Experimental Investigation of Nitrogen Species Distribution in Wood

Article pubs.acs.org/EF

Experimental Investigation of Nitrogen Species Distribution in Wood Combustion and Their Influence on NOx Reduction by Combining Air Staging and Ammonia Injection Kristina Speth,*,† Martin Murer,‡ and Hartmut Spliethoff†,§ †

Institute for Energy Systems, Technical University of Munich, Boltzmannstr. 15, 85748 Garching b. München, Germany Martin GmbH für Umwelt- und Energietechnik, Leopoldstraße 248, 80807 München, Germany § ZAE Bayern, Walther-Meißner-Straße 6, 85748 Garching b. München, Germany ‡

ABSTRACT: The formation of the nitrogen species HCN, NH3 (N-intermediates), and NO out of fuel-bound nitrogen has a major influence on NO chemistry. Experiments have been carried out on an entrained flow reactor with pulverized wood as fuel. Staged combustion establishes a fuel-rich primary zone, where both N-intermediates and NO exist. The introduction of NRP as the ratio of the N-intermediates to NO offers a parameter that describes the nitrogen distribution in the primary zone, whereas TFN describes the overall amount of nitrogen. Air staging is an effective method for NOx reduction; the main controlling parameter is the primary air ratio, which defines both NRP and TFN. In fuel-rich conditions, NRP exceeds 1; with increased oxygen availability and temperature, the N-intermediates are depleted and NO is formed (NRP < 1). Thus, the NRP can be increased by adding NH3. Conventional SNCR is strongly temperature-dependent; hence, with increased temperatures, the best operation point shifts to lower air ratios. A combination of air staging and ammonia injection directly in the primary zone furthers NOx reduction, as long as it is realized in almost stoichiometric conditions. Since the reduction efficiency increases at high temperatures, the technology is called selective high temperature reduction.

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INTRODUCTION Nitrogen oxides (NOx) have a detrimental impact on the environment like acid rain, photochemical smog, and destruction of ozone, which is why emission limits are steadily tightened. During thermal decomposition, mainly NO is formed, as well as smaller amounts of NO2.1,2 As NO reacts to NO2 in atmospheric conditions, both are calculated as NO2 and summarized to NOx. Although the established NOx reduction methods achieve good results, stringent emission limits strengthen the need to develop new technologies. As there are several overlapping reaction paths of NO formation and reduction, these mechanisms are not easily understood. Though research activities started years ago, there are still some key questions that remain to be answered. Especially, the release of fuel-bound nitrogen, and the distribution of nitrogen species to HCN, NH3, and NO during air-staged combustion have received comparatively little attention. This might be due to the challenges of in-furnace measurements under reducing conditions. Nevertheless, the impact of nitrogen distribution on the functionality of NOx reduction technologies is significant. Hence, the principles of NO-forming reactions and NOx reduction mechanisms must be taken into consideration to gather a better understanding of NO chemistry. Generally, three NO formation mechanisms are known (see Figure 1). For more detailed information on nitrogen chemistry in combustion, see Miller et al.3 An elaborate overview on fuel nitrogen conversion is given by Glarborg et al.4 Thermal NO5 originates from molecular air nitrogen, whose strong triple bond is broken up at high temperatures. Depending on oxygen availability and residence time, thermal NO starts to form at temperatures above 1570 K,1 and significant amounts are © 2016 American Chemical Society

Figure 1. Simplified mechanisms of NO formation based on Miller et al.3

formed at temperatures above 1800 K.6 Prompt NO is as well formed from air nitrogen, but in fuel-rich conditions including CHi-radicals.7 Both mechanisms are negligible in biomass incineration because of the lower temperatures. In this case, fuel NO is the dominating NOx source.4,8 According to fuel conversion, fuel-N conversion consists of several steps, beginning with devolatilization of the fuel particles, followed by oxidation of gaseous species and oxidation of tars and char (both neglected in Figure 1). During devolatilization, the molecular nitrogen bound in the fuel is partially released to light gaseous species and tars, and partially Received: April 19, 2016 Revised: June 1, 2016 Published: June 20, 2016 5816

DOI: 10.1021/acs.energyfuels.6b00943 Energy Fuels 2016, 30, 5816−5824

Article

Energy & Fuels

Figure 2. Simplified configurations of air staging, fuel staging, selective noncatalytic reduction (SNCR), and selective high temperature reduction (SHTR).

