Upgrading biomass fast pyrolysis liquids, Sci Zagadnieniami-poukladane-materialy, nieposegregowane [ Pobierz całość w formacie PDF ]
Upgrading Biomass Fast Pyrolysis Liquids
Anthony V. Bridgwater
Bioenergy Research Group, Aston University, Birmingham, United Kingdom; a.v.bridgwater@aston.ac.uk (for correspondence)
Published online 5 April 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11635
inorganics from soil during harvesting such as mud splashing
during rain and accumulation from dragging over soil. Other
contaminants include chlorine from sea-side locations and
biocide applications and sulfur from fertilizer applications.
As bio-oil is oleophobic, all feed water reports to the bio-
oil. Water is also formed from fast pyrolysis reactions, so to
maintain a reasonable water level in the bio-oil product of
typically 25%, feed water is usually limited to a maximum of
10 wt %. This level minimizes the potential for phase separa-
tion and gives a manageable viscosity.
In addition to hemicellulose, cellulose, and lignin biomass
can also contain minor organic components such as extrac-
tives, oils, and proteins. The extractives and oils can lead to
a separate phase at the top of the bio-oil. Proteins have a
high-nitrogen content and lead to a distinctive and unpleas-
ant smell.
A comprehensive examination is made of the characteris-
tics and quality requirements of bio-oil from fast pyrolysis of
biomass. An appreciation of the potential for bio-oil to meet
a broad spectrum of applications in renewable energy has
led to a significantly increased R&D activity that has focused
on addressing liquid quality issues both for direct use for
heat and power and indirect use for biofuels and green
chemicals. This increased activity is evident in North Amer-
ica, Europe, and Asia with many new entrants as well as
expansion of existing activities. The only disappointment is
the more limited industrial development and also deployment
of fast pyrolysis processes that are necessary to provide the
basic bio-oil raw material.
2012 American Institute of Chemical
Engineers Environ Prog, 31: 261–268, 2012
Keywords:
fast pyrolysis, bio-oil, upgrading
INTRODUCTION TO FAST PYROLYSIS AND BIO-OIL
Reactor and Liquid Collection
At the heart of a fast pyrolysis process is the reactor. Sev-
eral comprehensive reviews of fast pyrolysis processes for
liquids production have been published such as [1, 2, 5–7].
The key requirements are rapid heating, a carefully con-
trolled reaction temperature of typically 480–520
8
C depend-
ing on feed material, short hot vapor residence time of less
than 2 s, efficient char removal, and rapid quenching of the
vapors and aerosols. Variations outside these limits lead to
lower liquid yields and less stable bio-oil.
Introduction
Fast pyrolysis is now a well-established thermal process-
ing method for converting biomass into high yields of a liq-
uid known widely as bio-oil [1]. This bio-oil has several un-
usual characteristics of which the main ones are summarized
in Table 1. These are mostly a consequence of formation by
rapid quenching and thus ‘‘freezing" the intermediate prod-
ucts of flash degradation of hemicellulose, cellulose, and lig-
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,
suitable norms and standards are required. Exploration and
development of standard tests for bio-oil has led to certifica-
tion by CEN in Europe and ASTM in North America [8, 9].
The evaluation and development of test methods is very im-
portant in defining quality and setting standards for definition
and marketing.
Liquid Characteristics and Quality
The objective or purpose of upgrading bio-oil is to
improve its quality, i.e., to reduce or remove one or more of
its undesirable characteristics or properties. Table 2 lists the
characteristics with causes, effects, and possible solutions [2].
It is important to define the term ‘‘quality’’ since different
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,
chemically, and catalytically. This has been extensively
reviewed [1–3, 10]. A number of the characteristics listed in
Table 2 have attracted particular interest and concern and
these are considered in more detail below.
SIGNIFICANT FACTORS AFFECTING BIO-OIL CHARACTERISTICS
Feed Material
The composition of the biomass feed has a significant
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.
The potassium and sodium in biomass are catalytically
active in fast pyrolysis through cracking to water and CO
2
in
the vapor phase. Ash contents above around 2.5 wt % often
lead to a phase separated product with significantly lower
liquid yields [4]. Biomass can be contaminated by metals and
Acidity or Low pH
Bio-oil has a typical pH of around 2.5 from the organic
acids formed by degradation or cracking of the biopolymers
that make up biomass, particularly the cellulose and hemicel-
lulose. Hemicellulose can be preferentially reduced by wash-
ing in hot water or dilute acid and by torrefaction. Neither
method is very effective since conversion is relatively ineffi-
cient and cellulose is also affected in both methods.
2012 American Institute of Chemical Engineers
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,
for example, senesces over winter and a significant propor-
tion of the minerals in the ash return to the rhizome. In addi-
tion, ash will be leached from the standing crop during win-
ter from rainfall to potentially give ash contents of such
grasses below 2.5 wt %.
