University of Maryland MSW-MSO Project

proposed by, Bruce Hoglund, February 1998


Our Goal: The all-in-one “municipal stomach” to recycle Municipal Wastes




Background

“More than 209 million tons of MSW was generated in 1994. Paper and paperboard accounted for 81.3 million tons (38.9 percent) of the total waste stream, yard wastes 30.6 million tons (14.6 percent), plastics 19.8 million tons (9.5 percent), metals 15.8 million tons (7.6 percent), food 14.1 million tons (6.7 percent), glass 13.3 million tons (6.3 percent), and other 34.2 million tons (16.4 percent) (Figure 8).”1

“The generation of MSW has increased from 88 million tons in 1960 to 209.1 million tons in 1994. During that time, per capita generation of MSW increased from 2.7 pounds per person per day to 4.4 pounds per person per day (Figure 9). Per capita generation is expected to remain constant through 2000, when total MSW generation is expected to reach 223 million tons.” 2


1994 Municipal Solid Wastes (MSW) Composition



Pollution and wastes are really nothing more than misplaced chemicals. Recycling of these chemicals is usually limited by economics, convention, and regulations. These constantly shifting factors may now be favorable towards the nearly complete recycle of waste materials. Most recycling efforts center around fairly homogeneous materials such as steel, aluminum, paper, and glass. The more mixed these materials are with others, the less valuable they become. Methods such as magnetism for steel and density differences for glass and aluminum can be employed to further separate loosely mixed materials, although it is still best to separate these materials as early in the waste generation cycle as possible, e.g., curbside recycling programs. Except for a few specific examples (e.g., milk jugs, soda bottles, etc.) plastics do not lend themselves to recycling, even if a ‘pure’ stream of waste plastics is obtained. This is because plastics are not pure materials, but complex mixes of hydrocarbons (carbon and hydrogen) and various other radicals such as chlorine (e.g., PVCs), cyanide (CN, e.g., acrylics), and fluorine (e.g., Teflon). It is these radicals, when thermally broken down at high temperatures and then essentially quenched by the quick cooling that occurs during the combustion or incineration process that is responsible for the production and emissions of cyanide and the greatly feared dioxins and furans. By decreasing the peak temperatures, increasing the residence time at high temperatures, and providing a safe molecular ‘sink’ for the radicals (e.g., sodium) the production and emission of these materials can be eliminated. This is why Molten Salt Oxidation (MSO) continues to attract interest for the difficult to incinerate materials. Thus, MSO is not a competitor of incineration, as it is so often presented, but a complementary tool in the manipulation of chemicals and disposal of wastes.

As can be seen in the MSW composition graph above, much of MSW is “biomasses”. In fact, MSW is a already collected biomass that is largely going to waste. Furthermore, it is ideally colocated with the customers needing the energy, unlike most other biomass harvesting schemes.


Historical MSO and Today’s World

Commercial Molten Salt utilization began with the operation of the Hall-Héroult method of producing Aluminum over 100 years ago. Molten salts for oxidation were first considered at the turn of the century for the gasification of coal. The products are “syngas” which is a mixture of hydrogen (H2) and carbon monoxide (CO) gases. Syngas can be used directly as a fuel gas, as it has a heating value about 1/3 that of natural gas and cleanly burns to produce only carbon dioxide (CO2) and water (H2O). Syngas can also be the starting point for many chemicals, plastics, or liquid fuels (e.g., methanol). The gasification of coal application of MSO has been rekindled periodically, but most recently with the energy crisis of the late 1970’s and 1980’s. The main obstacle is the difficult economics of using a low value fuel (coal) to produce a slightly higher value product (syngas). These difficult economics are compounded by the high as of coal and the additional cost of removing the ash from the MSO. The ash issue is the greatest unknown in the MSO field. Ironically however, most of the MSW ash is less tightly bound and thus more easily pre-separated (e.g., removing cans and bottles) than coal.

Difficult economics, not technical difficulties, stopped the MSO gasification of coal. The ability to obtain the energy and materials in MSW for a revenue (tipping fees) instead of a cost will provide an obvious advantage over coal in economics of operation.

