Does seaweed offer a solution for bioenergy with biological carbon capture and storage?

Abstract

The concept of combining bioenergy with carbon capture and storage (BECCS) has been identified as one mechanism to achieve energy production with a net negative atmospheric carbon emission. Conventional geological carbon capture and storage involves large capital outlay and recurring expense. Therefore there is increased interest in coupling biological carbon capture and sequestration with biomass production. Systems such as low-input high-diversity native grasslands and biochar production have been heralded as carbon negative biofuels. Another potential production system for bioenergy with biological carbon capture and storage (BEBCCS) is the large-scale cultivation of seaweed (macroalgae) for biofuels. Marine biofuels offer considerable advantages over terrestrial first-generation biofuels in terms of their low land use, fresh water and fertilizer requirements. In addition, they also offer considerable potential in the field of BEBCCS. Macroalgae rapidly accumulate biomass and as part of this process, a significant proportion (estimates range up to 60%) of the carbon they fix photosynthetically is released into the water. A proportion of this released dissolved organic carbon (DOC) is highly labile and enters the bacterial loop and is rapidly remineralized back to CO₂. However, a proportion is known to be resistant to biological degradation and enters the refractory DOC pool. The marine DOC pool is the largest organic carbon pool on the planet and the refractory component is known to have a turnover period of 100s-1000s years. The release of refractory DOC from macroalgae combined with biofuel production offers the potential for large scale carbon negative fuel production. © 2012 Society of Chemical Industry and John Wiley & Sons, Ltd

Introduction

As governments search for a political solution to controlling atmospheric carbon dioxide concentrations¹ there is an increasing realization that the target of limiting average global temperature to a 2 °C rise will not be made solely through reducing emissions of greenhouse gases (GHGs) but will require the development of negative emissions technology to recapture some of the already released emissions. It has been estimated that to stabilize CO₂ at 400 ppm, the total carbon sequestration will need to be in the order of 120–500 GtC.2,3 One such negative emissions technology is the concept of combining energy production from biomass with carbon capture and storage – bioenergy with carbon capture (BECCS).4,5 In this system the biomass is converted to energy (either directly from biomass firing or conversion into a carrier such as ethanol) and the CO₂ is captured from the flue gases and stored. However, a number of economic issues have been highlighted with this approach including reduced energy efficiency, higher investment costs and increased production costs.⁶ In light of this and combined with technological and societal issues associated with geological storage of sequestered carbon,⁷ there is a growing interest in the potential of coupling biological carbon capture and sequestration with biomass production⁸ – bioenergy with biological carbon capture and storage (BEBCCS). There are a number of existing technologies that utilize this approach. These include the low-temperature pyrolysis of woody biomass which results in the production of energy and a carbon-rich stable biochar. The biochar is returned to the soil with the effect of sequestering a portion of the carbon therein,⁹ with concomitant improvements in soil water and nutrient retention. Another BEBCCS approach is through the use of low-input high-diversity (LIHD) grassland biomass with the above-ground biomass being used for energy production and a subsequent accumulation of carbon below ground in the roots and soils acting as a carbon sequestration mechanism.¹⁰ BEBECCS systems offer the advantages of BECCS without the additional large expense and risks associated with geological CCS and, in addition, often have associated benefits such as soil improvement in the case of biochar or increased biodiversity for LIHD grassland.

Macroalgae as bioenergy with biological carbon capture and storage

Another potential production system for BEBCCS is the large-scale cultivation of macroalgae for biofuels. Although the concept of macroalgal biofuels has been around since at least the early 1970s,¹¹ its potential as a BEBCSS energy production system has previously remained undescribed. Macroalgal biofuels offer a number of distinct advantages over terrestrial biofuels such as minimal or no freshwater or additional fertilizer requirements, no conflict with existing land use, and a larger potential production area. They also offer significant carbon sequestration potential both during their production and as a result of their energy conversion processes. Macroalgae naturally have very high productivity rates in terms of carbon capture during photosynthesis, with estimates of gross primary productivity of approximately 1600 gCm^–2y^–1.¹² This compares favorably to global net primary productivity of crop land of 470gCm^–2y^–1.¹³ As the seaweeds grow there is a release of photosynthate into the water column as dissolved organic matter (DOM). This released DOM is a complex mixture comprised of mainly carbohydrates entering the oceanic dissolved organic carbon (DOC) pool. The oceanic carbon pool is approximately 600GtC,¹⁴ comparable with the atmospheric pool in terms of size. When the macroalgae release DOC in to the water the majority of that DOC is labile (lDOC) and is rapidly utilized by the marine bacterial community and converted back to inorganic carbon (DIC) through bacterial respiration.¹⁵ However, a proportion of the released DOC is refractory (rDOC) which is resistant to microbiological degradation and will join the oceanic carbon pool. The age of the oceanic rDOC pool has been estimated at 4000–6000 years¹⁶ so carbon entering this pool as a result of macroalgal cultivation can effectively be considered sequestered. It has been proposed that this pathway is an important source of carbon sequestration on geological time scales for DOC produced by phytoplankton.¹⁷ However rDOC production has not been previously posited as a potential carbon sequestration mechanism for climate change abatement when coupled with macroalgal biofuel production.

