A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy of the University of London
by
Luis Javier Molero de Blas, MSc
under the supervision of
Prof Anthony R Burgess, PhD, ARCS, DIC, CEng, FInstE, CChem, FRSC
Department of Chemical Engineering
University College London
Torrington Place
LONDON WC1E 7JE
England
Decreasing quality and stricter environmental regulations impose potential limitations to the use of heavy fuel oils in combustion. Because of their low cost they are economical alternatives for power generation. However, they contain large amounts of nitrogen and sulphur, which form NOX and SOX during combustion and cause undesirable pollution. Additionally they often produce carbonaceous particulates.
A knowledge of the formation and interaction of nitrogen and sulphur compounds in combustion is necessary to establish pollution abatement strategies. In this thesis a study of those processes was performed using a number of heavy petroleum-based fuels.
An extensive literature review on relevant aspects of heavy fuel combustion was carried out, with particular emphasis on the formation of NOX, SOX, particulates and ash.
In the first stage of the experimental work, the ignition characteristics of the fuels were determined by means of the Single Suspended Droplet Technique. They were found to comply with most of the correlations for heavy fuel combustion established by Taylor and Burgess, relating the combustion characteristics of a fuel droplet to its dimensions.
The formation of NOX and SO2 was studied in a drop-tube furnace as a function of the stoichiometry, flame temperature and residence time. Additional information about other species formed was also obtained, as well as about the formation of thermal-NOX.
In a further stage, a numerical model was used to simulate mathematically the experimental results and study the mechanisms of interaction between N and S species. These calculations were aided by measurements of the flame temperature in the drop-tube furnace.
In the last stage of the experimental work, the interaction of sulphur species with NOX (ie NO and NO2) was studied experimentally by adding SO2-gas to the combustion system in the drop-tube furnace. Sulphur was found to influence the formation and emission of NO and NO2 in different ways according to the equivalence ratio. Nitrogen-sulphur interactions were also studied as a function of the residence time.
Firstly, I must thank Dr Fernando Bosch Bosque (now retired) for sheltering our research proposal initially. I would also like to thank Dr Jacinto Monge Gutiérrez, Dr Fernando Domínguez Vega, Mr Ángel López Ortega and Dr Ángel Nodar Blanco (also now retired) at Repsol S.A. for the technical, scientific and economic support which made this project viable.
Dr Michael Weekes and Dr Steve Graville (currently at BOC Ltd) were of invaluable help at re-building the drop-tube furnace and its software at UCL. So was Mr Andrew Pearce of Severn Furnaces Ltd. British Petroleum plc are to be acknowledged for providing us with a unique opportunity to acquire a unique piece of apparatus, the drop-tube furnace.
I should like to express my gratitude to Dr John Smart (currently at Fuel and Combustion Technology Int), Dr Nina Skorupska and Dr Gerry Riley at National Power plc for the fuel samples, analyses and economic support provided during the last part of the project.
At the Department of Chemical and Biochemical Engineering of University College London I would like to express my thanks to Dr David Bogle, the staff at the Departmental Office, the Store and the Electronics and Mechanical Workshops.
Back at home my gratitude goes to all of my friends, but especially to Emi, Lucas, Enrique and Pepe for their perseverance at writing the e-mail messages that made the distance so much shorter.
Thanks to June Burgess for the great fun at the conferences, dinners, banquets, History lectures... and especially for the Rock and Roll.
Great thanks are due to Dr Carlos I Fernandes for his friendship along the years, the great fun we had around the World and his priceless help when conjuring the demons of the numerical model. God bless!!
My fondest, greatest and dearest thanks go to all of my friends, residents and staff of
International Hall of the University of London.
