CAPACITY ENHANCEMENT STUDY OF AFBC BOILERS & CFD APPLICATION

This paper reveals the capacity enhancement study of power generation steam boilers to cater to additional steam load to generate more power. Boiler load consideration during the design stage is essential. This is mainly applicable for captive power units like steel, cement, chemical & textile industries since there will be future expansion. Due to unexpected expansion in some of the captive power industries boilers are being required to operate above MCR rating to fulfill their captive load. However, it needs an operational & design study to avoid extreme damage due to overloading of the boiler.

Here we analyzed each and every part of the boiler from a design point of view and suspected areas were analyzed with CFD.

INTRODUCTION

Generally, the boiler has a certain overload capacity in the design itself. One can say that the boiler can be operated for an additional 10% load without any compromise in trouble. However, with a continuous large thermal load, the life of the boiler equipment will degrade.

For a boiler, short-time overload operation is allowed. However, in practice some boilers have a relatively large thermal load and a small boiler capacity, blindly making the boiler under long-term overload conditions. This is extremely unfavorable to the device itself and will seriously affect the useful life of the device.

In overload conditions, in order to generate more steam, more fuel must be used to increase the flow rate of flue gas, and the convection coefficient of heat on the flue gas side is increased. At the same time, due to the increase in flue gas temperature, the heat transfer temperature difference is increased, thus convective overheating. 

The increase in the amount of heat absorbed by the device exceeds the increase in the load to increase the temperature of the outlet vapor, which is detrimental to the operation of the superheater. Due to the increase in the temperature level of the entire furnace, the working conditions of the heat-receiving surfaces deteriorate, and the long-term over-temperature operation may cause damage to the metal materials. Under overload conditions, high fly ash concentration and high flue gas flow rate cause intense wear.

VISIT TO A STEEL PLANT

A steel industry that has a 90 TPH AFBC boiler designed for the following two low GCV coals. 100% Indian coal with GCV of 3500 kcal/kg & 3000 kcal/kg. During the visit, the boiler was mostly operated with different GCV mixes of Indian coal varying from 4100 kcal/kg to 3500 kcal/kg along with Dolachar (7 to 8% by weight) with GCV of 1000 kcal/kg. However, the load of the boiler is 10% more than the design capacity due to more power requirements on the process side. This operation was to be reviewed by us.

KEY REQUIREMENT

Fluidization velocity and bed coil area are the key design parameters for the generation capacity in the AFBC boiler. The furnace bed cross-sectional area decides the fluidization velocity. This plays a vital role in combustion and bed coil life. The required fluidization velocity in an AFBC boiler is 2.6 m/s

The challenge in an AFBC boiler is to maintain bed temperature within acceptable limits to avoid hot & cold clinker. During the design stage, the combustion temperatures of the AFBC process

are maintained below the fuel ash softening temperature, so low-grade fuels can be burned without the risk of slagging and fouling in the boiler. Considering low ash fusion coals, the combustion temperature should be limited between 870 to 920°c. 

However, due to additional load requirements, the boiler is being overloaded. In that case, fuel feeding will be and combustion temperature cross above the ash fusion temperature of coal which in turn reduces the life of boiler parts. Here redesigning of the bed coil is required to reduce the combustion temperature. This is achieved by lowering the bed coil slope without affecting boiler circulation.

OUR APPROACH

A. Study of fuel feeding system

B. Study of pressure parts like a water wall, bed coil, boiler bank, economizer, and superheater

C. Capacity of auxiliaries like, FD, PA, ID, BFP, etc

D. Study of tube metal temperature to avoid overheating

E. Air nozzle pressure drop

F. Fluidisation velocity

G. Evaporator capacity check

H. Resistance time for proper combustion

I. Velocity profile study in pressure parts & non-pressure parts

J. Pressure drop in superheater circuit

K. Duct velocity checks

L. Shutdown inspection of the boiler

A. Study of fuel feeding system

The mixing nozzle capacities were checked for 90 TPH & 102 TPH boiler loads with different fuel mixes. We concluded that the operating mixing nozzles can be used for an additional 10% load. However, if there is one feeder down then the PA line will be overloaded.

