Release of volatiles from the pyrolysis of a Victorian lignite at elevated pressures

A Loy Yang lignite sample was pyrolysed in a wire-mesh reactor at pressures from 1 to 61 bar. The char yield did not show considerable sensitivity to changes in pressure or heating rate and was mainly a function of temperature. However, the tar yield was sensitive to changes in pressure, holding time and heating rate. The tar yield at 1000 K s⁻¹ showed a minimum at around 6-11 bar. The tar yield at 1 K s⁻¹ increased slightly with increasing pressure from 1 to 20 bar. The UV-fluorescence spectroscopy of the tar samples indicated that the release of larger (three or more fused rings) aromatic ring systems was also greatly affected by increases in pressure. It is believed that increases in pressure have slowed down the bulk diffusion within meso- and macro-pores in the pyrolysing lignite/char particles. During the extended stay within the char particle, volatile precursors were thermally cracked to form mainly gaseous species as well as very small amounts of char. At very high pressures where the diffusion was very slow, the formation of light gases caused the pressure to build up within the particles, inducing the forced flow of volatile precursors out of the particles and leading to increased tar yield.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

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

Comprehensive study of the influence of total pressure on products yields in fluidized bed gasification of wood sawdust

Wood sawdust gasification experiments were performed in a steam fluidized bed at 800 °C between 2 and 10 bar. An evolution of gas yields with time was measured during the tests, and especially an increase of hydrogen and carbon dioxide yields. This test duration effect was ascribed to char build-up in the bed. As tests proceed, the contribution of char steam gasification to gas yield increases, and the catalytic effect of char on hydrocarbons and tar conversion and on water-gas shift reaction is enhanced.As total pressure increases from 2 to 10 bar, hydrogen, carbon dioxide and methane yields increase by 16%, 53% and 38% respectively, whereas carbon monoxide yield decreases by 33%. The changes in gaseous yields with pressure can be partly explained by the influence of pressure on gas phase reactions (acceleration of water-gas shift kinetics and change in hydrocarbon reactions). The increase of methane yield with pressure is rather suggested to be linked to a change in secondary pyrolysis reactions scheme under high pressure.     

Fundamentals of coal topping gasification: Characterization of pyrolysis topping in a fluidized bed reactor

Coal topping gasification refers to a process that extracts the volatiles contained in coal into gas and tar rich in chemical structures in advance of gasification. The technology can be implemented in a reactor system coupling a fluidized bed pyrolyzer and a transport bed gasifier in which coal is first pyrolyzed in the fluidized bed before being forwarded into the transport bed for gasification. The present article is devoted to investigating the pyrolysis of lignite and bituminite in a fluidized bed reactor. The results showed that the highest tar yield appeared at 823 to 923 K for both coals. When coal ash from CFB boiler was used as the bed material, obvious decreases in the yields of tar and pyrolysis gas were observed. Pyrolysis in a reaction atmosphere simulating the pyrolysis gas composition of coal resulted in a higher production of tar. Under the conditions of using CFB boiler ash as the bed material and the simulated pyrolysis gas as the reaction atmosphere, the tar yields for pyrolytic topping in a fluidized bed reactor was about 11.4 wt.% for bituminite and 6.5 wt.% for lignite in dry ash-free coal base.     

Study of the influence of pressure on enhanced gaseous hydrocarbon yield under high pressure-high temperature coal pyrolysis

