Preservation of sugar beets for biogas production
Andrea WAGNER1, Horst AUERBACH2, Carsten HERBES3, Friedrich WEISSBACH4
1 BAG Budissa Agroservice GmbH, D-02694 Malschwitz Kleinbautzen, Birnenalle 10 Phone: +49(0)35932-35630, Fax: +49(0)35932-35656
E-mail-address: andrea.wagner@budissa-bag.de
2 ADDCON EUROPE GmbH, D-53113 Bonn
3 NAWARO® BioEnergie AG, D-04105 Leipzig
4 Biogas consultant, D-18107 Elmenhorst Introduction
The use of sugar beets has attracted significant attention as potential substrate for biogas production since dry matter (DM) yield per hectare and gas production per kg DM are high.
However, storability of this crop is limited. Preservation is necessary for the use in spring and summer. Furthermore beets need to be cleaned and chopped before used in a biogas plant.
Therefore stones have to be separated. Practically this can only be done in combination with the washing process. This means, that all beets freshly harvested or stored in a pile have to be washed before they are used in the biogas plant.
Sugar beets in a silo release a high amount of nutritive effluent which should be essentially utilized. This would require storage containers that are gas- and water-tight. With a storage density of only 230 kg dry matter per m³ these containers would be unaffordable.
Practical trials showed that sugar beets can also be ensiled in large plastic bags, even without being chopped before. The bags are gas- and water-tight as long as they are undamaged. Whole sugar beets are stored in large plastic bags with a silo press for beets, a BUDISSA PUSH BAGGER® (Fig. 1). The machine is filled through a hopper with telescopic loaders or overhead loading wagons. The beets are pushed with a push blade into the PE- bags with a diameter of 1.95 m (6,5’) (PT 600) or 2.40 m (8’) (PT 800). Amounts of up to 75 tons or 240 tons per bag can be stored, that means 1.3 tons or 3.0 tons per metre respectively. The technical performance depending on the machine type is between 100 and 140 tons/hour. For emptying the bag a front loader, a cutting drum, or other devices can be used. The storage costs for this flexible method are 4.50 – 6.00 EURO/ton.
In a collaborative project of several companies a procedure for the preservation of sugar beets was developed and tested. The aim of the experiments was to answer questions like:
First, chopping and then storing the beets or vice versa? How to preserve the beets to maintain the Methane Yielding Potential as far as possible? Which gas yield potential can be expected in fresh and preserved beets? The idea was to treat the surface of washed whole sugar beets with a fungi inhibiting chemical additive and to store them in large plastic bags.
Fig. 1: Whole sugar beets in large plastic bags Materials and methods
Preliminary tests on the possibility of storing whole beets in plastic bags were carried out in 2007. Whole and crushed sugar beets, harvested in November 2008, were then stored in plastic drums (215 L, 4 replications) and left untreated or treated with KOFASIL® STABIL (containing sodium benzoate and potassium sorbate) to inhibit fungal development.
Whole beets were dipped in the liquid additive for about 2 min whereas 2 litres of the additive were added per ton to the chopped beet mass. Storage time was 4½ and 9 months. After anaerobic storage in the drums, the beets (whole and chopped) were homogenized by using a garden chopper. Beet silages and effluent were investigated for all volatile fermentation acids and alcohols by gas chromatography. Contents of DM, ash and ADF were determined.
The DM contents were corrected for volatiles and the Methane Yielding Potential of fresh beets, beet silages and effluent was calculated according to WEISSBACH (2009). Effluent release and fresh matter recovery were measured. From these data the losses of organic matter (OM) and methane yielding potential during ensiling were calculated. In addition, the aerobic stability of the beet silage was tested. Experimental data were submitted to statistical analysis by ANOVA.
Results
The whole sugar beets in large plastic bags undergo intensive fermentation as, under anaerobic conditions, beet cell tissue dies off and releases some juice which in turn is fermented. The fermentation process starts in the peripheral areas of the sugar beet, leading to production of high concentrations of fermentation products (Tab. 1) (Weissbach and Strubelt 2008). It shows, that the main fermentation product of sugar beet silage is alcohol.
Tab. 1: Concentration of saccharose and fermentation products in whole ensiled sugar beets (beets stored in a plastic tube from December 2007 to August 2008)
Parameter Section of the whole sugar beet
Outer section Mid section Core section
DMcorrected, g/kg1) 250 231 242
Saccharose, g/kg FM 62.6 78.9 88.4 Lactic acid, g/kg FM 10.2 8.9 7.8 Acetic products, g/kg FM 10.6 9.6 7.9 Ethanol, g/kg FM 35.6 40.0 40.5
pH 3.85 3.85 3.85
1) DMcorrected (g/kg)=DMuncorrected+0.95*volatile fatty acids (C1-C4, g/kg)+0.08*lactic acid (g/kg)+1.00*alcohols (C1-C4, including diols, g/kg); Weissbach and Strubelt, 2008, www.LANDTECHNIK-NET.com
During ensiling sugar beets the organic matter content of the silage decreases, in the same process the Methane Yielding Potential per kg organic matter increases (Tab. 2). This increase depends on the alcohol concentration of the beets. Alcohol shows a higher energy potential than sugar and therefore produces more methane. Methane content of biogas from carbohydrates is always 50 %. On the contrary, utilization of alcohols as substrates for biogas production results in significantly higher methane production per kg substrate than that from sugars. Methane concentration in biogas from primary alcohols is always 75 % (WEISSBACH, 2009). As the ultimate result of all fermentation processes, the average specific gas production potential of ensiled sugar beets is markedly higher than that of fresh sugar beets. Based on the fresh matter, silage and effluent as well as fresh sugar beets produce the same quantity of methane (Table 2).
