Determination of active surface areas of coal chars using a temperature-programmed desorption technique

A new dynamic method is described for measuring active surface areas (ASA) of carbons, involving oxygen chemisorption followed by temperature-programmed desorption of CO₂ and CO. The method gives similar results to the standard volumetric method. Under the chosen experimental conditions, it was shown that during chemisorption the carbon surface was saturated with oxygen and that during desorption a number of possible secondary reactions did not occur. ASA and total surface area (TSA) (measured by adsorption of argon at 77 K), are reported for a steam activated series prepared from a model char and a coal char. In the initial stages of gasification, the ASA/TSA ratio decreased and then remained constant for a wide range of burn-off. The initial decrease in the ASA/TSA ratio is attributed to a low value of TSA caus3d by activated diffusion of argon at 77 K into micropores.     

Inhibition of steam gasification of char by volatiles in a fluidized bed under continuous feeding of a brown coal

Steam gasification of a Victorian brown coal was performed in an atmospheric bubbling fluidized-bed reactor with continuous feeding of the coal. The gasification converted no more than 28, 51 and 71% of the nascent char (on a carbon basis) at 1120, 1173 and 1223 K, respectively. The char recovered from the fluidized bed was, nonetheless, gasified toward complete conversion when exposed to steam in another reactor, in which volatiles from the pyrolysis were absent while interaction between the char and products from the gasification was minimized. Atmosphere created in the fluidized bed thus prevented the char gasification from taking place beyond upper-limit conversion. In the absence of volatiles, nascent char underwent gasification catalyzed by inherent metallic species and non-catalytic gasification in parallel. The non-catalytic gasification was greatly decelerated by the presence of H₂ in the gas phase due to its dissociative chemisorption onto free carbon sites forming H-laden carbon. H₂ was, however, not a so strong inhibitor as to terminate the gasification. It was rather suggested that much more H-laden carbon was formed through dissociative chemisorption of volatiles and/or chemisorption of hydrogen radical from thermal cracking of volatiles in the gas-phase, which resulted in prevention of the non-catalytic gasification. It seemed that the char was converted in the fluidized-bed mainly by the catalytic gasification, while the conversion was limited due to deactivation of metallic species within the char matrix and their release from the char.     

Importance of carbon active sites in the gasification of coal chars

A demineralized lignite has been used in a fundamental study of the role of carbon active sites in coal char gasification. The chars were prepared in N₂ under a wide variety of conditions of heating rate (10 K min^?1 to 10⁴ K s^?1), temperature (975–1475 K) and residence time (0.3 s–1 h). Both pyrolysis residence time and temperature have a significant effect on the reactivity of chars in 0.1 MPa air, determined by isothermal thermogravimetric analysis. The chars were characterized in terms of their elemental composition, micropore volume, total and active surface area, and carbon crystallite size. Total surface area, calculated from C0₂ adsorption isotherms at 298 K, was found not to be a relevant reactivity normalization parameter. Oxygen chemisorption capacity at 375 K and 0.1 MPa air was found to be a valid index of char reactivity and, therefore, gives an indication, at least from a relative standpoint, of the concentration of carbon active sites in a char. The commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with their active surface areas. The importance of the concept of active sites in gasification reactions is illustrated for carbons of increasing purity and crystallinity including a Saran char, a graphitized carbon black and a spectroscopically pure natural graphite.     

Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms

In the present work, the mechanisms involved in NO–char heterogeneous reduction have been studied using a synthetic coal char (SC char) as carbon source. Another synthetic char (SN char) without nitrogen in its composition has also been employed in these studies. Isothermal reduction tests at different temperatures have been carried out. Two temperature regimes were considered: low temperature (T < 250 °C) where NO chemisorption takes place and high temperature (T > 250 °C) where NO–C reaction occurs. Step response experiments combining consecutive reaction stages with NO and ¹⁵NO were performed in order to determine the role of nitrogen surface complexes, C(N), in the reduction process. The results revealed N₂ and CO₂ to be the main reduction products under the experimental conditions employed in this work. NO chemisorption at lower temperatures results in N₂ emission and surface complexes (mainly oxygenated) formation, while char gasification by NO involves a direct NO attack on the char surface to form surface complexes. As a consequence of desorption of these complexes new sites of reaction are created.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part VI. Effects of gas atmosphere on the formation of NH3 and HCN

