NAWARO Bioenergie AG

Fundamentally changing economic conditions have made sugar beet quite an interesting substrate for biogas production recently. In order to use sugar beets beyond natural shelf-life, their preservation has attracted significant attention [2, 6]. Against this background, existing knowledge from previous investigations which aimed supplying sugar beets for feeding purposes all year around can be used [3, 4, 11].

Preliminary tests under practical largescale conditions have shown that whole sugar beets can be stored in the absence of air for a certain period of time [6]. Under anaerobic conditions, metabolic activity of beet tissue ceases resulting in cell death, release of liquid cell content and turning to fermentation as is known from ensiling chopped sugar beets.

To assess preserving technologies for sugar beets, balance trials for establishing the recovery of the biogas production potential from harvest to the biogas reactor are necessary. Inclusion of all volatile products in silages forms a precondition of such balances. The aim of this study was to determine the range of concentrations of individual volatile compounds which may be found in sugar beet silages, and to propose a substrate-specific equation for correction of DM for volatile compounds in these silages.

Materials and methods

Analytical results of 35 sugar beet silages from previous trials [4], which had been completely documented, could be used for this investigation. Those sugar beets were washed, chopped and stored in airtight plastic bags. Storage time varied between 2 weeks and 9 months. To control fermentation, in some of the silages potassium/sodium pyrosulphite was applied for suppressing lactic acid fermentation, in others sodium benzoate was used for inhibition of alcoholic fermentation. Lactic and acetic acids were determined individually, whereas the higher homologues of acetic acid were only analysed as the sum all other acids („butyric acid” according to Lepper-Flieg). Total content of alcohols was determined oxidimetrically and expressed as ethanol.

The analytical results from previous trials were amended by results on 9 samples taken from sugar beets preserved in plastic tubes within the scope of practical testing of this technology [6]. These sugar beets had not been washed and chopped. They were stored for 6 months (December 2007 to June 2008). These samples were submitted to gas-chromatographic analysis for all individual short chain fatty acids and alcohols.

Efforts to determine potentially volatile compounds in the drying residue were not successful as during extraction of drying residues significant amounts of solubilized pectins disturbed the chromatographic analysis.

Results and discussion

The average contents for sugar, fermentation acids and alcohols of ensiled sugar beets compared reasonably well with those stored unprocessed under air-exclusion in plastic tubes. Therefore, it was possible to combine all data and further use them as one data set.

The wide range of individual data, however, exclusively results from the previous trials. The reasons for the great variability of analytical data are the varying storage length and fermentation pattern of silages. On the contrary, data on the contents of individual low fatty acids (besides acetic acid) and alcohols (besides ethanol) were obtained only from the recently analysed samples. It could be shown that the concentrations of higher homologues of acetic acid and of ethanol are very small and that those do not have to be taken into consideration individually when correcting DM content. Butyric acid formation does not occur in sugar beet silages. Methanol which is regularly found in preserved sugar beets is most likely to be formed during the process of decomposition of pectins.

If sugar beet silages are stored for longer periods, the vast majority of the sugar is converted by fermentation into lactic and acetic acids, but mainly into ethanol. It is well known that, during fermentation, lactic acid formation goes on earlier than ethanol production [4]. Due to low buffering capacity of sugar beets, only relatively small concentrations of lactic acid are required to reduce pH to below 4, thereby ceasing further lactic acid production. Residual sugar is then converted into ethanol by yeasts which are known to be acid-tolerant. Extend of ethanol fermentation depends on storage length and conditions. The high variability in residual sugar content is associated with the enormous range of concentration of volatile fermentation products. There is an expected close relationship between residual sugar level and ethanol content.

Although volatility of each individual fermentation product could not be measured, it can well be derived from other investigations. As in maize silages, typical pH of sugar beet silages is below 4. Therefore, a volatility coefficient of the total of low fatty acids of 95% can be assumed [9]. Furthermore, volatility coefficient of lactic acid of 8% [1, 7, 8] can also be applied to sugar beet silages. As found in investigations with maize and grass silages [9, 10], alcohols with one hydroxyl group evaporate always completely. Since alcohols with two hydroxyl groups occur only in minute amounts in sugar beet silages, 100 % volatilization can presupposed also for the total of all alcohols here.

By means of these volatility coefficients, the DM figures obtained in the common way (DMn) were corrected for the loss of volatiles (DMc). The results are shown in the last lines of Table 1. On average, the error of DMn was found to be approximately 35%. The enormous variability of this error is vastly associated with differences in ethanol content, which is demonstrated in Figure 2on the basis of the quotient DMc/DMn.

Conclusions and recommendations

As dry matter of freshly harvested sugar beets is composed of sugar of about 70% and this is fermented at a variable extent, it follows that a vast and substantially varying proportion of the organic matter of sugar beet silages consist of volatile fermentation products. Therefore, information on substrate-specific biogas yield makes only sense if it is based on corrected DM and corrected organic matter contents, respectively. Complete chemical analysis of silage for volatile fermentation products is the crucial pre-requirement for this approach. It is recommended to correct the DM content determined in the common way (preliminary drying until constant weight at 60 to 65 °C, followed by final drying at 105 °C for 3 hours) for the loss of volatiles during this process by using the following equation:

DMc = DMn+ 0.95 FA + 0.08 LA + 1.00 AL [g kg-1FM],
where is:
FA = total content of low fatty acids (C2…C6)
LA = lactic acid content
AL = total content of alcohols (C1…C4, including diols).

