Biomass energy generation

June 30, 2008

With the seemingly ever-rising costs of electricity, coal and diesel it makes sense to look more closely at the use of forest biomass to provide for some of our energy needs. Forest harvesting residue, or slash, is a resource that should be exploited for this purpose. Sawdust waste from sawmills, which is an eyesore and a headache for sawmillers, is another source of suitable material. Plantation biomass is a carbon neutral source of energy that is ideal for replacing fossil fuels we so desperately rely on. In addition, replacing fossil fuels will not only help counteract global warming but could also qualify the user for carbon credits under the Clean Development Mechanism (CDM).

by Dave Dobson (ddobson@mweb.co.za)

Slash bundle
The slash bundle can be handled by conventional
forest equipment and transporters.

 

Carbon credits
South Africa is a participant in what is now known as the Koyoto Protocol. This is an agreement amongst member nations to reduce greenhouse gas emissions thereby attempting to curb global warming. South Africa's participation affords us access to the CDM through which it becomes possible to generate certified emission reduction credits (CER's). CER's are developed either through efforts to reduce emissions (substituting fossil fuel e.g. coal or diesel, with biomass) or by sequestering carbon in trees via new afforestation. These CER's can now be sold to other countries allowing them to emit an additional amount of CO2 or non-CO2 greenhouse gas equivalent to the particular CER credit.

Biomass availability in South African forests

South Africa harvests some 18 million tons of timber per annum. This operation is estimated to generate about 6.7 million tons of forest biomass.

Nutrient concentration is highest in the bark and the leaves, which should as a result be left on site to facilitate nutrient recycling. Consequently, only about 2.5 million tons of the original biomass component should actually be considered for biomass harvesting. Not all of this material is readily accessible and some estimates have put the available portion at 60% of the total i.e. 1.5 million tons of biomass annually. Viewed broadly this biomass could replace about 1 million tons of coal.

The moisture content of the biomass is also important. The average moisture content of the woody component of the plantation slash is generally of the order of 50%, ideal for biomass harvesting. Interestingly, moisture in biomass has both positive and negative consequences. The more moisture in the material the easier it is to chip. However, moist fuel has a negative effect on combustion, with losses in boiler effiency when the moisture content of the fuel exceeds 50%. In addition, storage of moist biomass results in losses as a result of decomposition. Ideally biomass should be stored at moisture contents of 30% or less since it is at this level that the material is too dry for the micro-organisms responsible for decomposition. Drying the biomass to 30% moisture will also result in heat gains when it is burnt.

Biomass collection
Work has been done in South Africa on biomass collection. However, much of this has tended to concentrate on manual collection methods. While understandable from a job-creation point of view, the costs of the operation don't always add up too well. As a job-creation exercise it's fine but for a viable business venture one needs to be cautious, which appears to be the case in practice. The problem with forest biomass collection is the fact that it has an extremely low bulk density, which, if not addressed has serious consequences for the transport component of the operation.

It is important to settle on the correct collection process if a successful biomass venture is to be undertaken. This has led to some innovative developments. Foremost is the development of a plantation slash bundler, which, like hay baling, enables the operator to collect and create a biomass bundle held together with twine. These compacted bundles, 70 cm in diameter and 2,4 m to 4.8 m in length are dropped infield once they have been made. From this point on traditional forestry equipment can be used to forward the bundles to roadside and to load them onto interlinks for transport to the chipping plant.

Conclusion
With the current trends in energy pricing and pressure to reduce reliance on fossil fuels it make sense, where possible, to switch to carbon neutral forest biomass. Of the various biomass collection methods the slash bundling option appears the most viable. It not only allows the operator to render the biomass more dense but the creation of the bundles makes it possible to slot the harvesting operation in with the conventional forwarding/long haul arrangements that exist in the industry.

Care must be taken when collecting forest biomass to ensure that enough nutrients are left on site to maintain long-term site productivity.

Published in May/June 2008

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