Amount Of Unburned Carbon Research Articles

A simple numerical model to estimate the effect of coal selection on pulverized fuel burnout

The amount of unburned carbon in ash is an important performance characteristic in commercial boilers fired with pulverized coal. Unburned carbon levels are known to be sensitive to fuel selection, and there is great interest in methods of estimating the burnout propensity of coals based on proximate and ultimate analysis--the only fuel properties readily available to utility practitioners. A simple numerical model is described that is specifically designed to estimate the effects of coal selection on burnout in a way that is useful for commercial coal screening. The model is based on a highly idealized description of the combustion chamber but employs detailed descriptions of the fundamental fuel transformations. The model is validated against data from laboratory and pilot-scale combustors burning a range of international coals, and then against data obtained from full-scale units during periods of coal switching. The validated model form is then used in a series of sensitivity studies to explore the role of various individual fuel properties that influence burnout.

The Fluidity of Cement Pastes with Fly Ashes Containing a Lot of Unburned Carbon

Fly ashes containing 6.1∼16.5 wt% of unburned carbon were treated thermally at 500℃ for 3h and thus, the content of unburned carbon was decreased below 2.1 wt%, the range of particle size distribution became narrower and the mean particle size became smaller. Besides, the properties of particles in fly ashes were improved, particularly the particle shape became close to a spherical type. The fluidity of cement pastes containing fly ashes treated previously at 500℃ for 3h was increased much than that of cement pastes containing original fly ashes. When the added amount of superplasticizer was over the saturation amount, there was no correlation between the amount of unburned carbon in fly ashes and the apparent viscosity of cement pastes actually. On the contrary, when the added amount of superplasticizer was below the saturation amount, there was a correlation.

234 熱分解ガス化溶融灰からのダイオキシン類生成に対する影響要因の解析

This paper describes de novo synthesis potential of gasification-melting system differ form that of conventional incinerator because the composition of fly ash is largely different. The fly ash from gasification-melting plant contained very little amount of unburned carbon which could serve as a carbon source for dioxins formation. In contrast, volatile heavy metals such as Pb, Zn and Cu in fly ash which could serve as catalyst were enriched. In this study, experiments were performed using various fly ashes from gasification-melting plants in order to clarify the effect of unburned carbon and metal components in fly ash on de novo synthesis.

00/00396 Computational investigation of carbon-in-ash levels for a wall-fired boiler after low-NO x combustion modifications

Many coal-fired boilers retrofitted with low NOX firing systems are experiencing significant operational difficulties due primarily to (1) increased carbon in the fly ash or (2) increased water wall wastage. This paper presents the results of a computational investigation of a wall-fired boiler that has recently been retrofitted with low NOx burners and overfire air. The focus of this paper is the effect of the retrofit on unburned carbon/NOx and the potential for applying operational and design modifications to minimize unburned carbon without adversely impacting NOx emissions. Although the magnitude of an increase in unburned carbon after a low NOx retrofit is system and coal dependent, it is often the case that reduction in emissions of nitrogen oxides is accompanied by a corresponding increase in the amount of unburned carbon in fly ash. Since low NOX firing systems increase the residence time of coal/char particles in a fuel rich environment, it might be expected that there is insufficient time under high temperature, oxidizing conditions to ensure complete carbon oxidation in a low NOx firing system. A relatively straight forward consideration of the effect of temperature and oxygen concentration on coal particle pyrolysis/oxidation can be used to provide a qualitative understandingmore » of this effect. However, the complex flow patterns and highly nonlinear physical and chemical phenomena in a boiler make it difficult to predict carbon-in-ash (c-i-a) levels without the use of advanced computational tools. The sensitivity of c-i-a to burnout is pointed out in a figure for a coal with 10% ash. Although c-i-a changes slowly for low burnout, it changes very rapidly at the extent of burnout typical of a boiler. When the extent of burnout drops only slightly, from 99.5 to 99 percent for example, the c-i-a doubles, from 4 to 8%, which in many situations would be unacceptable. The importance of fuel efficiency and ash disposal/recycle emphasizes the need for understanding and addressing this issue. The computational tools used by Reaction Engineering International (REI) have been developed to address the operational and design considerations of a wide range of combustion systems including utility boilers, pyrolysis furnaces, rotary kilns, waste incinerators, flash smelters, and smelting cyclones. The current models simulate reacting flows and particles, including gaseous diffusion flames, pulverized-coal flames, liquid sprays, coal slurries, injected sorbents, and other oxidation/reduction systems. In particular, emphasis has been placed on simulating coal combustion and pollutant formation.« less

Fireside corrosion of selected alloys by ash recovered from coal-water slurry combustion

The corrosion behavior of five selected stainless steels was evaluated in a simulated fireside corrosion atmosphere for a coal-water slurry system. Two types of ash, generated from actual combustion tests of coal-water slurries, were used as the fireside test deposits. The ashes differed in the amount of unburned carbon in each. The alloys selected were Types 304, 316, 321, 347 stainless steels and high-nickel 800 M. Tests were conducted for 48 h at temperatures of 600, 700, 800°C in a simulated flue gas, representative of the atmosphere expected in coal-water slurry combustion, and at 1000°C in inert atmosphere. Analyses of the test coupons after removal from the furnace included scanning electron microscopy, optical microscopy, and weight-loss measurements. Scanning electron microscopy showed the presence of fused corrosion deposits, proving that liquid phases were involved in the corrosion process. Microprobe analysis of selected samples detected no sulfur in the corrosion products, suggesting that complex sulfates were not being formed. The low-carbon ash was more corrosive toward the alloys than the high-carbon ash. The high-carbon ash tended to shift the temperature of maximum corrosion toward higher temperatures. Type 316 stainless steel showed the highest corrosion losses among the five alloys tested with the low-carbon ash at 600–800°C. For tests with the high-carbon ash, the corrosion losses for all the alloys, except for Type 347, were low. The differences in corrosion losses between the types of ashes relate to the differences in carbon and calcium contents of the two ashes. The overall corrosion losses were small when compared to literature values for fireside corrosion losses in pulverized coal combustion.

