A novel Upflow Anaerobic Solid State (UASS) reactor was used for biogas production from fibrous feedstock. Digestate from UASS reactor was hydrothermally carbonized into HTC biochar in a pressurized batch reactor. The integration of the two bioenergy concepts was applied in this study to increase overall bioenergy production.
Lignocellulosic biomass is one of the most abundant yet underutilized renewable energy resources. Both anaerobic digestion (AD) and hydrothermal carbonization (HTC) are promising technologies for bioenergy production from biomass in terms of biogas and HTC biochar, respectively. In this study, the combination of AD and HTC is proposed to increase overall bioenergy production. Wheat straw was anaerobically digested in a novel upflow anaerobic solid state reactor (UASS) in both mesophilic (37 °C) and thermophilic (55 °C) conditions. Wet digested from thermophilic AD was hydrothermally carbonized at 230 °C for 6 hr for HTC biochar production. At thermophilic temperature, the UASS system yields an average of 165 LCH4/kgVS (VS: volatile solids) and 121 L CH4/kgVS at mesophilic AD over the continuous operation of 200 days. Meanwhile, 43.4 g of HTC biochar with 29.6 MJ/kgdry_biochar was obtained from HTC of 1 kg digestate (dry basis) from mesophilic AD. The combination of AD and HTC, in this particular set of experiment yield 13.2 MJ of energy per 1 kg of dry wheat straw, which is at least 20% higher than HTC alone and 60.2% higher than AD only.
Finding renewable and sustainable energy sources are major concerns in the world’s energy sector. Recently, the United Nations reported that up to 77% of the world’s energy in 2050 will be expected from renewable sources1. Lignocellulosic biomass such as straw, grasses, rice hulls, corn cobs have no conflicts with the food versus fuel issue. Moreover, biomass is probably the only renewable energy source with structural carbon, compared to other renewable energy sources such as wind, solar, and water2. However, handling characteristics, lower bulk density, high ash content, and lower energy content hinder the use of lignocellulosic biomass for energy production2.
Anaerobic digestion (AD) is one of the prime examples of producing bioenergy from waste biomass.3 In general, there are four degradation steps involve in anaerobic digestion as shown in Figure 14. In first three consecutive steps, the polysaccharides of the biomass are converted into organic acids. In the final step, methanogenic organisms produce biomethane. Traditional AD is a time and energy consuming process. The continuous stirring reduces overall economics of AD, especially for AD of lignocellulosic biomass. A novel upflow anaerobic solid-state (UASS) reactor has the potential to overcome the stated shortcomings (Figure 2)4. Spontaneous solid-liquid separations is one of the significant advantages of UASS, since the designed facilitates biogas bubbles to lift unreacted solid residues upwards5. This eliminate the use of stirrer and therefore reduces the consumption of on-site power. Moreover, liquid circulation ensures distribution of microorganisms and metabolites throughout the reactor as well5. Compared to solid biofuels, biogas is easier to handle, and leaves little or no residue. In fact, the specific energy density of biogas is several times higher raw biomass4. However, AD favors simple polysaccharides like starch, fatty acids, and hemicellulose1. As a result, cellulose, and lignin, major portion of fibrous lignocellulosic biomass like wheat straw, remains as a solid digestate after AD5. Although, biogas production varies from the feedstock, type of microorganisms, reaction temperature, and reaction time, a huge amount of digestate is usually produced.
While biogas is used for energy, digestates (up to 90% water) are usually stored in a fermentation residue-depot to collect remaining methane emissions. Afterwards these are dried and spread on the cropland to improve soil fertility and water retention capacity. High inorganic content often hinder digestate directly for fuel, as high amounts of slag might corrode the equipment6. Hydrothermal carbonization (HTC) is a thermochemical treatment process especially designed for wet feedstock, where biomass (with 80-90% water) is heated up to 200-260 °C at water saturation pressure and hold for 0.5-6 h (Figure 3)7,8.Subcritical water has the maximum ionic product at 200-260 °C, which means water under these conditions is reactive and behaves as a mild acid and a mild base simultaneously9. Hemicellulose, along with other extractives, degrade around 180-200 °C, while cellulose reacts around 220-230 °C, and lignin reacts at relatively higher temperature (>250 oC), but much slower than cellulose and hemicellulose10. Due to significant dehydration and decarboxylation, HTC results solid product named HTC biochar, with mass yield (dry HTC biochar/dry feed) of 40-80%, liquor containing carboxylic acids, furan derivatives, phenolic substances, and sugar monomers, and 5-10% CO2 rich gaseous product11. During HTC, oxygen containing volatile substances are significantly reduced and thus leave a carbon-rich solid. HTC biochar is also stable, hydrophobic, and friable compare to raw moist feedstock12,13. Due to its hydrophobic characteristics, dewateribility of HTC biochar increases several times compared to raw digestate or even raw biomass.14-18 Moreover, HTC biochar has fuel values similar to lignite coal16,17. However, cellulose and lignin partially degrade in the HTC environment18.
