Application of gain scheduling for modeling the nonlinear dynamic characteristics of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> emissions from utility boilers

A hierarchical gain scheduling (HGS) approach is proposed to model the nonlinear dynamics of NO _x emissions of a utility boiler. At the lower level of HGS, a nonlinear static model is used to schedule the static parameters of local linear dynamic models (LDMs), such as static gains and static operating conditions. According to upper level scheduling variables, a multi-model method is used to calculate the predictive output based on lower-level LDMs. Both static and dynamic experiments are carried out at a 360 MW pulverized coal-fired boiler. Based on these data, a nonlinear static model using artificial neural network (ANN) and a series of linear dynamic models are obtained. Then, the performance of the HGS model is compared to the common multi-model in predicting NO _x emissions, and experimental results indicate that the proposed HGS model is much better than the multi-model in predicting NO _x emissions in the dynamic process. This paper was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 25–28, 2008.     

Trimerization of isobutene over WOx/ZrO2 catalysts

Trimerization of isobutene to produce isobutene trimers has been investigated over WO_x/ZrO₂ catalysts that were obtained by wet-impregnation and successive calcination at high temperatures. Very stable isobutene conversion and high selectivity for trimers are attained over a WO_x/ZrO₂ catalyst obtained by calcination at 700 °C. From the XRD study it can be understood that tetragonal ZrO₂ is beneficial for stable performance; however, monoclinic ZrO₂ is not good for trimerization. Nitrogen adsorption and FTIR experiments suggest that amorphous WO_x/ZrO₂ is inefficient catalyst even though it has high surface area and high concentration of acid sites. The observed performance with the increased selectivity and stable conversion demonstrates that a WO_x/ZrO₂ having tetragonal zirconia, even with decreased porosity and acid sites, is one of the best catalysts to exhibit stable and high conversion, high selectivity for trimers and facile regeneration.     

The effects of regeneration-phase CO and/or H2 amount on the performance of a NOX storage/reduction catalyst

The effects of regeneration-phase CO and/or H₂, and their amounts as a function of temperature on the trapping and reduction of NO_X over a model and a commercial NO_X storage/reduction catalyst have been evaluated. Overall, for both catalysts, their NO_X removal performance improved with each incremental increase in H₂ concentration. For the commercial sample, using CO at 200 °C, beyond a small amount added, was found to decrease performance. The addition of H₂ to the CO-containing mixtures resulted in improved performance at 200 °C, but the presence of the CO still resulted in decreased performance in comparison to activity when just H₂ was used. With the model sample, the presence of CO resulted in very poor performance at 200 °C, even with H₂. The data suggest that CO poisons Pt sites, including Pt-catalyzed nitrate decomposition. At 300 °C, H₂, CO, and mixtures of the two were comparable for trapping and reduction of NO_X, although with the model sample H₂ did prove consistently better. With the commercial sample, H₂ and CO were again comparable at 500 °C, but mixtures of the two led to slightly improved performance, while yet again H₂ and H₂-containing mixtures proved better than CO when testing the model sample. NH₃ formation was observed under most test conditions used. At 200 °C, NH₃ formation increased with each increase in H₂, while at 500 °C, the amount of NH₃ formed when using the mixtures was higher than that when using either H₂ or CO. This coincides with the improved performance observed with the mixtures when testing the commercial.     

The Effects of CO2 and H2O on the NO x Destruction Performance of a Model NO x Storage/Reduction Catalyst

The effects of CO₂ and H₂O on the NO _x storage and reduction characteristics of a Pt/Ba/Al₂O₃ catalyst were investigated. The presence of CO₂ and H₂O, individually or together, affect the performance and therefore the chemistry that occurs at the catalyst surface. The effects of CO₂ were observed in both the trapping and reduction phases of the experiments, whereas the effect of H₂O seems limited to the trapping phase. The data also indicate that multiple types of sorption sites (or mechanisms for sorption) exist on the catalyst. One mechanism is characterized by a rapid and complete uptake of NO _x . A second mechanism is characterized by a slower rate of NO _x uptake, but this mechanism is active for a longer time period. As the temperature is increased, the effect of H₂O decreases compared to that of CO₂. At the highest temperatures examined, the elimination of H₂O when CO₂ is present did not affect the performance.     

