Amount Of Unburned Carbon Research Articles

Numerical study on the effects of primary air ratio on ultra-low-load combustion characteristics of a 1050 MW coal-fired boiler considering high-temperature corrosion

Primary air significantly impacts the ultra-low-load combustion of coal-fired utility boilers. In this study, eleven combustion scenarios with varied primary air ratios (the flowrate ratio of primary air to the total air, PAR) were numerically investigated for a 1050 MW double-tangential circular coal-fired boiler under ultra-low load. The effects of PAR on temperature distribution, coal burnout, and gas species were thoroughly examined. Meanwhile, special attention was paid to the reducing atmosphere near furnace walls for alleviating the problem of high-temperature corrosion. Results show that a relatively large high-temperature region in the furnace is achieved at PAR = 0.153, contributing to combustion stability under ultra-low load conditions. Although the residence time tends to be shortened as PAR increases from 0.123 to 0.275, the amount of unburned carbon in coal particles has a general trend of decreasing because of more available oxygen from primary air. Due to the high excess air coefficient at ultra-low load, strong reducing atmosphere and high-temperature regions mostly concentrate around the operating coal burners, posing corrosion hazard there. As PAR increases to 0.275, the area of high CO concentration regions near furnace walls and the maximum CO concentration decrease noticeably, indicating less pronounced high-temperature corrosion at large values of PAR. NO emissions increase from 294.4 mg/Nm3 to 823.7 mg/Nm3 with increasing PAR from 0.123 to 0.275. This is shown to be related to the weakened reducing atmosphere as well as the reduced residence time of the coal particles as PAR increases.

Feasibility Study of Co-Firing of Torrefied Empty Fruit Bunch and Coal through Boiler Simulation

Torrefied empty fruit bunch (EFB) co-firing is a promising technology to reduce emissions from coal-fired power plants. However, co-firing can influence the combustion and heat transfer characteristics in a coal boiler. In order to study the feasibility of co-firing application of torrefied EFB (T-EFB) in boilers, the combustion characteristics, gas emissions and heat flux distribution were analyzed, respectively. First, the kinetic parameters of T-EFB devolatilization and char oxidation were obtained by experimental analysis. Second, the computational fluid dynamics (CFD) analysis was applied to the actual 500 MWe boiler simulation to further evaluate the differences in the co-firing performance parameters (combustion characteristics and emissions) of the T-EFB and the heat transfer characteristics within the boiler. Numerical results show that T-EFB co-firing can improve the ignition characteristics of pulverized coal, reduce the formation of unburned particles. When the blending ratio was increased from 10% to 50%, significantly NOx (oxides of nitrogen) reduction (levels from 170 to 98 ppm at 6% O2) was achieved. At a blending ratio above 40%, boiler combustion efficiency decreases as the total heat flux of the boiler decreases due to an increase in the amount of unburned carbon. In addition, T-EFB co-firing can affect the heat transfer characteristics of the boiler.

The Relationship between Unburned Carbon Levels in Coal Combustion Ash and Volatile Matter Content in Coal

ABSTRACT In this study, considering a specific coal fired power plant, fly ash samples were analyzed in terms of unburned carbon (UBC) and amount of UBC in these samples were compared to coal volatile matter content. UBC which can be used as an expression to tell efficiency of power plants is basically reasoned by either coal properties or by the conditions of combustion environment in boiler. Coal combustion boilers are utilized in terms of their design conditions and they are really hard to utilize them out of these ranged conditions. That is why in order to investigate the effect of these abovementioned parameters, either laboratory scale or pilot scale combustion tests should be conducted. Briefly in this study, a period of same conditions of same boiler utilization of different types of coals considered, and different amounts of UBC in fly ashes were correlated with high or low amount of volatile matter content in coal. Obtained correlation between coal volatile matter and UBC in ash showed that volatile matter content of coal is dominantly affecting the coal burnout.

