Characteristic chemical time scale analysis plays a key role in the understanding of turbulence–chemistry interaction in turbulent combustion research and is also an important basis for the selection or development of combustion models in turbulent combustion modeling. A new method named main direction identification (MDID) was developed based on the modification of the CTS-ID (chemical time scale identification) method to achieve the function of identifying the characteristic time scale. Direction weight factor combined with mole fraction limit were used as a criterion in MDID to determine the characteristic time scale. MDID was applied to study characteristic chemical time scales of a CH4–O2 inverse diffusion flame in hot syngas coflow, which is a model flame developed before to study the combustion process in partial oxidation reformers. Results show that the chemical time scale given by the MDID method is about 10–5 s in the combustion area and 10–2 s in the reforming area. The main reaction pathway was also analyzed using the MDID method. The new method was compared with three existing methods published in previous studies; the Damköhler numbers given by MDID are more consistent with the mild combustion nature of the flame compared with other methods. Then the MDID method was evaluated on a conventional oxy-fuel-type high temperature flame to assess its flexibility to different reaction regimes. The time scale variation accurately reflects the changes of reaction regimes, indicating that the MDID method performs well on reacting flows varying from fast reaction regime to slow reaction regime. The effect of mole fraction limit on this method was also studied.

•Rapid pyrolysis of Shenfu coal was conducted with residence time 0–750 ms.•Mass loss and gas yield were analyzed in both N2-based and CO2-based atmosphere.•Chemical structure evolution of char was investigated by FT-IR.Rapid pyrolysis of Shenfu bituminous coal was conducted with a wall temperature 1073–1473 K and residence time 0–750 ms in both CO2 and N2 atmospheres to study the pyrolysis mechanism in CO2 atmosphere. The interest was focused on mass loss, gas yield, element content of chars and a series of chemical structure parameters (CH2/CH3, Har/H, fa and (R/C)u) obtained by FT-IR analyses. The results show that varied impacts dominated on pyrolysis at different temperatures in CO2 atmosphere. Mass loss decreased at 1073 K but increased at 1273 and 1473 K. Both heterogeneous reaction of CO2 with char and homogeneous reactions of CO2 with volatile occurred during pyrolysis in CO2 atmosphere. The combined effect of enhanced polycondensation of aromatic rings and C-CO2 reaction has been found to increase the mass loss of char. More H-containing free radicals generated during polycondensation, which consequenctly increase combination possibility with other radical fragments to inhibit the cross-linking reaction.Download high-res image (128KB)Download full-size image

•A detailed kinetic mechanism was developed for oxidation of H2S-CH4 mixture.•The kinetic model was proved to be reliable by validation versus experimental data of H2S-CH4 with or without O2.•Simultaneous production of H2, S2 and CS2 can be expected from decomposition of H2S-CH4 under high temperature.•Rate of production analysis was performed on inspecting the pathway from CH4 to CS2.This study presents a detailed kinetic investigation into ultra-rich oxidation of H2S-CH4 under high temperature (900–1250 °C) and ambient pressure. Effects of temperature, initial H2S/CH4 ratio and equivalence ratio (Φ) on reactants conversions and products distributions were experimentally studied in a tubular flow reactor and kinetically analyzed by CHEMKIN software. A detailed kinetic mechanism involving 85 species and 515 reactions has been developed and validated using reference data for H2S-CH4 decomposition and results from extended experimental conditions involving the O2 addition. For H2S-CH4 system, conversion of H2S increased steady with the rising temperature while reactivity of CH4 was weak at temperature below 1000 °C. At temperature higher than 1000 °C, conversion of CH4 increased rapidly and devoted further formation of H2 and CS2 mainly via reacting with H2S decomposition products. The H2 production efficiency was negatively associated with initial H2S fraction as H2S decomposition was dominant H2 source within 1150 °C. The stoichiometric ratio for H2S/CH4 merely showed its advantage in H2 production at higher temperature under which CH4 reached its equilibrium conversion swiftly. Introduction of little amount of O2 (Φ = 6 or higher) accelerated the whole reaction process and triggered H2S partial oxidation and H2 formation at lower temperature. CH4 explicitly showed inferior position in oxidation competition with H2S and maintained poor conversion at temperature below 950 °C. The results of rate of production (ROP) analysis at condition without O2 showed that CH4 reactivity showed dependence on free S radical via S + CH4 = SH + CH3, and the formed CH3 was mainly converted via reacting with SH and H radicals. CH3 could be concurrently reverted to CH4 via reactions with H2S and H2. O2 activated the whole system by forming chain branching radicals O and OH. These radicals promoted H2S and CH4 conversions to form richer S, H and CH3 radicals. SH + CS = CS2 + H was important for CS2 formation and with presence of O2, CS2 was likely to be consumed via oxidation reactions. Finally reaction pathways for H2S, CH4 conversion and H2, CS2 formation were presented.

