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Introduction to Pyrolysis | Advantages,Classification, Modes Of Pyrolysis

Introduction to Pyrolysis | Advantages,Classification, Modes Of Pyrolysis

The word Pyrolysis is derived from Greek words 
Pyr: Fire
Lysis: Separating
Pyrolysis is one of the dry chemical processes. 
It is thermal decomposition occurring in the absence of oxygen. It is destructive distillation of wood, biomass in the absence of air or oxygen.
It is always also the first step in combustion and gasification processes where it is followed by total or partial oxidation of the primary products.

Among the thermo-chemical processes, pyrolysis has become an attractive alternative because of the ease of operation. 
Its suitability as fuel for energy production and as feedstock for chemical industries, 
relatively few polluting emissions,
CO2 neutral cycle and 
ease of reproduction make pyrolytic oil.

Classification of Pyrolysis
The pyrolysis can be classified as allothermal and autothermal.
Allothermal: The process in which heat is supplied from an external agency.
Autothermal: heating by fuel wood itself.

Allothermal Process
Pyrolysis is achieved by using an external heat source such as exhaust gas, electrical energy etc, However, the process is unsuitable for pyrolysis since
1) There is no control over the products in an integrated systems.
2) If a non-reacting medium such as N2 is used for heat transfer & uniform heating, there is the problem of purification at the gas collection points.
3) If the spatially differentiated system  of heating is adopted, the coefficient of heat transfer between the source & wood/biomass is reduced. This is because the presence of wall enables only conductive transfer instead of convection & radiation . This reduces the net energy efficiency.
4) Allothermal process may involve the use of a high grade source (electric energy) for producing a low grade product.

In this process a small amount of air or oxygen is supplied to the wood/biomass & combustion is initiated with sole objective of causing pyrolysis.
If combustion is excessive , fuel energy instead of going into the production of secondary fuel  is escaped as sensible heat of the fuel gas.
Therefore, combustion must be kept at minimum level.

Different modes of Pyrolysis
-The proportion of gas, liquid, and solid products depend very much on the pyrolysis technique used and on the reaction parameters.
-Depending on the operating conditions, the pyrolysis process can be divided into three sub classes.

Different modes of Pyrolysis
Fig. Different modes of Pyrolysis
Fast Pyrolysis
Pyrolysis is now a days more popular for producing oils known as bio-crude or bio-oils. 
pyrolysis oil, bio-oil, bio-crude-oil, bio-fuel-oil, wood liquids, wood oil, liquid smoke, wood distillates, pyroligneous tar, pyroligneous acid, and liquid wood
This process is known as fast or flash pyrolysis.
High heating rates, of up to a claimed 1000 °C/s , at temperature below about 650 °C and with rapid quenching, causes the liquid intermediate products of pyrolysis to condense before further reaction breaks down higher molecular weight species into gaseous products. 
The high reaction rates also minimize char formation, and under some condition no char is apparently formed. 
Pyrolysis at these high heating rates is known as fast, or flash pyrolysis according to the heating rate and residence time, although the distinctions are blurred. 
Fast pyrolysis occurs in a time of few seconds or less. 
o using small particles,
o transfer heat very fast only to the particle surface

In 1980s, several fast pyrolysis processes were developed, e.g., waterloo fast pyrolysis, Ensyn rapid thermal pyrolysis, Georgia tech entrained flow pyrolysis, and National Renewable Energy Laboratory vortex ablation   pyrolysis. 
The common feature of all these processes are high rate of heat transfer at moderate temperatures(450 oC–550 oC) and a short (< seconds) residence time of product vapour in the reactor. 
These reaction conditions result in a high yield conversion of  biomass (50%–75%) into a liquid product. This liquid has a high oxygen content, burns smoothly and cleanly, and has a potential to be used as a fuel oil substitute. 
Combustion tests have demonstrated that BPOs (biomass pyrolysis oils) could be burnt efficiently (pulsation free flame, low-hydrocarbons and carbon monoxide emissions) in standard or in slightly modified burners (Sims 1994).

The essential features of a fast pyrolysis process for producing liquids
Very high heating and heat transfer rates at the reaction interface, which  usually requires a finely ground biomass feed,
Care fully  controlled  pyrolysis  reaction  temperature  of  around  500  ºC  and  vapour phase temperature of 400-450 ºC,
Short vapour residence times of typically less than 2 seconds, and
Rapid cooling of the pyrolysis vapours to give the bio-oil product.

Typical process principle

Fast pyrolysis Process
Fig. Fast pyrolysis Process
A fast pyrolysis process includes
drying the feed to typically less than 10% water in order to minimise the water in the product liquid oil (although up to 15% can be acceptable), 
grinding the feed (to around 2 mm in the case of fluid bed reactors), to give sufficiently small particles to en sure rapid reaction, 
pyrolysis reaction, 
separation of solids (char), and 
collection of the liquid product (bio-oil). 

Types of reactors
Fluid bed 
Circulating fluid bed
Entrained flow
Rotating cone
Transported bed
Vacuum moving bed
Essential: very high heating and heat transfer rates; moderate and carefully controlled temperature; and rapid cooling

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