Page 34 - Synthetic Fuels Handbook

22 CHAPTER ONE

1.4 PRODUCTION OF SYNTHETIC FUELS

1.4.1 Thermal Decomposition
The production of fuels from alternate fuel sources usually (but not always) involves a
degree of thermal conversion. In a very general sense, thermal decomposition is often used
to mean liquid production by thermal decomposition but gaseous and solid product may
also be produced.
For example, cracking (pyrolysis) refers to the decomposition of organic matter by
heat in the absence of air. Thermal decomposition is frequently used to mean the same,
although it generally connotes the breakdown of inorganic compounds. The petroleum
industry tends to use the words cracking and coking for the thermal decomposition of
petroleum constituents.
When coal, oil shale, or tar sands are thermally decomposed, hydrogen-rich volatile
matter is distilled and a carbon-rich solid residue is left behind. The carbon and mineral matter
remaining behind is the residual char. In this regard, the term carbonization is sometimes
used as a synonym for coal pyrolysis. However, carbonization has as its aim the production
of a solid char, whereas in synthetic fuel production greatest interest centers on liquid and
gaseous hydrocarbons.
Thermal decomposition is one method to produce liquid fuels from coal, and it is the
principal method used to convert oil shale and tar sand bitumen to liquid fuels. Moreover, as
gasification and liquefaction are carried out at elevated temperatures pyrolysis may be
considered a first stage in any conversion process.
Of most interest in the production of synthetic fuels is the prediction of the rate and
amount of volatile yield and product distribution for a given raw material and pyrolysis
parameters. Among the important chemical variables are the elemental composition and the
functional compositions of the organic and inorganic matter, as well as the composition of
the ambient gas in which the pyrolysis takes place. Among the more important basic physical
variables are the final temperature, the time and rate of heating, the particle size distribution,
the type and duration of any quenching, and the pressure. An indication of the uncertainty
existing in this field is that at present there is no agreement on whether yield, that is, the loss in
mass of the raw material from pyrolysis, is changed with heating rate.
The best understood pyrolysis processes are the cracking and coking of petroleum,
(Chap. 3). However, the predictive capability for producing any fuel from an alternate fuel
source is very speculative, especially since the properties of that fuel source (even petroleum)
can vary with the origin.
First and foremost, assuming all process parameters are equal, the composition of the
raw material is important in determining the yield of distillable products. The principal material
property defining the yield is the atomic hydrogen-to-carbon ratio (derived from the elemental
analysis). On the other hand, the composition of the volatile products evolved during thermal
decomposition is largely determined by the raw organic material.
The reaction temperature affects both the amount and composition of the volatile yields.
When a fuel produces volatile products, the residence time of the products within the hot
zone and the temperature of the hot zone can markedly affect the distortion of the final
products. Secondary and tertiary products will be formed from the primary products and to
use the delayed coking process as an analogy (Chap. 3), the products may undergo several
thermal alternations before stabilization in the final molecular form.
Pressure will also affect the yield of distillable products since there is also a relationship
between pressure and residence time. Generally, higher pressures favor cracking reactions
and produce higher yields of lower molecular weight hydrocarbon gases, whereas lower
pressure will lead to larger tar and oil fractions.