Energy efficiency of hydrogen production via gasification from lignocellulosic residual biomass blend

Facing the challenges of environmental sustainability and energy security caused by anthropogenic carbon emissions, there is a need to adopt cleaner energy generation technologies, leveraging Colombia's existing national resources. In this context, hydrogen emerges as a promising source of renewable energy. Therefore, this project explores the use of a blend of residual lignocellulosic biomass as raw material for hydrogen production through gasification for energy purposes

Brazilian Journal of Animal and Environmental Research, Curitiba, v.7, n.2, p. 1-14, 2024 concentration were: gasification temperature of 707°C, oxygen flows of 484 kg/h, steam at 420 kg/h, and gasification pressure of 1 atm.These findings support the potential of the studied lignocellulosic biomass blend as an alternative for hydrogen production, while also offering an opportunity for the valorization of lignocellulosic residual biomass.

INTRODUCTION
In Colombia, the energy landscape has historically relied heavily on hydroelectric and thermal sources, which collectively account for 68% and 23% of energy production, respectively, as documented by (Zapata et al., 2022).However, there has been a noticeable recent trend toward diversifying this energy mix.The inclusion of renewable energies such as wind and bioenergy has seen their contribution to the national energy portfolio rise from a mere 1% to 3%, driven by a commitment to sustainable development goals (Ángel-Sanint et al., 2023).Nonetheless, this transition raises concerns about the ongoing significant greenhouse gas emissions, primarily due to the continued reliance on hydroelectric and fossil fuel facilities.Furthermore, the escalating energy demand in urban and rural areas, particularly in Santander department, where electricity access is constrained and costly due to population growth (Montalvo-Navarrete; Lasso-Palacios, 2024), underscores the urgent need for broader and more sustainable energy solutions.Hence, there is a critical requirement to reinforce the Regional Transmission System, expanding it with projects that prioritize sustainability and adhere to environmental regulations.Within this framework, the integration of renewable energy sources has emerged as a solution, serving as a pivotal ally in mitigating the environmental impacts associated with conventional energy generation methods, while advancing sustainable development objectives and diversifying the country's energy matrix.Non-Conventional Renewable Energy Sources (NCREs) offer several advantages across economic, social, and environmental dimensions, including CO2 reduction, diminished air pollution, reduced energy dependency, avoidance of invasive extraction practices, and extending electricity access to remote areas (Sagastume Gutiérrez et al., 2020).Among these, hydrogen production stands out as a highly promising avenue, with the National Planning Department of Colombia (DNP) projecting it to be the primary driver of the energy transition, potentially reducing carbon emissions by 25% by 2030 (MME, 2021).
Hydrogen production can draw from various resources, including biomass, which is abundant in Colombia given its agricultural and livestock activities (Calvo-Saad et al., 2023).
Notably, producing hydrogen from residual lignocellulosic biomass presents an opportunity to valorize substantial amounts of waste.For instance, the Atlas of Residual Biomass of Colombia indicates an annual production of nearly 72 million tons of vegetable waste, with Santander department contributing significantly, highlighting the energy potential of these residues (UPME, 2011).Utilizing such biomass not only addresses waste management challenges but also mitigates water contamination risks associated with their decomposition (Vargas-Corredor;Pérez-Pérez, 2018).
Therefore, the project aims to study a blend of residual lignocellulosic biomass for hydrogen Brazilian Journal of Animal and Environmental Research, Curitiba, v.7, n.2, p. 1-14, 2024 production through the thermochemical conversion technology of gasification in a prospective manner.For this reason, initially, the energy efficiency of the hydrogen present in the synthesis gas will be evaluated using a simulation of the process in the Aspen Plus® software.This presents an opportunity to revalue lignocellulosic waste and reduce the environmental impacts associated with fossil fuels, as recycling carbon dioxide and using vegetable biomass as raw material achieves low net greenhouse gas emissions, contributing to Colombia's commitment to carbon neutrality before the OECD and to the sustainable development goals set by the UN (Davies;saygin, 2023).

Biomass selection based on availability volume
A review of literature was undertaken to identify crops with significant potential for generating residual lignocellulosic biomass in Santander, Colombia.The study relied on statistical data sourced from AGRONET, the Colombian Ministry of Agriculture's information and communication network for the agricultural sector, which only covered information up to the year 2021.In light of the absence of more recent data, projections for the years 2022 and 2023 were extrapolated.This extrapolation was grounded in the Compound Annual Growth Rate observed in the available data.Subsequently, crop-specific residue factors were applied to identify the most suitable sources for gasification simulation using Aspen Plus Dynamics V.11® software.
Ultimately, a 50:50 blend of sugarcane bagasse and cocoa pod husk was selected for its favorable availability and physicochemical properties, both of which are essential for efficient gasification.

