Stockholm - Prospektiv livscykelanalys av storskalig biokolsproduktion och användning på mjölkgårdar

Vad var motivationen för studien?

Stockholm har ett stort fjärrvärmenät för att ge termisk komfort under vintertid. Under de senaste decennierna har bränslemixen i värmenätet blivit kraftigt dekarboniserad, och flyttat bort från kol och olja för att gynna biomassa, storskaliga värmepumpar och avfall. Vad kan vara nästa steg för att ytterligare minska klimatpåverkan från Stockholms energisystem? Är det klimatlämpligt att investera i en storskalig biokolanläggning eller ska vi investera i konventionell bioenergiteknik? Det var sammanhanget för denna fallstudie, utförd 2017-2019.

Research highlights

  • En storskalig pyrolysanläggning kan integreras i Stockholms fjärrvärmenät och fungera som baslastanläggning med hög drifttid (ca 80 % av året)
  • Jämfört med konventionell bioenergi producerar pyrolys mindre värme och kraft per enhet biomassa. Detta innebär en avvägning mot klimatförändringar som främst påverkas av vilken typ av el som finns tillgänglig.
  • Med ett fossilfritt elsystem blir bygge av en ny pyrolysanläggning ett bättre klimatalternativ än konventionell förbränning.
  • Det finns vissa osäkerheter för bedömning av effekterna av kaskadanvändning av biokol i djurhållning, men de skulle kunna ge 10–20 % mer reduktion än direkt biokolinförlivning i marken, via minskningar av utsläpp av lustgas och metan.

Resurser

Referens

Azzi ES, Karltun E, Sundberg C (2019) Prospective life cycle assessment of large-scale biochar production and use for negative emissions in Stockholm. Environmental Science & Technology 53:8466–8476. DOI: 10.1021/acs.est.9b01615

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Download pre-print as PDF

Presentationer

Studien har presenterats vid två konferenser och ett flertal workshops. Slides från presentationerna kan laddas ned nedan:

2019, Stockholm, Nordic Biochar conference

2018, Gothenburg, Negative emission conference

Modell

Microsoft Excel användes för modellering. Filen finns tillgänglig vid tillfrågning.

Inspelningar

Titta på en presentation av artikeln.

Vill du läsa mer?

Abstract

Several cities in Sweden are aiming for climate neutrality within a few decades and for negative emissions thereafter. Combined biochar, heat, and power production is an option to achieve carbon sequestration for cities relying on biomass-fuelled district heating, while biochar use could mitigate environmental pollution and greenhouse gas emissions from the agricultural sector. By using prospective life cycle assessment, the climate impact of the pyrolysis of woodchips in Stockholm is compared with two reference scenarios based on woodchip combustion. The pyrolysis of woodchips produces heat and power for the city of Stockholm, and biochar whose potential use as a feed and manure additive on Swedish dairy farms is explored. The climate change mitigation trade-off between bioenergy production and biochar carbon sequestration in Stockholm’s context is dominated by the fate of marginal power. If decarbonisation of power is achieved, building a new pyrolysis plant becomes a better climate option than conventional combustion. Effects of cascading biochar use in animal husbandry are uncertain but could provide 10–20% more mitigation than direct biochar soil incorporation. These results help design regional biochar systems that combine negative carbon dioxide emissions with increased methane and nitrous oxide mitigation efforts and can also guide the development of minimum performance criteria for biochar products.

Frequently asked questions [under construction]

A compilation of comments and questions arising from disucssing this article. Don’t hesitate to contact us if you have questions.

In the article, we used 3 time periods to describe the marginal electricity mix:

  • 2020: coal-dominated, at 1000 gCO2-eq kWh-1
  • 2030: coal and natural gas mix, at 500 gCO2-eq kWh-1
  • 2040: efficient natural gas, at 250 gCO2-eq kWh-1).

All these values are higher than the current average emission factor for electricity in Sweden, which is around 40-50 gCO2-eq kWh-1.

This was based on a report from Hagberg (2017) and their specific methodology. Altough both the modelling and the time frame suggested can be discussed, the conclusion that remains is that biochar is rarely preferable over conventional bioenergy in a fossil energy system. Despite carbon sequestration, it does not achieve as much climate-change mitigation as an efficient combined heat and power plant introduced in a fossil energy system.

Later, we recalculated these results with the current Swedish average mix for electricity (dominated by nuclear and hydro power). The climate-relevance of the energy-biochar trade-off is in that case much less important. This led to the following general conclusions for energy utilities.

Investing in biochar production capacity is a rather suitable climate decision if the four following conditions are met:

  • in both the short- and long-term, you foresee that energy services (e.g. heat and electricity) will have a climate change impact well-below the one from natural gas energy generation,
  • if you foresee that sustainable biomass is available in sufficient amounts, considering also other potential uses of biomass under development,
  • if you are confident that the biochar production process selected will lead to a form of biochar that is highly stable in soils,
  • if you can ensure that biochar will be used in applications that provide tangible socio-environmental benefits.

Biochar effects in agriculture are very variable because of both the diversity of agricultural systems and the diversity of biochar properties.

The three main effects considered in this study were:

  • Effect on soil N2O emissions
  • Effect on animal manure mangement
  • Effect on enteric fermentation

The estimates presented in the paper must be seen as explorative values. On-farm experimentation is needed as effective emission reductions are likely to be influenced by many contextual factors.