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Fuel Processing Technology 86 (2005) 1679–1693
www.elsevier.com/locate/fuproc
Utilization of macro-algae for enhanced CO2 fixation
and biofuels production: Development of a
computing software for an LCA study
Michele ArestaT, Angela Dibenedetto, Grazia Barberio
IAMC, Department of Chemistry and CIRCC, University of Bari, Campus Universitario, 70126 Bari, Italy
Abstract
A Life Cycle Assessment study was carried out for evaluating the potential of utilizing marine
biomass for energy production. Macro-algae obtained from the Adriatic and Jonian seas have been
selected and tested for our initial case. Different techniques (supercritical CO2, organic solvents, and
pyrolysis) were utilized in this study for the extraction of biofuel. Supercritical CO2 appears to be the
most effective. A computing software has been developed which allows to evaluate various options
and can be used with either aquatic or terrestrial biomass. It has been used in our studies to make an
energetic evaluation of selected marine macro-algae. The results of the energetic assessment are
presented here.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Macro-algae; CO2 fixation; Fuel production; Life cycle assessment (LCA)
0378-3820/$ -
doi:10.1016/j.
Abbreviati
MEA, monoe
SCFE, superc
energy associa
Esa, separatio
EdV, energy fo
supplied nutri
pretreatment; E
for processing
* Correspon
E-mail add
see front matter D 2005 Elsevier B.V. All rights reserved.
fuproc.2005.01.016
ons: LCA, life cycle assessment; LCI, life cycle inventory; scCO2, supercritical carbon dioxide;
thanolamine, HOCH2CH2NH2; SETAC, Society of Environmental Toxicology and Chemistry;
ritical fluid extraction; Enet, net energy; Eric, energy recovered as heat from the flue gases; Eb,
ted to the extracted biofuel; Ers, energy of the residual solid after extraction; Ets, transport energy;
n energy using MEA; Escr, separation energy using cryogeny; Ed, energy of distribution of CO2;
r the distribution of the flue gas; Ec, energy for algae cultivation; Enu, energy associated with
ents; Eh, energy for algae harvesting; Edr, energy for algae drying; Epr, energy for algae
sc, energy for extraction with scCO2; Eso, energy for extraction with organic solvents; Ees, energy
biofuel.
ding author. Tel.: +39 80 544 20 84; fax: +39 80 544 20 83.
ress: m.aresta@chimica.uniba.it (M. Aresta).
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–16931680
1. Introduction
Biomass, either terrestrial or aquatic, is considered a renewable energy source with
quasi zero-emission. Among alternative energy sources, biomass represents the most
ready to be implemented on a large scale without any environmental or economic
penalty. The photosynthetic efficiency of aquatic biomass results to be much higher (6–
8%, average) than that of terrestrial (1.8–2.2%, average). This makes the former more
adapt for an enhanced CO2 fixation to afford a high biomass production. Also, aquatic
biomass presents an easy adaptability to grow in different conditions, either in fresh- or
marine-waters, and in a wide enough range of pH [1]. The pond culture of algae
presents the advantage of assimilating carbon dioxide emitted by electric power plants
[2b,c,3,4], using wastewater that may supply the amount of required nutrients [5].
Either marine micro-algae or seaweed could be used for solar energy conversion and
biofuel production. Micro-algae have received so far more attention [6,7] with respect
to macro-algae [8,9] as agents for enhanced CO2 fixation due to their facile adaptability
to grow in ponds or bioreactors and the extended knowledge on several strains used for
fish feeding. Macro-algae are extensively grown and used as food in Asiatic Countries,
or as source of chemicals. They are usually collected from natural water basins where
they are seasonally available. Only recently they have been considered for energy
production, and the potential of some Pacific Ocean strains has been preliminarily
studied [8].
