Coal Ignition Research Articles

Numerical simulation of the influence of stationary louver and coal particle size on distribution of pulverized coal to the feed ducts of a power plant burner

One of the key requirements related to successful utilization of plasma technology as an oil-free backup system for coal ignition and combustion stabilization in power plant boilers is provision of properly regulated pulverized coal distribution to the feed ducts leading the fuel mixture to a burner. Proper regulation of coal distribution is deemed essential for achieving an adequate pulverized coal concentration in the zone where thermal plasma is being introduced. The said can be efficiently achieved by installation of stationary louver in the coal-air mixing duct ahead of the feed ducts of a burner. The paper addresses numerical simulation of a two-phase flow of air-pulverized coal mixture in the mixing ducts, analyzing the effects of particle size distribution on pulverized coal distribution to the burner feed ducts. Numerical simulation was performed using the FLUENT 6.3 commercial code and related poly-dispersed flow module, based on the PSI-CELL approach. Numerical experiments have been performed assuming a mono-dispersed solid phase with particle diameter ranging from 45 mm to 1200 mm. Distance between the louver blades and the resulting effect on the flow profile was analyzed as well. Results obtained indicate that the size of coal particles considerably influence the overall solid phase distribution. While fine particles, with diameters at the lower end of the above specified range, almost fully follow the streamlines of the continuous phase, coarser particles, which hit the louver blades, deflect towards the thermal plasma zone. In this manner, a desired phase concentration in the considered zone can be reached. For the said reason, installation of stationary louver have been deemed a very efficient way to induce phase separation, primarily due to more pronounced impact of the installed louver on discrete phase flow then the impact on the flow of the continuous phase.

Supercritical water oxidation of coal in power plants with low CO2 emissions

Abstract In this paper, the application of Super Critical Water Oxidation (SCWO) to direct combustion at low temperature of coal fine particles with pure oxygen for power generation is presented, including also a novel method for capturing and storing carbon dioxide as liquid. A detailed simulation model of a 100 MW th coal-fired SWCO plant with low CO 2 emissions characterised by a steam cooled membraned SC reactor has been developed using Aspen Plus software. According to the well-known Semenov's thermal-ignition theory, the coal particle ignition temperature in SCW conditions has been also evaluated and the results have been integrated within the Aspen Plus model. This has been tested under different operating conditions. The simulation results are presented and the effects of the main plant operating conditions, such as ignition temperature, coal particle size and combustion pressure on the plant performances are discussed. The gross and net thermodynamic efficiencies of the power plant have been estimated to be around 44% and 28%, respectively. The pure oxygen production process results the main energy penalty.

Particle imaging of ignition and devolatilization of pulverized coal during oxy-fuel combustion

Oxy-fuel combustion of coal is a promising technology for cost-effective power production with carbon capture and sequestration that has ancillary benefits of emission reductions and lower flue gas cleanup costs. To fully understand the results of pilot-scale tests of oxy-fuel combustion and to accurately predict scale-up performance through CFD modeling, fundamental data are needed concerning coal and coal char combustion properties under these unconventional conditions. In the work reported here, the ignition and devolatilization characteristics of both a high-volatile bituminous coal and a Powder River Basin subbituminous coal were analyzed in detail through single-particle imaging at a gas temperature of 1700 K over a range of 12–36 vol % O 2 in both N 2 and CO 2 diluent gases. The bituminous coal images show large, hot soot cloud radiation whose size and shape vary with oxygen concentration and, to a lesser extent, with the use of N 2 versus CO 2 diluent gas. Subbituminous coal images show cooler, smaller emission signals during devolatilization that have the same characteristic size as the coal particles introduced into the flow (nominally 100 μm). The measurements also demonstrate that the use of CO 2 diluent retards the onset of ignition and increases the duration of devolatilization, once initiated. For a given diluent gas, a higher oxygen concentration yields shorter ignition delay and devolatilization times. The effect of CO 2 on coal particle ignition is explained by its higher molar specific heat and its tendency to reduce the local radical pool. The effect of O 2 on coal particle ignition results from its effect on the local mixture reactivity. CO 2 decreases the rate of devolatilization because of the lower mass diffusivity of volatiles in CO 2 mixtures, whereas higher O 2 concentrations increase the mass flux of oxygen to the volatiles flame and thereby increase the rate of devolatilization.

