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Gasification of lignin-rich residues for the production of biofuels via syngas fermentation: Comparison of gasification technologies

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This paper reports the use of lignin-rich residues from second generation bioethanol production, to produce syngas that can be applied in the gas fermentation process. Three gasification technologies at a different scale were considered in this study. Fixed bed updraft gasification of about 30 kg/h solid feed, bubbling fluidized bed gasification of about 0.3 kg/h solid feed and indirect gasification of about 3 kg/h solid feed. Two lignin-rich residues with different properties were tested and the results were evaluated in terms of feedstock pretreatment (grinding, drying and pelleting) and syngas quality requirements for the fermentation process. The molar H 2 /CO ratio (ranging from 0.6 to 1.0) and the tar yield (18–108 g/Nm ³ ) obtained from the three gasification technologies was quite different. For the syngas fermentation process, low H 2 to CO ratio is preferred, as most of the organisms grow better on CO than H 2 . Furthermore, different contents of impurities that can reduce the fermentability of the gas (such as hydrocarbons, HCN, HCl, NH 3 , COS and other organic S- compounds) were detected in the product gas. The concentration of these compounds in the syngas is related to the content of the corresponding compounds in the original feedstock. The different characteristics of the lignin-rich feedstocks are related to the specific pre-treatment technologies for the (hemi)cellulose extraction. By tuning the pre-treatment technology, the properties of the feedstock can be improved, making it a suitable for gasification. Tar and unsaturated hydrocarbon compounds need to be removed to very low levels prior to the fermentation process. As a next step, the combination of the gasification and the appropriate product gas cleaning, with the syngas fermentation process for the production of bio-alcohols will be evaluated and the overall efficiency of the gasification-fermentation process will be assessed.
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Fuel
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Full Length Article
Gasication of lignin-rich residues for the production of biofuels via syngas
fermentation: Comparison of gasication technologies
E.T. Liakakou
a,
, B.J. Vreugdenhil
a
, N. Cerone
b
, F. Zimbardi
b
, F. Pinto
c
, R. André
c
, P. Marques
c
,
R. Mata
c
, F. Girio
c
a
ECN Part of TNO, Biomass & Energy Eciency Unit, Westerduinweg 3, Petten 1755 LE, Netherlands
b
ENEA, Energy Technologies Department, ss Ionica 106, 75026 Rotondella, Italy
c
LNEG, Estrada do Paco do Lumiar, 22, 1649-038 Lisboa, Portugal
ARTICLE INFO
Keywords:
Lignin gasication
Updraft gasication
BFB gasication
Indirect gasication
Biorenery
Syngas fermentation
ABSTRACT
This paper reports the use of lignin-rich residues from second generation bioethanol production, to produce
syngas that can be applied in the gas fermentation process. Three gasication technologies at a dierent scale
were considered in this study. Fixed bed updraft gasication of about 30 kg/h solid feed, bubbling uidized bed
gasication of about 0.3 kg/h solid feed and indirect gasication of about 3 kg/h solid feed. Two lignin-rich
residues with dierent properties were tested and the results were evaluated in terms of feedstock pretreatment
(grinding, drying and pelleting) and syngas quality requirements for the fermentation process. The molar H
2
/CO
ratio (ranging from 0.6 to 1.0) and the tar yield (18108 g/Nm
3
) obtained from the three gasication tech-
nologies was quite dierent. For the syngas fermentation process, low H
2
to CO ratio is preferred, as most of the
organisms grow better on CO than H
2
. Furthermore, dierent contents of impurities that can reduce the fer-
mentability of the gas (such as hydrocarbons, HCN, HCl, NH
3
, COS and other organic S- compounds) were
detected in the product gas. The concentration of these compounds in the syngas is related to the content of the
corresponding compounds in the original feedstock. The dierent characteristics of the lignin-rich feedstocks are
related to the specic pre-treatment technologies for the (hemi)cellulose extraction. By tuning the pre-treatment
technology, the properties of the feedstock can be improved, making it a suitable for gasication. Tar and
unsaturated hydrocarbon compounds need to be removed to very low levels prior to the fermentation process. As
a next step, the combination of the gasication and the appropriate product gas cleaning, with the syngas
fermentation process for the production of bio-alcohols will be evaluated and the overall eciency of the ga-
sication-fermentation process will be assessed.
1. Introduction
In many biorenery concepts, valorization of the lignin-rich re-
sidues is still a major issue. Second generation bioreneries for the
production of bioethanol use pre-treatment technologies to make the
polycarbohydrates accessible for enzymatic hydrolysis [1]. Un-
fortunately, in most pre-treatment processes, lignin ends up in a residue
together with unconverted bers, feedstock minerals, and process
chemicals like sulphates, enzymes and occulants. This type of residue
is usually burned for production of heat or electricity on site, which is a
rather low-added-value application. Having in mind that lignocellulosic
biomass generally contains 3040% lignin, the optimal valorization of
this residue is a key factor for the economic and environmental sus-
tainability of a biorenery [2]. The syngas obtained from gasication of
lignin-rich biorenery residues oers the potential to produce higher-
added-value products, such as liquid fuels and chemicals [3], but this
has been assessed only to a limited extend on bench and pilot scale
[46]. Gasication of lignin diers signicantly from gasication of
lignocellulosic biomass, mainly because lignin has dierent physico-
chemical characteristics. The structure and chemical composition of
lignin, that is an aromatic polymer with higher C/O ratio than lig-
nocellulosic biomass, favours tar formation (dened as all the con-
densable organic hydrocarbons of molecular weight higher than ben-
zene [7]). Tar content varies depending on gasier type, bed geometry
and gasication conditions (temperature, residence time, gasifying
agent). The condensed tar compounds may lead to problems such as
clogging and fouling of pipes and equipment [8], therefore the appro-
priate gasication conditions to minimize tar production should be
https://doi.org/10.1016/j.fuel.2019.04.081
Received 31 October 2018; Received in revised form 20 March 2019; Accepted 12 April 2019
Corresponding authors.
E-mail address: eleni.liakakou@tno.nl (E.T. Liakakou).
Fuel 251 (2019) 580–592
Available online 17 April 2019
0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
T
applied.
The pretreatment of the feedstock prior to gasication is a crucial
parameter for the process. The objective of the pretreatment is to make
biomass suitable for a specic gasication system. Lignin-rich feed-
stocks have also dierent physical properties compared to woody bio-
mass; they do not consist of bers and depending on the original
feedstock might have a powder-like consistency or comprise of big,
dense particles. Therefore, the pretreatment system can become a cri-
tical aspect of minimizing failure in the process (e.g. stickiness and
blocking problems during feeding) and usually includes size reduction,
drying and densication. Lignin-rich feedstocks usually have irregular
shape and size, with varying composition. Size reduction (usually
grinding and sieving) is required to obtain the desired particle size and
to provide a uniform size for gasication or for the next pretreatment
step. In general, smaller particles have larger surface areas, allowing
better heat transfer and higher reaction rates but the desired size also
depends on the type of the gasier used. For example, too large or dense
particles are not problematic for xed bed gasiers but may result in
low conversion or cause diculties in the feeding process in bubbling
uidized bed gasiers. Fine particles (< 0.5 mm), on the other hand,
are not desired because they also cause diculties in feeding, low
conversion, pressure drop in xed bed or entrainment in uidized bed
reactors and therefore should be sieved out. Furthermore, lignin-rich
residues have high moisture content and therefore drying is typically
required for reducing the moisture content to 1015%. If the lignin-rich
residues have a low bulk density, densication (e.g. torrefaction and
pelleting or briquetting) can be applied as a pre-treatment for use as a
solid fuel [9].
