Dark fermentative biohydrogen production using pretreated Scenedesmus obliquus biomass under an integrated paradigm of biorefinery

In recent times, biohydrogen production from microalgal feedstock has garnered considerable research interests to sustainably replace the fossil fuels. The present work adapted an integrated approach of utilizing deoiled Scenedesmus obliquus biomass as feedstock for biohydrogen production and valorization of dark fermentation (DF) effluent via biomethanation. The microalgae was cultivated under different CO₂ concentration. CO₂-air sparging of 5% v/v supported maximum microalgal growth and carbohydrate production with CO₂ fixation ability of 727.7 mg L^?1 d^?1. Thereafter, lipid present in microalgae was extracted for biodiesel production and the deoiled microalgal biomass (DMB) was subjected to different pretreatment techniques to maximize the carbohydrate recovery and biohydrogen yield. Steam heating (121 °C) in coherence with H₂SO₄ (0.5 N) documented highest carbohydrate recovery of 87.5%. DF of acid-thermal pretreated DMB resulted in maximum H₂ yield of 97.6 mL g^?1 VS which was almost 10 times higher as compared to untreated DMB (9.8 mL g^?1 VS). Subsequent utilization of DF effluent in biomethanation process resulted in cumulative methane production of 1060 mL L^?1. The total substrate energy recovered from integrated biofuel production system was 30%. The present study envisages a microalgal biorefinery to produce biohydrogen via DF coupled with concomitant CO₂ sequestration.     

Influence of inoculum cell density and carbon dioxide concentration on fed-batch cultivation of Nannochloropsis oculata

The marine microalgae Nannochloropsis oculata is a promising source of biofuel because of its high lipid content. For achieving high productivity of oil from microalgae, a high cell concentration before harvesting is beneficial. The present study investigated fed-batch cultures of N. oculata fed with vitamins and nutrient solutions and found that the biomass yield of N. oculata in the fed-batch culture was 1.25 times higher than that in batch culture. Fed-batch cultivation, especially at high illumination, decreased the inhibitory effect of high carbon dioxide (CO₂) concentration on the microalgal growth. The specific growth rate was directly proportional to the light intensity in the CO₂ environment. A light intensity of 40,000 Lux was able to achieve high specific growth rates in fed-batch cultivation at a CO₂ volume fraction of 2%–15%. The tolerance of N. oculata to CO₂ was enhanced by the daily feeding of nutrients in the fed-batch cultivation. At 2% CO₂, a final cell density of about OD₆₈₂ = 11.4 was achieved in the fed-batch culture in 30 days. Furthermore, a cell density of 14.4 g L⁻¹ was obtained by outdoor fed-batch cultivation in 27 days.     

Effect of carbon supply mode on biomass and lipid in CSMCRI's Chlorella variabilis (ATCC 12198)

CSIR-CSMCRI's Chlorella variabilis (ATCC 12198) was evaluated through autotrophic, mixotrophic and heterotrophic growth for lipid production. Autotrophic growth was assessed by providing sodium bicarbonate/sodium carbonate/CO₂ (air in a medium). Higher lipid productivity (115.94 mg L⁻¹ d⁻¹) with higher biomass productivity (724.98 mg L⁻¹ d⁻¹) of this strain was attained through bicarbonate and CO₂ sequestration in a photobioreactor. Ability to regulate the pH in favorable bicarbonate/carbonate ratio showed its potential in alkaline effluent based carbon sequestration system for biofuel generation. The simultaneous study was also conducted to understand the effect of elevated CO₂ (0.4, 1 and 1.2 g L⁻¹) in air on the culture to assess adaptation, growth and lipid in the closed chamber conditions. It was observed that CO₂ sequestration by the microalgae from the CO₂ enriched environment was optimum at 1 g L⁻¹ C. variabilis adapted to comparatively higher CO₂ (1 g L⁻¹) but grew better in low CO₂ (0.4 g L⁻¹). It was also observed that the growth, lipid content and fatty acid composition was significantly affected by CO₂ supply strategies. The effect of intermittently added sodium bicarbonate at different pH on microalgal lipid content and composition of fatty acids was observed which could affect the quality of biodiesel. The effect on fatty acid composition was observed in response to carbon supply mode during the microalgal growth at different pH dictating the properties of biodiesel.     

