Gasification of wood char and effects of intraparticle transport

Kinetic parameters for gasification of hybrid poplar spp. char have been measured. A differential reactor was used to obtain rate data for catalytic and non-catalytic reactions of small wood char particles (1–2 mm in size) at 100 kPa for temperatures in the range 400–700 °C, steam partial pressures between 45–100 kPa, and space velocities in the range 2.0–7.3 s^?1 During pyrolysis of wood without the addition of either K₂CO₃ or Na₂CO₃, the cellular structure of the wood was preserved. Additionally, this cellular structure remained intact during most of the gasification process. Addition of K₂CO₃ and Na₂CO₃ before pyrolysis caused a degradation of the regular cellular structure and an increase in the rate of gasification of the resulting char. Effectiveness factor calculations were made for particles of various sizes and results indicate that diffusion control of the gasification reaction becomes important for particles larger than 0.5 CM.     

Catalytic effects of potassium on lignin steam gasification with <Emphasis Type="Italic">γ</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> as a bed material

The effects of potassium on the reactivity of biomass-char steam gasification with the presence of a porous material were investigated by using a thermogravimetric reactor with high-heating rates. Lignin was employed as a char-rich biomass model compound. The potassium carbonate (K₂CO₃) was added to lignin and a mixture of lignin and γ-Al₂O₃ porous particles by means of aqueous impregnation. The effects of K₂CO₃ and γ-Al₂O₃ addition on pyrolysis of lignin and steam gasification of lignin-derived char were evaluated in terms of lignin conversion and the gaseous products. Results showed that K₂CO₃ slightly increased the steam gasification rate of lignin-derived char, but it did not influence the conversion in both the pyrolysis and steam gasification steps. In addition, tar was reduced by adding K₂CO₃ because of the increment of carbon conversion to gas product. The presence of γ-Al₂O₃ was found to induce the lower reactivity of resulting char after pyrolysis, reducing the gasification rate and conversion. A significant improvement in gasification conversion was observed with the presence of both K₂CO₃ and γ-Al₂O₃. Especially, almost complete gasification was achieved at a reaction temperature of 1,073 K.     

Synthesizing activated carbons from resins by chemical activation with K2CO3

We prepared activated carbons from phenol-formaldehyde (PF) and urea-formaldehyde (UF) resins by chemical activation with K₂CO₃ with impregnation during the synthesis of the resins. The influence of carbonization temperature (773-1173 K) on the pore structure (specific surface area and pore volume) and the temperature range at which K₂CO₃ worked effectively as an activation reagent, were investigated. The specific surface area and micropore volume of PF-AC and UF-AC increased with an increase of carbonization temperature in the range of 773-1173 K. We prepared activated carbon with well-developed micropores from PF, and activated carbon with high specific surface area (>3000 m²/g) and large meso-pore volume from UF. We deduced the activation mechanism with thermogravimetry and X-ray diffraction. In preparing activated carbon from PF, K₂CO₃ was reduced by carbon in the PF char. The carbon was removed as CO gas resulting in increased specific surface area and pore volume above 1000 K. In preparing AC from UF, above 900 K the carbon in UF char was consumed during the K₂CO₃ reduction step.     

Co-carbonization of Ashland A240 petroleum pitch with alkali metal carbonates and hydroxides

Petroleum pitch was co-carbonized with the carbonates and hydroxides of Li, Na, K, Rb and Cs, using 1–15 wt% of the salt. The resultant 1173 K carbons were examined by optical microscopy to assess quantitatively the components of the optical texture. The half-peak widths of the (002) diffraction bands were determined and the porosity and surface topography of fracture surfaces of the carbons were monitored by scanning electron microscopy. The aim of the study was to assess how the anisotropic carbon of pitch coke can be replaced by isotropic carbon via addition of alkali salts with a view to the optimization of commercial gasifiers. The effectiveness of the alkali carbonates was Li < Na < K< Rb < Cs, with hydroxides being more effective than carbonates. The optical texture changed abruptly from small domains to isotropic with residual anisotropic carbon in the isotropic matrix. The half-peak widths increased with increasing isotropic carbon content.     