•Oxidation of CH4 and H2S in a non-premixed flame was studied under oxygen deficient condition.•Partial oxidation of H2S was prior to that of CH4 and S2 but not H2 was the major product.•Formation of CS2 and COS occurred at the very initial oxidation process as the result of reaction between CH4 and sulfur-containing species.•By-produced combustibles such as H2, CO and higher hydrocarbons were formed and maintained especially under fuel rich condition.Oxidation process of gas mixture of hydrogen sulfide (H2S) and methane (CH4) in a non-premixed flame and formation of COS and CS2 has been studied under oxygen deficient condition. Premixed fuel consisting of H2S and CH4 with constant molar ratios (1:1) was injected and air was varied from the stoichiometric to the fuel rich condition (equivalence ratio, Φ = 2) to determine the influence of Φ upon the oxidation manner. The flame could be divided into three sections namely initial oxidation, intense combustion and post-flame section. Oxidation competition between H2S and CH4 was observed at the first section wherein H2S conversion, mainly via partial oxidation, was superior to that of CH4. Initial CH4 conversion involved partial oxidation and interaction with sulfur species which contributed to productions of COS and CS2. Formation of SO2 was witnessed at the second section together with significant increase in concentrations of CO, H2 and C2H2. However, the productions of these combustibles were favored by higher temperature and fuel rich condition and likely to be maintained under anoxic environment in the post-flame section. The CS2 and COS concentration profiles suggested a tight relationship between its conversion path and the O2 level downstream.

•A detailed kinetic mechanism was developed for describing H2S oxidation process and chemical role of CO2.•The comprehensive kinetic model has been validated via tube furnace experiment.•CO2 functions as oxidant medium via CO2 + H = CO + OH under oxygen deficient condition and high temperature.•Rate of production analysis was performed on studying the conversion of H2S, H and OH radical.This paper presents a detailed kinetic study of H2S oxidation with presence of CO2 under fuel rich condition. Effect of CO2 reactivity on the partial oxidation of H2S was particularly studied via tube furnace experiment and detailed kinetic analysis with CHEMKIN-PRO software. A detailed kinetic model involving 90 species and 596 reactions was developed and validated using experimental data with respect to production of H2, and conversions of CO2 and H2S under different conditions by altering the initial gas composition temperature and residence time. In the tube furnace experiment, it was found that decomposition of H2S and conversion of CO2 were promoted by increase in temperature while H2 production decreased and CO concentration evidently increased at temperature higher than 1450 K, which could be explained by CO2 + H = CO + OH. The degree of this reaction as features the major reactivity of CO2 is obviously dependent on residence time whilst H2S partial oxidation proceeds much swiftly at 1473 K or higher temperature. In addition to the experiment, rate of production (ROP) analysis for H2S, H and OH were performed by CHEMKIN-PRO. It was found for the partial oxidation of H2S, reactions S + SH = S2 + H and SH + SH = H + HSS are important in H producing while H2S + O = SH + OH and H2O2(+M) = OH + OH(+M) are key steps for OH producing. Presence of CO2 prolonged the OH production process via CO2 + H = CO + OH to the whole residence time which definitely differed from the condition without CO2. Thereby production selectivity of SO2 and H2O was promoted while that of H2 and S2 were degraded when CO2 was considerably present in the H2S partial oxidation scenario.

