# Greenhouse Gas Impact of Algal Bio-Crude Production for a Range of CO2 Supply Scenarios
## 3 Results and Discussion
However, the power plant in scenario 3 would be built for supplying CO\({}_{2}\) and energy to the biorefinery, and natural gas would be sourced for this purpose. The excess electricity produced from the NGCC unit is exported to the grid, assuming grid average GHG emissions of 500 g CO\({}_{2}\) eq./kWh as a credit. The GHG emissions associated with scenario 6 are 49 g CO\({}_{2}\) eq./MJ refined bio-crude. This scenario has slightly higher energy requirements as compared to fossil fuel based scenarios (scenario 1 and 2), due to additional natural gas utilization in the DAC plant for  
Figure 3: GHG emissions results (kg CO\({}_{2}\) eq.) for 1 MJ of refined bio-crude production for different CO\({}_{2}\) supply scenarios. the calciner as well as production of steam in the CHP unit. The powerplants assumed in scenarios 1 and 2 have all their emissions allocated to electricity production, and the emissions would end up in the atmosphere in the absence of the biorefinery. However, this is different for scenario 6, where the DAC plant is specifically built to supply the CO\({}_{2}\) and energy to the biorefinery. The excess electricity produced from the DAC unit is exported to the grid, assuming grid average GHG emissions of 500 g CO\({}_{2}\) eq./kWh. While scenario 1 captures emissions which would have normally been vented to the atmosphere, scenario 6 captures CO\({}_{2}\) directly from the air as well as from the combustion of natural gas in the CHP and calciner. Thus, the overall life-cycle emissions for 1 MJ of refined bio-crude production are lower when sourcing the CO\({}_{2}\) from a conventional coal plant when compared to a new DAC unit. A sensitivity analysis of different scenarios was also performed. The distance between the biorefinery and the conventional fossil-based power plants does affect the life cycle GHG emissions for the refined bio-crude production. The sensitivity assumes a pressure drop of 0.055 bar for each mile. The increase in the GHG emissions with increasing distance is presented in Figure 4. Scenario 1 is more sensitive to the transportation distance as compared to scenario 2. This is because scenario 2 employs carbon capture. The regenerator of the carbon capture unit operates at elevated pressures, and the flowrate is significantly smaller once the nitrogen and other gases are removed. The increased distance between biorefinery and the power plant can also add a considerable economic burden. The emissions from the production of refined bio-crude are sensitive to the GHG footprint of the grid to which electricity is exported. To better understand this variation, a sensitivity analysis is performed by varying the grid-average emissions from 50 to 750 g CO\({}_{2}\) eq./kWh electricity. The results, shown in Figure 5 are that scenarios 3 and 5 are most sensitive to the grid electricity GHG footprint, whereas scenario 2 is the least sensitive. This is because scenarios 3 and 5 derive the maximum benefit from the export of electricity. The results highlight the need for performing a techno-economic analysis to gain a better understanding of both scenarios. If boreal biomass with a biomass growth cycle of 75 years is considered, the GHG footprint of refined bio-crude production would be 51.5 g CO\({}_{2}\) eq./MJ refined bio-crude, which is very close to the GHG footprint of bio-crude produced from fossil-based CO\({}_{2}\) sources as well as the DAC process. The primary reason for this is the GHG footprint of 0.58 kg CO\({}_{2}\) eq./kg biomass for boreal biomass as compared to the GHG emissions of 0.138 kg CO\({}_{2}\) eq./kg biomass for 11-year pine plantation thinnings. Thus, not all biomass feedstocks would provide similar GHG emissions benefits. Furthermore, the utilization of relatively efficient processes such as biomass gasification must be studied. Figure 4: Sensitivity of power plant based CO\({}_{2}\) supply with increasing distance from the biorefinery. In previous work, [10] we evaluated the CO\({}_{2}\) emissions of different CO\({}_{2}\) sources.