Microbial removal of sulfur dioxide from a gas stream

A study of the feasibility of utilizing the Thiobacillus ferrooxidans or Desulfovibrio desulfuricans bacterium for microbial removal of sulfur dioxide from flue gases has been carried out. Sulfur dioxide may be readily reduced to H₂S by contact with sulfate-reducing microorganisms in which Desulfovibrio desulfuricans were dominant in the first stage. The H₂S was then oxidized to sulfur by the ferric sulfate in a second stage where ferrous ions were regenerated. These results were compared to microbial oxidation of SO₂ from flue gases to sulfate by Thiobacillus ferrooxidans. The mechanisms for the reduction of SO₂ to H₂S in the presence of Desulfovibrio desulfuricans and the oxidation of SO₂ to H₂SO₄ in the presence of ferric sulfate and Thiobacillus ferrooxidans are discussed. Sulfuric acid or gypsum CaSO₄ · 2H₂O are byproducts from microbial flue gas desulfurization.     

Conversion of sulfur dioxide to sulfate and hydrogen sulfide by Desulfotomaculum orientis

The autotrophic, sulfate-reducing bacterium, Desulfotomaculum orientis, grew in batch culture with molecular hydrogen (H₂) as an energy source, carbon dioxide (CO₂) as a carbon source and sulfur dioxide (SO₂) as the terminal electron acceptor. At high H₂ partial pressure, SO₂ was stoichiometrically reduced to hydrogen sulfide (H₂S). At low partial pressures of hydrogen (< 0.025 atm), SO₂ was both oxidized to sulfate and reduced to hydrogen sulfide. These results indicated a new mode of sulfur metabolism for D. orientis.     

Microwave-assisted desulfurization of NOx storage-reduction catalyst

The activity of NO_x storage-reduction (NSR) catalysts is greatly reduced by sulfur poisoning, caused by the SO₂ present in the exhaust stream. Desorption of sulfur species from poisoned NSR catalysts occurs at temperatures in excess of 600 °C using reducing atmospheres and conventional heating. In this work, microwave (MW) heating has been used to promote desulfurization of poisoned NSR catalysts. The experiments were carried out by heating the catalyst with MW radiation and using hydrogen as the reducing gas. Desorption of H₂S at 200 °C was observed. Desorption at even lower temperatures (150 °C) was observed when water was introduced to the system. In the presence of water, sulfur species desorbed as both H₂S and SO₂. An overall reduction of sulfur species of about 60% was obtained. The use of MW heating proves to be an efficient way to achieve regeneration of poisoned NSR catalysts.     

The role of zeolite type on the lean NOx reduction by methane over Pd loaded pentasil zeolites

The lean selective catalytic reduction of NO_x by methane over protonic palladium loaded ZSM-5, FER and MOR, as well as, on bimetallic Pd–Pt-HMOR was examined. Special emphasis was paid on the combined effects of water and SO₂ in the feed stream. Under dry conditions and in the absence of SO₂, the degree of NO_x conversion at 450°C decreases as follows: Pd-HZSM-5>Pd-HMOR>Pd-HFER. Sulfur dioxide alone has no apparent effect on the activity for NO_x reduction, but the coexistence of water and SO₂ inhibits both NO_x and methane conversions. The extent of inhibition by water and SO₂ on NO_x reduction is Pd-HFER>Pd-HZSM-5>Pd-HMOR. Acid mordenite doped with low levels of Pt and Pd leads to an active catalyst that is more tolerant to the presence of either water or SO₂ than the corresponding monometallic Pt- and Pd-HMOR. Nevertheless, NO_x reduction is also inhibited at temperatures below 450°C when SO₂ and water are both present. TPD experiments of water over calcined samples of protonic Pd supported pentasil zeolites, Pd/γ-Al₂O₃ and Pt–Pd-HMOR with and without pretreatment in SO₂+O₂ indicate that sulfation of the surface increases water chemisorption by the support. Therefore, the observed decrease of NO_x reduction on Pd-loaded zeolite catalysts when SO₂ and H₂O coexist in the feed stream may be due to enhanced water inhibition and presumably active site poisoning.     

