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The effects of particle grinding on the burnout and surface chemistry of coals in a drop tube furnace
Abstract
Grinding coals to a pulverised coal specification for blast furnace injection can be costly, which is why some iron manufacturers choose a larger granulated coal size specification. However, there is a concern that these coals may have lower burnout in the raceway region so there is a technical and economic balance with coal grinding. This paper investigates how the process of grinding alters the physical properties, plus the surface chemistry, of coals and their chars formed in a drop tube furnace; it was found that in many cases the larger particle size coals gave improved combustion burnout compared to smaller sizes. The physical properties of the chars, formed from grinding coals to different sizes, resulted in char swelling in the smaller particle sizes, compared to char fragmentation for the larger size classifications. Minerals phases associated with better coal reactivity were found to undergo higher conversion to other chemical forms with the larger size coals, suggesting a potential catalytic or synergistic contribution to their burnout. A closer look at the surface chemistry suggests that the action of grinding coals has an important effect on the surface chemistry. The XPS spectra of the chars, formed in a drop tube furnace, indicated that grinding the coals to a smaller particle size reduced the carbon-oxygen and carbon-mineral interactions compared to the larger sizes and correlated with the higher burnouts. An increasing trend was identified for the carbon sp2 bonding with larger size and higher rank coals which correlated with their burnout at low carbon conversions; however, this did not hold at higher conversions, suggesting other factors were more dominant. © 2015 Elsevier Ltd.
... Under certain conditions, coal is prone to physical changes including swelling, fragmentation, and agglomeration [15][16][17][18][19] all of which will go on to impact particle reactivity. Upon the initial heating of a coal particle, caking coals are prone to developing plasticity, often occurring simultaneously with devolatilisation. ...
... In order to simulate the injection of coal into the blast furnace, a drop tube furnace (detailed in [15,33] and shown in Fig. 1) was used. The drop tube furnace utilises high heating rates (10 4 -10 5°C /s) and short residence times (35 ms-700 ms) that can be adjusted to resemble the initial blast furnace hot blast and raceway environments. ...
... This is not likely to be a chemical effect (due to precautions taken in the grinding method), rather an impact of increased numbers of individual particles being injected at a given time. Additionally, the violent fragmentation effect that occurs during injection of larger granulated coals [15] will serve to separate individual particles during injection thus disrupting particle combinations. ...
Blast furnace iron manufacturers aim to reduce expensive coke usage through the injection of coal. This paper investigates contrasting agglomeration behaviour with a view towards optimising blast furnace operations and limiting furnace permeability issues.
A drop tube furnace (DTF) was used to investigate the performance of two coal particle size specifications that were representative of injection coal sizes: pulverised (100% < 300 μm, 50% < 75 μm), and granulated (100% < 1 mm, 50% < 250 μm). A range of coals was subjected to DTF testing with issues arising from the injection of caking coals. Results show these coals exhibit signs of agglomeration, a potentially problematic effect concerning blast furnace permeability. Considering gasification reactivity upon leaving the blast furnace raceway, it was found that the agglomerated coal chars do not suffer from poor reactivity and are more reactive than the non-agglomerated chars. Pre-treatment through oxidation was found to be an effective means of eliminating agglomeration in the DTF as a result of the reduction in caking properties.
... The effect of coal particle size on PCI process was investigated by Hutny et al. [13], who found lower burnouts for granular coals compared to pulverized samples. Conversely, Steer et al. [14,15] and Vamvuka et al. [11] observed that in some cases, larger particle size coals can improve combustion due to fragmentation of particles. The injection of granular coals is desirable since it could increase the mill production capacity, reduce problems related to the pneumatic transport of coal to the tuyeres and reduce the energy consumption and maintenance costs in the grinding unit [16]. ...
... The intermediate particle size (sample B2), however, yielded burnouts similar to the finest sample (B1) (both samples showed burnouts in the order of 63%). Such improvement on coal combustion could be attributed to the fragmentation of the larger particles, induced by high pressure and temperature gradients inside these particles and could generate smaller and more reactive fragments [11,14]. Since these are preliminary results in this work, such phenomenon must be better investigated with a greater range of coal rank and varied combustion conditions in the injection rig. ...
The effect of the coal volatile matter content and particle size has been investigated in a new lab-scale pulverized coal injection rig (PCI rig) in terms of combustion efficiency and pressure variation. Two coals typically used for blast furnace injection (a high and a low volatile bituminous coal) and their blends experienced combustion under pressurized conditions and extremely high heating rates and short residence times such as those experienced by coal particles in industrial process. Combustion tests were conducted for a low volatile coal prepared in the particle size ranges of 25–75 µm, 105–250 µm and 250–500 µm. Burnouts were lower for the larger particle size sample, but the intermediate particle size sample (105–250 µm) yielded similar conversion to that of the finer sample. The burnouts of the high and low volatile coal, as well as those of their blends were proportional to the volatile matter content of samples in the test conditions. The measurement of pressure variation in the reactor chamber indicated a displacement in the beginning of reactions to longer times as larger was the particle size of coal. The high volatile coal reached the maximum pressure variation earlier than the low volatile one and the combustion of this coal in the blends may have anticipated the reaction of the low volatile coal portion in the blends. Keywords: Blast furnace injection, Coal combustion, Lab-scale combustion test facility, Combustion pressure
... Recent results suggest that even the coal preparation process, e.g. milling or grinding, affects the coal conversion process by changing surface chemistry and mineral phases [5]. ...
... Effective diffusion coefficient of O 2 in air versus temperature and for different pressures according to Equation(5). Particle properties were assumed as e = 0.5, t = 0.6, and d pore = 10 −5 . ...
Identifying coals suitable for blast furnace injection has become increasingly important due to rising injection rates. This review of traditional pulverised coal reactivity testing equipment reveals that no agreed-upon evaluation standard exists and that different reactor types are employed for testing. Therefore, reference blast furnace conversion conditions are defined, followed by a discussion of their influence on the coal conversion process as illustrated by conceptual conversion models. Critical process parameters are temperature, heating rate and pressure, while other effects can be calibrated. Evaluating the currently employed test equipment with regard to these process parameters shows that only specially designed drop-tube furnaces and flow reactors provide conversion conditions near to blast furnace conditions. For consistent injection coal testing, special reactors complying with the previously defined critical process parameters must be established.
... In summary, the current literature base indicates that the assumption of equal emissivity across the spectral and temperature range as assumed in standard CFD codes [3,8,9] is not appropriate. Burn out effects, investigated in [15] , e.g. the reduction in carbon content with simultaneous enrichment of ash at the particle surface [18], changes of the porosity [19,20] and the carbon struc- ture [21,22] are also known to be influencing factors on the particle emissivity. Itaya et al. investigated the radiation properties of fine char particle clouds. ...
... Solomon et al. found typical values of 0.4–1.0 at 782 K. Table 4gives an overview of the used reference measurements for Fig. 13. The initial results presented here represent char properties in the early char burn out phase, as is indicated in Table 3. Potential burn out effects, which contain reduction in carbon content with simultaneous enrichment of ash at the particle surface [18], changes of the porosity [19,20] and the carbon structure [21,22] are not measured at high temperature conditions yet, which might be relevant for char particles in later conversion stages. Former investigations have shown, that particle emissivity changes with conversion [15], as the carbon-to-ash ratio and also the particle structure (size and porosity) are known to change with increasing conversion. ...
... Burn out effects, investigated in [15], e.g. the reduction in carbon content with simultaneous enrichment of ash at the particle surface [18], changes of the porosity [19,20] and the carbon structure [21,22] are also known to be influencing factors on the particle emissivity. Itaya et al. investigated the radiation properties of fine char particle clouds. ...
... The initial results presented here represent char properties in the early char burn out phase, as is indicated in Table 3. Potential burn out effects, which contain reduction in carbon content with simultaneous enrichment of ash at the particle surface [18], changes of the porosity [19,20] and the carbon structure [21,22] are not measured at high temperature conditions yet, which might be relevant for char particles in later conversion stages. Former investigations have shown, that particle emissivity changes with conversion [15], as the carbon-to-ash ratio and also the particle structure (size and porosity) are known to change with increasing conversion. ...
A newly designed experimental setup for the ‘‘in-flight” measurement of the emissivity of burning char particles in the spectral range from 1.25 µm to 5.5 µm is presented. Single coal particles (Colombian bituminous coal and Rhenish lignite) were burned in a flat flame burner under oxy-fuel conditions. As the spectral region from 850 nm to 2.5 µm is dominant for the radiative heat flux at temperatures typical for pulverized fuel combustion, the emissivity is measured spectrally resolved in this region by a fiber spectrometer. Additionally the total emissivity in the range from 2.4 µm to 5.5 µm is measured by an InSb-detector. As particle temperature and diameter are necessary for the experimental determination of the emissivity, two-color pyrometry with simultaneous particle size measurement was carried out in the visible wave length range. There are differences between the emissivities of both coal chars. These initial results represent the first emissivity measurements of pulverized coal char particles under combustion conditions.
... The heat output of a fuel is most closely correlated to its fixed carbon content, and its intrinsic combustion reactivity is mainly influenced by its volatile content, particle size, pore structure, and carbon chemical structure under the same process conditions. [25][26][27][28][29][30][31] Therefore, research on these properties of semi-coke is essential to evaluate its impacts on the smelting process. ...
