Near-extinction and final burnout in coal combustion

Abstract

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...

Modeling high-pressure char oxidation using langmuir kinetics with an effectiveness factor

“…For all of these three conditions, the temperature profiles all show a characteristic drop near the end of combustion (at about 85% dry ash free [daf] burnout). This temperature drop is consistent with the near-extinction behavior observation by Hurt and Davis [27]. The main interest of this study is the reaction rates before the near-extinction stage and, therefore, average reactivities were determined for the 10%-70% burnout region.…”

Combustion Aerosols: Factors Governing Their Size and Composition and Implications to Human Health

“…125 The models provide useful qualitative information but cannot predict quantitative trends with confidence. The fragmentation scales with the ratio of the volume fraction of ash in the char to the void fraction, Λ.…”

“…129,130,[187][188][189] Coal fly ash typically contains less than 5% unburned carbon, but problems with char burnout can result in much higher carbon values. 125,[190][191][192] Elemental balances on power plants show that ~1-4% of most metals in the fuel are emitted in the stack PM. 143,148,152,178,193 The major exceptions are Se and Hg, which escape as vapors.…”

SCT reaction kinetics model and diffusion for p.c. combustion in TGA

“…[11,12] It was presented that it is a two-step char burnout kinetics model described where about 50-80% of char depleted rapidly at high temperature, but the residual part is of deactivation and near extinction at lower temperature. [13] However, the role of internal surface of char is not sufficiently considered in the CBK model. Reaction kinetics and pore diffusion resistance are coupled to pseudo-kinetics model, accounting for the available surface area of char burning.…”