Investigation into the kinetics of pressurized steam gasification of chars with different coal ranks

The steam gasification of coal chars derived from three different ranks of typical Chinese coals was studied in a pressurized fixed-bed differential reactor at elevated pressure (up to 2.0 MPa). Three mathematical models [volumetric model (VM), grain model (GM), and random pore model (RPM)] for the gasification kinetics of different chars were validated, through which the kinetic parameters were obtained and discussed. The results show that the evolution trend of the coal char gasification rate with carbon conversion differs from coal ranks and has little change with pressure and temperature. The pressurized gasification process of the Shenmu sub-bituminous coal char (SM char) and the Jingcheng anthracite char (JC char) can be well-predicted by the RPM, while that of the Huolinhe lignite char can be better described by the VM. The pressure has little effect on the options of the reaction kinetic models for the three chars. The kinetic parameter E is almost a constant independent of pressure, while k ₀ changes with pressure, and it seems that k ₀ would be almost a constant over 1.0 MPa for SM and JC chars. The reaction order decreases with increasing the total system pressure and differs from different coal types.     

Thermogravimetric analysis of gasification reactivity of coal chars with steam and CO2 at moderate temperatures

Comparative study on the gasification reactivity of the three types of Chinese coal chars with steam and CO₂ at 850–1050 °C was conducted by isothermal thermogravimetric analysis. The effects of coal rank, pore structure, ash behavior, and gasification temperature on the gasification reactivity of coal chars were investigated. It is found that the gasification reactivity difference between different coal chars changes with reaction degree and gasification temperature, and has no immediate connection with coal rank and initial pore structure. Ash behavior plays an important role in the char reactivity, and changes with gasification temperature and reaction degree due to the variation in the compositions and relative amount. The influence of pore structure is more noticeable during a relatively moderate reaction process. The relative reactivity ratio of steam to CO₂ gasification generally decreases with the increasing temperature, and is related with the catalytic effect of inherent minerals. The characteristic parameters of the chars were analyzed, finding that the value of half reaction specific rate is approximate to the average specific rate under the same conditions. The nth-order distributed activation energy model is proposed to describe the coal char gasification process, and the results show that the activation energy increases with the increasing carbon conversion.     

Gasification characteristics of coal char under CO2 atmosphere

The char gasification characteristics and the composition of evolved gases in a CO₂ environment have been studied using a thermogravimetric analyzer (TG) coupled with a mass spectrometer. Three types of coal char were studied: lignite (TXL), sub-bituminous (PRB), and bituminous (KYB). TG results showed that the reactivities of TXL and PRB were higher than that of KYB, and the reactivity of TXL was higher than that of PRB. The characterization of the chars implied that the mineral content in the char plays an important role in the reactivity and that the surface area and pore volume may accelerate the reactivity of chars. The evolved gases from three chars were mainly CO and SO₂. SO₂ was slightly delayed by CO during gasification of TXL and PRB chars, but for KYB, SO₂ and CO formed in the same temperature range, but at higher temperatures compared with TXL and PRB. The CO production of KYB was the best, 0.98 mg mg^?1; and SO₂ was the least, 0.031 mg mg^?1. PRB and TXL chars had similar CO production, but SO₂ in TXL was higher.     

High-pressure gasification reactivity of biomass chars produced at different temperatures

The knowledge of biomass char gasification kinetics is of considerable importance in the design of advanced biomass gasifiers, some of which operate at high pressures. In the present work the effects of pyrolysis temperature, total pressure and CO₂ concentration on the gasification of biomass chars have been studied using the thermogravimetric approach. The chars were obtained by pyrolysis in a drop tube furnace reactor at temperatures of 1000 and 1400 °C. The gasification tests were carried out in a pressurized thermogravimetric analyser (PTGA) at different temperatures, pressures and CO₂ concentrations. The reactivity measurements were conducted under the kinetically controlled regime, and three nth-order kinetic models as well as the Langmuir–Hinshelwood model were applied to determine the kinetic parameters.     

Kinetics of coal and char oxycombustion studied by TG–FTIR

Coal and char oxycombustion is a complex process because of very high reaction rate of oxygen with coals and chars carbon. Very important process during oxycombustion is diffusion of O₂ to surface of coal and char grain. This process can be minimized using small samples and high flow of the gas, but it is also dependent on temperature. For this reason, it is impossible to eliminate diffusion processes which cause significant impact on calculated kinetic parameters. This paper describes the results of thermogravimetric studies of oxycombustion process with evolved gas analysis by FTIR. Ultimate and proximate analysis of coal and char were made. Thermogravimetric experiments of coal and its char oxycombustion were conducted using five heating rates, namely 2.5, 5, 10, 20 and 40 K min^?1, and gas mixture composed of 20 % O₂ in CO₂. Activation energies of coal and char oxycombustion were calculated by isoconversional methods: integral Vyazovkin and differential Friedman. Activation energies for three ranges of heating rates were calculated. This paper shows influence of heating rate on calculated activation energy. The reason of this phenomenon is due to change of the mechanism of coal and char oxycombustion from the chemical kinetic control regime to mixed chemical kinetic–diffusion control regime.     

