Potential Chemical Fuels

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There are specific criteria that must be satisfied by the chemical fuel selected to support the fusion energy system. To be useful the fuel must:

1. Provide a large energy storage capability per unit mass to minimize the amount of fuel material manufactured, handled, stored, and transported.

2. Be a fluid, either a gas or a liquid, for ease of transportation in pipelines and transfer to and from storage vessels.

3. Be non-toxic to plants and animals. (No matter how carefully the fuel is handled accidents, mistakes and natural disasters will result in spills.)

4. Provide only gaseous products when burned in air to allow disposal of the reaction products directly to the atmosphere.

5. When burned with air, yield non-toxic products harmless to the environment. (The reaction products must be totally non-toxic or the shift from hydrocarbons to the new fuel will just change one set of toxins for another.)

6. Be made of common chemical elements to ensure an abundant supply.

7. Be made from elements available in most locations to reduce the amount of shipping required in the production and use of the fuel.

8. Be easily manufactured by a low cost, process to be economically viable.

9. Be easy to use in existing power generation equipment to aid transition with a minimum of equipment modification.

These criteria will be applied to potential substitutes to determine which have the potential to serve as the manufactured fuel for use with the fusion energy system.

168 Williams, Laurence O., "Hydrogen Power", Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, 1980

To search for substances to replace the chemicals found in fossil fuels, all chemical elements and compounds with potential as fuels were examined. The nine criteria were used to judge each element and compound to determine if it was a suitable candidate.

There are 83 non-radioactive elements from which to choose. The elements from 1 to 18 and their compounds will be evaluated against each criterion to determine their suitability as fuels. The elements with an atomic number higher than 18 (Argon) are metals or semi-metals. They release little energy when they react with the oxygen. Because of their failure to meet criterion 1 they can be easily eliminated from consideration. Table 5.1 shows how elements 1 to 18 fare when evaluated against the nine criteria. All data regarding physical and chemical properties, abundance, toxicity, and reaction energies of elements and compounds were derived from values taken from the Handbook of Physics and Chemistry. 169

+ = Pass the Criterion - =

Fail the

Cr

teri

on

Criterion Numb

>er

1

2

3

4

5

6

7

8

9

Atom #

Element

State

Pass?

1.

Hydrogen

Gas

+

+

+

+

+

+

+

+

+

Yes

2.

Helium

Gas

-

+

+

+

+

-

-

-

-

No

3.

Lithium

Solid

+

-

-

-

-

+

-

+

-

No

4.

Beryllium

Solid

+

No

5.

Boron

Solid

+

-

+

No

6.

Carbon

Solid

+

-

+

+

-

+

+

+

-

No

7.

Nitrogen

Gas

-

+

+

+

-

+

+

+

-

No

8.

Oxygen

Gas

-

+

+

+

+

+

+

+

-

No

9.

Fluorine

Gas

-

+

-

+

-

-

-

-

-

No

10.

Neon

Gas

-

+

+

+

+

+

+

+

-

No

11.

Sodium

Solid

+

-

-

-

-

+

+

+

-

No

12.

Magnesium

Solid

+

-

+

-

+

+

+

+

-

No

13.

Aluminum

Solid

+

-

+

-

+

+

+

+

-

No

14.

Silicon

Solid

+

-

+

-

+

+

+

-

-

No

15.

Phosphorous

Solid

+

No

16.

Sulfur

Solid

-

-

+

+

-

+

+

+

-

No

17.

Chlorine

Gas

-

+

-

+

-

+

+

+

-

No

18.

Argon

Gas

-

+

+

+

+

+

+

+

-

No

Table 5.1 Elements Evaluated as Potential Fuels

The first criterion is the amount of energy produced on combustion. A measure of energy must be selected for comparison of the energy produced by various fuels. The technically proper measure of energy is the joule. A joule is a quantity of energy, as are British Thermal units (Btu) and calories. A watt is a measure of power, the potential for the delivery of energy. Since a watt is a measure of potential to provide energy, watts per unit time must be specified to provide a quantity of energy. The joule (a measure of quantity) is defined as 1.00 watt for one second. Thus, a 1.0-watt light bulb uses energy at a rate of 1.0 joule per second and a 100-watt bulb uses 100 joules per second.

