Fischer-Tropsch catalyst testing in a continuous bench-scale coal gasification system

A bench-scale oxygen-blown fluid-bed gasifier was coupled to a modular fixed-bed Fischer-Tropsch (FT) reactor system for testing an FT catalyst under syngas. Various blends of subbituminous coal, torrefied biomass, and untreated biomass were gasified at 22 bar absolute, 800°-860 °C, and 4 kg/h. Syngas exiting the fluid bed passed through a cyclone, candle filter, and sulfur sorbent to reduce fine particulate and H₂S to levels well below 1 ppmv. The syngas was cooled to condense out moisture and volatiles and then reheated to temperatures required for FT synthesis. The clean syngas then flowed into the FT reactor with a 5:1 ratio of recycled FT product gas-to-fresh syngas feed. A 70% overall conversion of CO and H₂ was achieved at 269 °C and 18.9 bar over an iron-based catalyst supported on gamma-alumina pellets.     

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.     

Syngas and a separate nitrogen/argon stream via chemical looping reforming - A 140 kW pilot plant study

A dual circulating fluidized bed pilot plant was operated in chemical looping reforming conditions at a scale of 140 kW fuel power with natural gas as fuel. A nickel-based oxygen carrier was used as bed material. The pilot plant is equipped with an adjustable cooling system. Three experimental campaigns have been carried out at 747 °C (1020 K), 798 °C (1071 K) and 903 °C (1176 K), respectively. In each campaign, the global stoichiometric air/fuel ratio was varied step-wise between 1.1 and the minimum value possible to keep the desired operating temperature when the cooling is finally switched off. The results show that the fuel reactor exhaust gas approaches thermodynamic equilibrium. The residual amount of methane left decreases with increasing fuel reactor temperature. Further, the oxygen in the air reactor can be completely absorbed by the solids as soon as the air reactor operating temperature is higher than 900 °C (1173 K). Even though no steam was added to the natural gas feed no carbon formation was found for global excess air ratios larger than 0.4.     

Isothermal and Morphological Studies of the Torrefaction of Spruce Wood

The replacement of fossil fuels by biofuel for decreasing the action of greenhouse gases on the global climate is encouraged in industrially developed countries. A promising trend in the refining of waste biomass is torrefaction—a mild pyrolysis process in which biomass is heated to 250–350°C without the access of oxygen at low heating rates; as a result, biocoal with improved chemical and physical properties is formed. The torrefaction (mild pyrolysis at 250–300°C) of spruce stem wood was studied in a fixed-bed reactor at different temperatures. The mass and energy yields of biocoal, its specific heat of combustion, and morphological changes in the biomass structure in the course of spruce wood torrefaction were determined. It was established that the torrefied samples began to decompose at higher temperatures, as compared with the nontorrefied biomass. The torrefied fuel had a higher heat of combustion, which increased with the temperature of torrefaction. Conclusions on the restructuring of test samples and the formation of a porous structure at different temperatures depending on exposure time were made.     

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.     

Pressurized gasification of woody biomass—Variation of parameter

Pressurized gas produced from biomass is a renewable resource that is attracting a great deal of attention due to its wide range of industrial applications, such as the production of hydrogen, chemicals or high grade fuels. Therefore, the Vienna University of Technology in cooperation with BioEnergy 2020+ is operating a bubbling pressurized gasification plant. The pressurized research unit (PRU) is able to perform the gasification of wood chips, wood pellets, coal and other solid fuels with gasification agents air, steam, oxygen or carbon dioxide. This paper gives the results of parameter variation at this plant with regard to the producer gas composition. The feedstock was wood pellets and as bed material olivine was used with an average particle size of 0.5 mm. The parameters varied were temperature (720-900 °C), pressure (1-5 bar), air ratio (0.2-0.4), gasification agent (air, steam, oxygen), biomass feed input (4.5-8 kg/h) and the fluidization conditions of the reactor fluidized bed (fluidization number (3-7)).     