Hydrotalcites for adsorption of CO2 at high temperature

Adsorption of carbon dioxide by hydrotalcites was investigated by using a gravimetric method at 450 ‡C. Hydrotalcites possessed higher adsorption capacity of CO₂ than other basic materials such as MgO and Al₂O₃. Two different preparation methods of hydrotalcite with varying Mg/Al ratio were employed to determine their effects on the adsorption capacity of CO₂. In addition, varying amounts of K₂CO₃ were impregnated on the hydrotalcite to further increase its adsorption capacity of CO₂. The hydrotalcite prepared by the high supersaturation method with Mg/Al=2 showed the most favorable adsorption-desorption pattern with high adsorption capacity of CO₂. K₂CO₃ impregnation on the hydrotalcite increased the adsorption capacity of CO₂ because it changed both the chemical and the physical properties of the hydrotalcite. The optimum amount of K₂CO₃ impregnation was 20 wt%. The hydrotalcite prepared by the high supersaturation method with Mg/Al=2 and 20 wt% K₂CO₃ impregnation has the highest adsorption capacity of CO₂ with 0.77 mmol CO₂/g at 450 ‡C and 800 mmHg.     

In situ diagnostics of Victorian brown coal combustion in O2/N2 and O2/CO2 mixtures in drop-tube furnace

Experimental investigation of the combustion of an air-dried Victorian brown coal in O₂/N₂ and O₂/CO₂ mixtures was conducted in a lab-scale drop-tube furnace (DTF). In situ diagnostics of coal burning transient phenomena were carried out with the use of high-speed camera and two-colour pyrometer for photographic observation and particle temperature measurement, respectively. The results indicate that the use of CO₂ in place of N₂ affected brown coal combustion behaviour through both its physical influence and chemical interaction with char. Distinct changes in coal pyrolysis behaviour, ignition extent, and the temperatures of volatile flame and burning char particles were observed. The large specific heat capacity of CO₂ relative to N₂ is the principal factor affecting brown coal combustion, which greatly quenched the ignition of individual coal particles. As a result, a high O₂ fraction of at least 30% in CO₂ is required to match air. Moreover, due to the accumulation of unburnt volatiles in the coal particle vicinity, coal ignition in O₂/CO₂ occurred as a form of volatile cloud rather than individual particles that occurred in air. The temperatures of volatile flame and char particles were reduced by CO₂ quenching throughout coal oxidation. Nevertheless, this negative factor was greatly offset by char-CO₂ gasification reaction which even occurred rapidly during coal pyrolysis. Up to 25% of the nascent char may undergo gasification to yield extra CO to improve the reactivity of local fuel/O₂ mixture. The subsequent homogeneous oxidation of CO released extra heat for the oxidation of both volatiles and char. As a result, the optical intensity of volatile flame in ∼27% O₂ in CO₂ was raised to a level twice that in air at the furnace temperature of 1273 K. Similar temperatures were achieved for burning char particles in 27% O₂/73% CO₂ and air. As this O₂/CO₂ ratio is lower than that for bituminous coal, 30-35%, a low consumption of O₂ is desirable for the oxy-firing of Victorian brown coal. Nevertheless, the distinct emission of volatile cloud and formation of strong reducing gas environment on char surface may affect radiative heat transfer and ash formation, which should be cautioned during the oxy-fuel combustion of Victorian brown coal.     

A model of char capture by molten slag surface under high-temperature gasification conditions

A simple model was proposed for char capture by molten slag surface under high-temperature gasification conditions. In this model, char particles were pneumatically conveyed onto the molten slag surface. The char particles were assumed to be captured if they reach the molten slag surface, whereas they were repelled if they reach the part that is covered by the unreacted char particles. Thus the probability of char capture was given by the balance of char feed rate per unit surface area of the slag and the rate of char consumption by the gasification reaction.Experiments were carried out to evaluate the probability of char capture by molten slag surface at 1350 °C. A ceramic tube whose bottom was closed was vertically placed in an electric furnace. Mixture of coal ash and flux (limestone) was placed at the bottom of the reactor. The reactor was heated up to a temperature higher than the melting point of the mixture of coal ash and flux, thus slag was formed at the bottom. Char particles were conveyed by gas stream from the top of the reactor to the molten slag surface. If the char particles were not captured at the reactor bottom, they were immediately conveyed out of the reactor by the gas stream. CO was produced by gasification reaction in pure CO₂ or CO₂ diluted by N₂. The conversion of carbon to CO decreased with increasing char feed rate. The effect of char properties such as particle size, density, and gasification rate, on the conversion of carbon to CO was evaluated. The theoretical results agreed well with the experimental results.     

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.     

Catalytic effect of potassium carbonate on carbothermic production of hexagonal boron nitride

Effect of potassium carbonate addition on the carbothermic formation of hexagonal boron nitride (hBN) was investigated by keeping the K₂CO₃ added B₂O₃+C mixtures in nitrogen atmosphere at 1400 °C for 40–160 min. K₂CO₃ amount was varied in the range of 10–60 wt% of the B₂O₃+C mixture. Products were subjected to XRD and quantitative analyses, SEM and TEM observations, and particle size measurement. Amount of hBN increased considerably with K₂CO₃ addition; also particle size and crystallinity improved. Catalytic role of K₂CO₃ was suggested as forming a potassium borate melt in which hBN particles form, in addition to carbothermic formation reaction. Effect of K₂CO₃ on increasing the hBN amount decreased when it was used over 40%. This was attributed to the rapid evaporation of the formed potassium borate liquid.     

