Critical review on third generation micro algae biodiesel production and its feasibility as future bioenergy for IC engine applications
Graphical abstract
Introduction
For the past decade, energy demand is increasing cumulatively in tandem with global pollution. As a result, global warming and a rapid decrease in fossil fuels are observed as they are the primary source for the automotive and energy sector. In order to maintain an aesthetic environment and address the fossil fuel depletion, alternative fuels which are sustainable, renewable and eco-friendly are widely being researched upon. Through the years, researches carried out on first and second biofuel production techniques and their respective engine testing revealed that the cultivation of these feedstocks on land was unsustainable. However, a significant reduction of NOX emissions was observable for most of the biofuels on engine testing. To tackle the problem of sustainable biofuel production, third-generation feedstocks such as micro algae and microbes have been trending in this field of research [1]. In recent years, significant advances in the field of fuel production have opened new perspectives on achieving sustainable biofuels. These advancements were explored to eliminate the flaws linked with the first and second-generation biofuels such as the utilization of edible feedstocks, excessive land usage and cost related to maintenance like pesticides, large quantities of water etc. Their constituent fuels which can be procured are biodiesel, alcohols and gases. Micro algae species are becoming the main source of feedstock in the production of biodiesel. This is because of its multiple advantages over conventional terrestrial feedstocks. Species such as Sp. Chlorella has the ability to grow throughout the year therefore ensuring a constant supply of micro algae oil with minimal water requirements. Moreover, most of the micro algae species have a tolerance to high carbon dioxide content and require no herbicides or pesticides to grow. Micro algae cells have more than 30 percent higher lipid composition compared to other sources such as soybeans and palm oils [2]. Furthermore, varying sources of wastewater from the industry or sewage, containing nutrients such as phosphorus and nitrogen can be exploited for micro algae culturing instead of providing any additional nutrients. More importantly, micro algae species have the innate ability to grow under harsh conditions like coastal seawater or brackish water, which usually attenuates older generation biofuel feedstocks [3].
Both micro and macro algae can be grown on a large scale in less time [4]. Fig. 1 represents the complete flow of process involved in converting micro algae feedstock into usable biodiesel for IC engine testing. Moreover, the full structure of this article is relative to this flowchart. After selecting the micro algae species which is to be cultivated, gradual multiplication of micro algae colonies is initiated. The environment for the cultivation of micro algae is crucial for the increased biomass yield. Therefore, growth factors such as light intensity, CO2, temperature and pH of medium influence the cultivation and yield rates drastically [5], [6], [7], [8]. Consequently, optimization techniques are utilized to identify the optimal ranges of these factors to reduce and cost and ultimately save time. Micro algae are cultivated in two modes such as open type photo bioreactor and the closed type photo bioreactor. The reactors have various design types within each domain which are constructed and used based on space and resources available. Their geometry and dimensions are usually optimized in such a way that maximum cultivation outputs can be expected. In the open type reactors, the control over some of the factors might be challenging, however, certain precautions can be taken to overcome the impact of these factors. Closed type is more effective than open type since the factors like light intensity can be modified and adjusted to make sure that the micro algae grow effectively. In both cases, CO2 can be supplied through external tanks or supply through flue gases or through bubbling [9]. Once the micro algae grow, the biomass is harvested by flocculation and the resultant sludge is collected. Heat or electricity is used to dry the biomass. Rarely, certain solvents are used as dehydrating agents in cold areas. The dry micro algae cake residue contains lipids which are used in biodiesel production with the help of extraction techniques such as solvent and mechanical and extraction. The lipids are extracted from the dry biomass using these techniques until a residual micro algae oil is procured. The extracted oil goes through the process of transesterification or hydrolysis which converts it to biodiesel with the help of certain solvents and catalysts.
At optimal physio-chemical properties of the extracted and processed biodiesel, optimized blends of this fuel can be prepared. These blends can be utilized in IC engines to access their efficiency based on engine output characteristics. To enhance engine efficiency, parameters such as exhaust gas recirculation, boost pressure, valve timing, injection pressure and mass can be calibrated while operating on different micro algae biodiesel fuel blends. On optimal calibrations, an engine performance map is acquired at the end of the calibration process. Based on the engine map, the output characteristics such as brake thermal efficiency, brake specific fuel consumption, heat release rate, cylinder pressure, NOX and hydrocarbon emissions and can be quantified for a range of operating factors [10]. These operating factors can be speed, load, compression etc. By employing an optimized engine test design, an ultimate engine map can be developed for a range of micro algae biodiesel blends which gives efficient engine output characteristics at specific operating factors [11]. Through the years, various researches have been carried out in the production and utilization of micro algae biodiesel in IC engines. Using these studies, a few consolidated articles are available. However, there are no articles which discuss both the production and engine testing aspects in the same article. To address this issue, this study, this article intents to provide an inclusive outlook on the various stages and techniques involved in micro algae biodiesel production and its application in IC engines. Furthermore, a critical interpretation is provided for most of the sections to suggest the most efficient methods involved in various stages of micro algae biodiesel production and engine testing. Also, a techno-economic assessment is drawn to assess the current market scenario of micro algae biodiesel. Furthermore, future scope such as research focusing on genetic engineering for the production of hybrid micro algae strains to ensure higher lipid yield rate are briefly discussed. Similarly, in the automotive sector, micro algae fuel based flex-fuels engines are yet to be researched upon. Therefore, a brief scenario is provided on the possibilities of micro algae biodiesel based application in IC engines.
