Phosphorus recovery from HTL processed macroalgae can generate positive ma­terial and energy costs and revenues

Replacing fossil fuels with sustainable fuel from biomass requires both innovative technological solutions and a feedstock that does not put too much strain on food production and land use. Hy­drothermal liquefaction is a technology for producing biofuels that has been gathering increasing interest, and by using seaweed (macroalgae) as a feedstock it is a promising option that fulfils both previously mentioned criteria. Using macroalgae has the added benefit of remediating eutrophic coastal waters since the macroalgae during marine cultivation absorb some of the excess nutrients, e.g. phosphorus (P), from the surroundings.

After the hydrothermal lique­faction of the macroalgae, the phosphorus can be recovered and used to produce struvite, a natural fertilizer that can replace the conventional mineral fertilizer. A newly finished f3 project, Phosphorus recovery in algae-based biofuels, has had as its purpose to iden­tify profitable and environmentally friendly technological solutions connecting phosphorous recov­ery with macroalgae processed with hydrothermal liquefaction (HTL) and at the same time diver­sify the products outcome of biofuel production.

Stavros Papakonstantakis, Chalmers, has lead the project that carried out a deep literature review and its citation network analysis (CNA) to investigate the possibility of integrating phosphorus (P) recovery with biofuel production from macroalgae hydro­thermal liquefaction (HTL). Many processes and technologies concerning phosphorus recovery from different waste products and streams were studied together with their potential synergies with hydrothermal liquefaction and with other HTL related waste streams treatments. Any process was assessed according to its technological maturity, complexity, material and energy requirements, cost and environmental impacts.

Results shows that P recovery could be effectively achieved from HTL waste streams treatments, diversifying the HTL product outputs. All the evaluated options resulted to generate a positive ma­terial and energy costs and revenues with net balance peaking around 20$/kg of dry feedstock. Nevertheless, the revenues were highly affected by the price of the feedstock macroalgae (which should be therefore further verified) accounting for more than 60% of the total costs.

The struvite production, peaking at 7.2 kg/t dry macroalgae, has a marginal impact on the profits, but pays back its related operating costs.

Results from the project are presented in an f3 report, available here, and have also been included in a conference proceeding prepared and accepted for PSE 2018 (Process Systems Engineering) in July i San Diego, USA. The proceeding will be linked by f3 after the conference.