Investigations of sulphur deactivation of NOx storage catalysts: influence of sulphur carrier and exposure conditions

The influence of SO₂, H₂S and COS in low concentrations on the deactivation of Pt/Rh/BaO/Al₂O₃ NO_x storage catalysts was investigated. Different samples of the catalyst were exposed to synthetic gas mixtures mimicking lean/rich engine cycling in a mixed lean application at 400 °C. The lean gas mixture contained 8 vol.% O₂, 500 vol-ppm C₃H₆ and 400 vol-ppm NO balanced to 100 vol.% with Ar. The rich excursions were performed by switching off the oxygen supply. Sulphur, 25 vol-ppm of either SO₂, H₂S or COS, was added to the gas flow either during the lean, the rich or both periods. This procedure aimed at investigating the influence of the exposure conditions and therefore the lean and rich periods were kept equally long (5 min). In addition, thermodynamical calculations for the prevailing conditions were performed.

It was concluded that all sulphur compounds investigated, i.e. SO₂, H₂S and COS, had similar, negative impact on the NO_x storage ability of the catalyst and that they all showed increased deactivation rates during rich exposure compared to lean. During lean exposure, all sulphur carriers showed similar behaviour, while H₂S and COS caused severe loss of noble metal activity during rich exposure.     

Multifunctional catalysts for de-NOx processes: The case of H3PW12O40·6H2O-metal supported on mixed oxides

The role of a multifunctional catalyst for de-NO_x process has been investigated. The NO_x storage capacity of H₃PW₁₂O₄₀·6H₂O (HPW) was improved by the presence of a noble metal (Pt, Rh or Pd). Both HPW and noble metal were deposited on a specific support (based on Zr–Ce or Zr–Ti). The presence of noble metal in several oxidation states, as evidenced by TPR and IR, involves the possibility of forming different catalytic sites: (i) M⁰ (zero-valent metal) and perhaps (ii) (metal–H)^δ+ from specific interactions between noble metal and the HPW proton. Supports were also able to adsorb and activate NO_x and to generate cationic catalytic sites (M^x+). These cationic sites seem to be the clue for their important activity toward NO_x reduction. This catalyst presents an outstanding resistance to SO₂ poisoning which can be related to NO and NO₂ absorption mechanism in HPW. The use of alternating short cycles of lean/rich mixtures allows us optimising the performance of this catalytic system in terms of both NO_x reduction capacity and NO_x storage efficiency: up to 48 and 84%, respectively (with a 2% CO + 1% H₂ mixture for reducing). Experimental results sustain two hypotheses: first, HPW-metal-support catalyst includes several (independent) catalytic functions required for a de-NO_x process to occur and second, the formation of oxygenate active species must be indispensable for NO_x reduction into nitrogen.     

Kinetic modelling of sulfur deactivation of Pt/BaO/Al2O3 and BaO/Al2O3 NOx storage catalysts

In this work, a kinetic model is constructed to simulate sulfur deactivation of the NO_x storage performance of BaO/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts. The model is based on a previous model for NO_x storage under sulfur-free conditions. In the present model the storage of NO_x is allowed on two storage sites, one for complete NO_x uptake and one for a slower NO_x sorption. The adsorption of SO_x is allowed on both of these NO_x storage sites and on one additional site which represent bulk storage. The present model is built-up of six sub-models: (i) NO_x storage under sulfur-free conditions; (ii) SO₂ storage on NO_x storage sites; (iii) SO₂ oxidation; (iv) SO₃ storage on bulk sites; (v) SO₂ interaction with platinum in the presence of H₂; (vi) oxidation of accumulated sulfur compounds on platinum by NO₂. Data from flow reactor experiments are used in the implementation of the model. The model is tested for simulation of experiments for NO_x storage before exposure to sulfur and after pre-treatments either with SO₂ + O₂ or SO₂ + H₂. The simulations show that the model is able to describe the main features observed experimentally.     

