Development of Digestate for Energy Purposes Using Excess Heat from Biogas Plants

Abstract

The paper presents an analysis of methods for utilizing digestate for energy purposes from two different biogas plants using different technologies. Biogas plant A used only cattle manure and corn silage as substrates, while biogas plant B used technology based on the utilization of food production waste. The analysis showed differences in the chemical, elemental, and thermogravimetric composition of both types of digestate. An analysis of the energy inputs required to produce fuel from digestate was also performed, along with energy balance calculations. The research and analysis led to the conclusion that both types of digestate are suitable for energy recovery. The possibilities of optimizing the process using excess heat from the biogas plant were also analyzed. In the case of digestate A, the combustion heat of digestate B was 17.20 MJ·kg⁻¹, while for digestate A, it was 14.80 MJ·kg⁻¹. The calorific value of digestate A at 8.79% moisture content was 13.40 MJ·kg⁻¹, while for digestate B at 6.03% moisture content, it was 15.80 MJ·kg⁻¹, respectively.

1. Introduction

1.1. Characteristics of Digestate

Digestate is a by-product of anaerobic methane fermentation, during which biogas is obtained from organic material. It is a residue of the microbiological conversion of organic matter and occurs in two main fractions: liquid and solid [1,2]. The liquid fraction mainly contains nitrogen compounds in the form of ammonium and easily accessible potassium, while the solid fraction is rich in organic substances with a lower degree of decomposition, including lignin and cellulose, as well as minerals such as phosphorus, calcium, and magnesium [3,4].

The chemical composition of digestate largely depends on the type of substrate used. In the case of animal manure fermentation, mineral fractions dominate, resulting in a high content of ash and elements such as Ca, Mg, K, P, and heavy metals (e.g., Zn, Cu) [5,6,7]. Digestate from plant residues and silage usually has a lower ash content, and its energy properties are more favorable for possible use in combustion [8,9]. Sewage sludge fermented in municipal biogas plants may contain organic contaminants and heavy metals, which limits its further use [10,11].

1.2. Energy Potential of Digestate

Immediately after fermentation, digestate contains between 70 and 90% moisture, which significantly limits its direct use as fuel [12]. It is therefore necessary to dehydrate and dry it to achieve a moisture content of less than 10%, allowing the material to be pelletized or briquetted. Studies show that after proper preparation (drying, forming), the calorific value of digestate can range from 15 to 17 MJ·kg⁻¹, which makes it comparable to other energy biomasses, such as firewood [1,13].

Ash content is also an important parameter in the combustion of digestate. Depending on the substrate, ash can account for between 8% and even 40% of dry matter. High ash content increases the risk of grate clogging and slag melting in boilers, and also generates more post-combustion waste [7,14,15]. Despite these limitations, the combustion of digestate is becoming an increasingly interesting alternative to its traditional use as fertilizer, especially in areas with high agricultural production density and limited demand for organic fertilizers [3,16,17].

1.3. Technologies for Utilizing Digestate

The most common way of dealing with digestate is to use it in agriculture as organic fertilizer, thanks to its high nitrogen and other macroelement content [6]. However, in many countries, environmental restrictions, seasonality of use, and high transport costs force the search for alternative methods of its management [18].

Among advanced technologies, thermal conversion of digestate is becoming increasingly important. In the first stage, it is necessary to use dehydration methods (mechanical and thermal), which can be implemented using a screw press, belt filters, or drum and belt dryers [1,12]. Drying can be powered by waste heat from cogeneration in biogas plants, which improves the energy efficiency of the entire process [3].

The next stage may be pelletization, which allows for the production of granular fuel material with standardized size and combustion properties. Pellets from post-fermentation can be used in biomass boilers, but due to their high ash and mineral content, equipment with increased resistance to corrosion and clogging is required [8,9].

In addition to direct combustion, pyrolysis and gasification methods are also being considered for post-fermentation. Pyrolysis produces biochar, which can be used as a soil improver, fertilizer carrier, or pollutant sorbent [14,15]. Gasification, on the other hand, enables the production of synthesis gas (syngas), which can be used for energy or chemical purposes, but this technology requires the raw material to have a relatively low moisture content and a high degree of process control [10,19].

The energetic use of digestate, a residue from the meta-new fermentation process, is an important direction for the management of this waste, especially in the context of digestate overproduction in regions with a high concentration of agricultural biogas plants. From an economic point of view, the combustion of digestate can bring energy benefits, provided that the material is properly prepared, primarily by dehydration and/or drying.

Immediately after fermentation, digestate contains between 70% and even 90% water, which completely disqualifies it as a fuel without prior processing [1,2,3]. To enable combustion or gasification, it is necessary to reduce the moisture content to below 10%, which can be achieved through mechanical dewatering (e.g., screw press) and thermal drying, often using excess heat from cogeneration in biogas plants [6,9].

The technical and scientific literature provides a wide range of values for the lower heating value (LHV) of dry digestate. Dziedzic, K. et al. [1,2] indicate that granulated (pelletized) digestate, prepared from corn silage and manure, can achieve a calorific value of 15–17 MJ·kg⁻¹ at a moisture content of 9–10%. In turn, Wang et al. [13] conducted experiments with the pelletization of pig digestate with the addition of corn straw, obtaining an LHV of 17.1 MJ/kg. Furthermore, enriching the raw material with biochar allowed for a further increase to 18.9 MJ·kg⁻¹, making such a product fully competitive with conventional wood pellets.

