Scott C. Hill

Modeling of nitrogen oxides formation and destruction in combustion systems

Abstract The formation of nitrogen oxides (NO X ) in combustion systems is a significant pollutant source in the environment, and the control of NO X emissions is a world-wide concern as the utilization of fossil fuels continues to increase. In addition, the use of alternative fuels, which are typically of lower quality, tends to worsen the problem. Advances in the science of NO X reactions, mathematical modeling, and increased performance of computer systems have made comprehensive modeling of NO X formation and destruction a valuable tool to provide insights and understanding of the NO X reaction processes in combustion systems. This technology has the potential to enhance the application of various combustion techniques used to reduce NO X emissions from practical combustion systems. This paper presents a review of modeling of NO X reactions in combustion systems, with an emphasis on coal-fired systems, including current NO X control technologies, NO X reaction processes, and techniques to calculate chemical kinetics in turbulent flames. Models of NO X formation in combustion systems are reviewed. Comparisons of measured and predicted values of NO X concentrations are shown for several full-scale and laboratory-scale systems. Applications of NO X models for developing technologies, in order to reduce NO X emissions from combustion systems are also reported, including the use of over-fire air, swirling combustion air streams, fuel type, burner tilt angle, use of reburning fuels, and other methods.

NOx control through reburning

Abstract 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.

Theory for NO formation in turbulent coal flames

A NO pollutant model has been formulated to be consistent with the assumptions madein current comprehensive models for fuel-rich turbulent pulverized-fuel flames. The model accounts for the effects of turbulent fluctuations. The pollutant species equations are decoupled from the overall velocity, temperature and major species field equations. The fuel nitrogen is assumed to evolve from the coal at a rate proportional to coal weight loss and to react quickly to HCN in the gas phase. The homogeneous rate of decay of HCN to NO or N2 includes the effects of turbulent fluctuations to obtain a time-mean reaction rate. Heterogeneous reduction of the NO is modeled by a slow (non-fluctuating) rate. Thermal and prompt NO are considered to be insignificant in these fuel-rich coal flames. The timemean reaction rates are obtained by assuming that the species mass fractions are linear functions of stoichiometry, and then integrating the resulting kinetic expression over the probability density functions for the appropriate mixture fractions. Computations are presented to demonstrate the applicability of the theory to practical coal combustion chambers.

Prediction of nitrogen oxide formation in turbulent coal flames

A model of nitrogen pollutant formation and destruction in pulverized coal reactors, whose foundations were presented recently, is briefly outlined and then evaluated by comparisons of predictions with measurements. The model incorporates effects of turbulence on the NO reactions and subsequent predictions show these effects to be important. Thermal and prompt NO were neglected. The model predicts: 1) An initial decrease followed by a gradual increase in NO emissions with increased swirl number as observed for the particular cases modeled; 2) an increase in NO emissions with increased stoichiometric ratio as observed; at higher values, the model underpredicts NO concentrations, partly because of higher thermal NO contributions which are not modeled, and 3) the observed increase in NO with a decrease in particle size and with increase in moisture percentage. Predicted NO emissions for the four test variables differed by an absolute average of 9% from measured values for 24 comparisons. Comparisons with local NO and HCN concentrations from pulverized-coal flames show some agreement except in regions immediately following ignition where oxygen depletion and devolatilization are overpredicted. Predicted NO effluent emissions for six profile cases differ by an absolute average of 24% from measured values. Results suggest that fuel nitrogen release during devolatilization and gas phase reactions of HCN and oxygen control NO formation, while fuel nitrogen conversion to HCN may be near quantitative and rapid. According to the predictions, NO reduction is dominated by HCN-NO reactions and not by the char-NO reactions, while turbulence has an important impact on the gaseous reactions.

Critical requirements in combustion research

Abstract This document summarizes a study to survey the state-of-the-art in combustion research and to discuss combustion-related research needs. In order to complete these tasks, information was obtained from three sources: (1) published, combustion-related review articles, (2) program plans from agencies sponsoring combustion-related research, and (3) a survey of experienced combustion researchers. In order to synthesize results from these sources, combustion research activities were divided into applied areas (coal gasification, explosions, fires, power generation, reciprocating engines, virginal and residual solid fossil fuels, solid propellants and turbine engines) and fundamental areas (chemistry of combustible materials, combustor modeling, detonations and explosions, diagnostics/instrumentation, droplet kinetics, flame ignition, stability and propagation, gas phase kinetics, particle/droplet dynamics, pollutant formation, radiative heat transfer, soot formation/reduction and turbulence/fluid mechanics). Approximately 160 combustion-related review articles published in the past five years were located which provide access to over 16,000 references in the combustion literature. At least three articles from each applied and each fundamental area were reviewed, and the most frequently identified key research needs in these articles were summarized. Combustion research program plans were sought from twenty-three agencies and received from twelve agencies. Research plans were then categorized according to relevant applied and fundamental combustion areas. A survey instrument with the above applied and fundamental categories was developed, and sent to fifty-five prominent combustion researchers. Survey responses were received from forty-three researchers. The survey provided opportunity for respondents to identify principal combustion research needs. Based on these three sources of information, the key combustion research needs identified in the applied areas were: clean and efficient combustion of low-grade liquid and solid fuels, and reduction of pollutants through combustion control. The key combustion research needs identified in the fundamental areas were: interactions of turbulence and kinetics, non-intrusive combustion diagnostics, computer model development, gas-phase kinetics, droplet/particle cloud combustion, soot formation and chemistry, flame structure and solid phase kinetics. Additional detail and discussion of these research needs are given in the text. The senior author further identified significant research needs in droplet/particle dynamics, explosions and chemistry of combustible materials. At the request of the senior author, this document was also independently reviewed by two prominent combustion researchers and their comments are included in this document.

A reduced kinetic model for NOx reduction by advanced reburning

Advanced reburning technology, which makes use of natural gas injection followed by ammonia injection, has proven to be an effective method for the removal of up to 85-95% of the NO in pulverized, coal-fired furnaces. This paper reports the development of a seven-step, 11-species reduced mechanism for the prediction of nitric oxide concentrations using advanced reburning from a 312-step, 50-species full mechanism. The derivation of the reduced mechanism is described, including the selection of the full mechanism, the development of the skeletal mechanism, and the selection of steady-state species. The predictions of the seven-step reduced mechanism are in good agreement with those of the full mechanism over a wide range of parameters, applicable to coal-based, gas-based, and oil-based combustion cases. Comparisons with three independent sets of experimental laminar data indicate that the reduced mechanism correctly predicts the observed trends, including the effects of temperature, the ratio of (NH 3 /NO) in , and concentrations of CO, CO 2 , O 2 , and H 2 O on NO reduction. The observed effects of CO on NH 3 slip were also reliably predicted. Mechanistic considerations provide an explanation for the roles of the important radicals and species. Also, parametric studies of the effects of CO 2 and H 2 O have been performed with the reduced mechanism. A maximum in NO reduction exists, which strongly depends on the concentrations of NO in , CO, and O 2 , the ratio of (NH 3 /NO) in , and temperature.