# Greenhouse Gas Impact of Algal Bio-Crude Production for a Range of CO2 Supply Scenarios
## 2 Methodology  
### Process Flowsheet  
The algae production, CO2 production and delivery, HTL reactor, CHG reactor, and hydro-treating is modeled in ASPEN Plus(r) simulation software. The software platform has been used in previous studies to model similar algal growth processes [17; 18]. The Redlich Kwong Soave (RKS) thermodynamic model has been utilized to estimate the thermodynamic properties of the different compounds modeled in the flowsheet. The algae production is modeled utilizing a methodology reported by NREL [17]. The model was validated against data reported by NREL, and, subsequently, the model was updated to incorporate the selected Algenol algae strain [16]. The algae are assumed to be grown in PBRs with a productivity of 25 g/m\({}^{2}\)/day [15] in a 2000 acre algae growth facility [19]. The CO\({}_{2}\) utilization efficiency of the PBRs is assumed to be 85% with the remaining 15% released back to the atmosphere in a gas purge. The harvested algae is used for the production of bio-crude through HTL occurring at 350 \({}^{\circ}\)C and 200 bar [15]. The HTL process leads to three components, namely the bio-crude, the aqueous phase, and the fuel-gas. The aqueous phase is subjected to CHG, leading to the production of syngas rich in methane. The syngas from the CHG and fuel gas from the HTL unit is combusted to provide the energy required for the operation of both the HTL and CHG units, thus making the thermochemical conversion process self-sufficient in terms of its heating requirement. The CO\({}_{2}\) produced from the combustion of these gases is recycled back to the PBRs. The only energy input to the HTL and CHG process is the electricity requirement by the different pumps used for pumping the algae slurry and the aqueous phase. The HTL and the CHG processes were simulated by updating the models proposed by PNNL [18]. The simulation incorporates separate day and night operations as well as recycling of CO\({}_{2}\), water, and nutrients. The separate day and night operations are needed to reconcile the daytime algae production limited by sunshine and the continuous day/night operation of the HTL and the CHG units. The HTL and CHG reactors are assumed to operate for 24 h/day, whereas the algae production is considered to occur for 12 h/day. Thus, the HTL units utilize half of the algae produced during the daytime, and half of it is utilized during night-time. The CO\({}_{2}\) recycle from syngas combustion takes place during the algae growth phase during the daytime. During night-time operations of the HTL and the CHG units, the CO\({}_{2}\) is vented to the atmosphere. The water recycle from the CHG unit, however, is assumed to take place continuously. The bio-crude produced from the HTL process needs to be upgraded to be utilized as a substitute to either gasoline range or diesel range fuels. This upgrading is accomplished by the hydrotreating and the hydrocracking processes. The study assumes that the hydrotreating and hydrocracking would be accomplished at a conventional crude refinery, and no separate infrastructure would be built for these catalytic conversion processes. The hydrogen utilized for these processes would be supplied through the steam methane reforming process at the conventional crude refinery [20]. A transportation distance of 100 km has been chosen to account for the transportation of bio-crude to the refinery and the transport of the diesel and gasoline range biofuels to the market. A mass balance for all scenarios along with the assumptions undertaken are presented in the Supplementary Information.