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DOI: 10.3303/CET2294094

Paper Received: 15 April 2022; Revised: 09 June 2022; Accepted: 17 June 2022
Please cite this article as: Dimande D., Wukovits W., Koch D., Mihalyi-Schneider B., Friedl A., 2022, Process Simulation and Life Cycle
Assessment of Dilute Sodium Hydroxide Pretreatment of Wheat Straw, Chemical Engineering Transactions, 94, 565-570
DOI:10.3303/CET2294094

A publication of

CHEMICAL ENGINEERING TRANSACTIONS

VOL. 94, 2022 The Italian Association
of Chemical Engineering
Online at www.cetjournal.it

Guest Editors: Petar S. Varbanov, Yee Van Fan, Jiří J. Klemeš, Sandro Nižetić
Copyright © 2022, AIDIC Servizi S.r.l.
ISBN 978-88-95608-93-8; ISSN 2283-9216

Process Simulation and Life Cycle Assessment of Dilute
Sodium Hydroxide Pretreatment of Wheat Straw

Diana Dimande*, Walter Wukovits, Daniel Koch, Bettina Mihalyi-Schneider,
Anton Friedl
Institute of Chemical, Environmental and Bioscience Engineering, TU Wien, Getreidemarkt 9/166, 1060 Vienna, Austria
diana.dimande@tuwien.ac.at

Wheat straw is an abundant lignocellulosic residue from wheat cultivation. It is composed of cellulose,
hemicellulose, and lignin and offers several possibilities for valorisation. Nowadays, a growing interest in
lignocellulosic biomass fractionation and lignin utilisation is observed. Alkaline pretreatment with NaOH is often
applied to obtain a high-quality lignin fraction under milder conditions. In this study, alkaline pretreatment of
wheat straw using a NaOH solution is simulated in Aspen Plus to determine mass and energy balances
producing a lignin fraction. These balances were used as input to perform a life cycle assessment to screen and
estimate the environmental impacts underlying 12 different scenarios. According to the impact assessment
results, pretreatment with 2 % NaOH solution, reactor temperature 105 ºC, and solid biomass as the thermal
energy source, as described in the base case, is the best compromise between low Global Warming,
Eutrophication, Human Toxicity potentials, Blue Water Consumption, and resource efficiency.

1. Introduction
Lignocellulosic biomass is the largest source of renewable organic material, mainly composed of three biological
polymers: cellulose, hemicellulose, and lignin. Lignin is a natural hydrophobic polymer responsible for biomass
rigidness and recalcitrance for further processing (Woiciechowski et al., 2020). It can be found in agricultural
residues, forestry waste, or municipal and industrial waste (Wertz and Bédué, 2013). Wheat straw, rice straw,
and corn stover are the most abundant agricultural residues and the primary lignocellulosic materials in this
category. Most of the pretreatment processes applied to biomass focus on cellulose and hemicellulose fractions,
but increasing interest in lignin as an abundant renewable resource with several applications is observed.
Technologies for the fractionation of lignocellulosic biomass into its main constituents are divided into four
categories: physical/mechanical, physicochemical, chemical, and biological treatments (Toquero and Bolado,
2014).To obtain lignin from lignocellulosic biomass, Organosolv or alkaline pretreatment is applied. While the
Organosolv process uses harsh conditions (high temperature, low pH) leading to the formation of by-products,
the use of alkaline solutions based on Ca(OH)2 (lime) or NaOH allows having a process under mild conditions,
avoiding lignin condensation and limiting the degradation of sugars to furfural and organic acids. Pretreatment
of biomass of various types with sodium hydroxide is the most studied alkaline treatment (Harmsen et al., 2010).
From an economic point of view, pretreatment is one of the most expensive steps within the biomass conversion
process, and it can affect upstream and downstream operations. Besides process efficiency and economic
considerations, the focus is nowadays on the (environmental) sustainability of pretreatment processes
(Mussatto and Dragone, 2016). According to the International Organization for Standardization – the framework
(2006a) and the guidelines (2006b), Life Cycle Assessment (LCA) is an international standardised method used
to study the potential environmental impacts of a product or a product system (Kl̈opffer and Grahl, 2014).
This study aimed to develop a simulation model of the alkaline pretreatment of wheat straw using a NaOH
solution and its underlying scenarios and finally assess the environmental impacts of the overall pretreatment
process.

