For as long as civilizations have generated solid wastes, the accepted disposal method has primarily been landfilling.  This has continued despite great advancements in technology, as well as environmental impacts to our air and water created by many landfills used for municipal solid waste.

By Dan Predpall

During the 1980s and 1990s, nearly 100 mass-burn incinerators were built in the U.S., providing a new method for processing MSW and producing electricity. However, air toxics emissions became a basis for public opposition to existing and new facilities. Advancements in air-emission control technologies resulted in greatly reduced emissions, to the point that air toxics emissions are generally below detection limits. This has not satisfied the many environmental groups that have opposed these technologies over the past 20 years. As a result, building additional facilities to handle growing volumes of MSW will be very difficult, if not impossible in most urban areas.

In Europe and Japan, new processes for treating MSW, called “conversion technologies,” are being widely implemented. Many of these facilities are in operation, and others are under construction. Conversion technologies use advanced thermal, biological, or chemical processes to convert the carbon-based portion of the MSW stream into useful products, including electricity, renewable or “green” fuels, or chemicals.

Conversion Technologies 101
Conversion technologies (CTs) include a wide range of processes that can be categorized into thermal, biological, and chemical technologies (some approaches involve combinations of these). Thermal CTs are well developed overseas, and include gasification, pyrolysis, and subsets of these, such as plasma gasification and processes that combine gasification and pyrolysis.

Pyrolysis is the thermal degradation of organic materials, using an indirect source of heat at 750-1,650 degrees F in the absence of oxygen, to produce a synthetic gas, leaving behind a carbon char.

Gasification is the thermal conversion of organic materials, using direct heat at 1,400-2,500 degrees F with a limited supply of oxygen, producing a syngas.

Biological CTs are best exemplified by anaerobic digestion (AD). In AD, biodegradable material is converted by a series of bacteria groups into methane and CO₂. This “biogas” is a medium-Btu gas containing 50 to 70% methane.

An example of chemical CTs is ethanol production, which consists of a series of chemical reactions starting with hydrolysis of the organic feedstock to sugars, followed by fermentation of the sugars to dilute ethanol, with a finishing step to produce fuel-grade ethanol. The hydrolysis step with MSW is still under development.

CTs typically have four components: a pre-processing system, a conversion unit, a post-processing system for product synthetic gas (or biogas) clean-up, and a back end for producing a marketable product.

Figure 1 shows a block diagram of a typical conversion facility.

Pre-processing typically includes sizing, separation and drying to provide a feedstock quality suitable for the conversion unit, and to remove recyclables.

The conversion unit may generate small quantities of byproduct that can be sold, and small quantities of solid or liquid wastes. The syngas or biogas typically will undergo a cleaning process prior to use for electricity generation or green fuel
production.

CTs include of a number of technologies that have been used with other types of feedstocks for decades, such as anaerobic digestion, gasification and pyrolysis. Also, there are many emerging technologies under various stages of development, including plasma gasification, thermal depolymerization, and hybrid systems that make ethanol via gasification, followed by either biological systems or chemical systems.

Why The Interest In CTs?
More counties and cities in the US are considering CTs as a solution to their MSW disposal problems. The most important drivers appear to be the increased cost of landfilling, and the lack of landfill capacity (these issues are, of course, related). For example, the first entities to seriously evaluate conversion technologies were capacity constrained, including the US Virgin Islands, Puerto Rico, and Hawaii. Soon after, several counties and cities in California began looking at CTs as a way to comply efficiently with increasing requirements for landfill diversion.

The cost of landfilling varies with location; however, typical costs in large metro areas are in the range of $15-$30 per ton. At these costs, CTs will not be able to compete. However, as these metro areas fill their remaining landfills and look to rail-hauling MSW to distant locations, landfill costs will rise rapidly, into a higher range of $50-$75 in the near term, and, likely greater than $100 by the year 2010. It is anticipated that MSW management using CTs will compete successfully with landfilling on a cost basis at these levels.

Other drivers include the strong desire to manage MSW locally, under control of local agencies, and increasing landfill diversion to prolong landfill capacity.