As extension of our studies on CO2 chemical utilization [2a], we have started a
Research Programme aimed at evaluating the potential of selected Mediterranean macro-
algae for biofuel production integrating the capture of CO2 from continuous point sources
(power plants, industries), its purification, distribution with uptake by macro-algae and
their processing. The Programme includes the macro-algae physiological characterization,
the definition of the best conditions for their growth in ponds, including the evaluation of
the resistance to CO2, NOx and SO2. In our study, the Life Cycle Assessment (LCA)
methodology has been applied to evaluate the energy production from macro-algal
biomass. We have developed a specific software (COMPUBIO) which can be used for an
energetic, environmental or economic evaluation. We discuss here its application to
estimate the energy balance for biofuel production from macro-algae, considering the
overall process from CO2 capture from power plants to algae production and conversion.
This study is aimed at establishing the feasibility of using seaweed as renewable-energy
source and enhanced CO2 utilization.
The flowchart includes the following steps: (1) flue gas (from power plants) recovery,
(2) CO2 separation, (3) transport of CO2 or of the whole flue gas, (4) distribution of gas,
(5) growing of algae, (6) collection of algae, (7) biofuel extraction from algae and
processing.
Although various separation technologies (monoethanolamine—MEA, physical
adsorption, cryogenics and membranes) are included into COMPUBIO, at this stage data
relevant to MEA populate the database. This technology is widely used for industrial
capture of CO2. It is based on the reversible uptake of CO2 by
2HOCH2CH2NH2 þ CO2VHOCH2CH2NHCOO�þH3NCH2CH2OH ð1Þ
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–1693 1681
HOCH2CH2NH2—MEA according to reaction (1). The CO2 uptake is favoured by high
pressure and low temperature (290–320 K, 1–5 MPa). CO2 can be released by lowering
the pressure (0.1 MPa) and increasing the temperature (400–430 K).
The transport of either pure CO2 or flue gas (whenever macro-algae are resistant to the
NOx and SO2 present in flue gas) is taken into account, and their distribution into algae
ponds. For the production of fuels from the marine biomass, several technologies such as
direct combustion, extraction with sc-CO2 or organic solvents, pyrolysis, gasification,
liquefaction, anaerobic fermentation are included in COMPUBIO and used as necessary.
These technologies are summarized below and the application of each of them is
highlighted.
Direct combustion (boiler and steam turbines) is conventionally adopted for producing
energy from biomass. Large biomass power generation systems have an efficiency
comparable with those of fossil fuel system, but the costs are higher because of the
moisture content of biomass [11].
Among biological processes, anaerobic digestion, which produces methane and CO2,
is best suited for high moisture-content herbaceous plants, marine crops and manure
[12].
Among thermochemical processes pyrolysis, gasification and direct liquefaction can be
used for all kinds of biomass, especially low-moisture herbaceous and woody.
Pyrolysis converts (750 K and 0.1–0.5 MPa, in absence of air) the dried biomass into
three phases: an oil-like liquid (bio-oil or biocrude), a carbon rich solid residue
(charcoal) and a hydrocarbon rich gas mixture. Changing the temperature or the heating
rate drives the reaction towards the promotion of charcoal, pyrolytic oil, gas or
methanol. Fast-flash pyrolysis (low temperature, high heating rate, and short gas
residence time) maximizes the yield of liquid products, while slow pyrolysis (low
temperature, low heating rate) increases char. Fuel gas is maximized running the
pyrolysis at high temperature, with low heating rate and long gas residence time.
Pyrolysis has been applied to many products: solid waste, wood, agriculture crops and,
more recently, aquatic biomass like micro-algae [13].
Gasification is a pyrolysis performed at high temperature in order to obtain only gas
(Syngas) as product. Commercial gasifiers are available in a range of size and are run on a
variety of fuels including wood, charcoal and agricultural waste. Recently, a low
temperature catalytic gasification of biomass with a high moisture content (also algae) has
been developed [14].
Liquefaction is a low temperature, high pressure thermochemical process which uses a
catalyst to produce a liquid product from wet materials. Liquefaction, which is more
expensive than pyrolysis, has been applied to recover liquid fuel from wet biomass and
micro-algal biomass [15].
