3. Prospects of Hydrogen Production from Biomass in Scale
To assess the previously mentioned processes, it is a necessity to examine the type of feedstock and pre-treatment that may be needed, the production materials/energy requirement and the purity of the gas output.
There are two forms of feedstock that can be utilized to provide the required amount of biomass to create hydrogen: lignocellulosic residue and dedicated crops. Because they are produced towards the end of the harvest season for crops like cereal wheat or during the transformation process, lignocellulosic waste is easily accessible and inexpensive. Dedicated energy from crops like sorghum, however, demands land utilization and growth time, which might be difficult. Clustering biomass kinds based on their chemical components (carbohydrates, lignin, and other components) can make the necessary pre-treatment easier [
10,
73].
Comparing the biological processes of biomass, dark fermentation seems to be the most competitive when considering scale-up possibilities. It manifests an efficiency of 60-80% [
12], yield between 0.004 - 0.044 kgH
2/kg biomass [
10] and has a TRL of 5 [
9,
10,
66]. It is a simple method that can produce H
2 without light, contributing to waste recycling. It is also CO
2-neutral and has no O
2 limitation [
12]. The main disadvantages of this process are fatty acids removal, low H
2 rates and yields, low conversion efficiency, and the requirement of large reactor volume [
12]. In addition, dark fermentation is limited by poor catalyst durability and product contaminants [
62]. The production cost is around 0.332 – 2.63 €/kgH
2 [
10,
66] but the numbers are indicative because there are few full plant cost estimations [
61]. There are no commercial scaled plants yet. Also, feedstock pretreatment is significantly expensive in order to be enzymatically fermentable. Thus, the industrial scale of the process development is limited [
10].
Photo fermentation is also a CO
2-neutral process that contributes to waste recycling and can use different organic wastes and wastewaters [
12] but, the light conversion efficiency is only 1–5% [
11], the production cost is 0.362 – 3.66 €/kgH
2 [
10,
66,
69] and the possibly achieved yield is 0.004 - 0.049 kgH
2/kg biomass [
9]. The method also requires sunlight, large reactor volume and is sensitive to O
2 and has a TRL of 4 [
9,
10,
12,
66]. Problems that might occur with industrial effluents being used for H
2 production are the color of the wastewaters, which will prevent light penetration, and heavy metals or other toxic compounds that might be carried in the wastewaters [
12,
22]. Lastly, there are difficulties in controlling the various bacteria [
10]. Compared to dark fermentation, it is thought to be less financially competitive because it achieves approximately the same yield while requiring higher production cost [
63].
The last biological process, biocatalyzed electrolysis, has an electrical requirement that remains far below that of commercial water electrolysis. Even when microbial metabolic and other energy losses increase the energy demand, it is expected to remain below 1 kWh/m
3 H
2. Biological anodes can easily be operated under non-sterile conditions because electrochemically active consortia can be naturally selected from a wide range of inocula. This process can theoretically produce hydrogen as a pure gas in the cathode chamber instead of a mixed gas output [
35]. The most significant barrier to these techniques in terms of viability is the low hydrogen production rate [
72]. In addition, enzymatic biocathodes are relatively unstable and they are not self-regenerating [
35]. There has not been thorough recent research on this method, especially from a techno-economic standpoint. Another challenge that this method faces is that the lignocellulosic biomass, a plentiful natural resource, cannot be directly processed by the microorganisms in the microbial electrolysis cell. In order to transform it into monosaccharides or low-molecular-weight molecules, it must first undergo fermentation [
77]. Therefore, biocatalyzed electrolysis emerges as a supplementary technology to fermentative processes. Their combination allows the recovery of the energy content in the substrate up to 90% [
36,
67].
