SO2 removal from flue gas by activated semi-cokes: 1. The preparation of catalysts and determination of operating conditions

The objective of this work was to use waste semi-coke as the raw material to prepare catalysts of industrial-scale size for SO₂ removal from flue gas and to find the optimal preparation methods. Results showed that lignite semi-coke was a suitable raw material, and that the catalyst, prepared by pre-activating in an autoclave, oxidizing with HNO₃, loading with CuSO₄ and finally calcining at 700 °C, exhibited the best desulfurizing property with a sulfur retention of about 9.6% SO₂/100 gC at a reaction temperature of 90 °C. Also, the effects of H₂O content in the flue gas, reaction temperature and space velocity on the desulfurizing property were investigated to determine optimum operating conditions. An H₂O content of 7% was appropriate for catalysts in this work. In the temperature range 80-120 °C, the catalyst showed good performance for SO₂ removal and was gradually deactivated at temperatures above 120 °C. Space velocity exhibited an optimal value of 830 h⁻¹. The kinetic behavior varied with space velocity and the desulfurizing property was controlled by diffusion at space velocities below 830 h⁻¹, and controlled by adsorption or catalytic reaction at space velocities above 830 h⁻¹.     

Supercapacitive properties of a nanowire-structured MnO2 electrode in the gel electrolyte containing silica

Nanowire-structured MnO₂ active materials were prepared by a chemical precipitation method and their supercapacitive properties for use in the electrodes of supercapacitors were investigated by means of cyclic voltammetry in an aqueous gel electrolytes consisting of 1 M Na₂SO₄ and fumed silica (SiO₂). The MnO₂ electrode showed a maximum specific capacitance of 151 F g⁻¹ after 1000 cycles at 100 mV s⁻¹ when using the gel electrolyte containing 3 wt.% of SiO₂, which is higher than 121 F g⁻¹ obtained when using the 1 M Na₂SO₄ liquid electrolyte alone.     

Formation of N-containing gas-phase species from char gasification in steam

The formation of N-containing products during char-steam gasification has been investigated in a laboratory scale fixed bed reactor. Experiments were conducted at 1000 °C, 0.1-1.0 MPa, and 6-46% of H₂O in He base flow. Two very different coal chars, which were prepared from the rapid heating of Australian bituminous and sub-bituminous coals, were studied. The nitrogen-containing products released during the gasification were measured using an FTIR spectrometer (NH₃, HCN and HNCO) and gas chromatography (N₂). The major N-containing products formed during char-steam gasification are NH₃, HCN and N₂. Reactions of HCN in the same reactor were also studied; these experiments were conducted with HCN alone, HCN/steam, and HCN/steam/char. The results are consistent with a mechanism in which HCN is the primary N-containing product of the char-steam reaction, and the additional products result from further reactions of HCN either in the gas phase or promoted by the surface of the reactor or the char. Increasing concentrations of steam significantly influence the distribution of char-N to N-containing gas-phase products, resulting in the increase of NH₃ at the expense of N₂. Some differences in char behaviour are also observed, particularly on the distribution of N-containing products at 0.1 MPa total pressure.     

Full-scale measurements of SO2 gas phase concentrations and slurry compositions in a wet flue gas desulphurisation spray absorber

Two measurement campaigns were carried out at ENERGI E2's Asnæs Power plant, unit 5. The unit has a capacity of 620 MW_e and is equipped with a wet flue gas desulphurisation (FGD) plant employing a counter-current spray absorber with five spray levels. In the first campaign, the power plant was firing Orimulsion^® with 2.85 wt% S resulting in a flue gas concentration of SO₂ exceeding 2000 ppmv. In the second campaign, the fuel applied was a low-S blended coal and the SO₂ concentration in the raw gas was around 400 ppmv. A novel probe for in situ sampling of gas phase concentrations in wet FGD spray absorbers was developed and applied for measuring axial profiles of the SO₂ gas phase concentrations in the absorber. The expected decrease in SO₂ concentrations along the height of the absorber was found in the spray section (from height 26.5 to 36.2 m) whereas the SO₂ concentration above the holding tank and below the gas inlet was quite low probably due to long local residence times in the region. Horizontal variations, due to somewhat different flow conditions near the column wall were investigated and the SO₂ concentrations were found to be higher near the wall. Measurements at different gross loads showed that the SO₂ gas phase concentration at a given position inside the absorber was roughly linearly related to the L/G ratio in the measuring interval. Turning off one of the lower spray levels, while burning coal with low S content, did not lower the overall removal efficiency of the absorber. However, the SO₂ gas phase concentration inside the lower part of the absorber was increased by a factor of 2-3. Measurements of slurry pH at different positions showed a decrease of approximately 0.5 units from the upper to the lower part of the absorber. The full-scale measurements provide a detailed set of experimental data for validation of mathematical models of a wet FGD spray absorber.     

