Conversion of waste biomass to hydrogen gas or thermal energy using
supercritical water
Takeshi Sako*, Idzumi Okajima, Hiroyuki Soma, Chisato Kagiyama and Kazuyuki Murakami
Department of Materials Science, Shizuoka University
Johoku 3-5-1, Hamamatsu, Shizuoka, 432-8561 Japan
Abstract
Utilization of large amount of waste biomass is very important. Environmentally friendly supercritical water is
used for the production of hydrogen gas or thermal energy from waste biomass efficiently.
The first topic is the supercritical water gasification and hydrogen production. The advantages of this method are
that hydrogen gas is produced much from any kind of waste biomass using supercritical water and wet biomass
can be used without drying process. The promising targets are livestock waste, garbage and so on. 1500-2000
cm3 of hydrogen production per gram on dry biomass basis is realized at 700 oC, 10 MPa, 20 min of reaction
time and 20 of molar ratio of water to organics in biomass. Alkali catalyst gives excellent performance and can
be used many times by simple regeneration.
The second topic is the supercritical water combustion of livestock excrement. It is incinerated completely and
safely to carbon dioxide, nitrogen and steam by this method. The toxic and bad-smelling ammonia is
decomposed rapidly and the toxic nitrogen oxide is not produced in supercritical water. The optimum condition
is determined to be 650oC, 15 MPa, 15 min and 1.2 times stoichiometric amount of oxygen. Furthermore the
obtainable thermal energy from the combustion of a ton of raw cow’s excrement containing 80% moisture is
estimated to be 2.87x106 kJ. It is equivalent to around 70 liter of C-heavy oil and corresponds to the reduction of
about 56 kg of carbon dioxide (on carbon basis).
Keywords: supercritical water, biomass, hydrogen, thermal energy
1. Introduction
Recently the recycling of wastes is expected strongly from the viewpoint of the environmental defense and the
effective use of resources. Among many kinds of wastes, waste biomass is focused the spotlight of attention as a
renewable resource. Supercritical water (Tc=374 oC, Pc=22.1MPa) is a promising benign solvent for the
decomposition of organic substances and is suitable for the treatment of waste biomass because the biomass
contains around 80% moisture (Goto et al., 1998; Kruse et al., 2000; Schmieder et al., 2000).
In this work, we investigate the applicability of supercritical water techniques to wet waste biomass of livestock
excrement, garbage and others. The first application is supercritical water gasification and hydrogen production.
A lot of hydrogen can be produced without drying process of wet organic wastes. The second is supercritical
water combustion to carbon dioxide, nitrogen, water and mineral acid. The rapid and complete decomposition
and the recovery of thermal energy are realized. Optimum treatment conditions are studied using batch-type
reactor and the possibility of continuous process is examined using flow-type reactor.
2. Experimental apparatus and procedure
2.1 Supercritical water gasification and hydrogen production
Batch-type and flow-type reactors were used in this experiment. Figure 1 shows the batch-type experimental
setup. The reactor was 316 stainless steel tube with 9 cm3 in inner volume, 1/2 inch in o.d. and 150mm long. The
experimental procedure is as follows. The waste biomass, distilled water and catalyst were loaded into the
reactor. Then the air in the reactor was replaced with argon gas. The reactor was sealed and put into the sand
bath heated at a reaction temperature. It took about 3min for the reactor to reach the setting reaction temperature
around 700oC. Then small amount of distilled water was added into the reactor using a high-pressure pump in
order to adjust the reaction pressure finally. After a given reaction time, the reactor was taken out of the sand
----------------------------------------------------------------------------------------------------------------------------- ----------*Authors to whom correspondence should be addressed: Mail: ttsako@ipc.shizuoka.ac.jp
bath and cooled quickly in water to stop the reaction as soon as possible.
Figure 2 shows the flow-type experimental setup. This apparatus used a tubular reactor made of 316 stainless
steel having 373 cm3 in inner volume, 1 inch in o.d. and 1000mm long. The mixed solution of reactant and alkali
catalyst was fed into the reactor using a high-pressure pump. The reactor was heated at a given reaction
temperature with electric furnaces. The temperature of the reactor was measured by three K-type thermocouples
mounted on the reactor’s outer wall. The reaction temperature was the average of these three thermocouples. The
pressure of the reactor was controlled by back pressure regulator. After the gasification, the effluent was cooled
to room temperature in water bath and depressurized through the back pressure regulator.
