Residual Ash / Carbon

Carbon Char and Bottom Ash

 

The RCR process uses steam to sanitize and size-reduce. The recyclables are removed for recovery and valorization on site. The remaining element of the treated waste stream has been derived from the organic fraction of the waste and will be a friable cellulose -based fiber.

 

It can be used as a virgin feedstock for composting, or more usually as a fuel for a dual fuel steam-raising boiler for the production of process steam to charge the autoclaves or to feed a steam turbine for the generation of electricity to meet the parasitic needs of the RCR plant and the surplus exported to the local electricity grid.

 

During the thermal decomposition process rubber crumb introduced into a vacuum chamber deposits carbon char at the base of the chamber. This is highly dangerous and volatile material as it can instantly combust if exposed to air at temperatures above 96C.

 

Dependent upon the ultimate destination whether it is to be carbon black for reuse in the colourisation of plastics or essential in the production of new vehicle tyres or activated carbon used essentially in the water filtration sector, 100% of the ash can be utilised either as carbon black or through the introduction of high temperature steam (1000°C) will quench the hot carbon char and release hydrogen and oxygen to produce an expanded and porous but non-volatile carbon filtrate material and as a

 

In the STAG process, where refuse derived fuel is introduced to a steam raising boiler, then the ash content following the combustion represents around 7% of the feedstock volume. This is equivalent and as fine as wood ash and is diverted to the Construction and Demolition waste recovery unit where it is micronised and mixed with homogenous C&D recyclable and there is zero ash destined for landfill since it is a component for the production of 60 cement-based DIY building products.

 

Incineration of MSW is not the panacea for waste disposal. It is merely a 75% disposal solution and the 25% residue is made up of highly toxic and volatile fly ash which is contaminated with dioxins, dibofurans, chlorine, heavy metals and must be comingled with either bitumen or concrete and then land filled. 22% or thereabouts is contaminated bottom ash that is a controlled hazardous waste and must be disposed of in a special hazardous landfill at considerable cost and must be monitored in perpetuity.

 

None of RCR’s processes produce any difficult ash that cannot be utilised to 100% of its volume.  RDF can be used as a virgin feedstock for composting, or more usually as a fuel for a dual fuel steam-raising boiler for the production of process steam to charge the autoclaves or to feed a steam turbine for the generation of electricity to meet the parasitic needs of the RCR plant and the surplus exported to the local electricity grid.

 

Water Use

 

RCR’s autoclaves used 20 tonnes of water for each 90 minute load, unload and treatment cycle. MSW typically contains as much as 45% moisture and in China and parts of Asia the moisture content is as high as 70%. When wet waste is introduced into the autoclave it comingles with the steam that condenses within the autoclave to water at 3 atmospheres (bar).

 

When the autoclave is depressurised the condensed steam reverts to steam and exists the autoclave only to be captured in the condensate vessels where the water is then subjected to a filtration process and a reverse osmosis to purify the water and remove any salts so that the water can be returned to the water jacket on the steam raising boiler or the steam or gas turbine. This warm water is raised in temperature to produce steam that is stored pending its use in the autoclave or the condensing steam turbine.

 

In all cases the water from within the MSW combined with the steam condensed and recovered through the condensate vessels results in a typical consumption within a large-scale STAG plant at around 30 tonnes of water. Water is also used within the decomposition / pyrolysis of mixed plastics and to convert the captured chlorine into  hydrochloric acid.

 

Water consumption is very low since it is augmented through the moisture content inherent within the MSW when frequently the severe rainstorms merely fill up garbage cans with rainwater that in turn is recovered from the autoclave process or from the condensing steam turbine.