Pretreatment of Corn Stover with Diluted Nitric Acid for the

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Pretreatment of corn stover with diluted nitric acid for enhancement of acidogenic fermentation Rui Zhang, Fengguo Liu, Hanqiao Liu, and Dianxin Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02596 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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Energy & Fuels

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Pretreatment of corn stover with diluted nitric acid for

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enhancement of acidogenic fermentation

3 4

Rui Zhang*, Fengguo Liu, Hanqiao Liu, Dianxin Zhang

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School of Energy and Safety Engineering, Tianjin Chengjian University, Tianjin 300384, China

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*Corresponding author: Rui Zhang

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E-mail: [email protected]

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Telephone: +86 22 23085277

9 10 11 12 13 14 15 16 17 18 19 20 21 22

1

ACS Paragon Plus Environment

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ABSTRACT: In this study, dilute nitric acid was selected as the catalyst

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to pretreat corn stover. The effects of nitric acid pretreatments on the

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efficiency of acidogenic fermentation and the acidogenic characteristic of

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prehydrolysates were evaluated. The results showed that about 97%

27

hemicelluloses were recovered at the optimal condition (150 oC, 0.6%

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HNO3, 2min). Acidogenic fermentation of prehydrolysate B (the highest

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yield of total sugar) obtained at the optimal condition proved to be more

30

efficient, compared to prehydrolysate A (no inhibitor) and prehydrolysate

31

C (the highest yield of inhibitors), the VFAs concentration can reach the

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highest level, and the distribution of VFAs exhibited butyric acid type

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fermentation, this was more suitable for subsequent methanogenic

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fermentation. This research provided an effective and suitable method for

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accelerate biogas production from corn stover.

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KEYWORDS: Pretreatment, Nitric acid, Corn stover, Acedogenic

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fermentation

38 39

1. INTRODUCTION

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Corn stover is the most abundant agricultural residues in china, and it

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represents an ideally renewable, cheap, widely available feedstock for the

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production of bioethanol, biogas, biohydrogen, and other chemicals.

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Among all of the technologies, conversion of corn stover to biogas is one

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of the most effective and cheapest methods, 1-3 which not only can supply 2


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sustainable energy resources, but also can reduce the environmental

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pollution. Besides, the Chinese government introduces a series of energy

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structure adjustment policies currently, such as the project of changing

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fuel from coal to natural gas, which makes the demand of natural gas

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increasing quickly. Therefore, biogas production from corn stover is

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promising. However, the main components of corn stover are

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polysaccharide (cellulose and hemicellulose) and lignin, which form

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complex three dimensional structures and are less available for

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microorganisms under normal fermentation. Therefore, to achieve

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efficient use of corn stover, pretreatment is the key point. Pretreatment

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prior to anaerobic digestion has been proven to be one of simple and

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effective methods to improve biodegradability of lignocellulosic

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materials.4-6

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A number of pretreatment methods have been investigated by former

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researchers, including dilute acid pretreatment, alkali, inorganic salt,

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liquid hot water, steam explosion, enzymatic pretreatment, and other

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methods.7-12 Among all the methods, dilute acid pretreatment is the most

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frequently studied process for corn stover, which can effectively convert

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most of hemicellulose (about 80-90%) into fermentable sugars.13 Dilute

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acid pretreatment has been considered as a suitable technology for

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industrial scale biogas production.14-16 Sulfuric acid is commonly

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employed as catalyst.17-19 As another strong acid, nitric acid has been 3


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studied by a few researchers. Xiao et al. found that nitric acid could

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accelerate the solubilization of ligin in the pretreatment of the newspaper

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for bioconversion to methane.20 Rodriguez-chong et al. reported the effect

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of nitric acid pretreatment of sugar cane bagasse, and confirmed that

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nitric acid needed a shorter time to ensure high sugar concentration and

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low concentration of inhibitors as compared with sulfuric acid and

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hydrochloric acid.21In our previous study,22 nitric acid was also used to

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pretreat corn stover, the results showed that 96% of the hemicelluloses

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were hydrolyzed under the optimum condition. Thus, nitric acid has been

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shown to be effective for hydrolysis of ligocellulosic materials.

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Furthermore, for methane production, nitric acid pretreatment was more

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suitable than sulfuric acid, by reason that sulfate was converted to H2S,

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which is a pollutant, while nitrate was converted to N2, which is

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environmentally friendly in the anaerobic fermentation. Therefore, nitric

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acid pretreatment is a favorable method for the conversion of corn stover

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to methane.

