Differences in reactivity of pulverised coal in air (O2/N2) and oxy-fuel (O2/CO2) conditions

The reactivity of four pulverised Australian coals were measured under simulated air (O₂/N₂) and oxy-fuel (O₂/CO₂) environments using a drop tube furnace (DTF) maintained at 1673 K and a thermogravimetric analyser (TGA) run under non-isothermal (heating) conditions at temperatures up to 1473 K. The oxygen concentration, covering a wide and practical range, was varied in mixtures of O₂/N₂ and O₂/CO₂ in the range of 3 to 21 vol.% and 5 to 30 vol.%, respectively. The apparent volatile yield measured in CO₂ in the DTF was greater than in N₂ for all the coals studied. Pyrolysis experiments in the TGA also revealed an additional mass loss in a CO₂ atmosphere, not observed in a N₂ atmosphere, at relatively high temperatures. The coal burnout measured in the DTF at several O₂ concentrations revealed significantly higher burnouts for two coals and similar burnouts for the other two coals in oxy-fuel conditions. TGA experiments with char also revealed higher reactivity at high temperatures and low O₂ concentration. The results are consistent with a char–CO₂ reaction during the volatile yield experiments, but additional experiments are necessary to resolve the mechanisms determining the differences in coal burnout.     

In situ diagnostics of Victorian brown coal combustion in O2/N2 and O2/CO2 mixtures in drop-tube furnace

Experimental investigation of the combustion of an air-dried Victorian brown coal in O₂/N₂ and O₂/CO₂ mixtures was conducted in a lab-scale drop-tube furnace (DTF). In situ diagnostics of coal burning transient phenomena were carried out with the use of high-speed camera and two-colour pyrometer for photographic observation and particle temperature measurement, respectively. The results indicate that the use of CO₂ in place of N₂ affected brown coal combustion behaviour through both its physical influence and chemical interaction with char. Distinct changes in coal pyrolysis behaviour, ignition extent, and the temperatures of volatile flame and burning char particles were observed. The large specific heat capacity of CO₂ relative to N₂ is the principal factor affecting brown coal combustion, which greatly quenched the ignition of individual coal particles. As a result, a high O₂ fraction of at least 30% in CO₂ is required to match air. Moreover, due to the accumulation of unburnt volatiles in the coal particle vicinity, coal ignition in O₂/CO₂ occurred as a form of volatile cloud rather than individual particles that occurred in air. The temperatures of volatile flame and char particles were reduced by CO₂ quenching throughout coal oxidation. Nevertheless, this negative factor was greatly offset by char-CO₂ gasification reaction which even occurred rapidly during coal pyrolysis. Up to 25% of the nascent char may undergo gasification to yield extra CO to improve the reactivity of local fuel/O₂ mixture. The subsequent homogeneous oxidation of CO released extra heat for the oxidation of both volatiles and char. As a result, the optical intensity of volatile flame in ∼27% O₂ in CO₂ was raised to a level twice that in air at the furnace temperature of 1273 K. Similar temperatures were achieved for burning char particles in 27% O₂/73% CO₂ and air. As this O₂/CO₂ ratio is lower than that for bituminous coal, 30-35%, a low consumption of O₂ is desirable for the oxy-firing of Victorian brown coal. Nevertheless, the distinct emission of volatile cloud and formation of strong reducing gas environment on char surface may affect radiative heat transfer and ash formation, which should be cautioned during the oxy-fuel combustion of Victorian brown coal.     

Exploring the possibilities of using Brazilian subbituminous coals for blast furnace pulverized fuel injection

The current study investigates the combustion and blast furnace injection performance of three Brazilian subbituminous coals (Mina do Recreio) and their beneficiation products using laboratory scale combustion tests. The coals have relative high ash yields (up to 40 wt%) that were reduced stepwise to levels as low as 12 wt%, dry basis. The reduction of ash yields is paralleled by a significant decrease in sulphur and inertinite contents.The combustion tests were performed in a drop tube reactor operating at 1300 °C using two different atmospheres (2.5 and 5% O₂). The chars exhibited preferentially rounded shapes with thick walls and abundant secondary porosity for the 2.5% O₂ chars, whereas the 5% O₂ chars showed very thin walls as a consequence of extensive burnout. The intrinsic reactivities of both set of chars were similar. The differences in conversion between the two working atmospheres were 24-37% and roughly tend to increase with increasing mineral matter content. Conversions as high as 76-81% were reached operating under 5% O₂ indicating that the coals are easy to burn. The small differences in burnout among the coals and their beneficiation products cannot be clearly attributed neither to mineral matter or inertinite content. A rough inverse relationship was found between the intrinsic reactivity of the chars and the inertinite content of the parent coal indicating that the char material derived from inertinite was intrinsically less reactive than that derived from vitrinite. These differences were no longer relevant at high temperature.Blast furnace injection performance was studied through thermobalance experiments using CO₂ atmosphere and 1050 °C temperature. It is apparent that the beneficiation process has no effect on the reactivity of the coals from Recreio Mine. The only exception is the low ash coal-2-LabB (11.5 wt%), for which a higher reactivity is indicated. The reactivity tests show also that the coals have adequate properties to be used together with imported coal blends in pulverized coal injection in the blast furnace (PCI).     

