Advanced Exergy, Exergoeconomic, and Exergoenvironmental Analyses of an Integrated Solar-Assisted Gasification Cycle

Context and Rationale for ISGC Systems

Integrated gasification combined cycles (IGCC) have gained significant attention as a sustainable methodology for the co-generation of power, steam, and high-value chemical co-products. A primary technical motivation for these systems is their inherent fuel flexibility; they are capable of processing a diverse range of feedstocks, including coal, biomass, municipal waste, and heavy liquid refinery residues. This versatility allows industrial operators to convert low-cost opportunity fuels and refinery wastes into valuable utilities, thereby improving the economic viability of processing plants. Despite these advantages, traditional IGCC systems encounter substantial hurdles. The construction and maintenance of such plants are technically demanding, often leading to high capital investment requirements, reduced system reliability, and lower availability compared to conventional power systems.

Environmental and Efficiency Drivers

The integration of parabolic trough collectors into gasification cycles defines the Integrated Solar-assisted Gasification Cycle (ISGC), a configuration designed to address both efficiency and environmental mandates. While conventional IGCC plants already offer higher thermal efficiencies than standard coal-fired plants—reaching up to 60% when fueled by natural gas—the addition of a solar energy field further enhances thermal performance. Specifically, the solar field can be utilized to produce superheated water vapor, which is then mixed with fuel before entering the gasifier. This synergy reduces the reliance on fossil-based thermal energy for process steam, thereby lowering the overall environmental footprint of the facility.

Emissions Control and Carbon Management

A critical rationale for pursuing ISGC technology is the pursuit of near-zero emission goals. Traditional gasification facilitates the removal of pollutants such as SOx, NOx, and particulates more effectively than direct combustion. However, the shift toward pre-combustion gas purification allows for the systematic separation of hydrogen sulphide and carbon dioxide. The technical framework of an ISGC supports the integration of carbon capture and storage (CCS) and gas switching combustion (GSC) technologies. By capturing CO2 at the pre-combustion stage, these systems provide a pathway to significantly mitigate greenhouse gas emissions, making them particularly attractive for oil-rich countries that possess both high solar irradiance and a surplus of heavy refinery residues.

The ISGC System Configuration

The proposed Integrated Solar-assisted Gasification Cycle (ISGC) is structured as a sophisticated tri-partite framework comprising a gasification unit, a purification cleanup train, and a combined cycle power block. The integration begins with the Air Separation Unit (ASU), which utilizes a membrane-based separation process to extract pure oxygen. This oxygen stream is fed directly into the oxygen-blown gasifier. By utilizing pure oxygen rather than ambient air, the system prevents nitrogen interference during the gasification process, which provides a critical mechanism for the reduction of toxic NOx pollutants.

Thermal integration is further enhanced by a specialized solar energy field utilizing parabolic trough collectors. This renewable component is designed to superheat water vapor, which is subsequently mixed with the heavy refinery fuel stream before the mixture enters the gasifier reactor. This configuration allows the plant to efficiently convert low-cost opportunity fuels—such as heavy liquid refinery residues—into high-value syngas while leveraging solar irradiance to decrease the carbon footprint of the thermal preparation stage.

Following gasification, the syngas undergoes a rigorous multi-stage purification process to ensure environmental compliance and turbine protection. The process flow is detailed as follows:

• Syngas Cooler: A heat exchanger that recovers thermal energy from the raw syngas to saturate water for the steam turbine cycle.
• H2S Remover: A dedicated unit that extracts hydrogen sulfide to meet pollutant emission standards.
• CO2 Absorber: A pre-combustion carbon capture unit that separates carbon dioxide from the fuel stream prior to power generation.
• Humidification and Expansion: The syngas is humidified and its pressure is synchronized with the compressor outlet via an expander, which simultaneously produces additional energy.

The final stage of the configuration involves a dual-pressure Heat Recovery Steam Generator (HRSG). This component is essential for maximizing thermal efficiency by utilizing heat from both the syngas cooler and the gas turbine exhaust flue gases. The HRSG generates high-pressure and low-pressure steam through a series of economizers, evaporators, and superheaters, which then drive a two-stage steam turbine (ST) system. This integrated approach ensures that waste heat from the combustion chamber and gasifier is reclaimed to maximize the net output power, which reaches approximately 319.92 MW under standard operating assumptions.

