Nitrogen-Free Co-Gasification of Fermentation Residues

Abstract

The limited usage of fermentation residues, due to increasingly stringent legal requirements, demands novel routes of utilization for these feedstocks. To the best of our knowledge, for the first time, a mixture of fermentation residues and wood chips is used as feedstock in a fixed-bed gasifier, using only O₂/CO₂ mixtures as gasifying agent. The maximum O₂ concentration achieved was 31.6 Vol.-%. Pronounced process stability was achieved with a cold gas efficiency of about 94%, possibly due to CO₂ conversion within the process. The heating value of the produced synthesis gas was 8.5 MJ/m³ _i.N.dry, with increased amounts of carbon monoxide and methane when compared to air-blown operations.

1. Introduction

While the efforts to lower anthropogenic CO₂ emissions globally increase, the utilization of biogas has also increased worldwide. Germany can be highlighted in this context, with about 9600 biogas plants nationwide providing 5600 MW of electricity, approximately 12.2% of the total amount of renewable electricity [1]. While the predominant feedstock consists of renewable raw materials such as corn, grass, and grain (76.6% energetic), the usage of manure (14.8% energetic), organic wastes (4.6% energetic), and other residues (3.9% energetic) is also possible [2].

As this is a major contribution toward the production of green energy, a major drawback is the resulting amount of fermentation residues, totaling approximately 82 Mt per year (the amount varies due to differing reference states) in Germany alone [3,4,5,6]. These fermentation residues are almost entirely used in agriculture as organic fertilizers [6]. According to German law, the use of these substrates is allowed with restrictions regarding the time and amount, as stated in the national fertilizer ordinance (“Düngeverordnung”) [7]. The goal of this regulation is to prevent the overfertilization of fields and the resulting eutrophication, but it also leads to higher costs in the usage of fermentation residues, as storage capacities and, in some cases, long transportation routes for application in other areas are required. Therefore, new technologies for the usage of these materials are of high interest.

A possible solution could be the gasification of the fermentation residues, transforming them into a product gas with major amounts of H₂ and CO. This gas can be either used for the production of electricity and heat with the existing gas engines that many of these facilities already possess or in separate gas engines designed for the synthesis of gas [8]. Additionally, the produced gas could be used as feedstock in carbon-based industries or added to the biogas plant to enable biological methanation, increasing the methane yield of the overall process [9,10,11,12,13]. When the produced biogas is fed into the gas grid and not used to produce electricity, the energy to heat the fermenter is usually missing. In these cases, the necessary heat can possibly be provided by the heat of the gasification process.

While only a few approaches of gasification of fermentation residues have already been published, they mainly focus on allothermal operations, mostly on a laboratory scale. Chen et al. reported on such gasification using a laboratory-scale downdraft fixed-bed gasifier with air as the gasifying agent. The gasifier was operated allothermally, with the obtained cold gas efficiencies (CGE) achieving values of between 35.9% and 67.0%, while a major amount of tars was observed (3.34–25.8 g/m³ _i.N.) [14]. The influence of steam-to-biomass ratios as well as the equivalence ratios was investigated by Koido et al., who also relied on an allothermal process in a pressurized fixed-bed gasifier. They reported a correlation between the steam-to-biomass ratio and the cold gas efficiency, obtaining a CGE of 89.5% for a steam-to-biomass ratio of 10. It was also reported that the CH₄ content in the product gas slightly increased with increasing steam rates [15]. Wisniewski et al. and Chang et al. reported on the allothermal gasification of fermentation residues using CO₂ as the gasifying agent on a laboratory scale [16,17]. Chang et al. reported a CO₂ conversion of approximately 28% at a temperature of 950 °C [16]. While all these approaches were based on small-scale gasifiers (fuel consumption in the range of a few grams per test), C. Freda et al. reported on the gasification in a rotary kiln with a fuel throughput of up to 27.5 kg/h using air and mixtures of air and steam. Despite the greater capacity of their plant, it also relied on an allothermal process, thus depending on electrical heating [18]. A significant development can be observed in the literature by Balas et al., Elbl et al., and Milcak et al., all of whom reported on the usage of different fermentation residues in an autothermal fluidized-bed reactor (“BIOFLUID 2”). With its thermal capacity of about 150 kW, it resembles one of the larger experimental plants. Balas et al. reported on the gasification of dry and wet fermentation residues as well as wood chips using air as the gasifying agent. While the tar content of the product gas was rather high, with values of 3.9 to 5.1 g/m³ _i.N., the highest CGE reported was 78.8% for wood chips [19]. The implementation of mixtures of air and steam in this reactor was reported on by Elbl et al., who demonstrated the general possibility of gasifying sewage sludge and fermentation residues. The tar contents of the experiments were in a similar range to the ones previously reported for this plant, ranging from 3.4 to 7.5 g/m³ _i.N. [20]. A comparison of air, air and steam, and oxygen and steam as the gasifying agents was reported on by Milcak et al., who used different mixtures of wood chips and digestates. It was reported that the CGE increased for the usage of oxygen–steam mixtures as the gasifying agents, reaching values of up to 71.8%. Furthermore, the influence of the equivalence ratio was investigated, showing that an increase in the equivalence ratio resulted in a lower CGE [21].

