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

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227

6.11 Performance and Operating Issues

Top gas

burner

Venturi

Air

Fuel

Gas

Ash pit

Pyrolysis

zone

Reduction

zone

FIGure 6.24 Gasiﬁer with an aspirator for cracking tar. Fresh air picks up the tar from the

gasiﬁer and injects it into the high-temperature combustion zone.

H = fractional of hydrogen in the fuel in dry basis (–)

HHV = higher heating value (kJ/kg)

HHV

= higher heating value of biomass on dry basis (MJ/kg)

HHV

daf

= higher heating value of biomass on dry ash–free basis (MJ/kg)

bed

= height of the bed (m)

= enthalpy of steam at gasiﬁcation temperature (kJ/kg)

= heat of the input gas (kJ)

O] = concentration of steam (–)

k = rate constant (s

–1

)

= pre-exponential constant in the Arrhenius equation (s

–1

)

LHV

= lower heating value of the biomass (MJ/kg)

LHV

daf

= lower heating value of biomass on dry ash-free basis (MJ/kg)

LHV

= lower heating value of the solid fuel (MJ/Nm

)

LHV

= lower heating value of the produced gas (MJ/Nm

)

m = mass-ﬂow rate of carbon or char (kg/s)

= theoretical air requirement for complete combustion of a unit of biomass (kg/kg)

= amount of air required for gasiﬁcation of unit mass of biomass (kg/kg)

M = fractional of moisture in the fuel (–)

daf

= moisture based on dry ash-free basis

= fuel ﬂow rate (kg/s)

= quantity of steam (kg/s)

= gas produced (kg/s)

n = order of reaction (–)

= number of moles of species i (–)

N = fractional of nitrogen in the fuel in dry basis (–)

total

= total number of moles

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chapter

6 Design of Biomass Gasiﬁers

O = fractional of oxygen in the fuel in dry basis (–)

= amount of char produced per nm

of product gas (kg/nm

)

= heating value of char (kJ/kg)

Q = power output of the gasiﬁer (MWth)

ext

= external heat addition to the system (kJ/Nm

)

= Lower heating value of the product gas from gasiﬁcation (MJ/Nm

)

gasiﬁcation

= heat supplied to gasify 1 mol of biomass (kJ/mol)

loss

= heat loss from the gasiﬁer (kJ/Nm

)

r = steam gasiﬁcation reaction rate (kg/s)

R = universal gas constant (0.008314

kJ/mol.K)

S = fractional of sulfur in the fuel in dry basis (–)

SC = steam to carbon molar ratio (–)

t = time (s)

T = temperature (K)

= gas temperature at the exit (°C)

= gas temperature (°C)

= gas temperature at the entrance (°C)

= ﬂuidizing velocity (m/s)

V = volume of the ﬂuidized bed (m

)

bed

= volume of the bed (m

)

daf

= volatile based on dry mass-free basis

= gas generation rate (m

/s)

= volumetric ﬂow rate of product gas (Nm

/s)

= volumetric fraction of gas species i (–)

W = total steam needed in Eq. 6.22 (kg/s)

= rate of the char moving in (kg/s)

out

= rate of the char moving out (kg/s)

char

= weight of the reacting char (kg)

X = fraction of char in the feed converted (–)

= ﬁxed carbon fraction in the fuel (kg carbon/kg dry fuel)

char

= char fraction in bed (–)

= fraction of steam used up in gasiﬁcation

ε = voidage of the bed (–)

= Lagrangian multiplier for species i (–)

= density of air at the opening temperature and pressure of the gasiﬁer (kg/m

)

θ = residence time of char in bed or reactor (s)

= bed density (kg/m

)

= density of bed solids (kg/m

)

gef

= gasiﬁer efﬁciency (–)

ceff

= cold gas efﬁciency (–)

= cold gas efﬁciency of the gasiﬁer (–)

= hot gas efﬁciency of the gasiﬁer (–)

net

= net gasiﬁcation efﬁciency of the gasiﬁer (–)

ΔH

= heat of formation at temperature T (kJ/mol)

Hydrothermal Gasiﬁcation

of Biomass

Chapter 7

Biomass Gasiﬁcation and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00007-6

229

7.1 IntroductIon

In the mid-1970s Sanjay Amin, a graduate student working at the Massachu-

setts Institute of Technology (MIT), was studying the decomposition of organic

compounds in hot water (steam reforming):

C H O H O CO H

6 10 5 2 2 2

7 6 12+ → + (7.1)

While conducting an experiment in subcritical water, he observed that in addi-

tion to producing hydrogen and carbon dioxide, the reaction was producing

much char and tars. Herguido et

al. (1992) also made similar observations in

the steam gasiﬁcation of biomass at atmospheric pressure.

