ACERC
Abstracts - 1998 |
ACERC
Abstracts - 1998 |
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Research Area 2: NOx/Pollutants |
Tree, D.R.; Black, D.L.;
Rigby, J.R.; McQuay, M.Q. and Webb, B.W.
Prog. Energy Combust. Sci., 24:355-83 (1998).
Energy conversion of fossil fuels or waste products to electricity and heat through clean and efficient combustion processes continues to be an issue of international importance. The Controlled Profile Reactor (CPR) is a small-scale (0.2-0.4 MW) combustion facility that has been used to obtain data for model validation, the testing of new combustion concepts, and the development of new combustion instrumentation. The CPR has a cylindrical, down-fired combustion chamber, 240 cm long and 80 cm in diameter. This review of the past ten years of research completed in the CPR includes a description of the reactor and instrumentation used, a summary of three experimental data sets which have been obtained in the reactor, and a description of novel tests and instrumentation. Measurements obtained include gas species, gas temperature, particle velocity, particle size, particle number density, particle-colored temperature profiles, radiation and total heat flux to the wall, and wall temperatures. Species data include the measurement of CO, CO2, NO, NO2, O2, NH3 and HCN. The three combustion studies included one with natural gas combustion in a swirling flow, and two pulverized-coal combustion studies involving Utah Blind Canyon and Pittsburgh #8 coals. Most, but not all of the above measurements were obtained in each study. The second coal study involving the Pittsburgh #8 coal contained the most complete set of data and is described in detail in Section 3 of the paper. Novel combustion instrumentation includes the use of Coherent Anti-Stokes Raman Sprectroscopy (CARS) to measure gas temperature. Novel combustion experiments include the measurement of NOx and burnout with coal-char blends. The measurements have led to an improved understanding of the combustion process and an understanding of the strengths and weaknesses associated with different aspects of comprehensive combustion models.
Smoot, L.D.; Hill, S.C.
and Xu, H.
Prog. Energy Combust. Sci., 24:385-408 (1998)
Reburning is a process whereby a hydrocarbon fuel is injected immediately downstream of the combustion zone to establish a fuel-rich zone in order to convert nitric oxide to HCN. The reburning fuel can be gaseous (e.g., natural gas), solid (e.g., coal char or wood) or liquid (e.g., residual oil). Typically, the amount of reburning fuel used is 10-30% of the total fuel. This technology is practiced commercially with nitric oxide reduction levels of 35-65%, depending on the type and scale of the boiler or combustion, the primary and reburning fuels and other variables. Current research and development are suggesting several advanced reburning concepts including injection of ammonia or urea aft of the reburning fuel injection. Nitric oxide reductions of over 90% are anticipated. In this mini-review, a review of reburning technologies, measurements and mechanisms is presented. Predictive methods for reburning are also discussed. Recent work on reburning, including development of a global reburning reaction rate, is summarized, and results of application of a comprehensive combustion model to reburning measurements are summarized.
Brown, A.L. and Fletcher,
T.H.
Energy & Fuels, 12:745-57 (1998).
A semiempirical model has been developed for predicting coal-derived soot. The main feature of the model is a transport equation for soot mass fraction. Tar prediction options include either an empirical or a transport equation approach, which directly impacts the source term for soot formation. Also, the number of soot particles per unit mass of gas may be calculated using either a transport equation or an assumed average. Kinetics are based on Arrhenius rates taken from published measurements. Radiative properties are calculated as a function of averaged optical constants, predicted gas temperatures, predicted gas densities, and the soot mass fractions. This model has been incorporated into a comprehensive coal modeling code and evaluated based on comparisons with soot, temperature, and NOx measurements for three experimental cases. Accurate predictions of soot yields have been achieved for both laminar and turbulent coal flames. Larger scale turbulent predictions illustrated that inclusion of a soot model changed the local gas temperatures by as much as 300 K and the local NOx concentration by as much as 250 ppm. These predictions demonstrate the necessity for an accurate soot model in coal combustion systems.
Mallampalli, H.P.; Fletcher,
T.H. and Chen, J.Y.
Journal of Engineering for Gas Turbines and Power, 120:703-12 (1998).
This study has identified useful reduced kinetic schemes that can be used in comprehensive multidimensional gas-turbine combustor models. Reduced mechanisms lessen computational cost and possess the capability to accurately predict the overall flame structure, including gas temperatures and key intermediate species such as CH4, CO and NOx. In this study, four new global mechanisms with five, six, seven, and nine steps based on the full GRI 2.11 mechanism, were developed and evaluated for their potential to model natural gas chemistry (including NOx chemistry) in gas turbine combustors. These new reduced mechanisms with five, six, seven, and nine steps based on the full GRI 2.11 mechanism, were developed and evaluated for their potential to model natural gas chemistry (including NOx chemistry) in gas turbine combustors. These new reduced mechanisms were optimized to model the high pressure and fuel-lean conditions found in gas turbines operating in the lean premixed code calculations, the five-step reduced mechanism was identified as a promising model that can be used in a multidimentional gas-turbine code for modeling lean -premixed, high-pressure turbulent combustion of natural gas. Predictions of temperature, CO, CH4, and NO from the five- to nine-step reduced mechanisms agree within 5 percent of the predictions from the full kinetic model for 1 < pressure (atm) < 30, and 0.6 < phi < 1.0. If computational costs due to additional global steps are not severe, the newly developed nine step global mechanism, which is a little more accurate and provided the least convergence problems, can be used. Future experimental research in gas turbine combustion will provide more accurate data, which will allow the formulation of better full and reduced mechanisms. Also, improvement in computational approaches and capabilities will allow the use of reduced mechanisms with larger global steps, perhaps full mechanisms.
