Biomedical Engineering Reference
In-Depth Information
cleaner and far less contaminated by traces of other gases. LFG may contain
a bewildering array of 'others', dependent on the exact nature of the waste
undergoing decomposition. The list includes the likes of 1,2 dichloroethene, alkyl-
benzene, butylcyclohexane, carbon disulphide, propylcyclohexan, methanethiol,
decane, dichlorobenzene, undecane, ethylbenzene, dodecane, trimethylbenzene,
tridecane, toluene, dimethyl disulphide, nonane and sulphur dioxide. Biogas, by
contrast, is relatively pure by comparison, since the bulk of the inorganic matter
and many potential pollutants are excluded from the bioreactor, either by source
or mechanical separation, as part of the waste preparation process. This obviates
the need for high temperature flaring, commonly used for LFG to destroy resid-
ual pollutant gases, since they are highly hostile to the fabric of any generation
equipment intended to be used.
The main cause for concern in this respect is hydrogen sulphide (H 2 S), which is
a metabolic by-product of sulphur-reducing bacteria. Unsurprisingly, the amount
present in the final gas produced depends largely on the relative abundance of
sulphur containing compounds in the original biowaste. H 2 S is acidic and this
poses a major corrosion risk to gas handling and electrical generation equipment.
Scrubbing hydrogen sulphide out of biogas is possible, but in practice it is more
common to use a high-alkalinity lubricant oil which is changed often.
Biogas utilisation involves burning it, with some of the energy being trans-
formed to electrical. There are three basic types of engine which are suitable
generating motors for biogas uses, namely turbine, dual fuel and spark ignition.
For each there are numbers of different manufacturers worldwide. While, clearly,
it lies far outside of the scope of this topic to discuss them, it is worth noting
that for any given application, the type of engine used will normally be decided
by a number of contextual issues. Hence, the quantity of the biogas produced, its
purity, the intended life of the plant, relevant pollution controls and other similar
site specific considerations will need to be considered.
Generation processes are generally relatively inefficient thermodynamically,
and often much of the available energy is effectively lost as heat. However,
the nature of engineered AD processes is such that there is a ready built-in
demand for thermal energy to elevate and maintain the digester temperature. This
may account for between 20 and 50% of the total energy produced, dependent
on system specifics, in a typical temperate facility, with the remainder being
available for other uses. A representative energy flow for gas engine generators
is shown in Figure 10.4.
Ethanol fermentation
Fermentative processes have been described earlier, both in the general wider
metabolic context and specifically in regard to their potential use in the treatment
of biowaste. Fermentation produces a solution of ethanol in water, which can be
further treated to produce fuel-grade ethanol by subsequent simple distillation,
to 95% ethanol, or to the anhydrous form by azeotropic co-distillation using
a solvent.
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