Coal Fired Boiler - Gas Cooling between Air Heater and EP

[threeS]

As our coal-fired boiler has operated for 20 years, the air heater efficiency drops, the gas temperature leaving the air heater has increased by around 15 degC. This deteriorates performace of Electrostatic Precipitator downstread of air heater.

Two possible solutions:-

1. Restore air heater efficiency (very difficult)
2. Add some equipment between air heater and EP to increase the gas temperature.

Anyone can provide some details in particular on the second solution.

Thanks

Eric

 

[yogibear1]

Eric some questions:
Is your second proposed solution turned around? You say the gas out is too high; and you still want to increase the temperature entering the ESP??? Shouldn't it be lowered or cooled? As I understand ESP's if they are too hot they don't perform adequately.
What style air pre-heater (APH) do you have, is it tubular or regenerative (as by CE/Ljungstrom)? Do you have an APH sootblower (steam or air) and how effective is it? Do you ever wash the APH, and what were the results? To what do you attribute or suspect is the loss of efficiency and increase in gas temp out of APH? Such as - Is there a significant loss of APH heat transfer surface or is it fouled? If it is a tubular unit do you have a number of tubes plugged? If you have a regenerative APH is the cold end average temperature above the dew point of the flue gas so as to mitigate against corrosion of the cold end baskets?

 

[davefitz]

The best solution is to restore the original air heater seal clearences and element cleanliness to original spec.

If you wish to increase the stack gas temp to higher values downsctreamof a wet scrubber , one solution used by Babcock Hitachi is to use an indirect heat exchanger. Transfer heat form economizer oultet gases to the indirect fluid, pump the fluid over to the outlet of the scrubber, then reheat the stack gases.

 

[threeS]

The second solution should be "cool outlet gas" instead of "heat it up".

For the questions raised by yogibear1, I will address later.

To davefitz, do you have any idea to cool down outlet gas? and why restoring the air heater performance is the best solution?

Thanks

 

[Heula]

Before introducing an equipment between Air heater and ESP , the following points are to be considered :
1, The layout/space availability to accomodate the equipment.
2. If wet scrubber is introduced , the outlet gas temp. will be around 85 to 100 deg. C which is not preferable for down stream equipments like ID fan , ducts , Chminey ( If Steel ) due to dew point temp. consideration.

Hence , best solution would be to retrofit the Air heater.

Hope , you will apprecaite the above points .
Francis Heula

 

[threeS]

Thanks to Heula, dew point is really a concern. Other than the wet scrubber, any other equipment available in the market.

 

[davefitz]

The outlet gas temp can be reduced by upgrading the air heater to original specifications - reducing leakage and cleaning out the baskets for original pressure drop and heat transfer properties. Because there is a 20% difference in the relative values of W*Cp between air and gas, the air heater heat transfer effectiveness should be calculated based on the air side conditions( since the air side has the lower value of W*Cp), but tradition has used the gas side properties instead. Based on the air side properties , a max air heater effectiveness of 0.85-0.90 might be achievable with a rotary air heater.

To get lower gas temperatures, one can use an extended surface LP economizer downstream of the air heater , which substitutes for some of the extraction heating of the LP heaters. Problems of corrosion of the tubes and stack ore difficult to solve- I understand that in germany they use rubber lines stacks and flues to countre corrodion for condensing sulphuric acid.

Anotehr method is to improve the HP economizer's effectiveness- convert the bare tube economizer to an H-plate exended surface economizer- but this canot be used for some fuels.

 

[TD2K]

If dewpoint isn't a concern in the stack gases and you've decided that repairing/restoring the air preheater isn't economical, have you considered using a desuperheater in the flue gases.

I wouldn't think it would take much water to drop the gas temperatures by the amount you needed. Run some quick calcs to see how much water would be needed and what the impact on the dew point would be from the additional water.

 

[Heula]

Recent boilers are designed with a provision to add one or two loops on economiser. Hence , You may look in to the possibility of adding coil length in economiser to bring down the temp. of flue gas. But , one has to study the corresponding heat transfers in all areas both upstream and dwon stream.
Heula

 

[threeS]

Thanks to all valuable advice.

Having taking a bit more study, I want to summary below:

Target of my task: To reduce the dust emission to ambient, or to reduce gas temperature from around 145 degC to 125 degC so as to increase EP efficiency.

Plant: 20-year 250MW coal fired boiler with regenerative air heater (Ljungstrom) and EP.

Possible Solutions:

1. Increase heating surface of ECO. The ECO has already been fin-type. This will affect steam-water cycle performance, resulting in recalculation/redesign of all steam parameters.

2. Modify RAH to incrase effectiveness of RAH. Installation of additional heating element will be subject to space constraint. Or we have considered to replace the existing heating element profile with a better DU profile.

3. Install a heat exchanger or water spray between RAH and EP. The space should not be a problem but I found no source to approach the relevant suppliers. Any suggestion.

4. Increase EP effciency. I am not familiar with EP so no details upto now.

Any other suggestion?

And do you have idea to download some user-friendly freewares for heat exchange calculation in ECO and RAH?

 

[davefitz]

You can install a "kettle boiler" or a LP boiler bank , saturation temperature = 110C. Tbese to be extended surface tube ; corrosion considerations of highest concern. Also, corrosion of flue to EP may be of concern. If you cannot find a use for the steam , you can condense the steam over an ambient air heater and simple vent the hot air.

The LP steam can be integrated to the power cycle, but if not convenient and you wish to try something completely different, you can heat a lithium bromide absorption chiller with the 125 C fluegas and use the chiller to further cool the circ water feeding the condenser waterbox.

 

[dberner]

Have you completed any investigative work around efficiency of the existing ESP? Such as comparing operating data to OEM predictions, CAVT,...

Is the RAH performing up to expectations? If not, why not? If it is, increasing the RAH effectiveness should include a review of the effect the priamry air temp increase will have on the unit (PA fans, burner velocities,...).

 

[LjungstromGuy]

Ericwong,

Sounds to me like you simply need an air preheater upgrade with modern heat transfer profiles with a higher heat transfer than the DU and NF6 that you are probably using.

http://www.airpreheatercompany.com

have a selection of new profiles that would increase your heat transfer and other methods of installing additional depths of baskets into an existing rotor.

 

[davefitz]

another possible source of problems is that the air flow split or gas flow split between the secondary air heater and the primary air heater is not correct.