The cost of implementing algae-based CO2 capture systems will be the major driving force behind changes, and for algae to be useful in this context, they must be price competitive with relatively conventional, non‐biological forms of carbon capture.
The costs of conventional CCS are about $30-50 per T of CO2 captured, but it is not entirely clear if the conventional methods are sustainable, especially from technological and societal perspectives.
Cost Components of Algae-based CO2 Capture for Biodiesel and co-product production
Algae-based CO2 capture (and partial sequestration) costs much higher than conventional CCS, but this process is more sustainable as it also produces products that can be monetized. Algae based CO2 capture might even present a profitable business proposition in future as the price of oil keeps rising.
For non-biological, conventional methods of CCS, predictions show the costs, in terms of energy penalties, can be approximately 13.5% for separation of CO2 and another 9% for compression (Herzog, 2009). Transport and storage costs are small in comparison (McCoy and Rubin, 2009). In economic terms, values in the region of $52 per ton of CO2 for capture and an additional $10 per ton of CO2 for transport and storage have been calculated (Hamilton et al., 2009).
In comparison, some studies done on costs of algae production using CO2 from power plants report that these could potentially be $250 per ton of CO2 in a photobioreactor system (Chisti, 2007) and $55 per ton of CO2 in a raceway pond system (Stepan et al, 2002). However the energy penalty associated with algae as a CCS technology would likely be zero or negative due to the production of large amounts of biomass that can be used as fuel.
- Cost for cultivation- Cost of nutrients- Cost of harvesting and extraction- Cost of conversion of extracted oil to biodiesel and other by-products.
The total cost of algae fuel = costs for (cultivation + nutrients + harvesting + oil extraction + costs for conversion to biodiesel and other by-products). In addition to biodiesel, the other value added products such as ethanol and algal extract. Lipid, carbohydrate and protein could be produced for better revenues.
To make the algae-based carbon capture method a commercial reality by solving the biological, technical and economic challenges, the following parameters have to be taken into consideration:
• Energy efficient technologies
• Easy scalability.
• Maximum utilization of the algae biomass as co-products
• Using the most economical sources / tools / concepts at each stageCost Components for Algae Cultivation
Ponds and PBR
: There are two extensively used methods for algae cultivation - open ponds and photobioreactors (PBR). More details about the cultivation techniques from here - http://www.oilgae.com/algae/cult/op/op.html
Mixing: The capital cost for mixing involves the installation of paddle wheels. The operating cost for mixing, primarily involves electricity cost. Paddle wheel has been considered due to its less energy consumption. Moreover, paddle wheel offers advantages like optimal flow rate, proper mixing and high productivity. For calculating the mixing costs, the lowest speed for the paddle wheel at which the productivity is high should be considered because the power consumption increases as a cube of speed of rotation of the paddle wheel.
Carbon Capture, Transportation and Storage: The cost for capture, transportation and carbonation is computed taking into account the installations and operations required for flue gas capture and processing. Cost for transportation should be computed for a distance of 100 km using pipeline with 20 inches diameters. The cost for carbonation includes specialized components such as sumps for controlled release of CO2 so as to ensure efficient capture of released CO2 by algae. Storage cost has been computed for specialized storage tank to store the CO2 during night time.
Nutrients: The cost for nutrient supply involves expenses towards supplementing algal cultures with iron, phosphorus, nitrogen and potassium. The use of processed industrial flue gas and sea water, offers the potential advantage of minimizing the expenses over nutrients.
Water: Negligible expenses are associated with usage of sea water, which has been considered in our model due to the advantage of availability and the presence of nutrients such as nitrogen and other trace metals required for algae growth.
Harvesting and Extraction : Expenses towards harvesting are computed considering the most economic, productive and energy efficient method.
Algae harvesting can be done using automated , semi-automated and manual harvesting techniques. Detailed description of each of the techniques can be obtained from here - http://www.oilgae.com/algae/har/har.html
Because the costs for drying algae will be prohibitive in the region of $500 per dry ton). As a result, some companies are working on direct extraction of algae biomass bypassing the drying step.
Direct extraction or Wet extraction basically involves subjecting the wet harvested algae into a extraction unit. Studies reveal that there will be a need of ultrasonication prior to wet extraction to homogenize the algae cells prior to solvent extraction. It must be noted however that the (homogenization + wet extraction) method is still evolving for oil extraction from wet biomass, and is not a completely proven technology.
Mechanical and chemical methods of algae biomass extraction are available here - http://www.oilgae.com/algae/oil/extract/extract.html
Transesterification : Depending on whether or not the algal oil has high FFA (free fatty acid content), transesterification costs can differ – oil with high FFA will require pretreatment before transesterification. For our baseline costing, we have considered pretreatment costs.
Ethanol Production: The marine microalgae that we have considered in the model are rich in carbohydrates, and hence we have used the traditional starch-to-ethanol process as the route for ethanol production while estimating costs for ethanol production.
High-value product from the left-over algae extract : The algae extract left over after oil and starch have been extracted is rich in proteins. This can be used as animal feed or as biofertilizer. We have assumed that the processing cost for this end product is negligible.Other cost components
: Labour costs vary from country to country. We have assumed that large scale algae cultivation that require significant labour will be feasible only when these are done in countries with low labour costs – typically developing or underdeveloped countries. We have assumed a labour cost of $2000 per year per person, with an estimate that the equivalent of one person will be required for 10 hectares for operational purposes.Biodiesel refining:
We have also considered the cost of refining the biodiesel resulting from transesterification. This process typically involves degumming and neutralization. Conclusion
Under conventional CCS methods, it costs about $30-50 to capture, transport and store 1 T of CO2. Under the algae-based carbon capture (and partial sequestration) route, while it costs much more to capture 1 T of CO2 (about $175 per T of CO2), under optimal conditions, not only are all the costs covered through the revenues generated from biodiesel and other by-products, the CO2 capture process might even provide an attractive business opportunity by actually being profitable. (Source: Oilgae, Feb, 2011)