Steam gasification reactivity of char from rapid pyrolysis of bio-oil/char slurry

A slurry of bio-oil and char originating from wood pyrolysis is a promising gasifier feed-stock because of its high energy density. When such a slurry is injected into a high temperature gasifier it undergoes a rapid pyrolysis yielding a char which then reacts with steam. The char produced by pyrolysis of an 80 wt% bio-oil/20 wt% char mixture at heating rates of 100-10,000 °C/s was subjected to steam gasification in a thermogravimetric analyzer. The original wood char from the bio-oil production was also tested. Gasification was conducted with 10-50 mol% steam at temperatures from 800 to 1200 °C. Reactivity of the slurry chars increased with pyrolysis heating rate, but was lower than that of the original chars. Kinetic parameters were established for a power-law rate model of the steam-char reaction, and compared to values from the literature. At temperatures over 1000 °C, the gasification rates appeared to be affected by diffusional resistance.     

Kinetics of gasification of Douglas Fir and Cottonwood chars by carbon dioxide

Arrhenius kinetic parameters have been determined for the CO₂ gasification of chars (heat treatment at 1000 °C) prepared from well-characterized samples of a hardwood, a softwood and a Montana lignite. The effects of pre-pyrolysis addition of inorganic salts of the alkali, alkaline earth and transition metal groups to the wood samples have also been determined. The reactivities of the chars of the cottonwood and lignite samples exceeded that of Douglas fir char by a factor of four to seven between 700 and 900 °C. The reactivity of the wood char was related to the inorganic content of the sample. There was very little difference in the reactivity of chars prepared from the hardwood and the softwood after treatment with similar quantities of inorganic salts. The inorganic content of the lignite char was more than five times greater than that of cottonwood char, but its reactivity was similar. The carbonates of sodium and potassium were equally effective gasification catalysts. The transition metal salts were the most effective catalysts initially, but they lost their activity well before the gasification was complete. The data indicate that treatment of wood with aqueous salts results in replacement of some of the natural minerals by ion exchange, and that these exchangeable ions play a major role in controlling reactivity of the chars.     

Changes in char reactivity and structure during the gasification of a Victorian brown coal: Comparison between gasification in O2 and CO2

Char reactivity is an important factor influencing the efficiency of a gasification process. As a low-rank fuel, Victorian brown coal with high gasification reactivity is especially suitable for use with gasification-based technologies. In this study, a Victorian brown coal was gasified at 800 °C in a fluidised-bed/fixed-bed reactor. Two different gasifying agents were used, which were 4000 ppm O₂ balanced with argon and pure CO₂. The chars produced at different gasification conversion levels were further analysed with a thermogravimetric analyser (TGA) at 400 °C in air for their reactivities. The structural features of these chars were also characterised with FT-Raman/IR spectroscopy. The contents of alkali and alkaline earth metallic species in these chars were quantified. The reactivities of the chars prepared from the gasification in pure CO₂ at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O₂ also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O₂ and pure CO₂) are oxidising conditions, the results indicate that the reaction mechanisms for the gasification of brown coal char at 800 °C in these two gasifying atmospheres are different. FT-Raman/IR results showed that the char structure has been changed drastically during the gasification process.     

Influence of carbonization conditions on the gasification of acacia and eucalyptus wood chars by carbon dioxide

Gasification rates of cubic shaped acacia and eucalyptus wood chars were measured thermogravimetrically in a carbon dioxide atmosphere at temperatures in the range 810–960 °C. The effects of wood species and carbonization conditions, such as temperature, heating rate and soaking time, were determined. Both reactivity and the activation energy for the gasification of wood chars were found to be strongly influenced by the carbonization conditions employed during their preparation and wood type. The reactivities of both the acacia and eucalyptus wood chars decreased with increasing preparation temperature; while the activation energy for their gasification increased. Slow carbonization (heating rate: 4 °C min⁻¹) led to the production of wood chars having lower reactivities and higher activation energies than those of the wood chars prepared under rapid carbonization (heating rate: 30 °C min⁻¹) at the same temperature. With increasing soaking time, at carbonization temperatures of 800 and 1000 °C, the reactivity of resulting wood chars was reduced. The results also show that the reactivities of acacia wood chars are higher than those of similarly prepared eucalyptus wood chars.     

