Energy efficiency improvement scheme of coaxial heat exchanger in Biomass Power generation

Source: 　 Time: 2026-01-07 09:40:17　Hit:

Biomass power generation uses agricultural and forestry waste (such as straw, sawdust, rice husks, etc.) as fuel, and generates steam through combustion to drive steam turbines for power generation. Its core objective is to enhance energy conversion efficiency (currently, the efficiency of mainstream power plants is approximately 25% to 35%, far lower than the over 40% of coal-fired power plants). The main bottlenecks in energy efficiency lie in the underutilization of waste heat resources (such as sensible heat from flue gas, latent heat from exhaust steam, waste heat from biomass drying, and waste heat from wastewater discharge, etc.) and the large irreversible losses in the thermodynamic cycle. The coaxial heat exchanger (tubular type), with its advantages such as compact structure, high heat exchange efficiency, good isolation between cold and hot media, and resistance to corrosion by complex media, can play a key role in the stepwise recovery of waste heat, optimization of thermal cycles, and system coupling in biomass power generation, and has become a core equipment for energy efficiency improvement. The following proposes specific energy efficiency improvement plans from four dimensions: application scenarios, key technologies, system integration, and intelligent control:

I. Energy Efficiency Bottlenecks of Biomass Power Generation and the Application Scenarios of coaxial heat exchanger

1. Core energy efficiency bottleneck

• Waste of flue gas waste heat: The flue gas temperature of biomass boilers is usually 120-250℃ (directly discharged by conventional power plants), and the sensible heat of flue gas accounts for 15%-25% of the total input energy. Failure to recover it leads to energy waste.

• Waste heat loss from exhaust steam: The exhaust steam from the steam turbine (with a pressure of 0.005-0.008MPa and a temperature of 32-41℃) is directly discharged into the condenser, and the latent heat (approximately 2400kJ/kg) is not utilized.

• High energy consumption in biomass drying: The moisture content of the fuel entering the furnace needs to be controlled at 15%-20% (if too high, it will affect the combustion efficiency), and the drying process consumes a large amount of thermal energy (accounting for 10%-15% of the fuel energy).

• Low thermal cycle efficiency: The traditional Rankine cycle (steam parameters 3.5-4.5MPa/435-450℃) has limited efficiency and lacks intermediate reheating or reheating optimization.

2. Adaptation scenarios for coaxial heat exchanger

The coaxial heat exchanger can achieve efficient recovery of waste heat and improvement of energy grade in the following links:

Application scenarios, medium characteristics, core functions, energy efficiency improvement goals

The high-temperature flue gas (150-300℃, containing ash, alkali metals, and acidic gases) at the tail end of the boiler is used to preheat the boiler feed water/air, or to generate low-pressure steam to reduce the flue gas temperature to 80-120℃. The recovered heat accounts for 60%-80% of the sensible heat of the flue gas

The waste heat from the steam turbine exhaust steam (32-41℃, pressure 0.005-0.008MPa) and circulating water (25-30℃) are used to drive absorption heat pumps /ORC for power generation, or to preheat the heating network water. The utilization rate of waste heat from the exhaust steam is increased by 30%-50%, and the power supply efficiency is enhanced by 2%-3%

The energy consumption for biomass drying is reduced by 20% to 30% by preheating the drying exhaust gas (60-80℃, containing moisture) and the drying material (40-60℃), as well as heating the drying medium (heat transfer oil)

The heat recovered from the boiler exhaust water (100-150℃) and steam turbine exhaust water (80-120℃) preheating deaerator feed water or domestic hot water accounts for 70%-90% of the heat recovered from the exhaust water

Ii. Key Technical Solutions: "Material-Structure-System" Collaborative optimization

1. Material selection: Resistant to corrosion by complex media and anti-slagging/wear

Biomass flue gas contains alkali metals (K, Na), chlorine (Cl), and sulfur (S), which can easily lead to metal corrosion (such as high-temperature chlorine corrosion and low-temperature sulfuric acid dew point corrosion) and slagging (melting and adhesion of alkali metal oxides). High-speed scouring of ash (SiO₂, Al₂O₃) is prone to cause wear. The material needs to take into account high-temperature resistance, corrosion resistance, slag resistance and wear resistance.

