustion because it increases the reaction rate by multiplying the reaction area Turbulence can increase the reaction rate by a factor of 100 Kaario, 013

40 Experimental flame Increased area compared to laminar flow

41 Diesel combustion Combustion is mainly controlled by the fuel spray Turbulent mixing is controlling and limiting the fuel consumption rate Also some effect from the flow field Strong influence from combustion conditions: gas density, temperature, EGR In high-speed engines swirl enhances mixing Ignition is characterized by chemistry

42 Diesel combustion High-pressure fuel spray Atomization, Breakup, Evaporation Vapor mixing Auto-ignition Combustion Lean overall mixture Reactions take place close to Lambda=1 High efficiency No pumping losses at low load Requires good ignition characteristics from fuel Nx and soot emissions

43 Schematic diesel flame J. Dec 1997

44 Std vs HCCI Diesel combustion

45 Gasoline engine combustion tto combustion Turbulent premixed combustion Stoichiometric or lean combustion Problems in efficiency due to low compression ratio and at low load due to pumping losses (choking) High combustion temperature à Nx emissions High octane fuel that resists autoignition and evaporates easily Catalyst removes nitrogen oxides Picture from optical gasoline engine

46 Flame speed in tto engines Flame speed in laminar combustion is close to m/s 3000 rpm, 90% of mixture has burned in about 3.5ms Laminar flame would have propagated about ~1.mm Real flame speed is in this case close to 40-times higher Higher rotational speed à Higher turbulence à Increased flame area à Faster combustion

47 Group work Watch videos about various engine combustion variants (see next slide) In groups, discuss what you saw (e.g. otto and diesel groups) Present your main observations based also on the lecture material. Focus on topics like Initial situation Mixture preparation Flame type How is the combustion proceeding in each case? Draw the combustion schematically at two time instances in D What controls the combustion and reaction rate Combustion phases

48 Videos 6 different engine combustion concepts are presented: 1. tto (gasoline) combustion. Diesel 3. HCCI (homogenous charge compression ignition) 4. PCI (premixed compression ignition) 5. MK (modulated kinetics) 6. RCCI (reactivity controlled compression ignition) Videos are from Sandia National Laboratories, USA

49 The ptical engine in the videos Fully optical (optical imaging possible through the piston and cylinder liner) All test points made with 100 rpm rotational speed Load load conditions (IMEP bar) Engine compression ratio is r c =10.75

50 tto combustion -No EGR -Fuel: Iso-ctane -High engine-out HC, C, and Nx emissions, but low tailpipe emissions (catalyst) -Peak thermal efficiency typically 30%, full load range possible

51 Diesel combustion -No EGR -Fuel: n-heptane - Normal fuel injection close to TDC -High engine-out and tailpipe PM and Nx emissions (3-way catalyst not possible) -Peak thermal efficiency typically 40-45%, full load range possible

52 HCCI combustion -No EGR -Fuel: Primary Reference Fuel (PRF) mixture PRF57. PRF is a mixture of iso-octane and n-heptane. -Very early fuel injection -Low engine-out PM and Nx emissions, high C and HC emissions -High efficiency (~50%), limited to lower loads unless EGR and/or high boost

53 Premixed Compression Ignition (PCI) combustion -Strong EGR used -Fuel: n-heptane -Early fuel injection (-5 cad) -Low engine-out PM and Nx emissions, high C and HC emissions -Wall-wetting can be problematic (with diesel fuel), limited to lower loads

54 Modulated Kinetics (MK) late injection -Strong EGR used -Fuel: n-heptane -Late injection (3 cad) -Low engine-out PM and Nx emissions, high C and HC emissions -Less wall wetting than PCI, lower efficiency (combustion phasing), limited to lower loads

55 Reactivity Controlled Compression Ignition (RCCI) -No EGR used -Fuel: PRF64. PRF is a mixture of iso-octane and n-heptane. -Injection in multiple steps (GDI type injection first) -Low engine-out PM and Nx emissions, high C and HC emissions -High efficiency (~50%), limited to low to medium loads (with EGR and/or boost)