Introduction to CCCHP and Solid Fuel Dynamics
The global energy landscape remains heavily dependent on solid fuels, with coal fulfilling 35% of global electricity demand in 2024. This reliance resulted in the production of 10,704 terawatt-hours of electricity, highlighting the massive scale of existing infrastructure. However, the environmental toll of fossil fuel combustion has led scientists and climate analytics experts to recommend a complete cessation of coal combustion by at least 2040. To mitigate environmental pollution and transition toward a carbon-neutral cycle, sustainable biomass—derived from wood, agricultural residues like bagasse, and forestry waste—is being positioned as the primary renewable alternative. The economic momentum for this transition is evident in the biomass market, which is projected to grow from a value of USD 46.98 billion in 2021 to USD 84.78 billion by 2030.
Mechanics of Combined Cycle Combined Heat and Power (CCCHP)
To maximize the utility of both renewable and non-renewable solid fuels, Combined Cycle Combined Heat and Power (CCCHP) systems represent a pinnacle of thermodynamic integration. These systems enhance energy efficiency by simultaneously generating electricity and heat from a single combustion source. The mechanical superiority of a CCCHP plant stems from its "combined cycle" architecture, which integrates two distinct thermodynamic cycles:
- The Brayton Cycle: A gas turbine system that utilizes high-temperature combustion exhaust to rotate an expander and drive an electric generator.
- The Rankine Cycle: A steam turbine system that captures waste heat from the gas turbine’s exhaust through a Heat Recovery Steam Generator (HRSG). This recovered energy produces high-pressure steam to drive additional turbines.
By capturing energy that would otherwise be rejected into the environment as waste heat, CCCHP systems can achieve total efficiency levels exceeding 60%. This is a significant advancement over single-cycle plants, as the overall electrical efficiency becomes the summation of both cycles. Furthermore, the "Combined Heat and Power" (CHP) aspect allows for the extraction of intermediate-pressure steam for industrial heating applications, further optimizing the fuel's chemical energy.
The Role of Biomass and Moisture Dynamics
Transitioning from coal to biomass involves complex dynamics, particularly regarding fuel moisture content. While coal typically possesses higher calorific values, biomass like bagasse introduces higher volatile matter and varying moisture levels (ranging from 10% to 40% in simulated models). In CCCHP systems, biomass moisture actually enhances the performance of the Rankine cycle by increasing the water vapor concentration in the flue gas. Because water vapor has a higher specific heat capacity than dry flue gases like CO2, it carries more latent and sensible heat into the HRSG. Research indicates that optimizing these dynamics can push biomass CCCHP energy efficiency to as high as 67.22%, provided the moisture content is managed to avoid cooling the flue gas below the dew point.
Simulation Framework and System Configuration
The research institute utilized Ebsilon Professional 16 software to design and optimize a Combined Cycle Combined Heat and Power (CCCHP) system. This simulation environment was selected for its robust steady-state modeling capabilities and its ability to handle complex thermal power plant configurations using a Gaus–Seidel algorithm to solve linearized characteristic equations. The simulation framework leverages standard thermodynamic libraries, including water–steam table IAPWS-IF97, to evaluate the performance of coal, biomass, and cofired combustion systems.
Design Objectives and Thermodynamic Cycles
The CCCHP system is configured to meet a total output target of 150 MW. This is partitioned into 125 MW of electrical power, chosen to ensure stability and integration with existing infrastructure, and 25 MW of heating energy intended for industrial applications. The design integrates two primary thermodynamic cycles:
- The Brayton Cycle: Utilized in the gas turbine assembly, which includes an air compressor, combustion chamber, and expander. The gas turbine pressure ratio is set at 10:1. The system assumes an isentropic efficiency of 85% for the compressor and 90% for the expander unit.
- The Rankine Cycle: Employed in the steam turbine system to recover waste heat from the gas turbine exhaust. The heat recovery process is facilitated by a Heat Recovery Steam Generator (HRSG).
Steam Generation and Turbine Partitioning
The HRSG is a critical interface in the combined cycle, consisting of an economizer, evaporator, and superheater. It is designed to produce high-quality steam at 575 °C and 120 bars, utilizing an inlet water temperature of 42 °C. To maximize work extraction and facilitate heat production, the steam turbine units are partitioned into three distinct stages:
| Stage | Inlet Pressure (Bar) | Inlet Temperature (°C) |
|---|---|---|
| High-Pressure (HPST) | 120 | 575 |
| Intermediate-Pressure (IPST) | 40 | 407 |
| Low-Pressure (LPST) | 5 | 172 |
The 25 MW heating component is achieved through a steam extraction unit designed to obtain heat energy specifically from the intermediate-pressure steam turbine (IPST) stage. The remaining steam continues to the low-pressure stage and a condenser operating at 0.05 Bar. This configuration allows for a comprehensive energy and exergy analysis, assessing how fuel moisture content affects the balance between gas turbine output and steam turbine recovery.
