Burner Management

[Mandar 0]

Hi, I did not know where to post this, so I have posted it here. I am looking for details on Boiler/Furnace burner management systems. Especially for coal fired boiler startup using oil or gas. Every search turns up the Honeywell/Fireye FSG systems. I am looking for details on the startup concepts since I need to develop the system using PLC. Any help will be appreciated. Thanks in advance. Mandar

[Rodney 21]

Mandar Not familiar with the FSG Flame safety relay however the Honeywell and Fireye I can highly recommend. With regard to replacing them with a P.L.C I am not sure what the present safety leigislation says about that. Rodney

[paulengr 44]

More details. Gas/oil is easy and relatively safe. Coal isn't. I'll give you a lot of details here but I truly hope that I scare you off from trying to go it alone. This is simply not something that a controls engineer should tackle alone. Even a good process engineer is treading on thin ice. I have plenty of experience with all 3 fuels as both a process and a controls engineer. Frankly, I don't believe there is ever any such thing as a truly safe coal system (as in a system where you don't require an operator or where you can prevent an operator from doing stupid things). I've just recently put a burner safety/management system in. Get yourself a copy of the relevant NFPA first before starting. They have an entire code dedicated to boilers. Check out www.nfpa.org. You can view (but not print) the code online as well if you just want to look first. They used to totally disallow PLC's at least as far as the flame safety portion of things goes. Now that safety PLC's are out there, those are allowed. The old way of doing it (and not a bad way) is that you feed all the inputs to both the flame safety module and to the PLC. The PLC monitors the process but does not directly control lightoff except perhaps for if the flame safety relay says "purge" or "preignition sequence", the PLC might be the one setting the air damper. The air damper would indicate position via limit switches (thus precluding any direct PLC interaction in terms of lightoff). A safety PLC is actually 2 PLC's with a separate "checker" in between. If the "checker" indicates a failure, then everything executes a fail safe sequence which is hardware based. I am not familiar with other manufacturers but the GuardLogix PLC from AB is an example. All burner safety equipment has to be safety rated. So this means Maxxon valves instead of just ordinary gate/butterfly valves, etc. Since you are dealing with a severe multifuel scenario, I'd suggest forgetting the Honeywell and Fireye UV flame scanners. Go for Ametek's flame scanners. The low end ones aren't that much more expensive and the high end ones can handle multiple burner scenarios firing into a common chamber where you get severe flame interaction problems to deal with (when you have more than one burner, the standard flame scanner may "see" more than one burner and fail to function properly). If this is a 24/7 operation (coal fire systems usually are), you must buy flame scanners with the self-checking option. If it's not 24/7, you have to verify externally. In your logic, check if the flame scanner indicates flame when there's no fuel and lock out the system if this happens since flame scanners can fail either way (false positives and false negatives). In terms of air/fuel ratio, I've found from working with lots of systems that a simple cross-connected pressure regulator and metering the oil/gas side of things works best for the "pilot" burner. The exact same safety system spec'd out in the NFPA is what I use. Whether you do it in a safety PLC or a burner safety relay is a matter of simple choice. Either way it is nothing but a line of multiple safety checks and a couple timers for the purge/flame detection circuit. Once switching over from gas/oil to coal, things are much trickier. There is no such thing as a truly safe coal system. For one thing, there's no such thing as a safety rated gas analyzer (O2 or CO). You need one for truly optimized burner management (one per burner although there are some compromise strategies). At a minimum, you need some sort of temperature sensor to tell you if you are above the autoignition point (1650) of coal or not. For burner optimization, you need the O2 analyzer so that you can run as close to 0.5-1% O2 as possible (assuming that this is your goal). If you are trying to run NOx control, there are even more requirements since then you have to somehow avoid