An experimental investigation into the gasification reactivity of chars prepared at high temperatures

The gasification activities of three kinds of Binxian chars with carbon dioxide were studied at 1000–1300 °C and under atmospheric pressure in self-made thermal balance. The specific surface area of coal or chars was determined with BET methods during gasification. The results showed that the reaction rate of two rapid pyrolysis chars increases at the beginning and decreases subsequently with increasing carbon conversion at relatively high temperatures. The heating rate of coal has a significant effect on the gasification process. The activation energy of slow pyrolysis char varies between 160 kJ/mol and 180 kJ/mol during gasification. The activation energy of the two rapid pyrolysis chars displays a linear trend when the carbon conversion is less than 40% and decreases slowly afterwards.     

碱金属对煤热解和气化反应速率的影响

通过对原煤、酸洗原煤、负载碱金属的酸洗原煤在800～1050℃热解制得焦样,用X射线衍射技术考察了碱金属对煤焦微晶结构的影响,在加压热天平（PTGA）上考察了煤样的热解过程,以及焦样的二氧化碳气化活性。结果表明：碱金属对煤的热解和气化阶段都有影响。在热解阶段,碱金属的存在抑制了煤焦的石墨化进程,降低了热解反应活化能,促进了热解反应的进行;在气化阶段,作为催化剂的碱金属,降低了气化反应活化能,延长了反应速率达到最大值的时间。修正的随机孔模型可以较好地描述煤焦-CO₂的气化反应过程。     

Modeling the influence of changes in aliphatic structure on char surface area during coal pyrolysis

The influence of changes in aliphatic structure on char surface area during coal pyrolysis was modeled, and the effect was introduced to a previous char surface area model for lignite pyrolysis established based on the chemical percolation devolatilization (CPD) model. The modified model can predict not only the N₂ and CO₂ char surface area during rapid pyrolysis of three lignites but also the CO₂ char surface area of two high-volatile bituminous coals; the agreement of the modified model with experiments is improved at high temperatures. The decrease in aliphatic chain length can reduce adsorption positions around aromatic core, and decrease char surface area. When mass release is more than 55% at about 1,100 K, the predicted N₂ char surface area starts to decrease with further generation of volatiles, and the increase of predicted CO₂ char surface area with increasing generation of volatiles also become slow at the end of mass release.     

A literature survey and an experimental study of coal devolatilization at mild and severe conditions: influences of heating rate, temperature, and reactor type on products yield and composition

Coal devolatilization studies to maximize the yield of condensable products by operating at elevated temperatures and heating rates have been published. The objectives of this study were to investigate the influences of relatively mild operating conditions (e.g. relatively low temperature and pressure) on product quality, by comparing devolatilized products obtained at various temperatures and heating rates. Fixed bed, fluid bed, and entrained flow reactor units were used to obtain pyrolysis products. In addition, literature data on tar yields in various reactor units at a range of temperatures and residence times were surveyed and compared with experimental data. The liquids were characterized by a number of techniques, including field ionization mass spectroscopy (f.i.m.s.), sequential elution solvent chromatography (s.e.s.c.) and elemental analysis. The results demonstrate that the quality and yield of liquids obtained at a rapid heating rate are functions of peak pyrolysis temperature. It was shown that at a rapid heating rate, the yields of heavier polyfunctional groups (i.e. hydrocarbons with greater mean molecular weight) are greater than those obtained in the fixed bed slow heating rate reactor. The liquids generated at a slow heating rate are of lower molecular weight, viscosity, and sulphur content, and of higher H/C atomic ratios compared with the liquids obtained in a rapid heating rate unit. The effect of increasing the maximum pyrolysis temperature (at a constant slow heating rate) was to increase the yield of light gases (mainly H) at the expense of char hydrogen content and char reactivity. The tar yield is not markedly influenced when the peak devolatilization temperature is increased at a relatively slow heating rate. However, the quality (as defined by the H/C (atomic) ratio) of the liquids, and the reactivity (in air) of char, was reduced when the peak pyrolysis temperature was increased. At a rapid heating rate, the primary products, which have many structural characteristics of the parent coal, are devolatilized. The quality of the liquids obtained at a rapid heating rate is, therefore, determined by the devolatilized primary coal fragments evolved at the devolatilization temperature. In a slow heating rate fixed bed unit, however, the primary coal fragments undergo additional cracking reactions which involve stabilization of free radicals by donatable hydrogen. This leads to the formation of low molecular weight hydrocarbons of relatively higher quality. In-situ (both intraparticle or extraparticle) stabilization of reactive coal fragments by donatable hydrogen may lead to a significant improvement in the overall quality of the pyrolysis liquids in a fixed bed system in which time-temperature history is conducive for such reactions.     

