# A Review of Urea Pyrolysis to Produce NH3 Used for NOx Removal

Authors: Denghui Wang; Ning Dong; Yanqing Niu; Shien Hui
Journal: Journal of Chemistry
 (2019)

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## Abstract

Urea pyrolysis, from which the denitration reactant ammonia is produced, plays an important role in the urea-based NOx removal process. Research into urea pyrolysis is mostly focused on three parts: urea pyrolysis pathway, catalytic hydrolysis of HNCO, and catalytic pyrolysis of aqueous urea solution. In this paper, detailed overview on research progress of urea pyrolysis was conducted. From the review, it could be concluded that although much research has been carried out, concentration was mainly in analysis of urea pyrolysis products and exploration of catalysts used to improve ammonia yields. Little work has been done on mechanism study and development of a kinetic model with high accuracy, which are still in great need nowadays.


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## Body

## 1. Introduction

With the sustained combustion of fossil fuels in boilers and diesel vehicles, massive harmful emissions of nitrogen oxides (NOx), mainly including NO and NO2, have been discharged into the atmosphere [1–5]. In recent decades, widespread serious environmental problems, such as urban smog, acid rain, and ozone depletion, have been caused from it [5–7]. To reduce the severe NOx emission, a number of techniques have been developed and utilized up to now, and the selective catalytic reduction (SCR) and the selective noncatalytic reduction (SNCR) stand out on account of the high denitration efficiency [8–10]. In SCR and SNCR processes, reducing agents are injected into the flue gas to reduce NOx into harmless nitrogen (N2). At present, the most popular reducing agent is ammonia (NH3), principally in forms of aqueous ammonia and liquid ammonia [11–13]. Related NOx reduction reactions are as follows:(1)4NH3+4NO+O2⟶4N2+6H2O(2)2NH3+NO+NO2⟶2N2+3H2OHowever, as a hazardous chemical, NH3 implies potential risk to safety operation. Its production, transportation, storage, application, and handling require strict safety and environmental regulations, which immensely restricts the convenience of its utilization [14, 15]. On this account, as a substitution, urea (NH2CONH2) has been suggested as the reducing agent nowadays because of its nontoxicity, innocuousness, impossibility of explosion, and capacity to be carried on board [16–18].In service, urea is firstly pyrolyzed to NH3, and then the generated NH3 reduced NOx as reactions (1) and (2). Therefore, the pyrolysis of urea significantly influences the following NOx removal. Ideally, 1 mole of urea experiences two-step reactions to form 2 moles of NH3: the direct thermal decomposition of urea into NH3 and isocyanic acid (HNCO) (3) and the hydrolysis of HNCO into NH3 and CO2 (4) [19, 20]:(3)NH2CONH2⟶NH3+HNCO(4)HNCO+H2O⟶NH3+CO2Unfortunately, in the actual urea-based NOx removal process, 1 mole of urea can hardly generate 2 moles of NH3. It is mainly due to the following two factors: (i) unignored side reactions in parallel with reactions (3) and (4) and (ii) the rather low reaction rate of (4) [21, 22]. These side reactions include the generation of macromolecular by-products such as biuret and cyanuric acid, which leads to reduction of NH3 production. The product of urea decomposition HNCO reacts with intact urea to produce biuret (5), and a small amount of HNCO can react with biuret to produce cyanuric acid (6) [23]:(5)NH2CONH2+HNCO⟶NH2CONHCONH2biuret(6)NH2CONHCONH2+HNCO⟶CYAcyanuric acid+NH3Therefore, the systematic study on urea pyrolysis mechanism and the methods to improve the reaction rate of (4) have been deeply focused on, in order to promote the NH3 production efficiency for NOx removal [17, 24–26]. In this paper, a review of urea pyrolysis is presented to summarize related studies.