In the frame of the EU-funded project Ambition [10],dierent ga-
sication technologies were tested for the valorization of lignin-rich
residues, obtained from the production of second generation bioe-
thanol. A key target of the project is to convert the solid residues into a
syngas which can be used in a biological process to produce bio-alco-
hols. Some anaerobic microorganisms, known as acetogens, can be used
as a biocatalysts for the microbial conversion of syngas into short-chain
organic acids and alcohols, from C2-compounds, acetate and ethanol, to
butanol, butan-2,3-diol and butyric acid [11]. The ability of these mi-
croorganisms to withstand some of the impurities contained in the
syngas and their exibility to use dierent mixtures of CO and/or CO
2
and H
2
makes them an attractive alternative to the chemical catalytic
processes. However, the integration of gasication with syngas fer-
mentation is still in an early stage of development, where many ques-
tions exist concerning the syngas quality needed in the fermentation
process. Current syngas fermentation eorts are predominantly focused
on ethanol production.
Syngas fermentation has been chosen as an attractive conversion
route by several companies for pilot-, demo- and near commercial-scale
cellulosic ethanol production [12]. LanzaTech is deploying two com-
mercial ethanol-producing facilities and has three commercial-scale
projects under development, using o-gases and syngas from orchard
wood and nutshells [13]. LanzaTech has also demonstrated the pro-
duction of acetone and isopropanol [14]. Coskata was addressing
ethanol production in a demo- unit, rst using syngas from biomass
gasication and later from methane reforming, but went out of business
in 2015 [14]. INEOS New Planet BioEnergy, developed a syngas-to-
ethanol process, but stopped the operations by 2016 due to the high
levels of hydrogen cyanide in syngas [15].
While these developments are promising, challenges associated with
the scale-up and operation of this novel process, such as low mass
transfer eciency and the presence of inhibitory compounds in syngas
still remain. According to the literature [1619], the main requirement
for syngas for fermentation is low contents of contaminants like tar,
ethylene and benzene, as they inhibit fermentation and adversely aect
cell growth. Most of the organisms grow better on CO than H
2
.Asa
result, the H
2
to CO ratio can be low, i.e. a water-gas shift reaction after
gasication is not needed. However, many of these requirements, such
as the tolerance to sulphur, will depend on the particular type of mi-
croorganisms used. The challenge is to dene the gasication condi-
tions that lead to lower tar production while keeping the H
2
/CO ratio at
values suitable for syngas fermentation.
In this work we compare the performance of three gasication
technologies: updraft xed bed, bubbling-uidised bed (BFB) and in-
direct gasication, for valorization of two kinds of lignin-rich residues
from second generation bioethanol production. The three technologies
chosen for gasication are very dierent and each one has unique
features, which allows its integration with the syngas fermentation
process.
Updraft xed bed gasication, is typically operated at medium and
small scale (1015 MW), therefore is a good match with the fermenta-
tion technology and has several advantages such as high overall energy
eciency and fuel conversion, simple structure, low investment cost
and easy maintenance. However, the tar compounds and other pyrolysis
products are not cracked in the combustion zone, since they are carried
by the gas ow to the gasier top [20]. Thus, a very crude gas is pro-
duced with signicant amounts of tar compounds, so the challenge
would be to reduce the tar content to the suitable levels for the fer-
mentation process.
Bubbling uidized bed (BFB) direct gasication is suitable for
medium to large scale applications. BFB gasiers are able to produce a
synthesis gas with relatively high heating value and can be operated at
constant temperature. Silica sand or a catalytic bed material (like lime,
dolomite and olivine) can be used as uidization medium to improve
the gasication process. This process is exible in feedstock and can be
considered a mature technology.
Indirect gasication allows high fuel conversion and better control
and process optimization. The combustion products (ue gas) and ga-
sication products (product gas or synthesis gas) are not mixed. This
means that the product gas is not diluted with N
2
coming from the air
used for combustion, and thus, is suitable for synthesis or fermentation
applications after proper cleaning and upgrading without the need for
an expensive air separation unit. N
2
dilution of the product gas in-
tended for fermentation would result in lower mass transfer eciency,
as well as higher energy demand to compress an inert gas and bigger
reactors and equipment downstream resulting in higher OPEX and
CAPEX costs. Furthermore, indirect gasication produces a high value
gas which contains compounds such as CH
4
,C
2
-C
4
gases (including
ethylene and acetylene), benzene, toluene and xylene (BTX), and tar.
The separation of the most valuable components of the product gas is a
good way to maximize the value from the feedstock via co-production
schemes [21].
Besides syngas productivity and energy balance, the important focus
points are the pre-treatment of the lignin feedstocks and the gas quality,
since the requirements for syngas fermentation are very dierent
compared to a chemical catalytic processes. The product gas from the
lignin-rich feedstock gasication, will be utilized in the fermentation
process for the production of bio-alcohols, after appropriate cleaning
and conditioning to remove impurities that can reduce the ferment-
ability of the gas (such as tar compounds, BTX, unsaturated hydro-
carbons, HCN, HCl, COS and other organic S-compounds).
The use of biomass derived syngas for fermentation is quite a new
subject, so limited information about the requirements of the syngas is
available. The required syngas composition also dependents on the type
of microorganism used in the fermentation process. It is dicult to
select which gasication technology is most suitable to produce syngas
to be used in fermentation, because the required syngas composition
depends on the nal fermentation product and hence the type of mi-
croorganism. The main objective of the work presented is to provide
information about the dierent syngas composition obtained from dif-
ferent gasication technologies. After knowing the requirements of a
certain fermentation process, the gasication technology will be chosen
to ensure the desired gas composition. As a result of this work, the total
eciency of the process, from the lignin residue until the nal biofuel
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
581
synthesis, will be assessed and the results will be reported in the near
future.
2. Experimental
2.1. Feedstock properties and pretreatment
Two technical lignins derived from steam explosion and enzymatic
hydrolysis were received from two bioreneries and will be referred
throughout the manuscript as lignin A and lignin B. Lignin A is origi-
nating from wheat straw and was further lter pressed, therefore con-
sists of big dense particles, while lignin B originates from softwood and
consists partly of large spherical particles and partly of small, brittle
particles and a lot of powder. The as received materials can be seen in
Figs. 1(a) and 2(a), respectively.