Исследование степени выгорания пылеугольного топлива при обеспечении предварительного контакта с горячим воздухом

A situation in which unburned carbon remains after combustion of pulverized coal in the furnaces of power-generating boilers is due to the fact that large particles do not have enough time to burn out completely during their dwelling in the furnace space. Typically, the high-density coal-air mixture conduits (HDCC) used at coal-fired thermal power plants are directly connected to the main burners and cut in the vertical hot air ducts. This connection arrangement has a number of drawbacks; in particular, pulsations of pulverized coal-air mixture pressure arise in the pulverized coal conduits, and it is impossible to secure early ignition of pulverized coal due to its being insufficiently heated upstream of the burner. For solving this problem (i.e., making sure that pulverized coal more promptly comes in contact with hot primary air) under the conditions of the Starobeshevo thermal power plant, a new pulverized coal feeding arrangement was tried out in 2010, according to which the HDCC is cut into the primary air channel near the pulverized coal bunker. With this arrangement put in use, it became possible to completely avoid the need to support the pulverized coal flame with gas at a load of 130 MW and to decrease the amount of unburned carbon by about 1%. The pulverized coal fuel transformations in the course of its being in contact with hot air upstream of the burner inlet were investigated. To this end, fuel samples were taken directly under the pulverized coal bunker and upstream of the burners located at different distances from the bunker. It has been determined that in the course of fuel being in contact with hot air for a period from 0.5 to 0.9 s, the total moisture content in it decreased by 4...48%, and the content of volatiles decreased by 6...25%. Simplest numerical assessments have shown that the release of such amounts of volatiles and their combustion immediately after entering the furnace yield a heat release sufficient for heating the fuel and primary air by several hundred degrees. The revealed regularity explains the physical nature of the results in terms of decreasing the amount of unburned carbon that have been achieved after re-cutting the pulverized coal conduits and taking measures for pulverized coal to more promptly come in contact with hot air.

Comparative study of coal combustion residues from pulp and paper mills of Assam

ABSTRACTIn the present work, the physicochemical, mineralogical, and morphological characteristics of coal combustion residues (CCRs) collected from two different Pulp and Paper Mills of Assam, India, have been investigated. CCRs from both the sources were mechanically sieved to obtain various size-classified fractions and their physical properties were determined using standard methods. X-ray diffraction (XRD), Fourier Transformation Infrared (FTIR) Spectroscopy, Scanning Electron Microscopy (SEM) etc. techniques were employed to study the mineralogical and morphological characteristics of each size classified fraction. The loss on ignition (LOI) values of the coarser fractions were found to be unexceptionally high, implying the presence of a huge amount of unburned carbon, and indicating the incomplete combustion of feed coal. From the utilization point of view, coarser fractions may be recycled (as domestic fuels) and less-coarse fractions may be used as adsorbents of toxic organic chemicals. Due to the larger water-holding capacities (WHCs), the coarser fractions may be used for soil amendment.

Eluted Soluble Silica Content in Rice Husk Charcoal Produced by Rice Husk Burner

This study aims to compare burner combustion method with conventional method from the viewpoint of total amount of eluted soluble silica in rice husk charcoal (RHC). RHC was prepared in an electric furnace by controlling the combustion temperature and time. The amount of eluted soluble silica in RHC was measured by the molybdenum blue absorptiometric method. The amount of unburned carbon and the crystallization of silica were measured by weight change between RHC production and by the specific surface area and pore volume of RHC, respectively. As a result, RHC combusted at 900 °C for 3 min as a developed rice husk burner contained 60% of the amount of eluted soluble silica in RHC combusted by conventional method at 400°C for 60 min. With regard to the mechanism of silica elusion, in short-term combustion assuming a developed rice husk burner, high temperature was suitable for increasing the amount of eluted soluble silica because it was determined by eliminating the plant fiber covering silica in rice husk. On the other hand, in long-term combustion assuming conventional method, low temperature combustion showed the possibility for increasing the amount of eluted soluble silica because it was determined by the crystallization of silica.

