Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant–experimental study

Vít FREISLEBEN, Zdeněk JEGLA, Jaroslav BARTÁK, Jan ZABLOUDIL: Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant – Experimental study Patronem tohoto čísla je WASTen Centrum expertů Klastr inovativních firem – www.expert.wasten.cz WASTE FORUM 2023, číslo 1, strana 13 Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant–experimental study Vit FREISLEBENa, Zdenek JEGLAa, Jaroslav BARTAKb, Jan ZABLOUDILa aInstitute of Process Engineering, Brno University of Technology, Technicka 2896/2, 616 69 Brno, Czech Republic, e-mail: [email protected] b EVECO Brno, ltd., Brezinova 1608/42, 616 00 Brno, Czech Republic Abstract This paper presents the results of experimental research on the modernization of technology for the removal of nitrogen oxides (NOx) from flue gas in hazardous waste incineration plant (waste-toenergy unit). The investigated technological solution is a combination of traditional SNCR (selective noncatalytic reduction) method, and a modern technology based on urea hydrolysis. A semi-operating hydrolysis reactor is applied to produce gaseous ammonia, which is subsequently used to remove NOx from the flue gas in a hazardous waste incineration plant. Both the NOx reduction efficiency and the operating parameters of the semi-operating device are observed. The results show that the proposed technological solution is very efficient in terms of NOx reduction, where concentrations around 60 mg/scm can be easily achieved. Due to the simplicity of the proposed solution, the semi-operating device is also very reliable. However, in case of insufficient heating of the transport paths of the generated ammonia, there is a risk of deposits and clogging, as presented in the paper. Keywords: urea hydrolysis, flue gas denitrification, selective non-catalytic reduction, waste-to-energy Introduction Incineration of waste and its energy recovery is an effective way for economical and environmentally friendly waste removal1. Combustion processes, however, produce nitrogen oxides (NOx), in particular nitric oxide (NO), nitrogen dioxide (NO2), and others. Nitrogen oxides, are environmentally harmful and form toxic gases associated with acid rain and smog formation, respiratory irritation, or reduced oxygenation of the body2. In order to minimize the environmental impact of waste incineration, flue gas denitrification technology is often applied. The most common denitrification technology is the selective non-catalytic reduction (SNCR), which is based on the injection of the reducing reagent to the hot flue gas stream at temperatures of 850-1100 °C (i.e., to the primary or secondary combustion chamber). In this process, the dominant reaction is the reduction of NO by the NH2radical, see equation (1) below3. (1) Alternatively, selective catalytic reduction (SCR) technology is applied, where the reagent is injected into the flue gas stream at temperatures of 180 - 290 °C and the NOx is subsequently removed on the catalytic bed4. This method is very efficient, but the investment costs are considerably higher compared to SNCR. The most frequently used reagents are aqueous urea solution, ammonia solution or pure liquified ammonia. Urea solution is safe to store and handle, but the NOx removal efficiency is lower, reaction with NOx slower and it has higher propensity to corrosion of heating surfaces due to the isocyanic acid formation during the urea decomposition in the flue gas5. Generally, the ammonia reagents (ammonia solution or pure ammonia) are better in terms of NOx removal efficiency and can be used at lower flue gas temperatures6. However, as the ammonia is flammable and toxic substance, there are substantial operational risks and excessive costs for handling and storage. The reduction of NO by urea is shown in equation (2) and by ammonia in equation (3). (2) (3) Vít FREISLEBEN, Zdeněk JEGLA, Jaroslav BARTÁK, Jan ZABLOUDIL: Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant – Experimental study Patronem tohoto čísla je WASTen Centrum expertů Klastr inovativních firem – www.expert.wasten.cz WASTE FORUM 2023, číslo 1, strana 14 The modern approach is to produce gaseous ammonia from urea solution by hydrolysis reaction, while the ammonia gas is directly injected into the flue gas. In this way, high NOx removal efficiency could be achieved with minimal safety requirements during reagent handling. The urea hydrolysis is an endothermic reaction taking place at elevated temperatures (above 120 °C) according to the equation (4). (4) First, the process of urea hydrolysis was investigated in a batch7 and semi batch8 reactor. For industrial applications, however, the continuous production of ammonia is necessary, so the results of the experimental study were published on the use of a continuous