Chapter II. Experiments with the single suspended droplet technique
Appendix I: List of References
Appendix II: Analyses of the fuels used in this thesis
Appendix III: Reaction mechanism used in the numerical model
Appendix IV: Modified "CONP.FOR" source code used in numerical model
Figure 1: Sulphur and nitrogen contents in vacuum residues according to their geographical origin
Figure 2: Evolution of proven worldwide reserves of crude oil and natural gas
Figure 3: Percentage of heavy fuel oils in total worldwide production of petroleum products
Figure 4: Refinery layout for minimum heavy oil production by means of visbreaking
Figure 5: World reserves of extra-heavy hydrocarbons and bitumen
Figure 6: Asphaltene molecular shape, as proposed by Yen
Figure 7: The pre-ignition delay period
Figure 8: The flame period
Figure 9: The coke-ignition period
Figure 10: Soot residue left after combustion
Figure 11: Diagram of the free droplet apparatus used by Malik and Burgess
Figure 12: Breakdown of thermal power generation in twelve European countries in 1993
Figure 13: Gas turbine cycles
Figure 14: Typical design of a gas turbine combustor
Figure 15: Gas turbine cycle with steam generator and heat recovery as in Patent Application GB 2,187,273
Figure 16: Combustor chamber arrangement
Figure 17: Gas turbine cycle as in Patent Application 1,140,757
Figure 18: Plan view of the single suspended droplet furnace
Figure 19: Electric circuit diagram of the single suspended droplet apparatus
Figure 20: General view of the single suspended droplet apparatus
Figure 21: (from left to right) Light projector and single suspended droplet furnace
Figure 22: (from left to right) Light projector and single suspended droplet furnace
Figure 23: Photo-diode, furnace and control probe thermocouple
Figure 24: Combustion sequence of a droplet of fuel M2 of do = 0.88 mm burned at 750 °C furnace temperature (horizontal axis: time, s; vertical axis: output voltage (temperature, light), µV)
Figure 25: Calculated Pre-ignition Delay for a standard 1 mm initial diameter droplet
Figure 26: Generic boiling range for heavy vacuum gas oils and heavy coker gas oils
Figure 27: Graph showing ln 1/ti vs 1/TF for droplets of do = 1 mm of all fuels under study
Figure 28: Calculated Flame Time for a standard 1 mm initial diameter droplet
Figure 29: Calculated Total Combustion Time for a standard 1 mm initial diameter droplet
Figure 30: Graph showing the Ignition Temperature from experiments with fuel M3 at 800 °C vs do
Figure 31: Relationship between the Peak Temperature and the square of the initial droplet diameter from experiments with fuel M2 at 750 °C
Figure 32: Graphs showing the evolution of the Normalised Droplet Diameter with respect to the Normalised Pre-ignition Delay
Figure 33: Twin-fluid atomiser used in experiments in the drop-tube furnace
Figure 34: The drop-tube furnace. Below, description of numbered parts
Figure 35: Flow diagram of the drop-tube furnace
Figure 36: Sample treatment facility and gas analysers in drop-tube furnace
Figure 37: DTFSYS.XLS spreadsheet for data presentation (values are not representative)
Figure 38: Graph showing the volume of air used for combustion and the volume of wet combustion gases formed, according to the net calorific value of the fuel and the equivalence ratio
Figure 39: NO concentrations from fuel M1 at 900 °C
Figure 40: NO concentrations from fuel G1 at 900 °C
Figure 41: NO concentrations from fuel M1 at 1,100 °C
Figure 42: NO concentrations from fuel G1 at 1,100 °C
Figure 43: NO concentrations from fuel M1 at 1,200 °C
Figure 44: NO concentrations from fuel G1 at 1,200 °C
Figure 45: Fuel-N to NO conversion at exhaust from all experiments performed (open symbols: fuel M1; solid symbols: fuel G1)
Figure 46: NO2 concentrations from fuel M1 at 900 °C
Figure 47: NO2 concentrations from fuel G1 at 900 °C
Figure 48: NO2 concentrations from fuel M1 at 1,100 °C
Figure 49: NO2 concentrations from fuel G1 at 1,100 °C
Figure 50: NO2 concentrations from fuel M1 at 1,200 °C
Figure 51: NO2 concentrations from fuel G1 at 1,200 °C
Figure 52: Fuel-N to NO2 conversion at exhaust from all experiments performed (open symbols: fuel M1; solid symbols: fuel G1)
Figure 53: NOX concentrations from fuel M1 at 900 °C
Figure 54: NOX concentrations from fuel G1 at 900 °C
Figure 55: Conversion of fuel-N to NOX from fuel M1 at 900 °C
Figure 56: Conversion of fuel-N to NOX from fuel G1 at 900 °C
Figure 57: NOX concentrations from fuel M1 at 1,100 °C
Figure 58: NOX concentrations from fuel G1 at 1,100 °C
Figure 59: Conversion of fuel-N to NOX from fuel M1 at 1,100 °C
Figure 60: Conversion of fuel-N to NOX from fuel G1 at 1,100 °C
Figure 61: NOX concentrations from fuel M1 at 1,200 °C
