Improving net energy gain in fermentative biohydrogen production from glucose

Our previous studies had demonstrated enhanced fermentative hydrogen production from sucrose in batch reactors with dairy manure as a supplement providing nutrients, buffering, and hydrogen-producing organisms. In this study, manure leachate is evaluated as a supplement in glucose fermentation in batch and continuous flow reactors at 25 °C without any nutrient supplements, initial pH adjustments, buffering, or stirring. Hydrogen yields found in this study are comparable to or better than those reported at higher temperatures. When the heat energy expended to maintain the test temperatures is considered, positive net energy gain of ∼10 kJ/L of reactor volume was achieved while most literature reports translated to negative net energy gain. Anaerobic digestion (AD) and microbial fuel cells (MFC) were evaluated as follow-up processes to extract additional energy from the end products of dark fermentation (DF). This evaluation showed that DF followed by MFCs to produce electricity to be a more energy-efficient approach.     

Fermentative biohydrogen production: Evaluation of net energy gain

Most dark fermentation (DF) studies had resorted to above-ambient temperatures to maximize hydrogen yield, without due consideration of the net energy gain. In this study, literature data on fermentative hydrogen production from glucose, sucrose, and organic wastes were compiled to evaluate the benefit of higher fermentation temperatures in terms of net energy gain. This evaluation showed that the improvement in hydrogen yield at higher temperatures is not justified as the net energy gain not only declined with increase of temperature, but also was mostly negative when the fermentation temperature exceeded 25 °C. To maximize the net energy gain of DF, the following two options for recovering additional energy from the end products and to determine the optimal fermentation temperature were evaluated: methane production via anaerobic digestion (AD); and direct electricity production via microbial fuel cells (MFC). Based on net energy gain, it is concluded that DF has to be operated at near-ambient temperatures for the net energy gain to be positive; and DF + MFC can result in higher net energy gain at any temperature than DF or DF + AD.     

Modeling and simulation of net energy gain by dark fermentation

For dark fermentation (DF) to be accepted as a sustainable process for biohydrogen production, the net energy gain should be positive and as high as possible. A theoretical approach is proposed in this study to evaluate the net energy gain possible from hydrogen generated by the DF process as well as from the end products of DF via anaerobic digestion (AD) and microbial fuel cells (MFC). Experimental data on hydrogen evolution and aqueous end products formation from sucrose and from sucrose/dairy manure blends were used to validate the proposed approach for estimating net energy gain via DF, DF + AD, DF + MFC. Good agreement was found between the experimental and predicted net energy gain values, with overall correlation coefficient of 0.998. Based on the results of this study, DF + MFC is recommended as the best combination to maximize net energy gain.     

Fermentative hydrogen production from wastewaters: A review and prognosis

Biohydrogen is a promising candidate which can replace a part of our fossil fuels need in day-to-day life due its perceived environmental benefits and availability through dark fermentation of organic substrates. Moreover, advances in biohydrogen production technologies based on organic wastewater conversion could solve the issues related to food security, climate change, energy security and clean development in the future. An evaluation of studies reported on biohydrogen production from different wastewaters will be of immense importance in economizing production technologies. Here we have reviewed biohydrogen production yields and rates from different wastewaters using sludges and microbial consortiums and evaluated the feasibility of biohydrogen production from unexplored wastewaters and development of integrated bioenergy process. Biohydrogen production has been observed in the range of substrate concentration 0.25–160 g COD/L, pH 4–8, temperature 23–60 °C, HRT 0.5–72 h with various types of reactor configuration. The most efficient hydrogen production has been obtained at an organic loading rate (OLR) 320 g COD/L/d, substrate concentration 40 g COD/L, HRT 3 h, pH 5.5–6.0, temperature 35 °C in a continuously-stirred tank reactor system using mixed cultures and fed with condensed molasses fermentation soluble wastewater. The net energy efficiency analysis showed vinasse wastewater has the highest positive net energy gain followed by glycerin wastewater and domestic sewage as 140.39, 68.65, 51.84 kJ/g COD feedstock with the hydrogen yield (HY) of 10 mmol/g COD respectively.     

