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thesis entitled

A Kinetic Study of the Role of Oxygen in
Hydrogasification of Saran Char and Coal Char

presented by

Michael Gerard Lussier Jr.

has been accepted towards fulfillment
of the requirements for

M. S . degree in Chem. Engr .

 

 

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Major professor

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MSU Is An Affirmative Action/Equal Opportunity institution
ammo-o1

A KINETIC STUDY OF THE ROLE OF OXYGEN
IN HYDROGASIFICATION OF SARAN CHAR AND
COAL CHAR

BY

Michael Gerard Lussier Jr.

JK'THIHIIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

NUUETTHRiOF'EMSIEflflEE

Department of Chemical Engineering

1992

I") p 171/),
3-- 34.41

67

ABSTRACT

A KINETIC STUDY OF THE ROLE OF OXYGEN IN
IHYDEKXNNSIFIIHUIIONICN? SAEUUEiCHAJLJUMD CKHUL CEHUR

BY

Michael Gerard Lussier Jr.

The effect of several sample treatments on the
hydrogasification rate of an Illinois'6 coal char and
Saran char were investigated. Uncatalyzed rates at 725%:
and 3.3MPa H2 decayed rapidly with. conversion for both
chars. Hydrogen pretreatment was shown not to affect
rate, and rate was shown not to correlate well to total
surface area within the first 15-25% carbon conversion.
Outgassing was shown to decrease the reaction rate, due to
annealing, by an order of magnitude. Partial combustion
in air was used to fix oxygen functional groups on char
surfaces. Oxidation of fresh chars did not measurably
increase hydrogasification rates at 725°C, but oxidation
of outgassed chars or chars previously reacted with
hydrogen increased rate by a factor of two to three.
Potassium carbonate catalyzed hydrogasification of all
chars, but was poisoned by coal ash. Three separate
reaction stages during hydrogasification have been

proposed based on the findings in this investigation.

I dedicate my masters thesis to my parents Michael

and Roseann Lussier, and my sister Amy Lussier.

ACKNOWLEDGEMENTS

I would like to thank Dr. Dennis J. Miller, my
graduate advisor, for his strong support and guidance
during the course of my Masters research and thesis work.
Thanks must also go to Dr. Martin C. Hawley and Dr.
Lawrence T. Drzal for accepting the task of serving on my
committee.

I would like to thank Captain Martin E. Toomajian for
the hard work put into sample analysis, as well as Mark
Benedict.

Finally, I would like to thank the United States
Department of Energy for their financial support through

grant.'DE-F622-89PC89777.

glgrxgu
Chapter 1.

1.1.
1.2.

Chapter 2.
2.1.
2.2.

TABLE OF CONTENTS

Introduction

Background

Literature Review

1.2.1. Uncatalyzed Hydrogasification
1.2.1.1. General Reaction

Phenomena

1.2.1.2. Role of Oxygen

1.2.2. Catalyzed Gasification

Previous Research

Research Objectives

Experimental

Starting Materials

Pyrolysis and Oxidation

2.2.1. Quartz Tube Reactor
Specifications

2.2.2. Pyrolysis

2.2.3. Partial Oxidation

2.2.3.1. Pretreatment

2.2.3.2. Intermittent Treatment

PAGE

12
15
16

17
17

18

18
19
21
22
22

Chapter 3.
3.1.
3.2.
3.3.

Demineralization
2.3.1. Method I
2.3.2. Method II
Catalyst Impregnation
Hydrogasification
2.5.1. Gasification Reactor
Specifications
2.5.1.1. Gas Chromatograph
3.5.1.2. Sample Mount
2.5.2. Hydrogasification Kinetic
Experiments
2.5.2.1. Conditions
2.5.2.2. Procedure
2.5.3. Outgassing
Surface Analysis
2.6.1. Total Surface Area
2.6.2. Active Surface Area
2.6.3. Surface Oxygen Concentration

2.6.4. Surface pH

Results

Effect of Temperature
Base Case Char Reactivity
Outgassing

3.3.1. Saran Char

3.3.2. Coal Char

vi

23
23
24
25
26

26
26
27

29
29
30
31
32
32
32
33
34

35
35
37
39
39
39

3.5.

Chapter 4.
4.1.

Oxidative Treatments
3.4.1. Oxidative Pretreatments
3.4.1.1. Saran Char
3.4.1.2. Coal Char
3.4.1.3. Demineralized Coal
Chars
3.4.2. Intermittent Oxidative Treatment
3.4.2.1. Saran Char
3.4.2.2. Coal Char
Hydrogen Addition at Steady State
Reaction Temperature
Reduced Gasification Temperature
Hydrogen Pretreatment
Catalyst Addition
3.8.1. Catalyzed Base Cases
3.8.2. Preoxidation of Catalyzed Char
3.8.3. Intermittent Oxidation of

Catalyzed Chars

Discussion

General Observations

4.1.1. Temperature Effects
4.1.2. Total Surface Area

4.1.3. Active Surface Area
4.1.4. Oxidation Effects

4.1.5. Catalyst Effects

vii

42
42
45
45

49
49
49
53

57
57
60
63
63
66

66

70
70
70
72
72
79
86

4.2. Mechanistic Considerations 87

4.2.1. Proposed Model 88

4.2.2. Supporting Observations 90

Chapter 5. Conclusions/Recommendations 93
5.1. Conclusions 93

5.2. Recommendations 98
Appendix A. Hydrogasification Rate Calculations 99
List of References 102

viii

LIST OF TABLES

Ultimate Analysis of Various Chars

Active Surface Area of Intermittently
Oxidized Saran Char - Two Different Runs

Surface Analysis of Various Chars

Base Case Coal Char Hydrogasification Rate
Data and Calculations

ix

20

78
80

101

10

11

12

LIST OF FIGURES

Various Edge Configurations on the Graphite
Aromatic Basal Plane

Newly Designed Sample Holder

Arrhenius Plot of Coal Char at 7%
Conversion (EA = 76 kcal/mol)

Hydrogasification of Various Chars at 725%:
and 500 psi B,

Effect of Outgassing on Hydrogasification
of Saran Char

Effect of Outgassing on Hydrogasification
of Coal Char

Weight % Burnoff of Various Chars in Air
at 375°C

Weight % Burnoff of Individual Trays of
Various Chars in Air at 375%:

Effect of Oxidative Pretreatment at 375%:
on Hydrogasification of Saran Char

Effect of Oxidative Pretreatment at 375%:
and 725°C on Hydrogasification of Coal
Char

Effect of Oxidative Pretreatment at 375%:
on Hydrogasification of Coal Char

Effect of Oxidative Pretreatment at 375%:
on Hydrogasification of Demineralized
Coal Char (Method I)

PAGE

28

36

38

40

41

43

44

46

47

48

50

13

14

15

16

17

18

19

20

21

22A

228

23

24

25

26

27

Effect of Oxidative Pretreatment at 375%:
on Hydrogasification of Demineralized
Coal Char (Method II) 51

Effect of Intermittent Oxidation at 375%:
on Hydrogasification of Saran Char 52

Effect of Intermittent Oxygen Chemisorption
on Hydrogasification of Saran Char 54

Effect of Intermittent Oxidation at 375%:
on Hydrogasification of Coal Char 55

Effect of Intermittent Oxidation at 725%:
on Hydrogasification of Coal Char 56

Effect of Addition of 500 psi H2 at Steady
State Reaction Temperature (725°C) on
Coal Char 58

Hydrogasification of Saran, Coal, and Pre—
Oxidized (375°C) Coal Char at 600°C and
500 psi H2 59

Effect of 725°C Hydrogen Pretreatment on
Hydrogasification of Coal Char 61

Effect of 600°C Hydrogen Pretreatment on
Hydrogasification of Coal Char 62

Effect of KARL Catalyst on Hydrogasification
of Various Chars (linear plot) 64

Effect of K2003 Catalyst on Hydrogasification
of Various Chars (semi-log plot) 65

Effect of KfiXh Catalyst on Hydrogasification
of Pre-Oxidized Coal Chars 67

Effect of Intermittent Oxidation at 3754:
on Hydrogasification of Kth Catalyzed
Chars 68

Total Surface Area of Chars at Various %
Conversion in Hydrogasification 73

Instant Rate x 103/Unit Total Surface Area
for Various % Conversion in
Hydrogasification of Chars 74

Active Surface Area of Chars at Various %
Conversion in Hydrogasification 75

xi

28

29A

29B

30A

30B

Instant Rate/Unit Active Surface Area for
Various % Conversion in Hydrogasification
of Chars

Hydrogasification of Saran Char - Rate
Normalized from 600°C to 725°C via
Activation Energy (linear plot)

Hydrogasification of Saran Char - Rate
Normalized from 600°C to 725°C via
Activation Energy (semi-log plot)

Hydrogasification of Coal Chars - Rate
Normalized from 600°C to 725°C via
Activation Energy (linear plot)

Hydrogasification of Saran Char - Rate

Normalized from 600°C to 725°C via
Activation Energy (semi-log plot)

xii

77

82

83

84

85

CHAPTER 1

INTRODUCTION

1.1. BACKGROUND

Coal hydrogasification is the conversion of coal to
methane via reaction with hydrogen at elevated
temperatures and pressures, causing complete enrichment of
carbon with hydrogen. There are many reasons why coal
hydrogasification is beneficial. Synthetic natural gas is
a much cleaner burning fuel than coal, and is capable of
being transported by pipeline. The current world supply
of natural gas is not a renewable resource, and is rapidly
being depleted. Even though coal is also a nonrenewable
resource, conversion of it to natural gas may bridge the
time lag between depletion of natural gas and the
development of alternative energy sources such as nuclear,
solar, and biological.

