Ignition Characteristics Of Coal Research Articles

Ignition predictions of isolated coal particles by different ignition criteria and devolatilization models

The prediction accuracy of coal ignition characteristics depends on the sub-model selections. A detailed modeling study on the influences of ignition criteria and devolatilization models on the ignition characteristics of isolated coal particles is conducted using a one-dimensional transient ignition model. The assessed ignition criteria include the thermal explosion theory (TET), the transient adiabatic criterion (TAC) and the flammability limit criterion (FLC), while the assessed the devolatilization models include the chemical percolation devolatilization (CPD) one and two competing reaction (TCR) one. Under each combination, the ignition characteristics under different oxygen concentration, particle size, and surrounding temperature are studied. The prediction results are compared with microgravity experimental data obtained from the 3.6 s drop tower in National Microgravity Laboratory Center of China (NMLC). Modeling results reveal that the particle center temperature is more reasonable to be used as the characteristic temperature in the ignition study. When devolatilization process is described by CPD model, for homogenous ignition FLC is more accurate to predict ignition temperature and time than TAC at a higher XO2 and a high Tw, while TAC is more accurate to predict ignition temperature at a large particle size. Using FLC, against TAC, the transient of ignition mode from homogenous to combined happens at a ∼ 80 K higher temperature and ∼ 50 μm larger particle size, while that from combined to heterogenous happens at 250 μm rather than 400 μm. When FLC is used as the ignition criterion, selection of devolatilization model has more notable effect on the prediction of ignition temperature and time. Furthermore, the combination of FLC and CPD is more accurate to predict ignition temperature and time at a higher XO2. Two diagrams are respectively given to guide the selection the ignition criterion and identify the ignition mode at a wide range of conditions. In addition, the mechanisms of the associated discrepancy are discussed.

Laboratory experimental study on water-soaked–dried bituminous coal’s thermal properties

In the process of multi-seam mining, the measure of water exploration is usually employed for the upper coal seam to prevent and control water accumulation. The residual coal immersed in water for a long time in the upper layer is dried under the action of air leakage, leading to the enhancement of the risk of coal spontaneous combustion. Therefore, it is necessary to study the water soaking–drying process (WSD) on the spontaneous ignition characteristics of coal. In this study, a laser thermal conductivity analyzer was used to investigate the changes in thermal properties of bituminous coal collected from mines in western China after WSD. The results show that compared with the raw (unsoaked) coal samples, the thermal diffusivity, specific heat capacity, and thermal conductivity of the bituminous coal are considerably reduced after WSD; the maximum reductions in the three parameters are 11.11%, 6.64%, and 12.39%, respectively. These reduced thermal parameters indicate that after WSD, bituminous coal is more prone to self-heating, resulting in an increase in the rate of coal–oxygen compound reactions and a significantly increased risk of spontaneous combustion.

Analysis on Spontaneous Combustion Characteristics of Adding Associated Sulfur Minerals in Coal

ABSTRACT In order to further study the influence of associated sulfur minerals on spontaneous combustion and ignition characteristics of coal, four coal samples with different sulfur contents were prepared by adding different sulfur minerals to raw coal, and through Thermogravimetric Analysis (TG) experiment, Differential Scanning Calorimeter (DSC) test and X-ray diffraction (XRD) analysis, the influence of associated sulfur minerals on spontaneous combustion and ignition characteristics of coal was studied. Experimental results show that with the increase of associated sulfur minerals, the characteristic temperature of coal decreases, while the oxygen absorption, flammability and incombustibility stability index increase gradually, indicating that the coal is more prone to spontaneous combustion after being mixed with the associated sulfur minerals. Based on Coats–Redfern method to calculate the activation energy of coal during combustion stage with different associated sulfur minerals in coal, the calculation shows that the activation energy of coal combustion stage decreases with the increase of the associated sulfur minerals in coal and makes it easily to burn. Through the XRD and DSC analysis of associated sulfur minerals, it is found that the main components of the associated sulfur minerals are melanterite and copiapite, which can react with water and oxygen at room temperature, and the exothermic reaction makes coal more prone to spontaneous combustion. The associated sulfur minerals are exothermic after 200°C and reach an exothermic peak at 565°C, which makes the activation energy of coal in combustion stage decrease and coal easily to burn.

