In addition to that, access to carbon dioxide and water areessential. Even though microalgae can produce in the presence of saline water,fresh water is needed in a raceway pond system to compensate for the evaporativeloss depending on the wind velocity, air temperature, and humidity level of thelocation. Temperature is an important element in biomass cultivation.
Most algaegrow better in warmer climates ranging from 25-40?. Tropical locations with auniformly warm temperature throughout the year (Chisti, 2016), can act as perfectlocations for algaculture as the temperature doesn’t have to be monitored at alltimes, and the algae can adapt to local conditions.There are however some drawbacks while using raceway pond systems, thatrender them sometimes ineffective.
Since, carbon dioxide is required to acceleratethe production of microalgae, an accumulation of oxygen can act as a hindrance tothe process. There is no known mechanism in a raceway pond, that helps curb thisaccumulation of oxygen. Peak sunlight hours during the day can hamper with thephotosynthesis, as the level of oxygen may increase to up to three times of the levelin saturated water. For this reason, smaller raceway ponds achieve better resultsthan larger ponds with respect to oxygen removal, and in turn better productivity.
Another issue with raceways is the contamination due to exposure to rain, dust andother debris. Smaller ponds may be placed inside, but that can’t be said for largerponds. Filtration can help inhibit infestations and contamination of the ponds, but thatis an expensive process.The production cost of biomass with raceways is considered to be the leastexpensive option.
The cost of a pond depends on the type of facility it is built in,plastic lined earthen raceways are the least expensive alternatives with their totalcost of construction amounting to be approximately $70,000 per hectare, whereasponds enclosed in greenhouses or covered facilities are more expensive as theyprotect from contamination. Raceways require least amount of capital investmentand therefore remain the system of choice, despite their low productivity anddrawbacks.
A photo-bioreactor is a closed equipment which provides a controlledenvironment and enables high productivity of algae.
PBRs curb all the problems thatare faced in raceways ponds, like carbon dioxide supply, temperature, optimaloxygen levels, pH levels etc. There are two types of photo-bioreactors- flat-plate andand tubular. Both PBRs are made of transparent materials for maximum solar lightenergy absorption. Flat-plate PBRs are suitable for mass cultivation of algae,because high photosynthetic efficiencies can be achieved. Tubular PBRs aresuitable for outdoor cultivation, and are constructed with either glass or plastic tubes.
Systems covering large areas outdoors, consist of tubes exposed to sunlight and canbe operated either in batches or continuously. Photo-bioreactors usually have a4water pool as a temperature control system in order to prevent the tubes fromoverheating as they act as solar receptors. They also have built in cleaning systemfor the tubes without stopping production.
Fundamentally, using photo-bioreactorsare more advantageous than using raceways for many reasons, like cultivation ofalgae under controlled environments resulting in higher productivity, protection fromcontamination, space-saving and larger surface to volume ratio. However there aresome limitations attached to PBRs; the capital cost is very high which is impedingthe progress of microalgae biofuel production, in spite of larger production levels.
Also, data from the past two decades has shown that the productivity in an enclosePBR is not much higher than that achieved in open-pond cultures.3. Environmental Limitations of Microalgae CultivationAs with all large scale productions, wide scale microalgae biofuel productioncould have diverse environmental impacts. Water is a critical element of the biofuelproduction processes, in both raceway-ponds and PBRs.
With the current globalwater crisis, using large amounts of fresh water to compensate for evaporation inopen ponds or to cool PBRs, renders the system economically unviable. Seawater orbrackish water may be used in these functions, but have to be filtered in order toprevent infestation of bacteria, and contamination. Recirculating water is onealternative to curb the usage of water, but that has risks of virus infestations, and theresidues of previously destroyed algae cells.
Filtration systems are expensive, andfactor in with the lack of cost effectiveness of these systems.Most microalgae production farms have to be located close to the equator inorder to ensure high levels of production due to the uniformity of the climate, andadequate amount of solar radiation. Another factor is the type of land and terrain thefarm is located in, for instance to install a large raceway pond, a relatively flat land isrequired. The addition of nutrients and fertilisers like nitrogen and phosphorus is alsoessential for algaculture.
The amount of nutrients and fertilisers to be usedadditionally depends on the soil porosity and permeability of the land. Algalcultivation requires a lot of fertilisers to make up for the compensation for fossil fuels.Researching and budgeting nutrients and fertilisers is a key concern in research anddevelopment of microalgae cultivation.
