Piqiang Tan,Xiaoyu Li,Shiyan Wang,Zhiyuan Hu,Diming Lou

School of Automobile Studies,Tongji University,Shanghai 201804,China

Keywords:SCR Low NH3-NOx ratio Purification Hydrothermal Urea crystallization

ABSTRACT An insufficient amount of NH3(ammonia)will reduce the conversion efficiency of NOx,which may lead to excess NOx emissions,resulting in NH3-SCR failure.In this article,SCR failure caused by a low NH3-NOx ratio is studied systematically by experiments.The main reasons for a low NH3-NOx ratio in SCR include insufficient urea injection,hydrothermal aging of catalysts and urea crystallization.It was found from an insufficient urea injection experiment that with the increase of NH3-NOx ratio,the NOx conversion efficiency of the SCR system increased,but the ammonia leakage also increased.The main influencing factors of NOx conversion efficiency are different under different NH3-NOx ratios.A flow reactor system was used in the catalyst hydrothermal aging experiment to investigate the effect of hydrothermal aging on catalyst activity.After a 24 h hydrothermal aging experiment at 800°C,the NOx conversion efficiency of the copper-based zeolite catalysts decreased significantly at the boundary of medium and low temperature regions.And the NO2-NOx ratio in the mixture had a significant effect on the catalytic performance.Thermogravimetry coupled to Fourier transform infrared spectroscopy (TG-FTIR) was used to analyze the composition of urea deposits in a urea deposits analysis experiment.It was found that the main components of urea deposits were urea and isocyanic acid (HNCO).Preventing HNCO polymerization,especially the formation of CYA,can decrease the formation of urea deposits.

1.Introduction

NOxfrom diesel engines can cause substantial damage to the environment and human health.NO is irritating and toxic and can cause damage to the human central nervous system in high concentrations.Although the content of NO2is relatively small,it is more harmful to the human body than NO.It stimulates the eyes and respiratory tract,and it combines with hemoglobin in the body,reducing oxygen delivery capacity.In addition,NOxforms highly toxic photochemical smog with HC under the intense ultraviolet radiation of sunlight,posing a serious threat to the environment and human health [1–4].Increasingly stringent regulations have been implemented to limit NOxemissions.For example,the NOxlimit for cars and light-duty trucks was reduced to 35 mg﹒km-1by the China VI-b standard and to 0.4 g﹒(kW﹒h)-1by the Euro VI standard [5].

Selective catalytic reduction of NOxwith NH3(NH3-SCR) has become the most promising aftertreatment purification technology to reduce NOxin diesel exhaust because of its high NOxconversion efficiency and good fuel economy[6–9].In the NH3-SCR,NOxreacts with NH3(ammonia)in the presence of catalysts and converts into harmless N2and H2O[10,11].NOxin diesel engine exhaust usually consists of more than 90%NO and only a minor fraction of NO2.At this point,the reaction taking place is the so-called standard SCR reaction,as shown in formula (1).One problem of SCR is the poor activity at low temperatures where most NOxis produced,e.g.,cold start-up and on short traveling distances.Except for raising the temperature of the exhaust and catalyst,the most commonly used method to improve the NOxconversion efficiency of SCR at low temperature is to increase the NO2-NO ratio.When NO2∶NO=1,the reaction is dominated by a rapid SCR reaction,shown in formula (2),which is much faster than the standard SCR reaction[12,13].If the fraction of NO2exceeds 50%,the SCR reaction shown in formula(3)takes place[8].This reaction with pure NO2is slower than the standard SCR reaction[14].However,since NH3is difficult to store and handle in mobile applications as a reactive toxic gas,a urea-water solution (32.5% urea by weight,UWS) is used as the NH3source for SCR in diesel vehicles since it is nontoxic and can be stored much more easily and safely[15–18].After the injection of the urea-water solution into the diesel exhaust upstream of the SCR catalyst from the nozzle,the water in the urea-water solution evaporates,and the urea pyrolyzes first to produce equimolar amounts of gaseous NH3and isocyanic acid (HNCO) [19,20].In the subsequent reaction,HNCO hydrolyzes to form NH3and CO2.The series of reactions are shown in formulas (4) and (5):