Thermodynamic study of combining chemical looping combustion and combined reforming of propane

Existing energy generation technologies emit CO₂ gas and are posing a serious problem of global warming and climate change. The thermodynamic feasibility of a new process scheme combining chemical looping combustion (CLC) and combined reforming (CR) of propane (LPG) is studied in this paper. The study of CLC of propane with CaSO₄ as oxygen carrier shows thermodynamic feasibility in temperature range (400-782.95 °C) at 1 bar pressure. The CO₂ generated in the CLC can be used for combined reforming of propane in an autothermal way within the temperature range (400-1000 °C) at 1 bar pressure to generate syngas of ratio 3.0 (above 600 °C) which is extremely desirable for petrochemical manufacture. The process scheme generates (a) huge thermal energy in CLC that can be used for various processes, (b) pure N₂ and syngas rich streams can be used for petrochemical manufacture and (c) takes care of the expensive CO₂ separation from flue gas stream and CO₂ sequestration. The thermoneutral temperature (TNP) of 702.12 °C yielding maximum syngas of 5.98 mol per mole propane fed, of syngas ratio 1.73 with negligible methane and carbon formation was identified as the best condition for the CR reactor operation. The process can be used for different fuels and oxygen carriers.     

Mechanisms and kinetics of homogeneous secondary reactions of tar from continuous pyrolysis of wood chips

The change of mass and composition of biomass tar due to homogeneous secondary reactions was experimentally studied by means of a lab reactor system that allows the spatially separated production and conversion of biomass tar. A tarry pyrolysis gas was continuously produced by pyrolysis of wood chips (fir and spruce, 10-40 mm diameter) under fixed-bed biomass gasification conditions. Homogeneous secondary tar reactions without the external supply of oxidising agents were studied in a tubular flow reactor operated at temperatures from 500 to 1000 °C and with space times below 0.2 s. Extensive chemical analysis of wet chemical tar samples provided quantitative data about the mass and composition of biomass tar during homogeneous conversion. These data were used to study the kinetics of the conversion of gravimetric tar and the formation of PAH compounds, like naphthalene.It is shown that, under the reaction conditions chosen for the experiments, homogeneous secondary tar reactions become important at temperatures higher than 650 °C, which is indicated by the increasing concentrations of the gases CO, CH₄, and H₂ in the pyrolysis gas. The gravimetric tar yield decreases with increasing reactor temperatures during homogeneous tar conversion. The highest conversion reached in the experiments was 88% at a reference temperature of 990 °C and isothermal space time of 0.12 s. Hydrogen is a good indicator for reactions that convert the primary tar into aromatics, especially PAH. Soot appears to be a major product from homogeneous secondary tar reactions.     

An experimental study of biomass ignition

An experimental study has been conducted to determine the ignition temperature of biomass at 21% O₂, both under pulse ignition conditions and under thermogravimetric conditions. In the pulse ignition experiments, samples of about 2 g wheat straw were placed in an isothermal reactor. The ignition temperature was determined from the transient CO and CO₂ profiles to approximately 255 °C at a superficial gas velocity of 14 cm/s (STP). The ignition temperature increased for decreasing superficial gas velocity.Thermogravimetric experiments at 20% O₂ and heating rates of 5 °C/min with finely milled biomass indicated ignition temperatures of approximately 220 °C for wheat straw, 235 °C for poplar wood, and 285 °C for eucalyptus wood. These values are significantly lower than values obtained for coal under similar conditions and confirm the relationship between volatile matter content and ignition temperature previously reported for coal.A mechanistic model for ignition of biomass is proposed. We believe that the ignition process is initiated by oxidation reactions on the straw surface. These reactions raise the surface temperature above that of the surrounding gas and promote ignition of the volatiles. Once ignited, the volatiles may form a homogeneous diffusion flame away from the particle surface. The superficial gas velocity affects the particle heating rate as well as the transport of oxygen to the surface. For this reason the ignition process is not entirely controlled by kinetics at low temperatures.     

