Basu P. Biomass Gasification and Pyrolysis: Practical Design and Theory
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247
7.5 Application of Biomass Conversion in SCWG
7.4.7 pressure
Experiments by Van Swaaij et al. (2003) in their microreactor over the range
of 19 to 54
MPa, those by Kruse et al. (2003) in a stirred tank (30–50
MPa,
500 °C), and those by Lu et al. (2006) in a plug-flow reactor (18–30
MPa,
625 °C) showed no major effect of pressure on carbon conversion or product
distribution. Nor did Mettanant et
al. (2009) see much effect in their tempera-
ture and pressure range, although they noted a clear positive effect of pressure
at 700 °C. This issue needs further exploration.
7.4.8 reactor type
The reactors used so far for SCWG research have been either batch or continu-
ous (flow). Depending on their type of mixing, they can be further divided as
follows:
Autoclave
Tubular steel
Stirred tank
Quartz capillary tube
Fluidized bed
A batch reactor is simple, does not require a high-pressure pump and can be
used for almost all biomass feedstock. However, its reaction processes are not
isothermal and it needs time to heat up and cool down. During heat-up many
reactions occur that cause transformation of the feedstock; this does not happen
in a continuous-flow reactor.
Reactor type has an important effect on the influence of feed concentration.
The drop in gasification efficiency with feed concentration, noted in tubular
reactors, was not found in the stirred-tank reactor studied by Matsumura et
al.
(2005). However, the reactor used was exceptionally small (1.0
mm in diam-
eter), so validation of this finding in a reasonably large reactor (Matsumura and
Minowa, 2004) is necessary. The process development of SCW gasifiers is
lagging laboratory research because of engineering difficulties and the high cost
of pilot plant construction.
7.5 applIcatIon oF BIomaSS converSIon In ScWG
Three major areas of application for biomass SCWG are: (1) energy conversion,
(2) waste remediation, and (3) chemical production.
7.5.1 energy conversion
All three of the followng important feedstocks for the energy industry can be
produced by biomass conversion in supercritical water:
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Bio-oil: Potential use in the transport sector
Methanol: Though a chemical feedstock, may be used for combustion
Hydrogen: Potential use in fuel cells
The overall efficiency of an energy conversion system depends on the
technology route, on the wetness of the biomass, and on many other factors.
Yoshida et
al. (2003) compared the effect of moisture content on the net effi-
ciency of seven options for electricity generation, including an SCWG com-
bined cycle. Interestingly, the SCWG-based system shows a total efficiency
independent of moisture content, while for all other systems, total efficiency
decreases with increasing moisture. Total electricity generation efficiency is
even higher than that for conventional combustion-based systems. Integrated
gasification combined cycle (IGCC) efficiency is higher than that of SCWG for
biomass containing less than 40% moisture. Above 40%, its efficiency drops
below that of SCWG (Figure 7.9).
Yoshida et al. (2003) also compared the total heat utilization efficiency of
seven energy conversion processes:
Direct combustion of biomass
Combustion of biomass-oil produced by liquefaction or pyrolysis
0
5
10
15
20
25
30
35
40
0
Moisture content (%)
Total efficiency (%)
direct combustion
thermal gasification
combined cycle
methanol combustion (from
SCWG)
methanol combustion (from
thermal gasification)
SCWG combined cycle
biomass-reforming fuel cell
anaerobic digestion gas
combustion
10 20 30 40 50 60 70 80
FIGure 7.9 Dependence of net electricity generation efficiency of different biomass-based
processes on biomass moisture content. (Source: Adapted from Yoshida et
al., 2003.)
249
7.5 Application of Biomass Conversion in SCWG
Combustion of methanol produced by thermal gasification
Combustion of methanol produced by SCWG
Combustion of biogas produced by thermal gasification
Combustion of methanol produced by supercritical water gasification
Anaerobic digestion
Supercritical water gasification has the distinction of easily separating CO
2
from the product gas. This makes it an optimal technology for generation of
electricity and heat from biomass when CO
2
emission limits become binding.