Droplet ignition of coal–water slurries prepared from typical coal- and oil-processing wastes

The requirements for stable ignition (and subsequent combustion) of fuel suspensions prepared from typical coal- and oil-processing wastes are studied experimentally. Attention focuses on the differences between the ignition characteristics of coal–water slurries (containing petrochemicals) obtained on the basis of filter cakes containing T, K, SS, Zh, D, and G coal. To eliminate the influence of the droplet holder (traditionally, thermocouple junctions, ceramic rods, or metal wire) on the ignition characteristics of the fuel droplet, the experiments employ a special model combustion chamber and a device for introducing a single drop of suspension. The ignition time and the minimum temperature of stable ignition of a droplet of coal–water slurries suspended in an oxidant flux are established. The influence of the following factors on the initiation of fuel combustion is determined: the oxidant temperature, the droplet size, the size of the coal dust, and the properties and concentrations of the components. The compositions of the coal–water slurries corresponding to optimal ignition (minimum inertia) are identified.

Spontaneous ignition characteristics of coal in a large-scale furnace: An experimental and numerical investigation

A comprehensive understanding of the spontaneous combustion characteristics of coal in various surroundings is necessary for developing reliable test platform and predictive models. In this study, the characteristics of oxidation and self-heating combining various gas flow equations in loose coal were investigated separately and used to simulate the experimental procedure of spontaneous combustion. The main focus was to investigate the effect of thermal boundary on temperature profiles as well as spontaneous combustion period. The results showed that the numerical approach was validated by comparison with the test data. Furthermore, the model based upon Brinkman equation showed a higher accuracy, which indicated that airflow behavior influences the balances of coal oxidation and heat dissipation, thus impacts the temperature profiles of loose coal. The areas of high temperature zones would be evidently expanded and the spontaneous ignition time would be significantly accelerated if the thermal exchange between the coal and its surroundings decreased. Our results, especially for the field of engineering, have substantial effects for grasping and controlling coal spontaneous combustion disaster.

Ignition Characteristics of Coal–Water Slurry Containing Petrochemicals Based on Coal of Varying Degrees of Metamorphism

This paper examines a coal–water slurry containing petrochemicals (CWSP) based on lignite, bituminous coal, anthracite, and typical waste—carbon residue obtained from the pyrolysis of tires. Waste motor oil is used as a liquid fuel component of the slurry. The study of CWSP ignition is performed via placing a fuel droplet on a low-inertia thermocouple junction (junction diameter is 0.1 mm; thermal inertia is less than 1 s) into the hot oxidant flow. The temperature and velocity of the oxidant flow vary in the range of 600–1200 K and 0.5–5 m/s, respectively. These ranges are typical for power plants in the large- and small-scale energy industry. The paper specifies the minimum oxidant temperatures which are sufficient for sustainable ignition, as well as the ignition delay times, the times for complete burnout of CWSP droplets (radii of 0.5–2 mm), and the maximum temperatures in the fuel combustion area. We discuss how the oxidant temperature and the component composition of CWSP influence the parameters l...

Differences in the ignition characteristics of coal–water slurries and composite liquid fuel

The processes of the inert heating, ignition, and combustion of the drops of typical coal–water slurries and promising composite liquid fuel were experimentally studied with the use of high-speed (to 105 frame/s) video recording facilities. The particles of brown and black coals with of sizes 80–100 μm were used as the basic components of the coal–water slurries and composite liquid fuel. Spent automobile oil (from an internal combustion engine) was also added to the composite liquid fuel (relative mass concentration, 0–15%). The characteristic stages of the processes of the inert heating and evaporating of liquid components and the ignition and combustion of coal–water slurries and composite liquid fuel (the initial radii of drops varied from 0.5 to 2 mm) were established. The ignition delay and complete combustion times of the drops of fuel compositions were determined under changes of the temperature of an oxidizing agent (air) in a range from 600 to 900 K at fluid velocities from 0.5 to 5 m/s. Representative temperatures at the centers of coal–water slurry and composite liquid fuel drops were measured at all of the established stages of the combustion initiation process. The necessary and sufficient conditions for the steady ignition of the drops of the test fuel compositions were recognized.