高级氧化法在烟气脱硫脱硝脱汞中的应用研究进展

近年来,面对大气污染日益严峻的现状,用于燃煤烟气中SO₂、NO_x和Hg⁰的控制技术的研发显得尤为关键。鉴于高级氧化法在污水处理领域较好的应用效果,对于将其用于烟气处理的研究也已展开。本文主要从高级氧化法用于单独脱硝、脱汞和同时脱硫脱硝以及三者联合脱除这4个方面进行了较为详细的综述。重点分析了光催化氧化、蒸发Fenton试剂、多相催化剂、UV/Fenton技术等氧化法在烟气处理领域的研究现状。分析表明,通过对诸多影响因素以及催化剂选择和改性等方面的试验与调控,可使SO₂、NO_x和Hg⁰的脱除率达到90%左右甚至更高的水平,但仍然存在着·OH自由基量难控制以及脱除效率难维持等问题。由此指出,在今后对于将高级氧化法用于燃煤烟气联合脱除的研究中,需要进一步尝试基于多方法融合的完整、稳定、高效脱硫脱硝脱汞的新思路。     

Simultaneous SO2/NOx removal by a powder-particle fluidized bed

Simultaneous dry removal of SO₂ and NO_x from flue gas has been investigated using a powder-particle fluidized bed. In a process of flue gas desulfurization by use of solid sorbents such as FeO (dust from a steel plant) and CuO, the smaller the particle size of sorbents, the higher the expected SO₂ conversion. In a powder-particle fluidized bed (PPFB), fine particles less than 40 μm in diameter fed into the bed are fluidized with coarse particles. But only the fine particles are entrained from the bed, and their residence time in the bed is remarkably long.

The reduction of NO_x with NH₃ in the fluidized bed is catalyzed by coarse particles or both coarse and fine particles. In this study, PPFB was applied to simultaneous dry SO₂/NO_x removal process, and several kinds of sorbents or catalysts were evaluated in a PPFB. Using the selected sorbents and catalysts, kinetic measurements were made in the temperature range of 300 to 600°C. SO₂ removal efficiencies were affected by reaction temperature, sorbent/S ratio, and static bed height. NO_x removal efficiencies in excess of 95% were achieved at NH₃/NO_x mole ratio of 1.0. When FeO was used as sorbent, SO₂ conversion increased with increasing temperature and reached 80% at 600°C.     

CO2, NOx and SO2 emissions from the combustion of coal with high oxygen concentration gases

The emissions of CO₂, NO_x and SO₂ from the combustion of a high-volatile coal with N₂- and CO₂-based, high O₂ concentration (20, 50, 80, 100%) inlet gases were investigated in an electrically heated up-flow-tube furnace at elevated gas temperatures (1123–1573 K). The fuel equivalence ratio, φ, was varied in the range of 0.4–1.6. Results showed that CO₂ concentrations in flue gas were higher than 95% for the processes with O₂ and CO₂-based inlet gases. NO_x emissions increased with φ under fuel-lean conditions, then declined dramatically after φ=0.8, and the peak values increased from about 1000 ppm for the air combustion process and 500 ppm for the O₂(20%)+CO₂(80%) inlet gas process to about 4500 ppm for the oxygen combustion process. When φ>1.4 the emissions decreased to the same level for different O₂ concentration inlet gas processes. On the other hand, NO_x emission indexes decreased monotonically with φ under both fuel-lean and fuel-rich combustion. SO₂ emissions increased with φ under fuel-lean conditions, then declined slightly after φ>1.2. Temperature has a large effect on the NO_x emission. Peak values of the NO_x emission increased by 50–70% for the N₂-based inlet gas processes and by 30–50% for the CO₂-based inlet gas process from 1123 to 1573 K. However, there was only a small effect of temperature on the SO₂ emission.     

