I have built a few of these - lets have a look!
[The gas and pipework characteristics are -
Biogas - 60% CH4, 40% CO2, traces of NOX, SOX & HCL]
OK - the gas analysis is impossible. The CH4 and CO2 are OK for a normally operating digester. But it is An-aerobic and without air there is no oxygen available for the byproducts NOx, SOx and I doubt the Hydrochloric acid. My guess is that if the digester is operating normally the sulphur will appear as H2S, hydrogen sulphide and the only possible other trace gas will be NH4, ammonium. Since this is soluble in water it will be removed in condensate. Good anaerobic digester gas. If the digester is overloaded, the CO2 will increase to 60% and the boiler will be hungry for the methane fuel component, now only 40%. This is important to know later for the setting up of the flow meter.
Temperature variability from digester temperature to just above freezing is normal.
[The pressure at source is about 20 mbar and ideally the same at consumption. ] Hmmm - no allowance for pressure drop due to gas velocity. We can look at this later.
[The pipework is 100m total length has 2 90 degree bends, the diameter changes after each bend it goes from 200mm to 300mm to 250mm.] Each bend is worth about a couple of meters of pipe in gas flow resistance. The valves a bit more.
[ Theres a condensate trap at the end of the final leg of pipe.] Excellent!
[ The whole pipeline falls consistently at 2 degrees.] If this gets any better I am going to throw a party!
[ There are 2 gate and 2 butterfly valves on the system. Theres 1 gas flow meter and 2 gas pressure switches on the system. The pressure switches dont protrude into the pipeline they have their own 2" take off legs but the gas flow meter is an endress & hauser t mass and im not sure how it fits into the pipe.] Cool, lets party! The thermal mass meter is normally a hot wire anemometer located on the centerline of the pipe out of the condensate running down the pipe - top entry required normally.
[ Please can someone tell me how to work out the pressure drop and maximum velocities with some working out. Im interested to know what happens if we increase the pressure at source along the system.] OK lets take it one step at a time.
[The system is set up to supply two gas engines running big alternators producing electricity. At the same time as electric production they heat water that is used to heat 2 anaerobic digesters fed with waste to around 35C which is its optimal for biogas production. The digesters are both large covered tanks with 5000m3 capacity and 10m Diameter. The pressure in the system is produced by the digestion process as the gas is released from the sludge mixture held within.
The roofs of the digesters are floating type so they rise and fall to collect gas from the system.]
This is conventional. The rising dome digesters give you a constant pressure at source - any excess pressure causes them to move upwards and reduce the pressure until the upward force of the gas equals the mass of the dome.
[The engines are sized at 600kWe each and theyre about 34% electrically efficient.] That is about right for modern engines.
[The biogas has a CV of 35.8mj/m3 ] OH NO IT DOESN'T!!
The calorific value of biogas as described is between 18 and 22 mJ/m3. There are huge problems in obtaining a practical CV for biogas. These relate to moisture content, altitude, but most of all to the amount of methane fuel in the gas. The methane percentage varies depending on the loading rate of the digester and its previous history. Biological related changes give Methane % as 60 plus or minus 20%. Also methane contains hydrogen and this oxidises to water. The H2S oxidised in the combustion chamber of the engine = Sulphuric Acid to which is added the water from combustion of the hydrogen. This acidic exhaust water eats engines. Therefore in biogas engines you cannot use the higher calorific value of the methane, only the lower calorific value of the methane which allows for a loss of energy due to not condensing the exhaust of the engines. The exhaust should be say 280 deg C, but check with the engine manufacturer or loose the engines in a year or less.
Get the digester system settled down and you will obtain 1.74kwh of electricity from one cubic meter of biogas plus 2kwh of hot water for the heat exchangers.
To supply the whole system of 2 engines x 600kWe you will need 689 m3 biogas per hour - twice what you are proposing. The error due to gas quality would be expected to give a working range of about 500 to 900 m3 per hour.
[I have a new boiler on the system using the gas aswell and when the boiler is running the engines are starved of gas. I know they are because their output goes down and bounces around.]
OK so if your digesters are producing 10,000 to 15,000 m3 of gas a day, then the engines will run for 14 to 21 hours a day.
[So I suppose what im trying to work out is what happens if I fluctuate the pressure at the top of the system or if I start to draw more than can be supplied down the pipeline.]
Since the digesters are constant pressure devices, if you draw off more than the digester produces the gas flames go out in the boiler and the engines starve of fuel, condense as they are running sub - optimally and they corrode and break down.
[I assume there must be a maximum that can be drawn down a leg at a certain diameter and pressure. A bit like if you try and breath only through your mouth with a straw and you get out of breath because you cant get enough down the straw.] This is not a problem, I have not done the calculations on my Mears slide rule but 250mm is a 0.05m2 pipe and that gives a velocity in a clean pipe of:
700m3 / (60x60) x (0.05) = 4 m/sec. That is just about ideal if the pipe is clean. It never is though, condensation on the bottom, foam around the edges etc, but I don't think this is your problem. The big problem is the engines are most likely oversized for the digesters unless these are being fed 7% total solids sludge low in Surplus Activated Sludge. Operating on thicker sludge will increase gas flow and reduce the amount of heat required to heat the digester feedstock.
Calibrate the flow meter on air in the factory and use this as a guide for measuring gas flow. The density of the biogas will oscillate around the density of air as the CO2 level varies. Since you cannot predict the percentage to calibrate on, then all the instruments will be calibrated with the same error if they are calibrated against air. Its cheaper and reproducible.
Turn off one of the engines and keep it warm with warm water in the heating jacket. The duty engine can be changed over once a day. Since one engine should run two digesters you should be able to turn off the boiler. If you cannot then look at the total solids concentration into the digester and reduce the amount of water you have to heat up each day. Ensure the hot water to the sludge heat exchangers does not exceed 70deg.C in the heat exchanger in contact with the sludge. This is to prevent bacteria dying on the surface of the heat exchangers. If this happens, the efficiency of the heat exchangers will reduce and you will need to supplement the heat from the engines with biogas boiler heat to maintain the digester temperature.
Run the engines at 100% capacity even 1% harder to maintain overall efficiency (speak to the manufacturer first). Then if you have spare gas, turn on two engines during the day time only when electricity is most valuable.
I hope this is of some help.
Best Regards
Les. Gornall