In both experiments, the gaseous product was collected into a gas sampling bag and a small quantity of it was
analyzed using gas chromatographs (Shimadzu GC-8A with Molecular Sieve 5A for the analysis of hydrogen,
and GC-8A with Porapak Q for the analysis of hydrocarbon and CO 2) equipped with thermal conductivity
detectors. The remaining gaseous product was introduced to a big gas syringe and the volume was measured.
The liquid and solid products in the reactor were collected with distilled water and filtered. The TOC in the
filtrate was analyzed by TOC meter (Shimadzu TOC-VCSN) to determine the organic carbon dissolving in
water.
P
T
4
2
5
3
1
1. Sand bath
2. Reactor
3. Gas sampling bag 4. High-pressure pump
5. Distilled water
T. Thermometer
P. Pressure gauge
Figure 1. Batch-type experimental setup for supercritical
water gasification.
3
2
P
T-1
T-2
7
T-3
5
1
8
4-1
4-2
4-3
6
1. Balance 2. Reactant + alkali solution 3. High-pressure pump
4-1~4-3. Electric furnaces 5. Reactor 6. Water bath
7. Back pressure regulator 8. Gas sampling bag
T-1~T-3. Thermometers P. Pressure gauge
Figure 2. Flow-type experimental setup for supercritical water gasification.
The solid was stirred in 12M hydrochloric acid about 3 hours to dissolve the catalyst into the solution. The
remaining solid was the residue. After the filtration and washing with pure water, the residue was dried at 60 oC
in an oven about 12 hours and weighed in order to determine the decomposition efficiency of waste biomass.
Table 1 shows the compositions of samples used in this work. Pig’s excrement, cow’s excrement, paper sludge,
garbage and strained lees of distilled spirits were used for the SCW gasification. Ni-5132P nickel catalyst
provided by N.E.Chemcat Co., guaranteed grade of KOH, NaOH, K 2CO3, Na2CO3, KCl provided by Wako pure
Chemicals Co. were used for a catalyst.
Table 1. Composition of samples
Pig’s excrement
Cow’s excrement
Paper sludge
Garbage
Strained lees of
distilled spirits
Component [wt%]
Water
Organics
Inorganics
73.6
22.7
3.7
82.0
16.0
2.0
55.0
29.6
15.4
78.9
18.4
2.7
74.6
24.0
1.4
C
43.6
43.4
46.2
42.9
46.4
Element [wt%]
H
N
6.3
3.2
5.5
2.5
3.6
1.4
5.2
3.4
5.2
7.1
O
35.9
37.4
48.0
37.4
33.0
2.2 Supercritical water combustion
The optimum condition of clean and complete incineration of livestock excrement was determined with a batchtype reactor similar to that in Figure 1. 0.1g of livestock excrement, a given weight of 30wt% hydrogen peroxide
aqueous solution and pure water were loaded into the reactor. The both side of the reactor was sealed by caps
and heated in the sand bath during a certain reaction time for supercritical water combustion. Then the reactor
was taken out from the sand bath and cooled in water to stop the reaction as soon as possible. The products and
unchanged reactants were recovered from the reactor with pure water. The weight of the solid residue, the
amount of TOC and the concentration of ammonium ion were measured.
3. Results and discussion
3.1 Supercritical water gasification and hydrogen production
The main reactions of the gasification of waste biomass with supercritical water are given by
[ C, H, N, O ] + SC-H2O
CO + SC-H2O
CO + 3H2
CO + H2 + H2O + NH3 + N2
CO2 + H2
CH4 + H2O
(1)
(2)
(3)
where [C, H, N, O] is waste biomass consisting of carbon, hydrogen, nitrogen, oxygen atoms and SC-H2O
represents supercritical water (Okajima et al., 2004).
