Utilization of Iron Additives for Advanced Control of NOx Emissions

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Utilization of Iron Additives for Advanced Control of NOx Emissions from Stationary Combustion Sources Vitali V. Lissianski,* Peter M. Maly, and Vladimir M. Zamansky General Electric Energy and Environmental Research Corporation, Irvine, California 92618

William C. Gardiner University of Texas, Austin, Texas 78712

A novel NOx control technique based on the utilization of iron-containing compounds was studied. Iron-containing additives can be injected in small amounts either into the combustion zone or, if reburning is used, into the reburning zone to reduce NOx emissions. Tests in a 300-kW Boiler Simulator Facility demonstrate that iron additives achieve 20-30% NOx reduction without reburning, whereas in conjunction with reburning, they increase NOx reduction from about 60%, typical for conventional reburning, to about 80-85%. The additives tested included powdered metallic iron, iron oxides, iron waste, Fe(CO)5, char, and ash. Up to 1000 ppm (atomic Fe basis) was added to the flue gas. Experimental variables studied include additive composition and amount, injector location, and reburning heat input. Depending on the additive, the effect is dominated by either heterogeneous or homogeneous reactions. Introduction Limitations on NOx emissions from stationary combustion sources are becoming increasingly stringent in industrialized countries. Current and near-future regulations require 65-80% NOx reduction from coal-fired boilers and furnaces. Two NOx control technologies can achieve this level at acceptable cost: selective catalytic reduction (SCR) and combinations of combustion modification techniques. Well-known combustion modification methods include low NOx burners (LNBs), airstaging, and fuel-staging (reburning). Advanced reburning1-3 (AR) is a recently developed combustion modification technique in which reburning is combined with N-agent injection. Reburning is one of the most promising low-cost NOx control technologies. However, reburning alone cannot provide the levels of NOx reduction required by new regulations. Integration of reburning with other lowcost NOx control technologies (for example, LNB) can achieve the target 65-80% reduction in NOx emissions in many cases. Therefore, even a small increase in reburning efficiency would be very important. It would allow low-cost NOx control technologies to provide the required level of control in many units. It has been recently discovered4 that injection of ironcontaining compounds into the combustion and reburning zones (the latter process is referred to as promoted reburning) of conventional gas- and coal-fired combustors can increase the efficiency of NOx reduction by up to 20% in comparison with that can be achieved by basic reburning. Injection of metal compounds does not require a separate injector for the additive: the additive can be co-injected with the main or reburning fuel. The increase in ash loading as a result of additive injection is not significant relative to the inherent ash loading * Corresponding author: Vitali Lissianski, GE EER, 18 Mason, Irvine, CA 92618. Phone: (949) 859-8851. Fax: (949) 959-3194. E-mail: [email protected].

in the combustion of solid fuels. The cost of promoted reburning is lower than that of other technologies with similar levels of NOx reduction (for example, AR) because iron-containing additives are inexpensive (Fe waste, byproduct of the metallurgic industry, might have a negative cost) and the same level of NOx reduction can be achieved without N-agent injection. There are no reports in the literature of NOx reduction by iron-containing compounds under reburning conditions. It has already been reported5-7 that, in a fluidized-bed combustor, NO can be catalytically reduced by CO over an Fe2O3 surface; in summary, the heterogeneous reactions

3CO + Fe2O3 f 3CO2 + 2Fe

(1)

2Fe + 3NO f 1.5N2 + Fe2O3

(2)

giving an overall conversion stoichiometry CO + NO f CO2 + 0.5N2, were proposed to account for the observed NO reduction. This chemistry, however, might correctly describe the observations under fluidized-bed conditions and might not be appropriate at all for describing the observed effect of iron-containing compounds at the higher temperatures and lower iron concentrations typical of reburning. In this paper, we describe an investigation undertaken to characterize the effects of process conditions on NOx reduction by iron injection into combustion and reburning zones and to gain a chemical understanding of the process. Experimental Section Experiments with injection of iron-containing additives were conducted in a 300-kW Boiler Simulator Facility (BSF) described earlier3 and shown in Figure 1. The BSF is designed to provide accurate subscale simulation of the flue-gas composition and temperatures in a full-scale boiler. It consists of a burner, a vertically down-fired radiant furnace with a 56-cm inside diameter and a 7-m height, a horizontal convective pass,

