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Table 5.2.2 gives the analysis of emissions of particulates and NO^ from the combustion of LIFO emulsion. The ash contents which indicate the extent of burn-out are quite different from the RVBR results. The ash varies, depending on the equivalence ratio, from 9.4 %wt under rich conditions to 53.7 %wt under lean conditions. The present work shows that the equivalence ratio is more important than in heavy residue combustion. The temperature had no effect on the ash content of the particulates collected, although it did increase the C/H ratio. Therefore the emulsion particulates collected have undergone more cracking and dehydrogenation reactions. The emulsion ash level shows that the emulsion particulates are further down the coke bum-out path and are therefore more likely to have a higher C/H ratio.

Nitrogen and sulphur

The nitrogen content of the emulsion particulates is slightly lower than the heavy residue particulates, even though the emulsion particulates are more bumt out and LIFO contains more fuel-nitrogen. The sulphur content of the emulsion particulates is similar to the heavy residue particulates when the original sulphur concentration in the fuels are taken into account.

Structure

Some of the particulates collected from emulsion combustion are shown in the SEM photographs in Figure 5.2.29. Typical small sized high ash content particulates can be observed in these

the morphology of the particulates. The only significant difference between the particulates is the build up of soot under the rich conditions. In all the photographs large balloon-type particulates were observed with a similar appearance to those observed with heavy residue.

5.2.2 DISCUSSION 5 2.2.1 Heaw residue

A. Effect of equivalence ratio

The NO^ emissions from this work are similar to those given in the literature at the equivalence ratios used (Miller and Bowman 1989). The thermodynamic equilibrium as calculated by Chemix (CSIRO 1986) in Table 5.2.3 suggests that the reductions in fuel NO^ observed in the DTF as a result of decreasing the equivalence ratio are orders of magnitude too low. The equilibrium data shows that NO^ emissions under rich conditions are insignificant. The table also shows that at the low fumace wall temperatures used in the experimental work, relatively small amounts of NO^ are formed, suggesting that the actual flame temperatures in DTF1 are much higher than the fumace wall temperatures.

In the early stages of combustion before perfect mixing has taken place, fuel-nitrogen will be converted to NO where lean conditions prevail and nitrogen where fuel-rich conditions prevail. If the overall combustion is lean it is most likely that the NO which has formed will be emitted in the exhaust, but if the combustion is rich, reduction reactions will limit the NO emitted. Reduction of NO under rich conditions by gaseous reducing agents (eg hydrogen, carbon monoxide) occurs readily in the presence of hydrocarbon radicals (Chen et al 1988). The hydrocarbon radicals provide a route for recycling NO into species that can then be reduced to molecular nitrogen. This is discussed in section 2.2.2 2 of the review. The equilibrium predicted by Chemix differs from a real system in that the calculations are carried out at a pre-determined stoichiometry under perfectly-mixed conditions. The difference in results between this model and a real system show that the formation and reduction of NO^ in a real system is controlled by the mixing of fuel and air and the local stoichiometries that exist in the flame.

B. Effect of temperature

The results show that increasing the temperature from 873 K to 973 K had no effect on the fractional conversion of fuel-nitrogen to NO^. This is in agreement with Pershing et al (1979), although the temperatures used in this work are much lower. There is no evidence to show that the flame temperature increases with increase in fumace wall temperature, so until work in DTF2 under isothermal conditions, the effect of temperature on fuel NO ^ remains unanswered.

c. Effect of residence time

All the NO^ profiles obtained from this work remain remarkably steady as the residence time varies. One exception to this is Figure 5.2.3 which shows a gradually decreasing NO^ concentration with residence time. The conversion of nitrogen to NO^ gradually decreases from 45 % close to the atomiser to 19 % at the exit of the fumace. The likely reasons for this relationship are similar to those given for the effect of equivalence ratio, in that the NO^ decreases as the CO and hydrocaitons increase as the equivalence ratio increases. Several authors have reported that the presence of solid particles in the flame and in its combustion products gives rise to the existence of heterogeneous NO reduction on the gas/solid interface (Wendt et al 1979, Levy et al 1981, Cheng et al 1981, DeSoete 1990). It is possible that solid-bound hydrogen, caiton or nitrogen reduce NO to molecular nitrogen, or to NHg or HCN, intermediates which can further react to form molecular nitrogen. The particulates formed In heavy fuel oil combustion enhance the likelihood of NO reduction because of the large surface areas available for heterogeneous reactions to take place. If these mechanisms of NO reduction by hot carbonaceous particulates do occur significantly they may offer a potentially useful mechanism for enhanced NO^ control. A certain amount of scepticism should be applied in considering the NO^ reductions occurring in the post-flame region of Figure 5.2.3. This is because the work was carried out with a stoichiometric mixture, a difficult experimental condition to maintain, and a slight shift towards rich or lean conditions has a large effect on the NO^ emissions.

