Impact of heterotrophically stressed algae for biofuel production via hydrothermal liquefaction and catalytic hydrotreating in continuous-flow reactors

Abstract

Two algal feedstocks were prepared for direct comparison of their properties when converted to liquid hydrocarbon fuel. The first feedstock was prepared by growing an algal strain phototrophically using a bio-film based approach. The second feedstock employed the same algal strain but was stressed heterotrophically to significantly increase the lipid concentration. The algal feedstocks were converted to liquid hydrocarbon fuels. First, the whole algae (i.e. not defatted or lipid extracted) were converted to an intermediate biocrude using continuous hydrothermal liquefaction (HTL) at 350°C and 3000 psig. The biocrudes were subsequently upgraded via catalytic hydrotreating (HT) at 400°C and 1500 psig to remove oxygen and nitrogen as well as increase the hydrogen-to-carbon ratio. The yield and composition of the products from HTL and HT processing of the feedstocks are compared. A techno-economic analysis of the process for converting each feedstock to liquid fuels was also conducted. The capital and operating costs associated with converting the feedstocks to finished transportation fuels are reported. A fuel minimum selling price is presented as a function of the cost of the algal feedstock delivered to the HTL conversion plant. Heterotrophic stressing of the algae significantly increased the concentration of lipids compared to the phototrophically grown algae. The high lipid concentration resulted in a doubling of the yield to biocrude, and hence diesel fuel blendstock. Although heterotrophic stressing of algae is costly, results presented in this study suggest that the significant increase in fuel yield over phototrophic growth could more than offset increased feedstock production costs. Water for recycle 6,560 kg/h Steam 11,400 kg/h Water for recycle 12,490 kg/h Water for offgas NH 3 scrubbing 9,810 kg/h Natural gas 740 kg/h To Process heating Key modeled performance results are shown in Table 7. The assumed scale is metric tons per day of algae on a dry and ash free basis, consistent with Jones et al.[12] Thus the algae slurry feed water flow rates differ for the two cases, reflecting the difference in the whole algae ash content.

Continuous Hydrothermal Liquefaction of Biomass: A Critical Review

“…The authors however remark that this could be also caused by the fact that oxygen is determined by difference, so this could lead to errors in its determination. Another paper from the same group, authored by Albrecht et al [43], showed similar results on heterotrophically stressed algae biocrude.…”

“…[42], macroalga Saccharina spp. [44] and Chlorella, with both standard and high lipid content [43]. Some experience was also gained in the processing of agricultural waste, such as grape pomace [45], and of wastewater solids [46].…”

“…In general, the tests conducted at PNNL are carried out at temperatures around 350 • C and cover a wide range of dry matter concentrations, ranging from 5% up to 34.4% in the case of algae slurry. Yields are also quite different and they go from a minimum of 8.7% for macroalgae [44] to a maximum of 71% for Chlorella [43].…”

A Comparative Technoeconomic Analysis of Algal Thermochemical Conversion Technologies for Diluent Production

“…The product values for HTL range from %1.39 to 2.72 $ L À1 for different algal biomass feedstocks. [30,60,93,94] Orfield et al studied an algal biorefinery concept using HTL and reported a product cost of 1.64 $ L À1 . [94] Another HTL study on microalgae using different cultivation systems showed product costs of 1.66-2.20 $ L À1 for biomass costs from 510 to 673 $ t À1 .…”

Hydrothermal liquefaction of biomass for the production of diluents for bitumen transport

“…In addition, the total acid number (TAN) of stabilized biocrude is significantly low; the low TAN will help avoid corrosion during bitumen pipeline transportation. The properties of stabilized biocrude from HTL and of diluents are summarized in Table …”