Among extraction techniques, the solvent extraction by using Soxhlet involves a
repeated solvent distillation through a solid sample to remove the analyte of interest.
This technique is sometimes very slow and requires the use of organic solvents which
pose the problem of their disposal or recovery. A relatively new technique is the
supercritical fluid extraction (SCFE). We have used supercritical carbon dioxide
(scCO2) as extraction solvent. The scCO2 extraction is quite advantageous, as CO2 is
not toxic and its critical temperature is close to room temperature (304 K), and is
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–16931682
suitable for the extraction of thermolabile compounds. Because of the low permeability
and resistance of the cell membrane of algae, the efficacy of the scCO2 extraction
depends on the strain used. Some of them require a pretreatment consisting on milling
the algae for an efficient extraction of chemicals, biofuel, or lipids. Only scCO2 can be
used for extraction or a co-solvent (methanol) can be added (373 K and 8 MPa)
[9a,b,c]. In the former case, a dry residual mass is obtained which can be used for
other purposes.
COMPUBIO allows to assess the economic cost of CO2 utilization, the energetics of
fuel production and to quantify the reduction of the atmospheric CO2 loading
associated to the enhanced production of macro-algae as a source of biofuel. The
software is of more general applicability and, by substituting some computing-blocks,
could be used for assessing the potential of any kind of biomass, as renewable energy
source.
2. Life cycle assessment
The LCA methodology has been applied to compare several options for the enhanced
fixation of CO2 for the production of algal biomass, using different kinds of algae (micro-
and macro-) and various conversion technologies in order to select the process with best
environmental, economic and energetic performance.
2.1. Methodology
The SETAC (Society of Environmental Toxicology and Chemistry) guidelines have
been used [10]. SETAC defines four phases for an LCA assessment: goal definition and
scoping, inventory, impact assessment, interpretation (improvement) analysis. The LCA
tool is under further development and improvement for including all kinds of biomass and
possible extraction technologies. The life cycle considered includes the following steps:
capture of CO2 from flue gases generated in power plants; transport of the whole flue gas
or separated CO2; distribution of flue gas/CO2; algae production; algae harvesting; algae
conversion to produce biofuel.
2.2. Goal definition and scoping
Our study is aimed at assessing the potential of algal biomass as biofuel source and to
evaluate the economic, energetic and environmental convenience. Therefore, the goal of
the study is to establish the energetic benefits associated to the enhanced CO2 fixation in
macro-algae, considering as boundary of the system the energy input–output. The
functional unit has been fixed at 1 MJ of energy produced from algae. Our study allows a
comparative evaluation of different production and conversion technologies using various
algal strains with extension to other different kinds of biomass.
The following assumptions have been made: the algae cultivation pond is situated at the
seaside, preferably associated to a fishery; gas transport is in a range of 100 km; nutrients
can be supplied with recycled wastewater from the fishery or from another source at a
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–1693 1683
maximum distance of 5 km from the pond. The allocation close to a fishery avoids
environmental impact issues and the integrated technology allows space (sea/land) optimal
utilization. Moreover, the clean-up of water allows its recycle, with credits for the algae
production process.
2.3. Energy analysis
The flowchart traces the pathway along which the study will move; it contains
per each stage the matter and energy balance. The life cycle stages are presented in
Fig. 1.
2.3.1. Description of flowchart
The flowchart integrates all processes from CO2 capture to biofuel extraction. The
first stage is the CO2 emission from power plant and its capture. Power plants of
various size (100–600 MW) fed with various fuels (coal, oil, and natural gas) have
been considered so that COMPUBIO can be used with different production-systems.
Fig. 1. Flowchart of the analyzed system.
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–16931684
For CO2 distribution in ponds, two options have been considered: either direct flue-gas
injection, or CO2 separation–compression and transport.
The CO2 separation technologies included into COMPUBIO are: chemical absorption
(MEA), physical adsorption, cryogenic separation, use of membranes. At the moment
the software is populated with data relevant to the use of MEA.
Next stage is algae production (COMPUBIO can be used with either micro-algae or
macro-algae). The computing blocks include:
– Cultivation with temperature and irradiance control, aeration of ponds and stirring (only
for micro-algae).
– Fertilisation. Different options have been taken into consideration:
(i) nutrients supply,
(ii) nutrients recovery from wastewater effluents,
(iii) nutrients recycling from the wastewater solutions when gasification is used as the
algae-conversion technology.
The last two options make available an amount of nutrients sufficient for the algal
growth. Effluent water from aquaculture plants, some municipal wastewater or breeding
water can be used. Since the concentration of nutrients after algae growth is small enough
to allow the water discharge into natural basins, the nutrient recovery is important not only
for the internal energy/cost balance, but also for reducing the environmental impact of
water-flows from another anthropic activity (fisheries, for example), with a credit to the
algae-growing process.
– Harvesting of algae implies a quite different technology for micro- or macro-algae.
– Drying. It could be not required by some of the conversion technologies such as
anaerobic digestion or liquefaction. In this study, it has been assumed that the drying,
when necessary, is made by using solar energy.
– The final stage is the conversion of biomass. Different conversion technologies have
been included into COMPUBIO, spanning from the direct combustion to other
thermochemical or biochemical processes, making possible the selection of the best
option with the biomass under study.
2.3.2. Collection and treatment of data
Data relevant to alternative processes for algae treatment have been gathered, for a
comparison of technologies (not all the technologies have had so far an application to
aquatic biomass). In the inventory phase (Life Cycle Inventory, LCI), it is very important
to describe the inputs and outputs for each step and each process. In the course of this
study, we have verified that either data are not available for all processes or a discrepancy
may exist between sources. Data for biofuel extraction from macro-algae, using either
organic solvents or sc-CO2, have been collected through experiments carried out in our
research group. The collected data can be summarized as reported below:
– Data on CO2 emission and capture have been obtained from databases [16], relevant
research institutes [17,18] and scientific literature [19,20a,b,21].
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–1693 1685
– Data on algae production have been collected from agricultural research institutes
[22a,b,c], farmer associations [23], scientific literature [24,26–30] and personal
communication of Italian research institutes [25,31].
– Data on conversion of biomass have been provided by relevant industries [32], by
literature [33–39] and by experimental projects performed within our research group
[41].
During the inventory stage, it has been necessary to ascertain the data quality in
terms of geographical validity, time frame, and truthfulness of information. The
quality of the data in LCI strongly affects the reliability of the whole study.
2.3.3. Processing of the collected data
Usually the inventory process generates a long list of data which interpretation
may be difficult. In this study, the data are presented in Inventory Tables of energy
balance per each step of the production cycle. Calculations were carried out with
Excel using the data reported in Table 1. In addition, we have developed a more
complex computing model, using the software Visual BasicR 6.0 (see Figs. 2 and 3)
which requires the presence of a writing interface for the application of the
flowchart. After the compilation of the application in the bform codeQ mode using
Table 1
Relevant data collected for energy balance
Process Energy
consumption (MJ)
Energy
produced (MJ)
References
Gas transport Direct injection 0.0799a [20]
CO2 separation 1.672a [21]
CO2 distribution 0b
Algae production Cultivation 2.15c [15,22,34]
Nutrient supply 4.55c [35]
Harvesting 0.85d, 5.5e [34,31]
Drying 0f [this work]
Conversion technology Gasification 5.95d [35]
Pyrolysis 2.5g 15–20g [32,33]
Liquefaction 6.7–11.9g 35f [15,34]
Anaerobic digestion 2.66h [38,39]
Combustion 11.9d [35]
The emission of power plant is assumed to be 0.9159 kg CO2/MJ if fed with coal; 0.7557 kg CO2/MJ if fed with
oil; 0.5630 kg CO2/MJ if fed with natural gas.a Per kg CO2.b The energy consumption is zero as pressurized gas is used.c Per kg algae.d Per kg micro-algae.e Per kg macro-algae, referred to lagune harvesting.f Solar energy is used.g Per kg oil.h Per kg biomass.
a) b)
d)c)
Fig. 2. The case of micro-algae: external nutrient supply—(a) CO2 recovery; (b) direct flue gas injection; nutrient
recovery; (c) CO2 recovery; (d) direct flue gas injection.
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–16931686
the specific language, it is possible to run the programme in the bform objectQ mode.
With this model, it is possible to manage a large number of data making easy their
interpretation.
a) b)
c) d)
Fig. 3. The case of macro-algae: external nutrient supply—(a) CO2 recovery; (b) direct flue gas injection; nutrient
recovery; (c) CO2 recovery; (d) direct flue gas injection.
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–1693 1687
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–16931688
The final value is calculated according to the choices in the writing interface and,
changing the inputs, it is possible to compute the energy produced under different
conditions. COMPUBIO manages the following options:
– CO2 emission: it is possible to choose the size and the capacity of the power plant.
– Gas used: it is possible to switch from the direct injection of flue gas to CO2 separation,
compression and transport.
– CO2 distribution: two options of the ratio (CO2 fixed/amount of algae) are pre-set.
Anyway, it is possible to vary the excess of CO2 pumped in the outdoor ponds over a
wide range.
– Algae production: it is possible to select the type of algae (micro- or macro-) and the
mode of nutrients supply for their growth (external nutrient supply, nutrient recycled,
nutrient recovery from aquaculture, breeding and municipal wastewater).
– Conversion technology: data are available about the different conversion technologies
of algal biomass. Table 1, Figs. 2 and 3 show the data elaboration process in the
inventory analysis. Table 1 indicates the data collected for the energy balance. Here we
have reported the most relevant data for each phase object of the study.
Figs. 2 and 3 are examples of Visual BasicR 6.0 application to the calculation of net
energy produced by growing algae.
In Figs. 2 and 3 the micro-algae and macro-algae cases are presented, respectively. For
both cases, a 600 MW power plant fed with coal is considered. Either CO2 separated using
MEA (Figs. 2a and 3a) or flue gas is injected (Figs. 2b and 3b) in ponds; the ratio CO2
fixed/algae is 1:1 and the quantity pumped in ponds is in excess of five times; the
fertilisation occurs either through nutrient supply (Figs. 2a,b and 3a,b) or nutrient recovery
(Figs. 2c,d and 3c,d); the algae conversion process illustrated in the figures is the
gasification for micro-algae and the anaerobic digestion for macro-algae, respectively.
The calculated data (see Figs. 2a,b and 3a,b) show that the distribution of separated
CO2 is energetically more favourable than the distribution of flue gas, which is in line with
other studies [20b]. Also, if fresh nutrients have to be supplied, then the whole process will
hardly produce energy, most likely it will consume energy (compare data in Figs. 2 and 3,
a–c for CO2 capture and b–d for direct injection of flue gas).
3. Fuel extraction from macro-algae
In our experimental work we have selected some macro-algae typical of the Adriatic or
Jonian sea. In particular, the study has been carried out on two algae, Chaetomorpha linum
(O.F. Mqller) Kqtzing (Cladophorales, Cladophoraceae) and the Pterocladiella capillacea
(S.G. Gmelin) Santelices et Hommersand (Gelidiales, Rhodophyta). C. linum is dominant
species in the bentopleustophytic population of Mar Piccolo in Taranto (Jonian sea), Italy,
and very much present in the estuary of the Galeso river where it can reach a density of 3.6
kgdw/m2. P. capillacea, a very good agarophyte, can be found on the rocky substrates in
the South Adriatic sea close to Bari, Italy and can reach a density of 0.15 kgdw/m2. These
algae were never cultivated in vitro so far and, thus, we have ascertained the possibility of
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–1693 1689
growing them on a large scale in a pond. The effects of several parameters such as: the
dependence of the biomass production on nitrogen availability, ratio biomass/volume,
salinity, temperature and irradiance were considered [40]. The two species have been
selected because of their easy availability and low cost harvesting technology, their
presence and vegetation all the year long, their ability to grow on a large-scale pond and
high percent of compounds with a potential use as biodiesel [40]. Two extraction
technologies were used such as solvent extraction and scCO2. Particular attention was
dedicated to the preparation of samples, in order to ameliorate the efficiency of the
process, and to the characterization of the lipidic content.
The methodology for the extraction of biodiesel from algae using supercritical carbon
dioxide was developed. A qualitative quantitative comparison of the extracts with those
obtained using the organic solvent extraction was also carried out [40]. The scCO2 was
shown to be more efficient and less costly. In a SITEC apparatus operated in batch at 313–
323 K and 25–30 MPa, the extraction of oil from algae was carried out using either scCO2
alone or added with methanol (1 mL) as co-solvent. The equipment can be used in
continuous using only scCO2.
In order to have an efficient extraction, it was necessary to pretreat the algae. In fact, if
they were used as collected, no oil was extracted. Among the various techniques,
grounding of dried (at 308 K) algae in liquid nitrogen was the most effective. The very fine
solid obtained was extracted under the conditions specified above. Per each sample the
amount of extracted oil was determined per kg of dry matter. The oil content varied from
7% to 20%. Then the oil was analyzed by GC-MS and its composition was determined. All
extracted products were identified by GC-MS and the mass spectrum of each product was
compared with that of an authentic sample used as standard. This allowed to identify the
components of the oil and to calculate the heat content expressed as MJ/kg oil. Such value
was checked through a combustion test. Such studies have shown that the morphological
difference of the two algae is associated to a different lipid content, both quantitative and
qualitative. In fact, in C. linum methyl myristate, methyl palmitate, methyl linoleate and
methyl oleate are found, while P. capillacea contains besides methyl myristate and methyl
palmitate, methyl arachidonate and methyl-all-cis-5,8,11,14,17 eicosanpentaenoate as
major components. Studies are still in progress [41] for the complete characterization of
the extract from algae grown under different light and temperature conditions and with
different CO2 and nutrient supply.
4. Energetic balance
LCA studies on biofuel production from terrestrial biomass (rapeseed oil) have
already shown a convenient energy balance, despite the energy consumption in the
cultivation and processing stages, and much lower emissions than fossil fuels [42,43].
The energetic yield of micro-algae has also been reported [20]. Similar studies on
macro-algae are not available.
In this study, we have considered in particular macro-algae and developed a software
that may be used for several options. The global balance of the process of production of
biofuel takes into account all the energy inputs and the produced amounts of energy.
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–16931690
Separate calculation modules deal with the amount of energy relevant to the six sections
illustrated in Fig. 1. As reported in Eq. (2), the net energy (Enet) produced is the sum of a
number of terms relevant to the gained and used energy specified in the second member of
the equation.
Enet ¼ Ericð Þ þ Ebþ Ers� Ets� Esa Escrð Þ � Ed EdVð Þ � Ec� Enu� Eh
� Edr � Epr � Esc Esoð Þ � Ees ð2Þ
The first three terms of the second member of Eq. (2) represent the gained energy, while
others indicate the spent energy. The first term, Eric, is the energy that may be recovered as
heat from the hot flue gases emitted from a power plant. If cooled gases are considered,
this amount will be zero. Eb is the energy associated to the extracted biofuel and Ers is the
energy of the residual solid biomass (or tars). The other terms are, in general, relevant to
energy spent in the process. Following the flowchart in Fig. 1, the section bseparation and
transportQ includes the separation energy using MEA or cryogeny (in Eq. (2), Esa,
indicates the energy when MEA is used as separation phase, Escr is the energy of
separation by cryogeny). Ets is the transport energy. Section 3 of Fig. 1 is relevant to the
distribution: either separated CO2 or the entire flue gas are considered here. In Eq. (2), Ed
represents the energy of distribution of separated CO2 and EdV is the energy for the
distribution of the entire flue gas.
