Ambient-pressure thermogravimetric characterization of four different coals and their chars

Ambient-pressure thermogravimetric characterization of four different coals and their chars was performed to obtain fundamental information on pyrolysis and coal and char reactivity for these materials. Using a Perkin-Elmer TGS-1 thermobalance, weight loss as a function of temperature was systematically determined for each coal heated in helium at 40 and 160 °C/min under various experimental conditions, and for its derived char heated in air over a temperature range of 20 to 1000 °C. The results indicate that the temperature of maximum rate of devolatilization increases with increasing heating rate for all four coals. However, heating rate does not have a significant effect on the ultimate yield of total volatiles upon heating in helium to 1000 °C; furthermore, coupled with previous data⁹ for identical coal samples, this conclusion extends over a wide range of heating rate from 0.7 to 1.5 × 10⁴ °C/s. Using the temperature of maximum rate of devolatilization as an indication of relative reactivity, the devolatilization reactivity differences among the four coals tested that were suggested by this criterion are not large. For combustion in air, the overall coal/char reactivity sequence as determined by comparison of sample ignition temperature is: N. Dakota lignite coal ≈ Montana lignite coal > North Dakota lignite char > III. No. 6 bituminous coal ≈ Pittsburgh Seam bituminous coal > Montana lignite char > III. No. 6 bituminous char > Pittsburgh Seam bituminous char. The reactivity differences are significantly larger than those for devolatilization. The reactivity results obtained suggest that coal type appears to be the most important determinant of coal and char reactivity in air. The weight loss data were fitted to a distributed-activation-energy model for coal pyrolysis; the kinetic parameters so computed are consistent with the view that coal pyrolysis involves numerous parallel first-order organic decomposition reactions.     

Continuous rapid carbonization of powdered coal by entrainment, and response surface analysis of data

A study was made of rapid carbonization of coal in entrainment by composite factorial design of test-runs. From the data obtained through the operation of a 4 in. diameter flash carbonizer, the effects and interactions of the variables were determined and represented by three-dimensional models called response surfaces. These surfaces indicated the essential variables for each product yield or quality and showed the trend towards optimum values of these responses. Internal heating was shown to be superior to external heating from the standpoint of producing a uniformly devolatilized char with higher throughput rates. Illustrating the method and the results obtained, the quality of the char, defined as the amount of volatile matter (VM) remaining in it, and the char yield for the internally and the externally heated carbonizer are shown as a function of the temperature, coal-feed rate and composition of the entraining gas. Optimum char quality of 15% VM with the highest throughput rate, 750 g/h, was achieved by means of internal heating at 1900°F, which resulted in a char yield of 1075 lb/ton of coal carbonized.     

Flash pyrolysis of coals. 1. Devolatilization of a Victorian brown coal in a small fluidized-bed reactor

The devolatilization behaviour of finely-ground (< 0.2 mm) Loy Yang brown coal was investigated under rapid heating conditions using a small-scale fluidized-bed pyrolyser. The pyrolyser operated continuously, coal being fed at rates of 1–3 g/h directly into a bed of sand fluidized by nitrogen. Particle heating rates probably exceeded 10⁴ °C/s. The yields of tar, C₁-C₃ hydrocarbons and total volatile matter are reported for a pyrolyser-temperature range of 435 to 900 °C. A maximum tar yield of 23% w/w (dry ash-free coal), 60% more than the Fischer assay, was obtained at 580 °C. Yields of C₁-C₃ hydrocarbons increased with increasing temperature, reaching 8% at 900 °C. Elemental analyses showed that the composition of the tar and char products was strongly dependent on pyrolysis temperature. The effects on the devolatilization behaviour of the coal produced by the moisture associated with the coal, by hydrogen, and by the replacement of the sand by a fluidized bed of petroleum coke were investigated.     

