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Oxidation of ammonia over a copper oxide-containing solid oxygen carrier with oxygen uncoupling capability
Combustion and Flame 165 (2016) 445–452
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Oxidation of ammonia over a copper oxide-containing solid oxygen carrier with oxygen uncoupling capability Fredrik Normann a,∗, Mao Cheng b, Dongmei Zhao c, Zhenshan Li b,∗, Ningsheng Cai b, Henrik Leion c a
Department of Energy and Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden Key Laboratory for Thermal Science and Power Engineering of Ministry of Education Department of Thermal Engineering, Tsinghua University, Beijing 100084, China c Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden b
a r t i c l e
i n f o
Article history: Received 24 September 2015 Revised 28 December 2015 Accepted 29 December 2015 Available online 27 January 2016 Keywords: Solid oxygen carrier Copper oxide Oxygen uncoupling Ammonia oxidation Nitrogen oxides
a b s t r a c t Measurements and modelling of the oxidation of ammonia over copper oxide (CuO) under conditions relevant to chemical-looping combustion with oxygen uncoupling show that CuO fully converts NH3 into NO or N2 under most of these conditions. Our experiments demonstrate that considerable amounts of NO are formed when the oxygen carrier is fully oxidised. Decreasing the degree of oxidation of the oxygen carrier affected the selectivity of the NH3 oxidation towards NO. Ageing of the oxygen carrier had a similar effect. Modelling suggests that these results reflect a change in the rate of oxygen release from the CuO particles. © 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction In chemical-looping combustion (CLC), the fuel is oxidised by a solid oxygen carrier rather than by a direct reaction with air. As the gas that is generated from the fuel conversion is free of airborne nitrogen, a flue gas that is highly concentrated in carbon dioxide (CO2 ) is generated. The primary objective in developing CLC is therefore to capture CO2 . A CLC system consists of two interconnected reactors, an air reactor and a fuel reactor. Within the air reactor, the oxygen carrier particles absorb oxygen from the air. These particles are then transported to the fuel reactor where they are used to convert the fuel. For more details of the CLC process, see the review of Adanez et al. [1]. Oxygen carriers come in many forms, but the most common types by far are particles that comprise of a chemically active metal oxide, such as NiO, Fe2 O3 , Mn2 O3 , or CuO. A support material is often added to provide physical stability, so that the particle can survive the conditions of a fluidised bed. Examples of such supports are Al2 O3 , ZrO2 , TiO2 and other chemically inert materials. The active part of the carrier provides oxygen by transitioning from an oxidised state (Me–O) in the air reactor to a reduced ∗
Corresponding authors. E-mail addresses: [email protected] (F. Normann), [email protected] (Z. Li).
state (Me) in the fuel reactor, as shown in Reaction (1). The carrier is thereafter regenerated, according to Reaction (2), in the air reactor.
Cn Hm + (2n + ½m)Me–O → nCO2 + ½mH2 O + (2n + ½m)Me
(1)
Me + ½O2 → Me–O
(2)
In Reaction (1), the fuel reacts directly with the metal oxide. However, most of the fuels used worldwide are solids, e.g., biomass, coal or waste, and solid–solid reactions between fuel and oxygen carriers will not take place at a sufficiently high rate for efficient fuel conversion [2]. With an oxygen-carrier that, according to Reaction (3), releases gas-phase O2 inside the fuel reactor the fuel can instead react with the released oxygen through the regular combustion process (Reaction (4)). This version of CLC is called Chemical-Looping with Oxygen Uncoupling or CLOU [3], and it dramatically expands the usefulness of oxygen carriers. While several oxygen carrier systems are available for CLOU [4, 5], the most commonly discussed is the CuO/Cu2 O system used in the present work.
Me–O → Me + ½O2
(3)
Cn Hm + (n + ¼m)O2 → nCO2 + ½mH2 O
(4)
http://dx.doi.org/10.1016/j.combustflame.2015.12.029 0010-2180/© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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in CLC) have been the subject of investigations (e.g., Ohtsuka et al. [12], Leppälathi et al. [13], Zhong and Tang [14], and Wang et al. [15]). The general conclusion drawn from these investigations is that the presence of catalytic metal oxide surfaces reduces the formation of NOx , although the mechanism underlying this reduction remains to be elucidated. The metal-based oxygen carriers interact with nitrogen in both oxidised and reduced states. In a well-mixed furnace, it can be expected that oxidised and reduced metal oxides will be present simultaneously. It is feasible that the metal oxide will provide a surface for catalytic oxidation:
N + 1/2O2
surface
−→ NO
(5)
The mobility of oxygen in the oxide would make it possible for non-catalytic reactions to occur:
Me − O + N → Me + NO Fig. 1. Global mechanisms of NO and N2 O formation and reduction during combustion. Vol-N denotes volatile nitrogen-containing compounds, which are typically HCN or NH3 .
Ammonia may also be catalytically decomposed to N2 and H2 through the following reaction:
2NH3 In the design of systems for thermal power generation, it is important to consider the formation of nitrogen oxides (NOx ), and it has been shown to be especially important for carbon capture schemes [6]. Since CLC proceeds at moderate temperatures (800– 1000 °C) and without a flame, no thermal NOx is formed [7]. However, solid fuels (e.g., coals or biomass) contain nitrogen, which may form NOx in the fuel reactor. The present work investigates the formation of NOx under conditions relevant to solid fuel conversion in CLOU processes [4]. Given its industrial importance, the nitrogen chemistry during combustion and technical solutions for NOx control are widely studied (reviewed by Glarborg et al. [8], Molina et al. [9], and Normann et al. [6]). The main reaction paths in NOx chemistry are summarised in Fig. 1. The fate of nitrogen species in CLC has not been investigated to the same extent. Here, both the homogeneous gas-phase effects and heterogeneous effects of solid oxygen carriers affect the nitrogen chemistry during combustion. During fuel decomposition or pyrolysis, the nitrogen chemistry is dominated by the evolution of volatile nitrogen compounds in the fuel. The fuel-bound nitrogen is split into volatile nitrogen and char-bound nitrogen. While the type and relative level of released nitrogen volatiles are a matter for discussion [8], the general opinion is that hydrogen cyanide (HCN) predominates for high-rank fuels (e.g., coal), while a higher level of ammonia (NH3 ) is formed from fuels of lower rank (e.g., biomass). In a second stage, the released volatile nitrogen compound undergoes oxidation. This oxidation step is critical, as it directs the levels of NO and N2 formation. The reducing potential of the combustion atmosphere establishes the selectivity between the nitric oxide (NO)- and nitrogen (N2 )-forming reactions. The presence of solid oxygen carriers will have a considerable effect on the combustion atmosphere. Most importantly, the atmosphere will contain less free oxygen, whereby the ability to uncouple oxygen determines the level of free oxygen. The resulting atmospheres range from the extreme without any gas-phase oxygen to one with a constant oxygen concentration of a few percentage points. Heterogeneous reactions that take place between solids (metal oxygen carriers, char and soot) and nitrogen species during combustion are important for the formation of NOx . Much work has been performed to determine the mechanisms, as well as the kinetics of the interactions between NOx species and char during combustion. Research groups that have performed extensive studies on NOx -char interactions include Beér et al. (e.g., [10]), and Tomita et al. (e.g., [11]). In addition, the interactions between nitrogen and transition metal oxides (often used as oxygen carriers
(6)
surface
−→ N2 + 3H2
(7)
Reaction (7) has been shown to be catalysed by metals, such as Fe-dolomite, iron sinter, and nickel. There is also the possibility that the reduced metal oxide reacts with nitrogen oxides to form N2 through the following overall reaction:
2Me + 2NO → N2 + 2Me − O
(8)
The oxygen carrier may also catalyse the reduction of NO by the fuel, for example, carbon monoxide (CO). The importance of the fuel may be attributed to the regeneration of active sites or to direct reduction, catalysed by a surface, as follows:
CO + Me − O → CO2 + Me surface
NO + CO −→
1 N2 + CO2 2
(9)
(10)
Recently, Cheng et al. [16] investigated the oxidation of NH3 over ilmenite, which is an oxygen carrier without oxygen uncoupling capability. In their study, they summarised the recent studies performed with CLC that included NOx measurements. In summary, a couple of studies have measured NOx emissions from technical-scale CLC units, out of which a few has looked at the CLOU case [17,18]. The investigations performed to date have been rather unfocussed and the process of formation of NOx under conditions relevant to CLC and CLOU systems is still poorly understood. The present work targets the conversion of ammonia by copper oxide (CuO), which is an oxygen carrier with the ability to perform oxygen uncoupling. Ammonia is of interest as a common precursor of NOx , since the low-rank coals that are used preferentially for CLC/CLOU are expected to release considerable amounts of ammonia. The results are relevant to the gas-phase chemistry of NOx under conditions relevant to CLOU. The aim is to map and explain the tendencies to form NO or N2 in a laboratoryscale fluidised bed reactor. To interpret the experimental results, some modelling of the concerned gas-phase chemistry was performed. 2. Experimental The experiments regarding NH3 oxidation over oxygen carriers were carried out in a set-up that consisted of a gas feeding system, a reaction system, and a gas analysis system, as illustrated in Fig. 2. The reaction system is based on a fluidised bed reactor with a height of 870 mm and an inner diameter of 22 mm. A porous quartz plate is located within the reactor at a height of 370 mm. The temperature is measured 5 mm below and 25 mm above the
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Fig. 2. Overview of the experimental set-up.
