Coal Fired Boiler - Gas Cooling between Air Heater and EP
Coal Fired Boiler - Gas Cooling between Air Heater and EP
(OP)
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





RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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?
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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.
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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.
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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.
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
Heula
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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?
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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.
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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,...).
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP
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.
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.
RE: Coal Fired Boiler - Gas Cooling between Air Heater and EP