Our results indicate that the gas atmosphere surrounding coal/char particles can greatly affect the formation of NH₃ and HCN through its influence on the availability of H radicals. Based on our results, it is believed that the chemisorption of CO₂ on the nascent char surface can consume H radicals or block the access of N-sites by H radicals for the formation of NH₃ and HCN. For the chars whose thermal cracking generates little H radicals, the gasification of char by CO₂ can also generate additional H radicals, enhancing the formation of NH₃. However, even gasification of char in CO₂ at 950 °C does not lead to the formation of HCN. The oxidation of coal with 4% O₂ at low temperatures (400-600 °C) leads to the formation of HCN as well as NH₃ due to the enhanced formation of (H) radicals. The gasification of coal with 15% H₂O drastically enhances the formation of NH₃ due to the greatly enhanced availability of H as an intermediate between the reactions of H₂O and char. These results support our reaction mechanisms proposed previously, emphasising the importance of H on the formation of NH₃ and HCN during pyrolysis, which can also be extended to the conversion of coal-N during gasification.     

Enhanced catalysis of K2CO3 for steam gasification of coal char by using Ca(OH)2 in char preparation

A novel approach has been proposed for mitigating the potassium deactivation in the K₂CO₃-catalyzed steam gasification of coal char by addition of Ca(OH)₂ in the char preparation. It was experimentally found that the Ca(OH)₂-added char had higher reactivity for the catalytic gasification than the raw char. Ca(OH)₂ played a role in suppressing the interactions of K₂CO₃ with acidic minerals in coal during the gasification and also probably in forming more active oxygenated intermediate on the char surface. The distribution of gaseous products was examined during the catalytic gasification. An oxygen transfer and intermediate hybrid mechanism is applied for understanding of the rate and selectivity of the catalytic gasification.     

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.     

Modeling of catalytic gasification kinetics of coal char and carbon

Calcium- and potassium-catalyzed gasification reactions of coal char and carbon by CO₂ are conducted, and the common theoretical kinetic models for gas-carbon (or char) reaction are reviewed. The obtained experimental reactivities as a function of conversion are compared with those calculated based on the random pore model (RPM), and great deviations are found at low or high conversion levels as predicted by theory. Namely, calcium-catalyzed gasification shows enhanced reactivity at low conversion levels of <0.4, whereas potassium-catalyzed gasification indicated a peculiarity that the reactivity increases with conversion. CO₂ chemisorption analysis received satisfactory successes in both interpreting catalytic effects and correlating the gasification reactivity with irreversible CO₂ chemical uptakes (CCU_ir) of char and carbon at 300 °C. In details, calcium and potassium additions led to significant increases in CCU_ir and correspondent high reactivities of the char and carbon. Furthermore, CCU_ir of char and carbon decreased with conversion for calcium-catalyzed reaction but increased for potassium-catalyzed one, corresponded to the tendency of their reactivity. The RPM is extended and applied to these catalytic gasification systems. It is found that the extended RPM predicts the experimental reactivity satisfactorily. The most important finding of this paper is that the empirical constants in the extended RPM correlate well with catalyst loadings on coal.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VIII. Catalysis and changes in char structure during gasification in steam

The purpose of this study is to investigate the major factors influencing the Na-catalysed and non-catalysed gasification reactivity of a Victorian brown coal in steam. An acid-washed (H-form) sample and a Na-exchanged (Na-form) sample prepared from the same Loy Yang brown coal were gasified in 15% steam in a novel two-stage fluidised-bed/fixed-bed reactor. All C-containing species in the gasification product gas were converted into CO₂ that was monitored with a mass spectrometer continuously to determine the in situ gasification reactivity. While the volatile-char interactions were responsible for the volatilisation of Na when the coal was continuously fed into the reactor, the physical entrainment by gas of agglomerated Na-containing crystalline species (likely to be Na₂CO₃ or Na₂O) from char surface was the main mechanism for the loss of Na during char gasification. The Raman spectroscopy of char showed the preferential release of smaller aromatic ring system to be more significant during the non-catalysed char gasification than the Na-catalysed gasification. The dispersion of Na in char appeared to deteriorate with the enrichment of large aromatic ring systems in char, greatly affecting the char gasification reactivity. The char gasification reactivity showed a maximum with increasing conversion with the maximum to shift towards lower conversion with increasing temperature. Increasing temperature does not always lead to increases in the in situ char gasification reactivity.     

