Combustion simulations were conducted to investigate the parameters controlling the formation of unburned carbon in fly ash from coal reburning in a coal-fired boiler. Unburned carbon (UBC or Loss on ignition, LOI) was generally caused by particles flowing through fuel-rich regions and/or spending insufficient residence time in the furnace. LOI contributions by each individual coal source were identified and quantified. The LOI from the main burners was found to depend mainly on the availability of combustion air in the burner zone. However, the LOI from the reburning jets depended on both the amount of air in the reburning jets and the available over-fire air (OFA) downstream. Moving some air from the lower burners to the upper burners to compensate for the shorter residence time was found to significantly reduce the overall LOI without adverse impacts on the NOx emissions in this study.

Combustion simulations were conducted to evaluate the technical feasibility of using a waste-based syngas as a supplemental fuel in a 70,000 kg/h-steam coal-fired boiler. The syngas was either co-fired with coal at the burners or injected downstream as a reburning fuel under both fuel lean and conventional (fuel rich) reburning configurations. Sensitivity of syngas heat input and furnace stoichiometry were examined. Results indicated that the syngas was an effective reburning fuel although it contained less than 6% hydrocarbons. NOx reductions from 12–46% were predicted for different reburning configurations; the highest NOx reduction of 46% was achieved with 23% heat input under a conventional reburning configuration. Furnace LOI increased over baseline values for all but one reburning configuration. The highest LOI came from the top burner row in all cases. Results suggested the LOI increase due to reburning might be mitigated by biasing more air to the upper burners to enhance particle burnout. Predictions indicated co-firing syngas at the burner centerline was not beneficial as it resulted in poor combustion of the coal particles and high LOI and CO emissions. Simulation results also indicated that minor changes to the syngas composition do not significantly affect the performance of syngas reburning.

This test program was designed to study the formation and emission of PCDD/PCDF during Refuse Derived Fuel combustion. Tests were conducted in a pilot scale RDF incinerator. Incinerator fly ash hold up for one hour at a temperature of 570-degrees-F resulted in an 8 to 20 fold increase in PCDD/PCDF content. Reductions in RDF load were found to decrease PCDD/PCDF emissions. Fluctuations in RDF feed rate produced increases in PCDD/PCDF emissions. Test results suggest the dominant PCDD/PCDF formation mechanism(s) in the incinerator involve organic compounds in entrained fly ash. A combustion strategy for PCDD/PCDF emissions control has been developed. Natural gas co-firing allows RDF load reduction while smoothing temporal and spatial fluctuations in the furnace.

The effectiveness of combustion modifications, including staged combustion and reburning, for the control of nitrogen oxide emissions from coal or oil fired combustors is most often limited by problems due to carbon burnout or flame impingement. This paper presents a new data on the use of selective reducing agents, such as ammonium sulfate, 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 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.

AbstractA new method for reducing NO and NO2 emissions formed by the combustion of coal and heavy oil consists in a selective reduction process using cyanuric acid (II).

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 paper summarizes an experimental study which was conducted to investigate the chemical constraints of the reburning process and identify reburning configurations for optimal NOx reduction in coal-fired boilers. Tests were performed on a bench scale tunnel furnace to characterize and optimize the fuel-rich reburning zone and the fuel-lean burnout zone independently. Detailed measurements ofunburned hydrocarbons, CO, NH3, and HCN were made at the reburning zone exit. The influence of the concentrations of reactive species was examined as were temperature effects for both the reburning and burnout zone. Results indicated that reburning zone chemistry was not rate limiting. The impacts of temperature and burnout zone oxidation were of major importance. Integration of the optimum reburning and burnout zone configurations resulted in increased NOx reduction. Over 85 percent reduction in NOx emissions was achieved with ammonium sulfate injection in the burnout zone under optimum reburning conditions.

Injection of calcium-based sorbents is currently being considered as a potential method of reducing SO2 in existing coal-fired boilers. This study investigated the ability of small concentrations of additives to enhance sulfur capture with calcium-based sorbents. The interaction between the mineral ash (produced from coal combustion) and the sorbent-additive combination was also studied. Two furnace facilities and an X-ray diffractometer were used in the experimental study. The furnaces used included a 300 kW down-fired furnace capable of simulating time and temperature profiles for a variety of boilers, and a bench-scale 18 kW drop-tube furnace. Addition of known promoters with sorbents has effectively increased sulfur capture. Both alkali metals and chromium react with calcium to increase the accessibility of CaO sites by particle fragmentation, creation of large pores, and the presence of a liquid phase. In general, the chromium eliminated by ash interaction was small, while the ash effectively prevented a significant portion of sodium from contacting and reacting with the calcium sorbent.

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.

The overall objectives of this project are to provide a basic understanding of the principal processes that govern fine particulate formation in pulverized coal flames, and develop procedures to predict the levels of emission of fine particles from pulverized coal combustors. (VC)