10.1021/ie010019q CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001

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Figure 1. Boiler Simulation Facility (BSF).

and a baghouse. Ports located along the axis of the furnace provide access for injectors and sampling probes. A suction pyrometer is used to measure furnace gas temperatures. Natural gas and pulverized coal were tested as the main fuels. For the natural gas tests, the initial NOx concentration was set at 600 ppm by adding ammonia to the combustion air. Previous work with the BSF has shown that this generates a controllable concentration of NOx with negligible ammonia slip. With coal, no effort was made to control the initial NOx concentration. Natural gas was used as the reburning fuel in all tests. The L-shaped reburning fuel injector was located on the centerline of the furnace, aligned countercurrent to the furnace gas flow. Overfire air (OFA) was injected through a similar injector to burn out combustibles generated in the reburning zone. The iron compounds were transported pneumatically as powders to the furnace and injected through a radial injector. Temperature decreases along the BSF reactor at a linear rate of ∼300 K/s. Injection of the reburning fuel and OFA occurred at 1700 and 1450 K, respectively. Iron compounds were added with the main fuel, with the reburning fuel, or into the reburning zone at 1590 K. A continuous emissions monitoring system (CEMS) was used for on-line flue-gas analysis. It consisted of a heated sample line, a sample conditioning system to remove moisture and particulates, and gas analyzers. The species monitored included O2, NOx, N2O, CO, and CO2. The estimated uncertainty of the composition measurements made in the BSF was (5% for reburning only and (10% for promoted reburning. Surface area measurements of iron-containing particles were made using the conventional nitrogen BET physical adsorption method. Test Results The following additives were tested: metallic iron, iron oxides, iron waste, Fe(CO)5, char, and ash. Three test series were conducted. The first two, in which natural gas was fired as the main fuel, involved screening iron additives under constant baseline conditions

and parametric evaluation of process variables. In the third series, coal was fired as the main fuel to provide conditions representative of industrial combustors. These studies involved screening the performance of four additives, including ferrous and ferric oxidation states and metallic iron itself, different particle sizes, and an industrial iron waste product tested as an example of a waste available at low or no cost. [The sample tested, a byproduct of the steel processing industry, consisted nominally of about 80% Fe2O3 and 20% impurities, primarily Ca(OH)2.] Iron metal powder, 100% smaller than 10 µm in diameter, was studied at 1000 ppm mole fraction in the flue gas; Fe2O3 powder, 100% smaller than 5 µm, over the range 0-1300 ppm; Fe3O4 (FeO‚Fe2O3), 100% smaller than 5 µm, at 1000 ppm; and iron oxide waste, 80% smaller than 50 µm in diameter, at mole fractions ranging from 0 to 1000 ppm. The amount of metal in the flue gas is expressed here (and throughout this paper) as the number of Fe atoms per 106 molecules present, calculated as if the entire flue-gas content, aside from ash, were present as gas-phase molecules. The initial screening tests involved co-injecting each of the four additives (metallic iron, iron oxides, and iron waste), together with the reburning fuel, at reburning heat inputs of 18 and 25% from the total heat input, in the amounts needed to provide 1000 ppm of iron in the flue gas. Figure 2a summarizes the results. Reburning without additives provided 60-66% NOx reduction; iron waste and Fe2O3 provided the greatest NOx reductions, up to about 19% more than reburning alone. The maximum NOx reductions observed were 77 and 85% for reburning heat inputs of 18 and 25% from the total heat input, respectively. Iron metal and Fe3O4 provided 3-9% of improvement. Emissions of N2O in tests with and without additives were less than 3 ppm. After the screening tests, more detailed studies were performed to parametrically evaluate the process variables injection mode (with the main fuel, with the reburning fuel, and into the reburning zone), additive concentration, and reburning heat input. The parametric tests focused on Fe2O3 and the iron waste product, which showed the greatest effects in the screening tests. Figure 2b shows the effect of iron waste compounds coinjected with the main fuel. In the absence of reburning, approximately 23% NOx reduction was achieved by iron injection. Reburning without additives resulted in 60% NOx reduction for 18% reburning and 66% NOx reduction for 25% reburning. Additive injection improved the process efficiency by 8-10%. Figure 2c shows the performance for Fe2O3 injection with the main fuel, with the reburning fuel, and into the reburning zone at 1590 K. The best performance was obtained for Fe2O3 coinjection with the reburning fuel, which improved NOx reduction by about 20%. Injection into the reburning zone was least effective. Figure 2d shows the effect of iron oxide waste co-injected with the reburning fuel as a function of additive concentration. In the absence of additive, conventional reburning again gave 66% NOx reduction. NOx reduction levels increased as the iron concentration increased from 0 to 600 ppm, after which further increases had little effect. About 86% NOx reduction was achieved at 600-770 ppm of additive in flue gas, that is, 21% greater than the baseline condition. Reburning heat input was then varied from 10 to 25% of the total heat input with and without Fe2O3 additive.

