Turning Biomass into Power or Fuel

This morning I ran across a news item indicating that Dow Chemical was installing a biomass cogeneration unit at its facility in Aratu, Brazil to provide process steam with minimal greenhouse gas emissions. It's a good example of another way to convert biomass into energy that hasn't attracted nearly as much interest as advanced biofuels have. That's somewhat surprising, since biomass power shares most of the logistical limitations but few of the technical challenges that have made the production of biofuel from non-food biomass so difficult. Perhaps the relative neglect of biomass power results more from motivation than outcomes.

I'm sure I paid more attention to this story because of Dow's choice of eucalyptus as the biomass source. I grew up under the spreading limbs of a giant eucalyptus tree in California--limbs that periodically fell off in storms, including a 9-ton monster that practically cut our house in half. In the years before that tree was finally cut down I raked up enormous quantities of the eucalyptus leaves and nuts that bombarded our yard. It would be fair to say that I developed a strong distaste for the species, at least for the ornamental and wind-break purposes for which many Californians had chosen this Australian import. However, many of these same features, including its fast growth and dense, oily wood, seem to be good attributes for biomass supply.

As noted in a recent Wall St. Journal article, the Achilles heel of biomass power is logistics. The lower the energy density of the biomass, relative to the fossil fuels it is intended to replace, the closer the source must be to the facility where it will be used, before transportation erodes any cost benefits, even after considering emissions reductions. Wood chips provide about 2/3 as much energy per pound as bituminous coal, but they can take up more than six times as much volume, unless they are first dried and turned into pellets. As is the case for cellulosic biofuels, these supply-chain considerations limit the scale of biomass power application and impose an additional constraint of sustainability: It doesn't pay to build a biomass power plant (or a cellulosic biofuel plant) unless you can be sure of a long-term supply of the raw material. The Journal article included examples of projects that paid a high price for miscalculations in this regard. One strategy for mitigating this limitation is co-firing, which relies on biomass for only a portion of a power plant's fuel needs.

The lower energy density of biomass also makes it essential to extract as much energy as possible from each pound or cubic foot. One of the reasons for the high efficiency of the Brazilian ethanol industry is that many of its mills turn the bagasse, the waste left over after extracting the juice from sugar cane, into process heat and power and need little or no fossil energy. Burning biomass in a high-efficiency combined heat and power application, as the Dow project appears to do--based on the scant information I could find--provides another way to get the most bang for the biomass buck.

That brings us back to motivation. One of the main justifications for the pursuit of cellulosic biofuels is that we have relatively few practical, cost-effective alternative fuels that could replace more than a small fraction of our petroleum use. On the other hand, we have many ways to generate electricity, including more than a few that emit little or no greenhouse gas, one of the main benefits of biomass power--though this point is not without controversy. However, I can't help wondering whether in the long run the best way to turn non-food biomass into energy for vehicles is to turn it into electricity first, rather than working so hard to break down plant structures that have evolved over millions of years to resist easy conversion into chemical energy. Resolving that dilemma depends on a lot more than engineering considerations, however, since we still don't know much about how consumer preferences will play into it. In the meantime, projects like Dow's provide another option for reducing emissions from facilities that must meet increasingly stringent sustainability criteria.