While these approaches showed the general possibility of utilizing fermentation residues in gasification, we were unable to find suitable literature about fixed-bed gasifiers using feedstock in a nitrogen-free manner on a larger scale. It is assumed that this is due to the undesirable characteristics of fermentation residues, usually containing high amounts of moisture and ashes with low melting points. The water content of the fermentation residues depends on the process configuration and can reach values of about 70% even after mechanical dewatering. Due to this high-water content, a pre-drying of the material upstream of the gasifier is usually necessary. The ash content is of greater importance, especially when the composition is leading to low melting points (usually between 900 and 1100 °C). On the one hand, it can increase the pressure loss within fixed-bed gasifiers, limiting the gas flow, on the other hand, it can lead to sintering and slagging within the reactor, causing serious technical problems (e.g., blocking or restricting movement of the ash grate, blocking the gas outlet, etc.). Molten or sintered ashes usually have to be removed manually from the reactor after cooling down, resulting in shutdowns and high operational costs.

As previously reported, the implementation of nitrogen-free gasification using mixtures of O₂ and CO₂ has been successfully implemented for a readily available fix-bed gasifier [22]. While this setup was able to convert wet and dry wood chips with high efficiency, the implementation of more complex feedstocks such as fermentation residues remains of interest. It is assumed that fermentation residues could be used for this process, as the required energy density within the system can be adjusted by the O₂ concentration in the gasifying agent used. While the usage of this feedstock leads to operational problems in air-blown operations (increased tar formation, slagging, etc.) due to significantly lower reactor temperatures and inhomogeneities within the reactor, the implementation of O₂/CO₂ mixtures as the gasifying agent is expected to resolve this problem due to increased reactor temperatures. To the best of our knowledge, the gasification of wood chips and fermentation residues in a fixed-bed gasifier using O₂ and CO₂ is tested here for the first time. This approach could potentially provide a novel route of utilizing these residues. Furthermore, it is expected to reduce operating costs of gasification, as costly wood chips can be replaced by residues.

2. Materials and Methods

The experimental setup used in this study was identical to a previously used setup, as published in [22], while using a different type of feedstock. In the following section, a short overview is provided, highlighting the most important features.

2.1. Feedstock

The feedstock used in the experiment consisted of a mixture of dried, commercially available coniferous wood chips (size category P 45, approximately 5% water content (wet basis)) and fresh fermentation residues (approximately 70% water content (wet basis)) from a biogas plant using manure with significant amounts of straw as feedstock. The wood chips and the fermentation residues were combined in a mixture with a volumetric ratio of 1:1. Based on a rough estimate, by measuring how much feedstock passes through the gasifier double-slide lock, the bulk density of the feedstock was determined to be approximately 350 kg/m³. The ultimate and proximate analyses of the feedstock used is shown in

Table 1. Ultimate and proximate analyses of feedstock used in this study (ar: as received).

