Sách Ebook: Coal gasification and its applications - david a bell, brian f towler, maohong fan

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
between these technologies is shown in Figure 0.1. The gasification process begins with
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,
but rather by the steam reforming of natural gas. In this process, steam and natural gas are
fed to catalyst-packed tubes, which are held inside a furnace to provide the endothermic
heat of reaction. Figure 0.1 also shows other gases, which can be blended with syngas for
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.
Just as coal is not the only feedstock for gasification, gasification is not the only use of
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
a chemical reaction system. Although this chapter may look complex, our knowledge of
the chemistry of gasification is far from complete. Chapter 4 covers several gasifier designs.
These designs were selected because they are now in commercial use or development, or
because they illustrate interesting concepts. One gasification approach is sufficiently
different that it deserves its own chapter, underground coal gasification, covered in
Chapter 5. Instead of mining coal and transporting it to a gasifier, the coal is left in place
underground, and the reactant gases are brought to the coal. Deeply buried coal seams,
which are uneconomic to mine, may be exploited by underground coal gasification.
Syngas leaving the gasifier contains numerous impurities. The inorganic fraction of
the feedstock leaves as solid ash or molten slag. Ash or slag removal is usually an integral
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. Highertemperature
gasifiers do not produce significant tar. The syngas also contains sulfur in the
form of H2S, with lesser quantities of COS. Sulfur must be removed from syngas either
to prevent emission of SO2 when syngas is burned, or to prevent catalyst poisoning in
downstream reactors. Sulfur removal is described in Chapter 6.
Carbon dioxide removal can occur either as a part of impurity removal, or after water
gas shift, as shown in Figure 0.1. The traditional carbon dioxide removal techniques are
closely related to sulfur removal, and are described in Chapter 6. The ability to remove
carbon from syngas and sequester it in a geological formation is one of the major
attractions of coal gasification. This allows coal to be used while minimizing greenhouse
gas emissions. A major objection to this approach is that carbon capture and sequestration
are expensive. This prompted a great deal of research into new carbon dioxide separation
technologies, and which is described in Chapter 10.
Syngas contains a number of minor impurities, and one of the more significant is
mercury, a neurotoxin. Removal of mercury is discussed in Chapter 9.
For some applications, a nearly pure hydrogen stream is desired. In others, such as
methanol synthesis, a specific ratio of carbon monoxide to hydrogen is required. In either
case, the gasifier usually produces a higher ratio of carbon monoxide to hydrogen than
desired. This ratio needs to be shifted towards a greater hydrogen content. The usual way
to do this is through the water gas shift reaction in which carbon monoxide reacts with
steam to form hydrogen and carbon dioxide, as described in Chapter 7. Hydrogen can
then be burned in a turbine to generate electric power, an application known as integrated
gasification combined cycle. This is a means of producing electric power from
coal with minimal greenhouse gas emissions.
Hydrogen is also a potential transportation fuel. The usual approach is to produce
electric power from hydrogen in a fuel cell, and then use that power in an electric motor.
One of the main technical obstacles is a practical means of storing hydrogen in a vehicle.
Chapter 9 explores hydrogen storage for this application.
Nearly all synthetic nitrogen chemicals start as ammonia, synthesized from hydrogen
and nitrogen gas. Nitrogen fertilizers are, by far, the largest volume synthetic nitrogen
chemicals. Chapter 11 describes ammonia synthesis and some of the more common
nitrogen fertilizer compounds.
Methanol is a major commodity chemical made from syngas, as described in
Chapter 12. Methanol is an intermediate used to make a wide range of products. One of
these, dimethyl ether (DME), is especially interesting. DME can be used as a fuel or
converted to hydrocarbons, including gasoline and olefins for polymer production.
Chapter 13 describes the direct conversion of syngas to hydrocarbons, including
substitute natural gas (methane) and Fischer-Tropsch liquid, a synthetic crude oil. The
Fischer-Tropsch liquid is then refined to meet petroleum product specifications.
Coal is an inexpensive feedstock, but gasification-based plants tend to have very high
capital construction costs. In concept, one could build a single plant that would
incorporate all of the elements shown in Figure 0.1, but such a complex plant would be
extraordinarily expensive to build. Instead, gasification-based plants have a more limited
set of features dictated by economics and the regulatory environment.
There are two major trends that prompt current interest in coal gasification. The first
is the widely held belief that conventional petroleum supplies are declining, while
demand for transportation fuels continues to rise. This has led to heightened interest in
alternative energy supplies, including coal. The second major trend is concern about
global warming. Gasification offers a relatively cost-effective means of using coal while
minimizing greenhouse gas emissions.