Experimental study of wet flue gas desulphurization with a novel type PCF device

Scrubbers are being widely used to remove the dust, sulphur dioxide and other harmful gases from coal-fired boilers. In this paper, a novel ‘wet-type’ desulphurization absorber, the PCF device (Chinese LOGO), was developed and studied through an experimental method. The mixture of air and SO₂ was used as simulated flue gas and CaCO₃-in-water suspension was used as absorbent. The results show that the PCF device has a good overall performance for FGD. Under moderate conditions employed, the content of SO₂ in outlet flue gas can achieve a level much lower than that permitted, while the pressure drop is very small due to co-flows in preliminarily treating chamber and no venturi structure in inlet tube. Guide plates and self-excitation chamber can improve the SO₂ removal efficiency by intensifying the mass-transfer and second purification. Some feasible process parameters are as follows: slurry pH value = 5.6-6.0, liquid-gas ratio = 8.7-10.4 L/m³, superficial gas velocity in inner cylinder = 3.5-4.5 m/s, and addition of Cl⁻ (in the form of CaCl₂) to the slurry (25 g/L) decreased the degree of SO₂ removal about 13.12%.     

TiO2 nanotube arrays annealed in CO exhibiting high performance for lithium ion intercalation

Anatase titania nanotube arrays were fabricated by means of anodization of Ti foil and annealed at 400 °C in respective CO and N₂ gases for 3 h. Electrochemical impendence spectroscopy study showed that CO annealed arrays possessed a noticeably lower charge-transfer resistance as compared with arrays annealed in N₂ gas under otherwise the same conditions. TiO₂ nanotube arrays annealed in CO possessed much improved lithium ion intercalation capacity and rate capability than N₂ annealed samples. At a high charge/discharge current density of 320 mA g⁻¹, the initial discharge capacity in CO annealed arrays was found to be as high as 223 mAh g⁻¹, 30% higher than N₂ annealed arrays, ∼164 mAh g⁻¹. After 50 charge/discharge cycles, the discharge capacity in CO annealed arrays remained at ∼179 mAh g⁻¹. The improved intercalation capacity and rate capability could be attributed to the presence of surface defects like Ti-C species and Ti³⁺ groups with oxygen vacancies, which not only improved the charge-transfer conductivity of the arrays but also possibly promoted phase transition.     

Nanosized α-LiFeO2 as electrochemical supercapacitor electrode in neutral sulfate electrolytes

In this work we have explored the electrochemical properties of two lithiated iron oxide powders for supercapacitor purposes. These samples mainly consisted of α-LiFeO₂ in nanosized or micrometric form. Electrolyte was an aqueous 0.5 M Li₂SO₄ solution and voltage range studied was between 0 and −0.7 V vs. a Ag/AgCl reference electrode. As expected, electrochemical performance was dependent on the particle size. When electrolyte was deaerated a stable capacitance of ≈50 F g⁻¹ is provided by the nanosized sample for several hundred cycles. Other sulfate based salts (Na₂SO₄, K₂SO₄, Cs₂SO₄) were investigated as electrolytes but only Li₂SO₄ leads to a stable capacitance upon cycling, probably due to lithium intercalation. An hybrid cell consisting of this sample and MnO₂ as negative and positive electrodes, respectively, delivered 0.3 F cm⁻² (10 F g⁻¹). Although these values are lower than reported for other aqueous hybrid cell, α-LiFeO₂/MnO₂ asymmetric capacitor is interesting from both, an economic and an environmental point of view.     