Table 1 Reduction gas composition for given ratios of syngas to NH3 (the remainder being N2 ). Case
Temperature (°C)
Syngas/NH3 ratio
NH3 fraction (vol%)
Syngas fraction (vol%)
Time (s)
1 2 3 4
850, 900, 950 900 900 850, 900, 950
0 70, 140, 400, 800, – 0, 10, 20, 30, 70 –
1 0.6, 0.3, 0.1, 0.05, 0 0.6 0
0 40 0, 5, 10, 20, 40 0
350 150 150 150
quartz plate using thermocouples enclosed in quartz shells. The temperature measured at 25 mm above the quartz plate is used to control the temperature at ±5 °C of the set temperature. The pressure drop is observed with Honeywell pressure transducers at a frequency of 20 Hz. The oxygen carrier material, in a batch of 15 g, is placed on the quartz plate before the experiment starts. The gases are mixed and injected from the bottom of the reactor through the quartz plate. Mass flow controllers regulate the gas flows. The experiments are performed in cycles that consist of an oxidation phase and a reduction phase, separated by an inert phase. The reducing phase is of primary interest for this investigation. During the reducing phase, a gas mixture of NH3 , N2 , and synthesis gas (50% CO and 50% H2 ) is introduced into the reactor. During the oxidation phase, 5% O2 in N2 for temperatures in the range of 850–900 °C and 10% O2 in N2 for a temperature of 950 °C is introduced into the heated reactor until stable concentrations of O2 are reached at the outlet (>1000 s). An inert phase of pure N2 for 180 s follows each oxidation phase and reduction phase, in order to flush the reactor. A flow rate of 900 mL/min is maintained for all the gas mixtures over the entire cycle. The set temperature of the reducing phase is varied between 850 °C and 950 °C. Table 1 presents the four test cases. Cases 1–3 examined the oxidation of NH3 under the influences of reactor temperature, NH3 inlet concentration, and presence of syngas. Case 4 studied the possible reaction between NO and Cu2 O, whereby 2000 ppm NO in N2 was introduced into the reactor after the syngas had reduced the CuO-particles into Cu2 O. Full conversion of the copper oxide to Cu2 O could be assumed since there was no release of O2 from the oxygen carrier during the inert phase. Subsequent XRD
analysis confirmed that there was no Cu or CuO present at this stage. A cooler dries the gas – the small amount of condensing water in the cooler was confirmed not to have any significant influence on the measurement of NH3 . The dried flue gas was analysed by the Rosemount NGA-2000 gas analyser for CO, CO2 and O2 . The chemiluminescence detector CLD 700 EL by ECO Physics was used to detect NO and NO2 . The concentration of N2 O, which was measured by gas chromatography on several occasions, was negligible. Residual NH3 was absorbed in deionised water mixed with sulphuric acid, whereby NH3 was converted into NH4 + , which was quantified using the DIONEX ICS-900 ion chromatograph. NH3 absorption was performed before introduction of the sample into the chemiluminescence detector, to avoid any influence of NH3 on the NOx measurement. The residence time of the reactor system was around 30 s, and the NH3 absorption process had a residence time of around 30 additional seconds between the logging time of the gas analyser and the NOx measurement. Compensation for these residence times has been made in the figures. The distributions of Cu on the particle surfaces before and after the reaction cycles were visualised by ESEM-EDX (FEI QUANTA 200F). The oxygen carrier, which comprised 40% copper oxide supported on 60% ZrO2 , originated from a batch that had been successfully tested under CLOU conditions [19]. The carrier was manufactured through spray drying by VITO in Belgium. After spray drying, the particles underwent calcination in air for 4 h at 1100 °C. All the oxygen carrier materials were sieved to yield a particle size in the range of 125–180 μm. The key properties of the oxygen carrier are listed in Table 2.
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Table 2 Properties of the oxygen carrier material. Chemical composition
40% CuO, 60% Zrx Y1 − x O2 (YSZ)
Particle size range (μm) Specific surface area (BET) (m2 /g) Pore volume (cm3 /kg) ˚ Pore size (A)
125–180 0.9834 5.346 86.49 0.6 1700
Crushing strength (N) Bulk density (kg/m3 )
Fig. 4. Calculated outlet concentrations of NO in relation to the oxygen release rate and syngas-to-ammonia ratio. The reactor temperature is 940 °C. Overall, the stoichiometric ratio of oxygen released to fuel is 1.5.
the experimental reactor, while excluding catalytic and direct gas– solid reactions. A similar approach has previously been applied in investigations of nitrogen chemistry in oxy-fuel combustion [21]. 4. Results Fig. 3. Illustration of the model. The blue field (light grey field in greyscale print) illustrates the plug-flow reaction zone with gradual introduction of oxygen from the oxygen carrier.