Sanjay interestingly noted that when he raised the water above its “critical

state,” the tar that formed in the subcritical state disappeared entirely (Amin

et al., 1975). This important ﬁnding kick-started research and development on

supercritical water oxidation (SCWO) for disposal of organic waste materials

(Tester et

al., 1993), which has now become a commercial option for disposal

of highly contaminated organic wastes (Shaw and Dahmen, 2000).

Biomass in general contains substantially more moisture than do fossil

fuels like coal. Some aquatic species, such as water hyacinth, or waste products,

such as raw sewage, can have water contents exceeding 90%. Thermal gasiﬁca-

tion, where air, oxygen, or subcritical steam is the gasiﬁcation medium, is

very effective for dry biomass, but it becomes very inefﬁcient for a high-

moisture biomass because the moisture must be substantially driven away

before thermal gasiﬁcation can begin; in addition, a large amount of the extra

energy (~2260

kJ/kg moisture) is consumed in its evaporation. For example,

Yoshida et

al. (2003) saw the efﬁciency of their thermal gasiﬁcation system

reduce from 61 to 27% while the water content of the feed increased from 5 to

75%. So, for gasiﬁcation of very wet biomass, some other means such as

anaerobic digestion (see Section 2.2) and hydrothermal gasiﬁcation in high-

pressure hot water are preferable because the water in these processes is not a

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chapter

7 Hydrothermal Gasiﬁcation of Biomass

liability as it is in thermal gasiﬁcation. Instead it serves as a reaction medium

and a reactant.

The efﬁciencies of these processes do not decrease with moisture content.

For anaerobic digestion and supercritical gasiﬁcation, Yoshida et

al. (2003)

found the gasiﬁcation efﬁciency to remain nearly unchanged, at 31% and

51%, respectively, even when the moisture in the biomass increased from 5

to 75%.

A major limitation of anaerobic digestion is that it is very slow, with a rela-

tively low efﬁciency and, most important, it produces methane only, no hydro-

gen. If hydrogen is the desired product, as is often the case, an additional step

of steam reforming the methane (CH

+ H

O = CO + 3H

) must be added to

the anaerobic digestion process.

Hydrothermal gasiﬁcation involves gasiﬁcation in an aqueous medium at

very a high temperature and pressure exceeding or close to its critical value.

While subcritical water has been used effectively for hydrothermal reaction,

supercritical water has attracted more attention owing to its unique features.

Supercritical water offers rapid hydrolysis of biomass, high solubility of inter-

mediate reaction products, including gases, and a high ion product near (but

below) the critical point that helps ionic reaction. These features make super-

critical water an excellent reaction medium for gasiﬁcation, oxidation, and

synthesis.

This chapter deals primarily with hydrothermal gasiﬁcation of biomass in

supercritical water. It explains the properties of supercritical water and the

biomass conversion process in it. The effects of different parameters on SCW

gasiﬁcation and design considerations for the SCW gasiﬁcation plants are also

presented.

7.2 SupercrItIcal Water

Water above its critical temperature (374.29 °C) and pressure (22.089 MPa) is

called supercritical (Figure 7.1). Water or steam below this pressure and tem-

perature is called subcritical. The term water in a conventional sense may not

be applicable to SCW except for its chemical formula, H

O, because above the

critical temperature SCW is neither water nor steam. It has a waterlike density

but a steam like diffusivity. Table 7.1 compares the properties of subcritical

water and steam with those of SCW, indicating that SCW’s properties are

intermediate between the liquid and gaseous states of water in subcritical

pressure; descriptions of each follow the table.

Figure 7.1 shows that the higher the temperature, the higher the pressure

required for water to be in its liquid phase. Above a critical point the line sepa-

rating the two phases disappears, suggesting that the division between the liquid

and vapor phases disappears. Temperature and pressure at this point are known

as critical temperature, and critical pressure, above which water attains super-

critical state and hence is called supercritical (SCW).

