Effect of CaO on NOx Reduction by Selective Non-Catalytic Reduction under Variable Gas Compositions in a Simulated Cement Precalciner Atmosphere

High-concentration CaO particles and gas compositions have a significant influence on NOx reduction by selective non-catalytic reduction (SNCR) in cement precalciners. The effect of gas composition on NOx reduction by SNCR with NH3 was studied in a cement precalciner atmosphere with and without CaO at 700–1100 °C. It was found that CaO significantly lowers NOx reduction efficiency between 750 °C and 1000 °C, which is attributed to the catalytic oxidation of NH3 to NO. Although increasing NH3 concentration was advantageous to NOx reduction, the existence of CaO led to the opposite result at 750–900 °C. Adding H2O can suppress the negative effect of CaO on NOx reduction. Decreasing O2 content from 10% to 1% shifts the temperature range in which CaO has a significant effect from 750–1000 °C to 800–1050 °C. CO has a variety of influences on the CaO effect under different experimental conditions. The influences of NH3, H2O, O2, and CO on the effect of CaO can be attributed to the impacts of the gas compositions on gas-phase NH3 conversion, gas-solid catalytic NH3 oxidation, or both processes. A proposed pathway for the effect of gas compositions on NOx reduction in CaO-containing SNCR process was developed that well predicted the CaO-containing SNCR process.


Introduction
Air pollution is now one of the most serious environmental problems worldwide. Nitrogen oxides (NO x , including NO and NO 2 ) emitted from cement rotary kilns are major contributors to acid rain, photochemical smog and haze [1]. In China, 1.7 Mt NO x was emitted by the cement industry in 2015, accounting for 9.1% of total NO x emissions from anthropogenic sources [2]. The cement industry has become the third-largest NO x emission source in China, after the thermal power industry and vehicles. Meanwhile, as the largest cement-producing country in the world, "Emission Standard of Air Pollutants for Cement Industry" (GB4915-2013), which is regarded as the strictest in history, has been issued by the Chinese government to reduce NO x , SO 2 , NH 3 and Hg 0 emissions.
Current NO x emission control technologies in the cement industry include low-temperature sintering, staged combustion, low NO x burners, over-fire air, SNCR and selective catalytic reduction (SCR) [3]. By far, SNCR is the predominant deNO x technology in cement rotary kilns due to its suitable, efficient, and cost-effective performance. Additionally, since SNCR is typically conducted in the cement precalciner, where the temperature ranges from 850 • C to 1200 • C and conforms to the temperature window of SNCR, some countries have introduced full-scale SNCR into the cement industry. However, as opposed to SNCR for electric power generation, NO x reduction efficiencies have industry. However, as opposed to SNCR for electric power generation, NOx reduction efficiencies have varied from 15% to 80% between projects, and even within individual projects, for cement kilns [4,5]. This variability is mainly due to the complicated environment in the cement precalciner, including temperature, gas composition and Ca-based particles (95% composed of CaO) from raw material calcinations. Therefore, the influences of multiple factors on NOx reduction by SNCR in a simulated cement precalciner atmosphere have been researched extensively.
Previous researchers, including our team, have investigated the effects of O2, H2O, SO2, CO, etc. on the SNCR process over various temperature ranges [6][7][8][9]. In addition to temperature and gas compositions, CaO particles of high concentration from raw material calcinations have a significant influence on NOx reduction in a cement precalciner. Actually, catalytic reactions involving NH3 and NO can occur on the CaO surface [10,11]. To date, researchers have investigated catalytic NH3 decomposition, NH3 oxidation, and NOx reduction on the CaO surface. In the absence of O2, CaO can catalyze NH3 decomposition to N2 [12,13]. CaO also catalyzes reactions between NH3 and NO in an O2-free atmosphere [5,14]. When O2 is present, these reactions do not apply, and the main effect of CaO is to catalyze NH3 oxidation with high NO selectivity [15,16].
Actually, gas-phase reactions coexist with the gas-solid catalytic reactions of the SNCR process in cement precalciners. However, previous research has only focused on the gas-solid catalytic reactions, not how these reactions interact with the gas-phase SNCR process. In addition, the temperature and gas conditions used in previous research did not match the conditions in cement precalciners. As a result, it is unclear how CaO affects overall NOx reduction efficiency within the temperature range and under the variable gas conditions of a cement precalciner. Thus, the present work attempts to investigate the influence of CaO on NOx reduction by SNCR with NH3 under variable gas compositions, using a fixed-bed reactor. Both gas-phase and gas-solid catalytic reactions were considered. The influences of gas compositions on the effect of CaO were also studied by varying NH3, H2O, O2, and CO contents. Pathways for the effect of gas compositions on NOx reduction in CaO-containing SNCR process were proposed.

