Thermal Treatments

Incineration:

Incineration is the process by which waste is burned in the presence of oxygen. The waste is fed into a furnace that is maintained at 1000 degrees Celsius by the additional burning of fuel oil jets. Air is drawn through using fans to provide the oxygen.

The grate moves to keep the falling ash and clinker passing into the quenching bunker beneath. This is bottom ash that is taken to an outside ash processing plant and if found to be inert can be used as aggregate other wise it must be landfilled in a hazardous landfill site.

The hot gasses from the top of the furnace pass into a boiler where they turn water into steam and this in turn is used to power a turbine and generator. The hot gasses and light debris pass through a scrubber where lime removes some of the acid oxide gasses and catalytic carbon removes some of the heavy metals.

The remainder passes trough a series of filters that trap the larger particles. The gasses which have not been scrubbed out plus the finer particles pass up a chimney stack and exit in the form of an emission plume. The residue in the filters plus that in the scrubbers constitutes fly ash that is highly toxic.

Incinerators do not normally treat agri-waste or sewage sludge. They are also not seen as suitable for treating food waste. In order to be energy efficient they must consume items that would otherwise be a recoverable asset such as wood paper and plastic.

Pyrolysis:

The simplest form of Pyrolysis is frequently called destructive distillation. Once again the process depends upon the breaking of energy rich hydrocarbon chains found in food waste, wood paper and plastic which could all be recovered.

In this case the process takes place in sealed chambers where there is no oxygen. This can be achieved using a vacuum or by using an atmosphere saturated with nitrogen. In either case the operating temperature is about 1000 degrees Celsius. The hydrocarbons formed vary from very heavy tars and oils through light fuel oils to gases.

The process is actually a form of thermal depolymerisation (see catalytic depolymerisation). The residue left in the chamber is high in chlorine and contains PCBs (Dioxins) and Furans. There are also chlorine derivatives in the gas such as hydrochloric acid gas. The tendency to produce NOx is reduced in the Nitrogen atmosphere process.

Some of the fuel oils and gases can be burned to produce power, but some of the others are very corrosive and hazardous and require special disposal. The efficiency of  fuel production and therefore energy potential is higher than incineration.

Gasification:

This process is very similar to the one that used to be used in Gas Works to produce "Town Gas" when it was decided that Coal Gas was too dangerous.

Carbonaceous material is heated in the presence of steam with a very limited supply of oxygen. In this kind of reducing atmosphere, Hydrogen, Carbon Monoxide and a small amount of Methane are produced. This gas product is known as Synthetic Gas or Syngas.

The heat to drive the process can be generated either by plasma arc or microwaves and varies between 1000 and 4000 degrees Celsius. As with other thermal processes the system works best on energy rich carbon compounds such as food waste, paper, wood and plastics, which would have recovery potential.

The advantage that this process has over incineration is that the volatile oxides known as NOx and SOx  are not produced neither are PCB's (Dioxins) and Furans. However, as matter cannot be destroyed there are a number of hazardous substances that are produced and are trapped in the ash. These include heavy metals and tars.

In the different forms of this technology, a bed of reactive carbon called char is used to generate the gas and this absorbs the  hazardous waste and must be disposed of eventually. In some systems the hazardous waste is absorbed onto other substances such as Dolomite, leaving the spent char clean and capable of being used as a soil enhancer. The energy content of the gas means that it can be used to power generators which have a higher output than that produced by incineration.

Biological Treatment

Anaerobic digestion:

Anaerobic digestion does not require the consumption of plastic, wood, paper or other recoverable items in order to function.

Therefore these can be removed at the start of the process. The main feed stock is biomass derived from food waste or similar organic matter. This portion is mixed into a liquid slurry and is heated by energy generated later in the process.

Once the conditions in the tank are right the atmosphere is sealed to exclude oxygen and the mixture is seeded with special bacteria that rapidly multiply.

They then feed on the waste. As they consume it they produce heat and liberate gas. This gas is called biogas and is a mixture of Methane and Carbon Dioxide.