Application scenarios, medium temperature, recommended material combinations, core performance requirements

Boiler tail flue gas waste heat recovery 150-300℃ inner tube (feed water/heat transfer oil) : ND steel (09CrCuSb, resistant to sulfuric acid dew point corrosion); Outer tube (for flue gas flow) : 310S stainless steel (25Cr-20Ni, anti-oxidation) + SiC ceramic coating (anti-alkali metal slagging), resistant to Cl⁻ corrosion (≤1000mg/L), anti-slagging (ash melting point > 1200℃)

Inner tube for waste heat recovery of steam turbine exhaust steam at 32-41℃ (for exhaust steam condensate water) : 316L stainless steel (resistant to wet chlorine gas corrosion). Outer pipe (for circulating water) : Duplex stainless steel 2205 (resistant to erosion and wear), resistant to low-temperature wet corrosion and water flow rate impact (≤3m/s)

Biomass drying waste heat recovery at 60-80℃ : Overall: aluminum alloy (6061-T6, lightweight + thermal conductivity 167W/m·K) or fiberglass reinforced plastic (FRP, resistant to moisture corrosion), resistant to wet heat aging and biomass dust adhesion

Waste heat recovery from drainage water at 100-150℃ inner tube (for drainage water) : titanium alloy TA2 (resistant to chloride ion pitting corrosion). Outer pipe (for cold water) : 304 stainless steel, resistant to high concentration of salt (TDS > 5000mg/L) and stress corrosion

Innovation direction

• Anti-slagging coating: The inner wall of the outer tube is sprayed with a nano-Al ₂O₃-TiO₂ composite coating (50-100μm thick), which reduces ash adhesion through the "low surface energy effect".

• Gradient functional material (FGM) : The inner and outer tubes are made of "heat-resistant steel + ceramic" composite tubes (such as 310S stainless steel for the outer tube + SiC ceramic for the inner tube), which alleviates the coupling damage caused by thermal stress and corrosion.

2. Structural design: Enhanced heat transfer, anti-clogging, and low-resistance optimization

In view of the characteristics of biomass media such as "high ash content, easy scaling and large load fluctuations", the structural design should focus on heat transfer enhancement, anti-clogging and wear resistance, and adaptability to variable loads

(1) Enhanced heat transfer design of the flow channel

• Spiral groove + fin composite structure: Spiral grooves are processed on the outer wall of the inner tube (depth 0.5-1mm, pitch 10-15mm), and sawtooth-shaped fins are added to the inner wall of the outer tube (height 3-5mm, spacing 15-20mm). The heat transfer coefficient is increased by 40%-60% compared with smooth tubes (the K value on the flue gas side can reach 50-80W/m²·K).

• Multi-flow diversion and variable cross-section flow channels: The high-temperature flue gas side adopts "segmented parallel flow channels" (each section length ≤2m) to avoid local overheating and slagging. On the low-temperature side (such as the feed water side), a "gradually shrinking flow channel" (inlet cross-sectional area > outlet) is adopted, with the flow rate controlled at 1.5-2.5m/s (taking into account both heat transfer and clogging prevention).

• Spoiler insert: A helical spring spoiler (with a diameter slightly smaller than the ring gap) is set in the annular gap to enhance fluid turbulence, suitable for low-flow-rate flue gas (< 5m/s).

(2) Anti-clogging and wear-resistant design

• Large pipe diameter and self-cleaning structure: The outer diameter of the inner pipe is ≥32mm (to avoid ash blockage), and the outer pipe is installed in a line vibration device (frequency 1-2Hz), and the accumulated ash on the outside of the pipe is regularly removed.

