Kinetic model considering catalyst deactivation for the steam reforming of bio-oil over Ni/La2O3-αAl2O3 (original) (raw)
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Fuel, 2018
Hydrogen-rich gas production by steam reforming (SR) of the raw bio-oil was studied in a continuous two-step system, with the first unit of thermal treatment (at 500°C) used for retaining the pyrolytic lignin. The remaining volatile stream was reformed in the second unit (fluidized bed reactor) over a Ni/La 2 O 3-αAl 2 O 3 catalyst at 700°C. The effect of space-time (0.04-0.38 g catalyst h/g bio-oil) and steam-to-carbon ratio (S/C) (1.5-6) on bio-oil conversion and product yields was assessed. Temperature programmed oxidation (TPO) was used to analyze the coke deposited on the Ni/La 2 O 3-αAl 2 O 3 catalyst. It was found that a raise in both the space-time and the S/C ratio contribute to increasing the H 2 yield and to decreasing that of CO, CH 4 and C 2-C 4 hydrocarbons. Catalyst deactivation is highly attenuated by raising space-time because of the lower deposition of encapsulating coke, which is directly related to the concentration of bio-oil oxygenates in the reaction medium. Space-time does not affect the formation of filamentous coke (less responsible for deactivation). The S/C ratio has less influence on total coke content than space time. For 700°C, 0.38 g catalyst h/g bio-oil and S/C = 6, a hydrogen-rich gaseous stream (66 vol% H 2) is obtained, with the H 2 yield being 93% based on the bio-oil entering the catalytic reactor (or 87% based on the raw bio-oil fed into the two-step system), which decreases to 70% after 7 h time on stream as a consequence of the low catalyst deactivation. (3) Under usual operating conditions, the H 2 yield obtained is lower than the stoichiometric maximum because of side reactions, such as thermal decomposition of bio-oil oxygenates (Eq. (4)), methanation (Eqs. (5) and (6)), Boudouard reaction (Eq. (7)), and thermal decomposition of CH 4 (Eq. (8)). These reactions lead to the formation of byproducts (CO, CO 2 , CH 4 and light hydrocarbons) and to carbon (coke)
Fuel Processing Technology, 2013
A study was carried out on the effect of temperature (in the 500-800°C range) and space-time (between 0.10 and 0.45 g catalyst h(g bio-oil ) −1 ) on Ni/La 2 O 3 -αAl 2 O 3 catalyst deactivation by coke deposition in the steam reforming of bio-oil aqueous fraction. The experiments were conducted in a two-step system, provided with a thermal step at 200°C for the pyrolytic lignin retention and an on-line step of catalytic reforming in fluidized bed reactor. Full bio-oil conversion and a hydrogen yield of around 95% (constant for 5 h) were achieved at 700°C, S/C (steam/carbon) ratio of 12 and space-time of 0.45 g catalyst h(g bio-oil ) −1 . The results of catalyst deactivation were explained by mechanisms of coke formation and evolution, which are established based on kinetic results and coke analysis by temperature programmed combustion. At 700°C the coke is gasified and Ni does not undergo sintering. The results of hydrogen yield were compared with those obtained in the literature using different reaction technologies.
Chemical engineering transactions, 2017
This work aimed to establish a suitable O2 feeding strategy for the hydrogen production by oxidative steam reforming (OSR) of raw bio-oil in a reaction system with two-steps: thermal treatment (at 500 °C, for the controlled deposition of pyrolytic lignin) followed by the reforming of the volatile stream in a fluidized bed reactor. Specifically, the effect of co-feeding O2 before or after the thermal step was analyzed for oxygen-to- carbon molar ratio (O/C) in the 0.34-0.67 range. The catalytic step was kept at 700 °C, steam-to-carbon molar ratio (S/C) = 6.0, and space-time = 0.6 gcatalyst h(gbio-oil)-1. When O2 is co-fed before the thermal step, there is a partial combustion of both, pyrolytic lignin and oxygenates, thus resulting a lower amount of oxygenated compounds entering the reforming reactor, although the composition of these oxygenates is not affected by the presence of O2 in the thermal step. As a result, a noticeable lower H2 yield was obtained when O2 is fed before the t...
