A systems analysis of biodiesel production from wheat straw using oleaginous yeast: process design, mass and energy balances

Abstract

BackgroundBiodiesel is the main liquid biofuel in the EU and is currently mainly produced from vegetable oils. Alternative feedstocks are lignocellulosic materials, which provide several benefits compared with many existing feedstocks. This study examined a technical process and its mass and energy balances to gain a systems perspective of combined biodiesel (FAME) and biogas production from straw using oleaginous yeasts. Important process parameters with a determining impact on overall mass and energy balances were identified and evaluated.ResultsIn the base case, 41% of energy in the biomass was converted to energy products, primary fossil fuel use was 0.37 MJprim/MJ produced and 5.74 MJ fossil fuels could be replaced per kg straw dry matter. The electricity and heat produced from burning the lignin were sufficient for process demands except in scenarios where the yeast was dried for lipid extraction. Using the residual yeast cell mass for biogas production greatly increased the energy yield, with biogas contributing 38% of total energy products.ConclusionsIn extraction methods without drying the yeast, increasing lipid yield and decreasing the residence time for lipid accumulation are important for the energy and mass balance. Changing the lipid extraction method from wet to dry makes the greatest change to the mass and energy balance. Bioreactor agitation and aeration for lipid accumulation and yeast propagation is energy demanding. Changes in sugar concentration in the hydrolysate and residence times for lipid accumulation greatly affect electricity demand, but have relatively small impacts on fossil energy use (NER) and energy yield (EE). The impact would probably be greater if externally produced electricity were used.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0640-9) contains supplementary material, which is available to authorized users.

Techno‐economic analysis (TEA) of microbial oil production from waste resources as part of a biorefinery concept: assessment at multiple scales under uncertainty

“…There are a number of complex factors affecting scale‐up performance, and reliance on laboratory scale data leads to uncertainty in cost analysis results and subsequent evaluation of economic viability. The performances used in the study are indicative of those assumed elsewhere for oleaginous yeast There is substantial variability across feedstocks in their ability to be broken down and hydrolyzed to form a fermentable hydrolysate.…”

“…There is limited description in the literature of industrial lipid extraction from yeast. Given this, extraction was based on previously published techno‐economic data for yeasts (100 t year –1 ), and on larger scale wet algal extraction (10 000 t year –1 ). There is uncertainty on the ability to extract 95% lipid from yeast biomass using hexane at the industrial scale, and the energy inputs required to adequately disrupt and break apart the cells, and remove water and hexane following the extraction step. The literature to date has focused on the use of microbial lipids to produce biodiesel .…”

“…To date, the majority of TEA studies for microbial biorefineries have focused on phototrophic microalgae . A smaller number of TEAs also have been performed on yeast and heterotrophic algae, with all studies assessing the use of SCOs for biodiesel rather than for food or other terrestrial oil product replacement . Amongst all TEA studies conducted on SCO routes to bioproducts, none have fully accounted for uncertainties in their modelling.…”

“…A detailed process and cost model for acid pretreatment and enzyme hydrolysis for the production of bioethanol has been previously produced by Humbird et al . However, aerobic conversion of hydrolysates at larger scales is poorly documented in the literature, with only a small number of TEA studies evaluating heterotrophic organisms . Within these, agitation and aeration during fermentation were suggested to be the most significant areas of electricity use within the whole process; however, changes in the extraction of lipid and drying gave the greatest impact on mass and energy balance …”

Greenhouse gas performance of biochemical biodiesel production from straw: soil organic carbon changes and time-dependent climate impact

“…This study builds on a previous study describing the process design and energy balance of biodiesel production from straw using oleaginous yeast [ 35 ]. That study included straw harvesting and transport, SOC changes due to straw harvesting and replacement of nitrogen removed with the straw, processing the straw in a biorefinery and production of biorefinery inputs.…”