Page 44 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 1 Introduction 23

2. A gasified fuel can be used in a wider range of application than solid
fuel. For example, sensitive industrial processes such as glass blowing
and drying cannot use dust-laden flue gas from combustion of coal or
biomass, but they can use heat from the cleaner and more controllable
combustion of gas produced through gasification.
3. Gas can be more easily carried and distributed than a solid fuel.
Transportation of synthetic gas, or the liquid fuel produced from it, is
less expensive as well as less energy intensive than transportation of
solid fuel for combustion.
4. The concentration of CO 2 in the flue gas of a gasification-based
plant is considerably higher than that of a combustion-based plant, so
it is less expensive to separate and sequestrate the CO 2 in an IGCC
plant.
5. SO 2 emissions are generally lower in an IGCC plant (Table 1.5). Sulfur
in a gasification plant appears as H 2 S and COS, which can be easily
converted into saleable elemental sulfur or H 2 SO 4 . In a combustion
system, sulfur appears as SO 2 , which needs a scrubber producing ash-
mixed CaSO 4 , which has less market potential.
6. Gasification produces less NO x per unit energy output than does a com-
bustion system (Table 1.5). In gasification, nitrogen can appear as
NH 3 , which washes out with water and as such does not need a selective
catalytic reducer (SCR) to meet statutory limits. A PC combustion plant,
on the other hand, requires expensive SCR for this purpose.
7. The total solid waste generated in an IGCC plant is lower than that gen-
erated in a comparable combustion system (Table 1.5). Furthermore, the
ash in a slagging entrained-flow gasifier appears as glassy melt, which
is much easier to dispose of than the dry fly ash of a PC system.
8. For generation of electricity in a small remote location or for distrib-
uted power generation, a power pack comprising a gasifier and a
compression ignition engine is more convenient and economic than
a combustion system comprising a boiler, a steam engine, and a
condenser.
9. The producer gas from a gasifier can be used as a feedstock for the
production of fertilizer, methanol, and gasoline. A gasification-based
energy system has the option of producing value-added chemicals as
a side stream. This polygeneration feature is not available in direct
combustion.
10. If heat is the only product that is desired, combustion seems preferable,
especially in small-scale plants. Even for a medium-capacity unit such
as for district heating, central heating, and power, combustion may be
more economical.
11. A gasification based system can generate power using a combined cycle
(IGCC). Example 1.1 shows that such a system is more efficient than a
combustion based one.