## 2. Discussion

### 2.1. Urea Pyrolysis Pathway

To make the complex urea pyrolysis pathway clear, abundant experimental research was carried out, and possible conversion pathways were proposed based on the experimental results.Chen and Isa firstly used simultaneous thermogravimetry/differential thermal analysis/mass spectrometry (TG/DTA/MS) to measure the gaseous products during urea pyrolysis [27]. They used TG-DTA technology to heat and decompose urea at a heating rate of 4°C·min−1 in 25–500°C. The urea melting point was measured to be 132.5°C, and no gaseous urea was detected during the whole pyrolysis process, indicating that the decomposition reaction of urea completed after melting and before gasification. The whole pyrolysis process was divided into four stages, corresponding to 66%, 13%, 18%, and 3% weight loss, respectively. This was the first time that the pyrolysis process of urea was artificially divided into four stages in open literature. Simultaneously, the synthesis pathways of biuret and other macromolecular products as well as the decomposition route of biuret at high temperature were also proposed in this research. In addition, related reaction formulas (7)–(12) were hypothesized to explain the whole urea pyrolysis process.(7)(8)(9)(10)(11)(12)In order to accurately determine the formation and decomposition of relevant macromolecular products during urea pyrolysis, Schaber et al. adopted thermogravimetric analysis (TGA), liquid chromatography analysis (HPLC), ammonium ion electrode analysis (ISE), and Fourier transform infrared spectroscopy (FTIR) to analyze the substance types and corresponding quality of the residues in the reactor where 100.0 g urea was heated to different temperature under sand bath in an open reaction vessel [23]. Through their measurement and analysis, the whole process of urea pyrolysis was also divided into four weight loss stages (room temperature to 190°C, 190–250°C, 250–360°C, and above 360°C), corresponding to different typical reactions via which different macromolecular products formed and decomposed, as shown in Figure 1. Similar to the experimental results of Chen and Isa [27], Schaber et al. also divided the urea weight loss process into four stages, and the weight loss ratio in each stage was approximately equal to that proposed by Chen and Isa. However, Schaber et al. adopted more abundant measurement methods to determine the types of reaction products and the mass values of related products in each stage. Meanwhile, Schaber et al. carried out a detailed analysis process and proposed hypothetical representative reactions in each stage, so the completeness of the theoretical system was initially achieved. Although these hypothetical reactions still need further meticulous verification, their experimental results could help to provide essential literature support for later researchers.Figure 1
HPLC Mass Plot: urea pyrolysis reaction.Lundström et al. used differential scanning calorimetry (DSC) for the first time to measure the value of heat exchange during the thermal decomposition of solid urea in quartz crucible at the heating rates of 10°C·min−1 and 20°C·min−1, respectively [28]. Chemical analysis methods were mainly used to focus on the measurement and analysis of products in the pyrolysis process as reported in the literature [23, 27], while Lundström et al. applied thermal analysis methods to research urea pyrolysis [28]. In addition, in order to determine the specific reactions ascribed to corresponding caloric values as accurately as possible, they measured and analyzed the DSC curves of biuret and cyanic acid. The production of HNCO in urea pyrolysis was also measured by FTIR, but the measurement accuracy was not too high. The experimental results of Lundström et al. indicated that, during pyrolysis of solid urea in quartz cup at a heating rate of 20°C·min−1, the production of NH3 peaked only at around 250°C, while the production of HNCO peaked at around 250°C and 400°C. The experimental results were consistent with the results of the above literature [23, 27]: biuret began to decompose at 190°C and consumed a lot in the range of 190–250°C, which indicated that the decomposition reaction of biuret (9) produced NH3, resulting in that the production of NH3 peaked at 250°C. The production of HNCO that peaked at 250°C was due to the decomposition reaction of urea, while the production of HNCO that peaked at 400°C may be due to the decomposition reaction of cyanuric acid (7).Bernhard et al. studied the effect of temperature increase rate and heating time on the urea gasification process under normal pressure by a temperature-programmed desorption method, and the pyrolysis products were analyzed by using HPLC and FTIR techniques qualitatively and quantitatively [29]. The experimental results showed that the urea solution could be successfully heated and converted into gaseous state under normal pressure. Gaseous urea molecules could be obtained up to 97% under the experimental conditions of high evaporation area and high carrier gas flow rate, and the massive production of other byproducts could be effectively avoided. This discovery verified the fact that a considerable amount of urea did not decompose after the urea solution was completely vaporized, and it proved that urea still existed in the reactor in the form of fixed gaseous urea molecule. At present, there are few experimental and simulation studies on urea gasification. Considering the gasification of urea, the degree of urea pyrolysis reaction (3)–(12) could be determined more accurately, which is beneficial to improve the computational modeling of urea-based NOx removal systems.The contribution of urea solution to denitration was the generation of NH3, and the yield of NH3 was very important for the efficient denitration. Mahalik et al. studied the yield of NH3 generated by aqueous urea solution heated by a stable heat source at standard atmospheric pressure in a self-made semicontinuous reactor [30]. The effects of reaction temperature, reaction time, stirring rate, and initial urea concentration on ammonia generation rate in the reactor were systematically researched. In their experiments, the urea concentration in the reactor, rather than the reaction product NH3, was