http://www.mdpi.com/2306-5354/4/4/92
[illustrations in on-line article]
Techno-Economic Analysis of Biofuel Production from Macroalgae (Seaweed)
Mohsen Soleymani 1
and Kurt A. Rosentrater 2,*
1
Department of Biosystems Engineering, Shahid Chamran University of
Ahvaz, Ahvaz 61357-8315, Iran
2
Department of Agricultural and Biosystems Engineering, Iowa State
University, 3327 Elings Hall, Ames, IA 50011, USA
*
Author to whom correspondence should be addressed.
Academic Editor: Daniel G. Bracewell
Received: 14 August 2017 / Revised: 21 November 2017 / Accepted: 22
November 2017 / Published: 26 November 2017
Abstract
A techno-economic evaluation of bioenergy production from macroalgae was
carried out in this study. Six different scenarios were examined for the
production of different energy products and by-products. Seaweed was
produced either via the longline method or the grid method. Final
products of these scenarios were either ethanol from fermentation, or
electricity from anaerobic digestion (AD). By-products were digestate
for AD, and animal feed, or electricity and digestate, for the
fermentation pathway. Bioenergy breakeven selling prices were
investigated according to the cost components and the feedstock supply
chain, while suggestions for potential optimization of costs were
provided. The lowest production level of dry seaweed to meet 0.93 ($/L)
for ethanol fuel and 0.07 $/kW-h for electricity was found to be 0.68
and 3.7 million tonnes (dry basis), respectively. At the moment, biofuel
production from seaweed has been determined not to be economically
feasible, but achieving economic production may be possible by lowering
production costs and increasing the area under cultivation.
1. Introduction
Since 1900 the amount of global carbon emissions has increased, due to
increasing use of fossil fuels for transportation, industry, and private
enterprises. Additionally, the rate of emissions has increased in recent
decades; emissions increased by over 16 times between 1900 and 2008, and
about 1.5 times during 1990–2008 alone [1]. Environmental challenges of
fossil fuel use, along with other issues, such as dynamic swings in
crude oil prices and challenges in energy security, to name a few, have
made the replacement of these environmentally harmful and unsustainable
fuels by renewable and sustainable alternatives necessary [2,3].
Bioethanol, which is considered a renewable energy source, potentially
can reduce transportation emissions in addition to replacing a portion
of the petroleum-based fuel supply [4,5], even though its current
production is not enough to meet all of current fuel demand. The present
substrates for bioethanol production (predominantly corn and sugarcane)
compete directly with human foods by using arable lands, water,
fertilizer, and other resources, and arguably may have negative effects
on food prices [5,6]. Therefore, much attention is now focused on
producing biofuels from lignocellulosic biomass, agricultural wastes,
and other biological materials. Although these feedstocks do not compete
directly with human food resources, they can compete indirectly if they
are cultivated in available arable lands [7]. Also because cellulosic
biomass has high lignin content, its conversion into biofuels can be
difficult and cost-prohibitive [5].
Since algae grow in marine waters [5,8], algal biofuels, which are
considered “third-generation biofuels” [2], may help change the food vs.
fuel argument. Yes, it is true that large-scale adoption of this
approach can potentially have negative effects, but it can allow highly
productive land to be used for food production as opposed to crops for
biofuels. Macroalgae, or seaweed, has no lignin but high moisture
(70–90%) and ash (21.5–33.4%) levels [3,9]. Low lignin in macroalgae
makes it well matched to biogas production in anaerobic digesters [10].
On the other hand, easily fermentable carbohydrates, including laminarin
and mannitol, especially in brown macroalgae, are suitable for
bioethanol conversion [5,8,11].
In spite of valuable food and medical products [12,13], which are
produced from seaweeds, their profitability as energy crops has not yet
been commercially confirmed. Seaweed cultivation can be very labor
intensive, and also can require expensive equipment [5,14]. The
potential profit of the seaweed-based renewable energy industry will
hopefully be high enough to offset these high costs [15]. It may be
possible to achieve this level of profitability, but only by increasing
the efficiency and scale of current production [8].
Cultivation costs will vary according to the geographical origin,
cultivation methods, cultivation scale, yield per unit area,
technologies used, transportation methods, and various pretreatment
operations [8,10,16,17,18]. For example, the net profit for a farmer
with a four-person family in 2012 in Mexico and Indonesia was only just
higher than the International Poverty Line [18]. [19] suggested that to
have a profitable seaweed farm, the products should be sold at higher
price (for example €2/kg wet basis), or the farm should be expanded by
production of other valuable products (e.g., scallops) in order to have
supplemental income.
