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Agricultural Biogas Plant as a Thermodynamic System: A Study of Efficiency in the Transformation from Primary to Secondary Energy

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20 September 2023

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21 September 2023

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Abstract
Using a wide range of organic substrates in the methane fermentation process enables efficient biogas production. Nonetheless, in many cases, the efficiency of electricity generation in biogas plant cogeneration systems is much lower than expected, close to the calorific value of the applied feedstock. This paper analyses energy conversion efficiency in a 1 MWel agricultural biogas plant fed with corn silage or vegetable waste and pig slurry as a feedstock dilution agent, depending on the season and availability. Biomass conversion studies were carried out for 12 months, during which substrate samples were taken once a month. The total primary energy in substrates was estimated in laboratory conditions by measuring the heat of combustion in a ballistic bomb calorimeter (17,760 MWh·year-1), and in the case of pig slurry, biochemical methane potential (BMP, (201.88±3.21 m3·Mg VS-1). Further, the substrates were analysed in terms of their chemical composition — from protein, sugar and fat content to mineral matter determination, among other things. The results obtained during the study were averaged. Based on such things as the amount of biogas produced at the plant, the amount of chemical (secondary) energy contained in methane as a product of biomass conversion (10,633 MWh·year-1) was calculated. Considering the results obtained from the analyses, as well as the calculated values of the relevant parameters, biomass conversion efficiency was determined as a ratio of chemical energy in methane to (primary) energy in substrates, which was 59.87%, as well as electricity production efficiency, as a ratio of electricity produced (4,913 MWh·year-1) to primary energy, with a 35% cogeneration system efficiency. Full energy conversion efficiency, related to electricity production, reached a low value of 27.66%. This article provides an insightful, unique analysis of energy conversion in an active biogas plant as an open thermodynamic system.
Keywords: 
Subject: Environmental and Earth Sciences  -   Waste Management and Disposal

1. Introduction

Energy carriers used in global transportation today are primarily derived from fossil fuels. This contributes to a significant increase in greenhouse gas emissions [1,2,3,4]. Europe is striving to become a greenhouse gas-neutral continent with policies oriented toward a modern economy. Accordingly, the European Union has for many decades supported the development of renewable energy sources, including solar, biofuels, hydropower and wind power. In Poland, the organic waste energy recovery sector, in principle, has been developing since around 2010 [5,6,7].
Currently, the main feedstock for biogas production for energy purposes is waste from agricultural and food production, including livestock production, as well as corn silage [8]. For the most part, this waste contains all the components necessary for microbial growth, such as carbohydrates (cellulose, hemicelluloses, starch, sugars), proteins, fats, as well as biogenic elements, micronutrients and vitamins [9,10]. If left unprocessed, it may cause sanitation hazards and specific environmental problems. Given the need to dispose of agricultural waste from an environmental standpoint, as well as its natural origin and chemical composition, the most viable and economical methods of degrading this waste are biotechnological methods, including the anaerobic digestion process, which allows organic waste to be converted into energy and valuable products such as feed, fertiliser, etc. Microorganisms transform many organic compounds under anaerobic conditions [11,12]. By means of specific fermentation or anaerobic respiration, they produce biogas, which consists mainly of methane (50–65%), carbon dioxide (30–45%) and other gases in small quantities, including ammonia and hydrogen sulphide [13,14,15]. From a biochemical point of view, anaerobic processes break down sugars, protein and fat.
Equation (1) illustrates the decomposition reaction of an organic compound in methane fermentation. The subscripts c, h, o, n, s, and y, x denote the number of atoms present in the chemical compound molecule and/or involved in the anaerobic biodegradation reaction [14,16].
CcHhOoNnSs + yH2O → xCH4 + (c − x)CO2 + nNH3 + sH2S
It must be noted, however, that despite the high potential of the Polish market in terms of waste and agricultural substrate availability, investors are still keen to grow maize for energy purposes. In times of a global energy crisis, energy carriers must be conserved, and low-cost alternatives to maize must be used. These alternatives, as highlighted in this paper, are all kinds of waste materials, including biodegradable organic matter. Anaerobic technologies offer high potential in managing a wide range of bio-organic wastes [17,18]. Vegetable waste is mainly hulls, oil cake or whole plants that do not meet the quality requirements. Due to its composition, including mainly simple and complex sugars, this material should be processed as a substrate for biogas production. Plants that use agricultural and food production waste as feedstock operate both in Poland and elsewhere across the world [19,20]. Such solutions enable the optimum use of the plant resources harvested.
Biogas is typically produced continuously under suitable environmental conditions at a pH of around 7 [21,22]. Its composition and quantity depend on the kind of chemical compounds undergoing biodegradation. This process is usually based on a one- or two-stage system, separating hydrolysis and acid fermentation from methanogenesis in a varying number of digesters, depending on biogas plant capacity. To efficiently carry out anaerobic digestion and maximise the production potential of the individual bacterial groups, it is imperative to prepare suitable feedstock for the plant and to create the correct environmental conditions [23]. Using a continuous process when processing large volumes of waste is more advantageous. Further, a process carried out at temperatures suitable for thermophilic microflora runs faster and enables the use of smaller reactor volumes [24]. Yet, it should be mentioned that the increased biochemical reaction rate, which follows an increase in temperature, does not comply with the Arrhenius rule or Van't Hoff's Rule (a temperature increase of 10 °C doubles the rate of a chemical reaction). Thus, the transition from mesophilic to thermophilic conditions should not be expected to bring a two- or threefold increase in process speed.
This paper analyses the efficiency of primary to secondary energy conversion in an anaerobic digestion process. Primary energy is a naturally occurring energy form that has not undergone any man-made conversion process. It exists as non-renewable energy (fuel chemical energy) and renewable energy (solar, hydro, geothermal and biomass, including organic waste, among other things). Secondary energy is the result of converting primary energy into carrier form [25,26,27]. In the anaerobic degradation process, the intermediate secondary energy carrier is methane, which forms part of biogas as the main product of the process and is subsequently combusted in a to cogenerate electricity and heat [28,29]. Both forms are secondary energy. Losses occur at every stage of the energy chain. Converting the total chemical energy stream contained in the waste into chemical energy concentrated in methane may sometimes prove inefficient, and it requires the consideration of many process factors [30]. The next stage in the chain, which involves converting chemical energy contained in methane into electricity, proceeds relatively poorly. When biogas is burned in a CHP engine, the generated heat can be recovered to provide additional energy. When heat and losses are not managed, the efficiency is estimated to be around 35% [31].
Figure 1 shows a simple diagram of the conversion of one energy type into another, as occurring in a thermodynamic system. The output (secondary) energy is always less than the input (primary) energy. This means that the energy efficiency, as the degree of energy conversion in a process, is always less than 1, which is associated with the occurrence of losses and reduced fuel efficiency [32,33]. In the case of a biogas plant, this is related to the incomplete or low conversion of the chemical energy contained in the substrates and, subsequently, in the methane.
This study aimed to analyse energy transformation efficiency in methane fermentation carried out on a technical scale using substrates in the form of maize silage and agricultural and food waste. The study considered data on the value of the primary energy accumulated in the biomass and the secondary energy contained in the main product of the process — methane. The final stage of this study involved estimating electricity generation efficiency as the efficiency of full energy conversion of the biogas plant.