depends on the type of fuel, temperature, and air ratio and has a major influence on the overall conversion of fuel-N to NOx. Several technical approaches have been established to lower NOx emissions, whereby technical methods can be classified into combustion engineering methods applied directly to the region where the fuel is burned, and secondary methods applied downstream.2,19 Secondary methods lead to better reduction efficiencies but generally have high investment costs. In the following, air staging and selective noncatalytic reduction (SNCR) are discussed in detail, as the study at hand introduces a combined technology termed selective high temperature reduction (SHTR). Further information on other NOx reduction technologies can be found elsewhere;2,20−22 a schematic overview of some technologies is given in Figure 2. To evaluate and compare different operation points, we propose the introduction of the NOx reduction potential (NRP) calculated by eq 1. The differences compared to the common total fixed nitrogen (TFN) calculated by eq 2 are discussed later. Both are calculated based on species concentrations in parts per million (ppm).

remains in the solid phase (char). The fraction of nitrogen that remains in the char as well as the fraction of nitrogen that is transformed to NO depends on fuel properties and operating parameters like temperature and residence time.4 A lot of experimental data has been published during the last years with a focus on nitrogen conversion in coal combustion, whereas less data is available for biomass. A brief overview of nitrogen evolution is given by Leppälahti et al.9 Literature data show that the fraction of volatile-N increases with increased temperature and that biomass tends to release the nitrogen species at lower temperatures than coal. Storm et al. found that about 80% of the fuel-N is released at temperatures above 900 K during pyrolysis of straw and wood;10 according to Winter et al., the distribution of volatile-N to char-N during the combustion of beech at 1070 K is 62% and 38%, respectively, whereas char-N mainly produces NO.11 These data are supported by Darvell et al., who achieved a volatile-N percentage of 79−91%, and found out that char causes mainly NOx and N2.12 The main light gaseous species are HCN and NH3,4 whereas mainly HCN is released during the pyrolysis of bituminous coal,9,13 and mainly NH3 is released during pyrolysis of low rank coals9,13 and biomass.9 Bassilakis et al. stated that slower heating rates increase the NH3 concentration, whereas fast heating rates increase the concentration of HCN.14 They assume that HCN is transferred to NH3 as it diffuses through the particle,14 which is supported by Leppälahti.15 Furthermore, the influence of alkaline species on nitrogen species distribution has been discussed elsewhere.16 It has to be admitted that experimental data of inert pyrolysis might not be comparable to those under oxidizing conditions,17 as the atmosphere has a huge impact on nitrogen evolution.18 This work does not distinguish between gaseous nitrogen compounds that are released with the volatiles, out of tars, or out of char, as the focus is not on the investigation of fuel properties. In the following, HCN and NH3 are termed as Nintermediates. The N-intermediates are subsequently converted into NHi-radicals4 that either can be oxidized to NO in oxygenrich conditions or can themselves work as a reduction agent and reduce already formed NO to elementary nitrogen; hence, NHi-radicals are the driving force in NOx reduction. Further information on nitrogen species distribution and radical chemistry can be found in the literature. As both HCN and NH3 can finally lead to the formation of either NO or N2, the often used term “NO precursors” is misleading; hence, Nintermediates seems to be an appropriate denomination. The conversion of fuel-bound nitrogen to the intermediate species

NRP = (HCN + NH3)/NO

(1)

TFN = HCN + NH3 + NO

(2)

Air staging is one of the most economical23 combustion engineering methods that has proven to effectively reduce NOx and has been widely discussed both for gas flames24−26 and solid fuels.8,13,27−34 Herein, the furnace is clearly divided into two zones. In the primary zone, reduction conditions prevail due to a lack of oxygen. In the burnout zone, additional air is injected and full fuel conversion is reached. The NOx emissions are strongly dependent on the air ratio and the residence time in the primary zone. Emissions decrease with a longer residence time in the primary zone35 and reach their minimum, as long as the primary air ratio is set to the point where TFN is minimal.2 It is suggested that the optimal air ratio is independent of fuel type, source of fuel nitrogen, and residence time and that the best NOx reduction occurs at λ1 0.75−0.85.2 Taking into account these two parameters, air staging achieves a NOx reduction of 50−80% (depending on the nitrogen content of the fuel),36,37 but can lead to slightly increased CO concentrations and unburned carbon.2 In some cases, air staging does not meet existing legal requirements due to fuel properties (e.g., the distribution of volatile-N and char-N), high initial NO concentrations, plant-specific limits in the reduction of the primary excess air level, or too short residence times in 5817


