A simpler method of energy recovery may be facilitated by anaerobic di­gestion of algal biomass providing a promising source of bio-energy in the form of biogas. The process was considered a potential source of useful energy recovery from algal cultivation near the start of modern research [2]. Anaerobic digestion is a process that has been used for hundreds of years to provide a source of energy from low value organic matter with minor energetic inputs. In the case of algal biomass, all the carbohydrates, proteins and fats can be converted into methane and carbon dioxide, al­though some components provide greater methane yields than others. It follows therefore that there is slightly less necessity to cultivate particular strains of algae for increased yields.

Table 6, taken from a study by Sialve et al. [85], displays the methane potential of each biomass component. Research has been conducted inves­tigating the potential of various strains of algal biomass and Sialve et al. [85] used the methane potential to calculate yields for a number of strains. Their results can be viewed in Table 7 which compares theoretical results with experimental results from literature. Table 7 suggests that the values of methane yield can vary between species due to compositional make-up and that the yield depends very much upon the growth conditions as this can have a great impact upon the composition of the biomass. Comparing the actual yields with the theoretical yields shows a realistic conversion efficiency loss of about 50% in the majority of cases.

It is important therefore that in further studies investigating potential yields, exaggerated or over-optimistic yields are not used as these may not reflect real performance.

As opposed to direct conversion, anaerobic digestion can alternatively be used to recover energy from the waste biomass following extraction of the more valuable components from the biomass cells. In their life-cycle assessment, Lardon et al. [77] calculated that the only feasible way of pro­ducing a positive energy balance of algal biodiesel was to recover further energy using anaerobic digestion of the residual waste. In fact, in normal culture conditions, they found that the energy produced from anaerobic digestion would be greater than that from extracted biodiesel. In their in­vestigation of biogas from algae, Sialve et al. [85] suggest that at lipid contents below 40% it is unlikely to be worth recovering the lipids using current methods and the biomass should simply be digested to recover the maximum energy yield. In their LCA study of algae digestion Collet et al. [86] found the environmental impacts of biogas from algae to be poor in comparison to algal biodiesel using results from the study conducted previously by Lardon et al. [77]. The study compared the results for 1MJ of energy produced in a combustion engine. The difference in impacts was mainly due to electricity consumption and assuming anaerobic di­gestion is also applied following the biodiesel extraction in the biodiesel scenario. The figures used in the mentioned study provided high values of energy consumption which contrast with those used in other studies [87], the impacts may therefore not be as adverse as suggested. Collet et al. [86] concluded that the impacts can be improved with reduced energy consumption and a combined process of lipid extraction and anaerobic digestion may provide the optimal solution.

The biogas produced through anaerobic digestion differs from bio­diesel and bio-ethanol in that it is not a fuel that can be used directly for combustion in vehicle engines. There are two options for biogas, one is combustion within a co-generator to produce electricity with possible heat recovery. The alternative is to refine the biogas removing the CO2 and the methane can then be used as a fuel within a gas engine [90]. Fur­ther energy is required to upgrade the gas to a useable transport fuel and this is often ignored in studies with the energetic content of the gas is only considered. Further research is necessary to investigate the impact of downstream processing if comparison to the alternative biofuel types as a transport fuel is desired.

Anaerobic digestion is one of the methods of recovering energy that seems to provide a positive net energy balance due to the low inputs re­quired [77]. The results may however be optimistic as real yields are much

TABLE 7: Theoretical and actual methane from different algal species. Algae Species Proteins (%) Lipids (%) Carbohydrates(%) CH, (L/g VS) (Theoretical) [85] CH4 (L/g VS) (Experimental) Refs. Euglena gracilis 39-61 14-20 14-18 0.52-0.8 — [85] Chlamydomonas reinhardtii 48 21 17 0.69 0.59 [88] C Morelia pyrenoidosa 57 2 26 0.8 0.17-0.32 (CMorella — Scenedesmus) [2] CMorella vulgaris 51-58 14-22 12-17 0.63-0.79 0.24 [89] Dunaliella salina 57 6 32 0.68-0.74 0.44-0.45 (.Dunaliella) [85] Spirulina maxima 60-71 6-7 13-16 0.63-0.74 0.31-0.32 (jSpirulina) [85] Spirulina platensis 46-63 4-9 8-14 0.47-0.69 0.31-0.32 (jSpirulina) [85] Scenedesmus obliquus 50-56 12-14 10-17 0.59-0.69 0.17-0.32 (CMorella — Scenedesmus) [2]

lower than theoretical calculated yields. Additionally the biogas may re­quire further processing to be useful as a fuel and this will affect the en­ergy consumption and environmental impacts. Nevertheless the process is capable of recovering energy from all strains of algae regardless of the composition and therefore can be very useful as part of a flexible approach.

2.3 LIMITATIONS

Despite having been researched for over 50 years now, there are still only a few companies that are growing algae for fuel on a large or commercial scale. The economics of producing algae for fuel do not currently justify the intensity of the numerous processing stages and current practicalities. Cultivating algae with high productivity year round is a challenging task unless grown in controlled conditions, however, this itself, is not a vi­able solution. Attempts have been made to cultivate pure strains of algae in environmental conditions but with little success. In most cases local strains of algae come to dominate, out-competing the selected strain. This section reviews the limitations currently facing biofuel production from algal feedstock.