Emission of trace toxic metals during pulverized fuel combustion of Czech coals

A study of the trace elements emission (As, Se, Cd, Co, Cr, Cu, Zn, Hg, Tl, Pb, Ni, Sn, Sb, V, Mn and Fe) from pulverized coal combustion has been made at six heating and power stations situated in the Czech Republic. The amount of chlorine in coal has considerable influence on volatilization of some elements such as Zn, Cu, Pb, Hg and Tl, which is explained by the formation of thermodynamically stable compounds of these elements with chlorine. Generally, the affinities for Cl follows the order Tl > Cu > Zn > Pb > Co > Mn > Sn > Hg. The experimental data indicates enrichment of some of the trace toxic elements in the emissions (Cu, Zn, As, Se, Cd, Sn, Sb, Hg and Pb) and good agreement was obtained by thermodynamic equilibrium calculations with a few exceptions. In the case of Fe, Mn, Co, Cr and Sn calculated values are overestimated in the bottom ash and there are zero predicted amounts of these elements in the fly ash. In comparison, the results from experiments show up to 80% of these elements retained in fly ash. This implies that there exist additional steps leading to the enrichment by Fe, Mn, Co, Cr and Sn of small particles. Such mechanisms could include the ejection during devolatilization of small inorganic particles from the coal of bottom ash particles, or disintegration of the char containing these metals to small particles of fly ash. On the other hand, there are slightly overestimated or similar values of relative enrichment factors for As, V, Cu, Cd, Sb, Tl and Pb in the fly ashes and zero predicted values for bottom ashes. Our experimental results show about 5% or less of these elements are retained in bottom ashes, so they probably remain in the bottom ash inside unburned parts of coal. Copyright © 2003 John Wiley & Sons, Ltd.     

Influence of CeO2 morphology on WO3/CeO2 catalyzed NO selective catalytic reduction by NH3

WO₃/CeO₂ catalysts with different support morphologies were fabricated by incipient wetness technique and applied to selective catalytic reduction of NO by NH₃ (NH₃-SCR). WO₃/CeO₂ rod (WCR) displayed higher catalytic activity and resistance to SO₂ and H₂O compared with WO₃/CeO₂ polyhedron (WCP) and WO₃/CeO₂ cube (WCC). N₂-BET, XRD, Raman, H₂-TPR, TEM, HRTEM, NH₃-TPD, XPS and in situ DRIFTS were conducted to investigate the physicochemical properties of the catalysts and the adsorption of NH₃ and NO_x species on the catalytic surface. These characterization results demonstrated that the larger BET surface area, the smaller CeO₂ particle size, the higher surface acidity, the more oxygen defects, the better redox performance, and the higher Ce³⁺ and O_α ratios of the catalysts played critical functions in obtaining more outstanding NH₃-SCR catalytic performance. All of these characterization results were also closely related to the CeO₂ morphology. The results of the in situ DRIFTS showed that the WCR had the highest intensities of the adsorbed NO_x and NH₃ species among these three catalysts. The reactions between adsorbed species attributed to NO_x and NH₃ on the catalyst surface can also be a key factor in the NH₃-SCR catalytic performance enhancement.     

CeO2 modified Fe2O3 for the chemical hydrogen storage and production via cyclic water splitting

The chemical hydrogen storage (hydrogen reduction) and production (water splitting) behaviour of Ce-modified Fe₂O₃ mixed oxides were investigated. Fe_1−xCe_xO_2−δ (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 1) oxides prepared by chemical precipitation were characterized by XRD (X-ray diffraction), H₂-TPR (hydrogen temperature-programmed reduction) and H₂O-TPO (steam temperature-programmed oxidation) tests. XRD results showed that two kinds of Fe–Ce–O solid solutions (Ce-based and Fe-based) coexisted in Fe–Ce mixed oxides. H₂-TPR experiment suggested that Ce addition could reduce hydrogen reduction temperature while H₂O-TPO experiments over reduced oxides showed that Fe–Ce mixed oxides could split water to produce hydrogen at a lower temperature and complete in a shorter time. Both redox reactions (hydrogen reduction and water splitting) were sensitive to the temperature and active at a high temperature. The successive redox cycles were carried out over the Fe_0.7Ce_0.3O_2−δ mixed oxide at 750 °C. It kept a stable production of hydrogen in the successive redox process at the condition of serious agglomeration of the materials. The highest hydrogen storage amount was up to 1.51 wt% for the Fe–Ce sample with a 30% substitution of Ce for Fe.     