In another study, Kratzeisen et al. [20] determined the calorific value of pelletized digestate to be 15.0–15.8 MJ·kg⁻¹, which also falls within the lower range of values for typical wood biomass. These studies confirm that digestate, when properly dewatered and formed, can be a valuable solid fuel in biomass combustion systems.

The higher heating value (HHV) of dry digestate is also similar to that of other forms of biomass. Combustion modeling conducted by Zhou et al. [12] indicates an HHV of dry digestate of approximately 18.3 MJ·kg⁻¹, while other analyses have achieved values of up to 18.6 MJ·kg⁻¹ [21]. These energy parameters place digestate in the same category as coniferous wood pellets (approx. 18–19 MJ·kg⁻¹).

In studies conducted on digestate from Croatian biogas plants, an LHV of approx. 13 MJ·kg⁻¹ was recorded for the dry fraction without granulation [22]. In Polish conditions, with the use of biological drying (biodrying), the calorific value of the digestate increased from approx. 9 MJ·kg⁻¹ (moisture content ~44%) to approx. 17.5 MJ·kg⁻¹ after reaching a moisture content of less than 10% [21].

Ash content remains an important technological aspect. Depending on the feedstock (manure, sewage sludge, plant biomass), ash can account for between 8% and even 40% of the dry matter of the digestate [1,12]. High ash content not only reduces the energy value of the fuel, but also increases the risk of clogging, sintering, and corrosion in boilers, requiring the use of appropriately selected materials and combustion technologies.

A review of the literature shows that the most effective way to use digestate for energy is to dry and pelletize it, which produces a fuel with high energy density, good combustion parameters, and ease of transport and storage. On the other hand, the use of raw or only mechanically dehydrated digestate fraction is much less effective due to its low calorific value and operational problems.

In light of the available data, it can be concluded that the conversion of digestate into standardized biomass fuel is technically feasible and energetically justified. However, this requires the use of integrated technologies: from mechanical and thermal drying, through forming (pelletizing or briquetting), to appropriately adapted combustion systems. Only then is it possible to achieve a positive energy and environmental balance while limiting emissions and technological risks.

The review shows that the energy use of digestate has significant potential, especially in processed form (dried and pelletized). However, the use of digestate as fuel requires appropriate preparation technologies and combustion installations adapted to its high ash and mineral content. The biggest challenge remains the effective and economical drying of the material and the control of emissions of undesirable compounds (e.g., heavy metals) during combustion.

In Poland, the main regulation governing the use of digestate is the Act on Fertilisers and Fertilisation of 10 July 2007 [23]. According to this act, digestate can be classified as an organic fertilizer and a soil improver.

In the case of the EU, the most important document is the Waste Framework Directive (2008/98/EC) [24], which defines the differences between waste, products and secondary raw materials. In this directive, digestate is classified as biowaste that can be used for agricultural purposes if it meets quality requirements such as the presence of E. coli < 1000 CFU/g and contaminants (plastic, glass, metal) < 0.5 g/kg.

In order to use digestate for energy purposes, e.g., combustion, it is necessary to obtain an ‘end-of-waste’ decision from the provincial governor in Poland. In order to obtain this decision, it is necessary to carry out quality tests on the digestate and prove its safety, repeatability of composition and utility value. In order to also be able to market fuel from digestate, the product must also be registered as a biofuel or alternative fuel in accordance with the requirements of the RED II/RED III directive and certification [25].

During the literature review, a significant lack of comparison of digestates that differ significantly in terms of morphology, with a simultaneous comparison of their energy potential as fuel, was noted. There is also a lack of a comprehensive approach in the literature that would involve analyzing the implementation of biofuel production technology from a specific post-fermentation product within a biogas plant using excess heat.

The aim of this study was to analyze the possibilities of energy utilization of digestates of various origins. To achieve this goal, the study was limited to analytical research on two types of digestate. The first digestate (A) came from a biogas plant characterized by technology based on the use of cattle manure and corn silage. The second digestate (B) came from a biogas plant with a different technology based on the utilization of agricultural and food production waste, such as fruit and vegetable waste and feed residues.

The digestate samples were analyzed for moisture content, composition based on thermogravimetric analysis, elemental composition, and biochemical composition, and the combustion heat and calorific value of the dried digestate were determined.

The entire study was complemented by a simulation of the energy balance of the technology for producing fuel from digestate, including its preparation and combustion.

2. Materials and Methods

To achieve the objective of the study, selected samples of digestate were collected from two biogas plants that differed in terms of the feedstock used. In the case of biogas plant A, the substrate used consists exclusively of cattle manure and corn silage, while biogas plant B uses technology based on the utilization of agricultural and food production waste, such as fruit and vegetable waste and feed residues. This choice was made in order to compare and find potential differences in the energy potential of digestate depending on the type of substrate from which it is produced.