低阶煤低温热解半焦在模拟高炉喷吹条件下的燃烧性能

采用自制固定床热解装置在隔绝空气的条件下制备神木长焰煤热解终温分别为400℃、450℃、500℃及550℃的热解半焦,利用管式沉降炉模拟高炉喷吹条件研究神木长焰煤低温热解半焦的燃烧性能,并考察了热解终温、半焦喷吹粒径以及燃烧反应温度对半焦燃烧性能的影响。研究表明：低温热解半焦的燃烧性能优于实验所选用无烟煤的燃烧性能,半焦的燃烧性能与其燃料比之间存在负相关关系,即燃料比越高,燃烧性能越差;降低热解终温、减小半焦喷吹粒径以及提高燃烧反应温度均能改善半焦的燃烧性能,当热解终温为400℃、喷吹粒径100～200目、燃烧反应温度为1100℃时半焦的燃尽度最佳为96%。本实验半焦制备及燃烧条件与现有低温热解和高炉喷吹工艺相符,且热解半焦各项性能均符合喷吹用煤指标。     

Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures

The present paper was addressed to mineral transformations and ash formation during O₂/CO₂ combustion of pulverized coal. Four Chinese thermal coals were burned in a drop tube furnace to generate ashes under various combustion conditions. The ash samples were characterized with XRD analysis and ⁵⁷Fe Mössbauer spectroscopy. The impacts of O₂/CO₂ combustion on mineral transformation and ash formation were explored through comparisons between O₂/CO₂ combustion and O₂/N₂ combustion. It was found that, O₂/CO₂ combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. The differences observed in the ashes formed in two atmospheres were attributed to the impact of the gas atmosphere on the combustion temperatures of coal char particles, which consequently influenced the ash formation behaviors of included minerals.     

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

The aim of the present investigation is to examine differences between O₂/N₂ and O₂/CO₂ atmospheres during devolatilization and char conversion of a bituminous coal at conditions covering temperatures between 1173 K and 1673 K and inlet oxygen concentrations between 5 and 28 vol.%. The experiments have been carried out in an electrically heated entrained flow reactor that is designed to simulate the conditions in a suspension fired boiler. Coal devolatilized in N₂ and CO₂ atmospheres provided similar results regarding char morphology, char N₂-BET surface area and volatile yield. This strongly indicates that a shift from air to oxy-fuel combustion does not influence the devolatilization process significantly. Char combustion experiments yielded similar char conversion profiles when N₂ was replaced with CO₂ under conditions where combustion was primarily controlled by chemical kinetics. When char was burned at 1573 K and 1673 K a faster conversion was found in N₂ suggesting that the lower molecular diffusion coefficient of O₂ in CO₂ lowers the char conversion rate when external mass transfer influences combustion. The reaction of char with CO₂ was not observed to have an influence on char conversion rates at the applied experimental conditions.     

Effects of operation parameters on NO emission in an oxy-fired CFB combustor

Oxy-fuel Circulating Fluidized Bed (CFB) combustion technology, a very promising technology for CO₂ capture, combines many advantages of oxy-fuel and CFB technologies. Experiments were carried out in a 50 kW_th CFB facility to investigate how operation parameters influence the NO emission in O₂/CO₂ atmospheres. The simulated O₂/CO₂ atmospheres were used without recycling the flue gas. Results show that NO emission in 21% O₂/79% CO₂ atmosphere is lower than that in air atmosphere because of lower temperature and higher char and CO concentrations in the dense bed. Elevating O₂ concentration from 21% to 40% in O₂/CO₂ atmosphere enhances fuel-N conversion to NO. Increasing bed temperature or oxygen/fuel stoichiometric ratio brings higher NO emission in O₂/CO₂ atmosphere, which is consistent with the results in air-fired CFB combustion. As primary stream fraction increases, NO emission increases more rapidly in O₂/CO₂ atmosphere than that in air atmosphere. Stream staging is more efficient for controlling NO emission in oxy-CFB combustion than that in air combustion. Oxygen staging provides an efficient way to reduce NO emission in oxy-CFB combustion without influencing the hydrodynamic characteristic in the riser.     