231

7.2 Supercritical Water

22.1

374

0.1

611 (Pa)

0.01 100

Ice

Water

Vapor

Critical point

Temperature (°C)

Pressure (MPa)

Supercritical

water

FIG

ure

7.1

Phase diagram of water showing the supercritical region.

TABLE 7.1 Properties of Supercritical and Subcritical Water

Property

Subcritical

Water

Supercritical

Water

Supercritical

Subcritical

Steam

Temperature (°C) 25 400 55 150

Pressure (MPa) 0.1 30 28 0.1

Density, kg/m

997* 358* 835 0.52*

Dynamic viscosity, µ

(kg/m.s)

890.8 × 10

−6

43.83 × 10

−6

* 0.702 × 10

−6

14.19 × 10

−6

Diffusivity of small

particles (m

/s)

~1.0 × 10

−9

** ~1.0 × 10

−8

** ~1.0 × 10

−5

Dielectric constant*** 78.46 5.91 1.0

Thermal conductivity,

λ (w/m.k)

607 × 10

−3

* 330 × 10

−3

* 28.8 × 10

−3

Prandtl number,

µ/λ

6.13 3.33 0.97

*Haar et al., 1984; **Serani et al., 2008; ***Uematsu and Franck, 1980.

Subcritical water (T < T

sat

; P < P

). When the pressure is below

its critical value, P

, and the temperature is below its critical value, T

, the

ﬂuid is called subcritical. If the temperature is below its saturation value,

the ﬂuid is known as subcritical water, as shown in the lower left block of

Figure 7.1.

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chapter

7 Hydrothermal Gasiﬁcation of Biomass

Subcritical steam (T > T

sat

; P < P

. Note: T may be above T

). When water

(below critical pressure) is heated, it experiences a drop in density and an

increase in enthalpy; this change is very sharp when the temperature of the

water just exceeds it saturation value, T

sat

. Above the saturation temperature,

but below the critical value, the ﬂuid (H

O) is called subcritical steam. This

regime is shown below the saturation line in Figure 7.1.

Supercritical water (T > T

; P > P

). When heated above its critical pres-

sure, P

, water experiences a continuous transition from a liquidlike state

to a vapor like state. The vaporlike, supercritical, state is shown in the

upper right block in Figure 7.1. Unlike in the subcritical stage, no heat of

vaporization is needed for the transition from liquidlike to vaporlike.

Above the critical pressure, there is no saturation temperature separating

the liquid and vapor states. However, there is a temperature, called

pseudo-critical temperature, that corresponds to each pressure (>P

) above

which the transition from liquidlike to vaporlike takes place. The pseudo-

critical temperature is characterized by a sharp rise in the speciﬁc heat of

the ﬂuid.

The pseudo-critical temperature depends on the pressure of the water. It

can be estimated within 1% accuracy by the following empirical equation

(Malhotra, 2006):

T P

F P P

c c

* *

( )

= + −

( )

= =

0 1248 0 01424 0 0026

. . .

;

(7.2)

where T

sat

is the saturation temperature at pressure P; P

sat

is the saturation

pressure at temperature T; P

is the critical pressure of water, 22.089 MPa;

is the critical temperature of water, 374.29 °C; and T

is the pseudo-

critical temperature at pressure P (P > P

7.2.1 properties of Supercritical Water

The critical point marks a signiﬁcant change in the thermophysical properties

of water (Figure 7.2). There is a sharp rise in the speciﬁc heat near the critical

temperature followed by a similar drop. The thermal conductivity of water

drops from 0.330

W/m.K at 400 °C to 0.176

W/m.K at 425 °C. The drop in

molecular viscosity is also signiﬁcant, although the viscosity starts rising with

temperature above the critical value. Above this critical point, water experi-

ences a dramatic change in its solvent nature primarily because of its loss of

hydrogen bonding. The dielectric constant of the water drops from a value of

about 80 in the ambient condition to about 10 at the critical point. This changes

the water from a highly polar solvent at an ambient condition to a nonpolar

solvent, like benzene, in a supercritical condition.

233

7.2 Supercritical Water

The change in density in supercritical water across its pseudo-critical tem-

perature is much more modest, however. For example, at 25

MPa it can drop

from about 1000 to 200

kg/m

while the water moves from a liquidlike to a

vaporlike state. At subcritical pressure, however, there is an order of magnitude

drop in density when the water goes past its saturation temperature. For example,

at 0.1

MPa or 1

atm of pressure, the density reduces from 1000 to 0.52

kg/m

as the temperature increases from 25 to 150 °C (refer to Table 7.1).

