Pyrolysis is a thermochemical decomposition of biomass into a range of useful products, either in the total absence of oxidizing agents or with a limited supply that does not permit gasification to an appreciable extent. It is one of several reaction steps or zones observed in a gasifier if we use gasification application. During pyrolysis, large complex hydrocarbon molecules of biomass break down into relatively smaller and simpler molecules of gas, liquid, and char.
Pyrolysis has similarity to and some overlap with processes like cracking, devolatilization, carbonization, dry distillation, destructive distillation, and thermolysis, but it has no similarity with the gasification process, which involves chemical reactions with an external agent known as gasification medium. Pyrol-ysis of biomass is typically carried out in a relatively low temperature range of 300 to 650 °C compared to 800 to 1000 °C for gasification. Other review the difference between pyrolysis and gasification, please click here.
The product of pyrolysis depends on the design of the pyrolyzer, the physical and chemical characteristics of the biomass, and important operating parameters such as
- Heating rate
- Final temperature (pyrolysis temperature)
- Residence time in the reaction zone
Besides these, the tar and the yields of other products depend on (1) pressure, (2) ambient gas composition, and (3) presence of mineral catalysts (Shafizadeh, 1984).
By changing the final temperature and the heating rate, it is possible to change the relative yields of the solid, liquid, and gaseous products of pyrolysis. Rapid heating yields higher volatiles and more reactive char than produced by a slower heating process; slower heating rate and longer residence time result in secondary char produced from a reaction between the primary char and the volatiles.
Type of Pyrolysis
Based on heating rate, pyrolysis may be broadly classified as slow and fast. It is considered slow if the time, theating, required to heat the fuel to the pyrolysis temperature is much longer than the characteristic pyrolysis reaction time, tr, and vice versa. That is:
-Slow pyrolysis: theating is bigger than tr
-Fast pyrolysis: theating is smaller tr
These criteria may be expressed in terms of heating rate as well, assuming a simple linear heating rate (Tpyr/theating, K/s). The characteristic reaction time, tr, for a single reaction is taken as the reciprocal of the rate constant, k, evaluated at the pyrolysis temperature (Probstein and Hicks, 2006, p. 63).
There are a few other variants depending on the medium in and pressure at which the pyrolysis is carried out. Given specific operating conditions, each process has its characteristic products and applications. In the following list, the first two types are based on the heating rate while the third is based on the environment or medium in which the pyrolysis is carried out: (1) slow pyrolysis, (2) fast pyrolysis, and (3) hydropyrolysis.
Slow and fast pyrolysis are carried out generally in the absence of a medium. Two other types are conducted in a specific medium: (1) hydrous pyrolysis (in H2O) and (2) hydropyrolysis (in H2). These types are used mainly for the production of chemicals.
In slow pyrolysis, the residence time of vapor in the pyrolysis zone (vapor residence time) is on the order of minutes or longer. This process is used primarily for char production and is broken down into two types: (1) carbonization and (2) conventional.
In fast pyrolysis, the vapor residence time is on the order of seconds or milliseconds. This type of pyrolysis, used primarily for the production of bio-oil and gas, is of two main types: (1) flash and (2) ultra-rapid. Carbonization produces mainly charcoal; fast pyrolysis processes target production of liquid or gas.
Carbon is a preferred product of biomass pyrolysis at a moderate temperature. Thermodynamic equilibrium calculation shows that the char yield of most biomass may not exceed 35%. See table below gives the theoretical equilibrium yield of biomass at different temperatures. Assuming that cellulose represents biomass, the stoichiometric equation for production of charcoal (Antal, 2003) may be written as :
Charcoal production from biomass requires slow heating for a long duration but at a relatively low temperature of around 400 °C. An extreme example of a pyrolysis or carbonization is in the coke oven in an iron and steel plant, which pyrolyzes (carbonizes) coking coal to produce hard coke used for iron extraction. This is an indirectly pyrolyzer that operates at a temperature exceeding 1000 °C and for a long period of time to maximize gas and solid coke production.
The best biochar for improving soil quality (agricultural application) can be produced with slow pyrolysis process, more review on this, please click here. The best charcoal for activated carbon production also can be produced with this process, more explanation please click here. We can also produce high fixed carbon charcoal with this technology, read more click here. In simple words we will produce charcoal as you wish.