•Oxidation of acid gas in a non-premixed flame was studied under oxygen enriched condition.•Reactivity of CO2 was significantly promoted in an O2 enhanced flame.•COS could be initially formed in the flame inner core by reaction involving CO2 and sulfur species.•Formation of COS and CS2 is promoted under O2 enriched combustion.O2 enriched combustion is applied to a non-premixed acid gas (AG) flame, in which AG and air are injected separately into a vertical reactor via a coaxial burner at atmosphere pressure. Equilibrium predictions of AG oxidation were performed using Aspen Plus and lab-scale experiments with particular focus on formation of COS and CS2 were undertaken under different combustion conditions. The effects of equivalence ratio (Φ = 0.8, 1.0 and 3.0) and initial O2 concentration (OC) in air (21, 30 and 50 vol%) have been studied and the flame is interpreted by analyzing the axial temperature and species concentration distributions along the reactor.AG diffusion flame could be divided into three zones, namely AG decomposition, oxidation and complex reaction zones, among which decomposition zone is tightly associated with formation of COS and CS2. It is shown that Φ generally determines the flame temperature and controls the H2S oxidation degree and production rate of H2 and CO. Reactivity of CO2 primarily expressed via CO2 + H = CO + OH during the fuel rich flame. COS is primarily produced in the flame inner core via reactions jointing CO2 and sulfur species, consequently its formation shows a low sensitivity to Φ in the air-supplied flame. O2 enrichment basically contributes to higher flame temperature, accelerated H2S oxidation and advanced CO production. Also an increasing tendency for H2S to decompose into H2 and S2 can be observed. And this factor directly triggers the formation of COS and CS2 via increasing the presence of CO and sulfur species. COS is formed within extended channels involving the primary production by reaction of SH + CO2 and the secondary by reactions between CO and sulfur species. The CS2 formation is more complex, mainly comprises of reactions involving CS intermediate and evidently it is enhanced under O2 enriched combustion.

•The volume change of high temperature ash (HTA) and low temperature ash (LTA) was investigated by heating stage microscope.•The mineral matter variations of HTA and LTA samples during the heating process were observed.•The HTA and LTA samples shrink, melt and then spread in the heating process.•The shrinkage of HTA sample is different from that of LTA sample.•Those mineral matter variations result in the obvious volume change of HTA and LTA samples.The flow behavior of high-temperature ash (HTA) and low-temperature ash (LTA) from room temperature to 1350 °C was studied. The change of volume in the heating process was investigated by the heating stage microscope. The mineral matter composition and microstructure characteristics of HTA and LTA samples during the heating process were observed by X-ray diffraction and scanning electron microscope. The HTA and LTA samples shrink at the first heating stage, and then melt. With the increasing temperature, the minerals in HTA and LTA samples change and the flowability improves. The HTA and LTA samples show significantly spreading property. However, the shrinkage of HTA sample is different from that of LTA sample. The volume change of HTA sample is not obvious when the temperature is lower than 800 °C, while the volume of LTA sample presents the continuous decrease. At the temperature range of 800–1000 °C, the main minerals formed in LTA and HTA samples are augite and albite, respectively. Those mineral matter variations result in the obvious volume change of HTA and LTA samples. In addition, the microstructure of the samples shows that the shapes of particles in LTA are irregular and the surface of particles is coarse at lower temperature. With the increasing temperature, the attachments on the particle surface agglomerate, melt and accumulate. The variation of particles validates the change of flow behavior in the heating process.

Experiments were carried out to investigate the rapid pyrolysis of Nei-meng (NM) lignite, Shen-fu (SF) bituminous coal, and Jin-cheng (JS) anthracite with the duration time of 0–500 ms and the temperature of 1173–1773 K using a high-frequency induction furnace. Interest was centered on the primary fragmental characteristics of particles, including the changes of mass loss, particle density, and size distribution, during the pyrolysis. A pair of fragmental parameters, i.e., dimensionless particle diameter δ and particle distribution Sf, was proposed to characterize the fragmentation during the different stages of pyrolysis. The result showed that the pyrolysis progress increases with time and temperature. The fragmentation extent is also positively related to the time and temperature. The progress of primary fragmentation is lignite ≈ bituminous coal > anthracite. However, the particle morphology changes little during the pyrolysis fragmentation. Evidence reveals that major fragmentations of NM and SF occur at the outer zone of the particle and the coarse fragmentation of JS is insignificant compared to the exfoliation.

