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Using Gasification to Process Municipal Solid WasteBy Joseph W. Schilli

 Volume 12, Number 4 • Winter Publication of the Environmental and Resource Management Group of HDR

The United States leads the world in

generating municipal solid waste

(MSW) with a per capita average of 

about 4.6 pounds per day. Consequently,

determining what to do with all of that garbage

grows ever more important. The country is

making strides first by setting the pace in

recycling and also by being a leader in

converting waste to energy through specialized

combustion processes. Now, there is

movement to adapt an industrial technology

called gasification to serve as yet another 

option in solid waste management.

Put simply, MSW is the garbage produced by

residential households, commercial

 businesses, industrial operations such as

factories and institutional facilities like schools

and hospitals. With annual MSW production

topping 229 million tons, gasification can ease

the burden on landfills by processing solid

waste and at the same time serve as a valuable

fuel resource through the production of power 

and other commercial materials.

Gasification Explained

The basic chemical processes behind

gasification have been known for years. In

fact, facilities that used gasification to produce

oil from coal date back to the early 1900s.

Gasification can be generally defined as a

 process for the production of gaseous or liquid

fuels from organic material within the feedstock. Dependent on the

feedstock, materials such as metals may also be recovered.

There are two fundamental stages in the gasification process:

• Organics in the feedstock are broken down into compounds

consisting largely of carbon and hydrogen.• These compounds then undergo reactions to

form a liquid or gas with a significant energy content.

There have been significant research and development efforts

related to processing solid waste with gasification and related

technologies since the 1970s. For example, pyrolysis, a technology

similar to gasification, attained commercial operation in a facility in

Baltimore during the late 1970s and early 1980s. Unfortunately, the

facility experienced persistent technical problems and eventually

had to cease the pyrolysis operation.

Today, close to 100 firms claim to have an operational gasification

technology and/or a research and development effort related to

gasification.

Finding Alternatives for Solid Waste Management

The processing of MSW in general has changed substantially duri

the past two decades. Vast numbers of small, poorly controll

landfills are being replaced with fewer but larger and high

engineered facilities.

But just as important as improving the efficiency of landfills

reducing the amount of materials going into them. It begins wi

source reduction – a strategy of modifying how products are ma

and used in a way that reduces the volume and toxicity of was

Recycling also is a crucial component of solid waste manageme

and has more than doubled since 1990. Today, the U.S. diverts abo

30 percent of MSW away from landfills with those raw materi

 being converted back into usable products.

Another 15 percent of the country’s MSW now goes throu

thermal processing before entering the landfill. But combusti

facilities have met with stricter operating standards intended

lessen the impact on air quality and control disposal of a

 byproduct. Some facilities were closed but others implement

 Levels of carbon monoxide, nitrogen oxides, particulate matter, sulfur dioxide and dioxin present in air 

emissions from the Thermoselect process all fall well within U.S. standards.

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cleaner processes and continue to operate

today. Thermal processing remains a

functional piece of the MSW puzzle today

 because of its ability to reduce feedstock by

an average of 90 percent in volume and 75

 percent in weight and because the heat

generated can be used to convert water into

steam and thereby produce energy.

Gasification is the next evolution in the thermal

 processing of MSW. This process further 

reduces the amount of solid waste and

 produces off-gases that can be used to generate

energy or feedstock for a manufacturing

operation. But to work properly, gasification

requires strict air and oxygen control. Pyrolysis

failed primarily because MSW tends to have a

“fluffy” consistency that entraps indeterminate

quantities of air, making it difficult to control

oxygen levels.

But a breakout technology in gasification

called Thermoselect addresses the issue of 

air and oxygen control by using a horizontal

ram system to compact MSW to a density of 

approximately 2,100 pounds per cubic yard,

thus regulating the amount of air trapped

within the feedstock that enters the

degasification chamber. It also uses a

metered injection of pure oxygen that mixes

with gases from the degasification chamber 

in a high-temperature (2,200 degrees

Fahrenheit) reaction chamber. This creates a

 byproduct called Syngas, which, on a dry basis, contains 25 to 42 percent carbon

monoxide, 25 to 42 percent hydrogen, 10 to

25 percent carbon dioxide and 3 to 4 percent

nitrogen and other constituents.