Semi-coke is a product of low-temperature pyrolysis by low-rank coal, with a composition similar to that of anthracite for pulverized coal injection (PCI). Herein, we investigated the differences in grindability and combustibility between semi-coke and anthracite, analyzed the compositional and microstructural characteristics related to the performance of semi-coke, and assessed the impacts on grinding efficiency and blast furnace operation after replacing anthracite for injection with two types of semi-coke. Semi-coke is rich in high-hardness quartz that is tightly bound to the carbon matrix, making the semi-coke particles very hard, with a high Hardgrove grindability index (HGI) and high abrasion index. The addition of semi-coke reduced the grinding efficiency of the mill and afforded large-sized milled particles. The developed pore structure of semi-coke can enhance kinetic diffusion, and semi-coke is less ordered than coal, thereby providing more reactive sites for combustion reactions. These two reasons cause the ignition temperature of semi-coke to be significantly lower than that of coal. The addition of semi-coke increased the PCI ratio, decreased the fuel ratio, improved the permeability of the blast furnace, decreased the sulfur content in pig iron and carbon content in blast furnace dust. The difference in grinding productivity between semi-coke and coal widens as grinding time increases, suggesting that the HGI method may overestimate the actual grindability of semi-coke. The feasibility of reducing the grinding energy by optimizing particle sizes of semi-coke and improving the grindability of semi-coke by using selected pyrolytic coal and adjusting the pyrolysis temperature was proposed.
... Proximate analysis of the samples was performed according to ASTM standard methods D3172a, E871 and E872. Mass loss of a sample during the pyrolysis process was determined via the ash-tracer method [10,11]. ...
The evaluation of the pyrolysis behaviour of solid fuels is commonly performed by use of thermogravimetric analysis (TGA) instruments. However, most TGA instruments simulate packed bed slow pyrolysis. The results generated by use of such instruments might need further mathematical manipulation in order to be applied in the design and operation of continuous biomass pyrolysis reactors that are being developed nowadays. The purpose of this study is to evaluate the pyrolysis behaviour of algal biomass by use of a laboratory scale rotary kiln pyrolyser. This study managed to show that it is possible to study the pyrolysis behaviour of algal biomass in a rotary kiln reactor. The main decomposition temperature of algal biomass was found to be in the temperature range 200-500 ᵒC.
... The grindability parameter can be measured with a standard laboratory apparatus known as Hardgrove Grindability Index (HGI) machine, which offers a measure of relative grindability or ease of pulverization of coal compared to standard coals. Most of the pulverization operations are designed so that 70% of the output coal passes through a 75-µm sieve (Mesh 200) (Steer et al. 2015). Thus, the HGI apparatus is standardized similarly to grind coal sample in 60 revolutions, and the final products are sieved through a 75-µm for the determination of HGI value (Sahoo et al., 2011). ...
Low-grade coals are blended with high-quality coals to meet economic, environmental, and quality specifications. Hence, the grindability and abrasiveness of coal blends are crucial economic and operational parameters. This work evaluates, analyzes, and predicts the grindability and abrasive behavior of coal blends. Three binary coal blends with common low-grade coal were first prepared at various ratios. Blends 1 and 2 were composed of identical and similar ranks, whereas Blend 3 was composed of different ranks. The blends were analyzed using proximate, ultimate analyzers, and a Bomb calorimeter. The grindability and abrasive behavior of the blends were measured using Hardgrove grindability index (HGI) and Yancey, Geer, and Price methods, respectively. Further, the coarser (+75μm) and finer (−75μm) fractions of HGI experiment were characterized using proximate, ultimate and heating value analyses. The additivity of HGI values was observed for Blend 1 and Blend 2, whereas, the non-additive behavior was observed in Blend 3. Further, the blends’ mineral matter contents and abrasiveness index were found to be additive. Several existing models were found to be inaccurate for HGI predictions. Therefore, a new cross-validated model using multi-linear regression was proposed. The model exhibited better HGI predictions of coal blends with a coefficient of determination R² = 0.9416.
... Chars from DTF1 were collected and analysed for proximate and ultimate analysis. The burnout (%) is calculated using the ash tracer method, which assumes no loss of metals and ash species through volatilisation, as given in Eq. (3) [36]. Nitrogen partitioning was calculated from the char yields and N-contents of fuels and chars. ...
This work investigated the effects of different ash co-products on the combustion of solid fuels, in particular the fuel-nitrogen behaviour: The fuel-ash additive combinations investigated were: Firstly, biomass ashes added to bituminous coals, representative of those used in power stations; Secondly, a low reactivity coal; Thirdly, a high-N biomass (olive cake) was chosen as a high reactivity fuel and studied with a power-station pulverised coal fly ash as an additive. These five solid fuels have a wide fuel ratio FR (i.e. the ratio of fixed carbon to volatile matter content). The ash additives were a pulverised fly ash (PFA) and a furnace bottom ash (FBA) from wood pellet combustion in a UK power station. Fuels (with and without additives) were studied for nitrogen partitioning during (i) devolatilisation and for (ii) NOX formation during combustion, using two different electrically heated drop tube furnaces (DTF) operating at 1373 K. Devolatilisation was also studied via ballistic-heated thermogravimetric analysis (TGA). The extent of impact of additives on volatile yield under devolatilisation conditions was dependent on fuel ratio, high FR has the greatest increase in volatile release when co-feeding the additive. Under devolatilisation conditions, there is a correlation between volatile nitrogen and carbon conversion for all the fuels tested. Thus, additives liberate more volatile-nitrogen from the coals and also deliver enhanced carbon conversion. A mechanism is proposed whereby ultra-fine particles and vapours of reactive compounds from the additives interact with the reacting fuel/char particle and influence N-release during both devolatilisation and char burn-out. The enhanced conversion of fuel-nitrogen to volatile-nitrogen and the reduction of char-nitrogen can lead to reductions of NOX emissions in emissions-controlled furnaces. This approach could assist fuel-flexible power stations in achieving their NOX emission targets.
... Moreover, these authors observed a reduction rate in the burnout performance of 1.7% for flames with fine particles and 0.7% for flames with coarse particles. Steer et al. [3] examined how the process of grinding alters the physical properties, plus the surface chemistry, of coals and their chars formed in a drop tube furnace (DTF) and found that in many cases the larger particle size coals (<1000 lm) gave improved combustion burnout compared to smaller sizes (<106 lm). Ninomiya et al. [4] investigated the effect of the pulverized coal particle size on the particulate matter (PM) emissions in a DTF and concluded that PM emissions increase with a decrease in the coal particle size, and that for coal particle sizes below 63 lm a bimodal mode distribution of PM was formed, in http://dx.doi.org/10.1016/j.enconman.2017.03.012 0196-8904/Ó 2017 Elsevier Ltd. ...
Milling and grinding biomass fuels for pulverized combustion in industrial furnaces can be very expensive. This study aims to evaluate the effect of the particle size on the burnout and emissions of particulate matter from the combustion of agricultural residues (wheat straw and rice husk) in a drop tube furnace. Initially, both agricultural residues were crushed and sieved below 1 mm and the resulting particle size distributions formed one size class here named <1000 μm. In addition to this wide size class, three narrow particle size classes were prepared for each residue; namely, size classes 100–200 µm, 400–600 µm and 800–1000 µm. Subsequently, all size classes of both agricultural residues were burnt in a drop tube furnace at 1100 °C. The data reported include profiles of temperature, particle burnout and particulate matter concentration and size distribution measured along the drop tube furnace. The main conclusions from this study are: (i) for both agricultural residues the size class 100–200 µm presents the highest burnout values, followed by the size classes 400–600 µm and 800–1000 µm; (ii) the burnout values for the rice husk are higher than those for the wheat straw, and the total particulate matter emissions are rather similar for both agricultural residues, regardless of the size class; (iii) during the last stages of the combustion of the wheat straw occur particle fragmentation and the size classes 400–600 µm and 800–1000 µm are those that most contribute to this phenomenon, but particle fragmentation was not observed during the combustion of the rice husk; (iv) the wheat straw size classes 100–200 µm, 400–600 µm and <1000 µm present a bimodal particulate matter size distribution, while the class size 800–1000 µm presents a unimodal one, but the rice husk size classes show all a unimodal particulate matter size distribution.
... They placed a char sample on a holder which was heated up to 1273 K and they also discovered non-gray behavior. Burnout is expected to influence the emissivity as the reduction of carbon content with simultaneous enrichment of ash at the particle surface [17], changes of the porosity [18,19] and the carbon structure [20,21] take place. Itaya et al. found, among others, a greater extinction efficiency for a higher carbon content investigating the radiation of particle clouds [22]. ...
The investigation of burnout effects on the char particle emissivity in the spectral range from 1.25 to 5.5 micro metre is presented. Single coal particles of a bituminous coal were combusted in a flat flame burner under oxy-fuel conditions (20.1 mol%). The emissivity was determined using a test-rig, which also mea- sures particle temperature and diameter, both being necessary input parameters for single particle emis- sivity, in the visual spectral range. The infrared radiation was measured spectrally resolved with a fiber spectrometer (1.25–2.25 micro metre) and integrated with an InSb detector (2.4–5.5 micro metre). The emissivity decreases clearly with progressing burnout: E.g. at 1255 nm the emissivity decreases from 0.49 to 0.38. The effect is larger in the spectral range from 1.25 to 2.25 mm, but still visible in the longer wave length range.
... Hence, fluidized-bed jet mills are important tools to produce fine particles of scrap tire rubber by ambient or cryogenic grinding. However, it should be noted that the published literature is mainly concerned with the grinding behavior of brittle materials [15][16][17], but very little has been carried out on the kinetics of batching grinding of polymeric materials like tire rubber under various operational processes in a fluidized-bed jet mill. ...
