Page 26 - Piston Engine-Based Power Plants

Fuels and Energy Resources for Reciprocating Engines 19

LPG is almost exclusively derived from fossil fuel sources. Its com-
ponents can be isolated during the refining of crude oil and they are
also present in natural gas as it emerges from the ground. The liquid
has a slightly higher energy content than gasoline but its energy density
is lower. Since its boiling point is below room temperature it must be
pressurized to provide a liquid.

When it is used as an engine fuel, LPG is often called autogas. It is
sold in many countries across the globe but the five largest markets are
Turkey, South Korea, Poland, Italy and Australia. Between them,
these five account for around half of the vehicles using this fuel. LPG
is the third most popular fuel after gasoline and diesel but only
accounts for around 3% of the global market for vehicles. It is used for
stationary power generation too, but again the use is limited.

NATURAL GAS

Natural gas is an important fuel for stationary power generation and is
used widely where gas pipeline systems are available. The gas, as it
emerges from the ground, contains a mixture of smaller and larger chain
hydrocarbons. The primary component is usually methane and this will
provide up to 90% of pipeline gas. Other components include ethane,
propane and butane as well as carbon dioxide. Reciprocating engines
that burn natural gas are spark ignition engines and they are usually
modified to optimize them for this fuel. The gas can also be used in
road vehicles but there are only limited numbers of these operating
across the globe. Natural gas can also be pressurized and liquefied.

There are other sources of methane rich gas in addition to natural
gas. Landfill waste sites produce methane by a process of anaerobic
digestion and this gas can be collected and used to provide heat and
power using a gas engine. Animal and plant wastes can also be pro-
cessed in digesters to provide a similar gas. This gas is usually referred
to as biogas.

ENERGY SOURCES FOR EXTERNAL COMBUSTION ENGINES

External combustion engines, in which the heat to drive the engine
cycle is provided from outside the engine, can generate energy from a