Hierarchical methods to improve the performance of the SiC-FET as SO2 sensors in flue gas desulphurization systems

Abstract

Experiments were performed both in the laboratory and a desulfurization pilot unit in order to improve the SiC-FET sensor performance using two-step data evaluation. In both cases, a porous Pt-gate enhancement type SiC-FET was utilized in a temperature cycled operation (TCO). Liner Discriminant Analysis (LDA) was chosen as the method for multivariate data analysis. Hierarchical methods with two-step LDA worked quite well in the laboratory tests with SO 2 concentrations varied from 25-200 ppm. The same data evaluation was also applied to tests in the desulfurization pilot unit, with higher gas flow and a larger SO 2 concentration range (up to 5000 ppm). The results from the SO 2 quantification showed a significantly improved fit to corresponding reference instrument (FTIR) values. 2 IntroductionSO 2 has a significant impact on human health and the environment due to its ability to cause acid rain and form acid particulates. The majority of the emission comes from the energy sector [1]. The European Environmental Agency (EEA) has reported that in the last decade the SO 2 emission has decreased by about 74% and the majority of the reduction has come from power generation and distribution (58%) [2]. The EEA also stated [2] that the reduction in SO 2 has been the result of use of fuels with lower sulfur content and the increase in efficiency of desulfurization systems. However, these measures are not sufficient. The goal for future SO 2 emission reduction has been outlined according to stricter regulation. One way to achieve this goal is to improve the efficiency of the sulfur removal unit even more.In thermal power plants, SO 2 is produced as an unwanted by-product from the combustion of sulfur containing fuel, and it needs to be removed before venting the flue gas to the atmosphere. SO 2 is removed in a desulfurization system. Lime (Ca(OH) 2 ) is mixed with ash and injected into the dry desulfurization system to react with SO 2 in the gas phase. The resulting products are separated from the flue gas by a fabric filter before venting the flue gas to the atmosphere [3]. In this process, sensors play an important role, not only in detecting the concentration of SO 2 at the outlet of the unit, but also in monitoring the concentration profile throughout the duct. The latter application is important for maintaining concentration uniformity of the flue gas, and thus, for improving the efficiency of the desulfurization unit.SiC based sensors are suitable candidates for the desulfurization application due to the inert behavior of SiC in high temperature, oxidizing environments [4][5][6]. Moreover, SiC-FET sensors have shown promising results in industrial environments for CO [7,8] and NH 3 [6,9].The sensors are based on commercial transistor devices with porous catalytic metal gate on top of the gate oxide [4]. The target gas is adsorbed, and then decomposed upon interaction with oxygen ions on the catalytic metal surface. The interaction between the target gases and the metal surface changes the balance of the surf...

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