Grinding plays an important role in achieving scrap tires recycling. The batch grinding kinetics of scrap tire rubber particles in a fluidized-bed jet mill was studied based on population balance modeling. The selection and breakage functions were obtained according to the first Kapur function. It is found that these two functions were affected by the operating parameters such as the inlet gas pressure, feed load and grinding temperature. The breakage behaviors of scrap tire rubber particles under different operating conditions in the jet mill were discussed. Furthermore, a cubic function was proposed to predict the relation between the Kapur function and the particle size. The model prediction agrees well with the experimental data obtained under various operational conditions.
... While traditionally coal particles have been used directly to measure the char reactivity in drop-tube reactors (e.g. [14][15][16][17][18][19]), more recent studies have been based on char generated externally [2,6,12,20,21]. ...
As an alternative to direct use in char burnout kinetics experiments, coal and other solid fuels may be devolatilized and converted to char in a separate step in order to eliminate the influence of volatile release and combustion on measured char conversion properties. In this study, the effects of char preparation conditions on char burnout characteristics during pulverized coal combustion are investigated. Untreated particles of a Co- lombian high-volatile bituminous coal aswell as three chars fromthat coal, which were externally produced in three different reactors, were tested in a combustion-driven entrained flow reactor. The reactivities of the chars were quantified as pre-exponential factors for commonly employed single-film burnout models, which were estimated fromopticallymeasured temperatures and sizes of individual burning particles. The results indi- cate that char production with devolatilization under heating rates greater than 2 × 104 K/s yields suitable ma- terials for experimental research on char consumption kinetics. The characteristics of high-heating-rate production methods appear to affect char reactivities only in the early stages of the consumption process (less than 20–25% conversion of the initial char mass).
... The common gasification agents are air, steam, air−steam, and carbon dioxide. Coal-CO 2 gasification has been extensively investigated using thermogravimetric analyzers, 14,15 fluidized bed reactors, 16−19 drop tube furnaces, 20,21 and fixed bed reactors. 22 To the best of our knowledge, molecular level study on the interaction between a char−minerals macromolecule and a CO 2 small molecule has seldom been published so far. ...
The effect of inorganic compounds (e.g. Na2CO3, CaCl2, FeCl3 and H3BO3) on the gasification reactivity was investigated. On the one hand, coal and CO2 gasification experiments were performed using a thermogravimetric analyzer. On the other hand, five models (Na2CO3-char, CaCl2-char, FeCl3-char, char and H3BO3-char) were established in Materials Studio software due to that the char gasification is the rate-determining section. Then, the adsorption mechanism of gasification agent (CO2) in above five models was simulated using the grand canonical Monte Carlo method. After that, diffusion coefficient and adsorption activation energy were estimated. From the results of experiments, the catalytic ability followed the sequence of Na2CO3-coal > CaCl2-coal > FeCl3-coal > coal > H3BO3-coal. By comparing the activation energy of CO2 adsorption in simulation, it may be noted that the sequence of CO2 adsorption ability was the same as the inorganic compound catalytic ability. Therefore, it was inferred that the CO2 adsorption process was the determining step in gasification process.
Pulverised coal injection (PCI) is used in ironmaking to replace expensive and energy intensive coke with coal, reducing overall costs and greenhouse gas emissions. As the coke making process removes some of the sulphur present in coal, the utilisation of PCI results in the admission of greater levels of sulphur into the blast furnace. The increased sulphur levels could potentially lead to an increase in costs and energy usage related to hot metal sulphur removal processes. In an increasingly volatile market, the ability to make use of higher sulphur coals is also of both financial and logistical relevance.
This work aims to produce a more thorough understanding of the transformation of sulphur introduced through PCI into the blast furnace, leading to changes in coal selection, blending, or mitigation efforts.
The volatilisation of sulphur from four coals used in PCI was investigated using a drop tube furnace (DTF) to produce conditions similar to a blast furnace raceway. Chars and flue gases were analysed using a range of techniques, including X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, and wet chemistry methods.
The coal’s burnout was found to be the biggest factor in coal sulphur volatilisation. It was found that a coal’s volatile matter content was a key indicator into the volatility of a coal’s sulphur content; it influences the rate of a coal’s burnout, the availability of hydrogen for the formation of H2S, and the reactivity of the produced chars. Coals with higher volatile matter contents are more likely to volatilise their sulphur component at shorter residence times than coals with lower volatile matter contents. The bonding of volatilised sulphur to nascent char was seen in coals with lower volatile matter contents. The sulphur forms present in the initial coal samples were shown to influence the sulphur forms found in the collected chars.
Soil organic matter (SOM) is considered as a pivotal factor influencing the adsorption of pollutants. However, few prior quantitative investigations of the SOM functional group distribution to the contaminants' fate have been conducted. In this paper, the SOM cluster method based on COSMO-RS theory has been conducted to illustrate the chemical composition variables of SOM that affect the polycyclic aromatic hydrocarbons (PAHs) fate in quantitative terms. In the theoretical simulations, the contributions of carbonyl, carboxyl, aromatic, oxyalkyl and aliphatic groups in SOM to phenanthrene (Phe) and pyrene (Pyr) adsorption are evaluated by calculating the partition coefficients (LogP). The results show that the increase in oxyalkyl content leads to a decrease in LogP. Inversely, carbonyl and carboxyl groups of SOMs positively associated with Phe adsorption. The changes in aromatic and alkyl components have a similar magnitude of influence on LogP. Moreover, the effect of non-carbon-based functional groups in SOM on the Phe partitioning has been examined for the first time. The increase of sulfur and nitrogen content in SOM hinder Phe adsorption, while the rise of phosphorus content promotes the adsorption. In soil adsorption experiments, four natural soils, characterized by X-ray photoelectron spectroscopy (XPS) and Diffuse reflectance infrared Fourier transform (DRIFT), are selected to verify the influence of SOM functional group distribution. Comparing the experimental SOM-water partition coefficient (LogKoc) with the simulation predicted LogP suggests that the COSMO-RS based SOM cluster method can predict PAHs adsorption ability in SOM.
Концентрація напружень в однорідній пластині з круговим отвором, підкріпленим включенням із функціонально-градієнтного матеріалу // Інформаційні технології в металургії та машинобудуванні ІТММ’2021: Матеріали міжнародної науково-технічної конференції (16–18 березня 2021 р., Дніпро). – Дніпро: НМетАУ, 2021. – С. 110–115, DOI: https://doi.org/10.34185/1991-7848.itmm.2021.01.013
Blast furnace coal injection is a vital part of modern ironmaking, reducing the amount of coke reductant required in the process and increasing its efficiency. However the injection of different coals or their blends, into the raceway formed by the hot blast, has technical issues due to the very short particle residence times and the limited availability of oxygen in this region. This makes complete burnout difficult and limits the range of coals suitable for this application, leading to partially burnt chars being carried out of the raceway into the blast furnace shaft and potentially into the off-gas system.
This paper explores the fate of these chars, from a range of different coals, looking at how this influences the selection for injection and the implication of these on the blast furnace. In particular, we have looked beyond the limitations of selecting coals based on proximate analysis alone by examining in more detail other physical and chemical properties and their potential effect on the process. A drop tube furnace (DTF) has been used to synthesise chars in a high heating rate environment, and although burnout and volatile loss values suggest suitability of some coals for blast furnace injection, additional problematic effects have been identified and measured such as char swelling and agglomeration which may impact the gas permeability of the furnace. A TGA/DSC has been used to measure the gasification of chars by the Boudouard reaction and compare the thermal impact of more reactive samples.
While other studies have concentrated on the combustion of injection coals to determine their suitability, this one focuses on the implications of the partially burnt chars formed by incomplete reaction in the raceway.
Char emissivity of burning particles is an important factor for heat transfer calculations in pulverized fuel boilers. As the chemical composition is known to influence the emissivity in general, a coal sample has been prepared by a leaching method to reduce the mineral content. A flat flame burner was used for the combustion of the particles in oxy‐fuel atmosphere, providing boundary conditions comparable to pulverized coal applications. The burnout‐dependent emissivity of the sample was measured in the spectral range from 1.25 to 5.5 µm, and compared with data for an unleached sample of the same coal, indicating that the mineral content has minor effect for the investigated conversion levels, although clear changes in the emissivity show that conversion in general is not negligible.
The measurement and analysis of reaction behavior, kinetics quantification, and mechanism clarification for gas–solid reactions are essential to the research and development of process technologies in fields of chemical engineering, energy production, mineral processing, etc. By far, different approaches and instruments have been applied for gas–solid reaction analysis and measurement, but a recent development appears distinctive because of its use of a micro fluidized bed (MFB) in implementing the so-called isothermal differential analysis of gas–solid reactions at the Institute of Process Engineering, Chinese Academy of Sciences (CAS). Systematic work has been reported on hydrodynamic studies in MFBs and the standardized development as well as its utilization of the so-called micro fluidized bed reaction analyzer (MFBRA) for investigating various particle-involved reactions, such as pyrolysis, combustion, gasification, reduction, catalysis, absorption, etc. The obtained results well demonstrated the distinctiveness of MFBs from the common large-size fluidized beds and the obvious advantages of the MFBRA for reaction analysis in aspects of rapid heating of the reactant sample, minimization of diffusion limitation to the reaction, real-time online capture of isothermal product formation characteristics, etc. It was shown that MFBRA is particularly capable of analyzing rapid complex reactions and providing some exceptional functions, such as steam atmosphere, online particle sampling, and decoupling of complex reactions in series. A series of applications well verified that the MFBRA provides actually a fundamental approach and the kind of instruments that are complementary to those of thermogravimetric analysis (TGA) for gas–solid reaction measurement and analysis.