The limiting value of ash content in biomass to reduce or
avoid this catalytic effect is believed to be around 2.5 wt %,
although this depends on other process parameters and the
composition of the ash.
Washing biomass with water or dilute acid is feasible to
remove ash but is costly in financial and energy terms both
for washing and subsequent drying. However, a further
advantage of acid washing is the potential removal of hemi-
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
require careful evaluation.
Physical property
Typical value
Moisture content
25%
pH
2.5
Specific gravity
1.20
Elemental analysis
C
56%
H
6%
O
38%
N
0–0.1%
HHV as produced
17 MJ/kg
Miscibility with hydrocarbons
Very low
Viscosity (40C and 25% water)
40–100 cp
Solids (char)
0.1%
Vacuum distillation residue
up to 50%
There has been only a little work on corrosion of metals
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,
which is still at an early stage of development [2].
Char
Char acts as a vapor cracking catalyst so rapid and effec-
tive separation from the pyrolysis product vapors is essential.
Cyclones are the usual method of char removal, however,
some fines always pass through the cyclones and collect in
the liquid product where they accelerate aging and exacer-
bate the instability problem. A more effective but more diffi-
cult method is hot vapor filtration, which gives a higher
grade product at the expense of liquid yield reduction of
around 15% [13–15]. Pressure filtration of the liquid for sub-
stantial removal of particulates (down to
<
1
l
m) is very diffi-
cult because of the complex interaction of the char and pyro-
lytic lignin, which appears to form a gel-like phase that rap-
idly blocks the filter. Modification of the liquid microstructure
by addition of solvents such as methanol or ethanol that
solubilize the less soluble constituents can address this prob-
lem and contribute to improvements in liquid stability.
Aging
Aging or instability affects most bio-oils. It is caused by
continued reaction of the degradation products from fast py-
rolysis, which has been frozen in the rapid quenching that is
an essential feature of fast pyrolysis [10]. It shows as a slow
increase in viscosity at ambient temperature and sometimes
as phase separation. Heating bio-oil will increase the aging
effect through a tendency for chemical reactions to continue.
Up to around 55
C, the changes seems to be reversible so
8
preheating to 50
C or less will usually have no adverse
effects on oil quality or behavior. Above around 60
8
C the
changes are increasingly less reversible and prolonged expo-
sure to higher temperatures causes increased viscosity and an
increased propensity for phase separation. Around 100
8
C,
bio-oil separates into a heavy bitumen type phase mostly
from the pyrolytic lignin and a low-viscosity aqueous frac-
tion, but both are different to phase separated bio-oil at am-
bient conditions. The heavier material will hinder heat trans-
fer and as temperatures increases, it will eventually carbonize
to form a coke layer. This is what happens in tempts for
distillation.
Polar solvents have been used for many years to homoge-
nize and reduce the viscosity of biomass oils. The addition of
solvents, especially methanol, showed a significant effect on
the oil stability, for example, Diebold and Czernik [12] found
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.
Temperature is widely used to control viscosity in com-
bustion applications, but for bio-oil this needs to be carefully
considered. In-line preheating immediately prior to combus-
tion works well, but recirculation of a heated bio-oil
for example in some engine designs needs to be managed
carefully.
8
Distillability
Pyrolysis liquids cannot be completely vaporized once
they have been recovered from the vapor phase. If the liquid
is heated to 100
8
C or more to try to remove water or distil
off lighter fractions, it rapidly reacts and eventually produces
a solid residue of around 50 wt % of the original liquid and
some distillate containing volatile organic compounds and
water. The distillate contains those compounds that are vola-
tile together with thermally cracked products from higher
temperatures.
High Viscosity
Bio-oil viscosity is important particularly for direct com-
bustion applications, where it needs to be atomized such as
in burners, engines, and turbines. Testing of bio-oil in
engines is reviewed in [3, 16, 17] and in burners in [3, 18,
19]. For engines, the preferred maximum viscosity is typically
17 cS above, which pressure requirements become excessive.
Preheating is discussed under ‘‘Aging’’ above. Viscosity is
most affected by water content and temperature and is thor-
oughly covered in Diebold’s review [10].
Polar solvents have been used for many years to homoge-
nize and reduce the viscosity of biomass oils. The addition of
solvents, especially methanol, showed a significant effect on
the oil stability. Diebold and Czernik [12] found that the rate
of viscosity increase (‘‘aging’’) for the oil with 10 wt % of
methanol was almost 20 times less than for the oil without
additives.