Impact of torrefaction on syngas production from wood

Torrefaction is a way to treat biomass before transportation or thermochemical conversion. It can be used to increase the energy content of wood or to facilitate grinding. The purpose of this paper was to quantify the impact of such a treatment on the behaviour of wood during gasification by steam at high temperature to produce syngas. The aspects of both gas yields and reaction kinetics were considered.Beechwood was submitted both to light torrefaction and severe torrefaction, using a specially designed crossed fixed bed reactor. The initial wood and the torrefied woods were first characterised, then gasified in a new laboratory high-temperature entrained flow reactor (HT-EFR) at 1400 °C for 2 s in an atmosphere containing 20 vol% steam in N₂. The syngas produced was then analysed. The experiments were modelled using a thermo-dynamical equilibrium approach.It was confirmed that torrefaction decreased the O/C ratio. The quantity of syngas produced increased with the severity of the torrefaction. The equilibrium approach describes the results satisfactorily.Gasification experiments carried out at a lower temperature - 1200 °C - indicated that the chars from torrefied woods are less reactive towards steam than the char from wood.     

The influence of pyrolysis conditions on the subsequent gasification of lignocellulosic chars

Prune pit chars prepared by pyrolysis at heating rates of 1 and 15°C/min to 500, 700 and 900°C were subsequently gasified by exposure to CO₂ at 900°C for various lengths of time. Gasification rate was found to be dependent on the conditions during pyrolysis: slow heating below 500°C and prolonged exposure to high temperatures (~900°C) during pyrolysis in an inert atmosphere lead to lower rate gasification. Despite differences in gasification rate, the pore structure developed for a given mass loss due to the gasification reactions was apparently independent of the char preparation conditions. Pore volume in the gasified char (expressed on an absolute basis) passed through a maximum at 40–50% burnoff, apparently due to mass loss from the exterior of the particles.     

Catalytic gasification of char from co-pyrolysis of coal and biomass

The catalytic gasification of char from co-pyrolysis of coal and wheat straw was studied. Alkali metal salts, especially potassium salts, are considered as effective catalysts for carbon gasification by steam and CO₂, while too expensive for industry application. The herbaceous type of biomass, which has a high content of potassium, may be used as an inexpensive source of catalyst by co-processing with coal. The reactivity of chars from co-pyrolysis of coal and straw was experimentally examined. The chars were prepared in a spout-entrained reactor with different ratios of coal to straw. The gasification characteristics of chars were measured by thermogravimetric analysis (TGA). The co-pyrolysis chars revealed higher gasification reactivity than that of char from coal, especially at high level of carbon conversion. The influence of the alkali in the char and the pyrolysis temperature on the reactivity of co-pyrolysis char was investigated. The experimental results show that the co-pyrolysis char prepared at 750 °C have the highest alkali concentration and reactivity.     

The CO2-gasification and kinetics of Shenmu maceral chars with and without catalyst

The CO₂ gasification of maceral chars was performed using CAHN TG-151 pressurized thermobalance under different conditions. The effect of mineral in macerals and catalyst on the gasification reactivity of maceral chars and the gasification kinetics were systematically investigated. The results showed that the apparent gasification rate of maceral chars depends on the temperature, pressure, BET surface area of chars and the gasification extent. With increasing temperature and pressure, the gasification rate of maceral chars all increase. After demineralization, the gasification reactivity of maceral chars all decrease. The gasification reactivity of maceral chars greatly increases with loading catalyst. And the loading method of catalyst has great effect on the gasification reactivity. The maceral chars loaded with catalyst by ultrasonic treatment have higher gasification reactivity than that by impregnation. The comparison of gasification reactivity of maceral charas demineralized maceral chars and maceral chars with and without catalyst showed that vitrinite chars always have higher gasification reactivity than inertinite chars. The kinetic results by distributed activation energy model showed that inertinite char has higher activation energy than vitrinite char, and the addition of catalyst greatly minimizes the activation energy and enhances the gasification rate.     

