Selective Catalytic Reduction Unit Research Articles

Cold-Start NOx Mitigation by Passive Adsorption Using Pd-Exchanged Zeolites: From Material Design to Mechanism Understanding and System Integration.

It remains a major challenge to abate efficiently the harmful nitrogen oxides (NOx) in low-temperature diesel exhausts emitted during the cold-start period of engine operation. Passive NOx adsorbers (PNA), which could temporarily capture NOx at low temperatures (below 200 °C) and release the stored NOx at higher temperatures (normally 250-450 °C) to downstream selective catalytic reduction unit for complete abatement, hold promise to mitigate cold-start NOx emissions. In this review, recent advances in material design, mechanism understanding, and system integration are summarized for PNA based on palladium-exchanged zeolites. First, we discuss the choices of parent zeolite, Pd precursor, and synthetic method for the synthesis of Pd-zeolites with atomic Pd dispersions, and review the effect of hydrothermal aging on the properties and PNA performance of Pd-zeolites. Then, we show how different experimental and theoretical methodologies can be integrated to gain mechanistic insights into the nature of Pd active sites, the NOx storage/release chemistry, as well as the interactions between Pd and typical components/poisons in engine exhausts. This review also gathers several novel designs of PNA integration into modern exhaust after-treatment systems for practical application. At the end, we discuss the major challenges, as well as important implications, for the further development and real application of Pd-zeolite-based PNA in cold-start NOx mitigation.

Nonlinear control of a selective catalytic reduction unit for a bio-fueled cogeneration plant

The paper puts forward a method for control of selective catalytic reduction (SCR) units. First, a dynamic model of SCR performance including heat transfer, ammonia adsorption, NOx reduction, and ammonia oxidation is developed. The model is then more simplified to only include medium time scale dynamics which can be shaped using control. The obtained model is utilised in the design of a nonlinear control strategy for the regulation of the NOx emissions in the system. A robust observer of the system state is developed and exploited for control design. Numerical simulations verify the effectiveness of the proposed control system.

Reinforcement learning for online adaptation of model predictive controllers: Application to a selective catalytic reduction unit

This work presents a novel application of reinforcement learning (RL) for online dynamic tuning of model predictive controllers (MPC). Applying a state-action-reward-state-action (SARSA) algorithm for temporal difference learning with a control-specific reward function improves the error tracking performance of a standard MPC formulation. The proposed RL approach is also readily adaptable to other MPCs, or entirely different control approaches. Practical details for the implementation of the RL-MPC algorithm are also presented. The proposed algorithm is applied to a case study of controlling nitrogen oxide (NOx) emissions in an industrial selective catalytic reduction (SCR) unit, a control problem characterized by significant nonlinearity and time delay. Along with an RL-MPC formulation for NOx control, another MPC is proposed to mitigate ammonia slip and decrease ammonia consumption in the SCR. Results showing the efficacy of the RL-MPC for NOx control through learning and implementation on the nonlinear SCR dynamic model are presented.

Model-based fault diagnosis of selective catalytic reduction for a smart cogeneration plant running on fast pyrolysis bio-oil

The paper puts forward a method for the detection of hydrothermal aging of selective catalytic reduction (SCR) units. First, a dynamic model of SCR performance including heat transfer, ammonia adsorption, NOx reduction, and ammonia oxidation is developed. The model then is parameterized and tuned with respect to published experimental data of two SCR units before and after hydrothermal aging. Such tuning also determines the target parameters of the SCR dynamic model most relevant to hydrothermal aging. The constant terms in SCR reaction and ammonia oxidation rates are the identified target parameters. Next, a model-based detection algorithm is proposed which is based on non-linear parameter estimation techniques. The performance of the proposed detection algorithm is tested via numerical simulation scenarios generated by high fidelity dynamic model of a cogeneration plant running on fast pyrolysis biooil. Satisfactory performance and robustness of fault detection are illustrated when it is subject to measurement noise and varying operating conditions.

