Tải bản đầy đủ - 0 (trang)
3 Reactivity and Conversion Options

3 Reactivity and Conversion Options

Tải bản đầy đủ - 0trang

296



Chemistry of Sustainable Energy



TABLE 8.2

Biomass Conversion Options

Type of Conversion

Thermochemical



Method

Combustion (excess air)



Applications

Heat

Steam → boiler → electricity



Gasification (partial air)



Heat (CHP)

Gas turbine → electricity

Syngas → chemicals, fuels



Biochemical

Mechanical



Pyrolysis (no air)



Heat, fuel



Digestion



Biogas → electricity



Fermentation



Fuel (ethanol)



Extraction



Fuel (biodiesel)



Source: Adapted from Turkenburg, W.D. (Ed.), 2000. Renewable energy technologies. In World Energy

Assessment: Energy and the Challenge of Sustainability, edited by J. Goldemberg, New York:

UNDP/UN-DESA/World Energy Council, Chapter 7, Figure 7.1, p. 223.



8.3.2  General Reactivity Patterns

Given the complexity of the various materials being used as feedstocks, the reactions

involved in biomass energy conversions are far ranging and complex. This diversity

is amplified by the huge variability in process conditions, from gas phase to aqueous

phase, room temperature to thousands of degrees, and so on. Nevertheless, some

general patterns emerge and it is worthwhile to engage in an overview of the reactivity of some typical biomass functionality here.

Understandably, sugars and their derivatives (including the celluloses shown

above) play a large role in biomass energy conversion. A simple carbohydrate, such

as sucrose, can be considered to be a “polyhydroxyaldehyde,” a name that is revealing: the hydroxyaldehyde (or hydroxyketone) open-chain form of the sugar is in

equilibrium with the hemiacetal (or hemiketal) closed form (Figures 8.8 and 8.9).

The transformations associated with most biomass energy conversion processes

are directly related to the presence of the carbonyl and hydroxyl functionality. For

example, aldol (Figure 8.10) and conjugate additions (Figure 8.11a and b)—as well

as their associated retro reactions—are commonplace and can lead to unwanted side

products and degraded products.

Biomass energy conversions are not limited to carbohydrates, however. Plant and

animal biomass also contain large amounts of oils (lipids) and proteins. The central

reaction relevant to these biomass components is hydrolysis, as shown in Figure 8.12.

As we will see later in this chapter, these reactions can be readily accomplished biochemically by enzymatic catalysis.

Because of the harsh operating conditions of thermochemical conversions (gasification and pyrolysis), reactivity is much more erratic. Hemicellulose, cellulose,



HO



OH



O



α-D-glucopyranose



H



H



O



H



H



H



B



B



HO

HO

H



H OH

H



OH



OH



O



H



HO



OH



OH



OH



OH



H



Furanose

formation



O

HO



FIGURE 8.8  Equilibrium of glucose illustrating pyranose, furanose, and open-chain forms. The anomeric carbon is circled.



HO



H OH



H



O



α-D-glucofuranose



H



HO



OH



OH



H

OH



H



Biomass

297



298



Chemistry of Sustainable Energy



HEMI



Hemiacetal



Acetal/ketal

(note OH functionality on

anomeric carbon)



Hemiketal



H



O



O



OH



H



Acetal



R

OH



R



Ketal



O



O



OR'



Acetals



OR'



Ketals



(note H on anomeric

carbon)



(note alkyl group on

anomeric carbon)



FIGURE 8.9  The structural differences between acetals, ketals, hemiacetals, and

hemiketals.



O

R



O

H



H



H



:base



H



O



R



H



H



O



H



R



O

H2O



R



R



O



R



H

R



H



FIGURE 8.10  The aldol condensation.



and lignins decompose to give off mainly small molecules such as water, carbon monoxide, carbon dioxide, and low-molecular-weight oligomers. Cellulose

degrades in the range of 240–350°C to give dehydration products and levoglucosan, often by radical processes as depicted in Figure 8.13. Lignins, while not likely

to dehydrate, are quite susceptible to radical cleavage reactions at the ether bonds

at temperatures of 280–500°C, yielding a complex mixture of phenols (Soltes and

Elder 1981).



8.4  BIOMASS BEGINNINGS: HARVESTING AND PROCESSING

We are interested in biomass because of its great potential for sustainability, yet the use

of biomass presents some unique challenges. First, it comes in a wide variety of forms,

from wood chips to rice husks to animal manure, items with wildly different material

handling properties. But we do not grow wood chips, rice husks, or animal manure—

we grow trees, and rice, and . . . animals. Biomass requires significant processing even

after it has been harvested (not to mention the fact that cultivated biomass needs to

be planted, nurtured, and grown prior to harvesting). Furthermore, with regard to harvesting, crops clearly grow in seasonal domains. A constant supply of feedstock is a



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



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

3 Reactivity and Conversion Options

Tải bản đầy đủ ngay(0 tr)

×