Mechanism of the Reaction between HNCO and CaO in the Urea

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Mechanism of the Reaction between HNCO and CaO in the UreaSelective Non-catalytic Reduction deNOx Process Shi-long Fu, Qiang Song,* and Qiang Yao Key Laboratory of Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: CaO decreased the performance of the urea-selective non-catalytic reduction deNOx process by affecting the conversion of urea pyrolysis products. The effect of CaO on HNCO conversion was studied in a pyrolysis/gas−solid reaction system along with diffuse reflectance infrared Fourier transform spectroscopy and scanning electron microscopy. CaO was found to react with HNCO, and HNCO conversion and product selectivity changed with the temperature and O2. Mechanism analysis showed that CaO first reacted with HNCO to produce Ca(NCO)2. Ca(NCO)2 decomposed to CaCN2 and CO2 at high temperatures. CaCN2 stably existed because of its high thermal stability. In the presence of O2, Ca(NCO)2 decomposed and was also oxidized to CaO, N2O, and CO2. In addition, produced CaCN2 was oxidized to CaO, NO, and CO. CaO was reproduced in Ca(NCO)2 and CaCN2 oxidation; thus, the reaction between CaO and HNCO was accelerated and HNCO conversion was increased. The selectivity of products was determined by Ca(NCO)2 decomposition, Ca(NCO)2 oxidation, and CaCN2 oxidation.

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INTRODUCTION Nitrogen oxides (NOx) are among the most important air pollutants. Cement kilns and circulating fluidized-bed boilers (CFBBs) are important emission sources of NOx in China. The selective non-catalytic reduction (SNCR) deNOx process is the most suitable technology for cement kilns and CFBBs in terms of cost and deNOx efficiency.1−4 Urea is usually used as the reducing agent and is injected into the furnace in the form of urea−water−solution (UWS). With the increasing temperature, UWS droplets undergo water evaporation, urea melting, and pyrolysis.5−7 The products of urea pyrolysis are NH3 and HNCO, and HNCO reacts with H2O to form NH3 and CO2.8,9 CaO particles of high concentration in cement kilns and CFBBs catalyze the oxidation of NH310,11 and affect HNCO formation and conversion,12 thus decreasing SNCR deNOx efficiency.13 The influence of CaO on urea pyrolysis and product conversion must be considered when urea-SNCR technology is applied in cement kilns and CFBBs. Although the catalysis of CaO on NH3 oxidation has been clarified,10,11,14 the influence of CaO on HNCO conversion remains unclear. Liu et al.15 studied sewage sludge pyrolysis and found that HNCO emission was decreased in the presence of CaO. Ren et al.16 studied the effect of CaO on the pyrolysis of wheat straw, rice straw, and corn cob and also found that HNCO production was inhibited by CaO. Li et al.17 studied the catalytic effect of metal oxides on urea alcoholysis and found that isocyanate was formed in the presence of alkali metal oxides during urea decomposition and the activity order was ZnO > CaO > MgO > La2O3. Li et al. indicated that alkali metal oxides can catalyze urea decomposition, which is the key step of urea alcoholysis. Kilimova et al.12 used thermogravimetry to study the effect of CaO on urea pyrolysis and used Fourier transform infrared spectroscopy (FTIR) to measure the infrared absorbance of gas products. Results showed that CaO © XXXX American Chemical Society

decelerated urea pyrolysis and decreased the amount of HNCO production. NO and N2O were also produced in the presence of O2. Kilimova et al. explained that CaO reacted with HNCO to produce Ca(NCO)2, but the further conversion of Ca(NCO)2 was not studied. Previous studies showed that HNCO conversion was affected by CaO, but the reaction and mechanism of CaO on HNCO conversion were unclear. A pyrolysis and gas−solid reaction experimental system was established in this work to study the effect of CaO on HNCO conversion. The solid products were analyzed using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The mechanism of the reaction between CaO and HNCO was proposed.

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EXPERIMENTAL SECTION

A pyrolysis and gas−solid reaction system was established, as shown in Figure 1, which produced HNCO online to react with CaO, considering that compressed HNCO is too corrosive to store in a gas tank. Argon was used as the carrier gas; the total flow rate was 1.5 L/min; and the O2 concentration was 5% in the presence of O2. Cyanuric acid (CYA) pyrolysis,18 as eq 1, occurred in a quartz reaction chamber inserted into furnace 1 to produce HNCO online. The pyrolysis temperature was 380 °C, and 5 mg of CYA was used in each experiment.

CYA → 3HNCO

(1)

Produced HNCO entered the gas−solid fixed-bed reactor in furnace 2, which had an external quartz pipe and an internal quartz crucible. A total of 20 mg of CaO (particle size was 80−100 μm) was loaded on quartz wool in the crucible to provide excess CaO relative to produced HNCO. The feed gas flowed through the CaO sample bed, and the outlet gas was analyzed by FTIR (Nicolet 6700). PolytetrafluoroReceived: January 6, 2017 Revised: April 14, 2017 Published: April 16, 2017 A

DOI: 10.1021/acs.energyfuels.7b00046 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Diagram of the pyrolysis/gas−solid reaction system. ethylene (PTFE) pipes heating to 150 °C were used to connect FTIR and the reactors to reduce HNCO condensation. HNCO decomposition and oxidation can be ignored in the temperature range of 600−900 °C because of the stability of HNCO at high temperatures19 and short residence time (less than 50 ms) in the reactor, which was manifested by blank experiments. Scanning electron microscopy (SEM, Zeiss Merlin) and DRIFTS (Pike Accessory) were used to analyze the solid products. The resolution of the DRIFTS spectra was 4 cm−1, and each spectrum was obtained after 100 scans. Given that H2O was a residual in CYA, the pyrolysis product of CYA was HNCO, along with small amounts of NH3, CO2, and N2O. Unreacted HNCO and produced CO, CO2, NO, and N2O were measured after the reaction with CaO. The amount of gas products can be obtained by multiplying their concentrations by the flow rate and then integrating with respect to time. The amount of produced HNCO in furnace 1 can be obtained by the mass of added CYA, which was equal to the inflow amount to furnace 2. Thus, HNCO conversion can be calculated as eq 2, and the selectivity of CO, NO, and CO2 can be defined as eq 3. N2O selectivity can be defined as eq 4 nHNCO,out αHNCO = 1 − × 100% nHNCO,in (2)

SCO/NO/CO2 =

SN2O =

nCO/NO/CO2,out − nCO/NO/CO2,in nHNCO,in − nHNCO,out

(n N2O,out − n N2O,in) × 2 nHNCO,in − nHNCO,out

to produce CO2 and N2O. HNCO conversion and product selectivity are shown in Figure 2. The reaction rate between

Figure 2. HNCO conversion and gas product selectivity in the absence of O2.

CaO and HNCO accelerated with the increasing temperature; thus, HNCO conversion increased from 6.6% at 600 °C to 45.5% at 900 °C. CO2 selectivity increased from 64.0% at 600 °C to 70.6% at 900 °C, whereas N2O selectivity increased slightly. Parts of the carbon and nitrogen elements in reacted HNCO were fixed in the solid products because both CO2 and N2O selectivities were less than 100%. The ratio of carbon atom/oxygen atom in HNCO was 1, whereas that in CO2 was 2. Thus, the production of CO2 demonstrated that CaO did not catalyze HNCO decomposition but directly reacted with HNCO. O2 exists in the flue gas in cement kilns and CFBBs. Thus, the influence of O2 on the reaction between CaO and HNCO was studied. In the presence of O2, the amounts of HNCO, CO2, N2O, as well as CO and NO were measured in the exhaust gas. CYA decomposition produced a small amount of NH3, and it was detected by FTIR; however, in the presence of

× 100% (3)

× 100% (4)

where n (mol) is the amount of the gas products.

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RESULTS AND DISCUSSION Influence of CaO on HNCO Conversion. Previous studies9,15−17 indicated that HNCO can react with CaO, but no direct evidence was provided. Thus, the reaction between CaO and HNCO was studied in the temperature range of 600− 900 °C in the absence of O2. In the exhaust gas, the amounts of unreacted HNCO and CO2, NH3, and N2O were measured. NH3 was produced in CYA pyrolysis and decomposed to N2 and H2 on the CaO surface;10 thus, HNCO reacted with CaO B

DOI: 10.1021/acs.energyfuels.7b00046 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels CaO and O2, no NH3 was detected in the exhaust. A study by Johnsson20 showed that CaO catalyzed NH3 oxidation to NO and produced NO influenced the analysis of the experimental results. Thus, produced NO from NH3 oxidation was eliminated using the proposed kinetic model by Fu et al.10 to make the experimental results more precise. HNCO conversion and product selectivity in the reaction between HNCO and CaO in the presence of O2 are shown in Figure 3. HNCO

Figure 4. Surface morphology of the products in the reaction between CaO and HNCO.