Influence of homogeneous decane oxidation on the catalytic performance of lean NO x catalysts

Extensive homogeneous gasphase reactions were observed when decane was used as the hydrocarbon reductant for the selective reduction of NO _x . The catalytic performance of a SnO₂/CoO _x /Al₂O₃ catalyst was found to be strongly dependent on the extent of the homogeneous reaction in the precatalytic volume. The effect of the homogeneous reaction on the catalytic performance also depended on whether SO₂ was present in the feed. By filling the precatalytic volume with 25–35 mesh irregularly shaped quartz chips, gasphase reaction was suppressed significantly. This methodology was used to evaluate the inherent catalytic performance of SnO₂/CoO _x /Al₂O₃ and SnO₂/Al₂O₃ catalysts with decane as a reductant. It was found that in the absence of SO₂, SnO₂/Al₂O₃ was a better catalyst than SnO₂/CoO _x /Al₂O₃, but in the presence of 30 ppm of SO₂ the latter was a far better catalyst.     

Advanced De-SOx catalyst: Mixed solid solution spinels with cerium oxide

Mixed solid solution spinels impregnated with cerium, Ce/MgO·MgAl_2-xM_xO₄ (M=Fe, V, Cr, x≤0.4), were studied for controlling the SO_x emission from the fluid catalytic cracking (FCC) regenerator. An insufficient sulfur release problem inherent to the earlier De---SO_x catalyst, Ce/MgO·MgAl₂O₄, was effectively overcome by incorporating a transition metal into the spinel structure. Studies of the SO_x pick-up, temperature profile for the sulfate reduction, the thermal analysis, and the De---SO_x cycle test in the batch as well as the automated continuous reactor are discussed to define the role of a transition metal in the mixed spinels for the De---SO_x performance. These advanced De---SO_x catalysts have led to a commercial success for the simultaneous control of SO_x and NO_x emissions from the FCC regenerator.     

Preparation and electrochemical properties of Cr-doped Li9V3(P2O7)3(PO4)2 as cathode materials for lithium-ion batteries

Cr-doped Li₉V_3−xCr_x(P₂O₇)₃(PO₄)₂ (x = 0.0–0.5) compounds have been prepared using sol–gel method. The Rietveld refinement results indicate that single-phase Li₉V_3−xCr_x(P₂O₇)₃(PO₄)₂ (x = 0.0–0.5) with trigonal structure can be obtained. Although the initial specific capacity decreased with Cr content at a lower current rate, both cycle performance and rate capability have excited improvement with moderate Cr-doping content. Li₉V_2.8Cr_0.2(P₂O₇)₃(PO₄)₂ compound presents the good electrochemical rate and cyclic ability. The enhancement of rate and cyclic capability may be attributed to the optimizing particle size, morphologies, and structural stability during the proper amount of Cr-doping (x = 0.2) in V sites.     

NOx Storage-Reduction Three-Way Catalyst with Improved Sulfur Tolerance

The NO _x storage-reduction catalyst (NSR catalyst) is poisoned by SO₂ caused by fuel sulfur, thus its activity is reduced. In order to improve the NSR catalyst, the sulfur poisoning phenomenon has been analyzed. Based on this result, we developed TiO₂ and Rh/ZrO₂ to promote the sulfur desorption. The developed catalyst has made remarkable progress in its sulfur tolerance, about 50% improvement in NO _x purification performance compared with the conventional one.     

Activity of oxygen reduction reaction on small amount of amorphous CeOx promoted Pt cathode for fuel cell application

Pt on ceria (CeO_x) particles supported on carbon black (CB) were synthesized using the combined process of hot precipitation and impregnation methods. During 30 cycles of cyclic voltammetry pre-treatment in the potential ranging from −0.2 to 1.3 V (V vs. Ag/AgCl), it was observed that a small amount of CeO_x, which consisted of the interface region between Pt and CeO_x, remained on Pt particles. Other free CeO_x particles were dissolved into H₂SO₄ aqueous solution. To develop the Pt-CeO_x/CB catalyst, the surface chemical states, the net chemical composition, morphology and electrochemical behavior in H₂SO₄ aqueous solution were characterized. Our microanalysis and electrochemical analysis indicate that the active CeO₂ with high specific surface area provides the continuous amorphous cerium oxide (Ce³⁺, Ce⁴⁺) layer with pores on the surface of Pt particles. It is concluded that the amorphous cerium oxide layer on Pt inhibits the oxidation of Pt surface and contributes to enhancement of the activity on Pt cathode. The single cell performance was also improved using the Pt-CeO_x/CB cathode. Based on all data, it is expected that the design based on characterization of the interface between Pt and small amount of amorphous cerium oxide layer could help in preparation of more active Pt catalyst.     

