Char Combustion Rate Research Articles

Numerical study of coal particle cluster combustion under quiescent conditions

Behavior of ignition and combustion of coal particle cluster under a quiescent condition was numerically simulated by solving balance equations of mass and enthalpy with combustion kinetic models of volatiles and char. Two-flame structure, one flame penetrating into the cluster and the other moving out of the cluster, was predicted during the combustion of coal particle cluster. Effects of radiative heat transfer, group number, ambient temperature, coal particle size, and oxygen concentration on ignition and combustion of coal particle clusters were also analyzed. Simulations indicated that the gas volume fraction of coal particle cluster increases with time after devolatilization. Gas velocity passing through the cluster surface varied significantly at volatile liberation. The ignition time delay was reduced with the increase of ambient temperature. The cluster devolatilization rate and char burning rate increased while the ignition time delay decreased with the increase of ambient oxygen concentration.

Comparison of fractal properties of porous structures during coal devolatilization

This paper investigates fractal property changes of pore structures during coal devolatilization. Similar to char pores, coal pores can also be classified as micro pores and macro pores based on their fractal dimensions. The specific surface area and fractal dimension of micro pores in coal particles are basically unchanged after devolatilization. However, the specific surface area and fractal dimension of macro pores, which are key factors in char combustion, are increased after devolatilization. In fact, the fractal dimensions are basically doubled. These parameters will affect another fractal geometrical factor β in char pores that is correlated to char combustion rate. Since the rate of char combustion can be predicted from their fractal pore properties, it may be possible to predict char combustion directly from the properties of their parent coal pores in the future.

NUMERICAL SIMULATIONS OF GAS DIFFUSION AND REACTION IN FRACTAL PORES DURING CHAR COMBUSTION

Classic molecular gas dynamics and fractal pore models are used to simulate gas diffusion and reaction within char pores in combustion. Through numerical simulations it is found that the char combustion model based on Thiele modulus can be well used in regular pores, but is not well used in real char particles that have fractal pore structures. When oxygen concentrations are the same in different char pores, which is often happened in initial stage of combustion, fractal dimensions of pores will affect char combustion rates in exponential form. These numerical simulations can partly explain the previous experimental results.

MODELING OF NOx EMISSIONS FROM FLUIDIZED BED COMBUSTION OF HIGH VOLATILE LIGNITES

A comprehensive model, previously developed and tested for prediction of behavior of continuous fluidized bed combustors is extended to incorporate NOx formation and reduction reactions and applied to the simulation of Middle East Technical University (METU) 0.3 MWt Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) burning lignites with high Volatile Matter/Fixed Carbon (VM/FC) ratios in their own ashes. Favorable comparisons are obtained between the predicted and measured temperatures and concentrations of gaseous species along the combustor. Results show that determination of partitioning of coal nitrogen into char-N and volatile-N, char combustion rate, and amount of volatile nitrogen released along the combustor are found to be the most important parameters that affect NO formation and reduction in bubbling fluidized bed combustors.

Effect of raw material composition in woody biomass pellets on combustion characteristics

Char yield, char combustion time and char combustion rate of pellets made from different tree parts of Norway spruce and industrially made stem wood pellets of Norway spruce and Scots pine were studied. The pellets were incinerated in a laboratory scale oven at various temperatures, gas flows and oxygen concentrations. It was found that the combustion time for a single pellet mostly depended on the raw material composition and to a minor extent on the density. Pellets made of bark had up to a 50% longer char combustion time compared to that of stem wood pellets, due to differences in char yield. Industrially made stem wood pellets of pine and spruce sawdust were found to have small differences in combustion characteristics. The variations in combustion characteristics of pellets are discussed in relation to composition of raw material.