2. Materials and Methods

2.1. Substrates

The biogas plant under study, located on a farm in Poland's Wielkopolskie Voivodeship, mainly used maize silage (MS) as feedstock. For economic reasons and depending on the season and availability, vegetable waste was used as an MS substitute at 50% of the total solids stream. The remaining feedstock was MS. The waste stream included onion (ONI), carrot (CAR), potatoes (POT), celery (CEL), leek (LE) and parsley (PAR). The maize silage used at the plant was produced "on-site" – on the farm – while vegetable waste was supplied from a nearby production facility. Further, the plant was fed with pig slurry (PS), sourced from the same farm, whose function was to hydrate the feedstock. Thus, from a logistical and economic perspective, the most favourable solutions were used.
Table 1 shows the percentage of each feedstock fraction. The percentage content was determined by the weight of the individual wastes applied to the digesters.

2.2. Physicochemical and chemical analysis of materials

The energy value (EV) of the test materials (except for pig manure, which is explained later in this section) was determined by burning dried and crushed samples of the test substrates in an oxygen atmosphere using a CB 370 ballistic bomb calorimeter (Gallenkamp, Cambridge, United Kingdom) in a specialised laboratory. The bomb calorimeter consisted of a sealed vessel made of acid-resistant stainless steel with reinforced walls, making it possible to burn the fuel placed inside it. The vessel was placed in a calorimeter, which was used to measure the amount of heat released from the initiation of the reaction until thermal equilibrium. The bomb used in the experiment to measure the heat of solids combustion was equipped with a bottom that enabled the burnt sample to be placed inside the bomb, as well as a valve for introducing oxygen and contact electrodes. Combustion heat was measured based on the volume and temperature of the air escaping from the calorimeter (kJ·100 g-1).
Substrate and sample physicochemical analyses were carried out using the methods and procedures described in the following standards: pH – potentiometric analysis with Elmetron CP-215, Zabrze, Poland (PN-EN 12176:2004); total solids, TS (drying residue) – gravimetric analysis; weight analysis – measurement by drying at 105 °C (Zalmed SML dryer, Zalmed, Łomianki, Poland), PN-EN 12880:2004 – method used to simultaneously determine the water content of the materials tested; volatile solids, VS (roasting residue) – gravimetric analysis, measurement by combustion at 550 °C (MS Spectrum PAF 110/6 oven, Warsaw, Poland), PN-EN 12879:2004 [1,2].
The quantification of protein, fat, minerals (insoluble ash), as well as starch and total dietary fibre, was carried out according to the procedures described below.
Protein – calculated from TKN (total Kjeldahl nitrogen) using a conversion factor of 6.25 for crude proteins; AOAC 920.87 [34]; TKN—titration, Kjeldahl method, 0.1 n HCl, Tashiro’s indicator; PN-EN 13342, EN 15104:2011;
  • Fat – Soxhlet method, extracted with hexane using a Soxhlet automatic extractor, model B-811 BUCHI, (Büchi Labortechnik AG, Flawil, Switzerland); AOAC 920.85 [35].
  • Mineral matter – ash, range: (0.02–40%), gravimetric analysis [36];
  • Starch – Luff-Schoorl titration method; the determination principle is based on the reduction reaction of Cu+2 ions contained in the Luff fluid by the reducing saccharides present in the solution tested. The reaction takes place in an alkaline environment (pH of about 9.5), at the boiling point. The Luff fluid consists of copper(II) sulphate (VI), sodium carbonate and citric acid [37];
  • Dietary fibre method – a chemical method in which fibre is determined as the fraction remaining after fermentation with standard solutions of 0.25 N sulphuric acid and 0.25 N sodium hydroxide under strictly controlled conditions, AOAC 962.09) [38].
A gas chromatography method (GC-2014 gas chromatograph, Shimadzu, Kyoto, Japan) was used to determine glucose, fructose and sucrose content. To this end, non-volatile saccharides were converted into more volatile derivatives, such as trimethylsilyl. Once the column had been appropriately selected through chromatography and separated into individual sugars, the saccharides were identified by comparing the retention times of the analysed compounds with those of the benchmarks and their quantitative analysis was conducted based on the chromatographic peak areas.
Table 2 shows the analytical results for protein, fat and total sugars, as well as ash, total solids and volatile solids, for all substrates except pig manure.
Table 3 summarises the analysis results concerning the different types of sugars – including glucose, fructose, sucrose, starch and dietary fibre – for the same substrates.
The micro- and macronutrient content of the materials used during the study (see Table 4) was analysed by atomic absorption spectrometry, ASA (ZA3300 ASA spectrometer, Hitachi, Tokyo, Japan). This instrumental analytical method determines trace amounts of elements in samples of different natures, including in clinical trials. It belongs to the optical spectroscopic methods and examines the impact of UV and VIS radiation on atoms. The method itself uses the atomic absorption phenomenon.
In the case of pig manure, only the necessary parameters were determined due to the different methodologies for determining energy value compared to most of the substrates tested (see Table 5).

