Dec 14, 2009 - Fast pyrolysis technology development. RH Venderbosch, BTG Biomass Technology Group BV, Enschede, the Netherlands. W Prins, Faculty of ...
Review
Fast pyrolysis technology development RH Venderbosch, BTG Biomass Technology Group BV, Enschede, the Netherlands W Prins, Faculty of Bioscience Engineering, University of Ghent, Belgium and BTG Biomass Technology Group BV, Enschede, the Netherlands Received October 21, 2009; revised version received November 30, 2009; accepted December 14, 2009 Published online in Wiley InterScience (www.interscience.wiley.com); DOI: 10.1002/bbb.205; Biofuels, Bioprod. Bioref. 4:178-208 (2010) Abstract: While the intention of slow pyrolysis is to produce mainly charcoal, fast pyrolysis is meant to convert biomass to a maximum quantity of liquids (bio-oil). Both processes have in common that the biomass feedstock is densified to reduce storage space and transport costs. A comfortable, more stable and cleaner intermediate energy carrier is obtained, which is much more uniform and well defined. In this review, the principles of fast pyrolysis are discussed, and the main technologies reviewed (demo scale: fluid bed, rotating cone and vacuum pyrolysis; pilot plant: ablative and twin screw pyrolysis). Possible product applications are discussed in relation to the bio-oil properties. General mass and energy balance are provided as well, together with some remarks on the economics. Challenges for the coming years are (1) improvement of the reliability of pyrolysis reactors and processes; (2) the demonstration of the oil’s utilization in boilers, engines and turbines; and (3) the development of technologies for the production of chemicals and biofuels from pyrolysis oils. One important conclusion in relation to biofuel production is that the type of oxygen functionalities (viz. as an alcohol, ketone, aldehyde, ether, or ester) in the oil should be controlled, rather then merely focusing on a reduction of just the oxygen content itself. © 2010 Society of Chemical Industry and John Wiley & Sons, Ltd Keywords: pyrolysis, technology, review, bio-oil, biomass
Introduction nvironmental concerns and possible future shortages have boosted research into alternatives for fossil-derived products. Biomass is abundantly available worldwide and considered to be renewable. Despite its complexity, the use of biomass is rapidly expanding. Agriculture, petrochemical industries, and individual entrepreneurs, meanwhile, have developed a significant production of socalled first-generation biofuels from vegetable oils (biodiesel), sugar and starch (bioethanol). The scale of production of
E
these first-generation fuels (95
© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:178–208 (2010); DOI: 10.1002/bbb
Review: Fast pyrolysis technology development
about 1200 kg/m3, which is significantly higher than that of fuel oil. It has a distinctive acid, smoky smell, and can irritate the eyes. The viscosity of the oil varies from 25 up to 1000 cP, depending on the water content and the amount of light components in the oil. It is important to note that oil properties depend on feedstock and operating conditions, but may change during storage, a process indicated as ‘ageing’, which (by lack of definition) is usually noticed by an increased viscosity in time and a possible phase separation of the oil in a watery phase and a viscous organic phase. Due to the presence of large amounts of oxygenated components, the oil has a polar nature and does not mix readily with hydrocarbons. In general, it contains less nitrogen than petroleum products, and almost no metal and sulfur components. However, some of the nitrogen is transferred to the oil product as well: feed materials with a high nitrogen contents yield oil with higher pH-values and larger amounts of nitrogen in the oil. Degradation products from the biomass constituents include organic acids (like formic and acetic acid), giving the oil its low pH of about 2 to almost 4. The oil attacks carbon steel,
RH Venderbosch, W Prins
and storage of the oils should be in acid-proof materials like stainless steel or poly-olefins. Water is an integral part of the single-phase chemical solution. The (hydrophilic) bio-oils have water contents of typically 15–35 wt.%, and water cannot be removed by conventional methods like distillation. This high water content is a serious drawback if considering the heating values: the higher heating value (HHV) is below 19 MJ/kg (compared to 42–44 MJ/kg for conventional fuel oils). Above a certain water-content level, viz. in the range of 30 to 45 wt.%, phase separation may occur. Depending on the type of feedstock and process conditions the ratio of oil over aqueous phase varies from 50:50 to 30:70, and the presence of these two phases can complicate the oil’s application. In many cases, sufficient drying of the biomass feedstock material prior to pyrolysis prevents phase separation. Applying different condensation temperatures will yield oils with different water contents (affecting the oil yield as well), and there is quite some room for optimization.23 Usually the choice of the feedstock and process (characteristics) will determine the ‘oil’ quality and possible phases. Beneficial effects of the water content have also been reported, viz. in case of combustion. It causes a decrease in viscosity of the oil (facilitating transport, pumping, and atomization); it improves ‘stability’; it lowers the combustion 80 oil
Oil and char yield (wt.%)
70 60 50 40
Trendline oil (Chariamonti et al. 2007)
30 char
20 gas
10 0 0
1
2
3
4
5
6
7
8
Ash content (wt.%) Figure 4. Relationship between oil yield and ash content in the biomass. The solid lines represent trend lines taken from literature,20 while the broken line is a trend line based upon more than 20 data Figure 3. Fast pyrolysis oil.