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Energy & Fuels Table 1. Ultimate and Proximate Analysis of the Wooden Fuel (% dry ash-free basis)

(% raw)

volatiles

C

H

N

S

O

ash

water

O/N

84.9

48.85

6.15

0.15

0.18

44.85

0.99

6.6

300

NO of 1.9 to determine the influence of stoichiometry on NOx reduction. • Air staging (AIRST) experiments with different air ratios λ1 in the primary zone at wall temperatures of 1270 and 1670 K to measure the N-intermediates distribution and to determine the NRP. The data are used to explain the functionality of NOx reduction mechanisms in the gas phase. • Selective high temperature reduction (SHTR) experiments at a wall temperature of 1270, 1570, and 1670 K to investigate the interrelation of the NRP and NOx reduction. Variation of residence time τRed, momentum ratio J, and ratio β of injected NH3 to initial NO to determine their influence. The research background of this study is the investigation of NOx formation and reduction in grate-based waste incineration plants. Hence, the aim of the study is not the in-depth discussion of the thermodynamic and chemical processes in the flame, nor the detailed measurement of nitrogen species, nor the discussion of radical chemistry. Rather, the focus is on the investigation of NOx formation during air staging and the practical limitations of NOx reduction processes utilizing ammonia. The entrained flow reactor enables an easy control of temperature and the adjustment of distinct boundaries between the combustion zones according to Figure 2. Milled wood is used because of its availability and suitability for the experimental setup. The fuel studied had a mean particle size of 120 μm. The elemental analysis is shown in Table 1. A schematic overview of the experimental reactor is shown in Figure 3.

the reduction zone. Therefore, additional measures need to be applied. Alternatively, during SNCR as a secondary method, the NHi concentration is increased by adding a reduction agent to the furnace downstream the burning zone (see SHTR in Figure 2). The reduction agent can be either ammonia or urea, whereas both are normally injected to the furnace in aqueous solution. The reduction efficiency of SNCR is highly variable for different operation conditions and applications; hence, best practice efficiencies released in the literature vary drastically. Reduction rates of 65−84%38,19,39,40 have been indicated. In general, the reactions are strongly temperature-dependent, which is why the injection has to take place at 1100−1300 K.3 Lower temperatures cause ammonia slip and inhibit reduction reactions; higher temperatures lead to oxidization of the reduction agent which results in the formation of additional NOx.41 As both air staging and SNCR are limited reduction technologies, this paper investigates the combination of air staging and SNCR by injecting ammonia as a reduction agent directly to the primary zone (see Figure 2). NH3 is a combustible molecule; hence, ammonia injection can be seen as a method of fuel staging, even though the stoichiometry is not significantly influenced. Despite the fact that similar ideas42−46 are fairly old, only few experimental data47,48 are available for a combination of ammonia injection and air staging, whereas the ammonia injection in combination with fuel staging (termed advanced reburning),47,49−52 has gathered more attention. A combination of fuel staging, air staging, and ammonia injection has been discussed theoretically53 and experimentally.41 Air and fuel staging (see Figure 2) are comparable in terms of providing a fuel-rich zone; hence, it can be assumed that ammonia injection triggers comparable reactions. The optimum temperature generally shifts to lower temperatures as long as the reduction agent is injected in oxygen-rich conditions when CO increases.41 The lower temperature might hinder burnout and CO oxidation; thus, the temperature dependency is very distinct.41 As the oxygen concentration decreases to only a few hundred ppm, the reduction agent cannot be oxidized, which is why this method achieves good reduction efficiencies at higher temperatures.41,47 For that reason, the method is referred to as selective high temperature reduction (SHTR). To understand the ongoing chemical reactions more completely, the N-intermediates HCN and NH3 are measured. Particles in the primary combustion stage have not been analyzed as not the fundamental combustion processes, but the gas-phase functionality of the NOx reduction mechanism is emphasized.

