Upgrading Biomass Fast Pyrolysis Liquids
Anthony V. Bridgwater
Bioenergy Research Group, Aston University, Birmingham, United Kingdom; [e-mail] (for correspondence)
Published online 5 April 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11635
A comprehensive examination is made of the characteris- inorganics from soil during harvesting such as mud splashing
during rain and accumulation from dragging over soil. Other
tics and quality requirements of bio-oil from fast pyrolysis of
contaminants include chlorine from sea-side locations and
biomass. An appreciation of the potential for bio-oil to meet
biocide applications and sulfur from fertilizer applications.
a broad spectrum of applications in renewable energy has
As bio-oil is oleophobic, all feed water reports to the bio-
led to a significantly increased R&D activity that has focused
oil. Water is also formed from fast pyrolysis reactions, so to
on addressing liquid quality issues both for direct use for
maintain a reasonable water level in the bio-oil product of
heat and power and indirect use for biofuels and green
chemicals. This increased activity is evident in North Amer- typically 25%, feed water is usually limited to a maximum of
10 wt %. This level minimizes the potential for phase separa-
ica, Europe, and Asia with many new entrants as well as
tion and gives a manageable viscosity.
expansion of existing activities. The only disappointment is
In addition to hemicellulose, cellulose, and lignin biomass
the more limited industrial development and also deployment
can also contain minor organic components such as extrac-
of fast pyrolysis processes that are necessary to provide the
tives, oils, and proteins. The extractives and oils can lead to
basic bio-oil raw material. Ó 2012 American Institute of Chemical
a separate phase at the top of the bio-oil. Proteins have a
Engineers Environ Prog, 31: 261 268, 2012
high-nitrogen content and lead to a distinctive and unpleas-
Keywords: fast pyrolysis, bio-oil, upgrading
ant smell.
INTRODUCTION TO FAST PYROLYSIS AND BIO-OIL
Reactor and Liquid Collection
At the heart of a fast pyrolysis process is the reactor. Sev-
Introduction
eral comprehensive reviews of fast pyrolysis processes for
Fast pyrolysis is now a well-established thermal process-
liquids production have been published such as [1, 2, 5 7].
ing method for converting biomass into high yields of a liq-
The key requirements are rapid heating, a carefully con-
uid known widely as bio-oil [1]. This bio-oil has several un-
trolled reaction temperature of typically 480 5208C depend-
usual characteristics of which the main ones are summarized
ing on feed material, short hot vapor residence time of less
in Table 1. These are mostly a consequence of formation by
than 2 s, efficient char removal, and rapid quenching of the
rapid quenching and thus   freezing" the intermediate prod-
vapors and aerosols. Variations outside these limits lead to
ucts of flash degradation of hemicellulose, cellulose, and lig-
lower liquid yields and less stable bio-oil.
nin. The liquid thus contains many reactive species, which
contribute to its unusual attributes.
Norms and Standards
For bio-oil to successfully become a traded commodity,
Liquid Characteristics and Quality
suitable norms and standards are required. Exploration and
The objective or purpose of upgrading bio-oil is to
development of standard tests for bio-oil has led to certifica-
improve its quality, i.e., to reduce or remove one or more of
tion by CEN in Europe and ASTM in North America [8, 9].
its undesirable characteristics or properties. Table 2 lists the
The evaluation and development of test methods is very im-
characteristics with causes, effects, and possible solutions [2].
portant in defining quality and setting standards for definition
It is important to define the term   quality  since different
and marketing.
applications have different requirements in terms of charac-
teristics, most of which have been reviewed [3].3
BIO-OIL UPGRADING
Bio-oil can be upgraded in a number of ways physically,
SIGNIFICANT FACTORS AFFECTING BIO-OIL CHARACTERISTICS
chemically, and catalytically. This has been extensively
reviewed [1 3, 10]. A number of the characteristics listed in
Feed Material
Table 2 have attracted particular interest and concern and
The composition of the biomass feed has a significant
these are considered in more detail below.
effect on both the yield and quality of the resulting bio-oil of
which the main parameters are ash, contamination, composi-
tion, and water content. Acidity or Low pH
The potassium and sodium in biomass are catalytically
Bio-oil has a typical pH of around 2.5 from the organic
active in fast pyrolysis through cracking to water and CO2 in
acids formed by degradation or cracking of the biopolymers
the vapor phase. Ash contents above around 2.5 wt % often
that make up biomass, particularly the cellulose and hemicel-
lead to a phase separated product with significantly lower
lulose. Hemicellulose can be preferentially reduced by wash-
liquid yields [4]. Biomass can be contaminated by metals and
ing in hot water or dilute acid and by torrefaction. Neither
method is very effective since conversion is relatively ineffi-
Ó 2012 American Institute of Chemical Engineers cient and cellulose is also affected in both methods.
