Bioenergy

Bioenergy or biofuel is a term used to describe renewable energy generated from biological materials. Microalgae are increasingly being recognized as a sustainable feedstock for biofuel production due to their rates of production and the significant fraction of the biomass that is made up of useful lipids and carbohydrates. algal biofuels have seen considerable reasearch effort and investment from both academic and industrial sectors.

Algae are well suited to biodiesel production as many species have high lipid contents that can readily extracted and converted to biodiesel by transesterification on an industrial scale.  Microalgae are also an ideal candidate for bioethanol production due to their high content of fermentable sugars, lack of the structural component lignin and again high productivity.

However, algae can also generate a whole suite of bioenergy products. Methane can be generated from microalgal biomass by gasification, pyrolysis or anaerobic digestion. This can then either be combusted directly for generation electricity and heat or alternatively as biogas as a liquid fuel replacement or for domestic purposes. Microaglae can also be utilized for photo-biological hydrogen production for use in fuel cells, essentially generating a sustainable, emission-free hydrogen gas from sunlight and water. A number of other technologies exist for the production of a range of biofuels from microalgae, including biobubtanol and biogasoline for use as liquid fuels. However, significant technological challenges remain to be answered. 

Nutrition

Algal Biomass

Fresh or dried microalgae are being increasingly used for animal and human nutrition. Algae have a high protein content and significant quantities of omega 3 and 6 oils with proven health benefits. Annual production of biomass for human consumption has seen dramatic increases over the past decades, especially in Asian regions where it is sold either as capsules, tablets or health drinks. 

Recent studies suggest that inclusion of microalgae in animal diets can have several health-promoting effects and offers could offer a cheaper and more sustainable replacement for fish meal. With the aquaculture industry seeing a huge increase in output, (an estimated 43 tons annually in 2004) there is now a substantial market for live and dried algae to supplement the diets of juvenile fish and shellfish. 

Polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFAs) are essential parts of our diets and key for child development. Only plants are able to synthesise PUFAs, and microalgae supply aquatic food chains with these vital components. Fish which feed on algae provide oils that are the common sources of PUFAs for human nutrition, but with increasing worries about accumulation of toxins and cost, microalgal PUFAs therefore have a promising market for food and feed. One prime example is the enrichment of infant formulas with microalgal EPA and DFA. 

High-value Products

Pigments and Antioxidants

Microalgae naturally contain large quantities of photosynthetic pigment and antioxidant molecules, which improve light energy utilization and help protect against damaging free-oxygen within the cell. These components are increasingly in demand and represent a high-value product that can be readily obtained from microalgal cultures. 

Many pigments such as chlorophyll, the main photosynthetic pigment, find use as chemical dyes. Another important group are the carotenoids, which contain a diverse number of molecules including β-carotene and astaxanthin and have a myriad of uses, most interesting though is their nutritional and therapeutic benefits. Some carotenoids act as vitamin precursors that have intrinsic anti-inflammatory and antioxidant properties, giving them several pharmaceutical applications associated with protection against oxidative damage.

A number of pigments and antioxidants are being considered by the cosmetics industry for inclusion in their products both to improve preservation but also because of their valuable UV-absorption and sun-protecting qualities. Other groups such as the phycobiliproteins have unique qualities make them ideal for diagnostic applications and as highly sensitive fluorescent reagents and dyes.

Phycocolloids

These polysaccharides derived from the cell walls of seaweeds are economically one of the most important products from microalgae. The major phycocolloids are alginates, agar and carrageenans and are used in a wide range of industries, in particular the food industry, for their gelling, viscosifying and emulsifying properties. Many algal polysaccharides are also of considerable pharmacological importance due to their intrinsic antioxidant, antiviral and immune-activating activities.

Stable Isotopes

As cultivation of microalgae can be tightly controlled, they are ideal candidates for the production of stable isotopically labeled compounds. Stable isotopes (13C, 15N and 2H) can be incorportated into highly valued organic molecules such a amino acids, carbohydrates, lipids and nucleic acids. These have application in scientific investigation and potentially for clinical purposes. 