measured to reflect the depth of the reaction and the urea conversion rate. The experimental results showed that the increase of reaction temperature, the extension of reaction time, and the increase of stirring rate effectively increased the urea conversion rate. The increase of the initial concentration of urea solution also led to the increase of urea conversion rate. According to the experimental results, they simulated the reaction process by Fluent and obtained the temperature field distribution and concentration field distribution in the reaction process [31]. The simulation results were compared with experimental data and analyzed in detail, based on which the specific expressions of the reaction rate and reaction activation energy in the pyrolysis of aqueous urea solution were calculated. The activation energy was determined to be 59.85 kJ·mol−1, and the pre-exponential factor was determined to be 3.9 × 106·min−1.Our group focused on the effect of different oxygen concentration and heating rates on the mass loss and the formation of gaseous products (NH3, N2O, and CO) during solid urea pyrolysis [21]. Experimental results showed that there were two sharp endothermic peaks in the process, reflecting urea melting and its decomposition, respectively. Little NH3 was detected below 140°C, confirming that little urea decomposed before urea melting. This was consistent with the results of the literature by Chen and Isa [27]. Above the urea melting point, NH3 production dramatically increased. The presence of O2 lowered the highest NH3 yield, possibly as a result of the oxidation of partial generated NH3, but O2 accelerated the formation of N2O and CO. At the same condition, the formation amount of N2O was approximately twice that of CO.Although some progress was made in urea pyrolysis, and some important experimental results were obtained by diverse measurement and analysis methods, these studies mainly focused on analysis of urea pyrolysis products. Too many experimental data were acquired on urea pyrolysis, but too few literatures were published on the reaction kinetics analysis. Through specific experiments, Brack et al. calculated the reaction order, reaction pre-exponential factor, and reaction activation energy of urea pyrolysis [32]. They focused their research on the formation pathways of ammonia, isocyanic acid, urea, biuret, cyanic acid, and ammelide in the pyrolysis process of solid urea, and a simplified reaction mechanism including 15-step elementary reactions (shown in Table 1) was proposed through the combined use of FTIR-TGA and HPLC analysis. Besides, Ebrahimian et al. proposed a condensed-phase kinetic scheme containing 12 reactions for urea thermal decomposition, as shown in Table 2 [33]. The mechanisms were validated by thermogravimetric experiments including gaseous data as well as solid-phase concentration profiles. Compared with the corresponding experimental data, it was confirmed that both the two models had good qualitative consistency and certain application reference value, but the errors of quantitative calculation were debatable. Aoki et al. performed urea thermal decomposition experiments using a laminar flow reactor and the urea decomposition and NH3 and HNCO formation rates were presented [34]. They assumed the following decomposition pathways, which have been widely adopted up to now.(13)NH22CO⟶k1NH3+HNCO,k1=1.2676×104exp−1.554×104cal/mol/RT(14)NH22CO+H2O⟶k22NH3+CO2,k2=9.925×103exp−2.098×104cal/mol/RTTable 1
Kinetic reaction scheme of urea decomposition.
Reaction
γ
j
A
E
A (kJ/mol)
I
CYA(s) ⟶ 3HNCO(g)
0
1.001 × 103 mol/s
118.42
II
Biuret(m) ⟶ urea(m) + HNCO(l)
1
1.107 × 1020 1/s
208.23
III
Urea(m) + HNCO(l) ⟶ biuret(m)
1/1
3.517 × 1011 ml/mol s
75.45
IV
Urea(m) ⟶ HNCO(l) + NH3(g)
0.3
2.000 × 104 mol0.7/ml0.7 s
74.00
V
2 biuret(m) ⟶ ammelide(s) + HNCO(l) + NH3(g) + H2O(g)
2
3.637 × 1026 1/s
257.76
VI
Biuret(m) + HNCO(g) ⟶ CYA(s) + NH3(g)
1/1
9.397 × 1020 ml/mol s
158.68
VII
Biuret(m) + HNCO(g) ⟶ triuret(s)
1/1
1.091 × 1015 ml/mol s
116.97
VIII
Triuret(s) ⟶ CYA(s) + NH3(g)
1
1.238 × 1018 1/s
194.94
IX
Urea(m) + 2HNCO(l) ⟶ ammelide(s) + H2O(g)
1/2
1.274 × 1020 ml2/mol2 s2
110.40
X
Biuret(m) ⟶ biuret(matrix)
1
8.193 × 1026 1/s
271.50
XI
Biuret(matrix) ⟶ biuret(m)
1
3.162 × 1009 1/s
122.00
XII
Biuret(matrix) ⟶ 2HNCO(g) + NH3(g)
1
5.626 × 1024 1/s
266.38
XIII
Urea(s) ⟶ urea(m)
1
1.000 × 1015·T1.5 1/s
160.00
XIV
Ammelide(s) ⟶ ammelide(g)
1
1.000 × 1014 ml/mol s
201.67
XV
HNCO(l) ⟶ HNCO(g)
Herz–Knudsen equation
Note. States of aggregation: (s): solid, (m): molten, (g): gaseous, (l): liquid/dissolved, (matrix): solid matrix. Triuret: NH2-CO-NH-CO-NH-CO-NH2. For reactions with two educts the reaction order γj of the second educt is specified as second value.Table 2
Condensed-phase kinetic scheme for urea thermal decomposition.
Reaction
A
i (s−1)
E
i (kJ mol−1)
(1) Urea ⟶ NH4+ + NCO−
8.50 × 106
84
(2) NH4+ ⟶ NH3(g) + H+
1.50 × 102
40
(3) NCO− + H+ ⟶ HNCO(g)
6.57 × 102
10
(4) Urea + NCO− + H+ ⟶ biuret
7.87 × 1014
115
(5) Biuret ⟶ urea + NCO− + H+
1.50 × 1024
250
(6) Biuret + NCO− + H+ ⟶ CYA + NH3(g)
2.81 × 1018
150
(7) CYA ⟶ 3NCO− + 3 H+
1.50 × 1019
260
(8) CYA + NCO− + H+ ⟶ ammelide + CO2
3.48 × 105
35
(9) Ammelide ⟶ 2NCO− + 2H+ + HCN(g) + NH(g)
6.00 × 1014
220
(10) Urea(aq) ⟶ NH4+ + NCO-
1.20 × 108
84
(11) NCO− + H+ + H2O(aq) ⟶ NH3 + CO2(g)
5.62 × 109
59
(12) Urea(aq) + NCO− + H+ ⟶ biuret
3.93 × 1014
115
(g): gaseous; (aq): aqueous.For urea pyrolysis pathways, Eichelbaum et al. drew the whole diagram, as shown in Figure2 [35]. It clearly showed the generation and decomposition of macromolecular products in detail.Figure 2
Schematic of all major reactions, intermediate and final products for the pyrolysis and hydrolysis of neat urea in the temperature range between 20 and 750°C.Much work has been done on urea pyrolysis mechanism, and valuable research results have been achieved. The urea pyrolysis network has been accomplished already, which includes not only the direct thermal decomposition reaction of urea and the hydrolysis reaction of HNCO, but also the generation path and decomposition path of macromolecular products such as biuret and cyanuric acid during urea pyrolysis. Even so, the detailed mechanism with accurate prediction ability still needs more study.