Experimental Setup
The experimental setup consists of a gas feeding unit, a high-temperature fixed bed experimental system, and gas analytical instruments. A schematic diagram is shown in Figure 1.  A quartz cylinder with a 25 mm inner diameter, 1100 mm length and two gas inlets was used as the SNCR reactor. A quartz sieve was correctly plated at the constant temperature zone of the electric furnace to support Ca-based particles. The inner temperature of the reaction zones was detected online by a thermal couple. The simulated cement precalciner gas stream containing N 2 , NO, CO, CO 2 and air was introduced into the reactor from the lower inlet, the secondary stream with NH 3 (diluted by N 2 ) and water were fed into the reactor from the upper inlet. The two streams passed through respective preheating zones, and were then mixed in the reaction zones; the gas outlet was at the top of the reactor. The distance between the stream mixing point and the sieve plate was 50 mm.

Samples
CaO particles were prepared by decomposing analytical-grade CaCO 3 with a particle size of 38-45 µm. In the experiments without CaO, SiO 2 particles with a size of 38-45 µm were used as the background solid material due to its sluggishness. Prior to the experiments, SiO 2 or CaCO 3 was introduced into the reactor, distributed uniformly on the sieve plate, and then a preheating process was carried out: (1) The heating rate was set to 15 • C min −1 and the solid materials were gradually heated from room temperature. Meanwhile, the air flow rate was set to 0.1 dm 3 min −1 to take away the CO 2 generated from CaCO 3 calcinations. During this stage, the sintering of the solid particles proceeded gradually; (2) When the temperature of the solid material was about 850 • C, the air-flow rate was set to 0.5 dm 3 min −1 ; (3) When the temperature reached 1000 • C, the flow rate was set to 2 dm 3 min −1 . This stage lasted for 30 min; (4) The heating process was stopped, and the solid materials were cooled down to the target experimental temperature in N 2 atmosphere. After the preheating process, CaCO 3 completely decomposed to CaO, and the particle materials were sintered to a porous block structure to avoid being blown out by the gas stream.
The properties of the CaO samples are listed in Table 1.

Experimental Methods
The experiments were performed at atmospheric pressure. The basic cement precalciner gas was composed of 10% O 2 , 20% CO 2 , 3% H 2 O, 4000 ppm CO, 400 ppm NO, and 400 ppm NH 3 with N 2 used as balance gas, and the total flow rate was 2 dm 3 min −1 . The effects of both CaO amount and basic gas composition on NO x reduction by SNCR were investigated. The concentrations of O 2 , CO, NO, and NO 2 were monitored by using a Testo 350-pro flue gas analyzer. NH 3 slip was sampled by a bubble column absorber containing dilute sulfuric acid solution and then analyzed by Nessler's reagent spectrophotometry. NO x reduction efficiency, abbreviated as NO x reduction, is measured based on inlet and outlet NO x concentration. The temperature window is defined as the temperature region in which the NO x reduction efficiency exceeds 80% of the maximum value. The NO x reduction efficiency is defined as: where C NO x ,in and C NO x ,out represent the inlet and outlet NO x concentrations, respectively.

Effect of CaO Amount on NO x Reduction
The effect of CaO amounts on NO x reduction by SNCR was studied under the basic gas compositions, and the results are illustrated in Figure 2.

