Biological CO2 capture and utilization with algae has the potential to mitigate major environmental problems associated with greenhouse gas emissions and excess wastewater nutrient discharges, and at the same time, generate a valuable biomass product that can be used for biofuels and/or animal feed. However, there are important practical limitations in currently available systems and technology that have limited pilot demonstration and applications for this technology. This project addressed critical challenges to practical demonstrations of biological CO2 capture systems and subsequent thermochemical conversion of biomass to biofuels. First, the capability to harvest and store actual power plant flue gas samples in pressurized cylinders was developed, and these samples were then used to study acclimation in algae cultivation systems dosed with flue gas. Second, this project demonstrated the use of anaerobic digestion to recover residual energy from the aqueous byproduct of hydrothermal liquefaction (HTLaq), which is generated during the conversion of algae or other organic feedstocks to biofuels. Algae cultivation experiments showed that a mixed culture of algae is capable of using CO2 from power plant flue gas without a negative impact on the algal growth rate. In fact, the algal biomass productivity was up to 67% higher with flue gas injection than that from control cultures. The CO2 removal efficiency was between 18 to 25%, and there is room for further improvement. A heavy metal analysis of algal biomass cultivated with flue gas inputs showed that algae can overaccumulate certain heavy metals (Zn, Pb, and Cu) that could limit its use for some animal feed products. Further study is needed to identify the factors controlling heavy metal uptake and develop mitigation strategies. In the second part of this study, we demonstrated anaerobic treatment of HTLaq combined with sewage sludge from municipal wastewater treatment at both the lab and full-scale operations. The lab-scale experiments showed that compared to a control digester with sewage sludge only, 18% more biogas was produced when HTLaq was dosed at 12% of the total organic loading to the digester. This dosing level is substantially higher than reported in other literature. The higher dosing level was accomplished via gradual acclimation of the anaerobic cultures to increasing amounts of HTLaq. Full-scale testing was conducted in the anaerobic digesters at a local wastewater treatment plant, which was dosed with up to 0.4% of the organic loading from HTLaq. This experiment was limited by the amount of HTLaq available, but showed successful anaerobic digestion without any evidence of inhibition or negative impacts on biogas production. Future work should investigate higher loading rates of HTLaq at both the lab and full-scale operations to further enhance bioenergy production. A techno-economic analysis performed showed that bioenergy production at a typical wastewater plant could be increased by up to 70% by integrating HTL conversion of sewage sludge upstream of anaerobic digestion. The total annualized cost for this combination was also lower than anaerobic digestion alone for a greenfield land application.
|Name||TR Series (Illinois Sustainable Technology Center)|
- Carbon capture
- Power plants -- Environmental aspects
- Technology demonstration
- Algal biomass