Reducing NOx Emissions Using Fuel Staging, Air Staging, and

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Reducing NOx Emissions Using Fuel Staging, Air Staging, and Selective Noncatalytic Reduction in Synergy Edgardo Coda Zabetta,*,† Mikko Hupa,† and Kari Saviharju‡ A° bo Akademi Process Chemistry Centre, Piispankatu 8, FI-20500 Turku, Finland, and Andritz Oyj, Kraft Mill Systems, P.O. Box 500, FI-48601 Kotka, Finland

Fuel staging (FS), air staging (AS), and selective noncatalytic reduction (SNCR) are techniques for abating nitrogen oxides (NOx ) NO + NO2) from boilers and engines. Each of these techniques has a limited range of applicability, within which 50% to 70% NOx reduction is attained. Higher reductions are achieved by methods that use the aforesaid techniques in sequence, thus cumulating their reduction ability, but also collecting their respective limits. In this paper, we describe a new method that we call “combined staging” (CS). This method combines FS, AS, and SNCR in synergy rather then in sequence. In CS, the fuel is first staged for converting NOx precursors to hydrogen cyanide (HCN). Then, the air is staged for reducing HCN to N2. Further reduction is achievable by optional SNCR. In the followings the basics of FS, AS, SNCR, and their sequential applications are reviewed first. Then, the combined staging is introduced, its chemical details are elucidated via kinetic modeling, and options for its application are illustrated. Finally, assumptions and limits of the kinetic models are discussed. The present work reveals that CS can reduce over 40% NOx at lower temperatures and within shorter residence time than required by other techniques and methods. Thus, CS could reduce NOx effectively in devices where other techniques fails, e.g., in kraft recovery boilers, fluidized bed combustors, low-grade fuel combustors, small and domestic boilers, and fast engines. 1. Introduction Nitrogen oxides (NOx ) NO + NO2) are pollutants that form during most combustion processes. Nitrogen oxides form from the nitrogen contained in fuel and air via a number of routes that depend on fuel and process.1-7 Fuel staging (FS), air staging (AS), and selective noncatalytic reduction (SNCR) techniques were introduced decades ago for reducing NOx from combustion devices such as furnaces and engines. Alone, each technique reduces NOx by some 50 to 70%, but only within well-defined applicability limits. To achieve higher reductions, methods have been developed for using two or all three techniques in sequences. These methods optimize individually each component technique (FS, AS, SNCR). This way, the NOx reduction by each component cumulates to outstanding overalls, i.e., over 90%. However, also the limits of each component adjoin, thus restricting the applicability of sequential methods, as it is the case for hybrid reburn (HR) and advanced reburn (AR). In short, all these techniques and methods require high temperatures and few seconds to complete their NOx reducing task. Differently, in the combined staging (CS) here proposed NOx reducing techniques are combined in synergy rather than in sequence, thus achieving significant NOx reductions while avoiding the accumulation of applicability limits from each component technique. The chemical details and the applicability limits of FS, AS, SNCR, HR, and AR are listed in the following subsections. The CS method, subject of this paper, is * To whom correspondence should be addressed. Tel: +358 2 215 4930. E-mail: [email protected]. † A ° bo Akademi Process Chemistry Centre. ‡ Andritz Oyj, Kraft Mill Systems.

described in Section 2 where the focus is placed on the conceptual, chemical, and appliance aspects that distinguish the CS from the methods described in Section 1. Because the CS is described in this paper as predicted by models, our model assumptions and their effects on the predictions are discussed in Section 3. However, as the scope of this paper is to illustrate a new reduction method and not to discuss the tools used for its formulation, the reader is readdressed to the referred literature for any more concise information on the models adopted for this work. 1.1. Fuel Staging (FS) and Reburning. To the best of our knowledge, the earliest reference to an industrial fuel staging technique is that of Fernandez et al. in 1966.8 The technique consists of staging the combustion fuel in a number of streams, which are delivered to the furnace at convenient locations. The simplest FS features a sequence of fuel streams that are located along the furnace so as to set a progressive increase of the stoichiometry ratio from extremely fuel-lean (SR . 1) to the nominal excess air that warrants complete combustion (SR > 1). The reactions inset by each fuel stream provide precursors and radicals that participate in the reduction of previously formed NO. While this setup leads to decent NO reductions, better results are achieved with a variant of FS known as “reburning”. Reburning was proposed by Wendt et al.9 in 1973 (Figure 1a). In reburning a “primary fuel” (fuelI) is burned to completion with an excess of “primary air” (airI), then “reburn fuel” (fuelII) is added for resetting reducing conditions, and finally combustion is completed with excess “burnout air” (airIII).10-19 Accordingly, three zones are set in the furnace: “primary combustion”, “reburn”, and “burnout” zone. The purpose of primary combustion is to promote the efficient conversion of fuel to energy. However, the conditions that ensure high

10.1021/ie050051a CCC: $30.25 © 2005 American Chemical Society Published on Web 05/24/2005

Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005 4553 SRI > 1

N(fuel,air) + O2 98 NO + ... (primary combustion) (1, global) NO + CHi, HCCO, H

0.85 < SRII < 0.98

98 TII >> 1600 K (or 1500 K)

N2 + ...

