Page 290 - Synthetic Fuels Handbook

276 CHAPTER NINE

acids into hydrogen, carbon dioxide, and acetate. Finally, the methanogenic bacteria pro-
duce biogas from acetic acid, hydrogen, and carbon dioxide.
The process of anaerobic digestion occurs in a sequence of stages involving distinct
types of bacteria. Hydrolytic and fermentative bacteria first break down the carbohydrates,
proteins, and fats present in biomass feedstock into fatty acids, alcohol, carbon dioxide,
hydrogen, ammonia, and sulfides. This stage is called hydrolysis (or liquefaction).
Next, acetogenic (acid-forming) bacteria further digest the products of hydrolysis into
acetic acid, hydrogen, and carbon dioxide. Methanogenic (methane-forming) bacteria then
convert these products into biogas.
The combustion of digester gas can supply useful energy in the form of hot air, hot
water, or steam. After filtering and drying, digester gas is suitable as fuel for an inter-
nal combustion engine, which, combined with a generator, can produce electricity. Future
applications of digester gas may include electric power production from gas turbines or fuel
cells. Digester gas can substitute for natural gas or propane in space heaters, refrigeration
equipment, cooking stoves, or other equipment. Compressed digester gas can be used as an
alternative transportation fuel.
Thus, there are three principal byproducts of anaerobic digestion: (a) biogas, (b) acido-
genic digestate, and (c) methanogenic digestate.
Biogas is a gaseous mixture comprising mostly of methane and carbon dioxide, but also
containing a small amount of hydrogen and occasionally trace levels of hydrogen sulfide.
Biogas can be burned to produce electricity, usually with a reciprocating engine or micro-
turbine. The gas is often used in a cogeneration arrangement, to generate electricity and use
waste heat to warm the digesters or to heat buildings.
Since the gas is not released directly into the atmosphere and the carbon dioxide
comes from an organic source with a short carbon cycle, biogas does not contribute to
increasing atmospheric carbon dioxide concentrations; because of this, it is considered
to be an environment friendly energy source. The production of biogas is not a steady
stream; it is highest during the middle of the reaction. In the early stages of the reaction,
little gas is produced because the number of bacteria is still small. Toward the end of the
reaction, only the hardest to digest materials remain, leading to a decrease in the amount
of biogas produced.
The second by-product (acidogenic digestate) is a stable organic material comprised
largely of lignin and chitin, but also of a variety of mineral components in a matrix of dead
bacterial cells; some plastic may be present. This resembles domestic compost and can be
used as compost or to make low-grade building products such as fibreboard.
The third by-product is a liquid (methanogenic digestate) that is rich in nutrients and
can be an excellent fertilizer dependent on the quality of the material being digested. If the
digested materials include low-levels of toxic heavy metals or synthetic organic materials
such as pesticides or polychlorobiphenyls, the effect of digestion is to significantly concen-
trate such materials in the digester liquor. In such cases further treatment will be required
in order to dispose of this liquid properly. In extreme cases, the disposal costs and the
environmental risks posed by such materials can offset any environmental gains provided
by the use of biogas. This is a significant risk when treating sewage from industrialised
catchments.
Nearly all digestion plants have ancillary processes to treat and manage all of the
byproducts. The gas stream is dried and sometimes sweetened before storage and use. The
sludge liquor mixture has to be separated by one of a variety of ways, the most common
of which is filtration. Excess water is also sometimes treated in sequencing batch reactors
(SBR) for discharge into sewers or for irrigation.
Digestion can be either wet or dry. Dry digestion refers to mixtures which have a solid
content of 30 percent or greater, whereas wet digestion refers to mixtures of 15 percent
or less.