Integrated Taguchi method and response surface methodology to confirm hydrogen production by anaerobic fermentation of cow manure

In the present study, we evaluated the feasibility of integrating the Taguchi method and the response surface methodology (RSM) to predict and optimize fermentative hydrogen production of cow manure (CM) slurry, a mixture of CM and tap water that was equivalent to 6% of the volatile solid (VS) content. Batch vial tests were first conducted in accordance with an experimental design using the Taguchi method L₁₈ orthogonal array that selected the significant influencing factors (temperature and pH) of hydrogen production, and then the RSM with a central composite design was used for the following experiments based on the aforementioned factors. Finally, fermentation experiments in triplicate were carried out in a 2-L semi-continuously stirred tank reactor (semi-CSTR) with a fixed organic loading rate (OLR), solid retention time (SRT) and varying temperatures and pH under a steady-state operation. Through a series of investigations conducted in this study, our experimental data confirmed that the optimal conditions were 60 °C with pH 5.20 ± 0.21, resulting in hydrogen content (HC) of 54.64 ± 11.45%, volumetric hydrogen production (VHP) of 405.54 ± 193.61 ml-H₂/l/d, and specific hydrogen yield (SHY) of 10.25 ± 4.96 ml-H₂/g-VS. This study demonstrates a good performance of the Taguchi method with pretests and the prediction of the response surfaces methodology. The confirmed experimental results show the behavior of anaerobic fermenters’ treating in significant factors, which will comply with management strategies for treatment of relative organic wastes in the future.     

Effect of sawdust addition and composting of feedstock on renewable energy and biochar production from pyrolysis of anaerobically digested pig manure

Pyrolysis experiments were conducted on the separated solid fraction of anaerobically digested pig manure (SADPM). The aim of these experiments was to investigate the influence of (1) sawdust addition and (2) composting the feedstock, on the products of pyrolysis and on the net energy yield from the pyrolysis process. Mixtures of SADPM and sawdust were made to give the following treatments; manure only, 4:1(w/w) and 3:2(w/w). These mixtures were pyrolized at 600 °C both before and after aerobic composting. The yields of the biochar, bio-liquid and gas were influenced by the addition of sawdust to the SADPM and by composting of the feedstock. With the addition of sawdust, biochar and gas higher heating values (HHV) increased, while bio-liquid HHV decreased. More than 70% of the original energy in the feedstock remained in the biochar, bio-liquid and gas after pyrolysis, increasing as the proportion of sawdust increased. The HHV of the biochar decreased, while the HHV of the bio-liquid increased, after the feedstocks were composted. The energy balance showed that increasing the rate of sawdust addition to SADPM resulted in an increased net energy yield. The addition of a composting stage increased the net energy yield for the manure only feedstock only. However, with increasing sawdust addition, composting of the feedstock reduced the net energy yield.     

Design of a novel biohythane process with high H2 and CH4 production rates

A biohythane process based on wheat straw including: i) pretreatment, ii) H₂ production using Caldicellulosiruptor saccharolyticus, iii) CH₄ production using an undefined consortium, and iv) gas upgrading using an amine solution, was assessed through process modelling including cost and energy analysis. According to simulations, a biohythane gas with the composition 46–57% H₂, 43–54% CH₄ and 0.4% CO₂, could be produced at high production rates (2.8–6.1 L/L/d), with 93% chemical oxygen demand (COD) reduction, and a net energy yield of 7.4–7.7 kJ/g dry straw. The model was calibrated and verified using experimental data from dark fermentation (DF) of wheat straw hydrolysate, and anaerobic digestion of DF effluent. In addition, the effect of gas recirculation was investigated by both wet experiments and simulation. Sparging improved H₂ productivities and yields, but negatively affected the net energy gain and cost of the overall process.     

