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

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2.6 Other Gasiﬁcation-Related Properties of Biomass

The wet-basis moisture is

W W

wet

wet dry

wet

−

(2.21)

The wet basis (M

wet

) and the dry basis (M

dry

) are related as

dry

wet

−1

(2.22)

Fixed Carbon

Fixed carbon (FC) in a fuel is determined from the following equation, where

M, VM, and ASH stand for moisture, volatile matter, and ash, respectively.

FC M VM ASH

= − − −

1 (2.23)

This represents the solid carbon in the biomass that remains in the char in

the pyrolysis process after devolatilization. With coal, FC includes elemental

carbon in the original fuel, plus any carbonaceous residue formed while heating,

in the determination of VM (standard D-3175).

Biomass carbon comes from photosynthetic ﬁxation of CO

and thus all of

it is organic. During the determination of VM, a part of the organic carbon is

transformed into a carbonaceous material called pyrolytic carbon. Since FC

depends on the amount of VM, it is not determined directly. VM also varies

with the rate of heating. In a real sense, then, ﬁxed carbon is not a ﬁxed quantity,

but its value, measured under standard conditions, gives a useful evaluation

parameter of the fuel. For gasiﬁcation analysis, FC is an important parameter

because in most gasiﬁers the conversion of ﬁxed carbon into gases determines

the rate of gasiﬁcation and its yield. This conversion reaction, being the slowest,

is used to determine the size of the gasiﬁer.

Char

Char, though a carbon residue of pyrolysis or devolatilization, is not pure

carbon; it is not the ﬁxed carbon of the biomass. Known as pyrolytic char, it

contains some volatiles and ash in addition to ﬁxed carbon. Biomass char is

very reactive. It is highly porous and does not cake. This noncaking property

makes it easy to handle.

2.6.3 thermogravimetric analysis

Because of the time and expense involved in proximate analysis by ASTM

D-3172, Klass (1998) proposed an alternative using thermogravimetry (TG) or

differential thermogravimetry (DTG). In these techniques, a small sample of

the fuel is heated in a speciﬁed atmosphere at the desired rate in an electronic

microbalance. This gives a continuous record of the weight change of the fuel

sample in a TG apparatus. The DTG apparatus gives the rate of change in the

chapter

2 Biomass Characteristics

weight of the fuel sample continuously. Thus, from the measured weight loss-

versus-time graphs, we can determine the fuel’s moisture, volatile mater, and

ash content. The ﬁxed carbon can be found from Eq. (2.23). This method,

though not an industry standard, can quickly provide information regarding the

thermochemical conversion of a fuel. Table 2.10 compares results of proximate

analysis (dry basis) of some biomass from the ASTM and TG methods.

TG analysis provides additional information on reaction mechanisms,

kinetic parameters, thermal stability, and heat of reaction. A detailed database

of thermal analysis is given in Gaur and Reed (1995).

2.6.4 Bases of Expressing Biomass composition

The composition of a fuel is often expressed on different bases depending on

the situation. The following four bases of analysis are commonly used:

 As received

 Air dry

 Total dry

 Dry and ash-free

A comparison of these is shown in Figure 2.14.

As-Received Basis

When using the as-received basis, the results of ultimate and proximate analy-

ses may be written as follows:

Ultimate: %C H O N S ASH M

+ + + + + + =

100 (2.24)

Proximate: %VM FC M ASH

+ + + =

100 (2.25)

where VM, FC, M, and ASH represent the weight percentages of volatile matter,

ﬁxed carbon, moisture, and ash, respectively, measured by proximate analysis;

and C, H, O, N, and S represent the weight percentages of carbon, hydrogen,

As-received basis

Air-dry basis

Total-dry basis

Dry and ash-free basis

Coke

A ash O oxygen M

inherent moisture H hydrogen

N nitrogen M

surface moisture C carbon S sulfur

Volatile

VM M

SNOHC

FCA

FIG

urE

2.14

Bases for expressing fuel composition.

2.6 Other Gasiﬁcation-Related Properties of Biomass

oxygen, nitrogen, and sulfur, respectively, as measured by ultimate analysis.

The ash and moisture content of the fuel is the same in both analyses. As

received can be converted into other bases.

Air-Dry Basis

When the fuel is dried in air its surface moisture is removed while its inherent

moisture is retained. So, to express the constituent on an air-dry basis, the

amount is divided by the total mass less the surface moisture. For example, the

carbon percentage on the air-dry basis is calculated as

−

100

% (2.26)

where M

is the mass of surface moisture removed from 100 kg of moist fuel

after drying in air. Other constituents of the fuel can be expressed similarly.

Total-Dry Basis

Fuel composition on the air-dry basis is a practical parameter and is easy to

measure, but to express it on a totally moisture-free basis we must make allow-

ance for surface as well as inherent moisture. This gives the carbon on a total-

dry basis, C

−

100

(2.27)

where M is the total moisture (surface + inherent) in the fuel: M = M

+ M

Dry Ash–Free Basis

Ash is another component that at times is eliminated along with moisture. This

gives the fuel composition on a dry ash–free (DAF) basis. Following the afore-

mentioned examples, the carbon percentage on a dry ash–free basis, C

daf

, can

be found.

M ASH

daf

− −

100

(2.28)

where (100 − M − ASH) is the mass of biomass without moisture and ash.

The percentages of all constituents on any basis totals 100. For example:

C H O N S

daf daf daf daf daf

+ + + + =

100% (2.29)

2.6.5 heating Value of Fuel

The heating value of biomass is relatively low, especially on a volume basis,

because its density is very low.

chapter

2 Biomass Characteristics

Higher Heating Value

Higher heating value (HHV) is deﬁned as the amount of heat released by the

unit mass or volume of fuel (initially at 25 °C) once it is combusted and the

products have returned to a temperature of 25 °C. It includes the latent heat of

vaporization of water. HHV can be measured in a bomb calorimeter using

ASTM standard D-2015 (withdrawn by ASTM 2000, and not replaced). It is

also called gross caloriﬁc value (GCV). In North America the thermal efﬁ-

ciency of a system is usually expressed in terms of HHV, so it is important to

know the HHV of the design fuel.

Lower Heating Value

The temperature of the exhaust ﬂue gas of a boiler is generally in the range 120

to 180 °C. The products of combustion are rarely cooled to the initial tempera-

ture of the fuel, which is generally below the condensation temperature of

steam. So the water vapor in the ﬂue gas does not condense, and therefore the

latent heat of vaporization of this component is not recovered. The effective

heat available for use in the boiler is a lower amount, which is less than the

chemical energy stored in the fuel.

The lower heating value (LHV), also known as the net caloriﬁc value

(NCV), is deﬁned as the amount of heat released by fully combusting a speci-

ﬁed quantity less the heat of vaporization of the water in the combustion

product.

The relationship between HHV and LHV is given by

LHV HHV h

H M

= − +









100 100

(2.30)

where LHV, HHV, H, and M are lower heating value, higher heating value,

hydrogen percentage, and moisture percentage, respectively, on an as-received

basis. Here, h

is the latent heat of steam in the same units as HHV (i.e.,

970

BTU/lb, 2260

kJ/kg, or 540

kCal/kg).

Many European countries deﬁne the efﬁciency of a thermal system in terms

of LHV. Thus, an efﬁciency expressed in this way appears higher than that

expressed in HHV (as is the norm in many countries, including the United

States and Canada), unless the basis is speciﬁed.

Bases for Expressing Heating Values

Similar to fuel composition, heating value (HHV or LHV) may be also expressed

on any of the following bases:

 As-received basis (ar)

 Dry basis (db), also known as moisture-free basis (mf)

 Dry ash–free basis (daf), also known as moisture ash–free basis (maf)

2.6 Other Gasiﬁcation-Related Properties of Biomass

If M

kg of fuel contains Q kJ of heat, M

kg of moisture, and M

ash

kg of ash,

HHV can be written on different bases as follows:

HHV

M M

HHV

M M M

f w

daf

f w ash

−

( )

− −

( )

kJ kg

(2.31)

Estimation of Biomass Heating Values

Experimental methods are the most reliable means of determining the heating

value of biomass. If these are not possible, empirical correlations like the

Dulong-Berthelot equation, originally developed for coal with modiﬁed coef-

ﬁcients for biomass, may be used. Channiwala and Parikh (2002) developed

the following uniﬁed correlation for HHV based on 15 existing correlations and

50 fuels, including biomass, liquid, gas, and coal.

HHV C H S O N ASH= + + − − −349 1 1178 3 100 5 103 4 15 1 21 1. . . . . . kJ kg (2.32)

where C, H, S, O, N, and ASH are percentages of carbon, hydrogen, sulfur,

oxygen, nitrogen, and ash as determined by ultimate analysis on a dry basis.

