(397b) Energy Optimization of Bioethanol Production Via Gasification of Switchgrass

The
limited energy resources in the form of fossil fuels, the lack of security in
their supply, and the increasing demand have led to consider alternative and
renewable energy sources.  Among the different possibilities only biomass
provides an alternative fuel that can be implemented in the short-term for the
transporting sector, which is the most challenging due to the need for high
density energy sources. Thus, ethanol from biomass
has become one of the most important alternatives to gasoline due to its
compatibility with current automobile engines (Cole 2007) and to the fact that
it can take advantage of the existing supply chain of liquid fuels that is already
well established and thus, its production has been supported by governmental
policies in the US and the EU. However, the volume of production of ethanol
that is required to meet these policies and the competence of the first
generation ethanol with the food supply chain has raised questions regarding
its technological feasibility as an alternative fuel. Thus, the so called
second generation of biofuels has received worldwide attention in order to
improve the yield and reduce the consumption of utilities in the production
process while using a raw material not related to food. However, currently no
production process of ethanol from lignocellulosic raw materials has been
implemented due to existing technical, economic
and commercial barriers (Huang 2008).

Two
types of process technologies can be used to transform lignocellulosic raw
materials into ethanol. The first one is based on the hydrolysis of the raw
material to break down the physical and chemical structure of the crops to
expose the sugars that are fermented to ethanol. Due to its similarity with the
current production of ethanol and the expected lower capital cost, this
technology has received the attention of many researchers, e.g. Hamelinck
et al. (2005); Cardona & Sánchez (2006);
Zhang et al. (2009); Keshwani
& Cheng (2009). The main disadvantage of the
hydrolysis of the lignocellulosic material is the fact that lignin cannot be
processed and thus a part of the carbon source of the raw material cannot be
used to obtain ethanol. The second technology is based on the gasification of
the raw material into syngas, which is used to obtain ethanol either via
Fisher-Tropsch based catalytic reaction or via fermentation of the syngas
(Phillips et al, 2007; Huhnke, 2008;  Piccolo
and Bezzo, 2009; Zhu et al 2009).

In this
paper we present the bioethanol production process from switchgrass that
features optimal energy use. The process consists of three different parts. The
first one is the gasification of the raw material. Two different technologies are
considered: (1) indirect low pressure gasification with steam where the
combustion of char in a parallel equipment (combustor) provides the energy for
the gasification of the biomass by heating sand which is fed back to the
gasifier; or (2) direct high pressure gasification of the raw material with
steam and pure oxygen to avoid the dilution of the gas.

The
second part comprises the technologies to clean up the gas from solids as well
as other compounds like hydrocarbons, NH₃, CO₂ or H₂S
and to adjust the gas composition. The hydrocarbons are partially removed in
the tar where they are either reformed with steam, or partially oxidized. In the case of high
pressure, the solids are removed in a ceramic filter and next the gas will be
expanded generating energy. If the indirect lower pressure method of
gasification is used, the solids are removed together with NH₃ in a
wet scrubber and compressed. In both cases, the last traces of hydrocarbons are
removed in a Pressure Swing Adsorption (PSA) system with a bed of silica gel.
Then, the composition of the gas is adjusted to a molar ratio of CO:H₂
of 1. In order to accomplish this, three technologies such as water gas shift
reactor, bypass and hybrid membrane / PSA for removal of H₂ (with a
bed of oxides) are considered. The selection depends on the performance of the
gasifier and the tar reformer.
Sour
gases such as CO₂ and H₂S are next removed. In case of using
catalytic synthesis of ethanol, the H₂S must be completely removed
from the gas due to its poisoning effect on the catalysts. In contrast,
fermentation with bacteria can handle up to 2.5% in volume of H₂S. The three
technologies considered for removing the sour gases are: (1) the absorption of
the sour gases in Monoethylamine (MEA), (2) a PSA system with a bed of Zeolite
5A, and (3) the use of a membrane permeable to CO₂ and also using
MEA as carrier. Selexol is another possible technology but the solvent is more
expensive and more suitable for higher pressures than the ones used in the
flowsheet.

 Finally,
two
synthetic paths are considered. The first option is the fermentation path where
the syngas is fermented in a stirred tank reactor. The unreacted gases are
recycled to the gas cleanup section of the process. The water must be
removed from the ethanol-water solution to obtain fuel quality ethanol. A beer
column greatly reduces the amount of water in the diluted solution of ethanol.
In the next step four technologies are evaluated to dehydrate the ethanol: (1)
distillation to the azeotropic point, (2) adsorption in corn grits, (3) the use
of molecular sieves with a bed of Zeolite 13 X, and (4) pervaporation. The
second path for the synthesis of ethanol is the high alcohols synthesis
production .The
light hydrocarbons and the unreacted gases are recycled back to the cleanup
section of the process
The purification of ethanol is carried out using a sequence of distillation
columns. Two sequences are evaluated, direct and indirect.

 The
optimization of the system is formulated as a Mixed-Integer Non-linear
Programming problem where all the units are modeled using short cut methods,
design equations and mass and energy balances. The problem is solved through a
special decomposition scheme. We relax the binary variables related to
gasification, reforming and synthetic path generating 8 subproblems that can be
solve as NLP's.  For each of the subproblems we optimize the energy needed for
the cleanup and purification stages. Once the structure of the process is
determined, multieffect columns replace distillation columns and heat
integration is performed. Finally an economic evaluation is carried out
accounting for labor, maintenance, chemicals, raw material and utilities.

The optimal process consist of direct gasification followed by
steam reforming, removal of the excess of hydrogen to be sold while the sour
gases are removed in sequence using PSA and MEA  in this order . Finally the catalytic
synthetic path is selected followed by direct distillation sequence to purify
ethanol to fuel grade. This process reports a promising production cost of 0.42
$/gal when the excess of hydrogen in sold as a byproduct.

References:

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Sánchez. O.J.
(2006) Energy consumption analysis of integrated flowsheets for production of
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Keshwani, D.
R., Cheng, J.J. (2009) Switchgrass for bioethanol and
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Hamelinck,
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Huang,
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Huhnke, R. L. (2008) Cellulosic
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for a Sustainable World

http://www.articlearchives.com/energy-utilities/renewable-energy-biomass...

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(2009) 478 ? 491

Zhu,
Y., Gerber, M.A., Jones, S.B., Stevens, D.J (2009) Analysis of the effects of
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