2.3. Biogas Production at Laboratory Scale

Pig slurry, used in biogas plant operation as a feedstock diluting agent, was the only substrate for which Biochemical Methane Potential (BMP) was determined on a laboratory scale. In this case, the BMP is an intermediate parameter in estimating a material's energy value. Determining the energy value by measuring combustion heat using the calorimetric bomb was impossible due to the low solids content of the pig slurry (see Table 4).
The slurry's biochemical methanogenic potential was determined in an anaerobic bioreactor working in batch mode (see Figure 1), under mesophilic conditions. In their previous publications, the authors of this study have also provided a detailed schematic and description of the construction and operation of the micro digesters [10,20,39]. According to German DIN 38 414-S8 Standard [40], the experiment was run until the daily biogas production of all bioreactors fell below 1% of the total biogas production. The biogas volume obtained from the slurry was measured every 24 hours. Methane, carbon dioxide, hydrogen sulphide, ammonia and oxygen concentrations in the biogas were measured using a Geotech GA5000 gas analyser (Geotech, Bydgoszcz, Poland). Biogas yields (in m3·Mg-1) from dry matter and dry organic matter were estimated based on experimental data. The specific biogas production from the substrate (depending on study duration) was calculated in stages — from one reading to the next.
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Figure 2. Anaerobic bioreactor used in the biogas production experiment: 1 – water heater, 2 – water pump, 3 – insulated heating medium tubes, 4 – water jacket (39 °C), 5 – bioreactor (1.4 L), 6 – slurry sampling valve, 7 – biogas transport pipe, 8 – graduated biogas tank, 9 – gas sampling valve.
Figure 2. Anaerobic bioreactor used in the biogas production experiment: 1 – water heater, 2 – water pump, 3 – insulated heating medium tubes, 4 – water jacket (39 °C), 5 – bioreactor (1.4 L), 6 – slurry sampling valve, 7 – biogas transport pipe, 8 – graduated biogas tank, 9 – gas sampling valve.
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2.4. Biogas Production at Technical Scale

A technical-scale biomass conversion study was run for 12 months at a biogas plant in the Wielkopolskie Voivodeship (the exact location is not given at the owner's request), equipped with a 1 MWel cogeneration system. The plant comprised three digesters (two primary digesters and one secondary digester, which also acted as a digestate tank). The primary digesters contained three paddle agitators with adjustable pitch, operating in interval mode (triggered at specific time intervals). The mixing time, 20 min·hour-1 in the case of the biogas plant analysed, was determined based on observations and practical experience, considering substrate properties, the size of the digester tanks and the propensity to form scum. Notably, the interval mode is by far the most popular in biogas plants due to the high energy intensity of the mixing equipment.
The substrates (feedstock) – maize or vegetable waste silage – were sampled once a month to determine their energy value (as stated in section 2.1.). At least three samples were taken and tested each time. Analysed for representativeness, the material was then subjected to testing. Based on the analyses performed, the uncertainty of the results was also calculated as a numerical value indicating the degree to which the obtained measurement result can be regarded as correct. In estimating measurement uncertainty, this study used procedures compliant with Polish and German standards [41,42]. The energy value results obtained for the samples tested over the year were averaged.
As previously reported, maize silage was being replaced by vegetable processing waste. On average, the vegetable processing waste stream replaced 25 Mg of maize silage each year. The initial plans provided for the plant to be fed with 50 Mg of maize silage each day. Hence, half of this amount was successfully replaced by vegetable waste. This approach has made it possible to use waste that is a valuable calorific material for methane fermentation. The amount of biogas produced (average values per day were used for the calculations) was read by the operator using an ST51 thermal gas mass flow meter (Introl, Sp. z o.o., Katowice, Poland), located upstream of the cogeneration unit. The biogas plant in question is a testament to a successful regenerative system that minimises raw material consumption, waste volume, emissions and energy losses by creating a closed process loop (i.e. circular economy).
The data values required to calculate the plant performance parameters (biomass energy conversion efficiency, electricity generation efficiency) specified in the paper's aim were determined step-by-step based on well-known chemical and physical relationships, as presented in the text, and summarised in a logical sequence in Table 6, Table 7, Table 8, Table 9 and Table 10.