Proximate and Ultimate Analysis of the Biomass blend for Simulation
To accurately determine the proximate and ultimate analysis values of the selected biomasses, these parameters were estimated on a dry basis.This involved conducting material balances with a calculation base of 100 kg for each biomass type.The proximate and ultimate analyses were constructed based on cellulose, hemicellulose, and lignin analyses reported in a previous study conducted by (Wu et al., 2013)

Model Development and Assumptions
For the simulation in Aspen Plus®, the Peng-Robinson thermodynamic model with Boston Matías Modification (PR-BM) was selected.Additionally, the developed model is based on Gibbs free energy minimization.As gasifying agents, oxygen and steam were chosen.The following assumptions were made: It was presumed that all the sulfur present in the biomasses transforms into hydrogen sulfide.It was also assumed that all the fixed carbon present in the biomasses is gasifiable.

Aspen plus model flowsheet
The simulation was carried out in four main phases, aiming to accurately replicate the complex physical and chemical processes involved in biomass gasification.These stages are illustrated in figure 1, which depicts the flow diagram of the simulation in Aspen Plus®.The simulation is described as follows: Phase 1 -Biomass Drying: begins with two types of lignocellulosic residual biomass, sugarcane bagasse and cocoa pod husk.Both are introduced as independent inputs and are incorporated in equal flows of 1000 kg/hr to later mix and obtain a homogeneous composition.This material undergoes a drying procedure, using hot air as the agent.The hot air is mixed with the biomass to remove moisture, thus obtaining dry biomass.The resulting stream is separated into two phases: 1) dry biomass and 2) exhaust air stream containing the moisture removed during the process.
It is worth mentioning that, in reality, these blocks in the simulation represent a dryer, possibly a rotary dryer, depending on the biomass particle size.
Phase 2 -Pyrolysis and Gasification: once the biomass is dried, it undergoes a thermal decomposition process at 1 atm and 707°C, known as pyrolysis.In this stage, the unconventional components derived from the proximate and ultimate analyses are transformed into conventional components, which can be managed by the Aspen Plus database.During this step, the sulfur present in the stream is separated from the remaining flow and converted into H2S through the reaction S + H2 → H2S.This component is separated because it does not participate in the chemical equilibrium that occurs in the gasifier, so it is removed to prevent it from disturbing this equilibrium.Next, the remaining flow is gasified in a RGIBBS gasifier, using 484 kg/h of oxygen at 25°C and 420 kg/h of steam at 200°C as gasifying agents.
Phase 3 -Ash Removal and Reintegration of H2S: after the gasification process, the ash is separated from the gaseous products.And the H2S produced from the sulfur that had been separated in the previous stage is reintegrated with the gaseous products.This reintegration is necessary to complete the composition of the synthesis gas.It also helps promote methane reforming, prevents coke formation, and favors hydrogen production.
Phase 4 -Syngas Cooling: finally, since the produced synthesis gas is at a temperature of 706.4°C, it is subjected to a cooling process through three heat exchangers to ensure efficient and controlled cooling.Through this process, the syngas reaches a temperature of 25°C, making it available for use.

Model validation
In order to validate and evaluate the prediction accuracy of the simulated model using Aspen Plus software, a comparison with respect to experimental results reported by (Raheem et al., 2019) Given the differences between the conditions of the referenced study and the proposed simulation, it was necessary to adjust the simulation to the mentioned parameters.This decision was based on the premise that, to achieve effective validation, the simulation must replicate the experimental conditions.Subsequently, the results were compared, particularly the fractions of the synthesis gas components, to ascertain whether the model's predictions reflected trends and behaviors that significantly correlated with the experimental values reported in the literature.The details of this comparison are presented in figure 2. The model predictions generally exhibit a reasonable agreement with the experimental data.
For instance, the primary components, H2 and CO, display a discrepancy of 1.1% and 3.8%, respectively, between the reference study and the calibrated model, indicating that the proposed model is capable of capturing fundamental trends and behaviors of the lignocellulosic biomass gasification process, which is consistent with expectations and the degree of uncertainty inherent in gasification experiments.
On the other hand, differences in the concentrations of CO2 and CH4 were observed to be 1.01%and 1%, respectively.These components, although present in smaller proportions, are essential for assessing the efficiency of the process and the quality of the synthesis gas.The similarity in their percentages suggests that the model has an accurate representation of the dynamics of the formation of these compounds.Overall, the discrepancies observed across all components remained below the 4% threshold.These slight variations indicate that the model was able to capture with notable accuracy the production of synthesis gas and the relative proportions of its components.