changed the conversion of the products. As shown in Figure 4d, the structure of the product layer was similar to that at 600 °C and the product cluster was larger and more round at 900 °C. Thus, the conversion of the products was changed in the presence of O2 at a high temperature. DRIFT Analysis of Solid Products. The solid products in the reactions between CaO and HNCO were characterized by DRIFTS and compared to the spectra of CaO, CaCO3, and NaNCO [Ca(NCO)2 was unavailable and NaNCO has the same acid radical]. As shown in Figure 5a, the adsorption band of CaO was at 1600−1250 cm−1 and the bands of CaCO3 were at 2000−2800 and 1750−1250 cm−1. The bands of CaO and CaCO3 were partly coincident but can be easily distinguished by the peak shape. The adsorption bands of isocyano (NCO) were at 3500−3200, 2520, 2213, and 2046 cm−1. DRIFTS spectra of the products between CaO and HNCO at 600 and 900 °C in the absence of O2 are shown in Figure 5b. The bands of isocyano at 3346, 3245, and 2520 cm−1 were observed, which meant that Ca(NCO)2 was formed in the reaction between CaO and HNCO. The bands of isocyano and carbodiimide (CN2) were also observed in the range of 2280− 1918 cm−1,21 which showed that CaCN2 was also formed. According to Klimova et al.,12 CaCN2 was the product of Ca(NCO)2 decomposition. Weak bands of CaCO3 at 2986 and 2876 cm−1 showed that CaCO3 was produced in the conversion of Ca(NCO)2. The band of excess CaO was also observed at 1640−1276 cm−1. At 900 °C, the above-mentioned bands were all observed and no new bands appeared. The bands of isocyano at 3346 and 3245 cm−1 and the bands of CaCO3 at 2986 and 2876 cm−1 were weakened; however, the bands of carbodiimide in the range of 2280−1918 cm −1 were strengthened. Experimental results showed that the reaction between CaO and HNCO and the conversion of the products at 900 °C followed the same mechanism as those at 600 °C. On the basis of the weakened isocyano band and the strengthened carbodiimide band, the conversion of Ca(NCO)2 accelerated at 900 °C. DRIFTS spectra of the products between CaO and HNCO at 600 and 900 °C in the presence of O2 are shown in Figure 5c. At 600 °C, the isocyano bands at 3346, 3245, and 2520 cm−1 showed that Ca(NCO)2 was formed in the reaction

Figure 3. HNCO conversion and product selectivity in the presence of O2.

conversion increased from 52.9% at 600 °C to 86.3% at 900 °C. In comparison to HNCO conversion in the absence of O2, O2 promoted the reaction between CaO and HNCO. With the increasing temperature, CO2 selectivity decreased from 95.0 to 71.1%, whereas CO selectivity increased from 5.1 to 27.4%. The sum of CO2 and CO selectivities was almost 100%, which meant that all carbon elements in reacted HNCO were converted to CO2 or CO and no carbon elements were left in the solid products. NO selectivity increased from 60.7% at 600 °C to 93.3% at 900 °C, whereas N2O selectivity decreased from 35.4 to 1.9%. The sum of NO and N2O selectivity was slightly less than 100%, which showed that a little nitrogen element was converted to N2. Experimental results showed that O2 affected the reaction between CaO and HNCO. In the presence of O2, the carbon element in HNCO can be converted to CO2 and CO, while the nitrogen element can be converted to NO, N2O, and N2. The competition of the reaction paths determined the product selectivity. The morphologies of the products in the reaction between CaO and HNCO are shown in Figure 4. The original CaO particles were in polyhedron shape with a smooth surface. After the reaction at 600 °C in the absence of O2, CaO particles were in an irregular shape and covered with the product cluster, as shown in Figure 4a. At 900 °C, the product layer around the particle was changed to a more porous structure, as shown in Figure 4b. Gaseous product release during solid decomposition is helpful for porous structure formation. HNCO conversion increased with the increasing temperature, which meant that more CaO reacted with HNCO, more CO2 was produced during the product decomposition, and then a more porous structure was formed. The morphology of the product cluster was the same at 600 and 900 °C, which implied that the production and conversion of the solid products might follow the same mechanism. As shown in Figure 4c, at 600 °C and in the presence of O2, the coverage of the product cluster was larger and the porous structure was more complex. Thus, O2 accelerated the reaction between CaO and HNCO and C