Formation of oxygen structures by ozonation of carbonaceous materials prepared from cherry stones: II. Kinetic study

In this second part, the kinetics of the ozonation process of a char prepared from cherry stones (CS) is investigated. The char was obtained by heat treatment of CS at 600°C for 2 h in nitrogen. The effects of reaction time, partial pressure of ozone, and mass transport phenomena on the formation of oxygen complexes are studied. The surface chemistry of the samples was examined by FT-IR spectroscopy and the elemental chemical analysis was also determined for some samples. Results showed that the ozonation of the char led to oxygen chemisorption and to carbon gasification. The amount of oxygen complexes formed in the chemisorption stage (i.e., OH groups, CO structures, and ether structures) was found to be very sensitive to the increase in the ozonation time. The type of oxygen complexes was also time dependent. Ozonated products with relatively high concentrations of CO groups and ether structures were prepared by applying high ozone doses, whereas the formation of OH groups was favored at low ozone contents. The particle size did not influence the surface chemistry of the ozonated products. Only when the gas flow rate was lower than 40 l h⁻¹, restrictions to ozone mass transport developed. For kinetics of the char ozonation process, a mechanism based on the Langmuir-Hinselwood adsorption-desorption model was proposed, and the intrinsic reaction rates were calculated as a function of ozonation temperature. The activation energy for the ozonation stage of the char was equal to 41.6 kJ mol⁻¹.     

Gasification reaction kinetics on biomass char obtained as a by-product of gasification in an entrained-flow gasifier with steam and oxygen at 900-1000 °C

The purpose of this study was to investigate the gasification kinetics of biomass char, such as the wood portion of Japanese cedar char (JC), Japanese cedar bark char (JB), a mixture of hardwood char (MH) and Japanese lawngrass char (JL), each of which was obtained as a by-product of gasification in an entrained-flow type gasifier with steam and oxygen at 900-1000 °C. Biomass char was gasified in a drop tube furnace (DTF), in which gasification conditions such as temperature (T_g), gasifying agent (CO₂ or H₂O), and its partial pressure (P_g) were controlled over a wide range, with accompanying measurement of gasification properties such as gasification reaction ratio (X), gasification reaction rate (R_g), change of particle size and change of surface area. Surfaces were also observed with a scanning electric microscope (SEM). By analyzing various relationships, we concluded that the random pore model was the most suitable for the biomass char gasification reaction because of surface porosity, constant particle size and specific surface area profile, as well as the coincidence of R_g, as experimentally obtained from Arrhenius expression, and the value is calculated using the random pore model. The order of R_g was from 10⁻² to 10⁻¹ s⁻¹, when T_g = 1000 °C and P_g = 0.05 MPa, and was proportional to the power of P_g in the range of 0.2-0.22 regardless of gasifying agent. Reactivity order was MH > JC > (JB, JL) and was roughly dependent on the concentration of alkali metals in biomass feedstock ash and the O/C (the molar ratio of oxygen to carbon) in biomass char.     

Gasification performance of torrefied Timothy hay and spruce wood chars in a CO2 environment

Timothy hay abundantly available in New Brunswick, Canada, is mostly used for animal feed and bedding. Upgrading biomass using Torrefaction method can offer benefits in its waste management, energy density and energy conversion efficiency. Temperature and residence time play an important role in the torrefaction process. Meanwhile, CO₂ gasification is also a promising thermochemical conversion process due to its potential to reduce net GHG emissions and tune syngas composition. This study investigates the impact of torrefaction parameters on isothermal and non-isothermal CO₂ gasification of Timothy hay and spruce chars. Timothy hay chars exhibited higher CO₂ gasification reactivity than chars from spruce. The physicochemical properties analysis indicated that higher reactivity of Timothy hay char was mainly attributed to the high amount of alkali and alkaline earth metal (AAEM) content, relatively large BET surface area, a high number ofactive sites, and a low crystalline index. Moreover, in both experimental cases, char derived through a high heating rate and high residence time conditions exhibited improved gasification performance, which was attributed to the generation of large amounts of AAEM (Ca and K) and high specific surface area. Co-gasification results during non-isothermal processes under CO₂ showed the presence of larger interactions in coal char/Timothy hay char blends than that of coal char/spruce char blends. For both experimental conditions, interactions were enhanced once the char prepared from high heating rate and high residence time was gasified with coal char. Thus, the proposed approach is a sustainable way of conversion of Timothy hay under CO₂ environment.     

