Higher Gasification Reactivity Research Articles

Optimizing the gasification reactivity of biochar: The composition, structure and kinetics of biochar derived from biomass lignocellulosic components and their interactions during gasification process

Biochar can become a renewable substitute to coal in the future’s energy mix. This work aims to investigate the optimization of reactivity of biochar samples derived from biomass (Miscanthus) and its lignocellulosic components (i.e. cellulose, hemicellulose and lignin) by studying the material characteristics, composition, thermal behavior and interaction during the CO2 gasification process. For biochar production, the yield increased with lignin content in the biomass, but high lignin content reduced biochar’s porous structure. The conversion characteristic was gradually stimulated by increasing the gasification temperature and heating rate, and the lignin biochar (LB) reactivity was the most promoted. High lignin content was favorable for improving the biochar reactivity. The reactivity of the xylan biochar (XB) was attributed to the high content of alkali and alkaline earth metals, while the activity of the cellulose biochar (CB) was due to the rich porous structure. The activation energy of biochar samples gasification reaction ranged from 109.8 to 196.1 kJ/mol, and the values decreased with the increase of carbon conversion rate. Modelled biomass with 32.5% cellulose, 15% xylan and 52.5% lignin was considered the optimal feedstock to make biochar because of the its low activation energy and high gasification reactivity.

Preparation of coke from biomass char modified by vapour deposition of tar generated during pyrolysis of woody biomass

ABSTRACT In this study, in order to use woody biomass for coke production, carbon/carbon composites are first prepared using the vapour deposition (VD) method, whereby gaseous-tars generated by the pyrolysis of pine sawdust is deposited onto the pyrolysed pine sawdust char by a vertical flow-type fixed-bed reactor with pyrolysis and VD zones. Then, coke properties and gasification reactivity of composite and composite/coal blends are investigated to clarify the binder effect of biomass tar on the properties of prepared coke. The carbon/carbon composites could be prepared with a high yield at a low VD temperature. High-strength and high-reactivity coke was produced from the composite. The properties of the coke produced from composite/caking coal blending showed that the optimum blending amount depends on the type of caking coal, but high-strength and high-gasification reactivity coke can be produced. Additionally, it was found that 3–15wt-%-biomass could be added as char for coke production.

CO2 Gasification Reactivity of Char from High-Ash Biomass

Biomass char produced from pyrolysis processes is of great interest to be utilized as renewable solid fuels or materials. Forest byproducts and agricultural wastes are low-cost and sustainable biomass feedstocks. These biomasses generally contain high amounts of ash-forming elements, generally leading to high char reactivity. This study elaborates in detail how chemical and physical properties affect CO2 gasification rates of high-ash biomass char, and it also targets the interactions between these properties. Char produced from pine bark, forest residue, and corncobs (particle size 4–30 mm) were included, and all contained different relative compositions of ash-forming elements. Acid leaching was applied to further investigate the influence of inorganic elements in these biomasses. The char properties relevant to the gasification rate were analyzed, that is, elemental composition, specific surface area, and carbon structure. Gasification rates were measured at an isothermal condition of 800 °C with 20% (vol.) of CO2 in N2. The results showed that the inorganic content, particularly K, had a stronger effect on gasification reactivity than specific surface area and aromatic cluster size of the char. At the gasification condition utilized in this study, K could volatilize and mobilize through the char surface, resulting in high gasification reactivity. Meanwhile, the mobilization of Ca did not occur at the low temperature applied, thus resulting in its low catalytic effect. This implies that the dispersion of these inorganic elements through char particles is an important reason behind their catalytic activity. Upon leaching by diluted acetic acid, the K content of these biomasses substantially decreased, while most of the Ca remained in the biomasses. With a low K content in leached biomass char, char reactivity was determined by the active carbon surface area.

