INTERNAL COMBUSTION ENGINES
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
December 1979
€1,0}^ * ^ •
ACKNOWLEDGMENTS
To Dr. H. W. Parker, chairman of my committee, I extend my deepest
appreciation for his patience and guidance throughout this project. I
would also like to thank the rest of the committee. Dr. S. R. Beck and
Dr. R. W. Tock, for their suggestions and criticisms.
Appreciation is also extended to the Texas Energy Advisory Council,
the Water Resources Center, and the Center for Energy Research for funds
used in this research. Celanese Chemical Corporation is also recognized
for their contribution of packing material for the cooling towers.
I would also like to thank Richard Mergenhagen, Eric Sattler, and
Terry Presley for their help on the project. A very special thanks to
Ms. Sue Willis for her assistance in preparing this paper.
n
Gasification Reactions 3
PAGE
Table 1 . E q u i l i b r i u m Constants f o r Generator Gas Reactions
Table 2. Results of Updraft Gasifier Experiments 63
Table 3. Results of Downdraft Gasifier Experiments 65
I V
PAGE
Figure 4. WOOD DOWNDRAFT GASIFIER 21
Figure 5. UNIVERSITY OF CALIFORNIA-DAVIS GASIFIER 25
Figure 6. UPDRAFT GASIFIER DETAILS 27
Figure 7. PILOT PLANT UPDRAFT GASIFIER 28
Figure 8. SCHEMATIC OF GASIFIER FOR RUN III 31
Figure 9. GASIFIER FOR FIBROUS MATERIAL 36
Figure 10. COTTON GIN TRASH GASIFIER 38
Figure 11. PILOT PLANT DOWNDRAFT GASIFIER 39
Figure 12. DETAILS OF HEARTH 41
Figure 13. DETAILS OF COOLING TOWER AND HUMIDIFIER 43
Figure 14. RESULTS OF RUN Fl 46
Figure 15. GIN TRASH MOVING MECHANISM FOR RUN F4 51
CHAPTER I
Fuels developed from biomass are an important alternative in the
search for new energy sources. In the United States alone there is ap­
proximately 10 BTU/yr available in various forms of biomass. If mar­
ginal lands were used, 10 to 15 times that number could be produced
(Ward, 1977). In using these figures, biomass is considered to be in
the form of wood, agricultural residue and animal and municipal waste.
Energy from aquatic biomass will also play an important role in the near
future.
Combustion of biomass on a small scale for home heatina and on a
large scale for steam and the drying of forestry products is common prac­
tice. Small scale production of mechanical power using biomass is more
difficult, and this is the broad concern of this investigation.
Several methods have been developed to transform biomass into a con­
venient fuel form. At the present time, the most visible method is the
fermentation of biomass to produce alcohol. This product is then mixed
with gasoline to produce "gasohol." Gasohol enjoys considerable popular­
ity, as it is a product which can be seen and used immediately. However,
fermentation may not be economically feasible when compared to other
methods of utilizing biomass. Whereas fermentation alcohol must be pro­
duced in large quantities, air gasification is a method of producing a
low BTU gas from a controlled combustion reaction for individual engines.
This type system should be particularly useful in converting biomass to
mechanical work on a small scale.
1
The form of biomass chosen for this study is cotton gin trash. This
is an agricultural residue which is inadvertently harvested with cotton,
and must be separated from the cotton at the cotton gin. Once collected,
the cotton gin trash is disposed of in a limited number of ways. The
farmer could either spread the gin trash over a field as humus, or feed
it to his cattle as a source of roughage. Any excess is simply treated
as waste. Cotton gin trash consists of small parts of the cotton plant,
cotton burrs, and some cotton. When present in large amounts, gin trash
has a low density and is slightly glutinous.
This investigation is concerned with the possibility of using cotton
gin trash to operate a gas generator, which, in turn, will be used to
fuel a stationary internal combustion engine. This type of operation
would enable the farmer to grow the fuel needed to power his irrigation
pumps. The objective of this investigation is to design, build, and
operate a gasifier which can be adapted to use fibrous agricultural
residues such as cotton gin trash.
CHAPTER I I
Gasif icat ion Reactions
The process of gas i f ica t ion involves several reactions between so l id
carbon and an oxygen-containing gas stream, usually a i r . Among the pro­
ducts from these reactions are the non-combustible gases COp, H^O, and
N2, plus the combustible gases CO, H2, and CH^. In order to maximize the
amount of combustible gases produced, gasi f icat ion takes place u t i l i z i n g
excess carbon. This is in d i rect contrast to the combustion process,
which takes place wi th excess, or at least theore t i ca l , oxygen.
As gas i f ica t ion proceeds, there are two d i s t i nc t reaction zones
formed in the gas i f ie r . The f i r s t zone formed, v^hich involves the com­
plete combustion of the fuel to carbon dioxide and steam, is known as the
oxidation zone. This zone is characterized by the fol lowing react ions:
C + Oo - ^ COo AH = -172.8 ^^^ 2 s — 2 ' " • " lb-mole
C .H^O ^ H^^CO AH = 5 2 . 7 ^ ^ ! i y ^
H ^ . 1/2 02 ^ H2 ^" = - ^ 0 3 . 7 ^ 3 ^
CO + 1/2 O2 ; ^ CO2 AH = -50.9 j ^ ^
The amount of heat given o f f in th is zone is dependent upon the nature
of the f ue l . The nature of the fuel also determines i f other components,
such as other hydrocarbons, t a r s , or su l fur compounds, are present in the
product stream from the oxidation zone. The products from the oxidation
zone then enter an adjacent reduction zone, which is the second main
reaction zone in the gas i f i e r . I t is in th is zone that the combustible
gases are formed, as shown by the fo l lowing equations:
COp + C - ^ 2C0 AH = 70.9 T K ^ ^ ^ ' ^— lb-mole
H 0 + C — ^ Hp + CO AH = 52.7 , .^^^. ^ "^— ^ lb-mole
The f i r s t equation is known as the Boudouard react ion, and the second
equation is the heterogeneous water-gas react ion. These reactions are
related by the homogeneous water-gas s h i f t react ion:
COp + H - ^ CO + H«0 AH = 18.2 ,.^'^^, ^ 2 ^5— 2 lb-mole
In add i t ion , methane may be formed in the reduction zone by a var iety of
react ions, the simplest being:
C + 2H^ — ^ CH. AH = -35.8 T T T S T T
c ^— 4 lb-mole
Methane formation, however, rarely occurs at low pressure. Of course
there are many other reactions which take place in both zones. The equ­
ations which have been given are considered the main react ions, as the
products of the other reactions are present only in small quant i t ies
under normal gas i f ie r operation.
As the thermodynamic data i l l u s t r a t e s , heat is produced in the ox i ­
dation zone by the combustion of carbon, hydrogen, and carbon monoxide.
The amount of heat produced in th is zone greatly affects the ef f ic iency
of the gas i f i e r , as th is is the only heat source for the endothermic
Boudouard and heterogeneous water-gas reactions. The methane formation
reaction is weakly exothermic, however the heat given o f f by th is reac­
t ion is ins ign i f i can t compared to that given o f f by the oxidation reac­
t ions.
The heat effects in the reduction zone can be understood by apply­
ing Le Chatelier's principle. This implies that an increase in tempera­
ture causes a shift toward the side of heat absorption in order to de­
crease the temperature. Since the Boudouard and the heterogeneous water-
gas reactions are both endothermic, an increase in temperature would
cause a shift in the reactions favoring formation of more CO and H2.
The same increase in temperature would tend to discourage formation of
CH^, as the methane formation reaction is exothermic.
A comparison of the products produced by the three reactions is
shown in Figure 1 (Hottel and Howard, 1971). Clearly, it is more advan­
tageous to maintain the reduction zone at high temperatures, since more
combustible gas is produced. Also note that the heterogeneous water-gas
reaction produces more combustible gas, at equilibrium, than does the
Boudouard reaction at identical conditions. This implies that, at high
temperature, it might be advantageous to inject steam into the gasifier
along with air. It also implies that with increasing temperature, the
CO increase is more pronounced than the increase of H2.
Figure 1 only shows the relationship of the product streams in the
reduction zone. It cannot be used to predict composition of final gas
from a gasifier, as the three reactions must be considered to proceed
simultaneously, and therefore influence each other (Gumz, 1950).
