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RESEARCH REVIEW
Pyrogenic carbon capture and storage
Hans‐Peter Schmidt
1
|
Andrés Anca‐Couce
2
|
Nikolas Hagemann
1,3
|
Constanze Werner
4
|
Dieter Gerten
4,5
|
Wolfgang Lucht
4,5
|
Claudia Kammann
6
1
Ithaka Institute, Hamburg, Germany
2
Institute of Thermal Engineering, Graz
University of Technology, Graz, Austria
3
Environmental Analytics, Agroscope,
Zurich, Switzerland
4
Potsdam Institute for Climate Impact
Research (PIK), Research Domain I:
Earth System Analysis, Potsdam,
Germany
5
Humboldt‐Universität zu Berlin,
Geography Department, Berlin, Germany
6
Department of Applied Ecology,
Hochschule Geisenheim University,
Geisenheim, Germany
Correspondence
Hans‐Peter Schmidt, Ithaka Institute,
Hamburg, Germany.
Email: schmidt@ithaka-institut.org
Funding information
Bundesministerium für Bildung und
Forschung, Grant/Award Number:
01LS1620A, 01LS1620B
Abstract
The growth of biomass is considered the most efficient method currently available
to extract carbon dioxide from the atmosphere. However, biomass carbon is easily
degraded by microorganisms releasing it in the form of greenhouse gases back to the
atmosphere. If biomass is pyrolyzed, the organic carbon is converted into solid (bio-
char), liquid (bio‐oil), and gaseous (permanent pyrogas) carbonaceous products.
During the last decade, biochar has been discussed as a promising option to improve
soil fertility and sequester carbon, although the carbon efficiency of the thermal con-
version of biomass into biochar is in the range of 30%–50% only. So far, the liquid
and gaseous pyrolysis products were mainly considered for combustion, though they
can equally be processed into recalcitrant forms suitable for carbon sequestration. In
this review, we show that pyrolytic carbon capture and storage (PyCCS) can aspire
for carbon sequestration efficiencies of >70%, which is shown to be an important
threshold to allow PyCCS to become a relevant negative emission technology. Pro-
longed residence times of pyrogenic carbon can be generated (a) within the terres-
trial biosphere including the agricultural use of biochar; (b) within advanced bio‐
based materials as long as they are not oxidized (biochar, bio‐oil); and (c) within
suitable geological deposits (bio‐oil and CO
2
from permanent pyrogas oxidation).
While pathway (c) would need major carbon taxes or similar governmental incen-
tives to become a realistic option, pathways (a) and (b) create added economic value
and could at least partly be implemented without other financial incentives. Pyroly-
sis technology is already well established, biochar sequestration and bio‐oil seques-
tration in soils, respectively biomaterials, do not present ecological hazards, and
global scale‐up appears feasible within a time frame of 10–30 years. Thus, PyCCS
could evolve into a decisive tool for global carbon governance, serving climate
change mitigation and the sustainable development goals simultaneously.
KEYWORDS
biochar, bio‐oil, carbon sequestration, climate mitigation, permanent pyrogas, pyrolysis, tCDR
1
|
INTRODUCTION
To keep global warming in the range that has sustained
civilization during the past millennia, the carbon balance
between emissions to the atmosphere and carbon
accumulation in the terrestrial system has to return to an
equilibrium by 2050 at the latest (Obersteiner et al., 2018;
Rockström et al., 2016, 2009 ). To achieve this, greenhouse
gas (GHG) emissions need to be reduced by at least 90%
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
© 2018 John Wiley & Sons Ltd
Received: 27 March 2018
|
Accepted: 4 August 2018
DOI: 10.1111/gcbb.12553
GCB Bioenergy. 2019;11:573–591. wileyonlinelibrary.com/journal/gcbb
|
573
with the world economy becoming climate neutral by 2050
(Rogelj et al., 2015, Bertram et al., 2015; Sanderson,
O’Neill, & Tebaldi, 2016; Schleussner et al., 2016). How-
ever, even if the most ambitious scenarios of global GHG
emission reductions were implemented within this time
frame, the additional need for large‐scale atmospheric car-
bon dioxide removal (CDR) to prevent overshooting the
1.5°C temperature threshold remains (Boysen, Lucht,
Schellnhuber, et al., 2016; Hansen et al., 2017; Smith
et al., 2015). Thus, most recent scenarios from integrated
assessment models (IAMs) include the large‐scale deploy-
ment of so‐called negative emissions technologies (NETs)
to achieve CDR (Fuss et al., 2014; Hansen et al., 2017;
Riahi et al., 2015; Rogelj et al., 2015; Rogelj, McCollum,
Reisinger, Meinshausen, & Riahi, 2013; Van Vuuren et al.,
2013). Technical solutions to extract CO
2
from the atmo-
sphere like direct air capture (Kumar et al., 2015; Sanz‐
Pérez, Murdock, Didas, & Jones, 2016), enhanced weather-
ing (Koeve, Keller, & Oschlies, 2017; Moosdorf, Renforth,
& Hartmann, 2014), and artificial ocean alkalinization
(Montserrat et al., 2017) are promising geoengineering
approaches, although they are, to different degrees, either
not yet mature or available at the needed scale, or the risk
of large‐scale implementation may still be considered too
high (Hansen et al., 2017). An increase in terrestrial and
marine biomass production, combined where possible with
the sequestration of a significant part of its accumulated
carbon, is therefore considered a CDR strategy that may be
implemented most rapidly, and with the lowest risk of
other geological and ecological processes (Hansen et al.,
2017; Rockström et al., 2017; Smith et al., 2015).
Beside afforestation, bioenergy production with carbon
capture and storage (BECCS) is the only NET included in
the mitigation scenarios of the Intergovernmental Panel on
Climate Change's (IPCC) Fifth Assessment Report (Allen
et al., 2014). The BECCS scenario anticipates that biomass
combustion could become a major pathway for clean
energy production and that capturing the emitted CO
2
would become a complementary technology. Besides the
known and unknown risks of fossilizing CO
2
and its high
costs (>150 USD/t CO
2
(Klein et al., 2014; Smith et al.,
2015), BECCS is increasingly being recognized for poten-
tially harming ecosystem services and the integrity of the
biosphere with its extended implementation (Boysen,
Lucht, Gerten, & Heck, 2016; Burns & Nicholson, 2017;
Heck, Gerten, Lucht, & Popp, 2018).
Alternatively, CDR could be achieved through increased
net primary productivity (NPP) of the biosphere, combined
with the extension of the mean residence time (MRT) of the
biogenic carbon (i.e. net C sequestration) (Erb et al., 2017;
Smith et al., 2015). Increasing soil organic matter is one way
forward to extend MRTs (Lal, 2011; Minasny, Malone,
et al., 2017; Zomer, Bossio, Sommer, & Verchot, 2017).
However, the capacity of soils to accumulate organic carbon
is likely limited (Barré et al., 2017; Minasny, Arrouays,
et al., 2017) and requires large amounts of nitrogen (van
Groenigen et al., 2017). Maintaining increased soil organic
matter (SOM) stocks will depend on agronomic methods and
may further be hampered by the effects of climate change
(Sierra, Trumbore, Davidson, Vicca, & Janssens, 2015). The
MRT of SOM is 50–80 years at maximum (Schmidt et al.,
2011; Wang & Chang, 2001). Thus, increasing SOM is
clearly an important CDR with extensive ecological cobene-
fits; however, its long‐term potential may not be sufficient as
sole CDR technology (Boysen, Lucht, Schellnhuber, et al.,
2016; Smith et al., 2015; Soussana et al., 2017; van Groeni-
gen et al., 2017). Moreover, the conversion of plant residues
(shoots, roots, and root‐derived C) into SOM has low C‐effi-
ciencies of 10%–15% (Bolinder, Angers, Giroux, & Laver-
dière, 1999) not considering SOM saturation (Six & Jastrow,
2002). A complementary way forward to extend the MRT of
biogenic C in the terrestrial system and to increase C‐effi-
ciencies is the pyrolytic treatment of biomass with subse-
quent sequestration in the bio‐, geo‐, and anthroposphere.