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In this study, diluted nitric acid was employed as a catalyst to hydrolyze

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corn stover to transfer as many of the effective components (mainly

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sugars) of the corn stover as possible into the liquid phase

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(prehydrolysate), then the prehydrolysate was used to produce volatile

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fatty acids (VFA). The prehydrolysate is primarily composed of small

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molecule compounds that are more accessible to microorganisms. 4


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Therefore, acidogenic fermentation of prehydrolysate could be efficient,

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which will accelerate biogas production from corn stover. In addition, the

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solid residue is also very valuable for containing cellulose and lignin, it

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could be used to prepare carbonaceous solid acid, which showed excellent

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catalytic hydrolysis performance for the lignocellulosic biomass.23-25

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Therefore, the corn stover could be utilized efficiently.

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However, the components of prehydrolysates obtained at different

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pretreatment conditions were different, which influence the efficiency of

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acidogenic fermentation greatly. So far, the comparison of acid

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production capacity of different type of prehydrolysate obtained by nitric

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acid pretreatment is relatively little. In this study, the contents of

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prehydrolysates including sugars, acetic acid, and inhibitors (furfural and

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HMF) were determined to evaluate the pretreatment effect. Then, three

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types of prehydrolysates from nitric acid pretreatments were fermented to

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produce volatile fatty acids (VFAs) and the acid production capacity of

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sugar and inhibitors (furfural and HMF) in the prehydrolysate was

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investigated.

106 107

2. MATERIAL AND METHODS

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2.1. Raw materials

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Corn stovers were harvested at a local farm and leaves were discarded.

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The corn stover was air-dried, milled, screened to select the fraction of 5


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particles with a size lower than 0.5 mm, and homogenized in a single lot

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and stored until needed. The initial composition of corn stover was

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(weight percent on dry basis): glucan, 36.2±0.62%; xylan, 19.0±0.36%;

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araban, 2.9±0.43%; acetyl group, 4.4±0.59%; Klason lignin, 14±

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0.28%; ash, 7.4±0.37%; and 16.1% others which are mainly protein,

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extractives, and non-structural sugars. These values were in the range

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reported by other researchers for corn stover.26-29

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2.2. Nitric acid-catalyzed hydrothermal pretreatment

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Pretreatment was performed in glass media bottles at 120 oC and in a

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laboratory-scale pure titanium reactor with a total volume of 500 ml at

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150 oC and 180 oC. The glass media bottles were heated to 120 oC with

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autoclave. The pure titanium reactor was heated to 150 oC and 180 oC

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with an electric heater. When the desired temperature inside the reactor

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was reached, the treatment time was started to be counted. After the target

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treatment time was reached, the reactors were cooled to below 50 oC with

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room-temperature water. Then the pretreatment product was filtered by

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vacuum pump, obtaining a solid phase and a liquid phase for further

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analysis. Nitric acid concentrations were 0.1%, 0.6% (w/w). The

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liquor/solid ratio was 9 g liquor/g solid corn stover in all experiments.

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2.3. Anaerobic acidogenic fermentation

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Three kinds of specific prehydrolysates under different experimental

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conditions were selected for further anaerobic acidogenic fermentation. 6


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The first kind of prehydrolysate (prehydrolysate A) is obtained when the

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total sugars were relatively low, but the inhibitors (furfual and HMF)

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were zero. The second kind of prehydrolysate (prehydrolysate B) is

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obtained when the total sugars reached the maximum. The third kind of

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prehydrolysate (prehydrolysate C) is obtained when the inhibitors (furfual

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and HMF) reached the maximum.

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The acidogenic fermentation of the prehydrolysates took place in

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anaerobic bottles which were first pruged by nitrogen to remove the air in

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the headspace of bottles prio to the experiments, and then these bottles

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were placed in a rotary shaker (150 rpm) and incubated at 37 oC (±1 oC).

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The volume of each bottle was 250 ml, and the working volume was 200

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ml. The seeding sludge was obtained from a municipal sewage treatment

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plant located in Tianjin, china. Before acidogenic fermentation, the

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prehydrolysate was diluted to meet the feed to microbes ratio (F/M ratio)

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of 1.0 g of COD/g VSS.30,31 Assay with seed sludge alone was performed

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as a control. VFAs produced from acidogenic fermentation of the

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prehydrolysate were subtracted from the assay.