Current interest in curbing carbon dioxide (CO2) greenhouse gas emissions via capture and sequestering and establishment of a hydrogen economy may also be aided by developing MSO as it can produce concentrated CO2 and H2 gases.



Physical Compositions of Individual Solid Wastes3

 

 

Percent by

Moisture
content

Dry Weight

lb (based on 100 lb

Percent Dry

Weight

Typical Density

Density, Typical

kg/m^3

Component

Weight

percent

or kg sample)

(%)

lb/ft^3

lb/ft^3 * 16.019 = kg/m^3

Food wastes

0.15

0.7

4.5

5.8

18

288

Paper

0.4

0.06

37.6

48.1

5.1

82

Cardboard

0.04

0.05

3.8

4.9

3.1

50

Plastics

0.03

0.02

2.9

3.8

4

64

Textiles

0.02

0.1

1.8

2.3

4

64

Rubber

0.01

0.02

0.5

0.6

8

128

Leather

0.01

0.1

0.5

0.6

10

160

Garden trimmings

0.12

0.6

4.8

6.1

6.5

104

Wood

0.02

0.2

1.6

2

15

240

Glass

0.08

0.02

7.8

10

12.1

194

Tin cans

0.06

0.03

5.8

7.4

5.5

88

Nonferrous metals

0.01

0.02

1

1.3

10

160

Ferrous metals

0.02

0.03

1.9

2.5

20

320

Dirt, ashes, bricks, etc.

0.04

0.08

3.7

4.7

30

481




Chemical Composition of Individual Solid Wastes4

Percent by weight (dry basis)

Component

Carbon

Hydrogen

Oxygen

Nitrogen

Sulfur

Ash

Food wastes

48.0

6.4

37.6

2.6

0.4

5

Paper

43.5

6.0

44.0

0.3

0.2

6

Cardboard

44.0

5.9

44.6

0.3

0.2

5

Plastics

60.0

7.2

22.8

--

--

10

Textiles

55.0

6.6

31.2

4.6

0.2

2.5

Rubber

78.0

10.0

--

2

--

10

Leather

60.0

8.0

11.6

10

0.4

10

Garden trimmings

47.8

6.0

38.0

3.4

0.3

4.5

Wood

49.5

6.0

42.7

0.2

0.1

1.5

Dirt, ashes, bricks, etc.

26.3

3.0

2.0

0.5

0.2

68


Current Opportunities for MSO

The most studied opportunity for MSO is the destruction of hazardous wastes including radioactive and chemical weapons (since the 1950s by Rockwell). Unfortunately, although commercial molten salt technology is over 100 years old, its use for oxidation of wastes is still relatively new, especially when compared with the millenia old burning technology upon which incineration is based. It is our belief that the immediate application of MSO to the most hazardous wastes is the equivalent to attempting to run before you learn to walk. We wish to develop practical, inexpensive MSO technology for the more common wastes (e.g., “garbage” or MSW) or biomasses, and the production of valuable gases and products before the more difficult, niche services area of hazardous, radioactive, and chemical weapons is attempted.


Basic MSO of MSW Concept





The main processes of interest occuring within the MSO are the following:

CxHyOz + heat --> z CO + y H2 + (x-z) C

C + H2O + heat --> CO + H2 (the “Water Gas reaction”)

CO + H2O --> CO2 + H2 + heat

C + CO2 + heat --> 2 CO


We can roughly say, from a rough heat balance perspective:

4 C + 6 H2O + O2 = 6 H2 + 4 CO2

‘2 1/4 parts water to 1 part carbon’, or slurry 69% water.



The actual MSO-MSW process can be ‘black boxed’ and considered by the following rough flows out of the MSO facility and the following facts:

Salt facts:
Heat content about equal to water on volumetric basis.
Density of about 2.3 that of water.
Conducts electricity
Corrosive
Excellent Solvent
Viscosity at operational temperature similar to water’s
Catalytic Properties

MSO Facts:
Thermal efficiency: 70%
Energy Product Produced: Hydrogen gas
All carbon is ultimately oxidized to CO2

Questions That Need Answers


What uses do the 2 gas streams (CO & H2 and N2 & CO2) have? What markets (profitable uses)?