Figure 62: NOX concentrations from fuel G1 at 1,200 °C
Figure 63: Fuel-N to NOX conversion at exhaust from all experiments performed (open symbols: fuel M1; solid symbols: fuel G1)
Figure 64: SO2 concentrations from fuel M1 at 900 °C
Figure 65: SO2 concentrations from fuel G1 at 900 °C
Figure 66: SO2 concentrations from fuel M1 at 1,100 °C
Figure 67: SO2 concentrations from fuel G1 at 1,100 °C
Figure 68: SO2 concentrations from fuel M1 at 1,200 °C
Figure 69: SO2 concentrations from fuel G1 at 1,200°C
Figure 70: O2 concentrations from fuel M1 at 900 °C
Figure 71: O2 concentrations from fuel G1 at 900 °C
Figure 72: O2 concentrations from fuel M1 at 1,100 °C
Figure 73: O2 concentrations from fuel G1 at 1,100 °C
Figure 74: O2 concentrations from fuel M1 at 1,200 °C
Figure 75: O2 concentrations from fuel G1 at 1,200 °C
Figure 76: CO2 concentrations from fuel M1 at 900 °C
Figure 77: CO2 concentrations from fuel G1 at 900 °C
Figure 78: CO2 concentrations from fuel M1 at 1,100 °C
Figure 79: CO2 concentrations from fuel G1 at 1,100 °C
Figure 80: CO2 concentrations from fuel M1 at 1, 200 °C
Figure 81: CO2 concentrations from fuel G1 at 1,200 °C
Figure 82: CO concentrations from fuel M1 at 900 °C
Figure 83: CO concentrations from fuel G1 at 900 °C
Figure 84: Traverse and position measurements of CO from fuel M1 at 900 °C and 
= 0.833
Figure 85: Traverse and position measurements of CO from fuel G1 at 900 °C and = 0.833
Figure 86: CO concentrations from fuel M1 at 1,100 °C
Figure 87: CO concentrations from fuel G1 at 1,100 °C
Figure 88: CO concentrations from fuel M1 at 1,200 °C
Figure 89: CO concentrations from fuel G1 at 1,200 °C
Figure 90: N2O concentrations from fuel M1 at 900 °C
Figure 91: N2O concentrations from fuel G1 at 900 °C
Figure 92: N2O concentrations from fuel M1 at 1,100 °C
Figure 93: N2O concentrations from fuel G1 at 1,100 °C
Figure 94: N2O concentrations from fuel M1 at 1,200 °C
Figure 95: N2O concentrations from fuel G1 at 1,200 °C
Figure 96: Flame temperature and readings of NO and NO2 from fuel M1 at = 1.000 and 900 °C
Figure 97: Axial distribution of the [NO2/NOX] ratio and flame temperature profile from experiments at = 1.200 and 900 °C
Figure 98: Formation of SO2 and disappearance of O2 for fuel M1at = 1.200 and 900 °C furance wall temperature
Figure 99: Equilibrium concentrations of sulphur species according to the equivalence ratio calculated using CHEMKIN (1.10 % wt sulphur, 1,527 °C)
Figure 100: NO emissions from diesel fuel
Figure 101: NO2 emissions from diesel fuel
Figure 102: Total NOX (NO + NO2) from diesel fuel
Figure 103: Conversion of fuel-N to NOX from diesel fuel
Figure 104: Flame temperature profile from diesel fuel at = 0.833 and 900 °C furnace wall temperature corrected for heat losses
Figure 105: Flame temperature profile from fuel M1 at = 0.833 and 900 °C furnace wall temperature corrected for heat losses
Figure 106: Flame temperature profile recorded from Orimulsion at = 0.833
Figure 107: Flame temperature profile recorded from fuel M1 at = 0.833
Figure 108: Flame temperature profile recorded from fuel M1 at = 1.000
Figure 109: Flame temperature profile recorded from fuel M1 at = 1.200
Figure 110: Flame temperature profile from Orimulsion at = 0.833
Figure 111: Flame temperature profile from the heavy coker gas oil M1 at = 0.833
Figure 112: Flame temperature profile from the heavy coker gas oil M1 at = 1.000
Figure 113: Flame temperature profile from the heavy coker gas oil M1 at = 1.200
Figure 114: Calculated concentrations of NO from fuel M1 at various equivalence ratios
Figure 115: Calculated concentrations of NO from Orimulsion at = 0.833
Figure 116: Calculated concentrations of NO2 from fuel M1 at three equivalence ratios
Figure 117: Calculated concentrations of NO2 from Orimulsion at = 0.833
Figure 118: Calculated concentrations of HO2 radicals from fuel M1 at three equivalence ratios
Figure 119: Evolution of species involved in NO2 reactions from fuel M1 at = 1.200
Figure 120: Calculated concentrations of SO2 from fuel M1 in air at three equivalence ratios
Figure 121: Calculated concentrations of SO2 from fuel G1 and three equivalence ratios
Figure 122: Calculated concentrations of sulphur dioxide and various reduced sulphur species from fuel M1 burned in fuel-rich conditions
Figure 123: Numerical model calculations of NO formation from fuel M1 at = 0.833 and various degrees of fuel-S addition
Figure 124: Effect of SO2 on NO emissions from fuel M1 at various equivalence ratios (experimental results: solid symbols; numerical simulation: open symbols)
Figure 125: Effect of SO2 on NO emissions from Orimulsion at = 0.833 (experimental results: solid symbols; numerical simulation: open symbols)