Fermentative biohydrogen production II: Net energy gain from organic wastes

Several reports have demonstrated the feasibility of hydrogen production by dark fermentation (DF). However, most reports had resorted to mesophilic or thermophilic conditions to increase hydrogen yield, overlooking the energy input to the process and hence, loss of net energy gain. For net positive energy gain, energy input to the process should be minimized and additional energy should be harvested from the aqueous end products of DF. Our previous study presented an approach to assess the potential for net energy gain from the hydrogen produced by DF, and from the end products of DF via anaerobic digestion (AD) or microbial fuel cells (MFC). In this study, that approach is extended to identify the most promising process configuration and operating conditions to maximize net energy gain possible from liquid and particulate organic wastes. Based on this analysis, DF followed by MFC appears to result in higher net energy gains.     

Biohydrogen production from rice straw: Effect of combinative pretreatment,modelling assessment and energy balance consideration

Biohydrogen production from agro waste biomass through combinative pretreatments is an emerging cost effective, alternative energy technology. The present study aimed to ascertain the extent to which the combinative dispersion thermochemical disintegration (DTCD) enhances the cost effective and energy efficient biohydrogen production from rice straw. The efficiency of the combinative pretreatment was evaluated in terms of degree of disintegration and biohydrogen generation. The optimal conditions for combinative pretreatments are pH 10, temperature 80 °C, rpm 12000 and disintegration time 30 mins. A higher degree of disintegration of about 20.9% was achieved through DTCD pretreatment when compared to dispersion thermal disintegration (DTD) (13.2%) and disperser disintegration (DD) (9.5%). The specific energy spent to achieve maximal degree of disintegration for the three pretreatments were in the following order: DD (1469 kJ/kg Rice Straw) > DTD (1044 kJ/kg Rice Straw) > DTCD (742 kJ/kg Rice Straw). Hence, a considerable amount of energy could be saved through this combinative pretreatment. First order kinetic model (exponential rise to maximum) of biohydrogen production is helpful in deriving the two parameters of uncertainty: substrate biodegradability and hydrolysis rate constant. These two parameters evaluate the maximal biohydrogen yield potential of rice straw through combinative pretreatments. As expected, a higher biohydrogen yield of about (129 mL/g COD) was observed in DTCD when compared to DTD (81 mL/g COD) DD (58 mL/g COD) and Control (8 mL/g COD). To gain insights into the feasibility of implementing the pretreatment at large scale, scalable studies are essential in terms of energy balance and cost. A higher positive net energy of about 0.39621 kWh/kg rice straw was achieved for DTCD when compared to others.     

Characterization of animal manure and cornstalk ashes as affected by incineration temperature

Incineration has been proposed as an alternative technology to reuse animal manure by producing energy and ash fertilizers. The objective of this study was to assess the impact of incineration temperature on the physical (ash yield) and chemical (nutrient) properties of ashes for different types of animal manure and cornstalk. The source materials were incinerated in a temperature-controlled muffle furnace at the temperature of 400, 500, 600, 700, 800 or 900 °C and the properties of the resultant ashes were determined following the procedures set by China National Standards. The results indicated that ash yield (AY, %), total nitrogen (TN) recovery and total potassium (K₂O) recovery all decreased with increasing incineration temperature. The ranges of AY, ash TN and K₂O recovery were, respectively, 43.6–30.2%, 6.9–0.6%, and 80–61% for laying-hen manure; 34.3–32.1%, 18.8–15.4%, and 95–56% for cattle manure; 25.3–20.7%, 14–0%, and 78–57% for swine manure; and 8.4–7.5%, 2.1–1.4%, and 37–19% for cornstalk. However, total phosphorus (P₂O₅) content of the ashes increased with incineration temperature, being 20.7–24.0% for swine manure, 4.5–7.5% for layer manure, and 2.7–3.4% for cornstalk. Animal manures have greater TN and P₂O₅ volatilization but less K₂O and total sodium (Na₂O) volatilization as compared to the cornstalk. The results provide a basis for incineration as an alternative means to reuse animal manures and cornstalk and suitability of the resultant ash co-product for different applications.     

