Capnophilic lactic fermentation and hydrogen synthesis by Thermotoga neapolitana: An unexpected deviation from the dark fermentation model

The heterotrophic bacterium Thermotoga neapolitana produces hydrogen by fermentation of organic substrates. The process is referred to as dark fermentation and is typically complemented by production of acetic acid. Here we show that synthesis of products derived by reductive metabolism of pyruvate, mainly lactic acid, occurs to the detriment of acetic acid fermentation when the cultures of the thermophilic bacterium are flushed by saturating level of CO₂. Sodium bicarbonate in a very narrow range of concentrations (∼14 mM) also causes the same metabolic shift. The capnophilic (CO₂-requiring) re-orientation of the fermentative process toward lactic acid does not affect hydrogen productivity thus challenging the currently accepted dark fermentation model that predicts reduction of this gas when glucose is converted into organic products different from acetate.     

Enhancement of hydrogen production rate by high biomass concentrations of Thermotoga neapolitana

The objective of this study was to enhance the hydrogen production rate of dark fermentation in batch operation. For the first time, the hyperthermophilic pure culture of Thermotoga neapolitana cf. Capnolactica was applied at elevated biomass concentrations. The increase of the initial biomass concentration from 0.46 to 1.74 g cell dry weight/L led to a general acceleration of the fermentation process, reducing the fermentation time of 5 g glucose/L down to 3 h with a lag phase of 0.4 h. The volumetric hydrogen production rate increased from 323 (±11) to 654 (±30) mL/L/h with a concomitant enhancement of the biomass growth and glucose consumption rate. The hydrogen yield of 2.45 (±0.09) mol H₂/mol glucose, the hydrogen concentration of 68% in the produced gas and the composition of the end products in the digestate, i.e. 62.3 (±2.5)% acetic acid, 23.5 (±2.9)% lactic acid and 2.3 (±0.1)% alanine, remained unaffected at increasing biomass concentrations.     

Hydrogen production from carrot pulp by the extreme thermophiles Caldicellulosiruptor saccharolyticus and Thermotoga neapolitana

Hydrogen was produced from carrot pulp hydrolysate, untreated carrot pulp and (mixtures of) glucose and fructose by the extreme thermophiles Caldicellulosiruptor saccharolyticus and Thermotoga neapolitana in pH-controlled bioreactors. Carrot pulp hydrolysate was obtained after enzymatic hydrolysis of the polysaccharide fraction in carrot pulp. The main sugars in the hydrolysate were glucose, fructose, and sucrose.     

Enhancement of fermentative hydrogen production from green algal biomass of Thermotoga neapolitana by various pretreatment methods

Biomass of the green algae has been recently an attractive feedstock source for bio-fuel production because the algal carbohydrates can be derived from atmospheric CO₂ and their harvesting methods are simple. We utilized the accumulated starch in the green alga Chlamydomonas reinhardtii as the sole substrate for fermentative hydrogen (H₂) production by the hyperthermophilic eubacterium Thermotoga neapolitana. Because of possessing amylase activity, the bacterium could directly ferment H₂ from algal starch with H₂ yield of 1.8–2.2 mol H₂/mol glucose and the total accumulated H₂ level from 43 to 49% (v/v) of the gas headspace in the closed culture bottle depending on various algal cell-wall disruption methods concluding sonication or methanol exposure. Attempting to enhance the H₂ production, two pretreatment methods using the heat-HCl treatment and enzymatic hydrolysis were applied on algal biomass before using it as substrate for H₂ fermentation. Cultivation with starch pretreated by 1.5% HCl at 121 °C for 20 min showed the total accumulative H₂ yield of 58% (v/v). In other approach, enzymatic digestion of starch by thermostable α-amylase (Termamyl) applied in the SHF process significantly enhanced the H₂ productivity of the bacterium to 64% (v/v) of total accumulated H₂ level and a H₂ yield of 2.5 mol H₂/mol glucose. Our results demonstrated that direct H₂ fermentation from algal biomass is more desirably potential because one bacterial cultivation step was required that meets the cost-savings, environmental friendly and simplicity of H₂ production.     

Biohydrogen production from untreated and hydrolyzed potato steam peels by the extreme thermophiles Caldicellulosiruptor saccharolyticus and Thermotoga neapolitana

Production of hydrogen by the extreme thermophiles Caldicellulosiruptor saccharolyticus and Thermotoga neapolitana was studied in serum flasks and in pH-controlled bioreactors with glucose, and hydrolyzed and untreated potato steam peels (PSP) as carbon sources. Two types of PSP hydrolysates were used: one in which the starch in the PSP was liquefied with alpha-amylase, and one in which the liquefied starch was further hydrolyzed to glucose by amyloglucosidase.     

