Coal Gasification and Its Applications

411 Pages·2010·11.838 MB·English

by  David A. Bell, Brian F. Towler, Maohong Fan I

COAL GASIFICATION
AND ITS
APPLICATIONS
DAVID A BELL
BRIAN F TOWLER
MAOHONG FAN
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INTRODUCTION
This is a book about coal gasification and its related technologies. The relationship
betweenthesetechnologiesisshowninFigure0.1.Thegasificationprocessbeginswith
a viable feedstock. In this book, we focus on one of those feedstocks that must go
through the gasification process, coal. The nature of coal, including its properties and
availability, are described in Chapter 1. Petcoke, petroleum coke, a solid, high-carbon
byproduct of petroleum refining, can also be gasified. Gasifiers designed for coal,
especially high temperature, entrained flow gasifiers, are used for this application.
Biomass gasification has a great deal in common with coal gasification, but biomass
gasifiers are optimized for biomass feedstock.
The product of gasification is syngas, which is primarily a mixture of carbon
monoxide and hydrogen. Most syngas, however, is not currently made by gasification,
butratherbythesteamreformingofnaturalgas.Inthisprocess,steamandnaturalgasare
fedtocatalyst-packedtubes,whichareheldinsideafurnacetoprovidetheendothermic
heatofreaction.Figure0.1alsoshowsothergases,whichcanbeblendedwithsyngasfor
further processing. One such gas under consideration is hydrogen, which can be
produced by electrolyzing water using off-peak power from a nuclear power plant. In
a few cases, carbon dioxide from an external source may supplement the carbon
monoxide in syngas.
Justascoalisnottheonlyfeedstockforgasification,gasificationisnottheonlyuseof
coal. Most coal is burned to produce electric power. Chapter 2 describes a few of the
non-gasification uses of coal.
Gasification is described in Chapters 3, 4, and 5. Chapter 3 describes gasification as
achemicalreactionsystem.Althoughthischaptermaylookcomplex,ourknowledgeof
thechemistryofgasificationisfarfromcomplete.Chapter4coversseveralgasifierdesigns.
Thesedesignswereselectedbecausetheyarenowincommercialuseordevelopment,or
because they illustrate interesting concepts. One gasification approach is sufficiently
different that it deserves its own chapter, underground coal gasification, covered in
Chapter5.Insteadofminingcoalandtransportingittoagasifier,thecoalisleftinplace
underground, and the reactant gases arebrought to the coal. Deeply buried coal seams,
whichareuneconomictomine,maybeexploitedbyundergroundcoalgasification.
Syngas leaving the gasifier contains numerous impurities. The inorganic fraction of
thefeedstockleavesassolidashor moltenslag.Ashorslag removalisusuallyanintegral
part of the gasifier design. If the gasification occurs at relatively low temperatures, then
tar will be produced. Tar removal is also an integral part of gasifier design. Higher-
temperaturegasifiersdonotproducesignificanttar.Thesyngasalsocontainssulfurinthe
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