The biogas is collected and used as fuel in a gas engine that drives a generator. The heat from the exhaust gas is used to warm the digestion tanks.

The Methane burns to form Carbon Dioxide and water. Most of the Carbon Dioxide is trapped and used under pressure to split the biomass apart earlier in the process so that digestion can be speeded up.

The passage of biomass through the plant is continuous because a moving plug is inserted, with the digested material in front, and the next load following behind.

Provided that all unwanted material has been filtered out, the residue of the digestion process is compost, and therefore nothing has to be landfilled.

Aerobic Digestion:

Aerobic treatment is the process that occurs in the garden compost heap but on a much bigger scale. The process does not require the consumption of plastic or paper, so these can be removed along with other recyclable materials.

The remaining waste is mainly wood and food waste. This is shredded and mixed and arranged into long heaps called windrows.

Bacteria can be introduced into the heaps or they can be naturally left to decay.

As the composting process progresses heat is given off and air is required to reach the more dense parts of the heaps.

A tractor with a rotary attachment drives down the windrows and mixes the material allowing in air.

The process gives off Methane, but this cannot be trapped and used because the heaps must be open as air is required for decay to occur.

In some systems pipes run under the windrows to absorb some of the heat, but otherwise no energy is recovered by aerobic digestion.

The Methane and Carbon Dioxide liberated by decay are both greenhouse gasses, but the compost is a valuable residue provided it has been screened to prevent pieces of glass or plastic from being included.

Physical and Chemical Treatment

Catalytic Depolymerisation:

This process is not suitable for all forms of waste, only those that contain long chain molecules of hydrocarbons such as most forms of plastic.

Plastic is originally made from oil molecules which have been joined together, and this process splits them apart again.

Fuel crops such as rape can also be treated. The treatment needs moderate heat (about 200 degrees Celsius) which is achieved by using a heated oil bath.

The plastics are mixed with a complex metallic catalyst based on silicate salts, and continuously agitated. As the chains are spilt the oils rise to the surface and are passed into fractionating columns. The lighter fuel oils are collect and cracked off by distillation.

There are residues from the system including spent catalyst and some oils that are volatile, corrosive and hazardous, (see also pyrolysis which is a form of thermal depolymerisation). The fuel oils can be used to run generators to produce electricity.

Fractionation:

This process has a limited application in waste management.

It relies upon the separation of mixed materials into different lots (fractions) as a result or differences in their physical properties. The simplest example might be to use differences in weight/ bouyancy.

Mix waste with water, some substances will float, others will sink, and so they can be separated. The most common use is in recovering usable oil from waste oil collections by using a fractionating column that separates the oils by their viscosity.

An other form of oil treatment is used to recover vegetable oils such as palm oil.

Here the different melting points of the oils are used. As heat is applied the lighter oils begin to melt and rise to the surface and so they can be drained off.

Other techniques include the differences by which substances react to solvents such as alcohol. An application might be the fractionation of fuel oil from a mixture of heavy oils (as might possibly occur in garage waste or chip shop fat), so that the it can be used to power generators to produce electricity.

Materials Recovery Facility (MRF):

This is a waste treatment system that can be added to the front end of certain other systems.

These systems are the ones that do not require recyclable items to be consumed to provide energy during the process. The items recovered are usually glass, metals and plastic. In some cases wood and paper can be added to these.

The methods by which the items are removed from the waste stream vary, but can be roughly divided into clean or dirty.

Clean MRF's use a steam steriliser called an autoclave to "cook" the waste. This prevents dust and smell, kills all bacteria and discourages rodents.

The other difference between systems is that some are simple and others are automated.

In the simple process, lines of workers stand beside the conveyors and hand pick items out of the waste. In the simplest automated systems, the waste turns in a drum called a trommell that has slots in the sides thus allowing material to be sorted by size and weight.

In the more complex systems waste is sorted by infra-red and laser detectors.