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Techno-economic analysis of biofuel from seaweed

November 27, 2017
AlgaeIndustryMagazine.com

Overview of the supply chain and biofuel production methods. Orange rectangles indicate the main steps, blue rectangles indicate the final products in each scenario, and yellow indicate the by-products in each scenario. The red dashed box indicates the system boundary.

In spite of valuable food and medical products 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. The potential profit of the seaweed-based renewable energy industry will hopefully be high enough to offset these high costs.

The cultivation of seaweed consists of four stages:

  1. collection and settlement of zoospores on seed strings
  2. production of seedlings
  3. transplantation and outgrowing of seedlings
  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. At present, nearshore farms are the most common, while offshore farming is often only experimental.

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.

Separation of costs by major component for 95 ML ethanol production annually and anaerobic digestion of residuals. Negative costs actually mean profit (e.g., digestate and electricity).

Mdpi.com has recently published a study evaluating the techno-economics of bioenergy production from macroalgae. 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 tons (dry basis), respectively.

Several suggestions for economic optimization of the seaweed bioenergy supply chain emerged from the study:

  1. 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.
  2. 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.
  3. 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).
  4. 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.

Changes in BESP (breakeven electricity selling price) ($/kW-h) and BFESP (breakeven fuel ethanol selling price, $/L, red axis) as a function of scale of seaweed production (million tons dry seaweed per year).

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.

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 tons 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).

Source: Soleymani, M.; Rosentrater, K.A. Techno-Economic Analysis of Biofuel Production from Macroalgae (Seaweed). Bioengineering 2017, 4, 92.

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