Coal Ignition Research Articles

Resolved flow simulation of pulverized coal particle devolatilization and ignition in air- and O2/CO2-atmospheres

A resolved laminar flow simulation approach is used to investigate the effect of enhanced oxygen levels on single coal particle ignition, comparing the numerical results against experimental data for well-defined conditions (Molina and Shaddix, 2007). Devolatilization is described by a generic boundary condition at the particle surface that accounts for both convective and diffusive phenomena during pyrolysis. The heating rate history of the particle is obtained by solving for intra-particle heat transfer and heat exchange between the particle and its surroundings. The time evolution of volatile release is captured by using the particle mean temperature to calculate the devolatilization rate from a single kinetic rate law with CPD-fitted parameters. The assumed volatile composition includes both light gases and larger hydrocarbons to represent tars. A skeletal kinetic mechanism for pyrolysis and oxidation of hydrocarbon and oxygenated fuels containing 52 species and 452 reactions is used to accurately describe homogeneous chemistry. Particle heat-up, pyrolysis, ignition and envelope flame stabilization are characterized in four gas atmospheres differing in oxygen content and the use of either N2 or CO2 as balance gas. In agreement with the experimental evidence, enhanced oxygen levels shorten ignition delay time τign and result in a higher intensity of the combustion process according to temperature and radical production peaks for all studied mixtures. For the studied oxy-mixtures the presence of CO2 in substitution of N2 delays ignition. The observed behavior is coherent with the different thermo-physical properties of the gas mixtures. The sensitivity of predicted ignition delay to a set of uncertainties is also discussed. It is found that while the absolute values of predicted ignition delay time are functions of potential particle preheating, particle Reynolds number and the chosen criterion to extract ignition delay, the relative trends among the gas mixtures remain in line with the experimental evidence.

Numerical study on the ignition behavior of coal dust layers in air and O2/CO2 atmospheres

Understanding the fundamental basis of coal dust layers ignition behavior is of interest for the mitigation of industrial fires and explosions. This paper develops a 2D numerical method to investigate the self-ignition of coal dust layers with an emphasis on the roles of the oxygen mole fraction and diluent gas. A global one-step 2nd-order reaction kinetic model considering both coal density and oxygen density is used to estimate the reaction rate of self-heating process until ignition. In addition, the overall heat transfer coefficient is evaluated based on a steady state method and the kinetic parameters are estimated by fitting experimental and numerical temperature evolution figures. This model has been found to be valid to predict the transient temperature and concentration profiles until ignition. The numerical results of the minimum ignition temperature of dust layers (MITL) are in close agreement with experimental observations. The ignition delay time and the most sensitive ignition position (MSIP) of dust layer are found to vary with varying ambient gas conditions, hot plate temperatures and ambient temperatures using the verified model. The model provides a satisfactory explanation for the dependence of ignition characteristics of coal dust layer in oxygen-enriched oxy-fuel atmospheres.

Experimental study on ignition characteristics of pulverized coal under high-temperature oxygen condition

The high-temperature oxygen ignition technology of pulverized coal, which can replace the oil gun and achieve oil-free pulverized coal ignition by mixing the high- temperature oxygen and the pulverized coal stream directly, was proposed and a relevant ignition experimental system was built. The ignition characteristics of pulverized coal under high-temperature oxygen condition were investigated: the ignition process was described and analyzed, the influence of relevant parameters on the pulverized coal stream ignition were obtained and analyzed. The results showed: when the oxygen heating temperature is over 750 °C, the pulverized coal stream could be ignited successfully by high-temperature oxygen; increasing the pulverized coal concentration, primary air temperature and oxygen volume flow rate or decreasing the primary air velocity is helpful for the ignition and combustion of the pulverized coal stream.

The influences of moisture on particle ignition behavior of Chinese and Indonesian lignite coals in hot air flow

The effect of moisture content on ignition and combustion behavior of Chinese (HL) and Indonesian (YN) lignite were investigated. Particles with a size range of 75–105μm with different moisture contents were injected in a bench-scale, electrically heated transparent reactor and the combustion of individual particles was observed with a high-resolution high-speed camera. Direct ignition observations indicated that most of the HL lignite particles underwent extensive fragmentation during ignition. Fragmentation was attributed to the explosive diffusion of volatiles and water vapor to the particle surface as a result of fast heating rate. Fragmentation reduced the particle size and increased the possibility of heterogeneous ignition of individual fragments. YN lignite particles on the other hand, underwent one-mode whole particle ignition upon heating. Higher moisture content caused a significant ignition delay in both lignite samples. 10% and 20% moisture in lignite samples resulted in around 83 and 160ms delay in ignition for both coals. Higher intensity of fragmentation of HL particles during combustion compared to YN lignite resulted in shorter total particle combustion time at higher moisture contents. The findings of this study advanced the knowledge of the effects of moisture on ignition and combustion of low-rank coals.

