The influence of fly ash physical and chemical properties on electrostatic precipitation process

Flue gases emitted from coal fired power plants are mainly cleaned with electrostatic precipitators (ESP). Increased awareness of the effects of atmospheric pollution and tightening legislation force precipitator producers to increase their ESP efficiency, especially when collecting very fine particulates (PM2.5). The fly ash characteristic parameters are formatted during combustion process and its depend on the coal type as well as boiler parameters and combustion conditions. Due to that many series of tests were done to investigate the influence of fly ash physical and chemical properties on ESP operation.     

Laboratory studies of back-discharge in fly ash

The back-discharge is a type of discharge that takes place in the presence of corona discharge and occurs at an electrode covered with a dielectric layer of resistivity higher than about 10⁸ Ω m. Back-discharge can be observed in electrostatic precipitators when dust covering the collection electrode has low conductivity. In this paper, the studies of back-discharge generated in ambient air, in point-to-plane geometry with the plate electrode covered with fly ash are presented. The discharge is characterised in terms of its visual forms, current–voltage characteristics, and light emission spectra. Three forms of back-discharge were investigated: glow discharge, streamers, and low-current back-arc discharge. The current of the back-arc discharge was only a few milliamps. The discharge was stabilised by a high series resistance. It was noted that the voltage of ignition of the back-discharge for negative polarity is lower than for a positive one. Spectroscopic measurements of emission spectra provided information on elements present in the discharge column. The elements present in the fly ash, including toxic metals, can be re-entrained into the gas as particles or can be emitted as ions or neutrals during the discharge, and can decrease the collection efficiency of electrostatic precipitators. These elements were detected in the emission spectra. The effect of the discharge on the fly ash layer was also discussed. It was observed that sinter-like leftovers remain in the dust layer after a back-arc discharge.     

Experimental study on electrostatic removal of high-carbon particle in high temperature coal pyrolysis gas

A high-temperature electrostatic precipitator (ESP) presents a good solution for hot gas cleaning, which can remove fly ash from pyrolysis gas at temperatures higher than the tar dew point. In this paper, the characteristics of negative DC corona discharge in air and simulated coal pyrolysis gas were studied. The removal of coal pyrolysis furnace fly ash (ash A) was investigated and compared with that of coal-fired power plant fly ash (ash B) in ESP with a temperature ranging from 300?K to 900?K. The current density of simulated gas was higher than that of air under the same discharge voltage and at different temperatures. The simulated gas also had a higher spark voltage and a lower onset voltage compared with air. The fractional collection efficiency of ash A was lower for particles with diameters of larger than 0.1?µm at high temperature, compared with ash B. A lower collection efficiency in simulated gas was obtained for particles with diameters of less than 0.1?µm compared with air. The collection efficiency of submicron particles in simulated gas was usually higher than it in air, especially for particles with diameters of less than 0.04?µm. In simulated gas, the overall collection efficiency of ash A was obviously lower than that of ash B, especially at high temperature. From 300?K to 700?K, the collection efficiencies of both ash samples were as high as above 93%, but the collection efficiency of ash A in simulated gas decreased to 78.7% at 900?K.     

Agglomeration of sub-micron particles by a non-thermal plasma electrostatic precipitator

Non-thermal Plasma agglomeration is presented as a promising process to reduce the number concentration of sub-micron particles in an acrylic duct, which included a saw-tooth electrode and a wire-plate electrostatic precipitator (ESP). The generated plasma by pulse-energized ESP, the particle agglomerations were controlled under operating conditions such as pulse voltages, pulse frequencies, dust loadings, and gas velocities. When gas velocity increased from 0.5 to 1 m s⁻¹ at 45 kV_p, 20 kHz, it was found that efficiency was increased. At gas velocity of 1 m s⁻¹, the sub-micron particle number reduction efficiency for all particle sizes was over 90% in ESP.     

