# Greenhouse Gas Impact of Algal Bio-Crude Production for a Range of CO2 Supply Scenarios
## 3 Results and Discussion  
The life-cycle greenhouse gas emissions for production of algal biocrude are 43, 50, 86, \(-\)19, \(-\)57, and 49 g CO\({}_{2}\) eq./MJ for CO\({}_{2}\) sourced from coal, natural gas, on-site purpose-built natural gas combined cycle, biomass combustion, biomass gasification, and direct air capture, respectively. The contributions to the GHG emissions for the six different scenarios described above are presented in Figure 3. The CO\({}_{2}\) supply refers to the avoided emission of CO\({}_{2}\) to the atmosphere, which would have been released to the atmosphere in the absence of the biorefinery. The algae conversion process refers to algae production and the HTL, CHG, hydrotreating, and hydroracking conversion processes. The majority of emissions under this category are due to the CO\({}_{2}\) losses from the PBRs during the growth of algae. The PBRs are assumed to have a CO\({}_{2}\) utilization efficiency of 85% [15]. The CHG night-time emissions refer to the emissions from the combustion of fuel gases from the HTL and the CHG processes, which are used to provide the necessary heat for the HTL and the CHG reactors. The daytime emissions are recycled to the PBRs. Bio-oil transportation refers to the transportation of the bio-crude to the refinery as well as the transportation of the refined bio-crude to the end-user. The emissions from the combustion of refined bio-crude at the end-use are represented by bio-oil combustion. Both the scenarios result in the production of excess electricity, which is exported to the grid at grid average emissions. The emissions associated with the construction of PBRs are also shown in Figure 3. The nitrogen and phosphorous-based fertilizers utilized during the algae growth process are the major nutrients considered in the present study. Hydrogen is produced through the traditional steam reforming process and utilized during the hydrotreating and hydroracking of bio-crude. Sourcing CO\({}_{2}\) from a biomass-based plant is expected to yield the best results in terms of life-cycle GHG emissions and renders the bio-crude production process carbon negative. The biomass-based CO\({}_{2}\) supply scenario benefits from the carbon intake during biomass growth as well as the export of the excess electricity produced. The gasification process being more efficient than the biomass combustion process could significantly increase the production and export of electricity. This makes biomass gasification the lowest emission pathway for CO\({}_{2}\) delivery from the viewpoint of carbon footprint. However, the economic considerations of gasification as compared to combustion must also be considered. The conventional power plant-based scenarios, namely, scenarios 1 and 2, have life-cycle GHG emissions of 43 g CO\({}_{2}\) eq./MJ refined bio-crude and 50 g CO\({}_{2}\) eq./MJ refined bio-crude, respectively. The energy requirement of scenario 2 is slightly lower as compared to scenario 1. The reason for this lower energy requirement is the production of pressurized CO\({}_{2}\) from the carbon capture unit. This, in turn, would reduce the compression energy required from the transportation of CO\({}_{2}\) over 2 miles. Additionally, a higher capacity NGCC plant would be required in scenario 1 to meet this demand. Consequently, scenario 1 would have a higher amount of electricity exported during the night. Other emissions are similar for scenarios 1 and 2. The life-cycle GHG emissions are expected to be the highest for scenario 3 where the CO\({}_{2}\) is sourced from a purpose-built NGCC plant. Alternately, in scenarios 1 and 2, the CO\({}_{2}\) production is deemed to be equal to sequestrating CO\({}_{2}\) from the atmosphere. In the case of the conventional power plants (scenarios 1 and 2), all the CO\({}_{2}\) emissions are allocated to electricity production. This assumption is based on the fact that, in the absence of the biorefinery, all the emissions would end up in the atmosphere.