Thermal decomposition of coal and coal-enrichment wastes

The thermokinetic parameters of the thermal decomposition of coal and its processing wastes (filter cake) used in the preparation of coal–water and organic coal–water fuels are determined experimentally by thermogravimetry in an inert atmosphere. The dependence of the thermokinetic parameters of decomposition on the coal’s metamorphic stage is established; the activation energy of decomposition varies from 39.1 kJ/mol (lignite) to 109.4 kJ/mol (anthracite). There is practically no difference between coal and its processing wastes in the thermokinetic parameters of thermal decomposition. The results provide the starting point for an experimental database that may be used in modeling the ignition and combustion of coal and its derivatives: coal–water and organic coal–water fuels.

The investigation of the coal ignition temperature and ignition characteristics in an oxygen-enriched FBR

Oxygen-enriched circulating fluidized bed (CFB) technology is considered as one of the low carbon emission power generation technologies. By recycling the flue gas into the furnace, the CO2 concentration in the exhaust gas can reach over 90%. The coal ignition temperature and ignition characteristics are important factors for boiler design and for choosing the feeding temperature during the transition from air-firing to oxy-firing. The ignition and combustion processes of five types of coal under four different atmospheres (air, O2 27%, O2 40%, O2 53%) were measured in a laboratory scale fluidized bed reactor (FBR) with an under-bed preheat system. Using thermocouples and a gas analyzer, the changes in bed temperature and the concentrations of different components, such as O2, CO2 and CO, in flue gas were directly measured to determine the coal ignition temperature, TiF. It was found that TiF decreases with increasing O2 concentration. At lower bed temperatures, two-stage ignition processes were observed in certain ranges of initial bed temperature and oxygen concentration. The influence of initial bed temperature on volatile release and the SO2 emission in the ignition processes were also discussed.

Study Governing the Impact of Long-Term Water Immersion on Coal Spontaneous Ignition

Oxidation of air-dried coal in mined-out areas is one leading factor contributing to coal spontaneous ignition during exploitation. Although some related work has been done, there is still some space for improvement. We screened out three coal samples with different ranks of coal and immersed them in distilled water for 200 days. Afterward, we identified some changes in pore structure and oxygen consumption characteristics of coal samples after water immersion. Additionally, we analyzed the composition of dissolved substance, both organic and inorganic. The results demonstrated that some parameters of three different residual coal samples increased, including average pore size, specific surface area and pore volume. In regard to the composition of dissolved substance, we identified Na+ and Cl−, which accounted for the largest proportion, along with some organic functional groups. We found that higher rank leads to a smaller total amount of dissolved substance. Additionally, we performed some experiments concerning coal oxidation rate and the amount of CO, CO2 and C2H4 generated during oxidation. The results showed that water immersion does play a significant role, along with coal rank. Considering all results and analysis, long-term water immersion is responsible for pore structure development in coal and the degree of reduction in spontaneous-ignition-proof substance, which helps to boost coal spontaneous ignition.

Ignition of coal-water fuel particles under the conditions of intense heat

The dependence of the coal-water fuel (WCF) particles ignition delay time has been determined experimentally in this work. It depends on the characteristic size of the latter in the conditions corresponding to the intensity of heat supply by combustion space of the boiler units. The physical and mathematical models have been formulated according to the analysis and generalization of experimental data that describe complex jointly proceeding thermal and thermal chemical processes (inert heating, evaporation of water, thermochemical interaction of the latter with carbon coke, thermal decomposition of the organic part of coal particle ignition of WCF) in the induction period of time. The process of the heat sink on the wire of the thermocouple has been put into the consideration in order to maximize the approximation of the mathematical model to the conditions of the experiments, in which the particle of WCF has been fixed in the experiments. There is a good compliance of certain theoretical delay times of WCF ignition with those recorded in the experiments.