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2. Methodology
Process simulation in Aspen Plus (Aspen Plus V10, 2017) was used to calculate the mass and energy balances
of lignin isolation from wheat straw, applying alkaline pretreatment with NaOH based on a lab-scale study taken
from literature. The process flowsheet included wheat straw pretreatment, solid/liquid separation, washing,
precipitation, and neutralisation steps. The obtained balances formed the basis for LCA analysis using the
software GaBi (GaBi ts 9.1, 2019).

2.1 Process description

The core step of the investigated process is the reactor to fractionate wheat straw into its main constituents -
lignin, cellulose and hemicellulose. Data for alkaline pretreatment with NaOH are taken from Barman et al.
(2012), describing the dissolution of lignin and the remaining solid fraction of biomass as a function of the
concentration of sodium hydroxide, at 105 °C, with a solid-liquid ratio for the extraction solution of 10 g wheat
straw/100 mL NaOH solution, during a 10 min pretreatment time (Table 1). In the following step, the remaining
solid phase, mainly consisting of cellulose, is separated from the liquid phase and then washed with water.

Table 1: NaOH concentration effect on lignin and hemicellulose removal (Barman et al., 2012)

NaOH conc. (wt%) Lignin removal (%) Hemicellulose removal (%)
0 13.7 ± 2.9 8.8 ± 2.8
0.5 32.1 ± 3.2 18 ± 3.1
1.0 37.2 ± 4.0 24.9 ± 3.2
1.5 56.2 ±3.7 46.1 ± 3.5
2.0 70.3 ± 2.9 68.2 ± 3.8

After cellulose separation, the resulting lignin-containing liquid phase is combined with the obtained washing
solution. Lignin is precipitated by adding acidified water (5 % H2SO4) as a counter-solvent until a pH value of 2
is reached. Precipitated lignin is separated from the remaining solution, mainly containing hemicellulose. The
solid lignin is washed. Finally, the washing solution and the remaining liquid solution from the solid-liquid
separation step after lignin precipitation are neutralised with Ca(OH)2 before going to a water treatment unit and
being released into the environment.

2.2 Process simulation

Process simulation in Aspen Plus (Aspen Plus V10, 2017) is used to provide the mass and energy balances for
LCA analysis. The process is scaled to 12.5 t/h wheat straw – based on an annual wheat straw production of
1,600 kt/y in Lower Austria, of which about 20 % is available for various use. To guarantee a stable wheat straw
supply, 100 kt/y are assumed for lignin production at an operating time of the plant of 8,000 h/y. The composition
of wheat straw used in the process simulation was the same as used by Paul (2018): 28.61 wt% cellulose,
19.76 wt% hemicellulose and 14.17 wt% lignin at a water content of 12 wt%. NRTL was set as the general
property method, and the elecNRTL method was used to accurately calculate the pH value for the precipitation
and neutralisation steps. An extended NREL database for biomass components is used to represent the key
biomass components such as lignin, cellulose, and hemicellulose (Wooley and Putsche, 1996).
The pretreatment step is modelled as a stoichiometric reactor defining the dissolution of different constituents
of wheat straw following Barman et al. (2012). For the base case of simulation (reaction temperature 105 °C,
sodium hydroxide concentration 2 wt%, solid loading of 10 % w/v), 84.5 % of lignin is solubilised to allow a lignin
recovery, as stated in Table 1. It is assumed that no cellulose is dissolved under these conditions.
The lignin precipitation step is modelled in a stoichiometric reactor. Due to lacking information about lignin
solubility in a water-sodium hydroxide solution, it was assumed that the precipitation only depends on the
solubility of lignin in water. 87 % of lignin precipitation is assumed following a correlation for lignin solubility in
ethanol-water solutions at pH=2 (Santos, 2019) based on solubility data from Silva (2012) – recovery, and
Huijgen et al. (2010) – fractionation.
Solid/liquid (S/L) separation to remove cellulose after the pretreatment step and solid lignin after the precipitation
step is simulated using a simple splitter, assuming that all solid components are found in the solid stream at a
dry matter content of 30 %. Both washing steps (cellulose, lignin) are implemented as mixer/splitter combinations
assuming counter-current washing of the solids, replacing the solution in solids with an equal amount of water.
In the neutralisation step, the addition of calcium hydroxide until reaching pH 7 is controlled via a design
specification. Calculation of pH (lignin precipitation, neutralisation) is based on the property method elecNRTL.
The wastewater treatment step is not included in the Aspen Plus simulation but is considered in the GaBi model.

