Transport Biofuels from Aquatic Biomass

Fossil fuel inputs in producing microalgae tend to be high. When microalgae are grown in bioreactors, outputs are unlikely to energetically outperform inputs (Wijf- fels 2008; Reijnders 2008). A claim has been made for ultrahigh bioproductivity from algae in thin channel ultradense culture bioreactors indirectly irradiated by the sun (Gordon and Polle 2007). The cultures are irradiated with pulsed light emitting diodes, powered by photovoltaic cells. The efficiency of converting solar radiation into biomass is probably below 0.2%, and the corresponding energetic yield is likely to be exceeded by fossil fuel inputs (Wijffels 2008).

As to producing microalgal biofuels in open ponds, it is a remarkable aspect of several recent publications strongly advocating algal transport biofuels (e. g. Chisti 2007; Huntley and Redalje 2007; Chisti 2008a; Dismukes et al. 2008) that inputs of fossil fuels are not addressed. Two less recent studies are available that looked at energy inputs and outputs in open pond cultures of microalgae. They did not take account of all inputs, though. For instance, fossil fuel input into the handling and clean-up of discharges from ponds (which will probably be necessary in view of the extreme pH and/or salt concentrations and high nutrient levels in algal ponds) was considered by neither of the studies. Sawayama et al. (1999) studied operational life cycle energy inputs in growing and processing Dunaliella tertiolecta to sup­ply bio-oil. Processing was by thermal liquefaction (also Yang et al. 2004). Oper­ational energy inputs (fossil fuels) exceeded energetic output by 56% when microal­gal yield was 15 Mg ha-1 year-1. Hirano et al. (1998) studied Spirulina production and processing to supply methanol (via synthesis gas). Here the assumed yield was approximately 110 Mg ha-1 year-1. Both fossil fuel inputs in infrastructure and op­eration were considered. The energetic output exceeded the life cycle fossil fuel input by 10%. At more realistic estimates of Spirulina yield, which are in the order of 10-30 Mgha-1 year-1 (Vonshak and Richmond 1988; Jimenez etal. 2003), fossil fuel inputs would have exceeded energetic outputs. Chisti (2008b) has argued that the energetic inputs used in the studies of Hirano et al. (1998) and Sawayama et al. (1999) are ‘grossly overestimated’. However, even at Chisti’s (2008b) estimate, the fossil fuel input energetically would equal an output of approximately 30 Mg dry weight algal biomass ha-1 year-1, which is at the upper end of the range for the commercial production of Spirulina (Jimenez et al. 2003).

Though experimentally, yields have been demonstrated that may energetically exceed fossil fuel inputs (Hirano et al. 1998; Chisti 2008b), it is far from certain that such yields can be achieved in actual commercial practice. Large differences between experimental yields and average commercial yields are also common in the production of terrestrial crops, as will be explained in Sect. 2.4.1.

A ‘high yield’ has furthermore been claimed for oil from Haematococcus plu — vialis produced by a combination of a closed bioreactor and 1.3 days in a pond (Huntley and Redalje 2007). This yield probably corresponds with a photosynthetic efficiency in producing biomass of just over 1% and a photosynthetic efficiency in producing algal oil of roughly 0.6% (Vasudevan and Briggs 2008). No data have been published about the cumulative energetic inputs in this type of culture, but from the above, it would seem unlikely that the energetic value of algal oil would much exceed the cumulative energy input into the infrastructural and operational inputs.

Studies regarding algal production of H2 suggest that the cumulative energy de­mand for algal H2 production is probably of the same order of magnitude as the energetic output, when the solar energy conversion efficiency does not exceed 1% (Burgess and Fernandez-Velasco 2007).

On the other hand, it may be that the yield of microalgae grown in water satu­rated by CO2 from power stations may exceed fossil fuel inputs when there is no allocation of the fossil fuel input into electricity production to these algae. However, whether this application will actually become operational is unclear, as algal perfor­mance has so far been disappointing, and sequestration of CO2 in abandoned gas and oil fields and aquifers has a higher efficiency (Benemann et al. 2003; Vunjak- Novakovic et al. 2005; Odeh and Cockerill 2008).

The emergence of some saltwater and freshwater macroalgae and macrophytes as pests offers scope for their conversion into transport biofuels. Only for one of the macrophytes (water hyacinth) are data available about the overall energy efficiency of conversion into ethanol. These data suggest a negative energy balance (Gunnars — son and Petersen 2007).

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *