John C. Kramlich

Towards the development of a chemical kinetic model for the homogeneous oxidation of mercury by chlorine species

The potential for regulation of mercury emissions from coal-fired boilers is a concern for the electric utility industry. Field data show a wide variation in the fraction of mercury that is emitted as a vapor vs. that retained in the solid products. The reason for this variation is not well-understood. Near the end of the flue gas path, mercury exists as a combination of elemental vapor and HgCl2 vapor. The data show that HgCl2 is more likely to be removed from the flue gas. Thus, the degree of oxidation is considered to be a critical factor that tends to reduce emission. Mercury is certain to exist as elemental vapor in the flame, with the oxidation occurring at some point in the post-flame environment. At present, the mechanism promoting this oxidation is not quantitatively known, particularly under the low chlorine concentrations afforded by many coals. In the present work, we measure mercury oxidation from a furnace operating between 860°C and 1171°C. These data are compared with similar results from the literature. The possible elementary reactions that may lead to oxidation are reviewed and a chemical kinetic model is proposed. This model yields good qualitative agreement with the data and indicates that mercury oxidation occurs during the thermal quench of the combustion gases. The model also suggests that atomic chlorine is the key oxidizing species. The oxidation is limited to a temperature window between 700°C and 400°C that is defined by the overlap of (1) a region of significant superequilibrium Cl concentration, and (2) a region where oxidized mercury is favored by equilibrium. Above 700°C, reverse reactions effectively limit oxidized mercury concentrations. Below 400°C, atomic chlorine concentrations are too low to support further oxidation. The implication of these results are that homogeneous oxidation is governed primarily by (1) HCl concentration, (2) quench rate, and (3) background gas composition.

Nitrous oxide behavior in the atmosphere, and in combustion and industrial systems

Tropospheric measurements show that nitrous oxide (N2O) concentrations are increasing over time. This demonstrates the existence of one or more significant anthropogenic sources, a fact that has generated considerable research interest over the last several years. The debate has principally focused on (1) the identity of the sources, and (2) the consequences of increased N2O concentrations. Both questions remain open, to at least some degree. The environmental concerns stem from the suggestion that diffusion of additional N2O into the stratosphere can result in increased ozone (O3) depletion. Within the stratosphere, N2O undergoes photolysis and reacts with oxygen atoms to yield some nitric oxide (NO). This enters into the well known O3 destruction cycle. N2O is also a potent absorber of infrared radiation and can contribute to global warming through the greenhouse effect. A major difficulty in research on N2O is measurement. Both electron capture gas chromatography and continuous infrared methods have seen considerable development, and both can be used reliably if their limitations are understood and appropriate precautions are taken. In particular, the ease with which N2O is formed from NO in stored combustion products must be recognized; this can occur even in the lines of continuous sampling systems. In combustion, the homogeneous reactions leading to N2O are principally NCO + NO → N2O + CO and NH + NO → N2O + H, with the first reaction being the most important in practical combustion systems. Recent measurements have resulted in a revised rate for this reaction, and the suggestion that only a portion of the products may branch into N2O + CO. Alternatively, recent measurements also suggest a reduced rate for the N2O + OH destruction reaction. Most modeling has been based on the earlier kinetic information, and the conclusions derived from these studies need to be revisited. In high-temperature combustion, N2O forms early in the flame if fuel-nitrogen is available. The high temperatures, however, ensure that little of this escapes, and emissions from most conventional combustion systems are quite low. The exception is combustion under moderate temperature conditions, where the N2O is formed from fuel-nitrogen, but fails to be destroyed. The two principal examples are combustion fluidized beds, and the downstream injection of nitrogen-containing agents for nitrogen oxide (NOx) control (e.g., selective noncatalytic reduction with urea). There remains considerable debate on the degree to which homogeneous vs heterogeneous reactions contribute to N2O formation in fluidized bed combustion. What is clear is that the N2O yield is inversely proportional to bed temperature, and conversion of fuel-nitrogen to N2O is favored for higher-rank fuels. Fixed-bed studies on highly devolatilized coal char do not indicate a significant role for heterogeneous reactions involving N2O destruction. The reduction of NO at a coal char surface appears to yield significant N2O only if oxygen (O2) is also present. Some studies show that the degree of char devolatilization has a profound influence on both the yield of N2O during char oxidation, and on the apparent mechanism. Since the char present in combustion fluidized beds will likely span a range of degrees of devolatilization, it becomes difficult to conclusively sort purely homogeneous behavior from potential heterogeneous contributions in practical systems. Formation of N2O during NOx control processes has primarily been confined to selective noncatalytic reduction. Specifically, when the nitrogen-containing agents urea and cyanuric acid are injected, a significant portion (typically > 10%) of the NO that is reduced is converted into N2O. The use of promoters to reduce the optimum injection temperature appears to increase the fraction of NO converted into N2O. Other operations, such as air staging and reburning, do not appear to be significant N2O producers. In selective catalytic reduction the yield of N2O depends on both catalyst type and operating condition, although most systems are not large emitters. Other systems considered include mobile sources, waste incineration, and industrial sources. In waste incineration, the combustion of sewage sludge yields very high N2O emissions. This appears to be due to the very high nitrogen content of the fuel and the low combustion temperatures. Many industrial systems are largely uncharacterized with respect to N2O emissions. Adipic acid manufacture is known to produce large amounts of N2O as a by-product, and abatement procedures are under development within the industry.