Biological hydrogen production by extremely thermophilic novel bacterium Thermoanaerobacter mathranii A3N isolated from oil producing well

Hydrogen producing novel bacterial strain was isolated from formation water from oil producing well. It was identified as Thermoanaerobacter mathranii A3N by 16S rRNA gene sequencing. Hydrogen production by novel strain was pH and substrate dependent and favored pH 8.0 for starch, pH 7.5 for xylose and sucrose, pH 8.0–9.0 for glucose fermentation at 70 °C. The highest H₂ yield was 2.64 ± 0.40 mol H₂ mol glucose at 10 g/L, 5.36 ± 0.41 mol H₂ mol – sucrose at 10 g/L, 17.91 ± 0.16 mmol H₂ g – starch at 5 g/L and 2.09 ± 0.21 mol H₂ mol xylose at 5 g/L. The maximum specific hydrogen production rates 6.29 (starch), 9.34 (sucrose), 5.76 (xylose) and 4.89 (glucose) mmol/g cell/h. Acetate-type fermentation pathway (approximately 97%) was found to be dominant in strain A3N, whereas butyrate formation was found in sucrose and xylose fermentation. Lactate production increased with high xylose concentrations above 10 g/L.     

Assessment of flexible energy vectors poly-generation based on coal and biomass/solid wastes co-gasification with carbon capture

This paper evaluates biomass and solid wastes co-gasification with coal for energy vectors poly-generation with carbon capture. The evaluated co-gasification cases were evaluated in term of key plant performance indicators for generation of totally or partially decarbonized energy vectors (power, hydrogen, substitute natural gas, liquid fuels by Fischer–Tropsch synthesis). The work streamlines one significant advantage of gasification process, namely the capability to process lower grade fuels on condition of high energy efficiency. Introduction in the evaluated IGCC-based schemes of carbon capture step (based on pre-combustion capture) significantly reduces CO₂ emissions, the carbon capture rate being higher than 90% for decarbonized energy vectors (power and hydrogen) and in the range of 47–60% for partially decarbonized energy vectors (SNG, liquid fuels). Various plant concepts were assessed (e.g. 420–425 MW net power with 0–200 MW_th flexible hydrogen output, 800 MW_th SNG, 700 MW_th liquid fuel, all of them with CCS). The paper evaluates fuel blending for optimizing gasification performance. A detailed techno-economic evaluation for hydrogen and power co-generation with CCS was also presented.     

Feasibility of biohydrogen production at low temperatures in unbuffered reactors

Feasibility of biohydrogen production by dark fermentation at two temperatures (22 °C and 37 °C) in unbuffered batch reactors was evaluated using heat-treated compost as inocula and sucrose as substrate, without any initial pH adjustment or inorganic nutrient supplements. Gas production was quantified by two different pressure release methods – intermittent pressure release (IPR) and continuous pressure release (CPR). Hydrogen production (47.2 mL/g COD/L) and sucrose-to-hydrogen conversion efficiency (53%) were both found to be highest at the lower temperature and IPR conditions. Hydrogen production was higher at the lower temperature irrespective of the pressure release condition. The high yield of 4.3 mol of hydrogen/mole of sucrose obtained in this study under IPR conditions at 22 °C is equivalent to or better than the literature values reported for buffered reactors. Even though literature reports have implied potential inhibition of hydrogen production at high hydrogen partial pressures resulting from IPR conditions, our results did not show any negative effects at hydrogen partial pressures exceeding 5.0 × 10⁴ Pa. While our findings are contrary to literature reports, they make a strong case for cost-effective hydrogen production by dark fermentation.     

Comparative study on bio-hydrogen production from corn stover: Photo-fermentation,dark-fermentation and dark-photo co-fermentation

In this study, three different fermentation methods, such as photo-fermentation (PF), dark-fermentation (DF) and dark-photo co-fermentation (DPCF) for bio-hydrogen production from corn stover were compared in terms of hydrogen production, substrate consumption, by-products formation and energy conversion efficiency. A modified Gompertz model was applied to perform the kinetic analysis of hydrogen production. The maximum cumulative hydrogen yield of 141.42 mL·(g TS)⁻¹ was achieved by PF, DF with the minimum cumulative hydrogen yield of 36.08 mL· (g TS)⁻¹ had the shortest lag time of 4.33 h, and DPCF had the maximum initial hydrogen production rate of 1.88 mL· (g TS)⁻¹·h⁻¹ and maximum initial hydrogen content of 44.40%. The results also indicated PF was an acid-consuming process with a low total VFAs concentration level of 2.90–4.19 g·L⁻¹, DF was a process of VFAs accumulation with a maximum total VFAs concentration of 12.66 g·L⁻¹, and DPCF was a synergistic process in which the total VFAs concentration was significantly reduced and the hydrogen production efficiency was effectively improved compared with DF. The energy conversion efficiency of PF, DF and DPCF were 10.12%, 2.58% and 6.45%, respectively.     

