Environmental Engineering Reference
In-Depth Information
exceeding 0.1mg/Nm 3 (Persson et al., 2006). A high concentration of
ammonia in biogas is a problem because combustion of ammonia in gas
engines leads to the formation of nitrous oxide (NO x ). Most engines can
accept ammonia concentrations of 100mg/Nm 3 (Persson et al., 2006).
Ammonia formation can be avoided by controlling the biogas process, as
ammonia is formed at high pH and temperature. Adjusting the C/N ratio of
the feedstock can also avoid ammonia formation. The removal of ammonia
is usually combined with other biogas cleaning procedures.
Oxygen and nitrogen
The presence of O 2 and N 2 in biogas can lower the heating value of the gas
and cause corrosion in gas pipelines and other equipment. Normally, O 2 and
N 2 are not present in biogas from sewage and dedicated AD plants, as
methane is formed under anaerobic conditions (without O 2 ). On the other
hand, landfill gas contains O 2 (1-3% vol) and N 2 (1-17% vol) as some air
may be sucked in together with the landfill gas through the underpressure
collection system (Rasi, 2009). Moreover, a high O 2 content in the biogas
(6-12%) can lead to an explosion due to presence of combustible CH 4 in the
biogas mixture (Vandeweyer et al., 2008). Finally, the removal of O 2 and N 2
- if present in large quantities - can be costly and impede the use of biogas
for vehicle fuel or grid injection. Oxygen in biogas is generally removed
during the desulphurisation process. Other applicable methods include
adsorption processes (e.g. with activated carbon or molecular sieves).
17.3 Utilisation of biogas for the generation of electric
power and heat in stationary applications
Over the years, biogas collection and utilisation technologies have
improved. Several technologies that convert biogas to more useful forms
of energy are now available. For direct substitution of biogas for natural
gas, the biogas has to be cleaned of impurities (especially H 2 S, siloxanes,
water vapour etc.) and the equipment has to be slightly modified to prevent
corrosion and maintain the right gas feed pressure and fuel-to-air ratio in
order to ensure flame stability.
Table 17.2 presents a comparison of typical power generation units with
their capacities, efficiencies, fuel consumption and heat recovery rates. The
performance and characteristics of some of these technologies have
improved in recent times due to increased experience of using of biogas
for electricity generation.
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