Factors influencing the transformation of minerals during pulverized coal combustion

The transformation of minerals and dispersed inorganic constitutents during pulverized coal combustion has been examined by burning utility sized coals (70% less than 200 mesh) in a laboratory-scale combustor. Experiments were conducted with four U.S. coals possessing different mineralogies. Size and composition of the initial minerals and the resulting ash were measured by a variety of techniques, including computer controlled SEM, low temperature ashing, deposition on a cascade impactor, and optical (Malvern) particle size analysis. Results for a Kentucky # 11 coal with a large amount of fine, included silicate minerals suggest that coalescence amongst illite, kaolinite, and quartz minerals was the dominant process, with occasional iron incorporation into the resulting glass. Pyrite was found to fragment to a limited extent. Illinois # 6 bituminous coal, possessing a similar mineralogy, yielded similar results. For a Beulah lignite coal containing large pyrite grains, mineral fragmentation was inferred from the data, increasing with increasing oxygen level. A high ash content San Miguel Texas lignite containing zeolite minerals demonstrated little mineral interaction during combustion. Differences in results obtained for the different coals highlighted the importance of understanding individual mineral transformations in predicting the formation and behavior of ash.     

Study of the flue gas characteristics and gasification reaction of pulverized coal combustion in O2/CO2/H2O atmosphere

The impacts of O₂ and H₂O on the combustion characteristics of pulverized coal in O₂/CO₂/H₂O atmosphere were studied. The gasification reaction ratio was calculated from the components of flue gas. The competition exists between C-CO₂ and C-H₂O reactions under rich CO₂ atmosphere. At various H₂O concentrations, the differences were found in generation amounts of CO and H₂ in flue gas. At O₂ concentrations <10%, C-CO₂ reaction decreased while C-H₂O reaction increased with increasing H₂O concentration; while at O₂ concentration >30%, H₂O showed no obvious specific patterns of effects. The gasification ratio was reduced as coal ranks increase.     

Cyclic molecular designed dispersion (CMDD) of Fe2O3 on CeO2 promoted by Au for preferential CO oxidation in hydrogen

A novel cyclic molecular designed dispersion (CMDD) method was employed to uniformly deposit 0.0–6.5 wt% Fe on CeO₂. The gold catalysts (0.0–3.8 wt%) supported on Fe₂O₃-CeO₂ were tested for CO preferential oxidation (PROX). As the CMDD method involved grafting of Fe (acac)₃ onto the surface –OH groups, 2.7 surface –OH/nm² was determined for the CeO₂ by using the Grignard reagent. The performance of CMDD catalysts was compared with the corresponding catalysts prepared by impregnation. The catalysts were characterized by XPS, XRD, TPR, HRTEM-EDX, BET and ICP. The results suggested that Au/Fe₂O₃-CeO₂ was significantly more active and selective than Au/CeO₂. The CMDD-prepared catalysts with 1.4–2.5 wt% iron showed the highest activity and selectivity, especially at temperatures as low as 323 K. The formation of Ce-Fe solid solution for CMDD catalysts promoted the dispersion of both iron and gold. Highly dispersed gold nanoparticles mainly as small as 2.2 nm were observed in HRTEM.     

NOx and H2S formation in the reductive zone of air-staged combustion of pulverized blended coals

Low NO_x combustion of blended coals is widely used in coal-fired boilers in China to control NO_x emission; thus, it is necessary to understand the formation mechanism of NO_x and H₂S during the combustion of blended coals. This paper focused on the investigation of reductive gases in the formation of NO_x and H₂S in the reductive zone of blended coals during combustion. Experiments with Zhundong (ZD) and Commercial (GE) coal and their blends with different mixing ratios were conducted in a drop tube furnace at 1200°C–1400°C with an excessive air ratio of 0.6–1.2. The coal conversion and formation characteristics of CO, H₂S, and NO_x in the fuel-rich zone were carefully studied under different experimental conditions for different blend ratios. Blending ZD into GE was found to increase not only the coal conversion but also the concentrations of CO and H₂S as NO reduction accelerated. Both the CO and H₂S concentrations inblended coal combustion increase with an increase in the combustion temperature and a decrease in the excessive air ratio. Based on accumulated experimental data, one interesting finding was that NO and H₂S from blended coal combustion were almost directly dependent on the CO concentration, and the CO concentration of the blended coal combustion depended on the single char gasification conversion.Thus, CO, NO_x, and H₂S formation characteristics from blended coal combustion can be well predicted by single char gasification kinetics.     

