Patent Application
20060008757
This invention relates generally to operating combustion systems and, more particularly, to methods and systems for operating combustion systems to facilitate reducing NOx emissions.
Typical boilers, furnaces, engines, incinerators, and other combustion sources emit exhaust gases that include nitrogen oxides. Nitrogen oxides include nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). Total NO+NO2 concentration is usually referred to as NOx. Nitrogen oxides produced by combustion are mainly in the form of NO. Some NO2 and N2O are also formed, but their concentrations are generally less than approximately 5% of the NO concentration, which generally ranges from 200 to 1000 ppm for coal-fired applications. Nitrogen oxide emissions are the subject of growing concern because they are alleged to be toxic compounds and precursors to acid rain and photochemical smog, and contributors to the greenhouse effect.
Several commercial technologies are available to reduce NOx emissions from combustion sources. Currently, Selective Catalytic Reduction (SCR) is a commercial technology that is frequently used to facilitate NOx control. With SCR, NOx is reduced by reactions with Nitrogen Reducing Agents (N-agents, such as ammonia, urea, etc.) across the surface of a catalyst. Known SCR systems operate at temperatures of approximately 700° F. and routinely are able to achieve approximately 80% NOx reduction. However, several inherent drawbacks of SCR, and most importantly, its high cost, may prevent it from being an all-encompassing solution to the problem of NOx removal. Moreover, SCR requires the installation of a large amount of catalyst in the exhaust stream, and SCR catalyst life is limited. Specifically, catalyst deactivation, due to a number of mechanisms, generally limits catalyst life to about four years for coal-fired applications. Costs associated with system modifications, installation and operation, combined with the cost of catalyst material, render SCR quite expensive pollutant control technology. Furthermore, because the spent catalysts are toxic, the catalysts also present disposal problems at the end of lifetime.
To facilitate reducing costs compared to the SCR technology, the reaction of N-agents with NOx can proceed without a catalyst at a higher temperature. This process is called the Selective Non-Catalytic Reduction (SNCR). SNCR is effective over a narrow range of temperatures, or “temperature window” centered about 1800° F. wherein the N-agent forms NHi radicals that react with NO. Under ideal laboratory conditions, deep NOx control may be possible; however, in practical full-scale installations, the non-uniformity of the temperature profile, difficulties of mixing the N-agent across the full combustor cross section, limited residence time for reactions, and ammonia slip (unreacted N-agent) may limit SNCR's effectiveness. Generally, NOx control via SNCR is limited to between approximately 40% and approximately 50%. However, since SNCR does not require a catalyst and therefore has a relatively lower capital cost compared to SCR, it is a valuable option for NOx control with a lower efficiency of NOx control compared to SCR systems.
Other known combustion systems include combustion modifications such as Low NOx Burners (LNB), reburning, and over-fire air (OFA) injection control of NOx emissions via combustion staging. These technologies provide relatively moderate NOx control of between approximately about 30% and approximately 60%. However, their capital costs are low and, since no injection of N-agents is required, their operating costs are generally reduced in comparison to SCR or SNCR systems. NOx control in reburning is achieved by fuel staging wherein a main portion of the fuel, for example, approximately 80% to approximately 90% is fired through the conventional burners with a normal amount of air, for example, approximately 10% excess. A certain amount of NOx is formed during the combustion process, and in a second stage, the remainder of the fuel (reburn fuel) is added into the secondary combustion zone, called the reburn zone, to maintain a fuel-rich environment. The reburn fuel can be coal, gas or other fuels. In the reducing atmosphere within the fuel-rich zone, both NOx formation and NOx removal reactions occur. Experimental results indicate that within a specific range of conditions (equivalence ratio, temperature, and residence time in the reburn zone), NOx concentrations may typically be reduced by approximately 50% to approximately 60%. Part of the reburn fuel is rapidly oxidized by oxygen to form CO and hydrogen, and the remaining reburn fuel provides a fuel-rich mixture with certain concentrations of