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Conversion technologies
02-NOV-2005

A new alternative for MSW management

Thermal conversion of MSW produces syngas
which has a wide range of energy and industrial applications, and can achieve
landfill diversion rates approaching 100%. It is not surprising then that thermal
conversion technologies are the subject of increasing interest in North America. 

Conversion technologies have been used to treat a wide range of feedstocks
(including coal, petroleum coke and wood waste) for over 100 years. Most of
these feedstocks were fairly homogeneous, and commercial-scale application of
conversion technologies to municipal solid waste (MSW) did not occur until the
1980s and 1990s. Most of these facilities were installed in Europe and Japan
in response to regulations to reduce MSW disposal to landfills. In North America,
many cities and counties have evaluated, or are currently evaluating, conversion
technologies as part of MSW management planning¹.

Why the sudden interest in conversion technologies in North America? Reasons
include:

• reduced landfill capacity


• difficulties in developing new landfills


• desire to manage MSW locally


• laws or regulations requiring increased diversion from landfills


• difficulty in permitting new waste-to-energy (WTE) facilities


• incentives for renewable energy or fuels.

OVERVIEW OF THERMAL CONVERSION TECHNOLOGIES

A major advantage of conversion technologies is the production of an intermediate
product ¿ synthesis gas or syngas ¿ which is used in a wide range of energy
generation and chemical manufacturing processes. This distinguishes conversion
technologies from WTE, which directly combusts the MSW. With no intermediate
product, WTE is limited to the production of steam and/or electricity.

A typical conversion technology facility contains three subsystems (Figure
1). The first is a pre-processing subsystem for removing additional recyclables
and, often, for size reduction and drying; for many conversion technologies,
the more homogeneous the feedstock, the better. The processed MSW then enters
the conversion unit, where it is thermally converted ¿ not combusted ¿ to syngas.
This subsystem may include syngas cooling and cleaning, as well as removal of
additional by-products and residuals. The syngas then enters the energy production
subsystem, which may be for power generation or further processing to produce
chemicals. This final subsystem may include flue gas emission controls.

The most common thermal conversion technologies are:

• pyrolysis


• conventional gasification (fixed or fluidized bed)


• combined pyrolysis and gasification


• plasma gasification.

These four technologies are outlined below.

Pyrolysis

Figure 2 shows a process flow diagram for a pyrolysis facility. In pyrolysis,
carbon-based materials are thermally degraded using an indirect, external source
of heat, at temperatures of 400°¿900°C in the absence of free oxygen. The volatile
portion of the feedstock is thermally decomposed, producing syngas composed
primarily of hydrogen and carbon monoxide.

If the syngas is cleaned of contaminants, it can be combusted in a reciprocating
engine, producing electricity. Otherwise, it can be combusted in a boiler, producing
steam for power generation, with the flue gas treated in an emission control
system.

Non-volatile organic materials are left behind as char, typically disposed
of with the ash. Pyrolysis has been in use for hundreds of years, primarily
for producing charcoal. MSW pyrolysis, with power generation, has been in successful
operation in Europe for almost 20 years.

Conventional gasification

Figure 3 shows a process flow diagram for a gasification facility. Gasification
converts carbonbased materials in the presence of direct heat at temperatures
of 760°¿1650°C and with a limited supply of oxygen to a syngas composed primarily
of hydrogen and carbon monoxide. This is a chemical process, not combustion.
As with pyrolysis, if the syngas is cleaned of contaminants, it can be combusted
in a reciprocating engine, producing electricity. Otherwise, the syngas can
be combusted in a boiler, producing steam for power generation.

The char that would have been left behind in pyrolysis is converted to additional
syngas. Inorganic materials end up as either bottom ash (low-temperature gasification)
or as an inert, glassy slag (high-temperature gasification).

Gasification has been in use for more than 200 years. It has been used worldwide
to gasify coal to produce `town gas¿ for heating, cooking and lighting. MSW
gasification has been implemented worldwide in the past few years, with some
facilities producing steam and others electricity.