Section snippets
Objective and motivation of the study
Over the years, multiple studies have researched micro algae based biodiesel production and its application in IC engines. However, any researcher who wishes to study the process behind the production of micro algae biodiesel and its applications will have to read through the dearth of literature available on this subject. Even though a collection of studies is accessible for a small part of micro algae biodiesel production, no review article is available on the multiple techniques and
Overview of third generation biofuels
Third-generation biofuels are alternative fuels that are derived from micro-organisms which are photosynthetic such as micro algae and microbes. In recent years, third-generation biofuels have been widely researched upon as it is considered as a viable alternative resource as it overcomes the disadvantages incurred from producing first and second-generation biofuels [12]. First and second-generation biofuels are manufactured from edible and non-edible plant parts to process products like pure
Methodology followed for processing micro algae strain into micro algae biodiesel
Certain micro algae species have significant properties which enable them to be used as potential feedstocks for biodiesel production. Firstly, the cultivation of micro algae does not require terrestrial land as compared to other generational feedstocks. During the production of micro algae biodiesel, no other food sources are compromised. Secondly, micro algae species can grow and multiply at a rapid rate. Also, most of the micro algae species are rich in lipid content. For instance, Chlorella
Role of optimization applications in the production of micro algae biodiesel
In the past decade, processing and utilization of biodiesel from micro algae biomass have sparked an interest among the scientific and industrial sectors due to its sustainability. Through the studies carried on in these years, it is proclaimed that third generation biofuel is a perfect carbon neutral alternative for older generation biofuels [105]. Moreover, third generation biofuels are highly capable of providing the current demand for transportation fuels. Despite the huge advantages
Influence of physio-chemical properties of micro algae biodiesel on engine operation
This section deals with the impact of physio-chemical properties of micro algae biodiesel on engine output characteristics. It is important to know the fuel properties affect the factors influencing performance, combustion and emission. Also, these properties are responsible for the tribological characteristics like wear and tear on engine operation. Table 5 illustrates various physical properties of micro algae biodiesel from multiple works of literature and compares it with ASTM D6751
Performance characteristics of micro algae biodiesel
In engine performance study, brake thermal efficiency and brake specific fuel consumption are the important parameters to be assessed. The important observations made on the performance characteristics of biodiesel derived from different micro algae species are reported in Table 7.
Combustion characteristics of micro algae biodiesel
The effect of micro algae derived biodiesel on the combustion characteristics was reported in several studies. The most important combustion parameters investigated were in-cylinder pressure, heat release rate and ignition lag or delay period.
Emission characteristics of micro algae biodiesel
In engine emission study, carbon monoxide, carbon dioxide, unburnt hydrocarbons, oxides of nitrogen, smoke density and particulate matter are the important emissions to be assessed as shown in Fig. 9. Furthermore, important inferences which have been acquired from multiple studies regarding each emission gases while operating on micro algae biodiesel are highlighted. Also, key observations made on the emission characteristics of biodiesel derived from different micro algae species is reported
Factors affecting performance, emission and combustion characteristics of micro algae biodiesel
The important factors affecting engine characteristics are engine load, speed, injection pressure, injection timing and compression ratio. Fig. 10 illustrates the frequently observed behaviours and influence these factors to have on engine output characteristics while operating on micro algae biodiesel. The effects of engine parameters on performance, emission and combustion characteristics of micro algae biodiesel reported in selected literature are listed in Table 10.
Techno-economic assessment of micro algae biodiesel production
In micro algae biofuel industry, the techno-economic assessment is a crucial step as it reveals information on baseline economics, environmental impacts and sustainability of the process. Comparing first, second and third generation biodiesel feedstocks, third generation based microalgae offers better features in terms of yield and land requirements. Unlike other feedstocks, microalgae can be genetically modified to increase oil content and maintain saturation-unsaturation levels to reduce
Conclusion and future perspectives
Micro algae biofuels are recognised as a capable substitute for the existing generation of biofuels due to their sustainable nature of production and remediation. As incurred from the studies in this article, a raceway pond is the best choice of photo bioreactor for cost-effective growth conditions as wastewater can be used as growth medium and sunlight can be the light source. More importantly, CO2 can be absorbed from the atmosphere as they are freely available in ambient air. Furthermore,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors wish to thank Department of Science Technology (DST), India for funding of this work under the scheme of ASEAN-India Science & Technology Development Fund (AISTDF) for the project (CDR/2018/000061).
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