Exergy optimisation of reverse combustion linking in underground coal gasification

Abstract Underground coal gasification (UCG) is a gasification process carried on in non-mined coal seams using injection and production wells drilled from the surface, which enables the coal to be converted into product gas. A key operation of the UCG is linking the injection and production wells. Reverse combustion linking (RCL) is a method of linking the process wells within a coal seam, which includes injection of an oxidant into one well and ignition of coal in the other so that combustion propagates towards the source of oxidant thereby establishing a low hydraulic resistance path between the two wells. The new theory of the RCL in typical UCG conditions has been recently suggested. The key parameters of the RCL process are determined using the technique of intrinsic disturbed flame equations. The present study is concerned with extending the results of the RCL theory to incorporate hydrodynamics of air injection and flow during RCL operation to derive mass flow rate of air to the combustion front a...

Effect of biomass blending on coal ignition and burnout during oxy-fuel combustion

Oxy-fuel combustion is a GHG abatement technology in which coal is burned using a mixture of oxygen and recycled flue gas, to obtain a rich stream of CO 2 ready for sequestration. An entrained flow reactor was used in this work to study the ignition and burnout of coals and blends with biomass under oxy-fuel conditions. Mixtures of CO 2/O 2 of different concentrations were used and compared with air as reference. A worsening of the ignition temperature was detected in CO 2/O 2 mixtures when the oxygen concentration was the same as that of the air. However, at an oxygen concentration of 30% or higher, an improvement in ignition was observed. The blending of biomass clearly improves the ignition properties of coal in air. The burnout of coals and blends with a mixture of 79%CO 2–21%O 2 is lower than in air, but an improvement is achieved when the oxygen concentration is 30 or 35%. The results of this work indicate that coal burnout can be improved by blending biomass in CO 2/O 2 mixtures.

A numerical study of pulverized coal ignition by means of plasma torches in air–coal dust mixture ducts of utility boiler furnaces

Paper presents selected results of numerical simulation of processes in air–coal dust mixture duct of pulverized coal utility boiler furnace with plasma-system for pulverized coal ignition and combustion stabilization. Application of the system in utility boiler furnaces promises to achieve important savings compared with the use of heavy oil burners. Plasma torches are built in air–coal dust mixture ducts between coal mills and burners. Calculations have been performed for one of rectangular air–coal dust mixture ducts with two opposite plasma torches, used in 210MWe utility boiler firing pulverized Serbian lignite. The simulations are based on a three-dimensional mathematical model of mass, momentum and heat transfer in reacting turbulent gas-particle flow, specially developed for the purpose. Characteristics of processes in the duct are analyzed in the paper, with respect to the numerical results. The plasma-system thermal effect is discussed as well, regarding corresponding savings of liquid fuel. It has been emphasized that numerical simulation of the processes can be applied in optimization of pulverized coal ignition and combustion stabilization and enables efficient and cost-effective scaling-up procedure from laboratory to industrial scale.

Prediction of ignition behavior in a tangentially fired pulverized coal boiler using CFD

Prediction of pulverized coal ignition behavior in a 40 MW tangentially fired commercial boiler is studied. Pulverized coal combustion simulation is performed considering radiation properties of particles. Coal devolatilization and char combustion are modeled and the first order spherical harmonic approximation is used to model the radiative transfer equation. To confirm the accuracy of the simulation method, the results are confirmed by available operating data, design data, and the ignition image in the boiler whose inside is observed by the developed high temperature resistant CCD video camera system. The work indicates that the simulation method can be applied to commercial boilers and predict the ignition behavior with considering not only coal properties but also boiler operating conditions.

Ignition characteristics of coal blends in an entrained flow furnace

Ignition tests were carried out on blends of three coals of different rank – subbituminous, high volatile and low volatile bituminous – in two entrained flow reactors. The ignition temperatures were determined from the gas evolution profiles (CO, CO2, NO, O2), while the mechanism of ignition was elucidated from these profiles and corroborated by high-speed video recording. Under the experimental conditions of high carbon loading, clear interactive effects were observed for all the blends. Ignition of the lower rank coals (subbituminous, high volatile bituminous) enhanced the ignition of the higher rank coal (low volatile bituminous) in the blends. The ignition temperatures of the blends of the low rank coals (subbituminous–high volatile bituminous) were additive. However, for the rest of the blends the ignition temperatures were always closer to the lower rank coal in the blend.

