Integrated biological hydrogen production

Biological systems offer a variety of ways by which to generate renewable energy. Among them, unicellular green algae have the ability to capture the visible portion of sunlight and store the energy as hydrogen (H₂). They hold promise in generating a renewable fuel from nature's most plentiful resources, sunlight and water. Anoxygenic photosynthetic bacteria have the ability of capturing the near infrared emission of sunlight to produce hydrogen while consuming small organic acids. Dark anaerobic fermentative bacteria consume carbohydrates, thus generating H₂ and small organic acids. Whereas efforts are under way to develop each of these individual systems, little effort has been undertaken to combine and integrate these various processes for increased efficiency and greater yields. This work addresses the development of an integrated biological hydrogen production process based on unicellular green algae, which are driven by the visible portion of the solar spectrum, coupled with purple photosynthetic bacteria, which are driven by the near infrared portion of the spectrum. Specific methods have been tested for the cocultivation and production of H₂ by the two different biological systems. Thus, a two-dimensional integration of photobiological H₂ production has been achieved, resulting in better solar irradiance utilization (visible and infrared) and integration of nutrient utilization for the cost-effective production of substantial amounts of hydrogen gas. Approaches are discussed for the cocultivation and coproduction of hydrogen in green algae and purple photosynthetic bacteria entailing broad utilization of the solar spectrum. The possibility to improve efficiency even further is discussed, with dark anaerobic fermentations of the photosynthetic biomass, enhancing the H₂ production process and providing a recursive link in the system to regenerate some of the original nutrients.     

An improved tri-reforming based methanol production process for enhanced CO2 valorization

The consistent rise of CO₂ concentration in the atmosphere is known to be significantly detrimental to the environment. Thus, mitigating CO₂ has become an urgent necessity. Current methods involving CO₂ mitigation can be broadly divided into two major categories which involve (i) CO₂ capture and sequestration (CCS) and (ii) CO₂ capture and valorization. Since, production of fuels/chemicals is an added feature along with mitigation in CO₂ valorization based methods, they could be economically favorable. An energy intensive CO₂ capture step is a common drawback of most CO₂ valorization methods that aim to mitigate CO₂ from major CO₂ emission sources (such as industrial flue gases). In this paper we employ and analyze a relatively new process called tri-reforming [1,2] which was developed to directly convert power plant based flue gases to synthesis gas, while avoiding the capture step. This paper is presented as an improvement over a tri-reforming coupled methanol production process as developed by Zhang et al. [3]. The process in Zhang et al. [3] involves utilizing tri-reforming process using flue gas and methane to produce synthesis gas which is then converted to methanol in the next step. The main contributions of this paper to the tri-reforming coupled methanol production process are: (i) proposition of a high pressure tri-reforming step to limit capital costs of the process (ii) establishment of steam input coupled with water separation step as a process improvement whose impact is shown to further amplify at higher tri-reformer pressures. The paper evaluates the process in terms of the profit generating and CO₂ valorization potential of the process as reflected by two parameters, gross margin (GM) and NPCV (net percentage of CO₂ valorized) respectively. In the proposed approach, higher pressures were utilized in the tri-reforming process to ensure economic feasibility of the process by limiting the reactor volume. The process improvements for the flowsheet containing the steam input combined with water separation (SWS) step over the one without these steps (termed as WSWS) are demonstrated in terms of an increase in GM/NPCV values at various pressures. The results indicate substantial improvements in GM and NPCV values (especially at higher tri-reformer pressures) ranging from 24.30 to 84.96% and 28.80–78.44% respectively in SWS cases over WSWS cases at various pressures. The simulations have been carried out in Aspen Plus V8.4 and are optimized using sensitivity analysis.     

