Benefits gained from the use of logging residues as a fuel depend more on the type of fossil fuel they replace than on the distance the residues have to travel, according to new research. Residues that replace coal produce the greatest reductions in CO2 emissions.
Logging residues are the side branches, leaves, bark, and other wood and tree waste removed during and after logging and are potentially a significant source of bioenergy. However, there are many possible systems of recovering, refining and transporting the residues and it is a challenge to find an affordable approach which leads to the greatest reduction in CO2 emissions when residues are used to replace fossil fuels. Based on the experience of forest residue recovery in north-central Sweden, the EU-funded research analysed the costs, energy produced and CO2 benefits of two different recovery systems, several different transport modes and distances, and the effect of replacing of different fossil fuels. However, the research did not consider any alternative uses of logging residues and potential implications for CO2 emissions. CO2 emissions from the biomass itself, e.g. if the residues were left in the forest to decompose, were also not considered in calculations, nor were the long-term greenhouse gas implications for the forest, which may suffer reduced growth and therefore sequestration capacity as as result of valuable nutrients and minerals being removed with the residues, which would otherwise replenish soil organic matter.
The two recovery systems were: (i) the traditional chip system, where residues are chipped at the roadside and (ii) the bundle system where residues are compressed into bundles and transported to a central facility where they are chipped. Local, national and international transport was considered, including the possibility of extended sea transport for international trade.
Unsurprisingly, delivering to a local end user was the least costly. However, there was little difference in cost between national and international destinations. National and international delivery for both types of recovery system cost roughly EUR20 per MWh of fuel. Local transportation cost about EUR13-14 per MWh of fuel, depending on the system. Overall the chip system was slightly more costly because more dry matter was lost on the roadside than in the bundle system.
The study analysed the amount of CO2 emissions avoided from using logging residues to replace three main types of fossil fuel: coal, oil and natural gas. It assumed that efficiency of converting the fuel into energy was reduced by 4 per cent when coal is replaced by logging residues, 2 per cent when oil is replaced by residues and unchanged when gas is replaced by residues. The amount of CO2 emissions avoided from using residues differed significantly according to which fossil fuel was replaced. The greatest reduction in CO2 emissions was achieved if the replacement fuel was coal, at around 10 to 11 tonnes of carbon per hectare for bundle systems. If oil or gas were to be replaced the reduction in emissions was lower, at about 8 and 6 tonnes of carbon per hectare, respectively. The benefit in terms of reductions in CO2 emissions was significantly better (by about 2 tonnes of carbon per hectare) for bundle recovery systems than chipping systems but there was little difference between the modes of transport, whether road, rail or sea.
The results indicate that compressing bundles on site and transporting to a central location for chipping is more cost-effective, produces more energy and has a larger benefit in terms of reductions in CO2 emissions. However, traditional chip systems might be more cost-effective for small-scale end users where the residue is used relatively near to where it is collected. If the residue is transported the mode of transport has a minor impact on CO2 emissions.
Greater reductions in net CO2 are achieved when logging residues are transported longer distances and used to replace coal than when residues are transported relatively locally and used to replace oil or gas. For example, when residues are transported internationally to replace coal they avoid around 50 per cent more CO2 emissions than when residues are transported locally to replace natural gas. This information could be useful when deciding how best to use logging residues to replace fossil fuels.
Source: Gustavsson, L., Eriksson, L. & Sathre, R. (2011) Costs and CO2 benefits of recovering, refining and transporting logging residues for fossil fuel replacement. Applied Energy. 88:192-197.