Diagnosis Of CFBC Efficiency & Frequent Failures

A Chinese-supplied CFBC (Circulating Fluidised Bed Combustion) boiler had been in operation for three years before our team was invited to conduct an inspection. The customer was experiencing multiple challenges related to boiler efficiency and availability. During our visit, we had the opportunity to inspect the internal condition of the CFBC boiler to determine the root causes of these persistent issues.

Our findings and recommendations are documented in this paper, with the aim of assisting operations and maintenance teams in identifying problem statements and their root causes effectively.

Identified Problems:

1. Frequent failures of the economiser top bank occur shortly after coil replacements.
2. Sustained operation at high bed temperatures.
3. Excessively high superheater steam temperatures & high erosion
4. Erosion in the furnace water wall

To investigate these issues, we employed advanced Computational Fluid Dynamics (CFD) tools to analyse gas flow patterns and recommended corrective engineering measures to resolve the failures.

This paper focuses specifically on the premature failure of the economiser coils and the remedial actions taken.

Economiser Design & Construction

The economiser was a counter-flow type with three coil banks arranged in a staggered layout. The coils had an outer diameter of 32 mm, with a transverse pitch of 80 mm and a longitudinal pitch of 50 mm.

Observed Issue

Despite coil replacement, the customer reported significant tube thickness reduction and failures within six months. In many cases, continuous operation could not be maintained for more than a month without experiencing issues.

Root Cause Analysis

An internal inspection revealed that the primary cause of failure was the uneven and preferential gas flow to the economiser top bank, a result of the existing flue gas inlet duct arrangement. This led to localised overheating and accelerated erosion of the economiser tubes.

CFD Analysis and Engineering Solution

CFD analysis was conducted to determine the required straight length in the flue gas duct to ensure uniform gas distribution across the economiser section. Based on our findings, we recommended relocating the top economiser bank to the bottom, which effectively provided an additional 1500 mm of straight duct length. This modification significantly improved gas flow distribution.

Outcome

Following the implementation of our design changes, the plant was restarted and has since been operating smoothly, without any further tube failures in the economiser section.

B. HIGH BED TEMPERATURE OPERATION

CFBC bed temperature is being controlled by the circulating ash content, and the evaporator heat transfer area provided in the furnace. Amount of ash is being controlled by cyclone design and loop seal performance.

Loop seal bed width should be equal to downcomer diameter, but provided here was less than the downcomer diameter. Due to insufficient ash re-circulation, heat transfer to the furnace wall was affected, and we did engineering work to improve ash collection in the cyclone and ash recirculation through the loop seal. 

We provided a loop seal construction drawing with a revised air nozzle for the loop seal for improved performance.

C. HIGH SUPERHEATER TEMPERATURE AND EROSION

As heat transfer to the furnace was affected, cyclone gas temperature increased to 1000 Deg C, which caused high heat pickup in superheater zone.

D. EROSION IN FURNACE WALL 

Not all erection engineers or erection contractors are clear on the quality requirements of the furnace panel assembly. Furnace wall panels are sent in parts due to transport and erection constraints. Field work demands stringent quality levels. Some of the serious defects and their effects are listed here.

1. Failure to remove the tube & fin buttweld beads leads to localised erosion patterns, which ultimately lead to tube failure at the fin-tube weldments.
2. Fin filler plates are to be properly placed in the plane. The welds should be full penetration welds, and the excess beads are to be ground off. Portable pencil grinders/carbide cutting tools are to be used to remove the weld beads on the fire side.
3. Panel fins, which are slit for alignment purposes, are to be sealed with full penetration welds. The beads are to be ground flush afterwards. Generally, during construction, the panel-to-panel weldments are carried out from outside the furnace. CFBC boiler demands scaffolding inside in order to carry out full penetration welds and to flush grind all the tube-to-tube butt weldments, fin-to-fin butt weldments, and fin closures at field joints.
4. The furnace wall panels have to be absolutely vertical in both Y-Z planes. Failure to maintain verticality leads to gross erosion of panels.
5. No scrap can be left on the fireside anywhere. Or else local failure will be encountered. Even a thermocouple inserted in the furnace will be subject to erosion. Not only would the thermocouple be cut, but also the panel tubes nearby will be eroded off.
6. Pressure tapping is permitted in the CFBC combustor. Again, the tapping should be flush with the fin.
7. Periodical replacement of panels may be required in CFBC boilers. The replacements may be warranted within a period of 5 years, depending on the coal ash particle abrasiveness and other construction defects. Usually, the replacement will be a length of 5 m from the bottom tapered panels. Such replacement panels are also to be fabricated using a mechanised panel manufacturing machine with auto SAW or auto MIG technology. 
8. Providing a shield is not permitted in the CFBC furnace.

Author:

R. Nagarajaprasath
Director
Boilertech Energy Services

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