How meaningful is the “biogas yield”?

No matter whether substrate supplier or plant operator, the enormous variety diversity in maize makes it difficult to decide year by year on the "best variety" from those potential 700 varieties on sale. Of course by now every biogas plant operator has enough of his own practical experience from long-term maize growing to name the appropriate or, more aptly, "working" variety for operating his plant.

However, every year the plant breeders offer new varieties - which criteria are crucial for the practitioner when selecting a new biogas variety/ changing a variety? If you ask the practitioners straight away for the suitable variety profile of a biogas variety, you will often get the following answers regarding the variety characteristics: Site suitability, yield reliability, stress tolerance, health and a variable harvest window - the characteristic gas yield only appears much further down the list. And that is although the parameter biogas yield is a known internal industry criterion for the evaluation and therefore suitability of a biogas variety.

Are the practitioners right in seeing this value as second grade? Why does it play a secondary role despite of the plant operation? Especially in new varieties, there is a lot of information available regarding theoretical gas yield and yield results. However, one has to analyse these critically if they are to be a sufficient measure for making correct conclusions regarding the variety specific "biogas performance" of the individual gas plant.

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The situations in the plants are very complex

Taking a little detour to the current plant situation will shed a bit more light on the matter:

1. Hydraulic retention time
The classical maize/slurry biogas plants stemming from the biogas plant boom years 2005 - 2009 in the 500 kW class have often developed into performance optimised highly efficient plants through the constantly necessary technical adaptations resulting from the political corrections to the various Renewable Energies Sources Act (EEG) amendments. It is not rare to find the plant today offering twice as much energy production capacity - with identical fermenter volume!

Consequently, the systems receive higher infeed amounts per time unit in order to be able to produce the necessary raw biogas amounts for running a second BHKW (block and heat power plant). While originally, for instance, 22 t maize + 6.6 m³ slurry were fed-in (28 t/day), nowadays 35 to 40 t/day are fed into the fermenter in order to achieve the additional 250 kW BHKW operation. This naturally reduces the hydraulic retention time and with it the available time span for the process biology for fermentation of the media. Earlier, an HRT of 100 days was calculated for the initial planning of the plant. Today, the substrate commonly passes through the gas formation phase, which is important for the process biology, in 30-40 days. If substrates of lower quality, e.g. with higher lignification and lower methane formation, are then used, gas production is impaired and the plant performance drops. This is one reason for the increased incorporation of technical processes for substrate disintegration into the plants in the last few years. Meanwhile a number of substrate-independent mechanical or thermal conditioning processes are applied, to make enzymatic digestion of the organic matter easier for the bacteria by later increasing the surface area. This increases the methane formation in amount and time, and thus optimises the absolute methane production per tonne of substrate (substrate mix) used!

2. Methane productivity opposed to theoretical biogas yield of the maize variety
The benchmark for evaluation of the biological process efficiency of the substrates in the plant is the methane productivity. This is the specific methane production rate, measured in Nm³ gas/m³ fermenter volume and day. In practice, this value varies within the range of 0.8 to 1.8 and is irrevocably linked to the effectivity of the process biology adapted individually to the plant and to the quality of the input materials used in the substrate mixture.

The process biology - which is the activity of the bacterial biomass - in turn is an individual, complex influenced system within the framework of the biogas process. There are many biochemical influencing factors, e.g. pH-value, fatty acid status, temperature, mesophile/thermophile mode of operation, viscosity and retention time. Against this background, the significance of the variety-specific theoretical biogas yield of a maize variety determined in a precision trial can only be low. As scientific trials of the LfL Bayern regarding methane productivity already demonstrated in 2007, the theoretical biogas yield of a variety also varies depending on the harvest time and with it in the usual harvest window of between 30 to 38 % whole plant dry matter by more than 100 g Sl/kg oDM (Kaiser & Dr. Gronau). Added to that are the site and yearly effects and the influencing factors on the possible gas yield multiply, which make it nigh impossible to classify the variety just by the theoretically determined biogas yield.

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Can you predict the gas yield?

This problem is known to the official consultation offices, so that many state offices of the Chambers of Agriculture point out this fact consciously and clearly during the variety recommendations for silo maize varieties. An example for this is the comment of LWK Lower Saxony to this years' biogas variety recommendation: "It is still not fully investigated how differences in biomass composition influence the gas yield. […] In practice there are in parts differences noticeable in the gas yield of different silages, which cannot clearly be traced back to concrete parameters."

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Individual experience remains the deciding factor

Finally, it can be stated: The stated methane yield of a variety is normally a calculated value, not a value gained from a fermentation test. These fermentation tests for biogas formation of individual maize varieties are quite elaborate and are only very rarely ordered as laboratory tests by working plants. Not the least because maize is very rarely the only substrate used. As these examples show, the real gas yield performance of a biogas plant depends on a multitude of factors and cannot be determined simply and only from the side of the substrate using the methane yield parameter of one maize variety. Therefore, the calculated methane yield should not be the first and foremost deciding factor for variety selection, as it cannot reflect the real operating conditions of the local plant. Especially since the experienced plant operator is able to qualitatively evaluate best how the maize varieties fare regarding their biogas suitability from year to year using measurable methane productivity within the context of the individually used substrate mix.

And at last there are then the arguments stated initially, which offer a fundamental basis for decisions regarding the specific variety question and selection for a new biogas variety: Site suitability, yield stability, stress tolerance (e.g. Sudrix) and healthy, safe ripening (e.g. Surprime). The necessary variety information can be gained, not only from the official trial publications, but also from the numerous regional field days and from the breeders' consultation media.

Stephan Weniger

Status: 11.10.16