Char Consumption Rate Research Articles

Bluff-body MILD combustion regime for semicoke and bituminous coal mixtures with effect of oxidizer O2 concentration in O2/N2 and oxy-fuel (O2/CO2) atmospheres

The present study numerically investigates the effects of the O2 content (XO2) in the oxidizer on the MILD combustion behaviours of large-proportion semicoke mixtures, such as those involving temperature, MILD regime recognition, char surface consumption, homo/heterogeneous Damkhöler number, and NOx emission, under O2/N2 and O2/CO2 atmospheres. The results indicate that by increasing XO2 from 15 % to 30 %, the peak temperature increases significantly due to the increased char consumption rate, the high temperature region shrinks, and the axial length of the MILD regime region slightly increases from ∼1 to ∼1.4 m because of enhanced flue gas recirculation under O2/N2 or O2/CO2 atmosphere. The reduction effects of the char-O2 reaction proportions and the promoted effects of the char-CO2 reaction proportions due to the increased XO2 are both obviously weakened when N2 is substituted for CO2. A high O2 content and an O2/N2 atmosphere reduce the char burnout time. With increasing XO2, the maximum value of the turbulent Damkhöler number Dat increases from 6.79 to 10.16 under the O2/N2 atmosphere and from 3.99 to 10.6 under the O2/CO2 atmosphere, implying that a low O2 dilution and a CO2 environment promote the establishment of the homo-MILD regime. The maximum values of the surface Damkhöler number (Das-CO2 and Das-H2O) of the two gasification reactions (Char-CO2 and Char-H2O) have the same order of magnitude and are much lower than those of the surface Damkhöler number (Das-O2) of the Char-O2 oxidation reaction, i.e., Das-H2O < Das-CO2 ≪ Das-O2 < 1. The maximum values of the three surface Damkhöler numbers all increase with increasing XO2, while they decrease with the replacement of N2 by CO2, indicating that a high O2 content and an O2/N2 atmosphere could facilitate the establishment of an ideal hetero-MILD combustion regime to some extent. Low NOx emissions can be realized under moderate O2 and CO2 dilution conditions by regulating the oxy-MILD combustion regime of semicoke blends.

Effect of coal particle size on gasification performance of two‐stage entrained‐flow coal gasifier

AbstractTo clarify the effect of coal particle sizes on gasification performance of an advanced two‐stage entrained‐flow coal gasifier for IGCC (integrated gasification combined cycle) application, a comprehensive three‐dimensional numerical model is established by incorporating the shrinking core model combined with the Langmuir‐Hinshelwood kinetic rate expression, which considers the inhibitory effect of CO on char‐CO2 reaction. The flow, temperature, and species distributions were obtained, and the results are consistent with the operating data. Results show that the helical flow and eight recirculation zones in the gasifier improve carbon conversion efficiency through extending the residence time of coal particles. Slow devolatilization of large particles caused by slow heating retards volatiles combustion, and thus char combustion and gasification. As a result, less char gasification and higher gas temperature appear in the injection region of the first stage. Higher inertia of larger particles produces higher slip velocity, which enhances heat transfer and mass diffusion to char particles and increases char consumption rate in diffusion‐limited regions. The regions of high inner wall temperature spread from locations around burners to the whole inner wall of the injection and bottom regions in the first stage with increase of coal particle sizes.

The competitive behavior for O2 and CO2 reaction during char oxy-fuel combustion: effects of temperature and inherent minerals