The influence of thermal annealing on oxygen uptake and combustion rates of a bituminous coal char

The effect of thermal annealing on the combustion reactivity of a bituminous coal char has been investigated with a focus on the role of the formation of surface oxides by oxygen chemisorption. The combined use of thermogravimetric analysis and of analysis of the off-gas during isothermal combustion of char samples enabled the determination of the rate and extent of oxygen uptake along burn-off. Combustion was carried out at temperatures between 350 and 510 °C. Char samples were prepared by controlled isothermal heat treatment of coal for different times (in the range between 1 s and 30 min) at different temperatures (in the range 900–2000 °C). Results indicate that oxygen uptake is extensive along burn off of chars prepared under mild heat treatment conditions. The maximum oxygen uptake is barely affected by the combustion temperature within the range of combustion conditions investigated. The severity of heat treatment has a pronounced effect on char combustion rate as well as on the extent and rate at which surface oxides are built up by oxygen chemisorption. Chars prepared under severe heat treatment conditions show negligible oxygen uptake and strongly reduced combustion rates. Altogether it appears that a close correlation can be established between the extent and the accessibility of active sites on the carbon surface and the combustion rate. Despite the investigation has been carried out at temperatures well below those of practical interest, results provide useful insight into the relationship existing between thermal annealing, formation of surface oxide and combustion reactivity which is relevant to the proper formulation of detailed kinetic models of char combustion.

Experimental Validation of a Coupled Solid- and Gas-Phase Model for Combustion and Gasification of Wood Logs

A solid-gas-phase model for thick wood gasification/combustion is extensively studied, after a re-examination of the kinetic constants for the char gasification reactions. The solid-phase model, which includes the description of all the relevant heat and mass transfer phenomena and chemical reactions, is coupled with a CFD code for the gas-phase processes. Both the gasification and combustion of single wood logs are simulated (log radius in the range of 0.06-0.1 m, initial moisture content, on a dry basis, 1-81%, inlet gas temperature 1253-1613 K, inlet gas velocity 0.5-1.0 m/s, and various compositions of the gaseous mixture). For comparison purposes, a solid-phase model, with global heat and mass transfer coefficients and a constant-property gas phase, is also considered. Although both models predict the mass loss dynamics to be qualitatively similar, the solid-phase model overestimates the total heat flux and underestimates the char combustion rate. Extensive experimental validation of both models is carried out in terms of conversion time and average mass-loss rates. Acceptable agreement is obtained for the comprehensive model, whereas in the other case, the conversion times are generally underestimated and the average mass loss rates are overestimated. However, improvements in the predictive capabilities of the solid-phase model could be achieved through the introduction of corrective factors for the external heat and mass transfer coefficients.

Simulation of coal combustion by AUSM turbulence-chemistry char combustion model and a full two-fluid model

An algebraic unified second-order moment (AUSM) turbulence-chemistry model of char combustion is introduced in this paper, to calculate the effect of particle temperature fluctuation on char combustion. The AUSM model is used to simulate gas-particle flows, in coal combustion in a pulverized coal combustor, together with a full two-fluid model for reacting gas-particle flows and coal combustion, including the sub-models as the k-ε- k p two-phase turbulence model, the EBU-Arrhenius volatile and CO combustion model, and the six-flux radiation model. A new method for calculating particle mass flow rate is also used in this model to correct particle outflow rate and mass flow rate for inside sections, which can obey the principle of mass conservation for the particle phase and can also speed up the iterating convergence of the computation procedure effectively. The simulation results indicate that, the AUSM char combustion model is more preferable to the old char combustion model, since the later totally eliminate the influence of particle temperature fluctuation on char combustion rate.

実験室規模の加圧流動層燃焼における窒素酸化物の生成特性―チャーによるＮＯｘ分解―

A laboratory scale pressurized fluidized bed combustion (PFBC) system was designed and constructed to clear the details of combustion of coal in PFBC. This PFBC system allows visual observation of hot fluid-ized bed by using a transparent electric furnace. To simulate the emission characteristics of coal combustion of the full scale PFBCs by this lab-scale PFBC, a new similarity rule was introduced. Volumetric heat input and mean gas residence time through fluidized bed in lab-scale PFBC were adjusted to the values matching those of full scale PFBCs. Motion of bubbles and burning of coal particles and volatiles were recognized by visual observations. Volatiles burned mostly in the bottom region of the fluidized bed. On the other hand, char particles burned uniformly in the bed. Ranges of conversion of fuel-N to NO agreed with those obtained in a full scale PFBC. Thus, it is verified that new similarity rule explained above is useful to simulate the combustion behaviors in full scale PFBC by a laboratory scale small PFBC.Effects of operating pressure and O2 partial pressure in flue gas on NO emission were examined. Linear relationship between O2 partial pressure and NO emission was observed. NO emission was decreased rapidly with increasing in operating pressure at constant O2 partial pressure. This may be caused by an enhance-ment of NO reduction by char particles because of increase in number density of char particle in the bed at elevated pressure. In order to confirm this assumption, initial combustion rate of char in the pressurized fluidized bed condition was measured. Initial combustion rate of char increased with 0.6th power of operating pressure. This means that char number density is increased in elevated pressure condition. One of the major reasons to achieve the low NOx emission in PFBC condition, should be the reduction of NOx by char particles through the bed.

Co-firing pulverised coal and biomass: a modeling approach

A CFD modelling study has been undertaken to examine the co-firing of pulverised coal and biomass with particular regard to the burnout of the larger diameter biomass particles. Computations were based on a research combustion facility that replicates an industrial coal-fired power station. Three percent, by mass, of pinewood was blended with a bituminous UK coal, and the effects of the wood particle size and shape on the burnout of the combined wood and coal char were investigated. The effect of varying the devolatilisation and char combustion rate constants for the biomass component in the blend was also investigated. It was concluded that the combustion of small (200 μm) wood particles was rapid but the rate of combustion of larger particles was dependent on their composition, size, and shape.

Combustion of lignite carbon under agitation

To get insights into the burning of large coal particles as agitated beds in the recently developed rotary chambers, combustion behaviour of eight Turkish lignites was studied in a laboratory-size rotating wire mesh basket. Single particles from 0.5 to 5 g average weight were loaded in the basket, which was then inserted into a preheated tube furnace. The net effect of removing ash continuously as a result of agitation on the combustion rates and times was searched for all the lignites, whereas the other parameters, such as rotation speed of the basket, particle size, temperature and air velocity, were investigated on only Çan lignite. Because both volatile and char combustion times and their confident prediction have important bearings in the design and capacity of coal combustion systems, experimentally determined char combustion rates and times were compared to those found by using double resistance theory. It was observed that combustion of lignite chars in the agitated beds proceed faster and lasted in shorter times as expected, mainly due to the removal of ash layer by agitation during the period of char combustion. The influence of the agitation on the combustion of chars from lignites with high-volatile matter content was especially noticeable. Combustion rates increased more as the speed of basket was increased. A combined control mechanism could be used to describe combustion rates of lignite chars.

STUDY ON THE TRANSIENT PROCESS OF WASTE FUEL INCINERATION IN A FULL-SCALE MOVING-BED FURNACE

ABSTRACT The transient process in a full-scale municipal solid waste incinerator was investigated to assess the effect of the grate movement and waste feeding cycles. Inside-bed measurements of temperature and O2 were carried out and an 18-hour record of furnace temperature, airflow, and flue gas compositions was analyzed using discrete Fourier transform. The transient burning process was also simulated numerically by solving the governing equations for both the solid and gas phases. It is found that fluctuations in the furnace temperature and flue gas O2 level correspond to the waste feeding and grate movement cycles, either in phase or out of phase (depending on the length of the cycles), while combustion inside the bed is dominated by a series of big spikes and dips in both local temperature and O2 level. Theoretical calculations indicate that each grate movement causes a surge (by 35%) in the volatile release rate but a depression (by 24%) in the char burning rate. The overall bed-burning rate fluctuates between 90 and 110% of its average.

Char characterisation and its application in a coal burnout model ☆

In this study, char image analysis techniques have been employed to investigate the morphology of chars obtained from a Drop-Tube furnace. Char image analysis results have been incorporated as inputs to a char burnout model based on Hurt's CBK model. It has been observed that the char combustion rate was strongly affected by char structural parameters and the inclusion of char morphology has led to a better prediction of char burnout. It has also been suggested by the model that the inclusion of ash inhibition overestimates the resistance attributed by ash film and the consideration of ash film resistance should be undertaken in a different way to give a better prediction at the later stages of char combustion.