Comparative study of the two-zone fluidized-bed reactor and the fluidized-bed reactor for oxidative coupling of methane over Mn/Na2WO4/SiO2 catalyst

In this work, oxidative coupling of methane over Mn/Na₂WO₄/SiO₂ catalyst is studied in a two-zone fluidized-bed reactor (TZFBR) and its performance is compared with a fluidized-bed reactor (FBR). Diluted oxygen in argon was introduced into the bottom of the TZFBR through a quartz ferrite and methane was entered at higher positions along the fluidized bed. The catalyst circulated between the oxygen-rich and methane-rich zones in the TZFBR reactor. The effects of the main operating variables including bed temperature, the methane/oxygen ratio (R_mo), and the height at which methane was introduced into the reactor (H_m) were investigated. It is found that under some operating conditions the TZFBR gives a higher C₂ selectivity than that obtained in the FBR reactor. Reaction of methane with lattice oxygen of the Mn/Na₂WO₄/SiO₂ redox catalyst in the methane-rich zone may have led to the higher selectivity.     

High-performance Li4Ti5−xVxO12 (0 ≤ x ≤ 0.3) as an anode material for secondary lithium-ion battery

Powders of spinel Li₄Ti_5−xV_xO₁₂ (0 ≤ x ≤ 0.3) were successfully synthesized by solid-state method. The structure and properties of Li₄Ti_5−xV_xO₁₂ (0 ≤ x ≤ 0.3) were examined by X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electronic microscope (SEM), galvanostatic charge–discharge test and cyclic voltammetry (CV). XRD shows that the V⁵⁺ can partially replace Ti⁴⁺ and Li⁺ in the spinel and the doping V⁵⁺ ion does almost not affect the lattice parameter of Li₄Ti₅O₁₂. Raman spectra indicate that the Raman bands corresponding to the Li–O and Ti–O vibrations have a blue shift due to the doping vanadium ions, respectively. SEM exhibits that Li₄Ti_5−xV_xO₁₂ (0.05 ≤ x ≤ 0.25) samples have a relative uniform morphology with narrow size distribution. Charge–discharge test reveals that Li₄Ti_4.95V_0.05O₁₂ has the highest initial discharge capacity and cycling performance among all samples cycled between 1.0 and 2.0 V; Li₄Ti_4.9V_0.1O₁₂ has the highest initial discharge capacity and cycling performance among all samples cycled between 0.0 and 2.0 V or between 0.5 and 2.0 V. This excellent cycling capability is mainly due to the doping vanadium. CV reveals that electrolyte starts to decompose irreversibly below 1.0 V, and SEI film of Li₄Ti₅O₁₂ was formed at 0.7 V in the first discharge process; the Li₄Ti_4.9V_0.1O₁₂ sample has a good reversibility and its structure is very advantageous for the transportation of lithium-ions.     

Catalytic Properties of Ag/Al2O3 Catalysts for Lean NOx Reduction Processes and Characterisation of Active Silver Species

A study of the lean NO _x reduction activity employing different reductants over Ag/Al₂O₃ samples prepared from reverse microemulsions or impregnation with EDTA-complexes is presented. A multitechnique approach is employed for characterisation of the samples and/or processes taking place in the course of the NO _x -SCR reaction with propene and propane. Results by in situ-DRIFTS reveal that, for the propene reductant, silver provides a new path for hydrocarbon activation involving generation of adsorbed acrylate species as a partially oxidised active intermediate, in line with previous proposals for other non-noble metal systems. It is shown, mainly on the basis of XAFS studies, that active silver species are related to well dispersed silver aluminate-like phases with tetrahedral local symmetry and a relatively high disorder in the oxygen first shell.     

Catalytic reduction of NH4NO3 by NO: Effects of solid acids and implications for low temperature DeNOx processes

Ammonium nitrate is thermally stable below 250 °C and could potentially deactivate low temperature NO_x reduction catalysts by blocking active sites. It is shown that NO reduces neat NH₄NO₃ above its 170 °C melting point, while acidic solids catalyze this reaction even at temperatures below 100 °C. NO₂, a product of the reduction, can dimerize and then dissociate in molten NH₄NO₃ to NO⁺ + NO₃⁻, and may be stabilized within the melt as either an adduct or as HNO₂ formed from the hydrolysis of NO⁺ or N₂O₄. The other product of reduction, NH₄NO₂, readily decomposes at ≤100 °C to N₂ and H₂O, the desired end products of DeNO_x catalysis. A mechanism for the acid catalyzed reduction of NH₄NO₃ by NO is proposed, with HNO₃ as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNO_x systems could help mitigate catalyst deactivation at low operating temperatures (<150 °C).     

Highly Active CNT-Promoted Cu–ZnO–Al2O3 Catalyst for Methanol Synthesis from H2/CO/CO2

With types of in-house-synthesized multi-walled carbon nanotubes (CNTs) and the nitrates of the corresponding metallic components, highly active CNT-promoted Cu–ZnO–Al₂O₃ catalysts, symbolized as Cu _i Zn _j Al _k -x%CNTs, were prepared by the co-precipitation method. Their catalytic performance for methanol synthesis from H₂/CO/CO₂ was studied and compared with the corresponding CNT-free co-precipitated catalyst, Cu _i Zn _j Al _k . It was shown experimentally that appropriate incorporation of a minor amount of the CNTs into the Cu _i Zn _j Al _k could significantly increase the catalyst activity for methanol synthesis. Under the reaction conditions of 493 K, 5.0 MPa, H_2/CO/CO₂/N₂ = 62/30/5/3 (v/v), GHSV = 8000 h^-1, the observed CO conversion and methanol formation rate over a co-precipitated catalyst of Cu₆Zn₃Al₁-12.5%CNTs reached 36.8% and 0.291 mol CH₃OH s^-1 (m²-surf. Cu)^-1, which was about 44 and 25% higher than those (25.5% and 0.233 mol CH₃OH s^-1 (m²-surf. Cu)^-1) over the corresponding CNT-free co-precipitated catalyst, Cu₆Zn₃Al₁. Addition of a minor amount (10–15 wt%) of the CNTs to the Cu₆Zn₃Al₁ catalyst was found to considerably increase specific surface area, especially Cu surface area of the catalyst. H₂-TPD measurements revealed that the CNTs and the pre-reduced CNT-promoted catalyst systems could reversibly adsorb and store a considerably greater amount of hydrogen under atmospheric pressure at temperatures ranging from room temperature to 573 K. This unique feature would be beneficial for generating microenvironments with higher stationary-state concentration of active hydrogen adspecies on the surface of the functioning catalyst, especially at the interphasial active sites since the highly conductive CNTs might promote hydrogen spillover from the Cu sites to the Cu/Zn interphasial active sites, and thus be favorable for increasing the rate of the CO hydrogenation reactions. Alternatively, the operation temperature for methanol synthesis over the CNT-promoted catalysts can be 15–20 degrees lower than that over the corresponding CNT-free contrast system. This would contribute considerably to an increase in equilibrium CO conversion and CH₃OH yield. The results of the present work indicated that the CNTs could serve as an excellent promoter.     

Variation of LiNiO<Subscript>2</Subscript> cathode properties with substitution of Ga,In and Tl by the combustion method

LiNi_1-y M_yO₂ (M = Ga, In and Tl, y = 0.010, 0.025 and 0.050) with small y were synthesized by the combustion method by calcining in an O₂ stream at 750 °C for 36 h. XRD analyses, SEM observation and measurement of the variation of discharge capacity with the number of cycles were carried out. All the samples had the Rm structure and LiNi_1-y In _y O₂ contained LiInO₂ phase as an impurity. Among LiNi_1-y Ga _y O₂ the sample with y = 0.025 had a relatively large first discharge capacity (172.2 mAh g⁻¹) and relatively good cycling performance (discharge capacity 140.3 mAh g⁻¹ at n = 20). For LiNi_0.975M_0.025O₂ (M = Ga, In and Tl), the first discharge capacity decreased in the order of the substituted element Ga, In and Tl. The variations of cation mixing and hexagonal ordering with the substituted element (decrease in I₀₀₃/I₁₀₄ and increase in R-factor from M = Ga through M = Tl) are considered to lead to the behavior of the first discharge capacity with the substituted element. LiNi_0.975Tl_0.025O₂ had the smallest degradation rate of the discharge capacity.     

