Global Warming Potential 87

GWP results from petroleum diesels and biodiesels (soybean, canola and microalgae) are presented in Figure 13. The life-cycle GWP of microalgal diesel in this study is 60%-100% higher than in other microalgal diesel studies due to the differences in system boundaries and inventories, as described in Table 7. The microalgal GWP is also higher than other studies’ on conventional diesel and soybean diesel results by approximately 90% and 98%, respectively.

Based on the results of this study, soybean diesel, as a conventional biofuel, meets the RFS2 life-cycle GHG emissions reduction threshold, which equals a 50% reduction relative to the life-cycle GHG (90 g CO2 eq/ MJ baseline) emissions from the petroleum diesel produced

and distributed in 2005, as mandated by EISA (Assessment and Standard Division Office of Transportation and Air Quality, 2010). Canola diesel, a qualified biomass-based diesel and advanced diesel as specified in RFS2, contributes a GWP slightly lower (~8%) than the mean value of the GWP of microalgal diesel of this study. However, all of the results from potential biomass-based diesels – microalgal and canola diesels – exceed the life-cycle GHG emissions reduction threshold.

diesel. Moreover, the production of soybean diesel does not require energy intensive processes such as harvesting, dewatering and drying as microalgae diesel (Uduman et al., 2010).

The harvesting, dewatering, and drying processes are the primary areas where the greatest environmental improvements to microalgal diesel production can be realized. Approximately 80% of the life-cycle GWP of microalgal diesel in this study resulted from the energy consumed in microstrainer and belt filter harvesting processes (more details on the life-cycle GWP are presented in Supporting Information). The selection of harvesting process should be appropriate to the size and properties of the microalgal cell, downstream processes and the final product from microalgal biomass (microalgal diesel in this study) (Mata et al., 2010; Singh et al., 2011; Uduman et al., 2010; Xu et al., 2011). The four primary harvesting methods for microalgal biomass are microstraining, belt filtering, flotation and sedimentation (Weissman, 1987). A flotation process is suitable for the harvesting of microalgae with high oil content since the microalgal cells tend to float. In addition, the flotation process is not as time consuming as a simple sedimentation, and requires lower operation cost (Brennan & Owende, 2010; Singh et al., 2011; Uduman et al., 2010). The drying process can be implemented by natural air or sun drying, however this method is time and area consuming and can potentially lose some bioreactive products (Li et al., 2008b; Vijayaraghavan & Hemanathan, 2009). Other common drying methods are drum-drying, freeze-drying and spray-drying, however, spray-drying is not economically feasible for low value product e.g. biofuels (Mata et al., 2010). There have also been efforts to avoid the drying processes, but it was found not to be cost-effective (Xu et al., 2011). The authors suggested a co-location of the drying process with the equipment that provides a controlled source of heat or airflow, e.g. vents or condenser units in industrial

facilities, to lower energy consumption for the drying process since it is one of the major contributions to GWP from microalgal diesel production (Uduman et al., 2010; Xu et al., 2011).

oil content since the microalgal cells tend to float. In addition, the flotation process is not as time

consuming as a simple sedimentation, and requires lower operation cost [13, 46, 49]. The drying process can be implemented by natural air or sun drying, however this method is time and area consuming and can potentially lose some bioreactive products [37, 50]. Other common drying methods are drum-drying, freeze-drying and spray-drying, however, spray-drying is not economically feasible for low value product e.g. biofuels [16]. There have also been efforts to avoid the drying processes, but it was found not to be cost-effective [47]. The authors suggested a co-location of the drying process with the equipment that provides a controlled source of heat or airflow, e.g. vents or condenser units in industrial facilities, to lower energy consumption for the drying process since it is one of the major contributions to GWP from

microalgal diesel production [13, 47].

!

Figure 2 GWP of Biodiesels. The probability distribution for microalgal diesel investigated in this study

is compared to ranges from other studies fuels. Data from other studies summarized in Table 1. 80% of the life-cycle GWP of microalgal diesel investigated in this study results from the energy consumed in microstrainer and belt filter harvesting processes.

3.2 Eutrophication potential

The life-cycle EP of conventional diesels are already so low (1.31!10-11 _{g N eq/MJ), that biofuels will} never achieve a reduction like the RFS2 GHG threshold. The EP impact from conventional biofuels can be lessened by utilizing agricultural management strategies such as reducing tillage, optimizing fertilizer application, constructing wetland buffers, cover cropping or planting perennials to reduce runoff [5, 51]. In contrast to other studies, the environmental impacts of microalgal diesel in this study were higher since they were evaluated from well to wheel. A high EP from microalgal diesel is mainly a result of the energy intensive harvesting process. Other comparative studies do not have similar system boundaries, for example the study by Clarens et al. [4] on a comparative LCA of dried microalgal biomass and other bioenergy feedstocks did not include the biomass upgrading into fuels and coproducts. Another study by

0 1E-11 2E-11 3E-11 4E-11 5E-11 6E-11 -4 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96 100 104 108 _{112 116} F re q u en cy GWP (!109 _{kg CO}

2 eq) per 50% of the RFS’s volume required from advanced biofuels in 2012

Soybean diesel Low-sulfur diesel Conventional diesel Microalgal diesel of other studies Canola diesel RFS baseline Microalgal diesel of this study Figure 13 GWP of Biodiesels.

The probability distribution for microalgal diesel investigated in this study is compared to ranges from other studies. Original data from other studies is summarized in Table 6. 80% of the life-cycle GWP of microalgal diesel investigated in this study results from the energy consumed in microstrainer and belt filter harvesting processes.