3. Results and Discussion

3.1. Substrate chemical composition

The starting point of the research carried out in this study was the analysis of total solids, volatile solids and mass content of biodegradable compounds, including protein, fat and total carbohydrates in all substrates except pig slurry (Table 2). The confirmed high moisture content of vegetable waste indicates that it is unsuitable for incineration or storage. Yet, as indicated by numerous literature sources [43,44], the composition of vegetables makes them suitable substrates for biogas plants. One limitation of their anaerobic digestion, like in the case of other plant wastes, is the potential for rapid acidification and increased production of volatile fatty acids (VFAs), which reduce the anaerobic reactor's activity [8,20]. At the biogas plant in question, the pig slurry used as a low-TS and pH-neutral diluting agent (see Table 5), as well as a source of nitrogen [45], also acted as a stabilising buffer for the system. Tests carried out for the same materials on monosaccharides (glucose, fructose and sucrose), as well as on starch and dietary fibre (Table 3), indicate that the simple sugar content (particularly fructose) in vegetables is lower than in fruit [43,46], which significantly limits the risk of an adverse pH drop. Essentially, carbohydrates are the building blocks for methane production, and in the case of vegetables, their sources are mainly dietary fibre and starch (see Table 3).
As substrates used in the biogas plant studied, maize silage and potatoes had the highest carbohydrate content: 23.5 ± 0.023 g·100g-1 and 20.5 ± 0.02 g·100g-1, respectively (Table 2). The least carbohydrates were found in celery (7.7 ± 0.076 g·100g-1) and leek (5.7 ± 0.006 g·100g-1). These values were confirmed by the results of analyses concerning individual sugars contained in the raw materials (Table 3). Maize silage contained a significant amount of starch, amounting to 123 ± 0.072 g·100g-1, as did potatoes, at 6.6 ± 0.097 g·100g-1. Carrots and onions had the least starch, at 0.002 g·100g-1 and 0.1 ± 0.001 g·100g-1, respectively. Celery and parsley had the highest fibre content, at 4.9 ± 0.029 g·100g-1 and 4.2 ± 0.025 g·100g-1, respectively. As for the group of simple sugars, the highest amount of sucrose was found in parsley at 4.8 ± 0.028 g·100g-1. The potato was characterised by a very low simple sugar content. By contrast, onions contained the most glucose, at 1.7 ± 0.01 g·100g-1, and fructose, at 1.5 ± 0.009 g·100g-1. The highest amount of protein was found in maize silage – 3.7 ± 0.013 g·100g-1 – and parsley – 2.6 ± 0.009 g·100g-1 (Table 2). Fat was also most abundant in maize silage (1.5 ± 0.011 g·100g-1).
The present study also included an analysis of the micro- and macronutrients contained in the substrates (see Table 4). Knowledge of the mineral matter content makes it possible to balance the feedstock nutrients. In the case of the present study, the mineral content was optimal for the functioning of the bacterial flora without inhibiting the process at the same time [47,48].
As shown by the above analysis, the chemical composition of maize silage is the most favourable compared to the raw materials used in the plant under study. In practice, maize silage remains the most commonly used material in biogas plants due to its widespread availability and high nutrient content. However, as highlighted in the paper's introduction, alternatives to maize silage – today's biogas market mainstay – should be sought due to the need for crop rotation and rising maize silage prices.
The results of the analyses presented in Table 2, Table 3 and Table 4 are intended to illustrate the chemical composition of the individual feedstocks in the substrate stream of the biogas plant in question. Analysis of protein, fat and sugar quantities enabled a theoretical verification of the substrate energy potential, which any professionally operated biogas plant should exploit optimally [10,14]. These data make it possible to estimate the maximum proportion of methane in biogas, considering the stoichiometry of the conversion occurring according to Equation 1. In principle, this study did not aim to identify such data. Nonetheless, when analysing biogas plant operation, including energy transformation efficiency, several pertinent issues must be raised.
Creating up-to-date studies of substrate chemical composition and their BMP is a key aspect of biogas plant operation [49]. Any biogas plant should be regarded as a professional plant for the processing of organic matter, including waste, which enables the production of biogas containing an energy carrier in the form of biomethane [1,50]. In practice, the biogas market, including in Poland, tends to overestimate the methane content of the biogas obtained and ignore the above indications. Neglect in terms of biogas plant technological processes is driving many operators involved in renewable energy production to bankruptcy. Due to a lack of competencies, Poland is unable to make efficient use of energy carriers (methane) from widely available waste materials, which forces it to import billions of cubic metres of methane.