Figure 3 shows the effect of the catalysts on the gas productivity from pig’s excrement at 700oC, 10MPa, 20min
and 20 of molar ratio of water to carbon in sample. The amount of the catalyst was 20wt% to the organic
component in the sample. The y-axis represents the volume of the gaseous products at 25 oC and 101.3kPa for 1
gram of the organic component in sample. Decomposition efficiency of waste biomass (D.E.) was calculated by
Weight of residue [g]
D.E. [%] =
1-
x 100
Weight of charged sample [g]
Using alkali catalyst of KOH or NaOH, the pig’s excrement was decomposed completely and 1550 cm3 of
hydrogen gas was produced. In the case of salt catalyst of K2CO3 or Na2CO3, the decomposition efficiency and
the volume of hydrogen gas were almost the same as those using alkali catalyst. In addition, KCl catalyst
produced 1240 cm3 of hydrogen gas, which corresponded to 80% of that using KOH. As a result, the pig’s
excrement including chloride atom could be gasified with alkali catalyst efficiently. On the other hand, nickel
catalyst formed 1080 cm3 of hydrogen gas from the pig’s excrement and lowest among six kinds of catalysts.
This was because a small amount of chlorine and sulfur atoms in the pig’s excrement deactivated nickel catalyst.
2500
99%
[ml/g-organic component in sample]
o
Volume of gaseous products at 25 C and 101.3kPa
Figure 4 shows the origin of hydrogen gas in the supercritical water gasification of the pig’s excrement using
KOH catalyst. The bar graph on the left hand side (Gaseous product) shows the virtual volume of hydrogen gas
from hydrogen, methane and ethane, when all hydrogen atoms in the product would be hydrogen gas. The bar
graph on the right hand side (Origin of hydrogen) shows the volume of the hydrogen gas produced from each
origin. The hydrogen gas of 36% was released from the hydrogen atom in the sample during the perfect
decomposition. However hydrogen gas of 64% needs other origin. Judging from the gasification reactions in
equations (1)–(3), the second origin might be water. In this case, active supercritical water reacted with carbon
monoxide to produce hydrogen gas.
Figure 5 shows the temperature dependence of the gas productivity at 10MPa, 30min and 20wt% KOH catalyst.
The decomposition efficiency slightly increased with temperature, but the volume of hydrogen gas drastically
increased from 370 cm3 at 500oC to 1560 cm3 at 700oC. High temperature was very important for the effective
production of hydrogen gas. The pressure dependence of the gas productivity was examined at 700oC, 30min and
20wt% KOH catalyst. The decomposition efficiencies were almost the same for pressure change. However the
volume of hydrogen gas decreased from 1560 cm3 at 10MPa to 630 cm3 at 30MPa, and the product yield of
99%
98%
98%
96%
98%
D.E.
2000
1500
1000
C2H6
CO2
500
CH4
H2
0
KOH
NaOH
K2CO3 Na2CO3
KCl
Ni
Volume of hydrogen at 25oC and 101.3kPa
[ml/g-organic component in sample]
Figure 3. Effect of catalysts on gas productivity from pig’s excrement in
supercritical water gasification. (700oC, 10MPa, 20min, 20 of H2O/C molar ratio,
20wt% catalyst)
2500
2000
1500
From H2O
1000
From sample
H2 in C2H6
H2 in CH4
500
H2
0
Gaseous product
Origin of hydrogen
Figure 4. Origin of hydrogen gas in supercritical water gasification of
pig’s excrement. (700oC, 10MPa, 20min, 20 of H2O/C molar ratio, 20wt%
KOH)
Volume of gaseous products at 25oC and 101.3kPa
[ml/g-organic component in sample]
hydrogen gas in total product gas also dropped from 77% at 10MPa to 55% at 30MPa. On the other hand, that of
methane gas rose from 12% to 24% with pressure. This was owing to the acceleration of reaction (3) by the
increase in pressure. The gasification reaction was fast at high temperature condition, and it reached the chemical
equilibrium within 20 min.