Calculation of the final gas composition should be possible if the
equilibrium constant is known for each of the simultaneous reactions.
The direct measurement of the equilibrium is very difficult as gas com­
position and temperature must be determined simultaneously. However,
the equilibrium constants can be determined by thermodynamic methods
60 ^
40 ..
30 ..
20 ..
10 ,.
Figure 1. PRODUCT EQUILIBRIUM COMPOSITIONS
which involve no equi l ibr ium measurements (Smith and Van Ness, 1975).
The dependence of the standard Gibbs free-energy change, AG°, on tempera­
ture may be wr i t ten
./AG°x ^^ RT ^ AH°
R = Universal gas constant
AH° = Standard enthalpy change.
One de f i n i t i on of the equi l ibr ium constant, K, may be wr i t ten
^ - - In K
d In K AH° '' RT2
This equation gives the effect of temperature upon the equilibrium con­
stant. This equation may be integrated, if the standard heat of reac­
tion is known as a function of temperature, as indicated by the equation
In K = i / ^ dT + I
where I is a constant o f in tegra t ion . However, AH° may be wr i t ten as
AH° = AH^ + /*AC° dT 0 »/ p
where AH is another integration constant, which is easily determined if
the standard heat of reaction is known at a single temperature. If each
8
C° is expressed as a power series in T,
where a ., ^. and y - are known constants, then the standard heat of reac­
tion may be written as
AH° = AH + AaT + ^ 4- ° ^ 0 d 3
This may be subst i tuted into the integral equation for the equi l ibr ium
constant to produce
Thermal data, usually in the form of a standard heat of reaction and a
standard Gibbs free-energy change of react ion, may then be used to deter­
mine I . This produces an equation which then relates the equi l ibr ium
constant to temperature.
This method has been applied in order to calculate the equi l ibr ium
constants of the three main reactions of gas i f icat ion (Gumz, 1950). The
equi l ibr ium constant of the Boudouard reaction may be expressed by
log K g = - ^ ^ ^ Y ' ^ ^ ^ " 2.295483 x log T - 0.001208714T
+ 0.153734 x 10"^ T^ + 3.26730
The equi l ibr ium constant of the heterogeneous water-gas reaction is given
by
+ 0.8255484 x 10"^ T^ - 33.45778
The reaction of methane formation has the fol lowing constant of equ i l i
brium:
log K = ^§^M + 3.034338 x log T - 2.09594 x 10'^ T
+ 0.38620 X 10"^ T^ - 13.06361
Values of these equi l ibr ium constants over a range of temperatures are
shown in Table 1 (Gumz, 1950).
Table 1. Equil ibrium Constants for Generator Gas Reactions
T °K log K 21 log K log K
300
500
700
800
900
1000
1100
1200
1300
-20.79759
- 8.74457
- 3.57358
- 1.96315
- 0.71568
+ 0.27808
+ 1.08638
+ 1.75585
+ 2.31863
-15.21416
- 6.56132
- 2.61853
- 1.35825
- 0.37227
+ 0.41694
+ 1.06373
+ 1.60064
+ 2.05385
+9.40146
+3.50016
+0.95261
+0.14392
-0.49209
-1.00754
-1.43432
-1.79367
-2.10013
These equilibrium constants may be used to calculate final gas com­
positions if equilibrium is attained in the gasifier. However, as stated
before, determination of when the equilibrium state exists is yery diffi­
cult. In fact, opinions differ as to whether equilibrium is actually at­
tained, as it may take hours, or even days for complete equilibrium to
be established (Rambush, 1923; Gumz, 1950).
10
The process of gasification must involve physical transport before
the chemical process can take place. In the oxidation zone, as the oxy­
gen carrying stream comes into contact with the burning carbon, all free
oxygen is exhausted to form a mixture of CO2, CO, H2, and H2O. This
free oxygen does not necessarily penetrate to the surface of the fuel,
but rather it is burned at the boundary layer to form more CO2 and H2O
from CO and H2 (Gumz, 1950). This theory is supported by an indirect
proof. One support for this view is the lack of slag formation in the
interior of the fuel bed. If the oxygen penetrated to the surface of
the fuel and, as the primary reaction, was burned to COp, the interior
of the bed, which is well insulated by the rest of the bed, would soon
show temperatures well above those required for slag formation (Gumz,
1950). These temperatures would be so much in excess that even an ex­
tensive humidification of the oxygen carrying stream would not be suf­
ficient to prevent slag formation (Gumz, 1950). Experience has shown,
however, that slag formation can be controlled completely using saturat­
ed air. This implies that the Boudouard and heterogeneous water-gas re­
actions take place at the surface of the burning fuel, since this type
of system would produce lower temperatures. Therefore, the combustion
mechanism by way of gasification occurring at the fuel surface in the
oxidation zone shows good agreement between theory and practice.
Immediately following the oxidation zone is the reduction zone.
Here, too, the gasification reactions are heterogeneous, therefore they
take place at the phase boundary between the solid fuel and gas. The
temperature at this phase boundary, which is very difficult to measure,
governs the rate of reaction. Accordingly, a higher quality gas is
11
formed at the beginning of the reduction zone, where the temperature is
still high. As the distance from the heat producing oxidation zone in­
creases, the temperature of the reduction zone decreases, which corres­
ponds to a slightly decreasing gas quality, as more CO2 is formed (Gumz,
1950). The gas formed at this boundary layer is mixed by diffusion and
convection with the gas of combustion leaving the oxidation zone.
Therefore, the total gas quality increases as the distance from the oxi­
dation zone increases. Heat transfer is analogous to the mass transfer
at the phase boundary. The gasification reactions finally stop (1) when
the CO2 concentration gradient between the gas phase and boundary layer
goes to zero, and (2) when the temperature driving force is evened out
and no more heat transfer takes place (Gumz, 1950).
According to the preceding discussion, it would appear that the
gasifier should be operated at the highest temperature possible in order
to produce a gas of the highest quality. However, the beneficial effects
that the higher temperatures have on the reaction rates of the endotherm­
ic gasifier reactions are soon offset by several important physical con­
siderations. First, such high temperatures are difficult to attain with­
out the added complexity of outside heaters, as heat loss becomes a major
factor. Secondly, at temperatures above approximately 1200 °C, the ashes
formed from the combustion of gin trash begins to melt and form slag
(Schacht and Le Pori, 1978).
Since the gasification process is composed of both chemical and
physical reactions, even the accelerated reaction rates at high tempera­
tures may not result in the optimum gas composition. In general, the
rate of chemical reaction is so fast above 900 °C that it is negligible
12
when compared to the transport phenomena. Below 600°C the rate of chem­
ical reaction is slow compared to the transport phenomena (Gumz, 1950).
Therefore, it appears that the optimum temperature inside the gasifier
is in the range of 700°C - 900°C (Gumz, 1950; Reed and Jantzen, 1979).
Types of Gasifiers
In order to reach these optimum reaction conditions, several reac­
tor types have been developed since the last century. The first type,
which was developed in the middle 1800's, is known as the updraft gasi­
fier (Rambush, 1923). Then came the downdraft gasifier, which soon be­
came quite popular. There is also a cross-flow gasifier, and several
variations on the previously mentioned gasifiers (Reed and Jantzen,
1979). This research utilized the two simplest designs, those being
the updraft and downdraft types.
The updraft gasifier was the first type used in this research (Fig­
ure 2). In this type of design, the air enters the gasifier near the
bottom of the fuel container. The air then enters the bed of fuel and
a burning zone is created, which contains both of the reaction zones
mentioned earlier. The gas formed then continues upward through the
unburned portion of the fuel bed. This hot gas has the effect of drying
and preheating the unburned fuel. As the burning proceeds, the entire
fuel bed is pulled down by gravity, thus creating a stationary combus­
tion zone. After the gas passes through this moving fuel bed, it exits
the gasifier.
The main disadvantage of the updraft type of gasifier is the ex­
cessive production of tar. The updraft gasifier yields tar-free gas
13
Drying &
Preheating
Reduction
•> Gas
14
only from fuels such as coke, anthracite, or charcoal (Reed and Jantzen,
1979). It therefore seems that the type of fuel used is one factor
which should be utilized in the design of a gasifier. Since the gas
used to operate internal combustion engines should be as clean as pos­
sible, the gas produced by an updraft gasifier would have to undergo
extensive cleansing before use in such an engine if the fuel used is
high in tar content.