Effect of CaO Amount on NOx Reduction
The effect of CaO amounts on NOx reduction by SNCR was studied under the basic gas compositions, and the results are illustrated in Figure 2. As shown in Figure 2, the NOx reduction curve without CaO is a unimodal shape. The maximum reduction efficiency of approximately 68% occurs at 850 °C. The temperature window of SNCR is 800-1050 °C. After CaO is added into the reactor, the NOx reduction efficiencies between 800 °C and 1000 °C decrease significantly, and these curves take on a bimodal shape. As CaO increases from 0.5 g to 5 g, the low-temperature peak value of NOx reduction drops from 61% to 38% with the corresponding temperature change from 800 °C to 750 °C. The high-temperature peak value, which is approximately 58% at 1000 °C, is hardly influenced by CaO content. The trough value of NOx reduction at 800-850 °C drops sharply with the increase of CaO content, and even a negative reduction efficiency, corresponding to the CaO of 5 g, appears at 850 °C.
As discussed in detail in our previous study [8], without CaO addition, the final NOx reduction efficiency by SNCR is determined by the competition between gas-phase NOx reduction and gas-phase NH3 oxidation processes: In a CaO-containing atmosphere, reactions (3) and (4) also occur catalytically on the CaO surface, which can lead to extra NOx formation, and may be responsible for the negative NOx reduction efficiency at 850 °C in the 5 g CaO group of Figure 2  As shown in Figure 2, the NO x reduction curve without CaO is a unimodal shape. The maximum reduction efficiency of approximately 68% occurs at 850 • C. The temperature window of SNCR is 800-1050 • C. After CaO is added into the reactor, the NO x reduction efficiencies between 800 • C and 1000 • C decrease significantly, and these curves take on a bimodal shape. As CaO increases from 0.5 g to 5 g, the low-temperature peak value of NO x reduction drops from 61% to 38% with the corresponding temperature change from 800 • C to 750 • C. The high-temperature peak value, which is approximately 58% at 1000 • C, is hardly influenced by CaO content. The trough value of NO x reduction at 800-850 • C drops sharply with the increase of CaO content, and even a negative reduction efficiency, corresponding to the CaO of 5 g, appears at 850 • C.
As discussed in detail in our previous study [8], without CaO addition, the final NO x reduction efficiency by SNCR is determined by the competition between gas-phase NO x reduction and gas-phase NH 3 oxidation processes: In a CaO-containing atmosphere, reactions (3) and (4) also occur catalytically on the CaO surface, which can lead to extra NO x formation, and may be responsible for the negative NO x reduction efficiency at 850 • C in the 5 g CaO group of Figure 2 Figure 3 demonstrates that NH3 can be oxidized to NOx by gas-phase reactions. In the 0 g CaO group, the outlet NOx concentration does not increase monotonically, as reported in other research [14], when temperature rises from 700 °C to 1100 °C. The relatively high level of outlet NOx at 700-800 °C can be attributed to the high concentration of CO in the gas stream. As stated in our previous study [8], CO can enhance the conversion of NH3 to NH2 and NH at low temperatures, thus promoting gas-phase NH3 oxidation. As temperature rises, CO is gradually consumed by O2 in the preheating zone of the reactor, which inhibits the oxidation of NH3 to NO. When the temperature goes above 950 °C, the conversion of NH3 to NH is accelerated without CO, raising the outlet NOx concentration. In the 5 g CaO group, the relationship of outlet NOx concentration with temperature is almost opposite from the group with no CaO. As temperature rises, the NOx concentration increases to its maximum at 850 °C and then gradually declines. There are two types of reaction process involving NH3 in the 5 g CaO group, namely, gas-solid catalytic NH3 oxidation and gas-phase NH3 conversion. For gas-solid catalytic NH3 oxidation, the initial and rate-determining step is the H-abstraction of adsorbed NH3 [17,18]: Then, deeper H-abstraction might occur with NH2(ad), forming NH(ad), generating NO: In the presence of O2 and NO, NH2(ad) can reduce adsorbed NO to form N2: As such, the NOx selectivity of catalytic NH3 oxidation is determined directly by the proportion of NH2(ad) involved in reactions (7) and (10).  Figure 3 demonstrates that NH 3 can be oxidized to NO x by gas-phase reactions. In the 0 g CaO group, the outlet NO x concentration does not increase monotonically, as reported in other research [14], when temperature rises from 700 • C to 1100 • C. The relatively high level of outlet NO x at 700-800 • C can be attributed to the high concentration of CO in the gas stream. As stated in our previous study [8], CO can enhance the conversion of NH 3 to NH 2 and NH at low temperatures, thus promoting gas-phase NH 3 oxidation. As temperature rises, CO is gradually consumed by O 2 in the preheating zone of the reactor, which inhibits the oxidation of NH 3 to NO. When the temperature goes above 950 • C, the conversion of NH 3 to NH is accelerated without CO, raising the outlet NO x concentration. In the 5 g CaO group, the relationship of outlet NO x concentration with temperature is almost opposite from the group with no CaO. As temperature rises, the NO x concentration increases to its maximum at 850 • C and then gradually declines. There are two types of reaction process involving NH 3 in the 5 g CaO group, namely, gas-solid catalytic NH 3 oxidation and gas-phase NH 3 conversion. For gas-solid catalytic NH 3 oxidation, the initial and rate-determining step is the H-abstraction of adsorbed NH 3 [17,18]: Then, deeper H-abstraction might occur with NH 2(ad) , forming NH (ad) , generating NO: In the presence of O 2 and NO, NH 2(ad) can reduce adsorbed NO to form N 2 : As such, the NO x selectivity of catalytic NH 3 oxidation is determined directly by the proportion of NH 2(ad) involved in reactions (7) and (10).
For gas-phase NH 3 oxidation and NO x reduction, the rate-determining step is the conversion of NH 3 to NH 2 [19]: Figure 3 also shows that the outlet NH 3 concentration with CaO addition is lower than that without CaO below 850 • C. This indicates that reactions (5) and (6) proceed faster than reactions (12) and (13) below 850 • C. Because most NH 3 is oxidized in the gas-solid reaction process, the NO x concentration is mainly affected by the NO x selectivity of catalytic NH 3 oxidation. This leads to the escalation of outlet NO x concentrations up to 850 • C. According to the kinetic parameters in previous research, when the temperature rises from 700 • C to 1100 • C, the rate constant of catalytic NH 3 oxidation varies insignificantly [5], while the rate constants of gas-phase NH 3 conversion increase significantly [19]. As a result, above 850 • C, the gas-phase NH 3 conversion rate gradually surpasses the catalytic NH 3 oxidation rate. More NH 3 is oxidized in gas-phase reaction processes with a low NO x selectivity, diminishing the outlet NO x concentration. When the temperature rises to 1100 • C, the gas-phase NH 3 conversion is fast enough to prevent NH 3 involvement in catalytic reactions, and almost all NH 3 is consumed by the gas-phase process. In this condition, the catalytic NH 3 oxidation on the CaO surface can be ignored. When NO is introduced into the gas stream, NH 3 consumption by the gas-phase NO x reduction process further decreases the NH 3 in the catalytic oxidation. This narrows the temperature range with significant CaO effect to 750-1000 • C.