The properties of the coal-derived char play crucial roles in coal conversion reactions and the formation of air pollutants. The nascent char is highly reactive due to the existence of numerous free radicals, active sites, and organic functional groups on its surface. Here, we showed that a combination of nuclear magnetic resonance spectroscopy (NMR) and X-ray photoelectron spectroscopy (XPS) techniques is an effective and precise way to characterize the occurrence, distribution, and evolution of organic functionalities in coal chars. Using these methods, we explored detailed information about chemical features of superfine pulverized coal chars in different atmospheres, and we also discussed the influence of particle size on the evolutionary behavior of functionalities. Results indicate that, in both N2 and CO2 atmospheres, the content of CO species increases with the reduction in char particle sizes. This increment facilitates the heterogeneous reduction of NOx on char surfaces. The chemisorbed NO is susceptible to being incorporated into chars, and being transformed into pyridine-type nitrogen with the favor of adjacent oxygen-containing groups. Moreover, the significant increment in oxygen-containing groups with the reduction of particle size is further confirmed through ¹³C NMR analysis. It was shown that there is an excellent correlation between estimates derived from XPS and NMR for oxygen configuration. The findings from this work provide some new insights into NOx reduction mechanisms and shed light on the practical application of superfine pulverized coal in the future.
Coal in pulverized form has been utilized to generate heat for boiler operation, most of which is used to run power generation plants. Owing to the issues of the pollutant gas production within the pulverized-coal-fired boiler, the CO, NO, and unburned carbon (UBC)concentrations need to be reduced despite the formation of SO, which is also a potential pollutant. In order to study the pollutant generation mechanism of pulverized coal and predict its contents, the drop-tube furnace (DTF) experiment has been established and used extensively by many researchers worldwide. The DTF experiment itself, including the sample preparation processes, takes a long time to be performed. Therefore, in the present study, DTF modeling using one-dimensional numerical simulation is established for the rapid, good-accuracy prediction of the output gas and UBC concentrations. The DTF model is successfully established and found to provide results with trends similar to those of experimental results. Coal with high reaction rates is observed to have higher NO emissions than that with low reaction rates; this is attributed to a higher peak temperature in the combustion cloud around the coal particles, which leads to thermal NO formation. Suppression in the NO formation rate is observed in low-stoichiometric-ratio combustion, even under high-temperature conditions. The existence of a high CO gas concentration would assist in suppressing the NO formation during the combustion of coal char particles.
Diamond‐like carbon (DLC), a thin amorphous carbon film, has many uses in tribological systems. Exploiting alternative substrates and interlayers can enable the control of the hardness and modulus of the multilayer system and improve wear or friction properties. We used XPS and atomic force microscopy to examine DLC that had been concurrently coated on an epoxy interlayer and a steel substrate by plasma‐enhanced chemical vapour deposition. sp2/sp3 ratios were calculated both by the deconvolution of the XPS C1s line and by the analysis of the C KLL Auger spectrum. Altering the substrate causes changes in the carbon bonding configuration, evident with the same trend through both analysis methods, although with differing absolute values, related to hydrogen and oxygen content. There is significant variation in the microscale surface texture, exhibited by both average roughness values and size and uniformity of surface asperities. This suggests that alteration to the film surface structure is a factor to be considered in addition to interface adhesion, hardness and elastic modulus in investigating substrates and interlayers for tribological coatings. Examination of a DLC film separately produced on a steel substrate, in comparison with that produced concurrently with a DLC coating on epoxy, shows the possibility of effects on the chemistry of the film through transfer of material from adjacent samples within the plasma deposition, related to heating, outgassing or sputtering processes. The possibility of such contamination has implications in coating parameter design and coating of multiple samples with plasma‐enhanced chemical vapour deposition. Copyright © 2012 John Wiley & Sons, Ltd.
Coal burning profiles, as determined by thermogravimetric analysis, depend critically upon test procedures and apparatus design. Kinetic parameters from Arrhenius plots of such profiles cannot readily be related to any specific stage of combustion. Nevertheless some features of the profiles are clearly related to coal properties, in particular maceral composition. It is shown here that a correlation exists between unburnt carbon loss as predicted from high temperature oxidation rates and a characteristic temperature of the thermogravimetric profile. This suggests that burning profiles could provide a valuable, rapid laboratory method of ranking coals in terms of their burnout performance.
In the present work, a detailed spectroscopic investigation of carbon in different crystalline forms from diamond to graphite was carried out. The spectra of photoemission peak, plasmon losses and X-rays-excited C KLL were studied in order to characterize the carbon phase in different samples. In addition, a method for determination of sp(2)/sp(3) ratio from the first derivative of Auger C KLL spectrum was employed. The obtained results indicated that carbon photoemission peak for sp(2) and sp(3) configurations is characterized by the same value of binding energy. The peaks of plasmon losses and Auger KLL transition were more diagnostic than C 1s photoemission peak, permitting to distinguish the sp(2) and sp(3) configurations. For intermediate situation, which is present in diamond-like carbon (DLC), characterized by a mixture of sp(2) and sp(3) states, the spectra were very similar to the carbon-black, whereas single-wall carbon nanotubes (CNT) were containing about 50% sp(2) states.
One of the effective methods of reducing coke consumption is pulverized coal injection. The most important problems encountered in this method are reduced permeability, unburned and high ash content. To select the best coal for injection, suitable tests can be used. Therefore, experiments such as proximate and ultimate analysis, Rock-Eval and combustion tests were performed on four kinds of coals from different mines, including Sarakhs, Sangrood, Karmozd, and Tabas. The results of proximate and ultimate analysis indicated that although the sulfur content and ash content of selected coals were a little high, they were suitable for coal injection. The results of combustion experiments and Rock-Eval tests showed that Karmozd coal was the best one to be injected into blast furnace. The result indicated that the mixing of coals could improve the combustion properties of pulverized coals.
This book is an integrated approach towards the applications of coal (organic) petrology and discusses the role of this science in the field of coal and coal-related topics. Coal petrology needs to be seen as a continuum of organic (macerals) and inorganic (minerals and trace elements) contributions to the total coal structure, with the overprint of coal rank. All this influences the behavior of coal in utilization, the coal by-products, the properties of coal as a reservoir for methane or a sequestration site for carbon dioxide, and the relationships of coal utilization with health and environmental issues. The interaction of coal properties and coal utilization begins at the mine face. The breakage of the coal in mining influences its subsequent beneficiation. Beneficiation is fundamental to the proper combustion of coal and is vital to the preparation of the feedstock for the production of metallurgical coke. An understanding of basic coal properties is important for achieving reductions in trace element emissions and improving the efficiency of combustion and combined-cycle gasification. The production of methane from coal beds is related to the properties of the in situ coal. Similarly, coal bed sequestration of carbon dioxide produced from combustion is dependent on the reservoir properties. Environmental problems accompany coal on its way from the mine to the point of utilization and beyond. Health aspects related with coal mining and coal utilization are also included because, in planning for coal use, it is impossible to separate environmental and health issues from the discussion of coal utilization. The book is aimed at a wide audience, ranging from researchers, lecturers and students to professionals in industry and discusses issues (such as the environmental, and health) that are of concern to the general public as a whole. Key Features: - This book focuses on the applications of coal (organic) petrology to our modern society. - It is an integrated approach to help the reader appreciate the importance of coal quality and coal utilization. Coal composition (macerals, mineral, trace elements) and the overprint of coal rank are treated together. - The book synthesises all the possibilities of the organic petrology as a tool for coal utilization in conventional applications (mining and beneficiation, coal combustion, gasification, liquefaction, carbonization), as a precursor of carbon materials and as a petroleum source and reservoir rock. - The role of applied petrology in the characterization of solid by-products from coal utilization is also discussed. - In addition, this book describes the present status of environmental and health problems linked to coal utilization and the ways in which such problems might be overcome in the future.
The swelling characteristics of coal chars and their influence on the formation of residual ash particles between 1-5 μm were studied. A bituminous coal was classified into they size fractions. They were less than 63 μm, 63-100 μm and 100-200 μm, respectively. They were pyrolyzed and burnt at the temperatures of 1373 K, 1523 K and 1673 K in a laboratory drop tube furnace. The obtained products were analyzed by a laser diffraction particle size analyzer and a scanning electron microscope. The results showed that this bituminous coal was possessed of obvious swelling in pyrolysis due to its high content of vitrinite macerals. The finer the coal particle size was, the greater the relative degree of swelling and the more char cenospheres were formed. Coal chars with greater degree of swelling were easier to fragment during combustion and more residual ash particles between 1-5 μm were produced.