D. Effect on particulates Nitrogen content

The results show that the nitrogen in the fuel is released at a different rate to that of the carbon, hydrogen and sulphur; concentrating as the burn-out proceeds. Therefore the fuel-nitrogen is likely to be associated with the less volatile higher molecular weight compounds such as the asphaltenes. Data has been obtained for the evolution of fuel-nitrogen from a heavy fuel oil showing that less than 30 % of the nitrogen is evolved after 85% of the mass has been vaporised (Hanson et al 1982). In the particulates collected from the DTF the nitrogen remaining accounted for between 2 % and 5 % of the total nitrogen. A real combustion environment would have higher temperatures and higher combustion efficiencies (99.9 %), giving a low particulates burden to trap nitrogen. At 973 K and 0 = 1.0 the particulates collected at 370 ms had a nitrogen content of 1.5 %wt and those collected at 490 ms had a nitrogen content of 1.6 %wt. This implies that the nitrogen is concentrated during the volatilisation of the fuel droplet and is then oxidised at a similar rate to the carbon during the coke formation stage. The coke burn-out is

occur at a similar rate. However, at 873 K and 0 = 0.8 the particulates collected at 210 ms had a nitrogen content of 1.4 %wt and those collected at 420 ms had a nitrogen content of 1.9 %wt. This indicates that the fuel is still devolatilising and burning, concentrating the higher molecular weight nitrogen compounds as the coke forms.

Structure

Large thin-walled particulates with broken shells were observed (Figure 5.2.15) at a residence time of 210 ms. Similar structures have been observed from the combustion of coal (Sjogren 1971, Kohen et al 1990, Smith 1990, Witzel et al 1991). The walls are so thin, probably much less than 0.1 pm, that the structures collapse and tear easily. Indeed it is thought that this could occur during sample preparation and evacuation prior to the SEM examination, therefore it is not clear whether the balloons tear during combustion or not. An optical micrograph of the particulates shown in Figure 5.2.15 with specially careful preparation showed only complete spheres which were so thin as to be translucent. These balloons can bum very rapidly and fragmentation of them once a few holes have been bumt through will Increase the rate further. Calculations based upon the rate of bum-out of thin sheets under boundary layer diffusion control, that is flame conditions, show that bum-out within 1 ms is possible and is likely within 10 ms. It is presumed that the balloons are inflated by the evolution of volatiles within them and expand until that evolution ceases or the skin ruptures. If the skin solidifies before gas evolution ends then the balloons will fragment as the pressure cannot be contained. The skin must be elastic for the balioon to expand, yet the skin is also carbonising as it is heated and polymerising due to the presence of oxygen in the surrounding gas. In fact no surface blow holes were observed in these particulates; which suggests that the surface was too plastic to rupture into symmetrical holes. Thus the formation and size of a balloon involves a balance between gas evolution, increasing skin viscosity and ultimate solidification, rate of heating and oxygen availability.

EDAX examination of particulates from the DTF was carried out to establish if the impurities present in the fuel became concentrated during bum-out. The distribution of vanadium, sodium, calcium and sulphur appeared to be uniform and are known to be present in the oil in complex organic structures. In contrast, iron and silicon became concentrated in a small proportion of the particulates, but not in the same ones. It is possible that the silicon derives from the insulating material in the fumace or catalyst fines in the oil and so would be expected to be present as discrete concentrations. It is also possible that rust in the heated fuel system is a source of iron, along with the 16 ppm of iron in the fuel. Overall the results of the EDAX examination were inconclusive.

5.2.22 Emulsion