Evaluation of SO2 catalyzed steam pretreatment of corn stover
with adjusted dry matter content
Carl Gustav Mårtensson
Department of Chemical Engineering Lund University
Abstract
Corn stover is a residual material that is suitable for the production of bioethanol. The
material has to be pretreated to be able to convert the hydrocarbons in the material into
sugars. Steam pretreatment is a relatively cheap method where the material is exposed to
high-pressure steam in a closed reactor. If the material is impregnated with the catalyst
SO2 it is possible to perform the steam pretreatment at lower temperatures.
However, SO2 is a poisonous gas, which shouldn’t be used in excess. The addition of
SO2 is based upon the dry matter content of the material. If the dry matter contain of the
material can be increased prior to impregnation then less SO2 will be added. The effect
when adjusting the dry matter content of the material is studied within this project.
To investigate the effectiveness of the pretreatments enzymatic hydrolysis of pretreated
material was performed. The highest yield of glucose obtained in the hydrolysis was 92
% of theoretical for material that had a dry matter content of 38 % that had been
pretreated at 210 oC for five minutes. The highest yield of xylose was 74 %, which was
obtained from material with a dry matter content of 28 % that was pretreated at 190 oC
for five minutes.
The highest total yield of sugars was recovered from material pretreated at 190 oC for
five minutes with a dry matter of 28 %.
The fermentability of the hydrolysates was tested under standardized conditions. No
problems with the fermentability were observed in any of the samples fermented.
Introduction
The increasing amount of greenhouse
gases present in the atmosphere today is
recognized as a great problem. Carbon
dioxide from gasoline driven cars is a
major issue when discussing the
greenhouse effect. Scientists all over the
world are struggling to come up with other
more environmental friendly fuels than
gasoline and diesel. Bioethanol being a
renewable energy source is such a fuel.
Bioethanol is today already produced from
sugar canes and corn. Much research is
currently investigating the possibility to
produce bioethanol from other cheaper
residual products. When considering
residual products as a raw material it is the
cellulose that is to be converted to sugar
and then fermented into ethanol.
The process for doing that is more complex
and possibly more expensive than that used
for sugar canes and corn but the raw
material is much cheaper.
Biomass
Biomass is a group name for products
ranging from different sources but with a
similar chemical composition. Ethanol
produced from biomass is called
bioethanol. When biomass grows it
consumes CO2 via the photosynthesis,
which means that the net contribution of
CO2 to the atmosphere is less when using
bioethanol rather than gasoline as a fuel.
The three major components in biomass
are cellulose, hemi cellulose and lignin.
Cellulose is a polymer with D-glucose as
repeating unit. The polymer is mainly
crystalline and the strains are positioned in
layers, which are kept together with
hydrogen bonds and lignin. This gives the
molecule its stiff characteristics. Hemi
cellulose is a shorter more branched
polymer with substituted groups. The hemi
cellulose polymer can consist of different
pentoses for example xylose or manose.
Lignin is an amorphous mainly branched
fenolic polymer. It contributes to the stiff
quality of wood by acting as glue between
the cellulose strains.
The native structure of cellulose makes it
non susceptible to enzymes. To make the
cellulose more accessible it needs to be
pretreated. There are many kinds of
pretreatment methods but the common goal
for them all is to make the structure softer,
remove the lignin, increase the porosity
and split the cellulose chains into shorter
ones.
The project
The raw material used in this project is
corn stover, which is what’s left after the
milling process of corn. When the corn
stover enters the process it has a dry matter
content of about 96 %. The first step is
therefore to moisten the material. After this
is done the material has a dry matter
content of 20 %.
The pretreatment method used within this
project is steam explosion. During steam
pretreatment the material is exposed for
steam with a temperature around 200 oC.
Water inside the cells is evaporated
causing an increased pressure inside the
cell. When the time for the pretreatment
has expired the pressure is released. The
pressure inside the cells is then higher than
on the outside, which causes them to
explode. A problem related to this method
is that the high temperature enhances the
degradation of the cellulose and hemi
cellulose sugars. The degradation products
act inhibiting on the subsequentsteps .
Therefore the raw material is impregnated
with the catalyst SO2 before pretreatment,
which makes it possible to perform the
pretreatment during shorter times and at
lower temperatures.
Sulfur dioxide is a poisonous gas. It is
consequently important that no more gas
than necessary is used. The addition of SO2
is based upon the water content in the
material. If the dry matter content of the
material can be increased before the
impregnation then a smaller amount of SO2
can be used. The purpose of this project
was to evaluate the effect of a varied dry
matter content in the material in the
pretreatment.
Enzymatic hydrolysis
During enzymatic hydrolysis the cellulose
in the pretreated material is converted to
sugar. Common for all enzymes are that
they are specific and effective. The
enzymes used in this project were specific
against b-1,4-glucosidic bonds. Three
different enzymes, endoglucanase,
exoglucanase and b-glucosidase were used.
The endoglucanases are especially active
against the amorphous parts of the
cellulose and splits the chains into shorter
ones. The exoglucanases cleave units of
cellobiose from the ends of the cellulose
chains. A cellobiose unit is two linked
glucose molecules. Finally the cellobiose
unit is cleaved into two glucose molecules
by b-glucosidase. Since both cellobiose
and glucose inhibit the effect of the
enzymes a low concentration of the solid
material is chosen for the hydrolysis.