The fate of char structure and active groups in petroleum coke gasification in a drop tube furnace

A research on the fate of active groups from three typical petroleum cokes (petcokes) during gasification in a drop tube furnace have been presented. The results showed that the relative reduction (R) index can be used to assess the gasification reactivity of petcoke. The decomposition of side chain reaction of dealkylation is easier to proceed. Petcoke with a relative ordered degree of char structure has an energy barrier resulting in the transformation from the ordered graphitic carbon structure to amorphous carbon structure occurred only after the temperature reaches 1200 °C. It is recommended to control the petcoke gasification temperature above 1200 °C. A high temperature is beneficial for the conversion of oxygen-containing functional groups to hydroxyl groups. Petcoke with high ash content promotes the decomposition of functional groups to form active radicals, which favors the formation of amorphous carbon and, consequently, more active sites resulting in the high gasification reactivity. The high temperature gasification can make the stable active radicals (aromatic hydrocarbons groups) to break up the thermal barrier and convert to active radicals (aliphatic hydrocarbons and side chains on aromatic structure, oxygen functional groups and hydroxyl groups). The active radicals react with oxygen adsorbed on the carbon surface to generate CO, H2, CH4, etc.

In-situ study of effect of migrating alkali metals on gasification reactivity of coal char

The migrating and diffusion of alkali metals affect catalytic gasification of coal char. The paper investigated in-situ gasification behaviors of coal char by TG and hot stage microscope, and studied catalytic performance of single NaAlO2 particle and Na distribution by SEM-EDX. The results show that in the beginning stage, the curve of carbon conversion obtained by area method is consistent with that by TG method. In the ending stage, the ash in coal char acts as framework of the particles, as a result, area of the particles keeps unchanged. Meanwhile, the ash hinders diffusion of gasifying agent into coal char, and decreases the gasification rate, so carbon conversion rate calculated by area change is lower than that from TG. The single NaAlO2 particle has catalytic performance. The coal char particles, which are closer to NaAlO2, have higher gasification reactivity, because the amount of migrating alkali is higher. The migrating distance of NaAlO2 is higher than 840 μm at 900°C.

Hydrogen-rich gas production from steam co-gasification of banana peel with agricultural residues and woody biomass

Steam co-gasification of banana peel with other biomass, i.e., Japanese cedar wood, rice husk and their mixture, was carried out for the hydrogen-rich gas production in a fixed-bed reactor. For the co-gasification process, the banana peels were physically mixed with rice husk, Japanese cedarwood and their mixture respectively by different mixing weight ratios. The effects of reaction temperature and the addition amount of banana peel on the gas production yield were investigated by comparing the experimental data with the calculated ones based on the individual biomass gasification at the same condition. It was found that the banana peel with a high content of alkali and alkaline earth metal (AAEM) species exhibited not only high gasification reactivity but also a significant enhancing catalytic effect on the co-gasification process at the low temperature, especially with the biomass containing no silica species. The high content of silica species in the rice husk had a negative effect on the gasification reactivity of banana peel during the co-gasification since it could hinder the release of AAEM from the biomass and/or lead to the possible formation of inactive alkaline silicates. However, the combination of these three samples with the suitable weight ratio could improve the gasification performance at the low temperature due to the synergetic effect provided by high contents of potassium and calcium from banana peel and cedarwood respectively. Moreover, the addition of calcined seashells as the CaO source could further improve the gas production yield, especially the hydrogen gas yield at a relatively low gasification temperature of 750 ℃.

Gasification reactivity and characteristics of coal chars and petcokes

Gasification reactivity and characteristics of three coal chars and two petcokes were investigated in detail. The temperature programmed reaction technique were used to test the gasification reactivity. The characteristics of the various chars were analyzed by XRD, SEM, N2 adsorption technique and FT-IR. As expect, the coal chars and petcokes showed significantly different gasification reactivity. The reactivity of the two petcokes was much poorer than that of the three coal chars. The gasification reactivity of the coal char increased with the decreasing of coal rank. The gasification reactivity was closely related to the characteristics of samples. It was found that the lower degree of graphitization, less compact surface and more developed pore structure corresponded to higher gasification reactivity of chars. These characteristics of the carbon structure was more favorable to gas diffusion, and hence resulted in high gasification reactivity. In addition, coal char contained more oxygen-containing functional groups and active components such as Ca/Mg/Fe/K/Na compounds, which all had promoting effect on gasification reaction.