The digestate obtained from the biogas plant was in liquid form, so the moisture content of the samples was first determined, and then, for further analysis, the material was prepared for processing in an air-dry state using a special laboratory dryer with forced air circulation. Samples of digestate A and B are shown in Figure 1.

The moisture content of the raw digestate was 92.0% for digestate A and 96.1% for digestate B. It was determined using the oven-drying method. This method was chosen due to the significant water content in the raw digestate. After drying the digestates, air-dry samples were obtained for further analysis with a moisture content of 8.79% for digestate A and 6.03% for digestate B. Due to the nature of the research, an additional test was carried out on the possibility of mechanical separation of the digestate from moisture using a steel sieve with a mesh size of 1 mm. As a result of sieving, samples with a moisture content of 75.1% for digestate A and 80.2% for digestate B were obtained. The moisture content in the air-dry state was determined using a thermogravimetric analyzer, which is more accurate than the oven-weighing method. The moisture content results are presented in

Table 1. Moisture parameters of digestates A and B.

Sample	Moisture Content [m/m]
	Raw Condition	Mechanically Dewatered Digestate	Air-Dry Condition
Digestate A	92.0% ± 0.1	75.1% ± 0.1	8.79% ± 0.01
Digestate B	96.1% ± 0.1	80.2% ± 0.1	6.03% ± 0.01

.

The air-dry samples were subjected to a series of analyses, such as elemental, proximate, and biochemical analysis.

2.1. Proximate Analysis

Proximate analysis studies were performed using the TGA701 automated thermogravimetric analyzer. The measurement involved heating the sample to temperatures specified according to the standard while measuring the mass loss [26]. For biomass testing, ASTM E1131-08(2014) [27] was used to describe the steps involved in the thermogravimetric analysis performed with an automatic analyzer. The moisture content was determined by heating the sample to 105–110 °C and evaporating it. Determination of the volatile content was performed by heating the samples, placed in covered dishes, to a temperature of 850 ± 15 °C for about 7 min. The biomass sample was then incinerated at 550 °C.

2.2. Biochemical Analysis

The standard methods applied in analyses of plant biomass were used to test the chemical composition of the digestates. The extractive content was determined according to Soxhlet (TAPPI–T 204 cm-07) by using 96% ethanol [28]. The content of cellulose was analyzed by Seifert’s method. For this purpose, a mixture of acetylacetone and dioxane at an acidic pH was used [29]. Determination of lignin content by the Tappi method used 72% sulfuric acid (TAPPI–T 222 cm-06) [30]; For the holocellulose content estimation, sodium chlorite was used (TAPPI–T 9 wd-75) [31]. The theoretical content of hemicellulose was calculated mathematically as the difference between the holocellulose and cellulose contents. For the analysis of the amount of ash, the DIN 51731 standards were used [32]. According to the standard, the results were calculated in relation to the dry mass of the raw material, and the average of the 3 measurements was calculated. For determination of the moisture content the oven-dry method was used. The drying process was performed at a temperature of 103 ± 2 °C. The difference between the weighed values did not exceed 0.01%. Chemical analysis of the studied raw materials was performed courtesy of the Institute of Wood Chemical Technology at the Poznan University of Life Sciences. The research included analyses of the content of extractive substances, lignin, cellulose and holocellulose. Determination of the extractable substance content was based on extraction of resins, waxes and fatty acids from the tested material with ethyl alcohol. The extracted samples were suitable for further determination.

The determination of lignin content involved dissolving insoluble carbohydrate compounds in sulfuric acid to form soluble sugars. The remaining lignin was then separated by sieving through crucibles so that its mass could be determined. The determination of cellulose content was based on its separation by dissolving the lignin and other substances contained in the wood with a mixture of acetylacetone and dioxane, acidified with hydrochloric acid. Holocellulose was extracted by delignification with sodium chlorite in the presence of glacial acetic acid.

2.3. Elemental Analysis

Elemental composition analyses were carried out using automatic CHN628 and 628S elemental analyzers at the Department of Power and Transport Means at the University of Life Sciences in Lublin. Examination of the content of carbon, hydrogen and nitrogen using the CHN628 analyzer consisted of the combustion of a material sample of 0.1 g placed in a foil capsule, leading to complete oxidation of the abovementioned elements to carbon dioxide (CO₂), water (H₂O) and nitrogen oxide (NOx), respectively. The carbon and hydrogen content were determined using separate NDIR (non-dispersive infrared) cells based on the contents of water vapor and carbon dioxide. Nitrogen oxides (NOx) were reduced to pure nitrogen by using a copper-filled reduction tube. Carbon dioxide and water were removed from the gas portion. The nitrogen content was determined using a thermoconductive (TC) cell [22]. Determination of the sulfur content was performed using the 628S module. The analysis involved placing a 0.2 g sample in a special crucible, which was then introduced into a furnace where it was burned in an oxygen atmosphere at 1350 °C. The sulfur in the sample was released in the form of sulfur dioxide (SO₂) and its content was determined using the IR infrared detection objective [33].

