Flash pyrolysis of coals. Devolatilization of bituminous coals in a small fluidized-bed reactor

The devolatilization behaviour of ten bituminous coals was investigated under rapid heating conditions using a small-scale fluidized-bed pyrolyser. The pyrolyser operated continuously, coal particles being injected at a rate of 1–3 g h^?1 directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C₁–C₃ hydrocarbon gases, and total volatile-matter and an agglomeration index are reported for all coals. Maximum tar yields were obtained at about 600 °C and were always substantially higher than those from the Gray-King assay. Total volatile-matter yields were also substantially higher than the proximate analysis values. The maximum tar yields appear to be directly proportional to the coal atomic $\text{H}\text{C}$ ratio. The elemental analysis of the tar is strongly dependent on pyrolysis temperature. The tar atomic $\text{H}\text{C}$ ratio is proportional to that of the parent coal. The effect on the devolatilization behaviour of two coals produced by changes in the pyrolyser atmosphere and the nature of the fluidized-bed material were also investigated. Substituting an atmosphere of hydrogen, helium, carbon dioxide or steam for nitrogen, has no effect on tar yield and, with one exception, little effect on the hydrocarbon gas yields. In the presence of hydrogen the yield of methane was increased at temperatures above 600 °C. Tar yields were significantly reduced on substituting petroleum coke for sand as the fluid-bed material. A fluidized bed of active char virtually eliminated the tar yield.     

Flash pyrolysis of coals. 1. Devolatilization of a Victorian brown coal in a small fluidized-bed reactor

The devolatilization behaviour of finely-ground (< 0.2 mm) Loy Yang brown coal was investigated under rapid heating conditions using a small-scale fluidized-bed pyrolyser. The pyrolyser operated continuously, coal being fed at rates of 1–3 g/h directly into a bed of sand fluidized by nitrogen. Particle heating rates probably exceeded 10⁴ °C/s. The yields of tar, C₁-C₃ hydrocarbons and total volatile matter are reported for a pyrolyser-temperature range of 435 to 900 °C. A maximum tar yield of 23% w/w (dry ash-free coal), 60% more than the Fischer assay, was obtained at 580 °C. Yields of C₁-C₃ hydrocarbons increased with increasing temperature, reaching 8% at 900 °C. Elemental analyses showed that the composition of the tar and char products was strongly dependent on pyrolysis temperature. The effects on the devolatilization behaviour of the coal produced by the moisture associated with the coal, by hydrogen, and by the replacement of the sand by a fluidized bed of petroleum coke were investigated.     

Flash pyrolysis of coals: influence of cations on the devolatilization behaviour of brown coals

The influence of cations on the pyrolysis behaviour of brown coals under flash heating conditions was investigated by means of a small fluidized-bed pyrolyser. A stream of coal particles in nitrogen was injected at rates of 1–3 g coal/h directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C₁–C₃ hydrocarbons and total volatile matter from four Gelliondale brown coals and a Montana lignite were determined as a function of pyrolysis temperature. With all coals the maximum tar yield was obtained at 600 °C. Removal of cations present in the coals markedly increased the yields of tar and total volatile matter, with little effect on the yields of hydrocarbon gases. The converse was also observed in that the addition of Ca²⁺ to a cation-free coal decreased the yields of tar and total volatile matter. The extent of the reduction in tar yield at 600 °C in the presence of cations was found to be similar for all coals. After acid washing, tar yields appear to correlate with the atomic $\text{H}\text{C}$ ratios of the coals in a manner similar to that observed previously with bituminous coals.     

Flash pyrolysis of coals: comparison of results from 1 g h−1 and 20 kg h−1 reactors

The performances of 1 g h^?1 and 20 kg h^?1 flash pyrolysers are compared for three Australian coals: Loy Yang brown coal (Victoria), Liddell bituminous coal (New South Wales), and Millmerran sub-bituminous coal (Queensland). The two reactors gave comparable yields of tar, char and C₁–C₃ hydrocarbon gases over a range of operating conditions for each particular coal. The yield of total volatile matter from Millmerran coal was similar from both reactors, as were the compositions of chars from Loy Yang coal and tars from the Liddell and Millmerran coals. For Millmerran coal, the yields of tar, C₁–C₃ gases and volatiles from the large reactor below 650 °C, were slightly lower than for the small reactor, possibly owing to a shorter retention time of Millmerran coal particles in the large-scale reactor. At a temperature near 600 °C tar yields were independent of tar concentration in the effluent gas, over a range 0.0025–0.1 kg m^?3 for Liddell coal, 0.005–0.26 kg m^?3 for Millmerran coal and 0.0045–0.09 kg m^?3 for Loy Yang coal. The tar yields from Millmerran and Liddell coals at 600 °C in the large reactor, correlate directly with the atomic $\text{H}\text{C}$ ratio of the parent coal, in the same manner as that found for a wider range of bituminous coals in the small-scale reactor.     