As an alternative to the updraft gasifier, the downdraft gasifier
was developed (Figure 3). In this arrangement, the air is introduced
into the middle of the fuel bed by a central nozzle or a number of noz­
zles in an annular configuration. Immediately below the air inlet is
the hot oxidation zone. As previously stated, some of the heat produced
in this zone is used by the endothermic reactions in the reduction zone,
which is immediately beneath the oxidation zone. However, some of this
heat is utilized in a drying area which is above the oxidation area.
Charring of the fuel takes place in this area, which results in solid
carbon and a tar fog. These two products then pass into the hot oxida­
tion zone, where the carbon provides fuel for combustion, and the tar
fog is consumed by this combustion. The gases produced in the oxidation
zone then pass through the reduction zone, and then exits the gasifier
as a combustible gas.
Since the tars are consumed in the oxidation zone, the downdraft
gasifier may be operated using a variety of fuels. However, the reduc­
tion zone places two restrictions on the fuel composition (Overend,
1979). First, when burned, the fuel should form a fairly strong car­
bonized structure that is large enough to prevent excessive pressure
15
Air
16
drop through the reduction zone. Secondly, because of the large heat
effects associated with the production of water vapor, the water content
of the fuel should be limited to around 25 percent (Gumz, 1950).
History
The use of gasification technology to produce combustible gases is
not a new concept. Gasifiers were used as far back as 1861 to fire furn­
aces in the iron working industry (Rambush, 1923). Between the years
1879-1881, J. E. Dawson, in England, developed a cooling and cleaning
process, and showed that gas engines could be powered by a gasifier
(Rambush, 1923). Most of these early gasifiers were of the updraft var­
iety. However, a downdraft gasifier was described as far back as 1843
in Sweden by Gustaf Ekman (Reed and Jantzen, 1979). Also, most of the
early gasifiers used coal or coke as a fuel. It was not until the early
1920's that gasifier technology had grown sufficiently to include the
many different forms of cellulosic fuels (Horsfield, 1979).
The development of gasifiers in Sweden is an interesting case study.
The following material was taken from the text edited by SERI (Reed and
Jantzen, 1979). In 1918, Axel Swedlund of Sweden designed an updraft
charcoal generator, which was followed in 1924 by the first of his down-
draft designs. During 192o and 1924, several experiments were conducted
using updraft gasifiers on trucks, buses, and rail cars. One experiment
consisted of driving a truck 624 kilometers. However, due to the high tar
content of the gas, the engine had to be removed and cleaned after 320
kilometers. The results of these experiments showed that, although the
use of gasifiers to power internal combustion engines was possible, it
17
was not convenient at that time. In general, start-up was very difficult
when the gasifier was the only fuel available. Also, during that period,
the engines had relatively little power even when using liquid fuel.
The first downdraft charcoal gasifiers appeared on automobiles in
the late 1920's. Soon afterward an interest developed in wood as a gas­
ifier fuel. This fuel was tried out in trucks in the 1930's, but there
were considerable amounts of tar deposited in the motors. This led to a
lack of consumer interest in this type of arrangement.
Bills were introduced in 1930 in the Swedish Parliament to support
gasifier research. In the following year a government committee was ap­
pointed on the initiative of Ingeniorsvetenskapsakademien to perform
scientific and practical experiments with gasifiers. In 1932 the Parlia­
ment appropriated money for a loan fund for car owners who wished to in­
stall gasifiers. In addition, a tax break was offered for vehicles
powered by gasifiers. These measures led to a rapid increase in the
number of gasifiers installed over the next year. However, due to the
lack of experience of both manufacturers and operators, public opinion
regarding gasifiers rapidly declined. Even in cases where the gasifier
gave good performance, the convenience of liquid fuel made it more at­
tractive than operation with gasifiers.
With the increasing political tension of the late 1930's came an
increased interest in military preparedness. A new government committee
was formed in 1937 to review the progress made in gasifier design. In
general, it was found that significant improvements had been made in
both gasifier design and adaptation of engines for use with gasifiers.
18
I t was also found that the number of cars in Sweden had increased during
th is t ime.
This increase in the car f l ee t might have continued into the early
1940's but for the outbreak of World War I I , which was accompanied by a
commercial blockade. At th is t ime, the opportunity to buy l i q u i d fuel
was severely res t r i c ted . These circumstances led to an intensive gasi­
f i e r research program during the war. During the las t years of the war,
gas i f ie r technology had advanced to the point where i t was considered
an adequate subst i tu te fo r l i q u i d f ue l . When the war ended, however,
l i q u i d fuels once again became p l e n t i f u l , and i t was not economically
feasible to continue gas i f ie r research.
The downdraft type gas i f ie r was the most predominant type used in
Sweden during the war. Therefore, most of the research involved only
downdraft type gas i f i e rs . Such research led to a l i s t of general desir­
able features, which may be summarized as fo l lows:
1) Simple and safe design.
2) Low weight ( for non-stat ionary).
3) Simple and inexpensive i n s t a l l a t i o n .
4) Simple maintenance with easi ly accessible parts.
5) Capabi l i ty of easi ly changing parts.
6) Good energy requirements.
to both gas production and the requirement of clean
and tar-free gas during all load conditions as well as
prolonged idling.
motor efficiency is obtained.
9) High heat value of gas produced, and easy ignition
in an air mixture.
11) Easy adaptation to various motor efficiencies and
operating conditions.
12) Short start-up time.
The ease in which these requirements are attained is highly depend­
ent upon the type of fuel. The type of fuel also determines, to some
extent, the design of the gasifier. A charcoal gasifier is not design­
ed to crack the tars produced when wood is used as a fuel. Also, due
to the high heat produced when charcoal is burned, the exposed parts of
a charcoal gasifier must be resistant to high temperatures. Conversely
the most critical parts of a wood gasifier are the fuel container and
hearth (Reed and Jantzen, 1979).
The advantages of a charcoal gasifier are fairly obvious. Since
yery little tar is produced, cleaning of the gas is comparatively simple.
Also, gas produced from charcoal is relatively free from water and other
corroding components. However, it is necessary to use uniform, high
quality charcoal in order to achieve the best results from a charcoal
gasifier. This should be remembered when economics are considered.
The wood gasifier has the advantage of lower fuel costs. Indeed,
it is even possible for the owner to prepare his own fuel. However,
due to the detrimental substances found in wood gas, the design of a
20
wood gasifier is more complex. The deciding factor in wood gasifier de­
sign is the tar content of the fuel, as these tars must be consumed be­
fore the gas leaves the hearth (Reed and Jantzen, 1979). Since the
fuel chosen for this study more closely resembles wood than charcoal,
the design parameters of the wood downdraft gasifier will be considered
in more detail.
From a functional point of view, the wood gasifier consists of
three main sections: (1) an upper section, which is known as the fuel
container, where drying and charring takes place; (2) a lower section,
which contains the hearth, where both oxidation and reduction reactions
take place; and (3) a device for supplying primary air. The gas outlet
and ash collection area are located in the same zone as the hearth.
The fuel container usually determines the size of the gasifier.
Several elaborate schemes have been devised to provide for drying of the
fuel (Reed and Jantzen, 1979). As to whether these devices are really
efficient remains to be determined. In any case, the size of the fuel
container is an important parameter only in the case of non-stationary
gasifiers. Since this investigation involves only stationary gasifiers,
the size of the fuel container was really not that important.
After leaving the fuel bed, the charred fuel then enters the
hearth, where the important gasification reactions occur. The hearth
is located underneath the air nozzles, which are directed towards its
center. The hearth is usually conical in shape, and constricted down­
ward (Figure 4). It was found that this configuration promotes the high
temperature necessary for total combustion and cracking of the tar
gases (Reed and Jantzen, 1979).
21
22
is accomplished within a relatively small volume, since the temperature
is so high due to the close proximity to the oxidation zone. Associated
with the charring and oxidation processes is a reduction in the size of
the fuel. Ideally, the cone angle should be selected so that the fuel
moves smoothly from one zone to the next, in order to optimize gas
quality (Reed and Jantzen, 1979).