Characterization of Solid Residues from Entrained Flow Gasification of Coal Bio-Oil Slurry

Char and soot characterization was performed on samples obtained from pilot-scale entrained flow gasification of coal bio-oil slurry (CBS) and coal/water slurry (CWS) at different operating conditions. The compositions and gasification reactivity of char and soot have been investigated to reveal the difference of CBS and CWS slurries. The results show that both char and soot obtained from CBS gasification contain alkali and alkaline earth metals which come from the bio-oil. These CBS char and soot alkali-containing samples exhibit higher gasification reactivity than those from CWS gasification. For CBS gasification, soot has higher gasification reactivity compared with char. The gasification reactivity may be caused by the catalytic effect of KCl and K(2)SO(4 )existing in char and soot. The reactivity of char and soot is also influenced by the gasification conditions (such as H2O/C ratio), and high CO2 gasification reactivity is observed for chars exposed to high H2O/C ratios in the entrained flow reactor.

Synergistic Catalytic Effect of Inherent Minerals and Specific Structures on the Steam Gasification of Lignite and Anthracite

ABSTRACT The synergistic effects of inherent minerals and specific organic structures in coal on steam gasification were comprehensively investigated herein. The steam gasification of lignite, anthracite, and their corresponding demineralized samples were tested by a laboratory fixed-bed micro reactor system. Lignite showed much higher gasification reactivity than anthracite, which led to a significant reduction of gasification temperature. Moreover, it was proved that minerals could promote the release rate of H2 but decrease the release rate of CO, which resulted in high H2/CO of the syngas for lignite, but the effects of minerals on the steam gasification of anthracite were not so obvious. These gasification behaviors could be attributed to the existence of inherent minerals in lignite, which indicated that inherent minerals had a remarkable catalytic effect on steam gasification, but limited impact on that of anthracite. The inherent minerals and organic structures of coal were characterized by scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Raman). The results showed that interactions of inherent minerals with oxygen-containing functional groups such as carboxyl functional groups and the more amorphous carbon structures in lignite played a key role in the catalytic effect during steam gasification.

Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation

A new process integrating a circulating fluidized bed (CFB) reactor and an entrained bed reactor was proposed for gasification of preheated coal. The CFB reactor as a preheater was successfully used in clean coal combustion. In this study, gasification of preheated coal was tested in a bench-scale test rig, which consisted of a CFB preheater and a down flow bed (DFB) gasifier. The effects of operating parameters of the preheater and gasifier were revealed via thermodynamic equilibrium calculations. A stable preheating process was obtained in the CFB preheater at the O2/C molar ratio of 0.31 and higher gasification reactivity was gained in preheated char owing to the improvement in intrinsic reactivity, specific surface area and total pore volume. Effective gasification of preheated char was achieved in the DFB gasifier at 1100 °C and the total O2/C molar ratio of 0.67, meanwhile the CO + H2 yield and carbon conversion increased. Thermodynamic equilibrium calculations revealed when the gasification reaction rates varied little above 1100 °C and the same carbon conversion was achieve in gasifier, lowering the temperature would lead to an increase in cold gas efficiency and a decrease in O2 demand.