Combined pyrolysis and gasification

This process essentially follows the pyrolysis reactor shown in Figure 2 with
a separate gasification reactor as shown in Figure 3. At the end of the pyrolysis
section, the syngas exits the chamber and the carbon char left over from pyrolysis
is fed into the adjacent gasification chamber, producing more syngas. Together,
this integrated process results in high conversion to syngas. Plasma gasification
In plasma gasification, a plasma arc is used to create a high-temperature stream
of ions (plasma) using air, oxygen, nitrogen, steam or other gases at temperatures
up to 5500ºC. The plasma is used to heat the MSW to a gasification temperature
of 1100º¿1650ºC, producing syngas and melting the inorganic components into
slag. Metals can be segregated in molten form below the slag layer and reclaimed
in fairly pure form. Particulate matter removed downstream can be re-injected
into the reactor to produce slag.

Plasma gasification has been in use for years in steelmaking and is being used
to melt WTE ash to meet limits on dioxin/furan content. It has been installed
on a commercial scale in Japan for treating MSW and auto shredder residue (ASR).
Plasma gasification has the potential to be more efficient in terms of electricity
production than either conventional gasification or pyrolysis.

Evaluating the potential for conversion technologies in Los Angeles, California, US  URS Corporation recently prepared a detailed conversion technology evaluation for the City of Los Angeles.3 The City currently disposes of about 4000 tons/day of residential `black bin¿ waste to landfill (1 US short ton = 0.907 metric tonnes). URS evaluated the physical, thermal, biological and chemical conversion technologies for treating this waste stream. The City also asked for WTE to be included for comparison. Development of one or more commercial-scale facilities, each with a capacity of 100,000 tons/year, was set as a project objective. Starting with over 200 suppliers and technologies, URS conducted fatal flaw analyses to identify major technical issues that could preclude further development of the technology with the specified feedstock. It also used detailed ranking and weighting criteria to evaluate the technologies and suppliers. Based on their responses to a questionnaire, only 26 suppliers met the criteria for operating facilities treating MSW or similar feedstocks. None of the chemical conversion technologies met the criteria, and only a few WTE suppliers responded as the proposed 100,000 tons/year throughput was not an economic size. A `Request for Qualifications¿ asking for detailed technical and economic data on existing plants and for a proposed facility was sent to the 26 suppliers; 17 responses were received. URS evaluated the technologies, as well as the suppliers and their capabilities. In order to determine the economic advantages at higher throughputs, data for a facility processing 300,000¿400,000 tons/year were also requested. Substantial reductions in overall processing costs occurred at this higher level, with overall costs near or below those of WTE and landfilling. Evaluation criteria used by URS were: landfill diversion (> 85%) engineering the complete system operational experience (> 20 operational facilities worldwide) permitability (no unmanageable permitting difficulties identified) supplier credibility (supplier has extensive technical and financial resources to engineer and develop the project) ability to market by-products (experience in marketing electricity and recyclables in California was best) economics (worst-case break-even costs of < $29/short ton) visual impact of the facility (stack height < 8.8 metres or 29 feet). Following detailed technical, economic and environmental evaluations, the individual technologies and suppliers were ranked using these criteria. Data for the top five ranked suppliers at 300,000¿400,000 tons/year throughput are shown in Table A. Because the cost estimates provided by the suppliers were in response to a questionnaire, these data should only be used for comparisons between these suppliers and their technologies, and to show relative comparisons to WTE and landfill costs. None of the biological conversion technologies made this `short list¿, mainly due to cost and the implications of having to dispose of large amounts of compost if it is proved unsaleable. The study found that, with pre-processing to remove recyclables and with integration concepts common in the power industry (such as using engine exhaust heat for drying the feedstock), conversion technologies can provide efficiencies (in net kWh/ton) up to 50% greater than that of WTE. When the syngas is combusted in a reciprocating engine, power generation efficiency is higher than that provided by a conventional boiler/steam turbine¿generator cycle. Pre-processing, feedstock conversion and production of slag together provide diversion rates greater than 95%. The next step for this project is preparation of a detailed `Request for Proposal¿ to develop one or more conversion technology facilities in the City. A detailed public education and outreach programme has already begun, and citizens are becoming aware of the technical and financial advantages of conversion technologies. Los Angeles Councilman, Greig Smith, recently advocated the development of seven separate conversion technology facilities in his `R.E.N.E.W. LA¿ programme4 to address long-term MSW management and increased diversion from landfills. URS has just completed a similar study for the County of Los Angeles.5 TABLE A. Conversion technology evaluation for City of Los Angeles ¿ top five ranked suppliers Supplier Technology Pre-processing Marketable by-products Overall diversion rate (%) Capital cost ($/ton per year) Overall processing cost ($/ton)a Interstate Waste Technologies Thermoselect ¿ pyrolysis and gasification None Metals, slag, sulphur, metal salts ~ 100 582 40 Waste Recovery Seattle Inc. WTE None Bottom ash, metals, hydrochloric acid 98 474 56 Whitten Group International ENTECH Renewable Energy System ¿ gasification Removal of recyclable glass, metals, construction wastes Recyclables from pre- processing, bottom ash 98 560 43 WasteGen TechTrade ¿ pyrolysis and gasification Shredding and drying Bottom ash, metals 99 606 52 Renewable Resources Alliance Primenergy ¿ gasification Repmoval of recyclable glass, metals, paper Recyclables from pre- processing, bottom ash 85 137 0 a Includes capital recovery, interest and facility operation and management (O&M), offset by sale of recyclables, by-products and electricity