3. Numerical approach The oxidation of ammonia was evaluated through gas-phase chemistry modelling of the CO/H2 oxidation and nitrogen chemistry. The modelling examines the selectivity for the ammonia oxidation paths depending on the reactor conditions. The modelling results are compared to the experimental findings and used as a part of the interpretation of the results. However, heterogeneous and catalytic effects are not considered in the modelling work. The importance of the gas-phase chemistry for oxygen carriers, such as copper oxide, which has a strong CLOU effect, is of special interest and the comparison of the experiments and homogenous effects is therefore of interest. The detailed kinetic mechanism [20] applied includes the reactions of importance for the oxidation of light hydrocarbons (C1–C2), CO, and H2 , as well as the nitrogen chemistry relevant to NH3 oxidation. The principle of the modelling is illustrated in Fig. 3. Similar to the experiments, mixtures of ammonia and syngas in N2 are added to the reactor model. The reaction zone (blue field in Fig. 3) is modelled as a plug-flow. (As an example, the generated CO2 profile presented in Fig. 6a in the result section shows that the experimental reactor works at close to plug-flow conditions.) The release of oxygen from the oxygen carrier is simulated by gradual introduction of oxygen into the reacting flow. The oxygen release rate is constant throughout the reactor in all the modelled cases. Although the rate of oxygen release is investigated by changing the rate at which oxygen is introduced into the reacting flow between cases. This simple approach to describe the gas–solid interactions allows for evaluation of the homogeneous nitrogen chemistry in a mixture of reactants that is comparable to the gas atmosphere in
4.1. Modelling The modelling results are presented in Fig. 4. The calculated NO concentrations at the outlet of the reactor are shown for different assumed rates of oxygen release from the CuO particles. The rate of oxygen release is obviously important in relation to NO formation. When the release of oxygen is slower than the rate of fuel oxidation, ammonia oxidation will take place in an oxygen-lean environment and the ammonia will be almost completely oxidised to N2 . In contrast, when the release of oxygen is faster than the oxidation, NH3 oxidation will take place in an oxygen-rich environment and result in considerable formation of NO. The theoretical maximum concentrations of NO are 10,000, 5000, and 4000 ppm at the outlet for syngas-to-ammonia ratios of 20, 100, and 150, respectively. The extreme case is to assume instantaneous release of oxygen, which would represent perfect mixing of the fuel and oxygen; under such conditions, around 80% of the NH3 would be converted into NO by gas-phase reactions alone. The overall excess of oxygen or the overall stoichiometry of the mixture does not have significant effect on NO formation as long as the rate of oxygen release is kept constant. In practice, the availability of oxygen is of course strongly coupled to the rate of release from the oxygen carrier and the rate of consumption of oxygen by the fuel. If the overall stoichiometry is decreased below unity, the conversion of the syngas is incomplete. However, for incomplete NH3 oxidation, the stoichiometry must be decreased far below unity. Figure 5 compares the quantities of the relevant nitrogen species (NO, NO2 , N2 O, and NH3 ) at the outlet of the reactor for different reactor temperatures and oxygen release rates. The conversion of ammonia is complete for all the tested oxygen release rates. At reactor temperatures above 800 °C (as used for the experimental runs), NO totally dominates the other nitrogen oxides. In Fig. 5, the NO concentration peaks in the temperature range of 600–700 °C and decreases at higher temperatures. Again, the relationship between the rate of oxygen release from the CuO
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Fig. 5. Calculated outlet concentrations of important nitrogen species as a function of reactor temperature for two oxygen release rates, 0.002 mol/s (solid line) and 0.001 mol/s (dashed line). Observe that the NO concentration is divided by three to simplify the comparison. The syngas-to-ammonia ratio is 40. Overall, the stoichiometric ratio of oxygen released to fuel is 1.5.
particles and the reaction rate is important. As the temperature is increased, the reaction rates increase, while the oxygen release rate in the model is kept constant. The NH3 will, thus, be oxidised in a less-oxidising environment at higher temperatures. However, in reality, the oxygen release rate is also increased by temperature, which would increase the rate of NO formation, as illustrated by the dashed lines in Fig. 5 for which the oxygen release rate is doubled. N2 O formation peaks at 200 ppm at around 700 °C – there is no significant formation of N2 O at temperatures below 600 °C or above 800 °C. The formation of NO2 increases at lower temperatures and is insignificant at temperatures above 800 °C. The rates of formation of both N2 O and NO2 appear to be unaffected by the oxygen release rate in the investigated range. 4.2. Laboratory measurements Figure 6 compares the measured outlet concentrations of CO, CO2 , O2 and NO over CuO during one reduction phase with an inlet of 0.5% NH3 , 25% H2 , and 25% CO in N2 . Figure 7 shows the
449
measured reactor temperature during the same reduction phase. During the reduction of CuO, the temperature increased by almost 40 °C from the set temperature of 900 °C, as the overall reaction is exothermal. The CLOU effect of the CuO was observed in the presence of O2 at the reactor outlet. After around 100 s, all the CuO had been reduced to Cu2 O, which does not release oxygen, and O2 was no longer detected at the outlet. The end of oxygen release also caused a small increase in the CO2 concentration (observed after 100 s in Fig. 6a), as the total outlet flow was decreased. The CO was, however, still fully oxidised in a heterogeneous reaction with Cu2 O. The Cu2 O was fully reduced and CO was no longer oxidised after a total time period of around 230 s. NO formation is most pronounced at the onset of the reduction phase. Mixing events in the system caused the initial gradient of NO concentration, as confirmed by a test with NO in the inlet gas stream and an inert bed material of quartz sand. When the CuO was fully oxidised almost 50% of the NH3 was oxidised to NO. However, the formation of NO is rapidly decreased as the oxygen carrier is reduced. The amount of unconverted NH3 was low for all the cases over the entire reduction phase. This indicates that the oxidation of NH3 is complete until the very end of the reduction phase, at which point the oxygen carrier is fully reduced to metallic Cu. The residual nitrogen can be assumed to form N2 , which is supported by the modelling (cf. Fig. 4). The concentration of N2 O was measured at several time-points and concluded to be negligible, and this again was supported by the modelling. Given these premises, Fig. 8 shows the selectivity of the oxidation of NH3 for the entire reduction phase of 17 consecutive cycles for Test Case 1 without syngas. There is a clear trend for the selectivity towards NO to decrease with the age of the oxygen carrier. On the other hand, the reactor temperature does not seem to affect the selectivity towards NO, even though the outlet oxygen concentration during the reduction phase from CuO decomposition is obviously increased in line with the reactor temperature. The outlet oxygen concentrations during the reduction phase were 0.4, 1.2, and 3.6 vol% at 850 °C, 900 °C, and 950 °C, respectively. These values are slightly lower than the thermodynamic equilibrium concentrations of 0.5, 1.5, and 4.5 vol%, respectively, at the same temperatures [3]. The age of the oxygen carrier did not influence the outlet oxygen concentration. Figure 9 shows the EDX maps of Cu distributed on the oxygen carrier particle surface before and after Test Case 1. It is clear that the Cu tends to agglomerate on the surface of the oxygen carrier particles after the reaction cycles. This
Fig. 6. The O2 , CO, CO2 and NO concentrations in the dried flue gas during the reduction phase with 0.6% NH3 plus 40% syngas reacting with CuO and the inert phase of Test Case 3.
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Fig. 7. Measured temperatures in the reactor during the reduction phase with 0.6% NH3 plus 40% syngas reacting with copper oxide and the inert phase of Test Case 3. The set temperature is 900 °C.
