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.     

Three‐stage well‐mixed reactor model for a pressurized coal gasifier

Three Canadian coals of different rank were gasified with air‐steam mixtures in a 0.1 m diameter spouted bed reactor at pressures to 292 kPa, average bed temperatures varying between 840 and 960°C, and steam‐to‐coal feed ratios between 0.0 and 2.88. In order to analyze gasifier performance and correlate data, a three‐stage model has been developed incorporating instantaneous devolatilization of coal, instantaneous combustion of carbon at the bottom of the bed, and steam/carbon gasification and water gas shift reaction in a single well mixed isothermal stage. The capture of H₂S by limestone sorbent injection is also treated. The effects of various assumptions and model parameters on the predictions were investigated. The present model indicates that gasifier performance is mainly controlled by the fast coal devolatilization and char combustion reactions, and the contribution to carbon conversion of the slow char gasification reactions is comparatively small. The incorporation of tar decomposition into the model provides significantly closer predictions of experimental gas composition than is obtained otherwise.     

Effects of operating factors in the coal gasification reaction

The effects of operating factors on a gasification system were reviewed by comparing a computational simulation and real operation results. Notable operation conditions include a conveying gas/coal ratio of 0.44, an oxygen/coal ratio of 0.715, a reaction temperature of 1,000 °C, and reaction pressure of 5bar in the case of Adaro coal; based on this, the cold gas efficiency was estimated as 82.19%. At the point of the reaction temperature effect, because the cold gas efficiencies are more than 80% when the reaction temperatures are higher than 900 °C, the gasifier inner temperature must remain over 900 °C. At high reaction temperature such as 1,400 °C, the reaction pressure shows little effect on the cold gas efficiency. The addition of steam into the gasifier causes an endothermic reaction, and then lowers the gasifier outlet temperature. This is regarded as a positive effect that can reduce the capacity of the syngas cooler located immediately after the gasifier. The most significant factor influencing the cold gas efficiency and the gasifier outlet temperature is the O₂/coal ratio. As the O₂/coal ratio is lower, the cold gas efficiency is improved, as long as the gasifier inner temperature remains over 1,000 °C. With respect to the calorific value (based on the lower heating value, LHV) of produced gas per unit volume, as the N₂/coal ratio is increased, the calorific value per syngas unit volume is lowered. Decreasing the amount of nitrogen for transporting coal is thus a useful route to obtain higher calorific syngas. This phenomenon was also confirmed by the operation results.     

High‐temperature pyrolysis and CO2 gasification of Victorian brown coal and Rhenish lignite in an entrained flow reactor

The low rank coals from Victoria, Australia, and Rhineland, Germany are of interest for use in entrained flow gasification applications. Therefore, a high temperature, electrically heated, entrained flow apparatus has been designed to address the shortage of fundamental data. A Victorian brown coal and a Rhenish lignite were subjected to rapid, entrained flow pyrolysis between 1100 and 1400°C to generate high surface area chars, which were subsequently gasified at the same temperatures under CO₂ in N₂ between 10 and 80 vol %. The Victorian coal was more reactive than the Rhenish coal, and peak char reactivity was observed at 1200°C. Char conversion and syngas yield increased with increasing temperature and plateaued at high CO₂ concentration. Ammonia and tar species were negligible and HCN and H₂S were present in parts per million (volume) concentrations in the cooled, filtered syngas. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2101–2111, 2016     

A new integrated approach of coal gasification: the concept and preliminary experimental results

The total carbon conversion of conventional fluidized bed gasifier is relatively low (<90%) mainly because of carbon loss in fly-ash. In this paper, a new concept of integrated coal gasification—fluidized bed+entrained flow is introduced. Within this process, large partition of coal with higher reactivity is converted in an ash agglomerating fluidized bed reactor under moderate temperature (~1000 °C). The remaining small partition of coal (fly-ash) with lower reactivity is converted in a small integrated entrained flow gasifier under higher temperature (1200–1400 °C). Low carbon content ash is withdrawn in dry mode by ash agglomerating, with no need to be melted. Preliminary experimental results show that the whole system can be operated steadily, total carbon conversion reaches >95%, efficient gas (CO+H₂) concentration is 78–82%. Heat exchange between two reactors has been realized, the high temperature gas from entrained flow gasifier can be cooled, and in the mean time the temperature of fluidized bed nearly keeps constant. The high-temperature ash from entrained flow gasifier can be cooled by the char in dense phase of the fluidized bed and then withdrawn in agglomerating mode. All these results prove the concept correct and feasible.     