Sensitivity Analysis of Oxygen and Steam Flows on Hydrogen Production
A sensitivity analysis was conducted with the primary objective of identifying the optimal oxygen and steam flows that would maximize hydrogen production in the gasification process.
Simultaneous modifications were made to the oxygen flows, within a range of 300 to 600 kg/h, and steam flows, within a range of 400 to 700 kg/h.This concurrent adjustment allowed for the examination of how these variations affected the hydrogen composition in the product stream.Both variables were manipulated at specific intervals, selecting 20 equidistant points within each range, which entailed a total of 400 calculations (20x20) to obtain the values of the "H2" variable in all possible combinations.
The results determined that a combination of an oxygen flow of 484 kg/h and a steam flow of 420 kg/h resulted in the maximum hydrogen concentration.To select these flows, operation was conducted within the region of the three-dimensional surface, known as the "red zone," visualized in Figure 3.This region corresponds to the area of maximum hydrogen concentration.]

Hydrogen Production in Synthesis Gas
The model determined a hydrogen molar proportion of 38.7% in the synthesis gas, a value that not only falls within the ranges described in the scientific literature for lignocellulosic biomass gasification processes, which typically vary between 10% and 80% (Pandey;Prajapati;Sheth, 2019), but also highlights the suitability of agro-industrial residues like sugarcane bagasse and cocoa pod husk for hydrogen production.This proportion surpasses that of Wood pellets, Pinewood sawdust, Almond shells, Palm shells, among others documented in previous studies (Cao et al., 2020;Mohammed et al., 2011;Yahaya et al., 2019).Therefore, the preliminary assessment of the biomass blend, composed of a 50:50 ratio of sugarcane bagasse and cocoa pod husk, suggests a clear potential.

Equivalence Ratio (ER)
Within the study's scope, the equivalence ratio (ER) was calculated to evaluate the balance between oxidative and reductive conditions during the gasification process.The ER obtained was 0.198.This outcome confirms that the oxygen supplied during the process is significantly less than what would be required for complete combustion, representing a characteristic feature of the reductive conditions present in a gasification process.Achieving these reductive conditions, clearly evidences the effective design and control of the simulated gasification process, ensuring the maximization of hydrogen production.

Hydrogen Yield in Synthesis Gas.
The hydrogen yield in the synthesis gas was calculated using the following equation: Where: H2 Yield = moles of H₂ obtained in syngas per kilogram of biomass introduced at the beginning of the gasification process, moles of H₂ in Syngas = total moles of H₂ present in the syngas at the conclusion of the gasification process, this is calculated by multiplying the molar fraction of H₂ in the syngas by the total molar flow of the syngas., Weight of the Biomass = total mass of the biomass introduced at the start of the process, measured in kilograms (kg).
The calculated hydrogen yield was 27 mol/kg, indicating that for each kilogram of biomass gasified, 27 moles of hydrogen are produced.This value serves as a efficiency indicator of the gasification process in hydrogen production.Considering the conversion rate of moles to kilograms, approximately 36.7 kg of biomass is required to produce 1 kg of hydrogen, providing an additional perspective on the process's efficiency.To understand the significance of this result, the energy potential of hydrogen compared to other fuels can be analyzed.According to Duarte (2016), 1 kg of hydrogen has an energy content equivalent to 2.78 kg of gasoline, 2.80 kg of diesel, 2.40 kg of methane, or 6.09 kg of methanol.The yield of 27 mol/kg achieved in this study, represents a significant advancement, especially considering that the agro-industrial residues used could have otherwise been discarded.
Comparing these results with previous research, it was determined that the yield achieved in the model has notable competitiveness.For example, scientific literature has documented ranges of molar hydrogen yield in the gasification of Pine saw dust from 15 to 20 mol H₂/kg of biomass (Parthasarathy;Narayanan, 2014).In contrast, gasifying cotton stalk and Palm kernel has been reported to achieve a molar hydrogen yield of between 8 and 14 mol H₂/kg of biomass (Khan;Alattab, 2022).In the gasification of Coffee bean husks, the molar hydrogen yield has 34 mol H₂/kg Brazilian Journal of Animal and Environmental Research, Curitiba, v.7, n.2, p. 1-14, 2024 of biomass (Pala et al., 2017), while in the gasification of coconut shells, such yield has demonstrated a fluctuation ranging from 30 to 38.6 mol H₂/kg of biomass (Alipour Moghadam et al., 2014).