[8] conducted a study to compare the cost of production of ethanol,
methane (then converted to gasoline via syngas and methanol), and
biodiesel derived from seaweed. This study found that fuel production
from seaweed was not economically feasible unless you considered the
production of valuable by-products such as alginates, mannitol, and
iodine, which could help offset the production costs.
The breakeven selling price for electricity generated from seaweed has
been estimated at around €120/MW-h ($154 if €1 = $1.28) [10]. This price
may be acceptable compared with some other renewable energy prices, such
as solar thermal ($251/MW-h), solar photovoltaic (157 $/MW-h), and
biomass-generated electricity (120.2 $/MW-h) [20].
Nonetheless, economic studies of biofuel production from seaweed are few
in number. Several, including [21], have however investigated the costs
to produce seaweed. Published papers which have examined the use of
seaweed to produce biofuels and/or bioenergy include [22] as well as
[23]. Relatedly, [24] assessed the costs for production of biofuels from
microalgae, not seaweed.
Consequently, due to the dearth of published studies, the economic
investigation of this emerging energy resource is necessary. Thus the
aim of this study was to investigate different methods of seaweed
cultivation and conversion into bioenergy, to determine the most
economical combination of these methods, and to determine the minimum
scale of economical seaweed cultivation.
2. Methods
2.1. Data Sources, Calculation Methods, Scenarios, and Cost Analysis
All economic analyses in this study were conducted using US dollars ($).
Any economic data found in other currencies (i.e., Euros) were converted
prior to analyses.
Six different scenarios (Table 1) were simulated for the production of
different energy products and by-products. The system boundary of the
production system that we analyzed is illustrated in Figure 1. Seaweed
was produced either via the longline method or the grid method. Final
products of these scenarios were either ethanol from fermentation, or
electricity from anaerobic digestion (AD) (which could be followed by an
integrated Combined Heat and Power (CHP) system). By-products were
digestate for AD, and animal feed, or electricity and digestate, for
fermentation. Figure 1 illustrates these scenarios.
This system begins with seaweed production, including hatchery and grow
out farms. Longlines and continuous culture grid units were the two
methods considered for seaweed biomass production in a typical offshore
farm. Mature seaweeds, after the growing season, are harvested by boats
and transported by barges or boats to the shoreline. To have a
continuous supply in the industrial portion of the supply chain,
seaweeds should be shelf stable for a long potential storage time.
Therefore, to prevent spoilage and assure an appropriate shelf life,
harvested seaweed must be dried to under 10% moisture content. For many
food and feed products, recommended moistures are less than 10%, in
fact. Moreover, dry seaweed requires lower space and fuel consumption
for transport than wet seaweed. In this study, it is assumed that all
land transportation is carried out by trucks.
To conduct a comprehensive techno-economic analysis, all capital and
operational costs were determined at multiple production scales (0–2
million dry tonnes of seaweed per annum) for the supply chain
illustrated in Figure 1. All equipment and operational data were taken
from published literature. The lifetime of all equipment was considered
to be 10 years. Equipment costs were assumed to be constant worldwide.
The based currencies were converted to US dollars, based on average
conversion rates in the original year and then all costs were converted
to US dollar in 2013 according to the inflation rate between the
original year and 2013. The original costs of small capacity equipment
and industries in literature were converted to costs of 95 ML capacity,
using a scaling equation (Equation (1)) [25]:
New cost=Original cost (New size(capacity)Original size (capacity))0.6
Annual fuel ethanol production rate was considered to be 95 ML (95%
ethanol and 5% gasoline, volumetrically) based on [25]. The annual
requirement of fresh and dry seaweed production was calculated according
to the ethanol production rate (75 kg ethanol per 1 ton dry seaweed,
[4], and moisture content of fresh seaweed (85%, mass based on [19]).
Energy (electricity, heat, and fuel) costs were based on USA average
prices in 2013. Labor cost was considered according to the average labor
earnings in USA in 2013.
Marketing prices for animal feed and digestate were considered to be 590
and 9.75 $/t [10], respectively. Also, the marketing price of
electricity as a by-product was considered to be 70 $/MW-h [20].
In terms of techno-economic analysis, we determined breakeven prices.
The breakeven selling prices for electricity and fuel ethanol as final
products were calculated using Equations (2) and (3), respectively, as
follows:
BESP=∑ni=1Ci−∑ni=1RiQ
where the BESP is the breakeven electricity-selling price ($/kW-h),
BFESP is the breakeven fuel ethanol selling price ($/L), Ci is the cost
of ith step, Ri
is the revenue of ith by-product, and Q is the quantity of produced
electricity (kW-h) or ethanol (L).
