Thermoplastic properties of coals at elevated pressures: 3. Effects of low-temperature preoxidation and subsequent heat-treatment on behaviour in H2 and He

The thermoplastic properties of a mildly preoxidized Lower Kittanning seam low volatile coal have been examined at elevated pressures of H₂ and He utilizing a high-pressure microdilatometer. It was observed that the maximum swelling parameter (V_s, vol%) of the preoxidized coal was significantly restored at elevated pressures of He. The thermoplastic properties of the preoxidized coal were even further restored at high pressures of H₂. The results indicate that carbonization of this coal at elevated H₂ pressures reduces the effect of preoxidation by removing some of the oxygen introduced during preoxidation and replacing it with reactive donatable hydrogen. It was shown that subsequent heat-treatment of the preoxidized coal at a relatively mild condition (in vacuum at 403 K) results in dramatic reductions in the thermoplastic behaviour of coal when subsequently carbonized at elevated pressures of H₂ or He.     

Coke reactivity: Effect of Fe2O3 and K2CO3 addition to the coal charge before carbonization

Six individual coals and one blended metallurgical coal were used in this investigation, and two additives, Fe₂O₃ (up to 1%) and K₂CO₃ (0.5%), were added to each coal before carbonization. Results showed that the additives strongly increase CRI (coke reactivity index) and decrease CSR (coke strength after reaction) of each coke, but there are no significant changes in coke microstructure.     

Feasibility of recycling KOH in chemical activation of oil‐sands petroleum coke

Although potassium hydroxide (KOH) is known to be effective in generating highly porous activated carbons, the mechanism of KOH activation has not been well elucidated. To develop porosity in carbon, a high KOH/carbon mass ratio must be maintained. Consequently, KOH, as the activating agent, represents a major part of the cost of the activation process. Focusing on the mechanism, particularly the activation products, the present work attempted to establish the technical feasibility of recycling KOH. Experiments revealed that the major products of KOH activation at 600–900°C are metallic K, K₂CO₃, CO and H₂, which is supported by thermodynamic analysis. The overall reaction may be written as 6KOH + 4C = K₂CO₃ + 4K + 3H₂ + 3CO. At temperatures over 900°C, K₂CO₃ becomes unstable and participates in activation reactions with carbon; a more suitable overall reaction would be KOH + C = CO + K + 0.5H₂. As potassium ion is reduced to metallic K which is readily converted into KOH and hydrogen gas upon reacting with water, KOH recycling is feasible. The reuse of KOH in chemical activation could substantially reduce the cost of activation process. © 2011 Canadian Society for Chemical Engineering     

Etch rate studies of base catalyzed hydrolysis of polyimide film. II

The influence of a potassium carbonate (K₂CO₃) additive on the base catalyzed hydrolysis of polyimide (Kapton?) film in aqueous potassium hydroxide (KOH) was determined experimentally. The etch rate is significantly greater for KOH/K₂CO₃ solutions compared with solutions composed of KOH only. The experimental order of the reaction with respect to the KOH concentration was found to be 1.5. In addition, the rate was found to increase linearly with respect to the K₂CO₃ concentration at a fixed KOH concentration. Visual observations of a thinner gel layer on the surface of the film, combined with increased solubility of the etch by-products in KOH/K₂CO₃ relative to KOH, help to explain the difference in etch rate. It appears the ability of K₂CO₃ to enhance the solubility of the hydrolyzed polyimide (polyamic acid salt) results in faster etch rates.     

Effect of alkali metal catalysts on gasification of coal char

A detailed study has been conducted of the effects of LiCl, NaCl, KCl, RbCl, CsCl, KOH, and K₂CO₃ on the steam gasification of char produced from a western sub-bituminous coal. Initial screening of results revealed that K₂CO₃ had the greatest catalytic activity for a fixed cation content in the char. Subsequent experiments were performed to determine the effects of K₂CO₃ loading and gasification temperature on the rate of gasification and the product-gas composition. The results show that gasification rate is enhanced with increasing K₂CO₃ loading and reaction temperature. Increasing K₂CO₃ loading causes CO to be formed in preference to CO₂ and H₂ and suppresses the production of CH₄. Increasing temperature also causes CO to be formed in preference to CO₂ and H₂ but enhances the production of CH₄. These results are discussed in the light of a mechanism to explain the unique catalytic behaviour of K₂CO₃.     