Steam gasification has been studied most extensively
and has been used commercially to convert coal to natural
gas. It does not require pressures as high as

hydrogasification, does not have a rate that declines

2
rapidly with conversion, and is not equilibrium limited at
higher temperatures as is hydrogasification. The main

reactions that take place during steam gasification are as

follows:
a] C + Hg)-———+iy + CO synthesis gas formation
b] C0 + 3(Hg -——)CH,-+iyo methanation
c] CO + Hg)-——-+iy + CO2 water shift reaction

The study of hydrogen gasification does have distinct
advantages over steam gasification, despite the fact that
it requires higher pressures and is limited by equilibrium
conditions at higher temperatures. The basic reaction

that takes place during hydrogasification is as follows:

Hydrogasification is a direct exothermic method of methane
production. Unlike reactions occurring in steam
gasification, it is strictly a surface reaction.

The reaction rate during hydrogasification declines
rapidly with conversion. This is a disadvantage from a
process perspective, but can lend a great deal of insight

to questions about rate kinetics and mechanisms.

3

Hydrogasification rate can be increased by addition of a
number of catalysts, including alkali and alkali earth
metals and salts, as well as transition metals. Oxygen
functional groups on the coal surface are thought to play
a key role in methane formation. The only oxygen present
during hydrogasification is that which is already
associated with the solid, so it can be studied closely.

Hydrogasification is an important step in several
coal conversion processes. It is the primary reaction in
the HYDROCARB process, which is a method of producing 1
very high purity carbon from a variety of coals, wastes,
and biomass materials.[1] It may be a significant means
of increasing methane yields in the Exxon gasification
process,[2] and is the primary route to methane formation
in systems that possess significant partial pressures of
hydrogen.[3] Even though hydrogen is currently too
valuable a commodity to be used in large-scale methane
production, it may be the best route if an abundant

hydrogen source is developed.

1.2L.ILITTEUHUURElIUFVIEMi

1.2.1. UNCATALYZED HYDROGASIFICATION

1.2.1.1. GENERAL REACTION PHENOMENA

Much of the pioneering work on coal hydrogasification
was done by J.D. Blackwood et al. They were able to
determine that reaction rate is proportional to hydrogen
partial pressure during initial stages of carbon
conversion,[4-7] and that rate is not a strong function of
char type.[4,8,9] Other researchers also determined that
methane formation rate is first order in hydrogen partial
pressure.[10] The uncatalyzed hydrogasification rate
decreases rapidly with carbon conversion,[5,7,11-14] which
led some researchers to claim that the initial rate is
diffusion controlled.[15] Activation energies reported
include values of 43 kcal/mol[4] and 27 kcal/mol,[10] but
the extent of carbon conversion where the rate data were
taken is not specified. Other researchers reported
activation energies of 36 kcal/mol initially to 51
kcal/mol at steady state,[15] and 15 kcal/mol initially to
50 kcal/mol at steady state.[16] Activation energies of
the cleavage of carbon-carbon bonds in steam and carbon
dioxide char gasification include values of 80.5 kcal/mol

[17] and 61.4 kcal/mol [18] respectively.

5

Almost all chars possess a high internal surface area
resulting from a complex pore network. The char base
structure can be thought of as a collection of small
randomly aligned graphite crystallites.[19] Porosity and
thus surface area is the result of poor crystallite
packing due to random alignment. Total char surface area
as determined by BET analysis, however, correlates very
weakly or does not correlate at all with gasification
rate. This has been shown for oxygen gasification,[20]
carbon dioxide gasification,[21,22] as well as
hydrogasification.[9,23] In the case of oxygen
gasification, developing pores include those in the micro-
and mesopore range, which restrict molecular
diffusion.[20] In the case of hydrogasification,
macropore surface area does not increase until about 55%
carbon conversion. Widening of the micro- and mesopores
is the main reason for the increase.[24]

Structure on the molecular level appears to be much
more important in determining hydrogasification rate.
Several two-stage reactions have been proposed by
researchers to explain the rapid rate decay in
hydrogasification. One claim is that the initial high
rate results from reaction of hydrogen with amorphous
carbon atoms which were deposited during pyrolysis.[8]
Prior to pyrolysis, most of the char internal volume is

filled with compounds that are volatile relative to the

6
char base structure. During pyrolysis, some of these
compounds completely volatilize, and some carbonize. The
carbonized char coats part of the char surface in an
amorphous form. This amorphous, or "secondary carbon,"
tends to be more saturated with hydrogen and less
saturated with chemical bonds in general. The low steady
state rate is due to reaction of hydrogen with the
graphite-like char base structure.[8] The same
researchers have proposed a similar model in which initial
high reaction rate results from gasification of carbons
associated with oxygen functional groups.[25] Other
researchers used unpyrolyzed chars and make the claim that
high initial rate is due to pyrolysis of aliphatic side
chains and oxygenated functional groups followed by
hydrogenation of intermediate pyrolysis products. The low
steady state rate is due to direct attack of residual
aromatic base char structures.[26]

A strong preference for reaction of hydrogen with
edge carbon atoms, as opposed to basal plane carbon atoms,
has also been shown.[27] This is because of unpaired
sigma electrons on edge carbon atoms which make them more
reactive than those of the basal plane. It may also
result in part from diffusion of impurities which serve as
catalysts or leave vacancies if they desorb from edge
carbons.[28] Further probing into the reactivity of edge

carbon atoms by etch pit analysis of graphite shows that

7
hydrogen greatly prefers to attack the "armchair" edge
carbons (<112l> edge plane) over the "zig-zag" edge
carbons (<101l> edge plane) in hydrogasification and steam
gasification,[29-32] while oxygen and carbon dioxide show
no edge preference in gasification.[29,32] Etch pits
formed during hydrogasification and steam gasification
were hexagonal, oriented such that all edges were composed
of zig-zag configurations. Etch pits formed during oxygen
and carbon dioxide gasification were round, indicating
both configurations on the pit edge. Although coal is not
graphite, it is highly aromatic, and can be assumed
graphitic on a local level.[19] Various edge and pit
configurations are illustrated in Figure 1.

Understanding the way in which hydrogen adsorbs onto
the char surface is also an important part of determining
what takes place during reaction. Several researchers
have suggested successive dissociative hydrogen
Chemisorption onto adjacent carbon atoms.[16,33] Some
have gone further and suggested that the breakage of the
bond between adjacent carbons, which is caused by the
adsorption of the third pair of hydrogen atoms, is the
rate limiting step.[30] Associative hydrogen
Chemisorption has also been suggested as a possible
methanation mechanism. Two hydrogen molecules
successively adsorb onto the same carbon atom, completely

enriching it.[34] It has also been suggested that

      

 

  
   
    

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Figure 1: Various Edge Configurations

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9
dissociative hydrogen Chemisorption occurs first followed
by associative Chemisorption, with the dissociative step
being rate limiting.[6]

Understanding the role of hydrogen in gasification of
chars is also important because it has been shown to
inhibit steam and carbon dioxide gasification.[35-41] It
may also be part of the cause of rapid deactivation of
hydrogasification rate due to dissociative adsorption
[42].

With the idea of associative hydrogen Chemisorption,
these researchers have also introduced the concept of an
active site.[6] It is generally defined as a carbon atom
on the char surface that is not completely saturated with
chemical bonds, or a carbon with free or unpaired
electrons.[9] An active site is where hydrogen
associatively adsorbs, and when the carbon is saturated
with hydrogen and cleaved from the char surface another

active site is be generated on an adjacent carbon.

1.2.1.2. ROLE OF OXYGEN

It is generally believed that the major source of
active sites in all gasification reactions comes from the
desorption of oxygen functional group from the char
surface. These groups desorb in the form of carbon

monoxide and carbon dioxide when samples are heated to

10
reaction temperature,[7,43-50] and in the form of water
during reaction.[7] The desorption of carbon monoxide and
carbon dioxide is considered the rate limiting step in
oxygen gasification,[Sl] and determines the rate of this
reaction.[23] Hydrogasification rate has been shown to be
a strong function of the oxygen content of various
chars,[4,7,52] and initial rate a strong function of
oxygen surface concentration.[48,53] It has also been
shown to be a strong function of char preparation
temperature,[4,8,9,54] because char oxygen content is a?_.
strong function of preparation temperature.[4,7]

There are several methods of fixing oxygen onto char
surfaces in an attempt to increase the active surface
area, and therefore the gasification rate.[46,47] One
method is partial combustion in oxygen before carbon
dioxide gasification [55,56] and hydrogasification.[48-
50,57,581 Chemical methods include treatment with nitric
acid,[48,59,60] peracetic acid,[61] or hydrogen
peroxide.[62] Continuous oxidation can be performed by
addition of low concentrations of oxygen or carbon dioxide
to the reactant gas in hydrogasification.[63]

The most widely used general description of oxygen
functional groups on char surfaces is the pH of these
sites, which is determined by their formation
conditions.[64] Groups generally thought of as acidic

include carboxyl and phenolic hydroxyl groups [64][65] and

11

lactone groups.[66-69] Acidic groups are formed during
partial combustion in oxygen at temperatures ranging from
300°C to 450°C, [70] or to 500°C. [70-73] Some researchers
have claimed that the optimum oxidation temperature is
400%:.[72,74] Acidic groups generally desorb as carbon
dioxide during sample heatup.[70] Nitric acid treatments
generally fix acidic functional groups onto char
surfaces,[50] but basic groups can be fixed as
well.[62,75] Among the groups fixed by nitric acid are
carboxyl, phenol, amine, carbonyl, and ester groups.[76]
Hydrogen peroxide can also form a mixture of functional
groups.[62,75] Groups generally thought of as basic
include pyrone-like structures and chromenes,[65,68] while
neutral groups include carbonyls.[71] Basic groups are
formed during partial combustion in oxygen at temperatures
ranging from 700°C to 800°C, [62] or at 750°C +. [77,78]
Basic and neutral groups generally desorb as carbon
monoxide during sample heatup.[65,71] It is possible that
the chromine groups, which oxidize to lactone groups,[68]
may cause the low char reactivity during the later stages
of sample conversion during hydrogasification up to
temperatures of 870%:.[66]

Char reactivity can be decreased by heating samples
to temperatures near 1000%:. This has been shown to
greatly anneal char surfaces,[79] and decrease active

surface area.[80] Outgassing, which involves heating

12
chars to high temperatures under vacuum, cleans carbon
black of all acidic functional groups.[58] It has also
been shown to decrease hydrogasification rates by a factor
of six and reduce surface oxygen concentrations to nearly

zero.[57]