Features of vapor-grown cone-shaped graphitic whiskers deposited in the cavities of wood cells

Vapor-grown graphitic whiskers in wood charcoal formed during heat treatment above 2000 °C are discussed through electron microscope observations. The whiskers were composed of conical stacked hexagonal carbon layers, with a cone apex angle of 136°. This angle probably helps the successive helical growth of the whiskers. To grow these whiskers, pyrolyzed gas from the walls of wood cells could be used as the carbon source, and the wood cell cavities functioned as a reactor that can store the pyrolyzed gas and concentrated it to supersaturation levels. Whiskers were produced under various conditions, and their features were compared using electron microscopy to examine the growth mechanism. Samples were prepared by precarbonizing wood along with SiC via a secondary heating process at 2000-2700 °C. The precarbonizing temperature controlled the potential supply of pyrolysate gas. Whisker growth began when the gas concentration reached the supersaturation level, and this was regulated by the secondary heat treatment temperature. The gas concentration can determine the size, features, and yield of the resulting whiskers. Since the carbon source was internal, originating from the surrounding cell walls, the regulation of the gas concentration was due entirely to the heat treatment and the features of the cell tissue.     

Pyrolysis of wood at high temperature: The influence of experimental parameters on gaseous products

The pyrolysis of wood was carried out in an Entrained Flow Reactor at high temperature (650 to 950 °C) and under rapid heating conditions (> 10³ K s^− 1). The influence of the diameter and initial moisture of the particle, reactor temperature, residence time and the nature of the gaseous atmosphere on the composition of the gaseous products has been characterised. Particle size, between 80-125 and 160-200 μm, did not show any impact. Pyrolysis and tar cracking essentially happen in very short time period: less than 0.6 s; the products yields are only slightly modified after 0.6 s in the short residence times (several seconds) of our experiments. Higher temperatures improve hydrogen yield in the gaseous product while CO yield decreases. Under nitrogen atmosphere, after 2 s at 950 °C, 76% (daf) of the mass of wood is recovered as gases: CO, CO₂, H₂, CH₄, C₂H₂, C₂H₄ and H₂O. Tests performed under steam partial pressure showed that hydrogen production is slightly enhanced.     

NMR analysis of the transformation of wood constituents by torrefaction

The injection of biomass in a pressurised entrained flow reactor is challenging. Biomass preparation by torrefaction before gasification could be a suitable option to improve it. Transformation of the material induced by this treatment lead to interesting features: increased brittleness, improved fluidisation properties of the powder, hydrophobicity, higher energy content. The major biomass constituents, cellulose, hemicelluloses and lignin are variously affected by torrefaction, depending on their respective reactivity. The objective of this work is to investigate the transformation of the biomass constitutive polymers induced by this thermal treatment. For that purpose, both solid-state NMR and EPR investigations have been performed on wood samples (beech) torrefied at different temperatures ranging from 200 °C to 300 °C. The results of these investigations have been compared with data obtained on untreated wood. These characterizations have brought to light different transformations of the polymers: de-acetylation of hemicelluloses, demethoxylation of lignin, changes in the cellulose structure. Furthermore, the temperature at which depolymerisation of the different components begins to occur has been identified.     

Pyrolysis of waste-derived fuel mixtures containing PVC

This paper describes an experimental analysis of the pyrolysis of PVC and mixtures of PVC with wood (Finnish pine) and LDPE (low density polyethene) in nitrogen at 250-400 °C. The aim is to optimise the temperature range for producing low-chlorine or chlorine-free fuel in a dehydrochlorination reactor without pyrolysing any of the other combustible fractions. Results are presented for various process temperatures for PVC, PVC/wood and PVC/LDPE mixtures. It was found that the PVC tested is dehydrochlorinated at approximately 350 °C, and that secondary pyrolysis is suppressed when LDPE is present.     

Experimental study on CO2 capture conditions of a fluidized bed limestone decomposition reactor

In this study, the decomposition conditions of limestone particles (0.25-0.50 mm) for CO₂ capture in a steam dilution atmosphere (20-100% steam in CO₂) were investigated by using a continuously operating fluidized bed reactor. The results show that the decomposition conversion of limestone increased with the steam dilution percentage in the CO₂ supply gas. At a bed temperature of 920 °C, the conversions were 72% without steam dilution and 98% with 60% steam dilution. The conversion was 99% with 100% steam dilution at 850 °C of the bed temperature. Steam dilution can decrease not only the decomposition temperature of limestone, but also the residence time required for nearly complete decomposition of CaCO₃. The hydration and carbonation reactivities of the CaO produced were also tested and the results show that both the reactivities increased with the steam dilution percentage for decomposing limestone.     