Mathematical modelling of lignin pyrolysis

Seven lignins from different sources were pyrolysed (i) isothermally in vacuum over the temperature range 300–1300 °C and (ii) at a constant heating rate of 30 °C min^?1 and a pressure of 0.1 MPa over the temperature range 150–900 °C. The mass fraction of each product—char, tar and gas species—and the elemental composition of the char and the tar were determined for the flash pyrolysis experiments. The evolution rates of the gas species and the tar versus the dynamic temperature of pyrolysis were determined for the constant heating rate pyrolysis experiments. Although the amount of each product species varied from lignin to lignin, the evolution rates were insensitive to the lignin source and the extraction process. To model the data, modifications were made to a recently developed model of coal pyrolysis. The model proved to be successful in simulating both the data from vacuum flash pyrolysis and constant heating rate pyrolysis of Iotech lignin.     

Effects of thermal pretreatment in helium on the pyrolysis behaviour of Loy Yang brown coal

A wire-mesh reactor capable of multi-step heating/holding and minimising secondary reactions of volatiles was used to investigate the effects of thermal pretreatment in inert gas on the subsequent rapid pyrolysis behaviour of Loy Yang brown coal. Our results indicate that the presence of small amounts (<10 wt%) of moisture in brown coal has little influence on the tar and char yields from the pyrolysis of brown coal at 1000 K s⁻¹. While the hydrogen bonds between the moisture and the O-containing functional groups in the brown coal have little effects on its pyrolysis behaviour, the hydrogen bonds among the O-containing functional groups tend to induce cross-linking reactions to reduce the tar yields. Preheating the brown coal at >250 °C leads to reduced tar and increased char yields. However, the characterisation of tars using UV-fluorescence spectroscopy indicates that significant decreases in the release of larger aromatic ring systems are only observed after preheating at >380 °C for 30 min. The presence of ion-exchangeable cations (e.g. Ca²⁺) in the brown coal tends to stabilise the carboxylate groups and only preheating at >350 °C would result in changes in pyrolysis yields during the subsequent pyrolysis at 1000 K s⁻¹. These results may be explained by considering the formation of cross-links involving peripheral groups at low preheating temperatures and the formation of cross-links involving aromatic ring systems at elevated temperatures.     

Comparative investigations of coal pyrolysis under inert gas and H2 at low and high heating rates and pressures up to 10 MPa

Five German hard coals of 6–36 wt% volatile matter yield (maf) were pyrolysed at pressures up to 10 MPa, using two different apparatuses, which mainly differ in the heating rates. One consists of a thermobalance where a coal sample of ≈ 1.5 g is heated at a rate of 3 K min ^?1 under a gas flow of 3 I min^?1. The other apparatus is constructed for rapid heating (10²?10³ K s^?1) of a small sample of ≈10 mg of finely-ground coal distributed as a layer between the folded halfs of a stainless-steel screen, heated by an electric current. The product gas composition was determined by quantitatively analysing for H₂, CH₄, C₂H₄, C₂H₆, CO, CO₂ and H₂O. The amounts of tar and char were measured by weighing. The heating rate, pressure and gas atmosphere were varied. Under an inert gas atmosphere, high heating rates result in slightly higher yields of liquid products, e.g. tar. The yields of light hydrocarbon gases remain the same. With increasing pressure, the thermal cracking of tar is intensified resulting in high yields of char and light hydrocarbon gases. Under H₂, pyrolysis is influenced strongly at elevated pressure. Additional amounts of highly aromatic products are released by hydrogenation of the coal itself, particularly between 500 and 700°C. This reaction is less effective at higher heating rates because of the shorter residence time and diffusion problems of H₂. The yield of light gaseous compounds CH₄ and C₂H₆ increases markedly under either heating condition owing to gasification of the reactive char.     