Modeling and simulation of an integrated power-to-methanol approach via high temperature electrolysis and partial oxy-combustion technology

The Power to methanol (PtMeOH) approach based on water electrolysis and CO₂ capture is studied. The novelties are the integration of solid oxide electrolysis, partial oxy-combustion capture technology and methanol synthesis process, and the development and validation with experimental data of a SOEC system model using Aspen Custom Modeler. Simultaneously, CO₂ capture bench-scale unit and methanol synthesis process have been modeled and experimentally validated using Aspen Plus. These three systems have been thermally integrated in a final model to assess its high-performance operation. As a result, a clean, synthetic methanol is produced, which can be used as fuel or energy storage. In this lab-scale, a SOEC system of 1.2 kW and a synthetic flue gas of 7 l/min (40% CO_2, 60% N₂) are considered to obtain a methanol flow of 0.16 kg/h, with an overall efficiency of 29% for the integrated scenario. The SOEC system with an optimized BoP has the highest energy consumption, mainly due to water electrolysis, with 44% of total required energy. The novel power-to-methanol integrated in this work achieves around 20% reduction of the energy penalties estimated for the base case and makes use of oxygen from electrolysis for partial oxy-combustion and the water by-product of methanol synthesis in water electrolysis. The integrated model of the overall process is considered a useful tool for future works focused on further scale-up of the process.     

Techno-economic assessment on the fuel flexibility of a commercial scale combined cycle gas turbine integrated with a CO2 capture plant

Post-combustion carbon capture is a valuable technology, capable of being deployed to meet global CO₂ emissions targets. The technology is mature and can be retrofitted easily with existing carbon emitting energy generation sources, such as natural gas combined cycles. This study investigates the effect of operating a natural gas combined cycle plant coupled with carbon capture and storage while using varying fuel compositions, with a strong focus on the influence of the CO₂ concentration in the fuel. The novelty of this study lies in exploring the technical and economic performance of the integrated system, whilst operating with different fuel compositions. The study reports the design of a natural gas combined cycle gas turbine and CO₂ capture plant (with 30 wt% monoethanolamine), which were modelled using the gCCS process modelling application. The fuel compositions analysed were varied, with focus on the CO₂ content increasing from 1% to 5%, 7.5% and 10%. The operation of the CO₂ capture plant is also investigated with focus on the CO₂ capture efficiency, specific reboiler duty and the flooding point. The economic analysis highlights the effect of the varying fuel compositions on the cost of electricity as well as the cost of CO₂ avoided. The study revealed that increased CO₂ concentrations in the fuel cause a decrease in the efficiency of the natural gas combined cycle gas turbine; however, rising the CO₂ concentration and flowrate of the flue gas improves the operation of the capture plant at the risk of an increase in the flooding velocity in the column. The economic analysis shows a slight increase in cost of electricity for fuels with higher CO₂ contents; however, the results also show a reduction in the cost of CO₂ avoided by larger margins.     

Microalgal biomass production and carbon dioxide sequestration from an integrated ethanol biorefinery in Iowa: A technical appraisal and economic feasibility evaluation

Microalgae present some advantageous qualities for reducing carbon dioxide (CO₂) emissions from ethanol biorefineries. As photosynthetic organisms, microalgae utilize sunlight and CO₂ to generate biomass. By integrating large-scale microalgal cultivation with ethanol biorefineries, CO₂ sequestration can be coupled with the growth of algae, which can then be used as feedstock for biodiesel production. In this case study, a 50-mgy ethanol biorefinery in Iowa was evaluated as a candidate for this process. Theoretical projections for the amount of land needed to grow algae in raceway ponds and the oil yields of this operation were based on the amount of CO₂ from the ethanol plant. A practical algal productivity of 20 g m⁻² d⁻¹ would require over 2,000 acres of ponds for complete CO₂ abatement, but with an aggressive productivity of 40–60 g m⁻² d⁻¹, a significant portion of the CO₂ could be consumed using less than 1,000 acres. Due to the cold temperatures in Iowa, a greenhouse covering and a method to recover waste heat from the biorefinery were devised. While an algal strain, such as Chlorella vulgaris, would be able to withstand some temperature fluctuations, it was concluded that this process is limited by the amount of available heat, which could maintain only 41 acres at 73 °F. Additional heating requirements result in a cost of 10–40 USD per gallon of algal oil, which is prohibitively expensive for biodiesel production, but could be profitable with the incorporation of high-value algal coproducts.     

