100 MW锅炉NaOH颗粒生成的数值模拟

煤粉燃烧过程中矿物质的气化、成核、凝结等过程是炉膛中亚微米颗粒形成的主要途径。本文运用CFD软件针对某100 MW锅炉内NaOH颗粒形成进行了数值研究,计算得到了炉内的温度分布、氧浓度分布和亚微米颗粒数量浓度和质量浓度分布。结果显示,亚微米颗粒的生成数量与温度具有强烈的相关性,温度较高的区域亚微米颗粒数量浓度较大,温度较低的区域则较小;而在炉膛的高温区内,NaOH颗粒的质量浓度并不是最高,而是最低,同时随着炉膛高度的增加,NaOH颗粒的质量浓度逐渐增加。计算结果为今后数值研究燃煤过程中亚微米颗粒的形成与演化奠定基础,为研究温度等燃烧条件对亚微米颗粒形成的影响、抑制其排放等方面提供了一种有效研究手段。     

粉煤燃烧过程中细粒子形成机理的研究

煤是一种有机沉淀岩 ,主要包括有机碳化合物和无机矿物。在燃烧的高温条件下 ,煤中的矿物质首先经历热分解和气化 ,当烟气冷却时 ,它们经过冷凝成核、凝结和团聚等作用 ,形成大量细粒子。这些细粒子的排放会对大气环境和人类健康造成严重的影响。研究燃烧过程中细粒子的形成机理将为其污染防治提供理论基础和科学依据。本文系统地综述了煤燃烧过程中矿物质和痕量元素的气化、冷凝成核、表面凝结、团聚的机理、模拟和预测方法以及细粒子形成机理的研究进展。     

Size dependence of in situ pH in submicron atmospheric particles in Hong Kong

Some studies have reported that small submicron atmospheric particles are more acidic than large submicron particles; other studies demonstrated a reversed trend. In this study, the size dependence of in situ pH in submicron particles in Hong Kong was investigated. The equivalent ratios of [NH₄⁺]_measured to [SO₄²⁻]_measured in submicron particles were found to be generally less than unity and size dependent, suggesting the possibility of incomplete gas–aerosol equilibrium. The Aerosol Inorganic Model-II (AIM-II) model using measured ionic compositions with the gas–aerosol partitioning disabled was used to estimate the in situ pH in different sized particles. The estimated in situ pH of different sized submicron atmospheric particles was between −2.5 and 1.5 and it generally decreases with increasing submicron particle size. At such low in situ pH, the estimated HSO₄⁻ equivalent concentrations were 3.2 times (on average) of those of H⁺ in different sized particles. The trends of the size dependence of the [NH₄⁺]_measured to [SO₄²⁻]_measured ratio and pH under different regimes of relative humidity are discussed.     

Organonitrate group concentrations in submicron particles with high nitrate and organic fractions in coastal southern California

During wintertime measurements in coastal southern California, organonitrate groups accounted for up to 10% of organic mass (OM) in submicron particles. In this study, we report the calibrated absorptivity, the uncertainties in the calibrations, the detection limits for 12 and 24 h ambient sampling, and the multipeak retrieval algorithm for the method developed. Organonitrate groups were observed when both submicron particle-phase nitrate and OM concentrations exceeded 1 μg m^?3. These high concentrations were associated with a mixed urban fossil fuel combustion source type that had potential source regions near Riverside and the South Coast Air Basin. The high frequency of these organonitrate observations contrasts with a number of studies of aerosol particles in other regions with more humid conditions, in which organonitrate groups were not detected and submicron sulfate concentrations exceeded those of nitrate. Our results suggest both that organonitrates form and/or exist in significant concentrations during polluted urban conditions and that their lifetime may be limited by hydrolysis in the particle phase.     

Wet Electrostatic Precipitators for Control of Submicron Particles

The application of wet electrostatic precipitators is rapidly gaining popularity. The emission regulations are becoming more and more stringent and they are being enforced. The emphasis on removal of fine particles and organic liquid droplets in the submicron range is increasing. In order to meet the required outlet loadings and opacities, the pressure drop that has to be applied across a conventional scrubber has increased exponentially. The wet electrostatic precipitator is emerging as an economic alternative by virtue of its very low power consumption and its potential for removal of submicron particles with efficiencies in the high nineties. Several applications of the wet electrostatic precipitator will be reviewed in this paper.     

