Tests were conducted with a naturally staged, axial jet burner firing pulverized coal to evaluate near burner combustion modification alternatives for controlling NOx emissions to ultra low levels in a combustor operating with hot, running slag walls. The study focused on the use of small amounts of natural gas rather than swirl air to aid the stabilization of the axial pulverized coal flames. The relative importance of primary jet velocity and primary air stoichiometry were determined for these gas stabilized flames. Decreases in primary jet velocity had a dramatic effect on near burner NO production while primary air stoichiometry appeared to have a negligible effect. Another near burner modification, the use of a vitiated secondary air stream, did not significantly affect NO levels. Ultra low NO levels (below 90 ppmv) were achieved in this hot-walled combustor by optimizing the performance of the gas stabilized burner in conjunction with staged combustion and reburning.

A reduced chemical kinetic mechanism for the prediction of selective noncatalytic reduction (SNCR) chemistry has been developed and incorporated into a three-dimensional, CFD-based turbulent reacting flow model. The model can be used for prediction and investigation of thermal and mixing effects as well as the influence of CO on the SNCR process in practical systems. The model accurately describes the SNCR chemistry as indicated by comparisons of NO reduction efficiency, ammonia slip, and NaO emissions with predictions using a complete chemical mechanism (70 species, 327 reactions) and experimental measurements from independent investigators. The reduced mechanism (6 species, 7 reactions) and individual rate constants are provided so that the mechanism could be incorporated into any CFD-based computer code.The effects of thermal environment (injection temperature and quench rate), reactant ratios (initial WO and NH3/NO), and reagent mixing on NO reduction by ammonia were investigated in the presence and absence of CO using a pilot-scale facility. Comparison with the coupled CFD/chemistry model predictions indicated good agreement, and both suggest that SNCR effectiveness is critically influenced by (1) finite-rate chemistry; (2) imperfect reagent dispersion, (3) mixing delay times, (4) local CO concentrations, and (5) nonisothermal temperature profile.

Research completed to date suggests that the use of waste as a substitute fuel in cement kilns is fundamentally sound in theory, and in a number of cases, has been demonstrated sound in practice. Analysis of the database generated in certificate of compliance tests clearly demonstrate that cement plants burning waste-derived fuel (WDF) can employ existing technology to meet and in some cases exceed current emission regulations. With the exception of cement kilns using raw materials with high levels of hydrocarbons, the emission of organics can be controlled by applying well-established good-combustion practices. Metals emissions from cement plants burning WDF can be predicted based upon input metal flow rates, provided that plant-specific information on the collection efficiency of air pollution control equipment and by-pass flow rates are available.

Rotary kilns are used to dispose of many solid wastes and sludges and to thermally treat contaminated soils. In this communication the fates of hydrocarbon and metal species are examined with a view toward optimization of new kiln designs and maximizing existing unit throughout while minimizing pollutant emissions. Initially, process fundamentals are considered to characterize the controlling phenomena. Pilot- and large-scale data are then examined to define practical system complexities. Finally, techniques for data scale-up and performance prediction are summarized. Temperature is clearly the most important parameter with respect to the fate of both metal and hydrocarbon species; hence, heat transfer is often rate limiting. High temperatures favor hydrocarbon evolution, but can also enhance the formation of toxic metal fumes. Both the solid composition and the moisture content can significantly influence the time at temperature required for hydrocarbon destruction and metal vaporization. Improving bed mixing helps contaminant release but can also aggravate puffing tendencies with batch charging. Full-scale performance predictions currently require a combination of small scale data and computer modeling. Future work needs to focus on verification of large-scale predictions for complex mixtures and sludges so that expensive trial burns can be minimized.

The report addresses the evaluation of a technology which is a combination of two technologies used to control the atmospheric emission of NOx by stationary sources: (1) combustion modification (controls flame temperature and maximizes fuel-rich residence time to minimize NOx formation); and (2) flue gas cleaning (uses a reducing agent with or without a catalyst to remove NOx from combustion products). The combined technology uses fuel as a reducing agent to remove NOx. The process (referred to as in-furnace NOx reduction, reburning, and staged fuel injection) can be applied to many types of combustion systems. In fact, reburning is the process which allows the 'in-furnace NOx reduction' to take place.

