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2 - Pyrolysis

Pyrolysis is the thermochemical conversion process during which a (biomass) feedstock is heated in the partial or total absence of oxygen. While pyrolysis is the main technique for producing biochar, other thermochemical conversion techniques exists for carbonising biomass such as torrefaction, hydrothermal carbonisation, and gasification.

Biomass pyrolysis yields three main products: non-condensable gases, liquid oil or tars, and solid char. Often, the pyrolysis gases and oil are directly combusted and recovered as energy. In more advanced cases, pyrolysis oil and gas can be recovered, upgraded and used later for energy or chemical products.

In environmental systems analysis, pyrolysis is described as a multi-functional process or multi-product process since it delivers several useful products or services. Note that if waste biomass is used in pyrolysis, another service can be defined, namely waste treatment.

A continuum of thermochemical conversion…

The thermochemical conversion of biomass is a continuum of transformation processes during which biomass is heated up to a certain temperature, for a certain time, at a certain speed, under given atmospheric conditions.

For a given biomass, thermochemical conversion processes distinguish themselves by 3 main parameters:

  • Highest Treatment Temperature (HHT, in °C), also known as pyrolysis temperature. It varies between 300°C and 1200°C.
  • Residence Time (RT, in min) at the highest treatment temperature. It can vary from seconds up to several hours or even days.
  • Heating Rate (HR, in °C/min). It can vary from few °C/min to several hundred °C/min.

In addition, the pressure and the composition of the atmosphere in the reactor also distinguish thermochemical processes.

Categories of thermochemical conversions

  • Slow & intermediate pyrolysis: corresponds to low heating rates, with temperatures in the range of 200 to 900°C. It is the main process used to make biochar. Mass propertions of biochar, liquids, and gases are roughly equal to 1/3 each, with variability introduced by biomass type & process conditions.

  • Fast & flash pyrolysis: corresponds to very high heating rates, at high temperature, with very short residence time. It is the main process for making pyrolysis oil, with mass yields up to 75% of the input dry biomass.

  • Gasification: corresponds to treatment at very high temperature, up to 1200°C, sometimes in the partial presence of an oxidant like oxygen. Gases are the main products. Biochar mass yields rarely exceed 15% under gasification conditions, which imply that gasification chars have relatively high ash contents.

  • Torrefaction: corresponds to a special kind of pyrolysis, in a temperature range between 200 and 300°C. The purpose of torrefaction is to maximise the production of solids, lightly carbonised biomass, and to retain most of the biomass' energy in the solids. Torrefaction is sometimes used to improve the properties of a biomass fuel, e.g. in terms of storage convenience or grindability. However, long-term carbon sequestration is deemed not possible with char from torrefaction.

  • Hydrothermal carbonisation: it takes place in an aaqueous environment, at high pressure and temperature. It is suited for the production of hydrochar from wet biomass. The name hydrochar is used to differentiate it from biochar produced from other processes, due to its different chemical composition and structure. Long-term carbon sequestration is not either deemed possile with hydrochar.

Reading more about thermochemical conversion pathways of biomass in:

Weber, K.; Quicker, P. Properties of Biochar. Fuel 2018, 217, 240–261. https://doi.org/10.1016/j.fuel.2017.12.054

Installed pyrolysis plants

The number of pyrolysis plants (for biochar production) installed in Europe is increasing. In 2020, the European Biochar Institute estimated that there were 72 plants installed, with a total production capacity of 20 000 tonnes of biochar per year (EBI, 2021).

We maintain a survey of installed biochar production capacity [coming soon]

*Installed biochar production capacity in Sweden over time* [Coming soon: EU data & interactive figure]
Installed biochar production capacity in Sweden over time [Coming soon: EU data & interactive figure]

LCA of biomass pyrolysis

The environmental impact from operating a pyrolysis plant can be decomposed in the following terms:

  • Manufacturing and disposal of the pyrolysis reactor and surrounding equipment (e.g. dryer, silos, feeder, combustion, flue glas cleaning, concrete slab, as well as maintenance operations)

  • The supply and use of industrial products needed for operation of the pyrolysis reactor (e.g. electricity, quenching water, reaction additives, or other consumables)

  • Management of side-stream, mostly waste such as ash and fly ash collected in flue gas cleaning system, or wastewater sent to treatment

  • Other fuel and equipment use for handling of the biomass on site (e.g. wheel-loaders)

  • Other fuel and equipment use for handling of the pyrolysis products (biochar, and pyrolysis gas and oil if stored rather than directly combusted)

  • Direct environmental emissions from the operations, like dust from handling of biochar or air pollutant emissions from the stack of the pyrolysis reactor

In the LCA of pyrolysis processes, these terms can be very different and have varying importance for different environmental impact categories. The importance of the terms are also affected by the context:

  • Reactor type: the type of reactor (low-tech or high-tech, electricity-heated or syngas-heated pyrolysis) affects for instance the direct environmental emissions and the amount of inputs needed to operate the reactor.

  • Context dependence: an electricity-heated reactor supplied with electricity from coal will not have the same environmental performance than one supplied with hydropower.

  • Environmental impact categories: a low-tech reactor may have higher direct emissions, which are relevant for human health impacts but also climate change. A high-tech reactor running on nuclear energy may have a low climate change impact, but a high impact in ionising radiations.

Click on the pictures for details

Data for direct environmental emissions

Data on direct environmental emissions from pyrolysis reactor is rather scarce. However, we compiled data from 3 sources for different pyrolysis reactors.

The inventory data is scaled for 1 kg of biochar produced. It can be used in LCA, however, it should be noted that (i) the scope of environmental stressors considered is different in the three datasets, and that (ii) the measurements were performed for different biomass feedstocks, which has a influence on the kind of emissions.

Further descriptions & references are provided in the dataset:

Download dataset as Excel file

Data for technosphere inputs

Data on manufacturing, disporal, and operation requirements of pyrolysis reactor is rather scarce as well. Through modelling, we compiled some inventory data for different reactors.

Further descriptions & references are provided in the dataset:

Download dataset as Excel file

Cradle-to-gate climate impact of several biochar supply-chains

In our Uppsala case study, we presented cradle-to-gate emission factors for biochar products with different properties (carbon content, bulk densities), pyrolysis reactors (E: electricity-heated, S: syngas-heated, M: mobile-syngas heated), and different biomass (WP: wood pellet, GW: garden waste, LR: logging residues).

The figures below present the climate change impact, from biochar production and the initial carbon sink (i.e. the amount of carbon contained in the biochar, without any assumption regarding its long-term stability). Some parameter values can be changed to see how results are affected.

The values presented are not to be used as LCA data for product footprinting without further analysis & verification of our underlying modelling assumptions, which are numerous.

Meaning of contributions (colors):

  • Biochar C-sink: initial amount of C sequestred (100% stability assumed here)
  • Reactor: manufacturing and disposal of reactor
  • RLBU: reference land or biomass use (what would have happened to the biomass, if not used for biochar)
  • Transport: all transport from biomass supply to biochar handling
  • Pyrolysis: energy inputs for pyrolysis & direct air emissions
  • Biomass production: energy, machinery, fertilser use
  • Energy substitution: displacement of heat from alternative source, by combustion of pyrolysis co-products

For 1 tonne of biochar:

Results in mass units can be converted to volume units by multiplying by the bulk density of biochar. This is done below:

For 1 cubic meter of biochar: