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

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chapter

3 Pyrolysis and Torrefaction

(a) (b)

Fluidizing

gas

Air

Biomass

Gas and oil vapors

Solid

and

char

Flue

gas

Screw

feeder

Pyrolyzer

Hot

solid

Loop seal

Combustor

Freeboard

Char, gas,

and oil

vapors

Heat

Biomass

Distributor

plate

Fluid bed

Screw

feeder

Fluidizing gas (input)

(e)

Spinning

disk

Pressure

applied

to wood

Bio-oil liquid

released

from wood

Hot solid

Biomass

Pyrolysis

vapor

Air

Char

Reactor head

Aluminum

insert

Pressure

tap

Reactor

body

Biomass feed

Product quenching

Heat-carrier

solid

FIGurE 3.9 A variety of pyrolyzer designs: (a) bubbling ﬂuidized bed, (b) circulating ﬂuidized

bed, (c) ultra-rapid, (d) ablative, (e) rotating cone, and

3.6 Pyrolyzer Types

The product may ﬂow out of the pyrolyzer because of volume expansion while

the char remains in the reactor. In some designs a sweep gas is used for effec-

tive removal of the product gas from the reactor. This gas is necessarily inert

without oxygen. The main product of this type is char owing to the relatively

slow heating rate and the long residence time of the product in the pyrolysis

zone.

3.6.2 Bubbling-Bed Pyrolyzer

Figure 3.9(a) shows a bubbling ﬂuidized-bed pyrolyzer. Crushed biomass

(2–6

mm) is fed into a bubbling bed of hot sand. The bed is ﬂuidized by an

inert gas such as recycled ﬂue gas. Intense mixing of inert bed solids (sand is

commonly used) offers good and uniform temperature control. It also provides

high heat transfer to biomass solids. The residence time of the solids is consid-

erably higher than that of the gas in the pyrolyzer.

The required heat for pyrolysis may be provided either by burning a part of

the product gas in the bed, as shown in Figure 3.5, or by burning the solid char

in a separate chamber and transferring that heat to the bed solids (Figure 3.9b).

The pyrolysis product would typically contain about 70 to 75% liquid on dry

wood feed. As shown in the ﬁgure, the char in the bed solids acts as a vapor-

cracking catalyst, so its separation through elutriation or otherwise is important

if the secondary cracking is to be avoided to maximize the liquid product. The

entrained char particles are separated from the product gas using single- or

multistage cyclones. A positive feature of a bubbling ﬂuidized-bed pyrolyzer

is that it is relatively easy to scale up.

Vapor

Condenser

Scrapper driver

Liquid

200 °C

400 °C

Biomass feed

Vacuum pyrolysis

reactor

(f)

FIGurE 3.9 Continued

(f) vacuum.

chapter

3 Pyrolysis and Torrefaction

3.6.3 circulating Fluidized-Bed Pyrolyzer

A circulating ﬂuidized-bed (CFB) pyrolyzer, shown in Figure 3.9(b), works on

the same principle as the bubbling ﬂuidized bed except that the bed is highly

expanded and solids continuously recycle around an external loop com

prising a cyclone and loop seal. The bed operates in a special hydrodynamic

regime known as fast bed. It provides good temperature control and is uniform

around the entire height of the unit. The superﬁcial gas velocity in a CFB

is considerably higher than that in a bubbling bed. High velocity combined

with excellent mixing allows a CFB to have large throughputs of biomass.

Here, gas and solids move up the reactor with some degree of internal reﬂuxing.

As a result, the residence time of average biomass particles is longer than that

of the gas, but the difference is not as high as it is in a bubbling bed. A

major advantage of this system is that char entrained from the reactor is easily

separated and burnt in an external ﬂuidized bed. The combustion heat is trans-

ferred to the inert bed solids that are recycled to the reactor by means of a

loop seal.

Rapid thermal pyrolysis (RTP), a commercial process developed by Ensyn,

probably originated from the ultra-rapid ﬂuidized-bed pyrolyzer developed at

the University of Western Ontario in Canada. RTP uses a riser reactor. Here,

biomass is introduced into a vessel and rapidly heated to 500 °C by a tornado

of upﬂowing hot sand; it is then cooled within seconds. The heating rate is on

the order of 1000 °C/s, and the reactor residence time is from a few hundredths

of a millisecond to a maximum of 5 seconds, which gives a liquid yield as high

as 83% for wood (Hulet et

al., 2005).

