This preview shows page 1. Sign up to view the full content.
Unformatted text preview: ble, fruit,
and garden waste, organic wet fraction of municipal solid wastes, manure, and
sewage). The thermochemical route comprises biomass gasification in supercritical
water, co-production of Fischer-Tropsch-SNG from biomass, and biomass gasification
with downstream methanation. While anaerobic digestion and supercritical gasification
processes are applied for conversion of wet biomass (70-95 wt.% water), the
gasification / methanation processes are applied for conversion of relatively dry
biomass streams (10-15 wt.% water). ECN-RX--04-085 4 ANAEROBIC DIGESTION
Anaerobic digestion is a biological process in which organic wastes, in absence of air
(anaerobic), are converted to biogas, i.e. a mixture of methane (55-75 vol.%) and
carbon dioxide (25-45 vol.%). During anaerobic digestion, typically 30-60% of the
solid input is converted to biogas, The co-products consist of an undigested residue and
various water-soluble substances. Depending on the digestion system (wet or dry), the
average residence time is between 10 days and 4 weeks. A simplified stoichiometric
reaction for anaerobic digestion of biomass is:
C6H10O5 + H2O → 3 CH4 + 3 CO2
Anaerobic digestion is a proven technology, generally available on a commercial
basis, and being applied for small-scale decentralised treatment of “wet” organic
wastes at their origin. The produced biogas, either raw or usually after some
enrichment in CH4, can be used to generate heat and power through prime movers, at
capacities up to 10 MWe, with excess power fed into the grid. In a gas engine, electric
efficiencies of 30-35% (on LHV basis), and total efficiencies of 85% can be achieved.
Higher electric efficiencies of up to 50% might be reached by using fuel cell systems
instead of gas engines. Generally, the produced heat in a prime mover can be used
optimally within the digestion process, and for other purposes such as space heating.
The simultaneous demand for heat and power is the crucial factor for an efficient use of
biogas in co-generation. Otherwise upgrading, i...
View Full Document
This essay was uploaded on 03/18/2014 for the course ENG 316K taught by Professor Kruppa during the Spring '08 term at University of Texas.
- Spring '08
- Chemical Engineering