### 2.2. Catalytic Hydrolysis of HNCO

During urea pyrolysis, HNCO was an important intermediate product, as its hydrolysis reaction (12) played an important role in NH3 production [16–18]. The HNCO formed from urea decomposition is quite stable in the gas phase, so its hydrolysis reaction does not proceed as a homogeneous reaction. To accelerate it, various oxides have been applied as catalysts.Kleemann et al. studied the reaction rate and ammonia yield of the HNCO hydrolysis reaction under the catalysis of commonly used SCR catalysts (TiO2, V2O5/TiO2, V2O5-WO3/TiO2) [36]. And the catalytic effects of catalysts with different vanadium and tungsten contents on ammonia production from HNCO hydrolysis were compared in 140–475°C. The experimental results showed that HNCO had a higher hydrolysis reaction rate under the catalysis of pure TiO2 than under V2O5/TiO2 and V2O5-WO3/TiO2, while the addition of V2O5 or WO3 reduced the reaction rate to some extent. The activation energy of the HNCO hydrolysis reaction was determined to be about 13 kJ·mol−1, indicating that the overall reaction rate was mainly affected by the mass transfer rate.Piazzesi et al. carried out experimental research on the catalytic effect of Fe-ZSM5 and V2O5-WO3/TiO2 catalysts on the HNCO hydrolysis reaction and also compared the influence of different V2O5 contents on the catalytic effect of V2O5-WO3/TiO2 catalysts [37]. Experimental results showed that Fe-ZSM5 had a very high catalytic effect on the hydrolysis process of HNCO. The conversion efficiency of HNCO increased to 100% at about 260°C, and the perfect conversion rate was stably maintained until 450°C. The V2O5-WO3/TiO2 catalyst also exhibited a high catalytic effect on HNCO hydrolysis, and the catalytic effect of 1% V2O5 content was more significant than that of 3% V2O5 content. It indicated that, for the V2O5-WO3/TiO2 catalyst, the increase of V2O5 content resulted in the decrease of catalytic efficiency, which was consistent with the experimental results of Kleemann et al. [36].Czekaj and Kröcher studied the catalytic effect of TiO2 and γ-Al2O3 on HNCO hydrolysis by Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFTS) [38]. The experimental results proved that both TiO2 and γ-Al2O3 had significant catalytic effects on HNCO hydrolysis under the same reaction conditions and TiO2 had better catalytic performance than γ-Al2O3. Additionally, Czekaj and Kröcher used density functional theory to calculate the adsorption energy required for HNCO adsorbed on the catalyst surface during the reaction. The calculation results showed that the adsorption energy required for HNCO adsorption on TiO2 was lower than that on γ-Al2O3, which scientifically explained why the two catalysts differed in catalytic performance from the perspective of energy.Hauck et al. specifically carried out experimental research on the catalytic effect of TiO2 on HNCO hydrolysis, as well as the thermodynamic catalytic mechanism [39, 40]. In the self-made experimental system, mass spectrometry was used to determine the production of different atomic groups, and the reaction rate of the elemental reactions that might occur on the TiO2 surface was calculated by quantitative analysis. After calculation and analysis, they obtained the conclusion that HNCO hydrolysis was affected by mass transfer rate, which was consistent with the research conclusion of Kleemann et al. [36]. Moreover, Hauck et al. found that HNCO catalytic hydrolysis over TiO2 was restricted by the external mass transfer rate. However, the calculation by Hauck et al. rested on the reaction rate constant, but further activation energy calculation was not involved. It was worth mentioning that NO, NH3, and NO2 were added to the carrier gas stream, and the influence of the presence or absence of these gases on HNCO hydrolysis was investigated. Through the comparison and analysis of the experimental results, it was found that the existence of NO, NH3, and NO2 all inhibited HNCO hydrolysis to different degree, following the order of NO < NH3 < NO2. The main reason for the inhibitory effect was that the gases could be adsorbed on the surface of TiO2, thus reducing the adsorption of HNCO.Chen et al. measured and compared the similarities and differences of catalytic effects ofγ-Al2O3 and CuO/γ-Al2O3 on HNCO hydrolysis and calculated the activation energy of HNCO hydrolysis over the two catalysts, respectively [41]. They found that the catalytic effect of γ-Al2O3 was slightly higher than that of CuO/γ-Al2O3. According to their experimental data and calculation results, the activation energy of the HNCO hydrolysis over the two catalysts changed with the temperature range. When the temperature was higher than 200°C, the activation energy of HNCO hydrolysis over γ-Al2O3 was 13 kJ·mol−1, while over CuO/γ-Al2O3 it was 16 kJ·mol−1. The activation energy of HNCO hydrolysis measured by Chen was close to that measured by Kleemann et al. [36]. Such low activation energy reflected that the HNCO hydrolysis over γ-Al2O3 and CuO/γ-Al2O3 was mainly controlled by external mass transfer. When the temperature was lower than 200°C, the calculated activation energy obviously increased. The activation energy over γ-Al2O3 was calculated to be 25 kJ·mol−1, while over CuO/γ-Al2O3 it was calculated as 26 kJ·mol−1. It indicated that the reaction below 200°C was mainly dominated by mass transfer inside the catalyst pores. In addition, no matter what the temperature range was, the value of reaction activation energy over γ-Al2O3 was lower than that over CuO/γ-Al2O3, consistent with the phenomenon that γ-Al2O3 showed stronger catalytic effect than CuO/γ-Al2O3.It could be concluded from the analysis of the above literature [33–38] that the HNCO hydrolysis over metal oxides catalysts could be basically determined as a reaction process under mass transfer control. However, the determination of whether external or internal mass transfer is still controversial now. It is also a recognized fact that the hydrolysis reaction is slow without the assistance of relevant catalysts. All the catalysts used in the study were metal oxides, and they were mainly concentrated on the catalysts commonly used in SCR technology. Among them, TiO2 performed prominent catalytic effect, while the addition of vanadium and tungsten could hardly improve the catalytic efficiency.