淖毛湖煤慢速热解过程官能团相互作用

采用滴管炉,在短停留时间下,制备具有一定低温反应活性而消除主要低温交联位点的淖毛湖煤（NMHcoal）快速热解半焦（NRPchar）,再将NMHcoal和NRPchar混合进行慢速热解,研究官能团间的相互作用。热重分析结果表明,NMHcoal/NRPchar混合比为5∶5,温度为500℃热解时具有较强的负协同作用。固定床热解结果表明,NMHcoal热解生成的挥发物部分扩散至NRPchar中,?CH₃与芳碳自由基以及?O有更多的结合概率与时间,使焦油中含甲基的萘、酚类增多,半焦中烷基化邻氧芳碳结构与醚类结构增加。析出的酚类增多,使半焦中连氧芳碳结构减少。NRPchar中生成较多的多环芳烃前体,它们与酚类物质发生反应生成多环芳烃和CO,使共热解焦油中5、6环化合物含量增加,而另一部分滞留在半焦中使其比表面积降低。     

Gasification rate analysis of coal char with a pressurized drop tube furnace

Two coal chars were gasified with carbon dioxide or steam using a Pressurized Drop Tube Furnace (PDTF) at high temperature and pressurized conditions to simulate the inside of an air-blown two-stage entrained flow coal gasifier. Chars were produced by rapid pyrolysis of pulverized coals using a DTF in a nitrogen gas flow at 1400°C. Gasification temperatures were from 1100 to 1500°C and pressures were from 0.2 to 2 MPa. As a result, the surface area of the gasified char increased rapidly with the progress of gasification up to about six times the size of initial surface area and peaked at about 40% of char gasification. These changes of surface area and reaction rate could be described with a random pore model and a gasification reaction rate equation was derived. Reaction order was 0.73 for gasification of the coal char with carbon dioxide and 0.86 for that with steam. Activation energy was 163 kJ/mol for gasification with carbon dioxide and 214 kJ/mol for that with steam. At high temperature as the reaction rate with carbon dioxide is about 0.03 s⁻¹, the reaction rate of the coal char was controlled by pore diffusion, while that of another coal char was controlled by surface reaction where reaction order was 0.49 and activation energy was 261 kJ/mol.     

Effect of iron on the gasification of Victorian brown coal with steam:enhancement of hydrogen production

This paper reports the significant enhancement of hydrogen production during the gasification of Victorian brown coal with steam using iron as a catalyst. Iron was loaded into the acid-washed Loy Yang brown coal using ferric chloride aqueous solution. Gasification experiments were carried out using a quartz reactor at a fast particle heating rate. The yield of char was determined by directly weighing the reactor before and after each experiment. Gases were analysed using a GC with dual columns. The overall gasification rate of a char increases greatly in the presence of iron. The transformation of iron species during pyrolysis and gasification was examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that both reduced-iron (α-Fe and γ-Fe) and magnetite (Fe₃O₄) highly dispersed in a char can catalyse the gasification of the char with steam. In particular, the char from iron-loaded coal samples gives much higher yields of H₂ than a char from the acid-washed coal under similar conditions. The mechanism for the enhancement of hydrogen production in the presence of iron is discussed.     

Effect of pyrolysis temperature on the char micro-structure and reactivity of NO reduction

A phenol-formaldehyde resin (PFR) and a bituminous coal (SH) were pyrolyzed at various temperatures. The structure and the char-NO reactivity were analyzed in order to examine the effect of pyrolysis temperature on the micro-structure of the resulting char and further on the reactivity towards NO. Micro-structure of the char samples was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Raman spectroscopy. It was indicated that the micro-structure of PFR char and coal char experienced remarkable changes during pyrolysis, which resulted in the decrease of phenolic OH, aromatic hydrogen and more ordered structure. The pyrolysis temperature showed a weak impact on the reactivity of PFR char but comparatively remarkable impact on that of coal char at lower reaction temperature. Mineral matter in coal char presented a weak effect on the reactivity. This paper was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, China, June 26–28, 2008.     

Mineral matter effects on the rapid pyrolysis and hydropyrolysis of a bituminous coal. 1. Effects on yields of char,tar and light gaseous volatiles

The effects of minerals on product compositions from rapid pyrolysis of a Pittsburgh Seam bituminous coal were investigated. Whole, demineralized, and mineral treated samples of pulverized coal were heated in 100 KPa helium or 6.9 MPa hydrogen at 1000 K s^?1 to temperatures of up to 1300 K. Yields of char, tar and individual gaseous products were determined as a function of time-temperature conditions. Clays, iron-sulphur minerals, and quartz had few effects on pyrolysis in helium. Calcium minerals decreased yields of hydrocarbon products and increased yields of CO in helium pyrolysis. Calcite and clays decreased yields of CH₄ from hydropyrolysis, whereas iron-sulphur minerals had little effect on pyrolysis at 6.9 MPa H₂. Whole coal results were similar to demineralized coal results under all conditions.     

