Ngai Ting Lau

Microwave-assisted catalytic combustion of diesel soot

Abstract The concept of oxidizing diesel soot in a microwave-assisted catalytic trap was demonstrated. Comparisons were made to experiments using electric heating with and without catalysts to understand the influence of microwave irradiation on catalysis. The complex permittivities of the three materials (diesel soot, catalyst and support) involved in such a system were measured and the feasibility of using this combination of materials in diesel soot burn-off was evaluated, based upon these measured data. It was found that iron and copper were the most active catalysts in lowering the ignition temperature of diesel soots, while palladium was a necessary component in achieving more complete combustion. The iron-containing catalyst was also very effective and energy-efficient at low microwave input. A non-thermal or microwave enhancement effect was observed which further decreased the ignition temperature by more than 200°C. It was also found that the more vigorous burning of diesel soot by microwave heating led to an increase in carbon monoxide in the combustion products, because of the difference in the heating mechanism. However, when palladium was used, the same completeness of combustion as in electric heating could be achieved.

Growth and Shrinkage of New Particles in the Atmosphere in Hong Kong

Grown nucleated particles > 50 nm in diameter are an important source of cloud condensation nuclei (CCN) and when the size is > 100 nm, they can also have direct influence on the climate. In this study, the nucleation and growth of new particles in the atmosphere in Hong Kong were investigated during dry season (monthly averaged RH < 75%). The maximum size of grown nucleated particles was generally less than 40 nm during new particle burst and growth events. The exception, accounting for ∼ 20% of all burst and growth events, was those induced by strong photochemical reactions, in which subsequent particle shrinkage occurred. Temporal particle and gas concentration variability and meteorological conditions support the occurrence of particle shrinkage. The shrinkage rate calculated (∼ 8 nm h–1) was close to the growth rate. The observation of particle shrinkage sheds new light on particle transformation dynamics and it would add to the understanding of particle behavior in the atmosphere.

The catalytic reduction of SO2 by CO over lanthanum oxysulphide

Abstract Lanthanum oxysulphide was found to be an effective catalyst for the reduction of SO 2 by CO to elemental sulphur. Over 98% in SO 2 conversion and selectivity to elemental sulphur can be achieved under the following conditions: temperature above 500°C, stoichiometric CO SO 2 feed ratio, and a space velocity 21,600 cc g −1 h −1 . COS in the low ppm level was detected as a by-product similar to our earlier work with perovskite. The catalyst is resistant to oxygen and water vapour. In addition, it was recognised that the lanthanum oxysulphide is bifunctional, i.e., not only is it active in the reduction of SO 2 by COS [J.A. Baglio, Ind. Eng. Chem., Prod. Res. Dev., 21 (1982) 38], but also its lattice sulphur atoms are mobile enough to form COS with CO. This suggests that lanthanum oxysulphide functions as a catalyst for the reduction of SO 2 by CO via the intermediate COS.

Use of Stationary and Mobile Measurements to Study Power Plant Emissions

Abstract Chak K. Chan is with the Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong. This paper presents a technique to study air pollution by combining high spatial resolution data obtained by a mobile platform and those measured by conventional stationary stations. Conventional stations provide time-series point data but cannot yield information that is distant from the sites. This can be complemented or supplemented by mobile measurements in the vicinity of the conventional sites. Together, the combined dataset yields a clearer and more precise picture of the dispersion and the transformation of pollutants in the atmosphere in a fixed time frame. Several experiments were conducted in the years 2002–2003 to track the impact of power plant plumes on ground receptors in the immediate vicinity (within a radius of 30 km) of the plants, using a combined mobile and stationary dataset. The mobile data allowed the identification of emissions from coal-fired and gasfired power plants. Coal-fired power plants were the major source of sulfur dioxide (SO2), whereas nitrogen oxides (NOx) emitted from the gas-fired power plant played an important role in the formation of ozone (O3) at ground level. The mobile data showed that two particle size distribution regimes were detected: one had a dominant accumulation mode at 0.40–0.65 μm and the other at 0.65–1 μm. The existence of particles characterized by their mode at 0.65–1 μm and formed by in-cloud processes suggests that vehicular emissions were not the important source. Other local sources, such as power plants (elevated emission), were the likely sources, because Hong Kong does not have much manufacturing industry.

Comparison of daytime and nighttime new particle growth at the HKUST supersite in Hong Kong.

