Lifecycle Analysis of the Nutrient Cycle of Biofuels
If Canada or any other nation is to address its long-term transportation energy needs by massively expanding the use of biofuels, we must be sure that such an approach is truly sustainable. There are two main routes for biofuels: biological (such as fermentation of corn to produce ethanol) and thermochemical (such as the gasification of biomass to produce syngas). Each technique has its own strengths and weaknesses.
Gasification is the primary technology used in the thermochemical route, in which pulverized biomass is gasified at high temperatures into syngas, which contains H2, CO, and wastes such as CO2 and H2O. After cleaning, the H2 and CO can then be combined into more useful energy products such as diesel, gasoline, methanol, dimethyl ether, fuel-grade hydrogen gas, or burned for electricity. Since biomass is a renewable fuel, this process has very low net CO2 emissions and can even have net negative CO2 emissions if CO2 capture and sequestration techniques are employed.
However, many gasifiers (such as the entrained downward-flow variety) produce a solid waste called slag, containing the non-volatile components of the biomass (primarily metals in their oxide form). Currently, this is primarily used as an ingredient in asphalt. Although the carbon (i.e. CO2) in the biofuel is renewable since it perpetually cycles between plant and atmosphere, the metals (valuable plant nutrients such as phosphorus, iron, magnesium, etc.) are not returned to the earth in a useable form since they are sequestered in the slag. As a result, any massive effort to use slagging gasifiers to extract energy from biomass for any purpose will necessarily result in the gradual depletion of soil nutrients. These can be replaced with fertilizers which will then have some other affect, such as increased algal blooms, depletion of some other mineral resource, etc.
The question, then, is how much of an effect will this be? Is this a meaningful amount of soil nutrition being depleted from the biosphere, or is it vanishingly small such that there is no real concern? To answer these questions, a life cycle analysis on a biomass-to-liquids process must be performed which considers all of the cradle-to-the-grave effects and their secondary impacts on other contributions. For example, the increased amounts of processing for fertilizers in the long run may cause increased CO2 emissions, pollution, or resource depletion which could potentially offset the environmental gains achieved by a thermochemical biomass-to-liquids approach to our energy infrastructure.