In pyrolysis processes, shredded biomass is heated under
oxygen‐deficient atmospheres to temperatures between 350
and 900°C (EBC, 2012) converting the biomass into a solid
(biochar), a liquid (bio‐oil), and a permanent pyrogas frac-
tion. In most current pyrolysis systems, the pyrolysis liquids
and gases are combusted for the production of thermal and
electric energy releasing the carbon as GHG back to the
atmosphere. While this is often considered as carbon neutral,
that is “green energy”because it is made from biomass, it
can only sequester the biochar carbon but not the carbon
from the gaseous and liquid phases which is more than half
of the initial biomass carbon. To optimize the carbon
sequestration potential, the following review is focused on
the material use of all pyrogenic carbon species (i.e. solid,
liquid, gaseous), and to reduce the combustion of pyrolytic
products to a minimum. We specifically investigate the con-
ditions and capacity of pyrogenic carbon capture and storage
(PyCCS) as a complete negative emission system, quantify-
ing the C‐sequestration potential of all three pyrolysis prod-
ucts including their use as bio‐based materials and
agronomic amendments, but also for geologic carbon stor-
age. We review literature on biochar, bio‐oil, and pyrolysis
systems for the required properties of pyrolytic products and
summarize production conditions optimized for material use
and C sequestration. We then compare PyCCS with BECCS
in terms of carbon and nutrient efficiency, costs, environ-
mental risks, and time frame for technology implementation.
2
|
PYROLYSIS TECHNOLOGY
Pyrolysis is the thermal conversion of carbonaceous materi-
als (e.g. biomass) in a low oxygen atmosphere. During
574
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SCHMIDT ET AL.
pyrolysis, biomass evolves into biochar (solid product) and
releases volatiles, the so‐called pyrolytic gas or pyrogas.
Cooled to ambient temperature, pyrogases mostly condense
into a liquid, the bio‐oil. The remaining gas is called per-
manent pyrogas referring to the volatiles of the pyrolysis
gas that do not condense at ambient temperature. The qual-
ity and properties of the three products and their yield frac-
tion depend on the feedstock and its properties (e.g. woody
vs. herbaceous biomass, soft‐vs. hard‐wood, moisture con-
tent, particle size) and on the pyrolysis process conditions.
These include heating rate, highest treatment temperature
(HTT), residence time of the solid, residence time of the
volatiles as determined by the gas flow rate in the pyrolysis
reactor, direction of gas flow and pyrogas pressure, residual
oxygen contact, and for biochar also quenching/activation
procedures (Boateng, Garcia‐Perez, Masek, Brown, &
Campo, 2015; Hagemann et al., 2018; Weber & Quicker,
2018).
The solid pyrolysis product, biochar, is a highly porous
material consisting mainly of polyaromatic hydrocarbons
and inorganic species originally contained in the biomass.
Bio‐oil is a dark brown, oil‐like liquid composed of water
and more than 100 oxygenated condensable hydrocarbon
species including acetic acid, methanol, aldehydes, ketones,
phenols, oligomeric sugars, and water‐insoluble lignin‐
derived compounds (Sanna, Li, Linforth, Smart, & a,
Andrésen JM, 2011). The remaining permanent pyrogas
includes CO, CO
2
,CH
4
,H
2
, and C
x
H
y
; it is highly inflam-
mable. To date, a multitude of different pyrolysis technolo-
gies exists that are usually optimized for one of the
products, for example fast or flash pyrolysis for bio‐oil,
slow pyrolysis for biochar, or gasification for pyrolytic
gases.
3
|
BIOCHAR CARBON
PERSISTENCE
During the last decade, biochar was increasingly discussed
as a tool for carbon sequestration in soil (Fowles, 2007;
Laird, 2008; Lehmann, Gaunt, & Rondon, 2006; Matovic,
2011; Woolf, Amonette, Street‐Perrott, Lehmann, &
Joseph, 2010; Woolf, Lehmann, & Lee, 2016). It is the
most resistant form of organic carbon in soils as well as in
lake and ocean sediments (Cotrufo et al., 2016; Lehmann
et al., 2015; Reisser, Purves, Schmidt, & Abiven, 2016;
Zimmerman & Gao, 2013). In the absence of very long‐
term (1,000 years) field experiments in various environ-
ments and climatic zones, the MRT can only be approxi-
mated by (a) short‐term (<10 years) experimental data,
combined with mathematical decay models and temperature
normalization (Kuzyakov, Bogomolova, & Glaser, 2014;
Lehmann et al., 2015), by (b) bottom‐up assessments of
fire frequencies and pyrogenic carbon (PyC) leftovers from
natural fires (Cheng, Lehmann, & Engelhard, 2008; Santín
et al., 2017), or by (c) archeological evidence (Braadbaart,
Poole, & Brussel, 2009; Criscuoli et al., 2014).
The MRT of biochar is strongly related to its H/C
org
ratio (which depends mainly on the pyrolysis temperature)
and to the environmental conditions of its storage (Braad-
baart et al., 2009; Camps‐Arbestain, Amonette, Singh,
Wang, & Schmidt, 2015). In a meta‐analysis of 111 experi-
ments on biochar persistence, Lehmann et al. (2015) esti-
mated that biochars with a H/C
org
ratio of <0.4 have, when
applied to soil, a MRT of >1,000 years, corresponding to
half‐life (T
½
) times of ~700 years. Similar estimates were
obtained in a review by Zimmerman and Gao (2013).
Another meta‐analysis by Wang, Xiong, and Kuzyakov
(2016) based on 24 studies with stable (
13
C) and radioac-
tive (
14
C) carbon isotopes found MRTs of 556 years inde-
pendent of the H/C
org
ratio. However, the soil environment
seems to play a dominant role in PyC persistence. While
the majority of field and laboratory studies reveal centen-
nial MRTs of >100 years (Lehmann et al., 2015; Rasse
et al., 2017), some studies show that under certain soil
(biota) conditions, biochar can be degraded biologically
with much shorter MRTs (de la Rosa, Miller, & Knicker,
2018; de la Rosa, Rosado, Paneque, Miller, & Knicker,
2018). In their recent assessment of the worlds’stocks of
PyC, Reisser et al. (2016) found that soil PyC stocks aver-
age 13.7% (i.e. 200 Pg C globally) of the overall soil
organic carbon stocks down to 2 m depth. Soil clay content
and pH were the variables with the strongest predictive
value for high PyC contents in soils. However, native, fire‐
derived PyC in soils is chemically different to biochar pro-
duced from the same feedstock; the chemical characteristics
of biochar produced from the same material point toward
greater stability of the latter (Santín et al., 2017).
Once mixed into soil, PyC particles shatter into micro‐
and nanoparticles (Zimmerman & Gao, 2013). This is
mainly a physical process, that is the chemical structure
and the amount of carbon stored remain unchanged (Spo-
kas, et al., 2014). Fine particles can be transported verti-
cally to deeper soil horizons, groundwater, or horizontally
into adjacent soils or waterbodies where it may be pro-
tected against degradation. In fact, deep soil horizons are
the most important compartments for sequestration of soil
organic carbon on timescales of decades to centuries (Lor-
enz & Lal, 2014). Over longer timescales, groundwater and
sediments may be more relevant sequestration compart-
ments due to micronized biochar–carbon species being
even better preserved there from microbial and chemical
degradation (Zimmerman & Gao, 2013). Micron‐sized and
nanofractured PyC may ultimately reach the ocean (Foer-
eid, Lehmann, & Major, 2011; Hockaday, Grannas, Kim,
& Hatcher, 2007; Jaffe et al., 2013; Major, Lehmann, Ron-
don, & Goodale, 2010). In the ocean floor sediments, PyC
SCHMIDT ET AL.
|
575
MRTs of >6,000 years were calculated (Coppola, Ziolk-
owski, Masiello, & Druffel, 2014). However, to the best of
our knowledge, no published study experimentally proved
and quantified the transfer of PyC from topsoil applications
to waterbodies and sediments (Maestrini et al., 2014).
