Many municipalities and counties in the United States and abroad are grappling with the challenge of how to manage the portions of their municipal solid wastestreams that cannot be diverted from landfills by other means.

By Daniel Predpall

The MSW portion under discussion here is mostly organic, consisting of such materials as restaurant fats and greases, wood, grass clippings, construction debris, and residues from material recovery facilities that cannot be composted effectively. The implications are significant:

• Managing this waste will reduce the quantities going to landfills, which in turn will increase landfill life and postpone the need to locate and permit new landfills (always a difficult and costly undertaking).
• The remaining organic wastestreams can be viewed as a feedstock for producing a variety of products, including energy and chemicals, as well as other useful materials.
• The residues from managing this wastestream will be inert, thus reducing the environmental impacts from landfills on air and water. Rather than emitting methane to the atmosphere from landfills, this greenhouse gas can be captured and used to produce valuable products.
• MSW management can potentially achieve sustainable waste management and become environmentally, economically, and socially more acceptable through better resource management.

How can the remaining wastestream be managed to achieve some or all of these benefits? A new class of technologies is emerging that might offer real alternatives to landfill disposal. Incidentally, these technologies are different from the mass-burn incineration systems that were primarily built during the 1980s.

These technologies, a mix of new, emerging, and existing, have been collectively called "conversion technologies" or "advanced MSW treatment technologies." This group of technologies includes pyrolysis, gasification, anaerobic digestion, and ethanol fermentation.

For the most part, these conversion technologies are not "new." Some of these technologies have been in use for years in other industries, but the feedstocks have been limited to relatively homogeneous streams. Some have been used overseas, but not yet in the US. Others are at the demonstration stage and have not been designed for large waste volumes. Importantly, most conversion technologies have not been developed for use in a municipal environment; that is, they have not been subject to myriad state and local regulations; designed to handle a very heterogeneous, mixed wastestream; and optimized in terms of overall energy and materials production.

Because these technologies are only recently being applied to solve MSW disposal problems, a number of barriers to their implementation remain. These barriers fall into four main categories:

Regulatory: Since these technologies have not yet been applied at large scale for MSW management, new permitting regulations may be needed to address the technologies and their emissions. Existing regulations might not specify whether the use of these technologies will earn diversion credit (similar to recycling).

Environmental/social: Because the public is not familiar with these technologies and their potential impacts to the community and the environment, public education programs are needed to explain how these systems operate and how they are different from and more advanced than conventional technologies. Developers will need to clear up public misconceptions based on older combustion/incineration technologies. Impacts of these technologies are not yet well understood because of a lack of operational and emissions data.

Technical: The lack of experience with most of these technologies in the US means that we have to carefully evaluate design and operational issues with regard to feedstock characteristics, process integration, and emissions controls.

Economic: Again, due to the lack of US experience, there are few cost data for these systems. In addition, cost-benefit analysis is needed to explore the use of conversion technologies versus other alternatives.

In this article, these conversion technologies will be examined in terms of their genesis, the types of processes involved, which technologies are being evaluated by cities and counties, the criteria used to rank technologies for a specific application, and performance and implementation issues that must be considered.

Overview of the Concepts

Broadly speaking, we can categorize MSW conversion technologies into three groups:

• Physical
• Biological
• Thermal

Physical technologies can be described as material conversion, densification, or the making of a refuse-derived fuel (RDF). The purpose of this technology is to prepare the MSW for incineration (combustion) or as a supplementary fuel source. Essentially, the physical characteristics of the material are altered. These technologies consist of several unit operations that can include sizing/screening, shredding, magnetic separation, wet separation, picking, drying, pressing, grinding, baling, crushing, and pelletizing.

Biological technologies can treat the substantial organic fraction of the MSW stream. Biological processes typically involve natural metabolic functions of microorganisms (and sometimes larger organisms) that are exploited to treat the degradable fraction of the wastestream. Examples of biological technologies include anaerobic digestion (composting) and the production of ethanol and biodiesel.

Thermal technologies involve the thermal breakdown of solid materials into a gaseous constituent (synthetic gas, or syngas), and in some cases, a solid char residue and/or liquid (oil). The process energy is provided either in a reactor in the presence of some oxygen (gasification) or in a reactor in the absence of oxygen (pyrolysis). Some technologies utilize both methods. An advantage of these technologies is that the syngas produced can be utilized in boilers or low-profile reciprocating engines for generating electricity more cleanly and efficiently than conventional incinerators. With that, the city or county can help solve its MSW disposal problem, as well as generate revenues from the sale of electricity.

Most conversion technologies can be described as having three separate and distinct components: (1) front-end MSW preprocessing, (2) the conversion unit, and (3) the energy/chemicals production system.

This type of system is portrayed in Figure 1.

Front-end preprocessing is used to prepare the MSW for treatment by the conversion unit and separate out any recyclables. The energy production module can be a gas turbine, boiler, or reciprocating engine for power production. Alternatively, ethanol or other chemicals could be produced.