Particles larger than 50-100 nm in diameter have been considered to be effective cloud condensation nuclei (CCN) under typical atmospheric conditions. We studied the growth of newly formed particles (NPs) in the atmosphere and the conditions for these particles to grow beyond 50 nm at a suburban coastal site in Hong Kong. Altogether, 17 new particle formation events each lasting over 1 h were observed in 17 days during 8 Mar-28 Apr and 1 Nov-30 Dec 2011. In 12 events, single-stage growth of NPs was observed in daytime when the median mobility diameter of NPs (Dp) increased up to ∼40 nm but did not increase further. In three events, two-stage particle growth to 61-97 nm was observed at nighttime. The second stage growth was preceded by a first-stage growth in daytime when the Dp reached 43 ± 4 nm. In all these 15 events, organics and sulfuric acid were major contributors to the first-stage growth in daytime. Ammonium nitrate unlikely contributed to the growth in daytime, but it was correlated with the second-stage growth of ∼40 nm NPs to CCN sizes at nighttime. The remaining two events apparently showed second-stage growth in late afternoon but were confirmed to be due to mixing of NPs with pre-existing particles. We conclude that daytime NP growth cannot reach CCN sizes at our site, but nighttime NP growth driven by organics and NH4NO3 can.

Reduction of SO2 by CO and COS over La2O2S - a mechanistic study

Abstract The mechanism of the catalytic reduction of SO 2 by CO and COS over lanthanum oxysulfide was studied using a combination of temperature-programmed reaction coupled with mass spectrometry (TPR/MS) and feed-perturbation step-analysis techniques. The results showed that COS interacts more readily with lanthanum oxysulfide than CO does. The interaction of COS with the lanthanum catalyst was different from that of CO. In addition, COS preferentially reacted with SO 2 to form CO 2 and sulfur instead of following the disproportionation (to form CO 2 and COS) and decomposition (to form CO and sulfur) reactions. The redox mechanism is not the main reaction route for the reduction of SO 2 by CO and COS over lanthanum oxysulfide. SO 2 was strongly adsorbed by the oxysulfide and was retained in the oxysulfide even when heated in an inert gas stream, while COS was very reactive and did not remain long in the oxysulfide. These results suggest that COS has access to certain specific active sites in the oxysulfide and the catalytic reduction of SO 2 by COS possibly proceeds via adsorbed COS reacting with pools of SO 2 adspecies to form CO 2 and sulfur.

Simultaneous catalytic reduction of sulfur dioxide and nitric oxide

The present work reports a catalytic system for the simultaneous reduction of SO2 and NO using CO as a reducing agent. The catalyst contains lanthanum oxysulfide and cobalt sulfides. Experimental results showed that at temperature above 450°C, SO2 and NO conversions are greater than 98 and 99%, respectively.

Application of coal gasification technology as a flue gas pre-conditioning step for the catalytic reduction of acid gases

Abstract Typical flue gas contains an excess amount of oxygen, which can deactivate the reduction catalyst for NO X and SO 2 , such as the lanthanum oxysulfide-based catalyst. The reductant available in a flue gas stream rich in oxygen is usually scarce and not sufficient for the reduction. Coal gasification was applied to pre-condition the flue gas to remove the excessive oxygen and co-generate carbon monoxide for the reduction of NO X and SO 2 in this study. Coal was carbonized to porous semi-coke to prevent clogging caused by the condensation of volatiles before being gasified. The reactivity of the semi-coke with simulated flue gas was found to be the same as activated carbon. The semi-coke samples prepared from various coal sources proved to be effective in removing O 2 (over 90%) from the flue gas and a sufficient amount of CO was co-generated for the conversion of NO and SO 2 over a supported lanthanum oxysulfide catalyst in a subsequent reduction reactor. NO and SO 2 in the flue gas were also reduced in the gasification process, contributing to the overall denitrification and desulfurization efficiency. An overall NO and SO 2 removal efficiency of over 96% was achieved for a sequential coal gasification and catalytic reduction process, and the selectivity to elemental sulfur was as high as 98%.

Infected zone model II: Analyses of published experimental data

Abstract In a previous paper, a catalyst deactivation model for chemical poisoning was developed based on the concept that a poison influences more than the site (s) it occupies when adsorbed on a catalyst surface. The sites which are influenced but not occupied are called infected sites, and a collection of these sites is termed an infected zone. The mathematical formulation of the Infected Zone Model takes into account all of the various sites on the surface: normal, completely deactivated, and infected. To describe the deactivation of a reactive system, material balances of the flow streams and kinetics data are also needed. In this work, published experimental data from three different sources were analyzed using the infected zone model equations. The results indicate that better fits can be obtained in some cases, and confirm that the number of sites affected by a poison is larger than the number of sites actually occupied.

Infected Zone Model I: A Concept for Catalyst Deactivation by Poisoning

Abstract An Infected Zone Model (IZM) was developed to correlate the decay in activity with the amount of poison covering a catalyst under conditions of chemical poisoning. This model is based on the transformation of normal sites into completely deactivated and infected sites when poison interacts with sites. “Normal” sites are not under the influence of poison. “Completely deactivated” sites are those occupied by poison, while “infected” sites are those influenced but not occupied by poison; both contribute to the total loss in activity. The following equation, which is obtained through balancing the activities of these sites on a catalytic surface, correlates the normalized activity a of catalysts with poison coverage θ. a = I )[1−(1−θ) δo ] I is the average residual activity coefficient of infected sites, and δ o is the effective infected zone size.