An Efficient Method for Predicting Unburned Carbon in Boilers

Fiveland, W. A.; Jamaluddin, A.S.

doi:10.1080/00102209208951799

Cited by 13 publications

(3 citation statements)

References 2 publications

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“…The combination of in situ optical measurements and independent captive particle image sequences provide dramatic illustration of the asymptotic nature of the char burnout process. Single particle combustion to complete burnout comprises two distinct stages: (1) a rapid high-temperature combustion stage, consuming approximately 70% of the char carbon and ending with near-extinction of the heterogeneous reactions due to loss of global particle reactivity, and (2) a final burnout stage occurring slowly and at lower temperatures.…”

Section: And Conclusionmentioning

confidence: 99%

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Near-extinction and final burnout in coal combustion

Hurt

Davis

1994

Symposium (International) on Combustion

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The authors prefer an oral presentation and publication in the proceedings volume E. Experimental Data and Intet-pretation; D. Experimental Methodologies (6.2) Coal and Organic Solids Combustion Word count , body 3501 wds nomenclature table 126 wds (18 lines x 7 wds/line) references 336 wds (48 lines x 7 wds/line) dk ABSTRACTThe late stages of char combustion have a special technological significance, as carbon conversions of 99% or greater are typically required for the economic operation of pulverized coal fired boilers. In the present article, two independent optical techniques are used to investigate near-extinction and final burnout phenomena for Illinois #6 and Pittsburgh #8 bituminous coals. Captive particle image sequences, combined with in situ optical measurements on entrained particles, provide dramatic illustration of the asymptotic nature of the char burnout process. Single particle combustion to complete burnout is seen to comprise two distinct stages: (1) a rapid high-temperature combustion stage, consuming about 70% of the char carbon and ending with near-extinction of the heterogeneous reactions due to a loss of global particle reactivity, and (2) a final burnout stage occurring slowly and at lower temperatures. For particles containing mineral matter, the second stage can be further subdivided into: (2a) late char combustion, which begins after the nearextinction event, and converts carbon-rich particles to mixed particle types at a lower temperature and a slower rate; _d (2b) decarburization of ash u the removal of residual carbon inclusions from inorganic (ash) frameworks in the very late stages of combustion. This latter process can be extremely slow, requiring over an order of magnitude more time than the primary rapid combustion stage. For particles with very little ash, the loss of global reactivity leading to early near-extinction is clearly related to changes in the carbonaceous char matrix, which evolves over the course of combustion due to simultaneous oxidation and heat treatment. Current global kinetic models used for the prediction of char combustion rates and carbon burnout in boilers do not predict the asymptotic nature of char combustion. More realistic models accounting for the evolution of char structure are needed to make accurate predictions in the range of industrial interest. DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Governmen...

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Section: And Conclusionmentioning

confidence: 99%

“…NOx abatement [1]. While there is an extensive literature on ignition [2], and char combustion [3][4][5], little attention has been paid to extinction in the important late stages of combustion, in part due to the difficulty of making scientifically meaningful measurements on highly reacted samples in which inorganic material (ash) is the majority constituent.…”

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confidence: 99%

Near-extinction and final burnout in coal combustion

Hurt

Davis

1994

Symposium (International) on Combustion

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“…and Chen et al, calculated char conversion in a 500 MW (electric) utility boiler, identifying four contributions to carbon loss: unburned particles settling into the ash hopper, less than optimum distribution of coal among the burner levels, large particles in the feed, and high particle concentrations near the walls, where oxygen was depleted. However, when time and resources are limited, a one-dimensional approach is still useful for examining the sensitivity of carbon loss to coal properties, furnace design, burner adjustment, and combustion conditions, , especially in the face of practical problems, such as air inleakage, not amenable to detailed treatment.…”

Section: Introductionmentioning

confidence: 99%

Analysis of Carbon Loss from a Pulverized Coal-Fired Boiler

Walsh

1997

Energy Fuels

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Despite the large body of excellent work that has been done on the quantitative description of char combustion, our ability to specify the conditions to be met in order to maintain the carbon loss from pulverized coal-fired boilers below a desired value is still limited. The problems arise, in part, from the large number of fuel, combustion, and equipment characteristics that may influence carbon burnout. Measurements of the concentration and size distribution of carbon in fly ash from a 30 MW (electric) utility boiler have been used to test assumptions regarding the factors controlling carbon conversion. The principal explanation for unexpectedly high carbon losses under the conditions investigated was air leakage into the furnace and convection sections, which caused the furnace gas to be richer than expected from the flue gas analysis. Assuming that the inleaking air made no contribution to combustion, a calculation of carbon burnout, using the mean char combustion rate of Hurt and Mitchell [Twenty-Fourth Symposium (International) on Combustion, 1992Combustion, , pp 1243Combustion, -1250 and the char reactivity distribution of Hurt, Lunden, Brehob, and Maloney [Twenty-Sixth Symposium (International) on Combustion, 1996, pp 3169-3177], reproduced the carbon loss and size distribution of unburned particles and was within a factor of 2 of the Babcock & Wilcox correlation of carbon loss with stoichiometric ratio [Steam/Its Generation and Use, 37th ed., 1963, p 17-21]. According to the calculations, the distribution of char reactivities has a significant influence on the carbon loss and abundance of small particles in the unburned carbon size distribution.

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Evolutionary Changes in Furnace Combustion Conditions Which Affect Ash Deposition in Modern Boilers

FitzGerald¹

1996

Applications of Advanced Technology to Ash-Related Problems in Boilers

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An Efficient Method for Predicting Unburned Carbon in Boilers

Cited by 13 publications

References 2 publications

Near-extinction and final burnout in coal combustion

Near-extinction and final burnout in coal combustion

Analysis of Carbon Loss from a Pulverized Coal-Fired Boiler

Evolutionary Changes in Furnace Combustion Conditions Which Affect Ash Deposition in Modern Boilers

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