Intelligent monitoring system of unburned carbon of fly ash for coal fired power plant boiler

Coal fired power plant becoming preferable power plant type to support electricity demand mainly in Asia due to stable coal price and low maintenance. However, most coal fired plant operator struggle with condition where coal undergo incomplete combustion and produced unburned carbon where can be found in ashes especially in fly ash. Higher percentage of unburned carbon in fly ash reflects the lower efficiency of furnace and contributes to financial loses for plant operators. This problem also leads to technical issues such as slagging and clinkering and further reduces the efficiency of furnace. The plant operator determines the amount of unburned carbon by using conventional method and this proves be a challenge to identify and rectify the problem on day basis due time constraint to obtain results of unburned carbon. Thus in this paper, best Artificial Neural Network model was derived to develop intelligent monitoring system to predict unburned carbon level on more daily basis. By this model, the power producer can predict the unburned carbon level by using data in power plant to predict the unburned carbon level in short period of time.

Thermal Stability and Adsorption of Mercury Compounds in Fly Ash

Fly ashes were collected from five power plants under two loads in China. The ashes were heated at four different temperatures, and mercury speciation was determined based on the release regular pattern of mercury with temperature. The mercury concentration, unburned carbon content and mean ash particle sizes were measured. The correlation of mercury capture and unburned carbon content, mean ash particle sizes were analyzed. Results indicate that the amount of unburned carbon and mercury adsorb is significantly positively correlated in fly ash; the smaller the mean ash particle size, the more mercury particles are captured. There was little HgO and HgSO4, and the main form of mercury compounds in fly ash were HgCl2and HgS. The high element Cl content can result in high HgCl2ratio in particular mercury and element S play an important role in adsorbing mercury.

Solid Reactant-Based Geopolymers from Rice Hull Ash and Sodium Aluminate

High carbon rice hull ash and solid sodium aluminate were used as silica, alkali and alumina sources to synthesise one-part “just add water” geopolymer binders. Three binders with different Si/Al ratios and different water contents were studied. Due to the high carbon content of the samples, using a high amount of water is required to satisfy the workability of the binders. Similar to traditional geopolymer systems, high water content increases the crystallinity, decreases the reaction rate and negatively affects the microstructure of samples. In high carbon rice hull ash system, silica concentration is not a suitable indication of the silica availability for reaction, and the amount of unburned carbon in ash particles affects silica release rate. Increasing the silica content of raw materials leads to higher amount of Si/Al ratio in the final geopolymer binder and improves the mechanical and microstructural properties of samples. All samples studied here successfully made geopolymer binders. The highest strength achieved was 22 MPa after 3 weeks.

Size-Related Evaluation of Unburned Carbon

ABSTRACTThe unburned carbon (UBC) content of ash from coal-fired thermal power stations and plants that use massive amount of coal can sometimes be 6–7% or more; this simply means loss of carbon and decrease in efficiency. Carbon loss not only causes a decrease in efficiency but is also an economical issue. Decreasing the carbon content would provide the usage of ash from thermal power stations as cement additives. Many factors cause low combustion efficiency and a high amount of UBC. In the course of this study, fly ash and bottom ash were divided into size fractions and accumulation of UBC at coarser size fractions was observed. In this study, a ground coal sample from a mill was sieved, and the accumulation of UBC at coarser size fractions was associated to a lack of grinding of pulverized coal. Ground coal has a size distribution where 54.45% was below 75 µm, and 45.55% was still over this critical size. A significant amount (35.48%) of the UBC accumulated in the size fraction over 75 µm in fly ash, whereas almost 70% accumulated in the size fraction over 0.5 mm in bottom ash. Coal and ash samples were observed under a microscope to deduce origins of UBC.