All analytical data have to be fitted in this equation in the dimension g per kg fresh matter (FM).

As a consequence of correcting the DM content, all analytical parameter which are expressed as part of the DM have to be corrected as well. Those which are directly measured in the dried sample and usually expressed as percent of DMn(e. g. crude ash) must be multiplied with the quotient DMn/DMc. Difference fractions (e. g. organic matter) have to be calculated once more by using the figures expressed as percent of DMc.

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The need of correction of dry matter (DM) content of silages for the loss of volatile compounds during drying has been used for a long time in feed analysis. However, this procedure has not been common in the evaluation of silages, used as substrate for biogas production. In the VDI-guideline 4630 “Fermentation of Organic Materials” [3], which was published in 2005, the remark is given that the content of organic dry matter determined in the conventional way and used without consideration of the content of volatile compounds “…constitutes a falsified reference variable with regard to the determined biogas potential…”. But this VDIguideline [3] does not offer sufficiently exact recommendations for complete determination of these losses.

Recent experimental data published by Mukengele and Oechsner [2] showed that the often reported apparent increase of the biogas yield in silage, compared to green forage which it was made from, can mainly be explained by missing or incomplete correction of the DM content of silage. Some experimental results, which described an increase of the biogas yield as a consequence of the use of certain silage additives, might also be explained in a similar way.

The authors [2] used equations for the correction of DM content given by Weissbach and Kuhla [5]. Those are based on experimental data previously published by Berg and Weissbach [1, 4]. However, the measure of volatility of individual alcohols could not be determined in these investigations, due to a lack of appropriate analytical methods for separation (gas chromatography).

Special bacterial silage inoculants have recently been recommended for improving aerobic stability of maize silages, in particular of those intended to be used in biogas production. These silage additives modify the ensiling fermentation pattern. Beside higher acetic acid concentrations, treated silages also contain higher levels of 1,2-propanediole than untreated silages. This study is aimed at elucidating the volatility of alcohols (especially such with two hydroxyl groups) which are formed in silage, and at proposing an improved equation for the correction of DM content of maize silage.

Materials and methods

In total 117 samples of maize silages were taken from farm silos and analysed. All silages had been stored in silos for at least six months. Silages were analysed for all potentially volatile fermentation acids and alcohols as well as for pH. DM content was determined by the official German method used in feed evaluation. According to this method, samples are subjected to preliminary drying at 60…65 °C to constant weight and subsequent final drying at 105 °C for three hours.

From the farm silage pool, 20 samples were selected, which had relatively high contents of 1,2-propanediol and 2,3-butanediol, respectively. In those silages, all potentially volatile fermentation products were measured also in the residues after final drying (three hours at 105 °C). Subsequently, volatility was calculated from the difference between the contents of each fermentation product in the fresh and in the dried samples.

Results and discussion

All silages were of good fermentation quality. DM content ranged between 224 and 492 g kg-1(mean = 337 g kg-1). Almost all silages had a pH between 3.5 and 4.0 (mean = 3.8). Table 1summarizes means and parameters of variability for all individual potentially volatile compounds which were detected in maize silages.

Acetic acid was determined to represent the vast proportion of all short chain fatty acids, which are know to be highly volatile. However, its content varied extremely. Propionic acid was also detected in all silages, but at markedly lower concentrations. Butyric acid and other higher homologues (valeric and caproic acids) were found in traces only. Lactic acid, which is known to be less volatile, was measured at typical levels but showed unexpected high variability. Alcohols were mainly composed of ethanol. Its content was also extremely variable. Other simple alcohols (propanol and butanol) occurred in very low concentrations and were detected at low frequency. This applies also for 2,3-butanediol, which was found in traces only. On the contrary, 1,2-propanediol was detected in more than 90 % of all maize silages, and in some at substantial concentration.

The analytical data of the 20 selected silages and their drying residues are shown in Tables 2 and 3. In order to enable the comparison between fresh matter (FM) and drying residue, all data are expressed in g per kg FM. The results on volatility of acids are in good agreement with earlier studies [1, 4]. Acetic acid was found to be volatile at 95 %. This value can be considered as practically constant and can be used for all other shortchain fatty acids, produced in maize silage with its typical low pH. Under the drying conditions described above, the mean volatility of lactic acid was found to be 8 %, which agrees with earlier findings [1, 4]. The assumption that all simple alcohols are fully volatile [5] could also be confirmed.

For the first time it was possible to determine the volatility of 1,2-propanediol. The average volatility of this compound was found to be 77 %. It varied in a relatively small range and proved to be unaffected by the concentration of this alcohol in silages. The concentrations of 2,3-butanediol were too small for determining its volatility.