3.6.4 ultra-rapid Pyrolyzer

High heating rate and short residence time in the pyrolysis zone are two key

requirements of high liquid yield. The ultra-rapid pyrolyzer, shown in Figure

3.9(c), developed by the University of Western Ontario provides extremely

short mixing (10–20

ms), reactor residence (70–200

ms), and quench (~20

ms)

times. Because the reactor temperature is also low (~650 °C), one can achieve

a liquid yield as high as 90% (Hulet et al., 2005). The inert gas nitrogen is

heated 100 °C above the reactor temperature and injected at very high velocity

into the reactor to bombard a stream of biomass injected in the reactor. The

reactor can also use a heat-carrier solid like sand that is heated externally and

bombarded on a biomass stream through multiple jets. Such a high-velocity

impact in the reactor results in an exceptionally high heating rate. The biomass

is thus heated to the pyrolysis temperature in a few milliseconds. The pyrolysis

product leaves the reactor from the bottom and is immediately cooled to sup-

press a secondary reaction or cracking of the oil vapor. This process is therefore

able to maximize the liquid yield during pyrolysis.

3.6 Pyrolyzer Types

3.6.5 Ablative Pyrolyzer

This process, shown in Figure 3.9(d), involves creation of high pressure between

a biomass particle and a hot reactor wall. This allows uninhibited heat transfer

from the wall to the biomass, causing the liquid product to melt out of the

biomass the way frozen butter melts when pressed against a hot pan. The

biomass sliding against the wall leaves behind a liquid ﬁlm that evaporates and

leaves the pyrolysis zone, which is the interface between biomass and wall. As

a result of high heat transfer and short gas residence time, a liquid yield as high

as 80% is reported (Diebold and Power, 1988). The pressure between biomass

and wall is created either by mechanical means or by centrifugal force. In a

mechanical system a large piece of biomass is pressed against a rotating

hot plate.

3.6.6 rotating-cone Pyrolyzer

In this process, biomass particles are fed into the bottom of a rotating cone

(360–960

rev/min) together with an excess of heat-carrier solid particles (see

Figure 3.9(e)). Centrifugal force pushes the particles against the hot wall; the

particles are transported spirally upward along the wall. Owing to its excellent

mixing, the biomass undergoes rapid heating (5000

K/s) and is pyrolyzed

within the small annular volume. The product gas containing bio-oil vapor

leaves through another tube, while the solid char and sand spill over the upper

rim of the rotating cone into a ﬂuidized bed surrounding it, as shown in Figure

3.9(e). The char burns in the ﬂuidized bed, and this combustion helps heat the

cone as well as the solids that are recycled to it to supply heat for pyrolysis.

Special features of this reactor include very short solids residence time (0.5

seconds) and a small gas-phase residence time (0.3

seconds). These typically

provide a liquid yield of 60 to 70% on dry feed (Hulet et

al., 2005). The absence

of a carrier gas is another advantage of this process. The complex geometry of

the system may raise some scale-up issues.

3.6.7 Vacuum Pyrolyzer

A vacuum pyrolyzer, as shown in Figure 3.9(f), comprises a number of stacked

heated circular plates. The top plate is at about 200 °C while the bottom one

is at about 400 °C. Biomass fed to the top plate drops into successive lower

plates by means of scrapers. The biomass undergoes drying and pyrolysis while

moving over the plates. No carrier gas is required in this pyrolyzer. Only char

is left when the biomass reaches the lowest plate. Though the heating rate of

the biomass is relatively slow, the residence time of the vapor in the pyrolysis

zone is short. As a result, the liquid yield in this process is relatively modest,

about 35 to 50% on dry feed, with a high char yield. This pyrolyzer design is

complex, especially given the fouling potential of the vacuum pump.

chapter

3 Pyrolysis and Torrefaction

3.7 PyrolyzEr dEsIGn consIdErAtIons

This section discusses pyrolyzer design considerations in the production of

liquid fuel and charcoal through pyrolysis.