间热径向流反应器料层厚度对煤热解特性的影响

考察了方形径向流固定床煤热解反应器中变化煤层厚度对料层升温速度及煤热解产物分布特性的影响。随着料层厚度增加,导致煤热解反应要求的时间增长,热解水和气的产率相应增加,焦油和半焦收率逐渐降低,但焦油中轻质组分(沸点低于360℃组分)含量呈升高趋势,半焦和煤气热值稍许降低。如,加热壁温度900℃、从45 mm至105 mm增加煤料层厚度时,焦油产率从7.17%(质量,下同)下降到6.26% (相对干基煤),但焦油中的轻焦油组分含量则从67%升至72.7%,半焦产率由80.0%降至77.0%,热解水和煤气产率分别由6.96%和5.91%增至8.85%和7.90%,煤气热值则由24348.5 kJ·m^-3下降至20649.2 kJ·m^-3。所得半焦的热值径向上由高温侧向低温侧逐渐降低,煤料层越厚、热值降幅越大,而相同煤料层厚度处与加热壁平行的同一轴向平面上的半焦热值基本相同。针对研究的反应器,气相热解产物在反应器内沿径向(横向)由高温料层区向低温料层区流动。在该过程中伴随着热解产物对远离加热壁的低温煤料的传热、热解生成重质组分的冷凝和在煤/半焦颗粒表面的吸附截留,进而在低温料层进一步升高温度时发生二次裂解等物理化学过程。反应器内煤层厚度越大,上述各种伴随的物化作用越显著,从而明显影响煤料层的升温及热解特性。     

温度梯度与产物流动对先锋褐煤热解产物分布的影响

通过两段固定床反应器分离了煤热解和二次反应过程,模拟了传统外热式固定床反应器和外热式内构件固定床反应器内挥发性热解产物流动与固相反应颗粒间的相互作用,研究了不同温度的半焦对先锋褐煤热解产物分布和品质的影响。结果表明：当半焦的温度在500～900℃时,焦油发生剧烈的二次反应,热解气收率增加,焦油收率大幅下降,H/C同时大量降低且均低于空白样,油品质下降;在100～400℃半焦作用下,焦油发生轻微的二次反应,热解气收率略微增加,焦油收率缓慢下降,但H/C均高于空白样,油品质提升。该结果验证内构件外热式固定床反应器热解产生的焦油具有更高的收率及品质。     

Synthesis gas and char production from Mongolian coals in the continuous devolatilization process

Devolatilization of Mongolian coal (Baganuur coal (BC), Shievee Ovoo coal (SOC), and Shievee Ovoo dried coal (SOC-D)) was investigated by using bench-sized fixed-bed and rotary kiln-type reactors. Devolatilization was assessed by comparing the coal’s type and dry basis, temperature, gaseous flux, tar formation/generation, devolatilization rate, char yield, heating value, and the components of the raw coal and char. In the fixed bed reactor, higher temperatures increased the rate of devolatilization but decreased char production. BC showed higher rates of devolatilization and char yields than SOC or SOC-D. Each coal showed inversely proportional devolatilization and char yields, though the relation was not maintained between the different coal samples because of their different contents of inherent moisture, ash, fixed carbon, and volatile matter. Higher temperatures led to the formation of less tar, though with more diverse components that had higher boiling points. The coal gas produced from all three samples contained more hydrogen and less carbon dioxide at higher temperatures. Cracking by multiple functional groups, steam gasification of char or volatiles, and reforming of light hydrocarbon gas increased with increasing temperature, resulting in more hydrogen. The water gas shift (WGS) reaction decreased with increasing temperature, reducing the concentration of carbon dioxide. BC and SOC, with retained inherent moisture, produced substantially higher amounts of hydrogen at high temperature, indicating that hydrogen production occurred under high-temperature steam. The continuous supply of steam from coal in the rotary kiln reactor allowed further exploration of coal gas production. Coal gas mainly comprising syngas was generated at 700–800 °C under a steam atmosphere, with production greatest at 800 °C. These results suggest that clean char and high value-added syngas can be produced simultaneously through the devolatilization of coal at lower temperature at atmospheric pressure than the entrained-bed type gasification temperature of 1,300–1,600 °C.     