Effect of steam on ignition of pulverized coal particles in oxy-fuel combustion in a drop tube furnace

Oxy-fuel combustion is recognized as a promising technology to reduce CO2 emissions from coal-fired power plants. Typical oxy-fuel combustion is O2/CO2 recycled combustion, in which the dilute gases are the mixture of O2 and CO2. Wet flue gas recycle combustion (O2/CO2/H2O atmospheres) should be promoted due to some merits over dry flue gas recycle combustion (O2/CO2 atmospheres). In this study the ignition of bituminous coal was studied using experimental and numerical methods under O2/CO2/H2O atmospheres. The experiments were conducted in a drop tube furnace using a high-speed camera with oxygen mole fractions of 21%, 30% and 40%. The substitution of CO2 by 5%, 10%, 20% and 30% steam were implemented to evaluate the effect of the mole fractions of steam on the ignition of coal in O2/CO2/H2O atmospheres. The experimental results reveal that the ignition distances are barely affected when low mole fraction of steam (5%, 10% and 20%) was added to O2/CO2 atmospheres. As the mole fraction of steam is increased to 30%, the ignition distance is significantly decreased. The effect of steam on the ignition of coal in O2/CO2/H2O atmospheres was particularly analyzed using a proposed CFD model. The numerical results indicate that the physical properties of steam lead to the ignition delay of coal when CO2 is replaced by steam in O2/CO2 atmospheres. The lower specific heat and higher diffusivity of steam compared to CO2 lead to the advanced ignition of coal, and the higher thermal radiation of steam leads to the ignition delay of coal. The chemical properties of steam are beneficial to the ignition. The steam shift reaction played a more important role in the advanced ignition of coal than the steam gasification reaction.

High quality syngas production via steam-oxygen blown bubbling fluidised bed gasifier

This paper presents experimental and modelling results of fuel gas obtained in a steam-oxygen blown fluidised bed pilot scale (500 kWth) coal gasifier that is part of the ZECOMIX (Zero Emission of CarbOn with MIXed Technologies) research infrastructure run by ENEA. As there is poor information about start up and steady state operation of fluidised bed gasifiers at such a scale, in this work we investigated the influence of some important parameters such as coal ignition temperature, feeding rate of particulate bed material (olivine sand) and transition from oxidant to reducing environment. The experimental runs confirmed the crucial importance of S/O (steam/oxygen) ratio on the product gas composition. High quality syngas (low content of methane, below 1.0 v/v %, and high content of H2 and CO: CO + H2 61.0 v/v%) was obtained with S/O = 1.1.A shortcut model of the gasifier was also formulated, considering instantaneous coal pyrolysis, kinetics of coke steam reforming reactions in the fluidised bed, and achievement of thermodynamic equilibrium for the water gas shift reaction. The model takes into consideration the average residence time of coke particles into the gasifier: since this is a characteristic parameter of the reactor, the performance of different fluidised bed gasifiers can be simulated.

Influence of steam on ignition of Victorian brown coal particle stream in oxy-fuel combustion: In-situ diagnosis and transient ignition modelling

In this paper, we have investigated the ignition behaviour of Victorian brown coal under the steam-rich oxy-fuel combustion conditions. Two differently ranked coal, sub-bituminous and bituminous coal, were also tested for comparison. The ignition experiments were conducted in O2/N2/H2O and O2/CO2/H2O atmospheres, in an entrained flow reactor coupled with flat flame burner and in-situ optical diagnosis tools. A transient ignition model and 1-D single film model were also developed and used to predict particle ignition and the extent of char-steam gasification reaction, respectively. As has been confirmed, the injection of external steam to the furnace is beneficial in accelerating the ignition of Victorian brown coal in both air and oxy-firing modes with 21% O2. This effect is even more pronounced for the combustion of dense particle stream. In the oxygen-enriched condition, the ignition time was similar regardless of the steam concentration in the bulk gas. The transient modelling prediction confirmed that the homogeneous water–gas shift reaction (CO+H2O↔CO2+H2) was the main cause for the accelerated ignition for brown coal. It produced a highly concentrated flammable gas mixture of CO and H2 in coal particle cloud, which in turn accelerated the homogenous ignition of volatiles. This promotion effect is the most dominant for brown coal, because its volatile is rich in light hydrocarbons including gaseous CO that can readily react with the external steam via water–gas shift reaction. Moreover, the external steam triggered the heterogeneous char-steam gasification reaction for the brown coal, which further improved the overall char burnout rate. Regarding another two high-rank coals, their changes on both ignition and char burnout rate are however marginal upon the introduction of external steam, due to the shortage of gaseous volatiles (thus no water–gas shift reaction triggered) and the difficulty for their char to undergo steam gasification reaction under the experimental conditions observed here.