Fig. 8. Selectivity of NH3 oxidation over CuO towards different nitrogen species for the complete reduction phase. The results are based on measurements of the outlet concentrations of NO, NO2 , and NH3 . The remainder is assumed to be N2 .
might be a consequence of its complete reduction to metallic copper. During moderate reduction, no such surface agglomerations of Cu were observed [3]. When related to the modelling results, the experimental results may be explained by the rate of oxygen release from the oxygen carrier, which controls the local availability of oxygen in the gas phase. According to this theory NO is favoured when the release rate of oxygen is higher or of the same order of magnitude as the reaction rates, while N2 formation is favoured by a release rate that is lower than the reaction rates. Thus, the initial release of oxygen during the reduction phase of every cycle is high and the selectivity towards NO is strong. The release rate decreases gradually as the oxygen carrier is reduced and N2 formation is favoured instead. In a similar way, a decrease in oxygen release over the life-time of the oxygen carrier would explain the decrease in selectivity towards NO. The total release of oxygen from the carrier is, however, not affected by the age of the carrier and there is sufficient oxygen for complete oxidation of the syngas and the NH3 in all the
cases. It should be noted that changes in the heterogeneous chemistry that are not considered in the modelling might also explain the effect of particle life-time on NO formation. As evident in the EDX maps (Fig. 9), the amount of copper present on the particle surface increased during the test runs. The increase in surface copper might, for example, cause increased decomposition of NH3 to N2 (Reaction 7), which could also explain the decreased selectivity towards NO. Figure 10 shows the results of Test Case 2 with an inlet composition of 40% syngas and NH3 concentrations that varied between 0% and 0.6%. The NO outlet concentration profile (Fig. 10a) is similar in all the cases. The peak concentration was, as expected, higher when more NH3 was introduced into the reactor. However, there was also a significant change in selectivity towards NO (Fig. 10b), giving NO to NH3 ratios that ranged from around 0.65 for the full reduction phase at 0.05% inlet NH3 to 0.35 at 0.6% inlet NH3 . This difference was even more pronounced for the peak formation of NO. At the peak, more than 90% of the ammonia was
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Fig. 9. EDX map of Cu distributions on the oxygen carrier particle surfaces before (top row) and after (bottom row) Test Case 1.
Fig. 10. NO formation during the reduction phase with 40% syngas and various NH3 inlet concentrations. (a) Outlet NO concentration. (b) Selectivity of NH3 oxidation over CuO towards different nitrogen species for the complete reduction phase. Based on measurements of the outlet concentrations of NO, NO2 , and NH3 . The remainder is assumed to be N2 .
converted to NO at low inlet concentrations of NH3 , while conversion to NO peaked at around 40% for the high inlet concentration of NH3 . These results indicate that the N2 -forming reactions, i.e. foremost the catalytic decomposition and NO-NH3 reactions, are more dependent on the NH3 concentration than the NO-forming reactions. Figure 11 shows the results for Test Case 3, in which there was a constant inlet concentration of 0.6% NH3 and various syngas concentrations, ranging from 0% to 40%. During the initial part of the reduction phase (for around 50 s) the apparent NO formation rate was independent of the syngas concentration (Fig. 11a). However, after the initial part, NO formation clearly decreased with increasing syngas concentration, although the conversion of NH3 was complete in all cases. The reactor temperature was increased with
the inlet syngas concentrations due to the exothermal reactions (cf. Fig. 7). The outlet concentrations of oxygen was close to the corresponding temperature-dependent equilibrium value in all cases. In only the 40% syngas case was oxygen carrier reduced to such an extent that all CuO was reduced and oxygen release ceased, and even in this case the oxygen release ceased first at the end of the reducing phase. When related to the gas-phase modelling, this indicates that the presence of the syngas makes the critical decrease in oxygen release rate occur earlier. Figure 11b shows the selectivity of NH3 oxidation towards different nitrogen species dependent on the inlet concentration of syngas. The selectivity towards NO decreased at high syngas concentrations. The oxygen carrier used in Test Case 4 was Cu2 O. During the test, an inlet flow with 2000 ppm NO was introduced into the
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Fig. 11. Apparent NO formation during the reduction phase with 0.6% NH3 and various syngas inlet concentrations. (a) NO formation rate. (b) Selectivity of NH3 oxidation over CuO towards different nitrogen species for the complete reduction phase. Based on measurements of outlet concentration of NO, NO2 , and NH3 . The balance is assumed to be N2 .
reactor, which was loaded with the reduced oxygen carrier and heated to temperatures of 850 °C, 900 °C, and 950 °C, respectively. NO was not reduced over Cu2 O under any of the conditions tested. 5. Conclusions We performed measurements of the oxidation of NH3 in a mixture with syngas over copper oxide. The results were interpreted using additional gas-phase modelling of the combustion and nitrogen chemistry. The outcomes relate to the formation of NOx in the fuel reactor with solid fuel chemical-looping combustion with oxygen uncoupling. The experimental results show that CuO mediates efficient conversion of NH3 . Considerable amounts of NH3 are oxidised to NO when the oxygen carriers are fully oxidised. However, the formation of NO is drastically decreased as the oxygen carrier becomes more reduced. Ageing of the oxygen carrier also decreases the selectivity of NH3 oxidation towards NO. The experiments also show that the conversion of NH3 to NO is sensitive to the presence of syngas. The modelling results suggest that the observed effect can be explained by the gas-phase chemistry. In this case, it is the rate of oxygen release that changes the selectivity towards NO or N2 as the oxygen carrier is reduced. However, heterogeneous reactions and catalytic effects cannot be ruled out. Acknowledgments This work was supported by National Natural Science Foundation of China (51376105, 51561125001), Chalmers University of Technology, Tsinghua University, and The Swedish Research Council Formas. References [1] J. Adanez, A. Abad, F. Garcia-Labiano, P. Gayan, L.F. De Diego, Progress in chemical-looping combustion and reforming technologies, Prog. Energy Combust. Sci. 38 (2) (2012) 215–282. [2] H. Leion, T. Mattisson, A. Lyngfelt, The use of petroleum coke as fuel in chemical-looping combustion, Fuel 86 (12–13) (2007) 1947–1958. [3] T. Mattisson, A. Lyngfelt, H. Leion, Chemical-looping with oxygen uncoupling for combustion of solid fuels, Int. J. Greenh. Gas Control 3 (1) (2009) 11–19.