Generation of harmful gases such as dioxin

compounds is inhibited by the pressure,

temperature and reducing conditions of the

degasification chamber and the high

temperature reaction chamber. Furthermore,

the process has been developed to control

metal and inorganic pollutants as usable

 byproducts rather than off-gas pollutants.

For example, sulfur compounds are reduced

to produce elemental sulfur, which can be

recycled as a marketable byproduct. As a

result, it never enters the off-gas stream in

significant concentrations.

Additionally, the Thermoselect technology

was developed to produce molten byproducts

rather than ash. The molten byproducts arerapidly cooled in a water quench, which

causes the formation of metal and silica-

 based granules. The granules are formed

separately due to their different physical

 properties. These granules, having different

densities, are separated into distinct streams

 by a density media separation process.

Concentrated metal alloy granules might be

further refined into separate metals, and

mineral granules can be used as substitutes

for sand or gravel.

 Water Requirements and EnvironmentalImpactThe gasification process described above

has three primary water requirements: the

shock cooling vessel; the quenching process;

and either cooling tower makeup if the

Syngas is being used to generate electricity;

or feedwater if it is being used to create

 process steam.

The Syngas generated during the gasification

 process has a BTU content of between 225

and 250 BTUs per cubic foot, making it

useful in applications similar to those of 

natural gas. For example, existing

Thermoselect facilities currently use Syngas

to:

• Power an internal combustion engine for 

an electric generator 

• Provide fuel for a steel manufacturing

 process

• Generate steam for a downtown heating

system

Research underway now could eventually

lead to more uses for Syngas, including:• Feedstock for producing pure hydrogen

• Fuel for fuel cells

• Feedstock for producing methanol and

ammonia

While gasification facilities offer clear 

environmental benefits, there are some

environmental concerns presented by

gasification facilities, such as wastewater 

and the solid byproducts that remain after all

usable materials have been recovered. Air 

quality has not been a concern as the

Thermoselect facilities currently in operati

have produced significantly less harmful

emissions than what is allowed by U

regulatory standards and less than tradition

combustion technologies. Wastewater

treated by a system contained within t

gasification facility that incorpora

 buffering, precipitation and reverse osmos

The treated water then is recycled in

 process water that can be reused in tfacility. Any additional treatment is dictat

 by the potential end use.

As for the solid byproducts, much of it c

 be recycled. Markets are available for t

metal and mineral pellets as well as the sul

 but have not been identified yet for the sa

and metal hydroxides.

Future of Gasification

Gasification technologies, like Thermoselec

have been evolving. Private firms a

municipal governments appear committed establishing improved gasification technolo

as a viable solid waste management alternativ

The attractiveness of this option likely w

increase given its environmental record a

its ability to produce hydrogen-based energ

There are facilities operating in Europe a

Asia with more planned. There also are

number of municipalities in North Ameri

actively assessing gasification as a so

waste processing tool, including Toron

Los Angeles and municipalities in northe

Puerto Rico.

Several factors must be considered

determine the net cost of a Thermosele

gasification system. For example, wh

revenues might be attained by selli

energy? In the end, this new process can

a cost-competitive solution for solid wa

disposal when compared with more tradition

alternatives like landfills. This is particula

true in areas where landfills are not in clo

 proximity to where the solid waste is bei

 produced, and where the cost of electricityhigh.

In the end, these economic factors may al

 be mitigated by communities looking for

more environmentally sound waste reducti

alternative and a reliable local energy source

 Joseph W. Schilli can be reached at HDR’s Mia

 Lakes, Fla., office at (305) 728-7400 or e-m

 joe.schilli@hdrinc.com.

 A facility with Thermoselect gasification technology in

 Mutsu, Japan, has an internal combustion engine/ 

 generator set to convert Syngas into electrical power.