Submicron Fly Ash Research Articles

Synergistic catalytic enhancement of chemical oxygen demand removal using Mn-Bi doped submicron fly ash carrier: Mechanistic insights and environmental adaptability

This study presents a novel approach to enhancing the removal efficiency of Chemical Oxygen Demand (COD) from water using a submicron fly ash carrier (MFA) doped with manganese (Mn) and bismuth (Bi). The synthesis process involved mechanical ball milling and alkali activation of original fly ash (FA) to obtain MFA, followed by impregnation calcination to prepare the Mn-Bi@MFA composite. The synergistic effect of Mn and Bi facilitated the formation of abundant oxygen vacancies on the catalyst surface, significantly reducing the band gap width. The Mn-Bi@MFA composite exhibited remarkable properties with a specific surface area and pore structure reaching 15.17 m2/g and 0.062 cm3/g, respectively, surpassing FA by 7.16 and 20.67 times, respectively. Utilizing the Mn-Bi@MFA/O3 system, the COD removal amount reached 562.73 mg/g under specific conditions (reaction time = 60 min, pH=7.0, catalyst dosage = 1.0 g/L, ozone dosage = 2.0 g/h), exceeding that of the ozone-alone system by 51.62 %. Moreover, Mn-Bi@MFA demonstrated excellent environmental adaptability and repeatability. An investigation into the contribution order of different free radicals revealed •OH>O2•- >1O2 in COD removal, elucidating the mechanisms of ozone oxidation, catalytic ozonation, and photocatalysis. This research provides valuable insights into the development of efficient and sustainable water treatment strategies employing advanced nano-material-based catalysts.

An experimental study on the removal of submicron fly ash and black carbon in a gravitational wet scrubber with electrostatic enhancement.

It is of great significance to adopt a cost-effective and highly efficient method to capture submicron particles produced by small-scale industrial boilers. In this study, a middle-scale wet electrostatic scrubbing (WES) setup was built to investigate its performance in the removal of both fly ash particles and black carbon, with special attention to the submicron size range. Major factors including the particle properties and charging conditions were expatiated in detail to popularize this method. The results showed that the efficiency increase in black carbon is significantly higher than that of fly ash particles at the charging condition. For the case of droplet charging, the highest efficiency increase in black carbon in the submicron size range is up to 60%, while that of fly ash is only 40% under same conditions. In comparison with particle charging, droplet charging plays a more significant role in removing both fly ash particles and black carbon, which is beneficial for reforming conventional wet scrubbers. Moreover, more small particles could adhere to the surface of large fly ash particles after scavenging while this phenomenon was not found for black carbon due to the characteristic of fractal agglomerates.

Crude glycerol combustion: Particulate, acrolein, and other volatile organic emissions

Crude glycerol is an abundant by-product of biodiesel production. As volumes of this potential waste grow, there is increasing interest in developing new value added uses. One possible use, as a boiler fuel for process heating, offers added advantages of energy integration and fossil fuel substitution. However, challenges to the use of crude glycerol as a boiler fuel include its low energy density, high viscosity, and high autoignition temperature. We have previously shown that a refractory-lined, high swirl burner can overcome challenges related to flame ignition and stability. However, critical issues related to ash behavior and the possible formation of acrolein remained. The work presented here indicates that the presence of dissolved catalysts used during the esterification and transesterification processes results in extremely large amounts of inorganic species in the crude glycerol. For the fuels examined here, the result is a submicron fly ash comprised primarily of sodium carbonates, phosphates, and sulfates. These particles report to a well-developed accumulation mode (0.3–0.7μm diameter), indicating extensive ash vaporization and particle formation via nucleation, condensation, and coagulation. Particle mass emissions were between 2 and 4g/m3. These results indicate that glycerol containing soluble catalyst is not suitable as a boiler fuel. Fortunately, process improvements are currently addressing this issue. Additionally, acrolein is of concern due to its toxicity, and is known to be formed from the low temperature thermal decomposition of glycerol. Currently, there is no known reliable method for measuring acrolein in sources. Acrolein and emissions of other volatile organic compounds were characterized through the use of a SUMMA canister-based sampling method followed by GC–MS analysis designed for ambient measurements. Results indicate crude glycerol combustion produces relatively small amounts of acrolein (∼15ppbv) and other volatile organic compounds, with emissions comparable to those from natural gas combustion.

