Techno-Economic Analysis of Corn Butanol Process

Introduction

Biofuels production worldwide is continuing to grow at a very rapid pace.  Energy policy in the form of a renewable fuel standard (EISA 2007)¹ has helped to maintain strong markets for bioethanol and biodiesel as first generation biofuels.   However, recent market price fluctuations for feedstocks have been quite dramatic.  Average corn prices during this time have also ranged from $2 per bushel to $4.20 per bushel (USDA 2008)² with spot prices rising over $8 per bushel.  Butanol is considered a second generation biofuel that is better for the existing infrastructure. It has a higher heating value than ethanol and it is compatible with gasoline at high concentrations without engine modification. It is also more hydrophobic than ethanol and can be shipped via existing pipelines and distributed through the existing petroleum infrastructure. It becomes increasingly important to understand butanol production economics as a potential biofuel. Production cost data for non-existing or non-commercial biofuels processes is less readily available and comes largely from detailed techno-economic models and evaluations.  Therefore, detailed techno-economics models and evaluations are applied to several current and future biofuel processes in the paper. This work presents detailed comparative analysis on the production economics of butanol via biological conversion using corn.  Our objectives include demonstrating the impact of key parameters on the overall process economics (e.g. plant capacity, raw material pricing, yield, etc), and comparing how next-generation technologies and fuels will differ from today's technologies. 

Methods

Developing process economics of this sort requires experimental data, modeling toolsets, vendor equipment information, and engineering expertise. The first step is to develop a conceptual process design from information on the products, feeds, and proposed chemical and physical processing steps (Douglas 1989)³. The effective use of conceptual design methods in the early stages of process design can have a large impact on overall process design and development. Material and energy balance and flowrate information for a given process design are then generated using process simulation software packages. For these particular applications, Aspen Plus (Aspen Plus 2006)⁴ was used. This software contains physical property and thermodynamic data for a large number of chemical compounds. The material and energy balance data generated by these models are used to size and cost process equipment, which are fed into spreadsheets built for capital and operating cost estimation.

Results and Discussion

Fermentation of sugar-containing substrates to acetone, butanol, and ethanol (ABE) is well known.  With the onset of rising petroleum prices and new biotechnology, biological butanol production may become the more cost effective technology once again. In the long run, the butanol production via biomass may be more economical than petrochemical industry as a transportation fuel with much larger scale than in the past. 

Process description. While several designs and process variations have been explored in literature, very little modeling has been done using rigorous physical property models.  Simple assumptions of how components are expected to fractionate and separate, for example, can often be proven wrong through such an approach.  After analyzing several conceptual designs, NREL developed its own conceptual design for ABE production.  The USDA corn dry mill ethanol model was used as a basis for modeling.  The ethanol fermentation and downstream recovery was replaced with a butanol fermentation using Clostridium.  A complex sequence of distillation steps was then added to separate and purify the acetone, butanol, and ethanol components from the water.  Non-random two-liquid (NRTL) modeling package was used to more accurately predict the liquid/liquid interactions that would take place in a system such as this.  As with corn ethanol, the byproduct from this process is animal feed.  While DDGS from yeast-based production is common, bacterial-based DDGS would have to undergo animal feeding trials.  H₂ is also produced from Clostridial strains of this sort.  This is collected and purified using commercial pressure-swing adsorption (PSA) technology.  In this fashion, 5 total products are produced:  ethanol, hydrogen, DDGS, acetone, and butanol.

Corn is milled first then sent to liquefaction.  After liquefaction, microorganism is added to ferment the glucose to acetone, butanol and ethanol mixture.  Total residence time in the fermentors is 72 hours. The gas stream from the fermentors goes through a PSA unit to recover hydrogen, which is sold as co-product. The whole beer is degassed and sent to distillation systems. About 85% water is removed from the dehydration column then recycled back to liquefaction. Downstream distillation columns further separate acetone, ethanol and n-butanol from residual water, combined with process options of extractive distillations, molecular sieve or pervaporation membrane units.