1.2.2. CAIALYZED GASIEICATION

There are a number of catalysts that can be used for
increasing the gasification rate of chars. Catalysts
include alkali carbonates and most of the transition
metals in groups IIIB-VIIIB. Transition metal catalysts
are more effective than alkali carbonates,[81-84] but they
are also much more expensive.[37,85,86] Examples of
transition metals used include nickel and iron,[87-89]
copper,[90] and platinum.[91,92]

Potassium appears to be the best choice among the
alkali metals. Rubidium and cesium are more active than
potassium, but are very expensive. Sodium is cheaper than
potassium, but the greater catalytic activity of potassium
more than compensates for this.[93] The advantage of
potassium over alkali earths such as calcium is the fact
that potassium will evenly disperse itself over the char
surface due to its high mobility, while calcium must be

atomically dispersed to create comparable activity.[94]

13

Several mechanisms have been proposed for potassium
carbonate catalysis of steam and carbon dioxide
gasification. A number of researchers have suggested a
redox cycle with decomposition and reformation of the
carbonate[40,41,95-98]. Intermediates of alkali hydroxide
in steam gasification, and alkali oxide in carbon dioxide
gasification are included. Other researchers have
proposed that an alkali metal non-stoichiometric oxide
serves as a center for the capture of oxygen from the gas
phase and electrons from the char, which greatly increases
the reaction of adsorbed oxygen atoms.[99-101] Potassium
hydroxide has been suggested as an intermediate during
this mechanism.[40] Another mechanism involving
potassium-intercalation compounds has been proposed.[102]

Further research in this area has indicated that
surface oxygen is essential for catalyst activity,[3] and
is therefore involved in the mechanism.[52,53,103] Some
researchers have claimed that the potassium to carbon
ratio is what determines gasification rate,[103] but
others have shown that the surface oxygen concentration is
what determines the amount of retained potassium on the
char.[52] The proposal of C-O-K complexes determining
gasification rates has been supported by many
researchers.[104,105] These complexes have been shown to
be stable to 700°C and directly proportional to steam

gasification rate.[39] A redox cycle has been proposed

14
that involves the complex.[106] A possible mechanism for
carbon dioxide gasification that includes the C-O-K
complex consists of an alteration of the active catalyst
between the complex and potassium metal. Carbon and
oxygen are exchanged with the gas phase.[107]

Much less research has been done in the area of
potassium carbonate catalysis in char hydrogasification.
Significant rate enhancements have been
observed,[58,108,109] but application of the above
proposed mechanisms to catalyzed hydrogasification is
limited because the oxidizing reactant gases, which
support redox cycles, are not present.[93] A mechanism
has been proposed which includes formation of the C-O-K
complex by interaction between the catalyst and some basic
surface groups.[58]

Another obstacle that must be overcome in catalyzed
hydrogasification of coal chars is the strong inhibition
caused by mineral matter in the ash. Catalyst poisoning
with small amounts of sulfur has been observed during
hydrogasification using potassium, as well as transition
metals.[110] Aluminosilicates, mostly in the form of the
clay minerals illite and kaolinite, poison potassium

catalyzed steam gasification.[111,112]

15
1.33. PTUFVIKNUS IUESEIEMIH

Prior work by others in this research group focused
on the role of oxygen in hydrogasification of uncatalyzed
and potassium carbonate catalyzed chars. Zoheidi and
Miller [50,58] concluded thermal decomposition of oxygen
surface groups into CO and C02 leaves "nascent" active
sites, therefore enhancing char reactivity with hydrogen.
They observed rate enhancements with preoxidation of
carbon black by oxygen Chemisorption. Oxidation at 400%:
produced mainly acidic groups which increased the initial
rate about 100%, while oxidation at 800°C produced mainly
basic groups which resulted in only minor rate
enhancements. They also concluded stable rate enhancing
complexes, which probably take the form C-O-K, are formed
by the interaction of basic surface functional groups with
the catalyst.

Treptau and Miller [48] showed by XPS analysis the
surface oxygen concentration during uncatalyzed
hydrogasification of carbon black and coconut char is
essentially zero, and correlated initial rate to surface
oxygen concentration. They observed uncatalyzed rate
enhancements of about 70% with oxidation by HNO3
pretreatment of carbon black, and 50% with HNO3
preoxidation of coconut char. They also concluded basic

surface functional groups interact with the potassium

16
carbonate catalyst during hydrogasification to form stable

rate enhancing complexes, which may take the form C-O-K.

1 . 4 . RESEARCH OBJECTIVES

The coal hydrogasification reaction is very complex,
and not well understood. It is known that steam and
carbon dioxide gasification rates do not decay abruptly
with conversion, like the rates in hydrogasification. The
major objective of this research is to gain a more
fundamental understanding of the reaction, and enhance the
rate based on this knowledge. This focus of this research
is the study of the role of oxygen in hydrogasification of
coal, the investigation of the reaction inhibition by

hydrogen, and the poisoning of the reaction by coal ash.

CHAPTER 2

EXPERIMENTAL

2.1L. SIHURTIJHB HDUEERUHELS

The starting materials used in this investigation .n
include a Dow Saran (R) MA 127 Resin and an Illinois '6
PSOC-l493 HVC Rank Coal. The Saran has been chosen as a
model compound because it chars to a relatively pure
carbon with an extremely low ash content and displays
reactive and structural properties very similar to the
coal chosen for this investigation. The coal has been
selected because it is very well characterized and been

widely used in other studies.

17

18
2.2. PYROLYSIS AND OXIDATION

2.2.1. QUARTZ TUBE REACTOR SPECIFICATIONS

The quartz tube reactor was used for pyrolysis of
starting materials and low pressure oxidative treatments.
It consists of a 4.8 cm ID quartz tube placed in a 1400
watt Lindberg Electric Furnace (Model 54232). The tube is
91 cm in length with a 0.6 cm nipple on the upstream end
and a removable flange on the downstream end. Temperature
is controlled with an Omega Series CN-2010 Programmable
Temperature Controller. Firebrick plugs inside the quartz
tube at both ends help minimize heat loss. Gases used in
sample preparation include AGA 99.99% Nitrogen which
passed through an Airco Single Stage Argon-Nitrogen-Helium
Regulator for pressure control, and AGA Compressed Air
U.S.P. which passed through a Rego Single Stage Regulator
for pressure control. Gas flow rate was controlled by two
rotameters in series. For high flow rate purging a 2-S-
150 Fisher Scientific Laboratory Flow Meter was used,
while for tightly controlled flow rates a Cole Parmer
Model G Rotameter with a 420 cc/min maximum flow rate was
used. Exhaust gas was sent through two water-filled 1000
ml Erlemeyer side-arm flasks in series to trap potentially
hazardous products, then into a laboratory fume hood. The

samples were placed in Coors U.S.A. Alumina Chemical-

19
Porcelain Ware Ceramic trays (4.0cm x 3.0cm x 0.5cm) or
boats (10.4cm x 2.5cm x 1.5cm) during the reactions and
were stored under nitrogen in capped glass bottles inside

a desiccator.

2.2.2. PYROLYSIS

All starting materials were pyrolyzed because
gasification of char is the rate limiting step in the
overall gasification of coal. Charring rids starting
materials of volatile components that may interfere with
gasification. Saran was pyrolyzed mainly to expel
chlorine and to enrich the carbon content of the sample.
Coal was pyrolyzed mainly to expel tar-like hydrocarbons
which can clog reactor lines, and to rid the sample of
some sulfur which can poison the reaction, especially in
the presence of catalysts. Ultimate analyses of Saran
char and coal char are given in Table 2.1.

Samples were pyrolyzed by heating from 25°C to 900°C
at 10°C/min, soaking at 900°C for 30 minutes, and cooling
from 900°C to 100°C at 10°C/min. The actual cooling time
was several hours because of the heavy insulation and
large thermal mass of the furnace. Nitrogen flow rate was
kept constant at 400 cc/minute and atmospheric pressure
throughout all runs. With a reactor volume of 1650 cc,

purge rate was about one reactor volume every four

20

Table 2.1: ULTIMATE ANALYSIS OF VARIOUS
CHARS[116]

(Weight % on Dry Basis)

 

Saran Coal Demin. I Demin.II
Element Char Char Char Char
% Carbon 96.36 75.30 88.59 92.00
% Hydrogen 0.53 0.53 0.69 0.39
% Nitrogen 1.04 1.34 1.40 3.50
% Sulfur 0.43 3.55 2.32 1.21
% Ash 0.08 17.331 0.662 1.36
% Oxygen (diff) 1.56 1.95 6.34 1.54
100.00 100.00 100.00 100.00
% Chlorine 0.25
Fluorine Ug/g 163
BTU/lb 14073 11300 12847 13471

1. Correct dry ash content is 20.81% because 3.48% sulfur
is retained in the ash.

2. Correct dry ash content is 2.94% because 2.28% sulfur
is retained in the ash.

21
minutes. An average of 51 grams of coal per run was
loaded into the quartz tube reactor, with a yield of 61%
+1%. An average of 12 grams of Saran powder was loaded
per run with an average yield of 25.5% +0.5%. The Saran
forms a greatly expanded foam-like structure of carbon

when charred, which must be ground into a powder before

use .

2.2.3. PARTIAL OXIDATION

Oxygen functional groups on char surfaces have an
effect on char reaction rates. One of the main objectives
of this investigation is to study this effect, so partial,
combustion in air was used as the main method of oxidizing
char surfaces. All oxidations were performed in the
quartz tube reactor using slightly different methods to
achieve varying degrees of oxidation. The soak
temperature used for most chars was 375°C, while 725°C was
used in a few runs. Chars were held at the soak
temperature for 0.5-3 hours and were subject to various
combinations of air and nitrogen flow rates, or stagnant
conditions, depending upon the desired conversion and the
amount of char to be treated. Sacrificial carbon was used
in some cases so not all of the oxygen will react with the
sample of interest. Oxidation was used as a pretreatment

or an intermittent treatment for some hydrogasifications.

