Net energy analysis of the production of biodiesel and biogas from the microalgae: Haematococcus pluvialis and Nannochloropsis

Microalgae have been proposed as possible alternative feedstocks for the production of biodiesel because of their high photosynthetic efficiency. The high energy input required for microalgal culture and oil extraction may negate this advantage, however. There is a need to determine whether microalgal biodiesel can deliver more energy than is required to produce it. In this work, net energy analysis was done on systems to produce biodiesel and biogas from two microalgae: Haematococcus pluvialis and Nannochloropsis. Even with very optimistic assumptions regarding the performance of processing units, the results show a large energy deficit for both systems, due mainly to the energy required to culture and dry the microalgae or to disrupt the cell. Some energy savings may be realized from eliminating the fertilizer by the use of wastewater or, in the case of H. pluvialis, recycling some of the algal biomass to eliminate the need for a photobioreactor, but these are insufficient to completely eliminate the deficit. Recommendations are made to develop wet extraction and transesterification technology to make microalgal biodiesel systems viable from an energy standpoint.     

Development of suitable photobioreactor for algae production - A review

Microalgal species are recently in the spotlight for biofuels production like biodiesel, bioethanol and biohydrogen. Algae are also used as a biofertiliser, source of nutrient and for controlling pollution. Algae being a photosynthetic organism are produced in the photo bioreactors. Hence the design and development of photobioreactors for maximum production of algae is very important. Apart from maximum production, other factors such as design, cost effectiveness of the bioreactor, purity of the algae produced, user friendly, low maintenance and space convenience need to be optimized. The bioreactors which are used for the purpose of growing algae are bubble column photobioreactor, airlift photo bioreactor, flat panel bioreactor, horizontal tubular photobioreactor, stirred tank photobioreactor etc. These bioreactors have their own advantages and disadvantages. Work is on for developing hybrid type of bioreactors which may overcome the limitations of the developed photobioreactors. This paper covers the salient features, limitations of developed photobioreactors and recent developments in the field of photobioreactors.     

Flue gas waste heat affects algal liquid temperature for microalgal production in column photobioreactors

To explore the effects of waste heat (50–170°C) from steel plant flue gas on the column photobioreactor algal liquid temperature for microalgal production, a flue gas-microalgal liquid heat transfer model was developed that simulated the microalgal growth environment for flue-gas carbon dioxide (CO₂) fixation. The simulation results showed that the influence of high-temperature flue gas weakened with the increasing microalgal liquid temperature due to enhanced evaporation and heat dissipation. Increasing the flue gas temperature and aeration rate resulted in a higher microalgal liquid temperature up to a maximum increase of 4.16°C at an ambient temperature of 25°C, an aeration rate of 2 L/min, and a flue gas temperature of 170°C. In an experiment on the effect of incubation temperature on the growth rate of microalgae, at an optimal temperature of 35°C, the Chlorella sp. PY-ZU1 growth rate exhibited a remarkable increase of 104.7% compared to that at 42.5°C. Therefore, modulating the flue gas conditions can significantly increase the microalgal growth rate for CO₂ fixation, making it a promising approach to increase biomass production for efficient carbon utilization.     

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.     

Study on the destabilization mixing in the flat plate photobioreactor by means of CFD

One destabilizing structure applied in the flat plate photobioreactor was designed, and the flow field and particle trajectory in it were simulated by means of CFD (computational fluid dynamics) to optimize structural parameters of photobioreactor. The results showed that the vertical velocity along the light path was produced in the condition of destabilization, which helps to achieve homogenous mixing of medium needed for microalgae growth. The fluid micelle in the flow field waved regularly, which made the algae cell transmitted between light and dark area and enhance the efficiency of photosynthesis. This destabilizing structure had a great potential to be used in the photobioreactor design.     