Coal O2/CO2 combustion is a promising carbon capture and storage technology for coal-fired power plant. Char consumption rate prediction is essential for the boiler design, but challenging for O2/CO2 combustion since char reaction behavior within dual reactive gas media is complex. There is also a doubt of whether or not the O2 reaction and CO2 reaction occur independently, whereby the overall rate would be simply additive, or the two reactions compete for the same char actives, in which case the rate prediction could be more complicated. Here, the competitive behavior for O2 and CO2 reactions of char active sites was investigated via thermogravimetric approach, with a focus on the effect of temperature and inherent mineral composition. For char combustion within O2/CO2 environment with molar fractions of 30/70, the contribution of O2 to char consumption is the same as the O2 reaction alone. However, as being out-competed by O2, only 37% CO2 could occupy char active sites and react. Consequently, the overall char reaction rate within O2/CO2 is only 83% of the sum of that for individual gases at 900 °C. With increasing temperature, this ratio becomes even smaller, as the contribution of CO2 that being out-competed (to the sum of individual gases) is larger. With increasing O2 fraction, it occupied more active sites, thus further inhibiting CO2 reaction. The inherent alkali and alkaline earth metal provides the char with more active sites, thus muting the competition and inhibition for CO2 gasification.

CFD simulation of the steam gasification of millimeter-sized char particle using thermally thick treatment

A detailed char gasification model is developed using a multiphase Eulerian–Lagrangian algorithm and thermally thick treatment. The model is first validated by both gasification and combustion experiments of a millimeter-sized char particle. Temperature and mass loss histories as well as the particle morphology evolution correspond well with the existing results. Then the steam gasification of a 5 mm char particle is simulated and detailed physical and chemical conversion processes inside the particle are explored. During gasification, three distinct layers, i.e., the outer ash layer, the intermediate layer and the core layer, are identified based on the intraparticle porosity distribution. Simulation results show that the highest H2O and CO2 mass fractions locate in the ash layer, while the intermediate and core layers contain the highest H2 and CO mass fractions, respectively. Moreover, effects of several parameters are also explored. It is found that the Stefan flow caused by the mass transfer plays a key role in determining the diffusion and convection behavior during gasification. The strength of the Stefan flow in the intermediate layer appears to be two orders of magnitude smaller than that of the inflow and has an influence on the shifting from a kinetically-controlled mode to a diffusion-controlled mode. In addition, the char consumption rate in the intermediate layer increases with an increase in steam mass fraction, gasification temperature and inflow velocity while it decreases with increasing particle diameter. Meanwhile, the char consumption rate caused by CO2 is much smaller than that due to steam.

Competitive or additive behavior for H2O and CO2 gasification of coal char? Exploration via simplistic atomistic simulation

Here, a large-scale char atomistic structure (41,438 carbon atoms) and simple simulation approaches explored char gasification behavior for H2O and CO2 — comparing their mixture with one gas being passive (non-reactive) and the dual reactive mixture. Reactivity was captured through simplistic atomistic simulations via an automated sequence of gas diffusion, close-contact determination for gas (es) to reactive edge sites, then “reaction” via atom deletion. The higher reactivity of H2O was captured by a reaction probability function. For the reactive mixture (where both gases are reactive), the char consumption rate was 14% higher than with H2O alone, but lower (∼80%) than the sum of the individual gases (where only one gas is reactive), demonstrating competitive behavior. The H2O out competed the CO2 molecules and contributed ∼83% to the char consumption — with the reaction rate being similar to that of H2O independently. The pore size development for individual gases also differed with H2O favoring development in the smaller pore sizes in comparison to CO2. With fewer gas molecules (using 10% to capture a lower pressure), the competitive behavior was muted and became much closer to additive behavior. These simple simulations are consistent with the emerging rationalization of contributing factors to char gasification.

A kinetic study on char oxidation in mixtures of O2, CO2 and H2O

Flue gas is usually recycled to a boiler during oxy-fuel combustion. Elevated levels of CO2 and steam in the recycled flue gas may affect coal combustion. To investigate the effect of H2O and CO2 on char oxidation, two kinds of Chinese coals, including YQ anthracite and SH lignite, were combusted in the mixtures of O2, CO2 and H2O atmosphere. The char conversion rates in different atmospheres were measured at the temperature of 1273 K. Further, H218O isotopic tracing method was adopted to clarify the reaction path of H2O. The results show that the shrinking core model is applicable to predict both combustion and gasification reactions in TGA. For both YQ and SH chars, char-O2 reaction is independent but char overall oxidation rate is promoted by the increase of H2O partial pressure. The active sites for char-O2 and char-H2O reactions are partly independent and partly at the same active sites for YQ anthracite, but they share common active sites for SH lignite. Char-H2O reaction is inhibited by char-O2 reaction but the 18O in H2O is still transferred to CO2 even in oxygen-rich atmosphere. The possible reaction may be C + H218O + 16O2 = C16O18O + H216O. However, CO2 has little influence on YQ char oxidation, but it has a slight promotion on SH char oxidation in low oxygen atmosphere at 1273 K. Also, the active sites for char-O2 and char-CO2 reactions are partly independent and partly at the same active sites for SH lignite, though the active site for char-CO2 reactions is few. Finally, a reaction kinetic model is purposed to predict char consumption rate in the mixtures of O2, CO2 and H2O.