Tab. 2: Content of organic matter (OM) and Methane Yielding Potential
substrate OM-content
g/kg FM
Methane Yielding Potential litres/kg OM m³/t FM fresh beets, after short
storage in November
231 (226…236) 361 (360…361) 83 (82…85) fresh beets, stored in
piles till March
221 (218…225) 363 (361…364) 80 (79…82) ensiled beets, stored in
silos till August
212 (198…231) 383 (357…403) 81 (77...86) effluent released from
beets, undiluted
199 (177…214) 385 (374…410) 77 (68…80)
Methane yielding potential of the effluent was found to be approximately as high as that of fresh beets and beet silage (Table 2). Since the vast proportion of DM in sugar beets silages is composed of soluble compounds (saccharose, fermentation products), DM (corrected for loss of volatiles during drying) of effluent is nearly as high as of silage. Consequently, all effluent must be collected and used in biogas facilities.
Whole sugar beets produce significantly less effluent than chopped beets (Fig. 2). In this way the risk of nutrient losses through uncontrolled effluent leakage is markedly reduced, which in turn makes it possible to store beets in large plastic bags. Only a small undiscovered leakage can cause the loss of the effluent. This amount can be kept in the bags much easier and can be drained after a few weeks of storage.
0 5 10 15 20 25 30 35 40 45 50
untreated treated untreated treated
%
effluent produced
loss of Methane Yielding Potential through effluent
chopped sugar beets whole sugar beets
Fig. 2: Silage effluent and potential loss by leakage of effluent after 9 months storage
Even though the activity of yeasts causes the production of the energetic metabolite alcohol, energy is used for this process. Even if the silo is well sealed there are still losses by continuing respiration, until the oxygen is consumed. Therefore it was analysed, how much of the organic matter and Methane Yielding Potential is lost through the biological process in the silo.
0 2 4 6 8 10 12 14 16 18
untreated treated untreated treated
losses in %
OM-content Methane Yielding Potential
whole sugar beets chopped sugar beets
Fig. 3: Losses by fermentation and residual respiration after 9 months storage
The voids between the whole beets cause a higher risk of losses during the fermentation process compared to chopped sugar beets (Fig. 3). During a long storage period in the plastic bags also the pressure compensation between the voids and the atmosphere may cause a slight penetration of oxygen and microbial activities.
The application of the chemical additive KOFASIL® STABIL could inhibit fungal development and significantly reduce these losses. The loss of Methane Yielding Potential of the treated chopped and whole sugar beets was almost equal and at a very low level. With regards to the losses the disadvantage of not chopping the beets can be compensated with the use of the additive.
The voids between the whole beets also cause a higher risk of aerobic losses upon emptying the bags. After opening the bag the carbon dioxide in the voids can flow out easily and is replaced by inflowing air. Aerobic deterioration involves heat generation, the loss of sugars and the evolution of CO2.
15 18 21 24 27 30 33 36 39 42 45 48
0 1 2 3 4 5
temperature in C°
length of exposure to air (days) untreated
treated
ambient temperature range ↕
Fig. 4: Influence of surface treatment of sugar beets with KOFASIL®STABIL on the heating
The application of a chemical additive can improve aerobic stability, because it slows down the reheat significantly. This could be proven in stability tests. To analyse the aerobic stability the beets were homogenized and stored under defined aerobic conditions. These simulated conditions can’t be transferred to the conditions of whole sugar beets stored in a plastic bag - the temperature increase in this test starts much earlier.
Nevertheless the test shows the differences in sensitivity towards air penetration between different treatments (Fig. 4). It is evident, that the treatment with a chemical additive retards the reheat of the ensiled sugar beets. Therefore a slower feed rate can be realized without risking higher losses of Methane Yielding Potential.
To reduce these losses without an application of chemical additives, the bag should be emptied within a few days especially in the summer time.
Conclusions
The storage of sugar beets beyond March needs a preservation system that is characterised with minimal losses and low costs. One suitable possibility is the ensiling of washed (free from stones) whole sugar beets in large plastic bags. An accurate effluent-management is essential for this process. All effluent has to be collected and utilized in the biogas plant.
Preserving sugar beets without chopping before ensiling reduces the production of effluent considerably and allows controlling the process. However it causes higher losses through respiration in the initial aerobic phase and during fermentation. It also caues as a higher risk of reheat during the feed out period. Fermentation losses and secondary fermentation can be reduced effectively through surface treatment of whole sugar beets. The experiments pointed out a feasible process to preserve sugar beets with a total loss of only about 5% methane yielding potential. This enables the utilization of sugar beets as a substrate for biogas production throughout the year.
References
Weissbach, F. und C. Strubelt (2008): Die Korrektur des Trockensubstanzgehaltes von Zuckerrübensilagen als Substrat für Biogasanlagen. Landtechnik 63, H. 6. S. 354-355
Weissbach, Friedrich (2009): Gas production potential of fresh and ensiled sugar beets in biogas production. LANDTECHNIK 64 (2009), no. 6, p. 394-397.
