2. COMBUSTIBLE FUELS
Learning Objective: Understand the types and characteristics of combustible gases and fuel oils used in heating systems.
If electricity and coal are disregarded, the fuels most commonly used with heating equipment are either gas or petroleum. Next, we will take a brief look at the types and characteristics of combustible gases and fuel oils used for heating.
TYPES OF GASES
Gaseous fuels are usually classified according to their source that, in turn, determines their chemical composition. The heat valve (Btu per cubic foot) varies with the types of gas and determines the quantity required for a specific heating requirement. The types principally in use are natural gas, manufactured gas, and liquid petroleum gas.
Natural gas is a mixture of combustible gases and usually small amounts of inert gases obtained from geologic formations. While the composition of natural gas varies with the source, methane (CH4) is always the major constituent. Most natural gases also contain some ethane (C2H6) along with small amounts of nitrogen and carbon dioxide (CO2). Natural gas is colorless and odorless in its natural form; however; a distinctive odor is usually added as a safety factor for detecting leaks. Natural gas mixes readily and completely with combustion air and thus is substantially free from ash and practically smokeless. These characteristics contribute to good environmental pollution control. From a standpoint of trouble-free performance, ease of handling, and control, natural gas offers many advantages that make it the most desirable of all heating fuels.
The common manufactured gases are carbureted water gas, oil gas, and producer gas. These gases are roughly one-half hydrogen and one-third methane, plus small amounts of carbon dioxide, nitrogen, and oxygen. They are made by converting low-grade liquid or solid fuels to the gaseous form by destructive distillation (cracking) of oil or coal, by the steam-carbon reaction, or by a combination of both processes. These gases are ordinarily used at or near the production point because of high manufacturing costs rule out the added expense of distribution.
Liquefied Petroleum Gas
Liquefied petroleum gases are hydrocarbon gases normally obtained as a by-product of oil refineries or by stripping natural gas. These compounds are normally gaseous under atmospheric conditions; however, they can be liquefied by moderate pressure at normal temperatures.
The principal LPG products are propane (C3H8) and butane (C4H10). Propane, the most common, is available by the bottle or cylinder and in bulk form. Its boiling point is -44°F (note that this is very close to that of refrigerant R-22).
Butane is generally available in bulk form. It boils or vaporizes at 32°F. In other words, if the temperature of butane is 32°F or lower, at atmospheric pressure, it remains a liquid, and heat must be applied to bring it to the gaseous state. Note in table G, appendix II, the high heating. values of propane and butane.
Fuel oils are derived from crude oil, which consists primarily of compounds of hydrogen and carbon (hydrocarbons), and smaller amounts of oxygen, nitrogen, and depending on the source, sulfur. Practically all fuel oil is either a product or a by-product of refining crude oil by the fractional distillation process or by cracking.
The Bureau of Standards, United States Department of Commerce, standardizes commercially used fuel oils. The oils are numbered in grades 1, 2, 4, 5, and 6 and are titled commercial standard grades (CSG). These grades are identified in the Navy by military specifications and are intended for use in oil-burning equipment for the generation of heat in furnaces for heating buildings, for the generation of steam, and for other purposes.
Questions for Lesson 2
- What are the principal types of gases used in heating?
- Fuel oils consist primarily of what compounds?