How to specify supplementary firing equipments for HRSG 1

[threeS]

Hi, anybody can tell key points to specify the supplementary firing equipments for a HRSG.

The HRSG can be a vertical or horizontal type. The supplementary firing is limited to duct firing. The location can be at HRSG inlet duct or between first and second tube blocks counting from the gas turbine exhaust side.

 

[nambi]

For key points:
You have to specify the fuel used (gas or liquid fuel) and approx calorific value.

You have to mention in your specs about Excess air available from GT exhaust and Flue gas temp at GT exhaust.

Specify whether u require a continous pilot or a startup pilot, in case if u use a pilot.

Your burner specs has to meet NFPA saftey requirements. So the shutdown system should meet the same. In case you use a PLC, it should meet the saftey class. NFPA will prescribe you the number of shutdown valves.

For example if you have 3 duct burners fired by natural gas then each line to the burner will have a individual trip valve and a vent valve apart from two trip valves and a vent valve in the main line.
Each pilot will have individual solenoid valves apart from a trip valve and a vent valve.

Each burner should have a continuous flame sensing system can be one flame scanner and better to have two scanners.

 

[MJCronin]

ericwong,

I feel that the situation that you describe is one where the specifier can get into trouble...

Adding supplemental firing to a waste heat boiler is a change that can affect all aspects of boiler/plant operations and safety.

I feel that the best course of action is know the thermal requirements of what you are trying to do, then call in a couple of vendors of HRSGs. Ask them about the best location of supplemental burners. Ask them about the structural, mechanical, emissions and performance impacts on your existing boiler..........and then request proposals

My thoughts only.........

MJC

 

[athomas236]

eric,

In Europe we have been supplementary firing HRSG's for years, its not difficult just needs care in the specification. My comments are:

1. Specify the fuels, what is that main fuel, what is the back up.

2. Specify if you just want to maintain boiler output at low gas turbine loads and if so is this just on main fuel or back up or both.

3. Specify if you want the HRSG to be able to be able to achieve its MCR rating even when the gas turbine is shutdown.

4. When the gas turbine trips what do you want to happen, the HRSG shuts down or maintains output so an independent forced draft system is required.

I have no experience of supplementary firing being at other than the inlet ducting, otherwise it does not matter if boiler is vertical or horizontal although horizontal is more common.

I would avoid specifying the number of burners or their type. Leave it to the supplier then it will be a clearer case of responsibilty in court.

Going for lunch

More later

Athomas236

 

[athomas236]

eric,

Back again, where did we get to.

From recent experience based on an arbitration case that I was involved in as a expert witness in with an American boiler maker there are a number problems that can happen.

1. With a small number of large burners, flame impingement can occur that overheats the stainless steel liner that protects the duct insulation and causes increased heat flux on tubes of first tube bank.

2. Difficulty in selecting flame scanners that can detect and discriminate between oil and gas flames.

3. Difficulty in getting boiler tuned so than when fuel gas system trips, boiler can transfer to oil firing without tripping.

Hope some of this helps.


athomas236

 

[threeS]

To athomas236

Thanks your advice

let me clarify the suitation I face. I am writing a specification for a combined cycle plant. According to suppliers' preliminary documents, our required power output may not be achieved without a supplemenary fired HRSG. So some of suppliers have recommended to install a limited supplementary firing, either at HRSG inlet duct or after the final superheater/reheater tube block.

So I have put the following in the specification:-
1. the supplementary firing will only be required in meeting the peak load.
2. the main fuel (natural gas) and backup fuel (light oil) specification.
3. the gas turbine exhaust gas specfication
4. no additional air fan
5. design of supplementary firing equipment should follow NFPA.

I did not specify the type, number and materials of burners, flame scanners and ignitors. I let free hand to suppliers.

Anymore you think necessary to include in the specification.

 

[athomas236]

ericwong,

Seems like a good start, let me know what suppliers say.

athomas236