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2.4. Combined severity factor

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Combined severity factor (CSF) has been reported to evaluate the effect

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of the hydrolysis conditions on the concentration of total sugars and

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inhibitors, which combines catalyst concentration, reaction time, and

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temperature into a single variable.32-35 The combined severity factor was 7


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defined as,

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  T − 100   CSF = log  t ⋅ exp    − pH  14.75   

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Where t is the reaction time (min), T is the reaction temperature (oC), and

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pH is the acidity of aqueous solution determined by the acid

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concentration.

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2.5. Analytical method

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The chemical composition of corn stover was determined according to the

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National Renewable Energy Laboratory analytical methods for biomass.36

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Sugars and VFAs were determined using HPLC (LabAlliance, USA) with

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a

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5-hydroxymethyl-2-furfural (HMF)) with a UV detector by using a

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column (BioRad Aminex HPX-87H, 300×7.8 mm) at 65 oC and 5 mM

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H2SO4 as the mobile phase at a flow rate of 0.6 ml/min.

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COD was determined with a standard method.37

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3. RESULTS AND DISCUSSION

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3.1. Optimization of pretreatment conditions on the contents of

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prehydrolysates

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Figure 1 shows the yields of xylose, glucose and arabinose obtained from

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the prehydrolysates. Figure 1 showed that the yields of soluble xylose

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increased with HNO3 concentration and reaction time at 120 oC, and the

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xylose yield reached 88.9% at 120min and 0.6% HNO3.

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As the temperature rose to 150 oC, as shown in Figure 1, the yield of

refractive

detector

and

inhibitors

8


(furfural

and

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soluble xylose increased all the time when the HNO3 concentration was

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0.1%, while it decreased gradually with time when the HNO3

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concentration was 0.6%, which can be attributed to the further

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degradation of xylose. These results indicated that increasing acid

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concentration can accelerate the degradation of xylan if the temperature is

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high enough and prolonging the time results in the degradation of soluble

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xylose.

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As the temperature rose to 180 oC, the yields of xylose increased with a

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narrow range in the first 10 min with 0.1% HNO3 and in the first 5 min

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with 0.6% HNO3 and then decreased gradually. Meanwhile, the yield of

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xylose obtained at 180 oC, 0.1% HNO3 was higher than that obtained at

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150 oC, 0.1% HNO3, while the yield of xylose obtained at 180 oC, 0.6%

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HNO3 was lower than that obtained at 150 oC, 0.6% HNO3, the results

190

implies that higher temperature can accelerate the degradation of xylan

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when the acid concentration is low, but it can result in undesirable

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decomposition of xylose when the acid concentration is high enough. The

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lowest yield of xylose was 7.8% under harshest condition (180 oC, 0.6%

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HNO3, 60min). The maximum soluble xylose yield in this study was

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96.9% (150 oC, 0.6% HNO3, 2min), which is higher than that from

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H2SO4-catalyzed hydrolysis reported by Liu et al (2004) (90.0%),38

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Fe(NO3)3-catalyzed hydrolysis reported by Sun (2011) (91.8%)

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dilute formic acid catalyzed hydrolysis reported by Xu (90.9%)40. The 9


39

and

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higher xylose yield implies that HNO3 is a more efficient catalyst than

200

sulfuric acid, Fe(NO3)3 and fomic acid.

201

A similar trend was found with arabinose, as shown in Figure 1. The

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degradation of arabinan can be accelerated by increasing temperature and

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HNO3 concentration. The maximum arabinose yield of 100% was

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obtained at the condition of 120 oC, 0.6% HNO3, 120min, which was

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higher than that of xylose, besides, the maximum arabinose yield was

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obtained at a lower temperature than that of xylose. These results indicate

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that it is much easier to obtain arabinose than xylose.

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Figure 1 illustrates the effects of HNO3 concentration, temperature and

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reaction time on the glucose yield. The trends in Figure 1 suggested that

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the glucose yield increased first then decreased with temperature when

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the HNO3 concentration was 0.1%. However, when the HNO3

212

concentration increased to 0.6%, the glucose yield increased quickly with

213

increasing temperature. These results show that the acid concentration is

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very important for glucan hydrolysis. As shown in Figure 1, the reaction

215

time had little effect on glucose yields, prolonging time can result in

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degradation of glucose to HMF when the temperature rose to 180 oC. The

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maximum glucose yield was 18.6% obtained at 180 oC with 0.6% HNO3

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after 10min reaction, which is lower than that of xylose and arabinose.

