Selasa, 16 Juni 2009

Bioenergy

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The purpose of this paper is to provide the reader with comprehensive knowledge of the biomass energy sector. Biomass is plant matter and animal waste that can be harvested to create bioenergy in the form of electricity, heat, steam and fuels.

Biomass has great potential to contribute considerably more to the renewable energy sector. Already, in the U.S., residues from mill operations are the largest source of biomass for power plants and combined-heat-and-power projects. Photo Credit: NREL biomass research website

Agricultural residues such as orchard prunings and nut hulls as well as forest residues are also important contributors to power plants in combined heat and power (CHP) operations, particularly in California. Landfill gas projects are growing steadily, while animal waste digestion projects and energy crop plantations are still at an early stage of commercialization. [1]

In Europe, urban wood waste is an important source of bioenergy. In developing nations, a major source of biomass is timber cut by the rural poor specifically for heating and cooking. [1]

Biomass Basics and Environmental Impact

Introduction

Biomass is any organic matter, particularly cellulosic or lingo-cellulosic matter, which is available on a renewable or recurring basis, including trees, plants and associated residues; plant fiber; animal wastes; industrial waste; and the paper component of municipal solid waste [2].

Plants store solar energy through photosythesis in cellulose and lignin cells. Cellulose is defined as a polymer, or chain, of 6-carbon sugars; lignin is the substance, or “glue,” that holds the cellulose chain together [2]. When burned, these sugars break down and release energy exothermically, giving off CO2, heat and steam. The byproducts of this reaction can be captured and manipulated to create electricity, commonly called biopower, or fuel known as biofuel. (Both short for "biomass power" and "biomass fuel" respectively) [3].

Biomass is considered to be a replenishable resource—it can be replaced fairly quickly without permanently depleting the Earth’s natural resources. By comparison, fossil fuels such as natural gas and coal require millions of years of natural processes to be produced. Therefore, mining coal and natural gas depletes the Earth’s resources for thousands of generations. Alternatively, biomass can easily be grown or collected, utilized and replaced.

Moreover, using biomass to create energy has positive environmental implications. Carbon dioxide is a naturally occuring gas. Plants collect and store carbon dioxide to aid in the photosynthesis process. As plants or other matter decompose, or natural fires occur, CO2 is released. Before the anthropomorphic discovery of fossil fuels, the carbon dioxide cycle was stable; the same amount that was released was sequestered, but it has since been distrupted. In the past 150 years, the period since the Industrial Revolution, carbon dioxide levels in the atmosphere have risen from around 150 ppm to 330 ppm, and are expected to double before 2050! (please see diagram below)


An overwhelming majority of scientists now link carbon dioxide with rising temperatures in the atmosphere and other issues associated with climate change. Scientists are predicting a rise in average temperature 2-10 degrees Celsius. This change may seem insignificant, but note that the former ice age resulted from an average of 5 degrees Celsius drop in temperature [4]. This small shift in average temperature has huge implications for melting ice sheets, which would raise global water levels up to 30 feet, flooding the coastal cities in which most of the world currently resides. Additionally, more extreme weather patterns are predicted to occur, as well as habitat loss, spread of disease and a whole host of other problems. The amount of CO2 pumped into the atmosphere today will remain for at least a hundred years, since the half life will outlive all of us.

In order to curb CO2 emissions, we must take active strides to reduce our emissions. At present, the United States is responsible for 25% of the world's emissions, and is currently dedicated to a policy which actually encourages the release of more carbon dioxide into the atmosphere, claiming it to be an indication of economic growth. Burning biomass will not solve the currently unbalanced carbon dioxide problem. However, the contribution that biomass could make to the energy sector is still considerable, since it creates less carbon dioxide than its fossil-fuel counterpart. Conceptually, the carbon dioxide produced by biomass when it is burned will be sequestered evenly by plants growing to replace the fuel. In other words, it is a closed cycle which results in net zero impact (see diagram below). Thus, energy derived from biomass does not have the negative environmental impact associated with non-renewable energy sources. [5]


Biomass is an attractive energy source for a number of reasons. First, it is a renewable energy source as long as we manage vegetation appropriately. Biomass is also more evenly distributed over the earth's surface than finite energy sources, and may be exploited using less capital-intensive technologies. It provides the opportunity for local, regional, and national energy self-sufficiency across the globe. It provides an alternative to fossil fuels, and helps to reduce climate change. It helps local farmers who may be struggling and provides rural job opportunites. [6]


Bioenergy ranks second (to hydropower) in renewable U.S. primary energy production and accounts for three percent of the primary energy production in the United States [7]. For a brief biomass history, please click here.

Biomass Energy Conversion

Bioenergy conversion requires a comparison with other energy sources that are displaced by the bioenergy. Thus, biomass for power must be compared to coal, natural gas, nuclear, and other power sources including other renewables. While comprehensive data is not available, one study by REPP shows that emissions from biomass plants burning waste wood would release far less sulfur dioxide (SO2), nitrogen oxide (NOx) and carbon dioxide (CO2) than coal plants built after 1975. The comparison with combined cycle natural gas power plants is more ambiguous, since biomass releases far more sulfur dioxide, similar levels or greater levels of nitrogen oxide, but far less carbon dioxide than combined cycle natural gas plants.


Life-cycle impacts

Several studies by the National Renewable Energy Laboratory examined the “life-cycle” impact of bioenergy for power. That is, the studies examined the air, land and water impacts of every step of the bioenergy process, from cultivating, collecting, and transporting biomass to converting it to energy. One study found that a bioenergy operation featuring biomass gasification with combined-cycle power plant technology would release far less SO2, NOx, CO2, particulate matter, methane and carbon monoxide than coal power plants meeting new federal air pollution standards.



Sources Cited:

[1] Center For Renewable Energy and Sustainable Technology (CREST). Biomass FAQs. Discussion Section. www.repp.org.

[2] "What is Biomass?" American Bioenergy Association. http://www.biomass.org/index_files/page0001.htm May 12, 2005

[3] "Biomass FAQs." Office of Energy Efficiency and Renewable Energy. Department of Energy. http://www.eere.energy.gov/biomass/biomass_basics_faqs.html#biomass. July 2005.

[4] "History of Climate Change." Athena Curriculum Earth, an affiliate of NASA. Available Online at http://vathena.arc.nasa.gov/curric/land/global/climchng.html, as of June 24, 2005.

[5] "Bioenergy." http://www.montanagreenpower.com/renewables/bioenergy/ May12, 2005.

[6] Kirby, Alex."UK Boost for Biomass Crops." BBC News Science and Nature. http://news.bbc.co.uk/1/hi/sci/tech/3746554.stm. Oct 19, 2004.

[7] See Footnote 3

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