2.2. Hatchery and Grow out Systems
The cultivation of seaweed consists of four stages [26]: (1) collection
and settlement of zoospores on seed strings; (2) production of
seedlings; (3) transplantation and outgrowing of seedlings; and (4)
harvesting. The hatchery provides a protected area for young seedlings
and facilities to establish grow out arrays before transferring to the
main farm. Seaweeds can be cultivated in offshore/nearshore coastal
farms as well as land-based ponds. Pond culture requires high investment
and currently is used for specialty markets, and generally with
integration and production of other aquatic products [8]. At present,
nearshore farms are the most common, while offshore farming is often
only experimental [14]. Offshore farming was considered in this study
due to the potential of this method for large-scale farms [19].
Technical and economic data (capital, electricity, fuel, labor,
consumables, etc.) of hatchery and grow out farm were taken from [19].
It was assumed that harvesting vessels and barges or boats to transfer
harvested seaweed to the shore must be hired, similar to [19]. Thus the
harvesting costs included the leasing cost of boats and barges, as well
as labor and fuel consumption.
2.3. Drying Systems
The harvesting season, especially in cold regions with short growing
seasons, is often too short. So the large volume of harvested seaweeds
must be stored to continuously feed the ethanol process equipment.
However, the high moisture content is an obstacle to safe and effective
storage. Chemical treatments such as adding formalin or other additives
(for fresh storage) have a negative effects on fermentation yields.
Therefore, we assumed that the seaweed must be dried to achieve moisture
content below 22% suitable for long-term storage [5]. On the other hand,
dry material transportation is more efficient than wet, from both the
energy and cost point of view.
The heat energy needed to dry seaweed was obtained from Equation (4):
H=WRHR×(MCi−MCo)
(4)
where H is the total heat required to dry one tonne of wet seaweed (MJ),
WRHR is the seaweed water removal heat requirement (4000 MJ/t, [5]), MCi
is the seaweed initial moisture content (85%, Kg/Kg), and MCo
, is the seaweed final moisture content (22%, Kg/Kg).
The costs of the drying operation, in addition to heat, were the costs
of labor and the dryer facilities. The capital price of one typical
3-layer dryer with a thermal capacity of 1 t/h, was 60,000 $. This cost
was converted to the cost at the desired scale using Equation (1).
2.4. Transportation Systems
To avoid additional costs and energy required to dry and transport the
seaweed to the conversion system, the optimal situation would be to
establish all drying, energy conversion, and by-product processing
facilities integrated together, near the shorelines. It was assumed that
the drying equipment is installed near the shore, so that the harvested
seaweeds are delivered directly into the dryer equipment. Dried seaweed
was then transferred by trucks to the conversion plants. Transportation
costs for a 25 tonne truck were 2.6, 1.45, and 1.27 $/km for a 40, 160,
and 320 km transportation radius, respectively [27]. The average
distance of transportation between the dryer and final product
conversion equipment was considered to be 40 km (25 miles). Also it was
assumed that the labor demand for the transportation and drying steps
were equal to the labor demand in the ethanol plant [25].
2.5. Conversion Systems
After delivery, two energy conversion methods were considered, as follows:
AD (anaerobic digestion) integrated with the CHP system: Biogas
produced in an AD is burned in a CHP system to produce electricity. The
waste product (digestate) from AD was used as fertilizer.
Ethanol production through fermentation: Ethanol is the main
product in this method, and fermentation by-products are used as animal
feed, digestate, or electricity production, based on the selected
process method. Fermentation residuals can be converted into animal feed
or can be digested to produce biogas and thus electricity. Specifically,
the by-products in this method were animal feed or electricity and
digestate as bio fertilizer, or the combination of these three products.
According to [5], the rate of animal feed per liter of ethanol
production is 1.21 kg. The amount of digestate production in residual
fermentation followed by AD was equal to the amount of fresh seaweed
fermentation in AD, but electricity production was reduced to 64%
compared to scenario 1 (based on [10]).
2.5.1. Fermentation
Potentially, the production of liquid biofuels from brown algae is high,
due to the unique content of laminarin, mannitol, and alginate [8,16].
These structural polysaccharides and sugar alcohols should be broken
down into their fundamental monomers before fermentation [14].
Saccharomyces cerevisiae, Zymomonas mobilis, glucanases, mannitol
dehydrogenize, laminarinase, and cellulase are relatively common
microorganisms and enzymes which are used for industrial fermentations
[3,11,28,29,30]. To date, seaweed-based ethanol has been produced only
on an experimental scale, so data for these processes must be estimated
for large scale [5]. We assumed that the process of ethanol production
from seaweeds may be similar to the process for corn ethanol conversion
[5]. Therefore, with few exceptions, data of these processes, including
energy and labor demand, equipment, by-products processing, and so on,
were taken from [25].