Cannon, S.M.; Brewster,
B.S. and Smoot, L.D.
Combustion & Flame, 113:135-146 (1998).
The ability to use reduced CH4-air chemical mechanisms to predict CO and NO emissions in premixed turbulent combustion has been evaluated in a Partially Stirred Reactor (PaSR) model. CO emissions were described with reduced 4-, 5-, and 9-step mechanisms and a detailed 276-step mechanism. NO emissions from thermal, N2O-intermediate, and prompt pathways were included in the 5-, 9-, and 276-step mechanisms. Molecular mixing was described with a deterministic, Interaction-by Exchange-with-the-Mean (IEM) submodel. Random selection and replacement (without repetition) of fluid particles were used to simulate through-flow. The evolution of mean and root mean square (rms) temperature, CO, and NO in the PaSR was accurately described with the 9-step mechanism over a wide range in mixing frequency and equivalence ratio. Also, the 9-step mechanism provided accurate instantaneous reaction rates and concentrations for a broad region of the accessed composition space in the PaSR. The 5-step mechanism performed less reliably than the 9-step mechanism at phi = 1.0 but performed similarly to the 9-step mechanism at phi = 0.65. The 4-step mechanism underpredicted mean CO values and overpredicted instantaneous temperature reaction rates, most likely due to its inferior parent mechanism, partial equilibrium assumption for OH, and unallowed dissociation of neglected radical species. The detailed and reduced mechanism predictions of the accessed composition space in the PaSR covered only a small fraction of the allowable composition space, thus facilitating the use of an efficient in situ chemical look-up table for multidimensional, pdf-method calculations.
Adam, W.C. and Tree, D.R.
Western States Section of the Combustion Institute, Seattle, Washington,
October 26-27, 1998.
An experimental program has been completed where detailed measurements of a pulverized coal flame with reburning and advanced reburning have been obtained. Maps of species (CO, CO2, O2, NO, HCN, and NH3), temperature and velocity have been obtained which consist of approximately 60 measurements across a cross sectional plane of the reactor. Two maps at a single operating condition have been obtained and are compared. In addition to the mapping data, effluent measurements of gaseous products were obtained for various operating conditions.
Advanced reburning was achieved in the reactor by injecting natural gas downstream of the primary combustion zone to form a reburning zone followed by a second injection of ammonia downstream of reburning to form an advanced reburning zone. Finally, downstream of the ammonia injection, air was injected to form a burnout or tertiary air zone. The amount of natural gas injected was characterized by the reburning zone stoichiometric ratio. The amount of ammonia injected was characterized by the ammonia to nitrogen stoichiometric ratio or NSR and by the amount of carrier gas used to transport and mix the ammonia. A matrix of operating conditions where injector position, reburning zone stoichiometric ratio, NSR, and carrier gas flow rate were varied and NO reduction was measured in addition to two maps of data at one operating condition.
The data showed advanced reburning was more effective than either reburning or NH3 injection alone. At one advanced reburning condition over 95% NO reduction was obtained. Ammonia injection was most beneficial when following a reburning zone which was lightly lean, S.R. = 1.05, but was not very effective when following a slightly rich reburning zone, S.R. of 0.95. In the cases where advanced reburning was most effective (reburning S.R. = 1.05), higher NSR values improved NO reduction but the NSR was secondary to NH3 injector location. The optimal location for injection was found to coincide with changes in the temperature field.
The mapped temperature, species and velocity data for advanced reburning showed that the largest drops in NO occurred in a region where the O2 concentration was between 0.7 and 3.0%, NH3 was between 0 and 2961 ppm, and temperatures were between 1274 and 1343 K. These are similar to optimal conditions known for SNCR. Significant NO reductions were seen when NSR values were near one, suggesting NH3 was very effective at NO reduction when surrounding temperature and species conditions were favorable. Because this was only one detailed set of data, it is difficult to conclude that these conditions are optimal or need to exist for optimal NO reduction. More detailed mapping data at other operating conditions would be useful in identifying optimal advanced reburning conditions.
Fisher, J.; Pisharody, S.;
Wignarajah, K.; Lighty, J.S.; Burton, B.; Edeen, M. and Davis, K.A.
28th International Conference on Environmental Systems, Danvers, Massachusetts,
July 13-16, 1998
Over the last three years, the University of Utah (U of U), NASA Ames Research Center (ARC), and Reaction Engineering International (REI) have been developing an incineration system for the regeneration of components in waste materials for long-term life support systems. The system includes a fluidized bed combustor and a catalytic flue gas clean up system. An experimental version of the incinerator was built at the U of U. The incinerator was tested and modified at ARC and then operated during the Phase III human testing at NASA Johnson Space Center (JSC) during 1997. This paper presents the results of the work at the three locations: the design and testing at U of U, the testing and modification at ARC, and the integration and operation during the Phase III tests at JSC.
Walker, A.C.
A Mechanistic Study of the Reduction of Nitric Oxide by Methane on ZMS-5
Supported Rhodium and Palladium Catalysts, M.S./BYU, April 1998. Advisor:
Hecker
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