Hydrogen production with the microalga Chlamydomonas reinhardtii grown in a compact tubular photobioreactor immersed in a scattering light nanoparticle suspension

A new photobioreactor design (110 l) for the biological production of hydrogen with the microalga Chlamydomonas reinhardtii is presented. The photobioreactor (PBR) was made up of 64 tubes (i.d., 27.5 mm, length, 2 m) arranged on an 8 × 8 square pitch cell connected by 64 U-bends for a total length of 133 m. The PBR was contained in a rectangular parallelepiped tank (2.5 × 2 × 2 m) made with isotactic polypropylene, except for the opposite square faces which were made of transparent Plexiglas. The tubes were immersed in a thermostatic water bath and continuously illuminated with artificial light. The culture was circulated with a peristaltic pump. To attain a uniform distribution of light to the cells, we used a suspension of silica nanoparticles that scattered the light supplied by the light bulbs (2 × 2000 W) from the opposite square sides of the photobioreactor. Growth experiments carried out with C. reinhardtii CC124 strain, showed a 23% net increase in the final chlorophyll concentration when the nanoparticle suspension was used. Hydrogen production with the C. reinhardtii strain CC124 was investigated with the new photobioreactor design and carried out using a direct inoculum of sulfur-limited cultures having a residual sulfate concentration below 1 mg l⁻¹. The mean hydrogen output was 3121.5 ± 178.9 ml. The reactor fluid dynamic was investigated, and a tri-dimensional light profile inside the PBR is reported.     

Parameters influencing the design of photobioreactor for the growth of microalgae

Carbon dioxide sequestration using microalgae is the most promising method for combating global warming. Growth of microalgae is influenced by the availability of carbon dioxide, number of photons, initial concentration of microalgae and nutrients. The transfer of carbon dioxide from flue gas and absorption of photons from sunlight are influenced by the surface area/volume ratio of photobioreactor. The growth rate of microalgae follows lag, log, deceleration and stationary phases. The rate of growth increases with concentration of microalgae till an optimum concentration of algae is reached and then decreases for any fixed operating conditions and selected microalgae. At an optimum concentration the rate is the highest always. Operating a photobioreactor at this optimum concentration with highest surface area to volume ratio would require the smallest size of photobioreactor for a given production rate. Based on the review on the performance of various existing photobioreactors and the growth mechanism of microalgae it is observed that the design and operation of an efficient photo bioreactor system should consider (1) providing highest spread area to volume ratio (2) maintaining optimum concentration matching the highest rate (3) harvesting the excess microalgae formed over the optimum concentration to maintain the optimum concentration and (4) adding nutrients to the growth medium to maintain nutrient concentration at a constant level.     

Biodiesel production by microalgal biotechnology

Biodiesel has received much attention in recent years. Although numerous reports are available on the production of biodiesel from vegetable oils of terraneous oil-plants, such as soybean, sunflower and palm oils, the production of biodiesel from microalgae is a newly emerging field. Microalgal biotechnology appears to possess high potential for biodiesel production because a significant increase in lipid content of microalgae is now possible through heterotrophic cultivation and genetic engineering approaches. This paper provides an overview of the technologies in the production of biodiesel from microalgae, including the various modes of cultivation for the production of oil-rich microalgal biomass, as well as the subsequent downstream processing for biodiesel production. The advances and prospects of using microalgal biotechnology for biodiesel production are discussed.     