[4] Q. Imtiaz, D. Hosseini, C.R. Müller, Review of oxygen carriers for chemical looping with oxygen uncoupling (CLOU): thermodynamics, Mater. Dev. Synth. Energy Technol. 1 (11) (2013) 633–647. [5] M. Rydén, H. Leion, T. Mattisson, A. Lyngfelt, Combined oxides as oxygencarrier material for chemical-looping with oxygen uncoupling, Appl. Energy 113 (2014) 1924–1932. [6] F. Normann, K. Andersson, B. Leckner, F. Johnsson, Emission control of nitrogen oxides in the oxy-fuel process, Prog. Energy Combust. Sci. 35 (5) (2009) 385– 397. [7] M. Ishida, H. Jin, A novel chemical-looping combustor without NOx formation, Ind. Eng. Chem. Res. 35 (7) (1996) 2469–2472. [8] P. Glarborg, A.D. Jensen, J.E. Johnsson, Fuel nitrogen conversion in solid fuel fired systems, Prog. Energy Combust. Sci. 29 (2) (2003) 89–113. [9] A. Molina, E.G. Eddings, D.W. Pershing, A.F. Sarofim, Char nitrogen conversion: implications to emissions from coal-fired utility boilers, Prog. Energy Combust. Sci. 26 (4) (2000) 507–531. [10] L.K. Chan, A.F. Sarofim, J.M. Beér, Kinetics of the NOcarbon reaction at fluidized bed combustor conditions, Combust. Flame 52 (1983) 37–45. [11] T. Suzuki, T. Kyotani, A. Tomita, Study on the carbon-nitric oxide reaction in the presence of oxygen, Ind. Eng. Chem. Res. 33 (11) (1994) 2840–2845. [12] Y. Ohtsuka, Z. Wu, Effects of metal cations present naturally in coal on the fate of coal-bound nitrogen in the fixed-bed pyrolysis of 25 coals with different ranks: Correlation between inherent Fe cations and N2 formation from lowrank coals, Energy Fuels 23 (10) (2009) 4774–4781. [13] J. Leppälahti, P. Simell, E. Kurkela, Catalytic conversion of nitrogen compounds in gasification gas, Fuel Process. Technol. 29 (1-2) (1991) 43–56. [14] B.J. Zhong, H. Tang, The catalytic effect of Fe on char-NO reactions at hightemperatures, Int. J. Chem. Reactor Eng. 6 (2008). [15] X.B. Wang, H.Z. Tan, C.L. Wang, Q.X. Zhao, T.M. Xu, S.E. Hui, Effect of metal oxide on the emission of N2 O and NO in fluidized bed temperature range using pyridine as a nitrogenous model fuel, in: Proceedings of the 20th International Conference on Fluidized Bed Combustion, 2009, pp. 1017–1021. [16] M. Cheng, F. Normann, D. Zhao, Z. Li, N. Cai, H. Leion, Oxidation of ammonia by ilmenite under conditions relevant to chemical-looping combustion, Energy Fuels 29 (12) (2015) 8126–8134. [17] I. Adánez-Rubio, T. Mendiara, A. Abad, P. Gayan, F. Garcia-Labiano, L.F. De Diego, J. Adanez, Pollutant emissions during coal combustion in iG-CLC and CLOU process, Proceedings of the 3rd International Conference on Chemical Looping Göteborg, Sweden, 2014, Göteborg, Sweden, (2004). [18] T. Song, W. Guo, L. Shen, Fuel nitrogen conversion in chemical looping with oxygen uncoupling of coal with a CuO-based oxygen carrier, Energy Fuels 29 (6) (2015) 3820–3832. [19] M. Rydén, D. Jing, M. Källén, H. Leion, A. Lyngfelt, T. Mattisson, CuO-based oxygen-carrier particles for chemical-looping with oxygen uncoupling – Experiments in batch reactor and in continuous operation, Ind. Eng. Chem. Res. 53 (15) (2014) 6255–6267. [20] T. Mendiara, P. Glarborg, Ammonia chemistry in oxy-fuel combustion of methane, Combust. Flame 156 (10) (2009) 1937–1949. [21] D. Kühnemuth, F. Normann, K. Andersson, F. Johnsson, B. Leckner, Reburning of nitric oxide in oxy-fuel firing-the influence of combustion conditions, Energy Fuels 25 (2) (2011) 624–631.
Contents lists available at ScienceDirect
Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame
Oxidation of ammonia over a copper oxide-containing solid oxygen carrier with oxygen uncoupling capability Fredrik Normann a,∗, Mao Cheng b, Dongmei Zhao c, Zhenshan Li b,∗, Ningsheng Cai b, Henrik Leion c a
Department of Energy and Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden Key Laboratory for Thermal Science and Power Engineering of Ministry of Education Department of Thermal Engineering, Tsinghua University, Beijing 100084, China c Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden b
a r t i c l e
i n f o
Article history: Received 24 September 2015 Revised 28 December 2015 Accepted 29 December 2015 Available online 27 January 2016 Keywords: Solid oxygen carrier Copper oxide Oxygen uncoupling Ammonia oxidation Nitrogen oxides
a b s t r a c t Measurements and modelling of the oxidation of ammonia over copper oxide (CuO) under conditions relevant to chemical-looping combustion with oxygen uncoupling show that CuO fully converts NH3 into NO or N2 under most of these conditions. Our experiments demonstrate that considerable amounts of NO are formed when the oxygen carrier is fully oxidised. Decreasing the degree of oxidation of the oxygen carrier affected the selectivity of the NH3 oxidation towards NO. Ageing of the oxygen carrier had a similar effect. Modelling suggests that these results reflect a change in the rate of oxygen release from the CuO particles. © 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction In chemical-looping combustion (CLC), the fuel is oxidised by a solid oxygen carrier rather than by a direct reaction with air. As the gas that is generated from the fuel conversion is free of airborne nitrogen, a flue gas that is highly concentrated in carbon dioxide (CO2 ) is generated. The primary objective in developing CLC is therefore to capture CO2 . A CLC system consists of two interconnected reactors, an air reactor and a fuel reactor. Within the air reactor, the oxygen carrier particles absorb oxygen from the air. These particles are then transported to the fuel reactor where they are used to convert the fuel. For more details of the CLC process, see the review of Adanez et al. [1]. Oxygen carriers come in many forms, but the most common types by far are particles that comprise of a chemically active metal oxide, such as NiO, Fe2 O3 , Mn2 O3 , or CuO. A support material is often added to provide physical stability, so that the particle can survive the conditions of a fluidised bed. Examples of such supports are Al2 O3 , ZrO2 , TiO2 and other chemically inert materials. The active part of the carrier provides oxygen by transitioning from an oxidised state (Me–O) in the air reactor to a reduced ∗
Corresponding authors. E-mail addresses: [email protected] (F. Normann), [email protected] (Z. Li).
state (Me) in the fuel reactor, as shown in Reaction (1). The carrier is thereafter regenerated, according to Reaction (2), in the air reactor.
Cn Hm + (2n + ½m)Me–O → nCO2 + ½mH2 O + (2n + ½m)Me
(1)
Me + ½O2 → Me–O
(2)
In Reaction (1), the fuel reacts directly with the metal oxide. However, most of the fuels used worldwide are solids, e.g., biomass, coal or waste, and solid–solid reactions between fuel and oxygen carriers will not take place at a sufficiently high rate for efficient fuel conversion [2]. With an oxygen-carrier that, according to Reaction (3), releases gas-phase O2 inside the fuel reactor the fuel can instead react with the released oxygen through the regular combustion process (Reaction (4)). This version of CLC is called Chemical-Looping with Oxygen Uncoupling or CLOU [3], and it dramatically expands the usefulness of oxygen carriers. While several oxygen carrier systems are available for CLOU [4, 5], the most commonly discussed is the CuO/Cu2 O system used in the present work.
Me–O → Me + ½O2
(3)
Cn Hm + (n + ¼m)O2 → nCO2 + ½mH2 O
(4)
http://dx.doi.org/10.1016/j.combustflame.2015.12.029 0010-2180/© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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in CLC) have been the subject of investigations (e.g., Ohtsuka et al. [12], Leppälathi et al. [13], Zhong and Tang [14], and Wang et al. [15]). The general conclusion drawn from these investigations is that the presence of catalytic metal oxide surfaces reduces the formation of NOx , although the mechanism underlying this reduction remains to be elucidated. The metal-based oxygen carriers interact with nitrogen in both oxidised and reduced states. In a well-mixed furnace, it can be expected that oxidised and reduced metal oxides will be present simultaneously. It is feasible that the metal oxide will provide a surface for catalytic oxidation:
N + 1/2O2
surface
−→ NO
(5)
The mobility of oxygen in the oxide would make it possible for non-catalytic reactions to occur:
Me − O + N → Me + NO Fig. 1. Global mechanisms of NO and N2 O formation and reduction during combustion. Vol-N denotes volatile nitrogen-containing compounds, which are typically HCN or NH3 .
