Experimental Study of Sea Salt Particles (SSP) Filtration Performance for On-Board SOFCs

Abstract

This work described in the dissertation mainly focuses on the filtration of sea salt particles (SSP), which flow into the cathode side of Solid Oxide Fuel Cells (SOFCs) on the ships. Currently, SOFCs are being considered as power source for ships due to their high energy efficiency and low waste emissions. However, different types of air contaminants existed in the air could cause harmful effects on the performance of those SOFCs and decrease their useful life time. The impurities contain gaseous contaminants, such as sulfur compounds and particulates, like sea salt particles and smokes. Salt particles are all over around the sea because of the wake and bow spray. They could lead to corrosions and fouling of fuel cell systems and clog the finely divided and highly tortuous flow channels in fuel cell stacks as well. Therefore, it is significantly important to remove them from air stream before entering SOFCs. Usually, filtration is the most commonly applied approach due to the high efficiency and low cost. As for filtration system design, there are mainly two aspects. One is filter packaging configuration design and the other one is filter media design. Filter media is the most basic component of filter which does the filtration work. Depending on the type of filter media and how the filters are packaged, filter system could be designed to show different filtration performances for sea salt particles and meanwhile, an optimization could be achieved according to different air conditions and specifications. Chapter I mainly describes the background, history and motivation of sea salt particles filtration. Three important filtration parameters are introduced briefly, including pressure drop, filtration efficiency and particles loading capacity. These three parameters are highly significant during filter design that needs to be considered as main criteria. Moreover, in Chapter I, filter media manufacturing is described, which contains two main different processes, dry formed and wet laid process. In my work, all filter media tested are made using wet laid method due to its convenience and low cost. Sea salt particles filtration using pleated filters in novel packaging configurations is described in Chapter II. Commercial filters (24’’by 24’’) are tested for all the experiments. For single filter, the effects of filter depth and filter pleat count on three filtration parameters mentioned in Chapter I are discussed, including pressure drop across the filter, particles removal efficiency and loading capacity and compressor parasitic power. Meanwhile, MESA, which stands for Multi-Element Structured Array, is introduced and tested for slat particles filtration. MESA is a novel filter packaging configuration invented by Center of Microfibrous Materials Manufacturing (CM3), Auburn University, which integrates several single filters in one single filtration system to become a three dimensional array of filters and could extend the filtration area and reduce the pressure drop. The experiment results of MESAs are compared with single filters under same air conditions and conclude that MESAs are able to increase both particles removal efficiency and loading capacity and lower filter pressure drop and compressor parasitic power simultaneously, which results in much better filtration performance for salt particles. In Chapter III, filter media is the major concern. Comparison of filtration performance for sea salt particles between nonwoven and woven activated carbon fiber media is described in this chapter. Activated carbon fiber is applied in this part of work because it can remove particulates and gas contaminants, such as VOCs simultaneously, which can lower filtration costs. Woven and nonwoven media are currently two most widely applied types of filter media. Due to their different structures, they can show different filtration performances for salt particles. And to evaluate the advantages and disadvantages of each type of media is of high significance when the media are applied in real circumstances. Therefore, experiments of pressure drop, filtration efficiency and particles loading capacity are conducted for both nonwoven and woven filter media. The results have shown that nonwoven ACF media are much better for salt particles filtration due to the homogeneous structure. Moreover, since relative humidity (RH) during the sea environment is much higher, it is very necessary to investigate the effect of RH on filter media performance for salt particles. Activated carbon fiber belongs to hydrophilic material, which can absorb water into fiber pores. Experiment results show that higher RH is able to enhance filtration efficiency due to the improved capillary tension between particles and fiber surface. Besides experimental study, theoretical study is also of highly importance during filtration system design. Since what we conclude from Chapter III is that nonwoven filter media is superior compared to woven filter media when applied in salt particles filtration, Chapter IV mainly describes theoretical analysis of various nonwoven microfibrous filter media, including pressure drop across the filter media, filtration efficiency and particles loading capacity. Several different filter media are tested using bench scale setup and the experiment results are compared with estimation results in order to verify if the estimation equations are correct. Further modified PMP (Porous media permeability) and filtration efficiency equations according to various filter media type are used for pressure drop and particles removal efficiency estimation separately. Moreover, a new estimation approach for particles loading capacity is developed and verified with experiment results. From comparisons, what we can see is that estimation results match quite well with experiment results. Chapter V mainly describes sea salt particles filtration of various nonwoven filter media. A new type of nonwoven filter media, VGCF (carbon nanofiber) entrapped nonwoven polymer media is evaluated in terms of the filtration performance, including pressure drop, filtration efficiency, particles loading capacity and compressor parasitic power. This media is manufactured using wet-laid approach, which could lead to uniform three dimensional nanofiber nonwoven filter media with high filtration efficiency and low pressure drop. Besides, particles loading performance is compared between different types of nonwoven filter media, including surface media and depth media. The results show that surface media is better for initial filtration application and depth filter media is more suitable for long term particles filtration and loading. In the end, effect of relative humidity on hydrophobic filter media is discussed and what can be concluded is higher RH is able to lower filtration efficiency due to the occupation of interstitial fiber space by water droplets.