In summary, it can be stated that the percentages of volatilization of the individual fermentation products in maize silage – in contrast to their concentrations – are nearly constant at defined drying conditions. Therefore, the mean percentage of volatilization of the individual compounds can be generalized and used for correction of DM content on the basis of analysed fermentation pattern of maize silage. The validity of percentages of volatilization determined here, however, strictly depends on drying conditions as described in this study.

Conclusions and recommendations

Maize silages contain substantial amounts of volatile organic compounds, which have gas forming potential and must not be neglected in measuring the specific gas yield potential. Concentrations of these volatile acids and alcohols in silages are extremely variable between silages. Therefore, a complete analysis for fermentation products in silages is absolutely necessary if the specific gas yield is determined by fermentation tests. Simplified methods of DM correction in silages based on average concentrations of volatile compounds, which are only influenced by the DM level, are inappropriate for this purpose.

The following equation for calculating the corrected DM content (DMc) of maize silages based on the non-corrected DM content (DMn) measured by oven drying is recommended:

DMc = DMn + 0.95 FA + 0.08 LA + 0.77 PD + 1.00 OA [ g kg -1FM]
where is:
FA = fatty acids (C2 … C6)
LA = lactic acid
PD = 1,2-propanediol
OA = other alcohols (C2 … C4, including 2,3-butanediol)

All values are to be put into the equation in the dimension g per kg fresh weight.

The equation is valid only for defined drying condition (preliminary drying at 60…65 °C; final drying three hours at 105 °C).

As a consequence of DM correction, all other analytical parameters, which are expressed on DM basis, have also to be corrected. Those which are directly measured in the dried sample and usually expressed as percent of DMn(e. g. crude ash) must be multiplied with the quotient DMn/DMc. Difference fractions (e. g. organic matter) have to be calculated once more by using the values expressed as percent of DMc.

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Knowledge on the degree of volatility of all the individual fermentation products during drying under defined drying (volatilization coefficients) is required in order to enable the correction of DM content of silages. Data on volatilization coefficients have already been published earlier [1, 4]. It was shown in these studies that the volatilization coefficient of an individual compound can normally be determined by measuring and comparing the contents in the fresh silage sample and in the respective drying residue. The only exception is lactic acid. Due to a specific chemical reaction of lactic acid (formation of lactide) which is facilitated through dehydration by heat treatment and affected by various drying conditions, the decrease of lactic acid content during drying can be higher than its transfer into gas phase.

In those experiments, volatilization of certain alcohols remained unclear as these compounds could not be separated out by chromatography at that time [5]. This applies especially to alcohols with more than one hydroxyl-group. As a consequence of the use of special silage additives, enhanced formation of 1,2-propanediol is caused in silages. A volatilization coefficient of 77 % for 1,2propanediol which frequently occurs in maize silage has recently been determined [6], whereas data are still lacking for 2,3butanediol which can be found at a high incidence in grass silages.

The aim of the study was to determine the range of concentration of volatile fermentation products in grass silages as well as their volatilization coefficients in order to propose an improved equation for the correction of DM content of grass silages with special regard to mentioned two alcohols.

Material and Methods

A set of 182 grass silage samples from commercial farms was used, representing a wide range of DM level and substantial differences in fermentation quality. Information on use of silage additives and storage length upon sampling were not available. All silages were analysed for potentially volatile fermentation products in fresh and dried samples.

DM content was determined by preliminary drying at moderate temperature and subsequent final drying of the milled sample at 105 °C for 3 hours. In previous investigations [1, 4, 6], drying residues were analysed after final drying at 105 °C for 3 hours. In the experiments described here, however, the air-dry milled samples were used for analy- sis of fermentation products after had been submitted to an additional drying process at 70 °C over night (approximately 16 hours).

Results and discussion

DM content of the analysed silages ranged between 179 and 597 g kg-1(mean: 428 g kg-1), and pH varied between 3.8 and 6.1 (mean: 4.8).

As in maize also in grass silages, acetic acid was found to represent the vast propor- tion of short chain fatty acids, which are known to be highly volatile. But in addition to acetic and propionic acid, butyric acid and their higher homologues (valeric and capro- ic acid) were determined here at substantial frequency and sometimes in substantial con- centrations as well. It has been known that volatility of all these acids during drying de- pends on pH. The lower pH of the silage, the higher is the volatility [1, 5]. The high varia- bility of pH explains the high standard de- viation of the volatilization coefficients for these acids in grass silages. The variability of the volatilization coefficient was at least markedly higher than recently reported for maize silages [6]. The error of the use of a generalized mean volatilization coefficient can be avoided, if the silage pH is taken into account. Volatilization coefficient (VC) for the total of low fatty acids in grass silages can be estimated by using the following sub- strate-specific regression:

VC [%] = 105 – 0.059
pH (sR= 5.7)

Volatility of lactic acid does not depend on pH and is generally low. The average volatilization coefficient for lactic acid in this study was 10 % which agrees reasonably well with previous investigations from which a mean value of 8 % was generalised [1].