CFD modelling of oxy-coal combustion in an entrained flow reactor

Oxy-fuel combustion is seen as one of the major options for CO₂ capture for both new and existing coal fired power stations. Coal is burned with a mixture of oxygen and recycled flue gas to obtain a rich CO₂ stream ready for sequestration. Computational fluid dynamics (CFD) tests for coal combustion under different O₂/CO₂ (21-35% vol O₂) atmospheres in an entrained flow reactor (EFR) were carried out using three coals of different volatile matter content. The temperature profiles, burning rates, burnout and concentration of major species, such as O₂, CO₂, CO, were predicted and compared with an air reference case. A decrease in gas temperature and burning rate was observed for 21% O₂/79% CO₂ environment in comparison to the air reference case due to the difference in gas properties between N₂ and CO₂. Experimental coal burnouts obtained in the EFR, were used to test the accuracy of the CFD model. The numerical results showed a decrease in coal burnout when N₂ was replaced by CO₂ for the same oxygen concentration (21%), but an improvement in the O₂/CO₂ atmosphere for an oxygen concentration higher than 30%. The numerical results for oxy-coal combustion were in good agreement with the experimental results.     

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, 1073 K and 1273 K, 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.     

随机孔模型研究煤焦O2/CO2燃烧动力学特征

在热重分析仪上对焦作无烟煤焦和云浮烟煤焦O₂/CO₂条件下燃烧特性进行研究。确定在不同温度下不同煤焦O₂/CO₂燃烧的特征。利用随机孔模型(RPM)表征两种煤焦反应速率与碳转化率的关系,同时与未反应缩核模型(Model Ⅰ)和混合模型(Model Ⅱ)的拟合结果进行比较。研究表明,在不同反应条件下,随机孔模型具有最佳的拟合效果,相关系数都在0.986以上。比较RPM、ModelⅠ和Model Ⅱ计算结果发现,焦作无烟煤焦的O₂/CO₂等温燃烧的活化能比云浮烟煤焦的高,且同一煤种燃烧反应温度越高反应速率常数越大。由于随机孔模型的结构参数ψ可以很好地表现孔结构变化对煤焦燃烧反应的影响,因此随机孔模型能更加准确地描述煤焦O₂/CO₂燃烧特征。     

Optical Coal Particle Temperature Measurement under Oxy‐Fuel Conditions: Measurement Methodology and Initial Results

Coal combustion under oxy‐fuel conditions shows significant differences to combustion in air. Examinations on the single‐grain level give detailed insight into the combustion phenomena of ignition, volatile combustion, and char burnout and, therefore, provide the fundamentals for the development of large‐scale oxy‐fuel facilities. The combustion of a hard coal in a size fraction of d_p = 90–125 μm was investigated in a laminar flow reactor at a temperature of 1500 K. The gaseous fuel oxidizer contained 3 % O₂ by volume and CO₂ or N₂ as diluents. A third measurement in a CO₂‐rich atmosphere containing 9 % O₂ is also presented to show the influence of O₂ concentration. Particle temperatures were measured for three residence times with an imaging two‐color pyrometer.     

煤粉O2/CO2燃烧时PM2.5及其Fe、S的生成特性

沉降炉实验研究了煤粉O2/CO2燃烧时PM2.5的生成特性和主要成灰元素中危害较大的Fe、S元素生成特性.实验用DT烟煤、NMG褐煤和XLT褐煤,实验温度1300℃,在O2/CO2=1:9,1:4,3:7和O2/N2=1:4气氛下燃烧.低压撞击器(DLPI)按不同粒径大小从0.03～9.8μm共分为13级,分别收集燃烧...     

