Bio-based building products in the Dutch Environmental Database (NMD). Part 1, Proposal for crediting biogenic carbon storage

Samenvatting

The Dutch Government targets to reduce CO2 emissions by 49% in 2030, and to virtually net zero CO2 emissions by 2050, in order to combat climate change. The building and construction sector can contribute significantly to this goal by applying bio-based and circular building products. By using sustainably sourced bio-based materials, CO2 which has been taken up recently from the atmosphere will be stored as biogenic carbon1 in materials for as long as these are incorporated in a building and eventually in subsequent recycled products. This storage of biogenic carbon actually means a temporary yet long term negative greenhouse gas (GHG) emission, thus buying time to transform our society into a virtually GHG emission neutral economy and creating space to reach climate goals. However, the value of long term biogenic carbon storage is not included in standard LCA standards and guidelines, nor credited when determining the environmental impact of buildings (in Dutch ‘Milieu Prestatie Gebouwen’, MPG).2 The objective of this study is to review scientific approaches for quantifying the effect of biogenic carbon storage in building materials on global warming potential (GWP), which is the LCA impact parameter for climate change, and to draft a generally applicable methodology to valorise the GWP benefit of materials in durable building applications, in-line with the Determination Method (see footnote). The GWP benefit arises from the long term reduction of CO2 concentration in the atmosphere, which is a result of (most recent) sequestration of CO2 from the atmosphere as biogenic carbon in plant materials and subsequent long term application of this material in e.g. buildings. Therefore, all long term biogenic carbon storage presents a GWP benefit. The different scientific approaches for determining the effect of biogenic carbon storage on GWP described in literature exhibit only small differences for the GWP benefit at given storage times. The ‘dynamic LCA method’ is the most accurate in describing the effects over time and is therefore selected as the basis for crediting the value of biogenic carbon storage in building materials. The GWP benefit can be calculated as the product of: 1. the mass of bio-based material; precondition is that the bio-based material is sustainably sourced. 2. the biogenic carbon content of that material; initial experimental analysis shows that this value relates to the chemical composition of the materials, ranging from about 430 g carbon per kg of dry lignocellulose fibres (e.g. cotton, flax, hemp) to about 500 g/kg of dry wood and up to 685 g/kg of dry expanded cork. 3. the ratio between the molecular masses of CO2 and carbon, being 3.67. 4. the GWP benefit factor, which relates to the service life of the building product and the time horizon for which GHG effects are considered. This time horizon is a policy decision which needs to be long enough to avoid passing on problems to the future (i.e. long enough to develop adequate solutions to cope with the climate situation by that time) and short enough to achieve productive action now. Most standards, experts and governments have adopted a time horizon of 100 years. 5. the net biogenic storage ratio. This factor depends on bio-based feedstock origin and the application conditions, and ranges from 1 for virgin bio-based materials in new-built houses and extension of houses to 0 for using reused or recycled materials and for renovation if scrapped material is decomposed with CO2 emission to the atmosphere. Biogenic carbon storage presents a GWP benefit which is additional to both standard LCA calculations as well as to environmental impact calculation tools like e.g. the Determination Method. Therefore, no double counting is involved when crediting the temporary yet long term biogenic carbon storage. It is proposed to credit the GWP benefits of temporary biogenic carbon storage in bio-based building materials via one of several options: • In module D of the Dutch Determination Method. • As an additional parameter in the MPG tool, for consideration otherwise than including in the single score MPG calculation. • In carbon credit certification schemes. Fossil based products may also contain carbon, however, this carbon is derived from CO2 extracted from the atmosphere long time ago, and consequently does not deliver a reduction of CO2 concentration in the atmosphere at present. The proposed methodology applies equally for all building materials. However, bio-based materials do contain biogenic carbon, whereas fossil-based materials do not. Biogenic carbon storage is a long term yet temporary GWP benefit. Further action is required for the development and implementation of technologies and other measures to achieve permanent GHG emission reductions and climate change adaptation.