Methods of Improving Combustion by Electrical
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METHODS OF IMPROVING COMBUSTION BY ELECTRICAL MEANS By John Clifford Hilliard, B.Sc. M. R. I. C. C. Chem. A Thesis submitted for the Degree of Doctor of Philosophy Department of Chemical Engineering and Chemical Technology Imperial College of Science and Technology London SW7 November 1979 ABSTRACT Methods of interacting electrical energy with various combustion systems that exhibit both diffusion and chemical kinetically controlled heat release rates were considered from the point of view of over- coming the mutual exclusiveness of economy and emissions for such s ystems. The thesis covers situations involving the application of low electrical powers, which fall into two regimes determined by the v-i characteristic, and are broadly classified under 'Electric Fields' (high voltage) and 'Electric Arcs or Plasmas' (low voltage). Consideration was given to each of the three possible interaction stages, i) Mixture preparation stage ii) Conversion from reactants to products iii) Product discharge staige for either the high or low voltage interaction. Experimental and theoretical results for electrostatic carbura- tion and atomisation of fuels are presented and the practical signifi- cance discussed with respect to the unique qualities of alcohols. The principle of combustion control through the production of non-Maxwell-Boltzmann distribution of species is presented with reference to nitrogen, hydrogen and oxygen atoms, the merits of photochemical and plasma modes of generating these distributions is presented, with emphasis on the key role played by Il-atoms in con- trolling heat release rates. The importance of excited states in such systems is also considered. A highly efficient novel plasma generator that enables the pro- duction of atoms and radicals from virtually any feedstock (with no noble gas additives) resulted from this work, and the design para- meters are presented. Spectroscopic studies using the novel generator provided evidence to support the theories proposed and also demonstrated the potential for electronic state selectivity in the device. The practical significance of this work is discussed with respect to the weakness of present day energy conversion systems. It is much easier to recognise error than to find truth, error is superficial and may be corrected; truth lies hidden in the depths. Goethe, Spruche in Prosa Nescit vox missa reverti i CONTENTS Page ACKNOWLEDGEMENTS iv PICTURE CREDITS v LIST OF FIGURES vi LIST OF TABLES xii SYMBOLS AND NOMENCLATURE xiii CHAPTER 1 - INTRODUCTION 1 CHAPTER 2 - AIMS AND OBJECTIVES 10 CHAPTER 3 - THEORY AND METHODS OF CONTROLLING REACTION AND HEAT RELEASE RATES 12 Introduction 12 Heat Addition with Product Dilution 14 Heat Addition without Product Dilution 16 Photochemical Methods 18 O-atoms 19 H -atoms 24 N-atoms 2 9 Argon 3 3 Ozone 35 Peroxides 37 Inhibitors 38 Plasma Methods 39 CHAPTER 4 - PRELIMINARY PLASMA JET DEVELOPMENTS 44 Introduction 44 Nitric Oxide Removal by N-atoms 44 Flame Stabilisation by Plasma Generated Atoms 52 Power Requirements 56 Plasma Injection for Soot Removal 60 Conclusions 64 11 CHAPTER 5 - ELECTRIC FIELD INTERACTIONS WITH COMBUSTION 66 Introduction 66 Section I - Mixture Preparation 67 Application to Otto Engines 69 Theoretical 72 % Change in Charge Volume 75 Temperature Drop on Evapo rat ion 76 Electrostatic Carburation 78 Minimum Powers Required 86 Diesel Applications 87 Re suits 94 Interpretation of Results 94 Conclusions 99 Section IT - Product Discharge 10 5 a) Electron Manipulation 107 b) Ion Manipulation 118 Conclus ions 127 CHAPTER 6 - DESIGN CONSTRUCTION AND PROPERTIES OF A NOVEL PLASMA GENERATOR 13 1 Design 132 Electrode Configuration 134 Arc Characteristics 138 Operating Pressure 139 Rotation Frequency 141 Operating Powers 143 Stability 144 Feedstock Flows and Cooling Losses 145 Experimental 145 Results 147 Conclusions 149 CHAPTER 7 - SPECTROSCOPIC STUDIES 172 Introduction 172 Experimental 172 Nitrogen 174 Hydrogen Cyanide 17 6 Hydrogen 183 Argon 184 State Selectivity 184 Temperature Measurements 184 Conclus ions 186 iii CHAPTER 8 - CONCLUSIONS 189 Lean Barn 190 Plasma Jet Ignition 191 High Chemical Activity of Incomplete Combustion Products 194 Stratified Charge Combustion 195 Ignition by Radiation 197 Applications of Electrical Discharges in Combustion Systems 198 Hydrogen Addition 199 Overall Efficiency 200 Diesel Versus Otto 205 APPENDIX 1 Calculations relevant to nitrogen purification and extraction from air. 208 APPENDIX 2 Efficiency of idealised Diesel cycle in the limit as cut-off ratio tends to unity, and its s ignificance. 2 10 APPENDIX 3 Calculations relevant to excess enthalpy combustion. 