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep July 2012 261
Table 1. Typical properties of wood-derived crude bio-oil Woody feeds typically contain up to 1 wt % ash while
grasses can range up to 8 wt %. A key factor in the ash con-
tent for energy crops is time of harvesting since miscanthus,
Physical property Typical value
for example, senesces over winter and a significant propor-
Moisture content 25%
tion of the minerals in the ash return to the rhizome. In addi-
pH 2.5
tion, ash will be leached from the standing crop during win-
Specific gravity 1.20
ter from rainfall to potentially give ash contents of such
Elemental analysis
grasses below 2.5 wt %.
C 56%
The limiting value of ash content in biomass to reduce or
H6%
avoid this catalytic effect is believed to be around 2.5 wt %,
O 38%
although this depends on other process parameters and the
N 0 0.1%
composition of the ash.
HHV as produced 17 MJ/kg
Washing biomass with water or dilute acid is feasible to
Miscibility with hydrocarbons Very low
remove ash but is costly in financial and energy terms both
Viscosity (40C and 25% water) 40 100 cp
for washing and subsequent drying. However, a further
Solids (char) 0.1%
advantage of acid washing is the potential removal of hemi-
Vacuum distillation residue up to 50%
celluloses from which are derived aldehydes and related deg-
radation products that create the peculiar small of bio-oil and
contribute to the aging effect. So as with many other charac-
teristics of bio-oil, alkali metal effects are complex and
There has been only a little work on corrosion of metals
require careful evaluation.
in bio-oil [11]. The general view is that polyolefins and stain-
less steel are acceptable materials of construction. High acid-
ity can be managed in several ways including esterification,
Char
which is still at an early stage of development [2].
Char acts as a vapor cracking catalyst so rapid and effec-
tive separation from the pyrolysis product vapors is essential.
Aging
Cyclones are the usual method of char removal, however,
Aging or instability affects most bio-oils. It is caused by some fines always pass through the cyclones and collect in
continued reaction of the degradation products from fast py- the liquid product where they accelerate aging and exacer-
rolysis, which has been frozen in the rapid quenching that is bate the instability problem. A more effective but more diffi-
an essential feature of fast pyrolysis [10]. It shows as a slow cult method is hot vapor filtration, which gives a higher
increase in viscosity at ambient temperature and sometimes grade product at the expense of liquid yield reduction of
as phase separation. Heating bio-oil will increase the aging around 15% [13 15]. Pressure filtration of the liquid for sub-
effect through a tendency for chemical reactions to continue. stantial removal of particulates (down to <1 lm) is very diffi-
Up to around 558C, the changes seems to be reversible so cult because of the complex interaction of the char and pyro-
preheating to 508C or less will usually have no adverse lytic lignin, which appears to form a gel-like phase that rap-
effects on oil quality or behavior. Above around 608C the idly blocks the filter. Modification of the liquid microstructure
changes are increasingly less reversible and prolonged expo- by addition of solvents such as methanol or ethanol that
sure to higher temperatures causes increased viscosity and an solubilize the less soluble constituents can address this prob-
increased propensity for phase separation. Around 1008C, lem and contribute to improvements in liquid stability.
bio-oil separates into a heavy bitumen type phase mostly
from the pyrolytic lignin and a low-viscosity aqueous frac-
Distillability
tion, but both are different to phase separated bio-oil at am-
Pyrolysis liquids cannot be completely vaporized once
bient conditions. The heavier material will hinder heat trans-
they have been recovered from the vapor phase. If the liquid
fer and as temperatures increases, it will eventually carbonize
is heated to 1008C or more to try to remove water or distil
to form a coke layer. This is what happens in tempts for
off lighter fractions, it rapidly reacts and eventually produces
distillation.
a solid residue of around 50 wt % of the original liquid and
Polar solvents have been used for many years to homoge-
some distillate containing volatile organic compounds and
nize and reduce the viscosity of biomass oils. The addition of
water. The distillate contains those compounds that are vola-
solvents, especially methanol, showed a significant effect on
tile together with thermally cracked products from higher
the oil stability, for example, Diebold and Czernik [12] found
temperatures.
that the rate of viscosity increase (  aging  ) for bio-oil with 10
wt % of methanol was almost twenty times less than for the
oil without additives.
