alcohol combustion chemistry

Transcription

1 alcohol combustion chemistry S. Mani Sarathy, Patrick Oßwald, Nils Hansen, Katharina Kohse-Ho inghaus Clean Combustion Research Center, KAUST, Saudi Arabia Institute of Combustion Technology, DLR Stuttgart, Germany Combustion Research Facility, Sandia National Laboratories, USA Chemistry Department, Bielefeld University, Germany Contact

2 A comprehensive synthesis of fundamental experimental and theoretical studies on alcohol combustion chemistry. KAUST King Abdullah University of Science and Technology 2

3 Alcohol fuel origins A Syrian mercenary drinking beer in the company of his Egyptian wife and child, c BC. Photograph: Bettmann/Corbis KAUST King Abdullah University of Science and Technology 4

4 Alcohol fuel origins First written account of boiling wine and observing flammable vapors is found in the writings of Jabir ibn Hayyan (c CE). Al-Kindi (c CE) later described the distillation of wine in his Kitab al-taraffuq fi al- itr (The Book of the Chemistry of Perfume and Distillations). 5

5 Alcohol fuel origins Bernton H, Kovarik W, Sklar S, Griffin B, Woolsey RJ. The forbidden fuel. Bison Books; THE INTERNAL-COMBUSTION ENGINE und in the author's fuel research engine when using petrol and hyl alcohol under precisely similar conditions as to temperature, &c., d at a compression ratio of 5: 1. In both cases a careful series measurements was made at mixture strengths ranging from per cent weak to 25 per cent over-rich. In the case of fuels whose volatility is very low, such as kerosene, tyl alcohol, &c., advantage cannot be taken of the latent heat of aporation, because it then becomes necessary to add an excessive ount of heat before entry to the cylinder, in order to prevent ndensation in the induction system. For this reason alone the wer output obtainable from kerosene is actually some 15 per cent Ricardo HR. The high-speed IC engine. London Glasgow: Blackie Son Ltd; Methanol (i.e., wood alcohol), and ethanol were lamp fuels prior to First IC engines, including those by Samuel Morey, Nikolaus Otto, and George Brayton, utilized ethanol as the fuel. Henry Ford and Charles Kettering were strong proponents of ethanol as a fuel for internal combustion engines in the USA. Sir Harry R. Ricardo studied alcohol fuels in engines. It is perfectly well known that alcohol is an excellent fuel 6

6 Alcohol production technologies 8

7 Research questions What are the effects of molecular structure on fuel properties, combustion, and emissions? Petroleum Fuels Alcohol fuels (crude oil) ILLUSTRATION: BEBOY/FOTOLIA 9

8 Alcohol fuel properties 10

9 Alcohol engine combustion He B-Q, Liu M-B, Yuan J, Zhao H. Fuel Dec J. SAE technical paper series; Widespread alcohol fuels reduces soot, but may increase health concerns due to increased emission of carbonyls and unburned alcohols. Challenging to draw broad scientific conclusions regarding emissions from vehicle engine studies. Fundamental chemical kinetic combustion experiments and simulations at engine-relevant conditions are better for studying combustion science. premixed laminar flame speed homogenous ignition delay time combustion product species profiles 12

10 Articles reporting on alcohol combustion chemistry Early works ( ) by chemists had a fascination on organic molecule oxidation/pyrolysis. Not stated as a fuel. First fuel combustion studies were conducted in 1970s onwards using shock tubes, flow reactors, and premixed flames. Sharp increase in research since in C 4 and larger alcohols are focus of recent interest. Comparably very little data on the isomers of propanol. 14

11 1970 Methanol Temperature (K) D plots Propanols 1970 Temperature (K) Ethanol Temperature (K) log Pressure (atm) Equivalence Ratio log Pressure (atm) Equivalence Ratio Butanols Higher alcohols Temperature (K) log Pressure (atm) Equivalence Ratio Temperature (K) log Pressure (atm) Equivalence Ratio log Pressure (atm) Equivalence Ratio 15

12 Combustion chemistry of C 1 -C 5 alcohols Developed a comprehensive and consistent chemical kinetic model to study the combustion chemistry of C 1 -C 5 alcohols Methanol, ethanol, and two isomers of propanol Four isomers of butanol (n, iso, 2-, tert) n-pentanol and 3-methylbutanol (iso-pentanol) Tested against homogeneous gas-phase ignition delay, premixed laminar flame speed, and jet-stirred reactor species profiles. KAUST King Abdullah University of Science and Technology 16

