Search

Fine coal reject samples (FCRs) derived from the South African (SA) feed-coal preparation are unsuitable for gasification/combustion. In this novel investigation, two SA FCRs were taken from a thermal-export plant (TEP) and steam-station (STEM) for the coal-property comparison study and pyrolysis/gasification reactivity. H2S/SO2/CO2/Cadmium/Arsenic/volatile organic compounds (VOCs) emissions/dust/particulate-matter pollution linked to pyrite-spontaneous combustion and the interaction of organic sulfur and calcium in FCRs are serious threats to the global human health and atmospheric environment. Numerous investigations primarily focused on SA unoxidized FCR (STEM sample) rather than oxidized FCR (TEP sample) and beneficiated fractions. Furthermore, no Raman/X-ray diffraction (XRD)/density-separation/thermogravimetric results for the TEP (40–400 µm; 83% recovery) and STEM (111–500 µm; 20% recovery) samples have been previously reported. FCRs/their beneficiated fractions were characterized by the different coal analyses. Results indicate that the FCRs have similar properties compared to gasification/combustion feed coals with the exception of higher mineral matter (39% versus 29%), higher vitrinites (43% against 24%) and higher alkali index (8 versus 5) in the TEP sample. Thermogravimetric gasification experiments illustrate higher reaction rates/higher carbon conversion for the TEP sample than the STEM sample. Raman/XRD detected aromatic/aliphatic hydrocarbons/minerals, which positively contribute to the coal's reactivity. Results provide a baseline toward FCRs utilization in pyrolysis/gasification applications on the basis of their reactivity, and a minimization of higher disposal-costs/health-hazards/air-pollution/fines volume. [ABSTRACT FROM AUTHOR]

To evaluate the effect of active alkali and alkaline earth metals (AAEMs) on the co-gasification process, this work studied the gasification/oxidation/combustion reactivity of co-gasification chars acquired at different gasification times by four mixtures: raw coal and raw corn straws (RSM + RCS), acid-washing coal and raw corn straws (HSM + RCS), raw coal and acid-washing corn straws (RSM + HCS), and acid-washing coal and acid-washing corn straws (HSM + HCS). The results indicate that the char contents of the raw/acid-washed samples and mixed samples in the first 5 min of gasification are mainly affected by influence-1 (active AAEMs promote the increase of the char contents during pyrolysis). And after gasification for 10 min, influence-2 (active AAEMs catalyze the reaction of char, resulting in the reduction of char content during gasification) occupies the core position. The order in the effect of active AAEMs on gasification reactivity of char is active AAEMs in RSM and RCS > active AAEMs in RCS > active AAEMs in RSM > no active AAEMs. Among them, the effects of active AAEMs in RCS and active AAEMs in RSM and RCS are very significant. Besides, the ratios of experimental value to theoretical value of residual carbon contents for RSM + RCS chars and HSM + RCS chars range from 19 to 57%, while those for RSM + HCS chars and HSM + HCS chars range from 96 to 103% during oxidation reaction. Active AAEMs in RCS have a significant catalytic effect on oxidation/combustion reactivity of char. The maximum oxidation rate of RCS char is 314 times higher than that of HCS chars. The maximum combustion rate of RCS chars is 15–20 times higher than that of HCS chars. However, the maximum oxidation/combustion rate of RSM chars is less than 2 times that of HSM chars. • Effects of active AAEMs on in-situ pyrolysis-gasification coupling are explored. • The maximum oxidation rate of RCS char is raised by 300 times due to RCS-AAEMs. • The maximum combustion rate of RCS char is raised by 15–20 times due to RCS-AAEMs. • Active AAEMs in RCS can change combustion reactivity of large/small rings in chars. [ABSTRACT FROM AUTHOR]

