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- The additional time required for vaporization,
- The more complex hydrocarbons involved in the fuel,
- Poorer mixing of the fuel vapor with the air,
- Higher localized temperatures (leading to higher NOx),
- And the formation of deposits in the combustor which can adversely affect combustion ‘good-
ness’ by affecting flame shape and burner aerodynamics.
Also, some liquid fuels contain sulfur, organic nitrogen, and mineral elements, which lead to
additional pollution. Generally, distillate fuel oil and diesel fuel are low in these compounds, and gaso-
line and aviation fuel are even lower yet in these compounds. Other (heavier) fuel oils are relatively high
in these compounds. Sulfur in the fuel reacts essentially completely to sulfur dioxide (SO 2 ) upon com-
bustion. Also, a small fraction of the sulfur oxidizes to sulfur trioxide (SO 3 ), which is a problem because
it will readily form sulfur acid mists if the exhaust temperature is too low. Thus, in order to prevent
corrosion of the energy system equipment, the exhaust temperature may be maintained higher than it
would be without the sulfur. Thus, thermodynamic efficiency is degraded, and the ratio of CO 2 to unit of
electrical energy produced increases. Sulfate particulate emission can also result from the sulfur. (Within
the 3-way catalyst of an automotive engine, the small amount of SO 2 present can be reduced to hydrogen
sulfide (H 2 S). Ever smell this?) Organic nitrogen in the fuel, also called fuel-bound nitrogen, will end up
partly as NOx upon combustion and partly as N 2. Staged combustion generally promotes the N 2 end
point over the NOx end point. Mineral matter in the oil can cause several problems, including deposits
and flyash emissions. The deposits can promote reactions, which affect the other pollutants, such as
SO 2.
About 65% of the oil used in the US is used by the transportation sector that about 25% is used by
industry, and that most of the balance is used for residential and commercial heating. Very little is used
in the US for electrical generation. Most of the discussion in the paragraph immediately above pertains
to industrial, utility, and R/C burning of oil. However, the biggest sector involved in the control of
emissions from oil burning is the transportation sector, because the fuels burned by this sector (i.e.,
gasoline and diesel fuel) are mainly derived from oil. Thus, advancements in engine combustion tech-
nology and catalytic exhaust treatment are very important to the control of emissions from oil burning. A
major challenge is the development of automotive exhaust treatments which work well for fuel-lean
engine combustion. This is important because engines are thermodynamically more efficient when op-
erated with excess air. However, the present 3-way catalyst has a poor NOx reduction efficiency when
the engine is operated lean. Another major challenge is control of NOx and soot particulate from diesel
engines. The oil problem is being addressed with advanced combustion technology and exhaust particle
traps.
13.13 POLUTION DUE TO SOLID FUEL
The solid fuel of primary interest is coal. In power plant generally
coal is burned for electrical generation.
Coal is burned in several ways, depending on coal particle size. Lumps of coal, including coal
particles larger than about 0.25 inch in diameter, are spread on a grate and burned. Conceptually, this is
not unlike the burning of wood logs in a fireplace, though industrial and utility coal grate burners are
substantially more sophisticated than a fireplace. There is a limit to the size of a grate, (around the order
10 meters square is maximum). Grate coal burners are called Stokers. Stokers come in several forms,
and are used for small and medium size coal combustion systems.
