PYROLYSIS: A RENEWABLE ENERGY SOURCE
This short description underlines some topics to sustain pyrolysis gas “synthesis gas or syn-gas” or also called “ gas from molecular dissociation”, as renewable energy source with low impact on the environment and remarkable benefits for the community.
Pyrolysis in history
Pyrolysis (improperly called molecular dissociation) has been known since Ancient Egyptian time, as a method for charcoal production . The coal miners in the coal bunkers, famous thanks to Cassola, used pyrolysis gas as alarm . As long as pyrolysis process was active, pyrolysis gases were produced and kept the bunker vault in tension, dispersing in the air through a little hole in the vault; once the process ended and all the firewood turned into charcoal, the gas pressure failed and the bunker imploded.
On the contrary, modern pyrolysis plants have, as main production, that gas which turned out to be an excellent fuel.
A recent example is the self-cleaning oven in our kitchen, where pyrolysis process gasifies the greasy oven incrustations, which disperse in the air when the oven is opened.
The pyrolysis and the molecular dissociation
A particular attention is necessary to the definitions of the words to avoid misunderstandings. Nowadays we speak of molecular dissociation, but the term is improper because all-comprehensive. Chemical reactions are the result of molecular association and/or dissociation: the molecules separate to compound other molecules or to create new ones. “Molecular dissociation” is a generic and no suffice term to describe the phenomena developing during a pyrolysis process.
After combustion, chemical reactions belong to the same category, so that a molecular dissociation plant could be compared to an ordinary incinerator. Pyrolysis process, based on induced reactions with no oxygen, is often confused with better known gasifiers using important quantities of oxygen ( air or steam added depending on the stoichiometric needs of the reactions) to accelerate the reaction of matter dissociation. Gasification needs controls and calibrations depending on the type of input materials, which are very often heterogeneous.
Pyrolysis technology is not influenced by the heterogeneity of the input materials ( although same organic derivation and low wet percentage must be granted ). Processing in total absence of oxygen, pyrolysis technology enables to remove, from the ecosystem larger quantities of CO2, when compared to conventional technologies.
The pyrolysis gas and the landfill site gas
We all recognize that waste, notably organic waste, left in landfill site produces gases, and in particular methane which has a potential greenhouse effect 20 times superior to CO2.
Unlike landfill site gas, with uncontrolled and free emissions into atmosphere , pyrolysis gas production is monitored in pyrolysis reactor, minimizing pollution risks, and optimizing the storage of the produced gas.
Pyrolysis gas and waste
One person produces approx. 1,46Kg of waste a day. Approx. 0,4 Kg of RDF (prior appropriate refining) is left after the sorting for waste recycling (except pyrolysis that can process all waste with a low wet quantity).
A pyrolysis plant of 3,5 MWh processes 37.500 tons of RDF per year, equal to the average RDF production of a town with 250.000 inhabitants and a production of 26.250 MW equal to 2.250 TOE (Tonne of Oil Equivalent) spared. Moreover, pyrolysis plant has no processing waste : all input waste is processed, so there is no waste overloading landfill sites. the final waste, mostly composed of unused CO2, is glazed.
The glazed inerts are used in road constructions and other civil engineering. The result is a closed energy cycle,capable of processing waste in situ, not depending on landfill sites situated in other countries, and using a short chain, planning more political information for the people and more conscience for waste recycling
The pyrolysis gas and the European Directive
The European Directive April,5, 2006/12/CE 5, gives precise information about waste disposal in order to advantage innovative and effective processes, suitable for the technological changes in the waste production.
Each Member State of European Parliament has to:
- promote the value of waste as raw material
- reach independence for waste process
- reduce waste transports
- optimize waste disposal process minimizing environmental impact
An European Commission, called “Refuse Derived Fuel, Current Practice and Perspectives”, has underlined the advantages of replacing fossil fuels with RDF, provided that the limits of the produced emissions are granted.