Ammonia may also be catalytically decomposed to N2 and H2 through the following reaction:
2NH3 In the design of systems for thermal power generation, it is important to consider the formation of nitrogen oxides (NOx ), and it has been shown to be especially important for carbon capture schemes [6]. Since CLC proceeds at moderate temperatures (800– 1000 °C) and without a flame, no thermal NOx is formed [7]. However, solid fuels (e.g., coals or biomass) contain nitrogen, which may form NOx in the fuel reactor. The present work investigates the formation of NOx under conditions relevant to solid fuel conversion in CLOU processes [4]. Given its industrial importance, the nitrogen chemistry during combustion and technical solutions for NOx control are widely studied (reviewed by Glarborg et al. [8], Molina et al. [9], and Normann et al. [6]). The main reaction paths in NOx chemistry are summarised in Fig. 1. The fate of nitrogen species in CLC has not been investigated to the same extent. Here, both the homogeneous gas-phase effects and heterogeneous effects of solid oxygen carriers affect the nitrogen chemistry during combustion. During fuel decomposition or pyrolysis, the nitrogen chemistry is dominated by the evolution of volatile nitrogen compounds in the fuel. The fuel-bound nitrogen is split into volatile nitrogen and char-bound nitrogen. While the type and relative level of released nitrogen volatiles are a matter for discussion [8], the general opinion is that hydrogen cyanide (HCN) predominates for high-rank fuels (e.g., coal), while a higher level of ammonia (NH3 ) is formed from fuels of lower rank (e.g., biomass). In a second stage, the released volatile nitrogen compound undergoes oxidation. This oxidation step is critical, as it directs the levels of NO and N2 formation. The reducing potential of the combustion atmosphere establishes the selectivity between the nitric oxide (NO)- and nitrogen (N2 )-forming reactions. The presence of solid oxygen carriers will have a considerable effect on the combustion atmosphere. Most importantly, the atmosphere will contain less free oxygen, whereby the ability to uncouple oxygen determines the level of free oxygen. The resulting atmospheres range from the extreme without any gas-phase oxygen to one with a constant oxygen concentration of a few percentage points. Heterogeneous reactions that take place between solids (metal oxygen carriers, char and soot) and nitrogen species during combustion are important for the formation of NOx . Much work has been performed to determine the mechanisms, as well as the kinetics of the interactions between NOx species and char during combustion. Research groups that have performed extensive studies on NOx -char interactions include Beér et al. (e.g., [10]), and Tomita et al. (e.g., [11]). In addition, the interactions between nitrogen and transition metal oxides (often used as oxygen carriers
(6)
surface
−→ N2 + 3H2
(7)
Reaction (7) has been shown to be catalysed by metals, such as Fe-dolomite, iron sinter, and nickel. There is also the possibility that the reduced metal oxide reacts with nitrogen oxides to form N2 through the following overall reaction:
2Me + 2NO → N2 + 2Me − O
(8)
The oxygen carrier may also catalyse the reduction of NO by the fuel, for example, carbon monoxide (CO). The importance of the fuel may be attributed to the regeneration of active sites or to direct reduction, catalysed by a surface, as follows:
CO + Me − O → CO2 + Me surface
NO + CO −→
1 N2 + CO2 2
(9)
(10)
Recently, Cheng et al. [16] investigated the oxidation of NH3 over ilmenite, which is an oxygen carrier without oxygen uncoupling capability. In their study, they summarised the recent studies performed with CLC that included NOx measurements. In summary, a couple of studies have measured NOx emissions from technical-scale CLC units, out of which a few has looked at the CLOU case [17,18]. The investigations performed to date have been rather unfocussed and the process of formation of NOx under conditions relevant to CLC and CLOU systems is still poorly understood. The present work targets the conversion of ammonia by copper oxide (CuO), which is an oxygen carrier with the ability to perform oxygen uncoupling. Ammonia is of interest as a common precursor of NOx , since the low-rank coals that are used preferentially for CLC/CLOU are expected to release considerable amounts of ammonia. The results are relevant to the gas-phase chemistry of NOx under conditions relevant to CLOU. The aim is to map and explain the tendencies to form NO or N2 in a laboratoryscale fluidised bed reactor. To interpret the experimental results, some modelling of the concerned gas-phase chemistry was performed. 2. Experimental The experiments regarding NH3 oxidation over oxygen carriers were carried out in a set-up that consisted of a gas feeding system, a reaction system, and a gas analysis system, as illustrated in Fig. 2. The reaction system is based on a fluidised bed reactor with a height of 870 mm and an inner diameter of 22 mm. A porous quartz plate is located within the reactor at a height of 370 mm. The temperature is measured 5 mm below and 25 mm above the
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Fig. 2. Overview of the experimental set-up.
Table 1 Reduction gas composition for given ratios of syngas to NH3 (the remainder being N2 ). Case
Temperature (°C)
Syngas/NH3 ratio
NH3 fraction (vol%)
Syngas fraction (vol%)
Time (s)
1 2 3 4
850, 900, 950 900 900 850, 900, 950
0 70, 140, 400, 800, – 0, 10, 20, 30, 70 –
1 0.6, 0.3, 0.1, 0.05, 0 0.6 0
0 40 0, 5, 10, 20, 40 0
350 150 150 150
quartz plate using thermocouples enclosed in quartz shells. The temperature measured at 25 mm above the quartz plate is used to control the temperature at ±5 °C of the set temperature. The pressure drop is observed with Honeywell pressure transducers at a frequency of 20 Hz. The oxygen carrier material, in a batch of 15 g, is placed on the quartz plate before the experiment starts. The gases are mixed and injected from the bottom of the reactor through the quartz plate. Mass flow controllers regulate the gas flows. The experiments are performed in cycles that consist of an oxidation phase and a reduction phase, separated by an inert phase. The reducing phase is of primary interest for this investigation. During the reducing phase, a gas mixture of NH3 , N2 , and synthesis gas (50% CO and 50% H2 ) is introduced into the reactor. During the oxidation phase, 5% O2 in N2 for temperatures in the range of 850–900 °C and 10% O2 in N2 for a temperature of 950 °C is introduced into the heated reactor until stable concentrations of O2 are reached at the outlet (>1000 s). An inert phase of pure N2 for 180 s follows each oxidation phase and reduction phase, in order to flush the reactor. A flow rate of 900 mL/min is maintained for all the gas mixtures over the entire cycle. The set temperature of the reducing phase is varied between 850 °C and 950 °C. Table 1 presents the four test cases. Cases 1–3 examined the oxidation of NH3 under the influences of reactor temperature, NH3 inlet concentration, and presence of syngas. Case 4 studied the possible reaction between NO and Cu2 O, whereby 2000 ppm NO in N2 was introduced into the reactor after the syngas had reduced the CuO-particles into Cu2 O. Full conversion of the copper oxide to Cu2 O could be assumed since there was no release of O2 from the oxygen carrier during the inert phase. Subsequent XRD
analysis confirmed that there was no Cu or CuO present at this stage. A cooler dries the gas – the small amount of condensing water in the cooler was confirmed not to have any significant influence on the measurement of NH3 . The dried flue gas was analysed by the Rosemount NGA-2000 gas analyser for CO, CO2 and O2 . The chemiluminescence detector CLD 700 EL by ECO Physics was used to detect NO and NO2 . The concentration of N2 O, which was measured by gas chromatography on several occasions, was negligible. Residual NH3 was absorbed in deionised water mixed with sulphuric acid, whereby NH3 was converted into NH4 + , which was quantified using the DIONEX ICS-900 ion chromatograph. NH3 absorption was performed before introduction of the sample into the chemiluminescence detector, to avoid any influence of NH3 on the NOx measurement. The residence time of the reactor system was around 30 s, and the NH3 absorption process had a residence time of around 30 additional seconds between the logging time of the gas analyser and the NOx measurement. Compensation for these residence times has been made in the figures. The distributions of Cu on the particle surfaces before and after the reaction cycles were visualised by ESEM-EDX (FEI QUANTA 200F). The oxygen carrier, which comprised 40% copper oxide supported on 60% ZrO2 , originated from a batch that had been successfully tested under CLOU conditions [19]. The carrier was manufactured through spray drying by VITO in Belgium. After spray drying, the particles underwent calcination in air for 4 h at 1100 °C. All the oxygen carrier materials were sieved to yield a particle size in the range of 125–180 μm. The key properties of the oxygen carrier are listed in Table 2.