22

2.2.3.1. PRETREATMENT

One procedure has been used for oxidative
pretreatment of most chars. Five of the 0.5 cm deep
ceramic trays were loaded into the quartz tube reactor,
each containing 0.5 gram of Saran char or 1.0 gram of coal
char. The reactor was heated from 25°C to 375°C at a rate
of 10°C/min and held at 375°C for a designated time
period. The reactor was purged with air at a flowrate of
50 cc/min during the heat ramp and soak, and then 250
cc/min of nitrogen during the time it took the reactor to
cool down to room temperature, which was usually
overnight. One batch of char was oxidized at 725°C, but
the air was kept stagnant to prevent excessive conversion
of the char. Shallow trays and small amounts of char were
used to avoid large oxidation gradients within individual
trays that would be caused by mass transfer limitations.
Some conversion gradients were caused between trays
because of the parabolic temperature profile within the

heated zone of the reactor.

2.2.3.2. INTERMITTENT TREATMENT

The air flow rate for most intermittently oxidized
samples was kept stagnant because the total mass of these

samples was much less than that of the preoxidized chars.

23
Intermittently oxidized samples had weights less than 0.3
gram in the case of Saran char and 1.0 grams in the case
of coal char, compared to 2.5 grams of Saran char and 5.0
grams of coal char for oxidative pretreatments. Two to
three grams of sacrificial char were included in the
reactor with most intermittent oxidations to prevent

excessive sample conversion during oxidation.

2 . 3 . DEMINERALIZATION

Some of the coal samples in this study were
demineralized in order to study the effect of ash on both
uncatalyzed and catalyzed coal char hydrogasification.
Two methods of demineralization were used on the starting

materials.

2.3.1. METHOD I

The first method was based on one taken from a paper
on determination of coal mineral matter by Bishop and
Ward,[114] and has been designated as Demineralization
Method I. It included a hydrofluoric acid bath and a
hydrochloric acid bath of previously pyrolyzed coal.
Eighty grams of coal was mixed with 500 ml of a 49%

hydrofluoric acid solution at 60%: for 1 hour while being

24
stirred every 5 minutes. The slurry was vacuum filtered,
then the filter cake was combined with 1000 ml of a 37%
hydrochloric acid solution under the same conditions. The
new slurry was then vacuum filtered, washed with 5 L of
distilled water, and left overnight in 2000 ml of
distilled water under constant agitation. The next day it
was vacuum filtered, washed with 5 L of distilled water,
and dried at IJTPC under nitrogen overnight. On the third
day it was pyrolyzed for a second time. This method did
not work well for the coal used in this set of
experiments. This may be due to ash particle
encapsulation during initial pyrolysis and/or pyrites that
resist both hydrofluoric and hydrochloric acids. An
ultimate analysis of Demineralization Method I coal char

is in Table 2.1.

2.3.2. METHOD II

The second method was developed to achieve a more
complete demineralization than the first, and has been
designated as Demineralization Method II. It contains
most of the steps found in the first method, and also
includes a nitric acid bath to dissolve any pyrite that
may be left by the other acids. The other major
difference is that the coal was not pyrolyzed before the

demineralization treatment in Method II to avoid

25

encapsulation of ash particles. An ultimate analysis of
Demineralization Method II coal char is in Table 2.1.

The second coal demineralization method began with
coal that has not been charred. It included the same
concentrated hydrofluoric and hydrochloric acid baths as
in the first method. The third bath of demineralization
method II was a 1000 ml 18% nitric acid bath at 60%L
lasting for 1 hour while being stirred every 5 minutes.
The resulting filter cake was washed with 10 L of
distilled water, stirred overnight with 2 L of distilled

water, washed again and pyrolyzed.

2.41.[CAJHUUYSTPIUMPRIKHEATIIHI

Potassium carbonate catalyzes the hydrogasification
of coal chars particularly well. A 10 wt% catalyst
loading was used on all catalyzed chars to achieve a K/C
ratio of 0.02. This was done by combining 0.5-10 grams of
sample with the appropriate amount of 0.1 M KNEE solution
in a drying oven at 90%:. The samples were stirred every
45 minutes and removed when they appeared dry and no

longer lost weight due to water evaporation.

26
2.5. HYDROGASIEICATION

2.5.1. GASITICATION REACTOR SPECIFICATIONS

The hydrogasification reactor used for these
experiments is an externally heated differential fixed bed
reactor. The pressure vessel is made of Haynes-25 Cobalt
alloy with a quartz liner inside. Samples were collected
with a system of solenoid valves and a silica gel trapping
medium, and analyzed with a gas chromatograph and chart
recorder. The reactor, sample collection, and analysis
systems are described in detail by Zoheidi.[93]
Hydrogasification rate calculations are given in Appendix

A.1.

2.5.1.1. GAS CHROMATOGRAPH

One of two modifications on the existing
hydrogasification equipment during the course of this
experimentation was the replacement of the F&M Model 810
Research Chromatograph with a Varian Analytical
Instruments Model 3300 Gas Chromatograph. This
chromatograph uses a thermal conductivity detector with a
reference cell, and a Supelco 80/100 Carbosieve S—II
stainless steel column, 5'x1/8“, in parallel with a blank

column. It also has an auto zero function which

27
automatically establishes a baseline for the chart
recorder. This makes interpretation of the raw data
charts much easier. The column oven heating cycle used
for all sample analyses consisted of an initial soak at
500C: for 2 minutes, an increase from 50°C to 175°C at
20°C/min, and a final soak at 175°C for 2 minutes for a

total time of 10.25 minutes.

2.5.1.2. SAMPLE MOUNT

The other modification to the existing equipment was
a newly designed sample holder. The sample holder used in
past studies was a ceramic hemispherical trough, 2.6 cm
long, with a bed depth of 3 mm. It required a 325 mesh
316 stainless steel screen to be placed over the sample to
help prevent blowout by high gas velocities.
Disadvantages of this design included possible mass
transfer limitations, small sample size, potential sample
blowout, and metal in contact with the sample.

These problems were minimized or eliminated by using
a cylindrical quartz chamber capped at both ends by 40-60
micron porous quartz frits. A diagram of the new sample
holder is given in Figure 2. No metal parts come in
contact with the chars. The reactant and purging gases
are able to flow through the sample bed instead of over

it, and cannot blow char out of the holder. A quartz wool

28

Porous Quartz Frit (40 - 60 micron pore range)

Leg—“fl \\¢

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COMPONENT '2 - Fits over the wide end of
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Figure 2: Newly Designed Sample Holder
Material of Construction - Quartz

(figure not drawn to scale)

29
gasket seals the ring of contact between the two quartz
components to prevent sample from being entrained. The
components are held together by springs to ensure a tight
seal.

The sample chamber volume is over four times greater
than that of the ceramic sample mount, is variable, and
does not require metal parts to come in contact with the
chars. Insertion of small quartz frits and tubes in the
chamber allows sample mass to vary greatly without causing
empty spaces in the chamber. The only disadvantage is
that chars must be sieved to greater than 200 mesh size so
that smaller sample particles do not clog or pass through
the frits. This eliminates roughly half of the prepared

char mass that can be used for hydrogasification.

2.5.2. HYDROGASIEICATION KINETIC EXPERIMENTS

2.5.2.1. CONDITIONS

All gasification experiments were conducted at 725%:
and 500 psig under pure hydrogen at a flowrate of 300
cc(STPi/min unless otherwise indicated. A temperature of
725°C was used because temperatures lower than this result
in reaction rates that are too slow, while temperatures
higher than this can result in mass transfer limitations

for this reaction system. A pressure of 500 psig was used

30
because pressures below this result in slow reaction
rates, while pressures above this approach the safety
limitations of the reaction vessel, and methane may not
remain the greatly favored product.[7] A flowrate of 300
cc(STP)/min ensures that fractional conversion of hydrogen
will be low enough to observe intrinsic kinetics, and the
influence of methane on the reaction can be minimized.
Calculations of the fractional conversion of hydrogen at

these conditions have been done by Zoheidi.[93]

2.5.2.2. PROCEDURE

Samples were weighed and loaded into the reactor
which was then purged with helium before gasification.
After 15-20 minutes the reactor was evacuated to 30"Hg
vacuum and the furnace was turned on to a setpoint of
600°C. Another helium purge was started after the
evacuation. During this time the sample traps and gas
chromatograph columns were cleaned, then a calibration run
was made. The calibration gas used for all runs was 4.8%
CH" 4.9% CO, 4.9% C02, and balance He. The furnace
typically reached a temperature of about 525°C by the end
of the calibration, at which time it was set to 725%:.
The helium purge was stopped, the reactor was filled with
hydrogen when it reached a temperature of 600%:, and the

first sample was taken at 650%3. Most effluent gas sample

31
collection times were one minute with 15-20 minutes
between collections, but times varied depending upon the
rate of methane formation during a particular reaction.
Other procedures varied slightly depending upon
gasification temperature, pressure, and intermittent
treatments. At the end of a designated time period the
furnace and hydrogen were both shut off simultaneously,
and the reactor was purged with helium for 15-20 minutes
and allowed to cool to room temperature overnight. Most
gasifications proceeded for 2.25 hours, but some proceeded

longer for higher conversions.

2.5.3. OUTGASSING

Outgassing was used as a pretreatment or an
intermittent treatment to remove oxygen functional groups
from Saran char and coal char surfaces. This was done to
study the reactivity of the bulk char and the effects of
surface annealing. Ten to fifteen grams of sample were
set inside the hydrogasification reactor, which was purged
with helium. The reactor was then placed under 30"Hg
vacuum and heated.to 1000%: for 16 hours, during which
vacuum was continuously being applied. It was then
followed by another helium purge. Outgassing was used as
a pretreatment or an intermittent treatment for some

hydrogasifications.