When fuels burn they release a discrete quantity of energy depending of the mass of fuel reacted. The joule is a relatively small unit of energy. In this Chapter, units of 1 million joules per kilogram mass will be used (that is, megajoules per kilogram (MJ/kg)). If a fuel produces 10 MJ/kg and the energy is converted to electricity at 50% efficiency, burning 1 kilogram would produce 5 megajoules or

169 Weast; Robert C. Editor, "Handbook of Physics and Chemistry" CRC Press Inc. Boca Raton, Florida 33431, 67 Edition 1987

5,000,000 joules of electricity. Five million joules would light one hundred, 100-watt light bulbs for 50,000 seconds (13.88 hours).

Our primary method of energy production is combustion of fossil fuels in air. Complete combustion of methane with air or oxygen generates 57.8 MJ/kg. Gasoline or fuel oil are mixtures of several substances and have a variable heat of combustion ranging from 42. to 46. MJ/kg dependent on their composition. Coal is even more variable than oil. Coal's energy production potential depends on its moisture and ash content. It is also dependent on the ratio of hydrogen to carbon. It has energy of combustion ranging from 16. to 30. MJ/kg. These values provide the standard against which candidate fuels can be compared to determine if they meet the first criterion.

When burned in air, several of the 18 pure elements release an amount of energy greater than that released when coal is burned. The largest amount of energy, 142.0 MJ/kg is released by the combustion of hydrogen. Others are: beryllium, 67.9 MJ/kg, boron, 58.2 MJ/kg, lithium, 43.6 MJ/kg, carbon, 32.7 MJ/kg (the major element in coal), silicon, 31.4 MJ/kg, aluminum, 31.0 MJ/kg, magnesium, 26.0 MJ/kg, and phosphorus, 24.3 MJ/kg. These elements are all solids and fail the second criterion of being fluid for ease of handling.

On combustion, all but carbon produce solid oxides and thus fail the fourth criterion with reaction products that cannot be vented into the air. Carbon passes the fourth criterion, but fails the fifth because combustion results in highly toxic carbon monoxide when burned with insufficient oxygen, and carbon dioxide (the major greenhouse gas) when burned with excess oxygen. Two of the remaining elements, sodium and sulfur burn in air releasing 9.09 MJ/kg and 9.30 MJ/kg respectively. These energy releases are less than coal making them unsuitable because of the failure to meet the first criterion. Both are solids and fail the second criterion. Sodium produces a solid oxide causing it to fail the fourth criterion. Sulfur dioxide, the product of the combustion of sulfur, is a very toxic gas causing sulfur to fail the fifth criterion. This accounts for eleven of the first eighteen elements. The remaining seven elements: chlorine, nitrogen, oxygen, fluorine, helium, neon, and argon do not burn in air and fail the first criterion.

A number of the chemical elements were rejected because they were solids. If methods of handling were developed, that would allow the use of solid fuels in all applications, the second criterion could be ignored and the solid fuel adopted. Unfortunately, all of the energetic elements, save hydrogen and carbon, produce solid oxides. Use of a fuel that produces a solid oxide will require a solid oxide return system capable of handling two or more times the mass handled in the fuel supply system. More than tripling of the mass of material handled will be an extremely large burden in both cost and logistics.

Some of the chemical elements that release large amounts of energy when burned in air combine with hydrogen, carbon and sulfur to produce hydrides, carbides and sulfides respectively. These hydrides, carbides and sulfides will burn in air with energy release. They must be evaluated as potential fuels.

The hydrides, compounds comprised of hydrogen and some other element, deserve special attention because our current fuels are chemical compounds made up of hydrogen and carbon, the hydrocarbons. Table 5.2 shows how each of the hydrides of the first 18 elements fare when examined against the nine criteria.