Grass silages on average contain smaller amounts of alcohols than maize silages. But ethanol was shown to be the major alcohol also in grass silages, and its concentration can reach substantial levels. Propanol and in particular butanol occur at markedly lower frequency. All these alcohols with one hydroxyl-group evaporate completely during drying. Regarding 1,2-propanediol, which was found to occur in grass silages in similar concentrations like in maize silages, an average volatility of 77 % could be confirmed. As expected, incidence of 2,3Butandiol was higher in grass silages than previously found in maize silages. This study enables for the first time to determine the volatilization coefficient of this compound. On average, 87 % of the 2,3-butandiol initial present in silage are lost during drying.

In summary, contents of potentially volatile fermentation products in grass silages are extremely variable, whereas the variation of volatilization percentage of the individual compounds under defined drying conditions is relatively small.

Conclusions and recommendations

Grass silages may, as reported for maize silages, contain substantial concentrations of volatile organic compounds, which possess biogas forming potential and therefore must not be neglected in measuring the specific gas yield potential. Therefore, a complete chemical analysis of grass silages is absolutely necessary, if the specific gas yield is determined by fermentation tests. The use of simplified methods for DM correction without determination of individual fermentation products, e.g. methods based on the relationship between DM content and losses of volatiles during drying of silage samples can result in big errors.

Figure 1 illustrates the calculated losses of volatiles in the 182 grass silage samples used in this study which were derived from the direct comparison of the content of fermentation products in fresh silage and its respective dried sample. Losses averaged about 4 %, but were found to be as high as 16%. On account of differing fermentation intensity in silos which results in variation in the content of fermentation products and is affected by DM content of the ensiled grass, a clear tendency could be observed towards higher losses at lower DM levels. In individual cases, however, losses in silages of the same DM concentration can differ by up to 10% as consequence of different fermentation pattern.

Based on the complete analysis of fermentation products, the following equation for calculating the corrected DM content (DMc) of grass silages from the non-corrected DM content (DMn) measured by oven drying is recommended:

DMc = DMn + (1.05 – 0.059 pH) FA + 0.08 LA + 0.77 PD + 0.87 BD + 1.00 OA [g kg-1FM]
where is:
FA = total of low fatty acids (C2…C6)
LA = lactic acid
PD = 1,2-propanediol
BD = 2,3-butanediol
OA = total of other alcohols (C2…C4)

Concentrations for all individual compounds have to be fitted in the equation in the dimension g per kg fresh matter (FM).

The equation is valid for defined drying conditions (preliminary drying at 60 - 65 °C until constant weight and subsequent final drying at 105 °C for 3 hours).

As a consequence of DM correction, all other DM-based concentrations, e.g. nutrient contents, have to be corrected as well. All parameters which are typically subjected to direct analysis in the dried sample and usually expressed as percent of DMn (e.g. crude ash), must be corrected by multiplication of the value with the quotient of DMn/DMc. All fractions which are obtained by difference calculation, such as organic DM (oDM), must be calculated once more using the values expressed as percent of DMc.

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When substrates for biogas production are characterized, it is currently usual to eliminate the effects of differing contents of crude ash (XA) by subtracting XA from DM and expressing the substrate-specific biogas yield per kg of organic matter (OM). However, the substrate-specific gas yield per kg of OM is an extremely varying parameter. In substrates of plant origin, the main cause for this variability mostly is not the differing content of the three major nutrient fractions: protein, fat and carbohydrates, which form different volumes of biogas per kg [2, 10]. Even more important seems to be the proportion of OM which can be biologically utilized. A very close relationship between „digestibility of OM in the biogas fermenter” and the methane yield per kg OM has been found by Kaiser [6] recently.

Therefore, for characterization of substrate-specific biogas yield, it could be useful not only to subtract XA from DM but also that part of OM which cannot be utilized biologically. This would result in a new parameter for characterization of substrates, which could be named the content of „fermentable organic matter“(FOM). The study reported here aims at clarifying the pre-requirements and the opportunities of such an assessment of renewable primary products for biogas production.

Materials and methods

A method for calculation of expected biogas yield which, has been employed yet [7] is based on analysing all substrates according to Weende Feed Analysis and using digestibility coefficients from the DLG feed tables for ruminants. Contents of the individual nutrient fractions are multiplied with the respective digestibility coefficients and values for the specific biogas forming potential: The latter are taken over from a paper published by Baserga [2]. According to this author, specific biogas forming potential for carbohydrates, fats and proteins are 790, 1250 and 700 litres per kg, respectively; methane contents in biogas of 50%, 68% and 71% were assigned to carbohydrates, fats and proteins, respectively. There is evidence provided that the general validity of these values must be questioned [10]. Apart from this and other weaknesses, which cannot be discussed in this paper, the main constraint to this method lies in the fact that the calculation of biogas yields gives substantially lower values than those obtained from laboratory fermentation experiments on the same substrate.