SCR is a complex system with high NOxconversion efficiency.However,it also has a variety of failure modes,which greatly affect the durability of SCR.These failure modes include hydrothermal aging of catalysts,toxic aging of catalysts,carrier structure damage,and urea crystallization.The purification effect of SCR is largely related to the NH3-NOxratio.An insufficient amount of NH3will reduce the NOxconversion efficiency,whereas an excessive amount of NH3will increase the consumption of urea and cause the leakage of NH3and thus pollution of the environment[21,22].At present,it is more urgent to meet the requirements of NOxconversion efficiency imposed by increasingly strict emission regulations.Furthermore,an ammonia slip catalyst (ASC) can be installed behind the SCR to capture ammonia gas when the ammonia leakage is serious.Therefore,the SCR failure caused by a low NH3-NOxratio is more serious.

There are many reasons for a low NH3-NOxratio in SCR.The first reason for low NH3-NOxratio in SCR is that the actual urea injection volume is insufficient due to insufficient injection pressure or pipeline blockage.The insufficient actual urea injection volume mentioned here is entirely due to physical factors.Wang et al.investigated NOxsensor reading correction in diesel engine selective catalytic reduction (SCR) system applications to ensure the accuracy of the NH3-NOxratio in SCR [22].

Another reason for a low NH3-NOxratio in SCR is hydrothermal aging of catalysts.An important role of catalysts in SCR is to catalyze the pyrolysis and hydrolysis reactions of urea since these reactions are rather slow at typical diesel exhaust temperatures.As emission regulations become more stringent,a catalytic diesel particulate filter (CDPF) is required to be installed in front of SCR to trap particles.Thus,the SCR system periodically undergoes high temperature conditions,approaching 1000°C,accompanied with water vapor,due to the regeneration progress of the CDPF[10,23].The SCR catalyst will be hydrothermally aged after longterm service in this high-temperature and humid environment,which will lead to the reduction of the NH3-NOxratio in SCR[21].Fan et al.investigated the hydrothermal stability of Cu/SAPO-34 catalysts under different treatment times (3 h,6 h,12 h)at 950°C.Their H2-TPR and EPR results proved that the number of Cu2+species decreased and the location of Cu2+species changed after this treatment,which were mainly responsible for declined SCR reaction rates[24].Wu et al.used a flow reactor experimental system for the hydrothermal aging treatment of a one-pot method synthesized Cu-SSZ-13 catalyst.Through a series of activity measurements,such as standard SCR,NH3oxidation and NH3-TPD,it was found that the catalyst framework could be crashed by an 850°C hydrothermal treatment,losing most of its SCR activity.After a 750°C hydrothermal treatment,the positions of some copper active sites changed and copper oxide was generated,which reduced catalytic activity [25].

Urea crystallization is also one of the reasons for the low NH3-NOxratio failures of SCR.Ideally,after the injection of the ureawater solution into the diesel exhaust upstream of the SCR catalyst,urea is decomposed into NH3,CO2and H2O gaseous products,but urea decomposition is not always perfect depending on different physical conditions.Some solid by-products can form as well,such as complex polymers like melamine,biuret,ammeline and cyanuric acid [26–29].Therefore,urea crystals and these solid byproducts,called urea deposits,can accumulate on the exhaust pipe wall and catalyst surfaces,thus reducing the activity of catalysts and the NH3-NOxratio.Dong et al.measured urea thermolysis using thermal gravimetric analysis (TGA) and Fourier transform infrared spectroscopy(FTIR)and found that urea thermolysis exhibits three decomposition stages.The corresponding mass losses of the three stages and the composition of urea deposits were studied.However,the low NH3-NOxratio failure of SCR caused by urea crystallization needs further study,especially the composition of urea deposits [8,30].

At present,many studies have been conducted on various failure modes of SCR.These studies mainly focus on the influencing factors of NOxconversion efficiency and the catalyst failure mechanism,which are topics of great significance to the understanding and prevention of the SCR failure process.However,there are few systematic studies on SCR failure caused by low NH3-NOxratio,so there is still a lack of comprehensive understanding of SCR failure caused by low NH3-NOxratio.In this article,several typical reasons that can cause SCR failure due to low NH3-NOxratio are studied.Low NH3-NOxratio in SCR caused by insufficient urea injection is explored through an insufficient urea injection experiment.A catalyst hydrothermal aging experiment is conducted to explore the catalysts inactivation at high temperature.A urea deposit analysis experiment is conducted to explore the composition of urea deposits in the process of urea crystallization.This experimental study on the low NH3-NOxratio will be very useful for future studies on precisely controlling the actual NH3-NOxratio in SCR under various working conditions.