The composition of the exuded DOC from macroalgae will determine the proportions of DOC that enter either the lDOC pool (to be respired back to DIC) or the rDOC pool. Incubation experiments have shown that DOC exuded from kelp plants is a mixture of carbohydrates and phenols.18–20 The percentage of DOC consisting of carbohydrates varied between 5.5% and 64% while the phenol content varied between 7.5% and 19%. DOC released by macroalgae is relatively refractory when compared to phytoplankton DOC and the phenolic and humic-like components of the released DOC form part of the rDOC pool.²¹ If we use average figures from previous studies we can construct a mass balance model for the carbon flow for a bioenergy system based on the cultivation of macroalgae and conversion to methane through anaerobic digestion. The model can be broken down into three basic components (Fig. 1).

• 1.
  Production of seaweed. The biomass production of cultivated macroalgae of 1 hectare of sea area has been estimated to be in the region of 200t wet weight/hectare (w.w./ha).22,23 This approximates to 20t dry weigh (d.w.)/ha of which 6t/ha is carbon (using values from Gevaert et al.²⁴). As approximately 40% of net primary productivity is lost as DOC,18,19 then a total of 2.4tC is released into the water column per hectare of macroalgal cultivation per year. Of this approximately 15% is refractory18,21 and enters the rDOC pool and is effectively sequestered, totaling 0.36tC. The remainder is respired back to DIC through the microbial loop.¹⁵
• 2.
  The biomass is transferred to the energy conversion facility. In this model it is anaerobic digestion, as this is the conversion technology for which we have the best data. Industrial digestion of brown seaweeds have yielded 11m³ methane per tonne w.w. macroalgae.²⁵ So, for a hectare, this would be a total of 220m³ of methane equating to 110 kg of carbon as methane and given a 60/40% methane/CO₂ mix in the biogas produced,²⁵ 60kgC as CO₂. The remainder of the carbon is retained in the digestate or in the liquor that is produced.
• 3.
  One suggested use for this digestate is as a soil improver. If used as such, a proportion of the digestate will be respired back to CO₂ by microbial action in the soil. However a proportion of this carbon will be refractory and resist biological degradation, and so carbon will effectively become stored in the soils. Kelp species are known to contain humic like substances,²⁶ and it has been estimated that approximately 7% of the dry biomass of some kelp species is refractory fiber²⁷ so another 0.42tC is stored as refractory organic material in the soil.

Figure 1. The carbon flows through a macroalgae to methane bioenergy system – numbers in circles refer to descriptions in text (DIC – dissolved inorganic carbon, OC – organic carbon, DOC – dissolved organic carbon, rDOC – refactory dissolved organic carbon, CH₄ – methane, rOC – refactory organic carbon)

Within this model for every one hectare of the macroalgae that is used to produce biofuels a total of 0.78t of carbon is stored either into the marine rDOC pool or into the soil rOC pool, and 161GJ of energy is generated (using the conversion ratio of 55.7 kJ/g methane). A recent life cycle assessment (LCA) of this production system gave a GHG reduction of 21% compared to fossil fuel use,²⁸ however, these figures were based on the assumption that fuel production was carbon neutral instead of carbon negative as is posited here. It is important to note that this model is poorly constrained and there is a need for empirical research to better define its terms. However the principle that macroalgal cultivation for biofuel production can be a carbon negative biofuel and a system for BEBCCS is robust.