Modeling of unburned carbon in fly ash and importance of size parameters

Unburned carbon (UBC) percentage in coal ash can be considered as an economic and environmental constraint since it is not only an important characteristic in terms of combustion efficiency but also it becomes more of an issue of further utilization of ash. In this study, particle size distribution (PSD) effect on UBC in fly ash (FA) was discussed and efficient size parameter of PSD in terms of UBC formation was determined. At the first stage of the study, regarding ash formation, percentage of any set of coal particles which are more likely to form fly ash (FA) was theoretically determined. For this purpose, a formulation including the size parameters was proposed. Secondly, with the help of a 3-day long systematical control of power, the effect of size distribution on UBC was obtained. The size distributions of these samples showed that an increase in D32 size parameter causes an increase in the amount of UBC as well. Relationships between the amount of UBC and mean values of size parameters (D10, D50, D90, D32, D43) were all statistically meaningful for both units of power plant concerned but D32 size parameter has a better fit among all the size parameters. In summary, D32 & UBC have a regression coefficient of 0.90 (R2~0.9), while other size parameters (D10, D50, D90, D43) & UBC have regression coefficients of 0.2 (R2~0.2) for both units. Finally, a model including these size parameters was proposed to predict UBC in FA. The model proposed was in good agreement with the measured UBC in FA.

The removal of unburned carbon from fly ash by kerosene extraction

The recycling process was developed to remove and recover unburned carbon from fly ash using kerosene extraction. The effects of experimental factors such as shaking time, initial unburned carbon content, and organic to suspension (O/S) ratio on the removal of unburned carbon were investigated to obtain the optimum conditions. The results indicate that the removal ratio of unburned carbon increased with increasing shaking times and decreasing the initial unburned carbon content, but the residual amount of unburned carbon was independent of O/S ratio. The content of unburned carbon decreased successfully to less than 3% under the following conditions; 15 min in shaking time, 10% in pulp density, 30 °C in temperature, 200 rpm in shaking speed, and 0.5 in O/S ratio.

Plasma assisted power coal combustion in the furnace of utility boiler: Numerical modeling and full-scale test

This work presents modern plasma technology for solid fuel ignition and combustion. It promotes more effective and environmentally friendly low-rank coal incineration. To implement this technology at coal fired thermal power plants plasma-fuel systems (PFSs) were developed. PFS is a pulverized coal burner equipped with arc plasmatron producing high temperature air stream of 4000–6000°C. Basis of technology PFS is plasma thermo-chemical preparation of coal for burning. It consists in plasma heating of air–coal mixture up to temperature of coal volatiles release and char carbon partial gasification. In PFS air–coal mixture is deficient in oxygen therefore carbon is oxidized mainly to carbon monoxide. As a result, at the PFS exit a highly reactive mixture is formed of combustible gases and partially burned char particles, together with products of combustion, while the temperature of the mixture is around 1050°C. Further mixing with air promotes intensive ignition and combustion of the prepared in the PFS fuel.PFS have been tested for boilers start up and pulverized coal flame stabilization at 30 power boilers of 75–950t/h steam productivity. The boilers were equipped with different types of pulverized coal burners: direct flow, muffle and swirl burners. At tests of the PFS power coals of all ranks (lignite, bituminous, anthracite and their mixtures) were incinerated. Volatile content of them was in range of 4–50%, ash varied from 15% to 48% and heat of combustion was from 6690 to 25100kJ/kg.To show advantages of the plasma technology of coal combustion before conventional one numerical investigation of plasma ignition, gasification and thermo-chemical preparation of air–coal mixture for incineration in a power boiler was fulfilled. The numerical modeling was performed for low-rank bituminous coal of 40% ash content incinerated at a boiler of 420t/h steam productivity. Both analysis of the numerical modeling and experience of PFS industrial use showed ecological efficiency of the plasma technology. When plasmatrons operate in the regime of plasma stabilization of pulverized coal flame, NOX emission is reduced twice and amount of unburned carbon is reduced four times. The PFSs reduce the temperature at the exit of the furnace. Tests of the PFS for the boiler BKZ-420 of Almaty TPP-2 cold startup confirmed the possibility of high-ash Ekibastuz coal ignition in the cold furnace without heating of primary air.