Table 1 summarises the main thermochemical properties of the
fuels. The detailed ash composition, determined by ICP-AES, can be
found in the Appendix. As can be seen, the amount of volatile matter is
lower for both tested materials compared to beech wood which is
around 81 wt% [22], especially for lignin A. What stands out is the high
ash content of lignin A (14 wt%) that mainly consists of silica (5.4 wt%
of the feedstock), with minor amounts of calcium (0.5 wt%) and po-
tassium (0.3 wt%). This could lead to agglomeration and corrosion is-
sues at high temperatures (above 900 °C) especially to the BFB and
indirect gasier, due to alkali-silicate melt phase formation on the bed
material [9,23]. Furthermore, the sulphur and nitrogen content of both
lignin-rich feedstocks is relatively high compared to woody biomass,
which can lead to high S- and N- compounds in the product gas (such as
H
2
S, COS, NH
3
, etc.). The higher ash, lower volatile, lower carbon and
the higher potassium and calcium content of lignin A compared to
lignin B are attributed to the original feedstocks [22].
The materials were received with high moisture contents, 36 wt%
for lignin A and 52 wt% for lignin B, as shown in Table 1 and with
evident mold on the surface, probably because of the sugar content and
the mild environmental temperature. For this reason, drying of the
materials was required prior to the gasication tests.
For the updraft gasication, the larger pieces were broken down to
suitable size (2050 mm) and were dried indoors on canvas. The nal
moisture content of the feedstock as gasied is shown in Table 2. The
two feedstocks, as used in the updraft gasication tests, are shown in
Figs. 1(b) and 2(b). The bulk density was 427 kg/m
3
for lignin A and
335 kg/m
3
for lignin B.
For the BFB gasication, the feedstocks were roughly ground and
dried at 60 °C for 72 h. After drying to the nal moisture content (see
Table 2), the samples were further milled and sieved to obtain the
desired size (210 mm) for the gasication tests. The feeding point was
inside the bed and the aforementioned particle size range was selected
to ensure stable feeding and avoid entrainment of the particles out of
the reactor, which is usually the case with very small particles. Lignin A
and lignin B are shown as used in the gasication tests in Figs. 1(c) and
2(c), respectively.
For the indirect gasication, the feedstocks were dried at 90 °C for
48 h at a nal moisture content of 2 wt%. Lignin A was ground using a
Retsch SM300 cutter mill at 750 rpm and sieved using a 6 mm round
screen to obtain the suitable size (0.5 6 mm). Lignin A, as used in the
indirect gasication test, is shown in Fig. 1(d). Lignin B, after drying,
had a powder-like consistency which proved dicult to feed due to the
high amount of small particles (< 0.5 mm). In order to increase the
density of lignin B, it was pelletized with the addition of steam, and the
pellets were ground using a 10 mm screen. The sample obtained after
milling consisted of denser particles bellow 10 mm size but still a
considerable amount of nes was present, as shown in Fig. 2(d). The
nal moisture content of the feedstocks is shown in Table 2.
2.2. Description of the experimental set-ups and product analysis
2.2.1. Updraft gasier
The updraft gasication tests were carried out using the pilot plant
(a) (b)
(d)
(c)
Fig. 1. Lignin A (a) as received, as used in the updraft gasication test (b), as used in the BFB gasication test (c) and as used in the indirect gasication test (d).
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
582
PRAGA (uP drAft GAsication) at ENEA Research Center of Trisaia. The
rig and the main components are shown in Fig. 3. The core of the plant
is the xed bed updraft gasier which is operated slightly above at-
mospheric pressure. The plant is equipped with a real time measure-
ment of non-condensable gases (N
2
,H
2
, CO, CO
2
,CH
4
,O
2
) by online GC
analysis. Moreover, the syngas is sampled at the exit of the gasier for
oine analysis of water and organic volatiles, following the CEN/TS
15439:2006 procedure. The feedstock was fed into the gasier by
screws in a semi continuous mode, in batches of 4.24.5 kg, under N
2
atmosphere at intervals of about 12 min and completed in few seconds.
More details about the updraft gasier and the chemical analysis are
provided elsewhere [5,24]. Lignin A and lignin B were introduced in
the reactor at the ambient humidity contents of 7.8 wt% and 8.3 wt%,
respectively. The gasication conditions used can be seen in Table 2.
The biomass was fed into the gasier by screws in a semi continuous
mode, in batches of 4.24.5 kg, under N
2
atmosphere at intervals of
about 12 min, and completed in few seconds. The gaseous streams,
serving as gasication media, were injected at the bottom with constant
rate.
2.2.2. Bubbling uidized bed gasier
The BFB gasication tests were carried out in the bench scale in-
stallation at LNEG research center, in Portugal. A scheme of the BFB
gasier is shown in Fig. 4 and a detailed description can be found
elsewhere [6]. The BFB reactor is operated at atmospheric pressure and
is placed inside a furnace which is electrical heated. Steam and oxygen
were used as gasication agents, they were mixed in the windbox lo-
cated below a gas distributor at the base of the reactor. Equivalent ratio
(ER) values from 0 to 0.23 (0.3 g of oxygen/g daf feedstock) were used
to study the eect of this parameter. ER is the ratio between the oxygen
used and the stoichiometric amount required for complete combustion.
The eect of temperature was studied at a range of 750900 °C and the
eect of the steam ow rate was studied at a range of 00.9 (g of steam/
g daf of feedstock). The feedstocks were continuously fed into the ga-
sier through a screw feeder. To help the feeding and to avoid gas back
ow, a small nitrogen ow was used in the feeding system, which was
also water cooled to avoid clogging. Silica sand was used as the ui-
dization bed. Each experiment lasted between 90 and 120 min, de-
pending on the time necessary to collect all the samples at stable con-
ditions. Isopropanol was used for tar sampling, using the CEN/TS
15439:2006 procedure.
(a) (b)
(c) (d)
Fig. 2. Lignin B (a) as received, as used in the updraft gasication test (b), as used in the BFB gasication test (c) and as used in the indirect gasication test (d).
Table 1
Thermochemical properties of lignin-rich feedstocks.
