Optimization of biohydrogen production from sweet sorghum syrup using statistical methods

This study employed statistically based experimental designs to optimize fermentation conditions for hydrogen production from sweet sorghum syrup by anaerobic mixed cultures. Initial screening of important factors influencing hydrogen production, i.e., total sugar, initial pH, nutrient solution, iron (II) sulphate (FeSO₄), peptone and sodium bicarbonate was conducted by the Plackett–Burman method. Results indicated that only FeSO₄ had statistically significant (P ≤ 0.005) influences on specific hydrogen production (P_s) while total sugar and initial pH had an interdependent effect on P_s. Optimal conditions for the maximal P_s were 25 g/L total sugar, 4.75 initial pH and 1.45 g/L FeSO₄ in which P_s of 6897 mL H₂/L was estimated. Estimated optimum conditions revealed only 0.04% difference from the actual P_s of 6864 mL H₂/L which suggested that the optimal conditions obtained can be practically applied to produce hydrogen from sweet sorghum syrup with the least error.     

Modeling of rotating drum bioreactor for anaerobic solid-state fermentation

Solid-state fermentation (SSF) has received more attention and has been applied to production of different products in recent years, especially biofuel production. The major problems to overcome in large-scale SSF are heat accumulation and heterogeneous distribution in a complex gas–liquid–solid multiphase bioreactor (or fermenter) system. In this work, a mathematical model of a rotating drum bioreactor for anaerobic SSF is developed considering the radial temperature distribution in the substrate bed. Validation experiments were conducted in a 5 m³ pilot plant fermenter for production of fuel ethanol from milled sweet sorghum stalks. The model that was developed fit well with the experimental data. From these results, it was concluded that this mathematical model is a powerful tool to investigate the design and scale-up of an anaerobic SSF fermenter in the application of bioethanol production using cellulosic materials such as sweet sorghum stalks.     

Potential reduction of nonstructural carbohydrate losses by juice expression prior to ensiling sweet sorghum

Both the nonstructural carbohydrate (sugar) and structural carbohydrate (fiber) components of sweet sorghum must be collected and utilized, if it is to be competitive as an ethanol feedstock. Sugar collected in juice expressed from chopped whole stalks equaled 42% of whole-stalk sugar. Potential increase in press performance resulting from removal of a rind-leaf function was investigated. A fractionation ratio of 0.25 (rind-leaf fraction equal to 25% whole-stalk mass separated prior to juice expression) gave maximum collection of sugar in the juice. Collected sugar was 52% of whole-stalk sugar. Extraction ratio (mass of juice divided by input mass) increased from 0.36 for chopped whole stalks to 0.60 for the 0.25 fractionation ratio (F_r = 0.25) sorghum. For higher fractionation ratios, the increase in extraction ratio was not sufficient to offset sugar loss due to rind-leaf removal. Assuming press capacity (Mg h⁻¹) is the same for both chopped whole-stalk and F_r = 0.25 sorghum, press performance (juice sugar collected per h) will increase by 67%. Removal of juice sugar reduced expected sugar conversion during ensiling from 92% of initial sugar (chopped whole stalk) to 40% (combination silage produced from residues from F_r = 0.25 processing). An expression was derived to calculate nonstructural carbohydrate (NC) content of combination silage produced by combining the pith presscake and rind-leaf fractions. Minimum NC occurred for F_r = 0.25.     

利用厨余垃圾制备生物燃料乙醇的工艺技术

针对我国厨余垃圾的资源环境问题,本文对厨余垃圾转化为燃料乙醇过程,如厌氧发酵法、直接发酵法、分步糖化发酵技术、同步糖化发酵技术、联合生物加工以及固定化细胞技术等工艺技术路线进行了分析,并对利用餐厨垃圾制备生物燃料乙醇未来的发展前景进行了展望。     

Coproduction of hydrogen and volatile fatty acid via thermophilic fermentation of sweet sorghum stalk from co-culture of Clostridium thermocellum and Clostridium thermosaccharolyticum