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Figure 2. Test data on NO reduction by injection of iron-containing compounds. (a) Co-injection of 1000 ppm of different additives with reburning fuel. Shaded bars, 18% reburning; open bars, 25% reburning. 1, reburning only; 2, Fe metal; 3, Fe waste; 4, Fe2O3; 5, Fe3O4. (b) Iron waste co-injection with the main fuel. 1, 18% reburning alone; 2, 18% reburning with 1000 ppm waste; 3, 25% reburning alone; 4, 25% reburning with 1000 ppm waste; 5, 1000 ppm iron waste, no reburning. (c) Injection of 1000 ppm Fe2O3 at different locations for 18% reburning. 1, reburning alone; 2, co-injection with main fuel; 3, co-injection with reburning fuel; 4, injection into reburning zone. (d) Effect of iron waste co-injection with the reburning fuel at 25% reburning. (e) Effect of reburning heat input on Fe2O3 co-injection with reburning fuel. O, 1000 ppm Fe2O3 added; 0, without Fe2O3 addition. (f) Effect of iron waste co-injected with reburning fuel during coal combustion. O, 800 ppm waste added; 0, without waste addition. The initial level of NOx was 1200 ppm.

As shown in Figure 2e, NOx reduction appeared to increase with increasing reburning heat input. Fe2O3 additive increased NOx reduction by 11% at 10% reburning and by 21% at 25% reburning. Figure 2f shows the effect of iron oxide co-injected with the reburning fuel during combustion of coal. A bituminous Utah coal containing 0.67 wt % sulfur and 11.8 wt % ash on a dry basis was used. The initial uncontrolled NOx concentration it generated was 1200 ppm corrected to 0% O2, dry basis. The iron additive caused NOx reduction to increase by 6-9%. The maximum NOx reduction with coal firing was 84%. Tests also showed that addition of an iron compound in both the main and reburning zones

could provide higher NOx reduction than injection of the same amount of additive in one zone. Many minerals, including iron compounds, are present in coals. The flue-gas concentration of iron from the coal used (if all of it were present in atomic form) would have been about 200 ppm in these tests. Tests in which ironcontaining ash and char were injected showed only small effects, ∼1-2%. This finding can be explained by the difference in the chemical nature of the iron compounds in the additives compared to those in the coal/char/ash. Although metal is reported in traditional coal, char, and ash mineral analyses as the oxides, these are not the actual forms of metals in coal. Instead,