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Figure 3. Experimental setup of the entrained flow reactor. The entrained flow reactor consists of a ceramic tube, which can be electrically heated to temperatures of up to 1770 K. The given temperatures during the discussion of experimental results refer to average wall temperatures, since set-point temperature and wall temperature can differ when the set point is set to temperatures below 1270 K. Higher flame temperatures increase the gas temperature and wall temperature; thus, the mean wall temperature exceeds the setpoint temperature. The pulverized fuel is fed to the fuel inlet via vibrating chutes; it enters the reaction zone together with an air stream for means of transportation. Another air stream is injected through a concentric tube enclosing the fuel-carrying tube, which is swirled at the burner outlet. Both air streams are summarized as primary air. In air staging experiments, the air ratio in the primary zone λ1 can be influenced by

EXPERIMENTAL SECTION

Four types of experiments were carried out on an entrained flow reactor: • Combustion experiments with overall air ratios λtot of 1−1.6 at wall temperatures of 1270 and 1670 K in order to find out the initial amount of NOx emissions. • Selective noncatalytic reduction (SNCR) experiments at 1170, 1270, and 1670 K at a constant ratio β of injected NH3 to initial 5818


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Energy & Fuels adjusting the ratio of fuel to primary air. The total air ratio λtot is adjusted by secondary air that is injected through one of the ports vertical to the gas flow direction. Ammonia injection also occurs through one of the ports; hence, N2 is utilized as a carrier gas to improve mixing. Simultaneously, N2 is used to adjust the momentum ratio J of the flue gas flow and the injected cross flow at a constant value; hence, the ratio can be calculated according to Østberg et al.54 The ports are numbered in gas flow direction. The relevant residence times during the different experiments are estimated based on gas residence times from the fuel inlet to either the port where the gas probe is extracted (in combustion experiments), to the port where ammonia is injected (in SNCR experiments), or the port where secondary air is injected (in AIRST and SHTR experiments). In SHTR experiments, ammonia is injected through the first port, secondary air through the second, and the last probe is used for gas extraction. The main focus of this investigation is the distribution of the nitrogen species NO, NH3, and HCN to determine the NOx reduction potential at different operation points. The measurement setup was most complex for the measurements of HCN and NH3; hence, this setup is explained in the following. As the measurement configuration should not only be used in the lab but also in real waste incineration plants under very challenging conditions, a very robust and simple technology was needed. Hence, a wet chemical sampling method was chosen, even though more sophisticated measurement technologies are available. Independent of the analytical method, NH3 and HCN probe sampling is ambitious due to the very high solubility of the molecules and the interaction of ammonia with particles. Both mechanisms cause slip and lead to an underestimation of the quantities. Krüger et al. stated that temperatures above 670 K have to be adhered to during the whole gas probe extraction and particle separation to prevent an underestimation of gaseous NH3.55 Broer et al. discussed problems occurring in wet chemical HCN probe sampling.56 They found out that NH3 and HCN must not be measured in a row since HCN is soluble in acid solutions and that HCN is underestimated in most of the in-furnace measurements in general. Previously, we stated that the influence of the gas flow rate has a huge impact on the absorption efficiency, whereby NH3 is less sensitive and HCN is entirely absorbed as long as the gas flow rate does not exceed 1 L/min.57 It has to be admitted that these investigations are representative for the specific measurement setup and are not generally valid as the size of the bottles, the pore size of the frits, and the amount of absorption solution affect the absorption. A heated steel probe was designed to extract gas samples from the reactor (see Figure 4). The electrical heating guarantees tempering of

concentration of NH4+, we assume that this overestimation of NH3 is negligible. Each bottle was filled with a 100 mL absorption dissolution. The bottles were connected in rows of three, respectively. The gas flow through each line of bottles was set to approximately 0.95 L/min, the gas volume was measured by a Desaga (type 312 or 212), and each sampling lasted 10 min. The analysis of the dissolutions was carried out afterward in a laboratory using cuvette tests by Hach which were analyzed in a photometer (Cadas 100 by Dr. Lange). Downstream of the ammonia absorption, the O 2 and NO concentrations were measured online with paramagnetic and nondispersive infrared sensors, respectively. Regular flue gas measurements were carried out with a comparable setup, but with reduced temperatures and without washing bottles. It is worth mentioning that a similar, but scaled up, measurement setup has been designed and tested during in-furnace measurements in a conventional waste incineration plant with good results.