The influence of pressure on the yield of gaseous hydrocarbon products derived from pyrolysis of Fushun and Xianfeng coals have been investigated in an anhydrous and confined system. Pyrolysis was performed in sealed gold tubes at 380 °C and under the pressures ranging from 50 to 250 MPa for 24 h. The results show that the effect of pressure on coal pyrolysis and product generation should not be ignored. For the Fushun and Xianfeng lignite, the yields of gaseous hydrocarbon generation increase by 9.1% and 12.7% when the pressure increases from 50 to 250 MPa, respectively. However, the yields of hydrogen gas decrease greatly with pressure. The hydrogen gas yields of Fushun and Xianfeng lignite decrease by 76.5% and 75.9%, respectively, when the pressure increases from 50 to 250 MPa. Yields of carbon dioxide gas of Fushun and Xianfeng coals were enhanced with increasing pressure by 7.4% and 8.9% respectively. Data of stable carbon isotope compositions reveal that the methane and ethane carbon isotope values are also affected by pressure, as they become heavier by approximately 1.2‰ (PDB) when the pressure is increased from 50 to 250 MPa. Simultaneously, the hydrogen isotope compositions of methane and ethane increase by 10.3‰ and 7.1‰, respectively. Our experimental results suggest that the increase in gaseous hydrocarbon yield is resulted from synthesis of carbon dioxide and hydrogen and pressure serves to facilitate the synthetic process.     

Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char

Pine wood was pyrolyzed in a fixed bed reactor at a heating rate of 10 °C and a final temperature of 700 °C, and the resultant volatiles were allowed to be secondarily cracked through a tubular reactor in a temperature range of 500-700 °C with and without packing a bed of char. The thermal effect and the catalytic effect of char on the cracking of tar were investigated. An attempt was made to deconvolute the intermingled contributions of the char-catalyzed tar cracking and the char gasification to the yields of gaseous and liquid products. It was found that the wood char (charcoal) was catalytically active for the tar cracking at 500-600 °C, while at 650-700 °C, the thermal effect became a dominant mode of the tar cracking. Above 600 °C, the autogenerated steam gasified the charcoal, resulting in a marked increase in the yield of gaseous product and a significant change in the gas composition. An anthracite char (A-char), a bituminous coal char (B-char), a lignite char (L-char) and graphite also behaved with catalytic activities towards the tar cracking at lower temperature, but only L-char showed reactivity for gasification at higher temperature.     

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.     

Influence of carrier gas flow and heating rates in fixed bed hydropyrolysis of coal

Fixed bed hydropyrolysis experiments on a UK bituminous coal (82% dmmf C) at 580–650 °C and pressures up to 300 bar have indicated that tar yields depend strongly on the velocity of the hydrogen carrier gas relative to the static coal particles. Tar yields increase with increasing pressure provided that the superficial gas velocity is not reduced. Otherwise, tar yields can actually decrease because the beneficial hydrocracking reactions that occur are no longer sufficient to counter the increased char formation resulting from the slower rates of intra-particle diffusion and devolatilization of tar molecules. While raising the heating rate from 1 to 20 °C s⁻¹ had little effect on overall conversions, hydrocarbon gas yields increased significantly at the expense of tar. Moreover, the higher heating rate gave more aromatic tars, and the available evidence strongly suggests that the primary volatiles are hydrocracked before escaping from the coal particles as well as in the vapour phase.     

A two-stage fixed-bed reactor for direct hydrotreatment of volatiles from the hydropyrolysis of biomass: effect of catalyst temperature, pressure and catalyst ageing time on product characteristics

This investigation involved the hydropyrolysis of biomass (eucalyptus globulus) and the immediate catalytic hydrocracking of pyrolytic oils in the second stage of the reactor. The effects of temperature, pressure and the catalyst ageing time on the final product tar have been studied using the catalyst Zeolite H-ZSM5. The catalytically hydrocracked tar/oil products were characterised and compared with the hydropyrolysis product from the first stage of the reactor to determine the effect of catalytic hydrocracking. The carbon deposition on the catalyst has been examined using thermogravimetric analysis. The tar yields after catalytic hydrocracking decreased with increasing pressure and temperature of the cracking stage. The tar yields at 10 bar pressure were greater than those at 40 bar pressure. The fresh zeolite catalyst trapped more than 40% of the product from the hydropyrolysis stage and TGA evidence indicated that this was not as carbon deposition but as volatiles trapped in the zeolite matrix. Reuse of the catalyst resulted in little more uptake of volatiles; however, extended use of the catalyst did not result in increased yields of liquid products but in increased production of light volatiles or gas. The H-ZSM5 catalyst appeared to act as a more active cracking catalyst rather than to promote hydrogenation or deoxygenation of the liquids produced in the hydropyrolysis stage. Characterisation of the liquids by SEC and UV fluoresence indicated that structural changes were relatively minor despite the significant changes in yields of liquids with process conditions. Available reaction routes do not appear to allow specific deoxygenation pathways to predominate without disintegration of parent molecules to lighter volatiles, under the conditions used here.     

