Basu P. Biomass Gasification and Pyrolysis: Practical Design and Theory

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237

7.3 Biomass Conversion in SCW

 The product gas of SCWG automatically separates from the liquid contain-

ing tarry materials and char if any.

7.3 BIomaSS converSIon In ScW

There are three major routes for SCW-based conversion of biomass into energy

as follows:

Liquefaction: Formation of liquid fuels above critical pressure (22.1 MPa)

but near critical temperature (300–400 °C).

Gasiﬁcation to CH

: Conversion in SCW in a low-temperature range (350–

500 °C) in the presence of a catalyst.

Gasiﬁcation to H

: Conversion in SCW with or without catalysts at higher

(>600 °C) temperatures.

Here we discuss only the last two gasiﬁcation options.

7.3.1 Gasiﬁcation

Supercritical biomass gasiﬁcation takes place typically at around 500 to

750 °C in the absence of catalysts, and at an even lower (350–500 °C) tempera-

ture with catalysts. The biomass decomposes into char, tar, gas, or other inter-

mediate compounds, which are reformed into gases like CO, CO

, CH

, and

. The process is schematically shown in Figure 7.4. If the biomass is repre-

sented by the general formula C

, the gasiﬁcation process may be described

by the following overall reaction:

m n w x y zC H O H O H CH CO CO

6 12 6 2 2 4 2

+ → + + + (7.3)

Gasiﬁcation in SCW involves, among other reactions, hydrolysis and oxida-

tion reactions. A brief description of these reactions follows.

7.3.2 Hydrolysis

Hydrolysis (meaning “splitting with water”) is the reaction of an organic com-

pound with water. Here, a bond of an organic molecule is broken, and the water

molecule is also broken into [H

] and [OH

−

]. The organic molecules are cleaved

into two parts by the water molecule: One part gains the [H

] ion; the other

Biomass

Thermal

decomposition

Reforming

Char

Tar

Gas, etc.

FIGure 7.4 Biomass gasiﬁcation process.

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chapter

7 Hydrothermal Gasiﬁcation of Biomass

part, the [OH

−

] ion. Hydrolysis reactions are generally catalyzed by acid or base

catalysts. Water near its critical point (at high temperature and pressure) has a

high ion product, so the hydrolysis reaction is catalyzed by the water itself.

A simpliﬁed representation of the reaction scheme is shown in Figure 7.5(a)

with polyethylene terephthalate (PET) as an example. The hydrolysis of PET

into terephthalic acid and ethylene glycol in SCW is a better option than other

reactions (e.g., methanolysis or glycololysis) because it does not require sol-

vents and catalysts like others. Here, water near its critical point is used to

accomplish this reaction in a shorter time. Additionally, SCW avoids the need

to recover and dispose of external solvents or catalysts. Figure 7.5(b) is a

photograph of polyethylene terephthalate in ordinary water before and after

hydrolysis in SCW into ﬁne particles of teraphthalic acid in ethylene glycol

solution.

(b)

(a)

Terephthalic acid

Ethylene glycol Terephthalic acid

Hydrolysis

PET (polyethylene terephthalate)

O H

O C C C C

O O O

HO C

O O O O

C OH+HO CH

OH+HO C C OH+HO OH

Ethylene glycol

FIGure 7.5 (a) Hydrolysis of PET in supercritical water; photograph of PET pellets in subcriti-

cal water and (b) that of its product in SCW after hydrolysis. (Source: Adapted from Kobe Steel.)

239

7.3 Biomass Conversion in SCW

7.3.3 Supercritical Water oxidation

Supercritical water that exhibits complete miscibility with oxygen is a homo-

geneous reaction medium for the oxidation of organic molecules. This feature

of SCW allows oxidation of harmful or toxic substances at low temperature in

a process known as supercritical water oxidation (SCWO) or cold combustion.

In a typical SCWO unit, the entire mixture (water, oxygen, and waste) remains

as a single ﬂuid phase with no interphase transport limitations. This allows very

rapid and complete (>99.9%) oxidation of the organic wastes to harmless

lower-molecular-weight compounds like H

O, N

, and CO

. Unlike thermal

incineration, SCWO produces toxic by-products such as dioxin. This method

of waste treatment is especially attractive for highly dilute toxic wastes in water.

One important shortcoming of this process is the production of highly cor-

rosive liquid efﬂuents because chlorine, sulfur, and phosphorous, if present in

the waste, are converted into their corresponding acids (Serani et al., 2008).

The destruction of polychlorinated biphenyls (PCBs) in supercritical water,

producing carbon dioxide and hydrochloric acid, may be represented by the

following simple reaction:

C H Cl PCB O H O CO HCl

12 10 2 2 2

19 2 5 12

−

( )

+ +

( )

+ −

( )

= +

m m

m m m (7.4)

Conventional thermal incineration uses very high temperature to destroy by-

products like dioxin, which results in the production of another pollutant, NOx.

