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In this paper, a mathematical model of the fly ash deposition in a 660 MW supercritical once-through boiler was developed. The mechanism of particle-particle and particle-wall collisions has been considered. Particle collision is modeled by a direct simulation Monte Carlo (DSMC) method. Three submodels (impaction, thermophoresis, and sticking) were included in the mathematical model. In the ash deposition model, the critical velocity model was used to calculate the ash sticking rate during initial deposition, and the viscosity model was used for the outer layer deposition. An ash deposition probe was inserted into the peephole and ash deposits were collected. The morphology of these samples was analyzed using SEM. The deposit thickness and the distributions of adhesion particle size were obtained by mathematical model. The mean deposit thickness calculated by the DSMC method agreed with the value measured by the ash deposition probe with the relative error within 3.79%. The simulation results show that the deposition inner layer consists of fine particles less than 10 μm in size and sticky fine particles were selectively captured during the initial deposition stage. The DSMC method may be a practical alternative for studies of ash deposit formation. [ABSTRACT FROM AUTHOR]

There has been a controversy about the reactivity of coal/char combustion under oxyfuel and air conditions. The conversions of NCP anthracite and DT bituminous coal chars were studied under different atmospheres, using simultaneous thermogravimetry and differential scanning calorimetry (TG–DSC). A model of the intrinsic reaction rate was developed to obtain the variable activation energy at different temperatures and char conversion ratios in terms of a TG–DSC curve only. In the endothermic stage, the chemisorption obeys the same mechanism in O 2 /CO 2 , O 2 /N 2 and O 2 atmospheres, in which the reaction order is about 0.48 in the n th-order model, while in CO 2 is a zeroth-order reaction; both of them are not affected by coal rank. The chemisorption activation energy of CO 2 is less than that of O 2 . The chemisorption activation energies in O 2 /CO 2 are more than that in either 100% CO 2 or 100% O 2 , showing that both O 2 and CO 2 compete for the same active sites. The char-O 2 chemisorption is inhibited by the char-CO 2 chemisorption, resulting in a lower reactivity in O 2 /CO 2 . Subsequently, in the exothermic stage, the NCP and DT char combustions obey different mechanisms, in which the reaction orders are 1.65 and 1, respectively. According to the significant differences in variable activation energies obtained from the TG and the DSC data, the char reaction can be divided into a multi-step chemisorption oxidation stage and a one-step burning stage. Compared with TG method, DSC method is more suitable for describing the reactive characteristics of the multi-step stage. The results show that the char-O 2 /CO 2 chemisorption partially shares common active sites with a complicated multi-step mechanism. The trends of variable activation energies with char conversion ratio at different O 2 concentration in O 2 /CO 2 are similar. The activation energy is not significantly affected by CO 2 , and is about 178 ± 11 kJ/mol. The intrinsic reaction mechanisms of DT char combustion in O 2 /CO 2 and O 2 /N 2 have no fundamental difference; however, the NCP char combustion is affected by both CO 2 and oxygen partial pressure. [ABSTRACT FROM AUTHOR]

• An experimental design was used to investigate the effects of different torrefaction conditions on the properties of biomass. • Two spectrometers were simultaneously used to capture high SNR signals of continuous spectrum and characteristic spectrum. • In-situ detection of the temperature and K release concentration of torrefied biomass pellet combustion was achieved. • Changes in combustion characteristics and K release characteristics of torrefied biomass were evaluated. The orthogonal experiment designed in this paper investigated the effect of changes in experimental parameters such as biomass species, torrefaction temperature, torrefaction time, torrefaction oxygen concentration on the physicochemical properties and combustion characteristics of the torrefaction products. Two optical fiber spectrometers were used to capture the spontaneous emission spectra of the combustion flame of compressed pellets of torrefied biomass, and the in-situ measurement of pellet combustion temperature and K release concentration at high heating rate was realized by multi-spectral analysis algorithm. The experimental results indicate that for biomass, torrefaction will lead to a decrease of volatile and moisture content and an increase of fixed carbon and ash content in components, a decrease of O and H and an increase of C content in elements, a certain loss of biomass quality, an increase of high heat value, and each indicator move closer to coal. The effects of various experimental parameters on the volatile content, mass yield, high heat value increase rate and energy yield in biomass torrefaction products have similar laws, i.e., biomass species / torrefaction oxygen concentration > torrefaction temperature > torrefaction time. The mild torrefaction product pellets have a shorter ignition delay, increased volatile combustion flame height and peak temperature, longer volatile and char combustion duration, but lower peak char combustion temperature, among which the selected torrefaction products of rice straw and pine wood showed an increase in peak temperature of 0.56 % (11 K) and 0.61 % (12 K) for the volatile stage respectively, and a decrease of and 1.29 % (20 K) and 1.29 % (21 K) for the char stage respectively. In addition, the peak K release concentration during combustion of the mild torrefaction product pellets was reduced, among which the peak K release concentrations during the volatile and char stage were reduced by 43.22 % (3.89 ppm) and 52.93 % (51.55 ppm) of the rice straw torrefaction product respectively, and by 73.14 % (1.28 ppm) and 62.08 % (15.01 ppm) for the pine wood torrefaction product respectively. Biomass torrefaction products have fuel properties closer to coal, less K released from the combustion process, which have the potential to become an alternative to coal. [ABSTRACT FROM AUTHOR]

• The radiative preheating of the fuels at pressures from 1 to 100 atm was studied. • The critical strain rate was calculated for eight C 1 -C 6 hydrocarbon species. • The radiation time scale would dominate at elevated pressures. The radiative preheating of the unburned hydrocarbon fuels by the burned combustion products in counterflow diffusion flame at pressures from 1 to 100 atm was investigated. The radiative characteristics of hot H 2 O, CO 2 and cold hydrocarbon gases were calculated from HITEMP 2010, CDSD-4000 and HITRAN 2016 databases respectively. A simplified radiation reabsorption model was given based on the properties of the radiative reabsorption energy at T = 1500–2000 K, P = 1–100 atm. By using this model, the critical strain rate was calculated for eight C 1 -C 6 hydrocarbon species at T = 2000 K, P = 1–100 atm. The results show that radiation reabsorption should be taken into account when the strain rate is less than 2.9 s−1 at P = 100 atm. The comparison results of the radiation time and the residence time in the CH 4 counterflow diffusion flame shows that the radiation time scale would dominate at elevated pressures. [ABSTRACT FROM AUTHOR]