Pyrolysis oil gasification

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Gasification is a well-known process in which a combustable gas is produced from carbon containing fuels. Production of gas from coal dates back to 17th century England[1], while the first commercial scale gas production can be traced back to 1812 with the foundation of the London Gas, Light and Coke Company[2]. In the last two centuries, various gasification technologies are developed and implemented converting a range of fuels to gaseous (intermediate) products.

Via gasification, pyrolysis oil can be converted to produce Syngas for chemical / biofuel synthesis, pure Hydrogen gas and Fuel gas for heat and power production.


Gasification Technologies

Gasification processes generally take place at high temperatures (typically > 700°C). The heat required can be generated externally (allothermal) such as in a tubular reformer where the fuel is heated indirectly by firing a secondary fuel to heat the outside of the tubes where the gasification process takes place inside the tubes. Alternatively, heat can be generated within the gasifier by allowing partial oxidation reactions to take place (autothermal system). In these autothermal systems, part of the fuel is combusted by mixing it with air/oxygen.

Pyrolysis oil can be gasified using various technologies, the desired gas composition often dictates which gasification technology is most suitable. In the table below, an overview is given of various pyrolysis oil gasification technologies. Following the development of the fast pyrolysis technology, a lot of research has been performed on the gasification of pyrolysis oil. Due to limited availability of pyrolysis oil, as well as it's deviating properties, attempts to gasify pyrolysis oil on pilot / demonstration scale are still scarce. Below details and the current status of the various technologies are presented.

Various gasification techniques
Technology Oxidant Temperature Target product
Entrained Flow Gasification Oxygen 1200-1500 °C Syngas
Catalytic Partial Oxidation Oxygen 800-950 °C Syngas
Steam Reforming Steam 600-800 °C Hydrogen


Entrained Flow Gasification

In Entrained Flow (EF) gasification processes, the fuel is mixed with pure oxygen to obtain high temperatures (> 1200°C) and subsequently high reaction rates and fuel conversion can be achieved. Several companies have developed entrained flow gasifier systems, well-known designs include those of:


Entrained flow gasification of pyrolysis oil is at the moment the technology closest to commercial application of all pyrolysis oil gasification systems. Pilot scale test work has been performed by several consortia. Research activities on smaller scale are also available, but often focus on the influence of individual process parameters, such as temperature, on the process perfomance. Results obtained at a small scale are often difficult to extrapolated to predict full scale behaviour due to the complex interaction between chemical reactions and heat and mass transfer limitations.

Back in 2002, the gasification of pyrolysis oil in an oxygen blown entrained flow gasifier was demonstrated by Shell and BTG in a 1 MWth gasifier owned by Future Energy GmbH (former Choren) in Freiberg. At a pyrolysis oil feed capacity of 140 kg/hour, a gasification run was performed lasting 10 hours, after which the test was terminated because all available pyrolysis oil was consumed. At an oxygen equivalence ratio of ~0.4, a temperature of 1250°C was obtained. In line with expectations the gas product was primarily composed of CO, CO2, H2 and H2O. The CH4 concentration was with 2.5 vol.% almost ten times higher than calculated for the thermodynamic equilibrium. No significant tar concentrations (aromatic of phenolic) were found in the product gas. For further information on these tests, see[3]

In 2012 and 2013, tests are performed in the pilot scale entrained flow gasifier of ETC in Sweden in the Suprabio project. Pyrolysis oil derived fom clean wood as well as from wheat straw was produced by BTG and gasified under various equivalence ratio's. At a thermal load of 0.4 MW, pressures up to 4 barg and temperatures of 1250-1300 °C syngas was produced. Typical dry gas compositions of 46% CO, 30% H2 and 23% CO2 were obtained. Gas phase contaminants included about 2% CH4, and ppm levels of H2S, COS and benzene. Details on these tests were presented at the 2013, and 2014 European Biomass Conference and Exhibition. A full paper is also available [4]


