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4 Biomass Beginnings: Harvesting and Processing

4 Biomass Beginnings: Harvesting and Processing

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HO



H



H



O



+



OCH3



B



O



OCH3



R



H



O



OCH3



HO



+



O

OCH3



O

OCH3



H



B

HO



H



R



O



O



O



OCH3



OCH3



OCH3



H



B



H3CO



FIGURE 8.11  Conjugate addition to α,β-unsaturated carbonyl compounds. (a) The Michael reaction, (b) conjugate addition.



(b)



OR



(a)



R



O



R



O

OCH3



Biomass

299



300

(a)



Chemistry of Sustainable Energy

Generic protein hydrolysis

R1

N

H



(b)



O



O



O



H

N



R1



Catalyst



NH2



H2O



R2



O

OH



+



R2



OH

NH2



NH2

O



Generic lipid hydrolysis

HO



O

O



OH



(CH2)10CH3



O

O



Catalyst



(CH2)14CH3



OH + HO



H2O



O



OH



(CH2)7CH=CH(CH2)7CH3



O



O



O



HO



FIGURE 8.12  Hydrolysis of amides (a) and esters (b).



CH2OH



H HO





H O



O

HO



H



O





O



OH



H



O



O



H



HO

n



Cyclization and

H



HO



H-atom abstraction



OH

O



OH

OH

Levoglucosan



FIGURE 8.13  Thermal decomposition of cellulose with radical pathway to levoglucosan.



desirable, if elusive, ideal for biomass. On the other hand, some waste products (MSW,

food processing waste, etc.) can be reliably produced without regard to seasons. All of

these factors are pertinent when contemplating biomass as a sustainable energy source.

Almost all biomass feedstocks must undergo some sort of pretreatment before use

in an energy conversion process, from harvesting and washing to drying and densification. The goals of pretreatment are to improve the storage, handling, and transport

properties as well as the conversion efficiency of the biomass feedstock. First, after

harvesting, the desired material must be separated from unwanted residues. This

could be as simple as washing soil residue from harvested plants or screening to

remove gravel, and so on. The next step is to dry the biomass, if necessary.



8.4.1 Drying

The heating value of biomass changes dramatically with moisture content, as we

have already seen. Freshly harvested plant biomass may have a moisture content as

high as 60%, so for thermochemical energy conversion processes the material must

be dried to a moisture content of about 10–15%; otherwise, valuable heat is wasted in

the reactor for the evaporation of moisture (recall lower vs. higher heating values as



Biomass



301



discussed in Chapter 3). The ideal drying agent is solar energy, of course, but drying

with waste heat is an efficient alternative. While temperatures above 150°C cause

chemical changes in biomass, torrefaction (“roasting” the biomass at a temperature

of about 225–300°C in the absence of oxygen) is sometimes desirable—it increases

the carbon content of woody biomass as hemicellulose decomposes and volatile

compounds evaporate. The material is also made more brittle as the hemicellulose is

broken down, making subsequent preconversion treatments easier.



8.4.2  Comminution

Perhaps the most important step in biomass pretreatment is to then comminute it:

mechanically alter it so that, instead of a wide variety of shapes, sizes, and thicknesses, a conveniently manipulated, uniform solid feedstock is made. This is not done

solely for the purpose of convenience in handling. By modifying the particle size and

shape and the porosity and surface area, heat and mass transport properties are all

changed—all of which greatly impact the behavior (and quality) of the biomass in

subsequent energy conversions. Some of the more common processes of comminution are chipping, milling, or grinding until the preferred particle size is obtained.



8.4.3 Densification

Once a small particle size is obtained, it can be further modified by compressing or

pelletizing in a process that is known as densification. The optimum size of biomass

pellet, briquette, or particle depends on both the process and the equipment involved

and must take into account not only the handling but also the conversion process:

a finely distributed powder may be ideal for delivery via a screw auger, but the fine

particulate may present major problems later in the conversion process. In any case,

transforming the biomass into a pellet or powder is an energy-intensive process that

adds to the overall cost of the energy produced.