Fuel cells have the highest energy conversion efficiency for electricity
generation, but they need hydrogen as their fuel. For hydrogen production,
from very wet biomass, SCW gasification could be an attractive route. However,
the capital costs of a fuel cell and that of a gasification plant have an important
bearing on the economic viability of this generation option.
7.5.2 Waste remediation
Waste treatment is another SCWG application. As explained in Section 7.3.3,
in supercritical water even highly toxic wastes can be oxidized to harmless
disposable residues. The agricultural industry produces large volumes of non-
toxic but unhealthy products such as animal extracts and farm wastes that need
to be disposed of productively. Many of these contain so much moisture that
economical combustion or thermal gasification is not possible. Anaerobic
digestion is a widely used alternative, especially in developing countries for
production of useful gas (mostly methane) from animal extracts. Along with
methane, anaerobic digestion produces fermentation sludge, which can be used
as fertilizer.
Nevertheless, anaerobic gasification is orders of magnitude slower than
thermal and other gasification processes, even with the use of catalysts. As a
result, this makes large-scale commercial operation of anaerobic digesters dif-
ficult. Furthermore, the attractiveness of this method depends on the price of
fertilizer, which can vary as a result of over- or undersupply in the market
(Matsumura, 2002).
SCWG or SCWO is an alternative suitable for waste treatment because it
does not depend on the production of sludge and is much faster than anaerobic
digestion. Matsumura (2002) noted that supercritical water gasification has
better energy efficiency, cheaper gas production, and faster CO
2
payback time
(64.8%, 3.05 yen/MJ, and 4.19 years, respectively) in comparison with bio-
methanation (49.3%, 3.74 yen/MJ, and 5.05 years, respectively).
7.5.3 chemical production
Solvents are an important component of many chemical reactions. SCW acts
as a solvent, but can also be a reactant and/or a catalyst. Ordinary subcritical
water is popular as a solvent for reactions, especially because it is inexpensive
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7 Hydrothermal Gasification of Biomass
and easily disposed of. Many organics, however, do not react efficiently in it.
For these reactions, acid or base solvents are needed, which are good for syn-
thesis reactions but, unless they can be efficiently recovered, are expensive and
hazardous to dispose of.
Owing to its unique properties, SCW can act as a solvent for some reactions.
Based on their studies of the following reactions Krammer et al. (1999) noted
that many hydration, dehydration, as well as hydrolysis reactions can take place
in supercritical water with good selectivity and high space/time yield, with no
acids or bases as support materials.
Dehydration of 1,4-butandiol and glycerine
Hydrolysis of ether acetate, acetonitrile, and acetamide
Reaction of acetone cayano hydrine
Production of useful chemicals from biomass is another use for SCW gas-
ification. During its degradation in SCW, biomass produces phenols. Phenol
production increases with feed concentration (Kruse et
al., 2003). Because
phenol is an important feedstock for the green resin, wood composite, and
laminate industries, SCW provides an effective medium for green chemistry.
7.6 reactIon KInetIcS
Limited information is available on the global kinetics of SCW gasification.
Lee et
al. (2002) studied the kinetics of glucose (used as the model biomass)
in SCWG with a plug-flow reactor.
C H O H O CO H
6 12 6 2 2 2
6 6 12+ = + (7.5)
We define the reaction rate, r, as the depletion of the biomass carbon fraction,
C, with time. Assuming pseudo-first-order kinetics, we can write
r
dC
d
k C
g
= − =
τ
(7.6)
where k
g
is the reaction rate constant.
The fraction of carbon converted into gas, X
c
, may be related to the current
carbon fraction, C, and the initial carbon fraction, C
0
, in the fuel:
X
C
C
c
= −1
0
(7.7)
Now replacing the carbon fraction in Eq. (7.6) and integrating we get:
k
X
g
c
= −
−
( )
ln 1
τ
(7.8)
Table 7.4 presents some data on the global kinetics for SCWG of model com-
pounds. The rates measured by Metanant et
al. (2009), Lee et
al. (2002), and
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7.7 Reactor Design
Kabyemela et al. (1997) show how the reaction rate decreases with increasing
solid carbon in the feed.