3.2. Slurry BMP and calorific value of other substrates

The pig slurry used in the biogas plant analysed was characterised by poor biogas yields from fresh matter (FM) due to its low TS content of 5.6 ± 0.06% (see Table 5). The amount of biogas produced in relation to FM, as indicated by the data collated in Table 6, was 18 ± 0.24 m3 · Mg FM-1, while in terms of VS it was 395.85 ± 5.66 m3·Mg VS-1, which is in line with literature data [45,51]. Since the proportion of methane in the biogas obtained from PS was 51%, the BMP of this material is 201.88 ± 3.21 m3·Mg VS-1.
Table 7 shows the energy value (also called calorific value) of the individual fractions of the substrate stream fed daily into the plant. The calorific values (more precisely — combustion heat), obtained for individual samples using the calorimetric bomb and expressed in kJ·kg-1, were converted and presented in useful units (also for further calculations), including kWh·kg-1, MWh·day-1 and MWh·year-1.
The suitability of plant biomass for energy purposes is largely determined by the heat of combustion, heating value and chemical composition [51,52]. These features have a major impact on the technological conditions of processing and the quality of the product obtained, and in the methane fermentation process under discussion – on the quality of biogas [53]. The main component of biomass is coal, and it is the energy contained in it that is released during combustion [52]. As mentioned earlier in the article, simple and complex sugars are the main source of carbon in plant waste, including vegetable waste.
The first column of Table 7 shows the average daily amounts of substrates fed. The most caloric feedstock turned out to be maize silage. The daily value of energy provided by MS was 33,819 MWh·day-1, while the annual value was 12,344 MWh·day-1. The next substrate in terms of caloric value was potatoes, providing 7,708 MWh·day-1 calories daily, and 2814 MWh·year-1 annually. The substrates of the lowest calorific value were celery 0.868 MWh·day-1 and leek 0.672 MWh·day-1 (see Table 7). These results correlate with the results of the carbohydrate content of the mentioned materials (Table 2 and Table 3).
The above analysis of the energy value of the substrates used in the plant in question, provides information on the values of primary energy provided by each substrate to the system. The total value of primary energy accumulated in the biomass applied to the plant, after summing up the relevant data in Table 7 (columns 6-8), was: 48,659 kWh·day-1, 48,659 MWh·day-1 and 17,760 MWh·year-1.
Currently, the dominant source of primary energy on earth is the chemical energy of fossil fuels [25,26]. However, the prospects of depletion of these fuels and the threat to the state of the environment intensify interest in renewable energy sources (RES). Biomass is one of the oldest and most widely used RES today, and as a result it constitutes the world's third-largest natural energy (primary energy) source [3,54]. The heating value, as the basic energy parameter of biomass, is usually lower than that of conventional fuels. As confirmed by the values shown in Table 7, a characteristic feature of this parameter is the relatively wide dispersion of its values, which is due to the different chemical composition of the materials forming the harvested biomass. Important differentiating factors are: plant species, place of growth, weather conditions, growing season and others.

