Page 151 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: GTY/GWT P2: GLM Final
Encyclopedia of Physical Science and Technology EN006K-933 June 29, 2001 12:14

Fuel Chemistry 261

Some of the advantages of CFB boilers include fuel
flexibility (any fuel such as coal, peat, wood chips, agri-
cultural waste, Refuse Derived Fuel (RDF), and oil, with
sufficient heating value to rise the fuel and the combus-
tion air to above the ignition temperature of the fuel can be
successfully burnt in a CFB boiler), high combustion effi-
ciency, smaller furnace cross section: good load following
capabilities, higher turn down, and more environmentally
friendly.

A. Behavior of Ash in Boiler Furnaces

The mineral matter in coal probably is the factor most re-
sponsible in considering coal a “dirty” fuel as compared
to oil or gas. Because of the dominant role played by coal
mineral matter in boiler performance, the need for a better
understanding by R&D on the behavior of this impurity is
now more important than ever. The most commonly ob-
served problems associated with mineral matter in coal are
slagging and fouling. Slagging is the most serious problem
FIGURE 7 A schematic of a circulating fluidized bed boiler.
with pulverized-coal-fired steam generators, particularly
wall slagging, the buildup of slag deposits on water wall
tubes. This has several undesirable aspects:
characteristics, volume of solids, and the geometry of the
system create a hydrodynamic condition known as “fast 1. It decreases heat transfer appreciably.
bed” or “dilute phase refluxing.” As a result of this con- 2. It can interfere with gas flow when massive deposits
dition solid agglomerates are continuously formed, dis- accumulate near burners.
persed, and reformed again. This motion also increases 3. It results in masses of slag that can cause physical
the slip velocity between the gas and solid phases, which damage to the furnace bottom when they become
increases the attrition or fragmentation of char and sorbent dislodged.
particles. The air required for the combustion is usually 4. It plugs normal ash handling facilities with
admitted in two streams: as primary and secondary air. excessively large masses of bottom ash.
In CFBC boilers, as shown in Fig. 7, the furnace can be 5. It provides a cover to encourage tube wastage by
conceptually divided into three sections: (1) the lower fur- external corrosion.
nace zone, below the secondary air level where the solids
flow regime is similar to a bubbling or turbulent bed and The standard methods that are used to predict the slagging
the particles range in size from approximately 1000 to and fouling behavior depend on the ash analysis, which
500 µm; (2) the upper furnace zone, above the secondary is reported as oxides of various inorganic elements. Slag-
air level where fast fluidization occurs and the flow pat- ging characteristics of coal ash can be predicted to some
tern approaches almost plug flow and particles from 500 extent using the silica percentage. Generally, ash with a
to 150 µm are distributed in the circulating zone; and the low silica percentage will cause the most problems with
(3) cyclone, where the material coarser than the cut point slagging, and coals with a high silica percentage will be
of the cyclone is recycled back into the combustor. The the least troublesome. Thus coal benefeciation to control
larger particles remain in the system for an extended pe- silica percentage provides one way of modifying slagging
riod of time, ranging from hours for the dense phase in behavior, although the effectiveness of coal washing will
the lower zone to several minutes for the circulating phase depend on the chemical composition of the ash in the
in the upper zone of the furnace. Some of the fines make product. However, there are still a lot of factors that cause
only one pass through the combustor and exit the cyclone, slagging (boiler operating parameters) that are not well
so that their residence time is similar to that of the gas. understood.
The heating rate of a coal particle in an FBC boiler is Fouling of superheaters and reheaters causes problems
3
approximately 10 –10 C/sec depending on the particle such as plugging of gas passages, interference with heat
4◦
size and the total average residence time of a particle in transfer, and erosion of metal tubes. Of the factors lead-
the combustion chamber is estimated to be 20 min. ing to such deposits, the physical ones involving particle