Integrated Biorefinery Based on Algal Biomass

98 Pages·2010·1.99 MB·English

DEPARTMENT OF CHEMICAL ENGINEERING, LUND UNIVERSITY, FACULTY OF
ENGINEERING
Integrated Biorefinery
Based on Algal Biomass
A Feasibility Study

Final Report

2010-06-03

Authors: Tutors:
Johan Bergqvist Christian Hulteberg, Lund University
Kristina Henriksson Hans T Karlsson, Lund University
Karolina Johansson Børre Tore Børresen, Statoil ASA
Marcus Svensson
Oscar Tenfält
Disclaimer

This study was performed as a project in the course Feasibility Studies on Industrial Plants (KET050)
at the Department of Chemical Engineering, Faculty of Engineering, Lund University, Sweden in co-
operation with Statoil ASA New Energy and New Ideas.

Neither Lund University nor the authors of this report or Statoil ASA may be held responsible for the
effect following from using the information in this report. Neither the authors, Lund University nor
Statoil ASA make any warranty, expressed or implied, or assume any legal liability or responsibility
for the accuracy or completeness of this information.
Acknowledgements

The authors would like to thank all the persons who have contributed to the completion of this report.

Firstly, we would like to thank our tutors, Hans T Karlsson and Christian Hulteberg at the Department
of Chemical Engineering at Lund University and Børre Tore Børresen at Statoil ASA for their support.
Christian Hulteberg has contributed with most of the information about the Biofuel-Solution process,
some information about prices for chemicals, with estimations and with good advice. Hans T Karlsson
has performed calculations regarding enzyme consumption and capital cost for the transesterification
process, given advice on capital cost estimates and given many valuable tips. At our first meeting with
Børre Tore Børresen, he gave a fine description of the project and of the R&D concerning renewable
energy sources at Statoil ASA and he has given us interesting tips during the work with the feasibility
study.

Secondly, we would like to thank the following persons at the Department of Chemical Engineering at
Lund University. Prof. Guido Zacchi has answered answered numerous questions about ethanol pro-
duction and given good advice regarding calculation methods. Ola Wallberg (PhD) helped us with the
simulations in Aspen Plus each time there was a problem. Additional help and tips regarding Aspen
Plus were given by Mats Galbe (PhD). The membrane filtration gave rise to many questions but Prof.
Ann-Sofi Jönsson always had a good answer or a suitable approximation that saved the day. Since
information about how fermentation by-products can be recovered from the stillage is scarce, it is
thanks to Prof. Bernt Nilsson that the method involving ion exchange chromatography came to our
knowledge. Prof. Stig Stenström guided us among the vast number of books in the department library
to suitable sources of physical data. Regarding the composition of the algae and the salt tolerance of
yeast, Prof. Gunnar Lidén provided valuable literature. Ass. Prof. Lovisa Björnsson and PhD-student
Nges Ivo Achu, both at the Department of Biotechnology, has contributed with important information
regarding biogas production. Profs. Olle Holst, at the Department of Biotechnology, and Peter Råd-
ström, at Applied Microbiology, gave important advice about how to find the composition of the al-
gae.

Thirdly, we would like to thank all persons outside the university, from different companies who have
answered our questions regarding different products and process solutions. Per Munk Nielsen at No-
vozymes A/S has provided much valuable information about enzymes for transesterification and suit-
able process design for the same. Christina Bång at SinChriJo gave information on the price of and
sent much practical information about the usage of molecular sieves from Zeochem. A correct price
for fermentation yeast, based on the consumption at the biorefinery, was supplied without delay by
Pierre Basuyaux at Fermentis. Mattias F. Nilsson at Alfa Laval gave us tips and information about
membrane filtration for our application.

i
Abstract

A feasibility study on a biorefinery, based on biomass from microalgae, has been performed by a
group of students at the Department of Chemical Engineering at Lund University, Faculty of Engi-
neering by the order of Statoil ASA. The main product is pre-defined as biodiesel, in this case an ethyl
ester of algal oil.

Microalgae of the species Nannochloropsis salina are to be cultivated outdoors in photobioreactors and
harvested by flocculation followed by gravity sedimentation. The algal oil will be recovered by phase
separation in a stirring settling tank after the cells have been ruptured by cavitation. Degumming will
be used for removal of phospholipids. The degummed oil is to be transesterified with ethanol to bio-
diesel in an enzymatic process. The glycerol produced in the transesterification will be converted to
the valuable co-product n-propanol.