Influence of Initial NH 3 Concentration
In Figure 2, there is a slight increase in NO x reduction efficiency at 700-800 • C in the 0.5 g CaO group compared with the group with no CaO. This improvement may be related to the NH 3 concentration involved in gas-phase NH 3 oxidation and NO x reduction. To verify this, the effect of initial NH 3 concentrations on SNCR was investigated, and the results are shown in Figure 4. For gas-phase NH3 oxidation and NOx reduction, the rate-determining step is the conversion of NH3 to NH2 [19]: Figure 3 also shows that the outlet NH3 concentration with CaO addition is lower than that without CaO below 850 °C. This indicates that reactions (5) and (6) proceed faster than reactions (12) and (13) below 850 °C. Because most NH3 is oxidized in the gas-solid reaction process, the NOx concentration is mainly affected by the NOx selectivity of catalytic NH3 oxidation. This leads to the escalation of outlet NOx concentrations up to 850 °C. According to the kinetic parameters in previous research, when the temperature rises from 700 °C to 1100 °C, the rate constant of catalytic NH3 oxidation varies insignificantly [5], while the rate constants of gas-phase NH3 conversion increase significantly [19]. As a result, above 850 °C, the gas-phase NH3 conversion rate gradually surpasses the catalytic NH3 oxidation rate. More NH3 is oxidized in gas-phase reaction processes with a low NOx selectivity, diminishing the outlet NOx concentration. When the temperature rises to 1100 °C, the gas-phase NH3 conversion is fast enough to prevent NH3 involvement in catalytic reactions, and almost all NH3 is consumed by the gas-phase process. In this condition, the catalytic NH3 oxidation on the CaO surface can be ignored. When NO is introduced into the gas stream, NH3 consumption by the gas-phase NOx reduction process further decreases the NH3 in the catalytic oxidation. This narrows the temperature range with significant CaO effect to 750-1000 °C.

Influence of Initial NH3 Concentration
In Figure 2, there is a slight increase in NOx reduction efficiency at 700-800 °C in the 0.5 g CaO group compared with the group with no CaO. This improvement may be related to the NH3 concentration involved in gas-phase NH3 oxidation and NOx reduction. To verify this, the effect of initial NH3 concentrations on SNCR was investigated, and the results are shown in Figure 4. The NOx reduction efficiency without CaO increases with initial NH3 concentration above 750 °C. However, there is an adverse result below 750 °C, suggesting that the lower initial NH3 The NO x reduction efficiency without CaO increases with initial NH 3 concentration above 750 • C. However, there is an adverse result below 750 • C, suggesting that the lower initial NH 3 concentration improves NO x reduction at low temperatures. Therefore, when 0.5 g CaO is added, the NH 3 involved in gas-phase NH 3 oxidation and NO x reduction decreases due to NH 3 consumption through catalytic reactions, resulting in a slight increase in NO x reduction efficiency at 700-800 • C.
It can also be seen from Figure 4 that increasing the initial NH 3 concentration results in lower NO x reduction efficiencies at 750-900 • C when CaO is added. At this temperature, gas-solid catalytic NH 3 oxidation is more sensitive to NH 3 concentration changes than gas-phase SNCR process. The initial NH 3 concentration has a minor influence on the low-temperature peak value of NO x reduction, but significantly augments the high-temperature one. Conversely, as shown in Figure 2, CaO content affects the low-temperature peak value notably, but has almost no effect on the high-temperature one.
In an actual SNCR operation in a cement precalciner, NH 3 addition is more controllable than CaO, so the temperature range of 950-1050 • C is more favorable for obtaining high reduction efficiency.