Release of alkali and alkaline earth metallic species during rapid pyrolysis of a Victorian brown coal at elevated pressures

Possible reaction mechanisms responsible for the release of Na and Mg during pyrolysis at elevated pressures are described in this paper. In order to evaluate these mechanisms a Victorian brown coal, Loy Yang coal, was pyrolysed in a wire-mesh reactor at pressures up to 6.1 MPa at a heating rate of 1000 °C s⁻¹. Release of Na and Mg were quantified as functions of temperature and pressure. The results demonstrated that increasing pressure suppresses or promotes release of Na and Mg depending on the combination of pressure and temperature. The results obtained have been explained qualitatively by the proposed reaction mechanisms. At temperatures of 600 °C and lower, the release of Na and Mg from the pyrolysing coal/char particles, as light carboxylates, other organic salts and/or metals, was controlled by their diffusion through the pore system of the particles and, therefore, was suppressed by increasing pressure. At higher temperatures, the release of Na and Mg seems to be affected by the changes in intra-particle mass transfer mechanism due to increasing pressure as well as by chemical reactions responsible for the formation of volatile Na and Mg species.     

Synergies in co-pyrolysis of Thai lignite and corncob

The results from TGA experiments at the temperature range of 300–600 °C evidently distinguished the different pyrolysis behaviours of lignite and corncob; however, no clear synergistic effects could be observed for the mixture. The investigation of co-pyrolysis in a fixed-bed reactor, however, found significant synergies in both pyrolysis product yields and gas product compositions. The solid yield of the 50:50 lignite/corncob blend was much lower (i.e. 9%) than expected from the calculated value based on individual materials under the range of temperatures studied, and coincided with the higher liquid and gas yield. The synergistic effect in product gas composition was highly pronouncing for CH₄ formation, i.e. three times higher than the calculated value at 400 °C. Possible mechanisms were described including the interaction between corncob volatiles and lignite particles, and the effect of the heat profiles of lignite and corncob pyrolysis on the temperature dependent reactions. The enhanced devolatilisation of the blend was explained by the transfer of hydrogen from biomass to coal as well as the promotion of low-temperature thermal decomposition of lignite by exothermic heat released from corncob pyrolysis. Moreover, water, which was one of the major components in corncob volatiles produced mainly at around 200–375 °C, can also be expected to act as a reactive agent to promote the secondary tar cracking producing more CH₄.     

Rapid devolatilization and hydrogasification of bituminous coal

Rapid devolatilization and hydrogasification of a Pittsburgh Seam bituminous coal were studied and an appropriate coal conversion (weight loss) model was developed that accounts for thermal decomposition of the coal, secondary char-forming reactions of volatiles, and homogeneous and heterogeneous reactions involving hydrogen. Approximately monolayer samples of coal particles supported on wire mesh heating elements were electrically heated in hydrogen, helium, and mixtures thereof. Coal weight loss (volatiles yield) was measured as a function of residence time (0–20 s), heating rate (65–10000 °C/s), final temperature (400–1100 °C), total pressure (0.0001–7 MPa), hydrogen partial pressure (0–7 MPa), and particle size (70–1000 μm). Volatiles yield under these conditions increases significantly with decreasing pressure, decreasing particle size, increasing hydrogen partial pressure and increasing final temperature, but only slightly with increasing heating rate. The data support the view that coal conversion under these conditions involves numerous parallel thermal decomposition reactions forming primary volatiles and initiating a sequence of secondary reactions leading to char. Intermediates in this char-forming sequence can escape as tar if residence time in the presence of hot coal surfaces is sufficiently short (e.g. low pressures and small particles well dispersed). Hydrogen at sufficiently high partial pressure can interrupt the char-forming sequence thereby increasing volatile yield. Rate of total product generation is largely controlled by coal pyrolysis while competition between mass transfer, secondary reactions, and rapid hydrogenation affects only the relative proportions of volatile and solid products formed.     