Fuel Composition and Calorific Values
The chemical and energetic characteristics of the fuels employed in the 16 simulated Combined Cycle Combined Heat and Power (CCCHP) systems serve as the foundation for evaluating thermodynamic performance. The study utilizes coal and biomass (specifically bagasse) as primary fuel sources, each defined by distinct Net Calorific Values (NCV) and elemental compositions. Coal is assigned a high NCV of 35,590 kJ/kg, reflecting its high energy density as a non-renewable source. In contrast, bagasse is assigned an NCV of 20,000 kJ/kg, representing the renewable but lower-density energetic profile of agricultural residues.
Coal and Biomass Specifications
The coal used in these simulations is characterized by a stable chemical composition derived from the Ebsilon library database. It comprises 64% carbon, 0.25% sulfur, 0.11% ash, and 10% moisture content (w/w). Additional minor constituents include 0.05% hydrogen, 0.07% oxygen, and 0.005% nitrogen. These properties determine the stoichiometric air requirements and the resulting flue gas characteristics during the Brayton cycle phase of the combined system.
A critical component of the research is the sensitivity analysis regarding fuel moisture content, particularly for biomass. The simulation explored a range of moisture levels for bagasse from 10% to 40% (w/w). This variation is significant because moisture impacts the enthalpy of the flue gas; high moisture content results in a higher concentration of water vapor in the exhaust stream, which carries significant latent and sensible heat. This energy is subsequently recovered in the Heat Recovery Steam Generator (HRSG) to drive the Rankine cycle.
Cofiring Configuration
For the simulated cofiring systems, a hybrid approach was adopted to evaluate the transition from coal to renewable alternatives. These systems utilized a fixed coal quantity of 5 kg/s, which was combined with varying biomass quantities and moisture levels to meet a constant total power output of 150 MW. The relationship between these fuel properties is summarized in the table below:
| Combustion System | Moisture Content (w/w) | Coal Feed Rate | Biomass Feed Rate |
|---|---|---|---|
| Coal Standard | 10% | 100% (7.43 kg/s) | 0% |
| Biomass 1-7 | 10% to 40% | 0% | 100% |
| Cofiring 1-8 | 10% to 25.56% | 5 kg/s (Fixed) | Software-derived value |
These fuel properties directly influence system efficiencies. While coal requires the lowest total mass flow rate due to its high NCV, the biomass and cofiring systems show improved energy efficiency as moisture levels increase toward an optimal point (30% for biomass and 25.56% for cofiring), due to the enhanced heat recovery potential from the resulting flue gas.
Impact of Moisture on Flue Gas Enthalpy
The moisture content of biomass fuels significantly dictates the thermodynamic behavior and heat transfer potential of the flue gas within a CCCHP system. When biomass with a 30% moisture content (w/w) is combusted, it releases a substantial volume of water vapor into the exhaust stream. This increase in water vapor concentration fundamentally alters the flue gas energy content, as water vapor possesses a higher specific heat capacity compared to dry flue gas components such as CO2 and SO2. Consequently, despite the energy consumed during the combustion process to evaporate fuel-bound water, the resulting flue gas carries a higher overall enthalpy due to the significant latent and sensible heat retained by the steam.
Heat Transfer to the Rankine Cycle
The elevated enthalpy levels in the flue gas play a critical role in the heat recovery steam generator (HRSG) performance. As moisture increases the energy content of the exhaust, higher concentrations of water vapor lead to increased heat transfer from the flue gas to the water cycle. This mechanism shifts the balance of power generation within the combined cycle: while the power produced in the gas turbine (Brayton cycle) decreases with higher fuel moisture, the energy delivered to the steam turbine (Rankine cycle) increases. This heat recovery process allows the system to capture energy that would otherwise be lost, directly influencing the overall energetic efficiency of the plant.