Thermoplastic properties of coal at elevated pressures: 1. Evaluation of a high-pressure microdilatometer

Thermoplastic behaviour of a Pittsburgh seam hvA coal (PSOC1099) was characterized by the use of a high-pressure microdilatometer. Phenomena such as softening, swelling, final resolidification, and the temperatures at which they occur were measured as functions of heating rate (25 ° and 65 °C min^?1), particle size (= 75 μm and 250 × 425 μm), gaseous atmosphere (N₂, H₂, $\text{CO}\text{H}{}_2$) and applied gas pressure (atmospheric to 2.8 M Pa). The results obtained illustrate several important aspects of thermoplastic properties of this coal under the conditions utilized. It is observed that pressure alone can play a major role in determining its overall thermoplastic behaviour. Compared to that at atmospheric pressure, swelling is significantly reduced at 2.8 MPa of pressure for any given heating rate or particle size. In these experiments, the chemical composition of the gaseous atmospheres ($\text{CO}\text{H}{}_2$, H₂ and N₂) does not appear to alter significantly the plastic phenomena at any given pressure. Increasing the heating rate or decreasing the particle size results in increased swelling at all applied pressures and atmospheres.     

The thermoplasticity of coal and the effect of K2CO3 addition in relation to the reactivity of the char in gasification

The thermoplastic properties of a medium-volatile and a high-volatile A bituminous coal have been studied by means of high-pressure dilatometry as a function of the heating rate (10 and 65 K min⁻¹), particle size (< 44 μm, < 75 μm, 106–200 μm and 212–400 μm) and gas pressure (1–28 bar). The thermoplastic properties of the coals are significantly different at elevated pressures from those at atmospheric pressure. At atmospheric pressure the volume swelling increases strongly with increasing heating rate and, at 10 K min⁻¹, with increasing particle size. At a pressure of 28 bar however, the swelling is nearly independent of heating rate and particle size. The effect of addition of K₂CO₃ (20% by weight) was investigated at 65 K min⁻¹ and turned out to depend on the gas pressure and particle size. At atmospheric pressure, K₂CO₃ reduces the dilatation of the coals almost completely. This reduction decreases with increasing pressure, especially for the larger particle size fraction (212–400 μm). A detailed mechanism for the interaction of alkali metal carbonates with the coal is suggested. The softening and swelling of coal particles has consequences for the available and accessible surface area of the char formed and thus for the reactivity of the char in gasification. Results of reactivity measurements in a CO₂ atmosphere in a thermobalance that illustrate this effect are presented and related to the morphology of the char.     

Preparation of porous carbons from petroleum coke by different activation methods

Porous carbons were prepared from Shengli petroleum coke (SPC) and Minxi petroleum coke (MPC) by different activation methods with H₂O, KOH and/or KOH+H₂O as active agents. The porous carbons were characterized by nitrogen adsorption at 77 K. It has been found that activation method and component of petroleum coke, of which different kinds of transitional metals on petroleum coke are crucial for preparing high quality porous carbons. Under the identical experimental conditions, the co-activation with KOH and H₂O as active agents in the same activation process, which has been rarely reported in literature, is the easiest method for the preparation of porous carbons with high surface area. The sequence of active agents in terms of difficulty in the preparation of porous carbons with high surface area is as follows: KOH+H₂O>KOH>H₂O. A drawback of KOH+H₂O activation in the preparation of porous carbon in this work is found to be its low carbon yield in comparison to KOH activation. Compared with the SPC coke, the MPC coke with higher contents of transitional metal and carbon and lower content of nitrogen is more suitable for making high surface area porous carbons, which is believed to be mainly due to the difference in the contents of transitional metals. Porous carbon with surface area around 2500–3000 m²/g and carbon yield about 25–30% has been obtained from MPC coke by KOH+H₂O activation with less KOH and shorter activation time in comparison to the traditional methods.     

