Waste Conversion Technologies
Waste conversion technologies (WCTs) are non-incineration technologies that are used to convert the non-recyclable portion of the municipal solid wastestream to electricity, fuels, and/or industrial chemical feedstocks. Interest is growing in the demonstration and commercialization of WCTs due to their potential role in addressing renewable energy mandates and green jobs initiatives as well as meeting local solid waste management needs.
There are a number of important reasons why SWANA members and other solid waste managers will benefit by being knowledgeable and current with respect to the progress of WCT demonstration projects in North America:
- The federal government has provided almost $500 million in grants and loan guarantees to WCT demonstration projects in the US.
- WCTs offer the potential of generating significantly more revenues than traditional waste-to-energy (WTE) technologies. For example, if the fuel from a WCT facility is valued at $3 per gallon, this would provide a WCT facility with $150 per ton in energy revenues compared with $27.50 per ton (550 kilowatt-hours at $0.05 per kilowatt-hour) of revenues from traditional waste-to-energy facilities.
- WCTs convert post-recycled MSW to such alternative fuels as syngas or ethanol rather than directly incinerating MSW. These alternative fuels may offer the promise of lower emissions when compared with the emissions associated with the combustion of MSW in traditional waste-to-energy facilities.
- WCT project and/or technology developers have proliferated in recent years and have been actively targeting loca0l elected officials who, in turn, rely on their local solid waste managers to provide them professional opinions regarding the proposed projects, often on short notice.
This past year the Solid Waste Association of North America (SWANA) has taken an active role in the evaluation and commercialization of WCTs in North America. Recent SWANA activities include these:
- The development of a report on WCTs by the SWANA Applied Research Foundation
- The conduct of the inaugural WCT Showcase in conjunction with WASTECON 2011 in Nashville
The purpose of this article is to present a summary of the WCT report as well as highlights from the WCT Showcase.
SWANA Report on Waste Conversion Technologies
The SWANA Applied Research Foundation (ARF) has developed a report to provide solid waste managers with up-to-date technical and programmatic data and information regarding waste conversion technologies (WCTs).
This research need, which was submitted by the Metro Waste Authority of Des Moines, Iowa, and voted on for selection by the SWANA Applied Research Foundation's FY2011 Disposal Group, was described as follows:
“Solid Waste Agencies continually get unsolicited offers by individuals offering the ‘latest and greatest’ system as an alternative to landfilling. Most include plasma torch or some other waste-to-energy technology that, though technically feasible, is not a practical solution.
“What are these new technologies? What phase of development—theory, bench scale, pilot, or full operation? A white paper would give agencies with limited resources the ability to differentiate those technologies that are feasible and practical (operationally and economically) from those needing more development.”
The research—which was conducted by the SWANA ARF staff with input and guidance provided by the ARF Disposal Group Subscribers—consisted of a review and assessment of recently published literature, reports, and presentations.
The waste conversion technologies (WCTs) targeted for investigation are designed to process mixed municipal solid waste (MSW) for energy and materials recovery and are based on the following six processes:
- Plasma arc gasification
- Hydrolysis/fermentation (waste-to-ethanol)
- Anaerobic digestion
- Autoclave/mechanical processing
A description of each of the technologies is provided below, followed by a summary table comparing the technical viability, sustainability, environmental impacts, and economics of each technology. For the sake of brevity, the references for the information presented in this article have been omitted. However, they are included in the full study report. The SWANA ARF has also developed a database on WCT pilot, demonstration, and commercial projects in North America. To obtain an updated copy of the database, please contact the SWANA Applied Research Foundation.
Gasification is a commercially proven manufacturing process that converts such hydrocarbons as coal, petroleum coke, and biomass to a synthesis gas (syngas), which can be further processed to produce chemicals, fertilizers, liquid fuels, hydrogen, and electricity.
In a gasification facility, hydrocarbon feedstock is injected with air or oxygen and steam into a high-temperature, pressurized reactor until the chemical bonds of the feedstock are broken. The resulting reaction produces the syngas. The syngas is then cleansed to remove such impurities as sulfur, mercury, particulates, and trace minerals.
Because the syngas is cleaned before combustion, gasification plants produce significantly fewer quantities of such criteria air pollutants as nitrogen oxides and sulfur dioxide. The cleaned syngas—which is composed primarily of carbon monoxide and hydrogen—can be used to generate steam and/or electricity. It can also be used to produce methanol, ethanol, and other fuel liquids and chemicals through an add-on chemical process.
Waste gasification technologies vary in a number of important ways:
- Feedstock preparation—Some gasification technologies require waste to be processed for size reduction and/or removal of materials, while others process waste on an as-received basis.