Often, a proportion of an applied biochar are just missing
after several years without having left the soil system as
CO
2
efflux (e.g. Major et al., 2010).
Based on the available data, it is hard to provide a
globally valid leakage rate of biochar applied to soil after
80 years. Biochars produced at pyrolysis temperatures
>400°C show MRTs, based on extrapolations from degra-
dation assessments of at least three years, that range from
201 years (Singh, Cowie, & Smernik, 2012) to 4,419 years
(Lehmann et al., 2015; Zimmerman & Gao, 2013), depend-
ing on the models, degradation methods, and biochar char-
acteristics. Under a conservative estimation and
considering a double‐exponential decay model (Lehmann
et al., 2015; Zimmerman & Gao, 2013), we assume aver-
age C‐leakage rates for the first 100 years after soil appli-
cation (BC
+100
) between 10% at molar H/C
org
ratios below
0.4% and 20% above. However, caution is recommended
as some soils, for example with low SOC and frequent
harvest‐residue fires or frequent frost–thaw cycles, may
turn out to be less suitable for biochar carbon sequestration
causing lower MRTs (Rosa, Rosado, et al., 2018). While
more research about mechanisms of PyC degradation and
the fate of the degradation products are clearly needed,
three meta‐analyses and all studies on biochar persistence
in soil (based on >120 experiments) confirm that biochars
are much more recalcitrant than their precursor materials
and natural SOM and that MRTs exceed the centennial
scale.
4
|
BIOCHAR USES AND FURTHER
CLIMATE‐RELEVANT EFFECTS
Biochar can not only be used as soil amendment but also
in multiple ways for industrial products like construction
materials (Gupta & Kua, 2017), for wastewater treatment
(Mohan, Sarswat, Ok, & Pittman, 2014), and for electron-
ics (Gu et al., 2014). In all of these products, PyC can
serve as a C‐sink, as long as the product is not thermally
degraded and oxidized during use, recycling, or disposal.
When biochar is not used as a soil amendment but, for
example, as a composite building material or stored in a
long‐term deep soil repositories, it can likely be protected
more efficiently from microbial and chemical degradation
to reach MRTs comparable to those of PyC in sediments
(Coppola & Druffel, 2016).
To date, biochar is mainly used as a soil amendment, as
animal feed ingredient, for manure treatment in agriculture
(Schmidt & Shackley, 2016) or as a compost or planting
substrate additive (Jaiswal, Elad, Cytryn, Graber, & Fren-
kel, 2018; Kern et al., 2017). When applied to soil, biochar
can, but does not always, improve parameters like soil
nutrient retention, water retention, soil pH, cation exchange
capacity, or microbial activity, with effects strongly
depending on both soil and biochar characteristics (Atkin-
son, Fitzgerald, & Hipps, 2010; Cornelissen et al., 2013;
Lehmann et al., 2011) and plant species grown on it (e.g.
Guereña et al., 2015; Schmidt, Pandit, Cornelissen, &
Kammann, 2017). In the longer term, for example with
repeated additions of biochar organic blends, biochar will
modify the original soil chemical and physical properties
and cause bulk density decreases, improved water infiltra-
tion, and increased stocks of water or plant‐available nutri-
ents. Such modifications are frequent in PyC‐rich
anthropogenic dark earth soils worldwide (Glaser & Birk,
2012; Solomon et al., 2016) or at old charcoal‐making
(kiln) places (Borchard et al., 2014; Criscuoli et al., 2014).
Biochar can have contrasting effects on nonpyrogenic
soil organic carbon stocks (Wang et al., 2016). As shown
in some studies, biochar soil application may enhance the
accumulation of plant‐derived carbon entering the soil (lit-
ter, root exudates) and thus increase the content of soil
organic carbon beyond the addition of biochar's PyC
(Amendola et al., 2017; Bruun, Petersen, Hansen, Holm, &
Hauggaard‐Nielsen, 2014; Ventura et al., 2014; Weng
et al., 2017). In some cases, however, decomposition of
already present nonpyrogenic soil organic carbon was
accelerated (Wardle, Nilsson, & Zackrisson, 2008; Whit-
man & Lehmann, 2015). Thus, biochar can have positive
or negative priming effects; particularly, the interaction
with plant roots is not well understood (Cross & Sohi,
2011; Wang et al., 2016). To trace C flows, C sources
must be traced by stable isotopes to three CO
2
–C sources:
plant‐derived C, soil organic matter‐derived C, and bio-
char‐derived C. Due to the complexity of such experiments,
the number of published studies is still low.
Several meta‐analyses of biochar field trials indicate
average yield increases of 10%–20% following amendments
of more than 5 t/ha of pure, not‐nutrient‐enriched biochar
(Crane‐Droesch, Abiven, Jeffery, & Torn, 2013; Jeffery,
Abalos, Spokas, & Verheijen, 2015). While yield increases
(with pure biochar use) are low or absent in temperate
soils, biochar application increased yields in tropical and
subtropical soils by 25% on average, according to the latest
extensive meta‐analysis (Jeffery et al., 2017). The increase
also enhances accumulated C from the atmosphere by the
same percentage; if the same practice is maintained over
decades, the sustained increase in NPP increases the terres-
trial carbon pool by the corresponding amount. Such a “re-
turn of investment”of biochar use, that is the increase in
non‐PyC SOC and NPP, may be as important for PyCCS
as the PyC sequestration by the biochar itself.
576
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SCHMIDT ET AL.
However, the use of large amounts of pure biochar may
not become the final pathway of soil application in the
future. Biochar may serve as raw material for different
agricultural (fertilizer) products rather than being a final
amendment product in itself. For example, biochar made
from invasive weed and harvest residues improved fertilizer
efficiencies and boosted yields in fertile soils in Nepal by
on average 100% when used as a nutrient carrier at low
dosages (<1–2 t/ha) (Schmidt et al., 2017). Moreover, bio-
char can be used as a tool to further reduce agronomic
GHG emissions (Kammann et al., 2017) as it has been
shown to reduce methane emissions in saturated soils, for
example in rice agriculture (Han et al., 2016; Jeffery, Ver-
heijen, Kammann, & Abalos, 2016), or N
2
O emissions
from fertilized soils by 40%–50% on average (meta‐studies:
Cayuela et al., 2014; Schirrmann et al., 2017).
5
|
BIO‐OIL, THE LIQUID PRODUCT
OF PYROLYSIS
While biochar is well established as a means of carbon
sequestration (Brassard, Godbout, & Raghavan, 2016), bio‐
oil was not yet discussed as such. Bio‐oil, also known
under the names pyrolysis liquids, pyroligneous acid, or
(more historically) wood vinegar, is a very complex liquid
mixture composed of hundreds of different oxygenated
hydrocarbons and water. It is an obligate by‐product of all
kinds of pyrolysis. When bio‐oil is the main intended
pyrolysis product, optimal production parameters are fast
(<1 s) pyrolysis at 500°C (Xie, Xu, Fang, Luo, & Ma,
2013).
Fast pyrolysis, bio‐oil is typically viscous but free‐flow-
ing and dense (~1.2 kg/L), has usually a dark brown color,
is acidic (pH ~2.5), and has a low net calorific value (NCV
<20 MJ/kg) due to the high oxygen content and moisture
(Mohan, Pittman, & Steele, 2006; Oasmaa, Beld, Saari,
Elliott, & Solantausta, 2015). It is a micro‐emulsion, in
which pyrolytic lignin and other water‐insoluble organic
compounds are dispersed. Pyrogenic organic compounds
dissolved in the aqueous phase serve as emulsifiers, though
it has a strong tendency for phase separation. A decanted
heavy oil phase at the bottom, rich in nonpolar compounds
(mainly pyrolytic lignin), can be distinguished from an
upper aqueous phase rich in polar compounds (light
organic compounds and heavy sugars) (Oasmaa, Fonts,
Pelaez‐Samaniego, Garcia‐Perez, & Garcia‐Perez, 2016).