(reburn in presence of hydrocarbons) (2′, global) NO + H + M f N2 + ... (reburn in absence of hydrocarbons) (2′′, global) SRIII > 1

N2 + O2 98N2 (+ NO) + ... (burnout) (3, global)

Figure 1. Schematic presentation of nitrogen conversion in (a) reburning, (b) advanced reburn, and (c) combined staging. Concentrations normalized to cancel dilution effect. Disappearing nitrogen converts to N2 (not shown). Recommendations given on temperature (T), residence time (τ), and air-to-fuel stoichiometry ratio (SR). Only chemical kinetics and no mixing accounted for (chemistry time). Note the significantly shorter time in (c).

efficiency also favor the accumulation of oxy and hydroxyl radicals (O, OH), which ultimately contribute forming NO. Reburn is intended to reduce this NO: the excess of fuel forms methyl, ketenyl, or hydrogen radicals (CHi(i)0,1,2,3), HCCO, H) that drive the partial reduction of NO to molecular nitrogen (N2).10-17,20-25 Finally, burnout must complete the combustion process while minimizing the slip of polluting N-compounds or the reformation of NOx. The complex chemistry of reburning and the conditions for its success are summarized in global Reactions 1-3, where N(fuel,air) is any N-species carried by fuel and air, and M is any third body.16,19

In reburning the reduction of NOx is affected by the conditions in both reburn and burnout zones,21 and especially by temperature, pressure, stoichiometry, presence of hydrocarbons and their speciation, presence of N-compounds other than NO, and concentration of carbon monoxide.10-17,20,21,26 In particular, reduction occurs only within narrow ranges of temperature, whose boundaries are affected by the other variables.10-17,19-21 Two practical ranges have been found thus far: one is well above 1600 K and is known as “high-temperature reburning” (HR),17,25,27 the other one is from just below to just above 1500 K and is known as “low-temperature reburning” (LR).10-17,20,21 Interestingly, between these two ranges reburning fails. The HR is used in hot power plant furnaces where it contributes to 50-70% NOx reduction, while the LR has been recently introduced in municipal waste incineration as well as in glass and steel industries, where it leads to 45-55% reduction. Concerning the fuels, early applications employed natural gas as the reburn fuel in coal-operated boilers. In current applications, the reburn fuel can be same as the primary fuel, and enlists natural gas,28-32 pulverized coal,31-34 micronized coal,31 oil,32 and gasified biomass.18,35 Studies have been conducted to elucidate the effect of fuels on reburning. Methane (CH4), ethane (C2H6), and blends of light hydrocarbons that model fossil fuels have been tested in high-temperature reburning.17 Methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), propene (C3H6), propane (C3H8), n-butane (C4H10), mixtures simulating natural gas (CH4-C2H6), or gasified biomass (CO-H2 and CH4C2H4-C2H2) have been tested in low-temperature reburning.10-16,21 Conclusions from these studies disagree, however, some claiming that natural gas is the best reburn fuel,20,35-37 and others stating that all fuels are alike.19,31 Nonetheless, all studies agree that the main features of reburn fuels are their content of hydrocarbons, N-compounds, and carbon monoxide. We finally emphasize that in reburning NOx are reduced to N2 with least accumulation of other Ncompounds in the reburn zone.25 In particular, reburn fuels carrying little hydrocarbons have been suggested for minimizing the formation and accumulation of hydrogen cyanide (HCN).19 Alternatively, longer reburn zones have been suggested for promoting the destruction of HCN before the burnout zone,21 where it supposedly oxidizes back to NO. 1.2. Air Staging (AS). Basic air staging was introduces many centuries ago. Modern AS consists of staging the combustion air in a number of streams,