Swelling and plastic behavior of coal devolatilized at elevated pressures of air in the absence and presence of calcium oxide

Swelling and plastic properties of an lv bituminous coal (PSOC 1197) devolatilized at elevated air pressures (at 60 or 150 K/min) were monitored using a high-pressure microdilatometer. It was observed that the maximum swelling parameter (V₈) of the coal was markedly reduced when devolatilized at elevated air pressures, provided that the heating rate of the coal was sufficiently low (60 K/min to 923 K). In marked contrast, at a relatively higher heating rate (e.g., 150 K/min), the swelling of coal at elevated air pressures was closer to V_s obtained in N₂ pressures. An implication of this finding is that coal swelling can be very high at actual utilization conditions (rapid heating of coal in air). The reduction in coal V_s at a relatively slow heating rate is attributable, at least in part, to the preoxidation (chemisorption of oxygen to form oxygen crosslinks) of coal during initial heat-treatment in air (before there is significant devolatilization). Devolatilization of coal in the presence of CaO, however, markedly reduced the swelling of coal at various air pressures and heating rates. This reduction in coal swelling is attributed to crosslinking reactions of pyrolyzing coal that have been catalyzed by CaO.     

Coal devolatilization studies in support of the Westinghouse fluidized-bed coal gasification process

Coal devolatilization studies were carried out with a pressurized laboratory fluidized-bed reactor. The tests showed the behaviour of caking and noncaking coals during rapid heating in a fluidized bed. Small samples of coal were injected into a hot bed of char; the off-gas was sampled and analysed; and the char and agglomerates produced were examined and characterized. Data obtained on the variation of gas evolution rates with time were used to predict the duration of the plastic or sticky phase during transition of the coal to char. All tests were conducted at 1.013 MPa (10 atm) pressure; bed temperature and coal particle size were varied.     

Modeling a pilot-scale fluidized bed coal gasification reactor

A steady-state model has been developed to simulate the North Carolina State University pilot-scale fluidized bed coal gasification reactor. The model involves instantaneous devolatilization of coal at the top of the gasifier (freeboard region) and char combustion and gasification in the fluidized bed. A two-phase (emulsion-dilute gas) representation of the fluidized bed incorporates the phenomena of jetting, bubbling, slugging, and mass and heat transfer between phases, and enables the prediction of individual species flow rates and temperature profiles within the bed. The model has been successfully used to simulate the gasification of a devolatilized Western Kentucky bituminous coal and a New Mexico subbituminous coal and to predict effects of various operating parameters on key gasifier performance variables.     

Captive sample reactor for kinetic studies of coal pyrolysis and hydropyrolysis on short time scales

An apparatus has been designed for kinetic studies of coal devolatilization under closely controlled heating rates, temperatures, and pressures. The reactor system employs a wire-mesh pyrolysis furnace driven by an electronically-controlled power supply to ensure uniform heating rates and precise crossover to a variable length isothermal period; a rapid quenching scheme permits resolution of kinetic processes in 0.1 s intervals, extending the sensitivity of kinetic measurements from the initial stage of pyrolysis through to completion of primary devolatilization. Time-resolved kinetic studies on a bituminous coal in vacuo (13 Pa) and under helium (0.22 MPa) illustrate the operation of the reactor.     