Coal remains an important fossil fuel resource for many nations due to its large remaining resources, relatively low production and processing cost and potential high energy intensity. Certain issues surround its utilisation, however, including emissions of pollutants and growing concern about climate change. The coal handbook: Towards cleaner production Volume 2 explores global coal use in industry. Part one is an introductory section which reviews the social and economic value of coal, emissions from coal utilisation, the handling, impact and utilisation of coal waste, and an exploration of emerging and future issues around industrial coal utilization. Chapters in part two highlight coal resources, production and use in established markets as well as the emerging markets of Brazil, the Russian Federation, India, Indonesia, and China. Part three focuses specifically on coal utilisation in industry. Chapters consider thermal coal utilisation, coal use in iron and steel metallurgy, advances in pulverised fuel technology, and the evaluation of coal for thermal and metallurgical applications. Further chapters explore coal utilisation in the cement and concrete industries, coal gasification and conversion, and value-in-use assessment for thermal and metallurgical coal. A final chapter summarises the anticipated future pathway towards sustainable, long-term coal use, suggesting transitions that will be needed to ensure cleaner utilisation for many decades to come.
The injection of granular coal has proved a successful practice at Scunthrope Works blast furnaces. Over 38,000 tonnes of granular coal has been injected at Queen Mary blast furnace using a pilot plant and over 10,000 tonnes have been injected at Queen Victoria blast furnace in less than three months which included the time of commissioning and Christmas shut down period. Furnace operation has been smooth since the introduction of coal injection at both furnaces, driving is good with hanging and slipping virtually eliminated. Queen Victoria has been operating above previous record output levels as the coal injection was introduced. The granular coal has yielded replacement ratios close to unity which are comparable to those obtained by practices injecting pulverised fuel. The replacement ratios obtained, the rate of flue dust, the carbon content of the flue dust and the sludge and hydrogen content of the top gas indicate that granular coal gassification is completed in the furnace.
Large uncertainties exist in the apparent and intrinsic rate of char combustion. Estimates of the rate of char combustion within the range of uncertainties can result in over designed or under designed boilers. A review (2) has shown that one of the major uncertainties in the rate of char combustion is the catalytic influence of mineral matter. This paper reviews the available information on the catalytic effects of mineral matter and concludes that many coals contain sufficient sodium and calcium to increase the rate of char combustion by a factor of 100 at low temperatures. However, insufficient information is available to assess the influences of catalysis by mineral matter at combustion temperatures.
Coal injection plays an important role to the economic success of ironmaking by substituting a portion of the coke input and improving the blast furnace productivity. Manufacturers are looking at opportunities to increase their coal selection options by using higher proportions of technically challenging lower volatile matter content coals; this paper investigates the kinetics, devolatilisation and burnout of these in granulated coal blends using thermogravimetric analysis (TGA) and a drop tube furnace (DTF).
The combustion behavior of coal depends upon not only the coal characteristics but also the combustion conditions. The current understanding about the combustion of coal char during the extreme conditions of pulverized coal injection into blast furnaces is incomplete. To increase the understanding, this research aims to compare the combustion performance of chars obtained from a drop-tube furnace (DTF) and a pulverized coal injection (PCI) rig, thereby giving an indication of the development of reactivity with the heating rate and combustion temperature. Two coals with similar volatile contents were selected, namely, VH, a vitrinite-rich coal, and VL, an inertinite-rich coal. Chars were prepared from these coals in either a DTF or a PCI rig. Different combustion performances of the two coals were observed in the two combustion test rigs. The high-vitrinite coal exhibited similar burnout in the two rigs, but the high-inertinite coal had significantly lower burnout in the DTF than in the PCI rig. Both the chars produced in the DTF had higher reactivity at temperatures over 750 °C than the corresponding chars produced in the PCI rig. Coal maceral composition, char microstructure, and combustion conditions appear to contribute to this phenomenon, with the testing of additional coals being necessary to clarify influence.
In this investigation, XPS and SEM were used to indicate the changes in surface properties of bituminous coal during natural weathering processes. Natural weathering processes of bituminous coal were conducted on the roof. XPS results showed that the content of C C and C H groups on bituminous coal surface decreased but the content of C-0, C=0, and 0=C-0 groups increased after natural weathering processes. Meanwhile, the contents of Al and Si elements on bituminous coal surface were also increased. SEM results showed that the surface roughness of bituminous coal was increased after natural weathering processes. Natural weathering processes not only changed surface chemical composition of bituminous coal but also changed surface topography of bituminous coal. In addition, natural weathering processes made bituminous coal surface more hydrophilic.
Due to the lack of knowledge regarding the combustion of granular coal injected into a blast furnace, injection characteristics of granular coal were first studied through proximate analysis, element analysis, and research of explosivity, ignition point, meltability of ash, grindability, calorific value, etc. Using a sampling device in the raceway combined with petrographic analysis, during the combustion process of granular coal with high crystal water and volatile in raceway, cracks and bursts were found, leading to a reduction of particle size. Based on a model of mass control and dynamic theory of particle combustion, the transition dynamic model for cracking in combustion of granular coal was found, and the critical value of cracking ratio (ΩP) for granular coal combustion in the raceway was calculated. Finally, the utilization ratio and energy efficiency of granular coal used in the blast furnace were discussed, offering theoretical foundation and technical support for intensifying granular coal combustion and promoting granular coal injection.
This paper describes the practical combustion issues encountered with biomass co-firing on a large scale trial in a 500 MW down fired utility boiler at Aberthaw power station. It also investigates and discusses the effect of biomass particle size and physical properties on devolatilisation; flame stability; and slagging by using the biomass energy crop miscanthus.During large scale biomass co-firing, the air flow around the injectors plays an important part in the fuel combustion and stable boiler operation. Secondary air flow at the point of biomass injection was adjusted during the experiments to provide higher airflows around the biomass injectors, hence the mixing, and air available for biomass combustion, was improved producing less unburnt material in the ash and reducing potential filter blockages. However, higher biomass air flows disrupt the temperature distribution in the boiler causing a wider variation of superheater temperatures compared to the baseline condition firing coal only. This reduces the combustion efficiency and can lead to localised hot spots, in the superheaters, with the potential to cause equipment damage. The trials indicated during the experiments that a secondary biomass air flow of 50% of its maximum, gave an optimum balance between air/biomass mixing and variations in the superheater temperatures.
Victorian brown coal represents a significant resource for power generation in Australia. The typical ash content for these coals can be as low as 1% (dry basis). While low ash content is obviously a major advantage for the utilisation of these coals, it is a problem for researchers in that the low ash content leads to large inaccuracies when using the ‘ash tracer’ method to determine coal conversion in flow reactors. To address this problem, a modified ash tracer method has been developed to determine coal conversion which involves adding perlite as an artificial ash to the coal. This method has been successfully applied to measure coal conversion during the pyrolysis of Loy Yang coal in a drop tube furnace.
This paper discusses the demonstration of the British Steel/CPC-Macawber Blast Furnace Granular Coal Injection (BFGCI) technology that was installed on the blast furnaces at Bethlehem Steel's Burns Harbor Plant in Indiana as a highly successful Clean Coal Technology project, cofunded by the U.S. Department of Energy. In the BFGCI process, granular coal (10%–30% through a 200-mesh screen) is injected into a blast furnace as a fuel supplement to decrease coke requirements, thus reducing costs. Tests run to determine the effect of process variables on furnace operations showed that granular coal works as well as pulverized coal and is easier to handle and cheaper to produce because of reduced grinding costs.
Environmental legislation has had significant impact on coal utilization, especially coal combustion for power generation, in limiting emissions of potentially hazardous materials to the environment. For the most part, such emissions derive from the inorganic constituents in coal. However, as such legislation becomes ever more encompassing, it has increased the need to understand better the behavior of the inorganic species in coal processing to ensure, in part, that such legislation is not unduly burdensome. Consequently, it has led to significant development of new models for the behavior of inorganics in coal combustion and a complementary enhancement of many analytical methods for determining inorganics in coal.
Two fractions, representing organic-rich particles and organic particles with included minerals, were separated from each of four Chinese coals of different rank. They were then devolatilized at 900°C in a drop tube furnace (DTF) and a muffle furnace, respectively. The chars were subjected to thermogravimetric analysis, SEM observation and a nitrogen adsorption study. It was found that when three conditions were all met, high mineral content, high volatile content and high heating rate, the included minerals show inhibition on combustion. Otherwise, the included minerals show promotion. Mechanism analysis shows that this is the result of an interaction between the liquid phase and mineral matter during devolatilization. In cases when there is no, or little, liquid phase the included minerals act as pore producer in the coal matrix. Otherwise, the included minerals dilute and absorb the liquid phase, leading to the formation of a char structure with fewer voids and more ordered structure, thus, lower reactivity.
Surface composition of two of pyrite-free coals characterized by a high organic sulphur content subjected to oxidation by peroxyacetic acid, oxygen in 0.5N Na2CO3 aqueous solution and air for 7 days at 125°C were studied. Oxidation process followed the route: diffusion→chemisorption (formation of oxidized surface species)→further oxidation of surface species→gasification. The extent of each stage was dependent on the structure of initial coal. PAA acid was found to be the most efficient oxidizing and desulphurizing agent. The role of pyrite was discussed. The removal of pyrite influenced the results of the oxidation process following it. The oxidation ability of the used media formed a similar sequence (PAA>O2/Na2CO3>air/125) for coal samples with or without FeS2, but the numerical values of several parameters (the surface content of elements C, O, S, N; the type and number of oxygen-or sulphur-containing surface species) differed in both cases. The presence of FeS2 increased oxidation rate but the overall effect depended on the starting amount of oxygen-containing surface species which in turn was influenced by the procedure of pyrite removal.