Factors affecting coal particle ignition under oxyfuel combustion atmospheres

A set of 13 coals of different rank has been tested for ignition propensity in a 20-L explosion chamber simulating oxyfuel combustion gas conditions. Their char residues were also analysed thermogravimetrically. The effects of coal type, coal concentration (from 100 to 600 g/m³), O₂ in CO₂ atmospheres (up to 40% v/v) and particle size were investigated.The higher rank coals were significantly more difficult to ignite and mostly required higher energy chemical igniters (1000 or 2500 J) whereas the lower rank coals could be ignited with a 500 J igniter even at low coal dust concentrations.The minimum explosibility limit/ignition concentration in air varied slightly around a value of 200 g/m³, a little higher for low volatile coals and a little lower for high volatile coals.The ignition limit changed significantly, however, with O₂ concentration in CO₂, where coals required more oxygen to ignite. Most coals failed to ignite at all in 21% v/v O₂ in CO₂, but an increase to 30 or 35% v/v O₂ gave ignition patterns similar to those in air. In addition, the minimum ignition concentration decreased with increase in O₂. However, a further increase to 40% v/v O₂ did not generally affect the minimum ignition concentration.Particle size had a non-linear effect on coal ignition. The fine particles (<53 μm) behaved almost identical to the whole coal. However, the larger size fraction (>53 μm) was generally more difficult to ignite and exhibited a much lower weight loss.     

Combustion characteristics of coals using a drop-tube furnace

Fourteen coals were selected for char refiring tests using a drop-tube furnace (DTF) in order to compare with previous tests on a 1 MW combustion facility. Each coal was sieved into two size fractions (53-75 and 106-125 μm) and characterised using proximate analysis and conventional petrographic tests as well as a test for % unreactives using image-analysis. The coal fractions were then pyrolysed at 1300 °C for 200 ms in 1 vol% oxygen in nitrogen. The reactivity, morphology and surface area of the chars were evaluated using thermal, optical and adsorption techniques. Each char fraction was then passed through the DTF at 1300 °C using a residence time of 600 ms and a furnace atmosphere of 5 vol% oxygen in nitrogen to evaluate burnout propensity. The characteristics of the coals, the chars and the residues after refiring were compared to determine whether any links exist between burnout, intermediate char products and coal composition. The link between % unreactives and burnout was confirmed for high volatile bituminous coals. Results obtained from two low volatile coals confirmed that their burnout was better than predicted from their properties. For the S. American (Guasare) coal poorer than expected burnout was obtained, as in previous work, but only for the larger size fraction.     

O2/CO2气氛下煤燃烧SO2/NO析出特性

在水平管式炉上研究了O₂浓度、CO₂浓度、温度及石灰石添加等各参数对O₂/CO₂气氛下徐州烟煤和龙岩无烟煤燃烧过程中SO₂/NO排放特性的影响。结果发现,O₂/CO₂气氛下,烟煤和无烟煤燃烧SO₂/NO的析出规律与空气气氛下不同,同等O₂浓度下析出量比空气气氛下小。O₂/CO₂气氛下,随着O₂浓度的提高,烟煤和无烟煤SO₂/NO排放量均增大;随着CO₂浓度的升高, SO₂/NO排放量均减小。O₂/CO₂气氛下,石灰石添加对SO₂排放的抑制作用低于空气气氛下;石灰石添加对NO的排放有一定减排作用。对煤灰的元素分析显示O₂/CO₂燃烧对SO₂的抑制主要是由于煤灰的自固硫能力增强,而对NO的减排作用则是促进燃料N向其他含N气体的转换。     

Sulfur fate during bituminous coal combustion in an oxy-fired circulating fluidized bed combustor

To clarify the sulfur transformation behavior during oxy-fired circulating fluidized bed (CFB) combustion, experiments on SO₂ emission characteristics were carried out in a 50 kWth CFB combustor. Results show that SO₂ emission is quite dependent on the bed temperature in different atmospheres without limestone injection. With Ca/S=2.5, SO₂ emission in 21%O₂/79%CO₂ atmosphere is smaller than that in air atmosphere, but SO₂ emission decreases with the increase of O₂ concentration. The calcium forms in the ash prove the combination of calcination/carbonation and direct sulfation mechanism of limestone under oxy-combustion conditions. And the desulfurization efficiency of limestone (as deducting the self-retention efficiency from the total sulfur removal efficiency) increases from 40% to 52% as the O₂ concentration increases from 21% to 40%.     

