Biofuels from microalgae—A review of technologies for production, processing, and extractions of biofuels and co-products

TLDR: In this article, the authors reviewed the technologies underpinning microalgae-to-bio-fuels systems, focusing on the biomass production, harvesting, conversion technologies, and the extraction of useful co-products.

Abstract: Sustainability is a key principle in natural resource management, and it involves operational efficiency, minimisation of environmental impact and socio-economic considerations; all of which are interdependent. It has become increasingly obvious that continued reliance on fossil fuel energy resources is unsustainable, owing to both depleting world reserves and the green house gas emissions associated with their use. Therefore, there are vigorous research initiatives aimed at developing alternative renewable and potentially carbon neutral solid, liquid and gaseous biofuels as alternative energy resources. However, alternate energy resources akin to first generation biofuels derived from terrestrial crops such as sugarcane, sugar beet, maize and rapeseed place an enormous strain on world food markets, contribute to water shortages and precipitate the destruction of the world's forests. Second generation biofuels derived from lignocellulosic agriculture and forest residues and from non-food crop feedstocks address some of the above problems; however there is concern over competing land use or required land use changes. Therefore, based on current knowledge and technology projections, third generation biofuels specifically derived from microalgae are considered to be a technically viable alternative energy resource that is devoid of the major drawbacks associated with first and second generation biofuels. Microalgae are photosynthetic microorganisms with simple growing requirements (light, sugars, CO 2 , N, P, and K) that can produce lipids, proteins and carbohydrates in large amounts over short periods of time. These products can be processed into both biofuels and valuable co-products. This study reviewed the technologies underpinning microalgae-to-biofuels systems, focusing on the biomass production, harvesting, conversion technologies, and the extraction of useful co-products. It also reviewed the synergistic coupling of microalgae propagation with carbon sequestration and wastewater treatment potential for mitigation of environmental impacts associated with energy conversion and utilisation. It was found that, whereas there are outstanding issues related to photosynthetic efficiencies and biomass output, microalgae-derived biofuels could progressively substitute a significant proportion of the fossil fuels required to meet the growing energy demand.

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Figures 3.11 and 3.12 show the pattern in the production of biogas from microwave treated maize straw. Biogas production increased from the day of the first measurement untill around the 16th day for both pretreatment conditions. Production then stabilised for the following week before it started falling around the 24th day. The samples that were treated for four minutes yielded the highest amount of biogas with a sum total of 539.46mL which is an increment of 208% compared with that of the raw maize straw. The samples pretreated for two minutes had a biogas production of 514.07mL of methane. This represents a 194% increase in biogas output with respect to the raw samples. Biogas

Table 2.3 summarizes typical physicochemical characteristics of cattle manure obtained from previous works. Table 2. 3 Physicochemical properties of cattle manure.

Table 1.7: Acid and alkaline pretreatment methods for lignocellulosic feedstock

Fig. 3.21 portrays the effects of sodium hydroxide pretreatment on biomethane production from cattle manure taken as the average amount of biogas produced by samples from each pretreatment condition i.e. the concentration of the basic solutions. It also highlights the effectiveness of different pretreatment conditions that were applied on the cattle manure. It can be seen that the pretreatment condition which led to the highest biogas production was the 0.3M NaOH solution with a maximum daily output of

Figure 3.4 shows the relation between the reducing sugar production and cumulative biogas production from untreated and pretreated cattle manure. It can be seen that both

Table 3.8 shows the effects of liquid hot water treatment on methane yield from maize straw. This pretreatment method was very effective in eliminating the resistance posed by lignin, cellulose and hemicellulose for maize straw as it produced relatively high amounts of biogas when compared to other pretreatment methods and the untreated maize straw samples. It is also a very economically feasible and environmentally friendly method because it does not involve the purchase, handling and disposal of chemicals that are both costly and potentially harmful to the environment. Liquid hot water pretreatment does not lead to the production of inhibitory by products. Biogas production showed a steady rise within the first two weeks until it reached maximum production after three

Frequently asked questions

Q1. What is the main advantage of acid pretreatment?

