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Research advances in preparation of biological diesel oil by microalgae method
Abstract
The research advances in preparation of biological diesel oil by microalgae method at home and abroad are summarized in this paper. The general situation of the microalgae is analyzed at first. Microalgae as raw material to produce biological diesel oil, is an environmentally protective and renewable energy. Then, the current situation of biodiesel production by microalgae is further elaborated. The production technology of biodiesel is still not mature, and is needed to be further study. Finally, the problems of algae biodiesel are summed up and the future research priorities and future prospects are proposed.
This study focuses on the ultrasonic pretreatment of bio-oil produced from the hydrothermal liquefaction (HTL) of microalgae. Ultrasonic pretreatment would increase the viscosity of the bio-oil. Time was the most influential factor affecting the viscosity of the bio-oil and increasing the time significantly increased the viscosity of the bio-oil. Ultrasonic pretreatment could increase the H/C molar ratio of the bio-oil. Ultrasonic frequency has little effect on the properties of the bio-oil. Basically, the pretreated bio-oil produced at a higher temperature, longer reaction time, higher frequency, and higher power contained lower N. However, contrary results were observed for the O content.
The study of top-coal drawing characteristics is key to improving the top-coal recovery ratio and to reducing the waste of coal resources in longwall top-coal caving (LTCC) mining. This paper introduces a Boundary-Body-Ratio (BBR) based research system established by the authors following a long-term detailed study of the top-coal drawing mechanism. For BBR system, a simplified parabolic model is proposed to describe the development process of the boundary of top-coal in normal top-coal drawing cycles, where the drawing body is a cut variant ellipsoid (CVE) that can be described by the modified Bergmark-Ross model. A method for improving the recovery ratio and reducing the rock mixed ratio of top-coal by selecting reasonable drawing technique parameters and controlling the shape of the boundary of the top-coal is given for the BBR system. Based on the BBR research system, the drawing mechanism for loose top-coal in sublevel top-coal caving (SLTCC) in steeply inclined coal seams is studied for different sublevel heights and drawing direction conditions using distinct element numerical calculations and loose top-coal drawing experiments. The results show that the boundary curve of top-coal can be fitted by a parabola and that the drawing body remains as a cut variation ellipsoid in SLTCC, as is the case for a flat coal seam. The convex point of the boundary of the top-coal moves toward the goaf with increasing sublevel height, which would result in the incomplete development of the top-coal drawing body. The drawing extent index decreases linearly with increasing sublevel height, and the top-coal recovery ratio decreases after an initial increasing phase. Approximately 90% of the residual coal in the current sublevel will not be extracted during the drawing process of the next sublevel. Drawing from the floor to the roof expands the overlap area between the drawing body and the boundary of the top-coal, improving the recovery of the top part of the top-coal. Such a drawing sequence decreases the coal loss near the floor by 38.2% compared to drawing from the roof to the floor and greatly increases the resource recovery ratio.
In order to recover the intracellular blue-green pigment, marennine, synthesized by the microalga Haslea ostrearia, a continuous flow high-pressure disrupter was evaluated in the range: 30–270MPa. Cells were partly broken from 30MPa, but a pressure of 100MPa (1 cycle) was required to obtain optimal pigment release. The latter was directly linked to the physical cell breakage dependent upon the applied pressure and the number of disintegration cycles. Granulometric analysis by laser diffraction technology (0.04–2000m) revealed a size reduction of cell fragments when increasing these two operating parameters.
Biodiesel production based on microalgae as feedstock is associated with a high demand of nutrients, respectively nitrogen and phosphorus. The production of 1l biodiesel requires between 0.23 and 1.55 kg nitrogen and 29-145 g of phosphorus depending of the cultivation conditions for microalgae. The supply of nutrients can be expected to severely limit the extent to which the production of biofuels from microalgae can be sustainably expanded. The nutrient demand can be reduced if the nutrients in the residual algae biomass after oil extraction are reused for algae cultivation. This modeling work illustrates that for the investigated process chains and scenarios the nutrient recycling rates are in the range from 30% to 90% for nitrogen and from 48% to 93% for phosphorus. The highest recycling values can be achieved by hydrothermal gasification of the oil-free residues.
The generally held view that fat accumulation is characteristic of certain classes of algae is based on inadequate evidence.
In this investigation the fatty acid, unsaponifiable lipoid, total cell nitrogen, and hydrolysable polysaccharide contents of the following algae have been determined at various stages during growth in pure culture: Chlorella vulgaris, Scotiella sp., Euglena gracilis var. bacillaris, Tribonema aequale, Monodus subterraneus, Navicula pelliculosa, Porphyridium cruentum, Anabaena cylindrica, and Oscillatoria sp.
Using the results obtained, the capacities of the algae to accumulate fat have been compared in the following ways: (I) on a percentage dry-weight basis; (2) by means of the differential growth factor relating the fat content of cultures to dry weight of algal material; (3) by consideration of the proportion which fats form of the total reserve material (taken as fat plus hydrolysable polysaccharide) as a function of total cell nitrogen content.