Products from rapid heating of a brown coal in the temperature range 400–2300 °C

The devolatilisation behaviour of Yallourn brown coal was investigated under rapid heating conditions using two different flash pyrolysers: a fluid-bed reactor giving coal particle heating rates of 10⁴ °Cs^?1 with a gas residence time of about 0.5 s and a shock tube which generated heating rates of the order of 10⁷ °Cs^?1 and a 1 ms reaction time. Yields of products are reported covering pyrolysis temperatures in the range 400–2300 °C. Hydrocarbon gas yields reached maximum values which were remarkably similar for both reactors although occurring at different temperatures. Carbon oxide production was also similar for both reactors with CO yields reaching 30% wt/wt daf coal. These high yields of CO are very different from those reported for slow heating conditions. It appears that on flash heating, coal decomposition pathways change in a manner which increases CO yields at the expense of H₂0 and to a lesser extent C0₂, resulting in the volatilisation of additional carbon from the coal.     

Synthesis gas and char production from Mongolian coals in the continuous devolatilization process

Devolatilization of Mongolian coal (Baganuur coal (BC), Shievee Ovoo coal (SOC), and Shievee Ovoo dried coal (SOC-D)) was investigated by using bench-sized fixed-bed and rotary kiln-type reactors. Devolatilization was assessed by comparing the coal’s type and dry basis, temperature, gaseous flux, tar formation/generation, devolatilization rate, char yield, heating value, and the components of the raw coal and char. In the fixed bed reactor, higher temperatures increased the rate of devolatilization but decreased char production. BC showed higher rates of devolatilization and char yields than SOC or SOC-D. Each coal showed inversely proportional devolatilization and char yields, though the relation was not maintained between the different coal samples because of their different contents of inherent moisture, ash, fixed carbon, and volatile matter. Higher temperatures led to the formation of less tar, though with more diverse components that had higher boiling points. The coal gas produced from all three samples contained more hydrogen and less carbon dioxide at higher temperatures. Cracking by multiple functional groups, steam gasification of char or volatiles, and reforming of light hydrocarbon gas increased with increasing temperature, resulting in more hydrogen. The water gas shift (WGS) reaction decreased with increasing temperature, reducing the concentration of carbon dioxide. BC and SOC, with retained inherent moisture, produced substantially higher amounts of hydrogen at high temperature, indicating that hydrogen production occurred under high-temperature steam. The continuous supply of steam from coal in the rotary kiln reactor allowed further exploration of coal gas production. Coal gas mainly comprising syngas was generated at 700–800 °C under a steam atmosphere, with production greatest at 800 °C. These results suggest that clean char and high value-added syngas can be produced simultaneously through the devolatilization of coal at lower temperature at atmospheric pressure than the entrained-bed type gasification temperature of 1,300–1,600 °C.     

Release of chloroform-extractable materials from a bituminous coal after mild heating

Samples of Linby coal were heated to ⩽ 400 °C and afterwards extracted with chloroform in a Soxhlet apparatus. Extract yields were observed to pass through a minimum around 350 °C, suggesting loss of potential extract near 350 °C, in contrast to a coking coal for which extract yields increased monotonically with temperature. Between 350 and 400 °C bond rupture produced small amounts of tar and increases in extract yield. Samples impregnated with ZnCl₂ prior to heating yielded less extract and released larger amounts of molecular hydrogen and lower hydrocarbons. At the slow heating rate used (5 °C/min) the catalyst appeared to promote condensation and de-alkylation reactions, the effect becoming more pronounced above 350 °C.     

Development of a technology for coal conversion in the presence of coal tar

A new process for the hydrogenation of coal in the presence of wide-cut coal tar was proposed; it involves cavitation treatment, mixing with catalytic additives, and heating the resulting mixture at an elevated pressure in an atmosphere of hydrogen. The yields of hydrocarbon fractions to 300°C and gas condensate were evaluated.     

A preliminary account of char structures produced from Liddell vitrinite pyrolysed at various heating rates

Liddell-seam vitrinite particles were heated to 1000 °C in nitrogen at uniform rates ranging from 10^?1 °C s^?1 to 10⁴ °C s^?1. Little melting or swelling was observed when the particles were heated at 10^?1 °C s^?1 even though the vitrinite is from a coking coal of high-volatile bituminous rank. Particle size (100 μm) and loose packing were probably major influences on the plasticity. Vitrinite particles heated at rates faster than 10^?1 °C s^?1 showed an increase in plasticity with heating rate but the effects related to plasticity and volatile evolution appeared to be approaching a limit. Simple cenospheres (primary vesicles) were formed and preserved at a heating rate of 1 °C s^?1. At a heating rate of 10 °C s^?1 secondary vesicles were produced and preserved in the walls of the primary vesicles. At faster heating rates only secondary and tertiary vesicles were preserved. At a heating rate of 10⁴ °C s^?1 the vesicles preserved were very small.     