The replacement of fossil fuels with RDF has important environmental and economic benefits, as well, when it is compared with waste direct combustion, in incinerators for instance.
But what is RDF ?
European Commission on July 2003 used this definition: “ RDF, Refuse Derived Fuel, is composed of all waste with high calorific power which has no value and would be sent to landfill sites.
After a process carried out with criteria, rules and specific technology, these waste are turned into secondary fuel, used in energy production plants for the production process”,
So RDF is the final product of the recycling chain ( no more directly recyclable), but it can turn into another product, when again specifically processed..
It is clear that raw waste, if not processed, would be sent to landfill sites or to incinerators in spite of the opposite advice of the European Commission. RDF, as secondary fuel, can be used in many ways (however burnt with another fuel called main fuel):
- Used to produce energy in plants where the main fuel, fossil or biogas, is only used to keep the combustion temperature over the limit of dioxin formation.
- Used in cement factory with a percentage ratio related to the main fossil fuel.
- Used in industrial units as a supplement for the main fossil fuel.
Pyrolysis plant is introducing a new process for RDF, totally different from the above mentioned ones.
This further process is turning RDF from waste into resource: pyrolysis leads to the molecular decomposition of input materials (RDF).
This way, the raw material (RDF) is no more considered a secondary fuel but a raw material to be turned into pyrolysis gas, with a composition similar to biogas.
This further refining of the material, gives a product which is a primary fuel, totally different from the raw waste, even in its molecular structure.
Compared with biogas, syn-gas is richer in hydrogen and contains hydrocarbons in simple and cyclic chains. Although syn-gas has a lower calorific power than ordinary fossil fuels, due to a more difficult starter reaction, through the evolution of the process, it is now possible to use syn-gas straight, without starter.
The result is a less polluting energy compared to other fossil fuels.
The large reduction of polluting agents was not obtained adding filters or difficult devices, but simply removing the origin of the pollutants: dangerous and polluting molecules are heavier than hydrogen, carbon and oxygen, so they are not turned into gas but they remain solid and glazed after the end of the molecular transformation process.
There are no ashes, nanoparticles, non-combusted fumes, dioxins, furans, liberated in the atmosphere at the end of the transformation cycle.
Obviously, since the process declares to be ecologically sustainable, it must be technologically advanced and requires management control and machinery much more developed and sophisticated than a simple incinerator or oven.
The pyrolysis gas and the pollution
Pyrolysis plant is a valid choice, compared to an ordinary incinerator, for the reduction of CO2 emissions and for the low environmental impact. The following table, showing that polluting agents are all below the threshold values, is what really qualifies “ecologically sustainable” the energy produced from syn-gas:
COMPARISON OF FUME EMISSIONS with LEGISLATIVE PROVISIONS
| Polluting agents
n.124 dangerous waste
n. 503 MSW
|Guide line dm
|Directive 89/369/ CEE RSU||Analysis of emissions from pyrolysis gas|
|Cd, TI, Hg||0,05||0,05||0,2||0,05||0,05||0,02||0,01|
|Total of other metals||0,5||0,5||5||0,5||0,5||5||0,01|
The syn-gas and the environmental impact
In the previous paragraphs, we already underlined the advantages of pyrolysis plants relative to greenhouse gas emissions and other polluting agents, the reduction of road waste transport , the lower use of fossil fuels and the consequent valorisation of waste.
The pyrolysis gas and the society
These are the main benefits from the use of pyrolysis gas for energy production:
- decrease of dependence on fossil fuels, with a positive effect on balance of payments due to lower fuel imports .
- compliance of European Directive for a lower use of road transport, reduction of the pollution, improvement of life quality and environment without exerting a strict policy to reduce consumerism, even if necessary.
- increase of employment (a pyrolysis plant of 3,5 MWh processing 120 t/d of RDF needs approx. 12 employees).
Pyrolysis gas is a clean and eco-sustainable energy source, and it is a valid alternative as an energy source for the environment , the community and their economy.