Roles of inherent metallic species in secondary reactions of tar and char during rapid pyrolysis of brown coals in a drop-tube reactor

Two pairs of raw and acid-washed coal samples were prepared from Yallourn and Loy Yang brown coals, and subjected to rapid pyrolysis in a drop-tube reactor at 1073-1173 K in a stream of N₂ or H₂O/N₂ mixture. Examinations were made on the roles of the inherent metallic species in the secondary reactions of nascent tar and char that were formed by the intraparticle primary reactions. The experimental results revealed that the inherent metallic species were essential for vary rapid steam reforming/gasification of the nascent tar/char and simultaneous suppression of soot formation. In the absence of the metallic species, the soot formation from the tar accounted as much as 15-19 and 6-13% of the carbon in coal in N₂ and H₂O/N₂, respectively. The metallic species reduced the yield of soot to 6-8% in N₂ by enhancing the reforming of tar by H₂O generated from the pyrolysis of coal. In the H₂O/N₂ stream, instead of soot formation, a net gasification conversion up to 17% within 4.3 s was observed in the presence of the metallic species as a result of catalytic gasification of the nascent char. Moreover, the metallic species catalyzed the steam reforming of the nascent tar, giving its conversion up to 99%. Over the range of the conditions employed, a one-to-one stoichiometry was established between the steam consumption and the yield of carbon oxides formed by the steam reforming/gasification and water-gas-shift reaction.     

The physical character of coal char formed during rapid pyrolysis at high pressure

Rapid pyrolysis was conducted in a drop tube reactor using seven coals under various operating conditions. In addition to dense char, porous chars (network char and cenospheric char) were formed by the rapid pyrolysis under certain conditions. Porous char was mainly composed of film-like carbon and skeleton carbon. The pyrolyzed coal char particles were characterized in detail. Morphology and bulk density of porous char were quite different from the dense char formed under the same conditions, but elemental composition and BET surface area were similar to each other. CO₂ gasification reactivity of porous char was lower than dense char in the later gasification stage, and this was ascribed to the low reactivity of skeleton carbon.     

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.     

Structural analysis of chars generated from South African inertinite coals in a pipe-reactor combustion unit

An inertinite-rich medium rank C bituminous South African coal was utilized to generate chars in a pipe-reactor combustion unit. This unit generates chars at atmospheric pressures and temperature was controlled with N₂ to a maximum of 1250 °C. Chemical structural changes were investigated at different reaction zones identified in the pipe-reactor combustion unit. A combination of FTIR, XRD and Solid State NMR experiments were used to characterize the coal/char/ash fractions produced in the reactor. These techniques revealed that the coal structure becomes disordered in the drying zone as well as in the beginning of the pyrolysis zone in the reactor. As the temperature increases towards the base of the reactor the coal structure becomes more ordered and well aligned until char is formed and converted. Major structural changes were seen to occur in the drying to the pyrolysis zones. Structural changes within the molecular core were observed with FTIR and XRD results obtained from samples taken from the drying zone to the combustion zone. However, ¹³C CP/MAS and dipolar dephasing experiments were not able to corroborate these structural changes of the coal/char/ash fractions produced in the reactor occurring in the reduction and combustion zones.     

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.     

Evaluation of structural features of chars from pyrolysis of biomass of different particle sizes

The structural features of chars derived from pyrolysis of mallee wood of different particle sizes in a novel fluidized-bed/fixed-bed reactor have been investigated. Raman spectroscopy was used for structural evaluation of chars. Spectra were curve-fitted with 10 Gaussian bands representing typical structural features of the chars. The temperature had a significant influence on the evolution of char structure and thus the total Raman peak area between 800 and 1800 cm^− 1 is seen to decrease significantly with increasing pyrolysis temperature for all chars. On the other hand, the ratio I_D/I_{(Gr + Vl + Vr)} between the band intensities of condensed aromatic ring systems (> 6 rings) and amorphous char structures with small aromatic ring (3-5 rings) systems is seen to increase with increasing temperature. The particle size of biomass has a great role in char structure at fast heating rate (> 1000 °C/s) pyrolysis although it has no effect on char structure at slow heating rate pyrolysis (0.17 °C/s). However, in the bigger biomass particle, the structure of char prepared under fast heating rate pyrolysis is similar to that of the structure of char prepared under slow heating rate pyrolysis.     