3.7.1 Production of liquid through Pyrolysis

Pyrolysis is one of several means of production of liquid fuel from biomass.

The maximum yield of organic liquid (pyrolytic oil or bio-oil) from thermal

decomposition may be increased to as high as 70% (dry weight) if the biomass

is rapidly heated to an intermediate temperature and if a short residence time

in the pyrolysis zone is allowed to reduce secondary reactions. Table 3.2 earlier

in the chapter shows how heating rate, pyrolysis temperature, and residence

time affect the nature of the pyrolysis product. These ﬁndings may be sum-

marized as follows:

 A slower heating rate, a lower temperature, and a longer residence time

maximize the yield of solid char.

 A higher heating rate, a higher temperature, and a shorter residence time

maximize the gas yield.

 A higher heating rate, an intermediate temperature, and a shorter residence

time maximize the liquid yield.

There is an optimum pyrolysis temperature for maximum liquid yield. The

yield is highest at 500 °C and drops sharply above and below this temperature

(Boukis et

al., 2007). The residence time is generally in the range of 0.1 to 2.0

seconds. These values depend on several factors, including the type of biomass

(Klass, 1998). We can use a kinetic model for a reasonable yield assessment.

The one proposed by Liden et

al. (1988) is successful in predicting pyrolysis

liquid yields over a wide range of conditions.

Heat transfer is a major consideration in the design of a pyrolyzer. The heat

balance for a typical pyrolyzer may be written as

Heat released by char combustion Heat in incoming stream

[ ]

[[ ]

[ ]

Heat required for pyrolysis Heat loss

(3.10)

Assessing heat loss accurately is difﬁcult before the unit is designed. So, for

preliminary assessment, we can take this to be 10% of the heat in the incoming

stream (Boukis et

al., 2007, p. 1377).

Fast, or ﬂash, pyrolysis is especially suitable for pyrolytic liquefaction of

biomass. The product is a mixture of several hydrocarbons, which allows pro-

duction of fuel and chemicals through appropriate reﬁning methods. The heating

value of the liquid produced is in the same range (15–19

MJ/kg) as that

of the

parent biomass. The pyrolytic liquid contains several water-soluble sugars and

polysaccharide-derivative compounds and water-insoluble pyrolytic lignin.

3.7 Pyrolyzer Design Considerations

Pyrolytic liquid contains a much higher amount of oxygen (~50%) than does

most fuel oil. It is also heavier (speciﬁc gravity ~1.3) and more viscous. Unlike

fuel oil, pyrolytic oil increases in viscosity with time because of polymerization.

This oil is not self-igniting like fuel oil, and as such it cannot be blended with

diesel for operating a diesel engine.

Pyrolytic oil is, however, a good source of some useful chemicals, like

natural food ﬂavoring, that can be extracted, leaving the remaining product for

burning. Alternately, we can subject the pyrolytic oil to hydrocracking to

produce gasoline and diesel.

3.7.2 Production of charcoal through Pyrolysis

Carbon is a preferred product of biomass pyrolysis at a moderate temperature.

Thermodynamic equilibrium calculation shows that the char yield of most

biomass may not exceed 35%. Table 3.6 gives the theoretical equilibrium yield

of biomass at different temperatures. Assuming that cellulose represents bio

mass, the stoichiometric equation for production of charcoal (Antal, 2003) may

be written as

C H O C H O CO CH

6 10 5 2 2 4

3 74 2 65 1 17 1 08→ + + +. . . . (3.11)

Charcoal production from biomass requires slow heating for a long duration

but at a relatively low temperature of around 400 °C. An extreme example of

a pyrolysis or carbonization is in the coke oven in an iron and steel plant, which

pyrolyzes (carbonizes) coking coal to produce hard coke used for iron extrac-

tion. This is an indirectly heated ﬁxed-bed pyrolyzer that operates at a tempera-

ture exceeding 1000 °C and for a long period of time to maximize gas and solid

coke production.