The influence of CO2 gas concentration on the char temperature and conversion during oxy-fuel combustion in a fluidized bed

In spite of the extensive theoretical and experimental work carried out on coal/char oxy-combustion in a fluidized bed (FB), the effect of changing the atmosphere from O2/N2 to O2/CO2 for a high O2 concentrations is not entirely understood. In this work, experiments with single char particles are conducted in a bi-dimensional FB at 800 and 850 °C, varying the O2 concentration from 11 to 50%v/v in N2 or CO2. The FB reactor has a quartz window for visual observation, allowing the measurement of temperature and tracking the char conversion process by pyrometry with a digital camera. The method is shown to overcome the inherent limitations of other methods used in FB, such as thermocouples or pyrometry with an optical probe. Results indicate that the transfer of O2 from the bulk gas of the bed to the surface of a char particle controls the overall rate of char conversion in O2/N2 and in O2/CO2. In the latter gas mixture, the carbon consumption by gasification is significant even at a relatively low char temperature (850 °C). This additional carbon consumption makes the apparent char consumption rate in both atmospheres roughly equal (at the same O2 concentration) for char temperatures below 925 °C, and higher in O2/CO2 than in O2/N2 for char temperatures above 925 °C. Moreover, during the time in which the char stays in the emulsion phase, its temperature is roughly the same in both atmospheres, but when the char is in the bubble or splash zone its temperature is much higher than that in the emulsion phase. As a result, the difference in char conversion rate, observed in both atmospheres, is mainly controlled by the time in which the char particle is out of the emulsion phase. These results underline the importance of paying attention to the movement of a char particle through the different phases of the bed in order to improve the understanding of the oxy-fuel behavior in FB.

Influence of Torrefaction on Biomass Gasification Performance in a High-Temperature Entrained-Flow Reactor

In the present work, the gasification performances of two pairs of raw and torrefied biomasses, including raw forest residue and torrefied forest residue, and raw spruce and torrefied spruce in a high-temperature entrained-flow reactor, are numerically examined and compared to each other using a Eulerian–Lagrangian CFD model developed based on OpenFOAM. Moreover, the sensitivities of three important operating parameters (excess air ratio, steam/carbon molar ratio, and biomass particle diameter), which vary in the range of practical significance, are also tested. The calculated results are analyzed both qualitatively and quantitatively by five indicators: isothermal profiles, char consumption rate, gas compositions, species yield, and carbon conversion along the reactor length. The obtained results show that torrefied biomass can increase the maximum temperature in the reactor as compared to its raw parent fuel. During gasification, CO, H2, and CO2 are the major species in the product gas and CH4 accounts ...

Emission of Organically Bound Elements during the Pyrolysis and Char Oxidation of Lignites in Air and Oxyfuel Combustion Mode

This study aims to clarify the abundance of individual elements, particularly those in trace concentrations in lignites, and their emission dynamics during pyrolysis and char oxidation in both air and oxyfuel combustion modes. For this laboratory-scale study, the emission dynamics was represented by element release from the coal/char particle during thermal treatment in a drop-tube furnace. The main coal sample studied is a Victorian brown coal (VBC), which was compared with a Chinese lignite. Irrespective of elemental type, the VBC is rich in organically bound elements, which partly dissociated during the initial flash pyrolysis step. This dissociation extent varied broadly with elemental type. For element release during char oxidation, As release rates in both N2 and CO2 bulk gases were slower than char surface consumption rate, because of internal diffusion limitations and scavenging of a portion of As by Ca/Al/Fe-bearing discrete minerals. In contrast, the release rates of Pb from the char surface were faster than the carbon consumption rate. Releases of the remaining elements were simply in linear proportion to the char consumption rate for the two lignites studied, despite their differences in properties with no observable (element release) difference between air and oxyfuel combustion mode.