The microalgae derived hydrogen process in compact photobioreactors

This paper investigates the feasibility of a microalgae derived hydrogen process at a pilot scale. For that, a general transient mathematical model for managing microalgae derived hydrogen production, with temperature dependence of the cultivation medium is developed. The tool allows for the determination of the resulting whole system temperature, and mass fractions distribution. The simplified physical model combines principles of classical thermodynamics, mass, species and heat transfer, resulting in a system of differential equations which are discretized in space using a three-dimensional cell-centered finite volume scheme, namely a volume element model (VEM). A Michaelis–Menten type expression is proposed for modeling the rate of H₂ production with dependence on O₂ inhibition. Tridimensional simulations are performed in order to determine the mass fractions distributions inside a compact photobioreactor (PBR), under different operating conditions. A relatively coarse mesh was used (6048 volume elements) to obtain converged results for a large compact PBR computational domain (2 m × 5 m × 8 m). The largest computational time required for obtaining results was 560 s, i.e., less than 10 min. The numerical results for microalgal growth are validated by direct comparison to experimental measurements. Hydrogen production simulations are conducted to demonstrate PBR intermittent operation (aerobic and anaerobic stages) feasibility and adequate species evolution trends in an indirect biophotolysis approach. Therefore, after experimental validation for a particular H₂ production system, it is reasonable to state that the model could be used as an efficient tool for PBR systems thermal design, control and optimization for maximum H₂ production.     

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.     

Chlorella sp.: A new strain with highly saturated fatty acids for biodiesel production in bubble-column photobioreactor

The biodiesel production from a naturally isolated strain of Chlorella in 2 L bubble-column photobioreactor was studied. The microalgal strain was isolated from the rice paddy-field soil samples during a screening program. After 17 days, at the end of exponential phase of growth, the total content of the lipids was extracted. The extracted fatty acids were first esterified and then identified using GC/MS analysis. Several types of fatty acid methyl esters (FAMEs) were identified in the isolated microalga and the presence of saturated fatty acids in Chlorella sp. MCCS 040 was approved. The composition of fatty acids in the studied species of microalga was mainly palmitic acid methyl ester, myristic acid methyl ester, stearic acid methyl ester and undecanoic acid methyl ester. This strain because of its highly saturated fatty acids content can be an ideal candidate for biodiesel production.     

Transient analysis and performance studies of two tubular photobioreactors for outdoor culture of spirulina

A mathematical model to make a transient thermal analysis and to estimate the incident solar energy for two designs of tubular photobioreactor installed outdoors is presented here. In the first photobioreactor design the tubes were arranged in one plane, whereas in the second the tubes were arranged in two planes. The model was validated by comparing the experimental data and predicted values of culture temperature. Both the input solar energy and culture temperature in a tubular photobioreactor may be predicted with a reasonable degree of accuracy by employing the model. The performance of the two photobioreactors for mass culture of Spirulina was also studied in relation to their design and culture temperature. The average biomass yield obtained in one-plane and two-plane photobioreactors were (dry weight) 23.7 g m^?2 day^?1 and 27.8 g m^?2 day^?1 respectively. Such biomass yields corresponded to a volumetric productivity of (dry weight) 0.466 g litre^?1 day^?1 in the one-plane reactor and 1.5 g litre^?1 day^?1 in the two-plane reactor. We further observed that biomass yield could be increased by about 21% when the culture temperature was maintained at the optimal value of 35°C compared to another culture in which temperature changed according to the ambient temperature from 20 to 39°C during the day.     

Hydrogen production by immobilized Rhodopseudomonas palustris in packed or fluidized bed photobioreactor systems

The production of biohydrogen via photofermentation has been shown to have a low environmental impact and can often be integrated into wastewater treatment systems. However, currently, photofermentation has low production rates in comparison to industrial hydrogen production processes, and therefore requires improvement. One route for enhancing hydrogen productivity is the development of improved photobioreactor (PBR) systems. The aim of this study was to compare the hydrogen productivity of Rhodopseudomonas palustris under planktonic, and immobilized cell conditions, with the reactor operating either as a packed bed or a fluidized bed. The fluidized bed PBR achieved a maximum specific hydrogen production rate and substrate conversion efficiency of 15.74 ± 2.2 mL/g/h and 43% respectively, outperforming the conventional planktonic culture and the packed bed PBR. This work demonstrates a significant improvement in productivity over planktonic photofermentation, as well as demonstrating the use of immobilized cells under reactor conditions not usually associated with photosynthetic systems.     