ENVIRONMENTAL BENEFITS

Conversion technologies have a number of environmental benefits.

• Conversion technologies often incorporate pre-processing subsystems to produce
  a more homogeneous feedstock. This provides the opportunity to recover chlorine-containing
  plastic (as a recyclable), which could otherwise contribute to the formation
  of organic compounds or trace contaminants.


• Syngas produced by thermal conversion technologies is a much more homogeneous
  and cleaner burning fuel than MSW.


• Conversion technology processes occur in a reducing environment, so that
  formation of unwanted organic compounds or trace contaminants is precluded
  or minimized.


• Conversion technologies are closed pressurized systems such that there are
  no direct air emission points. Contaminants are removed from the syngas and/or
  from the flue gases before being exhausted from a stack.


• The volume of syngas produced in the conversion of the feedstock is considerably
  lower than the volume of flue gases formed by WTE facilities. Smaller gas
  volumes are easier and less costly to treat.


• Pre-cleaning of syngas is possible, thus reducing the potential for corrosion
  in power generation equipment and reducing overall air emissions. Sulphur
  compounds can be removed by commercially available equipment and recovered
  as marketable sulphur or gypsum.


• Syngas produced by thermal conversion technologies is a much more homogeneous
  and cleaner-burning fuel than MSW.


• Methane emissions from landfills are significant even with energy recovery.
  Using a conversion technology to convert the carbon content of the MSW to
  combustible syngas, instead of allowing it to degrade in a landfill to methane,
  eliminates this environmental impact.


• The inert, glassy slag recovered from high-temperature gasification is similar
  to that produced from steel mills and coal-fired power plants. It can be used
  for making roofing tiles and as sandblasting grit or asphalt filler.

RENEWABLE ENERGY AND FUELS

In creating electricity and useful by-products, conversion technologies typically
reduce the amount of MSW by more than 95%.²
The use of MSW for power generation results in significant savings in the use
of fossil fuels and their associated life-cycle impacts from the mining and
transportation of the fuels.

Conversion technologies typically reduce the amount of MSW by more than 95%

Another advantage of conversion technologies is the production of renewable
energy. MSW is often classified as a renewable fuel under state Renewable Portfolio
Standards (RPS) programmes in the US. `Green power¿ from MSW and other renewable
sources commands a premium price in some states.