219

The result indicates that cellulose is more difficult to degrade as

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compared to hemicellulose. 10


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Figure 1. Effects of hydrolysis temperature and nitric acid concentration on sugar yield

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Acetic acid was generated in the hydrolysis of acetyl groups of the

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hemicelluloses. Figure 2 shows the variation of acetic acid concentration

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for different temperature, acid concentration and response times. As

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shown in Figure 2, the acetic acid concentration increased quickly with

228

temperature and time when HNO3 concentration was 0.1%. However,

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when HNO3 concentration rose to 0.6%, the acetic acid concentration

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increased quickly first while increasing temperature from 120 oC to 150

231

o

C, and then the rate of release was very slow but the acetic acid

232

concentration was not decreased, indicating that no decomposition

233

reaction took place. Acetic acid produced in the hydrolysis process not

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only can promote further hydrolysis of corn stover, but can be used as a

235

good substrate in the methane production process.14

236 237

Figure 2. Effects of hydrolysis temperature and nitric acid concentration on the

238

concentration of acetic acid

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Furfural was generated as a degradation product from xylose and

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arabinose, HMF was the decomposition of glucose. Furfural and HMF are

241

furans compounds. Figure 3 shows the concentration variation of furfural

242

and HMF for different temperatures, HNO3 concentration and response

243

time. The result suggested that the concentrations of furfural and HMF 11


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were both increased with the increased temperature, acid concentration

245

and reaction time. The decomposition reaction did not occur under

246

condition of 120 oC, 0.1% HNO3. However, when the temperature rose to

247

180 oC, the decomposition reactions were accelerated. The maximum

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concentrations of furfural and HMF were 12.3 g/L and 7.9 g/L

249

respectively, which were obtained in the experiment performed with 0.6%

250

HNO3 at 180 oC with the reaction time of 60 min. This result implied that

251

most of the monosaccharide can be decomposed under the harshest

252

condition.

253 254 255

Figure 3. Effects of hydrolysis temperature and nitric acid concentration on the concentration of furfural and HMF

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Both hexose and pentose can be used as substrates for methane

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production by a mixture of microorganisms, but sugar degradation

258

compounds (furfural and HMF) are fermentation inhibitors. Therefore, to

259

evaluate the efficiency of hydrolysis conditions comprehensively, the

260

concentrations of sugars and furans (furfural and HMF) were compared at

261

various CSF (0.27-3.09). The results were shown in Figure 4, and it

262

indicated that the highest concentration (30.9 g/L) of total sugars was

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obtained when CSF was fixed at about 0.73, corresponding to the

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condition of 150 oC, 0.6% HNO3, and 2 min. The highest concentration

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(20.2 g/L) of total furans when CSF was fixed at 3.09, corresponding to

266

the condition of 180 oC, 0.6% HNO3, and 60 min. 12


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Energy & Fuels

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Figure 4. Concentration of total sugars and furans as a function of combined severity

269

factor (CSF)

270

In order to evaluate the pretreatment of corn stover with nitric acid

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completely, a simple economical analysis was made. The cost of the nitric

272

acid pretreatment mainly depends on the dosage and price of catalyst

273

according to our analysis. Hence, the cost of nitric acid pretreatment was

274

calculated according to HNO3. The optimum condition was taken for

275

example. At this condition, the HNO3 concentration is 0.6% (w/w), which

276

was prepared by 65% (w/w) HNO3, thereby, 0.083g 65% (w/w) HNO3

277

was needed when 1g corn stover was pretreated. The price of 65% (w/w)

278

HNO3 was $166 per ton according to market price. Therefore, the cost of

279

the nitric acid pretreatment is $ 0.014/Kg corn stover. Although the price

280

of HNO3 is higher than H2SO4 and HCl, the negative influence of HNO3

281

on anerobic fermentation is lower than H2SO4 and HCl, therefore, HNO3

282

has advantage. A new and high efficient method was needed to reduce its

283

pretreatment cost, which could increase its competitiveness as a catalyst.

284

3.2. Effect of prehydrolysate to the amount and composition of VFAs

285

Three kinds of prehydrolysate were chosen to be the substrate for

286

anaerobic acidogenic fermentation. The compositions of prehydrolysate A,

287

prehydrolysate B and prehydrolysate C are shown in Table 1.

288

Prehydrolysate A was obtained when nitric acid concentration was low, 13


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total sugar concentration was almost equal to that of prehydrolysate C,

290

and

291

Prehydrolysate B was obtained when total sugar concentration reached

292

the maximum, prehydrolysate C was obtained when the total furans

293

concentration reached the maximum. The three kinds of prehydrolysates

294

were selected to compare the effect of different concentration of sugar

295

and furan on the acidogenic fermentation process.

the

furans

(furfural

and

HMF)

concentration

was

zero.

296 297

Table 1 Pretreatment condition of three kinds of prehydrolysates and their

298

compositions

299 300

Figure 5 shows the variation of VFAs concentration in prehydrolysate A,

301

prehydrolysate B and prehydrolysate C fermentations. As shown in

302

Figure 5, VFAs concentration in prehydrolysate B fermentation was

303

higher than that in prehydrolysate A and prehydrolysate C at any time in

304

the whole process. The maximum VFAs concentration in prehydrolysate

305

A, B and C digestions were 1110.8 mg COD/ L, 2513.4 mg COD/ L, and

306

1095.4 mg COD/ L, respectively. This result implies that sugar was easier

307

to be utilized by bacteria than furfural and HMF. In addition, VFAs

308

concentration in prehydrolysate A fermentation was higher than that of

309

prehydrolysate C after 10h. Although the initial COD was the same, total

310

sugar was almost equal, while the inhibitor (furfural and HMF) 14


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concentration in prehydrolysate C was higher than that in prehydrolysate

312

A. This indicated that furfural and HMF were harder to be utilized by

313

acid-producing bacteria than sugar.

314 315

Figure 5. Variation of VFAs concentration during the acidogenic fermentation with

316

different prehydrolysates as substarte

317

Figure 6-8 show the VFAs distribution patterns with the addition of

318

different prehydrolysates. As shown in Figure 6, acetic acid and propionic

319

acid accounted for 67-98% of the VFAs from 2 to 72 h. Then the

320

proportion of propionic acid decreased to zero while the proportion of

321

acetic acid increased. The acids were only acetic acid and valeric acid

322

from 96 to120 h. The results indicated that the fermentation of

323

prehydrolysate A exhibited

324

fermentation before 72 h, which can result in low efficiency of

325

methanogenic phase due to the low acetogenic rate of propionic acid.

326

However, the final acid products of prehydrolysate A were suitable for

327

subsequent acidogenic fermentation. As shown in Figure 7, acetic acid

328

and propinic acid accounted for most of the VFAs before 48h, and then

329

butyric acid exceeded propionic acid. Furthermore, the proportion of the

330

sum of acetic acid and butyric acid was about 80% during 48 to 120h in

331

prehydrolysate B, exhibiting the feature of butyric acid fermentation type,

332

which was proved to be favorable to the subsequent biogas production.

the feature of propionic acid-type

15


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As shown in Figure 8, the proportion of the sum of acetic acid and

334

propionic acid were 65-90% in prehydrolysate C during the acidogenic

335

fermentation process, and propionic acid accounted for most of VFAs,

336

exhibiting propionic acid fermentation type.

337 338 339 340 341

Figure 6. VFAs distribution patterns during the acidogenic fermentation with prehydrolysate A as substrate Figure 7. VFAs distribution patterns during the acidogenic fermentation with prehydrolysate B as substrate

342

Figure 8. VFAs distribution patterns during the acidogenic fermentation with

343

prehydrolysate C as substrate

344

4. CONCLUSION

345

In this study, pretreatment of corn stover by dilute nitric acid and

346

acidogenic fermentation with different prehydrolysates were investigated.

347

The results show that nitric acid hydrolysis can be efficient. The VFAs

348

produced by prehydrolysate B was higher than that of prehydrolysate A

349

and C all the time during the acidogenic fermentation process, and the

350

maximum VFAs produced by prehydrolysate B was twice as much as that

351

of prehydrolysate A and C. In addition, the distribution of VFAs produced

352

by prehydrolysate B is more suitable for subsequent methanogenic

353

fermentation. Therefore, prehydrolysate B obtained at the condition of

354

150 oC, 0.6% HNO3 and 2 min, can be used as good substrates for the

355

acidogenic fermentation process.

356

AUTHOR INFORMATION 16


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Energy & Fuels

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Corresponding Author

358

*E-mail: [email protected]

359

Notes

360

The authors declare no competing financial interest.

361

ACKNOWLEDGEMENT

362

This research was funded by National Key Technology R&D Program

363

(Grant NO. 2007BAD75B07), Doctoral Scientific Research Foundation

364

of Tianjin Chengjian University (Grant NO. 60-1302), and The Basic

365

Scientific Research Sponsored Project of Universities in Tianjin (Grant

366

NO. 2016CJ05).