This correlation is valid within the range:

 0 < C < 92%; 0.43 < H < 25%

 0 < O < 50; 0 < N < 5.6%

 0 < ASH < 71%; 4745 < HHV < 55,345 kJ/kg

Ultimate analysis is necessary with this correlation, but it is expensive and

time consuming. Zhu and Venderbosch (2005) developed an empirical method

to estimate HHV without ultimate analysis. This empirical relationship between

the stoichiometric ratio (SR) and the HHV is based on data for 28 fuels that

include biomass, coal, liquid, and gases. The relation is useful for preliminary

design:

HHV = ×3220 Stoichiometric ratio kJ kg, (2.33)

where the stoichiometric ratio is the theoretical mass of the air required to burn

1 kg fuel.

Stoichiometric Amount of Air for Complete Combustion

Noting that dry air contains 23.16% oxygen, 76.8% nitrogen, and 0.04% inert

gases by weight, the dry air required for complete combustion of a unit weight

of dry hydrocarbon, M

, is given by

M C H O S

= + −

( )

[ ]

0 1153 0 3434 8 0 0434. . . kg kg dry fuel (2.34)

chapter

2 Biomass Characteristics

where C, H, O, and S are the percentages of carbon, hydrogen, oxygen, and

sulfur, respectively, on a dry basis.

2.6.6 composition of the Product Gas of Gasiﬁcation

The product gas of gasiﬁcation is generally a mixture of several gases, including

moisture or steam. Its composition may be expressed in any of the following

ways:

 Mass fraction, m

 Mole fraction, n

 Volume fraction, V

 Partial pressure, P

It may also be expressed on a dry or a wet basis. The wet basis is the composi-

tion gas expressed on the basis of total mass of the gas mixture including any

moisture in it. The dry basis is the composition with the moisture entirely

removed.

The following example illustrates the relationship between different ways

of expressing the product gas composition.

Example 2.2

The gasiﬁcation of a biomass yields M kg/s product gas, with the production of

its individual constituents as follows:

Hydrogen—M

, kg/s

Carbon monoxide—M

, kg/s

Carbon dioxide—

, kg/s

Methane—

, kg/s

Other hydrocarbon (e.g., C

)—M

, kg/s

Nitrogen—M

, kg/s

Moisture—

H O

, kg/s

Find the composition of the product gas in mass fraction, mole fraction, and other

fractions.

Solution

Since the total gas production rate, M, is

M M M M M M M M

H CO CO CH H N H O

= + + + + + +

2 4 2

kg s (i)

the mass fraction of each species is found by dividing the individual production

rate by the total. For example, the mass fraction of hydrogen is m

= M

/M.

The mole of an individual species is found by dividing its mass by its molecular

weight:

Moles of hydrogen mass molecular weight of H, n m

H H

= =

2 (ii)

The total number of moles of all gases is found by adding the moles of i species

of gases, n = Σ (n

) moles. So the mole fraction of hydrogen is x

= n

/n. Similarly

for any gas, the mole fraction is

2.6 Other Gasiﬁcation-Related Properties of Biomass

x n n

i i

= (iii)

where the subscript refers to the ith species.

The volume fraction of a gas can be found by noting that the volume that

kmol of any gas occupies at NTP (at 0 °C and 1

atm) is 22.4

. So, taking the

example of hydrogen, the volume of 1

kmol of hydrogen in the gas mixture is

22.4

at NTP.

The total volume of the gas mixture is V = summation of volumes of all con-

stituting gases in the mixture = Σ[number of moles (n

) × 22.4] nm

= 22.4 n. The

volume fraction of hydrogen in the mixture is volume of hydrogen/total volume

of the mixture:

V n n n n x

H H i H H

( )

= =22 4 22 4. . Σ (iv)

Thus, we note that

Volume fraction mole fraction=

The partial pressure of a gas is the pressure it exerts if it occupies the entire

mixture volume, V. Ideal gas law gives the partial pressure of a gas component,

i, as

P n RT V

i i

= Pa

The total pressure, P, of the gas mixture containing total moles, n, is

P n RT V= Pa

So we can write

i i i

= = = (v)

Partial pressure as fraction of total pressure mole fracti= oon volume fraction=

The partial pressure of hydrogen is P

= x

The molecular weight of the mixture gas, MW

, is known from the mass frac-

tion and the molecular weight of individual gas species.