2.4. Testing of Combustion Heat and Determination of the Calorific Value of Digestates

The moisture content of fresh digestates was determined prior to the study of the combustion heat. Due to the high water content, the digestates was dried to an air-dry state, enabling the determination of combustion heat and calorific value. Natural factors were used for drying. The date of the knotweed harvest was in the summer months, which meant that it was possible to dry the knotweed spontaneously by leaving it exposed to the atmospheric factors of temperature and wind. Four days with temperatures slightly above 20 °C and no rainfall were sufficient to obtain humidity below 10%. Samples of digestates were ground for analysis and then were sieved to obtain a representative sample with a 0.2 mm fraction. The combustion heat tests were carried out at the Department of Power and Transport Means at the University of Life Sciences in Lublin using a AC600 computer calorimeter based on DIN 51900-2:2003-05 standards [34]. The test involved complete combustion of the sample in an oxygen atmosphere in a calorimetric bomb immersed in a water jacket, with simultaneous measurement of the rise in temperature of the water. To determine the calorific value, it was necessary to determine the moisture content. This parameter was determined using the dryer-weight method according to ISO 18134-3:2015 [35].

2.5. Calculations Simulating the Technological Process of Production and Use of Fuel from Digestate

The results obtained in individual studies on moisture content and calorific value were used to calculate simulations of the technology for removing moisture and forming fuel from different types of digestate. The technological diagram is presented in Figure 2. It was assumed that it was necessary to select processes for dewatering and drying the digestate to a state enabling the production of fuel formed in the form of briquettes or pellets. The simulation also took into account the energy yield from the combustion of the fuel produced. The aim of the simulation was to check whether the energy obtained from the post-fermentation fuel is higher than the energy expenditure for fuel production. All calculations were performed in a way that allows for their later use by presenting them in relation to units of mass: kg and Mg. The results are given in MJ∙kg⁻¹ and kWh∙Mg⁻¹.

2.5.1. Simulation of Drying/Moisture Removal

Due to the fact that the digestate contains significant amounts of water, the simulation assumed the need for mechanical separation prior to drying the digestate in order to reduce energy consumption. The calculations were based on the obtained moisture content of the digestate, and a screw press separator operating on the principle of filtration was assumed to be used. Among the available technical solutions, a separator with a power of 5 kW and a capacity of 5 m³ per hour was assumed. Separators of this type are capable of achieving a solid fraction moisture content of 65–70%. The capacity and power data used for the calculations were averaged from available reliable data on medium-power separators.

In the case of drying, the calculations were based on the moisture content obtained after sieving the digestate and the final moisture content after drying of 15%, as recommended for the pellet production process. The heat of vaporization of water at 60 °C, amounting to 2359 kJ·kg⁻¹, was used for the calculations. The drying efficiency was assumed to be 60%. The simulation assumes a plate heat exchanger with a hot medium inlet temperature of 90 °C. The heat exchange efficiency is assumed to be approximately 85%. The hot medium outlet temperature is estimated at 70 °C. Hot water for the dryer is assumed to be the heat receiving medium.

2.5.2. Simulation of Fuel Formation

To form fuel into pellets, the parameters of a medium-power pellet mill with a power of 36 kW and a capacity of 400 kg·h⁻¹ were used. These are average parameters available on the market for medium-power pellet mills.

2.5.3. Simulation of Combustion Digestate

The results of the calorific value were used to calculate the energy yield, and the average combustion boiler efficiency was assumed to be 80%.

2.6. Statistical Analysis of Research Results

It was assumed that a statistically significant difference means that the observed difference between groups or variables in the study is so large that it is unlikely to occur by chance.

The adopted research methodology and research tools allowed the assumption that the difference was not due to sampling error or random fluctuations. A level of statistical significance (0.05 or 5%) was adopted, which determined how large the difference must be to be considered as total.

Using the environment of the statistical test of comparison of variance between groups of data (ANOVA), an attempt was made to answer the question of whether there was a significant difference in the means of the groups of individual data.

The data validation process assumed:

Null hypothesis (H0): there is no significant difference in means between data groups,

Alternative hypothesis (H1): there is a significant difference in means between data groups.

We collected data of repetitions of the results of a given measurement (a minimum of 3 repetitions) and performed an analysis of variance. If the p-value obtained from the ANOVA analysis is less than a predetermined level of significance (for example, 0.05), the null hypothesis was rejected and it was inferred that the means of the data are significantly different.

3. Results

As mentioned earlier, chemical analyses were performed on samples that had been dried beforehand in an air-dry state. The moisture content of fresh digestate A was 92.0%, and that of digestate B was 96.1%.

3.1. Proximate Analysis

Proximate analysis showed that the moisture content in the digestate samples was 8.79% for digestate A and 6.03% for digestate B. In the case of ash content, there was a slight difference in ash content, which was 15% for digestate A and 18.01% for digestate B. The volatile and fixed carbon content was also similar. The volatile matter content was 65% for digestate A and 64.04% for digestate B. In the case of fixed carbon, the content was 11.21% for digestate A and 11.91% for digestate B, respectively (Figure 3).

