Fuel nitrogen conversion in solid fuel fired systems

https://backend.orbit.dtu.dk/ws/files/145750384/Nreview_revised.pdf

Modeling nitrogen chemistry in combustion

Nitrogen oxides in the atmosphere contribute to photochemical smog, to the formation of acid rain precursors, to the destruction of ozone in the stratosphere and to global warming. Over the past 150 years, global emissions of nitrogen oxides into the atmosphere have been increasing steadily. A significant amount of the nitrogen oxide emissions is attributed to combustion of biomass and fossil fuels. Increasingly stringent NOx emissions regulations are being implemented in a number of industrialized countries. These regulations have driven and continue to drive the development of NOx emissions control techniques. This paper reviews existing and some emerging technologies for reduction of NOx emissions from combustion sources and examines the prospects of these technologies for meeting stricter emissions regulations. Both combustion modification and post-combustion methods for NOx reduction are considered. The important role of research on the chemistry of nitrogen oxides in combustion gases in development and optimization of emissions control techniques is described.

Control of combustion-generated nitrogen oxide emissions: Technology driven by regulation

Identification and emission rates of molecular tracers in coal smoke particulate matter

Control of combustion-generated nitrogen oxides by selective non-catalytic reduction.

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.

Bench and pilot scale process evaluation of reburning for in-furnace nox reduction

Fate of coal nitrogen during combustion

Fifty coals from North America, Europe, Asia, South Africa and Australia were burned ina 21 kW t refractory-lined tunnel furnace to determine the influence of coal properties on the fate of volatile and char nitrogen. Excess air fuel NO emissions (as determined by combustion in Ar/O 2 /CO 2 ) ranged from 415 ppm to 1380 ppm with a premixed burner. These results correlated with total fuel nitrogen, inert pyrolysis HCN yield, and non-volatile nitrogen content, rather than with the geographic origin of the coal. Minimum staged NO emissions (at optimum first stage stoichiometry) ranged from 140 ppm to 380 ppm. Detailed in-flame measurements indicated that as first stage stoichiometry (air/fuel) was reduced, first stage NO formation decreased, but was ultimately offset by increases in oxidizable gaseous nitrogen species and solid phase nitrogen retention. TFN (NH 3 +NO+HCN) generally increased with increasing fuel nitrogen and the species distribution was dependent upon coal rank. In general, HCN was greater than NH 3 with bituminous coals, but less than NH 3 with subbituminous and lignite coals. Second stage TFN conversion to exhaust NO decreased as the TFN distribution was shifted in favor of HCN and NH 3 . Char nitrogen conversion was generally low (less than 20 percent). Exhaust emissions were correlated in terms of the gas phase TFN and the char nitrogen entering the second stage. Increasing heat extraction generally reduced exhaust NO emissions via a complex mechanism. Reducing second stage flame temperatures had little effect on the concentration and speciation of the XN species leaving the first stage, and on the conversion of solid phase nitrogen in the second stage. However, it dramatically decreased the second-stage conversion of gas phase TFN to exhaust NO. The controlling mechanism appears to be selective NO reduction by NH 1 species. The effectiveness of increased heat extraction rate was also found to depend heavily on the coal composition.

Influence of coal composition on the fate of volatileand char nitrogen during combustion

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 deNO x . 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: NH 3 +OH=NH 2 +H 2 O HNCO+H=NH 2 +CO The NH 2 is then available for reaction with NO to eventually yield N 2 . 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 NH 3 or HNCO reaction.

Advanced NOx reduction processes using-NH and -CN compounds in conjunction with staged air addition

This paper discusses recent investigations that indicate that a small addition of certain chemical reagents can enhance or inhibit the ability of calcium sorbents to capture sulfur. The need to develop a low-cost retrofittable SOx control technology for existing non-NSPS boilers has caused renewed interest in the injection of calcium-based sorbents; however, the removal of SOx at a calcium/sulfur ratio of 2 has been limited to about 50%. Initially, Cr{sub 2}O{sub 3} was found to dramatically improve the sulfur capture of limestone. Chromium (e.g., Cr203) combined with calcium-based sorbents increased sulfur capture by a factor of 1.5-3 at conditions simulating a coal-fired boiler. Subsequently, alkali metal compounds (Li, Na, K) gave similar results. While small amounts of alkali metals enhanced sulfur capture by the CaO, they were also found to react with SO{sub 2} to form alkali metal sulfates. Some refractory oxides and clays typically found in coals were shown to reduce the effectiveness of additives which improve sulfur capture. While calcium utilization for the advanced sorbents has increased capture efficiencies with the use of promoters, the mechanisms of promotion have not been identified. The investigation studied mechanistic processes by which additives promote the ability of sorbents to capture SO{submore » 2} in coal-fired boilers.« less

S.L. Chen

Papers

Bench and pilot scale process evaluation of reburning for in-furnace nox reduction

Fate of coal nitrogen during combustion

Influence of coal composition on the fate of volatileand char nitrogen during combustion

Advanced NOx reduction processes using-NH and -CN compounds in conjunction with staged air addition

Increased SO2 removal with the addition of alkali metals and chromium to calcium-based sorbents