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RESULTS Combustion experiments were carried out to determine the initial NO x concentrations in relation to the overall stoichiometry λtot. The results are shown in Figure 5; concentrations are related to 11% O2 on a dry basis. It can be seen that the NOx emissions

Figure 5. NOx emissions for different air ratios λtot at 1270 and 1670 K. NOx increases with temperature and oxygen availability.

increase with air ratio and temperature. The increase with oxygen availability shows an almost linear correlation and can be attributed to oxidizing reactions of the N-intermediates. At high temperatures and at high air ratios, the formation of thermal NO increases the total NOx emissions. Low NOx emissions compete with complete fuel burnout and increased CO emissions;32 hence, slightly fuel-lean conditions prevail in most combustion processes. On the basis of these fundamental combustion experiments, the reduction efficiency of SNCR was determined at different temperatures and air ratios. The ratio β of injected ammonia to initial NO was set to a constant value of 1.9 during all experiments unless it is noted. We want to emphasize that β and NRP do not necessarily have the same value, since we detected ammonia even under slightly fuel-lean conditions. Therefore, the actual NRP exceeds β. The residence time in the reduction zone τred was set to 1.7 s. The overall NOx reduction was calculated based on the initial NOx emissions given in Figure 5. The results in Figure 6 show the strong temperature dependency of NOx reduction by ammonia injection. It can be seen that the best reduction of 50% was achieved at a temperature of 1170 K and a stoichiometry λtot of around 1.3. At a higher stoichiometry, the amount of available oxygen increases and NO formation mechanisms are promoted instead

Figure 4. Measurement setup. HCN and NH3 are absorbed in liquid solutions; NO and O2 are detected by a nondispersive infrared (NDIR) and paramagnetic (PM) sensor, respectively. the gas sample to temperatures over 620 K. The gas sample passes the probe and reaches an electrically heated filter type FSS-3SS-H350 by M&C TechGroup. The filter temperature was set to the maximum of 620 K, which is still below 670 K recommended by Küger et al.58 On the basis of previous measurements, including particle analysis, we assume that the ammonia-particle interaction is negligible under the given circumstances even at these slightly decreased temperatures, especially since wood contains only a small amount of ash. The gas sample was divided into two streams after particle separation. One stream was led through washing bottles containing 0.05 M H2SO4 to absorb ammonia, and the other was led through 0.1 M KOH to absorb cyanide. While the hydrolysis of HNCO might increase the 5819


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Figure 6. NOx reduction with SNCR at a constant ratio of NH3 to initial NO β = 1.9 for different air ratios. Residence time of the ammonia τred = 1.7 s. NOx reduction shifts to lower air ratios by increasing the temperature.

Figure 8. Nitrogen species in the primary zone for different primary air ratios λ1 at 1670 K. Residence time in the primary zone τ1 = 1.7 s.

under the given circumstances, the concentration of both NH3 and HCN decreases with increasing stoichiometry. The amount of NO increases with increased oxygen availability. Nintermediates are depleted at higher temperatures; the concentration of NO at 1670 K is slightly higher at a high stoichiometry due to thermal NO formation and promoted oxidizing reactions. Aho et al. stated that the HCN/NH3 ratio during pyrolysis decreases with an increased ratio of fuel-O/fuel-N up to the value of 20 and remains roughly constant at higher ratios.60 Compared to their data, the fuel-O/fuel-N ratio in the study at hand is even greater (see Table 1), which indicates that the oxygen availability is very high and HCN might be decomposed. As opposed to the findings of Aho et al., Hansson et al. suggested that the distribution of nitrogen species is independent of fuel-O/fuel-N ratio, but rather depends on the functionality of the nitrogen species in the fuel.61 Winter et al. assumed that the HCN/NH3 ratio depends on the fuel-H/ fuel-N ratio.11 Ren et al. proposed interactions between the mineral matter and nitrogen-containing amino acids that have been used as model compounds for nitrogen-containing species in biomass.62 Further research is necessary to clarify the formation mechanisms, ensuring that issues associated with the measurement of N-intermediates are taken into account. On the basis of the results shown above, the resulting NOx emissions caused by air staging are discussed in detail taking into account TFN and NRP in the primary zone. Experiments were carried out at a total air ratio λtot of 1.15; consequently, the initial NOx emissions are 240 mg/Nm3 at 1270 K and 330 mg/Nm3 at 1670 K (see Figure 5). NOx emissions during air staging show a characteristic minimum at a specific primary air ratio λ1 of about 0.75−0.85.2 The results confirm these findings; the minimal emissions during 1270 K experiments were achieved at a primary stoichiometry λ1 of about 0.8 (see Figure 9). TFN decreases slightly and NRP decreases significantly with increased oxygen availability. The NRP is above 1 as long as λ1 is below 1. At the optimal air ratio of 0.8, NRP exceeds 4, whereas TFN is about 180 ppm. At higher temperatures (see Figure 10), the minimal NOx emissions were as well accomplished at a primary stoichiometry λ1 of about 0.8. TFN decreases slightly in fuel-rich conditions due to the radical-forming reactions mentioned above, but increases at higher air ratios due to thermal NO formation. A TFN minimum occurs at a stoichiometry λ1 of 0.85. The NRP decreases significantly with increased oxygen availability and remains above 1 as long as λ1 is below 0.9. At the optimal air ratio of 0.8, NRP exceeds 4, whereas TFN is about 120 ppm.