The ignition characteristics and combustion processes of the single coal slime particle under different hot-coflow conditions in N2/O2 atmosphere

The ignition characteristics and combustion processes of single coal slime particle in a vertical heating tube furnace in N2/O2 oxidant atmosphere were investigated under different hot-coflow conditions, including variations in coflow temperature (Tc = 923, 1073, and 1173 K), gas flow rate (V = 0–30 L/min), and oxygen concentration (O2% = 5%–100%). All investigated hot-coflow conditions exhibited three ignition behaviors, homogeneous ignition of volatiles, heterogeneous ignition of char, and heterogeneous ignition of coal. Additionally, three corresponding ignition regimes were observed in the oxygen concentration-coflow temperature plane. Critical conditions for the transitions of the three ignition mechanisms varied as flow was increased from 0 to 30 L/min. Various ignition mechanisms resulted in various combustion processes. Under medium-to-low oxygen concentration and low coflow temperature, the heterogeneous ignition of char resulted in flameless combustion. The ignition temperature and ignition delay decreased as coflow temperature and oxygen concentration were increased. The trends became more obvious when test conditions progressed in medium-to-low oxygen concentrations. As the flow rate increased, ignition temperature and delay became increasingly sensitive to oxygen concentration. Experimental results are then discussed in conjunction with the various observed trends influencing mechanisms of different hot-coflow conditions.

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.

Conditions and Characteristics of Coal Water Slurry Containing Petrochemicals Ignition by Hot Particle

Characteristics of CWS containing petrochemicals ignition by single hot particle in disk shape established experimentally. The main components of fuel composition are coal 45 %, water 45 %, used engine oil 10 %. The main parameters of local heat sources are: material – steel, diameter 10–12 mm, high 4–8 mm. As a result of experimental study is established limit (minimum) conditions for ignition of CWS containing petrochemicals by hot particle and the impact of local heat source parameters on the main process characteristic – ignition delay time.

Effect of the shape of particles on the characteristics of the ignition of coal–water fuel

A solution to the problem of the ignition of a coal–water fuel particle under the conditions of radiation-convective heat exchange with the environment was proposed. The mathematical simulation was carried out taking into account the combined occurrence of a set of physical and chemical processes (thermal conductivity, the evaporation of water, the thermal decomposition of the organic matter of a coal component, the filtration of water vapor and gaseous thermal decomposition products to the surface, and the chemical interaction of water vapor with the solid and gaseous products of the pyrolysis of coal) in the initial and slightly dried parts of the fuel. The effect of the configuration of a particle on the delay times of ignition was established based on an example of two typical shapes (sphere and cube).

Ignition behaviour and flame stability of different ranks coals in oxy fuel atmosphere

The aim of the research has been to investigate the impact of a type of atmosphere onto the ignition of fuel and the stability of dust–gas flames. Three black coals and two brown coals were applied in the research. It covered performing their physicochemical analyses and determining the induction time of ignition of single particles and dust–air cloud as well as parameters of dust flames and their stability. The research program referred to various aspects of the ignition of fuel, including: effects of its temperature, size of grains, type of atmosphere (air, O2/CO2), O2 share (5–30%) in the O2/CO2 atmosphere. The conducted research highlighted that the ignition of fuel is connected with a degree of its carbonisation, temperature and the type of atmosphere at which it takes place. The ignition of black coals is more difficult (hindered) than the one of brown coals. For its enhancement it is required to apply (among others): higher oxygen concentrations, higher dust concentrations, fined dust milling or higher ambient temperatures. A type of gas (N2, CO2) with which oxygen was mixed had a fundamental impact onto the characteristics of ignition. For all the examined coals, the induction times of ignition of dust cloud measured in the air atmosphere were shorter than the ones measured in the oxy atmosphere. The presented research demonstrated that – for the stability of flames and the parameters of fuel – the specific thermal capacity and density of main gases making up a mixture with oxygen are of fundamental importance.

Influence of inherent moisture on the ignition and combustion of wet Victorian brown coal in air-firing and oxy-fuel modes: Part 1: The volatile ignition and flame propagation

This study for the first time reported the volatile ignition and oxidation for wet Victorian brown coal in both air-firing and oxy-fuel modes. The aim is to quantitatively understand the evaporation of inherent moisture and its overlapping with the subsequent volatile ignition and oxidation steps during pulverised coal combustion. A wet Victorian brown coal sample with the moisture content up to 30wt.% and a size range of 63–104μm has been tested. Its ignition, volatile flame propagation and char oxidation have been diagnosed optically in a laboratory-scale flat-flame burner reactor (FFBR) coupled with a high-speed camera. 1-D modelling was also conducted to predict coal ignition based on the classic coal drying theory and coal devolatilisation rate. As has been confirmed, the release of moisture on the initial drying stage is incomplete prior to the subsequent volatile ignition. The fraction of moisture released upon drying only accounted for 10% of its original amount in wet brown coal prior to ignition. This is due to a firm residence of the moisture within coal capillaries, and a rapid heat feedback from hot gas to the dried particle surface that was ignited instantaneously prior to the release of the remaining moisture. The unevaporated moisture was observed to be released with the volatiles together to form a thick cloud layer in the vicinity of the coal particle, thereby enlarging the sizes of the volatile flame remarkably, which are approximately twice the size of the dried coal flame. However, the intensity of the wet coal flame was much weaker than dried coal, due to the dilution effect of the inherent moisture on the volatile cloud. In the oxy-fuel combustion mode, the inherent moisture interrelated with CO2, a tri-atomic gas with a larger specific heat capacity than N2 to significantly delay the ignition of the wet coal particle and its flame intensity and propagation velocity.

