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3 Analysis and characterisation of biogas feedstocks BERNHARD DRO S G, RUDO LF BRAUN, G U¨ N T H E R B O C H M A N N , University of Natural Resources and Life Sciences, Austria and T E O D O R I T A A L S A E D I , B I O S A N T E C H , Denmark

DOI: 10.1533/9780857097415.1.52 Abstract: The abundance and variety of possible feedstocks for biogas plants necessitate detailed characterisation and evaluation of specific feedstock types. Feedstock characterisation requires reliable feedstock analysis. This chapter describes in detail the different feedstock analysis methodologies. The following essential analyses are described in detail: pH, total solids/dry matter, volatile solids/organic dry matter, chemical oxygen demand, total Kjeldahl nitrogen, ammonia nitrogen and biochemical methane potential. Additional analyses on biogas feedstocks are described, including total organic carbon, trace element analysis, sulphur, phosphorous and continuous anaerobic fermentation tests. Important details for feedstock evaluation are described. Firstly, different approaches for estimating a realistic energy recovery potential are laid out. Secondly, the effect of the carbon oxidation state in a feedstock on methane concentration in the produced biogas is described. Thirdly, the availability of macro- and micronutrients is estimated and a short summary of possible inhibitory or toxic components in biogas feedstocks is given. Key words: biogas, anaerobic digestion, feedstock analysis, feedstock characterisation.

3.1

Introduction

There are many organic materials available in significant quantities that can be used as feedstock for anaerobic digestion, such as waste fractions, 52 © Woodhead Publishing Limited, 2013

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industrial by-products and energy crops. Waste fractions include the organic fraction of municipal solid waste, source-separated organic wastes, food and feed leftovers, kitchen waste and grass cuttings. In industrial processes, significant quantities of organic by-products are accumulated, including agro-industrial by-products (manure, harvest residues, etc.) and food processing by-products (e.g. slaughter house wastes, whey, brewers’ spent grains, distillery slops, fruit and vegetable wastes, sugar beet residues). High-strength industrial wastewaters can also be of interest as feedstock in biogas plants. Finally, purpose-grown crops for anaerobic digestion include maize, grass and beets. Not all waste products and crops are equally suitable for biogas production and in some cases biogas production might not be profitable. To assess the suitability and profitability of biogas feedstocks, a reliable way of characterising and analysing feedstocks is necessary. A preliminary assessment of a feedstock can be carried out using data available in the literature combined with feedstock process and production data. Legal issues should also be considered, such as environmental and safety laws regulating the use of waste products. If the preliminary assessment indicates that the feedstock might be suitable, a detailed laboratory analysis should follow. Concise information about the different analysis methods (such as total solids (TS), volatile solids (VS), nitrogen content, chemical oxygen demand (COD)) and their limitations are discussed in this chapter. Furthermore, the availability of macro- and micronutrients should also be evaluated, as well as the possibility of the accumulation of inhibitory substances (antibiotics, heavy metals, disinfectants, ammonia, hydrogen sulphide, etc.). An important test for the anaerobic degradability and acceptability of a feedstock is a batch test for the biochemical methane potential (BMP). The best information on the behaviour of a biogas feedstock in a biogas plant can be obtained from continuous fermentation trials. However, a major disadvantage of these continuous trials is their complexity and cost. After a detailed characterisation of the biogas feedstock has been carried out and if the results seem promising, a detailed economic evaluation should follow. This is essential before realising a biogas project. Feedstock analysis and characterisation allow estimation of the price of a substrate when sold to a biogas plant operator.

3.2

Preliminary feedstock characterisation

3.2.1 General suitability as feedstock for anaerobic digestion Using basic data such as water content and content of inorganic matter or bulky/fibrous material, a rough estimation of the suitability of a specific

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3.1 Overview of feedstock suitability for different treatment technologies.

material as a feedstock for biogas production can be carried out (Fig. 3.1). Feedstocks with considerable water content and a low amount of inorganic matter or bulky/fibrous material are ideal for anaerobic digestion. In contrast, if the amount of inorganic matter or bulky/fibrous material increases and water content is rather low, aerobic composting is generally preferred. If the inorganic matter or bulky/fibrous material is even higher, combustion (for energy recovery) or landfilling (for inorganic wastes) is preferable. In addition, for feedstocks with a very high water content, aerobic wastewater treatment is generally applied. This overview is a very simplified approach and more detailed substrate evaluation follows later in this chapter.

3.2.2 Feedstock production and process data Process data of a possible feedstock should be available if anaerobic digestion is to be integrated into an existing process such as high-strength wastewater treatment or if an industrial by-product is to be digested. These data could include quantity, water content, composition and temperature. However, the available information can be very limited in many cases since the available feedstocks are often of little value (e.g. waste products). Nevertheless, continuous process data can often be better than some simple samples because these data also account for changes in feedstock composition (e.g. after changes in the process or after a cleaning procedure). In the case of many organic wastes there can also be a lot of information available because many waste products have to be analysed and declared before being transported or sold.