Lignin A Lignin B
Moisture content 105 °C (wt.%, as received) 36 52
Ultimate analysis (wt.%, dry basis)
C 47.2 57.7
H 5.6 6.2
O 33.0 33.8
N 1.3 0.8
S 0.18 0.13
Cl 0.020 0.002
Proximate analysis (wt.%, dry basis)
Ash 550 °C 14.0 0.1
Volatile matter 64.6 72.1
Lower heating value (MJ/kg) 18.4 22.9
ICP-AES analysis (mg/kg, dry basis)
Al 380 17
Ca 4750 380
Fe 290 48
K 3250 210
Mg 385 68
Na 906 390
P 930 160
S 1750 1300
Si 54,000 < 30
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
583
2.2.3. Indirect gasier
The indirect gasication tests were carried out in the lab scale
30 kW MILENA indirect gasier at ECN part of TNO research center, in
the Netherlands. MILENA is an indirect gasication technology devel-
oped by ECN which consists of a riser where gasication takes place and
a bubbling uidized bed combustor in an integrated design [25].A
picture and a scheme of the installation is shown in Fig. 5, including the
two sampling points that were used to analyse the product gas (S1) and
the ue gas (S2). The gasication conditions used can be seen in
Table 2. Lignin A and lignin B were introduced in the reactor at a rate of
2.9 kg/h and 1.8 kg/h, on a dry basis, respectively and at the environ-
mental humidity contents of 2 wt% and 17 wt%. Fresh Austrian olivine,
a mineral based on an iron-magnesium orthosilicate structure
(FeMgSiO
4
), was employed as the bed material. The gasication tem-
perature was approximately 780 °C and 870 °C, for Lignin A and Lignin
B, and steam uidization was conducted at 1.3 kg/h and 1 kg/h, re-
spectively. Additional nitrogen (20 NL/min for lignin A and 17 NL/min
for lignin B gasication) was used in this lab scale test, to compensate
for reduced gas velocity in the riser, due to the low amount of volatile
matter of lignin A. The riser reactor of the lab scale installation is de-
signed for wood chips, therefore, in order to achieve the required ve-
locity for sucient circulation, higher lignin feeding rates would be
required but it is not possible due to the limited capacity of the after-
burner. Furthermore, neon and argon gases were injected at 0.02 NL/
min and 1 NL/min as tracer gases. The product gas ow after the ga-
sier was calculated from the tracer gases molar balance. The gasi-
cation system was operated at atmospheric pressure.
After the gasier, a slip stream of the product gas for analysis was
cooled down to 5 °C in order to remove the condensate (water and tars)
from the dry gas, thus protecting the gas analysis set. Online monitoring
of product gas (H
2
, CO, CO
2
,CH
4
) and ue gas (O
2
,CO
2
, CO, C
x
H
y
,
N
2
O, NO, NO
2
) was carried out. ABB CALDOS 17 Thermal Conductivity
Detector was used for H
2
, ABB URAS 14 Non Dispersive Infra-Red
Analyser (NDIR) for CO, CO
2
,CH
4
,N
2
O, SO
2
, ABB MAGNOS 16 Para-
Magnetic O
2
sensor for O
2
, Ratsch RS55 Flame Ionisation detector was
used for the trace hydrocarbons in the ue gas and ABB LIMAS 11 UV
detectors for NO and NO
2
. Complementary, the product gas composi-
tion was measured online using a micro-GC (Varian Micro-GC CP
Table 2
Optimum gasication conditions used by the dierent gasication technologies.
Updraft gasication BFB gasication Indirect gasication
Gasication agent Air Oxygen/Steam Steam
Lignin A Lignin B Lignin A Lignin B Lignin A Lignin B
T gasication (°C) 776 687 800 800 780 870
T combustion reactor (°C) –––805 905
Fuel, dry (kg/h) 28.8 27.9 0.35 0.30 2.9 1.8
Fuel moisture content (wt.%) 7.8 8.3 10.5 7.8 2.0 17.0
Steam (kg/h) 3.0 2.6 0.1 0.1 1.3 1.0
Carrier gas CO
2
(NL/min) –––44
Fluidization N
2
(NL/min) –––20.0 17.3
Air in gasier (kg/h) 31.4 36.2 ––– –
O
2
in gasier (NL/min) ––0.82 0.88 ––
ER (O
2
) 0.17 0.18 0.13 0.13 ––
ER (H
2
O) 0.10 0.07 ––– –
Tracer gas Ne (NL/min) ––0.02 0.02
Tracer gas Ar (NL/min) ––– 11
Combustion air (NL/min) –––100 100
Afterburner air (NL/min) –––400 400
Fig. 3. PRAGA updraft gasier.
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
584
4900). The product gas was also sampled at the exit of the gasier for
oine analysis of the trace hydrocarbons (GC-FID) and sulphur com-
pounds (GC-FPD). The determination of HCl, NH
3
, HCN in the product
gas was carried out by wet chemical analysis. Additionally, the tar
guideline method was used for the determination of the content and
composition of the tar compounds in the product gas, following the
CEN/TS 15439:2006 procedure, as well as the water content using Karl
Fischer titration.
3. Results and discussion
3.1. Updraft gasication
3.1.1. Lignin A
Fig. 6 reports the temperature prole at steady operating conditions
recorded along the vertical axis by the set of 11 thermocouples during
the updraft gasication test of lignin A. This thermal prole inside the
gasier bed appears complex because it depends on the equilibrium
between several exothermic and endothermic chemical reactions at the
solid-gas interface and in the gas phase as well as on heat and mass
transfer. Steady conditions were assumed when the thermal prole
inside the gasier was stable, except for the uctuations in the free-
board where the biomass was loaded. In the freeboard, at steady state,
the temperature was 280 °C, while in the bed it was 776 °C with a
maximum of 1040 °C at 0.728 m bed height, which is almost at the
middle of the reactive bed. The use of steam had positive eects on the
stabilization of the gasier, because the highest temperatures were
found in the middle of the gasier and not at the bottom where ashes
are at the pure state and could agglomerate at melted state [4]. The
heating rate of the particles moving downward from the top of the
Fig. 4. Bubbling uidized bed gasier.
Fig. 5. MILENA indirect gasier.
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
585
reactor is also shown in Fig. 6. Starting from the top of the gasier,
there are two zones where endothermic reactions prevailed and the
thermal gradient approached its minimum: at 1.6 m it can be ascribed
to the primary pyrolysis of the feedstock and at 1.0 m can be associated
with gasication and cracking reactions. The exothermic reactions
prevailed in correspondence of the maxima peaks: at 1.2 m due to the
contribute of WGS reaction and at 0.8 m associated with the combus-
tion of lignin that provided most of enthalpy for the endothermic re-
actions.
In Fig. 7, the concentration of the main product gas components
during the updraft gasication of lignin A, as measured by the online
GC, is presented. After the start up period, there was a period of steady
operation of the plant between 60 and 200 min. The uctuations of the
gas composition is attributed to the biomass feeding steps that was of
semi-batch type.
The data were averaged from 60 to 200 min and with other che-
mical analysis (tar content, water content) are reported in Table 3.At
steady conditions, the average composition of the gas on dry basis
was 26.0 vol% H
2
, 24.8 vol% CO, 9.5 vol% CO
2
, 3 vol% CH
4
, 36.4 vol
%N
2
and 0.2 vol% O
2
. The H
2
/CO ratio was slightly above 1. The total
content of condensable organics was found 80 g/Nm
3
, of which 67 g/
Nm
3
classied as tar after the CEN 15439; the main compounds that
were identied by the HPLC were Acetic acid, 5-HMF, single ring
aromatic molecules (Benzene, Toluene, substituted Phenols) and traces
of Naphthalene. HCl and NH
3
were measured in the syngas equal to 37
and 7900 ppmV, respectively, equivalent to 55 wt% and 70 wt% of the
Cl and N in the original feedstock. The lower heating value of the
product gas was 7.3 MJ/Nm
3
.