Hydrogen and methane production from lipid-extracted microalgal biomass residues

A two-stage process to produce hydrogen and methane from lipid-extracted microalgal biomass residues (LMBRs) was developed. The biogas production and energy efficiency were compared between one- and two-stage processes. The two-stage process generated 46 ± 2.4 mL H₂/g-volatile solid (VS), and 393.6 ± 19.5 mL CH₄/g-VS. The methane yield was 22% higher than the one in the one-stage process. Energy efficiency increased from 51% in the one-stage process to 65% in the two-stage process. Additionally, it was found that repeated batch cultivation was a useful method to cultivate the cultures to improve the methane production rate and reduce the fermentation time. In the repeated batch cultivation, the methane yield slightly decreased if the ammonia levels rose, suggesting that the accumulation of ammonia could affect methane production.     

Assessment of triacylglycerol content in Chlorella vulgaris cultivated in a two-stage process

Chlorella vulgaris cultivation in two-stage process was applied to increase the lipid productivity without compromising the biomass productivity. At the first stage, microalgae was cultivated under nutrient sufficient conditions to obtain a maximized cell density; at the second stage, nitrate conditions are changed to trigger the accumulation of TAG. During first stage, the maximum biomass productivity (32 mg L⁻¹ d⁻¹) was observed after 13 days under nutrient sufficient conditions with 1.21 g L⁻¹ NaNO₃ and 0.00449 g L⁻¹ K₂HPO₄. Maximum lipid content (25.4% DW), lipid productivity (7.5 mg L⁻¹ d⁻¹) and TAG content (41.3% in total lipids) were favored by the nitrogen starvation conditions for more 4 days, at the second stage. Oil extracted at the second stage contained lower percentage of PUFAs being more suitable for the biodiesel production when compared with the oil extracted at the first stage. This two-stage phototrophic process is promising to provide a more efficient way for on a large-scale production of algal biomass and biodiesel production.     

Towards the integration of dark- and photo-fermentative waste treatment. 4. Repeated batch sequential dark- and photofermentation using starch as substrate

In this study we demonstrated the technical feasibility of a prolonged, sequential two-stage integrated process under a repeated batch mode of starch fermentation. In this durable scheme, the photobioreactor with purple bacteria in the second stage was fed directly with dark culture from the first stage without centrifugation, filtration, or sterilization (not demonstrated previously). After preliminary optimization, both the dark- and the photo-stages were performed under repeated batch modes with different process parameters. Continuous H₂ production in this system was observed at a H₂ yield of up to 1.4 and 3.9 mole mole⁻¹ hexose during the dark- and photo-stage, respectively (for a total of 5.3 mole mole⁻¹ hexose), and rates of 0.9 and 0.5 L L⁻¹ d⁻¹, respectively. Prolonged repeated batch H₂ production was maintained for up to 90 days in each stage and was rather stable under non-aseptic conditions. Potential for improvements in these results are discussed.     

Sequential dark–photo fermentation and autotrophic microalgal growth for high-yield and CO2-free biohydrogen production

Dark fermentation, photo fermentation, and autotrophic microalgae cultivation were integrated to establish a high-yield and CO₂-free biohydrogen production system by using different feedstock. Among the four carbon sources examined, sucrose was the most effective for the sequential dark (with Clostridium butyricum CGS5) and photo (with Rhodopseudomonas palutris WP3-5) fermentation process. The sequential dark–photo fermentation was stably operated for nearly 80 days, giving a maximum H₂ yield of 11.61 mol H₂/mol sucrose and a H₂ production rate of 673.93 ml/h/l. The biogas produced from the sequential dark–photo fermentation (containing ca. 40.0% CO₂) was directly fed into a microalga culture (Chlorella vulgaris C–C) cultivated at 30 °C under 60 μmol/m²/s illumination. The CO₂ produced from the fermentation processes was completely consumed during the autotrophic growth of C. vulgaris C–C, resulting in a microalgal biomass concentration of 1999 mg/l composed mainly of 48.0% protein, 23.0% carbohydrate and 12.3% lipid.     