High Viscosity
Temperature is widely used to control viscosity in com-
Bio-oil viscosity is important particularly for direct com-
bustion applications, but for bio-oil this needs to be carefully
bustion applications, where it needs to be atomized such as
considered. In-line preheating immediately prior to combus-
in burners, engines, and turbines. Testing of bio-oil in
tion works well, but recirculation of a heated bio-oil
engines is reviewed in [3, 16, 17] and in burners in [3, 18,
for example in some engine designs needs to be managed
19]. For engines, the preferred maximum viscosity is typically
carefully.
17 cS above, which pressure requirements become excessive.
Preheating is discussed under   Aging  above. Viscosity is
Alkali metals
most affected by water content and temperature and is thor-
All biomass contains ash in which alkali metals notably oughly covered in Diebold s review [10].
potassium and sodium dominate. Potassium in particular is Polar solvents have been used for many years to homoge-
catalytically active and enhances secondary cracking reac- nize and reduce the viscosity of biomass oils. The addition of
tions during pyrolysis. This results in loss of liquid yield, solvents, especially methanol, showed a significant effect on
higher water (and carbon dioxide) production, and potential the oil stability. Diebold and Czernik [12] found that the rate
phase separation from higher water content and loss of natu- of viscosity increase (  aging  ) for the oil with 10 wt % of
rally derived surfactants that maintain the micro-emulsion of methanol was almost 20 times less than for the oil without
bio-oil. additives.
262 July 2012 Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
Table 2. Characteristics of bio-oil.
Characteristic Cause Effect Solution
Acidity (low pH) Organic acids from biopolymer degradation Corrosion of vessels and pipework Careful materials selection such as
polyolefins or stainless steel
Aging Continuation of secondary reactions Slow increase in viscosity from secondary Do not store for long periods
including polymerization reactions such as condensation Avoid exposure to air
Potential phase separation Add water
Add co-solvents
Alkali metals Nearly all alkali metals report to char Catalyst poisoning Pretreat feed to remove ash
so not a big problem Deposition of solids in combustion Hot vapor filtration
High ash feed Erosion and corrosion Process oil
Incomplete solids separation Slag formation Modify application
Damage to turbines
Char Incomplete char separation in process Aging of oil Improved cyclones
Sedimentation Multiple cyclones
Filter blockage Hot vapor filtration
Catalyst blockage
Engine injector blockage
Alkali metal poisoning
Chlorine Contaminants in biomass feed Catalyst poisoning in upgrading Include suitable cleaning processes
either upstream or downstream
Color Cracking of biopolymers and char Discoloration of some products such as resins Efficient char filtration
De-oxygenation
Contamination of feed Poor harvesting practice Contaminants notably soil act as catalysts Improve harvesting practice
and can increase particulate carry over. Wash biomass
Distillability is poor Reactive mixture of degradation products Bio-oil cannot be distilled maximum 50% None known
typically. Liquid begins to react at below
1008C and substantially decomposes above 1008C
High viscosity Gives high pressure drop increasing equipment cost Careful heating up to 508C, rapid in-line
High pumping cost heating to 808C is also possible
Poor atomization Add water
Add co-solvents
Low H:C ratio Biomass has low H:C ration Upgrading to hydrocarbons is more difficult Add hydrogen and/or remove oxygen
Materials incompatibility Phenolics and aromatics Destruction of seals and gaskets Careful materials selection
Miscibility with hydrocarbons Highly oxygenated nature of bio-oil Will not mix with any hydrocarbons so Upgrading by hydrotreating or cracking
is very low integration into a refinery is more difficult with zeolites
Nitrogen Contaminants in biomass feed Unpleasant smell Careful feed selection
High nitrogen feed such as proteins Catalyst poisoning in upgrading Feed blending
in wastes NOx in combustion Include suitable cleaning processes
Add NOx removal in combustion
applications
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
July 2012
263
Table 2. Characteristics of bio-oil (continued).
