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The differences in the gas thermophysical properties (including heat capacity, thermal conductivity and diffusivity) between char combustion in wet and dry environments have remarkable influences on the combustion process. We conducted a detailed experimental study on the thermal effects of H2O on the coal char combustion at 1773 K in a high-temperature drop tube furnace (HTDTF). An appropriate portion of Ar was introduced into the bulk gas (O2/H2O/N2) during combustion to eliminate the opposed thermal effect of H2O. Results indicate that the H2O thermal effect initially increased with higher H2O concentrations from 0 to 10 vol.%, but it declined with a further increase in H2O concentrations in the bulk gas. Meanwhile, its effect was dominant in the preliminary and middle stages of char burning at low H2O concentrations. The increased H2O concentration and residence time promoted the gasification reaction and the water-gas shift reaction so as to weaken the negative thermal effect of H2O and improve the carbon conversion. In the 15%O2/5%H2O/80%N2 environment, the relative contribution of thermal effects to carbon conversion ratios ranked up to −36% at the residence time of 0.3 s, but it decreased to −0.3% at 21%O2/30%H2O/49%N2 atmosphere and the residence time of 0.4 s. [ABSTRACT FROM AUTHOR]

The synthetic effect of H 2 O on the combustion characteristics of coal char is complicated due to its thermal (high volumetric heat capacity), chemical, and dilution (thereby low oxygen concentration) effects. To assess the impact of these factors on the carbon consumption process, we conduct a quantitative study on the net effect of H 2 O gasification through experimental and kinetic research. The experiment research is performed at 1773 K in a high-temperature drop tube furnace. The thermal effects of H 2 O are stripped off the net effect by introducing Ar into the bulk gas to neutralize its high heat capacity. We assess the chemical effects of gasification (gasification endothermicity, and its additional carbon consumption) for a 100 µm coal char particle reacting in environments of varying O 2 , H 2 O, and Ar using a self-developed char burning kinetics model. The relative contributions of H 2 O gasification and oxidation to carbon consumption are also discussed. Results indicate that the reduced carbon consumption through the negative effect of gasification (e.g., thermal effects and the gasification endothermicity) exceeds the additional carbon consumption from gasification reaction in the early combustion stages (< 0.3 s), but it is surpassed by the latter with char consumption. Increasing both the concentrations of H 2 O and O 2 favor the gasification endothermicity. The reduced carbon conversion ratio caused by the endothermicity effect increases as high to around 0.4 then drops down with char consumption. Besides, in the 15 vol.% O 2 case, an increase in H 2 O concentration from 5 vol.% to 30 vol.% improves the gasification contribution from 22.9% to 46.6% in the middle stage of combustion. It also widens the particle temperature gap by endothermicity effect to over 60 K, but the gap narrows in the burnout stage. Similar results were found in the air-fired case. The relative contribution from H 2 O gasification reaction is less influential upon the rise of oxygen concentration, which decreases by over 15% than in 15 vol.% O 2 case. [ABSTRACT FROM AUTHOR]

• This work studies char fragmentation and ash coalescence during coal char combustion. • Dense char structure disadvantages fragmentation but enhances mineral coalescence. • Perimeter fragmentation is dominant in diffusion-limited regime. • The ash content effect is more related to mineral distributions within char particles. Air pollution from fossil fuel combustion motivated the extensive and in-depth investigation on particulate matter (PM) to meet the stringent emission norms. The relationship between char fragmentation/mineral coalescence and PM formation is elucidated in this paper. To separately investigate their contributions to PM formation, the effects of particle bursting during devolatilization and fragmentation of excluded minerals have been stripped off. We prepared size-graded and density-fractionated chars with varying ash content levels and conducted combustion experiments in a drop-tube furnace at 1300/1600/1800 K. The mass particle size distributions (PSDs) of collected ash were measured by an Electrical Low Pressure Impactor (for PM 10) and a laser particle-size analyzer (for PM 10+). The micro- morphologies and elemental compositions analyses were also shown for particles of different sizes. The results indicate that char fragmentation significantly increases the number of ash particles in fine-fragmentation mode (FFM), and the ash peak in the coarse mode comes from the char particles that show fewer fragmentation behaviors. Dense char structure disadvantages char fragmentation but enhances coalescence of included minerals. The distribution of fly ash at 1600/1800 K exhibits bi-modal with two peaks around 10 and 105 μm, while the distribution at 1300 K/50%O 2 exhibits uni-modal with one peak. This can be well explained by the percolation theory that the char combustion in the kinetically-limited regime is dominated by uniform fragmentation, while particles at 1600/1800 K mainly undergo perimeter fragmentation. The effect of ash content is more related to the mineral grains distributions within the char particles. [ABSTRACT FROM AUTHOR]

