As long ago as the late 1970s, trials with pyrolysis on a scale of around 200 kg/h with various hydrocarbon-containing wastes such as scrap tyres, shredder wastes or plastics were successfully conducted for the purpose of extracting aromatic oils as raw materials for the petrochemical industryà.
Mannesmann Demag Energie- und Umwelttechnik (MDEU) has now developed the Pyropleq process for pyrolytic treatment of domestic refuse. It is based on experience with pyrolysis of industrial wastes and the use of externally heated rotary kilns for a variety of purposes.
Since then several plants for pyrolytic treatment of soil contaminated with organic chemicals or mercury with capacities ranging from 3.0 to 6.5 t/h have been built and put into operationá. Here too, the externally heated rotary kiln has proved reliable and trustworthy.
The Pyropleq process is characterized by the independent operation of the process stages:
Pyrolysis of the pre-treated domestic refuse
Thermal utilization of the resulting pyrolysis gas.
The system is naturally designed for throughputs in the small to medium capacity range. The Pyropleq process is therefore both an addition and an alternative to the traditional incineration route for disposal of domestic refuse.
In this article, ‘pyrolysis’ means the thermal decomposition of substances in the absence of air (i.e. in an atmosphere where no combustion takes place) with liberation of gases including gaseous hydrocarbons, leaving behind a solid, coke-like residue known as “char”. Basic research into pyrolysis of domestic refuse was carried out by Bureau of minesâ, Garrettã and Hitachi Research Laboratoryä more than 20 years ago.
Domestic refuse
The relative proportions of the elements in the hydrocarbons produced in pyrolysis of domestic refuse differ significantly from those in hydrocarbons from industrial wastes. In the case of pyrolysis of mixed toxic wastes, the oxygen content of the hydrocarbons recovered is well below one per centå. Garrettã gives the following elementary analysis for the hydrocarbons obtained in pyrolysis of domestic refuse:
Carbon: 57.5 per cent
Hydrogen: 7.6 per cent
Oxygen: 33.4 per cent
Others: 1.5 per cent.
The high oxygen content of the hydrocarbons produced in pyrolysis of domestic refuse means that they are thermally unstable and highly water-soluble. With pyrolysis of domestic refuse, therefore, the hydrocarbons are not recycled, but used exclusively as an energy source.
Pyrolysis reactors
Several types of pyrolysis reactor, e.g. with various alternative methods of heating, have been investigated as to their suitability for waste pyrolysis. The following have attained importance:
Fluidized bed reactors: Japanese developers used this type especially for low-temperature pyrolysis of plastic wastes.
Shaft furnaces: pyrolysis in a shaft furnace was tested for domestic refuse by the Danish firm Kroyeræ.
Rotary drum type reactors: rotary drum type reactors can be heated externally[8, 9] or internally with fire tubes[10, 11] or ceramic spheresë. This is the preferred type of reactor for pyrolysis of domestic refuse[11, 13].
For the Pyropleq process, MDEU has chosen to use an externally heated rotary drum. Besides the favourable experience of the former affiliate company MVU with the development of the Rotopyr processà and with pyrolysis of contaminated soilsá, the decisive reasons for this choice were:
The robust operation and high availability of this type of reactor are proven not only for pyrolysis of domestic refuse but also for drying, calcination and soil remediation
The external heating enables simple control of the hot gases without the problems of different rates of thermal expansion of individual components
Several tried and tested austenitic stainless steels with high thermal and chemical resistance are available as materials of construction. The chosen pyrolysis temperature of 450 to 550°C rules out the use of ferritic steels.
The Pyropleq process
In a pre-treatment stage, the domestic refuse is freed from undesirable matter and reduced in size, and then fed into the pyrolysis drum. The PLEQ System rotary kiln is surrounded by several fixed, separate heating chests which transmit thermal energy through the shell of the kiln to the waste inside by convection and radiation.
The pyrolysis reactor is thermodynamically designed in such a way that the waste is heated to around 450 to 550°C and pyrolysed in less than an hour. The resulting pyrolysis gas leaves the drum at a temperature of around 550°C. It consists, besides the vaporized water, mainly of carbon monoxide, hydrogen and methane together with higher hydrocarbons.
At the end of the pyrolysis drum is a screening section which divides the solid pyrolysis residue into two fractions. The mainly fine-particle pyrolysis char, which has an average carbon content of 25 per cent, is discharged dry with exclusion of air. The screen oversize consists of recyclable metallic residues and coarse inert matter and is discharged wet through a water seal.
Hot-gas filtration by means of ceramic filter cartridges ensures effective dedusting of the pyrolysis gas. A pulse injection system cleans the cartridges of adhering dust.