Gas turbine engine firing ammonia

Japan decided the sixth Strategic Energy Plan that show 1% hydrogen/ammonia 1% hydrogen/ammonia will be positioned in power generation mix in FY2030. Ammonia is a hydrogen carrier and carbon free fuel. R&D project on gas turbine engines firing ammonia are carried on. AIST tried R&D of 50kW micro gas turbine firing ammonia. Combustor and fuel supply system were remodeled. Electric power generation firing Kerosene-ammonia / methane-ammonia / only ammonia was succeeded with low NOx emission by Selective Catalytic Reduction (SCR). Low NOx combustor was also developed by rich-lean staged combustion concept and reduce NOx emission level to 1/5 and succeeded to reduce size of SCR unit. Ammonia is transported and stored as liquified gas. In the case of ammonia gas supply system is necessary as ammonia vaporizer. Spray combustion R&D of liquid ammonia spray combustion technology for gas turbine combustor is also tried to simplify fuel system. R&D of technology to start up gas turbine using ammonia is also tried.

Insight on photocatalytic oxidation of high concentration NO over BiOCl/Bi2WO6 under visible light

The exhaust gas from an SCR (Selective Catalytic Reduction) unit still has about 100 ppm NO. BiOX/Bi2WO6 catalysts were fabricated by a one-step ultrasound-assisted method and employed for photocatalytic oxidation of NO under visible light. The bandgap energy of BiOX (Cl, Br, I)/Bi2WO6 were estimated to be 2.65 eV, 2.35 eV, and 1.75 eV, respectively. Photocurrent density and EIS curves revealed that BiOCl/Bi2WO6 had the optimal charge transfer velocity. Simulated NO removal experiments of BiOX/Bi2WO6 catalysts were carried out in a fixed bed reactor, 20–30% NO (100 ppm inlet) removal rate was achieved under visible light while promoting by H2O vapor, and the maximum NO removal rate is up to 31% on BiOCl/Bi2WO6. The fitted L-H model revealed that NO oxidation followed two-centered adsorption, and scavenger experiments confirm that the main ROS (Reactive Oxygen Species) was •OH. In situ DRIFT spectra revealed that there were bridging nitrates, monodentate and bridging nitrites, and other intermediates formed on the BiOCl/Bi2WO6, these species can be scrubbed by seawater to regenerate the catalyst. The NO photocatalytic oxidation pathway was proposed. Good activity and stability of BiOCl/Bi2WO6 provide a new approach for scale-up application of NO oxidation under visible light.

NOx Reduction Using Pilot-Scale SCR Under Oxyfuel Conditions

This paper presents a comprehensive experimental study of selective catalytic reduction (SCR) of nitrogen oxides formed in the oxyfuel combustion of lignite coal in a 500 kWth pilot-scale bubbling fluidized bed (BFB) boiler using the V2O5 over TiO2 honeycomb catalyst and the ammonia as the reducing agent. The study also compares the performance of the SCR between oxyfuel and air combustion modes. The SCR unit is placed in the tail-end position of the flue gas processing scheme after the cyclone particle separator. Experiments were realized both in air- and oxyfuel combustion regime, evaluating the impact of relevant process conditions on the NOx reduction. The dependence of the NOx reduction efficiency on the catalyst temperature and on the normalized stoichiometric ratio (NSR) of the reducing agent molar flow to the molar flow of NOx were studied. The efficiency of the NOx reduction was evaluated based on the NOx concentration measured simultaneously before and after the SCR unit. In the off-gas, the concentration of unreacted reducing agent (ammonia slip) was measured as well. For evaluation of real NSR, ammonia and flue gas flow rates were measured during each experiment. The catalyst temperature was regulated within the experiments in the range of 260 to 300 °C and the NSR was chosen in the range of 0 to 1.2. From this experimental setup, the correlations of the NOx reduction efficiency related to the different NSR and catalyst temperature were evaluated. The results show the fundamental information about the differences between air and oxyfuel conditions in a real combustion environment. In general, the NOx conversion efficiency was significantly higher during air combustion. In order to get similar results of maximum NOx conversion as reached during air combustion, it was necessary to increase NSR up to 1.8 and more, however at the cost of significantly increased ammonia slip. Although the SCR performance is lower in oxyfuel combustion, it was possible to decrease NOX under 100 ppmv with NSR about 1 and without any significant ammonia slip, which is important prior to further CO2 treatment.