Evolution of biomass char structure during oxidation in O2 as revealed with FT-Raman spectroscopy

FT-Raman spectroscopy has been used to identify structural features and evaluate the structural evolution of biomass chars during gasification with air. Chars prepared from the pyrolysis of a cane trash sample with a fast particle heating rate in a novel fluidised-bed/fixed-bed reactor at 500, 700 and 900 °C were oxidised at 400 °C in air in a TGA. The data derived from the spectral deconvolution of Fourier Transform — Raman spectra suggest that the 500 °C char showed very different structural features after pyrolysis and during oxidation from the 700 and 900 °C chars, while the differences between the latter two chars were small. Preferential consumption by O₂ of smaller aromatic rings and structures of somewhat aliphatic characteristics left the char more enriched with larger aromatic ring systems. The changes in char structure are in agreement with the observed reactivity measured in O₂ in a thermogravimetric analyser.     

Mineral reaction and morphology change during gasification of coal in CO2 at elevated temperatures

Studies of the gasification of char in CO₂ at elevated temperatures are necessary for the development of IGCC technology. Experiments at high heating rates and elevated temperatures revealed that the temperature dependence of gasification reactivity was very different for low compared with high temperature ranges. To elucidate these mechanisms, the reaction of mineral matter and the change in morphology during gasification of a char at elevated temperatures were examined by char characterisation. CO₂ gasification experiments showed a large difference in gasification rate for chars prepared at higher temperatures compared to those prepared at lower temperatures. Changes in char particle morphology and mineral matter during gasification are also quite different. At higher carbonisation temperatures, mineral reactions during pyrolysis, which occurs in addition to ash fusion, appear to be one of the factors accounting for these differences. Certainly, a change of mechanism is involved. Graphite enrichment may also contribute to the decrease in char reactivity.     

Importance of carbon active sites in the gasification of coal chars

A demineralized lignite has been used in a fundamental study of the role of carbon active sites in coal char gasification. The chars were prepared in N₂ under a wide variety of conditions of heating rate (10 K min^?1 to 10⁴ K s^?1), temperature (975–1475 K) and residence time (0.3 s–1 h). Both pyrolysis residence time and temperature have a significant effect on the reactivity of chars in 0.1 MPa air, determined by isothermal thermogravimetric analysis. The chars were characterized in terms of their elemental composition, micropore volume, total and active surface area, and carbon crystallite size. Total surface area, calculated from C0₂ adsorption isotherms at 298 K, was found not to be a relevant reactivity normalization parameter. Oxygen chemisorption capacity at 375 K and 0.1 MPa air was found to be a valid index of char reactivity and, therefore, gives an indication, at least from a relative standpoint, of the concentration of carbon active sites in a char. The commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with their active surface areas. The importance of the concept of active sites in gasification reactions is illustrated for carbons of increasing purity and crystallinity including a Saran char, a graphitized carbon black and a spectroscopically pure natural graphite.     

Changes in char reactivity due to char-oxygen and char-steam reactions using Victorian brown coal in a fixed-bed reactor

This study was to examine the influence of reactions of char–O2 and char–steam on the char reactivity evolution. A newly-designed fixed-bed reactor was used to conduct gasification experiments using Victorian brown coal at 800 °C. The chars prepared from the gasification experiments were then collected and subjected to reactivity characterisation (ex-situ reactivity) using TGA (thermogravimetric analyser) in air. The results indicate that the char reactivity from TGA was generally high when the char experienced intensive gasification reactions in 0.3%O2 in the fixed-bed reactor. The addition of steam into the gasification not only enhanced the char conversion sig-nificantly but also reduced the char reactivity dramatical y. The curve shapes of the char reactivity with involve-ment of steam were very different from that with O2 gasification, implying the importance of gasifying agents to char properties.     