8.5  THERMOCHEMICAL PROCESSES

8.5.1 Introduction

Most thermochemical processes use plant biomass as the feedstock, although a mixed

feedstock of animal waste (e.g., poultry or feedlot manure) with plant biomass has

been used with some success. (Animal waste alone is generally not a suitable feedstock

for thermochemical conversion methods because of its high moisture content.) While

humans have combusted plant biomass for energy generation from our very beginnings,

the focus here will be on the two other major thermochemical conversion methods:

gasification of biomass to generate syngas and pyrolysis to make bio-oil. In comparing the specific processes, we will see that there is not a clean line of demarcation

between the two; pyrolysis forms some gases and gasification includes pyrolysis reactions. Nevertheless, gasification’s goal is syngas, and the goal of modern flash pyrolysis

is the production of liquid fuel. Furthermore, two similarities in these processes stand

out: first, the complexity of the biomass feedstock going in leads to complex product



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Chemistry of Sustainable Energy



mixtures coming out. Second, the many variables associated with each process (reactor

design, temperature, rate of heating, biomass particle size, gasification agent, catalyst,

moisture level, residence time, etc.) have a profound impact on the composition of the

product. While many of the basic reactions during the thermochemical conversion are

known, control of the reaction output is still very much an empirical science.



8.5.2 Pyrolysis

8.5.2.1 Introduction

Pyrolysis is ancient technology: heating biomass in the absence of oxygen was used

in ancient Egypt to produce pitch for embalming purposes and for waterproofing

boats (Ringer et al. 2006). Pine was once “distilled” to obtain turpentine, and methanol is also known as wood alcohol for a reason: wood heated in the absence of

oxygen produces methanol. Slow pyrolysis (over a period of days) is still the main

method for the production of charcoal, but the primary goal of modern flash pyrolysis—with a residence time of less than 2 s—is to convert solid, low-density biomass

into a convenient liquid fuel, usually referred to as bio-oil (not to be confused with

oil expressed out of other biomass, as in oilseeds). The bio-oil (or pyrolysis oil) so

obtained can be upgraded for subsequent use in diesel engines, turbines, and blending with other fuels. (N.B. Another process for conversion of biomass into liquids is

liquefaction, which is often confused with pyrolysis. Liquefaction is carried out at

a lower temperature and under a pressurized hydrogen atmosphere—the key difference being the reducing medium. Our focus is on pyrolysis, favored for commercialization due to the lower capital costs (Graỗa etal. 2012).)

8.5.2.2Process

The pyrolysis process is quite straightforward: in a hot (≈500–600°C) oxygen-free

environment, a flash of thermal energy infuses the biomass feedstock, pyrolyzing

it (literally “cleaving with heat”), then the pyrolysis product is cooled as quickly as

possible (quenched) to prevent further reaction of the components that would result

in the formation of tar. The precise nature of the product (i.e., the proportion of solid,

liquid, and gas) depends a great deal on the process variables.

The short residence time is key: this is fast (or flash) pyrolysis. Just how fast is

fast? The rate of heating has been estimated to be >1000°C/s amounting to a rate of

heat transfer between 600 and 1000 W/cm2 (Reed et al. 1980). Ideally, the residence

time should be only a few hundred milliseconds (Mohan et al. 2006). The bio-oil produced condenses out as a greenish, dark brown to reddish oil; most char that is formed

is collected at the bottom of the reactor. The high rate of heat transfer is needed to

chemically shatter the biomass, cleaving the chemical bonds to make smaller molecules from MW of 2 (H2) to 300–400 (more detail on the reaction chemistry is presented in Section 8.5.4.2). About 70% of the product is bio-oil (note that this includes

water), 10–15% is gaseous, and the remainder is solid char (Ringer et al. 2006).

Because the heat transfer must be extremely fast, the feedstock for pyrolysis

must be particularly fine (particle size of about 2 mm) and the convection good

(Bridgewater et al. 2001). Upon entering the pyrolysis zone any moisture content in



303



Biomass

Oxygenates

400°C



Ethers

500°C



Phenolics

600°C



Ethers

700°C



PAH

800°C



Larger PAH

900°C



FIGURE 8.14  The changes in pyrolysis composition with time. PAH refers to polyaromatic hydrocarbons. (Adapted from Elliot, D.C. 1986. Analysis and comparison of biomass

pyrolysis/gasification condensates. PNL-5943, Final Report. Pacific Northwest Laboratory.