Biomass electrolysis offers higher hydrogen production efficiency and lower ΔE
eq than water electrolysis because the oxidation of the biomass-derived material has lower thermodynamic requirements. The production cost when applying the current density range of 0.2-1.0 A/cm
2 in biomass-based organic molecules is approximately 8 - 10 €/kgH
2, but it can be reduced significantly considering the value-added chemical(s) that are co-produced [
55]. This process’s disadvantage is its slow kinetics due to the numerous electron transfer mechanisms [
56]. Electro-oxidation has a TRL of 2-4 [
10] and there has not been extended recent research on this method.
For thermochemical processes, the industrial design has already been defined [
10]. The approach was developed using comparable techniques used with biofuels like biomethane and adapted from steam methane reforming (SMR) [
38]. Gasification has an efficiency up to 50 %, a TRL of 7, production cost between 1.14 – 3.29 €/kgH
2 and produces the highest yields [
10,
66,
67,
68]. Steam gasification has a 40% H
2 percentage in the gas, higher H
2/CO ratio (1.6), reduced impurities compared to air gasification and produces 0.040 kg H
2/kg biomass without catalyst and 0.070 kgH
2/kg biomass with catalyst [
29,
76]. This process is thought to be the most suitable process for hydrogen production. Concerning the feedstock, steam gasification is feasible for wet biomass (moisture from 5 to 35 wt%), while air gasification requires a dry raw material [
37]. However, because of the production of tar and char, the processes are more vulnerable to catalyst deactivation and the gas products need to be separated and purified [
10,
76]. It was also seen that the gasification process was economically non-viable due to ash-related issues such as corrosion, erosion, agglomeration, and sintering [
2,
40].
Pyrolysis has shown efficiency up to 65% using HDPE as feedstock, yield 0.100 kg H
2/kg biomass & HDPE and 0.373 kg H
2/kg HDPE [
9] and TRL of 7 [
10,
66]. The production cost of this process is around 1.14 – 2.41 €/kg H
2 [
10,
66,
69]. Studies show that at the same temperature, fast pyrolysis of biomass releases more volatiles than slow pyrolysis [
53]. It has been observed that the presence of a catalyst raised H
2 gas yield while reducing CO, C2–C4, and CH
4 yield [
2]. On the contrary, because of the production of tar and char, the processes are more vulnerable to catalyst deactivation [
10]. There is less data for biomass-to-hydrogen yields and production costs in the scientific literature on techno-economic analyses of pyrolysis than of gasification. The gasification and pyrolysis of biomass use similar procedures to those used to treat fossil fuels. They are projected to develop and reach a TRL of up to 9 in the following two decades [
10]. However, to produce negative emissions, thermochemical processes that release CO
2 must be combined with carbon capture systems (CCS) [
10].
Comparing biogas from landfill and anaerobic digestion of biomass the performing efficiency is slightly higher when it comes to biogas from anaerobic digestion. When undergoing SR and ATR at 20 bar it is shown to achieve a maximum of 51.7% and 27.8%, respectively [
92]. As the temperature of the reforming process increases, so does the yield of H
2. It reaches a peak before slightly decreasing [
103]. Although this approach produces significant hydrogen yields, achieving the required high purity hydrogen, it involves complicated energy integration, costly heat exchangers (high temperature), and numerous process units. The amount of distinct process stages affects system efficiency, making scaling down uneconomical [
87]. The approximate production cost is between 4.21-4.29 €/ kg H
2 for SR and 6.41-6.60 €/ kg H
2 for ATR, when both processes are performed at 20 bar using biogas from anaerobic digestor and landfill respectively. As far as TRL, SMR reach a TRL of 9, and ATR reach a TRL of 8 when processing natural gas [
105]. The process’ model generates 0.29 kg H
2 per kilogram of bio-methane [
113,
116].
Antonini et al [
113], have conducted research that compares natural gas to biomethane and investigates CCS and storage systems. Results show that natural gas and biowaste-based biomethane have minor differences in performance when undergoing SR and ATR, despite their different composition. However, addressing the life cycle impacts on climate change, biomethane is clearly more sustainable as the CO
2 that is released from biomethane does not contribute to the carbon cycle in contrast with the contribution of natural gas to GHG.