2.5.2. Anaerobic Digestion (AD)
Because of the typically low lignin content in macroalgae, it may be
suitable for production of biogas in an anaerobic digester [10,14,31].
The overall conversion efficiency could be improved by integrating the
methane production system with a CHP unit [10]. Therefore, it was
assumed that the AD was integrated with a CHP unit. The inputs for
anaerobic digestion, in addition to seaweed slurry (seaweed + water),
included electricity (mainly for pumping) and heat (to heat the slurry
from ambient temperature to the desired temperature). Electricity was
supplied by the output electricity of gas engines. Recovered heat from
the gas engine was more than the AD requirement [4,10]. However, because
of the variability of AD requirements in different locations and
seasons, it was assumed that all the produced heat was used to fulfill
the AD requirement. Therefore the outputs of AD with CHP were
electricity and digestate (as fertilizer). The economic data for AD and
CHP were taken from [4] and [10]. Labor was assumed to be the same as
for the ethanol plant [25].
2.6. Techno-Economic Analysis
All capital and operational costs were accounted for at multiple scales
(up to 1.8 million tonnes of seaweed). The economic model was built
using MS Excel, and six scenarios were examined using this
spreadsheet—as depicted in Table 1. Breakeven sales prices for both
electricity and ethanol were determined using this model, and will be
discussed below.
3. Results and Discussion
3.1. Breakeven Price
Hatchery, drying and transportation methods were the same in all
scenarios. However, the different combinations of cultivation methods
(grid or longline), energy conversion methods (fermentation or AD) and
by-product processing (animal feed, electricity, or digestate) created
six different scenarios for analysis. Table 1 shows the breakeven price
for the various scenarios in this study. The best result for ethanol
resulted from ethanol produced via fermentation followed by anaerobic
digestion of the residuals (1.55 ($/L), in scenario 3). However, the
production costs in this scenario were partially compensated for by
sales of the anaerobic digestion products (electricity and digestate);
BFESP was about three times higher than 0.58 $/L in the study of [8].
One reason for this was the high cultivation costs (98 $/t dry) in the
current study compared to that of [8] (25 ($/t dry)). Also the ethanol
conversion rate in that study was very high (254 kg compared to 75 kg
per one dry tonne of seaweed). The breakeven price of electricity
produced via CHP was approximately 0.23 $/kW-h. This price is about
3-fold more than the electricity price in the market (0.07 $/kW-h) in
the USA in 2013 [20], and it is 1.4 times more than 0.16 $/kW-h, which
was obtained by [10]. Additionally, one of the most important issues
which caused this difference was due to the different labor costs in the
UK and USA. In addition to different cost components, the scale of
production has a significant effect on the final product selling price.
There was not much difference between grid and longline cultivation
methods. However, the longline one is often preferred due to the higher
productivity of this method (35 t/ha vs. 18 t/ha (wet basis)).
Furthermore, Figure 2 shows the embedded cost components for scenario 3
for 95 ML ethanol production annually. It is clear in this figure that
labor and energy (electricity, fuel, heat) are the most dominant cost
components for ethanol production. And, as denoted by negative costs,
sales of digestate and electricity actually are a result of product
sales. Therefore, it appears that ethanol production from seaweed may be
more cost efficient in the countries that have low energy prices and/or
low labor cost. The share of labor is more prominent than energy,
because firstly labor has the highest cost of all cost components, and
secondly, part of the share of higher energy cost can be compensated for
by the higher electricity selling price as a by-product. So it is
recommended that energy conversion technologies and equipment could be
established in countries such as China, Korea, and Indonesia where the
labor cost is low but also seaweed cultivation experience is high.
In all scenarios (Table 1), it was assumed that all residuals of
fermentation could be used as animal feed or digest in AD to produce
biogas. Another alternative could be that a portion of residuals be uses
as animal feed while the rest could be used as digest for AD. Table 2
shows the BFESP when this approach would be implemented. BFESP in all
cases was lower than 2 $/L. Because, the by-products produced in AD
(electricity and fertilizer) are more valuable than animal feed, the
BFESP decreases when the portion of fermentation residuals allocated to
digest in AD increases. On the other hand, in situations when the value
of animal feed compared to electricity and digestate increases, larger
amounts could be allocated to animal feed.
