OCR Text |
Show radicals at long residence times, resulting in a decrease of N O x emissions. Turbulence was also considered of importance as it can influence the distribution of radicals. In conclusion, the present review reveals a rather confusing picture with respect to the effect of sulphur on fuel-NOx. Increases and reductions of fuel-NOx have been found mainly in fuel-rich conditions, although other equivalence ratios have also been studied. A variety of mechanisms have been proposed, such as direct N-S interactions, radical rearrangements, interference with NOx-hydrocarbon reactions or the amine subsystem, etc. In addition, the recombination of radicals by the presence of sulphur species can affect the reduction of nitric oxide after its formation. The need to clarify the array of possible routes for the interaction of fuel-NOx and SOx compounds is thus clear. Experimental Atomising air 4n Injection probe Fuel Pump -flu Heated can Secondary Balance air The experimental work was carried out in a vertical tube furnace (see flow diagram in Figure 1), which consists of a one-metre long, eight-centimetre diameter, cylindrical combustion chamber of ceramic material. Isothermal conditions were easily established by means of electrical heating in ten separately controlled zones. Gases and particulates resulting from combustion were removed by a water-cooled, stainless steel probe, which could be moved along the furnace axis by means of a step motor. Samples could be removed at any distance greater than 100 m m from the atomiser nozzle. The fuels studied, three heavy gas oils and Orimulsion, a commercial oil:water emulsion, were pumped by a piston or reciprocating pump driven by a powerful and precise step motor, which ensured a constant mass flow rate of fuel. Pre-heating was required in order to achieve flow and atomisation of fuels G1 and G 2 due to their high wax content and thyxotropic properties. The fuels contained varying amounts of sulphur and nitrogen, and in the case of Orimulsion, also water (30 % by weight) and asphaltenes (7.90 % by weight). Partial specifications are shown in Table 1. Atomisation was performed by a coaxial twin-fluid atomiser, in which a stream of compressed air sprayed the fuel into the combustion chamber. A secondary stream of air, introduced around the atomisation nozzle, allowed the setting of the overall equivalence ratio. f n T o p of furnace Twin-fluid atomiser Movable sampling probe Bottom of furnace • Exhaust Solids system By-pass line IW| NO co2 SO? N NO? °2 CO 2° Gas analysis system 5 Filter \/Cyc Exhaust Cyclone Figure 1: Flow diagram of the vertical tube furnace Sampling of the combustion gases was performed at a distance of 500 m m from the atomiser. Previous work on these fuels 21 has shown that at such a distance all 6 |