热解压力对生物质焦结构及气化反应性能的影响

利用加压固定床反应器、吸附仪、X射线衍射仪、元素分析仪、电感耦合等离子原子发射光谱仪等考察了热解压力对生物质半焦(以下简称半焦)产率、物化结构、元素组成的影响规律。同时,利用热天平对不同热解压力下所制半焦的气化行为进行了考察。结果表明,随热解压力升高,半焦产率增大,当压力升至1.0 MPa后,半焦产率基本不变;半焦中C元素含量随热解压力的升高而增加,而H元素含量和BET比表面积则减小;此外,随热解压力升高,玉米秸秆焦和锯末焦的石墨化程度增强,而稻壳焦的石墨化程度则基本不受热解压力影响。气化反应的研究表明,玉米秸秆焦及锯末焦的平均气化反应速率随热解压力的提高而减小,而稻壳焦的平均气化反应速率基本不受热解压力的影响。热解压力对半焦BET比表面积及碳微晶结构的影响规律与气化反应速率变化规律的对比研究表明,热解压力引起半焦微晶结构的变化是造成热解压力对半焦气化反应速率影响的主要原因。     

煤中灰含量对气化反应活性的影响

本文分别以H_2O(g)、CO_2为气化剂,在固定床反应器中对沈北、大同、晋城煤进行了各种条件的气化实验。比较了在不同温度、压力及不同灰含量下各种煤焦的气化反应活性。实验结果表明,随气化温度、压力提高,各煤焦的气化反应活性均相应提高。脱灰后的大同、晋城煤焦的反应活性明显增加;沈北煤焦则是:随脱灰程度加深,其气化反应活性先降低而后增加。各煤焦水蒸汽气化反应的气相组成亦随温度的变化而变化。沈北、大同、晋城原煤焦水蒸汽气化反应的表观活化能分别为71.77kJ/mol,104.25kJ/mol,157.00kJ/mol。     

An experimental investigation into the gasification reactivity and structure of agricultural waste chars

The gasification reactivity as well as physical and chemical structure of chars generated from two kinds of agricultural waste (i.e. corn straw and wheat straw) were studied to better understand the role of lower pyrolysis temperatures and lower heating rates on the gasification characteristics of agricultural waste chars. Char samples were generated in a one-stage quartz fixed-bed reactor. The carbon dioxide (CO₂) gasification reactivity of chars was measured by thermogravimetric (TGA) analysis. Scanning electron microscopy (SEM) analysis, surface area (BET) analysis, Fourier transform infrared spectroscopy (FTIR) analysis and X-ray diffractometry (XRD) analysis were employed to determine the effect of operating conditions on the char structure. Char gasification reactivities decreased with increasing pyrolysis temperatures. The char particles generated under high pyrolysis temperatures had many smaller pores with thinner cell walls, larger surface areas, and some melting. Results indicated that many functional groups’ bands decreased and even disappeared with an increasing pyrolysis temperature. The chars’ microcrystalline became larger at high pyrolysis temperatures. The reactivity of wheat straw char is higher than corn straw char. The difference in the gasification reactivity of agricultural waste chars generated at different pyrolysis temperatures correlated well with the effect of pyrolysis temperatures on the agricultural waste char structure.     

Mass transfer limitation in thermogravimetry of biomass gasification

In order to determine the intrinsic reactivity behavior from thermogravimetry studies, the experimental conditions should be such that the reactions are not mass transfer limited. Biomass char usually has a higher reactivity than coal chars. Therefore, mass transfer limitations may be more problematic when studying biomass char reactivity. Chemical reaction kinetics and mass transfer processes present in thermogravimetry are used for modeling the overall reaction rate for spruce bark CO₂ gasification. Thermogravimetric experiments are carried out between 700 and 900 °C, and the CO₂ concentration is varied between 10 and 90 vol%. The intrinsic activation energy is found to be 120 kJ mol^?1. The transition temperature between regimes I and II is here defined when the fraction apparent to true activation energy equals 0.75. Higher external mass transfer (e.g., by decreasing the diffusion path through the crucible’s freeboard), decreasing the sample amounts, and higher CO₂ partial pressures for the Langmuir–Hinshelwood reaction type increase the transition temperature. The results show that the transition temperature between regimes I and II conditions is approx. 1,030 °C for 90 vol% CO₂.     