Catalytic activities of ion-exchanged nickel and iron in low temperature hydrogasification of raw and modified birch chars

Raw, HNO₃ oxidized and carboxymethylated birch woods loaded with nickel or iron by the ion-exchange method were carbonized at 500 °C in a flow of nitrogen, and the resulting chars were hydrogasified in a thermobalance to examine their reactivities below 700 °C. The amounts of ion-exchanged metals on raw char were too small to give high gasification reactivity. However, oxidized and carboxymethylated woods with increased ion-exchange capacity produced much more reactive chars. Both nickel and iron exhibited larger catalytic activities on carboxymethylated chars than on oxidized chars, because better metal dispersion could be achieved on carboxymethylated wood with its larger cation exchangeability. It was noteworthy that only 1 wt% loading of iron, as well as nickel, on carboxymethylated char was sufficient to attain a gasification of 90 wt% at 700 °C. It was also noted that the catalytic effect, up to 600 °C, of iron on the gasification of oxidized and carboxymethylated chars was larger than that of nickel. This is ascribed to two factors; greater catalytic activity of metallic iron formed during the gasification than that of nickel metal, and low ash level in the chars. Above 600 °C, however, serious loss of activity of the iron was observed in the absence of wood ash. This showed the different influence of wood ash on the catalysis of iron in the low and high temperature regions.     

CO2 gasification rate analysis of coal char in entrained flow coal gasifier

Coal chars of four coal types were gasified with carbon dioxide using a PDTF or TGA at high temperature and pressure. Test conditions of temperature and partial pressure of the gasifying agent were determined to simulate the conditions in air-blown or oxygen-blown entrained flow coal gasifiers. Coal chars were produced by rapid pyrolysis of pulverized bituminous coals using a DTF with a nitrogen gas flow at 1670 K. In gasification tests with the PDTF, gasification temperatures were 1670 K or below and partial pressures of carbon dioxide were 0.7 MPa or below. Carbon monoxide of 0.6 MPa or below was supplied for the gasification tests with the TGA.As a result, coal types showed a large difference in the char gasification rate with carbon dioxide, and this difference remained large without decreasing even in the high-temperature area when the gasification rate was controlled by pore diffusion the same as in entrained flow gasifiers. Inhibition of the gasification reaction by carbon monoxide was also observed. Reaction rate equations of both the nth order and Langmuir-Hinshelwood type were applied to the char gasification reaction with the random pore model and the effectiveness factor, and the applicability of these rate equations to air-blown and oxygen-blown entrained flow gasifiers evaluated. Gasification rate equations and kinetic parameters applicable to a pore diffusion zone at high temperature were obtained for each coal.     

Reactivity of heat-treated coals in steam

Reactivities of seventeen 40 × 100 mesh (U.S.) coals charred to 1000 °C have been measured at 910 °C in 0.1 MPa of a N₂H₂O mixture containing water vapour at a partial pressure of 2.27 kPa. Char reactivity decreases, in general, with increasing rank of the parent coal. The chars show a 250-fold difference in their reactivities. Results suggest that gasification of chars in air, CO₂ and steam involves essentially the same mechanism and that relative gasification rates are controlled by the same intermediate oxygen-transfer step. Removal of inorganic matter from raw coals prior to their charring or from chars produced from raw coals decreases the reactivities of lower-rank chars, whereas reactivities of higher-rank chars increase. Addition of H₂ to steam has a marked retarding effect on char reactivity in most cases. However, in a few cases H₂ acts as an accelerator for gasification. The effect of particle size, reaction temperature and water-vapour pressure on char reactivity is considered.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part VI. Effects of gas atmosphere on the formation of NH3 and HCN