Hydrogen and syngas production from sewage sludge via steam gasification

High temperature steam gasification is an attractive alternative technology which can allow one to obtain high percentage of hydrogen in the syngas from low-grade fuels. Gasification is considered a clean technology for energy conversion without environmental impact using biomass and solid wastes as feedstock. Sewage sludge is considered a renewable fuel because it is sustainable and has good potential for energy recovery. In this investigation, sewage sludge samples were gasified at various temperatures to determine the evolutionary behavior of syngas characteristics and other properties of the syngas produced. The syngas characteristics were evaluated in terms of syngas yield, hydrogen production, syngas chemical analysis, and efficiency of energy conversion. In addition to gasification experiments, pyrolysis experiments were conducted for evaluating the performance of gasification over pyrolysis. The increase in reactor temperature resulted in increased generation of hydrogen. Hydrogen yield at 1000 °C was found to be 0.076 g_gas g_sample⁻¹. Steam as the gasifying agent increased the hydrogen yield three times as compared to air gasification. Sewage sludge gasification results were compared with other samples, such as, paper, food wastes and plastics. The time duration for sewage sludge gasification was longer as compared to other samples. On the other hand sewage sludge yielded more hydrogen than that from paper and food wastes.     

Characteristics of biohydrogen fermentation from various substrates

The characteristics of biohydrogen production from sucrose, slurry-type piggery waste and food waste under the effects of the reactor configurations and operational pHs (6 and 9) were examined by using heat-treated anaerobic sludge as a seed biomass. When sucrose was used in the batch test, the maximum hydrogen yield was 0.12–0.13 g COD (as H₂)/g COD (1.41–1.43 mol/mol hexose) at pH 6. In contrast, 0.10–0.11 g COD (as H₂)/g COD (1.12–1.21 mol/mol hexose) hydrogen yield was achieved from the reactor at pH 9. On the other hand, hydrogen production was not observed in the continuous sequencing batch mode fermenters fed with sucrose. Profile analysis at each cycle revealed hydrogen production at the initial operation periods but eventually only methane at 36 days. When slurry-type piggery waste was used as the substrate, the upflow elutriation-type fermenters produced methane but not hydrogen after 30 days operation. The fermentation intermediate profile showed that the hydrogen produced might have been consumed by homoacetogenic or propionate producing reactions, and eventually converted into methane by acetoclastic methanogens. The downflow leaching bed fermenters using food waste produced 0.013 L H₂/g volatile solids (VS) (0.0061 g COD (as H₂)/g COD) at pH 6 with 54% VS reduction whereas 0.0041 L H₂/g VS (0.0020 g COD (as H₂)/g COD) was produced at pH 9 with 86% VS reduction. The results show that the hydrogen produced should be released rapidly from the reactor before it can be consumed in other biochemical reactions, and substrates with high pH level (>9.0) can be used directly to produce hydrogen without needing to adjust the pH.     

Production of biocrude-oil from swine manure by fast pyrolysis and analysis of its characteristics

All around the world research is being conducted in the field of renewable energy due to the depletion of fossil fuels and the problem of global warming. Fast pyrolysis, an optimal technology for converting biomass to liquid fuel, enables lignocellulosic raw materials such as wood, switch grass and rice straw to be converted to biocrude-oil. Even though many studies on these materials have already been conducted, the high production costs and unstable supply thereof have frequently been pointed out as significant problems. Thus, this study considers the use of another feedstock to solve such disadvantages and to raise the recycling rate of organic wastes simultaneously. Swine manure was selected as an alternative feedstock due to the existence of a stable supply from the livestock farming industry. A bubbling-fluidized-bed reactor was used in the present study for fast pyrolysis. The yield and characteristics of biocrude-oil were investigated at various reaction temperatures. The optimum temperature for maximum biocruce-oil yield was found to be 600 °C with the highest yield of 18.48 wt% and HHV of 13.59 MJ/kg. Due to its low yield and high water content, swine manure is suggested to be blended with other types of biomass as a means of higher yield and quality of biocrude-oil.     