Sub- and super-critical carbon dioxide flow behavior in naturally fractured black coal: An experimental study

A proper understanding of super-critical carbon dioxide (CO₂) flow behavior in coal is essential, as CO₂ normally exists in its super-critical state in deep coal seams and studies are lacking. The main objective of this study is to distinguish the permeability behavior of coal for sub-critical and super-critical CO₂ flows. Therefore, a series of triaxial experiments was conducted on naturally fractured black coal specimens. Permeability tests were carried out for 15, 20 and 25 MPa confinements at 33.5 °C temperature. Three test scenarios were conducted to investigate, (1) variation of the permeability behavior of coal with CO₂ phase condition, (2) the swelling effect on sub- and super-critical CO₂ permeability patterns, and (3) the potential of nitrogen (N₂) to reverse CO₂-induced swelling. According to the test results, the permeability of super-critical CO₂ is significantly lower than sub-critical CO₂ due to the higher viscosity and swelling associated with super-critical CO₂. Moreover, at super-critical state there is a higher decline of CO₂ permeability with increasing injecting pressure due to the higher increments in the associated viscosity and swelling. Although CO₂ adsorption-induced swelling causes permeability of both CO₂ and N₂ to be reduced at low injection pressures the poro-elastic effect becomes more dominant and may cause CO₂ permeability to increase for higher injecting pressures, because CO₂ flow behavior may transfer from super-critical to sub-critical after the swelling due to the decline of downstream pressure development. Moreover, N₂ has the potential to reverse some swelling effects due to CO₂ adsorption, and this recovery rate is higher at lower injecting pressures and higher confining pressures.     

Co-carbonization of coking coal with K2CO3: structure of the resulting chars

Oakdale coking coal was co-carbonized with up to 30 wt% of K₂CO₃ to 900 °C. The resulting chars were examined for optical texture and morphology by scanning electron microscopy. No changes in optical texture were observed with additions of < 1 wt% K₂CO₃. Increased additions created an isotropic, non-fusing layer of char around the particles and this prevented the formation of a coherent coke. The size of the remaining anisotropy was also reduced, some char fragments being composed of isotropic carbon. Severe fissuring occurred in the particles of char, causing fragmentation; this was presumably due to diffusion of potassium into the char structure. X-ray studies indicated increased peak half-widths of (002) diffractions for the isotropic carbon.     

Interaction of mineral and inorganic compounds with coal: The effect on caking and swelling properties

Small amounts of certain inorganic compounds are known to modify the behaviour of coal during pyrolysis and gasification. A quantitative study of the effect of additives on dilatometry measurements showed that the alkali metal carbonates (excluding Li) have large effects on the dilatation and this effect was virtually identical for a given number of moles of additive. The effect of various other minerals and inorganic compounds on the caking and swelling properties was also investigated. The effects of a series of sodium salts varied according to the nature of the anion in the following order: Na₂B₄O₇. 10H₂O > NaNO₃ > Na₂O ∼ Na₂CO₃ ∼ NaHCO₃ > NaCl > Na₂SO₄. Possible mechanisms for the interaction of the minerals and inorganic additives with the coal are discussed in detail. The results for the alkali metal carbonates are consistent with the hydroxyl functional groups in the coal reacting to form the corresponding sodium salts.     

Phase Equilibria in Water-Salt Systems Consisting of Potassium,Sodium, and Ammonium Carbonates and Anti-Icing Properties of Carbonate Compositions

Phase equilibria in K₂CO₃–H₂O, Na₂CO₃–H₂O, K₂CO₃–Na₂CO₃–H₂O, K₂CO₃–(NH₄)₂CO₃–H₂O systems under temperatures ranging from 0 down to ?36°C are investigated. The carbonate compositions forming low-temperature eutectics are revealed. Their melting ability with respect to ice under temperatures ?5 and ?10°C is determined. It was found that potassium carbonate is characterized by sufficient anti-icing properties. Potassium carbonate composition activity is determined with respect to metals. Efficient corrosion inhibiters are selected. It was found that potassium carbonate is aggressive with respect to cement concrete. Special protection is necessary, if potassium carbonate is used on cement concrete coatings.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

Quantitative gas evolution kinetics of coal primary pyrolysis at high heating rates is critical for developing predictive coal pyrolysis models. This study aims to investigate the gaseous species evolution kinetics of a low rank coal and a subbituminous coal during pyrolysis at a heating rate of 1000 °C s^− 1 and pressures up to 50 bar using a wire mesh reactor. The main gaseous species, including H₂, CO, CO₂, and light hydrocarbons CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, were quantified using high sensitivity gas chromatography. It was found that the yields of gaseous species increased with increasing pyrolysis temperature up to 1100 °C. The low rank coal generated more CO and CO₂ than the subbituminous coal under similar pyrolysis conditions. Pyrolysis of the low rank coal at 50 bar produced more gas than at atmospheric pressure, especially CO₂, indicating that the tar precursor had undergone thermal cracking during pyrolysis at the elevated pressure.     