Energy Efficiency of Hydrogen in Synthesis Gas
Energy efficiency was quantified using the ratio of the energy in the produced hydrogen to the energy initially present in the biomass, expressed as a percentage.The efficiency (η) was calculated using the formula: ) * 100% Where: η = Overall energy efficiency of the hydrogen production process through biomass gasification,   2 = is the energy contained in the produced hydrogen.This can be calculated as (hydrogen flow) * (lower heating value of hydrogen),   = Energy contained in the original biomass.This can be calculated as (biomass flow) * (lower heating value of the biomass).
The energy efficiency result was 43% indicates that almost half of the energy contained in the biomass is converted into hydrogen energy.This result is significant considering the potential of biomass as a renewable resource and its contribution to sustainability and the circular economy.
It is highlighted that efficiency can vary according to the specific nature of the biomass used, reflecting the diversity in chemical composition.
Comparing this result with previous studies, it is found that the 43% efficiency is consistent with other efficiencies reported in the literature for gasification processes of different types of biomass, ranging from 37% to 55% (Cerone;Zimbardi, 2021;Meher et al., 2022 ;Shayan et al., 2018) This underscores the variability of efficiency depending on the biomass and the specific conditions of gasification.
However, when compared to more efficient hydrogen production methods, such as water electrolysis, the 43% efficiency achieved through biomass gasification appears moderate (El-Shafie,

2023
).Yet, it's crucial to recognize the additional benefits this method offers.The gasification process's adaptability to diverse waste streams presents notable waste management advantages.
Moreover, the economic benefits, stemming from lower initial investments in equipment, underscore the method's practical appeal.
Under the conditions modeled, biomass gasification emerges as a feasible method for hydrogen production.It capitalizes on the use of renewable biomass and waste, contributing significantly to sustainability and the principles of a circular economy.Nevertheless, the journey towards optimizing this process is ongoing.Further research is imperative to evaluate the associated costs comprehensively, refine the process for enhanced efficiency, and ultimately establish biomass gasification as a sustainably and economically viable method for hydrogen production in the long term.

Analysis and Estimation of CO2eq Emissions in Process
The automatic estimation provided by report Aspen Plus Dynamics V.11® software report of the total CO2eq production throughout the modeled system was analyzed.It reported a value of 1435.3 kg/hr CO2eq.To analyze this figure, the amount of carbon stored in the biomass was calculated by multiplying the percentage of carbon present in the biomass by the total mass of biomass fed into the process.
Comparing the amount of carbon stored in the incoming biomass with the CO2eq production throughout the system, it was found that the amount of carbon stored (916 kg/hr) was less than the production at the system's output (1435.3kg/hr CO2eq).These results suggest that the modeled biomass gasification process is not carbon neutral, as CO2eq emissions exceed the amount of carbon stored in the used biomass.
However, it is important to note that this approach only provides an estimate, and there may be uncertainty associated with the carbon percentage values used, as these can vary depending on many factors such as the intrinsic diversity presented by the chemical composition of any type of biomass.Similarly, it is necessary to highlight that there may be other additional factors to consider for a more complete and accurate evaluation regarding the carbon neutrality of the lignocellulosic biomass gasification process.
Moreover, although this calculation provides an estimate of the amount of carbon stored in the biomass before gasification, it does not provide a complete picture of the carbon balance of the process.In biomass gasification, the carbon stored in the biomass is released as carbon dioxide, and there are other factors that can affect the total carbon balance, including greenhouse gas emissions at different stages of the process, such as collection, transport, biomass preparation, and the combustion of synthesis gas, if the entire lifecycle of the process is considered.
That said, the interpretation and specific evaluation of the CO2eq results generated by the software in this study are intrinsically linked to the perspective adopted in relation to the concept of carbon neutrality of the biomass gasification process, given the ambivalence of existing interpretations on the topic.

Figure 2 .
Figure 2. Comparison between the developed model in Aspen Plus and literature data.

Figure 3 .
Figure 3. Optimal Oxygen and Steam Flow Sensitivity Analysis for Maximized H2 Production.

Table 1 .
These calculations were carried out using software Mathcad 14.0.The proximate and ultimate analysis results of blend biomass is shown in Table 1.Proximate and ultimate analysis of sugarcane bagasse and cocoa pod husk Biomass 1: Sugarcane bagasse Biomass 2: Cocoa pod husk Proximate analysis (%) Brazilian Journal of Animal and Environmental Research, Curitiba, v.7, n.2, p. 1-14, 2024