Alternative approach to produce syngas from biomass via pyrolysis and entrained flow gasification is explored by KIT under the name Bioliq. In this concept, the solid by product (char) obtained in the pryolysis process is mixed with the pyrolysis liquid. The resulting slurry can be converted to syngas via EF gasification. A schematic overview of the process is presented here. Benefit is that higher energetic efficiencies can be obtained, since the char by product typically contains 20-30% of the original biomass energy content. Disadvantage is that unlike pure pyrolysis oil, the slurry contains most of the ash from the original biomass. These ash componets cause slag formation inside the gasifier, requiring the use of a so called 'slagging gasifier', which are considerably more expencive compared to the 'non-slagging' variant. The Bioliq consortia is currently furthest towards commercial implementation of syngas (and subsequent biofuel synthesis) production from biomass via fast pyrolysis. A 500 kg/h pilot plant for the production of biosyncrude (the pyrolysis liquid/char slurry) is build and operated in campaigns of typically 2 weeks at the Karslruhe Institure of Technology. Various biomass streams have been converted to biosyncrude in the pilot plant. The High Pressure Entrained Flow gasifier is build with an input capacity of 5MW, for the gasification of biosyncrude at 1200°C and 80 bar. Product gas compositions are roughly 30% H2, 40% CO and 20% CO2. In 2013 the high pressure gas cleaning and DME / synfuel synthesis plants were constructed and July 2013 the DME synthesis plant was succesfully tested using artificial syngas. [5] [6]

Catalytic Partial Oxidation

Catalytic Partial Oxidation (CPO) is quite similar to (non-slagging) Entrained Flow gasification, with the exception that a catalyst is added to the system. The use of a catalyst enables higher conversion ratio's at lower temperatures, thus increasing the overall efficiency because a smaller fraction of the fuel needs to be combusted to generate the heat required for the process. Traditionally catalytic partial oxidation systems consist of an inlet/mixing zone, where fuel and oxidant are mixed, after which the mixture is led over a fixed bed catalyst. Operational difficulties associated with particulate formation in the atomization of pyrolysis oil has led to the investigation of alternative catalyst systems, such as the use of monolithic catalysts [7] or fluidized bed catalyst. [8]

Gasification of pyrolysis oil in catalytic partial oxidation systems has not been demonstrated at a scale larger than a few kg's an hour. Results of laboratory scale investigations are published by various researchers in various scientific journals [9]

In an autothermal system using Rh-Ce catalyst with millisecond contact time, Rennard et. al. succeded in achieving autothermal conditions. The addition of CH4 as co-feed improved the stability of the system. Product gas compositions depended on input parameters, but were quite similar to the values reported for EF gasification. Van Rossum et. al. reported the production of a methane free syngas in a process comprixing of an inert fluidized bed followed by catalytic fixed bed reactor. Commercial, Nickel based, catalysts were used to reform the pyrolysis vapours at atmospheric pressure and a temperature of 800°C. [10]. Leijenhorst et. al. demonstrated the gasification of pyrolysis oil in an atmospheric, air blown gasifier containing Nickel and Platinum based monolithic catalysts. A virtually tar and methane free product gas could be obtained at temperatures below 900°C. [11]

Steam Reforming

Steam reforming aims to produce typically hydrogen or somethimes synthesis gas as intermediate product for further processing. Conventional steam reforming is a process where hydrocarbons react with steam at high temperatures. The steam reforming reaction(1) is accompanied by the water gas shift (2) and methanation (3) equilibrium reactions.

  1. CnHm + nH2O ↔ nCO + (n+m/2)H2
  2. CO + H2O ↔ CO2 + H2
  3. CO+ 3H2 ↔ CH4 + H2O


Because the reforming reaction is strongly endothermic, external energy is required for the steam reforming process. Optionally oxygen can be supplied to the system, generating partial oxidation reactions to supply the energy required. Alternatively, the energy is indirectly supplied by combusting a second fuel.

Steam reforming of fossil fuels are investigated and applied thoroughly. Often nickel based catalysts are employed to enhance the steam reforming reactions. The main difference with steam reforming of pyrolysis oil is the presence of oxygenates, as well as the polymerization behaviour of the pyrolysis oil.

Quite some research is performed on the steam reforming of model compounds of pyrolysis oil such as acetic acid to mimic pyrolysis oil steam reforming. With these investigations, the activity of various catalysts was determined. Main issue however is to prevent catalyst contaminations. Even for pure methane, carbon deposition in the form of whiskers is troublesome. For pyrolysis oil steam reforming, preventing catalysts contaminations is believed to be a great challenge. For further information on the catalytic steam reforming of model compounds. [12].