Steam gasification of coal char using alkali and alkaline-earth metal catalysts

The catalytic effect of alkali and alkaline-earth metal salts or oxides on the gasification of Chinese Linnancang coal char was investigated at atmospheric pressure and a temperature of 800 °C. The order of catalytic activity is K₂SO₄ or K₂CO₃ Na₂CO₃ KCl NaCl CaCl₂ or CaO. The effect of amount of catalysts added on catalytic activity was studied. The distribution of K₂CO₃ or CaO catalysts on the coal char surface for different methods of catalyst loading was examined by an electron microprobe analyser. The relation between the catalytic activity and distribution of catalysts were illustrated. The loading method of K₂CO₃ has little effect on its catalytic activity but that of CaO influences the activity significantly.     

Modeling of catalytic gasification kinetics of coal char and carbon

Calcium- and potassium-catalyzed gasification reactions of coal char and carbon by CO₂ are conducted, and the common theoretical kinetic models for gas-carbon (or char) reaction are reviewed. The obtained experimental reactivities as a function of conversion are compared with those calculated based on the random pore model (RPM), and great deviations are found at low or high conversion levels as predicted by theory. Namely, calcium-catalyzed gasification shows enhanced reactivity at low conversion levels of <0.4, whereas potassium-catalyzed gasification indicated a peculiarity that the reactivity increases with conversion. CO₂ chemisorption analysis received satisfactory successes in both interpreting catalytic effects and correlating the gasification reactivity with irreversible CO₂ chemical uptakes (CCU_ir) of char and carbon at 300 °C. In details, calcium and potassium additions led to significant increases in CCU_ir and correspondent high reactivities of the char and carbon. Furthermore, CCU_ir of char and carbon decreased with conversion for calcium-catalyzed reaction but increased for potassium-catalyzed one, corresponded to the tendency of their reactivity. The RPM is extended and applied to these catalytic gasification systems. It is found that the extended RPM predicts the experimental reactivity satisfactorily. The most important finding of this paper is that the empirical constants in the extended RPM correlate well with catalyst loadings on coal.     

The physical character of coal char formed during rapid pyrolysis at high pressure

Rapid pyrolysis was conducted in a drop tube reactor using seven coals under various operating conditions. In addition to dense char, porous chars (network char and cenospheric char) were formed by the rapid pyrolysis under certain conditions. Porous char was mainly composed of film-like carbon and skeleton carbon. The pyrolyzed coal char particles were characterized in detail. Morphology and bulk density of porous char were quite different from the dense char formed under the same conditions, but elemental composition and BET surface area were similar to each other. CO₂ gasification reactivity of porous char was lower than dense char in the later gasification stage, and this was ascribed to the low reactivity of skeleton carbon.     

FTIR studies of potassium catalyst-treated gasified coal chars and carbons

The catalytic effect of K₂CO₃, KHCO₃, KOH, K₂SO₄ and KCl during steam gasification of coal and carbon chars has been evaluated by gravimetric measurements at 750 °C. The order of catalytic activity found was KOH~K₂CO₃ >KHCO₃ >KNO₃ >K₂SO₄ with almost no activity for KCl. Transmission i.r. spectra were obtained on thin wafers of KBr mixed with carbon and coal char samples after impregnation, devolatilization and partial gasification. All catalytically activated salts presented two characteristic bands at 1050 and 1400 cm^?1 while no such bands appeared in the case of catalytically inactive KCl. Furthermore, such bands were also present in a different carbon substrate impregnated with K₂CO₃ and in an oxygen-exposed intercalate (C₈K). The bands appeared at the same frequencies as those of KHCO₃ in pure, carbon impregnated, devolatilized, and partially gasified form.     

Enhanced catalysis of K2CO3 for steam gasification of coal char by using Ca(OH)2 in char preparation

A novel approach has been proposed for mitigating the potassium deactivation in the K₂CO₃-catalyzed steam gasification of coal char by addition of Ca(OH)₂ in the char preparation. It was experimentally found that the Ca(OH)₂-added char had higher reactivity for the catalytic gasification than the raw char. Ca(OH)₂ played a role in suppressing the interactions of K₂CO₃ with acidic minerals in coal during the gasification and also probably in forming more active oxygenated intermediate on the char surface. The distribution of gaseous products was examined during the catalytic gasification. An oxygen transfer and intermediate hybrid mechanism is applied for understanding of the rate and selectivity of the catalytic gasification.     

