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• Coal structure, boundary condition and equilibrium state are classification criteria. • Coal permeability models are classified into equilibrium and non-equilibrium ones. • The exclusion of equilibration process within REV leads to model prediction errors. • Coal permeability should be treated as a variable of process instead of a quantity. The evolution of coal permeability is very complex under the influence of coupled multiple processes. This is the primary motivation why numerous coal permeability models have been developed over the last decades. Although great efforts have been made to evaluate these models, the root causes of discrepancies between lab/field observations and model predictions are still not identified. The objectives of this study are to address this problem from the perspective of model selves and explore their implications for further improvements. In this study, we collected all coal permeability models available in the literature. Through analysis, we conclude that all models can be characterized by a combination of internal structure of coal, boundary conditions and equilibrium state within the controlled volume. Different combinations lead to two major categories of coal permeability models. One category is structure-based equilibrium models including matchstick, cubic and rock bridge models. The other category is structure-based non-equilibrium models. The equilibrium models only serve as the upper and lower envelopes of experimental data while the non-equilibrium models can explain the data in-between. Further analysis concludes that if local equilibrium is achieved, gas pressure and its associated swelling strain distribute uniformly throughout the entire volume and that if not achieved, both pressure and swelling strain distribute non-uniformly. These conclusions suggest that the exclusion of equilibration process between fracture and matrix systems is the root cause of discrepancies between lab/field observations and model predictions and that future research work should integrate rock structure, boundary conditions and equilibration process in coal permeability model. This inclusion of transient process within the controlled volume represents a leap of knowledge from equilibrium to non-equilibrium theory and opens up a new realm for unconventional gas reservoir modelling. [ABSTRACT FROM AUTHOR]

In this work, the pulverized coal combustion of Zhundong lignite was conducted in an optically-accessible, downward Hencken flat-flame burner to investigate the incipient formation of ultrafine particulate matter (PM). The ultrafine PM were collected in - flame from distinct positions under 1800 K, 1500 K and 1200 K using a self-designed thermophoretic sampler, together with extensive conventional transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) examinations. A novel in situ low-intensity phase-selective laser-induced breakdown spectroscopy (PS-LIBS) was further introduced to diagnose the dynamic behavior of particle-phase sodium during the pulverized coal combustion. The primary particles in the collected ultrafine PM have mean diameter of 8–15 nm, with Si and Na as main mineral components based on TEM/HRTEM–EDS results. The formation of ultrafine PM is regarded as a process of multi-component flame syntheses of mineral precursors, in which the competition between the devolatilization and the coalescence of particles are of most significance. Finally, based on an approach of time scale analysis, three characteristic times of minimum mean diameter of primary particles, the initial increment point of particle-phase sodium and the maximum devolatilization are found to be highly correlated. [ABSTRACT FROM AUTHOR]

• A newly designed two-stage flat-flame burner was used to study coal combustion. • Na can promote soot formation rate by enhancing heterogeneous soot precursors nucleation. • The significant catalytic effect of Na on soot oxidation is verified. • Fe may play an indispensable role in promoting soot formation. Coal-combustion-generated soot can be a source of particulate emissions and significantly affects the temperature profile inside the practical boiler. In this work, we experimentally investigated the effects of sodium (Na) and iron (Fe) on soot formation in pulverized coal combustion. Two kinds of coal samples with similar properties were used, with one rich in Na while the other rich in Fe. The water washed sample for each coal was ultilized to further explore the mineral effects. A novel two-stage flat-flame burner was designed to flexibly simulate the reducing-to-oxidizing atmosphere which a real coal particle stream undergoes in the boiler. The ex-situ sampling indicates the existence of mineral species, including Na and Fe, doped in the mature soot agglomerates. In-situ Laser-Induced Incandescence (LII) diagnostics, incorporated with on-line measurements of the particle size distribution (PSD), gives quantitative concentrations of primary soot particles. The experiment results show a reduced soot yield after water washing for both coal samples, elucidating a promoting effect of Na on soot formation in pulverized coal combustion. A similar effect is inferred for Fe, since the water washed Fe-rich coal produces more soot than the water washed Na-rich coal even though their residual Na contents in ash are close. It is postulated that both Na and Fe form nano-sized particles at the early stage of coal combustion and serve as effective nuclei for the soot precursor, the tar. Besides, the catalytic effect of Na on soot oxidation is confirmed because of the significant soot reduction in the combustion of Na-rich coal when the oxygen mole fraction in the inner region increases. [ABSTRACT FROM AUTHOR]

In this study, the sensitivity of two different pyrolysis models on the simulative prediction of inorganic ultrafine Particulate Matter (PM) formation is investigated. For consideration of PM formation, the volume-conserved discrete-sectional Population Balance Model (PBM) is used. The volatile release is an input parameter of the PBM and thus influences the calculated PM formation. As a representative of an empirical pyrolysis model, the Fu-Zhang model (FZM) is used. In comparison, the Chemical Percolation Devolatilization model (CPD) is taken as a representative for sophisticated pyrolysis network models. The latter takes influences of fuel composition, heating rate and particle temperature into account. The evaluation of the pyrolysis model impact is determined for a pulverized ( d p = 65 – 74 μ m) Chinese lignite (Zhundong) since this fuel has a relatively high volatile content and thus favors the formation of particulate matter. To investigate the influence of particle heating rate in the models, two different gas temperatures (1200 K and 1500 K, respectively 5.28·104 K s−1 and 7.51 × 104 K s−1 calculated with the energy balance) are considered. Furthermore, these conditions are selected because experimental data of the PM particle size distribution (PSD) from previous studies are available. The modeling of PSD is performed and validated with the existing experimental PSD data. These experimental data were achieved in investigations with a Hencken burner. The results of the two models (FZM and CPD) show recognizable differences in predicting the PSD. This study indicates that the CPD model improves significantly the prediction of ultrafine PM formation. [ABSTRACT FROM AUTHOR]