Our results indicate that the gas atmosphere surrounding coal/char particles can greatly affect the formation of NH₃ and HCN through its influence on the availability of H radicals. Based on our results, it is believed that the chemisorption of CO₂ on the nascent char surface can consume H radicals or block the access of N-sites by H radicals for the formation of NH₃ and HCN. For the chars whose thermal cracking generates little H radicals, the gasification of char by CO₂ can also generate additional H radicals, enhancing the formation of NH₃. However, even gasification of char in CO₂ at 950 °C does not lead to the formation of HCN. The oxidation of coal with 4% O₂ at low temperatures (400-600 °C) leads to the formation of HCN as well as NH₃ due to the enhanced formation of (H) radicals. The gasification of coal with 15% H₂O drastically enhances the formation of NH₃ due to the greatly enhanced availability of H as an intermediate between the reactions of H₂O and char. These results support our reaction mechanisms proposed previously, emphasising the importance of H on the formation of NH₃ and HCN during pyrolysis, which can also be extended to the conversion of coal-N during gasification.     

NH3 and HCN formation during the gasification of three rank-ordered coals in steam and oxygen

A Victorian brown coal (68.5% C), a Chinese high-volatile Shenmu bituminous coal (82.3% C) and a Chinese low-volatile Dongshan bituminous coal (90% C) were gasified in a fluidised-bed/fixed-bed reactor at 800 °C in atmospheres containing 15% H₂O, 2000 ppm O₂ or 15% H₂O + 2000 ppm O₂. While the gasification of these coals in 2000 ppm O₂ converted less than 27% of coal-N into NH₃, the introduction of steam played a vital role in converting a large proportion of coal-N into NH₃ by providing H on char surface. The importance of the roles of steam in the formation of NH₃ in atmospheres containing 15% H₂O + 2000 ppm O₂ decreased with increasing coal rank. This is largely due to the slow gasification of high-rank coal chars, resulting in low availability of H on char surface. The gasification of chars from the high-rank coal appears to produce higher yields of HCN than that of lower rank coals, probably as a result of the decomposition of partially hydrogenated/broken/activated char-N structures during gasification at high temperature. The alkali and alkaline earth metallic species in brown coal tend to favour the release of coal-N as tar-N but have limited effects on char-N conversion during gasification.     

Observation of alkali catalyst particles during gasification of carbonaceous materials in CO2 and steam

Results of a microscopical examination of catalysed carbon gasification are reported. Both in CO₂ and steam, alkali catalysts show evidence of mobility. In the steam gasification of coal chars, the catalysts irreversibly combine with indigenous mineral matter. This is less pronounced in C0₂. The catalysed CO₂ gasification was observed by hot stage microscopy, where alkali carbonate catalysts achieve an apparently molten state during incipient gasification. For single crystal graphite, circular pitting, hexagonal pitting and channelling were observed. For coal chars, irregular morphologies tend to obscure direct observation of surface/catalyst interactions, though subsequent scanning electron micrographs reveal the consequences of extensive catalyst mobility.     

Catalysis of gasification of coal-derived cokes and chars

Calcium is the most important in-situ catalyst for gasification of US coal chars in O₂, CO₂ and H₂O. It is a poor catalyst for gasification of chars by H₂. Potassium and sodium added to low-rank coals by ion exchange and high-rank coals by impregnation are excellent catalysts for char gasification in O₂, CO₂ and H₂O. Carbon monoxide inhibits catalysis of the CH₂O reaction by calcium, potassium and sodium; H₂ inhibits catalysis by calcium. Thus injection of synthesis gas into the gasifier will inhibit the CH₂O reaction. Iron is not an important catalyst for the gasification of chars in O₂, CO₂ and H₂O, because it is invariably in the oxidized state. Carbon monoxide disproportionates to deposit carbon from a dry synthesis gas mixture (3 vol H₂ + 1 vol CO) over potassium-, sodium- and iron-loaded lignite char and a raw bituminous coal char, high in pyrite, at 1123 K and 0.1 MPa pressure. The carbon is highly reactive, with the injection of 2.7 kPa H₂O to the synthesis gas resulting in net carbon gasification. The effect of traces of sulphur in the gas stream on catalysis of gasification or carbon-forming reactions by calcium, potassium, or sodium is not well understood at present. Traces of sulphur do, however, inhibit catalysis by iron.