5 replies to this topic

[che_bud]

Good Day,
I work on an LNG plant as a process engineer for the past four years and I am currently looking at the fired heaters on the facility.

I came across the term "absolute combustion" while reading the book "A Working Guide to Process Equipment." (Page 294)

It mentions that "The point of absolute combustion represents the best achievable efficiency point of any such piece of equipment on any day of the week, at any hour or minute."

I have attached a graph which shows the relationship between increasing % O2 in the flue gas and increasing heater outlet temperature.

I know it is common practice to have 2% Oxygen in the flue gas as a good operating point but the term absolute combustion does not consider it at 2% but that it varies depending on air-fuel mixing efficiency.

I would like to know how many of you guys use this term when determining the best point to operate a heater.

che_bud

Attached Files

•  graph.pdf   18.08KB   31 downloads

[ankur2061]

che_bud,

Have a look at the following link. This should provide some pathforward:

http://www.engineeri...tion-d_399.html

Regards,
Ankur.

[Zauberberg]

Assuming that we have all the tools, process measurements, and control loops in place to monitor and optimize heater operation, what Lieberman says is perfectly correct: there is no such "universal rule" for setting up O2 in the flue gas at 2%, 3%, or any other value - simply because the real combustion process will depend on so many factors different for each and every heater worldwide, and that is:

- Fuel type
- Compositional (LHV) changes of fuel by time
- Burner design and efficiency
- Air flow and distribution control
- Mixing efficiency
- Heater insulation and presence of various in-leakage points
- Competence and knowledge of operating personnel

Just from these above, we can conclude that operating a heater in the way proposed and explained by Norm is a continuous effort - and is something that should be done, or at least I can speak for myself and confirm that I have tried to optimize heater operation in petroleum refinery by following these and similar guidelines.

Even without additional analyzers and control loops, it is possible to monitor and adjust heater performance based on a single process parameter: heater outlet temperature (process side). If the process itself is fairly steady (e.g. no significant changes in plant throughput, heater inlet temperature, and fuel composition/LHV), it is relatively easy to track heater performance and optimize operation by running it slightly shifted towards right from the optimum operating point. And this must be done for safety reasons as well, because moving to the left side will create a snowballing effect where lower heater outlet temperature will call for more fuel gas which has already been starving for Oxygen, creating explosive mixture inside the combustion chamber.

[che_bud]

Thanks for the responses guys.

Ankur,
That link gave me some of the information about Stoichiometric combustion and excess of air which I already knew. I was more interested in what Lieberman had to say in his book about absolute combustion which Zauberberg referred to.

Zauberberg,
Thanks for that response. I only got Lieberman's book recently and it is full of useful information.
I was looking online to determine if Absolute Combustion is a term used outside of Lieberman and I didn't get much information. The heaters on our facility are having a difficult time in achieving the process outlet temperature and they are currently being viewed as inefficient. When I saw the term Absolute Combustion in Lieberman, I started to wonder if we are not as close to this point as we should and we may have deviated from this point over the years.

I am going to initiate a project to determine the Absolute Combustion point on our heaters and probably generate a curve to know where we are at and where we should be.

From your experience Zauberberg, do you have any advice for me when conducting this experiment? Basically I am considering varying the flowrate of the combustion air and measuring the excess oxygen and duty from the heater.

Do you have any other advice for optimizing the performance of a heater?

Thanks for your help guys...

che_bud

[Zauberberg]

Put the heater controls on manual and try increasing the air flow - that will tell you straight away on which side of the BOP you are. Of course, make sure to prepare a detailed plan/procedure for heater testing in order to identify all potential safety issues and to address them properly. Work closely with Operations and the panel men as they are your best buddies.

I found some interesting materials, see below.

Attached Files

•  Fired Heater Optimization ISA AD.pdf   195.83KB   39 downloads

[ankur2061]

che_bud,

One of the best methods I had encountered in optimizing heater combustion performance was through proposed heater modifications on my existing heater. These modifications were deferred because of the high cost involved, but it was the unanimous conclusion of the operations team that these modifications would definitely improve combustion performance:

Following modifications were proposed:

1. Install O2 analyzer (zirconium oxide O2 analyzers from Rosemount was recommended) in the flue gas duct from the heater

2. Provide variable speed drive for the combustion air FD fan (our heater was having a FD fan)

3. Provide initially a set -point of 2% O2 for the O2 analyzer and take a "trim" signal from the O2 analyzer to the variable speed controller of the FD fan to control (trim) the combustion air flow.

4. Manually observe the flame characteristics and also monitor the flue gas temperature which will provide a fair indication of the combustion characteristics in the fire box.

5. In case incomplete combustion is observed, increase the set-point of the O2 analyzer to 3% which will increase the combustion air flow to the fire box by increasing the speed of the FD fan drive and repeat the monitoring process.

6. By trial and error of the flue gas O2 content within a range of 2-5% you should be able to arriive at the best combustion efficiency point for your heater.

Hope this helps.

Regards,
Ankur.