3.2. Biochemical Analysis

The digestate samples subjected to chemical analysis (Figure 4) showed significant differences between the individual parameters determined as a result of this analysis. Digestate B contained a higher content of extractable substances (3.28%, digestate A; 11.31%, digestate B) and hemicellulose (16.74%, digestate A; 21.39%, digestate B). On the other hand, digestate A had a significantly higher cellulose content (31.35%), while the content of this component in digestate B was 26.21%. The lignin content, on the other hand, was at a similar level (39.20%, digestate A; 41.09%, digestate B).

3.3. Elemental Analysis

As a result of elemental analysis (Figure 5), differences in carbon content were found, which was 44.81% for digestate B, while for digestate A it was lower at 39.31%. Differences were also found in hydrogen content (5.72% for digestate A; 6.60% for digestate B) and oxygen content (52.78% for digestate A; 46.57% for digestate B). In the case of sulfur content, the differences were minor, with digestate A containing 0.49% and digestate B containing 0.54%. Similarly, a slight difference was observed in the case of nitrogen content (1.70% for digestate A and 1.48% for digestate B).

3.4. Testing of Combustion Heat and Determination of the Calorific Value of Digestates

The results of calorimetric tests (Figure 6) showed that more energy could be obtained from digestate B than from digestate A. The calorific value of digestate B was 17.20 MJ·kg⁻¹, while that of digestate A was 14.80 MJ·kg⁻¹. The calorific value of digestate A at a moisture content of 8.79% was 13.40 MJ·kg⁻¹, while for digestate B at a moisture content of 6.03% it was 15.80 MJ·kg⁻¹.

3.5. Calculations Simulating the Technological Process of Production and Use of Fuel from Digestate

The energy balance takes into account the following processes: separation, drying, fuel production (pelletization), and combustion. The pelletization process increases the volume density of biomass, which reduces transportation and storage costs. In addition, it ensures better material feeding with less dusting [36]. Due to the fact that the moisture content of fresh digestate in both analyzed samples was different (digestate A 92%, digestate B 96%), it was necessary to determine the amount of energy needed to dry the sample to 15% moisture content. A moisture content of approximately 15% is necessary for the fuel formation process. For this reason, in order to reduce energy expenditure on drying, a mechanical separation simulation was carried out. For this purpose, the moisture content results obtained during the post-ferment drainage test were used. For digestate A, it was assumed that it was possible to obtain dehydrated digestate with a moisture content of 75%, while for digestate B, a moisture content of 80% was assumed. The separation process was determined on the basis of calculations to be low energy-intensive, as 0.026 MJ∙kg⁻¹ (7.35 kWh∙Mg⁻¹) was required for digestate A and 0.0225 MJ∙kg⁻¹ (6.25 kWh∙Mg⁻¹) for digestate B.

Calculations showed that drying digestate A from 75% to 15% moisture content requires 2.66 MJ∙kg⁻¹ (737.58 kWh∙Mg⁻¹). In turn, to dry digestate B from 80% to 15% moisture content, the calculations showed that 2.88 MJ∙kg⁻¹ (799.05 kWh∙Mg⁻¹). It was observed the high energy consumption in the drying process.

Energy consumption in the granulation process was determined at 0.32 MJ∙kg⁻¹ (90,00 kWh∙Mg⁻¹) for both types of digestate due to their similar structure and expected identical substrate moisture content prior to pelletization.

In the case of using the energy contained in the produced fuel, the use of a boiler with an average efficiency of 80% was simulated, and the tested calorific values of the digestates were used for the calculations. In the case of digestate A, it was shown that it was possible to obtain 11.84 MJ∙kg⁻¹ (3288.89 kWh∙Mg⁻¹), and from digestate B, 10.72 MJ∙kg⁻¹ (2977.78 kWh∙Mg⁻¹). The results concerning the energy balance are presented in

Table 2. Energy balance of production and combustion of biofuel from digestate A.

Ordinal Number	Process	Energy Expenditure		Energy Yield		Energy Balance [Energy Expenditure–Energy Yield]
		MJ∙kg−1	kWh∙Mg−1	MJ∙kg−1	kWh∙Mg−1
1.	Separation	0.026	7.35	-	-	MJ∙kg−1	kWh∙Mg−1
2.	Drying	2.66	737.58	-	-
3.	Fuel forming	0.32	90.00	-	-
4.	Combustion	-	-	11.84	3288.89
	Total	2.86	793.82	11.84	3288.89	8.83	2453.95

and

Table 3. Energy balance of production and combustion of biofuel from digestate B.

Ordinal Number	Process	Energy Expenditure		Energy Yield		Energy Balance [Energy Expenditure–Energy Yield]
		MJ∙kg−1	kWh∙Mg−1	MJ∙kg−1	kWh∙Mg−1
1.	Separation	0.0225	6.25	-	-	MJ∙kg−1	kWh∙Mg−1
2.	Drying	2.88	799.05	-	-
3.	Fuel forming	0.32	90.00	-	-
4.	Combustion	-	-	10.72	2977.78
	Total	4.40	1221.81	10.72	2977.78	7.50	2082.48

and in Figure 7 and Figure 8.

Based on the calculations, it was shown that in both cases it was possible to achieve a positive energy balance for both types of digestate. In the case of the technology using digestate A, it is possible to obtain 8.83 MJ∙kg⁻¹ (2453.95 kWh∙Mg⁻¹), and in the case of the technology based on the use of digestate B, 7.50 MJ∙kg⁻¹ (2082.48 kWh∙Mg⁻¹).