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which are delivered to the furnace at convenient locations. Aside few exceptions,38-42 air staging is intended to reduce the formation of NOx by limiting the availability of NO promoters such as O2, O, and OH. Air staging achieves some 50% NOx reduction. Likewise fuel staging, AS is affected by parameters such as temperatures and stoichiometry, but also by the number of stages. Air staging works best under highly fuel-rich conditions (SR < 1) and in a range of temperatures that depends on additional variables such as pressure, hydrocarbons availability, and the speciation of N-compounds.43-45 Concerning the optimal number of stages, the literature is somewhat controversial, but it appears that 3 to 5 stages is a favorable option.35,46 The literature also specifies that the stoichiometry within each stage should not be relevant, as long as staging preserves SR < 1. Concerning the applications, air staging was first used in pulverized coal and oil fired boilers, but with time the technique was adjusted and renamed for a variety of other applications, e.g., “late air-staging” for fluidized bed combustors40-42 and “rich-lean combustion” for gas turbines.44 As for the fuels, AS has been applied mainly to fossil fuels,41,46 biomass,40 gasified coal and biomass,44,45 and waste.47 Like in fuel staging, also in AS efforts are devoted to limit HCN, which easily increases at the fuel-rich conditions.44,45 1.3. Selective Noncatalytic Reduction (SNCR). Selective noncatalytic reduction consists of driving the reduction of NO to N2 via the addition of an agent in the combustion process. It usually leads to a 50% reduction.48,49 Operational conditions, fuel composition, and agent speciation are all variable that set the selectivity, and thus the efficiency of SNCR. Accordingly, many variants of this technique have been patented, among which: fuel-lean process with ammonia,50 fuelrich with ammonia,51 and fuel-rich with urea.52 The fuelrich process with ammonia (NH3) can be summarized with the global Reactions 4 and 5, where OH and H are radicals formed by the combustion process, and NHi are amino radicals. SR < 1

NH3 + OH, H 98 NHi(i)0,1,2) + ... (4, global) SR < 1

NHi + NO 98 N2 + ...

(5, global)

Other variants of SNCR include the addition of the reducing agent via different streams, e.g., with the reburn fuel or the air.21,53 Each variant works best under a well-defined range of conditions. For instance, fuel-lean SNCR works best in the range 1100-1400 K, while fuel-rich SNCR is best at higher ranges. Also, both ranges shift and narrow in the presence of carbon monoxide (CO). The existence of temperature ranges is explained by Reaction 5, which is inefficient at low and high temperatures, where it is slow and uncompetitive, respectively.22,54 As for the effects of stoichiometry and CO, both are explained via their influence over the radicals OH and H involved in Reaction 4.38,49 1.4. Hybrid Reburn (HR). Hybrid reburn consists of reburning and SNCR in sequence, and leads to 80-90% NOx reduction.18,25,38,49 The reburning portion is optimized for the reduction of NOx, and it is somewhat set to sustain the conditions for the SNCR process. Agents for the SNCR are ammonia (NH3), hydrogen cyanide (HCN), urea ((NH2)2CO), cyanuric acid (C3O3N3H3), or ammonium sulfate ((NH4)2SO4).32,38

Hybrid reburn has been tested in various configurations where the reducing agent is added (i) along with the reburn fuel, (ii) downstream of the reburn fuel injection, (iii) at the transition between reburn and burnout zones, and (iv) in the burnout zone. Configurations (i) and (ii) are the most used: the agent is thus introduced in a reducing zone where it decomposes to amino-radicals (NHi), which in turn reduce NO according to Reactions 4 and 5.55 Differently, in configurations (iii) and (iv) the reducing conditions of the reburn zone are used to accumulate CO, whose oxidation during burnout produces OH radicals, which finally endorse the SNCR process similarly to Reaction 4.38,49 Also in hybrid reburn all accumulations of HCN are considered detrimental for the process of NOx reduction.49 1.5. Advanced Reburn (AR). Advanced reburn consists of reburning, SNCR, and air staging in sequence (Figure 1b). Thus, AR can be regarded as an HR augmented with air staging. In advanced reburn the NO formed during primary combustion is reduced in a reburn zone, and the NO that reform during burnout is limited by air staging. More in detail, the burnout air is staged so as to delay the oxidation of CO that in turn, as described for the SNCR technique, affects OH radicals and finally the temperature range for the SNCR to occur similarly to Reaction 4.25,32,49 Advanced reburn leads to an outstanding NOx reduction of over 90%.32,35,56-58 Just like the three constituent techniques (FS, AS, and SNCR), also AR operates best within ranges of temperature affected by other variables.32,35 Many operating configurations have been suggested for AR, the most interesting being the one illustrated in refs 38 and 59, which is an early attempt of optimizing reburning, SNCR, and air staging in a synergy. There (Figure 1b) reburn and burnout operate very close to stoichiometric, i.e., at stoichiometry ratios of 0.99 and 1.02, respectively. Such setup allows formation of enough CO (and consequently OH radicals) to initiate SNCR as in Reaction 4, and yet not too many radicals that would deplete NHi, thus preventing Reaction 5. This process requires so fine control of the stoichiometry that hardly can be used in furnaces and engines. We finally note that also in advanced reburn it is presumed that HCN would be detrimental to the NOx reduction, and therefore its accumulation is minimized. 2. Combined Staging (CS) The techniques and methods overviewed in Section 1 are currently applied in a variety of furnaces and engines, where they achieve 30-90% NOx reduction. As seen, however, all these techniques would fail reducing NOx in devices operated beyond certain conditions, and particularly in furnaces operated below 1600 K, or in so small furnaces and so fast engines that the reduction processes could not complete within the given residence time. Combined staging60 is a novel method that reduces NOx where others fail. In this section, we will introduce the concept of combined staging, we will investigate its chemical reactions via kinetic models, and we will suggest options for its application. Assumptions and limits of the models will be discussed in Section 3. 2.1. Concept. Like all recent staging methods31,40,44 combined staging is conceived around a reburning design. In detail, it enlists a primary zone, a reburn zone where also SNCR agents can be supplied, and an air

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staged burnout zone (Figure 1c). While this zoned layout is identical to that of advanced reburn (Figure 1b) the purpose of each zone is substantially different. Here, reburn is adjusted to maximize the conversion of NO to HCN rather than N2, and the staged burnout is optimized for the reduction of HCN to N2. Thus, the novelty of combined staging is the endorsement of NOx reduction via HCN, in a synergy. Combined staging works best when the reburn temperature is between 1300 and 1700 K, which is lower than required by other methods based on reburning (Reaction 2′). Also, CS operates best in plainly reducing conditions (SRII < 1), with a reburn fuel that carries hydrocarbons, and relatively low burnout temperatures, i.e., 1100-1350 K (Figure 2). As noted in Section 1.2, we underline that the stoichiometry in the air-staged burnout is not crucial as long as it progressively increase while remaining below 1, except for the last stage where combustion must be finalized. In the conditions above the reburn hydrocarbons produce methyl- and ketenyl-radicals (CHi(i)0,1,2,3) and HCCO) that promote the conversion of NO to HCN via global Reaction 6 rather than to N2 via Reaction 2′.10-16,19,21,26 If a reducing agent is supplied, the SNCR process initiate in the reburn zone where the agent, e.g., NH3, can be introduced with the fuel (Section 1.4) and its effect is emphasized by the abundance of CO and thus OH as by Reaction 4. Then, staged air in relatively cold burnout forms enough radicals to sustain the reduction of HCN to N2 (Reaction 7) but not enough to oxidize it back to NO.19,21,25,26,32,33 The staged air also helps reducing the residual NH3 to N2 via Reactions 4 and 5. Note than in Figure 2, the N2 is not shown, but its formation is apparent from the decrease of the total fix nitrogen ([Nfix] ) ∑ [Nspecies] - [N2], i.e., all nitrogen but N2). According to the above, combined staging can be summarized with the global Reactions 1, and 4-7.

Figure 2. Model prediction of nitrogen conversion in combined staging (CS). Annotations as in Figure 1. In addition, total fix nitrogen [Nfix] ) Σ[Nspecies] - [N2], and reduction red. ) [1 - (Nfixout/ Nfixin)]*100. Composition at reburn inlet (mol fraction): 12.9% CO2, 27.7% H2O, 0.3% O2, 300 ppm CO, 100 ppm NO, rest N2. Composition of fuels: 5.9% C2H4, 25.6% CO, 28.5% H2, 39.9% H2O, 558 ppm NH3, rest N2. Molar flow ratio 12% fuelII/fuelI. Conditions relevant to, e.g., kraft recovery boilers. Note the progressive conversion of nitrogen species via HCN. Only NO emitted.

SRI > 1

N(fuel,air) + O2 98 NO + ... (primary combustion) (1, global) SR < 1

NH3 + OH, H 98 NHi(i)0,1,2) + ... (optional SNCR) (4, global) SR < 1

NHi + NO 98 N2 + ... (optional SNCR) (5, global) NO + CHi, HCCO

SRII < 1

98

HCN + ... (reburn)

TII ) 1300 - 1700 K

(6, global) HCN + O2,staged

SRIII > 1

98 TIII ) 1100 - 1350 K

N2 + ...

(air staged burnout) (7, global) Like other techniques and methods,25,61 combined staging does not lead to any significant N-emission other than NOx. In addition, CS carries three additional benefits: (i) it is applicable to furnaces where other techniques fail because of the operational conditions, (ii) it is applicable to smaller furnaces and faster engines as the time demanded for reducing NOx is shorter than with other techniques, and (iii) it is easier to control as all stages are operated well above and well below

Figure 3. Net flux diagrams of N-species for (a) reburn and (b) burnout of Figure 2. Each arrow represents reactions involving the species at the arrow extremities and side. Direction and thickness of each arrow account for the net conversion. Note how HCN accumulates during reburn and then consumes in burnout.