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Table 2 Properties of the oxygen carrier material. Chemical composition
40% CuO, 60% Zrx Y1 − x O2 (YSZ)
Particle size range (μm) Specific surface area (BET) (m2 /g) Pore volume (cm3 /kg) ˚ Pore size (A)
125–180 0.9834 5.346 86.49 0.6 1700
Crushing strength (N) Bulk density (kg/m3 )
Fig. 4. Calculated outlet concentrations of NO in relation to the oxygen release rate and syngas-to-ammonia ratio. The reactor temperature is 940 °C. Overall, the stoichiometric ratio of oxygen released to fuel is 1.5.
the experimental reactor, while excluding catalytic and direct gas– solid reactions. A similar approach has previously been applied in investigations of nitrogen chemistry in oxy-fuel combustion [21]. 4. Results Fig. 3. Illustration of the model. The blue field (light grey field in greyscale print) illustrates the plug-flow reaction zone with gradual introduction of oxygen from the oxygen carrier.
3. Numerical approach The oxidation of ammonia was evaluated through gas-phase chemistry modelling of the CO/H2 oxidation and nitrogen chemistry. The modelling examines the selectivity for the ammonia oxidation paths depending on the reactor conditions. The modelling results are compared to the experimental findings and used as a part of the interpretation of the results. However, heterogeneous and catalytic effects are not considered in the modelling work. The importance of the gas-phase chemistry for oxygen carriers, such as copper oxide, which has a strong CLOU effect, is of special interest and the comparison of the experiments and homogenous effects is therefore of interest. The detailed kinetic mechanism [20] applied includes the reactions of importance for the oxidation of light hydrocarbons (C1–C2), CO, and H2 , as well as the nitrogen chemistry relevant to NH3 oxidation. The principle of the modelling is illustrated in Fig. 3. Similar to the experiments, mixtures of ammonia and syngas in N2 are added to the reactor model. The reaction zone (blue field in Fig. 3) is modelled as a plug-flow. (As an example, the generated CO2 profile presented in Fig. 6a in the result section shows that the experimental reactor works at close to plug-flow conditions.) The release of oxygen from the oxygen carrier is simulated by gradual introduction of oxygen into the reacting flow. The oxygen release rate is constant throughout the reactor in all the modelled cases. Although the rate of oxygen release is investigated by changing the rate at which oxygen is introduced into the reacting flow between cases. This simple approach to describe the gas–solid interactions allows for evaluation of the homogeneous nitrogen chemistry in a mixture of reactants that is comparable to the gas atmosphere in
4.1. Modelling The modelling results are presented in Fig. 4. The calculated NO concentrations at the outlet of the reactor are shown for different assumed rates of oxygen release from the CuO particles. The rate of oxygen release is obviously important in relation to NO formation. When the release of oxygen is slower than the rate of fuel oxidation, ammonia oxidation will take place in an oxygen-lean environment and the ammonia will be almost completely oxidised to N2 . In contrast, when the release of oxygen is faster than the oxidation, NH3 oxidation will take place in an oxygen-rich environment and result in considerable formation of NO. The theoretical maximum concentrations of NO are 10,000, 5000, and 4000 ppm at the outlet for syngas-to-ammonia ratios of 20, 100, and 150, respectively. The extreme case is to assume instantaneous release of oxygen, which would represent perfect mixing of the fuel and oxygen; under such conditions, around 80% of the NH3 would be converted into NO by gas-phase reactions alone. The overall excess of oxygen or the overall stoichiometry of the mixture does not have significant effect on NO formation as long as the rate of oxygen release is kept constant. In practice, the availability of oxygen is of course strongly coupled to the rate of release from the oxygen carrier and the rate of consumption of oxygen by the fuel. If the overall stoichiometry is decreased below unity, the conversion of the syngas is incomplete. However, for incomplete NH3 oxidation, the stoichiometry must be decreased far below unity. Figure 5 compares the quantities of the relevant nitrogen species (NO, NO2 , N2 O, and NH3 ) at the outlet of the reactor for different reactor temperatures and oxygen release rates. The conversion of ammonia is complete for all the tested oxygen release rates. At reactor temperatures above 800 °C (as used for the experimental runs), NO totally dominates the other nitrogen oxides. In Fig. 5, the NO concentration peaks in the temperature range of 600–700 °C and decreases at higher temperatures. Again, the relationship between the rate of oxygen release from the CuO
F. Normann et al. / Combustion and Flame 165 (2016) 445–452
Fig. 5. Calculated outlet concentrations of important nitrogen species as a function of reactor temperature for two oxygen release rates, 0.002 mol/s (solid line) and 0.001 mol/s (dashed line). Observe that the NO concentration is divided by three to simplify the comparison. The syngas-to-ammonia ratio is 40. Overall, the stoichiometric ratio of oxygen released to fuel is 1.5.
particles and the reaction rate is important. As the temperature is increased, the reaction rates increase, while the oxygen release rate in the model is kept constant. The NH3 will, thus, be oxidised in a less-oxidising environment at higher temperatures. However, in reality, the oxygen release rate is also increased by temperature, which would increase the rate of NO formation, as illustrated by the dashed lines in Fig. 5 for which the oxygen release rate is doubled. N2 O formation peaks at 200 ppm at around 700 °C – there is no significant formation of N2 O at temperatures below 600 °C or above 800 °C. The formation of NO2 increases at lower temperatures and is insignificant at temperatures above 800 °C. The rates of formation of both N2 O and NO2 appear to be unaffected by the oxygen release rate in the investigated range. 4.2. Laboratory measurements Figure 6 compares the measured outlet concentrations of CO, CO2 , O2 and NO over CuO during one reduction phase with an inlet of 0.5% NH3 , 25% H2 , and 25% CO in N2 . Figure 7 shows the
449
measured reactor temperature during the same reduction phase. During the reduction of CuO, the temperature increased by almost 40 °C from the set temperature of 900 °C, as the overall reaction is exothermal. The CLOU effect of the CuO was observed in the presence of O2 at the reactor outlet. After around 100 s, all the CuO had been reduced to Cu2 O, which does not release oxygen, and O2 was no longer detected at the outlet. The end of oxygen release also caused a small increase in the CO2 concentration (observed after 100 s in Fig. 6a), as the total outlet flow was decreased. The CO was, however, still fully oxidised in a heterogeneous reaction with Cu2 O. The Cu2 O was fully reduced and CO was no longer oxidised after a total time period of around 230 s. NO formation is most pronounced at the onset of the reduction phase. Mixing events in the system caused the initial gradient of NO concentration, as confirmed by a test with NO in the inlet gas stream and an inert bed material of quartz sand. When the CuO was fully oxidised almost 50% of the NH3 was oxidised to NO. However, the formation of NO is rapidly decreased as the oxygen carrier is reduced. The amount of unconverted NH3 was low for all the cases over the entire reduction phase. This indicates that the oxidation of NH3 is complete until the very end of the reduction phase, at which point the oxygen carrier is fully reduced to metallic Cu. The residual nitrogen can be assumed to form N2 , which is supported by the modelling (cf. Fig. 4). The concentration of N2 O was measured at several time-points and concluded to be negligible, and this again was supported by the modelling. Given these premises, Fig. 8 shows the selectivity of the oxidation of NH3 for the entire reduction phase of 17 consecutive cycles for Test Case 1 without syngas. There is a clear trend for the selectivity towards NO to decrease with the age of the oxygen carrier. On the other hand, the reactor temperature does not seem to affect the selectivity towards NO, even though the outlet oxygen concentration during the reduction phase from CuO decomposition is obviously increased in line with the reactor temperature. The outlet oxygen concentrations during the reduction phase were 0.4, 1.2, and 3.6 vol% at 850 °C, 900 °C, and 950 °C, respectively. These values are slightly lower than the thermodynamic equilibrium concentrations of 0.5, 1.5, and 4.5 vol%, respectively, at the same temperatures [3]. The age of the oxygen carrier did not influence the outlet oxygen concentration. Figure 9 shows the EDX maps of Cu distributed on the oxygen carrier particle surface before and after Test Case 1. It is clear that the Cu tends to agglomerate on the surface of the oxygen carrier particles after the reaction cycles. This
Fig. 6. The O2 , CO, CO2 and NO concentrations in the dried flue gas during the reduction phase with 0.6% NH3 plus 40% syngas reacting with CuO and the inert phase of Test Case 3.
