Technical Report

A perspective on algal biogas

September 2015

Executive Summary

There is a lot of scientific literature available on liquid biofuel production from microalgae; less literature is available on biogas from microalgae. Prior to 2010 few academic papers dealt with biofuel production from seaweed; however since 2010 a significant number of papers have been published in the scientific press. This publication has an ambition of synthesising the literature, and providing a perspective, on production of biogas from algae.

The rationale for producing biogas from algae is driven by the food-fuel debate and indirect land use change (ILUC). The ethics in using our finite resources of arable land (0.2ha of arable land per head of population on a worldwide basis) for energy and not for food is dubious. Algae take bioenergy off agriculture land and onto our seas and oceans. Seaweed can be used to clean nutrient enriched water (associated with salmon farms for example) while microalgae may capture CO2 from power plants.

There are numerous species of seaweed that may be segregated or distinguished in a number of ways; for example colour. The genetic difference between green seaweed Ulva lactuca and the brown seaweed Fucus is larger than that between U.lactuca and an oak tree. U.lactuca contains a lot of sulphur and typically has a carbon to nitrogen (C:N) ratio of less than 10, making mono-digestion extremely difficult. This is not the case for brown seaweeds such as laminaria; typically the C:N ratio and the corresponding specific biomethane yield increases from winter to summer and achieves a maximum C:N ratio of over 20 in late summer. Seaweed may be collected as a residue (such as the algae bloom associated with the green seaweed U.lactuca); may be cast on beaches (such as Fucus sp. and Ascophylum nodosum) or may be cultivated in aquaculture systems (such as growing Laminaria sp. in association with salmon farms). A sustainable significant biofuel industry would probably require the scale associated with aquaculture. The economics of a seaweed biofuel industry are dubious as certain seaweeds are used for food and have high economic value. The authors believe that biogas from cast seaweed will have applications in the short term, however the quantities of seaweed required to match a significant portion of renewable energy are very large and it is as yet unknown as to how this can be achieved in a sustainable manner.

There are also numerous species of microalgae. Cultivation may take place in open ponds (which are open to contamination) or in closed photobioreactors (which are more expensive in terms of energy input and financial investment and operation). The C:N ratio tends to be lower than for seaweed, but the composition varies greatly from species to species and depends on the growing conditions and the availability of nutrients. For biodiesel production the ambition is to maximise lipid production for esterification. Lipids also yield high levels of biogas but microalgae with excess levels of lipids are not amenable to stable anaerobic digestion. The big advantage of anaerobically digesting microalgae is that neither a pure culture is needed, nor a specific compound (e.g. lipids for biodiesel) needs to be produced. Both these advantages can significantly reduce the costs of producing microalgae biomass. Microalgae may be used to capture CO2 produced by power plants. The microalgae may be digested to produce biogas; this however releases the CO2 when combusted. Therefore the benefit of capturing the CO2 from fossil fuel power plants is more in extending the work done by the original fossil fuel rather than sequestering the CO2. The scale of raceway ponds or photobioreactors for significant carbon capture is very large. The energy input in mixing, harvesting and conversion of microalgae to biogas is very significant and may be of a scale that more energy is used in the process than generated in the biogas. A microalgal biogas industry is far from commercialisation. Innovation is required in optimising microalgae systems. Ideally they should be cultivated, capturing the CO2 from renewable energy such as biogas facilities thereby reducing the need for biogas upgrading and thus improving the net energy return. Currently, the microalgae industry is focussed on high value products to offset high production costs. A more economic approach to producing biogas from microalgae is a cascade usage in a biorefinery concept: a high value product will yield the most significant revenue whereas the biomass residue would be transformed into biogas.

The authors consider that a viable seaweed or microalgae biofuel or biogas industry is a number of years away from providing significant quantities of renewable energy and much research is required in optimising prospective algal biogas systems.

IEA Bioenergy Task 37 addresses the challenges related to the economic and environmental sustainability of biogas production and utilisation. IEA Bioenergy is one of 40 currently active Implementing Agreements within the International Energy Agency and has the aim of improving cooperation and information exchange between countries that have national programmes in bioenergy research, development and deployment. IEA Bioenergy’s vision is to achieve a substantial bioenergy contribution to future global energy demands by accelerating the production and use of environmentally sound, socially accepted and cost-competitive bioenergy on a sustainable basis, thus ensuring increased security of supply whilst reducing greenhouse gas emissions from energy use.