stoichiometric conditions.19,35,49,59 In the next subsection, the details of the chemical reactions occurring in CS are illustrated. 2.2. Chemical Details. Details of the nitrogen chemistry in combined staging were deduced by kinetic modeling (Section 3). The related results are collected in Figure 3, where the net flux of N-species is shown for reburn (3a) and burnout (3b) in conditions relevant to CS. We find that during reburn a significant portion of the NO from the primary combustion converts to HCN (Reaction 8,9). The HCN accumulates till the airstaged burnout, where it first reacts to NHi(i)2,1,0) (Reaction 10-16) and then it forms N2 (Reaction 1719) or NO (Reaction 20,21). Further reduction of NO occurs then via Reactions 22-24, which reform NHi

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to undergo Reactions 10-21 again. No pollutants other than NO (e.g., HCN, HNCO, N2O...) are left after burnout in combined staging (see also Figure 2).

N(air, fuel) + O2 f NO + ... (primary combustion) (1, global) NO + HCCO f HCNO + CO (reburn) (8, elementary) HCNO + H f HCN + OH

(9, elementary)

HCN + OH f HOCN + H (burnout) (10, elementary) HOCN + H f HNCO + H

(11, elementary)

HNCO + H f NH2 + CO

(12, elementary)

NH2 + H f NH + H2

(13, elementary)

NH2 + OH f NH + H2O

(14, elementary)

NH + H f N + H2

(15, elementary)

NH + OH f N + H2O

(16, elementary)

Figure 4. Net flux diagram of N-species for nonstaged burnout. Annotations as in Figure 3. Conditions as in Figure 2, but burnout air delivered at once. Note how all HCN converts back to NO, failing the reduction process.

NH + NO f N2O + H (reduction to N2) (17, elementary) N2O + H f N2 + OH

(18, elementary)

N + NO f N2 + O

(19, elementary)

NH + O f NO + H (back to NO)

(20, elementary)

N + OH f NO + H

(21, elementary)

Figure 5. Net flux diagram of N-species for reburn at temperature higher than recommended in CS. Annotations as in Figure 3. Conditions as in Figure 2, but TII ) 1573 K. Note how HCN accumulation fades as compared to Figure 3a.

M + H + NO f HNO + M (further reduction) (22, elementary) HNO + H + M f H2NO + M

(23, elementary)

H2NO + H f NH2 + OH

(24, elementary)

We note here that air-staged burnout is instrumental for CS: if burnout was not staged all HCN would convert back to NO, as show in Figure 4. We also emphasize that the chemistry of CS differs profoundly from that of other techniques. For instance, Figure 5 shows how HCN accumulates less and reacts more at temperatures higher than for CS, i.e., in conventional reburning: our kinetic simulations indicate that the accumulation of HCN fades above 1500 K, and vanishes above 1700 K. Similarly, the HCN does not accumulate when the reburn stoichiometry is higher than required for CS, i.e., when stoichiometry is that of conventional reburning (Figure 6). 2.3. Foreseen Applications. In Section 2.1, we noted that combined staging carries three main benefits: it is applicable to furnaces where temperature and stoichiometry are adverse to other techniques, it is applicable to small furnaces and fast engines, and it is easy to control. Thus, CS could be used for NOx reduction in relatively cold devices such as kraft recovery boilers (RB), fluidized bed combustors, low-grade fuel combustors, and small devices such as compact industrial

Figure 6. Net flux diagram of N-species for reburn in conventional reburning. Annotations as in Figure 3. Conditions as in Figure 2, but TII ) 1573 K and SRII ) 0.94. Note how HCN accumulation fades as compared to Figure 3a.

boilers and domestic furnaces. In all application, though, care should be placed as the staged burnout of CS delays combustion, thus increasing the emissions of carbon monoxide and unburned hydrocarbons. Combined staging in RB is currently the most appealing application.62 Recovery boilers are used in the pulping industry for recovering energy and chemicals from the black liquor, a byproduct of pulping.63 Staging techniques have been adapted to such boilers via strategies that we refer to as: (i) the “quaternary air” where extra air is injected in the upper portion of the boiler,64 (ii) the “vertical air staging” in which air jets are fed into the furnace from nozzles located on several vertical levels,65-68 (iii) the “Mitsubishi Advanced Combustion Technology” (MACT) where a reducing agent (urea) may be added after staging,69 (iv) the “black


Figure 7. Model prediction of nitrogen conversion in modified CS. Conditions as in Figure 2, but extended reburn and shortened burnout. Note how the modifications lead to reduction similar to Figure 2, but within only half of the time.