Characteristic Cause Effect Solution
Oxygen content is very high Biomass composition Poor stability Reduce oxygen thermally and/or
Nonmiscibility with hydrocarbons catalytically
Phase separation High feed water Phase separation Modify or change process
High ash in feed Partial phase separation Modify or change feedstock
Poor char separation Layering Add co-solvents
Poor mixing Control water content
Inconsistency in handling, storage, and processing
Smell Aldehydes and other volatile organics, While not toxic, the smell is often objectionable Better process design and management
many from hemicellulose Reduction in hemicelluloses content of feed
Containment and/or venting to flare
Solids See also char Sedimentation Filtration of vapor or liquid
Particulates from reactor such as sand Erosion and corrosion
Particulates from feed contamination Blockage
Structure Rapid de-polymerization and rapid Susceptibility to aging such as viscosity increase None known
quenching of vapors and aerosols and phase separation
Sulfur Contaminants in biomass feed Catalyst poisoning in upgrading Include suitable cleaning processes
Temperature sensitivity Incomplete reactions Irreversible decomposition of liquid Store liquids at room temperature
into two phases >1008C preferably in absence of air
Irreversible viscosity increase above 608C Avoid heating for prolonged periods
Potential phase separation above 608C Avoid heating above <" 1208C
Toxicity Biopolymer degradation products Human toxicity is positive but small Health and safety precautions
Eco-toxicity is negligible
Viscosity Nature of bio-oil Fairly high and variable with time Water and/or solvent addition reduces
Greater temperature influence than hydrocarbons viscosity
Water Pyrolysis reactions Complex effect on viscosity and stability: Control water in feed
Increased water lowers heating value, density,
stability, and raises pH
Feed water Affects catalysts Optimize at 25% for consistency and
miscibility
Optimize for application
264
July 2012
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
Oxygen Content, Water Content, and Miscibility with
Hydrocarbons
Bio-oil approximates biomass in elemental composition
with typically 40 45 wt % oxygen from the diverse-oxygen-
ated organic compounds. This means that it is not miscible
with hydrocarbons but miscible with polar solvents like
methanol, ethanol, acetone, etc. Both upgrading to hydrocar-
bons for transport fuels or biofuels and recovery of chemicals
is discussed below.
Pyrolysis liquids can tolerate the addition of some water,
but there is a limit to the amount of water, which can be
added to the liquid before phase separation occurs, in other
words, the liquid cannot be dissolved in water. Water addi-
tion reduces viscosity, which is useful, reduces heating value,
which means that more liquid is required to meet a given
duty, and can improve stability. The effect of water is there-
fore complex and important.
Solids Figure 1. Overview of fast pyrolysis upgrading methods.
Cyclones are widely used for char removal but are not
effective with small particle sizes. Hot-vapor filtration can
reduce the ash content of the oil to less than 0.01% and con-
sequently the alkali content to less than 10 ppm, much lower
Figure 2. There is also interest in partial upgrading to a prod-
than reported for biomass oils produced in systems using
uct that is compatible with refinery streams to take advantage
only cyclones [15]. A consequence of hot vapor filtration to
of the economy of scale and experience in a conventional re-
remove char is the catalytic effect of the accumulated char
finery. Integration into refineries by upgrading through crack-
on the filter surface, which potentially cracks the vapors,
ing and hydrotreating has been reviewed by Huber and
reduces yield by up to 20%, reduces viscosity and lowers the
Corma [23]. The main methods are:
average molecular weight of the liquid product.
" Hydrotreating.
Diesel engine tests performed on hot-filtered oil showed a
" Catalytic vapor cracking.
substantial increase in burning rate and a lower ignition
" Esterification and related processes.
delay when compared with unfiltered bio-oil, due to the
" Gasification to syngas followed by synthesis to hydrocar-
lower average molecular weight for the filtered oil [20]. Hot
bons or alcohols.