MW x MW

m i i

[ ]

Σ (vi)

where MW

is the molecular weight of gas component i with mole fraction x

symbols and nomenclature

ASH = weight percentage of ash (%)

C = weight percentage of carbon (%)

= speciﬁc heat of biomass (J/g.K)

pθ

= speciﬁc heat of biomass at temperature θ

C (J/g.C)

= speciﬁc heat of water (J/g.K)

pore

= pore diameter (m)

rad

= emissivity in the pores (–)

chapter

2 Biomass Characteristics

FC = weight percentage of ﬁxed carbon (%)

G(x), F(x), H(x) = functions of the cell structure and its dimensionless length of the

biomass in Eq. (2.10) (–)

HR = heat of combustion or heat of reaction (kJ/mol)

HF = heat of formation (kJ/mol)

H = weight percentage of hydrogen (%)

HHV = high heating value of fuel (kJ/kg)

= latent heat of vaporization (kJ/kg)

eff

= effective thermal conductivity of biomass (W/m.K)

= thermal conductivity of the solid in dry wood (W/m.K)

= thermal conductivity of the moisture in dry wood (W/m.K)

= thermal conductivity of the gas in dry wood (W/m.K)

rad

= radiative contribution to the conductivity of wood (W/m.K)

LHV = low heating value of fuel (kJ/kg)

wet

= biomass moisture expressed in wet basis (–)

dry

= biomass moisture expressed in dry basis (–)

= moisture percentage (by weight, %)

M = weight percentage of moisture (%)

= mass of surface moisture in biomass (kg)

= mass of inherent moisture in biomass (kg)

= mass of fuel (kg)

= mass of moisture in the fuel (kg)

ash

= mass of ash in the fuel (kg)

= mass fraction of the ith gas (–)

MW = molecular weight of gas mixture (–)

n = number of moles (–)

= mole fraction of the ith gas (–)

N = weight percentage of nitrogen (%)

O = weight percentage of oxygen (%)

= partial pressure of the ith gas (Pa)

P = total pressure of the gas (Pa)

Q = heat content of fuel (kJ)

R = universal gas constant (8.314 J/mol.K)

sp.gr = speciﬁc gravity (–)

S = weight percentage of sulfur (%)

T = temperature (K)

wet

= weight of wet biomass (kg)

dry

= weight of dry biomass (kg)

VM = weight percentage of volatile matter (%)

V = volume of gas (m

)

= volume fraction of the ith gas (–)

ΔH

comb

= heat of combustion or reaction, kJ/mol

true

= true density of biomass (kg/m

)

apparent

= apparent density of biomass (kg/m

)

bulk

= bulk density of biomass (kg/m

)

= bulk porosity of biomass (–)

= porosity of biomass (–)

2.6 Other Gasiﬁcation-Related Properties of Biomass

σ = Steven-Boltzmann’s constant (5.67 × 10

–8

W/ m

)

θ = temperature (°C)

Subscripts

ad, ar, db = subscripts representing air dry, as-received basis, and dry basis

daf = dry ash–free basis

td = total-dry basis

i = ith component

m = mixture

Pyrolysis and Torrefaction

Chapter 3

Biomass Gasiﬁcation and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00003-9

3.1 IntroductIon

Pyrolysis is a thermochemical decomposition of biomass into a range of useful

products, either in the total absence of oxidizing agents or with a limited supply

that does not permit gasiﬁcation to an appreciable extent. It is one of several

reaction steps or zones observed in a gasiﬁer. During pyrolysis, large complex

hydrocarbon molecules of biomass break down into relatively smaller and

simpler molecules of gas, liquid, and char (Figure 3.1).

Pyrolysis has similarity to and some overlap with processes like cracking,

devolatilization, carbonization, dry distillation, destructive distillation, and

thermolysis, but it has no similarity with the gasiﬁcation process, which involves

chemical reactions with an external agent known as gasiﬁcation medium. Pyrol-

ysis of biomass is typically carried out in a relatively low temperature range of

300 to 650 °C compared to 800 to 1000 °C for gasiﬁcation.

Torrefaction is a relatively new process that heats the biomass in the absence

of air to improve its usefulness as a fuel. Interest in torrefaction is rising on

account of its several advantages.

This chapter explains the basics of pyrolysis and torrefaction. A brief dis-

cussion of the design implications of the two is also presented.

3.1.1 Historical Background

Charcoal from wood via pyrolysis was essential for extraction of iron

from iron-ore in the pre-industrial era. Figure 3.2 shows a typical beehive oven

used in early times to produce charcoal from biomass using a slow pyrolysis

process. This practice continued until wood supplies nearly ran out and coal,

produced inexpensively from underground mines, replaced charcoal for iron

production.

The modern petrochemical industry owes a great deal to the invention of a

process of kerosene production using pyrolysis. In the mid-1840s, Abraham

Gesner, a physician practicing in Halifax, Canada (Figure 3.3), began searching