The cleaned pyrolysis gas is burnt in a combustion chamber at over 1200°C with a residence time of over two seconds. The freedom from dust ensures complete combustion in a similar manner to natural gas. Furthermore, the absence of catalytically active dust prevents dioxin and furan formation. The hot gas from the combustion chamber heats the pyrolysis drum from the outside.
A fan extracts the cooled gases from the heating chests surrounding the rotary kiln and feeds them to the steam generator. Nitrogen oxide emissions are adequately suppressed (e.g. to within the stipulations of Germany’s clean air regulations) by spraying aqueous ammonia into the dust-free flue gas before it enters the steam generator (SNCR).
As the pyrolysis gas undergoes hot-gas dedusting immediately on leaving the pyrolysis reactor (a so-called ‘in-situ’ measure), it is not necessary to make allowances for corrosion and encrusted deposits when designing the steam generator. Therefore steam parameters higher than the 40 bar/400°C that is normal practice in combustion of domestic waste are feasible without major additional expenditure.
A mixture of highly reactive sorbents and activated carbon is added to the cooled flue gas according to a technology developed by MDEU. The sorbent mixture neutralizes the acidic pollutants present in the flue gas and the activated carbon adsorbs volatile heavy metals such as mercury. The subsequent final cleaning in a fabric filter ensures that statutory emission levels (e.g. those prescribed by Germany’s “17. BImSchV”) are met.
Recycling
In the course of mechanical pre-treatment of the delivered refuse, recyclable materials are separated and prepared for re-use before the refuse is charged into the pyrolysis reactor. The pyrolysis char (fine fraction) is attractive as a fuel in view of its calorific value of around 10 GJ/t.
The possible means of thermally utilizing the pyrolysis char include in-situ melting, e.g. with the Plasmox technology, in a high-temperature chamber operating independently of the waste treatment in order to produce a glass-like product which is non-leachable and therefore marketable for:
Co-combustion in power stations
Substitution of fossil fuels in, for example, cement kilns or blast furnaces.
In pyrolysis, the proportion of solids is generally reduced by around 70 per cent while the metal content is completely unchanged, with the result that the relative metal content of the solid pyrolysis products is trebled compared with the input. After separation of the fine fraction (approximately 60 per cent), the residue normally contains iron, copper and aluminium in a concentration and condition in which they can be sorted and marketed.
The thermal balance shows that after discharging the solid pyrolysis products and after combusting the high calorific value pyrolysis gases at 1200°C and using the resulting hot flue gases to heat the pyrolysis reactor (internal thermal cycle), approximately 80 per cent of the heat input is available for steam generation at a temperature of around 1000°C. It follows from this that in the pyrolysis of one tonne of domestic refuse, around 550 kW of electric power can be generated from the surplus energy.
This means that the power generation efficiency of this technology is similar to that of a modern waste combustion plant. In both cases, heat utilization is further improved through the export of district heating or process heat.
Advantages
The separation of pyrolysis and combustion in terms of the process engineering which is a feature of the Pyropleq process, together with the use of proven sub-systems, ensures a high degree of availability and operational reliability. Other advantages are:
A wide range of wastes can be handled, i.e. not only domestic refuse, but also special wastes or car shredder waste can by pyrolysed
Dust-free combustion of the pyrolysis gases due to hot-gas dedusting
Heating of the pyrolysis drum by using the thermal energy of the flue gases without imported or supplementary heating in regular operation
Physical separation of the crude pyrolysis gas from the heating gas stream
The volumes of flue gas are smaller than with direct incineration systems
There is an energy surplus in the form of steam and electricity when using normal domestic refuse
Use of simple, conventional boiler and flue gas cleaning technologies
Many years of experience with the sub-systems in commercial plants
The technology is available as a result of our own experience and joint ventures.
Complementary technology
The Pyropleq process is a forward-looking development in the field of thermal waste treatment and complements MDEU’s existing waste management technologies in terms of process engineering.
The process is based on reliable, practice-proven individual components and systems. Their interaction results in an innovative technology which is distinguished by high availability and operational reliability. It complements traditional incineration-based thermal waste treatment in the low to medium capacity range. At the same time, as an alternative technology it responds to the current demand situation in the waste management sector.
According to a market survey, in ten years’ time more than one in five new plants (21 per cent) built in central Europe will be based on pyrolysis and gasification. Added together over a forecasting period of ten years, pyrolysis and gasification of refuse will account for 14 per cent of all new plants, or, expressed as an absolute figure, 41 new treatment plants will have been brought into service by 2007í.
There is already a lively demand for this technology in central Europe. MDEU is engaged in several Pyropleq projects, including Ticino Canton in Switzerland.