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Fig. 7. Measured temperatures in the reactor during the reduction phase with 0.6% NH3 plus 40% syngas reacting with copper oxide and the inert phase of Test Case 3. The set temperature is 900 °C.
Fig. 8. Selectivity of NH3 oxidation over CuO towards different nitrogen species for the complete reduction phase. The results are based on measurements of the outlet concentrations of NO, NO2 , and NH3 . The remainder is assumed to be N2 .
might be a consequence of its complete reduction to metallic copper. During moderate reduction, no such surface agglomerations of Cu were observed [3]. When related to the modelling results, the experimental results may be explained by the rate of oxygen release from the oxygen carrier, which controls the local availability of oxygen in the gas phase. According to this theory NO is favoured when the release rate of oxygen is higher or of the same order of magnitude as the reaction rates, while N2 formation is favoured by a release rate that is lower than the reaction rates. Thus, the initial release of oxygen during the reduction phase of every cycle is high and the selectivity towards NO is strong. The release rate decreases gradually as the oxygen carrier is reduced and N2 formation is favoured instead. In a similar way, a decrease in oxygen release over the life-time of the oxygen carrier would explain the decrease in selectivity towards NO. The total release of oxygen from the carrier is, however, not affected by the age of the carrier and there is sufficient oxygen for complete oxidation of the syngas and the NH3 in all the
cases. It should be noted that changes in the heterogeneous chemistry that are not considered in the modelling might also explain the effect of particle life-time on NO formation. As evident in the EDX maps (Fig. 9), the amount of copper present on the particle surface increased during the test runs. The increase in surface copper might, for example, cause increased decomposition of NH3 to N2 (Reaction 7), which could also explain the decreased selectivity towards NO. Figure 10 shows the results of Test Case 2 with an inlet composition of 40% syngas and NH3 concentrations that varied between 0% and 0.6%. The NO outlet concentration profile (Fig. 10a) is similar in all the cases. The peak concentration was, as expected, higher when more NH3 was introduced into the reactor. However, there was also a significant change in selectivity towards NO (Fig. 10b), giving NO to NH3 ratios that ranged from around 0.65 for the full reduction phase at 0.05% inlet NH3 to 0.35 at 0.6% inlet NH3 . This difference was even more pronounced for the peak formation of NO. At the peak, more than 90% of the ammonia was
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Fig. 9. EDX map of Cu distributions on the oxygen carrier particle surfaces before (top row) and after (bottom row) Test Case 1.
Fig. 10. NO formation during the reduction phase with 40% syngas and various NH3 inlet concentrations. (a) Outlet NO concentration. (b) Selectivity of NH3 oxidation over CuO towards different nitrogen species for the complete reduction phase. Based on measurements of the outlet concentrations of NO, NO2 , and NH3 . The remainder is assumed to be N2 .
converted to NO at low inlet concentrations of NH3 , while conversion to NO peaked at around 40% for the high inlet concentration of NH3 . These results indicate that the N2 -forming reactions, i.e. foremost the catalytic decomposition and NO-NH3 reactions, are more dependent on the NH3 concentration than the NO-forming reactions. Figure 11 shows the results for Test Case 3, in which there was a constant inlet concentration of 0.6% NH3 and various syngas concentrations, ranging from 0% to 40%. During the initial part of the reduction phase (for around 50 s) the apparent NO formation rate was independent of the syngas concentration (Fig. 11a). However, after the initial part, NO formation clearly decreased with increasing syngas concentration, although the conversion of NH3 was complete in all cases. The reactor temperature was increased with
the inlet syngas concentrations due to the exothermal reactions (cf. Fig. 7). The outlet concentrations of oxygen was close to the corresponding temperature-dependent equilibrium value in all cases. In only the 40% syngas case was oxygen carrier reduced to such an extent that all CuO was reduced and oxygen release ceased, and even in this case the oxygen release ceased first at the end of the reducing phase. When related to the gas-phase modelling, this indicates that the presence of the syngas makes the critical decrease in oxygen release rate occur earlier. Figure 11b shows the selectivity of NH3 oxidation towards different nitrogen species dependent on the inlet concentration of syngas. The selectivity towards NO decreased at high syngas concentrations. The oxygen carrier used in Test Case 4 was Cu2 O. During the test, an inlet flow with 2000 ppm NO was introduced into the
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Fig. 11. Apparent NO formation during the reduction phase with 0.6% NH3 and various syngas inlet concentrations. (a) NO formation rate. (b) Selectivity of NH3 oxidation over CuO towards different nitrogen species for the complete reduction phase. Based on measurements of outlet concentration of NO, NO2 , and NH3 . The balance is assumed to be N2 .
reactor, which was loaded with the reduced oxygen carrier and heated to temperatures of 850 °C, 900 °C, and 950 °C, respectively. NO was not reduced over Cu2 O under any of the conditions tested. 5. Conclusions We performed measurements of the oxidation of NH3 in a mixture with syngas over copper oxide. The results were interpreted using additional gas-phase modelling of the combustion and nitrogen chemistry. The outcomes relate to the formation of NOx in the fuel reactor with solid fuel chemical-looping combustion with oxygen uncoupling. The experimental results show that CuO mediates efficient conversion of NH3 . Considerable amounts of NH3 are oxidised to NO when the oxygen carriers are fully oxidised. However, the formation of NO is drastically decreased as the oxygen carrier becomes more reduced. Ageing of the oxygen carrier also decreases the selectivity of NH3 oxidation towards NO. The experiments also show that the conversion of NH3 to NO is sensitive to the presence of syngas. The modelling results suggest that the observed effect can be explained by the gas-phase chemistry. In this case, it is the rate of oxygen release that changes the selectivity towards NO or N2 as the oxygen carrier is reduced. However, heterogeneous reactions and catalytic effects cannot be ruled out. Acknowledgments This work was supported by National Natural Science Foundation of China (51376105, 51561125001), Chalmers University of Technology, Tsinghua University, and The Swedish Research Council Formas. References [1] J. Adanez, A. Abad, F. Garcia-Labiano, P. Gayan, L.F. De Diego, Progress in chemical-looping combustion and reforming technologies, Prog. Energy Combust. Sci. 38 (2) (2012) 215–282. [2] H. Leion, T. Mattisson, A. Lyngfelt, The use of petroleum coke as fuel in chemical-looping combustion, Fuel 86 (12–13) (2007) 1947–1958. [3] T. Mattisson, A. Lyngfelt, H. Leion, Chemical-looping with oxygen uncoupling for combustion of solid fuels, Int. J. Greenh. Gas Control 3 (1) (2009) 11–19.
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