Larry William Swanson

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.

Isothermal Physical Flow Modeling of a Gas Turbine Simple-Cycle Selective-Catalytic-Reduction (SCR) System

A physical flow model of a gas turbine (GT) simple-cycle Selective-Catalytic-Reduction (SCR) system was constructed to a 1/16 geometric scale to validate computational fluid dynamics (CFD) predictions and examine the impact of tempering air injection on system performance. Repeatable velocity contours and tempering air dispersion profiles were developed for baseline (no tempering air), and 12- and 6-lance tempering air injector configurations. The conclusions from the study are: (1) relative to the no lance baseline case, the 12-lance configuration tends to force more of the inlet flow towards the top of the duct, whereas the 6-lance configuration does not affect the upstream profile significantly, (2) adding tempering air does not have a significant impact on the diffuser inlet velocity distribution and has a minor effect on the velocity and dispersion profiles at the NOX-catalyst inlet, (3) at the NOX-catalyst inlet, the 6-lance configuration with tempering air exhibits a slightly skewed flow toward the lower right corner of the duct with a coefficient of variation (COV) of 19.4%, which is slightly better than that for the 12-lance configuration, (4) at the NOX-catalyst inlet, the 12-lance configuration disperses tempering air best because its COV is 20.8% relative to a 27.3% COV for the 6-lance configuration, and (5) a comparison between the local mixing-cup temperature contours for both 12- and 6-lance configurations, based on tracer injection into the tempering air flow, confirms that the CFD model does a good job of qualitatively predicting the heat and mass transport processes in the GT simple-cycle SCR system.Copyright

Enhanced Selective Non-Catalytic Reduction (SNCR) for Refinery Applications: Pilot-Scale Test Data With a Hydrogen Promoter

Selective non-catalytic reduction technology (SNCR) is an effective and economical method of reducing NOX emissions for a wide range of industrial combustion systems. It is widely known that the traditional SNCR temperature window is centered around 1,200 to 1,255 K [1]. However, for some applications, the flue gas temperatures in boilers, oxidizers, and heaters range from 950 to 1150 K. At these lower temperatures, injection of an amine reagent into flue gas no longer actively reduces NOX , but instead passes through the system and exits as ammonia slip. Earlier studies have shown that at lower temperatures, hydrogen and other promoters can be added to the system to shift the SNCR window to a lower temperature range, enhancing or promoting SNCR NOX reduction performance [2–5]. This extended abstract describes pilot-scale test results for an enhanced SNCR process (ESNCR) that uses hydrogen as the SNCR promoter. The impacts of flue gas temperature, hydrogen concentration, CO concentration, and SO2 concentration on ESNCR NOX reduction performance are presented.© 2011 ASME

Fuel Flexible Biomass Reburn Technology

Biomass reburn is a low NOx alternative to cofiring that effectively uses the high volatility and high char reactivity of biomass for NOx reduction. In this paper, computational fluid dynamics (CFD) and thermal modeling, and a NOx prediction model were used to evaluate the impacts of sawdust/coal reburn on the performance of a 250 MW opposed-fired boiler burning bituminous coal as the primary fuel. The results showed that the reburn system maintained overall boiler performance with a 50 – 70 °F reduction in the furnace exit gas temperature. Predicted losses in thermal efficiency were caused by the lower biomass fuel heating value (similar to biomass cofiring) and increase in unburned carbon. The higher unburned carbon emissions were attributed to an order of magnitude larger biomass mean particle size relative to bituminous coal. Thus, LOI emissions can be improved significantly by reducing the biomass mean particle size. The NOx predictions showed that for reburn rates above about 19%, adding dry sawdust biomass to a coal reburn system can improve NOx reduction; i.e., using pure dry sawdust as reburn fuel at 30% of the total heat input can lead to NOx levels about 30% less than those for pure coal reburn under for similar firing conditions.© 2009 ASME

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.

CFD Study on Vermiculite Injection System Optimization in a Cyclone-Fired Boiler

Boilers firing low-rank coal generally experience high levels of slagging and fouling. To help manage convective pass fouling, various additives or conditioners can be injected into the boiler furnace high temperature region as physical disruptors of slag deposits, exhibiting a varying density or gas evolution, which physically breaks up slag or as chemical modifiers of the ash to increase its softening temperature. Vermiculite is one additive that has been applied with success; however, it is important to ensure that the injected material is placed in the most heavily slagged and fouled areas. The purpose of this study was to evaluate the effectiveness of a vermiculite injection system installed on a large cyclone-fired boiler and to identify improvements in the injection system that would permit more effective treatment of the areas of interest. During the study, a million-cell full boiler combustion model was developed. Typical features of the furnace flow and temperature field were obtained. Considering the particular operating conditions and features of the upper furnace flow field (biased gas velocities and rotational flow), optimized injection schemes were proposed. This study shows the usefulness of applying CFD to solve slagging and fouling issues for coal-fired boilers.© 2008 ASME