Energy and Exergy Analysis of Green Hydrogen Production

Context of Biomass Thermal Conversion

The global energy landscape is currently defined by a sharp rise in demand driven by increasing consumption trends and the steady depletion of finite fossil fuel reserves. Despite the growing urgency for sustainable transitions, traditional coal remains a dominant force in power generation, contributing approximately 40% of the total power generated worldwide. This reliance, coupled with the environmental challenges posed by inadequate solid waste treatment, has necessitated the exploration of renewable energy sources and circular economy solutions. Hydrogen energy systems have emerged as a primary pathway for sustainable energy production from solid waste, specifically aimed at reducing long-term reliance on fossil fuels.

Biomass as a Sustainable Feedstock

Biomass represents the largest sustainable feedstock available globally, currently accounting for approximately 15% of the world’s primary energy consumption. Its attractiveness as an energy source is largely due to its renewable nature and its potential to produce carbon-neutral fuels, which are essential for mitigating the impacts of greenhouse gas emissions in line with international initiatives like the Kyoto Protocol. Utilizing organic waste as a feedstock allows for the conversion of low-value solid waste into high-value energy carriers, integrating waste management with energy security.

Mechanisms of Thermochemical Conversion

To extract energy from organic matter, thermochemical conversion technologies such as gasification and pyrolysis are utilized. These processes involve highly endothermic reactions that transform organic wastes into various useful products, most notably syngas. Syngas is a versatile mixture typically composed of the following:

• Hydrogen (H2): A critical component for green energy systems.
• Carbon Monoxide (CO): A primary constituent of the combustible gas mix.
• Methane (CH4) and Higher Hydrocarbons: Contributors to the gas's heating value.
• Carbon Dioxide (CO2), Char, and Liquids (Tars): Byproducts of the thermal decomposition.

The efficiency of these routes is highly dependent on operational parameters, including the specific biomass source, the presence of gasification agents, retention time, and temperature. While gasification involves the addition of partial oxygen or agents like steam to convert carbon content into gas, pyrolysis focuses on thermal decomposition, often in the absence of such agents. At high temperatures, specifically 900°C, these thermochemical routes are identified as effective technologies for producing green hydrogen, contributing to a sustainable and circular energy economy.

Thermodynamic Modelling and Mass Balance

The mathematical framework for simulating the reactor processes at Exerginity relies on a stoichiometric chemical equilibrium model. This approach facilitates a comprehensive comparison between pyrolysis and gasification by examining the first and second laws of thermodynamics. To ensure the simulation remains physically grounded, the model employs several fundamental assumptions. The system is evaluated under steady-state conditions, where kinetic and potential energies are considered negligible. Furthermore, all thermodynamic properties are calculated relative to reference conditions defined at a temperature of 298K and a pressure of 1 atm.

Gibbs Free Energy and Component Prediction

A central mechanism in the modelling process is the utilization of Gibbs free energy, also referred to as the Gibbs function or free enthalpy. In this thermodynamic system, Gibbs free energy is used to measure the maximum amount of work achievable when temperature and pressure remain constant. Mathematically, the simulation estimates the mole numbers of various components—specifically CO₂, H₂, CH₄, and ash—by seeking the values that minimize the total Gibbs free energy of the system. This predictive method allows for the estimation of syngas composition and hydrogen yield without requiring exhaustive experimental data for every intermediate reaction step.

Mass Balance Equations

The conservation of matter within the pilot-scale reactor is maintained through rigorous mass balance equations. These equations account for every material stream entering and exiting the thermal conversion unit. The input side of the balance includes contributions from the biomass feedstock, the gasifying agent (air), and the moisture content within the materials. Conversely, the output side accounts for the resulting gas, solid char, and liquid tar, as well as other minor impurities. The general mass balance is expressed as follows:

∑ M_in = M_water + M_woodchip + M_air
∑ M_out = M_gas + M_char + M_tar + M_other

Feedstock Modelling and Stoichiometry

For the purposes of this thermodynamic simulation, wood chips serve as the primary biomass feedstock. To represent the organic complexity of wood within a stoichiometric framework, the wood chips are modelled with a simplified chemical structure of CH_1.5O_0.7. This representation is derived from ultimate and proximate analyses, which identify the elemental mass fractions of carbon, hydrogen, and oxygen. In the model, the biomass enters the gasifier at environmental conditions and atmospheric pressure, where it undergoes drying and pyrolysis. The mass balance of all resulting components is determined at the conclusion of these thermal conversions, providing the necessary data to calculate energy and exergy efficiencies.