This is not the case with SCWO owing to its low-temperature operation (450–

600 °C).

7.3.4 Scheme of an ScWG plant

A typical SCWG plant includes the following key components:

 Feedstock pumping system

 Feed preheater

 Gasiﬁer/reactor

 Heat-recovery (product-cooling) exchanger

 Gas–liquid separator

 Optional product-upgrading equipment

The feed preheating system is very elaborate and accounts for the majority

(~60%) of the capital investment in an SCW gasiﬁcation plant.

Figure 7.6 describes the SCWG process using the example of an SCWG

plant for gasifying sewage sludge. Biomass is made into a slurry for feeding.

It is then pumped to the required supercritical pressure. Alternatively, water

may be pressurized separately and the biomass fed into it. In any case, the

feedstock needs to be heated to the designed inlet temperature for the gasiﬁer,

which must be above the critical temperature and well above the designed

gasiﬁcation temperature because the enthalpy of the water provides the energy

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chapter

7 Hydrothermal Gasiﬁcation of Biomass

required for the endothermic gasiﬁcation reactions. This temperature is a criti-

cal design parameter.

The sensible heat of the product of gasiﬁcation may be partially recovered

in a waste heat-recovery exchanger and used for partial preheating of the feed

(Figure 7.6). For complete preheating, additional heat may be obtained from

one of the following:

 Externally ﬁred heater (Figure 7.6)

 Burning of a part of the fuel gas produced to supplement the external fuel

 Controlled burning of unconverted char in the reactor system (refer to

Figure 7.12 later in chapter)

After gasiﬁcation, the product is ﬁrst cooled in the waste heat-recovery unit.

Thereafter, it cools to room temperature in a separate heat exchanger by giving

off heat to an external coolant.

The next step involves separation of the reaction products. The solubility

of hydrogen and methane in water at low temperature but high pressure is

considerably low, so they are separated from the water after cooling while

the carbon dioxide, because of its high solubility in water, remains in the

liquid phase. For complete separation of CO

, the gas may be scrubbed with

additional water (refer to Figure 7.14 later in chapter). The gaseous hydrogen

is separated from the methane in a pressure swing adsorber. The CO

-rich liquid

is depressurized to the atmospheric pressure, separating the carbon dioxide

from the water and unconverted salts.

Combustion air

plus CH

Flue gas

Product

gas

Phase

separator

Clean

water

SCW

reactor

HET

exchanger

Product

Waste effluent pump

FIG

ure

7.6

Schematic of a pilot plant for supercritical water gasiﬁcation of biomass.

241

7.4 Effect of Operating Parameters on SCW Gasiﬁcation

7.4 eFFect oF operatInG parameterS

SIFIcatIon

The product of gasiﬁcation is deﬁned by its yield and composition, which are

inﬂuenced by a number of gasiﬁer design and operating parameters. For proper

design and operation of an SCW gasiﬁer, a good understanding of the inﬂuence

of the following parameters is important:

 Reactor temperature

 Catalyst use

 Residence time in the reactor

 Solid concentration in the feed

 Heating rate

 Feed particle size

 Reactor pressure

 Reactor type

7.4.1 reactor temperature

Temperature has an important effect on the conversion, the product distribution,

and the energy efﬁciency of an SCW gasiﬁer, which typically operates at a

maximum temperature of nearly 600 °C. The overall carbon conversion

increases with temperature; at higher temperatures hydrogen yield is higher

while methane yield is lower. Figure 7.7 shows the temperature dependence of

gasiﬁcation efﬁciency and product distribution in a reactor operated at 28 MPa

(30-s residence, 0.6-M glucose) (Lee et

al., 2002). We see that the hydrogen

yield increases exponentially above 600 °C, while the CO yield, which rises

gently with temperature, begins to drop above 600 °C owing to the start of the

shift reaction (Eq. 5.52).

Gasiﬁcation efﬁciency is measured in terms of hydrogen or carbon in the

gaseous phase as a fraction of that in the original biomass. Carbon conversion

efﬁciency increases continually with temperature, reaching close to 100%

above 700 °C. Hydrogen conversion efﬁciency (the fraction of hydrogen in

glucose converted into gas) also increases with temperature. It appears strange

that at 740 °C, the hydrogen conversion efﬁciency exceeds 100%, reaching

158%. This clearly demonstrates that the extra hydrogen comes from the water,

conﬁrming that water is indeed a reactant in the SCWG process as well as a

reaction medium.