CFD modelling of air-fired and oxy-fuel combustion of lignite in a 100 KW furnace

In this paper, a comprehensive computational fluid dynamics (CFD) modelling study was undertaken by integrating the combustion of pulverized dry lignite in several combustion environments. Four different cases were investigated: an air-fired and three different oxy-fuel combustion environments (25 vol.% O₂ concentration (OF25), 27 vol.% O₂ concentration (OF27), and 29 vol.% O₂ concentration (OF29) were considered. The chemical reactions (devolatilization and char burnout), convective and radiative heat transfer, fluid and particle flow fields (homogenous and heterogenous processes), and turbulent models were employed in 3-D hybrid unstructured grid CFD simulations. The available experimental results from a lab-scale 100 KW firing lignite unit (Chalmer’s furnace) were selected for the validation of these simulations. The aerodynamic effect of primary and secondary registers of the burner was included through swirl at the burner inlet in order to achieve the flame stability inside the furnace. Validation and comparison of all the combustion cases with the experimental data were made by using the temperature distribution profiles and species concentration (O₂, CO₂, and H₂O) profiles at the most intense combustion locations of the furnace. The overall visualization of the flame temperature distributions and oxygen concentrations were presented in the upper part of the furnace. The numerical results showed that the flame temperature distributions and O₂ consumptions of the OF25 case were approximately similar to the reference combustion case. In contrast, in the OF27 and OF29 combustion cases, the flame temperatures were higher and more confined in the closest region of the burner exit plane. This was a result of the quick consumption of oxygen that led to improve the ignition conditions in the latter combustion cases. Therefore, it is concluded that the resident time, stoichiometry, and recycled flue gas rates are relevant parameters to optimize the design of oxy-fuel furnaces. The findings showed reasonable agreement with the qualitative and quantitative measurements of temperature distribution profiles and species concentration profiles at the most intense combustion locations inside the furnace. These numerical results can provide useful information towards future modelling of the behaviour of pulverized brown coal in a large-scale oxy-fuel furnace/boiler in order to optimize the burner’s and combustor’s design.     

Sulfur release from coal in fluidized-bed reactor through pyrolysis and partial oxidation with low concentration of oxygen

Yima (YM) and Datong (DT) raw coal were pyrolyzed in a fluidized bed reactor under 0.6%O₂-N₂, 1.1%O₂-N₂ and 2.1%O₂-N₂ atmosphere, and a flue gas analyzer was used to check the SO₂ in pyrolysis gas. The product of sulfur removal and char yield is suggested to measure the efficiency of sulfur removal. For YM coal, sulfur removal generally has increasing trend with the increase of oxygen concentration in atmosphere. The char yield of YM coal has no remarkable decrease when the oxygen content is lower than 1.1%. However, in 2.1%O₂-N₂ less char yield is obtained. For DT coal more sulfur is removed in 0.6%O₂-N₂ than in N₂, and at the same temperature more SO₂ is released with increasing oxygen content. It is suggested that the atmospheres used selectively break the C-S bonds other than C-C bonds. Pyrolysis of coal in fluidized-bed reactor under low concentration of oxygen atmosphere is a promising method to greatly remove sulfur, and not remarkably decrease the char yield.     

O2/N2、O2/CO2和O2/CO2/NO气氛下煤粉燃烧NOx排放特性

利用滴管炉研究了O₂/N₂、O₂/CO₂和O₂/CO₂/NO气氛下煤燃烧过程中NO_x的排放特性。实验结果表明,在O₂/N₂和O₂/CO₂气氛下,高温或高O₂浓度均使NO排放量增加。O₂/CO₂气氛下NO排放量比O₂/N₂气氛下NO排放量低大约30%～40%。在O₂/CO₂/NO气氛下,温度不同时,O₂浓度变化对NO排放量的影响规律不同,对循环NO降解的影响规律也不同。高温不利于循环NO降解。随停留时间的延长NO排放量出现两个峰值。     

O2/CO2气氛煤焦的燃烧及其孔隙结构变化

采用热力工况与实际煤粉炉相近的沉降炉实验装置,制备了不同环境气氛下（O₂/N₂及O₂/CO₂气氛）、不同燃尽率的煤焦试样,并采用低温氮吸附仪和扫描电子显微镜测定了其孔隙结构和表面形貌。结果表明,在相同的操作条件下,相同O₂浓度的O₂/CO₂气氛下煤焦的燃烧速率较慢、燃尽率较低,各试样的孔比表面积和比孔容积均较小。两种气氛下燃尽过程孔结构参数（SBET和VBJH）均呈减小趋势,且在孔径变化较明显的区域内（<5 nm）在CO₂气氛下煤焦的孔径分布较小且与煤种相关。SEM图像显示CO₂气氛下的煤焦表面致密,孔隙较少,其定性结果与N₂吸附法的定量测量结果吻合较好。