Fatty acid methyl ester production from wet microalgal biomass by lipase-catalyzed direct transesterification

The aim of this work was to optimize the production of fatty acid methyl ester (FAME, biodiesel) from wet Nannchloropsis gaditana microalgal biomass by direct enzymatic transesterification. This was done in order to avoid the high cost associated with the prior steps of drying and oil extraction. Saponifiable lipids (SLs) from microalgal biomass were transformed to FAME using the lipase Novozyme 435 (N435) from Candida antarctica as the catalyst, and finally the FAME were extracted with hexane. t-Butanol was used as the reaction medium so as to decrease lipase deactivation and increase mass transfer velocity. A FAME conversion of 99.5% was achieved using wet microalgal biomass homogenized at 140 MPa to enhance cell disruption, a N435:oil mass ratio of 0.32, methanol added in 3 stages to achieve a total of 4.6 cm³ g⁻¹ of oil and 7.1 cm³ g⁻¹ oil of added t-butanol, with a reaction time of 56 h. The FAME conversion decreased to 57% after catalyzing three reactions with the same lipase batch. This work shows the influence of the polar lipids contained in the microalgal biomass both on the reaction velocity and on lipase activity.     

Application of experimental design methodology for optimization of biofuel production from microalgae

Optimization of biofuel productivity, in terms of lipid content, polysaccharide content, and calorific value, from microalgae was performed by varying four variables (temperature, light intensity, nitrogen content, and CO₂ addition) using a 2⁴ full factorial design. A statistical analysis showing the influence of each variable and their interactions was conducted. The selected variables all influence biofuel productivity, but their importance varies according to the sequence: CO₂ addition > temperature > nitrogen content > light intensity. Interactive effects of temperature with light intensity and nitrogen with CO₂ addition for lipid and polysaccharide productivities were identified, respectively. For calorific value, interactive effects of CO₂ addition with light intensity and nitrogen content were observed. The highest biofuel productivity was obtained at the following conditions: temperature (>25 °C), light intensity (>60 μmol photons m⁻² s⁻¹), nitrogen content (<50 mg L⁻¹), and CO₂ addition (>18 mL L⁻¹ d⁻¹). 10 days was found to be the most favorable cultivation time for lipid production under the investigated conditions.     

Microalgae cofiring in coal power plants: Innovative system layout and energy analysis

This paper investigates the smart integration of a 500 ha microalgae culturing facility with a large scale coal power plant (758.6 MW_e): a fraction of the CO₂ contained in the coal plant flue gases is used for the algal cultivation, a fraction of the low-temperature flue gas heat available is used for the biomass drying, finally the produced biomass is co-fired in the coal plant. The produced algal biomass represents approximately 1% of the boiler heat input.Through the solution of energy and mass balances of each plant component, the overall system performances in terms of net energy ratio (NER) and CO₂ emissions reduction are obtained. The computed NER (1.92) guarantees an energy harvest almost twice the energetic cost needed to produce the microalgal fuel. The total CO₂ emissions are reduced of approximately 0.48%, identifying microalgae cofiring as a solution able to reduce the environmental impact of electricity generation. A simplified economic analysis has allowed an estimate of the algal system investment cost (about 235 k€ ha⁻¹) and of the levelized cost of electricity (LCOE) (554.4 € MWh⁻¹). A set of sensitivity analyses is finally performed to investigate the influence of the initial hypotheses on the results.     

Influence of culture age,ammonium and organic carbon in hydrogen production and nutrient removal by Anabaena sp. in nitrogen-limited cultures

Hydrogen can be generated from cyanobacteria cultivation with light and organic carbon as an energy source. The aim of this study was to investigate the influence of ammonium and glucose concentration, and the culture age on the production of hydrogen and other products, and phosphate and organic carbon removal by Anabaena sp. (UTEX 1448) in batch anaerobic photobioreactors. Our results demonstrated an effect of culture age and ammonium concentration in hydrogen production, an average increase of 4.1 times (90.4 μmol H₂ mg Chl a⁻¹ h⁻¹ and 13.2 mmol mg Chl a⁻¹) in the conditions with younger biomass and without ammonium. Culture age also had an effect in phosphate removal, with 92% of removal efficiency, and ethanol production (an average increase of 2.9 times–97 mg L⁻¹), however the optimum conditions were obtained with older biomass. This study demonstrates efficient hydrogen production by this strain of Anabaena sp. fewer researched.     