This study was focused on investigating the potential of hydrogen and volatile fatty acid (VFA) coproduction. Sweet sorghum stalks (SS) were used as substrate along with Clostridium thermocellum and Clostridium thermosaccharolyticum as production microbes. Inoculation ratio of C. thermosaccharolyticum to C. thermocellum (0:1–1.5:1 and 1:0 v/v), substrate concentrations (2.5–15.0 g/L) and inoculation time intervals of C. thermosaccharolyticum followed by C. thermocellum (0–48 h) were investigated. Experimental data showed that higher yields of hydrogen and VFA were obtained in the co-culture than their individual cultures. The optimum conditions for the highest yield of products found as 1:1 inoculation ratio of both strains, 24 h of time gap between C. thermosaccharolyticum followed by C. thermocellum after the first inoculation and 5 g/L of substrate concentration. The maximum yield of products was observed as hydrogen (5.1 mmol/g-substrate), acetic acid (1.27 g/L) and butyric acid (1.05 g/L) at optimum conditions. The results suggest that SS can be used for simultaneous production of hydrogen and VFA employing co-culture of C. thermocellum and C. thermosaccharolyticum strains. This approach can contribute to the sustainability of biorefinery.     

Comparison of fixed versus variable biofuels incentives

We evaluated several variants of a variable biofuel subsidy and compared them with the fixed subsidy and Renewable Fuel Standard using two different modeling approaches. First we used a partial equilibrium model encompassing crude oil, gasoline, ethanol, corn, and ethanol by-products. Second, we used a stochastic simulation model of a prototypical ethanol plant. From the partial equilibrium analysis, it appears the variable subsidy provides a safety net for ethanol producers when oil prices are low; yet, it does not put undue pressure on corn prices when oil prices are high. At high oil prices, the level of ethanol production is driven by market forces. From the plant level stochastic analysis, essentially the same conclusions are reached. As with the fixed subsidy, the variable subsidy can increase the net present value (NPV) sufficiently to encourage investment, but with lower risk for the producer, lower probability of a loss from the investment, and often lower expected cost to government. Finally, in the US, the ethanol industry is up against a blending limit called the blend wall. If the blending wall remains in place and no way around it is found, it does not matter much what other policy options are used.     

Modeling of fermentative hydrogen production from sweet sorghum extract based on modified ADM1

The Anaerobic digestion model 1 (ADM1) framework can be used to predict fermentative hydrogen production, since the latter is directly related to the acidogenic stage of the anaerobic digestion process. In this study, the ADM1 model framework was used to simulate and predict the process of fermentative hydrogen production from the extractable sugars of sweet sorghum biomass. Kinetic parameters for sugars’ consumption and yield coefficients of acetic, propionic and butyric acid production were estimated using the experimental data obtained from the steady states of a CSTR. Batch experiments were used for kinetic parameter validation. Since the ADM1 does not account for metabolic products such as lactic acid and ethanol that are crucial during the fermentative hydrogen production process, the structure of the model was modified to include lactate and ethanol among the metabolites and to improve the predictions. The modified ADM1 simulated satisfactorily batch experiments although further modifications could be made in order to further improve the predictions for the hydrogenogenic process.     

Optimal design of transverse ribs in tubes for thermal performance enhancement

We conducted an optimization using the second-order response surface method to determine the transverse rib geometry required to achieve the highest cooling performance in a circular channel. The best rib geometry was based on three design variables; rib height, rib width, and rib pitch. The turbulent heat transfer coefficients and friction losses were first calculated and then used to determine the thermal performance. We constructed the response surfaces of the three design variables as functions of the average Nusselt number ratio, friction loss, and thermal performance. These functions led to the optimum design point at the highest heat transfer rate in the special case of an actual turbine cooling passage with a constant friction loss.     

Biofuel excision and the viability of ethanol production in the Green Triangle,Australia

The promotion and use of renewable energy sources are established priorities worldwide as a way to reduce emissions of Greenhouse Gases and promote energy security. Australia is committed to reach a target of 350 ML of biofuels per year by 2010, and incentives targeted to producers and consumers have been placed. These incentives include zero excise until 2011 for the ethanol produced in Australia and gradual increase of the taxation rates reaching the full excise of 0.125 AUD per litre by 2015. This paper analyses the viability of the second generation ethanol industry in the Green Triangle, one of the most promising Australian regions for biomass production, by comparing the energy adjusted pump prices of petrol and the produced ethanol under different taxation rates and forecasted oil prices. Major findings suggest that under the current conditions of zero fuel excise and oil prices around 80US$ per barrel ethanol production is viable using biomass with a plant gate cost of up to 74 AUD per ton. Moreover, the forecasted increase in oil prices have a higher impact on the price of petrol than the increased ethanol excise on the pump price of the biofuel. Thus, by 2016 feedstock with a plant gate cost of up to 190 AUD per ton might be used for ethanol production, representing a flow of 1.7 million tons of biomass per year potentially mitigating 1.2 million tons of CO₂ by replacing fossil fuels with ethanol.     