An X-ray photoelectron spectroscopic perspective for the evolution of O-containing structures in char during gasification

The purpose of this study is to investigate the evolution of O-containing structures of char during gasification. Mallee wood (4.75–5.60mm) from Western Australia was gasified in a fluidised-bed reactor at 600–900°C in O-containing (pure CO2, 15% H2O-Ar) and non-O-containing atmospheres (15% H2-Ar). X-ray photoelectron spectroscopy (XPS) was applied to obtain detailed information about the nature of oxygen bonding with carbon as well as the content of oxygen species in char. The similar O/C ratio of char from XPS and elemental analysis indicated the relative chemical uniformity between char surface and char matrix. The deconvolution results of the O 1s spectra showed that the reactivity of the inherent aromatic CO structure was much higher than that of the aromatic CO structure during gasification. The amount of aromatic CO structure left in char during gasification in non-O-containing atmosphere was lower than that in O-containing atmosphere while the consumption of aromatic CO structure was proportional to the progress of gasification, regardless of the atmosphere. The newly formed CO structure in char during the gasification in the O-containing atmosphere was likely to be responsible for the high gasification reactivity. The well-dispersed alkali earth metallic species could be carbonated to form CaCO3 and MgCO3 on char surface once the char was exposed to CO2 at 900°C.

Characteristics of high temperature C-CO2 gasification reactivity of Victorian brown coal char and its blends with high ash fusion temperature bituminous coal

This research paper explores the high temperature gasification reactivity of brown coal char and its potential for promoting the gasification of high-ash-fusion temperature bituminous coal under entrained-flow gasification conditions. Using thermogravimetric analysis (TGA), the C-CO2 gasification reactivity for a brown coal char and two bituminous coals with low/high ash contents and high ash fusion temperatures, as well as their blends were investigated over a temperature range of 900–1300°C and atmospheric pressure. The nth order reaction rate equations were fitted based on a random pore model of the gasification reaction. The results show that the brown coal char has a high reactivity compared to the two bituminous coals, which mainly due to its more porous physical structure, whereas the catalytic effect of its ash-forming elements is insignificant above 1100°C. The intrinsic activation energy was determined to be 154.5kJ/mol for the brown coal char, which is lower than the higher activation energy of the two bituminous coals of 189.61kJ/mol and 196.02kJ/mol. Upon increasing the bituminous coal fraction, the activation energy increases stably due to the increased ash amount and increased molten ash that inhibit the gas diffusion. For the low-ash bituminous coal blended with 20–40% brown coal char, its C-CO2 reactivity is maximised at 1200°C when the ash turns soft. Upon further increase of the temperature, its reactivity decreases due to the enhanced melting of the ash. Instead, for the high-ash bituminous coal, its addition of 20% to brown coal char is beneficial in decreasing the overall ash melting temperature. The C-CO2 gasification reactivity of 20% high-ash bituminous coal blended with brown coal char is maximised at 1100°C when the ash turns soft and is minimised at 1200°C when the ash is fully molten.

Characteristics of fine chars from fluidized bed gasification of Shenmu coal

The ash fusion characteristics, particle-size distribution and gasification reactivity of Shenmu coal (SM) fine chars from ash agglomerate fluidized bed gasification were explored by ash fusion point analyzer, scanning electron microscopy equipped with energy dispersive X-ray analysis (SEM-EDX), and thermo-gravimetric analyzer. The results show that the decrease in basic ingredients (e.g. iron and calcium) and the increase in acidic ingredients make the ash fusion temperature (AFT) of SM fine chars lower than that of SM. There is a wide particle-size distribution in SM fine chars, which shows a significant multi-peak distribution, and a large difference of the elemental distribution in fine char particles with different sizes. The moisture content is lower and ash content is higher in fine power than that in SM, respectively. SM fine char has higher surface area than that of SM char, and relatively rich in meso-pores and macro-pores, which results in its higher gasification reactivity than SM char.

Effects of alkaline metal on coal gasification at pyrolysis and gasification phases

A Chinese high-rank coal was acid-washed and ion-exchanged with Na and K to prepare the H-form, Na-form and K-form coals. After pyrolysis, H-form, Na-form and K-form chars and two additional H-form chars (acid washed Na-form and K-form chars) were prepared to investigate the effects of alkaline metal (AM) on coal gasification at the pyrolysis and gasification phases. The H-form char had the highest pryolysis rate; the H-form char had a relative low gasification rate. The AM loaded coals exhibited relative low pyrolysis rate, while the corresponding chars had high gasification reactivity. Acid-washing reduced the reactivities of Na-form and K-form chars. AM inhibited the progress of graphitization of the base carbon resulting in a more reactive char of less ordered crystalline carbon structure. A kinetic model incorporating AM-catalyzed gasification and non-catalytic gasification was developed to describe the gasification rate changes in the char conversion for AM-catalyzed gasification of chars.