Information on the steam reforming of actual pyrolysis oil is often limited to steam reforming of the aqueous phase only. Recently results were published for the catalytic, and non-catalytic steam reforming of the aqueous pyrolysis oil phase. [13] In addition, an experiment was performed in which the whole pyrolysis oil was used. Here, pyrolysis oil was steam reformed with S/C ratio of 7 at a temperature of 1400°C. The carbon conversion was close to 100%, with gas compositions close to the theoretical equilibrium. Other research on the steam reforming of aqueous fraction.[14] and the whole pyrolysis oil[15] performed by Czernik et al. Hydrogen yields of 70-80% of the theoretically achievable amount were obtained, however catalyst stability remains an important aspect for future process development.

Concluding Remarks

The gasification routes described above are at the moment in research stage, and further development is required before any of the routes can be implemented on commercial scale. Similar to alternative gasification processes (either fossil or biomass based) one of the key issues involves dealing with tar components. Especially for high value applications, limitations on maximum allowable tar content are strict, requiring very high conversion ratios.

Other important aspects are to supply the required energy to the system, while maintaining acceptable product yields. The entrained flow and steam reforming routes both require a lot of energy, due to the high operating temperature, and the large amounts of steam to be heated respectively. For both the catalytic partial oxidation, and the (catalytic) steam reforming, potential contamination / reactivation of the catalyst is an important aspect for further development work as well.

References

  1. Handbook Biomass Gasification, H.A.M. Knoef, Editor, ISBN 90-810068-1-9 (2005), Chapter 2, pp. 7
  2. Gasification, Chris Highman and Maarten van der Burgt, ISBN 0-7506-7707-4, p. 2
  3. Handbook Biomass Gasification Second Edition, H.A.M. Knoef, Editor, ISBN 978-90-819385-0-1, (2012), Chapter 8, pp. 219-250
  4. E.J. Leijenhorst, D. Assink, L. van de Beld, F. Weiland, H. Wiinikka, P. Carlsson, O.G.W. Ohrman. Entrained flow gasification of straw- and wood-derived pyrolysis oil in a pressurized oxygen blown gasifier. Biomass and Bioenergy (2015) Vol.79 p.166-176
  5. Dahmen et al. Energy, Sustainability and Society 2012,2:3
  6. http://www.bioliq.de/english/95.php
  7. E.J. Leijenhorst et. al. "Autothermal Catalytic Reforming of pine-wood-derived fast pyrolysis oil in a 1.5 kg/h pilot installation: Performance of monolithic catalysts" Energy and Fuels, Vol. 28, p5212-5221 (2014)
  8. G. van Rossum, "Steam Reforming and gasification of pyrolysis oil", PhD thesis, University of Twente.
  9. Rennard et al. "Production of synthesis gas by partial oxidation and steam reforming of biomass pyrolysis oils, International Journal of Hydrogen Energy, Vol. 35, p4048-4059 (2010)
  10. G. van Rossum et. al. "Staged Catalytic Gasification/Steam Reforming of Pyrolysis Oil". Ind. Eng. Chem. Res. Vol. 48, p.5857-5866 (2009)
  11. E.J. Leijenhorst et. al. "Autothermal Catalytic Reforming of pine-wood-derived fast pyrolysis oil in a 1.5 kg/h pilot installation: Performance of monolithic catalysts" Energy and Fuels, Vol. 28, p.5212-5221 (2014)
  12. Trane R, Dahl S, Skjoth-Rasmussen MS, Jensen AD. Catalytic steam reforming of bio-oil. Int J Hydrogen Energ. 2012 Apr;37(8):6447-72.
  13. Remón J, Broust F, Valette J, Chhiti Y, Alava I, Fernandez-Akarregi AR, et al. Production of a hydrogen-rich gas from fast pyrolysis bio-oils: Comparison between homogeneous and catalytic steam reforming routes. Int J Hydrogen Energ. 2014;39(1):171-82.
  14. Garcia L, French R, Czernik S, Chornet E. Catalytic steam reforming of bio-oils for the production of hydrogen: effects of catalyst composition. Appl Catal a-Gen. 2000 Jul 10;201(2):225-39.
  15. Czernik S, Evans R, French R. Hydrogen from biomass-production by steam reforming of biomass pyrolysis oil. Catal Today. 2007;129(3-4):265-8.