Size-segregated chemistry of particulate dicarboxylic acids in the Arctic atmosphere

Gas–particle interactions of low-molecular-weight dicarboxylic acids were studied at a coastal Arctic site during the summer. Size segregated measurements with a Berner low-pressure impactor displayed up to four modes for ionic compounds: an Aitken mode, an accumulation mode, and two supermicron modes. The lower supermicron mode was ascribed to sea-salt, whereas the upper mode consisted mostly of species associated with continental particles. All four modes could be identified for oxalic acid, with the lower supermicron mode being the dominant. Malonic acid displayed a supermicron mode but was not found in the submicron size range. Succinic acid had an accumulation mode and, in a few samples, a supermicron mode. Glutaric acid displayed sometimes and accumulation mode, sometimes a supermicron mode, and occasionally both. The most probable formation pathway for submicron oxalic and glutaric acid was condensation from the gas phase, even though production in cloud droplets cannot be ruled out either. A slightly different formation pathway may have been important for submicron succinic acid production. Supermicron oxalic acid was probably formed by condensation from the gas-phase, by heterogeneous reactions occurring on the surface of pre-existing sea-salt and continental particles, or in cloud droplets. A larger mass median diameter for supermicron malonic and glutaric acid might be indicative of liquid-phase production in aqueous sea-salt particles. Evidence on possibly substantial sampling artifacts related to measuring dicarboxylic acids using filters were also obtained.     

A New Mathematical Model of Air Pollution Concentration

Abstract

Submicron particles play a major role in soiling processes and contribute to corrosion, current leakage and shorts in electronic equipment. For more than a year, optical particle counters have been used to continuously measure the concentrations of submicron particles at a telecommunications facility in Southern California. Separate instruments have simultaneously sampled at four locations: the outdoor air intake, immediately upstream of the HVAC filters, immediately downstream of the HVAC filters, and inside the office. The indoor concentrations can be explained in the context of a one-compartment mass balance model. Key parameters in the model (e.g., the air exchange rate) were monitored throughout the sampling period. In the latter part of this study, the particle counters were used as feedback elements in the HVAC system. An estimate of the concentration of indoor submicron particles, based on measurements of outdoor submicron particles, has been used as a control variable. When this variable exceeds a preset value, the outdoor air damper is partially closed, reducing the amount of outdoor air entering the building. That is, the position of the damper is based on the concentration of outdoor particles as well as the outdoor temperature. As a consequence, the average indoor concentration of submicron particles has been significantly reduced within this facility.     

A Comparison of the Size Distribution of Particulates Emitted from Air,Mechanical, and Steam Atomized Oil Fired Burners

Particulate size distributions were obtained in the effluent gases from three steam generating boilers of a power station employing mechanical, steam, and air systems for heavy fuel atomization. Samples were obtained with an eight-stage cascade impactor. The air atomized burner produced the lowest particulate loading, the greatest percentage of particulates by weight in the submicron range, and particulates with the lowest combustible content.     

An efficient venturi scrubber system to remove submicron particles in exhaust gas

An efficient venturi scrubber system making use of heterogeneous nucleation and condensational growth of particles was designed and tested to remove fine particles from the exhaust of a local scrubber where residual SiH4 gas was abated and lots of fine SiO2 particles were generated. In front of the venturi scrubber, normal-temperature fine-water mist mixes with high-temperature exhaust gas to cool it to the saturation temperature, allowing submicron particles to grow into micron sizes. The grown particles are then scrubbed efficiently in the venturi scrubber. Test results show that the present venturi scrubber system is effective for removing submicron particles. For SiO2 particles greater than 0.1microm, the removal efficiency is greater than 80-90%, depending on particle concentration. The corresponding pressure drop is relatively low. For example, the pressure drop of the venturi scrubber is approximately 15.4 +/- 2.4 cm H2O when the liquid-to-gas ratio is 1.50 L/m3. A theoretical calculation has been conducted to simulate particle growth process and the removal efficiency of the venturi scrubber. The theoretical results agree with the experimental data reasonably well when SiO2 particle diameter is greater than 0.1 microm.     

