A cost-benefit analysis of the inclusion of polyimide in fabric filter bags
Fourie, Jeanne Helene
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Electric energy is central to modern society. Although there is a global shift towards renewable energy sources, coal-fired power production remains a widespread electricity generation method. Coal-fired power production, however, results in particulate matter (PM) emissions. These PM emissions hold significant implications for the environment and human health. Consequently, effective reduction of PM emissions from coal-fired power plants is of great importance, especially in developing countries that rely heavily on coal as primary energy source. Baghouses are a popular choice for abatement of PM emissions from coal-fired electricity generation. Baghouses capture PM in the flue gas exhausted from the electricity producing boiler system by means of numerous cylindrical bags constructed of specialised filtration fabric. This fabric, made from polymeric fibres, is integral to baghouse efficiency and cost. South African coal-fired power station baghouses typically use either polyacrylonitrile (PAN) or polyphenylene sulphide (PPS) based filtration fabrics, depending on the operating temperature of the baghouse. Both filtration fabrics types have the option of including polyimide (PI) in a blended surface layer of the fabrics. The inclusion of polyimide, however, increases the cost of the bags. In this study, the inclusion of PI in PAN- and PPS-based filtration fabrics has been comparatively evaluated. The evaluation was based on the triboelectric properties of the fabrics as well as fabric resistance to acid attack by sulphuric and nitric acids. The latter was assessed through tensile strength analysis of the fabrics following acid exposure. The triboelectric properties of filtration fabrics hold implications for the filtration efficiency and pressure drop experienced in a baghouse. The chemical resistance of the filtration fabrics impacts the durability of the fabrics during operation and greatly influences the bag life. In order to comparatively evaluate the costs associated with bag choices, a cost-benefit analysis (CBA) method is proposed which considers all life cycle costs associated with filtration fabric selection. The triboelectric and chemical resistance results were incorporated in the proposed CBA model. The experimental findings indicated that fabrics with PI incorporation developed a positive surface charge polarity after triboelectric contact. PAN- and PPS-based fabrics, however, were found to develop a negative polarity after triboelectric contact. Subsequently, a correlation between ash interaction with charged filtration fabric and fabric polarity was found. It was discovered that ash particles penetrate more deeply into fabric volumes with an increasingly negative surface charge. A filtration and pressure drop benefit may therefore be achieved through the incorporation of PI in PAN and PPS surface layers. From the acid exposure investigations, it was found that combined sulphuric and nitric acid has severely degradative effects on all fabrics considered. The inclusion of PI in PAN-based fabrics was found to be beneficial when nitric acid exposure was experienced and where combined nitric and sulphuric acid exposure was experienced. When PI was incorporated in PAN, however, the fabric was found to be highly susceptible to degradation after exposure to sulphuric acid. PPS-based fabrics were not significantly affected by sulphuric acid, but were severely degraded by nitric acid. Thermogravimetric analysis revealed a fundamental change in polymer degradation behaviour after nitric acid exposure. PI incorporation in PPS based fabrics was found to offer an advantage in sulphuric acid conditions, but was not beneficial when nitric or combined nitric and sulphuric acid exposure was experienced. To evaluate the cost-implications of the experimental findings, a CBA method is proposed which considers initial bag capital cost as well as costs associated with induced draught (ID) fan power and production losses due to early bag failure. Assumptions were made based on the technical investigations conducted as part of this study, however are not reflective of actual baghouse operation; the results are therefore only exploratory in nature. High-level application of the CBA to PAN-based fabrics revealed that incorporation of PI could offer a cost-benefit in operating scenarios where nitric acid or simultaneous nitric and sulphuric acid exposure are expected. Application of the CBA to PPS-based fabrics revealed that PI incorporation could offer a cost advantage in environments where sulphuric acid exposure is experienced. Finally, except in extreme cases of chemical degradation, it was found that PI incorporation in PAN- or PPS-based fabrics offers a cost advantage if a filtration benefit due to triboelectric interactions with the ash facilitates a reduction in pressure drop.
- Engineering