This is mainly caused by the false assumption that the apparent digestibility measured in sheep is identical to biological degradability of the nutrients. However, animal faeces do not only contain indigestible compounds of the feed intake, but also metabolic and endogenous matter arising from the process of digestion [9]. The truly biologically not utilizable proportion of nutrients can be calculated, if the metabolic nutrient excretion is known and if that will be subtracted from the total amount of excreted nutrients. But this is only possible if the procedure of digestibility trials is strictly standardized so that approximately constant metabolic nutrient excretions can be assumed [11]. This high level of standardization cannot be expected generally if feed table values are used.

For the study reported on in this paper, numerous results from digestibility trials were available, which meet the necessary high standardisation level [13, 14]. Data from the following number of digestibility trials carried out each with typically 4 individual sheep could be used for: 44 trials on grains and grain by-products, 63 trials on forage maize and different maize products, 72 trials on whole-crop cereals, 75 trials on lucerne, 52 trials on green rye, 41 trials on green oats as well as 135 trials on grass from different sward types.

Results

At first, it was tested as to whether different nutrient concentrations and biological degradability of OM affect biogas yield. Biogas yields were calculated from the content of true digestible nutrients for a wide range of different crops using the gas forming potential of nutrients according to Baserga [2]. Crops are listed in descending order of their FOM content. Additionally, fermentation coefficient (FC = DOM/OM) as an indicator for biological degradability of OM (analogous to the digestibility coefficient DC) is given.

It is shown that the calculated gas production yields do not differ between crops if values are based on FOM. The main reason for this finding is that the vast majority of fermentable compounds are composed of carbohydrates in all crops and that differences in other nutrients are insignificant. The average yield of biogas and methane per kg FOM was found to be about 800 litres and 420 litres, respectively. Error of prediction of substrate-specific gas yield, which can be expected for such method was – compared with the typical measurement error of some laboratory fermentation methods – unexpectedly small.

Subsequently, it was investigated as to which extend the non-utilizable proportion of OM can be estimated by use of basic laboratory analysis numbers. Previous studies provided evidence [11] that animal faecal excretion from crude protein (XP) and crude fat (XL) expressed as proportion of intake of feed dry matter – so to speak „the contents of indigestible nutrients“– varies insignificantly within a given kind of crop. Therefore, it is possible to use crop-specific average values for animal excretions of these two nutrients.

On the contrary, the carbohydrates (sum of crude fibre and nitrogen-free extract) excreted by animals with faeces is extremely variable and must be estimated by using at least one suitable laboratory parameter. Figure 1 shows the model, which we have used for that purpose.

The organic residue provided by certain laboratory hydrolyses methods (x), e.g. crude fibre (XF) content in DM, is analogous to the animal faecal excretion of carbohydrates (y), expressed as proportion of intake of feed DM. The relationship between these two parameters can be described by a simple regression function. Intercept “a” of this function represents the metabolic excretion, whereas the regression coefficient „b“ reflects the increase of excretion, e.g. by increasing crude fibre content. Product „b•x“ represents the amount of carbohydrates, which are truly indigestible and thus nonutilizable. These functions for most kinds of feed are not linear and request approximation of polynomial equations of second grade. For instance, regression curves for non-utilizable carbohydrates increase progressively with increasing XF content.

Under the standardized conditions of the used digestibility trials, a mean metabolic excretion of 35 g carbohydrates, 20 g crude protein and 5 g crude fat per kg feed dry matter was determined, which amounts to a total of 60 g OM per kg feed DM.

Deriving equations for prediction of FOM is now be described by using one example on forage maize. All laboratory parameters as well as FOM are given in the dimension g per kg DM. The mean excretion of XP and XL were 36 g and 5 g per kg DM, respectively (standard deviation sx= 4 g and 1 g per kg DM, respectively). Excretion of carbohydrates could be described by the following regression equation:

y = 35 + 0.47 (XF) + 0.00104 (XF)2
sR= 24 g/kg.

The model for prediction of FOM is:
FOM = 1000 – (XA) – 36 – 5 – [35 + 0.47 (XF) + 0.00104 (XF)2] + 60
from which finally follows:
FOM = 984 – (XA) – 0.47 (XF) – 0.00104 (XF)2

Equations for all investigated crops are summarized in Table 2. Crude fibre content was found to be in most crops a suitable analytical parameter for estimating biologically non-utilizable carbohydrates and non-utilizable OM, respectively. Using other fibre fractions, like NDF, ADF or ADL, did not improve the performance of the estimation significantly. The only kind of crop, where neither XF nor another named fibre fraction resulted in sufficiently accurate estimations was grass from different swards. Therefore, it is proposed to preferably use in the prediction of FOM the content of „enzyme-resistant organic matter“(EROM) of grasses and grass silages. EROM is the organic residue after hydrolyzing the sample by means of enzymes [13, 14]. It is expressed in g per kg DM and can be understood as analog of XF. The difference between is that hydrolysis is attained by treatment with digestive enzymes (pepsin and cellulase) at 40°C and not by boiling in acids and bases, which is done in the determination of XF.

All equations can be used for fresh forages and silages thereof as well as gently dried material. However, the crucial pre-requirement for the applicability of these equations to silages remains that DM is corrected for the loss of volatile fermentation products during sample drying [12, 15, 16].