Judging from the experimental results, the optimum conditions of the hydrogen gas production with supercritical
water were 700oC, 10MPa, 20min and 20 of molar ratio of water to carbon in sample. At the most suitable
condition, the volume of hydrogen gas was compared among 4 kinds of waste biomass. The result is shown in
Figure 6. In addition to pig’s excrement, paper sludge, garbage and waste of distilled spirits were employed. The
2000
1800
95%
97%
99%
D.E.
1600
1400
1200
1000
800
H2
600
CH4
400
CO2
200
C2H6
0
500
600
700
Temperature [oC]
Figure 5. Temperature dependence of gas productivity from pig’s
excrement in supercritical water gasification. (10MPa, 30min, 20
Volume of gaseous products at 25oC and 101.3kPa
[ml/g-organic component in sample]
of H2O/C molar ratio, 20wt% KOH)
3000
99%
94%
96%
100%
D.E.
2500
2000
1500
1000
C2H6
CO2
500
CH4
H2
0
Pig’s
excrement
Paper
sludge
Garbag
e
Waste of
distilled
spirits
Figure 6. Comparison of gas productivity of wet waste
biomass. (700oC, 10MPa, 20min, 20 of H2O/C molar ratio,
20wt% KOH)
Volume of gaseous products at 25 oC and 101.3kPa
[ml/g-glucose]
order of the volume of hydrogen gas was waste of distilled spirits > garbage > pig’s excrement > paper sludge.
Flow-type reactor in Figure 2 was used for the supercritical water gasification of glucose, which is water-soluble
and a model component of biomass. Figure 7 shows the change of volume of each gaseous product with passage
of time at 700oC, 10MPa, 20min of residence time. The volume of each gas product was stable after 2.5 hours.
The average volume of H2, CH4 and CO2 were 2170 cm3, 240 cm3 and 460 cm3 per 1 gram of glucose. Potassium
cation in the effluence was analyzed by ion-chromatograph and was not detected. As a result, KOH catalyst was
estimated to deposite in the reactor.
3000
H2
CH4
2500
CO2
2000
C2H6
1500
1000
500
0
0
1
2
3
4
5
6
7
8
Passage of time [h]
Figure 7. Supercritical water gasification of glucose using flow-type reactor.
(700oC, 10MPa, 20min, 10 of H2O/C molar ratio, 20wt% KOH)
3.2 Supercritical water combustion
We investigated the optimum conditions of temperature, pressure, reaction time and oxygen supply ratio on
clean and complete combustion of cow’s excrement by SCWO. When the livestock excrement is incinerated in
the air, a lot of harmful substances are produced and diffuses into the air. The major products are shown as
follows:
Livestock excrement
[C, H, O, N]
CO2 + H2O
+ O2
N2 + NH3 + N2O+ NO + NO2
Ammonia and three kinds of nitrogen oxides are problem. On the other hand, when the excrement is treated by
supercritical water combustion, there is the possibility to suppress the formation of toxic nitrogen-containing
compounds. In this work, the optimum condition of supercritical water combustion was determined using the
batch-type reactor.
Figure 8 shows the temperature dependence of the combustion efficiency of carbon and product yield of
ammonia at 15MPa, 15min, 1.2 of oxygen supply ratio and no catalyst. These parameters are defined by
Combustion efficiency(%) = 1-
TOC in water after SC-water combustion[mg]
Weight of carbon in charged excrement [mg]
x 100
Product yield (%) =
Number of moles of ammonia produced x 100
Number of moles of nitrogen atoms in charged excrement
Oxygen supply ratio =
Weight of oxygen charged into reactor
Stoichiometric weight of oxygen for complete combustion
98
80
96
60
94
40
92
20
90
20
Product yield of N2O [%]
100
Product yield of ammonia [%]
Combustion efficiency of carbon [%]
100
15
10
0
0
350
450
550
650
5
350
750
450
550
650
750
Temperature [oC]
Temperature [oC]
Figure 8. Temperature dependence of combustion
efficiency of carbon and product yield of ammonia in
Figure 9. Temperature dependence of product yield of nitrous
cow’s excrement. (15MPa, 15min, 1.2 of oxygen
oxide from cow’s excrement. (15MPa, 15min, 1.2 of oxygen
supply ratio, no catalyst)
supply ratio, no catalyst)
Concentration of O2 with PSA, QPSA
Heat of combustion, QB
Compression of O2, QO2
Air
908kg of H2O
15MPa, 8.4kmol
(650oC、15MPa)
(Reactor)
A ton of cow’s
650oC,15MPa
excrement (25oC,
80wt% moisture)
200kg of
organics
Thermal energy of H2O, QO
Compression, QF
Preheating, QR
Output energy: QB + QO =
7.17x106 kJ
Input energy : QPSA + QO2+ QF+ QR
=
4.30x106 kJ
Usable energy: (Output energy) - (Input energy) = 2.87x106 kJ
= 70.1L of C-heavy oil = 56kgCO2(Carbon basis)
Figure 10. Energy balance on supercritical water combustion of a ton of cow’s excrement
At 400 oC, the combustion efficiency of carbon was 93%. In this case, 93% of carbon was converted to CO2 and
the remaining existed as TOC in aqueous solution. The combustion efficiency increased linearly with
temperature and almost complete combustion was achieved at 600 oC.