Identification of the reaction zones occurring in a commercial-scale Sasol–Lurgi FBDB gasifier

Gasification behaviour is particle dependent, whilst gasifier (reactor) behaviour is an averaging process of individual responses of each particle. It was hypothesized, that if it were possible to extract and analyze particles from different reaction zones within a gasifier, it may be likely to enhance the understanding of the contribution that these particles make towards gasification. This better understanding of the particle-type compositional responses could act as an enabler to further manipulate and improve gasifier performance.The primary focus of this study was to evaluate a sequential (axial) sampling “turn-out” methodology of a quenched fixed-bed commercial-scale Sasol–Lurgi gasifier, in order to present samples that accurately describe operational aspects occurring in the reaction zones within the reactor. Characterization of the chemical properties of the sample increments were expected to deliver distinct profiles of the drying, pyrolysis, reduction and combustion (ash-bed) zones, which could be used to advance the kinetic modeling capability of the process. In order to interpret the coal property transformational behaviour occurring within the commercial-scale gasifier, the proximate, Fischer tar, ultimate, and coal char CO₂ reactivity analysis were conducted.The pyrolysis zone was found to be the largest reaction zone situated below the drying zone within the gasifier, followed by the reduction zone, and combustion (ash-bed) zones. Whilst the boundaries of the pyrolysis zone were very clearly defined by the residual volatile matter distribution profile, distinctive regional overlap with a “slow pyrolysis with gasification” region was observed in the bottom half of the pyrolysis zone, above which a “rapid de-volatilization” region existed. The reduction zone was found to also exhibit an overlap in zonal fronts, i.e. a gasification region occurred below the pyrolysis zone and co-existed in equal proportions, with an oxidation frontal region occurring above the combustion zone. The combustion zone was found to be very shallow, below which the ash-bed region existed.The findings clearly suggest that text book pictures showing axially-depicted reaction zones occurring within the fixed-bed gasifier, i.e. drying, pyrolysis, gasification and combustion, inadequately describe the “real” situation and in practice, overlap of reaction regions within zones indeed also transpire.     

粉煤气化条件下不同粒径半焦的表征与气化动力学

利用可模拟真实粉煤气化条件的平流火焰反应器制取了同一煤种的2种不同平均粒径的半焦,并对其进行了物理表征和气化反应动力学研究. 其物理表征显示,小颗粒半焦的平均比表面积远小于大颗粒半焦. 半焦的CO2气化实验表明,含很多小孔的大颗粒半焦的气化反应受内扩散影响显著,比表面积较小的小颗粒半焦具有更高的气化反应速率. 利用随机孔模型导出了反应速率Ri. 根据小颗粒半焦在不同温度下平衡态时的反应速率Ri,s值获得半焦气化的本征气化反应速率表达式为Ri=1.243′103exp(-19243.5/T) g/(m2×min).     

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。     

Chemical reduction of sulphur dioxide to free sulphur with lignite and coal. 2. Kinetics and proposed mechanism

The reactivity of lignite and different ranks of coal with sulphur dioxide has been investigated in a corrosive-gas, thermogravimetric reactor system. With all coals, the reaction occurred in two distinct stages. A rapid initial stage was controlled primarily by the devolatilization rate of the coal. The second stage limited the overall rate and was controlled by surface properties of the coal char. The portion of lignite associated with the second stage of reaction exhibited a much higher rate of SO₂ reduction than the corresponding material from all other coals. Correlation of the data showed an inverse relation between the reactivity of coal chars and the relative rank of the parent coal. Activation energies associated with the reduction of SO₂ by the coal chars increased slightly from 134 kJ mol^?1 for lignite char to 150 kJ mol^?1 for HVB bituminous coal char. The higher reactivity of lignite or lower-rank coals was due in part to entropy factors or available catalytic sites on the surface of coal. Formation of a thermally stable CS complex on the surface of coal appeared to poison the surface and thus limit further reaction. Alkali and alkaline earth metals in lignite served as active sites for catalysing the reaction of SO₂ with the CS complex and thus enhanced the rate of SO₂ reduction with lignite.     

Experimental study of NO reduction over biomass char

Some biomass fuels produce more NO_x than coal on the basis of heating value, giving rise to the necessity and importance of controlling NO_x emission in biomass combustion. The present study investigated the NO reduction over biomass char in a fixed bed quartz reactor in the temperature range of 973–1173 K. The reaction rates of three biomass chars (sawdust, rice husk and corn straw) with NO were compared with Datong bituminous coal char. The results show that the reaction orders of biomass chars for NO are of fractional order and independent of temperature. Biomass chars are more active in reducing NO than coal char. The characteristics of biomass char affect NO conversion. Biomass char formed at high pyrolysis temperature, especially large in particle size, is less active in reducing NO. To some extent, increase of reaction temperature and char loading enhance NO conversion. There exists an optimum bed height for the highest NO conversion. Moreover, NO reduction over biomass char is also enhanced in the presence of CO, O₂ and SO₂.