Variation on biomass yield and morphological traits of energy grasses from the genus Miscanthus during the first years of crop establishment

This study presents the results of investigations of variation, genotype × year interactions and genotype × year × location interactions for the yield and morphological traits of several selected clones of energy grasses of the genus Miscanthus. The analyses were performed on the best clones of selected hybrid plants, which were obtained within the species M. sinensis or are the result of interspecific hybridization of M. sinensis and M. sacchariflorus. Analyses were conducted on the basis of three-year field trials at two locations. The young plants produced from in vitro cultures were planted at a density of one plant per m². The early stages of plant development, from planting until peak yield in the third year of cultivation, were analysed. Statistical analyses performed on the yield and morphological traits as well as changes in these characteristics over the successive years of the study showed considerable genotypic variation for traits under study. Moreover, significant genotype × year interactions as well as genotype × year × location interactions were observed in terms of yield and morphological traits. Based on the collective results of the study, we suggest that apart from M. x giganteus particularly hybrids of M. sinensis × M. sacchariflorus, should be taken into consideration in genetic and breeding studies on the improvement of yield from energy grasses of the genus Miscanthus.     

Modeling and simulation of a 100 kWe HT-PEMFC subsystem integrated with an absorption chiller subsystem

A 100 kW_e liquid-cooled HT-PEMFC subsystem is integrated with an absorption chiller subsystem to provide electricity and cooling. The system is designed, modeled and simulated to investigate the potential of this technology for future novel energy system applications. Liquid-cooling can provide better temperature control and is preferable for middle-scale transport applications, such as commercial vessels, because stack cooling can be achieved within smaller volumes. A commercial ship requiring cooling and electricity is taken as the case study for the application of the proposed system. All system components are described and analyzed in detail, in terms of modeling assumptions and configuration topology. The results show the conceptual feasibility of the proposed system configuration, since high net electrical efficiencies are accomplished. The calculated net electrical efficiency is 43.8% for a net electrical power output of 100 kW_e. The heat exhausted to the absorption chiller subsystem is 107 kW and can satisfy a cooling duty of up to 128 or 64.5 kW for a LiBr–water double-effect system or a water–NH₃ single-effect system, respectively. Finally, the projected total cost is comparable to conventional systems, i.e., diesel engines integrated with vapor-compression chillers, and therefore justifies further development of the proposed system.     

Evaluation of low cost cathode materials for treatment of industrial and food processing wastewater using microbial electrolysis cells

Microbial electrolysis cells (MECs) can be used to treat wastewater and produce hydrogen gas, but low cost cathode catalysts are needed to make this approach economical. Molybdenum disulfide (MoS₂) and stainless steel (SS) were evaluated as alternative cathode catalysts to platinum (Pt) in terms of treatment efficiency and energy recovery using actual wastewaters. Two different types of wastewaters were examined, a methanol-rich industrial (IN) wastewater and a food processing (FP) wastewater. The use of the MoS₂ catalyst generally resulted in better performance than the SS cathodes for both wastewaters, although the use of the Pt catalyst provided the best performance in terms of biogas production, current density, and TCOD removal. Overall, the wastewater composition was more of a factor than catalyst type for accomplishing overall treatment. The IN wastewater had higher biogas production rates (0.8–1.8 m³/m³-d), and COD removal rates (1.8–2.8 kg-COD/m³-d) than the FP wastewater. The overall energy recoveries were positive for the IN wastewater (3.1–3.8 kWh/kg-COD removed), while the FP wastewater required a net energy input of −0.7–−1.2 kWh/kg-COD using MoS₂ or Pt cathodes, and −3.1 kWh/kg-COD with SS. These results suggest that MoS₂ is the most suitable alternative to Pt as a cathode catalyst for wastewater treatment using MECs, but that net energy recovery will be highly dependent on the specific wastewater.