Additionally, a distinctive upper layer rich in extractives
(mainly waxes, fatty acids, phenolics) may form.
A typical composition of bio‐oil made from woody bio-
mass by fast pyrolysis at 500°C includes, in mass percent-
age, 25% water, 20% pyrolytic lignin, 55% of organic
water‐soluble compounds (around 30% of sugars produced
from cellulose and hemicellulose and 20% of carbonyls and
alcohols), and small amounts of solid particles including
char, ash, or sand (Bayerbach & Meier, 2009; Oasmaa
et al., 2016, 2015 ).
Bio‐oil can be also obtained from slow pyrolysis pro-
cesses over the whole pyrolysis temperature range (350–
900°C) and from all types of biomass feedstocks (Sanna
et al., 2011). Thus, pyrolysis optimized for biochar produc-
tion still yields bio‐oil, though at considerably lower
amounts and with some distinct differences in composition.
At a given temperature, slow pyrolysis bio‐oil has similar
elemental composition on a dry basis, although with the
following differences:
The water content is increased, which leads to enhanced
phase separation.
The sugar content is significantly reduced due to sec-
ondary reactions (Anca‐Couce, Mehrabian, Scharler, &
Obernberger, 2014; Branca, Giudicianni, & Blasi, 2003).
The pyrolytic lignin yield is reduced.
The production of benzene, toluene, and xylene isomers
(BTX) and polycyclic aromatic hydrocarbons (PAH) is
enhanced to significant values, which can be a concern
for some applications due to their toxicity (Fagernäs,
Kuoppala, & Simell, 2012; Milhé et al., 2013).
6
|
BIO‐OIL USES AND ITS
SEQUESTRATION
Bio‐oil can be employed for heat and power generation in
gas turbines or furnaces and, after an upgrading, as biofuel
for internal combustion engines (Elliott, 2007; Fagernäs,
Kuoppala, Tiilikkala, & Oasmaa, 2012; Kauffman, Dumor-
tier, Hayes, Brown, & Laird, 2014). Here, however, we do
not consider the purpose of energetic use which is of
course possible but would release the carbon back to the
atmosphere. We rather consider applications and uses that
maintain the bio‐oil's carbon sequestered. Instead of com-
busting the nonsolid by‐products of biomass pyrolysis as
suggested by Woolf et al. (2016), the bio‐oil could be
either used in a biorefinery approach (Table 1) or seques-
tered in geological repositories as discussed below.
Before the advent of fossil fuel extraction in the early
20th century, main raw materials for the chemical industry
were produced by wood distillation, which is a historical
name for pyrolytic bio‐oil production. Besides tar and pitch
for building ships and roads, basic chemicals like naph-
thalene, anthracene, aniline, phenol, benzene, methanol,
acidic acid, and acetone were produced with pyrolysis sys-
tems for the nascent chemical industry (Aftalion, 2001).
Until 1950, methanol was still mainly produced through
wood pyrolysis, though by then the much cheaper and
more homogeneous fossil oils replaced the biomass materi-
als as important resource for the chemical industry (Soltes
SCHMIDT ET AL.
|
577
& Elder, 2017). Currently, 10% of annually produced fossil
oils are used as raw material by the chemical industry
(Keim, 2014). If humanity succeeds in limiting mean glo-
bal warming to 1.5°C or 2°C as specified in the Paris
Agreement (Schleussner et al., 2016, Rockström et al.,
2017), carbon emissions will have to be reduced by 90%
until 2050. Emission reduction of this order can only be
achieved with rapid and complete phasing‐out fossil fuels
within the next 30 years (Rogelj et al., 2015, Bertram
et al., 2015; Sanderson et al., 2016; Schleussner et al.,
2016). This, however, implicates that the 440 million tons
of fossil oil (OECD, 2018) that are currently extracted and
used as raw material for the chemical industry would have
to be replaced by biomass‐derived raw chemicals, that is
bio‐oil, cellulose, starch, lactic acid monomers.
Most chemical products that are currently produced
from fossil fuels can as well be produced from bio‐oils in a
biorefinery approach (see Table 1) though today (until CO
2
emissions are properly taxed) at higher costs.
The most pertinent industrial C‐storing use of bio‐oil is
certainly its blending to asphalt binder for road construc-
tion. Raman, Hainin, Hassan, and Ani (2015) have shown
that the addition of 5%–20% of bio‐oil to the asphalt mix-
ture improves asphalt quality like stiffness and durability.
If 10% bio‐oil by weight were added as binder additives
and/or aggregates to 1.8 billion tons of asphalt poured each
year, 180 million tons of carbonized material would be
entombed annually. With a carbon content of 55%, the
amount of sequestered C in asphalt could be 100 million
tons per year. Bio‐oil could further be added to concrete
and other building materials (Czernik & Bridgwater, 2004;
Xie et al., 2013), which would also become long‐term
carbon storage deposits, as those materials would end their
material life cycle in other construction works or landfills.
Those applications could be realized already within years,
that is short to mid term.
More challenging is the use of bio‐oil as raw material
for bioplastics and other types of composite materials
(Crombie & Mašek, 2014), which will need advanced
chemical engineering. Depending on the life cycle of the
plastic materials, carbon fibers and carbon composites, the
persistence of the carbon in the terrestrial system would
vary between short, mid, and long term. With the increas-
ing public and political pressure on recycling plastic mate-
rials (instead of waste combustion and landfill dumping),
the carbon contained in the advanced biomaterials could be
stored for much longer timescales in the terrestrial system,
so that the material use and recycling would become a
long‐term carbon sink.
Research and development in regard to biorefinery of
bio‐oil is, although based on a long history, currently only
at the beginning though, from the theory, it is rather clear
that bio‐oil could well become a decisive raw material for
the chemical industry once the production of fossil fuels
would be banned. Further possible fields of bio‐oil applica-
tion include agronomy, animal husbandry, pharmaceutical
industry, or wood preservation (Table 1). However, those
latter applications would have a rather short carbon storage
capacity.
As long as the chemicals or materials using bio‐oil car-
bon are not combusted or otherwise oxidized, the carbon
remains sequestered, augmenting the terrestrial carbon pool.
With advanced end‐of‐life product management, the carbon
of these bio‐oil based products could be recycled or
sequestered in protected deposits at the end of its lifetime,
then still serving as a C‐sink.
7
|
GEOLOGICAL STORAGE OF
BIO‐OIL
Compared to fossil crude oil, bio‐oil is characterized by a
higher moisture and oxygen content, lower viscosity, and
lower caloric value. However, both liquids are similar in
regard to pumping, transportation, and storage (Czernik &
Bridgwater, 2004; Gueudré et al., 2017; Staš, Chudoba,
Kubicka, Blazek, & Pospíšil, 2017). It can be assumed that
the environmental toxicity of fossil crude and bio‐oil is
similar (Fermoso, Pizarro, Coronado, & Serrano, 2017;
Louwes et al., 2017; Varma & Mondal, 2017; Zhang,
Chang, Wang, & Xu, 2007) though direct comparisons and
ecotoxicological assessments were not yet done. It is clear
that both should not be applied to soil. However, bio‐oil
carbon could, as crude oil was for millions of years, be
sequestered for the long term (>1,000 years) when pumped
into depleted fossil oil deposits (Werner, Schmidt, Gerten,
TABLE 1 Suggested and tested uses of bio‐oil‐based products
Bio‐oil‐based
product and use References
Bioplastics Crombie and Mašek (2014)
Asphalt amendment Raman et al. (2015)
Amendment for
building materials
Czernik and Bridgwater (2004) and Xie
et al. (2013)
Wood preservative Czernik and Bridgwater (2004) and
Tiilikkala, Fagernäs, and Tiilikkala (2010)
Biocide and crop
protection
Tiilikkala et al. (2010)
Feed additive Yamauchi, Ruttanavut, and Takenoyama
(2010), Cai, Jiang, and He (2011) and
Chu et al. (2013)
Plant growth
enhancer
Ratanapisit, Apiraksakul, Rerngnarong,
Chungsiriporn, and Bunyakarn (2009),
Tiilikkala et al. (2010) and Hagner,
Penttinen, Tiilikkala, and Setälä (2013)
578
|
SCHMIDT ET AL.