Sequentially prepared Mo-V-Based SCR catalyst for simultaneous Hg0 oxidation and NO reduction

Molybdenum (Mo)-vanadium (V)-based selective catalytic reduction (SCR) catalyst synthesized by the sequential impregnation of Mo and W followed by V was investigated for simultaneous elemental mercury (Hg0) oxidation and nitrogen oxide (NO) reduction in an existing SCR unit with respect to different TiO2 phases, calcination temperatures, flue gas constituents, gas velocities, and reaction temperatures. Anatase phase TiO2 and the calcination temperatures of 400 and 500 °C resulted in ∼69% Hg0 oxidation at 10 ppmv HCl and 350 °C. The high calcination temperature of 700 °C resulted in TiO2 phase transformation from anatase to rutile and agglomeration. The modified SCR catalyst prepared with the impregnation sequence of Mo and W followed by V using anatase TiO2 and calcination temperature 500 °C showed ∼99% Hg0 oxidation and 87% NO reduction conversions at an NH3/NO molar ratio of 0.9 under 350 °C and 5,000 hr−1 space velocity in typical sub-bituminous and lignite coal simulated flue gases. The effects of these parameters and conditions were further investigated using various characterization techniques including BET, TEM, XRD, NH3 TPD and XAFS.

The Reactions and Role of Ammonia Slip Catalysts in Modern Urea-SCR Systems

The modern mobile aftertreatment systems (ATSs) include ammonia slip catalyst (ASC) located after the selective catalytic reduction (SCR) catalyst. Mainly copper, iron and vanadium SCR catalysts are applied commercially. SCR systems are mostly located after diesel particulate filter (DPF), which is thermally a demanding location for SCR and ASC. ASC properties are integrated to control strategy together with the SCR catalyst to reach NOx conversions above 95%. Reactions by Pt loading, Pt layer radial location (bottom, top, full layer) and feed conditions were simulated experimentally in synthetic gases on Cu, Fe and V based SCR catalysts in this study. NH3 slip after SCR unit was simulated in controlled (NH3/NO ≤ 1) or uncontrolled (NH3 only or NH3/NOx >> 1) conditions. The axial location of Pt relatively to the flow direction dominated to the radial location of Pt in reaction rates. The results showed significant differences in ammonia conversions and reaction selectivity (to N2, NO, N2O) by the Pt layer location (bottom or top) together with a thick SCR layer. The Pt loading was optimized to the range of 2–5 g/cft to keep a good selectivity to N2 as well as to avoid N2O/NOx formation and the Pt contamination effect on SCR. Pt-ASC was necessary to remove remaining NH3 and CO after SCR but the prevention of N2O formation is an essential part of design. Experimental simulations with 2-layer ASC with a Cu-chabazite (CHA) catalyst resulted in a complete mapping by the Pt loading as a function of space velocity and temperature as fresh and hydrothermally aged.