As stated before, the hearth is cone shaped to promote high tempera­
tures in order to decompose tar gases and promote combustion. These
high temperatures place a great thermal strain on the material used to
construct the hearth, which should be considered during design of the
gasifier. The more pronounced the cross section reduction of the cone,
the greater the temperature increase (Reed and Jantzen, 1979). Thus the
tendency is to make the opening relatively small. This, however, creates
a greater flow resistance with the resulting greater pressure drop. The
development of a method to design a hearth which meets these criteria
was one of the contributions of the Swedish designers during World War II
(Reed and Jantzen, 1979). The discussion which follows was again taken
from the book edited by SERI.
In order to correctly dimension a downdraft wood gasifier, the con­
cept of a hearth load is introduced. The hearth load is the quantity of
prepared generator gas, reduced to standard cubic meters per hour, divided
by the smallest passage area of the hearth. It is customarily designat­
ed as B. and the units are expressed as a numerator and denominator
-D 3
(m /cm - h r ) . The imaginary velocity of the prepared gas in i t s normal
23
state through the smallest area of the hearth is designated as V. (m/s).
The following relations are obtained from the definition of the hearth
load.
3^ = 0.36 y^ (m^/cm^-hr)
B. F l e x i b i l i t y = -HJOM
h min
The hearth load provides a maximum loading, B. , and a minimum, ^ h max
^h min' ^^^^""^^ ^"^ which gas i f i e r performance is unacceptable. The re­
la t ion between B^ ^^^ and B^ ^^.^ is known as the f l e x i b i l i t y of the gas­
i f i e r . As great a f l e x i b i l i t y as possible is desirable, and the numeri­
cal value of the f l e x i b i l i t y becomes an operating qual i ty parameter.
Tests on V-hearths have shown that B. „^ reaches about 0.9 in con- h max
tinuous operat ion, while B. . has been less than 0.2. An eight hour
test wi th B, = 0.05 on a V-hearth wood downdraft gas i f ie r from the Swed­
ish Generator Gas Company during operation of a 2-cycle engine has been
carr ied out without abnormal ta r content in the gas. For th is t es t , a
f l e x i b i l i t y of 18.0 was obtained, which appears to be a unique value.
This f l e x i b i l i t y value shows the great adaptabi l i ty of th is type of
gas i f ie r .
Below the constr ic t ion in the hearth is the reduction zone. In
order to increase the time during which the gas is in the reduction zone,
there is usually a sudden increase in the cross-sectional area. A grate
is usually used as a support for the hot coals which make up the reduc­
t ion zone, although i t may not be required (Reed and Jantzen, 1979).
24
Contemporary Gasifiers
Due to the recent oil shortage, gasifier technology is enjoying a
renewed interest. Small, portable gasifiers are being developed with an
emphasis on simplicity and reliability of operation. Large stationary
gasifiers are being developed on pilot plant scale using different fuel
types.
A great deal of research has been carried out in the Department of
Agricultural Engineering, University of California - Davis, on a wood
downdraft gasifier. The pilot plant gasifier (Figure 5) is approximate­
ly six feet in diameter and eighteen feet tall (Goss, 1979). This gasi­
fier has been operated for a total of 705 hours using various fuels.
The continuous feed enabled the gasifier to be operated for approximately
329 hours continuously with good results (Goss, 1979).
25
Condensate Out
CHAPTER III
The Updraft Gasifier
The main objective of this investigation was to develop a simple
process to gasify cotton gin trash. The first design chosen was the up­
draft gasifier, because of its simple design and, large stationary gasi­
fiers have been successfully operated on crop residue (Goss, 1979).
The original design of the pilot plant updraft is shown in Fig­
ure 6. The gasifier is 3.3 m tall and 0.61 m in diameter. It is con­
structed from carbon steel, except the grate which is made of 304 stain­
less steel. It was not expected that carbon steel would be satisfactory
for the highest temperature portions of the gasifier for long-term oper­
ation, but it was adequate for short-term tests. Only the 0.5 m portion
of the gasifier body near the grate was insulated for the early experi­
ments. The remainder of the gasifier was left uninsulated so that the
gases could be partially cooled and a portion of the tar removed by gin
trash retained at the top of the gasifier by the removable horizontal
spikes.
The layout for the pilot plant facility is shown in Figure 7. The
gas passes through a cooler after leaving the gasifier, which makes
handling easier. The gas then enters the knockout pot to remove the
tar and water which condenses out in the cooler. The gas then passes
through a conventional pleated paper carburetor air filter in an effort
to remove any remaining traces of tar mist. The gas is then mixed with
the desired portion of air for engine operation. The engine is a
26
27
Insulation
Grate
28
29
6 cylinder, nominal 100 horsepower engine, Ford 200 CID. Attached to
the engine is a dynamometer which measures engine performance.
Instrumentation consists of Type K thermocouples in 6.3 mm inconel
sheaths. Pressure drops and flow rates through the segments of the ap­
paratus are measured manually.
During the first test, the engine was operated for approximately
20 minutes exclusively on producer gas at approximately 2000 rpm and
power of 10 to 20 horsepower. Subsequent examination of the engine re­
vealed that the tar removal system was entirely inadequate to protect
the engine from tar contamination. Also the gas cooler was inadequate.
Examination of the gin trash remaining in the gasifier showed that an
irregular channel about 100 mm in diameter was burned starting at the
grate and continuing upward almost to the exit pipe.
Due to the nature of the updraft gasifier, it was expected that tar
would be produced during the experiment. The older literature reported
that depth filtration through beds of fine biomass of the cooled smoke
was effective for the removal of tars (Rambush, 1923). Apparently, the
tar mist formed is so fine that the fog moved through the bed of gin
trash and the knockout pot with little effect. Also, it was felt that
the channeling of the combustion zone through the fuel bed was detri­
mental to the efficiency of the gasifier.
Several changes were made in the apparatus before Run II. First,
an air distributor was added to divide the inlet air into four streams.
It was felt that this type of arrangement would cause four separate
channels to form in the fuel bed. This excessive channeling would then
loosen the structure of the fuel bed, thus causing the bed to collapse
30
upon itself. Also, changes were made in the tar removing apparatus. A
more efficient condenser was placed near the gasifier to cool the outlet
gas. The gas then went through the knockout pot and into a particle im-
pinger in an effort to remove the extremely small tar droplets. The gas
then entered another bed of biomass composed of alternating cotton gin
trash and finely ground mesquite beds. It was felt that by adding the
layers of finely ground mesquite, the tortuous path of the gas was in­
creased, therefore removing a greater percentage of the tar mist than
just gin trash.
Lighting the gin trash at the start of Run II proved to be quite
difficult. In the previous run, a small torch was used to light the gin
trash. In this run, the air distributor effectively blocked a portion
of the gin trash from the torch. Therefore, some areas of the gin trash
were probably not ignited, which would influence the formation of chan­
nels. The gin trash was assumed to be ignited when smoke became visible,
and the ignition door was closed, and the air was turned up to approxi­
mately 50 cfm, which was considered operating conditions for the rest of
the experiments. The smoke issuing from the second bed of biomass was
observed to contain tar. This was observed by placing a piece of fiber­
glass insulation over the gas outlet. If the insulation changed colors
to a dark brown, it was assumed that the tar mist was present in the gas
stream. It was decided that, in order to protect the engine, the engine
would not be operated if the presence of tar in the gas was suspected.
A venturi was added for Run III. A schematic of the system is shown
in Figure 8. The purpose of the venturi was to accelerate the particles
of the tar mist in order that they might coalesce or hit other obstacles
Cooler
31
Impinger
Figure 8. SCHEfWIC OF GASIFIER FOR RUN I I I
32
and stick to them. The diameter of the venturi throat was 0.5 in. The
cotton gin trash and ground mesquite bed was exchanged for a thick bed
of fiberglass insulation. It was felt that since the fiberglass was
used to detect the presence of tar, fiberglass might make an effective
filter.
The propane torch was used to light the gin trash. This method
proved to be not only unsatisfactory but also unsafe due to gas accumu­
lation. The fiberglass filter proved to be ineffective, probably due to
excessive channeling up the sides of the bed. Inspection of the fuel bed
showed that most of the gin trash was consumed, although the effects of
channeling were clearly evident.
At this point it was learned that other research projects had been
successful in using very high pressure drop venturi scrubbers for the
removal of tars and smoke (Mosely, 1978). The reported pressure drop
was 10 pounds per square inch differential (psid). The venturi which
was installed for Run III was converted to a wet scrubber for Run IV.