Dynamics modeling of hydrogen production by sulfur-deprived Chlamydomonas reinhardtii culture in tubular photobioreactor

A heterogeneous hydrogen production system induced by light attenuation across the culture in a photobioreactor and the boundary conditions is studied by solving the advective-diffusive reaction equation (ADRE) used to describe the system. A uniform light intensity is prescribed on the cylindrical surface of the tubular bioreactor and attenuated by Chlamydomonas reinhardtii culture toward the center. The rate constants and the kinetics orders of the S-system based kinetics equations were determined by correlating with the available experimentally measured data. The photobioreactor was operated for 200 h and the dynamics behavior of O₂ evolution and H₂ production were analyzed. The effects of different initial chlorophyll concentrations and quantities of sulfur re-added to the sulfur deprived culture on H₂ production were studied. The results demonstrate that H₂ production decreases with the light attenuation along radial direction. The overall H₂ production increases with the initial cell concentration and the amount of re-added sulfur, respectively, within the simulated range. The modeled results indicate that optimal combination of the culture parameters under the given light intensity and the mixing condition may exist for high H₂ production.     

Experimental research on thermal characteristics of a hybrid thermocline heat storage system

Considering the high-temperature thermal utilization of solar energy as the research background in this paper and focussing on the heat storage process, a kind of hybrid thermocline heat storage method in multi-scale structure and relevant experimental systems are designed by using the mixed molten nitrate salt as the heat storage medium and two representative porous materials, i.e. zirconium ball and silicon carbide (SiC) foam, as the heat storage fillers. The fluid flow and heat storage performance of molten salt in multi-scale structure are experimentally investigated. The results show that the theoretical heat storage efficiencies amongst the three experimental heat storage manners are less than 80% because of the existence of thermocline layers. Comparing to the single-phase molten salt heat storage, the two hybrid thermocline heat storage manners with porous fillers lead to a certain decrease in the effective heat storage capacity. The presence of porous fillers can also help to maintain the molten salt fluid as ideal gravity flow or piston flow and partially replace expensive molten salt. Therefore, it requires a combination of heat storage capacity and economical consideration for optimization design when similar spherical particles or foam ceramics are employed as the porous fillers.     

Effects of culture parameters, light, and mixing on H2 production by sulfur-deprived Chlamydomonas reinhardtii in flat plate reactor

Based on advective-diffusive reaction equation for inhomogeneous biochemical system and an empirical equation for light attenuation coefficient, the interplay among culture parameters, light intensity and illumination condition, and mechanical mixing condition during O₂ evolution and H₂ production in a flat plate photobioreactor with sulfur-deprived Chlamydomonas reinhardtii culture is modeled in this work. Four initial chlorophyll concentrations, two light attenuation levels, and two illumination conditions were modeled to study their effects on the dynamics of O₂ evolution and H₂ production. The results indicate that two side illumination is the best design for light penetration into a flat plate reactor. While for single side illumination condition, an optimal combination of the initial cell concentration, light intensity, and reactor width may have to be considered for high H₂ production.     

Bioflocculation of photo-fermentative bacteria induced by calcium ion for enhancing hydrogen production

The applications of photo-fermentative bacteria (PFB) for continuous hydrogen production are generally subjected to a serious biomass washout from photobioreactor, resulting from poor flocculation of PFB. In this study, through reducing the absolute ζ-potentials of PFB, Ca²⁺ greatly decreased total interaction energy barrier of PFB based on DLVO theory, thus promoted the bioflocculation of Rhodopseudomonas faecalis RLD-53. Average floc size of PFB increased with the Ca²⁺ concentration, and reached maximum 30.07 μm at 4 mmol/l. Consequently, biomass retention capacity of photobioreactor significantly enhanced after 30 min settling with half working volume discharge. In the continuous photo-fermentative sequencing batch reactor, compared with the free cell culture, bioflocculation reached a higher steady-state hydrogen production rate of 879 ml H₂/l/d and hydrogen yield of 2.64 mol H₂/mol acetate, respectively. Therefore, bioflocculation promoted by calcium ion was an effective strategy for retaining PFB in photobioreactor to produce hydrogen continuously.     

