What are the capabilities of bioenergy technologies that make them unique as a means of reducing greenhouse gases?
Bioenergy comes from the processing of biomass. Biomass "refers to living and recently dead biological material that can be used as fuel or for industrial production. Most commonly, biomass refers to plant matter grown for use as biofuel, but it also includes plant or animal matter used for production of fibres, chemicals or heat." (Wikipedia)
The carbon cycle is the sustained transfer of carbon between the atmosphere, geosphere, and hydrosphere and then back again. The current concern about fossil fuels is that they are corrupting the carbon cycle by adding below ground carbon to the above ground cycle. NASA estimates the worldwide amount of fossil fuel carbon added to the atmosphere each year to be a colossal 5.5 billion metric tons (see red graphic above). These are the dread greenhouse gases blamed for global warming. This addition of subterranean carbon to the atmosphere is termed "carbon positive."
It is possible to use fossil fuels in the production of electricity, capture most of the carbon dioxide from the process, and inject it back into the ground. This is called carbon geo-sequestration and it is used primarily to increase oil production by using gas pressure to force deposits of oil to the surface. However, most carbon dioxide from fossil resources - car emissions for example - can't be captured and returned underground (see orange graphic representation above).
One characteristic of renewable energy is that they are, by definition, "carbon neutral" (see gray graphic) - neither adding nor reducing the amount of above carbon used in the carbon cycle. Most forms of renewable energy - solar, wind, hydroelectric, tidal, and geothermal - don't involve the use or conversion of carbon at all.
In contrast to other renewable energy processes, bioenergy involves the conversion and combustion of the carbon content of biomass. Through photosynthesis carbon dioxide is pulled out of the air and stored as sugars (see green cycle above). As long as the carbon stays in the plant or in plant products, the biomass is essentially a carbon sink, temporarily removing the carbon out of the carbon cycle. Through roots, which are not harvested, a good portion of each plant's carbon capture is left in the ground.
If the carbon emissions from combustion are captured and injected into oil fields, then this would be "carbon negative." Unlike fossil fuel emissions, any emissions from biofuel is "carbon neutral" because the carbon was derived from carbon already converted into sugars and cellulose in the biomass. The net effect of bioenergy is at worst carbon neutral but, through best practices of the forest products industry, often carbon negative.
One other option for subterranean sequestration is being researched by agronomists. There is an ancient practice of using the char from burned biomass as a fertilizer or soil amendment for growing crops. This is variously termed terra preta, biochar, or agrichar.
A recent article in the Biopact Blog discusses the four generations of biofuels. It describes recent research into ways to maximize the "carbon negative" impact of bioenergy:
In fourth generation production systems, biomass crops are seen as efficient 'carbon capturing' machines that take CO2 out of the atmosphere and lock it up in their branches, trunks and leaves. The carbon-rich biomass is then converted into fuel and gases by means of second generation techniques. Crucially, before, during or after the bioconversion process, the carbon dioxide is captured by utilizing so-called pre-combustion, oxyfuel or post-combustion processes. The greenhouse gas is then geosequestered - stored in depleted oil and gas fields, in unmineable coal seams or in saline aquifers, where it stays locked up for hundreds, possibly thousands of years.
According to scientists who looked at this concept of 'bio-energy with carbon storage' (BECS) within the context of a strategy to counter 'abrupt climate change', these systems, if applied on a global scale, can take us back to pre-industrial levels of atmospheric CO2. The concept would be more efficient than techniques that are limited to scrubbing CO2 out of the atmosphere without tackling the source of the problem: the combustion of fossil fuels. BECS intervenes at the source and replaces fossil fuels with renewable biomass; the systems scrub CO2 out of the atmosphere while delivering clean energy. As such, they are seen as one of the only low-risk geo-engineering methods that could help us tackle climate change without powering down our societies.
In summary, Bioenergy is the only renewable energy technology that can reduce greenhouse gases.
This article is the second in a series of five about the unique capabilities of bioenergy.
On November 28th I made a presentation at the Capitol Hill Club to the Biomass Coordinating Council (BCC) of the American Council on Renewable Energy (ACORE) titled "BioEnergy Can Do." My aim was to list what I considered to be the top five unique capabilities of bioenergy that should drive legislative action on Capitol Hill. The five capabilities are:
1. Bioenergy can convert solar energy into liquid fuel.
2. Bioenergy can reduce greenhouse gas emissions.
3. Bioenergy can remediate ecological disasters.
4. Bioenergy can revive depressed economies.
5. Bioenergy can expand energy freedom of choice.
technorati BIOconversion, bioenergy, biofuels, carbon, sequestration