The calculated values for biogas yield by using the equations given above and by as- suming 800 litres biogas and 420 litres me- thane per kg FOM, respectively, do not al- ways agree with published results from labo- ratory fermentation tests [1]. This may be caused by several factors. The findings com- pare reasonably well with results from Ho- henheimer Biogas Test [8], given that gas volumes were calculated for norm conditi- ons [1]. Data in Table 3support this statement exemplarily for forage maize samples of which information on measured biogas yield and nutrient contents were available. For both, magnitude of substrate-specific me- thane yields and differences in quality bet- ween samples, a reasonably well comparison between the two methods can be stated.

Conclusions

As demonstrated, the content of FOM is sui- table to characterize the gas production po- tential of renewable primary products. Using this parameter bears the advantage that it is not affected by influences of different proto- cols of fermentation tests in individual re- search facilities. In addition, it is much faster and cheaper to determine. Content of XA is already measured generally. Only by deter- mination of one additional parameter (XF or EROM) a substantial gain of information can be attained.

FOM is to be defined as that amount of OM which can potentially be metabolized by microorganisms under anaerobic conditions and which can therefore be utilized for bio- gas production under optimal process condi- tions and in sufficiently long process time. FOM is identical to the content of true di- gestible organic matter calculated from strictly standardized digestibility trials with sheep [11, 13, 14]. It should, however, pre- ferably be measured by suitable laboratory fermentation techniques in future.

Conversion of FOM contents into biogas or methane volumes has not to be carried out necessarily for assessing gas production po- tential of renewable primary products. The content of FOM per se is a good characteris- tic of the gas production potential of sub- strates. Where required, substrate-specific gas yields should be expressed as gas vol- umes per kg FOM rather than per kg OM.

It should be possible to use constant coef- ficients for calculating the volumes of bio- gas and methane per kg FOM of the most re- newable primary products as has been shown in this study. But these coefficients have to be qualified. The coefficients used so far are only based on numbers for gas forming po- tential of the individual nutrient fractions given by Baserga [2]. The validity of these coefficients has to be checked by further stu- dies.

For special substrates there may be also a need for using gas yields per kg FOM dif- ferent from average values. This may apply to e. g. sunflowers (due to its high fat con- tent) and for ensiled sugar beets (due to its high ethanol content). Using FOM as the ba- sal parameter for substrate-specific gas yield data eliminates the impact of differences in fermentability of OM. Thereby new oppor- tunities may arise for deriving and bioche- mically accounting for gas formation poten- tial values of nutrients and substrates by means of stoichiometric calculations [3, 4, 5].

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New Energy Finance believes that the fast expansion of biogas power in countries such as Germany can be replicated elsewhere, particularly in other European Union states, and in developing countries that offer CDM opportunities. Investors should look for the presence of secure feedstock supplies and a feedin tariff set for several years at least. Feedstock could eventually become the Achilles heel for biogas generation, but we are a long way from that point now.

Germany and UK in the front

Germany’s biogas success story, much like its booming solar industry, is founded on generous feed-in tariffs. Since the Renewable Energy Sources Act entered into force in 2001, Germany has seen a spectacular increase in biogas investment. Up to 2001 900 biogas plants were installed, whereas 800 were built in 2006 only. The law amendment of 2004 introduced a differentiated feed-in tariff system, and as a result small scale CHP plants have been in favour. The paradoxical result of this law is that plants are relatively expensive as companies do not give top priority to installing bigger and more efficient plants.

In 2006, newly installed biogas capacity in Germany was 550MW and will produce as much kWh electricity (3.9bn) as 2,280MW new installed wind turbines due to higher load factors. The German market, including exports, is predicted to grow 30%-40% annually until 2010. Share of export revenues will be around 15%; with emphasis on Italy, Benelux and the US.

The UK’s position in generating electricity from landfill gas (see Figure 1) is remarkable, since it contributes to 33% of total renewable energy utilisation (heat as well as electricity), compared with just 6% for wind. This form of electricity production is competitive with fossil fuel plants even before subsidies are factored in. But the level of landfill gas utilisation is expected to diminish in the longer term, as the Landfill Directive (1999/31/EC) stipulates that member countries must reduce the amount of biodegradable waste going to landfill.

In Germany, a plant of up to 5MW is eligible for a €0.08 per kWh feed-in tariff. An additional bonus is available for smaller plants, new technologies, for those using energy crops and combined heat and power plants. The maximum feed-in tariff is €0.21/kWh (see Figure 2). Another country with similar legislation, Italy, with a feed-in tariff of €0.199/kWh for all sizes of plants will have higher growth rates than Germany and is a promising market for German companies in coming years. Also, the Czech Republic and France, where agricultural biogas deposits have remained practically unexploited until now, offer good conditions regarding feed-in tariffs and agricultural structure. Most of the big German players in the field, for example Schmack Biogas, EnviTec and Biogas Nord have already established sales offices in those countries.