Ignition and devolatilization of pulverized bituminous coal particles during oxygen/carbon dioxide coal combustion

Oxygen/carbon dioxide recycle coal combustion is actively being investigated because of its potential to facilitate CO 2 sequestration and to achieve emission reductions. In the work reported here, the effect of enhanced oxygen levels and CO 2 bath gas is independently analyzed for their influence on single-particle pulverized coal ignition of a U.S. eastern bituminous coal. The experiments show that the presence of CO 2 and a lower O 2 concentration increase the ignition delay time but have no measurable effect on the time required to complete volatile combustion, once initiated. For the ignition process observed in the experiments, the CO 2 results are explained by its higher molar specific heat and the O 2 results are explained by the effect of O 2 concentration on the local mixture reactivity. Particle ignition and devolatilization properties in a mixture of 30% O 2 in CO 2 are very similar to those in air.

Investigation on catalyzed combustion of high ash coal by thermogravimetric analysis

Thermogravimetric analysis (TGA) has been used to obtain information on ignition and burnout characteristics of high ash coal (Ab. HAC) incorporating Fe 2O 3, MnO 2 and BaCO 3. Experiments were conducted from the ambient temperature to l273 K at a heating rate of 20 K min −1. Ignition index and burnout index had been put forward to describe further the ignition and burnout characteristics. It was indicated that compared with those of HAC, the relative active sequence of catalysts to the ignition characteristics could be described as follows: MnO 2 > BaCO 3 > Fe 2O 3; the relative active sequence of catalysts to the burnout could be described as follows: Fe 2O 3 > BaCO 3 > MnO 2. Catalyzed combustion mechanisms of the coal samples were also discussed. It was also demonstrated that the catalysis of the catalysts to the HAC combustion was embodied in the enhancement of the emission of volatile matters (VM) from HAC, which reduces the ignition temperature. Another role of the catalysts to the combustion of HAC could be a carrier of oxygen that impromotes the oxygen transfer to the char of HAC.

Study of coal and coke ignition in fluidized beds

Ignition tests were conducted with delayed and fluid petroleum cokes, a high volatile bituminous coal and their blends in bench- and pilot-scale fluidized bed combustors. In the bench-scale FBC tests, a visual inspection ignition criterion was developed based on the ‘CO–CO 2’ profiles obtained as a function of time and bed temperature. In the pilot-scale unit, the rapid increase in SO 2 levels was used as the indicator of fuel ignition. In addition to the fluidized bed ignition tests, thermogravimetric analysis (TGA) measurements were made on all fuels and their chars. The results showed that, while the fluid coke (which has the lowest volatile content) was the most difficult to ignite, ignition was not a simple function of volatile content. Further, different test methods and ignition criteria demonstrated significantly different ignition temperatures, with the largest variation arising between bench- and pilot-scale equipment. Finally, tests on mixtures of petroleum coke and coal showed that there was no interaction between these two fuels, and that the coke ignited only when it achieved its own ignition temperature.

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.

Ignition behaviour of different rank coals in an entrained flow reactor

An experimental study to determine the temperature and mechanism of coal ignition was carried out by using an entrained flow reactor (EFR) at relatively high coal feed rates (0.5 g min −1). Seven coals ranging in rank from subbituminous to semianthracite, were tested and the evolved gases (O 2, CO, CO 2, NO) were measured continuously. The ignition temperature was evaluated from the gas evolution profiles, and it was found to be inversely correlated to the reactivity of the coal, as reflected by the increasing values of the ignition temperature in the sequence: subbituminous, high volatile bituminous, low volatile bituminous and semianthracite coals. The mechanism of ignition varied from a heterogeneous mechanism for subbituminous, low volatile bituminous and semianthracite coals, to a homogeneous mechanism for high volatile bituminous coals. A thermogravimetric analyser (TGA) was also used to evaluate coal ignition behaviour. Both methods, TGA and EFR, were in agreement as regards the mechanism of coal ignition. From the SEM micrographs of the coal particles retrieved from the cyclone, it was possible to observe the external appearance of the particles before, during and after ignition. The micrographs confirmed the mechanism deduced from the gas profiles.