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metals are mainly present in the form of sulfides and in silicate and aluminosilicate matrixes, which apparently do not react with combustion radicals and have minimal effects on NOx reduction. In some tests, the iron oxide additive was co-injected with a small amount of the reburning fuel (about 6%). Because the total composition of the mixture was fuellean, no OFA air was added. NOx reduction increased from 32 to 38% as the additive amount increased from 0 to 1300 ppm. To evaluate the effect of atomic iron, a few tests were conducted with the injection of 450 ppm of iron pentacarbonyl together with the reburning fuel and downstream in the reburning zone. An increase in NOx reduction of 10-13% was obtained at 20% reburning heat input. The BET surface area measurement of a sample of the reagent-grade Fe2O3 gave a surface area of 3.0 m2/ g, whereas a sample of the Fe2O3 waste had a surface area of 7.2 m2/g. Modeling and Discussion The test results show that iron-containing additives in the main combustion and reburning zones are capable of reducing NOx emissions. Possible explanations for this include gas-phase reactions involving iron-containing species, NO reduction on the surface of ironcontaining particles, or a combination of homogeneous and heterogeneous chemistry. In the following sections, we consider which of these processes might contribute to the NOx reduction. First, the distribution of iron between gaseous and condensed phases is computed from the known thermochemistry of iron. Second, we examine possible gas-phase mechanisms. Finally, direct and indirect contributions of heterogeneous reactions on the surface of iron-containing particles are discussed. Distribution of Iron between Gas and Condensed Phases. At the temperatures of interest here, the more stable forms of Fe/O/H species are the solid and liquid phases. Condensed phases of the ironoxygen system have been carefully characterized up to 1870 K.9,10 Information about vapor-phase iron, iron oxides, and hydroxides is less extensive, but reliable high-temperature measurements have been made of the vapor pressure of pure iron,11 and indirect experimental thermochemistry, including in particular the enhancement of iron’s volatility by the presence of water vapor,12,13 has been reported for gas-phase Fe/H/O species with nominal valencies ranging from 1 (FeH) to 4 (FeO2) (cf. ref 14 for references to the experimental and computational literature). The inferences drawn from experiments have been supplemented and systematized thermochemically through molecular electronic structure calculations by Kellogg and Irikura,14 who also identified OFeOH as a stable gas-phase species. To estimate the concentrations of iron-containing species in the gas phase under reburning conditions, equilibrium compositions of the gas and condensed phases were computed using the thermodynamic data of McBride et al.15 and the program of Feitelberg.16 The total concentration of gas-phase iron-containing species at 1700 K and 1 atm for a total iron amount of 1000 ppm was computed to be ∼7 ppm at equilibrium, suggesting that, under reburning conditions, at least for the injection of powders, most of the iron is present as condensed phases.

Iron as a Gas-Phase Catalyst Reducing NOx. To estimate the contribution of homogeneous reactions to NOx reduction, kinetic modeling of the reburning process in the presence of Fe-containing gas-phase species was undertaken using the ODF (for “one-dimensional flame”) kinetics code.17 A previously developed model18 of the reburning process was modified to describe injection of iron-containing additives with the reburning fuel. The model18 describes reburning as a series of four plug-flow reactors, each describing a stage of the successive physical and chemical processes: addition of reburning fuel and iron-containing compound in the first reactor, NOx reduction resulting from reaction of iron in the second reactor, addition of overfire air in the third reactor, and oxidation of partially oxidized flue gas in the fourth reactor. This model, which combines a detailed description of the combustion chemistry with a simplified representation of mixing, successfully describes the conventional reburning18 and AR19 processes. The mixing of the reburning fuel and the iron compound with the main stream was described using “inverse mixing”: flue gas was slowly added to the stream of natural gas and iron. The inverse mixing approach was described in detail by Zamansky and Lissianski.18 Inverse mixing was shown18,19 to give a realistic representation of the fuel-rich environment of the mixing area in the reburning zone. All calculations reported here were done with a constant thermal cooling rate of -300 K/s; calculations for -250 K/s showed similar results. As in the experiments, the reburning fuel and OFA were added at 1700 and 1450 K, respectively. Iron was added in the amount of up to 1000 ppm either with the reburning fuel or into the reburning zone at 1590 K. Modeling was done using the Glarborg et al.20 mechanism, which comprises 447 reactions of 65 chemical species. NOx reduction in conventional reburning is well reproduced20 by this mechanism. The kinetics of iron reactions in flames is not yet well understood, and rate constants of potentially important elementary reactions of iron-containing species with flame radicals have not been measured directly. Most of our understanding of these reactions derives from studies done on hightemperature reactions initiated by Fe(CO)5.21-29 For modeling of the promoted reburning process, the C/H/ O/N chemistry was supplemented by the 47 reactions of 8 iron-containing gas-phase species [Fe, FeO, FeO2, FeOH, Fe(OH)2, OFeOH, FeH, and Fe(CO)5] reported by Rumminger et al.27 and Rumminger and Linteris;28 these authors found that this mechanism was sufficient to describe inhibition of methane-air flames by Fe(CO)5 successfully. Reactions of iron-containing particles on the surface were not considered. Comparisons of modeling predictions with experimental measurements are shown in Figure 3, where the experimental data indicated represent the scatter of all runs for 18% reburning with different iron additives. Modeling the iron-promoted reburning process with the gas-phase iron-containing species present in equilibrium amounts and with the liquid and solid phases considered to be inert showed negligibly small effects (line 1). Line 2 in Figure 3 shows the effect of the gas-phase ironcontaining species assuming that the additive evaporates completely upon injection and that no condensed phase is present in the reburning zone. Under this assumption, the predicted promotion is similar to that found experimentally.

