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| The problem with burning fossil fuels
Burning any carbon based fuel converts carbon to carbon dioxide. Unless it is captured and stored, this carbon dioxide is usually released to the atmosphere. Burning fossil fuels releases carbon that was removed from the amosphere millions of years ago by animal and plant life. This leads to increased concentrations of carbon dioxide in the atmosphere.
What is the Greenhouse Effect?
Carbon dioxide is one of a number of gases that transmit the visible light incident on the Earth from the Sun, but absorb the infra-red radiation emitted by the warm surface of the Earth, preventing its loss into space. This keeps the Earth around 33ºC warmer than it would otherwise be, and is known as the Greenhouse Effect as it is the same effect achieved by the sheets of glass in a greenhouse.
Greenhouse gasses
Gases that have the above property are known as Greenhouse Gases (GHG), and include:
- Carbon dioxide (CO2)
- Methane (CH4)
- Nitrous oxide (N2O)
- Water vapour (H2O).
Global warming
The increasing concentrations of greenhouse gases in the atmosphere since the industrial revolution and the resulting widespread use of fossil fuels, gives rise to an increase in the greenhouse effect, and an increase in the average global temperature, known as Global Warming. This is predicted to lead to widespread, unpredictable changes to the global climate.
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| Using biomass to achieve a carbon balance
The combustion (direct or indirect) of biomass as a fuel also returns CO2 to the atmosphere. However this carbon is part of the current carbon cycle: it was absorbed during the growth of the plant over the previous few months or years and, provided the land continues to support growing plant material, a sustainable balance is maintained between carbon emitted and absorbed.
(a) As trees in the energy plantation grow, they absorb carbon dioxide from the atmosphere.
(b) During photosynthesis the trees store carbon in their woody tissue and oxygen is released back to the atmosphere.
(c) At harvest, woodfuel is transported from the plantation to the heat or power generating plant.
(d) As the wood is burned at the heat or power generating plant the carbon stored in the woody tissue combines with oxygen to produce carbon dioxide, this is emitted back to the atmosphere in the exhaust gases.
The amount of additional biomass that grows over the course of a year in a given area is known as the annual increment. Provided the amount consumed is less than the annual increment its use can be sustainable and biomass can be considered a low carbon fuel and biomass CO2 absorption and emission is in balance.
For forestry in the UK, the annual timber increment is of the order of 20 million tonnes. On top of this is the increment of all the agricultural crops and other vegetation.
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| Why biomass is not carbon neutral
It is not, however, strictly true to say that biomass is 'carbon neutral'. No fuel or energy source is. It is a low carbon fuel, but carbon is emitted, usually as a result of energy use, as a result of planting, harvesting, processing, transport and often fertilizer and pesticide production and administration.
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| Benefits of using biomass as a sustainable fuel
Correctly managed, biomass is a sustainable fuel that can both offer a significant reduction in net carbon emissions compared with fossil fuels and also many ancillary benefits:
- Biomass can be sourced locally, from within the UK, on an indefinite basis, contributing to security of supply.
- UK sourced biomass can offer local business opportunities and support the rural economy.
- The establishment of local networks of production and usage, allows financial and environmental costs of transport to be minimized. There is no region in the UK that cannot be a producer of biomass, although some have greater productivities than others.
- Woodlands, forestry and agriculture are generally perceived to be an environmentally and socially attractive amenity by the UK population, providing opportunities for recreation and leisure activities.
- Many biomass fuels generate lower levels of such atmospheric pollutants as sulphur dioxide, that contributes to 'acid rain'. Modern biomass combustion systems are highly sophisticated, offering combustion efficiency and emission levels comparable with the best fossil fuel boilers.
- Biomass residues, arisings, co-products and waste not used for energy, or some other application may be consigned to landfill. This imposes costs for disposal, additional burden on limited landfill resources, and also contributes to global warming by the creation of landfill gas, including a high proportion of methane (CH4), a greenhouse gas 21 times more potent that carbon dioxide.
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