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SUPERFICIAL VELOCITY - THE KEY TO DOWNDRAFT GASIFICATION
1
T. B. Reed
a
, R. Walt
b
, S. Ellis
c
, A. Das
d
, S. Deutch
e
a
The Biomass Energy Foundation, 1810 Smith Rd., Golden, CO 80401;
b
Community Power Corporation,
Aurora, CO,
c
Colorado School of Mines, Golden, CO,
d
Original Sources, Boulder, CO,
e
The National
Renewable Energy Laboratory, Golden, CO.
ABSTRACT
The “superficial velocity” (hearth load) of a gasifier is the most important measure of its
performance, controlling gas production rate, gas energy content, fuel consumption rate,
power output, and char and tar production rate.
The superficial velocity, SV, of a gasifier is defined as:
SV = Gas Production Rate/Cross Sectional Area = (m3/s)/(m2/s) = m/s
It is easily estimated or measured by measuring gas production rate or fuel throughput and
gasifier dimensions. It controls the rate at which air, then gas, passes down through a
gasifier. This in turn exercises a primary effect on heat transfer around each particle during
flaming pyrolysis of the volatiles, combustion of the tars and gasification of the charcoal.
A
low
SV causes relatively slow pyrolysis conditions at around 600°C, and produces high
yields of charcoal - 20-30%, large quantities of unburned tars, and a gas with high
hydrocarbon content and high tar (volatile) content. A
high
SV causes very fast pyrolysis,
producing less than 10% char-ash at 1050 C and hot gases at 1200-1400 C in the flaming
pyrolysis zone. These gases then react with the remaining char-ash to yield tars typically less
than 1000 ppm, 5-7% char-ash and a producer gas with less energy.
These relationships have been investigated in a velocity controlled inverted downdraft
gasifier with a 7.5 cm diameter. As the superficial velocity was varied from 0.05 m/s to
0.26m/s , the gas production rate increased from 102 to 679 cm
3
/s, charcoal production
decreased from 13.0% to 4.7% and tar in the gas decreased from 8330 to 300 mg/kg (ppm).
At low Superficial Velocities (and low Biot numbers), the particles are heated slowly to
pyrolysis temperature and remain essentially isothermal. At high superficial velocities the
outside of the particle can be incandescent (> 800°C) while the center is still at room
temperature. This permits the escaping gases to react with the charcoal, thus reducing the
charcoal yield and increasing the gas yield. We call this phenomenon “simultaneous
pyrolysis and gasification”, SPG and believe that it is the fundamental reason why the
Superficial Velocity controls all other aspects of gasification.
In producing heat the “tars” in producer gas are a useful fuel, providing no cold surfaces
intervene. The “inverted downdraft” stoves provide potentially simple, clean cooking for
developing countries. At low SVs they also produce charcoal. In producing electric power,
tars are detrimental to engine operation, and so high SVs must be maintained to minimize tar
and char production.
*Presented at 4
th
Biomass Conference of the Americas; Oakland, CA, 8/29/99
Introduction
Clean cooking and distributed power are the major problems of half the world population.
Gasification of biomass can help provide solutions to both these problems; unfortunately very
little fundamental research has been done on gasification and most new designs are made “by
guess and by golly”. Over a million vehicles used gasified biomass successfully during
World War II. Since then there have been many attempts to gasify biomass, most
unsuccessful.
Biomass gasifiers can be divided into three main categories depending on the source of
heat for pyrolysis:
Charcoal burning gasifiers
(updraft, counter-flow …);
tar burning
gasifiers
(downdraft, inverted downdraft, crossdraft, co-flow, open top, topless, …); and
fluidized bed
(with some char, some tar burning and many varieties). Due to the very high
volatile content of biomass, the tar burning gasifiers are preferred when it is necessary to
produce a very clean gas for power generation. This paper deals primarily with tar burning
gasifiers, but many of the principles can be applied to other forms.
The “superficial velocity”, SV, of a gasifier is the most fundamental measure of the
expected behavior of a gasifier and controls most of the other aspects of gasifier operation as
shown here:
Gas Production Rate
Char Production
SUPERFICIAL
VELOCITY
Fuel Consumption Rate
Gas Energy Content
Tar Production
It is as important to gasifiers as engine rpm to an engine designer or metabolism rate to a
physiologist.
It is defined as:
SV = Gas Production Rate/Cross Sectional Area = (Nm3/s)/(m2/s) = m/s
(It is sometimes defined as the “Hearth load”, B
h
measured in Nm
3
/
cm
2
hr. B
h
= 0.36 SV.)
It is independent of gasifier size, and so permits comparison of gasifiers of very different
dimensions. It is easily estimated or measured from air/oxygen fuel throughput and gasifier
dimensions. It controls the rate at which air, then gas, passes through a gasifier. This in turn
exercises a primary effect on heat transfer around each particle during flaming pyrolysis of
the volatiles, on combustion of the tars and the on the degree of gasification of the charcoal.
While the SV is the most important measure of performance for design, it is not generally
recognized or discussed.
1.
HISTORICAL
In the book, “Gengas”
1
G. V. Nordenswan says:
“The concept of hearth load plays a very important role in dimensioning a wood gas
generator hearth. The hearth load is the quantity of prepared gas, … divided by the smallest
passage area of the hearth. Thus the hearth load is dimensionally a velocity…”. He goes on
to say that the practical range for SV is 0.8m/s (where tars are quite high) to 2.5 m/s (above
which charcoal dusting is unacceptable). These numbers only apply to constricted hearth
(Imbert) gasifiers. Table 5-2 in Ref. 2 gives design parameters for various Imbert gasifiers
ranging from 4 Nm3/hr to 230 Nm3/hr.
