Power Generation: Sustainability

[operator]

Hi,

 I run a old landfill gas to power facility using the older 399 turbo engines which are low pressure units. The gas always had high silicon concentrations. Since it is a smaller site, 1mw, my company tells me that it is more cost effective to change heads and turbo's rather than installing a gas cleaning system. The silicon creates a lot of other problems such as backfiring in the aftercoolers and detonation (ping ) and the only way I could run is very lean air-fuel mixtures. When the landfill was new, I was seeing silicon ppm around 800 in my oil samples after only 1500 hours. Now its down to 120 ppm but I'm still having some problems with valve burning and always a problem with aftercooler backfire and fire when running under 6.5% exhaust O2.

 I know it's probably been asked before but has Cat done any research on supercharging rather than turbo charging landfill gas engines? The inlet gas stream is always cleaner than the exhaust and the ash is like crushed stone that gets into the turbo bearings and causes more oil usage resulting in more ping and higher CO levels. The ash also coats the turbo wheels then chips off after shut down cooling causing the turbo's to become out of balance. The biggest problem is the exhaust manifold running back pressures that always exceeds the intake manifold boost pressure and the leaner I run the higher this delta P will become. During stack tests we have to run 398 nozzel rings in our turbo's to keep boost high enough to permit 600kw loads at air-fuel ratio's around 7.2% exhaust O2. The boost is running at 15psi at the intake manifold and everything needs to be very clean but at this pressure the exhaust manifold back pressure is around 20psi and it actually is blowing back into the intake manifold when the intake valve opens. As the turbo's foul from ash, things get worse and detonation / ping starts to occur than backfiring into the intake and igniting the aftercooler. The engine will not burn valves, detonate or backfire while running at 15psi boost and air-fuels at 7.2-7.5% exhaust o2 even at engine loads of 650kw sustained.  It will do all of the above if the mixture is richened below 6% exhaust O2 and at low boost and low engine loads of 300kw. Running this lean and with this high a boost pressure is the only way we can keep CO and NOX levels down to state permit requirements. I do have to run automotive high performance 3/4" reach iridium spark plugs during the test to keep hydrocarbons within permit levels.

 My point is that even though the engine is not taxed with a crankshaft load of spinning a supercharger, it is taxed with the high exhaust backpressure each time the pistons try to push out the exhaust in the cylinders. Even if these pressures equal or are slightly lower than intake manifold boost pressure, I would think it would require the same engine power weather its turbo charged or supercharged. The supercharger would require less overall maintanence and have better reliability under dirty fuel conditions. The engine would keep breathing good because of unrestricted exhaust and this is essential on landfill gas engines. Even if it would take a little more engine power to run the supercharger vs a turbo it would be still a plus. A damper valve could be installed in the exhaust stack to initiate and regulate some exhaust back pressure if needed.

[emtechalaska]

I know of a few operators that have been getting some impressive results on their 399's "also 3400 & 3500 series"  by using Emission Technologies "Ultraburn" www.emissiontech.com  which is technology of introducing a catalyst of platinum,rhodium and rhenium salts in vapor form "ppb" into air intake.   The catalysts, will work with all hydrocarbon based fuels and  react  for a more complete combustion of the available fuel.  This process extracts more heat energy out of combustion process and buy doing so emissions are reduced along with fuel consumption.  This is basically the same technology used in a catalytic converter, except now you are using the increased energy during the power stroke instead of wasting it in exhaust downstream.  This product has brought some older 399's into meeting their current emission requirements without the need of replacement.  It is the only product I know of that reduces soot and NOx at the same time.  Engine TBO life cycles have reported increases by as much as 50%.  Company does do  a certain amount of "no initial cost, return on investment analysis" for clients.  You can contact Ray Emig  ray@emtechnw.com for more info.

                                                                        Don

[mikel]

CAT had a supercharged engine many years ago, a D397.  Supercharging may provide relief for some of your issues, but let's be realistic here, untreated landfill gas in ANY engine is asking for trouble.  If you had a roots blower instead of a pair of turbo's, do you really think it would live much longer in that situation?  Look at some of the  blower issues on the 71 and 149 series DDA engines when used in dirty environments, like in drill rigs.  Unless you belt drive it the gear drive system would likely be pretty complex and costly.

Your gas contaminents are causing your problems, Siloxanes are always around to some degree in bio-fuel gases.  It's not the silicone compounds themselves that cause the problems, it's that these compounds allow other contaminnents, like oil ash, organic debris, carbon residue, and salts to combine together and form deposits.  Deposits in aftercoolers typically are the result of trying to compress lots of air, raising the compressor outlet temperatures, and them running them into a charge air cooler, as the charge air and fuel mixture cools back down, the crud condenses outlet, usually in the lower third on the cooler core.  Older style engines with the carb mixer downstream of the charge air cooler had fewer problems with cooler core plugging, but need higher fuel gas supply pressures to overcome boost and get the fuel gas into the engine.

Backfires are typically caused by deposits on the spark plugs, causing either a misfire or preignition due to glow effect.  The "pinging" is either preignition or detonation caused by excessive deposit levels increasing cylinder compression and lowering detonation margin.

CAT, several landfill operators, and other engine manufacturers have tried lots of things, most productive thing?  Clean up the gas.  Even reliatively simple solutions like carbon (activated charcol) filters and well designed moisture removal system can have pretty good results.  I know of a few sites that used fuel gas chillers with positive results.

The IMPCO carburators you are likely using have poor ability to maintain lean air fuel ratios across large portions of the operating range, also to run lean you need excess air, supercharger sounds like a great idea, but the engine will have other problems running below half load, like poor combustion performance, oil leakage around the valve guides causing plug and cylinder fouling, and poor control of air/fuel ratio.