A review of carbon dioxide capture and utilization by membrane integrated microalgal cultivation processes

The capture of carbon dioxide (CO₂) from the air for microalgal cultivation has received increasing interest since it allows advantages that do not only reduce the amount of CO₂ already added to the air, but it is also more economical due to the accessibility of air, there are no regeneration requirements and it is a safe method that can help enhance microalgal growth. In order to capture CO₂ from the air, it is necessary to deal with CO₂ emissions from all sources in an atmosphere. Interestingly, the capture unit and microalgal culture can be located at any favorable site. Although a number of photobioreactors have been proposed with a CO₂ distribution system, the consequence of CO₂ losses is still being ignored. Thus, capturing CO₂ from the air via an integrated separation process in a photobioreactor is required for microalgal cultivation. Among the four available separation technologies, the membrane separation process would offer a safe, reliable and low cost method for CO₂ capture. Thus, this method of separation can be considered as a key factor in accelerating the development of a CO₂ enrichment process from the air for microalgal cultivation.     

Capturing CO2 in flue gas from fossil fuel-fired power plants using dry regenerable alkali metal-based sorbent

CO₂ capture and storage (CCS) has received significant attention recently and is recognized as an important option for reducing CO₂ emissions from fossil fuel combustion. A particularly promising option involves the use of dry alkali metal-based sorbents to capture CO₂ from flue gas. Here, alkali metal carbonates are used to capture CO₂ in the presence of H₂O to form either sodium or potassium bicarbonate at temperatures below 100 °C. A moderate temperature swing of 120–200 °C then causes the bicarbonate to decompose and release a mixture of CO₂/H₂O that can be converted into a “sequestration-ready” CO₂ stream by condensing the steam. This process can be readily used for retrofitting existing facilities and easily integrated with new power generation facilities. It is ideally suited for coal-fired power plants incorporating wet flue gas desulfurization, due to the associated cooling and saturation of the flue gas. It is expected to be both cost effective and energy efficient.     

An integrated biological hydrogen production process based on ethanol-type fermentation and bipolar membrane electrodialysis

An integrated bio-hydrogen production system involving fermentative hydrogen production and product separation is proposed. In this process, microorganisms conduct ethanol-type fermentation and generate H₂ gas in anaerobic bioreactor, and acetate is removed from fermentation broth by using a two chamber bipolar membrane electrodialysis as separation unit. A comparative study of fermentative hydrogen production of Ethanoligenens harbinese B49 in the integrated system with traditional fermentation process was carried out. Compared to traditional process, accumulated H₂ elevated 23%, glucose utilization ratio increased by 135% and cell growth increased by 27% in the integrated system. The specific hydrogen production rate reached 2.2 mol H₂/mol glucose, indicating that separation of acetate from fermentation system has a great role in promoting hydrogen producing capacity. Bipolar membrane electrodialysis showed high acetate separation efficiency and low glucose loss rate. In the integrated system, pH could be used to direct electrodialysis operation, since it has an exponential correlation with acetate concentration in fermentation broth. These results provide a new method for achieving efficient and stable H₂ production with simultaneous glucose recovery and acetate inhibition release.     

High quality product gas from biomass steam gasification combined with torrefaction and carbon dioxide capture processes

Gasification is currently recognized as a mature technology to convert biomass into useful and versatile product gas for further energy and fuel applications. However, there are some remaining problems relating to the process operation and process efficiency due to inherent properties of biomass feedstock such as high moisture content, low energy density and high oxygen content. Strategies to improve the efficiency of biomass gasification as well as the quality of product gas are thus required. For this purpose, a combined process of torrefaction, gasification, and carbon dioxide capture is developed and simulated in a commercial simulator to investigate the performance of a biomass gasification coupled with a pre-treatment and a post-treatment processes. The results show that the quality of product gas is enhanced when combining gasification with a torrefaction and a CO₂ capture processes. The heating value of the product gas and the cold gas efficiency are both increased with additional torrefaction. The CO₂ capture process using monoethanolamine offers a CO₂ removal efficiency of about 83% and consequently increase the product gas heating value up to 27%.     

