Organic Fuels
A variety of organic (carbon-containing) fuels are commonly employed in high-energy compositions. In addition to providing heat, these materials also generate significant gas pressure through the production of carbon dioxide (C02) and water vapor in the reaction zone.
The carbon atoms in these molecules are oxidized to carbon dioxide if sufficient oxygen is present. Carbon monoxide (CO) or elemental carbon are produced in an oxygen-deficient atmosphere, and a "sooty" flame is observed if a substantial amount of carbon is generated. The hydrogen present in organic compounds winds up as water molecules. For a fuel of formula C x HyOz , x moles of C0 2 and y/2 moles of water will be produced per mole of fuel that is burned. To completely combust this fuel, x + y/2 moles of oxygen gas (2x + y moles of oxygen atoms) will be required. The amount of oxygen that must be provided by the oxidizer in a high-energy mixture is reduced by the presence of oxygen atoms in the fuel molecule. The balanced equation for the combustion of glucose is shown below
Only six oxygen molecules are required to oxidize one glucose molecule, due to the presence of six "internal" oxygen atoms in glucose. There are 18 oxygen atoms on both sides of the balanced equation.
A fuel that contains only carbon and hydrogen - termed a hydrocarbon - will require more moles of oxygen for complete combustion than will an equal weight of glucose or other oxygen-containing compound. A greater weight of oxidizer is therefore required per gram of fuel when a hydrocarbon-type material is used.
The grams of oxygen needed to completely combust one gram a given fuel can be calculated from the balanced chemical equation. Table lists the oxygen requirement for a variety of organic fuels.
To determine the proper ratio of oxidizer to fuel for a stoichiometric composition, the grams of oxygen required by a given fuel must be matched with the grams of oxygen delivered by the desired oxidizer. For the reaction between potassium chlorate (KC10 3 ) and glucose (C6H1206) 2.55 grams of KC1O 3 donates 1.00 grams of oxygen, and 0.938 grams of glucose consumes 1.00 grams of oxygen. The proper weight ratio of potassium chlorate to glucose is therefore 2.55: 0.938, and the stoichiometric mixture should be 73.1% KC10 3 and 26.9% glucose by weight. An identical answer is obtained if the chemical equation for the reaction between KC1O 3 and glucose is balanced and the molar ratio then converted to a weight ratio
The more highly oxidizedor oxygen rich - a fuel is, the smaller its heat output will be when combusted. The flame temperature will also be lower for compositions using the highly-oxidized fuel. Also, fuels that exist as hydrates (containing water of crystallization) will evolve less heat than similar, nonhydrated species due to the absorption of heat required to vaporize the water present in the hydrates.
Two "hot" organic fuels are shellac and red gum. Shellac, secreted by an Asian insect, contains a high percentage of trihydroxypalmitic acid - CH3(CH2)11(CHOH)3COOH [2]. This molecule contains a low percentage of oxygen and produces a high heat /gram value. Red gum is a complex mixture obtained from an Australian tree, with excellent fuel characteristics and a low melting point to aid in ignition.
Charcoal is another organic fuel, and has been employed in high-energy mixtures for over a thousand years. It is prepared by heating wood in an air-free environment ; volatile products are driven off and a residue that is primarily carbon remains. Shimizu reports that a highly-carbonized sample of charcoal showed a 91:3:6 ratio of C, 11, and 0 atoms.
The pyrotechnic behavior of charcoal may vary greatly depending upon the type of wood used to prepare the material. The surface area and extent of conversion to carbon may vary widely from wood to wood and batch to batch, and each preparation must be checked for proper performance. Historically, willow and alder have been the woods preferred for the preparation of charcoal by black powder manufacturers.
Charcoal is frequently the fuel of choice when high heat and gas output as well as a rapid burning rate are desired. The addition of a small percentage of charcoal to a sluggish composition will usually accelerate the burning rate and facilitate ignition.
Larger particles of charcoal in a pyrotechnic mixture will produce attractive orange sparks in the flame, a property that is often used to advantage by the fireworks industry.