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results in panels a and b of Figure 5 showed that O2 had no effect on the reaction between CaO and HNCO but changed the conversion of the solid products. Reproduction of CaO in the reaction cycle accelerated the reaction between CaO and HNCO; thus, HNCO conversion was increased. DRIFTS spectra of the products in Ca(NCO)2 calcination and CaCN2 oxidation are shown in Figure 6. As shown in

Figure 6. DRIFTS spectra of the solid products in Ca(NCO)2 calcination and CaCN2 oxidation: (a) product between CaO and HNCO at 900 °C in the absence of O2, (b) product in spectrum a calcined for 30 min in the absence of O2, and (c) Product in spectrum b calcined for 30 min in the presence of O2.

spectrum a of Figure 6, isocyano bands at 3346, 3245, and 2520 cm−1 and the carbodiimide band at 2280−1918 cm−1 were observed in the products of the reaction between CaO and HNCO at 900 °C in the absence of O2. After calcination for 30 min at 900 °C in the absence of O2, isocyano bands completely disappeared and only the band of carbodiimide can be observed at 2280−1918 cm−1 (spectrum b of Figure 6). Experimental results showed that Ca(NCO)2 decomposed to CaCN2 at a higher temperature and CaCN2 can stably exist in the products, owing to its high thermal stability. As shown in spectrum c of Figure 6, the band of carbodiimide completely disappeared after calcination of the product in spectrum b of Figure 6 for 30 min in the presence of O2. Thus, CaCN2 can be oxidized by O2. DRIFTS spectrum of the final product was the same as that of CaO, which meant that the product of CaCN2 oxidation was CaO. Experimental results in spectra a−c of Figure 6 showed that Ca(NCO)2 decomposed to CaCN2 at a higher temperature and CaCN2 was a stable product in the absence of O2. In the presence of O2, CaCN2 can be oxidized to CaO. Experimental results in Figure 6 showed that CaCN2 can be oxidized by O2. Furthermore, the oxidation of O2 on Ca(NCO)2 was studied and shown in Figure 7. As shown in spectrum a of Figure 7, Ca(NCO)2 and CaCN2 were formed in the reaction between CaO and HNCO in the absence of O2 at 600 °C. As shown in spectrum b of Figure 7, the bands of isocyano and carbodiimide still existed in the DRIFTS spectra after calcination for 30 min in the absence of O2 at 600 °C. Thus, Ca(NCO)2 and CaCN2 in the products were not changed. However, after calcination of the product in spectrum a of Figure 7 in the presence of O2 for 30 min, the bands of isocyano and carbodiimide both disappeared and the spectrum of the product was the same as that CaO, as shown in spectrum c of Figure 7. Experimental results showed that Ca(NCO)2 can be oxidized by O2 to produce CaO.

Figure 5. DRIFTS spectra of the solid products in the reaction between CaO and HNCO.

between CaO and HNCO in the presence of O2. The carbodiimide band in the range of 2280−1918 cm−1 showed that Ca(NCO)2 decomposed to CaCN2, and the CaCO3 band at 2986 and 2876 cm−1 showed that CaCO3 was also formed in the presence of O2. Experimental results showed that the products between CaO and HNCO were the same in the presence or absence of O2 at 600 °C. At 900 °C, isocyano bands at 3346, 3245, and 2520 cm−1 and the carbodiimide band at 2280−1918 cm−1 disappeared completely. Thus, the oxidation of O2 on Ca(NCO)2 and CaCN2 accelerated their conversion. The spectra of the solid products in the presence of O2 at 900 °C were the same as that of CaO, which meant that CaO was consumed and produced in circulation. Experimental D

DOI: 10.1021/acs.energyfuels.7b00046 Energy Fuels XXXX, XXX, XXX−XXX

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meant that the reaction rates of Ca(NCO)2 decomposition and oxidation were comparable. With the increasing temperature, NO selectivity became higher, while N2O selectivity became lower, which showed that the Ca(NCO)2 decomposition rate increased faster than the Ca(NCO)2 oxidation rate. Ca(NCO)2 + 2O2 → CaO + N2O + 2CO2

(8)

CaCN2 + 2O2 → CaO + 2NO + CO

(9)

CaCN2 + O2 → CaO + N2 + CO

CaO reacted with HNCO to form Ca(NCO)2. In the presence of O2, Ca(NCO)2 decomposed to CaCN2 and CO2 or was oxidized to CaO, N2O, and CO2. Produced CaCN2 can be oxidized to CaO, NO, and CO. Ca(NCO)2 decomposition and oxidation along with CaCN2 oxidation determined the product selectivity in the reaction between CaO and HNCO. At low temperatures, Ca(NCO)2 decomposition was slow and more Ca(NCO)2 was oxidized by O2, resulting in high N2O and CO2 selectivities. With the increasing temperature, Ca(NCO)2 decomposition was accelerated and more CaCN2 was produced and oxidized, thereby increasing the NO and CO selectivities and decreasing N2O selectivity.