2.Experiment

Three typical reasons of low NH3-NOxratio failure in SCR are insufficient urea injection,hydrothermal aging of catalyst and urea crystallization.The actual urea injection volume would be insufficient due to insufficient injection pressure,pipeline blockage or other urea injection system failure.Based on the physical reason of low NH3-NOxratio failure in SCR,the insufficient urea injection experiment was designed.An important role of catalysts in SCR is to catalyze the pyrolysis and hydrolysis reactions of urea to form NH3.Due to the hydrothermal aging of the catalyst,the actual amount of NH3in SCR is insufficient.Based on the chemical reason of low NH3-NOxratio failure in SCR,the catalyst hydrothermal aging experiment was designed.The formation of urea deposits consumes part of urea.At the same time,urea deposits attached to pipelines or nozzles would cause insufficient urea injection and cause low NH3-NOxratio failure in SCR at the physical level.If urea deposits attached to the surface of the catalyst,it would affect the performance of the catalyst and cause low NH3-NOxratio failure in SCR at the chemical level.Based on the physical and chemical reason of low NH3-NOxratio failure in SCR,the urea deposit analysis experiment was designed.

2.1.Insufficient urea injection experiment

Fig.1 presents a schematic of the primary components of the experimental system employed in the insufficient urea injection experiment.The experimental system is mainly composed of a test diesel engine,an Electric dynamometer (HORlBA Dynas3),an exhaust-gas analysis system (HORlBA MEXA-7100D),a diesel oxidation converter,a selective catalytic reduction,and an SCR control system.

Fig.1.Schematic diagram of insufficient urea injection experiment bench system.

An electronically controlled,high-pressure common rail,inline six-cylinder,four-stroke diesel engine is employed in the experiment,and its primary specifications are listed in Table 1.The primary specifications of the SCR employed in the experiment are listed in Table 2.The urea injection volume is controlled by an SCR urea control system through adjusting the urea injection pulse width.An exhaust-gas analysis system measures NOxemissions and NH3leakage.A temperature sensor is installed at the SCR entrance to measure temperature of the exhaust gas.Diesel fuel,which meets China V fuel standards,is employed in this experiment,and its primary specifications are listed in Table 3.The main purpose of the insufficient urea injection experiment is to study the effect of insufficient urea injection on NOxconversion efficiency.The selection of catalyst type has little influence on the experimental results,because both of vanadium-based catalyst and copper-based zeolite catalyst have great catalytic performance.Therefore,in order to simplify the preparation process of the experiment,the SCR of vanadium-based catalyst was directly used in this experiment.

Table 1 Specifications of the diesel engine

Table 2 Specifications of SCR

Table 3 Specifications of the fuel

The diesel engine runs at 1600 r﹒min-1at first in this experiment.Then we adjust the temperature in front of SCR to 450°C by changing the torque of the diesel engine (535 N﹒m at 1600 r﹒min-1).After that,we set the urea injection amount of SCR to zero and make the diesel engine run for a while.NOxemissions in front of SCR are measured when the diesel engine is running stably.The theoretical urea injection pulse width (NH3-NOxratio is 1.0) is computed by exhaust flow and NOxemission.The actual urea inject pulse width is adjusted to be 1.0,0.8 and 0.6 times the theoretical urea injection pulse width,and the NOxemissions and NH3leakage behind the SCR are measured for each condition.

After measuring,we adjust the temperature in front of SCR to 450°C,400°C,350°C,300°C,250°C,230°C and 200°C by changing the torque of the diesel engine while keeping the speed constant at 1600 r﹒min-1(the corresponding torques are 535 N﹒m,398 N﹒m,305 N﹒m,245 N﹒m,160 N﹒m,135 N﹒m and 57 N﹒m,respectively).We repeat the above experiment at every temperature.

2.2.Catalyst hydrothermal aging experiment

A flow reactor system is used in the SCR catalyst hydrothermal aging experiment to deactivate the catalysts at high temperature.Fig.2 presents the schematic of the flow reactor system.

