Researchers argue that the optimal use of biomass as a renewable resource is for production of select chemicals, not power, heat or fuels
|Total US oil consumption compared to potential and currently harvested nonfood biomass divided into its main uses. The area of each circle is proportional to the consumed amount. Vennestrøm et al. Click to enlarge.|
In an essay presented in the journal Angewandte Chemie, Esben Taarning and co-workers from the catalyst company Haldor Topsøe and the Lindoe Offshore Renewables Center (Denmark) argue that the most efficient use of biomass—which, although renewable is still a limited resource—is for the production of select chemicals, thereby effectively replacing petroleum.
Biomass differs from the other renewable resources, they note, since the energy it contains is stored as chemical bonds. This enables biomass to be used for several purposes apart from electricity and heat generation, such as the production of liquid fuels and chemicals. While to date most of the biomass used by industry has been burned to generate energy, in the long-term neither that use, nor the use of biomass to produce fuels, are optimal, the authors argue.
It is also not the most sensible solution to convert biomass into fuels. In the first place, the amount of biomass available does not meet the demand for fuels; in the second, the chemical characteristics of fuels and biomass are too different, so the processes would be too complex and uneconomical.
In contrast, it really makes sense to use biomass as the feedstock for chemical industry. The available biomass should suffice to replace the fossil feedstocks used in the production of chemicals. The chemical characteristics of biomass and many bulk chemicals are also very similar, so the processes should be more economical than those for the conversion into fuels.—Esben Taarning
Means of transportation should be gradually switched to batteries or fuel cells, says Taarning.
To explore the best utilization of biomass, the team used the effective H/C ratio: the ratio between hydrogen and carbon atoms in the molecule adjusted for heteroatoms.
Transportation fuels have an effective H/C ratio in the range of 1 to 2.3...which is close to the ratio of crude oil. This ratio implies a high energy density, and it is thus ideal for liquid fuel purposes. Commodity chemicals, on the other hand, span a much wider H/C ratio, which is more comparable to that of biomass. A wide gap in the effective H/C ratio between a resource and a target chemical implies that a lengthy process is needed for its conversion.
Sugars, for example, have a similar effective H/C ratio to many functionalized high-value chemicals and should therefore be a more ideal feedstock than fossil resources in some cases. By utilizing biomass as feedstock for the production of chemicals instead of fuels, the necessity for deoxygenation, which is one of the biggest challenges when making fuels from biomass, is partially or completely avoided. Oxygen-rich chemicals such as ethylene glycol, acetic acid, and acrylic acid are examples of chemicals that could be obtained more efficiently from biomass than is possible from fossil resources.
Since oxidation reactions typically involve product loss owing to overoxidation, it would be desirable if these reactions could be avoided, or at least their use minimized, when producing chemicals; an objective which seems simpler to achieve when starting from lignocellulosic biomass. Olefins, on the other hand, have an H/C ratio far from that of biomass, which implies that biomass is a poor starting point. It seems that the vast amounts of olefins produced by the chemical industry today will not be easy to directly replace by biomass-based olefins, and the ideal solution is to develop alternative materials with an effective H/C ratio in the range of 0 to 1, thereby indirectly replacing olefins.—Vennestrøm et al.
Switching to broad use of biomass for chemicals will require divergence from the established value chains, the authors say. Instead of using brute force to convert these raw materials into specific platform chemicals that were originally selected because of their easy accessibility when starting from fossil resources, it would be better to use the interesting chemical characteristics already available in the biomass resources themselves and to optimize the use of favorable catalytic reaction pathways.
Because the development costs will be high and the first processes inefficient, it makes sense to initially concentrate on high-value products, thereby allowing for faster widespread adoption.
Through the clever selection of target chemicals it is possible to significantly increase the value added.—Esben Taarning
Also, many primary products and by-products of the current biofuel industry could be interesting platform chemicals in themselves they note: for example, ethanol as a starting material for the production of acetic acid, ethylene, and ethylene glycol, or glycerol for conversion into acrylic acid, a polymer precursor.
The shift from a fossil-based chemical industry to one based on biomass poses many challenges, but the possibilities are also great: to develop a more sustainable chemical industry utilizing a more versatile feedstock supply and producing products with superior properties.—Esben Taarning
P. N. R. Vennestrøm, C. M. Osmundsen, C. H. Christensen, and Esben Taarning (2011) Beyond Petrochemicals: The Renewable Chemicals Industry. Angewandte Chemie International Edition 50, No. 45, 10502–10509 DOI: 10.1002/anie.201102117