367

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http://www.mrel.gov/docs/gen/fy08/42623.pdf.

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(37) Standard methods for the examination of water and wastewater, 4th

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ed.; China EPA, National Environment Protection Agency: Beijing,

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Energy

Laboratory:Golden,

443 444 20


CO,

USA,

2008;

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A List of Tables

446

Table 1 Pretreatment condition of three kinds of prehydrolysates and their

447

compositions

448 449

Table 1. Pretreatment condition of three kinds of prehydrolysates and their

450

compositions Nitric acid Prehydrolysate

Reacn

Treatment

Total sugars

Total furans

temp(oC)

time(min)

concn (g/L)

concn (g/L)

concn (%(w/w))

A

0.1

120

120

8.90

0

B

0.6

150

2

30.88

0.83

C

0.6

180

60

8.60

20.15

451 452 453 454 455 456 457 458 459 460 21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

461

A List of Figures

462

Figure 1. Effects of hydrolysis temperature and nitric acid concentration

463

on sugar yield

464


465

on the concentration of acetic acid

466


467

on the concentration of furfural and HMF

468

Figure 4. Concentration of total sugars and furans as a function of

469

combined severity factor (CSF)

470

Figure 5. Variation of VFAs concentration during the acidogenic

471

fermentation with different prehydrolysates as substarte

472

Figure 6. VFAs distribution patterns during the acidogenic fermentation

473

with prehydrolysate A as substrate

474


475

with prehydrolysate B as substrate

476


477

with prehydrolysate C as substrate

478

22


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479 480

Figure 1. Effects of hydrolysis temperature and nitric acid concentration on sugar

481

yield

482

(Xy 1: Xylose-120 oC, 0.1% HNO3; Xy 2: Xylose-120 oC, 0.6% HNO3; Xy 3: Xylose-150 oC,

483

0.1% HNO3; Xy 4: Xylose-150 oC, 0.6% HNO3; Xy 5: Xylose-180 oC, 0.1% HNO3; Xy 6:

484

Xylose-180 oC, 0.6% HNO3; Gl 1: Glucose-120 oC, 0.1% HNO3; Gl 2: Glucose -120 oC, 0.6%

485

HNO3; Gl 3: Glucose-150 oC, 0.1% HNO3; Gl 4: Glucose-150 oC, 0.6% HNO3; Gl 5: Glucose

486

-180 oC, 0.1% HNO3; Gl 6: Glucose-180 oC, 0.6% HNO3; Ar 1: Arabinose-120 oC, 0.1% HNO3;

487

Ar 2: Arabinose-120 oC, 0.6% HNO3; Ar 3: Arabinose-150 oC, 0.1% HNO3; Ar 4: Arabinose-150

488

o

C, 0.6% HNO3; Ar 5: Arabinose-180 oC, 0.1% HNO3; Ar 6: Arabinose-180 oC, 0.6% HNO3)

489

23


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490 491


492

concentration of acetic acid

493 494


495

concentration of furfural and HMF

496

(Fur 1: Furfural-120 oC, 0.1% HNO3; Fur 2: Furfural -120 oC, 0.6% HNO3; Fur 3: Furfural -150 24


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Energy & Fuels

497

o

C, 0.1% HNO3; Fur 4: Furfural -150 oC, 0.6% HNO3; Fur 5: Furfural -180 oC, 0.1% HNO3; Fur 6:

498

Furfural -180 oC, 0.6% HNO3; HMF 1: HMF -120 oC, 0.1% HNO3; HMF 2: HMF -120 oC, 0.6%

499

HNO3; HMF 3: HMF -150 oC, 0.1% HNO3; HMF 4: HMF -150 oC, 0.6% HNO3; HMF 5: HMF

500

-180 oC, 0.1% HNO3; HMF 6: HMF -180 oC, 0.6% HNO3)

501

502 503

Figure 4. Concentration of total sugar and furans as a function of combined severity

504

factor (CSF)

505

(PHA: Prehydrolysate A, PHB:Prehydrolysate B, PHC:Prehydrolysate C)

506

25


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507 508

Figure 5. Variation of VFAs concentration during the acidogenic fermentation with

509

different prehydrolysates as substarte

510 511


512

prehydrolysate A as substrate

513 26


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514 515


516

prehydrolysate B as substrate

517 518


519

prehydrolysate C as substrate

27


Pretreatment of Corn Stover with Diluted Nitric Acid for the

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