石油焦高温气化反应性

常压，1 200 ℃～1 500 ℃，在自制管式反应器中，以二氧化碳为气化介质，研究了石油焦以及石油焦与后布连煤焦掺混后形成的混合焦的气化反应性，借助于XRD分析了高温处理后石油焦与煤焦在碳结构有序化方面的区别。研究结果表明，当碳转化率高于0.7，气化超过1 300 ℃，石油焦的反应速率出现急骤下降，气化温度越高，相应石油焦速率下降越快。混合焦气化反应性既不同于纯石油焦也不同于纯煤焦。随石油焦掺入比变化而改变的拐点主要源于石油焦与煤焦的反应性之间差异。较高转化率下出现的拐点，主要源于石油焦本身随气化温度提高导致气化速率下降。XRD测定显示，高温处理后石油焦中碳有序化程度要明显高于煤焦。高气化温度下石油焦碳结构发生明显有序化是导致其反应活性急剧下降的重要原因。     

微型流化床反应分析仪中原位/非原位煤焦气化动力学研究

利用新开发的微型流化床反应分析仪(micro-fluidized bed reaction analysis, MFBRA) 考察了义马烟煤半焦的原位以及两种非原位半焦气化行为并测定了其动力学参数,其中,原位半焦气化是指煤热解温度和气氛与半焦气化过程一致,非原位半焦1气化是指煤在Ar气氛下热解,热态条件下直接在CO₂气氛下气化;非原位半焦2气化是指煤在Ar气氛下热解,冷却收集后再在CO₂气氛下气化。研究发现,原位半焦具有最大的比表面积和最小的平均孔径,石墨化程度最弱,且对CO₂的化学吸附能力最强,表面活性位点最多。在最小化气体扩散的实验条件下,原位半焦气化反应的反应速率明显比非原位半焦气化反应快,且求取的活化能数据较小。实验揭示了原位半焦和非原位半焦结构和反应性的差异,也证明了MFBRA对原位等温气化反应的适用性。     

不同彬县焦的水蒸气气化反应动力学研究

在常压,900℃～1050℃考察了彬县煤的三种焦样（常规方法制焦、快速热解焦和脱灰快速热解焦）在热天平上的水蒸气气化反应。考察了温度和焦种对水蒸气气化反应的影响。对比了三种焦的动力学参数和比表面积。结果表明,气化温度是影响煤焦气化反应速率的主要因素,提高50℃,反应速率增加一倍。快速热解焦的反应速率在相同反应条件下明显大于慢速焦。三种焦的表观活化能以快速焦最大,因而反应速率受温度的影响也最大,快速脱灰焦次之,慢速焦最小。     

烟煤焦在H2O和CO2气氛下的结构演变与气化反应性关联

本研究以烟煤在1000 ℃热解所制得的焦样为研究对象,考察了其在H₂O、CO₂及两者混合气氛下的结构演变,以及气化反应性的影响。为了探究焦样在气化过程中的结构演变,利用氮吸附、SEM和拉曼光谱等表征手段分析不同碳转化率下的焦样结构。结果表明,H₂O气氛对焦样结构的演变明显不同于CO₂气氛,揭示了焦样在两种气氛下的反应路径不同。因结构演变的不同,随碳转化率的增加,焦样在两种气氛下表现出不同的气化反应性能。在CO₂气氛下,焦样的气化反应速率随碳转化率的增加而逐渐降低,与H₂O气氛存在下变化趋势相反。在H₂O和CO₂共气化条件下,煤焦在H₂O和CO₂混合气氛下的反应速率高于单气氛下的反应速率的计算值,表现出一定的协同作用。这是因为焦样与H₂O反应能够产生较大的比表面积,为焦样与CO₂反应提供更多的反应场所,促进了焦样与CO₂的反应。     

Preparation and characterization of waste ion-exchange resin-based activated carbon for CO<Subscript>2</Subscript> capture

Waste ion-exchange resin was utilized as precursor to produce activated carbon by KOH chemical activation, on which the effects of different activation temperatures, activation times and impregnation ratios were studied in this paper. The CO₂ adsorption of the produced activated carbon was tested by TGA at 30 °C and environment pressure. Furthermore, the effects of preparation parameters on CO₂ adsorption were investigated. Experimental results show that the produced activated carbons are microporous carbons, which are suitable for CO₂ adsorption. The CO₂ adsorption capacity increases firstly and then decreases with the increase of activation temperature, activation time and impregnation rate. The maximum adsorption capacity is 81.24 mg/g under the condition of 30 °C and pure CO₂. The results also suggest that waste ion-exchange resin-based activated carbons possess great potential as adsorbents for post-combustion CO₂ capture.     

