Gasification and pyrolysis. Innovative technologies for the energy valorization of waste

The economic cost of producing energy from fossil fuels has now reached unsustainable values, making it necessary to search for new fuels and develop truly sustainable process and technological alternatives.

Among the "new" fuels which, following pre-treatment and/or transformation, can integrate traditional ones there are different categories of waste of various origins (urban or industrial). The development of processes in this direction arises from the need to combine more sustainable energy production with the need for more efficient waste management.

The objectives of current applied research

In recent years, research into new technological solutions has become increasingly strong which, using various processes, even in combination with each other, aim to guarantee efficient transformation of waste while promoting maximum recovery of material and energy and maximum reduction of gaseous, liquid and solid emissions.

Eco-sustainable waste management

The valorisation of waste as a basic material for producing valuable fuels, such as methanol and hydrogen , is the objective of applied research in all the most industrialized countries.

The use of waste not as fuel "as is" but as material to be transformed into products of greater quality or value allows us to move up the steps of the "pyramid of eco-sustainability".

The push towards an energy economy based on the conversion of gaseous or at most liquid fuels (methane, light hydrocarbons, oils) and hydrogen is linked to the possibility of achieving, thanks to them, cleaner and more efficient combustion.

The transformation of waste into these fuels is possible thanks to thermochemical processes such as pyrolysis and gasification, which induce a change in the chemical structure of the matter through the action of heat.

It is therefore not a question of carrying out "selection and pre-treatment" processes such as the production of solid fuels such as RDF but of carrying out actual chemical processes whose reliability, efficiency and cost must be carefully assessed.

Thermochemical processes

Pyrolysis : in which a thermal degradation of the material takes place in the total absence of air/oxygen through the direct or indirect input of heat. The calorific value of the products obtained is therefore very high.

Gasification : in which partial oxidation of waste occurs in an oxygen-deficient environment. The final products are not completely oxidized and therefore have a lower calorific value than the initial waste.

Combustion : in which the complete oxidation of the organic fraction of the waste/fuel is carried out, in the presence of an adequate excess of oxygen and with the result of obtaining completely oxidized products with no calorific value.

Energy production

“It achieves total and very rapid oxidation of the fuel fraction fed, in the presence of an excess of air which is greater the more difficult the oxidizer-fuel contact is.

The reaction is exothermic and is therefore accompanied by heat development which depends on the lower calorific value (PCI) of the fuel and the combustion efficiency.”

Thermochemical processes alternative to combustion:

Pyrolysis

It is a process that takes place in the absence of oxygen and at temperatures above 400°C, reached through the direct or indirect contribution of heat, during which exclusively a thermal degradation of the organic material takes place, possibly supported by the action of catalysts.

The main products of the process are pyrolysis fuel gases, organic liquids and a solid, non-vitrified residue containing the char and the inorganic fraction of the waste.

Pyrolysis of plastic waste

The composition of the pyrolysis products is extremely variable with the process temperature and with the presence of catalysts such as transition metals and materials containing acid sites such as silico-aluminates, zeolites, clays.

Catalysts can, as well as the increase in temperature, promote dehydrogenation, i.e. the loss of intramolecular hydrogen from the polymer chain with a consequent increase in the degree of unsaturation of the radicals obtained.

Dehydrogenation is inevitably accompanied by the high production of unsaturated and aromatic compounds (benzene, toluene, xylene, etc.) and amorphous or crystalline carbonaceous solids (graphite, micro and nano-fibres).

The possibility of breaking the molecular bonds of polymers through the action of heat (thermolysis) or through chemical attack (solvolysis) has opened the way to the use of the decomposition product as feedstock for the petrochemical industry (feedstock recycling).

Pyrolysis of biomass

The pyrolysis of biomass can be differentiated based on the residence time: a high residence time leads to the production of charcoal; a low residence time leads to the formation of liquids with high yields.

The production of bio oils (as biomass pyrolysis liquids are normally called) takes place at moderate temperatures, i.e. below 600°C.

Plasma pyrolysis of hazardous waste

Plasma pyrolysis occurs at very high temperatures (around 20,000°C) thanks to the action of the electric arc that forms between two electrodes.

The energy of the arc is so high that the gas present between the electrodes ionizes. The "destructuring" process of a plasma pyrolyzer is based on this principle.

In fact, in this system the arc is generated inside a chamber where the intense heat generated by the arc degrades the most resistant organic molecules (oils, paints, solvents) until the individual atoms are obtained (plasma).

In a subsequent process the atoms recombine to form non-dangerous gaseous compounds (carbon dioxide and water produced by oxidation in a bed of ceramic material) or solids.

The latter are totally vitrified and incorporate the metals that are no longer leachable: they are therefore reusable as a construction material.

The electrodes used are made of carbon and are continuously inserted without having to stop the process for maintenance.

Pyrolysis of municipal solid waste

Heterogeneous waste is composed of different combustible product categories which, however, with an extremely schematized process, can be traced back to polymers (plastics, rubbers, resins) and biomass (paper, cardboard, wood, organic fraction, textiles).

Pyrolysis technologies

The application of pyrolysis of municipal waste in Europe is still at a stage of development and has therefore not reached commercial maturity even if the push to comply with the provisions of the Kyoto Protocol has given rise to many demonstration projects.

If the use of pyrolysis as a process for the production of chemicals is still very limited, pyrolysis intended as a preliminary stage to a subsequent combustion or gasification stage is already applied on a large scale.

Among the most interesting processes that use pyrolysis as a transformation process for various wastes (mixed plastics, car demolition residues, electronic waste, municipal and special solid waste) we can point out those carried out by WasteGen (UK), Texaco, Compact Power and Ebara.

Conclusions

Most commercial pyrolysis processes take place at low temperatures, i.e. between 450 and 600°C in order to avoid having to pay an excessive cost in energy (and economic) terms, even if this involves an increase in the residence time in the reactor (which can even last 2 hours) and the reduction of the waste fraction completely degraded inside the oven.

To improve the overall energy efficiency of the process, the pyrolysis gas, and possibly also the char , are sent to a combustion process which allows, if this is carried out at temperatures higher than 1200°C, to fully exploit the adiabatic flame temperature of the pyrolysis gas.

Char from a pyrolysis process can:

• be sent to landfill after being stripped of metals which, following the process, are recoverable in non-oxidized form

• be sent for combustion possibly together with the pyrolysis gas; in this case it will not be possible to recover the metals (which in this way are oxidized)

• be sent to gasification (an option that allows the metals to be recovered in a non-oxidized form and to increase the CCE of the global system by transforming the fixed carbon of the char into further syngas).

Category: news - technology - plastic - recycling - pyrolysis - waste

Maria Laura Mastellone and Umberto Arena

Second University of Naples Environmental Sciences Department