Kinetics of CO2 gasification for coals of different ranks under oxy-combustion conditions

It is widely recognized that char-CO2 gasification reactions present in oxy-fuel processes may considerably alter the overall reaction rates in comparison to those attained in conventional combustion. In this paper, the results of an experimental study on the gasification kinetics of three coals of different ranks (from anthracite to lignite) under realistic conditions in an entrained flow reactor are presented; the work included also devolatilization and char oxidation tests in the same experimental facility. A particle combustion model, based on apparent kinetics, was used to derive kinetic parameters for each reaction with coal chars. A clear gradation of the char reactivity towards both O2 and CO2 with the coal rank was found. The same models were used to assess the relevance of gasification in oxy-combustion conditions; gasification may account for as much as 20% of the overall char consumption rate in the combustion of a low rank coal at low O2, high CO2 concentrations and offset (and exceed) the loss in the oxidation rate due to the change in diffusivity. These results clearly indicate the need to carefully consider the inclusion of gasification in the simulation of coal-fired boilers depending on the operating conditions.

The bioenergetics of density-dependent growth in Arctic char (Salvelinus alpinus)

We explored the mechanisms of density-dependent growth in Arctic char ( Salvelinus alpinus ) by comparing the energetics of growth, consumption, and activity obtained under three replicated density treatments in a large-scale enclosure (90 m2surface area) experiment. The enclosures permitted the entry of zooplankton and allowed char to feed on the bottom and at the surface of the lake. We found a negative (power) relationship between growth and density. Char consumption rate decreased linearly with increasing density. Growth efficiency was affected by fish density in a similar manner as growth rate. Finally, activity increased with fish density and was particularly high at high densities. Our findings illustrate the complexity of the relationships among consumption, activity, growth rates, and fish density and bring further evidence to the possible involvement of behavioural mechanisms in density-dependent processes, notably by modulating activity costs with density.

Experimental investigation of the combustion of bituminous coal in air and O2/CO2 mixtures: 2. Variation of the transformation behaviour of mineral matter with bulk gas composition

Combustion of a Chinese bituminous coal was carried out in a laboratory-scale drop tube furnace (DTF) to clarify the variation of ash properties with bulk gas composition. The combustion conditions tested include three bulk gases, air, 21% O2/79% CO2 and 27% O2/73% CO2, two furnace/gas temperatures close to the fluidized bed reactor temperature range, 1073K and 1273K, and three particle residence times. Apart from bulk properties analysis, individual ash particles and the original mineral species in coal were characterized using Computer – Controlled Scanning Electron Microscopy (CCSEM). The results indicate that, under the given experimental conditions, shifting bulk gas from air to O2/CO2 mixtures is insignificant in terms of the elemental composition of bulk ash, in agreement with the literature. However, changes in the properties of individual species/metals are noticeable, including the extent of the vaporization of volatile elements, ash particle-size distribution (PSD), crystallization extent of K alumino-silicate associate, pyrite decomposition and oxidation rate and formation propensity of liquidus in ash. These changes were mostly considered to be caused by the evolution of included mineral grains in the distinct char particles in the O2/CO2 environment. Reduction in char particle temperature with bulk gas shifting from air to O2/CO2 mixtures was primarily crucial, which, however, could be overweighed by the existence of a fairly strong local reducing condition on the char surface in O2/CO2. Consequently, vaporization of the volatile elements such as Na and P was promoted; formation of the crystalline leucite in air was in contrast inhibited. Furthermore, the extent of coalescence of included minerals and oxidation rate of pyrite (or its derivative, pyrrhotite) were also influenced by char consumption rate, i.e. the receding extent of char surface. These parameters exerted a combined effect on ash formation, requiring detailed mathematical modeling to describe the dynamics of the formation of oxy-fuel ash. This study also indicated that the differences of ash properties formed between air and O2/CO2 mixtures can be greatly reduced and eventually eliminated by increasing furnace temperature. Increase in the turbulence of gas flow should also benefit the elimination of the side effects of local reducing gases on char surface.

