Impact of pretreatment and downstream processing technologies on economics and energy in cellulosic ethanol production

Abstract

Background: While advantages of biofuel have been widely reported, studies also highlight the challenges in large scale production of biofuel. Cost of ethanol and process energy use in cellulosic ethanol plants are dependent on technologies used for conversion of feedstock. Process modeling can aid in identifying techno-economic bottlenecks in a production process. A comprehensive techno-economic analysis was performed for conversion of cellulosic feedstock to ethanol using some of the common pretreatment technologies: dilute acid, dilute alkali, hot water and steam explosion. Detailed process models incorporating feedstock handling, pretreatment, simultaneous saccharification and co-fermentation, ethanol recovery and downstream processing were developed using SuperPro Designer. Tall Fescue (Festuca arundinacea Schreb) was used as a model feedstock.

Results: Projected ethanol yields were 252.62, 255.80, 255.27 and 230.23 L/dry metric ton biomass for conversion process using dilute acid, dilute alkali, hot water and steam explosion pretreatment technologies respectively. Price of feedstock and cellulose enzymes were assumed as $50/metric ton and 0.517/kg broth (10% protein in broth, 600 FPU/g protein) respectively. Capital cost of ethanol plants processing 250,000 metric tons of feedstock/year was $1.92, $1.73, $1.72 and $1.70/L ethanol for process using dilute acid, dilute alkali, hot water and steam explosion pretreatment respectively. Ethanol production cost of $0.83, $0.88, $0.81 and $0.85/L ethanol was estimated for production process using dilute acid, dilute alkali, hot water and steam explosion pretreatment respectively. Water use in the production process using dilute acid, dilute alkali, hot water and steam explosion pretreatment was estimated 5.96, 6.07, 5.84 and 4.36 kg/L ethanol respectively.

Conclusions: Ethanol price and energy use were highly dependent on process conditions used in the ethanol production plant. Potential for significant ethanol cost reductions exist in increasing pentose fermentation efficiency and reducing biomass and enzyme costs. The results demonstrated the importance of addressing the tradeoffs in capital costs, pretreatment and downstream processing technologies.

Figure 1

Generic Cellulosic Ethanol Production Process. Figure illustrates the common unit operations in ethanol production plants. Separation of liquid and solid after pretreatment is not necessary for all pretreatment technologies.

Figure 2

Chemical composition of tall fescue grass straw.

Figure 3

Process model of ethanol production from grass straw using dilute acid pretreatment. Figure illustrates the all unit operations and equipments used in the model development.

Figure 4

Process model of ethanol production from grass straw using steam explosion pretreatment. Figure illustrates the all unit operations and equipments used in the model development.

Figure 5

Capital cost for ethanol production from grass straw using different pretreatment processes. Figure shows the capital cost (direct fixed cost) per liter of ethanol. Figure also illustrates the breakdown of installed equipment cost and other costs. Other includes piping, electrical, insulation, design work, and buildings and construction, engineering costs, contractors' fees and contingency.

Figure 6

Operating cost for ethanol production from grass straw using different pretreatment processes. Figure shows the operating cost per liter of ethanol. Figure also illustrates the breakdown of facility dependent, raw material and other costs. Other costs include labor, utilities and waste disposal.

Figure 7

Ethanol cost estimations from current models and previous techno-economic studies on ethanol production process (2010 prices). (1) Sendich et al. [53] - Consolidated bio-processing (CBP), (2) Sendich et al. [53] - Simultaneous saccharification and co-fermentation (SSCoF), (3) Aden et al. [19], (4) Eggeman and Elander [31] - Dilute acid pretreatment, (5) Eggeman and Elander [31] - Hot water pretreatment, (6) Wallace et al. [54], (7) Laser et al. [30] - Base case- dilute acid pretreatment, (8) Wingren et al. [55] - Separate hydrolysis and fermentation, (9) Wingren et al. [55] - Simultaneous saccharification and fermentation (SSF), (10) Wingren et al. [56], (11) Gnansounou et al. [32](12) Hamelinck et al. [57] - Long term technology (Hot water pretreatment, CBP), (13) Hamelinck et al. [57] - Short term technology (Dilute acid pretreatment, SSF), (14) Hamelinck et al. [57] - Middle term technology (Steam explosion pretreatment, SSCoF), (15) Kazi et al. [29] - Dilute acid pretreatment, (16) Kazi et al. [29] - Hot water pretreatment.

Figure 8

Effect of pentose fermentation efficiency on cost of ethanol for dilute acid and steam explosion pretreatment process.

Figure 9

Impact of process water treatment on energy use, capital cost and electricity production. Figure illustrates the tradeoff between energy use and capital cost as the liquid stream after filter press is distributed among waste water treatment and evaporator. Blue and red lines show the total process energy and capital cost in the process per liter of ethanol. Green line shows the electricity produced from the excess steam (after using process steam) produced from lignin energy.