Characteristics of particulate matter generated in pressurized coal combustion for high-efficiency power generation system

Characteristics of particulate matter in pressurized coal combustion for high-efficiency power generation system are investigated by using the percolation model and the results are compared with experiments performed using drop tube furnace facilities (DTF). The results show that conversion for Plateau coal (high volatile matter and low ash contents) rises earlier than that for Newlands coal (low volatile matter and high ash content). For both coals, volatile consumption rate increases with increasing the gaseous temperature, whereas the char combustion rate increases with increasing both the gaseous temperature and the pressure. At the char-combustion-dominant stage after devolatilization, the characteristics of the particulate matter such as specific surface and porosity area are not affected by the gaseous temperature, but they are affected by the pressure and the coal properties. These results are of general agreement with the experiment by DTF and hence the present percolation model is valid for the prediction of the characteristics of the particulate matter in coal combustion.

Effect of Pressure on NOx Emission from Char Particle Combustion

The effect of pressure on NOx emission during char particle combustion was examined in chemical-reaction control and diffusion control regimes. A fixed bed was used for batch testing under 0.1−1.6 MPa. It was found that with increasing pressure, NOx emission decreased extremely, when char combustion rate was controlled by reactant gas diffusion into the char particle at high temperature. The extent to which NOx was reduced in char particles strongly influenced the NOx emission from char pressurized combustion. Pressure increased residence time for diffusion of NOx throughout the char particle and consequently further increased the reduction of NOx in the char particle. Both pressure and temperature strongly influenced the conversion of fuel-N in char to NOx. When pressure was raised from 0.1 to 1.1 MPa, the conversion of fuel-N to NOx fell from 0.18 to 0.06 at 973 K, and from 0.68 to 0.13 at 1173 K. NOx emissions were lower when large char particles were combusted than when small ones were combusted. It was also observed that practically no N2O was formed in to any extent in the char particle.

A Chemical Reaction Engineering and Transport Model of Kraft Char Bed Burning

A numerical model for carbon burning in the char bed of a kraft recovery boiler was developed that predicts the rate of char burning and the char bed temperature locally within the furnace. The model describes the char bed chemistry, accounting rigorously for chemical element and energy balances. Char burning rates are based on established kinetic rate data, and diffusion of reactant gases within the bed is taken into account. The chemical model is coupled to the gas field and furnace thermal environment above the bed to account for convective mass and heat transfer from the gases immediately above the bed and for radiant heat transfer from the furnace cavity. This model predicts, within 7% on average, the experimentally measured char burning rates reported by others in laboratory char bed studies, as well as predicting their reported char bed surface temperatures with an average error of 18 °C. It also predicts quantitatively the effects of gas composition and velocity on those rates with an average error of 7%.

Combustion characteristics of waste material containing high moisture

In this paper, the effect of water content on combustion characteristics of watery waste material, such as kitchen garbage, was studied experimentally. In order to clarify the water content quantitatively, a permeable balsa wood was chosen as the sample material. Combustion tests of the small balsa pieces (0.1–0.5 g) containing water up to 70 wt% were conducted in a thermogravimetric furnace. Mass reduction during pre-heating, volatile matter combustion and char combustion were measured with a microbalance. Also, the variations of flame temperature and center temperature of the test piece during the combustion process were measured by means of R- and K-type thermocouples, respectively. In the case of the combustion of balsa with high water-content, it was found that the ignition started before the water was removed completely, namely the simultaneous vaporization of the remaining water and devolatilization of volatiles occurred during volatile matter combustion. The ignition delay and the retention time of volatile matter combustion increased with increasing water content. The flame temperature decreased when the water content of wet balsa exceeded 50 wt%. The combustion rate of the volatile matter was drastically reduced in proportion to the water content. On the other hand, the char combustion rate increased slightly with increasing water content since part of the char is burned during the long volatile matter combustion. It was found from this study that excess moisture remarkably affected the volatile matter combustion with a flame.