liquor staging” in which the liquor is fed from at least two levels in a furnace equipped with vertical air staging,70 (v) the “upper liquor staging” where additional black liquor is introduced in the upper portion of the boiler where oxygen is in excess,71 and (vi) the “higher liquor staging” in which the black liquor is introduced in the boiler above the usual level.72 All of these strategies prove substantial NOx reductions; however, they all require adjusting stoichiometry and temperature over the optimum for boilers, and all demand upsizing boilers. In contrast, our kinetic simulations indicate that CS reduces NOx from optimally operated boilers, even when small in size. Figure 2 shows the NOx reduction predicted in conditions relevant to RB. The modeled boiler is operated with black liquor (BL) as primary and reburn fuel, it is equipped with combined staging, and its SNCR agent is the ammonia (NH3) naturally present in BL. The figure displays a 36% reduction of N-pollutants. That is, 36% of the primary NO and SNCR agent are converted to molecular N2. In a real boiler, this would be achieved by adding few extra inlets of fuel and air along the furnace, and by operating it to its maximum efficiency. The simulation also shows that CS requires shorter time for reducing NOx as compared to other techniques (see also Figure 1). Therefore, RB equipped with CS can be built smaller, and thus more economically. Besides RB, the setup in Figure 2 can be applied also to small industrial boilers and domestic furnaces. Nonetheless, for such applications we foresee two modifications of value. The first modification is depicted in Figure 7, as predicted by kinetic modeling. It accounts for a slightly longer reburn zone and a significantly shorter burnout zone than in Figure 2. This way, more time is given for HCN to form during reburn via Reactions 8-9, and still enough time is available to achieve its reduction during burnout via Reactions 1024. This modification attains an overall reduction nearly as good as that in Figure 2, but within an almost halved residence time. The second modification is shown in Figure 8a. It accounts for a low burnout temperature (1173 K) so as to prolong the formation of HCN into that zone, finally improving the reduction to over 43%. The low burnout temperature, however, causes a slip of harmful byproducts such as HCN, HNCO, and N2O. For preventing this slip, the temperature must be higher, at least in the final portion of the burnout zone, as shown in Figure 8b. This way the slip is avoided, and the reduction is as good as in Figure 7 but with an even shorter residence time. Summarizing, the modifications

Figure 8. Model prediction of nitrogen conversion in modified CS. Conditions as in Figure 2, but a) lower burnout temperature and b) two levels burnout temperature. In (a) note the outstanding reduction but also the slip of HCN, HNCO, and N2O. In (b) note the reduction similar to Figure 2, here attained in less than half of the time.

in Figures 7 and 8 should allow applying CS in even smaller boilers and faster engines. Moreover, the burnout in the modifications is so rapid that actual air staging may not be needed, and in-furnace mixing delays could be exploited instead. A last remark concerns the application of SNCR in CS. Our simulations indicate that SNCR contributes only marginally to reducing NOx when less than 1000 ppm of agent are supplied, in which case less agent is supplied than what forms via Reactions 8-16 and 22-24. Nonetheless, SNCR remains an excellent option for consuming N-pollutants from the reburn fuel, i.e., by using them as reducing agents for the primary NOx. The wide range of stoichiometry and temperatures applicable to CS makes possible using any N-pollutant as SNCR agent. Moreover, if reburn fuels and agents can be selected, best option is the one where the availability of agent is delayed in respect to its delivery with the fuel. This would allow for all primary air to react with the fuel, thus avoiding undesirable oxidation of precious agent. As suggested in refs 25 and 49, this can be achieved with agent precursors that need time for evaporating or decomposing. 3. Model Assumptions The present paper introduces the combined staging (CS), a new method that combines reburning, SNCR, and air-staging in a synergy against NOx. The arguments and details proposed around CS in Section 2 and Figures 2-8 were deduced from kinetic models. In this section, we discuss what we consider being the most critical assumptions set in the models. However, the models used for this work will be not illustrated in detail here, and for more concise information the reader is readdressed to the referred literature. The fuel was modeled as a mixture of pure gases determined via an in-house procedure. The composition

