(587c) Hydrogen Production From Fermenting Mixtures of Pentose and Hexose Sugars Produced From the Steam Explosion of Switchgrass

Introduction    

    
Increasing demand for fossil fuel supplies, international conflicts, pollution,
energy security and greenhouse gas emissions have caused heighten concerns for
industrialized and non-industrialized nations to develop alternative energy
sources. Hydrogen (H₂) is emerging as a potential contender because
it is CO₂ neutral if it is produced from agriculture products and it
has a high energy yield. 

   
Among the different H₂ production technologies, the dark
fermentation route presents considerable advantages because the process is
robust, it can operate under moderate temperature conditions, a wide variety of
low value agriculture residues can be utilized and feedstock sterilization is
not required (Gomez et al.,
2011).
During dark fermentation, H₂ methane and carbon dioxide are the ultimately
end products.  Theoretically, only 33% of the electron equivalents from a
hexose sugar source is transformed into H₂ assuming acetate is the
only reduced carbon byproduct. This is equivalent to 4 mol_×mol^-1
glucose or 0.467 L H_2×g^-1 COD.  The remaining 66% is diverted
into the formation of the volatile fatty acids (VFAs) and alcohols (Bartacek et
al., 2007).

    
Low H₂ yields are attributed to the presence of H₂
consuming microorganisms (Weissman
and Benemann, 1977). Increasing the
H₂ yield can be accomplished by using microbial stressing agents
such as heat, pH, chemicals as well as engineering design parameters to
suppress or control the growth of H₂ consumers (Terentiew and
Bagley, 2003).
Previous studies have shown that pH, hydraulic retention time (HRT) and culture
treatment are important factors which can affect the H₂ yield (Lay,
2000;
Ueno et al., 1996).
 Long chain fatty acids (LCFAs) have been shown to reduce the growth of H₂
consumers.  According to Cata Saady et al. (2012), linoleic acid
(LA; (C18:2)), a H₂ consumer inhibitor, is able to increase the H₂
yield.

   
Hydrogen production from simple sugars such as glucose has been demonstrated in
continuous flow reactors (Hafez et al., 2009). However, using feedstocks
containing pure sugars is not a practical approach because of our dependence on
agriculture food products to produce fuels and the financial implications
related to the supply and demand of food products to consumers.  In comparison, 
H₂ production utilizing feedstocks consisting of hexoses plus
pentoses from low value lignocellulosic agriculture residues will likely contribute
to a sustainable H₂ economy and solve issues related to the over use
of agriculture food products. A fast growing crop such as switchgrass (SG) with
low nutrient requirements (McLaughlin et al., 1999) is considered to be a
suitable energy crop in North America and its production is likely to increase several
fold (Sokhansanj et
al., 2009).
 In this study, we examined H₂ production from a hemicellulose rich
fraction generated from the steam explosion of SG.  Hydrogen production from SG
was assessed using mixed anaerobic cultures in an up-flow anaerobic sludge
blanket reactor (UASBR) under varying pH and HRT conditions with and without a
methanogenic inhibitor (LA).

Methodology

    
The raw material SG was steam exploded at 190 ^oC for 10 min under
high pressure conditions. The steam exploded liquor was hydrolyzed with 2.5 % of
H₂SO₄ to produced monomeric sugars.  The steam explosion
reaction conditions were optimized to minimize the formation of furans.  The
liquor was withdrawn from the reaction vessel and treated with a resin (Amberlite
XAD-4, Rohm and Haas, PA) to remove microbial inhibitors such as furan
derivatives (furfural and 5- hydroxymethyl furfural) (Weil et al., 2002).  This
treated liquor was used as a fed to the reactor at a concentration of 5 g COD.L^-1. 

     
Experiments were conducted in UASBRs using mixed anaerobic cultures obtained
from a wastewater treatment facility located at a brewery in Guelph, Ontario. 
The UASBRs were operated in continuous mode and at 37 ^oC. All
experiments were conducted in duplicate.  Hydrogen and methane were analyzed
using a gas chromatograph (GC) configured with a thermal conductivity
detector (TCD). 
Volatile fatty acids (VFAs) and alcohols were measured using an ion
chromatograph (IC) (data not shown).  The GC and IC methods were performed in
accordance with Cata Saady et al. (2012).  The COD for the influent and
effluent products were measured according to Greenberg et al.
(1998). 
The microbial communities were characterized using nested-PCR of16S rRNA gene
using terminal restriction fragment length polymorphism (T-RFLP).  Details of
the protocol are described by Chaganti et al. (2012).

Results

      
The pretreated SG consisted of approximately 60% pentoses and 40% hexoses. The
major sugars present were xylose and glucose in the liquid hydrolysate.  Hydrogen
production from SG hydrolysate was investigated using mixed anaerobic cultures
in the UASBRs under continuous operational conditions.  A maximum H₂
yield of 0.31±0.01 L H₂.g COD^-1 corresponding to
67% of theoretical yield was obtained at pH 5.0 and an HRT of 12 h at an LA
concentration at 2,000 mg.L^-1. At an HRT at 12 h, decreasing the pH
from 7.0 to 5.0, compared with control (without LA addition), the LA treated
cultures showed an increased in H₂ yield to approximately 65% (Fig.
1 a). Whereas when decreasing the HRT from 16 to 8 h at a pH of 6.0
compared with control, a 77% increase in the H₂ yield was observed
in the LA treated cultures (Fig. 1 b). Among the factors examined, pH
and inhibition of the culture was more pronounced when compared to the HRT. 
The VFAs (filtered), alcohols (filtered), plus gas COD of the effluent,
accounted for approximately 80-100% of the influent COD.  Archaea
(Methanosarcina sp and  Methanobacterium sp.) were dominant at
pH 7.0 without any inhibitor; however, when reducing the pH and increasing the
LA concentration, methanogens were washed out and Clostridium sp were dominant.
Decreasing the HRT from 16 to 8 h caused a reduction in growth of homacetogens
such as Bacteroides sp., and Morella thermoaceticum. These
results show that the H₂ yield increases as the quantity of H₂
consumers are reduced or suppressed in the bioreactors.

Conclusions

     
Switchgrass is a potential low value biomass which could be used to produce H₂
using mixed anaerobic cultures. UASBR operational parameters (HRT and pH) and a
methanogenic inhibitor (LA) can be manipulated to increase the H₂ yield. 
Elevated H₂ yields were observed when the cultures were inhibited
with LA, the pH was decreased to 5.0 and the HRT was reduced to 8 h.  

Figure 1:  Hydrogen yield from steam
exploded switch grass liquor

(a) at constant HRT (12 h) (b) at
constant pH (6.0)

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