With the constant increasing energy demands with the increase of population and urbanization. Conventional energy resources like coal, petroleum, natural gas, etc. are at the brink of extinction. As you can see in the proportional usage of all energy types. But mostly we are using conventional energy sources that are non replenishable. It draws the attention of researchers. Search for renewable energy sources start from there. Then we choose renewable energy sources for our energy requirements like wind, water, solar energy, biomass, etc.
Biomass is considered one of the most suitable and eco-friendly options for renewable energy sources. By means of many thermo-chemical processes like combustion, gasification, liquefaction, hydrogenation and pyrolysis, conversion of biomass begins into various energy yielding end products. Pyrolysis and types of pyrolysis process is like a way to convert some of our waste to energy. If you are intrested you can also see our Waste V/s Byproduct blog.
Pyrolysis is still in the developing stage. But due to its quality to transform biomass directly into solid, liquid and gaseous end products it receives special attention. In this blog I’ll be informing you about types of pyrolysis processes. But, before that i’ll give you some rundown about pyrolysis as well. So, buckle up and let’s get started.
What Does Pyrolysis Mean?
Pyrolysis is a process of thermal decomposition at high temperature in an inert atmosphere. It is considered as the first step in gasification or combustion. It is different from combustion and hydrolysis as it does not involve addition of reagents like oxygen, water.
Pyrolysis of organic materials form volatile products and leaves char behind. Char is a solid carbon-rich residue. This process biomass transforms into solid (char), condensable liquid (tar) and non condensing gases. This technique offers efficient utilization of particular importance for agriculture countries with vastly available biomass by-products.
How Pyrolysis is Done
Pyrolysis is generally heating of matter above its decomposition point in absence of oxygen. This leads to breakage of chemical bonds and form small molecules. But it can combine and form residues of larger molecular mass. Pyrolysis temperature ranges from medium (300–800°C) to high temperatures (800–1300°C). A series of reactions takes place in biomass degradation by gradual increase of temperature.
Upto 100 degrees water, waxes, fat, sugar heat sensitive substances like proteins and vitamins degrade. At temperatures between 100-500 degree common organic compounds like lignin, cellulose, most sugar decomposes. A non-volatile carbon rich residue is formed which is black brown in colour. At 200-400 degree if oxygen is involved exothermic reaction starts remaining nitrogen, sulphur, chlorine. Arsenic, etc volatize in oxides. Combustion leaves, the powdery solid residue of oxidized inorganic materials. Now that you know the process of pyrolysis you must be thinking about where it is being used. Next heading will answer your questions.
Where Pyrolysis is Used
Pyrolysis is used in many processes. Let me list out some:
Charcoal, Char, Coke, Carbon
By this process many types of carbon are produced that are used as fuels, structural materials, reagents, etc. Charcoal and coke are less smoky fuel. Activated carbon is used as an absorbent of chemicals. Biochar is a residue of complete pyrolysis. Biochar is used for retaining soil quality and increasing fertility and quality of soil.
Carbon fibres are carbon filaments that are very strong and give strong yarns and textiles. Thomas Edison used pyrolysis carbon filament of bamboo splinters to make the bulb. Pyrolysis carbon coating has many applications like artificial heart valves.
Biofuels ( Liquid and Gaseous)
Pyrolysis is the base method for fuel making from biomass. Some of the examples are synthetic diesel, syngas, bio-oil and some of the low quality oils.
Methane pyrolysis for hydrogen
Pyrolysis is a non-polluting industrial process to produce hydrogen from methane. It is a high volume yield low cost process.
Ethylene is produced by pyrolysis. Resulting ethylene are used to make many other polymers like PVC, Ethylene glycol ( anti freezing agent), polystyrene and polyethylene.
Semiconductors of many volatile organometallic compounds are made through pyrolysis. Some examples of semiconductors are silane, Gallium arsenide, trimethylgallium and arsenide.
Plastic waste and municipal waste are treated by pyrolysis. Waste reduction is the main advantage.