•Inverse diffusion flame of CH4–O2 in hot syngas coflow is numerically studied.•Flame is stabilized far away from stoichiometric mixture fraction by autoignition.•Flame is established in MILD combustion regime.•The inverse diffusion configuration play a key role in achieving MILD combustion.•This provides a new understanding of the combustion process in natural gas reformer.The structure and combustion mode of inverse diffusion flame of CH4 and O2 in hot syngas coflow are numerically studied to gain a fundamental understanding of the flame in non-catalytic partial oxidation (NC-POX) reformer. The configuration is modified based on the burner system of Cabra et al. [Combust. Flame2005, 143 (4), 491–506] to make the flame representative of that in NC-POX reformer. The Eddy Dissipation Concept (EDC) model with the detailed GRI 3.0 mechanism is used to model the turbulence–reaction interactions. Results of the study show that the flame is stabilized by autoignition with a wide reaction zone located far away from the stoichiometric line. Analyses on combustion mode show that the flame is established in Moderate and Intense Low-oxygen Dilution (MILD) mode. The inverse diffusion flame configuration which ensures a fully dilution of oxygen plays a key role in achieving MILD combustion in fuel rich coflow. The Increase of coflow temperature or decrease of jet velocity within the range of this study can lead to an early autoignition, but doesn't change the combustion mode.

A novel coal extraction method using supercritical carbon dioxide (scCO2)/1-methyl-2-pyrrolidone (NMP) mixed solvent, which has the advantages of high efficiency, diffusibility, separability, and environmental friendliness, is proposed in this paper. The physical and chemical properties of the extracts and residues in scCO2/NMP mixed solvent extraction of coal were studied. The results showed that 8 bituminous coals had relatively higher extraction yields. The bituminous coal with 85.12% carbon content had the maximum observed extraction yield. The mixed solvent of scCO2 and NMP had a good extractability for hydroxyl-rich substances, oxygen, sulfur, and other heteroatom-containing compounds. The extracts of Bei Su (BS) and Qu Jin (QJ) coals with higher extraction yields contained more aromatic ring structures. Studies of coal structure revealed that BET surface area of residues were generally lower than that of raw coals, especially for low-rank coals. After extraction, BET and pore volume declined whereas the pore size increased with increasing pressure. It could also be concluded that stacking heights of four coals (QJ, BS, Nan Lu Tian, and Yang Chang Wan) decreased after the extraction procedure. The degree of decline of NLT lignite was larger than that of other coals according to the carbon minicrystal research. A microcrystalline structure similar to raw coal was formed in the process of the solvent evaporation, with a lower stacking height and a higher interplanar spacing. The gasification reactivities of the four coals have been improved by the extraction with scCO2/NMP mixed solvent, especially for QJ and BS coals, which have high extraction yields.

•A comprehensive 2D model of an industrial Natural gas (NG) non-catalytic partial oxidation reformer is established.•Both the modified EDC model and the PDF model are used to model the turbulence-chemistry interaction.•The results of the modified EDC model agree better with the industrial operating data.•The increase of pressure promotes CH4 conversion and a pressure higher than 3.0 MPa is suggested for industrial operation.•The O2/NG mole ratio range of 0.66–0.67 is optimal according to the yield of the H2 and CO.A comprehensive 2D model of a natural gas (NG) non-catalytic partial oxidation (NC-POX) reformer is established in this study. The simplified mechanism (GRI-mech 3.0) is applied to calculate the reaction rates involved in the reformer process. Both the modified Eddy-Dissipation-Concept (EDC) model and the PDF model are applied to calculate the chemistry and turbulence interaction. The results of the EDC model agree well with the operating data of industrial reformer. The effects of operating pressure, the O2/NG mole ratio and the steam/NG mole ratio on the performance of reformer are investigated by using the EDC model. The results indicate that the increase of pressure promotes the CH4 conversion and a pressure higher than 3.0 MPa is suggested for industrial operation according to the conversion of the CH4 in the range of this study. As the O2/NG mole ratio increases, the temperature increases and the concentration of CH4 in syngas decreases. The O2/NG mole ratio range of 0.66–0.67 is optimal according to the yield of the effective syngas compositions (H2 + CO) mole fraction in raw syngas and the consumption of oxygen. It is also confirmed that the decrease of highest temperature of flame in the reformer and the raise of the syngas H2/CO mole ratio can be observed with the increase of the steam/NG mole ratio.

