Researchers demonstrate sustainable integrated process for wastewater algae to biocrude via hydrothermal liquefaction
|Flow diagram of the solvent extraction and product recovery method used. Credit: ACS, Roberts et al. Click to enlarge.|
A team at the University of Kansas has demonstrated the feasibility of an integrated wastewater algae-to-biocrude process using hydrothermal liquefaction (HTL) that can sustainably cultivate algal biomass for biofuel production. A paper on their work is published in the ACS journal Energy & Fuels.
This study is the first of hydrothermal liquefaction of wastewater-derived microalgae, the team said. The municipal wastewater matrix and resultant mixed-culture biomass significantly influenced liquefaction product distribution, yielding a higher proportion of biochar, which may be a valuable co-product, they found.
...a recent report by the National Research Council (NRC) of the National Academies has identified several concerns about the sustainability of algal biofuels [earlier post]: limited supplies of water, nutrients, and appropriate land; greenhouse gas emissions and fossil energy-use over the life cycle of algal biofuels; introduction of genetically modified algae to natural water; and waste products generated by algal biofuel production. Algal biofuel technologies that do not address these concerns are likely to have limited viability.
Traditional conversion technologies of algal biomass to biofuels have mainly included lipid extraction or pyrolysis for biodiesel, hydrotreated renewable diesel, or bio-oil production. These technology pathways have significant drawbacks when using algal feedstocks. Lipid extraction requires extensive dewatering and organic solvents, diminishing the economics of fuel production and increasing environmental concerns.
Whole cell conversion pathways, such as pyrolysis, alleviate environmental concerns from solvent extraction but still require extensive dewatering prior to conversion. A wet whole cell conversion pathway, hydrothermal liquefaction (HTL), has gained popularity in the past few years. HTL uses subcritical water as the chemical driving force to convert biomass to a carbon-rich biocrude. Using microalgal feedstocks, biocrude yields from HTL processing are 5−30% greater than the initial algae lipid content because other cellular components (beyond only lipids) are converted to biocrude. Economic analyses of producing fuel oil, synthetic gasoline, and diesel from wood chips with 50 wt % moisture have shown that atmospheric flash pyrolysis is less costly than high-pressure liquefaction. However, for high water content biomass, such as algae, the energy costs of dewatering may offset the energy demand of liquefaction processes. Also, HTL of algae gives a biocrude with a composition and energy density that more closely resembles petroleum crude than bio-oil from pyrolysis.
...While municipal wastewater is a readily available water and nutrient source that can support mass algal biomass cultivation, wastewater effluent has dynamic nutrient concentrations that vary daily and seasonally. This variable nutrient source is entirely different from the nitrogen-limited growth conditions that are known to yield high-lipid content biomass, such as 40−70% lipid by weight. Wastewater-fed algae typically have low lipid [10−29% dry weight (dw)] and low carbon (30−37% dw) contents, and these characteristics can have substantial effects on the HTL product formation and biocrude composition.—Roberts et al.
The process developed by the researchers used pilot-scale algal cultivation ponds fed with municipal wastewater as the nutrient source. The open ponds were self-inoculated from the wastewater source, resulting in a mixed-culture microalgal community with distinct differences compared to laboratory-maintained and fertilized monocultures: 29.0% dry weight (dw) ash, 48.9% ash-free dry weight (afdw) carbon, 37.5% afdw oxygen, and 14.0% afdw lipid. (Microscopic analysis identified 18 different algal species in the open ponds.)
The harvested algae was processed using hydrothermal liquefaction at 350 °C (autogenous pressures up to 2000 psig) for 1 h using 3 g of freeze-dried algae and 50 mL of water. The yield of biocrude was 44.5 ± 4.7% afdw, with an elemental weight percent composition of 78.7% carbon, 10.1% hydrogen, 4.4% nitrogen, and 5.5% oxygen and an energy content of 39 MJ/kg.
GC/MS analysis of the biocrude contained a significant number of straight-chain and branched hydrocarbons and mono- and polyaromatics in addition to fatty acids.
Hydrothermal processing also resulted in the formation of 18.4 ± 4.6% afdw aqueous co-products (ACPs) and 45.0 ± 5.9% dw solid biochar. The ACPs contained 4550 ± 460 mg L–1 organic carbon, 1640 ± 250 mg L–1 total nitrogen, and 3.5 mg L–1 total phosphorus. The solid biochar product contained >20% dw carbon with an energy density between 8 and 10 MJ kg–1.
|Potential block flow diagram of the continuous algal biofuel pathway. Credit: ACS, Roberts et al. Click to enlarge.|
The co-products could greatly enhance sustainability and the value chain for algal biofuels, adding markets in carbon sequestration, soil amendments, absorbents, and fertilizers. This promising demonstration requires further work upon optimizing the energy balance of the conversion method in conjunction with the cultivation strategy and determining the efficacy of the identified co-product markets.—Roberts et al.
Griffin W. Roberts, Marie-Odile P. Fortier, Belinda S. M. Sturm, and Susan M. Stagg-Williams (2013) Promising Pathway for Algal Biofuels through Wastewater Cultivation and Hydrothermal Conversion. Energy & Fuels doi: 10.1021/ef3020603