32
2.6. SURFACE ANALYSIS

2.6.1. TOTAL SUREACE AREA

Carbon dioxide physisorption was used to determine
the total surface area of sample chars. Samples were
first outgassed to remove water and other contaminants
which occupy pore volume. Next, they were weighed under
vacuum in an electronic microbalance. The samples were
then exposed to carbon dioxide at predetermined pressure
increments and weighed at each increment. Data was
analyzed with the Dubinin-Radushkevitch method to
calculate total surface area. Greater detail on this and
other surface analysis techniques are given by

Toomajian.[113]

2.6.2..ACTIVE SUREACE AREA

Oxygen Chemisorption was used as a method to
determine the active surface area of various samples,
because desorption of oxygen from char surfaces prior to
and during hydrogasification is believed to create active
sites where gasification takes place. Samples were first
outgassed in an inert atmosphere at 725%: to clean them of
surface contaminants. They were then put through a

temperature programmed desorption to determine how clean

33
the char surfaces were and at what temperature various
oxygen groups desorb. Pulses of a known amount of oxygen
were then passed over the sample at 295°C and analyzed to
determine the amount that had chemisorbed. This
temperature was chosen because it is high enough to
facilitate Chemisorption, but not high enough to cause
excessive combustion of the char. Oxygen Chemisorption
has also been used as form of oxidative sample treatment

for some hydrogasifications in this study.[113]
2.6.3. SURIACE OXYGEN CONCENTRATION

X—ray photoelectron spectroscopy was used to
determine the oxygen to carbon ratio on char surfaces. A
monochromated magnesium x-ray source was used in early
tests, but better analysis was made possible with a
monochromated aluminum x-ray source which was used in
later tests. Samples were outgassed in a vacuum
pretreatment reactor prior to analysis to remove loosely
physisorbed surface contaminants. A survey scan was the
first analysis performed on a char sample, which
determined overall sample elemental composition by
analysis of data over a wide range of energy levels. The
second analysis performed on a char sample was a multiplex
scan, which determined bonding states of specific elements

by focusing on a narrow band of energy levels.[113]

34
2.6.4. SUREACE pH

This test, ASTM method D3838-80, determined the
acidity or basicity of the functional groups on char
surfaces. It consisted of immersion of the char in a 0.1
M potassium chloride solution for two hours under reflux.
After the solution was cooled to 23%:, surface pH was
measured and compared to the pH of a 0.1 M KCl standard
solution that had undergone the same treatment. A
positive change in pH (pH of sample solution minus pH of
standard solution) indicated basic functional surface
groups, while a negative change in pH indicated acidic
groups. This test only provided information on the
general nature of surface functional groups, it did not

provide information on individual types.[113,115]

CHAPTER 3

RESULTS

Results of gasification experiments are presented in
this chapter. After establishing that kinetic rate
measurements are dominated by reaction and not mass
transfer, the effects of outgassing, preoxidation, and
intermittent oxidation are reported. Following these,
results at lower temperature, rates following hydrogen

pretreatment, and effects of catalyst addition are given.

3.1. EFFECT OF TEMPERATURE

Figure 3 contains rate data on the hydrogasification
of coal char at 7% conversion. Temperature varies from
800°C at the far left to 700°C by increments of 25°C. A
conversion of 7% was chosen because the reaction does not
reach steady state until 6% conversion at 800°C, and
conversions higher than 7% do not give as large a

separation between rate data points because of the rapid

35

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37
deactivation of the reaction. The maximum temperature at
which diffusion limitations do not affect the reaction
rate is 775%:. The temperature of all hydrogasifications
performed in this investigation was at or below 725°C to
remain well within kinetic rate limitations. The
activation energy based on a slope which excludes the data
point on the far left is 76 kcal/mol carbon, which is
close to values reported for intrinsic gasification rates

by other investigators.[17,18]

3.2. BASE CASE CHAR REACTIVITY

Hydrogasification rate is shown as a function of
carbon conversion in Figure 4 for Saran char and three
coal chars. The Saran char and coal char were just
pyrolyzed, while the other coal chars were demineralized.
The Saran char has been gasified for 8 hours. The coal
char and Demin. II coal char have both been gasified for 9
hours, while the Demin. I coal char has been gasified 2.25
hours. Rate is reported as cubic centimeters of methane
(STP) per minute per gram carbon present. All chars react
at similar rates and have similar rate decay patterns.

The Demin. II char rate curve is shifted outward slightly
from the others. This may be due to residual effects of

rung oxidation [48] that survived pyrolysis, or a greater

38

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amount of active surface area exposed by the removal of

ash particles.

3.3. OUTGASSING

3.3.1. SARAN CHAR

The effect of outgassing on the hydrogasification
rate of Saran char is shown in Figure 5. The 8 hour base
case run was not pretreated, but has been included as a
basis for comparison. The outgassed Saran char was
gasified for 4 hours, at a rate that was about an order of
magnitude less than the base case Saran Char. The
intermittent treatment consisted of a flash desorption to
1000°C for 15 minutes in helium under a 30" Hg vacuum
after 2.25 hours of gasification. This treatment
decreased the reaction rate of the Saran char to about 1/5

the rate of the base case.

3.3.2. COAL CHAR

The effect of outgassing on the hydrogasification
rate of coal char is shown in Figure 6, and the results
are similar to those of Saran char. The outgassed coal

char reacted at a rate which was an order of magnitude

40

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less than the base case, and the intermittent flash
desorption decreases the reaction rate to about half that

of the base case.

33.4. (IKIEUUEIVT:EERIUUEMEDTTS

3.4.1. OXIDATIVE PRETREATMENTS

Figures 7 and 8 show the results of oxidative
pretreatment of various chars by partial combustion in
air. Figure 7 shows percent weight loss of chars as a
function of the length of time they have been exposed to
flowing air at 375%:. The percent weight loss is a
weighted average of all five trays which contain char
during oxidation. The relationship between time of air
flow and percent weight loss appears to be fairly linear
over the range of data present. Figure 8 shows the same
six partial oxidation runs, but the percent weight loss
for each run is broken down into individual trays. The
trays were numbered in order from one to five and arranged
so that the direction of air flow was always from tray"1
to tray'15. Most plots are parabolic in shape, with
maxima at tray"3. This results from the parabolic

temperature profile within the heated zone of the furnace.

43

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3.4.1.1. SARAN CHAR

Figure 9 gives the effects of oxidative pretreatments
on the hydrogasification rates of Saran char. Two samples
were partially oxidized in air at 375%:. One lost 3.1 wt%
upon partial combustion, while the other lost 7.7 wt%
because it was burned longer. The rate curves may be
shifted slightly, but there was basically no rate
enhancement in gasification resulting from these pre-
oxidations. The other sample was first outgassed, thengn
oxidized in air at 375°C before hydrogasification. The
outgassed Saran char sample that was not preoxidized is
included as a basis for comparison. The rate of the
outgassed, then preoxidized sample was several times lower
than the base case, but was 2-3 times higher than the

outgassed sample.

3.4.1.2. COAL CHAR

The effects of oxidative pretreatment upon the
hydrogasification rates of coal char can be seen in
Figures 10 and 11. The first oxidation method used was
partial combustion in air at 375%: for two different time
periods. Weight losses were 0.1 wt% and 5.7 wt% during
oxidation. The second method was partial combustion in

air at 725%: to 3.7 wt% burnoff. The third was an oxygen

46

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49
Chemisorption, while the fourth was an outgassing followed
by a partial combustion in air at 375%:. .As with Saran
char, preoxidation of coal char did not result in any rate
enhancement. The sample that had been outgassed and then
preoxidized in air at 375°C did show some
hydrogasification rate enhancement over the coal char that
had just been outgassed, but the enhancement was not as

great as that of the Saran char.

3.4.1.3. DEMINERALIZED COAL CHAR

The effect of oxidative pretreatment on the
hydrogasification rate of coal char demineralized via
Method I is shown in Figure 12, and via Method II in
Figure 13. All pretreated samples were partially reacted
in air at 375%: for varying lengths of time. Neither of
the demineralized coal chars show enhanced rates due to

oxidative pretreatment.

3.4.2. INTERMITTENT OXIDATIVE TREATMENTS

3.4.2.1. SARAN CHAR

Figure 14 shows the effect of intermittent oxidation

at 375°C on the hydrogasification rate of Saran char.

Gasification proceeded for 2.25 hours between successive

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53
oxidations. The first intermittently treated sample
gained 2 wt% upon oxidation, while the second sample was
intermittently treated longer and lost 1 wt% during each
oxidation. Figure 15 shows the effect of intermittent
oxygen Chemisorption on the hydrogasification rate of
Saran char. After all intermittent oxidations, the rates
increased 2—3 fold over the base case, but decayed back to

the base rate after about 5% carbon conversion.

3.4.2.2. COAL CHAR

Intermittent oxidative treatments are shown for coal
char in Figures 16 and 17. Gasification proceeded for
2.25 hours between successive oxidations. The samples in
Figure 16 were oxidized two times at 375°C during each
reaction. The first lost 7 wt% both times upon combustion
in air, and the second lost 5 wt% during each oxidation.
Figure 17 gives the hydrogasification rare curve for coal
char that has been partially burned in air at 725%:. This
sample lost 3 wt% during the oxidation. As with the Saran
char, all rates increased 2-3 times over the base case
following intermittent treatment and decayed back to the

base rate after approximately 5% carbon conversion.

54

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57
3.5. HYDROGEN ADDITION AT STEADY STATE REACTION

TERENERAJTHUI

An advantage of using the newly designed sample mount
during hydrogasification is having the option of adding
hydrogen to the reactor as quickly as possible and not
having some of the sample blow out of the holder before
500 psi is achieved. The standard procedure used in this
study for gasifying samples was developed before the
design of the new sample mount, making slow addition of
hydrogen starting at Gowmithe best method. Figure 18
shows the effect on hydrogasification rate of adding 500
psi hydrogen very quickly to the reactor when it has just
reached 725%:. The initial recorded reactivity of coal
char is almost twice as high as that of the base case, but
the rest of the rate points fall closely to those of the

base case.

