Modeling of single char combustion, including CO oxidation in its boundary layer

Abstract

AbstractsA model incorporating intrinsic surface reaction, internal pore diffusion, and external mass transfer was developed to predict a transient temperature profile during a single char particle combustion. This model provides useful information for particle ignition, burning temperature profile, combustion time, and carbon consumption rate.A gas phase reaction model incorporating the full set of 28 elementary C/H/O reactions was developed. This model calculated the gas phase CO oxidation reaction in the boundary layer at particle temperatures of 1250K and 2500K by using the carbon consumption rate and the burning temperature at the pseudo-steady state calculated from the temperature profile model, but the transient heating was not included. This gas phase model can predict the gas species and the temperature distributions in the boundary layer, the CO2/CO ratio, and the location of CO oxidation.These models were applied to the ignition temperature profile and the CO 2 /CO ratio obtained by Tognotti (Tognotti et al., 1990) in an electrodynamic balance for 180 gm Spherocarb particles and to the combustion rate data measured by Tullin (Tullin et al., 1993) in a fluidized bed combustor for 4 mm Newlands char particles.The temperature profile model reproduced the experimental measurement of Spherocarb combustion temperatures in an electrodynamic balance, including the dependence of particle ignition on the oxygen partial pressure. Increasing the oxygen partial pressure reduces the combustion time and increases the maximum temperature. The internal diffusion limitation could be treated by using macropore surface area. The particle diameter does not have a large impact on the maximum temperature rise, but has a considerable effect on the combustion time. The addition of mineral catalyst promotes heterogeneous CO 2 formation and raises the particle temperature.A gas phase reaction model was used to calculate the CO2/CO ratio measured by Tognotti. Different water vapor concentrations at low and high particle surface temperatures (1250K and 2500K) were used to see the effect of hydrogen containing species on CO oxidation in the gas phase. The particle diameter was 180 gm. The bulk gas was a mixture of 100% oxygen and water vapor.At a temperature of 1250K, there was no significant CO oxidation in the gas phase even with 3.5% water vapor due to the small size of the particle and the steep temperature gradient. Estimates of gas phase reaction in the macropore could not account for the high CO 2 /CO ratio indicating that the presence of water vapor may enhance the rate of heterogeneous oxidation of CO.At a high temperature of 2500K, without hydrogen containing molecules the gas phase oxidation of CO is negligible. The effect of the hydrogen containing component on 2 the CO oxidation was significant. Spherocarb particles have 0.74% of 'H' internally. This internal hydrogen source was enough to explain the observed CO2/CO ratio.A mechanistic heat and mass transfer model was added to the temperature profile model to predict ...

Simulation of the Influence of the Coal Volatile Matter Content on Fuel NO Emissions in a Drop-Tube Furnace

“…Subsequently, the CO 2 concentration gradually increases while the CO concentration decreases along the axis. This is consistent with the results of Lee . It is well-known that CO and CO 2 are the primary products of the heterogeneous reaction and that CO is further oxidized to CO 2 by subsequent reactions.…”

“…It is well-known that CO and CO 2 are the primary products of the heterogeneous reaction and that CO is further oxidized to CO 2 by subsequent reactions. Lee stated that the ambient temperature has a significant effect on CO oxidation via subsequent homogeneous reactions. The initial CO concentration obtained with the CPD submodel is higher than that obtained with the two-competing-rates submodel because of more rapid devolatilization near the top of the reactor.…”

Impacts of Oxygen Diffusion on the Combustion Rate of In-Bed Soot Particles

“…The temperature at the top of the particle layer ( T s ) could be first determined from the energy balance eq at the top surface of the layer as illustrated in Figure . Most of thermodynamic and transport properties used in the equation are taken from work on coal char combustion, , while the combustion rate measured in this study is used for ṙ c . To obtain the heat transfer rate at the surface ( Q̇ out ), the temperature profile of the gas phase from the top of the particle layer( x = x t ) to the top of the TGA crucible( x = L t ) should be calculated where both reaction-induced convection and conduction are considered with two known temperatures in one-dimensional coordinate. − Q̇ normalo normalu normalt + Ė normalg = 0 , ṙ normalc normalΔ h normalc = A tc B normalf false( x t − L t false) normale B normalf 1 − normale B normalf false( T normals − T ∞ false) B normalf = false( 1 − normalψ false) C p , normalm normalo normall ṙ normalc false( x normalt − L normalt …”