Also, a new filter replaced the filter bed. This filter is simply a
large mass of fiberglass insulation strapped over the gas outlet. An
electrical charcoal starter was purchased and installed to ignite the^
gin trash.
Run IV consisted of testing the wet scrubber without lighting the
gin trash. This test only showed that, with the air turned on to oper­
ating conditions, the pressure drop could be adjusted by varying the
rate of water flow into the scrubber.
No physical changes were made in the apparatus for Run V. The main
objective of this run was to test the effectiveness of the new electric
starter and wet scrubber. The electric starter seemed to work very
33
well, and its use was continued while the updraft gasifier was being used,
Some channeling probably occured due to localized heating. The reported
values for the wet scrubber were qualitatively confirmed at a produced
gas flow rate of approximately 40 cfm and a liquid circulation rate of
0.2 GPM. At a pressure drop of 8 psid there was obviously smoke present
at the gas outlet. However, at a pressure drop of 10 psid, there was no
obvious smoke present. The penalty paid for using a high pressure drop
venturi is that about 5 percent of the engine horsepower must be used to
power the scrubber. Also, a significant investment, one to two thousand
dollars at this time, must be spent for a small positive displacement
blower to force the gas through the scrubber. If the blower is operated
on air supplied to the gasifier, the entire gasifier must be operated
at a pressure of 10 psid. For all these reasons, the use of the high
pressure drop venturi is not particularly attractive.
One significant occurrence which occurred during Run V was that,
towards the end of the run, the gasifier began to glow cherry red in a
localized area. This area was approximately 1 foot in height and extend­
ed about a quarter of the circumference of the gasifier. This glowing
area began about 1.5 feet above the grate, thus indicating that the gin
trash was not flowing downward, and channeling v as taking place.
When the fuel bed was inspected, it was found that the gin trash
had indeed remained in place, with channels moving upward. In this
case, the channels formed by the air distributor combined, and therefore
produced a relatively small channel with a large amount of air movement.
The small channel and large volume of air produced the necessary tempera­
tures to heat the gasifier hot enough to glow.
34
The gin trash was not ignited for Run VI. This test involved a
demonstration of a variable throat venturi, which was added to further
improve the efficiency of the scrubber.
The gin trash was ignited for Run VII without any changes in the
apparatus from Run VI. No trace of tar was found at any time during
Run VII which indicated that the variable throat scrubber was performing.
Once again it appeared that severe channeling occurred, as the gasifier
was heated enough to glow red. A gas sample was collected to be analy­
zed by gas chromatography, and the experiment was terminated prematurely
in order to preserve a physical record of the combustion process.
Examination of the fuel bed revealed a large amount of unburned gin
trash at the top of the vessel. About 2 feet from the grate, a large
cavity was observed, which continued back down to the grate. The re­
sults from the gas sample taken showed that the gas was composed mainly
of CO2, O2, and N2, with insignificant amounts of Hp and CO.
The objective of Run VIII was simply to duplicate Run VII, plus
obtain more gas samples. Gas samples were collected during start-up,
immediately after peak temperatures were observed, and before the run
was terminated. The combustion process was not terminated prematurely.
Also, the gasifier did not get hot enough to glow on this run. There
were traces of tar present in the outlet gas.
When the fuel bed was inspected, very little unburned gin trash
was found. Although a large quantity of ash was present, no traces of
slag was found. All three gas samples contained significant amounts of
COp, O2, and N2, with traces of H2 and CO. These traces of combustible
35
gases were c lear ly not enough to consider using the gas i f i e r to power
the engine.
At th i s po in t , i t became obvious that ta r production and the chan­
nel ing of the combustion zone through the fuel bed would continue to be
a problem with the updraft gas i f i e r . Excessive ta r production is inher­
ent with th is type of design when a wood-like fuel is employed. There­
fo re , the main problem involving tar-production is the removal of the
ta r mist from the gas stream pr io r to in jec t ion in to the engine. This
proved to be considerably more d i f f i c u l t than ant ic ipated, due pr imar i ly
to the apparent sub-micron size of the ta r droplets. For th is reason,
the most common type of gas i f i e r for small engine applications is not
the updraft gas i f i e r , but the downdraft gas i f i e r , in which most of the
ta r and smoke are caused to flow through the hot gas i f icat ion zone and
thereby be destroyed. However, these downdraft gasi f iers are not s u i t ­
able fo r f ibrous biomass residues, such as gin t rash , since i t w i l l not
move f ree ly through the constr ict ions in the lower port ion of the normal
downdraft gas i f i e r . This f ibrous nature also aggrevates the tendency to
channel, as the gin trash is prevented from sloughing into the channel
as would granular so l ids .
Cons •'deration of these factors lead to the design of several con­
siderably more complex gas i f i e rs . One such system is shown in Figure 9.
This system would require that the gin trash be moved from a large stor­
age area by a conveyor bel t into a small channel where counterflow com­
bustion occurs. The resul t ing glowing coals are then pushed over the
rest ra in ing baf f le into an area where gas i f ica t ion takes place. Af ter
consideration of the mechanical d i f f i c u l t i e s , not to mention the
Air
37
economic aspects, of such a system, the sponsoring agencies and the in­
vestigators chose to remain with the simple batch gasifier and to make
additional efforts to adapt it to fibrous fuels.
Downdraft Gasifier
The next type of design considered was the downdraft gasifier. This
type of gasifier is known to be successful in operating small engines
when wood is used as a fuel. The conversion to a downdraft design in­
volved installation of air injection nozzles and a hearth zone in the
updraft gasifier used previously. The piping to the vessel was then al­
tered in order to have the gas outlet at the bottom (Figure 10).
Figure 11 illustrates how the hot gas exiting the gasifier will have
ash and char fragments removed from it, then cooled by direct contact with
water being circulated through the small packed tower. The resulting hot
water will be used to humidify the injected air. By saturating the in­
jected air, the temperature in the gasifier will be controlled so that
ash fusion will not occur. These techniques of cooling the gas and pre­
venting ash fusion were practiced in the operation of gasifiers at the
turn of the century (Rambush, 1923).
The ram shown in Figure 10 was the initial modification to assist in
the downward motion of the fibrous bed. It consisted of a 2 in. pipe
with movable aluminum arms attached in such a way as to be fully extend­
ed on the downward stroke of the ram, and lie flat on the upward stroke.
The reciprocating motion of the ram is achieved through the use of a
pneumatic cylinder. This cylinder was connected to an air line with 90
psig available, which enabled the cylinder to apply a force of
38
Plunger to assist downward movement of Gin Trash
Arms attached to plunger. Fold flat when plunger goes up and swing out when plunger goes down.
Gas Outlet
39
Mechanism to Reciprocate Plunger In Gasifier
Air and: Steam <
Ash
Pump
40
approximately 180 pounds. This type of construction was designed to
break up any cavities formed while ensuring a continuous downward motion
of the fuel bed.
The hearth diameter was chosen using the concept of the hearth load
outlined earlier. Using the reported values of B^ ^ and B. . for a
V-hearth, a mean value of B^ = 0.475 was chosen. This value showed that
the diameter of the opening in the hearth should be 7 in. An appropriate
cone angle was then chosen. A detailed drawing of the hearth and its
dimensions are given in Figure 12.
High temperature refractory concrete was chosen for the hearth
material. Refractory concrete has good insulating quality plus being
durable at the constant high temperatures. Most of all, it was easy to
form into the cone shape required for the hearth. Detrimental effects
due to thermal cycling in the refractory concrete during the course of
the investigation were not observed.
The diameter of the air nozzles was chosen to be 0.75 in. Four air
nozzles were placed approximately 4 in. above the top of the refractory
concrete at regular intervals around the circumference of the gasifier.
An attempt was made to make the air nozzles as symmetrical as possible
in order to ensure even air distribution into the combustion zone. Air
lines of equal length connected each air nozzle to an outlet in the humi­
difier. Not only did this help ensure symmetry, but this action also
prevented any high pressure from forming in the humdifier and inhibit­
ing the flow of water from the low pressure cooling tower.