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Figure 1 presents a summary of the process balance for the base case.

Figure 1: Process balance for a functional unit of 1 kg (dry matter) precipitated lignin at base case conditions
(reaction temperature 105 °C, sodium hydroxide concentration 2 wt%, solid loading of 10 % w/v)

2.3 Life cycle assessment

The LCA was performed according to the ISO 14040 and ISO 14044 standards (International Organization for
Standardization – the framework (2006a) and the guidelines (2006b). The technical system boundaries include
the biorefinery process (see description in section 2.1), the provision of raw materials and utilities (wheat straw,
NaOH, water, energy, H2SO4 and Ca(OH)2), (by)-product streams (lignin, cellulose), process emissions and the
residual stream from wastewater treatment.
The pretreatment process of wheat straw using a dilute sodium hydroxide solution and its underlying scenarios
are assessed in a “cradle to gate” approach, not considering further processing of product streams and transport
from the biorefinery. Since wheat straw is the stalk leftover after wheat harvesting and wheat grain is the most
valuable product of this cultivation, the allocation between wheat grain and wheat straw used in this study was
economic. Later, a comparison between economic and mass allocation is made to investigate its influence. The
cellulose residue is considered a by-product of this process. Since its processing possibilities are not in the
scope of this study, it will not be considered in further calculations. The functional unit (FU) is 1 kg (dry matter)
of recovered lignin. It is important to note that the functional unit is fixed, but the raw material demand is
dependent on the yield of the extraction.
Professional GaBi database (version 8.7, service pack 36) from Thinkstep AG provided the background data on
wheat growth (including the use of fertilisers and pesticides) and harvesting processes, production of the
chemicals used in the process, and the energy and water consumed according to the mass and energy balances
obtained from the simulation.
The characterisation method CML 2001 (Jan 2016) was used to conduct the Life Cycle Impact Assessment
(LCIA). Four impact categories were selected: (1) Global Warming Potential, (2) Eutrophication Potential, (3)
Human Toxicity Potential, and (4) Blue Water Consumption. These are commonly used in literature for LCA of
biorefinery concepts like Paul (2018), which assesses an Organosolv pretreatment of wheat straw.

2.4 Description of the case studies

Different scenarios are considered in process simulation and LCA to see their effect on the process, and
consequently, their impacts on the environment. The base case scenario corresponds to lignin extraction from
wheat straw using a 2 wt% NaOH solution at 105 °C, followed by lignin precipitation with 5 wt% H2SO4. Economic
allocation between wheat straw and wheat grain, and solid biomass as the thermal energy source are assumed.
Other investigated scenarios are divided into two main categories: process-performance related variations
(extraction temperature, extraction solution, and concentration of precipitation agent) considered in process
simulation and not performance-related variations (energy source, water demand, and feedstock allocation)
considered only in LCA. The parameters selected in the first category follow a value range experimentally
investigated in the literature. Since there was no literature on the quality of the antisolvent used after alkaline

10
kg/kg

water
kg/kg
79.9

Process
Process

water

Wheat
straw

NaOH

9.54
kg/kg

1.63
kg/kg MJ/kg

Thermal
energy

S/L
separation

treatment
Pre-

77
kg/kg

kg/kg

H2SO4

Cellulose
@ 30% dm

91
kg/kg

14
kg/kg

Process
water

Lignin

Washing

14
kg/kg

10
kg/kg

2.4
kg/kg

Washing

S/L
separation

45
kg/kg

Process
water

134
kg/kg

Precip-
itation

@ 30% dm

3.3
kg/kg

2.3
kg/kg

3.3
Neutrali-

zation 2.3
kg/kg

131
kg/kg

Waste
water

Ca(OH)2

134
kg/kg

0.4
kg/kg

567

pretreatments, a different concentration was analysed in a designated scenario. For the second category, the
goal was to assess how these changes would affect the overall system. Table 2 summarises the conditions
used in the base case and the investigated scenarios.

