Distributions of carbon and nitrogen in the products from hydrothermal liquefaction of low-lipid microalgae

Abstract

Microalgae are considered to be a promising alternative feedstock for next generation biofuels because of their rapid photosynthetic growth rates and less impact on land-use for food production compared with grain and other lignocellulosic biomass. In this study, a fast-growing, low-lipid, high-protein microalga species, Chlorella pyrenoidosa, was converted via hydrothermal liquefaction (HTL) into four products: bio-crude oil, aqueous product, gaseous product, and solid residue. The effects of operating conditions (reaction temperature and retention time) on the distributions of carbon and nitrogen in HTL products were quantified. Carbon recovery (CR), nitrogen recovery (NR) and energy recovery in the bio-crude oil fraction generally increased with the increase of reaction temperature as well as the retention time. The highest energy recovery of bio-crude oil was 65.4%, obtained at 280 C with 120 min retention time. Both carbon and nitrogen tended to preferentially accumulate in the HTL bio-crude oil products as temperature and retention time increased, but the opposite was true for the solid residual product. The NR values of HTL aqueous product also increased with reaction temperature and retention time. 65-70% of nitrogen and 35-40% of carbon in the original material were converted into water soluble compounds when reaction temperature was higher than 220 C and retention time was longer than 10 min. The CR of gas was less than 10% and is primarily present in the form of carbon dioxide. This study also introduces a novel treatment process (Environment-Enhancing Energy) that integrates algal growth for wastewater treatment with HTL of algal biomass, which provides synergistic recycling of carbon dioxide from the HTL gaseous product and the nutrients from HTL aqueous product to support multiple stages of algae production.

Life cycle comparison of hydrothermal liquefaction and lipid extraction pathways to renewable diesel from algae

“…Hydrothermal liquefaction (HTL) is a thermal process that converts biomass to several products including an oil portion with high heating value (Alba et al 2012;Biller and Ross 2011;Brown et al 2010;Duan and Savage 2011;Minowa et al 1995;Vardon et al 2011;Yu et al 2011). HTL offers several advantages compared to other approaches.…”

“…The HTL yield is defined here to be the mass of HTL oil recovered per dry mass of algae processed, sometimes written as a percentage. Numerous studies are reported in the literature and each reports different yields for the various fractions (Table 1) and different distributions of solids between oil and aqueous phases (Alba et al 2012;Biller and Ross 2011;Brown et al 2010;Duan and Savage 2011;Minowa et al 1995;Valdez et al 2011;Vardon et al 2011;Yu et al 2011). The product distribution may be influenced by many factors, including chemical composition of the algae, reaction temperature, retention time, feedstock solid content, and catalyst usage.…”

“…Many aspects of HTL have been studied including reaction temperature, pressure, retention time, feedstock species, influence of solvents, effects of catalysts, elemental distribution in products, and energy recovery. That work indicates that high temperatures (250°C to 350°C) and high pressures (approximately 10 to 20 MPa) are required and that the HTL oil can contain substantial amounts of nitrogen incorporated from the algal biomass (Alba et al 2012;Biller and Ross 2011;Brown et al 2010;Duan and Savage 2011;Minowa et al 1995;Valdez et al 2011;Vardon et al 2011;Yu et al 2011). From a life cycle perspective considering total energy demand and total emissions, these aspects suggest net energy demand may be higher than for lipid-extracted pathways because heat will be needed to establish the process conditions.…”

“…Yu et al (2011) examined carbon and nitrogen distributions in HTL products formed from low-lipid, highprotein Chlorella. At 300°C, 30 % of the feedstock nitrogen went to the oil and solid phases and 70 % went to the aqueous phase.…”

Combustion behavior and kinetics of low-lipid microalgae via thermogravimetric analysis

“…High-lipid microalgae mean a lipid accumulation in microalgae under a stressed condition such as nitrogen depletion. It will sacrifice biomass productivity, reduce the net energy yield, and make the process very sensitive to contamination (Yu et al, 2011). According to Williams and Laurens (2010), high lipid containing algae may not necessarily be the most favorable candidate organisms.…”