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of gases after primary combustion was calculated according to thermodynamic equilibrium, while reactions at reburn and burnout were calculated according to gasphase detailed chemical kinetics. Reaction kinetics were deduced from the detailed mechanism illustrated in ref 73. Kinetic equations and flow pattern were solved under ideal plug flow conditions with the software Senkin74 of the Chemkin-II suite.75 More details on the assumptions for fuel composition, reaction kinetics, and flow pattern are discussed in the three following subsections, respectively. In brief, and on the bases of independent model testing accrued in the past years,10-25,35,38,49,58,76,77 we estimate that no model uncertainty should invalidate the qualitative arguments around combined staging. However, no experimental verification has been conducted for the specific conditions of CS, and thus we cannot quantify the uncertainties. 3.1. Fuel Composition. The model fuel in this work was a synthetic mixture that mimics a variety of bioderived fuels, including black liquor (BL). Black liquor is a liquid byproduct of pulping that contains wood constituents and pulping chemicals. Consequently, BL contains complex organic molecules (O-C-H-N), moisture (H2O), sulfur compounds (S-), salts of sodium and potassium (Na-, K-), as well as compounds of calcium (Ca-), iron (Fe-), and other elements. When fed into a boiler, BL undergoes physical-chemical transformations that turn most of it into gas-phase, and then burns. Detailed kinetics were applied here only to the gasphase chemical reactions, while the liquid-to-gas conversion was accounted for via in-house correlations. These correlations estimate the amount of gases relevant to combustion and NOx, i.e., carbon monoxide (CO), hydrogen (H2), hydrocarbons (CxHy), water vapor (H2O), nitrogen (N2), ammonia (NH3), and oxygen (O2). Two key assumptions are made in such correlations: (i) all hydrocarbons are assumed in form of ethylene (C2H4), and (ii) no compounds of S, Na, K, Ca, or Fe are taken into account. The first assumption is based on the observation that, while hydrocarbons are crucial in NOx-reducing techniques,20,37 the effect of CxHy relates to their content of C rather than their speciation.17,35 Hydrocarbons speciation affects NOx reduction only when fuels do not ignite promptly and vigorously, which never occurred in any case modeled in this work. The second assumption is based on the fact that, while many oxides and salts of S, Na, K, and Ca are known to restrain NOx formation and to promote its reduction, their effect is reported negligible under the conditions in our simulations.20,25,28,67,78-80 As for Fe, recent studies suggest that the effect of iron might be not negligible56,81 and should be investigated further. 3.2. Reaction Kinetics. As indicated above, reaction kinetics were deduced from the gas-phase detailed mechanism in ref 73. This very mechanism has been used successfully for the past years to simulate high-temperature reburning, SNCR, and air staging,5,35,45,73,82,83 which encourages us to consider it a valid choice here. Nonetheless, we note the followings: (i) heterogeneous reactions are not accounted as we believe them negligible in the devices of interest here, although we cannot exclude that surface-to-gas reactions might occur,33,84 (ii) reactions NO + CO f ... are neglected here, although these have been suggested in some recent

studies,85 (iii) a little outdated kinetic coefficients are attributed to the reburning reactions CH2 + NO f ...,85 and (iv) kinetics for the reactions HNO + ... f NHi + ..., HCNO + ... f ..., and N2O + CO f N2 + CO2 are uncertain as poorly known to date.10,11,13,19,58,84,86-88 3.3. Flow Pattern. The gas flow was modeled as an ideal plug flow, where fuel and air mix instantaneously with the bulk of burning gases. Such simplistic assumption is known to be appropriate for simulating SNCR and air staging. As for burnout, the ideal mixing assumption is appropriate for the conditions relevant to the CS as proposed in this paper, while it is known to be inadequate for other burnout techniques, i.e., at air-to-fuel ratio very close to stoichiometric21,45,58,84,86 and temperature above 1500 K.17,19,30,53,57,89 In this case, models should account for a progressive mixing of the staged burnout air, thus allowing more accurate predictions of the selectivity of hydrogen cyanide (HCN) and residual amines (NHi) to form NO rather than N2.19,21,33 There, models such as the Zwietering approach,21,28,57,58,86,89,90 the complex networking of ideal reactors,45,91,92 or one of the emerging models that solve both fluid dynamics and chemical kinetics93,94 should be used instead of the instantaneous mixing model used in this work. 4. Conclusions Fuel staging (FS), air staging (AS), and selective noncatalytic reduction (SNCR) are established techniques for abating the emissions of NOx (NO + NO2) from boilers and engines (Section 1). Individually each technique attains 50-70% NOx reduction, but can be applied only within a limited range of conditions. Higher reduction is achieved by methods that use the aforesaid techniques in sequence, thus accumulating the reduction ability of each technique, but also collecting all their applicability limits. As a consequence, the reduction of NOx by established techniques and sequential methods is ineffective in a number of combustion devices. In this paper, we have described a new method that is based on the synergic combination of FS, AS, and SNCR. We call this method “combined staging” (CS) (Section 2). Combined staging differs from others since it pursues the reduction of NOx via hydrogen cyanide (HCN) as the key intermediate (Figure 1). This is achieved as follows. A primary fuel is burned with excess air, thus forming NO (primary combustion). Then, a secondary fuel is introduced to reset reducing conditions, under which the NO is converted to HCN (reburn). Finally, tertiary air is added in a staged manner, so as to maximize the reduction of HCN to N2 (staged burnout). Further NOx reduction is achievable by optional SNCR, whose reducing agent and point of feeding are widely flexible. The performance of CS was tested with detailed chemical kinetic models. According to our simulations, CS works best when the reburn portion is set at temperatures between 1300 and 1700 K, in plainly reducing conditions (SRII < 1), and the reburn fuel carries hydrocarbons (Figures 2 vs 5 vs 6). Moreover, the burnout portion is best operated at temperatures between 1100 and 1300 K (Reaction 8-24 and Figure 3). The models also indicate that CS cannot perform without staging the burnout air (Figure 4). In no case CS leads to any other N-emission than NOx. Models also indicate that CS can reduce over 40% NOx from devices and in conditions where other tech-