Experimental test on a novel dual fluidised bed biomass gasifier for synthetic fuel production

This article presents a preliminary test on the 150 kW_th allothermal biomass gasifier at Mid Sweden University (MIUN) in Härnösand, Sweden. The MIUN gasifier is a combination of a fluidised bed gasifier and a CFB riser as a combustor with a design suitable for in-built tar/CH₄ catalytic reforming. The test was carried out by two steps: (1) fluid-dynamic study; (2) measurements of gas composition and tar. A novel solid circulation measurement system which works at high bed temperatures is developed in the presented work. The results show the dependency of bed material circulation rate on the superficial gas velocity in the combustor, the bed material inventory and the aeration of solids flow between the bottoms of the gasifier and the combustor. A strong influence of circulation rate on the temperature difference between the combustor and the gasifier was identified. The syngas analysis showed that, as steam/biomass (S/B) ratio increases, CH₄ content decreases and H₂/CO ratio increases. Furthermore the total tar content decreases with increasing steam/biomass ratio and increasing temperature. The biomass gasification technology at MIUN is simple, cheap, reliable, and can obtain a syngas of high CO + H₂ concentration with sufficient high ratio of H₂ to CO, which may be suitable for synthesis of methane, DME, FT-fuels or alcohol fuels. The measurement results of MIUN gasifier have been compared with other gasifiers. The main differences can be observed in the H₂ and the CO content, as well as the tar content. These can be explained by differences in the feed systems, operating temperature, S/B ratio or bed material catalytic effect, etc.     

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.     

Gasification studies of onakawana lignite

Onakawana lignite was gasified in air, steam and an air + steam mixture in a fixed bed reactor. The extent of devolatilization was determined by pyrolysis in nitrogen. The composition of products, expressed in terms of H₂/CO ratio, was temperature dependent. The ratio decreased with increasing temperature. During steam gasification the ratio decreased from 4.6 to 2.6 when temperature increased from 700° to 990°C. The addition of air to steam resulted in a marked decrease of this ratio. Steam gasification reactivity of chars prepared from Onakawana lignite at 500°C and 800°C were studied in the temperature range of 650°C to 1000°C. The carbon conversion results were fitted into equations describing the continuous and shrinking core models. The char prepared at 500°C was much more reactive than the one prepared at 800°C. Product distribution expressed as the H₂/CO ratio, was favourable in the temperature range. For comparison, the Kentucky #9 coal and chars derived from this coal were used as referee materials. The reactivity of these chars was markedly lower than that of chars derived from Onakawana lignite.     

Effect of particle size on lignite devolatilization in a fixed-bed reactor

A fixed-bed coal gasification reactor was set up which specifically simulated the devolatilization zone in a gasifier. Samples (100 g) of lignite coal in three size ranges; ?2+1 mm, ?3+2 mm and ?4+3 mm, were devolatilized in the temperature range 350–550 °C with a steam-oxygen mixture, at 1 atm. The effect of these operating variables on tar yield and composition, melting point, viscosity, specific gravity, and molecular weight distribution was determined. A first-order reaction model was fitted to the experimentally observed total loss in weight of the lignite.     

Thermogravimetric study of the devolatilization of hydropyrolysis chars

The devolatilization of hydropyrolysis (HyPy) chars formed at 485–850 °C under 3 MPa and chars formed at 580 °C under 0–5 MPa of H₂ and 3 MPa He is investigated in a thermobalance coupled to two gas chromatographs. The H₂, CH₄ and CO₂ released are analysed every 4 min and all are analysed at the end of the experiment. The amount of residual volatile matter in the chars decreases rectilinearly with the HyPy temperature, whereas their decrease is asymptotic with the HyPy pressure. The char formed under He contains 45% more residual volatile matter than that formed in the same conditions under H₂. The HyPy temperature must be limited if the char is to be burned in a boiler. The CH₄ formation is strongly dependant of the HyPy temperature.     

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.     

Metallic iron as a tar breakdown catalyst related to atmospheric,fluidised bed gasification of biomass

Tar formation is a major drawback when biomass is converted in a gasifier to obtain gas aimed for utilisation in power production plants or for production of chemicals. Catalytic cracking is an efficient method to diminish the tar content in the gas mixture. In this study, the capability of metallic iron and iron oxides to catalytically crack tars has been experimentally examined. To obtain metallic iron, small grains of hematite (Fe₂O₃) were placed in a secondary reactor downstream the gasifier and reduced in situ prior to catalytic operation. The fuel used in the atmospheric fluidised bed gasifier was Swedish birch with a moisture content of approximately 7 wt%.The influence of temperature in the range 700–900 °C and λ values (i.e. equivalence ratio, ER) between 0 and 0.20 have been investigated. In essence, the results show that raising the temperature in the catalytic bed to approximately 900 °C yields almost 100% tar breakdown. Moreover, increasing the λ value also improves the overall tar cracking activity. The iron oxides did not demonstrate any catalytic activity.     