carbon-containing radicals: CH3, CH2, CH, C, HCCO, etc. These active species can either form NO precursors in reactions with molecular nitrogen or consume NO in direct reactions with it. Many elementary reaction steps are involved in NO reduction. The carbon-containing radicals (CHi) formed in the reburn zone are capable of reducing NO concentrations by converting it into various intermediate species with C—N bonds. These species, in turn, are converted into NHi species (NH2, NH, and N), which later react with NO to form molecular nitrogen. Thus, NO can be removed by reactions with two types of radicals, namely species: CHi and NHi. However, reactions of intermediate N-containing species with NO are typically slow in the absence of O2 and do not contribute significantly to NO reduction in the reburn zone. In the third stage OFA is injected to complete combustion of the fuel. Typically OFA is injected at a location where the flue gas temperature is about 1800° F. to about 2800° F. to facilitate achieving complete combustion. The temperature of the flue gas at a point where overfire air is injected is henceforth referred to as TOFA. The OFA added in the last stage of the process oxidizes remaining CO, H2, HCN, and NHi species as well as unreacted fuel and fuel fragments, to final products, which include H2O, N2, and CO2. At this stage, the reduced N-containing species react mainly with oxygen and are oxidized either to elemental nitrogen or to NOx. It is the undesired oxidation of N-containing species to NOx that limits the efficiency of the reburning process.
Generally, reburning fuel is injected at flue gas temperatures of about 2300° F. to about 3000° F. The efficiency of NOx reduction in reburning may increase with an increase in injection temperature because of faster oxidation of the reburning fuel at higher temperatures, resulting in higher concentrations of carbon-containing radicals involved in NOx reduction. For reburning fuel heat inputs up to about 20%, the efficiency of NOx reduction increases with an increase in the amount of the reburning fuel. With larger amounts of reburning fuel, the efficiency of NOx reduction flattens out and may even slightly decrease. Increasing residence time in the reburn zone also improves reductions in nitrogen oxides emissions by allowing more time for reburning chemistry to proceed.
Lastly, an Advanced Reburning (AR) process, which is a synergistic integration of reburning and SNCR, is also currently available. Using AR, the N-agent is injected along with the OFA and the reburning system is adjusted to facilitate optimizing NOx reduction with an N-agent. By adjusting the reburning fuel injection rate to achieve near-stoichiometric conditions, instead of fuel-rich conditions normally used for reburn, the CO level is facilitated to be controlled, and the temperature window for effective SNCR chemistry may be broadened. With AR, NOx reduction achieved from the N-agent injection is nearly doubled, compared with that of SNCR. Furthermore, with AR, the widening of the temperature window provides flexibility in locating the injection system and the NOx control should be achievable over a broad boiler operating range.
However, although the technologies described above are available and capable of reducing NOx concentrations from combustion sources, they are complex systems that are also expensive to install, operate, and maintain.
In one embodiment, a method for reducing nitrogen oxides in combustion flue gas is provided. The method includes combusting a fuel in a main combustion zone such that a flow of combustion flue gas is generated wherein the combustion flue gas includes at least one nitrogen oxide species, establishing a fuel-rich zone, forming a plurality of reduced N-containing species in the fuel rich zone, injecting over-fire air into the combustion flue gas downstream of fuel rich zone, and controlling process parameters to provide conditions for the reduced N-containing species to react with the nitrogen oxides in the OFA zone to produce elemental nitrogen such that a concentration of nitrogen oxides is reduced.
In another embodiment, a furnace having a reduced NOx emission is provided. The furnace includes a main combustion zone for combusting a fuel, a fuel rich zone located downstream from the main combustion zone, at least one over-fire air port for injecting over-fire air into a combustion flue gas stream at a respective OFA zone, a controller configured to control process conditions in the main combustion zone and the fuel rich zone such that a molar concentration of reduced N-containing species is approximately equal to a molar concentration of NOx when the combustion flue gas reaches said over-fire air zone.