Rapid devolatilization and hydrogasification of bituminous coal

Rapid devolatilization and hydrogasification of a Pittsburgh Seam bituminous coal were studied and an appropriate coal conversion (weight loss) model was developed that accounts for thermal decomposition of the coal, secondary char-forming reactions of volatiles, and homogeneous and heterogeneous reactions involving hydrogen. Approximately monolayer samples of coal particles supported on wire mesh heating elements were electrically heated in hydrogen, helium, and mixtures thereof. Coal weight loss (volatiles yield) was measured as a function of residence time (0–20 s), heating rate (65–10000 °C/s), final temperature (400–1100 °C), total pressure (0.0001–7 MPa), hydrogen partial pressure (0–7 MPa), and particle size (70–1000 μm). Volatiles yield under these conditions increases significantly with decreasing pressure, decreasing particle size, increasing hydrogen partial pressure and increasing final temperature, but only slightly with increasing heating rate. The data support the view that coal conversion under these conditions involves numerous parallel thermal decomposition reactions forming primary volatiles and initiating a sequence of secondary reactions leading to char. Intermediates in this char-forming sequence can escape as tar if residence time in the presence of hot coal surfaces is sufficiently short (e.g. low pressures and small particles well dispersed). Hydrogen at sufficiently high partial pressure can interrupt the char-forming sequence thereby increasing volatile yield. Rate of total product generation is largely controlled by coal pyrolysis while competition between mass transfer, secondary reactions, and rapid hydrogenation affects only the relative proportions of volatile and solid products formed.     

基于过程强化与反应调控的煤定向热解制高品质油气产物基础研究及中试验证

对比了现有煤热解制油气技术的特点,从反应工程“三传一反”的角度系统分析和概括了煤热解过程中挥发分在颗粒内生成和释放、颗粒间扩散和反应器中停留等关键步骤中的热量、质量传递和挥发分二次反应对油气品质的影响,揭示了目前碎煤热解制油气技术普遍存在的目标产品产率低、品质差、含尘量高等技术难题的根源,并总结出煤定向热解调控的有效措施,即在挥发分生成和半焦缩聚段采用高温加热和快速传递的传热方式,在挥发分扩散过程中利用半焦床层重整焦油和过滤灰尘,在反应器中设置气体通道导流挥发分的定向溢出。针对研究团队前期开发的内构件移动床定向热解理念,介绍了导热板和集气腔等内构件的作用机制,即通过导热板和中心集气腔等内构件进行传热强化、热解气流动的有序引导,实现热量和挥发分的同向扩散和传递;通过移动床中颗粒的缓慢运动和床层的过滤作用除尘;概述了1~5 kg/次基础实验、反应器结构内传热和流动模拟,100 kg/次模试分析和1000 t/a中试验证的研究结果,充分证实了该技术在同步提高油气质量与品质、降低油中尘含量等方面的优势和对碎煤原料的适用性;基于上述研究形成了内构件定向热解技术及基于该技术的热/电-油-气联产技术。     

Pyrolysis of western canadian coals in a spouted bed

Coal pyrolysis has been studied to determine conditions for maximum liquid yields from some Western Canadian coals. Gas, tar and char yields were determined for four coals in a 12.8 cm dia. reactor. A characteristic temperature for maximum tar yield existed for each coal at a fixed feed rate and particle size. A steady increase in tar yield was found as the average coal particle size was reduced from 2.28 to 0.65 mm. Composition of gas, and ultimate analyses of tar and char are presented as a function of operating temperature. A simple first-order devolatilization model adequately describes the effects of coal feed rate, reaction time, and temperature on the yield of volatiles, but is insufficient to describe particle size effects.     

移动床中内构件对煤热解反应过程调控作用

在外热式内构件（多级折流板和多段集气管）移动床反应器内研究了淖毛湖煤的热解特性,并与常规固定床反应器中煤热解行为进行对比,考察了两反应器内的传热速率以及热解温度对产物分布、热解气组成、焦油组成和品质等影响规律。结果表明：在450℃低温热解时,煤颗粒在内构件移动床内的升温时间比固定床缩短了60%以上,内构件具有显著提高反应器内颗粒间传热速率的作用。随着热解温度的升高,热解气中的C₂H₄/C₂H₆和C₃H₆/C₃H₈的比值变大,挥发分的二次反应程度加大,但裂解程度低于固定床。内构件移动床中的焦油产率随温度的升高先增加后降低,在550℃时达到最高为10.8%（质量分数）,比固定床增高约28.6%。当热解温度越高时,移动床所产焦油中的沥青质组分含量越低,在750℃时焦油中轻质组分质量分数达到85.17%,脂肪烃含量降低到了28.00%。通过与固定床对比,揭示了内构件（多级折流板和集气管）调控淖毛湖煤热解反应并提高热解焦油产率和品质的作用。     