Richland, WA.)



the biomass explosively vaporizes, shredding the biomass and releasing the volatile

organics to initiate the pyrolysis reactions that form the primary products. These

volatiles can condense out or react with one another to produce the secondary products, including tars. The constitution of the pyrolysis product changes with both time

and temperature as shown in Figure 8.14 (Elliot 1986). Even though the residence

time is remarkably short, there is continuing interaction between the primary products formed from the initial pyrolysis and the secondary products produced between

and among the primary and secondary products. If the pyrolysis temperature is too

high, the product distribution will tend toward char via oligomerization, dehydration,

decarbonylation, and decarboxylation reactions. Temperatures above 700°C can lead

to gasification and a low-quality syngas.

Another consideration with respect to the need for fast heat transfer is the reactor

design. Pyrolysis reactors incorporate some mechanism by which the fast heat transfer can be achieved at a uniform temperature, something that is generally accomplished by good mixing. A fluidized bed is one example of a reactor that works

effectively for pyrolysis. In a fluidized bed, the materials are vigorously mixed by

suspending them (under pressure) in a gaseous flow via bubbling or some other form

of circulation. As a result, the particulate within the reactor behaves like a fluid,

hence the name. This almost always requires the addition of an inert additive such

as sand. One problem that fluidized bed reactors exhibit is that their high velocity

carries char fines and ash with the product as it condenses.

8.5.2.3 Product

Pyrolysis of biomass gives three products: the permanent gases, the bio-oil, and

char. The process conditions are optimized to obtain the highest yield possible of the

desired bio-oil. It is a delicate balancing act: higher temperatures and/or longer residence times can lead to tar formation or the opposite—cracking to form more gases.

The physical properties of the pyrolysis oil depend to some degree on the feedstock

(in terms of type of feedstock but even its growth environment), but the heating value

(about 17 MJ/kg) is essentially the same regardless of the source (provided that the

water content of product is held constant (Ringer et  al. 2006)). It is worth noting

two comparisons: (1) the heating value of the starting biomass is roughly 18 MJ/kg,

hence pyrolysis simply converts the biomass from a low bulk density solid fuel into a

more convenient liquid fuel, not a fuel with a significantly higher heating value, and

(2) the heating value of crude bio-oil is only about 40–45% (by weight) that of liquid

hydrocarbon fuels (Mohan et al. 2006).

The bio-oil product is a free-flowing liquid that is a microemulsion containing

as much as 50% water (although typically more like 15–30%). It is not miscible



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Chemistry of Sustainable Energy



with nonpolar organic solvents, making it unsuitable for blending with hydrocarbon

fuels. It is corrosive due to the presence of carboxylic acids in the product, giving

a pH of 2–2.5. Furthermore, because the product components are formed without

being allowed to reach equilibrium, the resultant oil is unstable: further reactions

take place upon aging, leading to a higher-viscosity product. These reactions (aldol

and other condensation reactions leading to phase separation) are catalyzed by acid

and the presence of char fines in the bio-oil. As a result, the shelf life is weeks to a

few months at most. Finally, bio-oil suffers from low volatility, cold-flow problems,

and poor combustion characteristics (Wornat et al. 1994). As a result, upgrading of

bio-oil is required (vide infra).

The chemical makeup of bio-oil is, in a word, complex: over 400 compounds have

been identified, typically by GC-MS, but to try and characterize every compound in

bio-oil would be pointless (Graỗa etal. 2012; Soltes and Elder 1981). Volatiles (CO,

CO2, methanol, acetaldehyde, acetic and formic acids, and other small molecules)

are formed from the pyrolysis of all three major components of plant biomass (cellulose, hemicellulose, and lignin). The breakdown of cellulose and hemicellulose

leads to mostly levoglucosan (roughly 35–45% by weight) and a wide variety of

furans, hydroxyaldehydes, hydroxyketones, and sugars. Lignin breaks down into a

huge variety of complex phenols (see Table 8.3) and aromatics (Alén et al. 1996).