Combustion Characteristics of Watery Materials.

To clarify the effect of water content on combustion characteristics of watery materials, combustion test of a high water-permeable balsa wood was performed using a thermogravimetric furnace. The furnace temperature is between 650°C and 1000°C, and water contents in the small balsa pieces (0.3g) were varied up 70 wt%. Mass reduction during pre-heating, volatile matter combustion and char combustion were measured with a microbalance. Also, the variations of flame temperature and center temperature of the test piece during combustion process were measured. On the combustion of high water-contained balsa, it was found that ignition started before the water was evaporated completely. It means that two proceses, devolatilization of volatiles and vaporization of remaining water, are proceeding simultaneously during the volatile matter combustion. Ignition delay and retention time of volatile matter combustion increased with increasing the water content. The increases of flame temperature and center temperature of the test piece were prevented by the latent heat of water vaporization. The combustion rate of volatile matter was drastically reduced in proportion to the water content. Also, it was confirmed that a part of char burned during the latter stage of volatile matter combustion. The effect of water content on char combustion rate was small. From this study, it was concluded that the water content over 50 wt% affected remarkably to the volatile matter combustion.

00/03054 Understanding the behavior of Australian black coals in pressurized fluidized bed combustion

Ultimately, this study aims to predict the coal combustion efficiency in an industrial pressurized fluidized bed combustor (PFBC) for Australian black coals. This combustion efficiency depends predominantly upon the rate of elutriation of fine carbon particles, which is proportional to bed carbon loading in atmospheric experiments. The bed carbon loading is, in turn, dependent upon the rate of combustion of char particles within the PFBC. A novel batch-fed reactor has been designed, constructed and commissioned to enable separation and study of the mechanisms of coal devolatilization, char combustion and fine carbon particle elutriation in a PFBC and extraction of coal-specific parameters to describe these processes. The attrition and char combustion rates can only be determined experimentally and it is essential to match the environment around each coal particle, so that the results may be translated to the industrial scale. Therefore, the rig was designed for identical conditions of pressure, temperature, particle size and fluidizing velocity within the bed to those used industrially. The exhaust gas is analyzed continuously for oxygen, carbon dioxide, carbon monoxide and hydrocarbons as a function of time after coal injection, allowing separation and identification of the devolatilization and char combustion stages as well as measurement ofmore » the combustion rates. The elutriated carbon particles undergo minimal freeboard combustion and are collected in a cyclone and an in-line filter over any period of time during the experiment, for subsequent analysis. The sand bed containing the rig for collection and characterization of the partially burnt char particles. The rig is mostly computer-controlled and the design was subjected to a hazards analysis before construction. Results from the rig will be used in a mathematical model to predict the performance of the coals in industrial-scale PFBC.« less

Evolution and Distribution of Surface Voids Affecting Char Burning Rates at Diffusion-Limited Conditions

Research has indicated that the accurate calculation of char burning rates at diffusion-limited conditions requires consideration of the effects of voids (gaps in the continuous surface of burnable material that are deeper than their surface radius) on the particle burning area. Experimentally, void area distributions as a function of void size can be quantified using scanning electron microscopy and digital processing of quenched char particle images. Modeling void area distributions is problematic, even assuming voids behave as pores, because of initial surface imperfections and effects of combustion on surface morphology. This work examines the effects of factors that affect general pore formation for their effect on voids larger than 1 μm, such as heterogeneous combustion, devolatilization, and particle size and how these factors influence the calculation of diffusion limited burning rate from temperature and image data. Void distributions were predicted using an established pore distribution model, with and without the effects of pore combination. Results indicate that void distributions roughly follow expected pore behavior through devolatiltzation, but not after ignition, with void area fraction increasing with increasing coal volatile content and particle heating rate, but not significantly affected by oxygen concentration or initial particle size. Further, analytical pore models provide reasonable approximations for void distributions in the calculation of particle burning rates from temperature and image data.