hydrolysis reactor for flue gas denitrification in combination with SCR technology9. It was further confirmed that there is minimal risk of by-product formation in the process of urea hydrolysis if the liquid in the reactor and the produced gas are kept at sufficiently high temperatures9. As mentioned above, due to the lower investment costs the SNCR technology is preferred in many (especially smaller) incineration plants compared to SCR. Currently, no experimental studies have been published on the reduction of NOx emissions using SNCR technology in combination with hydrolysis of urea as a source of reagent for the denitrification process (DeNOx). To fill this research gap, an experiment based on a combination of hydrolysis technology and SNCR was conducted to observe the NOx reduction in flue gas produced in an industrial hazardous (medical) waste incinerator. In this paper, the results of the experiment are presented. Experimental part An experimental device (urea hydrolyser) for ammonia production by urea hydrolysis decomposition was set up. The heart of this device is a pressure vessel with 40 L liquid volume (hydrolysis reactor) into which a urea solution was fed by a membrane feed pump. The liquid in the reactor was maintained at elevated pressure and temperature by electrical heating from the surface of the reactor. Under continuous heat supply to the reactor, hydrolytic decomposition of urea into gaseous ammonia and CO2 (according to equation (4)) and further evaporation of excess water occurred. The produced gas was then injected into the flue gas using a sonic nozzle of own manufacture. The direct measuring of the flow rate of the generated gas would require an expensive Coriolis flow meter, so to reduce costs the flow rate was measured indirectly by measuring the gas pressure before entering the nozzle. With knowledge of the nozzle geometry (especially outlet diameter), the ammonia gas flow rate was then calculated using subsonic flow theory. A constant amount of liquid is maintained in the reactor during operation by a float level gauge. A schematic of the urea hydrolyser is shown in Figure 1 and the actual device is illustrated in Figure 2. The main parameters of semi-operational urea hydrolyser are summarized in the Table 1. The composition of feed urea solution and the produced ammonia gas is given according to the mass balance in Table 2. Table 1: Basic characteristics of experimental semi-operational urea hydrolyser. Temperature [°C] Pressure [barabs] Liquid volume [L] Heating Power input (max/min) [kW] Urea feed concentration [%wt] Ammonia production [kg/h] (max/min)* 130 – 155 6 40 electric 7.15/0.5 30 1.6/0.1 * The ammonia production range is valid for 30%wt urea solution feed. Greater ammonia production can be achieved with a more concentrated inlet urea solution. Vít FREISLEBEN, Zdeněk JEGLA, Jaroslav BARTÁK, Jan ZABLOUDIL: Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant – Experimental study Patronem tohoto čísla je WASTen Centrum expertů Klastr inovativních firem – www.expert.wasten.cz WASTE FORUM 2023, číslo 1, strana 15 Table 2: Composition of feed urea solution and produced ammonia gas according to mass balance Feed liquid [%wt] Product gas [%wt] urea H2O NH3 CO2 H2O 30 70 17 22 61 Figure 1: Technological layout of the urea hydrolyser and the WtE unit Figure 2: The semi-operational urea hydrolyser Vít FREISLEBEN, Zdeněk JEGLA, Jaroslav BARTÁK, Jan ZABLOUDIL: Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant – Experimental study Patronem tohoto čísla je WASTen Centrum expertů Klastr inovativních firem – www.expert.wasten.cz WASTE FORUM 2023, číslo 1, strana 16 The experiment was then carried out on waste-to-energy unit processing a hazardous waste consisting mainly of hospital waste. The product gas was injected into the cylindrical secondary combustion chamber of the WtE unit, where the highly turbulent tangential flue gas flow occurs, which promoted an effective mixing of the ammonia with the flue gas. The main parameters of the flue gas in the place of ammonia injection are summarized in Table 3. Currently, no NOx reduction technology is applied here, as the emission limits are complied with. However, with the emission limits tightening, the application of SNCR technology is here also expected in the future. The final experimental setup is illustrated in Figure 3. Table 3: Treated flue gas properties and composition. Flow rate [scm/h] (dry basis) Temperature [°C] Pressure [kPaabs] Composition [%vol] CO2 O2 H2O N2 NOx 2 750 908 °C 97 4,3 13,7 9,7 72,3 148.3 mg/scm* * NOx concentration is valid for dry flue gas and O2 reference concentration 11%vol. Figure 3: The experimental setup The aim of the experiment was to measure NOx reduction in two steady-state operating stages (high-rate and low-rate product gas injection) and to observe the transitional behavior between these two stages. The experiment can be divided into five sub-steps:  Normal operation – The urea hydrolyser was not running. The flue gas reference parameters were monitored. The results are shown in Table 3.  