	Unit	Feedstock [ar]	Wood Chips [dry]		Fermentation Residues [dry]	Method
Water	%	40.34 ± 3.5	-		-	DIN CEN/TS 14774, DIN 51718
Ash (815 °C)	%	2.45 ± 0.05	1.15 ± 0.09		11.83 ± 0.03	DIN EN ISO 14780, DIN 51719
C	%	28.90 ± 1.57	50.00 ± 0.01		45.92 ± 0.40	DIN 51732
H	%	3.62 ± 0.17	5.44 ± 0.04		5.07 ± 0.03	DIN 51732
N	%	0.41 ± 0.001	0.23 ± 0.03		1.59 ± 0.06	DIN 51732
S	%	0.08 ± 0.0008	<0.02	-	0.34 ± 0.02	DIN CEN/TS 15289
LHV	MJ/kg	11.59 ± 0.034	19.76 ± 0.063		-	DIN CEN/TS 14918

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As illustrated in Figure 1, the feedstock mixture contained numerous fine particles originating from the fermentation residue. As these fine particles were expected to lead to a significant pressure loss in the reactor, wood chips were added to increase the gas flow within the reactor. Additionally, they increased the heating value of the mixture due to their low water content (approximately 5%), easing the usage of the fermentation residues.

2.2. Gasifier

The gasifier used was a commercially available fixed-bed gasifier (HGW200, representing the gasifier unit of the HKW 50) provided by LiPRO Energy GmbH & CO KG (26203 Wardenburg, Germany). The gasifier unit was a downdraft double-fire gasifier, relying on an autothermal process. During air-blown operation, the system is rated at 100 kW_th. The process was based on a staged gasification in which the feedstock was pyrolyzed in a screw conveyor prior to its gasification, using the residual heat of the product gas (double-pipe design). After the feedstock was pyrolyzed in the screw conveyor, it was fed into the gasifier reactor, where the pyrolysis gases were converted by the addition of the gasifying agent. The resulting gases were then passed through a coke bed where tars were cracked, and the product gas was further upgraded through Boudouard and water–gas shift reactions. The hot product gases were discharged from the reactor and fed into the double jacket of the pyrolysis screw. There, the heat from the gas was transferred to the feedstock using the countercurrent principle. A schematic illustration of the gasifier is provided in Figure 2.

The gasifying agent was mainly added at the top of the gasification reactor above the pyrolyzed fuel bed; smaller amounts were added at the grate of the reactor at the bottom. The addition of the gasifying agent at the grate was used to achieve maximum usage of the feedstock and to heat the reduction zone in order to maintain good reaction conditions. Feedstocks used were usually wood chips and roadside greens.

In order to control the gasifier temperatures, a variety of thermocouples were implemented in the process control system of the gasifier. In case of temperatures exceeding a preset limit in the oxidation or reduction zone of the gasifier, a control valve (see Figure 2) decreased the amount of gasifying agent directed toward this zone, increasing the gas flow toward the opposite zone (reduction zone). In case of both zones exceeding the temperature limits, the process control system sounded an alarm before shutting down the plant.

The volume flows of the gasifying agents added were measured using a series of flowmeters (GF Piping Systems, Schaffhausen, Switzerland, Type 335 (CO₂) and Type 123 (CO_{2 Red.} and O₂); accuracy ± 5%) for each individual stream of gasifying agent. Prior to the experiments, the flowmeter was standardized to the application medium (CO₂: 10–40 m³ _i.N./h, O₂: 5–20 m³ _i.N./h, CO_{2 Red.}: 0–5 m³ _i.N./h). Furthermore, the gas pressure was constantly measured by pressure sensors (WIKA Alexander Wiegand SE & Co. KG, Klingenberg, Germany, 0–500/0–250 mbar, CL 1.6) to convert the flow rate to standard conditions. The amount of feedstock used in the experiments was calculated by multiplying the cycles of the double-slide lock performed during the experiment by an estimated feedstock throughput for each lock movement. The estimated throughput per lock movement was verified by a series of experiments, manually weighing the amount of feedstock transported through the lock with each movement.