Estimation of Emissions from Digestate Combustion

Based on the results of the studies presented above, the potential level of emissions resulting from digestate combustion was estimated (

Table 4. Estimated emissions from the combustion of digestate.

	Emission		Digestate A	Digestate B
NOX		g·kg−1	16.76	14.59
SO2		g·kg−1	9.31	10.26
HCl		g·kg−1	2.50	9.00
PM	Without dust collection	g·kg−1	45.00	54.00
	With dust collection	g·kg−1	7.50	9.00

). For NOx emissions, the result was 16.76 g·kg⁻¹ for digestate A and 14.59 g·kg⁻¹ for digestate B. For SOx, the emission for digestate A was lower than for digestate B, amounting to 9.31 g·kg⁻¹ and 10.26 g·kg⁻¹, respectively. There were noticeable differences in HCl emissions, with HCl emissions estimated at 2.50 g·kg⁻¹ for post-fermentation A and 9.00 g·kg⁻¹ for post-fermentation B. Dust emission variants without and with dust collection were also analyzed. Without dust collection, the result was 45.00 g·kg⁻¹ for digestate A and 54.00 g·kg⁻¹ for digestate B. With dust collection, significant dust capture can be expected, with PM emissions of 7.50 g·kg⁻¹ for digestate A and 9.00 g·kg⁻¹ for digestate B.

4. Discussion

Based on the conducted research, it is evident that the process of preparing fuel from digestate in both cases is a process with a positive energy balance. In the case of the technology using digestate A, it is possible to obtain 8.83 MJ∙kg⁻¹ (2453.95 kWh∙Mg⁻¹), and in the case of the technology based on the use of digestate B, 7.50 MJ∙kg⁻¹ (2082.48 kWh∙Mg⁻¹).

The highest energy expenditure was related to drying the digestate and amounted to 2.66 MJ∙kg⁻¹ (737.58 kWh∙Mg⁻¹) for digestate A from 75% to 15% moisture content. In turn, to dry digestate B from 80% to 15% moisture content, the calculations showed that 2.88 MJ∙kg⁻¹ (799.05 kWh∙Mg⁻¹) was required.

Czekała W. and others [37] analyzed pellets from digestate. They showed that this material had a calorific value of 16.41 MJ∙kg⁻¹. In turn, they determined the combustion heat to be 17.99 MJ∙kg⁻¹. Karaeva J.V. and others [38] have received higher heating value (HHV) of digestate was 18.6 MJ∙kg⁻¹. The HHV of the char residue was 19 MJ∙kg⁻¹. Kratzeisen M. [20] conducted research on pellets from digestate. Net calorific value of digestate pellets were between 15.8 MJ∙kg⁻¹ and 15.0 MJ∙kg⁻¹ with water content of 9.2% and 9.9%. Vaskina I. and others [39] obtained a calorific value of the digestate at a level of approximately 17 MJ∙kg⁻¹.

Analysis of the composition of digestate from biogas plants provides valuable information about its fertilizer value, energy potential and the risks associated with heavy metal content. Key data obtained by other authors are discussed below.

Zhou et al. [12] studied digestate obtained from the methane fermentation of cattle manure. They obtained the following elemental composition of dry matter: carbon (C): 41.9%, hydrogen (H) 5.27%, nitrogen (N): 2.14–2.22%, sulfur (S): 0.38–0.44%, chlorine (Cl): 1.01–1.11%. This confirms that the digestate contains significant amounts of energetically and biochemically active elements, which is important both for assessing its combustion potential and its use as a fertilizer. The above results obtained by Zhou et al. [12] were comparable to the results obtained for the digestate in the course of the analysis in this article.

Corn silage is rich in complex carbohydrates (mainly starch and cellulose) and organic matter with high energy potential. During methane fermentation, not all organic matter decomposes—some remains in the digestate as undecomposed biomass with high calorific value. In addition, manure contains fibers, fats and lignin, which are also partially undecomposed and can increase the calorific value of digestate A. On the other hand, fruit and vegetable waste (digestate B) contains more water, simple sugars and more easily biodegradable components, which are largely consumed in the methane fermentation process. The residues have a lower organic matter content and more minerals (ash), which has a negative impact on calorific value. Digestate B probably contains more minerals (e.g., calcium, potassium, sodium) from vegetables and fruit, which results in a higher ash content and, consequently, a lower calorific value. Corn silage and manure contain components that are more difficult to biodegrade (e.g., lignin, hemicellulose), which partially remain in the post-ferment, retaining high energy potential when burned. In contrast, fruit and vegetable waste does not contain large amounts of such structures—most of the energy is released in biogas, and the residues have a lower calorific value.

A molecular analysis of digestates of various origins (food waste, sewage sludge, silage) showed a very low C/N ratio (1.5–3.3) in the food fraction and a higher ratio (4.7–12) in other types. The composition of macronutrients (N, P, K) corresponds to that of crops, although nitrogen often needs to be supplemented. An increase in the concentration of N, P, K, and micrometals (Ca, Mo) is observed in the liquid after separation [40].