Transition region 1 – The product gas flow rate was determined by stoichiometric calculation based on the data in Table 2 and 3 in such a way that the stoichiometric excess of Ammonia was equal to 3 (standard value for SNCR applications). In the start-up process, the concentration of ammonia in the produced gas is, however, lower than in steady state9.  DeNOx stage 1 - The amount of injected product gas was kept constant and the NOx concentration in the flue gas was stable. It is assumed that chemical equilibrium in the reactor and steady state operation are achieved.  Transition region 2 - The amount of injected product gas was reduced to correspond to a stoichiometric excess of ammonia 1.1, assuming a gas composition according to Table 2. The ammonia content of the product gas was assumed to be initially higher than the values in Table 3 and gradually decreased until the chemical equilibrium was achieved. Vít FREISLEBEN, Zdeněk JEGLA, Jaroslav BARTÁK, Jan ZABLOUDIL: Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant – Experimental study Patronem tohoto čísla je WASTen Centrum expertů Klastr inovativních firem – www.expert.wasten.cz WASTE FORUM 2023, číslo 1, strana 17  DeNOx stage 2 – Chemical equilibrium and steady state operation is reached with ammonia excess equal to 1.1. During the experiment, the temperature and pressure of the product gas upstream of the sonic nozzle were measured to calculate the gas flow rate using the theory of subsonic flow through the nozzle. ammonia slip in the flue gas was also measured by spectrophotometry method. Results and discussion The duration of the experiment was 12 hours. During this time, among other things, the NOx concentration in the flue gas was continuously measured. The result of this measurement with the marked sub-steps (see Experimental part) is shown in Figure 4. Figure 4: NOx concentration in the flue gas As can be seen from Figure 4, the NOx concentration in the flue gas was significantly reduced during the experiment. Therefore, the hydrolysis technology can be used for flue gas denitrification in combination with SNCR technology. The experimental results are also summarized in Table 4. In addition to NOx reduction, ammonia slip in the flue gas was also monitored. In total, 4 half-hourly measurements were carried out and the results of which are summarized in Table 5. As expected, the highest ammonia slip occurred when more product gas was injected (during time 7:30-8:00). Surprisingly, the ammonia slip during the reduced ammonia gas injection (time 10:30-11:00) was higher than at the beginning of the experiment. This may be, for example, due to a lower concentration of ammonia in the produced gas in the initial phase of the experiment, as explained in Experimental Part. However, the values are within legal emission limits, and it can be assumed that most of the injected ammonia reacted with NOx or was thermally decomposed into N2 and H2O. There was a significant fluctuation in NOx concentration throughout the experiment. The periodic fluctuation (with a period of approximately 15 minutes) is due to the movements of the grate on which the waste is combusted, as these temporarily accelerate the combustion. Longer term fluctuations can also be observed, especially between the times 7:45-8:45. This significant deviation from steady state was not due to a change in the amount of product gas injected as this was constant. However, similar prolonged fluctuations are relatively common in this operation and are caused by the inhomogeneous nature of waste composition. As mentioned in the Introduction, SNCR technology commonly uses a liquid reagent injection into the flue gas. It is performed by two-phase nozzles where the reagent is atomized using pressurized air or steam. The advantage of this design is the intensive penetration of the reagent in the flue gas stream, which promotes a uniform distribution of the reagent in the flue gas even in large combustion units. However, the disadvantages of this design are the higher purchase cost of the nozzles and associated equipment and operational difficulties such as clogging, unstable atomization of the liquid reagent and the danger of local subcooling of the combustion chamber walls. Vít FREISLEBEN, Zdeněk JEGLA, Jaroslav BARTÁK, Jan ZABLOUDIL: Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant – Experimental study Patronem tohoto čísla je WASTen Centrum expertů Klastr inovativních firem – www.expert.wasten.cz WASTE FORUM 2023, číslo 1, strana 18 With the urea hydrolyzer, the gaseous reagent is injected