Ash Deposition Characteristics Research Articles

Study on Fly Ash Adhesion Characteristics and Deposition Mechanism in Shell Coal Gasification Process

This paper describes the characteristics and mechanism of fly ash deposited in Shell coal gasification process. The chemical composition, particle size distribution, surface topography and elemental composition of fly ash samples derived from coal A and coal B are studied by X-ray fluorescence spectrometer, JX-2000 microscopic image analyzer and computer controlled scanning electron microscopy (SEM/EDX) respectively. The results show that the differences in ash deposition characteristics result from the differences in surface properties and element composition of fly ash particles generated from different coal feed. The fly ash formation and deposition mechanism are discussed based on physicochemical changes in gasification, particle size distribution and surface composition enriched in Ca, Fe in fly ash particles. The results can serve as reference data for a variety of deposition studies.

Ash Deposition Behavior of Upgraded Brown Coal and Bituminous Coal

Ash with a low melting point causes slagging and fouling problems in pulverized coal combustion boilers. Ash deposition on the heat exchanger tubes reduces the overall heat transfer coefficient due to its low thermal conductivity. The ash composition in coal and operational conditions in boilers such as heat load greatly affect the ash deposition behavior. This study focuses on the relationship between ash deposition characteristics and ash melting characteristics. Upgraded brown coal (UBC) and two types of bituminous coal with different melting temperatures and ash compositions were used as samples. The deposition tests were conducted using a refractory furnace. A water-cooled tube was inserted into the furnace to make the ash adhere to it. The molten slag fraction in ash was estimated by means of chemical equilibrium calculations. The results showed that the ash deposition characteristics have a close relationship to the ash melting characteristics. UBC with a low ash melting point shows a relatively high rate of ash deposition. The molten slag fraction in ash obtained by the chemical equilibrium calculation correlates with the deposition fraction of ash obtained by experiments, even under coal blending conditions. Therefore, the molten slag fraction in ash obtained by the chemical equilibrium calculations is one of useful indices to predict the blending method with UBC to reduce the deposition fraction of ash.

Pilot‐scale combustion of fast‐pyrolysis bio‐oil: Ash deposition and gaseous emissions

AbstractFast pyrolysis is a promising method to transform solid biomass into a liquid product called “bio‐oil” with an energy density of four to five times greater than the feedstock. The process involves rapidly heating biomass to 450–600°C in the absence of air and condensing the vapor produced to give bio‐oil. Typically, 50–75% (weight) of the feedstock is converted into bio‐oil that has a number of uses, for example energy production or bio‐refinery feedstock. This study investigated the gaseous emissions and ash deposition characteristics resulting from bio‐oil combustion in a pilot scale combustion test rig at Cranfield University. A feeding system with heated lines and heated/stirred reservoir was used to feed a spray nozzle in the combustion chamber.Ash deposit samples were collected from the resulting flue gas using three air‐cooled probes that simulate heat exchanger tubes with surface temperatures of 500, 600, and 700°C. The deposits formed were analyzed using SEM/EDX and XRD techniques to assess the corrosion potential of the deposits. The results are compared to measured ash deposit compositions formed from biomass combustion. Thermodynamic modeling software was used to make predictions for the partitioning of a range of elements for bio‐oil combustion and the results compared to the measured data. © 2009 American Institute of Chemical Engineers Environ Prog, 2009

Spectroscopic research on infrared emittance of coal ash deposits

This paper deals with thermal radiation characteristics of ash deposits on a pulverized coal combustion boiler of an electric power plant. Normal emittance spectra in the near to medium infrared (2.5–25 μm) region and total normal emittances were measured on four kinds of ground ash deposits. Measurements were conducted in the 570–1460 K temperature range which is common for boiler furnaces, by both heating and cooling the ash samples, with the aim to study the effect of their thermal history. Dependence of emittance on wavelength, temperature and chemical composition was studied, too. Samples were tested for transparency (opacity) to verify the accuracy of results. It was determined that the thicknesses used for the ash powders are opaque for infrared radiation for thicknesses in the order of a millimeter. Tests have shown that spectral emittance increases with an increase of wavelength with a characteristic pattern common for all samples. Spectral normal emittance increases strongly with temperature at shorter wavelengths and remains high and unchanged at longer ones. Emittance spectra are not very sensitive to chemical composition of ashes especially beyond λ ≈ 5 μm. With an increase of temperature, total emittance of the powdered sample decreases to a minimum value around 1200 K. Further temperature rise induces an increase of total emittance due to sintering in the ash. On cooling, the emittance increases monotonically following the hysteresis. Quantitative directions for evaluating thermal radiation characteristics of ash deposits for the merits of the safety design of boiler furnaces were proposed. That comprises correlating the experimentally obtained emittance spectra with curves of simple analytical form, i.e., a continuous function of minimum emittance vs. wavelength. The proposed method can be extended to other specimens from the same furnace and used to determine correlations for thermal calculation of old and design of new furnaces – with similar geometry and combusting similar coal. The method is potentially applicable to completely different boiler furnaces combusting different coal, and the authors recommend running the tests with new deposit samples. The data will then be applicable to the thermal design of a whole new class of furnaces, having similar geometry and combusting similar coal. This is expected to greatly enhance the accuracy and precision of thermal calculation as well as the efficiency of thermal design of steam boilers.

