Despite ongoing efforts to limit the amount of waste generated there remains a certain portion that cannot be reduced, reused, recycled or composted. In many instances waste residues are disposed in controlled landfills where their inherent value is lost. But an emerging clean energy technology may help divert more non-recoverable waste residues from landfill sites.
Gasification — a low-emissions modular process — is making its Canadian debut in Sherbrooke, Quebec. This partial oxidation chemical process can convert the carbon and hydrogen in unrecoverable municipal waste residues into clean synthetic gas.
A commonly held definition of gasification describes it as a thermal upgrading process in which organic materials (containing carbon and hydrogen) are converted into a gaseous mixture of carbon monoxide, hydrogen, carbon dioxide and simple hydrocarbons (synthetic gas). The process couples fluid-bed reactors with advanced gas conditioning technologies to provide a clean synthetic gas that can then be used alone or in combination with natural gas. The synthetic gas may be used as a fuel to produce electricity and steam or as a chemical feedstock to produce other chemicals.
Major developments in gasification occurred in the 1970s and the 1980s in response to the two oil shortages. Lessons learned from the energy demonstration projects were applied to a waste-to-synthetic gas program in the early 1990s and led to the creation of Enerkem Technologies Inc. This group was formed as a spin-off company of one of the advanced research laboratories at the University of Sherbrooke working to commercialize a patented gasification technology called BIOSYN.
In late 1998 Enerkem Technologies licensed the BIOSYN technology to a company in Spain (EIE SL), which has recently completed a gasification plant to convert non-recyclable plastics into synthetic gas. The facility fuels a power plant commissioned to generate 6.8 MW of electrical energy for the local grid. A second plant to gasify non-recyclable plastics will open in fall 2002. Agreements are in place for three other plants based on mixed plastics and biomass/plastic mixtures.
Data shows that when coupled with the appropriate energy conversion device gasification offers energy recovery that exceeds conventional combustion/steam cycle or pyrolysis processes.
“The synthetic gas produced using the BIOSYN process and the related gas-conditioning technology is a clean burning gas,” says Dr. Esteban Chornet of Enerkem Technologies Inc., adding that emissions from the combustion of the gas are primarily carbon dioxide (CO2) and water vapour.
Emission levels of nitrogen oxide (NOx) are equal to or lower than those produced from burning natural gas since synthetic gas combusts at lower temperatures than natural gas. Particulate emissions are within regulatory limits by efficient gas conditioning prior to gas combustion. Appropriate burners and flue gas post-treatments control emissions of carbon monoxide (CO) to below regulatory levels.
“No dioxins, furans, sulfurous or chloride emissions are generated from burning the synthetic gas produced using the BIOSYN process,” says Dr. Chornet.
Municipal waste as feedstock
Using municipal waste residues as a gasification feedstock is a relatively new concept. But to date waste materials such as shredded tires, plastics, municipal wastes (including small amounts of metals and residual food), spent solvents and textiles as well as residue biomass have been used successfully as feedstock.
According to Dr. Chornet, in order for residues from municipal wastes to become appropriate feedstock material ferrous and aluminum metals and a significant fraction of inorganics (mainly glass) must be removed. The materials should also be dried to 15-20 weight per cent moisture and texturized to bring the material to the appropriate size and density.
Waste plastics residues are good feedstock candidates because many plastics, particularly polyolefins, have high calorific values and simple chemical structures composed primarily of carbon and hydrogen.
Costs of the gasification approach need to be considered within an integrated waste management context of which they will constitute a fraction, within the range of alternate approaches. The key advantage of gasification is its ability, as a process, to convert any organic residue into a clean gas that can be handled very much as natural gas.
Last year Enerkem began working with the City of Sherbrooke to convert waste into clean synthetic gas. Referred to as the BioSyngaz-Estrie project, the program received a $1.2-million investment from the federal and provincial governments as well as Enerkem and SOQUIP (Socit Qubcoise d’Initiatives Ptrolires, a subsidiary of the Socit Gnrale de Financement). The pilot unit was designed and constructed with the capacity to convert 2.5 tonnes of sorted municipal waste residue per day.
The plant has been in operation since fall 2001. With the pilot plant now running to capacity, Dr. Chornet and his team have begun collecting data. Complete data reports will be ready for fall 2002. Continuing work, aimed at technology optimization and alternate use of the gas for synthesis of basic chemicals, such as alcohols, will be done at the pilot plant under the auspices of public/private partnerships in an effort to develop pathways of increased sustainability using the residues as raw materials. There will be no sales of the gas produced at the pilot plant. With the data obtained, a feasibility study will be conducted to determine costs within an integrated regional or municipal waste management strategy.
Ed Hogan is manager of Thermochemical Conversion with Natur- al Resources Canada in Ottawa, Ontario.