Conclusions

The estimates of total amount of carbon sequestered by macroalgal biofuels are lower than those demonstrated for LIHD grassland (1.2tC/Ha¹⁰). The production figures used in the model presented here is based on an estimate of primary productivity of 6tC/ha/yr, however estimates of natural production are at least an order of magnitude higher²⁹ and there may be considerable scope for increasing the production of the cultivated system. This combined with the much greater availability of space for marine biofuels production compared to the terrestrial environment, then macroalgal biofuels offer a significant potential for carbon capture and storage. In terms of CSS targets, if we use a target of 320 GtC as a convenient mid-point for current estimates of realistic and desirable carbon sequestration,2,3 and give ourselves a generous 100 years to achieve this, and we say that macroalgae cultivation to methane production will account for 10% of this target then we must store 320MtC tonnes/year. If we use our initial estimates 0.78tC/ha/year from macroalgae to methane biofuels, this equates to 80tC/km²/year then we would need to farm a sea area of 4x10⁶km². This equates to a total of 17% of the global territorial sea area or approximately 4% of the global exclusive economic zone (EEZ), and 400 times the area of seas presently under aquaculture production.³⁰ It is also worth noting that in total this would amount to a 5% increase in the global marine rDOC pool. The consequences of this are poorly understood as are other impacts on the global carbon cycle, such as a reduction in phytoplankton productivity from shading and nutrient reduction in areas where the mass cultivation of macroalgae occurred. In addition it is known that rDOC greatly alters the optical properties of coastal seawater through light absorbance³¹ and is known to reduce free ions in seawater,32 and so the ecosystem effects of this would need to be better understood prior to development. Even so, macroalgae to methane BEBCCS offers a real possibility to make a significant contribution to the global need to reduce atmospheric carbon dioxide. However, such is the scale required that this approach may be classified as geo-engineering and would require development of a marine agrarian revolution and a reappraisal of humanity's relationship with the sea.

Acknowledgements

This work received funding from the MASTS pooling initiative (The Marine Alliance for Science and Technology for Scotland), MASTS is funded by the Scottish Funding Council (grant reference HR09011) along with contributing institutions, and from the European Union's Seventh Framework Programme (FP7/2007-2013) project ‘AT∼SEA’ under grant agreement n° 280860. Their support is gratefully acknowledged.

Biographical Information

Adam Hughes

Dr Adam D. Hughes is a lecturer in Sustainable Mariculture at SAMS. His research interests focus on the development and diversification of the aquaculture industry in order to increase economic and environmental sustainability. Included in this is the development of marine biofuels from macroalgae. He is involved in a number of national and international projects on this subject.

Maeve Kelly

Maeve S. Kelly is Principal Investigator in Macroalgal & Invertebrate Biology. Her research focuses on the reproductive and larval biology, growth, nutrition and ecology of commercially important marine invertebrates (sea urchins, abalone, and sea cucumbers) and more recently on the culture of macroalgae (seaweeds) for human food, for bioremediation and as a biofuel. Linking these themes is her interest in integrated aquaculture, and sustainability in marine systems. This includes researching the potential of farming biomass energy crops at sea: macroalgae for anaerobic digestion to biogas or methane.

Kenny Black

Dr Kenneth Black has been a researcher in Marine Science since 1991. He is a member of the Scottish Government Working Group on Aquaculture and an accredited PRINCE2 Practitioner. His current research interests include: sustainability, energy, food, society; integration of physical and biological models relating to disease and parasite management in aquaculture; recovery processes in fish farm sediments; indicators and models of pollution from aquaculture; macroalgae as a source of biofuel; and added value products.

Iona Campbell

Iona Campbell has an undergraduate degree in Marine and Freshwater Biology from the University of Glasgow. She has done research for Cornell University, New York studying shellfish aquaculture as a tool for coastal restoration. She is interested in the exploration of sustainable aquaculture practices and is currently a PhD research candidate at the Scottish Association for Marine Science of University of the Highlands and Islands. Current research interests include: harmful algal blooms relating to the shellfish industry, and seaweed cultivation as a viable biomass source for renewable biofuel.

Dr Keith Davidson

Keith Davidson is a marine microbial biogeochemist interested in coastal and harmful phytoplankton, trophic transfer of carbon and nitrogen within the marine microbial food web, and mathematical modelling. He is head of the department of Microbial and Molecular Biology at SAMS, a fellow of the Society of Biology and the senior scientist for the regulatory monitoring programme for biotoxin producing phytoplankton in Scottish waters.

Dr Michele Stanley

Dr Michele Stanley is a Principal Investigator in Microalgal Molecular Phycology. She worked on applied phycological projects for more than 12 years initially at the University of Birmingham before taking her current position in 2006. She is currently establishing a research group focusing on the fundamental aspects of phycology, which have potential commercial applications. Main areas of interest include the potential of algal biomass as a source of renewable energy, the functional genomics of algal biofouling and adhesion, and algal cell wall development.