Temperature Variations and Phase Changes
The presence of moisture exerts a strong influence on the exhaust gas temperatures across various fuel types and moisture levels. Simulation data highlights the following variations in flue gas release temperatures:
| Fuel Type | Moisture Content Range | Exhaust Gas Temperature Range |
|---|---|---|
| Coal | 10% | 680 °C |
| Biomass | 10% to 40% | 688 °C to 56 °C |
| Coal–Biomass Cofiring | 10% to 50% | 683 °C to 395 °C |
A critical limitation occurs as moisture content exceeds the 30% threshold. At these higher concentrations, the flue gas may cool to a point where water vapor condenses below the dew point. This phase change releases latent heat and results in negative enthalpy values in the final exhaust streamlines. According to the second law of thermodynamics, this cooling process indicates that a portion of the energy is lost as heat transfer occurs while simultaneously increasing entropy generation, which eventually leads to a decline in both electrical and CCCHP efficiencies beyond the 30% moisture peak.
Energy and Exergy Efficiency Outcomes
The quantitative analysis of the simulated Combined Cycle Combined Heat and Power (CCCHP) systems reveals a distinct correlation between fuel type, moisture levels, and overall thermodynamic performance. In the biomass-only configuration, energy efficiency exhibited a significant upward trend as moisture content increased from 10% to 30% (w/w). Specifically, the biomass CHP efficiency peaked at 67.22% at the 30% moisture threshold. This improvement is attributed to the enhanced flue gas energy and the subsequent optimization of the Rankine cycle performance within the heat recovery steam generator (HRSG) unit.
A similar positive trajectory was observed in the coal–biomass cofiring simulations. In these systems, energy efficiency improved from 56.94% to 62.37% as the moisture content rose to 25.56%. However, the study identifies a critical limitation: once moisture levels exceeded these respective peaks (30% for biomass and 25.56% for cofiring), efficiency began to decline. This suggests that while moderate moisture enhances energy transfer to the steam turbine system, excessive moisture leads to efficiency losses, likely due to the high energy required to evaporate water during combustion and the potential for flue gas to cool below the dew point.
Exergy analysis, which evaluates both the quantity and quality of energy, provided a deeper understanding of system irreversibilities. The maximum exergy efficiency reached 50.06% for biomass samples and 50.10% for the cofiring samples. These figures represent a notable advancement over the baseline coal performance. For comparison, the coal-only exergy efficiency remained consistent with previous research at approximately 47%, specifically measuring 47.59% in this simulation at a 10% moisture level.
| Fuel Type | Peak Energy Efficiency (%) | Moisture Level at Peak | Max Exergy Efficiency (%) |
|---|---|---|---|
| Biomass | 67.22% | 30% | 50.06% |
| Cofiring | 62.37% | 25.56% | 50.10% |
| Coal-only | 56.94% | 10% | 47.59% |
The transition from a coal-dominant system to a biomass or cofired system demonstrates that renewable integration does not merely offer environmental benefits, such as the reduction of CO2 and SO2 emissions, but also yields superior exergetic quality. While biomass requires a higher fuel feed flow rate to produce the required 150 MW output due to its lower calorific value compared to coal, the recovered energy through the Rankine cycle compensates for these requirements, resulting in higher overall system efficiencies under optimized moisture conditions.
Environmental Impact and Emission Reduction
The transition from traditional coal-based power generation to biomass and coal–biomass cofiring systems represents a critical strategy for mitigating the global environmental crisis. The urgency of this shift is underscored by the fact that global CO2 emissions from coal combustion reached 15.3 GT in 2021, representing over 40% of the world’s total carbon dioxide emissions. Coal is not only a primary driver of climate change through greenhouse gas release but also a significant source of SO2 and NOx, which contribute to acid rain and smog formation.
Quantification of Gas Reductions
The simulation results demonstrate that increasing the biomass fraction and optimizing fuel moisture content lead to substantial improvements in the environmental profile of CCCHP systems. In the biomass-only combustion study, the CO2 concentration was approximately 20%. While this figure is significant, biomass is considered a carbon-neutral fuel source because the CO2 released during combustion is roughly equivalent to the amount absorbed by the plant during its growth period, thereby neutralizing the carbon cycle. In contrast, coal combustion introduces additional non-renewable carbon into the atmosphere.
The study specifically quantified the reduction of acid-rain-causing gases within the cofiring framework. As fuel moisture content in the coal–biomass cofiring system increased from 10% to 25.56%, a measurable decrease in pollutants was recorded:
- Carbon Dioxide (CO2): Emission concentrations reduced from 22.42% (w/w) to 20.77% (w/w).
- Sulfur Dioxide (SO2): Emission concentrations dropped from 0.66% to 0.61%.
Mechanisms and Comparative Advantages
Biomass combustion results in significantly lower SO2 and NOx emissions compared to coal-only systems. This reduction is primarily due to the lower sulfur and nitrogen content inherent in biomass materials like bagasse compared to the chemical composition of coal. Furthermore, the simulation indicated that higher moisture content in biomass fuel correlates with lower flue gas exit temperatures, which can influence the final concentration of pollutants released into the environment.
