Page 56 - Synthetic Fuels Handbook

44 CHAPTER TWO

combusted more easily and in part because natural gas contains fewer impurities than any
other fossil fuel. However, the major constituent of natural gas, methane, also contributes
directly to the greenhouse effect through venting or leaking of natural gas into the atmosphere
(Speight, 2005b).
Purified natural gas (methane) is the cleanest of all the fossil fuels. The main products
of the combustion of natural gas are carbon dioxide and water vapor. Coal and petroleum
release higher levels of harmful emissions, including a higher ratio of carbon emissions,
nitrogen oxides (NO ), and sulfur dioxide (SO ). Coal and fuel oil also release ash particles
x
2
into the environment; substances that do not burn but instead are carried into the atmo-
sphere and contribute to pollution. The combustion of purified natural gas, on the other
hand, releases very small amounts of sulfur dioxide and nitrogen oxides, virtually no ash
or particulate matter, and lower levels of carbon dioxide, carbon monoxide, and other reac-
tive hydrocarbons.
Natural gas has no known toxic or chronic physiologic effects (i.e., it is not poisonous) but
it is dangerous insofar as an atmosphere rich in natural gas will result in death to humans
and animals. Exposure to a moderate concentration of natural gas may result in a headache
or similar symptoms due to oxygen deprivation but it is likely that the smell (through the
presence of the odorant) would be detected well in advance of concentrations being high
enough for this to occur.
In fact, in the natural gas and refining industries (Speight, 2005b), as in other industries,
air emissions include point and nonpoint sources. Point sources are emissions that exit
stacks and flares and, thus, can be monitored and treated. Nonpoint sources are fugitive
emissions that are difficult to locate and capture. Fugitive emissions occur throughout refin-
eries and arise from, for example, the thousands of valves, pipe connections, seals in pumps
and compressors, storage tanks, pressure relief valves, and flanged joints.
While individual leaks are typically small, the sum of all fugitive leaks at a gas process-
ing plant can be one of its largest emission sources. These leaks can release methane and
volatile constituents of natural gas into the air. Companies can minimize fugitive emissions
by designing facilities with the fewest possible components and connections and avoid-
ing components known to cause significant fugitive emissions. When companies quantify
fugitive emissions, this provides them with important information they can then use to
design the most effective leak repair program for their company. Directed inspection and
maintenance programs are designed to identify the source of these leaks and to prioritize
and plan their repair in a timely fashion. A reliable and effective directed inspection and
maintenance plan for an individual facility will be composed of a number of components,
including methods of leak detection, a definition of what constitutes a leak, set schedules
and targeted devices for leak surveys, and allowable repair time.
A directed inspection and maintenance program begins with a baseline survey to iden-
tify and quantify leaks. Quantification of the leaks is critical because this information is
used to determine which leaks are serious enough to justify their repair costs. Repairs are
then made only to the leaking components that are cost-effective to fix. Subsequent surveys
are then scheduled and designed based on information collected from previous surveys,
permitting operators to concentrate on the components that are more likely to leak. Some
natural gas companies have demonstrated that directed inspection and maintenance pro-
grams can profitably eliminate as much as 95 percent of gas losses from equipment leaks.

2.7 GAS PROCESSING

Gas processing (gas refining) usually involves several processes to remove: (a) oil;
(b) water; (c) elements such as sulfur, helium, and carbon dioxide; and (d) natural gas
liquids (Chap. 6). In addition, it is often necessary to install scrubbers and heaters at or