The following table,
Table 4, displays a summary of the most important features of each process from various literature references which were mentioned in the previous paragraphs. The table highlights each methods efficiency (%), yield (kg H
2/kg biomass), production cost (€/ kg H
2) and TRL. In order to compare them, the necessary unit conversions were made. In addition, data from water electrolysis are shown in
Table 3 to compare the previously mentioned methods of green hydrogen production to water electrolysis, another method of green hydrogen production. The most advanced methods of water electrolysis are alkaline electrolysis (AEL), proton-exchange membrane electrolysis (PEMEL), solid oxide electrolysis (SOEL) [
106].
The Figures (
Figure 2,
Figure 3,
Figure 4) that are following are based in
Table 2. The two colors on the bars represent the range of the values that are mentioned on
Table 2, with orange depicting the minimum value and yellow the maximum value.
In
Figure 2, the comparison of the method’s yield is presented. The chart clearly shows the advantages of thermochemical methods in aspects of yield. This conclusion is justified because of the various and extended research and implementations of these technologies in fields other than hydrogen production. Biocatalyzed electrolysis has a lead among the biological methods, even though their performance does not differ much. In general, two obstacles are preventing biological processes from evolving and expanding on a global scale. Even while dark fermentation has made biological methods more competitive, the H
2 yield and production pace are significantly lower compared to thermochemical methods [
10,
71]. Lastly, pilot-scale processes are limited by the need for pre-treatment throughout the synthesis of complex biomass [
10].
Figure 3 was created by comparing the cost of each method. This chart also indicates that the thermochemical methods are quite advantageous but not far more inviting than the fermentation processes in terms of production cost. Electrooxidation is more expensive than the previously mentioned methods but not as unviable as biocatalyzed electrolysis, which is far more expensive than any other method even if the data we have are from laboratory materials. Biogas reforming methods are costly compared to the other thermochemical and biological methods but have similar cost to water electrolysis. In general, we can conclude that thermochemical and biological methods are the most advantageous financially.
In
Figure 4, the TRL of each method is depicted. The purpose of this figure is to compare the TRL of hydrogen production methods based on biomass to other established methods such as SR and ATR of natural gas and water electrolysis via PEM and AE. It is clearly shown through
Figure 4 that the methods that are competitive against the dominant methods of SR and water electrolysis are gasification and pyrolysis. Biological and electrochemical methods do not present TRL above 5, which means that they are still under development while thermochemical methods are in a state of pre-commercial demonstration.
Thermochemical methods have shown an advantage both in yield and TRL aspects. However, the CO
2 emissions intensity and the CCS incorporation need to be taken into account to assess these methods’ potential to meet our goal to mitigate GHG emissions. The life cycle assessment conducted by Antonini et al [
113] which was previously mentioned, also indicates that the CO
2 emissions intensity with CCS for hydrogen production from bio-methane is a net-negative CO
2 emissions process. ATR performs higher CO
2 capture than SMR and the addition of a low-temperature WGS is generally improving the life cycle performance. It is safe to conclude that in general, adding CCS leads to clear benefits considering the implications of climate change. However, results manifest that these methods perform worse with, than without, CCS because the CCS integration is increasing the energy demand and consumption, which leads to other environmental pressures.
Table 5 [
116] displays the CO
2 emission that each thermochemical method produces when used for hydrogen production. In addition,
Figure 5 is presented based on
Table 5.
From
Table 5 [
116], we conclude that in every case, the TRL is lower when the process is combined with CCS. Nevertheless, biomass gasification and bio-methane SMR show significant results when CCS is included in the system. Comparing these two methods, as the two most inviting methods of green hydrogen production, with their non-green equivalent it is shown that SMR with CCS from fossil methane and SMR with CCS from bio-methane present the same TRL but a significant difference in CO
2 emissions. The latter demonstrates net-negative CO
2 emissions which is a very important factor considering a net zero economy that requires carbon absorption and negative emissions from the atmosphere by 2030.