Thermodynamic and Conventional 4E Performance

The integrated solar-assisted gasification cycle (ISGC) demonstrates significant thermodynamic potential when processing heavy refinery fuels. The system achieves a net output power of 319.92 MW, utilizing a fuel mass flow rate of 15.5 kg/s. This output corresponds to a thermal efficiency of 50.01%, a figure that compares favorably with coal-based integrated cycles despite the use of low-cost opportunity fuels. From a second-law perspective, the total exergy destruction rate is measured at 366.57 MW. This leads to an overall system exergy efficiency of 53.98%. While the air separation unit displays a relatively low efficiency of 38% due to the power requirements of membrane-based oxygen separation, the inclusion of pure oxygen injection helps maintain the gasifier’s exergy destruction share at a moderate 25% of the total system irreversibilities.

Exergoeconomic and Investment Analysis

The exergoeconomic evaluation yields a system cost rate of 446 $/min. A critical distinction in this configuration is that the total cost rate of exergy destruction (164.27 $/min) is lower than the total equipment investment cost rate (188.52 $/min). This is primarily due to the relatively low cost of the heavy refinery residues used as primary fuel. The investment segment is dominated by the gas turbines, which represent the highest capital expenditure. However, the combustion chamber accounts for the largest share of the exergy destruction cost at 46.17%. The gasifier follows as a major cost contributor, representing 43.46% of the equipment cost rate. The condenser, by contrast, represents the lowest investment cost rate among the primary components.

Exergoenvironmental Impact Assessment

The environmental performance, assessed through life cycle indicators, shows a total impact rate of 72,796 pts/min. This value is heavily influenced by the exergy destruction rate, which contributes 70,961 pts/min to the total, whereas the manufacturing-related impacts of the equipment account for 22.2 pts/min. The combustion chamber is the primary source of environmental concern, dominating the impact rate with a 37.67% share of the total exergy destruction effects. The heat recovery steam generator (HRSG) follows with a 24.02% share. Analysis of exergoenvironmental factors indicates that heat recovery equipment possesses high adverse effects relative to their exergy destruction, suggesting that structure-related improvements, such as weight reduction or enhanced solar integration to increase inlet water temperatures, are necessary to mitigate their environmental footprint.

Advanced Exergy and Component Prioritization

To move beyond the limitations of conventional thermodynamic assessments, advanced exergy analysis provides a granular view of irreversibilities within the Integrated Solar-assisted Gasification Cycle (ISGC). By categorizing exergy destruction into avoidable, unavoidable, endogenous, and exogenous parts, researchers can distinguish between losses inherent to current technological limits and those that can be mitigated through engineering optimization. This stratification is essential for prioritizing components that offer the most significant potential for efficiency gains, rather than focusing solely on units with the highest absolute exergy destruction.

Analysis of Irreversibility Types

The study reveals a complex distribution of irreversibilities across the plant's architecture. The analysis of endogenous and exogenous destruction identifies whether losses are inherent to the component itself or caused by the inefficiencies of interconnected units. Key findings regarding these metrics include:

• Endogenous Exergy Destruction: The highest rates of endogenous destruction were observed in the solar field and the combustion chamber. This indicates that the irreversibilities in these units are primarily a function of their internal processes—such as chemical reactions and heat transfer across large temperature gradients—rather than the performance of upstream or downstream equipment.
• Avoidable vs. Unavoidable Metrics: While many components show high total exergy destruction, much of it is classified as unavoidable due to physical and technological constraints. For example, the combustion chamber was identified as the highest priority for improvement based on its 43% share of total exergy destruction. However, the condenser exhibited the highest avoidable exergy destruction share, marking it as a primary candidate for practical efficiency enhancements.

Component Prioritization for Optimization

The prioritization for system modification is grounded in the avoidable exergy destruction and avoidable exergoeconomic metrics. Components with high avoidable shares suggest that significant gains can be achieved by changing the equipment type or optimizing operating parameters, regardless of current exergy destruction costs.

These findings suggest that while the gasification and combustion units dominate the system's total exergy loss, the most effective path toward increasing overall efficiency lies in addressing the avoidable losses in the condenser and syngas cooler. Conversely, the steam turbines and carbon capture units already operate near their practical thermodynamic limits, offering little return on further optimization efforts.

Model Validation and Sensitivity

To ensure the reliability of the integrated solar-assisted gasification cycle (ISGC) model, the thermodynamic results generated in MATLAB were benchmarked against simulations performed in Thermoflex software, which utilizes real plant data. The validation process focused on critical stream parameters and system-wide performance indicators to verify the accuracy of the thermodynamic relations and assumed operating conditions.

Comparative Analysis of Stream and System Parameters

The comparison of individual process streams demonstrated high fidelity between the two software environments. As detailed in the validation data, parameters for mass flow, temperature, and pressure across the majority of the system nodes exhibited error margins lower than 1%. Specifically, mass flow rates for key streams such as the air separation unit products and the heavy refinery fuel input showed negligible variance. While minor deviations were noted in specific high-temperature nodes—such as a 2.63% temperature error in the syngas stream (Stream 6)—the overall consistency across the 42 identified streams confirms the robustness of the MATLAB modeling framework.

At the component level, the validation of power outputs and heat transfer rates further supported the model's integrity. Results for the gas turbine (GT) pack, air compressors, and various heat recovery steam generator (HRSG) components remained within a 4.5% error margin. For instance, the air compressor power (WAC) and gas turbine power (WGT) showed errors of 2.78% and 3.07%, respectively, while heat transfer rates in the superheaters and economizers showed even higher agreement, often with errors below 1%.

Exergy Performance and Fuel Benchmarking

A significant finding of the validation phase involves the system's exergy destruction profile. The ISGC configuration achieved a ratio of exergy destruction to total input exergy of 53.98%. This represents a notable improvement over previous coal-fed models, which reported a higher ratio of 56.7%. This reduction in irreversibility is particularly relevant given the shift in feedstock; the analysis confirms that the use of heavy refinery fuel did not diminish thermal efficiency compared to coal-fed benchmarks. In fact, the model reached a thermal efficiency of 50.01%, slightly outperforming the 49.94% efficiency reported in coal-based research, as shown in the following table:

These results validate the assumption that low-cost refinery residues can serve as an effective substitute for traditional solid fuels without compromising the plant's energetic or exergetic viability. The inclusion of the solar energy field and pure oxygen injection via the air separation unit contributed to these favorable metrics by reducing the exergy destruction share of the gasifier to 25%.

Environmental and Strategic Implications

The implementation of an integrated solar-assisted gasification cycle (ISGC) offers a transformative strategic advantage for oil-rich nations. By utilizing heavy refinery fuels—typically regarded as low-cost environmental liabilities—as the primary feedstock, these regions can convert refinery waste into high-value utilities including 319.92 MW of electrical power and substantial steam output. This approach effectively addresses waste management challenges while simultaneously diversifying the energy portfolio of petroleum-dependent economies.

The geographic characteristics of oil-rich countries, often characterized by high global solar irradiance, make them ideal candidates for the solar integration featured in this system. The use of parabolic trough collectors to generate superheated water vapor for the gasification process significantly enhances the plant's overall efficiency. Data indicates that this integration allows for a thermal efficiency of approximately 50.01% and an exergy efficiency of 53.98%. Such performance metrics demonstrate that renewable energy can be successfully hybridized with traditional heavy fuel processing to reduce the carbon footprint of industrial operations.

Advanced exergoenvironmental analysis provides critical insights into the physical and structural improvements necessary for future deployments. The study highlights that heat recovery equipment often exhibits low exergoenvironmental coefficients, suggesting that their exergy destruction has a notable adverse effect on the environment. To mitigate these impacts, strategic redesign of heat recovery components is recommended, specifically focusing on reducing equipment weight and improving the manufacturing processes. Furthermore, the analysis indicates that the combustion chamber and gasifier remain primary sources of irreversibility, representing 43% and 25% of the system's exergy destruction respectively.

