(618c) 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