Modeling of pulverized coal combustion stabilization by means of plasma torches

Application of plasma-system for pulverized coal ignition and combustion stabilization in utility boiler furnaces promises to achieve certain savings compared to the use of heavy oil burners. Plasma torches are built in air-coal dust mixture ducts between coal mills and burners. Characteristics of processes in the ducts with plasma-system for pulverized coal combustion stabilization are analyzed in the paper, with respect to the modeling and numerical simulation of mass, momentum and heat transfer in two-phase turbulent gas-particle flow. The simulations have been performed for three different geometries of the air-coal dust mixture ducts with plasma torches, for TENTAI utility boiler and pulverized lignite Kolubara-Field "D". Selected results of numerical simulation of processes are presented. The plasma-system thermal effect is discussed regarding corresponding savings of liquid fuel. The results of numerical simulations have been analyzed with respect to the processes in the duct and especially with respect to the influence of the duct shape to a temperature field at the outlet cross section, as a basis for the duct geometry optimization. It has been emphasized that numerical simulation of processes can be applied in analysis and optimization of pulverized coal ignition and combustion stabilization and enables efficient and cost-effective scaling-up procedure from laboratory to industrial level.

Prevention, control and/or extinguishment of coal seam fires using cellular grout

Coal fires are difficult and persistent problems throughout the world wherever coal is at or near the surface. Coal fires can be associated with mining activities including underground and surface mine operations, coal stockpiles and coal waste piles, and they can also occur in unmined areas along coal seam outcrops. Environmental safety and health hazards are posed by coal fires in the form of ground surface subsidence, air pollution, forest and brush fires, and destruction of property and surface improvements, in addition to the potential massive loss of energy resources. While many causes exist, the most common source of coal fire ignition is spontaneous combustion as weathered, or broken/pulverized coal oxidizes and the temperature rises to the ignition point. This process can occur before any mining in the case of outcrop fires, or anytime during and after the mining cycle. The magnitude of coal fire problems varies worldwide depending on reserves available, extent of mining, population, political/economic situations, and a host of other influencing factors. Mining and environmental regulations imposed in the United States have heightened awareness regarding coal fire problems and resulted in efforts to control the most problematic existing fires and extinguish small fires before they escalate. Coal fires in other major coal-bearing countries, such as China and India, pose a significantly greater problem in terms of worldwide impact. Figures for China vary dramatically but estimates indicate that as much as 100–200 million tons of coal reserves are consumed annually as a result of coal fires. Similar estimates for the Jharia Coal Field in India indicate that about 40 million tons of coal reserves were burned during the 1990s. These data point to a significant source of global pollution, in addition to substantial economic losses. Goodson & Associates, Inc. (G & AI), a Lakewood, CO engineering firm, has developed a technology utilizing cellular (foam containing) grout to mitigate coal fires from the perspectives of prevention, control and extinguishment. The variable combination of portland cement, waste fly ash, aggregates and special foams create a highly flowable, high-heat resistant grout that is used to simultaneously address each of the three necessary fire elements: fuel, oxygen and heat. Coal fire prevention can be achieved by spraying the material on the exposed surface of coal seams following completion of strip mining. Control is enabled by grout injection into cracks, vents and cut-off trenches to stall or prevent continued growth. Extinguishment can be accomplished by grout injection directly into the fire zone, air intakes and exhausts. The benefits of using this technology are numerous. Cellular grout injection is safer at lower cost than conventional techniques such as complete excavation, while causing less environmental disruption and providing for quicker applications. The use of large quantities of fly ash in the process is economically and environmentally sound, by providing for disposal of a waste material while addressing the worldwide problem of coal fires.