Ethanol for the transesterification will be produced by fermentation of the carbohydrates in the algal
residues, the algal flour, in an SSF (simultaneous saccharification and fermentation) process. Separa-
tion of the ethanol is to be performed by distillation. The surplus ethanol that is to be sold, and a minor
part of the ethanol required for the transesterification, will be dehydrated to fuel-grade quality. Dehy-
dration of the ethanol will be performed in a column with molecular sieves. The stillage is to be an-
aerobically digested to biogas, which can be combusted in a boiler, thus generating heat and power for
the plant. Some fermentation by-products, i.e. organic acids and glycerol will be recovered from the
stillage by centrifugation and evaporation. The glycerol will be used for production of n-propanol and
the acids will be separated by ion exchange chromatography. Residues from the biogas production, i.e.
primarily ammonia, will be returned to the algae cultivation as nutrients. Carbon dioxide from the
fermentation will be supplied to the algae cultivation. Since the algae are of marine origin, cultivation
in salt water is required. The yeast used for fermentation is not halotolerant and therefore most of the
salt will be removed by diafiltration before the SSF.

The products to be sold, apart from the main product biodiesel, will be fuel ethanol, n-propanol, buta-
nediol, acetic acid, propionic acid and succinic acid. Almost all energy required will be produced on
site by combustion of the biogas. The final production price for the biodiesel will be €1.03/l. Since the
capital cost of the photobioreactors and the cost of the enzymes for transesterification contribute most
to this, a reduction of one or both could lower the production price substantially.

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Contents

1 Introduction ..................................................................................................................................... 1
2 Background ..................................................................................................................................... 2
2.1 Algae Cultivation .................................................................................................................... 2
2.1.1 Nannochloropsis salina .................................................................................................... 2
2.2 Oil Recovery, Oil Pre-treatment and Transesterification ........................................................ 4
2.3 Plant Location .......................................................................................................................... 5
2.4 Adjustment of the Process ....................................................................................................... 5
3 Biodiesel Production ....................................................................................................................... 5
3.1 Chemical Catalysis .................................................................................................................. 5
3.2 Enzymatic Catalysis ................................................................................................................ 6
3.2.1 Enzymatic Transesterification Process ............................................................................ 6
3.3 Recommendations ................................................................................................................... 7
4 Glycerol Processing ......................................................................................................................... 8
4.1 The Biofuel-Solution Process .................................................................................................. 8
4.1.1 Hydrogen Source ............................................................................................................. 9
4.2 Recommendations ................................................................................................................... 9
5 Algal Flour Processing .................................................................................................................... 9
5.1 Filtration ................................................................................................................................ 10
5.1.1 Membrane filtration ....................................................................................................... 11
5.1.2 Recommendations ......................................................................................................... 12
5.2 Ethanol Production ................................................................................................................ 13
5.2.1 Hydrolysis ..................................................................................................................... 13
5.2.2 Fermentation .................................................................................................................. 14
5.2.3 Separation and Purification of the Ethanol .................................................................... 16
5.2.4 Recommendations ......................................................................................................... 18
5.3 Fermentation By-Products ..................................................................................................... 19
5.3.1 Recovery of Glycerol .................................................................................................... 20
5.3.2 Recovery of the Acids in the Stillage ............................................................................ 20
5.3.3 Recommendations ......................................................................................................... 21
5.4 Biogas Production ................................................................................................................. 22
5.4.1 The Substrate ................................................................................................................. 22
5.4.2 The Reactors .................................................................................................................. 22
5.4.3 The Coupling of Reactors .............................................................................................. 22
5.4.4 Nitrogen Inhibition ........................................................................................................ 22
5.4.5 The Temperature of the Process .................................................................................... 22
5.4.6 The Residues ................................................................................................................. 23
5.4.7 The Energy Production .................................................................................................. 23
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5.4.8 Recommendations ......................................................................................................... 23
6 Process Description ....................................................................................................................... 24
7 Mass and Energy Calculations ...................................................................................................... 26
7.1 Flows by Mass and Volume .................................................................................................. 26
7.1.1 Transesterification ......................................................................................................... 26
7.1.2 Algae Cultivation, Oil Separation and Oil Pre-Treatment ............................................. 27
7.1.3 SSF ................................................................................................................................ 27
7.1.4 Membrane Filtration ...................................................................................................... 28
7.1.5 Distillation and Dehydration ......................................................................................... 29
7.1.6 Fermentation By-Product Recovery .............................................................................. 29
7.1.7 Biofuel-Solution Process ............................................................................................... 29
7.1.8 Anaerobic Digestion ...................................................................................................... 30
7.1.9 Summary of the Results................................................................................................. 30
7.2 Energy Calculations .............................................................................................................. 31
7.2.1 Energy Demand ............................................................................................................. 31
7.2.2 Heating and Cooling System ......................................................................................... 33
8 Cost Estimates ............................................................................................................................... 34
8.1 Assumptions .......................................................................................................................... 35
8.2 Capital Costs .......................................................................................................................... 35
8.2.1 Add on Factors .............................................................................................................. 35
8.2.2 Algal Oil Production Facility ........................................................................................ 35
8.2.3 Biodiesel Production Facility ........................................................................................ 36
8.3 Operating Costs ..................................................................................................................... 37
8.3.1 Costs for Chemicals Used in the Algal Oil Production Facility .................................... 37
8.3.2 Costs for Chemicals in the Biodiesel Production Facility ............................................. 38
8.3.3 Labor Costs .................................................................................................................... 39
8.3.4 Energy Costs .................................................................................................................. 39
8.4 Total Production Costs .......................................................................................................... 40
8.5 Sensitivity Analysis ............................................................................................................... 41
9 Conclusions ................................................................................................................................... 42
10 References ................................................................................................................................. 42
A Appendix A ...................................................................................................................................... I
B Appendix B....................................................................................................................................... I
C Appendix C....................................................................................................................................... I
D Appendix D ...................................................................................................................................... I
E Appendix E ....................................................................................................................................... I
F Appendix F ....................................................................................................................................... I
G Appendix G ...................................................................................................................................... I
G.1 Capital Cost for the Filtration Units ......................................................................................... I
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G.2 Capital Cost for the Molecular Sieves and the Dehydration Columns ..................................... I
G.3 Capital Cost for the Distillation Units ..................................................................................... II
H Appendix H ...................................................................................................................................... I
I Appendix I ........................................................................................................................................ I
J Appendix J ........................................................................................................................................ I
K Appendix K ...................................................................................................................................... I
K.1 Yeast Consumption .................................................................................................................. I
K.2 Consumption of Enzymes for Ethanol Production ................................................................... I
L Appendix L ....................................................................................................................................... I

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1 Introduction

One of today’s most important topics of the climate debate is the demand on replacements of fossil
fuels. The renewable fuel must be carbon dioxide neutral and sustainable as well as profitable. Bio-
diesel from vegetable oils can be used as fuel undiluted or mixed into diesel and there are already
vegetable alternatives available from crops such as oilseed rape and soybean. Compared to other vege-
table oils, the oil produced from algae has a high productivity per surface area and otherwise arid land
can be used. In contrast to soybeans, which produce 59,000 l of oil per km2, algae generate 2,500,000 l
of oil per km2 at medium productivity. (1) However the production of biodiesel from algae oil is
struggling with adjusting the process to be cost efficient and thereby increase the competitiveness to-
wards fossil fuels. The economy can be improved by refining the by-products to added-value products.

As a student’s assignment a feasibility study of an integrated biorefinery from algae has been per-
formed at the Department of Chemical Engineering at Lund University for the international energy
company Statoil ASA. The focus has been on the use of by-products in the biorefinery, such as glyc-
erol and algal flour, and on refining of these into value-added products. The main product is specified
as biodiesel. Requests specified in the project description are the use of an enzymatic transesterifica-
tion process (e.g. Novozymes) and production of ethanol, which can be used for the transesterification,
from the algal flour. An overview of the processes that the biorefinery is composed of is given below
in Figure 1.1.

Sunlight
Nutrients Algae Cultivation Algal Oil Biodiesel Process Biodiesel
CO
2
Algal Flour
Ethanol
Butanediol
Ethanol Ethanol
Process
Stillage
Glycerol
Acids Glycerol
Biofuel-Solution
Hydrogen Biogas Process Energy
Process
Acids n-Propanol

Figure 1.1: A simplified process flowsheet in which the major processes and streams are included.

The starting point for the work is the report Biodiesel Production from Microalgae –A Feasibility
Study (2) performed by students at Lund University 2008. The study focused on the algae cultivation
and pre-treatment of the algal oil. This feasibility study is a further development of this study com-
plementing the plant with a biorefinery.

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The report begins with a brief description of the process suggested in the former study and the adjust-
ments made in this process e.g. to facilitate further processing of the by-products. After that, different
alternatives for the processes to be added are presented. Each section covers a certain product, with
suggestions for production methods and recommendations regarding the choice of method. The trans-
esterification of the algal oil and the refining of the glycerol formed are discoursed first. Algal flour
processing, i.e. fermentation into ethanol and anaerobic digestion into biogas, is treated next. When
the chosen total process has been described, heat and mass balance calculations follow. Estimates of
capital and production costs are summarised in the subsequent section. The report is concluded with a
summary of the results and a discussion regarding the same.
2 Background

A short summary of the methods chosen for algae cultivation, oil recovery, oil pre-treatment and
transesterification in the former study is given below. For details, the actual report Lassing, Merit –
Mårtensson, Peter – Olsson, Erik – Svensson, Marcus (May 16, 2008), Biodiesel Production from
Microalgae – A Feasibility Study, Department of Chemical Engineering, Lund University, Faculty of
Engineering (2) is recommended. Some changes in this process have been made to better suit the sub-
sequent processing of oil and by-products, i.e. not all of the methods described in Background are to
be used in the proposed biorefinery. The changes, e.g. the substitution of the base catalysed esterifica-
tion process for an enzymatically catalysed process, are described last in this section.
2.1 Algae Cultivation

The algae are to be cultivated in tubular closed photobioreactors from the Dutch company AlgaeLink
which focuses on equipment for algae production (2) (3). This equipment is considerably more expen-
sive than open ponds but the cultivation conditions can be better controlled, the productivity is higher
and the risk of contamination by other microorganisms is very low. The chosen algae species is Nan-
nochloropsis salina which is described below; together with the difficulties of finding information
about its chemical composition. N. salina is the algae species that AlgaeLink uses for biodiesel pur-
poses and since information for the former study was received from the company, it was a suitable
choice of algae species. Other advantages are reported high lipid content and the fact that it is an alga
that lives in salt water, thus the need for fresh water in the process is significantly lowered. (2)

Harvesting of the algae is performed in two steps – flocculation followed by gravity sedimentation.
Flocculation is achieved by addition of potassium hydroxide which increases the pH to 11. This influ-
ences the ionisation of components on the surface of the cell walls, which in turn leads to flocculation.
It is estimated that 85% of the algae can be recovered by this process and the remaining solution is
neutralised by nitric acid and subsequently returned to the bioreactor. The potassium nitrate formed
will function as an additional source of nutrients. Some kind of neutralisation is most likely needed
after sedimentation but this is not mentioned. Flocculation is applied due to the small size of the algae
which leads to slow sedimentation. Sedimentation was chosen for its low cost and because alternative
methods have relatively high energy demands. (2)
2.1.1 Nannochloropsis salina
N. salina is a yellow-green unicellular microalga of the class Eustigmatophyceae. The cells are ellip-
soid and only a few micrometers across. A photograph of some algae of the species N. salina is shown
in Figure 2.1. (2)

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Figure 2.1: Cells belonging to the microalgae species Nannochloropsis salina (4).

When the by-products from the biodiesel production, i.e. the carbohydrates and the proteins of the
algae, are to be refined, information about the composition of the cells is vital. Since the cultivation
conditions strongly affect the chemical composition and the productivity of the algae, there are no
definite data. According to information provided by AlgaeLink for the former study, the lipid content
of N. salina is 50% of its dry weight (2). In a study by S. Boussiba et al. (5), a lipid content of 16-21%
of the dry weight is reported for outdoor cultivation in open ponds. A maximum of 70% lipids, after
nine days of nitrogen starvation, is mentioned in the introduction of the report but the experiments
performed by S. Boussiba et al. (5) does not show any significant increase of the lipid content due to
nitrogen starvation. According to the results in this report, the best way to optimize the lipid produc-
tion is to maintain steady-state algal growth and to keep the cell density rather low as to avoid shading
of algae near the centre of the tubes. The maximum lipid production rate is reported as 4.0 g·m-2·day-1
at a total biomass production rate of 24.5 g·m-2·day-1 (5). Optimum cultivation conditions for N. salina
in a laboratory, determined by S. Boussiba et al. (5), are given in Table 2.1.

Table 2.1: Optimum growth conditions for N. salina cultivated in a laboratory (5).
Variable Optimum
Temperature 28°C
pH 7.5-8.0
[NaCl] 0.6 M
[KNO ] 10 mM
3
[NaHCO ] 0
3

The dry weight (DW), the ash free dry weight (AFDW) and the chemical composition of the AFDW
for N. salina has been determined by C. J. Zhu and Y. K. Lee (6). The results showed that 93% of the
DW is ash free and that 91% of the AFDW is constituted by organic material, i.e. proteins, carbohy-
drates and lipids (6). A comparison between the results from the study performed by C. J. Zhu and Y.
K. Lee (6) and a similar study performed by J. K. Volkman et al. (7) is shown in Table 2.2.

Table 2.2: Chemical composition for Nannochloropsis sp. and N. salina from studies performed by Zhu and Lee and
Volkman et al., respectively (6) (7).
Study Zhu and Lee Volkman et al.
Protein 44.4% of DW 17.8% of DW
Carbohydrates 10.4% of DW 8.8% of DW
Lipids 29.9% of DW 16.9% of DW

These studies are not completely comparable since Volkman et al. (7) report the composition of the
total dry weight while Zhu and Lee (6) report the composition of the ash free dry weight. A more im-
portant difference is that the results attained by Zhu and Lee (6) is for an unspecified species of the
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genus Nannochloropsis while the results attained by Volkman et al. (7) are specific for N. salina. In
the study performed by Zhu and Lee (6), all of the AFDW is analysed. The study performed by Volk-
man et al. (7) focuses on the suitability of N. salina as feed for maricultures and, as indicated by the
following quote from the report, some of the organic material is missed due to the analytical methods
used. Volkman et al. (7) wrote: “These methods provide data on readily hydrolysable carbohydrates
rather than structural carbohydrates such as cellulose.” (7).

The conclusion is that there are no definite values for the chemical composition of N. salina since the
results depend on the analytical methods used at the same time as the composition can vary drastically
with the growth conditions. Therefore, a chemical composition is assumed for the calculations to be
made. 40% of the dry weight of N. salina is assumed to be constituted by lipids while the cell wall,
which is assumed to be practically pure cellulose, is assumed to account for 30% of the dry weight.
The rest of the dry weight is assumed to be divided equally between proteins and carbohydrates other
than cellulose, i.e. starch and sugar, thus giving 15% proteins and 15% readily hydrolysable carbohy-
drates. These values are uncertain and probably constitute the most important source of error in the
feasibility study. Table 2.3 summarises the assumptions described above.

Table 2.3: Summary of the assumptions made regarding the chemical composition of N. salina.
Component Share of DW
Protein 15%
Lipids 40%
Carbohydrates 45%
Cellulose 30%
Starch or sugars 15%
2.2 Oil Recovery, Oil Pre-treatment and Transesterification

After harvesting, the oil must be extracted from the algae. Hydrodynamic cavitation will be used for
disruption of the cell walls and the oil is to be recovered from the resulting slurry by phase separation
in a stirred settling tank followed by centrifugation. The use of a bead mill instead of cavitation is
given as a possible alternative but this method was rejected due to lack of information. The advantage
of hydrodynamic cavitation is that no solvent is needed which reduces both the operating costs and the
environmental impact. (2)

Algal oil contains phospholipids which must be removed since it disturbs the further processing by
formation of emulsions and since it lowers the efficiency of the catalytic converters in diesel vehicles.
Removal of phospholipids is called degumming and is performed in two steps. First, the phospholipids
are coagulated by addition of water, dilute acid or a dilute salt-solution at 80°C. Subsequent centrifu-
gation removes most of the phospholipids. The rest of the phospholipids are removed in the second
degumming step, where precipitation is induced by addition of a small amount of phosphoric acid.
Separation is followed by neutralisation with sodium hydroxide in a neutralisation column. Soap resi-
dues are washed out with a dilute water solution of citric acid. After drying, the formed citrate is re-
moved by bleaching. (2)

In the former study, Biodiesel Production from Microalgae – A Feasibility Study (2), the transesterifi-
cation of the algal oil was to be performed with methanol and a base catalyst, e.g. sodium hydroxide or
potassium hydroxide. The presence of free fatty acids (FFA) in the algal oil is a problem when base
catalysed transesterification is to be performed since the FFAs form soap together with the cation of
the base. Esterification of the FFAs with methanol using a heterogenic acid catalyst, e.g. WO /ZrO ,
3 2
was suggested for elimination of FFAs. (2)

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