Hydrogen impacts on performance and CO2 emissions from a diesel power generator

This work investigates the performance and carbon dioxide (CO₂) emissions from a stationary diesel engine fueled with diesel oil (B5) and hydrogen (H₂). The performance parameters investigated were specific fuel consumption, effective engine efficiency and volumetric efficiency. The engine was operated varying the nominal load from 0 kW to 40 kW, with diesel oil being directly injected in the combustion chamber. Hydrogen was injected in the intake manifold, substituting diesel oil in concentrations of 5%, 10%, 15% and 20% on energy basis, keeping the original settings of diesel oil injection. The results show that partial substitution of diesel oil by hydrogen at the test conditions does not affect significantly specific fuel consumption and effective engine efficiency, and decreases the volumetric efficiency by up to 6%. On the other hand the use of hydrogen reduced CO₂ emissions by up to 12%, indicating that it can be applied to engines to reduce global warming effects.     

Hydrogen production performance by Enterobacter cloacae KBH3 isolated from termite guts

Two out of six bacterial isolates obtained from the guts of Globitermes sp. termites were identified as hydrogen-producing bacteria. One isolate, Enterobacter cloacae KBH3, was characterised using the BIOLOG identification system and 16S rRNA gene analysis. In a batch fermentation study to evaluate its growth in defined medium, E. cloacae KBH3 produced 154 ml H₂ per litre medium with approximately 50% hydrogen content. The carbon utilisation results suggest that E. cloacae KBH3 have the potential to be a good hydrogen producer. This strain is also able to produce hydrogen within a wide range of temperatures (28–40 °C) and pH (4.5–8). In several fermentation runs, the pH of the culture dropped from 6.5 to 5.36 within the first 3 h, which was mostly due to the biosynthesis of formate. An increase of cumulative hydrogen production was recorded as well as a decrease in the concentration of formate, indicating the importance of the formate pathway for hydrogen production. The highest rate of hydrogen production of 180.74 ml H₂/l/h was achieved when lactate and acetate were at their highest concentrations. Most of the hydrogen gas was produced during the exponential growth phase, and the biogas continued to be produced during the stationary phase. The specific growth rate was calculated to be 0.224 per hour while the hydrogen yield was 1.8 mol of hydrogen per mol of glucose. At the end of the batch study, the highest cumulative hydrogen production was 2404 ml H₂ per litre of fermentation medium.     

The Ni/ZrO2 catalyst and the methanation of CO and CO2

The Ni/ZrO₂ catalyst is one of the most active systems for the methanation of CO to be employed in the hydrogen purification for PEMFC. This contribution aims to study the effect of ZrO₂ on the methanation of CO and CO₂. The catalytic behavior of Ni/ZrO₂, Ni/SiO₂, a physical mixture comprising Ni and ZrO_2, and a double-bed reactor were evaluated. The TPD of CO and CO₂, TPSR and the cyclohexane dehydrogenation reaction were carried out to describe the catalysts and the reactions. The high activity of Ni/ZrO₂ catalyst toward the methanation of CO is related to the presence of active sites on the ZrO₂ surface. The methanation of CO occurs on ZrO₂ due to its ability to adsorb CO and also because of the hydrogen spillover phenomenon. Apparently, the effect of ZrO₂ is less relevant for the methanation of CO₂. Ni/ZrO₂ is a very promising system for the purification of hydrogen.     

Biomass and lipid production of marine microalgae using municipal wastewater and high concentration of CO2

In order to reduce the cost of the production of microalgae for biodiesel, the feasibility of using the mixture of seawater and municipal wastewater as culture medium and CO₂ from flue gas for the cultivation of marine microalgae was investigated in this study. Effects of different ratios of municipal wastewater and 15% CO₂ aeration on the growth of Nannochloropsis sp. were examined, and lipid accumulation of microalgae was also studied under nitrogen starvation and high light. It was found that optimal growth of microalgae occurred in 50% municipal wastewater, and the growth was further significantly enhanced by aeration with 15% CO₂. When Nannochloropsis sp. cells were transferred from the first growth phase to the second lipid accumulation phase under the combination of nitrogen deprivation and high light, both biomass and lipid production of Nannochloropsis sp. were significantly increased. After 12 days of the second-phase cultivation, the biomass concentration and total lipid content increased from 0.71 to 2.23 g L⁻¹ and 33.8–59.9%, respectively. This study suggests that it is possible to utilize municipal wastewater to replace nutrients in seawater medium and use flue gas to provide CO₂ in the cultivation of oil-bearing marine microalgae for biodiesel.     