What is the current make up of the trash to a waste to energy plant (e.g., the Ogden Waste to Energy Plant in Alexandria, VA)?
How much MSW is processed there? What are its energy balances?
Do they presort the MSW they receive? What do they remove? Does the removed material have a value? If so, what are the materials and how much is obtained? What are the costs of the pre-sorting operations ($/tonne of ‘raw wastes’).
How much ash is removed? What is the composition of the ash? Does it have a value/use?

What are the best combination of methods of removing ash causing materials in the trash?

Aqueous? (Hydropulper, Density difference, etc.)
Air density? (Air Knife, etc.)
Mechanical (Tableing, Tromell, Screens, etc.)

What would be the losses of sodium if the salt removed is 20% ash and the ash is in the form of 90% albite (NaAlSi3O8) and 10% anorthite (CaAl2Si2O8).

What is the likely ash causing content of a pre-sorted MSW feed into the MSO?
What is the likely salt removal rate (tonne salt/tonne waste feed)?
What is the heat losses of the salt removal?

What are the ash costs (salt losses) of:

Tires
Magazines (kaolin coatings)
“Juice Boxes”


What are the values of the product gases; hydrogen, N2 & CO2?
Who are the biggest customers of these gases?




Question:
If we have a 1,000 tonne/day MSW plant receiving ‘typical’ garbage (Alexandria example?), what are the component flows? Example:
Gas Products production Hydrogen produced/day? CO2 & N2 produced/day?
Glass removed Glass entrained (fed into MSO)
Al cans removed Al entrained
Iron removed Iron entrained
Dirts & other unclassifieds removed Entrained

Total landfill requirements (tonnes/day)?


Salt recycle information:
Sodium is added as a NaOH & NaCl mixture, where the NaOH comes from electrolysis of brine and the main cost is the electricity.

Sodium Chloride costs: $50/tonne
Electricity cost: $0.03/kW-h
NaOH cost: 1 tonne 70% NaOH/2500 kW-h

Question:
What by-products of the NaOH production could be sold? How much byproduct(s) are produced? How much do the by-product(s) sell for? To whom is likely to buy these byproduct(s) {who are the main consumers now}?

MSO Ash Facts:
MSO Ash causing elements, in materials containing the following:

Silicon SiO2
Calcium CaCO3
Aluminum Al & Al2O3
Iron Fe & Fe2O3
Sulfur

 

Completely undesireable:

Mercury
Arsenic
Zinc?
Cadmium?


Ash is the key to economic MSO-MSW operation!

Main (MSO) ashes are:

How do we pre-remove the ash forming materials?

Tabling (Hand sorting)
Tromel Screens
Curbside recycling
Magnetic
Density differences
Air (knife)
Water (“floatation”)
Densified water (salt &/or 'sand' “Heavy Media”)
Biogas?
Mix post digested sewage with raw MSW & digest
Post separation of digested MSW
Heavy Media (insoluble MSO salt ash)
Light fraction feeds MSO
Fluid recycled


How about thinking entirely outside the box? (Wet & fouling are now good)

New delivery methods
Hydropulpers?
Sewers (existing, new higher capacity short runs)?
Select clients?
Small & local combined sewage & MSW MSO plants?
Do hydropulpers allow easier separation? At feed site, or pre-MSO feed?

Additional products
Sewage (raw, biogassed, or secondary digested)
Wood Pulping operations?
Refinery wastes?
Fouling wastes in general

 

1 Energy Information Agency (EIA) branch of Department of Energy (DOE), article, “Municipal Solid Waste Profile: Introduction”, URL: http://www.eia.doe.gov/solar.renewables/renwable.energy.annual/contents.html

2 ibid

3 Tables’ data from "SOLID WASTES: Engineering Principles and Management Issues", George Tchobanoglous, Hilary Theisen, Rolf Eliassen (1977).
Pages 59 - 63, Table 4-4 "Determination of Moisture Content for Solid Wastes Same in Example 4-1" Page 60, TABLE 4-6 Typical Densities..."

4 Ibid