Figure 7. DRIFTS spectra of the solid products in Ca(NCO)2 oxidation: (a) product between CaO and HNCO at 600 °C in the absence of O2, (b) product in spectrum a calcined for 30 min in the absence of O2, and (c) product in spectrum a calcined for 30 min in the presence of O2.

Mechanism of the Reactions between HNCO and CaO. According to the experimental results, the mechanism of the reaction between CaO and HNCO can be summarized as follows. HNCO in the gas phase directly reacted with CaO to produce Ca(NCO)2, as shown in eq 5. CaO + 2HNCO → Ca(NCO)2 + H 2O

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CONCLUSION CaO decreased the performance of the urea-SNCR deNOx process by affecting the thermal conversion of urea. The reaction between CaO and HNCO was studied in a pyrolysis/ gas−solid reaction system along with DRIFTS and SEM. CaO reacted with HNCO to produce Ca(NCO)2. In the absence of O2, Ca(NCO)2 decomposed to CaCN2 and CO2 at high temperatures. CaCN2 stably existed in the products because of its high thermal stability. Moreover, CO2 selectivity increased with the increasing temperature. In the presence of O2, Ca(NCO)2 decomposed and was also oxidized to CaO, N2O, and CO2. Produced CaCN2 was oxidized to CaO, NO, and CO. CaO was reproduced in Ca(NCO)2 and CaCN2 oxidation, thereby accelerating the reaction between CaO and HNCO and increasing the HNCO conversion. The selectivity of gas products was determined by Ca(NCO)2 decomposition, Ca(NCO)2 oxidation, and CaCN2 oxidation. NO and CO selectivities increased, whereas N2O and CO2 selectivities decreased with the increasing temperature.

(5)

With the increasing temperature, Ca(NCO)2 decomposed to CaCN2 and CO2, as shown in eq 6. Meanwhile, part of CO2 reacted with CaO to form CaCO3, as shown in eq 7. Ca(NCO)2 → CaCN2 + CO2

(6)

CaO + CO2 → CaCO3

(7)

(10)

At higher temperature and in the absence of O2, CaCO3 decomposed to CaO and CO2 and CaCN2 stably existed in the products because of its high thermal stability. The reaction between CaO and HNCO was accelerated with the increasing temperature. Thus, HNCO conversion was increased. With the increasing temperature, the Ca(NCO)2 decomposition degree increased and more CO2 was released, resulting in the increase of CO2 selectivity. In the presence of O2, both Ca(NCO)2 and CaCN2 can be oxidized. According to our studies on urea pyrolysis in the presence of CaO and the study by Klimova et al., Ca(NCO)2 started to be oxidized from 400 °C and the gas products were N2O and CO2, as shown in eq 8. At 500 °C, Ca(NCO)2 started to decompose to CaCN2 and the gas products of CaCN2 oxidation were NO, CO, and N2, as shown in eqs 9 and 10. Given that CaO was reproduced in Ca(NCO)2 and CaCN2 oxidation, more CaO participated in the reaction with HNCO, and HNCO conversion was increased in the presence of O2. The N-containing product of Ca(NCO)2 oxidation was N2O, while the N-containing product of CaCN2 oxidation was NO. CaCN2 was formed just from Ca(NCO)2 decomposition. Thus, if the reaction rate of Ca(NCO)2 oxidation was far higher than that of Ca(NCO)2 decomposition, the N2O selectivity would be far higher than NO selectivity. On the contrary, if the reaction rate of Ca(NCO)2 decomposition was far higher than that of Ca(NCO)2 oxidation, the NO selectivity would be far higher than N2O selectivity. Experimental results in Figure 3 showed that N2O and NO selectivities were comparable, which

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00046. Calculation of the reactant and product gas amount (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-010-62794298. E-mail: [email protected]. cn. ORCID

Qiang Song: 0000-0002-5484-3594 Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.energyfuels.7b00046 Energy Fuels XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by the fund from the National Key Technologies Research and Development Program (2015BAA05B01).

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DOI: 10.1021/acs.energyfuels.7b00046 Energy Fuels XXXX, XXX, XXX−XXX