Power Generation: Sustainability

[ea6mcu]

Hi,

We operate a power generation scheme that utilises landfill gas.  There are 4 engines installed at the plant - all G3516 SITA gen sets with Techject system.

The plant has been in for 11 years now.  When the plant was designed and built, there was no gas flow rate orifice plate installed on the main gas inlet to the plant.

Due to changes in the way the landfill site is licensed, the Environment Agency would now like us to have some way of measuring the gas flow rate (ie the gas extracted from the landfill measured in Nm3/hr).

I don't particularly want to dig up the gas compound to install a flow monitoring device.

Is there a way of calculating the gas flow rate from looking at the methane quality and engine loads?

Currently we are operating the plant at approx 45% Methane, 3 engines running at approx 95%.

Many thanks,

Michael

[KeyTech]

Hi Michael,

You have a few options, though if the EA are requiring gas flow records, you may need to bite the bullet and instal an independant 'Instromet' type meter to monitor the gas flow. Roughly speaking you will be consuming somewhere around 500M3/hr on a G3516, assuming this is a 1030eKW rated machine running at about 980eKW. This flow rate is greatly variable with gas quality, and machine type, and also the mixture that you are running at has a significant effect. I assume that you are running at around 500mg/m3 Nox.  Just out of interest, assuming you are using an EGS fuel system with your Tecjets, you can see the desired gas flow rate in Ltr/sec. This is reasonably accurate, and you can multiply this value up to give you an estimate of your fuel consumption in M3/hr. If the EA are requiring this data as part of your PPC permit, then you will probably need an independant meter, compensated for pressure and temperature.

Hope this helps

Regards,

Chris

[hiltnerj]

Hello,

     The answer to this question really depends on how accurate of a volume flow record is required.  If you are tracking the gas composition (heating value or methane concentration), you could potentially measure the airflow (simple insertion meter at compressor inlet) and use the exhaust O2 sensor to back out the fuel flow.  This will give you a flow rate that is +-10% of the actual fuel flow.  If you require a flow rate more accurate than this, you will need a fuel flow meter of some variety.  There are a number of relatively non-intrusive ways of doing this that probably wouldn't require any digging.

hiltnerj

[AZimen]

I have run into a similar Regulatory Agency for Southern California in the their rule 1110.2 where they require measurements of fuel usage to recorded over time.

The Tecjet valve is a fuel metering and measurement device for low pressure fuel systems.  It measures fuel temperature, pressure upstream and delta P, and valve position to determine fuel flow into the engine.  The engine controller tells the Tecjet how much fuel to put into the engine and the Tecjet opens and closes until that demand is met.

That being said getting the measure fuel flow by the Tecjet into the engine out of the Tecjet/Engine Controller can be tricky.  The Tecjet is usually driven over J1939 which means you could tie a J1939 monitoring device onto that bus and records fuel flow values. 

It really does depend on which controller and Tecjet you have, there are a couple generations of them out there and numerous combinations.  If I knew what your Tecjet and controller were I could tell you if using the Tecjet's onboard fuel measuring was possible.

Just shoot me an email if you want to discuss if its possible/appropriate to use your specific Tecjet for your permit.

Aaron.Zimenoff@Woodward.com

Sales Support Engineer

Woodward Governor

[zpsguy]

The engine itself is the best flow meter you are going to have.  See if the EA will allow you to use a conversion from kilowatts generated to BTU of landfill gas burned.  Your engine spec sheet will provide a LHV input and dividing by the full load output will provide a heat rate. If you operate at partial loads the spec sheets typically have 100%, 75%, and 50% values.  Landfill gas changes every day, so measuring the flow rate is only part of the picture.  You would also need a device to measure the btu content of the fuel.  These two devices together both have tolerances and I bet you in the end you will end up back calculating the BTU's burned based on KW production.  You most likely already have a watt transducer or can get an output from your multifunction relay or meter to provide you all the necessary data.

[Srivatsa]

Hi,

Using the Engine output is not a correct way of determining the gas consumption.. The Heat Rate or fuel consumption of any gas Engine is dependent on how it is tuned.. If you are running too lean then you would endue with a higher fuel consumption rate and hence for the same output you will consume more gas in cu M. The best way to do this is to use the data in the Techjet J1939 bus (i.e Delta P, temp of fuel) and use a flow meter software to calculate the flow.. The Techjet will constantly regulate the fuel into the Engine based on the programmed map and hence irrespective of the LHV of the gas you can measure the volume using DeltaP across the techjet and temperature..

Hope this will be of help...

Message Edited by woodag on 10-07-2009 09:19 AM

[operator]

In New Jersey I use a calculated formula to figure landfill gas flow to KW output. I also have a flow meter so I can check for differences between the calculated and actual flow meter readings. Since I have two engines, I have to add the calculated figures for each engine and compare them to the flow meter. There are a lot of factors such as engine tune, humidity baro pressure, air-fuel ratio that affects flow rates so the numbers between the actual flow meter and calculated formula's are never exactly the same but are close.

 My engines are old 399 turbo units but I use a 7618 btu to one hp per hour rating to figure the flows. This number is the number the N.J.D.E.P. uses on my air permit. I also use the 1000 btu per cu.ft. value for pure methane which is also the number the D.E.P. uses on my air permit. To determine the flow, first you need to know the landfill gas methane % so if you are running 45% ch4 you would have to multiply (.45 x 1000) which equals 450 btu's per cu.ft.

 Next you take the running load, say it is 800kw and multiply by 1.34 which is the kw to horsepower equivalent, (1.34 x 800 =1072hp per hour). Then multiply this figure by 7618 to get required btu's per hour, (1072 x 7618 = 8166496 btu's per hr @ 800kw).

 Now you have to convert the btu's required to run at 800kw from hours to minutes so you can then figure cfm which is in cu.ft. per min, so divide 8166496 by 60 mins in an hour.( 8166496/60 = 136108.27 btu's per minute).

 The last step is to take the 136108.27 and divide it by the btu's per cu.ft. of your landfill gas at 45% so (136108.27 / 450 = 302.46 cu.ft. per min).

 Your flow at 45% ch4 and 800kw should be 302 cu.ft. per min.  The math is in two parts

 (1)  figure landfill gas btu's.       45% of methane x 1000 =  450btu's

 (2) 800kw x 1.34 x 7618 divided by 60 divided by 450 = flow in cfm.

 This is the flow for one engine at 800kw and 45%ch4. You would need to calculate for each engine then add together. The flow will change according to changes in the gas and engine load.

 I'm not sure the required btu's per hp is the same for 399's as it is for 3516's. I would think the 3516's are more efficient but I have to run my 399's very lean to comply with the NJDEP.  These numbers are the numbers right out of the air compliance permit so they should be close. Also Fluid Components in CA. makes gas flow meters that compensate for temp, pressure changes in inlet gas to the plant. They also totalize the cfm for logging which the DEP may require.