ALTERNATIVE ENERGY:

I've harped about global warming on my science blog over and over again. To summarize, the planet is warming, due to human-caused activities; this warming is having many detrimental effects today, and the situation will probably get worse in the relatively near future.

What can be done about global warming? A short-term goal should focus on road transport, i.e. reducing it or cleaning it up (e.g. via battery-powered vehicles), because it will likely be the largest contributor to transit-based climate warming in the coming decades.

In the long-term, environmentally-friendly sources of energy and energy storage are needed, e.g. solar power, and hydrogen-based energy storage through ammonia borane or hydrazine borane. More exotic sources of energy, probably more useful for personal devices, are also in the works, e.g. from Shewanella or Escherichia coli bacteria.

Can anything be done to produce sustainable energy today? Biofuels, carbon-based fuel derived from plants, may be a stopgap measure towards a more sustainable future.

Note that I am not advocating corn-derived biofuels. Their limitations are well-known.

On the other hand, biofuels derived from cellulose, microalgae, or some other plant show much more promise, although water utilization needs to be considered, especially in water-limited regions. A drawback of using biofuels as combustible energy is that biofuel combustion is polluting.

Abhishek Tiwari (University of Manchester, United Kingdom) and Jeremy Colls (University of Nottingham, United Kingdom) have looked into minimizing pollution from biofuel combustion. The scientists' model shows that combustion of locally-harvested biofuels leads to acidic aerosol (particle and gas) emissions into the atmosphere, but the acidity can be most effectively ameliorated via the gasification of Miscanthus grass.

The scientists' model.

The scientists went to a great deal of effort to look at the big picture in their model, compiled with the aid of standard recommendations, realistic estimates, and data from the current technical literature. They didn't look only at one aspect of biofuels' environmental impact (e.g. energy efficiency).

Rather, they considered the entire history of a batch of biofuels, from planting to final combustion and waste production. The did exclude construction and demolition, because these two variables are of minimal impact, and they typically assumed local (within 50 kilometers) transport of biofuels to the factory.

They considered three methods of mitigating aerosol production, a consequence of acidic emissions and their interaction with ammonia from the crop harvesting sites. All of the mitigation methods are aimed at reducing pollutant production and/or reducing its interactions with the local environment.

One of their mitigation methods is gasification, reacting the biomass at very high temperatures in air. Another method is delaying the harvest of Miscanthus grass from late fall to early spring, facilitating nutrient recycling and ultimately improving fuel quality, at the cost of reduced crop yield.

The remaining mitigation method is increasing the distance from the cropland to the factory. Mitigation results were compared with the pollutants resulting from no mitigation.

Note that this kind of model is extremely difficult to construct. It would be easy for a skeptic to maliciously pick out something from the scientists' model to try to discredit the entire effort.

There are many variables that could be considered; the scientists put much time and thought into generating a realistic model. Estimates are often more realistic than exact values, which may not be the same depending on location, climate, and many other factors.

Highlights of the study.

The scientists found that, unless pollution amelioration is implemented, total pollution is roughly similar across a range of biomass. Combustion of different biofuels generates different pollutants, but one biofuel isn't clearly cleaner than the others.

Is one pollution mitigation strategy more effective than the others? Here, a winner was found: gasification of the plant matter, such as Miscanthus grass.

It reduces acidity, possibly almost entirely, but increases emissions of methane and carbon monoxide (from a factor of 2 to over 12, depending on the scenario), because these molecules are the end result of gasification. Optimizing the combustion process and utilizing special technology may help lower these emissions, but this will cost money in research, development, and implementation.

Gasification may be a realistic pollution mitigator, but more money needs to be shoveled into research aimed at improving the process, keeping pollution considerations in mind. The other mitigation strategies seem to increase pollution around the board, and are less realistic in the long-term.

Implications.

Further expansion of the scientists' model is needed, such as incorporation of regional climatic variables and more detailed chemical reactions. However, the overall conclusions have much merit.

Biomass combustion can be rendered less polluting, in terms of aerosol production, if it is performed via gasification. Given the advantages of gasification over other mitigation approaches aimed at reducing the pollution of biofuel combustion, research money should focus on reducing its cost and enhancing its long-term sustainability.

NOTE: The scientists' research was funded in part by the Engineering and Physical Sciences Research Council (United Kingdom).

for more information:
Tiwary, A., & Colls, J. (2010). Mitigating secondary aerosol generation potentials from biofuel use in the energy sector Science of The Total Environment, 408 (3), 607-616 DOI: 10.1016/j.scitotenv.2009.10.019