Kinetic relationship between NO/N2O reduction and O2 consumption during flue-gas recycling coal combustion in a bubbling fluidized-bed

Flue-gas recycling combustion of a sub-bituminous coal and its rapid pyrolysis char at 1120 K has been simulated experimentally in a bubbling fluidized-bed. O₂, CO₂ and H₂O, and NO or N₂O were pre-mixed and fed into the bed together with coal/char particles with the O₂ concentration in the exit gas maintained at 3.5 vol%. Increasing the inlet O₂ concentration, thus increasing the O₂ consumption rate and decreasing the flue-gas recycling ratio, caused the once-through conversion of fuel-bound nitrogen into N₂O to decrease while the conversion to NO to remain unchanged. The in-bed reductions of NO and N₂O were both first order with respect to the respective nitrogen oxide, with the rate constants to increase linearly with the rate of O₂ consumption in the bed and thus also with that of char/volatiles consumption. This finding, which indicated linear increase in the concentrations of reactive species involved in NO/N₂O reduction with the rate of O₂ consumption, enabled consideration that the homogeneous and heterogeneous reduction rates of NO and N₂O were proportional to the consumption rates of O₂ by the volatiles and char, respectively. The rate analysis of the kinetic data revealed the relative importance of burning volatiles and char as the agents for the reduction of NO and N₂O. While the reduction in the gas phase was fully responsible for the NO-to-N₂O conversion, the reactions over the char surface governed the NO-to-N₂ reduction. The volatiles and char had comparable contributions to the reduction of N₂O to N₂. The NO-to-N₂ and N₂O-to-N₂ reductions over the char surface were, respectively, accelerated and decelerated by increasing the H₂O concentration.     

Gasification reaction kinetics on biomass char obtained as a by-product of gasification in an entrained-flow gasifier with steam and oxygen at 900-1000 °C

The purpose of this study was to investigate the gasification kinetics of biomass char, such as the wood portion of Japanese cedar char (JC), Japanese cedar bark char (JB), a mixture of hardwood char (MH) and Japanese lawngrass char (JL), each of which was obtained as a by-product of gasification in an entrained-flow type gasifier with steam and oxygen at 900-1000 °C. Biomass char was gasified in a drop tube furnace (DTF), in which gasification conditions such as temperature (T_g), gasifying agent (CO₂ or H₂O), and its partial pressure (P_g) were controlled over a wide range, with accompanying measurement of gasification properties such as gasification reaction ratio (X), gasification reaction rate (R_g), change of particle size and change of surface area. Surfaces were also observed with a scanning electric microscope (SEM). By analyzing various relationships, we concluded that the random pore model was the most suitable for the biomass char gasification reaction because of surface porosity, constant particle size and specific surface area profile, as well as the coincidence of R_g, as experimentally obtained from Arrhenius expression, and the value is calculated using the random pore model. The order of R_g was from 10⁻² to 10⁻¹ s⁻¹, when T_g = 1000 °C and P_g = 0.05 MPa, and was proportional to the power of P_g in the range of 0.2-0.22 regardless of gasifying agent. Reactivity order was MH > JC > (JB, JL) and was roughly dependent on the concentration of alkali metals in biomass feedstock ash and the O/C (the molar ratio of oxygen to carbon) in biomass char.     

Modelling of NO and N2O emissions from biomass-fired circulating fluidized bed combustors

A model for NO and N₂O emissions from biomass-fired circulating fluidized bed (CFB) combustors has been developed and evaluated in this study. All the model parameters were chosen for a typical woody biomass-pinewood chips. Both drying and devolatilization of biomass particles were modelled with limited rates, which were selected from the literature based on woody biomass fuels. The partition of fuel-nitrogen between volatiles and char was also specifically chosen for pinewood based on available experimental data from the literature. Volatile nitrogen was assumed to consist of NH₃, HCN and N₂ with the distribution between three species as input parameters to the model. Twenty-five homogenous and heterogeneous global chemical reactions were included in the model, of which 20 reactions represents the global fuel-nitrogen reactions. Both gaseous and solid phase were assumed to be in plug flow. The model has been applied to the modelling of a 12 MW_th CFB boiler. The predicted N₂O emissions were always less than 5 ppmv for pinewood combustion, which was consistent with the experimental results. The predicted NO emissions increased with the total excess air of the riser and the fuel-N content while the predicted percentage conversion of fuel-N to NO decreased with increasing fuel-N content. The NO emissions were also predicted to decrease with increasing primary zone stoichiometry. These predictions agree with the experimental results. The predicted NO emissions decreased slightly with increasing bed temperature, whereas experiments showed that NO emissions slightly increased with bed temperature for birch chips combustion and did not change with bed temperature for fir chips combustion. Sensitivity analyses reveal that the reaction between NO and char is the key reaction to determine the NO emissions.     