The effectiveness of combustion modifications for the control of nitrogen oxide emissions from coal fired combustors is most often limited by problems due to carbon burnout or flame impingement. This paper presents new data on the use of selective reducing agents suggesting that a hybrid control scheme is possible which uses combustion modification to provide those conditions which optimize, the selective reduction process. Very low emission levels appear possible that can presently only be achieved by catalytic reduction. The experimental studies were conducted in a tunnel furnace which simulated the thermal environment within a pulverized coal boiler. Application of each of the agents (ammonia, urea, cyanuric acid, and ammonium sulfate) to an overall fuel lean environment, produced NO reduction behavior very similar to that of thermal deNOx. However, if the agent was added to the fuel rich zone of a rich/lean staged combustor, very high NO reductions were obtained after the leanout point. The result of the staging was to extend the effectiveness of the agent to lower temperatures relative to overall lean injection. Parametric variations indicated that, in addition to temperature, the most important variable was the rich zone stoichiometry. Kinetic modeling suggests that the rich zone acts primarily as a source of CO. At the rich/lean transition the CO is oxidized and excess OH is produced by the, usual chain branching reactions. For low initial CO concentrations the excess radicals, are consumed by: NH3+OH=NH2+H2O HNCO+H=NH2+CO The NH2 is then available for reaction with NO to eventually yield N2. The strong rich zone stoichiometry dependence is exerted mainly through the amount of CO, supplied to the lean zone. Insufficient CO will limit the extent of the initial NH3 or HNCO reaction.

This patent describes a process for selectively reducing nitrogen oxides in combustion effluent streams. It comprises the steps of: introducing a reducing agent into a gaseous decomposition zone. The reducing agent having at least one functional group selected from the group comprising -NH and -CN, wherein the decomposition zone is substantially oxygen deficient and is maintained at a temperature in the range of from approximately 300{sup 0}F. to approximately 2400{sup 0}F.; introducing the resulting mixture from the decomposition zone to a reaction zone containing combustion effluents, the reaction zone having an excess of oxygen; and allowing the mixture from the decomposition zone sufficient residence time within the reaction zone to allow the reduction of the NO/sub chi/ within the combustion effluents.

The combustion of coal and heavy oil produces significant quantities of NOx (NO + NO2), which ultimately participates in photochemical smog and acid rain. Combustion modification schemes, such as staged combustion1 and reburning2, or downstream injection of selective reducing agents, particularlarly ammonia3 and urea4, can significantly reduce NOx emissions, but their effectiveness may be limited by operating constraints. Perry and Siebers5 described a selective reduction process using cyanuric acid, reported to be effective over a broad temperature range. Here we report new data that indicate that the same reductions can be achieved without stainless steel to activate the reaction mechanism, if the process is combined with reburning or staged combustion (or slightly fuel-rich operation in internal combustion) to produce carefully controlled stoichiometry. The NOx reductions that can be achieved with cyanuric acid and other –NH- and –CN-containing compounds at relatively low temperatures exceed those possible with either combustion modifications or selective reduction techniques alone or in normal combination.

This paper describes a combined experimental and theoretical study which was undertaken to quantify the impact of fuel and process parameters on reburning effectiveness and provide the scaling information required for commerical application of reburning under highly varied industrial conditions. Initially parametric screening studies were conducted in a 25 KW refractory-lined tunnel furnace. These studies were supported by large scale testing in a 3.0 MW pilot scale facility. The work at both scales focused on the importance and the fate of the reactive nitrogen species within the reburning zone. The results of this study confirm the potential of the reburning process for significant NOx reductions, but they also demonstrate that a constant reburning effectiveness cannot be assumed under all conditions. The NOx reduction possible through reburning depends primarily on the NO concentration at the end of the primary zone; the stoichiometry, temperature, and residence time in the fuel-rich reburning zone; the mixing and stoichiometry of the reburning fuel jet; and the temperature in the final burnout zone. At the optimum reburning stoichiometry (SR2=0.9) the exhaust emissions correlate linearly with the sum of the primary NOx and the equivalent reburning fuel nitrogen. Optimum effectiveness requires adequate primary zone residence time to insure complete combustion of the primary fuel. Reburning zone residence times of at least 400 ms are desirable and high temperatures favor molecular nitrogen formation. Rapid mixing of the reburning fuel enhances the effectiveness of the NOx destruction process but it can potentially detract from the overall process efficiency with coal reburning due to increased conversion of reburning fuel nitrogen unless the fuel is transported with an essentially inert gas stream. Extremely low exhaust emission levels can be achieved with coal reburning if the final burnout zone can be operated at a temperature low enough to promote in-situ thermal de-NOx.