213 REFERENCES 222 BIBLIOGRAPHY 245 iv ACKNOWLEDGEMENTS • The author would like to thank Professor F. J. Weinberg for all his help and advice throughout the duration of this work and for the introduction to the fascinating area of electrical aspects of com- bustion. Dr. A.R. Jones is particularly thanked for stimulating conversations especially during the work described in Chapter 6, at which time it is appropriate to acknowledge the ingenuity of my colleague Tony Chan in the final construction of the plasma gene- rator described in Chapter 6. The author is especially grateful to Professor A.G. Gaydon F.R.S., who made available spectroscopic equipment and offered much guidance in the early phases of the study, and to Professor Derek Bradley for his constructive comments on the original manu- script. Messrs. D. Downs, R. W. Wheeler, J. G. G. Hempson, R. D. Cuthbertson and M. Thompson, of Ricardo and Company Con- sulting Engineers are thanked for their interest in the work and for making available equipment and research facilities where necessary, as well as constructing some of the prototype plasma generators and plugs that were used in the work contained in Chapter 4. The technical skills of Messrs. C. Smith and K. Grose of the glass workshop were warmly appreciated at every phase of the experi- mental work. Finally I would like to thank my wife who managed to endure sufficiently well to type the manuscript, her patience was an inspira- tion. During the period October 1974 to October 197 7 the author was supported on an SRC-Ricardo Industrial Studentship Award. V PICTURE CREDITS The photographs contained in this thesis as Figures 4.2 and 6. 5 were taken by Leonard Moulder A. R. P. S. , his photographic assistance was greatly appreciated and the author learned much from him. All other photographic figures were taken by the author, v/ho would like to thank the Imperial War Museum, London, for permission to photograph Bertelli's black pottery head of Mussolini shown as Figure 6.1. vi LIST OF FIGURES Page Figure 1. 1 The effect of emission standards on fuel economy. 3 Figure 1. 2 The effect of NO reduction, by injection timing retard, on the brake thermal effi- ciency of Diesel engines. 4 Figure 1. 3 Emissions and fuel consumption curves versus air/fuel ratio for an E6 engine. 6 Figure 3. 1 Schematic of various ways in which rates of heat release may be modified, i) Re- circulation of hot combustion products with simultaneous dilution, ii) Heating without dilution of reactants, iii) Addition of species important in the rate determining stage either in-situ photochemically or from a plasma generator. 16 Figure 3.2 A hypothetical Jablonski diagram of a pho- tochemical scheme involving two modes of energy transfer leading to dissociation fol- lowing biphotonic absoption. 20 Figure 3.3 Potential energy curves for oxygen. 2 1 Figure 3.4 Burning velocity vs. equilibrium concentra- tion of H-atoms. 26 Figure 3. 5 A comparison of the mole fractions of H, CH^, OIT and O in an inhibited and uninhibited flame. 27 Figure 3.6 Equilibrium fractional dissociation of hydro- gen into ground state atoms as a function of temperature and pressure. 28 Figure 3.7 Limiting flow rates in electrical augmenta- tion of combustion. 31 Figure 3. 8 Schematic of the arrangement for reaction rate control via injection of appropriate plasma generated molecular fragments. 42 Figure 4. 1 Comparison of rates of reaction 1 and 2. a) Temperature dependences of rate constants; b) Minimum nitric oxide concentration attain- able for O2 (i) 21%, air (leanest limit), for ex- ample, an unthrottled engine at low load; (ii) 10%, unthrottled engine at high load; (iii) 5%, throttled engine, low load, high air/fuel ratio; (iv) 1%, throttled engine, idle or high load. 48 vii Figure 4. 2 Photographs of an early plasma generator in a variety of flame stabilising and depollution role s. a) A non rotated early plasma generator operating on 90% Ar, 10% used in the following photographs. b) A Ricardo model gas turbine flame tube operating on propane/kerosine, with observation port installed. c) A high flow premixed propane/air flame stabilised on the Ar/N^ plasma issuing from the generator seen in Figure 4. 2a (see also Figure 4. 3). d) The same event as Figure 4. 2c, viewed from upstream. e) A luminous sooting diffusion flame con- tained within the flame tube of Figure 4.2b. f ) Cessation of sooting on injection of a 10% N^ in Argon plasma under the conditions of Figure 4. 2e . g) An argon plasma injected into fast flow atmospheric pressure synthetic exhaust gases. h) Reaction of N-atoms generated by addi- tion of N£ to the plasma resulting in the destruction of NO (3000 80 ppm at 4. 2 1 s present in the condition of Figure 4. 2g. (In this early experiment a pink emission from N^* was observed that is not clearly seen in this picture). 50 Figure 4. 3 Potential chemical power vs. linear flow velo- city for the experimental situation seen in Figures 4. 2c and d. 54 Figure 4.4 Minimum powers required for production of ground state -atoms to reduce to zero an initial value of WZ (NO concentration-flow pa- ramete r). 61 Figure 4. 5 Diagramatic of the Ricardo flame tube seen in Figures 4.