Principles of Coal Gasification

Carbonization of coal to produce coal gas is a relatively simple process to perform and is done in a retort in the absence of air. The composition of the gas being produced varies depending on the coal being used but is typically comprised of hydrogen (40-50%) and methane (30-40%), with minor amounts (2-10%) of nitrogen, carbon monoxide, ethylene, and carbon dioxide. The gas yield is approximately 10,000 scf per short ton of coal carbonized with a heating value of 550 to 700 Btu/scf. When carbonizing a bituminous coal, about 20% of the weight of the coal is converted to gas [22]. This gas is used as a fuel at coking operations.

Although carbonization of coal is a simple process, only a small fraction of the coal is converted to gas; consequently, processes to convert all of the carbon in the coal to gas were developed. In one of these processes, air is slowly passed through a hot bed of coal, converting most of the carbon to carbon monoxide, with some carbon dioxide being formed. Some of the carbon dioxide is then converted to carbon monoxide by reacting with hot fuel carbon. The reactions that occur are:

(Combustion of carbon: AH = +170.0 x 103 Btu/lb mole of carbon gasified)

(Boudouard reaction: AH = -72.19 x 103 Btu/lb mole of carbon gasified) resulting in:

(AH = +97. 81 x 103 Btu/lb mole of carbon gasified)

Reactions (5-3) and (5-4) are collectively sufficiently exothermic to sustain reaction with the reactants fed at ambient conditions.

The gas produced by this method is called producer gas, and when a bituminous coal is used the gas composition is typically 20 to 25% carbon monoxide, 55 to 60% nitrogen, 2 to 8% carbon dioxide, and 3 to 5% hydrocarbons [22]. Unfortunately, the producer gas is diluted with nitrogen, and the heating value of the gas is only about 100 to 150 Btu/scf. The yield of producer gas is 150,000 to 170,000 scf per short ton of coal. Producer gas was used in a variety of industrial applications such as open-hearth furnaces in steel mills, glass-making furnaces, and pottery kilns; however, the demand for producer gas has been reduced with the demise of open-hearth furnaces in the steel industry and the development of natural gas and electric furnaces.

The temperatures developed in the fuel bed during Reactions (5-3) and (5-4) can be very high, and, when the ash in the bed is fusible, the endothermic carbon-steam reaction must be imposed by adding steam to the air:

(Carbon-steam reaction: AH =—58.35 x 103 Btu/lb mole of carbon gasified)

This reaction moderates the temperature and yields hydrogen in the product gas. This mixture of carbon monoxide and hydrogen is also called water gas. The water gas process is cyclic and involves a gas-making period during which the fuel bed is blown with steam to produce carbon monoxide and hydrogen, followed by an air-blowing period during which the heat is generated in the fuel bed. A typical water gas contains 50% hydrogen, 40% carbon monoxide, and small amounts of carbon dioxide and nitrogen and has a heating value of 300 Btu/scf. When a water-gas generator is being blown with air to reheat the bed, producer gas is made from the reaction of the hot carbon with oxygen, yielding about 35,000 scf of water gas and 80,000 scf of producer gas from one short ton of coal [22]. Water gas is a useful starting material for synthesizing chemicals or liquid fuels and is a good source of hydrogen. Treating the water gas with steam oxidizes the carbon monoxide to carbon dioxide and increases the amount of hydrogen by the equation:

(Water-gas shift reaction: AH = +3. 83 x 103 Btu/lb mole of carbon gasified)

The carbon dioxide can be removed from the product stream, leaving reasonably pure hydrogen.

Hydrogen can also be reacted with carbon at elevated pressures by the carbon hydrogenation or hydrogasification reaction:

(AH = +39.38 x 103 Btu/lb mole of carbon gasified)

and whenever the carbon source generates volatile matter, further quantities of methane will form by thermal cracking.

0 0

Post a comment