3.1.2. Lignin B
During lignin B gasication, the temperature prole of the updraft
gasier was monitored and the average values, are reported in Fig. 8.As
can be observed, the thermal prole of lignin B updraft gasication was
signicantly dierent than lignin A (shown in Fig. 6). Indeed, the
average temperature in the bed was 687 °C and the peak temperature
was 948 °C, which is 89 °C and 102 °C lower than for lignin A, respec-
tively. The enthalpy for lignin A was H
in
= 530 MJ/h, while for lignin B
H
in
= 630 MJ/h, calculated from the lower heating value of the lignin
(shown in Table 1) and the fuel rate (shown in Table 2). From this data
we expected higher temperature during the gasication of lignin B but
the opposite was detected. The lower temperature of the gasier could
be explained by a higher shift of potential enthalpy from the solid to the
gas. The heating value of the product gas was 7.8 MJ/Nm
3
(shown in
Table 4), which is higher than lignin A and is a consequence of such
Fig. 6. Temperature inside the updraft reactor along the vertical axis (z) and the derivative (right) during the gasication of lignin A.
Fig. 7. Syngas composition from Lignin A obtained in air-steam gasication test with the ENEA updraft gasier PRAGA. Steady operation: 60200 min.
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
586
lower heat release in the bed.
The syngas composition in the two cases suggested that lignin A is
more reactive towards the water gas shift (WGS) reaction than lignin B.
Indeed, careful calculations considering the exothermicity of the WGS
(-41.7 kJ/mol) and the dierence in H
2
/CO led to a dierence of 104 °C
in two adiabatic systems (in the Appendix more details are provided
regarding the thermal calculation). In turn, the dierent degree of WGS
could be attributed to the higher potassium and calcium content in the
ash of lignin A, that catalyses the reaction [26].
In Fig. 9 the concentration of the main product gas components
during the updraft gasication of lignin B - as measured by the online
GC - is presented. After the start up period, there was a period of steady
Table 3
Product gas composition for lignin A gasication. Experimental conditions as
given in Table 2.
Updraft
gasier
BFB
gasier
Indirect
gasier
Units
Gasication agent Air Oxygen/
Steam
Steam
CO 24.8 28.0 13.9 Vol%
H
2
26.0 22.0 8.6 Vol%
CO
2
9.5 21.0 17.9 Vol%
CH
4
3.0 14.0 5.9 Vol%
N
2
36.4 10.0 45.1 Vol%
O
2
0.17 nd
C
2
H
2
nd nd 0.1 Vol%
C
2
H
4
nd 4.5 2.8 Vol%
C
2
H
6
0.05 0.3 0.3 Vol%
Benzene 0.10 nd 0.5 Vol%
Toluene 0.01 nd 0.1 Vol%
Ar tracer gas ––2.1 Vol%
Sum C
3
921 950 3115 ppmV
Sum C
4
-C
6
nd 800 921 ppmV
H
2
S nd 781 1099 ppmV
COS nd nd 29 ppmV
Thiophene nd nd 34 ppmV
Methylmercaptane nd nd 32 ppmV
Other S-organics nd nd 9 ppmV
NH
3
7900 7466 8765 ppmV
HCN nd nd 1290 ppmV
HCl 37 nd 8 ppmV
Ne tracer gas ––409 ppmV
Tar content
*
80 18 34 g/Nm
3
Water content 22 40 43 Vol%
Product gas ow (dry,
tar free)
1.7 0.9 1.0 Nm
3
/
kg
dry feedstock
Product gas LHV (tar
free)
7.3 12.2 9.8 MJ/Nm
3
Values are at Normal conditions at temperature of 0 °C (273.15 K) and absolute
pressure of 1 atm (1.01325 × 105 Pa).
nd: not determined.
*
Higher than toluene, on dry basis.
Fig. 8. Temperature inside the updraft reactor along the vertical axis (z) and the derivative (right) during the gasication of lignin B.
Table 4
Product gas composition for lignin B gasication. Experimental conditions as
given in Table 2.
Updraft
gasier
BFB
gasier
Indirect
gasier
Units
Gasication agent Air Oxygen/
Steam
Steam
CO 28.0 28.0 15.6 Vol%
H
2
20.1 20.0 14.1 Vol%
CO
2
7.7 15.0 18.0 Vol%
CH
4
5.1 15.0 6.9 Vol%
N
2
38.7 19.0 39.2 Vol%
O
2
0.17 nd
C
2
H
2
nd nd 0.3 Vol%
C
2
H
4
nd 2.7 2.0 Vol%
C
2
H
6
0.11 0.2 0.1 Vol%
Benzene 0.2 nd 0.7 Vol%
Toluene 0.02 nd 0.1 Vol%
Ar tracer gas ––3.6 Vol%
Sum C
3
800 840 355 ppmV
Sum C
4
-C
6
nd 270 583 ppmV
H
2
S nd 654 644 ppmV
COS nd nd 20 ppmV
Thiophene nd nd 18 ppmV
Methylmercaptane nd nd 2 ppmV
Other S-organics nd nd 3 ppmV
NH
3
6930 834 4164 ppmV
HCN nd nd 114 ppmV
HCl nd nd 12 ppmV
Ne tracer gas ––404 ppmV
Tar content
*
100 108 30 g/Nm
3
Water content 22 34 42 Vol%
Product gas ow (dry,
tar free)
2.1 1.0 1.6 Nm
3
/
kg
dry feedstock
Product gas LHV (tar
free)
7.8 13.0 8.6 MJ/Nm
3
Values are at Normal conditions at temperature of 0 °C (273.15 K) and absolute
pressure of 1 atm (1.01325 × 105 Pa).
nd: not determined.
*
Higher than toluene, on dry basis.
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
587
operation of the plant between 120 and 230 min, during which the
conditions were kept stable. The average composition of the gas on
dry basis was 20.1 vol% H
2
, 28.0 vol% CO, 7.7 vol% CO
2
, 5.1 vol%
CH
4
, 38.7 vol% N
2
and 0.2 vol% O
2
(Table 4). The use of steam as a
gasication agent increases the partial pressure of H
2
O inside the re-
actor, promoting the endothermic water gas reactions in the regions
with high temperatures. The lower steam addition during lignin B ga-
sication compared to lignin led to a lower hydrogen content: 26.0 vol
% for lignin A and 20.1 vol% for lignin B, for 3.0 kg/h and 2.6 kg/h of
steam, respectively. The H
2
/CO ratio resulted from gasication of lignin
B was lower than that for lignin A, 0.7 versus 1.0, even at similar op-
erating conditions (feeding rate, ER(O
2
)), as shown in Table 2. The
product gas composition appears less regular than lignin A throughout
the test, due to the irregular shape and size of lignin B that consisted of
small particles with the aptitude to produce powder (shown in
Fig. 2(b)). In this sense, lignin B was not an optimal feedstock for up-
draft gasication and a few pre-treatment steps (like compression,
drying and palletisation) would be required in the Biorenery to obtain
pellets with good properties.