To further boost the viability and performance of future installations, research directions are pointing toward expanded solar utility. One high-potential modification involves the preheating of Heat Recovery Steam Generator (HRSG) inlet water via solar collectors. By elevating the temperature of the water entering the system through renewable thermal energy, the plant can further reduce its reliance on internal fuel combustion for steam generation, thereby increasing the net utility output and further improving the system's environmental profile.

Would you like a summary of the next segment of this technical documentation?

Key findings

• Efficiency Improvement — Replacing coal with heavy refinery fuel improved thermal efficiency to 50.01% and reduced the exergy destruction ratio by 4.8%.
• Major Irreversibility Source — The combustion chamber is the primary site of exergy destruction, responsible for 43% of system-wide losses.
• Economic Viability — The system operates at an investment cost rate of 188.52 $/min, while total exergy destruction costs are 164.27 $/min.
• Avoidable Exergy Destruction — The condenser, syngas cooler, and gas turbine are the top three components prioritized for modification based on high avoidable exergy destruction.
• Net Power Generation — A fuel input of 15.5 kg/s yields 319.92 MW of net power, supporting large-scale industrial utility needs.

Method and assumptions

The study utilized a steady-state thermodynamic model developed in MATLAB and validated against Thermoflex software based on real plant data. The system boundary includes an ASU, gasifier, solar field, dual-pressure HRSG, and purification units. Exergy analysis applied the SPECO methodology to stratify physical and chemical exergy. Exergoeconomic analysis incorporated the Capital Recovery Factor (CRF) based on a 25-year plant life and 8000 annual operating hours. Exergoenvironmental analysis employed the Eco-Indicator 99 methodology through Life Cycle Assessment (LCA). Advanced exergy analysis further decomposed irreversibilities into endogenous/exogenous and avoidable/unavoidable categories to determine component-level improvement potentials.

Where it applies

• Refinery Waste Upcycling — Converting heavy refinery residues and petroleum coke into syngas for power and steam generation.
• Renewable Integration for Power Plants — Utilizing solar thermal energy in IGCC plants to reduce fossil fuel consumption and carbon footprint.
• Regional Industrial Development — Implementation in oil-rich regions with high solar irradiance to maximize economic and environmental returns.

Terms used

• Exergy Destruction — The loss of available energy or potential to do work due to internal irreversibilities like friction and chemical reactions.
• Endogenous Exergy Destruction — The portion of exergy destruction within a specific component that is independent of the performance of other system components.
• Avoidable Exergy Destruction — The part of exergy destruction that can be eliminated through technical improvements or optimized design.
• SPECO Methodology — Specific Exergy Costing, a systematic approach for calculating efficiencies and costs based on fuel and product definitions at the component level.
• Heat Recovery Steam Generator (HRSG) — An energy recovery heat exchanger that recovers heat from a hot gas stream to produce steam.

Questions and answers

What is the primary benefit of adding a solar field to a gasification cycle?

The solar field, specifically using parabolic trough collectors, provides superheated steam to the gasifier and HRSG. This reduces the primary fuel requirement for heating and improves the overall environmental impact rate and thermal efficiency.

How accurate was the simulation compared to industry-standard software?

The MATLAB model was validated against Thermoflex simulations, showing a mass flow error of less than 1% and a maximum power calculation error of 4.37% for the expander, which is considered highly accurate for steady-state analysis.

Which system component offers the most room for efficiency improvement?

According to the advanced exergy analysis, the condenser has the highest avoidable exergy destruction, followed by the syngas cooler. However, the combustion chamber remains the largest source of total exergy destruction, suggesting it is a critical target for optimization.

How does the environmental impact of this system compare to traditional plants?

The ISGC incorporates H2S and CO2 removal before combustion, significantly reducing SOx and carbon emissions. The use of pure oxygen in the gasifier also binds and reduces toxic NOx pollutants compared to air-blown systems.

How to cite

Jadidi, E.; Khoshgoftar Manesh, M.H.; Delpisheh, M.; Onishi, V.C. Advanced Exergy, Exergoeconomic, and Exergoenvironmental Analyses of Integrated Solar-Assisted Gasification Cycle for Producing Power and Steam from Heavy Refinery Fuels. Energies 2021, 14, 8409.