Another option with syngas produced using a conversion technology is its further
processing to produce methanol (a feedstock for many chemical processes) or
ethanol (a valuable transportation fuel).

BARRIERS TO THE IMPLEMENTATION OF CONVERSION TECHNOLOGIES

Despite the success of conversion technologies in Europe and Japan, no MSW
facilities have yet been constructed at a commercial scale in North America
¿ none of these lengthy procurement processes have ended with a `buy¿ decision.

Examples of barriers that appear to restrain further development of conversion
technologies include:

• lack of regulations needed for permits (as in California)


• financial risk


• technical risk


• economics (overall $/ton processing cost versus tipping fees for existing
  disposal solutions)


• reluctance of cities and counties to take on environmental groups or to
  commit to the public education required for successful siting and permitting.
  

Some of these barriers have precluded demonstrationscale projects from going
forward ¿ when economics was not even a primary consideration. This makes it
more difficult to move these promising technologies from small scale to commercial
scale, where costs will be much more competitive. With the exception of the
decision by the City of Edmonton in Canada to continue with its MSW gasification
demonstrationscale project, there is still a lot of looking but no buying going
on. This has been very frustrating and costly for conversion technology suppliers.

As cities and counties progress their evaluations and continue to face both
increasing costs from landfills and increasing pressure to make wiser MSW management
choices, removal of the regulatory and economic barriers is expected, allowing
implementation of conversion technologies to proceed.

OUTLOOK

More cities and counties in North America are evaluating conversion technologies
and their economic and environmental advantages. The economics of conversion
technologies depend greatly on throughput. Our evaluations have shown that increasing
throughputs to the range of 300,000¿400,000 tons per year results in significant
decreases in per ton processing costs. Although the costs provided in this article
are estimates based on the specifics of one evaluation questionnaire, this shows
that conversion technologies can be competitive with WTE and landfills.

Conversion technologies offer significant environmental benefits compared with
landfilling. MSW, which is largely organic, is diverted from landfill and is
converted into a fuel for electricity production and other usable by-products.
Only a small amount of residue, which is typically inert, may require landfilling.
Diversion rates approach 100%.

Implementation of conversion technologies is expected to proceed

A strong education programme is required so that the public will understand
the technical differences and the economic, environmental and societal benefits
of conversion technologies compared with conventional MSW management options.
This must occur before a facility enters the siting and permitting phase.

Implementation of conversion technology facilities is expected to proceed in
response to concerns about WTE and landfills, as communities in North America
become more familiar with their benefits, and as barriers to development are
lifted.

Stephen D. Jenkins is the Gasification Technology
Leader at URS Corporation in Tampa, Florida, US.

e-mail: steve_jenkins@urscorp.com

NOTES

1. International Solid Waste Association (ISWA). IWSA Directory of waste-to-energy
   plants. 2004. Available on-line at: www.wte.org/2004_Directory/IWSA_2004_Directory.html
   


2. Investigation into municipal solid waste (MSW) gasification for power
   generation. Prepared for Alameda Power & Telecom by Advanced Energy Strategies
   and URS Corporation, May 2004. Available on-line at www.alamedapt.com/newsroom/reports/finalgasification.html
   


3. Evaluation of alternative solid waste processing technologies. Prepared
   for the City of Los Angeles, Department of Public Works, Bureau of Sanitation
   by URS Corporation, July 2005. Available on-line at www.lacity.org/SAN/alternative-technologies.htm
   


4. Smith, G. Together we can R.E.N.E.W. LA. 27 July 2005. Available
   on-line at www.lacity.org/council/cd12/renewla/cd12renewla243131327_07272005.pdf
   


5. Conversion technology evaluation report. Prepared for the County
   of Los Angeles Department of Public Works and the Los Angeles County Solid
   Waste Management Committee/Integrated Waste Management Task Force¿s Alternative
   Technology Advisory Subcommittee by URS Corporation, August, 2005. Available
   on-line at http://ladpw.org/epd/tf/subs.cfm
   

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