gas filtration has not yet been demonstrated over a long-term
process operation. A little work has been performed in this
area by NREL [15], VTT, and Aston University [13], and very
little has been published. Hydrotreating
Liquid filtration to very low particle sizes of below around Hydro-processing rejects oxygen as water by catalytic
5 lm is very difficult due to the physic-chemical nature of reaction with hydrogen. This is usually considered as a sepa-
the liquid and usually requires high pressure drops and self rate and distinct process to fast pyrolysis that can therefore
cleaning filters. be carried out remotely. The process is typically high pres-
sure (up to 200 bars) and moderate temperature (up to
4008C) and requires a hydrogen supply or source [24]. Full
Toxicity
hydrotreating gives a naphtha-like product that requires
As bio-oil becomes more widely available, attention will
orthodox refining to derive conventional transport fuels. This
be increasingly placed on environment, health, and safety
would be expected to take place in a conventional refinery
aspects. A study was completed in 2005 to assess the ecotox-
to take advantage of know-how and existing processes. A
icity and toxicity of 21 bio-oils from most commercial pro-
projected typical yield of naphtha equivalent from biomass is
ducers of bio-oil around the world in a screening study, with
about 25% by weight or 55% in energy terms excluding pro-
a complete assessment of a representative bio-oil. The study
vision of hydrogen. Inclusion of hydrogen production, for
included a comprehensive evaluation of transportation
example by gasification of biomass, reduces the yields to
requirements as an update of an earlier study [21] and an
assessment of the biodegradability [22]. The results are
complex and require more comprehensive analysis but the
overall conclusion is that bio-oil offers no significant health,
environment, or safety risks.
CHEMICAL AND CATALYTIC UPGRADING OF BIO-OIL
Bio-oil can be upgraded chemically and catalytically and
has been recently reviewed [2]. A summary of the main meth-
ods for upgrading fast pyrolysis products and the products is
shown in Figure 1.
Catalytic Upgrading of Bio-oil
Upgrading bio-oil to a conventional transport fuel such as
diesel, gasoline, kerosene, methane, and LPG requires full
deoxygenation and some conventional refining, which can
Figure 2. Upgrading of bio-oil to biofuels and chemicals.
be accomplished either by catalytic pyrolysis or by
decoupled operation as summarized below and depicted in
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep July 2012 265
Table 3. Chemicals recovered from bio-oil.
Examples of chemicals investigated for recovery
Acetic acid Hydroxyacetaldehyde Phenol and
phenolics
Aldehydes Ketones Phenols
Anhydrosugars Levoglucosan Plastics
Bio-ashphalt Levoglucosenone Preservative
Biolime Liquid smoke and Resins and
related products adhesives
Fibre materials Nonaromatic alcohols Slow release
fertilizer
Furfural Organic acids Synthesis gas
Hydrogen Pharmaceuticals
Figure 3. Methods of upgrading fast pyrolysis products with
cracking catalysts.
Steam Reforming
The water-soluble (carbohydrate-derived) fraction of bio-
oil can be processed to hydrogen by steam reforming [2].
This has been accomplished in a fluidized bed process by
around 15 wt % or 33% in energy terms. If only the organic
several researchers using commercial, nickel-based catalysts
fraction of bio-oil after phase separation is hydrotreated, the
under conditions similar to those for reforming natural gas.
hydrogen required can be produced by steam reforming the
The process depends on a viable use for the organic lignin-
aqueous phase as discussed below. There is also a high cost
derived fraction of bio-oil, for example, use as a phenol
from the high-pressure requirement [25]. Catalyst deactivation
replacement in phenol formaldehyde resins [31] or for
remains a concern from coking due to the poor C:H ratio.
upgrading this organic fraction. This area has been reviewed.
Almost all reported work on bio-oil upgrading has been
carried out by universities and research organizations [2] with
only UOP in the USA known to be currently active in part- Other Upgrading Methods
nership with Envergent [26]. An increasing number of methods are being investigated
and developed for improving the quality of bio-oil without
substantial de-oxygenation. Properties that are mostly
Zeolite Cracking
addressed are water content, acidity, stability, and reactivity.
Zeolite cracking rejects oxygen as CO2, yielding mainly ar-
Processes include esterification, reactive distillation, solid
omatic hydrocarbons as product but with extensive coke dep-
acid catalysts, ionic liquids and more complex co-processing
osition on the catalyst. The coking problem associated with
using multiple methods [2]. None are believed to be suffi-
many catalytic processes acting on bio-oil is generally more
ciently developed to offer possibilities for commercial
prevalent with cracking catalysts. A variety of approaches
deployment in the foreseeable future.
have been investigated including integrated catalytic pyrolysis,
close coupled vapor upgrading; decoupled vapor upgrading
by volatilization of bio-oil, decoupled liquid bio-oil upgrading,
Hydrogen
and supercritical processing as summarized in Figure 3.