Thermal cleaning is the pyrolysis process to remove organic substance from parts, components and products. Molten salt baths, fluidized bed systems, burn-off ovens vacuum ovens are several types of thermal cleaning.
There are many more examples or pyrolysis. Lets learn about the type of pyrolysis processes.
Types of Pyrolysis Processes
Depending upon the process, parameters and conditions there are several types of pyrolysis processes. Appropriate reactors are chosen for different pyrolysis processes depending on their process.
1. Fast Pyrolysis
1.(a) What is Fast Pyrolysis?
Term fast pyrolysis refers to gradual rapid heating of biomass for a temperature range 800–1300 °C. Rate of heating ranges between 10–200 °C for a short life span between 1-10 sec. Fast pyrolysis is done for liquid products as it yields more liquid. The product distribution range is 60–75% biofuel liquid products, 15–20% solid biochars and 10–20% noncondensable gaseous products
1.(b) Reactors For Fast Pyrolysis
Different reactors are used on the basis of desired product, conditions, and biomass. Some of the most popular reactors for fast pyrolysis are:
- Entrained flow reactor,
- Wire mesh reactor,
- Vacuum furnace reactor,
- Vortex reactor,
- Rotating reactor,
- Circulating fluidized bed reactor, etc.
Other reactors can also be used but depending upon terms and conditions. Auger-type reactors are used for both fast and slow pyrolysis.
1.(c) Fast Pyrolysis is Used For
Fast pyrolysis is used in many areas. Some examples of fast pyrolysis are mentioned below.
- Fast pyrolysis of biomass
Most recent renewable energy source is biomass. Biomass is easily available. It can transform into a number of energy yielding products. It is a safe alternative for conservation of conventional energy sources.
- Fast pyrolysis of wood
Fast pyrolysis of wood is now quite popular for coke production. As coke is considered as one one of the clean fuels.
- Fast pyrolysis of lignin
Plant biomass have biopolymers like cellulose, xylans, and lignin. On fast pyrolysis of lignin a bio-oil having methoxyphenols, phenolic aldehydes/ketones, low-molecular-weight phenols, and light oxygenates forms. This bio-oil is a good fuel.
- Fast pyrolysis of plastic waste
Pyrolysis of plastics is done to convert them into energy and bio-oil. Pyrolysis of plastic waste is performed at 300–900°C. Which produces a liquid oil.
- Fast pyrolysis of bio-oils
Bio-oil is a liquid product of fast pyrolysis of biomass. It is used for chemical production. It is a high quality hydrocarbon fuel that is tested for use in engines, turbines and boilers. If you want to know more about waste from food stuff check our blog Bio fuels from food wastes.
2. Slow Pyrolysis
2. (a) What is Slow Pyrolysis?
It is a conventional type of pyrolysis. It involves long residing time and slow heating rate. In slow pyrolysis biomass is pyrolyzed for about 400–500 °C temperature, with a heating rate of 0.1–1 °C/s. Approximate time for slow pyrolysis ranges between 5-30 mins.
Slow pyrolysis leads to production of solid carbonaceous char. Liquid and gaseous products are formed but in low quantities.
2.(b)Reactors For Slow Pyrolysis
Terms and conditions are applied depending on the various factors in order to use suitable reactors. Some widely known reactors for slow pyrolysis are:
- Rotary kilns,
- Heinz retort,
- Tubular reactor
- Fixed bed reactor
Auger-type reactors are used for both fast and slow pyrolysis.
2.(c) Slow Pyrolysis is Used For
Slow pyrolysis is used in many fields to make solid biofuel. I’m mentioning some of the examples below:-
- Slow pyrolysis of biomass
- Slow pyrolysis of wood
- Slow pyrolysis of biochar
- Slow pyrolysis of cellulose
Slow pyrolysis of biomass mainly targets biochar (up to ∼60 wt%), together with 25-30 wt% of bio-oil and the balance as gas. Slow pyrolysis leads to a good amount of charcoal production. With controlled temperature, nitrogen ratio, results can be altered. With increasing the temperature solid, liquid, gas fuels and oils are formed respectively.