Each air nozzle extended to within 3 in. of the constriction in the
hearth. It was felt that a system such as this would concentrate the
41
42
extremely hot oxidation zone immediately below the air nozzles, at the
location of the thickest insulation in the hearth. All the air which
entered the gasifier would then stand a better chance of passing through
the oxidation zone if this zone occurred at the constriction in the
hearth. This gas formed would then have to pass through the layer of
glowing coals supported by the grate, and thus undergo reduction. The
thickness of the bed of coals may be adjusted by varying the level of
the grate.
The design of the cooling tower and humidifier was fairly straight­
forward. Experience had shown that a packed bed height of 3 ft. provid­
ed enough heat transfer to cool the gas sufficiently. The tower dia­
meter was determined by calculating the flooding rate. Air was assumed
to enter the humidifier at 75 SCFM, and water, at the maximum flow rate,
entered the cooling tower at a rate of approximately 3800 pounds per
hour. The maximum temperature of the gas produced was assumed to be
600°C. The initial calculations were carried out using data for 0.5 in.
Berl saddles, as these were readily available (Treybal, 1955). These
calculations produced a tower diameter of 1 foot. This calculation was
carried out only for the cooling tower, however, it was assumed that the
tower height and diameter were adequate for the humidifier. After the
tovyers had been constructed, it was found that the particle size of the
packing was too small for the tower diameter. Therefore 1 in. Berl sad­
dles were used instead. With the increased packing size, the calculated
tower diameter decreased. Therefore the tower diameter calculated for
the smaller Berl saddles was sufficient for the large Berl saddles.
One advantage of a cooling tower of this design is that it also
acts as a scrubber which is effective in removing any loose particulates
43
formed by combustion. A detailed drawing of the cooling tower and humidi­
fier is shown in Figure 13. There was only enough Berl saddles to have a
packed height of about 1.5 feet in the humidifier. It was felt that
this was still sufficient to adequately humidify the inlet air, since the
incoming water was heated.
Thermocouples were placed at three-inch intervals up one side of the
hearth. Using these thermocouples, it was possible to know the general
location around the perimeter of the constricted portion of the hearth
to show temperature variations in this area. One thermocouple was loca­
ted just above the grate, to measure the temperature of the reduction
zone, and one thermocouple was used to measure the outlet gasifier temp­
erature. The temperatures of the entering and exiting streams of the
cooling tower and humidifier were also monitored.
For each of the runs made with the downdraft gasifier, the air rate
was increased slowly during lighting procedures. When smoke became ap­
parent, it was assumed that the trash was ignited. The air was then ad­
justed to the required rate.
The objective of the first experiment with the downdraft gasifier,
Fl, was to test the performance of the new apparatus. The gin trash may
have been damp, as it was difficult to ignite. When ignition was ac­
hieved, the air rate was slowly increased to approximately 20 SCFM. The
temperatures in the hearth showed a slow increase which was expected.
However, a thermocouple near the top of the hearth showed the first
drastic increase, which definitely indicated that a channel was moving
up the side of the hearth, above the level of the air nozzles. The temp­
erature in this area rose to a maximum of 460°C and began to decline
rapidly. After this decline began, a maximum temperature of 680°C was
44
Hot Gas From Gasif ier
4 Outlets for A i r Entering Gasif ier From Humidifier
Initial Air 7
Initial H2O Supply
1.5 ft. void
45
observed in the reduction zone, which was followed by a rapid decline.
The temperature in the constriction of the hearth never rose above
280°C. The outlet gas temperature was below 150°C. It was obvious
that the temperatures were not sufficient for the gasification reactions
to proceed, and the run was terminated.
The ram was utilized when the temperatures began to decline. The
action of the ram caused a reversal of the declining trend for a short
time. However, temperatures began to decline regardless of the action
of the ram.
Inspection of the fuel bed revealed that wery little of the gin
trash was consumed. This was expected due to the short run time. The
gin trash appeared not to have been disturbed from the top of th,e re­
actor to about 2 feet above the constriction. It was at this point
that a large hole appeared in the area surrounding the ram. It was
apparent that the ram had dug a small hole in the fuel bed, then it
ceased to have any effect. A cavity was discovered on a level even
with the air nozzle outlets (Figure 14). It appeared that the gin
trash which had occupied this area, and the trash which was pushed
down by the ram, had undergone complete combustion. The ash formed
then passed through the grate into the ash bin.
The interface between the unburned gin trash and the cavity ap­
peared to be quite strong, which is probably the reason that no more
gin trash moved into the combustion zone. This boundary seemed to con­
sist of about 1 in. of blackened gin trash which appeared to be stuck
together possibly by condensed tars. VJhen it was attempted to remove
this boundary, it was observed that large sections of the gin trash
stuck tenaciously together, and had to be broken up by hand.
46
Plunger
47
The objective of Run F2 was to determine if the results of the
first trial could be duplicated. The gin trash was considerably easier
to light. This time the temperatures increased in the order expected.
Since the gin trash is ignited under the grate, the reduction zone temp­
erature should rise first, followed by the temperature in the constrict­
ed area of the hearth. This was the case in this trial. The ram was
operated continuously during this run, completing a cycle in approxi­
mately 2 minutes. Air was once again initially set at 20 SCFM once
ignition occurred.
The temperatures rose rapidly, after the air flow was increased,
to reach peak values, then they began to decline, much in the manner
as that seen in Run Fl. The area even with the air nozzles reached a
maximum temperature of 590°C. When this temperature began to decline,
the air flow was increased to approximately 30 cfm. This action caused
the temperature in the reduction zone to suddenly increase from 350°C
to 710°C. The sudden effect of the air blast soon diminished, and this
temperature began to swiftly decline. Soon after the rate of decline
had decreased, the temperatures of the hearth and reduction zone in­
creased suddenly. The temperature of the oxidation zone increased from
300°C to 590°C, the constricted area increased from 170°C to 370°C,
and the reduction zone increased from 250°C to 500°C. Here it became
apparent that this run was beginning to duplicate the previous one,
since all temperatures were declining, and the trial was terminated.
Although higher temperatures were attained in this run, they were clear­
ly not sufficient.
Inspection of the fuel bed revealed the same combustion pattern
as Run Fl. The cavity extended farther up into the fuel bed, but this
was probably caused by the increased air flow rate. The oscillatory
action of the temperature was probably caused by unburned gin trash
falling into the cooled combustion zone, thus providing more fuel for
oxidation. This creates more heat, which reforms the boundary between
the unburned gin trash and the cavity.
Since no fuel was moving into the cavity, it was felt that if the
size of the cavity was increased, it would collapse due to the weight
of the fuel bed. The increase in the cavity size was accomplished by
shortening the air nozzles to a length of approximately 2 in. This
put the end of the nozzle at about 2 in. above the edge of the hearth.
This would cause the cavity to extend the diameter of the hearth, thus
weakening the ceiling of the cavity. Another advantage to increasing
the size of the cavity is that the area which is available for the oxi­
dation zone is also increased, thus making it possible to have a larger
oxidation zone.
The objective of Run F3 was to study the effects of shortening the
air nozzles. Fiberglass insulation was applied to the exterior of the
gasifier to cut down on heat losses. In addition, only the temperatures
in the gasifier were recorded, since until these temperatures reach op­
erating conditions, the full performance of the cooling tower cannot be
evaluated. The ram was operated every 30 seconds at the beginning of
the trial.
The gin trash was easy to ignite for Run F3. After ignition was
confirmed, the air flow was increased to 25 SCFM and kept there for the
49
remainder of the run. Temperatures as a whole were higher than the pre­
vious runs, however, the fluctuating tendency seen in the previous runs
was again observed. The maximum temperatures were 620°C and 810°C at
the constriction of the hearth and the upper area of the hearth respec­
tively. These peaks did not coincide. However, the maximum tempera­
ture in the reduction zone of 560°C did coincide with each of these
peaks. The outlet gas temperature never exceeded 410°C.
Immediately following the temperature peak of the constriction,
when the temperatures were decreasing, the plunger was operated contin­
uously. This had a temporary beneficial effect. When temperatures
began to decrease once again, the sides of the gasifier were struck
with a hammer in an effort to cause the fuel to move into the cavities
which were obviously forming. Immediately following this action, the
maximum temperature of the upper hearth was attained. The temperatures
began to decrease at this time, and the run was terminated after a gas
sample was collected.
