Autoclaving involves the high-pressure sterilisation of waste by steam, which ‘cooks’ the waste and so destroys any bacteria in it. This process is widely used to treat clinical waste, but is increasingly being proposed as a treatment for municipal waste. The process creates a so-called ‘fibre material’ from the biodegradable portion of the waste, which is separated along with some recyclable materials. Although there are no facilities operating commercially to treat municipal waste in the UK, there are several plants being built or planned. There are currently no clear markets for this ‘fibre material’, which will consist of a wide range of materials e.g. food, paper etc. This material would also still biodegrade if landfilled, so would require further treatment (e.g. composting) prior to landfill. It is therefore likely that much of the output from autoclaves will end up being be burnt as ‘refuse derived fuel’.

How autoclaving works

Autoclaving of municipal waste is a form of ‘mechanical heat treatment’ (MHT) – a process that uses thermal treatment in conjunction with mechanical processing.

Waste may initially be screened for the removal of any large items, and possibly shredded. The unsorted waste is sealed in an autoclave, which is a large, enclosed vessel about the size of a long fuel tanker that rotates to agitate and mix the waste.

Using the ‘pressure cooker’ principle, steam is injected at pressure – raising the temperature up to 160°C (degrees centigrade). The pressure is maintained for between 30 minutes and one hour. This sterilises the waste, by destroying bacteria present. It reduces the volume of waste by about 60 per cent, and reduces the moisture content.

The cellulose in all the organic matter – the biodegradable waste including food and garden waste, paper and card – is broken down into a ‘mush’ of fibre, sometimes know as floc or fluff.

After autoclaving, the waste is discharged and processed by mechanical separation technologies, similar to those used in MBT systems. Metals will be extracted for recycling, and possibly also plastics for recycling and glass for re-use as aggregate. There is a residue or ‘reject fraction’ that needs to be landfilled.

Typical outputs are:

64% ‘organic fibre’

17.5% recyclables

2.5% aggregate

16% other materials suitable for landfill

9% mixed plastics

4% glass

3.5% steel

1% aluminium

The water used in the process will usually be trapped and recycled, and wastewater will be discharged as an effluent stream, which removes pollutants from the process.

Pyrolysis is the thermal degradation of a substance in the absence of added oxygen. The general characteristics of pyrolysis of a waste stream are as follows:
1)    No oxygen is present (or almost no oxygen) other than any oxygen present in the fuel;
2)    Low temperatures typically from 300 °C to 800 °C;
3)    Products are syngas (main combustible components being carbon monoxide, hydrogen,
methane and some longer chain hydrocarbons including condensable tars, waxes and oils)
and a solid residue (consisting of non-combustible material and a significant amount of
carbon);
4)    The general lack of oxidation, and lack of an added diluting gas, means that the NCV of syngas from a pyrolysis process is likely to be higher than that from a gasification process (provided substantial quantities of carbon are not left in the solid residues). Typical NCV for the gas produced is 10 to 20 MJ/Nm3;
5) The overall process generally converts less of the chemical energy into thermal energy than gasification. Pyrolysis also offers the potential option of more innovative use of the pyrolysis syngas other than immediate combustion to produce heat. Pyrolysis generally takes place at lower temperatures than for combustion and gasification. The result is less volatilisation of carbon and certain other pollutants such as heavy metals and dioxin precursors into the gaseous stream. Ultimately, the flue gases will need less treatment to meet the emission limits of WID. Any pollutant that is not volatilised will be retained in the pyrolysis residues and need to be dealt with in an environmentally acceptable manner. The emission benefits of low temperature processing are largely negated if the char subsequently
undergoes high temperature processing such as gasification or combustion.
The solid residues from some pyrolysis processes could contain up to 40% carbon representing a significant proportion of the energy from the input waste. Recovery of the energy from the char is therefore important for energy efficiency.

Gasification is the partial thermal degradation of a substance in the presence of oxygen but with insufficient oxygen to oxidise the fuel completely (i.e. sub-stoichiometric). The general characteristics of gasification of a waste stream are as follows:
1)    A gas such as air, oxygen, or steam is used as a source of oxygen and/or to act as a carrier
gas to remove the reaction products from reaction sites;
2)    Moderate temperatures typically above 750 °C;
3)    Products are gas (main combustible components being methane, hydrogen, and carbon
monoxide) and a solid residue (consisting of non-combustible material and a small amount of carbon);
4)    The overall process does not convert all of the chemical energy in the fuel into thermal energy but instead leaves some of the chemical energy in the syngas and in the solid
residues;
5)    The typical NCV (net calorific value) of the gas from gasification using oxygen is 10 to 15 MJ/Nm3;
6)    The typical NCV of the gas from gasification using air is 4 to 10 MJ/Nm3.
For comparison, the NCV for natural gas is about 38 MJ/Nm3.

Gasification offers at least the theoretical potential for innovative use of the product syngas other than immediate combustion to produce heat. Examples of innovative use would be firing of the syngas in gas engines/turbines, the displacement of fossil fuel in large combustion plants or as feedstock for chemicals or liquid fuel production.