Page 289 - Synthetic Fuels Handbook

FUELS FROM CROPS 275

success for production of chemicals and is being actively developed for producing liquid
fuels. Pyrolysis oil has been successfully tested in engines, turbines, and boilers, and has
been upgraded to high quality hydrocarbon fuels.
In the 1990s several fast pyrolysis technologies reached near-commercial status and
the yields and properties of the generated liquid product, bio-oil, depend on the feedstock,
the process type and conditions, and the product collection efficiency (http://www1.eere.
energy.gov/biomass/pyrolysis.html).
Direct hydrothermal liquefaction involves converting biomass to an oily liquid by con-
tacting the biomass with water at elevated temperatures (300–350°C) with sufficient pres-
sure to maintain the water primarily in the liquid phase (12–20 MPa) for residence times
up to 30 minutes. Alkali may be added to promote organic conversion. The primary prod-
uct is an organic liquid with reduced oxygen content (about 10 percent) and the primary
by-product is water containing soluble organic compounds. (http://www1.eere.energy.gov/
biomass/pyrolysis.html).
The importance of the provisions for the supply of feedstocks as crops and other bio-
mass are often underestimated since it is assumed that the supplies are inexhaustible. While
this may be true over the long-term, short-term supply of feedstocks can be as much a risk
as any venture.

9.2.4 Anaerobic Digestion

Anaerobic digestion is a natural process and is the microbiologic conversion of organic
matter to methane in the absence of oxygen (Fig. 9.2). The decomposition is caused by
natural bacterial action in various stages and occurs in a variety of natural anaerobic envi-
ronments, including water sediment, water-logged soils, natural hot springs, ocean thermal
vents, and the stomach of various animals (e.g., cows). The digested organic matter result-
ing from the anaerobic digestion process is usually called digestate.
Symbiotic groups of bacteria perform different functions at different stages of the diges-
tion process. There are four basic types of microorganisms involved. Hydrolytic bacteria
break down complex organic wastes into sugars and amino acids. Fermentative bacteria
then convert those products into organic acids. Acidogenic microorganisms convert the

Carbo-
Sugars
hydrates
Carbon acids
alcohols
Acid acetic
Fats Fatty acids carbon dioxide Methane
hydrogen carbon dioxide
Hydrogen
carbon dioxide
Proteins Amino acids ammonia

Hydrolysis Acidogenesis Acetogenesis Methanogenesis
FIGURE 9.2 Anaerobic digestion.