13 Comprehensive alcohol combustion model KAUST King Abdullah University of Science and Technology 19

14 Unique molecular reactivity features C-H Bond dissociation energies (BDEs) provide a basis for analogies with alkanes and between alcohols Four-center dehydration are specific to oxygenated fuels and important for high T reactivity H-bonded transition states in H-atom abstraction reactions by OH and HO 2 a-hydroxyalkyl+o 2 and Waddington reactions are unique and important in low T 20

15 Model testing and analysis Comprehensive testing of the model at range of pressures (1-80 atm), temperatures ( K), and equivalence ratios (lean-to-rich). Critical analysis of rate constants, reaction flux diagrams, and sensitivity analysis 21

16 Insights into engine-relevant phenomenon The acquired knowledge (and model) on fundamental combustion chemistry can enable an understanding of their performance in internal combustion engines. The comprehensive C 1 -C 5 alcohol kinetic model is utilized to predict the effects of alcohol fuel structure on laminar flame speed, homogenous ignition delay time, and combustion products. These are related to engine-relevant phenomena, specifically pre-ignition and super-knock, fuel autoignition (octane/cetane), and pollutant emissions. 22

17 Pre-ignition and superknock Downsizing (low speed 1800 rpm, higher torque) Consequence of downsizing (low speed preignition- LSPI Takeuchi et al., SAE Int J. Fuels Lubr. 5 (3): ) PI is an abnormal combustion wherein a flame propagates prior to the spark plug firing. May cause T and P of end-gas to increase faster than it would under conventional spark timing, and thus may lead to a heavy knock or super-knock event. PI requires a local hot followed by a propagating flame front. Probability of pre-ignition increases as as the fuel/air Kalghatgi, G. and Bradley, Int J. Eng Res, 13-1 (2012) 23

18 Laminar flame speed and PI Methanol is most reactive in flames. High T reactions produce formaldehyde leading to reactive H radicals. Ethanol also produces a lot of aldehyde. Normal C 3 -C 5 populate the H-radical pool at approximately same rate, so display similar laminar flame speeds. Substitution of methyl or OH reduces flame speed. These populate the H radical pool at slower rates because they form resonantly stabilized radicals. Methanol and ethanol are likely to increase PI propensity. Higher alcohols will not increase pre-ignition propensity compared to typical n- and isoalkanes found in conventional gasoline fuels. 24

19 Fuel autoignition (octane and cetane) SI knock occurs in the end-gas after spark firing. The cetane number of a fuel quantifies its autoignition propensity in CI engines. RON < 60 and CN > 30 for CI engines; RON > for LTC engines; and RON > 80 for SI engines. t id can be correlated with fuel octane rating. At 835 K and 20 atm the simulated t id correlates well with the measured RON. Methanol, ethanol, the propanol isomers, and tert-butanol are RON > 100 and t id longer than PRF100. Discrepancies for n-propanol and iso-pentanol require additional RON measurements and more fundamental data to further refine kinetic models. 25

20 Chemical origins alcohol autoignition features Hydrogen abstraction from is favored at the a-site due to lower BDE. a-hydroxyalkyl+o 2 reaction suppresses low-t reactivity, as it competes with branching/propagating reactions. The OH moiety s presence is not expected to affect low-t and NTC t id in 7-8 carbon atom alcohols. Substitution increases octane because increasing primary C- H sites suppresses chain propagating and branching reactions. KAUST King Abdullah University of Science and Technology 26

21 Alcohol combustion emissions C 1 -C 5 oxygenated (carbonyls and enols) emissions. Abstraction from a-site and b-scission forms ethenol. a-hydroxyalkyl+o 2 reaction directly forms carbonyls at lower T. Yield sooting index (YSI) increases with carbon number because larger alkenes are formed, enabling PAH growth. C 4 and larger alcohols have higher YSI compared to n-alkanes of same carbon number. Four-center unimolecular reactions produce alkenes and sequester the fuel bound oxygen atom into a water molecule. OH substitution increases sooting tendency because four-center decomposition becomes favored. 27

22 Outlook Alcohol combustion chemistry experiments and models enable practical understanding of these fuels Detailed Chemical Kinetic Models Test Against Exp Data Application to Practical Systems Alcohols are suitable replacements for SI, CI, and LTC engines. Isomerization enables tuning of fuel combustion performance. Unavoidable carbonyl emissions may be abated using after-treatment. Displacing large hydrocarbons with smaller alcohols with similar physical and kinetic properties can reduce emissions. Future alcohol combustion chemistry research should focus on enginerelevant experiments, refined modeling capabilities, uncertainty quantification, and investigations into additional engine phenomenon. 28

23 ??