of reduction mechanisms. Therefore, higher temperatures lead to lower NOx reduction efficiencies and the best operation point shifts to slightly lower air ratios. According to Miller et al., SNCR allows for low NOx emissions as long as the reduction agent is injected at temperatures between 1100 and 1300 K.3 Our results show that the reduction decreases significantly at a temperature of 1270 K; hence, the strong temperature dependency of the SNCR technology can be confirmed, which obviously results in difficulties during the operation of SNCR processes. Fluctuating temperatures in the furnace hinder controlling and require several zones of reduction agent injection. Otherwise, low NOx emissions can only be reached at the expense of an increased consumption of the reduction agent, which in turn can cause ammonia emissions. At a very high temperature of 1670 K, NOx reduction can only be achieved at almost stoichiometric conditions, since high temperatures in combination with high air ratios lead to additional NOx formation, which causes a negative NOx reduction. The steep increase in NOx reduction with lower air ratios raises the question, whether it is possible to further improve the NOx reduction at high temperatures by injecting ammonia under fuel-rich conditions. Fuel-rich conditions prevail in the primary zone of air staging, therefore the NOx reduction mechanisms of bare air staging are initially discussed. Compared to SNCR, air staging is a reasonably simple and economical technology to reduce NOx.23 To enhance the understanding of the conversion of gaseous nitrogen species during air staging, the main nitrogen species NO, NH3, and HCN were measured over a broad range of primary air ratios λ1. The general trends of the single species are comparable for both 1270 and 1670 K (see Figures 7 and 8) and are consistent with the literature.13,28,50,59 Even though HCN is negligible

Figure 7. Nitrogen species in the primary zone for different primary air ratios λ1 at 1270 K. Residence time in the primary zone τ1 = 1.7 s. 5820


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in the formula. The minimum of NOx emissions after secondary air injection can be explained by examining the nitrogen distribution in very fuel-rich and very fuel-lean conditions: • In fuel-rich conditions, the NRP is very high, which means that, arithmetically, there are distinctly more Nintermediates available than NO. Only a minor fraction of the N-intermediates can react with NO to N2; thus, the N-intermediates themselves are oxidized to NO during secondary air injection. Additionally to the gas phase reactions, the amount of nitrogen remaining in the char in fuel-rich conditions is higher than that in fuel-lean conditions.13 No char samples were analyzed during these experiments, but the findings of Winter et al. show that beech wood has a distribution of 68% volatile-N and 32% char-N.11 Therefore, char-N can form additional NOx during char burnout, but the contribution to the overall NOx emissions is assumed to be small in biomass combustion.9 • In fuel-lean conditions, the NRP is below 1, which means that, arithmetically, there are not enough N-intermediates available to reduce already formed NO. Consequently, NO cannot be reduced thoroughly and is transferred from the primary to the secondary zone without reaction. Furthermore, the findings concerning low temperature (1270 K) and high temperature air staging (1670 K) can be explained as follows: • Air staging at both temperatures achieves the lowest NOx emissions at a primary air ratio of about 0.8. The proper stoichiometry seems to be temperature-independent. • Identical emission levels can be reached for both 1270 and 1670 K at the optimal operation point. The NOx emissions during high temperature air staging increase distinctly at high air ratios, probably due to the formation of thermal NO. • The NRP slightly shifts to fuel-rich conditions at high temperatures due to promoted oxidizing reactions at higher temperatures. Therefore, the N-intermediates are distinctly depleted at high temperatures, especially as long as oxygen is available. The sharp drop of NRP strengthens the need for additional measurements with a higher resolution of stoichiometry, especially in the range of air ratios λtot from 0.7 to 1.15. Concluding SHTR experiments were carried out by combining air staging and ammonia injection to investigate the NOx reduction of both bare air staging (AIRST) and SHTR (see Figure 11) in dependency of the primary air ratio and the resulting NRP. The dashed lines indicate the stoichiometry where NRP = 1; the NRP increases at lower primary air ratios, and decreases at higher primary air ratios, as can be seen in Figures 9 and 10. Ammonia injection increases the NRP. First, the NOx reduction achieved by air staging is discussed (triangular symbols in Figure 11). At both temperatures, the NOx reduction is maximal at a primary air ratio λ1 of 0.8, whereby the reduction seems to be more sensitive to stoichiometry during high temperature combustion (filled symbols in Figure 11), as a steep decrease in reduction efficiency occurs. The reduction efficiency increases at higher temperatures, as the initial NOx concentration increases (see Figure 5), and the overall NOx emissions are comparable for both temperatures (see Figures 9 and 10). SHTR experiments