Romio C. et al. [41] report that the calorific value of the total digestate is 16.9 MJ∙kg⁻¹ dry matter, while the separated solid fraction (SFD) has a calorific value of 18.3 MJ∙kg⁻¹ dry matter. This confirms that separation increases the proportion of dry biomass with high energy potential. The elemental composition of the digestate indicates the possibility of pelletization and combustion, although technical implementations must take into account the ash and metal content. Ashes from biomass combustion, including solid fractions of digestate, are a potential source of secondary raw materials that can be used in both agriculture and the construction materials industry. They contain significant amounts of minerals such as calcium, potassium, phosphorus, magnesium and silicon, and their composition depends mainly on the type of material burned and the parameters of the combustion process [42,43]. In the case of combustion of digestate produced from plant or organic substrates, ash can be considered as a potential mineral fertilizer, especially in the context of limited phosphorus resources worldwide [44]. On the other hand, its physicochemical properties also allow it to be used in the production of building materials, e.g., as an additive to cement, concrete, geopolymer composites or lightweight aggregates [45].

The calculated carbon dioxide emissions amounted to 786.2 g·kg⁻¹ for digestate A and 851.4 g·kg⁻¹ for digestate B. In the case of biofuels, these emissions are considered zero due to their biological origin.

Chen Y. et al. [46] obtained comparable results regarding the lignin, cellulose, and hemicellulose content in the digestate. Analyses of the decomposition of components during fermentation in biogas plants showed that hemicellulose undergoes partial hydrolysis and fermentation (approx. 68–74% loss), cellulose to a lesser extent (58–65%), while lignin is highly recalcitrant (only 10–32% degradation). Furthermore, cellulose alone is difficult to hydrolyze (only 40.1%) without the addition of hemicellulose. A similar situation can be observed in the case of digestates A and B, where the lignin content is significant in dry matter.

It is assumed that approximately 30% of the thermal energy produced by a cogeneration engine in a biogas plant constitutes excess energy that can be utilized. Heat transport is usually unprofitable due to significant heat losses along the length of the pipelines transporting the heating medium. In addition, investment in a heat transmission system usually requires significant financial outlays. Furthermore, in the climate zone of Poland and Central Europe, the demand for heat varies over time and depends on the season, which further extends the return on investment period. Therefore, a solution may be to use waste heat from biogas plants to dry digestate for biofuel production. This biofuel can be stored, easily transported, and used when actually needed.

In this case, the energy input for producing fuel from digestate would be significantly lower than shown in the balance sheet and would amount to 11.49 MJ∙kg⁻¹ (3191.54 kWh∙Mg⁻¹) for digestate A, and for digestate B, 10.37 MJ∙kg⁻¹ (2881.53 kWh∙Mg⁻¹).

The electricity required for the process of separating water from digestate and for the pelletizing process can be renewable energy produced by a biogas plant. Its use can be optimized in relation to the use of energy that cannot be used by the power grid during, for example, night hours or other hours with reduced electricity consumption by consumers. By using and optimizing the thermal and electrical energy from biogas plants in this way, the energy use of digestate can improve the environmental impact of biogas plants.

In addition, post-fermentation dehydration can be a precursor to the use of further technologies for the management of the liquid fraction remaining after post-fermentation drainage [6,47]. It is true that organic substances responsible for COD and BOD, such as ammonia, fatty acids, and dissolved compounds, remain in the liquid fraction, which in turn means that after separation of the solid fraction, an increase in these parameters may be observed, but mechanical separation can be a precursor to further filtration using, for example, membrane processes. Separation of the solid fraction will improve ultrafiltration efficiency, extend the life of the membranes, and reduce the need for frequent cleaning, as would be the case without the pre-separation process. The liquid filtered by ultrafiltration can be successfully used, for example, for irrigating farmland [48,49,50,51].

The proposed technology can be successfully applied in various types of biogas plants, but according to the authors, the greatest potential is expected for medium and large biogas plants. In particular, it is expected to be implemented in biogas plants where there is a clear problem with the use of excess heat from the cogeneration system. In biogas plants of this type, if it is not possible to continuously collect heat, problems arise related to heat transfer and storage. In the case of a thermal energy storage system, it is well known that it is associated with specific losses. In the case of heat storage facilities that allow heat to be stored for a period of several months to seasons, efficiency can range from 50 to 70% [52]. In addition, there is the issue of utilizing this heat, i.e., its transmission, which generates further losses. In the case of the technology analyzed in this paper, there are no such losses. The proposed technology refers to cases where it is not possible to use excess heat from biogas plants, e.g., due to the significant distance of the installation from buildings and discontinuous demand for heat. The aim of this work was to demonstrate that in such a case it is possible to use excess heat and digestate to produce valuable fuel, even in the case of significant morphological differences in the digestate. The fuel produced in this way is easier to use for heating purposes and much easier to store, which makes it an important way to improve the efficiency of biogas plants. The problem indicated here may primarily concern medium and high-capacity biogas plants. Usually, high-capacity biogas plants are designed to utilize almost all excess heat, but it happens that, for example, in the summer, there is a significant excess of waste heat, which can be used, for example, to dry digestate.