NOx Emission Reduction Technology for Marine Engine Based on Tier-III: A Review

The development of maritime trade has greatly promoted the development of diesel engines. However, with the increasingly serious environmental problems, more and more attention has been paid to the exhaust emissions of high-power marine diesel engines. The restrictions on SOx have been implemented globally, and the limitation of the NOx will be the next priority. This paper illustrates (a) Principle and research progress of NOx emissions-reduction technology of marine diesel engine; (b) Summary of advantages and disadvantages among various reduction technologies and their reduction effects; (c) The application effect of mainstream technology on board. Firstly, since exhaust gas recirculation (EGR) can achieve Tier-III directly from Tier-I without considering the increased fuel consumption. It is deemed as the most promising technology to reduce emissions by controlling combustion condition. However, EGR has shortcomings of excessive increase in fuel consumption and generation of waste water, which need to be solved immediately. Secondly, selective catalytic reduction (SCR) is the most effective and straightforward means to achieve Tier-III. Despite of the continuous optimization of SCR unit volume, the problem of scrap catalyst seriously limits its wide application. How to match the supercharger more efficiently is a key factor in choosing between high and low pressure SCR. Thirdly, nature gas (NG) engines are capable of achieving a reduction in NOx, but in order to meet the requirements of Tier-III, it still needs to be assisted by other technologies. The emissions of hydrocarbon (HC) and CO in NG engines are huge defects that must be solved. Lastly, technologies such as the Miller cycle, Two-stage supercharging and mixed-water combustion can also reduce emissions but were rarely used alone. These technologies can be combined with EGR, SCR and NG engines to optimize the engines’ economy and emission characteristics.

Power Improvement and NOx Reduction Strategies for a Hydrogen-Fueled Multicylinder Internal Combustion Engine

Abstract The conventional operation of a hydrogen internal combustion engine (ICE) under lean conditions results in low NOx emissions, however, at the cost of power generated. In this study, the power output of a hydrogen-fueled ICE was increased while maintaining the NOx emissions at low levels. The power output was increased by turbocharging, relatively richer operation, and spark timing optimization, whereas a combination of exhaust gas recirculation (EGR) and H2-selective catalytic reduction (H2-SCR) aftertreatment was used to reduce NOx emissions. Turbocharging resulted in a maximum torque output of 168 N·m at 3200 rpm as compared to 70 N·m at 1600 rpm for the naturally aspirated operation. However, the turbocharger could not generate enough boost at low speeds and the equivalence ratio was increased to obtain a high power output which resulted in a substantial increase in the NOx emissions. The use of EGR resulted in an average reduction of 72% in the NOx emissions. Retarding of spark timing significantly reduced the NOx emissions too, but was limited by the adverse impact on the torque. Since hydrogen would be available onboard a hydrogen-fueled vehicle, we for the first time report external injection of H2 for use as a reductant in the selective catalytic reduction unit. Even under extremely oxidizing conditions, the efficiency of aftertreatment was found to be 35.4% averaged over various speeds. A maximum of 83.7% overall reduction in NOx emissions was achieved by using the combined EGR and H2-SCR strategies.

A Case Study for the Deactivation and Regeneration of a V2O5-WO3/TiO2 Catalyst in a Tail-End SCR Unit of a Municipal Waste Incineration Plant

In this work, we set out to investigate the deactivation of a commercial V2O5-WO3/TiO2 monolith catalyst that operated for a total of 18,000 h in a selective catalytic reduction unit treating the exhaust gases of a municipal waste incinerator in a tail end configuration. Extensive physical and chemical characterization analyses were performed comparing results for fresh and aged catalyst samples. The nature of poisoning species was determined with regards to their impact on the DeNOx catalytic activity which was experimentally evaluated through catalytic tests in the temperature range 90–500 °C at a gas hourly space velocity of 100,000 h−1 (NO = NH3 = 400 ppmv, 6% O2). Two simple regeneration strategies were also investigated: thermal treatment under static air at 400–450 °C and water washing at room temperature. The effectiveness of each treatment was determined on the basis of its ability to remove specific poisoning compounds and to restore the original performance of the virgin catalyst.