the above optimal position or else use some other dirty trick to avoid flame-induced NOx. With gas, you can always just shut your fuel off outright with no negative consequences. This is not as easy with coal. With a chain grate or a pulverized bed system, this is probably possible. I haven't worked with one so I'm speculating. But, with a pulverizer (ring-race style or ball mill), things are not so easy. A pulverizer typically uses offgasses from the burner chamber as a combination hot air source for drying the coal and for sweeping air through the classifier. Some of the better ones may give you two separate air streams for this which is much better. If the temperature in the pulverizer gets too hot (monitor via the outlet temperature and inlet temperatures), you can ignite the coal right inside the pulverizer. This means that simply shutting off the coal stream can do this since it is difficult (impossible actually) to calculate the correct setting on the inlet damper(s) to avoid overdrying the coal and letting the temperature get out of control. Injecting water doesn't really help much either, especially since this may or may not start oxidation to the point where you cause spontaneous combustion. Simply relying on the autoignition temperature doesn't help you since coal in various states of dryness can generate heat exothermically on its own. At best, you could try injecting Purple K when the situation arises. I've never tried it but I've heard things that suggest that this may work. Once everything is cleaned out, restarting generally isn't a problem. It's shutting down that gets hairy. Outside of that when it comes to coal, it seems to be easier to control the coal to a fixed set point and to modulate the air to achieve the proper air-fuel ratio rather than the other way around. This is directly the opposite of gas and oil where it is generally easier to control air flow and modulate the fuel to match the air-fuel ratio. Coal feeder systems are notorious for various things going wrong. It doesn't help that I haven't met an honest coal vendor yet, so you constantly have to contend with various problems of too much ash, low ash fusion temperature, bad sizing, wet coal, etc. At best, try to buy direct and stay away from coal brokers. In the East, Massey and Lodestar are generally about as reputable as you can get, which isn't saying much. Anyways...there are few fire codes for coal combustion. If you are messing with one, this is the point where unless you really understand what you are doing, I'd suggest bringing in some outside expertise. For that matter, if you are at all familiar with this stuff, defer to some good expertise out there. Process Heating magazine is a good source to find one. Also, I've never gone wrong with recommendations from Combustion Engineers or from North American. Sorry I can't be of much more help but this is a very esoteric and difficult area to work with. Gas/oil system procedures manuals for operators are generally very simple ("push start/stop. If the red light comes on, call maintenance"). Coal/coke systems have a myriad number of things that can go wrong and often require the operator to more or less control the system to some degree, and have a lot more procedures and policies involved. You can't stop everything. I thought we had some of the best procedures but an operator managed to blow $250,000 worth of brick (blew out a Detrick roof) early one morning during light off. The O2 meter read 0.0% and he decided that the O2 meter wasn't working. The operator began adding coal down around 1450-1500 degrees, well below the autoignition temperature. Every time the operator increased the coal feed rate, temperatures DECREASED...a sure sign of you know what. Eventually though, all that extra coal laying around in there did light off, all at once. All the hydraulics were incinerated. The ductwork was destroyed. The refractory roof in that area was destroyed. It was a mess. You can cause the same thing with a dual fuel gas/oil burner too. Depending on how it is set up, some of the more primitive systems allow you to put both oil and gas to the burner simultaneously. This sends your air fuel ratio to half of what it should be. CO becomes a huge issue. The oil just starts seeping/puddling into everything. Eventually it does light, too. It usually isn't as spectacular though since a puddle of fuel oil burns only at the surface and not simultaneously throughout the whole bed (can't get oxygen into the whole puddle at once). Edited 23 Apr 2008 by paulengr

[DanW 41]

I have a healthy respect for the expertise that goes into a commercial flame safeguard. What's interesting about the diagram below is all the angled arrows on the circled terminal numbers, connectors to opto-isolators on the front end of DI's to the uP (all internal to the flame safeguard controller. The unit does all sorts of self checks for feedback, initial conditions and whatnot. It isn't complicated logic, but reflects lots of man-years in combustion safety and knowing what to write the logic for.

[rlp122 4]

I have to echo what the others have said. There is a reason that Honeywell/Fireye are the leaders in this field. Burners are notoriously fickle creatures. They have a tendancy to explode if not operated correctly. A Honeywell/Fireye controller will help ensure that you don't have to stand up in court and explain to Fred's family why you went the cheap route and designed your own. It will also help keep the judge from throwing you in jail and taking all your assets as well when the burner does fail due to the PLC not working as you intended. Just my two cents...

[paulengr 44]

Three things about these statements that are highly misinformed. First, Coal systems are not so simple. The advice you are giving is very incorrect and highly misinformed because you haven't worked with a solid fuel system. The advice you are trying to give is 100% correct with just liquid and gaseous fuels. It doesn't serve any purpose or help someone with a solid fuel system. If you check your Honeywell/Fireye books, you will find relays for gas, oil, and gas/oil. You will not find anything for coal or if you do, it will be at best for a stoker, not a modern pulverizer system. Second, The codes are much more lax in India, which is the poster's stated location. Depending on the state, it often comes down to ethical concerns more often than legal ones in that country. Short sighted monetary concerns frequently trump everything else. So you need to sell safety systems because they are more reliable in the face of unskilled/untrained personnel, not because there's any fear of law suits. Third, burners by themselves are not fickle if you operate them in their stable region and if you use the right kind of burner and set your gas/oil train up correctly. Granted you are working with a turbulent system but that does not mean that it has to be inherently unstable. If you try to for instance run gas-only control (no ratio control), then you will tend to run all over the ratio map and have lots of stability problems. Unfortunately this is an extremely common configuration. If you try to just run an open pipe and not run a multiport injection burner, again, stability problems, AND a very real possibility of flash-back and explosion. This is not even possible with a multiport burner running gaseous/liquid fuels. As long as you follow the codes, and you are dealing with inherently stable burner designs, the burners are inherently safe. I like the burner safety codes not because they are overkill, but because if you follow them and also set up your burners reasonably well, you can run very stable, vary your firing control all over the place, and never ever have to worry about issues with stability, explosions, etc. Using a flame safety relay for gas or liquid fuels is relatively inexpensive and shifts all the liability to the flame safety manufacturer (as long as you follow the codes and their instructions), as far as light-off procedures. It does not cover you if you screw up the burner design since the codes only cover light-off procedures and basic safeties. For that you need to hire a burner company. BUT this really still doesn't help you at all in solid fuels. Fourth, the guy said gas, oil, AND coal. Cheap (ok, not so cheap) little Honeywell and Fireye flame safety relays simply are NOT going to do it with that scenario. You can get a dual fuel relay system but you cannot get one that covers coal. The system may or may not even have a pilot or spark ignition system. Frequently anyone burning coal doesn't shut down often and frequently doesn't bother with the extra expense and labor involved in maintaining those (coal's much higher temperatures and inherent "dirty" nature means burner maintenance is a much bigger pain). There are also numerous additional safety concerns that simply do not exist with liquid and gaseous fuels. For instance, there's no fire safety concern about the pulverizing system. I'm leaving storage and transport off the table because although there are additional safety concerns there. You can get coal to light off simply by placing it outside, leaving too many fines in it, and letting it get rained on (spontaneous combustion). Usually with a solid fuel system, the general rules are to not do anything too quickly, use a flame safety system to light off the pilot burner(s), then simply use flame scanners and temperature to detect the fact that the coal system is either still burning or that it will auto-ignite if operators try to restart after some kind of flame-out condition (jammed coal chutes being very common). Certain things that seem simple in gas are not simple with coal. For instance, in a flame-out situation (UV flame scanner indicates no flame), the standard procedure with gas/oil is to cut off the fuel supply. With coal/coke, this is very dangerous! The procedure is to slow the feeder down to a minimum rate and reduce/cut off the air flows around the pulverizer circuit and not cut off the fuel 100% if you lose flame because if you do this, you can cause temperatures to get so hot in the pulverizer that you will light it on fire, rather than burning the fuel where you want it (in front of the burner). I like using flame safety relays in spite of all the comments I just made for two reasons. First, yes, it does add a layer of liability protection. Second, you've got to think like maintenance/production. If you put everything in a PLC and the system is down because of a real problem such as too much/too little gas pressure, and they have access to the PLC (probably a good idea for troubleshooting) the first thing they will do is figure out how to bypass your system and force the valves open, and possibly destroy the gas train and/or kill themselves. If you have a safety relay involved and it is well integrated into the burner system, the extra layer of hardware-based logic means there are many more things to bypass to try to jumper out the safety system. It tends to force them to look for the easy route, that being troubleshooting the problem valve/sensor and fixing that rather than jumpering out the safety system. You can use a safety PLC if you want to meet codes and do it yourself in a PLC (or adopt/follow codes if you are in a state where these don't apply). However, even with a dual fuel, multi-burner system, safety relays are so cheap that the extra time and development costs make them worth the money to simply skip it and go with a generic (non-safety) PLC at least for the oil/gas trains. Once you are into a coal system, you are already going to be spending significant time doing PLC development and I could honestly see going either way. Edited 26 Apr 2008 by paulengr

[BobLfoot 302]

I've watched this post develop and waited since my experience has been mostly with gas/oil units where we used Flame Controllers and very little solid fuel experience. Ok I had a wood / oil furnace in my home but that is about it. Paul is very correct that solid fuels are a different creature than vapor fuels. Vapor Fuels handled correctly provide enough burnable mass for the moment and not much longer. Solid fuels on the other hand have mass, both physically and thermally. This makes them tricky. I realize you are in India and the need for safety may not be there legally, but for your own peace of mind I strongly suggest you do an FMEA or Risk Analysis document of your planned system. Asking the what ifs before you "strike the first match" will save you asking the "why did i miss thats". If you can swing the cost redundant monitoring of the process will gain you a level of safety and reliaility as already pointed out. And no I would not go for redundant PLC CPU's, but I would have a last ditch manual control safety method should the PLC die and you need to bring this thing to a halt.

[rlp122 4]

Ya see, this is why so many folks don't post in forums. I made a statement that was to the best of my ability and you slam me so hard I bounced a mile. Your post could have been made in a much more informative manner and not nearly so sarcastic. You are right, I mainly work with liquid and gaseous fuels. I have worked with solid fuel, but not very often. Usually it has been wood burning, never coal. I missed him saying coal. My bad. I'll try to refrain from posting in your areas of expertise Paulengr because obviously you are the master who is unable to teach.