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Behaviour of Condensing Gaseous Species by Injection of Liquid Adsorbents (HMDS) in Combustion Facility
Abstract
In this study, we were clearly identify the behaviour characteristics on particle size distribution of a condensing species by injection condition of HMDS (Hexa Methyl Di-Silazane, silica precursor that is one of liquid adsorbents) to remove condensing gaseous species as using pyroligneous liquor generated during carbonization process of biomass as precursor of condensing gaseous species. When using HMDS to remove the condensing gaseous species by growth machanism of particles, we could be controlled properly particles size such as amount of adsorbent injection, residence time, heating temperature and MFC flux. Especially, in case of using the silica precursor, in consideration of the physical and chemical properties of the boiling point, the specific gravity and the molecular weight, we found that the condensing species could be effectively controlled by particles granulation.
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Combustion of biomass as fuels in small scale heating appliances is a common source of particle emissions. Majority of the particles is less than 1 μm (micrometer) and emitted directly to the atmosphere from the combustion devices. There is a growing concern worldwide about particle emissions from biomass combustion. Types of particles, formation mechanisms, their characteristics and available measurement techniques are discussed in this paper. Particle emission characteristics on small scale biomass combustion have also been discussed. The review findings have been summarized into three categories to show how combustion appliances and fuel affect the mass concentrations, number concentrations, corresponding size distributions and chemical compositions. Mass concentrations of particles in the flue gas from small scale combustion appliances reported in the literature to be in the range of 7-67 mg/Nm3, while particle number concentrations in the range of 0.8 x 107-5.5 x 109 particles/cm3. The dominating chemical compositions of the particle emissions in the fine mode were potassium fraction followed by sulphur, chlorine and sodium and consequently dominating chemical compounds were K2SO4 (potassium sulphate) and KCl (potassium chloride). From the literature it is clear that particle emissions from biomass combustion are closely related not only to the fuel properties but also to the combustion conditions and combustion appliances.
The flue gases emanating from the combustion of fuels or gasification process invariably comprise particulate matter and many chemical species in vapor form. The temperature of the flue gases gradually reduces when passing through different sections of heat exchanger, such as the superheater, evaporator, and so on. If the temperatures of the heat exchanger tube surface and the gas phase are favorable for condensation, the chemical species in the vapor form will condense on the particles and on the tube surface. The particle deposition behavior under these conditions is drastically different from the one observed in dry particulate fouling. In order to model the particle deposition under such circumstances, it is important to evaluate the criteria for particle adhesion to the surface. Impaction experiments of particles impacting a surface coated with a thin liquid film and particles that are coated with a liquid film impacting over a dry surface are performed to evaluate the limiting parameters under which a particle sticks to the surface without rebounding. The effects of liquid viscosity, liquid film thickness, and interacting material properties are evaluated. The experimental results are compared to the results of existing models and a suitable model for fouling is proposed. Controlled fouling experiments are performed for varying liquid films coated over a deposition tube under various process conditions to mimic the condensation effects on fouling. The results are compared with detailed impaction experiments.
Using dried sludge as a secondary fuel of a coal-fired power plant is proposed as an alternative option for sludge disposal. Because elemental contents of sludge are different from those of coal, different levels of acidic gas emissions are expected from the co-combustion of sludge with coal. In this study, sludge samples were obtained from 7 sewage treatment plants in Korea. Each sludge sample was combusted together with coal in a lab-scale combustor, and the concentrations of nitrogen oxides (NO_x), sulfur dioxide (SO_2), hydrogen chloride (HCl), chlorine (Cl_2) in the flue gas were analyzed. Compared to the combustion of coal only, NO_x concentration was slightly higher in the flue gas from the co-combustion of coal and sludge. SO_2 emission increased with the combustion of sludge due to the higher content of sulfur in sludge than in coal. For most of the tested samples, the concentrations of HCl and Cl_2 were varied depending on the chlorine content in the sludge sample.
Soot and tar which were derived from combustion or pyrolysis processes in Puverized Coal(PC) furnace or boiler have been significantly dealing in a radiative heat transfer and an additional source of NOx. Furthermore, the increasing for the use of a coal with low caloric value gives rise to a lot of tar-soot yield and LOI in a recycled ash for using cement materials. So, the ash with higher tar-soot yield and LOI can not recycle due to decreased strength of concrete. In this study, tar-soot yields and flame structures were investigated using the LFR for a blending combustion with bituminous coal and sub-bituminous coal. Also, The investigation were conducted as each single coals and blending ratio. The coals are used in a doestic power plant. In the experimental results, sub-bituminous coal with high volatile contents shows longer soot cloud length than bituminous coal, but overall flame length was shorter than bituminous coal. Tar-soot yields of sub-bituminous coal is lower than those of bituminous coal. Combustion characteristics are different between single coal and blended coal. Therefore, finding an optimal coal blending ratio according to coal rank effects on tar-soot yields.