Removal of elemental mercury from simulated coal-combustion flue gas using a SiO2–TiO2 nanocomposite

A novel silica–titania (SiO₂–TiO₂) nanocomposite has been developed to effectively capture elemental mercury (Hg⁰) under UV irradiation. Previous studies under room conditions showed over 99% Hg⁰ removal efficiency using this nanocomposite. In this work, the performance of the nanocomposite on Hg⁰ removal was tested in simulated coal-fired power plant flue gas, where water vapor concentration is much higher and various acid gases, such as HCl, SO₂, and NO_x, are present. Experiments were carried out in a fix-bed reactor operated at 135 °C with a baseline gas mixture containing 4% O₂, 12% CO₂, and 8% H₂O balanced with N₂. Results of Hg speciation data at the reactor outlet demonstrated that Hg⁰ was photocatalytically oxidized and captured on the nanocomposite. The removal efficiency of Hg⁰ was found to be significantly affected by the flue gas components. Increased water vapor concentration inhibited Hg⁰ capture, due to the competitive adsorption of water vapor. Both HCl and SO₂ promoted the oxidation of Hg⁰ to Hg(II), resulting in higher removal efficiencies. NO was found to have a dramatic inhibitory effect on Hg⁰ removal, very likely due to the scavenging of hydroxyl radicals by NO. The effect of NO₂ was found to be insignificant. Hg removal in flue gases simulating low rank coal combustion products was found to be less than that from high rank coals, possibly due to the higher H₂O concentration and lower HCl and SO₂ concentrations of the low rank coals. It is essential, however, to minimize the adverse effect of NO to improve the overall performance of the SiO₂–TiO₂ nanocomposite.     

Effect of pore size distribution of coal-based activated carbons on double layer capacitance

A series of coal-based activated carbons representing a wide range of mesopore content, from 16.7 to 86.9%, were investigated as an electrode in electric double layer capacitors (EDLCs) in 1 mol l⁻¹ H₂SO₄ and 6 mol l⁻¹ KOH electrolytic solutions. The activated carbons (ACs) used in this study were produced from chemically modified lignite, subbituminous and bituminous coals by carbonization and subsequent activation with steam. The BET surface area of ACs studied ranged from 340 to 1270 m² g⁻¹. The performance of ACs as EDLC electrodes was characterized using voltammetry, galvanostatic charge/discharge and impedance spectroscopy measurements. For the carbons with surface area up to 1000 m² g⁻¹, the higher BET surface area the higher specific capacitance (F g⁻¹) for both electrolytes. The surface capacitance (μF cm⁻²) increases also with the mesopore content. The optimum range of mesopore content in terms of the use of ACs studied for EDLCs was found to be between 20 and 50%. A maximum capacitance exceeding 160 F g⁻¹ and a relatively high surface capacitance about 16 μF cm⁻² measured in H₂SO₄ solution were achieved for the AC prepared from a sulfonated subbituminous coal. This study shows that the ACs produced from coals exhibit a better performance as an electrode material of EDLC in H₂SO₄ than in KOH electrolytic solutions. For KOH, the capacitance per unit mesopore surface is slightly lower than that referred to unit micropore surface (9.1 versus 10.1 μF cm⁻²). However, in the case of H₂SO₄ the former capacitance is double and even higher compared with the latter (23.1 versus 9.8 μF cm⁻²). On the other hand, the capacitance per micropore surface area is the same in both electrolytes used, about 10.0 μF cm⁻².     