Influence of coal preoxidation and the relation between char structure and gasification potential

A study was carried out to ascertain the effects of coal preoxidation and carbonization conditions on the structure and relative gasification potential of a series of bituminous coal chars. Chars were prepared from two freshly mined bituminous coals and preoxidized samples derived from them. Carbonization conditions included a wide range of heating rate (0.2–10000K s^?1), temperature (1073–1273 K) and time (0.25–3600 s). Char properties were characterized in terms of analysis of char morphology, surface area, elemental composition, and gasification reactivity in air. Over the range of conditions used, preoxidation substantially reduced coal fluid behaviour and influenced macroscopic char properties (char morphology). Following slow heating (0.2 K s^?1), preoxidized coals yielded chars having higher total surface areas and higher reactivities toward gasification in air than did similar chars prepared from fresh coal. Following rapid heating (10000 K s^?1) and short residence times (0.25 s), chars prepared from preoxidized and fresh coals exhibited similar microstructural and chemical properties (surface area, $\text{C}\text{H}$ ratios, gasification rates). Carbonization time and temperature were found to be the critical parameters influencing char structure and gasification potential.     

A TPD study of coal chars in relation to the catalysis of mineral matter

The steam gasification mechanism of brown coal was studied by a temperature-programmed desorption (TPD)technique. A Morwell coal was devolatilized in N₂ and then gasified in steam at 1100 K. During the TPD of a partially gasified char, H₂O, CO₂ and CO evolved approximately at 640, 870 and 1020 K, respectively. The presence of mineral matter was found to be responsible for these gas evolutions, since essentially no gas evolution was observed during the TPD of the demineralized coal char. The comparison of the above TPD pattern with those determined for the cation-exchanged samples revealed which inorganic species is responsible for each TPD peak: H₂O evolution was due to Ca; CO₂ evolution to Ca and Mg; CO evolution to Na and/or Fe. The exchanged metal species like Ca and Na significantly catalysed the gasification reaction. The relation between the catalytic activity and TPD pattern was discussed in terms of surface oxygen complexes.     

On the validity of thermogravimetric determination of carbon gasification kinetics

Thermogravimetric analysis has been widely applied in kinetic studies of carbon gasification, with the associated temporal weight change profiles being used to extract kinetic information and to validate gasification models. However the weight change profiles are not always governed by the intrinsic gasification activity because of the effect of chemisorption and its dynamics. In the present work we theoretically determine the criteria under which weight change profiles can be used to determine intrinsic kinetics for CO₂ and O₂ gasification by examining the region in which the chemisorption dynamics can be assumed pseudo-steady. It is found that the validity of the pseudo-steady assumption depends on the experimental conditions as well as on the initial surface area of carbon. Based on known mechanisms and rate constants an active surface area region is identified within which the steady state assumption is valid and the effect of chemisorption dynamics is negligible. The size of the permissible region is sensitive to the reaction temperature and gas pressure. The results indicate that in some cases the thermogravimetric data should be used with caution in kinetic studies. A large amount of literature on thermogravimetric analyzer determined char gasification kinetics is examined and the importance of chemisorption dynamics for the data assessed.     

Low-temperature gasification of a woody biomass under a nickel-loaded brown coal char

Catalytic gasification of a woody biomass, Japanese cypress, was investigated under a prepared nickel-loaded brown coal (LY-Ni) char in a two-stage fixed-bed reactor. The nickel-loaded brown coal was prepared by ion-exchange method with a nickel loading rate of 8.3 wt.%. Nickel species dispersed well in the brown coal, and the LY-Ni char via devolatilization at 600 °C showed a great porous property with a specific surface area of 382 m² g^− 1.The LY-Ni char was confirmed to be quite active for the Japanese cypress volatiles gasification at a relatively low-temperature range from 450 to 650 °C. For example, at 550 °C, 16.6 times hydrogen gas and 6.3 times total gases were yielded from the catalytic steam gasification of Japanese cypress volatiles under the LY-Ni char, compared with the case of non-catalyst. The biomass tar decomposition showed a dependence on catalyst temperatures. When the catalyst temperature was higher than 500 °C, Japanese cypress tar converted much efficiently, high gas yields and high carbon balances were obtained.     

