Evaluation of fabric filter plant operating costs as a function of bag filter dimensions
Van Riel, H.W.
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Particulate emissions from industrial activities have been governed worldwide. In South Africa the Department of Environmental Affairs (DEA) has, under the mandate of the National Environmental Management: Air Quality Act, or NEM: AQA (Act no. 39 of 2004), implemented minimum particulate matter (PM) emissions standards. South African power stations also need to comply with the more stringent legislation. One reason for the high PM emissions levels is that the coal quality has deteriorated over the years and is no longer within the original design basis. As a result, the currently installed PM removal technologies at certain stations, which was also designed with specific PM removal efficiencies, is no longer adequate to comply with the more stringent emission limits, and therefore needs to be modified or replaced. Historically, electrostatic precipitators (ESPs) have been the dominant PM abatement technology used for PM removal in coal-fired power stations. However, fabric filter plants (FFPs) have been shown to consistently offer higher PM removal efficiencies and therefore the use of this technology has been increasing steadily. Currently, almost half of South Africa's coal fired power stations are equipped with FFPs and following the introduction of the latest minimum emissions standards some of the affected power stations where ESPs are installed could possibly be replaced with FFPs. Bag filters are the key component of FFPs, which provides the physical barrier that inhibits the passage of the particulates while allowing the flue gas to permeate through to the stack. Several aspects relating to bag filter dimensions, material and construction therefore need to be considered during FFP design. The bag filter dimensions play a key role in the volume of particulate laden flue gas that can be cleaned cost-effectively in an FFP per unit time and is therefore determined by the process conditions of a specific power station. The bag filter dimensions are also of key importance in the design of the cleaning system, and in pulse-jet FFPs the bag filter size determines the capacity of the compressor plant used to generate compressed air for the cyclic pulsing (cleaning) process. Similarly, the plant footprint, layout and construction are affected by the dimensions of the filter bags, which in turn also determine the number of bags required. Additionally, the bag filter dimensions, together with the gas permeability of the material, also influences the pressure drop that is achieved across the bag house, and this has significant implications on the induced draft (ID) fan power requirements. Power stations in South Africa typically have three different bag diameters in its fleet of FFP units, with all having roughly the same length. However, in-house expertise on selecting the optimum bag size, which would offer the best trade-off between performance and cost, is required to be established. A test set-up was consequently designed and built to study the influence of bag diameter and length on performance as characterised by the air-to-cloth ratio, relative to the pressure drop achieved across bag filter assembly. Additionally, pressure drop across the system was also measured as a function of ash load to correlate the rate at which the pressure drop increases as a function of the various bag dimensions using Darcy's equation. Three different bag diameters (135mm, 150mm, and 160mm) and four different lengths (0.8, 1.2, 1.6, and 2.0 m) were selected for evaluation in this study. The modelling results that were derived from the measurement results were subsequently incorporated in an economic process model. A present worth analysis were done for the specified design scenario to account for the associated capital expenditure (CAPEX) and the operating expenditure (OPEX). The plant parameters included in the present worth analysis were items such as compressed air requirements (pulsing), bags replacement, the replacement of cages and the pulse valves. The results indicated that larger bags, both in diameter and in length would provide the most cost-effective solution. It should, however, be kept in mind, that this would only be true if the pulsing system is able to generate the required pulse energy to effectively clean the bags.
- Engineering