Biomass fly ash deposition in an entrained flow reactor

Fly ash deposition on boiler surfaces is a major operational problem encountered in biomass-fired boilers. Understanding deposit formation, and developing modelling tools, will allow improvements in boiler efficiency and availability. In this study, deposit formation of a model biomass ash species (K₂Si₄O₉) on steel tubes, was investigated in a lab-scale Entrained Flow Reactor. K₂Si₄O₉ was injected into the reactor, to form deposits on an air-cooled probe, simulating deposit formation on superheater tubes in boilers. The influence of flue gas temperature (589 – 968°C), probe surface temperature (300 – 550°C), flue gas velocity (0.7 – 3.5?m/s), fly ash flux (10,000 – 40,000?g/m²h), and probe residence time (up to 60?min) was investigated. The results revealed that increasing flue gas temperature and probe surface temperature increased the sticking probability of the fly ash particles, thereby increasing the rate of deposit formation. However, increasing flue gas velocity resulted in a decrease in the deposit formation rate, due to increased particle rebound. Furthermore, the deposit formation rate increased with probe residence time and fly ash flux. Inertial impaction was the primary mechanism of deposit formation, forming deposits only on the upstream side of the steel tube. A mechanistic model was developed for predicting deposit formation in the reactor. Deposit formation by thermophoresis and inertial impaction was incorporated into the model, and the sticking probability of the ash particles was estimated by accounting for energy dissipation due to particle deformation. The model reasonably predicted the influence of flue gas temperature and fly ash flux on the deposit formation rate.     

The correlation between corona current distribution and collection of fine particles in a laboratory-scale electrostatic precipitator

An analysis of the electrostatic gas cleaning fundamental phenomenon shows an essential influence of discharge electrode construction on the gas cleaning process efficiency.In the physical model tests there were used rigid discharge electrodes with corona emitting elements of various geometries. Different constructions of discharge electrode were tested in the aspect of discharge current uniform distribution on collecting electrode surfaces. Measurements of discharge current distribution has been carried out for discharge electrodes with different spike shapes and in different electric field geometry. The research aim was to determine the optimal discharge electrode construction ensuring high collection efficiency of fine particles. Collection efficiency measurements of selected fly ash samples (from coal fired boilers) were carried out on a laboratory testing bench in a horizontal electrostatic precipitator model.     

Experimental study of the mass balance in a pilot industrial electrostatic precipitator

Simultaneous measurements of the mass collection rates by plates and hoppers have been realized along a pilot ESP in an industrial environment, together with on-line current, voltage, opacity measurements; they have been associated with on-site measurements of particle concentration and resistivity, and with records of the operating conditions of the both boiler and the electrostatic precipitator. Using these measurements, it has been possible to reconstruct the detailed mass balance across the precipitator, under different operating conditions, depending on corona current and rapping sequences. It has been quantified that the best performances are obtained under slow rapping conditions and for current levels around the back-corona threshold. The detailed mass balance combined with size distribution of the fly ash may be very useful for the validation of theoretical models, in much more detail than through previously available published data. Continuous mass transfer from the collecting plates into the hoppers, independent on rapping shots, has been also put in evidence.     

Electrospray with electrostatic precipitator enhances fine particles collection efficiency

Electrospray was combined with an electrostatic precipitator (ESP) to enhance the collection efficiency of monodisperse nanometer-sized particles. The electrospray of deionized water produced water droplets with sizes ranging from 10 to 300 μm. The combination of the ESP with the electrospray enhanced collection of particles by 21–36% depending on the particle size. The combination of the ESP and the electrospray was also found to reduce the energy consumption of the ESP.     

Formation of Polyvinyl Alcohol film with graphene nanoplatelets and carbon black for electrostatic discharge protective packaging

This research investigated the synergic effect of graphene nanoplatelets (GNPs) and carbon black (CB) as a blended conductive filler for polymer film used as electrostatic discharge (ESD) packaging materials. Various weight ratios of GNPs/CB and combined filler concentrations were mixed and processed into Polyvinyl Alcohol (PVOH) based film. The surface resistivity and volume resistivity of the resulting film was measured under three different humidity environments. The study found that the composite with GNPs/CB ratios of 10:90 and 30:70 resulted in a sharp drop in surface resistivity by 5–8 orders of magnitude at the filler loading 8-10 wt%. The volume resistivity of the resulting film exhibited steady and consistent ranges within 10⁸–10¹² Ω cm across all loadings. The difference in conductivity between surface and volume made the film possible to be used in protecting equipment against electrostatic discharges inside of a package. The high loading of GNPs in hybrid GNPs/CB had no effect on enhancing both surface and volume conductivity of the composite film.     

Novel wet electrostatic precipitator for collection of fine aerosol

A novel wet electrostatic precipitator (WESP) is designed for effective control of fine aerosol from humid gases. It operates on the principle of unipolar particle charging in the corona discharge and particle precipitation under the field of their own space charge. The new precipitator is characterized by high gas velocity in the ionizing stage. Tests were carried out for gas with (NH₄)₂SO₄, HCl and (NH₄)Cl aerosol at particle number concentration up to 5·10⁷#/cm³ and mass concentration 10–1000 mg/Nm³. For test conditions one-field WESP ensures mass collection efficiency 90–97% and two-field electrostatic precipitator up to 99%.     