"The boost is running at 15psi at the intake manifold and everything needs to be very clean but at this pressure the exhaust manifold back pressure is around 20psi and it actually is blowing back into the intake manifold when the intake valve opens". Unless you have an aftermarket cam I'm not sure how this can happen, the is no scavange overlap in a G300 series camshaft.  The pressure in the exhaust manifold is due to a combination of exhaust system restriction and load on the turbo compressor wheel.  The max allowable exhaust system restriction on these engines is 27"H2O, what is your restriction?

By running "extra lean" you compound some of your problems, the air demand goes up, requiring more air from the turbo compressor, which causes teh compressor outlet temp to go up, so when you cool it back down you condense lots of crud out of the fuel, if you aftercooler system is marginal, you may not cool it down enough, further decreasing detonation margin.  Also as you lean the engine out, the energy demand on your ignition system increases, leading to shortened spark plug and transformer life, which can lead to more misfire.

Old 6.25 bore engines are beasts, tough, durable and simple. But keep throwing heads and turbo's on them, and the holes in the block start to wear out, the head surfaces aren't flat anymore, you likely don't keep liners in the condition they need to be (I'd bet most of your cylinders are bore polished), the mounting surfaces on the hot side of the turbo's are likely all warped, unless you change the flower pot and manifold turbo adaptors everytime. To top it off, the 6.25 bore gas engine has been out production for quite some time, how much longer you think you'll be able to get parts?

My two cents worth, Mike L.

[CrankMyCat]

As far as getting parts for the old 6.25 bore engines, I know that CAT Classic has begun making some of the engine parts, but I don't know if they will be making pieces like manifold segments or flower pots (really expensive pieces).  Also, a dirty environment is a dirty environment.  I would stick to your original engine equipment and spend that money on a cleaner running environment... ie. filters, scrubbers, etc. My two cents.

-caleb

[operator]

You are all correct when you say keeping the gas clean is the best solution. Still I believe supercharging may be a better way to go. The intake is much cleaner before the combustion process so I would think components and bearings would last longer and stay less restrictive.

 My old 399 engines are not original cat setups. I'm running a low pressure gas system and aftermarket air to gas aftercoolers. The aftercoolers show some pressure loss at high boost levels and do not in my opinion cool the air fuel mix enough. I don't have any problems with dirt clogging as the pressure loss has been consistent for the last 20 years and is around 1lb at 15psi boost and full load of 600kw. The 600vfi impco mixers I'm using have been modified so that they track the exhaust o2 evenly through the load adjustment. I had to increase pipe size from the regulators to the mixers to 3" and also modify the gas regulator to increase pressure as the air flow thru the mixer increased. I did this by using the vac signal ( between turbo and mixer ) against the sensing gas pressure to control the regulator. In other words, the vac on the mixer is used to trick the gas regulator to increase gas pressure to the mixers at high flow rates. By doing this, it overcomes the common senerio of the air-fuel leaning out as the throttle opens and load increases. I have seen this problem with all Impco mixers including the3516's running on pipeline natural gas. As the throttle opens the mixture gets leaner and as the throttles close, the mixture gets richer. I found that this modification allows me to start the engine and bring it up to full load without any adjustment of the gas regulators. It also somewhat compensates for landfill gas changes and ambient temp changes in load keeping the air-fuel ratio more constant. I use a needle valve in the vac /pressure sensing line to control the air-fuel curve and had to remove the fixed  bullets and orfices in the mixers to help eliminate the lean-out and pressure drop as load increases. This system works great for me and holds air-fuel ratio and load much more at a constant.

 The exhaust back pressure on these 399's running at 7.0% exhaust o2 is excessive. I know this is not what the engine was designed for but the siloxane makes the problem at lot worse. At 15psi manifold boost needed to keep load at 600kw and 7.0 ex o2 which is needed to keep Co and NOX in state limits, the exhaust back pressure is over 20lbs in the exhaust manifold. There is enough exhaust pressure left in the cylinders after the exhaust stroke to blow back into the intake manifold as the intake valve opens. I see this as a rapid fluctuating spike in the intake manifold pressure each time the intake valve opens that disappears when the load and backpressure drops. I feel that at this point increasing boost to increase load is just chasing our tails and keeping CO down becomes more difficult since we are refilling cylinders with spent exhaust gas and CO.  Leaning out drops all levels of emissions except hydrocarbons but with good ignition even hydrocarbons are keep low and within state compliance.

 What I've found is that richening air-fuel ratio's increases detonation, ping and intake backfiring. Richer mixtures also increase valve burning. I have never burned a valve, backfired or seen any ping even on a engine with dirty cylinders running at 7.5% exhaust o2 and 650 sustained kw for 2 days. The engine stays in state emission compliance ( except for particulate ) even with dirty heads that have over 12,000 hours on them. The turbo's, nozzle rings and wastegates have to be very clean and in good condition so the engine can breath at 15lbs boost. When the turbo's start to foul with ash, exhaust back pressure goes up and load decreases even though the boost pressure is maintained. The engine will start pinging and almost immediately backfire usually taking out an exhaust valve when it backfires. A clean engine will handle a rich mixer without any problem but it will still need to run very lean with high boost to maintain load to pass state emissions. Clean gas is the best cure but still more needs to be done on engine breathing. Landfill gas cleaning systems still leave some dirt behind and unlike desel engine soot, Landfill gas silicone is dry and abrasive which leads to turbo un-balance and bearing failure and more cost/maintanence.