Most of the FBC boilers were commissioned before 2010, which were in operation. Boiler design and combustion mechanisms were designed based on fuel availability. But now boiler users have to utilise multiple fuels.
We came across the steel plant audit in which, the boiler was designed for 30% dolochar and 70% coal, but the boiler is being operated inversely. The present operation was 70% dolochar & 30% coal. Present operating char is with a high level of fines.
Boiler operations and combustion parameters were observed and auxiliary data was collected for detailed analysis, boiler duty was found to be ok, but residence time is less than the required 2.8 sec for high fines fuels in AFBC boilers.
People in operation used to compare the incombustible number in LOI form, this is not a real number. If you use high ash fuel, this will dilute and if the boiler is being operated with low ash fuel, this number will be high.
LOI vs Un-Burnt carbon loss:
For the same LOI number, Carbon loss will be 3.08% in high ash fuel and 1.96% in low ash fuel. So, the plant person should compare boiler performance through carbon loss not in LOI format.
Boiler performance review:
During our visit boiler was inspected a high level of superheater inlet gas temperature was found. This indicates that fuel burning is more prevalent in superheater areas. Earlier days they used to face superheater failures due to overheating.
During calculation, found that residence time is less, so if high fines are used then secondary combustion will not be eliminated.
We decided to do secondary air with high turbulence to control LOI & superheater temperature secondary air.
High fluidisation velocity:
The boiler is being operated with 70% of char & 30% coal, for this combination fluidization velocity is being calculated as 3.05m/sec.
This fluidization velocity is high for the AFBC boiler system, it should be in the range of 2.5-2.6 M/sec. If it goes high then, naturally elutriation of fines will be more. LOI will increase. Due to high elutriation, fines will fly away and burn in the superheater zone and high superheater temperature pickup and high furnace outlet temperature are experienced.
To have good quality of fluidization the bed particle size range is critical. By the presence of oversize particles, fluidization will be affected. Ideally, the bed ash should not have any oversized particles (+ 3 mm).
High iron levels are experienced due to iron coming in dolochar. Only iron causes erosion. Iron levels should be contained below 10%. This is possible only by draining and replenishing with iron-free bed material, right from the first day of startup. For reclamation of iron-free bed ash, a magnetic drum system is required.
Low furnace Volume:
In addition to the high fluidization velocity, the furnace volume is also calculated to be insufficient. Furnace volume is directly related to the residence time of the combustion system. Ideally, the combustion system should have a minimum of 2.5 seconds of residence time to complete combustion before the flue gas reaches the superheater zone. Once the gas reaches the superheater area, the gas temperature starts reducing, and combustion stops.
For high-fines operation, the boiler needs 3 seconds of residence time to complete combustion. However, in this case, the residence time is calculated to be 2 seconds. The high fluidization velocity and limited furnace volume are bottlenecks. Due to the high fluidization velocity and insufficient furnace volume, the LOI is high, and the furnace outlet temperature is also elevated.
To control the furnace temperature, the refractory height should be reduced by 500mm from its present position.
DESIGN CONCEPT OF SECONDARY AIR SYSTEM:
The secondary air volume is considered to be 12% of the total combustion air. To achieve good turbulence inside the furnace, secondary air nozzles were provided in alternate locations.
- The secondary air is tapped from the APH outlet hot air duct, with a dimension of 650 square millimetres.
- Secondary air nozzles were provided only on the front and rear water wall panels. The secondary pipes were selected as 60 NB. For the insertion of the secondary air pipe, one water wall tube was removed, and a seal box was provided.
IMPLEMENTATION AND RESULTS
After implementing the secondary air system design LOI was reduced from 6.5% to 3.5% by which carbon loss was reduced by 1.5% and boiler efficiency improved by 1.5%.
Author:
Mr R.Nagarajaprasath
Director