Enhanced sintering resistance of Fe2O3/CeO2 oxygen carrier for chemical looping hydrogen generation using core-shell structure

CeO₂-supported Fe₂O₃ is a satisfactory oxygen carrier for chemical looping hydrogen generation (CLHG). However, the sintering problem restrains its further improvement on redox reactivity and stability. In the present work, a core-shell-structured Fe₂O₃/CeO₂ (labeled Fe₂O₃@CeO₂) oxygen carrier prepared by the sol-gel method was studied in a fixed bed. The effect of the core-shell structure on the sintering resistance and redox performance was investigated with a homogenous composite sample of Fe₂O₃/CeO₂ as a reference. The results showed that the Fe₂O₃@CeO₂ exhibited much higher redox reactivity and stability than the Fe₂O₃/CeO₂ with no CO or CO₂ observed in the generated hydrogen, while the hydrogen yield for Fe₂O₃/CeO₂ decreased with redox cycles due to serious sintering. The satisfactory performance of Fe₂O₃@CeO₂ can be ascribed to its high sintering resistance, since the core-shell structure suppressed the outward migration of Fe cations from the bulk to the surface of the particles. On the other hand, the migration of Fe cations and their subsequent enrichment on the particle surface led to the serious sintering of Fe₂O₃/CeO₂. The crystallite size evolution of Fe₂O₃ and CeO₂ in redox cycles further demonstrated the higher sintering resistance of Fe₂O₃@CeO₂. Further, the particle size distribution (PSD) results indicated the agglomeration of Fe₂O₃/CeO₂ after cycles. In addition, the CeO₂ shell could facilitate the transport of oxygen ions between the iron oxide nanoparticle core and the shell surface. Therefore, the coating of nanoscale Fe₂O₃ with a CeO₂ shell did not reduce the redox reactivity and stability of Fe₂O₃@CeO₂, but rather promoted it, though less oxygen-ionic-conductive CeFeO₃ was generated.     

Effect of different conditions on the combustion interactions of blended coals in O2/CO2 mixtures

The combination of oxy-fuel and blended-coal combustion may be one of these effective methods to both reduce CO₂ emissions and improve energy utilization efficiency in coal-fired power stations. The aim of this study is to investigate oxy-fuel combustion interactions of blended coals under different conditions using a thermo-gravimetric analyzer. The results show that compared with those in an O₂/N₂ mixture, the promotive and inhibitive effect and the comprehensive interactions are considerably weaker in an O₂/CO₂ mixture. In the O₂/CO₂ mixture, both increasing the O₂ concentration and decreasing the particle size result in decreasing the promotive effect but increasing the inhibitive effect and the comprehensive interactions, which increase the non-additive combustion characteristics. Enhancement of the heating rate increases the promotive effect but decreases the inhibitive effect and the comprehensive interactions, which weaken the non-additive combustion characteristics. Of these factors, the effects of the oxygen concentration and heating rate on comprehensive interactions are greater than that of particle size. This study provides useful information for the design and optimization of thermo-chemical conversion systems of coal blends in the O₂/CO₂ atmosphere.     

Experimental study on the formation of ultrafine particulate matters (PMs) during pulverized coal (PC) char combustion in O2/N2 and O2/CO2 atmospheres

Under the force of increasingly strict emission standard of particulate matters (PMs) and serious haze in China, further understanding of the formation of ultrafine PMs during coal combustion is crucial. In this work, the formation characteristics of ultrafine PMs during pulverized coal (PC) char combustion in O₂/N₂ and O₂/CO₂ atmospheres were investigated through a high-temperature drop tube furnace (DTF) with a two-stage dilution sampling system and a 14-stage electrical low pressure impactor (ELPI+). Results showed that in both number-based and mass-based particle size distribution (PSD) the peaks of ultrafine PMs located nearby 0.2 μm under all experimental atmospheres including O₂21%/N₂79%, O₂21%/CO₂79%, O₂27%/CO₂73% and O₂33%/CO₂67% (O21N79, O21C79, O27C73 and O33C67). And the peak values increased with the increase of O₂ concentration in O₂/CO₂ atmospheres because of the high char combustion temperature at elevated O₂ level. However, the mass and number concentration of ultrafine PMs in O₂/CO₂ atmospheres reduced significantly compared with these in O₂/N₂ atmosphere. When O21N79 atmosphere switched to O21C79, O27C73, O33C67 atmosphere, the mass concentration of ultrafine PMs reduced by 80.90%, 76.58% and 14.31%, and the number concentration reduced by 74.16%, 63.17% and 10.50%, respectively. Furthermore, the total mass of the ultrafine PMs was determined by the mass of the ultrafine PMs with larger particle size, whereas the total number of ultrafine PMs was determined by the number of the ultrafine PMs with smaller particle size. In this work, the ultrafine PMs with the aerodynamic size smaller than 0.3 μm were collected on the 1st to 7th impactor surfaces of the ELPI+. Among the ultrafine PMs, the particles on the 7th impactor (PMs with the superior limit of ultrafine PMs size) accounted for more than 94.8% of the total ultrafine PMs by mass, but less than 2.3% of the total ultrafine PMs by number; The particles deposited on the 1st impactor (PMs with the detectable minimum size of ultrafine PMs) accounted for more than 79.6% of the total ultrafine PMs by number, but less than 0.4% of the total ultrafine PMs by mass. In addition, PM_0.1 which accounted for 96% of the total number of the ultrafine PMs was analyzed for chemical composition by Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Results showed that the prominent components of PM_0.1 were Ca, Na, and Si in both O₂/N₂ and O₂/CO₂ atmospheres. However, compared with O₂/N₂ atmosphere, Na, Si, Al, and Fe were more enriched in PM_0.1 in O₂/CO₂ atmospheres.     