3.¢i.IREEKflNED GHUKIFIIflUrICflITEENENNRAJKHRE

Figure 19 shows the effect of reducing
hydrogasification temperature to 600°C on the reaction
rate of Saran char, coal char, and pre-oxidized coal char.

The pre-oxidized coal char has been burned in air at 375%:

58

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60
and lost 5.7 wt% upon partial combustion. The Saran char
reactivity is about twice that of the coal char, while the
pre-oxidized coal char has a rate that is slightly higher
than the coal char initially. The reactivity of all chars
decreases rapidly in the same way it decreases during

hydrogasification at 725%:.

3.17. EDHIRfllflflN PEUETRIHVEMEnflP

Dissociative adsorption of hydrogen may poison the
reaction of hydrogen with chars. Hydrogasification at
reduced temperature and pressure and subsequent effects on
hydrogasification rate at 725°C and 500 psig are shown in
Figures 20 and 21. The duration of the pretreatment was
2.25 hours for all samples. This was the same length of
time of all hydrogasifications in these figures, except
the base case. The pretreatment shown in Figure 20 was
initial gasification at 725°C and 10 psi hydrogen. The
rate during the pretreatment conditions was much lower
than that of the base case, while the rate after standard
gasification conditions were imposed appears to have
become greater than that of the base case. The first
pretreatment shown in Figure 21 was gasification of coal
char at 600°C under 50 psi hydrogen, which did not produce

a rate high enough to accurately record. The gasification

61

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63
rate of this sample after pretreatment was slightly lower
than the base case. The final pretreatment was an initial
gasification at 600°C and 500 psi hydrogen, which gave a
low but measurable rate. Upon exposure to normal
gasification conditions, which were 725°C and 500 psi
hydrogen for this study, the sample reacted at a rate that
was close to that of the base case. Overall, the effects
of the hydrogen pretreatments on hydrogasification rate

were minor.

3.8. CAIALYST ADDITION

3.8.1. CAEALYZID BASE CASES

Figures 22A and 228 show the effect on
hydrogasification rate of loading 10 wt% potassium
carbonate onto the various chars. The catalyzed coal char
reacted at a rate almost double that of the uncatalyzed
coal char, and the rate decayed in a similar manner after
8 hours of gasification. The catalyzed Saran char and
catalyzed demineralized coal char began hydrogasification
atia rate slightly higher than that of the catalyzed coal
char, but their rates did not decay with time. The
catalyzed Saran char reacted at a steadily increasing rate

until about 75% carbon conversion, where the rate dropped

64

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66
abruptly. After 4 hours of hydrogasification, the
catalyzed saran char was over 90% converted. The
catalyzed demineralized coal char reacted at a rate that
increased rapidly until near completion after 2 hours, and

had a higher rate than the Saran char.

3.8.2. PRIOXIDAIION OF CAIALYZED CHARS

The effect on hydrogasification rate of preoxidizing
coal chars before catalyst impregnation is shown in Figure
23. The first char had not been oxidized, while the
second had been oxidized in air at 375°C and lost 5.7 wt%
upon partial combustion. The third char had been oxidized
in air at 725°C and lost 3.7 wt% upon partial combustion.
Both preoxidized-catalyzed chars were gasified for 2.25
hours. No rate enhancement is evident over that of the

catalyzed coal char base case.

3.8.3. INTIRMITTINT OXIDAIION O! CAIALYZID GEARS

The effect on hydrogasification rate of intermittent
oxidation of various catalyzed chars is shown in Figure
24. The catalyzed Saran char was oxidized at 375°C in air
after 2.25 hours of hydrogasification. The rate after
intermittent oxidation decreased to less than one third

that of the catalyzed base case, but the rates did not

67

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68

 

 

 

 

 

 

 

 

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9103 iuoisul

69
match up well before the intermittent oxidation. The
catalyzed coal char was oxidized.at 375%: in air after
2.25 hours of hydrogasification. It lost about 10 wt%
during partial combustion, which caused a large gap
between data points. During the second gasification it
started at a rate higher than that of the catalyzed base
case coal char, but dropped below it after 3% carbon
conversion. As with the catalyzed Saran char, the
catalyzed coal char reaction rates did not match up well

with each other before the intermittent oxidation.

Chapter 4

Discussion

‘4.1u (flflNIEUNL CHMEERNUUPICWHB

4.1.1. TEMPERATURE EFFECTS

It has been shown that the conditions chosen for
hydrogasification in this investigation result in kinetic
and not mass transfer rate limitations, with the exception
of the first 2-3% carbon conversion. The activation
energy calculated from the Arrhenius plot in Figure 3 is
consistent with those found in 1iterature.[17,18] The
temperature range in which diffusion limitations dominate,
which will be above a certain threshold, is indicated by
the section of the Arrhenius curve that has a slope which
gives half the activation energy of the reaction. In this
system, that transition occurs near 775%:. This is 50%:
higher than the gasification temperature chosen for this
investigation. An interesting phenomenon illustrated by

the Arrhenius plot is the apparent decrease in rate from a

70

71
gasification temperature of 775°C to 800°C.
Hydrogasifications to produce these data points have been
duplicated to ensure their validity. This unusual rate
decrease with increased reaction temperature may due to
the annealing effects at higher temperatures. Another
possibility is the proportional increase in rates of
reactions that are responsible for the consumption of
active sites over the rates of reactions responsible for
methane formation.

Gasification rates measured during runs with the
newly designed sample mount are consistent with rates
measured during runs with the original sample mount
design. This demonstrates that mass transfer limitations
are not taking place because of particle size, since
larger sample particle sizes are required for the newly
designed mount. A gasification run was made with the
original mount containing a sample weight that was 40% of
the usual starting weight to investigate the possibility
of mass transfer limitations because of the sample bed
depth. The rates curves are identical, indicating
diffusion limitations do not occur because of the bed

depth either.

72
4.1.2. TOTAL SURIACI ARIA

Figure 25 shows total surface area changing very
little with carbon conversion for Saran char and coal
char. Since the hydrogasification rate changes
significantly within the 0-20% conversion range, it
appears that rate is only a very weak function of total
surface area. Analysis of Figure 26, however, indicates
that this may not be the case in conversions above 20%.
The rate per unit total surface area appears to reach a
constant level for coal at about 15%, and may be leveling
off for Saran char at about 25% conversion. The
possibility of a mechanism in which at least two different
modes of hydrogasification occur is supported if the rate
is not a function of total surface area for the first 15-
25% carbon conversion, but it is for the rest of the

reaction.

4.1.3..ACTIVI SUREACI ARIA

Figure 27 shows the active surface area of Saran char
and coal char as a function of carbon conversion in
hydrogasification. The coal char results are erratic,
with the last data point being unusually high. Later
measurements of just the coal ash after exposure to

hydrogasification conditions revealed an active surface

73

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76

area of 74ufi/g, which is very high relative to the coal.
This cannot be compensated for in the active surface area
calculations because it is not known exactly how much of
the ash is exposed at various stages during gasification.
Thus, measurement of active surface area using oxygen
Chemisorption is not feasible for hydrogasified coal char.

The general trend of the Saran char data is similar
to that of the total surface area data. Figure 28 shows
the rate per unit active surface area as a function of
carbon conversion. The Saran char data indicate a
possible leveling off of rate per unit active surface area
in a manner similar to rate per unit total surface area in
Figure 27. This may also support a multi-stage mechanism.

It is apparent that the method used for active
surface area measurement in this investigation is not
indicative of the active surface area that is important
during hydrogasification. Table 4.1 shows the active
surface area of two parallel runs of intermittently
oxidized Saran char. The values given describe char
properties at the end of each stage. According to the
values given, the active surface area is lower at the
beginning of a hydrogasification stage than at the end of
that stage, and the active surface area is higher at the
beginning of an oxidation stage than at the end of that
stage. This is the inverse of what would be expected.

The active surface area should decrease with

'77

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Table 4.1:

Stages
During

Experiment
Starting:

Gasification

Oxidation #1:

Gasification

Oxidation #2:

Gasification

78

ACTIVE SURFACE AREA OF INTERMITTENTLY
OXIDIZED SARAN CHAR - TWO DIFFERENT

RUNS1

#1:2

#2:

#3:

% Conversion
Run A Run B
00.00 00.00
14.5 14.7
0.32 -2.51
19.9 21.0
-1.24 -1.75

NA 24.7

ASA (m2/g C)[115]

Run A Run B

4.0 4.0
7.8 NA
4.9 4.3
6.1 >5.1
4.0 3.4
NA 6.6

1. Values are reported at the end of each stage of

t reatment .

2. All gasifications are 2.25 hours at 725°C in 500 psi

H20

3..A11 oxidations are 1 hour at 375%: in stagnant air.

79
hydrogasification since the rate decreases, because rate
is thought to be a function of surface oxygen group
concentration. The active surface area should increase
after oxidation, since oxidation fixes oxygen functional
groups on the char surface. Explanations for this include
the possibility of oxygen binding more strongly to the
highly reduced char surface after hydrogasification, or
reaction of oxygen with surface hydrogen to produce water

after hydrogasification.