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together with other reactions, supports partial equilibrium in the pool of radicals. The model shows reburning fuel being oxidized during the early stages of reaction with or without additive. However, with iron present, the reburning fuel is oxidized more slowly. Fuel oxidation generates carbon-containing radicals that reduce NO to HCN.30

Figure 3. Comparison of experimental and gas-phase modeling results for iron injection at 18% reburning fuel. Line 1 shows the modeling result for equilibrium concentrations of iron-containing species in the gas phase, line 2 for complete evaporation of the additive. The error bar for pure reburning represents the estimated (5% accuracy of the BSF tests. The bars for promoted reburning represent the spreads of test data for metallic Fe, Fe2O3, and iron waste.

The modeling thus suggests that for a homogeneous mechanism to dominate, all of the injected iron must be present in the gas phase. Because iron metal, oxides of iron, and iron oxide waste were injected in the form of powder, however, and because the total equilibrium concentration of gas-phase iron species under reburning conditions is only a small fraction (∼7 ppm) of the total, the modeling suggests that these additives reduce NOx emissions through a heterogeneous mechanism, as discussed in the following section. A homogeneous mechanism of NOx reduction might allow additives that decompose at high temperatures to produce significant concentrations of Fe-containing gas-phase species. Fe(CO)5 is such an additive. Tests show that injection of 450 ppm of Fe(CO)5 along with 20% reburning fuel results in an additional 10-13% NOx reduction. At room temperature, Fe(CO)5 is a volatile liquid; at reburning temperatures, it dissociates in the gas phase to iron atoms and carbon monoxide. Thus, upon injection of Fe(CO)5, the iron is at least initially present in the gas phase. The modeling of reburning in the presence of Fe(CO)5 led to the following interpretation of the effect of gasphase iron on NOx reduction in the reburning process. As was suggested by Jensen and Jones,23 the presence of iron results in a decrease of the H-atom concentration through a catalytic cycle

FeOH + H f FeO+H2 + 62 kJ

(3)

FeO+H2O f Fe(OH)2 + 330 kJ

(4)

Fe(OH)2 + H f FeOH + H2O + 40 kJ ___________________________________

(5)

Net: H + H f H2

(6)

that causes a decrease of other radical concentrations (OH and O) in the reburning zone. (The thermochemical values reported are -∆rxnHo0 values from ref 14.) The OH and O concentrations decrease with the H concentration because the fast reaction

H+O2 T OH + O

(7)

CH + NO f HCN + O

(8)

CH3 + NO f HCN + H2O

(9)

HCCO + NO f HCN + CO2

(10)