New Fabrics and their Potential Application

Considering fabric filtration, particularly the fabrics and their ability to handle submicron particulate and fumes, it is noted that the value of new developments are measured in terms of both improved operational and economic efficiency. The types of available fibers dictate fabric production characteristics as well as the limits of application for the manufactured fabric.     

The Significance of Particulate Emissions

Fine participates are the subject of increasing concern as one of the major air pollutants. They contribute to smog formation. They are a health hazard because they bypass the respiratory filters and penetrate deep into the lungs, and because they may act synergistically with other pollutants. The sources of submicron particle size pollutants are discussed.     

Formation characteristics of aerosol particles from pulverized coal pyrolysis in high-temperature environments

The formation characteristics of aerosol particles from pulverized coal pyrolysis in high temperatures are studied experimentally. By conducting a drop-tube furnace, fuel pyrolysis processes in industrial furnaces are simulated in which three different reaction temperatures of 1000, 1200, and 1400 degrees C are considered. Experimental observations indicate that when the reaction temperature is 1000 degrees C, submicron particles are produced, whereas the particle size is dominated by nanoscale for the temperature of 1400 degrees C. Thermogravimetric analysis of the aerosol particles stemming from the pyrolysis temperature of 1000 degrees C reveals that the thermal behavior of the aerosol is characterized by a three-stage reaction with increasing heating temperature: (1) a volatile-reaction stage, (2) a weak-reaction stage, and (3) a soot-reaction stage. However, with the pyrolysis temperature of 1400 degrees C, the volatile- and weak-reaction stages almost merge together and evolve into a chemical-frozen stage. The submicron particles (i.e., 1000 degrees C) are mainly composed of volatiles, tar, and soot, with the main component of the nanoscale particles (i.e., 1400 degrees C) being soot. The polycyclic aromatic hydrocarbons (PAHs) contained in the aerosols are also analyzed. It is found that the PAH content in generated aerosols decreases dramatically as the pyrolysis temperature increases.     

Engineering Design of Fine Particle Scrubbers

A concise, quantitative picture of the state of the art of particle scrubbing is presented in the form of performance prediction methods. A new relationship between the particle diameter collected at 50% efficiency and scrubber pressure drop for several of the most common scrubber types is a design tool of great utility. Scrubber capability for the collection of submicron particles by diffusion is described in a graph for several scrubber types.     

Comparison of black carbon (BC) aerosols in two urban areas – concentrations and size distributions

In this study, the BC aerosol measured at two very different urban sites is compared in terms of concentration, seasonal variation, and size distribution. During a 14 month study, one impactor sample was performed each month on a day with typical meteorological conditions. One (Vienna) or three (Uji) filter samples were obtained during the sampling time of the impactors. BC concentration in both the filter and impactor samples was analyzed with an optical technique (integrating sphere technique), where a calibration curve obtained from commercial carbon black is used to convert the optical signal to BC mass. Gravimetric mass concentration was measured at both sites. The gravimetric mass size distribution was measured only in Vienna. At both sites, the yearly average of the BC concentration on the sampling days was around 5 μg m⁻³. In Vienna, some seasonal trend with high concentrations during the cold season was observed, while in Uji, no pronounced seasonal trend was found. The BC size distribution in Uji was distinctly bimodal in the submicron size range. Log-normal distributions were fitted through the impactor data. The average BC mass median diameters (MMD) of the two submicron modes were 0.15 and 0.39 μm. Each mode contained about the same amount of BC mass. In Vienna only one submicron BC mode (average MMD 0.3 μm) was found because of the low size resolution of the impactor. An analysis of humidity effects on the MMDs of BC (both sites) and gravimetric mass (Vienna only) indicates that the Vienna aerosol is partly mixed internally with respect to BC, while the Uji aerosol seems to be externally mixed.     

Research Areas for Improved Incineration System Performance

In the past few years, many combustion tests were conducted in full scale operating incinerators. Those test data indicate that certain issues need to be re-evaluated, and point out certain research areas for improved incineration system performance. The issues include incinerability ranking and its relationship with operating temperature; significance of CO monitoring; and significance of products of incomplete combustion emission. The research areas for improvement include waste feed control; oxygen control; expert system; hydrogen halide conversion; submicron particulate formation; and improved air pollution control devices.     

Chemical characterization of submicron organic aerosols in the tropical trade winds of the caribbean using gas chromatography/mass spectrometry

Chemical characterization of submicron (D_p<0.6 μm) organic aerosols in the tropical trade winds in the Caribbean was performed. Samples were collected with a Lundgren cascade impactor at different locations in and around the island of Puerto Rico. The majority of the samples were collected at the Lighthouse at Fajardo. The GC/MS results were compiled into a database. The compounds identified were consistently observed in most of the samples and seem to have a natural origin. The families of compounds most consistently detected were esters (fatty acid esters and phosphates), fatty acids, and normal as well as branched hydrocarbons. The modality of extracted ion from full-scan MS data did not show the presence of PAHs in any of the samples analyzed from the coastal and ocean sites. Results tend to indicate that a substantial fraction of the submicron organic aerosol found in the tropical trade winds of the Caribbean may be associated with natural oceanic emissions; therefore, any treatment of the total aerosol should consider the contribution of this natural background aerosol.     

Closing Comments

“Industrial fabric filtration, the sleeping giant, is stirring. It appears that we are about to experience a new period of innovation and development similar to the one that occurred during the 1950’s.” The above remarks by Professor First, as presented in the Symposium, unerscore the renewed interest in filtration as a means for controlling submicron particulate emissions. Bur-chard and others pointed out the significance of fine particulate emissions, especially with regard to health effects and visibility degradation, and Walsh discussed the data required to employ fabric filters appropriately for particulate emission control.     

Product Guide/1982

Abstract

An efficient venturi scrubber system making use of heterogeneous nucleation and condensational growth of particles was designed and tested to remove fine particles from the exhaust of a local scrubber where residual SiH₄ gas was abated and lots of fine SiO₂ particles were generated. In front of the venturi scrubber, normal-temperature fine-water mist mixes with high-temperature exhaust gas to cool it to the saturation temperature, allowing submicron particles to grow into micron sizes. The grown particles are then scrubbed efficiently in the venturi scrubber. Test results show that the present venturi scrubber system is effective for removing submicron particles. For SiO₂ particles greater than 0.1 μm, the removal efficiency is greater than 80–90%, depending on particle concentration. The corresponding pressure drop is relatively low. For example, the pressure drop of the venturi scrubber is ～15.4 ± 2.4 cm H₂O when the liquid-to-gas ratio is 1.50 L/m³. A theoretical calculation has been conducted to simulate particle growth process and the removal efficiency of the venturi scrubber. The theoretical results agree with the experimental data reasonably well when SiO₂ particle diameter is greater than 0.1 μm.     

Sizing of atmospheric particulates with a high volume cascade impactor

This article describes the development of a high volume cascade impactor with a sampling rate of 20 ft³/min that can be powered by high volume blowers in common use for air sampling. With this impactor it is now possible to collect enough sample of atmospheric particulates in a 24 hour period to conveniently determine an aerodynamic size distribution of suspended particulates by gravimetric analysis. The impactor has four stages and an after-filter and separates particles from the air stream by inertial impaction. The effective cutoff diameters for the stages are 7.0, 3.3, 2.0, and 1.1 microns. The after-filter traps the submicron material.     

Flux Force/Condensation Scrubbing

Considerations for the engineering design of flux force/condensation (FF/C) scrubbers are reviewed. Fine par-ticulate removal in multiple sieve plate FF/C scrubbers is predicted, using mathematical design models. Results of experimental studies of two multiple sieve plate scrubbers for the removal of submicron particles are given. The published experimental data on FF/C scrubber performance are summarized. A preliminary analysis of the economics of FF/C scrubbers, compared to the conventional high energy scrubbers, defines the most favorable operating conditions for the application of FF/C scrubbers.