### 2.3. Catalytic Pyrolysis of Aqueous Urea Solution

The aforementioned studies on urea pyrolysis mechanism and on catalytic hydrolysis of HNCO were conducted for the production of NH3, and effectively converting urea to NH3 with low energy consumption was decidedly pursued in actual operation. As aqueous urea solution is the most widely used agent in the urea DeNOx process, many researchers have focused their research on the catalytic pyrolysis of aqueous urea solution.Koebel and Struts conducted an experimental study on the synergistic process of urea pyrolysis and hydrolysis in SCR system of automotive internal combustion engines [42]. They focused their research on how to reasonably select the heating source for urea pyrolysis. In the paper, Koebel and Strutz listed the heat source selection of different ways in the urea pyrolysis process in detail and carried out specific energy consumption calculation, heat calculation, and experimental research for these selections, respectively. By analyzing and comparing, the following most energy-saving technical scheme was put forward: selecting part of the hot exhaust gas of the internal combustion engine to provide heat supply for urea pyrolysis, and improving the ammonia production efficiency of urea pyrolysis by rationally using the integral-type catalyst. This technical scheme could not only improve the ammonia yield, but also effectively utilize the exhaust heat of the internal combustion engine, which was in line with the principle of economy and practicability.Yim et al. carried out experimental research and theoretical calculation on NH3 production from urea pyrolysis in SCR system in 150–450°C [43]. The atomized urea solution particles were heated and decomposed in a self-made aluminum reaction system, and the effects of residence time and reaction temperature on the pyrolysis of urea solution were mainly investigated. Their experimental results demonstrated that the hydrolysis of HNCO occurred without catalysts when the reaction temperature was high enough (≥400°C). The prolongation of residence time was beneficial to urea pyrolysis and effectively promoted ammonia yield. In order to improve the ammonia generation rate, they also studied the catalytic effect of the high efficiency denitration catalyst CuZSM5 on pyrolysis of aqueous urea solution. The reaction rate and reaction activation energy of the two-step pyrolysis reactions of aqueous urea solution were calculated based on the obtained experimental results. However, an aluminum reactor was used in their experimental system, and it was likely that Al2O3 attached on the inner surface of the reactor might catalyze the reaction, so their conclusion results needed further verification.In order to compare the effects of different oxide catalysts on the pyrolysis reaction of aqueous urea solution and obtain detailed catalytic efficiency values, Kröcher and Elsener tested the catalytic effects of twenty potential catalyst materials on the pyrolysis of aqueous urea solution in a self-made fluidized bed reactor [44]. The results are listed in Table 3. By comparison, γ-Al2O3 was identified as the most suitable catalyst material in fluidized bed reactor. Although TiO2 showed the highest catalytic efficiency during the experiments, its wear resistance was far lower than that of γ-Al2O3. The catalytic efficiency of the catalysts was tested under the fluidized state, which was usually higher than that in fixed bed reactor, so it limited the application reference value to some extent.Table 3
Test of fluidized bed materials for the thermohydrolysis of 50% urea solution.
Bed material
Particle diameter (mm)
Reactor diameter (mm)
Volume of bed material (ml)
Air flow (LN/h)
Urea dosage type
Urea solution dosage (g/h)
Bed loading (g/(h·ml))
Fluidized bed temperature (°C)
Y
NH
3 (%)
YHNCO (%)
Urea slip (%)
γ-Al2O3 “Uetikon1”
0.2–0.315
20
15
150
From top
185
6.2
300
99
1
0.4
275
99
1
0.4
250
100
0
0.4
225
102
0
0.5
200
96
1
1.2
γ-Al2O3 “Uetikon1”
0.50–0.63
25
15
520
From top
185
6.2
300
97
4
1.2
275
96
5
1.3
250
84
12
2.6
225
73
16
3.9
γ-Al2O3 “compalox”
0.50–0.63
25
15
520
From top
185
6.2
300
96
0
1
275
95
0
0.7
250
96
1
0.9
225
93
2
2.3
γ-Al2O3 “puralox”
0.2–0.315
20
15
150
From top
185
6.2
300
96
5
0.4
275
99
5
0.3
250
96
4
0.4
225
98
4
0.4
200
99
4
0.9
γ-Al2O3 “CAT 250”
0.50–0.63
50
60
1860
From top
340
2.9
300
97
2
0.7
275
98
3
0.6
250
96
2
0.6
225
97
3
0.6
200
95
3
1.2
γ-Al2O3 “CAT 250”
1.0–1.25
50
60
3900
From top
350
2.9
300
96
0
1.9
275
95
0
1.8
250
94
1
1.6
225
83
5
2.4
200
66
15
5
γ-Al2O3 “CAT 250”
1.0–1.25
50
60
3900
From top
350
2.9
300
94
0
1.1
275
94
1
1.2
250
93
1
1.3
225
89
6
2.3
γ-Al2O3 “CAT 250”
1.0–1.25
25
15
1109
From top
186
6.2
300
101
0
1.9
275
100
2
1.7
250
99
3
1.8
225
84
12
3.2
γ-Al2O3 “condea”
1.0–1.25
20
15
554
From top
185
6.2
300
89
5
5.8
275
78
16
6.1
250
69
23
6.3
γ-Al2O3 “seralite 537”
1.0–1.25
50
60
3900
Into bed
350
2.9
300
94
3
1.9
275
91
5
2.2
250
85
7
3
225
72
13
5
α-Al2O3 “seralite 512 S”
1.6–2.0
50
60
4700
Into bed
290
2.4
300
47
31
9.1
α-AlOOH (boehmite)
0.2–0.315
20
15
150
From top
185
6.2
300
99
3
0.8
275
94
4
0.5
250
96
3
0.8
225
95
5
0.6
200
96
5
0.7
SiO2 “aerosil”
0.50–0.63
25
15
410
From top
185
6.2
300
70
25
4.2
AlSiOx (molochite)
0.22–0.50
20
15
150
From top
4.5
0.02
400
61
25
n.d.
350
55
34
n.d.
300
38
33
n.d.
250
24
20
n.d.
MgSiOx “florisil”
0.50–0.63
25
15
400
From top
185
6.2
300
90
8
2.3
275
86
7
2.1
250
83
9
3
TiO2 “macrosorb T”
0.2–0.315
20
15
150
From top
185
6.2
300
104
1
0.7
275
102
1
0.8
250
95
4
2.7
225
62
8
5.9
TiO2 “7702”
0.50–0.63
25
15
520
From top
185
6.2
300
98
0
1.5
275
99
0
1.2
250
99
0
1
225
99
1
1
200
98
2
1.6
V2O5/WO3-TiO2
0.315–0.50
25
15
356
From top
185
6.2
300
96
1
0.6
275
96
1
0.6
250
90
6
1.1
225
87
6
1.5
200
82
6
2.9
Molsieve 5 Å
0.2–0.315
20
15
150
From top
94
3.1
300
93
7
0.9
275
58
21
7.2
Molsieve “13X”
0.50–0.63
25
15
410
From top
185
6.2
300
93
2
0.7
275
76
13
1.2
250
43
20
5.5
H-MOR
ca.2.(2–4)
50
60
3900
From top
350
2.9
300
85
4
2.6
275
83
6
2.8
H-ZSM-5
ca.2.(2–4)
50
60
3900
From top
350
2.9
300
72
19
3.1
275
66
25
3.6
Dealuminated Y
ca.2.(2–4)
50
60
3900
From top
350
2.9
300
52
35
5
275
51
36
5.6In the above literature [42–44], the solvent of urea solution was water, which might affect the catalytic effect of catalysts. Perhaps being aware of it, Bernhard et al. carried out experimental tests and kinetic analysis on the catalytic efficiency of stationary catalysts soon afterwards [45]. In their experimental system, the catalytic effects of different characteristic sizes of TiO2, ZrO2, Al2O3, H-ZSM-5, and SiO2 on urea pyrolysis and HNCO hydrolysis were tested, respectively. Particularly, the catalytic effect of the above materials on urea pyrolysis under anhydrous conditions was a conspicuous highlight. In previous studies, catalytic efficiency was tested in experiments with water. Differently, Bernhard et al. used methanol and ethanol as solvents to avoid the presence of water in the experiments, which provided convincing experimental data for the catalytic research of oxides on urea pyrolysis without water. The experimental results showed that the presence or absence of water had a great influence on the catalytic effect of the catalysts. When the urea solvent was water, the order of catalytic effect of different catalysts was ZrO2 > TiO2 > Al2O3 > H-ZSM-5 > SiO2. Under anhydrous conditions, the order changed to TiO2 > H-ZSM-5≈Al2O3 > ZrO2 > SiO2. Additionally, the ammonia production efficiency over these catalysts under anhydrous conditions was significantly lower than that under hydrous conditions.Lundström et al. adopted differential scanning calorimetry and mass spectrometry to study the effects of TiO2, Al2O3, and Fe-Beta catalysts on the production of ammonia, isocyanic acid, and carbon dioxide from urea pyrolysis under anhydrous and hydrous conditions [46]. The differences of ammonia production between anhydrous and hydrous conditions were also detected. However, different from the reference [45], in this paper, researchers did not recognize that TiO2, Al2O3, and Fe-Beta catalysts could catalyze urea pyrolysis under anhydrous conditions. They attributed the difference to the absence of HNCO hydrolysis under anhydrous conditions. This conclusion contradicted the results of Yim and Bernhard et al. [43, 45] but lacks scientific basis.In the literature mentioned above, urea was pyrolyzed on the catalyst which was put into the reactor in the form of solution with water, methanol, or ethanol as solvent. In the experiment of Bernhard et al., N2 at 100°C was passed through honeycomb-like TiO2 cordierite filled with aqueous urea solution [47]. The formed mixed gas stream was determined by HPLC and FTIR. In this paper, Bernhard et al. first clearly analyzed and obtained the adsorption form of urea on the surface of TiO2 and calculated the energy value consumed during the adsorption process.The effect of oxygen concentration and different additives (Na2CO3 and NaNO3) on NH3 yields and N2O and CO production from pyrolysis of aqueous urea solution was studied by our group [22]. Without additives, the presence of oxygen made the NH3 yields drop rapidly in 550–800°C. The addition of Na2CO3 or NaNO3 not only increased the NH3 yields but also reduced the N2O and CO production via a series of chain reactions of sodium species, but it did not restrain the NH3 oxidation at high temperatures. The addition of sodium species probably provided plenty of NaO and NaO2, which might catalyze the HNCO hydrolysis to dramatically increase the NH3 yields. Moreover, the harmful gaseous by-products (N2O and CO) might participate in the chain reactions of sodium species, so most N2O and nearly all CO could be consumed.Various catalysts were selected for hydrolysis of aqueous urea solution, and some outstanding ones stood out for the excellent efficiency. Many studies have been carried out on the exploration and development of catalysts, while little attention has been paid to the catalytic mechanism investigation. Further work, especially the dynamic mechanism, was still needed to help to make better use of these catalysts.