Ru-Promoted CrO x /Al2O3 Catalyst for the Low-Temperature Oxidative Decomposition of Trichloroethylene in Air

A series of Ru-promoted CrO _x /Al₂O₃ as catalysts for the low-temperature oxidative decomposition of trichloroethylene (TCE) were characterized and evaluated in comparison with an unpromoted CrO _x /Al₂O₃ catalyst. Catalyst characterization was conducted by surface area measurement, X-ray diffraction and X-ray photoelectron spectroscopy. Catalyst performance in the TCE decomposition reaction was evaluated with respect to the initial catalytic activity, the rate of catalyst deactivation, and the product concentrations of CO and Cl₂ under dry or wet air conditions. The presence of a small amount of Ru, as much as 0.4 wt% in a CrO _x /Al₂O₃ catalyst, brought about several beneficial effects on the catalytic reaction performance. As compared with the unpromoted CrO _x /Al₂O₃, this Ru-promoted CrO _x /Al₂O₃ catalyst showed enhanced catalytic activity (249 versus 264 °C in terms of temperature at which 50% of TCE conversion occurred), a reduced concentration of CO (180 versus 325 ppm) in the product, and a decreased propensity to deactivation. Performance improvements of the Ru-promoted CrO _x /Al₂O₃ catalyst were thought to originate from its enhanced oxidation activity due to the coexisting highly-dispersed Ru oxides rendering less active Cr(III) to more active Cr(VI), and facilitating the process of supplying activated oxygen for the reaction system.     

Optimization of post-combustion CO2 process using DEA-MDEA mixtures

This paper presents optimal operating conditions for the post-combustion CO₂ capture process utilizing aqueous amine solutions obtained using a process simulator (HYSYS). Three alkanolamine solutions (Methyldiethanolamine MDEA, DiEthanolAmine DEA and MDEA-DEA mixture) are considered to study the performance of the capture process.The design problem addressed in this paper requires specifying the optimal operating conditions (inlet and outlet temperature of the lean solution stream on the absorber, CO₂ loading, amine composition and flow rates, among others) to achieve the given CO₂ emission targets at a minimum total annual cost. A detailed objective function including total operating costs and investment is considered.The influence of the variation of CO₂ reduction targets and the mixing proportion of amines on the total annual cost is analyzed in detail. Numerical results are presented and discussed using different case studies.The results demonstrate that process simulators can be used as a powerful tool not only to simulate but also to optimize the most important design parameters of the post-combustion CO₂ capture process.     

Low-temperature selective catalytic reduction of NO with NH3 based on MnOx-CeOx/ACFN

MnO _x -CeO _x /ACFN were prepared by the impregnation method and used as catalyst for selective catalytic reduction of NO with NH3 at 80°C-150°C. The catalyst was characterized by N₂-BET, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The fraction of the mesopore and the oxygen functional groups on the surface of activated carbon fiber (ACF) increased after the treatment with nitric acid, which was favorable to improve the catalytic activities of MnO _x -CeO _x /ACFN. The experimental results show that the conversion of NO is nearly 100% in the range 100°C-150°C under the optimal preparation conditions of MnO _x -CeO _x /ACFN. In addition, the effects of a series of performance parameters, including initial NH₃ concentration, NO concentration and O₂ concentration, on the conversion of NO were studied. __________ Translated from Chemical Industry and Engineering Progress, 2007, 27(1): 87–91 [译自: 化工进展]     

Catalyst’s Sulfur Displacement by Thiophene, as Monitored by Exchange of 35S Radiotracer

The isotopic exchange has been studied between catalyst radiosulfur and H₂S, formed in thiophene hydrodesulfurization (HDS) (named S-displace) on alumina supported molybdena, on CoMoO_x, PdMoO_x, PtMoO_x and on silica–alumina supported NiWO_x. S-displace was compared with radiosulfur exchange data between catalyst radiosulfur and gas phase H₂S (S^exc) determined previously. The extent of S^exc was higher than that of the S-displace for Mo, CoMo in and NiW, whereas the extent of S-displace from PdMoO and PtMoO was significantly higher, than that of S^exc. Thiophene HDS product distribution data are discussed in terms of increased C=C hydrogenation and C–C hydrogenolysis activity, explained by increasing H₂S production with longer circulation time of the thiophene/H₂ mixture, The C₁/C₃<1 ratios among C₄-hydrogenolysis products indicate some coke formation. The decrease of thiophene HDS activity is presumably a consequence of increasing site-blocking with the formation of more H₂S and coke with longer duration of thiophene treatment.