Evaluation of Combustion Characteristics of Chinese High-Ash Coals

The combustion characteristics of four Chinese high-ash coals were evaluated by thermogravimetric and surface analysis. Changes in surface area and porosity during combustion were investigated with original (as-received) coals, coal samples before ignition, and chars after burnout. To understand the roles of various ash components, comparative experiments were carried out with as-received, de-ashed, and impregnated coals. SiO2, Al2O3, CaCO3, MgO, Na2CO3, TiO2, K2CO3, Fe2O3, FeS2, FeSO4(NH4)SO4·6H2O, and NH4Fe(SO4)2·12H2O at different concentrations were used as chemical additives impregnated into the de-ashed coals to investigate their influence on coal combustion. Most of these minerals inhibited coal ignitability; only K and Ca were found to promote coal ignition. Fe, Al, Na, and Ca were found to hinder both ignition and burnout of coal. Other ash components demonstrated variable effects on coal ignition and burnout with changing concentrations and species. A full understanding of the different effects of mineral matter is essential to achieve improved performance with high-ash coal applications.

Burning low volatile fuel in tangentially fired furnaces with fuel rich/lean burners

Pulverized coal combustion in tangentially fired furnaces with fuel rich/lean burners was investigated for three low volatile coals. The burners were operated under the conditions with varied value N d , which means the ratio of coal concentration of the fuel rich stream to that of the fuel lean stream. The wall temperature distributions in various positions were measured and analyzed. The carbon content in the char and NO x emission were detected under various conditions. The new burners with fuel rich/lean streams were utilized in a thermal power station to burn low volatile coal. The results show that the N d value has significant influences on the distributions of temperature and char burnout. There exists an optimal N d value under which the carbon content in the char and the NO x emission is relatively low. The coal ignition and NO x emission in the utilized power station are improved after retrofitting the burners.

Continuous wave laser ignition thresholds of coal dust clouds

Laser-based instruments are used in areas where coal dust ignition presents a safety hazard. The National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory (NIOSH, PRL) is conducting a study to help determine when an optical beam may be considered a potential ignition source in underground coal mines or coal storage facilities. Researchers conducted experiments to determine threshold igniting powers for Pittsburgh seam bituminous coal dust clouds using an 803 nm continuous wave laser. For fine sized coal dust dispersed in air, concentrations ranging from 600 to 2000 g/m3 were the most easily ignited. A heated layer of coal dust that deposited on the fiber tip during dispersion ignited the dust clouds. Minimum observed igniting powers for laser beams delivered by 200, 400, and 800 μm core fiber optic cables and directed into coal dust clouds were 2.0, 3.0, and 5.0 W, respectively. Threshold igniting power was proportional to beam diameter, and threshold igniting power density decreased with larger fiber diameters. Ignition delay times averaged 0.6 s and did not vary significantly with laser power under initially turbulent test conditions and with flammable targets. Comparisons are made with the results of other researchers.

E155 石炭とバイオマスの混焼特性

Fundamentals of pyrolysis and combustion characteristics of several types of biomass were studied, using a thermogravimetric analyzer. In order to elucidate effect of the biomass addition on coal combustion behavior, combustion performance, NOx emission characteristics and ash formation were discussed, using an electrically heated drop tube furnace. The results obtained showed that pyrolysis and combustion characteristics of biomass depended on lignin and cellulose composition in the biomass. The biomass could enhance the ignition of the low-rank coal. However, NO and N_2O emission behaviors under the co-combustion condition resembled those under the coal combustion. When the biomass was burned alone, fine particulates of less than 2,, m are produced. Under the co-combustion condition, coal ash could capture the fine particulates from the biomass during combustion.

203 劣質炭とバイオマスの混焼基礎特性

Fundamental pyrolysis and combustion characteristics of several types of biomass were studied, using a thermogravimetric analyzer. An electrically heated drop tube furnace was employed to elucidate the effect of the biomass addition on the combustion behavior, NOx emission characteristics and ash formation. The results obtained show that pyrolysis and combustion characteristics of biomass depend on lignin and cellulose compositions in the biomass. The biomass can enhance the ignition of the low-grade coal. However, NO and N_2O emission behaviors under the co-combustion condition resemble those under the coal combustion condition. When the only biomass is burned, fine particulates of less than 2μm are produced. However, the coal ash captures the fine particulates from biomass during co-combustion.