The current dynamism of the overall sector is considerable, but all these efforts may not be enough to reach the objectives that the European Union White Paper set in 1997: 15 Mtoe by 2010. At the current rate of expansion, Europe seems likely to achieve 8.6 Mtoe by 2010, compared to 5.3 Mtoe now.

Today the average output of a newly constructed biogas plant is 320kW electric, representing a considerable increase on the first generation of biogas plants with capacity of less than 50kW. However, capacity per plant still varies considerably (not only) from country to country (see Figure 3) due to agricultural structures and also differentiated feed-in tariffs. It is likely to level out as bigger plants are more efficient and technology standardisation is taking place.

One of the first companies generating biogas on an industrial scale is NAWARO BioEnergie, whose first 20MW biogas park ‘Klarsee’ in Punkun will be fully commissioned in September 2007. The 40 units were delivered by EnviTec which specialises in the standardisation of 500kW modules. The first module begun operations in November 2006 and by now 15 modules are operating. The park is situated in Mecklenburg-Vorpommern, close to the German-Polish border, where large agricultural enterprises and farmers (50-60) from both countries deliver the feedstock. Approximately 300,000 tonnes of corn silage, 60,000 tonnes of manure and 20,000 tonnes of grain per year are required. By recycling the byproducts of energy production, the company also produces fertilisers, which contribute an additional €50-150 per tonne revenue. The vicinity of the Baltic Sea ports represents a reloading point for shipping the fertilisers to international markets. Two other parks of the same size are under development.

Equity providers

NAWARO’s financing is done by Doric Asset Finance, which set up the Geno Bioenergie 1 fund of around €100m for the Klarsee biogas park. Minimum investment was €10,000; the first distribution for investors is projected for November 2008 at 5.5% of the contribution, with further 9% payments per year from 2009 onwards.

The biogas sector in Germany has attracted more financial investors: in Trust has shifted its investments from wind to biogas energy and has invested €80m in biogas plants so far. Another 6 parks worth €80m are under development, which will lead to a biogas portfolio of 39MW. The company is experiencing an increased interest of institutional investors and utilities. CornTec was involved in the construction of 12 plants (6.7MW); MTV closed two BioEnergy funds with a volume of €17.3m and €3.4m respectively. Not surprisingly, Aufwind Schmack was one of the first companies establishing biogas funds and closed ‘Cash Cow III’ (€6m) at the beginning of this year. Austrian’s Raiffeisen daughter Renergie, founded at the beginning of this year, announced to invest up to €100m in biogas projects.

Beyond electricity

More companies are starting to feed biogas into the natural gas grid as electricity and heat can be directly utilised where needed. Schmack has commissioned its first plant feeding biogas into the grid in Pliening in December 2006 and about five others have been commissioned this year. Although one of the more attractive opportunities, the development of this kind of plants is still insecure, as the technology is relatively expensive and still under development. The price for feeding the biogas into the gas grid has to be negotiated with the utility. If the company builds an electricity generation plant using the gas from the grid, it will get the favourable feed-in tariffs.

Sweden started much earlier to feed biogas into the local grid and has about 30 plants in operation. The focus lies also on fuel for vehicles as biogas is economically viable due to high carbon taxes on fossil fuels. The country started in the nineties to use biogas in lorries and car fleets. With about 4,000 vehicles and 28 biogas refueling station, Sweden is the most advanced country of Europe. It is followed by Switzerland who has about 2,000 biogas vehicles running on a mix of biogas and natural gas. Last June, the first filling station in Germany was opened. In the same month, the Austrian government signed an agreement with the oil and gas company OMV to promote biogas. The latter has said it has plans to open 200 biogas filling stations.

Outside Europe

United States

Looking overseas, no country offers comparable incentives as Europe for biogas electricity generation plants. Although companies are entering the market: Schmack Biogas has set up a joint venture with the Kurtz Group in the United States in August 2006. With the experience of the recycling business of Kurtz, one of the founders of Schmack is looking especially into the anaerobic digestion of sewage sludge. As the US does not have a comparable renewable electricity generation feed-in tariff system, the feedstock has to be free or even better, offering payment to the generator, for the disposal to make economic sense. Other options are manure or distiller’s grain from the ethanol production. The latter seems to be an attractive option as bioethanol plants are big heat users as well.

The US just introduced the Biogas Production Incentive Act in April 2007 which amends the Farm Security and Rural Investment Act of 2002. The Commodity Credit Corporation fund will be used to make countercyclical payments to biogas producers and loans/grants to feedstock providers and for plant construction. The incentives would not apply to landfill gas and sewage. The North American market holds considerable appeal for project developers as local conditions such as farm size are favourable and the market is still in its early stages.

Fortistar became the largest developer of landfill gas projects in the US in December 2006 when it bought Gas Recovery Systems with a 210MW landfill gas portfolio. Fortistar now controls roughly 15% of the US landfill generation market.