Partitioning of arsenic, selenium, and cadmium during the combustion of Pittsburgh and Illinois #6 coals in a self-sustained combustor

An important environmental issue is the emission of semi-volatile toxic metals, such as arsenic, selenium, and cadmium, from the combustion of coal. These materials may vaporize in the hot portions of the combustor then return to the solid phase in cooler zones of the process downstream. Understanding the mechanisms by which toxic metals partition between the vapor and solid phases is an important step for predicting and mitigating the effect of these metals upon the environment. Particulate ash samples were withdrawn from a 17-kW pilot-scale downflow combustor in which pulverized coal was burned under self-sustaining conditions. The samples were size segregated in a Berner low pressure impactor, and then analyzed using neutron activation. This research approach has suggested mechanisms, which govern the partitioning of arsenic, selenium, and cadmium in practical pulverized coal combustion processes. The results suggest that volatilization and subsequent transfer of selenium to submicron particle surfaces appears to be an important post-combustion phase mechanism for Illinois #6 coal but not for Pittsburgh seam coal. Most of the selenium in the Pittsburgh submicron fly ash, cadmium in Illinois #6 submicron fly ash, and arsenic in both Pittsburgh and Illinois #6 submicron fly ash enters the post-combustion zone in the solid phase. The dominant heterogeneous partitioning mechanism for transformation to large, supermicron particles is the reaction of metal vapor on the surface or within the pores of an ash particle. The results also suggest that the rate of transformation is dominated either by an exterior surface reaction-controlled regime or a pore diffusion-controlled regime. A relationship between the concentration of solid phase arsenic, selenium, and cadmium to calcium in supermicron particles was also observed, suggesting the formation of As–Ca, Se–Ca, and Cd–Ca reaction products.

Pilot scale study of trace element vaporization and condensation during combustion of a pulverized sub-bituminous coal

Trace metal emissions from coal-fired power plants are largely associated with the fly ash. The work reported here is part of a larger effort to develop a fundamental model for transformation of trace metals in coal to air toxic emissions from coal-fired power plants. Because the time-temperature history of the combustion gases determines the condensation behavior of gaseous species, experimental study of these phenomena require a realistic post-combustion environment. Pilot-scale combustion of a Powder River Basin coal was conducted using realistic post-combustion conditions. Trace element distributions were measured in the submicron fly ash at the inlet to an electrostatic precipitator. Flame temperature had a dramatic impact on the amount of certain trace elements such as As and Se in the submicron ash, indicating that these elements vaporize during the combustion process. The amount of vaporization was not sensitive to coal grind. There is evidence for the reaction of As and Se vapors with the large (supermicron) ash in the post-combustion flue gas via a surface reaction. The correlation between arsenic and calcium in the ash suggests the formation of calcium arsenate. No such correlation was observed between selenium and calcium.

Ultra-fine Ash Particle Formation During Waste Sludge Incineration in Fluidized Bed Reactors

Ash formation during the bubbling fluidized bed (BFB) combustion of bark and pulp mill sludge has been studied on an industrial and bench scale. During co-firing in an industrial BFB a submicron fly ash mode was formed via condensation of volatilized K, Na, Sand Cl species at 0.05–0.3 μm. The submicron mass mode below 0.3 μm made up 2.2–5.0% of the fly ash, while the share of the supermicron mass fraction was 93.6–97.2%. Elements depleted in the ultra fine ash were Ca, Si, Al, Mg, Fe, Mn, P and Ti. The bench-scale test showed that the ultrafine particle concentration was increased by a higher bed temperature and decreased due to sludge moisture. As, Cd, Pb and Rb were enriched in the ultrafine ash on a bench scale, while Ba, Co, Sr and V were depleted. Cu and Zn were enriched in the ultrafine ash during the combustion of dried sludge, but not when wet sludge was fired. Micron-size ash particles composed of non-volatile species, Ca, Si, Mg, Al, P and Mn, adhered to the bed sand, presumably by surfaceforces, and sintering densified the ash layer.

