Page 227 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

7.3 Solid Fuel Combustion 203

• Sub-bituminous coal
• Bituminous coal, and
• Anthracite coal

Peat is considered as the youngest coal in Europe and Asia, but it is considered
as biomass in Canada. In Northern Ontario, the Ring of Fires, there is a great
amount of peat covered in the forest. The majority of its weight (about 95 %) is
water and it requires extensive energy to dry it before combustion, and the con-
sequent thermal efficiency is the lowest.
By underground heating and pressurizing over thousands of years, peat becomes
lignite. Lignite is also considered as an immature coal because of its high water
content and low heating value. As time passes, lignite becomes darker and harder as
sub-bituminous coal followed by bituminous and finally anthracite coals. This
terminal rank represents the ultimate maturation of coals.

7.3.2 Solid Fuel Combustion

Solid fuels, coal or biomass, are burned after size reduction. Pulverized coal or
biomass can be as small as few micrometers and most of them are smaller than
0.2 mm by mass. They are injected into a furnace where it is mixed with oxidants
for combustion.
The combustion process of these solid particles is modeled by
• devolatilization or pyrolysis,
• volatile combustion, and
• char combustion.

7.3.2.1 Devolatilization
Devolatilization or pyrolysis is a complicated process that involves heat and mass
transfer as well as chemical decompositions. In general, when a coal particle enters
a combustion chamber where the gas is hot, the heating results in the release of
volatiles from the pores of the coal particle. The greater the heating rate, the faster
the volatiles release. Meanwhile, the size of the particle changes too. Typical
5
4
heating rate in a pulverized coal combustor is about 10 10 K/s and the corre-
sponding local temperature may be as high as 2,100 K [14].
The simplest model for this process is a single step model based on a global
kinetics that applies to most solid fuels like coal, biomass, and plastic [48].

k f
Fuel ! xV þ 1 xð ÞCh ð7:15Þ
where V and Ch stand for volatiles and chars, respectively.