The most important feature of supercritical water is that we can “manipu-

late” and control its properties around its critical point simply by adjusting the

temperature and pressure. Supercritical water possesses a number of special

properties that distinguish it from ordinary water. Some of those properties

relevant to gasiﬁcation are as follows:

 The solvent property of water can be changed very strongly near or above

its critical point as a function of temperature and pressure.

 Subcritical water is polar, but supercritical water is nonpolar because of its

low dielectric constant. This makes it a good solvent for nonpolar organic

compounds but a poor one for strongly polar inorganic salts. SCW can be

a solvent for gases, lignin, and carbohydrates, which show low solubility in

ordinary (subcritical) water. Good miscibility of intermediate solid organic

compounds as well as gaseous products in liquid SCW allows single-phase

chemical reactions during gasiﬁcation, removing the interphase barrier of

mass transfer.

100

0 200 400 600 800 1000 1200

Temperature (°C)

Dielectric constant (–)

–20

100

120

Specific heat (kJ/kg.K)

dielectric constant specific heat

374 °C

FIG

ure

7.2

Speciﬁc heat of water above its critical pressure shows a peak at its pseudo-critical

temperature. Dielectric constant at 22.1

MPa, also plotted on this graph, shows rapid decline closer

to the critical temperature.

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chapter

7 Hydrothermal Gasiﬁcation of Biomass

 SCW has a high density compared to subcritical steam at the same tempera-

ture. This feature favors the forward reaction between cellulose and water

to produce hydrogen.

 Near its critical point, water has higher ion products ([H

][OH

−

]~10

–11

(mol/l)

) than it has in its subcritical state at ambient conditions (~10

−14

(mol/l)

) (Figure 7.3). Owing to this high [H

] and [OH

–

] ion, the water can

be an effective medium for acid- or base-catalyzed organic reactions (Serani

al., 2008). Above the critical point, however, the ion product drops

rapidly (~10

−24

(mol/l)

at 24 MPa), and the water becomes a poor medium

for ionic reactions.

 Most ionic substances, such as inorganic salts, are soluble in subcritical

water but nearly insoluble under typical conditions of SCW gasiﬁers. As

the temperature rises past the critical point, the density as well as the ionic

product decreases (Figure 7.3). Thus, highly soluble common salt (NaCl)

becomes insoluble at higher temperatures above the critical point. This

tunable solubility property of SCW makes it relatively easy to separate the

salts as well as the gases from the product mixture in an SCW gasiﬁer.

 Gases, such as oxygen and carbon dioxide, are highly miscible in SCW,

allowing homogeneous reactions with organic molecules either for oxida-

tion or for gasiﬁcation. This feature makes SCW an ideal medium for

destruction of hazardous chemical waste through SCWO.

 SCW possesses excellent transport properties. Its density is lower than that

of subcritical water but much higher than that of subcritical steam. This,

along with other properties like low viscosity, low surface tension (surface

tension of water reduces from 7.2 × 10

−2

at 25 °C to 0.07 at 373 °C), and

high diffusivity greatly contribute to the SCW’s good transport property,

1400 –10

–12

–14

–16

–18

–20

–22

–24

–26

–28

–30

1200

1000

800

600

400

200

0 200

Density

Ionic product

Drop of physical properties

Density (kg/m

)

Log ionic product (mol

–2

)

400

Temperature (°C)

24 MPa

at 24 MPa

38 MPa

at 38 MPa

600

FIGure 7.3 At a given temperature, both the density and the ion product of water increase with

pressure. Data plotted for 24 MPa show that these values reduce quickly at the critical temperature,

but are slower at 38

MPa. The ion product is plotted as negative log.kw. (Source: Adapted from

Kritz, 2004; used with permission.)

235

7.2 Supercritical Water

which allows it to easily enter the pores of biomass for effective and fast

reactions.

 Reduced hydrogen bonding is another important feature of SCW. The high

temperature and pressure break the hydrogen-bonded network of water

molecules.

Table 7.1 compares some of these water properties under subcritical and super-

critical conditions.

