Gas blending (natural gas / LNG for CCGT) 2

[tincap]

CCGT fired by natural gas. Natural gas will be supplemented, at some future date, with LNG (natural gas and LNG will be blended and fired after 'correction' of the blended gas properties).

Does anyone know of any CCGT power station that has moved from natural gas to natural gas/LNG blend (or to LNG) and if so were there any 'big issues' with the transition &/or blending.

 

[rmw]

tincap

Not all LNG is liquified at its source before first having the other natural gas components stripped out, so it can contain NGL or Natural Gas Liquids, propanes, butanes, and the like, which give it a higher btu rating than straight pipeline methane.

The ratio of pure methane to other NGL components may or may not be consistent.

You should find out if your LNG has been stripped or not. If it has, you should have no problems

As far as firing CCGT's with LNG, I can name at least one major north/south pipeline in the USA along which many CCGT's are connected, so that their fuel is NG vaporized from LNG from a certain north African country. They seem to run just fine on it. But I seem to remember that it is stripped, so it is straight methane.

rmw

 

[tincap]

Thanks RMW. The LNG has some higher h/c's. The main concern is that it is higher CV than the gas it will be displacing (therefore higher Wobbe Index) - to compensate for the higher CV, using the existing burners, the revaporised LNG needs to be preheated {to a much higher temperature than the existing gas} before firing in the gas turbines (and/or ballasted with N2) to lower its Modified Wobbe Index (assuming no burner changeout).

I was wondering if the much higher levels of preheat needed for the LNG would lead to knock on operational problems with the gas turbines (or should we look at changing out the burners).

Don't suppose you know if the CCGT's you refer to had any modifications to their burners.

 

[rmw]

No, sorry, I don't know of any instances of anything other than the standard fuel prep skid that comes with modern CCGT's. Most of these are less than 4-5 year old installations.

Try combing through this site for some info. I think I remember one of the papers being on the topic you are interested in.

http://www.gepower.com/prod_serv/products/tech_docs/en/all_gers.htm

rmw

 

[davefitz]

You can try SCANA Urquhart station , and McIntosh stations. They recieve alternating supplies of domestic nat gas and regasified stripped Trindidad LNG via Elba island. The range of HHV is 1120-1150 btu/lb, and the range of Wobbe Index is 1350-1381. Tne max butane content from Trindidad is about 0.35% molar. I heard that they have not noticed any performance changes, but then again they are not monitoring the combustor's dynamic pressures.

For cases where the gas quality is changing, I do not think that there will be any significant upsets in NOx or dynamic pressures as long as the change in Wobbe Index is within 2.5% AND the butane content is kept below 0.5%, which implies the LNG is stripped either at the source (Trindidad) or at the regas station ( all others except perhaps Algeria).

Units which fire rich unstripped LNG with high levels of propane C3 and butane C4 as well as lean nat nat gas will probably see increases in NOx and dynamic pressures above allowable if the change in quality exceeds the above limits AND the magical automatic dynamic tuning device never becomes a reality (ie, remains as vaporware). Note - the newly FERC approved LNG regas terminal at Corpus Christi will have C2+ stripping at the regas terminal.

Recently published 2005 tech papers by Siemens Westinghouse show that if the gas quality varies from lean nat gas to rich LNG , simulated by injected C3 and C4 in the lean gas, the NOx will increase by about +33% as the Wobbe Index increases by about 4% due to butane addition. The same increase in NOx is not observed if the Wobbe index is alterred by varying fuel gas temperature or by increasing ethan content ( as at Elba). There were also noted changes in dynamic pressures. There is some speculation that these changes can be largely offset by adding a new magica auto tuning device, not yet demonstrated as effective in this specific application.

 

[davefitz]

OOPs. The range of HHV is 1020-1050 btu/scf. Got stop typing so fast.

 

[tincap]

davefitz,

Have you got a reference to the Siemens Westinghouse paper please (I've tried the siemens website, but there are no tech papers listed after 2004).

Thank you.

 

[davefitz]

tincap:

There are 2 published papers. One was at the Apr 4 2005 "Electric Power conf" in Chicago (powerpoint), and one was a power point presentation at the gas turbine council ( Wash DC) in Mar 2005 ( 2nd PP presentation by Bruce Rising)