One-pot synthesis of Fe2O3/C by urea combustion method as an efficient electrocatalyst for oxygen evolution reaction

The hematite type Fe₂O₃/C catalyst for oxygen evolution reaction (OER) was prepared using a facile urea combustion method. The Fe₂O₃/C electrode showed excellent electrocatalytic ability towards OER in alkaline medium. The phase and morphology of the product were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The sintering temperature is 600 °C, and the calcination time is 3 h, which is the best preparation process condition. The particles were irregular spherical with the size of 10–30 nm and dispersed uniformly. The current density of the Fe₂O₃/C electrode was 147 mA cm⁻² at 0.6 V (vs. HgO/Hg) in 6 mol L⁻¹ KOH electrolyte at room temperature.     

Numerical simulation of particle size distribution evolution during pulverized coal combustion

Two numerical simulations of particle size distribution (PSD) evolution during ash-free char combustion are presented to help determine the sensitivity of measured coal PSD evolution to fragmentation. The first simulation is based on percolation theory, and it builds the PSD evolution from an ensemble of individual particle size time histories. The second simulation is population balance that operates on the entire distribution as a unit. Inputs to the simulations come from experimental data available in the literature, and results of the simulations are discussed in conjunction with these data.     

Comparative study of semi-industrial-scale flames of pulverized coals and biomass

Three p.f. flames have been studied in a semi-industrial furnace, using different fuels: a bituminous coal, a lignite, and a biomass (oak sawdust). The operating conditions were exactly the same for the two coals, and very similar to those for the biomass flame. The objective of the study was to evaluate the impact of differences in fuel composition on flame characteristics, through measurement of the spatial distribution of the main parameters: temperature and concentrations of O₂, CO, NO_x, unburnt hydrocarbons, and N₂O. The higher volatiles content in the lignite leads to higher temperatures and more intense combustion than the bituminous coal. Nevertheless, as might be expected, more marked differences are observed between the flames from the biomass and coals. The much higher volatiles content of the wood results in a more intense flame close to the burner, as indicated by visual observations and by concentrations of unburnt gases (CO and unburnt hydrocarbons) in that zone. It is remarkable that the combustion zone extends further for the biomass; while unburnt species were very low for the coals at an axial distance of 1 m, high values were detected for the pulverized oak. The measurements suggest that two stages can be distinguished in the biomass flame: a zone of intense combustion close to the burner, followed by a second region where the large biomass particles gradually devolatilize and are consumed.     

Hydrogen production via chemical looping steam reforming of ethanol by Ni-based oxygen carriers supported on CeO2 and La2O3 promoted Al2O3

This work studies the effects of Ce⁴⁺ and/or La³⁺ on NiO/Al₂O₃ oxygen carrier (OC) on chemical looping steam reforming of ethanol for hydrogen production - alternating between fuel feed step (FFS) and air feed step (AFS). Suitable amount of Ce- and La-doping increases OC carbon tolerance. The solubility limit is found at 50 mol% La in solid solution. At higher La-doping, La₂O₃ disperses on the surface and adsorbs CO₂ forming La₂O₂CO₃ during FFS. From the 1st cycle, 12.5 wt%Ni/7 wt%La₂O₃-3wt%CeO₂–Al₂O₃ (N/7LCA) displays the highest averaged H₂ yield (3.2 mol/mol ethanol) with 87% ethanol conversion. However, after the 5th cycle, 12.5 wt%Ni/3 wt%La₂O₃-7wt%CeO₂–Al₂O₃ (N/3LCA) exhibits more stability and presents the highest ethanol conversion (88%) and H₂ yield (2.7 mol/mol ethanol). Amorphous coke on the OCs decreases with increasing La³⁺ content and can be removed at 500 °C during AFS; nevertheless, fibrous coke and La₂O₂CO₃ cannot be eliminated. Therefore, after multiple redox cycles, highly La-doped OCs exhibits rather low stability.     