Hydrogasification of various carbonaceous sources using pressure change properties

Hydrogasification experiments were carried out in a batch reactor capable of operating at 800 °C and 8 MPa. Carbonaceous matters used in the experiments were bituminous and anthracite coal and sawdust. It was found that the decreasing rate of hydrogen gas pressure was closely related to the rate of gas production. This result was confirmed by the change of char conversion. The methane content in the gas products and char conversion rose with the increase of temperature and pressure. The addition of water activated the hydrogasification reaction until the proper level of water amount (up to 30 wt%), but an excess level of water inhibited the reaction. The activation energy of bituminous coal and sawdust char obtained by the Arrhenius plot was 187 KJ/mole and 77 KJ/mole, respectively. In case of loading of catalysts, all catalysts loaded to the char did not give a positive effect in hydrogasification, but the catalytic effect depended on type of catalyst metals and char. In the present hydrogasification of bituminous coal and sawdust, the order of activities for the catalysts tested was K₂CO₃>Na₂CO₃>Fe(NO₃)₂>Ni(NO₃)₂>FeSO₄. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Catalytic activity of calcium for lignite char gasification in various atmospheres

Gasification rates of a Texas lignite char, having calcium contents varying from 1.1 to 12.9 wt%, were measured in air, steam, CO₂, and H₂. Gasification rates increased rectilinearly with increase in calcium loading. The chemisorption of CO₂ at 298 K on the CaO present on the char surface was used to determine CaO particle dispersion. Knowing this dispersion, specific activities of calcium as a catalyst for the gasification reactions were calculated. The spread in specific activities in different atmospheres is large, it being a maximum when the char-air reaction is compared with the char -H₂ reaction.     

Low-temperature preparation and electrochemical capacitance of WC/carbon composites with high specific surface area

WC/carbon composites (WCCs) with high specific surface area were synthesized by the direct carbonization of a mixture of hydroxylpropyl cellulose, polyvinyl alcohol, K₂WO₄ and K₂CO₃ at 900 °C in flowing N₂. The resultant material was characterized using X-ray diffraction, thermogravimetric analysis, nitrogen sorption and scanning electron microscopy. The electrode performance of this material for use as a capacitor was studied using cyclic voltammetry and galvanostatic charge-discharge measurements. The BET specific surface area of the WCCs varied from 300 to 1000 m²/g depending on the amount of K₂CO₃ added during the preparation. Samples prepared with small amounts of K₂CO₃ contained a large amount of mesoporosity. Electrochemical characterization revealed that WC was slowly oxidized to tungsten oxy-hydroxides, and pseudocapacitance due to the redox reactions of tungsten oxy-hydroxides was superimposed on the double-layer capacitance of the carbon support. Consequently large specific capacitance was observed. Galvanostatic charge-discharge measurements of a WCC (ca. 5 wt% WC) resulted in total specific capacitances as high as 477 and 184 F/g at current densities of 20 and 1000 mA/g, respectively. The long-term cycle stability of WCC was also verified by a 5000 cycle charge-discharge test at 1 A/g.     

Potassium catalysed gasification of Kentucky oil shale char

The steam gasification of a Kentucky oil shale char has been studied in a semi-batch fixed bed reactor. The effects of temperature (973–1173 K), catalyst loading (0–15% K₂CO₃) and pressure (up to 0.65 MPa) on the gasification rates and make-gas composition have been determined. Although gasification rates were increased by a factor of about three in the presence of 10% K₂CO₃, they were still significantly lower than those previously measured with western shale chars. The reason for this could be attributed to the relatively high hydrogen concentrations produced in the fixed-bed configuration since severe hydrogen inhibition has been previously reported for a similar shale char. There was no significant effect of the potassium catalysis on either the make-gas composition or the quantities of sulphur released during gasification. The only significant influence of pressure was to increase the methane make and this was independent of the catalyst loading.     

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.     

Fractal characteristic of three Chinese coals

Experimental and theoretical investigation about coal/char structure is presented. Surface structures of parent coal and char with different burn-off ratios were analyzed. We introduced the fractal theory into Scanning Electron Microscopy image analysis and utilize the particle surface fractal dimension (D_ps) to quantitatively describe the surface character of coal/char particles. D_ps of three Chinese coals reach their maximum in the 35-45 wt% char burn-off interval and then decrease with increasing carbon burn-off ratio. The inner-pore information of coal/char particles was determined by N₂ isotherm adsorption/desorption. Using fractal BET model, internal surface fractal dimension (D_s) of coal/char particles was calculated. The D_s change trend of three Chinese coals is similar to their S_BET development. It means the D_s can quantitatively describe the inner pore structure character of coal/char particles.