Acid pretreatment contributes to the output of high amounts of reducing sugars with the application of mild temperatures leading to the hydrolysis of cellulose and hence increased biogas production. 

Q2. What are the major building blocks of lignocellulosic biomass?

There are three major building blocks of lignocellulosic biomass namely; hemicellulose, lignin and cellulose, but also minute quantities of other components. 

Q3. What are the main advantages of acid pretreatment?

The various chemical pretreatment types applied before anaerobic digestion in biogas production include; acid pretreatment, alkaline pretreatment, wet oxidation, catalysed steam explosion, oxidative pretreatment with peroxides and the use of ionic liquids. 

Q4. Why did the biomass production from untreated straw last longer?

Biogas production from untreated straw lasted for fewer days probably because there were lower amounts of reducing sugars available for the bacteria to feed on. 

Q5. Why is the production of 2nd generation biofuels not very fruitful?

The production of 2nd generation biofuels is generally not very fruitful because of the necessity of pretreatment due to the presence of lignocellulose which adds to the operating cost, cost of transportation of feedstock and the acquisition of capital equipment. 

Q6. What are the main types of biofuels?

Biofuels generally refer to compact materials (such as wood chips, pellets etc.), fluids like oils, biodiesel and ethanol, or gaseous fuels such as biohydrogen, biogas, and biosyngas that are mainly obtained from biomass sources (Cecilia Sambusiti 2013). 

Q7. What was the effect of the heat assisted acid pretreatment on maize straw?

heat assisted acid pretreatment proved to be more effective especially for maize straw because test samples treated with acid without the application of heat failed to produce biogas probably because of a decrease in pH. 

Q8. What countries have successfully implemented industries that process and produce first generation biofuels?

The US, EU and other developing countries like China, Brazil, Thailand, Colombia and Indonesia, have successfully implemented industries that process and produce 1st generation biofuels like bioethanol and biodiesel. 

Q9. What is the rate of hydrolysis in lignocellulosic biomass?

If the enzymes do not have access to the substrate (the case in lignocellulosic biomass) hydrolysis becomes the rate-limiting step (Karimi 2008). 

Q10. What are some examples of heavy metals that can cause disturbances in biogas plants?

Examples of heavy metals that can lead to disturbances in biogas plants are copper, nickel cadmium, zinc (Dieter D. and Angelika S. 2008). 

Q11. How many stages of biogas production are involved in the anaerobic digestion process?

The anaerobic digestion process is complex and includes a wide range of microbes acting in up to nine stages of transformation of organic matter. 

Q12. What is the main merit of using microalgae as a substrate for biofuel?

Major merits of using microalgae as a substrates for biofuel6production is its enormous oil content (approximately 40% on dry matter basis). 

Q13. How much maize straw is produced simultaneously?

Estimates show that in the production of 1 kilogram of corn grain, approximately one kilogram of maize straw is simultaneously produced (Koundinya, et al. 2017). 

Q14. Why is there a step up in the efficiency of the biomethane production process?

due to the little or no lignin and hemicelluloses in algae biomass, there is a step up in the efficiency of the biomethane production process (Saqib et al., 2013). 

Q15. What is the condition for acetogens to acquire the energy they need to survive and?

This means that the condition necessary for acetogens to acquire the energy they need to survive and grow is very low concentrations of hydrogen and as a result, they enter a symbiotic relationship with methane producing microorganisms which survive only in environments with high hydrogen partial pressure 

Q16. How many different concentrations of sodium hydroxide solutions were applied on the biomass?

Three different concentrations of dilute sodium hydroxide solutions while heating in an autoclave at 120oC for thirty minutes were applied on the biomass. 

Q17. What was the method used to determine the reducing sugar loads for the untreated and treated biomass?

The method used to determine the reducing sugar loads for both the untreated and treated biomass samples used in the study was the G. Lorenz Miller method (G. L. Miller 1959) using the Dinitrosalicylic acid reagent (DNS). 

Q18. What is the average methane production from the pretreatment of cattle manure?

As seen above, the pretreatment condition which led to the maximum production of biomethane 0.3M acid solution with a total methane production value of 162.2mLCH4/gVS representing a 20% more methane production than the raw cattle manure.