Synthesis of polybutadiene nanoparticles by emulsion polymerization: The effect of electrolyte and initiator type on particle size and reaction kinetics

Emulsion polymerization of the butadiene (Bu) was performed in the presence of disproportionate potassium rosinate (DPR) as anionic emulsifier, potassium hydroxide (KOH), and potassium carbonate (K₂CO₃) as electrolytes, and three different initiators including potassium persulfate (KPS), 2,2^′-azobisisobutyronitrile (AIBN) or 4,4′-azobis(4-cyanovaleric acid) (ACVA, also known as VAZO) at 70 °C. Latexes were prepared with a solid content of about 30 wt%. The particle size and its distribution were measured by dynamic light scattering (DLS) analysis, while the polymerization conversion was determined gravimetrically at different time intervals. Results on the emulsion polymerization of Bu in the presence of KOH and K₂CO₃ co-electrolytes showed that adding KOH to the reaction media decreases the polymerization rate. Positive effect of co-electrolytes on the control over polybutadiene latex (PBL) particles size and its distribution was also confirmed, where K₂CO₃ played roles as electrolyte and pH buffer and KOH served double roles as electrolyte and alkaline supplier of the reaction media. Complete solubility of the AIBN in Bu resulted in higher rate of polymerization in the presence of AIBN in comparison to other initiators, i.e., VAZO or KPS. The results showed that initiator type plays a significant role on the formation of PBL nanoparticles and kinetics of the polymerization. The kinetic studies revealed that emulsion polymerization of Bu follows case 1 (i.e., \(\bar{n}\) ?0.5, where \(\bar{n}\) indicates average number of the propagating chains per particle) of the Smith-Ewart kinetics.     

The effect of water on the activation and the CO2 capture capacities of alkali metal-based sorbents

Alkali metal-based sorbents were prepared by the impregnation either of potassium carbonate (K₂CO₃) or of sodium carbonate (Na₂CO₃) on the supports (activated carbon (AC) and Al₂O₃). The CO₂ absorption and regeneration properties were measured in a fixed bed reactor at the low temperature conditions (CO₂ absorption at 60 ‡C and regeneration at 150 °C). The potassium carbonate which was supported on the activated carbon (K₂CO₃/AC) was clarified as a leading sorbent, of which the total CO₂ capture capacity was higher than those of other sorbents. This sorbent was completely regenerated and transformed to its original phase by heating the used sorbent. The activation process before CO₂ absorption needed moisture nitrogen containing 1.3–52 vol% H₂O for 2 hours either at 60 ‡C or at 90 °C. The activation process played an important role in CO₂ absorption, in order to form new active species defined as K₂CO₃· 1.5 H₂O, by X-ray diffraction. It was suggested that the new active species (K₂CO₃·1.5H₂O) could be formed by drying the K₄H₂(CO₃)3·1.5H₂O phase formed after pre-treatment with excess water.     

Dry gas cleaning process by adsorption of H2S into activated cokes in gasification of carbon resources

Gasification of carbon resources including biomass and coal is one of promising energy production technologies. The R&D on effective and convenient gas cleaning processes for removal of contaminants as well as high efficient reliable gasifiers is essential for industrial application in broad fields. In this study, a dry process of synthesis gas cleaning by adsorption of H₂S into activated cokes was proposed as a candidate of desulfurization technologies in gasification. The H₂S adsorption performance of activated coke produced from coal, which are used industrially for de-SO_x and de-NO_x, was evaluated by the thermogravimetric analyses and the adsorption examination in a fixed bed under the atmospheric and high pressures. Activated coke was not only the most active at about 423 K for the H₂S adsorption rate but also regenerative over 573 K by H₂S desorption with a sufficient rate under an inert gas flow of nitrogen. The H₂S adsorption performance of the activated coke was not inhibited by the co-existence of CO₂ or COS but enhanced rather by the co-existence. The adsorbent was promisingly active for both H₂S and COS adsorption as well. These behaviors suggest that the activated coke are available for simultaneous desulfurization of H₂S and COS. The H₂S breakthrough examination in the fixed bed revealed that it was possible to remove H₂S to lower level than 1 ppm for a long time depending on the residence time of gas flow in the bed. When the adsorption operation was carried out under high pressures up to 0.6 MPa, the regeneration of activated coke by H₂S desorption took place under the pressure reduced to the atmosphere. As the results, it was implied that the present activated coke could be applicable to the desulfurization process in coal gasification.     

