Improved hydrogen production via thermophilic fermentation of corn stover by microwave-assisted acid pretreatment

A microwave-assisted acid pretreatment (MAP) strategy has been developed to enhance hydrogen production via thermophilic fermentation of corn stover. Pretreatment of corn stover by combining microwave irradiation and acidification resulted in the increased release of soluble substances and made the corn stover more accessible to microorganisms when compared to thermal acid pretreatment (TAP). MAP showed obvious advantages in short duration and high efficiency of lignocellulosic hydrolysis. Analysis of the particle size and specific surface area of corn stover as well as observation of its cellular microstructure were used to elucidate the enhancement mechanism of the hydrolysis process by microwave assistance. The cumulative hydrogen volume reached 182.2 ml when corn stover was pretreated by MAP with 0.3 N H₂SO₄ for 45 min, and the corresponding hydrogen yield reached 1.53 mol H₂/mol-glucose equivalents converted to organic end products. The present work demonstrates that MAP has potential to enhance the bioconversion efficiency of lignocellulosic waste to renewable biofuel.     

Modulation of crude glycerol fermentation by Clostridium pasteurianum DSM 525 towards the production of butanol

High production yields and productivities are requisites for the development of an industrial butanol production process based on biodiesel-derived crude glycerol. However, impurities present in this substrate and/or the concentration of glycerol itself can affect the microbial metabolism. In this work, the effect of crude glycerol concentration on the production of butanol and 1,3-propanediol (1,3-PDO) by Clostridium pasteurianum DSM 525 is studied. Also, the effect of acetate and butyrate supplementation to the culture medium and the culture medium composition are evaluated. The results showed a marked effect of crude glycerol concentration on the product yield. The competitive nature of butanol and 1,3-PDO pathways has been evident, and a shift to the butanol pathway once using higher substrate concentrations (up to 35 g l⁻¹) was clearly observed. Butyrate supplementation to the culture medium resulted in a 45% higher butanol titre, a lower production of 1,3-PDO and it decreased the fermentation time. Acetate supplementation also increased the butanol titre but the fermentation was longer. Even though glycerol consumption could not be increased over 32 g l⁻¹, when the concentrations of NH₄Cl and FeCl₂ were simultaneously increased, the results obtained were similar to those observed when butyrate was supplemented to the culture medium; a 35% higher butanol yield at the expense of 1,3-PDO and a shorter fermentation. The results herein gathered suggest that there are other factors besides butanol inhibition and nutrient limitation that affect the glycerol consumption.     

Exploring impacts of process technology development and regional factors on life cycle greenhouse gas emissions of corn stover ethanol

This paper examines impacts of regional factors affecting biomass and process input supply chains and ongoing technology development on the life cycle greenhouse gas (GHG) emissions of ethanol production from corn stover in the U.S. Corn stover supply results in GHG emissions from −6 gCO₂eq./MJ ethanol (Macon County, Missouri) to 13 gCO₂eq./MJ ethanol (Hardin County, Iowa), reflecting location-specific soil carbon and N₂O emissions responses to stover removal. Biorefinery emissions based on the 2011 National Renewable Energy Laboratory (NREL) process model are the single greatest emissions source (18 gCO₂eq./MJ ethanol) and are approximately double those assessed for the 2002 NREL design model, due primarily to the inclusion of GHG-intensive inputs (caustic, ammonia, glucose). Energy demands of on-site enzyme production included in the 2011 design contribute to reducing the electricity co-product and associated emissions credit, which is also dependent on the GHG-intensity of regional electricity supply. Life cycle emissions vary between 1.5 and 22 gCO₂eq./MJ ethanol (2011 design) depending on production location (98%–77% reduction vs. gasoline). Using system expansion for co-product allocation, ethanol production in studied locations meet the Energy Independence and Security Act emissions requirements for cellulosic biofuels; however, regional factors and on-going technology developments significantly influence these results.     

Bioconversion of residual glycerol from biodiesel synthesis into 1,3-propanediol and ethanol by isolated bacteria from environmental consortia

Residual (raw) glycerol originated from biodiesel synthesis is becoming of great environmental and economical concern due to its ever-growing surplus. In the present study, several bacterial strains were isolated and characterized for their ability to convert this raw glycerol into 1,3-propanediol (1,3-PD) and ethanol. The best producers of both 1,3-PD and ethanol were identified by 16S rDNA sequences to be Klebsiella pneumoniae and Pantoea agglomerans strains. Batch bioreactor cultivations under anaerobic and aerobic conditions were carried out in order to access the kinetics of glycerol consumption and product formation. Results showed that one isolated, K. pneumoniae BLh-1, was able to simultaneously produce up to 9.4 g/L of 1,3-PD with yields of 0.41 mol product mol⁻¹ glycerol, and 6.1 g/L of ethanol with yields of 0.14 mol product mol⁻¹ glycerol under anaerobic conditions, showing great potential for bioprocesses.     