Performance of a groove-type photobioreactor for hydrogen production by immobilized photosynthetic bacteria

The biofilm technique has been proved to be an effective cell immobilization method for wastewater biodegradation but it has had restricted use in the field of photobiological H₂ production. In the present study, a groove-type photobioreactor was developed and it was shown that a groove structure with large specific surface area was beneficial to cell immobilization and biofilm formation of the photosynthetic bacteria on photobioreactor surface as well as light penetration. A series of experiments was carried out on continuous hydrogen production in the groove-type photobioreactor illuminated by monochromatic LED lights and the performance was investigated. The effects of light wavelength, light intensity, inlet glucose concentration, flow rate and initial substrate pH were studied and the results were compared with those obtained in a flat panel photobioreactor. The experimental results show that the optimum operational conditions for hydrogen production in the groove-type photobioreactor were: inlet glucose concentration 10 g/L, flow rate 60 mL/h, light intensity 6.75 W/m², light wavelength 590 nm and initial substrate pH 7.0. The maximum hydrogen production rate, H₂ yield and light conversion efficiency in the groove-type photobioreactor were 3.816 mmol/m²/h, 0.75 molH₂/molglucose and 3.8%, respectively, which were about 75% higher than those in the flat panel photobioreactor.     

Bioprocess engineering of microalgae to produce a variety of consumer products

Microalgae biotechnology has recently emerged into the lime light owing to numerous consumer products that can be harnessed from microalgae. Product portfolio stretches from straightforward biomass production for food and animal feed to valuable products extracted from microalgal biomass, including triglycerides which can be converted into biodiesel. For most of these applications, the production process is moderately economically viable and the market is developing. Considering the enormous biodiversity of microalgae and recent developments in genetic and metabolic engineering, this group of organisms represents one of the most promising sources for new products and applications. With the development of detailed culture and screening techniques, microalgal biotechnology can meet the high demands of food, energy and pharmaceutical industries. This review article discusses the technology and production platforms for development and creation of different valuable consumer products from microalgal biomass.     

A comprehensive review on pretreatment of microalgae for biogas production

Use of microalgal biomass for renewable energy production has gained considerable attention in the world due to increasing global energy demand and negative environmental impacts of nonrenewable fossil fuels. Anaerobic digestion is one of the renewable technologies that microalgal biomass is converted into biogas by anaerobic archea. One of the main drawbacks of using microalgal biomass for biogas production is that certain types of microalgae has rigid cell wall characteristics, which limits accessibility of anaerobic archea to microalgal intracellular organic matter during hydrolysis phase. This limitation lowers efficiency of biogas production from microalgal biomass. However, introducing pretreatment methods prior to anaerobic digestion provides disruption of rigid microalgal cell wall and improve biogas yields from microalgal biomass. The objective of this paper was to review current knowledge related to pretreatment methods applied prior to anaerobic digestion of microalgal biomass. Efficiency and applicability of pretreatment methods mainly depend on type of microalgae, cell wall characteristics, and cost and energy requirements during pretreatment process. In this review, various type of pretreatment methods applied to microalgal biomass was discussed in detail with background knowledge and literature studies in their potential on maximization of biogas yields and their cost effectiveness, which is important for large‐scale applications. In the view of current knowledge, it was concluded that each pretreatment method has a relative contribution to improvement in biogas production depending on the type of microalgae. However, energy and cost requirements are the main limitations for pretreatment. So, further studies should focus on reduction of cost and energy demand by introducing combined methods, novel chemicals, and on‐site or immobilized enzymes in pretreatment to increase feasibility of pretreatment prior to anaerobic digestion in industrial scale.