Optimization of bioethanol production from glycerol by Escherichia coli SS1

Bioethanol is a promising biofuel and has a lot of great prospective and could become an alternative to fossil fuels. Ethanol fermentation using glycerol as carbon source was carried out by local isolate, ethanologenic bacterium Escherichia coli SS1 in a close system. Factors affecting bioethanol production from pure glycerol were optimized via response surface methodology (RSM) with central composite design (CCD). Four significant variables were found to influence bioethanol yield; initial pH of fermentation medium, substrate concentration, salt content and organic nitrogen concentration with statistically significant effect (p ≤ 0.05) on bioethanol production. The significant factor was then analyzed using central composite design (CCD). The optimum conditions for bioethanol production were substrate concentration at 34.5 g/L, pH 7.61, and organic nitrogen concentration at 6.42 g/L in which giving ethanol yield approximately 1.00 mol/mol. In addition, batch ethanol fermentation in a 2 L bioreactor was performed at the glycerol concentration of 20 g/L, 35 g/L and 45 g/L, respectively. The ethanol yields obtained from all tested glycerol concentrations were approaching theoretical yield when the batch fermentation was performed at optimized conditions.     

Bioethanol production from sweet sorghum: Evaluation of post-harvest treatments on sugar extraction and fermentation

Three experimental sweet sorghum varieties (M81, Topper and Theis) and three post-harvest conditions were evaluated for ethanol production: juices extracted by milling were obtained from the whole plant, plant without panicle, and stalk (plant without panicle and leaves), respectively. A linear relationship was found between the total fermentable sugar concentrations and Brix degrees of the juices, which can predict the potential ethanol yield by field analytical tests. The juice extractability presented different behavior among the sweet sorghum varieties with respect to the treatments studied. However such treatments did not affect the level of sugar concentration of the juices obtained and the fermentation efficiency. Topper and Theis showed the best performance in terms of ethanol concentration, fermentation efficiency and ethanol yield. The variety used and its post-harvest treatment should be appropriately selected in order to improve the ethanol production from sweet sorghum.     

Size optimization of a PV/wind hybrid energy conversion system with battery storage using response surface methodology

This paper aims to show the use of the response surface methodology (RSM) in size optimization of an autonomous PV/wind integrated hybrid energy system with battery storage. RSM is a collection of statistical and mathematical methods which relies on optimization of response surface with design parameters. In this study, the response surface, output performance measure, is the hybrid system cost, and the design parameters are the PV size, wind turbine rotor swept area and the battery capacity. The case study is realized in ARENA 10.0, a commercial simulation software, for satisfaction of electricity consumption of the global system for mobile communications (GSM) base station at Izmir Institute of Technology Campus Area, Urla, Turkey. As a result, the optimum PV area, wind turbine rotor swept area, and battery capacity are obtained to be 3.95 m², 29.4 m², 31.92 kWh, respectively. These results led to $37,033.9 hybrid energy system cost, including auxiliary energy cost. The optimum result obtained by RSM is confirmed using loss of load probability (LLP) and autonomy analysis.     

Media optimization for biohydrogen production from waste glycerol by anaerobic thermophilic mixed cultures

Media compositions affecting thermophilic biohydrogen production from waste glycerol were optimized using response surface methodology (RSM) with central composite design (CCD). Investigated parameters used were waste glycerol concentration, urea concentration, the amount of Endo-nutrient addition and disodium hydrogen phosphate (Na₂HPO₄) concentration. Waste glycerol concentration and the amount of Endo-nutrient addition had a significant individual effect on the cumulative hydrogen production (HP) (p ≤ 0.05). The interactive effect on HP was found between waste glycerol and urea concentration as well as waste glycerol concentration and the amount of Endo-nutrient addition (p ≤ 0.05). The optimal media compositions were 20.33 g/L of waste glycerol, 0.16 g/L of urea, 3.97 g/L of Na₂HPO₄ and 0.20 mL/L of the amount of Endo-nutrient addition which gave the maximum HP of 1470.19 mL H₂/L. The difference between observed HP (1502.84 mL H₂/L) and predicted HP was 2.22%. The metabolic products from the fermentation process were 1,3-propanediol (1,3-PD), ethanol, acetic, formic, lactic, butyric, and propionic acids. Results from polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis indicated that the hydrogen producers present in the fermentation broth was Thermoanaerobacterium sp.     