HCN is subsequently converted to NH and NH2, which then react with NO to form N2. Simultaneously, the carbon-containing radicals are deactivated as NOreducing agents by reactions with other active species. Because, in the presence of iron-containing species, the concentration of non-carbon radicals is smaller, more carbon-containing radicals can participate in the reactions with NO. Thus, modeling suggests that the effect of iron in the gas phase on NO reduction can be explained by iron-catalyzed removal of non-carbon radicals (H, O, and OH) via the cycle of eqs 3-5 and 7 that results in higher concentration of carbon-containing radicals and thus increases the rates of NO reactions with carbon-containing radicals. A possible explanation for higher-than-equilibrium concentrations of ironcontaining species in the gas phase for the injection of Fe(CO)5 is that condensation occurs on a time scale longer than that of reactions in the reburning zone.31-36 Iron as an Indirect Heterogeneous NOx Reduction Catalyst. As noted in the previous section, NOx reduction in the gas phase is computed to depend strongly on radical concentrations during reburning. It is therefore of interest to know how these concentrations are influenced by loss of gas-phase radicals in surfacecatalyzed reactions. If such an effect is to be anticipated, the observed reduction of NOx need not be ascribed to conjectural heterogeneous chemistry involving NO itself. The theory of gas-solid reaction rates under practical conditions is very complex, particularly in particle-laden hot flows, as it has to describe, in addition to the chemical reaction itself, the contributions of mass and energy transport by diverse mechanisms in both phases as well as at the surface.37 For modeling promoted reburning, however, some simplification is possible. First, the gas-solid reactions are only a minor part of the total chemistry and physics; second, we are interested in computing an upper limit to the rate of radical loss, not the actual rates of surface-catalyzed reactions. For these purposes a simplified theoretical approach proves to be sufficient. Consideration of the experimental basis for assuming rapid destruction of atoms and radicals on iron surfaces is given later. Two extreme models are commonly used to estimate upper limits for surface reaction rates: one based on the kinetic theory of gases and one based on diffusion rates. Both require an assumption about the effective surface area per volume of gas: one has to decide whether a geometric or an adsorption-based (BET) surface area, or something else, is the appropriate area. This distinction is discussed below. For preliminary comparison, the geometric surface area for Fe2O3 was based on a monodisperse sample of 5-µm-diameter spheres having the 5.2 g/cm3 density of bulk hematite. This choice implies a specific surface area of 0.23 m2/g,

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a factor of 13 less than the 3.0 m2/g BET value measured for reagent-grade Fe2O3. At 1700 K and 1000-ppm (on an Fe basis) Fe2O3 loading, this implies a reactive surface area of 1.3 × 10-7 m2 per cm3 of flue gas. The gas-kinetic flux of radicals from the gas phase onto a surface is given by the classical kinetic theory formula -dci/dt ) ciAxkBT/2πmi, where ci is the concentration of radical i, mi is its mass, A is the surface area accessible to 1 cm3 of flue gas, kB is Boltzmann’s constant, and T is the temperature. For 1000-ppm iron loading and 1700 K, conditions typical of the BSF tests, the maximum rate of H-atom loss to the surface of Fe2O3 particles is computed to be 620 s-1 in this model. In contrast to the well-accepted simple kinetic theory of gases model for the arrival rate of molecules from an ideal gas onto a solid surface, the theory of diffusionlimited reaction rates has generated abundant controversial discussion of the conceptual and physical limitations to the validity of various approaches (cf. refs 38 and 39 for critical reviews). Although these limitations apply even to isothermal irreversible reaction, they do not apply to the present case, where the role of one reactant molecule is played by a sphere of diameter much greater than molecular dimensions. Thus even though the basic steady-state bimolecular rate constant formula of diffusion theory, k ) 4πFABDAB, is controversial because of the difficulty of assigning precise meaning and values to the encounter distance FAB and the relative diffusion coefficient DAB, in the case at hand, FAB is just the radius of the spherical iron-containing particles, and DAB can be assigned to the diffusion coefficient of the gas-phase radicals in the flue-gas environment. For the case of H atoms, experimental values for diffusion in N2 are available;40,41 the more recent41 value of DH-N2 ) 1.35 cm2/s at atmospheric pressure and 294 K can be estimated to increase by a factor of (1700/294)1.75 to 29.1 cm2/s at 1700 K, which is taken as representative of H-atom diffusion for flue-gas conditions. Using these values in the bimolecular rate constant formula and converting to a unimolecular rate constant through multiplication by the number density of 5-µm-diameter Fe2O3 particles at 1000-ppm (Fe basis) iron loading leads to an upper-limit heterogeneous H-atom loss rate of 150 s-1, four times smaller than the value from gas-kinetic theory. For estimating maximum surface loss rates, it is appropriate to use the geometric surface area, ignoring capillarity and roughness at scales below the mean free path in the reburning gas of about 1 µm. Part of the surface roughness, however, as well as the real distribution of particle size, does serve to increase the surface area available for heterogeneous reaction beyond the geometric area computed for a monodisperse spherical powder. If the BET surface area applied to the computation of the upper limit of H-atom loss, then the upperlimit rates of the heterogeneous H-atom loss reactions would increase by an order of magnitude. Thus, it can be concluded that reactions on the surface of particles are controlled by the diffusion of radicals to the surface. The upper-limit heterogeneous H-atom loss rate can be compared with the homogeneous H-atom loss rate in reactions 3-5. This comparison suggests that the diffusive loss of H atoms to surfaces becomes comparable to gas-phase H atom loss due to the homogeneous reactions of iron compounds discussed in the previous section at Fe concentrations above 0.3 ppm. Because the tests show that additive concentrations below 1000 ppm

