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In recent years, Taiwan's power sector has remained a hot topic, with the Hsing Ta Power Plant being a focal point: the phased decommissioning of aging coal-fired units, the gradual commissioning of gas-fired recirculation units, and increasingly stringent emission standards. But what is the principle behind thermal power generation? Many people are unclear about how a "coal-fired machine" actually converts coal into electricity. What are its efficiency and emission levels? How does it differ from modern advanced power plants? And why has there been a continuous shift from coal-fired to gas-fired and low-carbon technologies in recent years? Using the Hsing Ta Power Plant as an example, we will focus on the core technology of coal-fired power generation, clearly explaining the working principle of the Rankine Cycle (steam cycle), and then discuss how current technologies should transform and move towards environmental protection.
How to turn coal into electricity: The principle of the Rankine cycle
Coal-fired power generation follows the Rankine Cycle. The boiler heats feedwater into high-temperature, high-pressure steam → the steam drives the turbine to do work → the generator outputs electricity → the steam is condensed and returned to the water in the condenser → the feedwater pump returns it to the boiler, forming a closed loop.
The key to efficiency in a loop is:
- Higher steam parameters result in better efficiency: increasing the main steam pressure/temperature and the number of reheat cycles (single or double reheat) can improve the turbine's work output and reduce the exhaust steam moisture content.
- The lower the cold end temperature, the better: Condenser vacuum (low back pressure) can improve cycle efficiency, but good cooling (seawater/circulating water) is required.
- Effective heat exchange in boilers: Economizer (ECO), superheater (SH), reheater (RH), and air preheater (APH) extract waste heat from flue gas, but the exhaust gas temperature must not be lower than the acid dew point to avoid low-temperature corrosion.
- Auxiliary equipment energy consumption should be reduced: induced draft fans, forced draft fans, coal mills, circulating pumps, etc. are key loads for net efficiency.
A simplified concept of thermal efficiency:
- ηₙₑₜ ≈ 3600 / Heat Rate (kJ/kWh). For example, heat consumption of 10,000 kJ/kWh ⇒ η ≈ 36%.
- Expressed in kcal: Heat Rate (kcal/kWh) ≈ 860 / η. η = 41% ⇒ approximately 2,097 kcal/kWh. Heat consumption is often used as a KPI in engineering projects because it reflects the energy consumption of both main and auxiliary equipment.
Common technical approaches and efficiency differences
| type | Typical parameters of main steam/reheat | Common unit capacity | Typical net efficiency (lower heating value, LHV) | Heat consumption (kJ/kWh) reference | Remark |
|---|---|---|---|---|---|
| Sub-critical (Sub-C) | P < 22.1 MPa, main steam ~540°C, reheat ~540°C | 300–600 MW | 34–38% | 9,500–10,600 | Traditional coal-fired power is the mainstream method, with relatively low material requirements. |
| Supercritical (SC) | ≥24 MPa, main/reheat ~565–590°C | 500–1,000 MW | 39–42% | 8,600–9,200 | Improve efficiency and reduce coal consumption |
| Ultra-supercritical (USC) | 25–30 MPa, ~600–620°C | 600–1,000+ MW | 42–45% | 8,000–8,600 | Mainly 9–12Cr heat-resistant steel |
| Advanced Ultra-Supercritical (A-USC) | 30–35 MPa, ~700°C | 700–1,000+ MW | 45–48% (planned value) | 7,500–8,000 | Requires nickel-based superalloys, high cost |
*Quick Explanation: Moving to the right (USC, A-USC) means hotter and higher-pressure steam, better efficiency, and lower coal consumption per kilowatt-hour, but the material, manufacturing, and maintenance thresholds are also raised accordingly.
What does the "standard configuration" of environmental protection equipment look like?
Coal-fired power plants must be equipped with a three-stage air pollution control system:
- Desulfurization (FGD, commonly wet limestone-gypsum process): SO₂ removal rate is commonly 90-95% (TP3T+); the key design factors are liquid-to-gas ratio (L/G), Ca/S equivalent ratio, and in-tower contact efficiency.
- Denitrification (SCR): NOx is reduced to N₂ and water by ammonia/urea catalyst in the range of 300±100°C; efficiency is 80–90 %, and ammonia emission needs to be controlled.
- Dust collection (ESP/bag filter/wet ESP): particulate matter levels down to single digits mg/Nm³. These devices increase auxiliary equipment energy consumption and pressure drop, so a trade-off must often be struck between high efficiency in the main unit (USC) and high removal efficiency at the end point.
Where does the old machine differ from the modern one in terms of progress?
- Materials and steam parameters: The transition from Sub-C to SC/USC is essentially due to the application of heat-resistant materials (9–12Cr steel, some nickel-based alloys), which allows for higher temperatures and less wet section loss.
- Cyclic design: primary reheat → secondary reheat, sliding pressure operation, variable steam inlet adjustment of the turbine, improving part-load efficiency.
- Waste heat recovery: tail-end flue gas heat recovery (GGH/cold end heat exchange), condenser vacuum optimization, and variable frequency auxiliary equipment to reduce internal losses.
- Digitalization: Online optimization is performed based on heat consumption, boiler excess air (O₂%), flue gas temperature, coal mill load, etc.; combined with coal quality sensing and combustion air distribution, incomplete combustion and coking/ash accumulation are reduced.
- Water and ash: Water conservation through water recycling and resource utilization of ash and slag (cement/building materials) to reduce by-product disposal costs.
Why is the world continuing its shift from coal-fired power to low-carbon energy?
- Gas-fired combined cycle (CCGT): gas turbine + waste heat boiler + steam turbine, with a net efficiency of 58–64% (LHV), good start-up and shutdown flexibility, and low NOx/PM, but it is affected by gas source price and gas supply security.
- Biomass energy or ammonia co-combustion with coal: 5–20% energy ratio can reduce fossil carbon emissions, but combustion characteristics, corrosion and supply chain need to be evaluated.
- IGCC (Integrated Gasification Combined Cycle): Coal is first gasified and then syngas is used to drive a combined gas-steam cycle; it has good efficiency and low pollution, but high investment and engineering complexity.
- CCUS (Carbon Capture and Recycling): Post-combustion amine washing can capture ~901 TP3T CO₂, but it will reduce efficiency by 6-8 percentage points, requiring thermal integration and economic incentives. Simply put: efficiency, carbon emissions, system resilience, and fuel supply are the four-axis trade-offs of transformation; the Xingda Power Plant's recent strategic focus has been on transitioning from coal-fired to gas-fired turbines to improve efficiency and reduce emissions, while leaving room for future lower-carbon options. We break down the core technology of coal-fired turbines into three things: the energy path of the Rankine cycle, the generational evolution of steam parameters and materials, and the integrated impact of three-stage environmental protection. You can use "heat consumption" as a line to connect the main unit, auxiliary units, and environmental protection units, and then use KPIs such as O₂, flue gas temperature, and vacuum for daily adjustments. Modern advanced methods have pushed coal-fired efficiency to the 42-451 TP3T level, but if the goal is carbon emissions and system scheduling, gas recirculation, co-combustion, and CCUS become the next key technologies.
