ARTICLE pubs.acs.org/IECR
Ammonia Storage and Slip in a Urea Selective Catalytic Reduction Catalyst under Steady and Transient Conditions Yanguang Zhao, Jing Hu, Lun Hua, Shijin Shuai,* and Jianxin Wang State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, China ABSTRACT: This paper aims to investigate quantitatively the characteristics of the storage and slip of ammonia in a full-size vanadium-based selective catalytic reduction (SCR) catalyst under typical steady and transient conditions on a diesel engine test bench. The effect of space velocity (SV) and exhaust temperature on the ammonia storage process and the storage capacity of the catalyst was studied. The ammonia slip in various transient processes was also investigated. The experimental results indicate that the increase of SV causes early appearance of ammonia slip. Then, the ammonia storage time is shorten to avoid the ammonia slip and the ammonia storage amount is consequently reduced which has an effect on the NOx conversion efficiency. The exhaust temperature is the key factor affecting the ammonia storage capacity of the catalyst which sharply decreases with the increase of temperature. The NOx conversion efficiency strongly depends on the amount of ammonia storage when the temperature is lower than 280 °C, and it is nearly linear to the amount of ammonia storage at 200 °C. If the ammonia storage is not controlled properly ammonia slip will occur under various transient conditions. Rapid heating of the catalyst at low temperature can cause serious ammonia slip. The initial temperature of the heating process is crucial to causing ammonia slip in transient processes while SV is not the key factor. The saturation level of the ammonia storage has a significant effect on ammonia slip in transient processes. There is an appropriate amount of ammonia storage which can be defined as the “slip-preventing limit” that maximizes the NOx conversion efficiency while keeping ammonia slip under 10 ppm to comply with the Euro 6 legislation for heavy-duty diesel engines.
’ INTRODUCTION Diesel engines are inherently more thermodynamically efficient than gasoline engines. However, their emission of NOx and particulate matter (PM) is high, especially when compared with gasoline engines equipped with three-way catalysts. With the release of more and more stringent emission regulations around the world, the diesel engine is facing great challenges due to its high NOx and PM emission.13 Aftertreatment technologies such as urea selective catalytic reduction (SCR) and diesel particulate filter (DPF) are indispensable to reduce the NOx and PM for meeting future stringent emission standards such as those of Euro 6 or EPA 2010 and beyond. Urea SCR is a primary technology to reduce the NOx emission of heavy-duty (HD) diesel engines,46 which involves the main reactions as follows. NH2 CO NH2 ðsolidorliquidÞ f NH3 þ HNCO
ðR1Þ
HNCO þ H2 O f NH3 þ CO2
ðR2Þ
4NH3 þ 4NO þ O2 f 4N2 þ 6H2 O
ðR3Þ
4NH3 þ 2NO2 þ 2NO f 4N2 þ 6H2 O
ðR4Þ
8NH3 þ 6NO2 f 7N2 þ 12H2 O
ðR5Þ
2NH3 þ 2NO2 f NH4 NO3 þ N2 þ H2 O
ðR6Þ
4NH3 þ 4NO þ 3O2 f 4N2 O þ 6H2 O
ðR7Þ7
4NH3 þ 5O2 f 4NO þ 6H2 O
ðR8Þ7
r 2011 American Chemical Society
Urea SCR technology has been used to diminish NOx from power plants and other stationary power sources.8 However, automotive diesel engines are operating with frequent changes of load and speed under running conditions. This causes frequent changes of flow rate and temperature of the exhaust gas and increases the difficulty of NOx control. Much work has thus focused on adapting the SCR process to the special demands of mobile applications, and the mechanism and chemical kinetics of SCR reactions have been extensively studied.921 Extensive work has also been devoted to solving the problems in practical applications of the urea SCR system in HD diesel engines, including urea-related deposits,22,23 urea decomposition and mixing uniformity,2427 and urea injection control strategies.28,29 Tronconi et al.10,12,13,16,3032 studied the mechanism and kinetics of SCR reactions using transient response techniques. The classical transient response method was modified by replacing the step changes of the inlet reactant concentration with linear variations in time.16 A dynamic kinetic model which unified standard and fast SCR reactions into a single redox approach for vanadium-based catalyst was finally proposed considering the ammonia adsorption and desorption.32 The ammonia storage capacity was estimated, and it was identified that NO2 was also stored in comparable amount in the form of nitrates.12 Kleemann et al.9 investigated ammonia adsorption behavior and illustrated that WO3 was essential for achieving low ammonia slip, which led to an increased amount of strongly adsorbed ammonia, thus improving the catalytic activity. Received: May 16, 2011 Accepted: September 8, 2011 Revised: September 7, 2011 Published: September 08, 2011 11863
dx.doi.org/10.1021/ie201045w | Ind. Eng. Chem. Res. 2011, 50, 11863–11871
Industrial & Engineering Chemistry Research
ARTICLE
Figure 1. Experimental setup.