Table 3.6 Thermodynamic Equilibrium Concentration of Cellulose

Pyrolysis at Different Temperatures

Product

(%)

Temperature (°C)

200 300 400 500 600

C 32 28 27 27 25.2

O 36.5 32.5 9.5 27 22.5

8.5 10 10.5 10 9

23.9 28 32 35 36

CO 0 0 0.1 1.2 4.5

Source: Derived from data in Antal, 2003.

chapter

3 Pyrolysis and Torrefaction

3.8 torrEFActIon

Torrefaction, a process different from carbonization, is a mild pyrolysis process

carried out in a temperature range of 230 to 300 °C in the absence of oxygen.

This thermal pretreatment of biomass improves its energy density, reduces its

oxygen-to-carbon (O/C) ratio, and reduces its hygroscopic nature. During this

process the biomass dries and partially devolatilizes, decreasing its mass while

largely preserving its energy content. The torrefaction process removes H

and CO

from the biomass. As a result, both the O/C and the H/C ratios of the

biomass decrease. In raw biomass, high oxygen content prompts its over-

oxidation during gasiﬁcation, increasing the thermodynamic losses of the

process. Torrefaction could reduce this loss by reducing the oxygen in the

biomass. Torrefaction also increases the relative carbon content of the biomass.

The properties of a torreﬁed wood depends on torrefaction temperature, time,

and on the type of wood feed.

A popular example of torrefaction is the process of roasting coffee beans.

As the green beans are heated to 200 to 300 °C, their surface darkens (www.

coffeeresearch.org/coffee). Figure 3.10 contains photographs of rice husk,

peanut husk, bagasse, and water hyacinth before and after torrefaction. The

color change is present in all biomass but to different degrees.

Torrefaction also modiﬁes the structure of the biomass, making it more

friable or brittle. This is caused by the depolymerization of hemicellulose. As

a result, the process of size reduction becomes easier, lowering its energy con-

sumption and the cost of handling. This makes it easier to co-ﬁre biomass in a

pulverized-coal ﬁred boiler or gasify it in an entrained-ﬂow reactor.

Torrefaction causes some reduction in the energy content of the biomass

because of partial devolatilization, but given the much higher reduction in mass,

the energy density of the biomass increases. Table 3.7 shows an example of

torrefaction. Here, we note that by losing only 11 to 17% energy, the biomass

FIGurE 3.10 Comparison of several biomass species before and after torrefaction. Top row: raw

biomass, rice husk, sawdust, peanut husk, bagasse, and water-hyacinth; bottom row: the same

species after torrefaction. (Source: Adapted from previous work of the author.)

3.8 Torrefaction

(bagasse) lost 31 to 38% of its original mass. Thus, there is a 29 to 33% increase

in energy density (energy per unit mass) of the biomass. This increases its

higher heating value (HHV) to about 20

MJ/kg. Even if we take into account

the energy used in the torrefaction process, we can see from Table 3.7 that there

is a net rise in the energy density of the fuel.

Another special feature of torrefaction is that it reduces the hygroscopic

property of biomass; therefore, when torreﬁed biomass is stored, it absorbs less

moisture than that absorbed by fresh biomass. For example, while raw bagasse

absorbed 186% moisture when immersed in water for two hours, it absorbed

only 7.6% moisture under this condition after torrefying the bagasse for 60

minutes at 250 °C (Pimchua et al., 2009). The reduced hygroscopic (or enhanced

hydrophobic) nature of torreﬁed biomass mitigates one of the major shortcom-

ings for energy use of biomass.

3.8.1 Advantages of torrefaction

Torreﬁed wood performs better than original wood (or another biomass) in both

gasiﬁcation and combustion. Major features and advantages of torrefaction are

as follows:

 It increases the O/C ratio of the wood, which improves its gasiﬁcation

efﬁciency.

 It reduces power requirements for size reduction, and improves handling.

 It offers cleaner-burning fuel with little acid in the smoke.

Table 3.7 Changes in the Bagasse Properties after Torrefaction at 250 °C

Property

Torrefaction Time (min)

15 30 45

Mass yield (%) 69 68.33 62

Energy yield (%) 88.86 91.06 83.23

Energy density (% energy yield/% mass yield) 1.29 1.33 1.34

Energy required (MJ/kg product) 2.34 2.58 2.99

Higher heating value (HHV) (MJ/kg product) 19.88 20.57 20.72

Rise in HHV (%) 22.35 24.96 25.51

HHV (MJ/kg raw material) 15.44 15.44 15.44

Net energy (MJ/kg product) 17.54 17.99 17.73

Note: Moisture absorption after 2 hr in water: Raw bagasse, 186%; torriﬁed bagasse, 7.63%.