Alkali metals
All biomass contains ash in which alkali metals notably
potassium and sodium dominate. Potassium in particular is
catalytically active and enhances secondary cracking reac-
tions during pyrolysis. This results in loss of liquid yield,
higher water (and carbon dioxide) production, and potential
phase separation from higher water content and loss of natu-
rally derived surfactants that maintain the micro-emulsion of
bio-oil.
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
including polymerization
Slow increase in viscosity from secondary
Do not store for long periods
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
and can increase particulate carry over.
Improve harvesting practice
Wash biomass
Distillability is poor
Reactive mixture of degradation products
Bio-oil cannot be distilled—maximum 50%
typically. Liquid begins to react at below
100
8
C and substantially decomposes above 100
8
C
None known
High viscosity
Gives high pressure drop increasing equipment cost
Careful heating up to 50
8
C, rapid in-line
High pumping cost
heating to 80
8
C 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
is very low
Highly oxygenated nature of bio-oil
Will not mix with any hydrocarbons so
integration into a refinery is more difficult
Upgrading by hydrotreating or cracking
with zeolites
Nitrogen
Contaminants in biomass feed
Unpleasant smell
Careful feed selection
High nitrogen feed such as proteins
in wastes
Catalyst poisoning in upgrading
Feed blending
NOx in combustion
Include suitable cleaning processes
Add NOx removal in combustion
applications
 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,
many from hemicellulose
While not toxic, the smell is often objectionable
Better process design and management
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
quenching of vapors and aerosols
Susceptibility to aging such as viscosity increase
and phase separation
None known
Sulfur
Contaminants in biomass feed
Catalyst poisoning in upgrading
Include suitable cleaning processes
Temperature sensitivity
Incomplete reactions
Irreversible decomposition of liquid
into two phases
>
100
8
C
Store liquids at room temperature
preferably in absence of air
Irreversible viscosity increase above 60
8
C
Avoid heating for prolonged periods
Potential phase separation above 60
8
C
Avoid heating above 120
8
C
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
viscosity
Greater temperature influence than hydrocarbons
Water
Pyrolysis reactions
Complex effect on viscosity and stability:
Increased water lowers heating value, density,
stability, and raises pH
Control water in feed
Feed water
Affects catalysts
Optimize at 25% for consistency and
miscibility
Optimize for application
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
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
than reported for biomass oils produced in systems using
only cyclones [15]. A consequence of hot vapor filtration to
remove char is the catalytic effect of the accumulated char
on the filter surface, which potentially cracks the vapors,
reduces yield by up to 20%, reduces viscosity and lowers the
average molecular weight of the liquid product.
Diesel engine tests performed on hot-filtered oil showed a
substantial increase in burning rate and a lower ignition
delay when compared with unfiltered bio-oil, due to the
lower average molecular weight for the filtered oil [20]. Hot
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.
Liquid filtration to very low particle sizes of below around
Figure 1. Overview of fast pyrolysis upgrading methods.
Figure 2. There is also interest in partial upgrading to a prod-
uct that is compatible with refinery streams to take advantage
of the economy of scale and experience in a conventional re-
finery. Integration into refineries by upgrading through crack-
ing and hydrotreating has been reviewed by Huber and
Corma [23]. The main methods are:
Hydrotreating.
Catalytic vapor cracking.
Esterification and related processes.
Gasification to syngas followed by synthesis to hydrocar-
bons or alcohols.
Hydrotreating
Hydro-processing rejects oxygen as water by catalytic
reaction with hydrogen. This is usually considered as a sepa-
rate and distinct process to fast pyrolysis that can therefore
be carried out remotely. The process is typically high pres-
sure (up to 200 bars) and moderate temperature (up to
400
8
C) and requires a hydrogen supply or source [24]. Full
hydrotreating gives a naphtha-like product that requires
orthodox refining to derive conventional transport fuels. This
would be expected to take place in a conventional refinery
to take advantage of know-how and existing processes. A
projected typical yield of naphtha equivalent from biomass is
about 25% by weight or 55% in energy terms excluding pro-
vision of hydrogen. Inclusion of hydrogen production, for
example by gasification of biomass, reduces the yields to
5
m is very difficult due to the physic-chemical nature of
the liquid and usually requires high pressure drops and self
cleaning filters.
l
Toxicity
As bio-oil becomes more widely available, attention will
be increasingly placed on environment, health, and safety
aspects. A study was completed in 2005 to assess the ecotox-
icity and toxicity of 21 bio-oils from most commercial pro-
ducers of bio-oil around the world in a screening study, with
a complete assessment of a representative bio-oil. The study
included a comprehensive evaluation of transportation
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
be accomplished either by catalytic pyrolysis or by
decoupled operation as summarized below and depicted in
Figure 2. Upgrading of bio-oil to biofuels and chemicals.
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
July 2012
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