2.3. Characterization of Syngas

The analysis of the product gas was based on various methods and instruments, aiming to determine its composition and tar content. All measurements were conducted in the cooled product gas (approximately 50 °C) after removing particulates to reduce interferences with the measurement equipment. Permanent gases such as O₂, H₂, CO, and CH₄ were quantified using a multi-gas measurement device (VISIT 03H, Messtechnik Eheim GmbH, 74193 Schwaigern, Germany) utilizing nondispersive infrared sensors (NDIR), thermal conductivity detectors (TCD), and electrochemical sensors. To safeguard the sensors, the sample gas underwent the following preparation steps: it was first cleaned with washing oil and water, then passed through silica gel, activated carbon, and several filters. The CO₂ content, along with selected components like hydrocarbons (C₁–C₆), acid gases, and nitrogen oxides, was measured using Fourier-transform infrared spectroscopy (FTIR, CX4000, Gasmet Technologies GmbH, 76131 Karlsruhe, Germany). Anticipating that the total concentration of IR-active components would be excessively high, the sample gas was diluted with pure nitrogen (grade: 5.0). This dilution aimed to avoid total absorption—a situation where an IR-active gas absorbs all the light from the FTIR source, hindering characterization. Additionally, smaller concentration components might not be detectable in undiluted samples due to peak broadening of CO, CO₂, and H₂O at elevated concentrations caused by overlapping signals. Consequently, a volumetric dilution ratio of 1:10 (sample gas to nitrogen) was employed. The exact dilution ratio was monitored via pressure sensors in an internal diluting device. A heated sample line (180 °C) connected to a series of heated quartz wool filters facilitated sampling while protecting the device from dust or condensing tars.

To measure the volume flow of the product gas, both static and dynamic pressures (Kalinsky Sensor Elektronik GmbH & Co. KG, Erfurt, Germany, DS1/DS2) within the product gas pipe were recorded using an S-Pitot tube. These pressure readings were logged by the central process management system of the gasifier every 30 s. Using the measured gas composition, density calculations for the gas were made. This information, combined with the recorded pressures, allowed for volume flow calculations based on Bernoulli’s equation.

Additionally, the experiment was assessed for the tar content following CEN/TS 15439 (Tar Protocol) [23]. A sample port was installed downstream of the particulate removal unit in the gasifier, where a stainless-steel cannula (inner diameter: 6 mm) was placed in the main gas flow. This setup connected to a series of seven impinger bottles via a Noroprene tube (G-60-A). The first and last bottles acted as a condenser and safety bottle, respectively, while bottles two through six contained 2-propanol. Bottles one through three were maintained in a cold bath around 0 °C, while bottles four to seven were cooled to −20 °C. Gas sampling continued as long as possible (limiting factors: e.g., fluctuations in pressure, volume, etc.) at a consistent flow rate of approximately 0.5 m³/h. The liquid collected from these impinger bottles was gathered after flushing all equipment with 2-propanol, before combining it with the sample. To prevent degradation during storage until analysis via gas chromatography–mass spectrometry (GC-MS), the samples were kept in brown glass bottles sealed with PTFE at −10 °C. Analyses targeted benzene, toluene, xylenes (BTX fraction), 16-EPA-PAK compounds, and other aromatic and non-aromatic hydrocarbons. The detection limits varied between 0.05 and 0.18 mg/m³ _i.N.dry depending on the overall sample conditions. A schematic representation of measurement positions can be found in Figure 3.

3. Experimental Procedure

The experiment was carried out with a “hot” gasifier, meaning that the reactor was operated in the conventional manner using wood chips as feedstock and air as the gasifying agent for at least three hours before transitioning to the desired mixture of O₂ and CO₂. This approach ensured that the gasifier achieved stable operating conditions prior to experimentation and minimized any inhomogeneities within the reactor. The transition of the feedstock from wood chips to the mixture of wood chips and fermentation residues was conducted through operations with O₂ and CO₂ mixtures as the gasifying agent. The volume flow of the mixture of O₂ and CO₂ was then gradually increased while increasing the O₂ concentration in the gasifying agent until it reached the target setpoint. The goal was to find suitable operating conditions that allowed for stable operating conditions of the plant with the highest possible O₂ concentration in the gasifying agent. Limiting factors for the O₂ concentration included the reactor temperatures in the oxidation zone. A maximum temperature of 1150 °C was established to prevent damage to the reactor; however, efforts were made not to exceed 1100 °C to maintain operation within defined temperature limits. Once the desired operating conditions with the gasifying agent mixture were achieved, the gasifier continued running until stable conditions were confirmed. Stability criteria included constant reactor temperatures and consistent gas composition concerning permanent gases. Upon reaching stable operation, measurements commenced to evaluate operating status. The experimental conditions used are summarized in

Table 2. Experimental settings. All values refer to standard conditions and represent the gasifying agent added to the specific zone of the gasifier.

Zone	Parameter	Value	Unit
Oxidation zone	${\dot{\mathrm{V}}}_{{\mathrm{C}\mathrm{O}}_2}$	24.09	${{\mathrm{m}}^3}_{\mathrm{i}.\mathrm{N}.}/\mathrm{h}$
	${\dot{\mathrm{V}}}_{{\mathrm{O}}_2}$	11.12	${{\mathrm{m}}^3}_{\mathrm{i}.\mathrm{N}.}/\mathrm{h}$
	${\mathrm{x}}_{{\mathrm{O}}_2}$	31.6	Vol.-%
Reduction zone	${\dot{\mathrm{V}}}_{{\mathrm{C}\mathrm{O}}_2}$	2.57	${{\mathrm{m}}^3}_{\mathrm{i}.\mathrm{N}.}/\mathrm{h}$
$\sum {\mathrm{C}\mathrm{O}}_2+{\mathrm{O}}_2$	-	37.78	${{\mathrm{m}}^3}_{\mathrm{i}.\mathrm{N}.}/\mathrm{h}$
-	Oxygen–fuel equivalence ratio *	0.27	-

*: The oxygen–fuel equivalence ratio was calculated based on the feedstock composition, its throughput, and the added amount of oxygen in the corresponding gasifying agent.

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4. Results and Discussion

The increased O₂ content in the gasifying agent (31.6 Vol.-%) enabled the use of the feedstock mixture of fermentation residues and wood chips for the gasification system used in this study. It is assumed that this was due to higher temperatures within the reactor compared to air-blown operations, as more exothermic reactions can occur with a higher O₂ content in the gasifying agent. Furthermore, it is believed that the dosage of CO₂ in the gasifying agent evened out the reactor temperatures, as no slagging of the ashes was observed, despite it usually occurring in air-blown operations. Possible reasons for this can potentially include an increase in Boudouard reactions in locally occurring hot spots. This assumption is supported by literature reporting higher carbon conversion rates when introducing CO₂ as a gasifying agent [16,24,25]. The gas composition of the produced synthesis gas is displayed in Figure 4.

Due to the high water content of the feedstock used, the water content in the product gas was relatively high at 20–22 Vol.-% compared to previous experiments (8–12 Vol.-%). As a result, the hydrogen content in the product gas was approximately 19–21 Vol.-% _dry, which is within the range of previous experiments using wet fuels or the dosage of steam in the gasifying agent. In contrast to that, the amount of CO decreased to approximately 27–34 Vol.-% _dry in comparison to other experiments using similar gasifying agents (40–43 Vol.-% _dry). The CH₄ content was approximately five times higher (5–8 Vol.-% _dry) when compared to previous experiments, in which it remained at approximately 0.5–1 Vol.-% _dry. Literature-known influences on the methane content are manifold, the most prominent being the gasifier temperature [26,27], the ratio of oxygen to fuel [28], and the steam-to-fuel ratio [26,29]. It is uncertain if the increase in CH₄ compared to previous experiments is due to the high water content in the feedstock or the slightly lower reactor temperatures; nonetheless, it indicates a decrease in the cracking of hydrocarbons.

A comparison of different operating conditions for this gasifier setup is displayed in

Table 3. Comparison of different operating conditions for the gasifier system used in this study.