- Furnace type—Gasification furnaces include fixed-bed and fluidized-bed reactors. In addition, some technologies rely on plasma arc heating units for gasifying the waste. (Plasma arc gasification is reviewed in the following section of this report.)
- Supplementary fuels—Some technologies require that supplementary fuels, such as coke, be injected into the furnace.
- Energy products—Some gasification technologies are designed to produce a cleaned syngas either for combustion or for conversion to liquid fuels, such as ethanol or methanol, while others are designed for syngas combustion only.
There are no commercially operating MSW gasification facilities in North America, but there are two demonstration-size facilities and four commercial facilities currently under construction. Tipping fees for North American MSW gasification facilities are likely to be in the range of $100 to $300 per ton.
Plasma Arc Gasification
The plasma arc technology is a heating method that can be used in both pyrolysis and gasification systems. This technology was developed for the metals industry in the late nineteenth century. Plasma arc technology uses very high temperatures (7,000 ºF) to break down the feedstock into elemental byproducts.
For applications in which MSW is processed, the intense heat actually breaks up the molecular structure of the organic material to produce such simpler gaseous molecules as carbon monoxide, hydrogen, and carbon dioxide. The inorganic material is vitrified to form a glassy residue. A main disadvantage of the plasma arc systems is that a large fraction of the generated electricity is required to operate the plasma torches, which reduces the net electrical output of the facility. For example, a proposal by a plasma arc vendor to the city of Honolulu indicated that the net power produced by the proposed facility available for sale would be 415 kilowatt hours per ton of waste processed—which is less than is sold by the city’s existing H-Power waste-to-energy facility, which uses a traditional combustion technology.
A significant requirement for the MSW plasma arc gasification process is that the MSW must be preprocessed before being fed into the plasma arc gasifier. As an example, for one process, the waste must be shredded to a 2-inch particle size. Also, the process can require the use of supplemental fuels to moderate and control the gasification process.
There are no commercially operating MSW plasma arc gasification facilities in North America and only one worldwide. One national consulting engineering firm has concluded that, for at least one MSW plasma arc technology, the vendor “has not yet proved their system will function in full-scale commercial application on MSW.” Tipping fees for MSW plasma arc gasification facilities in North America are likely to be similar to or higher than those estimated for conventional MSW gasification ($100–$300 per ton).
|DRANCO Anaerobic Digestion Facility in Vitoria, Spain, handles 330 tons per day of mixed MSW. The facility has been operating since 2006.
Pyrolysis is a process that involves the thermal decomposition of feedstock at high temperatures (750°F–1,500°F) in the absence of air. The resulting end product is a mixture of solids (char), liquids (oxygenated oils), and gases (methane, carbon monoxide, and carbon dioxide) with the proportions of each determined by operating temperature, pressure, oxygen content, and other conditions. The oils and fuel gases can be used directly as boiler fuel or refined for higher-quality uses such as engine fuels, chemicals, adhesives, and other products. The solid residue contains most of the inorganic portion of the feedstock as well as large amounts of solid carbon or char.
Gases produced during the pyrolysis reaction can be utilized in a separate reaction chamber to produce thermal energy. The thermal energy can be used to produce steam for electricity production. It can also be used to heat the pyrolytic reaction chamber or dry the feedstock entering the reaction chamber. If pyrolytic gases are combusted to produce electricity, emission control equipment is needed to meet regulatory standards.
As with other gasification technologies, MSW pyrolysis technologies appear to require a preprocessing step to remove nonorganic recyclables and to size-reduce the waste. They also require a drying process to evaporate moisture from the waste feedstock.
There are no commercially operating MSW pyrolysis facilities in North America. There are 12 commercial facilities in Japan and Germany that process Japanese municipal and industrial waste and are in the size range of 100 to 400 tons per day. One consulting firm has recently concluded that MSW pyrolysis facilities can be characterized as having “previous failures at scale, uncertain commercial potential; no operating experience with large scale operations.”
Tipping fees for MSW pyrolysis facilities in North America can also be expected to be in the range of $100 to $300 per ton.
Fermentation is an anaerobic biological process through which microorganisms metabolize sugars and produce alcohols as a byproduct. In addition to producing such alcohols as beer and wine for consumption, fermentation can be used to produce such fuel liquids as ethanol and other chemicals.
Cellulosic feedstocks, including the majority of the organic fraction of MSW, must undergo a pretreatment step, referred to as “hydrolysis,” to break down cellulose and hemicelluloses to simple sugars that can be metabolized by the yeast and bacteria for the fermentation process. MSW must first be processed through a MRF to separate, shred, and dry the cellulosic fraction.
