Microalgae-based biofuel production has become a major focus of renewable energy development. Much of this research has centered on biodiesel created from harvested algae. In this paper, Luo et al. (2010) investigated the life cycle energy use and greenhouse emissions for a different method of microalgae-based biofuel production in which ethanol synthesized via intracellular photosynthetic pathways is collected directly from un-harvested cyanobacteria (blue-green algae). This method is advantageous in that it has lower energy costs and water usage than that of harvest-based microalgae biodiesel production. Luo et al. used already available information for this process to complete the necessary engineering calculations and determine life cycle impacts. They found that this extraction method has significantly lower (67–87% less) greenhouse gas (GHG) emissions than gasoline, and that the net life cycle energy consumption can be as low as 0.20 MJ/MJEtOH, indicating that this process has great promise as a biofuel production method that can meet regulatory requirements and decrease environmental impact.—Karen de Wolski
Luo D., Hu Z., Choi D., Thomas V., Realff M., Chance R., 2010. Life cycle energy and greenhouse gas emissions for an ethanol production process based on blue-green algae. Environmental Science Technology 44, 8670–8677.
Luo et al. completed a life cycle analysis of blue-green algae-based ethanol production by calculating projected energy usage and GHG emissions using approximate parameters. The ethanol-producing blue-green algae are genetically enhanced photoautotrophic cyanobacteria that are cultivated in CO2 and fertilizer supplemented seawater in closed photobioreactors (PBR). Because the microalgae require CO2, the PBRs could be located near a fossil-fuel power plant or industrial facility that outputs CO2. The calculations in this paper are done under this assumption. When mature, the algae produce ethanol that diffuses into the growth culture. Dilute ethanol-freshwater solution is extracted from the growth medium and purified to fuel grade (99.7% pure) through a series of separation processes. One of the primary determinants of energy usage for this process is initial concentration of ethanol, as higher starting concentrations require less energy for purification. The researchers used concentrations between 0.5% and 5% (weight percent) as parameters for their calculations because 0.5% is too dilute to be cost effective and 5% would likely allow for economical recovery. The life cycle analysis additionally accounts for production and disposal of the PBRs, mixing in the bioreactors, biomass disposal, fertilizer production and transportation, ethanol separation (vapor compression steam stripping, vapor compression distillation, and molecular sieve), ethanol transportation, and combustion.
Because Luo et al. did their calculations based on a range of initial ethanol concentrations, they had to apply ethanol separation processes that would be efficient for the entire range. Standard column distillation would be applicable to the upper end of the range, but is inefficient below 5%. Therefore, they used vapor compression steam stripping (VCSS) as the first separation method, which would concentrate the ethanol between 5% and 30%. For the second separation step, they used vapor compression distillation (VCD), which would result in a 94% ethanol concentration. For the final step (99.7% purification), the researchers chose molecular sieve dehydration. They used an equation accounting for mass flow rate and heat of evaporation to calculate energy required for vaporization in a steam-stripping column. Assuming an 80% efficiency rate for heat exchange and a 1% initial concentration, they found that the net heat input would be 0.18 MJ/MJEtOH. The stripper column requires electrically-powered steam compression, and the researchers estimated the work required for this process through an equation accounting for the gas constant, temperature, pressure, and adiabatic coefficient. For the 1% concentration reference case and 38% efficient electricity production, this value was estimated to be 0.11MJ/MJEtOH. For the molecular sieves stage of purification, heat requirement was found to be between 1 and 2 MJ/kgEtOH. The purification process accounts for the highest amount of energy consumption of this entire ethanol production method.
The energy usage of several other processes was additionally estimated. PBRs must be mixed to ensure uniformity of algae and nutrients, and the energy requirement was estimated to be about 0.056MJ/MJEtOH for a 1% initial ethanol concentration. Compressor-mediated oxygen removal was calculated to require 0.0001 MJ/MJEtOH. Water must be pumped into the system and sterilized, and 3 mol of water are needed per 1 mol of ethanol. The energy requirements of water pumping and sterilization were therefore estimated. Energy consumed during delivery of CO2 into the PBRs, fertilizer transportation and production, bioreactor production, and waste biomass disposal were additionally calculated.
Luo et al. also performed a GHG emissions life cycle analysis of this ethanol production process. Cellulosic renewable fuels must have less than 40% GHG emissions than petroleum-derived fuels according to the U.S. Energy Independence and Security Act. However, the act allows for funding for those fuels that have less than 20% of the greenhouse emissions of petroleum-derived fuel, making this figure the goal for biofuel development. The researchers considered grid electricity and on-site combined heat and power (CHP) from gas turbines as two possible electricity sources. It was estimated that grid electricity would produce 700 g CO2e/kWh, which would mean 13.5 CO2e/MJEtOH for on-site electricity alone. If on-site heat process heat was provided via natural gas, it would mean an additional 11 g CO2e/MJEtOH. The researchers found that CHP could greatly reduce GHG emissions in comparison to grid electricity due to increased energy efficiency. Emissions could be further reduced significantly if solar power were to be used for process heat supply.
The researchers conclude that the 20% goal can be reached through this process if initial ethanol concentrations are sufficiently high. For a grid electricity and natural gas powered system, this value would have to be 4–4.5%. For a natural gas CHP system, the value could be 1.0–1.2%. The figure decreases to 0.8% if solar heating is used at 80% heat exchange efficiency. This manner of ethanol production has many advantages, including the ability to locate facilities on nonagricultural land, and the relatively low energy, fertilizer, and water requirements in comparison to harvest-based microalgae biofuel production. If initial ethanol concentrations can be made reliably high, the life cycle net energy input and GHG emissions are low enough to meet new regulatory standards.