Figure 9. NOx emissions, total fixed nitrogen (TFN), and NOx reduction potential (NRP) for different primary air ratios λ1 at 1270 K. Residence time in the primary zone τ1 = 1.7 s. NOx and TFN are related to 11% O2. Lines represent polynomial fittings.

Figure 10. NOx emissions, total fixed nitrogen (TFN), and NOx reduction potential (NRP) for different air ratios λ1 at 1670 K. Residence time in the primary zone τ1 = 1.7 s. NOx and TFN are related to 11% O2. Lines represent polynomial fittings.

The air staging experiments show that the primary air ratio is the main controlling parameter of NOx reduction. Independent of the temperature, the minimum of NOx emissions occurred at a primary air ratio of about 0.8 during these experiments. Both NRP and TFN result out of the prevailing conditions in the primary zone and are mainly influenced by the air ratio. At 1670 K, the NO minimum and the TFN minimum occur at almost the same air ratio, which has already been stated elsewhere,2 whereas, at 1270 K, no minimum of TFN can be seen. For both temperatures, the NRP at the optimal air ratio was around 4−5. A comprehensive comparison with other data is not possible due to a lack of in-furnace measurements of the Nintermediates distribution in the primary zone. Nevertheless, the data of Chen et al.13 indicate that the optimal air ratio depends on the type of fuel and that the NOx minimum does not necessarily occur in synergy with the minimum of TFN. During their experiments with bituminous coal and lignite, the NRP at the optimal air ratio was around 2 for both coals, even though the NO minimum for the bituminous coal occurred at an air ratio of 0.75 and the NO minimum of lignite occurred at an air ratio of 0.65. The differences between the NRPs of coal compared to wood might be due to fuel properties, e.g., particle size and composition, mixing effects, residence times, temperatures, and distribution between HCN and NH3. As the HCN concentration during these experiments was insignificant, the NRP is not affected by HCN. Others stated that HCN does not appreciably influence NOx reduction41 or is less effective compared to NH3;63 hence, further investigation is necessary to either weight the factors of NH3 and HCN in dependency of their contribution to NOx reduction or completely ignore HCN 5821


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Figure 11. NOx reduction with air staging (AIRST) and SHTR for different air ratios in the primary zone λ1 at 1270 and 1670 K. Residence time in the primary zone τ1 = 1.7 s. Dashed lines indicate the primary stoichiometry where NRP = 1. The NRP is below 1 at higher air ratios.