Influence of H 2 O
The influence of H 2 O contents on the effect of CaO was investigated, and the results are shown in Figure 5. concentration improves NOx reduction at low temperatures. Therefore, when 0.5 g CaO is added, the NH3 involved in gas-phase NH3 oxidation and NOx reduction decreases due to NH3 consumption through catalytic reactions, resulting in a slight increase in NOx reduction efficiency at 700-800 °C. It can also be seen from Figure 4 that increasing the initial NH3 concentration results in lower NOx reduction efficiencies at 750-900 °C when CaO is added. At this temperature, gas-solid catalytic NH3 oxidation is more sensitive to NH3 concentration changes than gas-phase SNCR process. The initial NH3 concentration has a minor influence on the low-temperature peak value of NOx reduction, but significantly augments the high-temperature one. Conversely, as shown in Figure 2, CaO content affects the low-temperature peak value notably, but has almost no effect on the high-temperature one. In an actual SNCR operation in a cement precalciner, NH3 addition is more controllable than CaO, so the temperature range of 950-1050 °C is more favorable for obtaining high reduction efficiency.

Influence of H2O
The influence of H2O contents on the effect of CaO was investigated, and the results are shown in Figure 5.  Figure 5 shows that when no CaO is present, H2O has a positive effect on gas-phase SNCR process, which was discussed in our previous research [8]. In the 5 g CaO group, the NOx reduction efficiency without H2O remains negative up to approximately 870 °C. When H2O is added, the efficiency increases significantly and the negative effect of CaO on NOx reduction is suppressed to a certain extent. This may be because adding H2O stimulates gas-phase NH3 conversion, decreasing NH3 involved in catalytic oxidation. Furthermore, H2O can compete for active sites, adsorbing NH3 on the CaO surface, and thus inhibiting subsequent catalytic NH3 oxidation [16,20]. Therefore, H2O addition benefits NOx reduction not only in a gas-phase reaction system, but also in a CaO-containing atmosphere.

Influence of O2
O2 content is not uniform in cement precalciners, and generally ranges from 0% to 13%. Figure 6 shows the influence of O2 concentrations on the effect of CaO on SNCR.
As shown in Figure 6, the results without CaO coincide with our previous research. With CaO addition, when O2 content is reduced from 10% to 1%, the low-temperature peak, the trough, and the high-temperature peak of NOx reduction curve shift to the higher temperatures of 800 °C, 950 °C, and 1050 °C, respectively. In addition, the trough value drops to a more negative one from −4% to −18%. This result suggests that decreasing O2 content shifts the temperature region in which CaO  Figure 5 shows that when no CaO is present, H 2 O has a positive effect on gas-phase SNCR process, which was discussed in our previous research [8]. In the 5 g CaO group, the NO x reduction efficiency without H 2 O remains negative up to approximately 870 • C. When H 2 O is added, the efficiency increases significantly and the negative effect of CaO on NO x reduction is suppressed to a certain extent. This may be because adding H 2 O stimulates gas-phase NH 3 conversion, decreasing NH 3 involved in catalytic oxidation. Furthermore, H 2 O can compete for active sites, adsorbing NH 3 on the CaO surface, and thus inhibiting subsequent catalytic NH 3 oxidation [16,20]. Therefore, H 2 O addition benefits NO x reduction not only in a gas-phase reaction system, but also in a CaO-containing atmosphere.

Influence of O 2
O 2 content is not uniform in cement precalciners, and generally ranges from 0% to 13%. Figure 6 shows the influence of O 2 concentrations on the effect of CaO on SNCR.
As shown in Figure 6, the results without CaO coincide with our previous research. With CaO addition, when O 2 content is reduced from 10% to 1%, the low-temperature peak, the trough, and the high-temperature peak of NO x reduction curve shift to the higher temperatures of 800 • C, 950 • C, and 1050 • C, respectively. In addition, the trough value drops to a more negative one from −4% to −18%. This result suggests that decreasing O 2 content shifts the temperature region in which CaO significantly impacts NO x reduction from 750-1000 • C to the higher value range of 800-1050 • C, which shows that the gas-phase SNCR process is very susceptible to the change of O 2 content, while the variation of O 2 content between 1% and 10% fails to influence catalytic NH 3 oxidation [11]. Hence, when O 2 content decreases, the SNCR process slows and requires higher temperatures to proceed fast enough to dominate the whole process. As NO x selectivity of catalytic NH 3 oxidation rises with temperature, the minimum NO x reduction efficiency drops. significantly impacts NOx reduction from 750-1000 °C to the higher value range of 800-1050 °C, which shows that the gas-phase SNCR process is very susceptible to the change of O2 content, while the variation of O2 content between 1% and 10% fails to influence catalytic NH3 oxidation [11]. Hence, when O2 content decreases, the SNCR process slows and requires higher temperatures to proceed fast enough to dominate the whole process. As NOx selectivity of catalytic NH3 oxidation rises with temperature, the minimum NOx reduction efficiency drops. O2 is necessary for the SNCR process, so when O2 is not present, the NOx reduction efficiency without CaO is trivial below 1000 °C. The slight efficiency increase above 1000 °C is due to H radicals, which are generated from H2O pyrolysis and help NH3 conversion [8]. In contrast, the NOx reduction efficiency of the 5 g CaO group remains trivial, even at high temperatures. Although CaO can catalyze NO reduction by NH3 in the absence of O2 [5,14], the catalyst cannot restore its activity without O2 reoxidation [18]. On the other hand, the competitive adsorption of H2O by CaO not only inhibits the catalytic NH3 + NO reaction, but also reduces gaseous H radicals. This weakens gas-phase NOx reduction in the absence of O2. Therefore, it is not possible to reduce NOx with NH3 in an O2-free zone in a cement precalciner.