7.7 reactor deSIGn
Because SCWG is likely to enter the market once its major development bar-
riers are removed this section discusses important considerations for design of
an SCWG reactor. The discussion is based on limited information available in
laboratory units and on the design of thermal gasifiers (see Chapter 6).
The major design parameters for an SCWG reactor are temperature, resi-
dence time, pressure, catalysts, and feed concentration. Important design con-
siderations for auxiliary or support equipment are (1) waste heat-recovery
exchanger and feed-preheating system, (2) the biomass feed system, and
(3) product separation. The following subsections present a brief discussion of
some of the design parameters.
7.7.1 reactor temperature
The temperature and pressure of an SCWG must be above the critical value of
374.21 °C and 22.089
MPa, respectively. As explained in Section 7.4.7, pres-
sure has a minor effect on biomass conversion, but the effect of gasification
temperature is a major one (see also Section 7.4.1).
TABLE 7.4 Global Kinetic Gasification Rate for Model Compound
in Supercritical Water
Blasi et al., 2007
Metanant
et al., 2009
Lee et al.,
2002
Kabeymela
et al., 1997
Feed Wastewater from
wood gasifier/TOC
Rice husk Glucose/COD Glucose
Reactor Plug Batch Plug Plug
Temperature (K) 723–821 673–873 740–1023 573–673
Residence time (s) 60–120 3600 16–50 0.02–2
Solid content 7.0–1.0
gm/l 9.4
mol/L 0.6
mol/l 0.007
mol/l
Pre-exponential
factor, A (s
−1
)
1018 ± 494
184
897 ± 29
Activation energy,
E (kJ/mol)
75.7 ± 22
77.4
71 ± 3.9
96
k
g
(s
−1
)
0.0002–0.006 0.01–0.55 0.15–9.9
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7 Hydrothermal Gasification of Biomass
Because feedstock (biomass and water) must be heated to the reaction
temperature using energy from an external source, the lower the designed
reactor temperature, the lower the energy required for feed preheat and the
more efficient the process. The gasification temperature should be above
600 °C for a reasonable hydrogen yield, but it can be lower if catalysts
are used.
For synthetic natural gas (SNG) production, high methane and low hydro-
gen are required; therefore, we can choose a reaction temperature of 350 to
500 °C, but catalysts are necessary for a reasonable yield. With catalysts,
methane-rich gas may be produced even just below the critical temperature
(~350 °C) (Mozaffarian et al., 2004).
7.7.2 catalyst Selection
The choice of catalyst influences reactor temperature, product distribution,
and plugging potential. Section 7.4.2 discussed the catalysts used in SCW
gasification. They are selected on the basis of the desired product. Catalyst
deactivation is an issue assigned to most catalyzed reactions because the deac-
tivated catalysts must be regenerated. If they are deactivated because of carbon
deposits, as happens in a fluid catalytic cracker (FCC), they can be combusted
by adding oxygen, preferably in a separate chamber. The combustion reaction
reactivates the catalysts and can additionally provide enough heat for preheat-
ing the feed.
7.7.3 reactor Size
Consider a simple reactor receiving W
f
of feed while producing W
p
of product
per unit of time. The product comprises a number of hydrocarbon components
represented by species i. The total carbon in the product gas is its total in the
individual gaseous hydrocarbons:
Total carbon production in the product gas kmol s=
∑
W C
p i i
α
(7.9)
where α
iτ
is the number of carbon atoms in component i in the gas product;
C
i
is mole fraction of i in the gas product; and W
p
is the product gas flow rate
(kmol/s). The amount of carbon in the feed is known from the feed rate, W
f
(kg/s), and its carbon fraction, F
c
. The carbon gasification yield, Y, is defined
as the ratio of gasified carbon to the carbon in the feed:
Y
W C
W F
p i i
i
f c
=
∑
12
α
(7.10)
where 12 is the carbon’s molecular weight (kg/kmol).