NOx and N2O in lean-premixed jet-stirred flames

Abstract This study examines NOx/N2O formation mechanisms that are relevant to lean-premixed combustion in practical high-intensity combustors. Both experiments and kinetic modelling are presented. The experimental system for examining high-intensity, lean-premixed combustion is a 1-atm jet-stirred reactor. The reactor burns CH4 and CO/H2 over a fuel-air equivalence range of 0.41 to 0.67, a reactor mean residence time of 1.7–7.4 ms, and measured reactor temperature of 1415 to 1845 K. The CO/H2 fuel is used to eliminate the effects of hydrocarbon attack on the nitrogen system. The NOx mole fraction (wet) measured for the CH4 experiments correlates well with measured temperature (K) and reactor mean residence time τ (s) as follows: XNOx = τ(1.28 × 104)exp(−27230/T). Because of the enhanced O-atom concentration, the NOx mole fraction for the CO/H2 combustion is about threefold higher than for the CH4 combustion. Furthermore, because the free radical behavior in the CO/H2 experiments is complex near blowout, a simple correlation is not available. For the CH4-air experiments, chemical reactor modeling indicates that the approximate percentages of NOx production by the nitrous oxide, Zeldovich, and prompt mechanisms vary from 65:25:10 at 1650 K to 35:50:15 at 1850 K. For the CO/H2-air experiments the nitrous oxide to Zeldovich contributions vary from 95:5 at 1500 K to 65:35 at 1725 K.

The selective reduction of SO3 to SO2 and the oxidation of NO to NO2 by methanol

The article reports the discovery of a new homogeneous gas phase reaction in which methanol converts SO3 to SO2. In the course of this reaction NO is converted to NO2. This new reaction is highly selective in that ppm concentrations of SO3 and NO are converted by equivalent quantities of methanol, even in the presence of large excesses of O2. Both the conversion of SO3 to SO2 and NO to NO2 are reversible in that at any given temperature there is an optimal reaction time; the use of longer reaction times causes decreasing conversion. For the optimal temperature the reaction is also rapid, capable of achieving better than 80% reduction in only 55 ms. The existence of this new reaction was predicted by computer modeling. Subsequent experiments verified the predicted modeling trends. The mechanism by which methanol simultaneously reduces SO3 and oxidizes NO involves methanol functioning as a source of HO2 free radicals, which then initiate the reactions NO + HO2 = NO2 + OH, SO3 + HO2 = HSO3 + O2, and HSO3 + M = SO2 + OH.

A design attribute framework for course planning and learning assessment

A new method for course planning and learning assessment in engineering design courses is presented. The method is based upon components of design activity that are organized into a design attribute framework. The learning objectives of design courses can be expressed within this framework by selecting from among these components. The framework can also be used to guide the development of survey instruments for use in assessment. These two uses of the design attribute framework are illustrated in the context of a freshman engineering design course.

Observations of chemical effects accompanying pulverized coal thermal decomposition

Abstract A thermal decomposition reactor is used to simulate the environment of a practical coal burner and assess the influence of fuel properties on the chemical behaviour that accompanies the thermal decomposition of pulverized coal particles. The present study complements and extends previous work that explored the physical and thermal behaviour. The rate of component burnout is determined for hydrogen, nitrogen and carbon as well as the evolution of NO and HCN, and the efficiency of fuel nitrogen conversion. The results demonstrate a significant dependency on coal particle size and coal type. Modelling demonstrates that the penetration of oxygen into the fuel-rich devolatilization cloud depends on particle size and coal properties and, as a result, controls to a large extent the evolution and transformation of the chemical constituents associated with the parent fuel.