Influence of CO
CO is an important constituent of flue gas in a cement precalciner. Similar to O2, it varies with location and operation conditions. It has been reported that CaO can catalyze NO reduction by CO, but the catalysis becomes negligible in the presence of either O2 or a high concentration of CO2 [21]. Previous research has investigated the influence of CO on gas-phase NOx reduction [8,22,23]; however, it remains unclear how CO influences the effect of CaO on NOx reduction with NH3. Therefore the influence of CO concentrations in a multi-phase reaction system was studied, and the results are seen in Figure 7. Figure 7a shows an improvement of gas-phase NOx reduction when adding CO at low temperatures. In the 5 g CaO group, the increase of CO content leads to a slight decrease of NOx reduction efficiency between 820 °C and 900 °C, implying that the effect of CaO is enhanced. This may be attributed to H2O consumption by CO [24]: 2 CO + OH CO H → + O 2 is necessary for the SNCR process, so when O 2 is not present, the NO x reduction efficiency without CaO is trivial below 1000 • C. The slight efficiency increase above 1000 • C is due to H radicals, which are generated from H 2 O pyrolysis and help NH 3 conversion [8]. In contrast, the NO x reduction efficiency of the 5 g CaO group remains trivial, even at high temperatures. Although CaO can catalyze NO reduction by NH 3 in the absence of O 2 [5,14], the catalyst cannot restore its activity without O 2 reoxidation [18]. On the other hand, the competitive adsorption of H 2 O by CaO not only inhibits the catalytic NH 3 + NO reaction, but also reduces gaseous H radicals. This weakens gas-phase NO x reduction in the absence of O 2 . Therefore, it is not possible to reduce NO x with NH 3 in an O 2 -free zone in a cement precalciner.

Influence of CO
CO is an important constituent of flue gas in a cement precalciner. Similar to O 2 , it varies with location and operation conditions. It has been reported that CaO can catalyze NO reduction by CO, but the catalysis becomes negligible in the presence of either O 2 or a high concentration of CO 2 [21]. Previous research has investigated the influence of CO on gas-phase NO x reduction [8,22,23]; however, it remains unclear how CO influences the effect of CaO on NO x reduction with NH 3 . Therefore the influence of CO concentrations in a multi-phase reaction system was studied, and the results are seen in Figure 7. Figure 7a shows an improvement of gas-phase NO x reduction when adding CO at low temperatures. In the 5 g CaO group, the increase of CO content leads to a slight decrease of NO x reduction efficiency between 820 • C and 900 • C, implying that the effect of CaO is enhanced. This may be attributed to H 2 O consumption by CO [24]: This series of reactions weakens the inhibitory effect of H 2 O on catalytic NH 3 oxidation, engaging more NH 3 in catalytic oxidation, leading to incremental NO x generation. During the experiments, it was observed that the outlet CO concentration with the addition of CaO was lower than that without CaO under the same conditions. This is because CaO can catalyze CO oxidation [14], which suggests that there exist active sites adsorbing CO onto the CaO surface. If the active sites for CO are the same as those for NH 3 , CO should inhibit the negative effect of CaO on NO x reduction similarly to H 2 O. However, the results suggest that the active sites adsorbing CO and NH 3 are distinct, and that CO does not directly influence catalytic NH 3 oxidation. Therefore, the only reason that adding CO would enhance NO x reduction efficiency below 820 • C in the 5 g CaO group is that CO can promote the gas-phase SNCR process. The influence of CO can be ignored above 1000 • C in the group with no CaO, due to CO consumption by O 2 in the reactor's preheating zone. In the 5 g CaO group, because CO oxidation is promoted by CaO, the influence of CO is already negligible at 900 • C. This series of reactions weakens the inhibitory effect of H2O on catalytic NH3 oxidation, engaging more NH3 in catalytic oxidation, leading to incremental NOx generation. During the experiments, it was observed that the outlet CO concentration with the addition of CaO was lower than that without CaO under the same conditions. This is because CaO can catalyze CO oxidation [14], which suggests that there exist active sites adsorbing CO onto the CaO surface. If the active sites for CO are the same as those for NH3, CO should inhibit the negative effect of CaO on NOx reduction similarly to H2O. However, the results suggest that the active sites adsorbing CO and NH3 are distinct, and that CO does not directly influence catalytic NH3 oxidation. Therefore, the only reason that adding CO would enhance NOx reduction efficiency below 820 °C in the 5 g CaO group is that CO can promote the gas-phase SNCR process. The influence of CO can be ignored above 1000 °C in the group with no CaO, due to CO consumption by O2 in the reactor's preheating zone. In the 5 g CaO group, because CO oxidation is promoted by CaO, the influence of CO is already negligible at 900 °C. As can be seen from Figure 7b, when CO is introduced into the reactor with the secondary stream, NH3 is actually injected into an O2-free atmosphere with a high CO content before mixing with the stream containing O2. In this case, a large amount of NH3 is converted to NH2 and NH radicals, which shifts the temperature window of SNCR to lower values and causes negative NOx reduction efficiencies at high temperatures [8]. Additionally, except that the NOx reduction efficiency of 5 g CaO group is a little lower than that of the 0 g CaO group below 800 °C, there is little difference between the results of the two groups with CO addition. This means that the negative effect of CaO on NOx reduction is almost completely suppressed. Consequently, it can be inferred As can be seen from Figure 7b, when CO is introduced into the reactor with the secondary stream, NH 3 is actually injected into an O 2 -free atmosphere with a high CO content before mixing with the stream containing O 2 . In this case, a large amount of NH 3 is converted to NH 2 and NH radicals, which shifts the temperature window of SNCR to lower values and causes negative NO x reduction efficiencies at high temperatures [8]. Additionally, except that the NO x reduction efficiency of 5 g CaO group is a little lower than that of the 0 g CaO group below 800 • C, there is little difference between the results of the two groups with CO addition. This means that the negative effect of CaO on NO x reduction is almost completely suppressed. Consequently, it can be inferred that NH 2 and NH radicals generated from gas-phase NH 3 conversion cannot be adsorbed by CaO to join in the catalytic reactions. The catalytic oxidation of NH 3 must be initiated by the adsorption of molecular NH 3 .
Although the injection method in Figure 7b can suppress the negative effect of CaO on NO x reduction, NH 3 is apt to be oxidized to NO x via gas-phase reactions. Therefore, in an actual SNCR operation for cement precalciners, NH 3 should not be injected into an O 2 -free area with high CO and then mixed with an O 2 -containing gas stream.

Discussion
Homogeneous reactions (gas-phase) play a predominant role in non-CaO SNCR in cement precalciners, and the rate-determining step is the conversion of NH 3 to NH 2 . For heterogeneous catalytic reactions (gas-solid), the rate-determining step is the H-abstraction of adsorbed NH 3 [17,18]. CaO has an inhibitory effect on the SNCR process, especially in the middle temperature zone of 750-1000 • C; the CaO contents are closely related to the inhibitory effect of CaO. CaO [8]; these radicals then stimulate gas-phase NH 3 conversion, decreasing the NH 3 involved in catalytic oxidation. Furthermore, H 2 O can compete for NH 3 active sites on the CaO surface, and thus reducing the occurrence of heterogeneous reactions; O 2 is indispensable for NO x reduction by SNCR, as O 2 is the primary reactant of NO production from the reaction of NH 2 with O 2 in the CaO-containing SNCR process, which causes a negative effect of CaO on NO x reduction. CO leads to a slight decrease of NO x reduction efficiency between 820 • C and 900 • C, and enhances the effect of CaO, which may be attributed to H 2 O consumption by CO [24]. The influence of CO is already negligible above 900 • C. The results of this study could provide a reference for engineering applications for optimizing NO x reduction processes by NH 3 -SNCR in cement precalciners. A pathway for the effect of gas compositions on NO x reduction in CaO-containing SNCR processes, based on conclusions derived from the experimental results and acknowledged reactions, is given in Figure 8. that NH2 and NH radicals generated from gas-phase NH3 conversion cannot be adsorbed by CaO to join in the catalytic reactions. The catalytic oxidation of NH3 must be initiated by the adsorption of molecular NH3. Although the injection method in Figure 7b can suppress the negative effect of CaO on NOx reduction, NH3 is apt to be oxidized to NOx via gas-phase reactions. Therefore, in an actual SNCR operation for cement precalciners, NH3 should not be injected into an O2-free area with high CO and then mixed with an O2-containing gas stream.