The GC data were averaged from 120 to 230 min and together with
the other chemical analysis (tar content, water content) are reported in
Table 4. The total organic condensable content was 100 g/Nm
3
for
lignin B, of which 89 g/Nm
3
classied as tar after the CEN 15439
,
which
is higher than lignin A. This is ascribed to the higher bed temperature
during gasication of lignin A (776 °C versus 687 °C for lignin B) that
favors the tar cracking reactions. Moreover, the tar production can be
correlated with the uid dynamic of the system and more specically
with the residence time of the syngas in the bed: long residence time led
to low tar content according to a zero order kinetics of tar decom-
position in the bed producing incondensable hydrocarbons [4,24]. The
use of a higher air quantity as gasication agent during the tests of
lignin B (36.2 kg/h versus 31.4 for lignin A) resulted in higher total
syngas ow with a corresponding decrease in the residence time,
leading to larger tar content in the gas.
3.2. Bubbling uidized bed gasication
Based on a previous study about the eect of the experimental
conditions on the syngas production from lignin by oxy-gasication [6],
the eect of the steam/lignin ratio, gasication temperature and
oxygen ow rate, was studied in the present work. Steam/lignin weight
ratios between 0 and 0.8, as well as temperatures in the range of
750900 °C were tested. The eect of the equivalent ratio (ER) was
studied in the range of 00.23, by varying the oxygen ow inside the
reactor. When ER was 0, only steam was introduced inside the reactor.
Each experiment lasted between 90 and 120 min, depending on the
time necessary to collect all the samples at stable conditions. The op-
timum experimental conditions that were used for the BFB gasication
of the two lignins can be found in Table 2.
During the BFB gasication of lignins, the presence of steam fa-
voured the steam reforming reactions, thus the conversion of tar and
hydrocarbons to CO, CO
2
and H
2
.H
2
concentration increased with in-
creasing steam/lignin ratio, however, CO content decreased due to its
conversion into CO
2
via the WGS reaction. The steam/lignin ratio of
0.35 was chosen as the optimum, in terms of tar content, product gas
yield and LHV and low H
2
/CO ratio.
The increase of temperature clearly favoured the formation of H
2
,at
the expense of CO, CH
4
, higher hydrocarbons and tar compounds
concentration. This is attributed to steam reforming and cracking re-
actions that led to an increase in syngas yield, accompanied by a de-
crease in the gas LHV. The use of lower gasication temperature pre-
vents problems associated with bed agglomeration and leads to H
2
/CO
ratio lower than 1, which is required for the syngas fermentation pro-
cess, but has the disadvantage of producing syngas with higher tar
contents, which is not favourable for further fermentation tests. For the
aforementioned reasons, the temperature of 800 °C was selected as the
optimum.
The increase of ER clearly favoured partial oxidation reactions and
the release of CO and CO
2
, at the expense of CH
4
, other gaseous hy-
drocarbons and tar compounds. Oxidation reactions also favoured the
formation of H
2
O. The conversion of tar into gases, led to an increase in
the gas yield but with a corresponding decrease in the LHV, as expected.
The optimum ER value was found to be 0.13 to ensure the H
2
/CO ratio
required for fermentation.
In Table 3 and Table 4, the average product gas composition for the
optimum conditions tested, during lignin A and lignin B gasication,
respectively, is presented. As it may be observed, similar values were
obtained for both lignins in relation to the main gaseous components
(CO, CO
2
,H
2
and hydrocarbons). The H
2
/CO ratio was 0.8 for lignin A
and 0.7 for lignin B. The heating value of the product gas was 12.2 MJ/
Nm
3
for lignin A and 13.0 MJ/Nm
3
for lignin B.
The tar content of the gas from the two lignin-rich feedstocks BFB
gasication was quite dierent and could be related to the dierent
composition of the feedstocks. The total tar content in the raw syngas
was found 18 g/Nm
3
for lignin A and 108 g/Nm
3
for lignin B.
Quantitative determination of the individual tar components was not
carried out. NH
3
and H
2
S concentrations for lignin A were 780 and
7470 ppmV, respectively. For lignin B the concentrations were quite
lower, 650 and 835 ppmV, respectively, which agrees with the lower N
and S contents in the original lignin (see Table 1).
Fig. 9. Product gas composition from Lignin B obtained in air-steam gasication test with the ENEA updraft gasier PRAGA. Steady operation: 120230 min.
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
588
3.3. Indirect gasication
3.3.1. Lignin A
In Fig. 10, the temperature monitored by four thermocouples placed
within MILENA indirect gasier, during gasication of lignin A, is
shown. After the initial start-up, stable conditions were achieved for
260 minutes, as indicated in the graph. The average temperature in the
gasication reactor - above the riser - during the steady operation,
stabilizes at around 780 °C. The average temperature in the combustor,
given by 3 thermocouples located at the bottom, middle and top part of
the combustor, was stable at 805 °C, indicating good bed material cir-
culation. The small deviations in temperature between the 240 and
280 min of the test are caused by a system disorder (instant pressure
increase due to hampered hydrodynamics of the bed material).
In Fig. 11, the concentration of the main product gas components
during the indirect gasication of lignin A, as measured by the gas
analyser, is presented. During the stable conditions the average com-
position of the gas on dry basis was 8.6 vol% H
2
, 13.9 vol% CO,
17.9 vol% CO
2
and 5.9 vol% CH
4
. However, 7.2 vol% of the CO
2
is due
to the CO
2
used as a carrier gas in the feeding screw and steam gen-
erator, as shown in Fig. 5. The H
2
/CO ratio was 0.6, which is on the low
side compared to a woody biomass gasication test [25], due to the use
of relatively inactive olivine. Over prolonged run times, the Fe com-
ponent in olivine is activated [25,27] and the K and Ca ash components
are incorporated into the bed material, forming a uniform Ca-layer and
K enrichments on the surface of the olivine [28]. It has been reported
that activated olivine enhances H
2
production and decreases CO and
CH
4
content compared to silica sand, due to its catalytic eect on the
reforming of hydrocarbons and tar and the promotion of the watergas
shift (WGS) reaction [2831]. Due to the presence of iron at the surface
of the material, olivine has the ability to transfer oxygen between the
combustion side and the gasication side of the indirect gasication
system [31,32]. The high CO and CO
2
content is attributed to this
ability and has a benecial eect in the application of the gas to the
fermentation process, since high CO and CO
2
concentrations are re-
quired.
In Table 3 the average gas composition of the product gas, during
indirect gasication of lignin A, is shown. What stands out in the overall
gas composition is the high concentration of S-species (in the form of
H
2
S, COS and other S-organic components, 1200 ppmV in total), NH
3
(8765 ppmV) and HCN (1290 ppmV) concentration. These high con-
centrations can naturally be explained by the high content of the cor-
responding compounds, shown in Table 1.
The high concentration of HCN, benzene and sulphur compounds
(especially thiophene) of the product gas might cause problems in the
gas application to the fermentation process [33]. Therefore, the
cleaning of product gas is essential before being utilized downstream.