Steam Reforming of the Bio-Oil Aqueous Fraction in a Fluidized Bed Reactor with in Situ CO 2 Capture
Industrial & Engineering Chemistry Research, 2013
The effect of CO 2 capture in hydrogen production by steam reforming of the bio-oil aqueous fraction was studied. The reforming and cracking activity of the adsorbent (dolomite) and the relationship between these reactions and those corresponding to the catalyst (reforming and water gas shift (WGS)) were considered. The experiments were conducted in a two-step system with the first step at 300°C for pyrolytic lignin retention. The remaining volatiles were reformed in a subsequent fluidized bed reactor on a Ni/La 2 O 3 −α-Al 2 O 3 catalyst. A suitable balance was stricken between the reforming and WGS reactions, on the one side, and the cracking and coke formation reactions, on the other side, at 600°C for catalyst/ dolomite mass ratios ≥0.17. At this temperature and space-time of 0.45 g catalyst h (g bio-oil) −1 , bio-oil was fully converted and the H 2 yield was around 99% throughout the CO 2 capture step. Catalyst deactivation was very low because the cracking hydrocarbon products (coke precursors) were reformed.
Steam Reforming of Raw Bio-oil in a Fluidized Bed Reactor with Prior Separation of Pyrolytic Lignin
Energy & Fuels, 2013
The effect that operating conditions (temperature, steam/carbon molar ratio, and space-velocity) have on the steam reforming of raw bio-oil has been studied in a two-step reaction unit. In the first step (operated at 500°C), a carbonaceous solid (pyrolytic lignin) deposits by repolymerization of certain bio-oil components, and the remaining volatiles are reformed in the second step (fluidized bed reactor) on a Ni/La 2 O 3 −αAl 2 O 3 catalyst. Under suitable reforming conditions (700°C, S/C = 9, space-velocity = 8000 h −1 ), the yields of H 2 and CO were 95% and 6%, respectively. Catalyst deactivation was very low, whereby the H 2 yield decreased by only 2% over 100 min of reaction. By using dolomite as adsorbent in the reforming reactor, CO 2 was effectively captured, and the raw bio-oil was reformed at 600°C without adding water (S/C = 1.1), thus avoiding its vaporization cost. The yields of H 2 and CO were 80−82% and 1%, respectively, for a space-velocity (G C1 HSV) of 7000 h −1 and catalyst/ dolomite ratio of 0.25, although a high yield of CH 4 (7%) was obtained due to the cracking capacity of the dolomite. The coke content on the catalyst was high (7.7 wt % in 2 h) because of the limited gasification of coke precursors under the operating conditions (low temperature and low S/C ratio) used in the process with CO 2 capture.
Fuel, 2018
The hydrogen production by steam reforming (SR) of raw bio-oil (obtained by fast pyrolysis of pine sawdust) has been studied in a continuous two-step process, which consists of a thermal treatment at 500°C, followed by SR in a fluidized bed reactor with Ni/La 2 O 3-αAl 2 O 3 catalyst. The effect of SR temperature on bio-oil conversion, product yields and catalyst deactivation was evaluated in the 550-700°C range. The bio-oil conversion and H 2 yield were significantly enhanced by increasing temperature. A H 2 yield of around 88% and low catalyst deactivation were achieved at temperatures above 650°C, for a S/C (steam/carbon) ratio of 6 and space-time of 0.10 g catalyst h/g bio-oil. The influence temperature has on product yields and catalyst deactivation was explained by the different nature of the coke deposited. The temperature-programmed oxidation (TPO) curves of coke combustion allow identifying two fractions: i) Coke I, which is the main responsible for deactivation (by encapsulating the Ni sites), whose formation depends on the concentration of bio-oil oxygenates; ii) Coke II, which has filamentous nature and CO and CH 4 as main precursors. The effect of temperature on the formation of both types of coke depends on the space-time. Thus, for low values (0.04 g catalyst h/g bio-oil) there is significant formation of both types of coke, with their content increasing with temperature. For higher values (0.38 g catalyst h/ g bio-oil), the increase in reaction temperature promotes the removal of coke I, and therefore this is the prevailing fraction at 550°C and is negligible at 700°C. This fact is of special relevance for attenuating the Ni/La 2 O 3-αAl 2 O 3 catalyst deactivation.