Lucht, & Kammann, 2018). Those deposits would have the
advantage that they are typically deeper than 1,000 m
below the surface, that is deep below the groundwater zone
(Philp, 1985). They would thus be protected from oxygen
and microbial degradation. Any risk of leaching would be
avoided. The bio‐oil carbon could thus, in theory, be
sequestered without any apparent environmental risk while
being protected from decomposition. The geological integ-
rity of the sequestration deposits was demonstrated by the
ability to hold fossil oil for millions of years without leak-
age. Risks of volatilization to the atmosphere, except for
the transportation to the final geological deposit, could be
assumed to be minimal considering that bio‐oil has a poor
volatility compared to fossil oil (Czernik & Bridgwater,
2004).
The carbon density of bio‐oil (590–750 kg C/m
3
(Neves, Thunman, Matos, Tarelho, & Gómez‐Barea, 2011))
is slightly lower than that of fossil oil (800 kg C/m
3
(Czer-
nik & Bridgwater, 2004)) though more than three times
higher than that of liquefied CO
2
(135–220 kg C/m
3
(IPCC, 2005)), which is proposed to be used for BECCS.
Geologic sequestration of bio‐oil would therefore take less
volume than CO
2
–CCS. Moreover, it would avoid most of
the known and unknown short‐and long‐term risks associ-
ated with geological storage of CO
2
(CO
2
leakage, alter-
ation of groundwater chemistry, seismic activity, etc.
(Vaughan & Gough, 2016; Burns & Nicholson, 2017)). In
Werner et al. (2018), the carbon leakage during transporta-
tion of the bio‐oil to the final repository and the millennial
underground storage was assumed to be 2% over 80 years
for the geological bio‐oil storage pathway.
Today, the geological storage of bio‐oil can only be
considered a thought experiment. It certainly seems coun-
terintuitive to replenish fossil oil reservoirs with bio‐oil,
when today's fossil oil extraction continues unabated at
4.4 Gt per year (OECD, 2018). However, it might become
one of the options of last resort when, as it appears today,
the phasing out of fossil fuels will be delayed by one or
more decades. To return atmospheric CO
2
concentrations
to a save level, humanity might be soon forced (Rockström
et al., 2016) not only to stop fossil fuel extraction but also
to reverse the direction of liquid‐C movement to geological
stores using the same pipelines. In the meantime, bio‐oil
may reduce the dependency on fossil fuels by being used
as energy carrier or raw material for the chemical industry.
The bio‐oil can either be stored as one mixed liquid
substance or it can be stored in different fractions, which
are obtained either by phase separation or by fractional
condensation. Fractional condensation of bio‐oil would be
controlled by a liquid–vapor equilibrium separating the
heavy tars (polar and nonpolar, including pyrolytic lignin
and heavy sugars) which condense at higher temperatures
and an aqueous phase with light organic molecules which
condense at lower temperatures (Oasmaa et al., 2016). The
composition of each phase can be controlled by the temper-
ature in the fractional condensation when cooling down the
pyrolysis volatiles or by stepwise distillation of biomass
(i.e. recovering volatiles when heating the biomass at 250,
350, 400°C). It would thus be possible to recover first
those parts of the pyrolytic oil that are, already today, eco-
nomically valuable for bio‐based chemical materials (C
sequestration in economically valuable products) and to
sequester the remaining fractions in geological deposits.
While the aqueous phase of the oil is well pumpable
but has a low pH (<3), the heavy tar fraction is not well
pumpable but has a more neutral pH, which would facili-
tate the intermediate storage of that fraction. The following
bio‐oil properties should further be considered when plan-
ning for transport and storage:
Bio‐oils are incapable of sustaining combustion and can
be classified as nonflammable liquids (Oasmaa et al.,
2012).
Bio‐oil is slightly corrosive, and toxicity increases from
fast to slow pyrolysis bio‐oils, which is associated with
a higher concentration of PAHs (Blin, 2005).
Bio‐oil has a high viscosity, commonly in the order of
tens of centistokes, and the viscosity increases during
storage (Mohan et al., 2006).
Short‐term storage should be conducted in the type of
containers recommended for fast pyrolysis oil (Oasmaa
et al., 2016), which are composed of corrosion‐resistant
steel and materials such as AISI 304, AISI 316, polyte-
trafluoroethylene (PTFE), high‐density polyethylene
(HDPE), and polyvinyl chloride (PVC).
It can be expected that dedicated research and engineer-
ing could solve the mentioned storage and displacement
issues. However, more research is clearly needed, espe-
cially for value‐generating and cascading uses of bio‐oil for
C‐sequestering and its environmentally safe long‐term stor-
age.
8
|
OXIDATIVE USE AND
SEQUESTRATION OF THE GASEOUS
CARBON (CO
2
–CCS)
Depending on the process parameters, the carbon contained
in the permanent pyrogas accounts for 15%–45% of the
total carbon of the initial biomass (Table 2). To ensure a
high C‐sequestration efficiency of PyCCS, the gaseous car-
bon should be included in the sequestration scenarios (Fig-
ure 1). Usually, the gas is combusted to produce thermal
energy to drive the pyrolytic process (Crombie & Mašek,
2014) or to produce electricity. However, in conventional
combustion where air is used as oxidizing agent, the flue
SCHMIDT ET AL.
|
579
gas contains beside CO
2
and H
2
O more than 50% N
2
.
Separating the flue gas from the N
2
to obtain pure CO
2
for
CCS purposes would be a complex and expensive process
increasing the cost of CO
2
–CCS heavily. It would therefore
only be economically viable in large‐scale industrial
devices (Reynolds, Ebner, & Ritter, 2006). As one impor-
tant advantage of PyCCS is the option for decentralized,
small‐scale production units (e.g. one unit per farm or vil-
lage), this may exclude CO
2
–CCS from gas combustion. A
recent alternative that could well be set up in small‐scale
devices is chemical‐looping combustion (CLC). In CLC, a
solid oxygen carrier is used for combustion instead of air
(Adanez, Abad, Garcia‐Labiano, Gayan, & Diego, 2012).
The oxygen carrier, most often based on nickel, iron, or
TABLE 2 Pyrolysis product properties for five PyCCS scenarios
Units
Slow pyrolysis
400°C
Slow pyrolysis
500°C
Slow pyrolysis
650°C
Slow pyrolysis
800°C
Fast pyrolysis
500°C
Ash content % d.b. 1 1 1 1 1
Initial moisture % w.b. 25 25 25 25 5
Char yield % w.b. 26.1 22.4 17.9 14.4 20.0
C char % 74.6 79.8 83.8 85.4 78.6
H char % 3.4 2.5 1.6 1.0 2.5
O char % 19.1 14.4 10.4 8.4 14.1
Ash char % 2.9 3.3 4.2 5.2 4.8
H/C ratio –0.54 0.38 0.23 0.14 0.38
Liquids yield % w.b. 52.8 56.1 54.3 49.2 61.2
C liquids % d.b. 56.3 56.3 59.9 63.6 56.3
H liquids % d.b. 6.1 6.1 6.4 6.7 6.1
O liquids % d.b. 37.6 37.6 33.6 29.7 37.6
Moisture liquids % w.b. 66.4 62.4 64.5 71.2 28.9
Permanent gas
yield
% w.b. 21.1 21.4 27.7 36.5 18.9
C gases % 37.1 35.1 38.6 44.0 37.5
H gases % 7.1 7.4 6.9 6.5 6.5
O gases % 55.8 57.5 54.5 49.5 56.0
LCV biomass MJ/kg
w.b.
13.4 13.4 13.4 13.4 17.7
LCV char MJ/kg 27.2 28.7 29.7 29.8 28.3
LCV liquids MJ/kg
w.b.