Effect of selective catalytic reduction denitrification on fine particulate matter emission characteristics

Selective catalytic reduction (SCR) may affect the emission characteristics of fine particulate matter due to NH3 slip during denitrification. In this study, an experimental facility equipped with a coal-fired boiler, SCR unit, electrostatic precipitator (ESP), and wet flue gas desulfurization (WFGD) unit was adopted to investigate the effect of SCR on final fine particulate matter emission characteristics. The migration and transformation of the generated ammonium sulfates and NH3 slip in the downstream ESP and WFGD were also investigated. The results indicate that SCR can increase the final emissions of submicron particles due to the formation of (NH4)2SO4 and NH4HSO4 aerosols. In the ESP system, the generated ammonium sulfate particles can be effectively captured by the electric field force, which can also improve the capture efficiency of ESP on fine particles. Nonetheless, the operation of SCR with an NH3 injection still leads to an increase in fine particle emission levels at the ESP outlet. In the WFGD system, ammonium sulfates and ammonia can be partly removed by desulfurization through slurry scrubbing. However, the accumulated NH4+ in slurry will generate new ammonium species and ammonia through the entrainment and evaporation of the slurry. Finally, although both ESP and WFGD have a good removal effect on the generated fine particles, the SCR operation with the NH3 slip still results in an increase in the emission of fine particles and NH4+. As the formation and transformation mechanism of ammonia slip and ammonium sulfates generated by SCR is complex and affected by many factors, further research will be required.

Corrosion Behavior of Candidate Materials Used for Urea Hydrolysis Equipment in Coal-Fired Selective Catalytic Reduction Units

Corrosion tests were performed in the laboratory in order to assess the corrosion resistance of candidate materials used in urea hydrolysis equipment. The materials to be evaluated were exposed at 145 °C for 1000 h. Alloys 316L, 316L Mod., HR3C, Inconel 718, and TC4 were evaluated. Additionally, aluminide and chromate coatings applied to a 316L substrate were examined. After exposure, the mass changes in the test samples were measured by a discontinuous weighing method, and the morphologies, compositions, and phases of the corrosion products were analyzed using scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction. Results indicated that continuous pitting and dissolution corrosion were the main failure modes for 316L stainless steel. 316L Mod. and HR3C alloy showed better corrosion resistance than 316L due to their relatively high Cr contents, but HR3C exhibited a strong tendency toward intergranular corrosion. Inconel 718, TC4, and aluminide and chromate coating samples showed similar corrosion processes: only depositions formed by hydrothermal reactions were observed. Based on these results, a possible corrosion process in the urea hydrolysis environment was discussed for these candidate materials and questions to be clarified were proposed.

MPC case study on a selective catalytic reduction in a power plant

In this paper, a model predictive control (MPC) case study is presented on the selective catalytic reduction (SCR) in a coal-fired power plant. The MPC is used for optimizing the volume of ammonia injection in order to meet the emission standard of nitrogen oxides (NOx) as well as to prevent ammonia slip. A gain scheduling model is applied to deal with the nonlinearity of the SCR unit. The model gain is developed by kernel density estimation and the dynamic model is estimated by the asymptotic method of identification. The MPC control result shows that the standard deviation of outlet NOx concentration can be reduced by two-thirds, which leads to about 25% reduction of reagent consumption and the prevention of ammonia slip.

NOx selective catalytic reduction (SCR) on self-supported V–W-doped TiO2 nanofibers

A nano-fibrous, self-supported and lightweight SCR unit endowed with well-connected and open porosity, high reactivity, and tunable chemistry is herein proposed.