Some representative mechanisms for the formation of a few of these compounds will

be examined in the next section.

8.5.2.4  Pyrolysis Reactions

As can be gleaned from the description of the chemical makeup of the pyrolysis product, the pyrolysis reactor is a den of wild abandon with respect to cleavage of carbon–carbon and, particularly, carbon–oxygen bonds. The energy needed to break these

bonds is relatively low (compared to, e.g., the reforming of methane to form hydrogen

(Ringer et al. 2006)). A plethora of possible reaction pathways exist, from retro-aldols,

intramolecular acetal and ketal formation, dehydrations, depolymerizations, and repolymerizations via condensation or radical reactions. Many reaction mechanisms have

been proposed for the formation of the molecules that make up bio-oil, although a

complete mechanistic unraveling is neither realistic nor necessary—the formation of

these products follows the same pathways seen in other organic fragmentation processes, both polar and radical. For example, a plausible mechanistic pathway for the

formation of furans from cellulose (via levoglucosan) is proposed in Figure 8.15 (recall

the formation of levoglucosan from cellulose shown in Figure 8.13) (ThangalazhyGopakumar et al. 2011). Retro-aldol reactions can lead to any number of smaller molecules, as shown in Figure 8.16a and b; a related fragmentation mechanism is shown

in Figure 8.17 (Lomax et al. 1991). Any of these smaller molecules could re-condense

into higher-molecular-weight compounds, so it is not difficult to envisage how a complex mixture of molecules can result under the harsh conditions of pyrolysis.

8.5.2.5  Upgrading Bio-Oil

Given crude bio-oil’s undesirable properties (corrosiveness, instability, low heating value, and poor combustion properties), upgrading is required to make a



OCH3



HO



HO



CH3O



CH3O



HO



CH3O



OH



OH



OH



O



CH3



CH3O



OH



OCH3



OH



HO



CH3O



H3C

OH



OH



CH2CH3



HO



H3C



Source: Adapted from Thangalazhy-Gopakumar, S. et al. 2011. Energy Fuels 25 (3):1191–1199.



Syringol/als



Guiacols



Catechols



TABLE 8.3

Some Phenolic Lignin Breakdown Products



HO



CH3O



OH



Biomass

305



O



HO



OH



OH



O



OH



HO



OH



OH



O



OH

HO



OH



FIGURE 8.15  Formation of 5-hydroxymethylfurfural from cellulose via levoglucosan.



OH

OH

Levoglucosan



O



OH



H+

O



OH



O



B:

H



–2H2O



O

O

5-Hydroxymethylfurfural



OH



306

Chemistry of Sustainable Energy



307



Biomass

(a)



OH

O



OR′



O



OR′



HO

OH



RO



OH



+



2



O



XO

X = R, R′



OH



OH



(b)



O



HO



RO



OH



OH

O



OH



OR′



OR′



O



O



OR′



OR′



OH



O



+

OH



RO



RO



OH

B:



OH

H



RO

O



O



FIGURE 8.16  Fragmentation mechanisms in pyrolysis. (a) A retro [2 + 2 + 2] cycloaddition

and (b) a retro-aldol reaction.



commercially viable product. Cleanup by extractive separation is one approach;

for example, the National Renewable Energy Laboratory (NREL) solvent method

(Figure 8.18) was developed for deriving a useful adhesive. But upgrading to make

a high-quality fuel has required extensive research into developing suitable catalytic processes to convert the bio-oil into a less viscous, more stable material that

more closely mimics crude oil. The presence of aldehydes, ketones, and other reactive oxygenated species is primarily responsible for the instability of the bio-oil, so

deoxygenation is required as a part of the upgrading. Viscosity reduction requires

lowering in the proportion of higher-molecular-weight compounds. The general

reactions associated with upgrading are given in Table 8.4.

Upgrading can take place in situ (i.e., during the pyrolysis process) or after the

product formation. Two of the more commonly used methods to achieve these goals

are hydrodeoxygenation (HDO) and cracking (specifically cracking with a zeolite

catalyst) (Mortensen et al. 2011). The HDO method borrows heavily from progress

made in hydrodesulfurization of diesel, gasoline, and other petroleum products.