Hydrogen is produced in the syngas from gasification of
Almost all reported work has been carried out by universities
bio-oil and bio-oil/char slurries as described above. There are
and research organizations with only KIOR and Exelus in the
also activities in both noncatalytic partial oxidation and cata-
USA known to be currently active [2].
lytic partial oxidation and catalytic steam reforming of both
whole bio-oil and the aqueous fraction after phase separation,
Other Methods for Chemical Upgrading of Bio-oil particularly to meet the hydrogen demands of a hydrotreating
This section includes nonphysical methods and those cat- process. Catalysts are usually based on nickel or precious met-
alytic processes not covered in hydrotreating and zeolite- als. The activities have been summarized and reviewed [2].
related processes.
Chemicals
Aqueous Phase Processing
The chemicals in bio-oil are derived from random thermal
This is a relatively new approach that was first proposed
decomposition of hemicellulose, cellulose and lignin. For many
by Dumesic and co-workers, who produced hydrogen and
centuries, wood pyrolysis liquids were a major source of chemi-
alkanes from aqueous solutions through aqueous phase
cals such as methanol, acetic acid, turpentine, tars, etc. At pres-
reforming and dehydration/hydrogenation [27, 28]. A large
ent, most of these compounds can be produced at a lower cost
fraction of bio-oil is water soluble and the compounds pres-
from fossil fuel feedstocks. Although over 400 compounds have
ent in its aqueous fraction are mainly oxygenated hydrocar-
been identified in wood fast pyrolysis oil [10], their concentra-
bons. In this presentation, then authors show that the
tions are usually too low to consider separation and recovery.
concept of aqueous phase processing can be used to produce
Up to now therefore, the development of technologies for
hydrogen and alkanes from the aqueous fraction of bio-oil.
producing products from the whole bio-oil or from its major, rel-
atively easy separable fractions is the most developed. Table 3
Mild Cracking summarises the main chemicals that been investigated for
An alternative to orthodox, zeolite-based cracking is mild recovery. A more-detailed review on this subject, including con-
cracking over base catalysts that address only the cellulose sideration of higher-value products, was published by Radlein
and hemicellulose derived products and aim to minimize [32] and a thorough review of the published reports on
coke and gas formation. Crofcheck at the University of production of chemicals utilizing whole oil, fractions of bio-oil,
Kentucky [29] has explored ZnO and freshly calcined Zn/Al and specific chemicals has been published [11]. A comprehensive
and Mg/Al-layered double hydroxides to upgrade a synthetic review of phenolics recovery and utilization has been also been
bio-oil based on earlier work in Finland [30]. published [33].
266 July 2012 Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
Strategies for separation or recovery of any of these chem- 7. Bridgwater, A.V. (2003). Renewable fuels and chemicals
icals need to consider market, value, costs, and process. The by thermal processing of biomass, Chemical Engineering
natural first step is to evaluate components with the highest Journal, 91, 87 102.
concentration since the processing will be easier and the 8. ASTM D7544 - 10 Standard Specification for Pyrolysis
costs lower. However, this may not prove the best strategy, Liquid Biofuel. ASTM International, 100 Barr Harbor
and methodologies need to carefully consider capital and Drive, PO Box C700, West Conshohocken, PA, 19428-
operating costs, product values and residue, or waste utiliza- 2959 USA.
tion or disposal. The opportunities for optimization are con- 9. Oasmaa, A., Elliott, D.C., & Müller, S. (2009). Quality
siderable and challenging, and are likely to involve a range control in fast pyrolysis bio-oil production and use, Envi-
of process generation, evaluation, and optimization tools ronmental Progress & Sustainable Energy, 28, 404 409.
including process synthesis and linear programming. 10. Diebold, J.P. (2002). A review of the chemical and physi-
cal mechanisms of the storage stability of fast pyrolysis
bio-oils. In A.V. Bridgwater (Ed.), Fast pyrolysis of bio-
mass, a handbook (Volume 2, pp 243 292), CPL Press,
CONCLUSION
Newbury, UK.