Experimental and Numerical Study of NOx Formation From the Lean Premixed Combustion of CH4 Mixed With CO2 and N2

This paper describes an experimental and numerical study of the emission of nitrogen oxides (NO x ) from the lean premixed (LPM) combustion of gaseous fuel alternatives to typical pipeline natural gas in a high intensity, single-jet, stirred reactor (JSR). In this study, CH 4 is mixed with varying levels CO 2 and N 2 . NO x measurements are taken at a nominal combustion temperature of 1800K, atmospheric pressure, and a reactor residence time of 3 ms. The experimental results show the following trends for NO x emissions as a function of fuel dilution: (1) more NO x is produced per kg of C H 4 consumed with the addition of a diluent, (2) the degree of increase in emission index is dependent on the chosen diluent; N 2 dilution increases NO x production more effectively than equivalent CO 2 dilution. Chemical kinetic modeling suggests that NO x production is less effective for the mixture diluted with CO 2 due to both a decrease in N 2 concentration and the ability of CO 2 to deplete the radicals taking part in NO x formation chemistry. In order to gain insight on flame structure within the JSR, three dimensional computational fluid dynamic (CFD) simulations are carried out for LPM CH 4 combustion. A global CH 4 combustion mechanism is used to model the chemistry. While it does not predict intermediate radicals, it does predict CH 4 and CO oxidation quite well. The CFD model illustrates the flow field, temperature variation, and flame structure within the JSR. A 3-element chemical reactor network (CRN), including detailed chemistry, is constricted using insight from spatial measurements of the reactor, the results of CFD simulations, and classical fluid dynamic correlations. GRI 3.0 is used in the CRN to model the NO x emissions for all fuel blends. The experimental and modeling results are in good agreement and suggest the underlying chemical kinetic reasons for the trends.

The Premixed Conditional Moment Closure Method Applied to Idealized Lean Premixed Gas Turbine Combustors

This paper presents the premixed conditional moment closure (CMC) method as a new tool for modeling turbulent premixed combustion with detailed chemistry. By using conditional averages the CMC method can more accurately model the affects of the turbulent fluctuations of the temperature on the reaction rates. This provides an improved means of solving a major problem with traditional turbulent reacting flow models, namely how to close the reaction rate source term. Combined with a commercial CFD code this model provides insight into the emission formation pathways with reasonable runtimes. Results using the full GRI2.11 methane kinetic mechanism are compared to experimental data for a backward-facing step burning premixed methane. This model holds promise as a design tool for lean premixed gas turbine combustors.

Investigation of NOx and CO formation in lean-premixed, methane/air, high-intensity, confined flames at elevated pressures

The coupling between NOx formation chemistry and the mixing/transport environment is of critical importance to the design of lean-premixed gas turbine combustors but is incompletely understood. In the present research, this problem was addressed via the study of NOx formation in a high-pressure jet-stirred reactor operating on lean-premixed methane/air. These experiments focused on the effects of residence time (0.5–4.0 ms), pressure (3.0, 4.7, and 6.5 atm), and inlet temperature (344–573K). The combustion temperature varied from 1815±5 K at the lowest residence times to 1910±30K at the largest residence times. The NOx was lowest at intermediate residence times, reaching higher values at the extremes. Increasing pressure and inlet temperature tend to reduce NOx concentrations. Concentration profiling in the reactor suggests two general environments: (1) a highly non-equilibrium reaction zone defined by high CO concentrations, and (2) a postflame environment. The NOx formation was concentrated in the region of strongly non-equilibrium combustion chemistry. The Damkohler number was 0.06≤Da≤1, and the ratio of turbulent intensity to laminar burning velocity was 28≤u′/SL≤356, indicating the combustion occurs in the high-intensity, chemical rate-limiting regime. The results were interpreted using a two-environment, detailed chemistry model in which the size and structure of the flame environment were established by matching the measured data, and which were independently verified using turbulent flame velocity/thickness correlations. The modeling suggests NOx formation is controlled by both the specific conditions in the non-equilibrium zone and by the size of the zone. Since both these features are influenced by the experimental parameters, a highly nonlinear scenario emerges with implications for minimizing NOx via combustor design. The modeling also suggests the unique case of well-stirred combustion for NOx at elevated pressure is obtained at low residence time conditions.

Behavior of N2O in staged pulverized coal combustion

An experimental study is reported, the objeN/sub 2/Ot of which was to examine the influence of NOx control technology concepts on N/sub 2/O emissions from pulverized coal flames. The work concentrated on the effects of burner design, staged combustion and thermal environment on nitrogen oxides. The results showed that N/sub 2/O emissions from pulverised coal flames can from a significant portion of the total nitrogen oxides (up to 25). The application of combustion modification technology to control NO emissions does not result in significantly higher levels of N/sup 2/O.