Guisu Liu

Thermal conductivity of coal ash and slags and models used

Abstract A one-dimensional heat transfer method was used to determine the thermal conductivity for a range of coal ash and synthetic ash samples at elevated temperatures. The effect of parameters such as temperature, porosity, and sintering time were investigated. The thermal conductivity of the samples was generally observed to increase with increasing temperature. During heating of the samples, softening of minerals and sintering reactions resulted in changes in the physical structure of the ash, which then altered the observed thermal conductivity. The thermal conductivity of sintered ash samples was found to be higher than that of unsintered samples. The sintering temperature and sintering time were found to increase the observed thermal conductivity irreversibly. A decrease in sample porosity was also observed to increase the thermal conductivity. Chemical composition was found to have little effect on the thermal conductivity, apart from influencing the extent of sintering. Predictions of the thermal conductivity of ash samples based on Rayleighs model are also presented. The thermal conductivity of slag and particulate structures was modelled by considering spherical pores distributed in a continuous slag phase. A particulate layer structure was modelled by considering solid particles dispersed in a continuous gas phase. The Brailsford and Major model of random distribution for mixed phases gives results within 20% of the measured values for a partially sintered sample.

Modelling of a pressurised entrained flow coal gasifier : the effect of reaction kinetics and char structure

A model for a pressurised entrained flow coal gasifier is presented, with the effect of pressure, reaction kinetics and char structure on the gasification reactions being outlined. A sensitivity analysis to reaction kinetics and char structure was performed, and model predictions are compared with published atmospheric and high-pressure gasification data. It was found that both reaction kinetics and char structure are important in predicting coal gasification. The initial surface area may be more significant than intrinsic reactivity for bituminous coal chars. Low-pressure gasification kinetics (i.e. pressure order) cannot be extrapolated to high-pressure conditions. A significant difference in predicted carbon conversion was observed between various char structural models. It is suggested that the random pore model gives a reasonable prediction. Volatile matter has significant effect on carbon conversion due to the formation of high-surface area char particles. Increasing vitrinite content in coal correlates with increased carbon conversion. The model predictions show reasonable agreement with published experimental data on both atmospheric and high-pressure gasifiers in terms of carbon conversion and gaseous product composition. Comparisons with previous models and sensitivity analyses suggest that it is necessary to include the effect of char structure, and more sophisticated reaction kinetics than single order rates are required when modelling coal gasification.

Modeling char combustion: The influence of parent coal petrography and pyrolysis pressure on the structure and intrinsic reactivity of its char

Chars were made from four Australian coals of varying vitrinite content at pressures of 5, 10, and 15 atm. The morphology of the chars was correlated with the petrography of the parent coal. The intrinsic reaction rates of the chars at high pressures were measured, and no systematic effect of pyrolysis pressure or maceral concentration was found. It is concluded that observed variations in conversion rates under process conditions are likely to be due to char structural properties and not a result of variation in the intrinsic reactivity of the carbon in the chars. Consequently, this paper presents a char structural submodel that is integrated into an existing char combustion model to account for the combustion behavior of char particles of different morphologies. The char morphology used in the model was predicted using the developed correlation with parent coal petrography, so that a petrographic analysis as well as the proximate and ultimate analyses is required for model input. Validation of the model shows that chars produced at high pressure with a high percentage of cenospherical types burn more rapidly under process conditions than those at low pressure, with model predictions matching measurements. It is suggested that incorporating the char structural submodel into the existing char combustion model improves its predictability.

Modeling the fragmentation of non-uniform porous char particles during pulverized coal combustion

Char fragmentation during pulverized coal combustion was studied using an Australian bituminous coal. The coal was combusted with air in a drop tube furnace at a gas temperature of 1300°C. The char samples were collected at different levels of char burnout, and their structure was examined using scanning electron microscopy. Approximately 40% of the char particles formed after pyrolysis were cenospheres with a highly non-uniform porous structure and a large central void. A large number of fine particles were also observed in the char samples with burnout levels between 30 and 50 vol%, which suggests that significant fragmentation occurs during the early combustion stage. A mathematical model was developed relating the fragmentation of cenospherical char particles with the macropores in the particle shell. The formation of these macropores partially results because of the carbon removal from the surface of the thin shell due to surface oxidation. A percolation model was used to simulate the char structural changes during combustion in regime III, and the predicted particle size distributions qualitatively agreed with the experimental measurements.

A mechanistic study on char structure evolution during coal devolatilization—Experiments and model predictions

Char structures evolved during the devolatilization process have been found to play a significant role in the subsequent processes (e.g., char combustion and gasification) and to influence the ash formation mechanisms. In the present paper, a mathematical model has been developed based on the multibubble mechanism to simulate the char structure evolution process. The model is the first to provide predictions of heterogenous char structures evolved during devolatilization (e.g. cenospheric char, foam structure, or dense char structure) as well as transient particle swelling ratios, based on the ultimate and proximate data of the given coal. The devolatilization process is divided into the preplastic stage, plastic stage, and resolidified stage. Bubble number conservation, mass and force balance are formulated during the plastic stage to predict the transient swelling ratio and resultant char structures. Experiments have been conducted using a single coal particle reactor (SPR) and a drop tube furnace (DTF) with density-separated coal samples prepared using the sink-float method. The SPR experiments confirm that bubble behavior is responsible for the swelling of the particles that develop plasticity on heating. The analysis of the DTF chars shows that the swelling ratio and porosity decrease with increasing the coal density. Chars from low-density samples are mainly Group I chars (porosity >80%), while high-density samples yield mainly Group III chars (porosity