There has been considerable expansion of activity in the
11. Darmstadt, H., Garcia-Perez, M., Alain Adnot, A., Chaala,
past decade exploring processes for production of more use- A., Kretschmer, D., & Roy, C. (2004) Corrosion of met-
ful and valuable products from bio-oil. Attention has focused
als by bio-oil obtained by vacuum pyrolysis of softwood
on improving the quality of bio-oil to reduce or avoid prob- bark residues. An X-ray photoelectron spectroscopy and
lems in direct use, which requires identification and specifi- auger electron spectroscopy study, Energy Fuels, 18,
cation of the qualities concerned and upgrading to a more
1291 1301.
conventional product that can be more readily assimilated
12. Diebold, J.P., & Czernik, S. (1997). Additives to lower
into existing fuel infrastructures. This includes transport fuels,
and stabilize the viscosity of pyrolysis oils during stor-
synthesis gas for transport fuels and chemicals, hydrogen and
age, Energy & Fuels, 11, 1081 1091.
commodity, and speciality chemicals.
13. Sitzmann, J., & Bridgwater, A.V. (2007). Upgrading fast
Although quality can be defined by over 25 characteristics
pyrolysis oils by hot vapour filtration, 15th European
of bio-oil, most concern relates to the aging phenomenon,
Energy from Biomass Conference, Berlin, 7-11 May.
which inhibits wider-spread usage; acidity which causes cor- 14. Hoekstra, E., Hogendoorn, K.J.A., Xiaoquan, W., Wester-
rosion; hydrophilicity, which inhibits co-utilization with
hof, R.J.M., Kersten, S.R.A., van Swaaij, W.P.M., & Groe-
orthodox hydrocarbons; nondistilability, which inhibits refin- neveld, M.J. (2009). Fast pyrolysis of biomass in a fluid-
ing and high-oxygen content, which requires extensive re- ized bed reactor: In situ filtering of the vapors, Industrial
moval to derive hydrocarbon-based biofuels.
& Engineering Chemistry Research, 48, 4744 4756.
The relatively low hydrogen content of bio-oil invariably
15. Diebold, J.P., Czernik, S., Scahill, J.W., Philips, S.D., &
results in coking of catalysts in catalytic upgrading processes.
Feik, C.J. (1994). Hot-gas filtration to remove char from
Some solutions have been sought in more sophisticated cata- pyrolysis vapours produced in the vortex reactor at
lyst systems that require less severe conditions and also in
NREL. In T.A.Milne (Ed.), Biomass Pyrolysis Oil Prop-
multi-stage upgrading where bio-oil is processed in a series
erties and Combustion Meeting (pp. 90 108), Boulder,
of steps to give a progressively upgraded product. Liquid
CO: National Renewable Energy Laboratory.
processing is generally preferred to avoid problems of vapor- 16. Leech, J. (1997). Running a dual fuel engine on pyrolysis
izing bio-oil with consequent loss of carbon as coke unless
oil. In M.Kaltschmitt & A.V.Bridgwater (Eds.), Biomass
oxidative processing is included to oxidize any carbon that is
gasification and pyrolysis, state of the art and future
formed.
prospects (pp. 495 497), Newbury: CPL Press.
The use of model compounds makes for easier fundamen- 17. Ormrod, D., & Webster, A. (2000). Progress in utilization
tal science, but it is doubtful if any single chemical or even
of bio-oil in diesel engines, PyNe Newsletter, 10, Aston
small number of chemicals can adequately reproduce the
University, Birmingham, p.15.
complexity of whole bio-oil with interactions between the
18. Oasmaa, A., & Czernik, S. (1999). Fuel oil quality of
constituent chemicals.
biomass pyrolysis oils State of the art for the end users,
Energy Fuels, 13, 914 921.
LITERATURE CITED 19. Oasmaa, A., Kytö, M., & Sipilä, K. (2001). Pyrolysis oil
1. Bridgwater, A.V. (2012). Review of fast pyrolysis of bio- combustion tests in an industrial boiler. In A.V.Bridg-
mass and product upgrading, Biomass and Bioenergy, water (Ed.), Progress in thermochemical biomass con-
38, 68 94. version (pp. 1468 1481), Blackwell Science: Oxford.