Design of CO2 absorption plant for recovery of CO2 from flue gases of gas turbine

The ongoing human-induced emission of carbon dioxide (CO₂) threatens to change the earth's climate. A major factor in global warming is CO₂ emission from thermal power plants, which burn fossil fuels. One possible way of decreasing CO₂ emissions is to apply CO₂ removal, which involves recovering of CO₂ from energy conversion processes. This study is focused on recovery of CO₂ from gas turbine exhaust of Sarkhun gas refinery power station. The purpose of this study is to recover the CO₂ with minimum energy requirement. Many of CO₂ recovery processes from flue gases have been studied. Among all CO₂ recovery processes which were studied, absorption process was selected as the optimum one, due to low CO₂ concentration in flue gas. The design parameters considered in this regard, are: selection of suitable solvent, solvent concentration, solvent circulation rate, reboiler and condenser duty and number of stages in absorber and stripper columns. In the design of this unit, amine solvent such as, diethanolamine (DEA), diglycolamine (DGA), methyldiethanolamine (MDEA), and monoethanolamine (MEA) were considered and the effect of main parameters on the absorption and stripping columns is presented. Some results with simultaneous changing of the design variables have been obtained. The results show that DGA is the best solvent with minimum energy requirement for recovery of CO₂ from flue gases at atmospheric pressure.     

Hydrogen production by thermo-catalytic decomposition of methane: Regeneration of active carbons using CO2

Thermo-catalytic decomposition of methane using carbons as catalyst is a very attractive process for free CO₂–hydrogen production. One of the main drawbacks for the sustainability of the process is catalyst deactivation. In this work, regeneration of a deactivated active-carbon catalyst has been studied using CO₂ as activating agent under different regeneration conditions. It has been stated that during the regeneration stage, a compromise between the regeneration of the initial properties of the catalyst and the burn-off is needed in order to keep the sustainability of the process. Three deactivation–regeneration cycles have been performed for two sets of regeneration conditions. A progressive decreasing in the burn-off, surface area and surface oxygenated groups after each decomposition/regeneration cycle is observed. It can be explained considering that the carbon removed during the regeneration steps is not the carbon deposited from methane but the remaining initial catalyst, which is less resistant to gasification. The implication is that after three cycles of decomposition/regeneration, most of the carbon sample consists of carbon formed during the process since the initial catalyst has been gasified.     

CO2 reforming of CH4 by binode thermal plasma

A binode thermal plasma is first applied to CO₂ reforming of CH₄ to investigate how to enlarge the process and lower energy consumption. Experimental study is conducted in two modes. One is to introduce feed gases (CH₄ and CO₂) only into discharge region between the first anode and the second anode as plasma-forming gas; the other is to introduce them not only into discharge region but also into the plasma jet from the exit of plasma generator. The experimental results show that, the former brings about higher conversion and selectivity but appreciably lower energy conversion efficiency due to its higher energy utilization, while the latter brings about higher energy conversion efficiency but somewhat lower conversion and selectivity due to its larger feeding of CH₄ and CO₂. Furthermore, during discharge in both modes, the oxidation on cathode and anode, or carbon deposition in plasma generator is not observed.     

Performance of Na2O promoted alumina as CO2 chemisorbent in sorption-enhanced reaction process for simultaneous production of fuel-cell grade H2 and compressed CO2 from synthesis gas

The performance of a novel thermal swing sorption-enhanced reaction (TSSER) concept for simultaneous production of fuel-cell grade hydrogen and compressed carbon dioxide as a by-product from a synthesis gas feed is simulated using Na₂O promoted alumina as a CO₂ chemisorbent in the process. The process simultaneously carries out the water gas shift (WGS) reaction and removal of CO₂ from the reaction zone by chemisorption in a single unit. Periodic regeneration of the chemisorbent is achieved by using the principles of thermal swing adsorption employing super-heated steam purge.     

Fixation of CO2 by carbonating calcium derived from blast furnace slag

Industrial waste materials, such as steelmaking slags, appear to be potential raw materials for reducing CO₂ emissions by carbonation. The suitability of applying a carbonation route based on acetic acid leaching to produce carbonates from blast furnace slag is presented in this study. The effect of solution pH, temperature, and CO₂ pressure on the precipitation of carbonates was experimentally studied. A simple thermodynamic model was used to verify our results. The feasibility of the process was also discussed, addressing energy input requirements and the consumption of chemicals.     