Hydrogen production through sorption enhanced steam reforming of natural gas: Thermodynamic plant assessment

A detailed and comprehensive simulation model of a H₂ production plant based on the Sorption Enhanced Reforming (SER) process of natural gas has been developed in this work. Besides thermodynamic advantages related to the shift of reforming equilibrium, SER technology features an intrinsic CO₂ capture that can be of interest in environmentally constrained economies. The model comprises natural gas treatment, H₂ and CO₂ compression, as well as H₂ purification with an adsorption unit that has been integrated within the SER process by using the off-gas for sorbent regeneration. A complete thermal integration has been also performed between the available hot gas streams in the plant, so that high pressure steam is generated and used to generate power in a steam cycle.     

Resource demand implications for US algae biofuels production scale-up

Photosynthetic microalgae with the potential for high biomass and oil productivities have long been viewed as a promising class of feedstock for biofuels to displace petroleum-based transportation fuels. Algae offer the additional benefits of potentially being produced without using high-value arable land and fresh water, thereby reducing the competition for those resources between expanding biofuels production and conventional agriculture. Algae growth can also be enhanced by the use of supplemental CO₂ that could be supplied by redirecting concentrated CO₂ emissions from stationary industrial sources such as fossil-fired power plants, cement plants, fermentation industries, and others. In this way, algae may offer an effective means to capture carbon emissions for reuse in renewable fuels and co-products, while at the same time displacing fossil carbon fuels to help bring about a net reduction in overall carbon emissions. Significant displacement of petroleum fuels will require that algae feedstock production reach large volumes that will put demands on key resources. This scenario-based analysis provides a high-level assessment of land, water, CO₂ and nutrient (nitrogen, phosphorus) demands resulting from algae biofuel feedstock production reaching target levels of 10 billion gallons per year (BGY), 20 BGY, 50 BGY, and 100 BGY for four different geographical regions of the United States. Different algae productivities are assumed for each scenario region, where relative productivities are nominally based on annual average solar insolation. The projected resource demands are compared with data that provide an indication of the resource level potentially available in each of the scenario regions. The results suggest that significant resource supply challenges can be expected to emerge as regional algae biofuel production capacity approaches levels of about 10 BGY. The details depend on the geographic region, the target feedstock production volume, and the level of algae productivity that can be achieved. The implications are that the supply of CO₂, nutrients, and water, in particular, can be expected to severely limit the extent to which US production of algae biofuel can be sustainably expanded unless approaches are developed to mitigate these resource constraints in parallel to emergence of a viable algae technology. Land requirements appear to be the least restrictive, particularly in the Western half of the country where larger quantities of potentially suitable classes of land exist. Within the limited scope and assumptions of this analysis, sustainable photosynthetic microalgae biofuel feedstock production in the US in excess of about 10 BGY will likely be a challenge due to other water, CO₂ and nutrient resource limitations. Developing algae production approaches that can effectively use non-fresh water resources and minimize both water and nutrient requirements will help reduce resource constraints. Providing adequate CO₂ resources for enhanced algae production appears the biggest challenge, and could emerge as a constraint at oil production levels below 10 BGY.     

Environmental implications of power generation via coal-microalgae cofiring

Electrical power plants are responsible for over one-third of the US emissions, or about 1.7 Gt CO₂ per year. Power-plant flue gas can serve as a source of CO₂ for microalgae cultivation, and the algae can be cofired with coal. The study objective was to conduct a Life Cycle Assessment (LCA) to compare the environmental impacts of electricity production via coal firing versus coal/algae cofiring. The LCA results demonstrate that there are potentially significant benefits to recycling CO₂ toward microalgae production. As it reduces CO₂ emissions by recycling it and uses less coal, there are concomitant benefits of reduced greenhouse gas emissions. However, there are also other energy and fertilizer inputs needed for algae production, which contribute to key environmental flows. Lower net values for the algae cofiring scenario were observed for the following using the direct injection process (in which the flue gas is directly transported to the algae ponds): SO_x, NO_x, particulates, carbon dioxide, methane, and fossil energy consumption. Lower values for the algae cofiring scenario were also observed for greenhouse potential and air acidification potential. However, impact assessment for depletion of natural resources and eutrophication potential showed much higher values. This LCA gives us an overall picture and impacts across different environmental boundaries, and hence, can help in the decision-making process for implementation of the algae scenario.     

