2006

Lyyränen, Jussi

The objective of this work was to study the formation of particles and their morphology and chemical composition in large-scale diesel engines operating with low-grade residual fuel oils. The effect of a Mg-based fuel oil additive on exhaust gas particles was also investigated. Particle characteristics were determined by means of the methods of aerosol technology, chemical analyses, and electron microscopy. As particle and deposit formation and characteristics play an important role in corrosion and erosion, the particle characterisation studies provided the necessary background information. The mass size distributions from the large-scale diesel engines were bimodal, with a main ("small") mode at 60-90 nm and a "large" mode at 7-10 µm. The small mode particles were formed by the nucleation of volatilised fuel oil ash species, which grew further by condensation and agglomeration. The large-mode particles were mainly agglomerates of different sizes consisting of small particles. These particles were re-entrained from deposits and fuel residue particles of different sizes. The number size distributions peaked at 40-60 nm. Agglomerates consisting of these primary spherical particles were also found. TEM micrographs revealed that these particles consisted of even smaller structures. On the basis of the mass and elemental size distributions, evidence that the fuel oil ash was highly volatile was found. The main causes for the differences in the aerosol size distributions were the engine type and fuel oil properties. By estimating the chemical compounds formed on the basis of ICP and EDS analyses at the corresponding mode in mass size distributions (about 0.1 µm), it was found that there was not enough oxygen in the particles to form only V2O5. Complete oxidation of vanadium into vanadium pentoxide was not favourable. This can be caused by many different factors, such as short residence times or soot particles acting as surface toxicants by blocking the active surface. However, the amount of sulphuric acid in the particles was high, about 27 wt. %. This required the formation of vanadium pentoxide to catalyse the formation of SO3 to form sulphuric acid. Doping the heavy fuel oil with a Mg-based additive caused another mode at about 2 µm in mass size distributions, making the size distributions trimodal. The 2-µm mode was generated by magnesium, together with some vanadium, nickel, and sulphur. Particle formation was not affected by the fuel oil additive. Deposition and corrosion studies on the surfaces of the Nimonic 80 A sample slabs were carried out on a laboratory-scale with a newly set-up deposition-corrosion apparatus (DCA). With this device the formation of the exhaust ash particles, gas composition, and deposition and corrosion on the sample slabs occurs in a similar way as in large-scale engines. Although corrosion studies have been carried out before, the formation of a corrosive ash layer when the particles deposit on the sample slabs has not previously been taken into account. Furthermore, the possible transformation of the deposited particles when they start to react to form a corrosive ash deposit has not been considered. In the deposition and corrosion experiments with SO2(g) and synthetic ash particle feeds, almost all of the particles observed looked like flat "pools" with small spherical particles in the middle of the "pool". Condensing sulphuric acid had dissolved the particles. Small (70-90-nm) spherical particles were also observed with an SO2(g) feed. On the other hand, hardly any S was found in the deposits. This indicated that S, in the form of SO2(g)/SO3(g), was transported through the deposit into the interface between the base material (pit area) and bottom of the deposit by molecular diffusion. The critical issue in the propagation of corrosion was the definition o