Regeneration of NOx trap catalysts

We have investigated the regeneration of a nitrated or sulphated model Pt/Ba-based NO_x trap catalyst using different reductants. H₂ was found to be more effective at regenerating the NO_x storage activity especially at lower temperature, but more importantly over the entire temperature window after catalyst ageing. When the model NO_x storage catalyst is sulphated in SO₂ under lean conditions at 650 °C almost complete deactivation can be seen. Complete regeneration was not achieved, even under rich conditions at 800 °C in 10% H₂/He. Barium sulphate formed after the high temperature ageing was partly converted to barium sulphide on reduction. However, if the H₂ reduced sample was exposed to a rich condition in a gas mixture containing CO₂ at 650 °C, the storage activity can be recovered. Under these rich conditions the S²⁻ species becomes less stable than the CO₃²⁻, which is active for storing NO_x. Samples which were lean aged in air containing 60 ppm SO₂ at <600 °C, after regeneration at λ=0.95 at 650 °C, have a similar activity window to a fresh catalyst. It is, therefore, important that CO₂ is present during the rich regenerations of the sulphated model samples (as of course it would be under real conditions), as suppression of carbonate formation can lead to sulphide formation which is inactive for NO_x storage.     

Sulphur dioxide interaction with NOx storage catalysts

The effect of SO₂ on the NO_x storage capacity and oxidation and reduction activities of a model Pt/Rh/BaO/Al₂O₃ NO_x storage catalyst was investigated. Addition of 2.5, 7.5 or 25 vol. ppm SO₂ to a synthetic lean exhaust gas caused deactivation of the NO_x storage function, the oxidation activity and the reduction activity of the catalyst. The degree of deactivation of the NO_x storage capacity was found to be proportional to the total SO₂ dose that the catalyst had been exposed to. SO₂ was found to be accumulated in the catalyst as sulphate.     

Activity and stability of Ag–alumina for the selective catalytic reduction of NOx with methane in high-content SO2 gas streams

In this work, we investigated the activity and stability of Ag–alumina catalysts for the SCR of NO with methane in gas streams with a high concentration of SO₂, typical of coal-fired power plant flue gases. Ag–alumina catalysts were prepared by coprecipitation–gelation, and dilute nitric-acid solutions were used to remove weakly bound silver species from the surface of the as prepared catalysts after calcination. SO₂ has a severe inhibitory effect, essentially quenching the CH₄-SCR reaction on this type catalysts at temperatures <600 °C. SO₂ adsorbs strongly on the surface forming aluminum and silver sulfates that are not active for CH₄-SCR of NO_x. Above 600 °C, however, the reaction takes place without catalyst deactivation even in the presence of 1000 ppm SO₂. The reaction light-off coincides with the onset of silver sulfate decomposition, indicating the critical role of silver in the reaction mechanism. SO₂ is reversibly adsorbed on silver above 600 °C. While alumina sites remain sulfated, this does not hinder the reaction. Sulfation of alumina only decreases the extent of adsoption of NO_x, but adsorption of NO_x is not the limiting step. Methane activation is the limiting step, hence the presence of sulfur-free Ag–O–Al species is a requirement for the reaction. Strong adsorption of SO₂ on Ag–alumina decreases the rates of the reaction, and increases the activation energies of both the reduction of NO to N₂ and the oxidation of CH₄, the latter more than the former. Our results indicate partial contribution of gas phase reactions to the formation of N₂ above 600 °C. H₂O does not inhibit the reaction at 625 °C, and the effect of co-addition of H₂O and SO₂ is totally reversible.     

The activity and characterization of sol–gel Sn/Al2O3 catalyst for selective catalytic reduction of NOx in the presence of oxygen

Catalytic performance of Sn/Al₂O₃ catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO_x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO₂ had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al₂O₃ (SG) catalyst, and the maximum NO₂ conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO_x desorption accompanied with O₂ around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H₂O and SO₂ at low temperature, and the temperature window was also broadened in the presence of H₂O and SO₂, however the NO_x desorption and NO conversion decreased sharply on the 300 ppm SO₂ treated catalyst, the catalytic activity was inhibited by the presence of SO₂ due to formation of sulfate species (SO₄²⁻) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al₂O₃ (SG) was not decreased in the presence of large oxygen.     