Several MSW conversion technologies are briefly described as follows:

PYROLYSIS

The thermal degradation of organic materials, through the use of an indirect, external source of heat, typically at temperatures greater than 925°F, in the absence or almost complete absence of oxygen, to produce pyrolysis char, pyrolysis oil, and a syngas composed primarily of hydrogen, carbon monoxide, carbon dioxide, methane, and complex hydrocarbons. The syngas can be utilized in boilers, gas turbines, or internal-combustion engines to generate electricity or be further processed into organic chemicals.

GASIFICATION

The thermal conversion of organic materials in the presence of heat at temperatures typically above 1,400°F and in a limited supply of oxygen (less than stoichiometric) to produce a syngas composed primarily of hydrogen and carbon monoxide, with inorganic materials converted to a solid, vitreous slag. The syngas can be utilized in boilers, gas turbines, or internal-combustion engines to generate electricity or be further processed into organic chemicals.

PLASMA GASIFCATION

The use of AC and/or DC electricity passed through graphite or carbon electrodes, with steam and/or oxygen/air injection (less than stoichiometric), to produce an electrically conducting gas (a plasma) typically at greater than 7,000°F that converts organic materials, including tars, oils, and char, to a syngas composed primarily of hydrogen and carbon monoxide, with inorganic materials converted to a solid, vitreous slag. The syngas can be utilized in boilers, gas turbines, or internal-combustion engines to generate electricity or be further processed into organic chemicals.

ANAEROBIC DIGESTION

Also called anaerobic composting or biogasification, this technology is the biological conversion of biodegradable organic materials in the absence of oxygen at temperatures lower than 200°F. The process is carried out by anaerobic microorganisms that convert carbon-containing compounds to a biogas (primarily methane and carbon dioxide). The residue is a stabilized organic material that can be used as a soil amendment. Anaerobic digestion is suitable for the biodegradables, including food wastes, yardwastes, animal wastes, and some paper fibers.

It is important in describing the pyrolysis and gasification technologies to note that they do not involve combustion and are very different from conventional incineration. While incineration requires more than stoichiometric amounts of oxygen to burn the MSW (creating carbon dioxide as a waste gas that must be exhausted through a tall stack), pyrolysis and gasification both produce a syngas product, which has many further uses.

Table 1 provides characteristics of several MSW conversion technologies that are becoming more common. These data were taken from various studies conducted by URS during the past year. View Table 1.

The data in this table demonstrate the variation that can be expected when evaluating technologies and suppliers. The cost data indicate a potential for conversion technologies to treat MSW at a cost of $40-$55/ton, which would be competitive with landfill disposal in some communities, particularly when considering future costs. More detailed studies will be needed to confirm this assumption.

Technology Evaluation Criteria

The evaluation of MSW conversion technologies is a real challenge because of these factors:

• Only a small number of commercial facilities
• A considerable uncertainty in the available data
• A wide variety of technologies based on a wide range of processes
• More than 100 vendors, most of which are overseas, having entered the marketplace
• Wide disparity in operational experience, from none to commercial

As a result, a multistage evaluation process is needed, typically consisting of two phases, each including a screening step followed by a ranking step. Following this process gradually increases the need for data as you progress through the phases.

At each step, specific criteria must be applied to make informed decisions about which technologies/vendors proceed to the next step. Two concerns must be balanced along the way. First, each step should address no more than about 20 (alternatives, to avoid data overload and the time and cost requirements). Second, many cities and counties in the US, Europe, and elsewhere are looking at new MSW disposal solutions. Therefore the existing vendors are overloaded trying to respond to requests for information (RFIs) and Requests for Proposals (RFPs). Because it is becoming increasingly difficult to attract good responses, lists should be kept as short as possible, and information requests should be kept as simple as possible during early screening and ranking steps.

Evaluation criteria typically fall into the following categories and subcategories:

TECHNICAL

• System flexibility
• Commercialization status and risk
• Ability to integrate the three subsystems (front-end preprocessing, conversion unit, energy/chemicals production)
• Need to scale up size for the application
• Process upset risks
• Water usage
• Conversion efficiency

ENVIRONMENTAL PERFORMANCE

• Regulated and toxic emissions
• Byproducts and residual wastes
• Visual impacts
• Nuisance impacts (odors and noise)
• Worker safety and health issues

SITING/PERMITTING

• Footprint required
• Stack/building height
• Proximity to msw sources
• Permitability
• Public acceptability
• Infrastructure requirements

COST

• Net cost (capital + operating - revenues)
• Financial strength of vendor
• Ability to market recyclables, byproducts, electricity, and chemicals
• Financial risks to public sector
• Performance guarantees and insurance issues

This evaluation involves multiple conflicting objectives, requiring tradeoffs between, for example, the environment and cost. Such techniques as Decision Analysis are needed to effectively compare alternatives. Decision Analysis introduces rules that ensure consistency and provides a mechanism for making the necessary tradeoffs between different levels of achievement of the various criteria.

Key Performance Issues

At the outset, we indicated that almost all MSW conversion technologies are in various stages of development, from early development to demonstration stage. Therefore studies of conversion technologies for MSW applications must consider several important issues today, including these:

Front  -end preprocessing: Conversion technologies often involve some type of preprocessing to create a feedstock that is compatible with the conversion unit. Examples include refuse-derived fuel (RDF) and post-recycled municipal biomass (PRMB). The front-end preprocessing must be designed to accommodate the specific requirements of the conversion unit and provide for variability in MSW supply (i.e., weekends).

