The study was pub­lished in 2011 and pro­vides an overview of the various process­es used to produce biochar (pyrol­y­sis process) and biochar (hydrother­mal car­bon­i­sa­tion) for soil appli­ca­tion. The focus is on tech­no­log­i­cal, eco­nom­ic and climate-rel­e­vant aspects of biochar pro­duc­tion. The authors eval­u­at­ed almost 30 sci­en­tif­ic studies.


Tech­ni­cal dif­fer­en­ti­a­tion

The fol­low­ing pro­duc­tion process­es for biochar and biochar were used by the authors for the analy­sis:

  • Tor­refi­ca­tion (pyrol­y­sis at approx. 250–300 °C, main product is coal)
  • Slow pyrol­y­sis (approx. 400 °C, main product is coal)
  • Medium speed pyrol­y­sis / inter­me­di­ate pyrol­y­sis (approx. 450–600 °C, main product is a pyrol­y­sis oil)
  • Fast pyrol­y­sis /flash pyrol­y­sis (approx. 450–600 °C, main product is a pyrol­y­sis oil)
  • Gasi­fi­ca­tion (approx. 800°C, partial oxi­da­tion, main product is a gas)
  • Hydrother­mal carbonisation/ HTC (approx. 180–220 °C, main product is a liquid)

Pyrol­y­sis and gasi­fi­ca­tion differ in the almost com­plete absence of oxygen in the pyrol­y­sis con­ver­sion process. Pyrol­y­sis tech­niques can be dis­tin­guished by the fol­low­ing para­me­ters

  • Reac­tion time (slow, fast process)
  • Heating process­es (com­bus­tion, elec­tri­cal, microwaves)
  • Tem­per­a­ture
  • Dwell time of the input mate­r­i­al

In hydrother­mal car­bon­i­sa­tion, the biomass is treated at around 180–220°C togeth­er with water and under pres­sure. Output are liquid and gaseous prod­ucts.
The authors could not make any state­ment about the tech­ni­cal maturity/marketability of the indi­vid­ual systems. The data sit­u­a­tion was not suf­fi­cient for this.

Eco­nom­ic via­bil­i­ty

The authors faced the fol­low­ing chal­lenge in eval­u­at­ing the eco­nom­ic data: Biochar is often pro­duced togeth­er with bio­genic energy sources (oil, syn­the­sis gas). An essen­tial part aims pri­mar­i­ly at the pro­vi­sion of energy (elec­tric­i­ty or heat) and not for the pro­duc­tion of biochar. Biochar is often only the by-product.
The authors also point out that the markets for renew­able ener­gies (biogas plants) and biochar compete for the same raw mate­r­i­al. The market for biochar as a soil improver is similar, com­pet­ing with prod­ucts such as peat or compost. This factor will also deter­mine the eco­nom­ic effi­cien­cy of a man­u­fac­tur­ing process in the future.
The pro­duc­tion costs vary strong­ly in the studies between 51 USD/t (pyrol­y­sis biochar from garden waste) and 386 USD/t (char­coal from retort process). However, accord­ing to the authors, a mean­ing­ful eco­nom­ic com­par­i­son of the methods is not pos­si­ble due to the low data sit­u­a­tion.

Green­house gas balance

Accord­ing to the authors, the fol­low­ing factors must be con­sid­ered for a com­plete CO2 balance or green­house gas balance (GHG) of the plant coal pro­duc­tion process:

  • Infor­ma­tion on the supply of raw mate­ri­als (includ­ing direct and indi­rect land-use changes)
  • Trans­for­ma­tion process itself
  • Use of by-prod­ucts
  • Biochar appli­ca­tion
  • Biochar sta­bil­i­ty in the soil
  • Influ­ence of biochar appli­ca­tion on soil-related N2O and CH4

The authors could only find reli­able data for pyrol­y­sis process­es. A pos­i­tive influ­ence on the GHG balance of the man­u­fac­tur­ing process­es is mainly due to the CO2 storage

effect of biochar in the soil. A study by Woolf et. al. (2010: Sus­tain­able biochar to mit­i­gate global climate change), this results in avoided GHG emis­sions of ‑1054 kg/CO2 per ton of dry biomass (with an average carbon content of 50%) per year.
However, even here the values vary greatly between the studies. Using a ref­er­ence sce­nario, the authors iden­ti­fy an oppo­site effect, namely increased green­house gas emis­sions of 123 kg CO2/t dry biomass feed­stock in the pro­duc­tion of biochar. Indi­rect land use changes can have a neg­a­tive impact on the balance. This means the effect caused by the dis­place­ment of exist­ing agri­cul­tur­al pro­duc­tion by the cul­ti­va­tion of energy crops on the same land, meaning new areas must be exploit­ed for agri­cul­tur­al pro­duc­tion.

Orig­i­nal article: Tech­ni­cal, Eco­nom­i­cal, and Climate-Related Aspects of Biochar Pro­duc­tion Tech­nolo­gies: A Lit­er­a­ture Review.
Author: Sebas­t­ian Meyer, Bruno Glaser, Peter Quicker
Pub­lished in: Envi­ron­men­tal Science & Tech­nol­o­gy 45, 22, 9473–9483, Amer­i­can Chem­i­cal Society 2011