Marine Estate Research Report - The Potential of Marine Biomass for Anaerobic Biogas Production

Concern over greenhouse gas emissions forcing climate change and dwindling oil reserves has focused debate and research effort on finding alternative sources of energy. Scotland has the capacity to generate much, or all, of its electrical energy needs from wind and hydropower and has the potential for offshore energy schemes generating from wind, waves and tidal streams. The route map to generating alternative transport fuels is less well defined. A relative shortage of good agricultural land, high rainfall and a low number of sunshine hours means there is little potential for producing biofuel (bioethanol or biodiesel) crops.

The Royal Commission on Environmental Pollution (RCEP) (2004a) however concluded that terrestrial biomass should play an important role in the renewable energy generation mixture. When energy crops are used as fuel the carbon does not contribute to net greenhouse gas emissions. Unlike most other renewable energy sources, biomass can be stored and used on demand to give controllable energy and is therefore free from the problem of intermittency, a particular problem for wind power. Also, unlike most other sources of renewable energy, biomass offers a source of heat as well as electricity. In fact in the RCEP (2004a) review, biomass is considered solely as a source of heat and electricity and not as a potential source of transport fuel; the RCEP report considers that there are three types of indigenous biomass only, forestry materials, energy crops e.g. willow and miscanthus, and agricultural residues. However the current document reviews the potential of another type of biomass, marine biomass, which has the additional benefit that it can be anaerobically digested to produce methane which, in turn, can be used to generate electricity, for heat or for transport.

Marine algae offer a vast renewable energy source for countries around the world that have a suitable coastline available. They are already farmed on a massive scale in the Far East and to a much lesser extent in Europe, primarily in France, and on a research scale in Scotland. Utilising marine as opposed to terrestrial biomass for energy production circumvents the problem of switching agricultural land from food to fuel production. In addition, the production of marine biomass will not be limited by ix freshwater supplies, another of the contentious issues of increasing terrestrial biofuel production.

Various forms of terrestrial biomass are routinely used as feedstock in anaerobic digesters for the production of methane. In the 1970‟s and 1980‟s researchers in the US began to investigate the potential of marine biomass (seaweeds), as opposed to terrestrial biomass, as a feedstock for methane production. These studies still provide much relevant data for the assessment of the industrial production of methane from marine macroalgae and showed that marine algae are as good a feedstock for anaerobic digestion (AD) processes as terrestrial sources. Marine algae contain no lignin and little cellulose, demonstrate high conversion efficiencies, rapid conversion rates and good process stability. The residues are suitable for use as nutrient supplements for agriculture.

If marine biomass is a serious contender for supplying even a small percentage of our energy needs and if these seaweeds are to be cultured, rather than harvested from the wild, then it has to be accepted that a larger portion of the seas will be „farmed‟. While culture operations must be subject to their own environmental impact assessment, seaweed farms offer the possibility of increasing local biodiversity as well as removing a proportion of the nutrients which can lead to eutrophication. There is the potential to improve biomass yield and quality through selective plant breeding and for further mechanisation of the culturing process to streamline production and reduce labour costs. Before Scotland can seriously assess the potential of marine biomass there is a need to establish larger (hectare or more) pilot-scale farms both to learn how to manage such systems and to better understand the limits on productivity.

This report describes the anaerobic digestion (AD) process (Section 1), reviews the historical harvesting and present production methods of seaweed biomass (Section 2), its conversion to methane (and to a lesser extent ethanol) (Section 3) and the options for wild harvest versus culture in a UK and Scottish context (Section 4). A number of case studies have been used to exemplify the current state-of-the-art in AD and possibilities for energy production (Section 5) and an attempt has been made to forecast the macroalgal biomass required to produce a similar methane x yield equivalent to one of the given examples, the South Shropshire Biogas facility. While Section 5.3 does include some projected figures on methane production, energy obtainable, nitrogen availability and the costs of farming, this is largely conjecture and it would be useful to obtain hard data from scale field trials. The report includes 27 recommendations for future work, including the need for practical, development and demonstration projects to carry forward some of the concepts and the need for a government/industry forum to launch the concept (Section 6).

The further research recommendations can be categorised as those relating to 1) obtaining the seaweed biomass, 2) then optimising the methane (or other energy carrying) output from that biomass and 3) the economic aspects of installing the infrastructure required to farm at sea and to process the biomass and the socioeconomics of large scale seaweed farms. As many of the factors in the first of these two categories will influence the last one, the emphasis in this report is on the research needs behind obtaining the biomass and optimising the methane output.