Effect of pyrite and pyrrhotite on free radical formation in coal

The effects of pyrite (FeS₂) and pyrrhotite (Fe₇S₈) on free radical formation in a coal sample (81% carbon content) have been investigated by electron spin resonance (e.s.r.) spectroscopy. Changes in the e.s.r. parameters (spin concentration g^-1, n, linewidth ΔH and g-value) were monitored in samples of coal, coal+8% FeS₂ and coal+8% Fe₇S₈, as these samples were heated in vacuum or in hydrogen from room temperature to 500 °C, in steps of 50 °C for a residence time of 30 min at each temperature. In vacuum heating, changes in n begin to occur at 400 °C, 350 °C and 300 °C respectively for coal, coal+8%Fe₇S₈ and coal+8% FeS₂ samples whereas in H₂, the corresponding temperatures are 250 °C, 200 °C and 150 °C. Changes in ΔH and g were also observed at these temperatures. The maximum increase in n occured for coal+8% FeS₂ samples whereas the minimum increase was observed for the pure coal sample. It is argued that enhancement in n is due to two mechanisms: the pyrite to pyrrhotite conversion and the presence of pyrrhotite itself. The detailed nature of the catalytic activity of pyrrhotite is not known.     

Pyrolysis of tire powder: influence of operation variables on the composition and yields of gaseous product

The pyrolysis of tire powder was studied experimentally using a specially designed pyrolyzer with high heating rates. The composition and yield of the derived gases and distribution of the pyrolyzed product were determined at temperatures between 500 and 1000 °C under different gas phase residence times. It is found that the gas yield goes up while the char and tar yield decrease with increasing temperature. The gaseous product mainly consists of H₂, CO, CO₂, H₂S and hydrocarbons such as CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈, C₄H₈ and C₄H₆ with a little other hydrocarbon gases. Its heating value is in the range of 20 to 37 MJ/Nm³. Maximum heating value is achieved at a temperature between 700 and 800 °C. The product distribution ratio of gas, tar and char is about 21:44:35 at 800 °C. The gas yield increases with increasing gas residence time when temperature of the residence zone is higher than 700 °C. The gas heating value shows the opposite trend when the temperature is higher than 800 °C. Calcined dolomite and limestone were used to explore their effect on pyrolyzed product distribution and composition of the gaseous product. It is found that both of them affect the product distribution, but the effect on tar cracking is not obvious when the temperature is lower than 900 °C. It is also found that H₂S can be absorbed effectively by using either of them. About 57% sulfur is retained in the char and 6% in the gas phase. The results indicated that high-energy recovery could not be achieved if fuel gas is the only target product. In view of this, multi-use of the pyrolyzed product is highly recommended.     

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.     

高效能两段组合式煤气化过程热态试验

针对现有气流床气化技术在显热回收方面的不足,华东理工大学洁净煤技术研究所创新性开发煤基两段组合式气化工艺。在所建立的两段组合式煤气化炉热态试验装置上,考察了二段处理煤量和一段出口煤气组成对出口煤气热值、有效气浓度、二段碳转化率、水蒸气和二氧化碳转化率的影响。试验结果表明此气化工艺能有效利用一段炉煤气中的显热,提高气化炉出口煤气热值;二段适宜加入褐煤量为1400g,是一段处理量的10%;二段加煤量过多会降低二段煤层反应温度和促使焦油的生成;随着一段气化炉出口煤气所含水蒸气、CO2等气化剂浓度的增加,其对显热回收的作用就更明显;该工艺能减少CO2排放,具有良好的环境效益。     

Brown coal conversion under the action of supercritical water

A test bench was developed and the conversion of the organic matter of coal (OMC) in supercritical water (SCW) was studied under conditions of a continuous supply of a water-coal suspension to a vertical flow reactor at 390–760°C and a pressure of 30 MPa. From 44 to 63% OMC was released as liquid and gaseous products from coal particles (from the water-coal supension) during the time of fall to the reactor. This stage was referred to as the dynamic conversion of coal. The particles passed through the stage of the dynamic conversion of coal did not agglomerate in the reactor in the subsequent process of batch conversion in a coal layer at T = 550–760°C. The volatile products of the overall process of the dynamic and batch conversion of coal included saturated hydrocarbons (CH₄ and C₂H₆), aromatic hydrocarbons (C₆H₆, C₇H₈, and C₈H₁₀), synthesis gas (H₂ and CO), and CO₂. At T < 600°C, CO₂ and CO were the degradation products of oxygen-containing OMC fragments, whereas they also resulted from the decomposition of water molecules at higher temperatures in accordance with the reaction (C) + H₂O = CO + H₂. The mechanisms were considered, and the parameters responsible for the dynamic conversion of coal were calculated.