As used herein, the terms “nitrogen oxides” and “NOx” are used interchangeably to refer to the chemical species nitric oxide (NO) and nitrogen dioxide (NO2). Other oxides of nitrogen are known, such as N2O, N2O3, N2O4 and N2O5, but these species are not emitted in significant quantities from stationary combustion sources, except N2O in some systems. Thus, while the term “nitrogen oxides” can be used more generally to encompass all binary N—O compounds, it is used herein to refer particularly to the NO and NO2 (i.e., NOx) species.
Reburn zone 16 may be supplied with a predetermined and selectable amount of a fuel 26. Although fuel 22 and fuel 26 are illustrated in
During operation, combustion by-products, including various oxides of nitrogen (NOx) may be formed in main combustion zone 14 and carried through furnace 12 to a furnace exhaust flue 30, and ultimately to ambient 32. Removal of the NOx emissions may be performed using a two-step process, henceforth referred to as in situ advanced reburning (AR) process. During a first step of the process, reburning fuel 26 may be injected into reburn zone 16 to provide a fuel-rich environment in which NOx is partially reduced to N2. Other reduced N-containing species including NH3 and HCN are formed in reburn zone 16 as a result of this process. An amount of reduced N-containing species formed depends on process conditions in combustion zone 14 and reburn zone 16, and on a chemical composition of main fuel 22 and reburning fuel 26. To facilitate optimizing NOx reduction using the in-situ-AR process, conditions in main combustion zone 14 and in reburn zone 16 may be selected such that a molar concentration of reduced N-containing species is approximately equal to a NOx concentration at the point of OFA injection. Reactions between reduced N-containing species such as NH3, HCN, and NO typically proceed relatively slowly in the fuel-rich environment of reburn zone 16. During a second step, OFA may be injected downstream of reburn zone 16. If OFA is injected into NO-containing combustion flue gas within a specific temperature range, a chemical reaction between NO and reduced N-containing species occurs, and NO is converted to molecular nitrogen. The reaction starts with formation of NH2 radicals in reactions of combustion radicals (OH, O and H) with NH3:
NH3+OH→NH2+H2O,
NH3+O→NH2+OH, and
NH3+H→NH2+H2.
The main elementary reaction of NO-to-N2 conversion is:
NH2+NO→N2+H2O.
Simultaneously, HCN is oxidized to NH3 and N-containing radicals that in turn react with combustion radicals as indicated above. In a conventional SNCR process, reaction between NH-forming reducing agents (N-agents) and NO occurs in a narrow temperature range (temperature window), typically about 1750° F. to about 1950° F. In the in-situ-AR process, oxidation of reburning fuel 26 in reburn zone 16 may not proceed to completion due to the lack of available oxygen. Accordingly, combustion flue gas exiting reburn zone 16 may contain relatively significant concentrations of unburned hydrocarbons, for example, H2 and CO. The presence of these species in the combustion flue gas shifts the conventional SNCR temperature window of NOx reduction toward lower temperatures. In the in-situ-AR process, the OFA is injected in combustion flue gas at temperatures relatively significantly lower than 1750° F. resulting in relatively significant additional NOx reduction. Downstream of the OFA injection zone the reduced N-containing species react mainly with NOx, producing elemental nitrogen. As such deeper NOx control is achieved as compared to traditional reburning, where the reduced N-containing species react mainly with oxygen downstream of the OFA injection zone.
Trace 406 illustrates that NO concentration at the end of reburn zone 16 depends on a relative heat input of the reburning fuel and decreases as relative heat input of the reburning fuel increases. For the range of relative heat inputs illustrated, the concentrations of NH3, trace 408, and HCN, trace 410 at the end of reburn zone 16 are considered. The TFN concentration, trace 412, at the end of reburn zone 16 is minimized at approximately 18% reburning fuel input. For the exemplary fuels and process conditions and 18% reburning fuel heat input, NO concentration, trace 406 at the end of reburn zone 16 is approximately equal to a sum of NH3 and HCN concentrations.
