COAL UTILIZATION

Coal production (and consumption) in the U.S. (1993) was 947 million short tons (1 ton=2000 lb), 8% of which was exported. Less than 1% was imported. By far the largest was the consumption of bituminous coal (about 60%). Subbituminous coal consumption was about 30%, that of lignite about 10% and anthracite less than 1%.

The three methods of coal utilization are:

COAL COMBUSTION is by far the largest and the most important use of coal. More than 85% is consumed in electric power plants (see price of coal), and almost 10% is consumed in industrial boilers and furnaces. Coal is combined with oxygen to release its heating value (7,000-15,000 BTU from every pound of coal consumed, depending on its rank). If it is intimately mixed with oxygen, carbon in coal is converted to carbon dioxide and all the coal is consumed. If the supply of oxygen is not adequate, some carbon is converted to carbon monoxide, which is a poison and a pollutant, or to soot, which is also a pollutant. So it is very important to ensure a permanent and adequate oxygen draft, as we know from our barbecuing experience, especially during startup. This is illustrated below.

The higher the 'volatile matter' content of coal, the easier it is to start its combustion. For example, it is easier to burn a bituminous coal than an anthracite. However, if the supply of oxygen is not adequate, some of the vapors, which form part of the volatile matter, will escape unburnt and condense on the colder walls of the furnace. This will cause a decrease in furnace efficiency. It also means that the furnace must be cleaned more often. For these and other reasons bituminous coals are preferred by industrial consumers and anthracites are preferred by residential customers. Lignites and subbituminous coals have been used less in the past because of their relatively low heating value (and high water content), but they are becoming increasingly interesting for the electric utilities because of their much lower sulfur content (and thus much lower pollution by sulfur dioxide).

COAL CARBONIZATION is the process that led to the industrial revolution of the nineteenth century. Prior to the use of this process, wood-based charcoal was used in the manufacture of iron from iron ore. When this charcoal was replaced by coke derived from bituminous coals, it became possible to produce iron and steel of much higher quality. The closeness of Pittsburgh to bituminous coal mines was responsible for its becoming the steel capital of the world.

In the carbonization process coal is heated to about 1000 degrees Celsius in the absence of oxygen. (If oxygen is not excluded from this process, combustion would occur and the only solid product would be ash.) Only its 'volatile matter' is thus removed and the resulting product (roughly corresponding to its 'fixed carbon') is called coke. Only certain bituminous coals (the so-called coking coals) produce coke of high quality.

Coke (C) is then used in the blast furnace to convert iron ore (FeO) to iron (Fe). This important chemical reaction can be represented as follows:

FeO + C = Fe + CO

Iron is then converted to steel, which was and still is (to a large extent) the most important product of an industrialized nation.

COAL CONVERSION is the process in which coal is converted to a cleaner and more convenient fuel. In order to understand this process, it is important to realize that coal is a hydrogen-deficient fuel when compared to petroleum and especially to natural gas. This is illustrated below:

When molecular hydrogen (H2) is added to coal at about 400 degrees Celsius, solid coal is converted to a liquid fuel that is quite similar to petroleum. This process is called coal liquefaction. Molecular hydrogen is expensive and this is a complex process. So it is not practiced anywhere in the world today. During World War II, Germany was cut off from petroleum fields and it produced all of its gasoline and jet fuel from coal. This is commercially available technology that is believed to become economically attractive when the price of a barrel of crude oil exceeds $30-35.

When coal is reacted with either steam (H2O) or molecular hydrogen (H2) at temperatures in excess of 700 degrees Celsius, it is converted to a gaseous fuel that is more or less similar to natural gas. This process is called coal gasification. It is also commercially available technology. It is more attractive economically than coal liquefaction, especially when steam is used in the following (simplified) reaction with coal:

C + H2O = CO + H2

The product (carbon monoxide, CO, and hydrogen, H2) can be used either as a clean gaseous fuel (having 1/3 of the heating value of natural gas) or as a raw material from which gasoline can be produced. The latter process is called 'indirect liquefaction'. That's how South Africa - with its abundant coal reserves and until recently suffering from an oil embargo - produces most of its gasoline.

The 'bottom line' of this brief discussion of 'synthetic fuels' from coal is that technology exists to convert the domestically abundant and dirty coal to more convenient, cleaner and more precious gaseous and liquid fuels.

lrr3@psu.edu (last revised 7/10/95)