3.3. Efficiency of methane and electricity production in a biogas plant: chemical energy and electricity

Secondary energy is the result of converting primary energy into carrier form. In the process of anaerobic degradation, methane is an intermediate carrier of secondary energy, which is part of biogas as the main product of the process. Biogas burned in a cogeneration system becomes a source of heat and electricity in one system based on internal combustion engines: the fuel burns in the engine and activates the generator, which converts mechanical energy into electricity [55,56].
Table 8 summarises the results of biogas production (annual, daily and, due to subsequent calculations, hourly) produced in the biogas plant under study. The amount of biogas produced annually (m3·year-1) was determined taking into account 365 days. The biogas plant operation was assumed to be 8,000 h·year-1, thus excluding activities related to plant operation, maintenance, etc. The daily production of biogas from pig slurry and the substrates used was 7,420 m3·day-1, the annual production was 2708,300 m3·year-1, and the hourly production was 338.84 m3·hour-1.
Subsequently, the capacity of the plant was estimated in relation to the biogas output of the given feedstock, in MW units (Table 9). Taking into account the hourly biogas production of 82.125 m3·hour-1 – for pig slurry and 256.41 m3·hour-1 - for other substrates, as well as: the average methane content in biogas (52 ± 1%), total chemical energy (bond energy) in m3 of methane (0.009968 MWh) and efficiency of the cogeneration system (35%), the power of the plant was obtained from pig slurry 0.15 MW and from other substrates 0.47 MW. The total power of the plant was 0.62 MW, where the designed capacity of the biogas plant is 1 MW. For the obtained power value, the amount of electricity produced was 4,913 MWh·year-1. The above-discussed results are shown in Table 9.
In turn, Table 10 presents the results of the efficiency of the process of converting the energy accumulated in the feedstock (primary), which was 17,760 MWh·year-1 (Table 7), into the energy contained in the methane produced (secondary, chemical), amounting to 10,633 MWh·year-1. The value of energy contained in CH4 was obtained by including the following in the calculation: annual biogas production of substrates excluding pig slurry (2,051,300 m3·year-1, see Table 8), average methane content in biogas (52 ± 1%) and total chemical energy in 1 m3 of methane (0.009968 MWh). The non-inclusion of pig slurry was due to the very low TS content (Table 5), which meant that it was mainly treated as a dilution factor in the study. The efficiency of biomass conversion in the plant under study (as the ratio of chemical energy in methane to primary energy in substrates, see Eq. 2) was 59.87%.
E F − b c = E   m e t h a n e E   s u b s t r a t e
where:
EF–bc – efficiency of biomass conversion (%),
E methane – chemical energy in methane (secondary), MWh·year-1;
E substrate – energy in substrates (primary), MWh·year-1.
In the last stage of the process, the efficiency of full energy conversion in the plant was determined. The energy efficiency of the biogas plant in this study is the degree of conversion of the primary energy contained in the biomass introduced to the plant during the year – into electricity. Thus, this parameter is to determine the efficiency of use of the fuel accumulated in the substrates. Knowing that the energy efficiency is the ratio of the amount of energy coming out of the process (the amount of electricity produced, estimated from the amount of biogas, including methane, and the capacity of the plant), which is 4,913 MWh·year-1 (Table 8 and Table 9), to the amount of energy introduced to the process (the cumulative energy value of the substrates, see Eq. 3), which is 17,760 MWh·year-1, the energy efficiency of full conversion is 27.66%.
E − E F = E l e c t r i c i t y E   s u b s t r a t e
where:
E–EF – energy efficiency, %;
Electricity – electricity produced by CHP system, MWh·year-1;
E substrate – energy in substrates (primary), MWh·year-1.
Taking into account the losses in the conversion of biomass into methane and the low efficiency of the cogeneration system (35%), included in the calculation of the total capacity of the plant (Table 9), the obtained low result of energy efficiency of the plant under study was considered reasonable and feasible.
Methane fermentation is a process that relatively efficiently converts the primary chemical energy contained in the waste into chemical energy contained in methane. With the methane content in the biogas at the level of 52%, as a result of biomass conversion, chemical energy concentrated in methane was obtained at the level of 59.93% - from the entire stream of chemical energy contained in the waste. However, steps can be taken to optimise the methane digestion process itself to increase the efficiency of the process of decomposing organic matter into biogas. To this end, the approach to biogas plants needs to change. They should be considered biochemical industrial plants that require efficient technological supervision, due to the presence of many important factors that affect the efficiency of the plant. These include pH, process temperature, type of mixing system, C:N ratio and others. It is in the interest of biogas plant owners to maximise biogas production, with the highest possible content of methane as an energy carrier [50]. In Poland, it is common to observe the implementation of commercial anaerobic digestion processes at capacities far below their optimum value, due to various irregularities, including, among others, lack of knowledge of the chemical composition of the materials used and, consequently, poor quantitative and qualitative selection of substrates and co-substrates, or failure to monitor key process stability parameters, etc [57]. When implementing optimisation measures in the biogas plant under study in this article, it is recommended to pay special attention to the environmental conditions prevailing in the digester, such as pH, buffer capacity or volatile fatty acid concentration.
If the efficiency of biomass conversion obtained in this work was considered to be slightly underestimated, and the process to be in need of optimisation, the conversion of chemical energy contained in methane into electricity in the plant under study must be assessed as definitely inefficient. The efficiency of the cogeneration system of the biogas plant under study, in terms of electricity production (the efficiency of the internal combustion engine is about 40% minus the efficiency of the generator), is only 35%. Biogas was burned in gas engines driving power generators, but waste heat was not used (except for technological purposes). It is worth emphasising that most biogas plants in Poland operate in this way, which is a major problem that generates energy losses and results in low conversion efficiencies [58,59]. This factor is the direct cause of poor utilisation of the primary chemical energy accumulated in the feedstock. The energy efficiency of full energy conversion in this study was only 27.66%.
Increasing the efficiency of the cogeneration system in biogas plants requires appropriate strategies, technologies and practices. The first recommendation is to select advanced internal combustion engines, gas turbines or other generators that can significantly improve the efficiency of the system, in addition to upgrading existing plants. Another important recommendation is to adapt the combustion process to the characteristics of biogas. Gas purification, precise regulation of the ratio of mixing gas and air, and control of the combustion temperature, are key factors in increasing and maintaining high efficiency of the system. Equally important factors for increasing the efficiency of the system is the effective use of the heat produced, not only in technological processes (to heat digesters, etc.), but also on a larger scale, for drying and heating purposes [60]. However, its transport over long distances is difficult and is accompanied by unavoidable losses. On the other hand, heat storage generates high costs. Therefore, it is important that cogeneration plants are located near places with high heat consumption, because otherwise, heat recovery is neither interesting nor cost-effective. The solution is to build cogeneration units near medium and large cities and enterprises where there is a demand for heat [61]. It would also have to be considered whether it would be more advantageous, in some situations, to burn the gas produced in boilers for heating-only purposes.
Undoubtedly, the priority in the operation of biogas plants should be obtaining biogas from waste and treating the process as the most environmentally friendly method of waste management. Present-day technologies allow the use of high-efficiency cogeneration. However, when it comes to the biogas plants under construction on the Polish market, the heat generated in the process is used only in typically technological processes, which constitutes its small contribution of 15–20%. The remaining part of the heat is undeveloped, which is contrary to the assumptions of sustainable development and efficient use of energy carriers. A clear depiction of the waste of energy is the value obtained in the presented study of the efficiency of all energy conversion in the process (that is, the efficiency of converting primary, chemical energy of the feedstock - into electricity), which amounted to 27.66%.

