The cultivation and harvesting of micro–algal biomass from the Hartbeespoort Dam for the production of biodiesel
Brink, Jacobus Petrus
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Renewable energy sources such as biomass are becoming more and more important as alternative to fossil fuels. One of the most exciting new sources of biomass is microalgae. The Hartbeespoort Dam, located 37 km west of South Africa’s capital Pretoria, has one of the dense populations of microalgae in the world, and is one of the largest reservoirs of micro-algal biomass in South Africa. The dam has great potential for micro-algal biomass production and beneficiation due to its high nutrient loading, stable climatic conditions, size and close proximity to major urban and industrial centres. There are five major steps in the production of biodiesel from micro-algal biomass-derived oil: the first two steps involve the cultivation and harvesting of micro-algal biomass; which is followed by the extraction of oils from the micro-algal biomass; then the conversion of these oils via the chemical reaction transesterification into biodiesel; and the last step is the separation and purification of the produced biodiesel. The first two steps are the most inefficient and costly steps in the whole biomass-to-liquids (BTL) value chain. Cultivation costs may contribute between 20–40% of the total cost of micro-algal BTL production (Comprehensive Oilgae Report, 2010), while harvesting costs may contribute between 20–30% of the total cost of BTL production (Verma et al., 2010). Any process that could optimize these two steps would bring a biomass-to-liquids process closer to successful commercialization. The aim of this work was to study the cultivation and harvesting of micro-algal biomass from the Hartbeespoort Dam for the production of biodiesel. In order to do this a literature study was done and screening experiments were performed to determine the technical and economical feasibility of cultivation and harvesting methods in the context of a new integrated biomass-to-liquids biodiesel process, whose feasibility was also studied. The literature study revealed that the cyanobacterium Microcystis aeruginosa is the dominant micro-organism species in the Hartbeespoort Dam. The study also revealed factors that promote the growth of this species for possible incorporation into existing and new cultivation methods. These factors include stable climatic conditions, with high water temperatures around 25oC for optimal Microcystis growth; high nutrient loadings, with high phosphorus (e.g. PO43-) and nitrogen concentrations (e.g. NO3-); stagnant hydrodynamic conditions, with low wind velocities and enclosed bays, which promote the proliferation of Microcystis populations; and substrates like sediment, rocks and debris which provide safe protective environments for Microcystis inoculums. The seven screening studies consisted of three cultivation experiments, three harvesting experiments and one experiment to determine the combustion properties of micro-algal biomass. The three cultivation experiments were conducted in three consecutively scaled-up laboratory systems, which consisted of one, five and 135-litre bioreactors. The highest productivity achieved was over a period of six weeks in the 5-litre Erlenmeyer bioreactors with 0.0862 g/L/d at an average bioreactor day-time temperature of 26.0oC and an aeration rate of 1.5 L/min. The three cultivation experiments revealed that closed-cultivation systems would not be feasible as the highest biomass concentrations achieved under laboratory conditions were too low. Open-cultivation systems are only feasible if the infrastructure already exists, like in the case of the Hartbeespoort Dam. It is recommended that designers of new micro-algal BTL biodiesel processes first try to capitalize on existing cultivation infrastructure, like dams, by connecting their processes to them. This will reduce the capital and operating costs of a BTL process significantly. Three harvesting experiments studied the technical feasibility and determined design parameters for three promising, unconventional harvesting methods. The first experiment studied the separation of Hartbeespoort Dam micro-algal biomass from its aqueous phase, due to its natural buoyancy. Results obtained suggest that an optimum residence time of 3.5 hours in separation vessels would be sufficient to concentrate micro-algal biomass from 1.5 to 3% TSS. The second experiment studied the aerial harvesting yield of drying micro-algal biomass (3% TSS) on a patch of building sand in the sun for 24 hours. An average aerial harvesting yield of 157.6 g/m2/d of dry weight micro-algal biomass from the Hartbeespoort Dam was achieved. The third experiment studied the gravity settling harvesting yield of cultivated Hartbeespoort Dam-sourced microalgae as it settles to the bottom of the bioreactor after air agitation is suspended. Over 90% of the micro-algal biomass settled to the bottom quarter of the bioreactor after one day. Cultivated micro-algal biomass sourced from the Hartbeespoort Dam, can easily be harvested by allowing it to settle with gravity when aeration is stopped. Results showed that gravity settling equipment, with residence times of 24 hours, should be sufficient to accumulate over 90% of cultivated micro-algal biomass in the bottom quarter of a separation vessel. Using this method for primary separation could reduce the total cost of harvesting equipment dramatically, with minimal energy input. All three harvesting