Hydrothermal gasiﬁcation of biomass has been divided into three broad

temperature categories: high, medium, and low with their desired products

(Peterson et al., 2008). Table 7.2 shows that the ﬁrst group targets production

of hydrogen at a relatively high temperature (>500 °C); the second targets

production of methane at just above the critical temperature (~374.29 °C) but

below 500 °C; and the third gasiﬁes at subcritical temperature, using only

simple organic compounds as its feedstock. The last two groups, because of

their low-temperature operation, need catalysts for reactions.

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chapter

7 Hydrothermal Gasiﬁcation of Biomass

Temperature (°C)

(a)

(b)

Gas yield (mol of gas/mol of glucose)

Hydrogen

Methane

Carbon dioxide Carbon monoxide

100

120

140

160

180

500 550 600 650 700 750 800

450 500 550 600 650 700 750 800

Temperature (°C)

Gasification efficiency (%)

Hydrogen

Carbon

Oxygen

FIGure 7.7 Effect of temperature on gas yield (a), and effect of temperature on gasiﬁcation

efﬁciency (b). (Source: Adapted from Lee et

al., 2002.)

243

7.4 Effect of Operating Parameters on SCW Gasiﬁcation

7.4.2 catalysts

An effective degradation of biomass and the gasiﬁcation of intermediate prod-

ucts of thermal degradation into lower-molecular-weight gases like hydrogen

require the SCW reactor to operate in the high-temperature range (>600 °C).

The higher the temperature, the better the conversion, especially for production

of hydrogen, but the lower the SCW’s energy efﬁciency. A lower gasiﬁcation

temperature is therefore desirable for higher thermodynamic efﬁciency of the

process.

Catalysts help gasify the biomass at lower temperatures, thereby retaining,

at the same time, high conversion and high thermal efﬁciency. Additionally,

some catalysts also help gasiﬁcation of difﬁcult items like the lignin in biomass.

Watanabe et al. (2003) noted that the hydrogen yield from lignin at 400 °C and

MPa is doubled when a metal oxide (ZrO

) catalyst is used in the SCW. The

yield increases four times with a base catalyst (NaOH) compared to gasiﬁcation

without a catalyst. The three principal types of catalyst used so far for SCW

gasiﬁcation are: (1) alkali, (2) metal, and (3) carbon-based.

An important positive effect of catalysts in SCWG is the reduction in

required gasiﬁcation temperature for a given yield. Minowa et al. (1998) noted

a signiﬁcant reduction in unconverted char while gasifying cellulose with an

catalyst at 380 °C. Base catalysts (e.g., NaOH, KOH) offer better

performance, but they are difﬁcult to recover from the efﬂuent. Some alkalis

(e.g., NaOH, KOH, Na

, K

, and Ca(OH)

) are also used. They, too,

are difﬁcult to recover.

The special advantage of metal oxide catalysts is that they can be recovered,

regenerated, and reused. Commercially available nickel-based catalysts are

effective in SCW biomass gasiﬁcation. Among them, Ni/MgO (nickel sup-

ported on an MgO catalyst) shows high catalytic activity, especially for biomass

(Minowa et

al., 1998).

Metal catalysts have a severe corrosion effect at the temperatures needed to

secure high yields of hydrogen. To overcome this problem, Antal et

al. (2000)

used carbon (e.g., coal-activated and coconut shell–activated carbon and maca-

damia shell and spruce wood charcoal). The carbon catalysts resulted in high

yields of gas without tar formation.

TABLE 7.2 Hydrothermal Gasiﬁcation Temperature Categories

Based on Target Product (T

~374.29 °C)

Temperature (°C) Catalyst Target Product

High (>500)

Not needed Hydrogen-rich gas

Medium (T

– 500)

Needed Methane-rich gas

Low (<T

)

Essential Other gases of smaller organic molecules

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chapter

7 Hydrothermal Gasiﬁcation of Biomass

7.4.3 residence time

A longer residence for the reactants in the reactor gives a better yield. Lu et al.

(2006) experimented with 2% (by weight) sawdust and 2% carboxymethyl

cellulose (CMC) in a ﬂow reactor at 650 °C and 25

MPa. Mettanant et

al.

(2009) experimented with 2% rice husk in a batch reactor under the same

conditions. Both found a steady increase in hydrogen and a moderate increase

in methane (Figure 7.8) when the residence time was increased by three times

and six times, respectively. Total organic carbon in the liquid product decreases

with residence time, whereas carbon and hydrocarbon gasiﬁcation efﬁciencies

increase. This implies that a longer residence time is favorable for SCW biomass

gasiﬁcation. The optimum residence time, beyond which no further improve-

ment in conversion efﬁciency is possible, depends on several factors. At a

higher temperature, the residence time required for a given conversion is

shorter.