Wood/plastic copyrolysis in an auger reactor: Chemical and physical analysis of the products

Previous studies observed that slow copyrolysis of wood and plastic in enclosed autoclaves produced an upgraded raw bio-oil with increased hydrogen content. We now demonstrate that fast simultaneous pyrolyses of 50:50, w/w, pine wood/waste plastics in a 2 kg/h lab scale auger-fed reactor at 1 atm, with a short vapor residence time, generates higher heating value upgraded bio-oils. Three plastics: polystyrene (PS), high density polyethylene (HDPE) and polypropylene (PP) were individually copyrolyzed with southern yellow pine wood at 525, 450 and 450 °C, respectively, to generate modified bio-oils upon condensation. These liquids exhibited higher carbon and hydrogen contents, significantly lower oxygen contents, higher heats of combustion and lower water contents, acid values and viscosities than pine bio-oil. The formation of cross-over wood/plastic reaction products was negligible in the oils. Simultaneous pyrolysis process design requires using a temperature at which the plastic’s thermal decomposition kinetics produce vapors rapidly enough to prevent vaporized plastic from condensing on wood chars and exiting the reactor.     

Catalytic steam gasification of Miscanthus X giganteus in fluidised bed reactor on olivine based catalysts

The Miscanthus X giganteus (MXG) presents many advantages (high yield, perennial crop, easy harvesting…) so it can be considered as a good candidate in terms of renewable energy sources. Several works have been carried out and were devoted to the MXG, especially in the agricultural field, but this study is the first which deals with gasification in order to produce syngas. The catalytic steam gasification of MXG in a fluidised bed reactor into presence of olivine based catalysts was investigated. Three parameters were studied, the temperature (800 °C and 900 °C), the pellets size (6 mm and 8 mm) and the nature of catalyst (olivine and Ni/olivine). Noteworthy is the efficiency shown by the Ni/olivine at 800 °C, which leads to the production of 1.7 m³ kg^− 1 daf of gas, containing 50% of H₂. Ni/olivine catalyst was characterised by XRD, TPR and SEM-EDX in order to monitor its structural changes during the process. Moreover, a solvent system of tar recovery was tested, which allows to obtain a more representative set of the whole tars. Then, the tars composition was determined by GC/MS. The identification of different compounds shows the presence of different PAHs, in majority naphthalene.     

Batch testing of solid fuels with ilmenite in a 10 kWth chemical-looping combustor

Batch experiments were conducted in a 10 kW_th chemical-looping combustor for solid fuels using ilmenite, an iron titanium oxide, as the oxygen carrier with two solid fuels: a petroleum coke from Mexico and a bituminous coal from South Africa. The purpose of these batch tests was to attain detailed information on fuel conversion, complementary to previous continuous operation of the unit. At steady-state, a fuel batch of typically 25 g was introduced in the fuel reactor and gas concentrations were measured at the outlet of both air and fuel reactors. The fuel reactor was fluidized with steam and the amount of bed material was typically 5 kg. The fuel introduced devolatilizes rapidly while the remaining char is gasified and the resulting syngases H₂ and CO react with the oxygen carrier. Operation involved testing at different fuel reactor temperatures from 950 to 1030 °C, and investigation of the influence of particle circulation between air and fuel reactors.The fuel conversion rate was increased at higher temperature: at 950 °C the instantaneous rate of conversion for petroleum coke averaged at 17.4%/min while at 1030 °C, the value was 40%/min. For the much more reactive South African coal, the averaged rate at 970 °C was 47%/min and increased to 101%/min at 1000 °C. For petroleum coke testing with particle circulation, the oxygen demand - defined as oxygen lacking to fully convert the gases leaving the fuel reactor - was typically 12-14% for the gasified char including H₂S, in line with previous experiments with the same unit and fuel. If only syngases are considered, the oxygen demand for char conversion was 8.4-11%. Similar or even lower values were seen for the char of South African coal. This is in line with expectations, i.e. that it is possible to reach fairly high conversion, although difficult to reach complete gas conversion with solid fuel. It was also seen that the volatiles pass through the system essentially unconverted, an effect of feeding the fuel from above. Moreover, the oxygen demand for char conversion decreased with increasing temperature. Finally, the CO₂ capture - defined as the proportion of gaseous carbon leaving the fuel reactor to total gaseous carbon leaving the system - decreased at higher particle circulation and a correlation between capture and circulation index was obtained.     

