## 3.4 Thermochemical conversion techniques

Definition

Thermochemical (e.g., pyrolysis, gasification, hydrothermal upgrading, combustion) processes occur where feedstock undergoes extreme conditions (high temperature and/or pressure, with or without a catalytic mean).

Example 1: Lignocellulose pretreatment

Lignocellulose is the most abundantly available raw biobased material on earth. Typical example of lignocellulosic feed stock are wood or straw. It is composed of carbohydrate polymers (cellulose, hemicellulose), and an aromatic polymer (lignin) (see chapter 3.3). A barrier to the production of ethanol or other compounds from biomass is that the carbohydrates necessary for fermentation are trapped inside the lignocellulose. Lignocellulose has evolved to resist degradation and to confer hydrolytic stability and structural robustness to the cell walls of plants. To extract the fermentable sugars, one must first disconnect the cellulose from the lignin.

Thus pretreatment is a crucial process step in the biochemical conversion of lignocellulosic biomass to fermentable sugars and finally to products like e.g. lactic acid or ethanol. Pretreatment, crucial for delignification, has been recognised as one of the most rate limiting and expensive processing steps. A complete process overview from lignocellulosic biomass to bio-ethanol is illustrated in the figure below.

Thermochemical pretreatments steps are for example (Harmsen et al., 2013):

Dilute or weak acid hydrolysis. This is one of the most effective pretreatment methods for lignocellulosic biomass. Acid (sulphuric acid, sulphur dioxide, carbonic acid) is added to the raw material and the mixture is held at elevated temperature (T > 160 °C) for a short period of time. Hydrolysis of hemicellulose occurs and hemicellulose removal increases porosity and improves enzymatic digestibility of cellulose.

Alkaline hydrolysis. The major effect of alkaline pretreatment is the removal of lignin from biomass, thereby improving the reactivity of the remaining polysaccharides, and decrystallisation of cellulose. As opposed to the acid-catalysed methods, the general principle behind alkaline pretreatment is the removal of lignin whereas cellulose and part of the hemicelluloses remain in the solid fraction.

Example 2: Fast pyrolysis

Fast pyrolysis is a process in which organic materials (biomass) are rapidly heated to 450 - 600 °C in the absence of air. Under these conditions, organic vapours, pyrolysis gases and charcoal are produced. The vapours are condensed to bio-oil. This pyrolysis oil can be used in several applications to produce heat, electricity and cooling. The process is explained in the following video in the other videos the application of pyrolysis by BTG in Hengelo in cooperation with Friesland Campina is shown.

https://www.youtube.com/watch?v=TRNIvoIlJ-Q

https://youtu.be/MDmW2BKaSPY