(587d) Comparative Techno-Economic Analysis of Algal Biofuel Production Via Hydrothermal Liquefaction: One Stage Versus Two Stages

Algal biofuels have been widely accepted as promising renewable energy resources and receive extensive investigations in four main research areas: algal feedstocks, cultivation methods, harvesting/dewatering technologies, and conversion processes. Conversion of algal biomass is the essential downstream step to realize high-efficient and cost-effective production of biofuels and associated co-products. Hydrothermal liquefaction (HTL) stands out in practically handling wet algal slurry and avoids toxic solvent usage as well as produces biofuels both in reasonable quantity and quality. Pacific Northwest National Laboratory (PNNL) developed a direct HTL (DHTL) and upgrading process to convert whole algal material to biofuels. This method could enable conversion of non-oil components in raw algae feedstocks such as polysaccharides and proteins to bio-crudes, thus improves overall bio-oil yield, which furthermore relieves constraints of strain-specific operation conditions. However, biofuels derived from DHTL contain high level content of nitrogen (1.5-6.0 wt.%) than crude oil (0.1 wt.%) operated at high temperature (350 ℃) and pressure (20.9 MPa). Subsequent denitrification process and high manufacture cost due to the sever reaction conditions would make DHTL process economically unfeasible.

To address the drawbacks of DHTL, a novel sequential hydrothermal liquefaction (SEQHTL) for simultaneous extraction of co-products and bio-oil from algal biomass has been proposed by our group. This process highlights recovery of proteins and polysaccharides at first stage and extraction of bio-oils at second stage at much lower temperature and pressure. The pre-removal of protein substances at first stage substantially decreases nitrogen content of bio-oil obtained at second stage by 30-55% and enhanced fatty acids exaction yields additionally.

A comprehensive techno-economic analysis (TEA) of DHTL of algae associated with oil upgrading was performed by PNNL, resulting in minimum fuel selling price (MFSP） $4.77 gal-1. However, this cost derived from DHTL cannot achieve the overall goal of 3 $/GGE established by the Bioenergy Technologies Office (BETO) by 2022. Based on sensitivity analysis, the total investment could play an important role in determining algal MFSP besides feedstock cost, thus decreases on investment cost could offer a desirable MFSP. In this case, SEQHTL process is expected to provide less investment cost due to the low operating temperature and pressure. Moreover, isolated proteins and polysaccharides can be regarded as co-products credits to further compensate production cost.

We have two phases analysis procedure. Our first goal is to compare our proposed SEQHTL process using Chlorella with DHTL process using another algae species, Nannochlorpsis. In this phase all the DHTL process data were retrieved from PNNL reports and SEQHTL data were retrived from pervious experimental work in our group. Preliminary results showed that HTL equipment cost of SEQHTL is only 60% of that in DHTL. The total capital investment is 150 million cheaper than DHTL. The sugar products indeed make some credits in operating cost. However, the final oil selling price of SEQHTL was still higher than DHTL because of the different strains applied (Oil conversion rate was 50% in PNNL, while our strain only have 30%). If we increase the oil yield up to the same value as DHTL, the selling price decreases to $4.21 gge. The second phase will be conducted to compare cost between SEQHTL and DHTL with same algal strain. Preliminay results showed that slimilar oil conversion rates were realized for these two methods, thus SEQHTL process is expected to be more economic to produce algal biofuels than DHTL in terms of capital costs, operating costs, and minimum fuel selling price.

In addition, sensitivity analysis indicated that four major factors that affecting MFSP in SEQHTL process were algal feedstock cost, bio-oil conversion rate, sugar selling price, internal rate of return. The cost of algae is the top reason that blocks the development of algal biofuels. If the cost could be lower to 70% of original price, the fuel selling price will be decreased by $1/gge. Realizing high bio-oil conversion yield is rather important, the more produced, the lower price could be sold. Additionally, the selling price of co-products plays an important role. If we could extract high value co-products as well as biofuels. The MFSSP could drop substantially.

The purpose of this study is to provide preliminary TEA result of SEQHTL process and demonstrate the potential of replacing conventional DHTL process with promising SEQHTL as a novel platform in conversion technology, especially with co-products extraction from natural biomaterials. Future R&D should be focused on enhancing bio-oil production yield and effective separation of co-products in the field of conversion technology as well as reducing the cost of algae feedstocks. Although uncertainty exists due to the lack of experimental data and scale-up experience of HTL facilities, SEQHTL process is a promising and competitive conversion technology to handle wet feedstocks and fractionate biomass into high valued co-products.