Energy and CO2 analysis of poplar and maize crops for biomass production in north Italy

The rising price of fossil fuel and the increasing environmental concern encourage the use of biomasses as energy sources. Aim of this study was to compare two poplar SRC and vSRC (6 and 3 years rotation cycle) with an annual crop (maize), used for biomass production in north Italy.The average of the biomass production was 13.9 Mg DM ha⁻¹ per year for the SRC and vSRC poplar and 19.2 Mg DM ha⁻¹ for the maize.The energy consumption for the poplar cultivations was about 15 GJ ha⁻¹ per year, which represented only the 6% of the energy biomass product (about 257 GJ ha⁻¹ per year).The input value of the maize was higher (26.8 GJ ha⁻¹ per year). In this case, the input value was about the 7% of the energy content in the biomass product (about 370 GJ ha⁻¹ per year).During the vSRC cultivation an amount of 8090 kg CO₂ eq ha⁻¹ was emitted, 6420 kg CO₂ eq ha⁻¹ for the SRC and 26,370 kg CO₂ eq ha⁻¹ for the maize.Compared to the maize, the poplar SRC (or vSRC) crops are interesting from an energetic point of view, while maize requires less manpower, but it has major problems related to the landscape biodiversity.     

Thermophilic hydrogen and methane production from sugarcane stillage in two-stage anaerobic fluidized bed reactors

This study evaluated the feasibility of H₂ and CH₄ production in two-stage thermophilic (55 °C) anaerobic digestion of sugarcane stillage (5,000 to 10,000 mg COD.L⁻¹) using an acidogenic anaerobic fluidized bed reactor (AFBR-A) with a hydraulic retention time (HRT) of 4 h and a methanogenic AFBR (AFBR-S) with HRTs of 24 h–10 h. To compare two-stage digestion with single-stage digestion, a third methanogenic reactor (AFBR-M) with a HRT of 24 h was fed with increasing stillage concentrations (5,000 to 10,000 mg COD.L⁻¹). The AFBR-M produced a methane content of 68.4 ± 7.2%, a maximum yield of 0.30 ± 0.04 L CH₄.g COD⁻¹, a production rate of 3.78 ± 0.40 L CH₄.day⁻¹.L⁻¹ and a COD removal of 73.2 ± 5.0% at an organic loading rate (OLR) of 7.5 kg COD.m⁻³.day⁻¹. In contrast, the two-stage AFBR-A system produced a hydrogen content of 23.9 ± 5.6%, a production rate of 1.30 ± 0.16 L H₂.day⁻¹.L⁻¹ and a yield of 0.34 ± 0.08 mmol H₂.g CODap⁻¹. Additionally, the decrease in the HRT from 18 h to 10 h in the AFBR-S favored a higher methane production, improving the maximum methane content (74.5 ± 6.0%), production rate (5.57 ± 0.38 L CH₄.day⁻¹.L⁻¹) and yield (0.26 ± 0.06 L CH₄.g COD⁻¹) at an OLR of 21.6 kg COD.m⁻³.day⁻¹ (HRT of 10 h) with a total COD removal of 70.1 ± 7.1%. Under the applied COD of 10,000 mg L⁻¹, the two-stage system showed a 52.8% higher energy yield than the single-stage anaerobic digestion system. These results show that, relative to a single-stage system, two-stage anaerobic digestion systems produce more hydrogen and methane while achieving similar treatment efficiencies.     

Conversion of microalgae to biofuel

This paper primarily presents an overall review of the use of microalgae as a biofuel feedstock. Among the microalgae that have potential as biofuel feedstock, Chlorella, specifically, was thoroughly discussed because of its ability to adapt both to heterotrophic and phototrophic culture conditions. The lipid content and biomass productivity of microalgae can be up to 80% and 7.3 g/l/d based on the dried weight of biomass, respectively, making microalgae an ideal candidate as a biofuel feedstock. The set-up of the system and the biomass productivity of microalgae cultivated in an open pond and a photobioreactor were also compared in this work. The effect of the culture condition is discussed based on the two-stage culture period. The issues that were discussed include the light condition and the CO₂, DO and N supply. The microalgal productivities under heterotrophic and phototrophic culture conditions were also compared and highlighted in this work. The harvesting process and type of flocculants used to aid the harvesting were highlighted by considering the final yield of biomass. A new idea regarding how to harvest microalgae based on positive and negative charges was also proposed in this work. The extraction methods and solvents discussed were primarily for the conventional and newly invented techniques. Conversion processes such as transesterification and thermochemical processes were discussed, sketched in figures and summarized in tables. The cost–benefit analysis of heterotrophic culture and the cultivation system was highlighted at the end of this work. Other benefits of microalgae are also mentioned in this work to give further support for the use of microalgae as a feedstock for biofuel production.     