have some promotional effects, these considerations suggest that heterogeneous chemistry must be considered, particularly when the promoters are injected as powders. Although no direct studies of iron-catalyzed elementary chemical reactions under reburning conditions have been reported, there is compelling evidence that they are very fast. First, one has the well-studied iron catalysis of ammonia synthesis and Fischer-Tropsch reactions, both of which involve surface reactions of H atoms42,43 and both of which proceed, under reducing conditions, hundreds of degrees below reburning temperatures. The speed of metal-catalyzed H-atom chemistry is by no means a novel discovery. Woods reported in 1922 that the Balmer spectrum of atomic hydrogen is quenched if traces of metalsshe tested Sn, Al, W, Pt, Cu, and a Th-Ce mixturesare present in spectroscopic discharge tubes,44 and Langmuir observed in 1915 that tungsten can be heated to incandescence by energy liberated from the surface recombination of H atoms.40 The possible mechanisms of heterogeneous NOx reduction would have to include direct catalytic reduction of adsorbed NO, as suggested for example by reactions 1 and 2, in addition to the indirect effects of radical loss described above. If such a direct mechanism is active, then it evidently requires the presence of both CO and Fe2O3, which would be consistent with the observation that Fe2O3 powder and iron waste (80% Fe2O3) showed higher levels of NOx reduction than iron metal powder and Fe3O4. The observation that the promoting effect on NOx reduction increases with the amount of reburning fuel would also be consistent with a direct reduction mechanism, as the CO concentration in flue gas increases with the amount of reburning fuel. We have no evidence, however, that direct catalytic reduction is a factor under the promoted reburning conditions tested in the BSF. Conclusions Judging from the observed effects of iron-containing compounds on NOx formation and destruction under flame, reburning, and flue-gas conditions, the following options for reducing NOx emissions can be suggested: (1) Injection of iron additives with the main fuel or into the main combustion zone, with or without reburning, can be used. Up to approximately 30% reduction of NOx can be obtained by this method. (2) Injection of iron additives with the reburning fuel or into the reburning zone, with or without overfire air, can be used. Up to about 20% of additional NOx reduction, compared to conventional reburning, can be achieved by this method. Iron-containing additives can be injected as solids or liquids (metal-organic compounds or as solutions of metal compounds in water or other solvents); they can also be components of the main fuel, the reburning fuel, or products of their pyrolysis or gasification. Injection of additives does not significantly increase the cost of the process as iron-containing additives are inexpensive and do not require a separate injector because the additives are injected with the main or the reburning fuel. Kinetic modeling suggests that surface reactions are responsible for the observed promotional effect of solid additives, whereas reactions of iron-containing gasphase species might account for the promotional effect of Fe(CO)5 on NOx reduction in the reburning zone.

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Received for review January 8, 2001 Revised manuscript received April 30, 2001 Accepted May 8, 2001 IE010019Q