While coal remains a highly polluting energy source, the integration of biomass through cofiring provides a pragmatic pathway toward sustainability. By replacing a portion of coal with renewable agricultural residues, the system effectively offsets greenhouse gas contributions and reduces the precursors for acid rain, supporting the international mandate to phase out coal consumption by 2040 to prevent further environmental degradation.
Key findings
- Optimal Moisture Content for Biomass — System efficiency peaks at 30% (w/w) moisture content for bagasse; exceeding this threshold triggers latent heat release and flue gas cooling that degrades performance.
- Shift in Cycle Dominance — Increasing moisture reduces Brayton cycle output (87.78 MW to 72.18 MW) but compensates through increased Rankine cycle power due to higher flue gas enthalpy.
- Exergy Efficiency Peaks — Exergy efficiency reached a maximum of 50.10% in cofiring systems, indicating higher quality energy utilisation compared to the 47.59% observed in pure coal systems.
- Environmental Performance Improvement — Cofiring biomass with 25.56% moisture reduced CO2 concentration from 22.42% to 20.77% (w/w).
Method and assumptions
The study utilised Ebsilon Professional 16 to model a CCCHP plant. The system boundary includes fuel and air input through to power and heat output. A total of 16 combustion systems were simulated: one coal-only (10% moisture), seven biomass-only (10-40% moisture), and eight coal–biomass cofiring (10-25.56% moisture). The Gaus–Seidel algorithm was used for solving linearised characteristic equations. Thermodynamic data was sourced from standard libraries including IAPWS-IF97 for water/steam and REFProp for fluids. Net Calorific Values for bagasse were determined experimentally using an automatic bomb calorimeter (Model 5E-C5508). The study assumed potential and kinetic exergy changes were negligible, focusing on chemical and physical exergy to determine system irreversibility.
Where it applies
- Industrial Power Generation Projects — Applicable to large-scale 150 MW renewable energy projects requiring stable integration with existing electrical grids and industrial heat demand.
- Biomass Pre-processing Optimization — Provides data for fuel suppliers to balance the costs of drying bagasse against the thermodynamic benefits of specific moisture levels.
- Sustainable Urban Heating — Useful for designing district heating systems that leverage waste heat from combined cycle plants to provide 25 MW of thermal energy.
Terms used
- Exergy — The maximum theoretical useful work obtainable from an energy conversion system as it comes into equilibrium with its environment.
- Brayton Cycle — A thermodynamic cycle that describes the workings of a constant-pressure heat engine, typically used in gas turbines.
- Rankine Cycle — A model used to predict the performance of steam turbine systems where a fluid continuously evaporates and condenses.
- Cofiring — The simultaneous combustion of two different types of fuel, such as coal and biomass, within the same power plant.
- Enthalpy — A thermodynamic property representing the total heat content of a system, equal to internal energy plus the product of pressure and volume.
Questions and answers
Why does biomass efficiency increase with moisture up to 30%?
Increased moisture generates higher concentrations of water vapor in the flue gas. Because water vapor has a higher specific heat capacity than dry gases like CO2, it retains and transfers more latent and sensible heat to the steam turbine system via the HRSG, improving Rankine cycle performance.
What happens if biomass moisture exceeds 30%?
Beyond 30%, the flue gas temperature drops significantly, often falling below the dew point. This causes water vapor to condense prematurely, releasing latent heat within the exhaust stream rather than transferring it to the working fluid, which reduces overall system efficiency.
How does biomass combustion affect the carbon cycle differently than coal?
Biomass is near carbon-neutral because it releases approximately the same amount of CO2 during combustion that the plant absorbed during its growth phase. In contrast, coal combustion introduces prehistoric carbon into the modern atmosphere, leading to a net increase in greenhouse gases.
Is biomass a viable total replacement for coal in these systems?
Yes, the study indicates biomass can achieve higher energy and exergy efficiencies than coal when moisture is optimised. However, biomass requires a higher fuel feed rate (e.g., 11.2 kg/s vs 7.43 kg/s for coal) to reach the same 150 MW output due to its lower calorific value.
How to cite
Wijesekara, D.; Amarasinghe, P.; Induranga, A.; Vithanage, V.; Koswattage, K.R. Energy, Exergy, and Environmental Impact Analysis and Optimization of Coal–Biomass Combustion Combined Cycle CHP Systems. Sustainability 2025, 17, 2363.