In this study, the effects of coal blending ratio, pyrolysis temperature, pyrolysis atmosphere and coal particle size on the quality of pyrolyzed char were investigated via a combination of orthogonal experimental design and grey relational analysis. Coal-XG and Coal-LL were pyrolyzed under various conditions to produce chars which were characterized to ascertain their properties such as chemical structure and reactivity. The results indicated that the pyrolysis temperature has the most significant impact on the quality of the char, followed by the pyrolysis atmosphere, coal blending ratio and coal particle size. The increase in pyrolysis temperature not only diminished char yield but also accelerated the consumption of oxygen-containing functional groups and the polycondensation of small aromatic ring structures throughout the reaction process. This led to a reduction in the reactivity of pyrolyzed char and the establishment of a more ordered structure. Meanwhile, the involvement of CO 2 and H 2 O prevented the condensation of small aromatic rings and fostered the generation of additional oxygen-containing functional groups, which resulted in lower yield and higher reactivity of the char under these two atmospheres. Through systematic analysis, only the Raman characteristic parameter I (Gr+VL+Vr) /I D exhibited a strong positive linear correlation with char reactivity, making it a reliable indicator for characterizing the reactivity of char. • The orthogonal design was used to study four response factors. • GRA was used to find optimal conditions and impact ranking of different factors. • Linear regression was applied to relate Raman/FTIR parameters to reactivity. • Blend experiments were conducted with two coking coals. [ABSTRACT FROM AUTHOR]

In order to better understand the impact of the carbon structure of char on its combustion reactivity, 14 types of organic model compounds were pyrolyzed under temperatures of 530 °C, 800 °C and 1028 °C at heating rates of 10 °C/min and 65 °C/s, respectively. The carbon structure and the oxidation reactivity of the produced char were determined by respective Raman spectroscopy and thermogravimetric analysis (TGA) methods. The relationships between different reactivity indexes and Raman spectral parameters have been assessed. The results reveal that changes in pyrolysis temperature and heating rate would result in certain evolution of the graphitization degree of char. Good correlations have been discovered between the Raman spectral parameters (A D /A ALL , A GL /A ALL , A (S+R+SL+VR+VL) /A D , and A (GR+VL+VR) /A D) and the combustion characteristic temperatures (T i , T b , T 50 , and T max). From the correlations above, Raman spectral parameter A (GR+VL+VR) /A D is found to be the best correlated to the char combustion characteristic temperature T b with R2 of around 0.76. • Char from 14 types of organic model compounds was studied. • Char yield rises with a higher pyrolysis temperature and a lower heating rate. • Char structure becomes ordered at a high temperature or slow heating rate. • Lower H/C and O/C reflect higher condensation degree of aromatic rings in char. • Higher ordered char structure leads to a higher oxidation temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] • High ash coal char reactivity assessment was performed in a packed bed reactor. • High ash coal char reactivity varies with particle size. • The transition of the conversion regime occurred at 1 mm char particle size. • A high H 2 proportion (∼67 %) was obtained by char-steam reaction in a packed bed. • However, H 2 reduces to 45% in oxy-steam char reaction in pilot scale studies. This paper focuses on char gasification in steam towards generating syngas as a part of two-stage gasification technology developed in-house to handle high-ash coal. Controlled experiments in a small-scale packed bed reactor are used to arrive at the dependence of char reactivity using parametric studies like the evolution of temperature, gas composition, flow rate, and carbon conversion rate with time and are compared for different particle sizes. The volume percentages of CO, CO 2 , CH 4, and H 2 were 7, 26, 1, and 66, respectively, in steam char experiments, over a particle size range from 1 mm to 4 mm. If the CO 2 is separated, the H 2 volume percentage reaches 90%. In small-scale experiments, the gas composition for all the particle sizes was invariable; however, the reactivity of small particles is higher than the bigger particles, i.e., 0.5 g/g-hr as against 0.38 g/g-hr for bigger particles due to the transition from diffusion to the kinetic regime, which establishes the suitability of small particles for pilot-scale operations. Finally, the experiments were carried out on pilot-scale char gasification in an oxy-steam environment, and the results were compared with simulated experiments for small-size particles. The consequences of adding oxygen with steam to keep the system auto-thermal are discussed. In the pilot scale under oxy-steam conditions, the volumetric concentrations of CO, CO 2 , CH 4, and H 2 were 10.8, 39.9, 4.7, and 44.6 %, respectively. In contrast, the reactivity increased from 0.46 to 0.90 g/g-hr with a decrease in the calorific value of gas from 9.7 to 8.6 MJ/Nm3. [ABSTRACT FROM AUTHOR]