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7.7 Reactor Design
From Eq. (7.8) the reaction rate is given in terms of conversion as
k
X
g
c
= −
−
( )
ln 1
τ
(7.11)
where τ is the residence time in a reactor of volume V.
For a continuous stirred-tank reactor,
τ
=
V
Volume flow rate of feed at reactor condition
s (7.12)
Thus, for a known reaction rate, k
g
, and a desired conversion, X
c
, we can esti-
mate the reactor volume required for gasification.
7.7.4 Heat-recovery Heat-exchanger design
A feedstock preheater is the second most important part of an SCW gasifier
system. The heat required to preheat the feedstock (water and biomass) is a
significant fraction of the potential heating value of the product gas. Without
efficient recovery of heat from the product gas, the external energy needed for
gasification may exceed the energy produced, making the gasifier a net energy
consumer. The feedstock should therefore obtain as much of its enthalpy as
possible from the sensible heat of the product. This is one of the most important
aspects of SCW plant design.
Figure 7.10 compares the capital costs of different components of an SCWG
plant. We can see that the heat-recovery exchanger represents 50 to 60% of the
total capital cost of the plant, which makes it a critical component.
Efficient heat exchange between the feed and the product is the primary
goal of an SCWG heat-recovery system. However, for supercritical water
intended for hazardous waste reduction (SCWO) or synthesis reaction (SCWS),
it may not be all that important since the primary purpose of these systems is
the production of chemicals, not energy as in a supercritical gasifier.
The heat-exchange efficiency, η, defines how much of the available heat in
the product stream can be picked up by the feed stream.
η
=
−
−
H H
H H
product out product in
feed in product in
- -
- -
(7.13)
where H is the enthalpy, and the subscripts define the liquid it refers to.
Theoretically, the heat-exchange efficiency can be 100% if no heat of vapor-
ization is required to heat the feed and an infinite heat-exchange surface area
is available. Of course, these conditions are not possible. Figure 7.11 shows
variations in heat-exchange efficiency with changes in tube surface area and
water pressure.
The specific heat of water rises sharply close to its critical point and then
drops equally sharply as the temperature increases (Figure 7.2). Thus, around
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7 Hydrothermal Gasification of Biomass
0
20
40
60
80
100
120
1 10 100 1000
Surface area (m
2
/kg.m of feed)
Heat-exchange efficiency (%)
20 MPa
22.5 MPa
25 MPa
30 MPa
FIGure 7.11 Calculated heat-exchange efficiency for water–water countercurrent heat exchanger
at different pressures when feed and product are entered at 24 °C and 600 °C, respectively. (Source:
Data taken from Knoef, 2005.)
500,000
1,000,000
1,500,000
2,000,000
2,500,000
3,000,000
3,500,000
4,000,000
4,500,000
5,000,000
Amos Amos (PSA) FZK Matsumura Gen. Atomics
Capital costs (€)
purification reactor heat exchanger feed preparation
FIGure 7.10 Investment cost of different SCW plant designs based on a throughput of 5000 kg/h
of sewage sludge. (Source: Adapted from Gasafi et
al., 2008.)
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7.7 Reactor Design
the critical point we may expect a modest temperature rise along the heat-
exchanger length.
Thermal conductivity in SCW is lower than that in subcritical water because
SCW’s intermolecular space is greater than that in liquid. A slight increase in
conductivity is noticed as the fluid approaches the critical point. This increase
is due to an increase in the agitation of molecules when the change from a
liquidlike to a gaslike state (SCW) takes place. Above the critical point, thermal
conductivity decreases rapidly with temperature.
The heat-transfer coefficient varies with temperature near its pseudo-critical
value (see Section 7.2) because of variations in the thermophysical properties
of water. As the temperature approaches the pseudo-critical value, conductivity
and viscosity decrease but specific heat increases. The drop in viscosity and the
peak of specific heat at the pseudo-critical temperature overcome the effect of
decreased thermal conductivity so as to increase the overall heat-transfer rate.
