Page 253 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

8.1 Fuel Cleaning 229

• Reduced SO 2 formation in the flue gas:
The reduction could be 10–40 % lower than burning coals without pre-cleaning.
• Decrease of ash content:
This allows the cleaned coal to be used for pulverizer and boiler. It also reduces
the load of downstream particle separators such as ESPs and bag houses.
• Lower maintenance costs for boilers:
The reduction of ash content leads to less wear and tear on coal preparation
before combustion and on boiler during combustion.
• Smooth operation:
Less operational problems such as boiler slagging and fouling.
Meanwhile, one has to bear in mind the disadvantages of coal washing. Intensive
energy is required for coal grinding and drying. And moisture added to the coal
may reduce the efficiency of the boiler and the entire plant. There may be 2–15 % of
energy loss to the preheating of coal.
In addition to these technical issues, one should also consider the environmental
regulations and the price of cleaned coal. In some countries, the price of coal is the
same regardless of its cleanness. In addition, the waste liquid stream from coal
cleaning may contain acidic toxic metals, which pose an extra challenge and cost in
waste treatment or disposal. More and more stringent regulations have been
introduced in the United States and China, for example, to prevent dumping of
these toxic acidic streams into the environment without prior treatment.

8.1.2 Oil and Gas Refinery

Crude oil and raw natural gas contain tens of thousands of kinds of hydrocarbon
compounds. By refinery, crude oil is decomposed into various fractions and
transformed into fuels including oil and gas and other products.
The oil and gas after refinery process contains specified amount of sulfur,
nitrogen, and ash contents. For example, sulfur compounds in the crude oil or raw
natural gas can poison many of the catalysts used for the treatment of hydrocarbons
in the petrochemical industry. More and more stringent environmental regulations
also require reduced sulfur compounds in the final petroleum products. The average
sulfur content in Canadian diesel was 350 ppm in 2000, and ultra-low sulfur diesel
with 15 ppm sulfur became mandatory in North America for highway vehicles in
2006. In 2009, all EU vehicles will run on 10 ppm sulfur diesel including off-roads.
Due to stringent regulation on sulfur content in fuels, a large amount of sulfur
compound is produced as a byproduct of oil refinery. The elemental sulfur is often
an important byproduct of oil refining and it is also a major raw material for the
productions of fertilizer and sulfuric acid.
Sulfur compounds are first separated from the refinery stream by absorption
using amine followed by another separation process to recover the amine and to