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Study finds HTL algal biofuels offer 50-70% lifecycle CO2 reduction compared to petroleum fuels; EROI and GHG comparable to or better than other biofuels
20 September 2013
A new life cycle analysis by a team led by researchers at the University of Virginia has concluded that biofuel produced from algae via hydrothermal liquefaction (HTL) can reduce life cycle CO2 emissions by 50 to 70% compared to petroleum fuels, and also has energy burdens and GHG (greenhouse gas) emission profiles that are comparable to or better than conventional biofuels, cellulosic ethanol and soybean biodiesel.
HTL algae-derived gasoline has a considerably lower GHG footprint and a better EROI relative to conventional ethanol made from corn on a per MJ basis, the team found. The data suggest that a shift to algae-derived gasoline could have immediate climate benefits even using existing technologies, the authors noted. In addition, given expected technological improvements, the benefits of algae-derived gasoline will likely improve.
The GHG emissions for HTL-derived algae fuels are also lower than other existing algae-to-energy processes based on transesterification, the researchers reported in their paper, published in the journal Bioresource Technology.
Furthermore, although the modeling of the energy return on investment (EROI) of pilot-scale HTL algal fuels is about 1, over time, that could increase to between 2.5-3. Both gasoline and diesel currently have EROI values of roughly 4-5, the team noted. In other words, the EROI of HTL algal fuels will be approaching that of the petroleum fuels, while offering a significant reduction in GHG.
(As a side note, petroleum-derived fuels once exhibited EROI values of about 100; as exploration and production becomes more difficult and remote, that EROI has dropped to the current values of 4-5.)
Although EROI has important limitations as a metric, the study authors noted, it still represents a valuable first estimate of the viability of different fuel production pathways relative to conventional benchmarks.
HTL. Industry has been exploring a number of different pathways to convert algae biomass into energy including lipid extraction (LE) and subsequent transesterification to produce biodiesel; direct ethanol production; heterotrophic metabolism; or thermochemical conversion of the entire algae cell.
Thermochemical conversion via hydrothermal liquefaction (HTL) is of particular commercial interest, the authors note, because it integrates with the existing petroleum refining infrastructure.
In HTL, wet algae biomass (water content of 85-90%) is converted through high pressure and temperature reaction processes into four streams:
non-aqueous biocrude (composed primarily of fatty acids, phenolic compounds, and long-chain alkanes) (20-60 wt%);
an aqueous phase containing organic acids and most of the nitrogen and phosphorus in the biomass (30-50%);
a gas phase containing CO2, CH4, and other volatile organic compounds (1-8 wt%); and
solid phase consisting primarily of biochar (~3 wt%).
The biocrude is separated from the aqueous phase using organic solvents and subsequently blended with petroleum crude to produce a variety of drop-in fuels in conventional refineries.
The study. In the study, the team used data collected at the Sapphire Energy pilot facilities in Columbus and Las Cruces, NM (Benjamin Saydah of Sapphire is one of the authors of the paper). They defined three scenarios (lab-, pilot-, and full- scale) to understand how development in the industry could impact the life cycle burdens.
They built a stochastic life cycle model using Microsoft Excel, enhanced with a plug-in enabling stochastic analysis using Monte Carlo simulation. Life cycle inventory data were from Ecoinvent database and CA-GREET model. System boundaries were created to account for all life cycle processes from the cultivation of algae biomass and all upstream burdens to the production of drop-in fuel at the refinery. They modeled cultivation in open ponds built on non-arable land and algae species are selected that can grow in brackish growth media.
The model developed here is the first to characterize the life cycle energy and greenhouse gas emissions associated with algae-to-energy processing using field data collected at a pilot-scale facility. It shows that existing, pilot-scale facilities have life cycle burdens on par with conventional biofuels, not as attractive from a life cycle perspective as many models based on lab-scale data would predict. But there exist significant opportunities for process optimization and improvement related to strain selection, biocrude yield from HTL, or heat recycle efficiency, among others, as captured in the full-scale scenario modeled here. Naturally, these projections about how technological improvements would impact the industry are inherently uncertain and should be interpreted carefully.
The results of this work show that pilot-scale cultivation of algae can already produce lower GHG emissions than petroleum and bio-ethanol benchmarks. Even though the EROI ratios of algae-to-energy production are not as favorable as petroleum fuels today, improvements in the short term will make algae liquid fuels competitive on an energy basis. The environmental burdens of this algae biofuel production system (under current field conditions) are comparable to crop-based biofuels such as corn ethanol and soybean biodiesel. Projections suggest that algae-based biofuels are set to surpass advanced biofuels (e.g., cellulosic ethanol) in terms of both EROI and GHG emissions.—Liu et al.
Xiaowei Liu, Benjamin Saydah, Pragnya Eranki, Lisa M. Colosi, B. Greg Mitchell, James Rhodes, Andres F. Clarens (2013) Pilot-scale data provide enhanced estimates of the life cycle energy and emissions profile of algae biofuels produced via hydrothermal liquefaction. Bioresource Technology, Volume 148, Pages 163-171 doi: 10.1016/j.biortech.2013.08.112
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