As the temperature further increases, beyond the pseudo-critical point, the
specific heat decreases sharply; the drop in thermal conductivity continues as
well, and therefore the heat-transfer coefficient reduces. For a given heat flux,
the wall temperature rises for the drop in heat-transfer coefficient. Generally,
for high heat flux and low mass flux, the heat transfer deteriorates, leading to
hot spots in the tube.
Heat Transfer in Supercritical Water
Table 7.5 illustrates the operation of a typical heat-recovery exchanger for
supercritical water gasification. The data are taken from a large operating near-
supercritical gasification plant. The fluid-to-wall heat-transfer coefficient in
clean supercritical water in the tube may be calculated by the correlation of
Yamagata et
al. (1972):
Nu = 0 0135
0 85 0 8
. Re Pr
. .
b b
(7.14)
Based on Yamagata’s experiments with isobutane, Hsu (1979) found that
the Sieder-Tate equation, as follows, is in better agreement:
TABLE 7.5 Sample Data for VERENA Pilot Plant Product-to-Feed
Heat Exchanger
Flow Rate (kg/h)
(Methanol %)
Product
in (°C)
Product
Out (°C)
Feed
In (°C)
Feed
Out (°C)
Reactor
Temperature (°C)
100 (10%)
561 168 26 405 582
90 (20%)
524 155 22 388 537
Note: Heat-exchanger surface area: 1.1 m
2
; heat-transfer coefficient: 920 W/m
2
C (Boukis et al.,
2005).
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7 Hydrothermal Gasification of Biomass
Nu =
( )
0 027
0 8 0 33
0 14
. Re Pr
. .
.
µ µ
b b
(7.15)
Heat transfer in SCWG may vary because of solids in the fluid. Thus, applica-
bility of these equations to SCWG is uncertain. Information on this aspect of
heat transfer is presently unavailable.
7.7.5 carbon combustion System
Because gasification and pyrolysis reactions are endothermic, heat from some
external source is required for operation of the reactor. In thermal gasification
systems, the reaction temperature is very high (800–1000 °C), so a large amount
of energy is required for production of fuel gases from biomass or other feed-
stock. This heat is generally provided by allowing part of the hydrocarbon or
carbon in the feed to combust in the gasifier, but then a part of the energy in
the feedstock is lost.
A SCW gasifier operates at a much lower (450–650 °C) temperature and
thus requires a much lower but finite amount of heat. Thermodynamically, the
heat recovered from the gasification product is inadequate to raise the feed to
the gasification temperature (450–600 °C) and provide the required reaction
heat. This shortfall is made up either by an external source or by combustion
of part of the product gas in a heater.
Both options are expensive. For example, a study of an SCWG design for
gasification of 120
t/day (5000
kg/h) of sewage sludge with 80% water showed
that 122 kg/h of natural gas is required to provide the gasification heat. This,
along with an electricity consumption of 541
kW, constitutes 23% of the total
revenue requirement for the plant (Gasafi et
al., 2008). A better alternative
would be controlled combustion of the unconverted char upstream of the gas-
ifier, which would make SCWG energy self-sufficient.
Although SCWG is known for its low char and tar production, in practice
we expect some char formation. Furthermore, as shown previously in Figure
7.7, gasification efficiency is low at lower temperatures. A low gasification
temperature is thermodynamically more efficient, but raises the char yield. If
this char can be combusted in SCW, it can provide the extra heat needed for
preheating the feed, thereby improving the efficiency of the overall system.
Combustion of char offers an additional benefit for an SCWG that some-
times uses solid catalysts, which are deactivated after being coated with uncon-
verted char in the gasifier. A combustor can burn the deposited carbon and
regenerate the catalyst. The generated heat is carried to the gasifier by both
solid catalysts and the gasifying medium (SCW and CO
2
).
Recycling of solid catalysts is an issue for plug-flow reactors. Special
devices such as fluidized beds may be used for these, as shown in Figure 7.12.
Here, the catalysts or their supports are granular solids, which are separated
from the product fluid leaving the reactor in a hydrocyclone operating in an
SCW state. The separated solids drop into a bubbling fluidized-bed combustor,