High energy syngas production by waste tyres steam gasification in a rotary kiln pilot plant. Experimental and numerical investigations

This paper presents experimental and numerical results on steam gasification of waste tyres in a rotary kiln pilot plant. Both the process performance and the gas features have been evaluated varying the feeding ratio (FR), defined as the steam/tyres mass ratio. First, several experimental tests have been performed. Then, the obtained experimental results have been used to verify the consistency of a numerical model developed with the aid of the commercial code ChemCAD®. Once done, the effect of increasing the FR on the gas energy content has been evaluated.Numerical results showed that the gas energy content increases as the FR increases as well, achieving a maximum value for FR = 0.33 that produced a gas which composition N₂ free is (H₂ = 52.7%_vol, CO = 18.1%_vol, CO₂ = 7.0%_vol, CH₄ = 22.2%_vol) in correspondence of which the lower heating value (LHV) is equal to 29.5 MJ kg_gas⁻¹. Higher FR values do not produce a further increase of the gas energy content, rather require a greater amount of input energy for heating the steam from the atmospheric to the process temperature.     

Investigation of flue-gas treatment with O3 injection using NO and NO2 planar laser-induced fluorescence

Direct ozone (O₃) injection is a promising flue-gas treatment technology based on oxidation of NO and Hg into soluble species like NO₂, NO₃, N₂O₅, oxidized mercury, etc. These product gases are then effectively removed from the flue gases with the wet flue gas desulfurization system for SO₂. The kinetics and mixing behaviors of the oxidation process are important phenomena in development of practical applications. In this work, planar laser-induced fluorescence (PLIF) of NO and NO₂ was utilized to investigate the reaction structures between a turbulent O₃ jet (dry air with 2000 ppm O₃) and a laminar co-flow of simulated flue gas (containing 200 ppm NO), prepared in co-axial tubes. The shape of the reaction zone and the NO conversion rate along with the downstream length were determined from the NO-PLIF measurements. About 62% of NO was oxidized at 15d (d, jet orifice diameter) by a 30 m/s O₃ jet with an influence width of about 6d in radius. The NO₂ PLIF results support the conclusions deduced from the NO-PLIF measurements.     

Removal of NO and SO2 by pulsed corona discharge process

Overall examination was made on the removal of NO and SO₂, by pulsed corona discharge process. The mechanism for the removal of NO was found to largely depend on the gas composition. In the absence of oxygen, most of the NO removed was reduced to N₂; on the other hand, oxidation of NO to NO₂ was dominant in the presence of oxygen even when the content was low. Water vapor was an important ingredient for the oxidation of NO₂, to nitric acid rather than that of NO to NO₂. The removal of NO only slightly increased with the concentration of ammonia while the effect of ammonia on the removal of SO₂ was very significant. The energy density (power delivered/feed gas flow rate) can be a measure for the degree of removal of NO. Regardless of the applied voltage and the flow rate of the feed gas stream, the amount of NO removed was identical at the same energy density. The production of N₂O increased with the pulse repetition rate, and the presence of NH₃ and SO₂ enhanced it. Byproducts generated from propene used as additive were identified and analyzed. The main byproducts other than carbon oxides were found to be ethane and formaldehyde, but their concentrations were negligibly small.     

Electrocatalytic reduction of bromate ion using a polyaniline-modified electrode: An efficient and green technology for the removal of BrO3 in aqueous solutions

The electrocatalytic reduction of bromate ion (BrO₃⁻) was investigated in a three-electrode system using polyaniline (PANI) as the electrode material. Bromate ion reduction and Br⁻ removal were observed during electrochemical treatment because of the catalytic and doping capabilities of the PANI film. BrO₃⁻ removal efficiency in the 0.10 mol L⁻¹ Na₂SO₄ supporting electrolyte achieved 99% at pH 7 in 25 min, with no bromide ion detected in the solution. Optimal removal was found in pH range 6-7, and the pH of the solution had a significant impact on bromate reduction. A reduction mechanism was also discussed by analyzing the cyclic voltammograms of the reduction process and X-ray photoelectron spectra of the main elements (N 1s and Br 3d) on the PANI surface. We propose that during the electrocatalytic reduction process, bromate is reduced to bromide because of the loss of electrons from the nitrogen atoms on the PANI chains. The doping of the resultant Br⁻ ions in the PANI film has an important role in avoiding further oxidation of Br⁻ to BrO₃⁻. The used PANI film can be regenerated by de-doping the Br⁻ ions with a 0.5 mol L⁻¹ H₂SO₄ solution. Thus the process can be considered efficient and green.     