into the flue gas using a single-phase nozzle. This has the disadvantage of lower penetration through the flue gas stream than two-phase nozzles but it is sufficient for smaller combustion plants. The great advantage then lies in the simplicity of design and operational reliability coupled with less tendency to fouling. In addition, the consumption of pressure air is minimal, as it only serves to cool the nozzle when product gas is not being injected. Even though very good results have been achieved and the urea hydrolyzes proved to be suitable technology for SNCR process, there are several aspects that must be carefully considered:  Sufficient penetration of product gas to the flue gas stream. As the combustion chamber is relatively large and the flue gas flows at high velocity, there is a risk of low product gas penetration to the flue gas stream resulting to insufficient degree of mixing, which leads to low DeNOx efficiency and high ammonia slip. To minimize this danger, ammonia gas was injected into the flue gas at a nearly sonic velocity (M=0.95) during the experiment. However, at a later stage of the experiment, when a lower amount of gas was injected, even a rate corresponding to M=0.38 proved to be sufficient.  Retention time. NOx reduction efficiency is dependent on the retention time of reagent in combustion chamber. In this experiment, the retention time was 1.36 seconds before the flue gas was cooled in the boiler, which also proved to be a sufficiently long period to reach good results.  Danger of reverse chemical reactions. In the transport pipeline between the hydrolyser and the nozzle the formation of ammonium carbamate (a corrosive crystalline substance that can clog the transport routes) is a risk when the temperature of the product gas drops below approx. 100°C. To avoid this danger, the transport pipeline was equipped by electrical heating to maintain the temperature of gas at around 130°C. However, there was a blind section in the flow path at the point of pressure measurement in the pipe without electrical heating, where ammonium carbamate was formed, which led to a gradual blockage as shown in Figure 5. Table 4: The experimental results Treactor [°C] preactor/nozzle [barabs] Mach no.* [scm/h]* Stoich. excess NOx [mg/scm] [%]*** [%]*** Transition region 1 138.9 6.0/1.8 0.95 1.94 0 – 3** 113.9 23.2 - DeNOx stage 1 138.5 6.0/1.8 0.95 1.94 3 60.2 59.4 19.8 Transition region 2 138.0 6.0/1.1 0.38 0.78 3 – 1.1** 70.4 52.5 - DeNOx stage 2 135.6 6.0/1.1 0.38 0.78 1.1 105.7 28.8 26.2 * The subsonic nozzle with the diameter 1.5 mm was used. The Mach number is then calculated applying the subsonic flow theory. From the Mach number, the product gas flowrate was determined. ** Stoichiometric excess in transition regions can´t be determined accurately as the hydrolysis reactor didn´t reach chemical equilibrium and thus the product gas composition isn´t known. *** – NOx removal efficiency is determined as the ratio of the measured NOx concentration to the reference concentration measured in normal operation stage (Table 3). – reagent efficiency determines the ratio of ammonia reducing NOx to the total amount of the ammonia injected to the flue gas. It is also the ratio of to the ammonia stoichiometric excess. Table 5: Ammonia slip (101 325 Pa, 0°C, dry basis). 4:30 – 5:00 h 5:30 – 6:00 h 7:30 – 8:00 h 10:30 – 11:00 h ammonia slip [mg/m3] 1.8 3.6 10.0 8.4 Vít FREISLEBEN, Zdeněk JEGLA, Jaroslav BARTÁK, Jan ZABLOUDIL: Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant – Experimental study Patronem tohoto čísla je WASTen Centrum expertů Klastr inovativních firem – www.expert.wasten.cz WASTE FORUM 2023, číslo 1, strana 19 Figure 5: Ammonium carbamate deposit in unheated section of the pipeline. Conclusions The paper presents the results of an experimental research focused on the use of the urea hydrolytic decomposition technology as a source of reagent (gaseous mixture of ammonia, CO2 and H2O) for the removal of nitrogen oxides from the flue gas produced in waste-to-energy unit processing a hazardous waste. The NOx reduction was performed by the SNCR method, where the reagent was injected into the secondary combustion chamber using a sonic nozzle. During the main part of the experiment (DeNOx stage 1), the NOx concentration was reduced from 148.3 mg/scm to 60.2 mg/scm (i.e., by 59.4%) at a stoichiometric excess of ammonia equal to 3. A significant NOx reduction of 28.8% was also achieved when a smaller amount of reagent with a stoichiometric excess of 1.1 was injected during the DeNOx stage 2. Thus, the results clearly indicate that very effective NOx reduction in flue gas can be achieved using urea hydrolysis technology in combination with SNCR method. Compared to the traditional technological approaches of the SNCR method, where the reagent is injected