When the fuel bed was inspected, it was found that more gin trash
had been burned than in previous runs. The gin trash seemed to move
downward at a uniform rate and was loosely packed. The plunging action
by the reciprocating ram apparently had a mixing effect, as pieces of
burned gin trash were found at the top of the fuel bed. Once again
there was a rather large cavity formed below the air nozzles, which
means that the cavity was somewhat larger than in previous runs. There
was again evidence that the plunger had moved the adjacent gin trash
into the reaction zone, and then ceased to be effective. This action
did not appear to have influenced the formation of the cavity. The
50
gas produced remained a dense, white fog throughout the run, possibly
indicating the presence of tar. In order to consume these tars, com­
bustion should be more controlled. The gas sample collected at the
end of Run F3 indicated large amounts of CO2, N2, and O2 when analyzed
by gas chromatography.
After consideration of these results, it was decided that the rea­
son that continuous high temperatures in the reaction zones are not
realized is that the gin trash does not flow freely into the zones of
combustion. A new stirring device was proposed to be used in conjunc­
tion with the ram (Figure 15), This device will have one extension
which will agitate the gin trash. It should also be able to rotate
360° and be able to move in and out of the gasifier to make room for
the ram. The temperature of the stirrer should be monitored.
The objective of Run F4 was to operate the stirring device and
study its effects. The air was again set at 25 SCFM, once ignition was
confirmed. Each thermocouple showed a series of peaks and depressions,
none of which appeared to be related. For example, the temperature at
the constriction of the hearth showed peaks early in the run of 600°C
and 720°C. However, the temperature of the reduction zone, which is
immediately below the constriction showed maximum temperatures before
the constriction early in the run of 580°C, and then \/ery late in the
run, of 800°C. This second peak did not appear to be any way related
to the second peak which occurred at the constriction in the middle of
the run. Overall, the temperatures appeared to rise and fall in an
arbitrary manner. Also, the overall range of the temperatures did not
appear to be affected by the alternating action of the ram and stirrer.
51
Air
Insulation
52
When the temperature of the reduction zone peaked at 900°C, at
which time it was the maximum temperature in the gasifier, the gas ob­
served at the outlet was characteristic of water gas in that it was a
light blue in color. This was in direct contrast to earlier in the
run where there was a dense white cloud issuing from the outlet. An
attempt to collect a gas sample was made at this time to test for the
presence of water gas. However, the sample pump was defective, and no
sample was collected. When all the temperatures began to decrease, the
run was ended.
It was found, upon disassembly, that all but about 6 in, of the
gin trash was consumed. An apparent mechanical failure in the new
stirring device prevented the combustion of the whole fuel bed. No
apparent cavities were formed, as char and ash were found in the area
below the unburned gin trash.
In an effort to produce high temperatures, it was decided to op­
erate the gasifier without humidifying the inlet air for Run F5. No
other equipment changes were made from the previous run. The only ap­
parent effect that this change produced was that the temperatures as
a whole increased at a more rapid rate. The temperature of the reduc­
tion zone increased wery rapidly after ignition to 890°C, and then
fell just as rapidly to around 500°C, around which it fluctuated for
the rest of the run. The temperature of the constricted area rose to
780°C. The area immediately above the constriction reached a maximum
of 850°C. It is interesting to note that these three maximum tempera­
tures occurred successively in the order mentioned. Immediately follow­
ing the peaks, the entire temperature field began to show a steady
53
decrease. Examination of the fuel bed showed that it had moved down­
ward in a uniform motion. No cavities vvere found.
The objective of Run F6 was to repeat Run F5 using an increased
air flow rate. After ignition of the trash was confirmed, the air rate
was increased slowly to 45 SCFM. During this increase, the action of
the ram and stirrer was continuous. The temperatures in the hearth re­
mained rather low during the period that the air flow was increasing,
which was characteristic of earlier trials. However, soon after the
air flow reached 45 SCFM the temperatures, on the average, more than
doubled. For example, the temperature of the constriction jumped from
values around 270°C to values around 800°C. The maximum temperature
before and after the sudden increase occurred in the area just above
the narrow part of the hearth. This temperature jumped from 500°C to
around 900°C, where it began a rapid decrease. The temperature of the
reduction zone after the increase centered around 700°C. This arrange­
ment of temperatures gave the appearance of a hot oxidation zone fol­
lowed by a cool reduction zone, which is an ideal situation for the
formation of combustible gases. Immediately after a gas sample was
collected, the reaction zones began to cool off rapidly. When the
gas sample was collected, light blue gas was observed at the gas outlet.
The gas sample contained no H2 and insignificant amounts of CO.
This would seem to indicate that the apparent oxidation zone - reduction
zone situation actually was not realized. Inspection of the fuel bed
revealed a large cavity in the lower part of the hearth, indicating
ram or stirrer failure.
54
The design of the stirring device was changed slightly for Run F7.
A longer, wider protrusion was attached to the end of the stirrer. It
was intended that a device of this kind would agitate the fuel bed more
effectively, thus destroying any cavities. However, early in Run F7
the welds holding the extension in place failed and the stirrer was
rendered useless. The maximum temperature of 880°C occurred in the re­
duction zone at the end of the run. A high temperature in this area
this late in the trial indicates that a large cavity was formed in the
hearth, through which burning gin trash fell into the area where the
reduction zone should have been located. Examination of the fuel bed
revealed such a cavity.
The stirring device was removed entirely for Run F8. An addition
to the ram was added which extended through the constriction in the
hearth.
The ram was put into operation as soon as the temperature in the
reduction area began to rise. When the temperatures in the hearth be­
gan to decline, the air rate was increased from 20 SCFM to 45 SCFM.
This caused an immediate increase in the temperatures of the reduction
zone and the area immediately above the constriction. However, these
temperatures began to decrease just as rapidly. After it became ap­
parent that a cavity had formed and was cooling off, the air flow was
interrupted while the' gin trash was tamped down through a small hole
in the top of the gasifier. The air flow was resumed, and the tempera­
tures increased for a short time, then began to decrease again. The
tamping process was repeated with the same results, only the increase
in temperature was more drastic this time. The maximum temperature of
55
940°C occurred in the constricted area. The outlet gas temperature cen­
tered around 700°C at this time. Once again the temperatures began a
rapid decline, and the run was discontinued as it was clear that the ram
was ineffective and a large cavity, which was found later, had formed.
For Run F9, the ram was removed and a new stirring device was in­
stalled. This stirrer had three narrow prongs arranged at staggered
intervals of 45°. The length of these prongs was such that they would
just miss any obstructions in the gasifier, therefore causing the great­
est amount of agitation possible. The stirrer was extremely difficult
to turn immediately after ignition. At this time, the temperature in
the reduction zone had reached a maximum and was beginning to decline.
This decline leveled off between 400°C and 500°C. When this happened,
the stirrer became easier to turn. This action did not seem to have any
beneficial effects on increasing the temperature. It soon became appar­
ent from previous experience that a cavity was forming, and the run was
ended. Inspection of the fuel bed revealed a very dense layer of gin
trash just under the reach of the stirrer, at the top of the refractory.
This gin trash formed a wery effective bridge, which prevented any gin
trash from moving into the hearth area. The stirring device appeared to
be entirely ineffective. The air flow rate for this trial was 20 SCFM.
An entirely different approach was used in Run FIO. Both the ram
and the stirrer were discarded. An air ejector pump was installed in
the outlet gas line to draw air and gas through the gasifier, as an ac­
tual engine might, and the top was left off of the gasifier. It
was reasoned that, by use of the ejector, the smoke level in the
56
gasifier could be controlled, thereby enabling the fuel bed movement to
be observed and the gin trash stirred as needed.
Both the inlet air and the ejector were operating during the igni­
tion process, which took an extraordinarily long time. It was finally
decided that the injected air was passing out of the top of the fuel
bed and bypassing the combustion zone altogether. The top of the gasi­
fier was put in place, and the temperatures began to show an immediate
increase. Gas was soon leaking around the top of the gasifier, which
had not been made airtight as in previous runs. Therefore the run was
terminated with the decision to build and install a better ejector
pump.
The objective of Run Fll was to test the performance of the new
ejector pump, which was constructed out of 0.5 in. pipe. Ignition of
the gin trash went smoothly. The inlet air flow rate was gradually
increased to 35 SCFM. Any air flow rate above this resulted in gas
exiting the opening in the top of the gasifier, even though the ejec­
tor pump was operating at full capacity. When the temperatures appear­
ed to be declining, the fuel bed was stirred manually. This stirring
action would usually cause a further decrease in temperature followed
by a rapid increase, which was then followed by another decrease. This
procedure was repeated several times with the results being the same.
