Pyrolysis destructive distillation

Pyrolysis is a general term for all processes whereby organic material is heated or partially combusted to produce secondary fuels and chemical products. The input may be wood, biomass residues, municipal waste or indeed coal. The products are gases, condensed vapours as liquids, tars and oils, and solid residue as char (charcoal) and ash. Traditional charcoal making is pyrolysis with the vapours and gases not collected. Gasification is pyrolysis adapted to produce a maximum amount of secondary fuel gases.

Destructive Distillation Models
Figure 11.5 Pyrolysis systems. (a) Small-scale pyrolysis unit. (b) Traditional charcoal kiln.

Various pyrolysis units are shown in Figure 11.5. Vertical top-loading devices are usually considered the best. The fuel products are more convenient, clean and transportable than the original biomass. The chemical products are important as chemical feedstock for further processes or as directly marketable goods. Partial combustion devices, which are designed to maximise the amount of combustible gas rather than char or volatiles, are usually called gasifiers. The process is essentially pyrolysis, but may not be described as such.

Efficiency is measured as the heat of combustion of the secondary fuels produced divided by the heat of combustion of the input biomass as used. Large efficiencies of 80-90% can be reached. For instance gasifiers from wood can produce 80% of the initial energy in the form of combustible gas (predominantly H2 and CO - producer gas), suitable for operation in converted petroleum-fuelled engines. In this way the overall efficiency of electricity generation (say 80% of 30% = 24%) could be greater than that obtained with a steam boiler. Such gasifiers are potentially useful for small-scale power generation (<150kW).

The chemical processes in pyrolysis are much related to similar distillations of coal to produce synthetic gases, tars, oils and coke. For instance the large-scale use of piped town gas (H2 + CO) in Europe, before the change to fossil 'natural' gas (mainly CH4), was possible from the reaction of water on heated coal with reduced air supply:

The following is given as a summary of the wide range of conditions and products of pyrolysis. The input material needs to be graded to remove excessive non-combustible material (e.g. soil, metal), dried if necessary (usually completely dry material is avoided with gasifiers, unlike boilers), chopped or shredded, and then stored for use. The air/fuel ratio during combustion is a critical parameter affecting both the temperature and the type of product. Pyrolysis units are most easily operated at temperatures less than 600 °C. Increased temperatures of 600-1000 °C need more sophistication, but more hydrogen will be produced in the gas. At less than 600 °C there are generally four stages in the distillation process:

1 ~ 100-120 °C. The input material dries with moisture passing up through the bed.

2 ~275 °C. The output gases are mainly N2, CO and CO2; acetic acid and methanol distil off.

3 ~280-350 °C. Exothermic reactions occur, driving off complex mixtures of chemicals (ketones, aldehydes, phenols, esters), CO2, CO, CH4, C2H6 and H2. Certain catalysts, e.g. ZnCl2, enable these reactions to occur at smaller temperature.

4 >350 °C. All volatiles are driven off, a larger proportion of H2 is formed with CO, and carbon remains as charcoal with ash residues.

The condensed liquids, called tars and pyroligneous acid, may be separated and treated to give identifiable chemical products (e.g. methanol, CH3OH, a liquid fuel). Table 11.6 gives examples and further detail.

The secondary fuels from pyrolysis have less total energy of combustion than the original biomass, but are far more convenient to use. Some of the products have significantly greater energy density (e.g. CH4 at 55MJkg 1) than the average input. Convenience includes easier handling and transport, piped delivery as gas, better control of combustion, greater variety of end-use devices and less air pollution at point of use. The following comments consider the solid, liquid and gaseous products respectively.

Table 11.6 Pyrolysis yields from dry wood

Approximate yields per 1000 kg (tonne) dry wood

Table 11.6 Pyrolysis yields from dry wood

Approximate yields per 1000 kg (tonne) dry wood


-300 kg

Gas (combustion 10.4 MJ m-3)

— 140 m3 (NTP)

Methyl alcohol

—14 litre

Acetic acid

—53 litre


—8 litre


—3 litre

Wood oil and light tar

—76 litre

Creosote oil

— 12 litre


— 30 kg

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  • ermias
    Is pyrolysis, destructive distillation?
    8 years ago
  • daniela
    How to build a distillation device?
    7 years ago
  • omar
    How is destructive distillation different to pryolysis?
    7 years ago

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