Synergy of CuO and CeO2 combination for mercury oxidation under low-temperature selective catalytic reduction atmosphere

Synergy for low temperature Hg0 oxidation under selective catalytic reduction (SCR) atmosphere was achieved when copper oxides and cerium oxides were combined in a CuO-CeO2/TiO2 (CuCeTi) catalyst. Hg0 oxidation efficiency as high as 99.0% was observed on the CuCeTi catalyst at 200°C, even the gas hourly space velocity was extremely high. To analyze the synergistic effect, comparisons of catalyst performance in the presence of different SCR reaction gases were systematically conducted over CuO/TiO2 (CuTi), CeO2/TiO2 (CeTi) and CuCeTi catalysts prepared by sol-gel method. The interactions between copper oxides and cerium oxides in CuCeTi catalyst yielded more surface chemisorbed oxygen, and facilitated the conversion of gas-phase O2 to surface oxygen, which are favorable for Hg0 oxidation. Copper oxides in the combination interacted with NO forming more chemisorbed oxygen for Hg0 oxidation in the absence of gas-phase O2. Cerium oxides in the combination promoted Hg0 oxidation through enhancing the transformations of NO to NO2. In the absence of NO, NH3 exhibited no inhibitive effect on Hg0 oxidation, because enough Lewis acid sites due to the combination of copper oxides and cerium oxides scavenged the competitive adsorption between NH3 and Hg0. In the presence of NO, although NH3 lowered Hg0 oxidation rate through inducing reduction of oxidized mercury, complete recovery of Hg0 oxidation activity over the CuCeTi catalyst was quickly achieved after cutting off NH3. This study revealed the synergistic effect of the combination of copper oxides and cerium oxides on Hg0 oxidation, and explored the involved mechanisms. Such knowledge would help obtaining maximum Hg0 oxidation co-benefit from SCR units in coal-fired power plants.

Effect of surfactant, HCl and NH3 treatments on the regeneration of waste activated carbon used in selective catalytic reduction unit

Deactivated waste activated carbon due to the deposition of boron in the exhaust gas from an incinerator was regenerated by the treatment with surfactant, HCl, and NH3. The catalysts were characterized using BET surface area, ICP, XPS and FT-IR spectra. Sequential treatment with surfactant and HCl removed the adsorbed boron very effectively. NH3 treatment increased the number of functional nitrogen groups in regenerated sample. For NH3 selective catalytic reduction (SCR) of NO, sequential treatments with surfactant, HCl, and NH3 showed the highest DeNOx activity, attributed to the increased metal removal efficiency, specific surface area and amount of nitrogen functional groups on the surface.

Particle and NOx emissions of a non-road diesel engine with an SCR unit: The effect of fuel

Diesel engines are challenged to reduce their emissions. Nitrogen oxides (NOx), hydrocarbons, particulate matter and carbon dioxide are most important regulated diesel emissions. The legislation is driving for the use of renewable fuels in vehicles but all the impacts of those on emission control technology are not clear yet. In this study the number and chemical composition of particles emitted from a non-road diesel engine with and without a selective catalytic reduction (SCR) unit were investigated. Common diesel fuel oil (DFO) and untreated rape seed oil (RSO) were used as fuels. The NOx reduction activity of the fresh SCR catalyst was also investigated. The results indicated that the fuel had a great impact on the particles emitted from the engine. The particle number was 10–100 times higher when RSO was used as fuel compared to DFO. The DFO particles contained carbon in significantly higher proportions compared to the RSO particles. Metals, such as Fe and Cu, were also found in the DFO particles. For both fuels the NOx reduction activity was 79% with a SCR converter. As a conclusion, the use of the SCR converter decreased the number of particles in general but increased the number of the nanoparticles.

Study on Metal Oxide Catalyst for Selective Catalytic Reduction of NOx at Low Temperature

The method of selective catalytic reduction (SCR) is now one of the mature applications of NOx removal technology, which can meet the practical requirements due to the more stringent laws and regulations on air pollutions. However, the active temperature of traditional SCR catalyst is usually in the temperature range of 300-400°C, which limits the condition of SCR unit. Therefore, low temperature SCR method has caused concern for the scholars in recent years. Catalyst is the key to SCR unit because its performance has a direct influence on the catalytic efficiency. This paper summarizes the research progress of metal oxide catalyst of low temperature SCR. In addition, the future research work is discussed.