Environmental Engineering Reference
In-Depth Information
A part from the physical upgrading of biomass for obtaining solid renewable fuels
(such as wood power, briquette and pellets), the energy production from biomass can
be realized by two basic types of processes: microbiological and thermochemical
methods. The choice of the specific process is effected evaluating the characteristics
of the initial biomass, in particular its carbon and nitrogen content (C/N ratio) and
humidity. A high nitrogen content implies green vegetable material, more suitable for
biochemical reactions, whereas a high carbon content indicates that the biomass can
be usefully exploited by combustion. Microbiological processes are involved in
obtaining a number of biofuels from different types of starting biomaterials:
bioethanol by alcoholic fermentation from any carbohydrate-containing materials,
biodiesel from plant oils or algae after transesterification of triglycerides with
methanol, biogas (a mixture of methane and carbon dioxide with traces of hydrogen,
nitrogen and hydrogen sulfide) by anaerobic digestion of complex organic materials,
such as animal dejections and by-products of several cultivations (C/N 15-30,
humidity higher than 50%), biohydrogen by conversion of protons to hydrogen gas
occurring during metabolism of many types of micro-organisms (hydrogenases) [
23
].
Unlike microbiological methods, the thermochemical processes permit the dry
biomass to be transformed, preferably with high C/N ratio, not only to chemicals
and fuels but also directly in heat and electricity, and can be divided in the
following groups: combustion, pyrolysis, and gasification. Combustion in the
presence of air is obviously the most simple and diffused method for converting
chemical energy in biomass to power (thermal and electric), being usually carried
out in furnaces, boilers, steam turbines and turbo-generators starting from dry
materials with humidity grade\50%. The other two methods are capable to supply
a variety of products as function of the operative conditions. Pyrolysis is the
incomplete thermal degradation in the absence of oxygen of any carbon-containing
material, it can occur already at temperatures lower than 400-500C providing
products containing carbonaceous residues, gaseous compounds but mainly bio-oil
(or pyrolysis oil), a liquid mixture of different substances, such as alcohols,
aldehydes, ketones, esters and phenolic compounds. At temperatures higher than
800C in the presence of air and water the complete gasification of biomass can be
observed. Coal or carbohydrates of different molecular length from biomass can
react with oxygen and steam to generate CO and H
2
(bio-syngas), which can be
directly used in gas turbines for power generation or converted to different fuels in
the presence of catalysts and at different operative conditions. In this contest the
Fischer-Tropsch process, based on the reaction between CO and H
2
to give water
and alkanes, in the presence of Fe based catalysts at 200-350C and 10-40 bar,
represents a possibility to produce biodiesel starting from biomass. Finally,
another route to obtain hydrogen from biomass is constituted by the transformation
of biogas (biomethane), deriving from anaerobic digestion, in bio-syngas by the
steam reforming or partial oxidation reactions (see
Sect. 2.1
).
Electricity production from biomass (direct combustion or derived by biofuels)
presents some advantages: very low cost of the starting material with respect to
fossil fuels, zero net CO
2
emissions, security of supply and reduction of transport
distances when electricity is produced close to the biomass production site.
