David Kelly Moyeda

Application of numerical simulation and full scale testing for modeling low NOX burner emissions

Design and application of low-NOX burners and other combustion modification technologies to coal-fired boilers rely largely on the designers expertise, garnered from previous experience and engineering intuition. Use of this ‘tried-and-true’ methodology for low-NOX technologies results in some risk when these technologies are applied to conditions or boiler designs that are outside of the normal experience base. GE Energy has found that the use of advanced design tools such as computational fluid dynamics (CFD) modeling in designing low-NOX burners can ensure an effective match between the burner design and the combustion system. However, the previous design experience also indicates that model validation against test data is essential to the success of applying CFD simulation in facilitating burner design. This paper summarizes the results of a study of a low-NOX burner for coal-fired boilers. The study was focused on validating the CFD predictions on NOX emission against a single burner full scale experimental measurements. The CFD predictions along with the full scale test results were also used to evaluate design changes in the burner to ensure optimal performance for the particular application.

The formation and control of PCDD/PCDF from RDF-fired combustion systems

Abstract During the combustion of municipal solid waste, incomplete destruction of chlorinated hydrocarbons and other precursors in the waste can lead to the formation and emission of polychlorinated dibenzo (p) dioxins and furans (PCDD/PCDF). Since regulations limiting these emissions are expected, the Gas Research Institute has been conducting research to develop advanced combustion systems for municipal solid waste (MSW) incinerators which involve the use of small amounts of natural gas to control PCDD/PCDF emissions and to improve unit operation. Characterization tests have been conducted on both full-scale and small-scale RDF fired combustion systems. The results of these tests have confirmed both furnace and downstream PCDD/PCDF formation and identified the importance of particulate related formation. These new insights into the sources of PCDD/PCDF emissions have lead to the development of advanced combustion control schemes.

Evaluation of Applying Low Calorific Fuel as Reburn Fuel in an Opposed Wall Fired Boiler

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.

Application of CFD for a low-NOx burner retrofit to a coal-fired utility boiler

The design and application of low-NOx burners and other combustion modification technologies to coal-fired boilers typically relies largely on designer know-how and previous experience. There is always some risk inherent in applying NOx control technologies to new applications. This risk is offset by the use of design tools such as computational fluid dynamics (CFD) to analyse and simulate system operation. This paper presents the results of a study designed to validate the application of a combined coal/fuel oil burner to a coal-fired boiler. The modelling study includes simulation of windbox air distribution and detailed modelling of the low-NOx burner alone, and as installed on the furnace. The results of the study validate the burner design for this new application and demonstrate the utility of CFD in studying such systems.

Utilization of Partially Gasified Coal for Mercury Removal

In this project, General Electric Energy and Environmental Research Corporation (EER) developed a novel mercury (Hg) control technology in which the sorbent for gas-phase Hg removal is produced from coal in a gasification process in-situ at a coal burning plant. The main objective of this project was to obtain technical information necessary for moving the technology from pilot-scale testing to a full-scale demonstration. A pilot-scale gasifier was used to generate sorbents from both bituminous and subbituminous coals. Once the conditions for optimizing sorbent surface area were identified, sorbents with the highest surface area were tested in a pilot-scale combustion tunnel for their effectiveness in removing Hg from coal-based flue gas. It was determined that the highest surface area sorbents generated from the gasifier process ({approx}600 m{sup 2}/g) had about 70%-85% of the reactivity of activated carbon at the same injection rate (lb/ACF), but were effective in removing 70% mercury at injection rates about 50% higher than that of commercially available activated carbon. In addition, mercury removal rates of up to 95% were demonstrated at higher sorbent injection rates. Overall, the results of the pilot-scale tests achieved the program goals, which were to achieve at least 70% Hg removal from baseline emissions levels at 25% or less of the cost of activated carbon injection.

A Thermal Model for Concentric-Tube Overfire Air Ports

A quasisteady multimode heat-transfer model for boiler concentric-tube overfire air ports has been developed that predicts the effect of geometry, furnace heat source and heat sink temperatures, axial injector wall conduction, and coolant flow rate on the tube wall temperature distributions. The model imposes a radiation boundary condition at the outlet tip of the ports, which acts as a heat source. The model was validated using field data and showed that both the airflow distribution in the ports and tube diameter can be used to control the maximum tube wall temperature. This helps avoid tube overheating and thermal degradation. For nominal operating conditions, highly nonlinear axial temperature distributions were observed in both tubes near the hot outlet end of the port.

ADVANCED BIOMASS REBURNING FOR HIGH EFFICIENCY NOx CONTROL AND BIOMASS REBURNING - MODELING/ENGINEERING STUDIES JOINT FINAL REPORT

This report presents results of studies under a Phase II SBIR program funded by the U. S. Department of Agriculture, and a closely coordinated project sponsored by the DOE National Energy Technology Laboratory (NETL, formerly FETC). The overall Phase II objective of the SBIR project is to experimentally optimize the biomass reburning technologies and conduct engineering design studies needed for process demonstration at full scale. The DOE project addresses supporting issues for the process design including modeling activities, economic studies of biomass handling, and experimental evaluation of slagging and fouling. The performance of biomass has been examined in a 300 kW (1 x 10{sup 6} Btu/hr) Boiler Simulator Facility under different experimental conditions. Fuels under investigation include furniture waste, willow wood and walnut shells. Tests showed that furniture pellets and walnut shells provided similar NO{sub x} control as that of natural gas in basic reburning at low heat inputs. Maximum NO{sub x} reduction achieved with walnut shell and furniture pellets was 65% and 58% respectively. Willow wood provided a maximum NO{sub x} reduction of 50% and was no better than natural gas at any condition tested. The efficiency of biomass increases when N-agent is injected into reburning and/or burnout zones, or along with OFA (Advanced Reburning). Co-injection of Na{sub 2}CO{sub 3} with N-agent further increases efficiency of NO{sub x} reduction. Maximum NO{sub x} reduction achieved with furniture pellets and willow wood in Advanced Reburning was 83% and 78% respectively. All combustion experiments of the Phase II project have been completed. All objectives of the experimental tasks were successfully met. The kinetic model of biomass reburning has been developed. Model agrees with experimental data for a wide range of initial conditions and thus correctly represents main features of the reburning process. Modeling suggests that the most important factors that provide high efficiency of biomass in reburning are low fuel-N content and high content of alkali metals in ash. These results indicate that the efficiency of biomass as a reburning fuel may be predicted based on its ultimate, proximate, and ash analyses. The results of experimental and kinetic modeling studies were utilized in applying a validated methodology for reburning system design to biomass reburning in a typical coal-fired boiler. Based on the trends in biomass reburning performance and the characteristics of the boiler under study, a preliminary process design for biomass reburning was developed. Physical flow models were applied to specific injection parameters and operating scenarios, to assess the mixing performance of reburning fuel and overfire air jets which is of paramount importance in achieving target NO{sub x} control performance. The two preliminary cases studied showed potential as candidate reburning designs, and demonstrated that similar mixing performance could be achieved in operation with different quantities of reburning fuel. Based upon this preliminary evaluation, EER has determined that reburning and advanced reburning technologies can be successfully applied using biomass. Pilot-scale studies on biomass reburning conducted by EER have indicated that biomass is an excellent reburning fuel. This generic design study provides a template approach for future demonstrations in specific installations.