5.6 6.6 6.8 5.6 14.7
LCV gases MJ/kg 13.9 13.4 14.5 16.4 13.5
C‐density of char kg/m
3
239 298 235
C‐density of
liquids
kg/m
3
672 590 734
C in char % 52.1 48.0 40.3 32.9 33.2
C in liquids % 26.8 31.9 31.0 24.1 51.9
C in gases % 21.0 20.2 28.7 43.0 15.0
Leakage BC
+100
%20 10 10 10 10
CSE without CO
2
%70 74 67 53 81
CSE % 88 94 94 94 95
Note. BC+100: biochar carbon that remains after 100 years after soil amendment; CSE without CO
2
: CSE of the PyCCS pathway with biochar and bio‐oil seques-
tration only; CSE: total carbon sequestration efficiency of the complete PyCCS pathway; d.b.: based on dry matter; LCV: low caloric value; w.b.: based on wet mat-
ter; PyCCS: pyrolytic carbon capture and storage.
Biomass feedstock for all considered scenarios is woody biomass. The correlations are derived from pyrolysis process parameters, and they are based on the assump-
tions and formula presented in the SI.
580
|
SCHMIDT ET AL.
manganese oxides, is moved through the permanent pyro-
gas stream in a countercurrent process where the metal‐
bound oxygen reacts with the gas. Once the oxygen carri-
ers are reduced, they are conveyed out of the combustion
reactor into another reactor where they get reoxidized with
air before being reused. This chemical looping can be
repeated for countless cycles without losing its efficiency
(Adanez et al., 2012). The reaction products of permanent
pyrogas CLC are nearly pure CO
2
and water, which can
easily be separated by condensation. The CO
2
could by
then be treated as in conventional CO
2
–CCS scenarios
(Zakkour, Kemper, & Dixon, 2014).
Alternatively, the gases could also be treated with a
chemical‐looping reforming (CLR) process that results in
CO
2
and H
2
as reaction products (Adanez et al., 2012; de
Diego et al., 2008). While the resulting H
2
will be a car-
bon‐neutral energy carrier, the pure CO
2
could be pro-
cessed for geological sequestration. As this process would
not generate the heat needed for pyrolysis, the required
energy input needs to be generated from renewable electric-
ity (i.e. E‐pyrolysis). This approach should be investigated
as a new option for power to gas.
CLC and CLR are considered as the most promising
alternative to reduce the costs of CO
2
capture (Kerr, 2005)
and are fitting technical opportunities to increase the C‐
transformation efficiency of the PyCCS technology. How-
ever, CLC and CLR in combination with pyrolysis are not
yet widely tested or mature technologies. Hence, they need
further development and up‐scaling before being imple-
mented at larger scales.
Following Kemper (2015) and Vaughan and Gough
(2016) and assuming that captured CO
2
is harder to contain
than bio‐oil and biochar, we assume leakage rates of 5%
over 80 years for the geological CO
2
storage.
9
|
MINERAL NUTRIENT
BALANCES
Biomass contains a wealth of elements beyond CHO.
Although present only in lower concentrations, they are of
high relevance due to their role as plant macro‐and
micronutrients. After harvest, essential nutrients bound in
biomass should be returned to biomass‐producing agroe-
cosystems. Otherwise, soil fertility deteriorates over time
(Hänsch & Mendel, 2009; Ingerslev, Skov, Sevel, & Ped-
ersen, 2011). Already today, the mineral depletion of
agronomically used soils is a point of concern, affecting
global food security (Jones et al., 2013). This would be
further accelerated when biomass is extracted without
returning the minerals it contains to the soil. The latter is
the case in BECCS scenarios where biomass is combusted
to produce energy, and the resulting ashes cannot or can
only partially be returned back to soils due to heavy
metal, PAH, and dioxin contamination (Ingerslev et al.,
2011) or to toxic ash transformation reactions during com-
bustion (Boström et al., 2012). On the other hand, return-
ing the ashes and minerals to the soil with the biochar
will likely deliver a “mineral weathering”CCS contribu-
tion to CDR, which would need to be further explored
and quantified.
FIGURE 1 General pyrolytic carbon capture and storage scheme for pyrolytic treatment of biomass, the pathways of solid, liquid, and
gaseous products, their use and sequestration scenarios, the respective C‐leakage rates, and the circular effect on carbon farming systems and
sustainable biomass production
SCHMIDT ET AL.
|
581
Nitrogen is an important nutrient of plants, and its avail-
ability is often limiting for plant growth. In biomass, it is
mainly present in the form of proteins. The main primary
pyrolysis product of proteins at temperatures around 500°C
is amides, which are present in the bio‐oil. Nitrogen is fur-
ther retained to a lower extent in the biochar but also
released in small amounts as NH
3
and HCN with the gas
phase (Nussbaumer, 2003). At higher temperatures, tars
crack, reducing the N‐content in the bio‐oil and enhance
the production of NH
3
and HCN in the permanent pyrogas.
NH
3
and HCN are precursors of nitrogen oxides (NO
x
),
which can form during combustion of the volatiles depend-
ing on the combustion technology employed (Vermeulen,
Block, & Vandecasteele, 2012). However, using a proper
combustion approach, harmful emissions can be minimized
(Nussbaumer, 2003) to near zero (i.e. N
2
release), in accor-
dance with the clean air regulations that are in place in
industrialized countries such as Germany.
Several elements such as Ca, Si, Mg, Al, Fe, or Ti are
almost entirely retained in the solid fraction due to their
nonvolatile nature. At pyrolysis temperatures of up to
700°C, alkali metals (K and Na) are mainly retained in the
char, but they are partially released to the gas phase above
700°C (van Lith, Jensen, Frandsen, & Glarborg, 2008).
Most of the chlorine and a considerable proportion of sul-
fur are released to the gas phase at temperatures above
400°C where it can be easily scrubbed off (Van Lith, Jen-
sen, Frandsen, & Glarborg, 2008). Cl and S‐bearing
organic compounds are further present in the bio‐oil. How-
ever, the sulfur and chlorine content of bio‐oil made from
woody biomass are lower than 100 ppm (Oasmaa et al.,
2015), which is considerably lower than the sulfur content
in crude oil.
Thus, most of the biomass nutrients are retained in the
biochar when slow pyrolysis technology is used at tempera-
tures below 700°C. The bio‐oil is mainly composed of car-
bon, hydrogen, and oxygen, with a minor content of
nitrogen, still smaller amounts of sulfur and chlorine, and
only traces (in the ppm range) of all other elements (Oas-
maa et al., 2015). The nutrient concentrations in biochar
and bio‐oil mainly depend on the initial biomass composi-
tion with woody feedstock having lower and annual harvest
residues have much higher mineral contents, particularly
graminoid species (e.g. switchgrass, miscanthus, or crop
residue of rice, wheat, maize, or others) with their compa-
rably high silicon content.
10
|
CARBON BALANCES AND
OPTIMIZED PYROLYSIS
PARAMETERS FOR PYCCS
The overarching aim of PyCCS is to enable low‐cost high
C‐transformation rates (i.e. minimum losses of carbon to
the atmosphere) and products with a long‐term C‐stability.
As biochar is the pyrolytic carbon product that can be
sequestered at the lowest costs and with large cobenefits,
high biochar yields are targeted. As the sequestration of the
carbon from the permanent pyrogases is the most expensive
and technically challenging C‐sequestration pathway, the
gas production should be minimized. While pyrolysis is
often optimized for the production of one product and the
other two products are considered either a waste or a barely
valued coproduct, in PyCCS scenarios all three pyrolysis
products have to be used to optimize the transformation of
the biomass carbon into pyrogenic forms for long‐term C
sequestration. Overall, an optimum between production
costs, robustness of the process, product distribution, qual-
ity and product value, sequestration potential (half‐life in
the terrestrial system), and the recycling of biomass
extracted nutrients should be achieved (Appendix S1).