7.4.4 Solid concentration in Feedstock

Unlike in other gasiﬁcation methods, solids in the feed have an important effect

on the gasiﬁcation in supercritical water. Thermodynamic calculations suggest

that the conversion of carbon to gases in SCW declines rapidly when the solid

content in a liquid feed exceeds 50% (Prins et

al., 2005), but experimental

results show this to occur for a much lower concentration. Experimental

data (Mettanant et al., 2009; Schmieder et al., 2000) indicate that gasiﬁcation

10 20

Gas yield (mol/kg)

30 40

Residence time (min)

CO CO

50 60

FIG

ure

7.8

Effect of residence time on the gasiﬁcation of 2% rice husk in supercritical water

at 650 °C and 30

MPa in a batch reactor.

245

7.4 Effect of Operating Parameters on SCW Gasiﬁcation

efﬁciency starts to decline when the solid concentration exceeds a value as low

as 2%.

Table 7.3 presents data (Mozaffarian et

al., 2004) that show the effect of

solid content in feed. Although experimental conditions and feedstock vary, we

can broadly classify these results into groups of low, medium, and high solid

feedstock. For a lower feed concentration (<2%), carbon conversion efﬁciency

is in the range 100 to 92% and reduces to 60 to 90% for an intermediate con-

centration (2–10%) and to 68 to 80% for a >10% concentration. An SCW

gasiﬁer thus needs a very low solid concentration in the feed for high carbon

conversion efﬁciency. This requires higher pumping costs and liquid efﬂuent

disposal, which may be a major impediment in commercialization of SCW

gasiﬁcation.

The reactor type also inﬂuences how solid concentration affects gasiﬁ

cation efﬁciency. For example, Kruse et

al. (2003) noted that a stirred reactor

shows opposite results—that is, higher gasiﬁcation efﬁciency at higher solid

content (1.8 to 5.4%) in feed. This contrasts with data from Schmieder

al. (2000) from tumbling and tubular reactors that indicate a decrease in

gasiﬁcation efﬁciency with solid content (0.2–0.6

M). In stirred reactors,

reactants are very well mixed, resulting in a heating rate that is faster than

achieved in other reactor types. This may be the explanation for the higher

gasiﬁcation efﬁciency where there is a higher solid content. The exact reason

for this decrease is not clear and is a major issue in the development of com-

mercial SCW gasiﬁers. Catalysts, high gasiﬁcation temperatures, and high

heating rates can avoid the drop in conversion of a high-solid-content feedstock

(Lu et

al., 2006).

7.4.5 Heating rate

Limited data obtained by Sinag et al. (2004) suggest that at a higher heating

rate the yield of hydrogen, methane, and carbon dioxide increases while that

of carbon monoxide decreases. Further investigation is needed to elucidate this

point.

7.4.6 Feed particle Size

The effect of biomass particle size is not well researched. With limited data,

Lu et

al. (2006) showed that smaller particles result in a slightly improved

hydrogen yield and higher gasiﬁcation efﬁciency. However, Mettanant et

al.

(2009) did not observe any effect when they varied the size of rice husk par-

ticles in the range of 1.25 to 0.5

mm. Even if the size effect is conﬁrmed with

further data, it remains to be seen if the extra energy required for grinding is

worth the improvement.

TABLE 7.3 Effect of Solid Content in Feed and Other Operating Parameters on Gasiﬁcation

Investigators

C < 2 wt.% 2 < C < 10 wt.% C > 10 wt.%

Holgate, 1995 Yu, 1993 Kruse, 1999 Hao, 2003 Xu, 1996 Kruse, 2003 Yu, 1993 Xu, 1996

Feedstock Glucose Glucose Wood Glucose Formic acid Baby food Glucose Glucose

Feed concentration

in SCW (%/weight)

0.01 1.8 1 7.2 2.8 5.4 14.4 22

Pressure (bar) 246 345 350 250 345 300 345 345

Temperature (°C) 600 600 450 650 600 500 600 600

Reactor type Flow reactor Tubular ﬂow

reactor

Autoclave Tubular ﬂow

reactor (9

mm)

Tubular ﬂow

reactor

SCTR Tubular ﬂow

reactor

Tubular ﬂow

reactor

Residence time (s) 6 34 7200 210 34 300 34 34

Carbon conversion

efﬁciency (%)

100 90 91.8 89.6 93 60 68 80

Gas composition:

61.3 61.6 28.9 21.5 49.2 44 25 11

36.8 29 48.4 35.5 48.1 41 16.6 5.7

CO — 2 3.3 18.3 1.7 0.4 41.6 62.3

1.8 7.2 19 15.8 1 14.6 16.7 16.5

2,3

- - 5.3 - - - 4.5

C = concentration of solid in feed

Source: Compiled from Mozaffarian et

al., 2004.