India

EnviTec, which announced an IPO for July this year, established a JV with the Indian power company Malavi Power Plant Ltd (MPPL). Currently the company has 9 plants under construction/development with an overall capacity of 16MW, more than double the size of plant as the standardised modules in Germany. A challenge for operating the plants is the kind of feedstock. According to Indian law the raw material must not be corn (food) and thus has to be waste like peels from fruits and oilseed millings. Experiments with different kinds of feedstock will indicate which plants offer the best gas yields. Due to the agricultural system manure is not available either. While the basic technology remains the same, the feeding system has to be adapted and the optimal combination of additives to provide good conditions for the bacteria has to be found.

Biogas Nord won two €1.8m contracts this year to plan and install biogas plants on the site of sugar factories in the Indian state of Maharashtra. The produced biogas from approximately 40,000 tonnes of press cake per plant per year will replace most of the fossil energy hitherto used to operate the sugar factories. India has already 4m biogas generators in place, but most of them are very small and for domestic use.

Usually the biogas is directly used for cooking; giving savings of about 50% when displacing LPG. Biotech, a NGO in Kerala, specialised in developing biogas digester using organic household, municipal waste and wastewater.

China

Chinese investment companies are also looking for European technology providers and investors. The German company Linde KCA will build a plant in Beijing using 73,000 tonnes of waste. Currently 4,000 medium to large scale biogas plants are in operation and China aims a target of 0.8GW capacity (4,700 units) by 2010 and 3GW by 2020 respectively. In the next five-year period, the investment from the Ministry of Agriculture is expected to be RMB 2.5bn ($310m) every year.

CDM can play an important role in the development of projects, as the IRR is likely to increase by 15% when issuing CERs. China’s government encourages foreign investment through a combination of income tax and VAT incentives and favourable import duties.

South East Asia

SE Asia’s largest biogas systems engineering company Asia Biogas is based in Thailand and operates also in the Philippines, Vietnam, Indonesia and Malaysia. The 3MW Korat plant is situated at the cassava processing plant Sanguan Wongse, where lots of heat and electricity is needed to dry the wet starch. Excess electricity is sold to the grid. The plant will be extended to 5MWe and 40MWth. Asia Biogas’ daughter firm CleanThai has developed and built Korat and 10 further plants at about the same size in Thailand. In early 2007, implemented biogas plants comprised capital investments of $20m.

Costs

Biogas plants cost around €2,500 – 4,000 (USD 3,375 to USD 5,400) per installed kW for agricultural farm applications and at this price with the coupled favourable feed-in tariffs it makes economic sense in a few European countries. Significant increases in the price of conventional energies associated with legislation that are more favourable to the biogas sector, have now opened up the way for energy production based in part on energy crops (notably corn) and not only on waste alone. But raw material security and price is also the biggest risk, as it amounts for about one third of production costs. While 2005 was a good year for biogas producers, in 2006 prices for agricultural commodities increased. At the beginning of 2006, operators paid €20-22 per tonne of maize, now it is about €25-27. Thus, farmers/operators are increasing the manure and grass part of the feedstock and trying to find the optimal mixture regarding costs and gas yield.

Favourable legislation, relative ease in securing a loan for the investment, ease in system O&M and an attractive pay back period makes biogas today a hot topic amongst farmers and investors.

20 megawatts of electricity is enough to meet the energy needs of a town of 50 000 people. The world’s largest biomass power station, due for completion in Penkun (Mecklenburg-Westen Pomerania) this summer, shows that such demand does not always have to be met using finite fossil fuels such as coal or gas. NAWARO Bioenergie AG produces biogas from corn silage here, in a fermentation process using liquid manure. The biogas is then coverted by combustion into electricity. Corn is particularly well suited as a renewable fuel for biogas as it contains more energy than most other feedstuffs.

The process is also beneficial for climate protection. The electricity produced is carbon-neutral: the amount of carbon-dioxide released is equal to the amount removed from the atmosphere by the corn as it was growing. The self-contained cycle of raw material and energy is also a worldwide first. Nearly all the fermentation residue produced is converted into high-quality depot fertiliser. All that is left is clear water, making the power station’s total efficiancy and environmental performance superior to that of conventional farm-based plants.

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Topping it all, the young company manages to include and use all products and by products (the latter to produce biofertilizer) till….literally nothing is left than pure water. All in all: a closed cycle.

Behind it stands the vision that electricity from biogas can play an integral part of the energy market worldwide – if produced the right and most efficient way.

The vision is backed up by numbers: German biogas units produced 2.9 billion kilowatt-hours of electricity in 2005, or about three times as much electricity as the amount supplied by photo tovoltaic solar cells. The new plant promises to push biomass energy to new levels - using all of its standardized modules it will generate electricity with a total capacity of 20 megawatt. That's the demand of a small city. The electricity generated at NAWARO is fed into the power grid.

The NAWARO concept appeals - nationally as well as internationally. But NAWARO plans first to complete the project phase in Germany - before conquering the worldwide market.

Read the article at clubofpioneers.com

The units will capture gas created during the fermentation of agricultural waste, including maize, crop residues and animal manure, and use it as a fuel. The Jenbacher engines will provide 20 megawatts of electricity and 22 megawatts in thermal output. No financial details were announced.