Optimization of key factors affecting hydrogen production from food waste by anaerobic mixed cultures

Key factors (inoculums concentration, substrate concentration and citrate buffer concentration) affecting hydrogen yield (HY) and specific hydrogen production rate (SHPR) from food waste in batch fermentation by anaerobic mixed cultures were optimized using Response Surface Methodology with Central Composite Design. The experiments were conducted in 120 ml serum bottles with a working volume of 70 mL. Under the optimal condition of 2.30 g-VSS/L of inoculums concentration, 2.54 g-VS/L of substrate concentration, and 0.11 M of citrate buffer concentration, the predicted maximum HY and SHPR of 104.79 mL H₂/g-VS_added and 16.90 mL H₂/g-VSS.h, respectively, were obtained. Concentrations of inoculums, substrate and citrate buffer all had an individual effect on HY and SHPR (P < 0.05). The substrate concentration and citrate buffer concentration had the greatest interactive effect on SHPR (P = 0.0075) while their effects on HY (P = 0.0131) were profound. These results were reproduced in confirmation experiments under optimal conditions and generated an HY of 104.58 mL H₂/g-VS_added and an SHPR of 16.86 mL H₂/g-VSS.h. This was only 0.20% and 0.24%, respectively, different from the predicted values. Microbial community analysis by PCR-DGGE indicated that Clostridium was the pre-dominant hydrogen producer at the optimum and worst conditions. The presence of Lactobacillus sp. and Enterococcus sp. might be responsible for the low HY and SHPR at the worst condition.     

Formation of ethanol from carbon monoxide via a new microbial catalyst

A recently discovered clostridial bacteria converts components of synthesis gas (CO, CO₂, H₂) into liquid products such as ethanol, butanol and acetic acid. Isolated from an agricultural lagoon, the stability and productivity characteristics of the bacteria were studied in a continuous 4.5 l bubble column bioreactor at 37°C using artificial blends of CO, CO₂, and N₂. Preliminary results on the rates of cell growth, substrate utilization, product formation, and yields of products and cells from CO are discussed. At steady state, apparent yields (mole C in products per mole CO consumed) of ethanol, butanol, and acetic acid were 0.15, 0.075 and 0.025, respectively, and the cell yield was 0.25 g/mol CO. The theoretical yield of ethanol is 0.33 if CO is only utilized for the production of ethanol. The experimental yield of CO₂ from CO was approximately 60% compared to the theoretical yield of 67% with ethanol as the sole product. As a comparison with another ethanol-producing bacteria, the results of a similar fermentation study using batch-grown Clostridium ljungdahlii showed yields of 0.062 for ethanol and 0.094 for acetic acid and a cell yield of 1.378 g/mol.     

Monetary value of the environmental and health externalities associated with production of ethanol from biomass feedstocks

This research is aimed at monetizing the life cycle environmental and health externalities associated with production of ethanol from corn, corn stover, switchgrass, and forest residue. The results of this study reveal current average external costs for the production of 1 l of ethanol ranged from $0.07 for forest residue to $0.57 for ethanol production from corn. Among the various feedstocks, the external costs of PM₁₀, NO_X, and PM_2.5 are among the greatest contributors to these costs. The combustion of fossil fuels in upstream fertilizer and energy production processes is the primary source of these emissions and their costs, especially for corn ethanol. The combined costs of emissions associated with the production and use of nitrogen fertilizer also contribute substantially to the net external costs. For cellulosic ethanol production, the combustion of waste lignin to generate heat and power helps to keep the external costs lower than corn ethanol. Credits both for the biogenic carbon combustion and displacement of grid electricity by exporting excess electricity substantially negate many of the emissions and external costs. External costs associated with greenhouse gas emissions were not significant. However, adding estimates of indirect GHG emissions from land use changes would nearly double corn ethanol cost estimates.