Applications

CO₂ and flue gas sequestration

One of the key advantages of using algae is that they are able to withstand high concentrations of CO₂ during culture. discrete, intensive microalgal growth platforms could be used for efficient carbon capture systems high-CO₂ content flue gases from power stations and otehr industry. It has also been found that many algae are able to remove nitrogen oxide and sulphur dioxide from gas streams, which are common causes of acid rain. These approaches could help to reduce carbon emissions and associated taxation, and help reduce costs of algal product manufacture. Additionally, residual biomass rich in carbon can be converted to a form used to enhance agricultural soil, a further CO₂ mitigation opportunity.

Remediation of waste water resources

Wastewater generated from agricultural, industrial and domestic sources often contains high concentrations of organic matter, nitrogen, phosphorous, heavy metals and is in plentiful supply. Processing of these wastes represents a serious environmental challenge and requires costly treatments to remove organic compounds and inorganic chemicals to prevent eutrophication in water bodies.

The use of algae in wastewater treatment has long been appreciated; their ability to efficiently accumulate nutrients and toxic metals when grown in nutrient-rich environments represents a low cost and more sustainable strategy compared to other treatment processes. For these reasons, the use of such wastewaters for the cultivation of microalgae is been suggested as a plausible route to economical biomass production, off-setting the cost of biomass production in particular for biofuels, offering savings on costly waste remediation, a reduction in the use of fresh water for algaculture and a cheap source of nutrients for growth.

Microalgae & Strain Selection

The term algae covers a wide range of diverse organisms that can be generally described as eukaryotic protists (also a very difficult group to define), that are distinct from plants but are typically photosynthetic and aquatic. They can either be microscopic single-celled microalgae or larger more complex multi-cellular macroalgae or seaweeds. They are distributed worldwide in both freshwater and marine habitats across a wide range of environments. Like plants, the majority of algae use photosynthesis to capture light energy to convert inorganic substances into simple sugars and then other molecules.

It is estimated that there are over 200,000 species of microalgae of immensely diverse structures that can vary in size from a micrometers to a few hundred micrometers. These organisms can be split into several groups including the green algae (Chlorophyta), red algae (Rhodophyta), and diatoms (Bacilliariophyta), as well as the prokaryotic cyanobacteria (blue-green algae).

Partly due to their simple structures, microalgae are highly efficient converters of solar energy, fixing five times more solar energy to chemical energy than most terrestrial plants. As a result they can achieve high growth rates, often doubling their biomass in a period as short as a few hours. These qualities make algae the ideal candidates for the production of lipid rich biomass and other metabolites over terrestrial sources. Additionally, despite growing in aqueous media, microalgae require less water than terrestrial crops and many valuable species can be grown in saline or brackish water.

Isolating and culturing algal species with optimal characteristics for biomass production or that of a particular metabolite is one of the biggest challenges. Identifying what your end product is, the cultivation method you want to use and the environmental conditions they will be exposed to all need to be taken into consideration when selecting a species to grow on an industrial scale. Although thousands of species have been characterised and studied, with more being added all the time, it may be that a more suitable species is lying undiscovered in the environment.

Cultivation Methods

Microalgae have been cultivated on an industrial scale for decades in East Asia, most typically grown in large open ponds or raceways for human and animal nutrition. However with the development of algal biotechnology and the desire to produce more specific products closed photobioreactors (PBRs) systems in a range of configuration have become increasingly common. Choosing the right cultivation method is critical and dependent on several factors including the desired end-product, algal species, local environmental condition and predictably cost. However, a significant number of commercial open pond systems are in operation globally.

Open Ponds

Open ponds could be simple lakes, lagoons or artificial containers for growth of large cultures. The most commonly used design is a large oval raceway. These systems are typically very simple, relatively cheap and straight-forward to operate, and benefit from free solar insolation at low latitudes. However, these systems often suffer from low productivities due to poor light utilisation, poor diffusion of gases and the requirement of large areas of land for cultivation. Additionally, due to them often being open the environment often become contaminated by predators and other fast-growing algal species, limiting the number of species that can be used to those that persist in extreme conditions.

Closed Photobioreactors Systems (PBRs)

Cultivation in closed PBRs is intended to overcome some of the limitations associated with open systems, while also enabling predictable and reproducible growths of high density microalgal cultures. These systems are flexible in design and could be tubes, panels, bags in a range of configurations made from an array of transparent materials. These could be intricately engineered or very simple in design; placed outdoors to utilise natural sunlight or indoors under artificial lighting. These systems allow much tighter control of growth conditions and can be optimised to suit the biological needs of a single species while reducing risk of contamination.