The distribution and characteristics of the solid, liquid,
and gaseous pyrolysis products depend mainly on the
pyrolysis temperature, particle size, residence time of the
pyrolysis volatiles in the kiln, pressure, and feedstock com-
position (Anca‐Couce, 2016). Slow pyrolysis at low tem-
peratures (350–450°C) has the highest solid yield, though
the carbon stability of the biochar is rather low, as is the
quality of the bio‐oil (Camps‐Arbestain et al., 2015; Fager-
näs, Kuoppala, & Simell, 2012). At a given pyrolysis tem-
perature, the biochar yield can further be increased with
slow heating rates, higher residence time of the volatiles,
larger particle size of the feedstock, and higher pressure
(Anca‐Couce, 2016).
At higher pyrolysis temperatures (700–850°C), the per-
manent pyrogas yield is significantly increased; conse-
quently, lower biochar and bio‐oil yields will be obtained
(Anca‐Couce, Sommersacher, & Scharler, 2017; Sadaka &
Boateng, 2009; Tripathi, Sahu, & Ganesan, 2016). The
resulting biochar contains graphite‐like carbon (instead of
amorphous carbon), which increases its recalcitrance, for
example in soil (Budai et al., 2014; Keiluweit, Nico, John-
son, & Kleber, 2010). At these high temperatures, the pyro-
lytic liquids show a higher carbon content and higher
aromaticity and contain a large spectrum of typical tertiary
tars (Anca‐Couce, 2016). Tertiary tars are molecules free of
oxygen, which can be stable at high temperatures having a
single aromatic ring (BTX) or several rings (PAH). Also,
high‐temperature pyrolysis oils have a high content of phe-
nols. Therefore, high concentrations of BTX/PAH and phe-
nols might pose an environmental concern for certain bio‐
oil use scenarios (e.g. when used in asphalt or for plant
protection), but not when sequestered geologically.
Fast pyrolysis is less suited for the purpose of carbon
sequestration because the overall energy expenditure is
high due to the necessary drying and milling of the feed-
stock and very high heating rates (>500°C/s) (Lédé &
582
|
SCHMIDT ET AL.
Authier, 2015). Moreover, the fast pyrolysis biochar yield
and quality are rather low (Brewer et al., 2012).
To provide an example for the selection of optimal
PyCCS parameters, we expose in Table 2 the results of an
exemplary calculation of carbon yields, distribution, and
composition for four slow and one fast pyrolysis scenarios
as a function of the pyrolysis temperature according to
Neves et al. (2011). Their comprehensive literature review
of pyrolysis experiments includes data from more than 60
biomass samples with different reactor configurations and
temperatures between 350 and 1,000°C.
Modifying pyrolysis conditions determine yield and
quality of biochar, bio‐oil, and permanent pyrogas. Summa-
rizing the above considerations, we suggest the following
general guidelines to optimize the pyrolysis process condi-
tions for PyCCS scenarios:
Produce a low portion of permanent pyrogas and high
portion of biochar and bio‐oil (avoid temperatures
>700°C);
Maximize the biochar yield compared to bio‐oil, using
rather slow than fast pyrolysis, as it is easier and less
cost‐intensive to sequester biochar, in soils or materials,
than bio‐oil;
Achieve low H/C ratios of <0.4 and thus a high recalci-
trance of biochar by using pyrolysis temperatures not
lower than 450°C.
Based on these guidelines, we suggest using slow pyrol-
ysis with a highest treatment temperature between 500 and
650°C for PyCCS‐based CDR. In this temperature range, a
compromise between high biochar yield and its recalci-
trance is achieved for a large spectrum of diverse biomass
feedstocks (Brassard et al., 2016; Weber & Quicker, 2018).
At these temperatures, the gas yield is low, and only few
minerals end up in the bio‐oil. When the resulting biochar
is applied to soil, either directly or via cascading use in
animal husbandry or prior material use (Schmidt, 2012),
most of the minerals (but only reduced portions of nitro-
gen) can be recycled within the biosphere.
However, the suggested temperature range does not
exclude the application of higher or lower pyrolysis tem-
peratures in individual cases. Depending on the economic
scenarios and intended uses of the pyrolysis products, the
production parameters can and sometimes should be
adapted, for example, lower temperatures are recommended
to produce biochar for peat replacement (Kern et al., 2017)
or for soilless media to enhance plant growth and/or dis-
ease suppression (Jaiswal et al., 2018). Higher temperatures
are necessary to produce activated biochar for remediation
of contaminated soils or for electric capacitors (Frack-
owiak, 2007; Hagemann et al., 2018). The latter pyrolysis
conditions will increase the yield of gas, but if the CO
2
after the oxidation of these gases is captured and stored,
the overall carbon efficiency can still be high.
To pyrolyze biomass, the feedstock has to be harvested,
transported, chipped, or milled. All these processes are cur-
rently executed by fossil fuel‐driven machinery though it
could equally be powered by carbon‐neutral electricity.
Except for the proposed concept of E‐pyrolysis for CLR,
pyrolysis itself has low electric consumption and the neces-
sary thermal energy is provided by the permanent pyrogas
oxidation (Crombie & Mašek, 2014). The postpyrolysis
processing of the products like quenching and milling of
biochar or the pumping of bio‐oil to geological storages
can be done electrically. The application of biochar to soil
necessitates land management machinery, although the car-
bon footprint of an estimated one liter of diesel per ton of
biochar is negligible and could in the near future be exe-
cuted by electric tractors (Lünenbürger, 2012; Tubiello
et al., 2013). Liquefaction and pumping of CO
2
after CLC
or CLR are equally a process that could be powered by
carbon‐neutral electricity (Kerr, 2005). The carbon footprint
for the production and maintenance of the machinery dedi-
cated to the preparation, pyrolysis, and postproduction pro-
cesses is compared to the amount of treated carbon
negligible, although detailed calculations are missing in the
literature. Based on these assumptions, we do not include
further C‐expenditures beside the C‐leakages of the applied
PyCCS products as considered in the chapters above
(10%–20%, 2%, and 5% over 80 years for soil‐applied bio-
char, geological bio‐oil storage, and geological CO
2
‐stor-
age, respectively).
11
|
IMPLEMENTATION
POTENTIAL AND ADVANTAGES OF
PYCCS IN COMPARISON WITH
BECCS
Biochar is suggested as a material for long‐term carbon
sequestration since the late 1990 s (Ogawa, 1991; Okimori,
Ogawa, & Takahashi, 2003). However, burning pyrolysis
liquids and gases and sequestering only the biochar result
in C‐sequestration efficiencies that are too low for making
it a competitive NET (Werner et al., 2018). This is espe-
cially the case when biochar is produced at higher tempera-
tures (>500°C), which is necessary to produce a
recalcitrant product well suited for most material uses.
In earlier studies, the potential of PyCCS was calculated
for harvest residues under the assumption that only the bio-
char is sequestered (Smith, 2016; Woolf et al., 2010, 2016
). Compared to the current practice of burning or decompo-
sition of these residues, pyrolysis would clearly be an
improvement, even if only 40% of the biomass‐C is eventu-
ally sequestered. However, the calculated NET potential of
0.7 Gt C
eq
/year (Smith, 2016) would, even when doubled
SCHMIDT ET AL.
|
583
under a complete PyCCS‐scenario, not be sufficient to ful-
fill current climate change mitigation targets (Fajardy &
Mac Dowell, 2017; Smith et al., 2015). Specific new and
complementary global biomass production would thus be
needed. When biomass production is undertaken for the
purpose of NET, the carbon sequestration efficiency is the
crucial factor that determines the size of the land area
needed for biomass production (Werner et al., 2018).
Increasing the carbon sequestration efficiency of a technol-
ogy will thus decrease the land requirements substantially
(Beringer, Lucht, & Schaphoff, 2011; Boysen, Lucht, Ger-
ten, et al., 2016).
The current benchmark for BECCS‐based NETs is a C‐
sequestration efficiency (CSE) of 90% (Klein et al., 2014),
which can theoretically be attained, but only at high eco-
nomic costs and environmental risks (Fajardy & Mac Dow-
ell, 2017; Smith et al., 2015). PyCCS systems can equally
theoretically attain CSEs of ~90% when the sequestration
of CO
2
–CCS is included and more realistically 70%–80%
(Table 2) when only biochar and bio‐oil are sequestered.
