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May 24, 2013

Nacogdoches is one of the world's largest purpose-built biomass-fuelled power generation plants

Credit: Southern Company

In the UK, biomass electricity generators have been required to report annually on their performance against sustainability criteria for the biomass feedstocks they use. The reporting was intended to develop knowledge ahead of the introduction of European sustainability standards, which have yet to materialise.

The two sustainability criteria are:

a minimum 60-per-cent greenhouse gas lifecycle emission saving for electricity generation, using solid biomass or biogas relative to fossil fuel

restrictions on using materials sourced from land with high biodiversity value or high carbon stock (primary forest, protected areas, peatland and wetlands)

From April 2013, generators are required to meet the sustainability criteria in order to receive support under renewable obligations. The UK government says that further work is underway to include sustainable forest management criteria in future.

Calculating emissions

The RSPB report says that the payback period for electricity generated from using whole conifer trees is 80 years, but other studies come up with lower figures. The difference lies in the assumptions used in making life-cycle assessments.

Establishing the life-cycle emissions of biomass fuel is a complex calculation. It must take into account such factors as: location of the source, location of combustion, the type and derivation of the source material, treatment or preparation, and transport of the material.

If the biomass comes from an energy crop planted specifically to produce biomass fuel, then the calculation may need to incorporate a calculation of the indirect land-use change impact of the biofuel if net carbon loss occurs when forests and grasslands are cleared for food production that has been displaced by biofuel plantations elsewhere.

A 2012 report by the US Southern Environmental Law Centre - using woody biomass for a modelled expansion of power generation in the southeast of the US - found that it will take 35-50 years to provide carbon reduction benefit. Biomass Supply and Carbon Accounting for Southeastern Forests analysed 17 existing and 22 proposed biomass facilities in the states of Alabama, Florida, Georgia, North Carolina, South Carolina, Tennessee and Virginia. Based on current trends in using wood in large power plants and exporting fuel pellets to Europe, biomass energy in the southeast of the US is projected to produce higher levels of atmospheric carbon for 35-50 years, compared with fossil fuels. After that, it will result in significantly lower atmospheric levels as regrowing forests absorb carbon.Biomass from composted waste or agricultural residues is an efficient way of reducing carbon emissions, but removing crop residues such as straw may deplete the soil's carbon stock, with resulting increases in fertiliser and irrigation use, and lower yields. Similarly, over-enthusiastic removal of forest residues and stumps for energy production will negatively affect soil properties and biodiversity.

Transport and combustion

Practical problems associated with the use of biomass feedstocks relate to the high bulk volume, high transport costs and large storage capacities. High moisture content can lead to biological degradation and block in-plant transport systems in freezing temperatures. Variations in moisture content complicate plant operation and process control.

Biomass can be processed or densified by turning it into pellets or briquettes. This reduces the moisture content, the transport costs and storage volume and makes for easier handling. Increased energy density and more homogeneous composition result in better combustion control, which offers higher energy efficiency during combustion. The major disadvantage is the cost of the pelleting process, which increases the price of the end product and adds to its emissions profile.

Wood has been used for heat and light since the dawn of history, making it the oldest and most universal fuel. Industrialisation saw biomass superseded by large-scale use of fossil fuels, but the pendulum has swung. Biomass is undergoing a renaissance as an emerging renewable energy source, the technology surrounding which is capable of generating reliable, flexible baseload electricity.

Sustainable biomass requires sustainable forests. Large-scale generation needs large-scale supplies. The development of commercial biomass operations must lead to increased emphasis on sustainable forest management to produce fuel with a reasonable carbon payback period.

During a life-cycle of around 300 years, unmanaged forests emit almost as much CO2 as they absorb. If carbon that would be released into the atmosphere during the unmanaged phase of decay is put to use in the form of fossil fuel substitute, then a managed forest acts as a carbon sink. Looking to the future, the lifecycle assessment of forest to furnace will be part of any biomass generation scheme. The mechanisms used to support biomass generation will align with sustainable forest-management policies.

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Posted: Friday May 24, 2013, 1:01 am
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