However, the costs associated with many of these systems in terms of initial outlay and running costs often make their use prohibitively expensive, arguably limiting the number of designs suitable for mass cultivation to just a few. It is expected with continued engineering advancements and optimisation of cultivation conditions for particular species these systems will become much more economical in the future when operated on large scales. One suggestion is use of a ‘hybrid’ system making use of both closed and open systems in a two-stage production strategy; first using closed systems to produce high-quality concentrated inoculums for much larger open systems to improve cost effectiveness. This strategy is being employed in the production of the commercially important pigment astaxanthin from the red alga Haematococcus pluvialis on a large scale. 

Harvesting and Extraction

This series of processes effectively requires the separation of algae from the liquid media it was cultivated in, drying the resultant biomass and processing it to obtain the desired products. These stages are often highly energy intensive depending upon the species grown and end product required; here we highlight some of the commonly employed techniques.

Harvesting

This process effectively involves concentrating dilute algal suspensions, removing liquid until a thicker algal paste is obtained. Choosing the most effective harvesting process is dependent on the strain of algae used, its properties and size. Normally algae can be harvested in a single step or two separate steps. Common techniques include:

Settling or Flocculation: this is often a precursor to other techniques were either through addition of chemical flocculants or by natural auto-flocculation, it causes cells to aggregate into larger clumps, making further processing easier.

Centrifugation: a rapid but energy intensive method employing a centrifuge that separates the biomass using sedimentation by application of a centripetal force to generate thick concentrated pastes. May only be suitable on a large scale and can also cause damage to sensitive algal cells. Variants on traditional equipment may make the process more effective and can be coupled with other devices to potentially separate cellular components e.g. lipids.

Filtration: A method of concentrating solutions by passing suspensions over micro-porous membranes under pressure. Can concentrate very low density cultures, while also avoiding damage to sensitive cells. Suffers from being limited to processing small volumes before the eventual clogging of membranes, however new techniques are improving efficiencies and processing volumes.

The KTC in collaboration with the Centre for Complex Fluid Processing in the Engineering department, which has considerable expertise in many of these techniques held a workshop discussing the practicalities of microalgal harvesting methods in March 2011. If you would like more information on this topic please get in touch.

Processing

In terms of processing a lot of attention is currently being paid to the extraction of oils from biomass as it is one of the areas where significant cost reductions could be made. A detailed description of extraction methods is beyond this introduction to algal biotechnology, but most methods can be broadly categorised into two methods:

Mechanical Methods: these generally involve crushing or breaking open cells by use of a mechanical force, this could be by use of a pressing device or through ultrasonic extraction. The latter method uses ultrasonic waves to generate bubbles in liquids which when they collapse cause cells to burst and release their contents. The selection of method is dependent upon the physical properties of the cell. These methods typically require the biomass to have been dried which is energy intensive.

Chemical Methods: Microalgal oils and other components can be extracted by application of chemicals to wet or dry biomass. Hexane is a popular solvent that can be highly efficient. Another alternative is supercritical fluid/CO₂ extraction, which is where CO₂ is liquefied under high pressure, heated to the point where is has the properties of both a liquid and a gas, which then works as an effective extraction solvent. These methods have the disadvantages of being both energy intensive and expensive while also presenting numerous health and safety issues.

These methods can often be used in conjunction with one another to achieve better results. 

Biorefinery Concept

In the production of biofuels from many terrestrial feedstocks such as sugarcane, all components are either used in the process of production, either to supply a product or as an energy source or sold as additional products. This kind of system is referred as having a biorefinery approach and can also be applied to microalgal biofuel production, helping to significantly reduce the cost of these technologies and improve their sustainability.

Some species are capable of producing a range of different components and intermediates for use for in a number of markets, by taking advantages this it helps maximise the value derived from the biomass feedstock. It could for example produce one low-value, but high-volume component such as lipids for biodiesel and several low-volume but high-value chemical products that enhances profitability. Furthermore, residual biomass can be used to generate methane via anaerobic digestion to produce electricity, which in turn can be used to power the cultivation and processing stages of biomass production. In combination with the use of waste nutrients and CO_2­, this approach can help to significantly reduce costs and green house gas emissions.