铁基复合载氧体煤化学链气化反应特性及机理

以水蒸气作为气化/流化介质,在流化床中研究了两种铁基复合载氧体的化学链气化反应特性及循环特性,并对气化过程中的反应机理、动力学方程进行了推断。结果表明:温度为920℃时,添加不同修饰物的铁基复合载氧体与煤焦气化的反应活性依次为Fe4Al6K1>Fe4Al6>Fe4Al6Ni1。在多次循环实验过程中,合成气成分保持稳定,表明Fe4Al6K1复合载氧体循环特性良好。XRD谱图分析表明,六次氧化还原实验后的铁基载氧体氧化态仍为Fe₂O₃。K⁺主要以铁酸钾形态存在,该结构有利于促进化学链气化反应。利用高斯函数对气化反应速率进行了峰拟合,拟合结果表明化学链气化主要分为3个阶段:化学链作用阶段、煤气化阶段以及Fe₃O₄向FeO转变的气化阶段。     

NH3 and HCN formation during the gasification of three rank-ordered coals in steam and oxygen

A Victorian brown coal (68.5% C), a Chinese high-volatile Shenmu bituminous coal (82.3% C) and a Chinese low-volatile Dongshan bituminous coal (90% C) were gasified in a fluidised-bed/fixed-bed reactor at 800 °C in atmospheres containing 15% H₂O, 2000 ppm O₂ or 15% H₂O + 2000 ppm O₂. While the gasification of these coals in 2000 ppm O₂ converted less than 27% of coal-N into NH₃, the introduction of steam played a vital role in converting a large proportion of coal-N into NH₃ by providing H on char surface. The importance of the roles of steam in the formation of NH₃ in atmospheres containing 15% H₂O + 2000 ppm O₂ decreased with increasing coal rank. This is largely due to the slow gasification of high-rank coal chars, resulting in low availability of H on char surface. The gasification of chars from the high-rank coal appears to produce higher yields of HCN than that of lower rank coals, probably as a result of the decomposition of partially hydrogenated/broken/activated char-N structures during gasification at high temperature. The alkali and alkaline earth metallic species in brown coal tend to favour the release of coal-N as tar-N but have limited effects on char-N conversion during gasification.     

Effects of volatile–char interaction on the formation of HCN and NH3 during the gasification of Victorian brown coal in O2 at 500 °C

In a coal gasifier interactions between volatiles and char are significant. The partly reducing conditions in a gasifier would mean the presence of high concentration of partial oxidation products and radicals surrounding the char particles. Currently, little is known about the effects of in situ volatile–char interactions on the conversion of char-N. This study examines the effect of in situ volatile–char interactions on the formation of HCN and NH₃ during the low temperature (500 °C) gasification of Loy Yang brown coal in oxygen. Two novel reactor systems were used. The reactor configurations allowed the quantification of HCN and NH₃ from char-N gasification, volatile-N oxidation and volatile–char interactions separately. Our results indicate that volatile–char interactions can have drastic effects on coal-N conversion during gasification by providing an important source of the radicals for the formation of HCN and NH₃ from char-N during gasification in 4% or 8% O₂ at 500 °C. In the presence of radicals and O₂, N-containing structures in the nascent char can be easily broken down to give HCN and NH₃ during the gasification of the char. In the absence of O₂, some of the nascent char-N structures may stabilise into structures less favourable for the formation of HCN and NH₃ and more favourable for the formation of other N-containing species such as NO_x.     

Correlation of gasification rates of various coals measured by a rapid heating method in a steam atmosphere at relatively low temperatures

Twenty-five kinds of coals (carbon content on dry ash-free basis, C[%], ranges from 65.0 to 92.8%) were pyrolysed and gasified simultaneously by use of a rapid heating method (heating rate ≈ 1600 K min⁻¹) in steam at temperatures between 750 and 850 °C to clarify the factors which control the gasification rates of various coals. The relationships were examined in detail between the reactivity of each coal, represented by the initial gasification rate − r_cm0, and various properties such as pore surface area of char, ultimate and proximate analyses of coal, reflectance of coal, contents of metals in char, and the amount of oxygen trapped by char. For gasification at 800 °C, the relation between − r_cm0 and the carbon content C[%] changed abruptly at C ≈ 75–80%. For higher rank coals (C > 75–80%), − r_cm0 was rather small and was well correlated by C[%]. On the other hand, the plot of − r_cm0versus C % scattered largely for the lower rank coals (C < 75–80%). For these coals, the rate of CO₂ formation was much greater than that of CO formation, and was almost proportional to − r_cm0. The CO₂ formation reaction is known to be catalysed by alkali or alkaline earth metals such as Na, K and Ca. Then the reactivities of lower rank coals were supposed to be controlled mainly by the catalytic effect of the minerals in the coal.