Using the latter pathway avoids the high costs, environ-
mental risks, and societal challenges of geological CO
2
storage. The technology for PyCCS is ready to be imple-
mented across small‐to‐industrial scales: Networks of small
production units could be installed avoiding the constraints
of using massive industrial power plants only (Schmidt &
Shackley, 2016). Rather than establishing vast industrial
biomass plantations with their monoculture drawbacks,
their trade‐offs with food production, and other SDGs or
their environmental side effects (Boysen, Lucht, Schellnhu-
ber, et al., 2016), a network of smaller scale PyCCS sys-
tems would allow to establish biomass as a desired and
cultivated coproduct of agriculture, for example in agro-
forestry, silvopasture, and perennial coppicing types of
agricultures (i.e. carbon farming approaches, (Toensmeier,
2016)). However, the logistics of such farm‐based PyCCS
systems producing biochar for local soil applications while
delivering the bio‐oil to centralized processing facilities are
challenging especially in tropical and subtropical countries
with poor infrastructure and low degree of economic orga-
nization.
If PyCCS generates sustainable tCDR that are paid for
by society (i.e. negative carbon taxes), economic incentives
could be provided for farmers to optimize their land man-
agement with combined crop and biomass production.
Thanks to the plant growth‐enhancing properties of bio-
char‐based fertilization, the same food and feed crop pro-
ductivity per cultivation area could be maintained, while
the focus on additional biomass production could increase
the NPP of agricultural land and thus the terrestrial carbon
accumulation. With biochar being applied back to agricul-
tural soils, preferentially nutrient‐loaded after cascading use
(Schmidt et al., 2017), most of the nutrients would be
recycled back to the soil, maintaining the soil fertility in
the long term. Thanks to the yield increasing value of bio-
char and the added value of bio‐oil and biochar, once a
market can be established for its material use, the C‐se-
questration costs of PyCCS may be expected to be signifi-
cantly lower than for BECCS. However, for the moment
this can only be assumed as it is not yet possible to pro-
vide a comprehensive cost‐benefit assessment based on
peer‐reviewed literature.
The expected 25% yield increases in tropical and sub-
tropical climates (Jeffery et al., 2017) following the amend-
ment of biochar to soil would lead to a higher production
of biomass and thus to more positive feedstock toward
PyCCS. Consequentially, the 70%–80% C‐sequestration
efficiency of the biochar and bio‐oil PyCCS pathway could
effectively rise to >90%. Moreover, biochar application
could reduce agricultural non‐CO
2
GHG emissions (Kam-
mann et al., 2017) and most likely increase nonbiochar soil
organic matter (Tian et al., 2016; Weng et al., 2017). Con-
sidering all these secondary climate mitigation effects of
PyCCS, the high C‐sequestration efficiencies would trans-
late into an even higher climate mitigation potential.
Another major argument for pursuing PyCCS instead of
BECCS is the fact that the supposed massive switch from
fossil fuel energy production to bioenergy production
(170 EJ/year (Smith et al., 2015)) is economically unlikely
at a large scale. Solar and wind energy production costs
are expected to decrease to <0.03 USD/kWh (Kumar Sahu,
2015), whereas prices for bioelectricity production without
CCS range from 0.06 USD with waste materials as feed-
stock to >0.12 USD for low‐cost feedstock and can go up
to 0.30 USD when pelletized biomass has to be purchased
(International Renewable Energy Agency, 2012). Although
the transformation of biomass into heat for, for example,
district heating systems in temperate climates may remain
economically viable at current prices of 0.02–0.04 USD/
kWh, the global need for heating is rather small compared
to the global electricity demand, particularly in tropical cli-
mates. Considering further that biomass will have to
become the principal source of chemicals for C‐based
materials when decarbonization (i.e. the reduction in
anthropogenic carbon emissions) eventually leads to phas-
ing out of fossil carbon (Honary & Singh, 2010), prices for
biomass feedstock and thus for bioenergy will increase
rather than decrease in the future. Hence, the long‐term
economic viability of biomass‐based electricity production
becomes increasingly unlikely, even if carbon‐offset pay-
ments would finance the extra costs for CCS. If the pro-
duction of thermal energy from biomass remains an
important factor for the future energy mix, at least in tem-
perate climates, the bio‐oil and permanent pyrogas of
PyCCS systems hold an interesting potential as flexible
energy carrier to produce heat on demand in the cold
584
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SCHMIDT ET AL.
season while it could be sequestered in the warm season or
when market prices for heat are low. Biomass pyrolysis
could also produce fuel, especially methane and hydrogen,
to replace fossil fuel though the carbon sequestration effi-
ciency of the system would be lowered.
Pyrolytic carbon capture and storage is mainly designed
to produce biomaterials that have multiple industrial and
agronomic uses besides sequestering carbon. This does,
however, not exclude that in some scenarios biofuels may
remain for a more or less long transitional period a valu-
able coproduct of PyCCS. While all pyrolytic products
already have an industrial value with a current market
price, the large‐scale adaptation of the technology will cer-
tainly depend on the price that the global community is
willing to pay per sequestered ton of carbon. The current
price paid by the market for biochar (300–600 Eur/t (Sch-
midt & Shackley, 2016)) and bio‐oil (150–400 Eur/t (Ali-
baba, 2018)) exceeds by far its energetic value, which is
about 135 and 33 Eur, respectively (based on a caloric
value of 29 GJ/t = 8.1 MWh/t for biochar and 6.8 GJ/
t = 1.9 MWh/t for bio‐oil; an average price of 60 Eur/
MWh electricity (Kost et al., 2013); the average energy
efficiency of 32% for coal‐fired power plants with CCS
(Goto, Yogo, & Higashii, 2013); and a 10% for investment
and operational costs). It is difficult to foresee how these
market prices for pyrolytic products may develop, once a
massive global expansion of the technology would occur,
although it can be assumed that a quickly increasing offer
would exert pricing pressure before new bio‐carbon‐based
markets develop. The evolution of PyCCS pricing will
mainly depend on the timing of phasing‐out fossil fuels
where (pyrolytic) biomass carbon will have to replace fossil
carbon as a base material for the chemical industry. Sub-
stantial carbon taxes, paying for carbon sequestration,
would have at least to pay for the economic contingencies
and assure that the pyrogenic carbon is sequestered in the
long term without (for economic reasons) being oxidized to
atmospheric CO
2
.
12
|
CONCLUSION
This review demonstrates that PyCCS holds the prospect,
when mediated by a substantial increase in sustainable bio-
mass production, to mitigate global climate change and to
curb the concentration of atmospheric carbon. In contrast
to BECCS, PyCCS is expected to provide additional
ecosystem services, to recycle biomass nutrients within the
biosphere, to be ready for implementation within a shorter
time frame, to be scalable also at the village and farm
levels, to provide advanced products for the bioeconomy,
and to avoid the high costs and risks of liquid CO
2
seques-
tration. The C‐sequestering use of bio‐oil and biochar as
raw materials for the chemical and construction industry
could increasingly replace fossil oil during the expected
phasing out of fossil fuels. Given the current economic and
political situation, it seems extremely unlikely that, anytime
soon, global governance may decide to set the incentives
to pump bio‐oil into geological storages. However, it may
become an option of last resort if the combustion of fossil
fuels will not be reduced quickly enough within the next
30 years. In the meantime, a mix of PyCCS and BECCS,
where pyrogas is used for methane and hydrogen fuel pro-
duction, and where biochar is applied to soil or used in
biomaterials, could evolve into a way forward to early
adoption of scalable, SDG‐supporting CDR technologies.
ACKNOWLEDGEMENTS
This study was conducted within the BMBF‐funded project
BioCAP‐CCS, grants no. 01LS1620A and 01LS1620B.
ORCID
Hans‐Peter Schmidt http://orcid.org/0000-0001-8275-
7506
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