Now hemicellulose and cellulose in the biomass contribute to biogas during AD, while cellulose and lignin mostly contribute to solid HTC biochar4,5. Thus, the combination of AD-HTC can potentially increase overall bioenergy yield. Hoffmann et al. simulated a similar combination but using AD and HTL (hydrothermal liquefaction) rather than AD-HTC19. HTL is a common method of liquefying biomass fraction and liquid product has high fuel value [43.1 MJ/kg]. However, HTL requires very high pressure (250 bar) compare to HTC (10-50 bar), which implies a high installation and operation costs than HTC. Again, the combination sequence of AD and HTC can be questioned as Wirth et al. recently reported AD of HTC process liquid20. However, an effective AD depends on the sugar concentration in the feedstocks. Sugars in HTC process liquid, produced during hydrolysis, often degrade rapidly under subcritical water. That’s why AD before HTC is more favorable in terms of bioenergy. However, AD of HTC process liquid can produce additional bioenergy, in which case, the combination sequence would be AD-HTC-AD.
The aim of the work was to evaluate the integration of AD and HTC processes for bioenergy production (Figure 3). The biogas production potential for thermophilic and mesophilic AD from UASS reactor was evaluated at a continuous operation of more than 200 days. Subsequently, HTC biochar production from digestate was also studied. The mass and energy balance of the cascaded AD-HTC was carried out and compared with the individual processes.
UASS reactors are capable to mitigate the shortcomings discussed in introduction. However, there is much room for improvement. Feeding system and digestate withdrawing are still manual. The UASS system faces problems handling feedstocks larger than 60 mm. The system works better with fibrous feedstocks as they float across the liquid, but other feedstocks like animal manure and sludge might not favor the UASS system. The UASS system is designed in such a way that the process liquor circulates from reactor to AF to the reactor again. However, even 2-5% solid in the circulating liquid was proven to be problematic, as they deposit in the AF or block the pipe entrance and hinder liquid circulation. Chemical analysis of the process liquid is important, as the production of free fatty acid and nitrogen can change the microbial system resulting in uncharacteristic biogas production. The UASS system is robust, and can run more than 200 days without showing any significant problems. The tubes connecting from pumps to reactors to AFs need to be replaced every alternative month. The water level in the waterbath needs to be checked on a weekly basis and refilled if necessary.
HTC of wet digestate is very effective for waste treatment as well as producing solid biofuel. The dewateribility of the solid product will also be facilitated by the HTC process as shown in the Figure 7. However, HTC of digestate needs to be performed as soon as possible, preferably the same day that the digestate is removed. Otherwise, the digestate starts degrading biologically, which is not favorable for HTC. As HTC is a high temperature (200-260 °C) and high pressure (20-50 bar) process, taking necessary precautions throughout the HTC procedure is very important. All the connections are checked at least once a month to make sure they are gas-tight. HTC process liquid has a higher concentration of furfural, 5-HMF, and phenolic compounds, which are rated as toxicants. So, it is recommended to use a face mask and gloves while handling HTC process liquid, especially when HTC process liquor is drained from reactor vessel to another container. Although HTC has many advantages for handling wet feedstock like digestate, it is still a batch process. In an economic evaluation, HTC batch process will be hard to justify. More research is thus required to facilitate continuous operation of HTC.
Elemental analysis is an effective method for homogeneous solid substrates, but not for heterogeneous substrates. As solid biofuel is usually heterogeneous and elemental analyzer only allows 5-10 mg of sample size, it is recommended to perform at least three replicates and use average. Another limitation of elemental analysis is measuring solid substrates with high ash content. Elemental analyzers only measure CHONS, and no other inorganics. So, elemental analysis of high ash solid substrates might not reveal the actual CHONS concentrations. Sample preparation in elemental analysis is vital, as sample needs to be wrapped precisely, otherwise, there will be an inconsistency in analyses. Fuel value of the solid fuel can be estimated from CHONS, but it is recommended to use a bomb calorimeter for precise fuel value determination.
About 92-161 L of methane was produced per kilogram of volatile solid in the feed. The volatile solid or organic total solid of the dry wheat straw was 86.9%. Dry digestate has lower atomic oxygen and hydrogen concentration, which is another indication of degradation of polysaccharides and simple sugar degradation during anaerobic digestion22,23. Moreover, lower H, and O concentrations increase the HHV of the digestate24. HHV of dry digestate is 22% higher than dry raw feedstock. The similar results are obtained with a detailed statistical analysis by Pohl et al23.