Biohydrogen production by a novel integration of dark fermentation and mixotrophic microalgae cultivation

Biohydrogen is usually produced via dark fermentation, which generates CO₂ emissions and produces soluble metabolites (e.g., volatile fatty acids) with high chemical oxygen demand (COD) as the by-products, which require further treatments. In this study, mixotrophic culture of an isolated microalga (Chlorella vulgaris ESP6) was utilized to simultaneously consume CO₂ and COD by-products from dark fermentation, converting them to valuable microalgae biomass. Light intensity and food to microorganism (F/M) ratio were adjusted to 150 μmol m⁻² s⁻¹ and F/M ratio, 4.5, respectively, to improve the efficiency of assimilating the soluble metabolites. The mixotrophic microalgae culture could reduce the CO₂ content of dark fermentation effluent from 34% to 5% with nearly 100% consumption of soluble metabolites (mainly butyrate and acetate) in 9 days. The obtained microalgal biomass was hydrolyzed with 1.5% HCl and subsequently used as the substrate for bioH₂ production with Clostridium butyricum CGS5, giving a cumulative H₂ production of 1276 ml/L, a H₂ production rate of 240 ml/L/h, and a H₂ yield of 0.94 mol/mol sugar.     

A life cycle assessment of pennycress (Thlaspi arvense L.) -derived jet fuel and diesel

Field Pennycress (Thlaspi arvense L.) is a member of the mustard family and may be grown as a winter crop between traditional summer crops to produce renewable biomass for renewable diesel and jet fuel. This paper estimated total annual biofuel production potential of 15 million cubic metres from rotation between corn and soybeans on 16.2 million hectares in the Midwest without impact on food production. This study also investigated the life cycle greenhouse gas (GHG) emissions and energy balance of pennycress-derived Hydroprocessed Renewable Jet (HRJ) fuel and Renewable Diesel (RD). Both system expansion and allocation approaches were applied to distribute environmental impacts among products and co-products along the life cycle of each biofuel. The life cycle GHG emissions (excluding land use change) for RD and HRJ range from 13 to 41 g MJ⁻¹ (CO₂ eq.) and −18 to 45 g MJ⁻¹ (CO₂ eq.), respectively, depending on how the co-products are credited. The majority of the energy required for each biofuel product is derived from renewable biomass as opposed to non renewable fossil. The fossil energy consumptions are considerably lower than the petroleum fuels. Scenario analyses were also conducted to determine response to model assumptions, including nitrogen fertilizer application rate, nitrogen content in crop residues, and sources of H₂. The results show that pennycress derived biofuels could qualify as advanced biofuels and as biomass-based diesel as defined by the Renewable Fuels Standard (RFS2).     

A comparative analysis of woody biomass and coal for electricity generation under various CO2 emission reductions and taxes

Mitigating global climate change via CO₂ emission control and taxation is likely to enhance the economic potential of bioenergy production and utilization. This study investigated the cost competitiveness of woody biomass for electricity production in the US under alternative CO₂ emission reductions and taxes. We first simulated changes in the price of coal for electricity production due to CO₂ emission reductions and taxation using a computable general equilibrium model. Then, the costs of electricity generation fueled by energy crops (hybrid poplar), logging residues, and coal were estimated using the capital budgeting method. Our results indicate that logging residues would be competitive with coal if emissions were taxed at about US$25 Mg⁻¹ CO₂, while an emission tax US$100 Mg⁻¹ CO₂ or higher would be needed for hybrid poplar plantations at a yield of 11.21 dry Mg ha⁻¹ yr⁻¹ (5 dry tons ac⁻¹ yr⁻¹) to compete with coal in electricity production. Reaching the CO₂ emission targets committed under the Kyoto Protocol would only slightly increase the price of fossil fuels, generating little impact on the competitiveness of woody biomass. However, the price of coal used for electricity production would significantly increase if global CO₂ emissions were curtailed by 20% or more. Logging residues would become a competitive fuel source for electricity production if current global CO₂ emissions were cut by 20–30%. Hybrid poplar plantations would not be able to compete with coal until emissions were reduced by 40% or more.     