Using an improved electrostatic precipitator for poultry dust removal

Pubic concerns related to particulate matter emissions from animal housing operations are increasing. The goal of this study was to custom develop a simple and low cost electrostatic precipitator (ESP) for poultry dust control. The performance of the improved electrostatic precipitator (iESP) to remove a test aerosol was evaluated under a series of operating voltages between ?60 kV and 60 kV. The mass and size distributions of the particles were measured by a cascade impactor. The overall dust removal efficiency ranged from 37% to 79% with the maximum efficiency obtained at ?30 kV. The iESP shows high removal efficiencies for particles less than 2.1 μm.     

The effect of an external DC electric field on bipolar charged aerosol agglomeration

A direct-current (DC) electric field was exerted in a bench-scale electrostatic precipitator (ESP) to induce the agglomeration of bipolar charged aerosol particles. The test aerosol particles were generated from water with an atomizer and their average diameter was 7.71 μm. A phase doppler anemometer (PDA) was used to measure the size distribution and the number concentration of the particles. Systematic experiments were conducted to investigate the agglomeration efficiency of the system. The percentage decrease in number of sub-micron sized particles was found to be about 10.7%.     

Structural,optical and electrical properties of tin oxide thin films by electrostatic spray deposition

Tin oxide (SnO₂) thin films were deposited by electrostatic spray deposition (ESD). The structural, optical and electrical properties of the films for different solvents were studied. The morphology of the deposited thin films was investigated by scanning electron microscopy. The optical transmission spectra of the films showed 66–75% transmittance in the visible region of spectrum. The electrical resistivity of thin films deposited using the different solvents ranged 1.08 × 10^?3–1.34 × 10^?3 Ω-cm. Overall, EG and PG were good solvents for depositing SnO₂ thin films by the ESD technique with stable cone jet.     

Experimental investigation on charging characteristics and penetration efficiency of PM2.5 emitted from coal combustion enhanced by positive corona pulsed ESP

The electrostatic precipitator (ESP) has been extensively used for collecting aerosol particles emitted from coal combustion, but its collection efficiency of PM2.5 (Particulate matter whose aerodynamic diameter is less than 2.5 μm) is relatively low due to insufficient particle charging. The positive pulsed ESP is considered to enhance particle charging and improve collection efficiency. A laboratory-scale pulsed ESP with wire-plate electrode configuration was established to investigate the particle charging and penetration efficiency under controlled operating conditions of different applied impulse peak voltages, impulse frequencies, dust loadings and residence times. The results show that most particles larger than 0.2 μm are negatively charged, while most particles smaller than 0.2 μm are positively charged. For a given operating condition, the particle penetration efficiency curve has the highest penetration efficiency for particles with a diameter near 0.2 μm, and there is always a negative correlation between the particle penetration efficiency and the average number of charges per particle. Under the same operating conditions, the particle penetration efficiency decreases with increasing impulse peak voltage and impulse frequency, but increases as the dust loading increases. The results imply that residence time of 4 s is optimum for particle charging and collection. PM2.5 number reduction exceeding 90% was achieved in our pulsed ESP.     

Preparation of PVOH coatings with graphene nanoplatelets for electrostatic discharge protective packaging

This study investigated the use of graphene nanoplatelets (GNP) as a conductive filler for electrostatic discharge (ESD) protective packaging. Various weight concentrations of GNP were mixed and sonicated with polyvinyl alcohol (PVOH). The resulting polymer solution was applied as a coating to corrugated board in order to form an ESD packaging. Surface resistivity, mechanical strength and coating adhesion were then measured. The study found that the electrical percolation threshold of the PVOH/GNP coating is 9–13wt% GNP. GNP incorporated PVOH coatings with surface resistivity of 10³–10⁸ Ω/sq. generally meet all of ESD packaging requirements. The humidity strongly affects the surface resistivity of the coatings below the percolation threshold, but the change of the surface resistivity with humidity is less significant above the percolation threshold.     