Char morphology and behaviour of Australian vitrinites of various ranks pyrolysed at various heating rates

This study concerns the relative importance of rank and heating rate on the development of plasticity during pyrolysis of several Australian vitrinites. Dispersed vitrinite particles, 100 μm in diameter, were heated to 1000 °C in nitrogen at linear heating rates ranging from 10^?1 to 10⁴ °Cs^?1 in a special electrical strip furnace. When pyrolysed at 10⁴ °C s^?1 all vitrinites became plastic and vesiculated except for vitrinite from anthracite, and gelinite from brown coal. The greatest plasticity developed in bituminous-rank vitrinite. Brown coal textinite and the lower-rank sample of the two subbituminous-rank vitrinites behaved similarly, whereas the behaviour of the higher-rank subbituminous coal resembled that of the semi-anthracite sample. At heating rates of 10^?1 °C s^?1 all the vitrinites retained their original morphologies. Cenospheres began to form in the vitrinites within the heating rate range of 1 °C s^?1 (for bituminous rank) to 10² °C s^?1 (for brown coal and semi-anthracite ranks). During pyrolysis, the differences in plastic behaviour attributable to rank could largely be eliminated by changing the heating rate by two orders of magnitude. The effects attributable to plasticity related to increasing heating rates reach a limit within five orders of magnitude of heating rate (for the conditions used in this study).     

Pyrolysis of peat: Product yield and characterization

Pyrolysis of peat obtained from Yeniça?a, Bolu, Turkey was conducted in a fixed-bed tube furnace under various conditions, and variations in the structure of the char, tar and gas products were examined. The chars produced were studied by proximate and ultimate analyses. The maximum tar yield of 20.41% was obtained at a heating rate of 20 °C/min, a temperature of 450 °C, a sweeping gas flow rate of 100 ml/min and a 0.5–2.0 mm size range. The chemical composition of the tar was examined by elemental analysis, FTIR spectroscopy, ¹H-NMR spectroscopy and column chromatography. The chemical composition of the tar with dense aliphatic structure was established to be CH_1.22O_0.25N_0.02. The composition of the gases obtained at a heating rate of 20 °C/min for the 0.5–2.0 mm size range was examined by gas chromatography.     

Devolatilization characteristics of high volatile coal in a wire mesh reactor

A wire mesh reactor was used to investigate the devolatilization process of coal particle during entrained flow gasification. Coal from Indonesia East Kalimantan mine, which has high moisture and high volatile matter, was chosen as a sample. Experiments were carried out at the heating rate of 1,000 °C/s and isothermal condition was kept at peak temperature under atmospheric pressure. The char, tar and gas formation characteristics of the coal as well as the composition of the gas components at peak temperatures were determined. The experimental results showed that devolatilization process terminated when temperature reached above 1,100 °C. Most of tar was formed at about 800 °C, while the rate of tar formation decreased gradually as the temperature increased. CH₄ was observed at temperatures above 600 °C, whereas H₂ was detected above 1,000 °C. The amount of formed gases such as H₂, CO, CH₄ and C _n H _m increased as the temperature increased. From the characteristics of devolatilization with residence time, it was concluded that devolatilization terminated within about 0.7 second when the temperature reached 1,000 °C. As the operating temperature in an entrained flow gasifier is higher than ash melting temperature, it is expected that the devolatilization time of high volatile coal should be less than one second in an entrained flow gasifier.     

Bio-oil from olive oil industry wastes: Pyrolysis of olive residue under different conditions

Olive residues were pyrolysed in a fixed bed reactor under different pyrolysis conditions to determine the role of final temperature, sweeping gas flow rate and steam velocity on the product yields and liquid product composition with a heating rate of 7 °C/min. Final temperature range studied was between 400 and 700 °C and the highest liquid product yield was obtained at 500 °C. Liquid product yield increased significantly under nitrogen and steam atmospheres. Liquid products obtained under the most suitable conditions were characterised by elemental analyses, FT-IR and ¹H-NMR. In addition, column chromatography was employed and the yields of the sub-fractions were calculated. Gas chromatography was achieved on n-pentane fractions. The results show that it is possible to obtain liquid products similar to petroleum from olive residue if the pyrolysis conditions are chosen accordingly.     