## 2.1. Urea Pyrolysis Pathway

To make the complex urea pyrolysis pathway clear, abundant experimental research was carried out, and possible conversion pathways were proposed based on the experimental results.Chen and Isa firstly used simultaneous thermogravimetry/differential thermal analysis/mass spectrometry (TG/DTA/MS) to measure the gaseous products during urea pyrolysis [27]. They used TG-DTA technology to heat and decompose urea at a heating rate of 4°C·min−1 in 25–500°C. The urea melting point was measured to be 132.5°C, and no gaseous urea was detected during the whole pyrolysis process, indicating that the decomposition reaction of urea completed after melting and before gasification. The whole pyrolysis process was divided into four stages, corresponding to 66%, 13%, 18%, and 3% weight loss, respectively. This was the first time that the pyrolysis process of urea was artificially divided into four stages in open literature. Simultaneously, the synthesis pathways of biuret and other macromolecular products as well as the decomposition route of biuret at high temperature were also proposed in this research. In addition, related reaction formulas (7)–(12) were hypothesized to explain the whole urea pyrolysis process.(7)(8)(9)(10)(11)(12)In order to accurately determine the formation and decomposition of relevant macromolecular products during urea pyrolysis, Schaber et al. adopted thermogravimetric analysis (TGA), liquid chromatography analysis (HPLC), ammonium ion electrode analysis (ISE), and Fourier transform infrared spectroscopy (FTIR) to analyze the substance types and corresponding quality of the residues in the reactor where 100.0 g urea was heated to different temperature under sand bath in an open reaction vessel [23]. Through their measurement and analysis, the whole process of urea pyrolysis was also divided into four weight loss stages (room temperature to 190°C, 190–250°C, 250–360°C, and above 360°C), corresponding to different typical reactions via which different macromolecular products formed and decomposed, as shown in Figure 1. Similar to the experimental results of Chen and Isa [27], Schaber et al. also divided the urea weight loss process into four stages, and the weight loss ratio in each stage was approximately equal to that proposed by Chen and Isa. However, Schaber et al. adopted more abundant measurement methods to determine the types of reaction products and the mass values of related products in each stage. Meanwhile, Schaber et al. carried out a detailed analysis process and proposed hypothetical representative reactions in each stage, so the completeness of the theoretical system was initially achieved. Although these hypothetical reactions still need further meticulous verification, their experimental results could help to provide essential literature support for later researchers.Figure 1
HPLC Mass Plot: urea pyrolysis reaction.Lundström et al. used differential scanning calorimetry (DSC) for the first time to measure the value of heat exchange during the thermal decomposition of solid urea in quartz crucible at the heating rates of 10°C·min−1 and 20°C·min−1, respectively [28]. Chemical analysis methods were mainly used to focus on the measurement and analysis of products in the pyrolysis process as reported in the literature [23, 27], while Lundström et al. applied thermal analysis methods to research urea pyrolysis [28]. In addition, in order to determine the specific reactions ascribed to corresponding caloric values as accurately as possible, they measured and analyzed the DSC curves of biuret and cyanic acid. The production of HNCO in urea pyrolysis was also measured by FTIR, but the measurement accuracy was not too high. The experimental results of Lundström et al. indicated that, during pyrolysis of solid urea in quartz cup at a heating rate of 20°C·min−1, the production of NH3 peaked only at around 250°C, while the production of HNCO peaked at around 250°C and 400°C. The experimental results were consistent with the results of the above literature [23, 27]: biuret began to decompose at 190°C and consumed a lot in the range of 190–250°C, which indicated that the decomposition reaction of biuret (9) produced NH3, resulting in that the production of NH3 peaked at 250°C. The production of HNCO that peaked at 250°C was due to the decomposition reaction of urea, while the production of HNCO that peaked at 400°C may be due to the decomposition reaction of cyanuric acid (7).Bernhard et al. studied the effect of temperature increase rate and heating time on the urea gasification process under normal pressure by a temperature-programmed desorption method, and the pyrolysis products were analyzed by using HPLC and FTIR techniques qualitatively and quantitatively [29]. The experimental results showed that the urea solution could be successfully heated and converted into gaseous state under normal pressure. Gaseous urea molecules could be obtained up to 97% under the experimental conditions of high evaporation area and high carrier gas flow rate, and the massive production of other byproducts could be effectively avoided. This discovery verified the fact that a considerable amount of urea did not decompose after the urea solution was completely vaporized, and it proved that urea still existed in the reactor in the form of fixed gaseous urea molecule. At present, there are few experimental and simulation studies on urea gasification. Considering the gasification of urea, the degree of urea pyrolysis reaction (3)–(12) could be determined more accurately, which is beneficial to improve the computational modeling of urea-based NOx removal systems.The contribution of urea solution to denitration was the generation of NH3, and the yield of NH3 was very important for the efficient denitration. Mahalik et al. studied the yield of NH3 generated by aqueous urea solution heated by a stable heat source at standard atmospheric pressure in a self-made semicontinuous reactor [30]. The effects of reaction temperature, reaction time, stirring rate, and initial urea concentration on ammonia generation rate in the reactor were systematically researched. In their experiments, the urea concentration in the reactor, rather than the reaction product NH3, was measured to reflect the depth of the reaction and the urea conversion rate. The experimental results showed that the increase of reaction temperature, the extension of reaction time, and the increase of stirring rate effectively increased the urea conversion rate. The increase of the initial concentration of urea solution also led to the increase of urea conversion rate. According to the experimental results, they simulated the reaction process by Fluent and obtained the temperature field distribution and concentration field distribution in the reaction process [31]. The simulation results were compared with experimental data and analyzed in detail, based on which the specific expressions of the reaction rate and reaction activation energy in the pyrolysis of aqueous urea solution were calculated. The activation energy was determined to be 59.85 kJ·mol−1, and the pre-exponential factor was determined to be 3.9 × 106·min−1.Our group focused on the effect of different oxygen concentration and heating rates on the mass loss and the formation of gaseous products (NH3, N2O, and CO) during solid urea pyrolysis [21]. Experimental results showed that there were two sharp endothermic peaks in the process, reflecting urea melting and its decomposition, respectively. Little NH3 was detected below 140°C, confirming that little urea decomposed before urea melting. This was consistent with the results of the literature by Chen and Isa [27]. Above the urea melting point, NH3 production dramatically increased. The presence of O2 lowered the highest NH3 yield, possibly as a result of the oxidation of partial generated NH3, but O2 accelerated the formation of N2O and CO. At the same condition, the formation amount of N2O was approximately twice that of CO.Although some progress was made in urea pyrolysis, and some important experimental results were obtained by diverse measurement and analysis methods, these studies mainly focused on analysis of urea pyrolysis products. Too many experimental data were acquired on urea pyrolysis, but too few literatures were published on the reaction kinetics analysis. Through specific experiments, Brack et al. calculated the reaction order, reaction pre-exponential factor, and reaction activation energy of urea pyrolysis [32]. They focused their research on the formation pathways of ammonia, isocyanic acid, urea, biuret, cyanic acid, and ammelide in the pyrolysis process of solid urea, and a simplified reaction mechanism including 15-step elementary reactions (shown in Table 1) was proposed through the combined use of FTIR-TGA and HPLC analysis. Besides, Ebrahimian et al. proposed a condensed-phase kinetic scheme containing 12 reactions for urea thermal decomposition, as shown in Table 2 [33]. The mechanisms were validated by thermogravimetric experiments including gaseous data as well as solid-phase concentration profiles. Compared with the corresponding experimental data, it was confirmed that both the two models had good qualitative consistency and certain application reference value, but the errors of quantitative calculation were debatable. Aoki et al. performed urea thermal decomposition experiments using a laminar flow reactor and the urea decomposition and NH3 and HNCO formation rates were presented [34]. They assumed the following decomposition pathways, which have been widely adopted up to now.(13)NH22CO⟶k1NH3+HNCO,k1=1.2676×104exp−1.554×104cal/mol/RT(14)NH22CO+H2O⟶k22NH3+CO2,k2=9.925×103exp−2.098×104cal/mol/RTTable 1
Kinetic reaction scheme of urea decomposition.
Reaction
γ
j
A
E
A (kJ/mol)
I
CYA(s) ⟶ 3HNCO(g)
0
1.001 × 103 mol/s
118.42
II
Biuret(m) ⟶ urea(m) + HNCO(l)
1
1.107 × 1020 1/s
208.23
III
Urea(m) + HNCO(l) ⟶ biuret(m)
1/1
3.517 × 1011 ml/mol s
75.45
IV
Urea(m) ⟶ HNCO(l) + NH3(g)
0.3
2.000 × 104 mol0.7/ml0.7 s
74.00
V
2 biuret(m) ⟶ ammelide(s) + HNCO(l) + NH3(g) + H2O(g)
2
3.637 × 1026 1/s
257.76
VI
Biuret(m) + HNCO(g) ⟶ CYA(s) + NH3(g)
1/1
9.397 × 1020 ml/mol s
158.68
VII
Biuret(m) + HNCO(g) ⟶ triuret(s)
1/1
1.091 × 1015 ml/mol s
116.97
VIII
Triuret(s) ⟶ CYA(s) + NH3(g)
1
1.238 × 1018 1/s
194.94
IX
Urea(m) + 2HNCO(l) ⟶ ammelide(s) + H2O(g)
1/2
1.274 × 1020 ml2/mol2 s2
110.40
X
Biuret(m) ⟶ biuret(matrix)
1
8.193 × 1026 1/s
271.50
XI
Biuret(matrix) ⟶ biuret(m)
1
3.162 × 1009 1/s
122.00
XII
Biuret(matrix) ⟶ 2HNCO(g) + NH3(g)
1
5.626 × 1024 1/s
266.38
XIII
Urea(s) ⟶ urea(m)
1
1.000 × 1015·T1.5 1/s
160.00
XIV
Ammelide(s) ⟶ ammelide(g)
1
1.000 × 1014 ml/mol s
201.67
XV
HNCO(l) ⟶ HNCO(g)
Herz–Knudsen equation
Note. States of aggregation: (s): solid, (m): molten, (g): gaseous, (l): liquid/dissolved, (matrix): solid matrix. Triuret: NH2-CO-NH-CO-NH-CO-NH2. For reactions with two educts the reaction order γj of the second educt is specified as second value.Table 2
Condensed-phase kinetic scheme for urea thermal decomposition.
Reaction
A
i (s−1)
E
i (kJ mol−1)
(1) Urea ⟶ NH4+ + NCO−
8.50 × 106
84
(2) NH4+ ⟶ NH3(g) + H+
1.50 × 102
40
(3) NCO− + H+ ⟶ HNCO(g)
6.57 × 102
10
(4) Urea + NCO− + H+ ⟶ biuret
7.87 × 1014
115
(5) Biuret ⟶ urea + NCO− + H+
1.50 × 1024
250
(6) Biuret + NCO− + H+ ⟶ CYA + NH3(g)
2.81 × 1018
150
(7) CYA ⟶ 3NCO− + 3 H+
1.50 × 1019
260
(8) CYA + NCO− + H+ ⟶ ammelide + CO2
3.48 × 105
35
(9) Ammelide ⟶ 2NCO− + 2H+ + HCN(g) + NH(g)
6.00 × 1014
220
(10) Urea(aq) ⟶ NH4+ + NCO-
1.20 × 108
84
(11) NCO− + H+ + H2O(aq) ⟶ NH3 + CO2(g)
5.62 × 109
59
(12) Urea(aq) + NCO− + H+ ⟶ biuret
3.93 × 1014
115
(g): gaseous; (aq): aqueous.For urea pyrolysis pathways, Eichelbaum et al. drew the whole diagram, as shown in Figure2 [35]. It clearly showed the generation and decomposition of macromolecular products in detail.Figure 2
Schematic of all major reactions, intermediate and final products for the pyrolysis and hydrolysis of neat urea in the temperature range between 20 and 750°C.Much work has been done on urea pyrolysis mechanism, and valuable research results have been achieved. The urea pyrolysis network has been accomplished already, which includes not only the direct thermal decomposition reaction of urea and the hydrolysis reaction of HNCO, but also the generation path and decomposition path of macromolecular products such as biuret and cyanuric acid during urea pyrolysis. Even so, the detailed mechanism with accurate prediction ability still needs more study.


