02-10-2011 08:53 PM
I look after a couple of 3516B's with Woodward controls & have run them with fuel/gas as low as 43%Ch4 with out any problems.
I have heard about others in the company running as low as 39%Ch4.
Ideally I would rather see a minimum of 47%Ch4 as the engines run a little smoother.
02-11-2011 12:52 PM
A gas 3600 series engine is controlled by the engine ECM with the air fuel and emissions done by mapping in the personality module on ESS engines or the flash file in an ADEM3 engine. The Woodward control is what? A DSLC, EasyGen, LSM??? The lowest BTU operating range is done with a combination of orifice plates and mapping. I know of units operting on fuels with a CH4 percentage of near 40%, but these units had a number of operating issues, mainly starting and maintaining emissions.
Older engnes with carburator type fuel systems can typically be field modified to operate to lower BTU's than engines with ECM based controls mainly because a reasonably experienced technician can "play" with the fuel system and timing on an older engine to get it to operate on the lower BTU fuel. Get outside the operating range of an ECM controlled gas engine and it shuts down, and the field adjustablity of these engines are limited.
I know in Austrailia there is a company who has removed the CAT controls from G3500 and G3600 engines and installed their own controls to improve emissions and fuel tolerance, but don't have any first hand knowledge of their success.
In general, lower BTU fuel values require more ignition energy, so ignition system maintenance, plugs, extenders, primary wiring and ignition module is critical for good operation.
You will see a higher misfire rate on low BTU fuels, this will affect engine mechanical connections and generator coupling life. Also makes synchronizing and load control difficult.
Startability is a huge issue on low BTU fuels, large air reserves, turbine type starters, well operating jacket water heaters, and lube oil heaters all help this issue.
Emissions are harder to control on low BTU fuels, typically CO is higher due to higher misfire rate, and NOx output are usually more unstable.
The methane percentage by itself isn't the only issue, overall makeup of the fuel gas, level of impurities, operating requirements, emissions regulations, maintenance budgets and plant staff expertise all have an impact on succesfull operation on lower BTU fuels.
Hope that helps, Mike L
02-14-2011 02:27 PM
I believe we still have the highest concentration of G3600's in the world in our area, so we have significant experience with the engines and commissioning of engines. We have been fairly lucky in not having to deal with poor quality gas, but have experienced some challenging sites. As mentioned by other replies the other components of the gas is a concern if the CH4 is low, C02, silicates, H2S, etc.
There is some info on low BTU G3600's but is limited. I believe there a 27 or possibly more running through out the world and some testing done on digester gas and landfill. The digester and landfill experience common failures trying to run on contaminated fuel. We have seen challenges with a methane number of less than 50, but again volume can offset the low methane percentage to a certain extent, it was always the other components in low methane fuel we experienced challenges with. Usually anything less than 50% is only seen in landfill applications.
I m going to some digging in our history files/commissioning reports and see if we have any useful examples.
Lower Heating Values
Natural Pipeline Gas 825–1200
Digester Gas 600–825
Landfill Gas 450–600
02-28-2011 02:44 AM
Gas Engines in general are designed for natural gas applications. When we encounter low energy gasses (like Bio gas, Landfill or Coal mine methane) the Engines are adapted to run for these applications. The basis of the system design is the Low heating value of the gas. The basic combustion principle is the KW input energy needed for the Engine for the combustion cycle. Typically a gas Engine will be 39 - 44 % Efficient, depending on the configuration, and hence 2.5 times the output is the approximane heat input is required to deliver the rated output. This heat input is directly related to the Low heat Value of the fuel. If the LHV of the fuel drops due to lowering of Methane content more volume of gas is needed to maitain the output. If more fuel is needed then more air is also needed so the Turbo charger system and the Fuel system will need to be resized. Also there is a limit on the Methane to air ratio and conbustibility limits. If the mixure is too lean the flame will not probogate inside the cylinder and combustion will not be complete. These are the factors that limit the min fuel LHV useable in an Engine and will vary for each model and configuration. We have G3520C Engines configured to run as low as 30% Methane for Coal mine Apllications. For a 3616 Natural gas Engine the typical design will be based on 34.1 MJ LHV fuel without modification. If used for Low LHV gasses it will depend on the Turbo Charger configuration and the Fuel system sizing. With a standard woodward system I presume it is a standard Natural gas Engine. Anothe important issue to note for natural gas is that if the Methane content is lower the other heavier hydrocarbons will increase. This will cause the LHV to increase not decrease and the fuel can cause detonation.
Hope this will be useful..
02-28-2011 03:12 AM
If it is G3516+ that you are talking about they have Woodward Techjet Air fuel ratio controls fitted after market. These units are mapped at site by Woodward.. If my memory serves me right the Min methane % is 50%. The Fuel system is sized for around 19 - 20 MJ min fuel LHV .. You can remap the Woodward Techjet to provide lower Methane Concentrations, but the range is limited. Based on the basic configuration of the G3516+ model there is not much room th go lower on the Methane content. The absolute limit would be around 45% with a lot of tuning and this would not be a very stable operation. A G3520C Engine can provide you better performance for lower Methane contents and lower LHV fuels of as low as 30%. Hope this will be useful..
05-19-2011 03:51 PM
I had experience with three 3616 gensets at 2 different sites in California. As mentioned below once you start increasing the air/fuel ratio the NOx tends to increase and you put yourself in a dangerous position with O2. I lost several exhaust valves due to the increased (spiking) O2 levels. In addition synchronizing and hunting are big issues for these largest of LFG machines.
Usually the Woodward unit (DSLC) does a pretty good maintain the load set point until you start operating outside of its deadband.