Biokolssystemanalys

Brief history of the Stockholm Biochar Project (SBP):

Biochar use in Stockholm was initiated in 2013 by the city’s tree officer to remediate tree growth problems in compacted urban soils and improve storm water management. For this purpose, trees had already started to be replanted in constructed soils, an arrangement of crushed rocks of various sizes filled with conventional peat-clay-sand soil. Biochar was introduced in the constructed soils to replace peat, and to further increase soil water retention and tree health. Biochar was initially imported from other European countries, mainly Germany.

In 2014, the SBP led by an independent consultant received a grant from the Mayors' Challenge to buy and operate a pyrolysis unit. The pyrolysis unit would convert the woody fraction of garden waste to biochar and district heat. Before installing the pyrolysis unit, the garden waste was sent to an incineration plant for combined heat and power production. Today, this project is being replicated in other cities across the world.

Functions

The biochar project here has several functions:

• Treatment of garden waste;
• District heating production;
• Trees planted in constructed soil & related effects (like stormwater management in constructed soil)

Reference situations

For each function, a reference situation is defined:

• The alternative to pyrolysis of the garden waste was incineration for combined heat and power production. This is the historic alternative, i.e. what was being done before the pyrolysis plant was introduced. Other alternatives could be: landfilling of the garden waste or chipping for use as mulch.
• The alternative to district heat produced from pyrolysis oil and gas was set here to production of district heat in another facility using biomass. Here, a large range of alternative fuels could be selected. Woodchips were selected because they are the major fuel used in Stockholm for district heating.
• The alternative to tree planting in biochar-macadam was assumed to be tree planting in a macadam with conventional soil.

With this picture set, one can start analysing environmental effects, benefits and trade-offs, from the Stockholm Biochar Project.

Biochar is often described as a carbon dioxide removal (CDR) technology with positive side effects. Here, we would like to stress that biochar side effects, or simply biochar effects, are always relative to a reference situation. This is made explicit in environmental systems analysis or life cycle assessment (LCA) by defining one or several reference situations.

Three types of environmental effects can be distinguished:

• Change in technosphere inputs: biochar entails a change in consumption of another product in the studied system
• Change in biosphere exchange: biochar modifies the environmental emissions from an activity
• Change in technosphere ouputs: biochar leads to an increase in production of something

More details are available in the article Assessing the diverse environmental effects of biochar systems: An evaluation framework.

Many stakeholders who use biochar in some product want to calculate the climate impact of their product. Like for other products they use, they would like to have an emission factor for biochar, i.e. an amount of kg CO₂-eq per kilogram of biochar or per cubic meter of biochar.

However, such an emission factor for biochar may be difficult to find in LCA research studies for several reasons:

• most of research studies assess a complete biochar production and use system
• biochar production is multi-functional and determining an impact for biochar requires to consider the pyrolysis co-products, the reference land or biomass use, and the carbon sink, if considered as a product in itself;
• there is not one unique value as it depends on the biomass used, the reactor used, and the energy context

Here, we would like to stress that to determine a biochar emission factor, there is a need to make subjective choices:

• delimiting the system boundaries (what is excluded from the modelling)
• deciding upon a reference land of biomass fate
• deciding how to handle co-products from pyrolysis, e.g. via allocation (which one?) or substition (which substituted product?)

In LCA theory, there is no unique & objective way of making these choices. Burden attribution and references are subjective. However, in practice, guidelines like the EU Product Environmental Footprints or EPDs PCR are set of rules where these subjectives choices have been made for specific categories of products.

More to read about this in our article about urban uses of biochar in Uppsala.