More recent gasifiers would have other values of SV as shown in Table I.
Table I – Reported Superficial Velocities in various gasifiers
2
GASIFIER
TYPE
PYROL. ZONE
SV m/s
CHAR ZONE
SV m/s
0.63
2.5
Imbert
Biomass Corp.
0.24
0.95
0.28
0.28
SERI Air
SERI Oxygen
0.24
0.24
1.71.
1.71.
Syn-Gas air
Syn-Gas oxygen
1.07
1.07
0.23
0.23
Buck
Rogers
(Chern)
Buck Rogers
(Wallawender)
0.13
0.13
The values in Table I are taken from literature reports and generally represent the highest
values reported there. The Stratified Downdraft gasifier has the same values of SV in the
pyrolysis and char zones since it uses a constant diameter gasification tube. Note that the
values differ widely from those recommended for Imbert gasifiers.
2.
EXPERIMENTAL
4.1
Apparatus
The apparatus shown in Figure 1 has been built to measure the superficial velocity over a
wide range in batch inverted downdraft gasification. (It can be configured with minor
modifications to operate in the conventional downdraft mode.) The apparatus is well
insulated with pressed fiber refractory (riser sleeve) to minimize heat loss. The air supply is
measured with a calibrated flowmeter and the pressure drop, temperature, and gas
composition, are measured during the runs.
In use the cylinder is filled with fuel to the gas outlet. The runs reported here were made
on mixed +1/4, -1/2 inch hardwood chips with a moisture content of 6.1% wet basis. The top
chips are saturated with alcohol and ignited and the cap is put on. The temperature and
pressure drop are monitored as a function of time.
The gas from the gasifier passes through the collection train
shown in Figure 2. The outlet pipe is heated with a torch to
prevent tar condensation at startup. The gas then passes through a
condenser in an ice bath to cool the gas and remove most of the
moisture and tar. At the end of the run the moisture recovered was
measured and the condenser was rinsed with acetone to collect the
majority of the tar. Residual moisture and tar can be removed after
the condenser and total gas flow can be measured with a gas meter
at the exit and the gas can be flared.
Tars were also measured using a filter to draw aliquot portions
directly from the gasifier during the runs. These results and new
methods will be reported later. After the run the amount of
charcoal remaining was measured.
While the experiments were performed on an “inverted
downdraft” gasifier, it is believed that the results will be similar
for conventional and new downdraft gasifiers. Further
experiments are planned.
10 cm
5cm
Cap
Gas Out
Gas Sample,
Pressure
Char from
Pyrolysis
40 cm
30 cm
Pyrolysis
Zone
Temperature
Unburned
Fuel
4"Pipe
Insulation
Pressure
Air In
Grate
Figure 1 – Gasifier for measuring
effects of superficial velocity change.
3.
RESULTS
The results of these measurements at four SVs are shown in Fig.
3 and reported in Table II
Flare,
ESP,
Flowmeter
Gas Out
Gas Sample,
Pressure
4.1
Flaming Pyrolysis
Tar burning gasifiers consume the tars by a process we call
“flaming pyrolysis.
2
The combustion of pyrolysis products in air
in sufficient supply (as in a match) is termed “flaming
combustion”. We apply the term “flaming pyrolysis” to the
combustion of the same volatiles in an inadequate supply of air so
that the products are largely CO and H
2
rather than CO
2
and H
2
O.
Tem
perature
Pressure
Tap
Compressed
Air, Flowmeter
Gasifier
0CIceBath
Fig. 2 - Gas Train
4.2
Charcoal and Tar Yield
It is seen in Table II and Figures 4 and 5 that charcoal and tar yields depend strongly on
the superficial velocity. The reduction in charcoal and tar with increasing SV is due to the
increased temperature during pyrolysis shown in Figure 3 and Table II. Tars were measured
both by total condensate in the condenser and by use of an absolute filter method.
3
4.3
Air/Fuel ratio
It is obvious in Table II that many other variables depend on SV. In Table II it can be seen
that the air/fuel ratio increases from 1.44 to 5.21 as more and more of the charcoal is gasified
and the process approaches complete combustion.
Table II – Results of operation of gasifier at 0.05, 0.19, 0.26 and 0.44 m/sec SV
Run #
5
6
8
9
MEASURED VALUES
Air Flow - l/m
4.1
18.8
26.0
46.4
Initial Fuel (g)
89.1
91.1
82.2
84.2
Final Charcoal (g)
11.6
6.8
3.8
0
Condensate (g)
16.7
13.8
11.8
13.8
Tar + Particulate – (g)
2.044
1.463
0.44
0.189
Average Pressure Drop – iwc
0.025
0.11
0.15
0.33
Time of Run – min
33.5
14.4
9.5
8
Pyrolysis Temp
770
993
1033
1045
Pyrolysis time – min
1.0
1.0
0.6
0.6
DERIVED VALUES
Superficial Velocity - m/s
0.052
0.187
0.260
0.437
0.206
0.423
0.387
0.548
Gas Produced – kg (m3)
102
489
679
1141
Gas production rate – cm3/sec
Tar, Particulate in Gas - mg/m
3
9941
3460
1138
346
8330
1330
300
10
Tar – mg/m3 -Filter Method
1.20
2.08
3.16
3.75
Fuel Velocity - cm/min
1.44
3.80
3.47
5.21
Air/Fuel Ratio
13.0%
7.5%
4.7%
0.0%
Charcoal Yield - %
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