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Amount and water content of feedstock An important piece of information about a feedstock is the amount produced or accumulated per year (or season). This information can be used to estimate if sufficient feedstock is available for treatment in a centralised plant. The water content of a feedstock (liquid, paste-like/semi-solid or solid) is also important first-hand information. Seasonal variation of feedstock accumulation and composition In general, a biogas plant is operated all year round. If the feedstocks are only seasonally available, they must be storable. Some feedstocks like crops can be ensiled for storage. However, in industrial processes that work in a campaign (e.g. sugar beet factories), the energy will also be needed during the campaign. In this case, a biogas process would have to be adapted to operate during the campaign and to stop for the rest of the year. Although this is not a state-of-the-art operation mode of biogas plants, it is a viable possibility. The composition of other waste fractions (municipal organic waste for example) can also vary due to seasonal changes. One example is organic waste produced in suburbs – its composition can depend on the seasonal change in gardening activities. For example, during the summer the waste may contain grass cuttings, which are replaced by hedge prunings in the winter. This has an obvious effect on the carbon to nitrogen (C:N) ratio. Feedstock temperature The temperature at which a feedstock will enter the biogas plant can be important information (Braun, 1982). This is especially the case if the biogas process is integrated into an existing process rather than being transported long distances or stored for a long time. In addition, the local climate is of relevance since the heating demand of the digester depends on the outside temperature. Very low temperatures of the feedstock combined with high water content cause a high heating demand. As a consequence, the net energy output of a biogas process can decrease substantially. Figure 3.2, shows the relationship between VS content, fermentation temperature and net energy for a substrate temperature of 58C. According to this figure, a slurry of 1% VS at 58C would not yield any energy when fermented at 408C. Heat exchangers can be applied to improve the energy balance, but this increases investment costs. Apart from low temperatures, if the feedstock temperature is too high, this can have a negative effect on the microbes in the digester. Therefore, feedstocks at very high temperatures occasionally will have to be cooled down before entering a biogas plant.

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3.2 Net energy yield of a feedstock at 58C depending on volatile solids content and fermentation temperature (source: Braun R (1982) Biogas – Methanga¨rung organischer Abfallstoffe. Springer, Berlin, Germany; Figure 37 on p. 100; with kind permission from Springer Science +Business Media B.V.).

Data on feedstock characteristics in the literature The literature provides detailed data tables for various biogas feedstocks. Although these data can differ significantly, they do provide very valuable first-hand information. A table of the characteristics of many different biogas feedstocks can be found in Chapter 2 of this book. Feedstock characteristics are also given by Braun (1982), Bischofsberger et al. (2005), Braun (2007) and Murphy et al. (2011).

3.2.3 Legal classification of feedstock Before using a certain substance as a feedstock in a biogas plant it is of importance to evaluate the legal consequences of using this feedstock. There are two main legal issues that should be stressed at this point – pollution control and biogas subsidies. Pollution control The utilisation of feedstocks considered as waste material or wastewater is strictly regulated due to pollution control measures. In particular, utilisation of the biogas by-product, the digestate, is regulated. It can be assumed that chemical contaminants (e.g. heavy metals, polycyclic aromatic hydrocarbons (PAHs), dioxins) if they enter a biogas plant via the feedstock will

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mostly remain in the digestate. The reason for this is that only a limited amount of chemical contaminants are anaerobically degradeable. In addition, the contaminant concentration based on TS will even increase, as TS are degraded in the biogas plant. Therefore, in order to increase the quality of the digestate, the input materials of biogas digesters should be tightly regulated. In many countries, there is an additional legislative issue. If substances that are legally considered wastes enter a biogas plant, the whole digestate is to be treated according to waste legislation. If, for example, municipal food waste is mixed with grass silage, which is not a waste pre-digestion, then the digestate from the mixture is then considered a waste. This applies even if there are no pollutants in the digestate. In countries such as Denmark, digestate produced from feedstock mixtures that comprise up to 25% organic wastes (with the exception of stomach and intestine contents from slaughterhouses, which are equivalent to animal manure) is considered animal manure, and its use is controlled by manure and slurry regulations. If the amounts of wastes co-digested are above this limit, the digestate is considered waste and its use (as fertiliser for example) is governed by sewage sludge regulations. Biogas subsidies Biogas is subsidised by some countries due to its high production costs. Different subsidies may be granted for crop digestion as compared to waste digestion. The main idea behind this strategy is that crops have to be purchased as substrates whereas gate fees are often received for waste material. Countries like Germany and Austria have lists of biogas feedstocks that are permitted for an extra subsidy in crop digestion. Although mixtures are permitted in some cases, the type of feedstock should generally fit to the corresponding subsidy scheme.

3.3

Essential laboratory analysis of feedstocks

The most important types of analysis for biogas feedstocks are now described. A summary of the different norms and standards for the analyses can be found in Section 3.7 at the end of this chapter.

3.3.1 Sample taking and preparation For biogas feedstocks, standards VDI 4630 and ISO 566713 give detailed information on different issues relating to sampling. The quality of many feedstocks can differ depending on the time and sampling location. Especially for heterogeneous feedstocks, the sample taker’s experience and knowledge of the overall process are of high importance. According to

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3.3 Influence of different steps in the analysis of a feedstock sample on the total error (source: Schwedt G (2007) Taschenatlas der Analytik. Wiley-VCH, Weinheim, Germany; Figure A on p. 19; copyright WileyVCH Verlag GmbH & Co. KGaA; reproduced with permission).

