In recent years, extensive studies were conducted to understand the potential implementation of oxy-fuel combustion with flue gas recycle in conventional boilers as an effort of mitigating CO2 emissions. The benefits of the technology include reduction of NO x and CO emissions by recycling more than 60% of the flue gas back into the boiler; generation of a flue gas stream with CO2 concentration higher than 90% on a dry basis, which can be easily recovered and stored; and utilization of the conventional boiler with minimum modifications. However, a practical implementation of the technology needs a thorough analysis of the mass and energy balance and their impacts on boiler thermal performance. This paper conducts a detailed process analysis on the oxy-fuel combustion with flue gas recycle in a conventional boiler. Key parameters such as the flue gas recycle ratio, boiler exit oxygen, and flue gas velocity and temperature distributions are investigated. Computational fluid dynamics (CFD) simulations are performed to help the understanding of the oxy-fuel combustion characteristics in a conventional boiler and in a single coal fired low NO x burner. The study shows that the ideal flue gas recycle ratio depends on boiler exit O2 and fuel properties and is generally around 0.7−0.75. Dry flue gas recycle helps to maintain the furnace temperature.

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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 furnace sorbent injection (FSI) process involves injection of dry sorbents into the upper furnace of a coalfired boiler for reduction of SO 2 emissions, forming solid products that are captured in the plant particulate control equipment. Key parameters that impact SO 2 removal rates are sorbent reactivity, flue gas temperature at the sorbent injection elevation, and degree of mixing between the sorbent and the flue gas. Several potential impacts on boiler performance, such as increased fouling and increased opacity, need mitigation during design and operation. An FSI demonstration project was recently conducted in a 188 Mega Watt generation (MWg), tangentially fired twin-furnace boiler. An integrated design approach combining computational fluid dynamics modeling, isothermal physical flow modeling, and boiler thermal modeling was applied to (1) understand boiler thermal profiles and flow patterns, (2) specify the optimum sorbent injection design, and (3) predict SO 2 removal rates. Field measurements and performance indicators during the test after the installation of the FSI system revealed that SO 2 reduction levels were generally consistent with modeling predictions. At the calcium-to-sulfur molar ratio of 2.5, SO 2 reduction ranged from approximately 40% to 55%. The highest SO 2 reduction achieved during the long-term trial was 72%, at a Ca=S ratio of 3.8.

Selective noncatalytic reduction (SNCR) technology is an effective and economical method of reducing NO x emissions from a wide range of industrial combustion systems. It is widely known that the SNCR process is primarily effective in a narrow temperature window, around 1200−1255 K, and that high CO concentrations can both shift the temperature window and limit the process’ effectiveness. To ensure proper design and application of SNCR technology, it is critical to understand the flow and temperature fields, SNCR kinetics, and species concentrations in the combustion system and to design an injection system that provides good mixing and distribution of the reagent with the furnace gases. The work summarized in this article developed and incorporated a reduced SNCR chemical mechanism into a commercial computational fluid dynamics (CFD) model. Three main results are reported: (1) the reduced mechanism is validated by comparisons to a detailed mechanism using a plug-flow reactor and a perfectly stirred reactor, (2) the SNCR modeling approach with the reduced mechanism is validated by comparing the three-dimensional modeling results with test data from a pilot-scale combustion furnace, and (3) the integrated CFD modeling approach is applied to designing an SNCR system for an industrial furnace. The SNCR system was installed and has been in operation for several years. The NO x reduction and ammonia slip performance for the full-scale system agreed well with the CFD predictions.

This project is designed to develop a family of novel NO control technologies, called Second x Generation Advanced Reburning which has the potential to achieve 90+% NO control in coal fired x boilers at a significantly lower cost than SCR. The sixth reporting period (January 1-March 31, 1997) included both experimental and modeling activities. New kinetic experimental data for hightemperature decomposition of sodium carbonate were obtained in a flow reactor at the University of Texas in Austin. Pilot scale combustion tests in a 1.0 MMBtu/hr Boiler Simulator Facility were continued with firing coal and using natural gas as reburn fuel. The results demonstrate that over 90% NO control is achievable by injecting one or two N-agents with sodium promoters into the reburning zone and with the overfire air. Advanced reburning technologies does not cause significant byproduct emissions. The AR kinetic model was updated to include chemical reactions of sodium carbonate decomposition. Modeling was conducted on evaluation of the effect of sodium on process kinetics in the reburning zone. This study revealed that increasing or decreasing radical concentrations in the presence of sodium can significantly affect the reactions responsible for NO reduction under fuel-rich conditions. The effect of mixing time on performance with sodium was also evaluated. Initial activities on engineering design methodology for second generation AR improvements are described.

Activated carbon injection (ACI) is an effective mercury control technology demonstrated in both short-term and long-term full-scale tests. The effectiveness of mercury capture by activated carbon depends on the mercury speciation, total mercury concentration, flue gas composition, method of capture, and activated carbon properties, such as pore size, type of carbon impregnation, and surface area, etc. It is also desired that an ACI system be designed to produce good mixing between the activated carbon and the flue gas. In recent years, General Electric Energy has conducted both short-term and long-term tests in large-scale coal-fired boilers for ACI mercury capture demonstration. The programs consisted of (1) combustion optimization to improve natural mercury capture by fly ash, (2) computational fluid dynamics (CFD) modeling of activated carbon injection to design ACI lances, (3) a short-term test to select the activated carbon type, and (4) a long-term test to evaluate the mercury capture performance. This paper presents the CFD modeling for an ACI demonstration in Sundance Station Unit 5. The CFD model developed describes the film mass transport, pore diffusion, and carbon surface adsorption and desorption phenomena for the prediction of the mercury capture rate. The model was applied to evaluate the lance design and to calculate the mercury capture rate. The test data were also presented for comparison with the model results.

Biomass has been considered as an alternative fuel to firing coal in utility boilers because of its vast availability and renewable nature. However, the use of biomass as a full or partial replacement for coal needs a careful evaluation of its impact on the boiler performance and the best approach for implementation. In the past, biomass has been implemented in a cofiring mode and is injected into the coal pipe providing a portion of the heat input. Another approach of utilizing biomass is through reburning. In this application, biomass can be directly injected above the burner zone as a reburn fuel, which utilizes the renewable energy and can lead to reduced NO X emissions. In addition, this approach reduces the requirements for mill and/or burner modifications. This paper reviews (1) the process conditions and burner flame structures for biomass cofiring as a function of the heat input and (2) the process requirements and impacts of biomass reburning on the boiler combustion performance. The study indicates that a successful implementation of biomass as an alternative fuel requires a case-by-case examination of the biomass properties such as its fuel factor, F, defined as the ratio of the air-to-fuel demand to the heating value, and its combustion moisture factor, M C , defined as the ratio of the fuel hydrogen content to the fuel carbon content.