Clarifying the influence of moisture on the ignition and combustion of wet Victorian brown coal in air-firing and oxy-fuel modes: Part 2: Contribution of gasification reaction to char oxidation rate

This paper is the second part of the study to clarify the influence of moisture in Victorian brown coal oxy-fuel combustion, with a focus on char oxidation and gasification reaction through experimental and modelling efforts. An in-situ high-speed two-colour pyrometer with the wavelength band of 0.85–1.05μm was employed to measure particle temperature in the flat flame burner reactor. The combustion stage of carbon particle was simulated for the transient phenomena with particle heating and radiative heat transfer. The multiple surface reaction single-film approach, including char–O2, char–CO2 and char–steam reactions, was employed and the contribution of individual reactions to carbon consumption rate was determined via matching with the measured particle temperature. Irrespective of the initial moisture content, the extent of char–steam gasification reaction was found to account for ~15% in the air-firing case. This reaction was mainly triggered by the external steam in the reactor, rather than the inherent moisture that resided preferentially as volatile cloud on char particle surface. The combined effect of both char–CO2 and char–steam gasification was significant in oxy-fuel combustion mode, especially for the wet coal. In the oxy-21 case, these two reactions have a total extent of around 8% and 18% on the burning char surface of dried and wet coal, respectively. The char–CO2 gasification is insignificant, because the char particle temperature was low. Increasing the oxygen percentage to 31% in CO2 enhanced the total extent of these two gasification reactions to 28%, based on the mass of total carbon. Such an extent is comparable with the literature. However, the steam gasification rate for brown coal char was far higher, ~26% relative to ~10% for high-rank bituminous coal reported in the literature. This substantiates the strong steam gasification reactivity of Victorian brown coal char. The contribution of inherent moisture to char–steam gasification reaction is crucial in the combustion of wet coal in the oxy-21 case, accounting for ~10%. This is due to the long residence of the unevaporated steam as a thick cloud on the char surface. Increasing the oxygen concentration in CO2 enhanced the char–O2 reaction, the release rate of volatiles and inherent moisture. Therefore, the char–steam reaction caused by the inherent moisture within coal matrix was minimised and eventually diminished in the oxy-31 case.

Numerical simulation of ignition of particles of a coal–water fuel

The problem of particle ignition of coal–water fuel has been solved. The simultaneous processes of water vaporization and thermal decomposition of the solid fuel are taken into account. The conditions and characteristics of particle ignition of coal–water fuel under typical furnace conditions were determined by numerical simulation. The obtained values of the ignition delay time are in good agreement with published experimental data.

Numerical study of H2O addition effects on pulverized coal oxy-MILD combustion

This paper numerically investigates the effects of H2O addition on pulverized coal oxy-MILD (moderate or intense low oxygen dilution) combustion based on our previous study, in which only O2/CO2 was treated as oxidizer. While in the present study, the initial H2O mole fraction in oxidizer is varied at a constant oxygen level (30% by volume) to represent four typical oxy-fuel operations, i.e. ideal dry recycle (0% H2O+70% CO2), practical dry recycle (5% H2O+65% CO2), wet recycle (15% H2O+55% CO2) and oxy-steam (70% H2O+0% CO2). It is revealed that, with the addition of H2O, the internal recirculation rate (KV) as well as the coal ignition is improved, however, due to the difference in physical effects between CO2 and H2O, the temperature peak is likewise promoted. CO2 is preferred than H2O to moderate the flame temperature and realize uniform temperature distribution under MILD combustion. Higher H2 and lower CO formations are obtained by adding H2O because of the competing char gasification reactions between CO2 and H2O under in-furnace low oxygen condition of MILD combustion. H2O addition tends to suppress the volatiles-N conversion while accelerate the char-N conversion in the upper furnace, respectively, and reduce the final NO emission by enhanced H2 formation. Moreover, H2O addition is beneficial to increasing heat transfer in both radiative and convective forms.