(359g) Efficient Resource Recovery to Enhance Biomass Conversion

Nitrogen is an essential nutrient for biological growth vital to the persistence of individuals and society in various aspects, from crop and livestock husbandry to microalgae cultivation for biofuel. Nitrogen is crucial for amino acids and protein synthesis, and it is commonly taken up in the inorganic forms of NO₃^- or NH₄⁺. In the US alone, 14 million metric tons (MMT) of NH₃ are utilized directly and indirectly as fertilizer; the volume of N nutrients demand is substantial.

The ammonia demand is 2.7 kg NH₃ per million BTU of renewable diesel (RD) for microalgae hydrothermal liquefaction (HTL), which implies that 3.3 million metric tons of ammonia are required to produce 10 billion gallons per year of RD by HTL. HTL of algae feedstock at 300°C led to the nitrogen distribution that roughly 30 % of the feedstock nitrogen in the oil and solid phases, and 70 % in the HTL aqueous phase (HTL-AP). HTL-AP is generally characterized by high concentrations of ammonium, phosphates, and organic carbon. Besides from the prospective recovery of NH4-N, the separation of other high-value organic compounds, i.e., acids, further improves the economic upside of algae biofuel production. HTL oil has a high heating value but requires significant upgrading before it can be used as RD for transportation, because of high oxygen, nitrogen, and sulfur contents, together with acidity and viscosity issues. Depending on the upgrading process of HTL oil, nitrogen incorporated into HTL oil, which roughly accounts 6 wt.%, maybe dissolved in the process wastewater, and can be further recovered.

Controlling energy and operating costs are the key to economic viability and commercialization of ammonia recovery from wastewater. The unit energy consumption for conventional stripping and production of (NH₄)₂SO₄ from wastewater is approximately 9 kWh/kg-N, while ammonia fertilizer production with the Haber–Bosch process is 10 kWh/kg-N. At a 80% removal ratio, the addition of chemicals for pH adjustment has a profound impact on operating costs. Batch operations of electrochemical separation processes, e.g. electrodialysis (ED) and electrodeionization (EDI), has reported removal ratios over 90% with energy consumptions around 5 kWh/kg-N. Furthermore, ED and EDI may eliminate chemical additives for pH control during ammonia removal/recovery because of water splitting reactions that produce proton and hydroxyl ions and enable in-situ pH adjustment under the electric field. Resin wafer electrodeionization (RW-EDI) immobilizes the loose resin beads of conventional EDI into a porous matrix material and improves unstable process performance, stack leakage, and flow shortcut that increase operating and maintenance costs.

>90% recovery of both ammonia and organic acids from the waste algae HTL aqueous phase was achieved using RW-EDI technology. >75% of TOC was removed from the treated waste aqueous phase. TEA for these resource recovery and potential economic benefits from their utilization will be discussed.