Flash pyrolysis of coals. Devolatilization of bituminous coals in a small fluidized-bed reactor

The devolatilization behaviour of ten bituminous coals was investigated under rapid heating conditions using a small-scale fluidized-bed pyrolyser. The pyrolyser operated continuously, coal particles being injected at a rate of 1–3 g h^?1 directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C₁–C₃ hydrocarbon gases, and total volatile-matter and an agglomeration index are reported for all coals. Maximum tar yields were obtained at about 600 °C and were always substantially higher than those from the Gray-King assay. Total volatile-matter yields were also substantially higher than the proximate analysis values. The maximum tar yields appear to be directly proportional to the coal atomic $\text{H}\text{C}$ ratio. The elemental analysis of the tar is strongly dependent on pyrolysis temperature. The tar atomic $\text{H}\text{C}$ ratio is proportional to that of the parent coal. The effect on the devolatilization behaviour of two coals produced by changes in the pyrolyser atmosphere and the nature of the fluidized-bed material were also investigated. Substituting an atmosphere of hydrogen, helium, carbon dioxide or steam for nitrogen, has no effect on tar yield and, with one exception, little effect on the hydrocarbon gas yields. In the presence of hydrogen the yield of methane was increased at temperatures above 600 °C. Tar yields were significantly reduced on substituting petroleum coke for sand as the fluid-bed material. A fluidized bed of active char virtually eliminated the tar yield.     

Modeling and experimental studies on single particle coal devolatilization and residual char combustion in fluidized bed

Single particle devolatilization followed by combustion of the residual coal char particle has been analyzed in a batch-fluidized bed. The kinetic scheme with distributed activation energy is used for coal devolatilization while multiple chemical reactions with volume reaction mechanism are considered for residual char combustion. Both the models couple kinetics with heat transfer. Finite Volume Method (FVM) is employed to solve fully transient partial differential equations coupled with reaction kinetics. The devolatilization model is used to predict the devolatilization time along with residual mass and particle temperature, while the combined devolatilization and char combustion model is used to predict the overall mass loss and temperature profile of coal. The computed results are compared with the experimental results of the present authors for combustion of Indian sub-bituminous coal (15% ash) in a fluidized bed combustor as well as with published experimental results for coal with low ash high volatile matter. The effects of various operating parameters like bed temperature, oxygen mole fraction in bulk phase on devolatilization time and burn-out time of coal particle in bubbling fluidized bed have been examined through simulation.     

流化床燃烧石油焦N_2O排放特性

通过在一小型流化床试验台上进行石油焦的燃烧试验 ,阐述了N2 O和NO形成与分解机理 ,模拟研究了N2 O的排放特性 .采用不同程度脱去挥发分的石油焦颗粒 ,研究脱挥发分的程度对N2 O形成的影响 ,脱挥发分的温度越高 ,即脱挥发分的程度越高 ,石油焦氮形成N2 O的量越少 ,这表明石油焦挥发分氮形成N2 O量高于相应石油焦焦炭氮燃烧产生的N2 O量 .燃料燃烧过程中 ,NO形成比较均匀 ,而N2 O形成比较复杂 ,燃料氮向NO的转化率随脱挥发分温度升高而增加 ,而向N2 O的转化率则有一临界脱挥发分温度点 .     

Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part II. Effects of experimental conditions on the yields of NOx and SOx precursors from the pyrolysis of a Victorian brown coal

A Victorian brown coal was pyrolysed in a quartz reactor. The reactor has some features of a drop-tube reactor and of a fixed-bed reactor, capable of operating at fast and slow heating rates. The yield of HCN was found to change with gas flow rate and coal feeding rate, indicating that HCN and/or its precursors could interact significantly with the nascent char to be incorporated into char as soot or to form N₂. Experimental results indicated that HCN does not significantly convert to NH₃, either on the char surface or in the gas phase, at least during the pyrolysis of the brown coal in this study. The yields of HCN and NH₃ were both sensitive to changes in heating rate. The reduction in the yields of HCN and NH₃ with decreasing heating rate is mainly due to the lack of radicals at the slow heating rate, which are required to initiate the opening of the N-containing rings. The carbonisation/condensation reactions also make the N-containing heteroaromatic ring systems increasingly stable during the extended holding at high temperatures at the slow heating rate. Experimental results appear to suggest that there are two types of organic sulphur-containing structures in the brown coal with very different thermal stability. The first type could be converted into H₂S at low temperatures (<600°C). The other type was stable at temperatures up to 1000°C. The changes in heating rate or coal feeding rate did not affect significantly the formation of H₂S.     

糠醛渣热解特性及热解挥发产物对其燃烧烟气原位控氮作用

通常,具有高含氮资源禀赋生物质在能源化利用过程中需控制NO_x排放。解耦燃烧是可适用于高含水、高含氮燃料的低NO_x燃烧技术,其对NO_x生成的抑制效果优于其他燃烧技术。为揭示解耦燃烧中热解挥发产物的原位控氮潜力、发展双流化床解耦燃烧技术,以糠醛渣为原料,借助固定床装置和双流化床装置,分别开展其热解特性和双流化床解耦燃烧近实际工况模拟研究。具体地,首先在固定床反应器中考察糠醛渣在不同温度下的热解产物分布,继而借助双流化床反应器考察了热解在线挥发产物对热解半焦同步燃烧烟气中NO_x的还原效果。结果表明：在500~700℃热解温度区间内,随温度的升高,半焦产率逐渐减少,从45.2%下降到39.8%;气体产率呈明显上升趋势,从12.4%上升到22.5%,CO、CH₄、H₂等还原性组分产率增加显著;焦油产率略有降低,从15.9%降低到12.9%;水分产率变化不大。双流化床解耦燃烧实验中,糠醛渣热解挥发产物对热解半焦同步燃烧所产烟气控氮效果良好,热解挥发产物对半焦燃烧烟气NO_x减排效果主要受热解温度、二次风占比影响,总过量空气系数ER=1.3,热解温度600℃、二次风过量空气系数ER₂=0.5时,糠醛渣热解挥发产物对相同热解条件下生成的半焦燃烧（900℃,过量空气系数ER₁=0.8）所产烟气原位控氮效果达到最优,NO_x减排率为54.80%。这表明,可通过控制热解挥发分产物产率、氧化程度,充分发挥挥发分的NO_x还原能力,从而明显改善解耦燃烧原位控氮效果。     

Kinetics of steam gasification of nascent char from rapid pyrolysis of a Victorian brown coal

Steam gasification of nascent char from rapid or slow pyrolysis of a Victorian brown coal was performed at 1073–1173 K in a novel drop-tube/fixed-bed reactor, in which steam-containing gas was forced to pass through an extremely thin bed of nascent char particles at sufficiently high velocity and large flux. The nascent char underwent parallel reactions consisting of non-catalytic gasification and catalytic one. The non-catalytic gasification followed first-order kinetics with respect to the fraction of unconverted carbon, and the rate constant was hardly influenced by operating variables such as heating rate for the pyrolysis, total pressure and even period of isothermal heating between the pyrolysis and gasification. The overall activity of inherent catalysts, alkali and alkaline earth metallic species, diminished due to volatilization and intra-particle deactivation, both of which were induced by the gasification. As a result, the catalytic gasification took place within a limited range of the char conversion up to 60–80%. The initial catalyst activity and the kinetics of activity loss largely depended on the operating variables as above and also partial pressure of steam.