Changes in char reactivity and structure during the gasification of a Victorian brown coal: Comparison between gasification in O 2 and CO 2

Char reactivity is an important factor influencing the efficiency of a gasification process. As a low-rank fuel, Victorian brown coal with high gasification reactivity is especially suitable for use with gasification-based technologies. In this study, a Victorian brown coal was gasified at 800 °C in a fluidised-bed/fixed-bed reactor. Two different gasifying agents were used, which were 4000 ppm O 2 balanced with argon and pure CO 2. The chars produced at different gasification conversion levels were further analysed with a thermogravimetric analyser (TGA) at 400 °C in air for their reactivities. The structural features of these chars were also characterised with FT-Raman/IR spectroscopy. The contents of alkali and alkaline earth metallic species in these chars were quantified. The reactivities of the chars prepared from the gasification in pure CO 2 at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O 2 also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O 2 and pure CO 2) are oxidising conditions, the results indicate that the reaction mechanisms for the gasification of brown coal char at 800 °C in these two gasifying atmospheres are different. FT-Raman/IR results showed that the char structure has been changed drastically during the gasification process.

Low Rank Coal Upgrading in a Flow of Hot Water

Simultaneous hydrothermal degradation and extraction at around 350{sup o}C using flowing solvent as a reaction/extraction medium were proposed for upgrading brown coal, more specifically, for converting brown coal into several fractions having different molecular weight and chemical structure under mild conditions. When an Australian brown coal, Loy Yang coal, was treated by water at 350{sup o}C under 18 MPa, the coal was separated into four fractions: gaseous product by 8% yield, water-soluble extract at room temperature (soluble) by 23% yield, extract precipitates as solid at room temperature (deposit) by 23% yield, and residual coal (upgraded coal) by 46% yield on daf basis. The separation was found to be realized by in situ extraction of low-molecular-weight substances released from coal macromolecular structure and/or those generated by hydrothermal decomposition reactions at 350{sup o}C. The solid products obtained, deposit and upgraded coal, were characterized in detail to examine the possibility of their effective utilization as solid fuel and chemical feed stock. The upgraded coal showed higher heating value and higher gasification reactivity than the parent coal, indicating that the upgraded coal can be a better solid fuel than the parent coal. The solid extract, deposit, was found to show thermoplasticity at less thanmore » 200{sup o}C, suggesting the possibility of utilizing the deposit as a raw material of high performance carbon materials. Several variables affecting the performance of the proposed method are also examined in detail in this paper. 12 refs., 8 figs., 3 tabs.« less

Influence of Pressure on Coal Pyrolysis and Char Gasification

Coal char structure varied greatly with pyrolysis pressure, which has a significant influence on the gasification reactivity. In this study, the influence of pressure on the behavior of coal pyrolysis and physicochemical structure and gasification characteristics of the resultant coal char was investigated using a pressurized thermogravimetric analyzer combined with an ambient thermogravimetric analyzer. First, the pyrolysis of Shenfu (SF) bituminous coal was performed in a pressurized thermogravimetric analyzer (TGA) at different pressures (0.1, 0.8, 1.5, 3, and 5 MPa). The volatile mainly evolved out at 400–800 °C. The gas products are mainly CO2, CO, CH4, and light aliphatics with some water. It was observed that the pyrolysis of coal was shifted to lower temperature (50 °C) with pressure increasing from ambient to 5 MPa, and the devolatilization rate of coal pyrolysis was decreased and the coal char yield was increased slightly. The structure of solid coal char was analyzed using FTIR, ASAP2020, and CNHS. In the solid char, the main organic functional groups are mainly C═O, C–C (alkane), C–H ar, C–O–C, and C═C ar. The carbon content was increased while H content decreased. Finally, the gasification of the solid char was preformed at ambient pressure with CO2 as gasify agent. The gasification process of coal char can be divided into postpyrolysis and char gasification. Higher pressure accelerated the initial stage of char gasification, and higher gasification reactivity was observed for char derived at 5 MPa.