Comparative Energy Analysis

The thermal conversion of biomass into green hydrogen requires a rigorous evaluation of energy yields to determine the efficiency of the first law of thermodynamics across different operating conditions. In this study, wood chips were subjected to both pyrolysis and gasification at temperatures of 500°C and 900°C. The efficiency results indicate a significant positive correlation between process temperature and energy recovery, as illustrated by the performance of both systems at elevated thermal levels.

Energy Efficiency Outcomes

In the pyrolysis reactor, the energy efficiency was recorded at 60% when operating at a low temperature of 500°C. However, as the temperature ramped to 900°C, the energy efficiency increased substantially to 94%. A nearly identical trend was observed in the gasification process, which yielded 61% energy efficiency at 500°C and reached a peak of 94% at 900°C. These findings suggest that at high temperatures, pyrolysis becomes as effective as gasification, achieving the same energy output in a simpler system without the requirement of gasification agents.

Syngas Quality and Calorific Values

The energy potential of the produced syngas is heavily influenced by the process temperature and the resulting chemical composition. The Lower Heating Value (LHV) of the syngas serves as a critical indicator of energy density. According to the mass balance and stoichiometric calculations, the LHV of syngas reached its maximum at 35.2 MJ/kg during gasification at 900°C. Comparative results for the thermal conversion processes are detailed below:

Performance Indicators and Mechanisms

To evaluate the thermal conversion systems comprehensively, the study utilized two primary performance indicators based on the first law of thermodynamics:

• Cold gas efficiency: Defined as the ratio of the chemical energy of the product gas to the total energy of the feedstock (including biomass and any agents).
• Sensible efficiency: Described as the ratio of the physical energy of the product gas to the total energy of the feedstock.

The increase in thermal efficiency at higher temperatures is partially attributed to the dissociation of carbon dioxide and the Boudouard reaction, which alters the Gibbs free energy of the system. While pyrolysis at 500°C produces higher levels of char, the gas content—and subsequently the energy yield—increases as temperatures rise, leading to more effective carbonization and higher hydrogen production rates at 900°C.

Exergy Performance and Irreversibility

The evaluation of biomass thermal conversion systems based on the second law of thermodynamics provides a more comprehensive assessment than energy analysis alone. By incorporating the concept of irreversible entropy increase, exergy analysis reveals the saving potential of a system by identifying internal irreversibilities. This diagnostic approach allows for the determination of the types and values of exergy destruction, serving as a critical criterion for approaching ideal thermodynamic conditions in hydrogen production.

In this study, total exergy (Ex) is defined as the sum of chemical exergy (Ex,ch) and physical exergy (Ex,ph). To maintain high precision in calculating the physical exergy of the resulting syngas components, specific heat capacity coefficients (a, b, c, and d) were utilized to model the behavior of H2, N2, CO, CO2, and CH4. These coefficients are applied within a polynomial equation for Cp, which is then integrated to determine the enthalpy and entropy differences between the process state and the reference conditions (T0=298K and P0=1 atm).

The exergy efficiency of the pyrolysis process demonstrates a significant dependence on operating temperature. At a low temperature of 500°C, the pyrolysis exergy efficiency is calculated at 52%. However, as the temperature increases to 900°C, the efficiency rises to 66%. This improvement is linked to the reduction of char formation and the increased production of combustible gases like hydrogen and carbon monoxide at higher thermal states.

When comparing the two conversion methods at the optimal high-temperature condition of 900°C, the performance metrics are closely aligned, though gasification shows a slightly higher exergy efficiency of 67%. Despite this marginal advantage for gasification, high-temperature pyrolysis remains a highly effective model because it achieves similar energy and exergy yields without the requirement for gasification agents, thereby simplifying the system architecture while maintaining effective potential for green hydrogen production.