The results above illustrate that significant concentrations of NH3 and HCN may be present in reburn zone 16. These species may react with NO and may facilitate substantially reducing NO emissions. A greater reduction in NO concentration may be realized when OFA is injected at combustion flue gas temperatures of approximately 1050° F. to approximately 1750° F. Because CO oxidation at lower temperatures of this range may not be complete, installation of downstream oxidation catalyst 202 may facilitate complete oxidation of CO.
The chemical kinetic code ODF, for “One Dimensional Flame” (Kau, C. J., Heap, M. P., Seeker, W. R., and Tyson, T. J., Fundamental Combustion Research Applied to Pollution Formation. U.S. Environmental Protection Agency Report No. EPA-6000/7-87-027, Volume IV: Engineering Analysis, 1987), was employed to model experimental data. ODF is designed to progress through a series of well-stirred or plug-flow reactors, solving a detailed chemical mechanism. The kinetic mechanism (Glarborg, P., Alzueta, M. U., Dam-Johansen, K., and Miller, J. A., Combust. Flame 115:1-27 (1998)) consisted of 447 reactions of 65 C—H—O—N chemical species.
The model was used to predict NOx reduction in natural gas reburning as a function of flue gas temperature at which OFA was injected (TOFA). Initial NOx (NOi) and the amount of reburning fuel were assumed to be 300 ppm and 18%, respectively. This amount of the reburning fuel was chosen for modeling because, as illustrated in
Process model output graph 800 includes a trace 806 that illustrates a prediction of NO concentration in the combustion flue gas decreasing as TOFA decreases. This NO reduction may be due to reactions of NO with NH3 and HCN. These reactions are similar to reactions that take place in a SNCR process. Optimum temperatures for the SNCR process are in the range of approximately 1750° F. to approximately 1950° F. without significant amounts of combustibles present in flue gas and decrease as CO concentration in flue gas increases. At temperatures higher than optimum some NH3 and HCN may be oxidized and form NO. At temperatures lower than optimum not all NH3 and HCN are consumed in reactions with NO and O2 resulting in “ammonia slip”.
A trace 808 illustrates a model prediction of CO concentration in flue gas at the end of reburn zone 16 at 18% reburning fuel heat input is about 2%. Optimum temperatures for the SNCR process at this CO concentration are in the range of approximately 1300° F. to 1400° F. A trace 810 of the model prediction illustrates that TFN reaches a minimum at a TOFA of about 1350° F. Although NO continued to be reduced further at temperatures below approximately 1350° F., not all NH3 and HCN were consumed in this process resulting in an increase in TFN.
Trace 808 illustrates a model prediction that CO was substantially completely oxidized to CO2 at a TOFA in a range of approximately 1350° F. to approximately 1900° F. The CO concentration in the combustion flue gas increased as TOFA decreased below approximately 1350° F. This may be due to low temperature CO oxidation becoming too slow and may not be substantially completed within time available in burnout zone 18.
Trace 810 illustrates a model prediction of OFA injection of approximately 1350° F. resulted in TFN reduction from 300 ppm to about 60 ppm. CO is substantially completely oxidized at TOFA of approximately 1350° F. and greater. When compared to empirical results the model results illustrated in graph 800 exhibited a close correlation.
It is contemplated that the benefits of the various embodiments of the invention accrue to all combustion systems, such as, for example, but not limited to, a stoker furnace, a fluidized bed furnace, and a cyclone furnace.
The above-described nitrogen oxide reducing methods and systems provide a cost-effective and reliable means for reducing nitrogen oxide concentration in combustion flue gas emissions without injecting N-reducing agents into the combustion flue gas stream. More specifically, empirical results show that significant concentrations of NH3 and HCN can be present in the reburn zone. These species may react with NO and significantly reduce NO emissions if OFA is injected at combustion flue gas temperatures of about 1050° F. to about 1750° F. Because CO oxidation at lower temperatures of this range is not complete, installation of a downstream oxidation catalyst may permit complete CO oxidation. Accordingly, controlling process conditions that promote the formation of N-containing agents and injecting OFA at temperatures in a range that facilitates the combination of NH3 and NO to form N2 provides a cost-effective methods and systems for reducing nitrogen oxide emissions.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
