AUTHORITY BOARD STUDIES WASTE CONVERSION TECHNOLOGIES
The solid waste management industry has seen a renewed interest in emerging
waste conversion technologies. In addition to incineration, or more commonly
referred to as “Waste-to-energy”, there are other new conversion
technologies that the Authority is reviewing that extract energy from
trash. A South County Landfill is but one alternative being explored,
to ensure they are not cornered into one position with no other waste
disposal alternatives. Below is a summary of those technologies being
studied.
Material Recovery Facility (MRF)
A solid waste facility where secondary materials are separated and sorted from mixed refuse, by hand or by use of machinery, for the purposes of recycling, composting or transformation. There are two types of MRFs – clean and dirty. A clean MRF accepts recyclable materials that have been separated from refuse. A dirty MFR accepts refuse and recyclable materials mixed together, and separation occurs at the plant. Plans are currently being finalized to construct a Salinas-based MRF, which will notably reduce the amount of trash needing to be placed in Salinas Valley landfills.Waste-to-Energy (incineration)
WTE involves the controlled direct burning of organic materials to produce heat, which in-turn is captured and used to generate steam to drive electricity-producing turbines. All inorganic materials are converted to ash.Composting
Composting is the decomposition of plant remains and other once-living materials to make an earthy, dark, crumbly substance that is excellent for enriching soil. Composting is a critical step in reducing the volume of garbage needlessly sent to landfills for disposal. The use of composting to turn organic wastes into a valuable resource is expanding rapidly in the United States and in other countries, as landfill space becomes scarce and expensive.Pyrolysis
Pyrolysis is the thermal degradation of organic materials through the use of an indirect, external heat source at temperatures typically above 925 degrees F, in the absence of oxygen. This thermal degradation produces pyrolysis char and oil (fuels), and a “syngas” (synthetic fuel) composed of hydrogen, carbon monoxide, carbon dioxide, methane and residual complex hydrocarbons. All inorganic materials are converted to ash. The resulting syngas is used to fuel conventional electrical generators and produce electricity.Gasification
Gasification is the thermal conversion of organic materials in the presence of heat at temperatures typically above 1,400 degrees F, and in an atmosphere with a limited supply of oxygen. The resulting degradation process produces a syngas composed primarily of hydrogen and carbon monoxide, with the inorganic materials converted to a solid, vitreous (glassy) slag. The resulting syngas is used to fuel conventional electrical generators and produce electricity.Plasma Gasification
Plasma gasification involves the use of electricity, which is passed through special electrodes in the presence of steam and oxygen to produce an electrically conducting gas (plasma) typically at temperatures in excess of 7,000 degrees F. The extremely high temperature plasma gas converts organic materials, including chars and oils into syngas composed of hydrogen and carbon monoxide, with the inorganic materials converted to a solid vitreous slag.Anaerobic Digestion
Anaerobic Digestion is the biological conversion of biodegradable organic materials in the absence of oxygen at temperatures under 200 degrees F. The process is carried out by anaerobic microorganisms that convert carbon-containing compounds to a biogas (primarily methane and carbon dioxide). The resulting biogas is used to fuel conventional electrical generators and produce electricity. The residue is a stabilized organic material that can be used as a soil amendment.Benefits of Conversion Technologies
- Waste materials are typically reduced in volume by up to 90%, significantly reducing the need for landfill space. In some cases the residual ash can be used in construction products such as concrete or brick production.
- Synthetic gas or methane produced by these processes is used to generate electricity.
- Co-locating these facilities with a comprehensive recycling and materials recovery operation assures that most inorganic materials and other recoverable items are removed for recycling or reuse prior to conversion processing. Advanced removal of inorganic items also reduces ash and other waste by-products requiring landfilling.
- Significant reduction in physical space requirements compared to landfills.
Challenges for Conversion Technologies
- Conversion technologies, with the exception of anaerobic digestion, will emit small levels of dioxins, furans and heavy metals even with the most advanced emission control equipment in use today.
- With the exception of WTE, these newer technologies have minimal experience in large-scale operations using municipal refuse as a feedstock.
- Projected costs for many of the conversion technologies, if expanded to handle large scale refuse systems, will be very high and may only be cost effective for communities with limited or no local landfill space.
- With the exception of WTE, the other conversion technologies have limited data on air emissions using municipal waste feedstock, though regulatory efforts are underway to establish air quality standards for these new technologies.
- Stabilized compost residues from anaerobic digestion can be difficult to market as a soil amendment due to the presence of even small amounts of contaminants in the feedstock such as heavy metals.