B. Pressure parts

The present arrangement of pressure parts was studied and bed coils were provided with inner and outer types, on each side 67 rows of inner bed coils & 58 rows of outer bed coils were provided which are with 4 deg slope. This lower slope is enabling good immersion presently and thus helping in an additional generation. In some other units, lower angle bed coil with plain bore fails due to steam blanketing which is caused by high heat flux. However, in our case, comers for bed coil inlet header are well distributed which is helping for good circulation and distribution as well.

The water wall is provided with a cross-sectional area of 10304mm x 4864mm. The furnace walls are refractory lined to a height of 2.8 m above the bed oil outlet header. Both primary and secondary superheaters were provided with a counterflow arrangement. The secondary superheater is provided in the convection zone which gives high-temperature residence time for combustion due to the absence of radiant heat pickup. This helps in the reduction of fly ash LOI. Since the water wall is refractory lined, there will be a good amount of heat pickup in superheaters.

A single-pass convection bank is provided at the outlet of the primary superheater. The second pass of the boiler is provided within the line economizer. There are 51 coils assembled in three sections. Economiser inlet & outlet temperatures were 135°c & 240°c respectively. For the operating pressure of 80 kg/cm2, the steaming tends to occur at 300°c. Hence, economiser hot end tubes are free from steam.

C. Capacity of auxiliaries

The fan capacities are worked out for additional boiler loads. Both FD fans, one PA fan and one ID fan are required to be in operation. Presently both ID fans are in operation due to increased flue gas quantity by false air ingress.

D. Study of tube metal temperature to avoid overheating

In general, the flue path temperature goes up on overloading of the boiler. The most vulnerable area is generally the furnace bed & superheater zone. Since the present bed coil configuration is capable of carrying additional heat input, the bed zone will not end up in a rise in temperature (Not more than 950°c). Since the furnace wall is refractory lined & the fuel with VM of 25% causes more flue gas temperature & thus heat pickup in the superheater zone.

Presently there is about 75°c is being de-superheated. The primary superheater outlet temperature itself reaches 450°c then it is de-superheated to 375°c to get the inlet of the secondary superheater. The height of the refractory in the water wall can be decreased in order to adjust heat pick up by furnace walls/superheater. This height can be reduced in steps. First, about 0.5 m height refractory shall be removed from the top to reduce the heat pick up by PSH or SSH.

The outlet tubes of 7 nos. in the primary superheater are provided with SA213 GR T11 material which is designed for a metal temperature of 524°c. For carbon steel tubes it lies below 450°c. With the present steam side temperature the coil is found to be safe.

E. Air nozzle pressure drop

The present fans are found to be adequate for the purpose with additional steam generation capacity. The distributor plate pressure drop without SA as per calculation is found to be 390 MMWC & 514 MMWC for the boiler load of 90 & 102 TPH respectively. This is high, thus if the air box pressure is read as 730 MMWC, the actual operating bed height is 314 (730-416) MMWC with SA 10%. The FD fan power consumption is high. If we are looking for a lower pressure drop and power saving, we need to replace all air nozzles.

With a revised number of holes of 24 from 20 and dia 3.5 mm at all 3350 air nozzles, the DB drop will be 289 MMWC with SA 10% and 357 MMWC without SA. That means we can expect a saving in pressure drop & thus a reduction in auxiliary power consumption.

F. Fluidisation velocity

The fluidization velocity generally considered is <3.0 m/s. Even with a design boiler load of 90 TPH with excess air of 25%, the fluidization velocity is found to be 3.2 m/s. For 102 TPH load, the velocity is lying at 3.7 m/s. This is high. The requirement of fluidization velocity is 3 m/s, and the available cross-sectional area is found to be short by 10.7 m3. Naturally, elutriation of fines will be more LOI will increase. Thus, if the present LOI reaches 8% even with 45% ash in fuel.

