Fouling of Egr Heat Exchangers – Investigation of Mechanisms Involved in Soot Particle Deposition

Cooled exhaust gas recirculation (EGR) is a very effective method to reduce nitrogen oxide emissions of diesel engines. However, due to particulate matter and other components present in diesel exhaust gas, EGR coolers are highly prone to fouling. As different soot deposition mechanisms are acting inside the coolers, various model cooler experiments were performed to clarify their contribution to cooler fouling. A model aerosol was used, containing soot particles either with or without hydrocarbon (HC), water and sulphuric acid (H2SO4) vapour added. The additive-free soot aerosol became deposited due to thermal forces under corresponding temperature conditions. Turbulent eddy diffusion increased size-dependent deposition efficiencies especially for particles smaller than 50 nm. However, the aerosol containing water, HC and H2SO4 vapour showed an even higher potential for deposit buildup. Thus we found that condensation and diffusiophoresis contribute essentially to EGR cooler fouling, as diesel exhaust always contains condensable components. INTRODUCTION The combustion process in diesel engines is a source for hazardous environmental pollutants such as nitrogen oxides (NOx) or particulate matter (soot). Future emission standards (e.g. Regulation (EC) N 715/2007 of the European Parliament and of the Council, 2007) require increased application of exhaust gas recirculation (EGR). EGR is a very simple though effective way to reduce NOx emissions in diesel exhaust. A fraction of the emitted flue gas is cooled in an EGR cooler, returned to the engine and mixed with fresh charge air. Due to the decreased oxygen content, NOx emission is eventually reduced (Ladommatos et al., 2000). Besides the usual combustion end products the recycled exhaust gas also contains unburnt hydrocarbons (HC), soot particles, metal oxides and traces of sulphuric acid (H2SO4) (Kittelson, 1998). These constituents are responsible for EGR coolers being very prone to fouling during vehicle lifetime. Due to various acting deposition mechanisms an insulating layer forms on the gas side of the EGR cooler which reduces heat transfer and thus the cooling efficiency of the device. As an additional consequence pressure losses are increased (Abd-Alla, 2002). Therefore, today’s EGR coolers are designed to meet the limitations for NOx output despite the efficiency decrease. In general, this means that EGR coolers are oversized to compensate the rate of fouling by cooling efficiency reserves. Deposition mechanisms The most important deposition mechanisms acting in EGR coolers are in alphabetical order: Diffusion, where small particles are transported to the cooler wall due to undirected Brownian particle motion. The effect of diffusion is strongly dependent on particle size, flow velocity and cooler geometry (Friedlander, 2000; Gormley and Kennedy, 1949). Diffusiophoresis, where the condensation of one or several gaseous components causes a vapour concentration gradient, inducing directed particle transport towards the surface on which condensation is happening. This effect depends on the condensate mass flow defined by the strength of the vapour concentration gradient as well as on the gas and particle velocities (Hirschfelder, 1954). It is known that diffusiophoresis may occur in exhaust gas cooler applications and enhance particle deposition. Lehtinen et al. (2002) and Gröhn et al. (2009) for example studied the effect of condensation on particle deposition. However, these authors only took the condensation of water vapour into account, whereas diesel exhaust gas also contains condensable diesel fuel components and sulphuric acid. Impaction, where inertia causes particles larger than a critical size to deposit in case of redirection of the gas flow. The critical particle size is strongly dependent on cooler geometry and gas flow velocity (Hinds, 1999). Thermophoresis, where a temperature gradient leads to transport of particles from the hot gas to the cold wall. For this mechanism temperature, flow conditions and cooler geometry are important (Messerer et al., 2003, Romay et al., 1998). This mechanism is often described as being the most important acting mechanisms in heat exchanger fouling (e.g. Abarham et al., 2010). The aim of this work is to gain more understanding of the mechanisms leading to the build-up of an insulating layer and all parameters influencing these mechanisms. Proceedings of International Conference on Heat Exchanger Fouling and Cleaning 2011 (Peer-reviewed) June 05 10, 2011, Crete Island, Greece Editors: M.R. Malayeri, A.P. Watkinson and H. Müller-Steinhagen Published online www.heatexchanger-fouling.com 82 H Müller-Steinh gen and A.P. Watkinso