The modeling results indicate that the irreversibilities in these systems are influenced by reaction mechanisms such as the Boudouard reaction. At elevated temperatures, the dissociation of carbon dioxide alters the Gibbs free energy, influencing the heat of reaction and the resulting exergy quality of the syngas. These findings underscore that optimizing temperature is the primary driver for enhancing the exergy performance of biomass conversion.

Syngas Quality and Hydrogen Yield

The chemical composition and quality of the produced syngas are highly dependent on the thermal regime and the specific conversion technology employed. In this study, the primary constituents monitored across all experimental runs include hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂). The variations in these components under different temperatures and process types reveal critical trade-offs between pyrolysis and gasification strategies for green hydrogen production.

Temperature-Dependent Reaction Mechanisms

Thermal conversion efficiency is largely driven by temperature increases, which favor specific chemical pathways. Higher temperatures promote the Boudouard reaction (C + CO₂ → 2CO) and the dissociation of methane, which significantly alters the final gas quality. For instance, the dissociation of carbon dioxide at 900°C changes the Gibbs free energy of the system, influencing the overall heat of reaction and syngas purity. Furthermore, carbonization occurs more homogeneously at elevated temperatures, whereas lower temperatures, specifically 500°C, result in significantly higher char production (6.6% in pyrolysis compared to 2.7% at 900°C gasification).

Hydrogen Concentration and Yield Comparisons

Hydrogen yield serves as a primary performance indicator for the viability of these thermal systems. The data indicates that high-temperature pyrolysis outperforms gasification in terms of raw hydrogen percentage within the syngas. At a stabilized reactor temperature of 900°C, the pyrolysis process achieved a 19% hydrogen yield, whereas gasification at the same temperature resulted in 14%. At the lower threshold of 500°C, both systems produced a identical hydrogen yield of 10%.

Syngas Composition and Carbon Dioxide Levels

The presence of oxygen in gasification systems leads to distinct differences in carbon oxide concentrations compared to the oxygen-starved environment of pyrolysis. The following table illustrates the comparative composition of the gas produced at both 500°C and 900°C:

A notable finding is the behavior of CO₂. In the gasification regime, the CO₂ content drops significantly from 15% at 500°C to a mere 0.01% at 900°C. This reduction is attributed to the reaction between carbon and oxygen, where carbon cannot fully convert CO₂ at lower temperatures but achieves near-total conversion at higher thermal levels. Consequently, the Lower Heating Value (LHV) of the syngas is maximized at 900°C for both technologies, with gasification reaching 35.2 MJ/kg and pyrolysis reaching 33.6 MJ/kg.

Feedstock Characteristics and Selection

The efficiency and chemical output of thermal conversion processes are fundamentally dictated by the physical and chemical properties of the biomass feedstock. In this research, wood chips were evaluated and ultimately selected as the primary raw material for green hydrogen production. This selection was based on a comparative analysis of wood chips against manure waste, focusing on energy potential, elemental composition, and byproduct profiles. The superior performance of wood chips in these categories makes them a more viable candidate for high-temperature pyrolysis and gasification models.

Comparative Energy Potential and Heating Values

A critical metric in feedstock selection is the High Heating Value (HHV), which quantifies the energy density of the material. Wood chips exhibit a significantly higher energy potential compared to manure, with measured values as follows:

The High Heating Value of 19.63 MJ/kg for wood chips provides a more robust energy foundation for the endothermic reactions required during thermal decomposition. Furthermore, the volatile content of wood chips (88.92%) exceeds that of manure (86.89%). Higher volatility is essential for maximizing the production of syngas components, such as hydrogen and carbon monoxide, during the initial stages of pyrolysis.

Impact of Moisture and Ash Content

The physical composition of the feedstock, particularly its moisture and ash levels, imposes practical limitations on reactor performance. In this study, the wood chip feedstock was measured at a moisture content of 16%. Controlling moisture is vital for maintaining the isothermal conditions of the pilot-scale reactor, as excessive water necessitates higher energy inputs for evaporation, potentially lowering the overall cold gas efficiency.