The product yield of ammonia was more than 50%, when the temperature was below 500oC. The product yield
decreased linearly with temperature and clean combustion without ammonia was realized at 600 oC. In this case,
most nitrogen atoms in the excrement might be converted to nitrogen gas and small part to nitrous oxide. Toxic
nitrogen monoxide and nitrogen dioxide did not produce below 800 oC, according to literature (Kamiya, 1978).
The optimum temperature condition was 600-800 oC.
The pressure dependence of the combustion efficiency of carbon and product yield of ammonia was investigated
at 600 oC of the optimum temperature, 15min, 1.2 of oxygen supply ratio and no catalyst. The combustion
efficiency had hardly the pressure dependence and was almost 100% above 10MPa. On the
other hand, the product yield of ammonia changed with pressure steeply and was zero above 15MPa. As a
result, the optimum pressure was more than 15MPa. The dependence of the combustion efficiency of carbon and
product yield of ammonia on the reaction time was examined at 600 oC and 15MPa of the optimum conditions,
and 1.2 of oxygen supply ratio and no catalyst. At 5 and 10min, there was a little amount of TOC in water
recovered. When the reaction time increased to 15 min, carbon in the excrement was converted to CO 2 almost
completely and the product yield of ammonia was zero.
Judging from the present results, the optimum conditions for the complete and ammonia-free combustion by
supercritical water combustion were 600 oC, 15MPa, 15min and 1.2 of oxygen supply ratio.
Figure 9 shows the temperature dependence of the product yield of nitrous oxide at the optimum conditions
determined above. Here the product yield of nitrous oxide is defined by the similar equation to that of ammonia.
It was zero at 400 oC, 13% of maximum at 600 oC, and zero at 650 oC again. The final optimum conditions for
complete and clean combustion of cow’s excrement without catalyst were 650 oC, 15MPa, 15min and 1.2 of
oxygen supply ratio.
Figure 10 shows the energy balance of supercritical water combustion of a ton of cow’s excrement with 80wt%
moisture. The input energy, which consisted of PSA concentration and compression of oxygen gas, charging to
the reactor and preheating of excrement, was 4.30x10 6 kJ/ton. On the other hand, the output energy, which
consisted of the heat of combustion of excrement and thermal energy of supercritical water, was 7.17x10 6 kJ/ton.
Usable energy, which was the difference between the output and input energies, was 2.87x10 6 kJ/ton and
corresponded to 70 liter of C-heavy oil.
4. Conclusions
Supercritical water gasification and hydrogen production of waste biomass is a promising technique. The
optimum conditions were 700oC, 10MPa, 20min and 20 of molar ratio of water to carbon in biomass. Hydrogen
gas of 1500-2300 cm3 was produced from 1 gram of dry waste biomass such as livestock excrement, garbage,
paper sludge and waste of distilled spirits.
Supercritical water combustion gave the clean, rapid and perfect incineration of livestock excrement. The
products were only harmless CO2, water, nitrogen and minerals. Furthermore there was the possibility to recover
the thermal energy generated from the combustion of the excrement.
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