Table 2: Summary of the scenarios

Scenario Extraction
Temperature

NaOH
concentration,
wt%

H2O
Cellulose
washing

H2SO4
concentration,
wt%

H2O
Lignin
washing

Allocation Thermal
energy source

Base case 105 2 - 5 - Economic Solid biomass
Parameters 180 130 1.5 1.0 0.5 ±50 %1 25 ±50 %1 Mass2 Natural gas2

1 - ±50 % of the amount of water used in the base case. 2 - Simulated directly in GaBi.

3. Results and discussion
All twelve scenarios of lignin extraction through alkaline pretreatment of wheat straw are compared to the base
case in a cradle-to-gate life cycle assessment. Each process contributor and its environmental impacts are
presented in a hotspot analysis to give information about their influence and contribution to the overall result.
The evaluation of the impact categories presented below provides insights into the environmental aspects and
shows the trends for process variations, making the analysis of different scenarios possible. The relative
difference between the total impacts of the scenarios and the base case is represented in Figure 2 and Figure
3, following different scales for the observed deviations in impacts. Total impacts for the base case: -1.02 x 101
[kg CO2 eq.] (GWP), 8.72 x 10-3 [kg (PO4)3- eq.] (EP), 6.06 x 101 [kg H2O] (BWC), and 4.30 x 10-1 [kg DCB eq.]
(HTP).

Figure 2: Relative difference between the total impacts of the base case and the scenarios for extraction
temperature, NaOH concentration, H2SO4 concentration, feedstock allocation and thermal energy source

The first impact category considered is the Global Warming Potential (GWP). These results are a sum of the
CO2 credits and the emissions to the environment. Since the credits are higher than the emissions, in terms of
absolute value, the total results are expressed as negative values and represent benefits to the environment.
The main contributors to the emissions in this category are the chemicals NaOH and H2SO4, resulting from the
high electricity consumption of their manufacturing processes. Wheat straw captures atmospheric CO2 during
its growth phase, reducing the net CO2 emissions and contributing to lower global warming. It is beneficial to
use it as feedstock. The lower the NaOH concentration, the higher the wheat straw needed to recover 1 kg of
lignin, increasing the CO2 credits and lowering the GWP at the cost of poor resource use.
The second impact category analysed, Eutrophication Potential (EP), is related to the excess nutrients. The
main process contributors affecting this category are wheat straw and thermal energy. Wheat straw cultivation
is responsible for nitrogen oxides and nitrate emissions to air and freshwater, due to ploughing, drilling, the use

5 %

30 %

32 %

49 %

16 %

-5 %

-20 %

14 %

56 %

76 %

10 %

3 %

-4 %

-10 %

42 %

13 %

53 %

71 %

11 %

3 %

-6 %

-9 %

50 %

-23 %

-82 %

-122 %

1 %

0
%

3 %

19 %

5 %

1.5 % NaOH

1 % NaOH

0.5 % NaOH

T=180 ⁰C

T=130 ⁰C

25 % H2SO4

Natural gas

Mass alloc

-140
%

-120
%

-100
%

-80
%

-60
%

-40
%

-20
%

0
%

20
%

40
%

60
%

80
%GWP EP BWC HTP

568

of fertilisers and pesticides, the harvesting and drying of the straw (Smith et al., 1999). When decreasing the
concentration of the sodium hydroxide solution, the total eutrophication impacts increase (Figure 2), since more
mass of straw would be necessary to recover the same amount of lignin. The change in the source of thermal
energy shows that renewable energy does not always mean a more environmentally friendly process, as the
use of biomass to produce energy has the same magnitude of emissions contributing to eutrophication as natural
gas.

Figure 3: Relative difference between the total impacts of the base case and scenarios on water demand for
cellulose and lignin washing

The main contributors to Blue Water Consumption (BWC) are process water demand and wastewater
treatment. The first accounts for all the freshwater inputs in the process, while the latter accounts for the water
recycled into the system after treatment. Wastewater treatment is beneficial to the whole process since it
reduces the freshwater inputs. Wheat straw also contributes to the impacts (~20 %), as wheat cultivation
requires water for irrigation. The base case counts 6.06 x 101 kg H2O of total BWC impacts. These impacts
increase up to 80 % when decreasing the sodium hydroxide concentration to 0.5 %, because of higher water
demand, especially preparing the extraction solution and the first washing step. As shown in Figure 3, using
more or less water for the washing steps causes only a small contribution to the impact on this category.
For Human Toxicity Potential (HTP), the total emissions in the base case correspond to 4.30 x 10-1 kg DCB
eq. (1,4- dichlorobenzene equivalents). Thermal energy used to run the extraction step is the major contributor
(corresponding to ca. 50 % of the total emissions), followed by the production of used chemicals. Solid biomass
is the thermal energy source for all scenarios, except for the scenario represented by Natural gas. The sensitivity
analysis reflects the share of thermal energy, as using a higher extraction temperature implies a significant
increase in the total impacts (Figure 2). For the impacts caused due to the use of chemicals, the literature data
analysis shows that the lower the concentration of sodium hydroxide, the higher the input for wheat straw, water,
and thermal energy. Their respective emissions also increase. Thermal energy production from solid biomass
emits various gases to the environment and requires large areas to grow the materials, limiting the location of
the power plants. As a result, it requires transportation and fuel. The high need and continuous supply of
resources may also lead to deforestation. According to (Singh et al., 2011), the infrastructure needed to produce
thermal energy may emit hazardous substances, like heavy metals, from construction to use. The increase of
the emissions in HTP due to wastewater treatment is a consequence of the rise in the water demand since it
requires a higher need for treatment afterwards. Using a more concentrated antisolvent reduces the impacts
caused by using calcium hydroxide prior to the wastewater treatment.

4. Summary and conclusion
LCA of NaOH pretreatment of wheat straw to produce lignin was performed based on mass and energy balances
obtained from process simulation, assuming a functional unit of 1 kg (dry matter) of recovered lignin.
Results show that the base case represents the best combination of low impacts for the four analysed categories
(-1.02 x 101 kg CO2 eq. (GWP), 8.72 x 10-3 kg (PO4)3- eq. (EP), 6.06 x 101 kg H2O (BWC), and 4.30 x 10-1 kg
DCB eq. (HTP)) and efficient resource use. The lower the NaOH concentration, the higher the resources
demand to recover the same amount of lignin due to a lower yield. It consequently affects the whole process
chain and the emissions to the environment.

-0.1 %

0.1 %

0.0 %

-5.4 %

5.4 %

-1.3 %

1.3 %

-0.1 %

0.1 %

0.0 %

-0.2 %

0.0 %

-0.1 %

-WCW

+WCW

-WLW

+WLW

-8.0 % -6.0 % -4.0 % -2.0 % 0.0 % 2.0 % 4.0 % 6.0 %
GWP EP BWC HTP

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The impact assessment shows that except for GWP, natural gas as the thermal energy source is responsible
for fewer impacts, namely -9 %, -10 % and -20 % for EP, BWC and HTP, when compared to solid biomass,
because the technologies used are already mature while renewable energy production is still in an early stage
of development. Considering the fossil fuel dependency and climate change, it is reasonable to consider shifting
to solid biomass as a thermal energy source. But it is crucial to optimise its extraction and processing to reduce
the environmental burdens.
Although the focus of the sensitivity analysis was on single parameter variations, evaluating combined
parameters such as NaOH concentration and temperature would be interesting. 2 % NaOH is the optimal
concentration for lignin removal among the four concentrations used. It is expected that higher NaOH
concentrations would increase lignin removal. But it may cause cellulose degradation since the pretreated
sample with 2 % NaOH already presents the lowest crystalline index among the four. Temperature is a critical
factor in lignocellulosic biomass pretreatment: high temperatures may also cause biomass degradation or even
enhance the formation of inhibitory compounds, not to mention energy consumption. Future work should be
focused on higher NaOH concentrations (up to 10 %) and lower extraction temperatures, with particular attention
to the precipitation and antisolvent quality.

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