Difference Between Fast and Slow Pyrolysis
Short upto few seconds to a minute
Long upto few minutes to hours
Relative big particles
Rotary kiln, fluidized
Fixed bed reactors
3. Flash Pyrolysis
3.(a) What is Flash Pyrolysis?
It is called a very fast pyrolysis. Its reaction time is several seconds. It involves very high temperature and a very high heating rate i.e. >1000 °C/s. It affords a very high yield of bio-oil with very low water content. This bio-oil conversion efficiency is approx 70%.
3.(b) Reactors For Flash Pyrolysis
Due to sudden high temperature special reactors are made. Biomass residence time in these reactors is less. Most commonly used reactors for flash pyrolysis are:
- Entrained flow reactor and
- The fluidized bed reactor
Some newly made specialized reactors are also available in markets.
3.(c) Flash pyrolysis is used for
Generally flash pyrolysis forms 60% biochar and 40% bio-oil and syngas.Bio-oil is the preferred product of this process. Bio-oils are extracted through several means some examples are:
- Flash pyrolysis of wood
- Flash pyrolysis of plastic
- Flash pyrolysis of biomass
- Flash pyrolysis of cellulose
With the cellulosic biomass, wood the organic oil yield is as high as 65–70%. The resultant liquid is acidic, stable and easy to pour. As its production cost from wood is competitive but it is a conventional ful or present time.
3.(d) Types of flash pyrolysis
Flash pyrolysis are of following types:
- Flash hydro-pyrolysis: This type of pyrolysis is done in the hydrogen atmosphere. 20 Mpa is the pressure at which it is carried out.
- Rapid thermal process: It is a particular heat transfer process. It has a very short heat resistance time i.e. between 30 ms and 1.5 s. Procedure is done at a temperature between 400-9500 degree C. Feedstock rapid depolymerization and cracking takes place. Rapid heat prevents additional reactions. It gives comparable viscosity as diesel also to the product.
- Solar flash pyrolysis: flash pyrolysis can also be performed by concentrated solar radiations. Solar towers, dish connectors, solar furnaces, etc. devices are used to concentrate solar radiation.
- Vacuum flash pyrolysis: this type of flash pyrolysis is done in vacuum. Due to vacuum, secondary decomposition reaction is inhibited. Which leads to high liquid-yield and low gas yield.
Due to vacuum condensable products are removed from the hot reaction. That prevents further cracking and re-condensation reactions.
4. Catalytic Biomass Pyrolysis
4.(a) What is Catalytic Pyrolysis?
From literature it can be depechired that liquid obtained from different pyrolysis processes cannot be used directly as biofuel. Due to high oxygen and water content it needs upgrading. Catalytic biomass pyrolysis is performed to improve the quality of bio-oil.
This procedure involves the use of catalysts to improve the quality of bio oil. So it can be used as transportation oil or alter its quality to meet several demands. Pre upgradation techniques like condensation and re-evaporation are not needed for this oil.
4.(b) Catalyst used
Various catalysts are used in this process. Use of catalyst depends upon product, procedure and biomass used. Mainly zeolites and basic materials are used as catalysts.
It is necessary to find ways that can change biomass into useful energy sources. We have to think especially for liquid fuel because diesel and petroleum are at a verge of extinction. Thermochemical processes are one of the best alternatives. As they are eco-friendly and suitable for achieving desirable requirements. Pyrolysis receives major attention because it produces the best quality product.
It can be depicted by the literature and my blog that slow pyrolysis yields good quality char. Fast and flash yield liquid oil to use as fuel. Catalytic biomass pyrolysis is an option to improve the quality of products.it can also be applied on gaseous products to improve its quantity and quality. Researchers also mention that it is a good alternative for waste to energy. Energy from biomass can be very beneficial for developing countries like India.
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