Emission analysis of Azolla methyl ester with BaO nano additives for IC engine

The aim of this work is to decrease emissions in diesel engines fueled with diesel and algae biodiesel blends and also addition of BiO nanoparticles. Azolla algae can be used to produce biodiesel, because of high oil content. The biodiesel was prepared by using Azolla algae non-edible oil through transesterification process. In the present study, the BaO nano additives into the algae oil-based methyl ester blend and its diesel blends are analyzed the emission characteristic at different load. Addition of BaO nanoparticle was a strategy to reduce emission (CO, HC, and O₂) of the biodiesel.     

Biomass integrated gasification combined cogeneration with or without CO2 capture – A comparative thermodynamic study

Fossil fuels presently cater to majority of energy demand of the world. However, due to the climate change problem capture of CO₂ emitted from the use of fossil fuels is emerging as a necessity. Alternately, developing technology with CO₂ neutral fuels may reduce green house gas emission. Possible even better solution may be combining both of these options, i.e., employing CO₂ capture process for energy efficient system using CO₂ neutral fuels, say biomass. In this paper, such cogeneration system with CO₂ capture using amine solution has been proposed. Thermodynamic modeling for the detail process has been implemented using ASPEN Plus^®. Comparative study of performance relative to a similar base case plant without carbon capture has been presented. Results show post combustion CO₂ capture process is better than pre-combustion CO₂ capture process for such plants with net negative CO₂ emission. Also degree of CO₂ capture has to be optimized on the basis of the overall performance of the plant as higher CO₂ capture affects thermodynamic and economic performance of the plant, specifically beyond certain value. In a net CO₂ negative emission plant extent of CO₂ capture is quite flexible and may be decided for optimum performance.     

Refining bioethanol from stalk juice of sweet sorghum by immobilized yeast fermentation

The relationship between total soluble sugar content and Brix in stalk juice of sweet sorghum was determined through one-dimensional linear regression. Meanwhile, bioethanol fermentation experiments were conducted in shaking flasks and 10 l fluidized bed bioreactor with stalk juice of Yuantian No. 1 sweet sorghum cultivar when immobilized yeast was applied. The experimental results in the shaking flasks showed that the order of influence on improving ethanol yield was (NH₄)₂SO₄>MgSO₄>K₂HPO₄, and the optimum inorganic salts supplement dose was determined as follows: K₂HPO₄ 0%, (NH₄)₂SO₄ 0.2%, MgSO₄ 0.05%. When the optimum inorganic salts supplement dose was used in fermentation in 10 l fluidized bed reactor, the fermentation time and ethanol content were 5 h and 6.2% (v/v), respectively, and ethanol yield was 91.61%, which was increased by 9.73% than blank. In addition, the results showed that the fermentation time was about 6–8 times shorter in fluidized bed bioreactor with immobilized yeast than that of conventional fermentation technology. As a result, it can be concluded that the determined optimum inorganic salts supplement dose could be used as a guide for commercial ethanol production. The fluidized bed bioreactor with immobilized yeast technology has a great potential for ethanol fermentation of stalk juice of sweet sorghum.     

Pathways to low-cost clean hydrogen production with gas switching reforming

Gas switching reforming (GSR) is a promising technology for natural gas reforming with inherent CO₂ capture. Like conventional steam methane reforming (SMR), GSR can be integrated with water-gas shift and pressure swing adsorption units for pure hydrogen production. The resulting GSR-H2 process concept was techno-economically assessed in this study. Results showed that GSR-H2 can achieve 96% CO₂ capture at a CO₂ avoidance cost of 15 $/ton (including CO₂ transport and storage). Most components of the GSR-H2 process are proven technologies, but long-term oxygen carrier stability presents an important technical uncertainty that can adversely affect competitiveness when the material lifetime drops below one year. Relative to the SMR benchmark, GSR-H2 replaces some fuel consumption with electricity consumption, making it more suitable to regions with higher natural gas prices and lower electricity prices. Some minor alterations to the process configuration can adjust the balance between fuel and electricity consumption to match local market conditions. The most attractive commercialization pathway for the GSR-H2 technology is initial construction without CO₂ capture, followed by simple retrofitting for CO₂ capture when CO₂ taxes rise, and CO₂ transport and storage infrastructure becomes available. These features make the GSR-H2 technology robust to almost any future energy market scenario.     

