Timber End-of-Life Impacts

[Sam Garton]

Hi there,

Noticed that a large impact comes from timber end of life – current assumption is wood landfill, and ~35,000m3 of CLT contributes 23,000 tCO2! If instead it was recycled or composted or even combusted (with or without heat rec/ elec gen) then this would result in less than 1000 tCO2.

Could you please help me understand why timber landfill has such a huge impact?

Also, out of interest, wondering if combusting really would be a wise choice or not – CO2 numbers suggests it has about the a similar impact as recycling and composting (all around 700 to 900 tCO2). One more thing, and you may not know this but I’m also wondering what method would be a viable option commercially for these timber end of life options (what services are available and how much they cost)?

Thanks!

[Fei Ngeow]

Hi Sam,

The dataset assumes industry average end-of-life for all materials. For timber, this is the ‘Wood Landfill’ which also assumes worse case conditions of anaerobic decomposition where methane is produced. Hence the industry average default assumption of high end-of-life impacts for timber.

When arranged according to highest to lowest impact for end-of-life disposal/recovery method for timber, it is as follows:

– Combustion (no recovery)
– Wood Landfill (default)
– Combustion (electricity generation)
– Composting (commercial composting conditions)
– Combustion (heat recovery)
– No disposal (assumes no disposal at the end-of-life however should only be modelled as a scenario and not as a given)

As best practice, due to the unpredictability of real life conditions at the end-of-life of buildings/products, we should always assume the most likely worse case scenario and model our LCA studies using the default settings for that material. All other disposal/recovery methods should be modelled as a scenario for consideration.

Hope that answers your question!

[Fei Ngeow]

Note that the latest version of the dataset (version 14) will have some significant changes to the end-life process impacts.

In order of impacts starting from worst to best:
– Combustion (no recovery)
– Combustion (heat recovery)
– Composting (commercial composting conditions)
– Combustion (electricity generation)

Please use the latest version of the dataset for all new/uncertified LCAs to ensure that the end-life-impacts for timber is correctly calculated.

[Richard Haynes]

Hi Sam,

Great question. If you’re interested in the deeper science around it I thought I’d comment. The underlying process creating the methane in landfill is anaerobic digestion (basically decomposing timber in an environment with no oxygen). In this environment we get CH4 (methane) production rather than CO2 essentially because there’s no O oxygen for the O2 bit of the chemical compound. One kilogram of methane has a global warming forcing factor of 34 to 80 times that of one kilogram of CO2 (depending on the time frame). So if a piece of timber that’s sucked up all this carbon from CO2 through photosynthesis is then decomposed without oxygen it can release way more global warming forcing affect than it originally sucked up (because of the potency of methane verse CO2). Fortunately only a small percentage of timber actually degrades in landfill, hence the impact is pretty small compared to what it could be if it all degraded. This is still an area of research so if you look through the historical eToolLCD datasets you’ll see big shifts in the impacts associated with landfill of timber. Fabiano Ximenes is the guru in this area I think, he’s been digging up old landfills to work out how much timber is decomposing and this information is feeding the LCI data.

Rich