Methods
An analysis of the raw material was
performed with a method developed by
National Renewable Energy Lab (NREL)
which determined the sugar and the lignin
content. The calculations of the total yield
of sugar were based on the values that
were determined with the NREL method.
After the material had been moisturized it
had a dry matter content of 20 % if this can
be increased then lower amounts of SO2
will be used. To increase the dry matter the
material was pressed. The material was
feed into a drum, which had a lid that
could be pressed down using a manual
press. The water, which was pressed out
could leave the material through a net in
the bottom of the drum. A problem related
to this process is that the material
afterwards can hold different dry matter
contents. To compensate for this the
pressed material was kept in a bucket with
a lid in a cold room for one day. After that
it was considered that the dry matter
content was homogenous in the material.
The pressed material was put in plastic
bags and SO2 was added. The addition of
SO2 was 3 weight% of the water content in
the material. The bags were shaken for five
minutes and then put in a ventilated space
for at least an hour to allow the gas to
penetrate into the material. In general
around 2 weight% was absorbed by the
material. The excess gas was removed
before the material was pretreated.
The pretreatment was carried out in a 2.4 l
reactor. Each pretreatment time was five
minutes but the temperature was varied
between 190 and 220 oC. When the time
for the pretreatment had expired the
material was released into a cyclone at
atmospheric pressure. The cyclone was
then manually cleaned and the material
was gathered in a bucket.
After the steam pretreatment the material
was filtered through a filter paper. To
speed up the process a vacuum suction was
used. The pretreated material was now
divided into hydrolysate and solid material.
The solid material was washed to remove
degradation products and sugars.
The solid material was then exposed to
enzymatic hydrolysis. The hydrolysis was
carried out in oneliter glass bottles at a
temperature of 40 oC. Acetate buffer was
added in such amount that a 2 weight-%
dry matter content solution was obtained.
The solution was autoclaved and when it
had cooled the enzymes were added and
the hydrolysis was started. Samples were
taken after 2,4,6,8,24,48,72 and 96 hours.
The samples were analyzed for their sugar
concentrations.
The glucose in the hydrolysate was
fermented to ethanol using ordinary
baker’s yeast. The microorganisms in this
yeast cannot ferment pentoses. The
purpose of the fermentation was to
evaluate the inhibiting effect of the
degradation products. Therefore a
reference solution containing nothing but
glucose and water was prepared. The
glucose concentrations in the samples and
the reference were adjusted so that it was
50 g/l of glucose in each one. The solutions
were then supplied with a nutrition source
for the yeast to grow in. The pH was
adjusted to 5.5. The samples and the
reference were then autoclaved and after
cooling the yeast suspension was added.
The yeast concentration in the solution was
5 g dry yeast/l. Samples were taken after
2,4,6,8 and 24 hours and analyzed for
ethanol, sugar and degradation products
content.
Discussion
After that a reasonable amount of material
had been pressed to a certain dry matter. It
was pretreated within a series of 190-220
o
C. After evaluating the result from the
material that had been pretreated at 220 oC
it was concluded that the loss when
exposing the material to such high
temperatures is to high. The higher dry
matter content, which means less SO2 but
also that there is less water to be vaporized
inside the cells can make the pretreatment
less effective and some parts of the
material may appear to be untreated
afterwards. To compensate for this effect
the material must be treated at higher
temperatures to avoid inhomogeneous
pretreatment but the higher temperatures
result in higher losses.
and the amount of yeast cells in the
solution a positive effect on the rate of
ethanol production has been found.
Before the enzymatic hydrolysis the solid
material was analyzed for its composition
with the NREL method. After the
hydrolysis was completed the sugar yield
within it was calculated.
To validate the results in this study more
pretreatments with different dry matter
contents have to be done, if possible in a
larger reactor to get a more homogenously
pretreated material.
Results and conclusions
References
When material is pretreated at high
temperatures and with high dry matter
content the losses tends to be bigger. Less
hydrolysate is to be expected after the
filtration if the material has been pretreated
with a high dry matter content. Raw
material pretreated with a dry matter
content around 30 % appears to be slightly
less homogenous afterwards than material
that has a dry matter content of 20 %
before pretreatment. However no negative
effect on the hydrolysis could be
established for high dry matter content raw
material that has been pretreated. The yield
in the hydrolysis step was 80-90 % for
glucose and 60-70 % for xylose. Material
pretreated with high dry matter content
could reach as high concentrations of
sugars as material pretreated with low dry
matter content.
The total yield of sugar was overall the
highest for material that had been
pretreated at 190 oC with a dry matter of
28 %. Also for the samples pretreated at
200 oC a dry matter content of 28 %
proved to give the highest total yield of
sugar. The yield of sugar is depending on
the loss of raw material. For the whole
series of material with a dry matter of 28 %
the losses in general were small compared
with series with both higher and lower dry
matter content.
The ethanol concentration in all the
samples that were fermented was higher
than it was in the reference. This is
explained by the presence of acetic acid in
the samples. At a certain ratio between the
concentration of undissociated acetic acid
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