Ignition behaviors of pulverized coal particles in O2/N2 and O2/H2O mixtures in a drop tube furnace using flame monitoring techniques

Oxy-steam combustion technology is regarded to be the next generation of oxy-fuel combustion technology. Because the physical and chemical properties of steam are very different from that of CO2 or N2, especially its reactivity, the ignition characteristics of coal in oxy-steam combustion are expected to be different from those in conventional combustion or O2/CO2 recycled combustion. This paper presents experimental investigations of the ignition behaviors of coal in O2/N2 and O2/H2O atmospheres using different oxygen fractions (21%, 30%, 35%, 40% and 50%). Ignition experiments were performed in a drop tube furnace. The ignition behaviors were recorded using a high-speed camera and a two-color pyrometer. According to the images of the ignition, the pulverized coals ignite homogeneously in all cases. The ignition of pulverized coal in the O2/H2O atmospheres occurs sooner than in the O2/N2 atmospheres at identical oxygen fractions. A high concentration of steam may enhance the steam gasification reaction (C+H2O→CO+H2) and the steam shift reaction (CO+H2O→CO2+H2) in O2/H2O atmospheres. These two reactions can form a large amount of H2 and CO in O2/H2O atmospheres in the gaseous mixture around the coal particle. This result is beneficial to reducing the ignition delay times of volatiles mixtures in O2/H2O atmospheres.

Experimental and numerical study of pulverized bituminous coal ignition characteristics in O2/N2 and O2/CO2 atmospheres

ABSTRACTAn unsteady‐state model of a pulverized coal cloud including the submodels of radiation and convection was used to study the ignition characteristics of a Datong bituminous coal under the conditions of O2/N2 and O2/CO2 atmospheres. An increase in the ignition temperature was calculated for the bituminous coal when air (21% O2 + 79% N2) was replaced with a mixture of 21% O2 + 79% CO2. With the increase of coal concentration, the ignition temperature reduced under O2/CO2 atmosphere. The ignition temperature of a Datong bituminous coal was investigated using thermogravimetric analysis (TGA) and a small furnace; experiment results showed a similar trend with the calculated results. Pulverized coal in ignition mode also appears very differently when the atmosphere changes. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.

Sensitivity analysis of different devolatilisation models on predicting ignition point position during pulverized coal combustion in O2/N2 and O2/CO2 atmospheres

Oxy-fuel combustion is considered as a promising solution to reduce greenhouse-gases and pollutant emissions. The main advantage of oxy-fuel combustion over other technologies for pollution reduction from pulverized coal combustion is that it can be applied to the existing coal-fired power plants. However, switching from conventional to oxy-fired coal combustion brings significant challenges. One of the most important is change of pulverized coal ignition characteristics. This paper presents the results of experimental and numerical analysis of ignition phenomena under oxy-fuel conditions. The main focus of the presented paper is to evaluate the effectiveness of the mathematical devolatilisation sub-model, in predicting the ignition point of pulverized coal flames under oxy-firing conditions. Regarding this, the performance of several devolatilisation models, from simple to more complex ones, in predicting ignition point position have been investigated. Numerically determined values of the ignition point position, and ignition temperature for various O2–N2 and O2–CO2 conditions were compared with experimental data from the laboratory ignition test facility. Obtained results pointed out that network devolatilisation models (CPD and FG) give more accurate results in comparison with standard devolatilisation models (single rate and two competing rates). The best performance is achieved using FG devolatilisation model. Thus, newly implemented FG model will be used for future numerical simulations of oxy-fuel pulverized coal combustion on 0.5MW pilot plant facility.

Coal ignition characteristics in CFB boiler

The coal ignition temperature in CFB boilers is an important parameter for designing the startup burner and for choosing operating method during the startup process. The combustion of seven different types of coal were measured in a laboratory scale fluidized bed with under-bed preheat system. The study analyzed the effects of particle size, furnace temperature, volatiles content and moisture content on the ignition temperature, T i F . T i F in the fluidized beds were measured at standard operating conditions. Comparison of the ignition temperatures and the feed temperatures in real CFB boilers shows that the T i F measured in fluidized beds can be used as a reference for the coal feeding process.