Species belonging to the same class have been found to resemble each other in the relative amounts of crude protein, fats, and hydrolysable polysaccharide which they contain. Marked differences in composition between species belonging to different classes have been found, but it is concluded that there are no funda mental differences in the physiological relations of fat accumulation in the algae belonging to the Chlorophycese, Euglenineae, Xanthophyceae, and Bacillario phyceae. Representatives of the Rhodophyceae and Myxophyceae, although they may under certain circumstances have moderately high fat contents, appear to differ in that in them fat accumulation is not associated with low cell nitrogen contents as it is in the other algae examined.
No well-defined correlations between the unsaponifiable lipoid and either the fatty acid or total cell nitrogen contents of the algae have been found. Euglena gracilis var. bacillaris has been found to have a rather higher content of unsaponifiable lipoid than the other algae.
An attempt to scale-up a biodiesel production plant from a heterotrophic culture of microalgae, together with an evaluation of feasibility is performed in this article. The plant employs the microalgae Chlorella protothecoides to obtain biomass. The subsequent oil extraction is done with supercritical carbon dioxide. Based on previous studies, it was possible to determine the mass and energy balances and to design the equipment of the main process. A non-conservative study reveals the no feasibility of the production plant unless the residues are sold (two different and real prices were chosen), providing with an investment recovery in both cases. On the other hand, the estimations from a conservative study, showed the non-viability of the process even if the residues are sold. This is mainly due to the big quantity of bioreactors required by the plant and the oil extraction yield. An alternative process with a complete extraction was also applied, which became viable in a conservative study only if the residues are sold at the highest price. All these results highlight the future potential of a plant with these characteristics in the current energetic context.
Microalgal biomass as feedstock for biofuel production is rapidly gaining appreciation in response to the increasing petroleum prices and the upsurge in global warming concerns. However, the process of creating concentrated biomass from microalgal cultures is limited by ineffective dewatering procedures. The economics of existing culture clarification unit operations make the process of creating biomass from microalgae unattractive for biofuel development. This work involves the comparison of the removal efficiency (recovery) of two marine microalgae species Chlorococcum sp. and Tetraselmis sp. by electrocoagulation (EC), a technique that has not been thoroughly explored in marine microalgae dewatering. High recovery efficiencies were obtained of up to 99 and 98% for Tetraselmis sp. and Chlorococcum sp., respectively. The effect of culture temperature and salinity on removal efficiency was also observed. A starting temperature of 60°C resulted in optimal recovery values of 96 and 94% for Chlorococcum sp. and Tetraselmis sp., respectively. Whereas a starting temperature of 5°C achieved optimal recovery of only 5 and 68% for Chlorococcum sp. and Tetraselmis sp., respectively. Increased salinity of microalgae culture showed increased microalgae recovery. Salinity of 20% gave optimal microalgae recovery values of only 6 and 9% for Chlorococcum sp. and Tetraselmis sp., respectively. Zeta potential (ZP) analysis was carried out to verify and further understand the charge neutralization mechanism due to Fe2+ cations.
Co-location of algae production facilities with cane sugar mills can be a technically advantageous path towards production of biodiesel. Algal biodiesel production was integrated with cane sugar production in the material and energy balance simulation program Sugars™. A model was developed that allowed comparison of production scenarios involving dewatering the algae to 20% ds (dry solids) or 30% ds prior to thermal drying. The net energy ratio, E(R) (energy produced/energy consumed) of the proposed process was found to be 1.5. A sensitivity analysis showed that this number ranged from 0.9 to 1.7 when the range of values for oil content, CO(2) utilization, oil conversion, and harvest density reported in the literature were evaluated. By utilizing available waste-resources from a 10,000 ton/d cane sugar mill, a 530 ha algae farm can produce 5.8 million L of biodiesel/yr and reduce CO(2) emissions of the mill by 15% without the need for fossil fuels.
A cell disrupter has been developed which can measure the forces required to disrupt both eukaryotic and prokaryotic cells. It operates a continuous process and will disrupt both large and small volumes. Shear forces are set up when a suspension under laminar flow conditions is released under high pressure through a short orifice. If the applied pressure is altered, the shear forces are simultaneously changed so that the amount of cell disruption can be compared under different known and repeatable conditions. The disrupter is now manufactured and supplied by Stansted Fluid Power Limited, Stansted, England.
Phase-contrast microscopy has shown that the disrupter will break a variety of organisms including Chlorella, Aspergillus fumigatis, Fusarium sp., Saccharomyces cerevisiae, Escherichia coli, Lactobacillus casei, Bacillus subtilis, Clostridium perfringens, Streptococcus faecalis, Streptococcus zooepidermicus and Staphylococcus aureus. The cells are not all broken at one pressure but a certain pressure must be applied before disruption starts which will then increase rapidly as the applied pressure is increased. The applied pressure required to disrupt half the population in a culture is different from one species to another, rods being disrupted more easily than spheres.
The ease of disruption seems to be related to the shape and chemical composition of the cell wall. Furthermore, the disrupting process, in an unsynchronized culture is not random and may be related to the statistical size distribution of the cells.