Effect of pressure on the hydropyrolysis of Manvers coal

Manvers weakly-coking coal was pyrolysed to 500 °C in a stirred autoclave under varying pressures of hydrogen and nitrogen. As expected the major changes produced by increase in nitrogen pressure were a decrease in tar yield accompanied by increases in gas and, to a smaller extent, in coke yields. Total pressures and hydrogen :coal ratios were altered to obtain maximum yields of tar, gases and liquor. All products were investigated. Tar fractions, separated into neutral, phenolic and basic components, were analysed by g.c.-m.s. Short-chain hydrocarbons were detected in the gas sample. Methanol densities and micropore surface areas the cokes were related to the conditions of pyrolysis. At the relatively low rates of heating employed, pressure had effects on tar composition similar to increasing the temperature of pyrolysis.     

Synergy in devolatilization characteristics of lignite and hazelnut shell during co-pyrolysis

Coal/biomass blends were prepared in the lignite/biomass ratios of 98:2, 96:4, 94:6, 92:8, 90:10, and 80:20 using a Turkish lignite from Elbistan region and hazelnut shell. Co-pyrolysis characteristics were investigated in a thermogravimetric analyzer (TGA) from ambient to 1173 K with a linear heating rate of 20 K/min under dynamic nitrogen flow of 40 ml/min. Char products from pyrolysis were investigated using XRD and SEM techniques. Devolatilization yields from the blends were evaluated in a synergistic manner and found that the overall yields for all the blends exceeded the expected yields which calculated from the additive behavior. As regards to devolatilization characteristics in given temperature intervals, it was concluded that there was significant synergy between 400 and 600 K, whereas additive behavior took place beyond 600 K. No evidence of synergy was observed in the activation energies. It was also concluded that the addition of hazelnut shell into lignite contributed to the sulfur fixing potential of char in the form of CaS and CaSO₄.     

The effect of steam on pyrolysis and char reactions behavior during rice straw gasification

Steam gasification of biomass can generate hydrogen-rich, medium heating value gas. We investigated pyrolysis and char reaction behavior during biomass gasification in detail to clarify the effect of steam presence. Rice straw was gasified in a laboratory scale, batch-type gasification reactor. Time-series data for the yields and compositions of gas, tar and char were examined under inert and steam atmosphere at the temperature range of 873-1173 K. Obtained experimental results were categorized into those of pyrolysis stage and char reaction stage. At the pyrolysis stage, low H₂, CO and aromatic tar yields were observed under steam atmosphere while total tar yield increased by steam. This result can be interpreted as the dominant, but incomplete steam reforming reactions of primary tar under steam atmosphere. During the char reaction stage, only H₂ and CO₂ were detected, which were originated from carbonization of char and char gasification with steam (C + H₂O→CO + H₂). It implies the catalytic effect of char on the water-gas shift reaction. Acceleration of char carbonization by steam was implied by faster hydrogen loss from solid residue.     

煤快速热解制油技术问题的化学反应工程根源:逆向传热与传质

从化学反应工程的角度分析了煤热解过程中挥发物逸出方向与传热方向相反的现象,阐述了反应温度和反应时间对挥发物反应(二次反应)的不同作用,指出提高热源温度加快热解速度的方法反而会促进挥发物二次反应,导致焦油损失与结焦增加,进而提出抑制挥发物二次反应的关键是降低挥发物在反应器中的温升幅度。     

A study on the pyrolysis of asphalt

The pyrolysis of asphalt has been studied using thermogravimetric analysis at atmospheric pressure and with nitrogen as the ambient gas. The heating rates ranged from 50 or 80 °C min⁻¹ to a final temperature of 650 °C. A two-stage first-order model is established to describe the pyrolysis of asphalt. In the model, the activation energy, E, is different for each stage, but is independent of the type of asphalt and its heating rate. The frequency factor depends on the heating rate and is independent of the asphalt. The final yield of volatiles depends on the type of asphalt. The modeled results agree with the experimental measurements, so the model is reasonable.     