Feedstock experience: Many of these technologies have been tested, or even proven, on other feedstocks, such as biomass or medical/industrial wastes. The ability to process and treat MSW might be transferable. For example, RDF produced in a preprocessing system might be able to provide a suitable MSW-based feedstock for conversion systems that have been used for years on biomass and agricultural solid wastes.

System scaling: Many municipal applications involve relatively large MSW flows (typically in excess of 300,000 tpy) because the larger cities have the more critical waste disposal problems. Many technologies will have to be scaled up significantly or require the use of numerous conversion modules to meet these flow rates. Some systems require upscaling of 10 times or more, which can introduce engineering and operation challenges and risks.

Process complexity and reliability: The reliability of these technologies is an important consideration because there might only be limited onsite area for storage of MSW. In addition, if the facility is designed to generate electricity, the ability to meet power production requirements will have a great impact on revenue. One must be concerned about the reliability of the overall system, which is a function of the reliability of each subsystem. Some conversion units, for example, require several conversion chambers, while others require just one. The same is true for the power generation subsystem, whether it be one or more boiler-turbine units or a series of small reciprocating engines. Tradeoffs must be made regarding increased number of units versus perceived increases in reliability.

Toxic air emissions control: This is a very important issue affecting public acceptability for technologies that operate at higher temperatures (above several hundred degrees Fahrenheit). The potential for dioxin and furan formation/re-formation, as well as the destruction, capture, and control of these substances, must be evaluated. Issues include operating temperatures, residence times, types of emissions control systems utilized, and the type of power production system used. Obtaining emissions testing data for many technologies is difficult. Tests might have been conducted using different protocols, making comparisons problematical.

System integration: Most conversion technologies are only recently being applied to MSW waste disposal in a municipal setting. Technology vendors generally are not experienced integrating the three subsystems: front-end preprocessing, conversion, and energy/chemicals production. One reason for this is that the vendor is typically experienced in either conversion or materials processing, and subsystem integration requires expertise in all three areas. System integration means not only an ability to make the three components work well together but also to optimize the system to increase efficiency and reduce operating costs.

Cost: Obtaining costs for MSW conversion systems can be difficult because the operating experience is typically short, and existing units might be somewhat different than what the vendor is presently offering. Also, vendors might not understand the issues involved in costing a system in the US. For example, does the vendor have a good appreciation for the marketability and pricing for byproducts produced? Often, prices for these materials have a significant impact on overall costs and revenue.

Evaluating MSW conversion technologies will involve focusing on a number of critical issues, particularly some complex technical issues and cost uncertainties. These issues should be identified early in the study to ensure proper consideration.

Who Is Evaluating MSW Conversion Technologies?

As we indicated above, most of the conversion technologies have been in existence for some time, but they were not used to process MSW. Europe and Australia were first to apply MSW type feedstocks to some of these technologies (primarily anaerobic digestion and pyrolysis). These applications were initiated in the late 1990s. During the past two years, selected North American cities and counties have begun evaluations as they confront landfill shortages or look for green power sources. The topic of conversion technologies and how they can fit into a solid waste management strategy is included in more local and regional MSW meetings and conferences.

As of this writing, URS is aware of evaluations that were conducted or are underway at the following locations:

• St. Croix, US Virgin Islands
• Collier County, FL
• Lake City, FL
• New York City, NY
• Toronto, ON, Canada
• Alameda, CA
• Santa Barbara County, CA
• City of Los Angeles, CA
• Los Angeles County, CA
• Southern California Association of Governments
• County of Hawaii, HI
• Catoosa County, GA

Conclusion

A number of cities and counties in the US are beginning to consider the merits of MSW conversion technologies. This trend is being driven by several issues:

• Shortages of landfill capacity
• Longer hauls to landfills (and higher per-ton transportation costs)
• Difficulty in permitting expansion of existing incinerators and siting/permitting of new incinerators
• Reduction of greenhouse gases and contamination from landfills
• Regulations requiring higher landfill diversion
• Generation of renewable power to satisfy renewable performance standards goals
• Desire to generate green power

Conversion technologies have the potential to recover energy and/or produce high-value organic chemicals from the portion of the MSW stream that cannot be recycled or composted (e.g. complex papers, plastics).

Vendors are realizing the potential for a new market. More than 100 vendors offer various types of conversion technologies. Therefore evaluations of technologies must not only consider the applicability of the technology to meet city/county solid - waste goals, but the evaluation must also include the ability of the vendor to successfully bring a project to market.

Developing an MSW conversion technology project presents major challenges. Although many barriers stand in the way, ranging from regulatory to technical to environmental to cost, none of these barriers is fatal. As more evaluations are conducted and demonstration projects proceed, these barriers will recede. The future appears attractive for including MSW conversion technologies in an overall approach to MSW management.

Daniel Predpall, P.E., is vice president of Power Business Line for URS Corporation in Santa Barbara, CA.

MSW - May/June 2004