There are no commercial hydrolysis/fermentation facilities in the world today, and therefore no basis on which to realistically estimate the tipping fees associated with this technology.
Anaerobic digestion is the bacterial breakdown of biodegradable organic material in the absence of oxygen. It can occur over a wide temperature range from 50°F to 160°F.
Anaerobic digestion of MSW can occur naturally, as in a landfill, or in a controlled environment, such as an MSW anaerobic-digestion facility. In the latter, MSW is first processed for removal of inorganic and recyclable components, reduced in size, and then placed in an airtight vessel called a digester, where the process occurs. Biogas can be used as fuel for engines, gas turbines, fuel cells, boilers, and industrial heaters. It can also be used in other processes and in the manufacture of chemicals.
There are no commercially operating MSW anaerobic digestion facilities in the US. However, there are a number of such facilities worldwide, including 13 in Spain, 16 in Germany and Switzerland, and one in Israel. Most of these facilities process source-separated or preprocessed MSW and other types of organic wastes.
Tipping fees for MSW anaerobic digestion facilities are likely to be in the $150 per ton range.
Autoclave, Mechanical Processing
The autoclave subjects the wastes to high temperature (usually with superheated steam) under high pressure for a sufficient length of time to kill all the bacteria and pathogens that might be in the waste. Some vendors then separate out the paper pulp for recycling as a raw material for paper manufacturers.
The process involves several steps. After minimal preprocessing, the wastes are shredded and ferrous metals are removed by magnets. These partially processed wastes are then fed to an autoclave to kill any pathogens. Nonferrous metals are removed after the wastes leave the autoclave, and the wastestream passes through a grinder to produce an end product—called “fluff”—which, as the name implies, is a light, dry, downy material. The fluff is relatively homogeneous and has a high organics content, although it also contains all the glass and plastics (and other nonorganic materials not removed by the ferrous and nonferrous metal separators) that were in the original wastestream.
There are no commercially operating MSW autoclave/mechanical processing facilities in the world today. One consulting study estimated that tipping fees for a US facility would be on the order of $85 per ton.
Federal Grants and Loan Programs
A number of federal programs have been created and/or used to support the development of WCT demonstration projects in the US.
The US Department of Energy (DOE) selected 19 integrated biorefinery projects to receive grants up to $564 million from the American Recovery and Reinvestment Act to accelerate the construction and operation of pilot, demonstration, and commercial scale facilities. Of this amount, $231.5 million is being used to support the development of four WCT demonstration projects.
In addition, three of these WCT demonstration projects have also received loan guarantees totaling $255 million.
Waste Conversion Technology Showcase
The Solid Waste Association of North America (SWANA) held its inaugural “Waste Conversion Technologies Showcase” at WASTECON 2011 in Nashville, Tennessee, on August 23–25, 2011.
The Waste Conversion Technologies (WCT) Showcase at WASTECON featured:
- WCT Pavilion—A separate portion of the WASTECON Exhibit Hall was dedicated to the presentation and exhibiting of WTC technologies as well as associated consulting and other services.
- WCT Technical Session Track—A separate track of technical sessions was developed to focus on WCT technologies and their associated planning and implementation issues.
Unlike other conferences, SWANA’s WCT Showcase focused on addressing the needs and perspectives of the public sector solid waste manager, who not only represents the local government that will purchase and implement the WCT, but who also must address critical implementation issues such as siting and community endorsement.
WCT exhibitors benefited from the fact that more public sector solid waste managers attend WASTECON than most other conferences. Companies that exhibited at the inaugural WCT Showcase included:
- Entec Biogas USA
- BIOFerm Energy Systems
- ICM Inc.
- Frontline Bioenergy
- W2E Organic Power/Eisenmann
SWANA wants to especially thank Harvey Gershman (GBB), Tim Raibley and Bruce Howie (HDR), and Jim Binder and Susan Higgins (ARI), who were instrumental in the development of the WCT Technical Sessions and who also served as session moderators.
SWANA will continue to provide SWANA members and other solid waste managers with up-to-date performance and cost information on the WCT demonstration projects being implemented throughout North America through its WCT Database. The SWANA report on WCTs summarized in this article will also be published by the end of the year and will be made available to ARF subscribers, SWANA members and the general public at nominal prices. Finally, plans are under way to conduct a second WCT Showcase in association with WASTECON 2012 in Washington, DC. If you would information on obtaining these resources and/or participating in the WCT Showcase at WASTECON as an exhibitor or speaker, please contact Jeremy O’Brien at email@example.com.
Author's Bio: Jeremy K. O'Brien, P.E., is director of applied research for the Solid Waste Association of North America.