6.2 Thermochemical processing
Thermochemical processes involve the use of heat, and possibly chemical reagents, to convert biomass into energetically more useful forms. The output may be heat or intermediate gaseous or liquid fuels.
Pyrolysis and torrefaction
Pyrolysis is one of the oldest and simplest methods of processing wood to produce a better fuel, charcoal. Charcoal is almost pure carbon with about twice the energy density of the original wood, making it easier and more efficient to transport and store. It burns at a much higher temperature, so it is much easier to design a simple and efficient stove for its use. However, the charcoal fuel cycle is potentially highly wasteful and polluting.
Torrefaction is the industrial-scale thermochemical treatment of biomass in the 200–340 °C range. The resulting solid ‘torrefied’ biomass, sometimes referred to as biocoal , has an approximately 30% higher heat energy content, increased density and reduced content of undesirable elements such as chlorine, in comparison to the original biomass.
Both pyrolysis and torrefaction release volatile components. The term pyrolysis is now normally applied to processes where the aim is to collect these volatile components and condense them to produce a liquid fuel or bio-oil.
Gasification is a thermochemical process where a gaseous fuel is produced from a solid fuel. There are two basic processes: pyrolysis and transesterification.
Pyrolysis is used to release the volatile matter from the heated solid, then the tars in the gas stream can be condensed out, leaving producer gas. The energy content of this gas is only about a tenth of that of natural gas from fossil fuels, but this is sufficiently good to run internal combustion engines, and higher quality gas can be made by using oxygen rather than air for combustion. A complete industrial gasification process using oxygen can produce a stream of carbon monoxide and hydrogen known as synthesis gas, or syngas, from which almost any hydrocarbon compound may be synthesized, including premium liquid fuels such as methanol.
Vegetable oils can be burned directly in some modern diesel engines, as in the car shown in Figure 4.9, either pure or blended with diesel fuel. Upgrading of vegetable oils to biodiesel results in a fuel that can blend with or replace petroleum diesel in cars with unmodified engines. The conversion process from vegetable oil to biodiesel is called transesterification.
European Union production of biofuel, around 80% of which is biodiesel, reached nearly 6 EJ in 2015 from almost nothing in 1990 though the rapid growth of the early 2000s has slowed considerably in recent years (Eurostat, 2017).
The final type of processing you'll look at in this course is biochemical.