Furthermore, wood chips were selected due to their lower ash content of 9.6%, compared to the 13.4% found in manure. Lower ash content is preferred in thermal conversion systems to reduce the accumulation of non-combustible residues, which can interfere with heat transfer and gas flow within a fixed-bed reactor. By prioritizing wood chips, the model achieves a more favorable balance of high energy yield and manageable byproduct formation, particularly at the optimal operating temperature of 900°C.

Mechanism of Selection

The choice of wood chips over manure was driven by the necessity for a feedstock that supports high calorific value syngas. The stoichiometric model utilized the ultimate and proximate analysis results to confirm that the carbon-heavy structure of wood chips (measured at 45% Carbon) provides the necessary elemental building blocks for effective carbon conversion. This selection ensures that the pyrolysis and gasification processes can reach their maximum energy efficiency potentials, which the study identifies as 94% under optimal high-temperature conditions.

Key findings

• Optimal Pyrolysis Condition — Pyrolysis at 900°C is identified as the optimal model condition due to low energy consumption and the absence of agent materials.
• Temperature Efficiency Correlation — Energy efficiency in pyrolysis shows a substantial increase from 60% at 500°C to 94% at 900°C.
• Hydrogen Production Yield — High-temperature pyrolysis (900°C) achieved a hydrogen content of 19%, outperforming gasification at the same temperature.
• Exergy Efficiency Gain — The exergy yield for pyrolysis increased from 52% to 66% as the temperature was raised from 500°C to 900°C.
• Agent Material Impact — The study suggests that at high temperatures, pyrolysis can match gasification performance without the need for a gasifying agent like air or steam.

Method and assumptions

The study utilises a pilot-scale fixed-bed reactor for both pyrolysis and gasification of wood chips. A stoichiometric chemical equilibrium model based on the minimisation of Gibbs free energy was developed to predict the product composition. The analysis is governed by the first and second laws of thermodynamics to calculate energy and exergy efficiencies. System boundaries are defined under steady-state and adiabatic conditions, with kinetic and potential energy changes neglected. Reference environment conditions were set at T0=298K and P0=1 atm. Input parameters were derived from proximate and ultimate analysis of wood chips (45% Carbon, 5.88% Hydrogen, 47.94% Oxygen). The model results were validated against existing literature and experimental data for syngas components like CO, H2, and CH4.

Where it applies

• Green Hydrogen Production — Providing a technical basis for designing high-temperature biomass pyrolysis plants to produce sustainable hydrogen.
• Waste-to-Energy Systems — Optimising the thermal conversion of wood waste and agricultural residues into high-calorific syngas.
• Industrial Decarbonisation — Utilising biomass-derived syngas as a carbon-neutral alternative to coal in power generation and chemical synthesis.

Terms used

• Exergy — The maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir.
• Pyrolysis — Thermal decomposition of organic material at elevated temperatures in the absence of oxygen.
• Gasification — A process that converts biomass into syngas by reacting the material at high temperatures with a controlled amount of oxygen or steam.
• Syngas — A fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide.
• Boudouard Reaction — The chemical equilibrium of a mixture of carbon monoxide and carbon dioxide at a given temperature in contact with solid carbon.
• Gibbs Free Energy — A thermodynamic potential that can be used to calculate the maximum reversible work that may be performed by a thermodynamic system at a constant temperature and pressure.

Questions and answers

Which process is more efficient for hydrogen production?

According to the study, high-temperature pyrolysis at 900°C is highly effective, yielding 19% hydrogen and 94% energy efficiency, comparable to gasification but simpler as it requires no gasifying agent.

How does temperature affect exergy yield?

Increasing the temperature from 500°C to 900°C increases the exergy yield from 52% to 66% in pyrolysis and from 63% to 67% in gasification, indicating reduced irreversibility at higher temperatures.

Why were wood chips preferred over manure in this study?

Wood chips were selected due to their higher volatile content (88.92%) and higher heating value (19.63 MJ/kg) compared to manure, making them more suitable for high-quality syngas production.

What are the primary assumptions of the thermal model?

The model assumes steady-state conditions, an isothermal reactor, negligible kinetic and potential energy, and that char is treated as pure solid carbon while tar is ignored.

How to cite

Büyük P. and Eryaşar A., “Energy and exergy analysis of green hydrogen production”, Journal of Polytechnic, 28(2): 461-468, (2025).