Reaction Chemistry and Engineering, 2020
A Computational Fluid Dynamic (CFD) model was derived and validated, in order to, investigate the hydrodeoxygenation 9 reaction of 4-propylguaiacol, which is a lignin-derived compound present in bio-oil. A 2-D packed bed microreactor was 10 simulated using pre-sulphided NiMo/Al2O3 solid catalyst in isothermal operation. A pseudo-homogeneous model was first 11 created to validate the experimental results from literature. Various operational parameters were investigated and validated 12 with the experimental data, such as temperature, pressure and liquid flow rate; and it was found that the CFD findings were 13 in very good agreement with the results from literature. The model was then upgraded to that of a detailed multiphase 14 configuration; and phenomena such as internal and external mass transfer limitations were investigated, as well as, reactant 15 concentrations on the rate of 4-propylguaiacol. Both models agreed with the experimental data, and therefore confirm their 16 ability for applications related to the prediction of the behaviour of bio-oil compounds hydrodeoxygenation. 17 33 obtained which has a heating value of approximately half of that 34 of conventional fuel oil averaging at about 30 MJ kg-1 4. Biomass 35 derived bio-oil has several disadvantages such as a low heating 36 value, high viscosity and a high oxygen content, which all restrict 37 its application as a liquid fuel. Therefore, further upgrading of 38 bio-oil by hydrodeoxygenation (HDO) is required 5. 39 40 The HDO process converts the oxygen containing compounds 41 such as acids, aldehydes, alcohols and phenol to oxygen-free 42 hydrocarbon fuels 6. Bio-oil obtained from the fast pyrolysis of 43 lignin contains approximately 39% of guaiacol and its 44 derivatives. Amongst these constituents, guaiacol is often 45 regarded as a representative model for bio-oil derived from 46 lignin because it has two types of CO bonds (Csp2OH and 47 65 phenolic compounds in the bio-oil is the origin of 66 polymerisation and coke formation during HDO at elevated 67 temperatures greater than 300 o C 3. 68 69 Lee et al. 8 studied the HDO of a model compound of lignin-70 derived bio-oil (guaiacol) because of its high potential to be 71 used as a substitute for conventional fuels. Platinum-loaded HY 72 zeolites (Pt/HY) with varying Si/AL molar ratios were used as 73 catalysts for the HDO of guaiacol, anisole, veratrole and phenol 74
Catalytic steam reforming of bio-oil
International Journal of Hydrogen Energy, 2012
Hydrogen and synthesis gas can be produced in an environmentally friendly and sustainable way through steam reforming (SR) of bio-oil and this review presents the stateof-the-art of SR of bio-oil and model compounds hereof. The possible reactions, which can occur in the SR process and the influence of operating conditions will be presented along with the catalysts and processes investigated in the literature. Several catalytic systems with Ni, Ru, or Rh can achieve good performance with respect to initial conversion and yield of hydrogen, but the main problem is that the catalysts are not stable over longer periods of operation (>100 h) due to carbon deposition. Support materials consisting of a mixture of basic oxides and alumina have shown the potential for low carbon formation and promotion with K is beneficial with respect to both activity and carbon formation. Promising results have been obtained in both fluidized and fixed bed reactors, but the coke formation appears to be less significant in fluidized beds. The addition of O 2 to the system can decrease the coke formation and provide autothermal conditions at the expense of a lower H 2 and CO-yield. The SR of bio-oil is still in an early stage of development and far from industrial application mainly due the short lifetime of the catalysts, but there are also other aspects of the process which need clarification. Future investigations in SR of bio-oil could be to find a sulfur tolerant and stable catalyst, or to investigate if a prereformer concept, which should be less prone to deactivation by carbon, is suitable for the SR of bio-oil.
Kinetics, thermodynamics and mechanisms for hydroprocessing of renewable oils
Applied Catalysis A: General, 2016
Intrinsic kinetics, diffusivity, energy calculations and reaction mechanism studies for the conversion of plant-oil triglycerides over Ni-W/SiO2-Al2O3 hydrocracking catalyst is reported. Specific insights into reaction mechanisms, are established using kinetic modeling and validated with experimental results. Diffusion studies and effectiveness factor calculations established diffusion-free intrinsic kinetics, with an activation energy of 115 kJ/mole required for triglycerides conversion. Thermodynamic calculations further established that the heat released during propane removal step (1.15 MJ/kg) was 8-times higher than that for hydrodeoxygenation step (0.14 MJ/kg) and lowest for hydrocracking reactions (0.08 MJ/kg). Such high exothermicity resulted in high temperature gradient across the catalyst bed (370°C above the reaction temperature).
Steam Reforming of Bio-oil Fractions: Effect of Composition and Stability
Energy & Fuels, 2011
The efficacy of steam reforming of the aqueous species in bio-oils produced from the fast pyrolysis of biomass is examined. A fractionating condenser system was used to collect a set of fractions of fast pyrolysis liquids with different chemical characteristics. The water-soluble components from the different fractions were steam-reformed using a nickel-based commercial catalyst in a fixed-bed reactor system. When reforming at 500°C, an overall positive effect in hydrogen yields was observed for the fractions with higher concentrations of lower molecular-weight oxygenates, such as acetic acid and acetol, while the heavier compounds, such as the carbohydrates, showed an opposite effect. In general, higher selectivity toward hydrogen correlated to a lower tendency toward carbon deposits. Overall, the bio-oil fraction corresponding to the light end performed the best with the highest activity toward hydrogen. A range of steam/carbon ratios was examined. Carbon accumulation in the reactor was clearly a main issue during steam reforming of all of the bio-oil fractions studied. Chemical changes caused by aging of aqueous bio-oil were found to have a detrimental effect on hydrogen production.