Enhanced electrocatalytic methanol oxidation properties by photo-assisted Fe2O3 nanoplates

In this paper, visible-light-driven two-dimensional (2D) Fe₂O₃ nanoplates with exposed (001) facets were first adopted to act as Pt support for photo-assisted electrocatalytic methanol oxidation. Under simulated solar light and visible light illumination Pt-2D Fe₂O₃ nanoplates displayed 2.32 and 1.30 times higher electrocatalytic activities for methanol oxidation than in dark condition, respectively. Besides, 2D Fe₂O₃ nanoplates owns much better electrocatalytic activities for methanol oxidation than Fe₂O₃ particles whether in dark, visible light or simulated solar light illumination. The nice photo-assisted electrocatalytic methanol oxidation activities of Pt-2D Fe₂O₃ nanoplates can be attributed to 2D structure enhanced the oxidation activities of photogenerated holes to oxidize OH⁻ to hydroxyl radical (·OH) as well as its large specific surface area. Our experimental results suggest that photo-assisted and two-dimensional strategy of semiconductors are promising ways to further improve the electrocatalytic activities for methanol oxidation in DMFCs.     

Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production

3%Ce- and 3%La-promoted 10%Cu/Al₂O₃ catalysts were synthesized via a sequential incipient wetness impregnation approach and implemented for ethanol CO₂ reforming (ECR) at 948–1023 K and stoichiometric feed ratio. CeO₂ and La₂O₃ promoters reduced CuO crystallite size from 32.4 to 27.4 nm due to diluting impact and enhanced the degree of reduction of CuO → Cu⁰. Irrespective of reaction temperature, 3%La–10%Cu/Al₂O₃ exhibited the highest reactant conversions, H₂ and CO yields followed by 3%Ce–10%Cu/Al₂O₃ and 10%Cu/Al₂O₃. The greatest C₂H₅OH and CO₂ conversions of 87.6% and 55.1%, respectively were observed on 3%La–10%Cu/Al₂O₃ at 1023 K whereas for all catalysts, H₂/CO ratios varying from 1.46 to 1.91 were preferred as feedstocks for Fischer-Tropsch synthesis. Activation energy for C₂H₅OH consumption was also reduced with promoter addition from 53.29 to 47.05 kJ mol⁻¹. The thorough CuO → Cu⁰ reduction by H₂ activation was evident and the Cu⁰ active phase was resistant to re-oxidation during ECR for all samples. Promoters addition reduced considerably the total carbon deposition from 40.04% to 27.55% and greatly suppressed non-active graphite formation from 26.94% to 4.20% because of their basic character and cycling redox enhancement.     

On the survivability and pyrosynthesis of PAH during combustion of pulverized coal and tire crumb

Results are presented on the emissions of semivolatile polycyclic aromatic hydrocarbons (PAH) from the combustion of a pulverized bituminous coal and ground waste automobile tires. Streams of fuel particles were injected at steady-state steady-flow conditions, and burned inside an isothermal drop-tube furnace, in air, at a gas temperature and gas residence time of 1150°C and 0.75 s, respectively. Combustion occurred under either very fuel-lean conditions (bulk equivalence ratio, φ < 0.5) or substantially fuel-rich conditions (φ = 1.6–1.9). Emissions from fuel pyrolysis, in the absence of oxygen, were also examined. The survivability of the fuel-PAHs during combustion/pyrolysis was assessed by examining the reactants (fuels) and the products of their oxidation/pyrolysis. The PAH species in the effluent of combustion were: 1) qualitatively compared with indigenous PAH constituents of the input fuels, and 2) quantitatively contrasted with known amounts of deuterium-labeled PAH standards, which were absorbed on the input fuels. No PAHs were detected in the effluent of combustion of either fuel under sufficiently fuel-lean conditions, e.g., φ < 0.5. This indicated that the PAH constituents of the input fuels, either indigenous or adsorbed, as well as those formed by pyrosynthesis in either the diffusion volatile flames or during the heterogeneous oxidation of the chars were destroyed. Significant amounts of PAHs were detected in the effluent of the combustion of both fuels under sufficiently fuel-rich conditions, e.g., φ > 1.6 and, especially, under pyrolytic conditions in N₂. These PAHs were mostly attributed to pyrosynthesis since none of the deuterated PAHs, adsorbed on the fuels, survived the combustion process. Small amounts of the labeled compounds, however, survived under purely pyrolytic conditions. These results were confirmed with separate experiments, where deuterium-labeled PAH standards were adsorbed on highly porous calcium/magnesium oxide or mullite particles. Again, small amounts of some PAHs survived in high-temperature pyrolytic conditions, but none in oxidative environments. These observations suggest that pyrosynthesis is the major contributing mechanism to the PAH emissions from the combustion of these fuels. Survivability of parent PAHs may be a minor mechanism at very high equivalence ratios.