4. Conclusions

Based on the results obtained in laboratory conditions (primary chemical energy accumulated in substrates, 17,760 MWh·year-1), on a technical scale (the amount of biogas produced, including methane) and the values of estimated parameters (secondary chemical energy contained in methane, 10,633 MWh·year-1), the biomass conversion efficiency was determined as the ratio of the secondary chemical energy of methane to the primary chemical energy of the substrates, was determined in this paper. The obtained value of 59.87% indicated a relatively efficient process of biomass conversion in the process of methane fermentation carried out in the plant under study, which, however, requires optimisation measures to increase energy conversion. An important and at the same time final stage of this study was the estimation of the amount of electricity produced (based on the biogas/methane produced and the power of the plant, including the efficiency of the cogeneration system of 35%), which amounted to 4,913 MWh·year-1, and then the efficiency of the full energy conversion in the plant, as the ratio of the electricity produced to the primary energy brought with substrates into the plant as an open thermodynamic system, exchanging both matter and energy with the environment. The efficiency of electricity production in relation to the total energy input (feedstock) reached a low value of 27.66%.
The article indicates the factors that reduce the total energy efficiency of the methane fermentation process. The main reason for the very low conversion efficiency of the primary chemical energy of the substrates was the low efficiency of the cogeneration systems of biogas plants operating in Poland, including the failure to utilise heat for broader purposes (beyond technological), including heating or drying. Limitations in the use of heat in areas distant from the location of the biogas plant (losses during transport and high storage costs). As a conclusion to the issues raised in the study and the results obtained, it was proposed to implement measures to increase the efficiency of cogeneration systems, including the full use of waste heat or the combustion of produced gas in boilers for heating purposes only. Following the principle of sustainable development, the authors of this paper emphasised the priority function of a biogas plant as a place that manages, in the most environmentally friendly way possible, organic waste of various origins.