VDI 4630, in order to obtain the best results, the rationale for and the methodology of sampling needs to be clarified in advance, to include . . . . . .

the aim of investigation origin of material expected sample characteristics variation of sample characteristics with time and location of sample taking parameters to be analysed need for security and protection measures for sample taker.

Sampling procedure A representative sampling procedure is essential for obtaining accurate data because many substrates are of inhomogeneous consistency. Therefore, to receive accurate data, a representative sampling procedure is a prerequisite. Details on sampling of biogas feedstocks in general can be found in VDI 4630 and details for sampling of sludges and wastewater are described in ISO 5667-13. Petersen (2005) gives a practical guide on sampling. Figure 3.3 shows the influence of the different steps of an analysis on the accuracy of a result. It can be seen that the biggest error occurs during sample taking. The second biggest influence on error is sample treatment and preparation. The analysis itself normally causes the smallest error. For homogeneous material, one sample is generally sufficient for a representative analysis. If the material shows very inhomogeneous phases, at

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3.4 Sampling with different flow rates and material composition (source: Schwedt G (2007) Taschenatlas der Analytik. Wiley-VCH, Weinheim, Germany; Figure E on p.19; copyright Wiley-VCH Verlag GmbH & Co. KGaA; reproduced with permission).

least one sample should be drawn from every phase. These samples can then be mixed together according to the quantity of the phases. If the material is very inhomogeneous and no phases can be located, samples should be drawn from different locations and depths of the material. They can also be put together as a mixed sample. For solid material, a representative sample can be obtained using the following procedure. With a spade or a sampling device, a large sample is taken from the material. This large sample is spread onto a clean surface and then mixed well. A cross is then drawn through the middle of the spread sample and two opposite quarters are removed. The remaining two quarters are spread and mixed again and again a cross is drawn and two quarters removed. This process is repeated until the required amount of sample is obtained. For liquid material, the liquid has to be stirred well before sampling. The sample bottle should then be submersed into the liquid for sampling. If a sampling valve is used, the first material leaving the valve should be rejected to allow cleaning of the sampling valve. If a sample is taken from a pipe where the material passes at different flow rates and with different composition, a sample proportional to flow rate or volume can be taken (Fig. 3.4). Automatic sampling devices can be used for this purpose. In

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addition, sampling in a pipe is preferably carried out in a vertical pipe or a pipe with turbulent flow so that problematic sediments can be avoided. Sample transport and storage Clean re-sealable sampling vials made of inert plastic, glass or steel should be used. After sampling, the vials have to be labelled. If poor biologic stability of the sample is assumed, samples must be cooled to 48C during transport. All samples should be stored in a cooling chamber at 48C until analysis. Obviously, short storage times before analysis are preferable to long storage times. If longer storage times are expected, samples can also be stored at208C, although this might produce changes in the degradability of the substrate. Sample preparation Physical impurities can be sorted out from the sample, but their amount and mass have to be documented. Samples are sometimes dried before analysis but this is only really suitable when non-volatile substances are being measured as it can cause the loss of some volatile components and therefore a false result. For some tests, milling or cutting the sample can improve analysis accuracy due to increased homogeneity of the sample, but milling of wet samples is often only possible after drying. An alternative way to mill organic fibres without drying (and without the associated losses) is during cooling with liquid nitrogen, but this is very rarely applied in the biogas field due to very high costs. Mixing in a blender is another alternative, although it is limited by high fibre content. Water can be added to the sample to improve blending performance.

3.3.2 pH value The pH value determines the acidity or basicity of an aquaeous solution. Its unit is the negative logarithm of the concentration of hydronium ions (H+). The pH value can be determined in a liquid feedstock with a standard potentiometric electrode (standards EN 12176 and APHA 4500-H+ B (see Section 3.7)). For semi-solid or solid feedstocks, the sample can be mixed with water and then analysed. Quite a wide range of pH values of biogas feedstocks is acceptable due to the usually high buffer capacity of the anaerobic digestion broth. The pH value in anaerobic fermentation is normally slightly above neutral. The buffer capacity depends mainly on CO2 concentration in the gas phase, ammonia concentration in the liquid phase and water content in general. If the pH in the feedstock is too high or too low so that the buffer capacity is exceeded and the pH in the reactor is

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changed significantly ( pH 7.5), it is preferable to have a neutralisation step before feeding to the biogas plant. If slight acidification occurs during anaerobic digestion, the pH can be increased artificially by adding base (e.g. Ca(OH)2, Na2CO3, NaOH) in the reactor (Bischofsberger et al., 2005).

3.3.3 Total solids (TS) and dry matter (DM) For the estimation of the water content of a feedstock, the TS or the DM are determined. Both parameters represent the same and are described in units of percent or grammes per litre. This analysis involves drying the sample to constant weight in a drying chamber at 103–1058C (standards EN 12880 and APHA 2540 B). ...