The Charcoal Vision: Producing Bioenergy while Simultaneously Enhancing Soil and Water Quality, Permanently Sequestering Carbon

Tom Miles

The Charcoal Vision: Producing Bioenergy while Simultaneously Enhancing Soil and Water Quality and Permanently Sequestering Carbon
David Laird, Science Magazine, August 30, 2007

Interpretive Summary: The US is rapidly pursuing development of a cellulosic ethanol industry. This strategy is of concern to agricultural scientists, farmers, and conservationists because harvesting biomass crops will have an adverse impact on soil and water quality. This report describes the Charcoal Vision, which is a scenario for processing biomass by pyrolysis to generate bio-oil and charcoal. The bio-oil could be used to offset fossil fuel oil and the charcoal could be returned to the soil from which the biomass was harvested. Returning the charcoal co-product of pyrolysis to the soil is anticipated to build soil quality, increase agricultural productivity, and improve water quality. National deployment of the Charcoal Vision could generate enough bio-oil to meet 25% of the current US consumption of fossil fuel oil. The scenario would simultaneously reduce net US emissions of carbon dioxide to the atmosphere by about 10%. This report will help policy makers develop strategies that simultenously benefit energy security, global change, environmental quality, and rural economies.

Technical Abstract: Processing biomass through a distributed network of fast pyrolyzers has many advantages relative to the cellulosic ethanol platform. Fast pyrolyzers thermally transform biomass into bio-oil, syngas, and charcoal. The syngas can be used to provide the energy needs of the pyrolyzer. Bio-oil is an energy raw material (17.0 MJ/kg) that can be burned to generate heat or electricity or shipped to a refinery for processing into transportation fuels. Charcoal should be returning the charcoal to the soils from which the biomass was harvested. Application of charcoal to soils is hypothesized to do several positive things for soils, including; supply nutrients, increase bioavailable water, build soil organic matter, enhance nutrient cycling, lower the bulk density, and act as a liming agent. Application of charcoal to soils is also anticipated to reduce the leaching of pesticides and nutrients to surface and ground water. The half-life of carbon (C) in soil charcoal is in excess of 1,000 years. This means that soil-applied charcoal will make both a lasting contribution to soil quality and the C in the charcoal will be removed from the atmosphere and sequestered in the soil for millennia. Assuming the U.S. can annually produce 1.1x10^9 Mg of biomass from harvestable forest and crop lands, then, national implementation of the Charcoal Vision would generate enough bio-oil to displace 1.91 billion barrels of fossil fuel oil per year or about 25% of the current U.S. annual oil consumption and thus offset 234 Tg of fossil fuel C emissions to the atmosphere per year. Furthermore, assuming that fixed C in the char is not biologically degraded, application of char to soils would sequester 139 Tg of C per year. The combined C credit for fossil fuel displacement and permanent sequestration, 373 Tg per year, is 10% of the average annual U.S. emissions of CO2-C.