Fig.2.Schematic diagram of the flow reactor system.1—Float flowmeter;2—Saturated steam generator;3—Integral flowmeter;4—Mixer;5—Heating furnace;6—Reactor;7—Thermocouple;8—K-type thermocouple;9—Gas analyzer;10—Computer.

The main purpose of the catalyst hydrothermal aging experiment is to study the effect of hydrothermal aging on the catalytic performance of the catalyst.Copper-based zeolite catalysts have been widely used because they have better hydrothermal stability than vanadium-based catalyst.Therefore,the copper-based zeolite catalyst sample (Cu/SSZ-13) with better hydrothermal stability was used in this experiment.A 24 h hydrothermal aging experiment is performed on the copper-based zeolite catalysts sample using a mixture of 10% H2O and 90% air with a flow rate of 1000 cm3﹒min-1at 800°C.The NOxconversion efficiency of the copper-based zeolite catalysts sample before and after the hydrothermal aging experiment is measured.In order to simulate the working conditions of the catalysts,we select the following experiment conditions in the NOxconversion efficiency measuring test:temperature:140–550°C,velocity of gas:50,000 h-1,mixed gas composition (volume fraction):600×10-6NH3,500×10-6NO+NO2,8% O2,5% H2O and N2.In order to explore the influence of NO2-NOxratio on NOxconversion efficiency,we use three kinds of mixtures (NO2-NOxratio=0,NO2-NOxratio=0.5,NO2-NOxratio=0.75) in the NOxconversion efficiency measuring test.

2.3.Urea deposits analysis experiment

Thermogravimetry coupled to Fourier transform infrared spectroscopy(TG-FTIR)is used to analyze the composition and component content of urea deposit.The urea deposit is heated by a thermogravimetric analyzer,and the weight change of urea deposit is measured during the heating process.Decomposition products of urea deposit during thermogravimetric analysis are studied by infrared spectroscopy using a Fourier transform infrared spectrometer.Fig.3 presents a schematic of the device employed in the urea deposits analysis experiment.

The experiment method is listed as follows:the urea deposit is heated with the thermogravimetric analyzer from 20°C at a rate of 20°C﹒min-1until the urea deposit decomposes completely.The mass changes of urea deposit are measured during the heating process,and the generated gas is passed into the Fourier transform infrared spectrometer for further infrared spectroscopy scanning.

3.Results and Discussion

3.1.Insufficient urea injection experiment

Fig.3.Schematic diagram of thermogravimetry coupled to Fourier transform infrared spectroscopy (TG-FTIR) device.

Fig.4.NOx emissions and exhaust temperatures at the SCR inlet at 1600 r﹒min-1.

NOxemissions in front of SCR,NOxemissions behind SCR and exhaust temperatures at the SCR inlet under 1600 r﹒min-1with NH3-NOxratios of 0.6,0.8 and 1.0 are shown in Fig.4.It can be seen that the NOxemissions in front of SCR increase with the increase of torque.This is mainly because as the torque increases,the fuel injection volume of the diesel engine increases,and the combustion temperature in the cylinder rises,which increases the generation of NOx.The NOxemissions behind SCR have different changes under different NH3-NOxratios.Overall,when the NH3-NOxratios increase,the NOxemissions decrease significantly in turn.This shows that as the NH3-NOxratio increases,the ammonia content increases and the NOxconversion efficiency increases.When the NH3-NOxratio is 0.6,there is no significant difference in NOxemission behind SCR at medium and low torque,whereas NOxemission behind SCR increases at high torque.When the NH3-NOxratio is 1.0,the NOxemission behind SCR tends to decrease first and then increase with the increase of torque.That is,the NOxemission is large under low torque and high torque,but is small under medium torque.This may be because the amount of NH3is the main factor affecting the NOxconversion efficiency of SCR when the NH3-NOxratio is 0.6.Because of the insufficient amount of NH3,the NOxconversion efficiency is relatively low under various working conditions.As the torque increases,the NOxemission in front of SCR increases,and the problem of insufficient NH3becomes more serious.When the NH3-NOxratio is 1.0,the amount of NH3is sufficient,so the exhaust temperature becomes the main factor affecting the NOxconversion efficiency of SCR.As the torque increases,the exhaust temperature at the SCR inlet increases from 200°C to 450°C.The NOxconversion efficiency is high under medium torque because the exhaust temperature at the SCR inlet is around 300°C,which is located in the reaction temperature window of catalysts.Low torque and high torque correspond to lower and higher exhaust gas temperatures,which are not conducive to SCR catalytic reaction.And as the amount of NH3increases,the effect of the exhaust temperature is more obvious.When the NH3-NOxratio is 0.8,the NOxconversion efficiency is affected by both the amount of NH3and the exhaust temperature.

The NOxconversion efficiencies of SCR at 1600 r﹒min-1with different torques and with NH3-NOxratios of 0.6,0.8 and 1.0 are shown in Fig.5.The NOxconversion efficiency curves under different NH3-NOxratios show similar trends.The NOxconversion efficiency increases first and then decreases with the increase of torque,and the NOxconversion efficiency is highest at medium torque.Because the exhaust temperature at medium torque is in the reaction temperature window of the catalysts,the NOxconversion efficiency under this condition is the highest.The highest NOxconversion efficiencies when the NH3-NOxratios are 0.6%,0.8% and 1.0% are 65.3%,85.8% and 98.1%,respectively.These results are very close to the theoretical reaction efficiencies of 60%,80% and 100%.The results show that the reaction of NH3is relatively complete at a suitable temperature.When the NH3-NOxratio is 0.6,the decrease of NOxconversion efficiency in the high-torque region is the least obvious.This may be because the main factor affecting the NOxconversion efficiency at this time is the insufficient amount of NH3rather than the exhaust temperature.

Fig.5.NOx conversion efficiency of SCR at 1600 r﹒min-1.

Fig.6.Ammonia leakage of SCR at 1600 r﹒min-1.

Fig.6 shows the amount of ammonia leakage behind SCR at 1600 r﹒min-1with different torques and with NH3-NOxratios of 0.6,0.8 and 1.0.It can be seen that when the NH3-NOxratio is 0.6,the amount of ammonia leakage is less than 5 ppm.There is almost no ammonia leakage because the amount of NH3injected is too small.When the NH3-NOxratio is 0.8,there is almost no ammonia leakage below 245 N·m.When the torque is above 245 N·m,the ammonia leakage increases,and the rate of increase gradually slows down.The maximum ammonia leakage is 37.7×10-6.When the NH3-NOxratio is 1.0,the ammonia leakage problem is further highlighted.As the torque increases,the ammonia leakage increases rapidly.The ammonia leakage reaches its maximum value of 127.4×10-6at medium torque.The reasons for these results may include the following:(1) as the torque increases,the amount of NOxgeneration increases,and the amount of NH3injected increases;(2) as the temperature of the exhaust increases,the volatility of urea increases;(3) the intake air flow rate and the fuel injection amount increase,so the flow rate of the exhaust gas increases,and the reaction time of NH3in the SCR is shortened.The exhaust flow and ammonia injection amount are shown in Fig.7.

From the insufficient urea injection experiment results,as the NH3-NOxratio increases,the ammonia content increases and the NOxconversion efficiency increases.But the ammonia leakage problem is more serious.At present,it is more urgent to meet the requirements of NOxconversion efficiency imposed by increasingly strict emission regulations.NH3-SCR has become the most promising aftertreatment purification technology to reduce NOxin diesel exhaust.So the technical route that we suggest is to provide sufficient NH3to meet NOxemission regulations.For heavy diesel engines,if ammonia emissions are excessive,ammonia slip catalyst (ASC) technology should be further studied to meet the regulations.

Fig.7.Exhaust flow and ammonia injection volume at 1600 r﹒min-1.

3.2.Catalyst hydrothermal aging experiment

Fig.8 shows the NOxconversion efficiency of copper-based zeolite catalysts before and after the hydrothermal aging experiment when the NO2-NOxratios are 0,0.5 and 0.75.According to the experiment results,the catalytic effect before and after the 200 h hydrothermal aging experiment show similar change trends.The NOxconversion efficiency in the low temperature region (140–200°C) is low,and the NOxconversion efficiency increases with the increase of temperature.In the medium temperature region(200–450°C),the NOxconversion efficiency is high.The NOxconversion efficiency in the high temperature region (450–550°C)decreases with the increase of temperature.

