INTRODUCTION TO EXPERIMENTAL WORK

Firstly, the physical and chemical characteristics of heavy liquid fuels have been reviewed, with focus on the refining processes (vacuum distillation, delayed coking, visbreaking, etc.) that increase the yield of lighter products and the changes in the characteristics of the heavy fuels that these processes can cause. The physical and chemical properties of liquid fuels were also reviewed.

The previous research on the combustion of heavy fuel droplets was examined in the next part of the literature survey. Two main techniques were reviewed, namely the single suspended droplet technique and the single droplet technique. The former technique consists of suspending a small droplet of fuel on a wire, which is then subjected to a source of heat or radiation. This technique will be used in the experimental part of the present thesis (see chapter II) to determine the basic ignition and burnout characteristics of the fuels under study. Also, the variation of the fuel droplet size during the pre-ignition period will be examined by means of video recordings.

The use of heavy liquid fuels for the generation of electricity by means of gas turbines was the object of the next section of the literature review. Atmospheric pollutants are formed in the generation of electricity by thermal means. These are, mainly, oxides of nitrogen (NO and NO₂), oxides of sulphur (predominantly SO₂) and particulate matter. The processes leading to the formation of these species were reviewed, as well as methods to reduce their formation and emission. In chapter III of this thesis the formation of these and other pollutants from the fuels under study will be examined experimentally in a drop-tube furnace. In this furnace, a spray of fuel and air was burnt at varying furnace wall temperatures and stoichiometric ratios, and the combustion gases were sampled at various distances from the atomiser. The data so obtained enabled the study of the formation of pollutants as a function of the flame wall temperature and stoichiometry of the combustion system.

This thesis is concerned with various aspects of the formation and emission of fuel-NO_X. Since thermal-NO_X is also a common occurrence at high flame temperatures in combustion processes, it was necessary to determine the amounts of thermal-NO_X formed in the drop-tube furnace. This was achieved by burning a fuel with a low nitrogen content, as the formation of thermal-NO_X is independent from the nitrogen content of the fuel, and is described in chapter IV .

The results from a numerical model are reviewed in chapter VI. The model was used to investigate further the reactions that take part in the formation of oxides of nitrogen. Some of these reactions are detailed in the literature survey reported in chapter I. In addition, the reactions that lead to interactions between sulphur and nitrogen compounds are studied in chapter VI. The model is aided by measurements of the flame temperature in the drop tube furnace, which are shown in chapter V.

Also in the literature survey, the interaction of oxides of sulphur in the formation and emission of oxides of nitrogen has been examined, with particular emphasis on the previous research on the effect of fuel-S on fuel-NO_X, ie oxides of nitrogen formed from the nitrogen contained in fuel. The fuels under study in this thesis were used to experimentally assess the effect of increased concentrations of fuel-S on the formation and emission of oxides of nitrogen. This was done at one furnace wall temperature and as a function of the stoichiometric ratio, so that fuel-lean, stoichiometric and fuel-rich conditions were investigated. Also, sampling at different distances from the atomisation nozzle was performed to investigate the stages of the combustion processes where N-S interactions occur. The experimental procedure and the results are detailed in chapter VII.

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