Page 185 - CW E-Magazine (11-6-2024)

Special Report

Catalysing change: Defossilising the chemical industry

Part 2: Biomass as feedstock

The Royal Society Biomass to chemicals processes acid, glycerol, fumaric acid and pro-
Processes that can be used for the pylene glycol. These can be widely
Types of biomass conversion of biomass products to used for pharmaceutical, polymers,
iomass can be considered as chemicals are either thermochemical, cosmetics, solvents and broader chemi-
“material of biological origin such as direct gasifi cation, or thermo- cal products. This offers the oppor-
Bexcluding material embedded catalytic and bio-catalytic, such as tunity to bypass the existing primary
in geological formations and/or fossi- hydrolysis. Thermochemical process- chemicals stage, which is benefi cial in
lised.” The types of biomass covered by ing is used to convert biomass into terms of energy use and manufacturing
this defi nition include: biomass crops; products such as biochar, bio-oil and complexity.
food crops, such as vegetable oils and syngas. Biochar and bio-oil contain
starches; agricultural residues; forest corrosive and unstable oxygenates and Biomass-sourced carbon is more
residues; horticultural residues; marine are therefore diffi cult to use. oxidised than all components of fos-
biomass; municipal food and garden sil carbon. This means that commo-
waste and the biogenic fraction of Enabling these to be valuable pro- dity chemicals with oxygen atoms in
municipal waste, such as paper and card. ducts for the chemical industry is an their structure require fewer process-
active area of research. A promising ing steps to be produced from biomass
Biomass has a highly heterogeneous conversion method is the direct gasi- than fossil feedstocks. This could
nature, as the biomass composition fi cation of biomass at high tempera- be a potential advantage over fossil
depends on the plant species, location and tures (>700°C) and low oxygen levels feedstocks. However, much further
year-to-year variability. Dry plant mat- to produce a syngas mixture contain- research and development is required
ter is known as lignocellulosic biomass. ing hydrogen, carbon monoxide, CO into known and potentially viable
2
This is made of polysaccharide carbo and methane. Another thermochemical routes to converting oxygenates into
hydrates – cellulose and hemicelluloses – route is pyrolysis (see Box 1). valuable chemicals. More research
and an aromatic polymer, lignin. Lignin is also needed into addressing disad-
makes plant cell walls, accounting for Biochemical processing, such as vantages such as high water content,
approximately one-third of lignocellu- hydrolysis and fermentation, leads to lower energy content and removing
losic biomass. chemical compounds such as adipic impurities.

Box 1: Biomass pyrolysis

Pyrolysis uses thermal degradation to convert feedstocks into solid, liquid and gaseous products. The products formed
are heavily dependent on the composition of the feedstock in question and process conditions. The type of reactor, heat
transfer, residence time, heating rate and temperature all impact product formation.

Oxygen contained within the structural components of biomass – lignin, cellulose and hemicellulose – leads to pyrolysis
oils. These oils contain oxygenated compounds that can be detrimental to oil stability and lead to undesirable properties.
Catalytic pyrolysis has advantages over non-catalytic pyrolysis, as it reduces the activation energy of feedstock degrada-
tion and provides control over product formation, helping to increase the purity, directing towards higher value products,
and promoting the removal of oxygen from pyrolysis oils.

Developments to catalytic pyrolysis seek to improve selectivity, promote deoxygenation reactions and reduce catalyst
degradation through coke formation. This is a major challenge for commercial catalytic pyrolysis. Commercial operations
usually seek valorisation through production of biochar or crude pyrolysis oils that can be used directly as fuels, as feed-
stocks in fuel production, or as feedstocks for production of other materials and chemicals. It is also possible to valorise
the gaseous products, but these are often combusted to provide process heat.

Chemical Weekly June 11, 2024 185

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