In the present study, two processes, thermal treatment and oxidation, were separated for a fundamental study of structural evolution during pyrolysis and combustion, as well as for the study of the influence of such evolution on char reactivity. Chars were prepared at different temperatures and heating rates from a size-graded low volatile bituminous coal. The reactivity of resultant chars was measured in Kinetic Regime I using a fixed bed reactor. The structure of fresh and partly burnt chars was characterized using quantitative XRD analysis (QXRDA), high-resolution TEM (HRTEM), high-resolution FESEM, and multi-point gas adsorption.Both QXRDA and HRTEM observations show that char structure becomes more ordered with increasing pyrolysis temperature and decreasing heating rate. Char structure was also investigated as a function of char burnoff. The QXRDA results show that the amorphous concentration of char decreases during combustion while the aromaticity and average crystallite size of char increase. As a result, char structure becomes more ordered during combustion. This is in agreement with HRTEM observations. Due to the low reaction temperature (about 673K), which is much lower than that for char preparation (1473K), it was believed that oxidation, instead of thermal effect, contributed to the structural ordering observed during combustion. The structural parameters obtained from QXRDA were then correlated to char reactivity. Structural ordering was found to be responsible for char deactivation during thermal treatment and oxidation. Since the amorphous concentration and aromaticity of char are two strongest indicators of char reactivity, a structural disorder index, DOI, was defined based on them to describe char structural evolution, and further correlated to char reactivity.
The fragmentation nature of large (1–4mm) granular anthracites and bituminous coal particles was examined experimentally in a high-temperature drop tube furnace. The influence of operating parameters such as furnace temperature, residence time and physical properties (carbon content, moisture content, ash level and volatile matter) of coal on the fragmentation process is reported. The extent of fragmentation increases with furnace temperature and particle size. Fragment counts indicate extensive fragmentation for short residence times. Further, the number of fragments tends to reduce with time for smaller particles, while the number of fragments remains high for larger particles. The differing rates of fragmentation and extinction between the different sized particles are found responsible for this behavior. Essentially, small particles tend to have higher extinction rates at later stages, while small fragments produced at an early stage burn out and are not replaced by further fragmentation of the bigger fragments produced from the initial fragmentation. A model based on the initial failure due to internal thermal stresses induced during heat up is proposed for these large particles. The location and time at which fragmentation occurs is determined theoretically by analysis of stresses within particles with failure occurring when stresses exceed particle-failure strengths. The model provides similar overall trends of fragmentation as found experimentally.
A range of coals from different parts of the world was studied to determine if there were any common relationships that could be determined to gain a clearer understanding of the distribution of coal properties within different particle-sizes. The properties examined were proximate analysis, maceral analysis and %Unreactives from image analysis. Each fraction was also pyrolysed in a drop-tube furnace at 1300°C, 1vol% oxygen and a residence time of 200ms and the resulting chars analysed for morphology using image analysis. There were substantial variations between the particle-size distributions of the different coal samples even though they were ground to the same specification for trials on a combustion rig. Ash distributions showed in all cases that the smallest particle size (−38μm) had either the highest ash level or was very close to it. However, the trends in ash level for increasing particle size showed variations between coals with some coals showing increases in ash towards the larger particles. Fusinite content did not necessarily concentrate in the smallest size fraction, however, liptinite content did increase with particle size. %Unreactives generally increases with particle size and is related to char morphology through an empirical parameter, the ACA [5]. In addition the ACA [5] parameter showed the effect of both particle size and %Unreactives on char morphology and clearly showed the significant influence of particle size on burnout. A parameter such as this could, therefore, be used in burn-out models and further correlated with %Unreactives and particle size.
An experimental apparatus has been developed in order to perform tests of primary fragmentation of solid fuels under severe heating conditions (up to 2200K and 12bar). Particles are laid on the strip and pyrolyzed under inert conditions, fragments are recovered and analyzed by a laser granulometer to assess the fragmentation propensity of the fuel.Experiments have been carried out at temperatures between 1400K and 1900K, heating rate of 5000K/s, pressure in the range 1–12bar. Four different coals have been studied: Gracem, Venezuelan, Omsky, and Kleincopje, classified respectively as anthracite, high and medium volatile bituminous coals.Results show that primary fragmentation at high heating rate and high temperature may result in the formation of relatively coarse fragments and sometimes in a multitude of fines. The probability of fragmentation and the propensity to form coarse versus small fragments varies from coal to coal. For a given coal fragmentation increases monotonously with temperature, whereas the effect of pressure is nonmonotonous.The role of different chemico-physical properties of coals on the pattern and the extent of primary fragmentation is discussed, in particular volatile matter content, ash melting point, rigidity and porosity of the carbon structure and swelling index.
Core-level photoemission spectra of graphite have been measured at very high resolution. The C 1s level is found to have a lifetime width with an upper limit of 165±15meV, which is considerably narrower than previously reported. The width is still substantially larger than that observed for carbon ionic states in small molecules. We argue that the additional broadening in the solid state is not due to a surface core-level shift or initial-state band formation, but may be due to unresolved final-state phonon broadening.
Contributions to the line shape of high-resolution Al Kα x-ray excited carbon 1s photoemission of highly oriented pyrolytic graphite are described and compared with previous work. A two-component system is used to model experimental results obtained from two photoelectron takeoff angles. The splitting between the two components is 0.47±0.02eV, with a variation in the intensity ratio consistent with that expected for the two takeoff angles. The photoemission lines are found to have a lifetime broadening ΓL of 0.128±0.006eV full width at half maximum (FWHM), an asymmetry α of 0.125±0.002, and additional broadening over that of the instrumental resolution of 0.28±0.04eV FWHM.
A Chinese bituminous coal was crushed in a jaw crusher and ground to micrometer size in a planetary ball mill. Three laboratory standard sieves, with a sieve size of 63, 100, and 200 μm, respectively, were used to obtain three coal fractions with different average diameter. The effect of particle size on the proximate composition and combustion properties of these samples was studied by proximate analysis, thermogravimetry (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC). Petrographic analysis showed that the content of vitrinite increased and that of inertinite decreased slightly with decreasing particle size, which was thought to contribute to increasing volatile yield (ad) for fine coal particles. When particle size decreased, the content of fixed carbon decreased and that of ash decreased and then increased. Combustion experiments in a thermobalance revealed that with increasing particle size, the whole burning profile shifted to higher temperatures, resulting in an increase in characteristic temperatures. It implied that finer coal particles exhibited higher reactivity. This was explained from the point of view of maceral enrichment effects, mass transfer effects, and different physical properties of coal particles and the resulting chars due to different particle size. The results also indicated that simultaneous reactions of volatiles and char might happen at low temperatures for small particles.
In a recent study, we discovered that oxygen from the gas phase, organic portions of the coal, and minerals in the coal have profound influence on the formation and desorption of stable surface oxides in the early stages of coal combustion. In an attempt to isolate the effects of minerals, demineralized coals (DMC) are oxidized in O2 with a contact time less than 1 s, and the amount and strength of stable surface oxides are characterized by temperature-programmed desorption (TPD) up to 1650 °C. Young chars derived from both demineralized lignite and bituminous coals show low and flat TPD profiles over a wide temperature range, signifying the minerals’ catalytic activities in forming stable surface oxides for both coals. Indeed, the oxidation rates of chars from both bituminous coals and lignite, estimated based on the O2 concentrations entering and exiting the Al2O3 reactor, were higher than their DMC counterparts. Moreover, graphite, containing no minerals and organically bound oxygen, has an even lower oxidation rate. Similar to those for the raw coals, the combined oxygen balance and elemental analysis of chars from DMC suggests that the oxygen in the organic portion of the lignite activates oxygen turnover and carbon oxidation during its combustion; neither chars from raw nor demineralized bituminous coals possess these properties. X-ray photoelectron spectroscopy (XPS) of raw and demineralized bituminous coals and their char show peaks at around 532.0 eV in the O(1s) difference spectrum, suggesting the possible existence of intercalated stable surface oxides.
Char samples are prepared in a drop tube furnace (DTF) in a nitrogen atmosphere using density-separated coal samples of two Australian coals. The structure and morphology of the char samples are analyzed under a scanning electron microscope (SEM). The porosities of individual chars are determined through image processing of the SEM images. The char particle size distribution (PSD) is analyzed using a Malvern Laser Sizer and through image analysis. The results demonstrate the heterogeneity of the char structures and morphology among the density fractions of the same coal. For both coals, porous chars with a cenospheric structure (Group I chars) are generated in the coal samples of low density (F1.25 and S1.25-F1.30). Solid char structures (Group III chars) are formed from the high density coal samples (S1.35-F1.50 and S1.50). The medium density samples contain different types of chars with a moderate porosity and wall thickness. The weight loss, swelling ratio, and the porosity decrease with increasing parent coal density. Results of size fraction samples suggest that a larger swelling occurs in DTF when the particle size decreases. Mechanisms for the formation of different char structures from different density fractions are discussed, and influences of the presence of ash on char structure formation, in addition to the dominant role of maceral compositions reported in previous works, are identified.
This study examines the effect of char burnout level and calcium content on the intrinsic char oxidation rates and physical properties of three series of chars. Three starting chars were prepared: untreated; acid-washed; Ca-reloaded. They were then oxidized to various levels of conversion. Low-temperature intrinsic oxidation rates were determined for each resulting sample using isothermal TGA. Other measured properties include burnout, N2BET and CO2DP surface areas and CaO surface area. It was shown that intrinsic oxidation rate decreased as burnout increased for the calcium-containing chars. For the acid-washed char (69% Ca removed), the intrinsic rate was independent of burnout. Activation energies for the chars were found to be independent of burnout level and calcium content. The other results are presented.
This study reports on the influence of mineral matter in parent coals on the formation and combustion of chars from these coals. Two Spanish high mineral matter coals were used, San Jose (SJ), 1.85% reflectance, and Santa Barbara (SB), 1.04% reflectance. Sizes of coal particles were 63-125 mu m (SJ) and 36-75 mu m (SB). These coal particles were separated, using a float-sink procedure, into six fractions with densities ranging from 1.35 to 1.85 g cm(-3). Ash contents within the particles range from 3.5 to 23 wt %. Combustion chars of intermediate burn-off were prepared from coal particles in a linear flow furnace in an atmosphere of nitrogen and oxygen at 1500 degrees C. Organic and associated mineral matter were characterized using optical microscopy, XRD, FTIR, and SEM-EDX. These chars were used to study the effect of mineral matter on further conversion. Results show that char reactivities at 500 degrees C (thermogravimetry) and combustibilities at 1200 degrees C (entrained flow) increase with increasing mineral matter content of the char, effects being more pronounced at the lower temperature. Surface combustion temperatures of coal particles at 1200 degrees C did not vary with ash content. The increase in reactivity is not attributed to catalytic effects of mineral matter or to differences in internal surface areas of the chars as measured from carbon dioxide isotherms. It is considered that the enhancement in rate is associated with an enhanced macroporosity within chars in association with the decomposed clay mineral illite, having a platelike morphology. The presence of the illite facilitates the formation of parallel wide pores within the char matrix and these enhance mass-transfer of oxygen to the carbon and of reactants from the carbon. The development of such macropores is not detected by the adsorption isotherm.
Diamond-like carbon (DLC) films were deposited on a Si substrate by electrolysis in a methanol solution at ambient pressure and low temperature. The morphology and microstructure of the resulting DLC films were analysed using atomic force microscopy, Raman spectroscopy, Fourier transformation infrared spectrometry, x-ray photoelectron spectroscopy (XPS), and x-ray excited Auger electron spectroscopy (XAES). The surface energy and mechanical properties of the DLC films were examined, and the growth mechanism of the DLC films in liquid phase electro-deposition is discussed as well. The results of the study show that the hydrogenated diamond-like carbon films are smooth and compact. The percentage of sp3 carbon in the DLC films is determined as 55–60%, based on the corresponding XPS and first-derivative XAES spectra of graphite, diamond, and the tested films. The DLC films show low surface free energy, good mechanical properties, excellent friction–reduction and wear-resistance. It is suggested that methanol dissociates to generate the active species of and C2H4 at high voltage applied to the electrode, followed by the generation of the alkyl chain [–CH2–CH2–]n whose C–C and C–H bond lengths and C–C–C and H–C–H bond angles are close to that of diamond. Subsequently, a diamond-like structure was formed by the ordered dehydrogenation of a short-chain [–CH2–CH2–]n in the electrolysis process.
Reactions of pulverized coal injection (PCI) in a blast furnace were simulated using a drop tube furnace (DTF) to investigate the burnout behavior of a number of coals and coal blends. For the coals with the fuel ratio ranging from 1.36 to 6.22, the experimental results indicated that the burnout increased with decreasing the fuel ratio, except for certain coals departing from the general trend. One of the coals with the fuel ratio of 6.22 has shown its merit in combustion, implying that the blending ratio of the coal in PCI operation can be raised for a higher coke replacement ratio. The experiments also suggested that increasing blast temperature was an efficient countermeasure for promoting the combustibility of the injected coals. Higher fuel burnout could be achieved when the particle size of coal was reduced from 60–100 to 100–200 mesh. However, once the size of the tested coals was in the range of 200 and 325 mesh, the burnout could not be improved further, resulting from the agglomeration of fine particles. Considering coal blend reactions, the blending ratio of coals in PCI may be adjusted by the individual coal burnout rather than by the fuel ratio.
This study has been undertaken to determine the extent of structural ordering and thermal deactivation which can occur when coal chars are heated to high temperatures. The chars have been prepared using the captive-sample wire-mesh reactor at temperatures up to 2200°C at very short hold times (500 ms), and the microstructures have been examined using high-resolution transmission electron microscopy (HRTEM). The char structures have been compared to those of natural graphites. Regions within the chars have been observed to show a similar degree of ordering to that present in the graphites. The reactivities for oxidation of the high temperature chars and graphites have been determined by TGA methods and compared to the reactivity of chars prepared at lower temperatures. The structural ordering and reactivity data indicates that thermal deactivation may account for the low reactivity of a proportion of the char present in utility boiler fly ash.
This paper presents an experimental method for studying the fragmentation of coal particles during coal combustion in a fluidized bed and the quantitative fragmentation indexes of 10 typical Chinese coal ranks. The influences of a variety of factors such as the bed temperature, the size of coal particles, the coal rank and the fluidizing medium on the fragmentation index of coal particles are also studied. The research results show that the main reason for the fragmentation of coal particles is the primary fragmentation, and that the volatile matter can drastically influence the degree of fragmentation of coal particles.
X-ray photoelectron spectroscopy (XPS), laser ionisation mass analysis (LIMA) and X-ray diffraction (XRD) have been used to investigate composition, occurrence and association of mineral species on polished, fractured and powder surfaces of an Australian bituminous coal from the Whybrow seam, Saxonvale Colliery, New South Wales. XPS on raw coal samples revealed composition and the likely mineral types present which were concentrated predominantly in different bands of the coal. This was further confirmed using a new rapid and selective LIMA experimental method which is devised and optimised for in situ mineral particle identification. A series of mineral species occurring as fine, distributed inclusions were rapidly identified using LIMA, including quartz, pyrite, and a number of clays and carbonates. In addition, evidence for organic and inorganic sulfur species and some trace elements were also obtained from individual particles. Mineral identifications were confirmed by ashing the samples and analysis using XRD. This work highlights the advantages of using XPS and LIMA as complementary methods for coal mineral study.
Volatiles release and particle formation for two kinds of pulverized coals (a high volatile bituminous coal and a low volatile bituminous coal) in a drop tube furnace are investigated to account for the reactions of pulverized coal injected in blast furnaces. Two different sizes of feed particles are considered; one is 100–200 mesh and the other is 200–325 mesh. By evaluating the R-factor, the devolatilization extent of the larger feed particles is found to be relatively poor. However, the swelling behavior of individual or two agglomerated particles is pronounced, which is conducive to gasification of the chars in blast furnaces. In contrast, for the smaller feed particles, volatiles liberated from the coal particles can be improved in a significant way as a result of the amplified R-factor. This enhancement can facilitate the performance of gas phase combustion. Nevertheless, the residual char particles are characterized by agglomeration, implying that the reaction time of the char particles will be lengthened, thereby increasing the possibility of furnace instability. Double peak distributions in char particle size are observed in some cases. This possibly results from the interaction of the plastic state and the blowing effect at the particle surface. Considering the generation of tiny aerosols composed of soot particles and tar droplets, the results indicate that their production is highly sensitive to the volatile matter and elemental oxygen contained in the coal. Comparing the reactivity of the soot to that of the unburned char, the former is always lower than the latter. Consequently, the lower is the soot formation, the better is the blast furnace stability.
A fresh char was prepared and reacted with oxygen under conditions similar to those prevailing in the raceway region of the blast furnace (BF) during pulverized coal injection (PCI), using a well-characterized drop-tube furnace (DTF). Char combustion under the present conditions was found to be controlled by the combination of pore diffusion and chemical reaction. Both the char density and size gradually decrease with burnoff, while the char surface area increases up to a burnoff of 40 to 50 pct due to the formation of a large amount of meso- and micropores, which were observed by high-resolution field-emission scanning electron microscopy (FESEM) and gas adsorption measurements. Despite the obvious increase in surface area, the char combustion reactivity decreases with burnoff. This is due to the loss of the intrinsic reactivity of char during combustion, as confirmed by fixed-bed (FB) measurements of fresh char and chars partly burnt in a DTF. The structural characterization by quantitative X-ray diffraction analysis (QXRDA) shows that the amorphous concentration (f
am
) of the char decreases during combustion, while the aromaticity (f
ar
) and the average crystallite size (L
002) of the char increase. The char becomes more ordered during combustion, which is in accordance with the observations made using high-resolution transmission electron microscopy (HRTEM). The char structural ordering observed was found to be responsible for the loss of char intrinsic reactivity during combustion. Based on the QXRDA, a char structure model has also been suggested to explain the char structural evolution observed during combustion. The implications of char structural evolution for char combustion during a PCI operation are also discussed.
The influence of carbon structure and mineral matter of three pulverized coals on their char characteristics including reactivity
was studied under a range of combustion conditions in a drop tube furnce (DTF) and thermogravimetric (TGA) furnace for PCI
application. Physical and chemical properties of coals and their combustion derivatives were characterized by automated reflectogram,
X-ray diffraction, scanning electron microscope, and BET N2 adsorption. The QEMSCAN technique was used to characterize the heterogeneous nature of minerals of discrete coal particles.
The TGA char reactivity was related to the proportion of coal particles displaying strong association of calcium/sulfur phases
with carbon matrix to highlight the catalytic influence of minerals on char reactivity at low temperatures. The study suggested
that during DTF combustion tests at 1200 °C, char reaction rates might have been catalyzed by coal minerals, particularly
due to illite and its association with carbon. Under the same combustion conditions, most of the coal minerals did not transform
significantly to slag phases. Coal burnout was found to improve significantly in a combustion temperature range of 1200 °C
to 1500 °C. The improvement of coal burnout with temperature appeared to be influenced by coal properties, particularly as
a function of the chemical nature of minerals, as well as the degree of associations with other minerals. The study implies
that coals with similar mineral compositions might not necessarily reflect similar combustion behavior due to the differences
in their associations with other phases. The study highlighted the significance of the characterization of the heterogeneity
of coal particles including mineral associations for a comprehensive and reliable assessment of the combustion performance
of PCI in an operating blast furnace.