Modeling char conversion under suspension fired conditions in O2/N2 and O2/CO2 atmospheres

The aim of this investigation has been to model combustion under suspension fired conditions in O2/N2 and O2/CO2 mixtures. Experiments used for model validation have been carried out in an electrically heated Entrained Flow Reactor (EFR) at temperatures between 1173K and 1673K with inlet O2 concentrations between 5 and 28vol.%. The COal COmbustion MOdel, COCOMO, includes the three char morphologies: cenospheric char, network char and dense char each divided between six discrete particle sizes. Both combustion and gasification with CO2 are accounted for and reaction rates include thermal char deactivation, which was found to be important for combustion at high reactor temperatures and high O2 concentrations. COCOMO show in general good agreement with experimental char conversion profiles at conditions covering zone I–III. From the experimental profiles no effect of CO2 gasification on char conversion has been found. COCOMO does however suggest that CO2 gasification in oxy-fuel combustion at low O2 concentrations can account for as much as 70% of the overall char consumption rate during combustion in zone III.

LFR 장치를 이용한 입자 크기 변화에 따른 저열량 아역청 미분탄의 연소특성에 관한 실험적 연구

In this study, the effect of particle size on the combustion characteristics of pulverized sub-bituminous coal was experimentally investigated. A laminar-flow-entrained reactor was designed and implemented to realize the desired heating ratio and temperature corresponding to the combustion atmosphere of a pulverized-coal-fueled furnace. The flame length and structure of burning particles according to different sizes were investigated. Coal combustion processes were clearly distinguished by direct visual observation of the flame structure. The onset point of volatile ignition is greatly affected by changes in the particle size, and the burning time of the volatiles is least affected by changes in the particle size. The length and instability of char flame also increase with the increase of the particle size. However, the char consumption rate within the residential time remains nearly constant.

국내 수입탄 촤의 산화반응률 측정을 위한 해석기법 비교

Char oxidation experiments were performed with a sub-bituminous roto-middle coal in the Drop Tube Furnace (DTF) at atmospheric pressure condition. While temperatures varied between 900, 1100, 1400 , particle ℃ size, mass, particle temperature, and CO/CO2 concentration were obtained to be used for kinetic analysis of the char oxidation. This study addresses several different methods to analyze the char consumption rate, which are classified as energy balance method, ash-traced mass method, flue-gas based method, and particle size based method. The char consumption rate obtained with such methods was compared with the results of Monson et al. (24)

Reactivity and combustion of coal chars

Seven industrial process chars were tested to determine their reactivities and/or combustion characteristics. Char densities, total surface areas, and elemental and proximate compositions were measured. Reactivity tests were conducted over three temperature ranges: thermogravimetric analysis (TGA) with air (550–750 K), with CO 2 in a fixed-bed (1070–1270 K) and with air in a drop-tube furnace (1300–2100 K). In TGA tests, reactivities among chars differed by almost two orders of magnitude at the same temperature. Fixed-bed CO 2 tests gave mass reactivities that differed, by about one order of magnitude, while mass reactivities at high temperature in the drop-tube furnace differed by only a factor of two. Parent coal type and char formation conditions are major factors influencing reactivity in low and intermediate temperature ranges. N 2 -BET surface area does not adequately explain observed reactivity in any temperature range while variation in external surface area does correlate reactivity at high temperature. High temperature reactivities compared well with predictions from a pore tree/char oxidation model, after adding char volatiles percentage. Only high temperature reactivity data characterized the char consumption rate in a laboratory combustor, but a significant residual char volatiles content appeared to be essential for flame stability.