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.
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)
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.