X-ray photoelectron spectroscopy and X-ray excited Auger electron spectroscopy were used to determine some of the properties of diamond like amorphous carbon films deposited using saddle field glow discharge of methane. By applying the chemical shift technique and curve fitting technique to the C1s peak in the X-ray photoelectron spectrum of the film, we were able to determine the sp3/sp2 bonding ratio of the films. The ratio from this method was in agreement (±1%) with that calculated from the X-ray excited Auger electron spectrum of the same film. The method was also applied to a-C:H films doped with different impurities (boron and phosphorus) in amounts varying from 1% to 20%. We found that the sp3/sp2 ratio of the film was dependent on the deposition parameters and on types of impurities and their concentrations.
During the oxidation of porous coal chars the internal surface area can increase as a function of the degree of conversion due to pore growth and the opening up of sealed internal pores or cavities. Consequently, rate expressions for their oxidation are more accurately described in terms of the intrinsic reactivity, where differences in surface area and porosity can be taken into account.The oxidative reactivity of coal chars is complicated by a number of different factors, which are explored in this paper. These include (i) the development of the pore structure during devolatilisation of the coal, (ii) the ash content, its distribution in the carbon matrix and effect on reactivity, (iii) the extent of the H and N functional groups present in the solid matrix, and their interrelation with residual volatile species which are present, (iv) the extent of the graphitic nature of the carbon surface and (v) the active surface area available for reaction.
A study was carried out to determine the effect of pf particle size distribution on coal burnout propensity in a 1 MW pulverised fuel burner. The specific aim of the work was to assess the improvement in combustion performance achievable by retrofitting commercially available high performance static or dynamic classifiers to existing plants. Two coals were used and were selected as representative of extremes in fuel characteristics experienced by coal importing utilities in Europe. Each coal was fired in the unit at a range of grind sizes to determine the overall impact of a variable performance from a mill. The levels of unburnt carbon in the resultant flyashes for the two coals showed significantly different behaviour. For the higher volatile coal, the unburnt carbon was found to be insensitive to grind quality. However, the coarser grinds of the other coal produced significantly lower unburnt carbon than expected when compared with the finest grinds. Generally the results indicate that the installation of improved classification technology, leading to a finer product, will help to lower unburnt carbon levels. Nevertheless, further work will be necessary to establish the levels of diminishing returns for grind size, burnout performance and grind costs.
Angle-resolved X-ray photoelectron spectroscopy (ARXPS) and ion-scattering spectroscopy (ISS) are utilized to investigate the surface composition of BASF type II carbon fibers as a function of progressive exposures to aqueous nitric acid. To the authors' knowledge this is the first such study of carbon fibers to incorporate both of these surface analytical techniques. Overall, the ARXPS and ISS spectra are shown to be highly synergistic and quite consistent with one another. The data indicate that the surface composition of the as-received fibers is heterogeneous and undergoes significant changes upon nitric acid exposure. The net effect of the nitric acid surface treatments appears to be the systematic and preferential formation of carbonyl and carboxyl groups, and the establishment of a more uniform surface composition as a function of depth. It is expected that these data will contribute toward a better understanding of the interphase in carbon fiber composites.
Analysis of hydrothermally-treated and weathered coals by X-ray photoelectron spectroscopy (XPS) was carried out, and the XPS C(1s) and N(1s) spectra obtained were curve-resolved into four peaks (C–C/CC/C–H, C–O, CO, and O–CO) and three peaks (pyridinic-N, pyrrolic-N, and quaternary-N), respectively. Upon hydrothermal treatment, the amount of carbon–oxygen forms decreased; while the ratio of pyridinic-N increased and quaternary-N decreased. On the other hand, some bituminous coals were subjected to natural weathering and laboratory oxidation, which gave opposite results compared to the hydrothermal treatment. The changes in the carbon–oxygen and organic nitrogen forms were discussed in terms of the effect of hydrothermal treatment and weathering (oxidation). Also, the XPS analysis of various kinds of coals (43 SS coals) was carried out, and the amounts of carbon–oxygen and organic nitrogen forms were discussed in terms of coal rank (carbon content).
Surface oxidation of samples from vitrain bands (low mineral content) in two Australian bituminous coals (Greta and Whybrow seams, Sydney Basin, NSW) were studied following thermal treatment in air at temperatures ranging from 20°C to 120°C for periods of up to 370 days. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) studies indicate clear changes in oxygen function group concentration at the surface of coal organic matter under different atmospheric oxidation conditions. The oxidation products observed include hydroxyl groups or ether linkages, carbonyl groups, and carboxyl groups. Increases in oxidation temperature accelerate the process of carboxyl group formation. It is found that the surface concentration of carboxyl groups can be used as an indicator of the extent of oxidation in low sulfur-content bituminous coals. This may be useful in coal preparation research.
The non-isothermal oxidation of coal chars was investigated by conventional thermogravimetric analysis (t.g.a.) over the temperature range 450–650 °C. For chars, produced by flame pyrolysis of pulverized low rank bituminous coals, three different components i.e. vitrinite-char, inertinite-char and soot, can be distinguished from one another by characteristic reactivity differences. Quantitative use of these differences has detected maceral segregation between different sized char fractions, and has demonstrated that vitrinitechar burns faster than inertinite-char, when oxidized by another combustion technique at 1050–1250 °C. Characteristic temperatures obtained from t.g.a. burning profiles of chars correlate well with their oxidation rates measured at these higher temperatures. However, the latter rates cannot be reliably determined by simple extrapolation of Arrhenius plots of the t.g.a. data.
The effect of heat treatment on the structure of an Australian semi-anthracite char was studied in detail in the 850–1150°C temperature range using XRD, HRTEM, and electrical resistivity techniques. It was found that the carbon crystallite size in the char does not change significantly during heat treatment in the temperature range studied, for both the raw coal and its ash-free derivative obtained by acid treatment. However, the fraction of the organized carbon in the raw coal chars, determined by XRD, increased with increase of heat treatment time and temperature, while that for the ash-free coal chars remained almost unchanged. This suggests the occurrence of catalytic ordering during heat treatment, supported by the observation that the electrical resistivity of the raw coal chars decreased with heat treatment, while that of the ash-free coal chars did not vary significantly. Further confirmatory evidence was provided by high resolution transmission electron micrographs depicting well-organized carbon layers surrounding iron particles. It is also found that the fraction of organized carbon does not reach unity, but attains an apparent equilibrium value that increases with increase in temperature, providing an apparent heat of ordering of 71.7 kJ mol−1 in the temperature range studied. Good temperature-independent correlation was found between the electrical resistivity and the organized carbon fraction, indicating that electrical resistivity is indeed structure sensitive. Good correlation was also found between the electrical resistivity and the reactivity of coal char. All these results strongly suggest that the thermal deactivation is the result of a crystallite-perfecting process, which is effectively catalyzed by the inorganic matter in the coal char. Based on kinetic interpretation of the data it is concluded that the process is diffusion controlled, most likely involving transport of iron in the inter-crystallite nanospaces in the temperature range studied. The activation energy of this transport process is found to be very low, at about 11.8 kJ mol−1, which is corroborated by model-free correlation of the temporal variation of organized carbon fraction as well as electrical resistivity data using the superposition method, and is suggestive of surface transport of iron.
To investigate the catalytic effects of minerals on the formation of diamondoids, a series of anhydrous and hydrous pyrolysis experiments were conducted at 340 °C for 72 h on six kerogens of four types (I, II, II-S, and III) in the presence and absence of different minerals (montmorillonite K10, acidic aluminosilicate, kaolinite, illite, CaCO3, CaSO4, and S0) and their mixtures in different ratios. Regardless of the accessibility of water, direct decomposition of kerogen upon thermal stress produces various quantities of diamondoids depending on the type of kerogen. Montmorillonite K10 and acidic aluminosilicate greatly promote the formation of diamondoids at 340 °C during kerogen pyrolysis because they are strong Lewis acids. Rearrangements of polycyclic carbonium ion intermediates at Lewis super-acid sites probably are responsible for the observed relatively large quantities of diamondoids produced at elevated temperatures in the presence of montmorillonite K10 or acidic aluminosilicate. In contrast, CaCO3 appears to inhibit the formation of diamondoids. Kaolinite is a less active catalyst because the yield of diamondoids is only slightly elevated, while no catalytic effect is observed from illite. The presence of elemental sulfur appears to “poison” the catalytic activity of montmorillonite K10, but it is counteracted to some extent by the presence of CaCO3. The yield of diamondoids was slightly improved in the anhydrite catalytic reaction with kerogen in the presence of water. The addition of elemental sulfur to the kerogen of any type may initiate the C–C bond cleavage and is not favorable for diamondoid formation. The yield of diamondoids is very sensitive to the amount of montmorillonite K10 mixed with the kerogen up to a montmorillonite K10/kerogen ratio of ca. 15:1. At higher montmorillonite K10/kerogen ratios (⩾15:1), the accessible Lewis sites at montmorillonite K10 might be completely saturated with the organic precursors of diamondoids from thermal degradation of kerogen.