niques and methods fail. Specifically, CS would be effective in devices too cold for applying reburning, e.g., kraft recovery boilers, fluidized bed combustors, and low-grade fuel combustors. Owing to its rapid chemistry, CS would also allow erecting smaller boilers, or could be applied in fast engines and to small domestic boilers (Figure 1). In the latter case, the method could be further improved by (i) optimizing the resident time for reburn and burnout (Figure 7) or (ii) cautiously setting the burnout temperature (Figure 8). We underline that the claims in this paper are the result of chemical kinetic simulations, whose validity carries uncertainties about the fuel composition, the reaction kinetics, and the mixing limitation in furnaces (Section 3). Acknowledgment This work is part of the activities of the Åbo Akademi Process Chemistry Centre, partly funded by the Academy of Finland within their “Centres of Excellence” program. Literature Cited (1) Zeldovich, Y. B. Acta Physiocochim. 1946, 21, 577 (in Russian). (2) Bowman, C. T. Kinetics of pollutant formation and destruction in combustion, Prog. Energy Combust. Sci. 1975, 1, 33-45. (3) Fenimore, C. P. Studies of fuel-nitrogen species in rich flame gases. Proc. Combust. Inst. 1978 17, 661-670. (4) Malte, P. C.; Pratt, D. T. Measurement of atomic oxygen and nitrogen oxides in jet-stirred combustion. Proc. Combust. Inst. 1974, 15, 1061-1070. (5) Coda Zabetta, E.; Kilpinen, P. Improved NOx submodel for in-cylinder CFD simulation of low- and medium-speed compression ignition engines. Energy Fuels 2001, 15, 1425-1433. (6) Winter, F.; Wartha, C.; Hofbauer, H. NO and N2O formation during the combustion of wood, straw, malt waste and peat. Biores. Technol. 1999, 70, 39-49. (7) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Some aspects of fuel nitrogen conversion in solid fuel fired systems. Proc. 6th Int. Conf. Technol. and Combust. for a Clean Environ. 2001, Oporto, Portugal, July 9-12. (8) Fernandez, J. H.; Sensenbaugh, J. D.; Peterson, D. G. Boiler emissions and their control. Proc. Air Pollution Control 1966. (9) Wendt, J. O. L.; Sternling, C. V.; Matovich, M. A. Reduction of sulfur trioxide and nitrogen oxides by secondary fuel injection. Proc. Combust. Inst. 1973, 14, 897-904. (10) Dagaut, P.; Lecomte, F.; Chevailler, S.; Cathonnet, M. Experimental and detailed kinetic modeling of nitric oxide reduction by a natural gas blend in simulated reburning conditions. Combust. Sci. and Technol. 1998, 139, 329-363. (11) Dagaut P.; Luche, J.; Cathonnet, M. Reduction of NO by n-Butane in a JSR: experiments and kinetic modeling. Energy Fuels 2000, 14, 712-719. (12) Dagaut, P.; Lecomte, F.; Chevailler, S.; Cathonnet, M. Experimental and kinetic modeling of nitric oxide reduction by acetylene in an atmospheric pressure jet-stirred reactor. Fuel 1999, 78, 1245-1252. (13) Dagaut, P.; Lecomte, F.; Chevailler, S.; Cathonnet, M. The Reduction of NO by Ethylene in a Jet-Stirred Reactor at 1 Atm: Experimental and Kinetic Modelling. Combust. Flame 1999, 119, 494-504. (14) Dagaut, P.; Luche, J.; Cathonnet, M. Experimental and kinetic modeling of the reduction of NO by Propene at 1 atm. Combust. Flame 2000, 121, 651-661. (15) Dagaut, P.; Luche, J.; Cathonnet, M. Reduction of NO by propane in a JSR at 1 atm: experimental and kinetic modeling. Fuel 2001, 80, 979-986. (16) Dagaut, P.; Lecomte, F. Experiments and kinetic modeling study of NO-reburning by gases from biomass pyrolysis in a JSR. Energy Fuels 2003, 17, 608-613.

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Received for review January 14, 2005 Revised manuscript received April 19, 2005 Accepted April 22, 2005 IE050051A

Reducing NOx Emissions Using Fuel Staging, Air Staging, and

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