Formation and behavior of submicron fly ash in pulverized coal combustion furnace

In this study, the formation mechanism and the behavior of submicron fly ash (SFA) (larger than 0.1 μm) in the industrial type pulverized-coal combustion furnace was examined experimentally by using two different models of furnace: a turbulent furnace, which was designed to realize practical combustion conditions, and a small, laminar flow furnace. Observations were made by an electron micrograph, x-ray microanalysis, and infrared spectroscopic analysis of the submicron particles sampled from the furnace, and measurements of the concentration distribution of the suspended and coagulated SFA were carried out. Both the electron micrographs and the infrared spectroscopic analysis showed that the submicron particles sampled near the concentration peak in the furnace closely resembled raw coal. The major components (aluminum, silicon, calcium, and iron) in the ash contained in the submicron raw coal fragment was compared with the SFA sampled at the flue for six different coals. The composition of the SFA was mostly compared to that of the raw coal fragment. There was no silicon condensation in these coals, with one exception. The measured concentration distribution of the SFA in the furnace showed that its peak concentration level in the furnace was two or three times higher than that calculated from the SFA content in coal, assuming that no formation or extinction of the SFA occurs in the region of coal combustion. From these results, we concluded that approximately half of the SFA in the furnace is simply carried over from the submicron coal fragment originally contained (3.5–9.5 wt%) in the pulverized raw coal, and that the remaining SFA is newly formed in the furnace through the breakup of larger coal particles in the devolatilization region. However, the breakup ratio was dependent on the primary air velocity at the burner exit. The percentage of the submicron coal fragment originally contained in the pulverized raw coal that became suspended in the furnace was estimated to be less than 10 wt% under actual flow conditions. The effect of the submicron coal fragment in the pulverized raw coal on the emission of suspended SFA was clearly shown using a small laminar furnace.

On the Fragility of Acoustically Agglomerated Submicron Fly Ash Particles

The concept and potential use of acoustic agglomeration as an aerosol conditioning device in a clean-up team is discussed. The experimental setup used for carrying out the room temperature agglomeration tests is briefly described. Photographic evidence is presented showing the survival of bonds holding the acoustically agglomerated particles together, after the particles were size separated in an (Anderson Mark II) inertial impactor. A theoretical analysis is developed which shows that the shearing stresses exerted on particles in the impactor are higher than would be experienced if they were being separated in a typical industrial cyclone. Thus, it is concluded that the acoustically agglomerated particles are “robust” enough to avoid breakup in a cyclone (a common industrial particle removal device).

Comparison of micron and submicron fly ash particles using scanning electron microscopy and x-ray elemental analysis

Fly ash particles from the Bull Run Power Station of TVA were studied in the submicron range (average size 0.6 ..mu..m) and in the micron range (2-7 ..mu..M) by means of a scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer for X-ray elemental analysis (EDX). It was found, for the fly ash studied, that the submicron particles were very similar to the particles in the micron range. Both types of particles were spherical and contained Si, K, Fe, Ti, and S as the main components. The submicron particles were distinguished only in some enrichment in sulfur content.

Oxidation of SO2 on the surface of fly ash particles under low relative humidity conditions

Submicron fly ash particles can accumulate sulfate by surface oxidation of SO2 in amounts between 9% and 45% of their mass at residence times between 0.3 and 12.5 hours at low humidities in the plume of a coal‐fired power plant. The accumulation is size dependent and rapid decrease in the amount of fly ash due to fallout and dispersion prevents efficient removal of sulfur from the atmosphere by this mechanism.

The oxidation of sulfur dioxide to sulfate aerosols in the plume of a coal-fired power plant

Instrumented aircraft have been used to determine the rate of oxidation of sulfur dioxide to sulfate aerosols within the dispersing plume of a coal-fired power plant. Measurements have been made in the stack and from 10 to 105 km downwind of a 2600-MW power plant. Sampling experiments were conducted during the winter of 1975–1976 and in the fall of 1976. Average temperatures and relative humidities varied from −5 to 17°C and 22 to 71% respectively. Data were also collected on sulfate production at night. An average of 1.1% and a maximum of 4.3% of the plume sulfur was in the form of sulfate. Most of the atmospheric oxidation that was observed appeared to occur in the immediate vicinity of the power plant. The average oxidation rate, beyond 10km, was found to be 0.2% h −1. Analyses of individual particles showed that the sulfur found in plume particles is usually associated with submicron fly ash particles.