Catalytic reduction of sulfur dioxide using hydrogen or carbon monoxide over Ce1−xZrxO2 catalysts for the recovery of elemental sulfur

This study focuses on the direct sulfur recovery process (DSRP), in which SO₂ can be directly converted into elemental sulfur using a variety of reducing agents over Ce_1−xZr_xO₂ catalysts. Ce_1−xZr_xO₂ catalysts (where x = 0.2, 0.5, and 0.8) were prepared by a citric complexation method. The experimental conditions used for SO₂ reduction were as follow: the space velocity (GHSV) was 30,000 ml/g_-cat h and the ratio of [CO (or H₂, H₂ + CO)]/[SO₂] was 2.0. It was found that the catalyst and reducing agent providing the best performance were the Ce_0.5Zr_0.5O₂ catalyst and CO, respectively. In this case, the SO₂ conversion was about 92% and the sulfur yield was about 90% at 550 °C. Also, a higher efficiency of SO₂ removal and elemental sulfur recovery was achieved in the reduction of SO₂ with CO as a reducing agent than that with H₂. In the reduction of SO₂ by H₂ over the Ce_0.5Zr_0.5O₂ catalyst, SO₂ conversion and sulfur yield were about 92.7% and 73%, respectively, at 800 °C. Also, the reduction of SO₂ using synthetic gas with various [CO]/[H₂] molar ratios over the Ce_0.5Zr_0.5O₂ catalyst was performed, in order to investigate the possibility of using coal-derived gas as a reducing agent in the DSRP. It was found that the reactivity of the SO₂ reduction using the synthetic gas with various [CO]/[H₂] molar ratios was increased with increasing CO content of the synthetic gas. Therefore, it was found that the Ce_1−xZr_xO₂ catalysts are applicable to the DSRP using coal-derived gas, which contains a larger percentage of CO than H₂.     

Transient analysis of the release and reduction of NOx using a Pt/Ba/Al2O3 catalyst

The release and reduction of NO_x in a NO_x storage-reduction (NSR) catalyst were studied with a transient reaction analysis in the millisecond range, which was made possible by the combination of pulsed injection of gases and time resolved time-of-flight mass spectrometry. After an O₂ pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al₂O₃ catalyst, the time profiles of several gas products, NO, N₂, NH₃ and H₂O, were obtained as a result of the release and reduction of NO_x caused by H₂ injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO₃)₂/cordierite plate, the release and reduction of NO_x on Pt/Ba/Al₂O₃ catalyst that stored NO_x took the following two steps; in the first step NO molecules were released from Ba and in the second step the released NO was reduced into N₂ by H₂ pulse injection. When this H₂ pulse was injected in a large amount, NO was reduced to NH₃ instead of N₂.

A only small amount of H₂O was detected because of the strong affinity for alumina support. We can analyze the NO_x regeneration process to separate two steps of the NO_x release and reduction by a detailed analysis of the time profiles using a two-step reaction model. From the result of the analysis, it is found that the rate constant for NO_x release increased as temperature increase.     

添加剂对秸秆成型燃料燃烧特性的影响

生物质成型燃料作为生物质能源的主要利用方式，优化其燃烧性能具有重要意义。在秸秆成型燃料中添加CaO、Ca(OH)₂、SiO₂、Al₂O₃、尿素、粉煤灰等常见添加剂，研究了其对SO₂、NO_x释放规律的影响。研究表明：提高钙硫比且降低含水量，能够降低SO₂、NO_x释放量。Al₂O₃的催化助燃作用增加了SO₂、NO_x的释放，粉煤灰与秸秆熔融烧结增强了异相脱硝。添加SiO₂的成型燃料促进了钙硅酸盐复合物的生成，从而减少SO₂的释放。通过分析不同SiO₂添加量时的燃烧情况发现，添加量5%时，脱硫脱硝效果最好。在成型燃料燃烧15 s后添加尿素，尿素高温热解产生的还原性气体能够与SO₂、NO_x反应从而减少硫、氮氧化物的释放。温度升高促进熔融反应和异相脱硝反应，阻碍了焦炭中N的燃烧使NO_x释放量大幅降低。     

