Pyrolysis / thermolysis is the chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except possibly steam.
This is made in hermetically sealed, indirectly heated rotary kilns, which chemically convert and to produce gas and coke.
Under normal process conditions the heat transfer will be done by radiation. This allows with same equipment sizes higher throughputs as done with torrefaction or LTC.
In conventionell technology the pyrolysis gas is incinerated in a combustion chamber to obtain hot flue gas for steam boilers. Due to the rotary kiln process parameters a high amount of condensates can be produced during colling down the pyrolysis gas.
Name
Pyro = Fire; better Thermo = Heat
Main target
The main intend of pyrolysis or thermolysis is the cracking of all carbon connections, to get mainly a pyrolysis gas of hydrogen and carbon monoxide.
To get these results you need process temperatures over 800 ºC!
With normally process temperatures between 400 and 750 ºC you have a lot of cracking and rebuilding process. Due to the process temperature you can earn more condensable oils or permanent gases from the pyrolysis process.
Due to the target you call the same process torrefaction, pyrolysis, carbonisation or gasification. There is a smooth transition, but all processes can be done in the (nearly) same rotary kiln!
Therefore rotary kiln systems are usable very universal!
Pyrolysis Applications
Tyre Recycling
End-of-life tyres contain a lot of chemical bound energy.
As a result of pyrolysis of waste tyres one obtains: high energy gas, hydrocarbon oils, char and steel. The ratios of the first three products depend on process conditions, mainly on temperature.
The high energy gas may be utilized as a source of energy for the pyrolysis process. The pyrolytic oil may be used as a fuel independently or mixed with diesel fuel in different ratios. The char after additional processing may be used as a carbon black substitute or precursor of carbon adsorbents.
Sewage sludge
Offen the efficiency of sludge pyrolysis will be discussed. The fact: the dryer the sludge the more energy efficient is the main pyrolysis.
Also the usage of undigested sludge is more energy efficient, because the energy content is nearly the double of digested sludge from the biogas plants.
Soil Cleaning
Soil, which is contaminated with oil, hydrocarbons, dioxins, furans or mercury can be purified by pyrolysis.
Direct and indirect heated kilns can be used for this application. The volatiles are converted to vapour, which is incinerated in a downstream combustion chamber.
Compared to a direct heated kiln an indirect heated kiln produces much less process gas so that the flue gas cleaning process is minimized, whereas the costs for the heating devices are more expensive.
Mercury has to be separated in a specially designed flue gas cleaning process.
Dioxins and furans are decomposed in absence of oxygen above approximately 500 ºC - for this application only an indirectly heated kiln is of use.
Pyrolysis of Biomass
Biomass with a high calorific value can be pyrolised to generate oil.
The most common application for rape-seed, wood, straw or other so-called energy plants is the flash pyrolysis which means, that the heat transfer is maximized, wheras the retention time in the rotary kiln is minimized.
Waste Pyrolysis
Waste, which is too rich in calorific value for classic incineration, is called refuse-derived fuel (RDF) or solid recovered fuel (SRF) and should be pyrolised.
Compared to classic incineration, the above mentioned process takes place at much less temperature and therefore produces considerably less pollutants such as nitrogen oxides (NOx).
In opposition to classic incineration the production of significant amounts of dioxins and furans have not to be feared.
Therefore the effort in flue gas cleaning of power plants which base on pyrolysis is minimized.