The physico-chemical characteristics and nanostructure of the aerosol samples from a coal-fired power plant, a charcoal kiln and diesel vehicles were investigated with focusing on black carbon (BC). Aerosols from the coal-fired power plant were mostly comprised of mineral ash spheres which are heterogeneously mixed. The main components of the aerosols from coal-fired power plant were calcium compounds, iron oxide, alumino-silicate without BC. The typical combustion-generated BC which shows the shape of bunch of grapes with 20~50 nm particles which were detected in aerosol particles from diesel vehicles. The nanostructure of each BC particle shows the shape of concentric circles which is comprised of closely-packed graphene layers. Aerosols from charcoal kiln were likely condensed organic carbon generated from the low-temperature combustion process.
Condensing species behaviour downstream of a combustor was discussed with particle size distribution in this study. The effects of operating conditions in a biomass combustion facility, i.e. concentration of condensing species, temperature gradient, residence time and injection of adsorbents, on particle size distribution were investigated. Pyroligneous liquid which was completely vaporized at the temperature higher than was used as a representative of condensing gaseous species. We found that particle size downstream of a combustor increased with increasing heating temperature (i.e. concentration of condensing species) and residence time. However, temperature gradient was not an important factor to control the particle size. The addition of precursor as an adsorbent could effectively prevent the particle formation by adsorbing condensing gaseous species on particles, and increased the particle size up to 300 nm, resulting in increasing particle removal efficiency in a conventional air pollution control device.
With the aim of studying the polycyclic aromatic hydrocarbon (PAH) emissions when high calorific waste materials are used as nonfossil fuels, waste tire combustion has been performed in a laboratory plant. The goal was to compare the PAH emissions with the ones obtained when a fossil fuel is burned at the same combustion conditions. PAH emissions have been analyzed in solids collected in two cyclones at the exit of the combustor and in the trap system composed of a condenser, a filter (20 w m), and an adsorbent. After PAH extraction with dimethylformamide (DMF) by sonication and fluorescence spectroscopy in the synchronous mode (FS) analyses, it can be concluded that the nonfossil fuel generates higher PAH atmospheric pollution.
This article addresses the differences in soot nanostructure based upon formation and growth conditions. Fuel structure effects can be masked or accentuated depending upon both temperature and rate of increase. Low temperature yields an amorphous soot for all fuels studied here, regardless of flow rate. High temperature yields different results depending upon the rate of increase. A rapid increase as realized by a high flow rate emphasizes pyrolysis kinetics that favor polyaromatic hydrocarbons (PAHs) with five-membered rings leading to soots with many shells and capsules: a highly curved nanostructure. Slower rates result in a different pyrolysis chemistry leading to graphitic soot, as characterized by extended graphitic segments oriented parallel to each other. Reflecting high thermodynamic stability, PAHs may resist decomposition and yield a less graphitic soot nanostructure using either a fast or a slow temperature increase. In contrast, ethanol produces a highly curved nanostructure using either temperature increase rate.
The pathways of cadmium species to form a submicrometer-sized aerosol in a combustion system exhaust were established. Cadmium oxide was the predominant species formed in the experiments and resulted in particles of a mean size of 26-63 nm with number concentrations in the range of 2-8 x 10(6) cm(-3). Two different nanostructured sorbents, a solid montmorillonite (MMT) and an in situ generated agglomerated silica, were used for capture of the cadmium species. The MMT sorbent was not stable at 1000 degrees C, and structural changes resulted. MMT did not suppress nucleation of cadmium species and partially captured it by weak physisorption as established by the leachability tests. In contrast, the in situ generated silica nanostructured agglomerates had a high surface area, suppressed nucleation of cadmium species vapors, and chemisorbed them effectively resulting in a firm binding, as compared to the MMT sorbent. There is an optimal temperature-time relationship at which the capture process is expected to be most effective. The leaching efficiency under these conditions was less than 3.2%. The nanostructured silica agglomerate size can be tuned for effective capture in existing particle control devices.
The reduction and control technology of tar during biomass gasification/pyrolysis: An overview
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