Source: Adapted from Pimchua et

al., 2009.

chapter

3 Pyrolysis and Torrefaction

 A fuel gas that has an enhanced heating value may be obtained through

gasiﬁcation.

 Torreﬁed wood absorbs less moisture when stored.

 One can produce superior-quality biomass pellets with higher volumetric

energy density.

Based on these features, we can easily speculate that torrefaction will allow

biomass to be used in entrained-ﬂow gasiﬁcation, direct combustion in a PC-

ﬁred boiler, and production of biopellet.

3.8.2 Mechanism of torrefaction

In biomass, hemicellulose is like the cement in reinforced concrete, and cel-

lulose is like the steel rods. The strands of microﬁbrils (cellulose) are supported

by the hemicellulose. Decomposition of hemicellulose during torrefaction is

like the melting away of the cement from the reinforced concrete. Thus, the

size reduction of biomass consumes less energy after torrefaction.

During torrefaction the weight loss of biomass comes primarily from the

decomposition of its hemicellulose constituents. Hemicellulose decomposes

mostly within the temperature range 150 to 280 °C, which is the temperature

window of torrefaction. As we can see from Figure 3.11, the hemicellulose

component undergoes the greatest amount of degradation within the 200 to

300 °C temperature window. Lignin, the binder component of biomass, starts

softening above its glass-softening temperature (~130 °C), which helps densi-

ﬁcation (pelletization) of torreﬁed biomass. Unlike hemicellulose, cellulose

100

200

Hemicellulose

Cellulose

Acid lignin

Milled wood

lignin

Wood

300 400 500

Temperature (°C)

Weight (%)

FIGurE 3.11 Weight loss in wood cellulose, hemicellulose, and lignin during torrefaction.

3.8 Torrefaction

shows limited devolatilzation and carbonization and that too does not start

below 250 °C.

Thus, hemicellulose decomposition is the primary mechanism of torrefac-

tion. At lower temperatures (< 160 °C), as biomass dries it releases H

O and

. Water and carbon dioxide, which make no contribution to the energy in

the product gas, constitute a dominant portion of the weight loss during

torrefaction. Above 180 °C, the reaction becomes exothermic, releasing gas

with small heating values. The initial stage (< 250 °C) involves hemicellulose

depolymerization, leading to an altered and rearranged polysugar structures

(Bergman et al., 2005a). At higher temperatures (250–300 °C) these form chars,

CO, CO

, and H

O. The hygroscopic property of biomass is partly lost in tor-

refaction because of the destruction of OH groups through dehydration, which

prevents the formation of hydrogen bonds.

3.8.3 design considerations for torrefaction

In a typical torrefaction process the biomass is heated gently to the desired

torrefaction temperature (θ

tor

), held there for a speciﬁed reaction time, and then

cooled down. The torrefaction temperature and the reaction time are two of the

most important parameters in this process. The torrefaction temperature θ

tor

generally reduces with reaction time t

heating

The design norm for torrefaction is

200 300

200

° < < °

−

< °

C C

C s

tor

heating

(3.12)

where θ

tor

is the torrefaction temperature in °C, and t

heating

is the heating time

above 200 °C. A typical reaction time is about 30 minutes. The properties of

torreﬁed wood depend on (1) the type of wood, (2) the reaction temperature,

and (3) the reaction time.

Torrefaction loses more oxygen and hydrogen than carbon. Hence, the H/C

and O/C ratios decrease. However, it should not be confused with carboniza-

tion, which takes place at a much higher temperature and produces charcoal

with even lower H/C and O/C ratios.

3.8.4 torreﬁed Pellet

The pelletizing process resolves some typical problems of biomass fuels: trans-

port and storing costs are minimized, handling is improved, and the volumetric

caloriﬁc value is increased. Pelletization may not increase the energy density

on a mass basis, but it can increase the energy content of the fuel on a volume

basis. For example, while the energy density on a mass basis for raw wood,

torreﬁed wood, wood pellet, and torreﬁed pellet was 10.5, 19.9, 16.2, and