A reduced-order model (ROM) is considered a promising solution for engineering simulation of a gasifier. In this study, a ROM of a commercial-scale opposed multiburner gasifier is developed based on a reactor network model (RNM). The RNM blocking for this gasifier is established and validated based on analysis of the gasifier flow field. The particle flow in the gasifier is characterized by the particle residence time in each reactor of the RNM. The random pore model and the “effective factor” method are employed to model the char gasification rate under high pressure and temperature. The interphase heat transfer and heat loss through the refractory wall are calculated. In model validation, the simulation results show well agreement with the industrial data. The model provides distributions of the gas temperature and compositions in the gasifier. Effects of the particle size on the particle temperature and carbon conversion are discussed quantitatively. It is observed that fine particles can be completely converted in the jet zone, while the large ones (>100 μm) are converted mainly in the impinging zone and impinging flow zone.

An experimental study on co-gasification of coal liquefaction residue and petroleum coke in carbon dioxide was investigated by thermogravimetric analysis. The temperature of the experiment was 1173–1323 K, and the isothermal (1273 K) kinetics were compared to evaluate the effect of loading of coal liquefaction residue on the gasification reactivity of petroleum coke. The gasification reactivity, X-ray diffraction, scanning electron microscopy images, and Brunauer–Emmett–Teller specific surface area were investigated. The results confirm that the gasification reactivity of petroleum coke was improved greatly by the catalytic components in coal liquefaction residue. The catalytic effect of the catalytic components in coal liquefaction residue was influenced by the temperature and loading. Under the condition of reaction kinetic control, higher temperature promoted the catalytic effect of coal liquefaction residue, and the catalytic effect also increased with the loading.

A non-catalytic POX of oven gas is proposed to solve the problem of secondary pollution due to solid wastes produced from the great amount of organic sulfur contained in oven gas in the traditional catalytic partial oxidation (POX) process. A study of the measurement of flow field and a thermodynamic analysis of the process characteristics were conducted. Results show that there exist a jet-flow region, a recirculation-flow region, a tube-flow region, and three corresponding reaction zones in the non-catalytic POX reformer. The combustion of oven gas occurs mainly in the jet-flow region, while the reformation of oven gas occurs mainly in the other two regions. Soot would not be formed by CH4 cracking at above 1200°C. Since there are very little C2+ hydrocarbons in oven gas, the soot produced would be very tiny, even if they underwent cracking reaction. The integrated model for entrained bed gasification process was applied to simulate a non-catalytic POX reformer. It indicated that the proper oxygen-to-oven gas ratio is 0.22–0.28 at different pressures in the oven gas reformation process.

On a laboratory-scale testing platform of impinging entrained-flow gasifier with two opposed burners, the detailed measurements of gas concentration distribution have been performed for carbonaceous compound (diesel oil) at atmospheric pressure. Under the condition of 1.48–2.36 O/C ratios (kg/kg), radial gas samples are collected at three axial positions and the syngas exit position with stainless steel water-cooled probes, the concentration distribution of the major gases (H2, CO, CO2, CH4 and O2) under stable operating state was determined with a mass spectrometry. These data are used to clarify mixing and reaction characteristics within the reactor, to give insight into the combustion process and provide a database for evaluating predictive mathematical models.

The effect of mechanochemical treatment during the grinding of petroleum coke on its gasification by CO2 was studied. An additive derived by drying the black liquor in papermaking industry is adopted in grinding process. Results show that the gasification reactivity of petroleum coke is effectively improved by grinding, and the activation by wet grinding is more noticeable than that by dry grinding. Besides, by wet grinding petroleum coke and additive together, the active metal species in additive are not easily volatilized in gasification, and retain a high catalytic reactivity to the coke–CO2 reaction throughout most of the conversion range. Changes in crystal structure of the petroleum coke induced by mechanochemical treatment is related to its gasification reactivity. In general, the crystalline–amorphous phase transition is the tendency of long time mechanical grinding, while a crystal structure re-formation stage is observed after wet grinding of petroleum coke with and without additive for some time. Similar phenomenon has also been found in the reported data, but not given attention. Some discussion is made in the paper, and more work should be undertaken to disclose the mechanism.

