A number of heavy distillate gas oils, in addition to Orimulsion, have been studied. The former fuels are by-products of refining operations (vacuum distillation and delayed coking), and contain varying amounts of nitrogen and sulphur. Orimulsion also contains 30 % water by weight.

In the first part of the experimental work, the ignition characteristics of the range of heavy distillate fuels studied were investigated with the single suspended droplet technique. The fuels were found to comply with the correlations that relate the ignition properties of a fuel droplet to its initial dimensions. However, no direct correlation was found between their pre-ignition delays and the data available for their distillation ranges. The groups of fuels studied (heavy vacuum gas oils and heavy coker gas oils) were also found to have different ignition temperatures. The single suspended droplet technique also helped determine other characteristics of their combustion, such as large soot formation, absence of coke particulates, or the size variation of fuel droplets prior to ignition, the latter assisted by video recordings.

In the next part of the experimental work, the formation of pollutants from two selected heavy distillate fuels was studied in the drop-tube furnace. This study was performed at various conditions of stoichiometry, furnace wall temperature and residence time. Oxides of nitrogen were found to be formed by the fuel-NO_X mechanism in amounts that varied with the flame temperature, the equivalence ratio and the distance from the atomiser. The thermal-NO_X and prompt-NO_X mechanisms were considered to be of lesser importance. The concentrations of NO detected were maximum under fuel-lean conditions, decreased in stoichiometric mixtures, and reached a minimum under fuel-rich conditions. Conversely, large amounts of nitrogen dioxide were detected in stoichiometric and especially fuel-rich conditions, but not under fuel-lean conditions. Hydroperoxyl radicals were thought to have a significant role in the formation of NO₂ under stoichiometric and fuel-rich conditions. SO₂ was the most important sulphur combustion product and accounted for almost 100 % of the total sulphur in fuel-lean conditions. However, in oxygen-deficient, fuel-rich conditions, SO₂ was transformed into other reduced sulphur species.

The preliminary information thus obtained about NO_X and SO_X formation was instrumental in choosing criteria for the subsequent study of fuel-N and -S interactions. It also provided experimental data for the validation of the numerical model used to study these interactions. The formation of negligible amounts of thermal-NO_X was confirmed with the use of a low-nitrogen diesel fuel. Also, information about the axial flame temperature was obtained by performing thermocouple traverse measurements.

Numerical simulation was performed with the CHEMKIN suite of FORTRAN codes. Two flow regimes in the furnace were simulated by different codes, namely "PSR" for the initial, short CSTR zone and "CONP" for the subsequent long PFR zone. The reaction system comprised reactions for the three NO_X formation mechanisms, ie fuel, thermal and prompt. The model was able to reproduce the experimental results of NO and NO₂ emissions at fuel-lean equivalence ratios. However, at stoichiometric or fuel-rich conditions the model failed to simulate the experimental measurements. It was thought that significant reactions for the formation of NO₂ in these conditions were absent from the model. The reduction of SO₂ under fuel-rich conditions, previously found in experiments, was also reproduced by the model.

Finally, the interaction of fuel-sulphur with oxides of nitrogen originating from the fuel-N content were investigated both experimentally and numerically. Orimulsion was included in this study. Experiments were performed with increased amounts of fuel-S simulated by the addition of SO₂-gas. The extent and direction of the interactions depended on the amount of sulphur dopant, the equivalence ratio and the residence time in the combustor. Addition of SO₂-gas yielded reductions of NO and increases of NO₂ concentrations at short residence times under fuel-lean conditions. The reduction of NO emissions was particularly effective in Orimulsion. Possible mechanisms include radical recombination, direct N-S interactions and interconversion of NO and SO₂ to yield NO₂ and SO₃. The numerical model also yielded reductions of fuel-NO_X at fuel-lean conditions, and the results were most accurate for Orimulsion. The emissions of NO₂ were largely increased in fuel-rich conditions by addition of SO₂. Recombination of H radicals by SO₂ can act by preventing the disappearance of NO₂. Also, SO₂ was found to interact with hydrocarbons present in the system, which may be related to increased efficiency of the formation of NO₂ by hydroperoxyl radicals.

Further work proposed

Further work to continue that reported in this thesis could comprise the effect of flame temperature on N-S interactions, as the experiments reported in this work were confined to one value of the furnace wall temperature. Detailed measurements of the flame temperature in the first 200 mm of the furnace and at higher furnace wall temperatures would also be beneficial for the numerical model.

The study of fuels of a heavier nature than those in this thesis may also provide useful information. Residual fuels may present a larger potential for NO_X reduction by SO_X, as the release of nitrogen during combustion is progressive.

Although SO₂ is the major sulphur combustion product, sulphur in fuel is normally in its reduced state. Experiments with addition of sulphur as H₂S or as an organic additive to the fuel, under similar conditions to those investigated in this thesis could provide comparative information and further insight into their interactions with fuel-NO_X. The analysis of gas samples for reduced sulphur species formed at stoichiometric and fuel-rich conditions could be instrumental at quantifying the reduction of sulphur dioxide in such conditions.

Since the numerical model was not able to reproduce the experimental results obtained under stoichiometric and fuel-rich conditions, an extension including larger species and reactions of hydroperoxyl radicals (RO₂) would be useful to widen its range of applicability.

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