The changes in NOxconversion efficiency before and after the hydrothermal aging experiment are compared under different NO2-NOxratios.Compared with fresh catalysts,the NOxconversion efficiency of aged catalysts does not change significantly in the middle and high temperature regions,regardless of the NO2-NOxratio.However,the catalytic performance of aged catalysts decreases significantly at the boundary between medium and low temperature (about 180–200°C.When the NO2-NOxratio is 0,the catalytic performance of the aged catalysts decreases in the range of 140–250°C,and the maximum value of catalytic performance reduction is 15.7% at 180°C.When the NO2-NOxratio is 0.5,the catalytic performance of the aging catalysts decreases in the range of 160–220°C,and the maximum value of catalytic performance reduction is 7.9% at 180°C.When the NO2-NOxratio is 0.75,the catalytic performance of the aged catalysts decreases in the range of 180–250°C,and the maximum value of catalytic performance reduction is 13.0%at 200°C.It can be seen that after the hydrothermal aging experiment,when the NO2-NOxratio is 0.5,the maximum decrease of the NOxconversion efficiency of the aged catalysts is the smallest,which may be due to the rapid SCR reaction occurring when NO2:NO=1:1.The high reactive rate masks the degradation of the catalytic performance.As the NO2-NOxratio increases,the difference in NOxconversion efficiency becomes gradually less obvious when the temperature is further reduced.When the NO2-NOxratio is 0,the difference between fresh and aged catalysts still exists at 140°C.But the difference of NOxconversion efficiency between fresh and aged catalysts does not exist at 160°C when the NO2-NOxratio is 0.5.And there is no difference in catalytic performance between fresh and aged catalysts at 180°C when the NO2-NOxratio is 0.75.

After hydrothermal aging,the catalytic activity of the copperbased zeolite catalysts decreased,especially the low-temperature deNOxactivity.The high temperature deactivation of copperbased zeolite catalysts is mainly related to the change of catalysts characterizations.The active Cu2+ions decrease while the CuO increase in catalysts after hydrothermal aging.It is widely accepted that the transformation of Cu2+ions to CuO occurs during the hydrothermal aging.CuO is inactive for NO reduction and promotes undesirable side reactions such as the nonselective NH3oxidation at high temperatures.More seriously,the CuO tends to destroy the zeolite structure in the hydrothermal aging.The collapse of CHA structure affects the catalytic activity of the zeolite catalysts.After hydrothermal aging,the surface acidity of catalysts reduces,due to dealumination.Dealumination decreases the storage capacity of NH3and causes the catalyst to enhance the nonselective catalytic reduction reaction,thereby reducing the catalytic efficiency.The LCu+ions,which have been proved beneficial for the low-temperature deNOxactivity,also decline after the hydrothermal aging.

Fig.8.NOx conversion efficiency of copper-based zeolite catalysts before and after hydrothermal aging experiment.

Fig.9.NOx conversion efficiency of copper-based zeolite catalysts before and after hydrothermal aging experiment.

Fig.9(a) shows the catalytic performance of the copper-based zeolite catalysts at different NO2-NOxratios before the hydrothermal aging experiment.Considering all the temperature ranges,the catalytic performance of the fresh catalysts is best when the NO2-NOxratio is 0.5.Compared with the NO2-NOxratio of 0.5,when the NO2-NOxratio is 0,there is no significant difference in NOxconversion efficiency above 160°C,but the NOxconversion efficiency decreases below 160°C.This may be because the standard SCR reaction and the rapid SCR reaction are both relatively rapid above 160°C,so they can both maintain high NOxconversion efficiency.Below 160°C,the standard SCR reaction rate is much slower than the rapid SCR reaction rate.When the NO2-NOxratio is 0.75,the overall NOxconversion efficiency is the lowest.Compared with the NO2-NOxratio of 0.5,the NOxconversion efficiency is lower when the NO2-NOxratio is 0.75 except for in the range of 350–400°C.This is because when NO2∶NO>1∶1,a reaction as shown in formula (3) occurs,and this reaction rate is the slowest.