Drivers that are expected in the near future include the desire to maximize the value of MSW residuals (material going to the landfill after source separation and recycling), recognition that CTs are a source of much-needed renewable energy, and the public’s desire to manage MSW in a more environmentally sustainable way. Once these drivers become more important, CT development will be rapid.

The Benefits of CTs
The benefits offered by CTs have not been publicized well. The benefits of CTs will be brought into public awareness as entities such as the city of Los Angeles and the county of Los Angeles continue to pursue development of CTs as a way to manage disposal of their residual MSW streams.

The key benefits of CTs are:

Increased Recycling. This benefit is not well recognized. The CTs under development today are being designed to process residuals from materials recovery facilities (MRFs) or trash (residuals after source separation).

These residuals still contain considerable amounts of recyclable materials that can be recovered in pre-processing. In addition, materials produced by the conversion units, such as slag and bottom ash, can be recycled. Some question why the operator would want to remove the recyclables from the input stream. The answer is simple: The facility will earn greater revenues by recycling additional metals, glass, and paper than it will by processing this material in the conversion unit, typically with a loss in overall efficiency. Further, “unrecyclable” plastics that would otherwise go to a landfill are excellent feedstocks for CTs.

Generation Of Renewable Energy. Processing MSW residuals to generate energy or green fuels qualifies in most states as a source of renewable energy. In addition, processing MSW to energy qualifies as a renewable energy under Renewable Portfolio Standards and can be used to create renewable energy credits in some states. If about one-third of the residential MSW collected annually in the city of Los Angeles were processed by CTs, about 50 MW of renewable energy could be produced.

Reduced Landfill Impacts. The byproducts created by CTs are typically very small in quantity, and inert. Therefore, the material that cannot be recycled in a CT and must be sent to a landfill will not result in impacts to the environment. The situation is quite different when MSW (or MSW residuals) are landfilled. Even modern landfills impact the environment via release of methane (a greenhouse gas) not captured by landfill gas collection, air emissions from equipment operating the landfill, and leakage through failures of landfill liners.

Offsets To Fossil Fuel Usage. CTs can generate electricity or green fuels by processing MSW. This in turn reduces the amount of fossil fuels needed to supply the energy requirement of a region. Energy savings result when the entire life cycle of the MSW collection and processing system is evaluated, and all of the energy usage and energy production and recycling benefits are considered.

The energy savings can be significant. For example, according to the California Integrated Waste Management Board’s (CIWMB) CT report to the legislature, energy savings in the Los Angeles region could be equivalent to a 150 MW power plant (this assumes treating about one-third of the total residential MSW collected annually in the City of Los Angeles).

Lower Air Emissions. The use of CTs can result in reductions in emissions of NOx, SOx, and particulates. For example, NOx emissions, which are precursors of smog, acid deposition, and reduced visibility, are primarily the result of fuel combustion processes. Through the use of CTs, NOx emissions can be avoided by displacing combustion activities and electricity production and increasing the recycling of materials.

Using similar data and assumptions as noted above, the NOx emissions avoided by building CTs in Los Angeles would be equivalent to those emitted from a 1,000-MW, gas-fired power plant.

Reduced Carbon Emissions. Carbon emissions contribute to the greenhouse effect, and, therefore, can lead to climate change. Carbon emissions result from the combustion of fossil fuels and the degradation of organics. Methane emissions from landfills represent a significant source of carbon emissions, since methane has a global warming potential about 21 times that of CO₂. The use of CTs can create offsets for carbon emissions through increased recycling, diversion of organics from landfills, and displacement of fossil fuels.

Based upon data in the CIWMB report to the legislature, processing about one-third of the residential MSW collected annually in Los Angeles would reduce carbon emissions by about 1,000,000 metric tons per year.

The overall benefit of CTs is that of increased environmental sustainability. In general, environmental sustainability involves a number of issues, including:

• Reliance on renewable energy
• Improving environmental quality
• Reducing waste
• Conserving natural resources
• Responsible consumption
• Long-term focus

Therefore, the use of CTs closely complies with the goals of environmental sustainability.

As the general public begins to realize the significant environmental advantages of CTs, the development of more CT facilities is expected.