The low-temperature performance of NOx storage and reduction catalyst

The catalytic performance and the behavior of NO_x storage and reduction (NSR) over a model catalyst for lean-burn gasoline engines have been mainly investigated and be discussed based on the temperature and reducing agents use in this study. The experimental results have shown that the NO_x storage amount in the lean atmosphere was the same as the NO_x reduction amount from the subsequent rich spike (RS) above the temperature of 400 °C, while the former was greater than the latter below the temperature of 400 °C. This indicated that when the temperature was below 400 °C compared with the NO_x storage stage, the reduction of the stored NO_x is somehow restricted. We found that the reduction efficiencies with the reducing agents decrease in the order H₂ > CO > C₃H₆ below 400 °C, thus not all of the NO_x storage sites could be fully regenerated even using an excessive reducing agent of CO or C₃H₆, which was supplied to the NSR catalyst, while all the NO_x storage sites could be fully regenerated if an adequate amount of H₂ was supplied. We also verified that the H₂ generation more favorably occurred through the water gas shift reaction than through the steam reforming reaction. This difference in the H₂ generation could reasonably explain why CO was more efficient for the reduction of the stored NO_x than C₃H₆, and hinted as a promising approach to enhance the low-temperature performance of the current NSR catalysts though promoting the H₂ generation reaction.     

Transient behaviour of catalytic monolith with NOx storage capacity

Transient behaviour of catalytic monolith converter with NO_x storage is studied under conditions typical for automobiles with lean-burn engines (i.e., diesel and advanced gasoline ones). Periodical alternation of inlet concentrations is applied—NO_x are adsorbed on the catalyst surface during a long reductant-lean phase (2–3 min) and then reduced to N₂ within a short reductant-rich phase (2–6 s). Samples of industrial NO_x storage and reduction catalyst of NM/Ba/CeO₂/γ-Al₂O₃ type (NM = noble metal), washcoated on 400 cpsi cordierite substrate, are used in the study. Effects of the rich-phase length and composition on the overall NO_x conversions are examined experimentally. Reduction of NO_x by CO, H₂ and unburned hydrocarbons (represented by C₃H₆) in the presence of CO₂ and H₂O is considered.

Effective, spatially 1D, heterogeneous mathematical model of catalytic monolith with NO_x and oxygen storage capacity is described. The minimum set of experiments needed for the evaluation of relevant reaction kinetic parameters is discussed: (i) CO, H₂ and HC oxidation light-off under both lean and rich conditions, including inhibition effects, (ii) NO/NO₂ transformation, (iii) NO_x storage, including temperature dependence of effective NO_x storage capacity, (iv) water gas shift and steam reforming under rich conditions, i.e., in situ production of hydrogen, (v) oxygen storage and reduction, including temperature dependence of effective oxygen storage capacity, and (vi) NO_x desorption and reduction under rich conditions. The experimental data are compared with the simulation results.     

Engineering practice and economic analysis of ozone oxidation wet denitrification technology

SO₂ and NO emitted from coal-fired power plants have caused serious air pollution in China. In this study, a test system for NO oxidation using O₃ is established. The basic characteristics of NO oxidation and products forms are studied. A separate test system for the combined removal of SO₂ and NO_x is also established, and the absorption characteristics of NO_x are studied. The characteristics of NO oxidation and NO_x absorption were verified in a 35 t·h^-1 industrial boiler wet combined desulfurization and denitrification project. The operating economy of ozone oxidation wet denitrification technology is analyzed. The results show that O₃ has a high rate and strong selectivity for NO oxidation. When O₃ is insufficient, the primary oxidation product is NO₂. When O₃ is present in excess, NO₂ continues to get oxidized to N₂O₅ or NO₃. The removal efficiency of NO₂ in alkaline absorption system is low (only about 15%). NO_x removal efficiency can be improved by oxidizing NO_x to N₂O₅ or NO₃ by increasing ozone ratio. When the molar ratio of O₃/NO is 1.77, the NO_x removal efficiency reaches 90.3%, while the operating cost of removing NO_x per kilogram is 6.06 USD (NO₂).