In the case of low-capacity biogas plants, significant heat surpluses suitable for this technology are not expected. However, heat surpluses may occur in such cases during the summer, but low-power agricultural biogas plants are usually used for disposal purposes, reducing greenhouse gas emissions, and the resulting digestate is mainly used as fertilizer.

5. Conclusions

The research and analyses conducted have led to the following conclusions:

• The research conducted has demonstrated the possibility of using digestates of various origins for energy purposes, as exemplified by digestate A, where the substrate used in the biogas plant consists exclusively of cattle manure and corn silage, digestate B originating from technology based on the management of agricultural and food production waste, such as fruit and vegetable waste and feed residues. In the case of digestate A, the combustion heat of digestate B was 17.20 MJ·kg⁻¹, while for digestate A it was 14.80 MJ·kg⁻¹. The calorific value of digestate A at 8.79% moisture content was 13.40 MJ·kg⁻¹, while for digestate B at 6.03% moisture content it was 15.80 MJ·kg⁻¹.
• The conducted research and analysis of the digestate showed significant differences in their elemental and biochemical composition (extractives: 8.03 pp.; cellulose: 5.14 pp.), which made it possible to verify the possibility of energy recovery from digestate for significantly different samples.
• The results of the proximate analysis were similar for both types of digestate. Both types of digestate were characterized by similar fixed carbon (11.21% digestate A, 11.91% digestate B) and volatiles (65.00% digestate A, 64.04% digestate B) results. An important result in terms of the energy use of digestate is the significant ash content in both types of digestate (15.00% digestate A, 18.01% digestate B). These ashes constitute a significant ballast in the fuel, and in such cases it is suggested to use chemically resistant boilers that limit heavy metal emissions into the environment.
• Biochemical analysis showed significant differences in the content of extractives (3.28% digestate A, 11.31% digestate B) and cellulose (31.35% digestate A, 26.21% digestate B). Differences was also observed in the content of hemicellulose (16.74% digestate A, 21.39% digestate B). In both cases, the main component was lignin, which is characteristic of digestate samples (39.20% digestate A, 41.09% digestate B). Lignin is not digested by methanogenic bacteria and therefore remains as ballast in the digestate. However, its significant share determines its potential as a fuel for combustion. The higher content of cellulose undigested in the methane fermentation process in the case of digestate A indicates the existence of a lignocellulose complex that did not decompose during fermentation. Perhaps it would be worth considering pre-treating the substrate for the biogas plant, which would facilitate the decomposition of this complex and increase biogas production.
• Elemental analysis showed a significant carbon content (39.31% in digestate A, 44.81% in digestate B) in the digestate. The nitrogen and sulfur content was low. This indicates the high potential of both types of digestate as fuel and the generation of low sulfur and nitrogen oxide emissions during digestate combustion. At the same time, it should be noted that compounds containing these elements are likely to be expected in the liquid phase of the digestate leachate.
• The energy balance carried out showed the energy benefit of using both types of digestate for combustion purposes. In the case of the technology using digestate A, it is possible to obtain 8.83 MJ∙kg⁻¹ (2453.95 kWh∙Mg⁻¹), and in the case of the technology based on the use of digestate B, 7.50 MJ∙kg⁻¹ (2082.48 kWh∙Mg⁻¹).
• A sensitivity analysis was performed for all key parameters at a level of ±3%. The key parameters included: initial moisture content, moisture content after separation, final moisture content, calorific value, boiler efficiency, separation process efficiency, and post-fermentation drying efficiency. The analysis revealed that the energy yield from combustion may decrease by 0.36 MJ∙kg⁻¹ (98.67 kWh∙Mg⁻¹) for digestate A and by 0.32 MJ∙kg⁻¹ (89.33 kWh∙Mg⁻¹) for digestate B. In turn, an increase in energy expenditure of 0.097 MJ∙kg⁻¹ (26.99 kWh∙Mg⁻¹) was predicted for digestate A and an increase of 0.104 MJ∙kg⁻¹ (28.75 kWh∙Mg⁻¹) for digestate B. The difference in the energy balance in this case was 0.45 MJ∙kg⁻¹ (125.66 kWh∙Mg⁻¹) for digestate A and 0.43 MJ∙kg⁻¹ (118.08 kWh∙Mg⁻¹) for digestate B.
• The emission calculations showed, above all, that the combustion of digestate should take place in installations that allow for the capture of ash. This ash can then be used for fertilizer purposes or for the production of building materials.
• The energy balance shows that the largest energy expenditure is related to the drying process. However, it is proposed to reduce this expenditure by using waste heat from biogas plants.
• The energy consumption for mechanical moisture separation and fuel formation is relatively low compared to the drying process. However, it is possible to optimize these processes by using renewable electricity produced by the biogas plant’s cogeneration engine.
• The use of excess heat from the biogas plant to dry the digestate and the use of excess electricity from the cogeneration unit to prepare biofuel can be a way of storing energy. Using heat that is not needed at a given time for the fuel production process, which can then be used when needed (e.g., in winter), will significantly improve the environmental impact of the biogas plant.