## 2.2. Catalytic Hydrolysis of HNCO

During urea pyrolysis, HNCO was an important intermediate product, as its hydrolysis reaction (12) played an important role in NH3 production [16–18]. The HNCO formed from urea decomposition is quite stable in the gas phase, so its hydrolysis reaction does not proceed as a homogeneous reaction. To accelerate it, various oxides have been applied as catalysts.Kleemann et al. studied the reaction rate and ammonia yield of the HNCO hydrolysis reaction under the catalysis of commonly used SCR catalysts (TiO2, V2O5/TiO2, V2O5-WO3/TiO2) [36]. And the catalytic effects of catalysts with different vanadium and tungsten contents on ammonia production from HNCO hydrolysis were compared in 140–475°C. The experimental results showed that HNCO had a higher hydrolysis reaction rate under the catalysis of pure TiO2 than under V2O5/TiO2 and V2O5-WO3/TiO2, while the addition of V2O5 or WO3 reduced the reaction rate to some extent. The activation energy of the HNCO hydrolysis reaction was determined to be about 13 kJ·mol−1, indicating that the overall reaction rate was mainly affected by the mass transfer rate.Piazzesi et al. carried out experimental research on the catalytic effect of Fe-ZSM5 and V2O5-WO3/TiO2 catalysts on the HNCO hydrolysis reaction and also compared the influence of different V2O5 contents on the catalytic effect of V2O5-WO3/TiO2 catalysts [37]. Experimental results showed that Fe-ZSM5 had a very high catalytic effect on the hydrolysis process of HNCO. The conversion efficiency of HNCO increased to 100% at about 260°C, and the perfect conversion rate was stably maintained until 450°C. The V2O5-WO3/TiO2 catalyst also exhibited a high catalytic effect on HNCO hydrolysis, and the catalytic effect of 1% V2O5 content was more significant than that of 3% V2O5 content. It indicated that, for the V2O5-WO3/TiO2 catalyst, the increase of V2O5 content resulted in the decrease of catalytic efficiency, which was consistent with the experimental results of Kleemann et al. [36].Czekaj and Kröcher studied the catalytic effect of TiO2 and γ-Al2O3 on HNCO hydrolysis by Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFTS) [38]. The experimental results proved that both TiO2 and γ-Al2O3 had significant catalytic effects on HNCO hydrolysis under the same reaction conditions and TiO2 had better catalytic performance than γ-Al2O3. Additionally, Czekaj and Kröcher used density functional theory to calculate the adsorption energy required for HNCO adsorbed on the catalyst surface during the reaction. The calculation results showed that the adsorption energy required for HNCO adsorption on TiO2 was lower than that on γ-Al2O3, which scientifically explained why the two catalysts differed in catalytic performance from the perspective of energy.Hauck et al. specifically carried out experimental research on the catalytic effect of TiO2 on HNCO hydrolysis, as well as the thermodynamic catalytic mechanism [39, 40]. In the self-made experimental system, mass spectrometry was used to determine the production of different atomic groups, and the reaction rate of the elemental reactions that might occur on the TiO2 surface was calculated by quantitative analysis. After calculation and analysis, they obtained the conclusion that HNCO hydrolysis was affected by mass transfer rate, which was consistent with the research conclusion of Kleemann et al. [36]. Moreover, Hauck et al. found that HNCO catalytic hydrolysis over TiO2 was restricted by the external mass transfer rate. However, the calculation by Hauck et al. rested on the reaction rate constant, but further activation energy calculation was not involved. It was worth mentioning that NO, NH3, and NO2 were added to the carrier gas stream, and the influence of the presence or absence of these gases on HNCO hydrolysis was investigated. Through the comparison and analysis of the experimental results, it was found that the existence of NO, NH3, and NO2 all inhibited HNCO hydrolysis to different degree, following the order of NO < NH3 < NO2. The main reason for the inhibitory effect was that the gases could be adsorbed on the surface of TiO2, thus reducing the adsorption of HNCO.Chen et al. measured and compared the similarities and differences of catalytic effects ofγ-Al2O3 and CuO/γ-Al2O3 on HNCO hydrolysis and calculated the activation energy of HNCO hydrolysis over the two catalysts, respectively [41]. They found that the catalytic effect of γ-Al2O3 was slightly higher than that of CuO/γ-Al2O3. According to their experimental data and calculation results, the activation energy of the HNCO hydrolysis over the two catalysts changed with the temperature range. When the temperature was higher than 200°C, the activation energy of HNCO hydrolysis over γ-Al2O3 was 13 kJ·mol−1, while over CuO/γ-Al2O3 it was 16 kJ·mol−1. The activation energy of HNCO hydrolysis measured by Chen was close to that measured by Kleemann et al. [36]. Such low activation energy reflected that the HNCO hydrolysis over γ-Al2O3 and CuO/γ-Al2O3 was mainly controlled by external mass transfer. When the temperature was lower than 200°C, the calculated activation energy obviously increased. The activation energy over γ-Al2O3 was calculated to be 25 kJ·mol−1, while over CuO/γ-Al2O3 it was calculated as 26 kJ·mol−1. It indicated that the reaction below 200°C was mainly dominated by mass transfer inside the catalyst pores. In addition, no matter what the temperature range was, the value of reaction activation energy over γ-Al2O3 was lower than that over CuO/γ-Al2O3, consistent with the phenomenon that γ-Al2O3 showed stronger catalytic effect than CuO/γ-Al2O3.It could be concluded from the analysis of the above literature [33–38] that the HNCO hydrolysis over metal oxides catalysts could be basically determined as a reaction process under mass transfer control. However, the determination of whether external or internal mass transfer is still controversial now. It is also a recognized fact that the hydrolysis reaction is slow without the assistance of relevant catalysts. All the catalysts used in the study were metal oxides, and they were mainly concentrated on the catalysts commonly used in SCR technology. Among them, TiO2 performed prominent catalytic effect, while the addition of vanadium and tungsten could hardly improve the catalytic efficiency.