The catalysts most frequently used are Co- or Ni-promoted molybdenum sulfide

OH



OH



H OH

O



O



RO



OH



O



H



HO



H+



OH



OH



O

HO



H

OH



OH



OR′



H



H



RO



O

OH



H OH

O



OR′



H

base



OH



OH

O



RO



OH

OH

OH



OH

O



O

+

HO



OH

OR′



O



FIGURE 8.17  Fragmentation via cleavage at the glycosidic linkage.



OH

OR′



308



Chemistry of Sustainable Energy



Whole oil

1. Dissolve in ethyl acetate (EtOAc; 1:1)

2. Filter



Residue

(char)



Filtrate

(let stand)

Wash with water



Aqueous phase



Organic phase

EtOAc-soluble

residues



Water-soluble

products



Extract with

5% NaHCO3

Aqueous phase



Organic phase

Phenols and

neutral

compounds



Carboxylates



1. Acidify with H3PO4 to pH 2

2. Saturate with NaCl

3. Extract with EtOAc

Aqueous phase

Water-soluble salts,

acid, and so on



Organic phase

Organic carboxylic

acids



FIGURE 8.18  Cleanup of bio-oil via solvent extraction. (Adapted with permission from

Chum, H. et al. 1989. Biomass pyrolysis oil feedstocks for phenolic adhesives. In Adhesives

from Renewable Resources, edited by R.W. Hemingway, A.H. Conner and S.J. Branham.

Washington, D.C.: Copyright 1989, American Chemical Society: 135–151.)



(MoS2) on Al2O3. The process is run under an atmosphere of hydrogen (ca. 1000–

12,000 kPa) at 250–450°C to reductively deoxygenate the bio-oil and generate water

as the by-product.

Elucidating the mechanism of the HDO process has been a matter of intense

research effort, particularly with regard to theoretical studies. While the specifics

are not yet well understood, several key steps can be reasonably anticipated (see

Figure 8.19):



309



Biomass



TABLE 8.4

Reactions in the Catalytic Upgrading of Bio-Oil





Type of Reaction



a



Cracking



b



Decarbonylation



Example

R



R′

O



R

c



Decarboxylation



Hydrocracking



e



Hydrodeoxygenation



O



–CO2



R′



OH



H



R



H



H2



H2



R′



H2



+



R



R



–H2O



Hydrogenation

R



R



OH



R



R

f



–CO



R′



H



R

d



+



R



R′



H



R



R′



Source: Reprinted from Appl. Catal. A Gen. 407 (1–2), Mortensen, P.M. et  al. A review of catalytic

upgrading of bio-oil to engine fuels. 1–19, Copyright 2011 with permission from Elsevier.



• The activation of the molybdenum–sulfur catalyst (1) presumably takes

place by oxidative addition of hydrogen gas followed by loss of hydrogen

sulfide (H2S) to form a vacant coordination site (2).

• If the oxygenated species is an alcohol, the compound coordinates in a σ

fashion to give intermediate (3).

• The actual deoxygenation step is not well understood; it has been postulated

to proceed by a carbocation intermediate (Romero et al. 2010) or by bimolecular nucleophilic substitution (Dupont et al. 2011). In the latter case, an

adjacent thiol attacks the coordinated alcohol (3), transferring the organic

group to the sulfur (4). Subsequent desulfurization (presumably by reductive elimination) yields the deoxygenated species.

• Regeneration of the catalyst likely takes place by hydrogenation and loss

of water (5).

Unraveling a basic understanding of the HDO mechanism should lead to the design

of improved catalysts for the process—a high priority, since the conversion of bio-oil

to higher-value fuels requires thorough deoxygenation.

Zeolite cracking of bio-oils is different from the HDO method in that hydrogenation is unnecessary so that the process can be run at atmospheric pressure.

Recall from Section 2.3 that the cage-like structure of the zeolites allows molecules to enter the pores and be “cracked” by acid catalysis at moderately high



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