The total tar (higher than toluene) concentration on dry basis in
the product gas is 34 g/Nm
3
. The main tar components formed from
lignin A gasication are aromatic (1-ring) components, such as xylene,
Fig. 10. Temperature in the riser and combustor bed zones of MILENA during lignin A indirect gasication. Steady operation: 130390 min.
Fig. 11. Product gas composition during lignin A indirect gasication (as measured by the online gas analyser). Steady operation: 130390 min.
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
589
styrene and toluene, together with heterocyclic aromatic compounds,
like phenol and cresol. Naphthalene was detected in considerable
amounts, along with small concentrations of other light and heavy
polyaromatic hydrocarbons (e.g. acenaphthylene, phenanthrene).
The total tar concentration of the lignin A indirect gasication was
higher than of lignocellulosic biomass [25] and is attributed to the
multi ring nature of the lignin molecule which favours tar formation, as
well as to the inactive olivine due to the small duration of the test
that is not able to crack the heavy tars eectively. In order to use the
product gas for fermentation, all the tar components must be removed
downstream, possibly using the OLGA unit [34]. The lower heating
value of the product gas was 9.8 MJ/Nm
3
.
The average composition of the ue gas at the exit of the combustor
reactor, during the steady operation of lignin A gasication, is shown in
Table 5. The small amounts of CO and hydrocarbons show that com-
plete combustion was achieved. However, the total NOx emissions are
quite high (574 ppmV, most of it being NO), compared to a woody
biomass gasication test [25], due to the high concentration of nitrogen
in the original feedstock.
3.3.2. Lignin B
In Fig. 12, the temperature in MILENA indirect gasier, during ga-
sication of lignin B, is shown. After the initial start-up, stable condi-
tions were achieved for 155 minutes, as indicated in the graph. The
average temperature in the gasication reactor above the riser
during the steady operation, stabilizes at around 870 °C. The average
temperature in the combustor, given by 3 thermocouples located at the
bottom, middle and top part of the combustor, was stable at 905 °C,
indicating good bed material circulation.
In Fig. 13, the concentration of the main product gas components
during the indirect gasication of lignin A, as measured by the gas
analyser, is shown. During the stable conditions the average
composition of the gas on dry basis was 14.1 vol% H
2
, 15.6 vol% CO,
18.0 vol% CO
2
and 6.9 vol% CH
4
. Again, 7.2 vol% of the CO
2
is due to
the CO
2
used as a carrier gas in the feeding screw and steam generator.
The H
2
/CO ratio was 0.9, which is higher than lignin A. The higher H
2
concentration is probably attributed to the water gas shift reaction that
is favoured by the higher gasication temperature and the higher steam
to carbon rate that is applied to lignin B gasication, compared to lignin
A (shown in Table 2).
In Table 4 the average gas composition of the product gas, during
indirect gasication of lignin B, is shown. The tar (higher than toluene)
content on dry basis was 30 g/Nm
3
, which is lower than lignin A and
is ascribed to the higher gasication temperature that favours the tar
cracking reactions. Unlike lignin A, lignin B produced primarily light
polyaromatic (2, 3-ring) compounds, like naphthalene, together with
acenaphthylene and phenanthrene. In smaller concentrations aromatic
and heavy polyaromatic compounds were also formed. The con-
centration of the total S-species was 690 ppmV, much lower than lignin
A and can be attributed to the lower S-content in lignin B. NH
3
and HCN
concentration in the product gas was 4160 ppmV and 115 ppmV, re-
spectively. Again, much lower than lignin A due to the lower N-content
in lignin B.
Regarding the concentration of lower hydrocarbons, CH
4
con-
centration is higher compared to lignin A as expected due to the higher
gasication temperature. Ethane, ethylene and C3+ hydrocarbons
concentration decreases compared to lignin A and this can also be at-
tributed to the increased gasication temperature which promotes
cracking reactions into methane and hydrogen (whose concentrations
are higher compared to lignin A). The high concentration of HCN,
benzene and sulphur compounds (especially thiophene) in the gas
might cause problems in the gas application to the fermentation pro-
cess. Therefore, cleaning of producer gas is essential before the down-
stream fermentation process. The lower heating value of the product
gas was 8.6 MJ/Nm
3
.
The average composition of the ue gas after the combustor, during
the steady operation of lignin A gasication, is shown in Table 5. The
small amounts of CO and hydrocarbons show that complete combustion
was achieved. The total NOx emissions was 370 ppmV (most of it being
NO), are lower than lignin A, due to the lower concentration of nitrogen
in the feedstock.
4. Conclusions
The gasication of lignin-rich feedstocks is technically feasible with
the three dierent gasication technologies considered in this study,
proving that it is possible to convert this kind of residue into a valuable
Table 5
Flue gas composition for lignin A and B during indirect gasication.
Component Concentration Units
Lignin A Lignin B
O
2
2.3 2.2 Vol%
CO
2
14.8 13.9 Vol%
CxHy 3.2 1.5 ppmV
CO 20.4 0 ppmV
NO 556 364 ppmV
NO
2
11 5 ppmV
N
2
O 7 < 1 ppmV
Fig. 12. Temperature in the riser and combustor bed zones of MILENA during lignin B indirect gasication. Steady operation: 75230 min.
E.T. Liakakou, et al. Fuel 251 (2019) 580–592
590
synthesis gas which can be used for the production of biofuels.
Lignin A was identied as an interesting candidate for syngas fer-
mentation, since it is not an easy feedstock for other applications.
Lignin B, on the other hand, is a more demanding feedstock in terms of
feeding, due to the irregular shape and size with the high amount of
nes and requires more pretreatment steps than lignin A. However it is
a feedstock with high purity (low sulphur and ash content) that could
be valorized via alternative pathways into high value products like
resins and phenol(ics).
The H
2
/CO ratio obtained from the three gasication technologies
was quite dierent and varied from 0.6 to 1.0. The optimal H
2
/CO ratio
required depends on the application of the product gas. For the syngas
fermentation process, low H
2
to CO ratio is preferred, as most of the
organisms grow better on CO than H
2
[12]. The tar content was also
very dierent between the three gasication technologies, as expected,
varying from 18 to 80 g/Nm
3
for lignin A and from 19 to 108 g/Nm
3
for
lignin B.
In the xed bed, the higher tar content produced from lignin B
gasication (100 compared to 80 g/Nm
3
for lignin A) is mainly attrib-
uted to the lower gasication temperature and the lower residence time
due to higher air quantity used as gasication agent. Fluidized bed
gasication led to the lowest tar content for lignin A (20 g/Nm
3
), which
was expected as this technology favours mass and energy transfer and
thus tar destruction. Lignin A led to lower tar than lignin B (108 g/
Nm
3
), because of the higher mineral matter content. In the uidized
bed, the lower tar content obtained from lignin B gasication (30
compared to 34 g/Nm
3
for lignin A) is an eect of the higher gasica-
tion temperature that favours tar cracking reactions, resulting in im-
proved gas yield and quality.