in liquid form using two-phase nozzles, the presented solution has proven to be both very reliable due to the simple nozzle design and economical, as a minimum amount of pressurized air is required for operation. The disadvantage of the presented solution is the danger of ammonium carbamate formation in the transport routes if they are not heated sufficiently. The objective of future research is to further optimize the urea hydrolysis technology and to perform further laboratory experiments and operational tests in waste-to-energy units and other combustion plants. Acknowledgment First, the authors would like to express their gratitude to their esteemed late colleague Dr. Ladislav Bebar for his valuable advisement in this research and for his outstanding lifelong scientific contribution. Further, this research has been supported by the project “Experimental Ammonia generator as an integrated device for efficient removal of nitrogen oxides from flue gas”, funded as project FV22-04 by Brno University of Technology. Also, this research has been supported by the EU project Strategic Partnership for Environmental Technologies and Energy Production, funded as project No. CZ.02.1.01/0.0/0.0/16_026/0008413 by Czech Republic Operational Programme Research, Development and Education. Vít FREISLEBEN, Zdeněk JEGLA, Jaroslav BARTÁK, Jan ZABLOUDIL: Urea hydrolysis as an efficient method for flue gas denitrification in waste-to-energy plant – Experimental study Patronem tohoto čísla je WASTen Centrum expertů Klastr inovativních firem – www.expert.wasten.cz WASTE FORUM 2023, číslo 1, strana 20 References 1. Chand Malav L., and 12 coauthors. A review on municipal solid waste as a renewable source for waste-to-energy project in India: Current practices, challenges, and future opportunities. J. Cleaner Prod. 277, 123227 (2020). doi: 10.1016/j.jclepro.2020.123227 2. ARNIKA: oxidy dusíku. [online]. 14. 12. 2010 [cit. 2022-10-11]. Available at: https://arnika.org/toxicke-latky/databaze-latek/oxidy-dusiku 3. Hou X., Pilawska M., Lu J., and Yue G. The formation of N2O during the reduction of NO by NH3. Fuel 87, 3271-3277 (2008). doi: 10.1016/j.fuel.2008.05.009. 4. Schnelle K. B., Dunn R. F. and Ternes M. E.: Air Pollution Control Technology Handbook, 2nd ed. CRC Press, Boca Raton, Florida, 2015. 5. Klinghoffer N. B., and Castaldi M. J.: Waste to energy conversion technology. Woodhead Publishing, Oxford, 2013 6. Pronobis M., Wejkowski R., Jagodzińska K., Kress T. Simplified method for calculating SNCR system efficiency. E3S Web Conf. 14, 2003 (2017). doi: 10.1051/e3sconf/20171402003. 7. Sahu J. N., Hussain S., Meikap B. C. Computational Fluid Dynamics Modeling for Urea Hydrolysis in a Batch Reactor for Flue Gas Conditioning. Chem Eng Technol. 34, 1347 – 1352 (2011). doi: 10.1002/ceat.201000482 8. Mahalik K., Sahu J. N., Patwardhan A. V., Meikap B. C. Kinetic studies on hydrolysis of urea in a semi-batch reactor at atmospheric pressure for safe use of ammonia in a power plant for flue gas conditioning. J. Hazard. Mater. 175 (1), 629-637 (2010). doi: 10.1016/j.jhazmat.2009.10.053 9. Zhang X., Zhang B., Lu X., Gao N., Xiang X., and Xu H. Experimental study on urea hydrolysis to ammonia for gas denitration in a continuous tank reactor. Energy 126, 677 – 688 (2017). doi: 10.1016/j.energy.2017.03.067. Hydrolýza močoviny jako účinná technologie pro denitrifikaci spalin ve spalovně odpadu – experimentální studie Vít FREISLEBENa, Zdeněk JEGLAa, Jaroslav BARTÁKb, Jan ZABLOUDILa aÚstav procesního inženýrství, Vysoké učení technické v Brně, Technická 2893/2, 616 69 Brno, Česká republika bEVECO Brno, s.r.o., Březinova 1608/42, 616 00 Brno, Česká republika e-mail: [email protected] Souhrn Článek předkládá výsledky experimentálního výzkumu zabývajícího se modernizací technologie pro odstraňování oxidů dusíku (NOx) ze spalin ve spalovacích provozech, jako například v jednotkách waste-to-energy. Zkoumaným technologickým řešením je kombinace tradiční technologie SNCR (selektivní nekatalytická redukce) a moderní technologie založená na hydrolýzním rozkladu technické močoviny. Poloprovozní hydrolýzní reaktor je zde aplikován k výrobě plynného amoniaku, který je následně využit k odstranění NOx ze spalin ve spalovně nebezpečného odpadu. Sledována je zde jak účinnost redukce oxidů dusíku, tak provozní parametry poloprovozního zařízení. Z výsledků vyplývá, že navržené technologické řešení je z hlediska redukce NOx velice účinné, kdy lze snadno dosáhnout koncentrací okolo 60 mg/Nm3. Díky jednoduchosti tohoto technologického řešení je poloprovozní zařízení také velice spolehlivé. V případě nedostatečného ohřevu dopravních tras generovaného plynu zde ale hrozí vznik úsad, jak je v článku prezentováno. Klíčová slova: Hydrolýza močoviny, denitrifikace spalin, selektivní nekatalytická redukce, waste-to-energy