Comparison of the Vickers Seerdrum with Autoclaving
The processing of municipal solid waste MSW using autoclaves has gathered interest in due to the increasing costs of disposal to landfill and the cost and public objection to incineration.
The beneficial aspect of autoclaving MSW is the separation of the organic fraction from the inorganics and the recovery of both. Exactly the same as the Seerdrum but at a much, much high cap ex and op ex.
As attractive as this seems, autoclaving of MSW has significant drawbacks and it is therefore not surprising to learn that there appears to be no commercial facilities in operation in the UK processing waste with autoclaves.
The disadvantages of autoclave technology for processing MSW are as follows:
High capital cost
High operating costs
Contamination of the Fibre
Production of difficult to treat condensate
High Capital Cost
Commercially sized autoclaves are very large, rotating, pressure vessels typically 2.4 m dia and 12 m long. These extremely heavy vessels,
manufactured and tested to achieve international pressure vessel certification, are supported on a base frame with suitable heavy bearing capacity to allow for rotation. The drive mechanisms are equally large and power hungry.
Each batch of MSW loaded into the autoclave is typically 15 to 20 tonnes. In order to achieve an 80% filling capacity, the autoclave must have its loading opening, and pressure vessel door, elevated. And after the batch is processed, the vessel must have its opening lowered for discharge. So, in addition to the vessel and base frame, very heavy duty lifting jacks must be incorporated.
Because the autoclaves must operate inside a building for odour reasons, the elevation height of the raised autoclave demand extended an roof height on the building.
None of the above comes cheaply, hence the high capital cost statement.
High Operating Costs
The autoclave process requires the addition of water to increase the moisture content of the MSW. This higher moisture content delivers the pulping action needed to produce the fibre from the paper, card and food waste within the MSW.
The water is added mostly as steam during each batch. At the start of each batch, the autoclave is cool because it has been vented. The initial steam injection condensates and provides the necessary moisture. Continues steam injection raises the pressure and temperature to the operating temperature of around 150 to160 deg C.
Steam raising boilers are very energy hungry and also demand boiler quality water, therefore, only good quality water can be used and this may also require pre treatment.
An autoclave is only useful if there are specific requirement to heat to these high temperatures for regulatory reasons.
Because the autoclaving is a batch process, vessel requires constant stopping, lowering, emptying, venting, elevating, filling, lowering to horizontal and pressurising. These process events for every batch contribute to the high operating costs per tonne processed.
Further, the vessel must be de-pressurised by venting the steam to a condensate tank. Because of the high operating temperatures, much of the volatile organic compounds, such as fats and oils in the food waste, evaporate and accumulate in the condensate tank, producing a high liquor with a very high COD concentration. This condensate cannot be re-used in the process and attracts high disposal costs. Also, these organics within the condensate are lost from the fibre, so, if the fibre is used to make biogas or ethanol, the fuel yields will be reduced from their real potential.
Autoclaves being pressure vessels must be operated to very high health and safety procedures under careful supervision adding to the labour costs of the business. Further, pressure vessels require annual inspections, again, adding to operating costs.
Contamination of the Fibre
One of the benefits of autoclaving MSW is the extraction of the organic fraction of paper, card and food waste as a cellulose fibre. Just like a Seerdrum.
The organics recovered will need to be cleaned further to separate the contaminants. This should deliver a clean organic fibre.
However, because the autoclave operates at 150 to160 deg C, a considerable amount of plastics melt and these contaminate the organic fibre. This compromises the fibre and makes it commercially impossible to refine the fibre further to a clean biomass fuel. Therefore, its value will be negative and there will be a cost attached to its disposal.
Autoclaves are commonly used in the medical profession for sterilisation. When adopted for processing MSW, sterilisation of the waste and extracted fibre was considered a benefit. However, MSW is not considered to be a hazardous material and therefore there is no reason to sterilise it. Sanitising it at the much lower pasteurisation temperature is sufficient.
The sterilisation came as a consequence of selecting high temperature autoclaving as a technology for treating MSW. Sterilisation was then promoted as a benefit but it is an unnecessary benefit.
It therefore follows, processing at 150 to160 deg C to produce the fibre is unnecessary. The fibre can be produced very efficiently at sub 100 deg C temperatures resulting in a sanitised fibre that is safe to handle. Crucially, at less than 100 deg C, no plastics melt and contaminate the fibre. And at this lower temperature, the volatile organics do not readily evaporate but remain in the fibre increasing its calorific value and biogas potential.
Processing at sub 100 deg C or at ambient temperatures as with the Seerdrum, is sufficient and delivers the biomass making the autoclave technology unnecessary.
Comparison of Autoclave Technology with the Vickers Seerdrum
The Vickers Seerdrum is an alternative to autoclaving and overcomes all of the disadvantages of autoclaving. Compared with autoclaving, the Vickers Seerdrum provides:
Much lower capital costs
Much lower operating costs
A continuous process
High calorific value fibre with little loss of volatile organics
No fibre contamination with melted plastics
No pressure vessel engineering
No highly polluted and odourous condensate disposal costs
No material odour problems from the process
Heat of autoclave shatters all glass to small fragments which pass out with the fibre
Low grade process water is acceptable in the Seerdrum but boiler quality water is needed for the autoclave.