As the combustion zone consumed more of the upper layer of gin trash,
a dense, white gas began to issue out of the top of the gasifier, and
the run was terminated.
The next run. Run F12, was different from the previous runs in
that a very small amount of gin trash was loaded into the gasifier.
This was done in an attempt to observe the development of a cavity.
57
Soon after ignition, glowing areas were observed immediately above
each air nozzle. This indicated that some air was moving upward through
the thin layer of gin trash. The ejector pump was operating at full
capacity while the inlet air was adjusted to a very low value of 5 SCFM.
When the channels appeared above each air nozzle, the reactor filled
with smoke, hindering the observations. Therefore the inlet air was
discontinued. The ejector pump then pulled most of the gas into the
outlet line.
A small section of the middle of the gin trash layer collapsed
while being prodded to expose a channel of glowing coals in the middle
of the fuel bed. This channel was about 3 in. in diameter and 12 in.
deep, extending to the grate below. When the channel was uncovered,
a small amount of unburned gin trash fell through to the grate and
immediately began to burn. When this was consumed, the channel as a
whole apparently began to cool off, as the glow of the charcoal began
to diminish. A small amount of unburned gin trash was again introduced
into the channel. As it burned, the channel became quite hot, with
some flames traveling downward toward the grate. This action obviously
caused an increase in the diameter of the channel, which caused more
unburned gin trash to fall into the channel, and the process was re­
peated. This continued until the channel was approximately 6 in, in
diameter, at which time the channel cooled so suddenly that further
combustion was impossible.
It was felt that this action was representative of what occurred
in the hearth during a normal trial only with four channels being form­
ed which corresponded to the four air nozzles. These channels may have
58
overlapped, thus creating a large cavity. Once this large cavity was
formed, the coals began to cool off, causing a decrease in temperature.
If any unburned gin trash falls into this area, the temperature of the
entire cavity, and therefore the hearth increased. If this phenomena
actually occurred, it might explain the erratic behavior of the tempera­
tures.
Channeling is definitely aggravated by the fibrous nature of the
gin trash, which prevents it from sloughing into the channel as granular
solids would. This causes the combustion front to move, instead of
staying stationary as it would if the gin trash moved freely. Heat
transfer in a countercurrent combustion process in a fibrous fuel bed
is therefore insufficient to produce the high temperatures necessary
for gasification reactions.
For Run F13, it was proposed to use a granular solid, charcoal
briquettes, as a fuel for use in comparison with gin trash. Normal
backyard barbequing briquettes were used. The gasifier was loaded to
the same level as that used with gin trash. Ignition took place
smoothly with the air flow rate being increased smoothly during the
process. A maximum air flow rate was reached at 45 SCFM, at which
time the temperatures of the outlet gas, constricted area of the
hearth, and the reduction zone began to increase sharply. The air
flow rate was reduced to 30 SCFM, which slowed the increase of the out­
let gas but had no effect on the temperatures in the hearth. These
temperatures continued their increase until the temperature in the
constriction zone was in excess of 1000°C. Temperatures of the area
above this constriction and the reduction zone leveled off at values
59
of 980°C and 950°C, respectively. The air flow rate was reduced further
to approximately 20 SCFM to reduce temperatures. The temperatures of
the reduction zone and the narrow portion of the hearth began to de­
crease, while the temperature of the area above the constriction in­
creased to values over 1000°C. This last temperature remained at values
over 1000°C for approximately 5 minutes, after which it too began to
decrease. After it became apparent that all temperatures were decreas­
ing at a steady rate, the air flow was again increased to 45 SCFM. This
action had no effect on any temperatures, which continued to decrease.
Run F13 greatly resembled previous runs with gin trash up to this
point. The temperatures were considerably higher, but the general trend
of a rapid increase in temperatures immediately after ignition followed
by a decreasing trend. However, in Run F13, this decreasing trend soon
leveled off, which indicated that the reactor had possibly attained
steady-state. The hottest location in the gasifier was the area dir­
ectly below the air nozzles at the top of the refractory concrete. The
temperature in this area varied between 830°C and 890°C during the ap­
parent steady-state operation, which lasted approximately 56 minutes.
The area between the top of the refractory concrete and its thickest
point contained the next hottest temperature of approximately 800°C.
This temperature did not vary appreciably. The temperature in the con­
stricted area varied from 650°C to 750°C. The reduction zone appeared
to be constant at 700°C. Outlet gas temperature was also steady at
560°C. Note that these temperatures seem to indicate a hot oxidation
zone at the top of the hearth followed by a cooling zone where reduc­
tion could take place. A gas sample taken during the steady-state
portion of the run gave the following analysis in mole percent:
60
H2 6.7
C02 13.7
02 5.1
N2 64.8
CO 9.6
The gasifier wall beneath each nozzle was hot enough to glow red
throughout most of the run. After approximately 2 hours, a small hole
appeared beside a bolt holding one of the air nozzles in place. Hot air
began to flow out of this opening and the run was terminated to prevent
any permanent damage to the gasifier. When the air flow was turned off,
the air Issuing out of the small hole ignited, producing a very hot,
light blue flame. This flame burned for several minutes very violently,
resembling a slightly contaminated hydrogen flame.
Inspection of the charcoal bed revealed that the level of charcoal
had dropped approximately 2 inches. Further examination showed that
several large masses of slag formed under each air nozzle despite the
fact that the inlet air was humidified. These took the shape of an
oval, being 6 in. across at the widest part.
In an attempt to repeat the success of Run F13 using gin trash, it
was proposed that for Run F14, a system be set up where the gin trash
bed would first be charred completely by a countercurrent combustion
process. The resulting charcoal could then be burned in the normal
manner.
In order to do this, the gasifier was filled completely with gin
trash, and the top was left off. The process was then carried out much
like a normal downdraft gasifier, only at reduced air flow rates. The
61
only source of air flow was provided by the suction from the ejector
pump. The gin trash was ignited at the grate, and the combustion front
was allowed to travel upwards through the gin trash. The air flow was
not increased in order to keep the temperatures down and promote forma­
tion of charcoal. This process took approximately 6 hours, after which
it appeared that there was a small channel forming. Soon after this
channel appeared at the top of the fuel bed, the top of the gasifier
was replaced and the inlet air turned on to about 30 SCFM to start the
gasification reactions. This caused the area under each air nozzle to
get quite hot, which was then followed by a general decrease in tempera­
tures. Inspection of the fuel bed showed a channel had formed along a
side of the gasifier, thus causing the air to bypass the unburned gin
trash. This action eventually caused the combustion zone to die out.
Methods should therefore be developed to convert the entire fuel bed
to charcoal uniformly, preventing any channels from forming at the same
time. This could be accomplished as shown in Figure 9 where the fibrous
gin trash is converted to charcoal while being added to the gasifier.
CHAPTER IV
RESULTS
Cotton gin trash was used as a fuel in an updraft and a downdraft
gasifier, in an attempt to produce a gas which would be used to power
an internal combustion engine in the South Plains of Texas. This en­
gine was operated once for approximately 20 minutes using the updraft
type, after which it became fouled with tar produced by the gasifier.
Several modifications were made in the apparatus in an attempt to re­
move the tars, the most successful being the high pressure drop venturi.
However, this method proved to be too expensive in terms of energy re­
quirements. The other methods were less effective. The gasifier was
then converted to a downdraft type, which normally produces less tars.
Difficulty was encountered in attaining sufficiently high temperatures
when the inlet air was humidified. When the temperatures did reach the
required values, gas samples were taken and analyzed. Insignificant
amounts of combustible gases were revealed.
Channeling and the formation of large cavities were problems en­
countered in eyery run using gin trash, despite repeated efforts to at­
tain even burning. Several mechanical devices were added to aid in the
movement of gin trash into the combustion zone, thereby preventing the
formation of cavities. This action, however, caused the temperatures
in this area to fluctuate between hot and cold. Therefore, the forma­
tion of cavities and channels may be the cause of not attaining suffi­
cient gasification temperatures in both the updraft and downdraft gasi­
fiers. For a summary of experimental results, see Tables 2 and 3.
62
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