Structural Design and Resistance Control of Bag Filters for Large Coal-fired Boilers

The flue gas treatment for large coal-fired boilers (65 t/h and above) presents three key challenges: extremely high air volume (100,000 – 500,000 m³/h), high dust concentration (20-50 g/m³), and long operational cycles (≥8000 hours annually). The supporting bag filter must therefore balance structural stability with high efficiency in its design. Simultaneously, precise resistance control (normal operating resistance of 1200-1500 Pa) is essential to ensure energy cost-effectiveness.

This article will explain the following aspects: core structural design points, the resistance control mechanism, and coordinated optimization strategies.

I. Core Structural Design Points

(A) Modular Casing Layout

Large bag filters require a “multi-unit modular” design. The air volume handled per single unit should be controlled between 50,000 and 80,000 m³/h. This prevents airflow maldistribution and structural deformation caused by an oversized single unit.

• Unit Division:

  • A 65 t/h boiler (air volume approx. 120,000 m³/h) uses 2 units.

  • A 300 MW unit (air volume approx. 400,000 m³/h) uses 6-8 units.

  • Connect these units with a manifold to ensure airflow distribution deviation ≤ ±5%.

• Casing Structure:

  • Use welded Q345R steel plates (thickness 12-16 mm).

  • Space columns ≤ 3 m apart.

  • Install reinforcing beams on the top (deflection ≤ L/500).

  • Ensure the structure can withstand the combined load from filter bags, dust, and maintenance (total load ≥ 5 kN/m²).

• Anti-corrosion Treatment:

  • Spray the inner wall with a polyurea coating (dry film thickness 1.2 mm).

  • Apply arc transition and reinforced anti-corrosion to weld seams.

  • Guarantee a corrosion-resistant life ≥ 5 years in sulfur-containing flue gas (SO₂ ≤ 3000 mg/m³).

(B) Adaptation of Filter Bags and Cleaning System

The filter bags and cleaning system must match high-flow operating conditions to balance filtration efficiency and cleaning energy consumption.

• Filter Bag Configuration:

  • Use Φ160 × 6000 mm filter bags (single bag filtration area 3.01 m²).

  • Select the material as 550 g/m² PPS+PTFE composite filter media (heat resistance 190°C).

  • Space the bags 350 mm apart to prevent dust bridging.

  • Calculate the number of bags per unit using this formula: Air Volume ÷ (0.8 m/min × 60 × Single Bag Area).

• Cleaning System:

  • Employ 3-inch solenoid pulse valves (response time ≤ 0.03 seconds).

  • Let each valve control 6 filter bags at a blowing pressure of 0.55-0.6 MPa.

  • Adopt compartmentalized offline cleaning. This means cleaning only one unit at a time while others operate normally. Consequently, it avoids system-wide resistance fluctuations.

(C) Integrated Flue Gas Pretreatment

For high dust loads, it is necessary to integrate efficient pretreatment devices at the filter inlet.

• Pre-dust Removal:

  • Connect a cyclone separator in series within the inlet duct (separation efficiency ≥ 85% @ 30 µm).

  • This removes 30-40% of coarse dust particles (≥ 50 µm), thereby reducing the load on the main filter media.

• Temperature Control Module:

  • Install a flue gas heat exchanger.

  • Cool the high-temperature flue gas (250-300°C) down to 160-180°C (the safe range for PPS filter media).

  • Ensure the heat exchange efficiency ≥ 90% and the temperature fluctuation ≤ ±10°C.

• Airflow Distribution:

  • Set up a three-stage flow guide at the inlet (diffusion section → guide plates → perforated flow equalization plate).

  • Use an equalization plate with an opening ratio of 35-40%.

  • Ensure the deviation in face velocity across the filter bags ≤ ±3%, which minimizes local abrasion.

II. Core Mechanisms for Resistance Control

(A) Resistance Composition and Critical Limits

The total resistance of a large bag filter consists of three parts:

1. Inherent Bag Resistance: 100-200 Pa

2. Primary Dust Cake Resistance: 300-500 Pa

3. Residual Resistance after Cleaning: 400-600 Pa

The target for total resistance control is 1200-1800 Pa. Exceeding 2000 Pa will cause a sharp increase in induced draft fan energy consumption.