Algal cultivation requires usage of fossil fuels continuously in a plethora ofways, ranging from electricity consumption during cultivation and natural gas used todry the algae for production. In PBRs, the temperature control for cooling the pipesfrom overheating increases the use of fossil fuels. This use of fossil fuels in algaebiofuel production is paradoxical to the cause and a dire need to optimise the systemto minimise the energy usage is established.
That being said, microalgae cultivationfaces a variety of environmental challenges, coming from the location to the type of5algae. Energy conservation and water management are two of the main challengesto be conquered to make the system sustainable in the future.4. Cost EffectivenessThe cost of algae biofuel production is essential to establish to know howsustainable this system can be in the future. The cost of biofuel production dependson a variety of factors, such as the the yield of the biomass, geographical location, oilcontent, scale of production systems etc.
Presently, microalgae biofuel production isstill more expensive than normal diesel fuels because of the ongoing R&D, and theambiguity of current knowledge. Chisti in 2007 approximated the cost of productionof algal-oils from a PBR with an annual production capacity of 10,000 tons per yearand estimated the cost of $2.80 per litre, considering the oil content to be 30% in thealgae used. This estimation is exclusive of the algal oil to biodiesel conversion costs,logistics, marketing costs and taxes.
Due to these high costs of algal-fuel, the utmostimportance during research should be given to cost-saving itself, in an attempt tomake biofuel from microalgae affordable enough to be commercialised in the nearfuture.Open pond systems would ideally be the most economically viable way tocultivate microalgae biofuel, but not without it’s set of intrinsic disadvantagesdiscussed earlier in this research paper.
As the technology gets increasinglyadvanced, the cost factor multiplies as well making the entire process a lot lesseconomical than what was started with first hand. Improved yield of biomass andnutrient oils (or lipids) would make the production costs drop rapidly.Moreover, to reduce the production costs alternative ways to manage energy andwater consumption have to be devised, a simplified design for PBRs is necessary.Substitutes for fresh water like wastewater and flue gases can contribute to lowercosts of production.
The rapid growth of environmental pollution by the usage of conventionalfossil fuels has sparked a lot of concern globally. The research and development foralternative fuels is one of the principal focuses for every country in an attempt for asustainable and promising future on this planet for all generations. Various optionsare available to us to help us make this shift, however to find a sustainable methodwhich is as promising as it is economically viable is a global challenge.
Currently,biomass derived fuels seem to be the most optimistic path.Various ways of harvesting algae have been discussed in this paper, the next step istypically to process the algae in a series of steps which differ from species to6species. One of the most important approaches in biomass production isHydrothermal Liquefaction or HTL.5.1 Hydrothermal LiquefactionHydrothermal Liquefaction employes “a continuous process that subjectsharvested wet algae to high temperatures and pressures” (Elliot, 2013).
Convertingsolid biomass to liquid fuels is not a spontaneous process. The liquid fuels derivedfrom fossil fuels on a large scale took thousands of years to convert biomass tocrude oil and gas. In present day, there are many modern conversion technologies toobtain liquefied fuels from various biomasses, these conversion technologies canfundamentally be classified into biochemical and thermochemical conversion.Biochemical mass usually has low energy density, high moisture content and doesnot have a very viscous physical form.
Thermochemical conversions in comparisonare much more viscous as they are converted at very high temperatures in highpressures in the presence of catalysts that make the conversions much more rapid.Simply, Hydrothermal Liquefaction is “the thermochemical conversion of biomassinto liquid fuels by processing in a hot, pressurized environment for sufficient time tobreak down into solid bio polymeric structure to mainly liquid components”(Gollakota, 2017).
Microalgae is, amongst all possible biomass sources, the most efficientand reliable source of wet biomass due to its high photosynthetic efficiency,maximum production levels, and its rapid growth in almost all environments. Overthe years, many thermochemical conversions have made their way, and while eachhas their pros and cons, HTL has come a long way as one of the most appropriateprocesses to tackle thermochemical conversion of wet biomass.
Many scientists overthe years have done extensive research pertaining to the development ofhydrothermal liquefaction, such as Beckmann and Elliott who studied the propertiesof oil obtained from HTL of biomass, and gave crucial inputs with respect to the kindof catalysts and other parameters are pertinent to the HTL process to ensuresignificant productivity.5.2 Process MechanismCurrently, the knowledge about HTL process mechanisms is qualitative andneeds a lot more space for research.