Process economics. Qureshi and Blaschek published ABE Butanol production costs at $1.56/gal based on $1.80 per bushel corn feedstock cost (Qureshi and Blaschek 2000)⁵. In this paper, by a different but thorough approach, the economics (annualized cost of production, not cash flow analysis) for corn ethanol production are calculated in an Excel spreadsheet.  The material and energy balance data obtained from the Aspen Plus simulation is used in the spreadsheet to size and cost the specified capital equipment.  It is also used to calculate the fixed (labor and supplies) and variable (raw materials) operating costs of the plant.  The economics have been updated to reflect year-$2007.  The specific costing data sources are originally gathered by USDA, and represent a combination of vendor specifications, costing program results, and chemical and utility list prices. Overall economic process assumptions are listed in details and the resultant process economics are reported in this paper with a total installed cost of equipments.

Cellulosic Butanol Potentials. Using corn grain to make butanol is a logical technology progression because much of the technology is already demonstrated and commercial. However, further increases in biofuel production (ethanol and butanol) to meet the goals of the renewable fuels standard can be produced from cellulosic biomass, such as corn stover, corn fiber, wheat straw, barley straw, energy crops like switchgrass and miscanthus.  It should be noted that Butanol-producing cultures are able to utilize a wide variety of carbohydrates, such as cellobiose, sucrose, glucose, fructose, mannose, lactose, dextrin, starch, xylose and arabinose (Qureshi and Thaddens 2008)⁶.  There is no reported cost data for cellulosic butanol production available yet.

Comparison on process economics of corn ethanol and corn butanol. The Total Project Investments (TPIs) for the corn ethanol and corn butanol processes are discussed in the paper for comparison at the plant scale of 45 MM gallon production per year. For TPI per gallon biofuel production, corn butanol costs more than corn ethanol process due to both low yield and high installed equipment cost under current technologies. However, once converted to a production cost with energy equivalence to gasoline, the modeled butanol production cost is becoming more attractive economically.

Conclusions

The techno-economic analysis of biobutanol process is presented with detailed comparison with commercialized corn ethanol processes.  Knowledge in techno-economic analysis of biofuels can also be applied to future development of biofuels processes that are not yet commercial.  This includes the cellulosic processes, but also advanced biofuels processes, such as butanol.  While feedstock costs are the single largest portion of the overall cost for cellulosic processes, capital costs are much higher for these processes because of the increased difficulty presented by deconstruction and utilization of these materials. The techno-economic analysis can be useful in determining which emerging technologies have highest potential for near to long term success. The results of this work can be also useful in directing research toward areas in which improvements will result in the greatest cost reduction, so that the advancement toward the final goal of commercialization can be measure.

Acknowledgement

NREL would like to thank the US Department of Energy (DOE) Office of the Biomass Program (OBP) for its continued leadership, support and collaboration in the biofuels arena.

References

(1)                 EISA (2007) EISA of 2007 Calls for Additional Production of Biofuels. http://www.renewableenergyworld.com/rea/partner/stoel-rives-6442/news/article/2008/01/eisa-of-2007-calls-for-additional-production-of-biofuels-51063

(2)                 USDA (2008) USDA Soybean Projections, 2008-17. http://www.ers.usda.gov/briefing/soybeansoilcrops/2008baseline.htm#ussoybean

(3)                 Douglas JM (1989) Conceptual Design of Chemical Processes. McGraw-Hill

(4)                 Aspen Plus^TM (2006) Release 2006.5, Aspen Technology Inc., Cambridge M

(5)                 Qureshi N, and Blaschek NP (2000) Economics of Butanol Fermentation using Hyper-Butanol Producing Clostridium Beijerinckii BA 101. Trans IChemE part c:78

(6)                 Qureshi N, Thaddeus CE (2008) Butanol, a Superior Biofuel Production from Agricultural Residues (Renewable Biomass): Recent Progress in Technology. Biofuels Bioproducts and Biorefining 2:319-330