Challenges in The US
Unfortunately, there remain a number of barriers to CT development in the US. The key barriers include:

No CTs Previously Operating. As of July 2005, there were no commercial-scale CTs operating in the US, using MSW as a feedstock. While a number of projects are in the development stage, many of these projects will not go forward due to development risks that apply to any new venture. Some counties and cities are risk averse, in the sense that they don’t want to be the first on the block with a CT; they would rather wait until a few of these systems are operating.

Financing Hurdles. As with any new venture, financing can be difficult. Adding to the problem is that most of the suppliers of CTs have limited resources. The larger corporations don’t appear to be involved in this business as yet; they will arrive, however, once the momentum of this fledgling industry increases.

Opposition. As more CT projects are being proposed, opposition from specific groups is growing. One is the global environmental organization that opposes mass-burn incineration. This group has typically opposed CT implementation on the grounds that CTs are actually “incinerators in disguise”. This is untrue; in fact, there are many significant technological differences between CTs and mass-burn incinerators. Another opposition group is the recycling industry. This industry sees CTs as a threat to its business because it claims that CTs will process all MSW, including recyclables. As mentioned above, this is unlikely because, A.) projects under development are using MSW residuals, and B.) the value of residuals as a recycled material is higher than its value for CT processing.

What is Needed?
In addition to overcoming the barriers mentioned above, two issues that are expected to be important are education and the place for CTs in the MSW management hierarchy.

Because all of the operating experience with CTs is overseas, detailed data and information about operating CTs is difficult to find in the US.

Agency personnel, environmental groups and the public need to be educated about CT design, operation and environmental impacts. Each project developer, whether pubic or private, must undertake a comprehensive public outreach program before trying to site or permit a CT facility. Otherwise, the agencies and the public likely will reject the project.

The first phase of the public outreach program must address education regarding CT systems. The second phase can then address siting and permitting. It is anticipated that the most controversial issues will include air emissions and local community impacts.

While project opponents will attack CTs by comparing their air emission profiles to incinerators, there are many design differences that will result in CT emissions below even the best-performing waste-to-energy facilities. Local community impacts will be important; advantages of CTs include the smaller profile and the ability to be sited within urban areas, where the MSW is generated.

In the 1990s, the USEPA published a solid waste management hierarchy of options that was meant to be a set of default guidelines. The hierarchy is:

1. Source reduction
2. Re-use, recycling, composting
3. Waste-to-energy
4. Landfill with energy recovery

With CTs now as an available option, and more analyses of life-cycle impacts of solid waste management, this hierarchy may no longer represent an order of options that will result in the lowest environmental impacts, or do the best to achieve a sustainable MSW management solution.

A new MSW management hierarchy might be arranged as follows:

1. Source reduction
2. Re-use, recycling, composting
3. Conversion (using CTs)
4. Waste-to-energy
5. Landfill with energy recovery

However, a problem with establishing such a hierarchy is that a total MSW management solution often must combine methods to be environmentally sound and cost effective. For example, as mentioned above, CTs increase recycling and efficiently convert the remaining MSW residuals to energy, green fuels, or chemicals.

Conclusion
It’s time for CTs to take their place in the MSW hierarchy, particularly if the goal is a sustainable MSW management approach that minimizes environmental burdens and seeks a cost-effective solution.

A sustainable approach to MSW management will involve the following issues:

• Consider the entire MSW management system, not just individual components
• Use a range of management options that maximize the value of MSW and minimize disposal in a landfill
• Balance environmental impacts, social issues, and costs (should the least-cost option always be selected?)

When the life-cycle impacts of all MSW management options are considered, CTs rank high because of the advantages discussed above.

The time may be approaching when CTs become a common element in an overall waste management solution for cities or counties that will experience higher disposal costs or disposal capacity constraints in the next several years.

A number of cities and counties are actively considering conversion technologies as a tool in solid waste management. For example, Riverside County, CA, has selected a biological CT to process waste now destined for landfilling.

The city of Los Angeles is studying CTs, recently releasing its 20-year plan for solid waste management, called RENEW LA, which calls for the development of several CTs.

The Los Angeles Bureau of Sanitation just released a detailed assessment of conversion technologies, which can be found at www.lacity.org/san/alternative-technologies.htm. 

MSW - May/June 2006