Reburning involves the injection of a secondary fuel above the main firing zone of pulverized coal-fired utility boilers to produce a reducing zone which acts to reduce NOx to molecular nitrogen. Overfire air is added above the reburn reducing zone to complete the combustion. Bench scale evaluations of the process carried out in a plug flow furnace at 23 KW have indicated that NOx reductions of up to 70 percent can be achieved depending on a number of process variables. The dominant variables include the initial NOx level that is to be reduced, the reburning fuel type (pulverized coal type or natural gas), and the residence time and temperature in the reducing zone. The reburning process has been combined with the injection of calcium-based sorbents (limestone) to investigate the potential for combined NOx and SOx reduction.

Incineration is a promising technique for the disposal of organic hazardous wastes. However, the waste destruction characteristics of turbulent spray flames have not been characterized. In the present research two reactors are used to simulate various aspects of liquid injection incinerator flame zones. The following questions are addressed: (1) Under what conditions do flames quantitatively destroy waste compounds, and (2) how must the flame be perturbed to cause it to fail to quantitatively destroy wastes. The two reactors operated on a simulated waste stream consisting of acrylonitrile, benzene, chlorobenzene, and chloroform. A microspray reactor was used to investigate destruction processes associated with individual droplets of waste compounds. A turbulent flame reactor used a heptane-fueled waste-doped turbulent spray flame to simulate incinerator flame-zone processes. The flames were found to be capable of quantitative waste destruction without the necessity of using common post-flame processes such as afterburners. Furthermore, the high waste destruction efficiency conditions corresponded to high combustion efficiency conditions (i.e., minimum CO and hydrocarbon emissions). Failure to achieve high destruction efficiency resulted from the perturbation of flame parameters. Failure conditions were identified with high and low theoretical air, low temperature, poor atomization quality, and flame impingement on a cold surface. Each failure condition also resulted in elevated CO and hydrocarbon emissions. Thus, the results suggest that CO and hydrocarbon measurements can be used as an indirect, continuous means of monitoring incinerator flame-zone performance.

Aerodynamics plays a dominant role in the performance of the VCC, and the objective of this study has been to produce the insight necessary for a design that has an optimum performance. In particular, given a specific firing rate and ash particle retention, our goal is to minimize the pressure loss. An important EER design tool has been isothermal and pilot-scale model testing and an additional objective of the study has been to predict the relationship between these laboratory tests and a full-scale device. The approach has been to (1) construct an aerodynamic model with all of the controlling physics, but simple enough to allow insight into the role of various design options; (2) use the existing EER laboratory data to validate the model; and (3) use the model to predict the performance of future designs and suggest design improvements. Predictions of the effect of various design parameters on both cold flow and hot flow pressure drop and particulate retention have been made and compared with laboratory data. The theory predicts many of the features and general trends of the data, but fails to properly predict the pressure drop for small exit diameter and small jets (the bench scale modelmore » being an exception). An adjustment in the particle size distribution is required to make reasonable particle retention predictions. Further experimentation should illuminate the reasons for these discrepancies. Finally, it is important to recognize that the pressure drop predictions are not inevitable pressure losses; and, in fact, are based on a loss-free theory. To the extent that the theory matches the data, pressure recovery through the use of a diffuser presents itself as a potentially extremely valuable design option. 24 figures.« less