• Normal Range: 1200-1500 Pa (dust cake thickness on bag surface: 1-2 mm). Here, filtration efficiency is ≥ 99.9%.

• Warning Range: 1500-1800 Pa. Strengthen the cleaning process at this stage.

• Emergency Range: > 1800 Pa. Shut down for inspection immediately to prevent fan overload.

(B) Tiered Resistance Control Strategy

Apply differentiated control based on the resistance stage.

(C) Optimization of Key Parameters

• Air-to-Cloth Ratio:

  • Strictly control it between 0.7 and 0.8 m/min (approx. 0.75 m/min for a 65 t/h boiler).

  • Remember, increasing the ratio by 0.1 m/min raises resistance by 150-200 Pa.

• Cleaning Sequence:

  • Implement “unit-rotational cleaning”.

  • Keep the interval between cleaning adjacent units ≥ 3 minutes.

  • This prevents total resistance fluctuations > 200 Pa.

• Air Supply Quality:

  • Treat compressed air with a “refrigerated dryer + precision filter” system.

  • Ensure the dew point ≤ -40°C and oil content ≤ 0.01 mg/m³.

  • This prevents abnormal resistance caused by moisture combining with dust.

III. Prevention and Adjustment of Resistance Anomalies

(A) Common Causes and Countermeasures for Resistance Anomalies

(B) Dynamic Adjustment System

Large bag filters need an intelligent resistance regulation system for real-time response.

• Monitoring Module:

  • Install differential pressure transmitters at the inlet/outlet of each unit (range 0-3000 Pa, accuracy ±1%).

  • Collect data every 10 seconds and trigger automatic alarms for anomalies.

• Adjustment Logic:

  • When resistance deviates ±200 Pa from the setpoint: Automatically adjust cleaning parameters (pressure ±0.05 MPa, cycle ±2 minutes).

  • When deviation reaches ±300 Pa: Activate the standby cleaning unit.

• Linkage Control (with Boiler Load):

  • When load > 80% (dust concentration rises): Lower the preset resistance upper limit by 100 Pa.

  • When load < 50%: Appropriately increase the air-to-cloth ratio to 0.85 m/min to avoid excessively low resistance.

IV. Application Case and Performance Validation

Design and Operational Data for a Bag Filter on a 300 MW Coal-fired Unit (Air Volume 420,000 m³/h):

• Structural Configuration:

  • 8 units (52,500 m³/h per unit).

  • 14,000 filter bags total (Φ160×6000mm, PPS+PTFE media).

  • 1,750 three-inch pulse valves.

• Resistance Control:

  • Normal operating resistance: 1300-1450 Pa.

  • Cleaning pressure: 0.55 MPa, cycle: 10 minutes.

• Operational Performance:

  • Outlet dust concentration: 3-5 mg/m³.

  • Induced draft fan power consumption: 8% lower than the design value.

  • Resistance fluctuation over 12 months of continuous operation: ≤ ±100 Pa.

  • Filter bag integrity rate: 99.8%.

V. Coordinated Optimization of Structure and Resistance

• Unit Quantity vs. Resistance:

  • Increasing unit count reduces load per unit. For example, 8 units show 100-150 Pa lower resistance than 6 units.

  • However, the initial investment increases by about 15%. Therefore, balance capital cost with energy consumption.

• Bag Length vs. Resistance:

  • A 6m bag provides 17% more filtration area and 8% lower unit resistance than a 5m bag.

  • But, ensure the cleaning airflow reaches the bag bottom (blowing tube should be 300 mm from the bottom).

• Airflow Velocity vs. Layout:

  • Optimize guide plate angles (30°) and equalization plate openings using CFD simulation.

  • This reduces local velocity deviation from ±8% to ±3%, improving resistance uniformity by 20%.

VI. Conclusion

The structural design of bag filters for large coal-fired boilers must focus on “modularity and high stability”. Concurrently, resistance control requires establishing a closed-loop system of “prevention – monitoring – adjustment”.

Practice proves that coordinated optimization of structural layout and resistance control achieves ultra-low emissions (dust ≤ 10 mg/m³). Furthermore, it confines the annual power consumption of the induced draft fan to within 2% of the boiler’s total energy consumption. Ultimately, this provides reliable support for the efficient and environmentally friendly operation of large coal-fired units.