Hello everyone,
i'm a mechanical engineer working as remote diagnostic for an Oil&Gas company, based in Florence, Italy. I'm going to ask some clarification about a topic treated several time on this forum; Premix mode for DLN-1 in GE units. I read many discussion about it, and one in particularly catched my attention ( leave here the link to retrive the discussion https://control.com/forums/threads/...eady-state-and-lean-lean-extended-mode.42564/ ). As you can read in the discussion, an user of this forum, talking about Premix mode in DLN-1 combustor say, quoting verbatim " ...The fuel in the Primary combustion zone is a very lean fuel-air mixture which is NOT burning with a diffusion flame; there is a temperature rise in the Primary combustion zone but there is no diffusion flame. While GE likes to say there is no "burning" going on in Premix mode, where there's a temperature rise there's combustion--but it's occurring at a much lower temperature which is what limits the formation of NOx."
This has pretty sense to me, althought i was surprised that also in Premix mode, Secondary zone worked with a Diffusion flame.
Now my questions;
1- There is any reference about this particular aspect that i can consult? Or maybe someone on this forum could be so kind to add more details?
2- In light of these considerations about the combustion w/out flame in Primary zone, could be correct refered to it as a Milde combustion?

Thank you so much to everyone.

Tommaso

 

@TomTom,

I'm very happy you have read prior threads about this topic; it's not often that posters do that as most people believe their question is unique. (Actually, the "mentality" in this day and age is, "I can post a technical problem to experts with little or no information and because they are experienced experts they will understand precisely what is happening and can tell me exactly what to do to solve the problem and I don't have to learn or understand anything! The Internet and forums are a wonderful thing, aren't they?!!?!?!?!)

NOx formation is related very closely to flame temperature. Diffusion flame temperatures in conventional combustion systems (i.e., non-DLN-I) GE-design heavy duty gas turbines can be as high as 3000 deg F at or near Base Load (that's why the combustion liners for conventional combustion systems have so many dilution slots and holes in the body of the liner--because the turbine nozzles and buckets can't last very long with such high temperatures so the high diffusion flame temperatures have to be cooled (diluted) with air before they enter the turbine section).

GE considers DLN-I technology to be proprietary, and as such doesn't really publish a lot of information about it as they spent millions of USD developing the technology and they don't want just anyone to copy what they've done simply by reading technical documents they wrote and published. Having said that, combustion technology for low NOx emissions is fairly standard across a lot of industries (industrial/utility boilers (both large and small); even residential water heaters (believe it or not!) the burn natural gas; etc.). I personally have never heard of MILD combustion technology but a quick search of the World Wide Web seems to indicate it shares some basic principles with GE's DLN-I technology. (I think a good GE document to have and to read (and re-read and re-re-read and re-re-re-read and make notes in!) is GER-3620. Unfortunately, it's difficult to read and understand--initially--because it tries be THE single document for all of GE's machines and combustion systems and while it does have some good nuggets of information it takes a LOT of reading to put them all together for a specific machine/combustion system.)

I was always taught for years that the fuel/air mixture in the primary combustion zone of GE DLN-I combustion systems was not actually "burning"--this from a couple of GE combustion engineers who were deeply involved in the development of the technology. I don't know why they said that, because just a few short years ago I was sternly corrected by a former member of that same team when I asked if it's really "burning" (combusting) or not. His response was, "YES, IT'S BURNING!!" I tried to ask him later for more details, but my impression was that I wasn't smart enough to be let in on the finer details of the technology (and he may have been right, who knows...).

So, it's as I always thought, and CSA seems to have believed also, that where there's a temperature rise with hydro-carbon based fuels there is combustion. It's just in the case of the premix fuel/air mixture, because it is so air-rich/fuel lean, that it doesn't result in the type of flame that is visible to the eye or a typical flame detector (Geiger-Mueller or Silicon-Carbide). It's burning, just at a lower temperature--which is what one wants to happen when trying to reduce NOx emissions without the use of water or steam in the combustion system. Personally, I think the original statements by GE combustion engineers that there was no combustion ("burning") happening in the primary combustion zone when operating in Premix Steady-State was because it was all kind of new to everyone and everyone was still learning about what's happening in the combustion system they were designing. (I'm told there were more than one combustion system/method that was experimented with at the time DLN-I was being developed, but that DLN-I became the standard because it was less expensive than some of the alternatives (which used precious metals and were NOT tolerant of the temperature swings in the combustion during starting and loading) and it was "simpler" (some people will argue with the simplicity of the system, but I'm told it was a lot simpler than some of the other designs which were considered).

I was always wondering why it was necessary to have a diffusion flame at all when operating in Premix Steady State combustion mode. I learned it was necessary for a couple of reasons--to help reduce the amount of CO emissions (CO is "burned" with higher flame temperatures) and the temperature was necessary to help establish the combustion of the lean air/fuel mixture in the Primary Combustion Zone during Premix operation. (When establishing Premix combustion mode the fuel to the Primary Combustion Zone is completely shut off--meaning there is NO combustion (burning of any kind) at all--and there is a need to have a high enough temperature in the combustor to get the lean air/fuel mixture to combust ("burn").)

But, again, most of what I know about DLN-I is from anecdotal information (word of mouth) and as an engineer all I needed to do was make sure all the components of the DLN-I system operated as they should, then a remote tuner would tell me what to do to tune the engine (if it was possible; often, the dilution holes in the DLN-I combustion liners had to be re-sized--something that involves removing the liners and sending them out to be modified and then re-installing them and redoing the tuning). [I, like others, never wanted to be a tuner because it's a thankless job--the Customer and often GE managers were unhappy with either the results of the tuning or the time required to perform the tuning. That's one reason why I think remote tuners are a really good idea--they are insulated from a lot of the trash-talking that goes on about the tuning process and the time/fuel required for the process.]

Anyway, thanks for using the Search feature of Control.com in your quest to learn and understand your question(s)! It's actually rare--and very much appreciated. There is a LOT of information here on Control.com about GE-design heavy duty gas turbine control systems and philosophies, compiled over more than two decades. And, a lot of people have posted to say if the information provided was helpful or not--which makes many of the past threads very helpful, if people will just read them. So, thank you for reading them!

 

@WTF?

Thank you very much for the reply. The site experience you reported is pure gold for who, like me, has a solid basic education about turbomachinery ( or, at least, i sincerely hope to have it), but never had to do directly with turbine or their components
(to make you laugh, i know why film cooling holes have a certain shape, but i never see a cooled blade. I know, there are photos and video, but it's not the same thing touch and see a real component).

Anyway, i agree with you; probably the only premixed combustion is not strong enough to complete the oxidation of CO in CO2, so we need a kind of re-boost by a diffusion flame to increase temperarure in combustion chamber in order to avoid excessive CO quenching (and unburned hydrocarbons entering in turbine, which are a problem not only for emission regulation, but it might be also for clogging of film cooling holes) and at the same time give stability to the all reaction.
At the same time, most of the fuel injected in combustion chamber is in Pimary zone, premixed, allowing a good temperature control and NOx reduction.

This makes sense to me from the point of view of combustion theory.

So, again, thank you very much for your reply and for sharing info about your experience.