Elsevier

Biomass and Bioenergy

Volume 115, August 2018, Pages 97-107
Biomass and Bioenergy

Research paper
Hydrogen assisted catalytic biomass pyrolysis. Effect of temperature and pressure

https://doi.org/10.1016/j.biombioe.2018.04.012Get rights and content

Highlights

  • Catalytic hydropyrolysis of beech wood was studied at 365–511 °C and 1.6–3.6 MPa.

  • Up to 22% mass yield of condensed organics and C4+ gases has been obtained.

  • Up to 53% of the energy in biomass was recovered in the condensed organics and C4+.

  • The organic liquid was essentially oxygen free.

  • The organic liquid consisted of a volume fraction of 20–40% naphtha and 60–80% diesel.

Abstract

Beech wood has been converted into a mixture of oxygen-free naphtha and diesel boiling point range hydrocarbons by using catalytic hydropyrolysis in a fluid bed reactor with a CoMoS/MgAl2O4 catalyst, followed by deep hydrodeoxygenation (HDO) in a fixed bed reactor loaded with a NiMoS/Al2O3 catalyst. The effect of varying the temperature (365–511 °C) and hydrogen pressure (1.6–3.6 MPa) on the product yield and organic composition was studied. The mass balance closed by a mass fraction between 90 and 101% dry ash free basis (daf). The yield of the combined condensed organics and C4+ varied between a mass fraction of 17 and 22% daf, corresponding to an energy recovery of between 40 and 53% in the organic product. The yield of the non-condensable gases varied between a mass fraction of 24 and 32% daf and the char yield varied between 9.6 and 18% daf. The condensed organics contained a mass fraction of 42–75% aromatics, based on GC × GC-FID chromatographic peak area, and the remainder was primarily naphthenes with minor amounts of paraffins. The condensed organics were essentially oxygen free (mass fraction below 0.001%) when both reactors were used. Bypassing the HDO reactor increased the oxygen concentration in the condensed liquid to a mass fraction of 1.8%. The results show that catalytic hydropyrolysis may be a viable way to process solid biomass into liquid and gaseous fuels.

Introduction

Recent research has shown that catalytic hydropyrolysis is an efficient process for producing diesel and gasoline hydrocarbons from biomass [1,2]. The reactive molecules formed by fast pyrolysis are immediately hydrogenated, thus inhibiting polymerization and other undesired properties of conventional fast pyrolysis bio-oil. In this process, the pyrolysis takes place at an elevated hydrogen pressure and in the presence of a HDO catalyst. The basic concept of hydropyrolysis share similarities with the Bergius process (high temperature and high hydrogen pressure) [[3], [4], [5]], however in the Bergius process coal and heavy oil is mixed into a slurry, while hydropyrolysis is a gas phase process. Steinberg et al. [6] showed in 1985 that using fast hydropyrolysis of wood at high temperatures (600–1000 °C) over 90% of the carbon can be converted into hydrocarbons, mainly methane and other gases. It has also been shown that catalytic hydropyrolysis of lignin can give oil mass yields up to 80% [7]. Despite that hydropyrolysis is not a new concept it has first in the recent years become a popular method for producing liquid fuels from biomass. Using a pyroprobe reactor, Melligan et al. [8] showed that conducting the pyrolysis of Miscanthus in an atmosphere of H2 instead of He decreased the concentration of ethanoic acids in bio-oil. It has also been shown that zeolites impregnated with reduced transition metals increase the hydrocarbon yield and decrease the molecular size of the phenols [[8], [9], [10]]. Other groups have been pursuing the high pressure, non-catalytic hydropyrolysis of biomass in an inverted cyclone or fluid bed reactors followed by downstream catalytic HDO of the product vapors prior to oil and water condensation [[11], [12], [13]]. Marker et al. [1,2] have proposed a process called Integrated Hydropyrolysis and Hydroconversion (IH). Their process consists of a fluid bed reactor, where the catalytic hydropyrolysis takes place, and a fixed bed hydroconversion reactor, where the deep HDO takes place. Different types of biomass were tested, and the yield of condensed organic liquid and C4+ hydrocarbons in the product gas phase varied between a mass fraction of 20.6 and 46.3%. The IH process has been able to run continuously for 750 h in a pilot plant with a biomass feeding rate of 50 kg per day. The composition of the catalyst used in the IH process has not been reported. Carbon footprint analysis of the IH process showed that producing liquid fuels from this process, when compared to conventional production from fossil fuels, decreases the emission of greenhouse gases with 67–86% [14]. Dayton et al. [[15], [16], [17]] also conducted several studies on catalytic hydropyrolysis using loblolly pine with a setup that did not include an additional HDO reactor after the fluid bed catalytic hydropyrolysis reactor. Using a commercially available hydrotreating catalyst gave an initial low oil yield (mass fraction below 5%), but the oil yield increased over time to a mass fraction of 12.5% as the catalyst deactivated [15]. Several experiments at different temperatures and hydrogen pressures with a commercially available NiMo hydrotreating catalyst have also been conducted with the same setup [16]. The catalyst was reduced in hydrogen and not sulfided prior to the experiments. Liquid organic yields between a mass fraction of 12.6 and 25.6% with an oxygen mass fraction between 2.4 and 11.9% and a char yield between 7.4 and 26% were obtained depending on temperature, total pressure and hydrogen partial pressure. The carbon recovery in the organic liquid and C4+ gases varied between 34.8 and 42.0%, thus being significantly higher than for zeolite based catalytic pyrolysis [16]. These results indicate that catalytic hydropyrolysis is a potential technology for converting solid biomass to liquid transportation fuels. The knowledge base on this type of process in the open literature is however still scarce. Often the catalyst composition is not reported and the liquid oil produced is not characterized in depth. Furthermore the combination of catalytic hydropyrolysis followed by HDO is not fully understood.