An Investigative Study on the Chemical, Morphological and Mineralogical Alterations of Dry Disposed Fly Ash During Sequential Chemical Extraction

The hazardous elements associated with various physicochemical forms in coal fly ash are of environmental concern due to their leaching potential and subsequent contamination of surface and groundwater in the vicinity of the ash dump. Selective sequential extraction was performed on dry disposed fly ash samples from a coal-fired power station in Mpumalanga province, South Africa. The alteration of the chemical, morphological and mineralogical species of weathered fly ash during the selective sequential extraction was investigated using X-ray fluorescence (XRF), Nano-scan electron microscopy (NANOSEM) and X-ray powder diffraction (XRD). Insoluble residue from the water-soluble fraction is composed of amorphous alumino-silicate. The residues from exchangeable carbonate and Fe and Mn fractions consisted mostly of amorphous alumino-silicate spheres with a lesser quantity of iron-rich spheres. The iron-rich spheres are surrounded by amorphous alumino-silicate spheres. The leaching behavior of trace metals (such as Ce, Y, Nb, Rb, U, and Tl) in weathered dry disposed fly ash was considered to have a dependency relationship with the components of SiO 2 , CaO, MgO, P 2 O 5 , and amount of unburned carbon. The decrease in the quantities of calcite with successive extraction could be considered as a marker of progress of sequential extraction technique. At the same time, the increase in the quantities of quartz could be also considered as an indicator of progress of the sequential extraction scheme. Trace elements bound to exchangeable or carbonate fraction during sequential chemical extraction were found associated with calcite. The elemental concentrations, as determined by electron dispersive x-ray spectroscopy (EDS), were consistent with XRF and XRD data. Therefore, the chemical extractant used in this study proved efficient for extraction of inorganic metals associated with various physicochemical forms in weathered fly ash. Key words: Coal fly ash; Selective sequential extraction; Major components; Trace elements; Morphology; Mineralogical composition

SHORT-TERM TESTS ON FIRING OIL SHALE FUEL APPLYING LOW-TEMPERATURE VORTEX TECHNOLOGY

The present paper provides experimental data from short-term, full-scale experiments of a high-pressure pulverized firing (PF) boiler, TP-67, firing oil shale after a retrofit to use vortex combustion (VC) technology. The essence of VC technology consists of the generation, at a lower part of the furnace, of a circulatory motion of gas relative to the horizontal axis (horizontal vortex) by rearranging the geometry of the combustion air injection and the fuel feeding into the combustion chamber. The tests were conducted at three boiler loads: 50% (160 t/h), 75% (240 t/h) and 100% (320 t/h). During the experiments, fuel samples of air-solids in the conduit and samples of bottom ash and fly ash from inertia dust collectors after the super heater (SH) and economizer (ECO), as well as fly ash from the electrostatic precipitator (ESP) of the first and second fields, were taken from both the left and the right sides of the boiler. The gas analysis was performed at the ESP exit. It was attempted to measure temperature distribution in the combustion chamber. Temperature measurements in the furnace using an infrared thermometer showed that the maximum temperature did not exceed 1150 °C, and there was slight temperature nonuniformity across the combustion chamber. During the tests, the ash distribution at different boiler ash discharge ports was obtained. Analysis of the bottom ash chemical composition showed a considerable increase in the amount of unburned carbon and marcasite Sp. The retrofit of the boiler to use VC technology did not result in a reduction in the amount of SO2 emissions, indicating an even weaker process of binding sulphur oxides in the furnace as compared to PF.