4.1.4. OXIDATION EFFECTS

It is evident that preoxidation via partial
combustion in air does not enhance the hydrogasification
rate of pyrolyzed chars at 725%:. However, it does
enhance the reaction rate of chars that have been
pyrolyzed, then outgassed. Table 4.2 shows surface
analyses of most of the chars used in this investigation.
It is clear that oxygen is being fixed onto the char
surfaces during partial combustion in air because the O/C
ratio increases in all but one case. The O/C ratio for
the outgassed samples is lower than the samples that have
not been outgassed. The decreased Saran char gasification
rate following outgassing is due to the decreased surface
oxygen concentration and not the collapse of pore

structure, since the total surface area increases slightly

80

Table 4.2: SURFACE ANALYSIS OF VARIOUS CHARS[113]

 

§am21§ TSA‘ ASA’ O/LRatig’ AEH‘
SARAN CHAR 816 4.0 0.066 -0.44
Oxidized 375°C, 3.1% Burnoff 815 6.1 0.108 -2.54
Oxidized 375%:, 7.7% Burnoff 876 9.9 0.152 -3.38
Outgassed 1000%: 870 1.5 0.043 +0.55
Outgassed 1000°C, then
Oxidized 375°C, 0.4% Burnoff 762 2.3 0.080 -2.42
Gasified 725%: 754 NA 0.018 +2.70
COAL CHAR 274 4.0 0.062 +4.55
Oxidized 375%:, 0.1% Burnoff 218 NA 0.052 +0.82
Oxidized 375°C, 5.7% Burnoff 195 NA 0.128 -1.66
Oxidized 725°C, 3.7% Burnoff NA NA 0.093 +2.88
Outgassed 1000%: 7 0.6 0.012 +3.71
Outgassed 1000°C, then
Oxidized 375°C, 0.5% Burnoff 87 0.9 0.100 +2.92
Outgassed 725%: NA NA NA +4.16
Outgassed 725°C, 02 Chemsorbed NA NA NA +3 . 88
Gasified 725%: 313 NA 0.052 +3.46
Gasified 725°C, Oxidized 3750C 316 6.3 NA +0.91

DEMIN. I COAL CHAR 322 1.3 0.066 -0.63
Oxidized 375°C, 2.3% Burnoff 354 6.9 0.094 -l.29
Oxidized 375°C, 6.3% Burnoff 376 6.7 0.092 -2.75
Gasified 725°C 363 NA NA +3.17

1. Units in mz/g Carbon
2. Units in mzlg Carbon

3. Oxygen in the form of metal oxides is not included in
this ratio (SnOz, A1203, Sioz, et.al.).

4. Units are difference in pH between a 0.1 M KCl solution
containing the char and a standard 0.1 M KCl solution,
both heat treated under reflux (ASTM method 03838-80).

81

both heat treated under reflux (ASTM method 03838-80).
during outgassing. The coal char loses much more surface

oxygen and total surface area upon outgassing. There may
be a contribution of both phenomena to the lowering of the
hydrogasification rate of coal char because less surface
oxygen means fewer functional groups, and collapse of the
pore structure may cause blockage of the reactant gas from
these groups.

There does seem to be some enhancement in rate for
gasifications performed on non-outgassed coal chars at
600°C during the first 2% carbon conversion, as seen in
Figure 19. This rate enhancement does not appear to occur
at 725°C because steady state conditions are not reached
until about 5% conversion, after the rate enhancement has
already died out. Figures 29A, 29B, 30A, and 308 show the
rate data at 600°C normalized to 725°C via activation
energy for Saran char, coal char, and preoxidized coal
char. The highly deactivating nature of the
hydrogasification reaction is very clearly illustrated.

Intermittent oxidation via partial combustion in air
does produce a noticeable hydrogasification rate
enhancement at 725%:. The rate after an intermittent
oxidation never reaches the rate measured at the beginning
of the first hydrogasification and only lasts for about 5%
carbon conversion before it decays back to the base level.
This seems to indicate that it takes roughly 5% carbon

conversion to consume the additional active sites formed

82

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86
during oxidation. The intermittent oxidation performed on
coal char at 725°C produced an enhancement comparable to
the intermittent oxidations at 375%:. This finding
suggests that the nature of the functional groups may not
be as important a factor as previously thought in
determining the hydrogasification rate of chars, since
acidic groups would be formed at 375%: and basic groups at

725°C.

4.1.5. CATALYST EFFECTS

It has been shown in this study that coal ash does
not catalyze or poison coal char that has not been treated
with potassium carbonate. It has also been shown that
potassium carbonate greatly catalyzes the
hydrogasification of chars that contain very little or no
mineral matter. In the latter stages of gasification,
there may be a rate increase up to three orders of
magnitude for Saran char and four orders of magnitude for
demineralized coal char. Coal ash, which contains sulfur
and aluminosilicates, strongly poisons the catalyst. Even
though the gasification rate of the catalyzed coal char is
twice the rate of the uncatalyzed coal char, the catalyzed
coal char rate does not increase with conversion like the

demineralized catalyzed coal char and the Saran char.

87
Preoxidation has no apparent effect on the catalyzed
char reaction rate at 725°C, which is the same result
found for uncatalyzed chars. Intermittent oxidation of
the catalyzed coal char produces a mild rate enhancement
that decays quickly. This result is also similar to that

observed for uncatalyzed chars.

4.2..MECEANISTIC CONSIDERATIONS

A multi-stage reaction and the concept of creation of
active sites by oxygen functional group desorption are
both supported by this investigation. Hydrogen inhibition
is not observed with any of the hydrogen pretreatments,
indicating that dissociative hydrogen poisoning may not be
the cause of the sharp hydrogasification rate decay with
sample conversion. Previous studies have proposed a
variety of two-stage reactions for char
hydrogasification.[8,25,26] The best explanation for the
phenomena observed in this study is a three-stage reaction

for hydrogasification.

4.2.1. PROPOSED MODEL

The dominant reaction during the first stage is the

rapid hydrogasification of amorphous "secondary" carbons

88
that are partially saturated with hydrogen, yet are
probably not saturated with chemical bonds. One category
of amorphous carbons is high molecular weight compounds
that are not chemically bound to the char surface, but
remain in char because they are not volatile enough to
enter the gas phase during pyrolysis. The other category
of amorphous carbons includes carbon structures that are
partially saturated with hydrogen and chemically bound to
the char base structure. If some of these groups gasify
via active sites, the sites may propagate to the base
structure during gasification.

The second stage involves hydrogasification of base
char carbons via active sites formed by oxygen functional
group desorption, and the preferential hydrogasification
of carbons located on the armchair edges of aromatic
planes. There is probably a great deal of overlap between
these two categories of carbon. It is also possible that
active sites propagate to the base char structure from the
chemically bound amorphous carbons and contribute to the
second stage gasification reaction.

Rate during the third stage of hydrogasification is
low because the carbons that react are primarily those on
the relatively unreactive zig-zag edges of the aromatic
planes. There are very few armchair edges because they
have been consumed during the second stage. The only

sources of surface oxygen functional groups are oxygen

89
migration to the char surface or the uncovering of oxygen
from the bulk char by removal of carbons. Because the
char surface has become very non-reactive and homogeneous,
the reaction rate becomes proportional to the total
surface area.

Preoxidation does not increase or decrease the
hydrogasification rate within the conversion range
measurable by the equipment used in this investigation
because two opposing effects roughly cancel each other out
with respect to the char's reactivity with hydrogen. One
force at work is a decrease in the ratio of amorphous
carbons to those of the basal aromatic planes by somewhat
preferential oxygen attack, and the other is the creation
of more oxygen functional groups on the aromatic base
structure.

Intermittent oxidation increases the
hydrogasification rate, and brings the reaction from the
third stage back to the second. No amorphous carbon
structures are formed upon intermittent oxidation, so the
rate will not be as high as that which is observed
initially. Oxygen functional groups are formed on the
char surface, which create active sites upon desorption.
Oxidation also increases the ratio of armchair carbons to
zig-zag carbons. This happens because oxygen, unlike
hydrogen, does not preferentially react with either type

of edge carbon, thus leaving both present following

90
oxidation. Since armchair carbons are more reactive
toward hydrogen than zig-zag carbons, their ratio is an
important parameter in determining the reactivity of a

char.

4.2.2. SUPPORTING OBSERVATIONS

There are several observations that support the
proposed three-stage hydrogasification reaction in this
study. Dissociative hydrogen poisoning and rate
dependence upon pore structure within the first 20% carbon
conversion have not been included in the proposed model.
This is because they have been shown not to affect rate or
correlate to rate within the first 20% carbon conversion
respectively.

Initial rate decreases rapidly with conversion and
cannot be matched with the intermittent treatments used in
this investigation. This indicates that there may be more
than one reaction stage present. Further, it supports the
claim that there is a small amount of highly reactive char
on the sample surface, which is consumed during the first
5% carbon conversion. If the high initial rate was due
only to the number of oxygen functional groups on the char
surface, then intermittent oxidation would restore the
functional groups and increase the hydrogasification rate

to its original value, but this is not observed.

91

Preoxidation neither increases nor decreases the
initial reaction rate as can be detected by the equipment
used in this investigation. The highly volatile material
contained in the starting coal is drawn out of the bulk
material upon pyrolysis, leaving pores. Most of the
volatile material is carried away in the gas phase, but
some carbonizes on the char surface. Because of the way
amorphous carbon structures are formed, it would seem
likely that most of the char total surface area would
initially be coated with them, so gases would have to
react with amorphous carbon structures before they could
react with carbons of the basal aromatic planes. From a
physical standpoint, it is likely that any reactant gas
would preferentially attack the amorphous carbons. This
proposed idea is supported by the facts that preoxidation
has been shown to increase the concentration of surface
functional groups, but it does not enhance the initial
hydrogasification rate.

This study shows that hydrogasification rate declines
rapidly and appears to become constant with total surface
area after about 20% carbon conversion. The O/C ratio is
low, and etch pit analysis performed by other researchers
shows the predominance of zig-zag carbon
configurations.[29-32] If oxygen functional groups are
uniformly distributed throughout the basal carbon matrix,

they should be uncovered by the consumption of overlying

92

carbons at a fairly uniform rate and be distributed fairly
evenly over the char surface. This means a roughly
constant ratio of total surface area to active surface
area caused by desorption of freshly uncovered oxygen
groups. Since the hydrogasification rate is thought to be
proportional to the active surface area, it should be
proportional to the total surface area during the third
gasification stage. Observations in this study appear to
indicate constant rate per unit total surface area during
the third stage of hydrogasification.

Intermittent oxidation has been shown to increase the
O/C ratio in this study. Other researchers have shown
oxygen to form round etch pits [29,32]. As the oxygen
attacks the zig-zag edges, it must leave both zig-zag and
armchair edges behind. This must increase the ratio of
armchair to zig-zag carbons and partially cover the edges
with oxygen functional groups, supporting the claim that
partial oxidation serves as a source for regeneration of
these configurations after they have been destroyed during

hydrogasification.

