This post reviews work on the anaerobic digestion of biomass to create biogas. In particular it looks at the use of marine sources of biomass - seaweed or macroalgae. Seaweed is seen as superior to other crops as an energy source as it does not compete with arable land for food production purposes. Many species of seaweed and macroalge can be anaerobically digested to create biogas. The level of gas produced compares favourably with other common sources for digestion. The bigoas can be burned in CHP plants or upgraded to biomethane and added to existing natural gas grids or used as a transport fuel. Seaweed can be harvested from the wild or cultivated in farms. As an island nation, Ireland has the potential to produce large quantities of seaweed. With the current price of oil, the price of producing biogas from seaweed may not be economically viable.
Methane is the main component of natural gas - one of the most common and useful fuel sources currently available. Methane can be burned in gas turbines to create electricity or used as a transport fuel. It is estimated that a mature biogas industry could satisfy 10% of Ireland’s energy in transport requirements 1.
Methane can be found in it’s natural state in underground in gas deposits. It is extracted and purified, and typically distributed by pipeline. The existing gas distribution grid and gas consumption technologies are major assets and have a significant role to play in the future world energy scenario.
Burning methane releases carbon dioxide. Although the level of released by burning methane is significantly less than the levels released from coal and oil, it still contributes to global warming. Like other fossil fuels, the deposits of natural gas are finite. If alternative, renewable, carbon neutral sources of methane can be developed, the existing infrastructure could be used to distribute and consume the gas.
One such alternative source is anaerobic digestion - the breakdown of organic matter in an oxygen free environment. Many sources of organic material can be processed in an anaerobic digester, including grass, food waste, manure and slaughterhouse waste. Another potential source of organic matter is seaweed or macroalgae.
A review of the chemistry and microbiology of the anaerobic digestion process is outside the scope of this review, but the process can be summarised as follows: “During the process of anaerobic digestion organic raw materials are converted to biogas, a mixture of carbon dioxide and methane with traces of other constituents, by a consortium of bacteria which are sensitive to or completely inhibited by oxygen” 8 The organic substrate matter is biochemically digested by bacteria. This process produces a quantity of methane and carbon dioxide, along with smaller quantities of other gases. The proportion of to depends on the original organic matter. The process also results in an effluent by-product of materials that cannot be digested. This effluent is high in nitrogen and can be used as a fertilizer.
The level of methane produced from a substrate is generally reported in terms of litres of methane per kilogram of volatile solid (L CH4/kg VS). For comparison, Chynoweth et al. 2 have studied a number of different substrates including seaweeds, grasses and municipal waste.
|Substrate||L CH4/kg VS||Macrocystis seaweed (Giant Kelp)||410</tr>|
|Napiergrass (Elephant Grass)||340|
|Municipal solid waste||220|
Range of biochemical methane potential data for biomass or waste feedstocks 2
Anaerobic Digestion of Seaweed
As seen in Table [substrates], some types of seaweed can produce levels of methane that compare well against other substrates commonly used such as municipal waste and grasses.
The level of methane produced varies widely between different studies. Comparisons across studies can be difficult, as the methodology used can vary considerably.
Vanegas and Bartlett 3 studied five species of seaweed found in Irish waters, co-digesting them with bovine slurry in small batches of 1L or less. The methane produced varied by 55%. Saccharina latissima performed best while Ulva performed worst. All produced worthwhile levels of methane except Fucus serratus, which did not produce enough methane to be worth studying further.
The levels reported for the same seaweed species can also vary considerably between studies. A number of papers studied Ulva or sea lettuce. The maximum level of methane reported from Ulva was 480 CH4/ kg VS while the lowest level was 162 CH4/ kg VS 4. McKennedy and Sherlock find that the methodology behind these studies varies. In some studies, the seaweed is preprocessed. The preprocessing can include washing, drying or chopping to a uniform size. Allen et al. 5 found that washed and dried Ulva had a higher biomethane potential than fresh, wilted or unwashed samples. Preprocessing increases the parasitic energy requirement for the process.
Some studies use an active reactor, while others start a new process. Some studies 3 co-digest seaweed with bovine slurry.