ABE fermentation from enzymatic hydrolysate of NaOH-pretreated corncobs

Acetone butanol ethanol (ABE) was produced from enzymatic-hydrolyzed corncobs by Clostridium saccharobutylicum DSM 13864. Pretreatment of corncobs was carried out with 0.5 mol L⁻¹ NaOH followed by enzymatic hydrolysis. The yield of total reducing sugars was 917 g kg⁻¹ pretreated (de-lignified) and washed corncobs. The hydrolysate was used without sediments removal for ABE fermentation. A solvent production of 19.44 g L⁻¹ with 12.27 g L⁻¹ butanol was obtained from 55.22 g L⁻¹ sugars, resulting in an ABE yield of 350 g kg⁻¹ and a production rate of 0.54 g L⁻¹ h⁻¹. A control experiment using 55.3 g L⁻¹ mixed sugars resulted in an ABE production of 16.81 g L⁻¹ with 10.26 g L⁻¹ butanol, corresponding to an ABE yield of 300 g kg⁻¹ and a production rate of 0.47 g L⁻¹ h⁻¹, indicating that the enzymatic hydrolysates may contain stimulating compounds that can improve the ABE fermentation.     

Hydrothermal pretreatment of switchgrass and corn stover for production of ethanol and carbon microspheres

Pretreatment of biomass is viewed as a critical step to make the cellulose accessible to enzymes and for an adequate yield of fermentable sugars in ethanol production. Recently, hydrothermal pretreatment methods have attracted a great deal of attention because it uses water which is a inherently present in green biomass, non-toxic, environmentally benign, and inexpensive medium. Hydrothermal pretreatment of switchgrass and corn stover was conducted in a flow through reactor to enhance and optimize the enzymatic digestibility. More than 80% of glucan digestibility was achieved by pretreatment at 190 °C. Addition of a small amount of K₂CO₃ (0.45-0.9 wt.%) can enhance the pretreatment and allow use of lower temperatures. Switchgrass pretreated at 190 °C only with water had higher internal surface area than that pretreated in the presence of K₂CO₃, but both the substrates showed similar glucan digestibility. In comparison to switchgrass, corn stover required milder pretreatment conditions. The liquid hydrolyzate generated during pretreatment was converted into carbon microspheres by hydrothermal carbonization, providing a value-added byproduct. The carbonization process was further examined by GC-MS analysis to understand the mechanism of microsphere formation.     

Geographic distribution of economic potential of agricultural and forest biomass residual for energy use: Case study Croatia

This paper provides methodology for regional analysis of biomass energy potential and for assessing the cost of the biomass at the power plant (PP) location considering transport distance, transport costs and size of the power plants. Also, methodology for determination of an upper-level price of the biomass which energy plant can pay to the external suppliers has been proposed. The methodology was applied on the case of Croatia and energy potential of biomass in the Croatian counties was calculated, using different methodologies, for wheat straw, corn stover and forestry residues, types of biomass considered economically viable at the moment. Results indicate that the average energy potential of wheat straw is 8.5 PJ, corn stover 7.2 PJ and forestry residues 5.9 PJ.     

Analysis of shaking effect on photo-fermentative hydrogen production under different concentrations of corn stover powder

High solid phase and easily congeal affect the mass transfer in the photo-fermentative biohydrogen production when taken straw biomass as substrate. Hence, oscillator was adopted to provide the shaking condition to enhance the mass transfer situation in this paper. Diverse shaking velocity (0, 80, 120 and 160 rpm) and substrate concentration (0, 2, 4, 6, 8 and 10 g) were studied, to evaluate the influence on the hydrogen yield capacity. The results showed that shaking could help to accelerate of gas release, shorten the fermentation time, and improve hydrogen production rate. Hydrogen yield was significantly enhanced at high substrate concentration under shaking condition. Highest hydrogen yield of 57.08 ± 0.83, 57.62 ± 1.37, 62.28 ± 0.84 mL/g-volatile solids (VS) were observed at shaking velocities of 80, 120 and 160 rpm with 6, 8 and 10 g corn stover powder, respectively. On the contrary, shaking significantly reduced the potential of hydrogen yield at a low substrate concentration, and the lower hydrogen yield obtained at the higher shaking velocity. As the lowest hydrogen yields of 27.68 ± 1.02 and 41.93 ± 0.40 mL/g VS were obtained at shaking velocity of 160 rpm with 2 and 4 g corn stover powder, respectively.