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Preprints 85703 g001
Figure 1. General scheme of energy conversion in a thermodynamic system, in a specific process (author’s own scheme).
Figure 1. General scheme of energy conversion in a thermodynamic system, in a specific process (author’s own scheme).
Preprints 85703 g001
Table 1. Percentage of each fraction in the feedstock.
Table 1. Percentage of each fraction in the feedstock.
Type of substrate ONI CAR POT CEL LE PAR MS
Content
(%)		 8 12 15 5 4 6 50
Explanation: ONI – onion; CAR – carrot; POT – potatoes; CEL – celery; LE – leek; PAR – parsley; MS – maize silage.
Table 2. Selected parameters of the materials tested, in relation to 100 g fresh weight.
Table 2. Selected parameters of the materials tested, in relation to 100 g fresh weight.
Sub. ONI CAR POT CEL LE PAR MS
Comp.
and unit		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)
Prot.
(g)		 1.4 0.005 1.0 0.004 1.9 0.006 1.6 0.005 2.2 0.008 2.6 0.009 3.7 0.013
Fat
(g)		 0.4 0.003 0.2 0.002 0.1 0.001 0.3 0.002 0.3 0.002 0.5 0.004 1.5 0.011
Carb.
(g)		 6.9 0.007 8.7 0.009 20.5 0.020 7.7 0.076 5.7 0.006 10.5 0.010 23.5 0.023
Ash
(g)		 0.5 0.004 0.4 0.003 1.0 0.008 0.9 0.008 0.9 0.007 1.1 0.009 0.5 0.004
Water(g) 90.8 0.090 89.7 0.09 76.5 0.080 89.5 0.88 90.9 0.730 85.3 0.680 70.8 0.570
TS
(%)		 9.2 0.074 10.3 0.083 23.5 0.189 10.5 0.084 9.1 0.073 14.7 0.118 29.2 0.243
VS (%) 8.7 0.070 9.9 0.080 22.5 0.181 9.6 0.077 8.2 0.066 13.6 0.109 28.7 0.238
Explanation: Sub. – substrates; Comp. – component; ONI – onion; CAR – carrot; POT – potatoes; CEL – celery; LE – leek; PAR – parsley; MS – maize silage; MU – measurement uncertainty; EV – energy value; Prot. – protein; Carb. – carbohydrates; TS – total solids; VS – volatile solids.
Table 3. Content of the different sugar types in the substrates tested, relative to 100 g fresh weight.
Table 3. Content of the different sugar types in the substrates tested, relative to 100 g fresh weight.
Sub. ONI CAR POT CEL LE PAR MS
Comp.
(g)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)
Gluc. 1.7 0.010 1.6 0.009 0.4 0.002 0.5 0.003 1.0 0.006 0.4 0.002 0.6 0.004
Fruc. 1.5 0.009 1.4 0.008 0.3 0.002 0.3 0.002 1.0 0.006 0.5 0.003 0.2 0.001
Sucr. 1.9 0.011 2.0 0.012 0.3 0.002 1.7 0.010 0.8 0.005 4.8 0.028 2.2 0.013
Stch. 0.1 0.001 0.3 0.002 16.6 0.097 0.4 0.002 0.1 0.001 0.6 0.004 12.3 0.072
Df. 1.7 0.010 3.6 0.021 1.6 0.009 4.9 0.029 2.7 0.016 4.2 0.025 3.3 0.019
Explanation: Sub. – substrates; Comp. – component; ONI – onion; CAR – carrot; POT – potatoes; CEL – celery; LE – leek; PAR – parsley; MS – maize silage; MU – measurement uncertainty; Gluc.– glucose; Fruc. – fructose; Sucr. – sucrose; Stch. – starch; Df. – dietary fibre.
Table 4. Selected minerals in the substrates tested, relative to 100 g fresh weight.
Table 4. Selected minerals in the substrates tested, relative to 100 g fresh weight.
Sub. ONI CAR POT CEL LE PAR MS
Comp.
(mg)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)		 Value MU
(±)
Na 6 0.040 82 0.550 7 0.050 86 0.560 6 0.040 49 0.330 7 0.050
K 121 0.810 282 1.890 491 3.280 320 2.140 248 1.660 399 2.670 283 1.890
Ca 25 0.160 36 0.240 4 0.030 40 0.270 48 0.320 43 0.290 6 0.040
P 14 0.090 32 0.210 61 0.410 80 0.540 52 0.350 77 0.510 102 0.680
Mg 8 0.050 16 0.110 23 0.150 19 0.130 11 0.070 27 0.180 37 0.250
Fe 0.50 0.003 0.50 0.003 0.60 0.004 0.50 0.003 1.10 0.007 1.10 0.007 0.80 0.005
Zn 0.24 0.002 0.34 0.002 0.35 0.002 0.56 0.004 0.69 0.005 0.60 0.040 0.40 0.003
Cu 0.06 – 0.10 0.001 0.14 0.001 0.05 – 0.13 0.001 0.14 0.001 0.04 –
Mn 0.17 0.001 0.19 0.001 0.10 0.001 0.20 0.001 0.18 0.001 0.58 0.004 0.20 0.001
Explanation: Sub. – substrates; Comp. – component; ONI – onion; CAR – carrot; POT – potatoes; CEL – celery; LE – leek; PAR – parsley; MS – maize silage; MU – measurement uncertainty.
Table 5. Physicochemical parameters of the slurry used.
Table 5. Physicochemical parameters of the slurry used.
pH Measurement uncertainty
(±)		 Total solids
(%)		 Measurement uncertainty
(±)		 Volatile
solids
(%)		 Measurement uncertainty
(±)
7.2 0.05 5.6 0.06 81.2 0.24
Table 6. Biogas generation efficiency, including methane from pig slurry.
Table 6. Biogas generation efficiency, including methane from pig slurry.
Biogas from FM
(m3·Mg FM-1)		 MU
(±)		 Biogas from TS
(m3·Mg TS-1)		 MU
(±)		 Biogas from VS
(m3·Mg VS-1)		 MU
(±)		 Methane
(%)
18 0.24 321.43 4.46 395.85 5.66 51.0
Explanation: FM – fresh matter; TS – total solids; VS – volatile solids; MU – measurement uncertainty.
Table 7. Energy value of daily feedstocks (excluding slurry).
Table 7. Energy value of daily feedstocks (excluding slurry).
Sub. Substrate amount (Mg·day-1) Energy value
(kJ·kg-1)		 Energy
value
(kWh·kg-1)		 Energy
value
(kWh·Mg-1)		 Energy
value
(kWh·day-1)		 Energy
value
(MWh·day-1)		 Energy
value
(MWh·year-1)
ONI 4 1,410 0.392 392 1,567 1.567 572
CAR 6 1,400 0.389 389 2,333 2.333 852
POT 7.5 3,700 1.028 1,028 7,708 7.708 2,814
CEL 2.5 1,250 0.347 347 868 0.868 317
LE 2 1,210 0.336 336 672 0.672 245
PAR 3 2,030 0.564 564 1,692 1.692 617
MS 25 4,870 1.353 1353 33,819 33.819 12,344
Explanation: Sub. – substrates; Comp. – component; ONI – onion; CAR – carrot; POT – potatoes; CEL – celery; LE – leek; PAR – parsley; MS – maize silage.
Table 8. Biogas production in the plant under study.
Table 8. Biogas production in the plant under study.
Substrates
type		 Daily
biogas production
(m3·day-1)		 Annual
biogas production
(m3·year-1)		 Hourly
biogas production(m3·hour-1)
Pig slurry 1,800 657,000 82.125
Other substrates 5,620 2,051,300 256.41
Sum 7,420 2,708,300 338.84
Table 9. The obtained capacity of the biogas plant (MW) and the electricity produced (MWh·year-1).
Table 9. The obtained capacity of the biogas plant (MW) and the electricity produced (MWh·year-1).
Substrates
type		 Hourly
biogas production(m3·hour-1)		 Power
(MW)		 Electricity produced (MWh·year-1)
Pig slurry 82.125 0.15 1,192
Other substrates 256.41 0.47 3,721
Sum 338.84 0.61 4,913
Table 10. Efficiency of biomass conversion under anaerobic conditions (excluding pig slurry) and efficiency of electricity production in the cogeneration system.
Table 10. Efficiency of biomass conversion under anaerobic conditions (excluding pig slurry) and efficiency of electricity production in the cogeneration system.
Energy accumulated in substrates
(MWh·year-1)		 Energy in methane produced (MWh·year-1) Biomass to
methane conversion efficiency (%)		 Electricity
production
efficiency
(%)
17,760 10,633 59.87 27.66
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