Optimum operating conditions for a two-stage gasification process fueled by polypropylene by means of continuous reactor over ruthenium catalyst

We studied fuel gas production by means of pyrolysis and steam reforming of waste plastics for applications in solid oxide fuel cells. More specifically, we evaluated the effects of pyrolytic gasification temperature, catalyst content, steam reforming temperature, and weight hourly space velocity for a Ru catalyst used in a 60 g h^− 1-scale continuous experimental apparatus, which consisted of a tank reactor for pyrolysis and a packed-bed catalytic reactor for steam reforming. Polypropylene (PP) pellets were used as a model waste plastic. Ru/γ-Al₂O₃ catalysts with two different Ru contents were investigated. To suppress residue formation, the optimum operating temperature of the pyrolyzer was 673 K. To ensure suppressed coke formation, sufficient carbon conversion to gaseous products, and minimized heat loss from the reactor, the optimum operating conditions for the reformer were determined to be 903 K and 0.11 g-sample g-catalyst^− 1 h^− 1 with a 5 wt.% Ru/γ-Al₂O₃ catalyst. The composition of the gas produced with the 5 wt.% catalyst was almost the same as that predicted by chemical equilibrium laws, and it was applicable for a direct hydrocarbon fuel cell.     

Coffee husks gasification using high temperature air/steam agent

Analyses made on the world's biomass energy potential show that biomass energy is the most abundant sustainable renewable energy. The available technical biomass energy potential surpasses the total world's consumption levels of petroleum oils, coal and natural gas. In order to achieve a sustainable harnessing of the biomass energy potential and to increase its contribution to the world's primary energy consumption, there is therefore a need to develop and sustain contemporary technologies that increase the biomass-to-energy conversion. One such technology is the high temperature air/steam gasification (HTAG) of biomass. In this paper we present findings of gasification experimental studies that were conducted using coffee husks under high temperature conditions. The experiments were performed using a batch facility, which was maintained at three different gasification temperatures of 900 °C, 800 °C, and 700 °C. The study findings exhibited the positive influence of high temperature on increasing the gasification process. Chars left while gasifying at 800 °C and 700 °C were respectively 1.5 and 2.4 times that for the case of 900 °C. Furthermore, increased gasification temperature led to a linear increment of CO concentration in the syngas for all gasification conditions. The effect was more pronounced for the generally poorly performing gasification conditions of N₂ and 2% oxygen concentration. When gasification temperature was increased from 700 °C to 900 °C the CO yield for the 2% O₂ concentration increased by 6 times and that of N₂ condition by 2.5 times. The respective increment for the 3% and 4% O₂ conditions were only twofold. This study estimated the kinetic parameters for the coffee husks thermal degradation that exhibited a reaction mechanism of zero order with apparent activation energy of 161 kJ/mol and frequency factor of 3.89 × 10⁴/min.     

Catalytic pyrolysis of biomass for biofuels production

Fast pyrolysis bio-oils currently produced in demonstration and semi-commercial plants have potential as a fuel for stationary power production using boilers or turbines but they require significant modification to become an acceptable transportation fuel. Catalytic upgrading of pyrolysis vapors using zeolites is a potentially promising method for removing oxygen from organic compounds and converting them to hydrocarbons. This work evaluated a set of commercial and laboratory-synthesized catalysts for their hydrocarbon production performance via the pyrolysis/catalytic cracking route. Three types of biomass feedstocks; cellulose, lignin, and wood were pyrolyzed (batch experiments) in quartz boats in physical contact with the catalysts at temperature ranging from 400 °C to 600 °C and catalyst-to-biomass ratios of 5-10 by weight. Molecular-beam mass spectrometry (MBMS) was used to analyze the product vapor and gas composition. The highest yield of hydrocarbons (approximately 16 wt.%, including 3.5 wt.% of toluene) was achieved using nickel, cobalt, iron, and gallium-substituted ZSM-5. Tests performed using a semi-continuous flow reactor allowed us to observe the change in the composition of the volatiles produced by the pyrolysis/catalytic vapor cracking reactions as a function of the catalyst time-on-stream. The deoxygenation activity decreased with time because of coke deposits formed on the catalyst.