Furthermore, dierent contents of impurities that can reduce the
fermentability of the gas (such as hydrocarbons, HCN, HCl, NH
3
, COS
and other organic S- compounds) were detected in the product gas. The
gasication results show that the concentration of these compounds is
related to the content of the corresponding compounds in the original
feedstock.
For updraft xed bed gasication, lignin A consisting of bid dense
particles is a better match than lignin B consisting of brittle particles
with powder-like consistency since a more stable thermal prole and
operating conditions were achieved. For BFB gasication, both lignins
seem to gasify without problems. However, it should be noted that the
big dense particles of lignin A might cause char build-up in the bed or
the very brittle particles of lignin B might lead to carbon loss due to
entrainment. For indirect gasication, lignin B showed more promising
results, despite the diculties in feeding. The lower ash content than
lignin A, allowed for higher gasication temperature, resulting in
higher gas yields that is benecial for the fermentation process.
The dierence in the lignin-rich residues characteristics is attributed
to the specic pre-treatment technologies for the (hemi)cellulose ex-
traction. By steering the pretreatment technology, the properties of the
feedstock can be improved, making it a suitable for gasication.
The main impurities in the synthesis gas from the lignin-rich feed-
stocks gasication, that need to be removed to very low levels are
unsaturated hydrocarbons and tar compounds. The eect of con-
taminants such as CH
4
, C2-C5 hydrocarbons, HCN, HCl, NH
3
and sul-
phur compounds on alcohol production and cell growth is not clear as it
has not been thoroughly studied.
The next step is going to be the combination of the gasication
process and the appropriate product gas cleaning, with the syngas
fermentation process for the production of bio-alcohols. There is a
limited number of studies focusing on the integration of the two tech-
nologies, so this study will contribute to the design and commerciali-
zation of the gasication-fermentation process and add to the economic
and environmental sustainability of a biorenery.
Acknowledgements
This work has received funding from the European Unions Horizon
2020 research and innovation programme under grant agreement No.
731263.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.fuel.2019.04.081.
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... • Aliphatic (iso)alkanes or cyclo-alkanes for jet fuels, gasoline and diesel [186] • Aromatics, mainly in the form of alkyl benzenes [170][171][172][173][174][175][176][177][178] that offer specific properties to fuels (i.e., octane boosters in gasoline or for improving the quality and properties of jet fuel, such as elastomeric swelling and lubricity characteristics) • Heavy fractions (low sulfur) of phenolic/aromatic lignin oils (oligomers) that can be blended with heavy marine (bunker) fuels [230][231][232][233][234][235][236][237] The integrated lignin conversion technologies may include [170][171][172][173][174][175][176][177][178][179][180][181][182][183][184][185][240][241][242][243][244][245][246][247][248][249][250]: ...
... The composition of syngas can vary depending on the gasification temperature and pressure, presence of steam and oxygen, heating rate, and composition of feed lignin [249]. Downstream gas cleaning and alcohol/hydrocarbon production via fermentation [250] or FT synthesis (coupled with hydrocracking in the case of wax production) utilizes the same technologies and processes as those used with the more conventional coal/biomass gas-to-liquid (GTL) processes. However, the need for gas cleaning makes this technology less mature than GTL processes. ...
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... Syngas is mainly composed CO, CO 2 and H 2 and is an organic waste product from industries such as steel mills or petroleum refineries (Karmann et al., 2019;Köpke et al., 2011). Syngas can also be produced by the gasification of lignocellulosic biomass, including lignin, but so far only a few plants use syngas for ethanol production (Liakakou et al., 2019;Straathof et al., 2019). ...
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The United Nations defined 17 Sustainable Development Goals (SDGs) in 2016 and agreed on fighting to confront the climate change and protecting the oceans and forests. Subsequently, the sustainable production of bioplastics is gradually gaining reputation and significance. With the usage of bioplastics such as biodegradable polyhydroxyalkanoates (PHAs) various SDGs would be tackled, but costs remain a crucial factor for competing against fossil-based plastics. Appropriate local feedstock selection can help to reduce the production costs and minimize transportation routes. In this work, four feedstock generations are introduced and respective conversion strategies to PHA are presented. Whilst the focus is on mapping the abundances of feedstocks and potential PHA production capacities in Europe, utilization of animal by-product streams is also highlighted as a rather unconventional but highly abundant feedstock for PHA production.
... A variety of gasifiers such as fluidized bed, fixed beds and entrained flow gasifiers are often used for this process (Kiang 2018). Different techniques have been applied for lignin-based biorefinery and syngas is most commonly produced, it can be used as for microbial bio alcohol production (Liakakou et al. 2019). Corn stover subjected to gasification at 350°C for 30 min evolution of phenolic compound reported from biomass (Nsaful et al. 2018). ...
Chapter
Currently, the overall population in the world is increasing continuously, thereby causing the depletion of fossil fuels. An alternative substitute for energy and renewables production is highly desirable. With an increasing population in the world, industrialization has a sharp boost that leads to a rise in waste streams. An accumulation of waste, specially agricultural waste, necessitates an appropriate waste management system. Researchers are exploring economic and successful management systems to recycle and reuse lignocellulose waste. Various conversion technologies may be implemented by efficiently converting lignocellulose biomass and organic waste to bioenergy and biochemicals. Bioethanol, biohydrogen, biodiesel, bioplastics, enzymes are potential renewable energy sources produced from lignocellulose biomass. Depending on the type of biomass, the conversion technologies are employed for efficient bioenergy and different biochemicals conversion. The present chapter discusses lignocellulose biomass-based conversion technologies, advantages and challenges for sustainable metabolites production.
... Gasification is a thermochemical method which uses a limited amount of oxygen (less than the oxygen content in air) in its reactions (Danish-Energy-Agency, 2018). The main outcomes (depending on the specific reactor and reaction conditions) are tar, char and synthesis gas or syngas (CO + H 2 ) (Liakakou et al., 2019). The gases that compose syngas are very valuable because they are used in the synthesis of many products, lowering the saturation risks in markets. ...
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Gasification of spent lignin pellets was used to obtain a gas suitable for energy production. Spent lignin was obtained from second-generation cellulosic ethanol demo plant using wheat straw as feedstock. Gasification of lignin did not give rise to any feeding problems, thus no significant changes were needed in the existing gasification installation. The rise of temperature and steam flow rate favoured the formation of H2, while hydrocarbons (CnHm) and tar contents decreased. The increase of equivalent ratio (ER) also decreased hydrocarbons and tar contents, but syngas higher heating value (HHV) was reduced. The use of natural minerals improved lignin gasification. The presence of dolomite led to the highest H2 and to the lowest CnHm and tar contents. Results obtained at bench-scale were confirmed at pilot-scale, as similar trends were obtained. However, as the residence time in pilot gasifier was higher, greater gas yields with higher H2 and CH4 concentrations were obtained, while tar contents decreased. After syngas hot cleaning and upgrading, the final syngas composition showed to be suitable for a wide range of applications (e.g. energy production and synthesis of chemicals), since it was substantially enriched in hydrogen, whereas tar and heavier gaseous hydrocarbons were completely destroyed.