Methane emissions in biogas production - The Biogas: Science, Production and Applications - page 256

maintained in the study of Liebetrau et al., which also explains the
difference in maximum emission numbers (i.e. 7% as opposed to 11%). In
greater depth, their report discusses the potential of addition of sodium
bicarbonate (NaHCO 3 ) to digesters in order to stabilize the process
conditions and optimize the conversion of organic material. By improving
the conversion it is expected that the residual gas potential in the digestate
will decrease.
Tests were performed with three laboratory-scale digesters (10.4 l), that
were given the same feed on a continuous basis (daily feed). One reactor
served as the control, while the other two were treated with different doses
of sodium bicarbonate. In order to demonstrate the stabilizing effect of the
addition, the reactors were subjected to a relatively high loading of organic
material (up to 6.5 kg/m 3 per day; generally, a value of 4 kg/m 3 per day is
applied). The tests showed that the residual biogas production in the three
weeks following active digestion was between 5.6% and 6.7% of the
captured biogas quantity during active digestion. This is an indication of the
amount of biogas that could be released in an open storage of digestate.
The results for the remaining gas potential have to be regarded as an
upper limit based on the following considerations.

. The reactors were deliberately loaded to stressful conditions, where

conversion of organic matter is not optimal.

. The applied loading rates were around 1.5 times as high as in normal

practice.

.

The conversion rates achieved were considerably lower than those
normally found in practice, with a methane production of 123Nm 3 /
kg OS input versus practical averages of around 370Nm 3 /kg OS input .
Mainly because of this last result, the representativeness of the reported data
for the practical situation of operational digesters appears to be limited.

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11.4 Overall methane emissions
In a peer-reviewed paper, Flesch et al. (2011) report the results of
measurements on overall methane emissions from a Canadian 1MW biogas
plant (a 100 ton/day co-digestion plant). The installation consisted of
feedstock hopper, gas-tight digester (with rubber cover), digestate separator,
digestate liquid lagoon, digestate solid fertilizer output tent, CHP, flare and
the piping around this equipment. The smaller 'secondary' emissions from
other sources outside this area (feedstock piles, runoff ponds and the offal
storage area) were estimated.
An inverse dispersion technique was used to measure the totality of
emissions. This is a micrometeorogical method that uses a downwind
concentration measurement to calculate the gas emission rate. Emissions