煤焦气化过程中比表面积和孔容积变化规律及其影响因素研究

煤焦的孔隙结构是影响气化反应的重要因素之一，本文通过测定部分气化焦样的比表面积及其孔隙结构，详细地研究了烟煤焦的孔隙结构在气化过程中的变化规律及其影响因素，结果表明，煤焦的孔隙结构在气化过程中的变化不但取决于原煤的性质，而且还取决于气化介质与气化温度；在相同条件下气化至相同气化率时总比表面积和孔体积大小顺序为彬县＞神木＞王封煤焦，总比表面积与微孔比表面积均随温度的升高而降低，在反应的前期CO2与H2O两种气氛下产生的总比表面积与微孔比表面积相当，但在反应后期CO2气氛下能够产生更多的总比表面积与微孔比表面积。     

胜利褐煤半焦冷却处理对其微观结构及反应性能的影响

采用不同的冷却方式对胜利褐煤热解"热"半焦进行冷却处理,考查了冷却方式对半焦微观结构及反应性能的影响.利用特制两段新型石英反应器,在高纯氩气、400、600、800 ℃的条件下,对褐煤热解30 min制得的"热"焦,分别浸入室温、干冰及液氮环境中得到冷态半焦,然后在15%水蒸气、900 ℃、10 min的条件下对冷态半焦进行非原位气化反应.为了比较,同样热解条件下制得的"热"半焦不经冷却,通过直接切换反应气氛为15%水蒸气进行原位气化(气化条件与非原位气化相同).利用TGA、SEM、BET和Raman光谱仪对半焦反应性和微观结构进行表征和分析.结果表明,"热"焦的水蒸气原位气化半焦产率低于冷态半焦的非原位气化.冷却处理对半焦的孔结构影响较大,随着冷却速率的增大,半焦比表面积及总孔容积显著降低,但对半焦的化学结构(芳环体系和含氧官能团)的影响非常小.冷却速率越大,半焦的反应性越低,冷态半焦反应性能降低主要由于冷却对半焦孔结构造成的不可逆转的破坏.     

The kinetics of CO2 gasification of coal chars

The gasification reactivities of three char samples derived from coals of varying ranks (“Turów” lignite, “Piast”, and “Wieczorek” sub-bituminous coals) toward CO₂ were investigated isothermally using thermogravimetric analysis. Kinetic behavior was studied at temperatures of 900, 950, and 1,000 °C under atmospheric pressure. Conditions for the chemical-controlled regime were established at these temperatures and pressure. In this paper, four kinetic models were applied to describe the varying conversion rate: volumetric model, grain model, modified volumetric model, and random pore model. From these models, only the random pore and the volumetric models positively corresponded to nearly the entire range of experimental results. Calculated values of activation energy for study samples were in the range of 180–250 kJ mol^?1, which is in accordance with other reported data. Moreover, the obtained results confirmed the significant impact of parent coal rank on its char reactivity, offering possibilities in the approximation of coal char kinetic behavior after further more detailed studies with a larger number of samples.     

晋城无烟煤加压快速热解特性及其对气化反应的影响

利用自行设计的加压热重固定床反应器进行了晋城无烟煤加压快速热解特性的研究,并结合热天平半焦等温热失重分析,考察了热解温度、停留时间和热解压力等外部操作条件对煤焦快速热解半焦特性的影响。结果表明,随热解温度的提高、停留时间的延长和热解压力的增大,所得到的半焦产率降低,气化反应性减弱,活化能提高;高温产生较小的比表面积,而停留时间的延长和压力的提高产生较大的比表面积,比表面积与气化反应速率无明显的依存关系。水蒸气气化速率是CO2的四倍左右。     

高温下煤焦的碳微晶及孔结构的演变行为

以贵州煤为原料,在热解温度950℃~1400℃制备了各种慢速和快速热解焦,主要对高温热解过程中煤焦的碳微晶和孔结构的演变行为进行了研究,同时也研究了高温气化过程中煤焦的孔结构变化规律。结果表明,慢速热解焦和快速热解焦的C和H含量明显不同;随热解温度的升高,煤焦的碳微晶结构向有序化方向发展,但慢速热解煤焦比快速热解煤焦的"石墨化"程度大;快速热解煤焦的微孔比表面积和微孔容积明显高于慢速热解煤焦,即快速热解煤焦的孔隙结构明显比慢速热解煤焦发达;在气化反应初期,煤焦的微孔比表面积下降,微中孔比表面积增加,反应后期煤焦的总比表面积快速下降。