Energy Fuels 2009, 23, 6275–6276 Published on Web 11/03/2009
: DOI:10.1021/ef9010912
Comments on “Thermochemical Catalytic Liquefaction of the Marine Microalgae Duanaliella tertiolecta and Characterization of Bio-oils” by Zou et al. Robert S. Weber* Sunrise Ridge Algae, 211 Seaton Glen, Suite 109, Houston, Texas 77094 Received September 25, 2009. Revised Manuscript Received October 26, 2009 In a recent publication in this journal, Zou et al.,1 described the use of acid-catalyzed alcoholysis as a means of converting biomass derived from an alga, Duanaliella tertiolecta, into a liquid that might serve as a feedstock for producing renewable transportation fuels. Their systematic investigation of process conditions resulted in identification of combinations of temperature, reagent ratios, and reaction time that converted, impressively, >95% of the original biomass into a combination of light and heavy bio-oils. Zou et al. reported neither a mass balance nor an energy balance on the conversion. Both are critical to the evaluation of a candidate process for producing a major commodity, such as fuel. Regrettably, a mass balance cannot be inferred directly from the information presented in the paper. The mass fraction of oxygen in the bio-oil is less than that in the starting algae and cannot be accommodated through the reaction stoichiometry of simple transetherification, implying that the alcoholysis involved not just depolymerization, by insertion of ethylene glycol, but also a significant amount of dehydration or decarboxylation. Neither did the authors report the energy content of the starting Duanaliella, but fortunately, that quantity can be estimated from the reported protein, lipid, and carbohydrate composition of the algae, as shown in Table 1. For comparison, Table 1 also presents measured and mass-fractionweighted estimates of the heating values of other algae to lend credence to this method of estimation. It will be seen that the estimated heating value of the Duanaliella alga is similar to that of other algae, viz., about 20 MJ/kg. Zou et al. reported that they were able to convert the algae nearly quantitatively into a bio-oil, whose specific heating value was 28.4 MJ/kg, i.e., nearly 50% greater than the estimated specific heating value of the starting algae. Because alcoholysis of biomass is an exothermic process,2 to be consistent with energy conservation, it therefore appears that some combustible moieties must have been incorporated into the product or some noncombustible moieties must have been removed. The composition and specific heating value of the bio-oil are in good accordance with the functional form developed 60 years ago by Spoehr and Milner,3 which relates the heat of combustion of biomass-derived materials to a quantity they call R, an approximation of the degree of reduction
of the material R ¼
ð1Þ
where C, H, and O are the mass fractions of carbon, hydrogen, and oxygen, expressed as percentages. There is a nearly linear relationship between R and the specific heat of combustion of a material (Figure 1), and the starting materials and the products fall close to the overall trend line. However, this Table 1. Heating Values of D. tertiolecta, Other Algae, and Algae Componentsa material
mass fraction
1
Duanaliella protein4 lipid4 carbohydrate4 estimated ΔHcomb measured ΔHcomb of bio-oil Chlorella sp.4 protein lipid carbohydrate estimated ΔHcomb measured ΔHcomb Scenedesmus and Chlorella5 protein lipid carbohydrate estimated ΔHcomb measured ΔHcomb
0.6132 0.0287 0.2169
DHcomb (kJ mol-1) 19.5 23.8 38.9 17.5 19.5 28.4
0.55 0.075 0.178
23.8 38.9 17.5 19.1 19.5
0.59 0.19 0.13
23.8 38.9 17.5 23.7 23.0
a The algae were estimated from the mass-weighted heating values of the components.
*To whom correspondence should be addressed. E-mail: bob.weber@ sunrise-ridge.com. (1) Zou, S.; Wu, Y.; Mingde, Y.; Li, C.; Tong, J. Energy Fuels 2009, 23, 3753. (2) Zou, X.; Qin, T.; Huang, L.; Zhang, X.; Yang, Z.; Wang, Y. Energy Fuels 2009, 23, 5213. (3) Spoehr, H. A.; Milner, H. W. Plant Physiol. 1949, 24, 120. (4) Milner, H. W. In Algal Culture from Laboratory to Pilot Plant; Burlew, J. S., Ed.; Carnegie Insitution: Washington, D.C., 1953; p 285. (5) Powell, R. C.; Nevels, E. M.; McDowell, M. E. J. Nutr. 1961, 75, 7. r 2009 American Chemical Society
2:664C þ 7:936H -O 3:989
Figure 1. Correlation by Spoehr and Milner3 between the heat of combustion (higher heating value) and degree of reduction of biomassderived materials. The points labeled with formulas represent two major products of the alcoholysis of D. tertiolecta, methyl hexadecadienoate and benzafuranone. For reference, the graph also includes a point for the solvent ethylene glycol (EG) employed by Zou et al.
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Energy Fuels 2009, 23, 6275–6276
: DOI:10.1021/ef9010912
correlation again only shows that a decrease in the concentration of oxygen in the bio-oil increases its specific heat of combustion. It is therefore still ambiguous how the alcoholysis described by Zou et al. served to beneficiate the bio-oil. In particular, is the alcohol serving as a reagent, in which case the renewable value of the final fuel is diminished, or is the ethylene glycol serving only as a possibly reusable reaction medium? Reviewers of this Comment raised two additional issues that ought to be included in future reports of the conversion of biomass to renewable fuels: (1) It should be clear whether the reported conversions use as-received or ash-free masses in the
conversion calculations. (2) It would also be informative to include the energy supplied by external heaters in the energy balance of the conversion process. Progress in the current wave of attention to the production of energy products from algae, the third wave since the 1950s, requires that all of the tools of reaction and reactor engineering be brought to bear, particularly, material and energy balances that can be used to quantitatively relate the practical value (e.g., energy productivity) to the characteristics of the underlying processes. Revisiting the approximations reviewed here may assist in capturing and, with good fortune, improving the energy yields of future processes.
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