Microalgae can grow in waste or sea water and offer vastly superior biomass yields per hectare; furthermore, CO₂ removed from the atmosphere during photosynthetic growth of the plant offsets CO₂ released during fuel combustion.

Since it is most likely that within the next decades the share of transport fuels from energy intensive unconventional oil resources will increase, the production of advanced biofuels from microalgae can only be a viable renewable fuel source if the energy intensity of the process can be managed and lowered accordingly. The production of advanced biofuels from algae-sourced biomass is heavily dependent on direct and indirect energy inputs, and is not environmentally feasible at the moment.

—Shirvani et al.

For the study, the team determined the life cycle energy balance and GHG emissions of producing microalgae (Chlorella Vulgaris) biodiesel compared to fossil diesel. Outputs of the process were biodiesel and co-products oilcake and glycerol. They estimated the fossil energy consumed and GHG emissions released at all stages of the production cycle, including feedstock farming (algae cultivation in open raceway ponds); biomass harvesting and drying; algae oil extraction; feedstock conversion (transesterification of algae oil into biodiesel); fuel distribution; and combustion by end user.

The team analyzed the cumulative fossil fuel demand relative to the energy production associated with the algae-to-biodiesel fuel using the fossil Energy Balance Ratio (EBR): Total fossil energy input (MJ) divided by Total energy output (MJ).

They estimated a yield of 851 GJ/ha/year biodiesel and coproducts in the form of oilcake (689 GJ/ha/year) and glycerol (89 GJ/ha/year), based on the assumption of an initial 30% oil content (22.5 tons/ha/year).

A hypothetical baseline assuming that all energy is produced by fossil fuels and that biodiesel is the sole fuel product, with both glycerol and oilcake as waste materials, yielded an EBR of 3.22, comparing unfavorably with an EBR of 1.20 for conventional diesel from the United States.

They noted that an EBR of 1.2 for conventional diesel reflects the minimum fossil energy requirements for oil-based diesel production, and that the industry has become more energy intensive with unconventional oil resources from oil shale and tar sands already accounting for 12% of US crude oil production in the year 2000. The larger uptake of unconventional resources is reflected in a higher fossil EBR of 1.65 and GHG emissions around 182 g CO_2eq/MJ_fuel.

To highlight the significant improvement in carbon footprint levels that could be achieved through co-product utilization, decarbonization of the electricity and heat grid as well as all other indirect energy sources in each step of the process, the researchers created four hypothetical cases.

• Case 1 is based on algal fuel production, with oilcake and glycerol as waste material. Under this assumption, the generation of algae-derived biodiesel is highly unfavorable and increase GHG emissions regions where the carbon intensity of the electricity grid is high to around 450 gCO_2eq/MJ_fuel range.

• Case 2 is more closely aligned with current production practices; here the oilcake product is utilized in a combined heat-and-power unit (CHP). Co-product utilization results in emission levels competitive with fossil fuels, when operated on a fully decarbonized electricity grid. By reducing the consumption of grid heat and electricity from 1.48 to 0.78 MJ/MJ and 0.9 to 0.2 MJ/MJ, the fossil EBR of 3.22 is lowered to 1.7. However, the remaining high energy intensity of the algae biodiesel production cycle still leaves a high carbon footprint.

• Case 3 assumes a zero-carbon energy source, such as geothermal or solar; this enables the algae-to-biodiesel production cycle to become a relatively low-carbon energy source. GHG emissions are considerably lower than petroleum-derived diesel figures, but are still limited by the high carbon intensity of fertilizer, chemicals, machinery and transport requirements.

• Case 4 operates the algae-to-biodiesel fuel cycle with fully decarbonized direct and indirect energy in addition to a smart utilization of oilcake residues within the process, finally overcoming the high carbon dioxide intensity of algae biodiesel.

They also ran six LCA studies for the UK, France, Brazil, China, Nigeria and Saudi Arabia to determine a regional production preference, based on heat and electricity grid compositions from different primary energy sources.

GHG emissions (CO2eq/MJfuel) of algae biodiesel production including co-product utilization via three different processes in 6 different countries. Data: Shirvani et al. Click to enlarge.

We conclude that the energy intensity of the production process puts a large caveat on the financial and environmental feasibility of current algae biodiesel production. The production cycle’s economic running costs can be partially lowered by displacing costly grid heat and electricity through the smart usage of oilcake residues via a CHP [combined heat and power] unit and the utilisation of glycerol as livestock feed. The carbon footprint of the algae-to-biodiesel carbon cycle can only be minimized through the successful decarbonisation of the heat and electricity grid and the sourcing of all indirect energy requirements for fertilizers, transport and building materials, from low-carbon energy sources.

As a priority, countries will need to defossilize primary energy sources used by their electricity grids, as only then can the transport sector move towards low GHG emissions. We have shown that countries such as China operating on a carbon-based electricity and heat grid, would eliminate the inherent environmental advantages of algal biodiesel, while Brazil and France which essentially operate on defossilized electricity grids, have the potential for biodiesel from algae to be a viable alternative to conventional diesel.

—Shirvani et al.

Resources

• Tara Shirvani, Xiaoyu Yan, Oliver R. Inderwildi, Peter P. Edwards and David A. King (2011) Life cycle energy and greenhouse gas analysis for algae-derived biodiesel. Energy Environ. Sci., doi: 10.1039/C1EE01791H