Fig.9(b) shows the catalytic performance of the copper-based zeolite catalysts at different NO2-NO ratios after the hydrothermal aging experiment.It can be seen from Fig.9(b) that after the hydrothermal aging experiment,when the NO2-NOxratio is 0.5,the NOxconversion efficiency of the copper-based zeolite catalysts is still the highest in most of the temperature ranges.Compared with the NO2-NOxratio of 0.5,the NOxconversion efficiency becomes lower in the range of 140–250°C when the NO2-NOxratio is 0,but more excellent catalytic performance is exhibited above 470°C.This may be because the performance degradation of the standard SCR reaction after the hydrothermal aging experiment is small in high temperature ranges,but it is large in low temperature ranges compared to the rapid SCR reaction.As for the NO2-NOxratio of 0.75,the overall NOxconversion efficiency is still the lowest,especially at 180–250°C.At low temperatures,the hydrothermal aging experiment is more detrimental to the performance of the reaction as shown in formula (3) than the rapid SCR reaction.

3.3.Urea deposits analysis experiment

Fig.10 shows the comparison of the thermogravimetric analysis results of urea deposit and pure urea.The decomposition process of urea deposit and pure urea can be divided into three reaction regions,in which different chemical processes result in mass loss during TGA.The temperature range from 190°C to 250°C is defined as the first reaction range.When the temperature is below 190°C,only a little mass loss is observed.This mass loss is related with urea vaporization and impurities such as water in the pure urea sample.The net mass loss in the first reaction range is mainly the result of urea decomposition,and the off gas is NH3.The urea decomposition process includes the following reactions:first,urea decomposes to release one NH3molecule and one HNCO molecule,as shown in formula (4).And then the HNCO reacts with the melted urea to produce biuret (NH(CO)2(NH2)2),as shown in formula (6).At this reaction range,the cyanuric acid (CYA,(HNCO)3)is conducted in many ways.Formula(7)shows the CYA production by urea decomposition.The reaction of biuret and HNCO to form CYA is shown in formula (8).The CYA could also be condensed directly and rapidly by HNCO,and the reaction is shown in formula(9).The temperature range from 250°C to 380°C is defined as the second reaction range.Most of the mass loss is related to the CYA decomposition,as shown in formula (10).And this reaction is just the reverse of the reaction shown in formula (9).As the temperature is increased higher than the CYA melting point of 360°C,the mass loss becomes more rapid.The temperature range beyond 380°C is defined as the third reaction range.The mass loss is related to the continued decomposition of CYA and the beginning decomposition of ammelide (410°C) and ammeline (435°C):

Fig.10.Thermogravimetric analysis of urea deposit and pure urea.

It can be seen from Fig.10 that the changes in the mass fractions of pure urea and urea deposit with temperature are similar,which suggests their components are nearly the same,but the content of each component is different.

The gas obtained from the decomposition of urea deposit at different temperatures is scanned by infrared spectroscopy.According to the infrared spectrum test data and the chemical mechanism of urea decomposition and crystallization process,the decomposition products of urea deposit at different temperatures are determined to be urea and cyanuric acid.The gases produced by urea deposit at different temperatures are denoted as urea deposit 1 and urea deposit 2.Fig.11 shows the infrared spectroscopy analysis of urea deposit 1 and pure urea,and Fig.12 shows the infrared spectroscopy analysis of urea deposit 2 and cyanuric acid.

It can be seen from Fig.11 that the infrared spectra of urea deposit 1 and pure urea have the same trend,their peaks appear in the same wave number segment,and their peak amplitudes are similar.This indicates that the main component of urea deposit 1 is urea crystals.Urea crystals are formed by direct condensation and deposition of urea water solution with insufficient pyrolysis and hydrolysis in the exhaust.It can be seen from Fig.12 that the infrared spectra of urea deposit 2 and cyanuric acid have the same trend,their peaks appear in the same wave number segment,but their peak amplitudes are different.This indicates that the main component of urea deposit 2 is CYA,but there are some other substances in urea deposit 2,such as cyanuric acid homologs and complicated macromolecular polymers.

According to the experiment results and related reactions,the urea deposit formation process is summarized,as shown in Fig.13.At first,the urea-water solution is ejected from the urea nozzle.Then the urea undergoes pyrolysis and hydrolysis reactions to generate HNCO and NH3.When pyrolysis or hydrolysis is insufficient,some urea molecules condense at the inner wall of the exhaust pipe and become one of the main components of the urea deposit.As time passes,some of the condensed urea molecules undergo further chemical reactions in the exhaust.These chemical reactions mainly include the following:urea reacts with HNCO to form biuret in the range of 150–200°C,as shown in formula (6).Biuret and HNCO react to form CYA in the range of 200–280°C,as shown in formula (8).The CYA can also be condensed directly and rapidly by HNCO,as shown in formula (9).Biuret has high activity and decomposes above 200°C,so biuret is not often deposited.But CYA has a high decomposition temperature and decomposes above 350°C.Therefore,some CYA remains and becomes one of the main components of the urea deposit.When the temperature is higher,cyanuric acid homologs and more complicated high molecular weight components such as ammelide,ammeline and melamine are formed.

