Condensation Of Aromatic Structures Research Articles

Experimental study on optimizing reaction path for promoting the gasification of coal in supercritical water

Coal gasification in supercritical water (SCW) has great potential in industrial application for converting coal into hydrogen-rich gas in a clean and efficient manner. However, when the reaction conditions are improperly coupled with reaction path, undesirable side reactions would take place, which is prone to reduce the efficiency of converting coal into hydrogen-rich gas. It is revealed that the condensation of aromatic structures and the formation of aromatic ether (Car-O) groups are the restrained reactions according to the reaction path obtained by studying the variation of gas–liquid-solid products during supercritical water gasification (SCWG) of coal. Methanol, Fe2O3 and Ni(CH3COO)2·4H2O are added to inhibit the restrained reactions. It is found that these all facilitate the cracking and gasification of aromatic structures. Fe2O3 and Ni(CH3COO)2·4H2O except methanol, reduce the formation of Car-O groups effectively. The optimal concentration of these corresponding to the greatest promotion is 7.5 wt% (methanol), 2.5 wt% (Fe2O3) and 2.5 wt% (Ni(CH3COO)2·4H2O), respectively. Ni(CH3COO)2·4H2O (2.5 wt%) has the highest promotion on the production of H2 and CO, as well as the cracking and gasification of aromatic structures. The corresponding maximum yields of H2 and CO increase by 0.29 and 1.85 times, respectively. Fe2O3 (2.5 wt%) has the greatest promotion on the decomposition of Car-O groups, and the relative content of Car-O groups decreases by 80.86%. This research provides the theoretical basis for regulating the ordered conversion of coal into hydrogen-rich gas during SCWG of coal.

Chemical character and structure of uraniferous bitumens (Vrchlabí, Czech Republic)

Black shales from the Permian rocks at Vrchlabí in the Krkonoše Piedmont Basin (Czech Republic) contain bitumens with uraninite grain inclusions and bulk uranium concentrations up to 4.8 wt%, determined by instrumental neutron activation analysis. The carbonaceous matter in the bitumens has been characterised by gas chromatography/mass spectrometry, attenuated total reflection Fourier transform infrared spectroscopy, scanning electron microscopy, and stable carbon isotopic composition. Progressive radiolytic alteration was documented by changes in the chemical composition and structure of both bulk samples and on the microscale. Condensation of aromatic structures, increase in content of oxygenated compounds, and enrichment in 13C were documented in the uranium-rich bulk samples. Micro-ATR-FTIR area mapping throughout the bright halos around uraninite inclusions revealed detailed structural changes, particularly a decrease in aliphatic bond intensities and an increase in the aromaticity of the structures close to the inclusions. A radiolytic alteration pathway has been suggested assuming a complex of reactions between organic matter and uranyl containing hydrothermal solutions, and oxidation by OH radicals produced in radiolysis of water. The organic matter transformations observed in the uraniferous bitumen correspond to the coalification processes, changing the bitumen composition from type I to type III and IV kerogens. Detailed characterisation of the radiation-induced chemical and structural changes in the naturally occurring uraniferous bitumens will contribute to the general understanding of the nature and behaviour of organic matter in other uraniferous environments or in the radioactive waste repositories.

Investigation of the conversion mechanism for hydrogen production by coal gasification in supercritical water

The technology of supercritical water gasification (SCWG) of coal has a great prospect because it converts coal into hydrogen-rich gas products efficiently and cleanly. However, there are bottlenecks affecting the complete gasification of coal in supercritical water (SCW) without catalyst under moderate conditions. This work is to explore the restricted factor for complete gasification of coal in SCW by investigating the conversion mechanism. The conversion mechanism of SCWG of coal with and without K2CO3 is proposed. Polycyclic aromatic hydrocarbons (PAHs) with graphite phase structures are formed by the condensation of aromatic structures at 550–750 °C. It is the restricted factor due to its characteristic of difficulty to be gasified. There is no condensation of aromatic structures in the process of SCWG of coal with K2CO3, which effectively inhibited the formation of PAHs with graphite phase structures. K2CO3 dramatically promoted the SCWG of coal, leading to carbon gasification efficiency (CE) reaching 98.43%.

Molecular reaction dynamics simulation of pyrolysis mechanism of typical bituminous coal via ReaxFF

A reasonable and effective macromolecular model of bituminous coal was established. The molecular dynamics method based on reactive force field (ReaxFF) was used to simulate the pyrolysis process of typical bituminous coal in the range of 1400–2600 K. The distribution of products and evolution of intermediate radicals were analyzed. Calculation results showed that with increase of pyrolysis temperature, yield of char firstly increased and then decreased, while the trend in tar production was opposite. Yield of pyrolysis gas increased monotonously with increasing temperature. The pyrolysis of coal at low temperature mainly experienced primary reaction with formation of tar free radical fragments and small molecular gases. At high temperature, the secondary reaction of tar fragments was remarkable, and char with more content but less quantity and small molecular gas with more content and quantity were produced. The temperature turning point from the primary reaction to the second one was 2000 K. Under the high temperature pyrolysis conditions, C and H in coal gradually migrated into char and tar, while oxygen-containing functional groups were more active, resulting in migration of O to pyrolysis gases. In the secondary reaction stage, comparing chemical properties of the three elements C, H and O, O was the most active, H was the second, and C was the most stable. H2O was firstly released during pyrolysis. NH3 mainly came from secondary reactions during which H2S was consumed and converted into other products. Yield of H2 was the highest, and increased with increasing pyrolysis temperature. A large amount of H2 was generated in secondary reactions, which was mainly from collision of hydrogen radicals generated from pyrolysis and condensation of aromatic structures. Based on ReaxFF simulation results, the weightless activation energy of coal pyrolysis was 39.45 kJ/mol.