Section 4 in Fig. 1 is relevant to the production of algae. The algae cultivation energy
is indicated as Ec in Eq. (2). Algae need nutrients (N, P, microelements) to which the
energy Enu is associated. A variance analysis has shown that if nutrients are added as fresh
chemicals to the pond, the overall energetic balance may be negative. Therefore, in order
to avoid such huge amount of energy input, either effluent water from aquaculture plants
should be used, or some selected municipal waters. The treated water can be either re-
circulated to the fishpond or emitted into natural basins without paying any penalties. Such
use of effluent water, by the way, generates a credit to the process that may be ultimately
taken into account in its economic evaluation. In the phase of growing, macro-algae do not
need a vigorous stirring as micro-algae. This difference introduces a credit for the macro-
algae system when compared to micro-algae. The energy associated to algae harvesting is
Eh. Macro-algae grow either on a solid substrate or free-floating in water. In the former
case it is necessary to cut the algae, that slightly rises the energy consumption. With free-
floating algae, harvesting can be made by simply rising a net installed in the pond, with a
large energy saving with respect to micro-algae, which need filtration for their separation.
The following section in Fig. 1 is relevant to algae treatment for biofuel production.
According to the conversion process, either wet or dry algae can be used. Some
technologies as anaerobic fermentation or liquefaction directly use wet algae. Drying, if
required, is made by using solar energy or recovered heat and the associated energy is
reported in Eq. (2) as Edr. Also, dried algae may or may not need to be pretreated for
biofuel extraction. In case pretreatment is needed (see above), the associated energy is
Epr in Eq. (2).
Among biofuel extraction techniques, the extraction with scCO2 (Esc) or organic
solvents (Eso) has been considered here. Ees is the energy for processing the extracted
biofuel. The utilization of scCO2 as solvent for biofuel extraction appears quite interesting,
M. Aresta et al. / Fuel Processing Technology 86 (2005) 1679–1693 1691
as captured carbon dioxide can be used to this end, making the whole process solvent-free,
and avoiding, thus, the production of waste solvents.
The whole balance shows that energy production from marine biomass is an attainable
target with the available technologies. In general, the obtained biofuel is too expensive yet
if it is compared to fossil fuel without emission capture [20b]. Should benvironmental
costsQ be considered enhanced CO2 fixation into marine biomass as a technology for
carbon recycling and energy production may become an economic route to reducing the
CO2 emission [44].
5. Conclusions
In this paper we have presented the preliminary results of an ongoing study on the
utilization of macro-algae for enhanced CO2 fixation for the production of biofuel. We
have developed a software (COMPUBIO) that allows to calculate the net energy of the
process using an LCA approach. The LCA study performed in the present work
demonstrates that there is a potential energy benefit associated to recycling carbon by
enhanced fixation of CO2 by macro-algae, if it is associated with the use of effluent water
as source of nutrients. The net energy gain depends on the conversion technology. In the
best case considered so far, macro-algae can generate a net energy of the order of 11,000
MJ/tdryalgae compared to 9500 MJ/t relevant to micro-algae gasification. Besides the energy
balance that is encouraging, the economic balance and the emissions are under evaluation
for completing the study. The ensemble of energetic, economic and environmental
assessments may be useful for supporting political decisions. Being preliminary results on
the energy balance encouraging, the inventory of emissions associated to biofuel
production is being completed with their classification into impact categories to assess
the global environmental impact of the whole cycle of production of biofuel. The
versatility of COMPUBIO that can be applied to different types of biomass and can take
into consideration various technologies is useful for performing a comparative LCA
between fossil fuel and biofuel derived from different renewable sources.
Acknowledgements
This work was done with the financial support of MIUR-FIRB 2001, Project
RBAU017RWX.
References
[1] IEA Report, Carbon Dioxide Utilization: evaluation of specific biological processes which have the
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