G0601-8-1 石炭酸素燃焼時の灰付着性(熱工学(8)燃焼(3))

Ash deposition characteristics in oxy-fuel combustion were investigated by using the portable device for monitoring ash deposition installed in the combustion test facility. The ash deposition tests and ash deposit analyses were conducted for two types of bituminous coal with different ash components. In comparison to air combustion tests, the result was obtained that the ash particle shape and size had no difference between oxy-fuel and air combustion. The distribution of metal element in ash for oxy-fuel combustion was almost same to air combustion. The results show that ash deposits properties in oxy-fuel combustion are equal to those in air combustion in bituminous coal firing.

Investigation of ash deposition in a pilot-scale fluidized bed combustor co-firing biomass with lignite

This study presents the results from investigation of ash deposition characteristics of a high ash and sulfur content lignite co-fired with three types of biomass (olive residue, 49wt%; hazelnut shell, 42wt%; and cotton residue, 41wt%) in 0.3 MWt Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustion (ABFBC) Test Rig. Deposit samples were collected on an air-cooled probe at a temperature of 500°C. Samples were analyzed by SEM/EDX and XRD methods. The results reveal that co-firing lignite with olive residue, hazelnut shell and cotton residue show low deposition rates. High concentrations of silicon, calcium, sulfur, iron, and aluminum were found in deposit samples. No chlorine was detected in deposits. Calcium sulfate and potassium sulfate were detected as major and minor components of the deposits, respectively. High sulfur and alumina-silicate content of lignite resulted in formation of alkali sulfates instead of alkali chlorides. Therefore, fuel blends under consideration can be denoted to have low-fouling propensity.

05/02485 Fundamental ash deposition characteristics in pulverized coal reaction under high temperature conditions

05/02485 Fundamental ash deposition characteristics in pulverized coal reaction under high temperature conditions

Fundamental ash deposition characteristics in pulverized coal reaction under high temperature conditions

Three types of coal with the different melting temperature and ash content were burned under the condition of high-temperature air pulverized coal reaction. A water-cooled tube was inserted into the furnace to make the ash adhere. Particle size and composition distributions of ash particles in both reacting coal particles and depositing layer were analyzed, using a Computer Controlled Scanning Electron Microscope, to study the deposition behaviors of ash particles. As a result, quantity of the ash deposition on the tube surface increases with a decrease of the melting temperature of coal ash. Index of fraction of the ash deposition depended on the coal type. For structure of the deposit layer, fine particles of size less than 3 μm mainly consisted of the initial layer for three types of coal, and the thickness was about 30 μm. Deposition of fine particulates of about 3 μm became a trigger of initial deposition at the stagnation point of tube even if irrespective of coal type is burned. The chemical compositions of ash particles in the reacting particles differed from those in the initial deposition layer. The deposition phenomenon relates to the particle size distribution of ash formed, the flow dynamics surrounding the probe, the chemical compositions in each ash particle and so forth.

Ash deposition characteristics in pulverized coal reaction under high temperature conditions

Ash deposition characteristics under high-temperature pulverized coal reaction conditions were studied using a horizontal pulverized coal reactor with pre-combustor to produce high-temperature vitiated air. The ash deposition experiments were performed by inserting a water-cooled tube into the reactor. Burning two types of coal with different melting temperature and ash content, and the ash deposition phenomena were visually and quantitatively elucidated. As a result, the quantity of ash deposition on the tube surface increases with an increase of the surface temperature. This is because the burning particles in the upper stream of the tube are radiatively cooled down under low temperature condition of the tube surface. Both the structure and quantity of ash deposition depend on coal types such as melting temperature and ash content. The deposition rate at the beginning of deposition, obtained by the peripheral width of deposition layer on the tube surface, is correlated as an Arrhenius-type equation since viscosity of the molten ash can be correlated as an Arrhenius-type equation. Hence, it suggests that the ash deposition phenomena may relate to the viscosity of molten ash.