Loss on ignition is a function of many parameters. Coals with a lower FC/VM ratio burn better. In addition, Inherent moisture decides the burn-up of the particles. This is due to the particle size reduction by steam released from particles. Finer particles have less time in the bed and hence it leaves the bed unburnt. Fines need to stay at combustion temperature for more periods. This happens in the CFBC boiler as the cyclone would capture +75-micron particles and recycle them to the furnace.

In AFBC, the turbulence is absent after the bed, the excess oxygen is not fully used up. If the bed expansion is less due to the heavy bed (iron/high bulk density/coarser particles) the LOI is higher. If the ash content is high, the unburnt carbon gets diluted and projects as if the combustion is good. In our case, Indian coal results in a 7.5 % LOI. However, the heat loss will be high in Indian coal due to the large amount of ash.

Bottom thermocouples are the key to keeping the bed healthier. Settling is identified by a drop in bed temperatures. Operators resort to draining the bed and make-up with the screened & reclaimed bed ash. This way the entire bed’s health can be maintained by inferring settling from thermocouple readings.

G. Evaporator capacity check

Bed coil heating surface area and coil arrangement will be the key to getting anything out of this boiler or else the bed coil erosion rate will be too high. The apparent gas velocity between the coils will decide the erosion rate. No shields allowed.

Bed coils are provided with 4 deg slope. This enables 90% coil immersion even with 900mm bed height from the DB plate. The bed coil HTA is sufficient for the additional capacity. The coil length required for the design boiler load of 90TPH is 1164m and the equivalent plain tube length available is 1299m. The bed coil studding pattern was found to be in two configurations. 

The bottom side of both the inner & outer coil is provided with a 4 x 3 pattern and the top side tubes are with a 3 x 2 pattern. With this, the present coil can have stud effectiveness 1.4. To achieve the boiler load of 102 TPH, the required coil length is 1192 m. With the present configuration, the additional capacity can be achieved. 

However, we have no limit on fluidization velocity. Since the boiler capacity is increased to operate at maximum load, almost every year the bed coil replacement will be required. The pitch between coil to coil is found to be 150 mm. With this apparent gas velocity, there are installations with a pitch of 120 mm. Based on the frequency of failure, complete replacement of coils has to be planned.

H. Resistance time for proper combustion

The furnace volume is 351 m3 measured from the top of the bed to the middle of the secondary SH. At 90 & 102 TPH load, this boiler has furnace gas residence time of 2.61 & 2.31 sec respectively for an average gas temperature of 800 deg C with 25% excess air for a fuel mix of 92% Indian coal + 8% Dolachar. 

This is quite low compared to the 3.5 sec adopted in another installation for achieving low LOI. However, the important point here is that a tall furnace with refractory lining will help to combust solid particles better.

The secondary air is useful in high VM coals which are above 20%. It helps to bring down the CO levels in the flue gas. Here the VM is found to be in the range of 25% in feed coal. Present SA ports are seen to be in line with requirements.

I. Velocity profile study in pressure parts & non-pressure parts

The gas velocities in pressure parts are checked for 25% excess air condition for the 92% Indian coal mix + 8 % Dolachar combinations for 90 TPH & 102 TPH boiler load.

The economizer tube inlet gas velocity is 9.39 m/s at 90 TPH load & 10.62 m/s at 102 TPH load. This is quite high which imparts a high rate of erosion when the gas is not uniformly distributed. The economizer topmost coils which are facing flue gas directly are to be provided with a shielding system to prevent the tubes from erosion. If there is uneven flow, the rate of erosion will be higher.

 flow, the rate of erosion will be higher.

Due to Improper Gas distribution, we found more erosion in economizer coils particularly on the ESP side. It was identified during shutdown inspection and the same is matching with CFD analysis.

The APH tube inlet gas velocity is 16.49 m/s at 90 TPH load and 18.66 m/s at 102 TPH load. Since there is a turn at APH inlet, there must be non-uniform distribution in gas which in turn causes a high erosion rate. Here ferrules are recommended at both top & bottom bank inlets. With this high velocity, it looks like almost every year there will be a need for ferrule replacement & part of tube replacement.