However, most research was conducted through simulated experiments using glass tubes as the reactor while the feed gas in the experiments was designed according to the composition of typical diesel exhaust gas. This was not effective for realistically simulating the variation of the flow rate, composition, and temperature of the exhaust gas under various transient conditions. Few studies addressed the characteristics of ammonia storage and slip of a full-size catalyst in real engine operating conditions. This paper investigated quantitatively the characteristics of ammonia storage and slip of a full-size vanadium-based SCR catalyst under various typical steady and transient conditions on a diesel engine test bench. The effect of exhaust temperature and SV on the ammonia storage capacity of the catalyst was investigated. Here, the ammonia storage capacity was defined as the quantity of the ammonia stored on the surface of the catalyst when the ammonia slip was less than 10 ppm. The effect of ammonia storage on NOx conversion efficiency was also investigated. The ammonia slip under various rigorous transient conditions was measured and estimated when the ammonia storage of catalyst was kept at different saturation levels.
’ EXPERIMENTAL SECTION Experimental Setup. The urea SCR system used in the experiment mainly consists of (1) a urea dosing control unit, (2) SCR catalyst, (3) dosing pump, (4) urea tank, (5) temperature sensor, and (6) NOx sensor. Figure 1 shows the experimental setup. The SCR catalyst is a commercial vanadium-based catalyst provided by the Haldor Topsoe company. The dimension of the monolith is i 267 mm 152 mm (i 10.5 in. 6 in.) with the channel density of 260 cpsi (channels per square inch), and two blocks of monoliths with a total volume of 17.0 L are used. In the experiment, the commercially available ureawater solution, 32.5% by weight complying with the DIN V70070 standard, is injected into the exhaust gas. Then the urea decomposes and hydrolyzes to provide the ammonia as the reductant. The specifications of the diesel engine used in the experiment are presented in Table 1. Gas Analysis. The concentration of NH3 and NOx (NO and NO2) in the exhaust gas is measured separately at the inlet and outlet of the catalyst through a Fourier transfer infrared (FTIR) gas analyzer (AVL SESAM-FTIR version 4.0). The sampling lines are heated to 190 °C, and then, the condensation of the H2O (important for NH3) is prevented to keep no less concentration in the sample.
Table 1. Specifications of the Diesel Engine engine model
YC6L240-40
cylinder number
6
cylinder configuration
in-line
displacement
8.4 L
bore and stroke
113 mm 140 mm
compression ratio
17.5
air induction fuel supply
turbocharged with charge air cooling electronic pump unit (EPU)
rated power
177 kW at 2200 rev/min
maximum torque
950 N 3 m at 12001700 rev/min
Urea Injection Rate and Ammonia Storage. The urea injection rate is calculated through the NH3:NOx ratio (1.2, in the experiment), the NOx concentration at the inlet of the catalyst, and the exhaust gas mass flow rate during the test. Then, the command of the urea injection rate is sent to the urea dosing pump. The ammonia storage is defined as the amount of the accumulated ammonia in grams on the surface of the whole catalyst. It is the integral of the difference between the ammonia addition rate and consumption rate. The ammonia addition rate is the calculated through the urea solution injection rate. And, the ammonia consumption rate is calculated through multiplying the exhaust gas mass flow rate by the difference of the NOx concentrations at the inlet and outlet of the catalyst. The ammonia would be oxidized at high temperature. Perhaps a small proportion of ammonia was oxidized at high temperature in the experiment. And, the oxidation of ammonia is not accounted for in the calculation of ammonia consumption.
’ RESULTS AND DISCUSSION Effect of Ammonia Storage on NOx Conversion Efficiency. The ammonia adsorbed on the surface of the catalyst has a significant effect on NOx conversion efficiency especially at low exhaust temperature. The relation between NOx conversion efficiency and ammonia storage at various exhaust temperatures and space velocities (SV) was investigated on the engine test bench using the full-size SCR catalyst for the heavy-duty diesel engine. The ammonia slip was kept below 10 ppm in the experiment. The effect of ammonia storage on NOx conversion efficiency at 11864
dx.doi.org/10.1021/ie201045w |Ind. Eng. Chem. Res. 2011, 50, 11863–11871
Industrial & Engineering Chemistry Research
ARTICLE
Figure 2. Effect of ammonia storage on the NOx conversion efficiency at different temperatures and SVs (17.0 L catalyst, ammonia slip