• Anti-wear lining: The inner wall of the outer pipe of the flue gas inlet section is surfacing with a WC-Co hard alloy layer (thickness 2-3mm, hardness HRC≥60) to resist ash erosion (wear rate < 0.05mm/ year).

• Pre-filtration and descaling: A cyclone separator (for removing large ash particles) + Y-type filter (80-120 mesh, to intercept fiber impurities) is installed at the medium inlet. Citric acid solution (2%-3%) is regularly introduced to clean the scale between the pipes online.

(3) Compact and low-resistance design

• Modular parallel layout: It adopts "multi-tube parallel modules" (with a single module heat exchange area of 50-100m²), and the number of modules can be flexibly increased or decreased according to the load (adjustment ratio 1:5), adapting to the load changes caused by moisture fluctuations of biomass fuel (50%-120%).

• Low-resistance flow field: A deflector cone (with an Angle of 30°-45°) is set at the inner pipe inlet, and the annular gap flow velocity of the outer pipe is ≤3m/s (pressure drop < 15kPa), avoiding additional power consumption of the fan/pump.

3. System Integration: Stepwise utilization of waste heat and multi-cycle coupling

The core of energy efficiency improvement is "temperature matching and stepwise utilization", matching waste heat of different grades to the optimal energy consumption link. The coaxial heat exchanger needs to be deeply integrated with the biomass power generation system:

(1) Stepwise recovery system for waste heat from flue gas at the tail end of the boiler

• High-temperature section (250-300℃) : The coaxial heat exchanger (with feed water in the inner tube and flue gas in the outer tube) preheats the feed water from 104℃ to 150℃, recovering heat to reduce the load on the boiler economizer.

• Medium temperature range (150-250℃) : Another coaxial heat exchanger (with heat transfer oil in the inner tube and flue gas in the outer tube) heats the heat transfer oil to 200℃, driving the ORC (Organic Rankine Cycle) unit to generate electricity (the ORC working medium is selected as R245fa, with a power generation efficiency of 8%-12%);

• Low temperature section (80-150℃) : The coaxial heat exchanger preheats the boiler intake air (from 25℃ to 80℃), enhancing the combustion efficiency (more complete release of fuel volatile matter).

(2) Deep recovery system for waste heat from steam turbine exhaust steam

• Absorption heat pump coupling The coaxial heat exchanger, as the core component of the "generator - condenser", the exhaust steam (32-41℃) enters the inner tube to release heat and condense, driving the lithium bromide absorption heat pump (COP=1.5-2.0) to heat the circulating water from 25℃ to 55℃, which is used for heating in the factory area or preheating the feed water of the deaerator.

• ORC power generation coupling: Exhaust steam is heated by a coaxial heat exchanger to form a low-boiling-point working medium (such as isopentane), generating steam to drive the ORC unit for power generation (single-unit capacity 50-200kW), and the utilization rate of exhaust steam waste heat is increased from 0 to 30%-40%.

(3) Closed-loop recovery system for waste heat from biomass drying

• Recovery of waste heat from drying exhaust gas: The coaxial heat exchanger (with the inner tube carrying the drying medium heat transfer oil and the outer tube carrying the drying exhaust gas) heats the heat transfer oil from 60℃ to 90℃, which is used to preheat the fresh air for drying (from 20℃ to 50℃), reducing the drying energy consumption by 25%.

• Waste heat recovery from drying materials: A coaxial heat exchanger (with cooling water in the inner tube and high-temperature materials in the outer tube) is installed at the material discharge port to cool the materials from 60℃ to 40℃, and the recovered heat is used to heat the boiler feed water.

Iii. Intelligent Control: "Dynamic adjustment + predictive maintenance" ensures efficient operation

The composition of biomass fuel (moisture and ash content) fluctuates greatly, and it is necessary to achieve dynamic matching between the coaxial heat exchanger and the system through intelligent control.