Discussion
Homogeneous reactions (gas-phase) play a predominant role in non-CaO SNCR in cement precalciners, and the rate-determining step is the conversion of NH3 to NH2. For heterogeneous catalytic reactions (gas-solid), the rate-determining step is the H-abstraction of adsorbed NH3 [17,18]. CaO has an inhibitory effect on the SNCR process, especially in the middle temperature zone of 750-1000 °C; the CaO contents are closely related to the inhibitory effect of CaO. CaO can catalyze NH3 decomposition. Moreover, CaO catalyzed NH3 oxidation in the presence of O2, which can lead to extra NOx formation.
The effects on NOx reduction efficiency in the presence of CaO with the increase of H2O, O2, and CO concentration are different. The addition of H2O benefits NOx reduction in homogeneous reactions, and suppresses the negative effect of CaO on NOx reduction to a certain extent. The thermal decomposition of H2O results in the regeneration of O and OH radicals [8]; these radicals then stimulate gas-phase NH3 conversion, decreasing the NH3 involved in catalytic oxidation. Furthermore, H2O can compete for NH3 active sites on the CaO surface, and thus reducing the occurrence of heterogeneous reactions; O2 is indispensable for NOx reduction by SNCR, as O2 is the primary reactant of NO production from the reaction of NH2 with O2 in the CaO-containing SNCR process, which causes a negative effect of CaO on NOx reduction. CO leads to a slight decrease of NOx reduction efficiency between 820 °C and 900 °C, and enhances the effect of CaO, which may be attributed to H2O consumption by CO [24]. The influence of CO is already negligible above 900 °C. The results of this study could provide a reference for engineering applications for optimizing NOx reduction processes by NH3-SNCR in cement precalciners. A pathway for the effect of gas compositions on NOx reduction in CaO-containing SNCR processes, based on conclusions derived from the experimental results and acknowledged reactions, is given in Figure 8.   Figure 8. Proposed pathways for the effect of gas compositions on NOx reduction in CaO-containing SNCR process.

Conclusions
CaO in a cement precalciner can inhibit NOx reduction by the SNCR process between 750 °C and 1000 °C due to the catalytic oxidation of NH3 to NO on the CaO surface. When CaO is added, NOx reduction efficiency follows a bimodal distribution against temperature. The low-temperature peak changes with CaO content, while the high-temperature peak is hardly influenced by CaO

Conclusions
CaO in a cement precalciner can inhibit NO x reduction by the SNCR process between 750 • C and 1000 • C due to the catalytic oxidation of NH 3 to NO on the CaO surface. When CaO is added, NO x reduction efficiency follows a bimodal distribution against temperature. The low-temperature peak changes with CaO content, while the high-temperature peak is hardly influenced by CaO content. The trough value between the two peaks decreases significantly as CaO increases. With a high CaO content, the trough value of NO x reduction at 850 • C is even negative. In CaO-containing cement precalciners, gas-phase NH 3 conversion coexists with gas-solid catalytic NH 3 oxidation. Below 750 • C, the NO x selectivity of catalytic NH 3 oxidation is low, so CaO does not significantly impact NO x reduction efficiency. Above 1000 • C, the gas-phase NH 3 conversion proceeds fast enough to prevent NH 3 from being oxidized catalytically. Although NO x reduction performance appears to improve by adding CaO at low temperatures, the improvement is slight, and likely results from the loss of NH 3 involved in gas-phase NH 3 conversion. NH 3 , H 2 O, O 2, and CO influences the effect of CaO by affecting gas-phase NH 3 conversion, or gas-solid catalytic NH 3 oxidation, or both processes. Although increasing NH 3 concentration is advantageous to NO x reduction, increasing NH 3 results in a decrease in NO x reduction efficiency at 750-900 • C when CaO is added. H 2 O can significantly suppress the negative effect of CaO on NO x reduction. As O 2 content decreases from 10% to 1%, the temperature region for a significant CaO effect shifts higher. With the addition of CO, the effect of CaO is reduced below 820 • C, while it is slightly enhanced at approximately 850 • C. When NH 3 is injected into an O 2 -free atmosphere with a high CO content, it is apt to be oxidized to NO x after mixing with an O 2 -containing stream. In this case, the effect of CaO on NO x reduction is almost eliminated.
Considering the effect of CaO on NO x reduction under variable gas compositions, it is recommended that NH 3 be injected into the O 2 -containing area with a low CaO concentration in the temperature range of 950-1100 • C to obtain a high NO x reduction efficiency by SNCR in a cement precalciner.