Catalytic activity of potassium halides in water vapour gasification of graphite

The catalytic activity of potassium halides in water vapour gasification of graphite was studied at 900 ° C and pressures up to 2 MPa. The initial step is the hydrolysis of the potassium halide which controls the catalytic activity: KF >KCl >KBr. Main steps of the catalysed gasification reaction are in good agreement with an oxygen transfer mechanism. The following general reaction scheme is proposed not only for the potassium halides, but also for K₂CO₃ and KNO₃. Initial reactions: K₂CO₃ + H₂O → 2KOH + CO₂; KNO₃ + H₂O → KOH + NO_x; KX + H₂O → KOH + HX. Intermediate step: KOH + C, H₂ → K + CO, H₂O. K-catalysed gasification reactions: K + H₂O → K(O) + H₂; K(O) + C → K + CO; K(O) + CO → K + CO₂.     

Effects of effective stress changes on permeability of latrobe valley brown coal

One of the key issues with geological sequestration of carbon dioxide in coal seams is change of permeability caused by carbon dioxide (CO₂) injection, and especially any resulting reduction in injectivity. Injection causes changes in pressure and effective stress, with further changes caused by coal matrix swelling associated with adsorption of CO₂. In this paper we aim to study how the change in effective stress and coal swelling may influence the gas permeability in brown coal using natural coal and reconstituted coal specimens. Tests were conducted at different confining pressures to represent conditions at different depths. Different gas injection pressures were also employed at each confining stress stage. The test results clearly depicted an exponential reduction of coal permeability to CO₂ gas when effective stress increases. Based on the experimental results, an empirical correlation to represent the effect of stress on permeability was developed. The results also showed that increase in pore pressure can induce further swelling of the coal specimens, and this can lead to further decrease in permeability which can have important impact on field injectivity. Test results for natural brown coal specimens were compared with results of tests on reconstituted coal specimens made from compaction of coal particles obtained from crushing of blocks of natural coal. Observed permeability behaviour of gas in reconstituted coal was similar to the natural coal specimen permeability trend, when effective stress increases.     

Swelling and plastic behavior of coal devolatilized at elevated pressures of air in the absence and presence of calcium oxide

Swelling and plastic properties of an lv bituminous coal (PSOC 1197) devolatilized at elevated air pressures (at 60 or 150 K/min) were monitored using a high-pressure microdilatometer. It was observed that the maximum swelling parameter (V₈) of the coal was markedly reduced when devolatilized at elevated air pressures, provided that the heating rate of the coal was sufficiently low (60 K/min to 923 K). In marked contrast, at a relatively higher heating rate (e.g., 150 K/min), the swelling of coal at elevated air pressures was closer to V_s obtained in N₂ pressures. An implication of this finding is that coal swelling can be very high at actual utilization conditions (rapid heating of coal in air). The reduction in coal V_s at a relatively slow heating rate is attributable, at least in part, to the preoxidation (chemisorption of oxygen to form oxygen crosslinks) of coal during initial heat-treatment in air (before there is significant devolatilization). Devolatilization of coal in the presence of CaO, however, markedly reduced the swelling of coal at various air pressures and heating rates. This reduction in coal swelling is attributed to crosslinking reactions of pyrolyzing coal that have been catalyzed by CaO.     

Effectiveness and mobility of catalysts for gasification of bitumen coke

The compounds K₂CO₃, KCl, Na₂CO₃, CaCO₃, CaO, and MgO were tested as catalysts for steam gasification of coke from oil sands bitumen at atmospheric pressure and 600-800 °C. Catalysts were added to liquid vacuum residue prior to coke formation and directly to the coke solids. K₂CO₃ and Na₂CO₃ were most effective, giving nearly complete conversion at 800 °C in 30 min, regardless of whether they were added before or after coke formation. Ca and Mg compounds did not promote catalytic reactions, nor did they interact physically with the coke based on SEM and EDX analyses. KCl was effective, to a lower extent than K₂CO₃ and Na₂CO₃ only at higher temperatures (800 °C). The alkali metal compounds showed high mobility within the coke phase, which was consistent with their catalytic activities.