The hydrides are excellent fuels on an energy basis. Lithium and beryllium hydrides are very good fuels but both are solids and both produce solid oxides. All beryllium compounds are very toxic. The hydrides of boron and carbon form many complex compounds containing multiple atoms of both boron and carbon. These compounds can be room temperature gases, liquids or solids depending on the exact chemical composition. The hydrides of boron have a high-energy content, but are extremely toxic making them non-candidates based on the third criterion. The carbon hydrides are the compounds that make up fossil fuels that we are trying to replace. Nitrogen forms two stable hydrides.

+ = Pass Criterion - = Fail the Cr

iteri

on

N =

Z o

Hyd

ride

Criterion Nu

mber

1

2

3

4

5

6

7

8

9

Atomic #

Element

State

Pass?

1.

Hydrogen

Gas

+

+

+

+

+

+

+

+

+

Yes

2.

Helium

N

3.

Lithium

Solid

+

-

-

-

-

+

-

+

-

No

4.

Beryllium

Solid

+

No

5.

Boron

S/L/G

+

+

No

6.

Carbon

S/L/G

+

+

+

+

-

+

+

+

+

No

7.

Nitrogen

Liquid

-

+

+

+

-

+

+

+

-

No

8.

Oxygen

Liquid

-

+

+

+

+

+

+

+

-

No

9.

Fluorine

Liquid

-

+

-

+

-

-

-

-

-

No

10.

Neon

N

11.

Sodium

Solid

+

-

-

-

-

+

+

+

-

No

12.

Magnesium

Solid

+

-

+

-

+

+

+

+

-

No

13.

Aluminum

Solid

+

-

+

-

+

+

+

+

-

No

14.

Silicon

Gas

+

+

-

-

+

+

+

-

-

No

15.

Phosphor.

Gas

+

No

16.

Sulfur

Gas

-

+

-

+

-

+

+

+

-

No

17.

Chlorine

Gas

-

+

-

+

-

+

+

+

-

No

18.

Argon

N

Table 5.2 Hydrides Evaluated as Potential Fuels

The nitrogen hydrides, ammonia and hydrazine, are potential fuels releasing 23.9 and 19.3 MJ/kg respectively. Ammonia boils at -33 degrees C and hydrazine at +111 degrees C. These temperatures would allow either to be handled as fluids. They fail criterion 3 because ammonia is toxic to animals with a maximum allowable concentration 50 parts per million and to plants at concentrations above 1%. Hydrazine is toxic (maximum allowable concentration 1 part per million) and is a suspected carcinogen. The hydrides of sodium, magnesium, aluminum and silicon are all excellent fuels but produce solid oxides when the burn. Silicon hydride, called silane, ignites spontaneously when it is exposed to air. Phosphorous hydride, called phosphine is a gas that that smells like decaying fish and is extremely toxic. Sulfur hydride, called hydrogen sulfide is the smell of rotten eggs, is very toxic and burns to produce toxic sulfur dioxide. The remaining hydride, chlorine hydride is called hydrochloric acid, a powerful toxic acid.

Metallic carbides and sulfides are solids; they produce solid and gaseous combustion products and release less energy than the pure metals. As a result, they fail to meet several of the criteria.

Carbon forms a nitride, cyanogen, which is related to hydrogen cyanide. Both cyanogen and hydrogen cyanide are potential fuels, but both are extremely toxic and can be rejected based on the third criterion. Carbon disulfide is a room temperature liquid with fuel properties. It is less toxic than cyanogen, but is sufficiently toxic to be rejected because of the third criterion. When it burns, it produces large amounts of toxic sulfur dioxide. Nitrogen sulfide is an explosive solid.

Boron carbide is a solid. It can be burned to release a large amount of energy, but it is a refractory solid, and on combustion, solid boron oxide is formed. Boron nitride is a solid and releases so little energy on combustion it cannot be considered a fuel.

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