One of the main advantages of using seaweed rather than other crops as a source for biomethane from anaerobic digestion is that it does not use land needed for food production. The seaweed resource can be harvested from the wild or cultivated in farms.
Some species of seaweed are problematic in seas and on beaches. An abundance of Ulva lactuca or sea lettuce can be caused by eutrophication of water by excess nitrogen. This phenomena is commonly known as a Green Tide. The excess seaweed typically washes up on beaches and rots, causing bad smells. The use of this nuisance resource was studied to assess the potential of using it to generate methane from anaerobic digestion 5. It was found to be a viable resource with a significant methane yield of 250 L CH4 /kg VS.
Allen et al. 5 also studied the co-digestion of Ulva lactuca with manure slurry. A number of positive affects were noted, including a 17% increase in biomethane production when compared to mono-digestion of the substrates.
Interestingly, the level of biomethane produced from manure slurry depends on the time of year. Allen et al. 5 found that yields from bovine slurry is higher during the winter. Ulva lactuca could be harvested when plentiful, stored and used in the digester when manure slurry is of lower quality for biomethane production.
The Irish Environmental Protection Agency 6 has identified 7 eutrophic sites around the country that experience ‘elevated nutrient concentrations’ and ‘accelerated growth of plants’. There were a further 9 sites identified as potentially eutrophic. These sites are likely to have a high level of Ulva lactuca that could be harvested for anaerobic digestion. Allen et al. 5 estimate that the biomethane resource available from one of these sites could power 264 cars, each travelling 15,000 km per annum.
Other studies do not consider the harvesting of wild seaweeds as a viable option for biofuel production. Hughes et al. state that ‘it would be impossible to justify harvest on the massive scale necessary to make a significant energy contribution’ due to the potential yield and environmental impacts associated 7. Instead they suggest cultivation of seaweed.
The farming of marine algae is well developed in Asia but in it’s infancy in Europe. Cultivation can provide predictable, quality controlled and relatively simple to harvest crop of seaweed. Seaweeds are typically grown on long tethers or nets. Growing seaweed requires high levels of nitrogen and other nutrients in the water. In Asia the waters are sometimes fertilized artificially @guiry_seaweed_1991, but this may not be viable at the scale required to grow seaweed as a fuel source. Some studies suggest seaweed cultivation and aquaculture as complimentary activities. The seaweed would benefit from the increased nitrogen levels due to fish farming. The scale of seaweed cultivation required to make an impact on energy demand is significant. Hughes et al. 7 estimate that an area 5440km^2^ would need to be cultivated to provide 1% of the United Kingdom’s total energy demand. This is 3% of the UK’s total waters. If the same level of biogas was to be produced from land based crops, they estimate it would require 18% of the UK’s crop land. Hughes et al. consider that cultivation and anaerobic digestion of seaweed has real potential for isolated coastal and island communities, such as the Isle of Mull in Scotland 7.
The price of producing biogas from seaweed is difficult to determine. As there are no known digesters running solely on seaweed, it is impossible to get real world costs. Chynoweth et al. 2 estimated the cost of methane production from seaweed to be $6-13 / GJ. This compares to $6-8 / GJ for grasses and $3-7 / GJ for wood. McKennedy and Sherlock 4 report a payback on investment time of 20 years. With natural gas prices currently low, anaerobic digestion is not viable option if economics are the main concern.
The levels of biogas produced varies significantly between different studies. Anaerobic digestion is a complicated process and biogas output is dependant on many interconnected factors. However, most studies agree that anaerobic digestion of seaweed can generate significant quantities of biogas and compare favourably with land based crops. Seaweed can be co-digested with other substrates, often resulting in an increase in gas production. The technology, while currently underdeveloped, exists and there are no major technical hurdles to overcome.
A huge advantage to biogas as a fuel is the ability to use it with existing technologies and infrastructure. It can be used directly in CHP systems or it can be upgraded to biomethane and used as a transport fuel and in national gas grids.
Coastal and island communities with the relevant skills and resources could benefit from energy independence from biogas production from cultivated and harvested seaweeds. However, generating biogas from seaweed at country scale would require huge areas of coastal waters. At current natural gas prices, anaerobic digestion of seaweed is not economically viable for most areas.