2. Bridgwater, A.V. (2011). Upgrading biomass fast pyroly- 20. Shihadeh, A.L. (1998). Rural electrification from local
sis liquids. Chapter 6. In R.C. Brown (Ed.), Thermo- resources: biomass pyrolysis oil combustion in a direct
chemical processing of biomass: conversion into fuels, injection diesel engine, Ph.D. Thesis, Massachusetts Insti-
chemicals and power. Wiley Series in Renewable Resour- tute of Technology.
ces (pp. 157 199), Wiley-Blackwell, UK. ISBN: 978-0- 21. Peacocke, G.V.C. (2002). Transport handling and storage
470-72111-7. of fast pyrolysis liquids. In Fast pyrolysis of biomass: A
3. Czernik, S., & Bridgwater, A.V. (2004). Overview of handbook (Volume 2, pp 293 338), CPL Press, Newbury,
applications of biomass fast pyrolysis oil, Energy & UK.
Fuels, 18, 590 598. 22. Blin, J., Volle, G., Girard, P., Bridgwater, A.V., & Meier,
4. Chiaramonti, D., Oasmaa, A., & Solantausta, Y. (2007). D. (2007). Biodegradability of biomass pyrolysis oils:
Power generation using fast pyrolysis liquids from bio- Comparison to conventional petroleum fuels and alter-
mass, Renewable and Sustainable Energy Reviews, 11, natives fuels in current use, Fuel, 86, 2679 2686.
1056 1086. 23. Huber, G.W., & Corma, A. (2007). Synergies between
5. Mohan, D., Pittman, C.U., & Steele, P. (2006) Pyrolysis bio- and oil refineries for the production of fuels from
of wood/biomass for bio-oil: A critical review, Energy & biomass, Angewandte Chemie International Edition, 46,
Fuels, 20, 848 889. 7184 7201.
6. Bridgwater, A.V. (2004). Biomass fast pyrolysis, Thermal 24. Elliott, D.C., & Baker, E. (1983). Hydrotreating biomass
Science, 8, 21 49. liquids to produce hydrocarbon fuels. In D.Klass (Ed.)
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep July 2012 267
Energy from biomass and wastes X (pp. 765 782), IGT, and Biological Engineers, Paper number 066035, 2006
Chicago, USA. ASAE Annual Meeting.
25. Cottam, M,-L., & Bridgwater, A.V. (1994). Techno-eco- 30. Nokkosmaki, M.I., Kuoppala, E.T., Leppamaki, E.A., &
nomic modelling of biomass flash pyrolysis and upgrad- Krause, A.O.I. (2000). Catalytic conversion of biomass
ing systems. Biomass and Bioenergy, 7, 267 273. pyrolysis vapours with zinc oxide, Journal of Analytical
26. Holmgren, J., Maringangeli, R., Nair, P., Elliott, D.C., & and Applied Pyrolysis, 55, 119 131.
Bain, R. (2008). Consider upgrading pyrolysis oils into 31. Mann, M.K., Spath, P.L., & Kadam, K. (1996). Technical
renewable fuels, Hydrocarbon Processing, 87, 95 103 and economic analysis of renewables-based hydrogen
27. Cortright, R.D., Davda, R.R., & Dumesic, J.A. (2002). production. In Proceedings of the 11th World Hydrogen
Hydrogen from catalytic reforming of biomass derived Energy Conference, Stuttgart, Germany, June 23 28.
hydrocarbons in liquid water, Nature, 418, 964 967. 32. Radlein, D. (1999). The production of chemicals from
28. Huber, G.W., & Dumesic, J.A. (2006). An overview of fast pyrolysis bio-oils. In Bridgwater A, Czernik S, Die-
aqueous-phase catalytic processes for production of bold J, Meier D, Oasmaa A, Peacocke C, Piskorz J,
hydrogen and alkanes in a biorefinery, Catalysis Today, Radlein D, (Eds.), Fast pyrolysis of biomass: A hand-
111, 119 132. book (pp. 164 188), Newbury: CPL Press.
29. Fisk, C., Crofcheck, C., Crocker, M., Andrews, R., Storey, 33. Amen-Chen, C., Pakdel, H., & Roy, C. (2001) Production of
J., & Lewis, S., Sr. (2006) Novel approaches to catalytic monomeric phenols by thermochemical conversion of bio-
upgrading of bio-oil. American Society of Agricultural mass A review, Bioresource Technology, 79, 277 299.
268 July 2012 Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep