utilization of macro-algae for enhanced co fixation and ...olli/b/aresta05.pdf · hoch 2ch 2nh...

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: [email protected] (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


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


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.


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].


– 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.


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.



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


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.


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,


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.

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