Steam Classification Recovers Fuel from Municipal Solid Waste

by the CADDET US National Team


Identifying alternatives to municipal solid waste (MSW) landfilling has become a priority for cities and counties across the United States. Waste Reduction Technologies, Inc. (WRT) of Covington, Kentucky has used a prototype steam classification system to process landfill waste into a pulped cellulosic matrix suitable for direct combustion, or as a component of an engineered alternative fuel.

Operation of WRT's steam pressure technique at a level of 10 tonnes/day successfully isolated 75% of the cellulosic material represented by garden waste, paper and other materials in the household waste. The company estimates that steam classification is economically viable at 200 tonnes/day.


Flow diagram of steam classification process

Project Background

Since the late 1980s, finding alternatives to conventional landfill disposal has been a subject of intense interest in Kentucky. Centralisation of fewer but larger facilities that now average 500 tonnes/day MSW input (the average amount produced by 200,000 residents) has increased waste transportation costs for some remote communities.

Steam classification was in a very early stage of development when individuals from Kentucky and Indiana began work in 1991 to improve methods of processing MSW. Grant assistance offered by the Kentucky Division of Energy through the US Department of Energy's Southeast Regional Biomass Energy Program (SERBEP) encouraged WRT to fabricate a system for testing at a landfill in Walton, Kentucky.

The primary objective was to model the process to project economic feasibility at 200 tonnes/day for recovery of waste metals, plastics and pasteurised cellulose. A secondary objective was to test the combustion qualities of the separated cellulose. A prototype unit was installed in 1993 at a landfill which serves several Northern Kentucky client communities. Cinergy Corporation, a regional coal-fired utility servicing Indiana, Ohio and Northern Kentucky, agreed to test-burn the extracted cellulose.

WRT views steam classification as an option to landfilling not a replacement for popular roadside recycling programmes. However, some waste audits have shown that cellulose content does not differ significantly in refuse obtained from roadside recycling containers compared to raw MSW.

Representatives from WRT are negotiating with various business interests to market the material as a pyrolysis feedstock for liquid fuel production or for gasification.
Other possibilities include pelletising the cellulose with a binder element or selling it to utilities for coal co-firing using pneumatic feeding.


The Project

Steam classification is a fairly straightforward process. The major components of the company's prototype unit consisted of an oil-fired steam boiler, a rotating steam pressure chamber, a rotary screen trommel and conveyor equipment.


The steam pressure cylinder

The rotating vessel is fitted with a rotary valve on one end and another valve in the centre of its door to purge cool, entrapped air during steam pressure build-up. A rotating internal helix agitates the MSW for even exposure to the steam after the vessel reaches its pressure and temperature parameters. After two hours of processing, the vessel is depressurised and steam released into a condensate tank for recycling back to the boiler feedwater supply. Reverse rotation of the helix expels the pulped MSW onto a conveyor leading to a rotary trommel where the cellulose falls through the screen. The remaining plastics, metals and glass exit the trommel onto a manual sorting conveyor.



Municipal waste input at about 20% moisture content was processed into a 75% fraction of cellulose shred with 30% moisture. Plastics and both ferrous and non-ferrous metals were also segregated. The process reduced the fraction of refuse remaining for landfill to 11% by weight.


The rotar screen trommel

In test burns, two different steam-classified waste streams raw MSW and rejected corrugated box liner from a packaging plant generating about 150 tonnes of waste daily were fed manually to Cinergy's boilers. The energy value of the cellulose was about half that of the coal fired at the utility's East Bend Power Plant. Cellulose combustion volatility was almost twice that of coal, increasing the chance of excessive friction. Further examination may show that the pulp is more appropriate for use in stoker boilers to make process steam.



Pilot unit capitalisation costs totalled $137,600 (where $ is the US dollar), including site preparation and installation. A $25,000 grant was contributed by SERBEP. Separation equipment represented the largest fraction of capital costs. At a 200 tonnes/day size, WRT's fabrication cost would be about
$2.5 million, with expenses for 6 12 employees comprising the bulk of operational expense.

Projections for a worst-case scenario assumed a 30% revenue reduction
for pulped cellulose, due to higher moisture and lower heat value; a 50% lower price for recycled aluminum; and operational expenses 20% higher than expected. This scenario returned an estimated cash flow of $259,000, or a 10% return on the investment.


Energy inputs and cellulose energy yields at 180 tonnes/day

For more information contact the  CADDET US National Team in Colorado.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.