in Wiley InterScience (www.interscience.wiley.com).As emissions regulations for coal-fired power plants become stricter worldwide, layering combustion modification and post-combustion NO X control technologies can be an attractive option for efficient and cost-effective NO X control in comparison to selective catalytic reduction (SCR) technology. The layered control technology approach designed in this article consists of separate overfire air (SOFA), reburn, and selective noncatalytic reduction (SNCR). The combined system can achieve up to 75% NO X reduction. The work presented in this article successfully applied this technology to NRG Somerset Unit 6, a 120-MW tangential coal-fired utility boiler, to reduce NO X emissions to 0.11 lb/MMBtu (130 mg/Nm 3 ), well under the US EPA SIP Call target of 0.15 lb/MMBtu. The article reviews an integrated design study for the layered system at Somerset and evaluates the performance of different layered-NO X -control scenarios including standalone SNCR (baseline), separated overfire air (SOFA) with SNCR, and gas reburn with SNCR. Isothermal physical flow modeling and computational fluid dynamics simulation (CFD) were applied to understand the boiler flow patterns, the combustible distributions and the impact of combustion modifications on boiler operation and SNCR performance. The modeling results were compared with field data for model validation and verification. The study demonstrates that a comprehensive process design using advanced engineering tools is beneficial to the success of a layered low NO X system. V V C 2009 American Institute of Chemical Engineers AIChE J, 56: 825-832, 2010
The alternative fuels, such as biomass, municipal wastes, and underground coal gasification gas, become attractive to the power plants as renewable energy sources or economical fuels. However, the alternative fuels usually have much lower heating value and different chemical compositions from those of coal and natural gas. Firing these alternative fuels in the boilers that are originally designed for coal firing or natural gas firing may cause unexpected boiler operating issues and/or thermal performance degradation. A careful evaluation study is often required prior to implementation. This paper presents the results of a study that evaluated the feasibility of using an underground coal gasification gas as a reburn fuel. The evaluation was done on Eskom’s Majuba Unit 5, a 710 MWe opposed wall-fired boiler, located in South Africa. The study utilized heat transfer analysis and computational fluid dynamics models to (1) evaluate the impacts of firing low calorific fuel on boiler efficiency and the boiler auxiliary system performance, (2) develop a conceptual gas reburn injection system, and (3) evaluate the impacts of gas reburn on the boiler thermal performance and boiler NOx emissions. The results indicate that the underground coal gasification gas can be an effective reburn fuel for the Majuba boiler with upgrades on the auxiliary systems.
This paper presents a method of real-time CNC interpolation for complex curved profiles which can be described with NURBS equations. The real-time interpolation algorithm is based on second order Taylor's expansion with the principle part being a formula for updating parametric value u after each sampling period. With the updated value of u, a new interpolated point is calculated based on the DeBoor's algorithm. Chord error caused by the interpolation algorithm depends on two main factors which are machining feedrate and radius of curvature of the NURBS curve. In this paper, an efficient method of limiting chord error is also presented with the basic idea of reducing machining feedrate at positions with a radius of curvature smaller than a critical value. Simulation results on Matlab software verify effectiveness of the proposed method.
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