3.2. Economic Analysis of the Production (Supply) Chain
Some suggestions for economic optimization of the seaweed bioenergy
supply chain are as follows:
Establish processing facilities and equipment in the closest
location to the beach/water as possible; this will minimize the cost of
transportation. Also, some of the seaweed can be consumed in fresh form
in AD or fermentation (i.e., during the harvest season) without the need
to dry and store the seaweed. Taking into account no transportation
between the shoreline and the conversion equipment, and use of 25% of
fresh seaweed, the BFESP and BESP can be reduced to approximately 1.17
($/L) and 0.23 ($/kW-h), respectively.
Reduce production costs. As shown in Figure 2, the most dominant
costs in the production chain are labor and energy inputs. So, with
better management of cost components, the BESP and BFESP can be reduced.
Considering the previous suggestion (establishment of integrated
facilities near the shore) and by decreasing the labor cost by 20 and 30
percent, the BFESP can be decreased to 1.02 and 0.95 ($/L),
respectively, and also BESP can be reduced to 0.16 ($/kW-h) and 0.15
($/kW-h), respectively.
Increase productivity per unit area. The seaweed production yield
in this study was only 5.25 and 2.7 (dry t/ha), respectively, for
longline and grid farms; however, the average global yield of seaweed
can range from 12 to 60 (dry t/ha) [17].
Extend the production scale. As shown in Figure 3, by increasing
the production scale, costs can be pro-rated, and BESP and BFESP will be
decreased.
3.3. Effect of Scale on Overall Cost
Figure 3 shows how these optimization procedures for BFESP in scenario 3
and BESP in scenario 5 (the lowest breakeven prices amongst all
scenarios) (based on suggestions mentioned above), decline as a function
of scale of seaweed production. It is clear in this figure that by
increasing the production quantity, costs will exponentially decline,
and BFESP and BESP will decrease. The required level to produce
bioenergy from seaweed depends on the BFESP and BESP values. Currently,
the marketing price of gasoline in the USA is 0.93 ($/L) [20]. To obtain
this price level via seaweed, the annual production of seaweed must be
5.7 million tonnes (dry basis). If, for example, 20% of the cost could
be subsidized by government policy (e.g., because of environmental
benefits of seaweed derived bioethanol), the required level of
production could be reduced to just 3.8 million tonnes (dry basis). This
level can be further reduced by some of the approaches explained above.
With regard to BFESP of 0.93 ($/L), the optimal level of production has
been determined to be 1.44, 1.11 and 0.97 million tonnes annually for
the optimized procedure, with 20% and 30% lower labor costs,
respectively. Also by considering subsidized options, this level can be
reduced to 1.0, 0.8, and 0.68 million tonnes for these options,
respectively.
The end use price of electricity for the industrial sector in the USA is
approximately 0.07 ($/kW-h) [20,32]. To achieve this price level by
seaweed, approximately 16.6 million tonnes (dry basis) seaweed must be
used. If it is assumed that 20% of the cost can be subsidized by
government policy, the optimal level will be reduced to 10.6 million
tonnes (dry basis). And, by adapting various management solutions, to
achieve BESP of 0.07 ($/kW-h), the required seaweed level will be
reduced to 8.9, 6.6, and 5.7 million tonnes (dry basis) annually for the
optimized procedures, with 20% and 30% lower labor costs, respectively.
Also, by considering options with subsidies, this level can be reduced
to 5.7, 4.3, and 3.7 million tonnes annually, respectively.
The economically feasible level of seaweed production to produce ethanol
is much lower than that to produce electricity. The principal reason is
that the management of by-products in ethanol production (at least as
assumed in this study) resulted in higher economic values than for those
in electricity production—it was assumed that residuals from ethanol
production were digested to produce fertilizer and electricity, which
were more valuable than animal feed.
4. Conclusions
Currently, the economical production of bioenergy from seaweed is not
possible. However, by better management practices, such as reducing
various cost components (especially labor) or improving the productivity
in each stage of the seaweed supply chain, it may be possible to achieve
a rational production cost. With the current situation, and applying the
suggestions mentioned in this study for cost reductions, the minimum
production of seaweed to have economically sustainable biofuel
production was determined to be 680,000 dry tonnes annually. To have
this quantity of production 129,500 ha needs to be cultivated. The cost
of ethanol production at this scale was 0.93 ($/L).
Author Contributions
Kurt Rosentrater conceived and designed the experiments; Mohsen
Soleymani built the computer model and performed the calculations; Kurt
Rosentrater and Mohsen Soleymani analyzed the data; Mohsen Soleymani
wrote the paper; while Kurt Rosentrater edited and revised the paper.
Conflicts of Interest
The authors declare no conflict of interest.
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