Natural emissions of CO2 from the geosphere and their bearing on the geological storage of carbon dioxide

Carbon dioxide (CO₂) capture and storage has the potential to reduce CO₂ emissions from fossil fuel combustion. Although leakage from monitored CO₂ injection sites has been minimal to non-existent, experience from the natural gas storage industry suggests that, if it becomes a widely deployed technology, leaks may be expected from some storage sites. Natural occurrences of CO₂ in the geosphere, some of which have been exploited, provide insights into the types of emissions that might be expected from anthropogenic CO₂ storage sites. CO₂ emission sites are commonly found in clusters in CO₂-prone geological provinces: the most common natural emissions sites in sedimentary basins consist of carbonated springs and mofettes. These represent at worst only a local hazard. In volcanic and hydrothermal provinces, more energetic emissions may occur due to active supply from degassing magma. These include rare, sudden emissions from fissures and craters that have caused fatalities. It is unlikely that such provinces would be considered for CO₂ storage Major lake overturn events such as occurred at Lake Nyos in 1986 are considered highly unlikely to occur as a result of CO₂ storage, not least because CO₂ levels in lake waters can be monitored and remediated. Natural CO₂ fields indicate that under favourable conditions CO₂ can be retained in the subsurface for millions of years. The main risk from man-made CO₂ storage sites that does not have any close analogy in nature is considered to be a well blowout. A blowout that took place at a natural CO₂ field provides some indication of the likely hazard.     

Enhanced H2 fermentation of organic waste by CO2 sparging

This study aimed to improve the productivity of dark fermentative hydrogen production from organic waste. An anaerobic sequencing batch reactor was used for hydrogen fermentation and it was fed with food waste (VS 4.4 ± 0.2% containing 27 g carbohydrate-COD/L) at various CO₂ sparging rates (40–120 L/L/d), hydraulic retention times (HRTs; 18–42 h), and solid retention times (SRTs; 18–160 h). CO₂ sparging increased the H₂ productivity by 5–36% at all the examined conditions, confirming the benefit of the replacement of headspace gas by CO₂. The maximum H₂ production was obtained by CO₂ sparging at 80 L/L/d, resulting in the H₂ productivity of 3.18 L H₂/L/d and the H₂ yield of 97.3 mL H₂/g VS_added. Increase of n-butyrate and isopropanol yields were concurrent with the enhanced H₂ yield by CO₂ sparging. Acidogenic efficiency, the sum of H₂, organic acid, and alcohol, in the CO₂-sparged reactor ranged from 47.9 to 56.0%, which was comparable to conventional acidogenesis. Thermodynamic analysis confirmed that both CO₂ sparging and CO₂ removal were beneficial for H₂-producing reactions, but CO₂ sparing has more profound effect than CO₂ removal on inhibiting H₂-consuming reactions.     

CO2 utilization options, part II: Assessment of dimethyl carbonate production

In this paper, the potential to reduce CO₂ emissions from dimethyl carbonate production by switching from the traditional phosgene-based production to a urea-based CO₂ utilization process is assessed. The total CO₂ emission for each process is estimated, including emissions related to the carbon content of the products, energy consumption in the production process, and energy consumption in the production processes of the required reactants. Implementation of the CO₂ utilization process probably will reduce total CO₂ emissions. However, in order to achieve substantially reduced CO₂ emissions, serious consideration must be given to the optimization and design of the CO₂ utilization process. Furthermore, the fuel-mix employed is one of the factors that influences the total CO₂ emission the most.     

CO2 reforming of CH4 by combination of thermal plasma and catalyst

Experiments on synthesis gas preparation from dry reforming of methane by carbon dioxide with thermal plasma only and cooperation of thermal plasma with commercial catalysts have been performed. In all experiments, nitrogen gas was used as the plasma gas to form a high-temperature jet injected into a tube reactor. A mixture of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$ was fed vertically into the jet. Both kinds of experiments were conducted in the same conditions, such as total flux of feed gases, the molar ratio of _CH4/_CO2${\mathrm{CH}}_4/{\mathrm{CO}}_2$, and the plasma power except with or without catalysts in the tube reactor. Higher conversion of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$, higher selectivity of _H2${\mathrm{H}}_2$ and CO, and higher specific energy of the process were achieved by thermal plasma with catalysts. For example, the conversions of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$ were high to 96.33% and 84.63%, and the selectivies of CO and _H2${\mathrm{H}}_2$ were also high to 91.99% and 74.23%, respectively. Both were 10–20%$10–20\%$ higher than those by thermal plasma only.