Fig.11.Infrared spectroscopy analysis of urea deposit 1 and pure urea.

Fig.12.Infrared spectroscopy analysis of urea deposit 2 and cyanuric acid.

Fig.13.Schematic diagram of urea deposit formation process.

In summary,the polymerization of HNCO with melted urea and biuret contributes to the formation of high molecular weight components,which have high decomposition temperatures and form stable deposits on catalyst surfaces.Preventing HNCO polymerization,especially the formation of CYA,can decrease the formation of urea deposits.This is desirable because urea deposits reduce the NH3-NOxratio,which can cause SCR failure.In addition,relevant measures can be taken to make the pyrolysis and hydrolysis reaction more sufficient to reduce the generation of urea deposit.The relevant measures are as follows:(1) Reduce urea injection quantity while ensuring compliance with emission regulations,to reduce NH3leakage and the urea deposit.(2) Adjust the nozzle in urea injection system.Increase the number of spray holes to reduce the average diameter of urea spray and accelerate the evaporation rate of urea.(3)Adjust the injection position to prevent the condensation of urea from being sprayed onto a cooler wall surface.(4) Improve the catalytic activity and hydrothermal stability of SCR catalyst to accelerate the pyrolysis and hydrolysis rates of urea.Thus reducing the co-existence time between urea and HNCO.

4.Conclusions

The SCR failure due to low NH3-NOxratio is studied in this article.The main causes of low NH3-NOxratio in SCR include insufficient urea injection,hydrothermal aging of catalysts and urea crystallization.Low NH3-NOxratio in SCR caused by insufficient urea injection is explored through an insufficient urea injection experiment.A catalyst hydrothermal aging experiment is conducted to explore catalyst inactivation at high temperature.A urea deposit analysis experiment is conducted to explore the composition of urea deposits in the process of urea crystallization.The main conclusions are as follows:

(1) Through the insufficient urea injection experiment,the low NH3-NOxratio in SCR caused by insufficient urea injection is investigated.The NH3-NOxratio is set as 0.6,0.8 and 1.It is found that with the increase of the NH3-NOxratio,the NOxconversion efficiency of the SCR system increases,but the ammonia leakage also increases.When the NH3-NOxratio is low,the NOxconversion efficiency is mainly affected by the amount of NH3.With the increase of torque,the NOxconversion efficiency increases and tends to be stable.When the NH3-NOxratio is high,the NOxconversion efficiency is mainly affected by exhaust temperature.The NOxconversion efficiency of SCR increases first and then decreases with the increase of torque.When the NH3-NOxratio is medium,the NOxconversion efficiency is affected by both the amount of NH3and the exhaust temperature.

(2) The hydrothermal aging experiment of the SCR catalysts is carried out to investigate the deactivation of catalysts at high temperature.After the 24 h hydrothermal aging experiment at 800°C,the NOxconversion efficiency decreases significantly at the boundary of medium and low temperature regions.The NOxconversion efficiency is different under different NO2-NOxratios.The NOxconversion efficiency is highest when the NO2-NOxratio is 0.5 and lowest when the NO2-NOxratio is 0.75.After the hydrothermal aging experiment,the NOxconversion efficiency of copper-based zeolite catalysts decreases more significantly when the NO2-NOxratio is 0 or 0.75 than when the NO2-NOxratio is 0.5.

(3) The low NH3-NOxratio in SCR caused by urea crystallization is investigated through urea deposit analysis.Urea deposits are mainly produced by physical crystallization of urea and related chemical reactions.By thermogravimetric-infrared spectrum analysis,it is found that the main components of urea deposits are urea and isocyanic acid.Preventing HNCO polymerization,especially the formation of CYA,can decrease the formation of urea deposits.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research&Development Program of China (No.2017YFC0211202).Authors would like to thank editors and anonymous reviewers for their suggestions to improve the paper.