1. Dynamic load regulation

• Sensor integration: PT100 temperature sensors, electromagnetic flowmeters, and pressure transmitters are installed at the inner/outer pipe inlet and outlet to monitor the medium flow, temperature, and pressure in real time.

• Variable flow control: The medium flow is regulated by a variable frequency water pump/fan (regulation ratio 1:3). When the flue gas volume increases, a backup casing module is connected in parallel to maintain the heat exchange efficiency.

• Multi-mode switching: Switch between "power generation mode" (prioritizing ORC power generation) and "heating mode" (prioritizing heat pump heating) based on season/load to maximize the comprehensive utilization efficiency of energy.

2. Predictive maintenance

• Corrosion/Wear monitoring: Install resistance probes (ER) and acoustic emission sensors (AE) to monitor in real time the thinning rate of the pipe wall (accuracy ±0.01mm/ year) and crack initiation.

• Slagging warning: The temperature distribution on the outer tube surface is monitored by an infrared thermal imager. Abnormal hot spots (temperature difference > 20℃) indicate slagging and trigger the shaking device.

• Digital twin optimization: Based on the CFD-FEM coupling model, input fuel composition and flue gas parameters, predict the optimal heat exchange area and flow channel structure, and dynamically optimize operating parameters.

Iv. Typical Case: Flue Gas Waste Heat Recovery Renovation of a 30MW Biomass Power Plant

• Before renovation: Flue gas temperature 220℃, sensible heat of flue gas not recovered, boiler efficiency 82%;

• Renovation plan:

• Equipment: 2 coaxial heat Exchangers (inner tubes made of ND steel for water supply and outer tubes made of 310S stainless steel +SiC coating for flue gas), with a single heat exchange area of 200m², featuring a spiral groove + fin structure;

• System: After passing through the heat exchanger, the flue gas temperature drops to 120℃. The recovered heat is used to preheat the feed water (from 104℃ to 145℃), and the excess heat drives the ORC unit (working medium R245fa, power generation capacity 500kW).

• Effect:

The flue gas temperature is reduced by 100℃, the boiler efficiency is increased to 87%, and 3,600 tons of standard coal are saved annually.

The annual increase in ORC power generation is 4 million kWh, with an overall energy efficiency improvement of 4.2% and an investment payback period of 3.5 years.

V. Future Development Trends

1. Material innovation: Promote ceramic matrix composites (CMC) and metallic glass (amorphous alloy), increase the temperature resistance to over 1000℃, and enhance the anti-slagging performance by 50%.

2. Miniaturization and distribution: Combining the microchannel coaxial heat exchanger (channel diameter 0.5-2mm), develop compact modules to adapt to small-scale biomass cogeneration projects;

3. Multi-energy complementary integration: Coupled with solar thermal collection and energy storage systems, it forms a comprehensive energy supply system of "biomass + new energy", further increasing energy efficiency by 5% to 8%.

4. Ai-driven design: Utilize machine learning to optimize the flow channel structure (such as genetic algorithm iteration of helical groove parameters), achieving automatic matching of "fuel characteristics - heat exchange performance - system efficiency".

Summary

The energy efficiency improvement of the coaxial heat exchanger in biomass power generation should be centered on "scene adaptation, material corrosion resistance, structural strengthening, system coupling, and intelligent regulation and control". By using composite materials and coatings resistant to complex media corrosion to break through material limits, the spiral groove/fin structure enhances heat transfer and is resistant to clogging and wear. It is deeply integrated with multiple cycles such as flue gas waste heat ORC and exhaust steam heat pumps to achieve stepwise utilization, and ultimately dynamically optimized operation through intelligent control. This solution can increase the efficiency of biomass power plants from 25% to 35% to 35% to 42%, while reducing carbon emissions by 15% to 20%. It is a key technical path for promoting the "cost reduction, efficiency improvement, and low-carbon transformation" of biomass power generation.

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