The overall impact of CO 2 on the coal char combustion is complicated due to the interplay between the CO 2 thermal effect and chemical effect (including the gasification endothermicity, and its direct additional carbon consumption). To assess the impact of these factors on the carbon consumption process, we conduct a quantitative study on the separate physicochemical effects of CO 2 through experimental and kinetic research. Two oxygen concentrations (15% and 21% O 2), representing industrial boiler and air combustion environments, were selected. The experiment research on its thermal effect (evaluated by carbon conversion ratios) is performed at 1773 K in a high-temperature drop tube furnace. The chemical effect of CO 2 is predicted for a 100 μm coal char particle using a self-developed char burning kinetics model. Results indicate that the carbon conversion ratios show a V-type distribution with higher CO 2 concentrations, as a result of its complex physicochemical effects, with minimal carbon conversion points around the CO 2 concentrations of 5–10 vol.%. The thermal effect initially increases with higher CO 2 concentrations from 0 to 10 vol.%, but it declines with a further increase in CO 2 concentrations, while the endothermicity effect of the gasification reaction increases with higher CO 2 concentrations. The relative contribution of the endothermicity effect on the char consumption is 20.4% in the 21%O 2 /10CO 2 /N 2 environment, lower than the thermal effect (29.9%), but it continuously increases and becomes the most influential inhibitory factor at higher CO 2 concentrations. The strongest suppression effect of CO 2 corresponds to the CO 2 concentrations of 10 vol.%. [ABSTRACT FROM AUTHOR]

The char reactivity loss during the burnout stage is closely related to the effects of ash minerals, which show complex transformation behaviors (ash vaporization, ash film, and ash dilution). To assess the impact of ash behaviors, we computed the char consumption characteristics in a 21%O 2 /79%N 2 environment using the Char Burning and Particulate Matter Kinetics (CBPMK) model with a user-design allocation of the ash film and dilution. Regretfully, the ash film fractions have not been experimentally studied due to the irregular ash structure and the high combustion temperatures. On basis of the research on ash formation mechanism, this paper aims to provide appropriately measured ash film fractions of chars as a function of carbon conversion ratio for further development of the kinetic model. The ash film fraction F increases exponentially with carbon conversion ratio N , and their relationship is summarized as F = aN 23.66Y (0< a ≤ 1, 0< N <1), where a is the coefficient related to the combustion temperature and mineral components, and Y denotes the initial ash content in coal char. The kinetics model CBPMK is then modified by integrating this self-derived function, and complex ash evolution is predicted during dynamic combustion process. Simulated char combustion characteristics (particle temperature and carbon conversion) are discussed in detail. With higher ash film fraction and a thicker ash film, both the particle temperature and carbon conversion rate decrease. For low to intermediate levels of burnout (N <0.6), the carbon conversion characteristics in the case of full ash dilution (F = 0) are identical to those in the normal combustion (F = f (N)). At the char burnout stage (N >0.8), the relative deviations from the normal carbon conversion ratio are more than +10% when ignoring ash film and -20% when ignoring ash dilution. [ABSTRACT FROM AUTHOR]

• Char burning kinetic model analyzes physical structure effects on combustion. • Internal surface area has positive and dominate effects on char combustion. • Bituminous burnout time shows V-type distribution with swelling ratio and porosity. • Low coal rank facilitates internal surface area formation and subsequent combustion. The combustion performance of high heating-rate chars is observably affected by the evolution of its physical structure during devolatilization. An effort has been devoted to building understandable relationships between the heating rates of different rank coals, char structure and subsequently detailed combustion characteristics. In this work, the physical structure parameters (internal surface area, porosity, and swelling ratio) of bituminous coals and lignites are qualitatively summarized as a function of heating rates (>104 K/s). Meanwhile, the physical structure effects on char combustion characteristics are investigated separately by using the self-developed kinetics model, where the transient char properties, ash distribution (ash film, ash dilution, and ash vaporization) and char combustion performances are taken into account. The relative importance of the three structure parameters on char burnout is also performed. The internal surface area of high heating-rate chars exerts positive effects on combustion with higher burning temperature and shorter burnout time, whereas the swelling ratio and porosity exert complex effects. The combustion rate initially increases then goes down with char conversion, owing to the integrated effects of formation and destruction of internal surface area. The importance of physical structure parameters on char combustion follows the order: internal surface area > swelling ratio > porosity. These results provide in-depth insight into the effects of physical structure during char combustion and suggest higher heating rates are favorable in industrial boiler applications. [ABSTRACT FROM AUTHOR]