Figure 126: Fuel-NO formation paths and the effect of SO2 on the concentration of the intermediate and final species at = 0.833
Figure 127: Experimental readings of NO from experiments of sulphur addition with the fuels investigated at = 0.833
Figure 128: Percentage change of NO emissions at = 0.833 from experiments of sulphur addition from all fuels investigated
Figure 129: Percentage change of NO emissions at = 1.000 from experiments of sulphur addition from the fuels investigated
Figure 130: Experimental readings of NO2 from experiments of sulphur addition from the fuels investigated at = 1.000
Figure 131: Nominal change in NO2 emissions at = 1.000 from experiments of sulphur addition from the fuels investigated
Figure 132: Experimental readings of NO2 from experiments of sulphur addition from the fuels investigated at = 1.200
Figure 133: Nominal change in NO2 emissions at = 1.200 from experiments of sulphur addition from the fuels investigated
Figure 134: Experimental readings of NOX from experiments of sulphur addition from all fuels investigated at = 0.833
Figure 135: Change in NOX emissions with addition of fuel-S at = 0.833 from all fuels investigated
Figure 136: Percentage change of NOX emissions at = 0.833 from all fuels investigated
Figure 137: Experimental readings of NOX from experiments of sulphur addition from the fuels investigated at = 1.000
Figure 138: Change in NOX emissions with addition of fuel-S at = 1.000 from the fuels investigated
Figure 139: Plot of [NO]/[NO2] vs percentage of fuel-S added for the fuels investigated at = 1.000
Figure 140: Experimental readings of NOX from experiments of sulphur addition from the fuels investigated at = 1.200
Figure 141: Percentage change of NOX emissions at = 1.200 from experiments of sulphur addition from the fuels investigated
Figure 142: Experimental readings of O2 from experiments of sulphur addition from the fuels investigated at = 0.833
Figure 143: Experimental readings of SO2 from experiments of sulphur addition from all fuels investigated at = 0.833
Figure 144: Experimental readings of SO2 from experiments of sulphur addition from the fuels investigated at = 1.000
Figure 145: Experimental readings of SO2 from experiments of sulphur addition from the fuels investigated at = 1.200
Figure 146: Experimental readings of N2O from experiments of sulphur addition from all fuels investigated at = 0.833
Figure 147: Experimental readings of N2O from experiments of sulphur addition from the fuels investigated at = 1.000
Figure 148: Experimental readings of NO from fuel M1 at three equivalence ratios, S addition and selected sampling distances. Solid symbols: air; open symbols: addition of 2.0 % S
Figure 149: Experimental readings of NO2 from fuel M1 at three equivalence ratios, S addition and selected sampling distances. Solid symbols: air; open symbols: addition of 2.0 % S
Figure 150: Experimental readings of NOX from fuel M1 at three equivalence ratios, S addition and selected sampling distances. Solid symbols: air; open symbols: addition of 2.0 % S
Figure 151: Nominal reduction of NO emissions at = 0.833 from experiments of sulphur addition from all fuels investigated
Figure 152: Nominal reduction of NO emissions at = 1.000 from experiments of sulphur addition from all fuels investigated
Figure 153: Experimental readings of CO2 from fuel M1 at three equivalence ratios, S addition and selected sampling distances. Solid symbols: air; open symbols: addition of 2.0 % S
Table 1: Physical and chemical properties of several typical heavy oils
Table 2: Typical nitrogen content of standard fuels
Table 3: Properties of heavy fuel oils as gas turbine fuels according to ASTM standard D 2880-90a
Table 4: Overall ignition activation energy and pre-exponential factor for the fuels studied
Table 5: Ignition Temperatures of the samples studied at 800 °C furnace temperature
Table 6: Experimental values of the Critical Diameter for Ignition
Table 7: Secondary air mass flow rates calculated for various equivalence ratios
Table 8: Maximum NOX emissions from the fuels studied
Table 9: Chemical and physical characteristics of diesel fuel
Table 10: Mole fractions of species in fuel feed of the numerical mode
Table 11: NO emissions as calculated from the numerical model and obtained experimentally from fuel M1
Table 12: NO2 reactions in the numerical model
Table 13: Qualitative effect of SO2 addition on emissions of nitric oxide from fuel M1 at various equivalence ratios
Table 14: Qualitative effect of SO2 addition on emissions of nitrogenous species at various equivalence ratios (font size denotes relative extent of interactions)
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