An extremely large magnitude eruption close to the Plio-Pleistocene boundary: reconstruction of eruptive style and history of the Ebisutoge-Fukuda tephra, central Japan

An extremely large magnitude eruption of the Ebisutoge-Fukuda tephra, close to the Plio-Pleistocene boundary, central Japan, spread volcanic materials widely more than 290,000km2 reaching more than 300km from the probable source. Characteristics of the distal air-fall ash (>150km away from the vent) and proximal pyroclastic deposits are clarified to constrain the eruptive style, history, and magnitude of the Ebisutoge-Fukuda eruption.Eruptive history had five phases. Phase 1 is phreatoplinian eruption producing >105km3 of volcanic materials. Phases 2 and 3 are plinian eruption and transition to pyroclastic flow. Plinian activity also occurred in phase 4, which ejected conspicuous obsidian fragments to the distal locations. In phase 5, collapse of eruption column triggered by phase 4, generated large pyroclastic flow in all directions and resulted in more than 250–350km3 of deposits. Thus, the total volume of this tephra amounts over 380–490km3. This indicates that the Volcanic Explosivity Index (VEI) of the Ebisutoge-Fukuda tephra is greater than 7. The huge thickness of reworked volcaniclastic deposits overlying the fall units also attests to the tremendous volume of eruptive materials of this tephra.Numerous ancient tephra layers with large volume have been reported worldwide, but sources and eruptive history are often unknown and difficult to determine. Comparison of distal air-fall ashes with proximal pyroclastic deposits revealed eruption style, history and magnitude of the Ebisutoge-Fukuda tephra. Hence, recognition of the Ebisutoge-Fukuda tephra, is useful for understanding the volcanic activity during the Pliocene to Pleistocene, is important as a boundary marker bed, and can be used to interpret the global environmental and climatic impact of large magnitude eruptions in the past.

Study on the Predicting Technique of Ash Deposition Tendency in the Entrained-Flow Coal Gasifier. 1st Report. Relationship between Ash Melting Characteristics and Deposition Characteristics.

In an entrained flow coal gasifier, the gas temperature in the combustor or combustion region higher than the ash fluidizing point in order to discharge the mineral matter in the coal as molten slag. Following coal combustion, the gas temperature decreases as the gasification reactions proceed, eventually to below the ash softening point at the exit of the gasifier. This study focuses on the relationship between the ash melting characteristics and the ash deposition characteristics at temperatures below the ash softening point in the gasifier. Ash fusibility tests were conducted with DTA and ash liquid phase ratios were calculated as continuous ash fusibility index. The deposition tests were conducted using a drop-tube furnace for five types of coal. It was shown that the deposition characteristics have a close relationship with the ash liquid phase ratio and the difference of the ash deposition characteristics among coal types was explained by the ash liquid phase ratio. Furthermore, it was obvious that initial ash deposition phenomena at temperature below ash softening point are greatly influenced by Ca and Fe-aluminosilicate melts.

Ash Deposition Characteristics in Pulverized Coal Reaction under High Temperature Conditions.

Ash deposition characteristics under high-temperature pulverized coal reaction conditions were studied using a horizontal pulverized coal reactor with pre-combustor to produce high-temperature vitiated air. The ash deposition experiments were performed by inserting a water-cooled tube into the reactor. Burning two types of coal with different melting temperature and ash content, and the ash deposition phenomena were visually and quantitatively elucidated.As a result, the quantity of ash deposition on the tube surface increases with an increase of the surface temperature. This is because the burning particles in the upper stream of the tube are radiatively cooled down under low temperature condition of the tube surface. Both the structure and quantity of ash deposition depend on coal types such as melting temperature and ash content. The deposition rate at the beginning of deposition, obtained by the peripheral width of deposition layer on the tube surface, is correlated as an Arrhenius-type equation since viscosity of the molten ash can be correlated as an Arrhenius-type equation. Hence, it suggests that the ash deposition phenomena may relate to the viscosity of molten ash.

98/02251 Investigation of ash deposition characteristics in pulverized coal firing boiler

98/02251 Investigation of ash deposition characteristics in pulverized coal firing boiler

96/03601 Development of a drop tube furnace based method for assessing the ash deposition characteristics of small samples of pulverized fuels

96/03601 Development of a drop tube furnace based method for assessing the ash deposition characteristics of small samples of pulverized fuels

Hydrological characteristics of new volcanic ash deposit.