CHAPTER 5

QQNCLQSIONS AND RECOMMENDATIONS

5 . 1 . CONCLUSIONS

The major objective of this investigation was to gain
a more fundamental understanding of the hydrogasification
reaction, and to determine ways to increase the reaction
rate based upon this knowledge. In order to ensure
kinetic and not mass transfer limitations, the proper
reaction temperature, 725°C, was established. It was also
determined that mass transfer limitations were not
occurring due to particle size or sample bed depth.

Hydrogasification rates of Saran char and coal char
were shown to decay rapidly with conversion, and were
similar in magnitude. Rate for the first 2-3% carbon
conversion could not be measured at the established
reaction temperature, but the sharp rate decay during the
rest of the reaction was evident. The sharp rate decay
was shown much more clearly by normalizing

hydrogasification rates measured at 600%: for the first 2-

93

94
3% carbon conversion. For uncatalyzed hydrogasification,
initial rate decays by a factor of five after 10-15%
carbon conversion, and a factor of ten after 20-25% carbon
conversion. Total surface area was found not to be a char
characteristic that correlates with hydrogasification rate
within the first 15-25% carbon conversion, but may
correlate well at conversions greater than this. This
indicates that the chemical nature of the char surface is
of fundamental importance in determining hydrogasification
rate. -

One focus of this investigation was to study hydrogen
inhibition. Several hydrogen pretreatments were done at
conditions less severe than those chosen for standard
gasification during this experimentation, so
hydrogasification rates would be low. Some pretreatments
were performed at either 725°C or 500 psi H,, but none
resulted in rate inhibition at standard gasification
conditions.

Another focus of this investigation was to study the
role of oxygen in hydrogasification. It was found that
partial combustion in air increases the char surface
oxygen concentration. These surface oxygen functional
groups desorb during heatup to hydrogasification
conditions and leave active sites where the reaction takes
place. Partial combustion in air increased the subsequent

hydrogasification rate of chars that had been outgassed or

95
previously gasified in hydrogen, but did not increase the
reaction rate of fresh chars. The hydrogasification rate
of chars that had been oxidized following outgassing or
hydrogasification was not restored to that shown initially
by a fresh char. Reaction rate following outgassing was
an order of magnitude lower than that of fresh char.

An attempt was made to accurately measure the
concentration of active sites on char surfaces in this
investigation. Coal ash was shown to have a very high
active surface area, which interfered with measurement.
The active surface area measurements for intermittently
oxidized Saran char were the opposite of what they should
have been. They showed a higher active surface area
following hydrogasification, and a lower active surface
area following intermittent oxidation. The opposite would
be expected, because intermittent oxidation has been shown
to increase the surface oxygen content and the
hydrogasification rate. A new method must be devised to
accurately measure char active surface areas.

The final focus of this investigation was to study
the poisoning of the reaction by coal ash. It was found
to neither poison nor catalyze the coal char that had not
been treated with potassium carbonate. The
hydrogasification rates of all chars used in this
investigation were catalyzed by potassium carbonate,

however it was found that coal ash greatly poisons this

96
catalyst. Catalyzed Saran char and catalyzed
demineralized coal char both had rate curves that started
2-3 times higher than the uncatalyzed chars and increased
with conversion. The catalyzed coal char started at a
rate that was about twice the uncatalyzed rates, and had a
rate decay pattern similar to that of the uncatalyzed
chars.

This set of observations supports a hydrogasification
reaction that is composed of three stages in which each
stage dominates during different ranges of carbon
conversion. During the first stage, hydrogen reacts
rapidly with the small amount of amorphous "secondary"
carbon. This carbon is highly reactive because it is
already partially saturated with hydrogen, but is not
completely saturated with chemical bonds. The second
stage involves hydrogasification of base char carbons via
active sites formed by functional group desorption, and
hydrogasification of carbons located on the armchair edges
of the base char, which is composed mainly of randomly
aligned graphite crystalites. Active sites formed during
the first reaction stage may propagate to the base char
and contribute to hydrogasification during the second
stage. Rate during the third stage of hydrogasification
is low because the carbons that react are primarily those
on the relatively unreactive zig-zag edges of the aromatic

planes. There are very few active sites or armchair edges

97
left. During the third stage, reaction rate is roughly
proportional to the char total surface area because the
surface is relatively unreactive and homogeneous, and the
only source of functional groups is oxygen trapped in the
bulk char.

Preoxidation does not change initial char reactivity
toward hydrogen within the measurable range of carbon
conversion in this investigation because two effects
roughly cancel each other out. One effect is the increase
in the number of oxygen functional groups, which increases
the overall char reactivity. The other effect is the
somewhat preferential attack of the "secondary" carbons by
oxygen, which decreases the overall char reactivity.

Intermittent oxidation increases char reactivities
for two reasons. It increases the number of oxygen
functional groups on the char surface, and it increases
the ratio of the armchair carbons to zig-zag carbons. The
rate increase from intermittent oxidation lasts for about
10% carbon conversion, after this the surface oxygen
concentration has declined back to that of the bulk char
and the carbons in the armchair configurations have all

been gasified.

98
5.2. RECOMMENDATIONS

There are several tasks that can be done in the
future to further the knowledge gained in this
investigation. The main concept that needs more
understanding is the cause of the sharp rate decay with
sample conversion during hydrogasification. Several
accomplishments would contribute to this understanding in
the future. One would be to find a technique that
measures the active surface area that is important during
hydrogasification. A very useful parameter for
characterizing char reactivity could result from the
implementation of such a technique. Another task should
be to modify the hydrogasification reactor and analysis
equipment so better initial resolution of rate data is
possible. This would aid in the investigation of initial
rate phenomena tremendously, and contribute to the next
task, transient product analysis. It is not possible to
get transient product analysis accurately with the present
instrumentation, but if it were it could lend great
insight into the phenomena of functional group desorption
which is essential for the formation of surface active

sites.

APPENDIX A

YDR SIFI TI N RATE LATIONS

The method used to calculate hydrogasification rate
of chars in this study involved 11 steps. Before an
experiment, the char was weighed and calibration data
collected. During a reaction, sample collections were
timed and taken at discrete intervals. After the
experiment, the rate was calculated by dividing the amount
of methane collected at each sampling by the collection
time. A plot of time vs. rate was drawn on graph paper,
and manually integrated by counting the number of squares
below the curve up to each individual data point. From
this, percent carbon conversion and instant rate were
calculated. The 11 steps are listed below, followed by an
example.

1. Initial char weight was taken, then the initial weight
of carbon in the char was determined by the following
equation:

Initial Wt. Carbon - (Initial Wt. Char)(% Carbon in Char)

2. A calibration was made to determine the area of a
chart recorder peak for a known amount of methane.
An electronic integrator reported the area under a
peak in terms of counts (# cts. cal.).

3. The amount of methane present in the full calibration
loop at STP was calculated by using the following

equation:
#cc ca, cal. = (1.81 cc loop) (4.9% CH.) (27; KKH740 mmHg)
295 760 mmHg
#cc CH, cal. = 0.0799 cc CH. (STP)

4. The total amount of methane taken in one sample during
the reaction was determined at STP once the
calibration was made and the attenuations were set:

99

10.

11.

100

#cc CH, (STP) = 799 s m l a en sam l
(# cts. cal.)(atten. cal.)

The absolute rate was calculated by dividing the total
amount of methane from a sample collection by the
collection time and the initial weight of carbon in
the sample:

Absolute Rate = 01% cc cg. (STP)]
(Coll. Time)(Initial Wt. C)

The time at which each sample collection was taken was
recorded during the reaction. A plot of absolute rate
vs. time was drawn on graph paper.

A curve was drawn through the data points. The
squares under the curve were counted between the
beginning of the reaction and a given data point, as a
means of manually integrating the curve to determine
the area under the curve that corresponds to the given
data point.

The value of one square on the graph was determined by
the chosen scale:

Block Value = (amount of time’( Abs. Rate ’
unit length unit lengt

The total amount of methane given off per unit weight
of carbon in the char up to a particular data point
was then determined by multiplying the number of
blocks under the curve by the value of each block:

(nu/Unit Wt. Carbon = (Block Value)(# Blocks)

This was converted to % carbon conversion by the
following equation:

% C Conv. = CH Unit Wt. C 12.011 C mol 100%
22,400 cc/mol(STP)

Once the % carbon conversion was known for the
different samples, the instant rate could be
calculated from the absolute rate:

Inst. Rate = (100%)(Abs. Rate)/(100% - % Carbon Conv.)

An example of the values calculated in determining the
instant rate as a function of % carbon conversion during
hydrogasification is given on the following page. The
example is of the base case coal char.

101

Table A.l: BASE CASE COAL CHAR HYDROGASIFICATION
RATE DATA AND CALCULATIONS

Initial Wt. Carbon = (0.19659 Char)(74.72% Carbon)
Initial Wt. Carbon = 0.1468g
# Counts Calibration = 160

Attenuation of calibration set at 8.

 

 

Time

into Counts

Sample Run Coll. of
Number (min) Time Atten. Counts

1 5 1 32 75

2 20 l 32 294

3 37 1 32 173

4 52 1 32 127

5 72 l 16 173

6 82 l 16 154

7 103 1 8 266

8 119 1 8 259

9 135 1 8 217

10 153 l 8 216

 

 

 

Block Value = (5.0 min) (0.1cc CH,/min'g C)

Block Value = 0.5 ccCH4/g C

 

 

 

 

 

Blocks
Sample Abs.1 to % Inst.2
Number Rate Point Conv. Rate
1 1.02 2 0.05 1.02
2 4.00 98 2.63 4.11
3 2.35 207 5.56 2.49
4 1.73 267 7.15 1.86
5 1.18 324 8.68 1.29
6 1.05 357 9.57 1.16
7 0.90 387 10.39 1.00
8 0.88 414 11.11 0.99
9 0.74 440 11.80 0.84
10 0.74 465 12.47 0.84

1. Units [=] cc CH‘/min'g Carbon Initial

2. Units [=] cc CH4/min'g Carbon Instant

10.
11.
12.
13.

14.

15.

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