The mechanism comprises of three majorsteps: depolymerisation, decomposition and recombination. The chemistry behind allthese processes is very complex as the biomass is a complex mixture ofcarbohydrates, proteins, oils etc. Each working mechanism of hydrothermalliquefaction is discussed below.5.2.1 Depolymerisation7In this process the macromolecules of the biomass are dissolves through theirphysical and chemical properties.
Depolymerisation makes it easier for the biomassto overcome it’s natural qualities and start behaving like fossil fuels. It mimics thegeological processes, that are involved in the production of conventional fossil fuels.The process first grounds the feedstock material into small chunks and mixes it withwater, if the feedstock is fry. This mixture is then put into a pressure vessel reactionchamber where it is heated at a constant volume at a temperature of 250?, themixture is held in these conditions for approximately 15 minutes at the end of whichthe pressure is released and most of the water is boiled off.
The resultant concoctionconsists of crude hydrocarbons and solid minerals. The minerals are removed andthe hydrocarbons are sent to the second stage.The disadvantage of this process is that it only breaks down long molecularchains into shorter ones, this implies that smaller molecules like carbon dioxide ormethane cannot be broken down further by depolymerisation.
Decomposition or Dehydration
The second stage of hydrothermal liquefaction involves the loss of the watermolecule, the carbon dioxide molecule and the acid content. Water at high pressuresand temperatures breaks down the hydrogen bonded structure of celluloses and inturn forms glucose monomers. This is how HTL provides an alternative processroute from microalgae biofuels to hydrocarbon liquid fuels.5.2.3 RecombinationThis is the last step in HTL which is reverse of the two previous processesbecause of the absence of the hydrogen compound.
The free radicals are largelyavailable which in turn recombine or repolymerise to form high molecular weight charcompounds.5.3 Hydrothermal Liquefaction of Microalgae:The main advantage of using HTL for microalgae is that it doesn’trequire the predrying of feedstock, yet ensuring a relatively high production. Theprocess of HTL applied to microalgae is similar to treating cellulose but with a fewdifferences, the major one being treating wed feedstock as opposed to dryfeedstock.
One of the principally researched issues that will ensure high productivityis a high lipid yield, which is necessary to convert microalgae into biodiesel. Theeffect of significant variables, such as temperature, pressure, volume, biomassconcentration and compositions of algae, catalysts et al. is still under research.During hydrothermal liquefaction of microalgae, a rational heat management system8must be put in place that ensures energy efficiency and separation of the endproduct.
Current Situation ; Future Viability:In present day, pertaining to all the advantages and disadvantages of HTL,there is sufficient proof that HTL has potential to become a commercialisedtechnology in the future.Biofuels produced using hydrothermal liquefaction are absent of carbon, thisimplies that there are no carbon emissions produced when the biofuel is burnt.Materials like algae use photosynthesis to grow, and therefore use the carbondioxide already present in the atmosphere.
The carbon imprint produced by biofuelsis exponentially lower than what is already being experienced by conventional fossilfuels. Hydrothermal Liquefaction is a clean process, which doesn’t harm theenvironment by producing harmful gases like ammonia or sulphur. If the technologyis mastered, HTL can pave the way for clean algal biofuels globally, although thereare still a number of challenges to be overcome.
The cultivation and production of microalgae biofuels is swiftly developing andis receiving attention and funding from global leaders. The rapid increase in worldpopulation, and hence the energy demand is a siren call to devise an alternativeenergy source. Microalgae’s versatile qualities make it a promising path to tread onwhen it comes to biofuels. There are various ways to derive biofuels from algae aswe saw in this paper, and also many challenges attached with them.
Bio-oil obtainedfrom various processes suffers from various drawbacks such as a high oxygencontent, instability etc, therefore an optimal technique to efficiently convert biomassto biofuel should be researched in order to be able to commercialise the use ofbiofuels in the near future. Making biofuels economically viable in the future is a bigchallenge in itself.
Even though, photo-bioreactors promise a bright future in terms ofbiofuel cultivation, the overhead costs attached from cultivating the biofuel to makingit market ready and selling it are still quite high. These high costs of biofuels ascompared to conventional fossil fuels are what render them unready forcommercialisation. However, even with theoretical development and research, abright future for microalgae fossil fuels presents itself.