NO reduction with H2 in the presence of excess O2 over Pd/MFI catalyst

The NO SCR (selective catalytic reduction) activity with H₂ in the presence of excess O₂ was investigated over Pd/MFI catalyst prepared by sublimation method. With GHSV=90?000 h⁻¹, a very high steady-state conversion of NO to N₂ (70%) is achieved at 100 °C. Significant reorganizations take place inside the catalyst upon its first contact with all reactants and products at the reaction temperature. Pd⁰, which has a significant role in the NO-H₂-O₂ reaction, is possibly the active site for NO reduction. The formation of Pd-β hydride deactivates the catalyst for NO reduction. Throughout the entire NO-H₂-O₂ reaction, no N₂O or NO₂ is formed; N₂ is the only N-containing product. The presence of O₂ inhibits the formation of undesirable NH₃. The rate of the NO+H₂ reaction is fast or comparable to that of the H₂+O₂ reaction. The oxidation of Pd⁰ and subsequent agglomeration of PdO are responsible for the decreased NO reduction activity at high temperature.     

Comparison of product selectivity during hydroprocessing of bitumen derived gas oil in the presence of NiMo/Al2O3 catalyst containing boron and phosphorus

A detailed experimental study was performed in a trickle-bed reactor using bitumen derived gas oil. The objective of this work was to compare the activity of NiMo/Al₂O₃ catalyst containing boron or phosphorus for the hydrotreating and mild hydrocracking of bitumen derived gas oil. Experiments were performed at the temperature and LHSV of 340-420 °C and 0.5-2 h⁻¹, respectively, using NiMo/Al₂O₃ catalysts containing 1.7 wt% boron or 2.7 wt% phosphorus. In the temperature range of 340-390 °C, higher nitrogen conversion was observed from boron containing catalyst than that from phosphorus containing catalyst whereas in the same temperature range, phosphorus containing catalyst gave higher relative removal of sulfur than boron containing catalyst. Phosphorus containing catalyst showed excellent hydrocracking and mild hydrocracking activities at all operating conditions. Higher naphtha yield and selectivity were obtained using phosphorus containing catalyst at all operating conditions. Maximum gasoline selectivity of ∼45 wt% was obtained at the temperature, pressure, and LHSV of 400 °C, 9.4 MPa and 0.5 h⁻¹, respectively, using catalyst containing 2.7 wt% phosphorus.     

Characterization of Chinese, American and Estonian oil shale semicokes and their sorptive potential

The primary byproduct of current oil shale oil extraction processes is semicoke. Its landfill deposition presents a potential threat to the environment and represents a waste of a potentially useable byproduct. Here we examine the sorptive characteristics of oil shale semicoke. Oil shale samples from Estonia, China and the United States were pyrolyzed at 500 and 1000 °C and their products analyzed for organic char content, surface area and porosity. Pyrolysis of the oil shales at temperatures of 500-1000 °C yields semicokes with organic char contents from 1.7% to 17.5% and BET surface areas of 4.4-57 m² g⁻¹, corresponding to 100-550 m² g⁻¹ of organic char. For comparison, the BET surface areas of class F coal fly ashes (combustion byproducts of bituminous coals) typically range from 2 to 5 m² g⁻¹, corresponding to 30-60 m² g⁻¹ of carbon while class C fly ash (from low rank coals) have carbon BET surface areas comparable to oil shale semicoke organic char surface areas.     

The effect of steam on pyrolysis and char reactions behavior during rice straw gasification

Steam gasification of biomass can generate hydrogen-rich, medium heating value gas. We investigated pyrolysis and char reaction behavior during biomass gasification in detail to clarify the effect of steam presence. Rice straw was gasified in a laboratory scale, batch-type gasification reactor. Time-series data for the yields and compositions of gas, tar and char were examined under inert and steam atmosphere at the temperature range of 873-1173 K. Obtained experimental results were categorized into those of pyrolysis stage and char reaction stage. At the pyrolysis stage, low H₂, CO and aromatic tar yields were observed under steam atmosphere while total tar yield increased by steam. This result can be interpreted as the dominant, but incomplete steam reforming reactions of primary tar under steam atmosphere. During the char reaction stage, only H₂ and CO₂ were detected, which were originated from carbonization of char and char gasification with steam (C + H₂O→CO + H₂). It implies the catalytic effect of char on the water-gas shift reaction. Acceleration of char carbonization by steam was implied by faster hydrogen loss from solid residue.