7.2.2 application of Supercritical Water in chemical reactions

Chemical reactions involve the mixing of reactants. If the mixing is incomplete,

the reaction will be incomplete, even if the right amounts of reactant and the

right temperature are available. The mixing is better when all reactants are

either in the gas phase or in the liquid phase compared to that when one reactant

is in the solid phase and the other is in the gas or liquid phase. The absence of

interphase resistance in a monophase reaction medium greatly improves the

mixing. The conventional thermal gasiﬁcation of solid biomass in air or steam

involves heterogeneous mixing, and therefore the gas–solid interphase resis-

tance limits the conversion reactions.

Supercritical water allows reactions to take place in a single phase, as most

organic compounds and gases are completely miscible in it. It is thus a superior

reaction medium. Because the absence of interphase mass transfer resistance

facilitates better mixing and therefore higher conversion, SCW can be an excel-

lent medium for the following three types of reactions:

Hydrothermal gasiﬁcation of biomass. SCW is an ideal medium for gas-

iﬁcation of very wet biomass, such as aquatic species and raw sewage,

which ordinarily have to be dried before they can be gasiﬁed economically.

SCW gasiﬁcation produces gas at high pressure and thus obviates the need

for an expensive product gas compression step for transport or use in

combustion.

Synthesis reactions. A variety of organic reactions like hydrolysis and

molecular rearrangement can be effectively carried out in SCW, which

serves as a solvent, a reactant, and sometimes a catalyst. There is no need

for acid or base solvents, the disposal of which is often a problem.

Supercritical water oxidation. Complete miscibility of oxygen in SCW

helps harmful organic compounds to be easily oxidized and degraded. Thus,

SCW is an attractive means of turning pollutants into harmless oxides.

7.2.3 advantages of ScW Gasiﬁcation over conventional

hermal

Gasiﬁcation

The following are two broad routes for the production of energy or chemical

feedstock from biomass:

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chapter

7 Hydrothermal Gasiﬁcation of Biomass

Biological: Direct biophotolysis, indirect biophotolysis, biological reac-

tions, photofermentation, and dark fermentation are the ﬁve major biologi-

cal processes.

Thermochemical: Combustion, pyrolysis, liquefaction, and gasiﬁcation are

the four main thermochemical processes.

Thermal conversion processes are relatively fast, taking minutes or seconds to

complete, while biological processes, which rely on enzymatic reactions, take

much longer, on the order of hours or even days. Thus, for commercial use,

thermochemical conversion is preferred.

Gasiﬁcation may be carried out in air, oxygen, subcritical steam, or water

near or above its critical point. This chapter concerns hydrothermal gasiﬁcation

of biomass above or very close to the water’s critical point to produce energy

and/or chemicals.

Conventional thermal gasiﬁcation faces major problems from the forma

tion of undesired tar and char. The tar can condense on downstream equip

ment, causing serious operational problems, or it may polymerize to a

more complex structure, which is undesirable for hydrogen production. Char

residues contribute to energy loss and operational difﬁculties. Furthermore,

very wet biomass can be a major challenge to conventional thermal gasiﬁcation

because it is difﬁcult to economically convert if it contains more than 70%

moisture. The energy used in evaporating fuel moisture (2257

kJ/kg), which

effectively remains unrecovered, consumes a large part of the energy in the

product gas.

Gasiﬁcation in supercritical water (SCWG) can largely overcome these

shortcomings, especially for very wet biomass or organic waste. For example,

the efﬁciency of thermal gasiﬁcation of a biomass containing 80% water in

conventional steam reforming is only 10%, while that of hydrothermal gasiﬁca-

tion in SCW can be as high as 70% (Dinjus and Kruse, 2004). Gasiﬁcation in

near or supercritical water therefore offers the following beneﬁts:

 Tar production is low. The tar precursors, such as phenol molecules, are

completely soluble in SCW and so can be efﬁciently reformed in SCW

gasiﬁcation.

 SCWG achieves higher thermal efﬁciency for very wet biomass.

 SCWG can produce in one step a hydrogen-rich gas with low CO, obviating

the need for an additional shift reactor downstream.

 Hydrogen is produced at high pressure, making it ready for downstream

commercial use.

 Carbon dioxide can be easily separated because of its much higher solubility

in high-pressure water.

 Char formation is low in SCWG.

 Heteroatoms like S, N, and halogens leave the process with aqueous efﬂu-

ent, avoiding expensive gas cleaning. Inorganic impurities, being insoluble

in SCW, are also removed easily.