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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:

… lead to a diversity of product distribution and qualities

  • Typical product distribution, …. in mass units …. in energy units…

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

UC David Biochar Database n.d. http://biochar.ucdavid.edu (accessed September 30, 2015).

Ippolito JA, Cui L, Kammann C, et al (2020) Feedstock choice, pyrolysis temperature and type influence biochar characteristics: a comprehensive meta-data analysis review. Biochar 2:421–438. https://doi.org/10.1007/s42773-020-00067-x

Mass and energy balance models for pyrolysis

Predicting the exact composition and the qualities of … is not easy may require advanced chemistry models (e.g. Aspen, …) or even actual experimentation and analysis of the products.

However, in the litterature, existing models for simpler models exists. Here, we present two of these.

This model applies to the simple, yet most common, case where pyrolysis oil & gas are directly combusted after pyrolysis to generate heat. It was developped and used in the case studies of this research project.

Description:

what is in the scope, what is excluded from it..: biomass drying, heat recovery, turbine Disclaimer: it’s to the user to make sure that combination of (mutually dependent) parameters make sense (e.g. if biochar yield is 10% or 30% probably also affects its carbon contents)

Model inputs:

  • q

Model outputs:

  • q

Sankey Widget

To do

Download as Excel Download as Python script

This second model is derived from the research article by Woolf and colleagues (2014). The authors described it as “an accurate-yet-simple empirical model for predicting yields, product compositions, and energy balances from biomass slow pyrolysis” helping to “quantify the trade-offs between energy and biochar yields in bioenergy–biochar systems”.

Description:

  • Model limited to pyrolysis, in particular slow-pyrolysis. It excludes gasification, where some oxygen or steam is allowed in the reactor.

  • Multiple pathways to produce biochar and biofuel (either gas or liquid fuel) are considered (see Table 1 in the article). To clarify, it includes biomass processing, pyrolysis, and upgrading of the pyrolysis oil and gas through several pathways. Gas pathways: water-gas shift reaction to produce hydrogen; catalytic methanation of pyrolysis gas. Liquid fuel pathways: catalytic methanol synthesis, Ljungdahl-Wood pathway, Fischer-Tropsch synthesis.

  • It assumes that all energy inputs needed by the process (heat and electricity) come from the pyrolysis products or from extra biomass. To clarify, this assumes no external inputs of e.g. natural gas or grid electricity.

  • Model inputs:

    • q

    Model outputs:

    • q

    Sankey Widget

    To do

Download as Excel Download as Python script

Woolf and colleagues (2021) released empirical equations to determine the biochar yield and the biochar carbon content of a given pyrolysis process based on the biomass ash content, the biomass lignin content, and the pyrolysis temperature.

Description:

The model is based on meta-analysis of data found in the literature. Two meta-analyses are cited:

  • Neves et al., 2011, with 128 measurements from 26 papers (R2 = 0.65), for biochar carbon content
  • Woolf et al., 2014, with 146 measurements from 18 papers (R2 = 0.65), for biochar yield

The model assumes that the ash content in the biomass is conserved during pyrolysis, and remains in the biochar.

Model inputs:

  • T → the pyrolysis temperature, in degree Celcius
  • Fa,bm → the ash contrent of the biomass, on a dry basis
  • L → the lignin content of the biomass, on a dry basis

Model outputs:

  • YBC → the dry-ash-free yield of biochar from pyrolysis
  • FC → the organic carbon mass fraction on a dry mass basis

Equations:

$$ Y_{BC} = 0.1261 + 0.5391 e^{-0.004T} + 0.002733L $$

$ F_C = (1 - \frac{F_{a,bm}}{F_{a,bm} + Y_{BC}} ) * (0.93 - 0.92 e^{-0.0042T}) $

 Download as Excel

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. To contribute, please answer the survey.

*Installed biochar production capacity in Sweden over time* [To do: EU data, plotly interactive figure]
Installed biochar production capacity in Sweden over time [To do: EU data, plotly 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

1_flamecurtain.png 2_gasifiercookstove.png 3_biomacon.png 4_biogreen.png 5_batch.png 6_pyreg.jpg 7_fpp40.jpg 7_fpp40_resttillbast.jpg

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

Source: our paper

How to read / Meaning of contributions

  • biochar C-sink: initial, no 100% stability assumed there
  • contributions meaning: RLBU
  • energy context & changing it…

Parameters: + disclaimer

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:


Resources

To explore the topic further, we recommend the following references:

  • [data]
  • [review]
  • [article] Woolf 2014
  • [article] Woolf 2016
  • pyrolysis data from asia (related to flame curtain)
  • pyrolysis data from kon tiki flame curtain schimdt and co
  • pyrolysis data from pyreg erlend sormo
  • Meyer review of technology
  • Weber & Quicker
  • Ippolito
  • Thermochemical pathways
  • NBN Map link
  • [link]