Production of syngas from steam gasification of three different ranks of coals and related reaction kinetics

The steam gasification properties of three different ranks of coals, Shengli lignite (SL), Shenhua subbituminous coal (SH), and Tavan Tolgoi anthracite (TT), were investigated using a lab-scale fixed-bed reactor, and the thermodynamic equilibrium constant and kinetics of the reaction were analyzed. The results showed that the aromaticity and condensation of aromatic structures in SL, SH, and TT became higher, and the maturity of organic substance became lower. The steam gasification reaction showed that the syngas from low-rank SL had a high H2/CO molar ratio, while the syngas from high-rank TT had relatively high CO content. The direct carbon gasification reactions for these three different ranks of coals were far from in equilibrium; the water gas shift reaction of SL was near equilibrium, and the degree of reaction for SL was higher than that of SH and TT. We studied a random pore model (RPM), shrinking core model (SCM), and hybrid model (HM), and the hybrid model was found to be the most suitable model of the three for fitting the steam gasification reactions of the three types of coal. It had high fitting correlation coefficient R2 values (ranging from 0.9939 to 0.9990) and small average error θ values (ranging from 0.009 to 0.016). The apparent activation energy E values of SL, SH, and TT fitted by HM were 179.10, 48.14, and 63.06 kJ/mol, respectively, and the corresponding pre-exponential factor k0 values were 3.14 × 107, 1.01, and 1.22 min−1, respectively. This study finds that the steam gasification of SL, SH, and TT coal samples consists of homogeneous phase reaction and shrinking core reaction.

Significant enhancement of the oxygen reduction activity of self-heteroatom doped coal derived carbon through oxidative pretreatment

Functional low-cost catalysts derived from earth abundant materials are vitally important for enabling mass implementation of renewable energy conversion systems, such as fuel cells and metal-air batteries. Herein, we report the activation of earth-abundant coal to realize very active and stable catalysts for the oxygen reduction reaction (ORR) through oxidative pretreatment followed by thermal treatment under ammonia forming self-doped heteroatoms and defects. The coal thermally pretreated in air at 350 °C for 30 min followed by treatment in ammonia for 2 h presented very good electrocatalytic ORR activity with onset potential of 0.978 V, half-wave potential of 0.871 V, Tafel slope of 57 mV dec−1, and predominant 4-electron reduction of O2 to OH−. The influence of the preoxidation step prior to ammonia treatment on the properties of the coal-derived carbon and hence the ORR has been specifically studied. The optimized catalyst also displayed stable performance during the ORR in 0.1 M NaOH for a test period of 35 h and exhibited excellent tolerance to methanol. The preoxidation step facilitates elimination of aliphatic and amorphous carbon, condensation of aromatic structures and the formation of initial pore structures, and subsequent removal of unstable heteroatoms from aromatic carbon rings to form surface and edge defects as potential active sites for promoting the ORR. Using pristine coal as raw material to produce functional ORR catalysts will be of great importance for real applications.

Insight into the effects of additive water on caking and coking behaviors of coal blends with low-rank coal

Additive water is usually needed to preserve 9–11% moisture content of blended coal in the stamp-charging coking process, which in fact is unfavorable for coking property. An insight into the improvement of coking property of coal blends with much low-rank coal (long flame coal SFC, lignite HLH) by regulating additive water was proposed in terms of its effects on the metaplast fluidity and the particle agglomeration behavior. The results indicate that the cohesiveness of gas coals (GC) can be properly enhanced by adding water. The restrictions on the condensation of aromatic structures and the break-up of –CH2– structure and the development of the polyhydroxy groups in aromatic structures are favorable to increase the fluidity of metaplast after adding water, as demonstrated by the optical texture and FTIR analysis of the semicoke (450 °C). The great difference in the hydrophilic properties for various rank coals leads to the particle agglomeration of gas coal and fat coal (GC and FC), which makes them difficult to mix uniformly with SFC after adding water, thus reduces the coking property. A reasonable approach of adding water suggests that H2O and SFC should be well mixed prior to blending with caking coals, which can lead to a uniform particle distribution and a better coking property. This approach promotes the direct and efficient utilization of low-rank coal (especially lignite) by the blending coking with no or less drying and dewatering processes.

Catalytic and Thermal Cracking of Coal-Derived Liquid in a Fixed-Bed Reactor

A coal-derived liquid, obtained from the Coal Technology Corp.'s mild gasification process, was cracked over char produced from Pittsburgh No. 8 coal mixed with Plum Run dolomite in the Foster Wheeler carbonizer. For the purpose of comparison, calcined Plum Run dolomite (PRD), char produced from Pittsburgh No. 8 coal, and silicon carbide (an inert material) were also studied. Coal liquid feed was analyzed by sulfur-selective gas chromatography (GC), liquid chromatography (LC), and proton nuclear magnetic resonance (NMR) and for elemental composition. The gaseous products of cracking were analyzed for hydrocarbons using GC. Most sulfur in the feed was present in molecules heavier than dibenzothiophene and was distributed in a variety of structures. The surviving coal liquid was analyzed by LC. The results indicated that deoxygenation of phenols, dealkylation of aromatic compounds (AR), and condensation of aromatic structures are some of the reactions occurring on the surface of bed materials. Energies of activation for homogeneous and for heterogeneous pyrolysis of the coal liquid were calculated after separating the rate of thermal cracking from the sum of rates of thermal and catalytic cracking.