Techno-economic, Energy, and Environmental Assessment of Bio-alcohol Production from Brown Algae

Abstract

This study evaluates the economics of biofuel production from brown algae through biochemical conversion pathways i.e. sugar platform (SP) and volatile fatty acids platform (VFAP). The processes are rigorously designed and simulated in Aspen Plus and optimized through heat integration analysis for utility requirements. The alternatives are analyzed in terms of product value (minimum ethanol selling price, MESP), biomass price (maximum dry seaweed price, MDSP), capital costs, energy consumption, and carbon footprint. Comprehensive sensitivity analyses were performed on process and economic parameters to elaborate bottlenecks and hidden uncertainties in each process. In chapter 3, a new simple pretreatment process is introduced for SP and economically validated and compared with the traditional acid thermal hydrolysis known as combined pretreatment at plant scales of 80,000 and 400,000 ton dry seaweed/year. MDSP for the simple and combined processes was calculated as 64.6 and 26 $/ton (80,000 ton/year) and 91.3 and 71.5 $/ton (400,000 ton/year), respectively. In addition, MESP for the simple and combined processes was determined as 2.39 and 2.85 $/gal (80,000 ton/year) and 2.08 and 2.33 $/gal (400,000 ton/year), respectively. Chapter 4 evaluates the economics of mixed alcohols production from brown algae through volatile fatty acids platform with a focus on economic impact of VFA recovery from fermentation broth. Two alternative processes, i.e., extraction/distillation (case 1) and hybrid pervaporation (PV) combined with extraction/distillation (case 2), are considered for recovery of VFAs from the dilute fermentation broth. The respective MESPs for cases 1 and 2 are calculated to be 1.24 and 3.61 $/gal, respectively. The results of the study showed that application of PV for dilute streams results in higher capital and energy costs primarily because of increased refrigeration, heating, and membrane costs. Chapter 5, in continue to chapter 4, evaluates the impact of alcohols recovery on the economics, energy efficiency, and CO2 emissions of the process. The three alternative processes of hybrid pervaporation/distillation (PV), hybrid vapor-permeation/distillation (VP), and classical molecular-sieves/distillation (classical) are considered for the dehydration and recovery of ethanol. MESPs for the PV, VP, and classical processes are calculated to be 1.06, 1.08, and 1.24 $/gal, respectively. Results show that the PV, VP, and classical processes have 2.0, 2.6, and 4.6 million$/year utility costs, respectively, for recovery of alcohols while producing 23,120, 30,190, and 62,160 ton CO2-eq/year greenhouse gases. Therefore, PV is identified as a more economical and environmentally friendly process. Chapter 6 evaluates the application of MixAlco® process through ketonization route (KR) and esterification route (ER) for mixed alcohols production from brown algae. The total energy costs of KR and ER are calculated to be 11.6 and 37.2 million$/y, respectively, while emitting 87,100 and 183,800 ton CO2-eq/year greenhouse gases. The higher utility and operating costs of the ER results in a negative value of net present value (NPV) and a present value ratio (PVR) less than one over 20 years of plant life. However, the NPV and PVR for the KR is calculated to be 61.1 million $ and 1.57, respectively, which shows the profitability of the KR. In addition, MDSP is calculated to compare the economics of MixAlco process with the PV process obtained as optimal process through chapter 4 and 5. An MDSP value of 114.7 and 90.0 $/ton is calculated for the PV and KR, respectively, which shows the superiority of the PV process over the KR. Chapter 7 evaluates the economics of a conceptual seaweed based biorefinery for biofuel and biochemical production. The economics of seaweed cultivation and biorefinery are evaluated for two cases of with and without incentives for CO2 mitigation. A minimum seaweed selling price (MSSP) of 63 and 73.3 $/ton dry seaweed are calculated for the cases with and without CO2 mitigation incentives for 10,000 ha cultivation area with 300 ton/ha.yr cultivation yield, respectively. In addition, ROI and payback time of the biorefinery are calculated to be 34.3% and four years for the case with incentives from CO2 mitigation, respectively, and 24% and 6.3 year for the case without CO2 mitigation incentives, respectively. The results show that brown algae have superior economics and environmental advantages over first and second generation biomass which meet all the sustainability and profitability goals. In addition, the results provide decision makers with strong knowledge toward taking solid steps for industrial implementation of brown algae based biorefinery.