Prediction of NO<SUB>x</SUB> and unburned carbon in ash in highly staged pulverised coal furnace using overfire air

Abstract NOx emissions from power plants burning pulverised coal are subject to increasingly stringent regulations, and this has a consequential effect on the amount of unburned carbon (UBC) produced. The prediction of both is important, and a number of modelling techniques are available, but there could be difficulties especially under highly staged combustion. This is because these regions have low oxygen concentrations where char gasification reactions might take place in addition to the normal combustion. The present paper discusses the role of these reactions mainly within a staged tangentially T fired furnace, which uses overfire air for NOx reduction, but it also considered a wall fired case. Predictions for UBC and NOx in these furnaces, which are represented by a series of slices, were made with and without the gasification equations and are compared with test data from furnaces. These results indicate that the char gasification reactions play only a small role even under highly staged conditions and only for the larger UBC particles.

Characterization of Low-temperature Coal Ash Behaviour under Atmospheric Pressure

The chemical composition of the virgin coal from Korba coal fields,South central India is determined with the help of Fourier transform Infrared spectroscopy. The process of burning of carbon rich coal is carried out from a temperature of 200˚C to 700˚C under atmospheric pressure and controlled air in a muffle furnace. The resultant coals are examined with FTIR. The result shows that the residence time of coal ash at high temperature has considerable influence on the composition of coal ash and little effect on the amount of unburned carbon. With increase of temperature the oxygenated groups are released mainly as H2O and CO, but also as aldehydes, alcohols and acids. The silicate and quartz mineral content remains the same throughout heating.

Characterization of low-temperature coal ash behaviors at high temperatures under reducing atmosphere

The coal ash obtained at 815 °C under oxidizing atmosphere was further treated at 1300 °C and 1400 °C under reducing atmosphere. The resultant ashes were examined by XRD, SEM/EDX and FTIR. The results show that the residence time of coal ash at high temperatures has considerable influences on the compositions of coal ash and little effect on the amounts of unburned carbon. The amorphous phase of mineral matters increases with the increasing temperature. The FTIR peaks due to presence of different functional groups of minerals support the findings of XRD, and supply additional information of amorphous phase which cannot be detected in XRD. The ash samples generated from a fixed bed reactor during char gasification were also studied with FTIR. The temperatures of char preparation are responsible for the different transformation of minerals during high temperature gasification.

Thermonuclear supernova simulations with stochastic ignition

We apply an ad hoc model for dynamical ignition in three-dimensional numerical simulations of thermonuclear supernovae assuming pure deflagrations. The model makes use of the statistical description of temperature fluctuations in the pre-supernova core proposed by Wunsch & Woosley (2004, ApJ, 616, 1102). Randomness in time is implemented by means of a Poisson process. We are able to vary the explosion energy and nucleosynthesis depending on the free parameter of the model which controls the rapidity of the ignition process. However, beyond a certain threshold, the strength of the explosion saturates and the outcome appears to be robust with respect to the number of ignitions. In the most energetic explosions, we find about of iron group elements. Other than in simulations with simultaneous multi-spot ignition, the amount of unburned carbon and oxygen at radial velocities of a few tends to be reduced for an ever increasing number of ignition events and, accordingly, more pronounced layering results.

Burn-out of pulverised coal and biomass chars ☆

The burn-out of carbon in pulverised fired power stations is commercially important. Interest in the burn-out of biomass chars is growing because biomass is increasingly being co-fired with coal to reduce the carbon dioxide emissions. The significance of carbon burn-out is that it is linked with the efficiency of the plant and the suitability of the coal ash for construction purposes. Residual carbon in ash has generally increased in recent years because of the influence of the lower temperatures and slower mixing resulting from the use of low NO x burners. The amount of unburned carbon is thus a function of the plant design and operating conditions but it is also linked to the ease of combustion of the coal and the char formed. These latter factors are related to the properties of the coal and this paper attempts to quantify the impact of certain coal and char properties on carbon-out. An approach for assessing biomass combustion performance is also discussed.