Pyrolysis, as the initial process of coal clean utilization, is essential to ensure the operation of pyrolysis-gasification decoupling technology. This paper aims to investigate the effects of temperature and oxygen content on coal pyrolysis product distribution and the char structure and reactive properties. Results demonstrate that in the range of 800–1100 °C, the increase in temperature is conducive to the release of volatiles and tar cracking, to enhance the gas phase yield and syngas concentration. However, as Raman spectroscopy analysis shows, higher temperature (>900 ℃) will intensify the graphitization of char, which is not conducive to the next gasification step. In addition, oxygen introduction at 900 ℃ can accelerate the release of volatiles and intensify the occurrence of depolymerization reaction, which increased the gas yields of CO and CO 2 in the gaseous product. Besides, the presence of trace oxygen in the pyrolysis process reduces the degree of coal char graphitization and improves the gasification reactivity. However, at high oxygen content (8 %) conditions, combustion occurs on the surface of the char as SEM results show, which destroys the structure and reduces the gasification activity of the char. Hence, the pyrolysis product distribution and post-pyrolysis char gasification activity can be adjusted by introducing trace oxygen in pyrolysis-gasification decoupling technology. • Fast pyrolysis and gasification decoupling characteristics of Yili coal is studied. • Oxygenated pyrolysis of Yili coal increases gas products yield and reduces tar. • Oxygenated pyrolysis char has lower condensation degree structure. • Coal char from pyrolysis at 900 ℃ is found to have higher gasification activity. [ABSTRACT FROM AUTHOR]

• The difference in gasification reactivity of digested solid char with and without AAEMs was systematically clarified. • The inherent AAEMs have promoting effect on gasification reactivity of char. • The E a value of the char after pickling increased 115.76 kJ·mol−1. • The maximum instantaneous CO concentration after char pickling was reduced by a factor of about three. Char reactivity usually determines the overall efficiency of the entire gasification process, while the presence of the alkali and alkaline earth metals (AAEMs) has a catalytic effect on char gasification. In this study, the influence of inherent AAEMs in the gasification behaviour of char was investigated. The char sample was prepared through the pyrolysis of solid digestate derived from anaerobic co-digestion of silage and dairy cattle slurry. The raw char and HCl-washed char were characterized by elemental analyzer, ICP-OES, SEM and XRD to explore their structural changes. HCl-char loses weight in the temperature range of 440–620 °C, and the weight loss percentage is significantly higher than that of char. Ash content of char is reduced by half after a pickling process. AAEMs are largely removed after char acid pickling, resulting in an increase in activation energy of the gasification reaction and a decrease in gasification reactivity, resulting in the gasification reaction time of HCl-char longer than char. Water-soluble AAEM and ion-exchange AAEM affect the evolution of carbon structure, which directly lead to char reactivity during the gasification reaction. As the carbon is consumed, the carbon microcrystalline structure of the residual carbon tends to be ordered, resulting in fewer active free carbon sites for the gasification reaction. Kinetic analysis showed that the loss and deactivation of AAEMs after char acid washing increased the average activation energy E a by 115.76 KJ/mol compared with the original char. [ABSTRACT FROM AUTHOR]

In this study, a theoretical model of biomass char gasification reactivity was developed, focusing on the catalytic effect of inorganic elements on the char gasification process. A comparison with previous research results shows that the catalytic ability of K in a fixed bed reactor is stronger than that of Ca, while the catalytic ability of Ca in a fluidised bed reactor is stronger than that of K. The migration and transformation of K and Ca in a fixed bed reactor and fluidised bed reactor are compared. In the fluidised bed reactor, a larger proportion of Ca is transformed into an ion-exchanged state than K, which is contrary to the experimental results in the fixed bed reactor. Then, according to the equivalent-volumetric impregnation method, the saturated loading ratio of K was determined to be 35%, and the full catalytic ratio of K was determined experimentally. Finally, eight typical biomass char samples were selected to perform experiments at different temperatures in the micro fluidised bed reactor to determine the char gasification reactivity, and the results were compared with the calculated values of the model. Results show that the model can effectively predict the char gasification reactivity both in trend and accuracy. [Display omitted] • Migration and transformation of K and Ca during pyrolysis process were clarified. • The catalytic ability of inorganic elements in fluidised bed reactor was quantified. • The order of catalytic ability in the fluidised bed reactor is Ca > K > Na > Fe > Mg. • The model considers catalytic/inhibitory effects of seven main inorganic elements. • The self-developed char reactivity model is suitable for the fluidised bed reactor. [ABSTRACT FROM AUTHOR]