The CO 2-gasification and kinetics of Shenmu maceral chars with and without catalyst

The CO 2 gasification of maceral chars was performed using CAHN TG-151 pressurized thermobalance under different conditions. The effect of mineral in macerals and catalyst on the gasification reactivity of maceral chars and the gasification kinetics were systematically investigated. The results showed that the apparent gasification rate of maceral chars depends on the temperature, pressure, BET surface area of chars and the gasification extent. With increasing temperature and pressure, the gasification rate of maceral chars all increase. After demineralization, the gasification reactivity of maceral chars all decrease. The gasification reactivity of maceral chars greatly increases with loading catalyst. And the loading method of catalyst has great effect on the gasification reactivity. The maceral chars loaded with catalyst by ultrasonic treatment have higher gasification reactivity than that by impregnation. The comparison of gasification reactivity of maceral charas demineralized maceral chars and maceral chars with and without catalyst showed that vitrinite chars always have higher gasification reactivity than inertinite chars. The kinetic results by distributed activation energy model showed that inertinite char has higher activation energy than vitrinite char, and the addition of catalyst greatly minimizes the activation energy and enhances the gasification rate.

04/00402 The influence of high-temperature crystallite growth and petrography of pulverized char on combustion characteristics: Xu, X. et al. Fuel, 2003, 82, (7), 853–858

Char reactivity is an important factor influencing the efficiency of a gasification process. As a low-rank fuel, Victorian brown coal with high gasification reactivity is especially suitable for use with gasification-based technologies. In this study, a Victorian brown coal was gasified at 800 °C in a fluidised-bed/fixed-bed reactor. Two different gasifying agents were used, which were 4000 ppm O2 balanced with argon and pure CO2. The chars produced at different gasification conversion levels were further analysed with a thermogravimetric analyser (TGA) at 400 °C in air for their reactivities. The structural features of these chars were also characterised with FT-Raman/IR spectroscopy. The contents of alkali and alkaline earth metallic species in these chars were quantified. The reactivities of the chars prepared from the gasification in pure CO2 at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O2 also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O2 and pure CO2) are oxidising conditions, the results indicate that the reaction mechanisms for the gasification of brown coal char at 800 °C in these two gasifying atmospheres are different. FT-Raman/IR results showed that the char structure has been changed drastically during the gasification process.

Analysis of pyrolysis and gasification reactions of hydrothermally and supercritically upgraded low-rank coal by using a new distributed activation energy model

Hydrothermal water treatments and supercritical (SC) water treatments of a lignite were performed to examine the feasibility of upgrading low-rank coals. The treatment below 400°C was found to be effective enough to keep high gasification reactivity at high temperature, as well as to suppress spontaneous combustion. The pyrolysis and gasification behaviors of raw and pretreated coals were examined by thermogravimetry (TG). The kinetic analysis was carried out based on a new distributed activation energy model (DAEM) presented by Miura [K. Miura, Energy & Fuels, (12), 864–869 (1998).]. According to this method, thermogravimetric curves measured at two or more different heating rates were needed to obtain the activation energy distribution function f( E) of a given coal sample. It was found that in the case of pyrolysis, the peak values of f( E) curves for upgraded coal samples are nearly 300 kJ/mol, whereas, the peak value of f( E) curve for their parent coal is about 200 kJ/mol. In the case of gasification, where only single reactions occur, the application of this new DAEM can give the changes of activation energy during reaction. Some interesting results occur, which may hint at some changes in the rate-controlling step of reaction or in the physical structure of coal during gasification.