The features of the opposed multi-burner (OMB) gasification technology, the method and process of the research, and the operation results of a pilot plant and demonstration plants have been introduced. The operation results of the demonstration plants show that when Beisu coal was used as feedstock, the OMB CWS gasification process at Yankuang Cathy Coal Co. Ltd had a higher carbon conversion of 3%, a lower specific oxygen consumption of about 8%, and a lower specific carbon consumption of 2%–3% than that of Texaco CWS gasification at the Lunan Fertilizer Plant. When Shenfu coal was used as feedstock, the OMB CWS gasification process at Hua-lu Heng-sheng Chemical Co. Ltd had a higher carbon conversion of more than 3%, a lower specific oxygen consumption of about 2%, and a lower specific coal consumption of about 8% than that of the Texaco CWS gasification process at Shanghai Coking & Chemical Corporation. The OMB CWS gasification technology is proven by industrial experience to have a high product yield, low oxygen and coal consumption and robust and safe operation.

Model compound pyridine was added to diesel oil to simulate the existence of coal-N. A laboratory opposed multi-burner (OMB) gasifier was used to investigate the axial and radial concentration distribution of HCN, NH3, NO and N2. The maximum concentrations of nitrogen pollutants (HCN, NH3 and NO) are produced at the nozzle plane and these concentrations decrease away from the plane. Moreover, N2 participates in gasification and the order of concentrations of nitrogen compounds in exit is N2>HCN>NH3>NO. NO and N2 increase as O2/fuel ratio increases. HCN and NH3 reach maximum when O2/fuel ratio is 1.3, but largely decrease at 1.7 and 0.9. Flow field distribution make radial concentration profiles uniform toward the exit region and low near the side-wall in other axial position. The steam addition leads to increased formation of HCN and NH3, whereas decreased formation of NO. The results may be useful for the further investigation of coal-N conversion mechanism.

•Rapid pyrolysis of different residence time was carried out in a special DTF.•Fragmental behavior of coals and biomasses was observed.•Viscous flow model was used in calculate the driving force of fragmentation.•Ohm principle was employed to analyse the fragmenting process.In order to study the primary fragmentation behavior of coals and biomasses, experiments of rapid pyrolysis were carried out. This work focused on the devolatilization and fragmentation characteristics including the solid/gas yield, particle density/morphology, particle size and fragmental probability (Sf). The effects of temperature, time and solid property were investigated. The viscous flow model was employed to characterize the pressure difference (ΔP), which was considered as the driving force of diffusion and fragmentation. The Ohm principle was used to establish the linear relation of devolatilization rate and fragmentation rate. The result showed that temperature and time have positive contribution to the fragmentation. The occurrence of fragmentation was observed more apparently with the decreasing of the ash content in the biomass. The pressure difference has a positive correlation with the fragmental rate, which shows the validity of application Ohm principle in the prediction of fragmenting process.Download high-res image (75KB)Download full-size image

•MILD combustion is studied in a natural gas partial oxidation gasifier.•MILD combustion is achieved in an inverse diffusion configuration.•MILD combustion is established due to the dilution of oxygen before ignition.•A barrier stream between oxygen and syngas is necessary for realizing MILD combustion in gasifiers.Moderate and intense low-oxygen dilution (MILD) is a promising combustion technology for the non-catalytic partial oxidation (POX) process of natural gas. This study investigates the establishment of MILD combustion in a bench-scale non-catalytic partial oxidation gasifier. Experiments under both inverse diffusion configuration (IDC) and normal diffusion configuration (NDC) are carried out using methane and pure oxygen as reactants. The flame appearances and temperature profiles are recorded using an imaging system and thermocouples, respectively. Results show that no visible flame can be observed in IDC case while a flame can be observed near the burner exit in NDC case. The effects of jet velocity and O2/CH4 ratio on the IDC flame are further studied. Numerical simulations of the experimental cases are also performed and the results agree well with the experimental results. It is found that the recirculation rate inside the furnace is sufficiently high to fully dilute the reactants. Combustion regime classification on grid scale shows that MILD combustion is established in the gasifier in IDC case. Analysis on the flame structure in IDC case shows that dilution of oxygen before ignition is the reason for the establishment of MILD combustion and the barrier effect of fuel between oxygen and hot syngas in the necessary condition for this process.