Typical yields from algal biofuel technologies

This is an examination of yields of "primary product" (algal oil/ethanol/biodiesel), expressed as barrels of oil equivalent/hectare of land area/year, from various algal (algae) biofuel technologies. I used data from company websites and press releases and converted the algal oil/ethanol production to a BOE/ha/year basis.
From the above figure, typical "yields" range from 100-1000 BOE/ha/yr. Compare this to Dr. Benemann's recent statement that the maximum algal yield without using genetically modified algae would be ~2000 gal algal oil/acre/year (101 BOE/ha/year).

Disclaimer: This is not meant to be a comparison of various processes or an endorsement/critique of a specific process. Utilization of and the value for the algal biomass and the biofuel determines the overall process economics. My assumptions and data are given below:

1. Algenol: 6000 gal EtOH/acre/year
2. Solix data from here.
3. GreenFuel, from a previous post
4. PetroAlgae: Assumed 200x current soybean oil yields (200x50 gal oil/acre/year).
5. GSPI: Link here
6. Theoretical maximum: from CircleBio's website, assuming 20,000 gal biodiesel worthy plant oil/acre/year.
7. I further assumed that 1 T of algae oil gives 1 T of biodiesel, unless mentioned otherwise on the company's website (the ratio is ~0.96 for soy oil).
8. Calorific value of 33MJ/L for biodiesel and 20 MJ/L for ethanol.

Related posts:
Analysis: Algae for CO2 capture - II
Analysis: Algae for carbon dioxide (CO₂) capture

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Analysis: Algae for CO2 capture - II

I evaluate the process economics of algal CO2 capture from cement plant, using the GreenFuel Holcim facility mentioned in a previous post. Internal rates of return (IRR) and payback periods for various scenarios are presented. As shown in the above figure, both increased yields as well as higher oil prices significantly influence the economics of algal CO2 capture.

Base case
Capital expenditure: 92 million $, CO2 fixed: 50,000 T/year (2011).
Algal oil production: 1.3 million gal/year.
Cost algal oil: 4 $/gal.
Price of CO2 offsets: 20 $/T CO2.
Timeline considered for IRR calculations: 10 years.

The rest of the scenarios are explained in the figure. Doubling the yields (and CO2 captured) does increase the IRR and lower the payback periods more than doubling the oil prices (mentioned in my last post). Moreover, CO2 trading plays only a minor role by itself, but results in higher IRRs and lower payback periods when considered along with other possibilities. The highest IRR and lowest payback occur when both yields as well as the oil prices are significantly higher than in the base case scenario.

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Comments: Marine cement production process

Recently, Calera proposed a process suitable for power plants and other major CO₂ emitters which converts CO₂ to calcium carbonate (CaCO₃). From the Scientific American:

"...The Calera process essentially mimics marine cement, which is produced by coral when making their shells and reefs, taking the calcium and magnesium in seawater and using it to form carbonates at normal temperatures and pressures. "We are turning CO2 into carbonic acid and then making carbonate," Constantz says. "All we need is water and pollution...
....Calera hopes to get over that hurdle quickly by first offering a blend of its carbon-storing cement and Portland cement, which would not initially store any extra greenhouse gases but would at least balance out the emissions from making the traditional mortar. "It's just a little better than carbon neutral," notes Constantz, who will make his case to the industry at large at the World of Concrete trade fair in February. "That alone is a huge step forward.".."

In the following, I discuss some of the critical challenges to make this a feasible operation. The roles of the abundance of calcium and magnesium in sea water and its pH are discussed in detail. The formation of calcium/magnesium carbonates from sea water requires energy to proceed. This energy could be supplied either in the form of a pH shift (by adding strong base) or by coupling the carbonate formation to other processes (algal photosynthesis). Why does blending calcium carbonate in cement make economic sense? The unit price for medium ground (4-9 um) CaCO₃ is around 95-100 $/T whereas portland cement retails at ~180 $/T . Each ton of CaCO₃ blended will save 80 $.

Sea water has 0.01 gram-mol Ca²⁺/kg-sea water and 0.05 gram-mol Mg²⁺/kg-sea water . The precipitation chemistry of calcium and magnesium carbonates is complicated, involving many minerals. If we precipitate all of the Ca²⁺ and Mg²⁺ as carbonates (CaCO₃ and MgCO₃), sequestering 1 T of CO₂ would require 360 T of sea water. Additionally, carbonate precipitation does not occur below a pH of ~10, whereas sea water has a pH of ~7.5-8.4 , which could be decreased by increasing the partial pressure of CO₂. Increasing the pH of the solution to favor carbonate precipitation likely requires the use of a base, such as sodium hydroxide, NaOH.

Apart from the large volumes of sea water to be handled to make carbonates, the manufacture of NaOH involves the electrolysis of brine and is fairly energy-intensive, producing 4.6 T CO₂/T NaOH. Depending on the extent of NaOH requirements, this could represent a significant source of CO₂ emissions. (Therefore, using NaOH to sequester CO₂ does not make much sense).

Switching gears to learning from nature, certain coralline algae form CaCO₃ from sea water. This CaCO₃ acts as a binder, stabilizing the coral reef. The mechanisms involved are fairly complex for my understanding, but an essential process is the equilibrium between the bicarbonate and carbonate ions, depending on the pH.

pH = 10.33 - log([HCO₃^-]/[CO₃^2-])

Under sunlight, algae photosynthesize dissolved CO₂ or HCO₃^- to form organic compounds (CH₂O). This leads to an increase in the local pH. An example of how this happens is given in a paper by John Dodson (subscription required). This increased pH combined with the presence of ion-selective membranes, which admit only certain ions might be how algae use photosynthesis to drive an endothermic CaCO₃ precipitation reaction.

Conclusions:

1. Large volumes of sea water are required to sequester CO₂ as calcium/magnesium carbonates. Membrane separations will likely play a critical role in reducing this requirement.
   
2. On the other hand, CO₂ sequestration in brines (by product of oil and gas production) also has been investigated. The advantage here is that these brines have high concentrations of calcium, iron and magnesium ions. Therefore, potentially less volumes of brine are needed compared to sea water to sequester the same amount of CO₂. The obvious problem here is to increase the pH of the solution to precipitate dissolved CO₂ as carbonate. Instead of using a strong base such as sodium hydroxide, folks at NETL used residues from bauxite (an ore of aluminium) processing to buffer the solution pH.
3. One needs a driver to precipitate calcium/magnesium carbonates from sea water. In the chemical process described above, it is the addition of sodium hydroxide.
4. The smarter way nature has found around that requirement is the formation of carbonates via the bicarbonate<->carbonate equilibrium, shifted towards the right by algal photosynthesis (a reaction converting bicarbonate/CO2 to organic carbon using sunlight, thereby increasing the pH).

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