Digestates from anaerobic digestion contains 80-90% water6. These are hydrophilic and water is partially bound in microbial or plant cells. As a result dewatering or drying of digestates is cumbersome and very energy intensive. For example, 2 kg of dry digestate binds 8 kg of water (80% wet), which requires 20.7 MJ of heat to dry digestate. Moreover, it tends to bio-degrade relatively quickly in ambient conditions, loses plant nutrients, and releases GHG (greenhouse gas) emissions such as N2O and CH4. So, despite higher energy potential, fresh digestate cannot be used directly as a solid fuel. It would need to be dried right after the digestion20.
From Table 1, it can be shown that the dry digestate has a similar atomic carbon content as raw straw, and they are visually similar before and after anaerobic digestion (Figure 6). This suggests that lignin and lignin-encrusted cellulose are mostly unreacted. However, a mass yield of 63% observed, meaning processed straw is 37% lighter than dry raw straw. Similar elemental carbon concentration means no carbonization occurred during anaerobic digestion22. As shown in Figure 7, HTC biochar from digestate (thermophilic) is very stable and soft. Because of the significant increase in hydrophobicity, it can literally submerge into water for months without its physical and chemical structure being affected12,25. The hydrophobicity also enhances dewatering of HTC biochar14. Structure of the straw is not discernable in the HTC biochar anymore, which means that cellulose might have been reacted. A significant carbonization is observed in HTC biochar along with the reduction of atomic oxygen. This is another indication of cellulose being reacted rather than lignin. Atomic carbon concentration in lignin is much higher than that of cellulose24-29. As a result, HTC biochar has an HHV of 29.6 MJ/kg, which are 61% higher than raw straw and 32% higher than dry digestate, respectively.
HHV of HTC processed straw is 28.8 MJ/kg, which is also similar to that of HTC processed straw digestate (29.6 MJ/kg). However, mass yield is 40.7% higher in HTC straw than that of HTC digestate with comparing to raw feedstock. As a result, if 1 kg of raw straw (18.4 MJ) is hydrothermally carbonized, HTC straw biochar will have the potential of 11.0 MJ. Otherwise, if same amount is applied to AD and HTC, a total 13.2 MJ bioenergy, in forms of biomethane (5.2 MJ) and HTC biochar from digestate (8.0 MJ), can be produced (Figure 8). Also, liquid phase of the UASS process is a potential liquid fertilizer. Moreover, HTC biochar might have higher potential on high value material use or use as soil amendment. For the carbon sequestration or carbon cycle point of view, material use of HTC biochar is more feasible that energy production.
Anaerobic digestion combined with hydrothermal carbonization can yield more bioenergy than the individual processes. However, a cascaded design is needed for better efficiency. The overall energy balance, followed by an economic evaluation, is required to validate this process. Future research should include use of HTC liquor and post-treatment (chemical or biological) of HTC biochar. Also, automation of both UASS and HTC systems will be needed. This study was carried on using a lab-scale UASS and HTC reactor, but scale-up of the process would be necessary if the process is to be commercialized.
The authors have nothing to disclose.
This research was supported by the German Federal Ministry of Research and Education to Project Management Julich (PtJ). The authors thank Mr. Ulf Lüder, for technical support in the biochar laboratory. The authors are also thankful to Ms. Maria Sanchez, and Mr. Jonas Nekat for their volunteer activities in the biogas, and analytical laboratory, respectively. Marcel Schmidt and Antje Schmidt are also acknowledged for their valuable efforts on videography and editing.
Name of Material/ Equipment | Company | Catalog Number | Comments/Description |
UASS reactor | Patented design | ||
Weighing machine | KERN | 440-55N | 0.2 g precision |
Biofilm carrier | RVT Process Equipment GmbH, Germany | Bioflow 40 | Establish 305 m2/m3 |
Heating bath | Lauda-Konigshofen, Germany | Lauda Ecoline 011 | Ensure mesophilic and thermophilic temperature |
Recirculation pump | Heidolph pumpdrive | 5201 | |
Wheat straw | Dittmannsdorfer Milch GmbH, Germany | 5-65 mm length | |
Biogas analyzer | Pronova, Germany | SSM 6000 | |
Gas meter | Ritter, Germany | Drum type | |
Process parameters | Mettler, Toledo, USA | InPro 4260 | Online |
HTC reactor | Parr instrument, Moline, IL, USA | Parr 4555 | 5 gallon volume |
HTC Temperature controller | Parr instrument, IL, USA | 4848 | K type thermocouple |
Weighing machine | KERN FKB | 0.1g precision | |
Heating system | Parr | A1600EEE | Band heater, 2 °C min-1 |
Software | SpecView | 32849 | Digital monitoring and programming interface |
Catalyst | Tungsten (VI) oxide | Elemental analyzer | |
Weighing machine | Mettler Toledo | SN-1128123281 | Precision 1 µg |
Sample pan | Elemental Analyssystem GmbH | Tin (Sn) 6x6x12 mm pan | Elemental analysis |
Drying oven | Binder GmbH, Germany | FP 115 | 105 oC oven |
Elemental analyzer | Vario | EL III | CHNS analyzer |