Unburned carbon loss from pulverized coal combustors

Because the amount of unburned carbon in fly ash from pulverized coal-fired boilers is such a small proportion of the combustibles in the fuel, its origin is usually in atypical properties of the fuel and furnace, rather than their average characteristics. Knowledge of the distributions of coal properties and combustion conditions is, therefore, one of the requirements for a useful simulation of carbon burnout. A reactor engineering approach to this problem, which accounts for the distributions of particle size, residence time, and mixedness of fuel and air has been adopted. The model was tested by comparison with measurements of combustibles in fly ash from a 136000 kg steam h −1 boiler firing medium (mvb) and high (hvb) volatile bituminous coals. Size distribution, as well as the concentration of unburned carbon in fly ash were used to test the accuracy of the simulation of char combustion. The key to the calculation is the allowance for distributions of residence time in the furnace and mixedness of coal and air, without which it is not possible to explain the presence of small unburned particles in fly ash. The coal and plant data required for the calculations are: proximate and ultimate analyses, heating value, screen analysis of the pulverized coal, the dimensions of the furnace, excess oxygen, air preheat, furnace exit gas temperature and a representative sample of the fly ash. The fly ash samples were screened and the carbon content determined for each size cut. The model reproduced the observed dependence of amount and size distribution of unburned combustibles on both grind and volatile matter. The principal adjustable parameters in the calculation are the rate coefficient for char combustion, the mixedness of fuel and air at the end of the volatiles flames, the characteristic mixing time in the post-flame region, and the average gas temperature in the furnace. Although no particle-to-particle variation of char reactivity was introduced, the slow char combustion rate required to fit the measurements suggests that unreactive char may have made a disproportionate contribution to the unburned. The model agrees with the dependences of unburned combustibles on volatile matter, furnace loading and coal fineness reported by Babcock and Wilcox. However, the model exhibits higher sensitivity to excess air than the Babcock and Wilcox correlation. The accuracy of the calculation might be improved by introducing a lower than first-order dependence of the char combustion rate on oxygen and by accounting for size reduction of char particles during burnout.

Unburned carbon loss from pulverized coal combustors

Abstract Because the amount of unburned carbon in fly ash from pulverized coal-fired boilers is such a small proportion of the combustibles in the fuel, its origin is usually in atypical properties of the fuel and furnace, rather than their average characteristics. Knowledge of the distributions of coal properties and combustion conditions is, therefore, one of the requirements for a useful simulation of carbon burnout. A reactor engineering approach to this problem, which accounts for the distributions of particle size, residence time, and mixedness of fuel and air has been adopted. The model was tested by comparison with measurements of combustibles in fly ash from a 136000 kg steam h−1 boiler firing medium (mvb) and high (hvb) volatile bituminous coals. Size distribution, as well as the concentration of unburned carbon in fly ash were used to test the accuracy of the simulation of char combustion. The key to the calculation is the allowance for distributions of residence time in the furnace and mixedness of coal and air, without which it is not possible to explain the presence of small unburned particles in fly ash. The coal and plant data required for the calculations are: proximate and ultimate analyses, heating value, screen analysis of the pulverized coal, the dimensions of the furnace, excess oxygen, air preheat, furnace exit gas temperature and a representative sample of the fly ash. The fly ash samples were screened and the carbon content determined for each size cut. The model reproduced the observed dependence of amount and size distribution of unburned combustibles on both grind and volatile matter. The principal adjustable parameters in the calculation are the rate coefficient for char combustion, the mixedness of fuel and air at the end of the volatiles flames, the characteristic mixing time in the post-flame region, and the average gas temperature in the furnace. Although no particle-to-particle variation of char reactivity was introduced, the slow char combustion rate required to fit the measurements suggests that unreactive char may have made a disproportionate contribution to the unburned. The model agrees with the dependences of unburned combustibles on volatile matter, furnace loading and coal fineness reported by Babcock and Wilcox. However, the model exhibits higher sensitivity to excess air than the Babcock and Wilcox correlation. The accuracy of the calculation might be improved by introducing a lower than first-order dependence of the char combustion rate on oxygen and by accounting for size reduction of char particles during burnout.

Sulphur dew‐point corrosion resistant mechanism of austenitic stainless steels

Sulphur dew‐point corrosion occurs in the low temperature parts (<200°C) of heavy oil fired boilers such as economisers, air preheaters, and chimneys. Its mechanis proposed by Kowaka et al suggests that corrosion proceeds in three steps as shown in Figure 1. The first step is corrosion in sulphuric acid at low temperature and concentration, the second step is in sulphuric acid at high temperature and concentration, and the third step is in boiler deposits containing sufficient amounts of unburned carbon and sulphuric acid at high temperature and concentration. The overall corrosion rate was suggested to be mainly determined by the third step due to its much longer time compared with those of first and second steps. Kowaka et al also proposed a test method in the mixture of concentrated sulphuric acid and activated carbon for developing sulphur dew‐point corrosion resistant low alloy steels, and the results were well correlated with the actual field tests. This report extensively covers the corrosion behaviours of austenitic stainless steels which might have practical applications in high temperature environments.