Emission analysis of Botryococcus braunii algal biofuel using Ni-Doped ZnO nano additives for IC engines

Microalgae biodiesel has been considered ?as a clean renewable fuel for diesel marine engines. This is due to its optimistic characterizations such as ?rapid growth rate, high productivity, and its ability to convert CO₂ into fuel. In this study, the use of microalgae biodiesel, obtained from Botryococcus braunii, as an alternative fuel for diesel marine engines has been investigated. The diesel engine is verified experimentally using Ni-Doped ZnO nano additive blends with algae biodiesel and neat diesel fuel. The results showed that doped nano additive blends? produce less emission compared to B20.     

Predominance of Bacilli and Clostridia in microbial community of biohydrogen producing biofilm sustained under diverse acidogenic operating conditions

Microbial community structure of acidogenic biofilm from long-term operated sequencing batch bioreactor producing biohydrogen was analyzed through culture independent technique. Bioreactor was operated under variable operation and substrate conditions for a period of 1435 days. Phylogenetic distribution showed a significant diversity and illustrated the presence of four dominant operational taxonomic units (OTUs) viz., Bacteroidia, Bacilli, Clostridia, Flavobacteria and Aquificae. Dominance of Clostridia and Bacilli classes were observed each with four OTUs. Majority of OTUs were found to produce fermentative H₂. Even at higher load and under diverse operating conditions bioreactor functioned without any process inhibition which indicates the robustness of sustained microbial community. Community structure of bioreactor was comparatively evaluated with other bioreactor producing H₂, operated with same parent culture and conditions but with different substrates, established the dynamics and shift of microbial diversity which corresponded to diverse substrates used for the bioreactor operation.     

Evaluation of power generation schemes based on hydrogen-fuelled combined cycle with carbon capture and storage (CCS)

IGCC is a power generation technology in which the solid feedstock is partially oxidized to produce syngas. In a modified IGCC design for carbon capture, there are several technological options which are evaluated in this paper. The first two options involve pre-combustion arrangements in which syngas is processed, either by shift conversion or chemical looping, to maximise the hydrogen level and to concentrate the carbon species as CO₂. After CO₂ capture by gas-liquid absorption or chemical looping, the hydrogen-rich gas is used for power generation. The third capture option is based on post-combustion arrangement using chemical absorption.Investigated coal-based IGCC case studies produce 400-500 MW net power with more than 90% carbon capture rate. Principal focus of the paper is concentrated on evaluation of key performance indicators for investigated carbon capture options, the influence of various gasifiers on carbon capture process, optimisation of energy efficiency by heat and power integration, quality specification of captured CO₂. The capture option with minimal energy penalty is based on chemical looping, followed by pre-combustion and post-combustion.     

The solar Cristina process for hydrogen production

Hydrogen production using the Cristina process coupled to a dedicated central receiver solar system has been studied. The Cristina process was originally conceived and developed at the Joint Research Center of the European Communities in Ispra to decompose the sulfuric acid and produce the sulfur dioxide necessary for hydrogen production. In the present study, the process has been adopted to an intermittently operating solar heat source to produce the sulfur dioxide during sunshine hours and operate in reverse as a sulfuric acid synthesis process at a required rate to produce high temperature heat during night operation by using a small part of the stored sulfur dioxide. In this manner, the chemical process is operated continuously, hence, thermal inertia and start-up problems have been eliminated.A system has been conceived to produce 10⁶ mole SO₂ per hour, which is coupled to a central receiver solar system producing 10⁶ GJ per year heat operating 2333 hrs per year. The system produces 0.62 × 10⁶ GJ hydrogen per year when coupled to a hydrogen producing step such as Mark 11 or 13 operating 7000 hrs per year and using electric energy supplied from outside.It has been found that the cost of sulfuric acid decomposition by the solar Cristina process is approximately 31 $ per GJ hydrogen. Including the cost of solar heat (approximately 32 $ per GJ hydrogen) and that of hydrogen producing step (approximately 5 $ per GJ hydrogen), the total cost has been estimated to be 68 $ per GJ hydrogen.