Finally, both fuels were mixed with powders of calcium magnesium acetate (CMA), calcium carbonate (CaCO₃), and calcium oxide (CaO), all of which are known sulfur reduction agents, at a molar Ca/S ratio of 1. Combustion of the fuels mixed with CMA or CaCO₃ generated enhanced amounts of PAHs, while combustion with CaO had no effect on the PAH emissions.     

Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion

Biomass is regarded as CO₂-neutral, while the high contents of potassium and chlorine in biomass induce severe particulate matter emission, ash deposition, and corrosion in combustion facilities. Co-firing biomass with coal in pulverized-combustion (PC) furnaces is able to solve these problems, as well as achieve a much higher generating efficiency than grate furnaces. In this work, the particulate matter (PM) emission from biomass co-firing with coal was studied in an entrained flow reactor at a temperature of 1623 K simulating PC furnace condition. PMs were sampled through a 13-stage impactor, and their morphology and elemental composition were characterized by scanning electron microscopy and electron dispersive X-ray spectroscopy. SO₂ emissions were measured to interpret the possibility of potassium sulfation during co-firing. Results show that PMs from the separated combustion of both biomass and coal present a bimodal particle size distribution (PSD). The concentration and size of fine-mode submicron particles (PM_1.0) from biomass combustion are much higher than those from coal combustion because of the high potassium content in biomass. For the co-firing cases, with the coal ratio increasing from 0% to 50%, the PM_1.0 yield is reduced by more than half and the PM_1.0 size becomes smaller, in contrast, the concentration of coarse-mode particles with the size of 1.0–10 μm (PM_1.0-10) increases. The measured PM_1.0 yields of co-firing are lower than the theoretically weight-averaged ones, which proves that during the biomass and coal co-firing in PC furnaces, the vaporized potassium from biomass can be efficiently captured by these silicon-aluminate oxides in coal ash. In the studied range of coal co-firing ratio (≤50 wt.%), the chlorides and sulfates of alkali metals from biomass burning are the dominating components in PM_1.0, and a certain amount of silicon is observed in PM_0.1-1. The analysis of chemical composition in PM_1.0, together with that of SO₂ emission, indicates a marginal sulfation of alkali metal chloride occurring at high temperatures in PC furnaces.     

Cu/Fe3O4 catalyst for water gas shift reaction: Insight into the effect of Fe2+ and Fe3+ distribution in Fe3O4

Two precursors, namely, p-CFO-T (tetragonal) and p-CFO-C (cubic), were fabricated by a sol-gel method via citric acid and poly(vinyl alcohol) complexation, respectively. After H₂-reduction, the two were converted to Cu/Fe₃O₄ catalysts of different complexions, which are named as CFO-CA and CFO-PVA, respectively. The distribution of Fe²⁺ and Fe³⁺ in the Cu/Fe₃O₄ catalysts was studied by Raman and XPS techniques. It was disclosed that the distribution of Fe²⁺ and Fe³⁺ in Fe₃O₄ has an effect on Cu–Fe₃O₄ interaction and catalyst surface basicity. Compared to CFO-PVA, CFO-CA has a larger amount of Fe³⁺, which mostly sits at the octahedral sites, leading to stronger Cu–Fe₃O₄ interaction, and a larger amount of catalyst surface sites that are of weak basicity. As a result, the critical elementary steps of WGS reaction, viz. water dissociation, –COOH decomposition and CO₂ desorption are promoted as reflected in the lower E_a and higher catalytic activity of CFO-CA.