From piggery wastewater nutrients to biogas: Microalgae biomass revalorization through anaerobic digestion

Microalgae grown in swine wastewater were used as a promising strategy to produce renewable energy by coupling wastewater bioremediation and biomass revalorization. The efficiency of a microalgae consortium treating swine slurry at different temperature (15 and 23 °C) and illumination periods (11 and 14 h) was assessed for biomass growth and nutrient removal at two NH₄⁺ initial concentrations (80 and 250 mg L⁻¹ NH₄⁺). Favourable culture conditions (23 °C and 14 h of illumination) and high ammonium loads resulted in higher biomass production and greater nutrients removal rates. The initial N–NH₄⁺ load determined the removal mechanism, thus ammonia stripping and nitrogen uptake accounted similarly in the case of high NH₄⁺ load, while nitrogen uptake prevailed at low NH₄⁺ load. Under favourable conditions, nitrogen availability in the media determined the composition of the biomass. In this context, carbohydrate-rich biomass was obtained in batch mode while semi-continuous operation resulted in protein-rich biomass. The revalorization of the resultant biomass was evaluated for biogas production. Methane yields in the range of 106–146 and 171 ml CH₄ g COD⁻¹ were obtained for the biomasses grown in batch and semi-continuous mode, respectively. Biomass grown under favourable conditions resulted in higher methane yields and closer to the theoretically achievable.     

Enhancing hydrogen production from biomass pyrolysis by dental-wastes-derived sodium zirconate

Sodium zirconate was synthesized via the solid-phase reaction from dental wastes with sodium carbonate. The dental-wastes-derived sodium zirconate (DW-SZ) dramatically enhanced hydrogen (H₂) production during biomass pyrolysis due to the presence of alkali metal (Na) and in-situ carbon dioxide (CO₂) capture. To be specific, the DW-SZ could maintain a maximum CO₂ uptake of 0.195 g-CO₂ g-sorbent⁻¹ at the 30th cycle under cyclic CO₂ absorption/desorption swings. The total CO₂ uptake in all 30 cycles reached 82% of the theoretical CO₂ uptake. The synthesized functional material was subsequently employed for H₂ production in the pyrolysis of three different biomass samples (i.e., municipal sludge, spirulina, and methylcellulose). The highest H₂ yield of 205 ml g⁻¹ was produced by spirulina in presence of functional material, followed by methylcellulose (197 ml g⁻¹), and municipal sludge (142 ml g⁻¹). Moreover, the DW-SZ was characterized by XRD, BET and SEM to help unravel and justify the underlying reaction mechanisms.     

Improvement of biohydrogen production from brewery wastewater: Evaluation of inocula,support and reactor

This work explores the production of biohydrogen from brewery wastewater using as inoculum a culture produced by natural fermentation of synthetic wastewater and Klebsiella pneumoniae isolated from the environment. Klebsiella pneumoniae showed good performance as inoculum, as evaluated using assays of between 9 and 16 cycles, with durations of 12 and 24 h, carbohydrate concentrations from 2.79 to 7.22 g L⁻¹, and applied volumetric organic loads from 2.6 to 12.6 g carbohydrate L⁻¹ day⁻¹. The best results were achieved with applied volumetric organic loads of 12.6 g carbohydrate L⁻¹ day⁻¹ and cycle length of 12 h, resulting in mean volumetric productivity of 0.88 L H₂ L⁻¹ day⁻¹, maximum molar flow of 10.80 mmol H₂ h⁻¹, and mean yield of 0.70 mol H₂ mol⁻¹ glucose consumed. The biogas H₂ content was between 18 and 42%, while the mean organic compounds removal and carbohydrate conversion efficiencies were 23 and 81%, respectively. The inoculum produced by natural fermentation was not viable.