热解温度及AAEM元素对生物质快速热解焦油的影响

生物质热解受热解温度、热解速率和碱金属及碱土金属(AAEM)元素影响显著。利用热裂解气相色谱质谱联用法(Py-GC/MS)针对热解温度及AAEM元素对生物质快速热解焦油的影响展开深入研究,通过样品热解前后的失重情况分析了热解温度及AAEM元素对生物质(稻壳和木屑、酸洗稻壳和酸洗木屑)热解特性的影响规律,利用气相色谱质谱仪(GC/MS)对热解焦油组分及含量进行了在线半定量分析,并对热解焦油组分分子量分布情况展开了讨论。结果表明生物质Py-GC/MS快速热解实验,酸洗脱除AAEM元素致使热解失重率减小。500~900℃范围内随温度的升高,大分子焦油成分逐渐减少,逐渐转化为轻质组分。AAEM元素限制了焦油前体的聚合,进一步抑制了含氧杂环类碳环(糠醛等)的生成。稻壳的热解焦油的相对分子质量主要分布在110~129。木屑快速热解焦油产率明显高于稻壳,且热解焦油中分子量分布广泛,含有更多较大分子量(150~209)的化合物成分。     

Continuous pyrolysis of waste tyres in a conical spouted bed reactor

Continuous pyrolysis of scrap tyres has been carried out in a conical spouted bed reactor and the results (yields, composition of the volatile fraction and carbon black properties) have been compared with those obtained operating in batch mode in a previous study. Continuous operation in the 425-600 °C range gives way to a yield of 1.8-6.8 wt.% of gases, 44.5-55.0 wt.% of liquid fraction (C₅-C₁₀ range hydrocarbons, with a maximum yield of limonene of 19.3 wt.% at 425 °C), 9.2-11.5 wt.% of tar and 33.9-35.8 wt.% of char. The main differences between the continuous and batch processes are in the yield of light aromatics, which is higher in the continuous process, and in that of the heavy liquid fraction or tar, which is higher in the batch process. These are the advantages of the continuous process, although hydrogenation of the liquid fraction is required even in this case in order to use it as fuel. The high yield of limonene, the flexibility in the operating conditions and the capacity for a continuous removal of the residual carbon black from the reactor are the advantages of conical spouted bed technology. The excellent performance of the conical spouted bed reactor for the tyre pyrolysis process is due to the solid cyclic movement, the good contact between phases, the high heating rate and the reduced residence time of the volatile products.     

Effect of pyrolysis pressure and heating rate on radiata pine char structure and apparent gasification reactivity

The knowledge of biomass char gasification kinetics has considerable importance in the design of advanced biomass gasifiers, some of which operate at high pressure. The char gasification kinetics themselves are influenced by char structure. In this study, the effects of pyrolysis pressure and heating rate on the char structure were investigated using scanning electron microscopy (SEM) analysis, digital cinematography, and surface area analysis. Char samples were prepared at pressures between 1 and 20 bar, temperatures ranging from 800 to 1000 °C, and heating rates between 20 and 500 °C/s. Our results indicate that pyrolysis conditions have a notable impact on the biomass char morphology. Pyrolysis pressure, in particular, was found to influence the size and the shape of char particles while high heating rates led to plastic deformation of particles (i.e. melting) resulting in smooth surfaces and large cavities. The global gasification reactivities of char samples were also determined using thermogravimetric analysis (TGA) technique. Char reactivities were found to increase with increasing pyrolysis heating rates and decreasing pyrolysis pressure.