## 2.3. Catalytic Pyrolysis of Aqueous Urea Solution

The aforementioned studies on urea pyrolysis mechanism and on catalytic hydrolysis of HNCO were conducted for the production of NH3, and effectively converting urea to NH3 with low energy consumption was decidedly pursued in actual operation. As aqueous urea solution is the most widely used agent in the urea DeNOx process, many researchers have focused their research on the catalytic pyrolysis of aqueous urea solution.Koebel and Struts conducted an experimental study on the synergistic process of urea pyrolysis and hydrolysis in SCR system of automotive internal combustion engines [42]. They focused their research on how to reasonably select the heating source for urea pyrolysis. In the paper, Koebel and Strutz listed the heat source selection of different ways in the urea pyrolysis process in detail and carried out specific energy consumption calculation, heat calculation, and experimental research for these selections, respectively. By analyzing and comparing, the following most energy-saving technical scheme was put forward: selecting part of the hot exhaust gas of the internal combustion engine to provide heat supply for urea pyrolysis, and improving the ammonia production efficiency of urea pyrolysis by rationally using the integral-type catalyst. This technical scheme could not only improve the ammonia yield, but also effectively utilize the exhaust heat of the internal combustion engine, which was in line with the principle of economy and practicability.Yim et al. carried out experimental research and theoretical calculation on NH3 production from urea pyrolysis in SCR system in 150–450°C [43]. The atomized urea solution particles were heated and decomposed in a self-made aluminum reaction system, and the effects of residence time and reaction temperature on the pyrolysis of urea solution were mainly investigated. Their experimental results demonstrated that the hydrolysis of HNCO occurred without catalysts when the reaction temperature was high enough (≥400°C). The prolongation of residence time was beneficial to urea pyrolysis and effectively promoted ammonia yield. In order to improve the ammonia generation rate, they also studied the catalytic effect of the high efficiency denitration catalyst CuZSM5 on pyrolysis of aqueous urea solution. The reaction rate and reaction activation energy of the two-step pyrolysis reactions of aqueous urea solution were calculated based on the obtained experimental results. However, an aluminum reactor was used in their experimental system, and it was likely that Al2O3 attached on the inner surface of the reactor might catalyze the reaction, so their conclusion results needed further verification.In order to compare the effects of different oxide catalysts on the pyrolysis reaction of aqueous urea solution and obtain detailed catalytic efficiency values, Kröcher and Elsener tested the catalytic effects of twenty potential catalyst materials on the pyrolysis of aqueous urea solution in a self-made fluidized bed reactor [44]. The results are listed in Table 3. By comparison, γ-Al2O3 was identified as the most suitable catalyst material in fluidized bed reactor. Although TiO2 showed the highest catalytic efficiency during the experiments, its wear resistance was far lower than that of γ-Al2O3. The catalytic efficiency of the catalysts was tested under the fluidized state, which was usually higher than that in fixed bed reactor, so it limited the application reference value to some extent.Table 3
Test of fluidized bed materials for the thermohydrolysis of 50% urea solution.
Bed material
Particle diameter (mm)
Reactor diameter (mm)
Volume of bed material (ml)
Air flow (LN/h)
Urea dosage type
Urea solution dosage (g/h)
Bed loading (g/(h·ml))
Fluidized bed temperature (°C)
Y
NH
3 (%)
YHNCO (%)
Urea slip (%)
γ-Al2O3 “Uetikon1”
0.2–0.315
20
15
150
From top
185
6.2
300
99
1
0.4
275
99
1
0.4
250
100
0
0.4
225
102
0
0.5
200
96
1
1.2
γ-Al2O3 “Uetikon1”
0.50–0.63
25
15
520
From top
185
6.2
300
97
4
1.2
275
96
5
1.3
250
84
12
2.6
225
73
16
3.9
γ-Al2O3 “compalox”
0.50–0.63
25
15
520
From top
185
6.2
300
96
0
1
275
95
0
0.7
250
96
1
0.9
225
93
2
2.3
γ-Al2O3 “puralox”
0.2–0.315
20
15
150
From top
185
6.2
300
96
5
0.4
275
99
5
0.3
250
96
4
0.4
225
98
4
0.4
200
99
4
0.9
γ-Al2O3 “CAT 250”
0.50–0.63
50
60
1860
From top
340
2.9
300
97
2
0.7
275
98
3
0.6
250
96
2
0.6
225
97
3
0.6
200
95
3
1.2
γ-Al2O3 “CAT 250”
1.0–1.25
50
60
3900
From top
350
2.9
300
96
0
1.9
275
95
0
1.8
250
94
1
1.6
225
83
5
2.4
200
66
15
5
γ-Al2O3 “CAT 250”
1.0–1.25
50
60
3900
From top
350
2.9
300
94
0
1.1
275
94
1
1.2
250
93
1
1.3
225
89
6
2.3
γ-Al2O3 “CAT 250”
1.0–1.25
25
15
1109
From top
186
6.2
300
101
0
1.9
275
100
2
1.7
250
99
3
1.8
225
84
12
3.2
γ-Al2O3 “condea”
1.0–1.25
20
15
554
From top
185
6.2
300
89
5
5.8
275
78
16
6.1
250
69
23
6.3
γ-Al2O3 “seralite 537”
1.0–1.25
50
60
3900
Into bed
350
2.9
300
94
3
1.9
275
91
5
2.2
250
85
7
3
225
72
13
5
α-Al2O3 “seralite 512 S”
1.6–2.0
50
60
4700
Into bed
290
2.4
300
47
31
9.1
α-AlOOH (boehmite)
0.2–0.315
20
15
150
From top
185
6.2
300
99
3
0.8
275
94
4
0.5
250
96
3
0.8
225
95
5
0.6
200
96
5
0.7
SiO2 “aerosil”
0.50–0.63
25
15
410
From top
185
6.2
300
70
25
4.2
AlSiOx (molochite)
0.22–0.50
20
15
150
From top
4.5
0.02
400
61
25
n.d.
350
55
34
n.d.
300
38
33
n.d.
250
24
20
n.d.
MgSiOx “florisil”
0.50–0.63
25
15
400
From top
185
6.2
300
90
8
2.3
275
86
7
2.1
250
83
9
3
TiO2 “macrosorb T”
0.2–0.315
20
15
150
From top
185
6.2
300
104
1
0.7
275
102
1
0.8
250
95
4
2.7
225
62
8
5.9
TiO2 “7702”
0.50–0.63
25
15
520
From top
185
6.2
300
98
0
1.5
275
99
0
1.2
250
99
0
1
225
99
1
1
200
98
2
1.6
V2O5/WO3-TiO2
0.315–0.50
25
15
356
From top
185
6.2
300
96
1
0.6
275
96
1
0.6
250
90
6
1.1
225
87
6
1.5
200
82
6
2.9
Molsieve 5 Å
0.2–0.315
20
15
150
From top
94
3.1
300
93
7
0.9
275
58
21
7.2
Molsieve “13X”
0.50–0.63
25
15
410
From top
185
6.2
300
93
2
0.7
275
76
13
1.2
250
43
20
5.5
H-MOR
ca.2.(2–4)
50
60
3900
From top
350
2.9
300
85
4
2.6
275
83
6
2.8
H-ZSM-5
ca.2.(2–4)
50
60
3900
From top
350
2.9
300
72
19
3.1
275
66
25
3.6
Dealuminated Y
ca.2.(2–4)
50
60
3900
From top
350
2.9
300
52
35
5
275
51
36
5.6In the above literature [42–44], the solvent of urea solution was water, which might affect the catalytic effect of catalysts. Perhaps being aware of it, Bernhard et al. carried out experimental tests and kinetic analysis on the catalytic efficiency of stationary catalysts soon afterwards [45]. In their experimental system, the catalytic effects of different characteristic sizes of TiO2, ZrO2, Al2O3, H-ZSM-5, and SiO2 on urea pyrolysis and HNCO hydrolysis were tested, respectively. Particularly, the catalytic effect of the above materials on urea pyrolysis under anhydrous conditions was a conspicuous highlight. In previous studies, catalytic efficiency was tested in experiments with water. Differently, Bernhard et al. used methanol and ethanol as solvents to avoid the presence of water in the experiments, which provided convincing experimental data for the catalytic research of oxides on urea pyrolysis without water. The experimental results showed that the presence or absence of water had a great influence on the catalytic effect of the catalysts. When the urea solvent was water, the order of catalytic effect of different catalysts was ZrO2 > TiO2 > Al2O3 > H-ZSM-5 > SiO2. Under anhydrous conditions, the order changed to TiO2 > H-ZSM-5≈Al2O3 > ZrO2 > SiO2. Additionally, the ammonia production efficiency over these catalysts under anhydrous conditions was significantly lower than that under hydrous conditions.Lundström et al. adopted differential scanning calorimetry and mass spectrometry to study the effects of TiO2, Al2O3, and Fe-Beta catalysts on the production of ammonia, isocyanic acid, and carbon dioxide from urea pyrolysis under anhydrous and hydrous conditions [46]. The differences of ammonia production between anhydrous and hydrous conditions were also detected. However, different from the reference [45], in this paper, researchers did not recognize that TiO2, Al2O3, and Fe-Beta catalysts could catalyze urea pyrolysis under anhydrous conditions. They attributed the difference to the absence of HNCO hydrolysis under anhydrous conditions. This conclusion contradicted the results of Yim and Bernhard et al. [43, 45] but lacks scientific basis.In the literature mentioned above, urea was pyrolyzed on the catalyst which was put into the reactor in the form of solution with water, methanol, or ethanol as solvent. In the experiment of Bernhard et al., N2 at 100°C was passed through honeycomb-like TiO2 cordierite filled with aqueous urea solution [47]. The formed mixed gas stream was determined by HPLC and FTIR. In this paper, Bernhard et al. first clearly analyzed and obtained the adsorption form of urea on the surface of TiO2 and calculated the energy value consumed during the adsorption process.The effect of oxygen concentration and different additives (Na2CO3 and NaNO3) on NH3 yields and N2O and CO production from pyrolysis of aqueous urea solution was studied by our group [22]. Without additives, the presence of oxygen made the NH3 yields drop rapidly in 550–800°C. The addition of Na2CO3 or NaNO3 not only increased the NH3 yields but also reduced the N2O and CO production via a series of chain reactions of sodium species, but it did not restrain the NH3 oxidation at high temperatures. The addition of sodium species probably provided plenty of NaO and NaO2, which might catalyze the HNCO hydrolysis to dramatically increase the NH3 yields. Moreover, the harmful gaseous by-products (N2O and CO) might participate in the chain reactions of sodium species, so most N2O and nearly all CO could be consumed.Various catalysts were selected for hydrolysis of aqueous urea solution, and some outstanding ones stood out for the excellent efficiency. Many studies have been carried out on the exploration and development of catalysts, while little attention has been paid to the catalytic mechanism investigation. Further work, especially the dynamic mechanism, was still needed to help to make better use of these catalysts.


## 3. Conclusion

Urea pyrolysis plays an important role in urea-based NOx removal technology. Research into urea pyrolysis mostly focused on three parts: urea pyrolysis pathway, catalytic hydrolysis of HNCO, and catalytic pyrolysis of aqueous urea solution. The network of urea pyrolysis has been basically established, which includes not only the direct thermal decomposition reaction of urea and the hydrolysis reaction of HNCO, but also the generation path and decomposition path of macromolecular products such as biuret and cyanuric acid during urea pyrolysis. The HNCO hydrolysis over metal oxides catalysts could be basically determined as a reaction process under mass transfer control. And it is also a recognized fact that the HNCO hydrolysis reaction is slow without the assistance of relevant catalysts. No matter whether in the presence or absence of water, TiO2 has relatively higher catalytic effect on urea pyrolysis and HNCO hydrolysis.Although much work has been done, concentration was mainly on analysis of urea pyrolysis products and exploration of catalysts used to improve ammonia yields. In future work, it is necessary to determine the detailed reaction mechanism and catalytic mechanism in the urea pyrolysis process more clearly and to establish a high-precision kinetic model.



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*Source: 6853638-2019-12-09.xml*