	Unit	This Study	Reference	Standard Operations
Feedstock	-	Wood chips + fermentation residues	Wet wood chips	Wood chips
Gasifying agent	-	O2 + CO2	O2 + CO2	Air
H2	Vol.-% dry	19–21	19–21	18–23
CO		27–34	40–43	24–26
CO2		30–34	35–40	24–26
CH4		5–8	0.5–1	0–2
H2O	Vol.-%	20–22	8–12	8–15
Tars	mg/m3 i.N.dry	2850	350	100–200
LHV	MJ/m3 i.N.dry	8.5	7–8	5.5

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The tar content measurements are displayed in Figure 5, showing significantly higher amounts of tars, at approximately 2850 mg/m³ _i.N.dry, when compared to air-blown operations (approximately 100–200 mg/m³ i_.N.dry) or similar experiments using dry wood chips as fuel (approximately 350 mg/m³ _i.N.dry). Nonetheless, 93.9% of the tars detected were benzene, toluene, ethylbenzene, and xylenes, representing a group of more volatile tars.

The heating value of the gas was calculated (including tar content) at 8.5 MJ/m³ _i.N.dry, which is significantly higher in comparison to that of air-blown operations (approximately 5.5 MJ/m³ _i.N.dry). This is due to the increased amounts of CO and CH₄ in the produced gas.

When focusing on the main reactor temperatures, as displayed in Figure 6, the temperatures show a steady behavior, indicating stable operating conditions. While smaller fluctuations in temperature can be observed, they can be explained by the process behavior (movement of the grate, addition of feedstock, etc.). The overall behavior is very similar to that observed during air-blown operations, with only minor deviations.

As the feedstock fed into the reactor could not be weighed during the experiments, it was estimated that approximately 8 kg was fed into the reactor with each movement of the double-slide lock of the gasifier. This assumption was based on previous measurements weighing the amount of feedstock added with each movement of the double-slide lock, but the actual amount may vary due to changes in the density of the feedstock used. Based on this assumption, the cold gas efficiency of the gasifier was calculated to be 94% (average value over the test period, approximately 100 min), a value significantly higher compared to that of air-blown operations (82–84%), yet in the range of similar experiments conducted by the authors using mixtures of O₂ and CO₂ as the gasifying agent (83.5–95.5%) using this gasification system. While this value is subject to certain errors (feedstock throughput, gas volume flow, gas composition, etc.), it serves as a good orientation for classifying the experiment and is, furthermore, within the range of values reported in the literature, ranging from 85% (fixed-bed gasifier using O₂/H₂O) [30] to 99% (allothermal fluidized-bed using O₂/CO₂) [31].

The content of nitrous oxides, ammonia, and sulfur dioxide in the product gas was measured by FTIR, as described above, and is displayed in Figure 7. While the use of wood chips, as in previous experiments, only resulted in minor amounts of nitrous oxides (approximately 10 mg/m³ _i.N.dry), ammonia (approximately 20–60 mg/m³ _i.N.dry), and sulfur dioxide (approximately 100–200 mg/m³ _i.N.dry), the concentration of these substances significantly increased with the use of feedstock in this experiment. This is due to higher concentrations of nitrogen and sulfur within the feedstock, originating in the fermentation residues used.

5. Conclusions

To the best of our knowledge, the gasification of fermentation residues and wood chips in a small-scale gasifier was successfully implemented for the first time using mixtures of O₂ and CO₂ as the gasifying agent. During and after the experiments, no technical problems were observed. The cold gas efficiency was high, at approximately 94%, and the heating value of the gas produced was 8.5 MJ/m³ _i.N.dry. A minor drawback was the comparably high tar content within the product gas (approximately 2850 mg/m³ _i.N.dry). While this amount is not expected to cause any problems when focusing on a thermal usage of the gas (for example, in gas engines or direct combustion), as it mainly consisted of volatile tars, it might be challenging for biological or material applications. The implementation of gas-cleaning technologies might be necessary for these applications. While the usage of this feedstock in conventional, air-blown operations leads to significant operational problems (e.g., slagging), which typically prevent the utilization of such materials, these problems were not observed during operations using O₂/CO₂ mixtures as the gasifying agent. It is assumed that this is due to the moderating effects of the CO₂ concentration, reducing temperatures in the coke bed due to heat consumption through suspected Boudouard reactions, and, as a consequence, preventing local temperature peaks that lead to the sintering of the ashes. While the results obtained in this study are the first of their kind, they seem to enable the usage of complex fuels in fixed-bed gasification. Nonetheless, verification of the results and additional experiments using other complex fuels are necessary to fully understand the impact of changing the gasifying agent.