After volcanic eruptions, debris flow can be triggered by minimal rainfall, which would other wise never cause a debris flow. This suggests that the infiltration rate of the volcanic ash covering the original soil surface is very low. Low infiltration rates of volcanic ash are anticipated due to the presence of clay. This research investigated the hydrological characteristics of volcanic ash deposits to observe the changes in the saturated hydraulic conductivity and infiltration rate under the influence of the presence of clay. Laboratory experiments were carried out on the three kinds of material: (1) volcanic ash from Mount Unzen, (2) volcanic ash from Mount Sakurajima collected at a nearby riverbed, and (3) a mixture of volcanic ashes from Mt. Sakurajima and Mt. Unzen. The results showed that the infiltration rate of the volcanic ash from Mt Unzen was low; approximately 14 mm/hr. It was approximately one third smaller than the infiltration rate of the volcanic ash from Mt. Sakurajima. Also as the clay content increased, the saturated hydraulic conductivity and constant final infiltration rate decreased. The clay content change of low-clay-content ash had a greater influence on changes in the saturated hydraulic conductivity and constant final infiltration rate than the clay content change of high-clay-content ash.

Ash deposition during biomass and coal combustion: A mechanistic approach

The variability in both inorganic and organic properties of biomass fuels is large. This paper discusses combustion-driven transformations and deposition of inorganic material found in solid fuels, with a focus on the formation of deposits and their properties. A small number of mechanisms is used to describe both the transformations and deposition. The discussion below outlines this mechanistic approach to describing the fate of inorganic material in solid fuels with a particular focus on the mechanisms of ash deposition. This mechanistic approach has the potential of embracing a large range of fuel variations, combustor types, and operating conditions without the need of developing extensive databases or testing procedures for each new situation. The approach has been successfully demonstrated for coal combustion, and examples from coal experiments will be used as illustrations. The same methodology and logic can be applied to biomass combustion. A comparison of coal and biomass is briefly presented, including the chemical structures and the modes of occurrence of inorganic material in the fuels. The major mechanisms of ash deposition during combustion of coal and biomass are related to the types of inorganic material in the fuel and the combustion conditions. The effects of fuel (biomass or coal) characteristics and combustor operating conditions on ash deposit properties such as tenacity, emissivity, thermal conductivity, morphology, strength, chemical composition, viscosity, and rate of growth are discussed. A mechanistic model describing ash deposition in solid-fuel combustors is presented and used to postulate characteristics of ash deposits formed in biomass combustors.

An analysis of radiant heat transfer in furnace chambers burning slagging coals

Results are presented which involve information on zoning calculation of heat transfer in furnaces burning slagging coals. New numerical methods are described which take into account the selectivity and scattering of radiation. Calculations of local heat transfer characteristics and of the processes of slagging on the surfaces of waterwalls under complex boundary conditions are presented. The spectral radiation characteristics of ash deposits and slags and of the fly ash particles were studied experimentally. Boilers with dry and wet systems of slag removal were investigated. Based on the results obtained some recommendations are given concerning rational combustion of slagging coals.

Turbulent flow in a model superheater

An experimental study was conducted of the turbulent flow in a model superheater using laser velocimetry. The primary objective of the model study was to understand aerodynamic behavior of gas flow through a full-scale superheater, in order to help explain heat transfer and ash deposition characteristics of the superheater. The one-quarter scale superheater model used air at standard conditions to obtain model tube and duct Reynolds numbers as close as practical to full-scale. Two-component vector velocity data were obtained with a dual Bragg cell laser velocimeter. Measurements of mean velocity, turbulence intensity, and velocity-time data were obtained at points spanning the flow at many stations along the flow path around and between the tubes. The data have significant practical value, to show design and analysis engineers the general characteristics of flow in multitube and tube-bank heat exchangers. The flow into the superheater model was nonuniform, which is typical of full-scale superheater installations. Flow nonuniformity is expected to reduce the efficiency of the superheater but these experiments showed that the superheater flow became relatively uniform downstream of the first tube bank. Wakes occurring behind each row of tubes increased turbulence intensity, which is expected to increase ash deposition in real superheaters. A frequency analysis of the data revealed weakly periodic flow oscillations behind the first row of tubes in the first tube bank but none downstream. Data analysis also showed that there was no large-scale flow unsteadiness or surging. Turbulence intensities were very high throughout the tube banks, on the order of 30–40% of the local maximum mean velocities.

Performance of a sonic burner system firing coal-oil mixtures

The development of coal—oil mixture technology is progressing at an accelerated rate and the aim of this work has been the development of a new burner system (termed the sonic burner) to increase lignite loadings in air-atomizing burner systems. The burner system developed incorporates a sonic velocity at the atomizing air throat and a fuel oil gun with a single, relatively large opening at its exit. Lignite loadings with No. 6 fuel oil have reached 60.0 wt%. The flame produced at these loadings was stable, compact and characteristic of a No. 6 fuel oil flame. Fuel line plugging did not occur at these high lignite loadings; however, pumping problems did preclude increased lignite loadings. Results of this study are directly applicable to higher rank coals. Although feasible from a combustion viewpoint, no inference should be made from these results concerning erosion, ash deposition, particulate control or pollution characteristics, the work reported being based solely on combustion characteristics.