J. Pressure drop in superheater circuit

Pressure drop calculations were performed for 90 TPH and 102 TPH loads in the superheater circuit. With capacity addition there is about 1.2 kg/cm2 rise is noted. The pressure drop in the superheater circuit for 90 TPH load is 6.2 kg/cm2. At 102 TPH load, it will be 7.4 kg/cm2. BFP load will be increased slightly.

K. Duct velocity check

The air/gas velocities in ducting are checked for 25% excess air condition for the 92 % Indian coal mix + 8 % Dolachar combinations for 90 TPH & 102 TPH boiler load.

For both loads, the PA fan discharge duct velocity is at 21.4 m/s. This velocity is high. If the duct is not enlarged at the fan outlet, there will be additional and unwanted pressure loss. This is to be addressed to reduce the velocity. 

The existing rectangular cross-section of 622 x 398 mm has to be changed to 622 mm sq to bring down the velocity to an acceptable limit. At fan discharge, an expander has to be provided then a 622 sq duct can be taken. If both the PA fans are in operation, then there is no need to change the ducting cross-sectional area. The velocity is under an acceptable limit.

For both loads, the FD fan delivery duct velocity is found to be above 16 m/s for the design supply. Then were modifications performed in ducting cross-section. With modified ducting cross-sectional area, the velocity is found to be within the limit.

For both loads, the FD fan compartment air duct velocity is found to be 21.1 m/s & 24.4 m/s. This is high. The present size diameter of 700 mm has to be switched over to a diameter of 900 mm along with the damper.

For both loads, the economizer outlet duct velocity is found to be 20.2 m/s & 22.9 m/s. Ducting modification is suggested here.

To impart good turbulence in the SA system, nozzles are generally designed for 60 m/s. The present arrangement is in line with the requirements.

L. Shutdown inspection of the boiler

Shutdown inspection by an expert is to be carried out essentially to know the condition of pressure part equipment in order to take preventive action.

The following are the observations:

  1. Bed coils were found to be eroded above the fuel nozzles; it can be minimized by removing 10 dia ring in the fuel cap & avoiding shields.
  2. Primary superheater tubes/ screen tubes/ boiler bank tubes were seen to be eroded preferentiality.
  1. At the economizer inlet, the tubes are protected by a protective layer made of scrap bed coils. Presently it is leading to erosion in economizer casing due to preferential gas flow.
  2. Economizer tubes were seen to be more eroding on the ESP side.
  3. A gas distribution baffle shall be provided, to control uneven gas flow in the economizer at the inlet & outlet. This needs to be studied by CFD.
  4. The economizer to the APH inter-connecting duct top casing plate is seen to be eroded more. CFD study has to be performed here.
  5. There was a temporary gas diversion plate provided in the flue gas inlet path of the economizer, which led to gas flow diversion.
  6. APH outlet to ESP inlet duct is eroded and air ingress is observed. It shall be attended.

CFD ANALYSIS IN GAS DUCTS

The Economizer inlet duct was analyzed with CFD and found that the gas flow is more at the ESP side.

Economizer inlet duct gas flow was analyzed with revision in duct geometry, here the uniform flow has been made across the cross-section and excess velocity, very low velocities, and recirculation zones have been removed.

CONCLUSION

The purpose of this paper is to describe some of the key areas that need attention when the boiler is overloaded above MCR condition. Fluidisation velocity, bed coil heating surface area and coil arrangement will be the key to getting anything out of this AFBC boiler. Or else the bed coil erosion rate will be too high. The apparent gas velocity between the coils will decide the erosion rate. Without compromising these factors, the boiler cannot be overloaded. If it is compromised and operated for the long term, the incident will be severe in view of the equipment’s life.

Author

R.Nagarajaprasath

Director

Boilertech Energy Services

www.boilertechenergy.com

    Leave Your Comment

    Your email address will not be published.*

    sixteen − two =