In this study, catalytic hydropyrolysis of beech wood has been performed in a fluid bed reactor with a sulfided commercial CoMo/MgAl2O4 catalyst followed by a HDO reactor loaded with a sulfided commercial NiMo/Al2O3 catalyst. The concept is thus similar to the IH process [1]. With this reactor combination it is expected that the biomass can be converted into a mixture of naphtha and diesel. It is well-known that the temperature is an important parameter in pyrolysis [18], thus the effect of the temperature is investigated in the range relevant for catalytic hydropyrolysis (365–511 °C). There is also a lack of knowledge of the effect of hydrogen pressure on catalytic hydropyrolysis of wood, and the effect of pressure is therefore investigated in the range 1.6–3.6 MPa. Furthermore equilibrium calculations indicate that the liquid product composition changes in the tested temperature and pressure range. This is to our knowledge the first study in the open literature of hydropyrolysis of wood using a sulfided hydrotreating catalyst. In order to get a comprehensive understanding of the effect of the temperature and pressure the produced oil is extensively characterized.

Section snippets

Biomass feedstock

Bark free beech wood supplied by Dansk Træmel (Product number: 10000251250390) was used as biomass. The particle size was approximately 200–700 μm. The moisture mass fraction was 6.72% (dried at 105 °C) and the ash mass fraction was 0.59% dry basis. The composition and the higher heating value (HHV) of the beech wood are shown in Table 1.

Catalysts

The catalyst used in the fluid bed reactor was a CoMo/MgAl2O4 catalyst supplied by Haldor Topsøe A/S. The active CoMo phase was chosen because it is an

Effect of temperature and pressure on the product distribution

The effect of operating conditions on catalytic hydropyrolysis of beech wood was studied by varying the total pressure and the temperature in the fluid bed and the HDO reactor as shown in Table 2. The temperature in the fluid bed was between 365 and 511 °C and the temperature in the HDO reactor was between 345 and 400 °C. The total pressure was varied between 1.6 and 3.6 MPa. In experiments 1 to 8 the H2S concentration was 460 ×106 mol fraction, while it was approximately 50 ×106 mol fraction

Conclusion

In this work, beech wood was converted into liquid fuels by catalytic hydropyrolysis in a fluid bed reactor with a sulfided CoMo/MgAl2O4 catalyst followed by a deep hydrodeoxygenation in a fixed bed reactor with a sulfided NiMo/Al2O3 catalyst. The char yield decreased and the gas yield increased with increasing fluid bed temperature, while the condensable organic yield was less affected by the temperature. Increasing the total pressure mainly increased the aqueous phase yield and decreased the

Acknowledgments

This work is part of the H2CAP project (Hydrogen assisted catalytic pyrolysis for green fuels) conducted at the CHEC research center at The Department of Chemical and Biochemical Engineering at DTU, Denmark. The work was supported by The Danish Council for Strategic Research (now Innovation Fund Denmark, project 1377-00025A), The Programme Commission on Sustainable Energy and Environment. Funding from DTU is also gratefully acknowledged. The authors would also like to thank Research Engineer

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