Bio-oil from olive oil industry wastes: Pyrolysis of olive residue under different conditions

Olive residues were pyrolysed in a fixed bed reactor under different pyrolysis conditions to determine the role of final temperature, sweeping gas flow rate and steam velocity on the product yields and liquid product composition with a heating rate of 7 °C/min. Final temperature range studied was between 400 and 700 °C and the highest liquid product yield was obtained at 500 °C. Liquid product yield increased significantly under nitrogen and steam atmospheres. Liquid products obtained under the most suitable conditions were characterised by elemental analyses, FT-IR and ¹H-NMR. In addition, column chromatography was employed and the yields of the sub-fractions were calculated. Gas chromatography was achieved on n-pentane fractions. The results show that it is possible to obtain liquid products similar to petroleum from olive residue if the pyrolysis conditions are chosen accordingly.     

Pilot Development of Polygeneration Process of Circulating Fluidized Bed Combustion combined with Coal Pyrolysis

A pilot polygeneration process of a 75 t h^–1 circulating fluidized bed (CFB) boiler combined with a moving bed coal pyrolyzer was developed based on laboratory‐scale experimental results. The process operation showed good consistency and integration between boiler and pyrolyzer. Some critical operating parameters such as hot ash split flow from the CFB boiler to the pyrolyzer, mixing of hot ash and coal particles, control of pyrolysis temperature and solid inventory in the pyrolyzer, and pyrolysis gas clean‐up were investigated. Yields of 6.0 wt‐% tar and 8.0 wt‐% gas with a heating value of about 26 MJ m^–3 at 600 °C were obtained. Particulate content in tar was restrained less than 4.0 wt‐% by using a granular filter of the moving bed. Operation results showed that this pilot polygeneration process was successfully scaled up.     

Mechanism of coal pyrolysis in relation to industrial practice

The validity of our earlier postulates of the mechanism of primary pyrolysis (at and up to 600 °C) is critically examined and it is indicated that the mechanism is strictly followed only under ideal conditions, e.g. in thin beds at rapid rates of carbonization, as in fluidbed and transport reactors. The departure of the Gray-King assay (600 °C) from the ideal path of pyrolysis, e.g. by yielding 20–30% less tar than the yield corresponding to hydroaromatic carbon content, is shown to be due to interaction between the potential tar-forming constituents and the incipient coke-forming substance. This appears to be a function of the thickness of the coal bed, the rate of heating, etc. The greater the thickness, the greater is the degree of interaction and consequent inhibition of tar formation, resulting in a proportionate increase in coke yield. Coke and tar yields are thus partly interconvertible, and the proportions of such interaction have virtually no effect on the proportion of carbon appearing as gas. In industrial high-temperature carbonization, the higher yields of coke and lower yields of tar are due to the same interaction, which occurs to a greater extent primarily because of the greater thickness and/or depth of the coal bed in coke ovens. The fixation of up to 75% of the ‘tar-forming’ carbon (hydroaromatic carbon according to the theory) does not appear to be due to cracking of tar after its formation, but is shown to be foreshadowed well within the primary stage of pyrolysis (below 600 °C), perhaps through condensation-polymerization reactions within the formative coke mass, the mechanism of which is ill-understood at present. The process appears to be very different from the cracking mechanism hitherto believed to explain it. This conclusion is also supported by a study of the distribution of carbon in the gas. Further, such comparative studies between laboratory and industrial conditions do not indicate any significant cracking of methane, hitherto believed to occur in coke ovens. Correspondingly, the reasons for carbon deposition on the exposed hot walls and other regions of coke ovens are discussed and doubt is thrown on the belief that it derives from the cracking of tar and gas.     

Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant

Treatment of plastic waste by gasification in fluidized bed with air using dolomite as tar cracking catalyst has been studied. The gasifier has a 1 m high bed zone (diameter of 9.2 cm) followed by a 1 m high freeboard (diameter of 15.4 cm). The feedstock is composed of blends of plastic waste with pine wood sawdust and coal at flow rates of 1–4 kg/h. Operating variables studied were gasifier bed temperature (750–880 °C), equivalence ratio (0.30–0.46), feedstock composition and the influence of secondary air insertion in freeboard. Product distribution includes gas and char yields, gas composition (H₂, CO, CO₂, CH₄, light hydrocarbons), heating value and tar content in the flue gas. As a result, a gas with a medium hydrogen content (up to 15% dry basis) and low tar content (less than 0.5 g/m_n³) is obtained.