point for wood.21
© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:178–208 (2010); DOI: 10.1002/bbb
183
RH Venderbosch, W Prins
Review: Fast pyrolysis technology development
temperature and, as a consequence, it may cause a reduction of the NOx emission. Generally speaking, the (organic) oil yield achieved in fast pyrolysis should be as high as possible. Besides, the oil should have a much higher (volumetric) energy content than the original biomass, and must be more stable towards biological degradation.
composition. GC injection includes vaporization of the feed, which is known to be difficult for pyrolysis oils and causing some coking in the injection part of the system. Moreover, chemical reactions occurring in the GC column cannot be excluded either, and it is questionable whether the components actually detected are really present in the feed oil. Other techniques of which the potentials for pyrolysis-oil analysis are being investigated are Gel Permeation Chromatography (GPC) and High Performance Liquid Chromatography (HPLC), but both methods need to be handled with great care. Fortunately, development of new techniques to footprint the oils is ongoing. As the oil can not be distilled without severe repolymerization and thus chemical degradation, a solvent fractionation technique, illustrated in Fig. 5, is developed to analyze the oil in an alternative way, and reveal the presence of certain fractions present in the oil:25
Composition and stability Pyrolysis oils are produced by the rapid quenching of fragmented biomass, these fragments being derived from the biomass constituents, cellulose, hemicellulose, and lignin. The largest fragments that are conveyed to the condenser in the vapor phase have a molecular mass that is far too high for being a gas component at temperatures around 500°C. They may be present in the vapor phase as aerosols, or are produced upon ‘freezing’ the vapors. This liquid product collected in the condenser includes the complete spectrum of oxygenated compounds, with molecular weights ranging from 18 to over 10 000 g/mol, the higher values probably caused by repolymerization of the biomass fragments. Whereas some researchers think the oil is a (micro-)emulsion of these compounds, there is also reason to believe the oil is a mixture of soluble components, likely with water as the solvent and polar sugar constituents behaving as bridging agents in the dissolution of hydrophilic lignin material.24 GC-analysis (including 2D-GC, GC-MS etc) appears, to a certain extent, valuable in the interpretation of the oil quality. However, the usefulness of GC data is limited due to the (unknown) destructive effect of the technique on the oil
• • • • • •
water solubles (acids, alcohols, diethylethers); ether solubles (aldehydes, ketones, lignin monomers, etc.); ether insolubles ((anhydo)sugars, hydroxyl acids); n-hexane solubles (fatty acids, extractives, etc.); DCM solubles (low molecular lignin fragment, extractives); and DCM insolubles (degraded lignins, high molecular lignin fragments, including solids)
The ether insolubles in particular (the sugar components, a syrup-like fraction) appear to have high oxygen contents (up to 50%) if compared to, for example, the DCM solubles and insolubles (25 to 30% oxygen). Ongoing research is aimed at Water (by KF titration) Solids (by MeOH-DCM extraction)
BIO-OIL water extraction
N-HEXANE SOLUBLES Extractives
WATER-SOLUBLES
WATER INSOLUBLES DCM extraction
ether extraction DCM SOLUBLES
DCM INSOLUBLES
LMM lignin (low molecular lignins, extractives)
ETHER SOLUBLES Ether solubles: (Aldehydes, ketons, lignin monomers)
HMM lignin (high molecular lignins, solids)
ETHER INSOLUBLES Sugars (anhydrosugars, anhydro-oligomers, Hydroxyacids (C500 W/m2K); intraparticle biomass heat transfer limitation should be avoided (requiring particles