show a generally comparable correlation (square symbols). Even though air staging achieves a better NOx reduction at lower primary air ratios, SHTR achieves a better NOx reduction at primary air ratios close to stoichiometry. On the basis of the findings above, it becomes evident that SHTR improves the NOx reduction as long as the NRP is below 1. At 1670 K, SHTR improves NOx reduction as long as the primary stoichiometry is above 0.9 and causes no benefit at 1270 K. The findings can be explained as follows: under fuelrich conditions, the NRP is already high enough to reduce NO, therefore further ammonia injection leads to additional NO formation. Therefore, the NOx reduction by SHTR decreases in comparison with the NOx reduction by air staging. Under near stoichiometric conditions, NRP is below 1; therefore, ammonia injection increases the amount of radicals and hence improves NOx reduction by SHTR compared to bare air staging. These findings are supported by the results shown in Figure 12 where the NOx reduction of SHTR is compared to the NOx reduction of AIRST at 1570 K in absolute terms. It can be seen that, for both residence times, SHTR has advantages only at close to stoichiometric conditions and that the functionality is very sensitive to changes in the stoichiometry. Ammonia injection under very fuel-rich conditions decreases NOx reduction, which means that additional NOx is produced. Higher β leads to increased NO formation under these conditions. The momentum ratio J seems to have no significant influence, which indicates that mixing does not limit the reduction in the investigated range. Longer residence times promote NOx reduction during SHTR; the NOx reduction can be improved by 10% compared to bare air staging.

Figure 12. NOx reduction by SHTR compared to AIRST at 1570 K in absolute terms. Variation of the ratio β and residence time τ1 in dependency of the primary air ratio λ1. Longer residence times promote NOx reduction.

these conditions; longer residence times in the primary zone improve the NO x reduction. This work indicates the functionality of SHTR, but the NOx reduction could only be increased by about 10%. In comparison with that, experimental data of Greul showed that the NOx reduction by SHTR can be improved by more than 40% compared to bare air staging.64 The differences between the study at hand and his investigations can be explained as follows: Greul used bituminous coal as a fuel, which leads to much higher initial NOx emissions compared to the combustion of wood. Additionally, the residence time in the reduction zone during his experiments has been 3 s. In accordance to Chen et al.,13 HCN was the main nitrogen species under fuel-rich conditions, and the concentration of HCN was comparable to the concentration of NH3 in the study at hand. The differences in the reduction efficiencies indicate that the ongoing reactions seem to be different for HCN and NH3. The global reaction mechanisms of NH3 lead to a much higher NO concentration than those of HCN,65 but NH3 has a greater contribution to NOx reduction than HCN.41,63 That leads to the assumption that, if NH3 is injected in a HCN-rich zone, both advantages can be utilized and NOx reduction can be improved.

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CONCLUSION The experimental results show that the release and distribution of nitrogen species is the key parameter in NOx reduction by means of air staging and injection of a reduction agent. Even though NOx reduction mechanisms have been associated with a minimum of TFN in the primary zone so far, NRP can be a suitable parameter to assess the NOx reduction potential of an operation point, especially when ammonia injection is utilized. The combination of air staging and ammonia injection improves NOx reduction, as long as the reduction agent is injected in the primary zone at slightly fuel-rich conditions that guarantee a lack of N-intermediates (NRP < 1). SHTR achieves a better NOx reduction than air staging as long as these conditions are met, but the temperature dependence is still given. The momentum ratio J has no significant influence under 5822


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Energy & Fuels

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Nevertheless, a detailed numerical simulation is inevitable for further investigation. Altogether, those findings lead to the conclusion that SHTR can be an alternative NOx reduction technology for combustion systems that utilize fuels with a high nitrogen content which release mainly HCN and are operated at relatively high temperatures and slightly fuel-rich conditions in the primary zone. As known from operation experiences of SNCR systems, reduction efficiency can be easily increased by increasing β,41 which, on the contrary, might increase NH3 emissions. This should be the same for SHTR; if mixing in the primary zone is not complete, ammonia might be transported from the primary to the burnout zone even if β is low. The main difference is that, during SHTR, ammonia is injected at high temperatures; hence, we assume that ammonia slip is completely decomposed in the burnout zone. Consequently, ammonia injection should not cause ammonia emissions, but might increase NOx emissions. Further SHTR experiments should focus on fuels with a higher nitrogen content to determine the influence of temperature and residence time τ at close to stoichiometric conditions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Bavarian Research Foundation through the project “Robust and efficient NOx reduction with ammonia”. The support of the TUM Graduate School is thankfully acknowledged.

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NOTE ADDED AFTER ASAP PUBLICATION This article published June 20, 2016 with a missing figure. The figure was added as Figure 7, and the subsequent figures and their respective citations renumbered. The corrected version published June 22, 2016.

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Experimental Investigation of Nitrogen Species Distribution in Wood

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