methods, which utilize the natural buoyancy of Hartbeespoort Dam microalgae, gravity settling, and a combination of sand filtration and solar drying, to concentrate, dewater and dry the micro-algal biomass, were found to be feasible and were incorporated into new integrated BTL biodiesel process. The harvesting processes were incorporated and designed to deliver the most micro-algal biomass feedstock, with the least amount of equipment and energy use. All the available renewable power sources from the Hartbeespoort Dam system, which included wind, hydro, solar and biomass power, were utilized and optimized to deliver minimum power loss, and increase power output. Wind power is utilized indirectly, as prevailing south-easterly winds concentrate micro-algal biomass feedstock against the dam wall of the Hartbeespoort Dam. The hydraulic head of 583 kPa of the 59.4 meter high dam wall is utilized to filter and transport biomass to the new integrated BTL facility, which is located down-stream of the dam. Solar power is used to dry the microalgae, which in turn is combusted in a furnace to release its 18,715 kW of biochemical power, which is used for heating in the power-intensive extraction unit of the processing facility. Most of the processes in literature that cover the production of biodiesel from micro-algal biomass are not thermodynamically viable, because they consume more power than what they produce. The new process sets a benchmark for other related ones with regards to its net power efficiency. The new process is thermodynamically efficient, exporting 20 times more power than it imports, with a net power output of 5,483 kilowatts. The design of a new integrated BTL process consisted of screening the most suitable methods for harvesting micro-algal biomass from the Hartbeespoort Dam and combining the obtained design parameters from these harvesting experiments with current knowledge on extraction of oils from microalgae and production of biodiesel from these oils into an overall conceptual process. Three promising, unconventional harvesting methods from Brink and Marx (2011), a micro-algal oil extraction process from Barnard (2009), and a process from Miao and Wu (2005) to produce biodiesel through the acid-catalyzed transesterification of micro-algal oil, were combined into an integrated BTL process. The new integrated biomass-to-liquids (BTL) process was developed to produce 2.6 million litres of biodiesel per year from harvested micro-algal biomass from the Hartbeespoort Dam. This is enough to supply 51,817 medium-sized automobiles per year or 142 automobiles per day of environmentally friendly fuel. The new BTL facility consists of three sections: a cultivation section where microalgae grow in the 20 km2 Hartbeespoort Dam to a concentration of 160 g/m2 during the six warmest months of the year; a harvesting section where excess water is removed from the micro-algal biomass; a reaction section where fatty acid oils are extracted from the microalgae and converted to biodiesel, and dry biomass rests are combusted to supply heat for the extraction and biodiesel units of the reaction section. The cultivation section consist of the existing Hartbeespoort Dam, which make up the cultivation unit; the harvesting section is divided into a collection unit (dam wall part of the Hartbeespoort Dam), a concentration unit, a filtration unit, and a drying unit; the reaction section consists of an oil extraction unit, a combustion unit, and a biodiesel unit. At a capital cost of R71.62 million (R1.11/L) (±30%), the new proposed BTL facility will turn 933,525 tons of raw biomass (1.5% TSS) into 2,590,856 litres of high quality biodiesel per year, at an annual operating cost of R11.09 million (R4.28/L at 0% producer inflation), to generate R25.91 million (R10.00/L) per year of revenue. At the current diesel price of R10.00/L, the new integrated BTL process is economically feasible with net present values (NPV) of R368 million (R5.68/L) and R29.30 million (R0.45/L) at discount rates of 0% and 10%, respectively. The break-even biodiesel prices are R5.34/L and R7.92/L, for a zero NPV at 0% and 10% discount rates, respectively. The cultivation of micro-algal biomass from the Hartbeespoort Dam is only economical if the growth is allowed to occur naturally in the dam without any additional cultivation equipment. The cultivation of micro-algal biomass in either an open or a closed-cultivation system will not be feasible as the high cost of cultivation will negate the value of biodiesel derived from the cultivated biomass. The utilization of the three promising harvesting methods described in this work is one of the main drivers for making this process economically feasible. At a capital cost of R13.49 million (R37.77/ton of dry weight micro-algal biomass) and a operating cost of R2.00 million per year (R210.63/ton of dry weight micro-algal biomass) for harvesting micro-algal biomass from the Hartbeespoort Dam, harvesting costs account for only 19% and 18% of the overall capital and operating costs of the new process, respectively. This is less than harvesting costs for other comparative processes world-wide, which contribute between 20 and 30% of the overall cost of biomass-to-liquids production. At current fuel prices, the cultivation of micro-algal biomass from and next to the Hartbeespoort Dam is not economical, but the unconventional harvesting methods presented in this thesis are feasible, if incorporated into the new integrated biomass-to-liquids biodiesel process set out in this work.
- Engineering