Experimental investigation of NOx emission and ash-related issues in ammonia/coal/biomass co-combustion in a 25-kW down-fired furnace

Co-firing ammonia in coal units is a promising approach for the phasedown of coal power. In this paper, we demonstrate the feasibility of burning ammonia with coal and biomass in a 25- kW down-fired furnace with a swirl-stabilized burner. Ammonia is injected from the central tube at thermal ratios ranging from 0 to30% and can be completely burnt out in most co-firing cases. We investigate the NO_x emission, unburnt carbon in fly ash, particulate matter formation and ash deposition behaviors when co-firing NH₃ with either SH lignite coal or the coal/biomass blend. With a fixed air staging ratio, the NO_x emission increases linearly with the NH₃ fuel ratio. By increasing the percentage of secondary air, the emitted NO_x can be reduced to 300 ppm with an NH₃ thermal ratio of 30%. The unburnt carbon is affected by NH₃ addition in a complex manner. With a 30% (thermal) NH₃ addition, the unburnt carbon increases from 0.4% to 5.6% for the SH coal mainly due to a temperature drop, but decreases from 2.2% to 0.7% for the SH coal/biomass blend. As for the ash-related issues, the addition of NH₃ to either coal or coal/biomass blend is found to alleviate both the fouling intensity and the ultrafine particulate matter formation ability. This is a major advantage over biomass combustion.     

Ash formation and deposition during combustion of pulverized torrefied wood and coal in a 100 kW downflow combustor

Torrefied wood originating from beetle-killed trees is an abundant biomass fuel that can be co-fired with coal for power generation. In this work, pulverized torrefied wood, a bituminous coal (Sufco coal) and their blended fuel with a mixing ratio of 50/50 wt.%, are burned in a 100-kW rated laboratory combustor under similar conditions. Ash aerosols in the flue gas and ash deposits on a temperature-controlled surface are sampled during combustion of the three fuels. Results show that ash formation and deposition for wood combustion are notably different from those for coal combustion, revealing different mechanisms. Compared to the coal, the low-ash torrefied wood produces low concentrations of fly ash in the flue gas but significantly increased yields (per input ash) of ash that has been vaporized. All the mineral elements including the semi- or non-volatile metals in the wood are found to be more readily partitioned into the PM₁₀ ash than those in the coal. The inside layer deposits sticking to the surface and the loosely bound outside deposits exposed to the gas both show a linear growth in weight during torrefied wood test. Unlike coal combustion, in which the concentration of (vaporized) ash PM₁ controls the inside deposition rate, wood combustion shows that the formation of porous bulky deposits by the condensed residual ash dominates the inside deposition process. Co-firing removes these differences between the wood and coal, making the blended fuel to have more similar fly ash characteristics and ash deposition behavior to those of the bituminous coal. In addition, results also show some beneficial effects of co-firing coal with torrefied wood, including reduction of the total deposition rate and the minimization of corrosive alkali species produced by wood.     

Electrohydrodynamic gas flow in a positive polarity wire-plate electrostatic precipitator and the related dust particle collection efficiency

In this paper, the results of the particle image velocimetry measurements of the flow velocity fields in an intermediate spacing wire-to-plate type electrostatic precipitator (ESP) with a single positive polarity wire electrode are presented. The observation plane was placed perpendicular to the wire electrode at its half-length. The investigation showed significant influence of the electric field and charge on the flow patterns in the intermediate spacing ESP under an extreme large electrohydrodynamic (EHD) number. The EHD forces cause the formation of strong vortex pairs in the upstream and downstream ESP regions for Ehd/Re²>1.     

Performance characteristics between horizontally and vertically oriented electrodes EHD ESP for collection of low-resistive diesel particulates

The novel electrohydrodynamically-assisted electrostatic precipitator (EHD ESP) was developed to suppress particle reentrainment for collection of low resistive diesel particulates. The collection efficiency was compared between vertically and horizontally oriented electrodes of the EHD ESP using 400 cc diesel engine. The particle size dependent collection efficiency was evaluated for the particle size ranging in 20 to 5000 nm using a scanning mobility particle sizer (SMPS) and a particle counter (PC). Both horizontally and vertically oriented EHD ESP showed an excellent suppression of particle reentrainment. However, the horizontally oriented electrode EHD ESP showed significantly improved for the particle size of 300–500 nm in comparison with vertically oriented electrode EHD ESP, resulting in more than 90% collection efficiency for all particle size range. The EHD ESP has high potential especially for highly concentrated marine diesel engine emission control.     

Low temperature electronic transport in sputter deposited a-IGZO films

We report on the electrical properties of a-IGZO thin films prepared by reactive sputtering. Without oxygen injection, dc resistivity measured at room temperature is ρ_300K = 1.22 × 10⁻³ Ωm. The lowest resistivity ρ_300K = 4.86 × 10⁻⁵ Ωm is obtained at a certain oxygen supply into the deposition process. Hall effect measurements of these films reveal a metallic-like behavior from mobility and carrier concentration vs. temperature in the range 15–300 K whereas films deposited without oxygen or for the highest oxygen flows behave as semiconductors. These enhanced electrical properties are connected to the oxygen vacancies and the local coordination structure around the In³⁺ cations.